Cycloguanil is a dihydrofolate reductase inhibitor, and is a metabolite of the antimalarial drugproguanil; its formation in vivo has been thought to be primarily responsible for the antimalarial activity of proguanil. However, more recent work has indicated that, while proguanil is synergistic with the drug atovaquone (as in the combination Malarone), cycloguanil is in fact antagonistic to the effects of atovaquone, suggesting that, unlike cycloguanil, proguanil may have an alternative mechanism of antimalarial action besides dihydrofolate reductase inhibition.
Although cycloguanil is not currently in general use as an antimalarial, the continuing development of resistance to current antimalarial drugs has led to renewed interest in studying the use of cycloguanil in combination with other drugs.
^Thapar, M. G.; Gupta, S.; Spindler, C.; Wernsdorfer, W. H.; Björkman, A. (May 2003). "Pharmacodynamic interactions among atovaquone, proguanil and cycloguanil against Plasmodium falciparum in vitro". Transactions of the Royal Society of Tropical Medicine and Hygiene97 (3): 331–337. doi:10.1016/S0035-9203(03)90162-3. ISSN0035-9203. PMID15228254.edit