Identification of Biological Factors Predictive of Response to Imatinib Mesylate in Aggressive Fibromatosis

Background: Imatinib induces responses and disease stabilisations in non-resectable patients with aggressive fibromatosis (AF). The precise target of imatinib in AF and predictive factors for response to treatment are unknown.

Methods: We investigated factors potentially predictive of response to imatinib in a series of 40 patients with progressive AF included in a phase II trial of imatinib: we tested the presence of KIT exon 10 variant (M541L), the expression of imatinib-sensitive kinases and cell cycle proteins by immunohistochemistry (IHC), and other clinical and biological factors.

Results: Of 10 patients for whom DNA could be extracted, 3 had a KIT exon 10 variant (30%), with no correlation with response or progression-free survival (PFS). The expression of other imatinib targets (PDGFRA/B, macrophage colony-stimulating factor receptor (M-CSFR)) and of downstream components of the cell cycle, cell proliferation and proliferation pathway (cyclin D1, ERK, MEK 1-2) did not correlate with PFS. Pre-treatment lymphopenia (<1500/microl) and tumour size >120 mm correlated with shorter PFS in univariate and multivariate analyses.

Conclusion: Our findings show that a baseline biological characteristic of the patient is the major parameter influencing response to imatinib in aggressive fibromatosis. Tumour characteristics, including the presence of a KIT exon 10 M541L variant, may influence tumour control but this needs to be confirmed and better explained.