Poloxamer 188 mechanism of action in ischemic heart failure.

DESCRIPTION (provided by applicant): This project was initiated to determine how Poloxamer-188 (P-188) improves cardiac function in ischemic heart failure (HF). While it is accepted that lack of oxygen (ischemia) results in tissue damage, the direct involvement of membrane injury in the progression of ischemic HF is a relatively novel concept. The long-term goal of this project is to establish a role for membrane tears in the progression of heart failure and to demonstrate that P- 188 interacts with damaged membranes to prevent unregulated entry of Ca2+ into failing cardiac muscle cells. In patients with acute decompensated HF the heart does not generate enough force to pump blood to meet metabolic demands of the patient. This is due, in part, to elevated intracellular Ca2+ levels that depress excitation-contraction coupling and increase myocardial stifnes. Ca2+ overload also leads to apoptosis, myocardial contracture and necrosis, which contribute to HF progression. Phrixus believes that Ca2+ overload results from unregulated entry of extracellular Ca2+ through microscopic membrane tears (microtears). The hypothesis to be tested in this proposal is that, in ischemic HF, cardiomyocytes develop contraction-induced, P-188 sealable, membrane tears as a result of adverse events that accompany the remodeling process (i.e. activation of calcium-activated proteases (calpain) and loss of dystrophin). Membrane sealing lowers intracellular Ca2+ levels, decreases calpain activity, and reduces cleavage of its substrates. Specific aim 1 will determine if membrane micro-tears contribute to the dysfunction of cardiomyocytes, isolated from the failing rat heart, and if P-188 treatment improves function. Cardiomyocytes from normal and failing hearts will be stretched over a range of lengths that are relevant to those the cells experience in the normal contraction cycle. During the stretch, the developed tension and the Ca2+ concentration in the cell will be measured in the absence and presence of P-188. If increases in tension and Ca2+ are seen in failing heart cells, compared with control, then a fluorescent, lipidic dye asay will be run to identify exposed hydrophobic regions in the membrane (microtears), and determine if P-188 can seal the tears. This aim will be done in Dr. Metzger's lab at the University of Minnesota. In the second aim, the consequences of increased Ca2+ in cardiomyocytes such as calpain activation, cleavage of dystrophin, dysferlin, and cardiac troponins I and T will be monitored as well as the ability of P-188 treatment, in vivo, to ameliorate these consequences. These studies will be the first steps towards establishing a role of membrane tears in HF progression as well as towards establishing the mechanism of action for P-188, a novel and exciting new class of therapy for HF. Phrixus has an open IND for a Phase II clinical trial with P-188 in ADHF patients. PUBLIC HEALTH RELEVANCE: This proposal is focused on determining the molecular mechanism of action by which Poloxamer-188 improves heart function in a rodent model of ischemic heart failure. Knowing the mechanism of action is important to predicting how a therapy will work and whether or not it will have unwanted effects. The FDA has cleared Poloxamer-188 for testing in heart failure patients.