Search

Bunch bean plants were treated with the pyrimidine analogues 2-thiouracil (TU) and 5-fluorouracil (FU). Plants showed a progressive stunting or retardation in both stem height and length of the longest adventitious root with increasing concentrations of analogue. Transverse sections of the stem at both the hypocotyledonary and epicotyledonary regions revealed a retardation of tissue development (a more juvenile appearance than that of the stem of the control plants), increased cambial activity, meristematic activity within the pith, and stimulation of adventitious root production. Sustained treatment with TU resulted in an earlier maturation of the root, as evidenced by secondary growth.

Bunch bean plants were treated with the pyrimidine analogues 2-thiouracil (TU) or 5-fluorouracil (FU). A progressive inhibition of the rate of leaf formation was observed with increasing concentrations of the analogues. External effects on the simple leaves included curling and a rugose appearance. Compound leaves exhibited asymmetry, distortion, and chlorosis. Treatment with the analogues also resulted in a reduction or promotion of leaflet number in compound leaves, but a true reversion to the juvenile chordate leaf type (simple) was not observed. Therefore, leaf segmentation is affected by TU and FU, but heteroblastic development is not. The internal effects were retardation of overall development with a persistence of the juvenile configuration of tissues and irregular local increases in the number of cell layers in the spongy mesophyll.

The effects of several base analogues and cycloheximide on RNA synthesis, protein synthesis, and cell elongation were studied in excised soybean hypocotyl. None of the pyrimidine analogues tested affected growth or protein synthesis; only 5-fluorouracil appreciably inhibited RNA synthesis. 8-Azaguanine and 6-methylpurine markedly inhibited RNA and protein synthesis and cell elongation. Cycloheximide effectively inhibited both cell elongation and protein synthesis. The results show that 5-fluorouracil selectively inhibited ribosomal and soluble RNA synthesis without affecting the synthesis of D-RNA. These results indicate that the requirement for RNA synthesis to support continued protein synthesis and cell elongation is restricted to the synthesis of D-RNA.5-Fluorouracil was incorporated into all classes of RNA in a form believed to be 5-fluorouridylic acid. Cycloheximide markedly inhibited the accumulation of ribosomal RNA, but the results indicate that CH did not inhibit, per se, the synthesis of ribosomal RNA. The accumulation of newly synthesized D-RNA was only slightly affected by cycloheximide. These results show that the inhibition of cell elongation by cycloheximide correlates with the inhibition of protein synthesis, but not with the effect on RNA metabolism.

Nucleoside diphosphokinase (NDP kinase) was identified in high concentrations in human erythrocytes, and a procedure was developed for the isolation of this enzyme. The best preparations had about 1400-fold greater specific activity (65 units per mg of protein) than that of hemolysates. Like crystalline yeast NDP kinase and partially purified NDP kinase preparations from various tissues, the erythrocytic enzyme is relatively nonspecific with regard to nucleotide substrates. Although there are differences in the kinetic constants, the di- and triphosphate nucleotides which contain natural purine and pyrimidine bases, and ribose or deoxyribose, and in addition a number of nucleotides which contain purine or pyrimidine analogues all serve as substrates. A kinetic analysis was performed which included initial velocity and alternative substrate studies, and study of inhibition by 5'-monophosphate nucleotides. The results of these studies were consistent with the concept that the erythrocytic NDP kinase follows a "ping-pong" mechanism. This permitted the postulation that the reactive intermediate is the phosphorylated enzyme.

The sensitivity of bacterial species to the lethal effects of radiations and alkylating agents is related to the base composition of their DNA; high guanine and cytosine (GC) content confers increased sensitivity to X rays and mustards and increased resistance to ultraviolet. Preliminary results suggest that differences in average GC content of different markers in Bacillus subtilis DNA may be detectable by radiosensitivity analysis. Purine starvation, thymine starvation, or incubation with certain purine and pyrimidine analogues may alter bacterial radiation responsiveness, as reflected in changes in the slope and/or shoulder of dose-survival curves; the responses differ characteristically with respect to such parameters as time of onset, reversibility, relative expression with X rays versus ultraviolet, oxygen-dependence, and glycerol-dependence. Available evidence pertaining to the molecular mechanisms of these radiosensitization phenomena is reviewed.

6-Azauracil caused a marked reduction in the number and size of local lesions on excisedNicotiana glutinosa leaves or leaf discs inoculated with tobacco mosaic virus or its nucleic acid. The amount of infectivity recovered from tobacco (N. tabacum ‘White Burley’) leaf discs floated on the pyrimidine analogue solution and subsequently ground and assayed 24, 48, 72, and 96 h after inoculation with intact virus was reduced in the 24, 48, and 72 h series but reached the same level as that of the water-treated control discs in the 96 h series.

SUMMARY: In growing cultures of Salmonella typhimurium, flagellar e-N-methyllysine (NML) can be isotopically labelled with either L-[Me-14C]methionine or L-[U-14C]lysine. This isotopic labelling of flagellar NML was unaffected by the addition of DL-NML to the incubation medium. The methionine analogues, D-methionine and DL-ethionine, competitively inhibited the incorporation of the methyl group from methionine into flagellar NML, but had no effect on the incorporation of L-[U-14C]lysine. Although it is probable that methylation of lysine occurs after it has been incorporated into the flagellin molecule, it has not been possible by using antibiotics and purine and pyrimidine analogues to differentiate between flagellar synthesis and the formation of flagellar NML. Attempts to obtain a cell-free preparation capable of synthesizing NML have so far been unsuccessful.

Flanagan, John F. (University of Pennsylvania, Philadelphia), and Harold S. Ginsberg . Role of ribonucleic acid biosynthesis in multiplication of type 5 adenovirus. J. Bacteriol. 87: 977–987. 1964.—The requirement for ribonucleic acid (RNA) biosynthesis in the multiplication of type 5 adenovirus was investigated by using radioactive phosphorus to label nucleic acids and two pyrimidine analogues, 6-azauridine and 5-fluorouracil or 5-fluorouridine, to inhibit synthesis of functional RNA. The data indicate that biosynthesis of RNA after infection is essential for production of virus-specific deoxyribonucleic acid, virus antigens, and infectious particles. The onset of essential RNA synthesis occurs 8 to 9 hr after virus infection and prior to the biosynthesis of other known virus-induced macromolecules.

Anticytomegalovirus experiments were carried out on 38 classes of compounds containing 320 materials of known or potential biological activity. All experiments were carried out in tubes using WI-38 cells with the test compounds added within minutes after the virus and then at additional times in medium changes 2 and 4 days later. Antiviral activity was determined by microscopic demonstration of inhibition of viral cytopathogenic effects. Thirty compounds were considered markedly active against the virus. These were the amino acid antagonists aminopterin and N-[3,5-dichloro-4-(2,4-diamino-6-pteridinyl-methylmethylamino)benzoyl]glutamic acid; the unsubstituted lactone, camptothecin; 10 purine analogs, including 8 thiopurines, 9-Β-D-arabinofuranosyladenine and purine-6-carboxaldehyde thiosemicarbazone; 13 pyrimidine analogs and 4 aldehyde thiosemicarbazones.

This compound was synthesized by Miss Yolanda A. Tota. It is stated to be at least as stable to heat as thiamine. I have tested this compound with three fungi, Phycomyces Blakesleeanus, Pythiomorpha gonapodyides and Phytophthora cinnamomi, each of which has a different kind of thiamine deficiency. Each organism was grown in 25-ml. quantities of a solution containing per liter 50 g. dextrose, 1.5 g. KH2PO4, 0.5 g. MgSO4-7H20, 2.0 g. asparagine and the following mineral supplements in p. p. m.: 0.005 B, 0.02 Cu, 0.1 Fe, 0.01 Ga, 0.01 Mn, 0.01 Mo and 0.09 Zn. To this solution various amounts of the analog, of thiamine, of thiazole or of pyrimidinel were added as shown in tables 1 and 2. The solutions were sterilized for 20 minutes at 12 pounds pressure and each treatment was carried out in triplicate. The temperature of incubation was 20?C. Phycomyces was grown for 8 days

The population dynamics of purine analog resistance have been studied. Two types of reconstruction experiments were performed: cloning numbers of azaguanine resistant, BUdR resistant, and sensitive cells were plated in the presence of a large number of the opposite genotype, which were made to act as a feeder layer by inactivating them with either the appropriate analog or the reverse selective HAT medium. These experiments demonstrate that there is no significant metabolic cooperation between drug resistant and drug sensitive cells, with the exception of BUdR sensitive cells, which at high densities inhibit the cloning ability of BUdR resistant cells in the presence of the drug. Thus the purine analog resistance system can be used for obtaining quantitative data concerning such parameters as mutation and selection, while at high cell densities the pyrimidine analog system appears to be unsatisfactory.——A second series of experiments indicates that, in artificial mixtures of sensitive and resistant cells grown in standard medium, the resistant cells are at a selective disadvantage and will be reduced eventually to the frequency which is observed in wild type populations.

The field of nucleosides has undergone rapid growth and development during the past decade'—3. Whereas, earlier studies were concerned with the development of methods for the synthesis of the nucleoside components of the nucleic acids, today several methods are available—each of which has its advantages and limitations. The host of "fraudulent" or "anomalous" nucleosides available today attests to the ability of chemists to cope with this interesting and rather complex group of compounds. Nor have their efforts been unrewarding. It is obvious to all that nucleosides have provided their fair share of interesting biochemicals useful in the elucidation of biosynthetic pathways and helpful both as anti-neoplastic and anti-viral agents. In addition, the increasing amount of "odd" nucleosides found as minor components of certain nucleic acids4, especially of transfer RNA, has further stimulated chemical efforts in this area. Finally, the continuing discovery of nucleoside antibiotics5 (of which there are now over twenty) has served to introduce an increasing number of natural-products chemists into this burgeoning field. This discussion deals with but one aspect of our research during the past few years in the area of the pyrimidine nucleosides and is not intended as a comprehensive review. Specifically, it covers that aspect of our endeavours involved in the use of preformed nucleosides, natural or anomalous, as precursors for the synthesis of new nucleosides. As with most workers in this area, our studies are motivated by biochemical or biological interests, and reference to such considerations will be made in this discussion. Nucleoside chemistry is replete with examples from many laboratories of nucleoside transformations which involved alteration only of the aglycon portion, that is, reactions in which the sugar moiety played a "passive" role. Halogenation, methylation, thiation, deamination, etc. of the main nucleosides of the nucleic acids (Figure 1) and of others are well documented'—3. Our main interest however was in alterations in the carbohydrate moieties of nucleosides. One of the significant developments in nucleoside chemistry was the earlier discoveries6 by Brown, Michelson, Todd and their associates at Cambridge who showed that pyrimidine anhydronucleosides (or "cyclonucleosides") could be formed which involve an oxygen bridge between carbon-2 of the aglycon and the sugar moiety. We will discuss the chemistry of these and other types of anhydronucleosides as tools for stereochemically-controlled reactions involving

SUMMARYTrimethoprim, a 2,4-diaminopyrimidine derivative which inhibits the growth of some bacteria by interfering with folic acid synthesis, inhibited the growth of several strains of the trachoma agent. Inhibition was most clearly demonstrated by measuring prolongation of mean death time of groups of chick embryos inoculated with a single lethal dose of agent. Over a certain range, prolongation was proportional to the logarithm of concentration of inhibitor; higher concentrations were toxic for the embryo. On a weight basis, trimethoprim was not as active as sulphafurazole. Inoculation in conjunction with sulphafurazole resulted in slight potentiation of activity. A related pyrimidine derivative, the antimalarial drug pyrimethamine, also significantly inhibited the growth of one strain of trachoma.In cell culture, trimethoprim decreased the number of inclusions formed by a suspension of the trachoma agent and induced morphological changes in the inclusions similar to those caused by sulphafurazole.Inhibition of the growth of the trachoma agent in the chick embryo was reversed by leucovorin calcium. It is concluded that, as with bacteria, the drug acts by blocking the folio acid cycle and that the trachoma agent most probably contains a dihydrofolate reductase.

Several purine and pyrimidine analogues which are sterically very similar to the normal nucleic acid bases have been tested for their mutagenic effect upon T4 phages. Besides 5-bromodeoxyuridine only 2-aminopurine and 2,6-diaminopurine were effective. Most of the base analogues tested inhibit bacterial growth when present in sufficiently high concentrations, and some of them show an additional effect upon phage growth. Thus 5-bromodeoxyuridine and 5-bromouracil inhibit phage growth (especially in the derivatives of bacteria K-12), and 2-aminopurine depresses lysis inhibition under certain growth conditions. The “mutability spectrum” of T4 rII-type phage mutants induced by 5-bromo-deoxyuridine was found to be essentially the same as that of 5-bromouracil-induced mutants studied by Benzer & Freese (1958) . In contrast, the mutability spectrum of 2-aminopurine-induced mutants was different from both spontaneous and bromouracil-induced mutants. The genetic coincidence, however, of some 2-aminopurine-induced mutants with some bromouracil-induced mutants indicates some mutagenic similarity between the two base analogues. The high frequency of phage mutants after growth in the presence of the base analogue is not due to selection of pre-existing mutants, as shown by a fluctuation test. Again, the differences between the mutability spectra are not due to any selection but are caused by preferential mutagenic actions at certain sites of the genome.

In order to study their antimetabolic activity, we have synthesized the 2-methyl-4-amino-, 2,4-diamino-, 2,6-diamino-4-hydroxy-, and 2,4-dihydroxypyrimidine analogs of folic acid from p-aminobenzoyl-L-glutamic acid and the corresponding substituted 5-bromomethyl-pyrimidines. The high antimetabolic activity of the 2,4-dihydroxypyrimidine analog in the inhibition of the reductases of folic and dihydrofolic acids and of the 2,4-diaminopyrimidine analog in the inhibition of the reductase of dihydrofolic acid has been shown.

SYNOPSIS Eighty-four purine and pyrimidine analogs were evaluated for growth inhibition of Tetrahymena pyriformis. The most toxic were 2-fluoroadenine, 2-fluoroadenosine, 6-methylpurine, a series of 5-fluoropyrimidines, and a series of adenine derivatives substituted in the 9-position. 2-Fluoroadenine was metabolized to the ribonucleoside triphosphate and was incorporated into nucleic acids; its inhibition of growth was reversed by high levels of adenine. 6-Methylthiopurine ribonucleoside was phosphorylated, but only to the monophosphate derivative. Contrasting T. pyriformis with mammalian cells gave clues to the mechanism of action of some of the agents. 6-Mercaptopurine, 6-methylthiopurine ribonucleoside, and 6-thioguanine, all potent pseudofeedback inhibitors of de novo purine biosynthesis in mammalian cells, are not toxic to T. pyriformis, which lacks the de novo purine pathway; this implies that inhibition of de novo purine biosynthesis by them underlies their growth inhibition of mammalian cells.

The development of resistance of animal tumour tissue in vivo toward pyrimidine and purine analogues is generally accounted for by a process of mutation and selection, although in s

Cytosine arabinoside inhibited the reproduction of L5178Y leukemic cells by inhibition of the reduction of cytidylic acid to 2′-deoxycytidylic acid. The specificity of the inhibitory action of cytosine arabinoside was indicated by nutritional studies and by the demonstration that the radioactivity of uridylic 2′-deoxyuridylic acid, thymidylic acid and RNA fractions of cells incubated with 3H-UR was little affected, and that the synthesis of DNA continued when 2′-deoxycytidine was added to cells otherwise inhibited by cytosine arabinoside. Data on the sensitivity of FUDR- and thymidine-resistant mastocytoma cells to cytosine arabinoside and FCDR and cross-resistance to other pyrimidine analogs are presented. Nutritional findings are described which suggest additional secondary sites of action of cytosine arabinoside, possibly at a kinase or phosphokinase level.

Various purine and pyrimidine analogs were amebicidal. The inhibition of Endamoeba histolytica by these compounds was reversed by purines and pyrimidines. Some new synthetic pyrimidine amides had greatest antiamebic activity. It is proposed that these analogs act as antimetabolites and block nucleic acid metabolism in the amebae.

The pyrimidine analog, 6-azauridine (AZUR), is an inhibitor of de novo pyrimidine biosynthesis and produces significant antineoplastic effects in experimental tumors. Use of this compound in the treatment of acute and chronic leukemia has resulted in a rapid and marked decline in circulating leukemic cells. Satisfactory responses in chronic leukemia, even in the "acute phase" of the disease, have occurred despite the development of temporary resistance. Similar responses in acute leukemia have been observed but no instances of remission have occurred in these short trials. Inhibition of the ability of leukocytes to decarboxylate orotic acid was found in patients receiving AZUR and correlated well with the clinical response in each case. The development of temporary clinical resistance to antileukemic effects was associated with decreased inhibition of this enzyme system in leukocytes removed from patients. The urinary excretion of orotic acid and orotidine in patients receiving AZUR did not correlate with the antileukemic effects and is probably the result of inhibition of pyrimidine metabolism in various body organs.

Biochemical effects of two new fluorinated pyrimidine analogs, 5-trifluoromethyluracil and 5-trifluoromethyl-2′-deoxyuridine, have been studied in Escherichia coli and its bacteriophage T4B. 5-Trifluoromethyluracil weakly inhibits the growth of E. coli B, substitutes very poorly for thymine in thymineless E. coli 15T−, but strongly inhibits the average viable burst size of bacteriophage T4B. 5-Trifluoro-methyl-2'-deoxyuridine induces forward mutations at the r locus in phage T4B to about the same extent as does 5-bromo-2′-deoxyuridine, and also causes reversion induction in three point-mutants of this phage. The DNA isolated from the phage grown in the presence of 5-trifluoromethyl-2′-[14C2]deoxyuridine had an increased buoyant density in cesium chloride and cesium sulfate. In preparative CsCl density gradient centrifugation, the radioactivity coincided exactly with the DNA. Hydrolysis revealed that the compound was incorporated into the DNA as such, the replacement of thymine amounting to up to 10%. The melting temperature of this DNA was lower than that of normal phage T4B DNA.

The pyrimidine analogs 3-deazauridine and 3-deazacytidine are effective inhibitors of the growth of various microbial and tumor cell lines in culture, and of some experimental tumors in vivo . As part of a study on their modes of action, the metabolic conversion of the analogs by intact Escherichia coli , leukemia L-1210 and Ehrlich ascites carcinoma cells, or by cell-free extracts derived from these or from Streptococcus faecium cells, was examined. Neither analog underwent cleavage of its glycosidic bond in the bacterial or tumor cell extracts, which catalyzed the cleavage of uridine but not of cytidine. Deazacytidine was not deaminated by extracts of E.coli , which effected the rapid deamination of cytidine. Both analogs were phosphorylated to the nucleoside triphosphate stage, as determined with Ehrlich ascites extracts, and Ehrlich ascites, L-1210 and E. coli cells. The possible formation of 2′-deoxyribonucleoside phosphate derivatives was examined with 3-deazacytidine, and a 2′-deoxyribonucleotide derivative of this analog was found in the acid-soluble fraction of E. coli cells. Neither 3-deazauridine nor 3-deazacytidine was incorporated into the RNA or DNA of Ehrlich ascites cells in vivo .

Shapira, Jacob (Consolidated Veterans Administration Hospital, Little Rock, Ark.), Lois Lowden, and Ralph Hale . Inhibition of a thymine-deficient mutant of Escherichia coli by 5-substituted uracils. J. Bacteriol. 83: 919–923. 1962.—Small inocula of well-washed cells of a thymine-requiring mutant of Escherichia coli were incubated in a thymine-containing glucose-salts medium with a variety of 5-substituted pyrimidines and pyrimidine ribosides. After a lag phase, the turbidity of the cultures increased appreciably which, in the case of 5-ethyluracil and 5-ethyluridine, was primarily due to an elongation of the cells. 5-Ethyluracil at low thymine concentrations increased the lag phase and decreased the rate and final amount of growth. At high thymine concentrations, it had less effect on the final turbidity of the cultures. The inhibition index for this compound was relatively constant, suggesting competitive inhibition. Several other pyrimidine analogues inhibited growth. The nucleosides of 5-bromouracil and 5-aminouracil were no more effective than the free bases. The ribosides of 5-ethyluracil and 5-butyluracil were appreciably more inhibitory than the free bases and were the most potent compounds tested. It is likely that the inhibition of growth is a reflection of the effect of these compounds on ribonucleic acid synthesis by the cells.

Previous evidence that anthocyanin formation in Spirodela is dependent on the synthesis of a complex nucleotide or nucleic acid has been extended through a survey of the effects of a variety of purine and pyrimidine analogs. Almost all such compounds have been found to inhibit anthocyanin formation. The group includes analogs of all four bases which make up ribonucleic acid (RNA): 8-azaguanine, 8-azaadenine, 6-azauracil, and 2-thiocytosine, as well as Antrycide (2,6′-dimethyl-6,4′-pyrimidylamino-4, 2′-diaminoquinoline 1,1′-dimethosulfate), an inhibitor of purine incorporation into nucleic acid. The most effective inhibitor is still, as previously reported, 8-azaguanine. It decreases the anthocyanin formation by 50% at 3 × 10 −7 M . Subinhibitory concentrations of azaguanine, azaadenine and Antrycide, but not in general of the natural bases, clearly promote pigment formation. The inhibitions are reversed in each case by the corresponding purine or pyrimidine, or its riboside or ribotide. There is a specific requirement for a purine, not satisfiable by pyrimidines, and conversely a requirement for a pyrimidine, not wholly satisfied by purines. There is indication of a requirement for both uracil and cytosine. Experiments using simultaneously two types of inhibitors, namely, analogs of both purines and pyrimidines, confirm that both purine and pyrimidine cooperate in the synthesis of anthocyanin. It has been demonstrated that the reversing compounds function within the plant, rather than by preventing the absorption of the antagonist. It is concluded that an unstable nucleic acid, which must be continually resynthesized, is the catalyst or limiting factor in the synthesis of anthocyanin in Spirodela .

The action of two pyrimidine analogs, 2-thio-cytosine and 2-thio-5-methyl-cytosine has been studied on Ehrlich adenocarcinoma, sarcoma 180 and Walker carcinosarcoma. The different action induced by these substances on the tumour growth is pointed out. Particular consideration is given to the different response of the tumours to 2-thio-5-methyl-cytosine and the behaviour of its inhibiting action during experiments is pointed out. Some hypotheses on the mechanism of action of these compounds are advanced, basing upon the different responses given to the same substances by microbiologic systems and by fibroblasts cultures from chick embryos.

Effects of purine and pyrimidine analogues on growth and amino acid accumulation were studied using Aerobacter aerogenes No. 19–35 which produces l-valine in phosphate-rich medium (PS-medium) and l-glutamic acid in phosphate-poor medium (PD-medium). The cell growth was inhibited in PS-medium containing purine and pyrimidine analogues. In PD-medium, 6-thiolpurine such as 6-mercaptopurine, 6-methylmercaptopurine and 6-thioguanine did not inhibit cell growth while other drugs did. The addition of 6-thiol-purine, especially 6-mercaptopurine, to PD-medium provoked the production of valine in place of l-glutamic acid. Cells are classified into two types by these characters: l-valine producing type and l-glutamic acid producing type. Such conversion phenomena seemed to occur only in strain No. 19–35, since other strains of A. aerogenes and E. coli producing valine did not show the conversion phenomenon when exposed to 6-thiolpurines.

An attempt has been made to evaluate the various cellular effects produced in a bacterial system by the pyrimidine analog, 5-fluorouracil (FU), to determine their interrelationships, and to clarify th

Data indicate that supplementation of a defined incubation medium (basal-casein) with pyrimidines, purines, or their analogues influences the developmental rate of Tribolium castaneum (Herbst). Larval survival and larval-pupation transition time were scored as a function of time following incubation of eggs in supplemented medium. The halogenated pyrimidines (iodo- and bromouracil) and halogenated nucleosides (iodo- and bromodeoxyuridine) exert a marked toxic effect on developing larvae. For these groups of analogues, toxicity is separable in terms of the degree and the time at which toxicity is manifest. There is a striking reduction in the duration of the larval period without any demonstrable toxicity for larvae grown in basal-casein medium containing nonhalogenated pyrimidine analogues, i.e., 5-aminouracil, 6-azauracil, and 6-azathymine. It is indicated that enrichment of the defined medium with thymine, uracil, guanine, or adenine decreases the time required for emergence. The latter purines prevent toxicity of 8-azaguanine as expressed by larval survival. Acceleration of developmental rate associated with pyrimidine, purine, or nonhalogenated pyrimidine analogues is less than when yeast was added to the incubation medium, but these developmental times are clearly less than those of Tribolium in unsupplemented medium.

Clinical and experimental studies have demonstrated that the pyrimidine analog, 5-fluorouracil (FU), may exert considerable antitumor effect (4). Further therapeutic advantages might be gained by combining this drug with another agent such as ionizing radiation. An important consideration in assessing the possible advantages of combination therapy, however, is the comparison of the efficacy of the individual agents with their effects when combined. The purpose of this investigation was to compare the effects of x-irradiation and FU when administered separately and in combination in the treatment of patients with advanced neoplastic disease. Methods Because the variability of tumors in different patients is great, the most suitable study was considered to be the measurement of changes in multiple metastases within individual patients. Experimentally controlled observations of comparable lesions treated with different modalities could thereby be achieved. The parameters were the regression rates of multiple...

Pyrimidine nucleotides are precursors of nucleic acids; several are also known to be co-enzymes in carbohydrate and phospholipid metabolism.1We have attempted to assess the physiological and metabolic roles of pyrimidine nucleotides in the central nervous system by the intrathecal administration of pyrimidine antimetabolites. Neurological disturbances have been reported with the use of several such substances. 6-Azauracil, an analogue of uracil, causes confusional states, progressing to coma and diffuse high-voltage, slow waves in the EEG in man2,3; these effects are reversible. Massive doses of this analogue produce ataxia and similar EEG changes in rats.45-Hydroxyuridine and 5-aminouridine, analogues of uridine, produce transient somnolence and seizures in rats upon intraventricular injection.5 Orotic acid is an effective precursor of acid-soluble and nucleic acid pyrimidines in many organs,6the central nervous system included.7,8Therefore, 5-fluoro-orotic acid (FO), a structural analogue of orotic acid (Fig. 1) was

Dihydroorotase (4,5- l -dihydroorotate amidohydrolase, EC 3.5.2.3) activity in sonicated preparation of the Novikoff ascites hepatoma was inhibited by a group of pyrimidines, pyrimidine analogues, and purine derivatives. Cytidine compounds were much more effective than the corresponding uridine derivative and among these CMP and 5-bromo-2′-deoxycytidine were potent inhibitors of the enzyme. The only exception to this rule occured with thymidine and TMP, which also were potent inhibitors of enzymic activity. Among the purine derivatives, noteworthy inhibition was obtained with adenosine, 2′-deoxyadenosine, ATP, ADP, deoxyguanosine, deGMP, and adenosine-N1-oxide. The latter was the most effective inhibitor of all the compounds tested. The inhibition was of the competitive and the Ki values have been determined and tabulated. Some of the implications of these inhibition are discussed. Although the substrate, carbamylaspartate, affords protection against heat inactivation of the enzyme, no such protection was observed with the inhibitors suggesting a different site of binding.