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O6-Methylguanine-DNA methyltransferase (MGMT) is a DNA repair enzyme that is overexpressed in certain tumors and is associated with resistance to the DNA alkylating agent temozolomide. MGMT inhibitors show potential in combating temozolomide resistance, but current assays for MGMT enzyme activity and inhibition, primarily oligonucleotide-based and fluorescent probe-based, are laborious and costly. The clinical relevance of temozolomide therapy calls for more convenient methodologies to study MGMT inhibition. Here, we extended the application of SNAP-Capture magnetic beads to develop a novel MGMT inhibition assay that demonstrated efficacy not only with known MGMT inhibitors, but also with the aldehyde dehydrogenase inhibitor, disulfiram. The assay uses standard fluorescence microscopy as a simple and reliable detection method, and is translationally applicable in drug discovery programs.

The function of direct DNA damage repair protein, namely MGMT in MM, and the impact of MGMT on melphalan treatment remains unclear. We showed a significantly higher MGMT mRNA expression in CD138+ myeloma cells than in matched CD138‐nontumorigenic cells derived from newly diagnosed and relapsed/refractory MM patients using qPCR. However, using gene expression databases, a similar expression of MGMT was observed during disease progression. MGMT depletion by its specific inhibitor lomeguatrib reduced myeloma cell viability, impaired S phase entry and DNA repair, and increased DNA damage and apoptosis. Apoptosis and DNA damage were further elevated by combined treatment with lomeguatrib and melphalan in RPMI 8226 cells. This is the first study demonstrating MGMT inhibition enhances DNA damage‐induced apoptosis and melphalan cytotoxicity in MGMT‐proficient MM cells and suggesting using lomeguatrib might have clinical importance in treating MM and overcoming melphalan resistance in MGMT‐proficient patients. [ABSTRACT FROM AUTHOR]

O 6 -Methylguanine-DNA methyltransferase (MGMT) is a DNA repair enzyme that is overexpressed in certain tumors and is associated with resistance to the DNA alkylating agent temozolomide. MGMT inhibitors show potential in combating temozolomide resistance, but current assays for MGMT enzyme activity and inhibition, primarily oligonucleotide-based and fluorescent probe-based, are laborious and costly. The clinical relevance of temozolomide therapy calls for more convenient methodologies to study MGMT inhibition. Here, we extended the application of SNAP-Capture magnetic beads to develop a novel MGMT inhibition assay that demonstrated efficacy not only with known MGMT inhibitors, but also with the aldehyde dehydrogenase inhibitor, disulfiram. The assay uses standard fluorescence microscopy as a simple and reliable detection method, and is translationally applicable in drug discovery programs.

Purpose: Glioblastoma multiforme (GBM) is a malignant high-grade glioma with a poor clinical outcome. Temozolomide (TMZ) is the first-line GBM chemotherapy; however, patients commonly develop resistance to its effects. Materials and methods: We investigated the antitumor activity of CAT3 in TMZ-resistant glioblastoma cell lines U251/TMZ and T98G. Orthotopic and subcutaneous mice tumor models were used to investigate the effects of various treatment regimes. Results: We found that PF403, the active metabolite of CAT3, inhibited proliferation of both cell lines. PF403 repressed the Hedgehog signaling pathway in the U251/TMZ cell line, reduced O6-methylguanine DNA methyltransferase (MGMT) expression, and abolished the effects of the Shh pathway. Moreover, PF403 blocked the Hedgehog signaling pathway in T98G MGMT-expressing cells and downregulated the expression of MGMT. CAT3 suppressed growth in the U251/TMZ orthotopic and T98G subcutaneous xenograft tumor models in vivo. We also demonstrated that inhibition of the Hedgehog pathway by PF403 counteracted TMZ resistance and enhanced the antitumor activity of TMZ in vitro and in vivo. Conclusion: These results indicate that CAT3 is a potential therapeutic agent for TMZ-resistant GBM. [ABSTRACT FROM AUTHOR]

Liposomal delivery is a well-established approach to increase the therapeutic index of drugs, mainly in the field of cancer chemotherapy. Here, we report the preparation and characterization of a new liposomal formulation of a derivative of lomeguatrib, a potent O 6 -methylguanine-DNA methyltransferase (MGMT) inactivator. The drug had been tested in clinical trials to revert chemoresistance, but was associated with a low therapeutic index. A series of lomeguatrib conjugates with distinct alkyl chain lengths – i.e. C12, C14, C16, and C18 – was synthesized, and the MGMT depleting activity as well as cytotoxicity were determined on relevant mouse and human glioma cell lines. Drug-containing liposomes were prepared and characterized in terms of loading and in vitro release kinetics. The lipophilic lomeguatrib conjugates did not exert cytotoxic effects at 5 μM in the mouse glioma cell line and exhibited a similar MGMT depleting activity pattern as lomeguatrib. Overall, drug loading could be improved by up to 50-fold with the lipophilic conjugates, and the slowest leakage was achieved with the C18 derivative. The present data show the applicability of lipophilic lomeguatrib derivatization for incorporation into liposomes, and identify the C18 derivative as the lead compound for in vivo studies. [ABSTRACT FROM AUTHOR]

Glioblastoma multiforme (GBM) is the most common form of malignant glioma. Current therapeutic approach to treat this malignancy involves a combination of surgery, radiotherapy and chemotherapy with temozolomide. Numerous mechanisms contributing to inherent and acquired resistance to this chemotherapeutic agent have been identified and can lead to treatment failure. This study undertook a metabolomics-based approach to characterize the metabolic profiles observed in temozolomide-sensitive and temozolomide-resistant GBM cell lines as well as in a small sub-set of primary GBM tumors. This approach was also utilized to explore the metabolic changes modulated upon cell treatment with temozolomide and lomeguatrib, an MGMT inhibitor with temozolomide-sensitizing potential. Metabolites previously explored for their potential role in chemoresistance including glucose, citrate and isocitrate demonstrated elevated levels in temozolomide-resistant GBM cells. In addition, a signature of metabolites comprising alanine, choline, creatine and phosphorylcholine was identified as up-regulated in sensitive GBM cell line across different treatments. These results present the metabolic profiles associated with temozolomide response in selected GBM models and propose interesting leads that could be leveraged for the development of therapeutic or diagnostic tools to impact temozolomide response in GBMs. [ABSTRACT FROM AUTHOR]

The treatment of anaplastic astrocytoma (AA) is controversial. New chemotherapeutic approaches are needed for AA treatment. Temozolomide (TMZ) is one of the chemotherapeutic drugs for the treatment of AA. The cytotoxic effects of TMZ can be removed by the MGMT (O(6)-methylguanine-DNA methyltransferase) enzyme. Then, chemotherapeutic resistance to TMZ occurs. MGMT inhibition by MGMT inactivators (such as lomeguatrib) is an important anticancer therapeutic approach to circumvent TMZ resistance. We aim to investigate the effect of TMZ-lomeguatrib combination on MGMT expression and TMZ sensitivity of SW1783 and GOS-3 AA cell lines. The sensitivity of SW1783 and GOS-3 cell lines to TMZ and to the combination of TMZ and lomeguatrib was determined by a cytotoxicity assay. MGMT methylation was detected by MS-PCR. MGMT and p53 expression were investigated by real-time PCR after drug treatment, and the proportion of apoptotic cells was analyzed by flow cytometry. When the combination of TMZ-lomeguatrib (50 μM) was used in AA cell lines, IC values were reduced compared to only using TMZ. MGMT expression was decreased, p53 expression was increased, and the proportion of apoptotic cells was induced in both cell lines. The lomeguatrib-TMZ combination did not have any effect on the cell cycle and caused apoptosis by increasing p53 expression and decreasing MGMT expression. Our study is a pilot study investigating a new therapeutic approach for AA treatment, but further research is needed. [ABSTRACT FROM AUTHOR]

Temozolomide (TMZ) is commonly used in the treatment of glioblastoma (GBM). The MGMT repair enzyme ( O-methylguanine-DNA methyltransferase) is an important factor causing chemotherapeutic resistance. MGMT prevents the formation of toxic effects of alkyl adducts by removing them from the DNA. Therefore, MGMT inhibition is an interesting therapeutic approach to circumvent TMZ resistance. The aim of the study was to investigate the effect of the combination of lomeguatrib (an MGMT inactivator) with TMZ, on MGMT expression and methylation. Primary cell cultures were obtained from GBM tumor tissues. The sensitivity of primary GBM cell cultures and GBM cell lines to TMZ, and to the combination of TMZ and lomeguatrib, was determined by a cytotoxicity assay (MTT). MGMT and p53 expression, and MGMT methylation were investigated after drug application. In addition, the proportion of apoptotic cells and DNA fragmentation was analyzed. The combination of TMZ and lomeguatrib in primary GBM cell cultures and glioma cell lines decreased MGMT expression, increased p53 expression, and did not change MGMT methylation. Moreover, apoptosis was induced and DNA fragmentation was increased in cells. In addition, we also showed that lomeguatrib-TMZ combination did not have any effect on the cell cycle. Finally, we determined that the sensitivity of each primary GBM cells and glioma cell lines to the lomeguatrib-TMZ combination was different and significantly associated with the structure of MGMT methylation. Our study suggests that lomeguatrib can be used with TMZ for GBM treatment, although further clinical studies will be needed so as to determine the feasibility of this therapeutic approach. [ABSTRACT FROM AUTHOR]

Background: The DNA repair protein O6-methylguanine-DNA methyltransferase (MGMT) reverses the O6-methylguanine (O6-meG) lesion induced by dacarbazine. Depletion of MGMT can be achieved using O6-meG pseudosubstrates. Herein, we report the first phase I experience of the novel O6-meG pseudosubstrate lomeguatrib, combined with dacarbazine.Methods: This is a phase I dose-escalation study to determine the maximum tolerated dose and recommended phase II dose (RP2D) of lomeguatrib combined with a single dose of dacarbazine on a 21-day schedule.Results: The vast majority of the 41 patients enrolled had metastatic melanoma (36/41) and most had no previous chemotherapy (30/41). The most frequent non-hematological adverse events (AEs) were nausea (52%), and fatigue (42%). The most frequent AEs of grade 3-4 severity were neutropaenia (42%), leukopaenia (17%), and thrombocytopaenia (12%). Only 1 patient had a partial response and 10 patients had stable disease.Conclusion: The RP2D of lomeguatrib was 40 mg orally twice daily for 10 days combined with 400 mg m(-2) of dacarbazine IV on day 2. Oral administration of lomeguatrib substantially increases the haematological toxicity of dacarbazine consistent with experience with other O6-meG pseudosubstrates. [ABSTRACT FROM AUTHOR]

Expression of the DNA repair protein O6-methylguanine-DNA methyltransferase (MGMT) correlates with resistance to irinotecan in colorectal cancer cell lines. This phase I study evaluated the maximum tolerated dose (MTD) of lomeguatrib, an inactivating pseudosubstrate of MGMT, in combination with irinotecan in patients with metastatic colorectal cancer and assessed the safety, toxicity and clinical pharmacology of combination treatment. Patients with metastatic colorectal cancer received lomeguatrib (10–80 mg PO) on days 1–5 with irinotecan (250–350 mg/m2 IV) on day 4 of a 21-day cycle. Twenty-four patients, pre-treated with a median of 2 lines of chemotherapy, received 104 cycles of treatment. The MTD was defined as 80 mg/day lomeguatrib with 300 mg/m2 irinotecan. The main toxicities observed were neutropaenia and diarrhoea. Lomeguatrib of 80 mg/day produced complete MGMT depletion in all available peripheral blood mononuclear cells (PBMCs) and paired tumour biopsies (one patient). There was no pharmacokinetic interaction between the drugs. In 22 patients assessable for tumour response, one achieved a partial response and 16 had stable disease. This study defined a tolerable dose of irinotecan in combination with lomeguatrib in patients with metastatic colorectal cancer. Combination treatment gave a similar response rate to irinotecan monotherapy in this heavily pre-treated patient group. [ABSTRACT FROM AUTHOR]

We evaluated the pharmacodynamic effects of the O(6)-methylguanine-DNA methyltransferase (MGMT) inactivator lomeguatrib (LM) on patients with melanoma in two clinical trials. Patients received temozolomide (TMZ) for 5 days either alone or with LM for 5, 10 or 14 days. Peripheral blood mononuclear cells (PBMCs) were isolated before treatment and during cycle 1. Where available, tumour biopsies were obtained after the last drug dose in cycle 1. Samples were assayed for MGMT activity, total MGMT protein, and O(6)-methylguanine (O(6)-meG) and N7-methylguanine levels in DNA. MGMT was completely inactivated in PBMC from patients receiving LM, but detectable in those on TMZ alone. Tumours biopsied on the last day of treatment showed complete inactivation of MGMT but there was recovery of activity in tumours sampled later. Significantly more O(6)-meG was present in the PBMC DNA of LM/TMZ patients than those on TMZ alone. LM/TMZ leads to greater MGMT inactivation, and higher levels of O(6)-meG than TMZ alone. Early recovery of MGMT activity in tumours suggested that more protracted dosing with LM is required. Extended dosing of LM completely inactivated PBMC MGMT, and resulted in persistent levels of O(6)-meG in PBMC DNA during treatment. [ABSTRACT FROM AUTHOR]

Lomeguatrib, an O(6)-methylguanine-DNA methyltransferase inactivator, was evaluated in an extended dosing regimen with temozolomide, designed according to pharmacodynamic data from previous studies. Patients with unresectable stage 3 or 4 cutaneous or unknown primary melanoma metastases were treated with lomeguatrib 40 mg, b.i.d. for 10 or 14 days and temozolomide 75-100 mg m(-2) on days 1-5. Drugs were administered orally with cycles repeated every 28 days, for up to six cycles. A total of 32 patients were recruited to the study. Lomeguatrib for 10 days with temozolomide 75 mg m(-2) was established as the optimal extended lomeguatrib dosing schedule, with haematological toxicity being dose limiting. There were two partial responses to treatment giving an overall response rate of 6.25%. Extending lomeguatrib administration beyond that of temozolomide requires a reduced dose of the latter agent. Only limited clinical activity was seen, suggesting no advantage for this regimen over conventional temozolomide administration in the treatment of melanoma. [ABSTRACT FROM AUTHOR]

Purpose. Metastatic melanoma and current treatments are reviewed. Summary. Despite the many advances in cancer treatment that have occurred over the last several decades, the prognosis for patients with advanced melanoma remains poor. The 5-year survival rate for patients with distant metastases is less than 10%. For these patients, surgery and radiation therapy are primarily used to palliate symptoms. Most patients with advanced melanoma receive systemic therapy. Single-agent cytotoxic chemotherapy with dacarbazine is the standard of care in community practice, although the response rate is generally low and few patients attain complete remission. Temozolomide is an orally active congener of dacarbazine that is at least as effective as dacarbazine when used as single-agent cytotoxic therapy. Low-dose extended temozolomide regimens may provide greater antitumor efficacy. Combinations of dacarbazine or temozolomide with other cytotoxic therapies have not markedly improved patient survival. Newer agents (e.g., lomeguatrib and decitabine) have been developed to overcome mechanisms of drug resistance. Biotherapy using high-dose interleukin-2 has been shown to induce durable responses lasting 5 years or more in some patients, although the overall response rate is not substantially better than that with dacarbazine. Interferon is also used for the treatment of metastatic melanoma, despite lack of approval by the FDA for this indication. Some evidence suggests that combining chemotherapy and biotherapy agents (biochemotherapy) increases the rate of treatment response but does not significantly extend overall survival. Conclusion. New strategies are needed to improve treatment response rates and duration of overall survival in patients with metastatic melanoma. [ABSTRACT FROM AUTHOR]

To evaluate the tumour response to lomeguatrib and temozolomide (TMZ) administered for 5 consecutive days every 4 weeks in patients with metastatic colorectal carcinoma. Patients with stage IV metastatic colorectal carcinoma received lomeguatrib (40 mg) and TMZ (50-200 mg m(-2)) orally for 5 consecutive days every 4 weeks. Response was determined every two cycles. Pharmacokinetics of lomeguatrib and TMZ as well as their pharmacodynamic effects in peripheral blood mononuclear cells (PBMC) were determined. Nineteen patients received 49 cycles of treatments. Despite consistent depletion of O(6)-methylguanine-DNA methyltransferase in PBMC, none of the patients responded to treatment. Three patients had stable disease, one for the duration of the study, and no fall in carcinoembryonic antigen was observed in any patient. Median time to progression was 50 days. The commonest adverse effects were gastrointestinal and haematological and these were comparable to those of TMZ when given alone. This combination of lomeguatrib and TMZ is not efficacious in metastatic colorectal cancer. If further studies are to be performed, emerging data suggest that higher daily doses of lomeguatrib and a dosing period beyond that of TMZ should be evaluated. [ABSTRACT FROM AUTHOR]

Abstract: Previous studies indicated that dacarbazine and Temozolomide could be highly effective against refractory acute leukaemia. Their activity relies mainly on the generation of methyl adducts at the O6-position of guanine in DNA. High levels of O 6-methylguanine-DNA methyltransferase (MGMT) or a defective mismatch repair (MMR) system, are associated with cellular resistance to triazenes. The MGMT inhibitor, O 6-(4-bromothenyl)guanine (Lomeguatrib), can restore in vitro sensitivity to Temozolomide in MMR-proficient blasts. In the early 1970s we discovered that, in vivo, triazene compounds induce the appearance of novel transplantation antigens in murine leukaemia (“Chemical Xenogenization”, CX). Non-self peptides presented by class I MHC molecules are generated by triazene-induced somatic mutations, affecting retroviral sequences that are detectable in the mouse genome. Moreover, preliminary experiments suggested that human cancer cells can also undergo CX. Therefore, we designed a chemo-immunotherapy strategy in leukaemic patients as follows: (a) cytoreduction and a hypothetical CX phase, i.e. treatment with Lomeguatrib (to suppress MGMT activity) and Temozolomide (to kill sensitive blasts and to presumably induce CX in resistant leukaemic cells); (b) immune response recovery phase using interleukin-2 (to possibly restore an immune response and take advantage of the hypothetical, triazene-induced CX). Here we present the results of pilot study which is in progress in patients with refractory/relapsed acute leukaemia. In all tested cases, Lomeguatrib suppressed MGMT activity in vivo. Six out of eight patients showed partial or complete disappearance of blast cells in peripheral blood or in bone marrow. We observed severe and long-lasting myelosuppression, accompanied by limited non-haematological toxicity. Up to now, two patients are alive (after 9 and 10 months, respectively), four died of opportunistic infections and two of progressive disease. This investigation confirms the potential role of triazenes in leukaemia and highlights the contribution of Lomeguatrib in overcoming drug resistance. Further studies are required to establish whether Temozolomide can induce CX in human leukaemia, and thus offer a new approach to control minimal residual disease. [Copyright &y& Elsevier]

Abstract: The DNA repair protein, O 6-alkylguanine-DNA alkyltransferase (MGMT) can confer resistance to the cancer chemotherapeutic effects of the class of DNA damaging drugs generally referred to as the O 6-alkylating agents. Inactivation of MGMT is thus a practical approach to improving the efficacy of such agents. An account is given of the collaboration between groups at Trinity College, Dublin and the Paterson Institute, Manchester which led to the development of the MGMT inactivating drug, Patrin™ (PaTrin-2, Lomeguatrib). The development of a simpler method of synthesis of O 6-arylmethylguanines opened up the way to make a series of O 6-heteroalkylmethyl analogues of the archetypal MGMT pseudosubstrate, O 6-methylguanine. Of these, the furfuryl and thenyl compounds were the most active against recombinant Human MGMT in an in vitro assay. The 4-bromothenyl derivative was chosen for clinical trial as the most active compound. The MGMT active site tolerates O 6-substituted guanines where the side chain can be quite large, but does not tolerate those with an aromatic or heteroaromatic ring with an ‘ortho’ substituent. [Copyright &y& Elsevier]

Background: Treatment resistance of glioblastoma multiforme to chemo- and radiotherapy remains a challenge yet to overcome. In particular, the O6-methylguanine-DNA-methyltransferase (MGMT) promoter unmethylated patients have only little benefit from chemotherapy treatment using temozolomide since MGMT counteracts its therapeutic efficacy. Therefore, new treatment options in radiotherapy need to be developed to inhibit MGMT and increase radiotherapy response. Methods: Lomeguatrib, a highly specific MGMT inhibitor, was used to inactivate MGMT protein in vitro. Radiosensitivity of established human glioblastoma multiforme cell lines in combination with lomeguatrib was investigated using the clonogenic survival assay. Inhibition of MGMT was analyzed using Western Blot. Cell cycle distribution and apoptosis were investigated to determine the effects of lomeguatrib alone as well as in combination with ionizing radiation. Results: Lomeguatrib significantly decreased MGMT protein and reduced radiation-induced G2/M arrest. A radiosensitizing effect of lomeguatrib was observed when administered at 1 µM and increased radioresistance at 20 µM. Conclusion: Low concentrations of lomeguatrib elicit radiosensitization, while high concentrations mediate a radioprotective effect. [ABSTRACT FROM AUTHOR]

Purpose: Glioblastoma multiforme (GBM) is a malignant high-grade glioma with a poor clinical outcome. Temozolomide (TMZ) is the first-line GBM chemotherapy; however, patients commonly develop resistance to its effects., Materials and Methods: We investigated the antitumor activity of CAT 3 in TMZ-resistant glioblastoma cell lines U251/TMZ and T98G. Orthotopic and subcutaneous mice tumor models were used to investigate the effects of various treatment regimes., Results: We found that PF403, the active metabolite of CAT 3 , inhibited proliferation of both cell lines. PF403 repressed the Hedgehog signaling pathway in the U251/TMZ cell line, reduced O 6 -methylguanine DNA methyltransferase (MGMT) expression, and abolished the effects of the Shh pathway. Moreover, PF403 blocked the Hedgehog signaling pathway in T98G MGMT-expressing cells and downregulated the expression of MGMT. CAT 3 suppressed growth in the U251/TMZ orthotopic and T98G subcutaneous xenograft tumor models in vivo. We also demonstrated that inhibition of the Hedgehog pathway by PF403 counteracted TMZ resistance and enhanced the antitumor activity of TMZ in vitro and in vivo., Conclusion: These results indicate that CAT 3 is a potential therapeutic agent for TMZ-resistant GBM., Competing Interests: Disclosure The authors report no conflicts of interest in this work.

Background: Glioblastoma multiforme (GBM) is often associated with a poor survival prognostic for patients. The main reason seems to be the acquired or inherent resistance to the chemotherapeutic agent used to treat the tumor, temozolomide (TMZ). To this day, the most recognized pathway of resistance is the DNA Direct Repair pathway by the means of the protein O6- methylguanine DNA-methyltransferase (MGMT)., Objectives: To design and synthesize a series of MGMT inhibitors that can sensitize GBM cells to TMZ., Methods: Twenty-five O6-alkyl, O6-aryl and O6-substituted-aryl guanine analogs including nine novel compounds were synthesized, characterized, analyzed by molecular docking and tested on the T98G GBM cells viability., Results: Following molecular modeling with MGMT, the newly designed compounds 19, 22, and 24 emerged as the most promising MGMT ligands and displayed modest cytotoxicity. Guanine analog (19), bearing a p-nitrobenzyl moiety, reduced considerably the O6-methylguanine DNAmethyltransferase expression level. When combined with TMZ (1), which is used as first line treatment for brain tumors, compounds 19, 22, and 24 decreased T98G cells proliferation by 32%, 68% and 50%, respectively. TMZ (1) displayed negligible effect on the proliferation of these cells further supporting the notion that this cell model is resistant to this alkylating agent., Conclusion: Overall, these results notably highlight a group of MGMT inhibitors that warrants further exploration in the development of therapeutic options to circumvent TMZ resistance in brain tumors., (Copyright© Bentham Science Publishers; For any queries, please email at epub@benthamscience.org.)