Potential Applications for Circulating Tumor Cells Expressing the Insulin-Like Growth Factor-I Receptor

Purpose: To detect insulin-like growth factor-IR (IGF-IR) on circulating tumor cells (CTC) as a biomarker in the clinical development of a monoclonal human antibody, CP-751,871, targeting IGF-IR. Experimental Design: An automated sample preparation and analysis system for enumerating CTCs (CellTracks) was adapted for detecting IGF-IR–positive CTCs with a diagnostic antibody targeting a different IGF-IR epitope to CP-751,871. This assay was used in three phase I trials of CP-751,871 as a single agent or with chemotherapy and was validated using cell lines and blood samples from healthy volunteers and patients with metastatic carcinoma. Results: There was no interference between the analytic and therapeutic antibodies. Eighty patients were enrolled on phase I studies of CP-751,871, with 47 (59%) patients having CTCs detected during the study. Before treatment, 26 patients (33%) had CTCs, with 23 having detectable IGF-IR–positive CTCs. CP-751,871 alone, and CP-751,871 with cytotoxic chemotherapy, decreased CTCs and IGF-IR–positive CTCs; these increased toward the end of the 21-day cycle in some patients, falling again with retreatment. CTCs were commonest in advanced hormone refractory prostate cancer (11 of 20). Detectable IGF-IR expression on CTCs before treatment with CP-751,871 and docetaxel was associated with a higher frequency of prostate-specific antigen decline by >50% (6 of 10 versus 2 of 8 patients). A relationship was observed between sustained decreases in CTC counts and prostate-specific antigen declines by >50%. Conclusions: IGF-IR expression is detectable by immunofluorescence on CTCs. These data support the further evaluation of CTCs in pharmacodynamic studies and patient selection, particularly in advanced prostate cancer.