Bisphosphonate Inhibition of the Exopolyphosphatase Activity of the <i>Trypanosoma brucei</i> Soluble Vacuolar Pyrophosphatase

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.