UPDATE 3-Alnylam says respiratory drug meets main goal

* Says mid-stage trial meets safety, tolerability goals

* Says found imbalances between study groups

* Imbalances not an issue - Alnylam COO

* Co to provide view on drug's development in H2

* Inhaled-drug study not powered for efficacy results

* Alnylam shares fall as much as 11 pct (Adds COO comment)

By Vidya L Nathan

BANGALORE, June 8 (Reuters) - Alnylam Pharmaceuticals (ALNY.O) and Cubist Pharmaceuticals (CBST.O) said their experimental drug for a respiratory infection met the main goal of a study, but reported imbalances between the study arms, sending Alnylam shares down as much as 11 percent.

The mid-stage study met its main goal of demonstrating safety and tolerability of the inhaled drug, ALN-RSV01, to treat respiratory syncytial virus (RSV) infection over 30 days after treatment in lung-transplant patients.

The companies also said the baseline viral load was higher and time from symptom onset to first dose was shorter in the dummy-drug group compared with the treatment arm.

"We believe that it would be inappropriate to draw any definite conclusions on efficacy from the specific study," Rodman & Renshaw analyst Simos Simeonidis said in a note.

However, Simeonidis said such imbalances were not unexpected in a small study, particularly in this specific disease population.

The current study was not powered for efficacy outcomes, the companies said, adding that the imbalances were not significant.

"This imbalance is by no means an issue. It would be another larger trial that would be powered to differentiate the drug and placebo, and show efficacy," Alnylam Chief Operating Officer Barry Greene said.

The trends are favourable, but specific inferences cannot be made, Greene said. The company would provide its view on the next steps on developing the drug in the second half, he said.

Rodman & Renshaw's Simeonidis said, "We do not view today's news as a breakthrough, but we believe that it does provide an additional, reassuring piece of evidence to move this program forward, and is thus an incremental positive development."