IN-VIVO BIODISTRIBUTION, PHARMACOKINETIC PARAMETERS, AND BRAIN UPTAKEOF O-6-METHOXY-5,6-DIHYDRO-3'-AZIDO-3'-DEOXYTHYMIDINE OR LO-6-ETHOXY-5,6-DIHYDRO-3'-AZIDO-3'-DEOXYTHYMIDINE DIASTEREOMERS AS POTENTIAL PRODRUGS OF 3'-AZIDO-3'-DEOXYTHYMIDINE

A new class of lo-6-alkoxy-5,6-dihydro-3'-azido-3'-deoxythymidine dias tereomers (5-X-6-OR-5,6-dihydro-AZTs X = I, Br, Cl; R = Me, Et) were e valuated as potential anti-AIDS prodrugs of 3'-azido-3'-deoxythimidine (AZT). In vivo regeneration of AZT from these 5-X-6-OR-5,6-dihydro-AZ Ts was examined in Balb/c mice after intravenous tail vein injection. The (5R,6R)-and (5S,6S)-5-bromo(or iodo)-6-methoxy-5,6-dihydro derivat ives of AZT (BMAZT, IMAZT) were rapidly converted to AZT, resulting in AZT plasma concentrations after a 144 mu mol/kg dose similar to those after an equivalent dose (144 mu g/kg, 38.5 mg/kg) of AZT, whereas AZ T was not detectable by HPLC after the same dose of the chloro diaster eomer (5R,GR)-CMAZT. The interaction of AZT and the 5-X-6-methoxy-5,6- dihydro-AZT diastereomers with the [(4-nitrobenzyl)thio]-9-beta-D-ribo furanosylpurine equilibrative-sensitive nucleoside transporter in muri ne erythrocytes was also studied. The (5R,GR)- and (5S,6S)-5-X-6-OMe-5 ,6-dihydro-AZT diastereomers demonstrated a high affinity (K-i = 0.2-0 .5 mM) for the transporter relative to AZT (K-i = 1.3 mM), with the ex ception of ro-6-methoxy-5,6-dihydro-3'azido-3'-deoxythymidine (CMAZT) which has a K-i value larger than 1.5 mM. [2-C-14]-Labeled (5R,6R)-and (5S,6S)-5-bromo-6-methoxy(or ethoxy)-5,6-dihydro-3'-azido-3'-deoxythy midines were synthesized by the regiospecific addition of methyl hypob romite or ethyl hypobromite to the 5,6-olefinic bond of [2-C-14]-AZT i n high radiochemical yield [(5R,6R)-BMAZT, 48%, and (5S,6S)-BMAZT, 33% ; (5R,6R)-BEAZT, 61%, and (5S,6S)-BEAZT, 15%), high radiochemical puri ty (>98%), and high specific activity (56 mCi/mmol)]. The amounts of r adioactivity in mouse brain after iv injection of [2-C-14]-labeled (5R ,6R)-BMAZT, (5S,6S)-BMAZT, or (5R,6R)-BEAZT were 2-4 fold higher than that for [2-C-14]-AZT (P < 0.05). The radioactivity remaining in blood after dosing with these 5-bromo-6-alkoxy-5,6-dihydro-AZTs was up to 2 0-fold higher than after injection of [2-C-14]-AZT at longer time inte rvals after injection. The amounts of radioactivity present in femoral bone following injection of [2-C-14]-AZT, or these 5-bromo-6-alkoxy-5 ,6-dihydro-AZTs, were similar. Subcellular and regional distributions of [2-C-14]-labeled AZT, (5R,6R)-BMAZT, or (5R,6R)-BEAZT in mouse brai n after jugular vein injection did not show preferential concentration in any particular subcellular fraction nor a marked preferential regi onal localization for either AZT or these 5,6-dihydro prodrugs of AZT.