2'-Deoxy-2'-methylenecytidine and 2'-deoxy-2',2'-difluorocytidine 5'-diphosphates: potent mechanism-based inhibitors of ribonucleotide reductase

Nucleosides and Nucleotides. 156. Chelation-Controlled and Nonchelation-Controlled Diastereofacial Selective Thiophenol Addition Reactions at the 2'-Position of 2'-[(Alkoxycarbonyl)methylene]-2'-deoxyuridines: Conversion of (Z)-2'-[(Alkoxycarbonyl)methylene]-2'-deoxyuridines into Their (E)-Isomers(1)

The Wittig reaction of 1-[3,5-O-(1,1,3,3-tetraisopropyldisiloxane-1,3-diyl)-beta-D-erythro-pentofuranos-2-ulosyl]uracil (4) with Ph(3)P=CHCO(2)R (R = ethyl or tert-butyl) exclusively gave (Z)-2'-[(alkoxycarbonyl)methylene] derivatives 5 and 13, respectively, in high yields. An unusual beta-facial selectivity of thiophenol addition to the 2'-[(alkoxycarbonyl)methylene] moiety of 5 and 13 was observed, and this facial selectivity was found to be influenced by both the thiolate counter cation and the bulkiness of the alkoxy moiety. Treatment of 2'-[(ethoxycarbonyl)methylene] derivative 5with LiSPh (1.5 equiv) in the presence of PhSH in THF selectively gave 2'beta-(phenylthio) derivative 11 in high yield along with a trace of 2'alpha-(phenylthio) derivative 10. On the other hand, when 2'-[(tert-butoxycarbonyl)methylene] derivative13 was treated with KSPh in the presence of PhSH in dioxane/DMF, the facial selectivity was reversed to selectively give the 2'alpha-(phenylthio) adduct14 (alpha:beta, 77:23) in 90% yield. Oxidation of 14 with m-chloroperbenzoic acid in CH(2)Cl(2) and subsequent pyrolysis of the resulting sulfoxides exclusively gave the (Z)-isomer 13 in 92% yield. The oxidativesyn-elimination of the (2'R)-2'-[(tert-butoxycarbonyl)methyl]-2'-deoxy-2'-thiophenoxy-5'-O-(triisopropylsilyl)uridine (17), which was obtained from 14 in two steps, exclusively gave the desired (E)-[(tert-butoxycarbonyl)methylene] derivatives 18 in 90% yield. Deprotection of 18 gave the (E)-(carboxymethylene)-2'-deoxyuridine (3). The (Z)-(carboxymethylene)-2'-deoxyuridine (2) was synthesized from 13 in a similar manner.

Nucleosides and nucleotides. 158. 1-(3-C-ethynyl-beta-D-ribo-pentofuranosyl)-cytosine, 1-(3-C-ethynyl-beta-D-ribo-pentofuranosyl)uracil, and their nucleobase analogues as new potential multifunctional antitumor nucleosides with a broad spectrum of activity

We previously designed 1-(3-C-ethynyl-beta-D-ribo-pentofuranosyl)uracil (EUrd) as a potential multifunctional antitumor nucleoside antimetabolite. It showed a potent and broad spectrum of antitumor activity against various human tumor cells in vitro and in vivo. To determine the structure-activity relationship, various nucleobase analogues of EUrd, such as 5-fluorouracil, thymine, cytosine, 5-fluorocytosine, adenine, and guanine derivatives, were synthesized by condensation of 1-O-acetyl-2,3,5-tri-O-benzoyl-3-C-ethynyl-alpha,beta-D-ribo-pentofur anose (6) and the corresponding pertrimethylsilylated nucleobases in the presence of SnCl4 or TMSOTf as a Lewis acid in CH3CN followed by debenzoylation. The in vitro tumor cell growth inhibitory activity of these 3'-C-ethynyl nucleosides against mouse leukemia L1210 and human nasopharyngeal KB cells showed that 1-(3-C-ethynyl-beta-D-ribo-pentofuranosyl)cytosine (ECyd) and EUrd were the most potent inhibitors in the series, with IC50 values for L1210 cells of 0.016 and 0.13 microM and for KB cells of 0.028 and 0.029 microM, respectively. 5-Fluorocytosine, 5-fluorouracil, and adenine nucleosides showed much lower activity, with IC50 values of 0.4-2.5 microM, while thymine and guanine nucleosides did not exhibit any activity up to 300 microM. We next evaluated the tumor cell growth inhibitory activity of ECyd and EUrd against 36 human tumor cell lines in vitro and found that they were highly effective against these cell lines with IC50 values in the nanomolar to micromolar range. These nucleosides have a similar inhibitory spectrum. The in vivo antitumor activities of ECyd and EUrd were compared to that of 5-fluorouracil against 11 human tumor xenografts including three stomach, three colon, two pancreas, one renal, one breast, and one bile duct cancers. ECyd and EUrd showed a potent tumor inhibition ratio (73-92% inhibition relative to the control) in 9 of 11 and 8 of 11 human tumors, respectively, when administered intravenously for 10 consecutive days at doses of 0.25 and 2.0 mg/kg, respectively, while 5-fluorouracil showed potent inhibitory activity against only one tumor. Such excellent antitumor activity suggests that ECyd and EUrd are worth evaluating further for use in the treatment of human cancers.

Nucleosides and Nucleotides. 151. Conversion of (Z)-2'-(Cyanomethylene)-2'-deoxyuridines into Their (E)-Isomers via Addition of Thiophenol to the Cyanomethylene Moiety Followed by Oxidative Syn-elimination Reactions(1)

The Wittig reaction of 1-[3,5-O-(1,1,3,3-tetraisopropyldisiloxane-1,3-diyl)-beta-D-erythro-pentofuranos-2-ulosyl]uracil (6) with Ph(3)P=CHCN afforded (Z)-2'-cyanomethylene derivative 7 exclusively. The (E)-isomer was accessed from its (Z)-isomer through a sequence of addition of thiophenol to the 2'-cyanomethylene moiety of the (Z)-isomer from the alpha-face, selectively, and stereoselective oxidative syn-elimination of the resulting adduct. The diastereofacial selectivity of the benzenethiolate addition to the cyanomethylene moiety was found to be influenced by participation of the 2-carbonyl group at the base moiety and steric hindrance of the sugar protecting groups. Although nucleophilic addition reactions at the 2'-position of 6 have been well-known to occur from the alpha-face selectively, treatment of 7 with LiSPh in THF unexpectedly afforded a mixture of alpha- and beta-phenylthio derivatives 8 and 9 in almost equal ratio. Furthermore, an unusual beta-facial selective addition was observed on treatments of 7 with PhSAlMe(2) in CH(2)Cl(2) or with LiSPh in the presence of Mg(ClO(4))(2) in THF. On the other hand, when (Z)-2'-(cyanomethylene)-5'-O-triisopropylsilyl derivative 10 was treated with LiSPh, the alpha-phenylthio derivative 13 was obtained predominantly (89:11). Oxidation of 8 with m-chloroperbenzoic acid (m-CPBA) in CH(2)Cl(2) and pyrolysis of the resulting sulfoxides afforded the (Z)-isomer 7 exclusively. Treatment of 13 with m-CPBA under the same conditions afforded the desired (E)-cyanomethylene derivatives 18 as a major product (E:Z = 14:1) in good yield. Deprotection of 18 by the standard procedures furnished (E)-2'-(cyanomethylene)-2'-deoxyuridine (5).

Gemcitabine and radiosensitization in human tumor cells

Gemcitabine is a nucleoside analogue with excellent clinical activity against solid tumors. Within the cell, gemcitabine is rapidly phosphorylated to its active di- and triphosphate metabolites. Cytotoxicity with gemcitabine appears to be related to multiple effects on DNA replication, where gemcitabine triphosphate can serve as both an inhibitor and substrate for DNA synthesis. Gemcitabine diphosphate inhibits ribonucleotide reductase, producing decreases in cellular dNTP pool levels in a cell-specific manner. These two major characteristics of gemcitabine, reduction in cellular dNTP pools and incorporation into DNA, are features of other antimetabolites antitumor agents which also exhibit radiosensitizing properties. Based on these favorable metabolic characteristics and the clinical activity of gemcitabine in tumor types which are commonly treated with radiation, the ability of gemcitabine to enhance X-radiation induced cytotoxicity was evaluated. Gemcitabine has been shown to be a potent radiosensitizer in a variety of tumor cell lines, including HT-29 colorectal carcinoma, pancreatic cancer, breast, non-small cell lung and head and neck cancer cell lines. Gemcitabine was most effective as a radiosensitizer when administered at least 2 hours prior to irradiation. For most cell lines, radiosensitization was evident at non-cytotoxic concentrations. The extent of radiosensitization increased with both increasing gemcitabine concentration and duration of exposure. Radiosensitization did not require redistribution of cells into a more radiosensitive phase of the cell cycle. The major metabolic effects observed under radiosensitizing conditions were the accumulation of high levels of gemcitabine triphosphate, and a selective decrease in the cellular dATP pool. The pattern of dATP decrease paralleled the increase in radiosensitization, whereas the level of gemcitabine triphosphate was not associated with the enhanced sensitivity to radiation. Compared to other radiosensitizers, the advantage of gemcitabine is that is can induce radiosensitization at concentrations that are 1000 times lower than typical plasma levels obtained with this drug. These studies will be used as guidelines for developing clinical trials of gemcitabine with radiation.

Inactivation of Escherichia coli ribonucleotide reductase by 2'-deoxy-2'-mercaptouridine 5'-diphosphate. Electron paramagnetic resonance evidence for a transient protein perthiyl radical

Ribonucleotide reductase catalyzes a key step in DNA biosynthesis and repair, supplying the cell with the four common deoxyribonucleotides. It is thus the target of antiproliferative agents. The enzyme consists of two subunits named protein R1 and protein R2. R1 provides the sites for the nucleotide substrates and redox-active cysteines required for catalysis. R2 harbors a tyrosyl radical essential for activity. We show here that 2'-deoxy-2'-mercaptouridine 5'-diphosphate, a substrate analog, is a very efficient inactivator of ribonucleotide reductase (Ki = 35 microM, Kinact = 0.18 s-1). Inactivation is due to specific scavenging of the protein R2 tyrosyl radical. This unique feature sets this compound apart from other mechanism-based inhibitors such as 2'-azido-or 2'-chloro-2'-deoxyribonucleotide which induce partial or total protein R1 inactivation. During reaction, a transient organic radical was detected by EPR spectroscopy. Its g anisotropy (gz = 2.0620, gy = 2.0265, and gx = 2.0019) and its hyperfine structure are consistent with a perthiyl RSS. radical. The loss of the hyperfine structure by deuterium labeling of the beta protons of R1 cysteines unambiguously shows that the perthiyl radical is located on protein R1. We thus conclude that inactivation of ribonucleotide reductase by 2'-deoxy-2'-mercaptouridine 5'-diphosphate is due to an irreversible transfer of the radical located on protein R2 to a cysteine residue of protein R1.

Inactivation of ribonucleotide reductase by (E)-2'-fluoromethylene-2'-deoxycytidine 5'-diphosphate: a paradigm for nucleotide mechanism-based inhibitors

Ribonucleotide reductase (RDPR) from Escherichia coli catalyzes the conversion of nucleotides to deoxynucleotides and is composed of two homodimeric subunits: R1 and R2. (E)- and (Z)-2'-fluoromethylene-2'-deoxycytidine 5'-diphosphate (FMCDP) are time dependent inactivators of this protein, with approximately 1.5 equiv being sufficient for complete loss of catalytic activity. Inactivation results from loss of the essential tyrosyl radical on R2 and alkylation of R1. Studies using electron spin resonance spectroscopy reveal that tyrosyl radical loss is accompanied by formation of a new, substrate-based radical. Experiments using [6'-14C]-(E)-FMCDP and [5-3H]-(E)-FMCDP reveal that alkylation of R1 is accompanied by release of 0.5 equiv of cytosine and 1.4 equiv of fluoride ion. When R1 is denatured subsequent to inactivation, approximately 1 equiv of label per R1 is observed only in studies carried out with [14C]FMCDP. Under these same conditions with [3H]FMCDP, 1.5 equiv of radiolabel is detected as cytosine. Inactivation of R1 thus results from alkylation by the sugar moiety of FMCDP. While studies to isolate the alkylated amino acid on R1 were unsuccessful, studies using a variety of site-directed mutants of R1 (C462S, C225S, C754/759S, C439S, and E441Q) indicate that E441 or possibly C439 is the modified residue. Inactivation is accompanied by rapid formation of a new chromophore with a lambda max at 334 nm. Dithiothreitol does not protect the enzyme against inactivation by FMCDP, although it does prevent chromophore formation. Two possible mechanisms are proposed to accommodate these experimental observations.

Synthesis and biological activity of 3- and 5-amino derivatives of pyridine-2-carboxaldehyde thiosemicarbazone

A series of 3- and 5-alkylamino derivatives, as well as other structurally modified analogues of pyridine-2-carboxaldehyde thiosemicarbazone, have been synthesized and evaluated as inhibitors of CDP reductase activity and for their cytotoxicity in vitro and antineoplastic activity in vivo against the L1210 leukemia. Alkylation of 3- and 5-amino-2-(1,3-dioxolan-2-yl)pyridines (1, 2) resulted in corresponding 3-methylamino, 5-methylamino, 3-allylamino, 5-ethylamino, 5-allylamino, 5-propylamino, and 5-butylamino derivatives (5, 6, and 11-15), which were then condensed with thiosemicarbazide to yield the respective thiosemicarbazones (7, 8, and 16-20). Oxidation of 3,5-dinitro-2-methylpyridine (21) with selenium dioxide, followed by treatment with ethylene glycol and p-toluenesulfonic acid, produced the cyclic ethylene acetal, 23. Oxidation of 2-(1,3-dioxolan-2-yl)-4-methyl-5-nitropyridine (26) with selenium dioxide, followed by sequential treatment with sodium borohydride, methanesulfonyl chloride, and morpholine afforded the morpholinomethyl derivative 30. Catalytic hydrogenation of 23 and 30 with Pd/C yielded the corresponding amino derivatives 24 and 31. Catalytic hydrogenation of 5-cyano-2-methylpyridine (33) with Raney nickel, followed by treatment with acetic anhydride, gave the amide derivative 35. N-Oxidation of 35, followed by rearrangement with acetic anhydride, produced the acetate derivative, 5-[(acetylamino)methyl]-2-(acetoxymethyl)pyridine (37). Repetition of the N-oxidation and rearrangement procedures with compound 37 yielded the diacetate derivative 39. Condensation of compounds 24, 31, and 39 with thiosemicarbazide afforded the respective 3,5-diaminopyridine-, 4-(4-morpholinylmethyl)-5-aminopyridine-, and 5-(aminomethyl)pyridine-2-carboxaldehyde thiosemicarbazones (25, 32, and 40). The most biologically active compounds synthesized were the 5-(methylamino)-, 5-(ethylamino)-, and 5-(allylamino)pyridine-2-carboxaldehyde thiosemicarbazones (8, 17, and 18), which were potent inhibitors of ribonucleotide reductase activity with corresponding IC50 values of 1.3, 1.0, and 1.4 microM and which produced significant prolongation of the survival time of L1210 leukemia-bearing mice, with corresponding optimum % T/C values of 223, 204, and 215 being obtained when administered twice daily for six consecutive days at dosages of 60, 80, and 80 mg/kg, respectively.

Radiosensitization of pancreatic cancer cells by 2',2'-difluoro-2'-deoxycytidine

PURPOSE

We have reported that the deoxycytidine analog 2',2'difluoro-2'-deoxycytidine (dFdCyd) is a potent radiosensitizer of HT29 human colon cancer cells probably through its effects on intracellular deoxyribonucleotide (dNTP) pools. Because dFdCyd has activity against pancreatic cancer in clinical trials, we wished to determine if dFdCyd would radiosensitize human pancreatic cancer cells.

METHODS AND MATERIALS

We assessed the effect of dFdCyd on radiation sensitivity of two human pancreatic cancer cell lines, Panc-1 and BxPC-3. To begin to investigate the mechanism of sensitization, we determined the effect of dFdCyd on dNTP pools and cell cycle distribution.

RESULTS

We found that dFdCyd produced radiation enhancement ratios of 1.7-1.8 under noncytotoxic conditions in both cell lines. Sensitization was not associated with intracellular levels of 2',2'-difluoro-2'-deoxycytidine triphosphate, the cytotoxic metabolite of dFdCyd, but occurred when dATP pools were depleted below the level of approximately 1 micromolar. Although both cell lines showed substantial cell cycle redistribution after drug treatment, the flow cytogram of the BxPC-3 cells would not, by itself, be anticipated to result in increased radiation sensitivity.

CONCLUSIONS

These findings demonstrate that dFdCyd is a potent radiation sensitizer of human pancreatic cancer cells and support the development of a clinical protocol using combined dFdCyd and radiation therapy in the treatment of pancreatic cancer.

Inhibitors of ribonucleotide reductase. Comparative effects of amino- and hydroxy-substituted pyridine-2-carboxaldehyde thiosemicarbazones

A new series of alpha-(N)-heterocyclic carboxaldehyde thiosemicarbazones (HCTs) was studied for their effects on L1210 cell growth in culture, cell cycle transit, nucleic acid biosynthesis and ribonucleotide reductase activity. 3-Aminopyridine-2-carboxaldehyde thiosemicarbazone (3-AP) and 3-amino-4-methylpyridine-2-carboxaldehyde thiosemicarbazone (3-AMP) were the most active compounds tested with respect to inhibition of cell growth and ribonucleotide reductase activity. 5-Aminopyridine-2-carboxaldehyde thiosemicarbazone (5-AP) and 4-methyl-5-aminopyridine-2-carboxaldehyde thiosemicarbazone (5-AMP) were slightly less active. 3-AP, 3-AMP, 5-AP and 5-AMP inhibited the incorporation of [3H]thymidine into DNA without affecting the rate of incorporation of [3H]uridine into RNA. The uptake and incorporation of [14C]cytidine into cellular ribonucleotides and RNA, respectively, were not decreased by 3-AP or 3-AMP; however, the incorporation of cytidine into DNA via ribonucleotide reductase was inhibited markedly. Thus, a pronounced decrease in the formation of [14C]deoxyribonucleotides from radioactive cytidine occurred in the acid-soluble fraction of 3-AP- and 3-AMP-treated L1210 cells. Consistent with an inhibition of DNA replication that occurred at relatively low concentrations of 3-AP and 3-AMP, cells gradually accumulated in the S-phase of the cell cycle; at higher concentrations of 3-AP and 3-AMP, a more rapid accumulation of cells in the G0/G1 phase of the cell cycle occurred, with the loss of the S-phase population, implying that a second less sensitive metabolic lesion was created by the HCTs. N-Acetylation of 3-AMP resulted in a compound that was 10-fold less active as an inhibitor of ribonucleotide reductase activity and 8-fold less active as an inhibitor of L1210 cell growth. N-Acetylation of either 5-AP or 5-AMP did not alter the inhibitory properties of these compounds. The results obtained provide an experimental rationale for the further development of the HCTs, particularly 3-AP and 3-AMP, as potential drugs for clinical use in the treatment of cancer.

2'-Deoxy-2'-methylene derivatives of adenosine, guanosine, tubercidin, cytidine and uridine as inhibitors of L1210 cell growth in culture

The 2'-deoxy-2'-methylene derivatives of adenosine (MdAdo), guanosine (MdGuo), tubercidin (MdTu), cytidine (MdCyd) and uridine (MdUrd) were synthesized as mechanism-based inhibitors directed at ribonucleotide reductase. It was shown that MdCyd 5'-diphosphate irreversibly inactivated ribonucleotide reductase from Escherichia coli (Baker et al., J Med Chem 34: 1879-1884, 1991). In studies reported here, MdAdo/EHNA, MdGuo and MdCyd inhibited L1210 cell growth with IC50 values of 3.4, 10.6 and 1.4 microM, respectively. Since MdAdo is a substrate for adenosine deaminase, the presence of EHNA was required to give maximal growth inhibition. 8-Aminoguanosine was not required to maximize the cytotoxic effects of MdGuo. The 2'-deoxy-2'-methylene derivatives of tubercidin and uridine did not inhibit L1210 cell growth at concentrations as high as 50 microM (MdTu) or 100 microM (MdUrd). L1210 cell lines resistant to hydroxyurea (directed at the non-heme iron subunit of ribonucleotide reductase) or deoxyadenosine (directed at the effector binding subunit of ribonucleotide reductase) were not resistant to MdCyd. An L1210 cell line that was highly resistant to dGuo due to the loss of a relatively specific deoxyribonucleoside kinase (Cory et al., J Biol Chem 268: 405-409, 1993) had a 6.6-fold increase in the IC50 value toward MdCyd, but showed only a 2-fold increase in resistance to MdGuo. Another L1210 cell line that was markedly deficient in adenosine kinase activity was highly resistant to MdAdo. Analysis by flow cytometry showed that MdCyd showed the transit of the cells through the G2/M phase of the cell cycle resulting in the buildup of the G2/M population. MdAdo, MdGuo and MdCyd inhibited the incorporation of [14C]cytidine into DNA without an effect on RNA synthesis or total cellular uptake of [14C]cytidine. The conversion of [14C]cytidine to deoxycytidine nucleotides was partially inhibited by MdGuo, but not by MdAdo or MdCyd. These data show that the 2'-deoxy-2'-methylene derivatives of adenosine, guanosine and cytidine are activated via specific nucleoside kinases and that the modes of action of these compounds are not identical.

Kinetic studies on 2',2'-difluorodeoxycytidine (Gemcitabine) with purified human deoxycytidine kinase and cytidine deaminase

Phosphorylation of cytosine analogs by deoxycytidine kinase (dCK) and deamination by cytidine deaminase (CDA) are two important processes in the activation and elimination of these drugs. We have investigated the kinetic parameters of 2',2'-difluorodeoxycytidine (dFdC) using purified enzymes from human cells. Deoxycytidine (CdR) and dFdC had Km values of 1.5 and 4.6 microM for dCK, respectively. Feedback inhibition of dCK by deoxycytidine 5'-triphosphate (dCTP) was also studied. Our results show that dCTP produced a greater inhibition of the phosphorylation of dFdC than CdR with concentrations of dCTP ranging from 1 to 25 microM. dFdC was a good substrate for CDA. Kinetic studies with this enzyme gave Km values for CdR and dFdC of 46.3 and 95.7 microM, respectively. The effect of competitive inhibitors of CDA on the deamination of dFdC was also investigated. Diazepinone riboside was a more potent inhibitor than tetrahydrouridine using either CdR or dFdC as the substrate. Inhibitors of CDA could be useful in clinical trials in patients with cancer to increase the chemotherapeutic effectiveness of dFdC.

Modulation of deoxycytidylate deaminase in intact human leukemia cells. Action of 2',2'-difluorodeoxycytidine

Cellular metabolism studies had demonstrated previously that low cellular concentrations of 2',2'-difluorodeoxycytidine (dFdC) nucleotides are eliminated by deoxycytidylate deaminase (dCMPD), whereas dCMPD activity is inhibited at high cellular dFdC nucleotide levels (Heinemann et al., Cancer Res 52: 533-539, 1992). An assay for measuring dCMPD activity in intact human leukemia cells has now been developed to permit investigations of the interactions of dFdC nucleotides with dCMPD in intact cells in which the regulated nature of this enzyme was not disrupted. Using [14C]dCyd as the substrate, radioactivity that accumulated in dTTP was quantitated after high-pressure liquid chromotography by a radioactive flow detector. The assay was first characterized using either the dCMPD inhibitor tetrahydrodeoxyuridine (H4dUrd) which directly inhibits dCMPD, or thymidine and 5-fluoro-2'-deoxyuridine (FdUrd) which indirectly inhibit and activate dCMPD, respectively, by affecting the cellular dCTP:dTTP value. Measured by this in situ assay, there was a strong correlation between dCMPD activity and dCTP:dTTP levels. Consistent with previous studies using partially purified enzyme, incubation of cells with dFdC resulted in a concentration-dependent inhibition of dCMPD in situ. The mechanism of modulation of dCMPD by dFdC, however, was clearly different from that of thymidine and FdUrd. In addition to the effect of dFdC on cellular dCTP:dTTP, our findings also suggested an additional inhibitory mechanism, possibly a direct interaction between dCMPD and dFdC 5'-triphosphate. Thus, results obtained using this direct assay of dCMPD in intact cells support the hypothesis that dCMPD is inhibited by nucleotides of dFdC.

8-Azidoadenosine and ribonucleotide reductase

Inhibitors of ribonucleotide reductase are potential antiproliferative agents, since they deplete cells from DNA precursors. Substrate nucleoside analogues, carrying azido groups at the base moiety, are shown to have strong cytostatic properties, as measured by the inhibition of the incorporation of thymidine into DNA. One compound, 8-azidoadenosine, inhibits CDP reduction in cytosolic extracts from cancer cells. The corresponding diphosphate behaves as a substrate for ribonucleotide reductase while the triphosphate is an allosteric effector.