Luxturna / Voretigene - FDA's first AAV Approval

    05/03/2018

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    When FDA first announced its approval of Luxturna (voretigene neparvovec-rzyl), sponsored by Spark Therapeutic, Inc in December 2017 the FDA Commissioner, Scott Gottleib made a press release about how gene therapy has become a breakthrough in the treatment of rare, intractable illnesses. It was also one of three gene therapy approvals in one incredible year for the agency, which has traditionally taken a cautious stance on novel, large molecule (and therefore) complex therapies. However, on close examination of Luxturna’s now public documents and advisory committee meetings, it seems the drug development process was given due diligence.

    Luxturna is a recombinant adeno-associated virus of serotype 2 (rAAV2) expressing the gene for human retinal pigment epithelial protein 65 kDa (hRPE65). It is indicated for the treatment of patients with bialleic RPE65 mutation-associated retinal dystrophy. The mutations occur in only 1000-2000 patients in the US who belong to a wider group of people suffering from genetic retinal disorders and includes the disease Leber congenital amaurosis (LCA). The human RPE65 protein is an essential protein for converting 11-cis-retinal to all-trans-retinal in the visual cycle that is critical for maintaining vision:

    Travis 2007

    People with these mutations mostly get affected early in life. They find themselves with severe vision impairments which progress to night blindness and loss of visual field.

    The AAV plasmid is designed with a CMV enhancer and a beta-actin promoter to drive robust expression, based on original research by Jean Bennet at UPenn. The pre-clinical and IND-enabling studies were done in mice and dogs modeled genetically on the human RPE65 mutation. The dogs in particular showed significant improvement after viral vector injection into the eye over a two and a half year period, provided they were dosed early in life.

    Luxturna was approved following a Phase 1 study and a Phase 3 study. The IND was submitted in 2007 by Children’s Hospital of Philadelphia and granted an Orphan Drug designation in the following year. The IND was then transferred to Spark Therapeutics, Inc in 2014 which became the sponsor up to its approval. In 2014 the FDA also granted Breakthrough Therapy Designation, which further expedited the development process.

    A brief summary of the clinical trials:

    Phase 1:

    The Phase 1 study was an open-label dose-exploration safety study on 12 patients who were first given the gene injected into one eye to observe any toxic effects or adverse events. Preliminary efficacy assessments were also obtained for visual function. This was followed by injection into the second eye a few years later.

    The inclusion criteria included patients who were 8 years or older, with visual acuity of no better than 20/160 (20/20 vision being perfect vision).

    The exclusion criteria included:
    Insufficient viable retinal cells
    Neutralizing antibodies to AAV at greater than 1:1000
    Pre-existing eye conditions that would interfere with surgery
    Ocular surgery within previous 6 months.

    Three dosing plans were set for the first eye: 1.5 X 10^10 vg/ 150uL, 4.8 X 10^10 vg/150uL and 1.5 X 10^11 vg/300 uL. For the second eye they used the highest dose since no adverse events were present in the first eye. To minimize immune response oral prednisone was given around the time of administration.

    Phase 3:

    The Phase 3 study was an open-label randomized study involving 31 patients. Again the gene was injected into one eye first before it was injected into the second eye - this time waiting for just a week or two. After one year the control patients and test group patients were crossed-over. Clinical assessments were done for safety and efficacy. A long-term follow up plan was designed for 15 years.

    Subretinal injection procedure:

    Phase 3 trial design and follow-up scheme:

    The efficacy test involved using the multi-luminescence mobility testing (MLMT) assessment in which patients were asked to navigate a course laid out be a number of arrows, turns and obstacles and at varied light illumination conditions. Patients were assessed at baseline, Day 30, 90, 180 and 365 using one treated eye, then the other treated eye. Every run of the test was recorded with video tape and assessed. Scores were given based on how well a person could complete the course in low light (high score for passing) or bright light (lower score for passing). A score change of 2 from before to after treatment was considered clinically meaningful.

    The inclusion and exclusion criteria for Phase 3 trials were much the same as Phase 1 except subjects were 3 years or older and they had to be able to perform MLMT testing.

    Dosing was set at 1.5 X 10^11 vg/300 uL injected subretinal.

    From the efficacy study it was found that patients with the treatment had an increase in MLMT score which was significantly more than in the untreated eye after one year. This was found in 51% of patients with treatment in both eyes and 71% of patients with treatment in just one eye after one year. Interestingly no statistical significance was found in patient visual acuity improvement but there was for sensitivity to light (light sensitivity threshold).

    The major adverse events discovered from the Phase 3 trial included Conjunctival hyperemia, intraocular pressure, cataract development and retinal tear. Two of the patients suffered serious adverse events with permanent visual loss.

    For each study safety assessments were done. They carefully looked at Interferon-gamma responses to AAV2 and to RPE65 by ELISPOT assay in peripheral blood mononuclear cells (PBMCs). For most measurements across the Phase 1 and 3 studies there was minimal, low or no change in antibody titers to AAV capsid and RPE65.

    Manufacturing

    HEK293 cells were triple transfected with rep/cap, helper and the RPE65 plasmids before the virus was harvested. The harvested virus was then purified through the downstream process. Using this manufacturing process they were able to produce up to 3e15 vector genomes necessary for Phase 1 trial and make them within GMP limits for quality. A detailed look at the upstream and downstream process can be found by in a paper published by James Fraser Wright in 2010:

    AAV Manufacturing Process Flow Through:

     

    Manufacturing Facility Diagram:

    Although retinal dystrophies are a complex disease characterized by both dysfunction and degeneration of photoreceptors, the Luxturna AAV seems to have overcome this problem by providing restored function in patients with the RPE65 mutation. It remains to be seen whether this treatment has long last benefits which stretch out beyond the 1-2 year period of analysis seen in patients so far, onto adult and old age when retinal degeneration becomes more common place.

     

    References:

    - Expert analysis of plasmid CHOP, Spark and UCL:

    http://www.bioterppartners.com/single-post/2015/05/07/AAV-RPE65-Gene-Therapy-More-Than-Meets-The-Eye

    - Advisory Committee Meeting:

    https://www.fda.gov/AdvisoryCommittees/CommitteesMeetingMaterials/BloodVaccinesandOtherBiologics/CellularTissueandGeneTherapiesAdvisoryCommittee/ucm574396.htm

    - Luxturna FDA Supporting Documents:

    https://www.fda.gov/biologicsbloodvaccines/cellulargenetherapyproducts/approvedproducts/ucm589507.htm

    - Original Research:

    Wright, J. F., Wellman, J. & High, K. A. Manufacturing and regulatory strategies for clinical AAV2 hRPE65. Curr Gene Ther 10, 341-349 (2010)

    Acland, G. M. et al. Gene therapy restores vision in a canine model of childhood blindness. Nat Genet 28, 92-95, doi:10.1038/88327 (2001)

    Amado, D. et al. Safety and efficacy of subretinal readministration of a viral vector in large animals to treat congenital blindness. Sci Transl Med 2, 21ra16, doi:10.1126/scitranslmed.3000659 (2010).

    Travis GH, Golczak M, Moise AR, and Palczewski K. Diseases Caused by Defects in the Visual Cycle: Retinoids as Potential Therapeutic Agents. Annu. Rev. Pharmacol. Toxicol. 2007; 47:469–512