Non-nucleoside inhibitors of mitochondrial thymidine kinase (TK-2) differentially inhibit the closely related herpes simplex virus type 1 TK and Drosophila melanogaster multifunctional deoxynucleoside kinase

Anti-flavivirus Activity of Different Tritylated Pyrimidine and Purine Nucleoside Analogues

A series of tritylated and dimethoxytritylated analogues of selected pyrimidine and purine nucleosides were synthesized and evaluated for their in vitro inhibitory activity against two important members of the genus Flavivirus in the Flaviviridae family, the yellow fever (YFV) and dengue viruses (DENV). Among all compounds tested, the 5′‐O‐tritylated and the 5′‐O‐dimethoxytritylated 5‐fluorouridine derivatives exerted potency against YFV. Interestingly in the series of purine analogues, the 5′O, N‐bis‐tritylated fludarabine derivative revealed strong inhibitory activity against DENV at μm concentrations, however significantly weaker potency against YFV.

siRNA knockdown of mitochondrial thymidine kinase 2 (TK2) sensitizes human tumor cells to gemcitabine

Nucleoside metabolism enzymes are determinants of chemotherapeutic drug activity. The nucleoside salvage enzyme deoxycytidine kinase (dCK) activates gemcitabine (2', 2'-difluoro-2'-deoxycytidine) and is negatively regulated by deoxycytidine triphosphate (dCTP). Reduction of dCTP in tumor cells could, therefore, enhance gemcitabine activity. Mitochondrial thymidine kinase 2 (TK2) phosphorylates deoxycytidine to generate dCTP. We hypothesized that: (1) TK2 modulates human tumor cell sensitivity to gemcitabine, and (2) antisense knockdown of TK2 would decrease dCTP and increase dCK activity and gemcitabine activation. siRNA downregulation of TK2 sensitized MCF7 and HeLa cells (high and moderate TK2) but not A549 cells (low TK2) to gemcitabine. Combined treatment with TK2 siRNA and gemcitabine increased dCK. We also hypothesized that TK2 siRNA-induced drug sensitization results in mitochondrial damage that enhances gemcitabine effectiveness. TK2 siRNA and gemcitabine decreased mitochondrial redox status, DNA content, and activity. This is the first demonstration of a direct role for TK2 in gemcitabine resistance, or any independent role in cancer drug resistance, and further distinguishes TK2 function from that of other dTMP-producing enzymes [cytosolic TK1 and thymidylate synthase (TS)]. siRNA knockdown of TK1 and/or TS did not sensitize cancer cells to gemcitabine indicating that, among the 3 enzymes, only TK2 is a candidate therapeutic target for combination with gemcitabine.

Assessment of Anticholinesterase Activity of Gelidiella acerosa: Implications for Its Therapeutic Potential against Alzheimer's Disease

The effect of various solvent extracts of Gelidiella acerosa on acetylcholinesterase (AChE) and butyrylcholinesterase (BuChE) activities was investigated. AChE and BuChE inhibitory activities were analyzed by spectrophotometric method. Phytochemical screening of the compounds present in the solvent extracts was done qualitatively. Characterization of the compounds present in the benzene extract of G. acerosa was done by GC-MS analysis. The results showed that, at 487.80 μg/mL, benzene extract showed significant (P < 0.05) inhibitory activity against both AChE and BuChE with the percentage of inhibition 54.18 ± 5.65 % (IC(50) = 434.61 ± 26.53 μg/mL) and 78.43 ± 0% (IC(50) = 163.01 ± 85.35 μg/mL), respectively. The mode of inhibition exhibited by benzene extract against the AChE and BuChE was found to be competitive and uncompetitive type of inhibition, respectively. Preliminary phytochemical analysis coupled with GC-MS illustrates that the benzene extract possesses high amount of terpenoids, which could be the reason for potential cholinesterase inhibitory activity.

Human mitochondrial thymidine kinase is selectively inhibited by 3'-thiourea derivatives of beta-thymidine: identification of residues crucial for both inhibition and catalytic activity

Substituted 3'-thiourea derivatives of beta-thymidine (dThd) and 5'-thiourea derivatives of alpha-dThd have been evaluated for their inhibitory activity against recombinant human cytosolic dThd kinase-1 (TK-1), human mitochondrial TK-2, herpes simplex virus type 1 (HSV-1) TK, and varicella-zoster virus TK. Several substituted 3'-thiourea derivatives of beta-dThd proved highly inhibitory to and selective for TK-2 (IC(50) value, 0.15-3.1 microM). The 3'-C-branched p-methylphenyl (compound 1) and 3-CF(3)-4-Cl-phenyl (compound 7) thiourea derivatives of beta-dThd showed competitive inhibition of TK-2 when dThd was used as the variable substrate (K(i) values, 0.40 and 0.05 microM, respectively), but uncompetitive inhibition in the presence of variable concentrations of ATP (K(i) values, 15 and 2.0 microM, respectively). These kinetic properties of compounds 1 and 7 against TK-2 could be accounted for by molecular modeling showing that two hydrogen bonds can be formed between the thiourea nitrogens of compound 7 and the oxygens of the gamma-phosphate of ATP. The importance of several active-site residues was assessed by site-directed mutagenesis experiments on TK-2 and the related HSV-1 TK. The low K(i)/K(m) ratios for compounds 1 and 7 (0.38 and 0.039 against dThd, and 0.75 and 0.12 against ATP, respectively) indicate that these compounds are among the most potent inhibitors of TK-2 described so far. In addition, a striking close correlation was found between the inhibitory activities of the test compounds against TK-2 and Mycobacterium tuberculosis thymidylate kinase that is strongly indicative of close structural and/or functional similarities between both enzymes in relation to their mode of interaction with these nucleoside analog inhibitors.

Structure, physiological role, and specific inhibitors of human thymidine kinase 2 (TK2): present and future

Human mitochondrial thymidine kinase (TK2) is a pyrimidine deoxynucleoside kinase (dNK) that catalyzes the phosphorylation of pyrimidine deoxynucleosides to their corresponding deoxynucleoside 5′‐monophosphates by γ‐phosphoryl transfer from ATP. In resting cells, TK2 is suggested to play a key role in the mitochondrial salvage pathway to provide pyrimidine nucleotides for mitochondrial DNA (mtDNA) synthesis and maintenance. However, recently the physiological role of TK2turned out to have direct clinical relevance as well. Point mutations in the gene encoding TK2 have been correlated to mtDNA disorders in a heterogeneous group of patients suffering from the so‐called mtDNA depletion syndrome (MDS). TK2 activity could also be involved in mitochondrial toxicity associated to prolonged treatment with antiviral nucleoside analogues like AZT and FIAU. Therefore, TK2 inhibitors can be considered as valuable tools to unravel the role of TK2 in the maintenance and homeostasis of mitochondrial nucleotide pools and mtDNA, and to clarify the contribution of TK2 activity to mitochondrial toxicity of certain antivirals. Highly selective TK‐2 inhibitors having an acyclic nucleoside structure and efficiently discriminating between TK‐2 and the closely related TK‐1 have already been reported. It is actually unclear whether these agents efficiently reach the inner mitochondrial compartment. In the present review article,structural features of TK2, MDS‐related mutations observed in TK2 and their role in MDS will be discussed. Also, an update on novel and selective TK2 inhibitors will be provided. © 2008 Wiley Periodicals, Inc. Med Res Rev, 28, No. 5, 797–820, 2008

[18F]FDG and [18F]FLT uptake in human breast cancer cells in relation to the effects of chemotherapy: an in vitro study

Increased 2'-deoxy-2'-[18F]fluoro-D-glucose (FDG) uptake is the most commonly used marker for positron emission tomography in oncology. However, a proliferation tracer such as 3'-deoxy-3'-[18F]fluorothymidine (FLT) might be more specific for cancer. 3'-deoxy-3'-[18F]fluorothymidine uptake is dependent on thymidine kinase 1 (TK) activity, but the effects of chemotherapeutic agents are unknown. The aim of this study was to characterise FDG and FLT uptake mechanisms in vitro before and after exposure to chemotherapeutic agents. The effects of 5-fluorouracil (5-FU), doxorubicin and paclitaxel on FDG and FLT uptake were measured in MDA MB231 human breast cancer cells in relation to cell cycle distribution, expression and enzyme activity of TK-1. At IC50 concentrations, 5-FU resulted in accumulation in the G1 phase, but doxorubicin and paclitaxel induced a G2/M accumulation. Compared with untreated cells, 5-FU and doxorubicin increased TK-1 levels by >300. At 72 h, 5-FU decreased FDG uptake by 50% and FLT uptake by 54%, whereas doxorubicin increased FDG and FLT uptake by 71 and 173%, respectively. Paclitaxel increased FDG uptake with >100% after 48 h, whereas FLT uptake hardly changed. In conclusion, various chemotherapeutic agents, commonly used in the treatment of breast cancer, have different effects on the time course of uptake of both FDG and FLT in vitro. This might have implications for interpretation of clinical findings.

Tracking cellular stress with labeled FMAU reflects changes in mitochondrial TK2

PURPOSE

Fluoropyrimidines like 1-(2'-deoxy-2'-fluoro-beta-D: -arabinofuranosyl)-thymine (FMAU) and 3'-deoxy-3'-fluorothymidine (FLT) accumulate in tumors and are being used as positron emission tomography tumor-imaging tracers. Proliferating tissues with high thymidine kinase 1 (TK1) activity retain FLT; however, the mechanism of selective accumulation of FMAU in tumors and certain other tissues requires further study.

METHODS

Retention of [(3)H]FLT and [(3)H]FMAU was measured in prostate cancer cell lines PC3, LNCaP, DU145, and the breast cancer cell line MD-MBA-231, and the tracer metabolites were analyzed by high-performance liquid chromatography (HPLC). FMAU retention, thymidine kinase 2 (TK2) activity, and mitochondrial mass were determined in cells stressed by depleted cell culture medium or by treating with oxidative, reductive, and energy stress, or specific adenosine monophosphate-activated protein kinase activator, or eIF2 inhibitor. TK1 and TK2 activities and mitochondrial mass were determined by FLT phosphorylation, 1-beta-D: -arabinofuranosylthymine (Ara-T) phosphorylation, and flow cytometry, respectively.

RESULTS

FMAU retention in rapidly proliferating cancer cell lines was five to ten times lower than FLT after 10 min incubation. HPLC analysis of the cellular extracts showed that phosphorylated tracers are the main retained metabolites. Nutritional stress decreased TK1 activity and FLT retention but increased retained FMAU. TK2 inhibition decreased FMAU retention and phosphorylation with negligible effects on FLT. Oxidative, reductive, or energy stress increased FMAU retention and correlated with mitochondrial mass (r (2) = 0.88, p = 0.006). FMAU phosphorylation correlated with increased TK2 activity (r (2) = 0.87, p = 0.0002).

CONCLUSION

FMAU is preferably phosphorylated by TK2 and can track TK2 activity and mitochondrial mass in cellular stress. FMAU may provide an early marker of treatment effects.

Human UMP-CMP kinase 2, a novel nucleoside monophosphate kinase localized in mitochondria

Enzyme deficiency in the salvage pathway of deoxyribonucleotide synthesis in mitochondria can cause mtDNA depletion syndromes. We have identified a human mitochondrial UMP-CMP kinase (UMP-CMPK, cytidylate kinase; EC 2.7.4.14), designated as UMP-CMP kinase 2 (UMP-CMPK2). The C-terminal domain of this 449-amino acid protein contains all consensus motifs of a nucleoside monophosphate kinase. Phylogenetic analysis showed that UMP-CMPK2 belonged to a novel nucleoside monophosphate kinase family, which was closer to thymidylate kinase than to cytosolic UMP-CMP kinase. Subcellular localization with green fluorescent protein fusion proteins illustrated that UMP-CMPK2 was localized in the mitochondria of HeLa cells and that the mitochondrial targeting signal was included in the N-terminal 22 amino acids. The enzyme was able to phosphorylate dUMP, dCMP, CMP, and UMP with ATP as phosphate donor, but the kinetic properties were different compared with the cytosolic UMP-CMPK. Its efficacy to convert dUMP was highest, followed by dCMP, whereas CMP and UMP were the poorest substrates. It also phosphorylated the monophosphate forms of the nucleoside analogs ddC, dFdC, araC, BVDU, and FdUrd, which suggests that UMP-CMPK2 may be involved in mtDNA depletion caused by long term treatment with ddC or other pyrimidine analogs. UMP-CMPK2 mRNA expression was exclusively detected in chronic myelogenous leukemia K-562 and lymphoblastic leukemia MOLT-4 among eight studied cancer cell lines. Particular high expression in leukemia cells, dominant expression in bone marrow, and tight correlation with macrophage activation and inflammatory response suggest that UMP-CMPK2 may have other functions in addition to the supply of substrates for mtDNA synthesis.

Novel selective human mitochondrial kinase inhibitors: Design, synthesis and enzymatic activity

Selective and effective TK2 inhibitors can be obtained by introduction of bulky lipophilic chains (acyl or alkyl entities) at the 2' position of araT and BVaraU, nucleoside analogues naturally endowed with a low TK2 affinity. These derivatives showed a competitive inhibitory activity against TK2 in micromolar range. BVaraU nucleoside analogues, modified on the 2'-O-acyl chain with a terminal N-Boc amino-group, conserved or increased the inhibitory activity against TK2 (7l and 7m IC(50): 6.4 and 3.8 microM, respectively). The substitution of an ester for a carboxamide moiety at the 2' position of araT afforded a consistent reduction of the inhibitory activity (25, IC(50): 480 microM). On the contrary, modifications at 2'-OH position of araC and araG, have provided inactive derivatives against TK2 and dGK, respectively. The biological activity of a representative compound, 2'-O-decanoyl-BVaraU, was also investigated in normal human fibroblasts and was found to impair mitochondrial function due to TK2 inhibition.

N1-substituted thymine derivatives as mitochondrial thymidine kinase (TK-2) inhibitors

Novel N1-substituted thymine derivatives related to 1-[(Z)-4-(triphenylmethoxy)-2-butenyl]thymine have been synthesized and evaluated against thymidine kinase-2 (TK-2) and related nucleoside kinases [i.e., Drosophila melanogaster deoxynucleoside kinase (Dm-dNK) and herpes simplex virus type 1 thymidine kinase (HSV-1 TK)]. The thymine base has been tethered to a distal triphenylmethoxy moiety through a polymethylene chain (n = 3-8) or through a (2-ethoxy)ethyl spacer. Moreover, substitutions at position 4 of one of the phenyl rings of the triphenylmethoxy moiety have been performed. Compounds with a hexamethylene spacer (18, 26b, 31) displayed the highest inhibitory values against TK-2 (IC50 = 0.3-0.5 microM). Compound 26b competitively inhibited TK-2 with respect to thymidine and uncompetitively with respect to ATP. A rationale for the biological data was provided by docking some representative inhibitors into a homology-based model of human TK-2. Moreover, two of the most potent TK-2 inhibitors (18 and 26b) that also inhibit HSV-1 TK were able to reverse the cytostatic activity of 1-(beta-D-arabinofuranosyl)thymine (Ara-T) and ganciclovir in HSV-1 TK-expressing OST-TK-/HSV-1 TK+ cell cultures.

Bromovinyl-deoxyuridine: A selective substrate for mitochondrial thymidine kinase in cell extracts

Cellular models of mitochondrial thymidine kinase (TK2) deficiency require a reliable method to measure TK2 activity in whole cell extracts containing two interfering deoxyribonucleoside kinases, thymidine kinase 1 (TK1) and deoxycytidine kinase. We tested the value of the thymidine analog (E)-5-(2-bromovinyl)-2'-deoxyuridine (BVDU) as a TK2-specific substrate. With extracts of OSTTK1- cells containing TK2 as the only thymidine kinase and a highly specific TK2 inhibitor we established conditions to detect the low TK2 activity commonly present in cells. With extracts of TK1-proficient osteosarcoma cells and normal human fibroblasts we showed that BVDU, but not 1-(beta-d-arabinofuranosyl)thymine (Ara-T), discriminates TK2 activity even in the presence of 100-fold excess TK1. A comparison with current procedures based on TK2 inhibition demonstrated the better performance of the new TK2 assay. When cultured human fibroblasts passed from proliferation to quiescence TK2 activity increased by 3-fold, stressing the importance of TK2 function in the absence of TK1.

Synthesis and evaluation of thymine-derived carboxamides against mitochondrial thymidine kinase (TK-2) and related enzymes

Based on the structure of our previously identified mitochondrial thymidine kinase (TK-2) inhibitors, three series of thymine-derived carboxamides have been synthesized and tested against TK-2 and related enzymes. The methodology employed has been a solution-phase parallel synthesis based on the coupling of three thymine-derived acids [4-(thymin-1-yl)butyric acid (I), [4-(thymin-1-yl)-butyrylamino]acetic acid (II) and 6-(thymin-1-yl)hexanoic acid (III)] with different commercially available primary amines that carry cyano and/or phenyl groups. The couplings were performed in good yields (from 60% to 90%), with the exception of those that incorporate the highly crowded triphenylmethylamine (e). From the new synthesized compounds, the N-trityl-6-(thymin-1-yl)hexanamide (IIIe) was the most active TK-2 inhibitor (IC(50)=19+/-2microM).

Deoxyribonucleotides and disorders of mitochondrial DNA integrity

Mitochondrial DNA (mtDNA) depends on numerous nuclear encoded factors and a constant supply of deoxyribonucleoside triphosphates (dNTP), for its maintenance and replication. The function of proteins involved in nucleotide metabolism is perturbed in a heterogeneous group of disorders associated with depletion, multiple deletions, and mutations of the mitochondrial genome. Disturbed homeostasis of the mitochondrial dNTP pools are likely the underlying cause. Understanding of the biochemical and molecular basis of these disorders will promote the development of new therapeutic approaches. This article reviews the current knowledge of deoxyribonucleotide metabolism in relation to disorders affecting mtDNA integrity.

Substrate specificity and phosphorylation of antiviral and anticancer nucleoside analogues by human deoxyribonucleoside kinases and ribonucleoside kinases

Structural analogues of nucleosides, nucleoside analogues (NA), are used in the treatment of cancer and viral infections. Antiviral NAs inhibit replication of the viral genome, whereas anticancer NAs inhibit cellular DNA replication and repair. NAs are inactive prodrugs that are dependent on intracellular phosphorylation to their pharmacologically active triphosphate form. The deoxyribonucleoside kinases (dNK) and ribonucleoside kinases (rNK) catalyze the first phosphorylation step, converting deoxyribonucleosides and ribonucleosides to their corresponding monophosphate form. The dNKs have been studied intensively, whereas the rNKs have not been as thoroughly investigated. This overview is focused on the substrate specificity, tissue distribution, and subcellular location of the mammalian dNKs and rNKs and their role in the activation of NAs.

Improving the selectivity of acyclic nucleoside analogues as inhibitors of human mitochondrial thymidine kinase: replacement of a triphenylmethoxy moiety with substituted amines and carboxamides

Two series of analogues of the novel human mitochondrial thymidine kinase inhibitor 1-[(Z)-4-(triphenylmethoxy)-2-butenyl]thymine were synthesized by replacing the triphenylmethoxy moiety by a variety of substituted amines and carboxamides. In all the cases, the selectivity against the mitochondrial enzyme was either maintained or improved, and several derivatives were almost as potent as the parent compound. A molecular model was built that can account for the observed selectivities.