Potentiation of adriamycin cytotoxicity by dipyridamole against HeLa cells in vitro and sarcoma 180 cells in vivo.

Dipyridamole (DP) is clinically prescribed for its vasodilating and antiplatelet effects. DP also inhibits nucleoside transport and enhances cytostatic activity of antimetabolites. We obtained evidence for augmentation of the cytocidal effect of Adriamycin (ADM) by DP, both in vitro and in vivo. Nontoxic levels of DP enhanced the cytotoxicity of ADM against HeLa cells, and the 50% effective concentration of ADM was decreased 2.4-fold by DP. DP also increased the activity of ADM in clonogenic assays. Intracellular levels of ADM in the case of concomitant exposure to ADM and DP were 1.5-fold higher than in the case of exposure to ADM alone, determined using high-performance liquid chromatography. Incorporation of ADM into the cells pretreated with DP was also increased (1.4-fold), while the efflux was little affected. The growth of Sarcoma 180 tumors was prominently suppressed by the combination of ADM and DP, compared to findings with ADM alone. DP also prolonged the survival of Sarcoma 180 tumor-bearing mice, when given in combination with ADM. While the enhancement of cytostatic activity of antimetabolites has been attributed to a decrease in utilization of the salvage pathway by DP, our data show that the synergic effects of DP with ADM were the result of increased intracellular levels of ADM.

Characterization of novel human leukemic cell lines selected for resistance to merbarone, a catalytic inhibitor of DNA topoisomerase II.

Merbarone is a catalytic inhibitor of DNA topoisomerase (topo) II that does not stabilize DNA-topo II cleavable complexes. Although the cytotoxicity of and resistance to complex-stabilizing topo II inhibitors, such as etoposide, is thought to be mediated through stabilization of these complexes, the mechanisms of cytotoxicity and resistance to catalytic inhibitors are not well known. To investigate this issue, we established 12 merbarone-resistant cell lines from human leukemia CEM cells, designated CEM/M70-B1 through -B12. Assessed by either growth inhibition or clonogenic assay, these cell lines are 3.5- to 6.6-fold resistant to merbarone, compared to the CEM parent cells. Karyotype analysis of three of the cell lines revealed that while CEM and drug-resistant cell lines had chromosome abnormalities in common, indicating a common origin, two of the merbarone-resistant lines (B1 and B8) each had unique structural markers. These novel cell lines are cross-resistant to complex-stabilizing topo II inhibitors, etoposide, teniposide, amsacrine, and doxorubicin, but not to other catalytic inhibitors, aclarubicin or SN-22995. Of considerable interest, these cell lines are cross-resistant to SN-38, a putative topo I inhibitor, but cross-resistance to other topo I inhibitors (camptothecin and topotecan) was lower and not seen in every cell line. In all 12 cell lines, there was a high correlation among drug resistance ratios between etoposide and teniposide and between merbarone and SN-38. By contrast, there was a low correlation between merbarone and etoposide and between SN-38 and other topo I inhibitors. These results suggest that resistance to merbarone and cross-resistance to etoposide might be through different mechanisms, whereas cross-resistance to SN-38 might be through a merbarone-related mechanism. Etoposide and SN-38 stabilized fewer DNA-topoisomerase complexes in CEM/M70-B cells than in CEM cells, but camptothecin stabilized more. Merbarone inhibited complex formation induced by etoposide in drug-sensitive and -resistant cells, but the degree of inhibition was lower in CEM/M70-B cells than in the parental cells. Moreover, merbarone did not affect complex formation stabilized by SN-38 or camptothecin. Immunoblot analysis of the CEM/M70-B cells showed decreased topo IIalpha, increased topo IIbeta, and no change of topo I protein, compared to CEM cells. We propose the hypothesis that decreased topo IIalpha may play a role in the resistance to merbarone that is different from that to complex-stabilizing drugs. Cross-resistance to catalytic inhibitors may be due to reduced complex formation as a consequence of decreased topo IIalpha. We also found that DNA-protein complexes stabilized by SN-38 might be different from those stabilized by topo II inhibitors and blocked by merbarone. Judging from both the high correlation of drug sensitivities and complex-formation assays, we postulate that mechanisms of cytotoxicity and cross-resistance of SN-38 in CEM/M70-B cells might be similar to those of merbarone. We believe that the CEM/M70-B cells are the first to be selected and characterized for resistance to a catalytic inhibitor of topo II. This study provides novel cell lines with characteristics of resistances to topo II and topo I inhibitors.

Differences in mutant p53 protein stability and functional activity in teniposide-sensitive and -resistant human leukemic CEM cells.

We examined p53 protein stability and DNA damage-induced p53-dependent responses in a human leukemic CEM cell line and two teniposide-resistant sublines, CEM/VM-1 and CEM/VM-1-5 ( approximately 40 and 400-fold resistant to teniposide, respectively). Although all cell lines contain the same p53 mutations at codons 175 (Arg-->His) and 248 (Arg-->Gln), the constitutive levels of p53 were progressively increased with the resistance of the cells to teniposide. By pulse-chase experiments, we found that the half-lives of mutant p53 protein were approximately 12, 17, and >30 h in CEM, CEM/VM-1, and CEM/VM-1-5 cells, respectively. The prolonged half-lives of p53 in these cells is consistent with the fact that the protein harbors the indicated mutations. Of note, however, is the fact that the increased p53 protein half-lives in the two drug-resistant cell lines corresponds to a proportional decrease in MDM2 protein levels but an increase in p53-MDM2 binding interactions. This suggests that MDM2-mediated p53 degradation may be altered in our leukemic cell lines. The DNA damage-induced p53 response is fully functional in the drug-sensitive CEM cells containing a mutant p53, but this pathway is attenuated in the drug-resistant cells. Specifically, while the mutant p53 was phosphorylated at serine-15 in response to ionizing radiation in all these cell lines, mutant p53 induction in response to teniposide or ionizing radiation and induction of the p53-target genes, p21 and GADD45 only occurred in the drug-sensitive CEM cells. As assessed by MTT cytotoxicity assay, CEM cells were also significantly more sensitive to ionizing radiation, compared to the drug-resistant cell lines, and this correlated with p53 induction. Collectively, these results suggest that changes in constitutive mutant p53 protein levels, p53-MDM2 binding interactions, and altered regulation of the DNA damage-inducible p53-dependent pathway may play a role in drug- and radiation-responsiveness in these cells.

Isolation and characterization of thrombomodulin from bovine lung.

Bovine thrombomodulin was isolated from the lung by Triton X extraction, affinity chromatography on diisopropyl phosphate-thrombin-agarose, and gel filtration on Ultrogel AcA-44. The final preparation was purified 6000-fold from the membrane extract with a yield of 21%. It showed apparent Mr of 78,000 and 105,000, before and after reduction, respectively, on polyacrylamide gel electrophoresis in SDS. The activity of the thrombomodulin was stable under the conditions of 1% SDS, 8 M urea, pH 2 and 10, and heat treatment at 60 degrees C for 30 min, but was unstable against treatment with 2-mercaptoethanol. Activation of protein C by thrombin in the presence of the thrombomodulin depended on Ca2+, and an equimolar complex formation between thrombin and thrombomodulin was required for the maximum rate activation. The rate of protein C activation by thrombin was increased 900-fold by thrombomodulin. Thrombomodulin inhibited the thrombin-induced fibrinogen clotting and platelet activation. However, it did not affect the inhibition of thrombin by antithrombin III with or without heparin, a protein C inhibitor or several synthetic inhibitors. These properties of bovine thrombomodulin were similar to those of rabbit thrombomodulin reported earlier.

Mechanism of inhibition of activated protein C by protein C inhibitor.

Protein C inhibitor isolated from human plasma inhibited thrombin, factor Xa, trypsin and chymotrypsin as well as activated protein C, but had very little effect on urokinase and plasmin. The inhibition constants (K1) of protein C inhibitor for activated protein C, thrombin and factor Xa were 5.6 X 10(-8) M, 6.7 X 10(-8) M and 3.1 X 10(-7) M, respectively. The second-order rate constant for inhibition of activated protein C by the inhibitor increased about 30-fold in the presence of an optimal heparin concentration (5-10 units/ml). The inhibition of activated protein C by plasma protein C inhibitor was also accelerated by heparin. When activated protein C (Mr = 62,000) was incubated with protein C inhibitor (Mr = 57,000), enzyme-inhibitor complexes with apparent Mr = 102,000 and 88,000 were observed in the nonreduced and the reduced samples, respectively, on SDS-polyacrylamide gel electrophoresis. In addition to these complexes, a band of unbound enzyme and a band with Mr = 54,000 were detected. When 125I-labeled protein C inhibitor was exposed to activated protein C, the inhibitor band was converted to bands with apparent Mr = 102,000 and 54,000 in the nonreduced samples, as determined by autoradiography after gel electrophoresis in SDS. The band with Mr = 54,000 also appeared when the inhibitor reacted with other serine proteases. The activated protein C was released from the inactive complex by treatment with 1 M ammonia or hydroxylamine. This phenomenon was found by SDS-polyacrylamide gel electrophoresis to represent the dissociation of the enzyme-inhibitor complex by ammonia or hydroxylamine into the free enzyme and the proteolytically modified inhibitor.(ABSTRACT TRUNCATED AT 250 WORDS)

Bovine plasma protein C inhibitor with structural and functional homologous properties to human plasma protein C inhibitor.

Bovine plasma protein C inhibitor was purified; it was then characterized in comparison with human protein C inhibitor. The specific inhibitory activity of the purified inhibitor for bovine activated protein C was 8,500 times that of the inhibitor in plasma. The purified inhibitor showed a single band with Mr 56,000 by SDS-PAGE at pH 7.0, and two bands at pH 8.8, a major one with Mr 56,000 and a minor one with Mr 105,000, under both unreduced and reduced conditions. The pI range of the inhibitor was between 4.4 and 6.1. The Mr of the inhibitor was reduced by treatment with neuraminidase, O-glycanase, and also with glycopeptidase-A, suggesting that the inhibitor has both Asn-linked and Ser/Thr-linked carbohydrate chains. Twenty-seven of the NH2-terminal 49 amino acid residues of the bovine inhibitor, which lacks the first 4 residues from the NH2-terminal amino acid sequence of human inhibitor, were identical to those of the human inhibitor. The bovine inhibitor inhibited bovine and human activated protein C, human thrombin, Factor Xa, Factor XIa, and plasma kallikrein with Ki = 1.0, 5.2, 2.6, 3.0, 1.3 X 10(-8) M, and 4.5 X 10(-9) M, respectively. The inhibitory rates for activated protein C and thrombin were accelerated significantly in the presence of heparin or negatively charged dextran sulfate. However, the acceleration by heparin or dextran sulfate for the inhibition of Factor Xa, Factor XIa, and plasma kallikrein was not significant. The bovine inhibitor did not inhibit human Factor XIIa or plasmin.(ABSTRACT TRUNCATED AT 250 WORDS)

Monoclonal antibodies to human protein C: effects on the biological activity of activated protein C and the thrombin-catalyzed activation of protein C1.

Thirteen monoclonal antibodies designated as MFC-1 to MFC-13 were obtained from hybridoma cells cloned after the fusion of mouse myeloma cells with spleen cells of mice immunized with purified human protein C. Studies were made to determine where the antibodies bound to the molecule of protein C and whether they affected the biological actions of protein C. By using the immunoblotting technique, six of these antibodies were shown to bind to the light chain of protein C, and five to the heavy chain of protein C and also activated protein C. The remaining two antibodies bound to neither the light chain nor the heavy chain, though both antibodies bound to the intact protein C. Antibodies specific for the light chain did not bind to the gamma-carboxyglutamic acid-domain. Two of the antibodies specific for the heavy chain (MFC-13 and -1) inhibited the amidolytic activity of activated protein C. The MFC-13 also inhibited the activity of bovine activated protein C, but not that of human Factor IXa, Factor Xa, or thrombin. In addition to these two antibodies, another one for the heavy chain (MFC-10) and two antibodies for the light chain (MFC-9 and -11) inhibited the inactivation of Factor Va by human activated protein C. One of the antibodies which inhibited the enzyme activity (MFC-1) blocked the inhibition of activated protein C by protein C inhibitor. Another one for the heavy chain (MFC-5) inhibited the activation of protein C by thrombin regardless of the presence or absence of thrombomodulin. Based on these results, we have established the positions of some monoclonal antibody-binding sites on the protein C molecule.

Gene structure of human thrombomodulin, a cofactor for thrombin-catalyzed activation of protein C.

The gene coding for human thrombomodulin, a thrombin receptor on endothelial cells and a cofactor for the activation of anticoagulant protein C zymogen, was isolated from a human genomic library by employing human thrombomodulin cDNA as a probe. The nucleotide sequences of the gene and the adjacent 5' and 3' flanking regions were then determined. The nucleotide sequence of this gene with approximately 3.7 kilobase pairs was identical to that of the cDNA, indicating that the gene for human thrombomodulin is free of introns. Hybridization data showed that there is only a single thrombomodulin gene in the human genome.

Heat enhances the cytotoxicity of cis-diamminedichloroplatinum(II) and its analogues cis-1,1-cyclobutane-dicarboxylato(2R)-2-methyl-1,4- butanediammineplatinum(II) and cis-diammine(glycolato)platinum in vitro.

cis-1,1-Cyclobutanedicarboxylato(2R)-2-methyl-1,4-butanediammin eplatinum(II) (NK121) and cis-diammine(glycolato)platinum (254-S), analogues of cis-diamminedichloroplatinum (II) (CDDP) with reduced nephrotoxicity, are under clinical phase trial in Japan. Since CDDP has been shown to be more cytotoxic under conditions of an elevated temperature, we tested the cytotoxicity and cellular uptake of these analogues at 37 degrees and 43 degrees C using EMT6/KU cells in vitro. The cytotoxicity of CDDP was enhanced at 43 degrees C, and that of NK121 and 254-S was also enhanced, in a dose- and time-dependent manner. The 90% cytotoxic concentration (IC90) of each drug was reduced 2.9-fold for CDDP, 2.5-fold for NK121, and 2.2-fold for 254-S. Cytotoxicity was maximal when the two modalities were used simultaneously for all three drugs. The intracellular platinum concentration was assayed using flameless atomic absorption spectrophotometry. When exposed to IC90 drug concentration at 43 degrees C for 2 h simultaneously, the intracellular platinum concentration increased to 0.095 +/- 0.007 micrograms/10(7) cells (a 1.9-fold increase) for CDDP, to 0.198 +/- 0.012 micrograms/10(7) cells (a 1.3-fold increase) for NK121, and to 0.090 +/- 0.014 micrograms/10(7) cells (a 1.3-fold increase) for 254-S; respectively, as compared with the level measured after drug exposure at 37 degrees C (P < 0.05 for all drugs). The elevation in platinum concentration may be one of mechanism related to a synergistic effect of the two treatment modalities. The concomitant use of CDDP analogues and heat shows potential for possible clinical application.

5-Fluorouracil's cytotoxicity is enhanced both in vitro and in vivo by concomitant treatment with hyperthermia and dipyridamole.

We obtained evidence that the cytotoxic effect of 5-fluorouracil (5-FU) is augmented when the drug is given in combination with hyperthermia (HYP) and dipyridamole (DP). Nontoxic levels of DP enhanced the combined cytotoxicity of 5-FU and HYP against B16 melanoma and human tumor cells in vitro as measured by the succinate dehydrogenase inhibition (SDI) test. Growth of B16 melanoma that had been subcutaneously implanted into the feet of C57 BL mice was inhibited by treatment with the combinations of 5-FU and HYP, of 5-FU and DP, and of 5-FU, HYP and DP as compared with the administration of 5-FU alone. Treatment with HYP plus DP did not alter the body weight of mice that received 5-FU. The administration of DP plus HYP seemed to render the tumor cells more sensitive to 5-FU. The combination of 5-FU, HYP and DP shows promise for the treatment of patients suffering from malignant disease.

Postoperative PSK and OK-432 immunochemotherapy for patients with gastric cancer.

We evaluated the effects of chemotherapy given postoperatively with and without immunomodulators on the survival of patients who had undergone resection for gastric cancer. We conducted a retrospective survey of data on 963 Japanese patients treated at our department of surgery between 1965 and 1987. Data related to the duration of postoperative survival were calculated for those who received chemotherapy, i.e. an individualized combination of various agents given with or without the immunomodulators PSK, a protein extract of the fungus Coriolus versicolor, and/or OK-432, a preparation of an attenuated strain of Streptococcus (immunochemotherapy). Postoperative immunochemotherapy was more often prescribed for patients with advanced disease. The survival of patients who received immunochemotherapy was shorter than that of patients who received only chemotherapy. In a subgroup of patients adjusted for disease stage, the survival of those on chemotherapy versus immunochemotherapy did not differ significantly at any stage. For optimal results, a protocol for postoperative immunochemotherapy needs to be designed and investigated prospectively and according to the stage of gastric cancer. The stage III gastric cancers seem amenable to a favorable response.

Microsatellite instability in Japanese colorectal carcinoma.

Recent studies have shown that microsatellite instability (MSI) play an important role in the development of various types of cancer. To clarify the clinicopathologic significance of MSI in colorectal carcinoma (CRC), the presence of MSI was examined in 54 Japanese cases of CRC using the polymerase chain reaction-based method. The incidence of MSI in CRC cases was 13 out of 54 cases (24%). CRC with MSI also showed a significant tendency not to have lymph node metastasis (P<0.05), although neither the survival nor the prognosis of the cases examined in this study were available due to the short period of follow-up. The present study showed that the incidence of MSI in Japanese CRC was 24% and suggests that CRC with MSI may behave in a less malignant manner.

Poorly differentiated human gastric carcinoma is more sensitive to antitumor drugs than is well differentiated carcinoma.

The chemosensitivities of 41 poorly differentiated gastric cancer tissues were compared with that of 16 well differentiated tissues, using the in vitro succinate dehydrogenase inhibition test. These human tissues obtained at the time of surgery were exposed to six different antitumor drugs: carboquone (CQ), adriamycin (ADM), mitomycin C (MMC), aclacinomycin A (ACR), cisplatin (DDP) and 5-fluorouracil (5-FU). The chemosensitivity was determined as positive when the succinate dehydrogenase (SD) activity of the drug exposed cells was decreased to below 50% of that of control cells, on day 3 of exposure. Decrease in SD activity was remarkable in the poorly differentiated tissues, compared to the well differentiated tissues, exposed to ADM, MMC, DDP and 5-FU. The sensitive rates were higher in the poorly differentiated tissues than in the well differentiated tissues, against all six antitumor drugs. Sixty-three per cent of the poorly differentiated tissues were sensitive to more than three antitumor drugs, in an identical tissue, but the rate was only 19% in the well differentiated tissues. The resistant rates to all drugs tested were 20% in the poorly differentiated and 31% in the well differentiated tissues. This would indicate that patients with a poorly differentiated gastric cancer will probably show a better response to antitumor drugs, compared to those with a well differentiated type.

Chemosensitivity differences between primary and metastatic lesions of clinical gastric cancer.

An in vitro chemosensitivity test, the succinate dehydrogenase inhibition (SDI) test, was used to examine 16 pairs of samples obtained simultaneously from primary and metastatic lesions of clinical gastric cancer. Concerning the metastases, 11 were in the lymph nodes and five in the liver. The chemosensitivities of metastatic lesions against six anti-tumour drugs, carboquone (CQ), adriamycin (ADM), mitomycin C (MMC), aclacinomycin A (ACR), and 5-fluorouracil (5-FU), differed from those in the primary lesions, and there were no correlations of chemosensitivities between the primary and the metastatic lesions against these drugs, except for DDP. The lymph nodes were more sensitive to CQ, ADM, MMC, DDP, ACR and 5-FU, while the liver was less sensitive than the primary lesions to CQ, ADM, MMC, DDP, and ACR. Our findings indicate that in patients with lymph node metastasis, there is a sensitivity to anti-tumour drugs, while in cases of liver metastasis, drug treatment may be less effective. We propose that chemosensitivity testing should be done when attempting to design anti-tumour drugs.

4'-O-tetrahydropyranyladriamycin has greater antineoplastic activity than adriamycin in various human tumours in vitro.

The sensitivities of 84 human tumor tissues (23 gastric, 8 colorectal cancers, 34 hepatomas, 6 breast, 6 lung cancers and 7 malignant lymphomas) to adriamycin (ADM) and 4'-0-tetrahydropyranyladriamycin (THP-ADM), a semisynthetic anthracycline glycoside, were determined using the in vitro succinate dehydrogenase inhibition (SDI) test. The succinate dehydrogenase (SD) activity of the tumor tissues was assayed following exposure to 6.9 microM of the drug for 3 days; sensitivity was considered positive when the SD activity decreased to below 50% of that of the control cells. In the case of exposure to THP-ADM, the SD activity in the tissue decreased and the decrease was more extensive in the gastric, colorectal cancers, hepatomas, lung cancers, with a statistically significant difference (P less than 0.001-P less than 0.05), but not in the breast cancers and malignant lymphomas. The rate of sensitivity was 70.2% for THP-ADM and 51.2% for ADM in the 84 tumor tissues. The percentage of tissues with a higher sensitivity to THP-ADM, compared to ADM, was 79.8%. As THP-ADM proved to be more cytotoxic than ADM to human tumors in vitro, this drug should be kept in mind when anti-cancer chemotherapy is designed.

Potentiation of 5-fluorouracil cytotoxicity by combining hyperthermia and dipyridamole in vitro.

Evidence was obtained for the augmentation of cytotoxic effects of 5-fluorouracil (5-FU) when hyperthermia and dipyridamole (DP) were combined in vitro. Nontoxic levels of DP enhanced the combined cytotoxicity of 5-FU and heat against HeLa and B16 melanoma cells, and the 50% effective concentration of 5-FU decreased 7.3 fold for HeLa cells and 3.0 fold for B16 melanoma cells, when exposed to heat plus DP. This combined effect did not depend on intracellular increases in the concentrations of 5-FU. Thus hyperthermia together with DP seems to improve the sensitivity of tumor cells to 5-FU. As the sensitivity of colorectal cancer tissue to drugs is low, 5-FU plus hyperthermia and DP shows promise for the treatment of such patients.

Dipyridamole potentiates adriamycin cytotoxicity by a mechanism other than inhibiting nucleoside uptake.

Nitrobenzylthioinosine (NBTI) and dipyridamole (DP) are competitive inhibitors of cellular nucleoside uptake. Although combined treatment of HeLa cells with adriamycin (ADM) and DP enhanced ADM cytotoxicity in cell growth and clonogenic assays, the combination of ADM and noncytostatic levels (less than 1 microM) of NBTI did not change the cytotoxic potential of ADM in vitro. DP enhanced the inhibition of clonogenicity by ADM, even in nucleoside-enriched mediums. These results suggest that the synergy between ADM and DP was hardly due to the inhibition of nucleoside uptake by DP, but was due to the enhancement of intracellular ADM accumulation by DP.

Postgastrectomy anticancer chemotherapy for stage IV gastric cancer equally tolerable for patients under and over age 65.

We compared the survival time and drug toxicity between patients with stage IV gastric cancer who were over and under 65 years. All had undergone gastric resection followed by treatment with mitomycin C and a fluorinated pyrimidine. There were no differences in prognostic factors or doses of drugs prescribed between the two groups, nor were there differences in survival rates or toxicities. As advanced chronological age is not sufficient justification to limit or withhold treatment with anticancer drugs, postoperative chemotherapy should be designed for patients with gastric cancer, regardless of age.

Cytotoxicity of N4-behenoyl-1-beta-D-arabinofuranosylcytosine through gradual conversion to 1-beta-D-arabinofuranosylcytosine in HeLa cells.

The metabolism of 1-beta-D-arabinofuranosylcytosine (ara-C) and N4-behenoyl-1-beta-D-arabinofuranosylcytosine (BH-AC) was studied in Hela cells. After the cells were exposed to ara-C at a concentration of 20 micrograms/ml or BH-AC at 46.5 micrograms/ml for 1, 3, 6, 12 or 24 hr, the level of ara-C was determined using the radioimmunoassay method, and the level of BH-AC and 1-beta-D-arabinofuranosyluracil (ara-U), using high-performance liquid chromatography. In the ara-C-treated cells, the intracellular ara-C increased to 1.26 micrograms/g cells after exposure for 6 hr, and ara-C was rapidly changed to ara-U in the cells and in the medium. In the BH-AC-treated cells, the intracellular BH-AC increased after exposure for 24 hr and BH-AC was gradually converted to ara-C in the cells: the intracellular level of ara-C was only 15% of that of BH-AC after exposure for 24 hr. BH-AC level in the medium persisted for 24 hr, at the initial concentration. Our findings show that BH-AC is stable compared to ara-C and gradually converts to ara-C. This conversion is presumably a critical step in the antineoplastic effect of BH-AC.

Modulation of cytotoxic effect of anticancer drugs by dipyridamole in HeLa cells in vitro.

Exponentially growing HeLa cells were treated with various antitumor drugs and dipyridamole (DP), and the cell growth inhibition ratio was determined. Enhanced growth inhibition was found in combined treatment with DP and 5-fluorouracil, 1-hexylcarbamoyl-5-fluorouracil, methotrexate, adriamycin, daunomycin, 4'-0-tetrahydropyranyl-adriamycin, actinomycin D and vincristine. In contrast, reduction of the cytotoxic effect of cytosine arabinoside and enocitabine was found when these were combined with DP.