Camptothecin resistance: role of the ATP-binding cassette (ABC), mitoxantrone-resistance half-transporter (MXR), and potential for glucuronidation in MXR-expressing cells

The mitoxantrone resistance (MXR) gene encodes a recently characterized ATP-binding cassette half-transporter that confers multidrug resistance. We studied resistance to the camptothecins in two sublines expressing high levels of MXR: S1-M1-80 cells derived from parental S1 colon cancer cells and MCF-7 AdVp3,000 isolated from parental MCF-7 breast cancer cells. Both cell lines were 400- to 1,000-fold more resistant to topotecan, 9-amino-20(S)-camptothecin, and the active metabolite of irinotecan, 7-ethyl-10-hydroxycamptothecin (SN-38), than their parental cell lines. The cell lines demonstrated much less resistance to camptothecin and to several camptothecin analogues. Reduced accumulation and energy-dependent efflux of topotecan was demonstrated by confocal microscopy. A significant reduction in cleavable complexes in the resistant cells could be observed after SN-38 treatment but not after camptothecin treatment. In addition to topotecan and SN-38, MXR-overexpressing cells are highly resistant to mitoxantrone and epirubicin. Because these compounds are susceptible to glucuronidation, we examined UDP-glucurono-syltransferase (UGT) activity in parental and resistant cells by TLC. Glucuronides were found at equal levels in both parental and resistant colon cancer cell lines for epirubicin and to a lesser extent for SN-38 and mitoxantrone. Low levels of glucuronidation could also be detected in the resistant breast cancer cells. These results were confirmed by analysis of the UGT1A family mRNAs. We thus conclude that colon and breast cancer cells have a capacity for glucuronidation that could contribute to intrinsic drug resistance in colon cancer cells and may be acquired in breast cancer cells. The lack of selection for higher levels of UGT capacity in the colon cells suggests that high levels of expression of MXR alone are sufficient to confer resistance to the camptothecins.     

Single-nucleotide polymorphism (SNP) analysis in the ABC half-transporter ABCG2 (MXR/BCRP/ABCP1)

Variations in the amino acid sequence of ABC transporters have been shown to impact substrate specificity. We identified two acquired mutations in ABCG2, the ABC half-transporter overexpressed in mitoxantrone-resistant cell lines. These mutations confer differences in substrate specificity and suggest that naturally occurring variants could also affect substrate specificity. To search for the existence of single nucleotide polymorphisms (SNPs) in ABCG2, we sequenced 90 ethnically diverse DNAs from the Single Nucleotide Polymorphism Discovery Resource representing the spectrum of human genotypes. We identified 3 noncoding SNPs in the untranslated regions, 3 nonsynonymous and 2 synonymous SNPs in the coding region and 7 SNPs in the intron sequences adjacent to the sixteen ABCG2 exons. Nonsynonymous SNPs at nucleotide 238 (V12M; exon 2) and nucleotide 625 (Q141K; exon 5) showed a greater frequency of heterozygosity (22.2% and 10%) than the SNP at 2062 (D620N; exon 16). Heterozygous changes at nucleotide 238 are in linkage disequilibrium with an SNP observed 36 bases downstream from the end of exon 2. No polymorphism at amino acid 482 was identified to correspond to the R to G or R to T mutations previously found in two drug resistant cell lines. Among 23 drug resistant sublines for which sequence at position 482 was determined, no additional mutations were found. Heterozygosity at amino acid 12 allowed us to identify overexpression of a single allele in a subset of drug resistant cell lines, a feature that could be exploited clinically in evaluating the significance of ABCG2 expression in malignancy. We conclude that ABCG2 is well conserved and that described amino acid polymorphisms seem unlikely to alter transporter stability or function.     

顺铂逆转鼻咽癌紫杉醇耐药细胞的耐药性

目的:观察顺铂是否能够逆转鼻咽癌紫杉醇耐药细胞HNE-2/taxol和5-8F/taxol的耐药性。方法:运用集落形成实验观察不同浓度顺铂对鼻咽癌亲本细胞(HNE-2及5-8F)和紫杉醇耐药细胞(HNE-2/taxol及5-8F/taxol)的生长抑制率,并且判断经低浓度顺铂预处理后,耐药细胞耐药指数的改变。通过等效剂量和联用指数分析,评价紫杉醇和顺铂联合用药的效果。运用FCM法检测不同浓度顺铂处理后鼻咽癌亲本细胞和耐药细胞的凋亡率变化。结果:两种鼻咽癌紫杉醇耐药细胞株对顺铂的敏感性均显著高于其亲本细胞(P<0.05)。顺铂与紫杉醇合用时,对鼻咽癌亲本细胞的增殖抑制能够产生相加作用,而对耐药细胞的增殖抑制则能够产生协同作用。经过低浓度顺铂预处理后,耐药细胞的耐药指数明显降低。在顺铂作用下,鼻咽癌紫杉醇耐药细胞的早期凋亡率显著高于亲本细胞(P<0.05)。结论:顺铂可以逆转鼻咽癌细胞的紫杉醇耐药性。顺铂和紫杉醇联合作用于鼻咽癌紫杉醇耐药细胞,能够产生协同效应。     

The acridonecarboxamide GF120918 potently reverses P-glycoprotein-mediated resistance in human sarcoma MES-Dx5 cells

The doxorubicin-selected, P-glycoprotein (P-gp)-expressing human sarcoma cell line MES-Dx5 showed the following levels of resistance relative to the non-P-gp-expressing parental MES-SA cells in a 72 h exposure to cytotoxic drugs: etoposide twofold, doxorubicin ninefold, vinblastine tenfold, taxotere 19-fold and taxol 94-fold. GF120918 potently reversed resistance completely for all drugs. The EC50s of GF120918 to reverse resistance of MES-Dx5 cells were: etoposide 7+/-2 nM, vinblastine 19+/-3 nM, doxorubicin 21+/-6 nM, taxotere 57+/-14 nM and taxol 91+/-23 nM. MES-Dx5 cells exhibited an accumulation deficit relative to the parental MES-SA cells of 35% for [3H]-vinblastine, 20% for [3H]-taxol and [14C]-doxorubicin. The EC50 of GF120918, to reverse the accumulation deficit in MES-Dx5 cells, ranged from 37 to 64 nM for all three radiolabelled cytotoxics. [3H]-vinblastine bound saturably to membranes from MES-Dx5 cells with a KD of 7.8+/-1.4 nM and a Bmax of 5.2+/-1.6 pmol mg(-1) protein. Binding of [3H]-vinblastine to P-gp in MES-Dx5 membranes was inhibited by GF120918 (K = 5+/-1 nM), verapamil (Ki = 660+/-350 nM) and doxorubicin (Ki = 6940+/-2100 nM). Taxol, an allosteric inhibitor of [3H]-vinblastine binding to P-gp, could only displace 40% of [3H]-vinblastine (Ki = 400+/-140 nM). The novel acridonecarboxamide derivative GF120918 potently overcomes P-gp-mediated multidrug resistance in the human sarcoma cell line MES-Dx5. Detailed analysis revealed that five times higher GF120918 concentrations were needed to reverse drug resistance to taxol in the cytotoxicity assay compared to doxorubicin, vinblastine and etoposide. An explanation for this phenomenon had not been found.     

Reversal of multi-drug resistance in human KB cell lines by structural analogs of verapamil

Several structural analogs of verapamil were studied for their ability to reverse multi-drug resistance (MDR) in human KB cell lines. D595, D792 and verapamil completely reversed resistance to colchicine and adriamycin. D595 and D792 had a higher reversing potency than verapamil. Devapamil, gallopamil, emopamil and D528 partially reversed MDR. The reversing potency of a drug did not correlate with its calcium antagonistic activity. No differences in reversing potency between (R)-isomers, (L)-isomers and the racemic forms were observed in the case of both verapamil and emopamil. (R)-verapamil, which has less calcium antagonistic activity and less in vivo toxicity than racemic verapamil, and D792, which has higher reversing potency and less in vivo toxicity than racemic verapamil, should be suitable for clinical applications to overcome drug resistance in cancer patients.     

卡维地洛逆转膀胱癌细胞株多药耐药性

目的：研究卡维地洛对人膀胱癌耐药细胞株BIU-87／ADM的多药耐药性逆转作用。方法：应用MTT法检测人膀胱癌耐药细胞株(BIU-87／ADM)的耐药性；将卡维地洛与多柔比星联合作用于B10-87和BIU-87／ADM细胞，检测细胞内多柔比星聚集量。结果：卡维地洛与多柔比星共同温育BIU-87／ADM细胞后．能使BIU-87／ADM细胞内多柔比星聚积量增加，与单用多柔比星比较，差异有显著性。结论：卡维地洛对多柔比星有增敏作用，卡维地洛可逆转人膀胱癌细胞株BIU-87／ADM的多药耐药性。     

Increased oral bioavailability of topotecan in combination with the breast cancer resistance protein and P-glycoprotein inhibitor GF120918.

PURPOSE: We discovered that breast cancer resistance protein (BCRP), a recently identified adenosine triphosphate-binding cassette drug transporter, substantially limits the oral bioavailability of topotecan in mdr1a/1b(-/-) P-glycoprotein (P-gp) knockout and wild-type mice. GF120918 is a potent inhibitor of BCRP and P-gp. The aim was to increase the bioavailability of topotecan by GF120918. PATIENTS AND METHODS: In cohort A, eight patients received 1.0 mg/m(2) oral topotecan with or without coadministration of one single oral dose of 1,000 mg GF120918 (day 1 or day 8). In cohort B, eight other patients received 1.0 mg/m(2) intravenous topotecan with or without 1,000 mg oral GF120918 to study the effect of GF120918 on the systemic clearance of topotecan. RESULTS: After oral topotecan, the mean area under the plasma concentration-time curve (AUC) of total topotecan increased significantly from 32.4 +/- 9.6 microg.h/L without GF120918 to 78.7 +/- 20.6 microg.h/L when GF120918 was coadministered (P =.008). The mean maximum plasma concentration of total topotecan increased from 4.1 +/- 1.5 microg/L without GF120918 to 11.5 +/- 2.4 microg/L with GF120918 (P =.008). The apparent bioavailability in this cohort increased significantly from 40.0% (range, 32% to 47%) to 97.1% (range, 91% to 120%) (P =.008). Interpatient variability of the apparent bioavailability was 17% without and 11% with GF120918. After intravenous administration of topotecan, coadministration of oral GF120918 had a small but statistically significant effect on the AUC and systemic clearance of total topotecan but no statistically significant effect on maximum plasma concentration and terminal half-life of total topotecan. CONCLUSION: Coadministration of the BCRP and P-gp inhibitor GF120918 resulted in a significant increase of the systemic exposure of oral topotecan. The apparent oral bioavailability increased from 40.0% without to 97.1% with GF120918.     

Increased oral bioavailability of paclitaxel by GF120918 in mice through selective modulation of P-glycoprotein.

Previous studies in mice with disrupted mdr1a P-glycoprotein genes have shown that the oral bioavailability of paclitaxel is very low because of the presence of this drug-transporting protein in the intestinal wall. Additional studies with cyclosporin A have shown that this P-glycoprotein-inhibiting agent is able to increase the bioavailability of paclitaxel in mouse models and in patients. However, the potential immune-suppressive side effects of cyclosporin A renders this compound less suitable for chronic use in cancer patients. In this paper we present the results obtained with GF120918, an experimental P-glycoprotein inhibitor, on the oral bioavailability of paclitaxel in both wild-type and mdrlab knockout mice. GF120918 (25 mg/kg) was administered p.o. by gavage 15 min or 2 h before oral or i.v. dosing of paclitaxel, respectively. Paclitaxel plasma levels were quantified by high-performance liquid chromatography. GF120918 increased the plasma values for areas under the concentration-time curve of oral paclitaxel in wild-type mice by 6.6-fold from 408 to 2701 ng x ml(-1) h. Calculated relative to their respective values for area under the concentration-time curve after i.v. administration, GF120918 increased the oral bioavailability of paclitaxel in wild-type mice from 8.5 to 40.2%. The plasma pharmacokinetics of paclitaxel in mdr1ab knockout mice was not altered by GF120918, whereas the pharmacokinetics of paclitaxel in wild-type mice receiving GF120918 became comparable with mdr1ab knockout mice. This result indicates that GF120918 at this dose-level selectively and completely blocks P-glycoprotein in the intestines and does not notably interfere in the elimination of paclitaxel by metabolism or other transporters. On the basis of this result, GF120918 has been selected for additional study in humans.     

Co-administration of GF120918 significantly increases the systemic exposure to oral paclitaxel in cancer patients

Oral bioavailability of paclitaxel is very low, which is due to efficient transport of the drug by the intestinal drug efflux pump P-glycoprotein (P-gp). We have recently demonstrated that the oral bioavailability of paclitaxel can be increased at least 7-fold by co-administration of the P-gp blocker cyclosporin A (CsA). Now we tested the potent alternative orally applicable non-immunosuppressive P-gp blocker GF120918. Six patients received one course of oral paclitaxel of 120 mg/m(2)in combination with 1000 mg oral GF120918 (GG918, GW0918). Patients received intravenous (i.v.) paclitaxel 175 mg/m(2)as a 3-hour infusion during subsequent courses. The mean area under the plasma concentration-time curve (AUC) of paclitaxel after oral drug administration in combination with GF120918 was 3.27 +/- 1.67 microM x h. In our previously performed study of 120 mg/m(2)oral paclitaxel in combination with CsA the mean AUC of paclitaxel was 2.55 +/- 2.29 microM x h. After i.v. administration of paclitaxel the mean AUC was 15.92( )+/- 2.46 microM x h. The oral combination of paclitaxel with GF120918 was well tolerated. The increase in systemic exposure to paclitaxel in combination with GF120918 is of the same magnitude as in combination with CsA. GF120918 is a good and safe alternative for CsA and may enable chronic oral therapy with paclitaxel.     

Circumvention of breast cancer resistance protein (BCRP)-mediated resistance to camptothecins in vitro using non-substrate drugs or the BCRP inhibitor GF120918.

This study was aimed at characterizing the role of BCRP/MXR/ABCP (BCRP) in resistance of the human ovarian tumor cell lines T8 and MX3 to camptothecins more extensively and investigating whether resistance can be reversed by inhibiting BCRP by GF120918. Camptothecins studied were topotecan, CPT-11, and its active metabolite SN-38, 9-aminocamptothecin, and the novel experimental camptothecins NX211, DX8951f, and BNP1350. Notably, DX8951f and BNP1350 appeared to be very poor substrates for BCRP, with much lower resistance factors observed both in T8 and MX3 cells than observed for the other camptothecins tested. In the presence of a nontoxic dose level of GF120918, the intracellular accumulation of topotecan in the T8 and MX3 cells was completely restored to the intracellular levels observed in the sensitive IGROV1 parental cell line. This resulted in almost complete reversal of drug resistance to topotecan and to most of the other topoisomerase I drugs tested in the T8 cell line and to complete reversal in the MX3 cells. However, coincubation of DX8951f or BNP1350 with GF120918 did not affect the cytotoxicity of either of these drugs significantly. From the combined data, we conclude that the affinities of topoisomerase I drugs for BCRP are, in decreasing order: SN-38 > topotecan > 9-aminocamptothecin approximately CPT-11 > NX211 > DX8951f > BNP1350. Furthermore, GF120918 appears to be a potent reversal agent of BCRP-mediated resistance to camptothecins, with almost complete reversal noted at 100 nM. Potential BCRP-mediated resistance to topoisomerase I inhibitors can also be avoided by using the BCRP-insensitive drugs DX8951f or BNP1350. This observation may have important clinical implications for future development of novel camptothecins.     

Reversal of MDR1-associated resistance to topotecan by PAK-200S, a new dihydropyridine analogue, in human cancer cell lines

Recent data suggest that expression of the membrane P170-glycoprotein (P-gp) may confer resistance to the topoisomerase-I-interactive agent topotecan. The present study describes the cellular effects of a new dihydropyridine analogue, PAK-200S, on P-gp-mediated resistance to topotecan in human breast and ovarian tumour cells. PAK-200S at a non-cytotoxic concentration of 2.0 microM completely reversed resistance to topotecan in P-gp-expressing MCF-7/adr (breast) and A2780/Dx5 (ovarian) tumour cells, respectively, with no effects on parental cells. Cellular pharmacokinetic studies by reversed-phase high-performance liquid chromatography analysis showed significantly lower cellular drug concentrations of the pharmacologically active closed-ring lactone of topotecan in multidrug-resistant cells than in parental cells. PAK-200S was effective in restoring the cellular lactone concentrations of topotecan in resistant MCF-7/adr cells to levels comparable to those obtained in parental cells. Furthermore, exposure of MCF-7/adr cells to topotecan in the presence of PAK-200S significantly increased the induction of protein-linked DNA breaks. PAK-200S did not alter nuclear topoisomerase I-mediated ex vivo pBR322 DNA plasmid unwinding activity and topoisomerase-I protein expression. These results suggest that reversal of P-gp-mediated resistance to topotecan by PAK-200S was related to the restoration of cellular drug concentrations of the active lactone form of topotecan rather than a direct effect on topoisomerase-I function.     

补骨脂素逆转人乳腺癌细胞多药耐药性的研究

目的研究中药补骨脂素对人乳腺癌细胞多耐药性的逆转作用.方法用MTT法测定药物的细胞毒性和IC50用流式细胞仪测定耐药细胞P170糖蛋白表达,并选异搏定作阳性对照,观察具有钙拮抗作用的补骨脂素对MCF-7/ADR多药耐药性的逆转作用.结果补骨脂素在非细胞毒性剂量下能使MCF-7/ADR对阿霉素的浓度升高,但对细胞表面的糖蛋白P-170却没有影响.结论补骨脂素具有逆转人乳腺癌MCF-7/ADR多药耐药性的作用.     

Reversal of multidrug resistance by verapamil and modulation by alpha 1-acid glycoprotein in wild-type and multidrug-resistant Chinese hamster ovary cell lines

Multidrug resistance can be reversed by a range of "calcium channel"-blocking drugs in vitro, of which verapamil is the most widely used. Verapamil is bound to alpha 1-acid glycoprotein (AAG) in vivo in humans but is absent from calf serum, used in tissue culture media. The effect of AAG on the ability of verapamil to alter Adriamycin cytotoxicity was assessed in parental Chinese hamster ovary cells (CHO-K1) and in a multidrug-resistant subline (CHO-Adrr). In both the parental and the resistant cells, there was a dose-related potentiation of Adriamycin cytotoxicity by verapamil. At 10 microM verapamil, there was a 5-fold decrease in the concentration of Adriamycin that caused 50% reduction in growth of CHO-K1 cells but a 15-fold decrease in CHO-Adrr cells. In the presence of increasing AAG concentrations within the range found in cancer patients, there was a concentration-related reduction in the effects of verapamil. In CHO-Adrr cells, there was complete reversal of the potentiating effect of 10 microM verapamil at 2 mg/ml AAG. In contrast, in CHO-K1 cells, AAG reduced the effects of verapamil by only 20% at a similar concentration. There was a much higher internal uptake of fluorescein-labeled AAG by CHO-Adrr cells than by CHO-K1 cells. These results suggest that, in addition to a plasma membrane site, there may be a major endosomal site of action of verapamil in multidrug-resistant cells. The implications are that verapamil in vivo in the presence of AAG may effectively reverse low levels of multidrug resistance, but not high levels. Thus selection of patients with low AAG levels may be appropriate for clinical studies.     

Gene expression in X-irradiated human tumour cell lines expressing cisplatin resistance and altered DNA repair capacity

Previous studies have indicated that excision repair genes,such as ERCC1, or early response genes, such as c-fos, may playa significant role in regulating cellular responses to cisplatin(CDDP) by mediating DNA synthesis and repair pathways. Thispresent study aimed to determine whether altered gene expressionmediated CDDP resistance expressed in two human tumour sublinesfollowing their in vitro exposure to fractionated X-irradiation,not to the drug itself. These sublines, designated SuSa/DXR10and SKOV-3/DXR10, established respectively from a testicularteratoma cell line (SuSa) or an ovarian carcinoma cell line(SKOV-3), expressed stable 3.1- and 2-fold levels of CDDP resistance,as judged by clonogenic assay. Both sublines expressed c-fos,c-myc and thymidylate synthase (TS) RNA constitutively, butat comparable levels to their parental counterparts. Whilstthe ovarian carcinoma cells inherently expressed markedly higherlevels (30- to 50-fold) of the excision repair gene ERCC1 thanthe teratoma cells, only the teratoma DXR10 subline showed anincreased level of expression of ERCC1 mRNA relative to theirparental cells. Expression of the ERCC3/XPB gene encoding arepair helicase, however, was similar in all the lines tested.The results suggest that CDDP resistance may be mediated bydifferent mechanisms in these DXR10 sublines from those previouslyidentified in drug-selected CDDP-resistant human ovarian A2780/DDPcells.     

Reversal of multidrug resistance by synthetic isoprenoids in the KB human cancer cell line

A colchicine resistant clone, Chr-24, derived from the human carcinoma KB cell line is extensively resistant to multiple drugs including vinblastine, vincristine, Adriamycin, actinomycin D, and daunomycin. In comparison with KB cells, very low accumulation of daunomycin or vincristine is observed in multidrug-resistant cells. Two isoprenoids with 9 to 10 isoprene chains (polyprenoids), N-(p-methylbenzyl)decaprenylamine and N-solanesyl-N,N'-bis(3,4-dimethoxybenzyl)ethylenediamine overcame the multidrug resistance almost completely in cultured Chr-24, whereas they only slightly sensitized the parental KB cells to anticancer agents. Both isoprenoids enhance the accumulation of vincristine or daunomycin in Chr-24, possibly by inhibiting efflux and also by enhancing influx of anticancer agents. A verapamil-like structure of N-solanesyl-N,N'-bis(3,4-dimethoxybenzyl)ethylenediamine is discussed in relation to its ability to overcome drug resistance.     

Porphyrin photosensitivity in cell lines expressing a heat-resistant phenotype

In vitro sensitivity to porphyrin-mediated photodynamic therapy (PDT) has been examined in cell lines resistant to hyperthermia. Parental (HA-1) and heat-resistant (3012) Chinese hamster fibroblasts as well as parental (RIF-1) and temperature-resistant (TR-4, TR-5, and TR-10) mouse radiation-induced fibrosarcoma cells were evaluated for thermal and PDT sensitivity. Quantitative survival curves were generated, and porphyrin uptake properties were obtained for all cell lines. Significant resistance to hyperthermia (45 degrees C for varying exposure periods) was documented for the 3012 and temperature-resistant RIF cell strains when compared with the parent lines. Normal and heat-resistant clones, however, exhibited comparable levels of porphyrin uptake and photosensitivity. Our results indicate that cross-resistance between hyperthermia and PDT is not observed and that members of the Mr 70,000 heat shock protein family (which are elevated in the thermal-resistant cells and which may be associated with the heat-resistant phenotype) do not play a significant role in modulating PDT sensitivity. Mechanisms of in vitro cytotoxicity appear to be different for PDT and hyperthermia even though possible subcellular targets (such as the plasma membrane) and types of damage (protein denaturation) may be similar for the two modalities.     

Overexpression of the ABC transporter TAP in multidrug-resistant human cancer cell lines.

Multidrug resistance (MDR) to anti-cancer drugs has been associated with the overexpression of P-glycoprotein (P-gp) and the multidrug resistance-associated protein (MRP), both being members of the ATP-binding cassette (ABC) superfamily of transporters. We investigated whether in addition to P-gp and MRP, another ABC transporter, the transporter associated with antigen processing (TAP), is associated with MDR. TAP plays a major role in MHC class I-restricted antigen presentation by mediating peptide translocation over the endoplasmic reticulum membrane. TAP1 and P-gp share a significant degree of homology among their transmembrane domains, which are thought to be the primary determinants of substrate specificity, and both can apparently mediate the translocation of peptides. Using immunocytochemistry and Western blot, TAP was overexpressed in parallel with MHC class I in several MDR human cancer cell lines. TAP was overexpressed more frequently in MRP-positive MDR cell lines (three out of three) than in P-gp positive MDR cells (two out of five). Reversal of resistance resulted in a decrease in TAP levels. Transfection of the TAP genes into TAP-deficient lymphoblastoid T2 cells conferred mild resistance to etoposide, vincristine and doxorubicin (2- to 2.5-fold). Furthermore, etoposide and vincristine inhibited TAP-dependent peptide translocation to the endoplasmic reticulum. Collectively, our results suggest that TAP may modestly contribute to the MDR phenotype, in particular in MRP- overexpressing MDR cells. Further insight into the role of TAP in MDR will require the study of other transfectants, as well as the investigation of TAP expression in P-gp and MRP-negative MDR cancer cell lines.     

A phase I, randomized, open-label, parallel-cohort, dose-finding study of elacridar (GF120918) and oral topotecan in cancer patients.

PURPOSE: Breast cancer resistance protein (ABCG2) substantially limits the oral bioavailability of topotecan. Coadministration with elacridar, an inhibitor of breast cancer resistance protein-mediated drug transport, increases the bioavailability of topotecan. The aim of this study was to establish the lowest effective dose of elacridar to obtain maximum oral bioavailability of topotecan and to determine the optimal schedule of coadministration of oral topotecan and elacridar. In the second part of this study, dose-limiting toxicities and maximum tolerated dose of oral topotecan coadministered with elacridar, at a daily times five regimen administered every 21 days, were established. EXPERIMENTAL DESIGN: In part I, 20 patients were randomized to receive 100, 300, 500, 700, or 1,000 mg of elacridar on days 1 and 8 1 h before or simultaneously with 2.0 mg oral topotecan, which was also randomized. On day 15, all patients were treated with 1.5 mg/m(2) i.v. topotecan. In part II of the study, patients were treated daily with oral topotecan and with the lowest effective dose of elacridar following from part I. The maximum tolerated dose and dose-limiting toxicity were determined in cohorts of three patients. Blood samples were taken on days 1, 8, and 15 of part I and on day 1 of cycles 1 and 2 of part II. RESULTS: Complete apparent oral bioavailability of topotecan (102 +/- 7%) for all treatment arms with elacridar in both schedules was seen in part I. In the topotecan dose escalation part, two dose-limiting toxicities were seen at the 2.5 mg topotecan dose level. CONCLUSION: The recommended schedule is 2.0 mg oral topotecan plus 100 mg elacridar administered concomitantly daily times five every 21 days.