Lack of Susceptibility of Bicyclic Nucleoside Analogs, Highly Potent Inhibitors of Varicella-Zoster Virus, to the Catabolic Action of Thymidine Phosphorylase and Dihydropyrimidine Dehydrogenase

Abstract

The susceptibility of the bicyclic nucleoside analogs (BCNAs), highly potent and selective inhibitors of varicella-zoster virus (VZV), to the enzymes involved in nucleoside/nucleobase catabolism has been investigated in comparison with the established anti-VZV agent (E)-5-(2-bromovinyl)-2Ј-deoxyuridine [BVDU; brivudine (Zostex)]. Whereas human and bacterial thymidine phosphorylases (TPases) efficiently converted BVDU to its antivirally inactive free base (E)-5-(2-bromovinyl)uracil (BVU), BCNAs showed no evidence of conversion to the free base in the presence of these enzymes. The lack of substrate affinity of TPase for the BCNAs could be rationalized by computer-assisted molecular modeling of the BCNAs in the TPase active site. Moreover, in contrast with BVU, which is a potent and selective inhibitor of dihydropyrimidine dehydrogenase (DPD) (50% inhibitory concentration; 10 M in the presence of a 25 M concentration of the natural substrate thymine), the free at a concentration of 250 M. Consequently, whereas BVU caused a dramatic rise of FU levels in FU-treated mice, the BCNAs did not affect FU levels in such mice. From our data it is evident that BCNAs represent highly stable anti-VZV compounds that are not susceptible to breakdown by nucleoside/ nucleobase catabolic enzymes and are not expected to interfere with cellular catabolic processes such as those involved in FU catabolism.

Highly potent and selective inhibition of varicella‐zoster virus replication by bicyclic furo[2,3‐d]pyrimidine nucleoside analogues

“…18). 30 Also, when the furopyrimidine nucleoside analogues Cf 1368 and Cf 1743 were administered to mice in combination with 5-FU, they did not increase the 5-FU plasma levels, while under the same experimental conditions BVU markedly raised the plasma levels of 5-FU ( Fig. 19).…”

“…Interestingly, the furopyrimidine nucleoside analogues Cf 1368 and Cf 1743 were found to be entirely resistant to the phosphorolytic cleavage by both human TPase (Fig. 15) 30 and bacterial TPase (Fig. 16).…”

“…16). 30 When modeled in the active site of E. coli TPase, 31,32 Cf 1743 would not be able to make an extensive network of hydrogen bonds as does thymidine since they have no free carbonyl at the corresponding position C(4) in the pyrimidine ring, and no hydrogen available on the corresponding N(3) position of the pyrimidine ring that may form a hydrogen bond with serine-186 in the TPase active site; more seriously, the bulky p-alkylphenyl side chain of Cf 1743 would clash with the main chain residues (176 and 177) of E. coli TPase (Fig. 17).…”

“…17). 30 It can be inferred therefore that TPase has no space available in its active site to reasonably accommodate Cf 1743 to afford optimal positioning for hydrolysis to the free base (aglycone). The free nucleobase of BVDU, (E)-5-(2-bromovinyl)uracil (BVU), is an efficient inhibitor of human dihydropyrimidine dehydrogenase (DPD).…”

Metabolic and Pharmacological Characteristics of the Bicyclic Nucleoside Analogues (BCNAs) as Highly Selective Inhibitors of Varicella-Zoster Virus (VZV)

“…In contrast with BVDU, the BCNAs are not converted by the human erythrocyte or E.coli thymidine phosphorylase (TPase) to their free base, and, in addition, the free base of the BCNAs is not inhibitory to human dihydropyrimidine dehydrogenase (DPD), [3] the enzyme that converts thymine and the anticancer drug 5-fluorouracil (5-FU) to their 5,6-dihydropyrimidine derivatives. Unlike BVDU, the BCNAs when administered together with 5-FU in mice, did not affect 5-FU plasma levels.…”