PCSK9 inhibition and LDL cholesterol lowering: the biology of an attractive therapeutic target and critical review of the latest clinical trials

PCSK9 is a serine protease expressed in the liver and the intestine. Gain-of-function mutations in PCSK9 are associated with autosomal dominant hypercholesterolemia, independently of mutations in apoB-100 or the LDL receptor (LDLR). Wild-type PCSK9 binds to the extracellular domain of the LDLR at the surface of hepatocytes, is internalized with the receptor and redirects it to lysosomes where the complex is degraded. Conversely, loss-of-function mutations in PCSK9 are associated with lower levels of LDL cholesterol (LDL-C) and reduced cardiovascular risk. Inhibition of PCSK9 binding to the LDLR with human monoclonal antibodies or reduction of PCSK9 expression with siRNA are the two main strategies to reduce LDL-C, which are actively pursued in the clinical development phases. In this article, the current evidence for PCSK9 as a target for reducing LDL-C and results of Phase I and II clinical trials with anti-PCSK9 therapeutic strategies are reviewed.