The cytotoxic and pro-apoptotic activities of the novel fluoropyrimidine F10 towards prostate cancer cells are enhanced by Zn(2+) -chelation and inhibiting the serine protease Omi/HtrA2

BACKGROUND

Intracellular Zn(2+) levels decrease during prostate cancer progression and agents that modulate intracellular Zn(2+) are cytotoxic to prostate cancer cells by an incompletely described mechanism. F10 is a new polymeric fluoropyrimidine drug-candidate that displays strong activity with minimal systemic toxicity in pre-clinical models of prostate cancer and other malignancies. The effects of exogenous Zn(2+) or Zn(2+) chelation for enhancing F10 cytotoxicity are investigated as is the role of Omi/HtrA2, a serine protease that promotes apoptosis in response to cellular stress.

METHODS

To test the hypothesis that the pro-apoptotic effects of F10 could be enhanced by modulating intracellular Zn(2+) we investigated cell-permeable and cell-impermeable Zn(2+) chelators and exogenous Zn(2+) and evaluated cell viability and apoptosis in cellular models of castration-resistant prostate cancer (CRPC; PC3, C4-2). The role of Omi/HtrA2 for modulating apoptosis was evaluated by pharmacological inhibition and Western blotting.

RESULTS

Exogenous Zn(2+) initially reduced prostate cancer cell viability but these effects were transitory and were ineffective at enhancing F10 cytotoxicity. The cell-permeable Zn(2+) -chelator tetrakis-(2-pyridylmethl) ethylenediamine (TPEN) induced apoptosis in prostate cancer cells and enhanced the pro-apoptotic effects of F10. The pro-apoptotic effects of Zn(2+) -chelation in combination with F10 treatment were enhanced by inhibiting Omi/HtrA2 implicating this serine protease as a novel target for prostate cancer treatment.

CONCLUSIONS

Zn(2+) -chelation enhances the pro-apoptotic effects of F10 and may be useful for enhancing the effectiveness of F10 for treatment of advanced prostate cancer. The serine protease Omi/HtrA2 modulates Zn(2+) -dependent apoptosis in prostate cancer cells and represents a new target for treatment of CRPC. Prostate 75:360-369, 2015. © 2014 Wiley Periodicals, Inc.

Genome-wide mRNA and microRNA profiling of the NCI 60 cell-line screen and comparison of FdUMP[10] with fluorouracil, floxuridine, and topoisomerase 1 poisons

A profile of microRNA (miRNA) and mRNA expression patterns across the NCI-60 cell-line screen was analyzed to identify expression signatures that correlate with sensitivity to FdUMP[10], fluorouracil (5FU), floxuridine (FdU), topotecan, and irinotecan. Genome-wide profile analyses revealed FdUMP[10] resembles FdU most closely and shows dissimilarities with 5FU. FdUMP[10] had the largest dynamic range of any of these drugs across the NCI-60 indicative of cancer cell-specific activity. Genes involved in endocytosis, such as clathrin (CLTC), SNF8, annexin A6 (ANXA6), and amyloid protein-binding 2 (APPBP2) uniquely correlated with sensitivity to FdUMP[10], consistent with a protein-mediated cellular uptake of FdUMP[10]. Genes involved in nucleotide metabolism were enriched for the three fluoropyrimidine drugs, with the expression profile for 5FU correlated to an RNA-mediated cytotoxic mechanism, whereas expression of glycosyltransferases (XYLT2) that use UDP sugars as substrates and the nucleoside diphosphatase and metastasis suppressor NM23 (NME1) were associated with FdUMP[10] sensitivity. Topotecan and irinotecan had significant negative correlations with miR-24, a miRNA with a high aggregate P(CT) score for topoisomerase 1 (Top1). Our results reveal significant new correlations between FdUMP[10] and Top1 poisons, as well as new information on the unique cytotoxic mechanism and genomic signature of FdUMP[10].

Sulfonyl-containing nucleoside phosphotriesters and phosphoramidates as novel anticancer prodrugs of 5-fluoro-2'-deoxyuridine 5'-monophosphate (FdUMP)

A series of sulfonyl-containing 5-fluoro-2'-deoxyuridine (FdU) phosphotriester and phosphoramidate analogues were designed and synthesized as anticancer prodrugs of FdUMP. Stability studies have demonstrated that these compounds underwent pH dependent beta-elimination to liberate the corresponding nucleotide species with half-lives in the range of 0.33-12.23 h under model physiological conditions in 0.1 M phosphate buffer at pH 7.4 and 37 degrees C. Acceleration of the elimination was observed in the presence of human plasma. Compounds with an FdUMP moiety (4-9) were considerably more potent than those without (1-3) as well as 5-fluorouracil (5-FU) against Chinese hamster lung fibroblasts (V-79 cells) in vitro. Addition of thymidine (10 microM) reversed the growth inhibition activities of only 5-FU and the compounds with an FdUMP moiety, but had no effect on those without. These results are consistent with thymidylate synthase as the target of the prodrugs.

Synthesis and anticancer activities of amphiphilic 5-fluoro-2'-deoxyuridylic acid prodrugs

Amphiphilic anticancer prodrugs of 5'-fluoro-2'-deoxyuridine-5'-monophosphate (5-FdUMP) were synthesized according to the hydrogen phosphonate method by coupling lipophilic cytosine derivatives or a phospholipid with 5-fluoro-2'-deoxyuridine (5-FdU). Studies within the in vitro Anticancer Screen Program of the National Cancer Institute have demonstrated high anticancer activities of the heterodinucleoside phosphates: N4-palmitoyl-2'-deoxycytidylyl-(3' --> 5')-3'-O-acetyl-5-fluoro-2'-deoxyuridine (dC(pam)-5-FdU(Ac), N4-palmitoyl-2',3'-dideoxycytidylyl-(5' --> 5')-3'-O-acetyl-5-fluoro-2'-deoxyuridine (ddC(pam)-(5' --> 5')-5-FdU(Ac), 5-fluoro-2'-deoxyuridylyl-(3' --> 5')-5-fluoro-N4-hexadecyl-2'-deoxycytidine (5-FdU-5-FdC(hex)), and of the new liponucleotide 1-O-octadecyl-rac-glycerylyl-(3 --> 5')-5-fluoro-2'-deoxyuridine (Oct1Gro-(3 --> 5')-5-FdU). The anticancer activities of these prodrugs are comparable to those of 5-FdU and the tumor specificities are modulated by their structures. The highest cytotoxic activity being even superior to 5-FdU was expressed by the dimer 5-FdU-5-FdC(hex).

A novel polypyrimidine antitumor agent FdUMP[10] induces thymineless death with topoisomerase I-DNA complexes

FdUMP[10], a 10mer of 5-fluoro-2'-deoxyuridine 5'-monophosphate (FdUMP), the thymidylate synthase inhibitory metabolite of 5-fluorouracil (FU), is most closely correlated with the DNA topoisomerase I (Top1) inhibitor camptothecin in the National Cancer Institute COMPARE analysis, but not with FU. FdUMP[10] exhibits more potent antiproliferative activity than FdUMP or 5-fluoro-2'-deoxyuridine (FdU) and is markedly more active than FU. Camptothecin-resistant P388/CPT45 cells lacking Top1 are cross-resistant to FdUMP[10] as well as to FdUMP, FdU, and the thymidylate synthase inhibitor raltitrexed (Tomudex). FdUMP[10] induces DNA single-strand breaks and cellular Top1-DNA complexes. Such complexes are also observed in response to FdUMP, FdU, raltitrexed, and FU. The FdUMP[10]-induced Top1-DNA complexes are not inhibited by the caspase inhibitor z-VAD-fmk and form independently of apoptotic DNA fragmentation, indicating that they do not correspond to apoptotic Top1-DNA complexes. In biochemical assay, Top1 is directly trapped at uracil and FdU misincorporation sites. We propose that FdUMP[10] damages DNA by trapping Top1 at uracil and FdU misincorporation sites resulting from thymidylate synthase inhibition and thymine depletion.

Relative free energies of binding to thymidylate synthase of 2- and/or 4-thio and/or 5-fluoro analogues of dUMP

Free energy perturbation calculations have been applied to evaluate the relative free energies of binding of 2'-deoxyuridine-5'-monophosphate (dUMP) and its 2- and/or 4-thio and/or 5-fluoro analogues to the wild-type E. coli thymidylate synthase (ecTS). The results accurately reproduce experimentally measured differences in the free energy of binding of dUMP versus 5-fluoro-dUMP to thymidylate synthase. They indicate that preferred binding of dUMP compared to 5-fluoro-dUMP in the binary complex is equally related to (i) more favorable electrostatic interactions of the dUMP molecule in the enzyme active site, and (ii) its less favorable solvation in the aqueous solution. The relative free energies of binding in the binary complex show moderate and qualitatively indistinguishable discrimination among the studied fluorinated and non-fluorinated 2- and/or 4-thio analogues of dUMP. The binding free energies of monothio analogues of dUMP and 5-fluoro-dUMP correspond quite well with experimentally measured activities of these nucleotides in the thymidylate synthase reaction. On the other hand, the binding free energies of both dithio analogues, 2,4-dithio-dUMP and 2,4-dithio-FdUMP, show lack of such correlation. The latter suggests that very low activities of the dithio analogues of dUMP and 5-fluoro-dUMP may relate more to the covalent reaction of these nucleotides within the ternary complex with TS and 5,10-methylenetetrahydrofolate, than to their pre-covalent binding. We speculate that a lack of substrate activity of 2,4-dithio-dUMP is related to the high aromaticity of its pyrimidine ring that prevents the Michael addition of the active site cysteine thiol to the pyrimidine C6 atom. A stronger affinity of the fluorinated analogues of dUMP to thymidylate synthase, compared to the non-fluorinated congeners, results from the fluorine substituent producing a local strain in the C6 region in the pyrimidine ring, thus sensitizing C6 to the Michael addition of the cysteine thiol.

Efficacy and safety of FdUMP[10] in treatment of HT-29 human colon cancer xenografts

Thymidylate synthase (TS) is the molecular target of fluoropyrimidine (FP) chemotherapy, and novel anticancer drugs effective against TS-overexpressing tumors are required to treat patients with FP-refractory solid tumors. We have evaluated the inhibition of cell proliferation in vitro and antitumor activity in vivo of FdUMP[10], an oligodeoxynucleotide 10mer in which 5-fluorouracil (5-FU) is the only nucleobase. FdUMP[10] is a pro-drug of FdUMP, the TS inhibitory metabolite of FPs. FdUMP[10] was 338-fold more potent than 5-FU at inhibiting cell proliferation in the NCI 60 cell line screen. The antitumor activity of FdUMP[10] was compared to 5-FU using H-T29 xenografts in female CD-1 athymic (nu+/nu+) mice. Treatment with FdUMP[10] as a single agent (40 mg/kg/daily x 5, i.v.) delayed tumor growth and resulted in a smaller mean tumor size (T/C value = 51%, p<0.001 compared with the control group). Treatment with 5-FU (25 mg/kg/daily x 5, i.p.) had similar results as single agent FdUMP[10] (T/C value = 65%, p=0.238 compared with the FdUMP[10] treated group. Simultaneous treatment of tumor-bearing mice with both drugs (FdUMP[10] plus 5-FU) further delayed tumor growth (T/C value = 36%; p=0.003 relative to 5-FU). The results from the combined treatment group were not, however, statistically significant relative to the group receiving single agent FdUMP[10] treatment (p=0.059). Histological examination revealed systemic damage was limited to the colonic epithelium in all treatment groups and was least extensive with single agent FdUMP[10] compared to the other treatment groups. The data support the concept that FdUMP[10] is a useful prototype of a novel type of FP that is likely to be more efficacious than FPs in clinical use.

Cytotoxic targeting of F9 teratocarcinoma tumours with anti-ED-B fibronectin scFv antibody modified liposomes

We prepared small unilamellar liposomes derivatised with single chain antibody fragments specific for the ED-B domain of B-fibronectin. This extracellular matrix associated protein is expressed around newly forming blood vessels in the vicinity of many types of tumours. The single chain antibody fragments were functionalised by introduction of C-terminal cysteines and linked to liposomes via maleimide groups located at the terminal ends of poly(ethylene glycol) modified phospholipids. The properties of these anti-ED-B single chain antibody fragments-liposomes were analysed in vitro on ED-B fibronectin expressing Caco-2 cells and in vivo by studying their biodistribution and their therapeutic potential in mice bearing subcutanous F9 teratocarcinoma tumours. Radioactively labelled ((114m)Indium) single chain antibody fragments-liposomes accumulated in the tumours at 2-3-fold higher concentrations during the first 2 h after i.v. injection compared to unmodified liposomes. After 6-24 h both liposome types were found in similar amounts (8-10% injected dose g(-1)) in the tumours. Animals treated i.v. with single chain antibody fragments-liposomes containing the new cytotoxic agent 2'-deoxy-5-fluorouridylyl-N(4)-octadecyl-1-beta-D-arabinofuranosylcytosine (30 mg kg(-1) per dose, five times every 24 h) showed a reduction of tumour growth by 62-90% determined on days 5 and 8, respectively, compared to animals receiving control liposomes. Histological analysis revealed a marked reduction of F9 tumour cells and excessive deposition of fibronectin in the extracellular matrix after treatment with single chain antibody fragments-2-dioxy-5-fluorouridylyl-N(4)-octadecyl-1-beta-D-arabinofuranosylcytosine-liposomes. Single chain antibody fragments-liposomes targeted to ED-B fibronectin positive tumours therefore represent a promising and versatile novel drug delivery system for the treatment of tumours.

Synthesis and biological studies of novel nucleoside phosphoramidate prodrugs

A novel approach to the intracellular delivery of nucleotides using phosphoramidate-based prodrugs is described. Specifically, we have developed phosphoramidate prodrugs of the anticancer nucleotide 5-fluoro-2'-deoxyuridine-5'monophosphate (FdUMP). These phosphoramidate prodrugs contain an ester group that undergoes intracellular activation liberating phosphoramidate anion, which undergoes spontaneous cyclization and P-N bond cleavage to yield the nucleoside monophosphate quantitatively. In vitro evaluation of 5-fluoro-2'-deoxyuridine phosphoramidate prodrugs 2a and 3b against L1210 mouse leukemia cells show potent inhibition of cell growth (IC(50) 0.5-3 nM). Cell-based thymidylate synthase inhibition studies show that, in contrast to FUdR, the nitrofuran compound 2a is of comparable potency in wild type vs thymidine kinase deficient LM cells. This result indicates that the activation of this novel prodrug occurs via the proposed mechanism of intracellular delivery. However, naphthoquinone 3b has an IC(50) value for thymidylate synthase inhibition that is comparable to FUdR in thymidine kinase deficient cells. Further studies revealed that 3b rapidly decomposes to the nucleotide in cell culture medium, suggesting that the naphthoquinone analogue is not sufficiently stable to function as a nucleotide prodrug.

Targeted drug delivery to chemoresistant cells: folic acid derivatization of FdUMP[10] enhances cytotoxicity toward 5-FU-resistant human colorectal tumor cells

Current chemotherapy protocols that include fluoropyrimidines, such as 5-fluorouracil (5-FU), are limited by the development of chemoresistance during the course of treatment. Our laboratory has developed a novel class of fluoropyrimidines, FdUMP[N], that are oligodeoxynucleotides (ODNs) composed of some number, N, of 5-fluoro-2'-deoxyuridine-5'-O-monphosphate (FdUMP) nucleotides. Novel synthetic procedures are described that permit conjugation of folic acid to the 5'-OH of FdUMP[10] via a phosphodiester linkage using automated synthesis. The synthetic methods developed are generally applicable for ODN conjugation with folic acid. The folic acid conjugate FA-FdUMP[10] showed improved cytotoxicity toward human colorectal tumor cells (H630), and 5-FU-resistant colorectal tumor cells (H630-10). Enhanced cytotoxicity was observed for FA-FdUMP[10] relative to nonconjugated FdUMP[10] for cells grown under folate-restricted conditions, consistent with cellular uptake being, in part, receptor-mediated. Folate receptor alpha (FRalpha) mRNA was shown by RT-PCR to be overexpressed 26.3-fold in 5-FU-resistant H630-10 cells relative to H630 cells. Thus, FA-FdUMP[N] may prove useful for the treatment of 5-FU-resistant malignancies.

Interleukin-2 and interleukin-4 increase the survival of retinal ganglion cells in culture

Natural cell death is a degenerative phenomenon observed during the normal development of the nervous system. The neuroprotective effects of cytokines produced by neuronal, glial or infiltrating cells on neurons have been extensively studied. In this work we studied the role of interleukin (IL)-2 and IL-4 on the survival of retinal ganglion cells (RGC) after 48 h in culture. Our results demonstrate that the effect of both ILs was dose-dependent and the treatment with either IL-2 (50 U/ml) or IL-4 (5 U/ml) induced a 2-fold increase in RGC survival. The effect of IL-4, but not of IL-2, was totally abolished by either 20 microM 5-fluoro-2'-deoxyuridine, an inhibitor of cell proliferation, or by 1 microM telenzepine, an inhibitor of M1 muscarinic receptor. Our results suggest that both cytokines could play an important role during the development of retinal tissue as well as during retina trauma.

The novel heterodinucleoside dimer 5-FdU-NOAC is a potent cytotoxic drug and a p53-independent inducer of apoptosis in the androgen-independent human prostate cancer cell lines PC-3 and DU-145

BACKGROUND

We analyzed the cytotoxic properties of the new heterodinucleoside phosphate dimer 5-FdU-NOAC, which is composed of the cytotoxic drugs 5-FdU and N(4)-octadecyl-1-beta-D-arabinofuranosylcytosine (NOAC) against human prostate tumor cells.

METHODS

5-FdU-NOAC effects on cell proliferation, cell cycle distribution, thymidylate synthase activity, and apoptosis were investigated in vitro in the two human prostate carcinoma cell lines DU-145 and PC-3 and compared to cells treated with the corresponding single drugs 5-FdU and NOAC.

RESULTS

Treatment of the cells with 5-FdU-NOAC resulted in IC(50) values of 3.9-5 microM and in a complete inhibition of cell proliferation at 200 microM after 96 hr compared to 5-FdU, where 10% of the cells remained resistant. Flow cytometric analysis revealed cell cycle perturbations in S-phase only in the DU-145 cells. 5-FdU-NOAC caused 50% inhibition of thymidylate synthase after 90 min at 0.6 microM in both cell lines. Apoptotic cell fractions in DU-145 (66%) and in PC-3 (34%) cells were found after treatment with 5-FdU-NOAC for 96 hr. DNA fragmentation further confirmed the induction of apoptosis.

CONCLUSIONS

5-FdU-NOAC inhibits thymidylate synthase and cell cycle progression causing proliferation arrest and apoptosis in DU-145 and PC-3 cells, suggesting a potential role of 5-FdU-NOAC for the treatment of prostate cancer.

Acyclic analogues of 5-fluoro-dUMP and 5-fluoro-2'-deoxyuridine: synthesis and inhibition of thymidylate synthase and tumour cell growth

1-[(2-Hydroxyethoxy)methyl]-5-fluorouracil (HEMFU) and 1-[(1,3-dihydroxy-2-propoxy)methyl]-5-fluorouracil (DHPFU) were prepared by alkylation of the di-O-TMS derivative of 5-fluorouracil and phosphorylated with the use of the wheat shoot phosphotransferase system to their monophosphates, HEMFUMP and DHPFUMP. 1-(2-Phosphonylmethoxyethyl)-5-fluorouracil (PMEFU) was obtained by condensation of diethyl-2-chloroethoxymethanephosphonate with 5-fluorouracil and cleavage of the alkylphosphoester with trimethylbromosilane. Inhibition of highly purified thymidylate synthase from mouse tumour Ehrlich carcinoma and leukemia L1210 cells by each of the nucleotide analogues, DHPFUMP, PMEFU and HEMFUMP, and of L5178Y mouse leukemia cell growth by the nucleoside (HEMFU) analogue, were studied. DHPFUMP proved to be the strongest inhibitor, non-competitive vs dUMP, with K(i)app 2.8 microM for time-independent interaction with the enzyme and N5,N10-methylenetetrahydrofolate (CH2H4PteGlu). In the presence of CH2H4PteGlu, DHPFUMP exhibited time-dependent inactivation of the enzyme, the inactivation rate plots being biphasic and pointing to Ki values in the microM range (10(3)-fold higher than for 5-fluoro-dUMP). HEMFUMP and PMEFU were much weaker inhibitors of the enzyme, with K(i)app values of 0.26 mM (non-competitive vs dUMP) and 30 mM (non-competitive vs dUMP), respectively. HEMFU, despite the weak interaction of its nucleotide analogue with the enzyme, proved to be a strong cell (L5178Y) growth inhibitor, with IC50 in the range 10(-5) M.

Inhibitory effects of fluorinated pyrimidines, 5'-DFUR, UFT and T-506, in a model of hepatic metastasis of mouse colon 26 adenocarcinoma-assessment of inhibitory activity and adverse reactions at the maximum tolerated dose

The effects of fluorinated pyrimidines, 5'-DFUR, UFT and T-506, on a mouse model of hepatic metastasis were assessed in regard to inhibitory activity and adverse reactions at the maximum tolerated dose. The model was prepared by injecting the mouse colonic cancer cell line, colon 26, into the portal vein of CDF1 mice. At the treatment regimens employed for 5'-DFUR (1.0 mmol/kg/day, p.o., daily from days 1 to 7), UFT (0.1 mmol/kg/day, p.o., daily from days 1 to 7), and T-506 (0.074 mmol/kg/day, i.v., days 1, 4, 7, and 10), complete inhibition of hepatic metastasis was obtained in six out of seven mice (85.7%) with 5'-DFUR, and in five out of six mice (83.3%) with T-506. Significant inhibition of hepatic metastasis was not achieved with UFT (3/7, 42.9%). 5'-DFUR and T-506 showed the highest rate of inhibition of hepatic metastasis, suggesting that these drugs would be effective for the prophylactic treatment of metastatic disease. 5'-DFUR and UFT exhibited mild adverse reactions such as loss of body weight.

Synthesis and interactions with thymidylate synthase of 2,4-dithio analogues of dUMP and 5-fluoro-dUMP

The 2,4-dithio analogues of 2'-deoxyuridine and 2'-deoxy-5-fluorouridine have been synthesized by thiation of the previously described 2-thio analogues, and then phosphorylated enzymatically or chemically to yield 2,4-dithio-dUMP and 2,4-dithio-5-fluoro-dUMP. In striking contrast to the 2-thio and 4-thio analogues of dUMP, which are good substrates of thymidylate synthase, 2,4-dithio-dUMP is not a substrate. But, surprisingly, it is a competitive inhibitor, relative to dUMP, of the purified enzymes from both parental and FdUrd-resistant L1210 cells, with K(i) values of 32 microM and 55 microM, respectively. Although 2,4-dithio-5-fluoro-dUMP behaved as a typical slow-binding inhibitor of the enzyme, its K(i) value was 10(3)-10(4)-fold higher than those for the corresponding 2-thio and 4-thio congeners. Similarly, 2,4-dithio-FdUrd was a much weaker inhibitor of tumour cell growth (IC50 approximately 10(-5)M) than FdUrd (IC50 approximately 10(-9)M), 2-thio-FdUrd(IC50 approximately 10(-7)M) or 4-thio-FdUrd (IC50 approximately 5x10(-8)M), while with 2,4-dithio-dUrd no influence on cell growth could be observed. Theoretical considerations, based on calculated aromaticities of the uracil and thiouracil rings, suggest that lack of substrate activity of 2,4-dithio-dUMP may result from increased pyrimidine ring aromaticity of the latter, leading to resistance of C(6) to nucleophilic attack by the enzyme active center cysteine.

Thymidylate synthases from Hymenolepis diminuta and regenerating rat liver: purification, properties, and inhibition by substrate and cofactor analogues

Comparative studies of thymidylate synthases, isolated from the tapeworm, Hymenolepis diminuta, and regenerating liver of its host, rat, aimed at a possibility of specific inhibition of the helminthic enzyme, are presented. While similar in structure (dimers with monomer molecular masses of 33.7 kDa and 34.9 kDa, respectively) and parameters describing interactions with substrates and products, the tapeworm and rat enzymes differed in the dependences of reaction velocity on temperature (Arrhenius plots biphasic and linear, respectively). The tapeworm, compared with the host, enzyme was less sensitive to the competitive slow-binding inhibition by 5-fluoro-dUMP and its 2-thio congener, but equally sensitive to inhibition by 4-thio-5-fluoro-dUMP, N4-hydroxy-dCMP and N4-hydroxy-5-fluoro-dCMP, the latter being more potent inhibitor of the parasite enzyme than 5-fluoro-dUMP. alpha-Anomer of 5-fluoro-dUMP behaved as a very weak competitive slow-binding inhibitor of both enzymes. Both enzymes differed markedly in sensitivity to inhibition by 10-propargyl-5,8-dideazafolate and its di- and triglutamates (pddPteGlu1-3), with pddPteGlu1 being stronger inhibitor of the mammalian enzyme, but pddPteGlu3 showing opposite specificity. Sulfonamidobenzoylglutamate analogue of pddPteGlu (pddPteSO2Glu) and 2-desamino-2-methyl derivative of this analogue (CH3pddPteSO2Glu) were weaker inhibitors of both enzymes than the parent compound. Substitution of the glutamyl residue in CH3pddPteSO2Glu with either norvaline or alanine increased inhibition potency, whereas similar substitutions with glycine, valine or phenylglycine were without a distinct effect with the host enzyme but weakened inhibition of the tapeworm enzyme.

5'-[4-(Pivaloyloxy)-1,3,2-dioxaphosphorinan-2-yl]-2'-deoxy-5-fluorouridine: a membrane-permeating prodrug of 5-fluoro-2'-deoxyuridylic acid (FdUMP)

5'-[4-(Pivaloyloxy)-1,3,2-dioxaphosphorinan-2-yl]-2'-deoxy-5 -fluorouridine (1c) was designed as a potential membrane-permeable prodrug of 2'-deoxy-5-fluorouridine 5'-monophosphate (FdUMP), a putative active metabolite of the antitumor drug 5-fluorouracil (FU). It was anticipated that 1c would be hydrolyzed in vivo by carboxylate esterase (E.C. 3.1.1.1) to the labile 4-hydroxy analogue 2a, which should penetrate cells by passive diffusion and ring open to the aldehyde 3a. Spontaneous elimination of acrolein from 3a would then generate the free nucleotide, FdUMP. 1c might also penetrate cells directly and undergo the same degradation sequence after hydrolysis by cellular esterases. 1c was prepared by condensing 2-hydroxy-2-oxo-4-(pivaloyloxy)-1,3,2-dioxaphosphorinane with 2'-deoxy-5-fluorouridine (FUdR) in the presence of triphenylphosphine and diethyl azodicarboxylate. 1c was moderately stable in aqueous buffers over the pH range 1-7.4 (T1/2 > 30 h). In the presence of carboxylate esterase, however, it was degraded, in a concentration-dependent manner, to FdUMP. No intermediates were detected in the incubation mixture. In mouse plasma, 1c was degraded first to FdUMP and then to FUdR. The latter is presumably formed by dephosphorylation of FdUMP by plasma 5'-nucleotidases or phosphatases. 1c and FU inhibited the growth of Chinese hamster ovary (CHO) cells in culture at a concentration of 5 x 10(-6) M. 1c was equally potent against a CHO variant that was 20-fold resistant to FU. Administered intraperitoneally for 5 consecutive days, 1c was as effective as FU at prolonging the life span of mice bearing P-388 leukemia. In the presence of 2-mercaptoehtanesulfonic acid, an acrolein scavenger, 1c was equally effective against a P-388 mutant cell line that was resistant to FU. Collectively, these data suggest that 1c acts as a membrane-permeable prodrug of FdUMP. This prodrug strategy may be generally useful for introducing dianionic phosphates and phosphonates into cells.

Synthesis and antitumor evaluation of bis[(pivaloyloxy)methyl] 2'-deoxy-5-fluorouridine 5'-monophosphate (FdUMP): a strategy to introduce nucleotides into cells

The bis[(pivaloyloxy)methyl] [PIV2] derivative of 2'-deoxy-5- fluorouridine 5'-monophosphate (FdUMP) was synthesized as a potential membrane-permeable prodrug of FdUMP. The compound was designed to enter cells by passive diffusion and to revert to FdUMP after removal of the PIV groups by hydrolytic enzymes. The most convenient preparation of PIV2FdUMP was by condensation of 2'-deoxy-5-fluorouridine (FUdR) with PIV2 phosphate in the presence of triphenylphosphine and diethyl azodicarboxylate (the Mitsunobo reagent). PIV2FdUMP was stable in the pH range 1.0-4.0 (t1/2 > 100 h). It was also fairly stable at pH 7.4 (t1/2 = 40.2 h). In 0.05 M NaOH solution, however, it was rapidly degraded (t1/2 < 2 min). In the presence of hog liver carboxylate esterases, PIV2FdUMP was converted quantitatively to the mono-[(pivaloyloxy)methyl] [PIV1] analogue PIV1FdUMP. After a 24 h incubation, only trace amounts of FdUMP (1-3%) were observed, indicating that PIV1FdUMP is a poor substrate for carboxylate esterases. In mouse plasma, PIV2FdUMP was rapidly metabolized, first to PIV1FdUMP and then to FdUMP. With continued incubation, FUdR was formed, presumably due to further catabolism of FdUMP by plasma phosphatases or 5'-nucleotidases. Since PIV1FdUMP is a poor substrate for carboxylate esterase, the cleavage of the second PIV group is most likely mediated by plasma phosphodiesterases. The rate of degradation of PIV2FdUMP in the presence of acid and alkaline phosphatase, 5'-nucleotidase, or spleen phosphodiesterase was the same as that in buffer controls, indicating that the compound is not a substrate for these nucleotide catabolizing enzymes. The concentration of PIV2FdUMP and its 3'-O-acetyl ester (PIV2 3'-O-Ac-FdUMP) required to inhibit the growth of Chinese hamster ovary (CHO) cells in vitro to less than 50 cells per colony was 5 x 10(-6) M, the same as that required for 5-fluorouracil (FU). Both nucleotide prodrugs showed the same growth-inhibitory potency against a mutant CHO cell line that was 20-fold resistant to FU (CHO/FU). Administered intraperitoneally at optimal dosage for 5 consecutive days, PIV2FdUMP and PIV2 3'-O-Ac-FdUMP were as effective as FU at prolonging the life spans of mice bearing intraperitoneally implanted P388 leukemia. Both prodrugs retained full therapeutic activity against a P388 subline resistant to FU. Collectively, these data indicate that PIV2FdUMP and PIV2 3'-O-Ac-FdUMP are effective membrane-permeable prodrugs of FdUMP.

Interaction of 2-thio-5-fluoro-dUMP and 4-thio-5-fluoro-dUMP with mammalian normal and tumour and helminthic thymidylate synthases: influence of C(4)-substituents on specificity for enzyme inactivation

To determine how 5-fluoro-dUMP modifications may affect its specificity, 2-thio-5-fluoro-dUMP and 4-thio-5-fluoro-dUMP were compared as inhibitors of thymidylate synthases isolated from parental and FdUrd-resistant mouse leukemia L1210 cells, human and rat colon adenocarcinomas, regenerating rat liver and the tapeworm, Hymenolepis diminuta, differing in sensitivity to time- and N5,10-methylenetetrahydrofolate-dependent inactivation by 5-fluoro-dUMP (Ki values ranging from 10(-9) to 10(-7) M). Inactivation by 2-thio-5-fluoro-dUMP, relative to 5-fluoro-dUMP, was 5-20-fold weaker, with specificity for inactivation of different thymidylate synthases paralleling that of 5-fluoro-dUMP. By contrast, 4-thio-5-fluoro-dUMP showed very different specificity, being as potent an inactivator for some enzymes as 5-fluoro-dUMP, but 45-85-fold weaker for others. The results suggest that an interplay between substituents at C(4) and C(5) of the pyrimidine ring may affect the specificity of thymidylate synthase inactivation.

[Phase I clinical study of TT-62. Research group of TT-62]

TT-62 is a new derivative of FdUMP, which is the active metabolite of 5-FU. A phase I clinical study of TT-62 was conducted by a cooperative study. The same patients received single and 2-week oral administration of TT-62. Starting from 60 mg/m2 (1n), the dose was escalated to 420 mg/m2 (7n). In the single administration, the maximum tolerated dose (MTD) could not be determined. In the 2-week administration, MTD was 420 mg/m2, and the dose limiting factor was gastro-intestinal disturbances such as anorexia, nausea, vomiting and diarrhea. Increases in GOT.GPT and a decrease in hemoglobin content were observed. After administration was stopped all side effects disappeared. TT-62 was detected mainly in the plasma, while trace amounts of 5-FU and FUdR were also detected. TT-62 was excreted mostly in the urine, as alpha-fluoro-beta-alanine (FBAL). The cumulative urinary excretion of FBAL was about 80% of the total dose, and the oral absorption of TT-62 was thus thought to be good.

[Development of fluorinated pyrimidines in Japan]

Four fluorinated pyrimidines that are in the stage of clinical trials at present in Japan were reviewed: BOF-A2, Ro09-1390, TT-62 and S-1. Both BOF-A 2 and S-1 are a compound of 5-FU derivative combined or mixed with an inhibitor of 5-FU degradation in order to prolong the blood 5-FU level as well as increase selective toxicity to tumor. Furthermore, an inhibitor of 5-FU phosphorylation in G1 tract contained in S-1 reduces G1 toxicity such as diarrhea etc due to prolongation of blood 5-FU level. Ro09-1390 is an improved compound of 5'-DFUR, which intends to reduce diarrhea caused by the latter. TT-62 is a FdUMP derivative and an active metabolite of 5-FU for oral formulation, which is superior to available 5-FU type anticancer agents in efficacy, and doesn't show cross tolerance to 5-FU.

Antitumor effect and tumor level of 5-fluoro-2'-deoxyuridylate following oral administration of tetradecyl 2'-deoxy-5-fluoro-5'-uridylate

The antitumor effect and tumor levels of 5-fluoro-2'-deoxyuridylate (FdUMP) following oral administration of tetradecyl 2'-deoxy-5-fluoro-5'-uridylate (TT-62) were compared with those attained following intravenous (i.v.) or intraperitoneal (i.p.) administration of 5-fluorouracil (5-FU) or 5-fluoro-2'-deoxyuridine (FUdR) in BDF1 mice bearing murine mammary adenocarcinoma 755 and athymic mice bearing the transplantable human colon adenocarcinoma LS174T. Oral administration of TT-62 showed a stronger antitumor effect against adenocarcinoma 755 than FUdR. The maximum effect of TT-62 was similar to that of 5-FU. However, TT-62 and FUdR treatments were more effective than i.v. administration of 5-FU against LS174T. Thus, oral administration of TT-62 showed marked antitumor activity in both tumor systems. The maximum tolerated dose of FUdR resulted in a much higher level of free FdUMP in the LS174T tumor than that obtained with 5-FU. After oral administration of TT-62 the levels of FdUMP in the tumor were about 10 times those attained with 5-FU, but significantly lower than the levels obtained following i.v. administration of FUdR. With TT-62 the levels of FdUMP in the tumor reached their peak at 60 min following the administration and gradually decreased thereafter. However, FdUMP levels after administration of FUdR decreased rapidly. Three hours after the administration of TT-62 and for up to 24 h the FdUMP levels in the LS174T tumor were almost the same as after administration of FUdR, i.e. effective levels of FdUMP were maintained for a long time with TT-62.

[Antitumor activity of T-506, a novel synthetic FUDR derivative, on murine colon cancer and its hepatic metastasis]

T-506 is a novel synthetic FUDR derivative which releases FUDR slowly in vivo. We studied antitumor activity of T-506 by i.v. injection against mouse colon cancer, colon 26. When T-506 was administrated to mice daily, from day 1 through day 10, or every 3 days, on days 1, 4, 7, and 11, after s.c. inoculation of the tumor, the survival period was expanded significantly. The subcutaneous tumor growth was also inhibited according to the dose levels. Then, we compared the therapeutic effects on the experimental hepatic metastasis of colon 26 between T-506, 5'-DFUR and UFT at each maximal tolerable dose; that is, T-506 (0.074 m mole/kg/day; i.v. on days 1, 4, 7, and 10), 5'-DFUR (1.0 m mole/kg/day; P. O. from day 1 to 7), UFT (0.1 m mole/kg/day; P. O. from day 1 to 7). T-506 and 5'-DFUR suppressed completely the metastases of 5 of 6 (83.3%) mice and 6 of 7 (85.7%), respectively. UFT did not show a significant inhibitory effect. However, since the loss of body weight was more marked in T-506 than in the other two drugs, the side effect was thought to be a serious problem. These data suggested that if the side effect could be overcome, T-506 would be useful clinically for the treatment of gastrointestinal cancers or hepatic metastases.

Evidence for acyloxymethyl esters of pyrimidine 5'-deoxyribonucleotides as extracellular sources of active 5'-deoxyribonucleotides in cultured cells

Cells commonly resist growth inhibition by purine and pyrimidine bases and nucleosides by restricting intracellular formation of the corresponding 5'-mononucleotides. Nucleotide derivatives that can act as effective membrane-transport precursors of the poorly membrane-permeable nucleotides have not been identified so far. We studied the bis(pivaloyloxymethyl)ester (I) of FdUMP (5-fluoro-dUMP) and a cyclic phosphodiester (II) of FdUMP derived from 1,3-dihydroxyl-1-C-(pivaloyloxy-methyl)propane which are active in vivo against a 5-fluoro-2'-deoxyuridine (FUdR)-resistant mouse leukemia and are attacked by carboxylic esterases under physiological conditions to produce FdUMP by elimination of formaldehyde and acrolein respectively. The assay for intracellular FdUMP was the inhibition of DNA synthesis due to inhibition of TMP synthetase in cultured mouse LM(TK-) fibroblasts genetically devoid of thymidine kinase (TK) and thus unable to convert FUdR directly to FdUMP. At 10(-6)M, I, II, or FUdR inhibited DNA synthesis in 2 hr by 99, 80, and 35% respectively; at 10(-5)M. maximal inhibition was attained after less than 15, 30 and 90 min respectively. Inhibition of DNA synthesis in TK+ cells by 10(-5) M I, II, or FUdR was reversed completely by 10(-5)M thymidine (TdR) but unaffected by 10(-5)M UdR, confirming TMP synthetase as the locus of inhibition. At 10(-5)M, bis(pivaloyloxymethyl) esters of phenyl phosphate or a p-substituted benzylphosphonic acid did not inhibit significantly DNA synthesis in TK+ cells. From this finding, and from effects produced by V (see below), we conclude that pivalic acid and CH2O arising from I contribute little to its above inhibitory effects. In TK- cells in which DNA synthesis is prevented by blockade of TMP synthetase with aminopterin, the bis(pivaloyloxymethyl) ester (V) of TMP, at 0.9 x 10(-4) M, induced a 4-fold faster rate of DNA synthesis than did 10(-3)M TMP, whereas 10(-3) M TdR did not affect the rate. After 3 hr the rate with V was 80% that in the absence of aminopterin. In the above systems the nucleotide diesters I, II and V appear to be acting as effective extracellular sources of active intracellular FdUMP and TMP, in processes that involve loss of the two esterifying groups.

Interaction of 5-fluoro-4-thio-2'-deoxyuridine 5'-phosphate with mammalian tumour thymidylate synthase: role of the pyrimidine N(3)-H dissociation

The role of the pyrimidine N(3)-H in binding of dUMP derivatives to thymidylate synthase was evaluated with the aid of a new dUMP analogue, 5-fluoro-4-thio-dUMP, synthesized by an improved thiation and enzymatic phosphorylation. The interaction of this analogue, and of 5-FdUMP, with the enzyme, and the pH-dependence of these interactions, were compared. Both were slow-binding competitive inhibitors of the enzyme from Ehrlich carcinoma, L1210 and CCRF-CEM cells, with Ki an order of magnitude higher for 5-fluoro-4-thio-dUMP than for 5-FdUMP. With both nucleotides, as well as the parent nucleosides, enzyme inactivation increased as the pH was lowered from 8 to 6. Maximum inactivation with 5-FdUrd was at pH 7.0, and with 5-fluoro-4-thio-dUrd at pH 6.0, in agreement with the higher pKa for the N(3)-H dissociation of the former, and pointing to participation of the N(3)-H as a hydrogen donor in binding to the enzyme.

Base analogue mutagenesis in yeast and its modulation by pyrimidine deoxynucleotide pool imbalances: incorporation of bromodeoxyuridylate and iododeoxyuridylate

Cells of the yeast, Saccharomyces cerevisiae, which are auxotrophic for thymidylate (tmpl) can also incorporate analogues of thymidylate. When the base analogue, 5-bromodeoxyuridylate, is incorporated into tmpl yeast cells it is lethal and mutagenic. Both lethality and mutation induction can be drastically altered by perturbation of the pyrimidine nucleotide pools. Analysis of mutation induction, bromodeoxyuridylate incorporation into DNA, and cell viability under various conditions revealed: (1) lethality and mutagenesis can be uncoupled, (2) thymidylate enhances mutagenesis and deoxycytidylate suppresses it, (3) mutation induction is not correlated with the magnitude of bromodeoxyuridylate incorporation into DNA. Therefore, in yeast, the pyrimidine nucleotide pools have a powerful effect on bromodeoxyuridylate mutagenesis. Both bromodeoxyuridylate and iododeoxyuridylate are extensively incorporated into the DNA of tmpl yeast cells; however, iododeoxyuridylate is non-mutagenic. Replication proceeds at the same rate in the presence of the natural substrate or either analogue. When cells are supplied with thymidylate and bromodeoxyuridylate together, there is no discrimination against bromodeoxyuridylate as a DNA precursor. However, in the presence of thymidylate and iododeoxyuridylate, there is a 3 to 1 discrimination against iododeoxyuridylate as compared to thymidylate.

Studies on the interaction with thymidylate synthase of analogues of 2'-deoxyuridine-5'-phosphate and 5-fluoro-2'-deoxyuridine-5'-phosphate with modified phosphate groups

The role of the phosphate moiety of dUMP, and some analogues, in their interaction with mammalian thymidylate synthase, has been investigated. Substrate and inhibitor activities, and the pH-dependence of these activities, of dUMP and 5-FdUMP, as well as analogues with modified phosphate groups, were compared. The methyl ester of dUMP was neither a substrate nor an inhibitor. By contrast, the methyl ester of 5-FdUMP was a slow-binding inhibitor of the enzyme from L1210, Ehrlich ascites carcinoma and CCRF-CEM cells, with Ki values in the micromolar range. Both 5-FdUrd and the newly synthesized 5'-methylphosphonate of 5-FdUrd were also slow-binding inhibitors of the Ehrlich carcinoma enzyme, but with Ki values in the millimolar range. The interaction of dUMP, 5-FdUMP, and the methyl ester of the latter decreased with increase in pH, whereas that of the 5'-methyl-phosphonate of 5-FdUrd remained unchanged. The results are discussed in relation to the role of the phosphate hydroxyls of dUMP in binding to the enzyme. 5-FdUMP and its analogues exhibited differing interactions with two binding sites on the enzyme molecule, consistent with cooperativity of binding. A convenient procedure is described for the synthesis of 5-fluoro-2'-deoxyuridine-5'-methylphosphonate, applicable also to the preparation of other 5'-methylphosphonate analogues.

Mechanism of action of 2',5-difluoro-1-arabinosyluracil

Results are described which demonstrate that the cytotoxic action of 2',5-difluoro-1-arabinosyluracil (FFara-Ura) involves conversion to the corresponding 5'-phosphate, FFara-UMP, and subsequent inhibition of thymidylate synthetase. The evidence for this is as follows: (a) cells lacking thymidine kinase are 120-fold more resistant to FFara-Ura; (b) FFara-Ura markedly inhibits the incorporation of 2'-deoxyuridine (dUrd) into DNA with little or no effect on 2'-deoxythymidine (dThd) incorporation; (c) FFara-Ura causes changes in deoxynucleoside triphosphate pool sizes, which are characteristic of specific inhibition of dTMP synthetase. Binding and spectroscopic studies demonstrate that FFara-UMP inactivates dTMP synthetase from Lactobacillus casei in a manner analogous to that described for FdUMP. Furthermore, FFara-Ura is not a substrate for the pyrimidine phosphorylases; the significance of this finding with regard to the possible chemotherapeutic utility of FFara-Ura is discussed.

Synthesis and biological evaluation of neutral derivatives of 5-fluoro-2'-deoxyuridine 5'-phosphate

5-Fluoro-5'-(2-oxo-1,3,2-oxazaphosphorinan-2-yl)-2'-deoxyuridine (1a) and 5-fluoro-5'-(2-oxo-1,3,2-dioxaphosphorinan-2-yl)-2'-deoxyuridine (1b) were prepared by reaction of 5-fluoro-2'-deoxyuridine (7a) and phosphoryl chloride with 3-amino-1-propanol and 1,3-propanediol, respectively. The thymidine analogues, 1c and 1d, were prepared similarly from thymidine. Compound 1b was synthesized in better yield from 13a and trimethylene phosphate with triphenylphosphine/diethyl azodicarboxylate as a condensing agent. Compounds 1a-d were resistant to degradation by 5'-nucleotidase, alkaline phosphatase, venom phosphodiesterase, and crude snake venom. None of these compounds were significantly biotransformed when incubated with mouse hepatic microsomal preparations in the presence of an NADPH-generating system. When administered intraperitoneally (ip) for 5 consecutive days, 1a was nearly as effective as 5-fluorouracil at prolonging the life spans of BDF1 mice implanted intraperitoneally with leukemia P-388. However, much larger dosages of 1a were required for optimal activity. Compound 1b administered similarly was only marginally effective. Neither 1a nor 1b was active against a P-388 mutant resistant to 5-fluorouracil.

Effects of retinoic acid on differentiation of choriocarcinoma cells in vitro

Choriocarcinoma cells maintain multiple hormonal functions in culture. We have found that these cells secreted no immunoreactive pregnancy-specific beta 1-glycoprotein (PS beta G), a placental protein. Choriocarcinoma cells can be induced to synthesize low levels of PS beta G by retinoic acid, 8-bromo-cAMP (8BrcAMP), cholera toxin, methyl-isobutylxanthine (MIX), and 5-bromo-2'-deoxyuridine (BrdUrd). The simultaneous addition of retinoic acid along with 8BrcAMP, cholera toxin, or MIX gave synergistic induction of PS beta G. The simultaneous addition of retinoic acid and BrdUrd failed to give even additive induction. In addition to stimulating PS beta G production, retinoic acid increased the production of hCG and its alpha-subunit (hCG alpha) by choriocarcinoma cells. The simultaneous addition of retinoic acid along with 8BrcAMP, cholera toxin, or MIX gave additive induction for hCG and hCG alpha. Passage of choriocarcinoma cells in medium containing retinoic acid induced a stable altered phenotype characterized by elevated levels of PS beta G, hCG, and hCG alpha. These retinoid-treated choriocarcinoma cells remained responsive to 8BrcAMP or compounds that increase intracellular cAMP concentrations and to BrdUrd; the production to PS beta G and hCG was greatly stimulated by 8BrcAMP, cholera toxin, or MIX, and the production of hCG and hCG alpha was greatly inhibited by BrdUrd. However, the production of hCG alpha was only slightly induced by these cAMP modulators, and the production of PS beta G was not increased by BrdUrd.

Synthesis and biological activity of 5-fluoro-2'-deoxyuridine 5'-phosphorodiamidates

Three 5'-phosphorodiamidate derivatives of 5-fluoro-2'-deoxyuridine (FdUrd), 5-fluoro-2'-deoxyuridine 5'-phosphorodiamidate (4a), 5'-phosphorodiimidazolidate (4b), and 5'-phosphorodimorpholidate (4c), were synthesized by aminolysis of 5-fluoro-2'-deoxyuridine 5'-phosphorodichloridate with the respective amine. In culture, these 5'-phosphorodiamidates inhibited the growth of murine leukemia (L5178Y) cells. 5-Fluoro-2'-deoxyuridine 5'-phosphorodiamidate (4a) was the most active derivative and, on a molar basis, produced a cytostatic effect comparable to that of FdUrd and 5-fluoro-2'-deoxyuridine 5'-monophosphate (FdUrd-5'-P). Compounds 4b and 4c were less active than 4a, with relative rates of activity 4a > 4b > 4c that corresponded to their rates of hydrolysis to FdUrd-5'-P. None of the 5'-phosphorodiamidates inhibited thymidylate synthetase of concentrations up to 1 mM.

5-Fluoro-2'deoxyuridine 5'-(p-azidophenyl phosphate), a potential photoaffinity label of thymidylate synthetase

5-Fluoro-2'-deoxyuridine 5'-(p-azidophenyl phosphate) (1), a potential photoaffinity labeling reagent for thymidylate synthetase from a methotrexate-resistant strain of Lactobacillus casei, has been synthesized and characterized. UV254 irradiation of mixtures of thymidylate synthetase with 1, containing 14C-labeled phenyl and 3H-labeled pyrimidine rings, in the presence of excess 5,10-methylenetetrahydrofolate, the cofactor for the reaction, produced two complexes, separable from the native enzyme by polyacrylamide gel electrophoresis, in which only the 3H-containing moiety was bound to the protein. When mixtures of enzyme and 1 were irradiated in the absence of cofactor, complexes separable from the native enzyme were not observed. However, the 14C-containing component of 1 was now bound to the protein in the absence of the 3H-containing portion. The results are discussed in terms of the topography of the enzyme active site.

Phosphonate analogue of 2'-deoxy-5-fluorouridylic acid

The phosphonate analogue (6) of 2'-deoxy-5-fluorouridylic acid has been prepared via a Pfitzner--Moffatt oxidation and Witting reaction. This compound was found to inhibit thymidylate synthetase from three sources and to be cytotoxic to H.Ep.-2 cells in culture.

Replication of bromodeoxyuridylate-substituted mitochondrial DNA in yeast

The DNA of several strains of Saccharomyces cerevisiae was labeled by growing the culture in medium supplemented with thymidylate and bromodeoxyuridylate. It was thus possible to follow the course of mitochondrial DNA replication in density shift experiments by determining the buoyant density distribution of unreplicated and replicated DNAs in analytical CsCl gradients. DNA replication was followed for three generations after transfer of cultures from light medium to heavy medium and heavy medium to light medium. Under both conditions, the density shifts observed for mitochondrial DNA were those expected for semiconservative, nondispersive replication. This was further confirmed by analysis of the buoyant density of alkali-denatured hybrid mitochondrial DNA. With this method, no significant recombination between replicated and unreplicated DNA was detected after three generations of growth.

Fluorescent inhibitors of thymidylate synthetase

Two fluorescent derivatives of 2'-deoxy-5-fluorouridine 5'-p-aminophenyl phosphate were prepared by treatment of this compound with fluorescein isothiocyanate in dimethyl sulfoxide or 5-(dimethylamino)naphthalene sulfonyl chloride in pyridine. The products of the reactions were isolated by diethylaminoethylcellulose chromatography and were shown to be homogeneous by polyacrylamide electrophoresis and TLC. Confirmation of the structure was provided by elemental analysis, absorption and fluorescence spectra, PMR measurements, and liberation of nucleotide upon hydrolysis with snake venom phosphodiesterase. The fluorescent derivatives are good competitive inhibitors (Ki approximately10(-6) M) of thymidylate synthetase from a methotrexate-resistant strain of Lactobacillus casei.