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We have employed cesium sulfate density gradient centrifugation to separate RNA and DNA of L1210 cells labeled with [3H]fluorodeoxyuridine. We have analyzed nucleotide and nucleoside digests of purified DNA from the [3H]fluorodeoxyuridine-labeled cells and demonstrate by reverse phase and anion exchange high pressure liquid chromatography the presence of tritium radioactivity co-migrating with fluorodeoxyuridine 5'-monophosphate or fluorodeoxyuridine. These observations demonstrate the internucleotide incorporation of fluorodeoxyuridine in DNA and suggest a new mechanism of action for this cytotoxic and mutagenic agent.

We have employed cesium sulfate density gradient analysis to monitor the incorporation of 1-beta-D-arabinofuranosylcytosine (ara-C) into L1210 cellular nucleic acid. This methodology permits a distinct separation of RNA and DNA under nondegrading conditions. The results demonstrate specific incorporation of ara-C into DNA with less than 1% of total incorporation into nucleic acid being detectable in RNA. Moreover, we have found a highly significant relationship (p < 0.0001) between the incorporation of ara-C into DNA with the loss of clonogenic survival. This relationship had not been established due to the previous use of alkaline conditions that degrade (ara-C) DNA. Our results suggest that the incorporation of ara-C into DNA is one mechanism responsible for producing lethal cellular events.

Recent work using isolated DNA polymerase-template complexes has shown that arabinofuranosyl derivatives con slow DNA synthesis by being incorporated into DNA. Our results suggest that these agents act by a similar mechanism in L1210 cells. The results demonstrate that inhibition of cellular DNA synthesis by cytosine arabinoside (ara-C) was significantly related to the extent of ara-C incorporation in DNA over a wide range of drug concentrations and times of exposure. Furthermore, treatment with increasing concentrations of ara-C resulted in a greater proportion of ara-C residues at the 3'-terminus of the elongating DNA chain. These observations suggest that ara-C incorporation results in poor primer termini for further chain elongation.

We recently demonstrated a highly significant relationship between the incorporation of 1-beta-D-arabinofuranosylcytosine (ara-C) into L1210 DNA and the loss of clonogenic survival. These studies have now been extended to the human promyeloblast (HL-60) cell line and myeloblasts from a patient with acute myelogenous leukemia. Our results demonstrate: (i) the specific internucleotide incorporation of ara-C into human myeloblast DNA; (ii) the lability of [3H]ara-C-labeled DNA to alkali which necessitates the use of nondegrading assay conditions; and (iii) a highly significant relationship (P less than 0.0001) between the loss of clonogenicity of these cells and the extent of ara-C incorporation. These findings suggest that the incorporation of ara-C into DNA is one of the initial events leading to cell lethality. This method is applicable to clinical samples of bone marrow and peripheral blood as an in vitro assay for studying the sensitivity of cell populations to this drug.

BCNU at a dose of 200 mg/m2 was administered in a 60-minute hepatic arterial (HA) infusion to three patients with liver-predominant neoplastic disease. Nitrosourea blood levels were measured during the 60-minute infusion and for 30 minutes after completion of the infusion in samples obtained simultaneously from a hepatic venous (HV) catheter and a peripheral venous (PV) site. Direct extraction of blood with diethyl ether was used in order to remove quantitatively the nitrosourea from blood and thereby stabilize it against in vitro breakdown in blood prior to colorimetric analysis. As determined by blood levels, steady-state was achieved within 50 minutes of infusion. Mean HV levels at steady-state were 2.5-fold higher in the HV compared to PV samples. The higher HV compared to PV levels must reflect higher tumor exposure to nitrosourea with HA compared to the usual iv route of administration. No drug-related hepatic toxicity was noted. Myelosuppression was noted in only one patient in whom reversible leukopenia (granulocyte nadir, 500/mm3) and thrombocytopenia (platelet nadir, 20,000/mm3) developed.

We have employed cesium sulfate density gradient analysis to monitor the incorporation of 1-beta-D-arabinofuranosylcytosine (ara-C) into L1210 cellular nucleic acid. This methodology permits a distinct separation of RNA and DNA under nondegrading conditions. The results demonstrate specific incorporation of ara-C into DNA with less than 1% of total incorporation into nucleic acid being detectable in RNA. Moreover, we have found a highly significant relationship (p0.0001) between the incorporation of ara-C into DNA with the loss of clonogenic survival. This relationship had not been established due to the previous use of alkaline conditions that degrade (ara-C) DNA. Our results suggest that the incorporation of ara-C into DNA is one mechanism responsible for producing lethal cellular events.

We have previously demonstrated that 1-beta-D-arabinofuranosylcytosine (ara-C) incorporates into leukemic cell DNA and that the extent of this incorporation correlates significantly with loss of clonogenic survival. These studies have now been extended by monitoring the incorporation of ara-C in DNA undergoing repair of UV-induced damage. The present studies have been performed using human foreskin diploid fibroblasts that undergo density-dependent growth arrest. The results demonstrate that ara-C incorporates specifically into DNA undergoing repair of damage by UV light. The extent of ara-C incorporation in AG1522 DNA during UV repair correlates significantly (p less than 0.0006) with cell lethality. These findings confirm that incorporation of ara-C into DNA undergoing either semiconservative or unscheduled DNA synthesis results in lethal cellular events. The present findings may provide the experimental basis for the development of new therapeutic approaches using ara-C in combination with agents that damage DNA.

Cytosine arabinoside (Ara-C) is the most effective agent in the treatment of acute myelogenous leukemia. This agent incorporates in leukemic cell DNA, and the extent of this incorporation correlates with loss of clonogenic survival. The incorporated Ara-C residue behaves as a relative DNA chain terminator, and the extent of (Ara-C)DNA formation correlates with inhibition of DNA synthesis. The incorporation of Ara-C into DNA requires the formation of Ara-CTP, and previous measurements of this metabolite have also been correlated with cytotoxicity. Because it is clinically relevant to define biochemical parameters predictive of Ara-C cytotoxicity, the present studies were undertaken to determine the relationship among Ara-CTP pools, formation of (Ara-C)DNA, and loss of clonogenic survival. The results demonstrate that the incorporation of Ara-C into DNA is the single most powerful predictor of cell lethality. Furthermore, although there is a correlation between Ara-CTP pools or continuous cellular exposure to Ara-CTP and cell kill, these relationships are less significant than that obtained with formation of (Ara-C)DNA. The extent of Ara-C incorporation into DNA can be predicted by the product of the Ara-CTP level and time (T), thus supporting the concept that Ara-C incorporation is dependent on continuous exposure to the triphosphate metabolite. These findings support the formation of (Ara-C)DNA as a highly predictive parameter of lethal cellular events.

A technique was developed for continuous iv infusion chemotherapy in an inbred rat model of acute myelogenous leukemia. Polyethylene tubing was inserted surgically into the internal jugular vein of adult WF rats, burrowed sc to the base of the tail, and connected to an infusion pump. A flexible spring was sutured at the base of the tail and fastened to the cage wall; it protected the infusion catheter and allowed movement of the rat within the cage. This technique was used to compare bolus with continuous infusion therapy with adriamycin, cytosine arabinoside, and neocarzinostatin. Only small differences were noted in host toxicity and in antitumor effect against tumor grown as a subcutaneous myeloblastoma. Nearly three times more neocarzinostatin was required by continuous infusion for an effect equivalent to that of bolus injection. In contrast, continuous infusion of methotrexate with concurrent thymidine infusion prevented toxicity, enhanced the antitumor effect, and prolonged survival. This infusion system should facilitate rapid preclinical evaluation of drugs considered for constant iv infusion therapy.

Although the biochemical and cytokinetic effects of thymidine (TdR) have been extensively studied in vitro, other parameters may be encountered in vivo that could alter its therapeutic efficacy. L1210 cells in culture are sensitive to the growth inhibitory effects of TdR, while TdR infusions in mice bearing L1210 ascites fail to prolong survival. Our results demonstrate that the TdR levels achieved in the serum and ascites fluid during sc and iv infusions range between 10(-4) and 10(-3) M. These levels are sufficient to inhibit the growth of bone marrow and intestinal mucosa cells, as evidenced by 32P incorporation studies. However, these TdR levels fail to exhibit the growth of L1210 ascites, despite the fact that these cells are sensitive to similar concentrations in vitro. The absence of growth inhibition in the L1210 ascites is shown to be due to increased endogenous CdR levels (approximately 2 X 10(-5) M) in the ascites fluid. The concomitant serum CdR levels were at least ten times lower than those achieved in the ascites. This factor probably accounts for the differential effect of TdR on normal cells as compared in vivo with agents such as TdR or even antimetabolites, such as cytarabine, may be modulated by increased CdR levels in the local tumor milieu. These studies are relevant in that TdR is currently being used in a variety of clinical protocols using the nucleoside alone at high doses or in combination with methotrexate, 5-FU, or cytarabine.

We have demonstrated previously that 9-β- d -arabinofuranosyl adenosine (ara-A), like 1-β- d -arabinofuranosyl cytosine (ara-C), incorporates exclusively into DNA, not RNA [Kufe et al., Cancer Res. 43 , 2000 (1983)]. We have also demonstrated that (ara-C)DNA is degraded by alkali [Major et al., Biochem. Pharmac. 31 , 861 (1982)], suggesting a structural instability of this abnormal nucleic acid. These findings have been extended by investigating the effects of various alkaline conditions on ara-A and (ara-A)DNA. The results indicate that the nucleoside was degraded under stringent conditions (0.4 M NaOH, 0.5 hr, 100°) and that a similar effect occurred following exposure of (ara-A)DNA. The results also indicate that milder alkaline conditions (0.4 M NaOH, 6 hr, 37°) resulted in strand scission of (ara-A)DNA at the 3'-carbon of the incorporated arabinosyl sugar without degradation of the ara-A moiety itself. These findings may be useful when attempting to purify (ara-A)DNA and confirm the structural instability of arabinosyl-containing DNA.

Thymidine (TdR) has been used to study the kinetics of in vitro cell proliferation and is currently being used clinically as a single agent at high doses. We have explored the in vivo cytokinetic effects of TdR on rapidly proliferating cell populations by continuous infusions in rats. The nucleoside was lethal at high doses when serum levels approached 10(-2) M.. At levels of 10(-3) M, TdR exposure for greater than 24 hours resulted in bone marrow hypocellularity and peripheral myelosuppression. Pathologic findings were also noted in the intestinal mucosa. Serum TdR levels of 10(-4) M were sufficient to induce arrest of cell growth in S phase by inhibition of DNA synthesis. Subsequent release after TdR exposure produced partial synchronization of the bone marrow and intestinal mucosa cell populations, as shown by microfluorometry and labeling studies to monitor DNA synthesis. A similar arrest of cell cycle traverse has been demonstrated in a tumor cell population by infusing rats bearing transplantable subcutaneous myeloblastomas. The inhibition of DNA synthesis, as determined by labeling studies, was comparable for bone marrow, intestinal mucosa, and myeloblastoma at serum TdR levels of 10(-3) M. This arrest of myeloblast cell growth was dependent on tumor burden and did not effect survival when maintained during 72-hour infusions. The continuous infusion of TdR provides an approach for studying cell kinetics in vivo, and findings similar to those described here have been observed in our clinical studies.

We have employed cesium sulfate density gradient centrifugation to separate RNA and DNA of L1210 cells labeled with [3H]fluorodeoxyuridine. We have analyzed nucleotide and nucleoside digests of purified DNA from the [3H]fluorodeoxyuridine-labeled cells and demonstrate by reverse phase and anion exchange high pressure liquid chromatography the presence of tritium radioactivity co-migrating with fluorodeoxyuridine 5'-monophosphate or fluorodeoxyuridine. These observations demonstrate the internucleotide incorporation of fluorodeoxyuridine in DNA and suggest a new mechanism of action for this cytotoxic and mutagenic agent.