Search

Trypanosoma brucei, the causative agent of African trypanosomiasis, contains a soluble, vacuolar pyrophosphatase, TbVSP1, not present in humans, which is essential for the growth of bloodstream forms in their mammalian host. Here, we report the inhibition of a recombinant TbVSP1 expressed in Escherichia coli by a panel of 81 bisphosphonates. The IC50 values were found to vary from ~2 to 850 μM. We then used 3D QSAR (comparative molecular field and comparative molecular similarity index; CoMFA and CoMSIA) methods to analyze the enzyme inhibition results. The R² values for the experimental versus the QSAR-predicted activities were 0.78 or 0.61 for CoMFA and 0.79 or 0.68 for CoMSIA, for two different alignments. The root-mean-square (rms) pIC50 error for the best CoMFA model was 0.41 for five test sets of five activity predictions, which translates to a factor of ~2.6 error in IC50 prediction. For CoMSIA, the rms pIC50 error and error factors were 0.35 and 2.2, respectively. In general, the most active compounds contained both a single aromatic ring and a hydrogen bond donor feature. Thirteen of the more potent compounds were then tested in vivo in a mouse model of T. brucei infection. The most active compound in vivo provided a 40% protection from death with no apparent side effects, suggesting that further development of such compounds may be of interest.

As a continuation of our project aimed at the search for new and safe chemotherapeutic and chemoprophylactic agents against American trypanosomiasis (Chagas' disease), several drugs structurally related to 4-phenoxyphenoxyethyl thiocyanate (4) were designed, synthesized, and evaluated as antiproliferative agents against the parasite responsible for this disease, the hemoflagellated protozoan Trypanosoma cruzi. This thiocyanate derivative was previously shown to be an effective and potent agent against T. cruzi proliferation. Several drugs possessing thiocyanate groups proved to be effective growth inhibitors of T. cruzi growth. Among the designed compounds, it is important to point out the extremely potent activity shown by 11, 23, 38, 53, 90, 99, and 117 against the epimastigote forms of the parasite. All of them exhibited IC50 values in the low micromolar range, and these values were comparable with those presented by our lead drug 4 and ketokonazole, a well-known antiparasitic agent. The activity displayed by the nitrogen-containing derivative 90 was very promising with IC50 values of 3.3 μM. Several other thiocyanate derivatives also proved to be very potent inhibitors of the multiplication of T. cruzi epimastigotes, such as compounds 28, 33, 43, 48, 56, 61, 66, 71, 76, and 124. Compound 43 resulted in being a promising drug because it was also very effective against amastigotes, the clinically more relevant form of the parasite. This compound was 3-fold more potent than 4, while 11 showed nearly the same activity as our lead drug against intracellular T. cruzi. It was very surprising that the experimental juvenoid 124, although fairly devoid of activity against epimastigotes, was very effective against intracellular amastigotes growing in myoblasts. The rest of the designed compounds showed a broad degree of inhibitory action, from moderately active drugs to drugs almost devoid of antiparasitic activity. Compound 43 is an interesting example of an effective antichagasic agent that presents excellent prospectives not only as a lead drug but also to be used for further in vivo studies.

We have investigated the effects in vitro of a series of bisphosphonates on the proliferation of Trypanosoma cruzi, Trypanosoma brucei rhodesiense, Leishmania donovani, Toxoplasma gondii, and Plasmodium falciparum. The results show that nitrogen-containing bisphosphonates of the type used in bone resorption therapy have significant activity against parasites, with the aromatic species having in some cases nanomolar or low-micromolar IC50 activity values against parasite replication (e.g. o-risedronate, IC50 = 220 nM for T. brucei rhodesiense; risedronate, IC50 = 490 nM for T. gondii). In T. cruzi, the nitrogen-containing bisphosphonate risedronate is shown to inhibit sterol biosynthesis at a pre-squalene level, most likely by inhibiting farnesylpyrophosphate synthase. Bisphosphonates therefore appear to have potential in treating parasitic protozoan diseases.