Phosphoramidates and phosphonamidates (ProTides) with antiviral activity

Following the first report on the nucleoside phosphoramidate (ProTide) prodrug approach in 1990 by Chris McGuigan, the extensive investigation of ProTide technology has begun in many laboratories. Designed with aim to overcome limitations and the key resistance mechanisms associated with nucleoside analogues used in the clinic (poor cellular uptake, poor conversion to the 5'-monophosphate form), the ProTide approach has been successfully applied to a vast number of nucleoside analogues with antiviral and anticancer activity. ProTides consist of a 5'-nucleoside monophosphate in which the two hydroxyl groups are masked with an amino acid ester and an aryloxy component which once in the cell is enzymatically metabolized to deliver free 5'-monophosphate, which is further transformed to the active 5'-triphosphate form of the nucleoside analogue. In this review, the seminal contribution of Chris McGuigan's research to this field is presented. His technology proved to be extremely successful in drug discovery and has led to two Food and Drug Administration-approved antiviral agents.

Nucleoside mono- and diphosphate prodrugs of 2',3'-dideoxyuridine and 2',3'-dideoxy-2',3'-didehydrouridine

Despite their close structural similarity to nucleoside analogues such as the anti-HIV drugs AZT and d4T, 2',3'-dideoxyuridine (ddU) and 2',3'-dideoxy-2',3'-didehydrouridine (d4U) are entirely inactive against HIV in their nucleoside form. However, it has been shown that the corresponding triphosphates of these two nucleosides can effectively block HIV reverse transcriptase. Herein we report on two types of nucleotide prodrugs (cycloSal and DiPPro nucleotides) of ddU and d4U to investigate their ability to overcome insufficient intracellular phosphorylation, which may be the reason behind their low anti-HIV activity. The release of the corresponding mono- and diphosphates from these compounds was demonstrated by hydrolysis studies in phosphate buffer (pH 7.3) and human CD4 (+) T-lymphocyte CEM cell extracts. Surprisingly, however, these compounds showed low or no anti-HIV activity in tests with human CD4 (+) T-lymphocyte CEM cells. Studies of the conversion of ddUDP and d4UDP into their triphosphate metabolites by nucleoside diphosphate kinase (NDPK) showed nearly no conversion of either diphosphate, which may be the reason for low intracellular triphosphate levels that result in low antiviral activity.

The DiPPro approach: synthesis, hydrolysis, and antiviral activity of lipophilic d4T diphosphate prodrugs

Bioreversible protection of the β-phosphate group of nucleoside diphosphates (NDPs) as bis(acyloxybenzyl)phosphate esters is presented. To investigate the structure-activity relationship of these potential NDP prodrugs (DiPPro drugs) a series of DiPPro compounds was synthesized bearing fatty acids of various lengths and d4T as a model nucleoside. For synthesis of the lipophilically modified diphosphate group, preformed phosphoramidites were allowed to react with nucleotides, and the β-P(III) moiety was subsequently oxidized. The chemical and enzymatic stability of these prodrugs was studied in different media such as phosphate buffer (pH 7.3) or CEM cell extracts. In all media, the hydrolysis rate was clearly dependent on the acyl moiety and decreased with increasing alkyl chain length. The compounds showed a markedly lower half-life in cell extracts than in pH 7.3 phosphate buffer due to the presence of enzyme-catalyzed cleavage. In all media, the DiPPro compounds released d4T diphosphate (d4TDP) as the main product beside d4TMP. In antiviral assays, the compounds proved to be at least as potent as d4T against HIV-1 and 2 in wild-type CEM/0 cells. As a proof of concept, compounds with longer acyl residues showed very good anti-HIV activities in thymidine-kinase-deficient cells (CEM/TK(-) ), indicating their ability to penetrate cell membranes and the delivery of phosphorylated metabolites.

Stereoselective synthesis and antiviral activity of methyl-substituted cycloSal-pronucleotides

Methyl-substituted cycloSal-pronucleotides of d4TMP were synthesized with high diastereoselectivities in satisfying chemical yields. The individual diastereomers were tested against HIV-1 and HIV-2 infected wild-type CEM/0 and HIV-2 infected thymidine kinase deficient CEM cells. All diastereomers tested showed significant antiviral activity in CEM/0 and strong activity in CEM/TK(-) cell cultures. The antiviral activities were strongly dependent on the chirality at the phosphate group and the position of the methyl-group(s) in the cycloSal moiety. In CEM/TK(-) cell cultures the difference in antiviral potency was found to be 7- to 20-fold. The stability of each diastereomer was studied in aqueous phosphate buffer and in CEM/0 cell extracts. Large differences in the half-lives were found. A comparison of the relative lipophilicity of the methyl-substituted cycloSal triesters was performed based on the retention times obtained by reversed phase HPLC. The results obtained clearly confirm the importance of a diastereoselective synthesis of cycloSal-pronucleotides.