Ribonucleotide-diphosphate reductases inhibition

Hydroxyurea is an oral agent that inhibits ribonucleotide diphosphate reductase and interferes with the synthesis of DNA, specifically the S phase of the cell cycle. Sinclair et al. have demonstrated in preclinical animal models that hydroxyurea may inhibit cells from leaving the Gj radiosensitive phase and entering the radioresistant S phase (9). Early studies have demonstrated little or no efficacy associated with the use of single-agent hydroxyurea (10). It has received FDA approval in the use of head and neck cancer when administered concomitantly with radiotherapy based upon promising results from earlier studies (11). 

Deoxy-2 -difluoromethylene nucleosides, also of interest in connection with the inhibition of ribonucleotide diphosphate reductase, have been made by a modified Julia synthesis as outlined in Scheme 13 the cytosine analogue was also reported. 0 OEt... 

Hydroxyurea (Fig. 13) inhibits the growth of a number of tumours, and has a rapid but temporary effect on DNA synthesis without concomitant effects on RNA or protein s3mthesis. This derivative appears to act by inhibiting ribonucleotide diphosphate reductase, which is responsible for converting ribonucleotide diphosphates to their corresponding deoxyribonucleotides. The evidence for this mechanism is that its effects can be reversed by deoxyuridine, deoxycytidine, and thymidine. 

Hydroxyurea suppresses DNA synthesis by inhibiting ribonucleoside diphosphate reductase, which catalyzes the reduction of ribonucleotides to deoxyribonucleotides. Hydroxyurea is used in chronic cases of granulocytic leukemia that are unresponsive to busulfan. In addition, it is used for acute lymphoblastic leukemia. Hydroxyurea may cause bone marrow depression. 

The control of ribonucleotide reductase activity is affected in the classic feedback fashion by cellular nucleotide concentrations. dATP inhibits the reduction of all four ribonucleoside diphosphates. dTTP inhibits the reduction of only CDP and UDP. ATP is the positive effector for the reduction of these two nucleotides, and both dTTP and dGTP stimulate the reduction of GDP and ADP. Hydroxyurea, an antitumor agent, inhibits ribonucleotide reductase, and this depletes the deoxyribonucleotide supply required for tumor DNA biosynthesis. 

Purine deoxyribonucleotides are derived primarily from the respective ribonucleotide (Fig. 6.2). Intracellular concentrations of deoxyribonucleotides are very low compared to ribonucleotides usually about 1% that of ribonucleotides. Synthesis of deoxyribonucleotides is by enzymatic reduction of ribonucleotide-diphosphates by ribonucleotide reductase. One enzyme catalyzes the conversion of both purine and pyrimidine ribonucleotides and is subject to a complex control mechanism in which an excess of one deoxyribonucleotide compound inhibits the reduction of other ribonucleotides. Whereas the levels of the other enzymes involved with purine and pyrimidine metabolism remain relatively constant through the cell cycle, ribonucleotide reductase level changes with the cell cycle. The concentration of ribonucleotide reductase is very low in the cell except during S-phase when DNA is synthesized. While enzymatic pathways, such as kinases, exist for the salvage of pre-existing deoxyribosyl compounds, nearly all cells depend on the reduction of ribonucleotides for their deoxyribonucleotide... 

Earlier data and those derived from these inhibition studies can be summarized in a model of ribonucleoside diphosphate reductase in which the active site is formed both from B1 and B2. It contains active dithiols contributed by Bl and a free radical contributed by B2. The active dithiols donate the electrons required for ribonucleotide reduction while participating in catalysis the function and nature of the free radical remain unknown. 

Gemcitabine is intracellularly activated by nucleoside kinases to diphosphate and triphosphate nucleosides. Gemcitabine diphosphate inhibits DNA synthesis by inhibiting ribonucleotide reductase while gemcitabine triphosphate competes with deoxycytidine triphosphate for incorporation into DNA. Gemcitabine is used for the treatment of non-small cell lung carcinoma and of adenocarcinoma of the pancreas. It has to be administred intravenously and is eliminated by metabolism with an elimination half-life of approximately 50 minutes. Its spectrum of adverse effects is comparable to that of 5-FU. 

Gemcitabine is phosphorylated initially by the enzyme deoxycytidine kinase and then by other nucleoside kinases to the di- and triphosphate nucleotide forms, which then inhibit DNA synthesis. Inhibition is considered to result from two actions inhibition of ribonucleotide reductase by gemcitabine diphosphate, which reduces the level of deoxyribonucleoside triphosphates required for the synthesis of DNA and incorporation of gemcitabine triphosphate into DNA. Following incorporation of gemcitabine nucleotide, only one additional nucleotide can be added to the growing DNA strand, resulting in chain termination. 

Inhibition of DNA synthesis is brought about by the action of dTTP as an allosteric inhibitor of ribonucleotide reductase (Reichard et al., 1961 Moore and Hurlbert, 1966 Brown and Reichard, 1969 Rummer et al., 1978). This enzyme is responsible for reducing all four ribonucleoside diphosphates (NDP) to the corresponding de-oxyribonucleoside diphosphates (dNDP). It is subject to a complex allosteric control which has been most studied with the bacterial enzyme. Most studies with the mammalian enzyme show it to be similar to the bacterial enzyme (Fig.11.7). 

Hydroxyurea also inhibits DNA synthesis by its action on the M2 subunit of ribonucleotide reductase, but in this case it is the reduction of the purine nucleoside diphosphates which is inhibited and the pool of dTTP rises slightly (Turner et al., 1966 Adams and Lindsay, 1967 Krakoff et al., 1968 Adams et al., 1971 Skoog and Bjursell, 1974 Thelander et al., 1984). What prevents the pool rising dramatically is not clear, but some mechanism comes into play to reduce turnover of the dTTP pool (Nicander and Reichard, 1985). Its action is most satisfactorily reversed by changing the medium for drug free medium. 

Pentostatin (Fig. 42.25) is a ring-expanded purine ribonuoleoside that inhibits adenosine deaminase and is used in the treatment of hairy cell leukemia. The elevated levels of deoxyadenosine triphosphate that result from inhibition of this degradative enzyme inhibit the action of ribonucleotide reductase (the enzyme that converts ribose diphosphate to deoxyribose diphosphate), thus halting DNA synthesis within the tumor cell. 

Hydroxyurea inhibits ribonucleotide reductase. By sequestering ferric ions, hydroxyurea destabilizes the organic bee radical in the R2 subunit of the enzyme. The inhibition of enzyme activity leads to a depletion of deoxyribonucleoside diphosphates, which are normally converted to deoxyribonucleoside triphosphates, the substrates for DNA synthesis. 

Mechanism of action Gemcitabine is a prodrug. Following entry into cells by nucleoside transporters, it is phosphorylated by deoxycytidine kinase and converted to active nucleosides, gemcitabine diphosphate (dFdCDP) and triphosphate (dFdCTP) [36 ]. These metabolites have a synergistic effect dFdCDP inhibits ribonucleotide reductase, which depletes the deoxyribonu-cleotide pools needed for DNA synthesis and enhances incorporation of dFdCTP into DNA [2 ] dFdCTP inhibits DNA polymerase and once incorporated into DNA results in chain termination incorporation of dFdCTP leads to evasion of detection by DNA repair enzymes. 

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