Shares of YM Biosciences Inc. fell 12 percent on news that the FDA placed a clinical hold on a Phase II study of AeroLEF in acute pain.

The agency, which previously cleared the company to start Phase II development in June, requested additional safety information, specifically data from certain patients in earlier studies who appeared to experience oxygen desaturation, a common effect of opioids such as fentanyl. YM's AeroLEF is an inhaled-delivery composition of free and liposome-encapsulated fentanyl.

Mississauga, Ontario-based YM said in a press release that the requested data are limited to a small group of patients and that the firm plans to conduct a subgroup analysis.

Meanwhile, no patients have yet been dosed in the planned Phase II study, and YM said no other trials involving AeroLEF are ongoing.

Prior to the setback, AeroLEF had been performing well in clinical testing. Results from a Phase IIb study, reported in July, showed the product met its primary endpoint, as determined by the summed pain intensity difference/pain relief scores during the four hours from the start of initial dose of study medication.

That trial involved patients in the post-anesthetic care unit after completion of surgery and evaluated them for up to 12 hours, in which they were allowed to self-administer AeroLEF to treat up to two additional pain episodes.

YM's shares (AMEX:YMI) lost 14 cents Thursday to close at $1.05.

Elsewhere in its pipeline, the firm continues to move forward with its lead drug, nimotuzumab, which is in late-stage studies, including a Phase III trial in adult glioma patients and a Phase II/III trial in advanced pancreatic cancer patients. That drug, a humanized monoclonal antibody targeting the epidermal growth factor receptor, also is in a European Phase III trial in children with inoperable brain cancer, the results of which are expected to support a marketing application overseas. Nimotuzumab is partnered in Europe with Wedel, Germany-based Oncoscience AG.