Alnylam Pharmaceuticals Inc. stock climbed after the Cambridge company reported positive early trial results for an experimental RNA interference drug, ALN-TTRsc. It’s designed to treat transthyretin familial amyloidosis, a rare genetic condition in which a starchy protein builds up in organs and tissues. ALN-TTRsc was evaluated for a form of the disease typically treated with a liver transplant. Alnylam will start a late-stage trial of the drug in 2014. It’s developing the drug in partnership with Genzyme.