A full spectrum rational approach to identify antiarrhythmic agents targeting IKs Channels

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Project Summary Acquired Long Q-T Syndrome (aLQTS) is a change in the electrocardiogram (EKG) due to lengthening of the ventricular electrical event, the action potential (AP). This lengthened AP predisposes to a lethal arrhythmia and sudden death. aLQTS is induced by many drugs approved by the FDA. However, the approved doses are limited by this side effect. The slow delayed rectifier, IKs, is a K+ current that can prevent aLQTS by shortening the APD. This proposal focuses on developing candidate agents that can eliminate the APD prolongation induced by drugs that cause aLQTS. There are 5 steps that regulate opening of the channel, voltage sensor activation, phosphatidylinositol 4,5-biphosphate (PIP2), Calmodulin, ATP and opening of the pore. Two of these, voltage sensor activation and PIP2 binding, have led us to two candidate compounds for aLQTS. Because many drug candidates fail due to cardiac toxicity, it is important to investigate all mechanisms controlling channel opening. The first aim, Aim 1, investigates how calmodulin and ATP binding and pore opening can be stimulated by novel agents to increase IKs. The systematic process by which the selection of these novel agents is achieved is called Full Spectrum Rational Drug Design. The structure of the channel is studied and the binding sites for compounds to modify particular steps in channel opening are determined (e.g., calmodulin and ATP interactions and pore opening). A computer program uses this structural information to screen a chemical library of more than 1 million compounds for those most likely to bind near to the structurally defined site. Multiple compounds have already been identified as potentially effective and preliminary data are provided from these compounds for each potential site. Aim 1 tests these compounds to determine their action on IKs, as well as their dose response curve and selectivity. The proposed mechanistic studies of each therapeutic site will provide insight into any antiarrhythmic drug designed to target IKs. The second aim, Aim 2, focuses on the effects of the hit agents on the ion channel’s function and action potential in canine ventricular and atrial myocytes. Each optimal drug candidate from Aim 1 will be studied on the biophysical properties of IKs. They will also be studied on the action potential at multiple concentrations, frequencies of stimulation, and in the absence and presence of a β agonist in both control conditions, and those that prolong the APD as a model for drug induced aLQTs. Atrial myocytes are included because the lead compound must not cause atrial arrhythmias as a measurement for safety.