Uptake and distribution of 2',3'-dideoxyinosine and its derivatives in a human monocytoid cell line.

The dideoxynucleoside analogue 2',3'-dideoxyinosine (ddI) has been used in the clinic as an alternative drug to zidovudine (AZT) in the treatment of patients with the acquired immunodeficiency syndrome (AIDS). However, it shows significant and variable toxicity in patients. It is known that various dideoxynucleoside analogues can cause the termination of the DNA chain following incorporation of the corresponding triphosphate metabolite by the polymerases. In the case of ddI, the presumed active metabolite is 2',3'-dideoxyadenosine 5'-triphosphate (ddATP). In order to understand the molecular basis for the toxicity of ddI, we evaluated the relationship between the intracellular formation of ddATP, its incorporation into cellular DNA and the effects on the growth of U937 cells, a human monocytoid cell line. Dideoxyinosine was not significantly toxic to U937 cells at concentrations as high as 500 microM in a 72 hrs. growth inhibition assay. The results of the uptake of 3HddI in this cell line showed a proportional increase in total metabolites with increasing concentrations of the drug (1-20 microM) after a 24 hrs. exposure. Incubation with 10 microM 3HddI resulted in the formation of low levels of ddATP within a period of 2 hrs. A significant amount of ddI-derived radioactivity was found in both DNA and RNA after exposure to 10 microM 3HddI for 24 to 72 hrs. However, no evidence of incorporation of ddATP into the cellular DNA fraction was obtained in these experimental conditions. Therefore, the lack of significant toxicity of ddI to U937 cells can be explained, at least in part, by its inability to incorporate ddATP into its cellular DNA at the doses studied.

Modulation of thymidine kinase activity: a biochemical strategy to enhance the activation of antineoplastic drugs.

Thymidine kinase is a key enzyme responsible for the activation of several anticancer and antiviral drugs. As the first enzyme in the salvage pathway of thymidine, it is regulated by the feedback inhibition exerted by the end-product of the pathway, namely thymidine 5'-triphosphate. 5'-Aminothymidine is a non-toxic analogue of thymidine capable of interfering with this regulatory mechanism. In fact, it has been shown that 5'-aminothymidine increases the cytotoxicity and metabolism of various thymidine analogues currently in use of the clinic as antineoplastic agents. This mini-review article focuses in the evidence supporting the role of 5'-aminothymidine as a potential prototype drug for a new class of anticancer agents: drugs which affect the regulation of key metabolic pathways that determine the efficacy of agents with cytotoxic activity. The mechanism of action, antineoplastic activities and basis for selectivity in tissue culture models are also described.

Low pH does not affect the dose response for 5'-amino-5'-deoxythymidine modulation of IdUrd DNA incorporation and radiosensitization in a human bladder cancer cell line.

We report that coincubation of 647V cells for one cell cycle with low concentrations (30 microM) of 5'-amino-5'-deoxythymidine increased IdUrd DNA incorporation and radiosensitivity at low extracellular pH (pHe 6.8) in a fashion similar to treatment at normal pHe. IdUrd DNA incorporation is inhibited by high (300 microM) 5'-AdThd concentrations at both normal and low pHe (7.4 and 6.8), resulting in no significant radiosensitization. These results at low pHe were not anticipated based on previously published studies of 5'-AdThd modulation of thymidine kinase (TK) activity and nucleoside cellular uptake. Our results suggest that regulation of intracellular pH (pHi) during the course of one cell cycle negates the 5'-AdThd dose-dependent modulation of TK activity demonstrated previously. Flow cytometric measurement of pHi in 647V cells showed that normal pHi (pH 7.4) was maintained in 647V cells over a 12- to 24-h exposure to low pHe (pH 6.8). Thus the concomitant use of IdUrd and high concentrations of 5'-AdThd (> 30 microM) is unlikely to result in selective in vivo radiosensitization of human tumors under conditions which are intermittently or chronically acidic. However, low concentrations of 5'-AdThd may prove to be an effective in vivo modulator of IdUrd radiosensitization of human tumors under both normal and acidic conditions.

Modulation of IdUrd-DNA incorporation and radiosensitization in human bladder carcinoma cells.

5' Amino-5'-deoxythymidine (5'-AdThd) has been demonstrated previously to antagonize dTTP-mediated feedback inhibition of purified thymidine kinase from 647V, a human bladder cancer cell line. Low concentrations of 5'-AdThd (3-30 microM) have also been shown to stimulate cellular uptake of iododeoxyuridine (IdUrd) in 647V cells at clinically relevant IdUrd concentrations (2 microM). We report that the combination of 30 microM 5'-AdThd plus 2 microM IdUrd results in a significant increase of IdUrd replacement of thymidine (dThd) (18%) in the DNA of 647V cells over that obtained by exposure to 2 microM IdUrd alone (7.9%). However, increasing the 5'-AdThd concentration to 300 microM inhibited the incorporation of IdUrd into DNA (3%). IdUrd-induced radiosensitization of 647V cells, as measured by clonogenic survival, was enhanced by coincubation with 30 microM 5'-AdThd, while 300 microM 5'-AdThd reduced the IdUrd radiosensitization. Additionally, radiation-induced single strand break generation when IdUrd was incorporated into 647V DNA, as measured by rapid alkaline elution, was also enhanced by coincubation with 30 microM 5'-AdThd, while 300 microM 5'-AdThd resulted in a decrease in the number of single strand breaks produced. In T24, another bladder cancer cell line, and SV-HUC-TT1, a tumorigenic cell line derived from SV-HUC, 3-10 microM 5'-AdThd was also able to enhance IdUrd replacement of dThd in DNA. However, no stimulation of dThd replacement by 5'-AdThd occurred in SV-HUC, a prototypic "normal" bladder urothelial cell line. Since 5'-AdThd is not a substrate for mammalian thymidine kinase and has little or no cytotoxicity in vitro and in vivo, it may be a selective modulator of IdUrd radiosensitization of human bladder carcinoma and should be tested in vivo.

Inhibition of herpes simplex virus type 2 growth by phosphorothioate oligodeoxynucleotides.

Phosphorothioate homo-oligodeoxynucleotides were found to be potent inhibitors of herpes simplex virus type 2 (HSV-2) but less potent for HSV-1 in cell culture studies. Oligomers with longer chain lengths were more active against HSV-2 than those with shorter ones. Of all the compounds examined, the 28-mer phosphorothioate homo-oligodeoxynucleotides were the strongest inhibitors of HSV-2. The degree of inhibition was related to the base moiety on the order of deoxycytidine = thymidine greater than deoxyadenosine. The inhibition of HSV-2 growth by S-dC28 was dose dependent with a 90% inhibitory dose of 1 microM. At 50 microM, S-dC28 inhibited HeLa S3 cell growth by less than 10%. The anti-HSV-2 activity was time and schedule dependent. The oligomer was most inhibitory to viral growth when present during the 1-h viral adsorption period, and this effect could be enhanced by continuous drug exposure after the adsorption period. S-dC28 was also an effective inhibitor of two HSV-2 drug-resistant mutants: a phosphonoformate-resistant mutant that induces an altered DNA polymerase and a 9-(1,3-dihydroxy-2-propoxymethyl)guanine-resistant mutant that does not induce the viral thymidine kinase. In drug combination studies, phosphonoformate was shown to potentiate the action of S-dC28 against HSV-2 growth. In conclusion, because of their potency and selectivity, phosphorothioate homo-oligodeoxynucleotides are a promising new class of anti-HSV agents.

Incorporation of 3'-azido-3'-deoxythymidine into cellular DNA and its removal in a human leukemic cell line.

3'-Azido-3'-deoxythymidine (AZT) is currently used in the treatment of patients with the acquired immunodeficiency syndrome (AIDS); this often, however, results in hematological toxicity. Although the mechanism of toxicity is not clear, it is thought to result in part from incorporation of AZT into DNA, which causes chain termination. In order to investigate the mechanism of AZT toxicity, the relationship between the presence of AZT in DNA of K562 cells, a chronic myelogenous leukemia cell line, and growth inhibition was examined. No growth inhibition was evident at less than 50 microM AZT, although incorporation of AZT into DNA was detected at 10 and 20 microM. This suggested that the presence of AZT in DNA was not sufficient to inhibit cell growth. Removal of AZT from the medium resulted in the removal of AZT from DNA of the cells, indicative of a cellular repair mechanism. Cellular DNA polymerases alpha, beta, gamma, and delta from human leukemic cells were inhibited by AZT trisphosphate to different degrees, polymerase alpha being the least potently inhibited. Furthermore, an enzyme with exonucleolytic activity, capable of removing AZT and dideoxycytidine from the correspondingly terminated DNA (in vitro), was obtained from these cells. In summary, AZT was incorporated into DNA at levels that were not toxic, and it could be removed by an exonuclease, which might play a key role in the susceptibility of cells to AZT.

Modulation of the feedback regulation of thymidine kinase activity by pH in 647V cells.

We have studied the effect of pH on the interactions between thymidine kinase, thymidine triphosphate, and 5'-amino-2',5'-dideoxythymidine (5'-AdThd) in purified preparations of the enzyme and in intact 647V cells, a human bladder cancer cell line. Thymidine kinase is competitively inhibited by 5'-AdThd. dTTP feedback inhibits in a noncompetitive fashion. However, 5'-AdThd partially reverses the inhibition produced by dTTP resulting in enhanced enzyme activity. We have found that dTTP (pKa = 7.5) is a much more potent inhibitor of purified preparations of thymidine kinase activity at low pH conditions. For example, 2.5 microM dTTP inhibited thymidine kinase activity by 50, 85, and 95% at pH values of 8.0, 7.5, and 6.5, respectively. The interaction of 5'-AdThd (pKa = 8.5) at either the active (competitive) or the regulatory (deinhibition) site is not altered significantly over a pH range of 6.5 to 9.5. To extend these findings to intact cells, we studied the perturbation of the uptake of thymidine by 5'-AdThd in 647V cells incubated in media buffered at various pH values. In cells exposed to media buffered at pH 8.5 or 7.5, 5'-AdThd maximally stimulated thymidine uptake about 250 and 300% at 10 and 30 microM, respectively. However, at pH 6.5, 300 microM 5'AdThd was required to produce maximal stimulation of about 500%. These observations are consistent with the greater sensitivity of thymidine kinase (in situ) to feedback inhibition by dTTP at the lower pH conditions. Intracellular dTTP pool sizes were not affected by variations in pH during the short time course of our experiments. However, after 1 h, the intracellular concentration of 5'-AdThd was twice that of the extracellular medium in conditions at pH 7.5 and 8.5 but was equimolar across the membrane at pH 6.5. This does not account for the differences in the perturbation of thymidine uptake by 5'-AdThd at various pH values. In general, our results indicate that regulation of thymidine kinase by dTTP is pH dependent, while its modulation by 5'-AdThd is not, and that regulation of thymidine kinase in situ is sensitive to alterations in pH.

Basis for the differential modulation of the uptake of 5-iododeoxyuridine by 5'-aminothymidine among various cell types.

We have previously reported that 5'-aminothymidine (5'-AdThd), an antagonist of the feedback inhibition exerted by dTTP that regulates thymidine kinase, enhances the uptake and cytotoxicity of 5-iododeoxyuridine in various human bladder cancer cell lines but not in normal human urothelial cells (HU) propagated in vitro. In this work we have analyzed the factors that could potentially account for the differential effect of 5'-AdThd among various cell types: 647V (a human bladder cancer cell line); HU; SV-HU (a SV40-transformed human urothelial cell line), and C3H/10T1/2 mouse embryo fibroblasts (10T1/2) cells. 5'-AdThd enhanced the uptake of IdUrd in SV-HU cells (greater than 400%), similar to what we have observed before for 647V cells. However, in 10T1/2 and HU cells, 5'-AdThd only minimally increased the uptake of 5-iododeoxyuridine (about 160%). Thymidine kinases purified from the different sources were similarly sensitive to inhibition by dTTP or 5'-AdThd and to deinhibition of the dTTP-induced regulation of enzyme activity by 5'-AdThd. Furthermore, [3H]-5'-AdThd permeated and accumulated intracellularly in all cell types. In none of these cultures was nucleoside phosphorylase activity detected, as indicated by the inability of the cells to produce thymine or iodouracil after exposure to the appropriate nucleosides. Also, 5'-AdThd did not affect the breakdown of dTMP by crude preparations of cytosolic 5'-nucleotidase from the different cells. We found that intracellular dTTP pools in the various cell types were substantially high (15-26 microM) compared to the sensitivity of thymidine kinase to inhibition by dTTP (IC50 2-4 microM). This suggests that thymidine kinase is in a strongly inhibited state in situ. To test the sensitivity of thymidine kinase (in situ) to regulation by dTTP we investigated: (a) the effect of depleting intracellular dTTP pools with methotrexate on the uptake of thymidine (dThd); and (b) the effect of pH on the uptake of dThd and its perturbation by 5'-AdThd, since the inhibition of thymidine kinase activity by dTTP is known to be pH dependent. We found that a 47% reduction of dTTP pools by methotrexate in 10T1/2 and HU cells did not result in an increase in thymidine kinase activity, as indicated by the lack of an effect on the uptake of dThd. However, we have previously shown that, under similar conditions, 647V cells show a substantial increase in dThd uptake.(ABSTRACT TRUNCATED AT 400 WORDS)

Regulation of the activation of fluorodeoxyuridine by substrate competition and feedback inhibition in 647V cells.

Fluorodeoxyuridine (FdUrd) is a cytotoxic analogue of thymidine which requires activation by thymidine kinase to FdUMP. FdUMP inhibits thymidylate synthetase and, thus, the synthesis of dTTP. 5'-Aminothymidine (5'-AdThd) can antagonize the feedback inhibition exerted by dTTP on thymidine kinase activity and thereby stimulate FdUrd phosphorylation. This provided a novel approach to assess the degree to which end product inhibition regulates the phosphorylation of FdUrd. We used 5'-AdThd to investigate the effects of dThd and IdUrd on the regulation of FdUrd uptake in intact 647V cells, a human bladder cancer cell line. Contributions from catabolic processes were found not to be important in our system. We detected no nucleoside phosphorylase activity in the 647V cells or any effect of 5'-AdThd on the breakdown of 5-fluorodeoxyuridine monophosphate to FdUrd by crude preparations from these cells. Thus, phosphorylation by thymidine kinase determined FdUrd uptake (phosphorylation). In the absence of added nucleosides the rate of FdUrd uptake increased in a time dependent fashion. Diminished feedback inhibition of thymidine kinase appeared to be an important factor, as evidenced by a decrease in intracellular dTTP pools and a time dependent loss in the ability of 5'-AdThd to stimulate FdUrd uptake. Thymidine and iododeoxyuridine inhibited FdUrd phosphorylation (uptake) by two mechanisms: competition for the active site of thymidine kinase and increased feedback inhibition. Increased feedback inhibition was indicated by stimulation of FdUrd uptake by 5'-AdThd. The effects of IdUrd on FdUrd uptake were also time dependent, presumably reflecting accumulation of iododeoxyuridine triphosphate and dTTP pools. FdUrd cytotoxicity was modulated by dThd, IdUrd, and 5'-AdThd in parallel to their perturbation of FdUrd uptake. Individually they reduced the growth inhibitory properties of FdUrd. These results show that the regulation of FdUrd uptake is critically dependent on the presence of dThd and IdUrd and emphasize the potential importance of circulating levels of these nucleosides in mediating FdUrd activation and cytotoxicity.

Enzyme regulatory site-directed drugs: study of the interactions of 5'-amino-2', 5'-dideoxythymidine (5'-AdThd) and thymidine triphosphate with thymidine kinase and the relationship to the stimulation of thymidine uptake by 5'-AdThd in 647V cells.

5'-Amino-2',5'-dideoxythymidine (5'-AdThd) is a nontoxic thymidine (dThd) analogue capable of antagonizing the feedback inhibition exerted by thymidine triphosphate (dTTP) on thymidine kinase (EC 2.7.1.21). In intact cells, this results in stimulation of thymidine uptake by 5'-AdThd. We have studied the interaction between 5'-AdThd and thymidine kinase purified from 647V cells. We found that 5'-AdThd inhibited competitively thymidine kinase activity (Ki of 0.5 microM) in the absence of dTTP whereas dTTP inhibited thymidine kinase activity in a noncompetitive manner. However, in the presence of dTTP, 5'-AdThd was able to stimulate enzyme activity in a mode that suggests competition with dTTP for the regulatory site. Altered interactions were observed at high substrate (dThd) concentrations, with dThd showing competitive kinetics with dTTP. In intact cells, we evaluated the hypothesis that antagonism of feedback inhibition could account for stimulation of dThd uptake by 5'-AdThd. If inhibition of thymidine kinase activity by dTTP is critical, then depletion of cellular dTTP by methotrexate should reduce the ability of 5'-AdThd to stimulate dThd uptake. Indeed, this was the case. If the dTTP pools were repleted by the addition of higher concentrations of dThd, the ability of 5'-AdThd to stimulate dThd uptake was restored. Furthermore, effects of 5'-AdThd on nucleoside phosphorylase or cytoplasmic 5'-nucleotidase activity (dTMP breakdown) could not account for the stimulation of dThd uptake in 647V cells. In summary, our results indicate that 5'-AdThd interacts with thymidine kinase at the dTTP-binding site, resulting in stimulation of enzyme activity and stimulation of dThd uptake in intact cells.

Preferential stimulation of iododeoxyuridine phosphorylation by 5'-aminothymidine in human bladder cancer cells in vitro.

5'-Aminothymidine represents a novel class of compounds capable of antagonizing the feedback inhibition which normally regulates thymidine kinase. As a consequence, the uptake of iododeoxyuridine, a substrate of thymidine kinase, can be substantially increased by 5'-aminothymidine. in this study the phosphorylation, uptake, and cytotoxicity of iododeoxyuridine was markedly enhanced by 5'-aminothymidine in three human bladder cancer cell lines, T24, HT1197 and 647V. In contrast, neither the uptake nor the toxicity of iododeoxyuridine was increased by 5'-aminothymidine in normal human urothelial cells propagated in vitro. Although 30 microM 5'-aminothymidine increased the uptake of 3 microM iododeoxyuridine 4- to 5-fold in the HT1197 and 647V cells and 2.5-fold in the T24 cells, no stimulation was produced in the normal urothelial cells. The modulation of iododeoxyuridine uptake was associated with parallel changes in the inhibition of cellular replication. The cytotoxicity of iododeoxyuridine was strongly augmented by 5'-aminothymidine in the HT1197 and 647V cells, modestly increased in the T24 cells, and unchanged in the normal urothelial cells. The degree to which iododeoxyuridine phosphorylation was stimulated did not correlate with cellular replication rates or with intracellular thymidine triphosphate concentrations. Iododeoxyuridine uptake was markedly increased in the HT1197 (doubling time = 52 h) and 647V (doubling time = 24 h) cells, moderately in the rapidly growing cells T24 (doubling time = 20 h), and minimally in the normal urothelial cells which doubled every 32 h. In exponentially growing cells the thymidine triphosphate pools were approximately 18 microM in the normal cells and about 21, 24, and 35 microM in the HT1197, T24, and 647V cells, respectively. The use of 5'-aminothymidine and other compounds capable of antagonizing feedback inhibition may provide a new means of increasing the efficacy of cytotoxic nucleosides.

Perturbation of thymidine kinase regulation: a novel chemotherapeutic approach.

The design of compounds which disrupt the regulation of key enzymes is a new strategy for drug development. 5'-AdThd, which can antagonize the feedback inhibition that normally regulates thymidine kinase, is representative of this novel class of agents. The ability of 5'-AdThd to deinhibit thymidine kinase markedly increases the capacity of some cells to phosphorylate thymidine and IdUrd and, thereby, enhance their cytotoxicity. This effect is evident in three human bladder cancer cell lines, but not in normal human urothelial cells. Thus, 5'-AdThd may be able to increase the therapeutic selectivity of IdUrd. Intracellular dTTP pools are critically involved in mediating the effects of 5'-AdThd, but differences in these pools from cell type of cell type may not account for the selective stimulation of IdUrd uptake produced in the cancer cells.