Active site competition is the mechanism for the inhibition of lipoprotein-associated phospholipase A2 by detergent micelles or lipoproteins and for the efficacy reduction of darapladib

Abstract

Lipoprotein associated phospholipase A2 (Lp-PLA2) has been characterized for its interfacial activation as well as inhibition by detergent micelles and lipoprotein particles. The enzyme has been shown to bind on the surfaces of hydrophobic aggregates, such as detergent micelles, lipoprotein particles and even polystyrene latex nanobeads. Binding to hydrophobic aggregates stimulates the activity of Lp-PLA2 but may not be the necessary step for catalysis. However, at higher concentrations, detergent micelles, latex nanobeads or lipoprotein particles inhibit Lp-PLA2 possibly by blocking the access of substrates to the active site. The competition mechanism also blocks inhibitors such as darapladib binding to Lp-PLA2 and reduces the efficacy of the drug. Darapladib has very low solubility and mainly exists in solutions as complexes with detergents or lipoprotein particles. The inhibition of Lp-PLA2 by darapladib is dependent on many factors such as concentrations of detergents or lipoproteins, incubation time, as well as the order of mixing reaction components. The in vitro Lp-PLA2 activity assays used in clinical studies may not accurately reflect the residual Lp-PLA2 activity in vivo. Darapladib has been found mainly bound on HDL and albumin when it is incubated with human serum. However, Lp-PLA2 is more sensitive to darapladib when bound on LDL and relatively resistant to darapladib when bound on HDL. Therefore, high cholesterol levels may decrease the efficacy of darapladip and cause the drug to be less effective in high risk patients. Our study will help to design better inhibitors for Lp-PLA2. The discoveries also contribute to understanding the mechanism of interfacial activation and inhibition for Lp-PLA2 and provide a new concept for researchers in building better kinetic model for interfacial enzymes.

Oxidized phospholipids and lipoprotein‐associated phospholipase A2 (Lp‐PLA2) in atherosclerotic cardiovascular disease: An update

“…A role in atherothrombosis has also been proposed for Lp‐PLA 2 which is present on rupture‐prone plaques but not on stable plaques. Unfortunately, the Lp‐PLA 2 inhibitor darapladib failed to demonstrate efficacy in two large clinical trials 79 …”

“…Unfortunately, the Lp-PLA 2 inhibitor darapladib failed to demonstrate efficacy in two large clinical trials. 79…”

Alternative Targets for sPLA2 Activity: Role of Membrane-Enzyme Interactions

“…It has recently been shown that subcutaneous administration of lysolipids to rats led to a decrease in the activity of sPLA2 in the blood [ 26 ]. In addition, the inhibitory effect of detergent micelles on lipoprotein-associated PLA2 has been shown [ 27 ]. In both cases we assume that micelles formed by lysolipids or detergents are responsible for PLA2 binding and subsequent enzyme take-off from the reaction.…”

Alterations of HDL’s to piHDL’s Proteome in Patients with Chronic Inflammatory Diseases, and HDL-Targeted Therapies

“…Since the majority of serum Lp-PLA2 is bound to LDL particles, the higher affinity of darapladib to HDL may provide an additional explanation for its failure. In case the hypothesis of different activity of HDL or LDL-bound Lp-PLA2 is a true, a more detailed understanding of this phenomenon could also help to clarify darapladib inefficiency [ 534 ].…”