A novel nortriterpene, termed correolide, purified from the tree Spachea correae, inhibits Kv1.3, a Shaker-type delayed rectifier potassium channel present in human T lymphocytes. Correolide inhibits 86Rb+ efflux through Kv1.3 channels expressed in CHO cells (IC50 86 nM; Hill coefficient 1) and displays a defined structure-activity relationship. Potency in this assay increases with preincubation time and with time after channel opening. Correolide displays marked selectivity against numerous receptors and voltage- and ligand-gated ion channels. Although correolide is most potent as a Kv1.3 inhibitor, it blocks all other members of the Kv1 family with 4-14-fold lower potency. C20-29-[3H]dihydrocorreolide (diTC) was prepared and shown to bind in a specific, saturable, and reversible fashion (Kd = 11 nM) to a single class of sites in membranes prepared from CHO/Kv1.3 cells. The molecular pharmacology and stoichiometry of this binding reaction suggest that one diTC site is present per Kv1.3 channel tetramer. This site is allosterically coupled to peptide and potassium binding sites in the pore of the channel. DiTC binding to human brain synaptic membranes identifies channels composed of other Kv1 family members. Correolide depolarizes human T cells to the same extent as peptidyl inhibitors of Kv1.3, suggesting that it is a candidate for development as an immunosuppressant. Correolide is the first potent, small molecule inhibitor of Kv1 series channels to be identified from a natural product source and will be useful as a probe for studying potassium channel structure and the physiological role of such channels in target tissues of interest.

Correolide is a novel immunosuppressant that inhibits the voltage-gated potassium channel K(v)1.3 [Felix et al. (1999) Biochemistry 38, 4922-4930]. [(3)H]Dihydrocorreolide (diTC) binds with high affinity to membranes expressing homotetrameric K(v)1.3 channels, and high affinity diTC binding can be conferred to the diTC-insensitive channel, K(v)3.2, after substitution of three nonconserved residues in S(5) and S(6) with the corresponding amino acids present in K(v)1.3 [Hanner et al. (1999) J. Biol. Chem. 274, 25237-25244]. Site-directed mutagenesis along S(5) and S(6) of K(v)1.3 was employed to identify those residues that contribute to high affinity binding of diTC. Binding of monoiodotyrosine-HgTX(1)A19Y/Y37F ([(125)I]HgTX(1)A19Y/Y37F) in the external vestibule of the channel was used to characterize each mutant for both tetrameric channel formation and levels of channel expression. Substitutions at Leu(346) and Leu(353) in S(5), and Ala(413), Val(417), Ala(421), Pro(423), and Val(424) in S(6), cause the most dramatic effect on diTC binding to K(v)1.3. Some of the critical residues in S(6) appear to be present in a region of the protein that alters its conformation during channel gating. Molecular modeling of the S(5)-S(6) region of K(v)1.3 using the X-ray coordinates of the KcsA channel, and other experimental constraints, yield a template that can be used to dock diTC in the channel. DiTC appears to bind in the water-filled cavity below the selectivity filter to a hydrophobic pocket contributed by the side chains of specific residues. High affinity binding is predicted to be determined by the complementary shape between the bowl-shape of the cavity and the shape of the ligand. The conformational change that occurs in this region of the protein during channel gating may explain the state-dependent interaction of diTC with K(v)1.3.

The discovery of novel therapeutic agents that act on voltage-gated sodium channels requires the establishment of high-capacity screening assays that can reliably measure the activity of these proteins. Fluorescence resonance energy transfer (FRET) technology using membrane potential-sensitive dyes has been shown to provide a readout of voltage-gated sodium channel activity in stably transfected cell lines. Due to the inherent rapid inactivation of sodium channels, these assays require the presence of a channel activator to prolong channel opening. Because sodium channel activators and test compounds may share related binding sites on the protein, the assay protocol is critical for the proper identification of channel inhibitors. In this study, high throughput, functional assays for the voltage-gated sodium channels, hNa(V)1.5 and hNa(V)1.7, are described. In these assays, channels stably expressed in HEK cells are preincubated with test compound in physiological medium and then exposed to a sodium channel activator that slows channel inactivation. Sodium ion movement through open channels causes membrane depolarization that can be measured with a FRET dye membrane potential-sensing system, providing a large and reproducible signal. Unlike previous assays, the signal obtained in the agonist initiation assay is sensitive to all sodium channel modulators that were tested and can be used in high throughput mode, as well as in support of Medicinal Chemistry efforts for lead optimization.

The voltage-gated potassium channel, K(v)1.3, is a novel target for development of immunosuppressants. Using a functional (86)Rb(+) efflux assay, a new class of high-affinity K(v)1.3 inhibitors has been identified. The initial active in this series, 4-phenyl-4-[3-(2-methoxyphenyl)-3-oxo-2-azaprop-1-yl]cyclohexanone (PAC), which is representative of a disubstituted cyclohexyl (DSC) template, displays a K(i) of ca. 300 nM and a Hill coefficient near 2 in the flux assay and in voltage clamp recordings of K(v)1.3 channels in human T-lymphocytes. PAC displays excellent specificity as it only blocks members of the K(v)1 family of potassium channels but does not affect many other types of ion channels, receptors, or enzyme systems. Block of K(v)1.3 by DSC analogues occurs with a well-defined structure-activity relationship. Substitution at the C-1 ketone of PAC generates trans (down) and cis (up) isomer pairs. Whereas many DSC derivatives do not display selectivity in their interaction with different K(v)1.x channels, trans DSC derivatives distinguish between K(v)1.x channels based on their rates of C-type inactivation. DSC analogues reversibly inhibit the Ca(2+)-dependent pathway of T cell activation in in vitro assays. Together, these data suggest that DSC derivatives represent a new class of immunosuppressant agents and that specific interactions of trans DSC analogues with channel conformations related to C-type inactivation may permit development of selective K(v)1.3 channel inhibitors useful for the safe treatment of autoimmune diseases.

The rational design and development of four-fold symmetrical ligands for potassium channels is described.