Abstract
Degradation products are unwanted chemicals that can develop during the manufacturing, transportation, and storage of drug products and can affect the efficacy of pharmaceutical products. Moreover, even small amounts of degradation products can affect pharmaceutical safety because of the potential to cause adverse effects in patients. Consequently, it is crucial to focus on mechanistic understanding, formulation, storage conditions, and packaging to prevent the formation of degradation products that can negatively affect the quality and safety of the drug product. In this sense, databases and software that help predict the reactions involving the pharmaceutically active substance in the presence of degradation conditions can be used to obtain information on major degradation routes and the main degradation products formed during pharmaceutical product storage. In some cases, when the presence of a genotoxic degradation product is verified, it is necessary to conduct more thorough assessments. It is important to consider the chemical structure to distinguish between compounds with toxicologically alerting structures with associated toxic/genotoxic risks and compounds without active structures that can be treated as ordinary impurities. Evaluating the levels of degradation products based on a risk/benefit analysis is mandatory. Controlling critical variables during early development of drug products and conducting a follow-up study of these impurities can prevent degradation impurities present at concentrations greater than threshold values to ensure product quality. The definition of the impurity profile has become essential per various regulatory requirements. Therefore, this review includes the international regulatory perspective on impurity documents and the toxicological evaluation of degradation products. Additionally, some techniquesused in the investigation of degradation products and stability-indicating assay methods are highlighted.
LAY ABSTRACT: Degradation products are impurities resulting from chemical changes that occur during drug manufacturing. They can also form during storage and transportation in response to changes in light, temperature, pH, and humidity, or due to inherent characteristics of the active pharmaceutical substance, such as their reaction with excipients or on contact with the packaging. The presence of these chemicals can affect product safety and quality. Therefore, it is necessary to know and follow the guidelines and standards regarding degradation products and existing regulatory environments to assess the toxicity and risk related to their presence in pharmaceutical products.
Introduction
Degradation products are impurities that result from chemical changes that occur during storage due to the effects of light, temperature, pH, humidity, reaction with excipients, or contact with primary packaging (1). Often, the terms degradation products and impurities are used as synonyms; however, there are some conceptual differences. Impurities consist of any organic (raw materials, sub-products, intermediates, degradation products, reagents, and catalysts) or inorganic (reagents and catalysts, heavy metals or residues of other metals, inorganic salts, and other materials used in synthesis) source materials in the active pharmaceutical substance or product. Degradation products are organic impurities resulting from the degradation of the active pharmaceutical substance and/or excipients (2).
Most of the existing regulatory guidelines are applicable to both impurities in pharmaceutical products and active pharmaceutical substances in general. In this work, some of these concepts will be extended to organic impurities resulting from the degradation of active substances present in the pharmaceutical product; this will be referred to hereafter as degradation products. The evaluation of degradation impurities for the purposes of drug registration is consolidated in international guidelines such as the International Conference on Harmonization (ICH) (1), which has been adopted by regulatory agencies in Europe, the United States of America, and Japan.
Other guidelines include those determined by other health authorities such as the Brazilian Health Surveillance Agency (ANVISA) (3), the Canadian Health Authority (Health Canada) (4), the Therapeutic Goods Administration of the Australian Health Authority (TGA) (1), and others. The regulatory concern is based on the fact that the presence of these chemicals, even in small amounts, can influence pharmaceutical quality and safety (5); some of these substances may have therapeutic or adverse activity at concentrations even lower than those of the active substance in the pharmaceutical product (6).
This review describes the existing toxicological evaluation of degradation products in pharmaceuticals and the relevant regulatory guidelines and standards.
International Regulatory Framework
In 1990, the ICH Q3A (R2) and Q3B (R2) guidelines were published; they addressed the monitoring and control of impurities in pharmaceutically active substances and products (7).
The ICH Q3B (R2) guideline directly addresses the evaluation of degradation products in pharmaceuticals but does not cover impurities due to the degradation of excipients, biopharmaceuticals, animal products, fermentation products, herbal products, peptides, radiopharmaceuticals, oligonucleotides, products used in developing clinical trials, enantiomeric impurities, polymorphic forms, and external contaminants (1, 8). The guidelines do not apply to the assessment of genotoxic or carcinogenic impurities (7). This guideline document suggests that the amount of degradation products in pharmaceutical products be within certain notification, identification, and qualification thresholds (1, 8).
The ICH M7 draft guideline highlights the identification, categorization, qualification, and control of mutagenic impurities that are DNA-reactive, in addition to those defined in the ICH Q3A (R2) and Q3B (R2) guidelines. The ICH M7 draft guideline emphasizes the importance of safety and quality risk management to establish levels of mutagenic impurities that are expected to have negligible carcinogenic risk (9).
According to the ICH, the reporting threshold is the maximum permissible concentration of a specific degradation product that does not need to be reported; anything above this value should be reported. The identification threshold is a limit above which a degradation product should be chemically identified. The qualification threshold is a limit above which it is necessary to acquire and evaluate data that establishes the biological safety of an individual degradation product or a given degradation profile (1). Based on these assumptions, the ICH Q3B (R2) guidelines specify numerical values for the notification, identification, and qualification thresholds shown in Table I.
Notification, Identification, and Qualification Thresholds
A degradation product is considered to be qualified if it meets one of the following conditions:
a) It is present in a marketed pharmaceutical product (whether derived from the drug product itself or not) in concentrations comparable to those that have been evaluated in clinical studies that ensure the safety of the pharmaceutical product, or
b) In situations where the amount is justified by the scientific literature or when the level and proposed acceptance criteria for impurities have been adequately evaluated using in vitro comparative studies for genotoxicity (1, 10).
The ICHM7 draft guideline emphasizes that post-approval alterations in marketed products that include changes in the drug substance chemistry, manufacturing, and controls (including synthesis routes, reagents, etc.) should be re-analyzed in terms of degradation products. The objective is to determine whether such modifications resulted in any new mutagenic impurities or exceeded the highest acceptance criteria for existing mutagenic impurities. In the same vein, post-approval submissions involving drug products (changes in composition, dosage form, etc.) should include the evaluation of the potential risk of any new mutagenic degradants or higher values of compounds that meet the acceptance criteria for previously identified mutagenic degradants in comparison to the original product (9).
Figure 1 shows a flow chart for making decisions regarding a degradation product above the qualification threshold. In addition to the ICH stability guidelines, which have been widely adopted by most regulatory agencies, other stability guidelines may also contain information regarding degradation products (Table II).
Decision tree for pharmaceutical degradation products. Figure adapted from ICH (1).
Regulatory Authorities and Economic Blocs That Include the Subject “Degradation Products” in Their Guidelines
Degradation Products: Toxicity and Loss of Efficacy
From the time when the ICH guidelines were first introduced, many changes have occurred, and the focus on the quality of pharmaceuticals is now on safety considerations regarding the presence of impurities and degradation products in finished pharmaceuticals products. These guidelines are applied by several countries for the evaluation of these drug products and represent a major concern of regulatory agencies worldwide (11).
The toxicological activity of degradation products can be observed in many pharmaceutically active substances and products and, in some cases, can compromise therapeutic action. Some examples of degradation situations that can affect the safety of drug products are listed in Table III.
Degradation Situations That Can Affect the Safety of Drug Products
Failures of products to comply with stringent limits on degradation products result in recalls of large quantities of pharmaceutical products from the market. Some examples of recall notifications from September 2011 to March 2012 issued by the Food and Drug Administration (FDA) are listed in Singh et al. (11). To solve problems related to degradation products, the industry today is forced to invest in tools that can predict the early formation of degradation products (11). Some of these tools also allow the evaluation of the toxicity of some structures.
Forced Degradation Studies and Stability-Indicating Assay Methods
A tool commonly used by pharmaceutical companies to predict the formation of degradation products is forced degradation or stress testing. These tests provide information on the major degradation routes and the main degradation products formed during storage of pharmaceutical products. Stress tests usually include subjecting pharmaceutical products to thermolytic, hydrolytic, oxidative and photolytic conditions, and pH changes (12–15).
Some databases may be useful in defining the degradation routes of the active pharmaceutical being investigated. An example is the Drug Degradation Database (16) created by Pfizer, Eli Lilly, and Amgen, which contains over 300 active pharmaceutical substances with registered degradation routes. Stress tests are important for defining and validating analytical methodologies as well for determining stability-indicating assay methods that are able to measure the amount of the active pharmaceutical substance in the presence of other contaminants such as degradation impurities (17, 18).
In a study by Garg et al. (19), a forced degradation of the active pharmaceutical ingredient (API) fentanyl, an opioid analgesic, was performed using light, acid, base, heat, and oxidation. Under acidic conditions, fentanyl degraded to N-phenyl-1-(2-phenylethyl)-piperidin-4-amine (PPA), a potentially genotoxic compound due to the aniline moiety in the structure. In this evaluation, a stability-indicating high-performance liquid chromatography (HPLC) method evaluating fentanyl and its related compounds was validated and demonstrated to be specific, precise, linear, accurate, sensitive, robust, and suitable for the intended use (19). Figure 2 depicts the path to define a stability-indicating methodology.
Flow chart for defining stability indicating assays. Source: Bakshi and Singh (85).
Many forced degradation studies use mass balance to assess the extent of degradation. This practice may only be acceptable in early or clinical development and not at the registration phase. In mass balance studies, the amount of active pharmaceutical substance is compared to that of the degradation products formed (20). Due to the lack of structural information regarding the degradation products and their unavailability in the early stages of new drug development, degradation is usually quantified using the normalization percent area (area %). In this technique, it is normal to use an identical response factor (area relative to amount) for the parent drug and the degradation product (1, 21). However, it is not always possible to obtain a suitable mass balance; in some cases, the degradation product is volatile and is lost before the analysis is completed. In other cases, there are differences in the molecular weight and the response depending on the detector used, among other limiting factors (21).
Investigation of Degradation Impurities: Technical Orthogonal Assessments
Pharmaceutical products need to be quantitatively and qualitatively assessed for impurities. These products may be evaluated using HPLC coupled with diode array detection (HPLC-DAD), liquid chromatography–mass spectrometry (LC-MS), or liquid chromatography coupled with nuclear magnetic resonance (LC-NMR), among other techniques. In many cases, it is necessary to use orthogonal analytical methods to be confident that impurity detection and identification are not confused by false positives caused by interference. For example, in the case of pramlintide, a 37-amino acid peptide used to treat people with type 1 and insulin-using type 2 diabetes, the complexity of quantifying low levels of structurally related impurities and degradation products requires the application of highly selective HPLC techniques and orthogonal separation modes (22).
In the case of hydrazine, the principal hydrolytic degradation product of the anti-tuberculosis drug isoniazid and a known degradant of hydralazine, a drug for treating hypertension, developing and validating methods for accurate determination at the parts-per-million level in the presence of API or drug product formulation excipients could be challenging due its polarity and low molecular weight combined with its susceptibility to oxidation.A wide variety of techniques can be employed in this assessment (23, 24).
Another example is clopidogrel hydrogen sulfate, an oral antiplatelet agent used to inhibit blood clots that develops a methyl-ester salt structure with various acidic counter ions such as HCl or besylates. These acids have the potential to react with the methyl ester either directly in an anhydrous environment or with residual methanol generated by hydrolysis when exposed to moisture. The resulting reaction products are methyl chlorideor methyl besylate, respectively, which are structures with genotoxicity concerns. The presence of genotoxic degradants at low levels is typically undetectable when conventional analytical tools for degradation studies are used. It is therefore prudent to consider more selective, sensitive detectors, such as mass spectrometry (MS), and conduct targeted analysis if the risk is high based on systematic risk assessment. Otherwise, risks from genotoxic degradants could remain undetected due to analytical limitations (23).
The concept of orthogonality covers a multidimensional separation system. Two analytical methods can be considered orthogonal when the separation mechanisms are independent such that the selectivity obtained using the first method is not correlated with that obtained using the second method (25).
Two strategies were reported by Argentine et al. (25) for the development of orthogonal analytical methods to fully characterize pharmaceutically active substances. The first strategy involves the use of reversed-phase liquid chromatography (RP-LC), which maximizes selectivity by evaluating chromatographic supports while using various organic solvents and mobile phase components that cover a wide pH range. The second strategy involves the use of different separation mechanisms, such as capillary electrophoresis (CE), supercritical fluid chromatography (SFC), thin layer chromatography (TLC), gas chromatography (GC), normal phase chromatography (NP-HPLC) and hydrophilic interaction liquid chromatography (HILIC). A three-dimensional impurity profile can be visualized if orthogonal detection principles are utilized (25). Reversed-phase chromatography (RP-LC) and a UV detector can be utilized in association with CE coupled to MS to obtain as much information as possible. This technique can be extended to other detection options such as photodiode array detection (PAD) and evaporative light scattering (ELS) (25).
Control of Genotoxic Impurities
In some cases, when the presence of a genotoxic degradation product is verified, it is necessary to conduct more thorough assessments. To ensure greater control of genotoxic impurities, the European Medicines Agency (EMA) published the 2006 “Guideline on the limits of genotoxic impurities”, which advised that any genotoxic impurities should be identified using both existing genotoxicity data and “structural alerts” (based on the chemical constituents of pharmaceutical drug substances known to be associated with particular types of toxic effects, e.g., mutagenicity) (7). This guideline was made available online at the start of 2007 and applies to new active substances and applications. It is also useful to evaluate variations of existing active substances when the assessment of the synthesis route, process control, and impurity profile does not ensure the prevention of the introduction of new or higher levels of genotoxic impurities compared with products containing the same active substances that are currently authorized in the Europe (26).
Genotoxic impurities are chemicals capable of causing changes in gene expression and direct or indirect damage to DNA or chromosomes (27, 28). To establish acceptable levels of exposure to genotoxic and/or carcinogenic compounds, the EMA guidelines consider the following categories of genotoxic impurities.
Genotoxic Compounds with Sufficient Evidence for a Threshold-Related Mechanism
Compounds in this class have an established level of exposure that can be genotoxic, and the same procedure defined for class 2 residual solvents described in the ICH Q3C(R5) guideline, which establishes permitted daily exposure (PDE) limits, is applicable (29). If the assessed values are above the threshold, it is necessary to reduce the compound in the pharmaceutical product to safe levels below the PDE.
Genotoxic Compounds without Sufficient Evidence for a Threshold-Related Mechanism
This category consists of compounds for which there are clearly defined genotoxic mechanisms, requiring pharmaceutical and toxicological evaluations to establish safe exposure levels. In general, pharmaceutical product evaluation should be guided by a policy of reducing substances to a level “as low as reasonably practicable” (ALARP) when it is not possible to avoid them. It is impossible to define a safe exposure level for genotoxic carcinogens without a threshold, and due to the difficulty of completely eliminating genotoxic impurities, it is common to define an acceptable risk level, that is, an estimate of daily exposure below which there is negligible risk to human health (30). Thus, the EMA and ICH M7 draft guideline adopts the concept of a “threshold of toxicological concern toxicity” (TTC) for genotoxic impurities (9, 30, 31). Figure 3 illustrates the decision tree used to evaluate genotoxic impurities.
Flow chart defining genotoxic-indicating assays. Source: CHMP (30).
1Impurities with a structural relationship to high potency carcinogens are excluded from the TTC approach.
2If carcinogenicity data are available: Does intake exceed the calculated 10–5 cancer lifetime risk?
3Case-by-case assessments should include the duration of treatment, indication, patient population, etc.
*Abbreviations: NOEL/UF, No Observed Effect Level/Uncertainty Factor; PDE, Permitted Daily Exposure; TTC, Threshold of Toxicological Concern.
** Use alternatives (different synthetic routes, formulations, starting materials, etc.).
An example of a compound in this category is hydrazine, which has been shown to cause gene mutations and chromosome aberrations and to induce cancer in some animal studies but is lacking any convincing evidence for carcinogenicity in humans. Hydrazine has been classified as a probable human carcinogen and has to be controlled with a staged TTC system (32).
The TTC can be defined as the dose (μg/day) of any unstudied chemical for which the probability of cancer incidence is insignificant. The TTC was initially set by the FDA at an estimated value of 0.5 ppb (equivalent to 1.5 μg/patient/day or 0.025 μg/kg/day) (33). This value corresponds to a 10–5 lifetime probability of developing cancer for pharmaceuticals that show some benefit with intentional exposure (28) and corresponds to a 10–6 risk in situations in which there is no theoretical benefit involved (1, 9). In line with the guidelines proposed, the Pharmaceutical Research and Manufacturing Association (PhRMA) has proposed a risk level of 10–6 for clinical trial therapies lasting fewer than 12 months that have no pharmacological benefit for volunteers. For studies lasting longer than 12 months, a risk of 10–5 has been defined because these studies usually have a drug benefit for volunteers (28). TTC values greater than 1.5 μg/person/day may be acceptable in situations such as short-term exposure to treatments when life expectancy is less than 5 years or where the impurity is a known substance for which much greater human exposure occurs from other sources, for example, food (1).
The TTC concept is applicable to impurities that have negligible risk of carcinogenic or toxic effects; it is not applied to compounds presenting risks of developing cancer even at doses below the TTC (30, 33–35). However, exceeding the TTC is not necessarily associated with an increase in cancer risk (9).
Classification of Impurities According to PhRMA
PhRMA envisions classifying genotoxic pharmaceutical impurities into five specific categories defined in a document that establishes procedures for testing, qualifying, and reporting toxicological risks (31). Figure 4 shows the five classes of impurities classified by PhRMA.
Classification of impurities by PhRMA. Source: Muller et al. (28), Dolan et al. (33), Dobo et al. (40, 86).
Class 1 compounds contain impurities that have high risks of genotoxicity. It is appropriate to eliminate these compounds or ensure their presence at levels below the safety threshold (e.g., TTC) (7). Class 2 compounds contain genotoxic impurities but have unknown carcinogenicity. Class 3 compounds contain a chemical alert group unrelated to the parent compound. Compounds in classes 2 and 3 must be at or below acceptable limits (generic or adjusted TTC) (9). Exceptions occur in cases where there is evidence for a threshold-related mechanism of genotoxicity, in which case the limit set by the PDE is used (28).
Compounds included in Classes 4 (with the same alert structures as the parent drug substance) and 5, in which the active pharmaceutical substance is not genotoxic, are treated as non-mutagenic impurities based on ICH Q3A (R2) and Q3B (R2) guidelines (9). Thus, the treatment of impurities involves the identification and classification of structural alert groups for the pharmaceutical active substance and its impurities; these are then grouped into one of the five classes defined above, and an appropriate threshold is set for each case (28).
In a summary, the alerting genotoxic drug degradant structure can come from a parent drug that already contains a genotoxic alert or from a parent drug with no alerting structures. Some examples follow. Oxybuprocaine has an alert for aromatic amines that form degradants via hydrolysis that have the same alert structure as the parent. This acid degradant can be qualified by standardized mutagenicity data obtained from the parent molecule. Acetaminophen contains a structural alert for N-acylatedaminoaryls, which form p-aminophenol, a compound that has a structural alert for an aromatic amine, which is different from the alert in parent structure. Propofol does not have any alert structures but degrades via oxidation into a dimeric degradation product with alerts for mutagenicity (36).
The existence of structural alert groups in an impurity alone is insufficient to trigger follow-up measures, unless it is a structure in the cohort of concern (a group of highly potent mutagenic carcinogens) or in the case of a new relevant impurity for which hazard data were generated after the overall control strategy and specifications for market authorization were established (9).
Databases and Software Used To Predict the Formation of Degradation Compounds and Alerting Toxic/Genotoxic Structures
Software that helps predict the reactions that occur with the pharmaceutically active substance in the presence of certain reagents, starting materials, and degradation conditions can be used to obtain information regarding degradation products. Zeneth (a degradation expert system) by Lhasa is a program that provides detailed descriptions of degradation pathways with supporting literature references (37).
In addition to programs that predict degradation reactions, programs are also used to evaluate alert structures for genotoxicity (Figure 5). Most pharmaceutical companies use these programs in combination with other suitable tests for predicting compound toxicity/genotoxicity (38). Assessments that use software programs to evaluate the toxic effects of compounds before synthesis or testing are referred to as in silico assessments. Software programs that use this principle are available as described by Muster et al. (39). DEREK (Deductive Estimation of Risk from Existing Knowledge), MCASE (Multiple Computer-Automated Structure Evaluation), and TOPKAT (TOxicity Prediction by Komputer-Assisted Technology) are among the most frequently used.
Chemical structures that have functional alert groups for genotoxicity. Figure adapted from Muller et al. (28).
A: Alkyl, Aryl or H
X: F, Cl, Br, I
EWG: Electron-Withdrawing Group (CN, C=O, ester, etc.)
DEREK recognizes the alert structures for genotoxicity in the tested agent using a bacterial mutagenicity assay, in vitro cytogenetic testing, and bibliographic references. MCASE is an organizational model based on data obtained from bacterial mutagenicity studies conducted on a series of compounds. TOPKAT is a program that uses data obtained from bacterial mutagenicity tests to evaluate similarities between the tested molecule and other molecules that exist in its database, excluding those that do not have significant evidence for mutagenicity (39). These programs are often used in combination to increase specificity while evaluating genotoxicity. For instance, the CASETOX (Computer Automated Structure Evaluation for Toxicology) programs, DEREK and TOPKAT, have specificities of 78% to 82%, respectively, when used alone, and 92% to 100% when used in combination (38). In Dobo et al. (40), eight companies were surveyed for their success rate in identifying non-mutagenic structures. The negative predictive value (NPV) of the in silico approaches was 94%. When human interpretation of in silico model predictions was conducted, the NPV substantially increased to 99%. This survey also suggests that the use of multiple computational models is not a significant factor in the success of these approaches with respect to NPV (40).
Other promising computational methods can be used in the toxicological assessment of genotoxic impurities. In this context, the application of promising computational methods, for example, QSARs (Quantitative Structure–Activity Relationships) and SARs (Structure–Activity Relationships) is needed, especially when very limited information on impurities is available (41). QSARs can identify structural alerts for known and expected impurities present at levels below qualified thresholds. It provides the information necessary to establish the practical use of a new in silico toxicology model to predict the Salmonella t. mutagenicity (Ames assay outcome) of drug impurities and other chemicals with high sensitivity (81%) and high negative predictivity (81%) based on external validation with 2368 compounds foreign to the model and with known mutagenicity (42). An important aspect of SARs as predictive toxicity tools is that they are derived directly from mechanistic knowledge. Mechanistic knowledge provides a basis for interaction and dialogue between model developers, toxicologists, and regulators, and it permits the integration of the QSARs results into a wider regulatory framework in which different types of evidence and data concur or complement each other and are used as a basis for making decisions and taking action (43).
The ICH M7 guideline describes the use of two QSARs prediction methodologies that complement each other; one should be expert rule-based and the second should be statistically based. The absence of structural alerts from two complementary QSARs methodologies is sufficient to classify the impurity as no concern (9).
The genotoxic and carcinogenic properties of various substances can also be found in databases such as TOXNET (TOXicological data NETwork), NIOSH (the National Institute for Occupational Safety & Health), GESTIS (information system on hazardous substances of the German), Discovery Gate (SYMX), and Pharmapendium (Elsevier) (44–48).
Toxicological Assays Used To Qualify Impurities
In the potential presence of chemical structures containing alerts for genotoxicity, risk should be evaluated by conducting toxicological tests. Currently, it is not possible to register a new medication without having information regarding its mutagenicity (49). Most existing guidelines on the subject adopted by regulatory agencies in Brazil (50), Japan, the United States and the European community support the necessity of a battery of tests for genotoxicity, which increases the sensitivity and broadens the spectrum of genetic events detected during the toxicity evaluation of a particular chemical (49). A standard battery of tests includes bacterial mutagenicity tests (51, 52), in vitro chromosomal damage with mammalian cells or in vitro mouse lymphoma thymidine kinase (TK) assays (53, 54), and in vivo tests for chromosomal damage using rodent hematopoietic cells (55). In vitro assays play an important role in genotoxic assessment because of their high sensitivity and rapid toxicity evaluation; in vivo tests are used to measure the effects of exposure route, treatment duration, metabolism, and the organs affected (49).
Negative test results from this battery are generally sufficient to ensure the absence of genotoxic activity. If a compound demonstrates positive results for any of these tests, depending on its therapeutic use, it is necessary to perform more than one test to determine its toxicity profile (56). In cases where a mutagenic compound is a non-carcinogen in a rodent bioassay, there would be no predicted increase in cancer risk (9).
Some of these tests were demonstrated in Vijayan et al. (57) to provide information about the genotoxic activity of the main degradation product of piperacillin, piperacillin impurity-A. This degradation product appears during manufacturing and storage processes of the antibiotic penicillin, and its level failed to pass the validation criteria of the computer-assisted toxicity prediction carried out by TOPKAT software. A bacterial mutagenicity test and an in vitro chromosomal aberration study were performed to establish the safety profile and qualification. The results of these studies indicated that piperacillin impurity-A is non-mutagenic in the Ames test and non-clastogenic in the chromosomal aberration study (57).
Because a single test is not capable of detecting all relevant genotoxic substances, some situations require changes in the set of genotoxicity tests used. This is especially critical for compounds that have one or more of the following characteristics: high toxicity, poor or no absorption, structural alerts but with negative results in the standard battery of genotoxic tests, showing evidence of tumor response or possessing structurally unique chemical classes (49).
Discussion
The presence of even small amounts of degradation products can affect pharmaceutical safety because of their potential to cause adverse effects in patients. For this reason, their assessment is necessary to determine shelf life and for medicine registration by national health authorities.
The United States Pharmacopoeia (USP) recently included the concept of degradation products under the topic “Impurities in Drugs and Pharmaceutical Forms” in USP 33–National Formulary (NF) 28, thus demonstrating the evolution of regulatory issues and concerns regarding this issue (58). Much of the existing regulatory material regarding degradation is contained in international guidelines such as those of the ICH, FDA, and EMA.
When the concentration of a degradation product is above the qualification threshold but qualifying data are lacking, it is necessary to conduct toxicological studies that take the maximum dose ingested, target population, and route and duration of drug administration into consideration (1). Degradation product evaluations should consider the chemical structure to distinguish between compounds with toxicological alert structures with associated toxic/genotoxic risks and compounds without alert structures that can be treated as ordinary impurities. Existing computer programs that perform predictive assessments based on chemical structure and compound activity are very useful in determining toxicological responses (38, 39).
Pharmaceutical industry research and development (R&D) has its own characteristics and requires significant investment. There is consensus that R&D consumes resources and is considered highly complex. In developing countries, R&D is hampered by a number of factors, such as the lack of supportive policies, incentives, infrastructure, and qualified staff. Moreover, the formation of specialized human resources in key areas of pharmaceutical production is often not a priority. All these factors coupled with negligible interaction between pharmaceutical companies and universities/research centers negatively affects degradation research in developing countries (59). Research that focuses on the assessment of degradation products is normally initiated during drug development and informs the choice of excipients, active ingredients, packaging material, and appropriate stability-indicating assay methods.
Once it has been determined that degradation products exist at concentrations above the limits set by the ICH or belong to a genotoxic category, the pharmaceutical development team should assess whether it is worthwhile to proceed with the proposed formulation. In most cases, the assessment of degradation products during development includes forced degradation studies, which allow the definition of factors, such as the selectivity of the analytical method used. It is not always possible to find reference standards for degradation products that need to be identified, quantified, and qualified. Often, it is necessary to characterize and synthesize the products to conduct the studies. Time and cost are limiting factors in this process.
When degradation products requiring qualification are present, it may be simpler and cheaper to reduce impurities to concentrations below the qualification threshold (6). In this way, applying the concept of quality by design is very desirable because it allows one to reduce to desired levels or eliminate the impurities that arise by controlling the critical parameters. Quality by design refers to the use of a systems approach to pharmaceutical development, that is, designing and developing formulations to ensure pre-defined parameters of quality. It includes the definition of a quality profile, the identification of critical quality attributes, parameters, and sources of variability to obtain consistent quality over time and thus allow the understanding and control of variables in pharmaceutical formulation (60). This concept can be applied to the evaluation of degradation impurities, including those that are genotoxic, using an integrated control strategy. The strategy focuses on mechanistic understanding, formulation, storage conditions, and packaging.
If, under the proposed packaging and storage conditions, it is anticipated that the degradation product will be formed at levels approaching the acceptable limit, the formation of degradation products must be controlled (9). Controlling critical variables and conducting a follow-up study of these impurities can exclude degradation impurities that are present at concentrations greater than threshold values, thus ensuring product quality. This approach can be applied during early development stages and involves appropriate identification methods (45). If it is anticipated that formulation development and packaging design options are unable to bring mutagenic degradant levels to less than the acceptable limits or to a level as low as reasonably practicable, the highest limit can be justified based on a risk/benefit analysis (9).
Investing in research is still the easiest and cheapest alternative for the pharmaceutical industry to ensure the development of robust formulations. When genotoxic degradation compounds are discovered after manufacturing, the drug design process may have been wasted, and the discovery may result in product recall. Such outcomes are detrimental for both the manufacturing industry and the exposed patients.
Conclusion
In the current regulatory environment, there is increasing overlap of drug registration rules among countries. Existing drug products are frequently simultaneously registered and marketed in more than one country, and regulatory agencies have sought to harmonize regulations that ensure pharmaceutical quality and safety. Stringent regulatory standards and an industry-wide commitment to the issue of degradation are essential to ensure pharmaceutical safety, efficacy, and quality.
Declaration of Interest Statement
The authors report no conflict of interest.
- © PDA, Inc. 2014
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