2 cloning and expression of mouse deoxycytidine kinase. Pure recombinant mouse and human enzymes show differences in substrate specificity

Activation of guanine-β-D-arabinofuranoside and deoxyguanosine to triphosphates by a common pathway blocks T lymphoblasts at different checkpoints

The deoxyguanosine (GdR) analog guanine-ß-d-arabinofuranoside (araG) has a specific toxicity for T lymphocytes. Also GdR is toxic for T lymphocytes, provided its degradation by purine nucleoside phosphorylase (PNP) is prevented, by genetic loss of PNP or by enzyme inhibitors. The toxicity of both nucleosides requires their phosphorylation to triphosphates, indicating involvement of DNA replication. In cultured cells we found by isotope-flow experiments with labeled araG a rapid accumulation and turnover of araG phosphates regulated by cytosolic and mitochondrial kinases and deoxynucleotidases. At equilibrium their partition between cytosol and mitochondria depended on the substrate saturation kinetics and cellular abundance of the kinases leading to higher araGTP concentrations in mitochondria. dGTP interfered with the allosteric regulation of ribonucleotide reduction, led to highly imbalanced dNTP pools with gradual inhibition of DNA synthesis and cell-cycle arrest at the G1-S boundary. AraGTP had no effect on ribonucleotide reduction. AraG was in minute amounts incorporated into nuclear DNA and stopped DNA synthesis arresting cells in S-phase. Both nucleosides eventually induced caspases and led to apoptosis. We used high, clinically relevant concentrations of araG, toxic for nuclear DNA synthesis. Our experiments do not exclude an effect on mitochondrial DNA at low araG concentrations when phosphorylation occurs mainly in mitochondria.

Novel potent inhibitors of deoxycytidine kinase identified and compared by multiple assays

Deoxycytidine kinase (dCK) phosphorylates deoxycytidine, deoxyguanosine, and deoxyadenosine and plays an important role in the salvage pathway of nucleoside metabolism. dCK is also required for the phosphorylation of several antiviral and anticancer nucleoside drugs, with resistance to these agents often being associated with a loss or decrease in dCK activity. Data also indicate a role for dCK in immune function, and dCK inhibitors may provide treatment for immune disorders. To identify novel dCK inhibitors, the authors evaluated 2 existing biochemical assays, adapted both to high-throughput screening, and identified several series of hits. They also compared the potency of the hits between purified recombinant and endogenous enzyme. Meanwhile, they also developed a novel cell-based assay that rests on the rescue of cells from dCK-dependent cytotoxic agents such as AraC. A large number of compounds were tested using the 3 assays, and a strong correlation in potency was observed between the biochemical assay using endogenous enzyme and the cell-based assay. The hits identified in these screens have proved to be good starting points for the synthesis of much more potent tool compounds to further investigate the physiological functions of dCK and potentially lead to the development of therapeutic agents.

Transcriptional regulation of the mouse deoxycytidine kinase: identification and functional analysis of nuclear protein binding sites at the proximal promoter

Deoxycytidine kinase (EC 2.7.1.74, dCK) catalyzes the phosphorylation of deoxynucleosides and several nucleoside analogues that are important in antiviral and cancer chemotherapy. The enzyme is predominantly expressed in lymphoid tissue by as yet poorly defined mechanisms. In this work, we have studied the mouse dCK regulatory region to understand the molecular details of the tissue specific expression of the enzyme. DNase I footprinting and electrophoretic mobility shift assays using nuclear extracts from mouse lymphocytes (EL-4, T cells; J558, B cells) and non-lymphoid cells (L929, fibroblasts) demonstrated the existence of at least six cis-acting elements (FP-1-FP-6) within the proximal promoter region. Functional analysis revealed that all the elements necessary to promote high level transcription of the mdCK gene are located downstream the transcription start site. 5'-Deletion and site-directed mutagenesis assays demonstrated the importance of four GC-rich regions, which bind Sp-1 and Sp-3 transcription factors. In addition, we identified a site (FP-3) located at the -282 to -310 nucleotide region of the promoter, which binds NF-1, only in B cells. Analysis of point mutations introduced at the different regions revealed functional differences in their role in mdCK transcription in the cell lines used.

Detection of an alternatively spliced form of deoxycytidine kinase mRNA in the 2'-2'-difluorodeoxycytidine (gemcitabine)-resistant human ovarian cancer cell line AG6000

Gemcitabine (2'-2'-difluorodeoxycytidine (dFdC)) is a deoxycytidine analogue that is effective against solid tumors, including lung cancer and ovarian cancer. dFdC requires the phosphorylation by deoxycytidine kinase (dCK) as a primary step in its activation. Deficiency of dCK is associated with resistance against this compound both in vitro in cancer cell lines and in clinical practice in acute myeloid leukemia and solid tumors. The human ovarian cancer cell line AG6000 is 100,000-fold resistant against dFdC compared to its parent cell line A2780. This cell line proved to be dCK deficient in enzyme activity assays and by Western blot analysis, but by RT-PCR, a normal and a truncated dCK mRNA was found. Sequencing revealed that exon 3 was deleted from the dCK cDNA, resulting in a 74-aa-long open-reading frame due to the generation of a premature stop codon. No gross genomic alteration was observed at the dCK locus, suggesting the involvement of post-transcription mechanisms. Transient transfection experiments indicated that the truncated dCK transcripts are not translated to protein. To study the functional role of the truncated dCK transcripts, both A2780 cells and AG6000 cells were stably transfected with human and rat dCK. The results indicated that over-expression of full-length dCK genes in AG6000 failed to completely reverse the sensitivity to dFdC or other drugs.

Activation of deoxycytidine kinase by gamma-irradiation and inactivation by hyperosmotic shock in human lymphocytes

Deoxycytidine kinase (dCK) is a key enzyme in the intracellular metabolism of deoxynucleosides and their analogues, phosphorylating a wide range of drugs used in the chemotherapy of leukaemia and solid tumours. Previously, we found that activity of dCK can be enhanced by incubating primary cultures of lymphocytes with substrate analogues of the enzyme, as well as with various genotoxic agents. Here we present evidence that exposure of human lymphocytes to 0.5-2 Gy dosage of gamma-radiation as well as incubation of cells with calyculin A, a potent inhibitor of protein phosphatase 1 and 2A, both elevate dCK activity without changing the level of dCK protein. When cells were gamma-irradiated in the presence of calyculin A, a more pronounced activation of dCK was observed. In contrast, both basal and stimulated dCK activities were reduced by hyperosmotic treatment of the cells. DNA repair determined by the Comet assay and by thymidine incorporation was induced by irradiation. Complete repair of gamma-irradiated DNA was detected within 1 hr following the irradiation along with dCK activation, but the rate of repair was not accelerated by calyculin A. These data provide evidence for the activation of dCK upon DNA damage and repair that seems to be mediated by phosphorylation of the enzyme, suggesting the role of dCK in DNA repair processes.

Identification of residues involved in the substrate specificity of human and murine dCK

Deoxycytidine kinase (dCK) is a salvage pathway enzyme that can phosphorylate both pyrimidine and purine deoxynucleosides, including important antiviral and cytostatic agents. Earlier studies showed that there are differences in kinetic properties between human and murine dCK, which may explain differences in toxic effects of nucleoside analogs. To determine if certain substitutions in amino acid sequences between human and mouse dCK give these differences in substrate specificity the 14 mutants and hybrid forms of human dCK were studied. All variants were characterised with dCyd, dAdo and dGuo as phosphate acceptors and ATP and UTP as phosphate donor. The relative activities with dCyd, dAdo and dGuo were about 70, 20, 30%, respectively, with UTP as compared to ATP for human dCK and 40, 60, 70% for mouse dCK. Among all tested mutants only the triple combination of substitutions Q179R-T184K-H187N (RKN) had a kinetic behaviour very similar to mouse dCK. The kinetic patterns with several important nucleoside analogs, such as AraC, CdA, ddC and AraG have also been studied. Results demonstrated 50-70% low relative capacities of the recombinant mouse and triple mutant RKN to phosphorylate this nucleoside analogs compare with human dCK. A model for dCK was used to try to explain the functional role of these amino acid substitutions. According to this model the triple mutant RKN have altered amino acids in a region necessary for conformational changes during catalyses. This may affects the substrate selectivity both for the nucleosides and the phosphate donors.

Conserved gene structure and transcription factor sites in the human and mouse deoxycytidine kinase genes

Deoxycytidine kinase (dCK) phosphorylates several anti-cancer and anti-viral nucleoside analogs. The enzyme is predominantly expressed in lymphoid tissues regulated by an unknown mechanism. We have cloned and sequenced the 20 kbp mouse dCK gene and approximately 1.7 kbp of the 5' flanking regions of both the human and mouse dCK genes. Five major inter-species conserved motifs were identified in the 5' region including the transcription initiation region, an SP1 site and two closely located putative octamer transcription factor sites. Luciferase reporter experiments showed that the human dCK 5' region efficiently initiated transcription but no tissue regulatory element could be identified.

Transduction of the human deoxycytidine kinase gene in rodent tumor cells induces in vivo growth retardation in syngeneic hosts

Deoxycytidine kinase (dCK) mediates the phosphorylation of nucleoside analogues that can be used as anti-cancer agents. We examined whether susceptibility of mouse colon carcinoma (Colon 26) and rat gliosarcoma (9L) cells to 1-beta-D-arabiofuranosylcytonsine (AraC), a chemotherapeutic agent, can be increased after the tumor cells were transduced with the human dCK gene. Expression of the dCK gene in both cell lines conferred increased sensitivity in vitro to AraC. Although their proliferation rates in vitro remained the same as those of parental cells, tumor growth of the transduced cells in syngeneic host animals was unexpectedly retarded compared with that of respective parental cells. In contrast, the growth of the transduced cells was not different from that of parental cells, when they were inoculated in T cell-defective nude mice. A histological examination revealed infiltration of eosinophils into the dCK gene-transduced but not into parental Colon 26 tumor. These data suggest that a therapeutic gene, when expressed in xenogenic animals, can be a tumor antigen which is recognized by a host defense system.

Cloning and characterization of mouse deoxyguanosine kinase. Evidence for a cytoplasmic isoform

Deoxyguanosine kinase (dGK) is a nuclear gene product that catalyzes the phosphorylation of purine deoxyribonucleosides and their analogues. The human enzyme is located predominantly in the mitochondria, as shown by biochemical fractionation studies and in situ localization of the overexpressed recombinant protein. Here we describe the cloning of mouse dGK cDNA and the identification of a novel amino-terminally truncated isoform that corresponds to about 14% of the total dGK mRNA population in mouse spleen. In situ fluorescence assays suggest that the new isoform cannot translocate into the mitochondria and thus may represent a cytoplasmic enzyme. Expression of mouse dGK mRNA was highly tissue-specific and differed from the tissue distribution observed in humans. Recombinant mouse dGK showed similar specific activity and substrate specificity as compared with the human enzyme. The broad specificity, restricted tissue distribution, and location of mouse dGK in multiple cellular compartments raise new considerations with respect to the role of the individual deoxynucleoside kinases in nucleotide metabolism.

Human thymidine kinase 2: molecular cloning and characterisation of the enzyme activity with antiviral and cytostatic nucleoside substrates

Based on amino acid sequence information from purified mitochondrial thymidine kinase (TK2), a cDNA of 1930 bp was cloned, containing an open reading frame encoding 232 amino acid residues starting with the N-terminal sequence determined from the native human protein preparation. Northern blot analysis with the cDNA coding region demonstrated several TK2 mRNAs, with 2 and 4 kb forms present in many tissues. We also characterised N-terminally truncated (starting at position 18) human TK2 with pharmacologically important antiviral and cytostatic nucleoside analogues. Results were highly similar to those with the native TK2 preparation. The anti-leukaemic drug arabinosyl cytosine is phosphorylated. The antitumour drug difluorodeoxycytidine and its metabolite difluorodeoxyuridine are good substrates, with K(m) values of 66 and 29 microM, respectively, and a relative Vmax of 0.6 compared to that of thymidine. Negative cooperativity was found with thymidine and the anti-HIV drug 3'-azidothymidine, but the reaction followed Michaelis-Menten kinetics with deoxycytidine, arabinosyl cytosine, and arabinosyl thymine. The results demonstrate a broad substrate specificity and complex kinetics, and suggest a role for TK2 in the activation of chemotherapeutic nucleoside analogues.

The intracellular localization of deoxycytidine kinase

Deoxycytidine kinase (dCK) catalyzes the rate-limiting step of the deoxynucleoside salvage pathway in mammalian cells and plays a key role in the activation of several pharmacologically important nucleoside analogs. Using a highly specific polyclonal antibody raised against a C-terminal peptide of the human dCK, we analyzed its subcellular localization by Western blots of biochemically fractionated nuclear and cytoplasmic fractions as well as by in situ immunochemistry. Native dCK was found to be located mainly in the cytoplasm in several cell types, and the enzyme was more concentrated in the perinuclear and cellular membrane area. In contrast, when dCK was overexpressed in the cells, it was mainly located in the nucleus. The results demonstrate that native dCK is a cytoplasmic enzyme. However, it has the ability to enter the nucleus under certain conditions, suggesting the existence of a cytoplasmic retention mechanism that may have an important function in the regulation of the deoxynucleoside salvage pathway.

Life on the salvage path: the deoxynucleoside kinase of Lactobacillus acidophilus R-26

In Lactobacillus acidophilus R-26, the synthesis of DNA precursor deoxynucleotides occurs exclusively by salvage of deoxynucleosides, beginning with phosphorylation by four deoxynucleoside kinases. Subunits bearing three of these activities are uniquely organized into two heterodimers, deoxyadenosine/deoxycytidine kinase (dAK/dCK) and deoxyadenosine/deoxyguanosine kinase (dAK/dGK), which, along with a distinct deoxythymidine kinase (TK), catalyze the parallel first committed steps of dNTP biosynthesis. Whereas TK is common to most prokaryotes (and eukaryotes), the other three activities that are the emphasis of this review are quite unusual in bacteria. Each activity is regulated in cis by its homologous end-product (dNTP) which is understood to act as a multisubstrate inhibitor capable of binding to both nucleoside and phosphate subsites. Conversely, the inactive dAK subunit is progressively activated by 1) association with a dGK or dCK subunit and 2) the conformationally driven heterotropic affect of dGuo or dCyd bound to the opposing subunit. Limited proteolysis has proven to be a powerful probe of conformational states. Further indication of conformational or structural differences between dAK and dGK (or dCK) is that the former follows an ordered kinetic path, while dGK or dCK exhibits rapid-equilibrium random kinetics. The multi-substrate behavior of end-product binding provides a convenient new diagnostic tool for distinguishing kinetic mechanisms. Tandem dak-dgk genes have been cloned from Lactobacillus DNA and expressed in Escherichia coli as dAK/dGK, utilizing the associated promoter. Sequence alignments reveal 65% identity in their DNA and 61% in their derived amino acid sequences. Encoded N-terminal sequences are identical for the first 18 residues, and both subunits share conserved sequences in common with adenylate kinase and viral TK. A more unusual conserved element, which appears to play a role in the activation of dAK, resembles the G2 loop of p21 ras. Remarkably, no homologous gene(s) for the dAK/dCK pair could be found. Comparisons of amino acid sequences, isoelectric pHs and subunit masses strongly indicated that native dCK and dGK are identical in sequence, except at their extreme N-termini (M-IVL for dCK and -TVIVL for dGK), suggesting that processing of a common precursor occurs in Lactobacillus. Accordingly, deletion of codons 2 and 3 from dgk resulted in the expression of dAK/dCK in the E. coli host; its kinetic properties are indistinguishable from those of native dAK/dCK. Subcloning the dgk or engineered dck gene resulted in expression of active dGK or dCK homodimers, each with a virtually unchanged Km toward its primary deoxynucleoside. However, in common with human dCK, dCK (or dGK) homodimer exhibits secondary activities with much larger Kms towards dAdo and dGuo (or dCyd). dCTP (or dGTP) is the best inhibitor of all three activities of the respective homodimer. Fully active heterodimers can be reconstituted simply by mixing a homodimer with independently expressed (inactive) dAK.

Intracellular metabolism of the N7-substituted acyclic nucleoside analog 2-amino-7-(1,3-dihydroxy-2-propoxymethyl)purine, a potent inhibitor of herpesvirus replication

We investigated the intracellular metabolism of S2242 (2-amino-7-(1,3-dihydroxy-2-propoxymethyl)purine), the only known antivirally active acyclic nucleoside analogue with the side chain substituted at the N7 position of the purine ring. Uptake of S2242 by CEM cells increased linearly with increasing extracellular concentrations of the compound and was blocked by inhibitors of nucleoside transport. S2242 was phosphorylated in a time- and concentration-dependent manner to its monophosphates, diphosphates, and triphosphates. Intracellular half-life of the diphosphates and triphosphates in CEM cells was approximately 3-6 hr. A strong correlation was found between the cytostatic action of the compound and its phosphorylation in different cell lines. In accord with the findings that (1) the cytostatic potential of S2242 is reversed by deoxycytidine (dCyd) and (2) the growth of deoxycytidine kinase-deficient (dCK-) cells is refractory to the inhibitory effect of S2242, the amount of metabolites formed from S2242 in the dCK- cell line was approximately one hundredth of that in the wild-type cells. The observation that purified dCK phosphorylates S2242 to its monophosphate further corroborates these results. The activity of S2242 against herpes simplex virus, varicella-zoster virus, and human herpesvirus type 6 was reversed by 50-100-fold on the addition of exogenous dCyd. Compound S2242 was not preferentially phosphorylated in herpes simplex virus 1-, varicella-zoster virus-, or human herpesvirus type 6-infected cells (Vero, human embryonic lung, and HSB-2 cells, respectively), and exogenously added dCyd reduced substantially the formation of S2242 metabolites in these cells. In human cytomegalovirus (HCMV)-infected human embryonic lung cells, a 5-25-fold increase in S2242 metabolite formation was observed compared with the noninfected cells, suggesting that an HCMV-encoded or -induced enzyme causes the specific phosphorylation of S2242. Exogenously added dCyd had little effect on the activity of S2242 against HCMV and on the phosphorylation of the compound in HCMV-infected cells. S2242 was not specifically phosphorylated by the HCMV-encoded UL-97 kinase in cells infected with a vaccinia/UL-97 recombinant. S2242 was found to be a substrate (K(m) = 90 microM) for purified human deoxyguanosine kinase; the latter enzyme was stimulated 3-4-fold in HCMV-infected cells.

Deoxycytidine kinase and deoxyguanosine kinase of Lactobacillus acidophilus R-26 are colinear products of a single gene

Three of the four deoxynucleoside kinases required for growth of Lactobacillus acidophilus R-26 exist as heterodimeric pairs specific for deoxyadenosine (dAK) and deoxycytidine (dCK) or dAK and deoxyguanosine (dGK). However, only two tandem genes, dak/dgk, are found, and are expressed only as dAK/dGK in transformed Escherichia coli. Sequencing peptides spanning 63% of the native dCK subunit revealed a sequence identical to that deduced from dgk (beginning MTVIVL...), except that dCK lacks residues 2 and 3 (dCK is M..IVL; dGK is .TVIVL). Also, mass spectrometry indicates that native dCK and dGK subunits are identical in mass adjusted for the first three residues. Furthermore, the native enzymes have identical isoelectric pH values, indicating an equal number of charged residues. To enable E. coli to express peptide having the native dCK sequence, codons 2 and 3 were deleted from the dgk portion of the tandem genes, resulting in expression of protein having the specificities and regulatory properties of native dAK/dCK, including heterotropic stimulation of dAK activity by deoxycytidine or dCTP (not deoxyguanosine or dGTP) and end-product inhibition of the respective activities by dATP and dCTP. Subcloning normal and mutant dgk yielded homodimeric dGK and dCK, respectively. The dCK homodimer strongly resembles human dCK, with a low K(m) for deoxycytidine, the ability to phosphorylate deoxyadenosine and deoxyguanosine at much higher K(m) values, and end-product inhibition by dCTP. Thus two distinct and specific enzymes evidently are derived from a single Lactobacillus gene. The mechanism by which this occurs in vivo has yet to be elucidated.

Cloning and expression of human mitochondrial deoxyguanosine kinase cDNA

Mammalian mitochondrial deoxyguanosine kinase (dGK) is responsible for phosphorylation of purine deoxyribonucleosides in the mitochondrial matrix. Using a RT-PCR-generated probe, based on amino acid sequence information from proteolytic fragments of purified bovine dGK, we have cloned a cDNA from a human brain cDNA library that encodes a 30 kDa protein. The deduced amino acid sequence of this protein included the sequence of all six peptides isolated and sequenced from purified dGK. Expression and purification of recombinant protein from induced Escherichia coli extracts revealed that it catalyses efficient phosphorylation of dGuo, arabinosyl guanine, dAdo, 2-chloro-2'-deoxyadenosine and dIno similar to purified dGK. Northern blot analysis demonstrated one dominant positive mRNA of 1.35 kb and it was found in several tissues at similar levels. The coding sequence of dGK showed 46% identity to the coding sequence of cytosolic deoxycytidine kinase, and conserved sequence motifs among the known deoxynucleoside kinase were identified.

On the phosphorylation of 2-chlorodeoxyadenosine (CdA) and its correlation with clinical response in leukemia treatment

The nucleoside analog 2-chlorodeoxyadenosine (CdA, Cladribine) is a chemotherapeutic agent for treatment of leukemias and lymphomas, most successfully used in hairy cell leukemia and B-cell chronic lymphocytic leukemia. CdA is phosphorylated intracellularly to its monophosphate derivative by the enzymes deoxycytidine kinase and deoxyguanosine kinase. Cell lines deficient in deoxycytidine kinase were shown to be resistant to CdA and a high deoxycytidine kinase level in combination with low 5'-nucleotidase has been proposed to partly explain the selectivity in CdA toxicity for lymphoid cells. In this report biochemical properties in CdA phosphorylation mediated by deoxycytidine kinase and deoxyguanosine kinase are reviewed and discussed in relation to the further metabolism of CdA 5'-monophosphate, the different possible mechanisms of action and the correlation with clinical response. It is concluded that much is known about the metabolism and mechanisms of action of CdA, but that the remarkable therapeutic effect in hairy cell leukemia has yet to be explicitly explained.

Differences in kinetic properties of pure recombinant human and mouse deoxycytidine kinase

Human and mouse deoxycytidine kinase (dCK) (EC 2.7.1.74) were cloned and expressed in Escherichia coli. Michaelis-Menten kinetics were determined for the purified enzymes with 2'-deoxycytidine (dCyd), 2'-deoxyadenosine (dAdo), 2-chloro-2'-deoxyadenosine (CdA), 2',3'-dideoxycytidine (ddCyd) and 9-beta-D-arabinofuranosylguanine (araG) as substrates and ATP and UTP as phosphate donors. Both human and mouse dCK showed highest affinity to dCyd with Km values of 0.05-0.2 microM. The anti-leukaemic compound CdA was the superior substrate of the nucleoside analogues tested. Both enzymes were able to efficiently utilize ATP and UTP as phosphate donors. However, the use of UTP instead of ATP as phosphate donor decreased Km values for all substrates investigated. The kinetic properties of mouse and human dCK differed in that the human enzyme showed higher affinity for the substrates dAdo, CdA, ddCyd and araG. The human enzyme also showed higher affinity for ATP and UTP. The ability to phosphorylate dCyd was, however, similar for both human and mouse dCK. At physiological concentration of the feedback inhibitor dCTP, mouse dCK showed lower activity than human dCK for all substrates investigated.

Adenosine-deaminase-deficient mice die perinatally and exhibit liver-cell degeneration, atelectasis and small intestinal cell death

We report the generation and characterization of mice lacking adenosine deaminase (ADA). In humans, absence of ADA causes severe combined immunodeficiency. In contrast, ADA-deficient mice die perinatally with marked liver-cell degeneration, but lack abnormalities in the thymus. The ADA substrates, adenosine and deoxyadenosine, are increased in ADA-deficient mice. Adenine deoxyribonucleotides are only modestly elevated, whereas S-adenosylhomocysteine hydrolase activity is reduced more than 85%. Consequently, the ratio of S-adenosylhomocysteine (AdoMet) to S-adenosyl homocysteine (AdoHcy) is reduced threefold in liver. We conclude that ADA plays a more critical role in murine than human fetal development. The murine liver pathology may be due to AdoHcy-mediated inhibition of AdoMet-dependent transmethylation reactions.

Mammalian deoxyribonucleoside kinases

The mammalian deoxyribonucleoside kinases are deoxycytidine kinase, thymidine kinase 1 and 2 and deoxyguanosine kinase. These enzymes phosphorylate deoxyribonucleosides and thereby provide an alternative to de novo synthesis of DNA precursors. Their activities are essential for the activation of several chemotherapeutically important nucleoside analogues. In recent years, these enzymes have been thoroughly characterised with regard to structure, substrate specificity and patterns of expression. In this review, these results are reviewed and furthermore, the physiologic metabolic role of the anabolic enzymes is discussed in relation to catabolic pathways. The significance of this information for the development of therapeutic protocols and choice of animal model systems is discussed. Finally, alternative pathways for nucleoside analogue phosphorylation are surveyed, such as the phosphotransfer capacity of 5'-nucleotidase.

Interspecies differences in the kinetic properties of deoxycytidine kinase elucidate the poor utility of a phase I pharmacologically directed dose-escalation concept for 2-chloro-2'-deoxyadenosine

2-Chloro-2'-deoxyadenosine (CdA, Cladribine), is a purine antimetabolite currently under investigation in phase II clinical trials for the treatment of lymphoid malignancies. Significant differences in CdA toxicity between mice and humans were observed during phase I clinical evaluation. For the elucidation of interspecies differences in drug toxicity the pharmacokinetics of CdA after subcutaneous injection and the kinetic properties of the CdA-phosphorylating enzyme, deoxycytidine kinase (dCK), were compared in mice and humans. The ratio of the dose lethal to 10% of mice (LD10) to the maximum tolerated dose (MTD) in humans was 50 and the ratio of the area under the curve obtained at approximately one-half the LD10 (AUCapprox. one-half the LD10)/AUC(MTD) was 49. A significant interspecies difference was observed in the kinetic properties of dCK, the main CdA-activating enzyme. With CdA as a substrate, the Michaelis constant (Km) of dCK in crude extracts of mouse thymus was 10 times higher than that in human thymus. An approximately 9-fold interspecies difference in maximum velocity (Vmax)/Km indicated a higher efficiency of dCK for CdA in humans than in mice. The peak plasma concentration was 210 times higher and exceeded the Km in mice. Initial and terminal half-lives were approximately 7 times shorter in mice and trough levels were similar in mice and humans. Thus, the differences in AUCs at equitoxic doses are largely explained by differences in the target enzyme properties and the pharmacokinetic pattern. The observed lower tolerance for CdA in humans as compared with mice confirms the view that antimetabolites may not be good candidates for pharmacokinetically guided dose-escalation schemes unless detailed information on interspecies variability in drug bioactivation is available.