Abstract

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 μM. Competitive antagonism was shown by a lack of voltage-dependence, Schild plot (K_b = 1.70 μM), and radioligand competition (K_i = 2.61 μM) with the 5-HT₃ receptor antagonist [³H]granisetron. Kinetic measurements (k_on = 4.0 × 10⁴ M⁻¹ s⁻¹_; k_off = 0.23 s⁻¹) were consistent with a simple bimolecular reaction scheme with a K_b of 4.35 μM. The metabolites CG and CPB similarly inhibited 5-HT₃ receptors as assessed by IC₅₀ (1.48 and 4.36 μM, respectively), Schild plot (K_b = 2.97 and 11.4 μM), and radioligand competition (K_i = 4.89 and 0.41 μM). At higher concentrations, CPB was a partial agonist (EC₅₀ = 14.1 μM; I/I_max = 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.