Red blood cells as a delivery system for AZT

Abstract

3′-Azido-3′-deoxythymidine 5′-phosphate (AZT-MP) was synthesized with the aim of checking whether it represents a prodrug of AZT. AZT-MP was then encapsulated in human erythrocytes by a procedure of hypo-tonic hemolysis and isotonic resealing and found to be dephosphorylated to the corresponding nucleoside (AZT), which was subsequently released outside the red cells. Encapsulated AZT-MP did not interfere with the major metabolic properties of erythrocytes. The dephosphorylation reaction had an apparent K_m of 1.6 mM, a pH optimum of 7.4, and was inhibited by 10^-5m Pb²⁺. ATP, TMP, and UMP had no effect on AZT-MP dephosphorylation. AZT permeated the erythrocyte membrane predominantly by nonfacilitated diffusion, but a slow release of AZT-MP was also observed. AZT-MP in human plasma was then dephosphorylated at a rate of 170 nmol/h/ml. Influx of AZT-MP in human erythrocytes was almost undetectable at concentrations below 1 mm. At 2 mri extraerythrocytic AZT-MP influx took place at a rate of 10 nmol/min/ml cells and was not inhibited by the nucleoside transport inhibitors dipyridamole and nitrobenzyl 6-thioinosine. Thus, AZTMP-loaded erythrocytes can perform as a slow delivery system for AZT, potentially avoiding the peaks of drug concentration commonly found after the oral or intravenous administration of nucleoside analogues.