Lyophilized Peptide and Protein Stability: What Storage Studies Report

Lyophilization (freeze-drying) is described in the formulation literature as a solidification strategy used because therapeutic peptides and proteins frequently show chemical degradation during handling in liquid dosage forms, with solid formulations investigated as a way to limit physical and chemical change during storage and transport [2]. Laboratory protocols for pure proteins describe lyophilization as a multi-stage process comprising freezing, primary drying, and secondary drying, and identify the glass transition temperature of the dried cake as a key design parameter measured by modulated differential scanning calorimetry [1]. The same methodological literature discusses stabilizing additives and the prevention of proteolytic degradation and microbial contamination as separate variables from the drying process itself [1]. Several solid-state studies have examined how the glassy matrix relates to measured degradation rates. In freeze-dried recombinant human interleukin-1 receptor antagonist, ten formulations with glass transition temperatures spanning roughly 20 to 56 °C were stored above and below their respective glass transition temperatures; in vitro analysis found deamidation and aggregation were greatly accelerated above the glass transition temperature but also occurred below it in some formulations, leading the authors to conclude that sub-glass-transition storage was necessary but not sufficient for long-term stability [3]. In the same in vitro work, formulations prepared with 1% or less sucrose showed an unfolded protein conformation in the initial dried solid, while formulations with 5% or more sucrose inhibited conformational change during lyophilization, and degradation at 50 °C varied inversely with sucrose concentration [3]. A separate in vitro study of lyophilized keratinocyte growth factor-2 formulated with sucrose or trehalose and hydroxyethyl starch measured degradation at 40, 50, and 60 °C and reported that degradation rates were highest where native secondary structure was most perturbed and where fast beta relaxations were fastest, but that the dominant correlate of degradation was the fraction of protein estimated to reside at the glass solid-air interface [4]. Residual moisture has been studied as a distinct variable. In vitro work on lyophilized insulin exposed to elevated temperature and moisture reported both covalent and non-covalent aggregation, with the covalent route characterised as intermolecular thiol-catalysed disulfide interchange following beta-elimination of a disulfide bridge; the extent of aggregation correlated directly with water uptake by the lyophilized powder [5]. Chemical degradation pathways can also differ between phases: an in vitro study of a model peptide (EVQLVESGGGLVQPGGSLR) formulated across apparent 'pH' 4 to 9 and stored at 50 °C found that the apparent 'pH' dependence of N-terminal pyroglutamate formation in lyophilized solids differed markedly from that in solution, and that in the apparent 'pH' 5.5–6 range the solid-state reaction rate exceeded the solution rate [6]. Excipient identity has repeatedly emerged as a measured determinant in solid-state stability screens. An in vitro study of a lyophilized monoclonal antibody reported that sucrose remained the primary stabilizing agent, that partially amorphous glycine was associated with a significant increase in stability, and that mannitol showed little stabilizing ability in that system despite producing crystalline cakes [7]. In a spin-freeze-drying study of a PEGylated peptide, variations in cooling and crystallization rates produced no consistent statistically significant effect on peptide or monomer content, whereas formulation composition dominated: trehalose-based samples remained stable at 2–8 °C and under 50 °C stress, mannitol-based samples showed moderate degradation at elevated temperature, and a 75:25 sucrose–mannitol system showed cake collapse, browning, and interference in peptide quantification attributed to sucrose hydrolysis [8]. Solid-state hydrogen-deuterium exchange mass spectrometry of four lyophilized granulocyte colony stimulating factor formulations correlated deuterium uptake with monomer content remaining after lyophilization and storage at −20 °C, ranking sucrose above phenylalanine, mannitol, and no excipient, while storage at 45 °C produced little difference between formulations [9]. Long-duration storage studies report time- and temperature-dependent outcomes for specific molecules. Lyophilized NOTA-single-domain antibody fragment precursors formulated with a mannitol–sucrose mix and polysorbate 80 showed in vitro integrity, absence of aggregation, and retained labeling efficiency for up to 12–18 months at 2–8 °C, and biodistribution in wild-type and tumour-bearing mice was comparable to non-lyophilized precursor [10]. A freeze-dried mucoadhesive matrix containing the antimicrobial peptide hLF 1-11 showed no detectable chemical degradation for at least 6 months in long-term stability testing, with antimicrobial activity in vitro reported at up to 15 months of storage [11]. Characterisation of yolkin, a hen egg yolk polypeptide complex, reported stability in lyophilized form with storage at 4 °C described as preferable in that in vitro assessment [12]. In an animal study, freeze-dried platelet-rich plasma and adipose stem cell-conditioned medium retained platelet-derived and vascular endothelial growth factor levels comparable to fresh material after freeze-drying and 90 days of storage, with a significant reduction measured at 180 days [14]. Review literature on erythropoietin similarly frames temperature fluctuation, light exposure, and interactions with other substances as instability factors, and lists excipients, lyophilization, and optimized storage conditions among the stabilization strategies studied [13]. Analyte stability during dried storage has also been measured outside peptide drug formulation. In human blood samples spotted on filter paper, thyroid-stimulating hormone measurements remained stable to day 30 when stored at 22 °C and to day 60 under refrigerated or freezer storage, with samples held at 22 °C showing a greater decrease than refrigerated or frozen samples over one year [15]. Taken together, the cited work treats glass transition temperature, retained secondary structure, specific surface area and solid-air interface fraction, residual moisture, excipient selection, apparent 'pH', and storage temperature as measurable, interacting variables in solid-state degradation, and the results are formulation- and molecule-specific rather than generalizable across peptides [2][3][4][8].