Contribution of dithiol ligands to in vitro and in vivo trypanocidal activities of dithiaarsanes and investigation of ligand exchange in an aqueous solution.

Twelve new dithiaarsanes were evaluated for their in vitro and in vivo trypanocidal properties in regard to their three parent molecules, 4-amino-phenylarsenoxide, melarsenoxide, and 4-dansylamino-phenylarsenoxide. The most potent dithiaarsane, compound 2b, had a minimum effective concentration of 1.5 nM after 48 h of incubation and at a dose of 0.39 micromol/kg of body weight (0.2 mg/kg) administered subcutaneously cured 100% of mice acutely infected with Trypanosoma brucei brucei CMP. With this model, the chemotherapeutic index of compound 2b was 512, compared to 256 for melarsamine dihydrochloride (Cymelarsan) under the same conditions. With a chronic infection produced by T. brucei brucei GVR, compound 2b cured 100% of mice after treatment at a dose of 25 micromol/kg (12.5 mg/kg) for 4 consecutive days, whereas melarsamine dihydrochloride and potassium melarsonyl (Trimelarsan) cured less than 50% mice at this dose. For both acute and late-stage infections, dithiaarsanes having a melaminophenyl ring exhibited the most-potent trypanocidal activity. Compound 2b is thus one of the most active organoarsenicals described in a mouse trypanosomiasis model. Considering that the main intracellular targets of organoarsenicals are thiol groups, we studied the possibility of ligand exchange between Cymelarsan and several dithiols. In aqueous solution, we observed a rapid exchange of cysteamine from melarsamine with free cysteamine and also with various dithiols always in favor of more stable cyclic derivatives. These ligand exchanges suggest the ability of trivalent organoarsenicals to react with targets such as trypanothione and dihydrolipoic acid. Among several ligands, a 1,3-dimercaptopropane moiety appeared the most suitable for trypanocidal activity.

Synthesis and in vitro anthelmintic properties of some new dithiaarsanes.

Fifteen new trivalent organoarsenicals were synthesized and evaluated for anthelmintic properties on three in vitro models, infective larvae of the filaria Molinema dessetae, infective larvae of an intestinal nematode, Nippostrongylus brasiliensis and adults and larvae of Rhabditis pseudoelongata a free living nematode. On the M. dessetae model, the most active compound after a 24 h incubation period had an EC50 of 0.02 mumol/l (compound 3a). Twelve compounds had an EC50 lower than 1 mumol/l whereas potassium melarsonyl exhibited an EC50 of 45.6 mumol/l. After 7 days incubation time, compound 1d had an EC50 of 2 nmol/l. On the N. brasiliensis model, compound 1d was also the most efficient after a 4 day incubation period (EC50 of 1 mumol/l). This compound was 100 times more active than potassium melarsonyl used as a reference compound. Nevertheless, no compound had an EC50 less than 100 mumol/l on Rhabditis pseudoelongata. Concerning the effect of dithiol ligands on the anthelmintic activity of these trivalent organoarsenicals on M. dessetae and N. brasiliensis, 2,2'-dimercaptodiethyloxide was more efficient as dithiol ligand than 1,3-dimercaptopropane which was more efficient than 1,2-dimercaptoethane. Moreover, the para-amino haptophore was more efficient than the melaminyl haptophore. These results showed that the use of new dithiol ligands for trivalent arsenicals enhanced greatly the anthelmintic activity compared with potassium melarsonyl.

The effect of spiroarsoranes on Trypanosoma brucei brucei and T. b. rhodesiense.

Topical application and intraperitoneal administration of spiroarsoranes were carried out to cure central nervous system (CNS) trypanosomiasis in the chronic Trypanosoma brucei GVR 35 mouse model. Topical application appeared more efficient than intraperitoneal injection. The periods of aparasitaemia after treatment were longer but none of the mice was permanently cured. Combination treatment with eflornithine (DFMO) and the spiroarsoranes failed to show any synergistic effect. In addition, spiroarsorane I was evaluated against the T. b. rhodesiense KETRI 2634 strain, whereby 60-mg/kg treatment produced a noticeable prolongation of the life span of trypanosome-positive animals. These in vivo results suggests that the spiroarsoranes have difficulty in crossing the blood-brain barrier (BBB) and clearing the parasites from the CNS or, alternatively, that these strains are less sensitive to pentavalent arsenicals than the T. b. brucei CMP fast strain, which in the present study was more sensitive to spiroarsoranes whose lipophilicity corresponded to a log-P value ranging from 2.5 to 3.7.