Dose selection and tolerability of anticancer agents evaluated by the European Medicines Agency in the period 2015-2020.

BACKGROUND: Novel anticancer agents are initially evaluated in a palliative setting in phase I studies. The benefit-risk applying the selected dose from these phase I studies can be considered acceptable at time of registration, however, it is unknown if the optimal dose has been selected during drug development. METHODS: The European Medicines Agency (EMA) European Public Assessment Reports (EPARs) overview was used to select anticancer agents evaluated between 2015 and 2020. The dose selection and tolerability data of EMA assessed anticancer agents was analysed to evaluate dose selection. RESULTS: Sixty EPARs were included for analysis. A dose-response relation was identified in five dossiers (8%). The maximum tolerated dose (MTD) was the selected dose for 15 anticancer agents (25%). The MTD was not determined in 27 out of 60 cases (59%). When the MTD was determined but not applied as final dose, the most frequently used dose selection criteria were the combination of toxicity, exposure response, pharmacokinetic data and pharmacodynamic data (in 7 out of 18 cases). Data on tolerability were analysed separately for protein kinase inhibitors and monoclonal antibodies as the dosing interval and mitigation of adverse events (AEs) differs. The median discontinuation, dose reduction and dose interruption rates due to AEs of protein kinase inhibitors were 10%, 26% and 45% for monotherapy and 13%, 47% and 55% for combination therapy, respectively. The median discontinuation rates due to AEs for monoclonal antibodies were 8% for monotherapy and 26% for combination therapy. CONCLUSION: The dose-response relationship has not been established for the majority of the registered anticancer agents. The selected posology is often poorly tolerable as reflected by the high discontinuation and dose reduction rates. Due to the absence of dose-response data, it is often unknown if the optimal dose has been selected for anticancer agents.

Lymph node harvest in breast cancer patients with and without preoperative scintigraphy

Background: Sentinel lymph node biopsy (SLNB) is the standard of care for women with node negative breast cancer. Tc99 nanocolloid is a popular tracer, facilitating preoperative lymphoscintigraphy to identify sentinel nodes. This study compares the number of sentinel lymph nodes identified at the time of surgery in women who had lymphoscintigraphy with those who did not.Method: All women with primary breast cancer undergoing a SLNB from 2003­2017 who were node negative and did not have neoadjuvant chemotherapy were retrospectively reviewed in this study.Results: 576 women who had Tc99 preoperative injection were included in the study. Three hundred patients (Group 1) underwent lymphoscintigraphy preoperatively and 276 (Group 2) had no lymphoscintigraphy. The mean age at diagnosis for Groups 1 and 2 were 56 and 58 years, respectively. Both groups were also found to be similar in terms of histological subtype, tumour location, size, and HER2- and Estrogen receptor (ER) status. The surgery for the breast primary was similar in groups 1 and 2. While both groups had a median number of 2 nodes identified, the mean number of nodes identified for Group 2 (2.12) was 10% higher than for Group 1 (1.92) (p = 0.0026).Conclusion: This is the largest series to show that a preoperative lymphoscintigram (scan) can safely be omitted when performing a SLNB. As newer tracers are being used (e.g. Iron Oxide), it is important to know that accuracy of SLNB is not compromised by omission of a scan

Pharmacogenomic information in drug labels: European Medicines Agency perspective.

Pharmacogenomics (PGx) has a growing impact on healthcare and constitutes one of the major pillars of personalised medicine. For the purpose of improved individualised drug treatment, there is an increasing effort to develop drugs suitable for specific subpopulations and to incorporate pharmacogenomic drug labels in existing and novel medicines. Here, we review the pharmacogenomic drug labels of all 517 medicinal products centrally approved in the European Union (EU) since the establishment of the European Medicines Agency in 1995. We identified all pharmacogenomic-related information mentioned in the product labels and classified it according to its main effect and function on drug treatment, that is, metabolism, transport and pharmacodynamics, and according to the place of the respective section of the Summary of Product Characteristics (SmPC). The labels are preferentially present in drugs having antineoplastic properties. We find that the number of drugs with pharmacogenomic labels in EU increases now steadily and that it will be an important task for the future to refine the legislation on how this information should be utilised for improvement of drug therapy.

Modeling and simulation in clinical pharmacology and dose finding.

The breakout session 2 of the European Medicines Agency/European Federation of Pharmaceutical Industries and Associations Modeling and Simulation (M&S) workshop focused on two topics: when and how M&S should be used and would be accepted by the authorities for the dose-regimen selection; and when and how M&S can be applied to register a dosing regimen without the need for a specific study. Each topic was introduced by an industry and regulatory perspective, followed by case examples for illustration (Table 1).CPT: Pharmacometrics & Systems Pharmacology (2013) 2, e29; doi:10.1038/psp.2013.5; advance online publication 27 February 2013.

Interchangeability of gabapentin generic formulations in the Netherlands: a comparative bioavailability study.

To investigate the so-called "drift" with generic-generic drug substitution, a single-dose, four-way crossover comparative bioavailability study was performed involving 24 healthy subjects and three generic and one branded formulation of a tablet containing 800 mg gabapentin as test medication. The results showed that the 90% confidence intervals (CIs) for the area under the drug concentration-time curve (AUC(0-t)) and for the peak drug concentration (C(max)) were within the acceptance range of 80-125% for all comparisons. The safety profiles of the different gabapentin formulations were comparable. To conclude, all three generic formulations of gabapentin were found to be bioequivalent with the branded formulation and with each other, indicating that the formulations are interchangeable. These results strongly indicate the absence of "drift" with gabapentin generic-generic substitution.

Generics: what is the role of registration authorities.

Substitution of branded medicines by cheaper generic medicines has been and is subject for debate in the Netherlands. One of the tasks of the Dutch Medicines Evaluation Board (CBG) is the evaluation of generic medicines. The way the CBG approves generics, as outlined in this paper, is based on assessment of the quality of the medicine and bioequivalence testing according to strict European guidelines. Registration of generic medicines in the Netherlands will only take place when bioequivalence has been demonstrated. Once bioequivalence has been demonstrated, the CBG is convinced that the generic has the same efficacy and safety as the branded medicine. Consequently, the CBG is of the opinion that the branded medicine can be safely exchanged with the generic medicine. However, for the acceptance of generics in daily practice adequate communication to the patient by prescriber, pharmacist, health insurance company and patient organisations is essential.

Adaptive intrapatient dose escalation of cisplatin in combination with low-dose vp16 in patients with nonsmall cell lung cancer.

The objective of this phase II and pharmacologic study was to explore the feasibility, toxicity and activity of adaptive intrapatient dose escalation of cisplatin in a dose-intensive weekly schedule using predefined levels of exposure, with the ultimate aim to improve the antitumour activity of the therapy in patients with nonsmall cell lung cancer (NSCLC). Platinum DNA-adduct levels in peripheral white blood cells during treatment were used as the primary parameter for adaptive dosing. If DNA-adduct levels were not available, the area under the concentration-time curve (AUC) of unbound platinum in plasma was used for dose adaptation. Target levels for DNA-adducts and AUC have been defined in a previously performed pharmacologic study. The feasibility of adaptive dosing was tested in 76 patients with stage IIIB and IV NSCLC, who were planned to receive 6 weekly courses of cisplatin at a starting dose of 70 mg m(-2), together with daily low oral dose of 50 mg VP16. In total, 37 patients (49%) who were given more than one course received a dose increase varying from 10 to 55%. The majority of patients reached the defined target levels by a dose increase during course two. Relevant grade 2 neurotoxicity was observed in eight (10%) patients and reversible ototoxicity grade 2 in 14 (18%) patients. The strategy of adaptive intrapatient dose adjustment of cisplatin is practically feasible in a research setting even when results for dose adaptation have to be reported within a short time-period of 1 week. The toxicity appeared to be manageable in this cohort of patients. In some patients, exposure after the standard dose was substantially lower than the defined target level and significant dose escalations of more than 50% had to be applied. The response rate (RR) was relatively high: overall 40% (29 out of 72 patients) partial remission (PR), in patients with stage IIIB the RR was 60% (15 out of 25 patients) and with stage IV 30% (14 out of 47 patients). Randomised studies are needed to determine whether the adaptive dosing strategy results in better efficacy than standard dosing.

Induction of breast cancer resistance protein by the camptothecin derivative DX-8951f is associated with minor reduction of antitumour activity.

DX-8951f (exatecan mesylate), a new water-soluble derivative of camptothecin, is currently being evaluated in phase II clinical trials. Resistance may be acquired when treating cancer patients with DX-8951f. Therefore, we selected a subline of the human ovarian cancer cell line A2780 for resistance against DX-8951f to investigate possible mechanisms of resistance. This DX-8951f-resistant subline, designated 2780DX8 (resistance factor=9.3), displayed a typical cross-resistance pattern including compounds, such as topotecan (resistance factor =34), SN-38 (resistance factor =47), mitoxantrone (resistance factor =59) and doxorubicin (resistance factor =2.9), which have previously been associated with the expression of breast cancer resistance protein. 2780DX8 cells did not show changes in the topoisomerase I gene, in topoisomerase I protein levels or catalytic activity. Overexpression of breast cancer resistance protein could be detected, both at the mRNA and protein level, while staining for Pgp, MRP1, or LRP was negative. GF120918, an inhibitor of breast cancer resistance protein, was able to reverse the DX-8951f-induced resistance in 2780DX8 cells. In vivo experiments in well-established 2780DX8 human tumour xenografts demonstrated that the growth inhibition induced by CPT-11 was more affected by breast cancer resistance protein expression than that of DX-8951f. These data indicate for the first time that DX-8951f is able to induce breast cancer resistance protein as a mechanism of resistance. Breast cancer resistance protein, however, results in only minor reduction of antitumour activity of DX-8951f which is an advantage over topotecan and CPT-11/SN-38.

Breast cancer resistance protein expression and resistance to daunorubicin in blast cells from patients with acute myeloid leukaemia.

Breast cancer resistance protein (BCRP) is a recently described member of the ATP binding cassette transporter superfamily. It has been shown to confer resistance to mitoxantrone, topotecan, doxorubicin and daunorubicin in human tumour cell lines. We describe a study of BCRP expression in blast cells derived from 20 patients with acute myeloid leukaemia (AML). Twelve samples were from patients who had received previous cytotoxic therapy. BCRP expression was measured by immunocytochemistry using the BXP-34 monoclonal antibody. In vitro drug sensitivity was assessed using the methyl thiazol tetrazoliumbromide assay. BCRP expression varied between patients, and six out of 22 (27%) samples had > 10% cells staining positively (median 37%, range 13-95%). BCRP positivity was seen in both de novo samples and those from previously treated patients. There was a marked variation in the effect of all drugs tested between patients. Although there was no correlation between BCRP positivity and the effect of mitoxantrone, topotecan or doxorubicin, the median daunorubicin LC(50) value of BCRP(+) cells was fourfold higher than that of BCRP- cells (0.89 micromol/l compared with 0.21 micromol/l, P < 0.05). These results suggest that BCRP may be involved in resistance to the agents commonly used in AML and may explain some of the anomalous results found when studying other membrane transporters, such as P-gp or MRP.

Adaptive intrapatient dose escalation of cisplatin in patients with advanced head and neck cancer.

The purpose of this study was to explore the feasibility and toxicity of intrapatient dose adjustment using predefined levels of exposure to cisplatin, with the ultimate goal to further improve the antitumor activity of the treatment. The primary parameter for adaptive dosing was the level of platinum DNA adducts in peripheral white blood cells (WBC) and the secondary parameter the area under the curve (AUC) of unbound platinum in plasma, which were determined during the applied courses. Target levels had been defined in a previously performed pharmacologic study. The concept of adaptive dosing was tested in 16 patients with locally advanced head and neck (H/N) cancer who would receive six weekly courses of cisplatin at a starting dose level of 80 mg/m(2), which was previously investigated in a phase II study. Forty-seven percent of patients received a dose increase varying from 10 to 40%. Only two patients had exposure levels significantly below the defined target levels for DNA adducts and AUC. The majority of patients reached the defined target levels by modest dose increases of 10-20% during course 2. Relevant but reversible ototoxicity (temporary grade 3 in two patients) and renal toxicity (temporary grade 2 in two other patients) were observed. The pattern and severity of the toxicity was comparable to that encountered in the previous phase II study in H/N cancer patients. We conclude that the strategy of intrapatient dose adjustment for cisplatin is practically feasible in a research setting even when a short turn around time of 1 week is the limit for reporting results. Although in some patients the dose increase that had to be applied to reach target levels was substantial (up to 40%), this approach in H/N cancer patients is not expected to improve the response rate significantly, because these significantly underdosed patients represented only a small percentage of the investigated population. The great majority of patients needed only limited (10-20%) dose increases which very likely will not improve the response rate to a clinically significant extent. The outlined concept is currently being explored in other tumor types and schedules of cisplatin.

Subcellular localization and distribution of the breast cancer resistance protein transporter in normal human tissues.

High expression of the Breast Cancer Resistance Protein (BCRP) gene has been shown to be involved in resistance to chemotherapeutic drugs. Knowledge of the localization of BCRP protein in normal tissues may help unravel the normal function of this protein. Therefore, we characterized the tissue distribution and cellular localization of BCRP in frozen sections of normal human tissues. For this purpose, we used the recently described monoclonal antibody BXP-34 and another independently developed monoclonal antibody directed against BCRP, BXP-21. Both monoclonal antibodies show specific BCRP plasma membrane staining on cytospins obtained from topotecan- or mitoxantrone-selected cell lines, as well as from BCRP-transfected cell lines. Immunoprecipitation experiments using either BXP-21 or BXP-34 yielded a clear M(r) 72,000 BCRP band from BCRP-overexpressing tumor cells. In the topotecan-selected T8 and mitoxantrone-selected MX3 tumor cell lines, BCRP turned out to be differentially glycosylated. In contrast to BXP-34, BXP-21 is able to detect the M(r) 72,000 BCRP protein on immunoblots and is reactive with BCRP in formalin-fixed, paraffin-embedded tissues. Using BXP-21 and BXP-34, prominent staining of BCRP was observed in placental syncytiotrophoblasts, in the epithelium of the small intestine and colon, in the liver canalicular membrane, and in ducts and lobules of the breast. Furthermore, BCRP was present in veinous and capillary endothelium, but not in arterial endothelium in all of the tissues investigated. In the tissues studied, the mRNA levels of BCRP were assessed using reverse transcription-PCR, and these corresponded with the levels of BCRP protein estimated from immunohistochemical staining. The presence of BCRP at the placental syncytiotrophoblasts is consistent with the hypothesis of a protective role of BCRP for the fetus. The apical localization in the epithelium of the small intestine and colon indicates a possible role of BCRP in the regulation of the uptake of p.o. administered BCRP substrates by back-transport of substrate drugs entering from the gut lumen. Therefore, it may be useful to attempt to modulate the uptake of p.o. delivered BCRP substrates, e.g., topotecan or irinotecan, by using a BCRP inhibitor. Clinical trials testing this hypothesis have been initiated in our institute.

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.

The HER tyrosine kinase inhibitor CI1033 enhances cytotoxicity of 7-ethyl-10-hydroxycamptothecin and topotecan by inhibiting breast cancer resistance protein-mediated drug efflux.

Because the activities of HER family members are elevated and/or aberrant in a variety of human neoplasms, these cell surface receptors are receiving increasing attention as potential therapeutic targets. In the present study, we examined the effect of combining the HER family tyrosine kinase inhibitor CI1033 (PD 183805) with the topoisomerase (topo) I poison 7-ethyl-10-hydroxycamptothecin (SN-38), the active metabolite of irinotecan, in a number of different cell lines. Colony-forming assays revealed that the antiproliferative effects of simultaneous treatment with CI1033 and SN-38 were synergistic in T98G glioblastoma cells and HCT8 colorectal carcinoma cells, whereas sequential treatments were additive at best. In additional studies examining the mechanistic basis for these findings in T98G cells, immunoblotting revealed that the inhibitory effects of CI1033 on epidermal growth factor receptor autophosphorylation were unaffected by SN-38. Likewise, CI1033 had no effect on topo I polypeptide levels, localization, or activity. Nonetheless, CI1033 markedly enhanced the number of covalent topo I-DNA complexes stabilized by SN-38 or the related agent topotecan (TPT). Analysis of intracellular SN-38 levels by high-performance liquid chromatography and intracellular TPT levels by flow microfluorometry revealed that CI1033 increased the steady-state accumulation of SN-38 and TPT by 9.4 +/- 1.9- and 1.8 +/- 0.2-fold, respectively. Further evaluation revealed that the initial rate of TPT uptake was unaffected by CI1033, whereas the rate of efflux was markedly diminished. Additional studies demonstrated that T98G and HCT8 cells express the breast cancer resistance protein (BCRP), a recently cloned ATP binding cassette transporter. Moreover, CI1033 enhanced the uptake and cytotoxicity of SN-38 and TPT in cells transfected with BCRP but not empty vector. Conversely, CI1033 accumulation was diminished in cells expressing BCRP, suggesting that CI1033 is a substrate for this efflux pump. These results indicate that CI1033 can modulate the accumulation and subsequent cytotoxicity of two widely used topo I poisons in cells that have no history of previous exposure to these agents.

Role of breast cancer resistance protein in the bioavailability and fetal penetration of topotecan.

BACKGROUND AND METHODS: Breast cancer resistance protein (BCRP/MXR/ABCP) is a multidrug-resistance protein that is a member of the adenosine triphosphate-binding cassette family of drug transporters. BCRP can render tumor cells resistant to the anticancer drugs topotecan, mitoxantrone, doxorubicin, and daunorubicin. To investigate the physiologic role of BCRP, we used polarized mammalian cell lines to determine the direction of BCRP drug transport. We also used the BCRP inhibitor GF120918 to assess the role of BCRP in protecting mice against xenobiotic drugs. Bcrp1, the murine homologue of BCRP, was expressed in the polarized mammalian cell lines LLC-PK1 and MDCK-II, and the direction of Bcrp1-mediated transport of topotecan and mitoxantrone was determined. To avoid the confounding drug transport provided by P-glycoprotein (P-gp), the roles of Bcrp1 in the bioavailability of topotecan and the effect of GF120918 were studied in both wild-type and P-gp-deficient mice and their fetuses. RESULTS: Bcrp1 mediated apically directed transport of drugs in polarized cell lines. When both topotecan and GF120918 were administered orally, the bioavailability (i.e., the extent to which a drug becomes available to a target tissue after administration) of topotecan in plasma was dramatically increased in P-gp-deficient mice (greater than sixfold) and wild-type mice (greater than ninefold), compared with the control (i.e., vehicle-treated) mice. Furthermore, treatment with GF120918 decreased plasma clearance and hepatobiliary excretion of topotecan and increased (re-)uptake by the small intestine. In pregnant GF120918-treated, P-gp-deficient mice, relative fetal penetration of topotecan was twofold higher than that in pregnant vehicle-treated mice, suggesting a function for BCRP in the maternal-fetal barrier of the placenta. CONCLUSIONS: Bcrp1 mediates apically directed drug transport, appears to reduce drug bioavailability, and protects fetuses against drugs. We propose that strategic application of BCRP inhibitors may thus lead to more effective oral chemotherapy with topotecan or other BCRP substrate drugs.

Breast cancer resistance protein is localized at the plasma membrane in mitoxantrone- and topotecan-resistant cell lines.

Tumor cells may display a multidrug resistant phenotype by overexpression of ATP-binding cassette transporters such as multidrug resistance (MDRI) P-glycoprotein, multidrug resistance protein 1 (MRP1), and breast cancer resistance protein (BCRP). The presence of BCRP has thus far been reported solely using mRNA data. In this study, we describe a BCRP-specific monoclonal antibody, BXP-34, obtained from mice, immunized with mitoxantrone-resistant, BCRP mRNA-positive MCF-7 MR human breast cancer cells. BCRP was detected in BCRP-transfected cells and in several mitoxantrone- and topotecan-selected tumor cell sublines. Pronounced staining of the cell membranes showed that the transporter is mainly present at the plasma membrane. In a panel of human tumors, including primary tumors as well as drug-treated breast cancer and acute myeloid leukemia samples, BCRP was low or undetectable. Extended studies will be required to analyze the possible contribution of BCRP to clinical multidrug resistance.

Transport of topoisomerase I inhibitors by the breast cancer resistance protein. Potential clinical implications.

The multidrug resistance protein BCRP (breast cancer resistance protein) is a member of the ATP-binding cassette family of drug transporters. Overexpression of BCRP caused by exposure of cells to mitoxantrone (MX) or doxorubicin/verapamil resulted in a resistance pattern that is different from what is generally seen in the case of P-glycoprotein and MRP1 overexpression. Recently, the BCRP gene has been described in ovarian, breast, colon, and gastric cancer and fibrosarcoma cell lines. Our human tumor cells T8 and MX3, derived from the ovarian cancer cell line IGROV1 by stepwise increased exposure to topotecan and MX, are resistant to topotecan, CPT11, SN38, and 9-aminocamptothecin as well as MX. Increased energy-dependent efflux of affected drugs was noted. BCRP is a very efficient transporter of topotecan. Our recent studies, using the monoclonal antibody (mAb) BXP34, revealed that BCRP is located in the plasma membrane of the T8 and MX3 cell lines. Preliminary results of staining of human tumor cells showed low or absent levels of BCRP in a panel of solid tumors and acute myeloid leukemia cells.

32P-postlabeling assay for the quantification of the major platinum-DNA adducts.

To allow more sensitive, selective, and routine analyses of platinum(Pt)-GG and -AG intrastrand cross-links we have significantly improved our quantitative (32)P-postlabeling assay (M. J. P. Welters et al. Carcinogenesis 18, 1767-1774, 1997). Instead of off-line scintillation counting we introduced an on-line flow radioisotope detector into the HPLC system. Furthermore, the isolation protocol for the adducts was significantly modified and optimized to reduce interfering background peaks that prevented quantification of low levels of the cisplatin-DNA adducts in white blood cells obtained from patients. Reduction of background signals was obtained by boiling the samples, followed by phenol/chloroform/isoamylethanol extraction after the DNA digestion step. The labeling efficiency for the adducts was increased by 40% by using Na-formate instead of NH(4)-formate for elution of the adducts from the strong cation-exchange columns. Finally, a calibration curve and quality controls were implemented. The labeling efficiencies were not different between the dinucleotides. The between- and within-run precision for the Pt-GG and Pt-AG adducts measured at the lower limit of quantification of 87 and 53 amol/microg DNA, respectively, was less than 20% CV. The adducts were stable in DNA stored for a 2-month time period at -80 degrees C. The assay is now routinely used for high-precision analyses of patient and cell line samples containing very low adduct levels.

Overexpression of the BCRP/MXR/ABCP gene in a topotecan-selected ovarian tumor cell line.

Topotecan- or mitoxantrone-selected cell lines (T8 and MX3, respectively), derived from the human IGROV1 ovarian cancer cell line, were resistant to the topoisomerase I inhibitors topotecan, SN-38 (the active metabolite of irinotecan), and 9-aminocamptothecin, as well as to the topoisomerase II drug mitoxantrone. In both resistant cell lines, decreased accumulation of topotecan and mitoxantrone was observed, caused by enhanced energy-dependent efflux of the drugs involved. In both cell lines, we found that the breast cancer resistance protein/mitoxantrone resistance/placenta-specific ATP binding cassette (BCRP/MXR/ABCP) gene was overexpressed. Furthermore, BCRP/MXR/ABCP expression levels in various partially revertant T8 cells correlated with the levels of resistance to topotecan, SN-38, and mitoxantrone, strongly suggesting BCRP/MXR/ABCP to be the transporter responsible for the enhanced efflux. Pharmacodynamic analysis demonstrated that BCRP/MXR/ABCP is a very efficient transporter of topotecan; in vitro, 70% of the intracellular topotecan pool was transported out of the T8 or MX3 cells within 30 s. In conclusion, we report for the first time that BCRP/MXR/ABCP can also be up-regulated upon exposure of tumor cells to the clinically important drug topotecan, and that BCRP-mediated efflux of topotecan is very efficient. This highly efficient efflux of topotecan by BCRP/MXR/ABCP may have clinical relevance for patients being treated with topotecan.

In vitro antagonistic cytotoxic interactions between platinum drugs and taxanes on bone marrow progenitor cell CFU-GM.

We have designed and used an in vitro bone marrow cell culturing system for investigating pharmacodynamic interactions between platinum anti-cancer drugs and taxanes. With this system, in which the bone marrow progenitor cell CFU-GM is proliferating and differentiating into granulocytes and monocytes, we could show a strong antagonistic cytotoxicity of the combination carboplatin and Taxotere, in three different schedules, and of the combination cisplatin and Taxol, in two out of the three schedules tested. Modulation of intracellular platinum drug accumulation in granulocytes and monocytes does not seem to be a plausible explanation for the observed antagonism. In vitro co-incubation of granulocytes/monocytes with the combination carboplatin and Taxotere did not reveal an effect of Taxotere on intracellular platinum accumulation. Although Taxol reduced intracellular cisplatin levels by 12%, this effect was not significantly different from the co-incubation of cisplatin with Cremophor EL, the solvent for paclitaxel in Taxol. The toxicity data obtained in this study seem to be in accordance with recent clinical trials where combination therapies with platinum drugs and taxanes resulted in marked reductions in myelosuppression in patients. Therefore, these types of assays could be useful as to the assessment of bone marrow toxicities of clinically important drug combinations.

Reduced cellular accumulation of topotecan: a novel mechanism of resistance in a human ovarian cancer cell line.

In order to unravel possible mechanisms of clinical resistance to topoisomerase I inhibitors, we developed a topotecan-resistant human IGROV-1 ovarian cancer cell line, denoted IGROV(T100r), by stepwise increased exposure to topotecan (TPT). The IGROV(T100r) cell line was 29-fold resistant to TPT and strongly cross-resistant to SN-38 (51-fold). However, the IGROV(T100r) showed only threefold resistance to camptothecin (CPT). Remarkably, this cell line was 32-fold resistant to mitoxantrone, whereas no significant cross-resistance against other cytostatic drugs was observed. No differences in topoisomerase I protein levels and catalytic activity as well as topoisomerase I cleavable complex stabilization by CPT in the IGROV-1 and IGROV(T100r) cell lines were observed, indicating that resistance in the IGROV(T100r) cell line was not related to topoisomerase I-related changes. However, resistance in the resistant IGROV(T100r) cell line to TPT and SN-38 was accompanied by decreased accumulation of the drugs to approximately 15% and 36% of that obtained in IGROV-1 respectively. No reduced accumulation was observed for CPT. Notably, accumulation of TPT in the IGROV-1 cell line decreased under energy-deprived conditions, whereas the accumulation in the IGROV(T100r) cell line increased under these energy-deprived conditions. The efflux of TPT at 37 degrees C was very rapid in the IGROV-1 as well as the IGROV(T100r) cell line, resulting in 90% efflux within 20 min. Importantly, the efflux rates of TPT in the IGROV-1 and IGROV(T100r) cell lines were not significantly different and were shown to be independent on P-glycoprotein (P-gp) or multidrug resistance-associated protein (MRP). These results strongly suggest that the resistance of the IGROV(T100r) cell line to TPT and SN-38 is mainly caused by reduced accumulation. The reduced accumulation appears to be mediated by a novel mechanism, probably related to impaired energy-dependent uptake of these topoisomerase I drugs.