Selective Tight Binding Inhibitors of Trypanosomal Glyceraldehyde-3-phosphate Dehydrogenase via Structure-Based Drug Design

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Abstract
Glyceraldehyde-3-phosphate dehydrogenase (GAPDH) from the sleeping sickness parasite Trypanosoma brucei is a rational target for anti-trypanosomatid drug design because glycolysis provides virtually all of the energy for the bloodstream form of this parasite. Glycolysis is also an important source of energy for other pathogenic parasites including Trypanosoma cruzi and Leishmania mexicana. The current study is a continuation of our efforts to use the X-ray structures of T. brucei and L. mexicana GAPDHs containing bound NAD+ to design adenosine analogues that bind tightly to the enzyme pocket that accommodates the adenosyl moiety of NAD+. The goal was to improve the affinity, selectivity, and solubility of previously reported 2‘-deoxy-2‘-(3-methoxybenzamido)adenosine (1). It was found that introduction of hydroxyl functions on the benzamido ring increases solubility without significantly affecting enzyme inhibition. Modifications at the previously unexploited N6-position of the purine not only lead to a substantial increase in inhibitor potency but are also compatible with the 2‘-benzamido moiety of the sugar. For N6-substituted adenosines, two successive rounds of modeling and screening provided a 330-fold gain in affinity versus that of adenosine. The combination of N6- and 2‘-substitutions produced significantly improved inhibitors. N6-Benzyl (9a) and N6-2-methylbenzyl (9b) derivatives of 1 display IC50 values against L. mexicana GAPDH of 16 and 4 μM, respectively (3100- and 12500-fold more potent than adenosine). The adenosine analogues did not inhibit human GAPDH. These studies underscore the usefulness of structure-based drug design for generating potent and species-selective enzyme inhibitors of medicinal importance starting from a weakly binding lead compound.