Selective inhibition of nicotinamide adenine kinases (NADKs) by compact NAD mimics

We have examined sequence diversity of 147 co-crystal structures of proteins utilizing NAD as a cofactor or as a substrate. Most of these proteins bind NAD in an extended conformations. However, there are clear exceptions to this general trend. Several bacterial reductases bind NAD in an extremely folded conformations. Although NAD can be accommodated in all the above conformations, it is likely that compact NAD mimics should not fit to the binding domains of the majority of NAD-utilizing enzymes that require an extended conformations of NAD.1 It may be expected, therefore, that such compounds could be selective against NADKs. Tiazofurin adenine dinucleotide (TAD) is a potent inhibitor (Ki = 100 nM) of NAD-dependent IMP-dehydrogenase (IMPDH). This NAD mimic binds at the cofactor binding domain in the extended conformation. As a proof of concept, herein we report the synthesis of tiazofurin-5-yl-adenosine-5’-yl disulfide 1 prepared by replacing the pyrophosphate (-O-P-O-P-O-) linkage by a short (-S-S-) disulfide bridge. We found that this compact disulfide analogue of TAD lost its activity against IMPDH and became a moderate inhibitor of M. tuberculosis (IC50 = 80 microM) NADK. We also found that di-(adenosine-5’-yl)disulfide (2) a byproduct in the synthesis of 1, showed slightly better activity against mycobacterium (IC50 = 45 microM) as well as human (IC50 = 87 microM) NADK. Introduction of a bromine at the C-8 of adenine ring results in restriction of conformation of 8-bromo-adenosine to the syn conformation. We prepared the corresponding 8-bromo disulfide (3) and found further improvement in the inhibitory activity against human (IC50 = 6 microM) and mycobacterium (IC50 = 14 microM) NADKs. Compound 3 is the most potent inhibitor of NADKs reported so far. Using a simple method for preparation of disulfide 2 we synthesized a number of nucleobase and sugar modified analogues that have been evaluated against some NAD-dependent enzymes. 1. Chen L et al Curr Med Chem 2008, 15, 650-670.