(Recasts first paragraph, adds Bristol-Myers hold, background, updates shares)
Sept 7 (Reuters) - U.S. health regulators stopped five blood cancer trials testing AstraZeneca Plc's immunotherapy Imfinzi in combination with Celgene Corp medicines from accepting new patients and halted another study entirely, the companies said on Thursday.
The Food and Drug Administration's decisions stem from safety concerns uncovered in similar blood cancer trials of Merck & Co Inc's rival Keytruda medicine in combination with widely used multiple myeloma drugs that Celgene sells.
The news follows Bristol-Myers Squibb Co's announcement on Wednesday that the FDA had placed similar partial clinical holds on three trials of the company's Opdivo immunotherapy in combination with multiple myeloma treatments.
The FDA placed a hold on three Merck multiple myeloma combination trials in July after safety monitors reported more deaths among patients who received Keytruda than those in the control group.
Keytruda, Opdivo and Imfinzi belong to a new high-profile class of drugs called PD-1 or PD-L1 inhibitors that work by blocking a mechanism tumors use to hide from the immune system. They have received numerous approvals for other cancers, such as lung cancer and melanoma.
Several companies are testing similar drugs against a wide variety of cancers. Further clinical hold announcements are possible if they are being tested against blood cancers as the FDA works "to better understand the true cause of the safety concerns," it said last week.
The FDA said it still believed the benefits of Keytruda and rival medicines in the class outweigh risks when taken for their approved uses.
Under the partial clinical holds placed on the Bristol-Myers and the AstraZeneca-Celgene studies, patients who were experiencing clinical benefits can continue treatment, but no new patients will be enrolled.
Patients in the study under full clinical hold will no longer receive the treatment, the companies said.
Most of the affected combination studies were for multiple myeloma. One of the trials under partial hold was testing Imfinzi, known chemically as durvalumab, in patients with lymphoma or chronic lymphocytic leukemia.
Celgene said it had not identified an imbalance in the risk/benefit profile in its trials, suggesting that the safety issue reported in the Keytruda studies had not cropped up in its trials with the AstraZeneca drug.
Celgene shares were up 1.3 percent, while AstraZeneca, which announced positive trial results for an asthma drug, gained more than 4 percent. (Reporting by Bill Berkrot in New York and Akankshita Mukhopadhyay in Bengaluru; Editing by Savio D'Souza and Lisa Von Ahn)