Altimmune, Inc. (ALT) shares plunged 61.61% in pre-market trading on Thursday, despite the company announcing positive topline results from its IMPACT Phase 2B trial of pemvidutide in the treatment of Metabolic dysfunction-Associated SteatoHepatitis (MASH). The dramatic stock decline caught many investors off guard, given the seemingly favorable clinical outcomes.

According to the company's announcement, the trial met its primary endpoint, achieving a 59.1% MASH resolution without worsening fibrosis. Additionally, the study demonstrated weight loss of up to 6.2% at 24 weeks. However, the market's severe negative reaction suggests that these results fell short of investor expectations in some critical areas.

The key disappointment appears to be the lack of statistically significant improvement in liver fibrosis. As noted by Citzensbank analyst Jonathan Wolleben, "missing on statistical significance for fibrosis will be hard for investors to look past." This shortcoming seems to have overshadowed the positive aspects of the trial results, leading to the sharp sell-off. The market's response underscores the high stakes and stringent expectations in the competitive field of MASH treatment development, where improvements in fibrosis are considered crucial for long-term patient outcomes.