Design, synthesis and evaluation of novel oxazaphosphorine prodrugs of 9-(2-phosphonomethoxyethyl)adenine (PMEA, adefovir) as potent HBV inhibitors

Synthesis and substrate properties towards HIV-1 reverse transcriptase of new diphosphate analogues of 9-[(2-phosphonomethoxy)ethyl]adenine

Background

The replacement of β,γ-pyrophosphate by β,γ-phosphonate moieties within the triphosphate chain of 5′-triphosphate nucleoside analogues was previously studied for various antiviral nucleoside analogues such as AZT and 2′,3′-dideoxynucleosides. Thus, it has been shown that these chemical modifications could preserve, in some cases, the terminating substrate properties of the triphosphate analogue for HIV-RT. Herein, we aimed to study such 5′-triphosphate mimics based on the scaffold of the well-known antiviral agent 9-[(2-phosphonomethoxy)ethyl]adenine (PMEA, Adefovir).

Methods

Synthesis involved coupling of a morpholidate derivative of PMEA with appropriate pyrophosphoryl analogues. The relative efficiencies of incorporation of the studied diphosphate phosphonates were measured using subtype B WT HIV-1 RT in an in vitro susceptibility assay, in comparison to the parent nucleotide analogue (PMEApp).

Results

Searching for nucleoside 5′-triphosphate mimics, we have synthesized and studied a series of diphosphate analogues of PMEA bearing non hydrolysable bonds between the and phosphorus atoms. We also examined their relative inhibitory capacity towards HIV-1 reverse transcriptase in comparison to the parent nucleotide analogue (PMEApp). Only one of them appeared as a weak inhibitor (IC₅₀ = 403.0 ± 75.5 µM) and proved to be less effective than PMEApp (IC₅₀ = 6.4 ± 0.8 µM).

Conclusion

PMEA diphosphoryl derivatives were designed as potential substrates and/or inhibitors of various viral polymerases. These modifications dramatically affect their ability to inhibit HIV-RT.

Prodrugs of phosphonates and phosphates: crossing the membrane barrier

A substantial portion of metabolism involves transformation of phosphate esters, including pathways leading to nucleotides and oligonucleotides, carbohydrates, isoprenoids and steroids, and phosphorylated proteins. Because the natural substrates bear one or more negative charges, drugs that target these enzymes generally must be charged as well, but small charged molecules can have difficulty traversing the cell membrane by means other than endocytosis. The resulting dichotomy has stimulated a great deal of effort to develop effective prodrugs, compounds that carry little or no charge to enable them to transit biological membranes, but able to release the parent drug once inside the target cell. This chapter presents recent studies on advances in prodrug forms, along with representative examples of their application to marketed and developmental drugs.

Voltammetric quantification of anti-hepatitis drug Adefovir in biological matrix and pharmaceutical formulation

Electrochemical reduction behavior of Adefovir was studied using Hanging Mercury Drop Electrode (HMDE) in Britton–Robinson (BR) buffer solution. Voltammetric study showed one well-defined reduction peak in the potential range −1.2 to −1.4 V (vs. Ag/AgCl) due to reduction of C=N bond of the imidazole ring. Solid-phase extraction and protein-precipitation techniques were employed for extraction of Adefovir from human plasma. The proposed method allows quantification of Adefovir in human plasma over the concentration range 0.50–5.00 μg/mL with the detection limit 0.17 μg/mL, whereas in pharmaceutical formulation 0.25–2.25 μg/mL with the detection limit 0.08 μg/mL.