Monophosphoryl lipid A: A novel nitric oxide-mediated therapy to attenuate platelet thrombosis?

Nitric oxide (NO) is a potent inhibitor of platelet aggregation. However, the benefits of NO-based therapies can be confounded by concomitant hypotension. Monophosphoryl lipid A (MLA) is a nontoxic derivative of endotoxin that purportedly increases nitric oxide synthase (NOS) activity and, presumably, NO production, yet has a hemodynamically benign profile. Thus our aims were to determine whether (a) MLA attenuates in vivo platelet aggregation in damaged and stenotic canine coronary arteries by a NO-mediated mechanism but without reductions in arterial pressure; and (b) the platelet inhibitory effects are manifest in vitro. To address the first aim, anesthetized dogs underwent coronary injury ± stenosis, resulting in cyclic variations in coronary blood flow (CFVs) caused by the formation/dislodgement of platelet-rich thrombi. In protocol I, dogs received MLA (100 μg/kg ± 40 μg/kg/h) or vehicle beginning 15 min before stenosis. Protocol II was identical, except the NOS inhibitor aminoguanidine was coadministered with MLA/vehicle. Coronary patency was assessed throughout the initial 3 h after injury ± stenosis. Infusion of MLA did not result in hypotension. However, in protocol I. the median nadir of the CFVs was higher (2.1 vs. 0.8 ml/min: p < 0.05), median duration of total thrombotic occlusion tended to be reduced (0 vs. 10.4 min; p = 0.1), and mean flow-time area, expressed as a percentage of baseline flow, was increased (53 ± 9% vs. 33 ± 3%; p < 0.05) in MLA- treated versus vehicle-treated dogs. In contrast, in protocol II, vessel patency was comparable in both groups. Finally, whole blood impedance aggregometry (protocol III) revealed a significant reduction in the in vitro platelet aggregation in blood samples receiving exogenous MLA, which was blocked by coadministration of exogenous aminoguanidine. Thus MLA attenuates platelet-mediated thrombosis in both damaged and stenotic canine coronary arteries and in vitro, possibly by an NO-mediated mechanism, but without concomitant hypotension.