Mechanisms of resistance of confluent human and rat colon cancer cells to anthracyclines: alteration of drug passive diffusion.

Two colon cancer cell lines, HT-29 (human) and DHD/K12/TRb (rat), were grown as monolayer cultures to various confluence degrees. The cytotoxic efficacies of doxorubicin and 4'-deoxydoxorubicin, evaluated by a survival assay, and the nuclear drug concentrations, measured by microspectrofluorometry, were shown to progressively decrease with the augmentation of confluence. This confluence dependent resistance (CDR) to anthracyclines was demonstrated independent of the multidrug resistance drug efflux mechanism. The cellular uptake of three compounds (sodium [51Cr]chromate, D-[14C]alanine, L-[14C]glucose) known to passively diffuse across the cell membrane as anthracyclines do was also reduced in confluent cells. After trypsin cell detachment, the kinetics of reversion of the sodium [51Cr]chromate uptake decrease and that of CDR were similar. Therefore, CDR may be attributed to a reduction of anthracycline cell intake due to a general alteration of passive diffusion across the cell membrane. However, CDR is only partly explained by this phenomenon since a reduced sensitivity of confluent cells was observed compared with nonconfluent cells for a similar amount of drug in their nuclei. CDR could explain the high resistance to anthracyclines of some solid tumors, such as colon tumors, in which cancer cells are tightly aggregated.

Cinchonine, a potent efflux inhibitor to circumvent anthracycline resistance in vivo.

Circumvention of multidrug resistance is a new field of investigation in cancer chemotherapy, and safe and potent multidrug resistance inhibitors are needed for clinical use. We investigated several analogues of quinine for their ability to increase anthracycline uptake in resistant cancer cells. Cinchonine was the most potent inhibitor of anthracycline resistance in vitro, and its activity was little altered by serum proteins. Serum from rats treated with i.v. cinchonine produced greater uptake of doxorubicin in cancer cells (DHD/K12/PROb rat colon cells and K562/ADM human leukemic cells) than did serum from quinine-treated rats (ex vivo assay). Cinchonine was more effective than quinine in reducing tumor mass and increasing the survival of rats inoculated i.p. with DHD/K12/PROb cells and treated i.p. with deoxydoxorubicin. Moreover, the acute toxicity of cinchonine in rats and mice was lower than that of other quinine-related compounds. The lower toxicity and greater potentiation of in vivo anthracycline activity produced by cinchonine are favorable characteristics for its use as an anti-multidrug resistance agent in future clinical trials.

The role of cell cycle regulation and apoptosis triggering in determining the sensitivity of leukemic cells to topoisomerase I and II inhibitors.

Topoisomerase (topo) inhibitors induce enzyme-linked DNA breaks. Resulting DNA damage can lead to cell cycle arrest and/or cell death by apoptosis. The sensitivity of five human leukemic cell lines to topo I (camptothecin or CPT) and topo II (etoposide or VP-16) inhibitors varied widely (100-fold for CPT and 30-fold for VP-16). Three cell lines were more sensitive (BV173, HL60, U937) and two cell lines were resistant (K562, KCL22) to both drugs. None of these cell lines were selected for drug resistance and overexpressed mdr1 gene. Their sensitivity was not related to their doubling time nor to cell cycle repartition. The initial DNA damage (cleavable complexes) induced by topo I and II inhibitors was measured as DNA-protein crosslinks (DPC) using alkaline elution. Neither DPC level induced by 30-min treatment with CPT or VP-16 nor the levels of topo 1, topo II alpha and topo II beta mRNA were related to sensitivity. Electron microscopy and DNA fragmentation measured by filter elution and agarose gel electrophoresis demonstrated that apoptosis was induced by both drugs in the five cell lines. The kinetics of DNA fragmentation was related to cell sensitivity. At drug concentrations higher than IC50, DNA fragmentation increased very rapidly in the three sensitive, compared with the two resistant, cell lines. Continuous exposure to both drugs induced cell cycle arrest in either G2 or S phase that was related both to cell sensitivity and drug concentration. Comparison between cell lines indicated that the ability of cells to arrest cell cycle in G2 or S phase was related to their drug sensitivity and increased with cell resistance. In a given cell line, cell cycle progression was observed to be progressively inhibited by increasing drug concentrations. Treatment of synchronized cells demonstrated that highly cytotoxic drug concentration induced a complete inhibition of cell cycle progression. Altogether, these data suggest that the ability of leukemic cell lines to regulate cell cycle progression and to trigger apoptosis is more indicative of their sensitivity to topoisomerase poisons than cleavable complexes induced by these drugs.

Comparative effects of quinine and cinchonine in reversing multidrug resistance on human leukemic cell line K562/ADM.

We have previously suggested that quinine and cinchonine could be good candidates for clinical circumvention of multidrug resistance (MDR) in hematological malignancies because of their tolerance and their retained efficacy in serum. In the present study, we have used the well-characterized multidrug resistant human leukemic cell line K562/ADM to compare the effect in vitro of quinine and cinchonine on doxorubicin, mitoxantrone, and vincristine uptake and cytotoxicity. In serum-free medium, quinine induced a dose-dependent increase of doxorubicin uptake reaching about 200% at 40 microM, while it had a slight and no effect on mitoxantrone and vincristine uptake respectively. In the same conditions, cinchonine induced a rapid and significant increase in the accumulation of the three drugs, reaching a plateau phase between 5 and 10 microM. Quinine and cinchonine induced both potentiation of doxorubicin, vincristine and mitoxantrone cytotoxicity in K562/ADM cells. However, quinine reached a plateau phase at 10 microM, while cinchonine had a maximal effect at 5 microM and was significantly more potent at low concentrations. When diluted in plasma, cinchonine was less bound to proteins than quinine. The free fraction of alkaloids was 37-55% for cinchonine and 20-30% for quinine. Cinchonine-induced enhancement of vincristine cellular accumulation was little modified by plasma proteins. When incubated in whole blood, the fraction of cinchonine trapped in red blood cells was rapidly and completely exchangeable with plasma. We conclude that cinchonine is a stronger inhibitor of MDR than quinine.

Phase I study of cinchonine, a multidrug resistance reversing agent, combined with the CHVP regimen in relapsed and refractory lymphoproliferative syndromes.

Overexpression of P-glycoprotein (P-gp) in cancer cells reduces intracellular accumulation of various anticancer drugs including anthracyclines and vinca alkaloids. This multidrug resistance (MDR) phenotype can be reversed in vitro by a number of non-cytotoxic drugs. We have identified the quinine's isomer cinchonine as a potent MDR reversing agent, both in vitro and in animal models. Here, we report an open phase I dose escalation trial in patients with refractory or relapsed malignant lymphoid diseases. Cinchonine dihydrochloride was administered by continuous i.v. infusion for 48 h and escalated over five dose levels ranging from 15 to 35 mg/kg/d. Cinchonine infusion started 24 h before i.v. doxorubicin (25 mg/m2), vinblastine (6 mg/m2), cyclophosphamide (600 mg/m2) and methylprednisolone (1 mg/kg/d) (CHVP regimen) and lasted for 24 h after chemotherapy infusion. Thirty-four patients received 87 cycles of CHVP/cinchonine. The MTD of cinchonine administered by continuous i.v. infusion was 30 mg/kg/d. Prolonged cardiac repolarization was the main dose-limiting toxicity. No ventricular arrhythmia including 'torsade de pointes' was observed. An MDR reversing activity was identified in the serum from every patient and correlated with cinchonine serum level. When infused at 30 mg/kg/d, cinchonine demonstrated a limited influence on doxorubicin pharmacokinetic. We conclude that i.v. infusion of cinchonine might be started 12 h before MDR-related chemotherapy infusion and requires continuous cardiac monitoring but no reduction of cytotoxic drug doses.

Multicentric evaluation of the MDR phenotype in leukemia. French Network of the Drug Resistance Intergroup, and Drug Resistance Network of Assistance Publique-Hôpitaux de Paris.

The wide discrepancies in the frequency of 'positive' samples for multidrug resistance (MDR) phenotype within the same type of tumor observed in the literature justified the need for the definition of consensus recommendations. To define standard techniques of MDR phenotype measurement, we ran a large multicentric evaluation of the different methods available. Thirty-six French centers participated in the study, and 742 samples of 2-10 x 10(6) viable cells were sent by overnight express mail between December 1993 and February 1996. The same batches of MRK16, 4E3 and UIC2 were used. Nineteen samples of leukemia (12 AML, 1 ALL, 6 lymphoproliferative syndromes) and six leukemic cell lines with different levels of MDR expression were tested. Five meetings reached agreement concerning the guidelines for each technique, except immunocytochemistry. The 19 fresh samples were tested by each center using one to four techniques among cytofluorometry, immunocytochemistry, functional tests and RT-PCR. Five samples were diagnosed as 'negative' according to local criteria, with few discordant results (0 to 16% of 'positive' results). For all the 14 remaining samples, large discrepancies were observed from center to center, and from one technique to another. No correlations could be found between techniques. Flow cytometric analysis of cells already exposed to MRK16 or control IgG2A, fixed in paraformaldehyde and sent to centers did not reduce the discrepancies between centers in two of the four samples with moderate expression, emphasizing the role of histogram interpretation. The use of alternative monoclonal antibodies (4E3 and UIC2) did not reduce the discrepancies observed. In a second step, the K562 parental cell line, a low resistant subline (K562/HHT100, x7 resistance index to DNR) and a high resistant subline (K562/HHT300, x125 resistance index to DNR) were sent blindly three times, with an increasing level of recommendations for flow cytometry. Dramatic improvements were observed in cytometric results when the result was expressed as the ratio of arithmetic mean of fluorescence of antibody (10 microg of MRK16)/arithmetic mean of fluorescence of control (10 microg IgG2A): the proportion of expected results increased from 61 to 100% for K562, and from 37 to 85% for K562/HHT100. For uptake and drug efflux measurements, the use of 1 h uptake of 0.1 microM of rhodamine, followed by 1 h efflux +/-10 microM of verapamil, permitted an increased reproducibility of the technique from 71 to 100% for K562 and K562/HHT100. Whatever the technique used, concordant results were obtained for K562/HHT300. The immunocytochemistry, using several antibodies (MRK16, JSB1 and C219) gave many non-interpretable results (44%), due to a frequent high background and discordant results between antibodies in the same centers, and discordant conclusions between centers. The group does not recommend this technique for circulating tumoral cells.

French multicentric evaluation of mdr1 gene expression by RT-PCR in leukemia and solid tumours. Standardization of RT-PCR and preliminary comparisons between RT-PCR and immunohistochemistry in solid tumours. French Network of the Drug Resistance Intergroup, and Drug Resistance Network of Assistance Publique-Hôpitaux de Paris.

Since there is no consensus on the techniques for multidrug resistance (MDR) phenotype evaluation, many discrepancies concerning the importance and frequency of mdr1 gene expression in leukemias and solid tumors are observed in the literature. In order to establish an inter-laboratory consensus in France, a multicenter study was carried out to propose further guidelines for MDR phenotype evaluation. The techniques used by the 38 laboratories participating in the trial were: immunodetection (immunohisto and/or cytochemistry, flow cytometry), functional tests, reverse transcription-polymerase chain reaction (RT-PCR) or Northern blot. We present the results obtained by 19 laboratories concerning the measurement of mdr1 gene expression assessed by RT-PCR or Northern blot in: (1)19 samples of tumor cells obtained from leukemic patients; (2) six solid tumor samples obtained at surgery; (3) eight cell lines exhibiting variable levels of resistance, and; (4)10 preparations of RNA and of cDNA obtained from solid tumors. Standardization of the RT-PCR technique and preliminary results comparing RT-PCR with immunohistochemistry in solid tumors are also reported.

Confluence dependent resistance (CDR) to doxorubicin and E-cadherin expression in murine mammary cells.

Confluence dependent resistance (CDR) is one of the principal mechanisms by which solid tumor cells resist anthracyclines. CDR is thought to be mediated by cell-cell contact which increases the fraction of non-proliferating resistant cells in a post confluence monolayer culture. As E-cadherin is a major Ca2+ dependent adhesion molecule, involved in cell-cell adhesion, differentiation and polarity of normal and cancerous epithelial cells, we decided to investigate its involvement in the CDR mechanism. In order to do this, we measured the intracellular accumulation and the cytotoxicity of doxorubicin (DXR) in four subclones, derived from the same parental murine mammary cell line (NMuMG), differing in their expression of E-cadherin. A significant reduction in DXR accumulation and cytotoxicity was observed in NM-f-ras-TD-CAMx, which expresses E-cadherin, suggesting that E-cadherin could play a role in the increase of drug resistance observed in confluent cancer cells.

mdr 1 gene-expression and villin synthesis in a colon cancer cell line differentiated by sodium butyrate.

Sodium butyrate (NaBu) but not dimethylsulfoxide (DMSO) induced the synthesis of villin, a protein of the brush border microvilli cytoskeleton, in a rat colon cancer cell line. Neither NaBu nor DMSO altered mdr 1-mRNA expression or multidrug resistance (MDR)--associated cellular transport of doxorubicin. These results show that mdr 1 gene expression and activity are independent of other brush border proteins induced by differentiating agents at the apical pole of the epithelial cell.

P27KiP1 overexpression inhibits the growth and doxorubicin sensitivity of HT29 human colon cancer cells in vivo.

We have previously shown that p27KiP1 plays a role in the tumor cell resistance of HT29 confluent monolayers to cytotoxic drugs in vitro. To determine whether p27KiP1 was a resistance factor to cytotoxic drugs in vivo we tested the effect of doxorubicin on p27KiP1-overexpressing HT29 tumors in nude mice. In this study we show that ectopic overexpression of p27KiP1 in HT29 human colon cancer cells decreases their tumorigenicity in vivo in nude mice. This decreased tumor growth was associated with increased p27KiP1 protein expression, studied by Western blotting in tumor extracts. Interestingly, the overexpressing-p27KiP1 tumors were significantly more resistant to intraveneous doxorubicin treatment than the control tumors. These results indicate that p27KiP1, which delays tumor growth could also increase tumor resistance to cytotoxic drugs in vivo.

Serum concentrations of amiodarone required for an in vivo modulation of anthracycline resistance.

We demonstrated previously that amiodarone is able to circumvent in vitro the inherent resistance to anthracyclines of the DHD/K12 rat colon cancer cell line. We have now determined in the rat the amiodarone seric concentrations required to enhance the in vitro cytotoxicity of 4'-deoxydoxorubicin (deoDX) against DHD/K12 cells. A maximal deoDX potentiation was obtained in vitro when anthracycline was diluted in the serum of rats receiving at least 75 mg/kg of intravenous amiodarone resulting in seric concentrations of more than 40 micrograms/ml. In patients treated with amiodarone, the mean serum concentrations were 0.9 +/- 0.1 microgram/ml after an one month's oral administration of 200 mg/day, 2.2 +/- 1.0 micrograms/ml after a 24 hr continuous infusion of 300 to 900 mg/day and 5.4 +/- 1.1 micrograms/ml after a brief 3 hrs infusion of 450 mg amiodarone. Such amiodarone concentrations in human serum are much lower than those necessary to produce a significant anthracycline potentiation. In rats receiving amiodarone at a maximal tolerated dose (100 mg/kg) minutes before the injection of 10 mg/kg doxorubicin (DX), we observed an increased accumulation of the anthracycline in the liver and kidney compared to rats receiving DX alone. The DX content was not modified by amiodarone in the other organs studied (heart, lung, spleen and pancreas). An amiodarone pretreatment accelerated the death of rats receiving 5 or 10 mg/kg DX did not provoke lethality for a lower dose of 2.5 mg/kg DX. The very high doses required and the risk of increased toxicity seem to preclude the use of amiodarone for the modulation of anthracycline resistance in cancer patients.

MDR1 and thymidylate synthase (TS) gene expressions in advanced breast cancer: relationships to drug exposure, p53 mutations, and clinical outcome of the patients.

To characterize the biological features of advanced breast cancer associated with poor chemotherapy response and worse prognosis, sequential tumor samples obtained from 75 patients receiving primary chemotherapy were analysed for MDR1 and TS gene expression before and after treatment. MDR1 gene expression was also analysed in 36 sequential normal samples. The levels of MDR1 and TS genes expression were determined by reverse transcription-PCR method, and examined in relation to p53 gene status, and the clinical outcome of the patients. After treatment, MDR1 expression levels were significantly enhanced in tumor (p = 0.0033) and normal (p = 0.0098) samples, whereas a significant decrease in TS expression was observed (p = 0.0054). There was no significant correlation between MDR1 or TS expressions and the presence of p53 mutations (detected in 24% of the cases), chemoresponsiveness, or survival. Only p53 mutations were associated with reduced disease-free survival (p = 0.0473). These results demonstrate that MDR1 and TS gene expressions were affected by drug exposure, but not by p53 gene status. Furthermore, the increase of MDR1 gene expression in normal and tumor tissues is in favor of an induced MDR1 expression rather than of a selection of resistant tumoral clones, which can be responsible for the absence of relationship of MDR1 expression with clinical outcome of advanced breast cancer patients.

Use of a specific monoclonal antibody for studying the liver metastatic invasion of a rat colon cancer.

A simple model for liver metastasis from colon cancer resulted from the intraportal injection of 2 x 10(7) highly tumorigenic DHD/K12/PROb cells into syngeneic BDIX rats. Early detection and development of cancer invasion were studied by conventional microscopy and immunoenzymatic staining using a specific monoclonal antibody. Metastases developed either from isolated cancer cells early disseminated in sinusoid network or from intraportal microthrombi. An intense immune reaction developed until day 15 after injection but decreased and disappeared at the latest stages of evolution.

[Preclinical evaluation of aromatase inhibitors antitumor activity]. / Evaluation préclinique del'activité antitumorale des inhibiteurs de l'aromatase.

Aromatase is an enzymatic complex responsible for the conversion of androgens into estrogens; these hormones are important in development, reproduction, but also in the growth of estrogen-dependent cancer. This enzyme is present in 60-70% of the breast cancer. The aromatase inhibitors are important drugs in the breast cancer treatment of postmenopausal women. In order to study their in vivo activity, animal models have been developed, e.g. rat with tumour induced by 7,12-dimethylbenz[a]anthracene, PMSG-primed immature rat or athymic nude mice with aromatase transfected MCF-7 xenograft. In this review, we were interested in preclinical results obtained with both classes: steroidal and nonsteroidal inhibitors. The former group, as substrate analogs formestane or exemestane, are irreversible, selective and long-lasting inhibitors of aromatase. The nonsteroidal molecules, such as letrozole or anastrozole, are reversible inhibitors with high affinity. Finally, knowledge of the enzyme active site, with molecular modeling and site-directed mutagenesis, could be useful to develop new inhibitor families, more specific and potent in vivo.

[Experimental chemotherapy of peritoneal carcinomatosis of colonic origin in rats]. / Chimiothérapie expérimentale d'une carcinomatose péritonéale d'origine colique chez le rat.

Animal models are useful in the evaluation of adjuvant or palliative treatment modalities of human colonic adenocarcinoma. In the present paper, the efficacy of 22 usual chemotherapeutic agents was evaluated in a model of peritoneal carcinomatosis of colonic origin in the BD IX rat. Mitomycin, cisplatine, carboplatine, cyclophosphamide, ifosfamide, and thiotepa were very effective agents on microscopic carcinomatosis (treatment given 3 days after an intraperitoneal inoculation of 1 x 10(6) DHD/K12/PROb cells). Intravenous administration was as effective as the intraperitoneal route, except for anthracyclines and 5-fluorouracil. Rats treated at early stages by thiotepa or cisplatin survived up to 4 months after cell injection and did not display tumor at autopsy. Administered late (15 days after cell injection), none of the drugs were able to cure the rats with carcinomatosis.

Cinchonine per os: efficient circumvention of P-glycoprotein-mediated multidrug resistance.

We have previously suggested that quinine and cinchonine could be good candidates for the clinical circumvention of multidrug resistance (MDR) in haematological malignancies because of their tolerance and their retained efficacy in serum. We have also shown that cinchonine was more efficient than quinine as an anti-MDR agent in vitro, ex vivo and in vivo after parenteral administration. Here, we report that cinchonine administered per os (po) is much more active than quinine po in circumventing MDR in rats bearing resistant colon tumours. The pharmacokinetics of cinchonine and quinine administered po in rat are shown to be very different. Cinchonine demonstrates a greater absolute bioavailability than quinine (44% versus 30%, respectively). Its serum concentration correlates with the anti-MDR activity measured ex vivo and in vivo. Cinchonine administered po does not significantly modify the pharmacokinetics of intravenous doxorubicin (DXR). However, cinchonine induces a significant increase of DXR uptake in organs which express the mdr1 gene (liver, kidney, lung). When associated with VAD (vincristine, adriamycin, dexamethasone) combined therapy in rats, cinchonine does not significantly increase the toxicity of the cytotoxic drugs. Based on these experimental data, a phase I clinical trial is currently in progress to test the tolerance of this potent MDR-reversing agent administered po.

Effects of cyclosporin at various concentrations on dexamethasone intracellular uptake in multidrug resistant cells.

BACKGROUND: The multidrug resistance phenomenon results from the expression of P-glycoprotein (P-gp), a drug-efflux pump. Corticosteroids are substrates for P-gp, whose function can be inhibited by cyclosporin. This study evaluates the ability of cyclosporin to modulate dexamethasone uptake in multidrug resistant cells. METHODS: The K 562 cell line, which does not express P-gp and a P-gp expressing clone, K562/ADM, were used. Cells were incubated with H3-dexamethasone in the absence or presence of cyclosporin at various concentrations. Then, cells were washed, lysed, and radioactivity was measured. RESULTS: The uptake of dexamethasone alone was higher in sensitive than in resistant cells. Addition of cyclosporin induced a dose dependent increase of dexamethasone uptake in resistant cells, whereas the drug did not influence dexamethasone uptake in parental cells. CONCLUSION: Cyclosporin, at therapeutic concentrations induces a moderate, but significant increase in dexamethasone accumulation in multidrug resistant cells. Thus, cyclosporin might increase the intestinal absorption of corticosteroids or their accumulation in mononuclear cells, or both, thereby increasing their therapeutic efficacy.

F11C antigen: a membrane marker able to distinguish two regressive and progressive variants from a rat colon adenocarcinoma.

Cell variants that differ in their tumorigenicity and immunogenicity have been isolated from a BDIX rat colon adenocarcinoma cell line, DHD/K12. One variant, PRO, and the clones derived from it, are poorly immunogenic and induce progressive and metastatic tumors; the other one, REG, and its clones, are highly immunogenic and induce regressive tumors. When looking for a membrane marker distinguishing between PRO and REG lines, we obtained monoclonal hybridomas by immunizing BALB/c mice with PROb or REGb cell clones. Hybridoma F11C, producing an IgM monoclonal antibody (MAb) was able to distinguish between the cell variants on membrane immunofluorescence. All REGb cells strongly express F11C membrane antigen. On PROb cells, F11C antigen expression is weak, as demonstrated by cytofluorimetric analysis, and limited to a fraction of the cell population. The F11C membrane antigen is highly specific for the DHD/K12 cell line and the variants derived from it, but is not expressed on cells dissociated from the DHD transplanted tumor, from which DHD/K12 was established, suggesting that F11C antigen emerged during cell culture. Fluorescence absorption on synthetic oligosaccharides demonstrated that F11C antibody cross-reacts with A type 3, A type 4 and A type 5 chain blood group tetrasaccharides.

Liposomal mitoxantrone for the local treatment of peritoneal carcinomatosis induced by colon cancer cells in mice.

Since liposomes are slowly resorbed from serous cavities, they may constitute a valuable tool for the treatment of peritoneal carcinomatosis. We prepared mitoxantrone (MXN)-liposomes with various lipid compositions and checked their antitumoral activity on a peritoneal carcinomatosis induced by a colon cancer cell injection (1 x 10(5) C51 cells) in BALB/c mice. MXN entrapment in liposomes was rapid and stable due to its high lipophilicity. MXN carried in phosphatidylcholine: cholesterol (2:1; G-liposomes) displayed a reduced toxicity in mice compared to the free drug. When tested at a non-toxic dose (2 mg/kg), MXN entrapped in G-liposomes proved to be as efficient as the free drug. At a higher MXN dose (3 mg/kg), both G-liposomes and phosphatidylcholine:cholesterol:dipalmitoylphosphatidylethanolamine (7:2:1) liposomes, loaded with MXN, significantly increased the life span of mice compared to the free drug and six other liposome formulations. Increase in the MXN therapeutic index, when used in the liposomal form, could then merit further clinical investigations in regard to patients with malignancies confined to serous cavities.

Circumvention of confluence-dependent resistance in a human multi-drug-resistant colon-cancer cell line.

Colorectal adenocarcinomas are inherently resistant to anthracyclines and other topoisomerase-II inhibitors. Resistance to doxorubicin of colon cancer cells (Caco2) depends on 2 main mechanisms. The first is typical multi-drug resistance, characterized by the mdr1 gene and its product the P170 membrane glycoprotein. P170 effluxes anthracyclines out of cancer cells and is antagonized in vitro by verapamil. The second mechanism, which develops when cell-culture density increases, we have designated confluence-dependent resistance. Confluence-dependent resistance depends on the reduced topoisomerase II content of the G0/G1-phase cells which accumulate in the confluent population. We show here that short treatments of confluent Caco2 cells with slightly toxic concentrations of DNA-damaging agents (cisplatin, melphalan or mitomycin C) produced a transient accumulation of cells in S- and G2/M-phases of the cell cycle. Concomitantly with the increase in the S-phase population, the topoisomerase II cellular level and the sensitivity of cells to doxorubicin were greatly enhanced. Overcoming confluence-dependent resistance through S-phase accumulation and inhibition of multi-drug resistance by verapamil were fully additive, and a nearly complete reversal of confluent Caco2 cells' resistance to doxorubicin was obtained when both strategies were combined.