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The Dark Side of ADC Therapeutics' Drug Zynlonta: A Mid-Stage Study Paused After Seven Deaths

ADC Therapeutics has paused enrollments to a mid-stage study evaluating its drug, Zynlonta, to treat a type of blood cancer after seven patients died and five others developed respiratory issues. The drugmaker's shares sank 21% after it said a review of the trial data of 40 patients indicated potentially excessive respiratory-related events. The trial was testing Zynlonta along with rituximab, a biosimilar drug for Roche's monoclonal antibody Rituxan, in unfit or frail patients with previously untreated diffuse large B-cell lymphoma.

The Safety Concerns

An investigator assessment showed 11 of the 12 events, including six of the seven deaths, were assessed as unlikely or unrelated to the study drug. The cancer patients who died were aged 80 years or above and had one or more severe respiratory, cardiac conditions such as pulmonary edema, and recent COVID-19 infection. ADC Therapeutics said that the cause is being further probed, and it has notified all study investigators and regulators, including in Europe where the drug is approved, of its decision to pause enrolment.

TD Cowen analyst Boris Peaker said in a note that the safety concerns could be due to the combination with rituximab and that a change in protocol such as lower dose and patient exclusion criteria could be necessary to continue the trial.

About Zynlonta

Zynlonta is an antibody-drug conjugate comprising an anti-CD19 monoclonal antibody attached to a pyrrolobenzodiazepine dimer toxin. The drug was granted accelerated approval by the U.S. Food and Drug Administration to treat adults with certain types of large B-cell lymphoma that has come back or that did not respond to previous treatment. The company is also testing the drug in an ongoing confirmatory trial to show its clinical benefits for full approval in the U.S.

ADC's Response

ADC Therapeutics said it does not expect to report more data from the mid-stage trial by the end of the year. The company has stated that it is committed to patient safety and is working closely with regulators and investigators to determine the appropriate next steps for the study.

The Impact on the Company

The pause in the mid-stage study is a significant setback for ADC Therapeutics, which was founded in 2011 and went public in May 2020. The company had a market capitalization of $7.6 billion before the news, which wiped out more than $1.5 billion in its market value.

The Competition

ADC Therapeutics is one of several companies that are developing new treatments for B-cell lymphomas, including Gilead Sciences, Bristol Myers Squibb, and Novartis. Gilead Sciences' Yescarta and Bristol Myers Squibb's Breyanzi are both FDA-approved CAR-T cell therapies for the treatment of certain types of large B-cell lymphoma.

The Future of Zynlonta

The pause in the mid-stage study for Zynlonta is likely to delay the drug's full approval in the U.S. and could lead to a decline in sales. The company is expected to provide an update on the situation in the coming months.


The pause in ADC Therapeutics' mid-stage study evaluating Zynlonta is concerning news for the company, which had high hopes for the drug. The safety concerns raised during the trial could delay the drug's full approval in the U.S. and could lead to a decline in sales. While the cause of the respiratory-related events is still being investigated, it is clear that the combination with rituximab needs to be further evaluated.


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