Abstract

The intranasal route of drug administration is noninvasive, convenient, and rapidly targets therapeutics to the central nervous system (CNS) using olfactory and trigeminal neural pathways connecting the nasal passages to the brain. The purpose of this research was to enhance intranasal drug targeting to the CNS by incorporating a vasoconstrictor [phenylephrine (PHE)] into nasal formulations containing therapeutic neuropeptides [hypocretin-1 (HC) or the dipeptide l-Tyr-d-Arg (d-KTP)]. Concentrations in CNS tissues, peripheral tissues, and blood were determined at 30 min following intravenous or intranasal administration of ¹²⁵I-labeled neuropeptides with and without PHE. Compared with intranasal controls, inclusion of 1% PHE in nasal formulations significantly reduced absorption into the blood for HC (65% reduction) and d-KTP (56% reduction), whereas it significantly increased deposition into the olfactory epithelium by ∼3-fold for both. PHE (1%) significantly increased delivery to the olfactory bulbs for HC (2.1-fold) and d-KTP (3.0-fold), whereas it significantly reduced concentrations in the trigeminal nerve for HC (65% reduction) and d-KTP (39% reduction) and in most remaining brain regions by ∼50% for both. The dramatic reduction in blood concentrations with PHE contributed to brain-to-blood concentration ratios that were significantly increased for HC throughout the brain (1.6–6.8-fold) compared with intranasal controls. For d-KTP, 1% PHE significantly increased ratios only in the olfactory bulbs (5.3-fold). With a 5% PHE formulation, d-KTP ratios were significantly increased to additional brain areas (1.5–16-fold). Vasoconstrictor nasal formulations may have particular relevance for CNS therapeutics with adverse side effects where it would be advantageous to limit systemic exposure.