AstraZeneca's high hopes for cancer immunotherapy were dented on Thursday as the recruitment of new patients with head and neck cancer into two clinical studies was put on hold, following instances of bleeding.

The drugmaker said the U.S. Food and Drug Administration (FDA) had placed a partial hold on enrolment into the final-stage Phase III trials involving two of its immune system-boosting drugs, although the studies are still continuing with existing patients.

Trials of durvalumab and tremelimumab in different cancer types are also progressing as planned. Durvalumab is being tested on its own and with tremelimumab in various cancers.

Expectations for the combination treatment have been building in particular in lung cancer, where the two-drug cocktail is being tested in a broad range of patients as an alternative to initial chemotherapy.

News of the problems in head and neck cancer first surfaced on the website clinicaltrials.gov, run by the U.S. National Institutes of Health, which said one of the trials, known as KESTREL, had been "suspended".

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"The trial is not suspended," an AstraZeneca spokeswoman said. "We have a pause or a partial clinical hold on enrolment of new patients."

Shares in AstraZeneca fell more than 4 percent in U.S. trading on concerns that the setback might signal wider problems for durvalumab.

AstraZeneca sought to play down concerns, however, stressing that pivotal data in lung cancer were still expected in the first half of 2017.

It also said that bleeding was a known complication in treating head and neck cancer, given the proximity of tumors to major blood vessels and use of prior cancer therapies, which may involve surgery and radiation.

Merck's rival immunotherapy drug Keytruda is already approved for head and neck squamous cell carcinoma, the same condition that AstraZeneca is testing for.

Bernstein analyst Tim Anderson said there was some reference in the medical literature to a decrease in blood platelets, which are needed for clotting, when patients were given so-called CTLA4 drugs like tremelimumab, although cases were rare.

Both Anderson and Deutsche Bank's Richard Parkes said it was unclear if bleeding constituted a real drug side effect or not.

AstraZeneca said it had submitted its analysis of bleeding events to the FDA for review and would work to provide the required information to resume new patient enrolment as soon as possible.