Abstract
Abstract: :
Purpose:To evaluate novel amino acid ester prodrugs of Ganciclovir, an antiviral agent used in Cytomegalovirus retinitis infection, with respect to their chemical stability, retinal pigmented epithelium (RPE) cell cytotoxicity and affinity toward the amino acid transporter(s) in ARPE – 19 cells. Methods:Four amino acid prodrugs of Ganciclovir (GCV) (Glu–GCV monoester, Glu–GCV diester, Phe–GCV monoester & Phe–GCV diester) were synthesized in our laboratory. Effects of pH on degradation rate constants of the prodrugs were studied to give an insight into the mechanism of catalysis and also yield some practical information about the drug stability. To determine the pH stability of these compounds, six pH values were selected at pH 1.2, 2.6, 4.2, 5.6, 7.4 and 9.0. Methanolic stock solutions of the prodrugs were used for the study. Stability was studied for 24 hours at 37°C with samples taken at appropriate time intervals. Solubility studies were carried out in pH 4.2 buffer at 37°C. All experiments were performed at least in triplicates. Stability samples obtained were assayed by HPLC with fluorescence detection. Cell proliferation assay using ARPE–19 cells was also carried out to compare the cytotoxicity of Phe–GCV and Glu–GCV mono and diesters, and Trifluorothymidine(TFT) with GCV. TFT is used as the positive control in these studies. Uptake studies across ARPE – 19 cells were performed using the appropriate substrates for the amino acid transporter(s). Experiments were performed on confluent monolayers at 37°C. Uptake samples were analyzed using liquid Scintillation counter. Results: All of the compounds exhibited maximal stability for 24 hrs at pH 4.2 with varying rates of hydrolysis at other pH values. Chemical degradation of these prodrugs was observed at neutral (7.4) and basic pH (9.0) whereas acidic pH (1.4 to 5.6) had a comparatively lesser effect. Phe–GCV monoester had the highest degradation rate constant (0.1487 hr –1) at pH 4.2 while Glu–GCV diester had the lowest rate constant (0.0845 hr –1). Different rates of inhibition of the amino acid transporter were observed with the prodrugs. Conclusions:Glu–GCV diester was observed to be most stable, while Phe–GCV monoester was least stable among the model prodrugs studied for their chemical stability. Cell cytotoxicity across ARPE – 19 indicate that, in the concentration range tested, the amino acid ester prodrugs of GCV exhibit equal or less toxicity in comparison with the parent compound GCV. These prodrugs while exhibiting varying affinities, demonstrated to be good substrates for the amino acid transporter(s) present in ARPE – 19.
Keywords: ion transporters • retinal pigment epithelium • retina