Activity of NBI-31772, a small-molecule inhibitor of the binding of insulin-like growth factor (IGF)-1 to IGF-binding proteins (IGFBPs), in primary cultured rabbit chondrocytes. Chondrocytes were incubated for 24 hours with or without IGF-1, IGFBP-3, or NBI-31772, as indicated, in the presence of 1.5 μCi/ml Na[35SO4]. Neosynthesized proteoglycans were measured by precipitation of nondialysable macromolecules with cetylpyridinium chloride, both in the secretion medium and in the cell layer, and by beta-counting. (a) Total proteoglycan synthesis and (b) cell-associated proteoglycans, calculated as the ratio of 35S-labeled proteoglycans remaining in the cell layer over total proteoglycans. Data are expressed as the means and SEM of four replicates from a representative experiment (out of five independent cultures). Statistical differences between groups containing NBI-31772 plus IGF-1 plus IGFBP-3 and the control group (IGF-1 plus IGFBP-3) were measured by Student's t-test. *P < 0.05; ***P < 0.001.