Use of In Vitro Human Skin Models to Assess Potential Immune Activation In Response to Biotherapeutic Attributes and Process-related Impurities.

Subcutaneous (SubQ) injection is a common administration route for biotherapeutics. However, limited tools are available for understanding the dynamic relationships between drug products and resident cells following injection. Advances in tissue engineering have enabled the production of in vitro skin models that recapitulate the morphological structure and functional activity of human skin. Here we explore the use of a commercially available skin model to investigate potential immune activation in response to subcutaneously injected biotherapeutics. Exposure to high levels of a mixture of process-related impurities (that are known potent immune system activators) induced a robust immune response from the skin model, as indicated by enhanced metabolic activity and increased secretion of 19 cytokines and chemokines. The skin model also responded to aggregated antibodies (generated by extreme mechanical stirring and pH-jump stress, which resulted in orders of magnitude higher particle numbers than that found in products), as shown by the secretion of several signature cytokines (GM-CSF, RANTES, and MCP-1). However, the magnitude of the responses to the aggregates were significantly lower than the response to the impurities. These results highlight the promising utility of in vitro skin models for investigating the potential immune response to process-related impurities and biotherapeutic attributes in a subcutaneous environment. The use of skin models for assessing drug safety may provide new insights to help guide drug product and process development, and potentially mitigate the risk of injection site reactions and systemic immunogenic responses that may compromise the safety and efficacy of subcutaneously administered drugs.

A Mass Spectrometric Characterization of Light-Induced Modifications in Therapeutic Proteins.

During the development of a therapeutic protein, its quality attributes that pertain to the primary structure must be appropriately characterized, commonly by LC-MS/MS peptide mapping experiments. Extracting attribute information from LC-MS/MS data requires knowledge of the attribute of interest. Therefore, it is important to understand all potential modifications on the therapeutic proteins. In this work, we performed UV and visible light irradiation experiments on several therapeutic proteins, with or without the presence of a photosensitizer. Light-induced modifications were detected and characterized by tryptic digestion followed by LC-MS/MS analysis. A list of potential light-induced modifications, with their respective mass changes, was obtained. These modifications are primarily on methionine, tryptophan, histidine, cysteine, tyrosine and phenylalanine residues. Many of these modifications have not been previously reported on therapeutic proteins. Our findings therefore provide a database of potential light-induced modifications that would enable the routine characterization of light-induced modifications on therapeutic proteins.

Characterization of Subvisible Particles in Biotherapeutic Prefilled Syringes: The Role of Polysorbate and Protein on the Formation of Silicone Oil and Protein Subvisible Particles After Drop Shock.

Subvisible particles (SbVPs) are a critical quality attribute for biotherapeutics. Particle content in prefilled syringes (PFSs) of a biotherapeutic can include protein particles and silicone oil particles (SiOP). Here, a real-world protein therapeutic PFS shows that although polysorbate is effective in preventing protein particle formation, it also leads to the formation of SiOP. PFSs of protein and buffer formulations in the presence and absence of polysorbate are subjected to a drop shock to generate SbVP and the effect of polysorbate and protein in generating SbVP is investigated. Particle characterization by light obscuration and flow imaging shows that polysorbate prevents protein particle formation as intended, but the presence of polysorbate substantially increases the formation of SiOP. The protein itself also acts as a surfactant and leads to increased SiOP, but to a lesser degree compared to polysorbate. In a separate companion study by Joh et al., the risk of immunogenicity was assessed using in vivo and in vitro models. Flow imaging distinguishes between SiOP and protein particles and enables risk assessment of the natures of different SbVP in PFSs.

Silicone Oil Particles in Prefilled Syringes With Human Monoclonal Antibody, Representative of Real-World Drug Products, Did Not Increase Immunogenicity in In Vivo and In Vitro Model Systems.

Silicone oil is a lubricant for prefilled syringes (PFS), a common primary container for biotherapeutics. Silicone oil particles (SiOP) shed from PFS are a concern for patients due to their potential for increased immunogenicity and therefore also of regulatory concern. To address the safety concern in a context of manufacturing and distribution of drug product (DP), SiOP was increased (up to ∼25,000 particles/mL) in PFS filled with mAb1, a fully human antibody drug, by simulated handling of DP mimicked by drop shock. These samples are characterized in a companion report (Jiao N et al. J Pharm Sci. 2020). The risk of immunogenicity was then assessed using in vitro and in vivo immune model systems. The impact of a common DP excipient, polysorbate 80, on both the formation and biological consequences of SiOP was also tested. SiOP was found associated with (1) minimal cytokine secretion from human peripheral blood mononuclear cells, (2) no response in cell lines that report NF-κB/AP-1 signaling, and (3) no antidrug antibodies or significant cytokine production in transgenic Xeno-het mice, whether or not mAb1 or polysorbate 80 was present. These results suggest that SiOP in mAb1, representative of real-world DP in PFS, poses no increased risk of immunogenicity.

Impact of Precipitation of Antibody Therapeutics After Subcutaneous Injection on Pharmacokinetics and Immunogenicity.

Antibody therapeutics with poor solubility in the subcutaneous matrix may carry unintended risks when administered to patients. The objective of this work was to estimate the risk of antibodies that precipitate in vitro at neutral pH by determining the impact of poor solubility on distribution of the drug from the injection site as well as immunogenicity in vivo. Using fluorescence imaging in a mouse model, we show that one such precipitation-prone antibody is retained at the injection site in the subcutaneous space longer than a control antibody. In addition, we demonstrate that retention at the injection site through aggregation is concentration-dependent and leads to macrophage association and germinal center localization. Although there was delayed disposition of the aggregated antibody to draining lymph nodes, no overall impact on the immune response in lymph nodes, systemic exposure of the antibody, or enhancement of the anti-drug antibody response was evident. Unexpectedly, retention of the precipitated antibody in the subcutaneous space delayed the onset of the immune response and led to an immune suppressive response. Thus, we conclude that precipitation due to poor solubility of high doses of antibody formulations delivered subcutaneously may not be of special concern in terms of exposure or immunogenicity.

Design of self-assembling transmembrane helical bundles to elucidate principles required for membrane protein folding and ion transport.

Ion transporters and channels are able to identify and act on specific substrates among myriads of ions and molecules critical to cellular processes, such as homeostasis, cell signalling, nutrient influx and drug efflux. Recently, we designed Rocker, a minimalist model for Zn2+/H+ co-transport. The success of this effort suggests that de novo membrane protein design has now come of age so as to serve a key approach towards probing the determinants of membrane protein folding, assembly and function. Here, we review general principles that can be used to design membrane proteins, with particular reference to helical assemblies with transport function. We also provide new functional and NMR data that probe the dynamic mechanism of conduction through Rocker.This article is part of the themed issue 'Membrane pores: from structure and assembly, to medicine and technology'.

De novo design of a transmembrane Zn²âº-transporting four-helix bundle.

The design of functional membrane proteins from first principles represents a grand challenge in chemistry and structural biology. Here, we report the design of a membrane-spanning, four-helical bundle that transports first-row transition metal ions Zn(2+) and Co(2+), but not Ca(2+), across membranes. The conduction path was designed to contain two di-metal binding sites that bind with negative cooperativity. X-ray crystallography and solid-state and solution nuclear magnetic resonance indicate that the overall helical bundle is formed from two tightly interacting pairs of helices, which form individual domains that interact weakly along a more dynamic interface. Vesicle flux experiments show that as Zn(2+) ions diffuse down their concentration gradients, protons are antiported. These experiments illustrate the feasibility of designing membrane proteins with predefined structural and dynamic properties.

Probing membrane protein unfolding with pulse proteolysis.

Technical challenges have greatly impeded the investigation of membrane protein folding and unfolding. To develop a new tool that facilitates the study of membrane proteins, we tested pulse proteolysis as a probe for membrane protein unfolding. Pulse proteolysis is a method to monitor protein folding and unfolding, which exploits the significant difference in proteolytic susceptibility between folded and unfolded proteins. This method requires only a small amount of protein and, in many cases, may be used with unpurified proteins in cell lysates. To evaluate the effectiveness of pulse proteolysis as a probe for membrane protein unfolding, we chose Halobacterium halobium bacteriorhodopsin (bR) as a model system. The denaturation of bR in SDS has been investigated extensively by monitoring the change in the absorbance at 560 nm (A(560)). In this work, we demonstrate that denaturation of bR by SDS results in a significant increase in its susceptibility to proteolysis by subtilisin. When pulse proteolysis was applied to bR incubated in varying concentrations of SDS, the remaining intact protein determined by electrophoresis shows a cooperative transition. The midpoint of the cooperative transition (C(m)) shows excellent agreement with that determined by A(560). The C(m) values determined by pulse proteolysis for M56A and Y57A bRs are also consistent with the measurements made by A(560). Our results suggest that pulse proteolysis is a quantitative tool to probe membrane protein unfolding. Combining pulse proteolysis with Western blotting may allow the investigation of membrane protein unfolding in situ without overexpression or purification.

Structural imperatives impose diverse evolutionary constraints on helical membrane proteins.

The amino acid sequences of transmembrane regions of helical membrane proteins are highly constrained, diverging at slower rates than their extramembrane regions and than water-soluble proteins. Moreover, helical membrane proteins seem to fall into fewer families than water-soluble proteins. The reason for the differential restrictions on sequence remains unexplained. Here, we show that the evolution of transmembrane regions is slowed by a previously unrecognized structural constraint: Transmembrane regions bury more residues than extramembrane regions and soluble proteins. This fundamental feature of membrane protein structure is an important contributor to the differences in evolutionary rate and to an increased susceptibility of the transmembrane regions to disease-causing single-nucleotide polymorphisms.

Similar energetic contributions of packing in the core of membrane and water-soluble proteins.

A major driving force for water-soluble protein folding is the hydrophobic effect, but membrane proteins cannot make use of this stabilizing contribution in the apolar core of the bilayer. It has been proposed that membrane proteins compensate by packing more efficiently. We therefore investigated packing contributions experimentally by observing the energetic and structural consequences of cavity creating mutations in the core of a membrane protein. We observed little difference in the packing energetics of water and membrane soluble proteins. Our results imply that other mechanisms are employed to stabilize the structure of membrane proteins.

Methods for measuring the thermodynamic stability of membrane proteins.

Learning how amino acid sequences define protein structure has been a major challenge for molecular biology since the first protein structures were determined in the 1960s. In contrast to the staggering progress with soluble proteins, investigations of membrane protein folding have long been hampered by the lack of high-resolution structures and the technical challenges associated with studying the folding process in vitro. In the past decade, however, there has been an explosion of new membrane protein structures and a slower but notable increase in efforts to study the factors that define these structures. Here we review the methods that have been used to evaluate the thermodynamic stability of membrane proteins and provide some salient examples of how the methods have been used to begin to understand the energetics of membrane protein folding.