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.
