Pharmacologic myocardial preconditioning with monophosphoryl lipid A (MLA) reduces infarct size and stunning in dogs and rabbits.

As the mechanism of ischemic preconditioning unfolds, various strategies for inducing pharmacologic preconditioning become apparent. Adenosine receptor agonists, KATP channel activators, and endothelial-neutrophil adhesion antagonists have enjoyed cardioprotective activity against ischemia/reperfusion injury in at least some preclinical models. Monophosphoryl lipid A (MLA), a structural derivative of the pharmacophore of endotoxin, enjoys an improved therapeutic index in relation to the parent biological product. MLA has found clinical application as a vaccine adjuvant and protects from sepsis and septic shock in the preclinical setting. In animal models of myocardial ischemia/reperfusion injury, pretreatment 12-24 hours prior to ischemia with a single IV bolus injection of MLA limits infarct size 50 to 75 percent in standard canine and rabbit models at doses of 10-35 micrograms/kg. Regional myocardial stunning following multiple 5-minute ischemic episodes as assessed by segment shortening is reduced in dogs pretreated 24 but not 1 hour prior to ischemia. Global cardiac function, as evaluated by pressure-volume constructs generated in dogs being weaned from cardiopulmonary bypass, recovers more quickly in animals pretreated with MLA. Cardiac protection in various models is associated with preservation of ATP during ischemia, induction of 5' nucleotidase and enhancement of calcium reuptake by SR during reperfusion. Limitation of infarct size by MLA in dogs and rabbits can be reversed by the administration of glibenclamide just prior to ischemia, suggesting a role for KATP channel opening during the first minutes of sustained ischemia. A clinical formulation of MLA (MPL-C) is currently undergoing clinical investigation in the Phase II setting in coronary artery bypass surgical patients. MLA may represent a novel means of inducing pharmacologic preconditioning, with potential for clinical application as a pretreatment before planned myocardial ischemia.