Commentary For reprint orders, please contact [email protected]
Gene therapy in the clinics: shifting into the next gear “Overall, gene therapy is now less surrounded by an aura of ‘just another fancy and highly complex treatment strategy, which will never result in an approved drug’, but rather becomes a vital option for otherwise incurable diseases.” Keywords: approved medicine n cancer n clinical trial n monogenetic disease n nonviral vectors n viral vectors
Gene therapy has faced several ups and downs since the first ideas about using gene vectors to selectively deliver genetic material into human cells (for a historic overview see [1]). The early 1990s brought a gene therapy hype, in 1992 even ‘The Economist’ titled a cover story ‘Changing your genes’, however, only 7 years later the deepest depression arose after the first death of a patient caused by a gene vector [2]. Up to now, the field clearly evolved, transforming the topic from rather experimental research with a low chance of ever being approved in the clinic into a realistic treatment option for so far incurable diseases, such as monogentetic disorders. Gene therapy in its narrow definition “is an approach to treating disease by either modifying the expressions of an individual’s genes or correction of abnormal genes” [101]. When expanded to nucleic acid-based therapies, this will also include the use of synthetic nucleic acids, such as oligonucleotides, RNAi technology or immune stimulating nucleic acids. There are >800 clinical patterns, which are caused by genetic aberrations, ranging from point mutations to relocations within chromosomes. For some of them, drugs have been developed to at least ease the symptoms. In addition, cell transplantation, for example, in the case of hematological disorders, are possible, but limited in the latter case by the availability of a suitable donor. Depending on the disease, a suitable gene vector has to be applied. For example, treating patients with a genetic defect within continuously dividing cells, such as within the hematopoietic system, a gene vector is necessary to ensure expression of the gene transferred also after many cell divisions. This led to the development of gene vectors integrating their transgene into the genome. On the contrary, when treating ocular diseases such as retinal disorders, genome integration is not the issue, as
these cells proliferate extremely slowly. Hence, the type of genetic disease treated dictates the necessary features of the gene vector.
10.4155/TDE.13.109 © 2013 Future Science Ltd
Ther. Deliv. (2013) 4(11), 1359–1363
Integrating viral vectors: lessons learned & improvements achieved The clinical trials on integrating retroviral vectors, almost 15 years ago, were characterized by great success, but also considerable drawbacks. Curing several infants from X-linked severe combined immunodeficiency, which untreated leads to death due to infections within the first or second year after birth, was the greatest achievement for gene therapy after decades of research and development [3]. The drawback followed promptly, with two cases of leukemia, which were caused by insertional mutagenesis [4]. This led to the initiation of several initiatives aiming at the analysis of the risks caused by integrating vectors and the development of safer approaches for gene delivery, for example, the German research network SPP1230 [102]. Extensive analysis of patient material revealed that the gene vector, a gammaretrovirus, inserted preferentially into the transcriptionally active site of the hematopoietic stem cells transduced ex vivo. The insertion into the gene LMO2 was responsible for all cases of leukemia observed (for a recent view on the risk of insertional mutagenesis by several gene vectors see [5]). After a general stop of all gene therapy trials worldwide, novel vectors have been developed that show less preference for integration into transcriptionally active sites. For example, the use of, so called, chromosomal isolators reduces position effects, certain viral elements are omitted and physiological promoters instead of viral promoters are used [6,7]. Recently, a clinical trial has been initiated at the Boston Childrens Hospital with such an optimized gammaretroviral vector termed pSRS11.EFS.
Manfred Ogris Department for Clinical Pharmacy & Diagnostics, Center for Pharmacy, University of Vienna, Althanstraße 14, A-1090 Wien, Austria Tel.: +43 1 4277 55551 Fax: +43 1 4277 9551 E-mail: [email protected]
ISSN 2041-5990
1359
Commentary | Ogris IL2RG, and parallel trials are also ongoing in Europe using the same vector. In contrast to the former, the novel vector contains a cellular promoter instead of a viral one, lacks viral elements that could otherwise lead to transactivation of neighboring cells and is self-inactivating – that is, after reverse transcription viral promoter/ enhancer elements are inactivated [8]. Up to 20 patients are now enrolled in a Phase I/II trial NIH X-SCID U01AI087628 [103]. Adeno-associated virus for effective treatment of hemophilia & eye disease & the first approved gene medicine in Europe As mentioned above, the gene vector should fit for the disease treated. Adeno-associated virus (AAV) is a DNA virus with low immunogenic potential capable of transducing both dividing and non-dividing cells. The virus genome replicates in the nucleus without significant integration (reviewed in [9]). This makes AAV an ideal vector for the transduction of non-dividing or slowly dividing cells, such as neuronal cells or muscle cells. Leber’s congenital amaurosis is an inheritable eye disease affecting the retina causing severe loss of vision at infancy. Disease symptoms include reduced eye sight at birth, retinal thinning and photophobia [104]. Mutations in the RPE65 gene are, besides others, a common cause for the disease. In a clinical Phase I/II study conducted by University College London, led by Robin Ali, patients with RPE65 mutation were treated with a single dose of an AAV vector encoding for functional RPE65, by sub-retinal (local) injection on one eye, and the improvement in vision monitored [10]. When passing a maze at very dim illumination, patients after treatment could avoid all obstacles, whereas, with the treated eye covered they bumped into obstacles and it was almost impossible to cross the maze (see also the freely available video on [105]). After treating patients on one eye in a similar study carried out at the University of Pennsylvania [11], three patients were re-treated on the other eye, up to 3 years after the first treatment, without observing any immunological reactions against the vector [12]. Besides improved vision on both eyes, scanning brain activity by MRI proved the functionality of the treatment. Due to its low immunological potential, AAV can also be administered via the blood stream. Hemophilia B, an X-linked blood disease characterized by excessive bleeding due to very low expression of the blood coagulation 1360
Ther. Deliv. (2013) 4(11)
factor IX (hFIX), is usually treated by frequent hFIX protein injections, is costly and, until the advent of recombinant protein expression technology, was risky due to potential viral contamination of human blood products. At the London Royal Free Hospital six volunteers suffering from severe hemophilia B were treated intravenously with escalating doses of AAV encoding for the hFIX gene [13,14]. Albeit initially designed as a safety study, in four of the six patients, levels of transgenic hFIX were high enough that the patients could suspend hFIX protein therapy. In the other two patients, protein dosing was required far less frequently. There were only minor side effects observed, such as a slight and temporary elevation of liver enzyme levels. The study was re-opened to treat a seventh patient, and the authors plan to treat up to 30 more patients. LPL deficiency is a rare (1/1,000,000) autosomal disease characterized by mutations in the lipoprotein lipase gene. This mutated LPL lacks the ability to bind and transport chylomicrons from the plasma to the tissue, which results in massive hypertriglyceridemia and in turn pancreatitis. Patients can only survive when adhering to a strict, extremely low fat diet (
Gene therapy in the clinics: shifting into the next gear “Overall, gene therapy is now less surrounded by an aura of ‘just another fancy and highly complex treatment strategy, which will never result in an approved drug’, but rather becomes a vital option for otherwise incurable diseases.” Keywords: approved medicine n cancer n clinical trial n monogenetic disease n nonviral vectors n viral vectors
Gene therapy has faced several ups and downs since the first ideas about using gene vectors to selectively deliver genetic material into human cells (for a historic overview see [1]). The early 1990s brought a gene therapy hype, in 1992 even ‘The Economist’ titled a cover story ‘Changing your genes’, however, only 7 years later the deepest depression arose after the first death of a patient caused by a gene vector [2]. Up to now, the field clearly evolved, transforming the topic from rather experimental research with a low chance of ever being approved in the clinic into a realistic treatment option for so far incurable diseases, such as monogentetic disorders. Gene therapy in its narrow definition “is an approach to treating disease by either modifying the expressions of an individual’s genes or correction of abnormal genes” [101]. When expanded to nucleic acid-based therapies, this will also include the use of synthetic nucleic acids, such as oligonucleotides, RNAi technology or immune stimulating nucleic acids. There are >800 clinical patterns, which are caused by genetic aberrations, ranging from point mutations to relocations within chromosomes. For some of them, drugs have been developed to at least ease the symptoms. In addition, cell transplantation, for example, in the case of hematological disorders, are possible, but limited in the latter case by the availability of a suitable donor. Depending on the disease, a suitable gene vector has to be applied. For example, treating patients with a genetic defect within continuously dividing cells, such as within the hematopoietic system, a gene vector is necessary to ensure expression of the gene transferred also after many cell divisions. This led to the development of gene vectors integrating their transgene into the genome. On the contrary, when treating ocular diseases such as retinal disorders, genome integration is not the issue, as
these cells proliferate extremely slowly. Hence, the type of genetic disease treated dictates the necessary features of the gene vector.
10.4155/TDE.13.109 © 2013 Future Science Ltd
Ther. Deliv. (2013) 4(11), 1359–1363
Integrating viral vectors: lessons learned & improvements achieved The clinical trials on integrating retroviral vectors, almost 15 years ago, were characterized by great success, but also considerable drawbacks. Curing several infants from X-linked severe combined immunodeficiency, which untreated leads to death due to infections within the first or second year after birth, was the greatest achievement for gene therapy after decades of research and development [3]. The drawback followed promptly, with two cases of leukemia, which were caused by insertional mutagenesis [4]. This led to the initiation of several initiatives aiming at the analysis of the risks caused by integrating vectors and the development of safer approaches for gene delivery, for example, the German research network SPP1230 [102]. Extensive analysis of patient material revealed that the gene vector, a gammaretrovirus, inserted preferentially into the transcriptionally active site of the hematopoietic stem cells transduced ex vivo. The insertion into the gene LMO2 was responsible for all cases of leukemia observed (for a recent view on the risk of insertional mutagenesis by several gene vectors see [5]). After a general stop of all gene therapy trials worldwide, novel vectors have been developed that show less preference for integration into transcriptionally active sites. For example, the use of, so called, chromosomal isolators reduces position effects, certain viral elements are omitted and physiological promoters instead of viral promoters are used [6,7]. Recently, a clinical trial has been initiated at the Boston Childrens Hospital with such an optimized gammaretroviral vector termed pSRS11.EFS.
Manfred Ogris Department for Clinical Pharmacy & Diagnostics, Center for Pharmacy, University of Vienna, Althanstraße 14, A-1090 Wien, Austria Tel.: +43 1 4277 55551 Fax: +43 1 4277 9551 E-mail: [email protected]
ISSN 2041-5990
1359
Commentary | Ogris IL2RG, and parallel trials are also ongoing in Europe using the same vector. In contrast to the former, the novel vector contains a cellular promoter instead of a viral one, lacks viral elements that could otherwise lead to transactivation of neighboring cells and is self-inactivating – that is, after reverse transcription viral promoter/ enhancer elements are inactivated [8]. Up to 20 patients are now enrolled in a Phase I/II trial NIH X-SCID U01AI087628 [103]. Adeno-associated virus for effective treatment of hemophilia & eye disease & the first approved gene medicine in Europe As mentioned above, the gene vector should fit for the disease treated. Adeno-associated virus (AAV) is a DNA virus with low immunogenic potential capable of transducing both dividing and non-dividing cells. The virus genome replicates in the nucleus without significant integration (reviewed in [9]). This makes AAV an ideal vector for the transduction of non-dividing or slowly dividing cells, such as neuronal cells or muscle cells. Leber’s congenital amaurosis is an inheritable eye disease affecting the retina causing severe loss of vision at infancy. Disease symptoms include reduced eye sight at birth, retinal thinning and photophobia [104]. Mutations in the RPE65 gene are, besides others, a common cause for the disease. In a clinical Phase I/II study conducted by University College London, led by Robin Ali, patients with RPE65 mutation were treated with a single dose of an AAV vector encoding for functional RPE65, by sub-retinal (local) injection on one eye, and the improvement in vision monitored [10]. When passing a maze at very dim illumination, patients after treatment could avoid all obstacles, whereas, with the treated eye covered they bumped into obstacles and it was almost impossible to cross the maze (see also the freely available video on [105]). After treating patients on one eye in a similar study carried out at the University of Pennsylvania [11], three patients were re-treated on the other eye, up to 3 years after the first treatment, without observing any immunological reactions against the vector [12]. Besides improved vision on both eyes, scanning brain activity by MRI proved the functionality of the treatment. Due to its low immunological potential, AAV can also be administered via the blood stream. Hemophilia B, an X-linked blood disease characterized by excessive bleeding due to very low expression of the blood coagulation 1360
Ther. Deliv. (2013) 4(11)
factor IX (hFIX), is usually treated by frequent hFIX protein injections, is costly and, until the advent of recombinant protein expression technology, was risky due to potential viral contamination of human blood products. At the London Royal Free Hospital six volunteers suffering from severe hemophilia B were treated intravenously with escalating doses of AAV encoding for the hFIX gene [13,14]. Albeit initially designed as a safety study, in four of the six patients, levels of transgenic hFIX were high enough that the patients could suspend hFIX protein therapy. In the other two patients, protein dosing was required far less frequently. There were only minor side effects observed, such as a slight and temporary elevation of liver enzyme levels. The study was re-opened to treat a seventh patient, and the authors plan to treat up to 30 more patients. LPL deficiency is a rare (1/1,000,000) autosomal disease characterized by mutations in the lipoprotein lipase gene. This mutated LPL lacks the ability to bind and transport chylomicrons from the plasma to the tissue, which results in massive hypertriglyceridemia and in turn pancreatitis. Patients can only survive when adhering to a strict, extremely low fat diet (
