Malignant pleural mesothelioma is a relatively rare malignancy with an annual incidence in the United States of approximately 3,000 cases. Based on asbestos exposure demographics, incidence should peak in the United States in the next 10 to 20 years. Peto et al have suggested that the incidence in Western Europe may continue to climb for substantially longer, possibly reaching as high as 1/100 among middle-aged men.[1,2] In developing countries (often bereft of asbestos regulation), the incidence is not known.
As Zellos and Sugarbaker from Brigham and Women’s Hospital in Boston aptly point out, the disease has been characterized by difficult and often late diagnosis, extreme local aggressiveness, and poor overall survival. The authors focus on multimodality therapy for locally advanced disease—an approach they and others have singled out as likely necessary in order to make substantial progress. At the same time, they touch upon novel therapeutic approaches including photodynamic therapy, immunotherapy, gene therapy, and intracavitary chemotherapy. Given the seemingly unique biology of this tumor, it may well provide additional interesting targets based on biological findings, which, in turn, may lead to breakthroughs in the treatment of this devastating disease.
Biology of Malignant Pleural Mesothelioma
Despite its low incidence, malignant pleural mesothelioma has nevertheless caught the attention of investigators as to its possible genetic and biological underpinnings. Abnormal expression of biologically active gene products has been observed, including platelet-derived growth factor (PDGF), cyclooxygenase (COX)-2, interleukin (IL)-6, fibroblast growth factor (FGF), and others. DNA array analysis has demonstrated that expression of as many as 26 genes is up-regulated and 13 down-regulated in malignant pleural mesothelioma cell lines, compared to progenitor mesothelial cells.[4,5]
Cytogenetic studies have demonstrated reproducible alterations in chromosome positions 1p21-22, 3p21, 6q15-21, and 9p21-22.[6,7] Although not all of these regions are completely characterized, some are known to harbor tumor-suppressor genes. One common genetic abnormality that has been elucidated is one protein product of the CDK4 locus, p16, which is located in the 9p21 region. Homozygous deletions of p16 have been noted with relatively high frequency in both malignant and premalignant lesions of the airway, and recently, in mesothelioma. This high incidence of p16 alterations in malignant pleural mesothelioma cell lines and other tumor tissues may be associated with a selective growth advantage.
Recently, an association between the SV40 virus and malignant pleural mesothelioma was noted, although this finding has been contended in multicenter evaluations.[9,10] Numerous publications have suggested that malignant pleural mesothelioma tumor tissue harbors SV40 sequences in 30% to 85% of cases, with normal tissue expression seen only approximately 10% to 20% of the time. In a compelling early report, Cicala et al demonstrated that malignant pleural mesothelioma could be induced in hamsters following exposure to the SV40 virus.
Taking Advantage of Biology
Given that malignant pleural mesothelioma remains a therapeutic problem, several groups, including ours, have sought to study genetic and molecular approaches to treatment. The first reported approach was the use of the adenoviral herpes simplex virus thymidine kinase (HSVtk)/ganciclovir (Cytovene) prodrug paradigm.[12,13] Promising preclinical studies led to the completion of a phase I clinical trial in which safety and gene transfer were documented. Although the feasibility of using the HSVtk/ganciclovir prodrug paradigm has been proven, its therapeutic potential in malignant pleural mesothelioma has not been convincingly documented.
Alluding to the common lack of p16 expression mentioned earlier, Frizelle et al recently described experiments in mice, involving malignant pleural mesothelioma xenografts to the flank and peritoneal cavity with an adenoviral vector capable of transferring the p16 gene. These investigators demonstrated transgene transfer in flank tumors, with decreased growth compared to controls. Although less impressive, a trend toward tumor reduction was also noted in animals receiving intraperitoneal tumor xenografts.
Another gene therapy strategy that has attracted investigators working in locoregionally advanced non-small-cell lung and head and neck cancers is to follow the induction of apoptosis with transfer of the tumor-suppressor p53 gene—an approach that is particularly effective when p53 gene expression is absent or down-regulated. However, abnormalities in p53 expression appear to be uncommon in malignant pleural mesothelioma.
Because apoptosis is a conserved pathway in cancer cells (which, when activated, can lead to cellular death), alternative approaches to the induction of apoptosis have been evaluated. Both overexpression and inhibition of several candidate genes in malignant pleural mesothelioma cell lines—including bak, bcl-xl, mda-7, and PTEN—have been examined by investigators from the department of thoracic surgery at M. D. Anderson.
Other Molecular Therapies
Pataer et al investigated an adenovirus-mediated transfer of the bak gene, a potent proapoptotic member of the bcl-2 family, into malignant pleural mesothelioma cell lines. The adenoviral vector was designed using a binary adenoviral vector system and had previously been shown to induce apoptosis in p53-sensitive lung cancer cells in a caspase 3-dependent fashion. The results of this experiment were interesting in that apoptosis was induced in both p53-positive and p53-negative malignant pleural mesothelioma cell lines.
In contrast to bak, the bcl-xl gene is one of the major antiapoptotic members of the bcl-2 family. Although other antiapoptotic proteins have been described, bcl-xl and bcl-2 are among the more potent and, when overexpressed, demonstrate equivalent ability to inhibit apoptosis. Several authors have noted that bcl-2 expression is uniformly attenuated or absent in malignant pleural mesothelioma, with expression of bcl-xl and proapoptotic bax and bak intact.[18,19] This lack of bcl-2 expression is important, as others have noted that antisense oligonucleotides directed at bcl-xl will only induce apoptosis when bcl-2 expression is absent or attenuated.
Some investigators have reported a relationship between bcl-xl expression and chemotherapy sensitivity. In one study, U-937 cells were stably transfected to overexpress bcl-xl, and apoptosis was decreased following exposure to vinblastine, paclitaxel, and cisplatin by 67%, 51%, and 55%, respectively. We have demonstrated that when malignant pleural mesothelioma cells are treated with sodium butyrate, bcl-xl is specifically down-regulated, and apoptosis is induced. This pharmacologic down-regulation of bcl-xl also primes malignant pleural mesothelioma cells for exposure to other antiapoptotic gene therapy such as ad-bak and ad-bax.[22,23] In addition, we recently demonstrated that specific down-regulation of bcl-xl with antisense oligonucleotides can lead to apoptotic cellular death.
Finally, in vitro experiments with adenovirus-mediated transfer of the mda-7 and PTEN genes to malignant pleural mesothelioma cells appear promising. In regard to the possible link between SV40 and this malignancy, Waheed et al demonstrated that antisense oligonucleotides directed at the SV40 early gene region were able to induce apoptosis in pleural mesothelioma lines that demonstrated positivity for large T antigen.
Integration of Novel Therapies
In a general solid tumor phase I trial, investigators found that the multitargeted antifolate pemetrexed (LY231514) exhibits surprising activity in patients with otherwise refractory malignant pleural mesothelioma. It is also encouraging to note that several biologically active substances overproduced by malignant pleural mesothelioma cells are potential targets for newer biological therapies such as kinase inhibitors.
As exciting as some of the early preclinical and clinical studies may appear, however, it is clear that unimodality therapy is highly unlikely to lead to significant progress in this disease, regardless of the approach (be it conventional, molecular, or biological). As such, the previous work by Sugarbaker and others—in which multimodality therapy was evaluated and resulted in long-term survival in some patients—deserves significant recognition. Approaches that have combined extrapleural pneumonectomy (or pleurectomy/decortication) with tolerable yet potent doses of radiation therapy have been particularly promising.
We are currently evaluating the combination of extrapleural pneumonectomy followed by intensity-modulated radiation therapy (IMRT) in selected patients. IMRT can greatly increase the radiation dose per unit volume over very discrete fields. In a pilot investigation, we demonstrated that the treatment is safe and leads to an apparent reduction in the rate of local recurrences.
In conclusion, the authors nicely cover substantial ground in their assessment of the staging, biology, and multimodality therapy of a devastating if relatively rare disease. Clearly, we need additional dedicated clinical and translational investigators working with progressive new approaches based on recently garnered biological information. Although past treatment failures have spawned nihilism among many researchers, we and others remain hopeful that the near future holds promise in the battle against this grim disease.
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