ABCG2 Substrates Research Articles

Afatinib circumvents multidrug resistance via dually inhibiting ATP binding cassette subfamily G member 2 in vitro and in vivo.

Multidrug resistance (MDR) to chemotherapeutic drugs is a formidable barrier to the success of cancer chemotherapy. Expressions of ATP-binding cassette (ABC) transporters contribute to clinical MDR phenotype. In this study, we found that afatinib, a small molecule tyrosine kinase inhibitor (TKI) targeting EGFR, HER-2 and HER-4, reversed the chemoresistance mediated by ABCG2 in vitro, but had no effect on that mediated by multidrug resistance protein ABCB1 and ABCC1. In addition, afatinib, in combination with topotecan, significantly inhibited the growth of ABCG2- overexpressing cell xenograft tumors in vivo. Mechanistic investigations exhibited that afatinib significantly inhibited ATPase activity of ABCG2 and downregulated expression level of ABCG2, which resulted in the suppression of efflux activity of ABCG2 in parallel to the increase of intracellular accumulation of ABCG2 substrate anticancer agents. Taken together, our findings may provide a new and useful combinational therapeutic strategy of afatinib with chemotherapeutical drug for the patients with ABCG2 overexpressing cancer cells.

Abstract 829: The novel camptothecin derivative and IAP inhibitor FL118 is an effective treatment for irinotecan-refractory colorectal cancer

Abstract The current standard care for metastatic colorectal cancer is combination of leucovorin, 5-fluorouracil, and irinotecan. Irinotecan is a semisynthetic analogue of the alkaloid camptothecin and functions by inducing DNA double-strand breaks through inhibition of topoisomerase I (Top1) activity. The camptothecin-derived Top1 inhibitors irinotecan and topotecan are effective anti-cancer agents that are currently utilized as first-line therapies in several human cancers. Unfortunately, resistance to these agents usually develops during treatment, often through upregulation of the drug efflux pump ABCG2, an ATP-binding cassette transporter. By increasing the rate of drug export, ABCG2 decreases the amount of irinotecan or topotecan that accumulates intracellularly, thereby protecting cancer cells from the cytotoxic effects of these drugs. Therefore, a camptothecin derivative that is also a poor substrate for ABCG2 could potentially overcome ABCG2-induced irinotecan resistance and restore therapeutic efficacy. Our lab recently reported on a novel camptothecin derivative, FL118, which shows strong anti-cancer activity both in vitro and in vivo. Here we report the ability of FL118 to overcome irinotecan resistance. Several irinotecan-resistant colon cancer cell lines derived from HCT116 were treated with either FL118 or SN38, the active metabolite of irinotecan. We observed a decrease in the potency of SN38 in irinotecan-resistant cells that overexpressed ABCG2 compared to cells that did not overexpress ABCG2. In contrast, this loss of potency was not observed for FL118. To confirm that the decrease in potency was ABCG2-dependent, HCT116-A2, an ABCG2 overexpressing cell line, was treated with SN38 or FL118 in the presence or absence of Ko143, an ABCG2 inhibitor. We observed that Ko143 could restore potency to SN38 in HCT116-A2 cells, confirming that irinotecan resistance in HCT116-A2 cells is mediated by ABCG2. However, Ko143 could not modulate the potency of FL118, further suggesting that FL118 is not affected by ABCG2 activity. Similar results were observed in studies with anti-ABCG2 shRNA. Based on these observations, we conclude that FL118 is not a substrate for ABCG2, and that further testing is warranted to determine whether FL118 would be an effective anti-cancer agent for patients with irinotecan-resistant colorectal cancer overexpressing ABCG2. Citation Format: David Westover, Xiang Ling, Xiaojun Liu, Hong Lam, Celine Gongora, Maguy Del Rio, Fengzhi Li. The novel camptothecin derivative and IAP inhibitor FL118 is an effective treatment for irinotecan-refractory colorectal cancer. [abstract]. In: Proceedings of the 105th Annual Meeting of the American Association for Cancer Research; 2014 Apr 5-9; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2014;74(19 Suppl):Abstract nr 829. doi:10.1158/1538-7445.AM2014-829

Abstract 1967: A-803467, a sodium channel blocker, reverses ABCG2-mediated MDR

Abstract The ATP-binding cassette, subfamily G, isoform 2 protein (ABCG2) is a vital member of the ABC transporter superfamily, which has been involved in multidrug resistance (MDR) in cancer. Its diverse range of substrates includes many antineoplastic agents such as doxorubicin and mitoxantrone. ABCG2 expression has been significantly increased in some solid tumors and hematologic malignancies, which is correlated to poorer clinical outcomes. In addition, ABCG2 expression is a distinguishing feature of cancer stem cells, whereby this membranous transporter imparts resistance to the chemotherapeutic drugs. To enhance the chemosensitivity of cancer cells, attention has been focused on MDR modulators. In this study, we investigated the ability of sodium channel blocker, A-803467 to reverse ABCG2-mediated MDR. We found that A-803467 at non-toxic concentration could significantly increase the cellular sensitivity to ABCG2 substrates in drug-resistant cells overexpressing either wild-type or mutant ABCG2. Mechanistic studies indicated that A-803467 (7.5 μM) significantly increased the intracellular accumulation resulted from inhibition of the efflux of mitoxantrone by inhibiting the transport activity without altering expression level of ABCG2 protein. Furthermore, ATPase analysis indicates that A-803467 stimulates the ATPase activity in membranes overexpressing ABCG2. Our findings suggest that A-803467 has the potential to be used in combination with ABCG2 chemotherapeutic substrates to augment the response in drug resistant cancers. Citation Format: Nagaraju Anreddy. A-803467, a sodium channel blocker, reverses ABCG2-mediated MDR. [abstract]. In: Proceedings of the 105th Annual Meeting of the American Association for Cancer Research; 2014 Apr 5-9; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2014;74(19 Suppl):Abstract nr 1967. doi:10.1158/1538-7445.AM2014-1967

Characterization of 3-methoxy flavones for their interaction with ABCG2 as suggested by ATPase activity

Breast Cancer Resistance Protein (BCRP/ABCG2) belongs to the superfamily of ATP binding cassette (ABC) transporters. Characteristic of some of these transporter proteins is the transport of a variety of structurally unrelated substances against a concentration gradient by using the energy of ATP hydrolysis. ABCG2 has been found to confer multidrug resistance (MDR) in cancer cells. Several anticancer drugs have been identified as ABCG2 substrates including mitoxantrone, etoposide and topotecan. As inhibition of the transporter is one of the strategies to overcome MDR, we have synthesized and tested several 3-methoxy flavones and investigated them for their ABCG2 inhibition. Among these, pentamethyl quercetin (compound 4) and pentamethyl morin (compound 5) were found to be fluorescent and hence screened for their possible transport by ABCG2 using confocal microscopy. This study showed that pentamethyl quercetin was far less accumulated in ABCG2 overexpressing MDCK BCRP cells as compared to MDCK sensitive cells, suggesting possible efflux of this compound by ABCG2. Pentamethyl morin showed no visible difference in both cell lines. Based on this observation, we studied several other fluorescent 3-methoxy flavones for their accumulation in ABCG2 overexpressing cells. To confirm the substrate or inhibitor nature of the tested compounds, these compounds were further investigated by ATPase assay. If stimulation of the transporter ATPase activity is detected, one can conclude that the compound is probably a transported substrate. All compounds except pentamethyl morin (compound 5) and tetramethyl quercetin (compound 6) were found to stimulate ATPase activity pointing to possible substrates despite being potent inhibitors of ABCG2.

ABCG2 AND MDR PROMOTE BRAINSTEM GLIOMAGENESIS AND LIMIT THE EFFICACY OF DASATINIB IN A DIFFUSE INTRINSIC PONTINE GLIOMA (DIPG) MOUSE MODEL

BACKGROUND: DIPG is an incurable tumor that arises in the brainstem of children. Our laboratory generates murine DIPGs by expressing PDGF-B and deleting p53 in nestin progenitors of the neonatal brainstem using the RCAS/tv-a system. Dasatinib, an inhibitor of PDGFR-A and SRC activation, has been evaluated in clinical trials for children with DIPG but with disappointing results. As dasatinib is a substrate of the ABC transporters ABCG2 and MDR, we hypothesized that ABCG2 and MDR may limit the efficacy of dasatinib in DIPG. METHODS: We generated murine DIPGs driven by PDGF-B and p53 loss in ABC WT (ABCG2+/+; MDRa+/+; MDRb+/+) and ABC KO (ABCG2-/-; MDRa-/-; MDRb-/-) mice. Next, we investigated whether dasatinib is efficacious in ABC KO and ABC WT DIPG-bearing mice. We treated ABC WT DIPG-bearing mice with dasatinib alone, elacridar (a dual ABCG2/MDR inhibitor) alone, and dasatinib in combination with elacridar. We also measured dasatinib levels in the tumor and plasma of all of the different tumor cohorts. Lastly, we evaluated the blood-brain-barrier (BBB) of our DIPG mouse model with Dextran Texas Red. RESULTS: Surprisingly, we observed a significant difference in tumor latency between untreated ABC KO and untreated ABC WT mice in this PDGF-B; p53 deficient DIPG model. Median survival of DIPG-bearing ABC KO was longer than DIPG-bearing ABC WT: 43 days vs. 34 days respectively (p = 0.02). We observed that dasatinib almost doubled the survival of DIPG-bearing mice in the ABC KO model: 44 days vs. 80 days (p = 0.0004). Dasatinib modestly prolonged survival in ABC WT mice: 33 days vs. 42 days (p < 0.0001). Co-treatment with elacridar significantly increased the efficacy of dasatinib in the ABC WT model 42 days vs. 59 days (p < 0.0001). However, elacridar alone did not significantly prolong survival in the ABC WT mice 33 days vs. 34 days (p = 0.38). Pharmacokinetic analysis demonstrated that genetic ablation of the ABC transporters or inhibition of ABC transporters with elacridar significantly increased dasatinib delivery into normal brain but not into tumor. Dextran Texas Red studies demonstrated a disrupted BBB in DIPG models independent of ABC transporters. CONCLUSIONS: 1. ABC transporters promote brainstem gliomagenesis 2. ABC transporters regulate dasatinib delivery into normal brain but not into the tumor as BBB is disrupted in our DIPG mouse model 3. ABC transporter activity limits the efficacy of dasatinib in vivo. Our results suggest that ABCG2 and MDR inhibitors may have a therapeutic role in the treatment of DIPG. SECONDARY CATEGORY: Preclinical Experimental Therapeutics.

Structural determinants of peripheral O-arylcarbamate FAAH inhibitors render them dual substrates for Abcb1 and Abcg2 and restrict their access to the brain

The blood–brain barrier (BBB) is the main entry route for chemicals into the mammalian central nervous system (CNS). Two transmembrane transporters of the ATP-binding cassette (ABC) family – breast cancer resistance protein (ABCG2 in humans, Abcg2 in rodents) and P-glycoprotein (ABCB1 in humans, Abcb1 in rodents) – play a key role in mediating this process. Pharmacological and genetic evidence suggests that Abcg2 prevents CNS access to a group of highly potent and selective O-arylcarbamate fatty-acid amidohydrolase (FAAH) inhibitors, which include the compound URB937 (cyclohexylcarbamic acid 3′-carbamoyl-6-hydroxybiphenyl-3-yl ester). To define structure-activity relationships of the interaction of these molecules with Abcg2, in the present study we tested various peripherally restricted and non-restricted O-arylcarbamate FAAH inhibitors for their ability to serve as transport substrates in monolayer cultures of Madin-Darby Canine Kidney-II (MDCKII) cells over-expressing Abcg2. Surprisingly, we found that the majority of compounds tested – even those able to enter the CNS in vivo – were substrates for Abcg2 in vitro. Additional experiments in MDCKII cells overexpressing ABCB1 revealed that only those compounds that were dual substrates for ABCB1 and Abcg2 in vitro were also peripherally restricted in vivo. The extent of such restriction seems to depend upon other physicochemical features of the compounds, in particular the polar surface area. Consistent with these in vitro results, we found that URB937 readily enters the brain in dual knockout mice lacking both Abcg2 and Abcb1, whereas it is either partially or completely excluded from the brain of mice lacking either transporter alone. The results suggest that Abcg2 and Abcb1 act together to restrict the access of URB937 to the CNS.

Functional characteristics of cancer stem cells and their role in drug resistance of prostate cancer.

Cancer stem cells (CSCs) have the ability to self-renew and differentiate to give rise to heterogeneous phenotype of the tumor cells. It is believed that these cells are involved in metastasis, recurrence and therapy resistance in various cancers. CSCs have been identified in prostate cancer (PCa), one of the most diagnosed malignancies in men over the world, for which chemotherapy resistance is a major problem in the treatment of castration-resistant advanced stages. Molecular signatures, gene expression and functional features have been reported for PCa CSCs. Most data come from cell lines which may not represent the actual tumor. In the present work, a CSCs enriched population obtained from PCa explants was functionally characterized and analyzed for drug resistance. Tumorsphere cultures positive for ABCG2 transporter, CD133, CD44, cytokeratins 5 and 18 (CK5 and CK18) and negatives for androgen receptor (AR) and prostate-specific antigen (PSA) showed higher clonogenic capacity, holoclone-forming ability, colony-forming capacity in soft agar and lower proliferative and apoptotic rate than control adherent cell cultures. Furthermore, exposing tumorsphere cultures to ABCG2 substrate drugs resulted in a high survival rate compared with control PCa cells. This high drug resistance was decreased using a selective inhibitor of ABCG2. According to these results, tumorspheres from PCa explants showed a functional stem phenotype and a marked drug resistance, probably mediated by high expression of the ABCG2 transporter, which might be considered as a suitable therapeutic target for CSCs.

Pelitinib (EKB-569) targets the upregulation of ABCG2 and potentiates cytotoxic effect of ABCG2 substrate anticancer drugs in hyperthermia

Pelitinib (EKB-569) targets the upregulation of ABCG2 and potentiates cytotoxic effect of ABCG2 substrate anticancer drugs in hyperthermia

FRI0105 Abcb1and ABCG2 Drug-Efflux Transporters Function and Its Association with Disease Activity in A Cohort of Patients with Rheumatoid Arthritis

BackgroundIs not uncommon to observe that an important number of patients with rheumatoid arthritis (RA) have to withdraw or change treatment because of lack or loss of efficacy. Several mechanisms...

Telatinib reverses chemotherapeutic multidrug resistance mediated by ABCG2 efflux transporter in vitro and in vivo

Multidrug resistance (MDR) is a phenomenon where cancer cells become simultaneously resistant to anticancer drugs with different structures and mechanisms of action. MDR has been shown to be associated with overexpression of ATP-binding cassette (ABC) transporters. Here, we report that telatinib, a small molecule tyrosine kinase inhibitor, enhances the anticancer activity of ABCG2 substrate anticancer drugs by inhibiting ABCG2 efflux transporter activity. Co-incubation of ABCG2-overexpressing drug resistant cell lines with telatinib and ABCG2 substrate anticancer drugs significantly reduced cellular viability, whereas telatinib alone did not significantly affect drug sensitive and drug resistant cell lines. Telatinib at 1μM did not significantly alter the expression of ABCG2 in ABCG2-overexpressing cell lines. Telatinib at 1μM significantly enhanced the intracellular accumulation of [3H]-mitoxantrone (MX) in ABCG2-overexpressing cell lines. In addition, telatinib at 1μM significantly reduced the rate of [3H]-MX efflux from ABCG2-overexpressing cells. Furthermore, telatinib significantly inhibited ABCG2-mediated transport of [3H]-E217βG in ABCG2 overexpressing membrane vesicles. Telatinib stimulated the ATPase activity of ABCG2 in a concentration-dependent manner, indicating that telatinib might be a substrate of ABCG2. Binding interactions of telatinib were found to be in transmembrane region of homology modeled human ABCG2. In addition, telatinib (15mg/kg) with doxorubicin (1.8mg/kg) significantly decreased the growth rate and tumor size of ABCG2 overexpressing tumors in a xenograft nude mouse model. These results, provided that they can be translated to humans, suggesting that telatinib, in combination with specific ABCG2 substrate drugs may be useful in treating tumors that overexpress ABCG2.

Drug interactions with sunitinib

Sunitinib is a tyrosine kinase inhibitor indicated for the treatment of gastrointestinal stromal tumor, advanced renal cell carcinoma, and pancreatic neuroendocrine tumors. The aim of this article is to describe the pharmacological interactions between sunitinib and commonly prescribed drugs. We reviewed available information on pharmacological interactions between sunitinib and concomitantly prescribed drugs. Drugs were grouped into different therapeutic groups according to the Anatomical Therapeutic Chemical (ATC) classification. Sunitinib interacts with CYP3A4 inducers or inhibitors and with P-glycoprotein and ABCG2 substrates. Pharmacodynamic interactions with drugs have also been found. Current information on drug interactions between sunitinib and other drugs is scarce and most of the times it is difficult to apply to clinical practice. Even so, this difficulty in managing drug interactions should not be a reason to ignore them as they can help to explain intolerances and treatment failures.

A novel MDCKII in vitro model for assessing ABCG2-drug interactions and regulation of ABCG2 transport activity in the caprine mammary gland by environmental pollutants and pesticides

The ABC efflux transporter ABCG2 represents the main route for active secretion of xenobiotics into milk. Thus, ABCG2 regulation by aryl hydrocarbon receptor (AhR) ligands including ubiquitously environmental pollutants is of great toxicological relevance. However, no adequate in vitro model is as yet available to study AhR-dependent ABCG2 regulation in dairy animals. In this study, we therefore systematically investigated the effect of various environmental contaminants and pesticides on ABCG2 efflux activity in MDCKII cells stably expressing mammary ABCG2 from dairy goats. The AhR-agonists TCDD, Aroclor 1254, prochloraz, and iprodione caused a dose- and time-dependent increase in EROD activity. Moreover, TCDD and prochloraz significantly stimulated ABCG2 transport activity through a dose- and time-dependent induction of transporter gene expression. AhR inhibitors like CH223191 significantly reversed TCDD- and prochloraz-induced stimulation of ABCG2 efflux activity. In contrast, non-AhR activators such as PCB 101 had no significant effect on EROD activity, ABCG2 gene expression or transporter activity. As we identified various anthelmintics including monepantel as potential ABCG2 substrates this regulatory mechanism may result in increased milk residues of potentially harmful xenobiotics. Thus, MDCKII-cABCG2 cells may represent a suitable in vitro model to study mammary ABCG2 secretory activity and its potential regulation by AhR-activating contaminants.

Interactions of tenofovir and tenofovir disoproxil fumarate with drug efflux transporters ABCB1, ABCG2, and ABCC2; role in transport across the placenta

Tenofovir (TFV) is used in pregnant women as a part of combination antiretroviral treatment to prevent mother-to-child transmission of HIV infection. We aimed to detect whether TFV and/or its prodrug, tenofovir disoproxil fumarate (TDF), are substrates of ATP-binding cassette (ABC) transporters that are functionally expressed in the placenta, namely P-glycoprotein (ABCB1/MDR1), Breast Cancer Resistance Protein (ABCG2/BCRP) and Multidrug Resistance-Associated Protein 2 (ABCC2/MRP2). We employed in-vitro cell-based assays and in-situ animal model to assess possible role of the efflux transporters in transplacental pharmacokinetics of TFV and TDF. In-vitro transport assays were performed in MDCKII cells transduced with human ABCB1, ABCG2 or ABCC2. To quantify the effect of these transporters on TFV/TDF transplacental passage, we employed the in-situ model of dually perfused rat term placenta in open and closed setup. In-vitro assays revealed that TDF is a dual substrate of ABCB1 and ABCG2 but not of ABCC2. In contrast, TFV transport was not influenced by any of these transporters. Applying concentration-dependent studies and selective inhibitors, we further confirmed these findings in situ on the organ level; both ABCB1 and ABCG2 limited mother-to-fetus transfer of TDF whereas TFV transplacental passage was not affected by these ABC transporters. We propose limited mother-to-fetus transport of both TFV and TDF. While placental transport of TFV is restricted passively, by physical-chemical properties of the molecule, mother-to-fetus passage of TDF is actively hindered by placental ABCB1 and ABCG2 transporters, pumping this compound from trophoblast back to maternal circulation.

Impact of genetic variability in the ABCG2 gene on ABCG2 expression, function, and interaction with AT1 receptor antagonist telmisartan

The ATP-binding cassette transporter ABCG2 plays a prominent role in cardiovascular and cancer pathophysiology, is involved in the pathogenesis of gout, and affects pharmacokinetics of numerous drugs. Telmisartan, a widely used AT1 receptor antagonist, inhibits the transport capacity of ABCG2 and may cause drug–drug interactions, especially in individuals carrying polymorphism that facilitate the telmisartan–ABCG2 interaction. Thus, the aim of this study was to identify ABCG2 polymorphisms and somatic mutations with relevance for the telmisartan–ABCG2 interaction. For this purpose, a cellular system for the conditional expression of ABCG2 was established. ABCG2 variants were generated via site-directed mutagenesis. Interaction of telmisartan with these ABCG2 variants was investigated in HEK293-Tet-On cells using the pheophorbide A efflux assay. Moreover, expression of ABCG2 variants was studied in these cells. Importantly, protein levels of the Q141K and F489L variant were significantly reduced, a phenomenon that was partly reversed by pharmacological proteasome inhibition. Moreover, basal pheophorbide A efflux capacity of S248P, F431L, and F489L variants was significantly impaired. Interestingly, inhibition of ABCG2-mediated pheophorbide A transport by telmisartan was almost abolished in cells expressing the R482G variant, whereas it was largely increased in cells expressing the F489L variant. We conclude that the arginine residue at position 482 of the ABCG2 molecule is of major importance for the interaction of telmisartan with this ABC transporter. Furthermore, individuals carrying the F489L polymorphism may be at increased risk of developing adverse drug reactions in multi-drug regimens involving ABCG2 substrates and telmisartan.

Dietary polyacetylenes of the falcarinol type are inhibitors of breast cancer resistance protein (BCRP/ABCG2)

Polyacetylenes of the falcarinol type are present in vegetables such as carrots and parsley. They display interesting bioactivities and hold potential as health-promoting and therapeutic agents. In this study, falcarinol, falcarindiol, falcarindiol 3-acetate and falcarindiol 3,8-diacetate were examined for their modulation on breast cancer resistance protein (BCRP/ABCG2), an efflux transporter important for xenobiotic absorption and disposition, and multidrug resistance in cancer. Falcarinol, falcarindiol, and falcarindiol 3-acetate were extracted from carrots and falcarindiol 3,8-diacetate prepared from falcarindiol. Their modulatory effects on ABCG2 were studied using three methods—mitoxantrone accumulation, vesicular transport, and ATPase assay. The polyacetylenes inhibited mitoxantrone (an ABCG2 substrate) efflux in ABCG2-overexpressing HEK293 cells. The inhibitory effect was confirmed in the vesicular transport assay, in which concentration-dependent inhibition of methotrexate (an ABCG2 substrate) uptake into ABCG2-overexpressing Sf9 membrane vesicles was observed (IC50=19.7–41.7µM). The polyacetylenes also inhibited baseline and sulfasalazine-stimulated vanadate-sensitive ATPase activities in ABCG2-overexpressing Sf9 membrane vesicles (IC50=19.3–79.3µM). This is the first report of an inhibitory effect of polyacetylenes on ABCG2. These results indicate a prospective use of polyacetylenes as multidrug resistance reversal agents, a possible role of ABCG2 in the absorption and disposition of polyacetylenes, and potential food-drug interactions between polyacetylene-rich foods and ABCG2 substrate drugs.

Abstract C98: Quizartinib (AC220) potentiates the antineoplastic activity of wild-type ABCG2 and ABCB1 substrates.

Abstract Quizartinib (AC220), a novel and potent class II receptor tyrosine kinase inhibitor that has better therapeutic and pharmacokinetic profile amongst its class. Quizartinib is currently under clinical trials for FLT3 ITD and wild-type AML and is tested in combination with chemotherapy. This study examined the effects of quizartinib on ABC transporter-mediated multidrug resistance (MDR). Cytotoxic studies showed that 0.75, 1.5, and 3 μM are the three less toxic concentrations for normal as well as cancer cells. Out of these three concentrations, quizartinib at 3 μM showed a significant reversal of MDR mediated by ABCG2 and ABCB1 in different MDR cell models, when compared to the positive controls, fumetrimorgin C and verapamil at 3 μM, respectively. Interestingly, quizartinib did not show a significant reversal of the MDR associated with the two mutants (R482T and R482G) ABCG2 overexpression. Results also showed that quzartinib at 3 μM significantly reversed wild-type ABCG2-mediated drug resistance by antagonizing the drug efflux function of wild-type ABCG2 and increasing the intracellular accumulation of [3H]-mitoxantrone in wild-type ABCG2-overexpressing cells. However, quizartinib at 3 μM did not reverse ABCC1- and ABCC10-mediated drug resistance in HEK/ABCC1 and HEK/ABCC10 cell lines. Hence these results conclude that quizartinib potentiates the antineoplastic activity of wild-type ABCG2 and ABCB1 substrates. Citation Information: Mol Cancer Ther 2013;12(11 Suppl):C98. Citation Format: Priyank Kumar, Rishil J. Kathawala, Hui Zhang, Nagaraju Anreddy, Yanglu Chen, Kanav Gupta, Louis D. Trombetta, Zhe S. Cheng. Quizartinib (AC220) potentiates the antineoplastic activity of wild-type ABCG2 and ABCB1 substrates. [abstract]. In: Proceedings of the AACR-NCI-EORTC International Conference: Molecular Targets and Cancer Therapeutics; 2013 Oct 19-23; Boston, MA. Philadelphia (PA): AACR; Mol Cancer Ther 2013;12(11 Suppl):Abstract nr C98.

Olomoucine II, but Not Purvalanol A, Is Transported by Breast Cancer Resistance Protein (ABCG2) and P-Glycoprotein (ABCB1)

Purine cyclin-dependent kinase inhibitors have been recognized as promising candidates for the treatment of various cancers; nevertheless, data regarding interaction of these substances with drug efflux transporters is still lacking. Recently, we have demonstrated inhibition of breast cancer resistance protein (ABCG2) by olomoucine II and purvalanol A and shown that these compounds are able to synergistically potentiate the antiproliferative effect of mitoxantrone, an ABCG2 substrate. In this follow up study, we investigated whether olomoucine II and purvalanol A are transported by ABCG2 and ABCB1 (P-glycoprotein). Using monolayers of MDCKII cells stably expressing human ABCB1 or ABCG2, we demonstrated that olomoucine II, but not purvalanol A, is a dual substrate of both ABCG2 and ABCB1. We, therefore, assume that pharmacokinetics of olomoucine II will be affected by both ABCB1 and ABCG2 transport proteins, which might potentially result in limited accumulation of the compound in tumor tissues or lead to drug-drug interactions. Pharmacokinetic behavior of purvalanol A, on the other hand, does not seem to be affected by either ABCG2 or ABCB1, theoretically favoring this drug in the potential treatment of efflux transporter-based multidrug resistant tumors. In addition, we observed intensive sulfatation of olomoucine II in MDCKII cell lines with subsequent active efflux of the metabolite out of the cells. Therefore, care should be taken when performing pharmacokinetic studies in MDCKII cells, especially if radiolabeled substrates are used; the generated sulfated conjugate may largely contaminate pharmacokinetic analysis and result in misleading interpretation. With regard to chemical structures of olomoucine II and purvalanol A, our data emphasize that even drugs with remarkable structure similarity may show different pharmacokinetic behavior such as interactions with ABC transporters or biotransformation enzymes.

The FLT3 Inhibitor Quizartinib Inhibits ABCG2 at Pharmacologically Relevant Concentrations, with Implications for Both Chemosensitization and Adverse Drug Interactions

The oral second-generation bis-aryl urea fms-like tyrosine kinase 3 (FLT3) inhibitor quizartinib (AC220) has favorable kinase selectivity and pharmacokinetics. It inhibits mutant and wild-type FLT3 in vivo at 0.1 and 0.5 µM, respectively, and has shown favorable activity and tolerability in phase I and II trials in acute myeloid leukemia, with QT prolongation as the dose-limiting toxicity. Co-administration with chemotherapy is planned. We characterized interactions of quizartinib with the ATP-binding cassette (ABC) proteins ABCB1 (P-glycoprotein) and ABCG2 (breast cancer resistance protein). Its effects on uptake of fluorescent substrates and apoptosis were measured by flow cytometry, binding to ABCB1 and ABCG2 drug-binding sites by effects on [125I]iodoarylazidoprazosin ([125I]-IAAP) photolabeling and ATPase activity, and cell viability by the WST-1 colorimetric assay. Quizartinib inhibited transport of fluorescent ABCG2 and ABCB1 substrates in ABCG2- and ABCB1-overexpressing cells in a concentration-dependent manner, from 0.1 to 5 µM and from 0.5 to 10 µM, respectively, and inhibited [125I]-IAAP photolabeling of ABCG2 and ABCB1 with IC50 values of 0.07 and 3.3 µM, respectively. Quizartinib at higher concentrations decreased ABCG2, but not ABCB1, ATPase activity. Co-incubation with quizartinib at 0.1 to 1 µM sensitized ABCG2-overexpressing K562/ABCG2 and 8226/MR20 cells to ABCG2 substrate chemotherapy drugs in a concentration-dependent manner in cell viability and apoptosis assays. Additionally, quizartinib increased cellular uptake of the ABCG2 substrate fluoroquinolone antibiotic ciprofloxacin, which also prolongs the QT interval, in a concentration-dependent manner, predicting altered ciprofloxacin pharmacokinetics and pharmacodynamics when co-administered with quizartinib. Thus quizartinib inhibits ABCG2 at pharmacologically relevant concentrations, with implications for both chemosensitization and adverse drug interactions. These interactions should be considered in the design of treatment regimens combining quizartinib and chemotherapy drugs and in choice of concomitant medications to be administered with quizartinib.

Effects of triclabendazole on secretion of danofloxacin and moxidectin into the milk of sheep: Role of triclabendazole metabolites as inhibitors of the ruminant ABCG2 transporter

ATP-binding cassette transporter G2/breast cancer resistance protein (ABCG2/BCRP) mediates drug–drug interactions that affect the secretion of drugs into milk. The aims of this study were: (1) to determine whether the major plasma metabolites of the flukicide triclabendazole (TCBZ), triclabendazole sulfoxide (TCBZSO) and triclabendazole sulfone (TCBZSO2), inhibit ovine and bovine ABCG2 and its Y581S variant in vitro, and (2) to examine whether coadministration of TCBZ with the ABCG2 substrates danofloxacin (a fluoroquinolone) and moxidectin (a milbemycin) affects the secretion of these drugs into the milk of sheep.TCBZSO and TCBZSO2 inhibited ruminant ABCG2 in vitro by reversing the reduced mitoxantrone accumulation and reducing basal to apical transport of nitrofurantoin in cells transduced with bovine variants (S581 and Y581) and the ovine variant of ABCG2. Coadministration of TCBZ with moxidectin or danofloxacin to sheep resulted in significantly reduced levels of moxidectin, but not danofloxacin, in the milk of TCBZ-treated sheep compared to sheep administered moxidectin or danofloxacin alone. The milk area under concentration time curve (AUC 0–48h) was 2.99±1.41μgh/mL in the group treated with TCBZ and moxidectin, and 7.75±3.58μgh/mL in the group treated with moxidectin alone. The AUC (0–48h) milk/plasma ratio was 37% lower in the group treated with TCBZ and moxidectin (7.34±1.51) than in the group treated with moxidectin alone (11.68±3.61). TCBZ metabolites appear to inhibit ruminant ABCG2 and affect the secretion of ABCG2 substrates into milk of sheep.

Overlapping Substrate and Inhibitor Specificity of Human and Murine ABCG2

ABCG2 (also known as breast cancer resistance protein) is an ATP-binding cassette (ABC) transporter localized to the plasma membrane where it mediates the efflux of xenobiotics, including potential therapeutics. Studies investigating Abcg2 function at the blood-brain barrier in mouse models are often compared with human ABCG2 function. It is critical to understand the nature of species differences between mouse and human ABCG2, since extrapolations are made from murine data to humans. Two independent drug-selected cell line pairs expressing human or mouse ABCG2 were compared for efflux of fluorescent substrates using flow cytometry. To this end, we developed and characterized a new mouse Abcg2-expressing subline that demonstrated efflux of known fluorescent ABCG2 substrates and increased resistance to mitoxantrone, which is reduced in the presence of the ABCG2 inhibitor Ko143. Our results indicate that the substrate specificity of human and mouse ABCG2 is very similar. We identified a new human and mouse ABCG2 substrate, a porphyrin analog, purpurin-18 (Pp-18), which is not a substrate for P-glycoprotein or multidrug resistance protein 1. The ability of inhibitors to block efflux activity of ABCG2 was assessed using Pp-18. Inhibitors also demonstrated similar effects on human and mouse ABCG2. Chrysin, benzoflavone, and cyclosporin A inhibited Pp-18 efflux in both human and mouse ABCG2. The similarity of the substrate and inhibitor specificity of human and mouse ABCG2 supports interpretation of mouse models in understanding the clinical, pharmacological, and physiologic roles of ABCG2.