Competition of NBI-31772, a small-molecule inhibitor of the binding of insulin-like growth factor (IGF)-1 to IGF-binding proteins (IGFBPs), with 125I-labeled IGF-1 binding to cartilage. Explants cut from a homogeneous area of osteoarthritic cartilage were separated in equal groups and incubated with 11.7 nM R3 IGF-1 (an analog of IGF-1 with a greatly reduced affinity for IGFBPs), 11.7 nM IGF-1, 1.5 M NaCl or 10 μM NBI-31772 in 1 ml of Dulbecco's phosphate-buffered saline (DPBS) for 24 hours at 4°C. 125I-labeled IGF-1 (20,000 dpm) was added to each incubation medium and the cells were incubated for 24 more hours. The medium was harvested and explants were washed three times with DPBS. The radioactivity in medium, lavage buffer, and cartilage explants was counted. The percentage of 125I-labeled IGF-1 remaining in cartilage was corrected for the weight of explants and divided by total counts. Data are the means of two experiments. Results are expressed as percentages of the control value (cartilage incubated with 125I-labeled IGF-1 alone).