Potency Test Failure Case Studies

Potency Test Failure Case Studies
Berenice Dethier, Research Chemist II

A potency test measures the amount of API in a formulation to ensure the product is within its specifications. Criteria for acceptance are set by the client and/or follow the industry benchmarks (generally set by USP).

Several factors can contribute to a failing potency result.  Investigating the root cause of the failing potency test is an important practice to characterize the true cause(s) of the failure. The goal of the out of specification (OOS) investigation is to identify what, how, and why the failure occurred. To answer all three questions, OOS investigations should include a comprehensive review of all aspects of the formulation procedure. The review typically includes everything between the acceptability of the raw materials to the validity of the potency test result. Understanding the factors that contributed to the potency failure allows the pharmacist a more strategic approach in eliminating future failures. The more specific the findings into the failure, the easier it is to properly address.

The OOS investigation first seeks to confirm the validity of the failing test result. This portion of the investigation includes a thorough review of the reference standard and sample preparations, the raw test data, the laboratory notebook write-up, test method trending, the analysts who performed the testing, and in some cases retesting of the sample to confirm the initial failing results. Once a failing potency result is confirmed, the investigation can then focus on the actual formulation process.

       1. Miscalculation in the formulation

Failure to consider the molecular weight conversion or calculation mistakes are typical causes for differences between the label concentration and the measured concentration. These causes for OOS results are identified by reviewing the formulation sheets with the client. Examples of situations prone to mistakes are:

  • Drugs compounded as free base vs. salt form
  • Drugs containing waters of hydration
  • Drugs absorbing water

For more information on how to perform calculations under these conditions, see the article about Drug Formulation.

Calculation issues can also arise when a formulation is scaled up or down. Proper training and careful review of the pharmacist’s calculations are keys to eliminate these errors.

       2. Improper or incomplete labeling

OOS results have been encountered when samples were labelled incompletely. Cases we have come across at ARL Bio Pharma include Amino Acids: the label indicated Lysine, the samples were out of specifications, but after looking at the formulation sheet we discovered that the concentration was calculated for Lysine HCl, and the samples were passing with the appropriate calculations. Other common situations are Fentanyl compounded from Fentanyl Citrate and Morphine Sulfate from Morphine Sulfate Pentahydrate. In these cases, we follow USP guidelines for reporting and OOS samples are rare.

Other problematic situations include labels stating a percentage with no other qualifiers, which is ambiguous: is it volume per volume, weight per volume or weight per weight concentration? This is typical for creams/ointments and for liquid active ingredients such as antimicrobial agents. Ointments are sampled by weight and solutions by volume. Consistency between how the sample amount is measured and how the concentration is expressed greatly decreases approximations on density and calculation mistakes, and avoids OOS results.

When the active ingredient is added to IV bags, the concentration should be expressed per bag rather than per volume to avoid OOS results, since adding solution increased the fill volume of the bag. For example, a sample labeled as “Vancomycin 1 mg/mL” can be prepared by adding 10 mL of 100 mg/mL Vancomycin to a 1000 mL bag, which may have a 10% overfill.  This results in an actual concentration of 0.9009 mg/mL.  In such a case the potency is more appropriately expressed as 1 g of Vancomycin per bag where the fill volume of the bag has been determined.

The labels should be self-explanatory and analysts should not need the formulation records to understand what they are working with.

       3. Improper compounding procedure

Oversights in compounding can affect potency at multiple levels. A trained pharmacist and a thorough Quality System should eliminate most sources of error.

A regular cause for potency failures stems from the mixing of triturated powder blends. ARL Bio Pharma often performs potency testing on triturated powder blends at 0.1% w/w or lower. At these concentrations, achieving a homogenous blending can be challenging. Out of Specification results are usually investigated hand in hand with the client (after intra-laboratory investigation has ruled out sample preparation or method problems). Often additional samples or aliquots are tested. If the sample results are sometimes high and sometimes low, we can conclude there is a lack of uniformity in the blend. Content uniformity also affects suspension formulations and creams/ointments should phase separation occur over time.  In other instances, the results are consistent but sub-potent indicating loss of API during the compounding process (for example issues with static electricity causing binding of the API to equipment).

More cases where compounding was the culprit in the OOS investigation include:

  • Container closure issues (example: an Alprostadil solution in Alcohol improperly closed, causing the samples to be super potent when the concentration increased upon solvent evaporation)
  • Improper handling or storage of drug causing the API concentration to be low (exposure to moisture, light, etc.)
  • Improper choice of filters: non-specific binding can occur, we recommend to test potency before and after filtration during formulation development

Successful potency out of specification (OOS) investigations identify why the sample did not pass. These investigations often require collaboration between the testing laboratory and the compounding pharmacy. If the two groups do not work together, assumptions are made and a root cause is typically not identified, resulting in repeated OOS test results. ARL Bio Pharma has years of experience in this collaborative approach and is happy to assist in troubleshooting complicated compounding procedures, upfront or following potency failures.