Dipeptide Nitrile CD34 with Curcumin: A New Improved Combination Strategy to Synergistically Inhibit Rhodesain of Trypanosoma brucei rhodesiense .

Rhodesain is the main cysteine protease of Trypanosoma brucei rhodesiense , the parasite causing the acute lethal form of Human African Trypanosomiasis. Starting from the dipeptide nitrile CD24 , the further introduction of a fluorine atom in the meta position of the phenyl ring spanning in the P3 site and the switch of the P2 leucine with a phenylalanine led to CD34 , a synthetic inhibitor that shows a nanomolar binding affinity towards rhodesain ( K i = 27 nM) and an improved target selectivity with respect to the parent dipeptide nitrile CD24 . In the present work, following the Chou and Talalay method, we carried out a combination study of CD34 with curcumin, a nutraceutical obtained from Curcuma longa L. Starting from an affected fraction (f a ) of rhodesain inhibition of 0.5 (i.e., the IC 50 ), we observed an initial moderate synergistic action, which became a synergism for f a values ranging from 0.6 to 0.7 (i.e., 60-70% inhibition of the trypanosomal protease). Interestingly, at 80-90% inhibition of rhodesain proteolytic activity, we observed a strong synergism, resulting in 100% enzyme inhibition. Overall, in addition to the improved target selectivity of CD34 with respect to CD24 , the combination of CD34 + curcumin resulted in an increased synergistic action with respect to CD24 + curcumin, thus suggesting that it is desirable to use CD34 and curcumin in combination.