Adequate tumour quinidine levels for multidrug resistance modulation can be achieved in vivo

Discovery of New 4-Indolyl Quinazoline Derivatives as Highly Potent and Orally Bioavailable P-Glycoprotein Inhibitors

The major drawbacks of P-glycoprotein (P-gp) inhibitors at the clinical stage make the development of new P-gp inhibitors challenging and desirable. In this study, we reported our structure-activity relationship studies of 4-indolyl quinazoline, which led to the discovery of a highly effective and orally active P-gp inhibitor, YS-370. YS-370 effectively reversed multidrug resistance (MDR) to paclitaxel and colchicine in SW620/AD300 and HEK293T-ABCB1 cells. YS-370 bound directly to P-gp, did not alter expression or subcellular localization of P-gp in SW620/AD300 cells, but increased the intracellular accumulation of paclitaxel. Furthermore, YS-370 stimulated the P-gp ATPase activity and had moderate inhibition against CYP3A4. Significantly, oral administration of YS-370 in combination with paclitaxel achieved much stronger antitumor activity in a xenograft model bearing SW620/Ad300 cells than either drug alone. Taken together, our data demonstrate that YS-370 is a promising P-gp inhibitor capable of overcoming MDR and represents a unique scaffold for the development of new P-gp inhibitors.

Future perspectives for the development of P-glycoprotein modulators

Resistance to chemotherapeutic agents constitutes one of the major obstacles to the successful treatment of cancer. While several mechanisms underlying drug resistance have been elucidated, the most widely studied mechanism involves the efflux of antineoplastic drugs from cancer cells by P-glycoprotein, the 170 kD glycoprotein product of the MDR-I gene. The observation that several compounds are able to inhibit P-glycoprotein in vitro created optimism that the problem of multidrug resistance in cancer could be quickly resolved by moving these compounds into the clinic. However, despite a large number of clinical trials with several different putative Pgp modulators, the value of Pgp modulation in clinical oncologic practice remains unresolved. While these initial trials have not answered the question of whether Pgp is an important mechanism of resistance in human cancers, or whether modulation of Pgp is likely to positively impact on the treatment of cancer, they have provided insights regarding the problems inherent in conducting trials of this nature. These clinical insights, along with knowledge gained from continued basic research on drug resistance mediated by Pgp and related transporters, will form a strong foundation for future research into the role of Pgp and Pgp modulation in the treatment of cancer. The ubiquitous nature of transporters and the high prevalence of transporter substrates among antineoplastic drugs, compel the development of modulators that can be used to prevent or reverse drug resistance.

Potent synergic effect between ibuprofen and azoles on Candida resulting from blockade of efflux pumps as determined by FUN-1 staining and flow cytometry

OBJECTIVES

Resistance to antifungals often relates to efflux pumps exporting drugs; several modulators may block them, reverting resistance. Verapamil, beta-oestradiol and progesterone, known efflux pump inhibitors of human neoplastic cells, and ibuprofen were tested as potential modulators of resistance of Candida spp.

METHODS

Forty-two clinical isolates of Candida (38 fluconazole-resistant), two ATCC type strains and two C. albicans strains with known mechanisms of fluconazole resistance were incubated with subinhibitory concentrations of the modulators. After exposure, MICs of fluconazole, itraconazole and voriconazole were re-determined. Simultaneously, yeasts exposed to modulators were stained with FUN-1 and analysed by flow cytometry. 3H-labelled itraconazole was also used to study efflux in the presence and absence of modulators.

RESULTS

Fluconazole MICs decreased in most strains after exposure to modulators, including control strains with documented efflux overexpression. No significant MIC variation was noticed for: all C. krusei strains tested, for the resistant strain by target change, for susceptible strains, and for a very few other clinical isolates. Reverted resistant phenotypes showed cross-resistance to itraconazole and to voriconazole, which was also reverted by the modulators. For these strains, an increase in FUN-1 staining and increased accumulation of 3H-labelled itraconazole were noticed after incubation with modulators.

CONCLUSIONS

Resistance related to overexpression of efflux pumps was common among clinical isolates and could be reverted by the assayed modulators, particularly ibuprofen. The mechanism of resistance in all tested C. krusei and in a few other strains seems, however, to be of a different nature. Ibuprofen is a promising compound in association with azoles, deserving future clinical trials. FUN-1 proved to be a good marker of efflux in Candida.

Optimizing chemotherapy by measuring reversal of P-glycoprotein activity in plasma membrane vesicles

The appearance of multidrug resistance (MDR) of cancer cells is a major obstacle to successful chemotherapy. Several proteins have been identified that pump chemotherapeutic drugs out of cells, thus bringing about MDR. One representative pump is the P-glycoprotein, whose function can be inhibited by blockers (also known as reversers, modulators or chemosensitizers). In clinical application, many of these blockers are often not effective because they become bound to the plasma of the patients. The extent of plasma binding of the blocker varies in different persons and we have developed a 96-well kit to assay such inter-person differences. The assay uses membrane vesicles isolated from a human lymphoblastoid cell line (CEM Col1000). Uptake of rhodamine into the vesicles was measured with different concentrations of the blockers verapamil and XR9576 in presence of human plasma. The reverser XR9576 is nearly 30 times more effective than the classical blocker verapamil, the relevant K(m) values ranging from 2.66 to 45 nM for XR 9576 and 0.7 to 5.5 microM for verapamil. An even greater difference between these two drugs, nearly 1,000-fold, could be shown also in intact cells by calcein AM uptake experiments.

Disposition of docetaxel in the presence of P-glycoprotein inhibition by intravenous administration of R101933

Recently, a study of docetaxel in combination with the new orally administered P-glycoprotein (P-gp) inhibitor R101933 showed that this combination was feasible. However, due to the low oral bioavailability of R101933 and high interpatient variability, no further attempts to increase the level of P-gp inhibition were made. Here, we assessed the feasibility of combining docetaxel with intravenously (i.v.) administered R101933, and determined the disposition of docetaxel with and without the P-gp inhibitor. Patients received i.v. R101933 alone at a dose escalated from 250 to 500 mg on day 1 (cycle 0), docetaxel 100 mg/m(2) as a 1-h infusion on day 8 (cycle 1) and the combination every 3 weeks thereafter (cycle 2 and further cycles). 12 patients were entered into the study, of whom 9 received the combination treatment. Single treatment with i.v. R101933 was associated with minimal toxicity consisting of temporary drowsiness and somnolence. Dose-limiting toxicity consisting of neutropenic fever was seen in cycles 1 and 2 or in further cycles at both dose levels. The plasma pharmacokinetics of docetaxel were not changed by the R101933 regimen at any dose level tested, as indicated by plasma clearance values of 22.5+/-6.2 l/h/m(2) and 24.2+/-7.4 l/h/m(2) (P=0.38) in cycles 1 and 2, respectively. However, the faecal excretion of unchanged docetaxel decreased significantly after the combination treatment from 2.5+/-2.1% to less than 1% of the administered dose of docetaxel, most likely due to inhibition of the intestinal P-gp by R101933. Plasma concentrations of R101933 were not different in cycles 0 or 2 and the concentrations achieved in the first 12-h period after i.v. infusion were capable of inhibiting P-gp in an ex vivo assay. We conclude that the combination of 100 mg/m(2) i.v. docetaxel and 500 mg i.v. R101933 is feasible, lacks pharmacokinetic interaction in plasma, and shows evidence of P-gp inhibition both in an ex vivo assay and in vivo as indicated by the inhibition of intestinal P-gp.

Interactions of racemic mefloquine and its enantiomers with P-glycoprotein in an immortalised rat brain capillary endothelial cell line, GPNT

The brain distribution of the enantiomers of the antimalarial drug mefloquine is stereoselective according to the species. This stereoselectivity may be related to species-specific differences in the properties of some membrane-bound transport proteins, such as P-glycoprotein (P-gp). The interactions of racemic mefloquine and its individual enantiomers with the P-glycoprotein efflux transport system have been analysed in immortalised rat brain capillary endothelial GPNT cells. Parallel studies were carried out for comparison in human colon carcinoma Caco-2 cells. The cellular accumulation of the P-glycoprotein substrate, [(3)H]vinblastine, was significantly increased both in GPNT cells and in Caco-2 cells when treated with racemic mefloquine and the individual enantiomers. In GPNT cells, the (+)-stereoisomer of mefloquine was up to 8-fold more effective than its antipode in increasing cellular accumulation of [(3)H]vinblastine, while in Caco-2 cells, both enantiomers were equally effective. These results suggest that racemic mefloquine and its enantiomers are effective inhibitors of P-gp. Furthermore, a stereoselective P-glycoprotein inhibition is observed in rat cells but not in human cells. The efflux of [(14)C]mefloquine from GPNT cells was decreased when the cells were incubated with the P-gp modulators, verapamil, cyclosporin A or chlorpromazine, suggesting that MQ could be a P-gp substrate.

Efflux of Rhodamine From CD56+ Cells as a Surrogate Marker for Reversal of P-Glycoprotein–Mediated Drug Efflux by PSC 833

The expression of high levels of P-glycoprotein (Pgp) in circulating mononuclear cells allowed us to use an ex vivo assay as a surrogate measure of Pgp antagonism. Efflux of rhodamine from CD56+cells was measured before the start of PSC 833 and at varying times thereafter. Patients receiving PSC 833 had decreased rhodamine efflux from their circulating CD56+ cells. Time course studies showed that following a single oral dose of PSC 833, decreased rhodamine efflux was found in some patients within 15 minutes of treatment. Maximal inhibition was observed at times ranging from 45 minutes to 60 minutes. A dose-response relationship was shown between the concentration of PSC 833 in the blood and the inhibition of rhodamine efflux, with an apparent plateau of the inhibition of rhodamine efflux at approximately 1,000 ng/mL. The Ki, defined as the concentration required for half-maximal inhibition of Pgp-mediated rhodamine efflux, was determined to be in the range of 29 to 181 ng/mL; although results in two patients were distinctly different, with Ki values of 914 and 916 ng/mL. MRK-16 staining was similar among all patients. We conclude that measurement of rhodamine efflux from CD56+ cells provides a surrogate assay with the potential for monitoring Pgp antagonism in clinical trials.

Efflux of Rhodamine From CD56+ Cells as a Surrogate Marker for Reversal of P-Glycoprotein–Mediated Drug Efflux by PSC 833

The expression of high levels of P-glycoprotein (Pgp) in circulating mononuclear cells allowed us to use an ex vivo assay as a surrogate measure of Pgp antagonism. Efflux of rhodamine from CD56+cells was measured before the start of PSC 833 and at varying times thereafter. Patients receiving PSC 833 had decreased rhodamine efflux from their circulating CD56+ cells. Time course studies showed that following a single oral dose of PSC 833, decreased rhodamine efflux was found in some patients within 15 minutes of treatment. Maximal inhibition was observed at times ranging from 45 minutes to 60 minutes. A dose-response relationship was shown between the concentration of PSC 833 in the blood and the inhibition of rhodamine efflux, with an apparent plateau of the inhibition of rhodamine efflux at approximately 1,000 ng/mL. The Ki, defined as the concentration required for half-maximal inhibition of Pgp-mediated rhodamine efflux, was determined to be in the range of 29 to 181 ng/mL; although results in two patients were distinctly different, with Ki values of 914 and 916 ng/mL. MRK-16 staining was similar among all patients. We conclude that measurement of rhodamine efflux from CD56+ cells provides a surrogate assay with the potential for monitoring Pgp antagonism in clinical trials.

Phase II trial of dexverapamil and epirubicin in patients with non-responsive metastatic breast cancer

Agents capable of reversing P-glycoprotein-associated multidrug resistance have usually failed to enhance chemotherapy activity in patients with solid tumours. Based on its toxicity profile and experimental potency, dexverapamil, the R-enantiomer of verapamil, is considered to be promising for clinical use as a chemosensitizer. The purpose of this early phase II trial was to evaluate the effects of dexverapamil on epirubicin toxicity, activity and pharmacokinetics in patients with metastatic breast cancer. A two-stage design was applied. Patients first received epirubicin alone at 120 mg m(-2) i.v. over 15 min, repeated every 21 days. Patients with refractory disease continued to receive epirubicin at the same dose and schedule but supplemented with oral dexverapamil 300 mg every 6 h x 13 doses. The Gehan design was applied to the dexverapamil/epirubicin cohort of patients. Thirty-nine patients were entered on study, 25 proceeded to receive epirubicin plus dexverapamil. Dexverapamil did not increase epirubicin toxicity. The dose intensity of epirubicin was similar when used alone or with dexverapamil. In nine intrapatient comparisons, the area under the plasma concentration-time curve (AUC) of epirubicin was significantly reduced by dexverapamil (mean 2968 vs 1901 microg ml[-1] h[-1], P= 0.02). The mean trough plasma levels of dexverapamil and its major metabolite nor-dexverapamil were 1.2 and 1.5 microM respectively. The addition of dexverapamil to epirubicin induced partial responses in 4 of 23 patients evaluable for tumour response (17%, CI 5-39%, s.e.P 0.079). The remissions lasted 3, 8, 11 and 11+ months. These data suggest that the concept of enhancing chemotherapy activity by adding chemosensitizers may function not only in haematological malignancies but also in selected solid tumours. An increase in the AUC and toxicity of cytotoxic agents does not seem to be a prerequisite for chemosensitizers to enhance anti-tumour activity.

Mutually co-operative interactions between modulators of P-glycoprotein

We measured the effects of combinations of verapamil, vinblastine, mefloquine, and tamoxifen, all being modulators of the multidrug resistance pump, P-glycoprotein, on the accumulation of labelled daunomycin into multidrug-resistant P388 leukemia cells at 37 degrees C. We found that, contrary to our initial expectations (based on Ayesh, Shao and Stein (1996) Biochim. Biophys. Acta 1316, 8), vinblastine, mefloquine, and tamoxifen all appeared to interact with one another synergistically, i.e. by the kinetics of a non-competitive interaction. A simple kinetic analysis showed that pairs of co-operating modulators can give apparent non-competitive behaviour, but refined kinetic analysis enables the two types of interaction to be distinguished. The modulators vinblastine, mefloquine, and tamoxifen thus appear to co-operate with one another in pairs to bring about reversal of P-glycoprotein. This may have important implications for the design of new modulators of P-glycoprotein.

Co-operative, competitive and non-competitive interactions between modulators of P-glycoprotein

We measured the effects of individual modulators and of pairs of modulators of the multidrug resistance pump, P-glycoprotein, on the accumulation of labelled daunomycin into multidrug-resistant P388 leukemia cells at 37 degrees C and developed a kinetic analysis which enables such data to be modelled in terms of co-operative, competitive or non-competitive interaction between pairs of modulators. The modulators verapamil, cyclosporin and trifluoperazine interacted with P-glycoprotein as single molecules, while vinblastine, mefloquine, dipyridamole, tamoxifen and quinidine displayed Hill numbers close to 2, suggesting that pairs of modulator molecules need to act together in order to bring about effective reversal of P-glycoprotein. When the modulators were presented to P-glycoprotein in pairs, we found examples of both competitive and non-competitive behaviour. We interpret these results on a model in which two modulatory sites exit on the MDR pump. To one of these, mefloquine, vinblastine and tamoxifen bind preferentially; to the other, verapamil, dipyridamole, trifluoperazine and quinidine bind (but mefloquine and tamoxifen only weakly if at all). Cyclosporin A can interact with both sites.

Decreased potency of MDR-modulators under serum conditions determined by a functional assay

A variety of agents are capable of overcoming P-glycoprotein-mediated multidrug resistance (MDR) in vitro. However, the clinical potential of these compounds is often limited due to high plasma protein binding. We compared the efficacy of several MDR-reversing compounds in serum-free culture medium and under serum conditions by means of a functional assay. Using flow cytometry the efflux of the fluorescent dye rhodamine 123 (Rh123) was measured from normal peripheral blood CD8+ T-lymphocytes which express low levels of P-glycoprotein. Inhibition of Rh123 efflux by R-verapamil, dexnigludipine-HCl, cyclosporin A, SDZ PSC833 and the protein kinase C (PKC) inhibitor CGP 41251 was determined in serum-free medium and in serum at concentrations from 0.1 to 50 mumol/l. With the exception of SDZ PSC833 all MDR modulators showed an insufficient or suboptimal modulation of P-glycoprotein under serum conditions at concentrations achievable in vivo. The highest potency under serum conditions demonstrated SDZ PSC833: even at a concentration of 0.5 mumol/l a sufficient inhibitory effect was observed. Subsequently this approach was applied to patients suffering from B-cell chronic lymphocytic leukaemia (B-CLL; n = 3) and acute myeloid leukaemia (AML; n = 2) which were positive in the Rh123 efflux assay. As for normal CD8+ T-lymphocytes, much higher drug concentrations were required under serum conditions to effectively inhibit Rh123 efflux from the leukaemic cells. Thus the interpretation of results of clinical 'modulator' trials should consider the decreased bioavailability of MDR-reversing agents.

Inhibition of intestinal P-glycoprotein and effects on etoposide absorption

P-glycoprotein (Pgp) actively pumps a number of antineoplastic drugs, such as etoposide, out of cancer cells and causes multidrug resistance. Pgp is also expressed at the brush-border membrane of the small intestine under normal physiological conditions. We hypothesized that inhibition of intestinal Pgp might decrease the efflux of etoposide from the blood into the intestinal lumen, thereby, increasing the bioavailability of etoposide. The absorption of etoposide was studied using everted gut sacs prepared from rat jejunum and ileum. The addition of C219, a monoclonal antibody of Pgp, at 100 ng/ml or of 0.2 M 5'-adenylylimidodiphosphate, a nonhydrolyzable adenosine triphosphate (ATP) analog, increased the absorption of etoposide. Quinidine, an antiarrythmic agent, has been demonstrated to circumvent multidrug resistance in cell lines, possibly by interfering with Pgp function. Adding quinidine at 1 mg/ml to the everted gut sac increased the absorption of etoposide. In vivo absorption of etoposide was also studied by intraluminal perfusion of the drug in the small intestine of anesthetized rats. Intravenous infusion of quinidine at either 1 or 2 mg/h increased the serum level of etoposide in a dose-dependent manner. Intravenous infusion of etoposide at 0.2 mg/h resulted in luminal exsorption of the drug in the small intestine. The intestinal clearance of etoposide was 41.7 +/- 7.2 ml kg-1, which decreased to 18.4 +/- 3.9 ml kg-1 with the infusion of quinidine at 1 mg/h. The present data confirm that intestinal Pgp mediates the efflux of etoposide and that the use of Pgp-inhibiting agents such as quinidine may increase the bioavailability of etoposide.

Identification of a multidrug resistance modulator with clinical potential by analysis of synergistic activity in vitro, toxicity in vivo and growth delay in a solid human tumour xenograft

Circumvention of multidrug resistance in vitro by resistance modulators is well documented but their clinical use may be limited by effects on normal tissues. We have compared four resistance modifiers, both in terms of modulation of doxorubicin sensitivity in vitro and toxicity in vivo, in order to determine whether it is possible to select agents with clinical potential. Verapamil, D-verapamil and quinidine are all maximally active in the multidrug resistant cell line at about 7 microM and are not cytotoxic at this concentration. The tiapamil analogue Ro11-2933 is a highly potent resistance modulator such that at only 2 microM sensitization is greater than is seen with the other modulators at 7 microM. Since the ID50 concentration for Ro11-2933 is 17.7 microM (5-12-fold less than the other modifiers) we have used isobologram analysis to demonstrate that the interaction with doxorubicin is supra-additive and cannot be explained by additive toxicity. This method of analysis also revealed that when resistance modulation is related to the cytotoxicity of the modulator itself, all four modulators show comparable activity. On the other hand, measurement of the acute toxicity in mice of the modulators did reveal differences. The LD10 for verapamil (51 mg/kg) was about one third of that for quinidine (185 mg/kg) and this is consistent with the known maximum tolerated plasma levels in patients. Furthermore, whilst epirubicin alone was unable to reduce the growth rate of a multidrug resistant human tumour xenograft, the addition of quinidine, but not verapamil, at the maximum tolerated dose did do so. D-Verapamil was only about half as toxic as racemic verapamil and this too is consistent with clinical observations. The LD10 for Ro11-2933 (152 mg/kg) was comparable with that for quinidine. In the human tumour xenograft model maximal growth inhibition was observed with the combination of epirubicin and Ro11-2933 (45 mg/kg) and this degree of growth inhibition was comparable to that obtained with epirubicin alone in the drug sensitive xerografts. Ro11-2933 had no measurable effects on the plasma or tumour pharmacokinetics of epirubicin. These results suggest that it is possible to predict the clinical potential of a resistance modulator. Furthermore, Ro11-2933 is a promising agent for use in the clinic since maximal resistance modulation in vivo is observed at about one third of the LD10 dose.

Differential effectiveness of a range of novel drug-resistance modulators, relative to verapamil, in influencing vinblastine or teniposide cytotoxicity in human lymphoblastoid CCRF-CEM sublines expressing classic or atypical multidrug resistance

A series of five potential modulators of resistance were tested for their relative ability, as compared with verapamil, to sensitize CEM lymphoblastoid leukemia drug-resistant tumor sublines expressing either the classic or the atypical multidrug-resistance (MDR) phenotype to vinblastine or teniposide. Maximal non-cytotoxic concentrations of each modulator were tested and sensitization induces (SIs) were derived by comparing the drug concentration required to inhibit growth by 50% in their presence or absence. Like verapamil (10 microM) itself, three of the other modulators tested, namely, S9788 (4 microM), flunarizine (20 microM) and quinidine (30 microM), resulted in 2- to 3-fold sensitization of vinblastine against the parental CEM cells, and comparable effects were noted in the CEM/VM-1 cells, which were not cross-resistant to vinblastine. In contrast, cyclosporin A (0.5 microM) and B859-35 (2 microM) did not enhance vinblastine growth inhibition in these lines. However, the greatest sensitization with all the modulators was noted in the classic MDR VBL1000 cells, with SIs ranging from 40- to 350-fold, except for cyclosporin A, which proved ineffective at the concentration tested (SI, 2.6). The greatest extent of differential sensitization of these VBL1000 tumor cells occurred with quinidine or B859-35, which proved significantly more effective than verapamil alone. Combinations of modulators resulted in additive effects, with B859-35 plus cyclosporin A proving superior to B859-35 plus verapamil. In contrast, none of these compounds proved effective as a sensitizer to teniposide. The growth-inhibitory effects of this drug were not modified significantly in either the 92-fold teniposide-resistant VM-1 cells or in the parental cells. Addition of verapamil itself also failed to modulate teniposide growth inhibition in the VBL1000 cells, which express significant cross-resistance to this drug (36-fold). However, SI values of 3- to 5-fold were obtained using quinidine or B859-35. These results serve (a) to emphasise the need to monitor the effects of modulators not only on drug-resistant cells but also on their drug-sensitive counterparts so as to ensure differential sensitization such that normal sensitive tissues are not likely to be adversely influenced and (b) to highlight the observation that the extent of modulation differs depending not only on the antitumor drug used but also on the mechanism of drug resistance expressed. This in vitro model system appears to provide a useful screening system for resistance modulators and certainly could be used in attempts to identify alternative agents that may influence teniposide sensitivity in these drug-resistant sublines.

Multidrug resistance in the laboratory and clinic

Multidrug resistance represents a major obstacle in the successful therapy of neoplastic diseases. Studies have demonstrated that this form of drug resistance occurs in cultured tumor cell lines as well as in human cancers. P-glycoprotein appears to play an important role in such cells by acting as an energy-dependent efflux pump to remove various natural-product drugs from the cell before they have a chance to exert their cytotoxic effects. Using the tools of molecular biology, studies are beginning to reveal the true incidence of multidrug resistance, as mediated by the MDR1 gene, in the clinical setting. It has been demonstrated, at least in the laboratory, that resistance mediated by P-glycoprotein may be modulated by a wide variety of compounds, including verapamil and cyclosporine A. These are compounds which, by themselves, generally have little or no effect on the tumor cells, but when used in conjunction with antineoplastic agents act to decrease, and in some instances eliminate, drug resistance. The mechanism(s) by which these agents act to reverse resistance is not fully understood. Clinical trials to modulate P-glycoprotein activity are now under way to determine whether such strategies will be feasible. The detection of the P-glycoprotein in patient samples is very important in the design of these studies, as it appears that drug-resistant cells lacking P-glycoprotein will be unaffected by agents such as verapamil. Clinical studies are needed in which patients are stratified into chemotherapy protocols based on levels of MDR1 mRNA or P-glycoprotein expression in the primary tumors. Several research areas have been identified that are important to the transfer of the discovery of the MDR1 gene and its protein product from the research laboratory to the clinical environment. There is an immediate need for comprehensive information on the prevalence and levels of expression of the human MDR genes and their protein products in human organs and tissues. Data are needed on P-glycoprotein levels in specific subpopulations (e.g., according to age, sex, race, and diet), and the study of the heterogeneity and variability of expression of P-glycoprotein in normal human tissues should be given high priority. Since early studies have indicated some successes in identifying patients with classic multidrug resistance who might be responsive to chemosensitization, it can be anticipated that clinical research will accelerate in this area. The next wave of clinical studies will provide clinical investigators with opportunities to develop and evaluate P-glycoprotein tests and correlate test results with clinical outcomes.

Clinical trials of agents that reverse multidrug resistance. A literature review

BACKGROUND

The discovery of the P-170 glycoprotein as a mediator of multidrug resistance (MDR) represents one of the most important research accomplishments in antineoplastic pharmacology during the last decade. Demonstration of P-170 in epithelial tissues, untreated and chemotherapeutically pretreated human malignancies, and identification of various agents capable of reversing resistance in vitro generated enthusiasm for clinical studies throughout the world. The authors provide an overview of the current status of clinical investigations of MDR1 reversing agents in hematologic and solid malignancies.

METHODS

The authors performed an extensive literature search and selected more than 70 articles concerning the potential clinical relevance of P-glycoprotein/MDR1 modulating agents. Information abstracted included type of reverting agent and chemotherapeutic regimen, number of patients, tumor type, histologic proof of P-glycoprotein expression, and objective response rates.

RESULTS

Proof of the involvement of MDR1 in clinical drug resistance has been slow to accumulate, primarily because of difficulties in adapting assays of MDR1 expression and in planning appropriate trials. Pilot studies have shown that verapamil, cyclosporine, and other chemosensitizers may reverse resistance in a subset of patients, but significant (cardiovascular) side effects are common. For leukemias, lymphomas, and multiple myeloma, response rates of 60-80% may be achieved with the potential for cure, whereas in solid tumors, only a few patients appear to benefit.

CONCLUSIONS

Because of predominantly negative results and unanswered fundamental questions regarding the biology of P-glycoprotein, additional clinical trials with less toxic modulators or their combination are appropriate to delineate optimal strategies for MDR1 reversal and to define the spectrum of responsive tumors. Additional attention also must be given to the coexistence of other resistance mechanisms that may offer separate opportunities for modulation.