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Antibody Trials Stopped After Deaths Reported

Chemotherapy vials

(National Cancer Institute)

Updated 4 January 2017. The U.S. Food and Drug Administration halted early-stage clinical trials of a monoclonal antibody drug for leukemia after four deaths occurred from liver toxicity. The drug, vadastuximab talirine — code-named SGN-CD33A — is made by biotechnology company Seattle Genetics in Bothell, Washington that sponsored the studies.

In the trials, Seattle Genetics is testing SGN-CD33A as a treatment for acute myeloid leukemia, where the bone marrow produces abnormal blood cells, which can affect red or white blood cells, as well as platelets. It is the most common type of acute leukemia in adults and requires early treatment to prevent quickly worsening symptoms. Acute myeloid leukemia usually begins in blood-forming stem cells that transform into immature white blood cells called myeloblasts and crowd out healthy blood cells.

SGN-CD33A is made with the company’s antibody-drug conjugate technology. That technology, says Seattle Genetics, is designed to act like monoclonal antibodies, synthetic proteins that aim for specific protein targets, in this case the antigen CD33 expressed on the surface of myeloid cells. Antibody-drug conjugates combine antibodies with cancer cell-killing compounds and linking agents that release the cancer-killing compounds inside the targeted cells.

One of the company’s clinical trials for SGN-CD33A reported 6 cases of liver toxicity, including veno-occlusive disease or blockages of veins in the liver, leading to four deaths. That trial is an early- and intermediate-stage study of SGN-CD33A combined with donated blood-forming stem cell transplants. The company says FDA put a full clinical hold on that trial, stopping it completely. FDA put partial clinical holds on 2 other early-stage trials, testing SGN-CD33A alone as a treatment for acute myeloid leukemia, and combined with standard chemotherapy drugs. Partial clinical holds allow current participants to continue their treatments after renewing their consent to take part, but stopping further enrollment.

Seattle Genetics says its late-stage clinical trial of SGN-CD33A with the chemotherapy drugs azacitidine and decitabine to treat acute myeloid leukemia can continue, as well as its early- and intermediate-stage study of SGN-CD33A as a treatment for myelodysplastic syndrome, a bone marrow disorder where blood-forming stem cells do not mature into full-functioning blood cells.

The company says in a statement it is working with FDA, “to determine whether there is any association between hepatotoxicity and treatment with SGN-CD33A, to promptly identify appropriate protocol amendments for patient safety and to enable continuation of these trials.”

Text updated to reflect SGN-CD33A is a monoclonal antibody, but does not invoke the immune system, thus not an immunotherapy.

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