On Thursday, MacroGenics Inc ( MGNX ) released interim data from the TAMARACK Phase 2 study of vobramitamab duocarmazine (vobra duo) that targets B7-H3, an antigen with broad expression across multiple solid tumors.

TAMARACK is being conducted in previously treated patients with metastatic castration-resistant prostate cancer (mCRPC).

MacroGenics ( MGNX ) shares plunged over 50% on Friday after the company reported five deaths (fatal outcome) during the trial and over 50% rate of grade 3 or worse adverse events in both dose cohorts (2 mg/kg and 2.7 mg/kg every four weeks).

On the data cut-off date of April 12, 2024, the rates of treatment-emergent adverse events (TEAEs) in the low (2.0 mg/kg) and high (2.7 mg/kg q4W) dose arms were 54.4% and 51.2%, respectively.

When the company reported early data last month, as of the cut-off date of Jan. 4, the rates of TEAEs were 25.3% and 31.4%, respectively.

MacroGenics' ( MGNX ) latest readout shows TEAE-related interruptions were up to 42.2% and 55.8% at the low and high doses, respectively, compared to 11% and 18.6% in earlier data readout.

Investigators determined that two cardiovascular deaths, one in each study group, were not related to the drug being tested. Additionally, three deaths at the higher dose are under investigation. Two of these patients died from pneumonitis.

The company reported overall response rates of 17.8% and 25.0%, respectively, at the low and high doses. Including patients with unconfirmed responses increased the figures to 24.4% and 43.8%.

Any prostate-specific antigen (PSA) reduction of over 50% was 50% and 50.7%, 2.0 mg/kg and 2.7 mg/kg arms, respectively. Confirmed PSA reduction was 43.9% and 36.6%, respectively.

Price Action: MGNX shares are down 73% at $3.92 at last check Friday.

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Image generated using artificial intelligence via Midjourney.