Proton MR spectroscopy and neuropsychological testing in adrenoleukodystrophy

Melatonin, the chief hormone secreted by the pineal gland, has been previously shown to inhibit human breast cancer cell growth at the physiological concentration of 1 nM in vitro. In this study, using the estrogen receptor (ER)-positive human breast tumor cell line MCF-7, we have shown that 10 microM L-buthionine-[S,R]-sulfoximine (L-BSO), an inhibitor of gamma-glutamylcysteine synthetase (the rate-limiting enzyme in glutathione synthesis), blocks the oncostatic action of 1 nM melatonin over a 5-day incubation, indicating that glutathione is required for melatonin action. The result was repeated with ZR75-1 cells, suggesting that the glutathione requirement is a general phenomenon among ER+ breast cancer cells. Addition of exogenous glutathione (1 microM) to L-BSO-treated groups restored the melatonin response in both cell lines. Further demonstration of the importance of glutathione was shown using the ER- breast tumor cell line HS578T, which is normally unresponsive to melatonin. Growth in this cell line was inhibited in the presence of 1 microM ethacrynic acid (an inhibitor of glutathione S-transferase) plus 1 nM melatonin, and this effect was blocked with 10 microM L-BSO. We also observed a steady decrease of intracellular glutathione in MCF-7 cells over a 5-day incubation, suggesting that these cells metabolize glutathione differently than do normal cells.     

Bipotential primitive-definitive hematopoietic progenitors in the vertebrate embryo

The effects of the diatomic radical, nitric oxide (NO), on melphalan-induced cytotoxicity in Chinese hamster V79 and human MCF-7 breast cancer cells were studied using clonogenic assays. NO delivered by the NO-releasing agent (C2H5)2N[N(O)NO]- Na+ (DEA/NO; 1 mM) resulted in enhancement of melphalan-mediated toxicity in Chinese hamster V79 lung fibroblasts and human breast cancer (MCF-7) cells by 3.6- and 4.3-fold, respectively, at the IC50 level. Nitrite/nitrate and diethylamine, the ultimate end products of DEA/NO decomposition, had little effect on melphalan cytotoxicity, which suggests that NO was responsible for the sensitization. Whereas maximal sensitization of melphalan cytotoxicity by DEA/NO was observed for simultaneous exposure of DEA/NO and melphalan, cells pretreated with DEA/NO were sensitized to melphalan for several hours after NO exposure. Reversing the order of treatment also resulted in a time-dependent enhancement in melphalan cytotoxicity. To explore possible mechanisms of NO enhancement of melphalan cytotoxicity, the effects of DEA/NO on three factors that might influence melphalan toxicity were examined, namely NO-mediated cell cycle perturbations, intracellular glutathione (GSH) levels and melphalan uptake. NO pretreatment resulted in a delayed entry into S phase and a G2/M block for both V79 and MCF-7 cells; however, cell cycle redistribution for V79 cells occurred after the cells returned to a level of cell survival, consistent with treatment with melphalan alone. After 15 min exposure of V79 cells to DEA/NO (1 mM), GSH levels were reduced to 40% of control values; however, GSH levels recovered fully after 1 h and were elevated 2 h after DEA/NO incubation. In contrast, DEA/NO (1 mM) incubation did not reduce GSH levels significantly in MCF-7 cells (approximately 10%). Melphalan uptake was increased by 33% after DEA/NO exposure in V79 cells. From these results enhancement of melphalan cytotoxicity mediated by NO appears to be complex and may involve several pathways, including possibly alteration of the repair of melphalan-induced lesions. Our observations may give insights for improving tumour kill with melphalan using either exogenous or possibly endogenous sources of NO.     

Sensitization to doxorubicin resistance in breast cancer cell lines by tamoxifen and megestrol acetate

Acquired drug resistance is a major factor in the failure of doxorubicin-based chemotherapy in breast cancer. We determined the ability of megestrol acetate and/or tamoxifen to reverse doxorubicin drug resistance in a doxorubicin-resistant breast cancer line (the human MCF-7/ADR). The cytotoxicity of doxorubicin, megestrol acetate, and/or tamoxifen was determined in the sensitive and resistant cell lines utilizing the sulphorhodamine B assay. Tamoxifen alone produced an IC50 (concentration resulting in 50% inhibition of control growth) of 10.6 microM, whereas megestrol acetate alone resulted in an IC50 of 48.7 microM in the MCF-7/ADR cell line. The IC50 of doxorubicin in MCF-7/ADR was 1.9 microM. Neither megestrol acetate alone nor tamoxifen alone at 1 or 5 microM altered the IC50 of doxorubicin. However, the combination of tamoxifen (1 or 5 microM) and megestrol acetate (1 or 5 microM) synergistically sensitized MCF-7/ADR cells. Additionally, megestrol acetate and tamoxifen inhibited iodoarylazidoprazosin binding to P-glycoprotein, and, in their presence, there was an increased doxorubicin accumulation in the MCF-7/ADR cells. Furthermore, the combination of tamoxifen and megestrol acetate had much less effect on the cytotoxicity of doxorubicin in MCF-7 wild-type cells. Clinically achievable concentrations of tamoxifen and megestrol acetate can largely sensitize MCF-7/ADR to doxorubicin. The combination of these three drugs in a clinical trial may be informative.     

Changes in radiation sensitivity and steroid receptor content induced by hormonal agents and ionizing radiation in breast cancer cells in vitro

Possible influences of tamoxifen and estradiol on in vitro radiation sensitivity and cellular receptor content after irradiation and/or tamoxifen treatment were studied in breast cancer cell lines; estrogen receptor (ER) and progesterone receptor (PgR) positive cell lines MCF-7 and MCF-7/TAM(R)-1 and the ER and PgR negative cell line MDA-MB-231. The tamoxifen resistant MCF-7/TAM(R)-1 cells were more resistant to ionizing radiation than the MCF-7 and MDA-MB-231 cells. Exposure to tamoxifen made the MCF-7 cells more radiation resistant, while estradiol made the MDA-MB-231 cells more radiation sensitive. A radiation dose of 6 Gy reduced the ER content in cytosol in both MCF-7 and MCF-7/TAM(R)-1 cells, but brought no alterations to the PgR content. In MCF-7/TAM(R)-1 cells tamoxifen exposure significantly increased the ER and reduced the PgR content, an effect not observed in the MCF-7 cells. To conclude, the present study indicates that irradiation and tamoxifen may modify the ER and PgR content in cytosol in breast cancer cells. Hormonal treatment may alter the radiation sensitivity, even in ER negative cells, suggesting that hormonal agents may act both via receptor and non-receptor binding mechanisms.     

Resistance to tomudex (ZD1694): multifactorial in human breast and colon carcinoma cell lines

ZD1694 (Tomudex; TDX) is a quinazoline antifolate that, when polyglutamated, is a potent inhibitor of thymidylate synthase (TS), the enzyme that converts dUMP to dTMP. Continuous exposure of MCF-7 breast and NCI H630 colon cells to TDX, with stepwise increases in TDX up to 2.0 microM, resulted in stably resistant cell lines (MCFTDX and H630TDX) that were highly resistant to TDX. Initial studies revealed 34-fold increase in TS protein levels in MCFTDX and a 52-fold increase in TS levels in H630TDX cell lines. Despite continued exposure of these cells to 2.0 microM TDX, TS protein and TS mRNA expression decreased to parental levels in H630TDX cells, whereas in MCFTDX cells TS mRNA expression and TS protein levels remained elevated. Southern blot analysis revealed a 20-fold TS gene amplification in the MCFTDX cell line. TDX uptake was 2-fold higher in resistant MCFTDX cells than in parental MCF-7 cells, whereas in H630TDX cells TDX uptake was 50-fold less than that observed in parental H630 cells. In contrast, no change in the transport of either leucovorin or methotrexate into H630TDX cells was noted when compared with the H630 parental cells. In H630TDX cells, folylpolyglutamate synthetase (FPGS) activity was 48-fold less compared to parent H630 cells; however, FPGS mRNA expression was similar in both lines. H630TDX cells were also highly resistant to ZD9331, a novel quinazoline TS inhibitor that does not require polyglutamation, suggesting that defective transport by the reduced folate carrier was also an important mechanism of resistance in these cells. In MCFTDX and H630TDX resistant cells, several mechanisms of resistance are apparent: one increased TS expression; the others evolved over time from increased TS expression to decreased FPGS levels and decreased TDX transport.     

Doxorubicin cellular pharmacokinetics plays no role in chemosensitizing effect of verapamil on Swiss-3T3 cells

AIM: To find whether or not the doxorubicin (Dox) cellular pharmacokinetics plays a role in chemosensitizing effect of verapamil (Ver) on drug sensitive cells. METHODS: Cytotoxicity and cellular Dox contents (during accumulation and retention periods) were measured in the absence and presence of verapamil in Swiss-3T3 cells and compared with those in multidrug resistant (MDR) MCF-7Adr cells and drug sensitive MCF-7WT cells. mdr-1 mRNA expression in Swiss-3T3 cells was analyzed. RESULT: Dox cytotoxicity was enhanced 2.0-fold in Swiss-3T3 cells by Ver (3 mumol.L-1) and 3.6-fold in MCF-7Adr cells by Ver (6 mumol.L-1), but not in MCF-7WT cells (Ver 6 mumol.L-1). Cellular accumulation of equi-effective concentrations of Dox increased at 6-h incubation in the presence of Ver in Swiss-3T3 (1.5-fold)i and MCF-7WT cells (2.1-fold) but decreased rapidly in MCF-7Adr cells by 20% to 50% compared to that in the absence of Ver. Cellular retention of Dox decreased after 10-min increase in the presence of Ver in Swiss-3T3 cells compared to that in the absence of Ver, that was similar to that in MCF-7WT cells, while the retention was augmented by Ver in MCF-7Adr cells. Slot blot analysis of RNA revealed no mdr-1 gene expression in Swiss-3T3 cells. CONCLUSION: Changes in cellular accumulation and retention of Dox did not account for the chemosensitizing effect of Ver on Swiss-3T3 cells.     

A multidrug resistance transporter from human MCF-7 breast cancer cells

MCF-7/AdrVp is a multidrug-resistant human breast cancer subline that displays an ATP-dependent reduction in the intracellular accumulation of anthracycline anticancer drugs in the absence of overexpression of known multidrug resistance transporters such as P glycoprotein or the multidrug resistance protein. RNA fingerprinting led to the identification of a 2.4-kb mRNA that is overexpressed in MCF-7/AdrVp cells relative to parental MCF-7 cells. The mRNA encodes a 655-aa [corrected] member of the ATP-binding cassette superfamily of transporters that we term breast cancer resistance protein (BCRP). Enforced expression of the full-length BCRP cDNA in MCF-7 breast cancer cells confers resistance to mitoxantrone, doxorubicin, and daunorubicin, reduces daunorubicin accumulation and retention, and causes an ATP-dependent enhancement of the efflux of rhodamine 123 in the cloned transfected cells. BCRP is a xenobiotic transporter that appears to play a major role in the multidrug resistance phenotype of MCF-7/AdrVp human breast cancer cells.     

Effects of sphingosine stereoisomers on P-glycoprotein phosphorylation and vinblastine accumulation in multidrug-resistant MCF-7 cells

To investigate the role of protein kinase C (PKC) in the regulation of multidrug resistance and P-glycoprotein (P-gp) phosphorylation, the natural isomer of sphingosine (SPH), D-erythro sphingosine (De SPH), and its three unnatural stereoisomers were synthesized. The SPH isomers showed similar potencies as inhibitors of in vitro PKC activity and phorbol binding, with IC50 values of approximately 50 microM in both assays. Treatment of multidrug-resistant MCF-7ADR cells with SPH stereoisomers increased vinblastine (VLB) accumulation up to 6-fold at 50 microM but did not alter VLB accumulation in drug-sensitive MCF-7 wild-type (WT) cells or accumulation of 5-fluorouracil in either cell line. Phorbol dibutyrate treatment of MCF-7ADR cells increased phosphorylation of P-gp, and this increase was inhibited by prior treatment with SPH stereoisomers. Treatment of MCF-7ADR cells with SPH stereoisomers decreased basal phosphorylation of the P-gp, suggesting inhibition of PKC-mediated phosphorylation of P-gp. Most drugs that are known to reverse multidrug resistance, including several PKC inhibitors, have been shown to directly interact with P-gp and inhibit drug binding. SPH stereoisomers did not inhibit specific binding of [3H] VLB to MCF-7ADR cell membranes or [3H]azidopine photoaffinity labeling of P-gp or alter P-gp ATPase activity. These results suggest that SPH isomers are not substrates of P-gp and suggest that modulation of VLB accumulation by SPH stereoisomers is associated with inhibition of PKC-mediated phosphorylation of P-gp.     

Expression of glucosylceramide synthase, converting ceramide to glucosylceramide, confers adriamycin resistance in human breast cancer cells

Multidrug-resistant cancer cells display elevated levels of glucosylceramide (Lavie, Y., Cao, H. T., Volner, A., Lucci, A., Han, T. Y., Geffen, V., Giuliano, A. E., and Cabot, M. C. (1997) J. Biol. Chem. 272, 1682-1687). In this study, we have introduced glucosylceramide synthase (GCS) into wild type MCF-7 breast cancer cells using a retroviral tetracycline-on expression system, and we developed a cell line, MCF-7/GCS. MCF-7/GCS cells expressed an 11-fold higher level of GCS activity compared with the parental cell line. Interestingly, the transfected cells demonstrated strong resistance to adriamycin and to ceramide, whereas both agents were highly cytotoxic to MCF-7 cells. The EC50 values of adriamycin and ceramide were 11-fold (p < 0.0005) and 5-fold (p < 0.005) higher, respectively, in MCF-7/GCS cells compared with MCF-7 cells. Ceramide resistance displayed by MCF-7/GCS cells closely paralleled the activity of expressed GCS with a correlation coefficient of 0.99. In turn, cellular resistance and GCS activity were dependent upon the concentration of the expression mediator doxycycline. Adriamycin resistance in MCF-7/GCS cells was related to the hyperglycosylation of ceramide and was not related to shifts in the levels of either P-glycoprotein or Bcl-2. This work demonstrates that overexpression of GCS, which catalyzes ceramide glycosylation, induces resistance to adriamycin and ceramide in MCF-7 breast cancer cells.     

Growth inhibition of hormone-responsive and -resistant human breast cancer cells in culture by N1, N12-bis(ethyl)spermine

Previous studies have documented differential sensitivity of human lung cancer and melanoma cell lines to the cytotoxic effects of N1, N12-bis(ethyl)spermine (BESpm). We show here that BESpm can significantly inhibit the growth of six human breast cancer cell lines with 50% inhibitory concentration in the microM range. The degree of inhibition does not correlate with estrogen receptor status. Detailed studies with estrogen receptor-positive MCF-7 and estrogen receptor- negative Hs578t cells show a similar dose-response curve with concentrations of 1-10 microM resulting in maximal growth inhibition. Growth inhibition in both lines is associated with an 8-12-fold induction of the polyamine catabolic enzyme, spermidine/spermine N1-acetyltransferase, and a progressive decrease in intracellular polyamine levels over 6 days even though steady-state levels of BESpm are achieved within 24 h. Similar studies on WTMCF7 and AdrRMCF7 cells show that the acquisition of resistance to hormonal or doxorubicin therapy is not associated with resistance to the growth-inhibitory effects of BESpm. These results suggest that BESpm exerts similar growth-inhibitory effects against both hormone-responsive and -unresponsive human breast cancer cells, a finding which has significance for the potential use of polyamine analogues in treating human breast cancer.     

Synthesis of tissue factor messenger RNA and procoagulant activity in breast cancer cells in response to serum stimulation

The procoagulant activity observed in many types of tissue and cultured cells is due to tissue factor, a 30 kd transmembrane protein. The mRNA for tissue factor is a 2.2-kb species, which in some non-cancer cells can be up-regulated or induced by cytokines or by serum stimulation. In this study, induction of procoagulant activity in cancer cells was evaluated using the breast cancer cell line, MCF-7, and an adriamycin resistant subline, AdrRMCF-7, which has increased tumorigenicity in nude mice compared to the parental cell line. Procoagulant activity was factor VIIa dependent and was inhibited by an anti-tissue factor antibody. MCF-7 cells had minimal tissue factor activity, while AdrRMCF-7 cells had an 10-fold increase compared to the parental line. This increase was not observed in MCF-7 cells transfected with the multi-drug resistant gene, which is associated with adriamycin resistance. Serum stimulation of quiescent MCF-7 cells increased tissue factor activity 5-fold over baseline level, but did not increase activity in cells grown in serum-replete medium. Tissue factor activity of AdrRMCF-7 quiescent cells and AdrMCF-7 cells grown in serum-replete medium was enhanced 2-fold by serum stimulation. The predominant tissue factor mRNA species in MCF-7 cells was a 3.2 to 3.4-kb band, which increased in response to serum stimulation of cells grown in serum-replete medium. The mature 2.2-kb tissue factor mRNA band was detected in quiescent MCF-7 cells within six hours of serum stimulation and remained present 24 hours after stimulation. Synthesis of the 2.2-kb tissue factor mRNA species in MCF-7 and AdrRMCF-7 cells correlated with appearance of procoagulant activity. Thus, while procoagulant activity correlates with the level of the 2.2-kb tissue factor mRNA species in these cancer cells, there are inherent differences in tissue factor activity, antigen, and mRNA levels, as well as in regulation of its synthesis between these cells.     

Reduced drug accumulation and multidrug resistance in human breast cancer cells without associated P-glycoprotein or MRP overexpression

MCF-7 human breast cancer cells selected in Adriamycin in the presence of verapamil developed a multidrug resistant phenotype, which was characterized by as much as 100,000-fold resistance to mitoxantrone, 667-fold resistance to daunorubicin, and 600-fold resistance to doxorubicin. Immunoblot and PCR analyses demonstrated no increase in MDR-1 or MRP expression in resistant cells, relative to parental cells. This phenotype is similar to one previously described in mitoxantrone-selected cells. The cells, designated MCF-7 AdVp, displayed a slower growth rate without alteration in topoisomerase II alpha level or activity. Increased efflux and reduced accumulation of daunomycin and rhodamine were observed when compared to parental cells. Depletion of ATP resulted in complete abrogation of efflux of both daunomycin and rhodamine. No apparent alterations in subcellular daunorubicin distribution were observed by confocal microscopy. No differences were noted in intracellular pH. Molecular cloning studies using DNA differential display identified increased expression of the alpha subunit of the amiloride-sensitive sodium channel in resistant cells. Quantitative PCR studies demonstrated an eightfold overexpression of the alpha subunit of the Na+ channel in the resistant subline. This channel may be linked to the mechanism of drug resistance in the AdVp cells. The results presented here support the hypothesis that a novel energy-dependent protein is responsible for the efflux in the AdVp cells. Further identification awaits molecular cloning studies.     

The small heat shock protein hsp27 is correlated with growth and drug resistance in human breast cancer cell lines

An emerging body of evidence suggests that the heat shock proteins (hsp) may be involved in drug resistance. When hsp are induced by elevated temperatures, resistance to doxorubicin (Dox), but not to other commonly used chemotherapeutic agents, is induced in breast cancer cells. To evaluate the role of hsp27 in this phenomenon, we have transfected MDA-MB-231 breast cancer cells, which normally express low levels of hsp27, with a full-length hsp27 construct. These hsp27-overexpressing cells now display a 3-fold elevated resistance to Dox. Anchorage-dependent proliferation and anchorage-independent growth were also increased 2-4-fold in these transfectants. We have also derived a MCF-7 breast cancer cell line with amplified endogenous hsp27 which is highly resistant to Dox. When these cells are transfected with an antisense hsp27 construct, they are rendered sensitive to Dox (3-fold) with anchorage-dependent as well as anchorage-independent growth, similarly decreased. These results suggest that hsp27 specifically confers Dox resistance in human breast cancer cells and, furthermore, that hsp27 may be involved in the regulation of cell growth.     

Transfection of glutathione S-transferase (GST)-pi antisense complementary DNA increases the sensitivity of a colon cancer cell line to adriamycin, cisplatin, melphalan, and etoposide

The goal of this study was to demonstrate that glutathione S-transferase (GST)-pi is directly involved in the intrinsic and acquired resistance of cancer cells to anticancer drugs. To this end, GST-pi antisense cDNA was transfected into the cultured human colon cancer cell line M7609, which expresses an innately high level of GST-pi and shows intrinsic drug resistance, and into an M7609 strain with acquired resistance to Adriamycin (ADR;i.e., M7609/ADR cells). The changes in the sensitivity of these transfectants to various anticancer drugs were investigated. The intracellular concentrations of GST-pi in M7609/anti-1 cells and M7609/anti-2 cells, two clones that were established by transfection of GST-pi antisense cDNA into M7609 cells, were decreased to approximately half of those detected in the parent cells (M7609) and in the control cells transfected with vector alone (M7609/pLJ). The sensitivities of the antisense transfectants in relation to ADR, cisplatin, melphalan, and etoposide were increased -3.3-fold, 2.3-fold, 2.2-fold, and 2.1-fold, respectively, compared with those of M7609 and M7609/pLJ. On the other hand, the sensitivities of the antisense transfectants to Taxol, vincristine, 5-fluorouracil, and mitomycin C were not significantly changed. Similarly, the transfection of antisense cDNA into M7609/ADR cells resulted in the reduction of intracellular GST-pi concentration (by about half) and an increased sensitivity to ADR (4.4-fold), but no increase in 5-fluorouracil sensitivity. Thus, GST-pi is considered to be a multidrug resistance factor that is responsible for both the intrinsic and acquired resistance of cancer cells to anticancer drugs such as ADR, cisplatin, melphalan, and etoposide.     

Role of cellular glutathione and glutathione S-transferase in the expression of alkylating agent cytotoxicity in human breast cancer cells

Glutathione (GSH) and glutathione S-transferases (GSTs) play an important role in the protection of cells against toxic effects of many electrophilic drugs and chemicals. Modulation of cellular GSH and/or GST activity levels provides a potentially useful approach to sensitizing tumor cells to electrophilic anti-cancer drugs. In this study, we describe the interactions of four representative alkylating agents (AAs), melphalan, 4-hydroperoxy-cyclophosphamide (4HC), an an activated form of cyclophosphamide, 1,3-bis(2-chloroethyl)-1-nitrosourea (BCNU), and cisplatin, with GSH and GST in the human breast cancer cell line MCF-7. Depletion of cellular GSH pools by approximately 80% by treatment of the cells with the GSH synthesis inhibitor buthionine sulfoximine (BSO) sensitized the tumor cells to each AA to a different extent, with dose-modifying factors of 2.39, 2.21, 1.64, and 1.27 observed for melphalan, 4HC, cisplatin, and BCNU, respectively. Treatment of the cells with the GST inhibitor ethacrynic acid (EA) failed to show any significant effects on the cytotoxicity of these AAs. However, EA did potentiate the cytotoxicity of melphalan when given in combination with BSO, an effect that may be due to a more complete depletion of cellular GSH levels by the combined modulator treatment. Following a 1-hr exposure to cytotoxic-equivalent concentrations of these AAs, GSH levels decreased substantially in the case of 4HC and BCNU, but increased by 30-50% in the case of cisplatin and melphalan. BSO pretreatment largely blocked this effect of cisplatin and melphalan on cellular GSH, while it further enhanced the GSH-depleting activity of both 4HC and BCNU. On the basis of these results, it is concluded that (a) GSH affects the cytotoxicity of different AAs to different extents, (b) basal GST expression in MCF-7 cells does not play a major role in AA metabolism, (c) EA can potentiate the enhancing effect of BSO on melphalan cytotoxicity in MCF-7 cells, and (d) depletion of cellular GSH by pretreatment with BCNU or cyclophosphamide may correspond to a useful strategy for enhancing the anti-tumor activity of other AAs given in a sequential combination.     

Human pharmacokinetic characterization and in vitro study of the interaction between doxorubicin and paclitaxel in patients with breast cancer

PURPOSE: We performed a pharmacologic investigation of paclitaxel (PTX) infused over 3 hours and bolus doxorubicin (DOX) to assess the role of sequence, interval between drugs, and duration of doxorubicin infusion on paclitaxel and anthracycline plasma disposition. We also explored possible mechanisms of pharmacokinetic interference involving the physiologic role of the multidrug resistance phenotype in anthracycline and taxane biliary excretion. PATIENTS AND METHODS: Pharmacokinetics was performed in 80 cycles and 36 women with previously untreated metastatic breast cancer. PTX, DOX, and their metabolites 6 alpha-hydroxyl-PTX (6 alpha OH-PTX) and doxorubicinol (DOL) were measured by high-pressure liquid chromatography (HPLC). Human breast cancer MCF-7 wild-type (WT) and resistant (TH) cell lines were cultured in whole human plasma to study anthracycline retention after treatment with different combinations of PTX, Cremophor EL (CEL) (PEG35 castor oil; BASF, Parsippany, NJ), and DOX. RESULTS: Pharmacokinetic interference between PTX and DOX was responsible for nonlinearity of DOX plasma disposition and increased concentrations of DOX and DOL. These effects were PTX dose-dependent, DOX concentration-dependent, and likely a result of interference at the level of liver elimination. In view of the physiologic role of P-glycoproteins (P-gp) in xenobiotic biliary excretion, retention of DOX was assessed in MCF-7 WT and MCF-7 TH cells. Intracellular was significantly higher in MCF-7 WT than MCF-7 TH (P < .05). However, concomitant exposure to DOX, PTX, and CEL caused similar DOX retention in both MCF-7 WT and TH cells. CONCLUSION: PTX, as clinically formulated in CEL, is responsible for a nonlinear disposition of DOX and DOL. Nonlinearity is PTX- and DOX-dependent, and possibly caused by competition for biliary excretion of taxanes and anthracyclines mediated by P-gp. Nonlinearity indicates that even minor modifications of dose and infusion duration of DOX and PTX may lead to unpredictable pharmacodynamic consequences. The postulated role of P-gp suggests that CEL is clinically active, and advises caution in designing combinations of PTX with other drugs that are substrate for P-gp.