Cagrilintide Solution Stability: Time-Course Degradation Studies in Lab Settings

 

Introduction:

Cagrilintide represents a significant advancement in long-acting amylin analogue therapeutics, engineered with an N-terminal C20 fatty-acid moiety that enables albumin binding and extends its half-life to 159–198 hours, supporting once-weekly dosing. This lipidation strategy, while pharmacologically advantageous, introduces unique stability challenges in aqueous solution. The hydrophobic fatty-acid chain increases the peptide's propensity for self-association, while the amyloidogenic nature of the amylin backbone creates risks for fibril formation that must be controlled throughout the product's shelf life. Time-course degradation studies conducted in laboratory settings provide the essential data needed to understand these risks and define formulation parameters that ensure physical and chemical stability.

The Core Stability Challenge: Amyloid Fibril Formation

The most extensively studied degradation pathway for cagrilintide in solution is the formation of amyloid fibrils. Cagrilintide is an analogue of human amylin, a hormone known for its ability to form fibrillar aggregates. In the context of injectable peptide pharmaceuticals, fibril formation is particularly concerning because such aggregates can provoke immunogenic reactions and create unpredictable pharmacokinetic profiles. The development programme for cagrilintide explicitly prioritised analogues with low fibrillation tendency, selecting a pramlintide-like backbone specifically to reduce this risk compared to native human amylin.

Laboratory time-course studies have employed Thioflavin T (ThT) fluorescence as a primary analytical tool for monitoring fibril formation. ThT exhibits enhanced fluorescence upon binding to amyloid fibrils, providing a quantitative measure of fibrillation over time under stressed storage conditions. These studies typically subject formulations to elevated temperatures or other stress conditions that accelerate degradation, allowing prediction of longer-term stability at recommended storage temperatures.

Buffer Selection: A Critical Determinant of Physical Stability

Time-course studies comparing different buffer systems have revealed that the choice of buffer has a profound effect on cagrilintide's physical stability. A systematic investigation at pH 4.0 compared glutamate, lactate, acetate, and benzoate buffers, each at 5 mM concentration, over a 28-day stressed storage period.

The results demonstrated a clear hierarchy of performance. The glutamate-buffered formulation maintained ThT fluorescence levels essentially unchanged from baseline throughout the 28-day study, indicating no or almost no fibril formation. Lactate-buffered formulations showed a modest increase, reaching approximately twice the baseline level by day 28. Acetate-buffered formulations showed a bigger increase, reaching four times the baseline by day 28. The increase was especially fast after day 21. Benzoate proved least suitable, with rapid and substantial fibril formation beginning as early as day 11 and reaching four times baseline by day 14, necessitating discontinuation of this experimental arm after day 21.

These findings led to the conclusion that 5 mM glutamate surprisingly displays a clear advantage over the other buffers tested for maintaining cagrilintide physical stability under stressed conditions. The buffer concentration is also constrained by practical considerations: it must be sufficient to maintain pH during storage but low enough to avoid compromising the stability of semaglutide when the two formulations are co-administered from a dual-chamber device.

pH and Formulation Design Considerations

The formulation pH of 3.5–4.5, with approximately 4.0 being preferred, was selected based on the chemical stability profile and the need to avoid crossing the isoelectric point during injection. Cagrilintide has a neutral isoelectric point, and its solubility is very low in the pH 6.0–8.0 range. At the injection site, as the formulation equilibrates toward physiological pH, there is a risk that the peptide could precipitate if solubility is insufficient. This concern was validated by early preclinical observations: at high subcutaneous doses in rats, severe injection site reactions with fibrillar precipitate were observed when the peptide was formulated at a pH that allowed the isoelectric point to be crossed.

The design strategy therefore focused on identifying analogues with improved solubility across the pH 4.0–7.4 range and low fibrillation tendency. The formulation at pH 4.0, combined with an appropriate buffer system, maintains cagrilintide in a soluble, non-fibrillar state during storage and provides sufficient solubility to avoid precipitation as the pH rises after injection.

Chemical Degradation Pathways

Cagrilintide is not only prone to physical aggregation, but it can also break down chemically. Analytical studies using trypsin digestion followed by LC-MS have identified deamidation as a detectable chemical modification pathway. Forced degradation at 50°C confirmed the identity of singly deamidated species and provided insights into additional modifications, with deamidation sites localised to specific tryptic fragments of cagrilintide. This bottom-up analytical approach, achieving >99.5% digestion efficiency, overcomes the challenges of analysing intact lipopeptides where the hydrophobic fatty-acid moiety complicates reversed-phase separation and impurity profiling.

The stability of the fatty-acid modification itself is a consideration. The γ-glutamic acid linker forms a non-peptide amide bond between the peptide and the C20 fatty acid, which is resistant to peptidase cleavage. This design feature ensures that the half-life extension mechanism remains intact during circulation and storage.

Practical Storage Implications

Time-course degradation data inform practical storage recommendations for research settings. Lyophilised cagrilintide powder is stable for at least one year at -20°C, with some sources indicating up to three years under appropriate conditions. Once reconstituted, however, solution stability is markedly reduced. In solvent, recommended storage at -80°C is limited to six months, while at -20°C the maximum recommended period is one month. Aliquoting into single-use volumes is essential to avoid repeated freeze-thaw cycles, which can accelerate both aggregation and chemical degradation.

For laboratory work with cagrilintide solutions, the time-course data support several practical recommendations: use a glutamate-based buffer at pH 4.0 where possible, store solutions at -80°C for extended periods, minimise time at ambient temperature, and monitor for signs of aggregation or precipitation, particularly if the solution's pH is inadvertently allowed to drift toward neutrality.

Conclusion

Time-course degradation studies have established that cagrilintide solution stability is governed primarily by buffer identity and pH. Glutamate buffer at pH 4.0 provides superior protection against amyloid fibril formation compared to lactate, acetate, or benzoate, with the difference becoming pronounced under stressed storage conditions. The pH of 4.0 maintains peptide solubility and avoids the isoelectric point range where aggregation risk is highest. While chemical degradation pathways such as deamidation exist, the dominant stability concern for cagrilintide solutions is physical aggregation, making buffer selection and pH control the most critical formulation parameters for laboratory and pharmaceutical applications alike.