Shares of Alnylam Pharmaceuticals Inc. fell 7.5% in premarket trade Wednesday after the company announced results of a Phase 3 study of givosiran, an RNA interference therapy for the treatment of acute hepatic porphyria, a family of rare genetic diseases that can cause potentially life-threatening attacks of severe abdominal pain, weakness and fatigue. The study met its primary endpoint -- reducing the yearly rate of porphyria attacks compared with placebo -- and hit five of nine secondary endpoints. But investors seemed spooked by the high percentage of serious adverse events reported by patients on givosiran compared with those on a placebo: 20.8% of those on givosiran reported serious adverse events compared to 8.7% of patients on placebo, though the difference in overall adverse events was not as stark, with 89.6% of patients on the drug reporting adverse events of any kind compared with 80.4% of patients on placebo. One patient discontinued treatment after seeing the amount of the liver enzyme alanine aminotransferase increase to more than eight times the normal number. Acute hepatic porphyria happens when a genetic defect leads to issues with one of the enzymes responsible for making heme in the liver. That causes a build-up of heme precursors, which are toxic to the body. Alnylam's givosiran targets an enzyme in the heme-making pathway called aminolevulinic acid synthase 1 (ALAS1). Alnylam said full study results will be presented in April and the company plans to submit a new drug application in mid-2019. Shares of Alnylam have gained 10.8% in the past 3 months, while the S&P 500 has gained 3.5%.