(E)-5-(2-bromovinyl)uridine requires phosphorylation by the herpes simplex virus (type 1)-induced thymidine kinase to express its antiviral activity

Antiviral Activity of Bicyclic Pyrimidine Nucleosides

A number of pyrimidine nucleosides, which may show two hydrogen bonding modes, have been prepared and tested for antiviral activity against a series of viruses. Whilst none of the compounds described showed significant activity against human immunodeficiency virus (HIV), the bicyclic 2′-deoxynucleoside, [2], derived from the base 6H,8H-3,4-dihydropyrimido[4,5-c][1,2]oxazin-7-one, was shown to inhibit herpes simplex virus type 1 (HSV-1) at similar concentrations as BVDU1 and ACV. Compounds 13, 6-(2-deoxyribofuranosyl)-6H,8H-2-methyl-3,4-dihydropyrimido[4,5-c][1,2]oxazin-7-one, and 14, N4-hydroxy-5-(2-chloroethyl)-2′-deoxyuridine, were as active as ACV against varicella-zoster virus (VZV).

(E)-5-(2-bromovinyl)-2?-deoxyuridine (BVDU)

(E)-5-(2-bromovinyl)-2'-deoxyuridine (BVDU, Brivudin, Zostex, Zerpex, Zonavir), now more than 20 years after its discovery, still stands out as a highly potent and selective inhibitor of herpes simplex virus type 1 (HSV-1) and varicella-zoster virus (VZV) infections. It has been used in the topical treatment of herpetic keratitis and recurrent herpes labialis and the systemic (oral) treatment of herpes zoster (zona, shingles). The high selectivity of BVDU towards HSV-1 and VZV depends primarily on a specific phosphorylation of BVDU to its 5'-diphosphate (DP) by the virus-encoded thymidine kinase (TK). After further phosphorylation (by cellular enzymes), to the 5'-triphosphate (TP), the compound interferes as a competitive inhibitor/alternate substrate with the viral DNA polymerase. The specific phosphorylation by the HSV- and VZV-induced TK also explains the marked cytostatic activity of BVDU against tumor cells that have been transduced by the viral TK genes. This finding offers considerable potential in a combined gene therapy/chemotherapy approach for cancer. To the extent that BVDU or its analogues (i.e., BVaraU) are degraded (by thymidine phosphorylase) to (E)-5-(2-bromovinyl)uracil (BVU), they may potentiate the anticancer potency, as well as toxicity, of 5-fluorouracil. This ensues from the direct inactivating effect of BVU on dihydropyrimidine dehydrogenase, the enzyme that initiates the degradative pathway of 5-fluorouracil. The prime determinant in the unique behavior of BVDU is its (E)-5-(2-bromovinyl) substituent. Numerous BVDU analogues have been described that, when equipped with this particular pharmacophore, demonstrate an activity spectrum characteristic of BVDU, including selective anti-VZV activity.

Discovery and development of BVDU (brivudin) as a therapeutic for the treatment of herpes zoster

This Commentary is dedicated to the memory of Dr. Jacques Gielen, the late Editor of Biochemical Pharmacology, whom I have known as both an author and reviewer for the Journal for about 25 years. This is, quite incidentally, about the time it took for bringing brivudin (BVDU) [(E)-5-(2-bromovinyl)-2'-deoxyuridine] from its original description as an antiviral agent to the market place (in a number of European countries, including Germany and Italy) for the treatment of herpes zoster in immunocompetent persons. BVDU is exquisitely active and selective against varicella-zoster virus (VZV) and herpes simplex virus type 1 (HSV-1). BVDU owes this high selectivity and activity profile to a specific phosphorylation by the virus-encoded thymidine kinase, followed by a potent interaction with the viral DNA polymerase. The (E)-5-(2-bromovinyl)-substituent can be considered as the hallmark for the activity of BVDU against VZV and HSV-1. Extensive clinical studies have indicated that BVDU as a single (oral) daily dose of 125 mg (for no more than 7 days) is effective in the treatment of herpes zoster, as regards both short-term (suppression of new lesion formation) and long-term effects (prevention of post-herpetic neuralgia). In this sense, BVDU is as efficient and/or convenient, if not more so, than the other drugs (acyclovir, valaciclovir, famciclovir) that have been licensed for the treatment of herpes zoster. There is one caveat; however, BVDU should not be given to patients under 5-fluorouracil therapy, as the degradation product of BVDU, namely (E)-5-(2-bromovinyl)uracil (BVU), may potentiate the toxicity of 5-fluorouracil, due to inhibition of dihydropyrimidine dehydrogenase, the enzyme involved in the catabolism of 5-fluorouracil.

Cellular transformation of the investigational new anticancer drug NB1011, a phosphoramidate of 5-(2-bromovinyl)-2'-deoxyuridine, results in modification of cellular proteins not DNA

NB1011 [E-5-(2-bromovinyl)-2'-deoxyuridine-5'-(L-methylalaninyl)-phenylphosphoramidate], a phosphoramidate prodrug of E-5-(2-bromovinyl)-2'-deoxyuridine-5'-monophosphate (BVdUMP), is an investigational new anticancer drug. NB1011 targets thymidylate synthase (TS), which catalyzes the transformation of BVdUMP into cytotoxic reaction products. Due to the elevated levels of TS expression in tumor cells compared to normal cells, these cytotoxic products are preferentially generated inside tumor cells, and, as expected, NB1011 is more toxic to cells with higher levels of TS expression. Therefore, NB1011 therapy should kill tumor cells without severely damaging normal cells. Radiolabeled NB1011 was used to determine the intracellular fate of NB1011 reaction products and, possibly, the mechanism of action of this investigational new drug. We found significant incorporation of the radiolabel into cellular macromolecules. In contrast to our expectations that NB1011 product(s) would be incorporated into DNA, we discovered that cellular proteins were the labeled macromolecular fraction. Herein, we report that the intracellular transformation of NB1011 involves formation of the corresponding monophosphate, TS-dependent transformation into highly reactive intermediates, and subsequent incorporation into cellular proteins. TS itself appears to escape irreversible inactivation. Our data suggest that protein modification not DNA incorporation accounts for the therapeutic effect of NB1011. The proposed mechanism is rather unexpected for a nucleotide analogue and could lead to the discovery of new cellular protein targets for future drug design.

Chemotherapy of varicella-zoster virus by a novel class of highly specific anti-VZV bicyclic pyrimidine nucleosides

(E)-5-(2-bromovinyl)-2'-deoxyuridine (BVDU) is a potent inhibitor of herpes simplex virus type 1 (HSV-1) and varicella-zoster virus (VZV). Its mechanism of action is based on a specific conversion to its 5'-mono- and 5'-diphosphate derivative by HSV-1- and VZV-encoded thymidine kinase, and after further conversion to its 5'-triphosphate derivative, inhibition of the viral DNA polymerase and eventual incorporation into the viral DNA. Recently, a new structural class of bicyclic pyrimidine nucleoside analogues (designated BCNAs) with highly specific and selective anti-VZV activity in cell culture has been discovered. The compounds need a long alkyl or alkylaryl side-chain at the base moiety for pronounced biological activity. This property makes these compounds highly lipophilic. They are also endowed with fluorescent properties when exposed to light with short UV wavelength. In striking contrast to BVDU, the members of this class of compounds are active only against VZV, but not against any other virus, including the closely related HSV-1, HSV-2 and cytomegalovirus. The most active compounds inhibit VZV replication at subnanomolar concentrations and are not toxic at high micromolar concentrations. The compounds lose their antiviral activity against thymidine kinase (TK)-deficient VZV strains, pointing to a pivotal role of the viral TK in their activation (phosphorylation). Kinetic studies with purified enzymes revealed that the compounds were recognized by VZV TK as a substrate, but not by HSV-1 TK, nor by cytosolic or mitochondrial TK. VZV TK is able to phosphorylate the test compounds not only to their corresponding 5'-mono- but also to their 5'-diphosphate derivatives. These data may readily explain and rationalize the anti-VZV selectivity of the BCNAs. There is no clear-cut correlation between the antiviral potency of the compounds and their affinity for VZV TK, pointing to a different structure/activity relationship of the eventual antiviral target of these compounds. The compounds are stable in solution and, in contrast to BVDU, not susceptible to degradation by thymidine phosphorylase. The bicyclic pyrimidine nucleoside analogues represent an entirely new class of highly specific anti-VZV compounds that should be further pursued for clinical development.

Different transmission rates of herpesvirus thymidine kinase reporter transgenes from founder male parents and male parents of subsequent generations

Previously we demonstrated that lines of transgenic mice carrying the herpes simplex type 1 virus thymidine kinase (HSV1-tk) reporter gene are male-sterile. Ectopic transcription of the HSV1-tk reporter in the testis was initiated downstream of the normal translation initiation codon and truncated proteins consistent with translational initiation at the second and third ATG codons were synthesized. Here we describe the effects on fertility 1) of converting the second and third ATG codons of the HSV1-tk reporter to CTG codons and 2) of utilizing the HSV type 2 thymidine kinase (HSV2-tk) reporter gene, in which the second ATG codon is located downstream of the ATP-binding pocket of the enzyme. Both reporters were coupled to the bovine thyroglobulin promoter (bTG-tk1 alpha and bTG-tk2 transgenes). The level of ectopic expression of these transgenes in the testis, relative to expression in the thyroid, was one to two orders of magnitude less than that of bTG-tk1. Sixty percent of male founders carrying the bTG-tk1 alpha and bTG-tk2 transgenes were fertile but did not transmit the transgene. In contrast, most males from subsequent generations were fertile and transmitted the transgenes at the expected frequency. This difference between founder males and male descendants is also observed with certain constructs in which the HSV1-tk reporter is coupled to other promoters. We attribute the effect to mosaicism among male founders, leading to competition between transgenic and nontransgenic spermatozoa and/or spermatogenic precursor cells and resulting in a lack of fertilization by transgenic sperm that would successfully fertilize eggs in the absence of competition.