1. Efficacy of albendazole and mebendazole against Hymenolepis microstoma and Hymenolepis diminuta
R.O. MeCraeken, K.B. Lipkowitz , and N.O. Dronen. Parasitol Res (1992) 78:108-111
Its inactivity in this study cannot be definitely ex-plained, but it is possible that adult H.microstoma are inherently less susceptible to MBZ treatment than adult H. diminuta. This interpretation would be in accord with the findings of Novak and her associates (Evans and Novak 1976; Novak and Evans 1978; Evans et al. 1979, 1980; Novak et al. 1982), who had previously reported that the developing cysticercoids of H. microstoma in Tribolium confusum are far less susceptible to treatment with MBZ and ABZ (and other benzimidazole anthelmintics as well) than are the cysticercoids of H. diminuta and H. nana. Our results, together with the reports cited above, suggest that there are major differences in the inherent susceptibility of hymenolepidid tapeworms to treatment with benzimidazole anthelmintics. In addition, it is likely that the preferred location of H. rnicrostoma in the bile duct of the host makes it much less vulnerable to chemotherapeutic attac with MBZ than enteral H.dirninuta. The metabolism and pharmacokinetics of MBZ in rats has been studied in detail by Allan and Watson (1982, 1983). After oral administration MBZ is absorbed, transported to the liver, metabolized to the secondary alcohol by carbonyl reduction of the benzoyl group at the 5-position of the benzimidazole carbamate molecule, conjugated to glucuronide or sulfate, and eliminated predominately in the bile. This, together with the absence of anthelmintic activity in the conjugated biliary MBZ metabolites (Meuldermans et al. 1976; Gottschall et al. 1990) suggests that the concentration of free MBZ at its site of action is very low. The ineffectiveness of MBZ against H. microstoma could be due to its extreme insolubility in water, poor drug absorption, and the consequent difficulty in achieving and maintaining effective (free) biliary drug concentrations. Further trials are needed to establish unequivocally whether H. microstoma is totally invulnerable to treatment with MBZ or, as with parbendazole (methyl 5-[butyl] benzimidazole-2-carbamate), if a different mode of oral administration (continuous [dietary] medication rather than Periodic gavage) would give the desired therapeutic response (Brody and Elward 1971; Kunkle and Theodorides 1971).
2. Development of drugs based on imidazole and benzimidazole bioactive heterocycles: recent advances and future directions
Monika Gaba • Chander Mohan. Med Chem Res (2016) 25:173–210
In 1961, Brown and his team at Merck Sharp & Dohme Laboratories discovered thiabendazole as a broad-spectrum anthelmintic (Brown et al., 1961). The introduction of thiabendazole against parasite infections of both humans and domestic animals provided a major breakthrough that opened up a new era to design further potent anthelmintics. Thiabendazole is the ﬁrst benzimidazole to be marketed over 50 years ago to combat helminthic infections. Although, it shows broad-spectrum activity against different helminths, it suffers from the limitation of being readily metabolized into inactive 5-hydroxythiabendazole, with very short half-life (Fisher, 1986). To prevent enzymatic hydroxylation of thiabendazole at 5-position, Merk scientists synthesized a variety of 5-substituted thiabendazoles, of which cambendazole showed promising activity with a longer half-life (Hoff et al., 1970; Hoff, 1982). Another milestone in the SAR of benzimidazoles was achieved at SmithKline Laboratory, where replacement of the thiazole ring of thiabendazole by thiocarbamate led to the discovery of parbendazole with high anthelmintic activity (Actor et al., 1967). The discovery of parbendazole stimulated a vigorous search for better benzimidazole anthelmintics in different pharmaceutical companies of the world. A number of benzimidazole-based broad-spectrum anthelmintics as derivatives of carbendazim came into the market that act by inhibiting the microtubule formation, such as mebendazole, ﬂubendazole, cyclobendazole, fenbendazole, oxfendazole (or fenbendazole sulfoxide), oxibendazole, nocodazole, albendazole, ricobendazole, (albendazole sulfoxide), and luxabendazole (Townsendand and Wise, 1990; Martin, 1997; Fig. 15). Albendazole, fenbendazole, and oxfendazole are the ﬁrst benzimidazoles to be successfully used for the treatment of all growth stages of gastrointestinal nematodes. These drugs may also be used in the treatment of lungworms, tapeworms, and adult stages of liver ﬂuke. The noncarbamate benzimidazole, triclabendazole, was later introduced as antihelmenthic agents for treatment of all stages of liver ﬂuke, but it is ineffective against nematodes. Luxabendazole is a benzimidazole sulﬁde used in the treatment of food-producing animal. The low solubility of benzimidazole sulﬁdes and sulfoxides leads to their low absorption from gut, resulting in low bioavailability. Therefore, netobimin and febantel, which are the prodrugs of albendazole and fenbendazole, respectively, have greater water solubility resulting in improved absorption and increased bioavailability, whereas other pro-benzimidazoles such as benomyl and thio- phanate, have found widespread use as fungicidal agents, which are precursors of carbendazim (Ozkay et al., 2010).