Stability-indicating Assays

Mark G. Klang, MS, RPh, BCNSP, PhD
Core Manager, Research Pharmacy
Memorial Sloan Kettering Cancer Center
New York, New York

Pharmacists will turn to published stability studies or conduct a new analysis to determine the shelf life of a medication and/or a compounded preparation. The goal of a stability study is to use sufficient stress that would be predictive of storage conditions through the life of the preparation. To evaluate a compounded medication for stability, it is important to define what parameters have the potential for change. What defines instability varies for each active pharmaceutical ingredient (API), excipient, concentration, formulation, container, and storage conditions.

Stability of Compounded Injectable Medications
The container material is very important in maintaining the stability of a compounded injectable medication. Injectables are supplied in glass ampules and vials and then repackaged into final containers for patient administration. A syringe is used when the drug is administered directly to the patient. Example issues are:

• Polyvinyl chloride, which contains a plasticizer, can be eluted when combined with lipophilic formulations.^1,2
• Polyolefin allows for a high rate of oxygen transport, which will contribute to higher instability in a drug sensitive to air exposure.³
• The syringe hub is coated with a silicone lubricant, which can react with proteins to cause aggregation.^4,5

Stability of Generic Products
Another issue in compounding is when the APIs are obtained from a finished generic product. Generic formulations are intended to approximate the bioavailability of a trade name product. The excipients used by a generic manufacturer are not the same as the trade name version or even other generics. The formulation, shelf life, and extended stability determined for one manufacturer may not apply to the other, requiring additional stability-indicating assays for compounded medications which use a different source than the original study.

FDA Requirements for Stability-indicating Assays
The FDA requests that a stability-indicating assay be used to evaluate the shelf life.⁶

Stability studies should include testing of those attributes of the drug substance that are susceptible to change during storage and are likely to influence quality, safety, and/or efficacy. The testing should cover, as appropriate, the physical, chemical, biological, and microbiological attributes. Validated stability-indicating analytical procedures should be applied. Whether and to what extent replication should be performed should depend on the results from validation studies.

This stability-indicating assay must be done in a manner so that the separation of the degradants does not interfere with the analysis of the API. The challenge is determining the extent of drug destruction, methods to achieve that end, and ensuring there is no overlap of these impurities with the original drug product.

There is no definition of what methods are acceptable or what extent of instability is intended by this guideline.

United States Pharmacopeia Standards
The United States Pharmacopeia (USP) provides detailed instructions with each pharmaceutical monograph on the procedure to identify the component listed. While these assays may be used to identify the isolated pharmaceutical entity, there is no validation stipulated in the text with regard to shelf life, breakdown components, formulation, or methods to evaluate stability.

USP Chapter <1191> Stability Considerations in Dispensing Practice states:

Stability is defined as the extent to which a product retains, within specified limits, and throughout its period of storage and use (i.e., its shelf-life), the same properties and characteristics that it possessed at the time of its manufacture.

This chapter goes further to state:

Because of potential unobservable problems with respect to sterility and chemical stability, all extemporaneous parenteral preparations should be used within 24 hours unless data are available to support longer storage.

A limitation of Chapter <1191> is that it considers that only manufacturers need to assess or evaluate the chemical stability of a product or combination.

Degradation
To create a stability-indicating assay, it is necessary to degrade the original substance. The recommended degradation is between 5% and 20%. For forced degradation studies, the International Conference on Harmonization QIB recommends exposure to acid HCl (0.1-1.0N), base NaOH (0.1-1.0N), H₂O₂, (3% to 35%), light (both UV and Visible), and heat (both dry and wet), all conducted over time.⁷ A critical review identified that few of the published studies stipulating "stability-indicating assay" in their title complied with all instability assays in their methods.⁸

The destruction should elicit changes in the original active ingredient and generate a new entity that can be measured. The most common method of stability assessment is reverse phase high-performance liquid chromatography (HPLC), with photo-diode array for UV detection. This method requires that the degradant exhibits π bonds, which can be detected by UV light. If all bonds become saturated during stability testing, the degradants will be invisible to this type of analysis. A more difficult outcome of stability analysis is when a degradant exhibits only a slight change from the parent compound and delivers a new result in the analysis that overlaps or is obscured by the results of the original ingredients. This type of complication requires an adjustment of the methodology to ensure separate identification of each degradant and original ingredients.

Mass spectroscopy allows for better identification of the breakdown products, but UV offers a wider range of mobile phase and column choices to scan for all possible degradants.

Conclusion
Stability assessment is challenging, as it requires evaluation and prediction of breakdown products as part of the analysis of the parent product.

References

1. Green R, Hauser R, Calafat AM et al. Use of di(2-ethylhexyl) phthalate-containing medical products and urinary levels of mono(2-ethylhexyl) phthalate in neonatal intensive care unit infants. Environ Health Perspect 2005; 113(9): 1222-1225.
2. Pearson SD, Trissel LA. Leaching of diethylhexyl phthalate from polyvinyl chloride containers by selected drugs and formulation components. Am J Hosp Pharm 1993; 50(7): 1405-1409.
3. Ezuki S, Kanno T, Ohto H et al. Survival and recovery of apheresis platelets stored in a polyolefin container with high oxygen permeability. Vox Sang 2008; 94(4): 292-298.
4. Sendo T, Otsubo K, Hisazumi A et al. Particle contamination in contrast media induced by disposable syringes. J Pharm Sci 1995; 84(12): 1490-1491.
5. Cohen S, Prieto N, Hunter CH. A foreign body in propofol syringe. Anesth Analg 2006; 103(3): 784.
6. U.S. Department of Health and Human Services. U.S. Food and Drug Administration. Guideline for Submitting Documentation for the Stability of Human Drugs and Biologics. [FDA Website.] 1987. Available at: www.fda.gov. Accessed May 5, 2014.
7. Alsante KM, Ando A, Brown R et al. The role of degradant profiling in active pharmaceutical ingredients and drug products. Adv Drug Deliv Rev 2007; 59(1): 29-37.
8. Bakshi M, Singh S. Development of validated stability-indicating assay methods: Critical review. J Pharm Biomed Anal 2002; 28(6): 1011-1040.

Become a fan of the IJPC Facebook page and share ideas, photos, and keep up to date with the latest compounding information - http://www.facebook.com/IJPCompounding

Learn about the Journal's new multi-media features and view our growing collection of educational and training videos at www.ijpc.com/videos or by subscribing to our Youtube channel at https://www.youtube.com/channel/UCT_3mOKDpffXaIUjB8iejSw.