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.