The alpha folate receptor is highly activated in malignant pleural mesothelioma

Malignant pleural mesothelioma: overview of the North American and European experience

MPM is an uncommon disease with limited treatment options. Early diagnosis, a standardized staging system, early referral to centers experienced in MPM, and efforts to develop collaborative multicenter trials are essential to improving treatment for patients with MPM. Efforts to manage this malignancy, which is projected to peak in the twenty-first century, constitute an important international health concern, particularly because the use of asbestos, despite successful regulatory efforts in many parts of the world, continues unabated in others.

Malignant Pleural Mesothelioma

The diagnosis and management of malignant pleural mesothelioma are major challenges that often frustrate both patient and clinician alike. Occupational asbestos exposure to crocidolite or amosite forms of the fiber is the most important known risk factor in North America and Western Europe. Other mineral fibers such as erionite, a naturally occurring fibrous zeolite crystal, are associated with mesothelioma in volcanic tuffs of the Cappadocia region of central Anatolia in Turkey. In addition, other possible factors such as the presence of simian virus 40 and genetic susceptibility have been associated recently with the development of mesothelioma in animal models. These latter findings are increasing our understanding of this disease. In addition, the discovery of elevated levels of various markers such as folic acid receptor alpha, cyclooxygenase 2, and multidrug resistance proteins 1 and 2 in mesothelioma tissue have opened up new areas of potential diagnostic and therapeutic importance. However, traditional treatment modalities--surgery, radiotherapy, and chemotherapy--have evolved slowly, and few gains in therapeutic efficacy have occurred. Recently, however, continuing research efforts have led to novel treatment strategies that are changing the way clinicians view a disease that has traditionally been managed with almost universal therapeutic nihilism. This review explores our current knowledge of this disease and presents current and novel therapeutic strategies.

Malignant pleural mesothelioma--an update

Exposure to asbestos is the most frequent, but not exclusive, cause of malignant mesothelioma. Clinical features include dyspnea, cough, nonspecific chest pain, weight loss and night sweats. Diagnosis may be complicated by histologic difficulties. Thoracoscopic techniques are proving beneficial, but no one method of imaging has proven superior, and disease staging is inconsistent. Conventional treatments such as chemotherapy, surgery, and radiotherapy have had variable impacts, although chemotherapy is useful in palliation and can improve both survival and quality of life. There is hope for new antimetabolite agents. The role of radical surgery is yet to be evaluated in a large trial. New radiotherapeutic techniques to improve local control are promising. Multimodality treatments appear to be the most successful for management of potentially resectable disease. It is likely that biological markers will improve accuracy in staging and prognosis. With new treatments based on better understanding of the biology of the disease, there is cautious optimism for the future for patients with malignant pleural mesothelioma.

Novel Combinations Using Pemetrexed in Malignant Mesothelioma

Malignant mesothelioma is an uncommon malignancy that is locally invasive and rapidly fatal. The majority of patients with mesothelioma are not candidates for curative surgical resection. Chemotherapy has yielded only modest results in these patients. Pemetrexed is a multitargeted antifolate that is being evaluated in many tumor types. Recent single-agent data and data in combination with cisplatin have suggested that pemetrexed has therapeutic benefits in patients with malignant mesothelioma. This article summarizes the data regarding pemetrexed in the treatment of malignant mesothelioma and the potential novel combinations involving the drug that may be used in the future for the treatment of the disease.

Pemetrexed Alone and in Combination with Platinum Compounds in the Management of Malignant Mesothelioma

Malignant pleural mesothelioma is an aggressive but rare malignancy with a dismal prognosis. It is traditionally resistant to chemotherapy. Antifolate agents have recently shown promising data in the treatment of this malignancy. Pemetrexed is a multitargeted antifolate inhibitor of thymidylate synthase and other folate-dependent enzymes that has emerged as one of the most active agents in this disease. Several phase I/II trials of pemetrexed as a single agent or in combination with a platinum drug have demonstrated considerable activity in mesothelioma. In a recently published phase III randomized study, pemetrexed/cisplatin showed a significant improvement in survival, response rate, and quality of life compared with single-agent cisplatin. In addition, several trials reported that folic acid and vitamin B12 supplementation significantly reduced the toxicity observed with the use of pemetrexed without affecting the efficacy of the drug.

The Role of α-Folate Receptor-Mediated Transport in the Antitumor Activity of Antifolate Drugs

PURPOSE

Raltitrexed, pemetrexed, lometrexol, and ZD9331 are antifolate drugs transported into cells via the ubiquitously expressed reduced-folate carrier. They display also high affinity for the alpha-folate receptor (alpha-FR), a low capacity folate transporter that is highly overexpressed in some epithelial tumors. The role of alpha-FR in the activity of the antifolates has been evaluated in two alpha-FR-overexpressing cell lines grown in a physiological concentration of folate (20 nM R,S-Leucovorin).

EXPERIMENTAL DESIGN AND RESULTS

A431-FBP cells (transfected with the alpha-FR) were 3-5-fold more sensitive to the antifolates than A431 cells. KB cells (constitutive alpha-FR overexpression) were less sensitive to the drugs when coexposed to 1 microM folic acid to competitively inhibit binding to the alpha-FR. Raltitrexed, pemetrexed, and lometrexol are polyglutamated in cells leading to drug retention, e.g., the raltitrexed 4- and 24-h IC(50)s in A431 cells were approximately 0.6 and 0.008 microM, respectively, compared with 0.003 microM for 72-h continuous exposure. A431-FBP cells were approximately 3-fold more sensitive to raltitrexed and pemetrexed at all exposure times. ZD9331 is not polyglutamated, and the 4- and 24-h IC(50)s in A431 cells were >100 and approximately 100 microM, respectively, reducing to 2 and 0.1 microM, respectively, in A431-FBP cells. The ZD9331 4- and 24-h IC(50)s in KB cells were 20 and 1 microM, respectively, and reversible by coaddition of 1 microM folic acid. An in situ thymidylate synthase assay demonstrated continued thymidylate synthase inhibition after ZD9331-treated A431-FBP and KB, but not A431, cells were placed in drug-free medium for 16 h. A model is proposed in which the antifolates accumulate in the alpha-FR/endosomal apparatus, leading to slow release into the cytoplasm. In particular, this leads to cellular retention of the nonpolyglutamatable ZD9331.

CONCLUSIONS

Antifolate drugs, particularly ZD9331, have the potential for increased efficacy in tumors that highly overexpress the alpha-FR.

Phase II study of pemetrexed with and without folic acid and vitamin B12 as front-line therapy in malignant pleural mesothelioma

PURPOSE

This phase II clinical study evaluated the efficacy of pemetrexed for the treatment of malignant pleural mesothelioma (MPM).

PATIENTS AND METHODS

Patients with a histologically proven diagnosis of MPM, chemotherapy-naive measurable lesions, and adequate organ function received pemetrexed (500 mg/m2) intravenously over 10 minutes every 3 weeks. After a protocol change, most patients also received folic acid and vitamin B12 supplementation to improve safety.

RESULTS

A total of 64 patients were enrolled. Nine (14.1%) of the 64 patients had a partial response. The Kaplan-Meier estimate for median overall survival was 10.7 months. Forty-three patients received vitamin supplementation for all courses of therapy, and 21 patients did not. Seven of the nine responders were vitamin supplemented. The median overall survival was 13.0 months for supplemented patients and 8.0 months for nonsupplemented patients. Vitamin-supplemented patients completed more cycles of therapy than nonsupplemented patients (median, six v two cycles, respectively). Grade 3/4 neutropenia (23.4%) and grade 3/4 leukopenia (18.8%) were the most common laboratory toxicities. Fatigue and febrile neutropenia were the most commonly reported nonlaboratory events (grade 3, 6.3%; grade 4, 0.0% each). The incidence of these toxicities was generally lower in the supplemented patients.

CONCLUSION

Single-agent pemetrexed for MPM resulted in a moderate response rate (14.1%) and median overall survival of 10.7 months. Patients supplemented with folic acid and vitamin B12 tolerated treatment better (less toxicity and more cycles of treatment) and had a 5-month greater median overall survival than nonsupplemented patients. These results indicate that patients with MPM could benefit from single-agent pemetrexed treatment combined with vitamin supplementation.

Future potential of thymidylate synthase inhibitors in cancer therapy

Thymidylate synthase (TS) catalyses the de novo synthesis of deoxythymidylate and is a key rate-limiting enzyme of DNA synthesis. The primary site of action of the classic antifolate methotrexate is direct inhibition of dihydrofolate reductase, but it also inhibits TS indirectly by diminishing levels of the TS cosubstrate 5,10-methylenetetrahydrofolate. Polyglutamated metabolites of methotrexate also directly bind and inhibit TS. The prototype fluoropyrimidine fluorouracil is metabolised to an irreversible inhibitor of TS and is the standard chemotherapy for gastrointestinal carcinomas. It is also frequently used in combination with other anticancer drugs against breast cancer and head and neck cancers. The clinical efficacy of fluorouracil is routinely increased by concomitant administration of the biomodulating compound leucovorin (folinic acid). Both the success and limitations of these early drugs led to a search for new, more efficacious TS inhibitors active against a broader range of neoplasms. Raltitrexed (ZD1694, Tomudex) is an antifolate TS inhibitor developed over the last decade that is similarly effective, yet better tolerated, than fluorouracil against colorectal cancer. Additional antifolate and fluoropyrimidine-based TS inhibitors continue to be developed. Many of these experimental drugs have been designed to exploit or thwart selective metabolism in neoplasms, including specific mechanisms of resistance. As the curative potential of relatively non-selective antiproliferative drugs like TS inhibitors is limited against most neoplasms, the future role of TS inhibitors will likely continue to be adjunctive in surgically resectable tumours and palliative in combination with other agents for non-resectable disease. Although TS inhibitors will eventually be supplanted by yet to be discovered agents targeting more tumour-specific cellular signalling pathways, they will probably remain important for the above uses for some time. Future advances in the effective use of TS inhibitors may be forthcoming in the form of improved dosing, fewer untoward effects and increased tumour selectivity with novel fluorouracil prodrug formulations. Furthermore, there is emerging evidence that some novel antifolate TS inhibitors are active against a broader range of neoplams, including lung carcinomas and mesothelioma, compared to classical TS inhibitors. Other possible advances to come include effective biomodulation of antifolate TS inhibitors with nucleoside transport inhibitors and individualised patient therapy based on tumour gene expression and resistance patterns (pharmacogenetics).

Folate-mediated delivery of macromolecular anticancer therapeutic agents

The receptor for folic acid constitutes a useful target for tumor-specific drug delivery, primarily because: (1) it is upregulated in many human cancers, including malignancies of the ovary, brain, kidney, breast, myeloid cells and lung, (2) access to the folate receptor in those normal tissues that express it can be severely limited due to its location on the apical (externally-facing) membrane of polarized epithelia, and (3) folate receptor density appears to increase as the stage/grade of the cancer worsens. Thus, cancers that are most difficult to treat by classical methods may be most easily targeted with folate-linked therapeutics. To exploit these peculiarities of folate receptor expression, folic acid has been linked to both low molecular weight drugs and macromolecular complexes as a means of targeting the attached molecules to malignant cells. Conjugation of folic acid to macromolecules has been shown to enhance their delivery to folate receptor-expressing cancer cells in vitro in almost all situations tested. Folate-mediated macromolecular targeting in vivo has, however, yielded only mixed results, largely because of problems with macromolecule penetration of solid tumors. Nevertheless, prominent examples do exist where folate targeting has significantly improved the outcome of a macromolecule-based therapy, leading to complete cures of established tumors in many cases. This review presents a brief mechanistic background of folate-targeted macromolecular therapeutics and then summarizes the successes and failures observed with each major application of the technology.

Phase I clinical and pharmacokinetic study of pemetrexed and carboplatin in patients with malignant pleural mesothelioma

PURPOSE

To determine the maximum tolerated dose (MTD) of pemetrexed and carboplatin given in combination, to derive a recommended dose for phase II studies, and to explore its efficacy. We assessed toxicities and explored the activity of the drug combination exclusively in patients with malignant pleural mesothelioma (MPM). The pharmacokinetics of both agents was investigated.

PATIENTS AND METHODS

Twenty-seven patients (23 male, four female) with MPM were treated on five escalating dose levels. Doses ranged from pemetrexed 400 mg/m(2) (as a 10-minute intravenous infusion), followed by carboplatin area under the plasma concentration-time curve (AUC) 4 mg/mL.min (as a 30-minute intravenous infusion) to pemetrexed 500 mg/m(2), carboplatin AUC 6 mg/mL.min. All patients had a World Health Organization performance status of 1. A total of 163 courses of treatment were administered (median, six; range, one to 10).

RESULTS

The main toxicity was hematologic, particularly neutropenia, although this was characteristically short-lived and caused few clinical problems. The MTD was pemetrexed 500 mg/m(2), carboplatin AUC 6, because three of the five patients treated at this dose level experienced a dose-limiting toxicity. Eight partial responses (in 25 assessable patients) were observed for a response rate of 32%. Seventy percent of patients noticed an improvement in symptoms, usually (84%) after only two courses. Median time to progression was 305 days, and median survival time was 451 days.

CONCLUSION

The MTD was pemetrexed 500 mg/m(2) and carboplatin AUC 6 mg/mL.min. The recommended phase II dose of the combination is pemetrexed 500 mg/m(2) and carboplatin AUC 5 mg/mL.min. The combination is both active and well tolerated in MPM and deserves further exploration.

Inhibitor of apoptosis protein-1 promotes tumor cell survival in mesothelioma

Malignant pleural mesothelioma (MPM) is a highly lethal pleural neoplasm that is often resistant to chemotherapeutic drugs, including cisplatin, and for which little is known regarding carcinogenic pathways. We used differential display to compare gene expression patterns in mesothelioma, normal pleura and normal lung, in order to better understand MPM pathobiology, and to search for genes that may facilitate drug resistance in this cancer. The human inhibitor of apoptosis protein-1 gene (IAP-1/MIHC/cIAP2) was discovered to be highly expressed in MPM. We confirmed overexpression of IAP-1 mRNA and protein in 39 additional human MPM tumor specimens and 3/5 (60%) MPM cell lines by multiple methods, including real time quantitative reverse transcription-PCR and western blot analysis. Using an antisense targeting approach, we found that attenuation of IAP-1 mRNA levels decreases baseline cell viability and increases the sensitivity of MPM cell lines to cisplatin by nearly 20-fold. Reduced IAP-1 gene expression also results in a concordant increase of the pro-apoptotic cleavage product of caspase 9 and a reduction in the number of viable tumor cells. Our observations strongly suggest that IAP-1 is at least partly responsible for promoting carcinogenesis and mediating resistance to cisplatin in many MPM tumors and that further study of this apoptotic pathway is warranted.

Despite some expression of folate receptor alpha in human mesothelioma cells, internalization of methotrexate is predominantly carrier mediated

OBJECTIVE

As a response to a published report documenting some expression of folate receptor alpha in human mesothelioma, studies were carried out examining the role of this receptor versus that of the reduced folate carrier in the internalization of folate analogs in this neoplasm.

METHODS

Influx measurements of tritiated methotrexate were carried out in 4 mesothelioma cell lines, and 2 additional cell lines were used as comparators. Relative gene-expression analysis for the carrier and receptor gene was done by using real-time reverse transcriptase-polymerase chain reaction in the above-mentioned cell lines and mesothelioma tumor tissues obtained from patients.

RESULTS

Internalization of tritiated methotrexate in mesothelioma cell lines grown at physiologic folate levels was carrier mediated rather than receptor mediated. Influx of this model permeant by these cells exhibited characteristics of carrier-mediated membrane transport and was only minimally reduced by the addition of 5 micromol/L folic acid, a concentration of this natural folate that would have completely inhibited influx by the folate receptor. Gene-expression analysis revealed prominent expression of the folate receptor in some but not all mesothelioma cell lines, and in only one case was expression of this receptor gene greater than expression of the reduced folate carrier gene. Similar analysis of human mesothelioma tumor tissue showed that, with few exceptions, receptor gene expression was substantially less than that for the carrier gene.

CONCLUSION

In view of the ongoing reduced folate carrier-mediated internalization that occurs in mesothelioma cells, these results would seem to argue against a role for the folate receptor in the internalization and cellular pharmacokinetics of methotrexate and other classic folate analogs in this neoplasm. Identifying the mediated route for internalization of these agents in tumor cells is a prerequisite for improving their structural design.

Pharmacogenetics and folate metabolism -- a promising direction

Folate metabolism is the target of two major drug groups: folate antagonists (e.g., methotrexate) and thymidylate synthase inhibitors (for example, 5-fluorouracil). These agents are widely used in cancer chemotherapy, as treatment for rheumatoid arthritis, and for other conditions. The administration of these drugs in cancer chemotherapy can induce a state of acute folate depletion with sometimes life-threatening toxic sequelae. Recent studies suggest that polymorphisms in folate-metabolizing enzymes may modify the therapeutic effectiveness and toxicity of drugs targeting folate metabolism. This review briefly summarizes major drugs targeting the folate pathway and describes common polymorphisms in folate-metabolizing enzymes and transport proteins. Pharmacogenetic studies investigating the relevance of these polymorphisms with respect to patients' response to antifolate chemotherapeutic agents are discussed. Investigating genetic variability in folate metabolism in the framework of pharmacogenetics is a promising field. Findings to date illustrate the potential for targeting therapy based on patients' genotypes with improved outcomes and reduced toxicity.

The emerging role of antifolates in the treatment of malignant pleural mesothelioma

Clinicians have long regarded malignant pleural mesothelioma as a chemoresistant neoplasm and as a result no standard chemotherapy regimen has emerged. Antifolates such as methotrexate are among the most active compounds in mesothelioma, albeit based only on phase II data. Recently two antifolate-based combinations with apparently higher efficacy than older regimens have emerged: the pemetrexed/cisplatin regimen and the raltitrexed/oxaliplatin regimen. In two phase I trials with pemetrexed combined with either cisplatin or carboplatin responses occurred in five of 11 and nine of 29 patients, respectively. In a phase I trial of raltitrexed/oxaliplatin, six of 17 patients (35%) with mesothelioma achieved a partial response. In a phase II trial of raltitrexed/oxaliplatin, 14 objective responses were confirmed in 72 patients (25%) with malignant pleural mesothelioma. Indeed, responses were seen in cisplatin-refractory patients. Based on the promising results from these combination trials, two large phase III studies have begun. The first study was a multicenter, multinational trial sponsored by Eli Lilly and Company, which randomized more than 430 patients with malignant pleural mesothelioma to cisplatin with or without pemetrexed. That trial completed enrollment in February 2001 and is the largest trial ever conducted in mesothelioma. The second trial is being conducted by the European Organization for the Research and Treatment of Cancer (EORTC) and compares cisplatin with or without raltitrexed with planned accrual of 240 patients. In both trials, survival is the main endpoint. These trials will help to define the role of these new antifolates in malignant pleural mesothelioma.