Peptide Compounding Potency Challenges: Key Considerations for Pharmacies
Recent regulatory shifts have put peptide compounding under increased scrutiny, while also prompting compounders to consider how these formulations should be approached from a quality and formulation perspective. Our recent review of the July 2026 FDA PCAC meeting examines what these developments could mean for compounders. But regulatory pathways are only the beginning of the discussion. Once a peptide is being considered for compounding, questions around formulation, potency, materials, processing, storage, and testing become equally important.
Many compounders approach peptide compounding as a variation of small-molecule compounding they have performed for years. Then a batch that passed potency testing at release begins to decline (or mysteriously increase) during stability testing. In other cases, a formulation that performed well during development shows variable concentrations during routine production.
Small molecule and peptide potency failures can feel frustrating because they often appear unpredictable. The first response is tempting to blame the testing laboratory. While this can be part of the problem in some cases, potency failures rarely have only one possible cause. The issues may also originate from the active ingredient, formulation design, processing conditions, packaging components, storage environment, and testing approach.
Rather than searching for who is at fault, a more useful approach is to ask when the measurable intact drug began to change. This requires looking at the entire system to find the answers.
Peptides have sequence-dependent susceptibility to certain reactions that warrant thoughtful formulation and process design. Peptide compounding (as with non-peptide compounding) can be successful when good pharmaceutical and compounding science are applied to the specific drug being prepared.
Small Molecules and Peptides: Shared Potency Framework
Although the focus of the framework we outline here are peptides, it also applies more broadly to small molecule drugs (such as epinephrine, lidocaine, testosterone, methylcobalamin, and others). The potency issues that may emerge are similar, and they can be investigated and addressed using a similar framework.
In addition, potency failure can apply to both a decrease and an unexpected increase in concentration. While out-of-specification potency may more commonly involve potency decreases, rare potency increases are also possible. Both scenarios require the same type of careful investigation.
Investigating Potency Requires Looking at the Whole System
One of the most important lessons in investigating peptide and small molecule potency challenges is that the final result reflects everything that happened before testing.
A potency failure may originate from:
Incorrect assumptions about the bulk drug substance, leading to incorrect starting potency calculations
Interactions with materials the bulk substance contacts, including surface binding
Chemical degradation driven by formulation conditions, excipients, packaging, or environmental exposure
Physical instability influenced by numerous factors, from the characteristics of the bulk substance and its concentration, to shipping, storage, and formulation specifics
Insufficiently controlled or understood compounding process
Interactions with the container closure system during storage and distribution
Incomplete or inappropriate analytical methods
The challenge is identifying where and why the change in concentration occurred.
A systematic root-cause approach therefore evaluates the full pathway:
Starting material → Formulation → Processing → Packaging → Storage → Sampling → Testing
How to Investigate Small Molecule & Peptide Potency Failures
1. Verify the Bulk Drug Substance
Potency problems can begin before compounding even starts. One of the first questions to ask is whether the intended amount of active drug was actually introduced into the batch.
An appropriately sourced, high-quality bulk drug substance is essential, but the word “purity” should not be used as a substitute for full characterization. A high purity number alone does not tell the complete story. A material reported as 99% pure by chromatography is not necessarily equivalent to 99% active drug by gross weight.
For a serious investigation, the pharmacy may consider verifying identity, chemical form, assay basis, water content, counterion or salt content, relevant impurities, molecular-weight assumptions, reference-standard basis, lot-to-lot differences, and the exact calculation used to determine the weighed quantity.
2. Compare Samples from Meaningful Process Points
The investigation should then compare samples from meaningful process points, such as the initial bulk solution, the solution after mixing, the post-filtration solution, the beginning of the filling run, the middle of the run, the end of the run, and retained finished units.
It may be necessary to evaluate more than one attribute. Depending on the suspected mechanism, that could include assay, related substances, pH, appearance, visible and subvisible particles, aggregation, fill volume, filter recovery, mass balance, and confirmatory LC-MS testing.
3. Examine the Variables Most Likely to Reproduce Failure
It is often useful to challenge the variables most likely to reproduce the failure. Those could include temperature excursions, light, agitation, freeze-thaw exposure, extended bulk hold time, alternative container closure systems, different filters, different tubing materials, and different concentrations.
4. Independently Evaluate the Analytical Process
The analytical process should be evaluated independently, including reference-standard assignment, sample preparation, sample-container recovery, dilution stability, filter compatibility, autosampler stability, integration, and method specificity.
The Foundation of Compounding Remains Good Pharmaceutical Practice
The foundation of compounding remains good pharmaceutical practice: know the material, understand the formulation, control the process, protect the preparation, and use a suitable test method. Small molecule drugs and peptides are no exception.
While peptides may present specific considerations, the core principles of quality compounding remain the same. Every compounded preparation depends on understanding the active ingredient, selecting appropriate formulation components, controlling critical process variables, ensuring compatibility with packaging and materials, and using analytical methods that accurately evaluate the final product.
The goal is not to treat peptides as exceptional in compounding. It is to apply sound pharmaceutical science to understand what each specific preparation requires to perform consistently.
Uncover the Hidden Culprits in Peptide Compounding
Maintaining the potency of peptides and small molecule drugs requires more than creating a formula that works once. It requires understanding the factors that influence performance throughout development, production, storage, and testing.
For compounders looking to examine the hidden causes behind peptide potency challenges in more detail, download our white paper, The Hidden Culprits in Peptide Compounding. Concepts described in the paper can also be applied to non-peptide potency trouble-shooting.
Disclaimer: This article is intended to provide general information on U.S. compounding. It should not be construed as legal, regulatory, or medical advice. Readers are encouraged to consult an attorney for guidance specific to their circumstances.