Phosphorolysis of (E)-5-(2-bromovinyl)-2'-deoxyuridine (BVDU) and other 5-substituted-2'-deoxyuridines by purified human thymidine phosphorylase and intact blood platelets

A thymidine phosphorylase-stable analogue of BVDU with significant antiviral activity

(E)-5-(2-Bromovinyl)isodideoxyuridine (BVisoDDU), synthesized on the basis of molecular modeling, is selectively active against HSV-1 (three different strains) but inactive against HSV-2. Unlike BVDU, BVisoDDU is completely resistant to cleavage by thymidine phosphorylase. BVisoDDU is also the first nucleoside analogue lacking OH groups at both the 2'- and 3'-position that shows pronounced activity against HSV-1 replication.

Stereoselective synthesis and antiviral activity of D-2',3'-didehydro-2',3'-dideoxy-2'-fluoro-4'-thionucleosides

As 2',3'-didehydro-2',3'-dideoxy-2'-fluoronucleosides have exhibited interesting antiviral effects against HIV-1 as well as HBV, it is of interest to synthesize the isosterically substituted 4'-thionucleosides in which 4'-oxygen is replaced by a sulfur atom. To study structure-activity relationships, various pyrimidine and purine nucleosides were synthesized from the key intermediate (2R,4S)-1-O-acetyl-5-O-(tert-butyldiphenylsilyl)-2,3-dideoxy-2-fluoro-2-phenylselenyl-4-thio-beta-D-ribofuranoside 8, which was prepared from the 2,3-O-isopropylidene-D-glyceraldehyde 1 in 13 steps. The antiviral activity of the synthesized compounds were evaluated against HIV-1 in human peripheral blood mononuclear (PBM) cells, among which cytidine 17, 5-fluorocytidine 18, adenosine 24, and 2-fluoroadenosine 32 showed moderate to potent anti-HIV activities (EC(50) 1.3, 11.6, 8.1, and 1.2 microM, respectively). It is noteworthy that 2-fluoroadenosine analogue 32 showed antiviral potency as well as high cytotoxicity (IC(50) 1.5, 1.1, and 7.6 microM for PBM, CEM, and Vero, respectively) whereas no other compound showed cytotoxicity up to 100 microM. The cytidine 17 and 5-fluorocytidine 18 analogues showed significantly decreased antiviral activity against the clinically important lamivudine-resistant variants (HIV-1(M184V)), whereas the corresponding D-2'-Fd4 nucleosides showed limited cross-resistance. Molecular modeling studies demonstrated that the larger van der Waals radius as well as the close proximity to Met184 of the 4'-sulfur atom of D-2'-F-4'-Sd4C (17) may be the reasons for the decreased antiviral potency of synthesized 4'-thio nucleosides against the lamivudine-resistant variants (HIV-1(M184V)).

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.

Lack of susceptibility of bicyclic nucleoside analogs, highly potent inhibitors of varicella-zoster virus, to the catabolic action of thymidine phosphorylase and dihydropyrimidine dehydrogenase

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)-2'-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 microM in the presence of a 25 microM concentration of the natural substrate thymine), the free base (Cf 1381; 6-octyl-2,3-dihydrofuro[2,3-d]pyrimidin-2-one) of BCNA (Cf 1368; 3-(2'-deoxy-beta-D-ribofuranosyl)-6-octyl-2,3-dihydrofuro[2,3-d]pyrimidin-2-one) and the free base Cf 2200 [6-(4-n-pentylphenyl)-2,3-dihydrofuro[2,3-d]pyrimidin-2-one] of BCNA (Cf 1743; 3-(2'-deoxy-beta-D-ribofuranosyl)-6-(4-n-pentylphenyl)-2,3-dihydrofuro[2,3-d]pyrimidin-2-one) did not inhibit the DPD-catalyzed catabolic reaction of pyrimidine bases (i.e., thymine) and pyrimidine base analogs [i.e., 5-fluorouracil (FU)] at a concentration of 250 microM. 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.

A mercapto analogue of 5'-noraristeromycin

5'-Noraristeromycin and its enantiomer have been found to possess a wide range of antiviral effects. In the search for analogues of and with improved activity, the synthesis of both enantiomers of 5'-mercapto-5'-deoxy-5'-noraristeromycin ( and ) has been accomplished. While (+)-7 was inactive, (-)- did show marginal activity against vaccinia virus, but not any other virus.

Novel 6-substituted uracil analogs as inhibitors of the angiogenic actions of thymidine phosphorylase

Thymidine phosphorylase (TP) catalyzes the reversible phosphorolysis of thymidine and other pyrimidine 2'-deoxyribonucleosides. In addition, TP has been shown to possess angiogenic activity in a number of in vitro and in vivo assays, and its angiogenic activity has been linked to its catalytic activity. A series of 5- and 6-substituted uracil derivatives were synthesized and evaluated for their abilities to inhibit TP activity. Among the most active compounds was a 6-amino-substituted uracil analog, 6-(2-aminoethyl)amino-5-chlorouracil (AEAC), which was a competitive inhibitor with a K(i) of 165 nM. The inhibitory activity of AEAC was selective for TP, as it did not inhibit purine nucleoside phosphorylase or uridine phosphorylase at concentrations up to 1 mM. Human recombinant TP induced human umbilical vein endothelial cell (HUVEC) migration in a modified Boyden chamber assay in vitro, and this action could be abrogated by the TP inhibitors. The actions of the inhibitors were specific for TP, as they had no effect on the chemotactic actions of vascular endothelial growth factor (VEGF). HUVEC migration was also induced when TP-transfected human colon and breast carcinoma cells were co-cultured in the Boyden chamber assay in place of the purified angiogenic factors, and a TP inhibitor blocked the tumor cell-mediated migration almost completely. These studies suggest that inhibitors of TP may be useful in pathological conditions that are dependent upon TP-driven angiogenesis.

Synthesis and antiviral evaluation of phosphoramidate derivatives of (E)-5-(2-bromovinyl)-2'-deoxyuridine

We report the design, synthesis and antiviral evaluation of a number of lipophilic, masked phosphoramidate derivatives of the antiherpetic agent (E)-5-(2-bromovinyl)-2'-deoxyuridine (BVDU), designed to act as membrane soluble prodrugs of the free nucleotide. The phosphoramidate derivatives of BVDU that contain L-alanine exhibited potent anti herpes simplex virus type 1 and varicella-zoster virus activity but lost marked activity against thymidine kinase-deficient virus strains. The phosphoramidate derivative bearing the amino acid alpha,alpha-dimethylglycine showed poor activity in all cell lines tested. It appears that successful kinase bypass by phosphoramidates is highly dependent on the nucleoside analogue, amino acid and ester structure, as well as the cell line to which the drugs are exposed.

Thymidine phosphorylase inhibitors with a homophthalimide skeleton

Several N-phenylhomophthalimide derivatives were prepared and their inhibitory activity on thymidine phosphorylase/ platelet-derived endothelial cell growth factor (TP/PD-ECGF) was assessed. Among them, 2-(2,6-diethylphenyl)-7-nitro-1,2,3,4-tetrahydroisoquinoline-1,3-dione (9) was found to be a more potent inhibitor than the classical inhibitor, 5-nitrouracil (1). Lineweaver-Burk plot analysis indicated that 9 shows mixed-type competitive inhibition of TP/PD-ECGF, while 1 is a competitive inhibitor.

Metabolism and mode of inhibition of varicella-zoster virus by L-beta-5-bromovinyl-(2-hydroxymethyl)-(1,3-dioxolanyl)uracil is dependent on viral thymidine kinase

A nonnaturally occurring L-configuration nucleoside analog, L-beta-5-bromovinyl-(2-hydroxymethyl)-1,3-(dioxolanyl)uracil (L-BVOddU) selectively inhibited varicella-zoster virus growth in human embryonic lung (HEL) 299 cell culture with an EC(50) of 0.055 microM, whereas no inhibition of CEM and HEL 299 cell growth or mitochondrial DNA synthesis was observed at concentrations up to 200 microM. L-BVOddU was phosphorylated by viral thymidine kinase but not by human cytosolic thymidine kinase, and the antiviral activity of this compound is dependent on the viral thymidine kinase. Unlike other D-configuration bromovinyl deoxyuridine analogs, such as E-5-(2-bromovinyl)-2'-deoxyuridine and 1-beta-arabinofuranosyl-E-5-(2-bromovinyl)uracil, this compound was metabolized only to its monophosphate metabolite. The di- or triphosphate metabolites were not detected. This suggested that the inhibitory mechanism may be unique and different from other anti-herpesvirus nucleoside analogs.

Novel nonsubstrate inhibitors of human thymidine phosphorylase, a potential target for tumor-dependent angiogenesis

Thymidine phosphorylase/platelet-derived endothelial cell growth factor (TP/PD-ECGF) is an enzyme involved in thymidine metabolism and homeostasis, and its catalytic activity appears to play an important role in angiogenesis. Here we describe the cloning and expression of a His-tagged human TP/PD-ECGF and its assay with uracil and thymine analogues. We present the design, synthesis, and biological evaluation of novel 6-(phenylalkylamino)uracil derivatives which, at micromolar concentrations, inhibit both catabolic and anabolic reactions of human TP in vitro. These base analogues are not converted by the enzyme into the nucleoside form, thus representing pure nonsubstrate inhibitors of the enzyme.

Structure-activity relationships of (E)-5-(2-bromovinyl)uracil and related pyrimidine nucleosides as antiviral agents for herpes viruses

A series of (E)-5-(2-bromovinyl)uracil analogues and related nucleosides was synthesized, and their antiviral activities were evaluated. (E)-5-(2-Bromovinyl)-2'-deoxy-L-uridine (L-BVDU, 2), 1-(beta-L-arabinofuranosyl)-(E)-5-(2-bromovinyl)uracil (L-BVAU, 4), (E)-5-(2-bromovinyl)-1-(2-deoxy-2-fluoro-beta-L-ribofuranosyl)uracil (L-FBVRU, 8) and (E)-5-(2-bromovinyl)-1-(2-deoxy-2-fluoro-beta-L-arabinofuranosyl)urac il (L-FBVAU, 10) were synthesized via appropriate 5-iodouracil analogues from L-arabinose. D- and L-Oxathiolane and -dioxolane derivatives 13, 16, 20, 21, and 29-34 were prepared by glycosylation reaction of the oxathiolane and dioxolane intermediates with silylated uracil analogues using TMSI as the coupling agent. The synthesized compounds were evaluated in cell cultures infected with the following viruses: varicella zoster virus (VZV), Epstein Barr virus (EBV), and herpes simplex virus types 1 and 2 (HSV-1 and HSV-2). Among the tested compounds, beta-L-CV-OddU (29), beta-L-BV-OddU (31), and beta-L-IV-OddU (33) exhibited potent in vitro antiviral activity against VZV with EC(50) values of 0.15, 0. 07, and 0.035 microM, respectively, and against EBV with EC(50) values of 0.49, 0.59, and 3.91 microM, respectively.

Structure and activity of specific inhibitors of thymidine phosphorylase to potentiate the function of antitumor 2'-deoxyribonucleosides

A new class of 5-halogenated pyrimidine analogs substituted at the 6-position was evaluated as competitive inhibitors of thymidine phosphorylase (TPase). The most potent member of the series was 5-chloro-6-(2-iminopyrrolidin-1-yl)methyl-2,4(1H,3H)-pyrimidine dio ne hydrochloride (TPI), which has an apparent K(i) value of 1.7 x 10(-8) M. TPI selectively inhibited the activity of TPase, but not that of uridine phosphorylase, thymidine kinase, orotate phosphoribosyltransferase, or dihydropyrimidine dehydrogenase. In vitro inhibition studies of TPI using a thymidine analogue, 5-trifluoromethyl-2'-deoxyuridine (F(3)dThd), as the substrate demonstrated that F(3)dThd phosphorolytic activity was inhibited markedly by TPI (1 x 10(-6) M) in extracts from the liver, small intestine, and tumors of humans, from the liver and small intestine of cynomolgus monkeys, and from the liver of rodents, but not from the liver or small intestine of dogs or the small intestine of rodents, suggesting that the distribution of TPase differs between humans and animal species, and that TPI could contribute to the modulation of TPase in humans. When F(3)dThd or 5-iodo-2'-deoxyuridine (IdUrd) was coadministered to mice with TPI at a molar ratio of 1:1, the blood levels of F(3)dThd (or IdUrd) were about 2-fold higher than when F(3)dThd (or IdUrd) was administered alone. In monkeys, the maximum concentration (C(max)) and the area under the concentration-time curve (AUC) after oral F(3)dThd alone were 0.23 microg/mL and 0.28 microg. hr/mL, respectively, but markedly increased to 15.18 microg/mL (approximately 70-fold) and 28.47 microg. hr/mL (approximately 100-fold), respectively, when combined with equimolar TPI. Combined oral administration of TPI significantly potentiated the antitumor activity of F(3)dThd on AZ-521 human stomach cancer xenografts in nude mice. In conclusion, TPI may contribute not only to inhibition of TPase-mediated biological functions but also to potentiation of the biological activity of various 2'-deoxyuridine and thymidine derivatives by combining with them.

Extending the lifetime of anticoagulant oligodeoxynucleotide aptamers in blood

We have investigated (123)I and (125)I DNA aptamer analogs of anticoagulant DNA aptamers to thrombin exosite 1 and exosite 2 for thrombus imaging potential. Two severe problems are rapid clearance from circulating blood and blood nuclease. With aptamers (unlike antisense) the nucleotide analogs used in polymerase chain reaction-selection cycles also must be used in the radiotracer. We investigated 3'-biotin-streptavidin (SA) bioconjugates of the aptamers to alleviate these problems. Blood nuclease assays and biodistribution analysis were used in the mouse and rabbit. We found that 3'-biotin protected the aptamers significantly from blood nuclease in vitro, but it did not slow in vivo clearance. In contrast, the 3'-biotin-SA bioconjugates were resistant to blood nuclease in vitro and were also longer-lived (10-20 times) in vivo. Bioconjugate aptamers retained affinity for thrombin. Two solutions emerge: 1) In noncirculating blood (within a thrombus) 3'-biotin extends aptamer lifetime, whereas 2) in circulating blood (the transport medium), where more aggressive clearance is encountered, 3'-SA extends aptamer lifetime.

Design, synthesis, and enzymatic evaluation of multisubstrate analogue inhibitors of Escherichia coli thymidine phosphorylase

A series of acyclic phosphonate derivatives of thymine has been synthesized and tested as multisubstrate analogue inhibitors of Escherichia coli thymidine phosphorylase. The compounds synthesized include 1-(phosphonoalkyl)thymines with six to nine methylenes (1-4, respectively); 1-[(Z)-4-phosphonomethoxy-2-butenyl]thymine (5) and its butyl and 2,3-cis-dihydroxybutyl derivatives (6 and 7, respectively); 1-[(Z)-(4-(phosphonomethoxy)methoxy)-2-butenyl]thymine (8) and also its butyl and 2,3-cis-dihydroxybutyl analogues (9 and 10); and 1-[((Z)-4-(phosphonomethoxy)-2-butenoxy)methyl]thymine (11). Evaluation of these compounds against E. coli revealed significant enzymatic inhibition by 2, 3, 4, 6, and 8 at a concentration of 1000 microM, 3 and 4 being the most potent. Replacement of the thymine base in 3 by 6-amino-5-bromouracil and 7-deazaxanthine afforded compounds 12 and 13, which showed a pronounced improvement of TPase inhibition, comparable to 7-deazaxanthine. When inorganic phosphate was used as a variable substrate, compounds 12 and 13 displayed competitive kinetics with respect to phosphate, indicating a direct interaction of these compounds with the phosphate binding site. Also compounds 12 and 13 were found to be competitive inhibitors of TPase against thymidine as a variable substrate. These results are consistent with the compounds being multisubstrate analogue inhibitors of E. coli TPase, and they represent the first example of such TPase inhibitors.

Determination of (E)-5-(2-bromovinyl)-2'-deoxyuridine in plasma and urine by capillary electrophoresis

(E)-5-(2-Bromovinyl)-2'-deoxyuridine is an antiviral drug used for treatment of infections with Herpes simplex virus type 1 as well as Varicella zoster virus. Two fast methods for the determination of the drug and its metabolite in plasma and urine by capillary electrophoresis have been developed. The plasma method can be used for measurement of total as well as unbound drug and metabolite. Plasma and urine samples are prepared for measuring by liquid/liquid extraction resulting in a limit of quantification of 40 ng/ml for total and 10 ng/ml for free BVdU in plasma and 170 ng/ml in urine. Inter- as well as intra-day precision were found to be better than 10% and both methods have been used for drug monitoring of patients.

7-Deazaxanthine, a novel prototype inhibitor of thymidine phosphorylase

7-Deazaxanthine (7DX) was identified as a novel inhibitor of thymidine (dThd) phosphorylase (TPase). It inhibited the TPase reaction in a concentration-dependent manner. At 1 mM, it almost completely prevented the TPase-catalysed hydrolysis of dThd to thymine. The 50% inhibitory concentration (IC50 of 7DX was 40 microM in the presence of 100 microM of the natural substrate dThd. 7DX is also endowed with a marked inhibitory effect on angiogenesis. It significantly prevents neovascularisation in the chicken chorioallantoic membrane during development. 7DX is the first purine derivative shown to be a potent inhibitor of purified TPase and angiogenesis.

Relaxed enantioselectivity of human mitochondrial thymidine kinase and chemotherapeutic uses of L-nucleoside analogues

Our discovery that Herpes virus thymidine kinase (TK) and cellular deoxycytidine kinase lack enantioselectivity, being able to phosphorylate both D- and L-enantiomers of the substrate, suggested the use of unnatural L-nucleoside analogues as antiviral drugs (Herpes, hepatitis and immunodeficiency viruses). Several L-nucleoside analogues have displayed a short-term cytotoxicity much lower than their corresponding D-counterpart. Since the delayed cytotoxicity of a drug often depends on its effects on mitochondrial metabolism, we have investigated the degree of enantioselectivity of human mitochondrial thymidine kinase (mt-TK). We demonstrate that mt-TK does not show an absolute enantioselectivity, being able to recognize, although with lower efficiency, the L-enantiomers of thymidine, deoxycytidine and modified deoxyuridines, such as (E)-5-(2-bromovinyl)-2'-deoxyuridine and 5-iodo-2'-deoxyuridine. Interestingly, the reported negative co-operativity of mt-TK phosphorylating beta-D-2'-deoxythymidine (D-Thd), disappears when the deoxyribose moiety has the inverted configuration, resulting in the preferential phosphorylation of d-Thd even in the presence of high concentrations of the L-enantiomer. This, coupled with the higher Km for beta-L-2'-deoxythymidine (L-Thd), makes mt-TK resistant to high concentrations of L-Thd and L-Thd analogues, minimizing the mitochondria-dependent delayed cytotoxicity that might be caused by the administration of L-nucleoside analogues as antivirals.

A Novel Synthesis of 2'-Modified 2'-Deoxy-4'-thiocytidines from D-Glucose(1)

Novel 2'-deoxycytidine antimetabolites, specifically several 2'-modified 2'-deoxy-4'-thiocytidines, were synthesized as potential new antineoplastic agents. Methyl 3-O-benzylxylofuranoside was converted to a 1,4-anhydro-4-thioarabitol 24. Protection of the primary alcohol of 24 gave a common intermediate (15) which was useful for the synthesis of various 2'-modified 2'-deoxy-4'-thionucleosides. Oxidation of the secondary hydroxyl group of 15, followed by the Wittig reaction or treatment with (diethylamido)sulfur trifluoride (DAST) produced 2-deoxy-2-methylene (26) and 2-deoxy-2,2-difluoro (34) derivatives, respectively. Unique Pummerer-type glycosylation between the corresponding sulfoxides and trimethylsilylated N(4)-acetylcytosine produced 2'-deoxy-2'-methylene- (10) and 2'-deoxy-2',2'-difluoro-4'-thiocytidines (11). On the other hand, treatment of 15 with DAST introduced a fluorine atom with retention of the 2'-stereochemistry, yielding 40. In contrast, the Mitsunobu reaction of 3-O-benzoyl derivative 53 which was obtained from 15 in five steps, using diphenylphosphoryl azide gave azide derivative 54 with inverted stereochemistry. These derivatives were converted to the corresponding 1-O-acetyl derivatives via the usual Pummerer rearrangement, which were in turn used to synthesize 4'-thiocytidines 12 and 58. Among the 2'-modified 4'-thiocytidines obtained, 2'-methylene (10) and 2'-fluoro (12) derivatives were found to have potent antineoplastic properties in vitro.

Platelet-derived endothelial cell growth factor thymidine phosphorylase in tumour growth and response to therapy

Angiogenesis plays an important role in the growth and metastasis of solid tumours. Platelet-derived endothelial cell growth factor (PD-ECGF) is known to be chemotactic for endothelial cells in vitro and angiogenic in vivo. It is also known as gliostatin, a factor promoting neuronal survival, and thymidine phosphorylase (dThdPase), which catalyses the reversible phosphorylation of thymidine to thymine and 2-deoxyribose-1-phosphate. This enzymatic activity is critical for angiogenic activity. PD-ECGF protein is highly expressed in tumours compared with most normal tissues and has been correlated with tumour growth, invasion and metastasis in clinical studies. In addition, dThdPase activity (by inference PD-ECGF) has been found to be a major determinant of the toxicity of 5-fluorouracil and its prodrugs, which are extensively studied clinically as anti-cancer agents. This review attempts to summarize recent gains in understanding the nature, location and action of PD-ECGF and its specific relevance to tumour biology.

Deglycosylation of antiherpesviral 5-substituted arabinosyluracil derivatives by rat liver extract and enterobacteria cells

A number of antiherpesviral 5-substituted derivatives of 1-beta-D-arabinofuranosyluracil (araU) were significantly resistant to phosphorolysis by rat liver extract (S-9), but were gradually deglycosylated in a 2% enterobacteria cell suspension. The relative order of the resistance conferred by the different C-5 substituents was: 5-propynyl > 5-(E)-2-bromovinyl > 5-(E)-2-chlorovinyl > 5-methyl > 5-iodo. The 2'-fluoro derivatives of araU were completely resistant to phosphorolysis by both liver extract and enterobacteria, whereas the corresponding ribofuranosyl and 2'-deoxyribofuranosyl nucleosides were easily phosphorolysed by S-9, and were immediately cleaved in a 1% enterobacteria cell suspension. These findings suggest that antiherpesviral 5-substituted araU analogues can be relatively stable in vivo, when injected intravenously, and that degradation of 1-beta-D-arabinofuranosyl-5-(E-2-bromovinyl)uracil (sorivudine) following oral administration is due primarily to the action of enterobacteria.

Lack of stereospecificity of some cellular and viral enzymes involved in the synthesis of deoxyribonucleotides and DNA: molecular basis for the antiviral activity of unnatural L-beta-nucleosides

Among enzymes involved in the synthesis of nucleotides and DNA, some exceptions have recently been found to the universal rule that enzymes act only on one enantiomer of a chiral substrate and that only one of the enantiomeric forms of chiral molecules may bind effectively at the catalytic site, displaying biological activity. The exceptions include: herpes virus thymidine kinases, cellular deoxycytidine kinase and deoxynucloside mono- and diphosphate kinases, cellular and viral DNA polymerases, such as DNA polymerase alpha, terminal transferase and HIV-1 reverse transcriptase. The ability of these enzymes to utilize unnatural L-beta-nucleosides or -nucleotides as substrate may be exploited from chemotherapeutic point of view.

Structure-activity relationships and conformational features of antiherpetic pyrimidine and purine nucleoside analogues. A review

A rational approach to the design of antiherpetic nucleoside analogues is based in part on the broad specificity of virus-coded thymidine kinases. Herpes virus thymidine kinase 'activates' many 5-substituted 2'-deoxyuridines, analogues of thymidine (e.g., idoxuridine, trifluridine, edoxudine, brivudine), 5-substituted arabinofuranosyluracil derivatives (e.g., 5-Et-Ara-U, BV-Ara-U, Cl-Ara-U), acyclonucleosides of guanine (e.g., aciclovir, ganciclovir, penciclovir), and purine nucleosides with the pentafuranosyl ring replaced by a cyclobutane ring (e.g., cyclobut-G, cyclobut-A). Activation involves selective, and frequently regiospecific, phosphorylation of these analogues to the 5'-monophosphates. These are further phosphorylated by cellular enzymes to the 5'-triphosphates, which are usually competitive inhibitors of the viral-coded DNA polymerases. Some analogues are also incorporated into viral, and to a lesser extent cellular, DNA. A recent, unusual, exception is human cytomegalovirus, which does not code for a thymidine kinase, but for a protein with the sequence characteristics of protein kinase and which phosphorylates ganciclovir to its 5'-monophosphate. The interaction of the analogues with cellular catabolic enzymes such as uridine and thymidine nucleoside phosphorylases is also discussed, as is the relationship between physicochemical properties (configuration, conformation, electronic and hydrophobic parameters) and antiviral activities, with particular reference to those drugs that are licensed, or under consideration, for clinical use.

Synthesis of brain-targeted 1-(2-deoxy-2-fluoro-beta-D-ribofuranosyl)-(E)-5-(2-iodovinyl)uracil coupled to a dihydropyridine <---> pyridinium salt redox chemical-delivery system

1-(2-Deoxy-2-fluoro-beta-D-ribofuranosyl-(E)-5-(2-iodovinyl)uracil (IVFRU) was coupled to a dihydropyridine <---> pyridinium salt redox chemical-delivery system (CDS) via a cleavable sugar-ester linkage as a site-directed approach to increase diffusion of the parent nucleoside into the central nervous system. Treatment of 1-(2-deoxy-2-fluoro-beta-D-ribofuranosyl)uracil with Bu(t)Me(2)SiCl in the presence of imidazole in DMF yielded the protected 5-O-tert-butyldimethylsilyl derivative. Subsequent reaction with nicotinoyl chloride hydrochloride in pyridine afforded 1-[5-O-tert-butyldimethylsilyl-2-deoxy-2-fluoro-3-O-(3-pyridylcarbony l )-beta-D-ribofuranosyl]uracil. Reaction with iodine monochloride in methanol simultaneously cleaved the silyl ether moiety and iodinated the uracil ring at the 5-position. Coupling with (E)-Bu(3)Sn-CH = CH-SiMe(3) in the presence of (Ph3P)2Pd2(II)Cl2 in THF gave 1-[2-deoxy-2-fluoro-3-O-(3-pyridylcarbonyl)-beta-D-ribofuranosyl]- (E)-5-(2-trimethylsilylvinyl)uracil. Quaternization with iodomethane in acetone yielded the N-methylpyridinium iodide salt. Ionation of the reactive (E)-trimethylsilylvinyl moiety with iodine monochloride in acetonitrile and reduction of the quaternary pyridinium iodide salt with sodium dithionite in the presence of sodium hydrogen carbonate was carried out as a one-pot procedure to afford 1-[2-deoxy-2-fluoro-3-O-(1-methyl-1,4-dihydropyridyl-3-carbonyl)-b eta-D-ribofuranosyl]-(E)-5-(2-iodovinyl)uracil (IVFRU-CDS). This synthetic strategy is readily amenable to the high specific-activity radioiodination of IVFRU.

Cellular metabolism of (-) enantiomeric 2'-deoxy-3'-thiacytidine

The metabolism of (-) enantiomeric 2'-deoxy-3'-thiacytidine (3TC) was examined in human immunodeficiency virus type 1 (HIV-1)-infected and mock-infected human cells. 3TC 5'-triphosphate levels accumulated comparably in HIV-1-infected and mock-infected phytohaemagglutinin-stimulated peripheral blood lymphocytes (PBL) and reached 40% or more of total intracellular 3TC metabolites after 4 hr. The rate of decay of 3TC triphosphate in HIV-1-infected and mock-infected PBL measured as a half-life (T1/2) ranged from 10.5 to 15.5 hr. 3TC did not significantly affect metabolism of deoxynucleotides in the U937 cell line, and was shown to be resistant to the action of human platelet pyrimidine nucleoside phosphorylase.

Relationship between Conformation and Antiviral Activity-II. 5-Methoxymethyl-2′-deoxycytidine and 5-methoxymethyl-N4-methyl-2′-deoxycytidine

5-methoxymethyl-N4-methyl-2′-deoxycytidine (N4-Me-MMdCyd) and 5-methoxymethyl-N4-methyl-2′-deoxycytidine-5′-monophosphate (N4-Me-MMdCMP) were synthesized to confer resistance to deamination by deaminating enzymes. N4-Me-MMdCyd and N4-Me-MMdCMP were inactive against Herpes simplex virus type 1 (HSV-1) and also nontoxic to VERO cells up to 1796 μM (highest concentration tested). 5-methoxymethyl-2′-deoxycytidine-5′-monophosphate (MMdCMP) was more potent than the nucleoside against HSV-1 in VERO cells. In HSV-infected VERO cells (10 PFU/cell), N4-Me-MMdCyd caused only slight perturbations of deoxyribonucleoside triphosphate pools. 5-methoxymethyl-N4-methyl-2′-deoxycytidine-5′-triphosphate (N4-Me-MMdCTP) was synthesized and the nature of interaction of N4-Me-MMdCTP and dCTP with DNA polymerase of Escherichia coli, HSV-1 and human α was investigated. N4-Me-MMdCTP was neither an effective substrate nor a strong inhibitor of Escherichia coli, HSV-1 or human α DNA polymerase.

The relationship between molecular conformation and antiviral activity for MMdCyd and N4-Me-MMdCyd is discussed. The conformation of the deoxyribofuranose ring in MMdCyd and N4-Me-MMdCyd are different. In N4-Me-MMdCyd, the exocyciic C(5′) side chain has the t conformation whereas MMdCyd has the g+rotomer conformation. The orientation of the N4-methyl group may also impede binding to the HSV-induced kinase by steric hindrance and/or by hindering hydrogen bonding between the enzyme and the lone pair of electrons at N(3). The results suggest that attempts to render resistance to deamination by alkylation at the N(4) position of the cytosine moiety is not likely to yield compounds with activity against HSV.

Metabolism of anti-herpes agent 5-(2-chloroethyl)-2'-deoxyuridine in mice and rats

The enzymatic splitting and metabolic elimination of anti-viral agent 5-(2-chloroethyl)-2'-deoxyuridine [CEDU] have been studied. For elucidation of structures of metabolites, several different kinds of extraction, purification and spectroscopic methods were used (Extrelut LC, TLC, HPLC, MS, NMR, IR, UV and CD). For mass spectral analysis, various ionization techniques (EI, CI and FAB-MS) were performed as complementary methods. After oral administration of [14C]-CEDU to mice and rats, the parent compound, 5-(2chloroethyl) uracil [CEU] and hydroxylated CEU metabolites were isolated and identified from urine and faeces by the above mentioned methods. The CEDU showed rapid phosphorolysis in vitro with thymidine phosphorylase Km 41.0 +/- 5.0; and uridine phosphorylase Km 10.0 +/- 1.5. The cleavage of the N-glycosidic bond of the nucleoside analogue and a new metabolic pathway of CEDU [stereoselective oxidation of 5-(2-chloroethyl) uracil] was observed in both species.

cis-trans-isomerization of [E]-5-(2-bromovinyl)-2,2'-anhydrouridine in vivo in rats

1. [E]-5-(2-bromovinyl)-2,2'-anhydrouridine [( E]BVANUR) has considerable antiviral activity against herpes simplex virus type 1 (HSV-1). 2. [E]BVANUR is not a substrate of pyrimidine nucleoside phosphorylases, but it is an inhibitor of uridine phosphorylase (Ki = 450 nM). 3. [E]BVANUR (trans-isomer, parent compound) undergoes isomerization to [Z]BVANUR (cis-isomer), the only metabolite in rat, which was identified by h.p.l.c., mass spectra and n.m.r. spectroscopy. 4. Absorption of the drug from the gastrointestinal tract after oral administration is minimal. Absorption of [E]BVANUR from the abdominal cavity after i.p. administration was slow.

High-performance liquid chromatographic analysis of 5-ethylpyrimidines and 5-methylpyrimidines in plasma

A high-performance liquid chromatographic (HPLC) method employing a C18 reversed-phase column, a mobile phase of sodium acetate and methanol, and an ultraviolet detector was developed for the analysis of 5-ethylpyrimidines and 5-methylpyrimidines in plasma. Samples were prepared for HPLC by sequential cation-exchange and anion-exchange column chromatography. Linear standard curves were obtained for samples containing 0.05-50 micrograms/ml 5-ethyl-2'-deoxyuridine and 5-ethyluracil, 0.05-10 micrograms ml 5-(1-hydroxyethyl)uracil, and 0.1-50 micrograms/ml thymidine, thymine and 5-hydroxymethyluracil. Applicability of the method to determination of the kinetics of 5-ethyl-2'-deoxyuridine elimination by the isolated perfused rat liver was demonstrated; clearance of the drug was 1.29 ml/min.

Antiherpes virus activity and effect on deoxyribonucleoside triphosphate pools of (E)-5-(2-bromovinyl)-2'-deoxycytidine in combination with deaminase inhibitors

The antiviral activity and cytotoxicity of (E)-5-(2-bromovinyl)-2'-deoxycytidine (BrVdCyd) against herpes simplex virus type 1 (HSV-1), singly and in combination with deaminase inhibitors was determined using rabbit kidney (RK-13), HEP-2, BHK-21 and VERO cells. BrVdCyd was a potent inhibitor of HSV-1 replication with ED50 values of 0.30 to 1.20 microM depending on the cell line used. In the presence of tetrahydrouridine or tetrahydrodeoxyuridine (H4dUrd), potency of BrVdCyd increased approximately two fold (ED50: 0.54 microM) in HSV-infected VERO cells. The combination of BrVdCyd and H4dUrd was also effective in decreasing virus yield. Dihydrodeoxyuridine (H2dUrd) reversed the activity of BrVdCyd (ED50: 6 to 7 microM). The effect of (E)-5-(2-bromovinyl)-2'-deoxyuridine (BrVdUrd), BrVdCyd and BrVdCyd in combination with H4dUrd on deoxyribonucleoside triphosphate (dNTP) pools was assessed in VERO cells infected with a high multiplicity of infection (10 PFU/cell). Significant differences in dNTP poll sizes (pmol/10(6) cell) were observed with different treatments. BrVdUrd and BrVdCyd treatment resulted in marked expansion of the dTTP pool (greater than 1200 pmol) compared to HSV-infected VERO cells (303 pmol). Exposure to H4dUrd resulted in a 12-fold expansion of the dCTP pool (326 pmol) and barely detectable levels of dTTP (less than 1.0 pmol). BrVdCyd plus H4dUrd treatment resulted in a slight expansion of the dTTP pool (515 pmol). These results indicate: (i) H4dUrd inhibits de novo dCyd/dCMP deaminase pathway and (ii) exposure to BrVdCyd plus H4dUrd puts a strain on viral DNA synthesis to such an extent that even though dTTP is being formed from alternative pathways, its eventual utilization as a substrate is reduced and hence it builds up.

Animal models for antiviral chemotherapy

Traditionally animal models have formed a vital part of the preclinical evaluation of new forms of antiviral therapy. A variety of models used in the past or potentially useful in the future are considered in this short review. Several valuable and complex questions concerning virus-drug interactions in vivo have been successfully addressed by means of animal models. Better understanding of drug modes of action and virus pathogenesis in the models enable even more accurate predictions to be made for the outcome of antiviral therapy in man. The complexity of virus infections in man is such that animals are likely to remain an important part in drug evaluation for many years. To this end, new developments such as improved techniques in the production of transgenic animals are opening up a variety of completely novel methods for studying inhibitors of a wider group of viruses in vivo including the human immunodeficiency virus. However, the correct interpretation of animal data requires the critical evaluation of animal models. This review will identify several important difficulties which confront those working on antiviral chemotherapy in animals and which must continue to be addressed if confidence in animal data is to be maintained.

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

(E)-5-(2-Bromovinyl)uridine (BVUrd), the riboside counterpart of (E)-5-(2-bromovinyl)-2'-deoxyuridine (BVdUrd), effected a dose-dependent inhibition of viral progeny formation and viral DNA synthesis in herpes simplex virus type 1 (HSV-1, strain KOS)-infected human (E6SM) diploid fibroblast cells. BVUrd was directly phosphorylated in HSV-1-infected cells, presumably by the virus-encoded thymidine kinase (TK), since (i) BVUrd was not phosphorylated by extracts of cells infected with a HSV-1 strain deficient in TK expression and (ii) the phosphorylation was inhibited by a polyclonal anti-HSV-1 antibody. Within the HSV-1-infected cells, BVUrd was incorporated into the viral DNA as BVdUMP (BVdUrd 5'-monophosphate). This incorporation may account for the antiviral action of BVUrd, and implies that, following its initial phosphorylation by the viral TK, BVUrd is converted to its 2'-deoxy counterpart, most likely at the 5'-diphosphate level (BVUDP----BVdUDP).

Enhancing effect of bromovinyldeoxyuridine on antitumor activity of 5'-deoxy-5-fluorouridine against adenocarcinoma 755 in mice. Correlation with pharmacokinetics of plasma 5-fluorouracil levels

5'-Deoxy-5-fluorouridine (DFUR), whether or not combined with (E)-5-(2-bromovinyl)-2'-deoxyuridine (BVDU) was pursued in BDF1 mice from both a pharmacokinetic viewpoint, following a single oral dose administration, and an anticancer viewpoint, following 5 daily oral doses in mice inoculated subcutaneously with adenocarcinoma 755 tumor cells. Half-life (t1/2) values for the elimination of DFUR and 5-fluorouracil (5-FU) from plasma following DFUR (100 mg/kg) administration were about 0.80 and 0.39 hr, respectively. Plasma 5-FU AUC (area under the curve) values following oral DFUR (100 mg/kg) was 0.224 micrograms.hr/ml. If DFUR (100 mg/kg) was combined with BVDU (10 mg/kg) the t1/2 and AUC values for 5-FU increased from 0.39 to 1.24 hr, and from 0.224 to 1.699 micrograms.hr/ml, respectively. Thus, BVDU significantly increased the plasma levels of 5-FU. It had no effect on the plasma levels of DFUR. At 100 mg/kg, DFUR did not show a significant antitumor activity. At 500 mg/kg it effected a 90% inhibition in tumor growth. When combined with BVDU (10 mg/kg), DFUR at 100, 200 and 300 mg/kg reduced tumor growth by 96, 100 and 100%, respectively. The antitumor activity achieved by DFUR, in the presence or absence of BVDU, correlated highly significantly with the AUC values for plasma 5-FU.

Local and systemic implications of thymidine catabolism by human keratinocytes

The human epidermis possesses a very active thymidine phosphorylase with the capacity to catabolize all of the thymidine available to the epidermis from the circulation and from the degradation of DNA in terminally differentiating keratinocytes. This high capacity of keratinocytes to catabolize thymidine could affect local levels of thymidine within the epidermis for DNA synthesis and could contribute to the regulation of the concentration of thymidine in the systemic circulation. Further work is needed to delineate the physiologic role of this keratinocyte enzyme. A practical consequence of the activity of epidermal thymidine phosphorylase is the role it may play in limiting the clinical efficacy of certain thymidine analogs, an important class of antiviral agents.

Inhibition of human hepatitis B virus DNA polymerase and duck hepatitis B virus DNA polymerase by triphosphates of thymidine analogs and pharmacokinetic properties of the corresponding nucleosides

Replication of hepadnaviruses involves a viral DNA polymerase containing both a DNA-dependent and an RNA dependent activity. This polymerase is a potential target for chemotherapy against hepatitis B. We have used human hepatitis B virus DNA-dependent DNA polymerase from human serum and duck hepatitis B virus DNA-dependent DNA polymerase from duck serum as well as RNA-dependent DNA polymerase activity from duck hepatitis B-infected duck liver. Triphosphates of thymidine analogs have been synthesized and tested for their inhibitory activities against these enzymes with the intention both to explore differences between these enzymes and structural requirements for inhibitors. The results showed that with the inhibitors tested, hepatitis B virus DNA-dependent DNA polymerase was the most sensitive enzyme and the triphosphate of 5-propenyl-2'-deoxyuridine was the most active inhibitor. In addition, the 5'-triphosphate of 5-propenyl-arabinofuranosyluracil also inhibited the hepadnavirus DNA-dependent DNA polymerases, and was a competitive inhibitor with respect to 2'-deoxythymidine triphosphate as showed by kinetic studies with duck hepatitis B virus DNA-dependent DNA polymerase from serum. Pharmacokinetic analysis showed 5-propenyl-2'-deoxyuridine to be well absorbed orally, but rapidly cleared from plasma. The arabinofuranosyl analog was also well absorbed but cleared less rapidly. Hence, these results indicate the potential of 5-propenyl-2'-deoxyuridine and 5-propenyl-arabinofuransyluracil for chemotherapy of hepatitis B.

Inhibition of uridine phosphorylase by pyrimidine nucleoside analogs and consideration of substrate binding to the enzyme based on solution conformation as seen by NMR spectroscopy

Some 3'- and/or 5'-substituted pyrimidine nucleosides, as well as anhydropyrimidine nucleosides, which have no flexibility about the N-glycosidic bond were studied as inhibitors of thymidine phosphorylase and uridine phosphorylase. The conformation of some analogs was also investigated in order to obtain information on substrate binding to the enzyme. The above compounds, including the potential anti-(human immunodeficiency virus) agent, 3'-azido-2',3'-dideoxy-5-methyluridine were not substrates for either thymidine phosphorylase or uridine phosphorylase. (The only exception was arabinofuranosyl-5-ethyluracil, which proved to be a poor substrate for uridine phosphorylase). The phosphorolysis of thymidine by thymidine phosphorylase was slightly or not at all altered by these pyrimidine nucloside analogs. The lowest Ki was obtained in the case of 3'-azido-2',3'-dideoxy-5-methyluridine and the highest in the case of 2'-deoxylyxofuranosyl-5-ethyluracil, when studying the analogs with flexible structure as inhibitors of uridine phosphorylase. The Ki for 2,3'- and 2,5'-anhydro-2'-deoxy-5-ethyluridine was 5-6 orders of magnitude higher than that for 2,2'-anhydro-5-ethyluridine. Competitive inhibition was observed in all cases. For these three molecules computer-aided molecular modelling predicts the following glycosidic torsion angles chi (O4,-C1,-N1-C2): 109 degrees for 2,2'-anhydro-5-ethyluridine, and 78 degrees and 71 degrees for 2,3'- and 2,5'-anhydro-2'-deoxy-5-ethyluridine respectively. These values are corroborated by high-resolution 13C- and 1H-NMR studies. 2'-Deoxy-5-ethyluridine is predicted to have a syn conformation with chi = 46 degrees and delta E about 2.5 kJ/mol over the minimum energy (in anti position, chi = -147 degrees). 1H and 13C data including homonuclear Overhauser enhancements complete the information about the solution conformation. Considering the Ki values obtained, it is likely that substrates of uridine phosphorylase will bind to the enzyme in the same conformation as 2,2'-anhydro-5-ethyluridine. The greater than 30 degrees deviation from the N-glycosidic torsion angle of 2,2'-anhydro-5-ethyluridine results in much higher Ki values.

Improved high-performance liquid chromatographic assay for the quantification of 5-bromo-2'-deoxyuridine and 5-bromouracil in plasma

A sensitive and specific procedure using high-performance liquid chromatography (HPLC) was developed for the quantification of 5-bromo-2'-deoxyuridine (BUdR) and 5-bromouracil (BU) in plasma. BUdR and BU were first extracted with a mixture of ethyl acetate and 2-propanol from plasma presaturated with solid ammonium sulfate. Following evaporation of the organic extract, the remaining residue was reconstituted in saturated ammonium sulfate solution, washed with a mixture of n-pentane-methylene chloride and re-extracted with the original solvent mixture. The organic extract was evaporated, reconstituted in mobile phase and chromatographed on a regular-bore ODS HPLC column using ultraviolet absorbance detection. The BUdR and BU quantification limits were both 0.1 microM, the mean intra-assay coefficients of variation were 5.0 and 5.6%, respectively, and the mean inter-assay coefficients of variation were 5.4 and 10.7%, respectively. This method was used to determine steady-state femoral arterial and hepatic venous plasma concentrations of BUdR and BU in a patient receiving a continuous intravenous infusion of BUdR (20 mg/kg per day).

Evaluation of (E)-5-(2-bromovinyl)- and 5-vinyl-1-beta-D-arabinofuranosyluracil (BrVaraU, VaraU) in the treatment of experimental herpes simplex virus type 1 keratitis in rabbits: comparison with (E)-5-(2-bromovinyl)-2'-deoxyuridine (BrVUdR)

The 5-substituted 1-beta-D-arabinofuranosyl (araU) analogues, (E)-5-(2-bromovinyl)-araU (BrVaraU) and 5-vinyl-araU (VaraU), which can be considered as structural analogues of (E)-5-(2-bromovinyl)-2'-deoxyuridine (BrVUdR), are potent and selective inhibitors of herpes simplex virus type 1 (HSV-1) replication in vitro. BrVaraU and VaraU have been compared with BrVUdR for their therapeutic effect on acute HSV-1 keratitis in rabbits. Both araU derivatives applied as 0.1% eyedrops suppressed the development of keratitis as monitored by the reduced number of herpes efflorescences. The healing effect of BrVaraU and VaraU was less pronounced than that of 0.1% BrVUdR eyedrops, the difference between BrVUdR and VaraU being statistically significant at the 10th day of treatment. As a further indication of the healing effect the number of cornea with opacities seen after cessation of drug treatment were 3.3, 7.4, 27.6 and 46.9% for the BrVUdR-BrVaraU-, VaraU- and placebo-treated eyes, respectively.

The role of blood platelets in nucleoside metabolism: assay, cellular location and significance of thymidine phosphorylase in human blood

The enzyme thymidine phosphorylase (thymidine: orthophosphate deoxyribosyltransferase, EC 2.4.2.4), which plays a crucial role in nucleic acid metabolism in both prokaryotic and eukaryotic cells by regulating the availability of thymidine, is present in mammalian blood. Here we describe a simple, rapid HPLC-based micromethod for the assay of blood thymidine phosphorylase. We have arbitrarily defined 1 unit of blood thymidine phosphorylase activity as the activity required to produce a 1-nM increment in the plasma concentration of thymine after incubation for 1 h at 37 degrees C with a saturating concentration of exogenous thymidine. In normal adults, whole (peripheral venous) blood thymidine phosphorylase activity with blood cells intact was 64 +/- 11 units (mean +/- S.D., n = 20, range 45-89). The apparent Michaelis constant for thymidine was of the order of 10(-4) M but varied nearly 5-fold between different individuals. Activity increased when blood cells were permeabilised or lysed with non-ionic detergents, implying that thymidine phosphorylase is an intracellular enzyme which may be influenced by exogenous as well as intracellular factors. When blood from normal donors was fractionated, thymidine phosphorylase activity consistently co-isolated with platelets. Whole-blood thymidine phosphorylase activity correlated well with platelet parameters. Although thymidine phosphorylase activity was also detected in plasma and serum, the small size and notorious fragility of platelets suggest its platelet origin. Blood from leukaemic donors showed significantly increased thymidine phosphorylase activity compared to normal controls (mean activity +/- S.D. was 96 +/- 27 units; range 58-140, n = 8). Thymine formation from thymidine was temperature- and pH-dependent in whole blood. 2'-Deoxyuridine and 3 of its 5-halogenated analogues (but not 3'-azido-3'-deoxythymidine (AZT), were catabolised by blood thymidine phosphorylase, even during blood clotting at room temperature. Assumptions about in vivo concentrations of these compounds should therefore be interpreted cautiously. In the presence of high concentrations of thymine and suitable deoxyribose donors, small amounts of thymidine were formed in some blood samples, so it is conceivable that thymidine catabolism may be reversible in vivo under some circumstances.

Human keratinocytes catabolize thymidine

Human neonatal foreskin keratinocytes incorporate exogenous thymidine into DNA and proliferate in vitro even after reaching confluence. Keratinocytes also catabolize thymidine, as reported for the first time below. Stratified cultures of keratinocytes reduced the amount of thymidine in the medium by more than 90% within 2 to 4 h. Consequently, the rate of incorporation of thymidine (0.2 microM, 4 microCi/ml) into DNA was linear for no more than 2 h. Linear incorporation of thymidine into DNA for at least 12 h could be achieved by continual addition of fresh radioactive thymidine to the culture medium. Different tissues have widely differing abilities to catabolize thymidine. Cutaneous catabolism of thymidine shows striking species differences. Soluble extracts from human neonatal foreskin and adult skin, as well as from cultivated human keratinocytes, actively catabolize thymidine. Soluble extracts of skin from mouse, rabbit, or guinea pig do not catabolize thymidine. Extracts from cultivated human fibroblasts and melanocytes have little or no ability to catabolize thymidine. Catabolism of thymidine by keratinocytes has important implications for the use of [3H]thymidine in studies of keratinocyte proliferation and for the use of thymidine analogs in therapy of cutaneous disease.