Impact of penetratin stereochemistry on the oral bioavailability of insulin-loaded solid lipid nanoparticles

Purpose: This study evaluated the stereoisomeric effect of L- and D-penetratin—cellpenetrating peptides (CPPs)-incorporated insulin-loaded solid lipid nanoparticles (INSSLNs) on the bioavailability (BA) of oral insulin (INS). Methods: Insulin-loaded solid nanoparticles, L-penetratin-INS-SLNs (LP-INS-SLNs), and D-penetratin-INS-SLNs (DP-INS-SLNs) were formulated by double emulsification. The developed SLNs were evaluated for particle size, zeta potential (ZP), and drug encapsulation and subjected to differential scanning calorimetry (DSC), Fourier transform infrared spectroscopy (FTIR), and evaluated for stability against enzymatic degradation in rat intestinal fluid. Finally, the SLNs were administered to rats to evaluate the BA of INS-SLNs that contained L- and D-penetratin. Results: The mean particle size, PDI, and ZP values of INS-SLNs, LP-INS-SLNs, and DP-INSSLNs ranged from 618.5 to 973.0 nm, 0.227 to 0.734, and-17.0 to-23.7 mV, respectively. The encapsulation efficiency (%EE) and drug loading (%DL) of INS-SLNs, LP-INS-SLNs, and DPINS-SLNs ranged from 59.03% to 67.42% and from 1.62% to 1.82%, respectively. Differential scanning calorimetry and FTIR analyses indicated that INS was successfully encapsulated in SLNs. Enzymatic degradation of DP-INS-SLNs was slower in intestinal fluid, and the half-life (t1/2) was significantly prolonged, compared to all other SLNs. The pharmacological availability (PA) and BA of orally administered LP-INS-SLNs, which were the most effective SLNs, were 13.1% and 15.7% relative to s.c. administration, respectively. Conclusion: Penetratin stereochemistry significantly impacted oral BA of INS-SLNs, which are promising carriers for oral INS administration.