Regulation of deoxycytidine kinase activity and inhibition by DFDC

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.

Ribonucleotide reductases: radical enzymes with suicidal tendencies

Ribonucleotide reductases catalyze a key step in DNA biosynthesis, using a diverse array of unprecedented metallo-cofactors to generate a transient protein radical that initiates nucleotide reduction. The new understanding of the chemistry and biochemistry of the system has allowed rational design of inhibitors of this process, which function as antitumor and antiviral agents.

Five-chlorodeoxycytidine and biomodulators of its metabolism result in fifty to eighty percent cures of advanced EMT-6 tumors when used with fractionated radiation

PURPOSE

To extend our findings in previous radiation and biochemical studies with five rodent tumors, in which we used one and occasionally two or three irradiations. The extent of control of the EMT-6 mammary adenocarcinoma was determined using fractionated radiation (12 irradiations) over a 3-week period using the radiosensitizer 5-chloro-2'-deoxycytidine (CldC) and biomodulators of its metabolism: N-(Phosphonacetyl)-L-aspartate (PALA), tetrahydrouridine and 5-fluoro-2'-deoxycytidine (FdC).

METHODS AND MATERIALS

Mammary adenocarcinoma EMT-6 tumors implanted 1 week prior to therapy in BALB/c mice were subjected to single daily doses of focused radiation, not exceeding a total of 60 Gy, on days 2-5 of each week. N-(Phosphonacetyl)-L-aspartate (PALA) was administered on the first day of therapy. Five-fluoro-2'-deoxycytidine and CldC were administered in the morning and afternoon, respectively, of the next 2 days, and CldC was administered on the fourth day. Tetrahydrouridine was always coadministered with FdC or CldC. Drug and radiation treatments overlapped for 3 weeks.

RESULTS

Fifty to 80% cures (usually 70%) were obtained with no apparent morbidity and the same moderate weight loss that occurs with radiation alone. Neither tumor regrowth delay nor cures were obtained with drugs or radiation alone. An apparent threefold dose increase effect was obtained with the end point: "days to reach 4 times initial tumor volume." Increasing the radiation dose threefold (without drugs) resulted in four out of five deaths; increasing the dose twofold (without drugs) resulted in extensive weight loss and hair loss in the entire ventral area and no cures. Increasing the dose of drugs or radiation 1.5-fold, in the complete protocol, did not result in increased morbidity. Comparative studies with Iododeoxyuridine demonstrate the heightened efficacy of CldC.

CONCLUSIONS

One cannot achieve the same results obtained with CldC and the modulators by merely increasing the dose of radiation. There is a significant window of safety in this approach. The evidence we have obtained with EMT-6, the fifth rodent tumor we have studied with CldC, as well as the demonstrated and proposed reasons for its superior efficacy over 5-Iododeoxyuridine (and 5-Bromodeoxyuridine), drugs in current use, indicate that CldC will allow more aggressive treatment of human tumors with radiation than is now feasible.

Mammalian deoxyribonucleoside kinases

The mammalian deoxyribonucleoside kinases are deoxycytidine kinase, thymidine kinase 1 and 2 and deoxyguanosine kinase. These enzymes phosphorylate deoxyribonucleosides and thereby provide an alternative to de novo synthesis of DNA precursors. Their activities are essential for the activation of several chemotherapeutically important nucleoside analogues. In recent years, these enzymes have been thoroughly characterised with regard to structure, substrate specificity and patterns of expression. In this review, these results are reviewed and furthermore, the physiologic metabolic role of the anabolic enzymes is discussed in relation to catabolic pathways. The significance of this information for the development of therapeutic protocols and choice of animal model systems is discussed. Finally, alternative pathways for nucleoside analogue phosphorylation are surveyed, such as the phosphotransfer capacity of 5'-nucleotidase.

Nucleoside transport and metabolism in lymphocytes, polymorphonuclear cells and cerebral synaptosomes

The salvage of nucleosides dominates over de novo biosynthesis in lymphocytes, polymorphonuclear cells (PMN) and in central neutral nervous system (CNS) in higher organisms. Earlier works in our laboratory have shown that the salvage of deoxycytidine (dCyd) did not correlate with DNA synthesis. The uptake and metabolism of dCyd was higher in undifferentiated germinal center lymphocytes and in follicles comparing to more differentiated cells. Recently we have compared the transport of thymidine (dThd), dCyd, uridine (Urd) and adenosine (Ado) in the three cell systems in which the salvage of nucleosides is dominating. It was found that dCyd was transported 30 times more effectively into lymphocytes than into PMN and synaptosomes, while Urd was transported about the same rate into the two cells and into synaptosomes. All transport processes could be inhibited by dipyridamole, NBRPR, papaverine and dilazep. The dCyd and dThd was phosphorylated even at 0 degrees C up to TTP and dCTP without incorporation into DNA and into liponucleotides. Our results show that the processes of transport-phosphorylation, as well as the processes of DNA-CDP-phospholipid synthesis are tightly coupled to each other in intact cells and organelles.

Targeted and non-targeted actions of anti-cancer drugs

(1) The currently used clinical anti-metabolites are targeted against-key enzymes of de novo purine and pyrimidine biosynthesis. However, the activities of salvage enzymes in each of the biosynthetic segments are markedly higher than those of the rate-limiting enzymes of de novo biosynthesis. Enzyme-pattern-targeted chemotherapy has been suggested to overcome the circumvention activity of salvage. Combination of inhibition of de novo and salvage pathways does provide a synergistic impact. Examples that enzyme-pattern-targeted drug treatment yields synergism include the following: tiazofurin (against IMP DH) and allopurinol (by raising serum hypoxanthine levels it inhibits GPRT); methotrexate or 5-FU lead to inhibition of the dTMP synthase reaction and AZT (a competitive inhibitor of thymidine kinase) or dipyridamole (a nucleoside transport inhibitor); acivicin, an inhibitor and inactivator of glutamine-utilizing enzymes in the de novo pathways of purine and pyrimidine biosynthesis, and dipyridamole. (2) Administration of MTX, 5-FU, tiazofurin or acivicin causes inhibition and/or inactivation of target enzymes. That these drugs are effective in spite of the presence of highly active salvage enzymes is now accounted for, at least in part, by new observations showing that these drugs markedly reduce (but do not eliminate) the activities (amounts) of CdR and TdR kinases, dTMP synthase and GPRT. This action is attributed to the rapid decay rate of these enzymes. (3) Studies on the bone marrow enzymic programs indicate that there is a window of opportunity for strengthening therapy and for the protection of bone marrow by administering salvage metabolites when the salvage enzymes are still present in high enough activities, i.e., 2-6 hr after administration of the blockers of de novo enzyme activities. (4) These results are a strong argument for discovering and utilizing inhibitors of purine and pyrimidine salvage enzymes to achieve more successful enzyme-pattern-targeted chemotherapy and to avoid development of resistant clones of cancer cells. (5) These approaches provide greater explanatory coherence than the previous accounts because recognition of (a) the importance of salvage and (b) rapid decay of key and salvage enzymes reveals a paradigm shift. The problem-solving process in chemotherapy should now be not only data-driven but also explanation-driven.