Tony Cundell, Ph.D. - Microbiological Consulting, LLC, Rye, New York
Introduction
In the absence of definitive guidance on microbial testing in R&D and Marketed Product Stability Programs for Drug Substances and Drug Products, the pharmaceutical industry makes inconsistent and sometimes misguided choices with respect to microbiology. These choices are further confounded by the fact that stability programs are almost always managed by analytical chemists or pharmacists without training and limited experience in the field of microbiology.
This article will review the current regulatory guidance and provide recommendations on the role of microbial testing as stability test parameters, and when other physicochemical tests will be preferred to microbiological tests. To be comprehensive, a product life cycle approach will be taken for drug substances and both sterile and non-sterile drug products.
Guidance Documents
Stability testing to establish the appropriate storage conditions and expiration dating are good manufacturing practice requirements as described in 21 CFR 211.137 Expiration Dating and 211.116 Stability Testing. The regulatory expectations are that each strength of a drug product and each primary package and closure system in which it is distributed throughout its shelf life must be supported by stability testing.
The content of the FDA good manufacturing practices is reproduced as follows:
21 CFR 211.137 Expiration dating
- To assure that a drug product meets applicable standards of identity, strength, quality, and purity at the time of use, it shall bear an expiration date determined by appropriate stability testing described in 211.166.
- Expiration dates shall be related to any storage conditions stated on the labeling, as determined by stability studies described in 211.166.
- If the drug product is to be reconstituted at the time of dispensing, its labeling shall bear expiration information for both the reconstituted and un-reconstituted drug products.
- Expiration dates shall appear on labeling in accordance with the requirements of 201.17 of this chapter.
21 CFR 211.166 Stability testing
a. There shall be a written testing program designed to assess the stability characteristics of drug products. The results of such stability testing shall be used in determining appropriate storage conditions and expiration dates. The written program shall be followed and shall include:
- Sample size and test intervals based on statistical criteria for each attribute examined to assure valid estimates of stability.
- Storage conditions for samples retained for testing.
- Reliable, meaningful, and specific test methods.
- Testing of the drug product in the same container[1]closure system as that in which the drug product is marketed.
- Testing of drug products for re- constitution at the time of dispensing (as directed in the labeling) as well as after they are reconstituted.
FDA Guidance for Industry provide information on how the U.S. regulation may be met including FDA Guideline for Submitting Documentation for the Stability of Human Drugs and Biologics, 1987; FDA Guidance for Industry Chemistry, Manufacturing, and Control (CMC) Information for Human Gene Therapy Investigational New Drug Applications, January 2020, FDA Guidance for Industry Container-Closure Systems for Packaging Human Drugs and Biologics, July 1999, and FDA Guidance For Industry Container and Closure System Integrity Testing in Lieu of Sterility Testing as a Component of the Stability Protocol for Sterile Products, February 2008.
The keystone guidance document is the International Conference on Harmonization (ICH), Harmonized Tripartite Guideline Q1A (R2) Stability Testing of New Drug Substances and Products while supporting documents include ICH Q5C Quality of Biotechnological Products: Stability Testing of Biotechnological/Biological Products and ICH Q6A Specifications: Test Procedures and Criteria of New Drug Substances, and Product.
Other useful ICH, WHO, and regional guidance documents include ICH Q1B Photo-stability Testing of New Active Substances and Medicinal Products; ICH Q1C Stability Testing: Requirements for New Dosage Forms; ICH Q1D Bracketing and Matrixing Designs for Stability Testing of Drug Substances and Drug Products; ICH Q1E Evaluation of Stability Data; World Health Organization (WHO) Technical Report Series, No. 863, 1996 Annex 5 Guidelines for stability testing of pharmaceutical products containing well-established drug substances in conventional dosage forms and Association of South East Asian Nations (ASEAN) Guideline on Stability Study of Drug Products.
Given the importance of water activity in the microbial stability of non-sterile drug products, and container-closure integrity for sterile products, the author would direct the reader to USP <922> Water Activity, <1112> Application of Water Activity Determination to Nonsterile Pharmaceutical Products, and USP<1207> Package Integrity Evaluation—Sterile Products.
USP <1112> Table 1 lists water activity (Aw) values required to support the growth of microorganism species. If water activity falls below that value, microbial growth is suppressed.2-4 USP and its companion chapters provides comprehensive guidance on the best application of container-closure integrity tests (CCIT) based on head space composition. CCIT is preferred to sterility and bacterial endotoxins tests as the microbiological stability does not change in a sterile product unless the container integrity is loss.
Next the author will review stability issues associated with drug substances, product development, and marketed products.
Drug Substances
Drug substances used to make drug products are diverse and range from synthesized chemical entities to mammalian cell-derived products to processed animal or plant-derived materials. The bioburden of these drug substances will reflect their origin and manufacturing process. For example, a synthesized chemical entity typically is recovered from organic solvents, crystallized, dried, and milled resulting in a low bioburden material, whereas animal or plant-derived materials may be potentially contaminated by fecal pathogens depending on the degree of processing. A chemical drug substance will typically have a low moisture content and if stored in lined fiber or plastic drums that protect the drug substance from elevated temperatures and humidity will have excellent microbiological stability as a result of its low water activity. Drug substances used for the manufacture of sterile drug products will require a low bacterial endotoxin content and if aseptically assembled must be sterile. Drug substances for the manufacture of non-sterile drug products must meet the absence of specified and objectionable microorganism requirements for the proposed dosage form, which is not usually included in compendial monographs. Although they do not have expiration dates, drug substances have re-certification dates at which they are re-tested against their specifications. Open-dish, accelerated, and long-term stability studies are conducted with drug substances to support product development.
Product Development
During product development, the formulation, product attributes, manufacturing processes, and packaging of the drug product are established. Accelerated and long-term stability studies are conducted to support successful product development. In early development, analytical methods may be qualified as fit for use, but may not be fully validated, and methods will be broader in scope than the eventual product specifications to fully characterize a product and investigate it shelf-life stability. For multiple-use aqueous drug products the antimicrobial preservative effectiveness of the product must be addressed during formulation development. After the first lot of production-scale product successfully passes preservative effectiveness testing at expiry revert to chemical assay as the test parameter, in place of the microbial test.
Microbiological stability will be promoted by a reduced water activity of the drug product and the protection from humidity of the container-closure system. For example, compressed tablets and liquid-filled and powder-filled capsules with a low water activity would not require microbial testing on stability.
Marketed Products
Drug products will have expiration dating that reflect their labelled storage conditions and primary packaging that are supported by long-term stability studies. Typically, one batch in each packaging style is entered into the stability program each year. The stability testing parameters often are a smaller subset than the release tests, which may not all be stability indicating. For example, content uniformity and absence of specified microorganisms which are release criteria would not be included in a stability study. Low water activity dosage forms like compressed tablets and powder-filled capsules protected from moisture by their packaging will have microbial stability hence would not be subject to microbial testing. Likewise, sterility and
bacterial endotoxin content in a sterile drug product protected by the packaging would not reflect the stability of the product and container-closure integrity may be substituted for these two tests.1,5
Critical Quality Attributes of Drug Products
The author recommends that the critical quality attributes with respect to microbiology be established for each dosage form and the appropriate stability test parameters selected from them. It should be re-emphasized that some release tests may not be considered stability indicating and may not be a stability test parameter. For example, with biotechnology products like monoclonal antibodies, identity, peptide mapping and sequencing, content uniformity, residual DNA, mycoplasma content, bacterial endotoxin, and sterility would not be stability tests.
The first division for discussion would be between non-sterile and sterile drug products.
Non-Sterile Drug Products
- Microbial enumeration and absence of specified microorganisms (Initial time point only)
- Antimicrobial Effectiveness (Aqueous, multiple-dose products only)
- Water Activity
- Container Closure Integrity
Sterile Drug Products
- Sterility
- Bacterial Endotoxins
- Antimicrobial Effectiveness (Aqueous, multiple-dose products only)
- Water Activity
- Headspace Composition
- Container Closure Integrity
- Reconstitution and Storage Studies
Tables 4 through 8 provide broad guidance for stability study protocols for a drug substance and different pharmaceutical dosage forms.
ICH Stability Storage Conditions and Time Intervals
In designing your stability protocols, it is important to select the stability temperature and humidity storage conditions and time intervals for testing. For the latter, for accelerated studies samples are pulled for testing at 1, 3, and 6 months whereas for long term studies samples are pulled at 3, 6, 9, 12, 18, 24 and 36 months. Release testing may be used as the zero-time point provide the samples are placed in the stability chamber promptly, so that the 36-month time point coincides with the projected expiration date.
USP <1112> Table 1 lists the water activity values required to support the growth of microorganism species. If water activity falls below that value, microbial growth is suppressed. Microorganisms likely to be found in pharmaceutical drug products will not grow at Aw less than 0.75, furthermore the compendial methods found in USP <61> and <62> will not detect halophilic bacteria, osmophilic yeast, and xerophilic molds due to the high-water activity of the microbiological culture media.
Role of Water Activity in Moisture Transmission of Blister Packaging
Tablets and capsules may be stored in blisters for the convenience of the patient especially when they are out of the house but the level of protection from humidity will vary by material of construction.
Moisture ingress of formed and sealed blisters (PVC, PVDC, Aclar Ultrx 2000 and cold form foil) when stored at 23°C/75% and 40°C/75% RH may be measured by weight gain or more functionally as an increase in water activity of the dosage form. USP <671> Containers – Performance Testing uses water-filled blister packs instead of desiccant-filled ones, which has been the standard test method since the 1970s, but the author believes that water activity determination of the stored drug products is a better choice in a stability program.
Conclusions
As marketed product stability protocols are binding regulatory commitments, companies should get them right and if necessary, discuss them with the national boards of health in advance of their finalization. With many dosage forms, microbial enumeration, bacterial endotoxin assay, and sterility testing are not stability[1]indicating tests and should be replaced by the more useful water activity determination (no-sterile products) and container-closure integrity testing (sterile products).
References
- Booth, C. 2016 Understanding Container Closure Integrity Testing Amer. Pharm. Review September 2016
- Cundell, A. M. 2009 Effects of water activity on microorganisms. In Water Activity Applications in the Pharmaceutical Industry A. M. Cundell and A. J. Fontana, Jr (editors) pp 175-204 PDA/DHI
- Showransky, L. 2011 Inhibition of microbial growth in solid dosages at ICH stability storage conditions. Euro. Pharm. Rev. Issue 4
- Cundell, T. 2015 The role of water activity in the microbial stability of non-sterile pharmaceutical drug products Euro. Pharm. Rev. 10 March Issue 1
- White, Edward. K., 2012 Container-Closure Integrity - The Aseptic Core. J. Valid. Technol. http://www.ivtnetwork.com/sites/default/fi les/IVTJVT0512_ Aseptic_FInal.pdf
Publication Detail
This article appeared in American Pharmaceutical Review:
Vol. 26, No. 8
Nov/Dec 2023
Pages: 18-22
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