There is an urgent need for the design and discovery of new and potent inhibitors for the DNA repair protein MGMT (O6-Methylguanine-DNA-methyltransferase) in glioma therapy. MGMT is highly expressed in brain tumors, and plays a primary role in conferring resistance to alkylating agents. The psuedosubstrates for MGMT such as the O6-benzylguanine have not been successful in the clinic due to prolonged inhibition of DNA repair in the bone marrow stem cells. Recently, we showed that the anti-alcoholism drug, disulfiram (DSF) inhibits MGMT activity in the same way as ALDH by conjugating with the active-site cysteine 145 (Carcinogenesis 35, 692, 2014). DSF, a symmetrical molecule, is metabolized and split in half to yield dithiocarbamate residues. Since the dithiocarbamates resulting from DSF decomposition are the ultimate reactive species that inactivate the aldehyde dehydrogenase and other signaling targets, we surmised that dithiocarbamate derivatives by themselves will be active as MGMT inhibitors and exert anticancer activities. Therefore, we tested various dithiocarbamates (pyrrolidine dithiocarbamate (PDTC); diethyldithiocarbamate (EDTC); dimethyldithiocarbamate (MDTC) and dibenzyldithiocarbamate (BDTC) on MGMT activity, protein levels, and other redox-sensitive proteins such as the NF-κB and GSTP1. The cytotoxicity of these dithiocarbamates against the MGMT-proficient SF-188 glioblastoma cells was comparable with that of DSF, with the MDTC being most effective and the benzyl derivative BDTC least potent. Western blot analysis in HT29 and SF-188 cells revealed a concentration-dependent degradation of MGMT by the dithiocarbamates, similar to DSF. All dithiocarbamates except the BDTC were superior to DSF in degrading MGMT protein. MDTC was the most potent followed by PDTC and EDTC in depleting the MGMT protein from tumor cells. Further, MDTC was also effective in reducing the cellular levels of NF-κB protein. We also established that MDTC binds to the active site cysteine145 in MGMT leading to its inactivation. Currently, we have developed the pegylated PLGA nanoparticles loaded with MDTC or zinc-chelated MDTC to target the glioblastoma and other cancers. The efficacy of these formulations in intracranial glioma models developed in nude mice is being evaluated. The lack of cytotoxicity and inability to bind with MGMT by BDTC suggests that the bulky group attached to the dithiocarbamate may hinder interaction with target proteins or interfere with their metabolism. Since MDTC has a good potential to cross the blood brain-barrier, possess reactive thiol groups that can interact with not only MGMT but also numerous signaling proteins, our strategy using this repurposed compound holds promise in glioma treatment (supported by grants from CPRIT [RP130266] and Carson-Leslie Foundation to KSS).
Citation Format: Hanumantha Rao Madala. Potent functional inactivation of the MGMT DNA repair protein by dithiocarbamate compounds increases the efficacy of temozolomide in human glioblastoma cells. [abstract]. In: Proceedings of the 107th Annual Meeting of the American Association for Cancer Research; 2016 Apr 16-20; New Orleans, LA. Philadelphia (PA): AACR; Cancer Res 2016;76(14 Suppl):Abstract nr 3722.