PURPOSE The therapeutic benefit of temozolomide (TMZ) in glioblastoma (GBM) is limited by resistance. The goal of this study was to elucidate mechanisms of TMZ resistance in GBM. EXPERIMENTAL DESIGN We developed an in vivo GBM model of TMZ resistance and used paired parental and TMZ resistant tumors to define the mechanisms underlying the development of resistance and the influence of histone deacetylation (HDAC) inhibition. RESULTS Analysis of paired parental and resistant lines demonstrated upregulation of MGMT expression in 3 of the 5 resistant xenografts. While no significant change was detected in MGMT promoter methylation between parental and derivative resistant samples, chromatin immunoprecipitation demonstrated an association between MGMT upregulation and elevated acetylation of lysine 9 of histone H3 (H3K9-ac) and decreased di-methylation (H3K9-me2) in GBM12 and GBM14. In contrast, TMZ resistance development in GBM22 was not linked to MGMT expression and both parental and resistant lines had low H3K9-ac and high H3K9-me2 within the MGMT promoter. In the GBM12 TMZ resistant line, MGMT re-expression was accompanied by increased recruitment of SP1, C-JUN, NF-kB and p300 within the MGMT promoter. Interestingly, combined treatment of GBM12 flank xenografts with TMZ and the HDAC inhibitor suberoylanilide hydroxamic acid (SAHA) favored the evolution of TMZ resistance by MGMT over-expression as compared to treatment with TMZ alone. CONCLUSION This study demonstrates, for the first time, a unique mechanism of TMZ resistance development driven by chromatin mediated MGMT upregulation and highlights the potential for epigenetically directed therapies to influence the mechanisms of resistance development in GBM.

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Veliparib statistically significantly enhances (P < .001) the efficacy of TMZ in tumors with MGMT promoter hypermethylation. Based on these data, MGMT promoter hypermethylation is being used as an eligibility criterion for A071102 (NCT02152982), the phase II/III clinical trial evaluating TMZ/veliparib combination in patients with GBM.

Cancer cells can resist the effects of DNA-damaging therapeutic agents via utilization of DNA repair pathways, suggesting that DNA repair capacity (DRC) measurements in cancer cells could be used to identify patients most likely to respond to treatment. However, the limitations of available technologies have so far precluded adoption of this approach in the clinic. We recently developed fluorescence-based multiplexed host cell reactivation (FM-HCR) assays to measure DRC in multiple pathways. Here we apply a mathematical model that uses DRC in multiple pathways to predict cellular resistance to killing by DNA-damaging agents. This model, developed using FM-HCR and drug sensitivity measurements in 24 human lymphoblastoid cell lines, was applied to a panel of 12 patient-derived xenograft (PDX) models of glioblastoma to predict glioblastoma response to treatment with the chemotherapeutic DNA-damaging agent temozolomide. This work showed that, in addition to changes in O6-methylguanine DNA methyltransferase (MGMT) activity, small changes in mismatch repair (MMR), nucleotide excision repair (NER), and homologous recombination (HR) capacity contributed to acquired temozolomide resistance in PDX models and led to reduced relative survival prolongation following temozolomide treatment of orthotopic mouse models in vivo. Our data indicate that measuring the combined status of MMR, HR, NER, and MGMT provided a more robust prediction of temozolomide resistance than assessments of MGMT activity alone.

Purpose Effective sensitizing strategies potentially can extend the benefit of TMZ therapy in GBM patients. We previously demonstrated that robust TMZ-sensitizing effects of the PARP inhibitor veliparib (ABT-888) are restricted to TMZ-sensitive GBM xenografts. The focus of this study is to provide an understanding for the differential sensitization in paired TMZ-sensitive and -resistant GBM models. Experimental Design The impact of veliparib on TMZ-induced cytotoxicity and DNA damage was evaluated in vitro and in vivo in models of acquired TMZ-resistance (GBM12TMZ-mgmtHigh, GBM12TMZ-mgmtLow, U251TMZ), inherent TMZ-resistance (T98G), and TMZ-sensitive (U251, GBM12). In vivo drug efficacy, pharmacokinetics, and pharmacodynamics were analyzed using clinically relevant dosing regimens. Results Veliparib enhanced TMZ cytotoxicity and DNA damage signaling in all GBM models in vitro with more pronounced effects in TMZ-resistant lines at 3-10 μM veliparib. In vivo, combined TMZ/veliparib, compared to TMZ alone, significantly delayed tumor growth and enhanced DNA damage signaling and γH2AX levels in the sensitive GBM12 xenograft line but not in the resistant GBM12TMZ lines. The pharmacokinetic profile of veliparib was similar for GBM12 and GBM12TMZ tumors with Cmax (~1.5 μM) in tissue significantly lower than concentrations associated with optimal in vitro sensitizing effects for resistant tumors. In contrast, robust suppression of PARP-1 expression by shRNA significantly increased TMZ sensitivity of U251TMZ in vitro and in vivo. Conclusions In vitro cytotoxicity assays do not adequately model the therapeutic index of PARP inhibitors, as concentrations of veliparib and TMZ required to sensitize TMZ-resistant cancer cells in vivo cannot be achieved using a tolerable dosing regimen.

Transforming growth factor β (TGFβ) has significant profibrotic activity both in vitro and in vivo. This reflects its capacity to stimulate fibrogenic mediators and induce the expression of other profibrotic cytokines such as platelet-derived growth factor (PDGF) and epidermal growth factor (EGF/ErbB) ligands. Here we address both the mechanisms by which TGFβ induced ErbB ligands and the physiological significance of inhibiting multiple TGFβ-regulated processes. The data document that ErbB ligand induction requires PDGF receptor (PDGFR) mediation and engages a positive autocrine/paracrine feedback loop via ErbB receptors. Whereas PDGFRs are essential for TGFβ-stimulated ErbB ligand up-regulation, TGFβ-specific signals are also required for ErbB receptor activation. Subsequent profibrotic responses are shown to involve the cooperative action of PDGF and ErbB signaling. Moreover, using a murine treatment model of bleomycin-induced pulmonary fibrosis we found that inhibition of TGFβ/PDGF and ErbB pathways with imatinib plus lapatinib, respectively, not only prevented myofibroblast gene expression to a greater extent than either drug alone, but also essentially stabilized gas exchange (oxygen saturation) as an overall measure of lung function. These observations provide important mechanistic insights into profibrotic TGFβ signaling and indicate that targeting multiple cytokines represents a possible strategy to ameliorate organ fibrosis dependent on TGFβ.

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