In vitro demonstration that pemetrexed is a good substrate for glucarpidase

4773 The objective of this work was to assess the ability of glucarpidase to hydrolyse pemetrexed and to compare the kinetics of the reaction with the hydrolysis of methotrexate (MTX) by glucarpidase. Studies were performed in vitro in tris buffer. Disappearance of pemetrexed and methotrexate in the presence of glucarpidase was measured spectrophotometrically, and results were used to estimate Km and k cat values. Pemetrexed was shown to be a good substrate for glucarpidase with comparable kinetic parameters (Km=25 µM, k cat =1808 s -1 ) to MTX (Km=10 µM, k cat =1039 s -1 ). Glucarpidase (formerly carboxypeptidase G2, CPG2) is an enzyme that hydrolyses the glutamate subgroup from MTX. It has consistently been shown to cause a marked and rapid reduction in systemic MTX concentrations in patients whose elimination of MTX is delayed following high-dose MTX treatment 1 . Like methotrexate, pemetrexed is a folate analogue that inhibits dihydrofolate reductase. Pemetrexed is licensed by FDA for the first-line treatment of malignant pleural mesothelioma and second-line, single-agent treatment of locally advanced or metastatic non-small cell lung cancer (NSCLC) 2 . The principal dose limiting toxicity found in Phase I was myelosuppression. In a randomized controlled comparison of pemetrexed against docetaxel, pemetrexed was as effective as docetaxel, but was associated with significantly fewer side effects 3 . Nevertheless, 5.3% of the pemetrexed treated patients had grade 3 or 4 neutropenia. The results of this investigation show that glucarpidase is able to hydrolyse pemetrexed with similar reaction kinetics to its hydrolysis of methotrexate. Glucarpidase thus has the potential to be used to control systemic exposure of patients to pemetrexed and may allow further enhancement of its antitumor efficacy/safety profile. 1. Adamson, P. C., Balis, F. M., Boron, M., Shalabi, A., O9Brien, M., Eby, M., Jayaprakash, N., Cole, D. E., Ivy, P., Widemann, B. C. Carboxypeptidase-G2 (CPDG2) and leucovorin (LV) rescue with and without addition of thymidine (Thd) for high-dose methotrexate (HDMTX) induced renal dysfunction. ASCO Meeting Abstracts 1 Jun 2005 23: 2076 2. Rollins, K. D., Lindley, C. Pemetrexed: a multitargeted antifolate. Clin.Ther. Sep 2005 27: 1343-1382 3. Hanna, N., Shepherd, F. A., Fossella, F. V., Pereira, J. R., De Marinis, F., von Pawel, J., Gatzemeier, U., Tsao, T. C., Pless, M., Muller, T., Lim, H. L., Desch, C., Szondy, K., Gervais, R., Shaharyar, Manegold, C., Paul, S., Paoletti, P., Einhorn, L., Bunn, P. A., Jr. Randomized phase III trial of pemetrexed versus docetaxel in patients with non-small-cell lung cancer previously treated with chemotherapy. J.Clin.Oncol. 1 May 2004 22: 1589-1597