A process for inhibiting a mammalian cancerous cell or virally infected cell includes providing a Trichomonas vaginalis purine nucleoside phosphorylase enzyme or a tail mutant purine nucleoside phosphorylase enzyme in proximity to the mammalian cancerous cell or the virally infected cell and exposing the enzyme to a purine nucleoside phosphorylase enzyme cleavable substrate to yield a cytotoxic purine analog. The process includes introducing to the cell a vector containing the phosphorylase enzyme, or a DNA sequence coding for the same and delivering to the cell an effective amount of the substrate such as 9-(²-D-arabinofuranosyl)-2-fluoroadenine (F-araA).
