A box of prescription drugs had been forgotten in a back room of a pharmacy for so long that some of the pills were older than the 1969 moon landing, and most had already passed their expiration date by 40 years. Possibly toxic, probably worthless. This situation provides an opportunity to answer an old question about the shelf life of medicines: Can these pills from the bell-bottom era still be used at all?
The shelf life, or expiration date, of a drug is the time frame in which a drug has been shown to be safe and effective despite exposure to various environmental factors such as temperature, humidity, and light. And to ensure shelf life, it is necessary to determine the stability of the drugs.
Pharmaceutical stability is defined by the World Health Organization as the ability of a pharmaceutical product to maintain its chemical, physical, microbiological, and biopharmaceutical properties within specified limits throughout its shelf life.
Drug manufacturers must assure regulators that therapeutic products dispensed to patients will meet approved specifications up to the expiration date indicated on the product container. This assurance is based on an increasing understanding of the capabilities of the manufacturing process and the sensitivity of the product to various chemical and physical degradation pathways, which are evaluated during product development through stability monitoring of representative materials manufactured for clinical trials and anticipated commercial distribution.
Pharmaceutical products are particularly sensitive to environmental influences. Stability testing is used to evaluate biopharmaceutical drugs under various environmental conditions over a period of time. The results are used to determine recommended storage and shipping conditions for drugs and products, as well as the appropriate shelf life or retesting period. In this article, we will discuss and reflect on strategies to ensure the stability of drugs and the innovations that arise in the process. Are we in a new era of drug stability?
The quality guidelines for drug stability studies:
Regulatory agencies, in accordance with the International Conference on Harmonization (ICH), are responsible for setting clear expectations for the design of stability studies for product registration. ICH has grown to 16 members and 32 observer health agencies since its inception in 1990. Examples of regulatory agencies that are part of ICH include the Food and Drug Administration (FDA-USA), the European Commission (EC), and ANVISA (Brazil).
These guidelines establish confidence in data analyses that justify the shelf life of commercial products. While most global health authorities accept strategies based on the ICH guidelines, some have additional regulations that effectively raise the barriers to the global delivery of innovative therapies.
Although guidance Q1A and Q5C from ICH prescribe the package of stability data required for the registration of a new product, it is permissible to submit stability protocols that deviate from the recommended design of the registration protocol, provided there is adequate scientific justification (including prior knowledge and sufficient supporting data). This guidance allows the use of reduced stability protocols for registration and for long-term commercial commitment studies (confirmatory and annual batches).
Crucial drug development steps:
Determination of shelf life follows science-based technical procedures as described in ICH Q1A guidance. The initial expiration date is based on the amount of real-time stability data from pilot batches for the drug product available at the time of marketing authorization. These initial dates may be subsequently extended if the manufacturer submits acceptable data based on accelerated stability studies and real-time stability data from the first three production batches.
Stability studies are performed in all phases of drug development. They begin in the preclinical phase and continue through the clinical trial phases to support formulation development and meet regulatory requirements for clinical trials.
Degradation studies:
The first stage usually takes the form of forced degradation studies such as stress testing. These studies help identify the ideal formulation from a variety of different candidates for further testing. The goal is to understand the primary degradation products of a molecule and help analysts select the best methods for further stability testing that mimic storage under real-world conditions in different regions of the world. Long-term stability studies on the drug product are then initiated and validated.
In drug development, forced degradation studies demonstrate the specificity of stability determination methods and provide insight into degradation pathways, which helps to elucidate the structure of degradation. These are the chemical behavior studies of the molecule, which in turn help in formulation and packaging development.
Photostability studies:
Another important stability test is photostability. Exposure of a drug to UV irradiation may affect its physicochemical properties. One drug that has been shown to be photosensitive is retinoic acid. A study conducted with a third-generation retinoid (tazarotene) showed that tazarotene underwent a degradation process under UV irradiation that resulted in two photodegradation products.
Knowing this degradation process, in silico studies were performed, which showed that five of the degradation products could be harmful and have a potential irritant effect. The cytotoxic properties of the degradation products were evaluated using the MTT assay on a group of human adherent cancer cells. The results demonstrate the importance of methods to analyze the photostability of tazarotene in topical formulations.
Factors that impact drug stability:
Loss of stability of a drug product may be directly related to loss of therapeutic effect or formation of toxic degradation products. The stability of pharmaceutical products depends on environmental factors such as temperature, humidity, light, and other factors related to the product itself, such as the physical and chemical properties of the active ingredients and pharmaceutical excipients, the pharmaceutical form and composition, the manufacturing process, and the nature and properties of the packaging materials.
The stability of pharmaceutical products is based on stability data obtained during the development phase of a drug and on the storage conditions that must be established. For example, light-sensitive drugs must be protected from exposure to light of certain wavelengths during their handling. The packaging should be specified taking into account the light sensitivity of the molecule. Oxygen-sensitive drugs also need to be handled in an inert atmosphere or may require an antioxidant. The sensitivity of the drug substance to environmental factors and the tendency to interact with other ingredients, excipients, and packaging materials should be evaluated through stability studies during the drug development cycle. Stability testing is therefore one of the critical procedures in the drug approval process.
Stability testing is an essential part of drug development to ensure that marketed products remain safe and effective under a variety of conditions. The results are also used to determine an appropriate shelf life for a drug product and recommend long-term storage. Selected and validated tests help pave the way for a drug to reach the market.
It is also important to define the climate zone used in the marketing of the drug, an important factor that interferes in the stability study. There are five different climate zones worldwide, categorized according to their recorded temperature and humidity histories:
Table: ICH stability zones.
| Worldwide Zone | Temperature Zone | Storage Condition |
| Zone I | temperature zone | 21 ºC / 45 % |
| Zone II | Mediterranean/subtropical zone | 25 ºC / 60% |
| Zone III | hot dry zone | 30 ºC / 35 % |
| Zone IVa | hot humid/tropical zone | 30 ºC / 65 % |
| Zone IVb | hot/higher humidity | 30 ºC / 75 % |
How is drug stability tested?
There is debate about the relative effectiveness of medications after the expiration date on the label. Expired medications have not necessarily lost efficacy, as the expiration date is only a guarantee that the stated efficacy will last at least until that time.
ICH Q1E guidelines suggest using linear or nonlinear regression and statistical modeling through poolability testing to determine the estimated shelf life of a drug product. This involves obtaining test results from at least three stability registration batches at predetermined storage times. For a simple linear regression model, the analysis is performed stepwise to determine which of the following alternative regression models is best suited to characterize bacterial response over storage time and estimate shelf life: (a) common intercept and slope, (b) separate intercepts and slope, or (c) separate intercepts or slopes.
The guidance states that shelf life is estimated based on the results of real-time stability testing and accelerated stability testing. In real-time stability testing, a product is stored and monitored under recommended storage conditions until it no longer meets product specifications. Accelerated stability testing involves storing a product under elevated stress conditions (e.g., high temperature and/or humidity). Degradation under the recommended storage conditions can then be predicted using known relationships between the acceleration factor and the rate of degradation.
Beyond chemical stability:
Nowadays, the stability of drugs takes into account more than just the expiration time as an output parameter. An interesting study (2018) has shown the importance of determining and understanding stability in the context of ligand binding. Based on high-content, high-throughput immunofluorescent single-cell detection, they determined target protein levels after heating adherent cells. This method allows high-throughput determination of target engagement in adherent cells based on the principle of altered thermal stabilization/destabilization in response to ligand binding and is, therefore, a valuable tool for small molecule drug development.
Another strategy to validate drug stability focuses on stabilizing the host immune microenvironment so the drug can work more effectively. In a 2018 study, the authors observed the possibility of inducing immune tolerance in high-risk corneal transplants. Using a drug delivery system, they found good biocompatibility and biodegradability of cyclosporine A, increasing its effectiveness in predicting corneal graft rejection. When an environment that facilitates the drug’s action is created, it becomes more stable and is able to bind to the receptor more effectively. It is not just a question of the drug being stable outside the body, but also of maintaining stability when ingested so that it can act efficiently. This is a more current point of view on drug-receptor interaction.
New generation of drug stability strategies:
A critical aspect of pharmaceutical development is demonstrating the long-term stability of the drug. Biopharmaceuticals such as proteins or peptides in liquid form are administered parenterally and should be stable throughout their shelf life, which generally includes a storage period (e.g., 2 years at 5 ºC) and a use period (e.g., 28 days at 30 ºC), if applicable.
Kinetic modeling:
A study conducted in 2021 analyzed the chemical degradation of SAR441255, a therapeutic peptide, in different formulations in combination with primary packaging materials under accelerated conditions to derive predictions of long-term stability for recommended storage conditions using advanced kinetic modeling. These in silico predictions served as an important decision parameter for entering clinical development. This strategy provided insights into stability within weeks that would otherwise take years when measured under long-term stability conditions. Kinetic models allow the prediction of the rate of degradation of a pharmaceutical product subjected to multiple temperature variations during storage, shipping, and distribution, and therefore could significantly support the life cycle management of pharmaceutical products.
New strategies as exemplified by SARS-CoV-2 mRNA vaccines:
Nowadays, the new types of medications are more robust and different from the original medications based only on chemical formulations. A good example of these new therapies are the mRNA vaccines against SARS-CoV-2. These vaccines became the pioneers in the fight against the pandemic, but the challenges associated with their formulation and stability are quickly becoming clear. The analytical tools used to verify their stability include the qualitative composition of the SARS-CoV-2 mRNA vaccine to analyze the identity, purity, potency, safety, and stability of the bioactive mRNA and mRNA-lipid/protein complex formations.
The techniques used were for monitoring the critical quality attributes of mRNA vaccines. Crommelin et al. also propose some additional tests to characterize properties related to formulation (e.g., lipid- and/or protein-containing). And analytical techniques for characterizing mRNA-lipid complexes are clearly lacking.
In 2021, a candidate vaccine against SARS-CoV-2 (ARCoV) was subjected to mRNA vaccine stability and evaluated using clinical batches produced under good manufacturing practice (GMP) conditions. After a kinetic storage period, all formulations were tested for mRNA purity, mRNA encapsulation efficiency, particle size, and size distribution. Finally, to determine immunogenicity, in vivo studies were performed using mice in which specific IgG antibodies were assayed.
Predictive studies to develop ideal sorbents:
Predictive studies consist of interesting strategies to help pharmaceutical companies develop the next generation of life-saving drugs and maintain product stability, namely working with companies to develop desiccants and other sorbents. Multisorb Technologies has been involved since 2009. The principle is to validate a technology that can quickly identify the sorbent required for the desired shelf life, optimize the sorbent format, and then combine the sorbent with dispensers that enable the most cost-effective manufacturing processes.
Drop-in sorbent solutions also continue to be a mainstay in the industry. The Multiform CSF canister is an excellent example of innovation in a classic drop-in solution. It consists of a silica gel desiccant canister. It is possible to eliminate a duplicate drop and parts and provide appropriate desiccant fillings in space-constrained packaging presentations.
Modeling the effects of climatic conditions:
On the other hand, Free Think Technologies has developed a software tool that can be used to model the effect of climatic conditions on the degradation of packaged pharmaceuticals over time. The modeling data can then be used to determine the shelf life of drugs and drug products. The software, called “ASAPprime,” includes a wizard that guides the user through questions aimed at creating an experimental design for an ASAP study on a particular product. It is based on knowledge of the product, the limitations of the planning space in terms of temperature and relative humidity, the data accuracy required, the time available, and the number of samples to be analyzed.
Once the study has been conducted, the results are entered into the main program to create a mathematical model of the product’s behavior. The statistical and scientific fitting process provides the user with the probability that a product will remain within stability-related specification limits at the intended shelf life, based on the storage conditions and packaging options selected. In other words, it is a prediction of shelf life rather than a test of the adequacy of an analytical method.
Conclusion:
The concept of stability is a big issue in the pharmaceutical world, as it includes chemical, physical, and biological stability. Stability is related to the quality of a pharmaceutical product. Evaluating the stability of drugs can prevent toxicity and increase the safety, efficacy, and quality of the final product. It is essential to understand the interplay of these attributes in formulation development and manufacture. Developing formulation strategies alongside stability testing methods will transform new drug molecules that emerge.
Despite the advances in the development of drugs, which are increasingly based on biological products, the stability tests are still the same, as they are standardized. There are some innovations resulting from the need for new formulations. But there is still a ways to go in applying new techniques that are reliable and have lower financial impacts.
