Selective antimitochondrial agents inhibit calmodulin.

Certain cationic-lipophilic compounds are known to selectively accumulate in tumor mitochondria and inhibit energy production. Since these substances bear a structural resemblance to known inhibitors of calmodulin, we studied whether rhodamine-123 or a bis-4-aminoquinaldinium could antagonize the action of calmodulin. Rhodamine-123 (IC50 = 58 microM) and dequalinium (IC50 = 1 microM) inhibited the activity of a calmodulin-stimulated cyclic nucleotide phosphodiesterase. Propylinium, a compound similar to dequalinium except for having a 3 rather than 10 carbon alkyl bridge connecting two non-substituted quinoline rings, had no inhibitory effect. Kinetic analysis showed that dequalinium competitively inhibited calmodulin's activation of phosphodiesterase. We also studied the antiproliferative effects of the compounds on the C6 astrocytoma cell line. Rhodamine-123 and dequalinium inhibited the proliferation of this cell line while propylinium had no effect. These studies demonstrate that rhodamine-123 and dequalinium are calmodulin-antagonists and inhibit cellular proliferation.