Ribo- and deoxyribonucleoside effect on 3'-amino-2',3'-dideoxycytidine-induced cytotoxicity in cultured L1210 cells

Effect of 3'-amino-2',3'-dideoxycytidine on DNA replicative intermediates

3'-Amino-2',3'-dideoxycytidine (3'-NH2-ddCyd) is a 3'-modified deoxycytidine analog that specifically inhibits DNA synthesis. Inhibition of chain elongation at the replication fork was examined utilizing a batch hydroxylapatite chromatography method. Exponentially growing cells were exposed to 3'-NH2-ddCyd and the diterpene aphidicolin for 9.5 hr at concentrations that inhibited DNA synthesis by approximately 60 and 90%, as determined by precursor uptake. Both agents demonstrated a concentration-dependent inhibition of pulse labeling of single-stranded DNA (ssDNA) and double-stranded DNA (dsDNA) generated by a limited alkaline lysis procedure. Upon removal of drug, the rate of elongation of pulse-labeled DNA was similar to that of untreated cells at both concentrations of aphidicolin and at the low concentration of the amino analog. Under these conditions, no reduction in cell survival was observed using the clonogenic assay technique. However, at the high concentration of 3'-NH2-ddCyd, the rate of elongation following drug removal was one-third that of untreated cultures, and a 50% loss in cell viability was observed. Furthermore, upon incubation of purified dsDNA with the Klenow fragment of Escherichia coli DNA polymerase I or purified ssDNA with calf thymus terminal deoxynucleotidyl transferase, only DNA from cells treated with the high concentration of 3'-NH2-ddCyd served as a poor template for further synthesis. The results indicate that 3'-NH2-ddCyd, in a concentration-dependent manner, inhibits DNA synthesis by reducing the rate of chain elongation at the replication fork, which subsequently leads to a functional blocking of 3'-ends in DNA. The data suggest that there may be a relationship between loss of cell viability and reduction in the number of 3'-ends available for DNA replication.

Relationship between Conformation and Antiviral Activity-II. 5-Methoxymethyl-2′-deoxycytidine and 5-methoxymethyl-N4-methyl-2′-deoxycytidine

5-methoxymethyl-N4-methyl-2′-deoxycytidine (N4-Me-MMdCyd) and 5-methoxymethyl-N4-methyl-2′-deoxycytidine-5′-monophosphate (N4-Me-MMdCMP) were synthesized to confer resistance to deamination by deaminating enzymes. N4-Me-MMdCyd and N4-Me-MMdCMP were inactive against Herpes simplex virus type 1 (HSV-1) and also nontoxic to VERO cells up to 1796 μM (highest concentration tested). 5-methoxymethyl-2′-deoxycytidine-5′-monophosphate (MMdCMP) was more potent than the nucleoside against HSV-1 in VERO cells. In HSV-infected VERO cells (10 PFU/cell), N4-Me-MMdCyd caused only slight perturbations of deoxyribonucleoside triphosphate pools. 5-methoxymethyl-N4-methyl-2′-deoxycytidine-5′-triphosphate (N4-Me-MMdCTP) was synthesized and the nature of interaction of N4-Me-MMdCTP and dCTP with DNA polymerase of Escherichia coli, HSV-1 and human α was investigated. N4-Me-MMdCTP was neither an effective substrate nor a strong inhibitor of Escherichia coli, HSV-1 or human α DNA polymerase.

The relationship between molecular conformation and antiviral activity for MMdCyd and N4-Me-MMdCyd is discussed. The conformation of the deoxyribofuranose ring in MMdCyd and N4-Me-MMdCyd are different. In N4-Me-MMdCyd, the exocyciic C(5′) side chain has the t conformation whereas MMdCyd has the g+rotomer conformation. The orientation of the N4-methyl group may also impede binding to the HSV-induced kinase by steric hindrance and/or by hindering hydrogen bonding between the enzyme and the lone pair of electrons at N(3). The results suggest that attempts to render resistance to deamination by alkylation at the N(4) position of the cytosine moiety is not likely to yield compounds with activity against HSV.

The role of deoxycytidine-metabolizing enzymes in the cytotoxicity induced by 3'-amino-2',3'-dideoxycytidine and cytosine arabinoside

The cellular metabolism of 3'-amino-2',3'-dideoxycytidine (3'-NH2-dCyd), a cytotoxic agent previously reported to be a poor substrate for purified Cyd/dCyd deaminase (dCydD), was compared with that of cytosine arabinoside (ara-C) in cells that displayed dCydD activity (HeLa) and in cells that did not (L1210). Growth inhibition induced by 3'-NH2-dCyd was dependent on the levels of anabolic enzymes, particularly dCyd kinase (dCydK), whereas cytotoxicity induced by ara-C was dependent on the expression of both anabolic and catabolic enzyme activities. Competition kinetics using purified enzyme revealed that the binding affinity of ara-C to dCydK was 5-fold that of the amino analog. However, this binding advantage is apparently offset in cells that contain high levels of dCydD, since the Ki values for this enzyme were 0.2 and 23 mM for ara-C and 3'-NH2-dCyd, respectively. This was reflected in the decrease in analog sensitivity observed between the two cell lines, whereby the concentrations of ara-C and 3'-NH2-dCyd required to inhibit growth by 50% were 200 and 7 times higher, respectively, in the dCydD-containing HeLa cells as compared with the dCydD-deficient L1210 cells. The metabolic stability and cytotoxicity of 3'-NH2-dCyd was independent of cell number. An unexpected finding was the extent to which the effectiveness of ara-C could be mitigated by the number of dCydD-containing cells. A completely cytotoxic concentration of ara-C was rendered nontoxic by a 10-fold increase in cell number. This observation was supported by an increase in I-beta-D-arabinofuranosyluracil (ara-U) formation, a decrease in ara-C 5'-triphosphate (ara-CTP) accumulation, and a rise in cell viability with increasing cell number. These findings indicate that unlike ara-C, the effectiveness of 3'-NH2-dCyd is independent of the level of deaminase, which suggests its possible utility in situations in which high levels of deaminase are manifest.

Specific inhibition of DNA biosynthesis induced by 3'-amino-2',3'-dideoxycytidine

3'-Amino-2',3'-dideoxycytidine (3'-NH2-dCyd) produced an S-phase-specific block in exponentially growing L1210 leukemia cells. The monophosphate and triphosphate forms of the drug were detected within a few hours of 3'-NH2-dCyd treatment of intact cells. No significant change in the deoxynucleoside triphosphate levels was observed during the early stages of treatment. However, by 24 h a 2-fold increase in the amount of the deoxynucleoside triphosphates was seen. The triphosphate form of the drug competitively inhibited dCTP incorporation into calf thymus DNA using highly purified DNA polymerase alpha. The Ki was determined to be 9.6 microM with respect to dCTP. Incorporation of the analogue into DNA was not detected. On the other hand, sucrose gradient analysis suggested that incorporation of the analogue into actively synthesized DNA may account for the biological activity of this compound. Treatment with 3'-NH2-dCyd induced single-strand breaks in actively synthesized DNA, but no double-strand breaks were observed in the presence of the analogue. The data indicate that 3'-amino-2',3'-dideoxycytidine specifically interferes with DNA replication at the level of DNA polymerase by inhibiting chain elongation.

Production of herpes simplex virus type 1 thymidine kinase in the presence of thymidine analogues

Treatment of herpes simplex virus type 1 (HSV-1) infected Vero, BHK, BHKtk- and LMtk- cells with 5-iodo-5'-amino-2',5'-dideoxyuridine (AIdUrd) caused increased synthesis of ICP36 and an increase in HSV-1 thymidine kinase (tk) activity at late times of infection. The overproduced ICP36 was identified as the HSV-1 encoded tk protein by immunoprecipitation. Whereas the thymidine analogue 5'-amino-5'-deoxythymidine (AdThd) caused an increase in HSV-1 tk synthesis and activity in wild type Vero and BHK cells, 5-iodo-2'-deoxyuridine (IdUrd) caused a similar increase only in tk- cells (LMtk-, BHKtk-). In vivo and in vitro stabilization studies using a [35S]methionine pulse-chase experiment or heat inactivation studies with purified HSV-1 tk revealed that stabilization of tk by the analogues could not account for the extent of the observed increase. Since overproduction of tk is observed only at late times of infection, it is suggested that the presence of these thymidine analogues in either the viral DNA or the cellular nucleotide pools is responsible for the observed differential effects.

Synergistic toxicity of pyrazofurin and cytidine in cytidine deaminase deficient lymphoid cells (Raji)

The intermediary metabolism of pyrimidine nucleosides was studied in a line of human B lymphoblasts (Raji) in which pyrimidine de novo synthesis deficiency was pharmacologically induced by pyrazofurin. It was found that Raji cells are cytidine deaminase deficient that cytidine has a synergistic effect on the toxicity of pyrazofurin towards these cytidine deaminase deficient cells, affecting both the proliferation and the viability of the cells. Indirect evidences suggest that this synergistic toxicity is not mediated by an effect on nucleoside diphosphate reductase nor on the first steps of pyrimidine de novo synthesis.

Cytidine deaminase activity of human normal and malignant lymphoid cells

Cytidine deaminase activity was determined by a radioisotopic assay in extracts of peripheral blood mononuclear cells of normal individuals and of patients with acute lymphoblastic leukaemia. The normal enzyme activity had a broad pH optimum between pH 6.5 and 8.0; apparent Km values for cytidine and deoxycytidine were 3.6 +/- 0.6 mumol/l and 26.5 +/- 3.5 mumol/l, respectively; the activity was resistant to heat inactivation; of the various effectors tested, only uridine, deoxyuridine and tetrahydroxyuridine had inhibitory effects. Cytidine deaminase activity was markedly decreased in lymphoblasts of patients with acute lymphoblastic leukaemia; enzyme activity was related to the percentage of circulating blast cells, and not to the clinical, cytological or immunological characters of the leukaemia.