Bifeprunox (DU-127090; 57) also has partial D2 and 5HT1A agonist activity with little activity at 5HT2A, 5HT2c, ^-adrenergic, and histamine H1 receptors. It is thus quite interesting in the context of the the hypothesis that 5HT1A agonism can substitute for 5HT2A antagonism.12 Bifeprunox appears to act as a partial D2 agonist in rodents and to exhibit activity consistent with other antipsychotic drugs in animal models of psychosis. This compound is currently in phase III clinical trials but little clinical information is currently available on its efficacy.
Another partial DA D2 agonist with clinical potential is OSU 6162 (PNU-96391A; 58), that has shown antidyskinetic and antipsychotic efficacy in small preliminary trials in patients suffering from Parkinson's disease and Huntington's disease. This NCE has also shown interesting activity as an add-on therapy in patients with refractory schizophrenia. Still, it remains to be demonstrated in larger studies that partial DA receptor agonism is sufficient to reliably elicit antipsychotic efficacy and whether these kinds of drugs are different enough to add to the antipsychotic pharmacopeia. This approach remains theoretical and needs empirical validation. Further, the optimum level of intrinsic partial agonism required for therapeutic benefit needs to be elucidated. In the end, the level of intrinsic activity may be quite different among individuals, making dose selection difficult. This has not proved to be the case for aripiperazole, where 15-20 mg day_ 1 has proved to be effective for almost all responsive patients.
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