NBTGR (p-Nitrobenzylthioguanosine) is a potent inhibitor of nucleoside transport; inhibits adenosine uptake with a K_i of 70 nM. IC50 & Target: Ki: 70 nM (adenosine uptake)^[1] In Vitro: A group of purine ribonucleosides with alkarylmercapto substituents at the purine 6-position are potent inhibitors of several aspects of nucleoside metabolism that involved the transfer of ribosyl groups. NBTGR is one of the compounds that inhibits nucleoside transport by human erythrocytes; initial rates of uridine uptake are reduced to zero upon exposure of cells to 1 μM NBTGR. The inhibitor is firmly bound because repeated washing cannot restore uridine transport capability to NBTGR-treated cells. NBTGR inhibits the influx of uridine, inosine, and cytidine, without inhibiting the uptake of the corresponding bases, or that of D-glucose or L-leucine. Uridine antagonizes the NBTGR inhibition of uidine transport in a concentration-dependent manner. NBTGR and related compounds appear to interact with the mechanism for the facilitated transport of nucleosides^[2].