The US Food and Drug Administration (FDA) has placed a clinical hold on three phase 3 multiple myeloma studies testing US drugmaker Merck’s Keytruda (pembrolizumab) in combination with standard multiple myeloma regimens including Celgene Corp drugs after more deaths were reported in patients.

Patients participating in the trials were taking Keytruda with Celgene’s Revlimid or alternatively its Pomalyst and dexamethasone, while others were taking the Celgene regimens without Keytruda.

In June, Merck said it had stopped enrolling patients in the trials. The FDA’s decision to put a hold on the tests altogether came after independent safety monitors observed more deaths in patients receiving the Keytruda combination than in the control groups in two of the studies.

Based on available data, the risks of combining Keytruda with either of the Celgene medicines outweighed any potential benefit for patients with multiple myeloma, the FDA said in explaining its move.

Other studies with Keytruda are not affected by the FDA ruling. The drug, which blocks a protein tumors use to avoid detection, is reportedly being tested in hundreds of combination trials in a wide variety of cancers.

Up to now, the Merck oncology drug has been approved solely for one type of blood cancer, classical Hodgkin lymphoma. However, the drugmaker has reported success in treating solid tumor cancers with approvals for advanced melanoma, advanced bladder cancer and a type of head and neck cancer, in addition to lung cancer. Success stories include former US President Jimmy Carter.

In May, Keytruda became the first cancer drug to win US approval based on a patient's specific genetic traits, regardless of where in the body the disease originated.