Residual Moisture and the Stability of Lyophilized Peptides and Proteins: What the Literature Reports

Residual moisture is one of the most frequently measured attributes of a lyophilized peptide or protein cake, and the published literature treats it as a variable rather than a single target. In an exploratory study of methionyl human growth hormone and tissue-type plasminogen activator, investigators used BET water sorption analysis to approximate a monolayer water content and reported that excipient-free tPA powders dried below that calculated monolayer showed opalescence on reconstitution, while samples at monolayer or multilayer water content showed greater loss of biological stability under temperature stress; the authors framed this as evidence that the "drier the better" assumption may not hold for that protein and that an optimum balances physical and biological stability (in vitro) [1]. A companion series on freeze-dried human growth hormone formulations characterised methionine oxidation, asparagine deamidation, and irreversible aggregation by HPLC, and reported that pulsed proton NMR indicated onset of greater solid-state mobility at a water content slightly above the BET monolayer level, with the qualitative effect of residual moisture on stability depending on the study temperature and on whether glycine/mannitol excipients and headspace oxygen were present (in vitro) [2]. Model peptides have been used to isolate moisture as a formulation factor. In a factorial study of a lyophilized Asn-hexapeptide (Val-Tyr-Pro-Asn-Gly-Ala), the pH of the bulk solution before drying had the dominant effect on deamidation rate and product distribution, while residual moisture level and temperature were reported as less significant main effects; a statistically significant two-factor interaction indicated that starting pH determined the extent to which peptide stability depended on moisture and temperature (in vitro) [3]. In the corresponding Asp-hexapeptide study, ANOVA of main effects indicated that residual moisture level, temperature, and especially the type of bulking agent (amorphous versus crystalline) had a significant impact on solid-state reactivity, with the importance of moisture again conditional on the excipient used (in vitro) [4]. A mechanistic follow-up placed the same Asn-hexapeptide in poly(vinylpyrrolidone) with glycerol as a plasticizer to vary glass transition temperature independently of water content, and reported that deamidation increased with both moisture and glycerol content, that rates correlated with Tg at constant water content in rubbery systems, and that in glassy formulations of similar Tg deamidation still increased with water content — interpreted as water acting both as a plasticizer and as a solvent/medium and chemical participant (in vitro) [5]. Several reports examine moisture alongside the physical state of excipients. A study of freeze-dried interleukin-6 assessed residual moisture by Karl Fischer titration together with DSC and X-ray measurements and reported that fully amorphous trehalose or sucrose systems prevented aggregation over nine months at 25 °C and 40 °C provided storage remained below the lyophilizate Tg, while crystallized glycine or mannitol was associated with destabilization (in vitro) [6]. In a fully human anti-IL8 antibody study, residual moisture was determined by coulometric Karl Fischer titration and DSC was used to measure cake glass transition temperatures, with increasing histidine in the bulk reported to limit high-molecular-weight species on lyophilization and storage (in vitro) [7]. Cycle-development work on recombinant interleukin-2 used residual moisture, RP-HPLC monomer content, and SDS-PAGE as cake quality measures and also sampled cakes from multiple chamber positions to check process uniformity (in vitro) [8]. Work on rDNA-derived cytokine reference preparations reported Karl Fischer moisture values of 0.5–4%, reduced to 0.1–0.5% after storage over phosphorus pentoxide, with biological activity monitored under accelerated degradation (in vitro) [9]. A quality-control review catalogued analytical methods for lyophilization-related degradation and compared methods for monitoring residual moisture levels in lyophilized proteins (in vitro) [10]. A gas pycnometry method paper reported that isolating the instrument from atmospheric moisture was the key to reproducible true-density measurement of hygroscopic freeze-dried amorphous solids, and proposed density as a candidate parameter related to mobility (in vitro) [11]. Residual moisture also appears as a routine release and stability attribute across process comparisons. A freeze-dried albumin-free recombinant human interferon alpha-2b formulation was monitored for residual moisture, pH, purity, and antiviral activity across accelerated (28 °C) and real-time (30-month) storage (in vitro) [12]. A comparison of freeze drying and spray drying of interleukin-8 variants reported that both routes gave powders with residual moisture below 5 wt% and that binding affinity and chemotaxis activity did not decrease after 12 weeks of storage, with oligomerization observed for the freeze-dried but not the spray-dried material (in vitro) [13]. Spray-drying process development for glucagon and glucagon–trehalose formulations used residual moisture as one of the off-line product attributes tied to operating conditions (in vitro) [14]. A lyophilized NOTA–single-domain-antibody precursor study tracked cake appearance, glass transition temperature, and residual moisture during formulation development, reported in vitro stability to 12–18 months at 2–8 °C, and compared biodistribution of lyophilized versus non-lyophilized precursor in wild-type and tumour-bearing mice (animal) [15]. Annealing studies on insulin-loaded PLGA nanoparticles reported residual moisture around 1% in all lyophilizates alongside FTIR-assessed insulin structural maintenance (in vitro) [16]. A PEG-precipitation and vacuum-drying process for interferon alpha-2a reported that excess mannitol gave low residual moisture and that stability after three months at 40 °C was comparable to a lyophilized counterpart (in vitro) [17]. Lyophilization of MPEG-hexPLA polymeric micelles reported a residual water content of about 2% by Karl Fischer titration for the sucrose-containing formulation (in vitro) [18]. A microdroplet vacuum-drying method optimised process parameters against residual moisture, protein stability, and long-term storage stability (in vitro) [19], and a chitosan–dipotassium orthophosphate lyophilizate carrying platelet lysate growth factors was characterised with residual moisture below 5% among its critical quality attributes over a six-month stability study (in vitro) [20].