Calcium-activated channel changer

Although the antifungal agent clotrimazole is a potent inhibitor of the intermediate-conductance, calcium-activated K⁺ channel (IKCa1), it has the drawback of also inhibiting human P-450 enzymes. Researchers originally believed that the two activities might be connected, but structural studies suggested that they could be teased apart. If so, the resulting molecule could be an immunosuppressant and an effective treatment for sickle cell disease and diarrhea.

Various pharmaceutical companies have pursued a selective inhibitor using combinatorial chemistry and high-throughput screens, says Heiki Wulff, a researcher at the University of California, Irvine. But Wulff took an old-fashioned approach by making clotrimazole derivatives one at a time and testing them with patch clamps. This method ultimately gave her an advantage over the big guns. “No pharmaceutical company would set up a patch-clamping screen. They would say you’re nuts, because it takes forever. It took me a year to synthesize 90 compounds and screen them,” she says.

Focusing mainly on clotrimazole’s heterocyclic rings, especially the imidazole, and replacing it with heterocycles that were similar in size and electron density, she eventually developed a selective IKCa1 inhibitor that had no effect on P-450 enzymes. The molecule, TRAM-34, is also 200- to 1500-fold more selective for IKCa1 than for other ion channels.

The molecule has potential in the treatment of sickle cell disease and diarrhea, but Wulff and her collaborators are excited mostly about TRAM-34’s immunosuppressant activity. It could be an attractive alternative to cyclosporin because IKCa1 is found primarily in T lymphocytes, and “it inhibits the [T cell] activation cascade at an early time-point, so you might prevent some of the side effects of cyclosporin,” says Wulff.

Touting TRAM-34’s use in the study of other potassium channels, Wulff added, “It’s a great pharmacological tool, because until now the only calcium-activated potassium channel inhibitor was clotrimazole.” This is especially true in P-450-rich epithelial tissues, where with existing inhibitors “you would never be able to distinguish [the two potassium channels].”