If at first you don't succeed, tweak your study design until you get it right.

Acadia Pharmaceuticals (NASDAQ:ACAD) finally got it right for its drug candidate pimavanserin to treat psychosis in patients with Parkinson's disease. The company announced that the latest phase 3 trial for pimavanserin met its primary and secondary endpoint.

The biotech had run two previous clinical trials testing pimavanserin in Parkinson's disease psychosis, but they succumbed to the placebo effect. Acadia tweaked a few things like adjusting the primary endpoint and running the trial exclusively in North America and ran a new trial.

The tweaks worked. In the last trial, the SAPS score, a measure of psychosis symptoms, dropped 5.9 points in patients taking placebo. In the new trial, the placebo group only saw their scores on the modified scale decline by 2.73 points. Those taking the drug saw their scores drop 5.79 points, a highly statistically significant change. Pimavanserin also improved nighttime sleep and daytime wakefulness, and didn't worsen motor function.

The safety profile looked clean, which should help it compete with atypical antipsychotics such as Johnson & Johnson's (NYSE:JNJ) Risperdal, Eli Lilly's (NYSE:LLY) Zyprexa, Bristol-Myers Squibb's (NYSE:BMY) Abilify, and Pfizer's (NYSE:PFE) Geodon that are used off label in Parkinson's patients, but have safety issues.

Now the big question is whether Acadia can repeat the feat. The Food and Drug Administration is going to want two trials before it'll approve pimavanserin. A second trial -- obviously with the same trial design -- is in the works. The first trial took two years to enroll even though it only measured treatment for six weeks. With solid data in hand, it'll presumably be easier to convince patients and their caregivers to enroll in the new trial.

That could also present a problem. If patients know they're potentially getting a drug that works well, it might increase the placebo effect in the next trial. Fortunately, the difference was so large in the last trial that Acadia has some leeway to see the placebo improve and still be statistically significant. That's good because tweaking again isn't an option.

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