Photolytic labeling to probe molecular interactions in lyophilized powders

Innovative Drying Technologies for Biopharmaceuticals

In the past two decades, biopharmaceuticals have been a breakthrough in improving the quality of lives of patients with various cancers, autoimmune, genetic disorders etc. With the growing demand of biopharmaceuticals, the need for reducing manufacturing costs is essential without compromising on the safety, quality, and efficacy of products. Batch Freeze-drying is the primary commercial means of manufacturing solid biopharmaceuticals. However, Freeze-drying is an economically unfriendly means of production with long production cycles, high energy consumption and heavy capital investment, resulting in high overall costs. This review compiles some potential, innovative drying technologies that have not gained popularity for manufacturing parenteral biopharmaceuticals. Some of these technologies such as Spin-freeze-drying, Spray-drying, Lynfinity® Technology etc. offer a paradigm shift towards continuous manufacturing, whereas PRINT® Technology and Microglassification^TM allow controlled dry particle characteristics. Also, some of these drying technologies can be easily scaled-up with reduced requirement for different validation processes. The inclusion of Process Analytical Technology (PAT) and offline characterization techniques in tandem can provide additional information on the Critical Process Parameters (CPPs) and Critical Quality Attributes (CQAs) during biopharmaceutical processing. These processing technologies can be envisaged to increase the manufacturing capacity for biopharmaceutical products at reduced costs.

Enhanced Binding and Reduced Immunogenicity of Glycoconjugates Prepared via Solid-State Photoactivation of Aliphatic Diazirine Carbohydrates

Biological conjugation is an important tool employed for many basic research and clinical applications. While useful, common methods of biological conjugation suffer from a variety of limitations, such as (a) requiring the presence of specific surface-exposed residues, such as lysines or cysteines, (b) reducing protein activity, and/or (c) reducing protein stability and solubility. Use of photoreactive moieties including diazirines, azides, and benzophenones provide an alternative, mild approach to conjugation. Upon irradiation with UV and visible light, these functionalities generate highly reactive carbenes, nitrenes, and radical intermediates. Many of these will couple to proteins in a non-amino-acid-specific manner. The main hurdle for photoactivated biological conjugation is very low yield. In this study, we developed a solid-state method to increase conjugation efficiency of diazirine-containing carbohydrates to proteins. Using this methodology, we produced multivalent carbohydrate-protein conjugates with unaltered protein charge and secondary structure. Compared to carbohydrate conjugates prepared with amide linkages to lysine residues using standard NHS conjugation, the photoreactive prepared conjugates displayed up to 100-fold improved binding to lectins and diminished immunogenicity in mice. These results indicate that photoreactive bioconjugation could be especially useful for in vivo applications, such as lectin targeting, where high binding affinity and low immunogenicity are desired.

A histological study of mouse tissues and water loss following lyophilization

Lyophilization is a practical method for product storage and transportation; it commonly is used in the food and pharmaceutical industries. Lyophilization also is used for preserving biological samples such as serum, plasma and animal tissues. We found that lyophilization does not affect the stability of RNAs and proteins in tissue samples. To investigate histological characteristics, we prepared lyophilized tissues for paraffin sectioning and hematoxylin and eosin (H & E) staining. We also measured water loss from organs during lyophilization. We used immunohistochemistry of frozen brain sections to identify potential protective effects of three concentrations of sucrose, glucose and trehalose against the effects of lyophilization. H & E staining revealed vacuoles in heart, lung, liver, kidney, spleen and brain after lyophilization without pretreatments, especially heart and kidney. We found that 10% solutions of sucrose, glucose and trehalose helped preserve tissue morphology. Immunohistochemistry of frozen brain tissue showed that 10% glucose and 30% sucrose preserved cellular characteristics and immunogenicity following lyophilization. Lyophilization removed > 70% of the water from organs, and lyophilized tissues without protectants were not suitable for histological study. Overall, we found that 10% glucose helped preserve both optimal tissue morphology and immunogenicity of freeze-dried tissue.

A Novel Photoreactive Excipient to Probe Peptide-Matrix Interactions in Lyophilized Solids

Excipients used in lyophilized protein drug products are often selected by a trial-and-error method, in part, because the analytical methods used to detect protein-excipient interactions in lyophilized solids are limited. In this study, photolytic labeling was used to probe interactions between salmon calcitonin (sCT) and excipients in lyophilized solids. Two diazirine-derived photo-excipients, photo-leucine (pLeu) and photo-glucosamine (pGlcN), were incorporated into lyophilized solids containing sCT, together with an unlabeled excipient (sucrose or histidine) at prelyophilization pH values from 6 to 9.9. Commercially available pLeu was selected as an ionizable photo-excipient and amino acid analog, while pGlcN was synthesized as an analog of sugar-based excipients. Photolytic labeling was induced by exposing the solids to UV light (365 nm, 30-60 min), and the resulting products were identified and quantified with liquid-chromatography mass spectrometry. The distribution of photo-reaction products was affected by the photoreactive reagent used, the type of unlabeled excipient, and the solution pH before lyophilization. When other components of the solid were identical, the extent and sites of labeling on sCT were different for pGlcN and pLeu. The results suggest that ionizable and nonionizable excipients interact differently with sCT in lyophilized solids and that photo-excipients can be used to map these interactions.

Photolytic Labeling To Quantify Peptide-Water Interactions in Lyophilized Solids

Interactions of a lyophilized peptide with water and excipients in a solid matrix were explored using photolytic labeling. A model peptide "KLQ" (Ac-QELHKLQ-NHCH₃) was covalently labeled with NHS-diazirine (succinimidyl 4,4'-azipentanoate), and the labeled peptide (KLQ-SDA) was formulated and exposed to UV light in both solution and lyophilized solids. Solid samples contained the following excipients at a 1:400 molar ratio: sucrose, trehalose, mannitol, histidine, or arginine. Prior to UV exposure, the lyophilized solids were exposed to various relative humidity (RH) environments (8, 13, 33, 45, and 78%), and the resulting solid moisture content (Karl Fischer titration) and glass transition temperature ( T_g; differential scanning calorimetry, DSC) were measured. To initiate photolytic labeling, solution and solid samples were exposed to UV light at 365 nm for 30 min. Photolytic-labeling products were quantified using reversed-phase high-performance liquid chromatography (rp-HPLC) and mass spectrometry (MS). In lyophilized solids, studies excluding oxygen and using H₂¹⁸O confirmed that the source of oxygen in KLQ adducts with a mass increase of 18 amu are attributable to reaction with water, while those with a mass increase of 16 amu are not attributable to reaction with either water or molecular oxygen. In solids containing sucrose or trehalose, peptide-excipient adducts decreased with increasing solid moisture content, while peptide-water adducts increased only at lower RH exposure and then plateaued, in partial agreement with the water replacement hypothesis.

Photolytic Labeling and Its Applications in Protein Drug Discovery and Development

In this mini-review, the major types of photolytic labeling reagents are presented together with their reaction mechanisms. The applications of photolytic labeling in protein drug discovery and development are then discussed; these have expanded from studies of protein-protein interactions in vivo to protein-matrix interactions in lyophilized solids. The mini-review concludes with recommendations for further development of the approach, which include the need for new and more chemically diverse photo-reactive reagents and better understanding of the mechanisms of photolytic labeling reactions in various media.

Quantitative Analysis of Peptide-Matrix Interactions in Lyophilized Solids Using Photolytic Labeling

Peptide-matrix interactions in lyophilized solids were explored using photolytic labeling with reversed phase high performance liquid chromatography (rp-HPLC) and mass spectrometric (MS) analysis. A model peptide (Ac-QELHKLQ-NHCH₃) derived from salmon calcitonin was first labeled with a heterobifunctional cross-linker NHS-diazirine (succinimidyl 4,4'-azipentanoate; SDA) at Lys5 in solution, with ∼100% conversion. The SDA labeled peptide was then formulated with the following excipients at a 1:400 molar ratio and lyophilized: sucrose, trehalose, mannitol, histidine, arginine, urea, and NaCl. The lyophilized samples and corresponding solution controls were exposed to UV at 365 nm to induce photolytic labeling, and the products were identified by MS and quantified with rp-HPLC or MS. Peptide-excipient adducts were detected in the lyophilized solids except the NaCl formulation. With the exception of the histidine formulation, peptide-excipient adducts were not detected in solution and the fractional conversion to peptide-water adducts in solution was significantly greater than in lyophilized solids, as expected. In lyophilized solids, the fractional conversion to peptide-water adducts was poorly correlated with bulk moisture content, suggesting that the local water content near the labeled lysine residue differs from the measured bulk average. In lyophilized solids, the fractional conversion to peptide-excipient adducts was assessed using MS extracted ion chromatograms (EIC); subject to the assumption of equal ionization efficiencies, the fractional conversion to excipient adducts varied with excipient type. The results demonstrate that the local environment near the lysine residue of the peptide in the lyophilized solids can be quantitatively probed with a photolytic labeling method.

Process and Formulation Effects on Protein Structure in Lyophilized Solids Using Mass Spectrometric Methods

Myoglobin (Mb) was lyophilized in the absence (Mb-A) and presence (Mb-B) of sucrose in a pilot-scale lyophilizer with or without controlled ice nucleation. Cake morphology was characterized using scanning electron microscopy, and changes in protein structure were monitored using solid-state Fourier-transform infrared spectroscopy, solid-state hydrogen-deuterium exchange-mass spectrometry, and solid-state photolytic labeling-mass spectrometry (ssPL-MS). The results showed greater variability in nucleation temperature and irregular cake structure for formulations lyophilized without controlled nucleation. Controlled nucleation resulted in nucleation at ∼(-5°C) and uniform cake structure. Formulations containing sucrose showed better retention of protein structure by all measures than formulations without sucrose. Samples lyophilized with and without controlled nucleation were similar by most measures of protein structure. However, ssPL-MS showed the greatest photoleucine incorporation and more labeled regions for Mb-B lyophilized with controlled nucleation. The data support the use of solid-state hydrogen-deuterium exchange-mass spectrometry and ssPL-MS to study formulation and process-induced conformational changes in lyophilized proteins.

Photolytic Cross-Linking to Probe Protein-Protein and Protein-Matrix Interactions in Lyophilized Powders

Protein structure and local environment in lyophilized formulations were probed using high-resolution solid-state photolytic cross-linking with mass spectrometric analysis (ssPC-MS). In order to characterize structure and microenvironment, protein-protein, protein-excipient, and protein-water interactions in lyophilized powders were identified. Myoglobin (Mb) was derivatized in solution with the heterobifunctional probe succinimidyl 4,4'-azipentanoate (SDA) and the structural integrity of the labeled protein (Mb-SDA) confirmed using CD spectroscopy and liquid chromatography/mass spectrometry (LC-MS). Mb-SDA was then formulated with and without excipients (raffinose, guanidine hydrochloride (Gdn HCl)) and lyophilized. The freeze-dried powder was irradiated with ultraviolet light at 365 nm for 30 min to produce cross-linked adducts that were analyzed at the intact protein level and after trypsin digestion. SDA-labeling produced Mb carrying up to five labels, as detected by LC-MS. Following lyophilization and irradiation, cross-linked peptide-peptide, peptide-water, and peptide-raffinose adducts were detected. The exposure of Mb side chains to the matrix was quantified based on the number of different peptide-peptide, peptide-water, and peptide-excipient adducts detected. In the absence of excipients, peptide-peptide adducts involving the CD, DE, and EF loops and helix H were common. In the raffinose formulation, peptide-peptide adducts were more distributed throughout the molecule. The Gdn HCl formulation showed more protein-protein and protein-water adducts than the other formulations, consistent with protein unfolding and increased matrix interactions. The results demonstrate that ssPC-MS can be used to distinguish excipient effects and characterize the local protein environment in lyophilized formulations with high resolution.

Mass spectrometric approaches to study protein structure and interactions in lyophilized powders

Amide hydrogen/deuterium exchange (ssHDX-MS) and side-chain photolytic labeling (ssPL-MS) followed by mass spectrometric analysis can be valuable for characterizing lyophilized formulations of protein therapeutics. Labeling followed by suitable proteolytic digestion allows the protein structure and interactions to be mapped with peptide-level resolution. Since the protein structural elements are stabilized by a network of chemical bonds from the main-chains and side-chains of amino acids, specific labeling of atoms in the amino acid residues provides insight into the structure and conformation of the protein. In contrast to routine methods used to study proteins in lyophilized solids (e.g., FTIR), ssHDX-MS and ssPL-MS provide quantitative and site-specific information. The extent of deuterium incorporation and kinetic parameters can be related to rapidly and slowly exchanging amide pools (N fast, N slow) and directly reflects the degree of protein folding and structure in lyophilized formulations. Stable photolytic labeling does not undergo back-exchange, an advantage over ssHDX-MS. Here, we provide detailed protocols for both ssHDX-MS and ssPL-MS, using myoglobin (Mb) as a model protein in lyophilized formulations containing either trehalose or sorbitol.