Capacity of deoxycytidine to selectively antagonize cytotoxicity of 5-halogenated analogs of deoxycytidine without loss of antiherpetic activity

Five-chlorodeoxycytidine, a tumor-selective enzyme-driven radiosensitizer, effectively controls five advanced human tumors in nude mice

PURPOSE

The study's goals were as follows: (1) to extend our past findings with rodent tumors to human tumors in nude mice, (2) to determine if the drug protocol could be simplified so that only CldC and one modulator, tetrahydrouridine (H4U), would be sufficient to obtain efficacy, (3) to determine the levels of deoxycytidine kinase and dCMP deaminase in human tumors, compared to adjacent normal tissue, and (4) to determine the effect of CldC on normal tissue radiation damage to the cervical spinal cord of nude mice.

METHODS AND MATERIALS

The five human tumors used were as follows: prostate tumors, PC-3 and H-1579; glioblastoma, SF-295; breast tumor, GI-101; and lung tumor, H-165. The duration of treatment was 3-5 weeks, with drugs administered on Days 1-4 and radiation on Days 3-5 of each week. The biomodulators of CldC were N-(Phosphonacetyl)-L-aspartate (PALA), an inhibitor of aspartyl transcarbamoylase, 5-fluorodeoxycytidine (FdC), resulting in tumor-directed inhibition of thymidylate synthetase, and H4U, an inhibitor of cytidine deaminase. The total dose of focused irradiation of the tumors was usually 45 Gy in 12 fractions.

RESULTS

Marked radiosensitization was obtained with CldC and the three modulators. The average days in tumor regrowth delay for X-ray compared to drugs plus X-ray, respectively, were: PC-3 prostate, 42-97; H-1579 prostate, 29-115; glioblastoma, 5-51; breast, 50-80; lung, 32-123. Comparative studies with PC-3 and H-1579 using CldC coadministered with H4U, showed that both PALA and FdC are dispensable, and the protocol can be simplified with equal and possibly heightened efficacy. For example, PC-3 with X-ray and (1) no drugs, (2) CldC plus the three modulators, (3) a high dose of CldC, and (4) escalating doses of CldC resulted in 0/10, 3/9, 5/10, and 6/9 cures, respectively. The tumor regrowth delay data followed a similar pattern. After treating mice only 11/2 weeks with CldC + H4U, 92% of the PC-3 tumor cells were found to possess CldU in their DNA. The great majority of head-and-neck tumors from patient material had markedly higher levels of dC kinase and dCMP deaminase than found in adjacent normal tissue. Physiologic and histologic studies showed that CldC + H4U combined with X-ray, focused on the cervical spinal cord, did not result in damage to that tissue.

CONCLUSIONS

5-CldC coadministered with only H4U is an effective radiosensitizer of human tumors. Ninety-two percent of PC-3 tumor cells have been shown to take up ClUra derived from CldC in their DNA after only 11/2 weeks and 2 weeks of bolus i.p. injections. Enzymatic alterations that make tumors successful have been exploited for a therapeutic advantage. The great electronegativity, coupled with the relatively small Van der Waal radius of the Cl atom, may result in CldC's possessing the dual advantageous properties of FdC on one hand and BrdU and IdU on the other hand. These advantages include autoenhancing the incorporation of CldUTP into DNA by not only overrunning but also inhibiting the formation of competing TTP pools in tumors. A clinical trial is about to begin, with head-and-neck tumors as a first target of CldC radiosensitization.

Molecular chlorine generated by the myeloperoxidase-hydrogen peroxide-chloride system of phagocytes produces 5-chlorocytosine in bacterial RNA

Myeloperoxidase, a heme enzyme secreted by activated phagocytes, uses H(2)O(2) and Cl(-) to generate the chlorinating intermediate hypochlorous acid (HOCl). This potent cytotoxic oxidant plays a critical role in host defenses against invading pathogens. In this study, we explore the possibility that myeloperoxidase-derived HOCl might oxidize nucleic acids. When we exposed 2'-deoxycytidine to the myeloperoxidase-H(2)O(2)-Cl(-) system, we obtained a single major product that was identified as 5-chloro-2'-deoxycytidine using mass spectrometry, high performance liquid chromatography, UV-visible spectroscopy, and NMR spectroscopy. 5-Chloro-2'-deoxycytidine production by myeloperoxidase required H(2)O(2) and Cl(-), suggesting that HOCl is an intermediate in the reaction. However, reagent HOCl failed to generate 5-chloro-2'-deoxycytidine in the absence of Cl(-). Moreover, chlorination of 2'-deoxycytidine was optimal under acidic conditions in the presence of Cl(-). These results implicate molecular chlorine (Cl(2)), which is in equilibrium with HOCl through a reaction requiring Cl(-) and H(+), in the generation of 5-chloro-2'-deoxycytidine. Activated human neutrophils were able to generate 5-chloro-2'-deoxycytidine. Cellular chlorination was blocked by catalase and heme poisons, consistent with a myeloperoxidase-catalyzed reaction. The myeloperoxidase-H(2)O(2)-Cl(-) system generated similar levels of 5-chlorocytosine in RNA and DNA in vitro. In striking contrast, only cell-associated RNA acquired detectable levels of 5-chlorocytosine when intact Escherichia coli was exposed to the myeloperoxidase system. This observation suggests that oxidizing intermediates generated by myeloperoxidase selectively target intracellular RNA for chlorination. Collectively, these results indicate that Cl(2) derived from HOCl generates 5-chloro-2'-deoxycytidine during the myeloperoxidase-catalyzed oxidation of 2'-deoxycytidine. Phagocytic generation of Cl(2) therefore may constitute one mechanism for oxidizing nucleic acids at sites of inflammation.

Methylation of HSV-1 DNA as a mechanism of viral inhibition: studies of an analogue of methyldeoxycytidine: trifluoromethyldeoxycytidine (F3mdCyd)

Although several hypomethylating agents such as 5-azadeoxycytidine and 5-fluorodeoxycytidine have been shown to activate transcription after incorporation into viral or cellular DNA, agents which selectively affect the methylation status of virus-infected cells have not been described. Studies on the antiviral effect of the methyldeoxycytidine (mdCyd) analogue trifluoromethyldeoxycytidine (F3mdCyd) showed significant antiviral activity against herpes simplex virus type 1 (HSV-1). This analogue of both dCyd and dThd is selectively incorporated into the DNA of herpesvirus infected cells due to the unique specificity of the herpesvirus thymidine kinase (TK) because the HSV-1 TK is both a dCyd and dThd kinase. In contrast, the deoxycytidine kinase of uninfected cells preferentially phosphorylates dCyd and has a poor affinity for F3mdCyd. F3mdCyd hemisubstituted M13 DNA displayed the same properties as mdCyd-substituted M13 DNA with respect to cleavage by restriction enzymes, and acted as an efficient template for eukaryotic DNA methyltransferase (S-adenosyl-L-methionine DNA (cytosine-5) methyltransferase: EC 2.1.1.37). Using the persistently infected CEM cell model system, the extent of DNA methylation was shown to increase in a dose-related manner when HSV-1-infected CEM cells were treated with increasing concentrations of F3mdCyd. Higher levels of methylation correlated with significant decreases in HSV-1 titers. Isoschizomer analyses followed by Southern blotting and hybridization with genomic HSV-1 DNA showed that DNA from HSV-1-infected, analogue-treated Vero cells was resistant to cleavage by restriction enzymes at a time when productive virus was not present in culture. We infer from these results that the methylation-like properties of the incorporated F3mdCyd occur concomitantly with, and appear to be involved in, the mechanisms of the analogue's antiviral effect towards HSV-1.

Sensitization to X ray by 5-chloro-2'-deoxycytidine co-administered with tetrahydrouridine in several mammalian cell lines and studies of 2'-chloro derivatives

5-Chloro-2'-deoxycytidine (CldC) + tetrahydrouridine (H4U) sensitizes mammalian cells (HEp-2, RIF-1, S-180) to X ray. This sensitization, as demonstrated previously with HEp-2 cells, is heightened when cells are pre-incubated with inhibitors of pyrimidine synthesis. CHO cells, which intrinsically lack both cytidine deaminase (CD) and deoxycytidylate deaminase (dCMPD), are sensitized to X ray by 5-chlorodeoxyuridine (CldU) but display no significant sensitization with CldC + H4U. The presence and level of these deaminases appears to correlate with X ray sensitization in cell culture. From experiments in cell culture, it can be inferred that one pathway of conversion, deoxycytidine kinase----dCMPD, or CD----thymidine kinase, may be sufficient for metabolizing CldC to a radiosensitizer. However, if both pathways are blocked, as in CHO cells, no X ray sensitization results. In addition to HEp-2 cells, which are extremely elevated in both CD and dCMPD activities, we have examined the sensitization of S-180 and RIF-1 cells to X ray by CldC + H4U. Both cell lines possess an enzymatic profile consistent with their sensitization to X ray by CldC + H4U. Dose enhancement ratios of 1.5 to 1.9 for cells treated with CldC + H4U and ratios of 2.0-2.7 for cells pre-treated with inhibitors of pyrimidine synthesis prior to CldC + H4U have been obtained. Based on reports of the marked X ray sensitization of bacteria by 2'-chloro-2'-deoxythymidine, we obtained 2',5-dichloro-2'-deoxycytidine and 5-bromo-2'-chloro-2-deoxyuridine and found these analogs to be X ray sensitizers of mammalian cells. The strategy that we propose with CldC + H4U and the related 2'-chloro derivatives, based on the elevation of CD and dCMPD in human tumors, offers a degree of selectivity that is not necessarily related to differences in cell kinetics; such that malignancies other than brain tumors may be amenable to this therapy.

Marked radiosensitization of cells in culture to X ray by 5-chlorodeoxycytidine coadministered with tetrahydrouridine, and inhibitors of pyrimidine biosynthesis

Our approach to overcome the problem of rapid catabolism and general toxicity encountered with 5-halogenated analogues of deoxyuridine (5-bromo, chloro or iododeoxyuridine), which has limited their use as tumor radiosensitizers, is to utilize 5-chlorodeoxycytidine (CldC) with tetrahydrouridine (H4U). We propose that CldC, coadministered with H4U, is metabolized in the following manner: CldC----CldCMP----CldUMP---- ----CldUTP----DNA. All the enzymes of this pathway are elevated in many human malignant tumors and in HEp-2 cells. In X irradiation studies with HEp-2 cells, limited to 1 or 2 radiation doses, we have obtained 3.0 to 3.8 apparent dose enhancement ratios (these represent upper limits) when cells are preincubated with inhibitors of pyrimidine biosynthesis: N-(Phosphonacetyl)-L-aspartate (PALA) and 5-fluorodeoxyuridine (FdU) or 5-fluorodeoxycytidine (FdC) + H4U. Optimum conditions for radiosensitization are: PALA (0.1 mg/ml) 18-20 hr prior to FdU (0.1 microM) or FdC (0.02 microM) + H4U (0.1 mM) followed 6 hr later by CldC (0.1-0.2 mM) + H4U (0.1 mM) for 56-68 hr. Viabilities of 10 +/- 4% to 15 +/- 1% (+/- S.E.) were obtained for drug-treated unirradiated cells. Enzymatic studies indicate that this toxicity may be tumor selective. CldC + H4U alone (at these concentrations) results in 20% substitution of CldU for thymidine in DNA (determined by HPLC analysis). Preliminary toxicity studies indicate that mice will tolerate treatment protocols involving a single dose of PALA (200 mg/kg) followed by a dose of FdU (50 mg/kg) and 3 cycles of CldC (500 mg/kg) + H4U (100 mg/kg) at 10 hour intervals, with marginal weight loss (4%). In this approach we seek to obtain preferential conversion of CldC to CldUTP at the tumor site by taking advantage of quantitative differences in enzyme levels between tumors and normal tissues.

Incorporation of 5-substituted analogs of deoxycytidine into DNA of herpes simplex virus-infected or - transformed cells without deamination to the thymidine analog

The incorporation into DNA of 5-bromocytosine and 5-iodocytosine, derived from their respective administered deoxyribonucleoside analogs, has been demonstrated in studies with cells infected with herpes simplex virus types 1 and 2 (HSV-1 and HSV-2) and in cells transformed with the thymidine kinase gene of HSV-1. No significant incorporation of iodocytosine or iodouracil occurred in the DNA of uninfected or nontransformed cells when the deaminating enzymes were inhibited, in accord with past studies in our laboratory with 5-bromodeoxycytidine and tetrahydrouridine. When 2'-deoxytetrahydrouridine, a potent inhibitor of cytidine deaminase and dCMP deaminase, was utilized, all the counts in DNA that were derived from [(125)I]iododeoxycytidine appeared as iodocytosine in HSV-infected cells. In the absence of a deaminase inhibitor, 32 to 45% of the counts associated with DNA pyrimidines appeared as iodocytosine, and 55 to 68% appeared as iodouracil in HSV-infected cells. Substantial incorporation of iodocytosine (16%) occurred in cells transformed with the HSV thymidine kinase gene, suggesting the importance of the specificity of cellular nucleoside kinases and the activity of the deaminases in presenting unmodified bases to an undiscriminating polymerase. Incorporation into DNA of bromocytosine derived from [(3)H]bromodeoxycytidine was demonstrated in HSV-2 infected cells; very little incorporation of bromocytosine compared with bromouracil could be demonstrated in these cells in the absence of inhibition of the deaminases (19% of the total counts associated with pyrimidines with deaminase inhibition and 1.5% without). Limited studies with 5-methyl[5-(3)H]deoxycytidine indicated essentially no (or very little) incorporation of this analog as such in the DNA of HSV-1- and HSV-2-infected and -transformed cells. This suggests an exclusion or repair mechanism preventing inappropriate methylcytosine incorporation in DNA. The addition of nucleoside and deoxyribonucleoside deaminase inhibitors, which leads to the incorporation of 5-halogenated analogs of deoxycytidine into DNA as such, does not impair their antiviral activity. We infer from studies with 4-N-alkyl (ethyl and isopropyl)-substituted analogs of iododeoxycytidine that they are incorporated as such into DNA without deamination and effectively inhibit the virus at concentrations that are marginally toxic. Among the several reasons presented for the heightened potential efficacy of analogs of deoxycytidine compared with those of deoxyuridine is that the former, as analogs of 5-methyldeoxycytidine, may impair viral replication by perturbing processes involving methylation and changes in the methylation of deoxycytidine in DNA which appear to be important for the process of HSV maturation. In addition, this capacity to perturb methylation may, in turn, be the key to their potential as agents affecting entry into or emergence from latency, a process in which dramatic changes in the postpolymer 5-methylation of deoxycytidine occur in the DNA of herpesviruses.