Doxorubicin loaded liposomes in hydrogels: rheological properties and drug release profiles

Hydrogels are versatile formulations that have gained over years a large attention for a wide range of applications in different health-related fields such as tissue engineering, regenerative medicine and drug delivery [1]. In drug delivery, hydrogels were widely investigated as aqueous formulations able to control the release of the drugs and active substances, entrapped inside their 3D network [2]. Diffusion, erosion or swelling-controlled mechanisms are well-known for several polymeric and fibrillary cross-linked networks, and many works have focused on how the hydrogel cross-linking is able to govern the release profile [3]. However, scarce information is available for the release of drugs loaded inside liposomes, which are incorporated inside a hydrogel. The so called “liposomes-in-hydrogel” formulations have been developed to sustain drug release longer at the administration site [4,5], but at which extent the liposomes membrane and the cross-linked network can have an impact on the drug release, it has not been fully elucidated. Therefore, the aim of the present work is to investigate the release properties of hydrogel systems incorporating doxorubicin (DOX)-loaded liposomes. For this purpose, different systems were prepared by thickening the commercial liposomal dispersion Doxil® (DOX encapsulation efficiency >95%) with hydroxypropylmethyl cellulose (HPMC) K4M and K100 types at the concentration of 2% w/w and Carbopol® 974 at the concentration of 0.5% w/w. Systems were prepared starting from Doxil® liposomal dispersion to have a final formulation, in which almost all drug (DOX used as a model) is initially encapsulated inside liposomes and it is not present in a free form inside the hydrogel network. Polymers (HPMC K4M, HPMC K100 and Carbopol® 974) were selected according to their different thickening potency and ability to form cross-linked systems in water. All prepared systems were characterised in terms of rheology by stress and frequency sweep test. Release studies were conducted at 37°C using as medium a 150 mM ammonium chloride buffer at pH 5.5 and 6.5, according to the “dialysis membrane” (using a Type II dissolution apparatus) or the ultracentrifugation method. The release properties of the liposomal dispersions were also investigated as a control. The release amount of DOX was quantified by UV-spectroscopy at 488 nm. The incorporation of DOX-loaded liposomes into HPMC-based hydrogels did not affect the rheological properties in terms of consistency and viscoelasticity. On the other side, the incorporation of DOX-loaded liposomes into Carbopol ® 974 based hydrogels led to a slight decrease in consistency without affecting the viscoelastic properties of the system. DOX release was strongly dependent from pH. Indeed, drug is release faster from liposomes at pH 6.5 than pH 5.5, reaching the 70% and 30% of drug release, respectively, after 48 hours. This effect was observed both for the liposomal dispersion and all the liposomes-incorporating hydrogels. Moreover, DOX release was also found dependent on the viscosity of the hydrogel. Specifically, less drug was released from the hydrogel having a higher consistency, as those prepared using HPMC K100 and Carbopol® 974 with the respect to those prepared with HPMC K4M at both tested pH values. However, the effect of the polymeric matrix in controlling the release is more evident at pH 6.5 than at pH 5.5 for all systems, since at a higher pH the passage of the drug across the liposomal membrane is favourite and a larger amount of drug is released from the core of the liposomes inside the matrix. A clear effect exerted by the different polymer was also observed for the hydrogel loaded with DOX as a free drug, used as a reference. By comparing the two release methods investigated (i.e. dialysis membrane and ultracentrifugation) for the liposomes incorporating hydrogels, a higher release was achieved using the ultracentrifugation since the formulation was in direct contact with the release medium, thereby better promoting the polymeric matrix dissolution. In conclusions, the release profiles of DOX are controlled by both the liposomal membrane and the polymeric matrix, as a function of the different diffusion rate of the drug across the phospholipid bilayer and the swelled hydrogel. References [1] Correa S. et al, Chem. Rev. 121, 18 (2021) [2] Li J. and Mooney D.J. Nat. Rev. Mater 16071, (2016) [3] Elsawy M.A. al, Biomacromolecules 23, 6 (2022) [4] Hurler J. et al. Int. J. Pharm. 456, (2013) [5] Billard A. et al, Carbohydr. Polym. 115, (2015)