Inhibition of Nucleoside Transport Proteins by C⁸-Alkylamine-Substituted Purines

4-Nitrobenzylthioinosine (NBTI, 1) is a well-known inhibitor for the nucleoside transport protein ENT1. Here we report on the synthesis and the biological evaluation of compounds that are less polar than NBTI. Compound screening in our laboratory indicated that introduction of an alkylamine substituent at the C⁸-position of N⁶-cyclopentyladenosine (CPA, 2) led to an increment in affinity for the transport protein. It was investigated whether this would also apply for NBTI derivatives. Two series of C⁸-alkylamine-substituted compounds were prepared, one in which the nitro group was absent (46−58) and another in which the ribose moiety was replaced by a benzyl group (72−75). Comparison of the biological data of these compounds with 6-benzylthioinosine (4, K_i = 53 nM) and 9-benzyl-6-(4-nitrobenzylsulfanyl)purine (59, K_i = 135 nM) confirmed the hypothesis. The K_i values improved upon elongation of the alkylamine chain from methylamine to n-hexylamine with an optimum for n-pentylamine (50, K_i = 2.3 nM). Substitution with 2-methylbutylamine (52), cyclopropylamine (53), cyclopentylamine (54, 72), and cyclohexylamine (55, 73) revealed that the presence of a bulky group enhanced the affinity. The presence of tertiary amines obtained by substitution with pyrrolidine, piperidine, and morpholine gave only poor results. For both series substitution with cyclopentylamine was most effective. Compound 54 (LUF5942) proved the most active, showing a comparable affinity (K_i = 0.64 nM) to NBTI but a significantly lower polar surface area.