Metabolism of 4'-thio-beta-D-arabinofuranosylcytosine in CEM cells

Because of the excellent in vivo activity of 4'-thio-beta-D-arabinofuranosylcytosine (T-araC) against a variety of human solid tumors, we have studied its metabolism in CEM cells to determine how the biochemical pharmacology of this compound differs from that of beta-D-arabinofuranosylcytosine (araC). Although there were many quantitative differences in the metabolism of T-araC and araC, the basic mechanism of action of T-araC was similar to that of araC: it was phosphorylated to T-araC-5'-triphosphate (T-araCTP) and inhibited DNA synthesis. The major differences between these two compounds were: (i) T-araC was phosphorylated to active metabolites at 1% the rate of araC; (ii) T-araCTP was 10- to 20-fold more potent as an inhibitor of DNA synthesis than was the 5'-triphosphate of araC (araCTP); (iii) the half-life of T-araCTP was twice that of araCTP; (iv) the catalytic efficiency of T-araC with cytidine deaminase was 10% that of araC; and (v) the 5'-monophosphate of araC was a better substrate for deoxycytidine 5'-monophosphate deaminase than was the 5'-monophosphate of T-araC. Of these differences in the metabolism of these two compounds, we propose that the prolonged retention of T-araCTP is a major factor contributing to the activity of T-araC against solid tumors. The data in this study represent another example of how relatively small structural changes in nucleoside analogs can profoundly affect the biochemical activity.

Metabolism of O6-propyl and N6-propyl-carbovir in CEM cells

The metabolism of O6-propyl-carbovir and N6-propyl-carbovir, two selective inhibitors of HIV replication, has been evaluated in CEM cells. Both compounds were phosphorylated in intact cells to carbovir-5'-triphosphate. The metabolism of these two agents was inhibited by deoxycoformycin and mycophenolic acid, but not erythro-9-(2-hydroxy-3-nonyl)adenine. No evidence of the 5'-triphosphate of either compound was detected in CEM cells.

Comparison of the mechanism of cytotoxicity of 2-chloro-9-(2-deoxy-2- fluoro-beta-D-arabinofuranosyl)adenine, 2-chloro-9-(2-deoxy-2-fluoro- beta-D-ribofuranosyl)adenine, and 2-chloro-9-(2-deoxy-2,2-difluoro- beta-D-ribofuranosyl)adenine in CEM cells

In an effort to understand biochemical features that are important to the selective antitumor activity of 2-chloro-9-(2-deoxy-2-fluoro-beta-D-arabinofuranosyl)adenine [Cl-F( upward arrow)-dAdo], we evaluated the biochemical pharmacology of three structurally similar compounds that have quite different antitumor activities. Cl-F( upward arrow)-dAdo was 50-fold more potent as an inhibitor of CEM cell growth than were either 2-chloro-9-(2-deoxy-2-fluoro-beta-D-ribofuranosyl)adenine [Cl-F( downward arrow)-dAdo] or 2-chloro-9-(2-deoxy-2, 2-difluoro-beta-D-ribofuranosyl)adenine [Cl-diF( upward arrow downward arrow)-dAdo]. The compounds were similar as substrates of deoxycytidine kinase. Similar amounts of their respective triphosphates accumulated in CEM cells, and the rate of disappearance of these metabolites was also similar. Cl-F( upward arrow)-dAdo was 10- to 30-fold more potent in its ability to inhibit the incorporation of cytidine into deoxycytidine nucleotides than either Cl-F( downward arrow)-dAdo or Cl-diF( upward arrow downward arrow)-dAdo, respectively, which indicated that ribonucleotide reductase was differentially inhibited by these three compounds. Thus, the differences in the cytotoxicity of these agents toward CEM cells were not related to quantitative differences in the phosphorylation of these agents to active forms but can mostly be accounted for by differences in the inhibition of ribonucleotide reductase activity. Furthermore, the inhibition of RNA and protein synthesis by Cl-F( downward arrow)-dAdo and Cl-diF( upward arrow downward arrow)-dAdo at concentrations similar to those required for the inhibition of DNA synthesis can help explain the poor antitumor selectivity of these two agents because all cells require RNA and protein synthesis.

Metabolism and metabolic actions of 6-methylpurine and 2-fluoroadenine in human cells

Activation of purine nucleoside analogs by Escherichia coli purine nucleoside phosphorylase (PNP) is being evaluated as a suicide gene therapy strategy for the treatment of cancer. Because the mechanisms of action of two toxic purine bases, 6-methylpurine (MeP) and 2-fluoroadenine (F-Ade), that are generated by this approach are poorly understood, mechanistic studies were initiated to learn how these compounds differ from agents that are being used currently. The concentration of F-Ade, MeP, or 5-fluorouracil required to inhibit CEM cell growth by 50% after a 4-hr incubation was 0.15, 9, or 120 microM, respectively. F-Ade and MeP were also toxic to quiescent MRC-5, CEM, and Balb 3T3 cells. Treatment of CEM, MRC-5, or Balb 3T3 cells with either F-Ade or MeP resulted in the inhibition of protein, RNA, and DNA syntheses. CEM cells converted F-Ade and MeP to F-ATP and MeP-ribonucleoside triphosphate (MeP-R-TP), respectively. The half-life for disappearance of HeP-ribonucleoside triphosphate from CEM cells was approximately 48 hr, whereas the half-lives of F-ATP and ATP were approximately 5 hr. Both MeP and F-Ade were incorporated into the RNA and DNA of CEM cells. These studies indicated that the mechanisms of action of F-Ade and MeP were quite different from those of other anticancer agents, and suggested that the generation of these agents in tumor cells by E. coli PNP could result in significant advantages over those generated by either herpes simplex virus thymidine kinase or E. coli cytosine deaminase. These advantages include a novel mechanism of action resulting in toxicity to nonproliferating and proliferating tumor cells and the high potency of these agents during short-term treatment.

Lack of mitochondrial toxicity in CEM cells treated with carbovir

Carbovir (CBV) is a guanine nucleoside analog with potent in vitro anti-HIV activity. A prodrug of CBV is currently being evaluated in clinical trials as a potential agent for the treatment of AIDS. The ability of CBV to inhibit mitochondrial DNA synthesis in intact CEM cells was evaluated in the present study, because most of the currently available anti-HIV nucleoside analogs have significant toxicities that result from their inhibition of mitochondrial DNA synthesis. No delayed cytotoxicity was observed in CEM cells treated with 50 microM CBV for 4 weeks. In addition, CBV at concentrations as high as 1 mM did not cause a decline in mitochondrial DNA levels and only minimally increased the concentration of lactic acid in the medium. In contrast to these results with CBV, treatment of CEM cells with 0.5 microM 2',3'-dideoxycytidine resulted in delayed cytotoxicity, a decrease in mitochondrial DNA content and increases in lactic acid levels in the medium. These results indicated that treatment of CEM cells with CBV did not result in the inhibition of mitochondrial DNA synthesis and suggested that treatment of AIDS patients with CBV, or a prodrug of CBV, would not result in some of the toxicities seen with the other anti-HIV nucleoside analogs.

Metabolism and metabolic actions of 4'-thiothymidine in L1210 cells

4'-Thiothymidine (S-dThd) is a potent inhibitor of L1210 cell growth and is active against P388 leukemia in mice. Because of these activities and its novel structure, we have begun studies of its metabolism and metabolic actions in L1210 cells in order to understand its mechanism of cytotoxicity, S-dThd inhibited the incorporation of radiolabeled precursors into DNA, but did not inhibit the incorporation of either uridine or leucine into RNA or protein, respectively, which indicated that the mechanism of its toxicity was due to its inhibition of DNA synthesis. S-dThd did not decrease the concentration of any of the natural deoxynucleoside triphosphates, which indicated that its cytotoxicity was not due to the inhibition of ribonucleotide reductase. S-dThd was readily phosphorylated and used as a substrate for DNA synthesis. Because the rate of incorporation of S-dThd into DNA was 20% that of thymidine, it is likely that the mechanism of action of S-dThd is not due to inhibition of DNA polymerases by the 5'-triphosphate of S-dThd, but instead to its incorporation into the DNA and its subsequent disruption of some function of DNA.

Metabolism of carbovir, a potent inhibitor of human immunodeficiency virus type 1, and its effects on cellular metabolism

Carbovir (CBV) [the (--)-enantiomer of the carbocyclic analog of 2',3'-dideoxy-2',3'-didehydroguanosine] is a potent inhibitor of human immunodeficiency virus type 1 (HIV) replication in vitro. We have characterized the metabolism of CBV and its effect on cellular metabolism in an effort to better understand its mechanism of action. CBV was primarily metabolized to the 5'-triphosphate of CBV (CBV-TP) to concentrations sufficient to inhibit HIV reverse transcriptase. Infection of CEM cells with HIV did not affect the metabolism of CBV. In CEM cells, there was no evidence of the degradation of CBV by purine nucleoside phosphorylase. The half-life of CBV-TP in CEM cells was 2.5 h, similar to that of the 5'-triphosphate of zidovudine (AZT). However, unlike the levels of the 5'-triphosphate of AZT, CBV-TP levels declined without evidence of a plateau. CBV did not affect the metabolism of AZT, and AZT did not affect the metabolism of CBV. A small amount of CBV was incorporated into DNA in intact CEM cells, and this incorporation was increased by incubation with mycophenolic acid, an inhibitor of IMP dehydrogenase. CBV specifically inhibited the incorporation of nucleic acid precursors into DNA but had no effect on the incorporation of radiolabeled precursors into RNA or protein. CBV did not decrease the level of TTP, dGTP, dCTP, or dATP. These results suggested that the cytotoxicity of CBV was due to the inhibition of DNA synthesis. Further studies are necessary to identify the target(s) responsible for growth inhibition.

Interference with HIV-1 reverse transcriptase-catalyzed DNA chain elongation by the 5'-triphosphate of the carbocyclic analog of 2'-deoxyguanosine

In an effort to better understand features in nucleotide analogs that result in the inhibition of HIV-1 reverse transcriptase, we have evaluated this enzyme with the 5'-triphosphate of the carbocyclic analog of 2'-deoxyguanosine (CdG-TP). CdG-TP was a reasonably potent competitive inhibitor of the incorporation of dGTP into DNA by HIV-1 reverse transcriptase using either a RNA or DNA template (Ki, 1 microM). CdG-TP was a good substrate for HIV-1 reverse transcriptase on both templates, but the DNA chain was poorly extended beyond the incorporation of CdG. These results indicate that substitution of ribose with a cyclopentane ring in nucleotides is not well tolerated by HIV-1 reverse transcriptase.

Incorporation of the carbocyclic analog of 2'-deoxyguanosine into the DNA of herpes simplex virus and of HEp-2 cells infected with herpes simplex virus

The carbocyclic analog of 2'-deoxyguanosine (CdG) is active against herpes simplex virus (HSV), human cytomegalovirus, and human hepatitis-B virus. In order to understand the mechanism of action of this compound against HSV, we have evaluated (a) the incorporation of [3H]CdG into viral and host DNA in HEp-2 cells infected with HSV and (b) the interaction of the 5'-triphosphate of CdG (CdG-TP) with the HSV DNA polymerase and human DNA polymerases alpha, beta, and gamma (EC 2.7.7.7). Incubation of HSV-1-infected HEp-2 cells with [3H]CdG resulted in the incorporation of CdG into both the HSV and the host cell DNA. These results indicated that CdG-TP was used as a substrate for HSV DNA polymerase and for at least one of the cellular DNA polymerases. Degradation of both viral and host DNA with micrococcal nuclease and spleen phosphodiesterase indicated that CdG was incorporated primarily into internal positions in both DNAs. The viral DNA containing CdG sedimented in neutral and alkaline sucrose gradients in the same way as did viral DNA labeled with [3H]thymidine, indicating that the HSV DNA containing CdG was similar in size to untreated HSV DNA. CdG-TP was a competitive inhibitor of the incorporation of dGTP into DNA by the HSV DNA polymerase (Ki of 0.35 microM) and the human DNA polymerase alpha (Ki of 1 microM). CdG-TP was not a potent inhibitor of either DNA polymerase beta or gamma. Using DNA-sequencing technology, CdG-TP was found to be an efficient substrate for HSV DNA polymerase. Incorporation of CdG monophosphate (CdG-MP) into the DNA by HSV DNA polymerase did not interfere with subsequent chain extension. These results suggested that the antiviral activity of CdG was due to its incorporation into the DNA and subsequent disruption of viral functions. In contrast, CdG-TP was not as good as dGTP as a substrate for DNA synthesis by DNA polymerase alpha, and incorporation of CdG-MP by DNA polymerase alpha inhibited further DNA chain elongation.

Effects of 2-chloro-9-(2-deoxy-2-fluoro-beta-D-arabinofuranosyl)adenine on K562 cellular metabolism and the inhibition of human ribonucleotide reductase and DNA polymerases by its 5'-triphosphate

2-Chloro-9-(2-deoxy-2-fluoro-beta-D-arabinofuranosyl)-adenine (Cl-F-ara-A) has activity against the P388 tumor in mice on several different schedules. Biochemical studies with a chronic myelogenous leukemia cell line (K562) grown in cell culture have been done in order to better understand its mechanism of action. Cl-F-ara-A was a potent inhibitor of K562 cell growth. Only 5 nM inhibited K562 cell growth by 50% after 72 h of continuous incubation. The 5'-triphosphate of Cl-F-ara-A was detected by strong anion exchange chromatography of the acid-soluble extract of K562 cells incubated with Cl-F-ara-A. Competition studies with natural nucleosides suggested that deoxycytidine kinase was the enzyme responsible for the metabolism to the monophosphate. Incubation of K562 cells for 4 h with 50 nM Cl-F-ara-A inhibited the incorporation of [3H]thymidine into the DNA by 50%. Incubation with 0.1, 1, or 10 microM Cl-F-ara-A for 4 h depressed dATP, dCTP, and dGTP pools but did not affect TTP pools. Similar inhibition of deoxyribonucleoside triphosphate pools was seen after incubation with 2-chloro-2'-deoxyadenosine. Both Cl-F-ara-ATP and Cl-dATP potently inhibited the reduction of ADP to dADP in crude extracts of K562 cells (concentration producing 50% inhibition, 65 nM). The effect of Cl-F-ara-ATP on human DNA polymerases alpha, beta, and gamma isolated from K562 cells grown in culture was determined and compared with those of Cl-dATP and 9-beta-D-arabinofuranosyl-2-fluoroadenine triphosphate (F-ara-ATP). Cl-F-ara-ATP was a potent inhibitor of DNA polymerase alpha. Inhibition of DNA polymerase alpha was competitive with respect to dATP (Ki of 1 microM). The three analogue triphosphates were incorporated into the DNA by DNA polymerase alpha as efficiently as dATP. The incorporation of Cl-F-ara-AMP inhibited the further elongation of the DNA chain, similarly to that seen after the incorporation of F-ara-AMP. Extension of the DNA chain after the incorporation of Cl-dAMP was not inhibited as much as it was with either Cl-F-ara-AMP or F-ara-AMP. Cl-F-ara-ATP was not a potent inhibitor of DNA polymerase beta, DNA polymerase gamma, or DNA primase.(ABSTRACT TRUNCATED AT 400 WORDS)

Mechanism of inhibition of human immunodeficiency virus type 1 reverse transcriptase and human DNA polymerases alpha, beta, and gamma by the 5'-triphosphates of carbovir, 3'-azido-3'-deoxythymidine, 2',3'-dideoxyguanosine and 3'-deoxythymidine. A novel RNA template for the evaluation of antiretroviral drugs

Carbovir (the carbocyclic analog of 2'-3'-didehydro-2',3'-dideoxyguanosine) is a potent inhibitor of human immunodeficiency virus type 1 (HIV-1) replication. Assays were developed to assess the mechanism of inhibition by the 5'-triphosphate of carbovir of HIV-1 reverse transcriptase using either RNA or DNA templates that contain all four natural nucleotides. Carbovir-TP was a potent inhibitor of HIV-1 reverse transcriptase using either template with Ki values similar to that observed by AZT-TP, ddGTP, and ddTTP. The kinetic constants for incorporation of these nucleotide analogs into DNA by HIV-1 reverse transcriptase using either template were similar to the values seen for their respective natural nucleotides. In addition, the incorporation of either carbovir-TP or AZT-TP in the presence of dGTP or dTTP, respectively, indicated that the mechanism of inhibition by these two nucleotide analogs was due to their incorporation into the DNA resulting in chain termination. Carbovir-TP was not a potent inhibitor of DNA polymerase alpha, beta, or gamma, or DNA primase. Given the potent activity of carbovir-TP against HIV-1 reverse transcriptase and its lack of activity against human DNA polymerases, we believe that further evaluation of this compound as a potential drug for the treatment of HIV-1 infection is warranted.

Comparison of the effect of Carbovir, AZT, and dideoxynucleoside triphosphates on the activity of human immunodeficiency virus reverse transcriptase and selected human polymerases

Carbocylic 2',3'-didehydro-2',3'-dideoxyguanosine (Carbovir; NSC 614846) is an antiretroviral agent which may be useful in the treatment of AIDS. We have synthesized the 5'-triphosphate of Carbovir and examined its ability to inhibit human immunodeficiency virus type 1 (HIV-1) reverse transcriptase (EC 2.7.7.49) and other retroviral reverse transcriptases, as well as human DNA polymerases alpha, beta, gamma (EC 2.7.7.7) and DNA primase (EC 2.7.7.6). Carbovir triphosphate emerges as a highly selective inhibitor of reverse transcriptases with little, if any, effect on the cellular enzymes. 3'-Azido-2',3'-dideoxythymidine (AZT) triphosphate and the two dideoxynucleoside triphosphates, ddTTP and ddGTP, inhibited HIV-1 reverse transcriptase to the same degree as Carbovir triphosphate, but were less selective in that they also inhibited DNA polymerases beta and gamma. We conclude that Carbovir is a highly selective antiretroviral agent.

Alterations in nucleotide pools induced by 3-deazaadenosine and related compounds. Role of adenylate deaminase

3-Deazaadenine, 3-deazaadenosine, and the carbocyclic analog of 3-deazaadenosine produced similar effects on nucleotide pools of L1210 cells in culture: each caused an increase in IMP and a decrease in adenine nucleotides and had no effect on nucleotides of uracil and cytosine. Concentrations of 50-100 microM were required to produce these effects. Although 3-deazaadenosine and carbocyclic 3-deazaadenosine are known to be potent inhibitors of adenosylhomocysteine hydrolase, the effects on nucleotide pools apparently are not mediated via this inhibition because they are also produced by the base, 3-deazaadenine, and because the concentrations required are higher than those required to inhibit the hydrolase. Cells grown in the presence of 3-deazaadenine or 3-deazaadenosine contained phosphates of 3-deazaadenosine (the mono- and triphosphates were isolated); from cells grown in the presence of the carbocyclic analog of 3-deazaadenosine, the monophosphate was isolated, but evidence for the presence of the triphosphate was not obtained. A cell-free supernatant fraction from L1210 cells supplemented with ATP catalyzed the formation of monophosphates from 3-deazaadenosine or carbocyclic 3-deazaadenosine, and a cell-free supernatant fraction supplemented with 5-phosphoribosyl 1-pyrophosphate (PRPP) catalyzed the formation of 3-deaza-AMP from 3-deazaadenine. Adenosine kinase apparently was not solely responsible for the phosphorylation of the nucleosides because a cell line that lacked this enzyme converted 3-deazaadenosine to phosphates. No evidence was obtained that the effects on nucleotide pools resulted from a block of the IMP-AMP conversion, but the results could be rationalized as a consequence of increased AMP deaminase activity. This explanation is supported by two observations: (a) coformycin, an inhibitor of AMP deaminase, prevented the effects on nucleotide pools, and (b) 3-deazaadenine decreased the conversion of carbocyclic adenosine to carbocyclic ATP and increased its conversion to carbocyclic GTP. The latter conversion requires the action of AMP deaminase and the observed effects can be rationalized by a nucleoside analog-mediated increase in AMP deaminase activity. Because these effects on nucleotide pools are produced only by concentrations higher than those required to inhibit adenosylhomocysteine hydrolase, they may not contribute significantly to the biological effects of 3-deazaadenosine or carbocyclic 3-deazaadenosine.(ABSTRACT TRUNCATED AT 400 WORDS)

Nitrosoureido nucleosides as potential inhibitors of nucleotide biosynthesis

Several nitrosoureido nucleosides (3a, 3b, 5a, 7a, 7c, and 10a) designed as inhibitors of enzymes that metabolize pyrimidine nucleotides have been prepared and their chemical and biological properties studied. The methylnitrosoureas 3a and 3b were not significantly cytotoxic to H.Ep.-2 and L1210 cells in vitro but showed moderate activity in the P388 mouse leukemia screen (79% ILS for 3a and 56% ILS for 3b). The (chloroethyl)nitrosoureas 7a and 7c inhibited proliferation of L1210 cells, were cytotoxic to H.Ep.-2 cells, and demonstrated good activity against P388 in vivo (135% ILS with one 30-day survivor for 7a and 191% ILS with two 30-day survivors for 7c). Overnight exposure of L1210 cells to 7a and 7c resulted in cell enlargement accompanied by cell lysis. Macromolecular synthesis in enlarged cells, particularly RNA and protein synthesis, was markedly increased relative to that in untreated control cells. The half-lives of each of the nitrosoureas in pH 7 buffer was determined and compared with biological activity.

Inhibition of utilization of hypoxanthine and guanine in cells treated with the carbocyclic analog of adenosine. Phosphates of carbocyclic nucleoside analogs as inhibitors of hypoxanthine (guanine) phosphoribosyltransferase

In cell cultures treated with the carbocyclic analog of adenosine (C-Ado, (+/-)-aristeromycin), the utilization of hypoxanthine and guanine has been observed to be blocked. In an attempt to define the mechanism of this inhibition, we have reexamined the metabolism of C-Ado and its effects on the metabolism of guanine and hypoxanthine. In cultures of L1210 cells, C-Ado at a concentration of 25 microM inhibited the utilization of hypoxanthine and guanine for nucleotide synthesis by more than 90% but produced little or no inhibition of the utilization of these bases in cultures of L1210/MeMPR cells which lack adenosine kinase and cannot phosphorylate C-Ado. In cultures of mammalian cells (L1210, HEp-2, and colon-26 cells), C-Ado was converted to the triphosphate (as previously observed) and also to the triphosphate of the carbocyclic analog of guanosine. The presence of coformycin in the medium at a concentration sufficient to inhibit AMP deaminase almost completely prevented the formation of carbocyclic GTP; thus, the deamination of C-Ado monophosphate is essential for the formation of phosphates of carbocyclic guanosine. Since hypoxanthine (guanine) phosphoribosyltransferase is known to be subject to end product inhibition, it was considered likely that phosphates of carbocyclic guanosine or carbocyclic inosine, present in C-Ado-treated cells, were responsible for inhibition of utilization of hypoxanthine and guanine. The 5'-phosphates of the carbocyclic analogs of inosine and guanosine were synthesized and found to be effective inhibitors of the phosphoribosyltransferase. Carbocyclic GMP was a better inhibitor than carbocyclic IMP and was also superior to GMP and IMP; the concentration of C-GMP that produced a 50% inhibition of GMP formation was approximately 1 microM. It is probable that the presence of phosphates of carbocyclic guanosine accounts for the inhibition of utilization of hypoxanthine and guanine in C-Ado-treated cells.

Metabolism and metabolic effects of 2-azahypoxanthine and 2-azaadenosine

The metabolism and metabolic effects of 2-azahypoxanthine and 2-azaadenosine were studied to elucidate the biochemical basis for their known cytotoxicities. 2-Azaadenosine is a known substrate for adenosine kinase. That 2-azahypoxanthine is a substrate for hypoxanthine (guanine) phosphoribosyltransferase is shown by the observations that, in cell-free fractions from HEp-2 cells supplemented with 5-phosphoribosyl-1-pyrophosphate, 2-azahypoxanthine inhibited the conversion of hypoxanthine to IMP but not the conversion of adenine to AMP, and hypoxanthine, but not adenine, inhibited the conversion of 2-azahypoxanthine to 2-azaIMP. [8-14C]2-Azahypoxanthine was synthesized from [8-14C]hypoxanthine via [2-14C]-4-amino-5-imidazolecarboxamide. In HEp-2 cells in culture, the principal metabolite of [8-14C]-2-azahypoxanthine was 2-azaATP; there was no detectable 14C in deoxynucleotides or in DNA or RNA fractions. 2-Azaadenosine was much more toxic than 2-azahypoxanthine, and, when used in the presence of an adenosine deaminase inhibitor, 2'-deoxycoformycin, was converted in HEp-2 cells to 2-azaATP in amounts that exceeded those of ATP in control cells. The pool of ATP was reduced by as much as 75% as 2-azaATP accumulated. In a short-term experiment (4 hr), 2-azaadenosine selectively reduced the pools of adenine nucleotides, whereas 2-azahypoxanthine reduced the pools of guanine nucleotides selectively. Both 2-azahypoxanthine and 2-azaadenosine inhibited the incorporation of formate into purine nucleotides and were without effect on the conversion of thymidine and uridine to nucleotides. 2-Azahypoxanthine inhibited the incorporation of thymidine into macro-molecules but not that of uridine or leucine; 2-azaadenosine inhibited the incorporation of all three of these precursors non-selectively. 2-AzaIMP inhibited IMP dehydrogenase competitively with IMP (Ki = 66 microM). The difference in effects of 2-azahypoxanthine and 2-azaadenosine perhaps may be due to the production, from 2-azahypoxanthine but not from 2-azaadenosine + 2'-deoxycoformycin, of 2-azaIMP, which inhibits synthesis of guanine nucleotides and thereby results in inhibition of DNA synthesis. Specific sites of action for 2-azaadenosine are yet undefined.

Comparison of the actions of 9-beta-D-arabinofuranosyl-2-fluoroadenine and 9-beta-D-arabinofuranosyladenine on target enzymes from mouse tumor cells

9-beta-D-Arabinofuranosyl-2-fluoroadenine (2-F-ara-A), a derivative of 9-beta-D-arabinofuranosyladenine (ara-A) that is resistant to deamination, selectively inhibits DNA synthesis and has activity against mouse leukemia L1210 comparable to that of ara-A plus the adenosine deaminase inhibitor, 2'-deoxycoformycin. To determine if these two nucleosides have similar modes of action, comparisons were made of their effects and those of their triphosphates on enzymes known to be inhibited by ara-A or 9-beta-D-arabinofuranosyladenine 5'-triphosphate. 9-beta-D-Arabinofuranosyl-2-fluoroadenine 5'-triphosphate was more effective than 9-beta-D-arabinofuranosyladenine 5'-triphosphate in inhibiting the reduction of adenosine 5'-diphosphate and cytidine 5'-diphosphate by ribonucleotide reductase from HEp-2 cells or L1210 cells. DNA polymerase alpha from L1210 cells was equally sensitive to 9-beta-D-arabinofuranosyl-2-fluoroadenine 5'-triphosphate and 9-beta-D-arabinofuranosyladenine 5'-triphosphate, and DNA polymerase beta from L1210 cells was much less sensitive to both triphosphates. S-Adenosylhomocysteine hydrolase from L1210 cells was inactivated by 2-F-ara-A and ara-A, but higher concentrations of the fluoro derivative were required. These results are consistent with 2-F-ara-A and ara-A inhibition of DNA synthesis by inhibition of ribonucleotide reductase and DNA polymerase alpha.

Carbocyclic analogues of 5-fluorouracil nucleosides

The carbocyclic analogues of 5-fluoro-2'-deoxyuridine (5-FdUrd, 1), 5-fluorouridine, and 5-fluoro-3 alpha-deoxyuridine were prepared by fluorination of the uracil nucleoside analogues with elemental fluorine. The 5-FdUrd analogue (C-5-F-2'-dUrd, 6) was enzymatically phosphorylated to the analogue of 5-FdUrd 5'-phosphate and inhibited the incorporation of 2'-deoxyuridine into DNA of murine colon 26 tumor cells and L-1210 cells in culture. Biochemical studies also indicated that C-5-F-2'-Urd (6) was a less potent inhibitor of DNA synthesis in tumor cells than was 5-FdUrd (1). C-5-F-2'-dUrd was cytotoxic (ED50 = 2.5 mcg/mL) to L-1210 cells in culture; the other two analogues were less cytotoxic. C-5-F-2'-dUrd was inactive--or, at best, borderline active--in tests against P-388 leukemia in vivo.

Studies with 2,5-piperazinedione, 3,6-bis(5-chloro-2-piperidyl)-,dihydrochloride. II. Effects on macromolecular synthesis in cell culture and evidence for alkylating activity

2,5-Piperazinedione, 3,6-bis(5-chloro-2-piperidyl)-,dihydrochloride (NSC-135758) inhibited DNA synthesis but not RNA and protein synthesis in Adenocarcinoma 755 cells in culture. The expression of such inhibition was delayed in time; it was necessary to expose tumor cells to NSC-135758 for 10-12 hours before measuring the macromolecular synthesis in order to demonstrate selective inhibition of DNA synthesis. Inhibition of DNA synthesis was demonstrated to be irreversible in human epidermoid carcinoma cells in culture. Exposure of cells in suspension culture to NSC-135758 or to melphalan for 1-4 hours, and then incubation of cells in the absence of an inhibitor for 20 hours, resulted in preferential inhibition of DNA synthesis; inhibition of RNA synthesis was observed under these conditions but was less pronounced. Chemical evidence for alkylating activity of NSC-135758 and of the bis-aziridine derived from it (NSC-201424) was obtained by demonstrating their reaction with 4-(p-nitrobenzyl)pyridine. NSC-135758 was more potent as a cytotoxic agent than was its derivative, a result which suggests that NSC-135758 is the active alkylating agent. Reaction of NSC-135758 with diethylamine was examined; the product obtained upon alkylation of diethylamine by NSC-135758 was identified from its proton magnetic resonance spectrum and by field desorption mass spectral analysis. These results support the view that NSC-135758 acts as an alkylating agent in inhibiting DNA synthesis and cell proliferation of tumor cells in culture.

Studies with 2,5-piperazinedione, 3,6-bis(5-chloro-2-piperidyl)-,dihydrochloride. III. Biochemical and therapeutic effects in L1210 leukemias sensitive and resistant to alkylating agents: comparison with melphalan, cyclophosphamide, and BCNU

2,5-Piperazinedione, 3,6-bis(5-chloro-2-piperidyl)-,dihydrochloride (NSC-135758) selectively inhibits DNA synthesis in L1210/0 leukemia and in cyclophosphamide-resistant L1210 (L1210/CPA). Melphalan (L-PAM) inhibits nucleic-acid synthesis but not protein synthesis in L1210/0 and L1210/CPA. CPA selectively inhibits DNA synthesis in L1210/0 but does not inhibit DNA synthesis in L1210/CPA. NSC-135758 is as active in vivo against L1210/CPA and BCNU-resistant L1210 (L1210/BCNU) as it is against L1210/0. It is inactive against ic implanted L1210/0. These data clearly indicate important differences in the biologic activity of this compound compared to either CPA or BCNU. L-PAM is similar to NSC-135758 in activity against L1210/CPA and L1210/BCNU.

The mechanism of action of 3-deazauridine in tumor cells sensitive and resistant to arabinosylcytosine

Deazauridine inhibited growth of tumor cells in culture and in culture and in vivo; this agent was significantly more effective against L1210/AraC than against the parent sensitive line. Inhibition of growth of tumor cells in culture was prevented by uridine and cytidine and was partially alleviated by deoxycytidine, but not by deoxyuridine or thymidine. DeazaUR inhibited nucleic acid synthesis but not protein synthesis in tumor cells in culture; deoxycytidine alleviated inhibition of nucleic acid synthesis. The labeling of pyrimidine ribonucleotides by 6-14C-orotic acid was inhbited by deazaUR. DeazaUR treatment of tumor cells in culture resulted in increased uptake of cytidine-3H into RNA, whereas uridine-3H uptake into RNA was inhibited. Labelling of DNA by uridine-3H/ and cytidine-H was inhibited by deazaUR. Pools of CMP, CDP, and CTP decreased markedly during deazaUR treatment of L1210 cells in culture and in vivo. These observations in growing cells pointed to deazaUR inhibition of the synthesis of cytidylic acid. Deazauridine 5'-triphosphate was found to be an inhibitor of the synthesis of CTP from UTP catalyzed by enzyme preparations from L1210 cells. This observation is in agreement with those of McPartland et al.19 that deazaUTP inhibited CTP synthetase purified from calf liver. Deazauridine treatment of L1210 cells in culture stimulated the uptake of deoxycytidine-3H into DNA while inhibiting the uptake of 3H-labeled deoxyuridine, thymidine, deoxyadenosine, and deoxyguanosine. Intracellular pools of dCTP were decreased by deazauridine treatment in L1210 cells in culture and in vivo. Deazauridine 5'-diphosphate inhibited the enzymatic reduction of pyrimidine ribonucleoside 5'-diphosphates to the corresponding deoxyribonucleotides. These results are consistent with the view that deazauridine, after its uptake and intracellular phosphorylation, strongly inhibits the formation of CTP. This is considered to be the primary metabolic effect of the analog. A secondary effect appears to be an inhibition of dCTP formation.