Uptake of 2-beta-D-ribofuranosylthiazole-4-carboxamide (tiazofurin) and analogues by the facilitated transport mechanism of erythrocytes.

Tiazofurin (TR), a new antitumor agent, enters human erythrocytes by utilizing their facilitated nucleoside transport system. TR competes with endogenous nucleosides for this transport mechanism, thereby reducing nucleoside uptake into the cells. Pre-incubation of erythrocytes for 10 min at 22 degrees C with 100 microM and 500 microM TR reduced the transport of 14C-uridine into the cells by 27% and 74%, respectively. Simultaneous exposure of cells to TR and [14C]uridine did not alter the inhibitory effect of TR. Furthermore, the transport inhibitory effect of TR was lost when cells were washed twice with Hanks basal salt solution following a 10-min pre-incubation with TR. The Km and Vmax (+/- S.E.) for radiolabeled TR transport into erythrocytes are 170 +/- 26 microM and 55 +/- 13 nmol/h per 10(6) cells, respectively, which is similar to the kinetic constants measured for uridine transport into erythrocytes (Km = 168 +/- 37 microM and Vmax = 61 +/- 16 nmol/h per 10(6) cells). The Ki (+/- S.E.) of TR for uridine transport is 178 +/- 11 microM and for thymidine transport is 102 +/- 59 microM. Three analogues of TR (its selenium isostere (SR), and Ara (Ara-TR) and Xylo (Xylo-TR) derivatives) were compared with TR for their ability to compete with and inhibit uridine transport, as these analogues were not available in a radiolabeled form for direct measurement of their transport into the cell. SR had similar kinetic characteristics of inhibition of uridine transport to TR (Ki = 145 +/- 15 microM) but Ara-TR had a Ki = 1.04 +/- 0.13 mM while Xylo-TR inhibited uridine transport with a Ki = 1.57 +/- 0.67 mM. Thus, TR is transported into erythrocytes with the same velocity and affinity for the carrier as uridine and competitively inhibits nucleoside transport into the cell. Of 3 other C-nucleoside derivatives examined, SR is of similar potency to TR but Ara-TR and Xylo-TR are much less effective at competing with uridine for the nucleoside transporter.