Effect of bond linkage on in vitro drug release and anti-HIV activity of chitosan-stavudine conjugates

Two kinds of chitosan-stavudine (d4T) conjugates, chitosan- O -isopropyl-5′- O -d4T monophosphate conjugate (Cs-P-d4T) with a phosphoramide linkage and chitosan-5′- O -succinyl-d4T conjugate (Cs-S-d4T) with a succinic spacer, were synthesized using an Atherton-Todd reaction and carbodiimide coupling reaction, respectively, and then structurally characterized. Their in vitro drug release behaviors and anti-human immunodeficiency virus (HIV) activity were investigated and compared. Both of the chitosan-d4T conjugates more strongly prefer to release corresponding d4T derivatives rather than free d4T in a prolonged manner but have different hydrolysis routes. The anti-HIV activity and cytotoxicity evaluated in the MT4 cell line revealed that the anti-HIV selectivity index was in the following order: Cs-P-d4T > d4T >> Cs-S-d4T since the released d4T-5′-( O -isopropyl) monophosphate from Cs-P-d4T can bypass the rate-limiting bottleneck of nucleoside phosphorylation, while the released 5′- O -succinyl-d4T from Cs-S-d4T has to be hydrolyzed to d4T and then successively phosphorylated to its active form to exert antiviral activity. The results suggested that constructing a chitosan-nucleoside reverse transcriptase inhibitor (NRTI) conjugate with a phosphoramide linkage may be an efficient approach for improving NRTI therapy efficacy in antiretroviral treatment.