Circulating levels of sTNF-R2 and their role in regulating the activity of NGR-mTNF and NGR-hTNF. (a) Serum levels of sTNF-R1 and sTNF-R2 in B16F1 tumor–bearing mice 1 hour after treatment with various doses of NGR-mTNF or mTNF. Animals (three mice per group) were treated at day 6. (b) Effect of the anti–sTNF-R2 mAb 6G1 on the antitumor activity of NGR-mTNF. The mAb 6G1 (100 μg) was administered to animals bearing B16F1 tumors at day 5 and 8. Each animal was treated 1 hour later with NGR-mTNF at the indicated doses, and 2 hours later with melphalan (90 μg, five mice per group). (c) Effect of NGR-hTNF and hTNF on the growth of RMA-T tumors. Mice were treated with various doses of each cytokine at day 11. NS, not significant (t test).