Effect of the PAF-receptor antagonist SM-12502 on human platelets

Understanding the regulation mechanisms of PAF receptor by agonists and antagonists: Molecular modeling and molecular dynamics simulation studies

Platelet-activating factor receptor (PAFR) is a member of G-protein coupled receptor (GPCR) superfamily. Understanding the regulation mechanisms of PAFR by its agonists and antagonists at the atomic level is essential for designing PAFR antagonists as drug candidates for treating PAF-mediated diseases. In this study, a 3D model of PAFR was constructed by a hierarchical approach integrating homology modeling, molecular docking and molecular dynamics (MD) simulations. Based on the 3D model, regulation mechanisms of PAFR by agonists and antagonists were investigated via three 8-ns MD simulations on the systems of apo-PAFR, PAFR-PAF and PAFR-GB. The simulations revealed that binding of PAF to PAFR triggers the straightening process of the kinked helix VI, leading to its activated state. In contrast, binding of GB to PAFR locks PAFR in its inactive state.

Nupafant, a PAF-antagonist prototype for suppression of ventricular fibrillation without liability for QT prolongation?

BACKGROUND AND PURPOSE

PAF antagonists inhibit ischaemia-induced ventricular fibrillation (VF) in animals. However, unfavourable ancillary actions (on QT interval and coronary flow) have been reported with the PAF antagonist, BN-50739. If these are class actions, they would preclude development of PAF antagonists as novel anti-VF drugs. Our purpose was to examine this proposition using the hitherto untested PAF antagonist, nupafant.

EXPERIMENTAL APPROACH

Two rat heart preparations (Langendorff and 'dual coronary' perfusion) were used to assay nupafant's effects on ischaemia-induced VF, coronary flow and QT interval, and to test for the site-selectivity necessary if any effects on VF are caused by PAF antagonism.

KEY RESULTS

Global (whole-heart) delivery of 10 microM nupafant, reduced the incidence of ischaemia-induced VF and widened QT interval without affecting coronary flow. Importantly, lower concentrations (0.1 and 1 microM) had no effect on VF, yet widened QT almost identically to 10 microM nupafant. When nupafant was delivered selectively to (and entrapped within) the involved region it partially protected against VF (P<0.05). This occurred without change in QT interval. Selective nupafant delivery to the uninvolved region was without effect.

CONCLUSIONS AND IMPLICATIONS

Nupafant protects against ischaemia-induced VF primarily by site-selective actions in the ischaemic region but, unlike BN-50739, the effect is unrelated to its QT widening action, and is not compromised by any effect on coronary flow. This establishes proof of concept that VF suppression by PAF antagonism need not invariably be associated with QT prolongation or vasodilatation, justifying further development of this drug class.

Protection against ventricular fibrillation by the PAF antagonist, BN-50739, involves an ischaemia-selective mechanism

The platelet-activating factor (PAF) antagonist BN-50739 can suppress certain cardiac arrhythmias. PAF is released from ischaemic myocardium and may contribute to initiation of ischaemia-induced ventricular fibrillation (VF). In this study we characterised the action of BN-50739 on left regional ischaemia-induced VF and examined whether effects are mediated within the ischaemic territory, or are nonspecific. In rat isolated Langendorff perfused hearts (n = 12/group), 10 microM BN-50739 reduced the incidence of ischaemia-induced VF from 75 to 17% (p<0.05). This was accompanied by QT widening and an increase in coronary flow. Heart rate and PR interval were not affected by the drug. In separate studies, isolated rat hearts were perfused by using a dual-lumen tube that allows independent delivery of solution to the left and right coronary beds. Successful regional localisation of drug delivery was confirmed by observing, before ischaemia, a regionally selective increase in coronary flow (p<0.05), measured by using two in-line flow meters. Protection against ischaemia-induced VF (p<0.05) was achieved by pretreatment with BN-50739, delivered selectively and entrapped within the involved region, but not when the drug was delivered to the uninvolved region. In conclusion, BN-50739 protects against ischaemia-induced VF by eliciting a pharmacologic action in the involved (ischaemic) myocardium. This supports the hypothesis that BN-50739 suppresses an arrhythmogenic effect of endogenous PAF released within the ischaemic tissue.