Design, synthesis and biological evaluation of aryloxy thiophosphoramidate triesters of anticancer nucleoside analogues

Aryloxy phosphoroamidate triesters, known as ProTides, are a class of prodrugs developed to enhance the physicochemical and pharmacological properties of therapeutic nucleosides. This approach has been extensively investigated in the antiviral and anticancer areas leading to three prodrugs on the market and several others in clinical stage. In this article we have prepared the P--S analogues of three ProTides that have reached the clinic as anticancer agents. These novel P--S ProTides were tested for their capacity in enzymatic activation and for their cytotoxic properties against a panel of solid and liquid tumor cell lines. As expected, the replacement of the P--O with a P--S bond led to increased metabolic stability albeit concomitant to a decrease in potency. Surprisingly, the intermediate formed after the first activation step of a thiophosphoramidate with carboxypeptidase Y is not the expected P--S aminoacyl product but the corresponding P--O aminoacyl compound.