Abstract A204: A cellular model of acquired resistance to rilotumumab (AMG 102) in glioblastoma.

Acquired drug resistance is a long-standing problem of cancer therapeutics, for example chemoresistance to temozolomide occurs in >90% of recurrent gliomas. The issue has become even more vexing with the development of highly selective targeted agents; e.g. agents targeting the epidermal and hepatocyte growth factor (EGF and HGF, respectively) pathways; resistance to gefitinib and erlotinib has already been frequently demonstrated in lung adenocarcinomas. Thus, anticipating acquired resistance and understanding its basis may help us develop clinical strategies to prevent or circumvent its occurrence. HGF, through its receptor tyrosine kinase Met, regulates mitogenesis, motogenesis, and morphogenesis in a range of cellular targets during development and homeostasis. HGF/Met signaling also contributes to oncogenesis and tumor progression in many prevalent human malignancies, including glioblastoma. Rilotumumab (AMG102) is a fully human neutralizing monoclonal antibody against HGF tested in multiple Phase 2 clinical trials, including mono therapy in renal cell carcinoma and Glioblastoma, as well as combination trials in gastric, colorectal and small cell lung cancers and castrate resistant prostate cancer.

To generate a cellular model of acquired resistance to rilotumumab, the HGF/Met dependent human glioblastoma-derived cell line U87-MG, was grown in continuous exposure to 600 nM rilotumumab for 120 days. Growth rate, HGF secretion, Met content and Met activation state were 10-fold, 10,000-fold, 10-fold and 80-fold higher than the parental cell values, respectively. The HGF and MET coding sequences and the HGF promoter DATE region, where truncation reportedly increases HGF expression level, were found to have normal sequence and length in both parental and resistant cell lines. Quantitative PCR studies to determine mRNA levels of all HGF isoforms revealed a dramatic increase in full-length HGF transcript. CGH array studies indicated amplification within both HGF and MET genes. Xenograft studies confirmed that tumor growth was resistant to rilotumumab, however the resistant cell line and tumors remained sensitive to a highly selective Met tyrosine kinase inhibitor suggesting that resistance was achieved via increased HGF/Met signaling rather than mutation or activation of alternate pathways. Microarray expression analysis demonstrated transcript profiles that were consistent with HGF/Met pathway activation. Thus the molecular basis of acquired resistance in this model differs from those prevalent in: [1] lung cancers treated with EGFR inhibitors and medulloblastomas treated with hedgehog inhibitors, where nearly all cases acquire secondary mutations in the targeted kinase; [2] breast cancers treated with HER2 inhibitors, where PTEN loss, p27 downregulation, and activation of other receptors are primary causes; or [3] malignant melanoma treated with BRAF inhibitors, where increased signaling by ARAF, CRAF, IGFR1, PDGFR, and other sources lead to PI3K- and/or MEK-mediated reactivation of the MAPK pathway. In addition to the importance of HGF/Met pathway activity in selecting glioblastoma patients for HGF-targeted therapeutics, our results suggest that monitoring Met pathway activity could provide early indications of acquired resistance to these agents, and that Met kinase inhibitors may still be efficacious when resistance occurs.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the AACR-NCI-EORTC International Conference: Molecular Targets and Cancer Therapeutics; 2011 Nov 12-16; San Francisco, CA. Philadelphia (PA): AACR; Mol Cancer Ther 2011;10(11 Suppl):Abstract nr A204.