Xencor has announced that the US Food and Drug Administration (FDA) has placed a partial clinical hold on its Phase I study of XmAb14045.

The drug is a bispecific antibody molecule being evaluated in patients with relapsed or refractory acute myeloid leukemia, along with other CD123-expressing haematologic malignancies.

The hold comes after recent safety reports by Xencor were submitted to the FDA on two patient deaths that were considered at least possibly related to XmAb14045.

One patient experienced cytokine release syndrome (CRS) after their first dose, and the other developed acute pulmonary edema following several doses of XmAb14045.

The hold will be effective until results form a pending review of additional details regarding these events, safety and efficacy information across the study, and satisfactory review of amendments to the study protocol and related documents are published.

Patients already enrolled in treatment may continue, but no new patients will be allowed to enroll in the study until the partial clinical hold is lifted by the FDA.

"Patient safety is Xencor's highest concern," said Bassil Dahiyat, president and chief executive officer at Xencor.

"We are working with the investigators and the FDA and will provide an update when more information about resuming enrollment can be shared. Our ongoing Phase I studies evaluating our other CD3 bispecific antibodies, XmAb13676 and XmAb18087, are not affected.”

XmAb14045 is a tumour-targeted antibody that contains both a CD123 binding domain and a cytotoxic T-cell binding domain (CD3)