[CANCER RESEARCH 44, 3355-3359, August 1984]

Enhancement of the Sensitivity of Human Colon Cancer Cells to Growth Inhibition by Acivicin Achieved through Inhibition of Nucleic Acid Precursor Salvage by Dipyridamole1

Paul H. Fischer,2 Rifat Pamukcu, Gerard Bittner, and James K. V. Willson2-3

Department of Human Oncology, University of Wisconsin School of Medicine, Madison, Wisconsin 53792 [P. H. F., R. P., J. K. V. W.J, and William S. Middleton Memorial Veterans Medical Center, Madison, Wisconsin 53705 [G. B., J. K. V. W.]

ABSTRACT mammary and lung xenografts (8). Although acivicin inhibits numerous enzyme reactions involving the transfer of nitrogen This study was undertaken to determine if salvage of nucleic from the 7-carboxamide of L-glutamine (10), including L-aspara- acid precursors might constitute a mechanism of resistance to gine synthetase (3), carbamoyl phosphate synthetase II (1, 10), acivicin in human colon cancer cells and, if so, to establish phosphoribosylformylglycinamidine synthetase (10), and amido- whether dipyridamole, an inhibitor of nucleoside and nucleobase phosphoribosyl transferase (4, 10), inhibition of 2 other L-gluta- transport, can block the salvage process and restore sensitivity mine-dependent enzymes, CTP synthetase (4,10,18) and XMP to acivicin. Acivicin inhibited the replication of human colon aminase (10), appears to be most closely associated with the cancer cells (VACO 5) in vitro in a dose- and time-dependent cytotoxicity of acivicin. Depletion of the CTP and GTP pools and fashion. In addition, marked cell lysis was evident after a 24-hr expansion of the DTP pool have typically been seen (10,13, 14, exposure to acivicin at concentrations greater than 1 ng/m\. The 30). primary metabolic effect of acivicin was depletion of the cytidine In rodent cancer cells, perturbation of both purine and pyrimi- triphosphate and guanosine triphosphate pools. Adenosine tri- dine metabolism is evidently critical, since cytosine and guanine phosphate levels were also reduced, but apparently as a con nucleosides antagonize acivicin cytotoxicity much more effec sequence of the guanosine triphosphate depletion. VACO 5 cells tively when they are used in combination than when they are exposed to acivicin (3 /iQ/ml) efficiently salvaged low levels (1 used alone (18,30). These data suggest the potentially important í¿m)ofcytidine, guanosine, and guanine and could, therefore, role which salvage of nucleic acid precursors might play in restore the depleted nucleotide pools. The combination of cyti modifying the actions of acivicin. In particular, studies on rat dine and guanosine, but not either nucleoside alone, provided hepatoma 3924A cells have documented the ability of nucleo significant protection against the growth-inhibitory properties of sides to circumvent the growth-inhibitory effects of acivicin (14, acivicin. Dipyridamole, at a noncytotoxic concentration (5 /¿M), 24,25,30). Furthermore, dipyridamole, an inhibitor of nucleoside blocked repletion of the cytidine triphosphate and guanosine transport (11, 19-21), antagonized the protection afforded by triphosphate pools in cells exposed to acivicin and the nucleic salvage mechanisms and substantially enhanced the cytotoxicity acid precursors. As a result, the growth-inhibitory effects of of acivicin (24, 25, 30). Since the antitumor activity of acivicin in acivicin were maintained. The salvage of cytidine was particularly humans is now being evaluated in Phase II trials, including colon sensitive to inhibition by dipyridamole, and no restoration of cancer, we felt that several important questions regarding the cytidine triphosphate pools was evident. The cellular uptake of actions of this drug in human cells should be addressed. Does a variety of nucleic acid precursors was differentially sensitive to acivicin inhibit the growth of a human colon cancer cell line, such inhibition by dipyridamole. The 50% inhibitory dose values ranged as VACO 5? Are the cytotoxic effects seen at clinically achievable from 0.01 to 2.5 fiM for cytidine and uridine, respectively. The concentrations and times of exposure to acivicin? Does salvage results of this study indicate that, although the replication of of nucleic acid precursors appear to be a likely mechanism of VACO 5 cells was inhibited by acivicin, low levels of nucleosides resistance to acivicin? Does the use of dipyridamole block reple and nucleobases can circumvent the cytotoxicity. Dipyridamole tion of the nucleotide pools and provide a reasonable therapeutic effectively blocked the salvage pathways and restored the sen approach for maintaining the growth-inhibiting properties of aciv sitivity of the cancer cells to the antiproliferative actions of icin? acivicin. MATERIALS AND METHODS INTRODUCTION Acivicin,4 a structural analogue of L-glutamine, is an antitumor Materials. Acivicin (NSC 163501) was obtained from the National Cancer Institute, Investigational Drugs division. Dipyridamole, nucleo- agent isolated from Streptomyces sviceus (6). The compound tides, nucleosides, and nucleobases were purchased from either Sigma has activity against several murine tumors, the L1210 and P388 Chemical Co. (St. Louis, MO) or P-L Biochemicals (Milwaukee, Wl). leukemias (6) and M5076 ovarian carcinoma (8), and human Aldrich Chemical Co. (Milwaukee, Wl) supplied the Alamine (tri-n-octyla- 1 Supported in part by CA 14520 and the Veterans Administration Medical mine) and Freon (1,1,2-trichlorotrifluoromethane). [8-3H]Guanosine (5 Ci/ mmol), [5-3H]cytidine (30 Ci/mmol), and [8-"C]guanine sulfate (51 nCi/ Research Service. Portions of this study have been presented at the Central Society for Clinical Research, November 2,1983, Chicago, IL (28). mmol) were purchased from Amersham/Searie Corp. (Arlington Heights, 2 To whom requests for reprints should be addressed. IL). [5-3H]Uridine (20 Ci/mmol) and [5-3H]deoxycytidine (25 Ci/mmol) 3 Recipient of an American Cancer Society Junior Faculty Clinical Fellowship. 'The abbreviation used is: acivicin, L-(aS,5S)-a-amino-3-chloro-4,5-dihydro-5- were obtained from Moravek Biochemcials, Inc. (Brea, CA). isoxazolacetic acid. Cell Culture. VACO 5 cells, a human colorectal cell line, were grown Received November 14,1983; accepted May 9,1984. in Eagle's minimal essential medium supplemented with 2 mM u-gluta-

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mine, 0.1 mu nonessential amino acids (Grand Island Biological Co., Grand Island, NY), insulin (2 //g/ml), transferrin (2 ¿jg/ml),and 7.5 nM sodium selenite (Sigma), gentamicin (50 ^g/ml) (Schering Co., Kenilworth, NJ), and 8% heat-inactivated fetal bovine serum, Lot 100374 (Hyclone, O.Ijig/ml Logan, UT). The contributions from the serum yielded final medium concentrations of less than 0.1 /JM guanosine and cytidine and 2.1 ^M guanine. Dissociation medium was identical to growth medium, except that a Ca2*- and Mg2+-free minimal essential medium was used and fetal bovine serum was excluded. VACO 5 cells were established from a poorly differentiated adenocarcmoma of the cecum and characterized as described previously (16). Stock cultures are passaged weekly and have maintained morphological, karyotypic, and drug sensitivity characteristics during the 12-month course of this study. VACO 5 cells were determined to be negative for Mycoplasme contamination, as demonstrated by culture under aerobic and anaerobic conditions (Wisconsin State Labo ratory of Hygiene, Madison, Wl). Cytotoxicity. Exponentially growing cells, initially plated at a density of 2 x 10* cells/ml in 25-sq cm tissue culture flasks, were maintained at 37°in a humidified 5% CO2 atmosphere. Medium containing the various additives was added 12 hr later. In the experiment described in Chart 1, 24 48 72 96 the exposure to acivicin was continued for an additional 2 to 96 hr. The EXPOSURE TO ACIVICIN (hr) cells were then washed twice, fresh medium was added, and the incubation was continued such that the entire growth period was 96 hr. Chart 1. Effects of acivicin on the replication of VACO 5 cells. Exponentially growing cells were plated at a density of 2 x 104 cells/ml and incubated for 12 hr For the experiment shown in Chart 3, the cells were exposed to acivicin, the nucleosides, or dipyridamole for 24 hr. The cells were then washed prior to the addition of acivicin. The cells were then exposed to acivicin at the indicated concentrations for increasing periods of time (2 to 96 hr). Following drug twice and incubated for 72 hr in fresh medium containing the nucleosides treatment, the cells were washed twice, and fresh medium was added. Incubation and dipyridamole as shown. Cells were harvested after a 1- to 2-hr was continued for a total of 96 hr for all conditions. The data are expressed as the exposure to a Ca2+ and Mg2* free dispersion medium. Cells excluding percentage of control cell number present at the end of 96 hr. trypan blue were counted using a hemocytometer. Measurement of Nucleoside Triphosphate Pools. Nucleoside tri- 200r UTP phosphate pool extractions were carried out on duplicate flasks in parallel with the cell growth studies. VACO 5 cells (2.5 to 12 x 106) were collected by centrifugation (10 min, 200 x g), the medium was removed, and the cells were resuspended in 1 ml of cold phosphate-buffered saline. The suspension was transferred to a 1.5-ml microfuge tube and centrifugea (30 sec, 11,500 rpm). The supernatant was removed, and the cell pellet was extracted in 0.1 ml of 0.5 M HCIO4 for 10 min at 4°. The extract was neutralized with an equal volume of 0.5 M Alamine in Freon (12). The neutralized samples were stored at -20° until analyzed 1 I 10 1 I 10 I 1 10 I I 10 by high-pressure liquid chromatography. Chromatography of the samples ACIVICIN(pg/ml) was performed on a system consisting of a Spectra-Physics 8700 solvent Chart 2. Effects of acivicin on ribonucleotide triphosphate pools in VACO 5 delivery system, a Whatman Partisil 10/25 SAX anion-exchange column cells. Exponentially growing cells were exposed to acivicin (0.1,1.0, and 10 >¡g/m\) for 24 hr prior to extraction with 0.5 M perchloric acid. The data are expressed as equipped with a guard column, a Kratos Spectroflow 773 UV spectre- the percentage of control based on either nmol/106 cells P) or nmol/flask (•)The photometer monitoring 254 nm, and a Hewlett-Packard 3380A integrator. control values (nmol/IO6 cells) were: CTP, 0.30; GTP, 0.71; ATP, 3.8; and UTP, Peaks were identified by retention times. Peak areas were linearly related 1.55. These data are from a single experiment; however, in other experiments in which the time of exposure and concentration of acivicin were varied, similar results to the amount injected over the range of 0.08 to 12 nmol, and extracted were obtained. standards were run on each day of analysis. Recovery of nucleotides by extraction averaged 93%. The buffer system used an isocratic elution of the nucleotides with 0.3 M sodium phosphate, pH 5.0, at a flow of 1 to was produced by acivicin. There were 20, 44, and 74% fewer 2 ml/min as determined by the age of the column. Typical retention times cells 24 hr after exposure to acivicin at 1.0, 3.0, and 10 /¿g/ml were: DTP, 6.6 min.; CTP, 9.0 min.; ATP, 11.3 min.; and GTP, 17.2 min. than were present at the time of drug addition. Such cell loss is an important parameter and must be considered when analyzing RESULTS AND DISCUSSION pool size data. In addition, acivicin produced marked changes in the cellular morphology, including a flattened cuboidal appear Effects on Cell Growth. The inhibition of VACO 5 cell repli ance and avid attachment to the plastic flasks (data not shown). cation by acivicin was influenced strongly by both the drug Effects of Acivicin on Ribonucleoside Triphosphate Pools. concentration and the duration of exposure (Chart 1). For ex Exponentially growing VACO 5 cells were exposed to increasing ample, following a 72-hr exposure, increasing concentrations of concentrations of acivicin for either 6 hr (data not shown) or 24 acivicin (0.1, 1.0, and 10 ng/m\) inhibited cell growth by 30, 82, hr (Chart 2) and then extracted for determination of ribonucleo- and 94.5%, respectively. Similarly, acivicin (3.0 ng/m\) reduced side triphosphate pool sizes by high-performance liquid chro cell replication by 34, 72, and 92% after exposures of 6, 24, and matography. A 6-hr exposure to acivicin, which did not signifi 72 hr. These concentrations are in the same range as the peak cantly alter cell numbers, produced a dose-dependent reduction plasma levels of 4.5 and 0.8 i¿g/m\achieved in the phase I trials in CTP and GTP pools and an expansion of the UTP pool. These in which acivicin was administered as a 24-hr (27) or 72-hr (5) effects were essentially the same as those seen in rodent cells continuous infusion. Cell lysis, as well as inhibition of replication, (10,13,14,18, 30). Since considerable cell loss was associated

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Downloaded from cancerres.aacrjournals.org on September 27, 2021. © 1984 American Association for Cancer Research. Inhibition of Nucleic Acid Precursor Salvage with a 24-hr exposure to acivicin, the results are presented both inhibition of GTP pool repletion was seen. as nmol/flask, which reflect overall changes in the cell population, The data presented in Table 1 indicate that VACO 5 cells can and as nmol/106 cells, which depict pool sizes in those cells readily restore depleted ribonucleoside triphosphate pools surviving the exposure to acivicin. The data, expressed as nmol/ through salvage mechanisms. As expected, however, cytidine flask (solid bars), illustrate a dose-dependent reduction in the did not replete GTP pools, and CTP pools were not expanded ribonucleoside triphosphate pools. Acivicin depleted the CTP by the purines. These results, as well as the findings with mouse pools most effectively, followed by the GTP, ATP, and UTP (18) and rat (14, 30) cells, suggested that a pyrimidine and a pools. The effects on CTP and GTP implicate CTP synthetase purine source would be necessary to replete both pools and to and XMP aminase as the most sensitive targets. These results effectively antagonize cytotoxicity. As shown in Chart 3, the are consistent with the relatively low K¡valuesreported for these growth-inhibitory effects of a 24-hr exposure to acivicin (3 M9/ reactions (13, 26). In addition, studies of the action of acivicin in ml) were strongly reduced by the simultaneous presence of vivo had shown rapid inactivation of CTP synthetase and deple cytidine and guanosine, whereas 10 tiM guanosine alone was tion of CTP and GTP pools in rat hepatomas (4, 26). When the ineffective. Similarly, cytidine alone reduced the inhibition pro results are expressed as nmol/106 cells (hatched bars), acivicin duced by acivicin only marginally, from 75 to 67 and 53%, (10 M9/ml) did not deplete the CTP, GTP, or ATP pools more respectively, when present at concentrations of 1 and 10 ßM than 1 fig/ml Thus, in the cells surviving a 24-hr exposure to (data not shown). The antagonism of acivicin toxicity produced acivicin, the nucleotide pool depletions were much less than in by a combination of cytidine (10 MM)and guanosine (10 MM)was, the population as a whole. It should be noted that, when plated however, fully blocked by dipyridamole (Chart 3). This effect was in drug-free medium, the cells surviving exposure to acivicin achieved at a concentration of dipyridamole, 5 /JM,which did not regrew with a normal population doubling time of 24 hr. The alter VACO 5 cell growth. The important therapeutic implication mechanisms underlying the relative insensitivity of the surviving is that blockade of either CTP or GTP repletion would maintain cells have not been established. Repletion of Ribonucleoside Triphosphate Pools and Mod 100- *

Table 1 Perturbation of nucleotide pools nmol/10«cells

ConditionControlACVC' (3)"0.07±0.007s ±0.04(3)0.28 (3)4.0±0.13 (3)5.9±0.27 "ACV (2)0.33+ 0.02e + 0.6(2)0.22 +0.6(2)3.8 +1.2(2)5.9 cytidineACV+ 1 fiu (3)1.2 ±0.03 (3)0.2±0.05 (3)2.8±0.38 ±0.6(3)4.6 cytidineACV+ 10 MM (1)0.036 (1)0.36 (1)2.6 (1)2.3 DPACV+ 1 «IMcytidine+ 5 ¡MÕ (3)0.07±0.007 (3)0.3±0.05 (3)3.2±0.62 (3)4.6±0.68 DPACV+ 10 MMcytidine + 5 ut* (1)0.046 (1)0.87 (1)4.4 (1)2.75 guanosineACV+ 1 ¡M (3)0.03±0.017 (3)2.2+ 0.18 ±0.9(3)5.3 (3)3.3±0.85 guanosineACV+ 10 MM (1)0.046 (1)0.41 (1)2.94 (1)2.5 DPACV+ 1 nu guanosine + 5 MM (3)0.15±0.02 (3)1.45±0.07 (3)7.1±0.62 (3)4.7±0.81 guanineACV+ 1 iiM (2)0.023+0.001 (2)2.1+ 0.16 (2)7.05+ 0.76 (2)4.5+ 0.04 guanineACV+ 10 iiM (3)0.05±0.003 (3)0.836 ±0.23 ±0.8(3)5.7 (3)5.23±0.52 DPACV+ 1 IM guanine + 5 MM (3)0.025±0.006 (3)0.88±0.21 ±1.2(3)4.1 (3)3.0±0.81 + 10 MMguanine+ 5 IM DPCTP0.28 + 0.01 (2)GTP1.2 + 0.08 (2)ATP5.7 +1.1 (2)UTP3.0 +1.5 (2) a Mean ±S.E. Numbers in parentheses, number of experiments. c ACV, acivicin; DP, dipyridamole. VACO 5 cells were exposed to acivicin (3 «¿g/ml)for6 hr. 8 Mean + range.

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Table 2 Repletion of ribonucleotide pools These data are the means derived from 2 or 3 separate experiments. In those cases in which n = 2, the range averaged 18% of the mean; in those cases in which n = 3, the coefficient of variation averaged 27%. CTPConditionControl cells0.54 cells1.7 cells5.3 cells2.3

ACV"' " 0.08 (12)c 0.3 (5o) (9.5) 0.6 (34) 0.9(14) 1.1 (20) 0.2 (7) 0.6 (26) ACV + 10 ¡Mguanosine 0.12 (18) 0.35 (65) 1.8 (86) 5.4(316) 4.5(69) 13.4(253) 1.7(61) 5.0(217) ACV + 10 ¿IMguanosine + 0.32(48)1.2 0.24(44)1.5 2.3(111)1.4 5.8(539)1.8(106) 5.3(82)1.5(23) 13.2(248)2.0 1.8(64)1.2(43) 4.5(197)1.6 1 ¡Mcy(¡dine ACV + 10 pM guanosine + (179) (278) (67) (38) (71)6.1 1OHM cytidine ACV + 10 ¡tuguanosine + 0.08 (12)nmol/IO"0.4 (74)GTPnmol/flask2.10.20.4 (19)nmol/10«2.1(123)ATPnmol/flask6.50.6 (9)nmol/10«2.9 (55)UTPnmol/flask2.81.2(43)nmol/10* (265) 5 MMDPnmol/flask0.67 a ACV, acivicin; DP, dipyridamote. '' VACO 5 cells were exposed to acivicin (3 /¿g/ml)for 24 hr. c Numbers in parentheses, percentage of control. the cytotoxicity of acivicin. The changes in ribonucleoside tri- 120- phosphate pools associated with the modulation of acivicin cy totoxicity were also determined. Pool sizes were measured in cells exposed for 24 hr to the experimental conditions described in Chart 3. Since these treatments were associated with large changes in cell number, the data were normalized on a per-cell Urd and per-flask basis (Table 2). The effects of acivicin on GTP and ATP pools were reversed by the addition of guanosine (10 fiM). In fact, on a per-cell basis, these pools were expanded 2.5 to 3 times over control values. Although 1 MMcytidine fully repleted the CTP pool following a 6-hr exposure (Table 1), only partial reversal was evident after a 24-hr treatment (Table 2). However, 10'9M 10'8M 10'7M IO'6M 10 MM cytidine was fully restorative, and CTP pools were ap proximately 2-fold larger than were controls. Although CTP and DIPYRIDAMOLE(M) GTP pools were repleted by the addition of cytidine (10 MM)and Chart 4. Effect of dipyridamole on the uptake of nucleic acid precursors. Ex guanosine (10 MM),cell growth was still somewhat less than the ponentially growing VACO 5 cells were exposed to 1 UM concentrations of the radtolabeted nucleic precursor and the indicated concentrations of dipyridamole for control rate (Chart 3). This may have resulted from the depressed 60 min. The data are expressed as the percentage of control nucleoside uptake. ATP pools associated with these conditions (Table 2). Dipyrida- The dose-response curves are the composites of several experiments (at least 3) done for each of the precursors. Cyö,cytidine; Quo, guanosine; Qua, guanine; mole entirely blocked the salvage of cytidine (10 MM)and restored dCyd. deoxycitidme: Uro, undine. the cytotoxicity of acivicin. All of the ribonucleoside triphosphate pools were reduced on a per-flask basis, whereas only minimal different precursors was apparent. Dipyridamole reduced cyti changes were evident on a per-cell basis. dine uptake by 50% at approximately 10 nw, whereas a similar Although a wide range of values have been reported, the levels effect on undine uptake required approximately 2.5 MM of the of most nucleosides in human plasma appear to be in the low inhibitor. Guanosine, guanine, and deoxycytidine were sensitive micromolar (0.1 to 1) range (7, 9, 17, 22, 23). In these experi to an intermediate degree. The influence of acivicin pretreatment ments, cytidine, 1 MMof guanosine, or guanine fully counteracted on the ability of dipyridamole to inhibit nucleoside uptake was the effect of acivicin on CTP or GTP pools during a 6-hr exposure also evaluated. The type of experiment described in Chart 4 was (Table 1). Thus, it would appear that physiological levels of repeated for several nucleosides using VACO 5 cells which had nucleic acid precursors are, in fact, potentially sufficient to facil been exposed to acivicin (3 fig/ml) for 6 hr, suggesting that pool itate the restoration of pool sizes by human cancer cells. size depletion does not alter the inhibitory effects of dipyridamole Effect of Dipyridamole on the Uptake of Nucleic Acid Pre on the uptake of cytidine, guanosine, or deoxycytidine (data not cursors. The uptake process involves both the transport and shown). metabolism of nucleosides and nucleobases (29). Since dipyri- Numerous studies have shown the feasibility of blocking nu damole inhibits the first step, transport, the uptake of both cleoside uptake both in vitro and in vivo with inhibitors of nucleo nucleosides and nucleobases can be altered as well. This action side transport (2, 15, 19-21). Dipyridamole, which inhibits the would, presumably, account for the ability of dipyridamole to transport of both nucleosides and nucleobases, is also clinically prevent the repletion of ribonucleotide pool sizes. Therefore, the used as a vasodilator and an antiplatelet drug. Because of our ability of dipyridamole to inhibit the uptake of a variety of these interest in conducting a clinical evaluation of acivicin in combi compounds into the 60% methanol-soluble and -insoluble por nation with a transport inhibitor, dipyridamole was chosen for tions of VACO 5 cells was assessed. Since the effects on use in this study. In the course of these experiments, several incorporation of the precursors into macromolecules and into the studies from the laboratory of G. Weber (24,25,30) reported on nucleotides were essentially identical, only data on the methanol- the interaction of acivicin and dipyridamole in rat hepatoma cells. soluble fraction are presented (Chart 4). Considerable variation Their data clearly showed that dipyridamole can block the ability in the degree to which dipyridamole inhibited the uptake of of nucleosides to reverse the cytotoxicity of acivicin. In addition,

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Downloaded from cancerres.aacrjournals.org on September 27, 2021. © 1984 American Association for Cancer Research. Inhibition of Nucleic Acid Precursor Salvage the replication of rat hepatoma cells was inhibited by dipyrida- 163501): a new amino acid antibiotic with the properties of an antagonist of L- glutamine. Cancer Chemother. Rep., 59: 481-491,1975. mole, with a 50% inhibitory dose of approximately 20 »M(30). 11. Kessel, D., and Hall, T. C. Effects of persantin on deoxycytidine transport by The results of this study, taken together with the data reported murine leukemia cells. Biochim. Biophys. Acta, 211: 88-94,1970. 12. Khym, J. X. An analytical system for rapid separation of tissue nucleotides at by others (24, 25, 30), suggest that the salvage of nucleic acid tow pressures on conventional aniónexchangers. Clin. Chem., 21:1245-1252, precursors is likely to antgonize the cytotoxic effects of acivicin 1975. in a clinical setting. Intervention with dipyridamole, as suggested 13. Loh, E., and Kufe, D. W. Synergistic effects with inhibitors of de novo pyrimidine synthesis, acivicin, and W-(phosphonoacetyl)-L-aspartic acid. Cancer Res., 41: by Weber er al. (25), would appear to be a realistic approach to 3419-3423,1981. this problem. A critical question is whether the anticancer effects 14. Lui, M. S., Kizaki, H., and Weber, G. Biochemical pharmacology of acivicin in of acivicin can be enhanced by dipyridamole without an equiva rat hepatoma cells. Biochem. Pharmacol., 31: 3469-3473,1982. 15. Lynch, T. P., Paran, J. H., and Paterson, A. R. P. Therapy of mouse leukemia lent increase in toxicity to normal tissues. As a first step toward L1210 with combinations of nebularine and nitrobenzylthioinosine-5'-mono- answering this question, we have initiated a phase I trial of phosphate. Cancer Res., 41:560-565,1981. 16. McBain, J., Meisner, L, Weese. J., Wolberg. W., and Willson, J. 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Downloaded from cancerres.aacrjournals.org on September 27, 2021. © 1984 American Association for Cancer Research. Enhancement of the Sensitivity of Human Colon Cancer Cells to Growth Inhibition by Acivicin Achieved through Inhibition of Nucleic Acid Precursor Salvage by Dipyridamole

Paul H. Fischer, Rifat Pamukcu, Gerard Bittner, et al.

Cancer Res 1984;44:3355-3359.

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Downloaded from cancerres.aacrjournals.org on September 27, 2021. © 1984 American Association for Cancer Research.