[Updated, 7/19/18, 6:25 p.m. See below.] A patient has died in a clinical trial testing Mersana Therapeutics’ lead cancer drug, and the FDA has put a partial hold on the early-stage study as a result.
The partial hold means that no new patients will be enrolled in the Phase 1 clinical trial. But patients who are already participating in the study can continue to receive the Cambridge, MA, biotech’s test drug, XMT-1522. Shares of Mersana (NASDAQ: MRSN) sank more than 38 percent after the company announced the news Thursday morning.
Mersana develops a type of cancer therapy called an antibody drug conjugate. By hitching cancer-fighting toxins to antibodies, these drugs are meant to deliver a more targeted strike to cancer cells that spares healthy tissue from the toxicity of the treatment. XMT-1522 was developed to target cancers whose cells overexpress HER2, a protein that promotes tumor growth. The drug is part of a wide-ranging partnership with Japan’s Takeda Pharmaceutical that could develop up to seven ADC therapies, potentially paying Mersana more than $1 billion if milestones are met.
Mersana has been testing XMT-1522 in a dose-escalation study intended to find the best dose to advance into Phase 2. The study was enrolling mostly breast cancer patients, though the company amended the study to also allow lung and gastric cancer patients. The study used a “3+3 design” in which at least three patients are treated at each dose level followed by a three week period for evaluating any toxic effects that would limit the dose. According to interim results from the study that were presented in May during the annual meeting of the American Society of Clinical Oncology, the most common side effects observed were fatigue, nausea, vomiting, anemia, and some elevated liver enzyme levels. When those results were announced, Mersana only had data for six dose levels. Enrollment of patients in dose level seven was ongoing.
[Paragraph updated with analyst comment.] The patient death disclosed Thursday was in dose level seven. According to Mersana, a physician involved in the study classified the patient death as “possibly drug-related.” In a research note, Leerink Partners analyst Jonathan Chang wrote that Mersana said the study had cleared dose level seven and started on level eight when the death occurred. The company added that it is still waiting to hear from the FDA. Chang said that a clean safety profile for XMT-1522 is crucial in order to differentiate Mersana’s drug platform from a competing program from Daiichi Sankyo. In a prepared statement, Mersana said it is working with the FDA and the clinical trial officials to review the death and resolve the clinical hold. Mersana added that the partial hold does not affect another drug, XMT-1536. That drug is in Phase 1 studies enrolling patients whose tumors express the protein NaPi2b, including ovarian and non-small cell lung cancer.
Here’s more on Mersana and its approach to cancer drugs.