Reversal of drug resistance by erythromycin: erythromycin increases the accumulation of actinomycin D and doxorubicin in multidrug-resistant cells

Transfected MDCK cell line with enhanced expression of CYP3A4 and P-glycoprotein as a model to study their role in drug transport and metabolism

The aim of this study was to characterize and utilize MDCK cell line expressing CYP3A4 and P-glycoprotein as an in vitro model for evaluating drug-herb and drug-drug of abuse interactions. MDCK cell line simultaneously expressing P-gp and CYP3A4 (MMC) was developed and characterized by using expression and activity studies. Cellular transport study of 200 μM cortisol was performed to determine their combined activity. The study was carried across MDCK-WT, MDCK-MDR1 and MMC cell lines. Similar studies were also carried out in the presence of 50 μM naringin and 3 μM morphine. Samples were analyzed by HPLC for drug and its CYP3A4 metabolite. PCR, qPCR and Western blot studies confirmed the enhanced expression of the proteins in the transfected cells. The Vivid CYP3A4 assay and ketoconazole inhibition studies further confirmed the presence of active protein. Apical to basal transport of cortisol was found to be 10- and 3-fold lower in MMC as compared to MDCK-WT and MDCK-MDR1 respectively. Higher amount of metabolite was formed in MMC than in MDCK-WT, indicating enhanced expression of CYP3A4. Highest cortisol metabolite formation was observed in MMC cell line due to the combined activities of CYP3A4 and P-gp. Transport of cortisol increased 5-fold in the presence of naringin in MMC and doubled in MDCK-MDR1. Cortisol transport in MMC was significantly lower than that in MDCK-WT in the presence of naringin. The permeability increased 3-fold in the presence of morphine, which is a weaker inhibitor of CYP3A4. Formation of 6β-hydroxy cortisol was found to decrease in the presence of morphine and naringin. This new model cell line with its enhanced CYP3A4 and P-gp levels in addition to short culture time can serve as an invaluable model to study drug-drug interactions. This cell line can also be used to study the combined contribution of efflux transporter and metabolizing enzymes toward drug-drug interactions.

Antitumor activity of erythromycin on human neuroblastoma cell line (SH-SY5Y)

Antitumor effects of erythromycin and the related mechanism were investigated in the present study. Neuroblastoma cells (SH-SY5Y) were exposed to erythromycin at different concentrations for different durations. Cell proliferation was measured by cell counting, and cell viability was examined by MTT assay. Cell cycle phase distribution and the cytosolic calcium level were detected by flow cytometry. Mitochondrial membrane potential was measured by the JC-1 probe staining and fluorescent microscopy. The expression of an oncogene (c-Myc) and a tumor suppressor [p21 (WAF1/Cip1)] proteins was analyzed by using Western blotting. Erythromycin could inhibit the proliferation of SH-SY5Y cells in a concentration- and time-dependent manner. The cell cycle was arrested at S phase. Mitochondrial membrane potential collapsed and the cytosolic calcium was overloaded in SH-SY5Y cells when treated with erythromycin. The expression of c-Myc protein was down-regulated, while that of p21 (WAF1/Cip1) protein was up-regulated. It was concluded that erythromycin could restrain the proliferation of SH-SY5Y cells. The antitumor mechanism of erythromycin might involve regulating the expression of c-Myc and p21 (WAF1/Cip1) proteins.

Cytochrome P450-mediated metabolism in the human gut wall

Objective

Although the human small intestine serves primarily as an absorptive organ for nutrients and water, it also has the ability to metabolise drugs. Interest in the small intestine as a drug-metabolising organ has been increasing since the realisation that it is probably the most important extrahepatic site of drug biotransformation.

Key findings

Among the metabolising enzymes present in the small intestinal mucosa, the cytochromes P450 (CYPs) are of particular importance, being responsible for the majority of phase I drug metabolism reactions. Many drug interactions involving induction or inhibition of CYP enzymes, in particular CYP3A, have been proposed to occur substantially at the level of the intestine rather than exclusively within the liver, as originally thought. CYP3A and CYP2C represent the major intestinal CYPs, accounting for approximately 80% and 18%, respectively, of total immunoquantified CYPs. CYP2J2 is also consistently expressed in the human gut wall. In the case of CYP1A1, large interindividual variation in the expression levels has been reported. Data for the intestinal expression of the polymorphic CYP2D6 are conflicting. Several other CYPs, including the common hepatic isoform CYP2E1, are expressed in the human small intestine to only a very low extent, if at all. The distribution of most CYP enzymes is not uniform along the human gastrointestinal tract, being generally higher in the proximal regions of the small intestine.

Summary

This article reviews the current state of knowledge of CYP enzyme expression in human small intestine, the role of the gut wall in CYP-mediated metabolism, and how this metabolism limits the bioavailability of orally administered drugs. Possible interactions between drugs and CYP activity in the small intestine are also discussed.

Modulation of Oral Drug Bioavailability: From Preclinical Mechanism to Therapeutic Application

Currently, more than one fourth of all anticancer drugs are developed as oral formulations, and it is expected that this number will increase substantially in the near future. To enable oral drug therapy, adequate oral bioavailability must be achieved. Factors that have proved to be important in limiting the oral bioavailability are the presence of ATP-binding cassette drug transporters (ABC transporters) and the cytochrome P450 enzymes. We discuss the tissues distribution and physiological function of the ABC transporters in the human body, their expression in tumors, currently known polymorphisms and drugs that are able to inhibit their function as transporter. Furthermore, the role of the ABC transporters and drug-metabolizing enzymes as mechanisms to modulate the pharmacokinetics of anticancer agents, will be reviewed. Finally, some clinical examples of oral drug modulation are discussed. Among these examples are the coadministration of paclitaxel with CsA, a CYP3A4 substrate with P-glycoprotein (P-gp) modulating activity, and topotecan combined with the BCRP/P-gp transport inhibitor elacridar. Both are good examples of improvement of oral drug bioavailability by temporary inhibition of drug transporters in the gut epithelium.

Exposure-response relationships and drug interactions of sirolimus

Sirolimus (rapamycin, RAPAMUNE, RAPA) is an immunosuppressive agent used for the prophylaxis of renal allograft rejection and exhibits an immunosuppressive mechanism that is distinct from that for cyclosporine and tacrolimus. The purpose of this manuscript is to discuss the exposure-response relationships and drug interactions of sirolimus. The various factors affecting sirolimus whole blood exposure included first-pass extraction, formulation, food, demographics, liver disease, assay method, and interacting drugs. Clinically significant effects caused by food, pediatric age, hepatic impairment, and interacting drugs require recommendations for the safe and efficacious use of sirolimus in renal allograft patients. An exposure-response model based on multivariate logistic regression was developed using the interstudy data from 1832 renal allograft patients. The analysis revealed an increased probability of acute rejection for sirolimus troughs <5 ng/mL, cyclosporine troughs <150 ng/mL, human leukocyte antigen (HLA) mismatches > or =4, and females. The outcomes suggested that individualization of sirolimus doses immediately after transplantation, based on HLA mismatch and sex, would likely decrease the probability of acute rejections in renal allograft recipients who receive concomitant sirolimus, cyclosporine (full-dose), and corticosteroid therapy. Sirolimus is a substrate for both Cytochrome P450 3A (CYP3A) and P-glycoprotein (P-gp) and undergoes extensive first-pass extraction. Drugs that are known to inhibit or induce these proteins may potentially affect sirolimus whole blood exposure. In healthy volunteers, cyclosporine, diltiazem, erythromycin, ketoconazole, and verapamil significantly increased sirolimus whole blood exposure, and rifampin significantly decreased sirolimus exposure. However, sirolimus whole blood exposure was not affected by acyclovir, atorvastatin, digoxin, ethinyl estradiol/norgestrel, glyburide, nifedipine, or tacrolimus. Among the 15 drugs studied, sirolimus significantly increased the exposures of only erythromycin and S-(-)verapamil.

The effect of erythromycin on the pharmacokinetics of rosuvastatin

RATIONALE OBJECTIVE

To examine in vivo the effect of erythromycin on the pharmacokinetics of rosuvastatin [an inhibitor of 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase]. Erythromycin is a potent inhibitor of CYP3A4 that markedly increases circulating levels of some other HMG-CoA reductase inhibitors.

METHODS

In this randomised, double-blind, two-way cross-over, placebo-controlled trial 14 healthy volunteers were given 500 mg erythromycin or placebo four times daily for 7 days. A single dose of 80 mg rosuvastatin was co-administered on day 4 of dosing. Plasma concentrations of rosuvastatin and active and total HMG-CoA reductase inhibitors were measured up to 96 h after dosing.

RESULTS

Eleven volunteers had data available from both dosing periods. There was no increase in rosuvastatin plasma exposure following co-administration with erythromycin compared to placebo. In fact, following co-administration with erythromycin, rosuvastatin geometric least square mean AUC((0-t)) and C(max) were 20% and 31%, respectively, lower than with placebo. Individual treatment ratios for AUC((0-t)) ranged from 0.48 to 1.17, and for C(max) ranged from 0.33 to 2.19. Essentially all of the circulating active HMG-CoA reductase inhibitors and most (>94%) of the total inhibitors were accounted for by rosuvastatin. Erythromycin did not affect the proportion of circulating active or total inhibitors accounted for by circulating rosuvastatin.

CONCLUSIONS

Erythromycin did not produce any increase in rosuvastatin plasma exposure. This indicates that CYP3A4 metabolism is not an important clearance mechanism for rosuvastatin, a result consistent with previous findings. The small decreases in rosuvastatin AUC((0-t)) and C(max) that occurred as a consequence of short-term treatment with erythromycin are unlikely to have relevance to long-term treatment with rosuvastatin.

Strategies to overcome simultaneous P-glycoprotein mediated efflux and CYP3A4 mediated metabolism of drugs

Cytochrome P450 3A4 (CYP3A4), abundant in both the liver and upper intestinal enterocytes, limits the systemic bioavailability of xenobiotics. P-glycoprotein (P-gp), the MDR1 gene product, is also known to reduce the oral bioavailability of the drug molecules. High cellular expression of P-gp and CYP3A4 in mature intestinal enterocytes and their similar substrate specificity suggest that the function of these proteins may be complementary and may form a co-ordinated intestinal barrier. Various ongoing preclinical and clinical studies have demonstrated that the oral bioavailability of various P-gp and/or CYP3A4 substrates can be increased by simultaneous administration of P-gp and/or CYP3A4 inactivators. The current review describes the background and summarises several proposed hypotheses in modifying oral bioavailability by various drug-inhibitor interactions.

Anti-inflammatory effects of macrolide antibiotics

Macrolides are widely used as antibacterial drugs. Clinical and experimental data, however, indicate that they also modulate inflammatory responses, both contributing to the treatment of infective diseases and opening new opportunities for the therapy of other inflammatory conditions. Considerable evidence, mainly from in vitro studies, suggests that leukocytes and neutrophils in particular, are important targets for modulatory effects of macrolides on host defense responses. This underlies the use of the 14-membered macrolide erythromycin for the therapy of diffuse panbronchiolitis. A variety of other inflammatory mediators and processes are also modulated by macrolides, suggesting that the therapeutic indications for these drugs may be extended significantly in future.

Cellular uptake of two fluoroketolides, HMR 3562 and HMR 3787, by human polymorphonuclear neutrophils in vitro

We analyzed the cellular accumulation of two new fluoroketolides, HMR 3562 and HMR 3787, by human polymorphonuclear neutrophils (PMN) in vitro. Both compounds were rapidly taken up by PMN, with a cellular-to-extracellular concentration ratio (C/E) of about 141 (HMR 3562) and 117 (HMR 3787) at 5 min, and this was followed by a plateau at 60 to 180 min, with a C/E of >300 at 180 min. Both ketolides were mainly located in PMN granules (about 75%) and egressed slowly from loaded cells (about 40% at 60 min), owing to avid reuptake. Uptake was moderately sensitive to external pH, and activation energy was also moderate (about 70 kJ/mol). As with other macrolides and ketolides, the existence of an active transport system was suggested by (i) the strong interindividual variability in uptake kinetics, suggesting variability in the number or activity of a transport protein; (ii) the saturation kinetics characteristic of a carrier-mediated transport system (V(max), about 2,300 ng/2.5 x 10(6) PMN/5 min; K(m), about 50 microg/ml); (iii) the inhibitory effects of Ni(2+) (a blocker of the Na+-Ca(2+) exchanger), phorbol myristate acetate (a protein kinase C activator), and H89 (a protein kinase A inhibitor). Although these two ketolides are more related to HMR 3647 (telithromycin), it is interesting that the presence of a fluoride gave these molecules a cellular pharmacokinetics more like those of HMR 3004 than those of HMR 3647. The macrolide transport system has not been yet elucidated, but our data confirm that, despite variations in chemical structure, all erythromycin A derivatives share a transmembrane transport system.

The effect of gut metabolism on tacrolimus bioavailability in renal transplant recipients

BACKGROUND

Tacrolimus, a substrate of CYP3A, has low and variable bioavailability similar to cyclosporine. Co-administration of ketoconazole, potent inhibitor of gut and hepatic CYP3A, has been shown to increase tacrolimus bioavailability in healthy volunteers. The purpose of this study is to assess the role of gut metabolism in the overall bioavailability of tacrolimus in a renal transplant population.

METHODS

We prospectively studied 19 adult renal transplant recipients who were receiving tacrolimus as part of a quadruple, sequential immunosuppression regimen. Each patient received tacrolimus (4-hr intravenous dose of 0.04 mg/kg between postoperative days 2 and 4). Whole blood samples were collected over 24 hr. After a 24-hr washout period, a single oral dose of ketoconazole (400 mg) was administered followed by the same intravenous dose of tacrolimus, and subsequent samples were obtained. Steady state oral pharmacokinetic profiles were obtained between 1 and 3 months after transplant while patients were receiving twice daily dosing of tacrolimus to maintain whole blood levels between 10 and 20 ng/ml. Two days later, 400 mg of ketoconazole was administered orally 2 hr before to the morning dose. Whole blood samples were collected over 12 hr.

RESULTS

In the absence of ketoconazole, 8.0% of the tacrolimus dose underwent first pass metabolism (E(H)), whereas in the presence of ketoconazole, first pass metabolism was 6.2% (P=0.01). Based on this difference in first pass metabolism, an increase of 2% in bioavailability is expected, but an increase of 47% is observed (P=0.001).

CONCLUSIONS

This indicates that the gut metabolism of tacrolimus is extensive and contributes significantly to its bioavailability.

Interference of antibacterial agents with phagocyte functions: immunomodulation or "immuno-fairy tales"?

Professional phagocytes (polymorphonuclear neutrophils and monocytes/macrophages) are a main component of the immune system. These cells are involved in both host defenses and various pathological settings characterized by excessive inflammation. Accordingly, they are key targets for immunomodulatory drugs, among which antibacterial agents are promising candidates. The basic and historical concepts of immunomodulation will first be briefly reviewed. Phagocyte complexity will then be unravelled (at least in terms of what we know about the origin, subsets, ambivalent roles, functional capacities, and transductional pathways of this cell and how to explore them). The core subject of this review will be the many possible interactions between antibacterial agents and phagocytes, classified according to demonstrated or potential clinical relevance (e.g., neutropenia, intracellular accumulation, and modulation of bacterial virulence). A detailed review of direct in vitro effects will be provided for the various antibacterial drug families, followed by a discussion of the clinical relevance of these effects in two particular settings: immune deficiency and inflammatory diseases. The prophylactic and therapeutic use of immunomodulatory antibiotics will be considered before conclusions are drawn about the emerging (optimistic) vision of future therapeutic prospects to deal with largely unknown new diseases and new pathogens by using new agents, new techniques, and a better understanding of the phagocyte in particular and the immune system in general.

Interference of antibacterial agents with phagocyte functions: immunomodulation or "immuno-fairy tales"?

Professional phagocytes (polymorphonuclear neutrophils and monocytes/macrophages) are a main component of the immune system. These cells are involved in both host defenses and various pathological settings characterized by excessive inflammation. Accordingly, they are key targets for immunomodulatory drugs, among which antibacterial agents are promising candidates. The basic and historical concepts of immunomodulation will first be briefly reviewed. Phagocyte complexity will then be unravelled (at least in terms of what we know about the origin, subsets, ambivalent roles, functional capacities, and transductional pathways of this cell and how to explore them). The core subject of this review will be the many possible interactions between antibacterial agents and phagocytes, classified according to demonstrated or potential clinical relevance (e.g., neutropenia, intracellular accumulation, and modulation of bacterial virulence). A detailed review of direct in vitro effects will be provided for the various antibacterial drug families, followed by a discussion of the clinical relevance of these effects in two particular settings: immune deficiency and inflammatory diseases. The prophylactic and therapeutic use of immunomodulatory antibiotics will be considered before conclusions are drawn about the emerging (optimistic) vision of future therapeutic prospects to deal with largely unknown new diseases and new pathogens by using new agents, new techniques, and a better understanding of the phagocyte in particular and the immune system in general.

Interactions of ofloxacin and erythromycin with the multidrug resistance protein (MRP) in MRP-overexpressing human leukemia cells

To investigate interactions between the multidrug resistance protein (MRP) and antimicrobial agents, we examined the effects of 12 agents on vincristine sensitivity and efflux of the calcein acetoxy-methyl ester (calcein-AM) of a MRP substrate in MRP-overexpressing cells. Only ofloxacin and erythromycin enhanced sensitivity with increased intracellular vincristine accumulation and inhibited the calcein-AM efflux. Our findings suggest that the two agents are possible MRP substrates and may competitively inhibit MRP function as a drug efflux pump.

Effect of diclofenac, disulfiram, itraconazole, grapefruit juice and erythromycin on the pharmacokinetics of quinidine

AIMS

In vitro studies suggest that the oxidation of quinidine to 3-hydroxyquinidine is a specific marker reaction for CYP3A4 activity. To assess the possible use of this reaction as an in vivo marker of CYP3A4 activity, we studied the involvement of cytochromes CYP2C9, CYP2E1 and CYP3A4 in the in vivo oxidative metabolism of quinidine.

METHODS

An open study of 30 healthy young male volunteers was performed. The pharmacokinetics of a 200 mg single oral dose of quinidine was studied before and during daily administration of 100 mg diclofenac, a CYP2C9 substrate (n=6); 200 mg disulfiram, an inhibitor of CYP2E1 (n=6); 100 mg itraconazole, an inhibitor of CYP3A4 (n=6); 250 ml single strength grapefruit juice twice daily, an inhibitor of CYP3A4 (n=6); 250 mg of erythromycin 4 times daily, an inhibitor of CYP3A4 (n=6). Probes of other enzyme activities, caffeine (CYP1A2), sparteine (CYP2D6), mephenytoin (CYP2C19), tolbutamide (CYP2C9) and cortisol (CYP3A4) were also studied.

RESULTS

Concomitant administration of diclofenac reduced the partial clearance of quinidine by N-oxidation by 27%, while no effect was found for other pharmacokinetic parameters of quinidine. Concomitant administration of disulfiram did not alter any of the pharmacokinetic parameters of quinidine. Concomitant administration of itraconazole reduced quinidine total clearance, partial clearance by 3-hydroxylation and partial clearance by N-oxidation by 61, 84 and 73%, respectively. The renal clearance was reduced by 60% and the elimination half-life increased by 35%. Concomitant administration of grapefruit juice reduced the total clearance of quinidine and its partial clearance by 3-hydroxylation and N-oxidation by 15, 19 and 27%, respectively. The elimination half-life of quinidine was increased by 19%. The caffeine metabolic index was reduced by 25%. Concomitant administration of erythromycin reduced the total clearance of quinidine and its partial clearance by 3-hydroxylation and N-oxidation by 34, 50 and 33%, respectively. Cmax was increased by 39%.

CONCLUSIONS

The results confirm an important role for CYP3A4 in the oxidation of quinidine in vivo, and this applies particularly to the formation of 3-hydroxyquinidine. While a minor contribution of CYP2C9 to the N-oxidation of quinidine is possible, a major involvement of the CYP2C9 or CYP2E1 enzymes in the oxidation of quinidine in vivo is unlikely.

Effect of clarithromycin on renal excretion of digoxin: interaction with P-glycoprotein

We present a digoxin-clarithromycin interaction in two patients in whom digoxin concentrations were unexpectedly increased. The ratio of renal digoxin clearance to creatinine clearance in one patient was lower during the concomitant administration of clarithromycin (0.64 and 0.73) than that after cessation of clarithromycin administration (1.30 +/- 0.20; mean +/- SD). Because P-glycoprotein could play an important role in the renal secretion of digoxin, we hypothesized that clarithromycin decreases renal digoxin excretion by inhibiting P-glycoprotein-mediated transport. Digoxin transport was evaluated with use of a kidney epithelial cell line, which expresses the human P-glycoprotein on the apical membrane by transfection with MDR1 complementary deoxyribonucleic acid. Clarithromycin inhibited the transcellular transport of digoxin from the basolateral to the apical side in a concentration-dependent manner and concomitantly increased the cellular accumulation of digoxin. These results suggest that clarithromycin may inhibit the P-glycoprotein-mediated tubular secretion of digoxin, and this interaction mechanism may contribute to an increase in the serum digoxin concentration.

Reduced intracellular activity of antibiotics against Listeria monocytogenes in multidrug resistant cells

Multidrug resistance is expressed not only by bacteria, but also by tumor cells and by some normal cells of the body. It enables eukaryotic cells to exclude not only cytostatic drugs but also non-cytostatic antibiotics. This was demonstrated in genetically engineered multidrug resistant (MDR) cells infected with the facultative intracellular bacterium Listeria monocytogenes for all macrolide antibiotics tested (azithromycin, clarithromycin, erythromycin, josamycin, roxithromycin and spiramycin). In these cells and in conventionally selected MDR cells higher concentrations of the macrolides were necessary to inhibit the growth of L. monocytogenes than in the respective parental cells. This effect was due to a reduced intracellular accumulation, which was shown with a biological assay for all macrolides tested. For azithromycin, the results of this test were confirmed by measurement of the intracellular concentrations with high-performance liquid chromatography (HPLC). Besides the macrolides, MDR cells excluded also antibiotics of other chemical groups which was shown for ciprofloxacin, clindamycin, rifampicin and the streptogramin derivative RP 59500. In addition, in conventionally selected cells higher concentrations of chloramphenicol, doxycyclin, ofloxacin and trimethoprim than in the respective parental cells were necessary to inhibit the growth of L. monocytogenes. In contrast, when using genetically engineered cells, no significant differences were found for these antibiotics. These differences might be due to a higher expression of multidrug resistance in the conventionally selected cells because these cells were also more effective in excluding rhodamine 123 in a flow cytometric assay. In conclusion, expression of multidrug resistance by eukaryotic cells leads to a reduced concentration of macrolides and other antibiotics in these cells and to an impairment of activity against intracellular bacteria.

Human MDR1 and mouse mdr1a P-glycoprotein alter the cellular retention and disposition of erythromycin, but not of retinoic acid or benzo(a)pyrene

The intracellular concentration of many steroids and xenobiotics is influenced by the membrane protein P-glycoprotein (Pgp). It has been inferred that the intracellular retention of many drugs that upregulate Pgp or modulate Pgp function might also be affected by Pgp. However, the ability of Pgp to influence the translocation of these drugs needs to be established to understand Pgp's influence upon their pharmacological effect. We utilized two approaches to determine the interaction of several agents with Pgp: (a) an in vitro system, LLC-PK1 cell lines and derivative LLC cell lines stably expressing on the apical membrane either mouse mdr1a or human MDR1 Pgp grown as polarized epithelium in transwell culture to measure translocation of radiolabeled drugs; and (b) an in vivo system, mdr1a nullizygous and wild-type animals, to compare the contribution of Pgp to in vivo distribution of radiolabeled drugs. In combination these complementary approaches identified erythromycin as a drug whose intracellular retention is influenced by Pgp, while the intracellular accumulation and tissue distribution of retinoic acid and benzo(a)pyrene were unaffected by Pgp.

Erythromycin is ineffective against Listeria monocytogenes in multidrug resistant cells

Multidrug resistance of tumor cells is a well-known phenomenon in oncology. Among the substances excluded from the cells are not only antineoplastic drugs but also certain antibiotics, e.g. erythromycin. To prove the hypothesis that this might render infections with intracellular bacteria untreatable with these antibiotics we used erythromycin to treat intracellular infection of multidrug resistant (MDR) cells with Listeria monocytogenes. Erythromycin was unable to restrict the growth of L. monocytogenes in KBV-1 MDR cells in concentrations of up to 25 micrograms/ml. In contrast, 0.049 micrograms/ml of erythromycin were sufficient to restrict the growth of the bacteria in nonresistant KB 3-1 cells. When verapamil was added to the supernatant of KBV-1 cells, erythromycin regained its effectivity on L. monocytogenes multiplying in these cells. The fact that MDR cells may render intracellular bacteria inaccessible to certain antibiotics might have important implications for the persistence of these bacteria in the host and for the treatment of patients with genetically engineered MDR cells.

Novobiocin modulates colchicine sensitivity in parental and multidrug-resistant B16 melanoma cells

The effect of the antibiotic agent novobiocin on the sensitivity of melanoma cells to colchicine and vinblastine was examined in drug-sensitive and drug-resistant B16 melanoma cells. A cell line COL/R was selected for colchicine resistance. The COL/R cell line (resistant to 80 ng/ml colchicine) was found to possess the multidrug-resistant (MDR) phenotype. The cells were shown to be cross-resistant to vinblastine and Adriamycin and to overexpress P glycoprotein. P glycoprotein activity was assessed by using the rhodamine 123 accumulation test. Rhodamine accumulation was markedly decreased in COL/R cells as compared to the parental B16 cells. Verapamil reversed drug resistance and increased rhodamine accumulation in COL/R cells. Novobiocin in combination with colchicine or vinblastine synergistically inhibited the proliferation of parental B16 cells. In COL/R cells, novobiocin markedly decreased colchicine resistance and increased rhodamine accumulation. These data show that novobiocin increases the sensitivity of both parental and MDR melanoma cells to microtubule-disrupting cytotoxic drugs.

P-glycoprotein-mediated multidrug resistance in normal and neoplastic hematopoietic cells

The multidrug transporter, P-glycoprotein (P-gp), is expressed by CD34-positive bone marrow cells, which include hematopoietic stem cells, and in other cells in the bone marrow and peripheral blood, including some lymphoid cells. Multidrug resistance mediated by P-gp appears to be a major impediment to successful treatment of acute myeloid leukemias and multiple myelomas. However, the impact of P-gp expression on prognosis has to be confirmed in several other hematopoietic neoplasms. The role of P-gp in normal and malignant hematopoiesis and clinical attempts to circumvent multidrug resistance in hematopoietic malignancies are reviewed. The recent transduction of the MDR1 gene into murine hematopoietic cells, which protects them from toxic effects of chemotherapy, suggests that MDR1 gene therapy may help prevent myelosuppression following chemotherapy.

Characterization of newly established adriamycin resistant human leukemic cell lines (KY-ADR1 and KY-ADR2)

New adriamycin (ADR) resistant human leukemic cell lines (KY-ADR1 and KY-ADR2) have been established. KY-ADR1 was selected from a cytosine arabinoside (Ara C) resistant cell line by gradually increasing the concentration of ADR and KY-ADR2 from the parental cell line, KY-821, by the same method. The IC50s of both cell lines were 4.3 x 10(-5) and 3.6 x 10(-5) M ADR, respectively. Both lines revealed a similar cross resistance to various anticancer drugs, but KY-ADR1 was resistant to Ara C, whereas KY-ADR2 was sensitive. MDR1 gene was over-expressed and P-glycoprotein was expressed on the cytoplasmic membrane in both lines. Neither verapamil nor cyclosporin A could completely reverse ADR resistance. In addition, no significant changes in topoisomerase II and glutathione-s-transferase levels were detected. These findings indicate that ADR resistance in both cell lines is mainly mediated by P-glycoprotein and some other mechanism may be present. Interestingly, growth of both cell lines was stimulated by natural IL-1 and not affected by TNF alpha and IFN gamma, whereas growth of parental KY-821 was inhibited by these factors. These cell lines will provide new biological aspects on drug resistant leukemic cells.

Pharmacologic circumvention of multidrug resistance

The ability of malignant cells to develop resistance to chemotherapeutic drugs is a major obstacle to the successful treatment of clinical tumors. The phenomenon multidrug resistance (MDR) in cancer cells results in cross-resistance to a broad range of structurally diverse antineoplastic agents, due to outward efflux of cytotoxic substrates by the mdr1 gene product, P-glycoprotein (P-gp). Numerous pharmacologic agents have been identified which inhibit the efflux pump and modulate MDR. The biochemical, cellular and clinical pharmacology of agents used to circumvent MDR is analyzed in terms of their mechanism of action and potential clinical utility. MDR antagonists, termed chemosensitizers, may be grouped into several classes, and include calcium channel blockers, calmodulin antagonists, anthracycline and Vinca alkaloid analogs, cyclosporines, dipyridamole, and other hydrophobic, cationic compounds. Structural features important for chemosensitizer activity have been identified, and a model for the interaction of these drugs with P-gp is proposed. Other possible cellular targets for the reversal of MDR are also discussed, such as protein kinase C. Strategies for the clinical modulation of MDR and trials combining chemosensitizers with chemotherapeutic drugs in humans are reviewed. Several novel approaches for the modulation of MDR are examined.

Effect of pyridoxine on tumor necrosis factor activities in vitro

Clinical trials with tumor necrosis factor (TNF) as an antitumor agent have so far given rather disappointing results. In this study we show that the naturally occurring vitamin B6 compound, pyridoxine, enhances TNF-induced cytolysis of three subclones of a mouse fibrosarcoma cell line (WEHI 164). The degree of pyridoxine-induced enhancement of TNF cytotoxicity seems to be dependent on the cells sensitivity to TNF, as the enhancement was much more pronounced in the relatively TNF resistant subclone act-R(cl.12)-WEHI 164, than in the very TNF sensitive subclone WEHI 164 clone 13. Furthermore, our study shows that pyridoxine, in contrast to its enhancing effect on TNF-induced cytotoxicity, rather inhibits TNF-induced growth of human FS-4 fibroblasts. Pyridoxine also enhances lymphotoxin (LT)-induced tumor cell killing and inhibits LT-induced fibroblast growth. Pyridoxine is a relatively non-toxic agent in vivo. Our results suggest that a combination of TNF and pyridoxine may be more efficient than TNF alone, in the treatment of cancer patients.

P-glycoprotein as multidrug transporter: a critical review of current multidrug resistant cell lines

MDR has been studied extensively in mammalian cell lines. According to usual practice, the MDR phenotype is characterized by the following features: cross resistance to multiple chemotherapeutic agents (lipophilic cations), defective intracellular drug accumulation and retention, overexpression of P-gp (often accompanied by gene amplification), and reversal of the phenotype by addition of calcium channel blockers. An hypothesis for the function of P-gp has been proposed in which P-gp acts as a carrier protein that actively extrudes MDR compounds out of the cells. However, basic questions, such as what defines the specificity of the pump and how is energy for active efflux transduced, remain to be answered. Furthermore, assuming that P-gp acts as a drug transporter, one will expect a relationship between P-gp expression and accumulation defects in MDR cell lines. A review of papers reporting 97 cell lines selected for resistance to the classical MDR compounds has revealed that a connection exists in most of the reported cell lines. However, several exceptions can be pointed out. Furthermore, only a limited number of well characterized series of sublines with different degrees of resistance to a single agent have been reported. In many of these, a correlation between P-gp expression and transport properties can not be established. Co-amplification of genes adjacent to the mdr1 gene, mutations [122], splicing of mdr1 RNA [123], modulation of P-gp by phosphorylation [124] or glycosylation [127], or experimental conditions [26,78] could account for some of the complexity of the phenotype and the absence of correlation in some of the cell lines. However, both cell lines with overexpression of P-gp without increased efflux [i.e., 67,75] and cell lines without P-gp expression and accumulation defects/increased efflux [i.e., 25,107] have been reported. Thus, current results from MDR cell lines contradict--but do not exclude--that P-gp acts as multidrug transporter. Other models for the mechanism of resistance have been proposed: (1) An energy-dependent permeability barrier working with greater efficacy in resistant cells. This hypothesis is supported by studies of influx which, although few, all except one demonstrate decreased influx in resistant cells; (2) Resistant cells have a greater endosomal volume, and a greater exocytotic activity accounts for the efflux.(ABSTRACT TRUNCATED AT 400 WORDS)

Multi-drug resistance genes in the management of neoplastic disease

P-gp can function as an ATP-dependent cytotoxic drug-efflux pump. In normal tissues, protein expression is localized to cell surfaces that face excretory lumina; hence, P-gp may function as a toxic-waste disposal system. Tumors that are derived from these tissues can be high expressors of P-gp, and these tumors tend to display intrinsic chemoresistance. Other non-expressing tumors can become P-gp positive after treatment or at relapse, suggesting that mdr may be involved in acquired resistance. The use of MDR-modifying agents has had some clinical success, and further trials of chemosensitizers are proceeding. P-gp overexpression does not explain how clinical resistance to anthracyclines, alkylating agents, and cis-platinum can arise simultaneously. In these cases, multiple genetic mechanisms of resistance may coexist. Eventually, mdr status can be used to select the most effective chemotherapy protocol for the individual. Currently, conversion of a previously mdr negative tumor to mdr expression, in the face of clinical resistance, justifies changing to a non-MDR drug protocol, or if not feasible, the use of MDR sensitizers.