Pharmacologically tuning membrane permeability to drug-sized organic cations through activation of KCa3.1 channels

PROJECT SUMMARY/ABSTRACT To reach their therapeutic targets within the body, most drugs need to have the ability to enter cells and traverse epithelial barriers. Achieving this hinges on efficient transportation across the cellular plasma membrane, which is especially crucial for drugs designed to act inside cells. For the many weak acid and weak

base drugs that can occupy both charged and uncharged states at physiological pH, it is generally assumed that simple diffusion of the uncharged species directly across the phospholipid bilayer of biological membranes is the major passive transport pathway. However, evidence has steadily accumulated supporting a larger role

of transport proteins, carriers, and channels, in aiding the passive movement of drugs, charged and uncharged, into and through cells. This is significant on two counts. First, it may be that for some drugs the expression and function of facilitated transport proteins may be as influential to the pharmacokinetics

governing absorption, distribution and elimination as the expression of active transporters such as P- glycoprotein. Second, whereas lipoidal diffusion is not a process that can be easily and selectively modified by administered agents, the activity of many membrane transporters, particularly channel proteins, can be readily

altered by pharmacological activators and inhibitors. Given that many such proteins exhibit preferential expression in certain cell types and tissues, there is the possibility of exploiting such pharmacologically tunable transport mechanisms for the purposes of influencing drug transfer and enhancing the potency and selectivity

of intracellularly active therapeutics. The current proposal is inspired by our recent discovery that activation of the intermediate conductance Ca2+-activated K+ channel (KCa3.1) increases the permeability of cells to the fluorescent nuclear stains, Hoechst 33258 and DAPI through a previously uncharacterized mechanism. KCa3.1

receptors are highly expressed in epithelial cells of the blood vessels and gut, where they could potentially influence the absorption and distribution of small cationic drugs. Additionally, they are reportedly upregulated in many cancers, where they might be exploited as a means of targeting tumor cells with small charged

chemotherapeutics. Our goal is to elucidate the mechanism underlying this KCa3.1-dependent, pharmacologically tunable drug transport pathway and reveal the structural characteristics that permit small organic cations to use it.