Pilocarpine hydrochloride liposomes: characterization in vitro and preliminary evaluation in vivo in rabbit eye

Liposomes containing either pilocarpine hydrochloride or pilocarpine free base were prepared by the sonication method. This manufacturing process yielded after removal of non-encapsulated solute, small multilamellar vesicles (MLV) as was confirmed by electron microscopy examinations. For an identical liposomal composition, the encapsulation capacity and the drug content of the liposomes were drastically higher for pilocarpine hydrochloride than for pilocarpine free base. Investigation of the preparative parameters revealed that increasing the initial amount of drug decreased the drug content and the encapsulation efficiency of the liposomes formed. Since fixed amounts of lipids were used, the volume sequestration rate decrease was attributed to a moderate viscosity increase of the dispersion medium. Increase of phospholipid concentration at a constant ratio of cholesterol and dicetylphosphate to phosphatidylcholine reduced the aqueous volume entrapped per mg of lipid and subsequently the pilocarpine content in the liposomes. Negatively charged liposomes gave larger rates of pilocarpine hydrochloride and aqueous volume encapsulation than neutral liposomes but, on the contrary, positively charged liposomes gave the lowest rates of pilocarpine hydrochloride and aqueous volume encapsulation. Thus, for drug carrying the same net charge as the phospholipids an increase in the surface charge density of the liposome was not only ineffective, but actually resulted in a lower drug encapsulation due to electrostatic repulsion. Preliminary in vivo results on rabbit eyes suggested that the liposomal vehicle was probably unable to improve sufficiently the corneal penetration of pilocarpine to reach satisfactory therapeutic levels when administered at lower concentrations than commonly used.