Nanotechnology in pulmonary drug delivery: Compressible proliposome formulations for nebulization

Abstract

Liposomes are promising carriers for the encapsulation of drug molecules and subsequent delivery to the respiratory tract. The advantages of liposomes as carriers have been widely reported in literature, there are however associated disadvantages such as stability issues when suspended in aqueous media. This has been remedied partially by proliposome formulations (i.e. dry formulation of lipid with carbohydrate carrier). The aim of this work was to develop a novel solid dosage form of "proliposome tablets" (a precursor of liposomes), entrapping beclometasone dipropionate (BDP) as a model hydrophobic drug for the treatment or prophylaxis of asthma.
A novel proliposome formulation method was developed in this study called the "Slurry" method, which is readily scalable to an industrial level. Soya Phosphatidylcholine (SPC) was utilised as the phospholipid component in combination cholesterol, in addition to four carbohydrate carriers (i.e. Lactose monohydrate (LMH), sucrose, sorbitol and D-mannitol), in three formulation ratios (i.e. 1:5, 1:10 and 1:15 w/w lipid to carrier) for the preparation of proliposome powders and tablets. Two dispersion media; deionised water (DW) or deuterium oxide (D2O) were used to generate liposomes from the formulated powders and tablets. Transmission Electron Microscopy studies confirmed that the generated liposomes from the proliposome formulations were multivesicular liposomes. In literature, the entrapment efficiency values in DW are recorded to be in excess of 80%. Using D2O, a novel separation method was developed, through which the entrapment efficiency of BDP in the generated liposomes was found to be in the range of 19.69 - 61.91%, showing great disparity in entrapment efficiency when compared to entrapment in DW. BDP entrapment efficiency was found in the following order of carriers used: Sorbitol > Sucrose > D-mannitol > LMH.
D-mannitol and sorbitol-based powder formulations were assessed in terms of flowability via tapped density (17.81 + 4.46%) and angle of repose (25.62 + 1.08°), classifying sorbitol as possessing good to excellent flowability. Upon tableting of sorbitol and D-mannitol, formulations at lipid to carrier ratios of 1:10 and 1:15 w/w; were viably compressible into tablets. Furthermore, sorbitol-based tablets at a ratio of 1:15 w/w were observed to be the only carrier-based formulation, using the automated function of the Minipress tablet machine (20 + 2 tablets/minute with a compression force of 3.00 - 4.50 kN). The manufactured tablets were tested in accordance with British and US Pharmacopeial standards. Nebulization studies of formulated proliposome powders and tablets (Sorbitol and D-mannitol-based) were conducted to identify the most efficient formulation for pulmonary delivery. Tablets manufactured from sorbitol-based proliposome powders (1:15 w/w ratio) offered the shortest nebulization and sputtering times (24.18 + 0.16 and 2.42 + 0.54 minutes), with a high output rate (180.20 + 0.31 mg/minute). Liposome size for the 1:15 w/w lipid to carrier ratio, prior to nebulization, for sorbitol tablets was 5.85 + 0.86 µm. Moreover, this size was smaller than the liposomes found in the nebulizer reservoir i.e. (10.38 + 1.11 µm). Based on entrapment efficiency, tableting and nebulization studies, sorbitol-based proliposome formulations at the 1:15 w/w lipid to carrier ratio; a six-month stability study was carried out under various conditions (Room, Fridge and Hot temperature i.e. 22 + 1°C, 6 + 1°C and 40 + 1°C), and testing (size, charge, entrapment efficiency and a set of quality control tests) at monthly intervals. Liposome size increased dramatically from the control sample size (5.9 + 0.7 µm), to (9.19 + 0.81 µm) at 40°C on month six. The entrapment efficiency dropped significantly at 40°C decreasing from 57.43 + 9.12% (control tablets) to 32.56 + 5.47% at the sixth month of Hot temperature storage. This was posed to be due to degradation of the lipid under high temperature.
These research findings established sorbitol-based proliposome formulations (in a 1:15 w/w lipid to carrier ratio) to be the most appropriate for tableting on an industrial scale. Proliposome formulation and entrapment efficiency studies identified a novel method of liposome separation and proliposome manufacture. Nebulization and stability studies indicated that proliposome tablets are a viable option for pulmonary drug delivery eliciting high stability when sorbitol was used carbohydrate carrier in proliposome.