Distribution and metabolism of intravitreal cidofovir and cyclic HPMPC in rabbits

Purpose. This study was designed to evaluate the intraocular distribution and metabolism of the antiviral nucleotide analogs cidofovir and cyclic 1-[(S)-3-hydroxy-2-(phosphonomethoxy) propyl]cytosine (HPMPC) in New Zealand white rabbits following intravitreal administration. Methods. Male rabbits received either C-14-cidofovir or C-14-cyclic HPMPC by intravitreal injection into both eyes (50 mu g/eye, 11 mu Ci/eye). Two animals per group were sacrificed at 24, 48, 72 or 240 h post-dose. Ocular tissues, kidney and liver were oxidized to determine total radioactivity and metabolites were determined by HPLC. Results. At 24 h post-dose, total radioactivity was 9.96 and 5.18 mu g-equiv/g for cidofovir and cyclic HPMPC, respectively, in vitreous and 20.9 and 3.54 mu g-equiv/g, respectively, in retina. Although the initial vitreal clearance was 2-fold faster for the cyclic analog, the estimated terminal elimination half-lives in vitreous (42 hr) and in retina (66-77 hr) were similar for both drugs. By 240 h post-dose, radioactivity in all ocular tissues was approximately ten-fold higher for cidofovir. Radioactivity in vitreous at 240 h after intravitreal dosing with either drug contained cidofovir, cyclic HPMPC and cidofovir-phosphocholine. Conclusions. The long retinal half-life observed presumably reflects formation of phosphorylated cidofovir within retinal cells. Cidofovir achieved a ten-fold higher level of phosphorylated drug in retina than cyclic HPMPC. Therefore, intravitreal cidofovir may be expected to suppress progression of retinitis for a longer period than an equivalent intravitreal dose of cyclic HPMPC. The intravitreal half-life of cidofovir was 20-fold longer than that of ganciclovir in the same animal model.