The effectiveness of a drug is dependent on accumulation at the site of action at therapeutic levels, however, challenges such as rapid renal clearance, degradation or non-specific accumulation requires drug delivery enabling technologies. Albumin is a natural transport protein with multiple ligand binding sites, cellular receptor engagement, and a long circulatory half-life due to interaction with the recycling neonatal Fc receptor. Exploitation of these properties promotes albumin as an attractive candidate for half-life extension and targeted intracellular delivery of drugs attached by covalent conjugation, genetic fusions, association or ligand-mediated association. This review will give an overview of albumin-based products with focus on the natural biological properties and molecular interactions that can be harnessed for the design of a next-generation drug delivery platform.

Background: Exosomes have been implicated in tumour progression and metastatic spread. Little is known of the effect of mechanical and innate immune interactions of malignant cell‐derived exosomes on endothelial integrity, which may relate to increased extravasation of circulating tumour cells and, therefore, increased metastatic spread.Methods: Exosomes isolated from non‐malignant immortalized HCV‐29 and isogenic malignant non‐metastatic T24 and malignant metastatic FL3 bladder cells were characterized by nanoparticle tracking analysis and quantitative nanomechanical mapping atomic force microscopy (QNM AFM) to determine size and nanomechanical properties. Effect of HCV‐29, T24 and FL3 exosomes on human umbilical vein endothelial cell (HUVEC) monolayer integrity was determined by transendothelial electrical resistance (TEER) measurements and transport was determined by flow cytometry. Complement activation studies in human serum of malignant and non‐malignant cell‐derived exosomes were performed.Results: FL3, T24 and HCV‐29 cells produced exosomes at similar concentration per cell (6.64, 6.61 and 6.46×10⁴ exosomes per cell for FL3, T24 and HCV‐29 cells, respectively) and of similar size (120.2 nm for FL3, 127.6 nm for T24 and 117.9 nm for HCV‐29, respectively). T24 and FL3 cell‐derived exosomes exhibited a markedly reduced stiffness, 95 MPa and 280 MPa, respectively, compared with 1,527 MPa with non‐malignant HCV‐29 cell‐derived exosomes determined by QNM AFM. FL3 and T24 exosomes induced endothelial disruption as measured by a decrease in TEER in HUVEC monolayers, whereas no effect was observed for HCV‐29 derived exosomes. FL3 and T24 exosomes traffic more readily (11.6 and 21.4% of applied exosomes, respectively) across HUVEC monolayers than HCV‐29 derived exosomes (7.2% of applied exosomes). Malignant cell‐derived exosomes activated complement through calcium‐sensitive pathways in a concentration‐dependent manner.Conclusions: Malignant (metastatic and non‐metastatic) cell line exosomes display a markedly reduced stiffness and adhesion but an increased complement activation compared to non‐malignant cell line exosomes, which may explain the observed increased endothelial monolayer disruption and transendothelial transport of these vesicles.

Albumin is the most abundant plasma protein involved in the transport of many compounds, such as fatty acids, bilirubin, and heme. The endothelial cellular neonatal Fc receptor (FcRn) has been suggested to play a central role in maintaining high albumin plasma levels through a cellular recycling pathway. However, direct mapping of this process is still lacking. This work presents the use of wild-type and engineered recombinant albumins with either decreased or increased FcRn affinity in combination with a low or high FcRn-expressing endothelium cell line to clearly define the FcRn involvement, intracellular pathway, and kinetics of albumin trafficking by flow cytometry, quantitative confocal microscopy, and an albumin-recycling assay. We found that cellular albumin internalization was proportional to FcRn expression and albumin-binding affinity. Albumin accumulation in early endosomes was independent of FcRn-binding affinity, but differences in FcRn-binding affinities significantly affected the albumin distribution between late endosomes and lysosomes. Unlike albumin with low FcRn-binding affinity, albumin with high FcRn-binding affinity was directed less to the lysosomes, suggestive of FcRn-directed albumin salvage from lysosomal degradation. Furthermore, the amount of recycled albumin in cell culture media corresponded to FcRn-binding affinity, with a ∼3.3-fold increase after 1 h for the high FcRn-binding albumin variant compared with wild-type albumin. Together, these findings uncover an FcRn-dependent endosomal cellular-sorting pathway that has great importance in describing fundamental mechanisms of intracellular albumin recycling and the possibility to tune albumin-based therapeutic effects by FcRn-binding affinity.

Fc-less bispecific T-cell engagers have reached the immuno-oncology market but necessitate continual infusion due to rapid clearance from the circulation. This work introduces a programmable serum half-life extension platform based on fusion of human albumin sequences engineered with either null (NB), wild type (WT) or high binding (HB) FcRn affinity combined with a bispecific T-cell engager. We demonstrate in a humanised FcRn/albumin double transgenic mouse model (AlbuMus) the ability to tune half-life based on the albumin sequence fused with a BiTE-like bispecific (anti-EGFR nanobody x anti-CD3 scFv) light T-cell engager (LiTE) construct [(t_½ 0.6 h (Fc-less LiTE), t_½ 19 hours (Albu-LiTE-NB), t_½ 26 hours (Albu-LiTE-WT), t_½ 37 hours (Albu-LiTE-HB)]. We show in vitro cognate target engagement, T-cell activation and discrimination in cellular cytotoxicity dependent on EGFR expression levels. Furthermore, greater growth inhibition of EGFR-positive BRAF mutated tumours was measured following a single dose of Albu-LiTE-HB construct compared to the Fc-less LiTE format and a full-length anti-EGFR monoclonal antibody in a new AlbuMus RAG1 knockout model introduced in this work. Programmable half-life extension facilitated by this albumin platform potentially offers long-lasting effects, better patient compliance and a method to tailor pharmacokinetics to maximise therapeutic efficacy and safety of immuno-oncology targeted biologics.

A poor pharmacokinetic profile due to inadequate distribution and rapid renal clearance limits site-specific target engagement and drug efficacy. The inherent properties of human serum albumin for ...

Harnessing the RNA interference pathway offers a new therapeutic modality; however, solutions to overcome biological barriers to small interfering RNA (siRNA) delivery are required for clinical translation. This work demonstrates, by direct northern and quantitative PCR (qPCR) detection, stability, gastrointestinal (GI) deposition, and translocation into peripheral tissue of nonmodified siRNA after oral gavage of chitosan/siRNA nanoparticles in mice. In contrast to naked siRNA, retained structural integrity and deposition in the stomach, proximal and distal small intestine, and colon was observed at 1 and 5 hours for siRNA within nanoparticles. Furthermore, histological detection of fluorescent siRNA at the apical regions of the intestinal epithelium suggests mucoadhesion provided by chitosan. Detection of intact siRNA in the liver, spleen, and kidney was observed 1 hour after oral gavage, with an organ distribution pattern influenced by nanoparticle N:P ratio that could reflect differences in particle stability. This proof-of-concept work presents an oral delivery platform that could have the potential to treat local and systemic disorders by siRNA.