Proprotein convertases subtilisin/kexin type 9 (PCSK9) is a member of the proprotein convertase (PC) family of subtilisin-like serine endoproteases that regulates low density lipoprotein receptor (LDLR) levels and function. Murine models and human genetic studies indicate that loss of PCSK9 protein increases LDLR levels while excess PCSK9 decreases LDLR levels. These changes in LDLR protein levels coincide with reciprocal changes in circulating levels of plasma LDL cholesterol (LDL-C). We have developed a highly potent RNA interference (RNAi) therapeutic, ALN-PCS, targeting both intra and extracellular PCSK9 for inhibition through an RNAi mechanism. Pre-clinical data in non-human primate models, indicate that a single intravenous dose of ALN-PCS results in rapid, dose dependent, and significant lowering of liver PCSK9 transcript, plasma PCSK9 protein and subsequently serum LDL-C and ApoB levels, without impacting serum HDL-C. Here we report on interim data from an ongoing Phase 1 trial of ALN-PCS being conducted as a randomized, single-blind, placebo-controlled, single-ascending dose study in healthy volunteer subjects with elevated baseline LDL-C (>116mg/dL) who are not on any lipid lowering therapy. The primary objective of the study is to evaluate the safety and tolerability of a single dose of ALN-PCS, with subjects being enrolled into sequential cohorts of increasing doses. Secondary objectives of the study include characterization of plasma and urine pharmacokinetics of ALN-PCS, assessment of pharmacodynamic effects of the drug on plasma PCSK9 protein levels, and evaluation of clinical efficacy as measured by LDL-C levels. Data from 20 subjects enrolled in five sequential dose cohorts ranging from 0.015 to 0.250 mg/kg in a 3:1 randomization of drug to placebo will be presented. ALN-PCS was safe and well tolerated in this study and there have been no serious adverse events related to study drug administration to date. There have been no drug-related discontinuations from the study and no liver enzyme elevations. A mild, transient rash that resolved spontaneously was observed in three subjects that received ALN-PCS, and in two that received placebo. To date, administration of ALN-PCS resulted in a rapid, dose-dependent, and durable silencing of PCSK9 protein levels in plasma of up to 66% relative to baseline, with a statistically significant mean reduction of 60% at day four in the current high dose group of 0.250 mg/kg (p<0.001). In addition, administration of ALN-PCS resulted in dose-dependent reductions in LDL-C of up to 50% relative to baseline, with a statistically significant mean reduction of 39% at day four (p<0.05) at the 0.250 mg/kg dose level. There was no significant decrease in high-density lipoprotein (HDL). Dosing of further cohorts at the current top dose (0.25 mg/kg) and at a higher dose is planned. Additional data will be presented as available.