Previous investigations from our laboratory have demonstrated that the covalent modification of a variety of proteins, including antioxidant enzymes, with the naturally occurring polyamines-putrescine(PUT), spermidine, and spermine-dramatically increases their permeability coefficient-surface area product (PS) at the blood-brain and blood-nerve barriers after parenteral administration. In the present study, we have covalently modified nerve growth factor(NGF) with PUT by targeting carboxylic groups for their graded modification by controlling the ionization of these groups with pH. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis, western, and isoelectric focusing analyses demonstrated conversion of NGF to its polyamine-modified derivatives at different pH values. Although the immunoreactivity of PUT-NGF determined by ELISA and western analysis decreased with decreasing pH, the biological activity of PUT-NGF was not affected at any pH as determined by survival and neurite extension of dorsal root ganglia and PC12 cultures. Plasma pharmacokinetics after a single intravenous bolus administration revealed intact PUT-NGF through 10 min and 73-82% intact protein at 15 min. The PS value for PUT-NGF was maximized and the residual plasma volume (Vp) of the protein in the blood vessels minimized when the pH of the modification reaction was >6.4. The biodistribution of PUT-NGF at 15 min showed 22-33% intact protein in different brain regions, which represented 0.4-5.9 ng of PUT-NGF in different brain regions, a physiological dose that is capable of eliciting a bioresponse. The design of this polyamine-modified NGF derivative that has enhanced permeability at the blood-brain and blood-nerve barriers with retained bioactivity may obviate the necessity to create small-molecule mimics of NGF and may be applicable to neurotrophins, engineered multifunctional chimeric neurotrophins, antioxidant enzymes, and other therapeutic proteins with specific clinical application to neurological diseases.