Comparison of Bioactivities of 5-Fluoro, 5-Iodo, 5-Iodovinyl, and 5-Fluorovinyl Arabinosyl Uridines against SR-39 TK-Transfected Murine Prostate Cancer Cells

Abstract

Due to the potency of gene-engineered techniques in drug development, various mutants of herpes simplex virus thymidine kinase (HSV-1 TK) have been generated. Among them, SR-39, 1) SR-26, 1) A-167Y, 2) A-167F 3) and A168H 3) have found to possess the highest affinity for acylic purine nucleosides e.g. Gancicrovir (GCV) or acyclovir (ACV). These bioactive mutants were screened from a pool of enzymes created by systematic swapping of the amino acid residues between 159 to 184 using wild-type HSV TK as a template. Of all the mutants being tested, residues 167 and 168 located in the active site have shown to play a critical role in tuning the enzyme catalytic activity. For instance, the affinity for purine-but not pyrimidine-containing nucleosides can be improved by replacing the natural alanine-167 or alanine-168 with aromatic-containing amino acids. Therefore, thymidine served as a natural substrate for human TK but exerted poor affinity for the mutant TKs, a character that could prevent from the undesired uptake during gene therapy. In spite of its improved substrate selectivity, the catalytic efficiency of mutant forms showed an overall reduction of 20-60-fold compared to the wild type.Although acyclic purine nucleosides rather than pyrimidine nucleosides generally show better substrate-specificity toward the mutant enzyme SR-39 TK, a 5-substituted pyrimidine nucleoside, 2Ј-fluoro-5-ethyl-arabinosyl uridine (FEAU), does not apply in this category. 4) Notably, not only SR-39 TK but also wild type HSV TK could efficiently phosphorylate FEAU. This unexpected activity revolutionized the current understandings that the unfavorable steric hindrance between the side chain at the 5-position of FEAU and the bulky aromatic ring of the catalytic site in SR-39 TK, in which the natural aliphatic-containing amino acids, isoleucine at residue 160 and alanine at residue 168, were replaced by an aromatic-containing amino acid, phenylalanine. Collectively, it is necessary to further investigate the steric environment of the active site in this enzyme.In contrast to the above strategy to create enzyme libraries for probing of specific substrate, an alternative approach by targeting SR-39 TK with a rational design of the pyrimidine nucleoside analogs or creation of a compound library can also provide useful information. One of the typical methods employed amino-substituted core compounds is achieved via amide-bond coupling.5) Besides, the synthesis of arabinosyl uridine analogs such as FaraU 1, IaraU 2 and FVaraU 3, IVaraU 4 have been achieved recently (Fig. 1).6-9) While arabinosyl thymidine analogs have been demonstrated to be excellent substrates for wild type HSV-1 TK,3) no relevant report of their corresponding activities against the mutant enzymes has been addressed. Additionally, the mutant enzyme SR-39 TK seemed to be more resistant to the structural variation of the pyrimidine nucleoside as described above.Combining the above considerations, we wish to undertake a cell survival assay to address whether these arabinosyl uri...