Synthesis and Evaluation of 2-Amino-9-(1,3-dihydroxy-2-propoxymethyl)- 6-fluoropurine Mono- and Diesters as Potential Prodrugs of Ganciclovir

A series of 2-amino-9-(1,3-dihydroxy-2-propoxymethyl)-6-fluoropurine mono- and diesters, 6a−h, were synthesized as potential prodrugs of ganciclovir and evaluated for their oral ganciclovir bioavailability in rats. Treatment of 2-amino-6-chloro-9-(1,3-dihydroxy-2-propoxymethyl)purine (4) with Me3N in DMF/THF (1/4) followed by the reaction of the resulting trimethylammonium chloride salt 5 with KF in DMF gave 2-amino-9-(1,3-dihydroxy-2-propoxymethyl)-6-fluoropurine (2) in 83% yield. Esterification of 2 with an appropriate acid anhydride (Ac2O, (EtCO)2O, (n-PrCO)2O, or (i-PrCO)2O) (6 equiv for 6a−d or 1 equiv for 6e−h) in DMF in the presence of a catalytic amount of DMAP produced the diesters 6a−d in 92−98% yields and the monoesters 6e−h in 37−44% yields. Of the prodrugs tested in rats, the monoisobutyrate 6h achieved the highest ganciclovir bioavailability (45%) that is 15-fold higher than that from ganciclovir (3%), followed in order by the diisobutyrate 6d (42%), the diacetate 6a (41%), the monobutyrate 6g (41%), the monopropionate 6f (39%), the dipropionate 6b (35%), the dibutyrate 6c (35%), and the monoacetate 6e (29%). The prodrugs 6e−h were found to be quite stable at pH 6.0 (t 1/2 = >29 days), 7.4 (t 1/2 = >7 days), and 8.0 (t 1/2 = >2 days) but had relatively short half-lives at pH 1.2 (t 1/2 = 60−83 min).