Abstract

The purpose of this investigation was to characterize the stability, activity, and interactions of recombinant human nerve growth factor (rhNGF) in various biological matrices in vitro and in vivo. rhNGF (10 μg/ml) remained stable in human plasma for up to 4 days at 37°C. There was a decrease in the recovery of rhNGF after incubation at lower concentrations (20 ng/ml) and for longer time periods (3 and 5 days at 37°C). Size exclusion HPLC analysis indicated that rhNGF forms high molecular weight (HMW) complexes after long incubation periods. We confirmed that α₂-macroglobulin (α₂M) is the major plasma component that binds to rhNGF. Furthermore, this interaction was considerably increased by treatment of plasma with primary amines such as CH₃NH₂. Changes in the pH environment did not affect the interaction of rhNGF with α₂M. We also determined that the binding of rhNGF to CH₃NH₂-treated pure α₂M or α₂M present in human plasma substantially diminished its immunoreactivity and bioactivity detection. The interaction of rhNGF with activated α₂M was reversed and inhibited by coincubation with dimethyl sulfoxide. Released rhNGF under these conditions was fully bioactive. ¹²⁵I-rhNGF also binds to α₂M by forming similar ¹²⁵I-rhNGF/HMW complexes in plasma after i.v. administration in rats and mice. Sixty minutes after dosing in rats, most of the labeled material was in the form of a ¹²⁵I-rhNGF/HMW complex. These studies have provided a better understanding of the nature of the interactions of rhNGF with plasma components as well as methods to enhance, reverse, and inhibit these interactions.