A phase I study of extended dosing with lomeguatrib with temozolomide in patients with advanced melanoma

Natural small molecule thymoquinone increases the chemosensitivity of glioblastoma to temozolomide through inhibiting Wnt/β-catenin signaling pathway to downregulate MGMT expression: In vitro and in vivo validation

Temozolomide (TMZ) is the only one oral first-line chemotherapeutic drug for glioblastoma treatment. However, O6-methylguanine-DNA methyltransferase (MGMT) can repair the lethal O6-methylguaine (O6-MeG) lesion produced by TMZ, thus imparting resistance to TMZ. Currently, the clinical utility of small molecule covalent MGMT inhibitors is limited by the occurrence of severe hematological toxicity. Therefore, developing new strategies for overcoming MGMT-mediated resistance is highly urgent. Here, we explored the feasibility that modulating Wnt/β-catenin signaling pathway in glioblastoma to inhibit MGMT expression to overcome TMZ resistance. From eight natural products or approved drugs with inhibitory effects on Wnt/β-catenin pathway, we found thymoquinone (TQ) completely suppressed MGMT expression in glioblastoma SF763 and SF767 cell lines within 24 h. As expected, TQ exhibited synergistic killing effects with TMZ in SF763 and SF767 cells, while in MGMT-negative SF126 cells only additive effect observed. Moreover, TQ remarkably enhanced the inhibition of TMZ on cell proliferation, clone formation, invasion and migration, and promoted cell apoptosis. In resistant SF763 mice tumor xenograft model, TQ significantly increased the suppression of TMZ on tumor growth, meanwhile maintaining good biosafety. Western blotting analysis indicated that TQ significantly inhibited the nuclear translocation of β-catenin and the expression of downstream proteins Cyclin D1 and MGMT. The addition of Wnt activator LiCl reversed the nuclear translocation of β-catenin and the expression of Cyclin D1 and MGMT induced by TQ. For the first time, our findings indicate that TQ can considerably increase the sensitivity of glioblastoma to TMZ by interfering Wnt/β-catenin pathway to downregulate MGMT expression.

Discovery of a novel Wnt inhibitor DK419: Reversing temozolomide resistance in glioblastoma by switching off Wnt/β-catenin signaling pathway to inhibit MGMT expression

Temozolomide (TMZ) remains the primary oral chemotherapeutic agent for glioblastoma, but its efficacy is hampered by resistance mechanisms involving O6-methylguanine-DNA methyltransferase (MGMT). MGMT repairs the TMZ-induced lethal O6-methylguanine (O6-MeG) lesions, leading to treatment resistance. Current small molecule covalent MGMT inhibitors have limited clinical application due to severe hematological toxicity when used with TMZ. Therefore, alternative strategies to overcome MGMT-mediated resistance are critically needed. Targeting the Wnt/β-catenin signaling pathway to suppress MGMT expression presents a promising approach. We synthesized and discovered that a novel Wnt inhibitor, DK419 (6-chloro-2-(trifluoromethyl)-N-(4-(trifluoromethyl)phenyl)-1H -benzimidazole-4-carboxamide), effectively suppressed MGMT expression within 12 h in TMZ-resistant SF763 and SF767 cell lines. DK419 demonstrated synergistic cytotoxic effects with TMZ in these cell lines, while only an additive effect was observed in MGMT-negative SF126 cells. Furthermore, DK419 significantly enhanced TMZ's inhibitory effects on cell proliferation, colony formation, invasion, and migration, while also promoting apoptosis. In a resistant mouse tumor xenograft model, DK419 significantly boosted TMZ's tumor growth suppression, maintaining good biosafety. Western blot analysis revealed that DK419 markedly inhibited the nuclear translocation of β-catenin and decreased the expression of its downstream targets, Cyclin D1 and MGMT. The addition of the Wnt activator LiCl reversed DK419-induced effects on β-catenin nuclear translocation and Cyclin D1 and MGMT expression. For the first time, our findings demonstrate that DK419 can significantly enhance glioblastoma sensitivity to TMZ by modulating the Wnt/β-catenin pathway to downregulate MGMT expression.

Unboxing the molecular modalities of mutagens in cancer

The etiology of the majority of human cancers is associated with a myriad of environmental causes, including physical, chemical, and biological factors. DNA damage induced by such mutagens is the initial step in the process of carcinogenesis resulting in the accumulation of mutations. Mutational events are considered the major triggers for introducing genetic and epigenetic insults such as DNA crosslinks, single- and double-strand DNA breaks, formation of DNA adducts, mismatched bases, modification in histones, DNA methylation, and microRNA alterations. However, DNA repair mechanisms are devoted to protect the DNA to ensure genetic stability, any aberrations in these calibrated mechanisms provoke cancer occurrence. Comprehensive knowledge of the type of mutagens and carcinogens and the influence of these agents in DNA damage and cancer induction is crucial to develop rational anticancer strategies. This review delineated the molecular mechanism of DNA damage and the repair pathways to provide a deep understanding of the molecular basis of mutagenicity and carcinogenicity. A relationship between DNA adduct formation and cancer incidence has also been summarized. The mechanistic basis of inflammatory response and oxidative damage triggered by mutagens in tumorigenesis has also been highlighted. We elucidated the interesting interplay between DNA damage response and immune system mechanisms. We addressed the current understanding of DNA repair targeted therapies and DNA damaging chemotherapeutic agents for cancer treatment and discussed how antiviral agents, anti-inflammatory drugs, and immunotherapeutic agents combined with traditional approaches lay the foundations for future cancer therapies.

DNA repair in personalized brain cancer therapy with temozolomide and nitrosoureas

Alkylating agents have been used since the 60ties in brain cancer chemotherapy. Their target is the DNA and, although the DNA of normal and cancer cells is damaged unselectively, they exert tumor-specific killing effects because of downregulation of some DNA repair activities in cancer cells. Agents exhibiting methylating properties (temozolomide, procarbazine, dacarbazine, streptozotocine) induce at least 12 different DNA lesions. These are repaired by damage reversal mechanisms involving the alkyltransferase MGMT and the alkB homologous protein ALKBH2, and through base excision repair (BER). There is a strong correlation between the MGMT expression level and therapeutic response in high-grade malignant glioma, supporting the notion that O⁶-methylguanine and, for nitrosoureas, O⁶-chloroethylguanine are the most relevant toxic damages at therapeutically relevant doses. Since MGMT has a significant impact on the outcome of anti-cancer therapy, it is a predictive marker of the effectiveness of methylating anticancer drugs, and clinical trials are underway aimed at assessing the influence of MGMT inhibition on the therapeutic success. Other DNA repair factors involved in methylating drug resistance are mismatch repair, DNA double-strand break (DSB) repair by homologous recombination (HR) and DSB signaling. Base excision repair and ALKBH2 might also contribute to alkylating drug resistance and their downregulation may have an impact on drug sensitivity notably in cells expressing a high amount of MGMT and at high doses of temozolomide, but the importance in a therapeutic setting remains to be shown. MGMT is frequently downregulated in cancer cells (up to 40% in glioblastomas), which is due to CpG promoter methylation. Astrocytoma (grade III) are frequently mutated in isocitrate dehydrogenase (IDH1). These tumors show a surprisingly good therapeutic response. IDH1 mutation has an impact on ALKBH2 activity thus influencing DNA repair. A master switch between survival and death is p53, which often retains transactivation activity (wildtype) in malignant glioma. The role of p53 in regulating survival via DNA repair and the routes of death are discussed and conclusions as to cancer therapeutic options were drawn.

The specific role of O6-methylguanine-DNA methyltransferase inhibitors in cancer chemotherapy

The DNA repair protein, O6-methylguanine DNA methyltransferase (MGMT), can confer resistance to guanine O6-alkylating agents. Therefore, inhibition of resistant MGMT protein is a practical approach to increase the anticancer effects of such alkylating agents. Numerous small molecule inhibitors were synthesized and exhibited potential MGMT inhibitory activities. Although they were nontoxic alone, they also inhibited MGMT in normal tissues, thereby enhancing the side effects of chemotherapy. Therefore, strategies for tumor-specific MGMT inhibition have been proposed, including local drug delivery and tumor-activated prodrugs. Over-expression of MGMT in hematopoietic stem cells to protect bone marrow from the toxic effects of chemotherapy is also a feasible selection. The future prospects and challenges of MGMT inhibitors in cancer chemotherapy were also discussed.

Temozolomide-perillyl alcohol conjugate downregulates O6-methylguanin DNA methltransferase via inducing ubiquitination-dependent proteolysis in non-small cell lung cancer

The DNA repair enzyme O⁶-methylguanin-DNA-methltransferase (MGMT) is able to remove products of alkylating agent such as O⁶-meG and emerges as a central determinant of cancer resistance to temozolomide (TMZ). Temozolomide–perillyl alcohol conjugate (TMZ–POH), a novel TMZ analog developed based on the conjugation of TMZ and POH, displayed strong anticancer potency in multiple cancer types, but seemed not to experience the chemoresistance even in cells with high MGMT expression unlike TMZ and other alkylating agents. In this study, we demonstrated TMZ–POH inhibited MGMT dependent on proteasomal pathway and this inhibition is a significant factor in its toxic effect in the non-small cell lung cancer (NSCLC) cells.

Preparation of PEGylated liposomes incorporating lipophilic lomeguatrib derivatives for the sensitization of chemo-resistant gliomas

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 O⁶-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.

Antineoplastic effect of decoy oligonucleotide derived from MGMT enhancer

Silencing of O(6)-methylguanine-DNA-methyltransferase (MGMT) in tumors, mainly through promoter methylation, correlates with a better therapeutic response and with increased survival. Therefore, it is conceivable to consider MGMT as a potential therapeutic target for the treatment of cancers. Our previous results demonstrated the pivotal role of NF-kappaB in MGMT expression, mediated mainly through p65/NF-kappaB homodimers. Here we show that the non-canonical NF-KappaB motif (MGMT-kappaB1) within MGMT enhancer is probably the major inducer of MGMT expression following NF-kappaB activation. Thus, in an attempt to attenuate the transcription activity of MGMT in tumors we designed locked nucleic acids (LNA) modified decoy oligonucleotides corresponding to the specific sequence of MGMT-kappaB1 (MGMT-kB1-LODN). Following confirmation of the ability of MGMT-kB1-LODN to interfere with the binding of p65/NF-kappaB to the NF-KappaB motif within MGMT enhancer, the efficacy of the decoy was studied in-vitro and in-vivo. The results of these experiments show that the decoy MGMT-kB1-LODN have a substantial antineoplastic effect when used either in combination with temozolomide or as monotherapy. Our results suggest that MGMT-kB1-LODN may provide a novel strategy for cancer therapy.

Inhibiting the DNA damage response as a therapeutic manoeuvre in cancer

The DNA damage response (DDR), consisting of an orchestrated network of proteins effecting repair and signalling to cell cycle arrest, to allow time to repair, is essential for cell viability and to prevent DNA damage being passed on to daughter cells. The DDR is dysregulated in cancer with some pathways up-regulated and others down-regulated or lost. Up-regulated pathways can confer resistance to anti-cancer DNA damaging agents. Therefore, inhibitors of key components of these pathways have the potential to prevent this therapeutic resistance. Conversely, defects in a particular DDR pathway may lead to dependence on a complementary pathway. Inhibition of this complementary pathway may result in tumour-specific cell killing. Thus, inhibitors of the DDR have the potential to increase the efficacy of DNA damaging chemotherapy and radiotherapy and have single-agent activity against tumours with a specific DDR defect. This review describes the compounds that have been designed to inhibit specific DDR targets and summarizes the pre-clinical and clinical evaluation of these inhibitors of DNA damage signalling and repair.

LINKED ARTICLES

This article is part of a themed section on Emerging Therapeutic Aspects in Oncology. To view the other articles in this section visit http://dx.doi.org/10.1111/bph.2013.169.issue-8.

Chemotherapeutic resistance in anaplastic astrocytoma cell lines treated with a temozolomide-lomeguatrib combination

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, IC50 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.

Challenging resistance mechanisms to therapies for metastatic melanoma

Melanoma is the most aggressive form of skin cancer and, if spread outside the epidermis, has a dismal prognosis. Before the approval of the anti-cytotoxic T lymphocyte-associated antigen 4 (CTLA-4) monoclonal antibody ipilimumab and the BRAF inhibitors vemurafenib and dabrafenib, no other agents had demonstrated better results in terms of overall survival than the DNA-methylating compound dacarbazine (or its oral analog temozolomide). However, most patients with metastatic melanoma do not obtain long-lasting clinical benefit from ipilimumab and responses to BRAF inhibitors are short lived. Thus, combination therapies with inhibitors of DNA repair (e.g., poly(ADP-ribose) polymerase [PARP] inhibitors), novel immunomodulators (monoclonal antibodies against programmed death-1 [PD-1] or its ligand PD-L1), targeted therapies (mitogen-activated protein kinase [MAPK]/extracellular signal-regulated kinase [ERK] kinase [MEK] or phosphatidylinositol 3-kinase [PI3K]/AKT/mammalian target of rapamycin [mTOR] inhibitors) or antiangiogenic agents are currently being investigated to improve the efficacy of antimelanoma therapies. This review discusses the implications of simultaneously targeting key regulators of melanoma cell proliferation/survival and immune responses to counteract resistance.

Safety and efficacy of decitabine in combination with temozolomide in metastatic melanoma: a phase I/II study and pharmacokinetic analysis

BACKGROUND

Temozolomide (TMZ) is widely used for chemotherapy of metastatic melanoma. We hypothesized that epigenetic modulators will reverse chemotherapy resistance, and in this article, we report studies that sought to determine the recommended phase 2 dose (RP2D), safety, and efficacy of decitabine (DAC) combined with TMZ.

PATIENTS AND METHODS

In phase I, DAC was given at two dose levels: 0.075 and 0.15 mg/kg intravenously daily × 5 days/week for 2 weeks, TMZ orally 75 mg/m(2) qd for weeks 2-5 of a 6-week cycle. The phase II portion used a two-stage Simon design with a primary end point of objective response rate (ORR).

RESULTS

The RP2D is DAC 0.15 mg/kg and TMZ 75 mg/m(2). The phase II portion enrolled 35 patients, 88% had M1c disease; 42% had history of brain metastases. The best responses were 2 complete response (CR), 4 partial response (PR), 14 stable disease (SD), and 13 progressive disease (PD); 18% ORR and 61% clinical benefit rate (CR + PR + SD). The median overall survival (OS) was 12.4 months; the 1-year OS rate was 56%. Grade 3/4 neutropenia was common but lasted >7 days in six patients.

CONCLUSIONS

The combination of DAC and TMZ is safe, leads to 18% ORR and 12.4-month median OS, suggesting possible superiority over the historical 1-year OS rate, and warrants further evaluation in a randomized setting.

6-Carboxyfluorescein and structurally similar molecules inhibit DNA binding and repair by O⁶-alkylguanine DNA alkyltransferase

Human O⁶-alkylguanine-DNA alkyltransferase (AGT) repairs mutagenic O⁶-alkylguanine and O⁴-alkylthymine adducts in single-stranded and duplex DNAs. These activities protect normal cells and tumor cells against drugs that alkylate DNA; drugs that inactivate AGT are under test as chemotherapeutic enhancers. In studies using 6-carboxyfluorescein (FAM)-labeled DNAs, AGT reduced the fluorescence intensity by ∼40% at binding saturation, whether the FAM was located at the 5' or the 3' end of the DNA. AGT protected residual fluorescence from quenching, indicating a solute-inaccessible binding site for FAM. Sedimentation equilibrium analyses showed that saturating AGT-stoichiometries were higher with FAM-labeled DNAs than with unlabeled DNAs, suggesting that the FAM provides a protein binding site that is not present in unlabeled DNAs. Additional fluorescence and sedimentation measurements showed that AGT forms a 1:1 complex with free FAM. Active site benzylation experiments and docking calculations support models in which the primary binding site is located in or near the active site of the enzyme. Electrophoretic analyses show that FAM inhibits DNA binding (IC₅₀∼76μM) and repair of DNA containing an O⁶-methylguanine residue (IC₅₀∼63μM). Similar results were obtained with other polycyclic aromatic compounds. These observations demonstrate the existence of a new class of non-covalent AGT-inhibitors. After optimization for binding-affinity, members of this class might be useful in cancer chemotherapy.

Inhibition of DNA repair with MGMT pseudosubstrates: phase I study of lomeguatrib in combination with dacarbazine in patients with advanced melanoma and other solid tumours

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.

The clinical development of inhibitors of poly(ADP-ribose) polymerase

A number of inhibitors of DNA repair have been evaluated or are undergoing development as potential cancer treatments. Inhibitors of poly(ADP-ribose) polymerase (PARP) are of particular interest in treating hereditary breast cancers occurring in patients who are carriers of BRCA1 or BRCA2 mutations. In vitro PARP inhibitors are highly cytotoxic to cell lines carrying BRCA mutations while only minimally toxic to cell lines without these mutations. This is thought to be due to a phenomenon known as synthetic lethality where the accumulation of single-strand breaks consequent on PARP inhibition are converted to double-strand breaks on cell division. Cancer cells in BRCA carriers are uniquely unable to repair the consequent double-strand breaks that result during cell division. PARP inhibitors were initially developed as possible chemo-potentiating agents but have now been evaluated clinically in BRCA-related tumors, showing remarkable single-agent activity. The potential future development and use is reviewed.

DNA repair pathways and their implication in cancer treatment

Many cytotoxic agents used in cancer treatment exert their effects through their ability to directly or indirectly damage DNA and thus resulting in cell death. Major types of DNA damage induced by anticancer treatment include strand breaks (double or single strand), crosslinks (inter-strand, intra-strand, DNA-protein crosslinks), and interference with nucleotide metabolism and DNA synthesis. On the other hand, cancer cells activate various DNA repair pathways and repair DNA damages induced by cytotoxic drugs. The purpose of the current review is to present the major types of DNA damage induced by cytotoxic agents, DNA repair pathways, and their role as predictive agents, as well as evaluate the future perspectives of the novel DNA repair pathways inhibitors in cancer therapeutics.

Targeting O⁶-methylguanine-DNA methyltransferase with specific inhibitors as a strategy in cancer therapy

O (6)-methylguanine-DNA methyltransferase (MGMT) repairs the cancer chemotherapy-relevant DNA adducts, O (6)-methylguanine and O (6)-chloroethylguanine, induced by methylating and chloroethylating anticancer drugs, respectively. These adducts are cytotoxic, and given the overwhelming evidence that MGMT is a key factor in resistance, strategies for inactivating MGMT have been pursued. A number of drugs have been shown to inactivate MGMT in cells, human tumour models and cancer patients, and O (6)-benzylguanine and O (6)-[4-bromothenyl]guanine have been used in clinical trials. While these agents show no side effects per se, they also inactivate MGMT in normal tissues and hence exacerbate the toxic side effects of the alkylating drugs, requiring dose reduction. This might explain why, in any of the reported trials, the outcome has not been improved by their inclusion. It is, however, anticipated that, with the availability of tumour targeting strategies and hematopoetic stem cell protection, MGMT inactivators hold promise for enhancing the effectiveness of alkylating agent chemotherapy.

Critical appraisal of temozolomide formulations in the treatment of primary brain tumors: patient considerations

Chemotherapy is assuming an increasingly important role in the treatment of malignant gliomas, of which temozolomide (TMZ) is a key part. TMZ belongs to a class of second-generation imidazotetrazinone prodrugs that exhibit linear pharmacokinetics and do not require hepatic metabolism for activation to the active metabolite. New intravenous (iv) TMZ formulations have recently been approved based on studies of bioequivalence between iv and oral TMZ. The efficacy of TMZ was initially evaluated in patients with recurrent disease but phase II and III trials in newly diagnosed gliomas are available. The results of a large phase III trial that compared RT alone vs RT concomitant with oral TMZ created a new standard of adjuvant treatment. Efficacy data for iv TMZ on which its approval was based are those extrapolated from clinical trials with oral TMZ. No comparative data are available on the differences in tolerability and patient satisfaction between oral and iv formulations of TMZ, or for quality of life. New oral formulations could encourage the adherence of patients to treatment. Although patients presumably would prefer oral treatment, iv formulations may be an alternative in noncompliant patients or patients for whom good adherence could not be expected.