Nose-to-brain delivery of phenytoin and its hydrophilic prodrug fosphenytoin combined in a microemulsion – formulation development and in vivo pharmacokinetics

Phenytoin is a low aqueous solubility antiepileptic drug, but its phosphate ester prodrug fosphenytoin is soluble, although less permeable. In a previous study, the intranasal administration of aqueous-based formulations of fosphenytoin led to high but delayed phenytoin bioavailability compared to the intravenous route. In this work, we hypothesized that formulating an association of the prodrug fosphenytoin and the drug phenytoin (the active and diffusible form), could result in a faster and/or more effective brain targeting.

Hence, nano or microemulsions containing both active drug and prodrug were developed and characterized regarding viscosity, osmolality, pH, mean size and in vitro drug release. Then, in vivo pharmacokinetics of a selected microemulsion containing fosphenytoin and phenytoin was evaluated in mice following intranasal administration and compared with a similar microemulsion containing fosphenytoin only. Both microemulsions led to higher brain drug levels at short time points than previously developed simpler aqueous based fosphenytoin formulations, likely due to the microemulsion’s permeation enhancing effect.

In addition, having a small amount of phenytoin in the formulation led to an equivalent maximum brain drug concentration and an overall higher absolute bioavailability, with a prolonged drug exposure. Hence, it can be concluded that if there is a need for a fast and prolonged therapeutic effect, a drug/phosphate ester prodrug combination in a microemulsion is ideal, but if a fast effect is all that is needed, having the prodrug alone could be enough, while considering a formulation with permeation enhancing components.

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Article information: Patrícia C. Pires, Ana C. Fazendeiro, Márcio Rodrigues, Gilberto Alves, Adriana O. Santos. Nose-to-brain delivery of phenytoin and its hydrophilic prodrug fosphenytoin combined in a microemulsion – formulation development and in vivo pharmacokinetics., European Journal of Pharmaceutical Sciences, 2021. https://doi.org/10.1016/j.ejps.2021.105918.