The plasma half‐life of recombinant human interleukin‐6 (rhIL‐6) was determined in rats by measuring the disappearance of the biological activity as well as of the radioactivity of 125 I‐rhIL‐6 from the circulation. The kinetics of clearance were biphasic. It consisted of a rapid initial disappearance corresponding to a half‐life of 3 min, and of a second slow one corresponding to a half‐life of about 55 min. By cellulose‐acetate electrophoresis it was shown that rhIL‐6 binds to a plasma protein resulting in a complex migrating in the β–γ region; 20 min after intravenous injection, about 80% of the 125 I‐rhIL‐6 that had disappeared from the circulation was found in the liver. 125 I‐rhIL‐6 was exclusively‐localized on the surface of parenchymal cells suggesting the existence of an interleukin‐6 receptor on the hepatocytes.
Castell et al. (1988) studied this question.