Comparative Efficacy of Different Mebendazole Polymorphs in the Treatment of Soil-transmitted Helminth Infections

Mebendazole has three polymorphic forms, identified as A, B and C. All of them are in accord with the US Pharmacopeia specifications (USP XXI) but they have distinct physiochemical characteristics (Himmelreich et al, 1977) and different therapeutic activities in experimentally infected mice with Trichinella spiralis infections (Rodriguez-Caabeiro et al, 1987). The original mebendazole tablets which were used to treat human infections had more than 90% of polymorph C (Van den Bossche et al, 1982), but most pharmacopeias currently do not specify the proportion of polymorph C that a tablet of mebendazole should contain, and the assay specified for measurement of the active ingredient measures all polymorphs together. There is some evidence to show that unlike polymorph C, polymorph A is ineffective in the treatment of hookworm and whipworm infections (Charoenlarp et al, 1993). The State Pharmaceutical Manufacturing Corporation of Sri Lanka produces both 500 mg and 100 mg tablets of mebendazole according to specifications laid down in the US Pharmacopeia. These tablets contain a mixture of polymorphs A and C. It is possible that increasing the content of mebendazole polymorph C in single dose tablets may improve cure rates and egg reduction rates, especially against hookworm and whipworm infections, where much variation in efficacy has been observed.