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Executive Summary

MERRELL DOWS' GAMMA VINYL GABA CLINICAL TRIALS SHOULD BE CONTINUED in patients with refractory complex partial seizures, FDA's Peripheral and Central Nervous Systems Drugs Advisory Cmte. recommended May 18. The cmte. also recommended that animal toxicity studies parallel human testing. In conducting further clinicals, the cmte. agreed that the firm should develop "species-specific metabolite data." Specific recommendations for test to be performed included: visual evoked potential, cerebrospinal fluid pressure and protein measurements, and the possible inclusion of cerebrospinal fluid electrophoresis for oligoclonal band determination. The cmte.'s recommendation to continue testing of the drug came in response to FDA concern about recent animal toxicity studies that found animals receiving the drug developed lesions of the brain and retina. FDAer Barry Rosloff, PhD, explained that "when these animal reports were submitted, clinical testing was well underway." Rosloff noted at the beginning of the meeting that "it is now shown that the lesions are widespread throughout the three species within the clinical use range." Based on the animal findings, FDA had recommended that no new subjects be admitted. The cmte. advised FDA that new patients be permitted to enter the clinicals. Merrell Dow, contended that recently-completed studies in rats and dogs demonstrate that the lesions are reversible. Summarizing presentations made on behalf of the firm by several investigators, Merrell Dow's Paul Schechter, MD/PhD, asserted: "Gamma vinyl GAGA has been associated with intramyelinic edema in non-primate brains, which is localized, non-progressive and reversible." Schechter also said that instances of retinopathy have been limited to albino rats. "No clinically significant signs in other animal species were seen," he told the cmte. FDA's Rosloff however, maintained that "although the brain vacuoles do appear to be reversible, other lesions were noted in the one-year rat study that were not reversible and in fact appeared to become more pronounced during the recovery period." Rosloff stated that "these lesions consisted of the presence in the cerebellum of what we term eosinophilic spheroids and mineralized microbodies."

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