In vitro antidiabetic activity of Spondias pinnata aqueous extract and encapsulated chitosan-TPP nanoparticles

Abstract

Spondias pinnata (L. f.) Kurz is a medicinal plant used in complementary medicine. Decoctions prepared using stem-bark of S. pinnata find applications in treating diabetes mellitus. However, low bioavailability of bioactive metabolites (polyphenols and flavonoids) and lack of appropriate release of metabolites delimit the antidiabetic activity of S. pinnata aqueous extract (SAE). Encapsulation of SAE with chitosan-tripolyphosphate (CS-TPP) could enhance its therapeutic potential and provide controlled release. The objective of this work to determine in vitro antidiabetic activity of S. pinnata stem-bark extracts and SAE-encapsulated CS-TPP nanoparticles using α-amylase inhibitory, α-glucosidase inhibitory, glucose uptake and glucose adsorption assay. The extracts were prepared by extracting dried and powdered stem-bark of S. pinnata into distilled water, acetone, ethyl acetate, dichloromethane under ultrasonication (40 kHz, 37 °C, 30 min) separately. The total phenol content (TPC) and flavonoid content (TFC) of the extracts were determined using Folin-Ciocalteu and aluminium chloride methods, respectively. Based on the results of α-amylase inhibitory assay, SAE was selected for the encapsulation with CS-TPP. The SAE had TPC of 4.18±0.02 mg gallic acid equivalents per gram of dry weight (GAE/g DW) and TFC of 0.37±0.01 mg quercetin equivalents per gram of dry weight (QE/g DW) and showed the highest α-amylase inhibitory activity (IC50 53.34±7.43 µg/mL). The acetone extract had TPC of 34.43±0.35 mg GAE/g DW and TFC of 4.06±0.05 mg QE/g DW and showed the highest α-glucosidase inhibitory activity (IC50 8.82±1.42 µg/mL). The highest glucose uptake and glucose adsorption were shown by acetone extract and aqueous extract, respectively. SAE-encapsulated nanoparticles were prepared from CS-TPP at varying concentrations (0.250, 0.375, 0.500 and 0.625% w/v) of SAE using ionic gelation method under magnetic stirring; the highest encapsulation efficiency (68.21% ± 0.66%) and loading capacity (0.79% ± 0.17%) were obtained at 0.625% w/v of SAE. Loaded nanoparticles were separated by centrifugation and free polyphenols were determined by Folin-Ciocalteu method. The Z-average particle diameter of SAE-encapsulated CS-TPP nanoformulations was 417±86 nm with polydispersity index of 0.574 and zeta potential of +20.63 mV. The IC50 values corresponding to α-amylase inhibitory activity and α-glucosidase inhibitory activity of SAE-encapsulated CS-TPP nanoparticles were 1.10±0.03 mg/mL and 3.16±0.15 mg/mL, respectively. Although the percentage of glucose uptake and adsorption in SAE encapsulated CS-TPP nanoparticles is lower than the crude extract, it had shown 11.59±1.03 % glucose uptake at 5 mM glucose concentration and 1.47 mmol/g glucose adsorption at 100 mM glucose concentration. The SAE, acetone extract and SAE-encapsulated CS-TPP nanoparticles showed higher antidiabetic activity than the positive control, acarbose. Further investigations on the releasing profiles of SAE-encapsulated CS-TPP nanoparticles would reveal the rates at which the active metabolites are released to the media during the timeframes of the conducted assays.