Transport and elimination of recombinant human NGF during long-term delivery to the brain

The gene for human nerve growth factor (NGF) has been cloned into a mammalian cell line and large quantities of recombinant human NGF (rhNGF) can now be produced for clinical use, but little is known about the fate of rhNGF following delivery to the brain. In this study, we implanted polymer matrices containing¹²⁵l-labeled rhNGF into the brains of adult rats and measured spatial distributions of the released protein for 8 weeks after implantation. NGF content in the tissue was determined by counting gamma radiation in thick (1 mm) sections and by autoradiography of thin (20 μm) sections. For the first several days, the rate of NGF release from the polymer matrix was high ( 100 ng/day); maximal NGF concentrations, measured at the polymer-tissue interface, were correspondingly high (> 20 μg/ml) through day 4. At later times, the release rate decreased (2–10 ng/day) and lower maximal concentrations were observed (1–10 μg/ml). NGF levels were always highest in the tissue sections closest to the polymer; during the 8 weeks of the experiment, NGF levels measured in thick sections decreased 100-fold, from 30 ng/section at day 2 to 0.3 ng/section at day 54. The first 10-fold decrease occurred during the first 10 days of the study; a further 6 weeks was required to achieve the second 10-fold decrease. Throughout the experiment, the majority of NGF remained within a restricted zone around the polymer at all times; the mass of NGF decreased to 10% of the maximal level within 2–3 mm of the polymer matrix. At early times (< 1 week), radiolabel corresponding to > 20 pg of NGF was also detected in regions of the brain further removed from the polymer. Comparison of local rhNGF concentration profiles with a simple mathematical model indicated that rhNGF diffuses through the brain interstitial space and is eliminated with a half-life of 45 min, although elimination appears to be substantially slower in white matter regions. This limited ability of NGF to penetrate and be retained within the brain tissue indicates that NGF will need to be delivered almost directly to the target tissue for efficacy.