Pharmacodynamic study using FLT PET/CT in patients with renal cell cancer and other solid malignancies treated with sunitinib malate.

To characterize proliferative changes in tumors during the sunitinib malate exposure/withdrawal using 3'-deoxy-3'-[(18)F]fluorothymidine (FLT) positron emission tomography (PET)/computed tomography (CT) imaging. Patients with advanced solid malignancies and no prior anti-VEGF exposure were enrolled. All patients had metastatic lesions amenable to FLT PET/CT imaging. Sunitinib was initiated at the standard dose of 50 mg p.o. daily either on a 4/2 or 2/1 schedule. FLT PET/CT scans were obtained at baseline, during sunitinib exposure, and after sunitinib withdrawal within cycle #1 of therapy. VEGF levels and sunitinib pharmacokinetic (PK) data were assessed at the same time points. Sixteen patients (8 patients on 4/2 schedule and 8 patients on 2/1 schedule) completed all three planned FLT PET/CT scans and were evaluable for pharmacodynamic imaging evaluation. During sunitinib withdrawal (change from scans 2 to 3), median FLT PET standardized uptake value (SUV(mean)) increased +15% (range: -14% to 277%; P = 0.047) for the 4/2 schedule and +19% (range: -5.3% to 200%; P = 0.047) for the 2/1 schedule. Sunitinib PK and VEGF ligand levels increased during sunitinib exposure and returned toward baseline during the treatment withdrawal. The increase of cellular proliferation during sunitinib withdrawal in patients with renal cell carcinoma and other solid malignancies is consistent with a VEGF receptor (VEGFR) tyrosine kinase inhibitor (TKI) withdrawal flare. Univariate and multivariate analysis suggest that plasma VEGF is associated with this flare, with an exploratory analysis implying that patients who experience less clinical benefit have a larger withdrawal flare. This might suggest that patients with a robust compensatory response to VEGFR TKI therapy experience early "angiogenic escape."