Journal of Steroid Biochemistry & Molecular Biology 145 (2015) 157–163
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Review
Targeting extra-gonadal androgens in castration-resistant prostate cancer Emily Grist a,b, * , Johann S. de Bono a,b , Gerhardt Attard a,b a b
Institute of Cancer Research, Cancer Therapeutics, 15 Cotswold Rd, Sutton, Surrey SM25NG, UK Royal Marsden NHS Foundation Trust, London, UK
A R T I C L E I N F O
A B S T R A C T
Article history: Received 11 July 2014 Received in revised form 4 September 2014 Accepted 6 September 2014 Available online 22 September 2014
Metastatic castration resistant prostate cancer (CRPC) is associated with a rise in PSA, suggesting an increase in transcription of steroid receptor regulated genes. The efficacy of the new anti-androgen therapies abiraterone and enzalutamide, that target extra-gonadal activation of androgen signaling, confirm CRPC’s addiction to genes regulated by the androgen receptor (AR). However, patients invariably progress and develop resistance. This review focuses on mechanisms of drug resistance associated with the AR and steroidogenesis in CRPC. Understanding this persistent dependency and adaptation to the androgen axis in CRPC will lead to an understanding of resistance to new licensed therapies and to novel drug discovery, ultimately improving clinical outcome in CRPC. This article is part of a Special Issue entitled ‘Essential role of DHEA’. ã 2014 Elsevier Ltd. All rights reserved.
Contents 1. 2. 3.
4. 5.
6.
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . The androgen axis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Novel endocrine treatments recently licensed for CRPC . . . . . . . . Abiraterone . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1. Enzalutamide . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2. Experimental agents targeting the androgen axis in clinical trials Mechanisms of endocrine resistance to novel agents . . . . . . . . . . The androgen receptor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.1. Androgen receptor mutations . . . . . . . . . . . . . . . . . . . . . . . 5.2. Steroidogenesis in CRPC . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.3. De novo steroidogenesis . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.4. Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Combination or sequential use? . . . . . . . . . . . . . . . . . . . . . . 6.1. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1. Introduction Prostate cancer remains the second commonest cause of male cancer-related mortality and accounts for 1–2% of male deaths [1]. Huggins and Hodges first identified that prostate cancer was
* Corresponding author. Tel.: +44 020 7352 8133. E-mail addresses: [email protected], [email protected] (E. Grist), [email protected] (G. Attard). http://dx.doi.org/10.1016/j.jsbmb.2014.09.006 0960-0760/ ã 2014 Elsevier Ltd. All rights reserved.
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hormone responsive in 1941 [2]. Treatments for metastatic disease are initially primarily directed at androgen deprivation by medical or surgical castration and can result in prolonged periods of clinical and biochemical remission [3]. Metastatic castration resistant prostate cancer (CRPC) develops after a median of 18 months, usually identified by a rise in prostate specific antigen (PSA) but in some patients, also increased clinical symptoms and/or radiological progression of disease [4]. This rise in PSA suggests increased transcription of steroid receptor regulated genes. The recent phase III trials demonstrating the efficacy of novel anti-androgen
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Fig. 1. Illustration of tumor volume and activity over time in metastatic prostate cancer, demonstrating the integration and sequencing of novel anti-androgen therapies, chemotherapy and radium-223 in castration resistant prostate cancer (*denotes sipuleucel T treatment licensed in the US but not the UK).
therapies abiraterone [5] and enzalutamide [6], highlight that CRPC remains ‘addicted’ to androgen receptor (AR) signaling. These agents are now licensed treatments for CRPC (Fig. 1). The various mechanisms maintaining CRPC’s reliance on the androgen axis will be discussed in greater detail in this review; these include amplification of the AR, epigenetic modifications affecting co-repressors and co-activators of the AR, AR mutations and alterations to key enzymes involved in steroidogenesis. Recognition of the involvement of these mechanisms in the development of castration resistance led to development of abiraterone and enzalutamide to target extra-gonadal androgen synthesis. Recent evidence suggests that re-activation of the AR is one of the mechanisms involved in resistance to abiraterone and enzalutamide in a proportion of patients. Moreover other members of the steroid receptor family, namely the glucocorticoid receptor [7], have also recently been implicated in the development of resistance.
3. Novel endocrine treatments recently licensed for CRPC 3.1. Abiraterone
2. The androgen axis
Abiraterone is a potent and irreversible inhibitor of CYP17 that was designed in the early 1990s at “The Institute of Cancer Research” (ICR) [12]. Abiraterone has been licensed in combination with prednisone 5 mg bid due to a syndrome of mineralocorticoid excess secondary to a rise in ACTH occurring in up to 80% of patients when abiraterone was used alone. The first phase III trial (COU-AA-301) recruited 1197 CRPC patients that had received docetaxel chemotherapy. Patients were randomised 2:1 to either abiraterone and prednisone (5 mg twice a day) or the same dose prednisone and placebo [5]. Abiraterone and prednisone significantly prolonged median overall survival compared to prednisone alone (14.8 months vs 10.9 months respectively, hazard ratio 0.65, p < 0.001). All secondary endpoints were significantly improved in the abiraterone arm. The regulatory authorities in North America and the EU granted licensing approval for abiraterone and prednisone for chemotherapy-treated patients
Medical and surgical castration as a first-line therapy does not suppress extra-gonadal precursors of androgens, notably androgenic steroids made by the adrenal glands. The leydig cells of the testes and the adrenal cortex convert cholesterol to pregnenolones via CYP11A1. These in turn are converted to the 19-carbon steroids DHEA, DHEA sulfate and androstenedione via CYP17 following 17a-hydroxylase and C17,20 lyase enzymatic activity, both of which are inhibited by abiraterone. DHEA or androstenedione are then converted to testosterone by several isoforms of 17b-hydroxysteroid dehydrogenase, or to dihydrotestosterone by 5a-reductase, to form high affinity AR ligands (reviewed in [8,9] and Fig. 2). The adrenal glands also convert progestagens to the C19-steroids cortisol, aldosterone and the androgenic steroids DHEA, DHEA-sulfate and androstenedione. Initial indications that adrenal androgens contributed to CRPC were observed when Adrenalectomy resulted in clinical responses in CRPC [10]. Subsequently clinical trials of the non-specific CYP inhibitor, ketoconazole reported significant anti-tumor activity that was associated with decreases in adrenal androgens [11]. However, significant toxicity limited its use and a rise in androgens at progression suggested loss of CYP17 inhibition led to resistance to this agent.
Fig. 2. Androgen biosynthesis pathway. Abiraterone inhibits the enzymatic activities of the two CYP17 A1 enzymes, 17a hydroxylase and C17,20-lyase. Source: Figure adapted from Eichloz A., Ferraldeschi R. et al. Putting the brakes on continued androgen receptor signaling in castration resistant prostate cancer. Molecular and Cellular Endocrinology. 360 (2012) 68–75.
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Table 1 Clinical trials for novel compounds acting on the androgen axis in CRPC from clinicaltrials.gov. Compound
Trial stage
Compound action
Recruitment
NCT number
VT-464 Orteronel plus prednisolone vs placebo plus prednisolone Galeterone ARN-509 vs placebo ODM 201 ODM 201
Phase Phase Phase Phase Phase Phase
Selective CYP17A1-lyase inhibitor CYP17A1 inhibitor Dual AR antagonist and CYP17A1 inhibitor AR antagonist AR antagonist AR antagonist
Recruiting Completed Recruiting Recruiting Completed Active, not recruiting
NCT02130700 Ref. [18] NCT01709734 NCT01946204 NCT01317641 NCT01429064
II III II III I/II II
in 2011. The COU-AA-302 trial followed and confirmed the efficacy of abiraterone and prednisone in chemotherapy naive CRPC [13,14]. This trial recruited 1088 patients and showed abiraterone improved survival (median 35.5 months vs 30.1 months, Hazard ratio 0.79, p = 0.0151) and improved radiographic progression free survival (16.5 months vs 8.2 months, hazard ration 0.52 p < 0.0001). 3.2. Enzalutamide Enzalutamide (MDV3100) is a rationally-designed, small molecule AR antagonist developed at the University of California, Los Angeles, to antagonize AR in bicalutamide-resistant pre-clinical models that over-expressed AR or had an AR mutation that was activated by bicalutamide. Enzalutamide does not require concomitant steroid administration. The AFFIRM trial was a phase III double-blind placebo-controlled trial with 1199 CRPC patients that had previously received docetaxel chemotherapy randomised 2:1 to enzalutamide or placebo. Overall median survival was 18.4 months in the enzalutamide arm and 13.6 months in the placebo arm (Hazard ratio for death 0.631, p < 0.0001) [6]. The PREVAIL clinical trial confirmed the efficacy of enzalutamide in treating chemotherapy naive CRPC patients [15]. In this trial, the rate of radiographic progression-free survival at 12 months was 65% among patients receiving enzalutamide as compared with 14% among patients receiving placebo (81% risk reduction, hazard ratio
in the enzalutamide group 0.19, p < 0.001) and a 29% reduction in the risk of death was seen (hazard ratio 0.71, p < 0.001). Whilst abiraterone and enzalutamide are successful new treatments in CRPC, resistance invariably emerges. Enzalutamide appears to be less effective when sequenced after abiraterone [16,17], and abiraterone treatment is less effective when given after enzalutamide [18,19]. These two treatments have led to a change in the CRPC treatment paradigm as demonstrated in Fig. 1. Whilst abiraterone and enzalutamide 105 are successful new treatments in CRPC, resistance invariably 106 emerges. Enzalutamide appears to be less effective when 107 sequenced after abiraterone [16,17], and abiraterone treatment 108 is less effective when given after enzalutamide [18,19]. Radium-223, an alpha-emitter that is taken up by osteoblasts, has been studied in a placebo-controlled randomised trial of 921 CRPC patients with symptomatic bone metastases. In this trial, radium-223 improved overall survival (Median 14.9 vs 11.3 months, HR 0.7, p < 0.001) [20]. Due to the different mechanism of action compared to abiraterone and enzalutamide, it could be an important adjunct to AR targeting treatments for CRPC. Docetaxel and cabazitaxel are both effective treatments for CRPC [21,22] and are currently often used after endocrine treatment due to improved tolerance of the former. Emerging evidence suggests the mechanism of action of taxanes is in part related to inhibition of AR translocation to the nucleus and
Fig. 3. Suggested mechanisms of resistance and disease progression in metastatic CRPC. (A) AR amplification, (B) mutations in the ligand binding domain leading to promiscuous activation by multiple ligands, (C) increased glucocorticoid receptor expression with bypassing of AR blockade, (D) constitutively active AR splice variants, (E) increased conversion of adrenal precursors to high affinity AR ligands and (F) de novo steroidogenesis and CYP17 upregulation.
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there could therefore exist cross-resistance between endocrine manipulations and taxanes. In one study, response to subsequent docetaxel treatment was diminished in abiraterone resistant patients and more data is required in support of this. In contrast, the activity of cabazitaxel appears maintained after prior abiraterone and enzalutamide treatment [23]. Biomarkers that identify patient sub-groups with enhanced sensitivity to taxanes are required to select patients who would derive greater benefit from docetaxel, prior to abiraterone or enzalutamide [23,24]. 4. Experimental agents targeting the androgen axis in clinical trials Significant research interest and drug development investment has been made in developing other novel agents or novel treatment combinations, targeting the androgen axis in CRPC (Table 1). To avoid the detrimental effects of glucocorticoid co-administration, new agents with postulated improved selectivity for C17, 20-lyase have been developed. One example is VT-464, which has been rationally designed as a new selective CYP17 lyase inhibitor [25] and is now being tested in phase II prostate cancer clinical trials (Table 1). The development of new agents targeting CYP17 inhibition with overall survival as a regulatory end-point is challenging given the generally unavoidable and recommended prior or subsequent treatment of patients with abiraterone. For example, orteronel (TAK-700), a CYP17 inhibitor demonstrated significant activity in phase I/II trials. However, in subsequent phase III trials in which it was also administered in combination with prednisone, a survival advantage was not demonstrated in CRPC patients. This could be explained by a cross over effect of patients in the placebo arm of the trial receiving effective AR targeting agents after discontinuing the trial [26]. Several other AR antagonists have been developed and two are currently in advanced clinical trials. ARN 509 has a similar structure and pre-clinical properties to enzalutamide with reported less blood-brain passage in mouse models [27]. ODM-201 is reputedly a structurally distinct AR antagonist with significant activity in abiraterone-naïve and enzalutamide-naïve CRPC but limited activity in patients that had prior abiraterone [28]. The challenge of developing these agents in patients previously treated with abiraterone or enzalutamide has led to the evaluation of ARN-509 in phase III trials recruiting CRPC patients with no radiologically detectable metastatic disease (clinicaltrials.gov NCT01946204). Other therapeutic approaches for targeting the AR merit further investigation. EZN-4176 is an anti-sense oligonucleotide designed to bind to and knock-down expression of full length AR. A phase I study demonstrated limited anti-tumor activity with EZN-4176 at the doses explored but the use of anti-sense as a strategy for inhibiting the AR remains an interesting approach [29]. Targeting of the amino-terminal domain of the AR also holds promise as a potential therapeutic strategy. EPI-001 is a small molecule that blocked transactivation of the amino-terminal domain of the AR. EPI-001 blocked the interaction of the AR with androgen response elements on target genes and its use resulted in regression of CRPC in xenograft models [30]. 5. Mechanisms of endocrine resistance to novel agents 5.1. The androgen receptor The AR gene is amplified at the genomic level in approximately one third of cases at development of castration resistance, and the AR protein is commonly overexpressed after castration and traditional androgen deprivation therapy [31,32] (Fig. 3A). Both of these changes are uncommon in untreated prostate cancer and are considered adaptive changes to castration and “traditional”
anti-androgen treatment. Increased AR mRNA was found to be associated with resistance to anti-androgen therapy in prostate cancer xenograft models and was sufficient to cause resistance to anti-androgen treatment [33]. Furthermore, androgen regulated genes were also found to be expressed at high levels in CRPC compared with benign prostate epithelia [34]. This suggests that transcriptional activity of the AR is reactivated in CRPC despite castrate levels of serum androgen. The mechanisms that underlie resistance to abiraterone and enzalutamide are still to be elucidated but early reports suggest that AR amplification is also associated with resistance to abiraterone and leads to adaptation to re-activation of AR signaling in some patients treated with novel endocrine treatments [35] [36]. In several preclinical models, activation of the AR in the absence of ligand occurs with AR splice variants that lack the LBD. Some studies have suggested that AR splice variants become more abundant in CRPC. AR V567es and AR V7 are the main ones to have been studied to date (Fig. 3D). Splice variants have been seen to arise in response to androgen deprivation therapy and abiraterone [37], and appear to be upregulated in prostate cancer progression [38,39]. AR-V7 in LNCaP xenograft models confers a castration resistant phenotype [40] and AR V7 was found to be predictive, in anti-androgen therapy naive disease, for biochemical recurrence following prostatectomy [39,41]. AR-V7 lacks a LBD and a recent study presented at ASCO 2014 suggested that detection of AR-V7 in circulating tumor cells from men with metastatic CRPC is associated with resistance to abiraterone and enzalutamide [42]. Disturbances in the co-regulators of the AR have also been hypothesised as a mechanism for progression of disease from a castration sensitive to a castrate resistant state [43]. For example, the steroid receptor co-activator (SRC) family may increase the ARs sensitivity to weak AR agonists DHEA and androstenedione [44]. SRC factors are higher in prostate cancer and have been found at even higher levels in CRPC [44]. Mutations have been identified in the AR, in particular in the hinge region, that result in stabilization of the AR by enhancing co-activator binding sites, or result in an inability for the AR to;1; bind to co-repressors (reviewed in [45]). 5.2. Androgen receptor mutations It is reported that AR mutations are prevalent in 10–20% of CRPC cases [46,47] (Fig. 3B). As with AR amplification, AR mutations are more commonly detected after treatment with anti-androgens, suggesting genomic adaptions are a mechanism of treatment resistance and in some cases lead to AR promiscuity resulting in inappropriate activation by several ligands. For example, the T877A point mutation of the AR in the LNCaP cell line resulted in increased AR activation by progestins, oestrogens and adrenal androgens [48,49]. Two decades ago point mutations in the LBD were identified in 5 out of 10 cases of CRPC, which were not identifiable in the archival diagnostic tissue collected at a castration sensitive stage of disease [47]. D879G or W741C AR mutations have been demonstrated to confer resistance to bicalutamide [47,50]. A withdrawal response can be seen in some patients stopping bicalutamide resulting in a PSA decline, suggesting these mutations result in bicalutamide gaining agonistic properties. In pre-clinical models using the LNCaP cell line, a F878L mutation in the LBD of the AR led to resistance to enzalutamide [51]. This AR genomic aberration changed the antagonistic action of enzalutamide to agonistic. This mutation was detected in 3 out of 29 CRPC patients with progressive disease receiving ARN-509, a drug similar in action to enzalutamide [52]. A double mutation has been identified in the AR, T877A and L702H, in a cell line derived from a patient that progressed through anti-androgen therapy and had received prior glucocorticoids as
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part of his treatment [7]. The mutated AR appeared to be activated by glucocorticoid with activation of androgen responsive genes, including a rise in PSA and in response to cortisol increased prostate cancer cell proliferation. Recently, it has been suggested in pre-clinical models, that increased GR expression may be a mechanism of resistance whereby GR bypasses enzalutamidemediated AR blockade without the need for any restored AR function [53] (Fig. 3C). This finding has important clinical consequences and has raised concern that corticosteroids commonly prescribed for prostate cancer patients could drive resistance. It has been established that in patients with malignancies, tumor aberrations can be detected from circulating tumor DNA. Circulating plasma tumor DNA (CtDNA) can be quantified and sequenced to identify point mutations and copy number changes. CtDNA sequential monitoring holds promise as a potential biomarker and as a non-invasive diagnostic tool improving out understanding of the unique genomic makeup of malignancies in order to personalize treatment approaches [36,54–57]. Collecting and sequencing sequential CtDNA will help us describe the evolution of genomic aberrations, as resistance to treatment develops and we believe, that in the future this will assist us in novel drug design and improve treatment strategies to delay or reverse resistance. 5.3. Steroidogenesis in CRPC One of the drivers of CRPC has been hypothesised as prostate cancers ability to utilize non-testicular sources of androgen in the castrate state with low circulating levels of androgen (Fig. 3E). Significant clinical benefit is observed when treating patients with first line anti-androgen therapies after relapse following castration [58–60], indicating the importance of continued AR ligand dependent activation in the castrate state, driving disease progression. Normal prostate tissue is able to convert DHEA and DHEA sulfate into high affinity AR ligands testosterone and dihydrotestosterone. A collection of evidence has suggested that in CRPC, levels of androgen in recurrent tumor are higher than in serum. Soft tissue metastases from castration resistant prostate cancers exhibited elevated levels of testosterone compared with untreated primary tumors [61,62]. An increase in enzymes responsible for steroid synthesis from adrenal androgens in CRPC tumor have also been reported [63]. AKR1C3 in CRPC bone marrow biopsies compared with primary prostate cancers was upregulated. This is an enzyme that converts DHEA and androstenedione into testosterone and dihydrotestosterone, supporting the hypothesis that CRPC cells are able to convert weak androgens to higher affinity AR ligands [64]. It has been demonstrated that dihydrotestosterone could be produced from precursors that completely circumvent testosterone via three enzymes SRD5A, 17b hydroxysteroid dehydrogenase (HSD) and 3b HSD [65]. Furthermore, a gain-of-function 367T mutation of 3b HSD has been described, which increased the conversion of DHEA to dihydrotestosterone through increased protein resistance to ubiquitination and degradation [66]. The authors describe this, resulting in ‘augmented 3b HSD activity due to increased abundance of the mutated protein, resulting in an increase in the proximal and otherwise rate-limiting step for dihydrotestosterone production’. 5.4. De novo steroidogenesis It remains speculative, partly due to the challenge with proving it, that CRPC cells are able to produce androgens directly from cholesterol de novo, independently of circulating adrenal androgens (Fig. 3F). This mechanism may explain in part the success of
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the CYP17 inhibitor abiraterone in CRPC. CYP17, is highly expressed in CRPC [62]. In CRPC xenograft models and in the LNCaP cell line, it was demonstrated that cholesterol synthesis is increased and that all enzymes necessary for de novo androgen synthesis were present [67]. Acetic acid, a precursor to cholesterol, was radiolabelled to [14C] acetic acid using LC-radiometric detection and was converted to dihydrotestosterone in CRPC LNCaP xenograft cells. However, it is not clear from this study whether this dihydrotestosterone production was adequate to activate the AR. The expression of enzymes involved in steroid biosynthesis in the VCaP cell line has been assessed. This identified high levels of CYP17 [68]. CYP17 is an enzyme involved in the rate-limiting step of converting pregnenolones to DHEA. Ketoconazole and abiraterone both decreased PSA expression in VCaP cells. This affect was reversed when a downstream androgen precursor androstenedione was added. The possible mechanisms of resistance to abiraterone were explored and CYP17 expression was increased in all relapsed VCaP xenografts, suggesting CYP17 upregulation provides a selective advantage to survival of tumor cells during abiraterone treatment, leading to treatment resistance, possibly due to increased intratumor de novo androgen production. Furthermore, tumor biopsies from CRPC patients treated with ketoconazole, a less selective CY17 inhibitor, also demonstrated markedly elevated levels of CYP17. The authors hypothesised that strong treatment selective pressures increase CYP17 and increase tumors dependency on de novo steroidogenesis. Contrary to this collection of evidence supporting intra tumor de novo steroidogenesis as a mechanism of CRPC progression and resistance to anti-androgen therapies, a further study identified low or absent mRNA expression of enzymes required for de novo steroid synthesis in normal prostate tissue, prostate cancer tissue and CRPC tissue. This included tissue from local disease and lymph node metastases [69]. Enzymes converting circulating adrenal steroid precursors androstenedione to testosterone (AKR1C3) were however identified in abundance. CYP17 essential for de novo steroid synthesis was expressed at only low levels. The authors concluded that the contribution of adrenal steroid precursors contribute more than steroids produced de novo to disease progression and treatment resistance. 6. Conclusion Resistance in CRPC to abiraterone and enzalutamide treatment could be due to an accumulation of changes in some patients that preserve activation of the androgen axis. Furthermore, CRPC patients progressing on abiraterone and enzalutamide demonstrate a rising PSA in the majority of cases, suggesting that repeated interventions targeting the androgen axis may be of clinical benefit to a subgroup of patients. 6.1. Combination or sequential use? Whilst enzalutamide’s affinity for the AR is high, it is less than dihydrotestosterone, and therefore outcompeting of enzalutamide at the AR may occur with increasing androgen synthesis [70]. Preliminary reports of the combination of abiraterone and prednisone with enzalutamide assessed in a phase I clinical trial in M.D. Anderson suggest this may be a well tolerated and effective strategy (clinicaltrials.gov NCT01650194) [71]. An industry sponsored randomised trial (PLATO) is evaluating the efficacy of continuing enzalutamide beyond progression and after initiation of abiraterone with prednisone compared to placebo with abiraterone and prednisone in CRPC patients, biochemically progressing on enzalutamide treatment (clinicaltrials.gov NCT01995513) and a combination strategy is also being assessed by a co-operative group in chemotherapy naïve CRPC patients
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(clinicaltrials.gov NCT01949337). Abiraterone and enzalutamide in combination vs abiraterone alone is also being assessed (clinicaltrials.gov NCT01946165). The benefit of combination treatment would need to offset the disadvantages of such a strategy which include cost, exposure to even more prolonged periods of a glucocorticoid and further treatment selective pressures which may lead to the outgrowth of different tumor cell clones adapted to evade later therapy. Whilst corticosteroid use in CRPC with abiraterone improves tolerability, evidence is emerging that it may also potentially drive resistance. Further research is required to understand the best combination of novel agents acting on elements of the androgen axis in CRPC and to understand in which sequence to deliver these therapies during different stages of disease. Whilst prostate cancer may become castrate resistant, it does not become hormone independent in a significant proportion of patients and further exploration of steroidogenesis and mechanisms targeting the AR axis leading to resistance could lead to newer therapies and better patient outcomes. Acknowledgements The Institute of Cancer Research developed abiraterone and, therefore, has a commercial interest in this agent. G.A. is on the ICR list of rewards to inventors for abiraterone. J.S.d.B. has received consulting fees and travel support from Amgen, Astellas, AstraZeneca, Boehringer Ingelheim, Bristol-Myers Squibb, Dendreon, Enzon, Exelixis, Genentech,GlaxoSmithKline, Medivation, Merck, Novartis, Pfizer, Roche, Sanofi-Aventis, Supergen and Takeda, and grant support from AstraZeneca and Genentech. G.A. has received honoraria, consulting fees or travel support from Astellas, Medivation, Janssen, Millennium Pharmaceuticals, Ipsen, Takeda and Sanofi-Aventis, and grant support from Janssen, AstraZeneca and Genentech. References [1] Cancer Research UK, Cancer Risk. 2012; Available from: http://www. cancerresearchuk.org/cancer-info/cancerstats/incidence/risk [2] C. Huggins, C.V. Hodges, Studies on prostate cancer. The effect of castration, or estrogen and of androgen injection on serum phosphatases in metastatic carcinoma of the prostate, Cancer Res. 1 (1941) 293–297. [3] F. Labrie, et al., Can combined androgen blockade provide long-term control or possible cure of localized prostate cancer? Urology 60 (1) (2002) 115–119. [4] B.A. Hellerstedt, K.J. Pienta, The current state of hormonal therapy for prostate cancer, CA Cancer J. Clin. 52 (3) (2002) 154–179. [5] J.S. de Bono, et al., Abiraterone and increased survival in metastatic prostate cancer, N. Engl. J. Med. 364 (21) (2011) 1995–2005. [6] H.I. Scher, et al., Increased survival with enzalutamide in prostate cancer after chemotherapy, N. Engl. J. Med. 367 (13) (2012) 1187–11977. [7] X.Y. Zhao, et al., Glucocorticoids can promote androgen-independent growth of prostate cancer cells through a mutated androgen receptor, Nat. Med. 6 (6) (2000) 703–706. [8] F. Labrie, Combined blockade of testicular and locally made androgens in prostate cancer: a highly significant medical progress based upon intracrinology, J. Steroid. Biochem. Mol. Biol. (2014) [Epub ahead of print]. [9] M.L. Auchus, R.J. Auchus, Human steroid biosynthesis for the oncologist, J. Investig. Med. 60 (2) (2012) 495–503. [10] E.M. Mahoney, J.H. Harrison, Bilateral adrenalectomy for palliative treatment of prostatic cancer, J. Urol. 108 (6) (1972) 936–938. [11] E.J. Small, et al., Antiandrogen withdrawal alone or in combination with ketoconazole in androgen-independent prostate cancer patients: a phase III trial (CALGB 9583), J. Clin. Oncol. 22 (6) (2004) 1025–1033. [12] S.E. Barrie, et al., Pharmacology of novel steroidal inhibitors of cytochrome P450(17) alpha (17 alpha-hydroxylase/C17–20 lyase), J. Steroid. Biochem. Mol. Biol. 50 (5–6) (1994) 267–273. [13] C.J. Ryan, et al., Abiraterone in metastatic prostate cancer without previous chemotherapy, N. Engl. J. Med. 368 (2) (2013) 138–148. [14] D.E. Rathkopf, et al., Updated interim efficacy analysis and long-term safety of abiraterone acetate in metastatic castration-resistant prostate cancer patients without prior chemotherapy (COU-AA-302), Urol. Eur. (2014) [Epub ahead of print]. [15] T.M. Beer, et al., Enzalutamide in metastatic prostate cancer before chemotherapy, N. Engl. J. Med. 371 (July (5)) (2014) 424–433.
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Contents lists available at ScienceDirect
Journal of Steroid Biochemistry & Molecular Biology journal homepage: www.elsevier.com/locate/jsbmb
Review
Targeting extra-gonadal androgens in castration-resistant prostate cancer Emily Grist a,b, * , Johann S. de Bono a,b , Gerhardt Attard a,b a b
Institute of Cancer Research, Cancer Therapeutics, 15 Cotswold Rd, Sutton, Surrey SM25NG, UK Royal Marsden NHS Foundation Trust, London, UK
A R T I C L E I N F O
A B S T R A C T
Article history: Received 11 July 2014 Received in revised form 4 September 2014 Accepted 6 September 2014 Available online 22 September 2014
Metastatic castration resistant prostate cancer (CRPC) is associated with a rise in PSA, suggesting an increase in transcription of steroid receptor regulated genes. The efficacy of the new anti-androgen therapies abiraterone and enzalutamide, that target extra-gonadal activation of androgen signaling, confirm CRPC’s addiction to genes regulated by the androgen receptor (AR). However, patients invariably progress and develop resistance. This review focuses on mechanisms of drug resistance associated with the AR and steroidogenesis in CRPC. Understanding this persistent dependency and adaptation to the androgen axis in CRPC will lead to an understanding of resistance to new licensed therapies and to novel drug discovery, ultimately improving clinical outcome in CRPC. This article is part of a Special Issue entitled ‘Essential role of DHEA’. ã 2014 Elsevier Ltd. All rights reserved.
Contents 1. 2. 3.
4. 5.
6.
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . The androgen axis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Novel endocrine treatments recently licensed for CRPC . . . . . . . . Abiraterone . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1. Enzalutamide . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2. Experimental agents targeting the androgen axis in clinical trials Mechanisms of endocrine resistance to novel agents . . . . . . . . . . The androgen receptor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.1. Androgen receptor mutations . . . . . . . . . . . . . . . . . . . . . . . 5.2. Steroidogenesis in CRPC . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.3. De novo steroidogenesis . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.4. Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Combination or sequential use? . . . . . . . . . . . . . . . . . . . . . . 6.1. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1. Introduction Prostate cancer remains the second commonest cause of male cancer-related mortality and accounts for 1–2% of male deaths [1]. Huggins and Hodges first identified that prostate cancer was
* Corresponding author. Tel.: +44 020 7352 8133. E-mail addresses: [email protected], [email protected] (E. Grist), [email protected] (G. Attard). http://dx.doi.org/10.1016/j.jsbmb.2014.09.006 0960-0760/ ã 2014 Elsevier Ltd. All rights reserved.
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hormone responsive in 1941 [2]. Treatments for metastatic disease are initially primarily directed at androgen deprivation by medical or surgical castration and can result in prolonged periods of clinical and biochemical remission [3]. Metastatic castration resistant prostate cancer (CRPC) develops after a median of 18 months, usually identified by a rise in prostate specific antigen (PSA) but in some patients, also increased clinical symptoms and/or radiological progression of disease [4]. This rise in PSA suggests increased transcription of steroid receptor regulated genes. The recent phase III trials demonstrating the efficacy of novel anti-androgen
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Fig. 1. Illustration of tumor volume and activity over time in metastatic prostate cancer, demonstrating the integration and sequencing of novel anti-androgen therapies, chemotherapy and radium-223 in castration resistant prostate cancer (*denotes sipuleucel T treatment licensed in the US but not the UK).
therapies abiraterone [5] and enzalutamide [6], highlight that CRPC remains ‘addicted’ to androgen receptor (AR) signaling. These agents are now licensed treatments for CRPC (Fig. 1). The various mechanisms maintaining CRPC’s reliance on the androgen axis will be discussed in greater detail in this review; these include amplification of the AR, epigenetic modifications affecting co-repressors and co-activators of the AR, AR mutations and alterations to key enzymes involved in steroidogenesis. Recognition of the involvement of these mechanisms in the development of castration resistance led to development of abiraterone and enzalutamide to target extra-gonadal androgen synthesis. Recent evidence suggests that re-activation of the AR is one of the mechanisms involved in resistance to abiraterone and enzalutamide in a proportion of patients. Moreover other members of the steroid receptor family, namely the glucocorticoid receptor [7], have also recently been implicated in the development of resistance.
3. Novel endocrine treatments recently licensed for CRPC 3.1. Abiraterone
2. The androgen axis
Abiraterone is a potent and irreversible inhibitor of CYP17 that was designed in the early 1990s at “The Institute of Cancer Research” (ICR) [12]. Abiraterone has been licensed in combination with prednisone 5 mg bid due to a syndrome of mineralocorticoid excess secondary to a rise in ACTH occurring in up to 80% of patients when abiraterone was used alone. The first phase III trial (COU-AA-301) recruited 1197 CRPC patients that had received docetaxel chemotherapy. Patients were randomised 2:1 to either abiraterone and prednisone (5 mg twice a day) or the same dose prednisone and placebo [5]. Abiraterone and prednisone significantly prolonged median overall survival compared to prednisone alone (14.8 months vs 10.9 months respectively, hazard ratio 0.65, p < 0.001). All secondary endpoints were significantly improved in the abiraterone arm. The regulatory authorities in North America and the EU granted licensing approval for abiraterone and prednisone for chemotherapy-treated patients
Medical and surgical castration as a first-line therapy does not suppress extra-gonadal precursors of androgens, notably androgenic steroids made by the adrenal glands. The leydig cells of the testes and the adrenal cortex convert cholesterol to pregnenolones via CYP11A1. These in turn are converted to the 19-carbon steroids DHEA, DHEA sulfate and androstenedione via CYP17 following 17a-hydroxylase and C17,20 lyase enzymatic activity, both of which are inhibited by abiraterone. DHEA or androstenedione are then converted to testosterone by several isoforms of 17b-hydroxysteroid dehydrogenase, or to dihydrotestosterone by 5a-reductase, to form high affinity AR ligands (reviewed in [8,9] and Fig. 2). The adrenal glands also convert progestagens to the C19-steroids cortisol, aldosterone and the androgenic steroids DHEA, DHEA-sulfate and androstenedione. Initial indications that adrenal androgens contributed to CRPC were observed when Adrenalectomy resulted in clinical responses in CRPC [10]. Subsequently clinical trials of the non-specific CYP inhibitor, ketoconazole reported significant anti-tumor activity that was associated with decreases in adrenal androgens [11]. However, significant toxicity limited its use and a rise in androgens at progression suggested loss of CYP17 inhibition led to resistance to this agent.
Fig. 2. Androgen biosynthesis pathway. Abiraterone inhibits the enzymatic activities of the two CYP17 A1 enzymes, 17a hydroxylase and C17,20-lyase. Source: Figure adapted from Eichloz A., Ferraldeschi R. et al. Putting the brakes on continued androgen receptor signaling in castration resistant prostate cancer. Molecular and Cellular Endocrinology. 360 (2012) 68–75.
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Table 1 Clinical trials for novel compounds acting on the androgen axis in CRPC from clinicaltrials.gov. Compound
Trial stage
Compound action
Recruitment
NCT number
VT-464 Orteronel plus prednisolone vs placebo plus prednisolone Galeterone ARN-509 vs placebo ODM 201 ODM 201
Phase Phase Phase Phase Phase Phase
Selective CYP17A1-lyase inhibitor CYP17A1 inhibitor Dual AR antagonist and CYP17A1 inhibitor AR antagonist AR antagonist AR antagonist
Recruiting Completed Recruiting Recruiting Completed Active, not recruiting
NCT02130700 Ref. [18] NCT01709734 NCT01946204 NCT01317641 NCT01429064
II III II III I/II II
in 2011. The COU-AA-302 trial followed and confirmed the efficacy of abiraterone and prednisone in chemotherapy naive CRPC [13,14]. This trial recruited 1088 patients and showed abiraterone improved survival (median 35.5 months vs 30.1 months, Hazard ratio 0.79, p = 0.0151) and improved radiographic progression free survival (16.5 months vs 8.2 months, hazard ration 0.52 p < 0.0001). 3.2. Enzalutamide Enzalutamide (MDV3100) is a rationally-designed, small molecule AR antagonist developed at the University of California, Los Angeles, to antagonize AR in bicalutamide-resistant pre-clinical models that over-expressed AR or had an AR mutation that was activated by bicalutamide. Enzalutamide does not require concomitant steroid administration. The AFFIRM trial was a phase III double-blind placebo-controlled trial with 1199 CRPC patients that had previously received docetaxel chemotherapy randomised 2:1 to enzalutamide or placebo. Overall median survival was 18.4 months in the enzalutamide arm and 13.6 months in the placebo arm (Hazard ratio for death 0.631, p < 0.0001) [6]. The PREVAIL clinical trial confirmed the efficacy of enzalutamide in treating chemotherapy naive CRPC patients [15]. In this trial, the rate of radiographic progression-free survival at 12 months was 65% among patients receiving enzalutamide as compared with 14% among patients receiving placebo (81% risk reduction, hazard ratio
in the enzalutamide group 0.19, p < 0.001) and a 29% reduction in the risk of death was seen (hazard ratio 0.71, p < 0.001). Whilst abiraterone and enzalutamide are successful new treatments in CRPC, resistance invariably emerges. Enzalutamide appears to be less effective when sequenced after abiraterone [16,17], and abiraterone treatment is less effective when given after enzalutamide [18,19]. These two treatments have led to a change in the CRPC treatment paradigm as demonstrated in Fig. 1. Whilst abiraterone and enzalutamide 105 are successful new treatments in CRPC, resistance invariably 106 emerges. Enzalutamide appears to be less effective when 107 sequenced after abiraterone [16,17], and abiraterone treatment 108 is less effective when given after enzalutamide [18,19]. Radium-223, an alpha-emitter that is taken up by osteoblasts, has been studied in a placebo-controlled randomised trial of 921 CRPC patients with symptomatic bone metastases. In this trial, radium-223 improved overall survival (Median 14.9 vs 11.3 months, HR 0.7, p < 0.001) [20]. Due to the different mechanism of action compared to abiraterone and enzalutamide, it could be an important adjunct to AR targeting treatments for CRPC. Docetaxel and cabazitaxel are both effective treatments for CRPC [21,22] and are currently often used after endocrine treatment due to improved tolerance of the former. Emerging evidence suggests the mechanism of action of taxanes is in part related to inhibition of AR translocation to the nucleus and
Fig. 3. Suggested mechanisms of resistance and disease progression in metastatic CRPC. (A) AR amplification, (B) mutations in the ligand binding domain leading to promiscuous activation by multiple ligands, (C) increased glucocorticoid receptor expression with bypassing of AR blockade, (D) constitutively active AR splice variants, (E) increased conversion of adrenal precursors to high affinity AR ligands and (F) de novo steroidogenesis and CYP17 upregulation.
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there could therefore exist cross-resistance between endocrine manipulations and taxanes. In one study, response to subsequent docetaxel treatment was diminished in abiraterone resistant patients and more data is required in support of this. In contrast, the activity of cabazitaxel appears maintained after prior abiraterone and enzalutamide treatment [23]. Biomarkers that identify patient sub-groups with enhanced sensitivity to taxanes are required to select patients who would derive greater benefit from docetaxel, prior to abiraterone or enzalutamide [23,24]. 4. Experimental agents targeting the androgen axis in clinical trials Significant research interest and drug development investment has been made in developing other novel agents or novel treatment combinations, targeting the androgen axis in CRPC (Table 1). To avoid the detrimental effects of glucocorticoid co-administration, new agents with postulated improved selectivity for C17, 20-lyase have been developed. One example is VT-464, which has been rationally designed as a new selective CYP17 lyase inhibitor [25] and is now being tested in phase II prostate cancer clinical trials (Table 1). The development of new agents targeting CYP17 inhibition with overall survival as a regulatory end-point is challenging given the generally unavoidable and recommended prior or subsequent treatment of patients with abiraterone. For example, orteronel (TAK-700), a CYP17 inhibitor demonstrated significant activity in phase I/II trials. However, in subsequent phase III trials in which it was also administered in combination with prednisone, a survival advantage was not demonstrated in CRPC patients. This could be explained by a cross over effect of patients in the placebo arm of the trial receiving effective AR targeting agents after discontinuing the trial [26]. Several other AR antagonists have been developed and two are currently in advanced clinical trials. ARN 509 has a similar structure and pre-clinical properties to enzalutamide with reported less blood-brain passage in mouse models [27]. ODM-201 is reputedly a structurally distinct AR antagonist with significant activity in abiraterone-naïve and enzalutamide-naïve CRPC but limited activity in patients that had prior abiraterone [28]. The challenge of developing these agents in patients previously treated with abiraterone or enzalutamide has led to the evaluation of ARN-509 in phase III trials recruiting CRPC patients with no radiologically detectable metastatic disease (clinicaltrials.gov NCT01946204). Other therapeutic approaches for targeting the AR merit further investigation. EZN-4176 is an anti-sense oligonucleotide designed to bind to and knock-down expression of full length AR. A phase I study demonstrated limited anti-tumor activity with EZN-4176 at the doses explored but the use of anti-sense as a strategy for inhibiting the AR remains an interesting approach [29]. Targeting of the amino-terminal domain of the AR also holds promise as a potential therapeutic strategy. EPI-001 is a small molecule that blocked transactivation of the amino-terminal domain of the AR. EPI-001 blocked the interaction of the AR with androgen response elements on target genes and its use resulted in regression of CRPC in xenograft models [30]. 5. Mechanisms of endocrine resistance to novel agents 5.1. The androgen receptor The AR gene is amplified at the genomic level in approximately one third of cases at development of castration resistance, and the AR protein is commonly overexpressed after castration and traditional androgen deprivation therapy [31,32] (Fig. 3A). Both of these changes are uncommon in untreated prostate cancer and are considered adaptive changes to castration and “traditional”
anti-androgen treatment. Increased AR mRNA was found to be associated with resistance to anti-androgen therapy in prostate cancer xenograft models and was sufficient to cause resistance to anti-androgen treatment [33]. Furthermore, androgen regulated genes were also found to be expressed at high levels in CRPC compared with benign prostate epithelia [34]. This suggests that transcriptional activity of the AR is reactivated in CRPC despite castrate levels of serum androgen. The mechanisms that underlie resistance to abiraterone and enzalutamide are still to be elucidated but early reports suggest that AR amplification is also associated with resistance to abiraterone and leads to adaptation to re-activation of AR signaling in some patients treated with novel endocrine treatments [35] [36]. In several preclinical models, activation of the AR in the absence of ligand occurs with AR splice variants that lack the LBD. Some studies have suggested that AR splice variants become more abundant in CRPC. AR V567es and AR V7 are the main ones to have been studied to date (Fig. 3D). Splice variants have been seen to arise in response to androgen deprivation therapy and abiraterone [37], and appear to be upregulated in prostate cancer progression [38,39]. AR-V7 in LNCaP xenograft models confers a castration resistant phenotype [40] and AR V7 was found to be predictive, in anti-androgen therapy naive disease, for biochemical recurrence following prostatectomy [39,41]. AR-V7 lacks a LBD and a recent study presented at ASCO 2014 suggested that detection of AR-V7 in circulating tumor cells from men with metastatic CRPC is associated with resistance to abiraterone and enzalutamide [42]. Disturbances in the co-regulators of the AR have also been hypothesised as a mechanism for progression of disease from a castration sensitive to a castrate resistant state [43]. For example, the steroid receptor co-activator (SRC) family may increase the ARs sensitivity to weak AR agonists DHEA and androstenedione [44]. SRC factors are higher in prostate cancer and have been found at even higher levels in CRPC [44]. Mutations have been identified in the AR, in particular in the hinge region, that result in stabilization of the AR by enhancing co-activator binding sites, or result in an inability for the AR to;1; bind to co-repressors (reviewed in [45]). 5.2. Androgen receptor mutations It is reported that AR mutations are prevalent in 10–20% of CRPC cases [46,47] (Fig. 3B). As with AR amplification, AR mutations are more commonly detected after treatment with anti-androgens, suggesting genomic adaptions are a mechanism of treatment resistance and in some cases lead to AR promiscuity resulting in inappropriate activation by several ligands. For example, the T877A point mutation of the AR in the LNCaP cell line resulted in increased AR activation by progestins, oestrogens and adrenal androgens [48,49]. Two decades ago point mutations in the LBD were identified in 5 out of 10 cases of CRPC, which were not identifiable in the archival diagnostic tissue collected at a castration sensitive stage of disease [47]. D879G or W741C AR mutations have been demonstrated to confer resistance to bicalutamide [47,50]. A withdrawal response can be seen in some patients stopping bicalutamide resulting in a PSA decline, suggesting these mutations result in bicalutamide gaining agonistic properties. In pre-clinical models using the LNCaP cell line, a F878L mutation in the LBD of the AR led to resistance to enzalutamide [51]. This AR genomic aberration changed the antagonistic action of enzalutamide to agonistic. This mutation was detected in 3 out of 29 CRPC patients with progressive disease receiving ARN-509, a drug similar in action to enzalutamide [52]. A double mutation has been identified in the AR, T877A and L702H, in a cell line derived from a patient that progressed through anti-androgen therapy and had received prior glucocorticoids as
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part of his treatment [7]. The mutated AR appeared to be activated by glucocorticoid with activation of androgen responsive genes, including a rise in PSA and in response to cortisol increased prostate cancer cell proliferation. Recently, it has been suggested in pre-clinical models, that increased GR expression may be a mechanism of resistance whereby GR bypasses enzalutamidemediated AR blockade without the need for any restored AR function [53] (Fig. 3C). This finding has important clinical consequences and has raised concern that corticosteroids commonly prescribed for prostate cancer patients could drive resistance. It has been established that in patients with malignancies, tumor aberrations can be detected from circulating tumor DNA. Circulating plasma tumor DNA (CtDNA) can be quantified and sequenced to identify point mutations and copy number changes. CtDNA sequential monitoring holds promise as a potential biomarker and as a non-invasive diagnostic tool improving out understanding of the unique genomic makeup of malignancies in order to personalize treatment approaches [36,54–57]. Collecting and sequencing sequential CtDNA will help us describe the evolution of genomic aberrations, as resistance to treatment develops and we believe, that in the future this will assist us in novel drug design and improve treatment strategies to delay or reverse resistance. 5.3. Steroidogenesis in CRPC One of the drivers of CRPC has been hypothesised as prostate cancers ability to utilize non-testicular sources of androgen in the castrate state with low circulating levels of androgen (Fig. 3E). Significant clinical benefit is observed when treating patients with first line anti-androgen therapies after relapse following castration [58–60], indicating the importance of continued AR ligand dependent activation in the castrate state, driving disease progression. Normal prostate tissue is able to convert DHEA and DHEA sulfate into high affinity AR ligands testosterone and dihydrotestosterone. A collection of evidence has suggested that in CRPC, levels of androgen in recurrent tumor are higher than in serum. Soft tissue metastases from castration resistant prostate cancers exhibited elevated levels of testosterone compared with untreated primary tumors [61,62]. An increase in enzymes responsible for steroid synthesis from adrenal androgens in CRPC tumor have also been reported [63]. AKR1C3 in CRPC bone marrow biopsies compared with primary prostate cancers was upregulated. This is an enzyme that converts DHEA and androstenedione into testosterone and dihydrotestosterone, supporting the hypothesis that CRPC cells are able to convert weak androgens to higher affinity AR ligands [64]. It has been demonstrated that dihydrotestosterone could be produced from precursors that completely circumvent testosterone via three enzymes SRD5A, 17b hydroxysteroid dehydrogenase (HSD) and 3b HSD [65]. Furthermore, a gain-of-function 367T mutation of 3b HSD has been described, which increased the conversion of DHEA to dihydrotestosterone through increased protein resistance to ubiquitination and degradation [66]. The authors describe this, resulting in ‘augmented 3b HSD activity due to increased abundance of the mutated protein, resulting in an increase in the proximal and otherwise rate-limiting step for dihydrotestosterone production’. 5.4. De novo steroidogenesis It remains speculative, partly due to the challenge with proving it, that CRPC cells are able to produce androgens directly from cholesterol de novo, independently of circulating adrenal androgens (Fig. 3F). This mechanism may explain in part the success of
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the CYP17 inhibitor abiraterone in CRPC. CYP17, is highly expressed in CRPC [62]. In CRPC xenograft models and in the LNCaP cell line, it was demonstrated that cholesterol synthesis is increased and that all enzymes necessary for de novo androgen synthesis were present [67]. Acetic acid, a precursor to cholesterol, was radiolabelled to [14C] acetic acid using LC-radiometric detection and was converted to dihydrotestosterone in CRPC LNCaP xenograft cells. However, it is not clear from this study whether this dihydrotestosterone production was adequate to activate the AR. The expression of enzymes involved in steroid biosynthesis in the VCaP cell line has been assessed. This identified high levels of CYP17 [68]. CYP17 is an enzyme involved in the rate-limiting step of converting pregnenolones to DHEA. Ketoconazole and abiraterone both decreased PSA expression in VCaP cells. This affect was reversed when a downstream androgen precursor androstenedione was added. The possible mechanisms of resistance to abiraterone were explored and CYP17 expression was increased in all relapsed VCaP xenografts, suggesting CYP17 upregulation provides a selective advantage to survival of tumor cells during abiraterone treatment, leading to treatment resistance, possibly due to increased intratumor de novo androgen production. Furthermore, tumor biopsies from CRPC patients treated with ketoconazole, a less selective CY17 inhibitor, also demonstrated markedly elevated levels of CYP17. The authors hypothesised that strong treatment selective pressures increase CYP17 and increase tumors dependency on de novo steroidogenesis. Contrary to this collection of evidence supporting intra tumor de novo steroidogenesis as a mechanism of CRPC progression and resistance to anti-androgen therapies, a further study identified low or absent mRNA expression of enzymes required for de novo steroid synthesis in normal prostate tissue, prostate cancer tissue and CRPC tissue. This included tissue from local disease and lymph node metastases [69]. Enzymes converting circulating adrenal steroid precursors androstenedione to testosterone (AKR1C3) were however identified in abundance. CYP17 essential for de novo steroid synthesis was expressed at only low levels. The authors concluded that the contribution of adrenal steroid precursors contribute more than steroids produced de novo to disease progression and treatment resistance. 6. Conclusion Resistance in CRPC to abiraterone and enzalutamide treatment could be due to an accumulation of changes in some patients that preserve activation of the androgen axis. Furthermore, CRPC patients progressing on abiraterone and enzalutamide demonstrate a rising PSA in the majority of cases, suggesting that repeated interventions targeting the androgen axis may be of clinical benefit to a subgroup of patients. 6.1. Combination or sequential use? Whilst enzalutamide’s affinity for the AR is high, it is less than dihydrotestosterone, and therefore outcompeting of enzalutamide at the AR may occur with increasing androgen synthesis [70]. Preliminary reports of the combination of abiraterone and prednisone with enzalutamide assessed in a phase I clinical trial in M.D. Anderson suggest this may be a well tolerated and effective strategy (clinicaltrials.gov NCT01650194) [71]. An industry sponsored randomised trial (PLATO) is evaluating the efficacy of continuing enzalutamide beyond progression and after initiation of abiraterone with prednisone compared to placebo with abiraterone and prednisone in CRPC patients, biochemically progressing on enzalutamide treatment (clinicaltrials.gov NCT01995513) and a combination strategy is also being assessed by a co-operative group in chemotherapy naïve CRPC patients
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(clinicaltrials.gov NCT01949337). Abiraterone and enzalutamide in combination vs abiraterone alone is also being assessed (clinicaltrials.gov NCT01946165). The benefit of combination treatment would need to offset the disadvantages of such a strategy which include cost, exposure to even more prolonged periods of a glucocorticoid and further treatment selective pressures which may lead to the outgrowth of different tumor cell clones adapted to evade later therapy. Whilst corticosteroid use in CRPC with abiraterone improves tolerability, evidence is emerging that it may also potentially drive resistance. Further research is required to understand the best combination of novel agents acting on elements of the androgen axis in CRPC and to understand in which sequence to deliver these therapies during different stages of disease. Whilst prostate cancer may become castrate resistant, it does not become hormone independent in a significant proportion of patients and further exploration of steroidogenesis and mechanisms targeting the AR axis leading to resistance could lead to newer therapies and better patient outcomes. Acknowledgements The Institute of Cancer Research developed abiraterone and, therefore, has a commercial interest in this agent. G.A. is on the ICR list of rewards to inventors for abiraterone. J.S.d.B. has received consulting fees and travel support from Amgen, Astellas, AstraZeneca, Boehringer Ingelheim, Bristol-Myers Squibb, Dendreon, Enzon, Exelixis, Genentech,GlaxoSmithKline, Medivation, Merck, Novartis, Pfizer, Roche, Sanofi-Aventis, Supergen and Takeda, and grant support from AstraZeneca and Genentech. G.A. has received honoraria, consulting fees or travel support from Astellas, Medivation, Janssen, Millennium Pharmaceuticals, Ipsen, Takeda and Sanofi-Aventis, and grant support from Janssen, AstraZeneca and Genentech. References [1] Cancer Research UK, Cancer Risk. 2012; Available from: http://www. cancerresearchuk.org/cancer-info/cancerstats/incidence/risk [2] C. Huggins, C.V. Hodges, Studies on prostate cancer. The effect of castration, or estrogen and of androgen injection on serum phosphatases in metastatic carcinoma of the prostate, Cancer Res. 1 (1941) 293–297. [3] F. Labrie, et al., Can combined androgen blockade provide long-term control or possible cure of localized prostate cancer? Urology 60 (1) (2002) 115–119. [4] B.A. Hellerstedt, K.J. Pienta, The current state of hormonal therapy for prostate cancer, CA Cancer J. Clin. 52 (3) (2002) 154–179. [5] J.S. de Bono, et al., Abiraterone and increased survival in metastatic prostate cancer, N. Engl. J. Med. 364 (21) (2011) 1995–2005. [6] H.I. Scher, et al., Increased survival with enzalutamide in prostate cancer after chemotherapy, N. Engl. J. Med. 367 (13) (2012) 1187–11977. [7] X.Y. Zhao, et al., Glucocorticoids can promote androgen-independent growth of prostate cancer cells through a mutated androgen receptor, Nat. Med. 6 (6) (2000) 703–706. [8] F. Labrie, Combined blockade of testicular and locally made androgens in prostate cancer: a highly significant medical progress based upon intracrinology, J. Steroid. Biochem. Mol. Biol. (2014) [Epub ahead of print]. [9] M.L. Auchus, R.J. Auchus, Human steroid biosynthesis for the oncologist, J. Investig. Med. 60 (2) (2012) 495–503. [10] E.M. Mahoney, J.H. Harrison, Bilateral adrenalectomy for palliative treatment of prostatic cancer, J. Urol. 108 (6) (1972) 936–938. [11] E.J. Small, et al., Antiandrogen withdrawal alone or in combination with ketoconazole in androgen-independent prostate cancer patients: a phase III trial (CALGB 9583), J. Clin. Oncol. 22 (6) (2004) 1025–1033. [12] S.E. Barrie, et al., Pharmacology of novel steroidal inhibitors of cytochrome P450(17) alpha (17 alpha-hydroxylase/C17–20 lyase), J. Steroid. Biochem. Mol. Biol. 50 (5–6) (1994) 267–273. [13] C.J. Ryan, et al., Abiraterone in metastatic prostate cancer without previous chemotherapy, N. Engl. J. Med. 368 (2) (2013) 138–148. [14] D.E. Rathkopf, et al., Updated interim efficacy analysis and long-term safety of abiraterone acetate in metastatic castration-resistant prostate cancer patients without prior chemotherapy (COU-AA-302), Urol. Eur. (2014) [Epub ahead of print]. [15] T.M. Beer, et al., Enzalutamide in metastatic prostate cancer before chemotherapy, N. Engl. J. Med. 371 (July (5)) (2014) 424–433.
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