Pemetrexed: Target gene expression in human solid tumors correlates with chemosensitivity patterns

Abstract

3128 Background: The novel antifolate ALIMTA (pemetrexed) exhibits clinical activity in a variety of solid tumors. Primary targets comprise thymidilate synthase (TS), dihydrofolate reductase (DHFR), and glycinamide ribonucleotide formyltransferase (GARFT). In the present study, target gene expressions were correlated with in vitro chemosensitivity of freshly explanted human tumor specimens. Such correlations in tumors taken directly from patients may help to rationally design subsequent clinical trials. Methods: Freshly biopsied tumor cells (solid tumors, pleural effusions, or ascites) were used for soft-agar cell cloning experiments. Cells were exposed to various concentrations of ALIMTA and clonogenic tumor growth was evaluated. Another part of the tumor specimens was shock frozen immediately after removal. Messenger RNA was isolated and reversely transcribed to cDNA followed by real-time multiplex PCR experiments. Results from these gene expression experiments were normalized against β-actin transcripts. Results: At present, 15 tumor samples from a variety of solid tumors are available for analysis of TS and DHFR and 14 samples for GARFT. No clear difference in TS or DHFR gene expression was observed between ALIMTA-sensitive and resistant specimens (mean ±SD: TS: 57 ±33 versus 76 ±89; DHFR 54 ±22 versus 61 ±21). GARFT expression was associated with sensitive specimens showing lower expression levels compared to resistant specimens (mean ±SD: 100 ±38 versus 331 ±212). GARFT expression levels and clonogenic survival after ALIMTA exposure were correlated (R=0,7810). Conclusions: These results indicate that low expression of GARFT might herald sensitivity to ALIMTA and that GARFT inhibition may be a clinically relevant mechanism of action. Supported in part by Eli Lilly & Company. No significant financial relationships to disclose.