The antimalarial drug proguanil is an antagonist at 5-HT3 receptors

Proguanil is an antimalarial prodrug that is metabolized to 4-chlorophenyl-1-biguanide (CPB) and the active metabolite cycloguanil (CG). These compounds are structurally related to meta-chlorophenyl biguanide (mCPBG), a 5-hydroxytryptamine 3 (5-HT3) receptor agonist. Here we examine the effects of proguanil and its metabolites on the electrophysiology and ligand-binding properties of human 5-HT3A receptors expressed in Xenopus oocytes and human embryonic kidney 293 cells, respectively. 5-HT3 receptor responses were reversibly inhibited by proguanil, with an IC50 of 1.81 microM. Competitive antagonism was shown by a lack of voltage-dependence, Schild plot (Kb = 1.70 microM), and radioligand competition (Ki = 2.61 microM) with the 5-HT3 receptor antagonist [(3)H]granisetron. Kinetic measurements (kon = 4.0 × 10(4) M(-1) s(-1) ; koff = 0.23 s(-1)) were consistent with a simple bimolecular reaction scheme with a Kb of 4.35 microM. The metabolites CG and CPB similarly inhibited 5-HT3 receptors as assessed by IC50 (1.48 and 4.36 microM, respectively), Schild plot (Kb = 2.97 and 11.4 microM), and radioligand competition (Ki = 4.89 and 0.41 microM). At higher concentrations, CPB was a partial agonist (EC50 = 14.1 microM; I/Imax = 0.013). These results demonstrate that proguanil competitively inhibits 5-HT3 receptors, with an IC50 that exceeds whole-blood concentrations following its oral administration. They may therefore be responsible for the occasional gastrointestinal side effects, nausea, and vomiting reported following its use. Clinical development of related compounds should therefore consider effects at 5-HT3 receptors as an early indication of possible unwanted gastrointestinal side effects.