Differences in dosing, duration and the disability of patients likely led to Pfizer Inc.'s phase II fizzle with its 5-hydroxytryptamine6 (5-HT6) serotonin receptor antagonist in Alzheimer's disease (AD), said Axovant Sciences Ltd. CEO Vivek Ramaswamy, who distinguished the pharma giant's experiment from the phase III effort by his firm with RVT-101, which also targets 5-HT6.