The future of antiviral chemotherapy

Ocular pharmacokinetics of acyclovir amino acid ester prodrugs in the anterior chamber: evaluation of their utility in treating ocular HSV infections

PURPOSE

To evaluate in vivo corneal absorption of the amino acid prodrugs of acyclovir (ACV) using a topical well model and microdialysis in rabbits.

METHODS

Stability of L-alanine-ACV (AACV), L-serine-ACV (SACV), L-isoleucine-ACV (IACV), gamma-glutamate-ACV (EACV) and L-valine-ACV (VACV) prodrugs was evaluated in various ocular tissues. Dose-dependent toxicity of these prodrugs was also examined in rabbit primary corneal epithelial cell culture (rPCEC) using 96-well based cell proliferation assay. In vivo ocular bioavailability of these compounds was also evaluated with a combination of topical well infusion and aqueous humor microdialysis techniques.

RESULTS

Among the amino acid ester prodrugs, SACV was most stable in aqueous humor. Enzymatic degradation of EACV was the least compared to all other prodrugs. Cellular toxicity of all the prodrugs was significantly less compared to trifluorothymidine (TFT) at 5mM. Absorption rate constants of all the compounds were found to be lower than the elimination rate constants. All the prodrugs showed similar terminal elimination rate constants (lambda(z)). SACV and VACV exhibited approximately two-fold increase in area under the curve (AUC) relative to ACV (p<0.05). Clast (concentration at the last time point) of SACV was observed to be 8+/-2.6microM in aqueous humor which is two and three times higher than VACV and ACV, respectively.

CONCLUSIONS

Amino acid ester prodrugs of ACV were absorbed through the cornea at varying rates (ka) thereby leading to varying extents (AUC). The amino acid ester prodrug, SACV owing to its enhanced stability, comparable AUC and high concentration at last time point (Clast) seems to be a promising candidate for the treatment of ocular HSV infections.

Antiretroviral therapy

Research into the biology of human immunodeficiency virus type 1 (HIV-1), the causative agent of the acquired immunodeficiency syndrome, has yielded valuable information about the replicative cycle of the virus. The steps involved are (1) attachment of the virus to a receptor protein on the surface of the target cell, (2) entry of the virus into the cell, (3) replication of the viral genome through a DNA intermediary, (4) entry of proviral DNA into the nucleus of the host cell, (5) integration of proviral DNA into the host cell genome, (6) transcription of proviral DNA to viral genomic and messenger RNA, (7) translation of viral messenger RNA to viral proteins, and (8) assembly of viral components into new virions that are released by budding from the host cell membrane. Each step in the HIV-1 replicative cycle offers a potential target for antiviral chemotherapy. Although many drugs have been developed, none appears singularly effective against all stages of HIV-1 infection. Many obstacles remain in the quest for an effective vaccine against HIV-1.