A phase II study of epidoxorubicin in colorectal cancer and the use of cyclosporin-A in an attempt to reverse multidrug resistance

Novel Potent ABCB1 Modulator, Phenethylisoquinoline Alkaloid, Reverses Multidrug Resistance in Cancer Cell

ATP-binding cassette (ABC) transporters, which are concerned with the efflux of anticancer drugs from cancer cells, have a pivotal role in multidrug resistance (MDR). In particular, ABCB1 is a well-known ABC transporter that develops MDR in many cancer cells. Some ABCB1 modulators can reverse ABCB1-mediated MDR; however, no modulators with clinical efficacy have been approved. The aim of this study was to identify novel ABCB1 modulators by using high-throughput screening. Of the 5861 compounds stored at Tohoku University, 13 compounds were selected after the primary screening via a fluorescent plate reader-based calcein acetoxymethylester (AM) efflux assay. These 13 compounds were validated in a flow cytometry-based calcein AM efflux assay. Two isoquinoline derivatives were identified as novel ABCB1 inhibitors, one of which was a phenethylisoquinoline alkaloid, (±)-7-benzyloxy-1-(3-benzyloxy-4-methoxyphenethyl)-1,2,3,4-tetrahydro-6-methoxy-2-methylisoquinoline oxalate. The compound, a phenethylisoquinoline alkaloid, was subsequently evaluated in the cytotoxicity assay and shown to significantly enhance the reversal of ABCB1-mediated MDR. In addition, the compound activated the ABCB1-mediated ATP hydrolysis and inhibited the photolabeling of ABCB1 with [¹²⁵I]-iodoarylazidoprazosin. Furthermore, the compound also reversed the resistance to paclitaxel without increasing the toxicity in the ABCB1-overexpressing KB-V1 cell xenograft model. Overall, we concluded that the newly identified phenethylisoquinoline alkaloid reversed ABCB1-mediated MDR through direct interaction with the substrate-binding site of ABCB1. These findings may contribute to the development of more potent and less toxic ABCB1 modulators, which could overcome ABCB1-mediated MDR.

Pharmacokinetics and Safety of Letermovir Coadministered With Cyclosporine A or Tacrolimus in Healthy Subjects

Letermovir is being developed for human cytomegalovirus infection treatment and prophylaxis. In patients receiving transplants, antivirals are coadministered with cyclosporine A (CsA) or tacrolimus (TAC) immunosuppressants. Therefore, we investigated the potential for letermovir-immunosuppressant interactions. In 2 phase 1 clinical trials either CsA 50 mg or TAC 5 mg was administered to healthy males. Following washout, letermovir 80 mg was dosed twice daily for 7 and 11 days in the CsA and TAC trials, respectively, with a second dose of immunosuppressant coadministered with letermovir at steady state. In addition, letermovir 40 mg twice daily was administered for 14 days, and either CsA 50 or 200 mg administered on days 7 and 14. Pharmacokinetics and tolerability were assessed. Letermovir increased CsA and TAC C_max by 37% and 70%, respectively, and exposure by 70% and 78%, respectively, compared with immunosuppressant alone; t_½ was also increased from 10.7 to 17.9 hours for CsA. CsA (50/200 mg) increased letermovir C_max,ss (109%/167%) and AUC_ss,τ (126%/237%) and decreased t_½ (4.33 to 3.68/3.04 hours) versus letermovir alone. TAC did not significantly affect letermovir pharmacokinetics. All treatments were well tolerated. Concomitant letermovir increased TAC and CsA exposure. CsA altered letermovir pharmacokinetics, whereas TAC did not.

APO866 Increases Antitumor Activity of Cyclosporin-A by Inducing Mitochondrial and Endoplasmic Reticulum Stress in Leukemia Cells

PURPOSE

The nicotinamide phosphoribosyltransferase (NAMPT) inhibitor, APO866, has been previously shown to have antileukemic activity in preclinical models, but its cytotoxicity in primary leukemia cells is frequently limited. The success of current antileukemic treatments is reduced by the occurrence of multidrug resistance, which, in turn, is mediated by membrane transport proteins, such as P-glycoprotein-1 (Pgp). Here, we evaluated the antileukemic effects of APO866 in combination with Pgp inhibitors and studied the mechanisms underlying the interaction between these two types of agents.

EXPERIMENTAL DESIGN

The effects of APO866 with or without Pgp inhibitors were tested on the viability of leukemia cell lines, primary leukemia cells (AML, n = 6; B-CLL, n = 19), and healthy leukocytes. Intracellular nicotinamide adenine dinucleotide (NAD(+)) and ATP levels, mitochondrial transmembrane potential (ΔΨ(m)), markers of apoptosis and of endoplasmic reticulum (ER) stress were evaluated.

RESULTS

The combination of APO866 with Pgp inhibitors resulted in a synergistic cytotoxic effect in leukemia cells, while sparing normal CD34(+) progenitor cells and peripheral blood mononuclear cells. Combining Pgp inhibitors with APO866 led to increased intracellular APO866 levels, compounded NAD(+) and ATP shortage, and induced ΔΨ(m) dissipation. Notably, APO866, Pgp inhibitors and, to a much higher extent, their combination induced ER stress and ER stress inhibition strongly reduced the activity of these treatments.

CONCLUSIONS

APO866 and Pgp inhibitors show a strong synergistic cooperation in leukemia cells, including acute myelogenous leukemia (AML) and B-cell chronic lymphocytic leukemia (B-CLL) samples. Further evaluations of the combination of these agents in clinical setting should be considered.

Improving cancer chemotherapy with modulators of ABC drug transporters

ATP-binding cassette (ABC) transporters, P-glycoprotein (P-gp, ABCB1) and ABCG2, are membrane proteins that couple the energy derived from ATP hydrolysis to efflux many chemically diverse compounds across the plasma membrane, thereby playing a critical and important physiological role in protecting cells from xenobiotics. These transporters are also implicated in the development of multidrug resistance (MDR) in cancer cells that have been treated with chemotherapeutics. One approach to blocking the efflux capability of an ABC transporter in a cell or tissue is inhibiting the activity of the transporters with a modulator. Since ABC transporter modulators can be used in combination with chemotherapeutics to increase the effective intracellular concentration of anticancer drugs, the possible impact of modulators of ABC drug transporters is of great clinical interest. Another possible clinical use of modulators that has recently attracted attention is their ability to increase oral bioavailability or increase tissue penetration of drugs transported by the transporters. Several preclinical and clinical studies have been performed to evaluate the feasibility and the safety of this approach. The primary focus of this review is to discuss progress made in recent years in the identification and applicability of compounds that may serve as ABC transporter modulators and the possible role of these compounds in altering the pharmacokinetics and pharmacodynamics of therapeutic drugs used in the clinic.

Generating inhibitors of P-glycoprotein: where to, now?

The prominent role for the drug efflux pump ABCB1 (P-glycoprotein) in mediating resistance to chemotherapy was first suggested in 1976 and sparked an incredible drive to restore the efficacy of anticancer drugs. Achieving this goal seemed inevitable in 1982 when a series of calcium channel blockers were demonstrated to restore the efficacy of chemotherapy agents. A large number of other compounds have since been demonstrated to restore chemotherapeutic sensitivity in cancer cells or tissues. Where do we stand almost three decades since the first reports of ABCB1 inhibition? Unfortunately, in the aftermath of extensive fundamental and clinical research efforts the situation remains gloomy. Only a small handful of compounds have reached late stage clinical trials and none are in routine clinical usage to circumvent chemoresistance. Why has the translation process been so ineffective? One factor is the multifactorial nature of drug resistance inherent to cancer tissues; ABCB1 is not the sole factor. However, expression of ABCB1 remains a significant negative prognostic indicator and is closely associated with poor response to chemotherapy in many cancer types. The main difficulties with restoration of sensitivity to chemotherapy reside with poor properties of the ABCB1 inhibitors: (1) low selectivity to ABCB1, (2) poor potency to inhibit ABCB1, (3) inherent toxicity and/or (4) adverse pharmacokinetic interactions with anticancer drugs. Despite these difficulties, there is a clear requirement for effective inhibitors and to date the strategies for generating such compounds have involved serendipity or simple chemical syntheses. This chapter outlines more sophisticated approaches making use of bioinformatics, combinatorial chemistry and structure informed drug design. Generating a new arsenal of potent and selective ABCB1 inhibitors offers the promise of restoring the efficacy of a key weapon in cancer treatment--chemotherapy.

How can we best use structural information on P-glycoprotein to design inhibitors?

This year marks the 30th anniversary of the discovery of the multidrug resistance (MDR) ATP-binding cassette (ABC) transporter P-glycoprotein (P-gp). Since then a considerable research effort has attempted to provide a greater understanding of the biological enigma of "multidrug" efflux. Moreover, the growing correlation between P-gp expression and a negative prognosis or poor outcome for chemotherapy has sparked significant interest in the generation of inhibitors. How close are we to overcoming the unwanted actions of P-gp in resistant cancer following 30 years of research? The initial inhibitors were pre-existing clinically used compounds and exploited the broad specificity of P-gp. Unfortunately, the concentrations required to inhibit P-gp meant that these compounds generated considerable toxicity. Pharmacological investigations progressed to rational design using the 1st generation compounds as a template structure. Inherent toxicity of the drugs was reduced; however, pharmacokinetic interactions with the anticancer drugs were unsustainable. Generation of the most recent of inhibitors employed combinatorial chemistry to produce a handful of potent and selective P-gp inhibitors. Some of these drugs have progressed to clinical trials with poor results or in some cases, undisclosed progress. There remains a clear need for the generation of P-gp inhibitors and this review describes the potential for a structure-based design to facilitate this undertaking. In particular, the plethora of functional data can provide important regions on the protein that could conceivably be exploited as inhibitor targets.

Approaches to multidrug resistance reversal

Multidrug resistance (MDR) is characterised by cross-resistance between unrelated anticancer drugs and is associated with the overexpression of a membrane bound high-molecular weight glycoprotein, named P-glycoprotein, which is able to actively expel the drugs out of the cells. In vitro, numerous compounds have demonstrated the ability to inhibit the transport activity of P-glycoprotein, resulting in enhanced intracellular drug accumulation and MDR reversal. Such compounds include drugs of current use in other therapeutic areas, such as verapamil, cyclosporin A, quinidine or tamoxifen. Clinical trials have been performed on these drugs with the aim of reversing drug-resistance, but their toxicity was often too high. Therefore pharmaceutical firms have preferred to evaluate either analogues of these drugs, or compounds specifically designed for resistance reversal. Drugs that have clearly shown a potential for sensitisation of resistant cancers with acceptable toxicity include dexverapamil one of the two enantiomers constituting verapamil, valspodar (PSC-833), an analogue of cyclosporine A, and original compounds, named VX-710 and GF-120918. Positive results have most often been obtained in haematological malignancies (myelomas, lymphomas and acute myeloblastic leukaemias), but sometimes also in solid tumours (breast and ovarian carcinomas). Randomised Phase III studies are ongoing for compounds showing a definite activity in Phase II studies, with the aim of analysing the benefits of the combination of an MDR reverter and conventional chemotherapy, in terms of patients' survival. However, drug-resistance is a multifactorial phenomenon, with MDR constituting only part of it. In addition, a rigorous clinical evaluation of MDR will have to be performed, which has not always been the case in early trials.

A phase I and pharmacologic study of the MDR converter GF120918 in combination with doxorubicin in patients with advanced solid tumors

BACKGROUND

Resistance to chemotherapy can partly be explained by the activity of membrane bound P-glycoprotein. Competitive inhibition of P-glycoprotein, by multidrug resistance (MDR) converters, may overcome this MDR. Previously studied MDR converters either have serious intrinsic side effects or considerably influence the pharmacokinetics of cytotoxic agents at concentrations theoretically required to convert MDR. GF120918 is a third-generation MDR converter with high affinity for P-glycoprotein and can be given orally. We performed a phase 1 study with escalating doses of GF120918 in combination with doxorubicin.

PATIENTS AND METHODS

The study group comprised 46 patients with advanced solid tumors. Doxorubicin was administered on day 1 (cycle 1), GF120918 on days 22-24 (cycle 2), and on days 29-33 with doxorubicin administered on day 31 (cycle 3). Pharmacokinetics of both GF120918 and doxorubicin were studied. The starting daily dose of GF120918 was 50 mg and was to be increased in subsequent cohorts until a steady state plasma level of 100 ng/ml was reached. The starting dose of doxorubicin was 50 mg/m2 and was to be increased after reaching the target dose level of GF120918.

RESULTS

In 37 of the 46 patients, full pharmacokinetic data from the three scheduled cycles were obtained. Pharmacokinetics of GF120918 showed a less than linear increase in Cmax with increasing doses, with considerable interpatient variation. The target steady-state plasma level for GF120918 was exceeded in 12 out of 19 patients who received 400 mg GF120918 alone twice daily and in 12 of 17 patients who received 400 mg GF120918 twice daily in combination with doxorubicin. GF120918 pharmacokinetics were not influenced by coadministration of doxorubicin. The doxorubicin AUC was only marginally influenced by GF120918 and only at the highest dose levels. In these patients there was a significant increase in the AUC of doxorubicinol in cycle 3 as compared to cycle 1. Hematologic toxicity mainly consisted of neutropenia and was more severe in cycle 3 than in cycle 1 (13 vs 5 patients with grade 4 neutropenia, P=0.003). Neutropenic fever was the dose-limiting toxicity at a doxorubicin dose of 75 mg/m2 with 400 mg GF120918 twice daily. The toxicity of GF120918 was limited to somnolence in eight patients and occasional gastrointestinal complaints.

CONCLUSION

GF120918 is an MDR converter with only minimal side effects at a dose level yielding concentrations able to convert the action of P-glycoprotein in vitro. A doxorubicin dose of 60 mg/m2 on day 3 in combination with 400 mg GF120918 twice daily on days 1-5 is an acceptable regimen for further clinical trials.

Identification of two distinct intracellular sites that contribute to the modulation of multidrug resistance in P388/ADR cells expressing P-glycoprotein

Although the ability of chemosensitizers to modulate P-glycoprotein (PGP)-based multidrug resistance (MDR) has been extensively studied, relatively little is known about the cellular pharmacology of the PGP inhibitors themselves in MDR cells. The studies described here have correlated the in vitro accumulation and retention properties of verapamil (VRP) in murine P388 (sensitive) and P388/ADR (MDR) cells with doxorubicin (DOX) uptake and cytotoxicity modulation characteristics in order to better understand VRP-tumor cell interactions that give rise to MDR modulation. VRP is rapidly taken up by DOX-sensitive and -resistant P388 cells where greater than 50% maximal VRP uptake occurs within 10 min of initial exposure at 37 degrees C. Whereas chemosensitization and DOX uptake in P388/ADR cells increase with increasing VRP concentration until a plateau is achieved at approximately 5 microM VRP, cellular modulator levels increase proportionally with increasing VPR concentrations beyond 20 microM. Subsequent to removal of noncell-associated modulator, VRP levels in both sensitive and resistant cells rapidly fall below 10% of those obtained at uptake equilibrium. However, a residual amount of VRP remains associated with the cells for extended time periods after the cells are washed. Pulse exposures of P388/ADR cells to high concentrations of VRP (50-100 microM) are capable of providing extended cell-associated VRP levels comparable to those obtained with continuous exposure at biologically active VRP concentrations (1-3 microM) and this leads to chemosensitization. These results are consistent with the existence of high- and low-affinity intracellular VRP pools in P388 MDR cells, both of which can contribute to the reversal of drug resistance. It is suggested that these properties should be taken into consideration during the design and evaluation of preclinical in vivo MDR models where pulsed exposure to high concentrations of resistance modulators often occurs. Special attention must be given to whether such high concentration pulses are desirable and/or achievable in relevant clinical settings.

Modulation of doxorubicin sensitivity by cyclosporine A in hepatocellular carcinoma cells and their doxorubicin-resistant sublines

BACKGROUND AND AIMS

Cyclosporine A (Cys) and verapamil (Ver) sensitize multidrug-resistant (MDR) cells to various anticancer drugs by interacting with membrane glycoproteins involved in the drug efflux. In the present study, we assessed the effect of Cys on the modulation of doxorubicin (DOR) sensitivity in hepatocellular carcinoma (HCC) cell lines, and their DOR-resistant sublines.

METHODS

The sensitivity to DOR and the chemosensitizing effects of Cys were assessed by using two human HCC cell lines, PLC/PRF/5 and Hep-3B, and their DOR-resistant sublines, PLC/DOR and 3B/DOR. The expression of multidrug resistance 1 (MDR1) and multidrug resistance-associated protein (MRP) mRNA in these cells were detected by using a RT-PCR. The HCC cell lines grown in individual wells of 24-well plates were incubated with DOR that were sequentially diluted in culture medium in combination with 5 micromol/L Cys for 24 h. The cell viability in each well was measured by using a 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) assay.

RESULTS

The mRNA of MDR1 and that of MRP were readily detectable in the HCC cell lines by RT-PCR. When 5 micromol/L Cys was added to the culture, the 50% inhibiting concentration (IC50) of DOR was reduced from 0.93 +/- 0.29 microg/mL to 0.32 +/- 0.10 microg/mL in PLC/PRF/5, and from 0.25 +/- 0.07 microg/mL to 0.09 +/- 0.04 microg/mL in Hep-3B. Furthermore, in the presence of 5 micromol/L Cys, the IC50 of DOR was reduced from 48.63 +/- 17.04 microg/mL to 0.49 +/- 0.14 microg/mL in PLC/DOR, and from 4.60 +/- 1.22 microg/mL to 0.15 +/- 0.06 microg/mL in 3B/DOR. The amounts of PCR products of MDR1 mRNA in PLC/DOR and 3B/DOR were greater than those in PLC/PRF/5 and Hep-3B, respectively.

CONCLUSIONS

In HCC, the amplification of MDR1 mRNA is probably the main mechanism underlying acquired DOR resistance. Cyclosporine is also indicated to be highly active in potentiating the anticancer activity of DOR in HCC cells and their DOR-resistant sublines.

Phospholipids as multidrug resistance modulators of the transport of epirubicin in human intestinal epithelial Caco-2 cell layers and everted gut sacs of rats

Phospholipids have been increasingly used as carriers for the delivery of a variety of drugs. Studies using cancer chemotherapeutic agents such as epirubicin encapsulated in liposomes, which are made of phospholipids and other ingredients, have generally shown reduced toxicity and enhanced therapeutic efficacy. The recent investigation of the role of P-glycoprotein (P-gp) in phospholipid translocation has opened a new area of research on the possible use of phospholipids as multidrug resistance (MDR) modulators. This study investigated the effects of liposomal encapsulation, empty liposome pretreatment, or free lipid pretreatment on the uptake and transport of epirubicin in the human colon adenocarcinoma cell line Caco-2 and in everted gut sacs of rat jejunum and ileum. Epirubicin uptake experiments, using a flow cytometer, showed that both liposomal encapsulation and empty liposome pretreatment increased the intracellular accumulation of epirubicin in Caco-2 cells significantly. These two treatments substantially increased apical-to-basolateral absorption of epirubicin across Caco-2 monolayers and markedly improved mucosal-to-serosal absorption of epirubicin in rat jejunum and ileum. Enhancement also was observed with both liposome encapsulation and empty liposome pretreatment in the reduction of basolateral-to-apical efflux of epirubicin across Caco-2 monolayers. However, because diffusion of free dipalmitoyl phosphatidylcholine (DPPC) or dipalmitoyl phosphatidylethanolamine (DPPE) lipids across the cell membrane is very slow, these free lipids showed marginal effects on absorption and/or secretion of epirubicin in both Caco-2 cells and rat gut sacs. The study suggests that inhibition of P-gp or other transporter proteins located in the intestines may be partially involved in the reduction of epirubicin efflux. In conclusion, the therapeutic efficacy of epirubicin may be improved by using phospholipids as excipients and MDR modulators in the formulations. Liposomal formulations may have important applications to circumvent drug resistance in cancer chemotherapy.

Multidrug resistance (MDR) in cancer. Mechanisms, reversal using modulators of MDR and the role of MDR modulators in influencing the pharmacokinetics of anticancer drugs

In recent years, there has been an increased understanding of P-glycoprotein (P-GP)-mediated pharmacokinetic interactions. In addition, its role in modifying the bioavailability of orally administered drugs via induction or inhibition has been also been demonstrated in various studies. This overview presents a background on some of the commonly documented mechanisms of multidrug resistance (MDR), reversal using modulators of MDR, followed by a discussion on the functional aspects of P-GP in the context of the pharmacokinetic interactions when multiple agents are coadministered. While adverse pharmacokinetic interactions have been documented with first and second generation MDR modulators, certain newer agents of the third generation class of compounds have been less susceptible in eliciting pharmacokinetic interactions. Although the review focuses on P-GP and the pharmacology of MDR reversal using MDR modulators, relevance of these drug transport proteins in the context of pharmacokinetic implications (drug absorption, distribution, clearance, and interactions) will also be discussed.

Development of multidrug-resistance convertors: sense or nonsense?

This review describes the clinical relevance of the two drug transporters P-glycoprotein (Pgp) and multidrug resistance-associated protein (MRP) and the in vitro phenomenon which is referred to as multidrug resistance (MDR). The attempts to try to block these resistance mechanisms are summarized with specific attention for the intentionally designed "second generation" MDR-convertors. Potential explanations of the limited clinical success rate are given and recommendations for the design of future studies provided.

Melphalan resistance and photoaffinity labelling of P-glycoprotein in multidrug-resistant Chinese hamster ovary cells: reversal of resistance by cyclosporin A and hyperthermia

The multidrug resistance phenotype is often associated with overexpression of P-glycoprotein, an energy-dependent efflux pump responsible for decreased intracellular accumulation of chemotherapeutic agents. The role of P-glycoprotein in the mechanism of cross-resistance to melphalan in multidrug-resistant Chinese hamster ovary cells (CH(R)C5) was investigated by photoaffinity labelling of P-glycoprotein using [3H]azidopine. We investigated whether the chemosensitiser cyclosporin A and hyperthermia, either used alone or combined, could reverse melphalan resistance and alter transport processes for [14C]melphalan in CH(R)C5 cells. Melphalan inhibited azidopine photolabelling of P-glycoprotein, implicating drug efflux mediated by P-glycoprotein in the mechanism of melphalan resistance in CH(R)C5 cells. Azidopine photolabelling also was inhibited by the chemosensitiser cyclosporin A, which binds to P-glycoprotein. Cyclosporin A alone reversed melphalan resistance in CH(R)C5 cells, but had no effect in drug-sensitive AuxB1 cells. Hyperthermia (40-45 degrees) alone increased melphalan cytotoxicity in both cell lines. When hyperthermia was combined with cyclosporin A, a large increase in melphalan cytotoxicity occurred, but only in CH(R)C5 cells. This effect increased with temperature and exposure time. Sensitisation to melphalan cytotoxicity by heat and cyclosporin A in CH(R)C5 cells appeared to be explained by altered drug transport processes. Lower accumulation of melphalan occurred in CH(R)C5 cells than in drug-sensitive cells. At 37 degrees, cyclosporin A increased drug accumulation in CH(R)C5 cells, but not in AuxB1 cells, by slowing drug efflux from cells. Heat alone increased both melphalan uptake and drug efflux for both cell lines. Our findings suggest that the combination of cyclosporin A and hyperthermia could be very useful in overcoming melphalan resistance by increasing intracellular drug accumulation in multidrug-resistant cells.

Multidrug resistance in oncology: diagnostic and therapeutic approaches

Resistance to anticancer drugs is often mediated by the overexpression of a membrane pump able to extrude many xenobiotics out of the tumour cells. The most frequently expressed of these pumps is called P-glycoprotein and is encoded by a gene called MDR1 (for multidrug resistance). There could be great clinical interest for investigating the expression of this gene or of its product in patients' tumours, as well as in developing ways of circumventing this mechanism of resistance. Multidrug resistance can be diagnosed in tumours by molecular biology techniques (gene expression at the mRNA level), by immunological techniques (quantification of P-glycoprotein itself) or by functional approaches (measuring dye exclusion). Numerous studies have tried to use the MDR status of tumours as a predictor of response to treatment, but they have not yet reached definitive conclusions to allow the use of this approach in routine determinations. This is because no consensus has emerged concerning the optimal technique and the best conditions for MDR determination. Continuous efforts are still required for defining appropriate standardization of the techniques. The development of MDR modulators for the treatment of resistant tumours is a promising approach requiring rigorous clinical trials with successive phase I, phase II and phase III studies. Phase I can be omitted when the reverter is already being used in therapeutics; phase II should be performed using a sequential design, in order to prove the inefficacy of the anticancer therapy before combining it to a modulator; and phase III must only be undertaken after the demonstration that responders can be recruited by the combination. However, the effect of some reverters on anticancer drug pharmacokinetics may hamper rapid evaluation. Several drugs are good candidates for MDR modulation, but definitive results are still lacking for the introduction of such combinations in standard therapeutic protocols.

Epirubicin has modest single-agent activity in head and neck cancer but limited activity in metastatic melanoma and colorectal cancer: phase II studies by the Eastern Cooperative Oncology Group

Epirubicin (4'-epidoxorubicin), a diastereoisomer of doxorubicin, has established activity in the treatment of many cancer types sensitive to doxorubicin. Its activity in other tumor targets such as melanoma, head and neck cancer, and recurrent colorectal cancer has been less well defined. Three concurrent phase II studies examined the efficacy and toxicity of epirubicin (90 mg/m2 given intravenously at 3-week intervals) in the treatment of 71 patients with the aforementioned cancers. Of 66 eligible patients who were assessable for response, one patient (with colorectal cancer) achieved a complete response and three patients (with head and neck cancer) achieved partial responses. The response rate in patients with head and neck cancer was 18% (95% confidence interval, 4-43%). Myelosuppression, alopecia, and nausea were the most frequent toxicities. Two patients died of neutropenic sepsis and grade IV leukopenia occurred in six patients (8%). Grade III toxicities were as follows: leukopenia (17%), anemia (10%), alopecia (8%), fever (1%), thrombocytopenia (1%). Grade I or II cardiac toxicity was noted in four patients at cumulative doses ranging between 375 mg/m2 to 1,283 mg/m2. Epirubicin is ineffective as a single agent at this dose and schedule in the treatment of patients with melanoma and colorectal cancer. In head and neck cancer, a modest response rate encourages further exploration of epirubicin and related anthracyclines in combination regimens.

Multiple resistance modulators combined with carboplatin for resistant malignancies: a pilot study

BACKGROUND

Chemotherapy resistance is probably multifactorial; hence, we assessed the feasibility of adding to carboplatin 6 concurrent resistance modulators in 53 patients with resistant cancers.

METHODS

Pentoxifylline and dipyridamole were added to carboplatin 400 mg/m2 in cohort 1, and metronidazole was also given in cohort 2. Mannitol and saline were administered in each cohort with the theoretical objective of improving carboplatin delivery to tumors by reducing blood viscosity. Because of excessive toxicity in cohort 2, cohort 3 received the same modulators as in cohort 2 but with a reduced dose of carboplatin (200 mg/m2). Subsequent patients had the following drugs added to those in the previous cohort: novobiocin (cohort 4), tamoxifen (cohort 5), ketoconazole (cohort 6). Cohort 7 patients received the 6 cohort 6 modulators along with carboplatin 300 mg/m2.

RESULTS

Thrombocytopenia was excessive in early cohorts with a carboplatin dose of 400 mg/m2, but was minimal at lower doses. Other toxicity was generally tolerable and reversible, particularly at carboplatin doses < or = 300 mg/m2, although gastrointestinal and neurological toxicity tended to worsen as additional modulators were added. No major responses (but 4 minor responses) were seen in this patient population with heavily pretreated or primarily resistant cancers.

CONCLUSIONS

Acceptable doses for phase II studies are carboplatin 300 mg/m2, 20% mannitol 250 ml plus normal saline 500 ml over 1 hr prior to carboplatin, pentoxifylline 700 mg/m2/day p.o. from 3 days before carboplatin to cessation of therapy, dipyridamole 100 mg/m2 p.o. q6h x 6 days starting 24 hr before carboplatin, metronidazole (750 mg/m2 p.o. 12 hr and immediately before, and 24 hr after carboplatin; 250 mg/m2 suppository p.r. 12 hr and immediately before, and 6 and 24 hr after carboplatin; and 500 mg/m2 i.v. right after carboplatin), novobiocin 600 mg/m2 p.o. q12h x 6 days starting 24 hr before carboplatin, and tamoxifen 100 mg/m2/day plus ketoconazole 700 mg/m2/day x 3 days starting the day before carboplatin, with oral dexamethasone and ondansetron as antimetics.

In vivo model systems in P-glycoprotein-mediated multidrug resistance

In this article we review the in vivo model systems that have been developed for studying P-glycoprotein-mediated multidrug resistance (MDR) in the preclinical setting. Rodents have two mdr genes, both of which confer the MDR phenotype: mdr 1a and mdr 1b. At gene level they show strong homology to the human MDR1 gene and the tissue distribution of their gene product is very similar to P-glycoprotein expression in humans. In vivo studies have shown the physiological roles of P-glycoprotein, including protection of the organism from damage by xenobiotics. Tumors with intrinsic P-glycoprotein expression, induced MDR or transfected with an mdr gene, can be used as syngeneic or xenogenic tumor models. Ascites, leukemia, and solid MDR tumor models have been developed. Molecular engineering has resulted in transgenic mice that express the human MDR1 gene in their bone marrow and in knockout mice missing a murine mdr gene. The data on pharmacokinetics, efficacy, and toxicity of chemosensitizers of P-glycoprotein in vivo are described. Results from studies using monoclonal antibodies directed against P-glycoprotein and other miscellaneous approaches for modulation of MDR are mentioned. The importance of in vivo studies prior to clinical trials is being stressed and potential pitfalls due to differences between species are discussed.

Reversal activity of cyclosporin A and its metabolites M1, M17 and M21 in multidrug-resistant cells

Cyclosporin A (CSA) is an effective inhibitor of the P-glycoprotein (P-gp) activity and has been shown to modulate multidrug resistance (MDR) in in vitro experimental models. During degradation of CSA, the metabolites arising from the parental compound reach high levels in the serum of patients, and it is not clear whether these metabolites maintain the reversal activity of the parental compound, like the metabolites of verapamil. In an in vitro experimental model, we compared the reversal activity of CSA and 3 CSA metabolites (M1, M17, and M21) in the range of concentrations obtained in whole blood during a clinical trial with CSA used as a revertant agent. As experimental model we used LoVo-resistant cells. Our in vitro studies indicated that the metabolic hydroxylation and demethylation of CSA lead to molecules that greatly differ from the parent drug in their reversal activity. In the range of concentration detected in the whole blood of the patients (1-3 microM), CSA had a significant reversal activity. It decreased the IC50 of antineoplastic drugs involved in MDR (vincristine, taxol, doxorubicin and etoposide) but not the IC50 of platinum or methotrexate. CSA increased intracellular doxorubicin content and inhibited P-gp 3[H]azidopine photolabeling. Conversely, CSA metabolite concentrations superimposable to those observed in the patients (0.5-2.2 microM) had no sensitizing effects on the cytotoxicity of MDR-related anti-neoplastic drugs, nor did they affect 3[H]azidopine photolabeling or doxorubicin uptake. This study demonstrates that, during degradation of CSA, metabolite derivatives arise that have a very different reversal activity from that of the parental compound.

HPLC method for monitoring SDZ PSC 833 in whole blood

P-glycoprotein (Pgp) is a 170-kDa membrane transporter that mediates drug efflux and is an effector of multidrug resistance. SDZ PSC 833 (PSC), a nonimmunosuppressive cyclosporine that potently modulates Pgp, is currently under clinical evaluation in patients with cancer. We have developed a reversed-phase HPLC assay for determining PSC blood concentrations that utilizes a step gradient with linear segments to resolve PSC into two distinct peaks (likely to be keto and enol isomers). To clinically validate the assay, PSC concentrations were obtained by HPLC from nine patients receiving oral doses of 5 mg/kg every 6 h. Values ranged from 0.91 to 5.4 mg/L during the dosing period, comparable with concentrations of PSC that modulate Pgp in vitro. In addition, we investigated the immunoreactivity of the Abbott TDx cyclosporin A (CsA) monoclonal whole-blood assay for PSC. The TDx CsA assay cross-reacts ∼17% with PSC as determined by adding known amounts of PSC to whole blood. When PSC concentrations obtained by the TDx CsA assay were divided by 0.17, we found agreement between the TDx CsA assay and the HPLC PSC assay for samples from nine patients.

Cyclosporin A as a multidrug-resistant modulator in patients with renal cell carcinoma treated with teniposide

Patients with refractory metastatic renal cell carcinoma (RCC) were enrolled in a phase II study with teniposide (VM26) and cyclosporin A (CSA) to investigate (1) the effect of CSA on the response rate to VM26; and (2) the effect of CSA on the pharmacokinetics and pharmacodynamics of VM26. Sixteen patients initially received VM26 alone (200 mg m(-2) day(-1) i.v.). No objective responses were observed and all patients crossed over to receive at least an additional two courses (range 2-5) of VM26 plus CSA (5 mg kg(-1) 2h(-1) followed by 30 mg kg(-1) 48h(-1) i.v.). At the end of the 2-h loading dose of CSA, whole-blood CSA levels ranged from 2250 to 3830 ng ml(-1), whereas at the end of the 48-h CSA infusion, CSA ranged from 1830 to 4501 ng ml(-1). CSA significantly (P<0.01) increased the area under the curve (AUC) of VM26. The variation in the paired AUC of VM26 was 50%. Terminal half-life of VM26 was significantly (P<0.01) increased (1.72-fold) after CSA administration, whereas the systemic clearance of VM26 was decreased by 1.4-fold (P<0.01). The nadir neutrophil count after VM26 plus CSA (median 700 microl(-1), range <100 to 2860 microl(-1)) was lower than after VM26 alone (median 1900 microl(-1), range 200 to 6000 microl(-1)). Increased haematological toxicity after CSA could be explained by the increase in the VM26 AUC and by inhibition of P-glycoprotein (P-gp) activity in haematopoietic precursor cells. Bilirubin concentrations in the serum were increased after VM26 plus CSA compared with VM26 alone (P<0.01). Among the 15 patients evaluable for response, one had a minor response, eight had stable disease, and six had progressive disease. In conclusion, the dose of CSA we used achieved plasma concentrations within the effective range for P-gp inhibition. CSA affected both the pharmacokinetics and pharmacodynamics of VM26 in the patients, principally by increasing the plasma concentrations of the antineoplastic drug and VM26 haemopoietic toxicity.

Expression of the low-density lipoprotein receptor, HMG-CoA reductase, and multidrug resistance (Mdr1) genes in colorectal carcinomas

Some malignant cells have elevated uptake of plasma low-density lipoprotein (LDL). We determined the expressions in colorectal cancers of the LDL receptor gene, of the gene for the rate-limiting enzyme in cholesterol synthesis, 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase, and of the multidrug resistance gene (mdr1) by quantitative RNA-RNA solution hybridisation. LDL receptor RNA levels in tumor tissue exceeded those in normal mucosa in 20 of 23 patients (2-11-fold higher in 17 of 23 patients), with a mean +/- SD of 7.8 +/- 5.8 copies/cell in tumor tissue vs 3.5 +/- 2.5 in normal mucosa (P = 0.002). The HMG-CoA reductase gene was similarly expressed in tumor and normal tissue, with means and SD of 2.0 +/- 1.3 copies/cell versus 2.2 +/- 1.9 (pi = 21). Mdr1 RNA was undetectable ( < 0.15 copies/cell) in 5 of 20 tumors, with a mean +/- SD of 1.0 +/- 1.1 copies/cell vs 1.6 +/- 1.7 in normal mucosa. Expression of all three genes was, in most cases, higher in normal liver than in liver metastasis of colorectal carcinomas or normal colon mucosa. The results may form the basis for using LDL as a drug carrier for treatment of colorectal carcinomas, and may indicate that drug resistance in these tumors is not due to overexpression of the mdr1 gene.

Molecular mechanisms of multidrug resistance in cancer chemotherapy

The occurrence of multidrug resistance (MDR) is one of the main obstacles in the successful chemotherapeutic treatment of cancer. MDR cell lines are resistant to the so-called naturally occurring anti-cancer drugs, such as anthracyclines, Vinca alkaloids and epipodophyllotoxins, but are not cross-resistant to alkylating agents, antimetabolites and cisplatin. So far, three separate forms of MDR have been characterized in more detail: classical MDR, non-Pgp MDR and atypical MDR. Although all three MDR phenotypes have much in common with respect to cross-resistance patterns, the underlying mechanisms certainly differ. Atypical MDR is associated with quantitative and qualitative alterations in topoisomerase II alpha, a nuclear enzyme that actively participates in the lethal action of cytotoxic drugs. Atypical MDR cells do not overexpress P-glycoprotein, and are unaltered in their ability to accumulate drugs. In this review we will focus on classical and non-Pgp MDR. The molecular mechanism of classical and non-Pgp MDR is transcriptional activation of membrane-bound transport proteins. These transport proteins belong to the ATP-binding cassette (ABC) superfamily of transport systems. The classical MDR phenotype is characterized by a reduced ability to accumulate drugs, due to activity of an energy-dependent uni-directional, membrane-bound, drug-efflux pump with broad substrate specificity. The classical MDR drug pump is composed of a transmembrane glycoprotein (P-glyco-protein-Pgp) with a molecular weight of 170 kD, and is, in man, encoded by the so-called multidrug resistance (MDR1) gene. Typically, non-Pgp MDR has no P-gly-coprotein expression, yet has about the same cross-resistance pattern as classical MDR. This non-Pgp MDR phenotype is caused by overexpression of the multidrug resistance-associated protein (MRP) gene, which encodes a 190 kD membrane-bound glycoprotein (MRP). MRP probably works by direct extrusion of cytotoxic drugs from the cell and/or by mediating sequestration of the drugs into intracellular compartments, both leading to a reduction in effective intracellular drug concentrations. For the classical MDR phenotype, evidence is accumulating that it plays a role indeed, in clinical drug resistance, especially in some hematological malignancies (acute myeloid leukemia, multiple myeloma and non-Hodgkin's lymphoma) and solid tumors (soft tissue sarcomas and neuroblastoma). The association of MRP with clinical drug resistance has not been elaborated, yet, and studies on MRP expression in human cancer have just begun. We found that overexpression of MRP, as determined by RNase protection assay as well as by immunohistochemistry, occurs in several human cancers, among which are cancer of the lung, esophagus, breast and ovary, and leukemias. Further studies are indicated to establish whether elevated MRP expression at diagnosis is an unfavorable prognostic factor for clinical outcome of chemotherapy.

Circumvention of multidrug resistance in neoplastic cells through scavenger receptor mediated drug delivery

A conjugate of the antineoplastic drug daunomycin (DNM) with maleylated bovine serum albumin (MBSA-DNM) was taken up with high efficiency by a multidrug resistant variant, JD100, of the murine-macrophage tumour cell line, J774A.1, through the scavenger receptors resulting in cessation of DNA synthesis. In contrast, free DNM at similar concentrations did not affect the incorporation of [3H]thymidine by these cells. These results suggest that receptor-mediated intracellular delivery of antineoplastic drugs could be a viable and new approach for overcoming the problem of multidrug resistance in chemotherapy of neoplastic diseases.

Cyclosporin A and doxorubicin-ifosfamide in resistant solid tumours: a phase I and an immunological study

In order to test whether circumvention of clinical resistance can be obtained in common solid tumours by targeting different drug resistance mechanisms, a phase I clinical and immunological study was designed. The purpose of the study was to determine the dose of cyclosporin A (CsA), in combination with doxorubicin (DOX) and ifosfamide (IFX), needed to achieve steady-state whole-blood levels of 2000 ng ml-1 and the associated toxicity of this combination. Treatment consisted of CsA 5 mg kg-1 as a 2 h loading infusion, followed by a CsA 3 day continuous infusion (c.i.) (days 1-3) at doses that were escalated from 10 to 18 mg kg-1 day-1. Chemotherapy consisted of DOX 55 mg m-2 by i.v. 24 h c.i. (day 2) and IFX 2 g m-2 i.v. over 1 h on days 1 and 3. Treatments were repeated every 4 weeks. Eighteen patients with previously treated resistant solid tumours received 39 cycles. Mean steady-state CsA levels > or = 2000 ng ml-1 were reached at 5 mg kg-1 loading dose followed by a 3 day c.i. of 16 mg kg-1 day-1 or greater. Haematological toxicity was greater than expected for the same chemotherapy alone. One patient died of intracranial haemorrhage due to severe thrombopenia. Other observed toxicities were: asymptomatic hyperbilirubinaemia (46% cycles), mild nephrotoxicity (20% cycles), hypomagnesaemia (72% cycles), mild increase in body weight (100% cycles), hypertension (15% cycles) and headache (15% cycles). Overall the toxicity was acceptable and manageable. No alterations in absolute lymphocyte number, the lymphocyte subsets studied (CD3, CD4, CD8, CD19) or CD4/CD8 ratio were observed in patients receiving more than one treatment cycle, although there were significant and non-uniform variations in the values of the different lymphocyte subsets studied when pre- and post-treatment values were compared. There was also a significant increase in the CD4/CD8 ratio. Tumour regressions were observed in two patients (epidermoid carcinoma of the cervix and Ewing's sarcoma). The CsA dose recommended for phase II trials is a 5 mg kg-1 loading dose followed by a 3-day c.i. of 16 mg kg-1 day-1 simultaneously with DOX and IFX at the doses administered in this study.

MDR1/P-glycoprotein expression in colorectal cancer

Drug resistance to multiple chemotherapeutic agents is considered a major cause of chemotherapy failure. An extensively studied and relatively well understood type of cellular drug resistance is P-glycoprotein (Pgp)-mediated multidrug resistance (MDR). Pgp acts as an energy-dependent drug efflux pump, thereby decreasing the intracellular drug concentration and causing drug resistance, in in vitro experiments. Colorectal cancer and colorectal mucosa generally express high levels of Pgp, and this may contribute to the general unresponsiveness of colorectal cancer to natural product, anticancer drugs. The controversies concerning the prognostic role of Pgp expression and its contribution to tumour aggressiveness, and possible reasons for the disappointing results of clinical MDR reversal trials in colorectal cancer are discussed.

The chemosensitizer cyclosporin A enhances the toxic side-effects of doxorubicin in the rat

The feasibility of using chemosensitizers in the circumvention of P-glycoprotein-mediated multidrug resistance has been shown in many studies. We recently reported on the chemosensitizing effect of cyclosporin A (CsA) on doxorubicin in a rat solid tumour model. Using the same experimental design we investigated the side-effects of the combination treatment. During the 35-day experiment doxorubicin treatment caused dose-dependent weight loss, which was enhanced by combination treatment with CsA. The main doxorubicin-related side-effects were myelosuppression (transient leucopenia and thrombopenia) and nephrotoxicity. Damage to the kidney was severe, leading to a nephrotic syndrome and resulting in ascites, pleural effusion, hypercholesterolaemia and hypertriglyceridaemia. These toxicities were enhanced by the addition of the chemosensitizer CsA. Mild doxorubicin-related cardiomyopathy and minimal hepatotoxicity were seen on histological examination. There were no signs of enhanced toxicity of the combination treatment in tissues with known high expression levels of P-glycoprotein, like the liver, adrenal gland and large intestine. CsA had a low toxicity profile, as it only caused a transient rise in bilirubin. In conclusion, the chemosensitizer CsA enhanced the side-effects of the anticancer drug doxorubicin without altering the toxicity pattern. There was no evidence of a therapeutic gain by adding CsA to doxorubicin, compared to single-agent treatment with doxorubicin in 25%-33% higher doses, because of the enhanced toxicity of the combination treatment.

Cyclosporin A enhances susceptibility of multi-drug resistant human cancer cells to anti-P-glycoprotein antibody-dependent cytotoxicity of monocytes, but not of lymphocytes

Cyclosporin A (CsA) was previously found to bind to P-glycoprotein expressed on multidrug-resistant (MDR) cancer cells. In the present study, the effect of CsA on anti-P-glycoprotein monoclonal antibody (mAb)-dependent cell-mediated cytotoxicity (ADCC) against human MDR cells was examined. The ADCC reaction was assessed by 4-h 51Cr-release assay. Highly purified lymphocytes (> 99%) and monocytes (> 99%) obtained from blood mononuclear cells (MNC) of healthy donors were used as effector cells. CsA decreased the cytotoxic activity of MNC against MDR cells, but enhanced their ADCC activity in the presence of anti-P-glycoprotein mAb MRK16. Lymphocyte-mediated ADCC and natural killer activity against MDR cells were also suppressed by addition of CsA. CsA induced a significant dose-dependent increase in monocyte-mediated ADCC activity. Interestingly, pretreatment of MDR cancer cells, but not of monocytes, with CsA significantly enhanced ADCC activity mediated by monocytes, but not by lymphocytes. A CsA analog (PSC833) and FK-506, but not verapamil also increased the sensitivity of MDR cells to ADCC by monocytes. CsA did not affect the binding of monocytes to MDR cells in the presence of MRK16 mAb. These results indicate that CsA may directly enhance the susceptibility of MDR cancer cells to the monocyte-mediated ADCC reaction.

Reversal of the human and murine multidrug-resistance phenotype with megestrol acetate

MA is an orally active PG derivative with an excellent safety profile that is used primarily for the treatment of carcinomas of the breast and endometrium. We investigated the potential application of MA as an MDR-reversal agent using cell culture and human tumor xenograft models. The reversing activity of MA in vitro was compared with that of PG and VER in two human MDR cell lines, the colon carcinoma HCT-116/VM46 and the breast carcinoma MCF-7/ADR, and in a murine cell line, J774.2. At concentrations as low as 3 microM, MA was capable of partially restoring sensitivity to Act D in the HCT-116/VM46 cells and sensitivity to DOX in the MCF-7/ADR cells. Although less effective than VER, MA was about 2.5 times more potent than PG in reversing MDR at equimolar concentrations. Increased accumulation of DOX in drug-resistant cells that were treated simultaneously with MA was observed by flow cytometry. In vivo, using established human colon and breast carcinoma xenografts implanted s.c. in athymic mice, the combined therapy with MA and DOX resulted in enhanced antitumor activity relative to that of DOX alone in the MDR sublines. These results suggest that MA may be a promising clinical MDR-reversing agent.

In vitro and in vivo chemosensitizing effect of cyclosporin A on an intrinsic multidrug-resistant rat colon tumour

Colon tumours are intrinsically resistant to chemotherapy and most of them express the multidrug transporter P glycoprotein (Pgp). Whether this Pgp expression determines their resistance to anticancer agents in patients is not known. We report here on the reversibility of intrinsic multidrug resistance in a syngeneic, solid tumour model. CC531 is a rat colon carcinoma that expresses Pgp, as was shown with the monoclonal antibody C-219. In vitro the sensitivity to doxorubicin, daunorubicin and colchicine was enhanced by the addition of the chemosensitizers verapamil and cyclosporin A (CsA), while the sensitivity to cisplatin was not enhanced. In a daunorubicin accumulation assay verapamil and CsA enhanced the daunorubicin content of CC531 cells. In vivo CsA was injected intramuscularly for 3 consecutive days at a dose of 20 mg kg-1 day-1. This resulted in whole-blood CsA levels above 2 mumol/l, while intratumoral CsA levels amounted to 3.6 mumol/kg. In a subrenal capsule assay the maximal tolerable dose of doxorubicin (4 mg/kg) significantly reduced tumour growth. Doxorubicin at 3 mg/kg was not effective, but in combination with CsA this dose was as effective as 4 mg/kg doxorubicin. These experiments show that adequate doses of the chemosensitizing drug CsA can be obtained in vivo, resulting in increased antitumoral activity of doxorubicin in vivo. The in vitro and in vivo data together suggest that the chemosensitization by CsA is mediated by Pgp. This finding may have implications for the application of CsA and CsA-like chemosensitizers in the clinical setting.

Epirubicin. A review of its pharmacodynamic and pharmacokinetic properties, and therapeutic use in cancer chemotherapy

Epirubicin is the 4' epimer of the anthracycline antibiotic doxorubicin, and has been used alone or in combination with other cytotoxic agents in the treatment of a variety of malignancies. Comparative and noncomparative clinical trials have demonstrated that regimens containing conventional doses of epirubicin achieved equivalent objective response rates and overall median survival as similar doxorubicin-containing regimens in the treatment of advanced and early breast cancer, non-small cell lung cancer (NSCLC), small cell lung cancer (SCLC), non-Hodgkin's lymphoma, ovarian cancer, gastric cancer and nonresectable primary hepatocellular carcinoma. Recently, dose-intensive regimens of epirubicin have achieved high response rates in a number of malignancies including early and advanced breast cancer and lung cancer. The major acute dose-limiting toxicity of anthracyclines is myelosuppression. In vitro and clinical studies have shown that, at equimolar doses, epirubicin is less myelotoxic than doxorubicin. The lower haematological toxicity of epirubicin, as well as the recent introduction of supportive measures such as colony-stimulating factors, has allowed dose-intensification of epirubicin-containing regimens, which is particularly significant because of the definite dose-response relationship of anthracyclines. Cardiotoxicity, which is manifested clinically as irreversible congestive heart failure and/or cardiomyopathy, is the most important chronic cumulative dose-limiting toxicity of anthracyclines. Epirubicin has a lower propensity to produce cardiotoxic effects than doxorubicin, and its recommended maximum cumulative dose is almost double that of doxorubicin, thus allowing for more treatment cycles and/or higher doses of epirubicin. In summary, dose-intensive epirubicin-containing regimens, which are feasible due to its lower myelosuppression and cardiotoxicity, have produced high response rates in early breast cancer, a potentially curable malignancy, as well as advanced breast, and lung cancers. Furthermore, there is evidence to suggest that improved response rates can improve quality of life in some clinical settings, but whether this leads to prolonged survival has not yet been determined. Recently implemented supportive measures such as colony-stimulating factors, prophylactic antimicrobials and peripheral blood stem cell support may help achieve other potential advantages of dose-intensive epirubicin-containing regimens such as reductions in morbidity and length of hospital admissions.

Pharmacologic interactions between the resistance-modifying cyclosporine SDZ PSC 833 and etoposide (VP 16-213) enhance in vivo cytostatic activity and toxicity

Cyclosporin A reverses multidrug resistance (MDR) and increases the in vivo cytostatic activity and toxicity of the anticancer agent etoposide (VP 16-213). SDZ PSC 833 (PSC 833), a non-immunosuppressive, non-toxic cyclosporin and very active modifier of P-gp 170-mediated MDR, elicits similar effects when administered with adriamycin. The underlying mechanisms, however, are not yet understood. The present pharmacological interaction study with PSC 833 and VP 16-213 was carried out to reveal the nature of this enhancement of cytostatic activity and toxicity. Rats pre-treated with either PSC-833 or solvent received a single dose of VP 16-213. Plasma levels of VP 16-213 were measured by high-performance liquid chromatography (HPLC). The resulting increase in cytostatic activity and toxicity of VP 16-213 mediated by PSC 833 was paralleled by marked changes in the pharmacokinetic parameters of VP 16-213 in vivo. Bioavailability and blood levels of VP 16-213 were significantly increased 30 min after administration if PSC 833 had been given before. The disappearance rate of VP 16-213 from the intravascular compartment was considerably slowed down by PSC 833. In drug-sensitive xenografts of human colon carcinoma, the PSC-833-induced pharmacologic changes in vivo could be counteracted by dose reduction of VP 16-213 while a full therapeutic potential was maintained. Doses of VP 16-213, 1.5 to 2 times smaller, combined with PSC 833, were as effective in terms of tumor-growth inhibition as the maximum tolerated dose of VP 16-213 alone. Thus, pharmacologic interactions between PSC 833 or other resistance modifiers and VP 16-213 and other cytostatic agents require careful attention if they are to be used in humans to overcome MDR.

Reversal of typical multidrug resistance by cyclosporin and its non-immunosuppressive analogue SDZ PSC 833 in Chinese hamster ovary cells expressing themdr1 phenotype

The new non-immunosuppressive cyclosporin derivative SDZ PSC 833 (PSC) is a potent agent used to overcome typical multidrug resistance (MDR) associated with overexpression of the mdr1 gene encoding for a P-170 glycoprotein. In the present study, the efficacy of PSC as compared with cyclosporin was determined in Chinese hamster ovary cell lines exhibiting different levels of resistance to colchicine (0, 0.1, 0.2 and 10 micrograms/ml, respectively). Low concentrations of PSC (8.2 nM) increased the cytotoxicity of colchicine in cell lines expressing low levels of drug resistance. The concentration resulting in 50% cell survival (LC50 value) found for colchicine alone or in combination with PSC in the CHO-A3 cell line that was resistant to 100 ng colchicine/ml decreased from greater than 500 to 200 ng/ml at 8.2 nM PSC and to less than 100 ng/ml at 82 and 820 nM PSC. In the CHO-A3 cell line that was resistant to 200 ng colchicine/ml, the LC50 values decreased from greater than 500 ng/ml for colchicine alone to 500 ng/ml for colchicine used in combination with 8.2 nM PSC and to less than 100 ng/ml for colchicine combined with 82 or 820 nM PSC. At a concentration of 82 nM PSC, the maximal effect in MDR reversal was observed in the cell lines exhibiting moderate resistance. In the highly resistant cell line, PSC (820 nM) also reversed colchicine resistance. In drug-accumulation experiments, we obtained a 4-fold increase in intracellular doxorubicin accumulation using 820 nM PSC. A comparison of PSC with cyclosporin revealed that a cyclosporin concentration 20-fold that of PSC was required to obtain the same sensitising effect. On the basis of these data, it may be concluded that PSC is a most promising chemosensitiser.

Cyclosporins as drug resistance modifiers

Cyclosporin A (CsA), a cyclic peptide of 11 amino acids isolated from the fungus Tolypoclodium inflatum Gams, is the principle drug used for immunosuppression in organ transplant patients. It is known to have a very specific effect on T-cell proliferation although the precise mechanism remains unclear. Following internalization, CsA binds to a cytosolic protein, cyclophilin, which has been shown to possess peptidyl-prolyl cis-trans isomerase activity. CsA is an effective modifier of multidrug resistance in human and rodent cells at doses in the range of 1 to 5 micrograms/mL. Although it reverses the drug accumulation deficit associated with multidrug resistance in some cell types, this is not always the case. CsA has P-glycoprotein binding activity but less specific membrane effects and inhibition of protein kinase C may also be involved in its resistance modifier action. A number of non-immunosuppressive analogues of CsA have been shown to have resistance modifier activity and some are more potent than the parent compound. One analogue from Sandoz, PSC-833, has been shown to be approximately 10-fold more potent than CsA and is expected to enter clinical trial in the near future. The use of such agents may allow a full test of the hypothesis that reversal of multidrug resistance will prove a useful clinical strategy.