Biodistribution of Neurotrophic Factors to the Optic Nerve and Retina Using Intranasal Delivery

Abstract

Purpose:: We recently reported the delivery of therapeutic levels of ¹²⁵I-insulin-like growth factor-1 (IGF-1) and ¹²⁵I-erythropoietin (EPO) to the optic nerve (ON) and retina using intranasal (IN) delivery. In this study, we examine the extent of distribution of intranasally administered ciliary neurotrophic factor (CNTF), brain derived neurotrophic factor (BDNF) and neurotrophin-4 (NT-4). Each of these factors has been shown to increase retinal ganglion cell survival following injury. The purpose of this study was to determine whether therapeutic doses of these neurotrophic factors (NTFs) can be achieved in the retina and ON via IN delivery.

Methods:: Similar methods were used as previously described. Briefly, 30 male Sprague-Dawley rats were anesthetized and treated in a supine position. Approximately 70 µg of ¹²⁵I-BDNF, ¹²⁵I-CNTF or ¹²⁵I-NT-4 (in 70 µl of saline) were intranasally administered over 18 minutes. Following a 25 min (n = 24) or 1 h (NT-4 and BDNF, n = 6) survival period, animals were perfused with 4% paraformaldehyde. Enucleated eyes were carefully dissected. Samples were weighed and radioactivity was measured using a gamma counter. Tissue concentrations of radiolabeled NTFs were determined in cornea, lens, sclera and retina, as well as anterior and posterior ON.

Results:: The IN delivery of each of these NTF resulted in significant ng levels in the retina and ON. Mean exogenous BDNF levels in the retina were 98.9 ± 18.3 ng at 25 min and increased to 246.7 ± 6.2 ng at 1 h. CNTF levels in the retina were 472 ± 84.7 ng at 25 min. NT-4 levels reached 308.4 ± 85.3 ng at 25 min and decreased to 283.7 ± 161.1 ng at 1 h. BDNF amounts in anterior optic nerve were 29.2 ± 12.6 ng at the 25 min time point and increased to 39.5 ± 6.2 ng at the 1 h survival time. NT-4 amounts were 78.0 ± 28.0 ng and 76.6 ± 33.8 ng at 1 h survival. Finally, CNTF amounts in anterior ON were 142.9 ± 29.5 ng at the 25 min time point.

Conclusions:: Most studies on the neuroprotective properties of NTFs have been conducted in vitro. Oshitari and Usami (2003) reported increased RGC survival in retinal explants when treated with 100 ng/ml of CNTF. Similar concentrations were found to increase RGC survival in BDNF-treated retinal cultures (Johnson et al, 1986). This in vivo study demonstrates the feasibility of delivering therapeutic levels of various NTFs to the retina and optic nerve. Work is currently being conducted to determine if cell survival pathways are upregulated upon IN administration of these NTFs.

Keywords: ganglion cells • growth factors/growth factor receptors • apoptosis/cell death