Solid-State Hydrogen-Deuterium Exchange Mass Spectrometry: Correlation of Deuterium Uptake and Long-Term Stability of Lyophilized Monoclonal Antibody Formulations.

Solid state hydrogen-deuterium exchange with mass spectrometric analysis (ssHDX-MS) has been used to assess protein conformation and matrix interactions in lyophilized solids. ssHDX-MS metrics have been previously correlated to the formation of aggregates of lyophilized myoglobin on storage. Here, ssHDX-MS was applied to lyophilized monoclonal antibody (mAb) formulations and correlated to their long-term stability. After exposing lyophilized samples to D2O(g), the amount of deuterium incorporated at various time points was determined by mass spectrometry for four different lyophilized mAb formulations. Hydrogen-deuterium exchange data were then correlated with mAb aggregation and chemical degradation, which was obtained in stability studies of >2.5 years. Deuterium uptake on ssHDX-MS of four lyophilized mAb formulations determined at the initial time point prior to storage in the dry state was directly and strongly correlated with the extent of aggregation and chemical degradation during storage. Other measures of physical and chemical properties of the solids were weakly or poorly correlated with stability. The data demonstrate, for the first time, that ssHDX-MS results are highly correlated with the stability of lyophilized mAb formulations. The findings thus suggest that ssHDX-MS can be used as an early read-out of differences in long-term stability between formulations helping to accelerate formulation screening and selection.

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.

Immunogenicity of Therapeutic Protein Aggregates.

Therapeutic proteins have a propensity for aggregation during manufacturing, shipping, and storage. The presence of aggregates in protein drug products can induce adverse immune responses in patients that may affect safety and efficacy, and so it is of concern to both manufacturers and regulatory agencies. In this vein, there is a lack of understanding of the physicochemical determinants of immunological responses and a lack of standardized analytical methods to survey the molecular properties of aggregates associated with immune activation. In this review, we provide an overview of the basic immune mechanisms in the context of interactions with protein aggregates. We then critically examine the literature with emphasis on the underlying immune mechanisms as they relate to aggregate properties. Finally, we highlight the gaps in our current understanding of this issue and offer recommendations for future research.

Thiol-Disulfide Exchange in Human Growth Hormone.

PURPOSE: Thiol-disulfide exchange was monitored in recombinant human growth hormone (hGH) and in model tryptic peptides derived from hGH to investigate the effects of higher-order structure on the reaction. METHODS: Different free thiol-containing peptides, varying in length and amino acid sequence, were used to initiate the reaction at pH 7.0 and 37°C in hGH. Protein samples were digested with trypsin and analyzed for native disulfides, scrambled disulfides and free thiols using LC/MS. The loss of native disulfide and disulfide exchange was compared with model peptides derived from hGH. RESULTS: Loss of native disulfide in cyclic (cT20-T21) and linear peptides (T20-T21pep) derived from the C-terminal hGH disulfide during the first 60 min of reaction was greater than loss of the C-terminal disulfide in hGH itself. Of the thiols tested, glutathione (GSH) was the most reactive, forming the highest percentage of mixed disulfides in intact hGH and in the model peptides. At longer reaction times (>240 min), native disulfides in both hGH and cT20-T21 were regenerated. The fastest rates of regeneration were observed for Cys and the di- or tripeptide containing an Arg residue adjacent to Cys, suggesting that they may be useful in refolding. CONCLUSIONS: Thiol-disulfide exchange reactions in hGH and related model peptides were influenced by higher order structure, by the size of the thiol reactant and by an Arg residue adjacent to Cys in the thiol reactant. Reduction of disulfide bonds in hGH did not affect higher order structure as measured by CD and HDX-MS.

Characterizing Protein Structure, Dynamics and Conformation in Lyophilized Solids.

The long-term stability of protein therapeutics in the solid-state depends on the preservation of native structure during lyophilization and in the lyophilized powder. Proteins can reversibly or irreversibly unfold upon lyophilization, acquiring conformations susceptible to degradation during storage. Therefore, characterizing proteins in the dried state is crucial for the design of safe and efficacious formulations. This review summarizes the basic principles and applications of the analytical techniques that are commonly used to characterize protein structure, dynamics and conformation in lyophilized solids. The review also discusses the applications of recently developed mass spectrometry based methods (solid-state hydrogen deuterium exchange mass spectrometry (ssHDX-MS) and solid-state photolytic labeling mass spectrometry (ssPL-MS)) and their ability to study proteins in the solid-state at high resolution.

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.

Structural transitions and interactions in the early stages of human glucagon amyloid fibrillation.

A mechanistic understanding of the intermolecular interactions and structural changes during fibrillation is crucial for the design of safe and efficacious glucagon formulations. Amide hydrogen/deuterium exchange with mass spectrometric analysis was used to identify the interactions and amino acids involved in the initial stages of glucagon fibril formation at acidic pH. Kinetic measurements from intrinsic and thioflavin T fluorescence showed sigmoidal behavior. Secondary structural measurement of fibrillating glucagon using far-UV circular dichroism spectroscopy showed changes in structure from random coil → α-helix → ß-sheet, with increase in α-helix content during the lag phase followed by increase in ß-sheet content during the growth phase. Hydrogen/deuterium exchange with mass spectrometric analysis of fibrillating glucagon suggested that C-terminal residues 22-29 are involved in interactions during the lag phase, during which N-terminal residues 1-6 showed no changes. Molecular dynamics simulations of glucagon fragments showed C-terminal to C-terminal interactions with greater α-helix content for the 20-29 fragment, with hydrophobic and aromatic residues (Phe-22, Trp-25, Val-23, and Met-27) predominantly involved. Overall, the study shows that glucagon interactions during the early phase of fibrillation are mediated through C-terminal residues, which facilitate the formation of α-helix-rich oligomers, which further undergo structural rearrangement and elongation to form ß-sheet-rich mature fibrils.

Predicting protein aggregation during storage in lyophilized solids using solid state amide hydrogen/deuterium exchange with mass spectrometric analysis (ssHDX-MS).

Solid state amide hydrogen/deuterium exchange with mass spectrometric analysis (ssHDX-MS) was used to assess the conformation of myoglobin (Mb) in lyophilized formulations, and the results correlated with the extent of aggregation during storage. Mb was colyophilized with sucrose (1:1 or 1:8 w/w), mannitol (1:1 w/w), or NaCl (1:1 w/w) or in the absence of excipients. Immediately after lyophilization, samples of each formulation were analyzed by ssHDX-MS and Fourier transform infrared spectroscopy (FTIR) to assess Mb conformation, and by dynamic light scattering (DLS) and size exclusion chromatography (SEC) to determine the extent of aggregation. The remaining samples were then placed on stability at 25 °C and 60% RH or 40 °C and 75% RH for up to 1 year, withdrawn at intervals, and analyzed for aggregate content by SEC and DLS. In ssHDX-MS of samples immediately after lyophilization (t = 0), Mb was less deuterated in solids containing sucrose (1:1 and 1:8 w/w) than in those containing mannitol (1:1 w/w), NaCl (1:1 w/w), or Mb alone. Deuterium uptake kinetics and peptide mass envelopes also indicated greater Mb structural perturbation in mannitol, NaCl, or Mb-alone samples at t = 0. The extent of deuterium incorporation and kinetic parameters related to rapidly and slowly exchanging amide pools (Nfast, Nslow), measured at t = 0, were highly correlated with the extent of aggregation on storage as measured by SEC. In contrast, the extent of aggregation was weakly correlated with FTIR band intensity and peak position measured at t = 0. The results support the use of ssHDX-MS as a formulation screening tool in developing lyophilized protein drug products.

Photolytic labeling to probe molecular interactions in lyophilized powders.

Local side-chain interactions in lyophilized protein formulations were mapped using solid-state photolytic labeling-mass spectrometry (ssPL-MS). Photoactive amino acid analogues (PAAs) were used as probes and either added to the lyophilized matrix or incorporated within the amino acid sequence of a peptide. In the first approach, apomyoglobin was lyophilized with sucrose and varying concentrations of photoleucine (L-2-amino-4,4'-azipentanoic acid; pLeu). The lyophilized solid was irradiated at 365 nm to initiate photolabeling. The rate and extent of labeling were measured using electrospray ionization/high-performance liquid chromatography/mass spectrometry (ESI-HPLC-MS), with labeling reaching a plateau at ~30 min, forming up to six labeled populations. Bottom-up MS/MS analysis was able to provide peptide-level resolution of the location of pLeu. ssPL-MS was also able to detect differences in side-chain environment between sucrose and guanidine hydrochloride formulations. In the second approach, peptide GCG (1-8)* containing p-benzoyl-L-phenylalanine (pBpA) in the amino acid sequence was lyophilized with various excipients and irradiated. Peptide-peptide and peptide-excipient adducts were detected using MS. Top-down MS/MS on the peptide dimer provided amino acid-level resolution regarding interactions and the cross-linking partner for pBpA in the solid state. The results show that ssPL-MS can provide high-resolution information about protein interactions in the lyophilized environment.