Pharmacokinetics and stability of the ch14.18-interleukin-2 fusion proteinin mice

The fusion protein formed from ch14.18 and interleukin-2 (ch14.18-IL-2), sh own to exhibit antitumor efficacy in mouse models, consists of IL-2 genetic ally linked to each heavy chain of the ch14.18 chimeric anti-GD2 monoclonal antibody. The purpose of this study was to determine the pharmacokinetics of ch14.18-IL-2 in mice and assess its stability in murine serum. Following i.v. injection, the fusion protein was found to have a terminal half-life of 4.1 h. Detection of IL-2 following injection of the ch14.18-IL-2 fusion protein showed a similar half-life, indicating that the fusion protein prol ongs the circulatory half-life of IL-2. Detection of human IgG1 following i njection of ch14.18-IL-2 showed a terminal half-life of 26.9 h. These data suggested that the native fusion protein is being altered in vivo, resultin g in a somewhat rapid loss of detectable IL-2, despite prolonged circulatio n of its immunoglobulin components. In vitro incubation of the ch14.18-IL-2 fusion protein in pooled mouse serum at 37 degrees C for 48 h resulted in a loss of its IL-2 component, as detected in enzyme-linked immunosorbent as say systems and in proliferation assays. Polyacrylamide gel electrophoresis and Western blot analysis of the fusion protein incubated in mouse serum a t 37 degrees C indicated that the ch14.18-IL-2 is cleaved, resulting in a l oss of the 67-kDa band (representing the IL-2 linked to the IgG1 heavy chai n) and the detection of a band of more than 50 kDa, slightly heavier than t he IgG1 heavy chain itself. This suggests that the fusion protein is being cleaved in vitro within the IL-2 portion of the molecule. These studies sho w that (1) ch14.18-IL-2 prolongs the circulatory half-life of IL-2 (compare d to that of soluble IL-2) and (2) the in vivo clearance of the fusion prot ein occurs more rapidly than the clearance of the ch14.18 antibody itself, possibly reflecting in vivo cleavage within the IL-2 portion of the molecul e, resulting in loss of IL-2 activity.