Response to imatinib in villonodular pigmented synovitis (PVNS) resistant to nilotinib

BACKGROUND: Pigmented villonodular synovitis (PVNS) is a rare locally aggressive tumor. PVNS is characterized in most cases by a specific t(1;2) translocation, which fuses the colony stimulating factor-1 (CSF1) gene to the collagen type VIa3 (COL6A3) promoter thus leading through a paracrine effect to the attraction of non-neoplastic inflammatory cells expressing CSF1-receptor. Imatinib is a tirosin-kinase inhibitors (TKI) active against CSF1-receptor whose activity in naïve PVNS was already described. We report on two PVNS patients who responded to imatinib after failure to nilotinib, another CSF1-receptor inhibitor. METHODS: Since August 2012, 2 patients with progressive, locally advanced PVNS resistant to nilotinib (Patient 1: man, 34 years; Patient 2: woman, 24 years) have been treated with second-line imatinib 400 mg/day. Both patients are evaluable for response. RESULTS: Both patients are still on treatment (7 and 4 months). Patient 1 had a dimensional response by MRI after 2 months from starting imatinib, together with symptomatic improvement. In Patient 2 a metabolic response was detected by [18F]fluorodeoxyglucose-positron emission tomography (PET) at 6 weeks coupled with tumor shrinkage by MRI, and symptomatic improvement. CONCLUSIONS: Imatinib showed antitumor activity in 2 patients with nilotinib-resistant PVNS. This observation strengthen the idea that targeted agent with similar profile can give a different clinical result, as already described for gastrointestinal stromal tumor (GIST) patients treated with the same agents. Molecular studies are needed to clarify the biologic mechanism(s) underlying the response.