Leishmaniasis is a major and increasing health problem in many parts of the world, with about 350 million people living in areas of disease endemicity and about 2 million new cases each year.1 About 500,000 of these are visceral leishmaniasis, which is nearly always fatal if left untreated.2 Chemotherapy remains the most effective control measure for this disease. The treatment options for leishmaniasis are limited and involve the administration of pentavalent antimonial as first line and amphotericin B and pentamidine as second line drugs.3, 4 These drugs are expensive and potentially toxic and require long-term treatment. In addition the development of drug resistance by the pathogens especially in HIV leishmania co-infected patients, has aggravated public health risks.5 The spread of drug resistance combined with other shortcoming of the available antileishmanial drugs emphasizes the importance of the development of new, effective and safe drugs against leishmaniasis.
A great number of natural and synthetic compounds have been tested in the recent years in antileishmanial assays.6, 7, 8
Also the use of 5-nitrofuran and 5-nitrothiophene as antibacterial and antiprotozoal is well established.9, 10 Although there is a large amount of experimental work on these heterocycles, they still remain an area of active research interest.11, 12 In this study we would like to report the synthesis and in vitro antileishmaniasis activities of some 2-(5-nitro-2-furyl) and 2-(5-nitro-2-thienyl)-5-substituted-1,3,4-thiadiazoles (5a–d and 6a–j).
The 2-amino-5-(5-nitro-2-furyl)-1,3,4-thiadiazole 3a was obtained by oxidative cyclization of 5-nitrofurancarboxaldehyde thiosemicarbazone 2a. Diazotization of 3a in hydrochloric acid in the presence of copper powder gave 2-chloro-5-(5-nitro-2-furyl)-1,3,4-thiadiazole (4a).13
Reaction of compound 4a with piperidine or morpholine in refluxing ethanol gave compounds 5a or b, respectively, in good yields. Similarly, the reaction of compound 4a with piperazine, N-methylpiperazine or N-phenylpiperazine gave the corresponding compounds 6a–c, respectively.
Starting from 2-chloro-5-(5-nitro-2-thienyl)-1,3,4-thiadiazole14 compounds 5c,d and 6f–h were prepared similarly.
Acetylation of 6a or f with acetic anhydride gave acetylated compound 6d and i, respectively. The reaction of 6a or f with benzoyl chloride yielded compounds 6e or j in high yield. The purity of the synthesized compounds was confirmed by thin layer chromatography (TLC), using various solvents of different polarities (Scheme 1).
-
Scheme 1
The in vitro efficacy of the synthesized compounds on promastigotes of L. major (ATCC J 774, HB-197) were assessed by a previously described method.15 Promastigotes (3 × 106) were cultured in medium 199 containing 10% heat-inactivated fetal calf serum. Incubation and growth of the parasite were carried out at 26 °C. Promastigotes were harvested on day four of the culture and used. The culture of parasite was diluted with the fresh medium to a final concentration of 5 × 106 parasites/mL. The compounds to be checked were dissolved in DMSO (15 mM) and further diluted to appropriate concentrations in culture medium as given in the Table 1. In a 96-well micro titre plate, 160 μL of the promastigotes suspension was added to 40 μL of various concentrations of each compound, medium alone, or pentavalent antimonial Pentostam as positive control. The cultures were incubated for 18 h and 3H-thymidine incorporation was measured and IC50 values of compounds possessing antileishmanial activity were calculated (Table 1).