Antimalarial quinones for prophylaxis based on a rationale of inhibition of electron transfer in Plasmodium

Knowledge of the biochemistry of Plasmodium is emerging as a new field. Previous studies showed that the parasite apparently requires electron transfer for energy, and techniques to study such energy mechanisms are available. The discovery of the existence of coenzyme Q(8) in Plasmodium implies an indispensable functionality for this redox entity in the electron transfer of the parasite, as coenzyme Q(n) similarily functions in other forms of life. Effective antimalarial activity in prophylaxis has been demonstrated in sporozoite-induced infections by Plasmodium gallinaceum in chicks by several representatives of 7-alkylmercapto-6-hydroxy-5,8-quinolinequinones. The absence of toxicity in this assay even at greatly elevated dosage underscores the achievement of selectivity and safety to the host for the potential utilization of antimetabolites of coenzyme Q(n) as medicinals. Seven new 7-alkylmercapto-6-hydroxy-5,8-quinoline-quinones were synthesized. The structural variations of the 7-alkylmercapto group in relationship to the antimalarial activities reveal substantial differences in biological activities, which can reflect molecular specificities of enzyme sites and which are not evident from the deceptively minor structural differences in the alkylmercapto groups. These analogs of coenzyme Q(8) having effective antimalarial activity are known to inhibit mammalian coenzyme Q(n) enzymes, and could be useful in elucidation of the basic electron transfer mechanisms of Plasmodium.