BAP1 loss induces mitotic defects in mesothelioma cells through BRCA1-dependent and independent mechanisms.

The tumour suppressor BRCA1-associated protein 1 (BAP1) is the most frequently mutated cancer gene in mesothelioma. Here we report novel functions for BAP1 in mitotic progression highlighting the relationship between BAP1 and control of genome stability in mesothelioma cells with therapeutic implications. Depletion of BAP1 protein induced proteasome-mediated degradation of BRCA1 in mesothelioma cells while loss of BAP1 correlated with BRCA1 loss in mesothelioma patient tumour samples. BAP1 loss also led to mitotic defects that phenocopied the loss of BRCA1 including spindle assembly checkpoint failure, centrosome amplification and chromosome segregation errors. However, loss of BAP1 also led to additional mitotic changes that were not observed upon BRCA1 loss, including an increase in spindle length and enhanced growth of astral microtubules. Intriguingly, these consequences could be explained by loss of expression of the KIF18A and KIF18B kinesin motors that occurred upon depletion of BAP1 but not BRCA1, as spindle and astral microtubule defects were rescued by re-expression of KIF18A and KIF18B, respectively. We therefore propose that BAP1 inactivation causes mitotic defects through BRCA1-dependent and independent mechanisms revealing novel routes by which mesothelioma cells lacking BAP1 may acquire genome instability and exhibit altered responses to microtubule-targeted agents.

Active symptom control with or without oral vinorelbine in patients with relapsed malignant pleural mesothelioma (VIM): A randomised, phase 2 trial.

Background: Currently, there is no US Food and Drug Administration approved therapy for patients with pleural mesothelioma who have relapsed following platinum-doublet based chemotherapy. Vinorelbine has demonstrated useful clinical activity in mesothelioma, however its efficacy has not been formally evaluated in a randomised setting. BRCA1 expression is required for vinorelbine induced apoptosis in preclinical models. Loss of expression may therefore correlate with vinorelbine resistance. Methods: In this randomised, phase 2 trial, patients were eligible if they met the following criteria: age ≥ 18 years, Eastern Cooperative Oncology Group (ECOG) performance status 0 or 1, histologically confirmed pleural mesothelioma, post platinum-based chemotherapy, and radiological evidence of disease progression. Consented patients were randomised 2:1 to either active symptom control with oral vinorelbine versus active symptom control (ASC) every 3 weeks until disease progression, unacceptable toxicity or withdrawal at an initial dose of 60 mg/m2 increasing to 80 mg/m2 post-cycle 1. Randomisation was stratified by histological subtype, white cell count, gender, ECOG performance status and best response during first-line therapy. The study was open label. The primary endpoint was progression-free survival (PFS), measured from randomisation to time of event (or censoring). Analyses were carried out according to intention-to-treat (ITT) principles. Recruitment and trial follow-up are complete. This trial is registered with ClinicalTrials.gov, number NCT02139904. Findings: Between June 1, 2016 and Oct 31, 2018, we performed a randomised phase 2 trial in 14 hospitals in the United Kingdom. 225 patients were screened for eligibility, of whom 154 were randomly assigned to receive either ASC + vinorelbine (n = 98) or ASC (n = 56). PFS was significantly longer for ASC+vinorelbine compared with ASC alone; 4.2 months (interquartile range (IQR) 2.2-8.0) versus 2.8 months (IQR 1.4-4.1) for ASC, giving an unadjusted hazard ratio (HR) of 0·60 (80% CI upper limit 0.7, one-sided unadjusted log rank test p = 0.002); adjusted HR 0.6 (80% CI upper limit 0.7, one-sided adjusted log rank test p < 0.001). BRCA1 did not predict resistance to ASC+vinorelbine. Neutropenia was the most common grades 3, 4 adverse events in the ASC +vinorelbine arm. Interpretation: Vinorelbine plus ASC confers clinical benefit to patients with relapsed pleural mesothelioma who have progressed following platinum-based doublet chemotherapy. Funding: This study was funded by Cancer Research UK (grant CRUK A15569).

Efficacy of CDK4/6 inhibitors in preclinical models of malignant pleural mesothelioma.

BACKGROUND: There is no effective therapy for patients with malignant pleural mesothelioma (MPM) who progressed to platinum-based chemotherapy and immunotherapy. METHODS: We aimed to investigate the antitumor activity of CDK4/6 inhibitors using in vitro and in vivo preclinical models of MPM. RESULTS: Based on publicly available transcriptomic data of MPM, patients with CDK4 or CDK6 overexpression had shorter overall survival. Treatment with abemaciclib or palbociclib at 100 nM significantly decreased cell proliferation in all cell models evaluated. Both CDK4/6 inhibitors significantly induced G1 cell cycle arrest, thereby increasing cell senescence and increased the expression of interferon signalling pathway and tumour antigen presentation process in culture models of MPM. In vivo preclinical studies showed that palbociclib significantly reduced tumour growth and prolonged overall survival using distinct xenograft models of MPM implanted in athymic mice. CONCLUSIONS: Treatment of MPM with CDK4/6 inhibitors decreased cell proliferation, mainly by promoting cell cycle arrest at G1 and by induction of cell senescence. Our preclinical studies provide evidence for evaluating CDK4/6 inhibitors in the clinic for the treatment of MPM.

Transcriptional perturbation of protein arginine methyltransferase-5 exhibits MTAP-selective oncosuppression.

We hypothesized that small molecule transcriptional perturbation could be harnessed to target a cellular dependency involving protein arginine methyltransferase 5 (PRMT5) in the context of methylthioadenosine phosphorylase (MTAP) deletion, seen frequently in malignant pleural mesothelioma (MPM). Here we show, that MTAP deletion is negatively prognostic in MPM. In vitro, the off-patent antibiotic Quinacrine efficiently suppressed PRMT5 transcription, causing chromatin remodelling with reduced global histone H4 symmetrical demethylation. Quinacrine phenocopied PRMT5 RNA interference and small molecule PRMT5 inhibition, reducing clonogenicity in an MTAP-dependent manner. This activity required a functional PRMT5 methyltransferase as MTAP negative cells were rescued by exogenous wild type PRMT5, but not a PRMT5E444Q methyltransferase-dead mutant. We identified c-jun as an essential PRMT5 transcription factor and a probable target for Quinacrine. Our results therefore suggest that small molecule-based transcriptional perturbation of PRMT5 can leverage a mutation-selective vulnerability, that is therapeutically tractable, and has relevance to 9p21 deleted cancers including MPM.

Clonal architecture in mesothelioma is prognostic and shapes the tumour microenvironment.

Malignant Pleural Mesothelioma (MPM) is typically diagnosed 20-50 years after exposure to asbestos and evolves along an unknown evolutionary trajectory. To elucidate this path, we conducted multi-regional exome sequencing of 90 tumour samples from 22 MPMs acquired at surgery. Here we show that exomic intratumour heterogeneity varies widely across the cohort. Phylogenetic tree topology ranges from linear to highly branched, reflecting a steep gradient of genomic instability. Using transfer learning, we detect repeated evolution, resolving 5 clusters that are prognostic, with temporally ordered clonal drivers. BAP1/-3p21 and FBXW7/-chr4 events are always early clonal. In contrast, NF2/-22q events, leading to Hippo pathway inactivation are predominantly late clonal, positively selected, and when subclonal, exhibit parallel evolution indicating an evolutionary constraint. Very late somatic alteration of NF2/22q occurred in one patient 12 years after surgery. Clonal architecture and evolutionary clusters dictate MPM inflammation and immune evasion. These results reveal potentially drugable evolutionary bottlenecking in MPM, and an impact of clonal architecture on shaping the immune landscape, with potential to dictate the clinical response to immune checkpoint inhibition.

BRCA1/MAD2L1 Deficiency Disrupts the Spindle Assembly Checkpoint to Confer Vinorelbine Resistance in Mesothelioma.

Mesothelioma is a universally lethal cancer lacking effective therapy. The spindle poison vinorelbine exhibits clinical activity in the relapsed setting, and in preclinical models requires BRCA1 to initiate apoptosis. However, the mechanisms underlying this regulation and the clinical implications have not been explored. Here, we show that BRCA1 silencing abrogated vinorelbine-induced cell-cycle arrest, recruitment of BUBR1 to kinetochores, and apoptosis. BRCA1 silencing led to codepletion of MAD2L1 at the mRNA and protein levels consistent with its status as a transcriptional target of BRCA1 Silencing of MAD2L1 phenocopied BRCA1 and was sufficient to confer resistance to vinorelbine. This was recapitulated in cell lines selected for resistance to vinorelbine, which acquired loss of both BRCA1 and MAD2L1 expression. Following ex vivo vinorelbine in 20 primary tumor explants, apoptotic response rate was 59% in BRCA1/MAD2L1-positive explants compared with 0% in BRCA1/MAD2L1-negative explants. In 48 patients, BRCA1 and/or MAD2L1 loss of expression was not prognostic; however, in a subset of patients treated with vinorelbine, survival was shorter for patients lacking BRCA1/MAD2L1 expression compared with double-positive patients (5.9 vs. 36.7 months, P = 0.03). Our data implicate BRCA1/MAD2L1 loss as a putative predictive marker of resistance to vinorelbine in mesothelioma and warrant prospective clinical evaluation.

Ganetespib in Combination with Pemetrexed-Platinum Chemotherapy in Patients with Pleural Mesothelioma (MESO-02): A Phase Ib Trial.

PURPOSE: Ganetespib, a highly potent, small-molecule Heatshock protein 90 inhibitor, has potential efficacy in malignant pleural mesothelioma (MPM) via activity on critical survival pathways and known synergies with antifolates and platinum chemotherapy. We conducted a dose-escalation study to identify the maximum tolerated dose (MTD) of ganetespib in patients with chemotherapy-naïve MPM. PATIENTS AND METHODS: MESO-02 (ClinicalTrials.gov: NCT01590160) was a nonrandomized, multicenter, phase Ib trial of 3-weekly ganetespib (100 mg/m2, 150 mg/m2, 200 mg/m2; days 1 and 15) with pemetrexed (500 mg/m2; day 1) and cisplatin (75 mg/m2; day 1) or carboplatin (area under concentration-time curve 5; day 1) in patients with MPM. Dose escalation was performed using the 3 + 3 design (cisplatin) and accelerated titration design (carboplatin). Secondary endpoints included best response, progression-free survival (PFS), and pharmacogenomic analyses. RESULTS: Of 27 patients enrolled (cisplatin, n = 16; carboplatin, n = 11), 3 experienced dose-limiting toxicities: grade 3 nausea (cisplatin, n = 1; carboplatin, n = 1) and grade 2 infusion-related reaction (carboplatin, n = 1). Ganetespib's MTD was 200 mg/m2. Partial response was observed in 14 of 27 patients (52%; 61% in 23 response-evaluable patients) and 13 of 21 (62%) with epithelioid histology. At the MTD, 10 of 18 patients (56%) had partial response, 15 of 18 (83%) had disease control, and median PFS was 6.3 months (95% CI, 5.0-10.0). One responder exhibited disease control beyond 50 months. Global loss of heterozygosity was associated with shorter time to progression (HR 1.12; 95% CI, 1.02-1.24; P = 0.018). CONCLUSIONS: Ganetespib can be combined safely with pemetrexed and platinum chemotherapy to treat patients with MPM. This class of agent should be investigated in larger randomized studies.

Loss of functional BAP1 augments sensitivity to TRAIL in cancer cells.

Malignant mesothelioma (MM) is poorly responsive to systemic cytotoxic chemotherapy and invariably fatal. Here we describe a screen of 94 drugs in 15 exome-sequenced MM lines and the discovery of a subset defined by loss of function of the nuclear deubiquitinase BRCA associated protein-1 (BAP1) that demonstrate heightened sensitivity to TRAIL (tumour necrosis factor-related apoptosis-inducing ligand). This association is observed across human early passage MM cultures, mouse xenografts and human tumour explants. We demonstrate that BAP1 deubiquitinase activity and its association with ASXL1 to form the Polycomb repressive deubiquitinase complex (PR-DUB) impacts TRAIL sensitivity implicating transcriptional modulation as an underlying mechanism. Death receptor agonists are well-tolerated anti-cancer agents demonstrating limited therapeutic benefit in trials without a targeting biomarker. We identify BAP1 loss-of-function mutations, which are frequent in MM, as a potential genomic stratification tool for TRAIL sensitivity with immediate and actionable therapeutic implications.

Crystal structure of EML1 reveals the basis for Hsp90 dependence of oncogenic EML4-ALK by disruption of an atypical ß-propeller domain.

Proteins of the echinoderm microtubule-associated protein (EMAP)-like (EML) family contribute to formation of the mitotic spindle and interphase microtubule network. They contain a unique hydrophobic EML protein (HELP) motif and a variable number of WD40 repeats. Recurrent gene rearrangements in nonsmall cell lung cancer fuse EML4 to anaplastic lymphoma kinase (ALK), causing expression of several fusion oncoprotein variants. We have determined a 2.6-Å crystal structure of the representative ∼70-kDa core of EML1, revealing an intimately associated pair of ß-propellers, which we term a TAPE (tandem atypical propeller in EMLs) domain. One propeller is highly atypical, having a discontinuous subdomain unrelated to a WD40 motif in place of one of its blades. This unexpected feature shows how a propeller structure can be assembled from subdomains with distinct folds. The HELP motif is not an independent domain but forms part of the hydrophobic core that joins the two ß-propellers. The TAPE domain binds α/ß-tubulin via its conserved, concave surface, including part of the atypical blade. Mapping the characteristic breakpoints of each EML4-ALK variant onto our structure indicates that the EML4 TAPE domain is truncated in many variants in a manner likely to make the fusion protein structurally unstable. We found that the heat shock protein 90 (Hsp90) inhibitor ganetespib induced degradation of these variants whereas others lacking a partial TAPE domain were resistant in both overexpression models and patient-derived cell lines. The Hsp90-sensitive EML4-ALK variants are exceptions to the rule that oncogenic fusion proteins involve breakpoints in disordered regions of both partners.

BAK and NOXA are critical determinants of mitochondrial apoptosis induced by bortezomib in mesothelioma.

Based on promising preclinical efficacy associated with the 20S proteasome inhibitor bortezomib in malignant pleural mesothelioma (MPM), two phase II clinical trials have been initiated (EORTC 08052 and ICORG 05-10). However, the potential mechanisms underlying resistance to this targeted drug in MPM are still unknown. Functional genetic analyses were conducted to determine the key mitochondrial apoptotic regulators required for bortezomib sensitivity and to establish how their dysregulation may confer resistance. The multidomain proapoptotic protein BAK, but not its orthologue BAX, was found to be essential for bortezomib-induced apoptosis in MPM cell lines. Immunohistochemistry was performed on tissues from the ICORG-05 phase II trial and a TMA of archived mesotheliomas. Loss of BAK was found in 39% of specimens and loss of both BAX/BAK in 37% of samples. However, MPM tissues from patients who failed to respond to bortezomib and MPM cell lines selected for resistance to bortezomib conserved BAK expression. In contrast, c-Myc dependent transactivation of NOXA was abrogated in the resistant cell lines. In summary, the block of mitochondrial apoptosis is a limiting factor for achieving efficacy of bortezomib in MPM, and the observed loss of BAK expression or NOXA transactivation may be relevant mechanisms of resistance in the clinic.

Phase II clinical trial of first or second-line treatment with bortezomib in patients with malignant pleural mesothelioma.

Based on promising preclinical efficacy of bortezomib in mesothelioma, a single-arm phase II trial (Ireland Cooperative Oncology Research Group 05-10 study), with Simon's two-stage design, was undertaken to assess efficacy of bortezomib monotherapy in the first-line (poor performance status) and second-line settings. The Bcl-2 homology domain 3-only protein Noxa has been implicated as a key inducer of apoptosis by bortezomib. Thus, in a biomarker research substudy, we hypothesized that deficiency in Noxa expression might correlate with resistance. In the second-line setting, 23 patients were enrolled. Partial response was confirmed in one patient (4.8%) who received four cycles of bortezomib. One patient had stable disease; however, progression occurred in the majority of patients within the first two cycles. Median progression-free survival and overall survival were 2.1 and 5.8 months, respectively. In the first-line setting, ten patients were accrued, and there was no evidence of objective response. In the tumor analysis, expression of Noxa was seen in all biopsies. Bortezomib monotherapy exhibits insufficient activity to warrant further investigation in unselected patients with mesothelioma.

Response to chemotherapy is predictive in relation to longer overall survival in an individual patient combined-analysis with pleural mesothelioma.

BACKGROUND: There is currently no early predictive marker of survival for patients receiving chemotherapy for malignant pleural mesothelioma (MPM). Tumour response may be predictive for overall survival (OS), though this has not been explored. We have thus undertaken a combined-analysis of OS, from a 42day landmark, of 526 patients receiving systemic therapy for MPM. We also validate published progression-free survival rates (PFSRs) and a progression-free survival (PFS) prognostic-index model. METHODS: Analyses included nine MPM clinical trials incorporating six European Organisation for Research and Treatment of Cancer (EORTC) studies. Analysis of OS from landmark (from day 42 post-treatment) was considered regarding tumour response. PFSR analysis data included six non-EORTC MPM clinical trials. Prognostic index validation was performed on one non-EORTC data-set, with available survival data. RESULTS: Median OS, from landmark, of patients with partial response (PR) was 12·8months, stable disease (SD), 9·4months and progressive disease (PD), 3·4months. Both PR and SD were associated with longer OS from landmark compared with disease progression (both p<0·0001). PFSRs for platinum-based combination therapies were consistent with published significant clinical activity ranges. Effective separation between PFS and OS curves provided a validation of the EORTC prognostic model, based on histology, stage and performance status. CONCLUSION: Response to chemotherapy is associated with significantly longer OS from landmark in patients with MPM.

Vorinostat eliminates multicellular resistance of mesothelioma 3D spheroids via restoration of Noxa expression.

When grown in 3D cultures as spheroids, mesothelioma cells acquire a multicellular resistance to apoptosis that resembles that of solid tumors. We have previously found that resistance to the proteasome inhibitor bortezomib in 3D can be explained by a lack of upregulation of Noxa, the pro-apoptotic BH3 sensitizer that acts via displacement of the Bak/Bax-activator BH3-only protein, Bim. We hypothesized that the histone deacetylase inhibitor vorinostat might reverse this block to Noxa upregulation in 3D. Indeed, we found that vorinostat effectively restored upregulation of Noxa protein and message and abolished multicellular resistance to bortezomib in the 3D spheroids. The ability of vorinostat to reverse resistance was ablated by knockdown of Noxa or Bim, confirming the essential role of the Noxa/Bim axis in the response to vorinostat. Addition of vorinostat similarly increased the apoptotic response to bortezomib in another 3D model, the tumor fragment spheroid, which is grown from human mesothelioma ex vivo. In addition to its benefit when used with bortezomib, vorinostat also enhanced the response to cisplatin plus pemetrexed, as shown in both 3D models. Our results using clinically relevant 3D models show that the manipulation of the core apoptotic repertoire may improve the chemosensitivity of mesothelioma. Whereas neither vorinostat nor bortezomib alone has been clinically effective in mesothelioma, vorinostat may undermine chemoresistance to bortezomib and to other therapies thereby providing a rationale for combinatorial strategies.

Vorinostat/SAHA-induced apoptosis in malignant mesothelioma is FLIP/caspase 8-dependent and HR23B-independent.

INTRODUCTION: Malignant pleural mesothelioma (MPM) is a rapidly fatal malignancy that is increasing in incidence. The caspase 8 inhibitor FLIP is an anti-apoptotic protein over-expressed in several cancer types including MPM. The histone deacetylase (HDAC) inhibitor Vorinostat (SAHA) is currently being evaluated in relapsed mesothelioma. We examined the roles of FLIP and caspase 8 in regulating SAHA-induced apoptosis in MPM. METHODS: The mechanism of SAHA-induced apoptosis was assessed in 7 MPM cell lines and in a multicellular spheroid model. SiRNA and overexpression approaches were used, and cell death was assessed by flow cytometry, Western blotting and clonogenic assays. RESULTS: RNAi-mediated FLIP silencing resulted in caspase 8-dependent apoptosis in MPM cell line models. SAHA potently down-regulated FLIP protein expression in all 7 MPM cell lines and in a multicellular spheroid model of MPM. In 6/7 MPM cell lines, SAHA treatment resulted in significant levels of apoptosis induction. Moreover, this apoptosis was caspase 8-dependent in all six sensitive cell lines. SAHA-induced apoptosis was also inhibited by stable FLIP overexpression. In contrast, down-regulation of HR23B, a candidate predictive biomarker for HDAC inhibitors, significantly inhibited SAHA-induced apoptosis in only 1/6 SAHA-sensitive MPM cell lines. Analysis of MPM patient samples demonstrated significant inter-patient variations in FLIP and caspase 8 expressions. In addition, SAHA enhanced cisplatin-induced apoptosis in a FLIP-dependent manner. CONCLUSIONS: These results indicate that FLIP is a major target for SAHA in MPM and identifies FLIP, caspase 8 and associated signalling molecules as candidate biomarkers for SAHA in this disease.

BRCA1 is an essential mediator of vinorelbine-induced apoptosis in mesothelioma.

Therapeutic options for malignant pleural mesothelioma (MPM) are limited despite the increasing incidence globally. The vinca alkaloid vinorelbine exhibits clinical activity; however, to date, treatment optimization has not been achieved using biomarkers. BRCA1 regulates sensitivity to microtubule poisons; however, its role in regulating vinorelbine-induced apoptosis in mesothelioma is unknown. Here we demonstrate that BRCA1 plays an essential role in mediating vinorelbine-induced apoptosis, as evidenced by (1) the strong correlation between vinorelbine sensitivity and BRCA1 expression level; (2) induction of resistance to vinorelbine by BRCA1 using siRNA oligonucleotides; (3) dramatic down-regulation of BRCA1 following selection for vinorelbine resistance; and (4) the re-activation of vinorelbine-induced apoptosis following re-expression of BRCA1 in resistant cells. To determine whether loss of BRCA1 expression in mesothelioma was potentially relevant in vivo, BRCA1 immunohistochemistry was subsequently performed on 144 primary mesothelioma specimens. Loss of BRCA1 protein expression was identified in 38.9% of samples. Together, these data suggest that BRCA1 plays a critical role in mediating apoptosis by vinorelbine in mesothelioma, warranting its clinical evaluation as a predictive biomarker.

MicroRNA regulation of core apoptosis pathways in cancer.

Recent research has demonstrated that microRNAs (miRNAs) are key regulators of many cell processes often deregulated in cancer, including apoptosis. Indeed, it is becoming clear that many miRNAs are anti-apoptotic and mediate this effect by targeting pro-apoptotic mRNAs or positive regulators of pro-apoptotic mRNAs. Conversely, many pro-apoptotic miRNAs target anti-apoptotic mRNAs or their positive regulators. We have reviewed the current knowledge in this area including evidence of miRNA involvement in cancer drug resistance.

MicroRNA signature of malignant mesothelioma with potential diagnostic and prognostic implications.

MicroRNAs (miRNAs) post-transcriptionally regulate the expression of target genes, and may behave as oncogenes or tumor suppressors. Human malignant mesothelioma is an asbestos-related cancer, with poor prognosis and low median survival. Here we report, for the first time, a cross-evaluation of miRNA expression in mesothelioma (MPP-89, REN) and human mesothelial cells (HMC-telomerase reverse transcriptase). Microarray profiling, confirmed by real-time quantitative RT-PCR, revealed a differential expression of miRNAs between mesothelioma and mesothelial cells. In addition, a computational analysis combining miRNA and gene expression profiles allowed the accurate prediction of genes potentially targeted by dysregulated miRNAs. Several predicted genes belong to terms of Gene Ontology (GO) that are associated with the development and progression of mesothelioma. This suggests that miRNAs may be key players in mesothelioma oncogenesis. We further investigated miRNA expression on a panel of 24 mesothelioma specimens, representative of the three histotypes (epithelioid, biphasic, and sarcomatoid), by quantitative RT-PCR. The expression of miR-17-5p, miR-21, miR-29a, miR-30c, miR-30e-5p, miR-106a, and miR-143 was significantly associated with the histopathological subtypes. Notably, the reduced expression of two miRNAs (miR-17-5p and miR-30c) correlated with better survival of patients with sarcomatoid subtype. Our preliminary analysis points at miRNAs as potential diagnostic and prognostic markers of mesothelioma, and suggests novel tools for the therapy of this malignancy.

Preliminary data suggestive of a novel translational approach to mesothelioma treatment: imatinib mesylate with gemcitabine or pemetrexed.

BACKGROUND: Malignant mesothelioma is a cancer which is refractory to current treatments. Imatinib mesylate is a selective inhibitor of tyrosine kinases such as bcr-abl, c-Kit, c-Fms and platelet derived growth factor receptor beta (PDGFRbeta). PDGFRbeta is often overexpressed in mesothelioma cells and is a therapeutic target for imatinib in some solid tumours. A study was undertaken to assess whether imatinib alone or combined with chemotherapeutic agents may be effective for treating mesothelioma. METHODS: Cultures from mesothelioma MMP, REN and ISTMES2 cell lines were treated with imatinib alone or in combination with a chemotherapeutic agent. RESULTS: Imatinib induced cytotoxicity and apoptosis selectively on PDGFRbeta positive mesothelioma cells via blockade of receptor phosphorylation and interference with the Akt pathway. Of the chemotherapeutic agents tested in combination with imatinib, a synergistic effect was obtained with gemcitabine and pemetrexed. CONCLUSIONS: This study provides a rationale for a novel translational approach to the treatment of mesothelioma which relies on enhancement of tumour chemosensitivity by inhibition of Akt.