Delivery of testosterone to the brain by intranasal administration: comparison to intravenous testosterone

Intranasal (i.n.) administration has emerged as a strategy to deliver therapeutics to the brain. Here, we compared i.n. and intravenous (i.v.) administration for testosterone. About 75% of the i.n. administered testosterone entered the blood. However, whole brain levels of testosterone were about twice as high after i.n. administration as after i.v. administration. About two-thirds of the testosterone entering the brain after i.n. administration did so by direct entry by nasal routes and the remainder indirectly by first entering the blood and then crossing the blood-brain barrier. All brain regions except the frontal cortex had higher levels of testosterone after i.n. administration than after i.v. administration, although the differences among brain regions varied much more for the i.n. route. The olfactory bulb, hypothalamus, striatum, and hippocampus had the highest levels after i.n. administration. The brain uptake pattern suggested a variety of distribution routes likely involving the cerebrospinal fluid, diffusion through brain tissue, and transport through nerve projections. Regional distribution patterns were similar after either i.n. or i.v. administration, suggesting that the dominant factor determining distribution/retention was the same for either route of administration. We conclude that the i.n. administration route delivers testosterone systemically and can target the brain, especially the olfactory bulb, hypothalamus, striatum, and hippocampus.