Shares of MacroGenics (MGNX -0.29%) were tanking 26.4% lower as of 11:39 a.m. EDT on Thursday. The big decline came after the company announced data from a cohort in a phase 3 study evaluating margetuximab in combination with retifanlimab in treating gastroesophageal adenocarcinoma, cancer of the stomach and esophageal junction with the stomach. MacroGenics also announced preliminary data from a phase 1 clinical study evaluating MGC018 in treating advanced solid tumors.
MacroGenics reported that 21 of 40 patients in the phase 3 cohort who received the margetuximab-retifanlimab combo achieved confirmed responses with tumor shrinkage observed at the first scan of 78% of patients. The company said that the median duration of response was 10.3 months. It also noted that the combo was tolerated relatively well with no life-threatening adverse events, although 19% of patients experienced severe adverse events.
The company also said that 21 of 39 patients in the prostate cancer expansion cohort of the phase 1 study of MGC018 achieved at least 50% prostate-specific antigen (PSA) reduction. Of the 16 patients evaluable for tumor response, 10 experienced some anti-tumor activity. Four of those patients had partial responses (tumor size reduction) -- two of which were confirmed and two were unconfirmed.
In the non-small cell lung cancer (NSCLC) cohort of the phase 1 study, 13 of 16 evaluable patients experienced some level of anti-tumor activity. Four of those patients achieved partial responses that were unconfirmed.
The company stated that 7% of patients discontinued the MCG018 study due to treatment-related adverse events. MacroGenecis reported that most of the adverse events have been "clinically manageable" so far. However, there were two fatalities -- one due to COVID-19 and another from an unidentified cause.
MacroGenics CEO Scott Koenig was upbeat about the data from both studies. However, investors clearly weren't so happy.
MacroGenics now plans to discuss the late-stage results for the margetuximab-retifanlimab combo with the U.S. Food and Drug Administration. The company is also enrolling patients in additional cohorts of the phase 1 study of MCG018. In the meantime, the biotech stock is likely to remain highly volatile.