Identification of anthracyclines and related agents that retain preferential activity over adriamycin in multidrug-resistant cell lines, and further resistance modification by verapamil and cyclosporin A

Combination therapy for KIT-mutant mast cells: targeting constitutive NFAT and KIT activity

Resistant KIT mutations have hindered the development of KIT kinase inhibitors for treatment of patients with systemic mastocytosis. The goal of this research was to characterize the synergistic effects of a novel combination therapy involving inhibition of KIT and calcineurin phosphatase, a nuclear factor of activated T cells (NFAT) regulator, using a panel of KIT-mutant mast cell lines. The effects of monotherapy or combination therapy on the cellular viability/survival of KIT-mutant mast cells were evaluated. In addition, NFAT-dependent transcriptional activity was monitored in a representative cell line to evaluate the mechanisms responsible for the efficacy of combination therapy. Finally, shRNA was used to stably knockdown calcineurin expression to confirm the role of calcineurin in the observed synergy. The combination of a KIT inhibitor and a calcineurin phosphatase inhibitor (CNPI) synergized to reduce cell viability and induce apoptosis in six distinct KIT-mutant mast cell lines. Both KIT inhibitors and CNPIs were found to decrease NFAT-dependent transcriptional activity. NFAT-specific inhibitors induced similar synergistic apoptosis induction as CNPIs when combined with a KIT inhibitor. Notably, NFAT was constitutively active in each KIT-mutant cell line tested. Knockdown of calcineurin subunit PPP3R1 sensitized cells to KIT inhibition and increased NFAT phosphorylation and cytoplasmic localization. Constitutive activation of NFAT appears to represent a novel and targetable characteristic of KIT-mutant mast cell disease. Our studies suggest that combining KIT inhibition with NFAT inhibition might represent a new treatment strategy for mast cell disease.

Interactions of Cyclosporin A with Breast Cancer Resistance Protein

The objective of this study was to investigate whether cyclosporin A (CsA) is a modulator for breast cancer resistance protein (BCRP). The interactions between CsA and BCRP were evaluated by using both membrane- and cell-based assays. CsA inhibited BCRP or BCRP R482T mutant-associated ATPase with an IC(50) of 26.1 and 7.3 microM (31,388 and 8779 ng/ml), respectively, indicating that CsA is a modulator for BCRP and its R482T mutant. The apparent permeability (P(app)) of CsA was not affected by the BCRP-specific inhibitor Ko143 in both apical-to-basolateral (A-to-B) and basolateral-to-apical (B-to-A) directions in hBCRP- or mBcrp-transfected MDCKII cells, whereas CsA at 50 microM significantly increased the A-to-B transport and decreased B-to-A transport of BCRP substrates, [(3)H]estrone-3-sulfate ([(3)H]E3S) and [(3)H]methotrexate ([(3)H]MTX), in hBCRP- and mBcrp1-trasfected MDCKII cells. Similar to cellular transport studies, CsA did not exhibit ATP-dependent uptake in BCRP-expressed membrane vesicles but inhibited the ATP-mediated E3S and MTX uptake in the same vesicles. The inhibitory constant (K(i)) of CsA toward BCRP was 6.7 microM (8507 ng/ml) and 7.8 microM (9380 ng/ml) when using E3S or MTX, respectively, as a BCRP substrate. The inhibitory potency of CsA on BCRP wild type or its R482T mutant was lower than that on P-glycoprotein. The present studies demonstrate that CsA is an inhibitor but not a substrate for BCRP, and has low potential to cause drug-drug interactions with BCRP substrate drugs due to its weak inhibitory effect on BCRP and BCRP R482T mutant at its normal therapeutic blood concentrations (200-400 ng/ml) (Blood 91:362-363, 1998).

Delivery of an anticancer drug and a chemosensitizer to murine breast sarcoma by intratumoral injection of sulfopropyl dextran microspheres

Intratumoral injection of controlled-release microsphere formulations of anticancer compounds has the potential to selectively increase tumour exposure to drugs. This work aimed to evaluate the therapeutic effect and toxicity of microsphere formulations containing the anticancer drug, doxorubicin, in a murine tumour model. The effect of co-administration of verapamil, a P-glycoprotein modulator or chemosensitizer, was investigated. Initial in-vitro studies confirmed the ability of verapamil to enhance the accumulation of both doxorubicin and [(99mTc)]sestamibi, also a P-glycoprotein substrate, in EMT6 murine breast sarcoma cells and a doxorubicin-selected multidrug-resistant variant, EMT6/AR1.0. Ex-vivo studies using confocal microscopy demonstrated release of doxorubicin from microspheres and diffusion of the drug through tissue. For in-vivo studies, EMT6 and EMT6/AR1.0 cells were grown in BALB/c mice. Following intratumoral injection of doxorubicin-loaded microspheres, alone or in combination with verapamil-loaded microspheres, the tumour diameter was measured serially as an indication of therapeutic effect, while the weight, appearance, and behaviour of the mice were monitored as an indication of general toxicity. Intratumoral injections of doxorubicin-loaded microspheres were tolerated much better than systemic administration of equivalent drug concentrations. There was a modest (up to 34%) delay of tumour growth compared with groups receiving no treatment or blank microspheres. Co-injection of verapamil microspheres with doxorubicin microspheres produced a moderate increase in toxicity but no further delay in tumour growth. Controlled-release microsphere formulations of anticancer agents administered intratumorally were an efficient way to deliver high drug doses to the tumour with little systemic toxicity.

Partial circumvention of P-glycoprotein-mediated multidrug resistance by doxorubicin-14-O-hemiadipate

Previously, we have reported partial circumvention of P-glycoprotein (Pgp)-associated resistance to doxorubicin (Dox) in MCF7/R human breast carcinoma and P388/R murine leukemia cell lines by doxorubicin-14-O-hemiadipate (H-Dox) [Povarov L.S. et al. (1995) Russian J. Bioorganic Chemistry 21: 797-803]. We felt that these changes were due to alterations in the cellular pharmacokinetics of the analog in multidrug (MDR) resistant cells, as compared to that of Dox. To address this hypothesis, we performed comparative studies of the accumulation, retention and intracellular localization of H-Dox and Dox in Dox-sensitive murine leukemia cell line P388/S and its Dox-selected. Pgp-positive drug resistant P388/R subline. These studies were performed in the presence or absence of cyclosporin A (CsA), a competitive inhibitor of Pgp. Flow cytometric analysis revealed significant differences in Dox and H-Dox accumulation in P388/R cells when compared to P388/S cells. In P388/R versus P388/S cells, there was a 38-fold decrease in Dox accumulation, but only a 5-fold decrease in H-Dox accumulation, indicating over a 7-fold increase in H-Dox buildup in resistant cells. CsA did not affect uptake or retention of either drug by sensitive cells. However, coincubation with CsA resulted in a 54-fold increase in Dox accumulation and only a 5-fold increase in H-Dox uptake in P388/R cells, restoring anthracycline levels in P388/R to 100% of that found in P388/S cells. Once internalized by the resistant cells, H-Dox was retained better than Dox regardless of presence or absence of CsA. Confocal microscopic analysis revealed the presence of H-Dox but no Dox in cellular nuclei of P388/R cells. Thus, increased activity of H-Dox toward P388/R cells was correlated with its enhanced ability to enter and be retained in these cells, and also with redistribution of H-Dox into the nuclei of the resistant cells as compared to Dox. Overall, our findings support our initial hypothesis and provide evidence that H-Dox, a 14-O-hemiadipate of doxorubicin, is affected by Pgp-mediated MDR to a lesser extent than parental Dox due to changes iin the cellular pharmacokinetics of the analog.

Modulation of Multidrug Resistance in Cancer by Immunosuppresive Agents. Preclinical Studies

This is a brief summary of the status of known immunosuppressive drugs describing their potential and mode of action to reverse the function of the MDR1 gene product, the P glycoprotein. Different aspects of these immunosuppressors have been reviewed in the recent literature. This summary will focus only on those studies which relate to the effect of these drugs on the P-glycoprotein. In addition, studies which may explain the mode of action, but do not deal directly with P-glycoprotein, are also summarized.

Doxorubicin-peptide conjugates overcome multidrug resistance

A well-known mechanism leading to the emergence of multidrug-resistant tumor cells is the overexpression of P-glycoprotein (P-gp), which is capable of lowering intracellular drug concentrations. To overcome this problem, we tested the capability of two peptide vectors that are able to cross cellular membranes to deliver doxorubicin in P-gp-expressing cells. The antitumor effect of peptide-conjugated doxorubicin was tested in human erythroleukemic (K562/ ADR) resistant cells. The conjugate showed potent dose-dependent inhibition of cell growth against K562/ADR cells as compared with doxorubicin alone. Doxorubicin exhibited IC50 concentrations of 65 microM in the resistant cells, whereas vectorized doxorubicin was more effective with IC50 concentrations of 3 microM. After treatment of the resistant cells with verapamil, the intracellular levels of doxorubicin were markedly increased and consequent cytotoxicity was improved. In contrast, treatment of resistant cells with verapamil did not cause any further enhancement in the cell uptake nor in the cytotoxic effect of the conjugated doxorubicin, indicating that the conjugate bypasses the P-gp. Finally, we show by the in situ brain perfusion method in P-gp-deficient and competent mice that vectorized doxorubicin bypasses the P-gp present at the luminal site of the blood-brain barrier. These results indicate that vectorization of doxorubicin with peptide vectors is effective in overcoming multidrug resistance.

Broad phase II and pharmacokinetic study of methoxy-morpholino doxorubicin (FCE 23762-MMRDX) in non-small-cell lung cancer, renal cancer and other solid tumour patients

The aim was to perform a broad phase II and pharmacokinetic study of methoxymorpholino-doxorubicin (MMRDX), a drug active against multidrug-resistant tumour cells in vitro when given by i.v. bolus at 1.5 mg m(-2) every 4 weeks, in metastatic or unresectable solid tumour patients with known intrinsic drug resistance. Patients received a maximum of six cycles. Plasma, urine and leucocyte MMRDX and its 13-dihydro metabolite pharmacokinetic analysis was performed in patients without liver metastases. Patients (n = 48, 21 NSCLC, 19 renal cell, three head and neck tumour, three cervical cancer and two adenocarcinoma of unknown primary) received 132 cycles of MMRDX. Common toxicity criteria (CTC) grade III/IV thrombocytopenia (12% of cycles) and neutropenia (27% of cycles) occurred with median nadir on day 22. Transient transaminases elevation > grade III/IV was observed in 7% of cycles, late and prolonged nausea > or = grade II in 34% and vomiting > or = grade II in 39%. In two patients, the left ventricular ejection fraction was reduced > or = 15%. Of 37 evaluable patients, one out of 17 NSCLC had a partial response. Mean (+/- s.d.) MMRDX AUC0-infinity calculated up to 24 h after dosing was 20.4 +/- 6.2 microg h l(-1) (n = 11) and t(1/2, gamma) was 44.2 h. Mean plasma clearance (+/- s.d.) was 37.2 +/- 7.3 l h(-1) m(-2) and volume of distribution 1982 +/- 64 l m(-2). MMRDX leucocyte levels 2 and 24 h after infusion were 450 to 600-fold higher than corresponding MMRDX plasma levels. In urine, 2% of the MMRDX dose was excreted unchanged, and 2% as metabolite. The main side-effects of 1.5 mg m(-2) every 4 weeks of MMRDX are delayed nausea and vomiting and haematological toxicity. MMRDX is characterized by extensive clearance and rapid and extensive distribution into tissues. A low response rate was observed in patients with tumours with intrinsic chemotherapy resistance.

Anthracyclines in haematology: preclinical studies, toxicity and delivery systems

The anthracyclines are widely used in the treatment of haematological and non-haematological malignancy and there is now more than 30 years' clinical experience with these agents but despite this, their mechanism of action is incompletely understood. The anthracyclines have been shown to intercalate with DNA and indirectly inhibit the activity of the enzyme topoisomerase II, resulting in DNA strand breaks. More recently, workers have focused on induction of apoptosis and have shown that daunorubicin stimulates production of the apoptotic mediator, ceramide and that the activity of doxorubicin can be blocked by inhibitors of CD95 (fas). One of the major problems with anthracycline therapy is the development of resistance which may be mediated by p-glycoprotein or by other mechanisms. Much recent research has concentrated on methods to modulate the drug-resistant phenotype and these include development of new analogues and use of specific reversal agents. The toxicity profile of the anthracyclines includes bone marrow suppression, severe local reaction following extravasation, radiation recall, alopecia, gastrointestinal and hepatic effects, development of secondary malignancies and significant cardiac toxicity. The risk factors for the development of anthracycline-related cardiac toxicity are well documented and several methods have been exploited in attempts at prevention. Finally, a number of drug delivery systems have been developed in order to improve therapeutic response and reduce toxicity to normal tissues, including the use of liposomal preparations.

Mechanisms for high methoxymorpholino doxorubicin cytotoxicity in doxorubicin-resistant tumor cell lines

Methoxymorpholino doxorubicin (MMRDX) is an anthracycline analogue that is able to overcome tumor cell resistance to classical anthracyclines. Mechanisms for increased MMRDX cytotoxicity were analyzed in a small cell lung carcinoma cell line (GLC4), its 300-fold doxorubicin-resistant and multidrug resistance-associated protein (MRP)-over-expressing subline (GLC4/ADR), an ovarian carcinoma cell line (A2780) and its 100-fold doxorubicin resistant and P-glycoprotein (P-gp)-overexpressing subline A2780AD. Cross-resistance, measured with the MTT assay at MMRDX concentration resulting in 50% growth inhibition, was 1.8-fold in GLC4/ADR and 4.5-fold in A2780AD compared to their respective parental cell lines. Cellular MMRDX accumulation was equal in GLC4 and GLC4/ADR and 2-fold lower in A2780AD compared to A2780. Doxorubicin fluorescence was analyzed with confocal laser scan microscopy. Fluorescence was nuclear in sensitive, and cytoplasmic in resistant, cell lines, while MMRDX fluorescence was found in the nucleus in all cell lines. Pre-incubation with the MRP blocker MK 571 restored in GLC4/ADR cells the nuclear doxorubicin fluorescence pattern, as observed in GLC4 cells. MMRDX, thus, can largely overcome cross-resistance in these P-gp- and MRP-overexpressing doxorubicin-resistant cell lines. Our results suggest that MMRDX is not a substrate for MRP-mediated resistance.

Cellular uptake, cytotoxicity, and transport kinetics of anthracyclines in human sensitive and multidrug-resistant K562 cells

Multidrug resistance in tumor cells is often associated with the presence of an approximately 170 kDa plasma membrane glycoprotein (Pgp) that acts as a drug-efflux pump and decreases intracellular antitumor drug concentration. We measured the uptake of seven anthracyclines (daunorubicin, doxorubicin, 4'-epi-doxorubicin, 4'-deoxy-doxorubicin, iododoxorubicin, 3'-(3-methoxymorpholino)-doxorubicin (FCE23762) and 4-demethoxy-daunorubicin) into K562 cells sensitive and resistant (K562/DNR) to daunorubicin. The K562/DNR subline expresses Pgp at the membrane surface, whereas its sensitive counterpart does not. Laser flow cytometry was used to quantitate intracellular anthracycline content. Uptake of daunorubicin, doxorubicin, 4'-epi-doxorubicin, and 4'-deoxy-doxorubicin was minimal in the K562/DNR subline as compared to their uptake in sensitive cells. On the contrary, iododoxorubicin, FCE23762, and 4-demethoxy-daunorubicin accumulate to nearly the same extent into sensitive and resistant K562 cells. Growth inhibition data indicated that the resistance factor for iododoxorubicin, FCE23762, and 4-demethoxy-daunorubicin is markedly decreased as compared to the other drugs. Fluorescence measurements were carried out to determine the kinetic parameters associated with the influx and efflux of the drugs into and out of K562 cells. Kinetic data indicated that iododoxorubicin, FCE23762, and 4-demethoxy-daunorubicin are not actively rejected from resistant cells, suggesting that they are poor substrates for Pgp-mediated transport. This observation is related to their ability to overcome the multidrug-resistant phenotype of K562/DNR cells in vitro.

The role of methoxymorpholino anthracycline and cyanomorpholino anthracycline in a sensitive small-cell lung-cancer cell line and its multidrug-resistant but P-glycoprotein-negative and cisplatin-resistant counterparts

The cytotoxic action of two morpholino anthracyclines, methoxymorpholino anthracycline (MRA-MT, FCE 23,762) and cyanomorpholino anthracycline (MRA-CN), was compared with the cytotoxicity of doxorubicin (DOX), the topoisomerase II inhibitor etoposide (VP-16), the topoisomerase I inhibitor camptothecin, methotrexate, and cisplatin in GLC4, a human small-cell lung-cancer cell line, in GLC4-Adr, its P-glycoprotein (Pgp)-negative, multidrug-resistant (MDR; 100-fold DOX-resistant) subline with overexpression of the MDR-associated protein (MRP) and a lowered topoisomerase II activity, and in GLC4-CDDP, its cisplatin-resistant subline. GLC4-Adr was about 2-fold cross-resistant for the morpholino anthracyclines and GLC4-CDDP was, relative to GLC4, more resistant for the morpholino anthracyclines than for DOX. Overall, MRA-CN was about 2.5-fold more cytotoxic than MRA-MT. The cytotoxicity profile of the morpholino anthracyclines in these cell lines mimicked that of camptothecin.

In vitro anthracycline cross-resistance pattern in childhood acute lymphoblastic leukaemia

Daunorubicin (DNR) is a major front-line drug in the treatment of childhood acute lymphoblastic leukaemia (ALL). Previously, we showed that in vitro resistance to DNR at diagnosis is related to a poor long-term clinical outcome in childhood ALL and that relapsed ALL samples are more resistant to DNR than untreated ALL samples. In cell line studies, idarubicin (IDR), aclarubicin (ACR) and mitoxantrone (MIT) showed a (partial) lack of cross-resistance to the conventional anthracyclines DNR and doxorubicin (DOX), but clinical studies in childhood ALL have been inconclusive about the suggested lack of cross-resistance. In the present study we determined the in vitro cross-resistance pattern between DNR, DOX, IDR, ACR and MIT in 48 untreated and 39 relapsed samples from children with ALL using the MTT assay. The relapsed ALL group was about twice as resistant to DNR, DOX, IDR, ACR and MTT as the untreated ALL group. Thus, resistance developed to all five drugs. We found a significant cross-resistance between DNR, DOX, IDR, ACR and MIT, although in some individual cases in vitro anthracycline cross-resistance was less pronounced. We conclude that IDR, ACR and MIT cannot circumvent in vitro resistance to DNR in childhood ALL. Clinical studies may still prove whether IDR, ACR or MIT has a more favourable toxicity profile than DNR.

Cellular pharmacology of the partially non-cross-resistant anthracycline annamycin entrapped in liposomes in KB and KB-V1 cells

The in vitro cytotoxicity, cellular pharmacology, and DNA lesions induced by the lipophilic anthracycline annamycin (Ann) were studied in KB and KB-V1 (multidrug-resistant) cells. Ann was tested in suspension in saline and 10% dimethylsulfoxide (DMSO: final concentration, 0.05%-0.5%) or entrapped in multilamellar liposomes (median size, 1.57 microns). Doxorubicin (Dox) was about twice as cytotoxic as Ann or liposome-entrapped Ann (L-Ann) against KB cells. Both Ann and L-Ann displayed a partial lack of cross-resistance with Dox (resistance indices: > 60 for Dox, 4.7 for Ann, 4.0 for L-Ann). Accumulation of Ann in KB and KB-V1 cells was consistently about 2-3 and 10-20 times higher, respectively, than that of Dox. Cellular retention of Ann in KB and KB-V1 cells was about 2 and 30 times higher, respectively, than that of Dox as a result of the different efflux patterns of the two drugs: Dox was not effluxed from KB cells but was significantly effluxed from KB-V1 cells (66% at 1 h, whereas Ann efflux was similar in both cell lines (about 50% at 1 h). Dox retention in KB-V1 cells was increased by a factor of 2 in the presence of verapamil or cyclosporine A, but Ann retention was not. In addition, accumulation of Dox in KB-V1 cells was enhanced by the metabolic inhibitor deoxyglucose/azide and the membrane carboxylic ionophore monensin, whereas accumulation of Ann was not affected by either agent. All these findings indicate significant differences in the cellular transmembrane transport systems between Dox and Ann and suggest that Ann efflux is not mediated by P-glycoprotein. Liposome entrapment reduced by a factor of 1.3-2.0 the cellular accumulation of Ann without affecting its cytotoxicity. As compared with Dox, both Ann and L-Ann induced 3 times more DNA double- and single-strand breaks in KB cells. In KB-V1 cells, Dox did not induce DNA damage, whereas the extent of DNA breaks induced by both Ann and L-Ann was similar to that induced by Dox in KB cells. Our results indicate (1) that the lack of cross-resistance between Ann and Dox is associated with a markedly enhanced accumulation and retention of Ann in KB-V1 cells and (2) that the type of liposomes used does not significantly affect the cellular effects of Ann.

Design and tumor targeting of anthracyclines able to overcome multidrug resistance: a double-advantage approach

A novel, 'double-advantage approach' to developing more effective chemotherapies will be reviewed. This approach is based on a presumption that analogs designed on a sound hypothesis, and combined with a rationally selected drug delivery system, will optimize antitumor activity by creating drugs that are more active and that can be more specifically targeted to tumors. In the design of drugs superior to doxorubicin, we have focused on typical multidrug resistance and new anthracycline analogs, whose uptake is not affected by P-glycoprotein. Analysis of structural elements of anthracyclines affecting activity against multidrug resistant tumors and affinity for liposomes will be discussed. Annamycin, a lipophilic anthracycline analog, was selected for further preclinical development as a liposomal formulation and demonstrated, in the initial biological evaluation, high activity against tumors resistant to doxorubicin.

The efflux of anthracyclines in multidrug-resistant cell lines

In order to address the association of enhanced drug efflux with the multidrug-resistant (MDR) phenotype, we have studied the cellular pharmacokinetics of anthracyclines in the P-glycoprotein (Pgp)-positive MDR cell lines H69/LX4 (human small cell lung cancer) and EMT6/AR1.0 (mouse mammary tumour). Both doxorubicin (DOX) and daunorubicin (DNR) were accumulated to a lesser extent and effluxed at a higher rate by MDR cells than by their drug-sensitive counterparts. In contrast, the 9-alkyl substituted compound, aclacinomycin A (ACL), was accumulated and effluxed from parent and MDR cells at an identical rate. In experiments designed to examine energy-dependent efflux, DOX and DNR were shown to be efficiently effluxed against the concentration gradient in the presence of glucose. However, in the same experiments the analogues ACL and Ro 31-3294 (9-alkyl and morpholinyl substituted), which have previously been shown to retain activity against MDR cell lines, were accumulated and effluxed at identical rates in parent and MDR EMT6 cells. Hence, 9-alkyl and morpholinyl substituted compounds appear to behave less favourably as substrates for energy-driven drug efflux by Pgp-positive MDR cells than do DOX or DNR. Resistance modifiers verapamil and cyclosporin A appeared to abolish energy-dependent efflux for DOX and DNR in both the EMT6 and H69 MDR lines whereas they had no effect on the cellular efflux of ACL. The altered cellular pharmacology in MDR cell lines may provide a rational basis for the use of modified anthracycline analogues (e.g. 9-alkyl and morpholinyl (substituted) and resistance of modifying agent in the treatment of tumours expressing a Pgp-mediated phenotype.

Examination by laser scanning confocal fluorescence imaging microscopy of the subcellular localisation of anthracyclines in parent and multidrug resistant cell lines

This study highlights the usefulness of laser scanning confocal microscopy in the examination of subcellular disposition of anthracyclines in tumour cell lines. The distribution of anthracycline compounds has been studied in two pairs of parental and multidrug resistant (MDR) cell lines. For the parental EMT6 mouse mammary tumour cell line EMT6/P treated with doxorubicin (DOX) the anthracycline fluorescence was shown to be predominantly nuclear but with some particulate cytoplasmic fluorescence and very low levels of plasma membrane staining. In the same experiments much fainter fluorescence was seen for the EMT6/AR1.0 MDR subline which hyperexpresses P-glycoprotein. The loss of nuclear fluorescence was comparatively greater than loss of cytoplasmic fluorescence. For the human large cell lung cancer line COR-L23/P cellular DOX disposition was markedly nuclear with nuclear membrane staining and diffuse cytoplasmic fluorescence. For the MDR line COR-L23/R, which lacks P-glycoprotein expression, DOX fluorescence was reduced in the nucleus compared with the parental line, but an intense area of perinuclear staining was seen consistent with localisation to the Golgi apparatus. The morpholinyl-substituted analogue MR-DOX achieved very similar subcellular distribution in both parental and MDR lines, consistent with its retention of activity in the latter. The presence of verapamil during anthracycline exposure increased the intensity of fluorescence in the MDR lines, particularly in the nucleus. Relatively little effect was seen in the parental lines. Confocal microscopy provides high resolution images of the subcellular distribution of anthracyclines in parent and MDR cell lines. Differences in drug disposition in various cell lines may provide insights into the mechanism of multidrug resistance and suggest strategies for its therapeutic circumvention.

The role of intracellular free calcium mobilization in the mechanism of action of antitumour ether lipids SRI 62-834 and ET18-OMe

Membrane-active antitumour ether lipids such as ET18-OMe (1-O-octadecyl-2-O-methyl-rac-glycero-3-phosphocholine) and SRI 62-834 ((+-)-2-(Hydroxy[tetrahydro-2-(octadecyloxy) methylfuran-2-yl] methoxyl phosphinyloxy)-N,N,N-trimethylethaniminium hydroxide) are selectively cytotoxic to tumour cells in vitro. Their precise mechanisms of action are unclear, but they are known to have effects on cell membranes and cell signalling. A previous report suggested that ether lipids cause a biphasic sustained rise in intracellular free calcium [Lazenby et al., Cancer Res 50: 3327-3330, 1990]. We show here that the second phase is an experimental artefact due to cell membrane permeabilization by ether lipids in serum-free buffers. In serum-free medium, the membrane toxicity of antitumour ether lipids was increased 50-60 fold, when compared to medium containing 10% serum. Membrane disruption was neither dependent on extracellular calcium, nor modulated by preloading cells with the calcium chelators bis(o-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid or 2-[[2-[bis(carboxymethyl)amino]-5-methylphenoxy]methyl]-6- methoxy-8-[bis(carboxymethyl)amino]quinoline. This indicates that the mechanism of membrane damage by ether lipids does not involve changes in calcium homeostasis. Using indo-1 and fura-2 as calcium probes, we established that lower concentrations of antitumour ether lipids do elicit a genuine monophasic and transient rise in intracellular free calcium, predominantly mobilized from internal stores. This acute calcium agonist activity of ether lipids is distinct from the inhibitory effects on cell signalling reported previously after more prolonged exposure. It appears that the calcium elevation induced by antitumour ether lipids is unlikely to be instrumental in their selective and potent antitumour activity.

Differing patterns of cross-resistance resulting from exposures to specific antitumour drugs or to radiation in vitro

This article reviews the patterns of cross-resistance identified in various P-glycoprotein-mediated and non-P-glycoprotein-mediated drug resistant mammalian tumour cell lines. The differing patterns of cross-resistance and the variable levels of resistance expressed are summarised and discussed. Although the mechanism by which P-glycoprotein can recognise and transport a large group of structurally-unrelated substrates remains to be defined, the recent evidence indicating that membrane associated domains participate in substrate recognition and binding is summarised, and other possible explanations for these variable cross-resistance patterns are considered. Amongst the non-P-glycoprotein-overexpressing multidrug resistant cell lines, two subsets are clearly identifiable, one lacking and the other expressing cross-resistance to the Vinca alkaloids. Resistance mechanisms implicated in these various sublines and possible explanations for their differing levels and patterns of cross-resistance are summarised. Clinical resistance is identified in patients following treatment not only with antitumour drugs, but also after radiotherapy. Experimental data providing a biological basis for this observation are summarised. A distinctive multiple drug resistance phenotype has been identified in tumour cells following exposure in vitro to fractionated X-irradiation characterised by: the expression of resistance to the Vinca alkaloids and the epipodophyllotoxins but not the anthracyclines and overexpression of P-glycoprotein which is post-translationally regulated, but without any concomitant overexpression of P-glycoprotein mRNA. Finally, the possible clinical relevance of these variable patterns of cross-resistance to the antitumour drugs commonly used in the clinic is considered.

Decreased resistance to N,N-dimethylated anthracyclines in multidrug-resistant Friend erythroleukemia cells

Doxorubicin-resistant Friend erythroleukemia cells, line F4-6 ADM2R, were selected by exposure of wild-type F4-6 cells to doxorubicin concentrations of up to 1 microgram/ml. In these cells, increased expression of multidrug resistance (MDR) genes was demonstrated by Northern blot analysis. The growth-inhibitory effect of doxorubicin, daunorubicin, N,N-dimethyldoxorubicin, N,N-dimethyldaunorubicin, morpholinodoxorubicin, and pyrromycin was comparatively investigated in resistant and wild-type cells. The doxorubicin-resistant F4-6 cells showed approx. 200-fold resistance to doxorubicin and about 100-fold resistance to daunorubicin with respect to the drug-sensitive counterpart. A dramatic decrease in resistance was observed for the N,N-dimethylated derivatives of doxorubicin and daunorubicin as well as for the N,N-dimethylated natural anthracycline pyrromycin and for morpholinodoxorubicin. Uptake studies using [14C]-daunorubicin and [14C]-N,N-dimethyldaunorubicin in resistant F4-6 cells showed a decreased accumulation of daunorubicin but no significant reduction in N,N-dimethyldaunorubicin accumulation as compared with the wild-type cells. Treatment with verapamil led to increased intracellular levels of daunorubicin in resistant cells, whereas an excess of N,N-dimethyldaunorubicin did not have this effect. Thus, the decreased resistance of the doxorubicin-resistant F4-6 cells to the N-alkylated anthracyclines may at least in part be due to a reduced affinity of these compounds for the efflux pump. The results indicate that the dimethylation of the amino group of the anthracycline sugar moiety and its incorporation within a morpholinyl ring may overcome MDR by similar mechanisms.

Organ distribution and tumor uptake of annamycin, a new anthracycline derivative with high affinity for lipid membranes, entrapped in multilamellar vesicles

Annamycin (Ann) is a lipophilic, non-cross resistant anthracycline antibiotic that is easily amenable to formulation in a wide variety of liposomal carriers. We studied the organ distribution and tumor uptake of Ann entrapped in multilamellar vesicles (L-Ann), free annamycin (F-Ann), and doxorubicin (DOX) in C57BL/6 mice bearing advanced subcutaneous B16 melanoma tumors. L-Ann was composed of DMPC: DMPG: Ann at a molar ratio of 7:3:0.7. Mean particle size was 1.88 +/- 0.89 microns, and the entrapment efficiency was 93.08% +/- 2.96%. F-Ann was prepared as a suspension (particle size < or = 0.2 microns) in 10% DMSO. Drug levels were measured by fluorescence spectrometry after extraction with chloroform. The extraction ratio ranged between 60% and 90% for both drugs in most tissues. Compared with those of DOX, organ AUCs of L-Ann were threefold higher in plasma and brain, twofold higher in liver and kidney, sixfold higher in lung, ninefold higher in spleen, and tenfold higher in B16 tumors. Compared with F-Ann, organ AUCs of L-Ann were twofold higher in plasma, liver, and B16 tumors and were twofold lower in brain. Heart AUCs were similar with all three drugs. Higher tumor uptake was associated with a faster penetration and more prolonged retention of Ann in tumor tissue compared with those of DOX. The results obtained indicate significant differences in organ distribution between L-Ann and DOX as a result of the higher affinity of Ann for lipid membranes and the use of the liposomes as a delivery system. The potential clinical relevance of the increased uptake of L-Ann in B16 tumors, lung, and brain is being investigated.

The influence of pH on the interaction of lipophilic anthracyclines with bovine serum albumin. Quantitative characterization by measurement of fluorescence quenching

We have investigated the interaction of the lipophilic anthracyclines 4'-iodo-4'-deoxydoxorubicin (IDX) and 4-demethoxy-daunorubicin (DDN) with bovine serum albumin by the quantitation of fluorescence quenching. The protein binding of IDX was extremely sensitive to the pH of the solution in which the complex was formed and paralleled the effect of pH on dimerization of the drug. The effect of pH on the protein binding and self-association of DDN was less extensive. Both compounds exhibited curvilinear Scatchard plots indicating apparent cooperativity in the binding process. Because of the self-association of the drugs in aqueous solution, we attempted to resolve this cooperativity in terms of the preferential binding of the dimer to the acceptor. However, we found that similar Scatchard plots could be simulated by using slightly erroneous estimates of the fluorescence yield of the complex, rendering any such analysis inconclusive. Consequently, the relationship between acceptor concentration and the fraction of ligand bound was considered to be fitted adequately in terms of a single acceptor site per albumin molecule. The pH dependence of the association constants for bovine serum albumin was described best by the hydrophobic interaction of neutral drug monomer with a binding site with titratable affinity. We postulate that the pH-dependent binding of some anthracyclines with albumin may lead to their enhanced uptake, relative to that of non-target organs, into tumours with an acidotic extracellular milieu.

Lack of enantio-selectivity in the in vitro antitumour cytotoxicity and membrane-damaging activity of ether lipid SRI 62-834: further evidence for a non-receptor-mediated mechanism of action

SRI 62-834 ([tetrahydro-2-(octadecycloxy)methylfuran- 2-yl]methoxylphosphocholine; CRC 86-05; NSC 614383) is a cyclic antitumour ether lipid (AEL) with a novel, but ill-defined, mechanism of action. AELs are believed to act on membranes and cell signals, but the precise mechanisms of selectivity are unclear. Receptor-mediated mechanisms can often be identified by the differential activity of the individual stereoisomers of a drug. We have therefore compared the R- and S-enantiomers of SRI 62-834 for: (1) cytotoxicity against the human HT29 colon carcinoma cell line using a tetrazolium dye reduction assay and (2) membrane-damaging effects monitored by 51Cr radiolabel release. The tetrazolium assay revealed near-identical mean ID50 values around of 2-3 microM for the R- and S-isomers as well as for the racemic mixture. Moreover, pre- and co-incubation of the cells with the potent platelet-activating factor (1-O-alkyl-2-O-acetyl-sn-glycero-3-phosphocholine;PAF) receptor antagonist WEB 2086BS (3-[4-(chorophenyl)-9-methyl-6H-thieno[3,2- f][1,2,4]triazolo-[4,3-a][1,4]-diazepin-2-yl]-1-(4- morpholinyl)-1-propanone) had no effect on the cytotoxicity of either isomer or the racemate. Short-term membrane damage was not evident at low micromolar concentrations and between 139 and 163 microM either lipid was required to release 50% of the incorporated 51Cr label. Again, there was no difference in potency between the enantiomers and the racemate. Coincubation with WEB 2086BS also failed to modulate the membrane-lytic potency of the AELs. These results indicate that the site(s) of cytotoxic action of SRI 62-834 is (are) not stereospecific and also appear to rule out the involvement of a conventional PAF receptor in the mechanism of action of SRI 62-834.

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.

Retention of activity by selected anthracyclines in a multidrug resistant human large cell lung carcinoma line without P-glycoprotein hyperexpression

A subline (COR-L23/R) of the human large cell lung line [corrected] COR-L23, derived by in vivo exposure to doxorubicin, exhibits an unusual multidrug resistant (MDR) phenotype. This subline shows cross-resistance to daunorubicin, vincristine, colchicine and etoposide but does not express P-glycoprotein. Interestingly, COR-L23/R [corrected] shows little or no resistance to a range of structurally-modified analogues of doxorubicin comprising 9-alkyl and/or sugar modified anthracyclines. We have previously identified these same compounds as effective agents against P-glycoprotein-positive MDR cell lines. In contrast to typical MDR cell lines, COR-L23/R [corrected] shows only minimal chemosensitisation by verapamil and no collateral sensitivity to verapamil. Compared to the parental cell line, COR-L23/R [corrected] displays reduced accumulation of doxorubicin and daunorubicin. Accumulation defects were apparent only after 0.5-1 h of incubation of cells with these agents. The rate of daunorubicin efflux was shown to be enhanced by COR-L23/R [corrected] and this efflux was demonstrated to be energy-dependent. The use of anthracyclines which retain activity in MDR cells thus appears to be a valid approach for the circumvention of MDR, not only in cells which express P-glycoprotein, but also where defective drug accumulation is due to other mechanisms possibly involving an alternative multidrug transporter.

P-glycoprotein drug efflux pump involved in the mechanisms of intrinsic drug resistance in various colon cancer cell lines. Evidence for a saturation of active daunorubicin transport

We studied the resistance of colon tumors to anticancer agents in vitro. Using daunorubicin (DN), a number of cellular parameters which normally indicate acquired or multidrug resistance (MDR), were compared for several human wild-type colon cell lines, i.e. HT29, SW1116 and COLO 320, and the murine colon cell line C-26. The sensitive/MDR human ovarian cancer cell line couple A2780/2780AD was used as a reference. The amount of P-glycoprotein (P-gp) was in the order HT29, A2780 less than or equal to SW1116 less than C26 less than or equal to COLO 320 less than 2780AD. The MDR modifiers verapamil, Cremophor EL, cyclosporin A and Ro 11-2933/001 had significant effects on DN cytotoxicity, total DN accumulation and efflux, only if P-gp was present. A flow-through system was used to study the mechanism of DN transport. For the first time, evidence for saturation of an active transport of DN from the cells is reported. We discussed the possible presence of cooperative activity between at least two binding sites on the protein responsible for DN efflux, likely to be P-gp.

Reduced nuclear binding of a DNA minor groove ligand (Hoechst 33342) and its impact on cytotoxicity in drug resistant murine cell lines

The reduced cellular uptake, and subsequent reduced nuclear availability, of cytotoxic agents is a factor in the resistance of mammalian cells to anti-cancer drugs that act by interaction with DNA. The whole cell uptake, nuclear binding and cytotoxicity of a DNA-specific ligand, Hoechst dye number 33342 (Ho342), has been studied in cytotoxic drug resistant variants of a murine tumour cell line. Cell lines showing various degrees of cross-resistance to adriamycin as a part of the phenotype of classical multi-drug resistance (MDR) demonstrated a reduction in intranuclear Ho342 content, up to a maximum of 35% of the level found in the parent as assessed by flow cytometry, despite similar levels of whole cell uptake determined using radiolabelled ligand. Ability to limit nuclear accessibility of Ho342 correlated closely with cellular resistance to Ho342 and to adriamycin. All drug resistant cell lines showed a significant increase in nuclear accessibility to Ho342 after verapamil treatment, including a methotrexate resistant cell line. The methotrexate resistant variant, not demonstrating MDR, showed reduced nuclear binding of Ho342 but increased cell kill associated with a propensity to develop a population of cells showing extra DNA replication in response to Ho342 exposure. Differences between cell lines in the relationship between Ho342-induced cell cycle perturbation and cell kill supported the conclusion that modulation of several pathways of response to cytotoxic agents had occurred in the development of drug resistance.

Mitoxantrone-DNA binding and the induction of topoisomerase II associated DNA damage in multi-drug resistant small cell lung cancer cells

The cytotoxicity anti-tumour intercalating agents such as the anthraquinone mitoxantrone is thought to relate to DNA binding and the trapping of DNA topoisomerase II complexes on cellular DNA. We have studied the uptake, nuclear location, DNA binding mode and DNA damaging capacity of mitoxantrone in a small cell lung carcinoma cell line (NCI-H69) compared with an in vitro-derived variant subline (NCI-H69/LX4) that exhibits "classical" multi-drug resistance (MDR). Variant cells maintained under doxorubicin selection showed reduced RNA levels that returned to control values within 7 days of growth under non-selective conditions. Variant cells released from selection stress showed resistance to DNA cleavage by doxorubicin, mitoxantrone, 4'-epidoxorubicin, 4'-deoxy-doxorubicin but reduced resistance to aclacinomycin A and a 9-alkyl substituted anthracycline in broad agreement with the cross-resistance patterns for cytotoxicity. Mitoxantrone treated NCI-H69 cells were found to accumulate DNA-protein crosslinks during a 4 hr post-treatment incubation period whereas variant cells maintained depressed levels of crosslinking. There was no apparent abnormality in the availability or drug sensitivity of topoisomerase II assayed in crude nuclear extracts of NCI-H69/LX4 cells. Whole cell uptake of radiolabelled mitoxantrone was depressed (50%) in NCI-H69/LX4 compared with NCI-H69, whereas assessment of nuclear-bound drug in individual cells by a fluorescence quenching technique showed at least a 10-fold greater level of target protection. The quenching results provide evidence of a high affinity, saturable mode of drug binding, favoured at low drug concentrations, that correlated with DNA cleavage capacity. We propose that the cytotoxic action of mitoxantrone is dependent upon a restricted and persistent form of binding to DNA that favours the long-term or progressive trapping of topoisomerase II complexes.

Mitomycin C cross-resistance induced by adriamycin in human ovarian cancer cells in vitro

We prepared Adriamycin-resistant cancer cells by exposing an ovarian serous cystadenocarcinoma cell line to the drug. The resistant cells also showed cross-resistance to a wide variety of other compounds, including vincristine, vinblastine, actinomycin D, daunorubicin, mitomycin C and carboquone. Against vincristine, the cells showed a greater than 5,000-fold increase in resistance, far surpassing their resistance to the selection drug. The resistant cells displayed a decrease in intracellular Adriamycin content and an increase in the mRNA of the mdr-1 gene coding for P-glycoprotein, with no amplification of the DNA. In revertant cells, resistance to Adriamycin was lost, but that to mitomycin C was maintained. Adriamycin resistance was partially overcome by the addition of verapamil or cyclosporin A, but cross-resistance to mitomycin C was not influenced at all. These results strongly suggest that the resistance to mitomycin C observed in our Adriamycin-resistant cells was due to some other mechanism than that causing multidrug resistance.

Chemosensitisation by verapamil and cyclosporin A in mouse tumour cells expressing different levels of P-glycoprotein and CP22 (sorcin)

The relationships between resistance to adriamycin, vincristine, colchicine and etopside, expression of P-glycoprotein and CP22 (sorcin), and resistance modification by verapamil and cyclosporin A have been studied in a panel of multidrug-resistant (MDR) mouse tumour cell lines. Whereas there was a generally good correlation between the degree of resistance and the amount of P-glycoprotein, no relationship between resistance and CP22 expression was seen. At 3.3 microM verapamil, the sensitisation of the MDR cell lines was no greater than that of the parent line. At 6.6 microM verapamil, however, sensitisation of the MDR lines generally exceeded that of the parent line, although the line CR 2.0, expressing very high levels of P-glycoprotein was an exception. Little sensitisation to etoposide was seen in any of the lines. When cyclosporin A was used as the sensitiser at either 2.1 or 4.2 microM, there was a greater effect in lines expressing moderate to high levels of P-glycoprotein than in the parent line, although this tendency was less for adriamycin than for the other cytotoxics. Sensitisation to etoposide was much greater with cyclosporin A than with verapamil. At low levels (less than 1 microM) of CsA, however, sensitisation to colchicine was greater in the parent line than in cell line CR 2.0. These studies indicate that chemosensitisation by verapamil and cyclosporin A is extremely complex, depending upon sensitiser dose, the particular cytotoxic and the cell line. At low doses of the sensitisers, the sensitisation may be greater in lines expressing low levels of P-glycoprotein than in lines showing high levels.

Patterns of cross-resistance in a multidrug-resistant small-cell lung carcinoma cell line

H69AR is a multidrug-resistant human small-cell lung carcinoma cell line that was selected in doxorubicin and has previously been shown to be cross-resistant to a variety of natural-product-type anticancer drugs. H69AR is unlike many other multidrug-resistant cell lines in that it does not overexpress P-glycoprotein. In the present study, the drug sensitivity and cross-resistance patterns of H69AR cells were further characterized. A total of 15 drugs belonging to a number of chemical classes were screened. These compounds included anthracyclines, DNA binders (anthrapyrazoles, benzothiopyranoindazoles, and pyrazoloacridines), and lipophilic antifolates. The alkylating agent melphalan and the antimetabolite cytosine arabinofuranoside (Ara-C) were also tested. In general, the drug sensitivity and cross-resistance profiles of H69AR cells were consistent with those reported by others using other drug-resistant cell lines. However, there were several unexpected instances of cross-resistance. Thus, the H69AR cell line was more resistant than its parent cell line to the potent 3'-deamino-3'-(3-cyano-4-morpholinyl) doxorubicin, bisantrene, the pyrazoloacridine PD 114541, Ara-C, and melphalan. In addition, no cross-resistance to the four lipophilic antifolates tested, including trimetrexate, was found. The absence of a consistent pattern among the various drug-resistant cell lines indicates that assumptions about the efficacy of anticancer drugs in multidrug resistance should be made with caution.

Characterisation of a mouse tumour cell line with in vitro derived resistance to verapamil

We have established a subline (EMT6/VRP) of the mouse tumour cell line EMT6/P with acquired resistance to the calcium transport blocker verapamil (VRP). The subline was 4-fold resistant to the cytoxicity of VRP alone compared with the parent line but of similar sensitivity to adriamycin, vincristine or colchicine. EMT6/VRP cells growing in 75 micrograms ml-1 VRP were morphologically different from and larger in diameter than EMT6/P cells, but these two parameters reverted almost to normal within 3 days of VRP removal, although resistance was retained. Expression of an mRNA coding for P-glycoprotein was similar in EMT6/VRP and the parent cell line, although considerable hyperexpression was seen in a multidrug resistant subline, EMT6/AR1.0. Cellular accumulation of both 3H-daunorubicin and 3H-VRP were greater in EMT6/VRP than in the parent line. Sensitisation to adriamycin by 3.3 micrograms ml-1 VRP was, however, somewhat reduced in EMT6/VRP (i.e. to 6.1-fold) compared with the 11-fold sensitisation seen in the parent line. It is clear that resistance to VRP seen in this cell line occurs via a different mechanism from the resistance to drugs such as adriamycin, vincristine and colchicine seen in multidrug resistant cell lines.

9-Alkyl, morpholinyl anthracyclines in the circumvention of multidrug resistance

The intramolecular combination of 9-alkyl substitution in the anthracycline A-ring plus incorporation of the amino group of the daunosamine sugar within a morpholinyl ring led to the retention of almost complete activity against P-glycoprotein positive, multidrug resistant variants of a mouse mammary tumour line and a human small cell lung cancer line. Resistance factors were close to unity. These structural elements may prevent efflux by the P-glycoprotein multidrug transporter. The use of 9-alkyl, morpholinyl anthracyclines with resistance circumvention properties may have clinical application.

Improved cellular accumulation is characteristic of anthracyclines which retain high activity in multidrug resistant cell lines, alone or in combination with verapamil or cyclosporin A

We have examined the cellular accumulation of anthracycline compounds, alone or in conjunction with resistance modifiers, in an attempt to identify mechanisms by which multidrug resistance (MDR) can be circumvented. This was facilitated by using the EMT6 mouse mammary tumour cell line EMT6/P and its MDR subline EMT6/AR1.0 with 30-fold resistance to Adriamycin (ADM), and the human small cell lung cancer line H69/P together with its MDR subline H69/LX4 with 100-fold resistance to ADM. Both MDR lines hyperexpress membrane P-170 glycoprotein. The accumulation of ADM was compared to that seen for the anthracycline analogues aclacinomycin A (ACL), Ro 31-1215 and 4'-deoxy-4'-iodo-Adriamycin (iodo-ADM). These analogues were selected because of their high activity against MDR sublines, including H69/LX4 and EMT6/AR1.0. Both MDR cell lines exhibited a deficiency in ADM accumulation compared to the parent lines. Smaller differentials were seen using Ro 31-1215 or iodo-ADM. Both resistant sublines were able to accumulate ACL in identical amounts to their respective parental sublines. Improved drug accumulation is likely to contribute to the improved activity of the analogues against MDR cell lines. However, the relative accumulation defects in the resistant lines did not correlate exactly with the degree of resistance to a particular compound. Cyclosporin A (5 micrograms/ml) or verapamil (3.3 micrograms/ml) caused a preferential increase in uptake in both MDR sublines, with a small or negligible effect for the parental line. A smaller effect was observed with iodo-ADM and Ro 31-1215, and levels of ACL were unchanged in the MDR lines in the presence of either resistance modifier. These results indicate two mechanisms for circumventing drug resistance due to reduced drug accumulation. Structurally modified derivatives can partially or completely eliminate uptake differentials between parent and drug resistant cell lines. Any residual uptake can be eliminated using resistance modifiers. The two mechanisms may both operate via inhibition or circumvention of P-170 mediated efflux. The situation is complex, however, and this study indicates the possible involvement of additional resistance mechanisms.