Alnylam Pharmaceuticals Inc., which is developing a treatment for a rare genetic disorder, TTR-mediated amyloidosis, rose the most ever after the drug showed promise in an early trial. The Cambridge company’s stock began trading in 2004. Alnylam said the drug cut levels of the protein that causes the disease by as much as 94 percent in 17 healthy volunteers. The treatment targets the mutated gene that causes a harmful accumulation of the protein in the heart, nervous system, and gastrointestinal tract. Alnylam has started a second of three phases of clinical trials usually needed for regulatory approval. It plans to develop the treatment without a partner.