Brain distribution study of [¹⁴C]-Riluzole following intranasal administration in mice

Amyotrophic lateral sclerosis (ALS) presents a substantial challenge due to its complex nature, limited effective treatment options, and modest benefits from current therapies in slowing disease progression. This study explores the potential of intranasal (IN) delivery to enhance the CNS delivery of riluzole (RLZ), a standard ALS treatment which is subject to blood-brain barrier efflux mechanisms. Additionally, the impact of elacridar (ELC), an efflux pump inhibitor, on IN RLZ CNS bioavailability was examined. To quantify RLZ in vivo in mice, [¹⁴C]-RLZ was synthesised using an optimised one-pot method. [¹⁴C]-RLZ yield was 21.3 ± 3.4 %, measured by High Performance Liquid Chromatography (HPLC), with a specific activity of 40.4 ± 3.9 µCi/mg measured by HPLC and liquid scintillation counting. RLZ synthesis was verified using proton nuclear magnetic resonance (¹H NMR), and liquid chromatography-mass spectrometry. IN RLZ (5 mg/kg) produced double the maximum brain levels (1.11 ± 0.34 % Injected Dose (ID)/brain) at 30 min as oral RLZ (5 mg/kg). The uptake of RLZ in the liver was reduced by half for intranasal administration compared to oral administration. Intravenous ELC (5 mg/kg) substantially increased brain levels of IN RLZ to 3.52 ± 0.62 % ID/g brain at 60 min post-administration, compared to 1.87 ± 0.33 % ID/g brain in the absence of the efflux pump inhibitor. However, increased concentrations were also observed in the liver and blood. These results indicate that intranasal delivery of RLZ enhances brain targeting and reduces liver accumulation compared to the oral route. Brain uptake of IN RLZ was enhanced further by ELC, although not selectively as accumulation in the liver or blood was also observed. Further metabolic research using Chromatography-Mass spectrometry (LC-MS) or NMR along with excretion studies are warranted for a more comprehensive understanding of the pharmacokinetics of IN RLZ and IN RLZ/ELC. Additionally, employing suitable ALS animal models is crucial for understanding RLZ's effects on disease progression, mechanism of action, efficacy, and potential side effects to aid further development.