Tumor-targeted inhibition of the DNA repair protein MGMT for enhanced chemotherapy overcoming tumor resistance and reducing systemic toxicity

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The DNA repair protein O6-alkylguanine-DNA alkyltransferase (MGMT) is the main factor in the resistance of tumor cells to the cytotoxic effects of O6-alkylating anticancer drugs such as temozolomide (TMZ), procarbazin, dacarbazin (DTIC), nimustine, semustine, carmustine and lomustine (CCNU). The cytotoxic effect alkylating agents on cells which express MGMT is drastically enhanced by pretreatment with MGMT inhibitors, which are quite selective without being toxic themselves. Clinically applied, however, MGMT inhibitors have thus far failed to produce the expected therapeutic results, the reason being that they inhibit MGMT in normal tissue as well as tumor tissue and thus lead to a general sensitization, which in turn requires a dose-reduction of anticancer drugs. MGMT inhibitors, which have thus far been used in clinical trials, are O6-benzylguanine (O6-BG) and O6-(4-bromothenyl)guanine (O6-BTG, lomeguatrib). Since these inhibitors increase the bone marrow toxicity of alkylating agents, we need to devise a tumor-targeting strategy of MGMT. A successful strategy for tumor-targeting of an MGMT inhibitor would also be significant for other substances that inhibit repair and could facilitate a selective reduction of chemo-resistance without adding to the systemic toxicity. Glucose derivatives are being used for tumor-imaging, facilitated by the fact that they accumulate in cells with high glucose consumption (most tumor cells). The applicants intend to follow a similar strategy by conjugating the MGMT inhibitors O6-BG and O6-BTG to ß-glucose (the conjugates are O6-BG-Glu and O6-BTG-Glu), in order to enrich tumor cells with these inhibitors and inhibit MGMT primarily in tumors. Previous work done by the applicant shows that glucose conjugates efficiently inhibit MGMT, are not toxic, and sensitise tumor cells to alkylating agents; at the same time it was shown that tumor cell lines can be sensitised to a large degree. Still unknown, however, is the identity of the transporter, which deposits and distributes the glucose conjugates within the organism, nor do we know the tumor response compared to normal tissue. The present follow-on project aims to decisively answer the following questions: 1) identify the transporters (in- and efflux) for O6-BG-Glu and O6-BTG-Glu; 2) is it possible to increase the anti-tumor-effect of TMZ and CCNU in a mouse xenograft model; 3) provide proof of the reduced systemic toxicity of TMZ and CCNU by applying glucose inhibitors and compare it with the non-conjugated inhibitors; 4) determine the distribution of O6-BG-Glu and O6-BTG-Glu within the organs of xeno-transplanted mouse; 5) once the transporter has been identified, determine the expression in tumor cell lines and tissue/tumors in order to identify those tumor groups, which are particularly susceptible to MGMT inhibition by Glu-MGMT-targeting.