Novel Therapeutic Mechanism of Action of Metformin and Its Nanoformulation in Alzheimer's Disease and Role of AKT/ERK/GSK Pathway.

Background: Insulin resistance in brain plays a critical role in the pathogenesis of Alzheimer's disease (AD). Metformin is a blood brain barrier crossing anti-diabetic insulin-sensitizer drug. Current study has evaluated the therapeutic and mechanistic role of conventional as well as solid lipid nanoformulation (SLN) of metformin in intracerebro ventricular (ICV) Aβ (1-42) rat-model of AD. Methods: SLN-metformin was prepared by the micro-emulsification method and further evaluated by zetasizer and scanning electron-microscopy. In the animal experimental phase, AD was induced by bilateral ICV injection of Aβ using stereotaxic technique, whereas control group (sham) received ICV-NS. 14 days post-model induction, ICV- Aβ treated rats were further divided into 5 groups: disease control (no treatment), Metformin dose of (50mg/kg, 100mg/kg and 150 mg/kg), SLN of metformin 50mg/kg and memantine 1.8mg/kg (positive-control). Animals were tested for cognitive performance (in EPM, MWM) after 21 days of therapy, and then sacrificed. Brain homogenate was evaluated using ELISA for (Aβ (1-42), hyperphosphorylated tau, pAKTser473, GSK-3β, p-ERK,) and HPLC (metformin level). Brain histopathology was used to evaluate neuronal injury score (H&E) and Bcl2 and BAX (IHC). Results: The average size of SLN-metformin was <200 nm and was of spherical in shape with 94.08% entrapment efficiency. Compared to sham, the disease-control group showed significantly higher (p≤0.05) memory impairment (in MWM and EPM), higher hyperphosphorylated tau, Aβ (1-42), and Bax and lower Bcl-2 expression. Metformin was detectable in brain. Treatment with metformin and its SLN form significantly decreased the memory impairment as well as decreased the expression of hyperphosphorylated tau, Aβ(1-42), Bax expression and increased expression of Bcl-2 in brain. AKT-ERK-GSK3β-Hyperphosphorylated tau pathway can be implicated in the protective efficacy of metformin. Conclusion: Both metformin and SLN metformin is found to be effective as therapeutic agent in ICV-AB rat model of AD. AKT-ERK-GSK3β-Hyperphosphorylated tau pathway is found to be involved in the protective efficacy of metformin.