What happened

Shares of AC Immune (ACIU -0.84%), a clinical-stage biopharmaceutical company, are exploding higher in response to clinical trial results. Investors excited about the company's Alzheimer's disease candidate drove the stock 50.1% higher as of 10:35 a.m. EDT on Tuesday.

So what 

AC Immune doesn't have any drugs to sell at the moment, so anything that affects the outlook for its lead candidate, semorinemab, will result in dramatic price swings. This biotech stock is shooting through the roof today thanks to phase 2 clinical trial data that was positive, but not entirely.

Two people smiling while looking at multiple computer monitors.

Image source: Getty Images.

After 49 weeks of either semorinemab or a placebo, patients with mild to moderate Alzheimer's dementia given semorinemab showed a significant reduction in their rate of cognitive decline compared with the placebo group. But only on one of several tests administered.

The rate of decline as measured by the Alzheimer's Disease Assessment Scale, Cognitive Subscale, 11-item Version (ADAS-Cog11) was 43.6% lower than the rate of decline measured among the placebo group. Unfortunately, this was just one of two co-primary endpoints for the study.

Now what

AC Immune stock would have climbed much higher this morning if semorinemab had hit both co-primary endpoints that investigators had set out to achieve. Scores on the Alzheimer's Disease Cooperative Study-Activities of Daily Living (ADCS-ADL) test weren't significantly better for patients who received semorinemab.

Treatment with semorinemab also failed to significantly improve rates of functional decline as measured by the Mini-Mental State Exam (MMSE) and the Clinical Dementia Rating-Sum of Boxes (CDR-SB) test. 

The company will wait for a scientific conference to present further details, but I'm not expecting any fireworks. AC Immune's collaboration partner, Roche, ran the phase 2 trial with semorinemab and today informed AC Immune about the mixed results. Roche still hasn't issued a press release to its own investors about the data AC Immune says is positive.