MacroGenics' stock plummets after 5 deaths in ADC trial

MacroGenics, a biotechnology company, faces significant setbacks in its phase 2 clinical trial. Despite reducing the dosage of its B7-H3-directed antibody-drug conjugate (ADC), Vobramitamab Duocarmazine (Vobra Duo), after observing high rates of adverse events in earlier phases, the company reported concerning safety outcomes. This clinical trial targeted patients with metastatic castration-resistant prostate cancer who had already been treated with either Xtandi or Zytiga. The trial used 2 mg/kg and 2.7 mg/kg doses every four weeks, lower than the high-frequency doses used in phase 1.

The initial phase 2 safety data of Vobra Duo caused a temporary surge in MacroGenics' stock last month. However, the positive traction was short-lived. After releasing further safety data, the biotech's shares plunged by over 70%, dropping from $14.67 to $4.37 per share in premarket trading on Friday.

Scott Koenig, M.D., Ph.D., the CEO of MacroGenics, provided a more optimistic interpretation of the data, despite negative market reactions. "While the safety data has accumulated more safety side effects, we believe that these are extremely manageable. In discussions with investigators, they are very comfortable with managing these patients and are very encouraged by both the safety and, obviously, the activity data they've seen to date," Koenig said during a May 9 earnings call.

In earlier data covering patients up to early January, the rates of treatment-emergent adverse events (TEAEs) were 25.3% for the lower dose and 31.4% for the higher dose. However, by mid-April, these figures had risen to 54.4% and 51.2%, respectively. Grade 3 TEAEs and other adverse measures also worsened. Initially, drug interruptions due to adverse events were noted at 11% for the low dose and 18.6% for the high dose. The latest data shows these rates surged to 42.2% and 55.8%. Similarly, discontinuation rates increased, although Koenig argued that these figures are comparable to other studies.

The trial also reported five patient deaths. Two cardiovascular deaths, one in each dose arm, were deemed unrelated to the study drug. The remaining three deaths, all in the higher dose group, are currently under investigation. Two of these patients succumbed to pneumonitis, an inflammatory lung condition. Koenig noted, "One of these cases was very complicated with other confounding matters. It's being further investigated right now, so I don't have a further decision with regard to a cause and effect. We have not seen, in the large number of patients here, any association with pneumonitis. That raises questions of what is the ultimate cause of these patients associated with the pneumonitis.”

Regarding efficacy, MacroGenics reported overall response rates of 17.8% for the low dose and 25.0% for the high dose. Including patients with unconfirmed responses, these figures increased to 24.4% and 43.8%. An important question that remains is the duration of these responses. As of the data cutoff, 43 patients were still undergoing treatment, evenly divided between the two dose groups. Koenig discussed the implications for radiographic progression-free survival (rPFS), the study’s primary endpoint. He stated, "What we had indicated was an rPFS baseline of 6 months or greater and obviously looking for 7, 8, 9, 10 or higher. I think that the data that we show today and the fact that these patients are still on therapy … there is no reason we can't meet some of the longer-lived rPFS values.”

As MacroGenics navigates these challenges, the biotech remains focused on managing side effects and analyzing the efficacy data to meet its clinical objectives.