Flavonoids and Related Compounds as Nucleoside Transporter Inhibitors

Mammalian nucleoside transporters can be classified into two main categories, namely, equilibrative nucleoside transporters (ENTs) and concentrative nucleoside transporters (CNTs). ENTs are ubiquitous, and mediate sodium-independent bidirectional facilitated diffusion nucleoside transport processes. CNTs on the other hand, are secondary active unidirectional transporters that are sodium-dependent. Both the equilibrative and the concentrative nucleoside transporters have several family members which are ENT1 to ENT4 and CNT1 to CNT6. Over the past two decades, important advances in the understanding of nucleoside transporter functions have been made. Identification and molecular cloning of the ENT and CNT families from mammals and protozoan parasites have provided much information about the structure, function, regulation, and tissue and cellular localization. Structure--function analyses of various nucleoside transporter chimeras and mutants have revealed important elements involved in substrate and inhibitor recognition and binding. However, the mechanisms that regulate nucleoside transporters in various tissues and cell types are just beginning to be understood. Because of the ability of these transporters to handle nucleoside analogues used in the treatment of patients with cancer and viral diseases, ongoing research should allow the design of more specifically targeted new compounds or improvements to existing drugs. New drugs are welcome not only in the treatment of cancer and viral diseases, but also in cardiovascular disorders and parasitic infections. Due to the absence of crystal structures and limited information regarding the active sites of nucleoside transporters, the designing of novel inhibitors is confined to ligand-based methods. In an effort to search for novel classes of inhibitors other than the existing ones, a series of 95 different flavone and flavone-like compounds was screened against concentrative nucleoside transporters (CNT 1, 2 and 3) and equilibrative nucleoside transporters (ENT 1 and 2). The results obtained in the form of IC50 values were further utilized to perform quantitative structure--activity relationship studies which indeed helped to understand the effects of different functionalities in the inhibition of nucleoside transporters. The validated 3D-QSAR models were used for design and activity prediction of new compounds. Pharmacophore hypotheses were also generated for hCNT3 using the PHASE pharmacophore mapping program to establish structural criteria for inhibitor design, and for database searching to find new hit molecules. Additionally, fifteen compounds were selected based on SAR and screened for equilibrative nucleoside transporter inhibition for validation of QSAR models. One novel compound, XI was designed with reduced complexity in further attempts to identify the ENT pharmacophore. But the synthetic route followed to prepare compound XI, resulted in the synthesis of compounds XII and XIII, which were evaluated as a mixture and exhibited substantial inhibitory activity against hENT1, but had no significant effect on hENT2 or hCNT3. This work has identified a novel class of CNT and ENT nucleoside transporter inhibitors and delineated structural determinants of potency and transporter subtype selectivity.