We reported positive initial results from our ongoing Phase 1 clinical trial with ALN-AS1, an investigational RNAi therapeutic targeting aminolevulinic acid synthase 1 (ALAS1) for the treatment of acute hepatic porphyrias. Study results showed that single subcutaneous doses of ALN-AS1 resulted in an up to 82% lowering of ALA and an up to 93% lowering of PBG in asymptomatic “high excreter” (ASHE) patients, who carry the genetic mutation of acute intermittent porphyria (AIP) and have elevated levels of aminolevulinic acid (ALA) and porphobilinogen (PBG), the toxic heme intermediates that mediate porphyria attacks. Furthermore, analysis of exosomal mRNA preparations from serum and urine revealed that treatment resulted in potent, dose-dependent, and durable silencing of liver ALAS1 mRNA. Importantly, ALN-AS1 was found to be generally well tolerated with no clinically significant drug-related adverse events to date. These data provide human proof-of-concept for ALN-AS1 as a potential therapy for AIP and other acute hepatic porphyrias.