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Rachel Brennan, Cori Bradley, Elizabeth Stewart, Jiakun Zhang, Matthew W. Wilson, Clinton Stewart, Fangyi Zhu, R. K. Guy, Michael A. Dyer; Epigenetic therapy targeting SYK in Preclinical Models of Retinoblastoma. Invest. Ophthalmol. Vis. Sci. 2012;53(14):1293.
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Inactivation of both copies of the RB1 gene is the initiating genetic lesion in retinoblastoma and is rate-limiting for tumorigenesis. Recent whole-genome sequencing of retinoblastoma demonstrated that inactivating RB1 deregulates multiple oncogenic and tumor suppressor pathways through epigenetic mechanisms. The proto-oncogene SPLEEN TYROSINE KINASE (SYK), which is not expressed in normal retinal cells, is upregulated through epigenetic mechanisms in retinoblastoma and is required for tumor cell survival. R406, an SYK inhibitor, has been developed for oral administration in adults with rheumatoid arthritis. This study tested the efficacy of subconjunctival R406 in preclinical models of human retinoblastoma (RB) and characterized the pharmacokinetics and toxicity profile.
We developed an ocular formulation using FDA approved adjuvants for ocular delivery. We performed pharmacokinetics of subconjunctival R406 and compared the vitreal penetration to that of systemic R406 as well as R788 (prodrug). Two genetic mouse models of retinoblastoma and a human orthotopic xenograft model were monitored with intraocular pressure (IOP), complete blood counts (CBC) and optomotry (vision) during chemotherapy. Eyes with progressive disease underwent enucleation. Response to therapy was documented by IOP, vision testing, diagnostic imaging (ultrasound/MRI), and ocular histopathology.
Comprehensive preclinical testing with R406 (18 weeks) in two different genetic RB mouse models and a human orthotopic xenograft were compared to the current standard of care while monitoring ocular and systemic toxicity. Subconjunctival delivery of R406 resulted in improved intraocular penetration of drug compared to oral or intravenous dosing. Administration of subconjunctival R406 once every 3 weeks resulted in little ocular or systemic toxicity in genetic or orthotopic xenograft models, as well as wild-type (C57/Blk6) controls. Efficacy in a human orthotopic xenograft, established from a human retinoblastoma tumor sample known to over-express SYK, was documented in comparison with standard of care agents.
Subconjunctivally delivered R406 is a promising epigenetic therapy for retinoblastoma. Our studies support a standardized approach to evaluate novel agents for incorporation into future clinical trials for retinoblastoma.
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