US pharmaceutical giant Merck has announced it is suspending new enrollments in two phase 3 multiple myeloma studies with its blockbuster immunotherapy drug Keytruda (pembrolizumab) in the wake of several deaths during the Keynote-183 studies.

The studies are examining the efficacy of a combination of Keytruda with Celgene’s Pomalyst and dexamethasone in previously treated patients and Keynote-185, which is combining Keytruda with Celgene’s Revlimid in certain patients not yet treated.

According to reports, the halt to enrollment came at the recommendation of an independent data monitoring committee. Patients already enrolled in the trials will continue to receive treatment, Merck said, stressing to analysts that “this is not a clinical hold.”

Other Keytruda studies continue unchanged, Merck said. The US drugmaker did not provide additional information about the number of deaths or reveal to what extent each of the studies was affected. Multiple myeloma is a type of aggressive and incurable blood cancer that develops in the bone marrow.

Keytruda belongs to a new class of drugs known as PD-1 inhibitors that help the immune system fight cancer by blocking a mechanism tumors use to evade attack. In May of this year, it became the first cancer drug ever approved by the US Food and Drug Administration (FDA) based on a patients' specific genetic traits.

The drug is also approved to treat lung cancer, bladder cancer, advanced melanoma, as well as refractory classical Hodgkin lymphoma.

The negative reports involving Keytruda follow a string of positive news. In recent weeks, Merck has received a number of landmark FDA approvals, including a combination of Keytruda with chemotherapy in first-line lung cancer, approvals in two separate bladder cancer applications and a priority review in stomach cancer.