Biomolecules for therapeutic use must be stabilized in their active, native forms right through to administration to the patient and delivery to their site of action. Stability screening of biopharmaceutical formulations at an early stage helps ensure that further investment is focused on those that demonstrate the best potential for further drug development. Differential scanning calorimetry (DSC) is a technique that can be used to rapidly determine optimum solution conditions for protein formulation stability, helping to identify the best formulation candidates and accelerate development. This application note describes Malvern MicroCal DSC technology and its use in formulation development.
An early decision in biopharmaceutical development is choosing whether a biopharmaceutical will be supplied as a liquid, or as a lyophilized (freeze-dried) powder. In general, liquid formulations are less expensive to manufacture and easier to use, but need to be stored under refrigerated conditions and tend to be less stable. Freeze-dried proteins are more costly to manufacture and require dissolution before administration to the patient, but can be stored at room temperature and may provide enhanced stability. Convenience for the end user is also a factor that needs to be considered (1,2). The formulation scientist must determine if the protein can maintain stability in solution for a sufficient period of time, or can only be kept stable in freeze-dried form.
A protein in aqueous solution is in equilibrium between its native (folded) and denatured (unfolded) conformations. Hydrophobic interactions and hydrogen bonding are the major stabilizing forces, and these must be overcome for a protein to unfold and denature. Conformational entropy weakens stabilizing forces, allowing the biopolymer to unfold (3). Proteins unfold upon heating or when denaturing chemicals (such as sodium dodecyl sulfate and guanidine hydrochloride) are added to the solution. Denatured proteins tend to be more susceptible (4-7) to irreversible chemical processes like proteolysis (8), oxidation (9), and deamidation (10), which can lead to inactivation. A denatured protein is also more likely to aggregate, and aggregation can likewise lead to loss of stability and to breakdown of the protein (11-14).
Prior to formulation development, the protein has to be characterized. This may include determining its molecular weight, amino acid composition, three-dimensional structure, presence of disulfide bonds, glycosylation, requirements of cofactors, inhibitors, solubility, thermodynamic parameters, functionality, isoelectic point, hydrophobicity and surface area. All this information is valuable for designing the optimal formulation of the protein. Using a rational drug design approach, bioengineered proteins can also be constructed for maximum stability and greatest efficacy in the solution of choice.
A protein's liquid formulation should be favorable for maintaining stability and bioactivity of the biopolymer during production, packaging, storage and shipping, until the ultimate delivery to the target site in the patient. Parameters to consider during formulation development include protein concentration, presence of additives (excipients), pH, temperature of storage, container, exposure to light, air and humidity.
Another factor in formulation development involves the drug delivery mechanism. For example, an intravenously delivered biopharmaceutical needs to be dilutable, and if the protein is poorly soluble, it can precipitate in the bloodstream of the patient. Also, if the drug is injected, the composition of the formulation should not cause tissue damage or pain to the patient. A further consideration is the potential adsorption of protein onto the container or device surface (syringe, pump, etc.).
Stability of a protein is typically determined using various analytical methods, including accelerated and real-time stability studies. The extent of aggregation/precipitation, oxidation, proteolytic degradation, and/or disulfide bond shuffling is also typically evaluated. Shipping conditions are tested to ensure the drug can be delivered without loss of bioactivity.
DSC and formulation development
Differential scanning calorimetry (DSC) is a microcalorimetry technique that is used to characterize the stability of a biomolecule directly in its native form. It does this by measuring the heat change associated with the molecule's thermal denaturation when heated at a constant rate. Measuring the thermal transition midpoint (Tm) provides a quick and easy indication of stability. The higher the Tm, the more stable the biomolecule.
A DSC instrument has a sample cell containing biomolecule plus buffer, and a reference cell filled with buffer only. Power is supplied to heaters to raise the temperature of the cells at a constant rate. During this temperature increase, the instrument monitors the temperature difference between the sample and reference cells. The difference in heat uptake between the cells required to maintain equal temperatures in both cells determines the apparent excess heat capacity (Figure 1). The temperature midpoint (Tm) of the transition for the enthalpy change occurs when the protein goes from native to denatured form. At the Tm, 50% of the protein is in the native state, and 50% is in the denatured state, assuming a two-state transition (Fig ure1). Some proteins with different regions of activity or more than one structural domain can have more than one Tm . The scientist can focus on one or two Tm values that show the greatest effects due to formulation changes.
Tm is an indicator of thermostability, and in general, the higher the Tm, the more stable the protein. With a higher Tm the protein is less susceptible to unfolding and denaturation at a lower temperature as well. By interrogating various conditions and additives, DSC can determine formulations with the highest Tm that will correspond to the optimal formulation(s) for stability (4,5,7,15,16).
During a chemical process heat is either released (exothermic) or absorbed (endothermic). The transition from native to denatured protein is generally endothermic. The change in enthalpy (∆H) during the conformational transition is measured by integration of the area under the transition (Figure 1). The heat capacity (Cp) of the denatured protein is typically higher than that of the native protein, resulting in a positive ∆Cp for thermal denaturation (Figure 1).
Figure 1: Typical DSC thermogram. This DSC scan was carried out on a dilute protein solution, where the protein undergoes a transition from a compact, native state at low temperature to an unfolded, denatured state at high temperature. The apparent excess heat capacity of the protein was measured, based on the difference in the heat capacity of the protein in buffer, and buffer alone. TheTm, ∆H and ∆Cp of the transition are calculated by fitting the data to a two-states transition model using non-linear least square regression analysis.
The Malvern MicroCal™ VP-Capillary DSC (Figure 2) and Malvern MicroCal VP-DSC systems are used in the study of biopolymers in solution. The Malvern MicroCal VP-Capillary DSC system is specifically designed for Tm screening of multiple formulations at high sample throughput (up to 50 samples in a 24 h period), with a fast scan rate (up to 250°C/h). A fully integrated autosampler allows unattended operation. See Table 1 for a comparison of features of the Malvern MicroCal VP-Capillary DSC and Malvern MicroCal VP-DSC systems.
##Figure 2: Malvern MicroCal VP-Capillary DSC system.
Table 1: Comparison of Malvern MicroCal VP-DSC and Malvern MicroCal VP-Capillary DSC systems
| ||MicroCal VP-DSC||MicroCal VP-Capillary DSC|
|Active cell volume||500 μl||130 μl|
|Typical minimum protein concentrations required||0.02 to 0.1 mg/ml||0.2 to 0.5 mg/ml (Tm)> 1.5 mg/ml (ΔCp and ΔH)|
|Maximum scan rate||90°C/h||250°C/h|
|Temperature range||-10°C to 130°C||-10°C to 130°C|
|Typical time per scan||60 to 150 min||35 to 55 min (depends on scan rate and temperatures)|
|Maximum scans per day||4 to 6 (manual) in 8 h||~ 50 (unattended) in 24 h|
|Automated cell filling and washing||No||Yes|
|Samples per 96 well plate||Not available||48|
Liquid formulation strategies
In formulation development, the main question is to find what solution conditions offer the greatest stabilization of native protein. The conditions that result in the highest Tm typically maintain the protein in its native state for the longest time, at lower temperatures as well. Using DSC, different pH and buffer conditions are screened first, followed by excipients and preservatives.
Buffer and pH optimization
In Figure 3, the Tm of protein CD40L was plotted against pH. The aggregation of CD40L was also determined after incubation at 37°C for 7 days. The Tm optimum correlated with the pH conditions where aggregation was minimized (16). This correlation between pH, Tm, and aggregation was also seen with macrophage colony stimulating factor (4).
Figure 3: Stability behavior of CD40 ligand (CD40L) correlating aggregation response (A) as determined by size exclusion chromatography (SEC), and (B) the Tm determined by DSC as a function of pH. The bracketed area represents the optimal pH range where Tm is maximized and aggregation is minimized. From (16).
Measuring Tm changes of proteins by DSC is a relatively simple process. Figure 4 shows Tm changes of chymotrypsinogen when pH is increased, as measured with the Malvern MicroCal VP-Capillary DSC. The Tm increased with increasing pH, indicating greater stability of the native form of chymotrypsinogen at higher pH.
Figure 4: Tm shift of chymotrypsinogen with pH. Chymotrypsinogen solutions (pH 1.96, 2.27, 2.57, and 3.02) were prepared and added to a 96-well plate. Five samples were used for each pH. Matched reference buffers were also placed in the 96 well plate. DSC scans were performed with Malvern MicroCal VP-Capillary DSC. The DSC data shown here are after buffer-buffer reference scan subtraction. The inset has the Tm data for each pH, and standard deviation.
Excipients are additives that can improve the stability of the protein. They include sugars, amino acids, antioxidants, polymers, alcohols, glycerol, and surfactants.
Once the optimum pH and buffer are determined, different excipients are added to the protein solution. If an excipient increases the Tm, the native form of the protein is more stable with the excipient than without it.
Excipient screening was used during the liquid formulation development of interleukin-1 receptor, IL-1R (5). There were two major transitions for IL-1R, one with Tm near 48°C and the other near 66°C. The transition with the Tm at the lower temperature was chosen to screen for excipients. The strategy was to look for excipients that would raise the Tm of the low temperature transition, indicating a positive change in native protein stability. Twenty-three excipients were screened (Table 2).
Table 2: Screening of excipients added to interleukin-1 receptor
|Excipient||Concentration (g/ml) in buffer||Molar ratio [Ms/Mp]*||Tm (°C)|
|PVP (MW 10 000)||0.01||7||48.9|
|PEG (MW 300)||0.0003||7||49.4|
|PEG (MW 1000)||0.001||7||49.1|
|PEG (MW 3350)||0.00335||7||48.7|
*Ms = moles of excipient / Mp = moles of protein
†Control buffer is 20 mM citrate buffer, pH 6.0. Excipients added to this buffer.
Table from Remmele, R.L. et al. (5).
The ionic strength of the buffer is adjusted to determine if an increase in Tm can be achieved by adding salt. For IL-1R, the addition of 100 mM NaCl had the greatest stabilizing influence, at modest ionic strength shifting the Tm from 48°C to 53°C. This stabilizing effect suggested a direct interaction between salt ions and charged groups of the protein (5). The Tm of IL-1R continues to increase with increasing salt, even when NaCl is 1500 mM, well above the concentration needed to saturate all the charged sites (Figure 5). These data suggest that salt ions affect water structure, which also plays a role in protein conformational stability. Both charge-charge interactions, and changes in water structure, add stability to native IL-1R structure.
Figure 5: Plot showing Tm of IL-1R with addition of NaCl. The 100 mM concentration is shown by the dashed line. From (5).
When a drug is supplied in a multidose format, preservatives are added to help prevent microbial growth. However, these preservatives may have a destabilizing effect on the protein. The effect of preservatives on the Tm of IL-1R was also examined (5). The preservatives meta-cresol, phenol, and benzyl alcohol destabilized IL-1R, based on the shift of both temperature transitions to lower temperatures (Table 3). The DSC data determined the order of the stability of IL-1R in the three preservatives - phenol produced the highest Tm, followed by meta-cresol and benzyl alcohol. The DSC stability data also correlated with aggregation of IL-1R, as measured by size exclusion chromatography (SEC). The higher the Tm, the less aggregation was observed after seven and 60 days.
Table 3: Effects of preservatives on IL-1R: Comparison of Tm and size exclusion chromatography
|7 days||60 days|
|Tm1 (°C)||Tm2 (°C)||Tm3 (°C)||Agg %||Native %||Agg %||Native %|
|0.9% Benzyl alcohol||45.2||48.5||63.6||2.93||96.92||16.46||83.09|
|Control solution: 20 mM sodium citrate buffer, pH 6.0, 100 mM NaCl. For size exclusion chromatography (SEC), IL-1R solution stored at 37°C for indicated time prior to chromatography. Agg % = % aggregation, determined by integration of high molecular weight protein peak after SEC; native % = % native IL-1R, determined by integration of main protein peak after SEC. From (5).|
The best formulation candidates, based on Tm screening, are then evaluated by accelerated stability studies. The protein is prepared in different formulations, then stored at 37°C. The amount of aggregation is determined by size exclusion chromatography at intervals during the accelerated stability study, and the protein is analyzed using SDS-PAGE to check for proteolysis.
Finally, the best formulation candidates must undergo real-time stability studies to determine the shelf life of the protein. Bioassays and analytical tests are performed throughout the study to ensure the protein remains active and viable.
DSC has been used as a screening tool to assess protein stability during formulation development for many years, and the use of the automated Malvern MicroCal VP-Capillary DSC allows faster screening with increased throughput as compared to the Malvern MicroCal VP-DSC (Table 1). Malvern MicroCal VP-Capillary DSC has been used to determine the Tm of proteins in different pHs and excipients, and the changes in the Tm correlated to the SEC data, supporting the use of DSC as a stability screening tool (17).
DSC data is useful in predicting the stability of a protein in solution. The Tm indicates thermostability, and Tm determination in different formulations is an approximate measure of the susceptibility to aggregation and other irreversible changes at lower temperatures. Formulations with the best thermostabilities are chosen for further stability, shelf life, and shipping studies. The use of DSC can save time and money in formulation development, and fully automated systems enable improved efficiency and productivity in this critical area of drug development..
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