Ciliary neurotrophic factor: pharmacokinetics and acute-phase response in rat.

Ciliary neurotrophic factor (CNTF) supports the survival of motoneurons in vitro and in vivo. Recombinant CNTF is an investigational drug for the treatment of amyotrophic lateral sclerosis. We determined the pharmacokinetics of radioiodinated CNTF after intravenous injection into rats. CNTF shows a biphasic clearance with an initial plasma half-life of 2.9 minutes and is removed from the circulation by the liver. No accumulation of radioactivity was detectable in nerve tissue or skeletal muscle after intravenous injection of 0.1 microgram and 0.5 microgram of CNTF. Radioactive degradation products accumulate in the skin. Liver cells express specific binding proteins for CNTF, and the incorporation and degradation of intravenously injected CNTF by the liver may occur after association of CNTF with the soluble CNTF receptor alpha in the circulation. Probably as a consequence of its binding to hepatocytes, CNTF induces acute-phase responses in liver. The short half-life and the inflammatory side effect may limit the clinical usefulness of systematically administered CNTF in the treatment of human motoneuron disorders.