Effect of Surface Charge Density of a w/o/w Emulsion on the Brain Targeting of Levodopa in Rats for the Treatment of Parkinson’s Disease

Background: Amidst levodopa being considered as the “Gold standard” in the treatment of Parkinson’s disease (PD), it still has critical therapeutic issues with its dose regimen and dosage forms leading to severe adverse drug effects, decreased drug efficacy during chronic use, and requires an enforced “drug holiday” in PD patients. Hence, in this study, we designed a novel levodopa and carbidopa water-in-oil-in-water (w/o/w) formulation for bioavailability improvement in the central nervous system (CNS). Methods: The new one-in-one embedment of the w/o/w levodopa and carbidopa emulsion formulation was obtained by a double emulsion technique. The plasma and brain levels following intravenous administration of the emulsions in rats were determined. Results: The incorporation of stearylamine (a cationic surfactant) considerably increased the surface charge density of the emulsion droplets. This formulation exhibited a narrow particle size distribution enabling parenteral administration. The formulation also provided a high drug loading capacity. In in vivo study, this novel formulation significantly increased the bioavailability of levodopa in the CNS (P < 0.001). The strong resistance to desorption (due to higher charge density) and the presence of positive charge on the particles upon dilution may be the main reason for enhanced brain levels of levodopa. Conclusion: Our current formulation F5 may decrease the dose of levodopa, leading to decreased adverse effects and dosing problems, thus appreciably benefit PD patients in the future.