Optimizing Central Nervous System Drug Development Using Molecular Imaging RJ Hargreaves1, J Hoppin2, J Sevigny1, S Patel1, P Chiao1, M Klimas3 and A Verma1 Advances in multimodality fusion imaging technologies promise to accelerate the understanding of the systems biology of disease and help in the development of new therapeutics. The use of molecular imaging biomarkers has been proven to shorten cycle times for central nervous system (CNS) drug development and thereby increase the efficiency and return on investment from research. Imaging biomarkers can be used to help select the molecules, doses, and patients most likely to test therapeutic hypotheses by stopping those that have little chance of success and accelerating those with potential to achieve beneficial clinical outcomes. CNS imaging biomarkers have the potential to drive new medical care practices for patients in the latent phases of progressive neurodegenerative disorders by enabling the detection, preventative treatment, and tracking of disease in a paradigm shift from today’s approaches that have to see the overt symptoms of disease before treating it. There is a huge unmet medical need for novel therapeutics that address the growing epidemics of chronic and complex illnesses especially neurological disorders and neurodegeneration associated with aging. Today, there is a paucity of disease modifying therapeutics for central nervous system disorders and many are served only by medicines that treat some of the symptoms of disease. For novel symptomatic treatments, the challenge is to demonstrate increased therapeutic benefits and differentiate from current standards of care, whereas for novel disease modifying agents, the demonstration of sustained efficacy in what are inevitably long clinical trials seems an even higher hurdle to clear. The costs of drug development have spiraled ever upward and clinical development times have increased significantly driving the need to rethink and reinvent the drug development process, especially in early development. Today, over $2 billion are spent on each new drug application 75% of which is lost on failure. Sadly, only 3 in 10 new products ever generate revenues that cover the average industry research and development costs. Overall, nearly 90% of molecules that enter the clinic fail, many ultimately for a lack of efficacy or failure to improve over current standards of care. The traditional approach to drug development, in which efficacy is first assessed in patients at phase IIA after extensive phase 1 testing in healthy human volunteers can substantially delay the first read on efficacy to more than two years after entry to humans.1,2 Given the high costs to reach phase II trials and the low phase II success rates for drugs with novel unvalidated mechanisms of action, deferring proof-of-concept to
late stage clinical trials is financially untenable. Obtaining a satisfactory return on research investments is essential if we are to continue to progress innovative drug discovery portfolios that can significantly improve clinical care and address unmet medical needs. Mervyn Turner, the former Chief Strategy Officer and Head of External Scientific Affairs at Merck and Co, commented3 that “. . .for a long time the biopharma industry felt that the old model was going to deliver. We kept expecting the arrival of a blockbuster drug. Over time, because the failure rate and costs of drug development were so high, it has really forced the recognition that (the current situation) represents something more fundamental than just another cycle.” Indeed, today, the blockbuster model has particularly high costs when there is late stage failure because of lack of efficacy as the clinical trials often require large patient populations to realize high sales potential. In mature therapeutic areas, termination for economics or market-related factors becomes increasingly likely if new product candidates do not differentiate meaningfully from established standards of care. The blockbuster model may be broken but it is clear that many pharmaceutical companies need “blockbuster” products to sustain them while on the rollercoaster ride of drug discovery and development. The question is then how can this be achieved? To date, the pharmaceutical industry has largely failed to improve performance or deliver a higher return on research investments despite the gymnastics of continual internal reorganizations, right sizing exercises, and perennial mergers and
1 Biogen, Cambridge, Massachusetts, USA; 2inviCRO, LLC, Boston, Massachusetts, USA; 3Merck Research Laboratories, West Point, Pennsylvania, USA. Correspondence: R Hargreaves ([email protected])
Received 12 March 2015; accepted 7 April 2015; advance online publication 13 April 2015. doi:10.1002/cpt.132 CLINICAL PHARMACOLOGY & THERAPEUTICS | VOLUME 98 NUMBER 1 | JULY 2015
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Figure 1 Potential uses of imaging at various phases in the drug discovery and development process.
acquisitions. The problem statement was framed by the Economist Intelligence Unit in 2011, “How do we re-invent how to innovate rather than just cut costs?” Central to their proposed solution was looking beyond internal research and development silos through open innovation and the integration of new tools and technologies to drive a mind-set that targets efficient early development decision-making as a metric of success.3 Indeed, Paul Matthews, the former Head of Imaging at GlaxoSmithKline, commented that “A fundamentally optimistic future scenario for the drug industry is to make drug discovery more successful by being smarter in our early “Go/No Go” decision-making through the disciplined use of biomarkers.4 Emerging drug candidates need to differentiate from existing standards of care to advance therapy or address new hypotheses for treatment of disease. Novel unprecedented mechanisms are often the means to reach these goals but carry a greater inherent risk of failure. De-risking strategies for early decision-making are therefore critical to cost-effective drug development and to sustaining the willingness to invest in research to produce truly innovative new medicines. Development of drugs for novel targets requires informative laboratory to clinic translational biomarkers to guide dosing in early experimental medicine proof of biology and clinical proofof-concept tests to drive early decision-making. The conundrum is, of course, that novel drug targets rarely have validated biomarkers—early innovation is inherently inversely proportional to validation. The use of a biomarker strategy poses disproportionate degrees of development rather than regulatory risk, dependent on when the biomarkers are used for making decisions. Preinvestment is required to ensure biomarkers are available at the right time, with the right fit-for-purpose level of analytical validation and clinical qualification. Biomarkers that reveal themselves too late have no significant impact on decisions and waste resources. The consistent use of biomarkers across drug 48
development portfolios has the promise to reinvigorate the drug discovery process through cost-effective decision making and improved success rates that help bring useful medicines to patients sooner.5 This review will discuss the value of using imaging technologies to generate biomarkers that can improve the efficiency of the CNS drug development process. The examples we have used, while focused on nuclear imaging and CNS drug development, illustrate guiding principles for the use of imaging biomarkers across therapeutic areas using the diverse preclinical and clinical imaging modalities available to scientists, clinical pharmacologists, and physicians today. MOLECULAR IMAGING—ADDING VALUE TO DRUG DISCOVERY AND DEVELOPMENT
The potential uses of imaging across the drug discovery and development spectrum are summarized in Figure 1. Imaging has distinct but complementary roles at different points in the drug discovery and development timeline with the initial goal of selecting the best molecule and dose to test the hypothesized therapeutic biology through to human dose selection and proof of clinical concept. Imaging can also be used to support patient selection and disease progression monitoring in late phase clinical trials and, if successful, after registration throughout the lifecycle of the drug.6,7 Enabling clinical proof-of-concept testing and dose selection
In the laboratory, molecular imaging can help validate therapeutic targets in models of disease and symptomatology by linking exposure to the degree of target engagement and the time-ontarget to preclinical measures of efficacy. Focusing research on those drug candidates that achieve the highest target engagement from the lowest exposure is critical to molecule selection as this maximizes the potential therapeutic index moving into clinical VOLUME 98 NUMBER 1 | JULY 2015 | www.wileyonlinelibrary/cpt
trials. Vertically integrated research strategies that encompass animal imaging models that are aligned with early experimental imaging paradigms in humans, and in turn guide subsequent clinical studies in patients, are becoming core translational research approaches today. To be optimally effective, simultaneous discovery efforts in medicinal chemistry and radiochemistry are required to ensure radiotracers reach the clinic at the same time as drug candidates. Imaging tracers that are ready for first-in-human studies enable the design of clinical protocols that can weave together imaging assessments of target engagement with first-in-human single or multiple dose safety and tolerability testing to define therapeutic windows for efficacy testing. In early clinical development, molecular imaging can be used to link target engagement to drug-induced biological changes that are expected to produce clinical benefit, so-called proof of mechanism or activity testing. Proof-of-concept can be declared when target engagement can be linked to a change in a clinically meaningful imaging endpoint. If a drug has adequate target engagement but does not produce the expected biological or clinical effects, the therapeutic concept is flawed and development can be stopped. In the later stages of drug development, imaging can also be used to define the relationship between drug dose/exposure and target engagement to enable dose selection for phase II clinical trials. Improving the efficiency of drug development
There is a growing interest in patient-specific, disease-specific, and outcome-specific imaging biomarkers that are independent of specific therapeutic mechanisms and molecules. Molecular imaging approaches that can be used to stratify patients for clinical trials will enable enrichment of clinical proof-of-concept studies potentially leading to shorter, smaller, and more definitive clinical trials. Stratification using molecular imaging may not only improve the drug development process but ultimately it could drive personalized precision medicine approaches to therapy, delivering the right drug at the right dose to the right patient. Many sporadic neurodegenerative diseases, such as Alzheimer’s dementia and Parkinson disease and amyotrophic lateral sclerosis (ALS), are being reclassified for drug development efforts into pathophysiology-based endophenotypes. The emergence of molecular imaging tracers to visualize tissues in life that previously could have been studied only at autopsy and thereby measure these molecular endophenotypes in vivo is one of the most exciting recent applications in neuroimaging biomarkers and is fundamentally changing drug development approaches to these diseases. The ability to observe neurodegenerative processes evolve over time and the correlation of these imaging measures with classical pathological evaluations is generating new unifying hypotheses that are being tested in current drug development efforts. One prominent theory proposes a time and age-dependent aggregation and spread of proteins such as beta-amyloid, tau, alpha-synuclein, superoxide dismutase 1, and TDP-43 along circuits of interconnecting cells within the brain. The abnormal build up of these proteins within brain disease pathways can CLINICAL PHARMACOLOGY & THERAPEUTICS | VOLUME 98 NUMBER 1 | JULY 2015
injure vulnerable CNS neurons that in turn leads to progressively worsening brain function. The various spreading proteinopathies can be accompanied by microgliosis and astrocytosis and each involves dysfunction in distinct cognitive and motor circuits that produces their characteristic regional pathology and pattern of the clinical presentation of disease. Because the build up of aggregated proteins and dysfunction of brain circuits starts to occur well in advance of clinical demise, functional imaging alongside molecular imaging offers additional hope for early evaluation of potential disease-modifying drugs and tracking the progression of these phenomena (Figure 2). MOLECULAR IMAGING TECHNOLOGIES
Improvements in imaging hardware and software over the past 15 years have bridged the gap between subcellular and wholebody imaging across a significant majority of the electromagnetic spectrum. This ever evolving molecular imaging toolbox (Figure 3) has enabled biodistribution studies and imaging translational biomarker endpoints to play an increasingly important role in the preclinical and clinical assessment of molecules, mechanisms, and therapeutic hypotheses.8 Molecular imaging uses a variety of imaging technologies and specialized imaging agents to visualize, characterize, and quantify anatomic structures and biological processes at the subcellular, cellular, and organ levels in a physiologically relevant tissue context in vivo with high spatial and biochemical resolution,8 often without the need for tissue dissection and processing. Each technology generates images in different ways and each has its strengths and weaknesses in terms of spatial resolution, sensitivity, and imaging probe characteristics (Figure 3a–d). All medical imaging modalities support some form of molecular imaging application. However, for contrast media or even “tagged” nanoparticles, direct detection of radionuclide and fluorophore labeled compounds continues to place nuclear and optical imaging as the primary biodistribution imaging modalities in drug discovery and development. A broad array of preclinical in vivo and ex vivo imaging systems9 is available commercially and these technologies continue to evolve. There are an estimated 500 nuclear micro-positron emission tomography (PET) and microsingle photon emission computed tomography (SPECT) compared to 2,000 commercial optical (bioluminescence and fluorescence) preclinical in vivo imaging systems in use today (data from the inviCRO Company database). The wide availability of optically active bioluminescent and fluorescent markers that can be genetically engineered offers many ways to image preclinically fundamental in vivo biological processes, such as gene expression, biochemical pathway activation, protein–protein interactions, cell trafficking, proliferation, transformation, and death. Moreover, the progressive miniaturization of optical imaging technologies to produce micro-optical coherence tomography10 and micro-confocal microscopy11 now promises to deliver in vivo histopathology in real-time that can be utilized in biomarker and biodistribution studies in drug discovery and development programs.12 To date, optical molecular imaging has its greatest impact in preclinical animal models of disease, given limitations in the depth of imaging that can be 49
Figure 2 Emerging science of neurodegenerative diseases. (a) Envisioned common pathogenesis steps beginning with (1) vulnerable neurons, which (2) succumb to age-driven accumulated stresses that result in synaptic dysfunction (shaded circles). Propagation of synaptic dysfunction (3) and cell death via excitotoxicity and spreading of intracellular and extracellular proteinopathy is envisioned to occur locally as well as along neural circuits with accompanying microgliosis and astrocytosis. (b) The involvement of specific neuronal circuits by spreading pathology is believed to account for the characteristic clinical manifestations of specific neurodegenerative diseases, such as dementia and motor problems. (c) Pathological changes are believed to occur well in advance of brain dysfunction and clinical symptoms and may be tractable by imaging and fluid biomarkers allowing for preventative therapy.
achieved.13 The recent development of clinical photoacoustic imaging14,15 promises, however, to increase the role of fluorescence imaging clinically that could, in turn, be useful in the drug development setting. Optical imaging currently transitions to the clinic largely using nuclear imaging modalities, such as PET and SPECT16 to visualize and characterize biological processes spatially and temporally at the molecular level in normal and pathophysiological conditions using various types of exogenously applied radioactive imaging probes. The most commonly used molecular imaging techniques visualize radiolabeled probes or radiotracers interacting with protein targets within or on the surface of cells. Radiotracers are versatile and sensitive, and can be designed to track the drug itself, image the drug target, or monitor key biochemical and physiological processes. Novel molecular tracer probes are the only way to quantitatively measure receptor populations and pharmacology at picomolar to nanomolar concentrations in vitro, ex vivo, and in vivo in both animals and humans (Figure 3), thus providing a unique bridge between the laboratory and the clinic. In vitro and ex vivo autoradiographic tissue imaging techniques using molecular probes labeled with beta emitting isotopes 14C, 3 H, or 35S are often used to map drug targets, pathology, and the 50
biodistribution of drug molecules into organs and tissues at high resolution as a prelude to studies in vivo. Tissue section autoradiography can also be used to screen potential PET imaging agents in the absence of cyclotron-derived isotopes.17,18 Preclinically and clinically in vivo, the key molecular imaging modalities are PET, which uses tracers labeled with positron-emitting radionuclides (most commonly 11C and 18F), and SPECT, which detects tracers labeled with gamma-emitting radioactive isotopes (e.g., 123I and 99mTc). Both PET and SPECT can be used to track small molecule and biologic therapeutics and PET and SPECT ligands can be used in vitro and ex vivo in autoradiographic studies. The use of structural imaging modalities, such as computed tomography (CT) and magnetic resonance imaging (MRI) in combination with PET or SPECT, enables precise spatial and anatomic localization of molecular activity. Moreover, the use of CT alongside nuclear imaging allows density assessment of tissues, such as the skull, thus allowing for attenuation correction and more accurate quantification of signal at a tissue source, such as the brain. Advances in the development of small animal tomographic cameras (microPET or microSPECT combined with CT) have facilitated translational bridging between preclinical and clinical research. SPECT has a greater number of radionuclides available for incorporation into imaging probes. SPECT has similar VOLUME 98 NUMBER 1 | JULY 2015 | www.wileyonlinelibrary/cpt
Figure 3 (a) A general sensitivity survey of in vivo molecular imaging techniques as a function of interrogation scale from subcellular to whole-body in humans. (b) An evaluation of the volumetric resolution for fluorescent imaging technologies, yielding a linear trend for resolution as a function of mass of tissue interrogated. The cluster of technologies in the upper right-hand corner correspond to in vivo “macro”-fluorescence technologies labeled “Fluorescence” in panel a. The cluster in the lower left-hand corner is comprised of both in vivo and ex vivo fluorescent microscopy technologies. The in vivo fluorescent microscopy techniques are captured in panel a under the heading “Intravital Microscopy.” (c) A translational nuclear imaging resolution plot from subcellular microautoradiography to whole-body SPECT imaging. Note the qualitative linear progression of volumetric resolution as a function of subject mass. (d) Standard injected radioactive dose per subject mass as a function of imaging technique and species. Note that preclinical techniques maintain the linear evolution of resolution across species, but require increased amounts of radioactivity because of a drop in sensitivity per volume in PET and SPECT.
sensitivity to PET and has had far wider adoption in clinical practice, but its use in drug development has fallen behind PET in North America and Western Europe because of its lower image resolution. Surveys of the clinical trials data base Clintrials.gov show clearly that nuclear imaging modalities, especially PET, have become powerful tools for drug discovery and development, with more than 250 studies per year being initiated by the industry in the 2008–2012 timeframe (Figure 4a) particularly in the neurosciences because of the inaccessibility of the brain and the general lack of CNS-specific markers to support accurate drug dosing and in oncology in which imaging is integral to clinical practice and the evaluation of response. The trade-offs between SPECT and PET are presented in Figure 4b. Recent developments in SPECT camera engineering and design have made SPECT technology more competitive with PET. SPECT can be viewed as more versatile in that it can image a wider range of long-lived radioisotopes to track the pharmacodynamics of biological drugs, such as antibodies and antisense molecules. Figure 4c illustrates the recent gains in resolution offered by multipinhole animal SPECT-CT systems, such as the CLINICAL PHARMACOLOGY & THERAPEUTICS | VOLUME 98 NUMBER 1 | JULY 2015
nanoSPECT. The images represent 99mTc-methylene diphosphonate bone scans in a mouse and a squirrel monkey. Each pair of images represents the same animal imaged first in older technology SPECT cameras and then in the nanoSPECT. The gain in resolution is made obvious by the ability to see single vertebrae, ribs, and teeth with the newer technology but not with the older technology. PET isotopes, while named for the primary nuclear particle emission, are ultimately tracked via the gamma emission generated through the collision of emitted positrons with electrons. These positron-derived gamma emissions have a characteristic emission energy signature common to all PET isotopes. SPECT isotopes, however, which emit gamma radiation directly, can have unique and distinct gamma emissions. These facts allow for the simultaneous imaging of multiple SPECT isotopes in a single scan, a feat that is not possible with PET isotopes. Such advances in SPECT resolution are greatly facilitating the study of molecules and biological processes relevant to drug development. Translation of these technical advances in SPECT to clinical scanners is imminent and promises to have a great impact on drug development. It is noteworthy that many radiotracer designs 51
Figure 4 Summary of SPECT vs. PET features. (a) Over the past decade, PET and SPECT imaging have become established as a staple in drug development efforts with dramatic increases in the number of trials using these imaging modalities (clinicaltrials.gov plot). (b) Although PET imaging is more widely used in research because of higher sensitivity and resolution, SPECT continues to be the workhorse of nuclear medicine around the world largely driven by the radiopharmaceutical distribution network and cost. The recent introduction of PET/MRI in the marketplace as well as improvements in preclinical and clinical SPECT imaging resolution continue to drive novel and improved applications in both modalities. Information in the table is from the Frost and Sullivan 2014 US Nuclear Medicine and PET Imaging Systems Market report and the inviCRO database. (c) A comparison of imaging resolution with previous generation vs. new multipinhole SPECT cameras. Images are of osteoblast activity using radiolabeled methylene diphosphonate (99mTcmethylene diphosphonate). Note the gain in resolution when using a multipinhole SPECT camera.
now incorporate the option of using SPECT as well as PET radionuclides for labeling. This may facilitate and accelerate multicenter clinical trials that require imaging for enrolment and disease tracking because of the wider availability of SPECT cameras scanners and the ease of SPECT ligand generation and distribution. CHALLENGES TO USING IMAGING IN DRUG DISCOVERY
A challenge to using imaging technologies to produce biomarker endpoints is to do so without adding time and cost to the drug development process, thereby producing a return on the investments that the biomarkers have themselves cost to develop. There is a “sweet spot” for the use of biomarkers for early “Go/No Go” decisions. The value generated through their use depends upon when the biomarker-enabled decision is made. The biomarker enabled “opportunity window” spans molecule selection at the research to development transition stage all the way into phase II clinical trials (Figure 1). Clearly, the most valuable markers have the greatest impact on the later more expensive phases of drug development and so produce the greatest return on the investment required to produce them. Recent calculations from Merck and Co (Gary Herman, personal communication cleared for public disclosure by Merck) have suggested that a two to threefold positive return on investment (the realized dollar value of biomarker decisions less the investment dollars required to produce them/biomarker-related dollar investment) can be expected through the use of biomarkers in early decision-making. It is 52
noteworthy that this analysis did not include the significant additional value realized by biomarkers that improve clinical endpoint determinations, dose selection, patient enrichment, and safety evaluation that can accelerate registration and approval in postphase IIB clinical trials. Next, and perhaps the biggest challenge for the use of medical imaging in drug development, is making the shift from its qualitative use to detect disease to a quantitative discipline that provides objective numerical measurements of tissue characteristics suitable for incorporation into hypothesis-testing clinical trial designs.19,20 In routine clinical practice, images are subject to qualitative radiographic interpretation and are used to facilitate diagnosis by identifying lesions or disease, trigger decisions to initiate therapy, and to monitor response to treatment. However, for drug development applications, rigorous quantification and reproducibility are most important to assess drug effects. As with all biomarkers, imaging markers undergo fit-forpurpose scientific validation and clinical qualification processes.6,21 However, there are diverse practical challenges that are specific to using imaging markers to characterize patients, identify responders, monitor drug actions, and define therapeutic outcomes. Imaging requires specialized equipment and trained individuals, but its successful routine use most importantly depends on assay validation through assessment of the true magnitude of effect and purposeful harmonization of data acquisition techniques, together with the development of “turn-key” applications using standardized tools for data collection and analysis. These VOLUME 98 NUMBER 1 | JULY 2015 | www.wileyonlinelibrary/cpt
Figure 5 Biomarker evolution. The left panel shows the classic drug development pipeline flow from initial discovery in the laboratory to its clinical use. The middle panel shows fit for purpose CNS biomarker evolution from initial identification of the potential biomarker through processes that include exploration, demonstration, classification, and its clinical qualification before it becomes a diagnostic in general medical and research use. Note that as progress toward a diagnostic use becomes more defined, increasing levels of evidence for the biomarker are required. The right panel shows the biomarker lifecycle from initial definition to adoption. An initial observation suggesting a potential CNS biomarker may be observed in a small study that then needs to be evaluated in a larger clinical trial that contributes to validation. Validation, demonstrating specificity and sensitivity of the biomarker assay, is followed by qualification (with demonstration of robust reproducibility) of the biomarker and then regulatory adoption. Once adopted, the process of continued biomarker evaluation defines its status and includes potential refinements as technologies and larger clinical data sets become available. Adapted from Hargreaves, R.J. et al. Expert Opin. Drug Discov. 6, 597–617 (2011).
requirements add complexity, especially in multicenter clinical imaging trials, in as much as detailed imaging manuals, standardized acquisition protocols (across different instruments from different manufacturers), data transfer, image reconstruction, and data processing algorithms (to annotate images with patientspecific information to permit confidential independent data review) are also required.22 Clinical qualification is a graded evidentiary process linking the biomarker to biology and clinical endpoints and is dependent upon the intended use of the biomarker (Figure 5). The intended use may be for internal, regulatory, or clinical trial design, or simply internal at-risk decision-making. However, qualifying imaging biomarkers for use in regulatory approval processes is especially difficult even when a biomarker is scientifically validated and well defined, ultimately requiring assessment in the context of clinical care or thorough integration into trials with proven active agents. This is clearly an issue when novel mechanisms and novel markers are developed together. Indeed, most biomarkers continue to be used at risk when driving decision-making during drug development and few ever reach the level of surrogacy where they can substitute for a clinical outcome. Proprietary molecular imaging biomarkers rarely have applicability across different targets even within the same disease area, and the cost of developing them has to be justified and accounted for in the context of specific drug discovery programs. In contrast, CLINICAL PHARMACOLOGY & THERAPEUTICS | VOLUME 98 NUMBER 1 | JULY 2015
disease imaging biomarkers can generally be considered platform technologies that can characterize patient populations, disease state, and therapeutic response. Therefore, they have a potentially broad cross-target utility with value to diverse therapeutic approaches within a common disease area. In this case, the barrier is that there is little incentive for any one company to bear the considerable cost of clinically qualifying the biomarker, so a different shared solution is required, in some cases entailing creation of public-private consortia. THE VALUE OF IMAGING BIOMARKERS Selecting and monitoring the right patients
The development of disease-modifying agents for Alzheimer disease is becoming increasingly focused on patients who are at risk for dementia but still in the prodromal asymptomatic stage of Alzheimer disease. The shift from trials in mild to moderate disease to early intervention has placed a huge emphasis on the use of imaging biomarkers that visualize amyloid to select patients and monitor treatment responses in clinical trials designed to test the amyloid deposition hypothesis. MRI is also being used alongside genetic risk factor status (Apo-E4) as a safety biomarker to detect amyloid-related drug-induced imaging abnormalities, such as edema (ARIA-E), in clinical trials of small molecule and immunotherapeutics designed to lower the brain amyloid load. 53
In a recent phase 1b study on the anti-amyloid antibody aducanumab, nearly 40% of patients meeting the clinical criteria of prodromal or mild Alzheimer disease did not manifest evidence of amyloidopathy by PET scanning with the amyloid PET tracer [18F]florbetapir and were thus not included in the treatment phase of the study.23 The study showed that aducanumab lowered amyloid deposits in the brain of patients with early Alzheimer’s disease (see cover picture) in a dose-dependent fashion with an associated benefit on clinical endpoints.24 The main safety finding ARIA-E was detected and monitored using MRI. The positive outcome of the aducanumab clinical trial suggests that the rigorous use of an amyloid PET tracer for cohort enrichment can serve as a powerful tool to reduce noise by ensuring that only patients with a pathophysiology relevant to the proposed therapeutic mechanism of action are included in the clinical trial. Moreover, this example illustrates how biomarkers generated using two imaging modalities—PET and MRI—can be used to provide key early data that can inform the design of future phase 3 clinical trials. Amyloid PET imaging and MRI can of course also be used to monitor the treatment response and safety of small molecule therapeutics that inhibit Ab-42 generation (BACE inhibitors) and other therapeutic approaches designed to decrease the amyloid load. By providing objective measures of the preservation, slowing, or reversal of amyloid pathology and drug safety, these imaging biomarkers can support subjective clinical outcome measures and claims for therapeutic disease modification. Future drug development efforts are aimed toward finding therapies to prevent the spread of other hallmark pathologies in Alzheimer’s disease, such as tau protein. It is not going to get easier. Anti-tau therapeutic programs testing tau hypotheses of Alzheimer disease face many of the same hurdles as those directed against amyloids. What species of phospho-tau to target: soluble or fibrillar? What therapeutic modality to use: immunotherapeutics or small molecules? Whom to treat, when to treat, and for how long? New biomarker approaches will be needed, and these will have to be developed alongside drug candidates. For example, as with Ab-42, directed therapeutics and amyloid tracers, novel tau imaging tracers25–28 could become early readouts for the potential benefits of tau-based drug therapies. Interestingly, because tau spreads in association with advancement of Alzheimer disease symptomatology, tau tracers may also be useful as true objective progression biomarkers for any disease-modifying mechanism, including amyloid directed therapies, in clinical proof-of-concept trials. The validation and qualification of patient and disease-based platform biomarkers that are independent of drug target and mechanism is now increasingly being addressed through formation of public-private consortia that share the risk and considerable cost of these long-term studies. Large consortia are developing and standardizing CNS imaging biomarker measurements in Alzheimer disease (Alzheimer Disease Neuroimaging Initiative; http://www.nia.nih.gov/Alzheimers/ResearchInformation/ ClinicalTrials/ADNI.htm) now in its third incarnation Alzheimer Disease Neuroimaging Initiative-3, Parkinson disease (Parkinson Progression Markers Initiative http://www.ppmi54
info.org), and Huntington disease (CHDI in collaboration with King’s College London University) to assess imaging markers that characterize disease status and progression in wellcharacterized patient populations to provide robust baselines for therapeutic trials. Natural progression data collected from these initiatives could therefore lead to the development of objective standard assessment tools to define the most appropriate patients to treat and how to monitor and assess the effects of potential disease-modifying drugs in clinical trials. Target engagement imaging
The goal of any early drug development program is always to test the mechanism and not the molecule in order to support additional research investments in late-phase clinical trials. Confirmation that drugs reach their targets in the brain using imaging markers of engagement and pharmacodynamics is central to successful clinical proof-of-concept testing in CNS drug development. All too often, suboptimal molecules that fail to test hypotheses have been advanced to the clinic and so confuse and complicate paths for future development of new drug entities, especially in neuropsychiatric disease. Indeed, the lack of an appropriate imaging “biomarker tool box” to prove adequate target engagement in proof-of-concept studies probably explains why so many neuroscience targets from the 1990s are still being pursued as they have not yet been invalidated. It is a sobering thought that many different companies may keep failing on the same targets because they lack the CNS biomarkers needed to make “Go/No Go” decisions on mechanisms or select appropriate doses for definitive clinical trials. In nuclear medicine, medicinal chemists together with radiochemists now play an increasingly important role in developing PET tracers to establish distribution, brain penetration, and target engagement of candidate drugs that can guide interpretation of preclinical experiments and help select doses for clinical trials. Simultaneous discovery efforts in medicinal chemistry and radiochemistry are required as unique radiotracers are required for each new protein target and radiotracers must reach the clinic at the same time as drug candidates to have an impact on decisionmaking processes. The design, in parallel with drug candidate synthesis, of structurally distinct precursor molecules suitable for rapid labeling at high specific activity with 11C and 18F radionuclides to produce imaging radioligands that have high target affinity, target selectivity, fast brain penetration, and physicochemical properties that minimize nonspecific binding to maximize signal-to-noise sensitivity is key to small molecule CNS neuroreceptor-focused drug development. Using target engagement to investigate therapeutic windows
A translational research strategy was used to link the laboratory and the clinic in the discovery and development of H3 receptor inverse agonist drugs that promote wakefulness and prevent excessive daytime sleepiness but also have the potential side effect of disrupting sleep.29–31 An H3 receptor PET tracer [11C]MK827832 was used in rats, monkeys, and humans to develop the relationship between receptor and pharmacologic and behavioral VOLUME 98 NUMBER 1 | JULY 2015 | www.wileyonlinelibrary/cpt
Figure 6 The use of a CNS PET tracer in translational research. The left panel shows the blockade of H3 histamine receptors in the rat, monkey, and humans with the PET tracer [11C]-MK-8728, the middle panel the similar effects of MK-249 an H3 receptor inverse agonist on wakefulness at the same central receptor occupancy and the right hand panel the corresponding alerting effects on EEG.
responses (Figure 6). The pharmacodynamic effects of the H3 receptor inverse agonist drug candidate MK-0249 were evaluated preclinically and clinically using sleep wake analysis and quantitative electroencephalography with significant wake promoting and high frequency EEG activity being apparent at receptor occupancies of 70% and higher. In a stimulant-referenced sleep deprivation model, MK-0249 had alerting activity that was statistically superior to placebo at doses associated with 90% receptor occupancy. However, using the Leeds Sleep Evaluation Questionnaire, subjects reported the adverse effects of difficulty getting to sleep and disruption of sleep when occupancy was estimated to be between 70% and 84%. Additionally, polysomnography during the recovery sleep period after sleep deprivation showed some evidence for sleep disturbance as long as 19–31 hours after dosing, when brain H3 receptor occupancy levels were between 78% and 69%. These studies indicated that, although a clinically significant alerting efficacy could clearly be attributed to the H3 inverse agonist mechanism, the pharmacodynamic profile of MK0249 did not allow the wanted alerting and unwanted sleep disrupting effects to be separated and the compound to be developed further. Nevertheless, these imaging studies guided future program goals by showing that the profile of an optimal H3 inverse agonist molecule for excessive daytime sleepiness would need to achieve high levels of receptor occupancy rapidly but have a much shorter half-life than MK-0249 so that the drug would be cleared from the brain when it came time to sleep. CLINICAL PHARMACOLOGY & THERAPEUTICS | VOLUME 98 NUMBER 1 | JULY 2015
PET imaging was also used to facilitate “Go/No Go” decisions on MRK-409 an a3-subunit-preferring GABAA agonist.33 MRK409 produced anxiolytic-like activity in rodent and primate unconditioned and conditioned models of anxiety with minimum effective doses corresponding to GABAA receptor occupancies ranging from 35–65%, depending on the particular model used. Interestingly, when MK-409 was dosed to achieve occupancies of >90%, animals showed minimal overt signs of sedation suggesting that it had the unique potential to be a nonsedating anxiolytic. Unfortunately, safety and tolerability studies in humans showed that there was pronounced sedation at a dose of 2 mg setting the maximum clinically tolerated single dose of MRK-409 at 1 mg. PET studies subsequently showed that [11C]flumazenil uptake following the 1 mg dose was comparable to that after placebo administration, indicating that occupancy of GABAA receptor benzodiazepine binding sites by MRK-409 was below the limits of detection (i.e.,
late stage clinical trials is financially untenable. Obtaining a satisfactory return on research investments is essential if we are to continue to progress innovative drug discovery portfolios that can significantly improve clinical care and address unmet medical needs. Mervyn Turner, the former Chief Strategy Officer and Head of External Scientific Affairs at Merck and Co, commented3 that “. . .for a long time the biopharma industry felt that the old model was going to deliver. We kept expecting the arrival of a blockbuster drug. Over time, because the failure rate and costs of drug development were so high, it has really forced the recognition that (the current situation) represents something more fundamental than just another cycle.” Indeed, today, the blockbuster model has particularly high costs when there is late stage failure because of lack of efficacy as the clinical trials often require large patient populations to realize high sales potential. In mature therapeutic areas, termination for economics or market-related factors becomes increasingly likely if new product candidates do not differentiate meaningfully from established standards of care. The blockbuster model may be broken but it is clear that many pharmaceutical companies need “blockbuster” products to sustain them while on the rollercoaster ride of drug discovery and development. The question is then how can this be achieved? To date, the pharmaceutical industry has largely failed to improve performance or deliver a higher return on research investments despite the gymnastics of continual internal reorganizations, right sizing exercises, and perennial mergers and
1 Biogen, Cambridge, Massachusetts, USA; 2inviCRO, LLC, Boston, Massachusetts, USA; 3Merck Research Laboratories, West Point, Pennsylvania, USA. Correspondence: R Hargreaves ([email protected])
Received 12 March 2015; accepted 7 April 2015; advance online publication 13 April 2015. doi:10.1002/cpt.132 CLINICAL PHARMACOLOGY & THERAPEUTICS | VOLUME 98 NUMBER 1 | JULY 2015
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Figure 1 Potential uses of imaging at various phases in the drug discovery and development process.
acquisitions. The problem statement was framed by the Economist Intelligence Unit in 2011, “How do we re-invent how to innovate rather than just cut costs?” Central to their proposed solution was looking beyond internal research and development silos through open innovation and the integration of new tools and technologies to drive a mind-set that targets efficient early development decision-making as a metric of success.3 Indeed, Paul Matthews, the former Head of Imaging at GlaxoSmithKline, commented that “A fundamentally optimistic future scenario for the drug industry is to make drug discovery more successful by being smarter in our early “Go/No Go” decision-making through the disciplined use of biomarkers.4 Emerging drug candidates need to differentiate from existing standards of care to advance therapy or address new hypotheses for treatment of disease. Novel unprecedented mechanisms are often the means to reach these goals but carry a greater inherent risk of failure. De-risking strategies for early decision-making are therefore critical to cost-effective drug development and to sustaining the willingness to invest in research to produce truly innovative new medicines. Development of drugs for novel targets requires informative laboratory to clinic translational biomarkers to guide dosing in early experimental medicine proof of biology and clinical proofof-concept tests to drive early decision-making. The conundrum is, of course, that novel drug targets rarely have validated biomarkers—early innovation is inherently inversely proportional to validation. The use of a biomarker strategy poses disproportionate degrees of development rather than regulatory risk, dependent on when the biomarkers are used for making decisions. Preinvestment is required to ensure biomarkers are available at the right time, with the right fit-for-purpose level of analytical validation and clinical qualification. Biomarkers that reveal themselves too late have no significant impact on decisions and waste resources. The consistent use of biomarkers across drug 48
development portfolios has the promise to reinvigorate the drug discovery process through cost-effective decision making and improved success rates that help bring useful medicines to patients sooner.5 This review will discuss the value of using imaging technologies to generate biomarkers that can improve the efficiency of the CNS drug development process. The examples we have used, while focused on nuclear imaging and CNS drug development, illustrate guiding principles for the use of imaging biomarkers across therapeutic areas using the diverse preclinical and clinical imaging modalities available to scientists, clinical pharmacologists, and physicians today. MOLECULAR IMAGING—ADDING VALUE TO DRUG DISCOVERY AND DEVELOPMENT
The potential uses of imaging across the drug discovery and development spectrum are summarized in Figure 1. Imaging has distinct but complementary roles at different points in the drug discovery and development timeline with the initial goal of selecting the best molecule and dose to test the hypothesized therapeutic biology through to human dose selection and proof of clinical concept. Imaging can also be used to support patient selection and disease progression monitoring in late phase clinical trials and, if successful, after registration throughout the lifecycle of the drug.6,7 Enabling clinical proof-of-concept testing and dose selection
In the laboratory, molecular imaging can help validate therapeutic targets in models of disease and symptomatology by linking exposure to the degree of target engagement and the time-ontarget to preclinical measures of efficacy. Focusing research on those drug candidates that achieve the highest target engagement from the lowest exposure is critical to molecule selection as this maximizes the potential therapeutic index moving into clinical VOLUME 98 NUMBER 1 | JULY 2015 | www.wileyonlinelibrary/cpt
trials. Vertically integrated research strategies that encompass animal imaging models that are aligned with early experimental imaging paradigms in humans, and in turn guide subsequent clinical studies in patients, are becoming core translational research approaches today. To be optimally effective, simultaneous discovery efforts in medicinal chemistry and radiochemistry are required to ensure radiotracers reach the clinic at the same time as drug candidates. Imaging tracers that are ready for first-in-human studies enable the design of clinical protocols that can weave together imaging assessments of target engagement with first-in-human single or multiple dose safety and tolerability testing to define therapeutic windows for efficacy testing. In early clinical development, molecular imaging can be used to link target engagement to drug-induced biological changes that are expected to produce clinical benefit, so-called proof of mechanism or activity testing. Proof-of-concept can be declared when target engagement can be linked to a change in a clinically meaningful imaging endpoint. If a drug has adequate target engagement but does not produce the expected biological or clinical effects, the therapeutic concept is flawed and development can be stopped. In the later stages of drug development, imaging can also be used to define the relationship between drug dose/exposure and target engagement to enable dose selection for phase II clinical trials. Improving the efficiency of drug development
There is a growing interest in patient-specific, disease-specific, and outcome-specific imaging biomarkers that are independent of specific therapeutic mechanisms and molecules. Molecular imaging approaches that can be used to stratify patients for clinical trials will enable enrichment of clinical proof-of-concept studies potentially leading to shorter, smaller, and more definitive clinical trials. Stratification using molecular imaging may not only improve the drug development process but ultimately it could drive personalized precision medicine approaches to therapy, delivering the right drug at the right dose to the right patient. Many sporadic neurodegenerative diseases, such as Alzheimer’s dementia and Parkinson disease and amyotrophic lateral sclerosis (ALS), are being reclassified for drug development efforts into pathophysiology-based endophenotypes. The emergence of molecular imaging tracers to visualize tissues in life that previously could have been studied only at autopsy and thereby measure these molecular endophenotypes in vivo is one of the most exciting recent applications in neuroimaging biomarkers and is fundamentally changing drug development approaches to these diseases. The ability to observe neurodegenerative processes evolve over time and the correlation of these imaging measures with classical pathological evaluations is generating new unifying hypotheses that are being tested in current drug development efforts. One prominent theory proposes a time and age-dependent aggregation and spread of proteins such as beta-amyloid, tau, alpha-synuclein, superoxide dismutase 1, and TDP-43 along circuits of interconnecting cells within the brain. The abnormal build up of these proteins within brain disease pathways can CLINICAL PHARMACOLOGY & THERAPEUTICS | VOLUME 98 NUMBER 1 | JULY 2015
injure vulnerable CNS neurons that in turn leads to progressively worsening brain function. The various spreading proteinopathies can be accompanied by microgliosis and astrocytosis and each involves dysfunction in distinct cognitive and motor circuits that produces their characteristic regional pathology and pattern of the clinical presentation of disease. Because the build up of aggregated proteins and dysfunction of brain circuits starts to occur well in advance of clinical demise, functional imaging alongside molecular imaging offers additional hope for early evaluation of potential disease-modifying drugs and tracking the progression of these phenomena (Figure 2). MOLECULAR IMAGING TECHNOLOGIES
Improvements in imaging hardware and software over the past 15 years have bridged the gap between subcellular and wholebody imaging across a significant majority of the electromagnetic spectrum. This ever evolving molecular imaging toolbox (Figure 3) has enabled biodistribution studies and imaging translational biomarker endpoints to play an increasingly important role in the preclinical and clinical assessment of molecules, mechanisms, and therapeutic hypotheses.8 Molecular imaging uses a variety of imaging technologies and specialized imaging agents to visualize, characterize, and quantify anatomic structures and biological processes at the subcellular, cellular, and organ levels in a physiologically relevant tissue context in vivo with high spatial and biochemical resolution,8 often without the need for tissue dissection and processing. Each technology generates images in different ways and each has its strengths and weaknesses in terms of spatial resolution, sensitivity, and imaging probe characteristics (Figure 3a–d). All medical imaging modalities support some form of molecular imaging application. However, for contrast media or even “tagged” nanoparticles, direct detection of radionuclide and fluorophore labeled compounds continues to place nuclear and optical imaging as the primary biodistribution imaging modalities in drug discovery and development. A broad array of preclinical in vivo and ex vivo imaging systems9 is available commercially and these technologies continue to evolve. There are an estimated 500 nuclear micro-positron emission tomography (PET) and microsingle photon emission computed tomography (SPECT) compared to 2,000 commercial optical (bioluminescence and fluorescence) preclinical in vivo imaging systems in use today (data from the inviCRO Company database). The wide availability of optically active bioluminescent and fluorescent markers that can be genetically engineered offers many ways to image preclinically fundamental in vivo biological processes, such as gene expression, biochemical pathway activation, protein–protein interactions, cell trafficking, proliferation, transformation, and death. Moreover, the progressive miniaturization of optical imaging technologies to produce micro-optical coherence tomography10 and micro-confocal microscopy11 now promises to deliver in vivo histopathology in real-time that can be utilized in biomarker and biodistribution studies in drug discovery and development programs.12 To date, optical molecular imaging has its greatest impact in preclinical animal models of disease, given limitations in the depth of imaging that can be 49
Figure 2 Emerging science of neurodegenerative diseases. (a) Envisioned common pathogenesis steps beginning with (1) vulnerable neurons, which (2) succumb to age-driven accumulated stresses that result in synaptic dysfunction (shaded circles). Propagation of synaptic dysfunction (3) and cell death via excitotoxicity and spreading of intracellular and extracellular proteinopathy is envisioned to occur locally as well as along neural circuits with accompanying microgliosis and astrocytosis. (b) The involvement of specific neuronal circuits by spreading pathology is believed to account for the characteristic clinical manifestations of specific neurodegenerative diseases, such as dementia and motor problems. (c) Pathological changes are believed to occur well in advance of brain dysfunction and clinical symptoms and may be tractable by imaging and fluid biomarkers allowing for preventative therapy.
achieved.13 The recent development of clinical photoacoustic imaging14,15 promises, however, to increase the role of fluorescence imaging clinically that could, in turn, be useful in the drug development setting. Optical imaging currently transitions to the clinic largely using nuclear imaging modalities, such as PET and SPECT16 to visualize and characterize biological processes spatially and temporally at the molecular level in normal and pathophysiological conditions using various types of exogenously applied radioactive imaging probes. The most commonly used molecular imaging techniques visualize radiolabeled probes or radiotracers interacting with protein targets within or on the surface of cells. Radiotracers are versatile and sensitive, and can be designed to track the drug itself, image the drug target, or monitor key biochemical and physiological processes. Novel molecular tracer probes are the only way to quantitatively measure receptor populations and pharmacology at picomolar to nanomolar concentrations in vitro, ex vivo, and in vivo in both animals and humans (Figure 3), thus providing a unique bridge between the laboratory and the clinic. In vitro and ex vivo autoradiographic tissue imaging techniques using molecular probes labeled with beta emitting isotopes 14C, 3 H, or 35S are often used to map drug targets, pathology, and the 50
biodistribution of drug molecules into organs and tissues at high resolution as a prelude to studies in vivo. Tissue section autoradiography can also be used to screen potential PET imaging agents in the absence of cyclotron-derived isotopes.17,18 Preclinically and clinically in vivo, the key molecular imaging modalities are PET, which uses tracers labeled with positron-emitting radionuclides (most commonly 11C and 18F), and SPECT, which detects tracers labeled with gamma-emitting radioactive isotopes (e.g., 123I and 99mTc). Both PET and SPECT can be used to track small molecule and biologic therapeutics and PET and SPECT ligands can be used in vitro and ex vivo in autoradiographic studies. The use of structural imaging modalities, such as computed tomography (CT) and magnetic resonance imaging (MRI) in combination with PET or SPECT, enables precise spatial and anatomic localization of molecular activity. Moreover, the use of CT alongside nuclear imaging allows density assessment of tissues, such as the skull, thus allowing for attenuation correction and more accurate quantification of signal at a tissue source, such as the brain. Advances in the development of small animal tomographic cameras (microPET or microSPECT combined with CT) have facilitated translational bridging between preclinical and clinical research. SPECT has a greater number of radionuclides available for incorporation into imaging probes. SPECT has similar VOLUME 98 NUMBER 1 | JULY 2015 | www.wileyonlinelibrary/cpt
Figure 3 (a) A general sensitivity survey of in vivo molecular imaging techniques as a function of interrogation scale from subcellular to whole-body in humans. (b) An evaluation of the volumetric resolution for fluorescent imaging technologies, yielding a linear trend for resolution as a function of mass of tissue interrogated. The cluster of technologies in the upper right-hand corner correspond to in vivo “macro”-fluorescence technologies labeled “Fluorescence” in panel a. The cluster in the lower left-hand corner is comprised of both in vivo and ex vivo fluorescent microscopy technologies. The in vivo fluorescent microscopy techniques are captured in panel a under the heading “Intravital Microscopy.” (c) A translational nuclear imaging resolution plot from subcellular microautoradiography to whole-body SPECT imaging. Note the qualitative linear progression of volumetric resolution as a function of subject mass. (d) Standard injected radioactive dose per subject mass as a function of imaging technique and species. Note that preclinical techniques maintain the linear evolution of resolution across species, but require increased amounts of radioactivity because of a drop in sensitivity per volume in PET and SPECT.
sensitivity to PET and has had far wider adoption in clinical practice, but its use in drug development has fallen behind PET in North America and Western Europe because of its lower image resolution. Surveys of the clinical trials data base Clintrials.gov show clearly that nuclear imaging modalities, especially PET, have become powerful tools for drug discovery and development, with more than 250 studies per year being initiated by the industry in the 2008–2012 timeframe (Figure 4a) particularly in the neurosciences because of the inaccessibility of the brain and the general lack of CNS-specific markers to support accurate drug dosing and in oncology in which imaging is integral to clinical practice and the evaluation of response. The trade-offs between SPECT and PET are presented in Figure 4b. Recent developments in SPECT camera engineering and design have made SPECT technology more competitive with PET. SPECT can be viewed as more versatile in that it can image a wider range of long-lived radioisotopes to track the pharmacodynamics of biological drugs, such as antibodies and antisense molecules. Figure 4c illustrates the recent gains in resolution offered by multipinhole animal SPECT-CT systems, such as the CLINICAL PHARMACOLOGY & THERAPEUTICS | VOLUME 98 NUMBER 1 | JULY 2015
nanoSPECT. The images represent 99mTc-methylene diphosphonate bone scans in a mouse and a squirrel monkey. Each pair of images represents the same animal imaged first in older technology SPECT cameras and then in the nanoSPECT. The gain in resolution is made obvious by the ability to see single vertebrae, ribs, and teeth with the newer technology but not with the older technology. PET isotopes, while named for the primary nuclear particle emission, are ultimately tracked via the gamma emission generated through the collision of emitted positrons with electrons. These positron-derived gamma emissions have a characteristic emission energy signature common to all PET isotopes. SPECT isotopes, however, which emit gamma radiation directly, can have unique and distinct gamma emissions. These facts allow for the simultaneous imaging of multiple SPECT isotopes in a single scan, a feat that is not possible with PET isotopes. Such advances in SPECT resolution are greatly facilitating the study of molecules and biological processes relevant to drug development. Translation of these technical advances in SPECT to clinical scanners is imminent and promises to have a great impact on drug development. It is noteworthy that many radiotracer designs 51
Figure 4 Summary of SPECT vs. PET features. (a) Over the past decade, PET and SPECT imaging have become established as a staple in drug development efforts with dramatic increases in the number of trials using these imaging modalities (clinicaltrials.gov plot). (b) Although PET imaging is more widely used in research because of higher sensitivity and resolution, SPECT continues to be the workhorse of nuclear medicine around the world largely driven by the radiopharmaceutical distribution network and cost. The recent introduction of PET/MRI in the marketplace as well as improvements in preclinical and clinical SPECT imaging resolution continue to drive novel and improved applications in both modalities. Information in the table is from the Frost and Sullivan 2014 US Nuclear Medicine and PET Imaging Systems Market report and the inviCRO database. (c) A comparison of imaging resolution with previous generation vs. new multipinhole SPECT cameras. Images are of osteoblast activity using radiolabeled methylene diphosphonate (99mTcmethylene diphosphonate). Note the gain in resolution when using a multipinhole SPECT camera.
now incorporate the option of using SPECT as well as PET radionuclides for labeling. This may facilitate and accelerate multicenter clinical trials that require imaging for enrolment and disease tracking because of the wider availability of SPECT cameras scanners and the ease of SPECT ligand generation and distribution. CHALLENGES TO USING IMAGING IN DRUG DISCOVERY
A challenge to using imaging technologies to produce biomarker endpoints is to do so without adding time and cost to the drug development process, thereby producing a return on the investments that the biomarkers have themselves cost to develop. There is a “sweet spot” for the use of biomarkers for early “Go/No Go” decisions. The value generated through their use depends upon when the biomarker-enabled decision is made. The biomarker enabled “opportunity window” spans molecule selection at the research to development transition stage all the way into phase II clinical trials (Figure 1). Clearly, the most valuable markers have the greatest impact on the later more expensive phases of drug development and so produce the greatest return on the investment required to produce them. Recent calculations from Merck and Co (Gary Herman, personal communication cleared for public disclosure by Merck) have suggested that a two to threefold positive return on investment (the realized dollar value of biomarker decisions less the investment dollars required to produce them/biomarker-related dollar investment) can be expected through the use of biomarkers in early decision-making. It is 52
noteworthy that this analysis did not include the significant additional value realized by biomarkers that improve clinical endpoint determinations, dose selection, patient enrichment, and safety evaluation that can accelerate registration and approval in postphase IIB clinical trials. Next, and perhaps the biggest challenge for the use of medical imaging in drug development, is making the shift from its qualitative use to detect disease to a quantitative discipline that provides objective numerical measurements of tissue characteristics suitable for incorporation into hypothesis-testing clinical trial designs.19,20 In routine clinical practice, images are subject to qualitative radiographic interpretation and are used to facilitate diagnosis by identifying lesions or disease, trigger decisions to initiate therapy, and to monitor response to treatment. However, for drug development applications, rigorous quantification and reproducibility are most important to assess drug effects. As with all biomarkers, imaging markers undergo fit-forpurpose scientific validation and clinical qualification processes.6,21 However, there are diverse practical challenges that are specific to using imaging markers to characterize patients, identify responders, monitor drug actions, and define therapeutic outcomes. Imaging requires specialized equipment and trained individuals, but its successful routine use most importantly depends on assay validation through assessment of the true magnitude of effect and purposeful harmonization of data acquisition techniques, together with the development of “turn-key” applications using standardized tools for data collection and analysis. These VOLUME 98 NUMBER 1 | JULY 2015 | www.wileyonlinelibrary/cpt
Figure 5 Biomarker evolution. The left panel shows the classic drug development pipeline flow from initial discovery in the laboratory to its clinical use. The middle panel shows fit for purpose CNS biomarker evolution from initial identification of the potential biomarker through processes that include exploration, demonstration, classification, and its clinical qualification before it becomes a diagnostic in general medical and research use. Note that as progress toward a diagnostic use becomes more defined, increasing levels of evidence for the biomarker are required. The right panel shows the biomarker lifecycle from initial definition to adoption. An initial observation suggesting a potential CNS biomarker may be observed in a small study that then needs to be evaluated in a larger clinical trial that contributes to validation. Validation, demonstrating specificity and sensitivity of the biomarker assay, is followed by qualification (with demonstration of robust reproducibility) of the biomarker and then regulatory adoption. Once adopted, the process of continued biomarker evaluation defines its status and includes potential refinements as technologies and larger clinical data sets become available. Adapted from Hargreaves, R.J. et al. Expert Opin. Drug Discov. 6, 597–617 (2011).
requirements add complexity, especially in multicenter clinical imaging trials, in as much as detailed imaging manuals, standardized acquisition protocols (across different instruments from different manufacturers), data transfer, image reconstruction, and data processing algorithms (to annotate images with patientspecific information to permit confidential independent data review) are also required.22 Clinical qualification is a graded evidentiary process linking the biomarker to biology and clinical endpoints and is dependent upon the intended use of the biomarker (Figure 5). The intended use may be for internal, regulatory, or clinical trial design, or simply internal at-risk decision-making. However, qualifying imaging biomarkers for use in regulatory approval processes is especially difficult even when a biomarker is scientifically validated and well defined, ultimately requiring assessment in the context of clinical care or thorough integration into trials with proven active agents. This is clearly an issue when novel mechanisms and novel markers are developed together. Indeed, most biomarkers continue to be used at risk when driving decision-making during drug development and few ever reach the level of surrogacy where they can substitute for a clinical outcome. Proprietary molecular imaging biomarkers rarely have applicability across different targets even within the same disease area, and the cost of developing them has to be justified and accounted for in the context of specific drug discovery programs. In contrast, CLINICAL PHARMACOLOGY & THERAPEUTICS | VOLUME 98 NUMBER 1 | JULY 2015
disease imaging biomarkers can generally be considered platform technologies that can characterize patient populations, disease state, and therapeutic response. Therefore, they have a potentially broad cross-target utility with value to diverse therapeutic approaches within a common disease area. In this case, the barrier is that there is little incentive for any one company to bear the considerable cost of clinically qualifying the biomarker, so a different shared solution is required, in some cases entailing creation of public-private consortia. THE VALUE OF IMAGING BIOMARKERS Selecting and monitoring the right patients
The development of disease-modifying agents for Alzheimer disease is becoming increasingly focused on patients who are at risk for dementia but still in the prodromal asymptomatic stage of Alzheimer disease. The shift from trials in mild to moderate disease to early intervention has placed a huge emphasis on the use of imaging biomarkers that visualize amyloid to select patients and monitor treatment responses in clinical trials designed to test the amyloid deposition hypothesis. MRI is also being used alongside genetic risk factor status (Apo-E4) as a safety biomarker to detect amyloid-related drug-induced imaging abnormalities, such as edema (ARIA-E), in clinical trials of small molecule and immunotherapeutics designed to lower the brain amyloid load. 53
In a recent phase 1b study on the anti-amyloid antibody aducanumab, nearly 40% of patients meeting the clinical criteria of prodromal or mild Alzheimer disease did not manifest evidence of amyloidopathy by PET scanning with the amyloid PET tracer [18F]florbetapir and were thus not included in the treatment phase of the study.23 The study showed that aducanumab lowered amyloid deposits in the brain of patients with early Alzheimer’s disease (see cover picture) in a dose-dependent fashion with an associated benefit on clinical endpoints.24 The main safety finding ARIA-E was detected and monitored using MRI. The positive outcome of the aducanumab clinical trial suggests that the rigorous use of an amyloid PET tracer for cohort enrichment can serve as a powerful tool to reduce noise by ensuring that only patients with a pathophysiology relevant to the proposed therapeutic mechanism of action are included in the clinical trial. Moreover, this example illustrates how biomarkers generated using two imaging modalities—PET and MRI—can be used to provide key early data that can inform the design of future phase 3 clinical trials. Amyloid PET imaging and MRI can of course also be used to monitor the treatment response and safety of small molecule therapeutics that inhibit Ab-42 generation (BACE inhibitors) and other therapeutic approaches designed to decrease the amyloid load. By providing objective measures of the preservation, slowing, or reversal of amyloid pathology and drug safety, these imaging biomarkers can support subjective clinical outcome measures and claims for therapeutic disease modification. Future drug development efforts are aimed toward finding therapies to prevent the spread of other hallmark pathologies in Alzheimer’s disease, such as tau protein. It is not going to get easier. Anti-tau therapeutic programs testing tau hypotheses of Alzheimer disease face many of the same hurdles as those directed against amyloids. What species of phospho-tau to target: soluble or fibrillar? What therapeutic modality to use: immunotherapeutics or small molecules? Whom to treat, when to treat, and for how long? New biomarker approaches will be needed, and these will have to be developed alongside drug candidates. For example, as with Ab-42, directed therapeutics and amyloid tracers, novel tau imaging tracers25–28 could become early readouts for the potential benefits of tau-based drug therapies. Interestingly, because tau spreads in association with advancement of Alzheimer disease symptomatology, tau tracers may also be useful as true objective progression biomarkers for any disease-modifying mechanism, including amyloid directed therapies, in clinical proof-of-concept trials. The validation and qualification of patient and disease-based platform biomarkers that are independent of drug target and mechanism is now increasingly being addressed through formation of public-private consortia that share the risk and considerable cost of these long-term studies. Large consortia are developing and standardizing CNS imaging biomarker measurements in Alzheimer disease (Alzheimer Disease Neuroimaging Initiative; http://www.nia.nih.gov/Alzheimers/ResearchInformation/ ClinicalTrials/ADNI.htm) now in its third incarnation Alzheimer Disease Neuroimaging Initiative-3, Parkinson disease (Parkinson Progression Markers Initiative http://www.ppmi54
info.org), and Huntington disease (CHDI in collaboration with King’s College London University) to assess imaging markers that characterize disease status and progression in wellcharacterized patient populations to provide robust baselines for therapeutic trials. Natural progression data collected from these initiatives could therefore lead to the development of objective standard assessment tools to define the most appropriate patients to treat and how to monitor and assess the effects of potential disease-modifying drugs in clinical trials. Target engagement imaging
The goal of any early drug development program is always to test the mechanism and not the molecule in order to support additional research investments in late-phase clinical trials. Confirmation that drugs reach their targets in the brain using imaging markers of engagement and pharmacodynamics is central to successful clinical proof-of-concept testing in CNS drug development. All too often, suboptimal molecules that fail to test hypotheses have been advanced to the clinic and so confuse and complicate paths for future development of new drug entities, especially in neuropsychiatric disease. Indeed, the lack of an appropriate imaging “biomarker tool box” to prove adequate target engagement in proof-of-concept studies probably explains why so many neuroscience targets from the 1990s are still being pursued as they have not yet been invalidated. It is a sobering thought that many different companies may keep failing on the same targets because they lack the CNS biomarkers needed to make “Go/No Go” decisions on mechanisms or select appropriate doses for definitive clinical trials. In nuclear medicine, medicinal chemists together with radiochemists now play an increasingly important role in developing PET tracers to establish distribution, brain penetration, and target engagement of candidate drugs that can guide interpretation of preclinical experiments and help select doses for clinical trials. Simultaneous discovery efforts in medicinal chemistry and radiochemistry are required as unique radiotracers are required for each new protein target and radiotracers must reach the clinic at the same time as drug candidates to have an impact on decisionmaking processes. The design, in parallel with drug candidate synthesis, of structurally distinct precursor molecules suitable for rapid labeling at high specific activity with 11C and 18F radionuclides to produce imaging radioligands that have high target affinity, target selectivity, fast brain penetration, and physicochemical properties that minimize nonspecific binding to maximize signal-to-noise sensitivity is key to small molecule CNS neuroreceptor-focused drug development. Using target engagement to investigate therapeutic windows
A translational research strategy was used to link the laboratory and the clinic in the discovery and development of H3 receptor inverse agonist drugs that promote wakefulness and prevent excessive daytime sleepiness but also have the potential side effect of disrupting sleep.29–31 An H3 receptor PET tracer [11C]MK827832 was used in rats, monkeys, and humans to develop the relationship between receptor and pharmacologic and behavioral VOLUME 98 NUMBER 1 | JULY 2015 | www.wileyonlinelibrary/cpt
Figure 6 The use of a CNS PET tracer in translational research. The left panel shows the blockade of H3 histamine receptors in the rat, monkey, and humans with the PET tracer [11C]-MK-8728, the middle panel the similar effects of MK-249 an H3 receptor inverse agonist on wakefulness at the same central receptor occupancy and the right hand panel the corresponding alerting effects on EEG.
responses (Figure 6). The pharmacodynamic effects of the H3 receptor inverse agonist drug candidate MK-0249 were evaluated preclinically and clinically using sleep wake analysis and quantitative electroencephalography with significant wake promoting and high frequency EEG activity being apparent at receptor occupancies of 70% and higher. In a stimulant-referenced sleep deprivation model, MK-0249 had alerting activity that was statistically superior to placebo at doses associated with 90% receptor occupancy. However, using the Leeds Sleep Evaluation Questionnaire, subjects reported the adverse effects of difficulty getting to sleep and disruption of sleep when occupancy was estimated to be between 70% and 84%. Additionally, polysomnography during the recovery sleep period after sleep deprivation showed some evidence for sleep disturbance as long as 19–31 hours after dosing, when brain H3 receptor occupancy levels were between 78% and 69%. These studies indicated that, although a clinically significant alerting efficacy could clearly be attributed to the H3 inverse agonist mechanism, the pharmacodynamic profile of MK0249 did not allow the wanted alerting and unwanted sleep disrupting effects to be separated and the compound to be developed further. Nevertheless, these imaging studies guided future program goals by showing that the profile of an optimal H3 inverse agonist molecule for excessive daytime sleepiness would need to achieve high levels of receptor occupancy rapidly but have a much shorter half-life than MK-0249 so that the drug would be cleared from the brain when it came time to sleep. CLINICAL PHARMACOLOGY & THERAPEUTICS | VOLUME 98 NUMBER 1 | JULY 2015
PET imaging was also used to facilitate “Go/No Go” decisions on MRK-409 an a3-subunit-preferring GABAA agonist.33 MRK409 produced anxiolytic-like activity in rodent and primate unconditioned and conditioned models of anxiety with minimum effective doses corresponding to GABAA receptor occupancies ranging from 35–65%, depending on the particular model used. Interestingly, when MK-409 was dosed to achieve occupancies of >90%, animals showed minimal overt signs of sedation suggesting that it had the unique potential to be a nonsedating anxiolytic. Unfortunately, safety and tolerability studies in humans showed that there was pronounced sedation at a dose of 2 mg setting the maximum clinically tolerated single dose of MRK-409 at 1 mg. PET studies subsequently showed that [11C]flumazenil uptake following the 1 mg dose was comparable to that after placebo administration, indicating that occupancy of GABAA receptor benzodiazepine binding sites by MRK-409 was below the limits of detection (i.e.,
