title: Multidrug Efflux Pumps Attenuate the Effect of MGMT Inhibitors.
abstract: Various mechanisms of drug resistance attenuate the effectiveness of cancer therapeutics, including drug transport and DNA repair. The DNA repair protein O(6)-methylguanine-DNA methyltransferase (MGMT) is a key factor determining the resistance against alkylating anticancer drugs inducing the genotoxic DNA lesions O(6)-methylguanine and O(6)-chloroethylguanine, and MGMT inactivation or depletion renders cells more susceptible to treatment with methylating and chloroethylating agents. Highly specific and efficient inhibitors of the repair protein MGMT were designed, including O(6)-benzylguanine (O(6)BG) and O(6)-(4-bromothenyl)guanine (O(6)BTG) that are nontoxic on their own. Unfortunately, these inhibitors do not select between MGMT in normal and cancer cells, causing nontarget effects in the healthy tissue. Therefore, a targeting strategy for MGMT inhibitors is required. Here, we used O(6)BG and O(6)BTG conjugated to β-d-glucose (O(6)BG-Glu and O(6)BTG-Glu, respectively) in order to selectively inhibit MGMT in tumors, harnessing their high demand for glucose. Both glucose conjugates efficiently inhibited MGMT in several cancer cell lines, but with different extents of sensitization to DNA alkylating agents, with lomustine being more effective than temozolomide. We further show that the glucose conjugates are subject to ATP-binding cassette (ABC) transporter mediated efflux, involving P-glycoprotein, MRP1, and BCRP, which impacts the efficiency of MGMT inhibition. Surprisingly, also O(6)BG and O(6)BTG were subject to an active transport out of the cell. We also show that pharmacological inhibition of efflux transporters increases the induction of cell death following treatment with these MGMT inhibitors and temozolomide. We conclude that strategies of attenuating the efflux by ABC transporters are required for achieving successful MGMT targeting.
authors.affiliation: Department of Toxicology, University Medical Center , Obere Zahlbacher Strasse 67, D-55131 Mainz, Germany.
authors.forename: Karl-Heinz
authors.initials: KH
authors.lastname: Tomaszowski
authors.affiliation: Montreal Neurological Hospital and Institute , 3801 University Street, Montréal, Quebec H3A 2B4, Canada.
authors.forename: Ralf
authors.initials: R
authors.lastname: Schirrmacher
authors.affiliation: Department of Toxicology, University Medical Center , Obere Zahlbacher Strasse 67, D-55131 Mainz, Germany.
authors.forename: Bernd
authors.initials: B
authors.lastname: Kaina
mesh_terms: D018528:ATP-Binding Cassette Transporters / Q000378:metabolism*; D018906:Antineoplastic Agents, Alkylating / Q000494:pharmacology; D017209:Apoptosis; D015153:Blotting, Western; D049109:Cell Proliferation; D015254:DNA Modification Methylases / Q000037:antagonists & inhibitors*; D045643:DNA Repair Enzymes / Q000037:antagonists & inhibitors*; D003606:Dacarbazine / Q000031:analogs & derivatives* / Q000494:pharmacology; D019008:Drug Resistance, Neoplasm; D004791:Enzyme Inhibitors / Q000494:pharmacology*; D005455:Fluorescent Antibody Technique; D005947:Glucose / Q000378:metabolism; D006801:Humans; D009369:Neoplasms / Q000188:drug therapy* / Q000378:metabolism / Q000473:pathology; D000077204:Temozolomide; D014407:Tumor Cells, Cultured; D025521:Tumor Suppressor Proteins / Q000037:antagonists & inhibitors*
chemical_list: D018528:ATP-Binding Cassette Transporters; D018906:Antineoplastic Agents, Alkylating; D004791:Enzyme Inhibitors; D025521:Tumor Suppressor Proteins; D003606:Dacarbazine; D015254:DNA Modification Methylases; C515491:MGMT protein, human; D045643:DNA Repair Enzymes; D005947:Glucose; D000077204:Temozolomide
keywords: ABC transporter; DNA repair; MGMT; drug targeting; inhibitors