Development of proliposomes for oral delivery of P-glycoprotein substrates

Abstract

The purpose of this study was to examine the feasibility of using proliposome delivery systems to improve the delivery of P-glycoprotein (P-gp) substrates via the oral route. Calcein AM and acyclovir were used as the hydrophobic and the hydrophilic models for P-gp substrates, respectively. Caco-2 cells were used as the intestinal epithelium model. Proliposomes were prepared by film deposition on carriers. Sorbitol particles were used as the carrier. The optimum proliposome compositions were determined from the physical properties (physical appearance, surface morphology, and flow properties) of the proliposome particles as well as the formation of liposomes upon hydration of the proliposomes with the aqueous phase. The ability of proliposomes to enhance the delivery of P-gp substrates was evaluated from the accumulation of the model substrates in Caco-2 cells. Intracellular accumulation of calcein AM was determined by the spectrofluorometric method at the excitation and emission wavelengths of 485 and 535 nm, respectively. Acyclovir was determined by the high performance liquid chromatographic method with a UV detector at 254 nm. The maximum amount of soybean phosphatidylcholine (SPC) that could be loaded onto the sorbitol particles was 0.20 mmol/g of sorbitol. The molar ratios of SPC:CHO that gave satisfactory proliposome preparations were 1:0 and 1:0.25. Calcein AM-loaded proliposomes, both with and without CHO, could significantly enhance the accumulation of calcein AM in Caco-2 cells by more than two fold when compared with the solution. The degree of enhancement of proliposomes without CHO was comparable to that of the liposomes extruded through 100-nm polycarbonate membranes. Inclusion of CHO in proliposomes further enhanced the uptake of calcein AM into Caco-2 cells. On the contrary, the expected benefits of proliposomes on acyclovir accumulation either via the bypass of P-gp function or by the increase in the uptake via endocytosis were not seen. Proliposome preparations could not enhance the intracellular accumulation of acyclovir in Caco-2 cells. Low entrapment efficiency (less than 5% of total acyclovir) might be partly responsible. These results suggested that the use of proliposome delivery systems to enhance the delivery of P-gp substrates might be feasible for hydrophobic molecules. In order to be successful with hydrophilic P-gp substrates, a strategy to increase entrapment efficiency might be necessary. The success of proliposome formulations in enhancing the delivery of hydrophilic P-gp substrates might also depend on the available mechanism(s) of uptake of each individual substrate. These issues should be addressed for further study on proliposomes.