Intranasal administration: a potential solution for cross-BBB delivering neurotrophic factors.

Neurotrophic factors (NTFs) are endogenous polypeptides that regulate the growth, survival, differentiation, and functioning of neurons. The neuroprotective effects of NTFs in experimental animals give strong rationale for developing therapies for neurological disorders. However, when NTFs are applied in clinical trials, great expectation leads to equal disappointment. NTFs are large molecular-weighted and hydrophilic proteins, which limits their access to the central nervous system (CNS) after systemic administration, principally due to poor blood-brain barrier (BBB) permeability and unfavorable pharmacokinetic profiles. Although intracerebral infusion may transport NTFs into the CNS, the invasiveness limits its clinical application. Intranasal administration has been under research for decades and presents promising outcomes in preclinical studies for brain delivering of NTFs. After intranasal delivery, NTFs gain direct and quick access into the CNS at concentrations high enough to elicit their biological effects, bypassing the BBB and minimizing systemic exposure. Due to its invasiveness and convenience, intranasal delivery is feasible for NTFs administration. Although direct evidence of nose-to-brain pathway in human is lacking due to ethical problems, the existence of the nose-to-cerebral spinal fluid pathway has been verified in men. Furthermore, there is abundant indirect evidence for the nose-to-brain pathway as determined by the efficacy of intranasally administered neuroproteins, such as insulin, oxytocin, and vasopressin in clinical trials. Based on the solid preclinical research supporting the efficacy of intranasal NTFs, and the successful clinical application of neuroproteins (not NTFs), it is time to evaluate clinical application of NTFs in treating both acute and chronic CNS diseases.