Active targeting to hepatocytes: how important is optimising the liposomes’ physicochemical properties and surface modification for targeting the hepatocytes.

Alsunbul, Maha Abdulaziz (2022). Active targeting to hepatocytes: how important is optimising the liposomes’ physicochemical properties and surface modification for targeting the hepatocytes. University of Birmingham. Ph.D.

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Abstract

The ability to selectively target drugs to their site of action is an increasingly important aspect in medicine. Using drug delivery systems and introducing active targeting strategies could maximise the accumulation of a drug to the desired site, while lowering its toxicity. Nanosized-drug carriers have been suggested to achieve this goal, and liposomes are one of the most studied nanocarriers. However, exploiting liposomes and other nanocarriers for delivery to the hepatocytes remains challenging, despite liver disease representing some of the most common diseases globally. In this work, different approaches have been taken to identify the best strategy for targeting the hepatocytes and to study the impact of composition on the physicochemical properties of liposomes.
First, a systematic review of the literature on hepatocyte targeting was conducted. Two databases (Medline and Embase) were searched for published primary research over the past 5 years. The initial search identified 6460 articles, out of which 142 were eligible for full-text screening and 25 were included in the review based on pre-defined inclusion/exclusion criteria. The impact of the liposomes size, PEGylation and active targeting on liver accumulation was probed using in vitro cellular uptake, and in vivo and clinical pharmacokinetics data. Overall, the results showed that a liposome size between 100-200 nm and using a ligand such as galactose, with a density no more than 20 mol% associated with higher cellular uptake and better pharmacokinetics profile. Adding PEG increases the circulation time but did not enhance the cellular uptake. However, using a dual modification of the liposome’s surface with PEG and galactose appeared to be a promising strategy to target the hepatocytes.
Secondly, three neutral liposomal formulations (100-200 nm) made from phosphatidylcholine derivatives (DSPC, soybean (SPC) and hydrogenated soybean (HSPC)) and cholesterol, using thin film hydration method. Initially, the aim was to test the impact of composition on liposome accumulation in different liver cells, but due to the pandemic, the full aim was not achieved. Still, the liposomes were evaluated in terms of size, zeta potential and membrane fluidity. Preliminary studies using SPC liposomes, suggested that a phospholipid: cholesterol ratio of 70:30 produced the best formulations; this ratio was then selected for all liposome formulations. Overall, liposomes size was below 200 nm with a narrow distribution index (PDI <0.3 a.u.). The zeta potential was within the range for neutral liposomes (-10 to 10 mV). Surface modification was performed using 2 or 5 mol% PEG (neutral, cationic, anionic). In general, PEGylation led to an increase in size, which was statistically significant for SPC liposomes (p-value<0.01). As expected, the surface charge was reduced compared to non-PEGylated vesicles. PEGylation efficiency varied between (22-85%), with DSPC liposomes achieving the highest efficiencies (85% when 2 mol% PEG was used). Initial PEG loading also affected PEGylation efficiency, with higher loadings achieved when 2 mol% PEG was used. Membrane permeability studies performed using calcein as a hydrophilic probe, showed temperature- and PEGylation-dependent patterns for SPC and HSPC liposomes (p-value<0.05). Finally, lyophilisation was performed on SPC liposomes using 150 mM sucrose using pre-and post-insertion method. In both cases, liposome size was maintained, but the post-insertion resulted in better homogeneity. Altogether, this study confirmed that composition must adjusted carefully when formulating liposomes, as this can affect the ease with which the surface can be modified and the release rate of an encapsulated compound.

Type of Work:

Thesis (Doctorates > Ph.D.)

Award Type:

Doctorates > Ph.D.

Supervisor(s):