Pemetrexed (Alimta) is a novel multitargeted antifolate antimetabolite that inhibits, among other enzymes, thymidylate synthase (TS), dihydrofolate reductase (DHFR), and glycinamide ribonucleotide formyl transferase (GARFT). The primary targets of pemetrexed(Drug information on pemetrexed) control pivotal steps in the de novo synthesis of pyrimidines and purines. The multitargeted nature of pemetrexed is confirmed by in vitro experiments demonstrating that both thymidine and hypoxanthine are required to prevent pemetrexed-induced cytotoxicity.[ 1] Three initial single-agent phase I studies were conducted to explore different treatment schedules of pemetrexed. These schedules comprised administration of the compound weekly * 4 every 6 weeks, daily for 5 days every 21 days, and once every 21 days. Pemetrexed was administered as a 10-mg/m² intravenous (IV) infusion over 10 minutes. Phase I Trials of Single-Agent Pemetrexed Rinaldi et al reported a single-agent phase I study with pemetrexed administered as a 10-minute infusion every week for 4 weeks. The treatment was repeated after 6 weeks in patients with advanced solid tumors.[2] A total of 25 patients received doses ranging from 10 to 40 mg/m²/wk. The doselimiting toxicity consisted of neutropenia that was completely reversible. Nonhematologic toxicities were mild, and no grade 3 or 4 nonhematologic toxicities were reported. The maximum tolerated dose was determined to be 40 mg/m²/wk and the recommended phase II dose utilizing this administration schedule was 30 mg/ m²/wk. Two patients with advanced colorectal cancer experienced minor responses. Based on the results of this study, Rinaldi and colleagues conducted a second phase I trial.[3] In this study, pemetrexed was administered as IV infusion over 10 minutes every 21 days. A total of 37 patients with advanced solid tumors received doses ranging from 50 to 700 mg/m². The maximum tolerated dose was found to be 600 mg/m² and the recommended dose for subsequent phase II trials was determined to be 600 mg/m². Neutropenia, thrombocytopenia, and cumulative fatigue were dose-limiting toxicities. Partial responses were noted in patients with advanced pancreatic cancer (n = 2) and advanced colorectal cancer (n = 2), with minor responses in patients with advanced colorectal cancer (n = 6). The third single-agent phase I trial with pemetrexed was conducted by McDonald and colleagues.[4] In this study, pemetrexed was administered as an IV infusion over 10 minutes daily for 5 days. The treatment was repeated every 21 days. Thirty-eight patients with advanced malignancies received pemetrexed with doses ranging from 0.2 to 5.2 mg/m². The maximum tolerated dose was found to be 4 mg/m², with neutropenia being the leading dose-limiting toxicity. One patient with metastatic non-small-cell lung cancer (NSCLC), one with metastatic colon cancer, and one with pancreatic cancer experienced minor responses. A comparison of the results from these three phase I studies indicated a relationship between the maximum tolerated dose treatment schedules; however, myelosuppression and epithelial toxicities as manifested by mucositis and/or diarrhea remained as predominant toxicities (Table 1). Additional nonhematologic toxicities included fatigue and rash. Because of the convenient administration schedule, the achievable dose intensity, and the extent of anecdotal antitumor activity observed, the every-21-day schedule was selected for further development of pemetrexed. The recommended phase II dose of 600 mg/m² was subsequently reduced to 500 mg/m² in order to further optimize the tolerability of pemetrexed and to prepare for early clinical trials with combination regimens. None of the single-agent phase I studies used supplementation of patients with folic acid(Drug information on folic acid) and vitamin B₁₂. The added value of vitamin supplementation to the safety of pemetrexed was only discovered after the agent had already entered its first registration phase III registration trial. Pharmacokinetic parameters of pemetrexed were evaluated in these phase I studies and yielded the following conclusions: at clinically used dose ranges, pemetrexed is eliminated from serum, with a mean terminal elimination half-life of 2 to 3 hours.[5] A linear relationship between area under the concentration-time curve (AUC) and dose was noted. Within 24 hours after administration of the compound, 70% to 90% of the administered dose is recovered in the urine. Hepatic metabolism of parent compound is minimal leading to negligible amounts of biologically inactive metabolites. There was no evidence for renal toxicities of pemetrexed in patients with normal creatinine clearance despite the fact that pemetrexed is renally eliminated. Nevertheless, in order to determine whether comedication with potentially nephrotoxic agents may lead to renal toxicities, a phase I trial was recently completed which evaluated the effects of combining ibuprofen(Drug information on ibuprofen) with pemetrexed in patients with advanced cancer. Preliminary pharmacokinetic data indicated that coadministration of these two agents did not affect creatinine clearance or pharmacokinetic variables of pemetrexed.[ 6] Combination Regimens With Pemetrexed Pemetrexed underwent extensive evaluation in preclinical studies to determine if the agent could be combined with other clinically used cytotoxic agents, including platinums, gemcitabine(Drug information on gemcitabine) (Gemzar), cyclophosphamide(Drug information on cyclophosphamide) (Cytoxan), the taxanes, doxorubicin(Drug information on doxorubicin), vinorelbine (Navelbine), and radiation.[7] Findings of synergistic or additive antitumor effects in human tumor xenografts and cell lines suggested that these combinations warranted further evaluation in clinical trials. Table 2 summarizes clinical studies that have been conducted with pemetrexed combined with a variety of other clinically relevant compounds, and details will be presented below. These trials have demonstrated that pemetrexed is a versatile and well-tolerated drug that can be combined at full doses with all compounds studied, laying the basis for a broad subsequent drug development program. Pemetrexed Combination With Gemcitabine
The cytotoxicity and potential underlying mechanisms of the combination of pemetrexed and gemcitabine have been evaluated in several preclinical studies involving a variety of tumors and using simultaneous and sequential administration.[8-14] Data have shown that the combination results in synergistic cytotoxicity when administered sequentially but antagonism with concurrent administration. Optimal synergy was observed with the pemetrexed → gemcitabine sequence in studies of HT29 colon carcinoma xenografts[8] and MIA PaCA-2, PANC-1, and Capan-1 pancreatic cancer cell lines.[9] In contrast, the highest level of synergy in a study of colon adenocarcinoma cell lines LoVo, WiDR, and LRWZ occurred when gemcitabine administration preceded that of pemetrexed, while the reverse sequence resulted in additive and synergistic effects.[10] In the latter trial, an increase in TS expression, which is associated with resistance to conventional antifolates, was noted in all cell lines. Experiments in HT29 colon cancer cells evaluated the cell-cycle-modulating effects of pemetrexed by flow cytometry as a potential mechanism to increase gemcitabine potency.[8] A decrease in HT29 proliferation rate correlated with an accumulation of cells in S phase after 12 to 24 hours of pemetrexed exposure. The authors concluded that synchronization of HT29 cells by pemetrexed was effecting a change in the nucleotide pools by inhibition of target enzymes- TS, GARFT, and DHFR- that in turn potentiated cytotoxicity of exposure to gemcitabine. Other studies have also shown S-phase cell synchronization after pemetrexed treatment.[9,10] In MIA PaCA-2, PANC-1, and Capan-1 pancreatic cell lines, Giovannetti et al demonstrated that pemetrexed treatment significantly enhanced gene expression and activity of deoxycytidine kinase (dCK), a key enzyme involved in pyrimidine salvage pathways and in the rate-limiting step in gemcitabine activation.[9] Rauchwerger et al studied the role of the equilibrative-sensitive nucleoside transporter (es-NT) in gemcitabine sensitivity.[11] Cellular uptake of gemcitabine requires transport across the plasma membrane by sodiumindependent (equilibrative) mechanisms (es-NT), the activity of which is a prerequisite for tumor growth inhibition by gemcitabine.[12] Thus, combining a nucleoside analog with agents that increase NT expression, such as TS inhibitors, would theoretically increase the potential for cell kill through depleting the nucleotide pool.[13,14] In experiments carried out using TS inhibitors (5-FU and raltitrexed [Tomudex]) with gemcitabine administered concurrently and sequentially in three human pancreatic and one human bladder cancer cell lines, TS inhibitor pretreatment significantly augmented cell kill relative to singleagent gemcitabine and significantly increased cell surface es-NT content over basal levels in two of the pancreatic cancer cell lines. Results were maximal when TS inhibitor treatment preceded gemcitabine administration.[11] Thus, potential mechanisms of synergy with the pemetrexed → gemcitabine sequence include TS inhibition, depletion of nucleotide pools, S-phase synchronization of cells, and activation of es-NT and dCK. Mechanisms for additive or synergistic effects observed with the reverse sequence are less clear as yet. Based on the demonstration of preclinical cytotoxic synergy, a phase I trial of pemetrexed in combination with gemcitabine was conducted in 56 patients with advanced solid tumors who had received at least one previous chemotherapy regimen.[15] Adjei et al used sequential administration of gemcitabine followed by pemetrexed, based on their in vitro clonogenic assays demonstrating cytotoxic synergy in cultured human colon carcinoma cells with this sequence but not the reverse sequence.[ 15] Patients in group I (n = 35) received gemcitabine at 1,000 or 1,250 mg/m² IV over 30 minutes on days 1 and 8, and pemetrexed on day 1 only, 90 minutes after gemcitabine, at escalating doses ranging from 200 to 600 mg/m² given IV over 10 minutes. Courses were repeated every 3 weeks. Because 57% of courses were associated with neutropenia that required reduction/omission of the day 8 gemcitabine dose, group II patients (n = 21) received the pemetrexed on day 8 instead of day 1. Neutropenia was the principal dose-limiting hematologic toxicity in both groups I and II and seemed to be dose related; no infections were noted in patients with severe neutropenia. The median neutrophil count nadir was on day 7 in group I and day 14 in group II patients, indicating a relationship between the nadir and pemetrexed administration. The maximum tolerated dose for group I was determined to be gemcitabine at 1,000 mg/ m² and pemetrexed at 500 mg/m² due to prolonged (> 5 days) grade 4 neutropenia in four of six patients receiving the 1,250-mg/m² gemcitabine dose. For group II, the maximum tolerated dose was gemcitabine 1,250 mg/m² and pemetrexed 500 mg/m² due to life-threatening and prolonged neutropenia seen at the higher pemetrexed dose of 600 mg/m². The primary nonhematologic toxicity was elevated hepatic transaminase level in 71% of treatment courses, most cases of which were mild to moderate and rapidly reversible. Other toxicities included nausea, fatigue, and rash. Patients receiving pemetrexed on day 8 (group II) had fewer and less severe toxicities and fewer dosage interruptions than those receiving pemetrexed on day 1 (group I). Among 55 assessable patients, objective responses were confirmed in 7 of 34 group I and 6 of 21 group II patients with tumors, including colorectal cancer (n = 3), non-small-cell lung cancer (n = 3), cholangiocarcinoma (n = 2), ovarian cancer (n = 2), mesothelioma (n = 1), breast cancer (n = 1), and adenocarcinoma of unknown primary site (n = 1). Twelve of these patients had partial responses and one was considered a mixed response, with response durations of at least 3 months. An additional 27 patients had stable disease, with durations of stable disease ranging from 1 to 11 cycles after the initial evaluation at cycle 2. Pharmacologic evaluations conducted in four group I patients at the maximum tolerated dose showed no alteration of pemetrexed pharmacokinetics based on gemcitabine pretreatment, although the sample size was small. Recommended dose and schedule of this regimen for phase II study was gemcitabine 1,250 mg/m² on days 1 and 8 with pemetrexed 500 mg/m² given 90 minutes after gemcitabine on day 8, every 21 days.[15] Phase II studies of the pemetrexed/ gemcitabine combination are being carried out in advanced-stage non- small-cell lung, breast, and pancreatic cancer, as described elsewhere in this supplement. Pemetrexed and Cisplatin(Drug information on cisplatin)
In a phase I study of pemetrexed and cisplatin, two administration schedules were investigated based on the hypothesis that because pemetrexed is primarily eliminated by renal excretion, hydration required for cisplatin administration may potentially modulate the clearance of pemetrexed and thus impact on antitumor activity or toxicity.[16] In order to investigate this hypothesis in a clinical setting, one patient cohort (n = 40) received pemetrexed followed by hydration and cisplatin on day 1 of a 21-day cycle. Another cohort (n = 11) was treated with pemetrexed on day 1 without any hydration, followed by hydration and cisplatin on day 2 of a 21-day cycle. In both cohorts, pemetrexed was administered as an IV infusion over 10 minutes. The maximum tolerated dose for both schedules was pemetrexed 600 mg/m² and cisplatin 100 mg/m², demonstrating that both compounds can be combined at fully active clinical doses. Dose-limiting toxicities consisted mainly of myelosuppression. Ten patients in cohort 1 experienced partial responses, and one patient with head and neck cancer had a complete response. In the second cohort, two patients experienced partial responses. Most notably, five of 11 patients with pleural mesothelioma developed confirmed and independently validated partial responses, indicating a profound antitumor effect of this combination in malignant pleural mesothelioma-a disease for which at that time no established treatment was available. Antitumor responses were also noted in other tumor types including NSCLC, colorectal cancer, melanoma, and cancer of unknown primary. The recommended doses for subsequent clinical studies were determined to be pemetrexed 500 mg/m² and cisplatin 75 mg/ m² with administration of both agents on day 1. Based on the provocative results of this study, pleural mesothelioma was chosen as the primary target tumor entity for approval, and additional studies, including a single-agent phase II trial and a phase I trial with pemetrexed and carboplatin(Drug information on carboplatin), were initiated. A courageous step was taken by initiating the ultimately successful pivotal phase III registration trial based on the results of the phase I combination trial of pemetrexed and cisplatin. Pemetrexed in Combination With Carboplatin
The combination of pemetrexed and carboplatin was evaluated in a phase I trial conducted by Hughes and colleagues.[17] Twenty-seven patients with MPM received escalating doses of pemetrexed (400 mg/m² to 500 mg/m²) and carboplatin (AUC 4 to 6). Pemetrexed was administered as a 10-minute infusion and carboplatin was administered as a 30-minute infusion, both on day 1 every 21 days. Pemetrexed at 500 mg/m² and carboplatin at AUC 6 was the maximum tolerated dose; three of five patients at this dose level experienced grade 4 neutropenia as the dose-limiting toxicity. Nonhematologic toxicities at the maximum tolerated dose included nausea, vomiting, and stomatitis. There were no grade 4 nonhematologic toxicities reported at this dose level. Two courses at all dose levels were complicated by grade 3 elevation of transaminase levels. Response to therapy was a secondary outcome and was measured in all patients. Of the 25 patients evaluable for response, there were eight confirmed partial responses, for an overall response rate of 32%. Five of the eight patients who experienced partial responses had stage IV disease, and five patients had mesothelioma of epithelial histology. All of the patients who received treatment with pemetrexed and carboplatin experienced cancerrelated symptoms at the start of chemotherapy. Nineteen (70%) of the original 27 patients accrued experienced relief in cancer-related symptoms while on study. Median overall survival was 451 days and median time to disease progression was 405 days. Figure 1 shows the response of a patient on this study. The recommended phase II dose for this combination was determined to be pemetrexed 500 mg/m² and carboplatin AUC 5, which allowed for administration of full doses of both agents.