Recent developments in the risk assessment of potentially genotoxic impurities in pharmaceutical drug substances.

Controlling the quality of medicines is just as important as demonstrating efficacy. The International Conference on Harmonisation has published general guidance on the quality and safety assessment of impurities in pharmaceutical drug substances and drug products. More recently, the European Medicines Evaluation Agency has published a guideline focusing on limits for genotoxic impurities. This is based on a Threshold of Toxicological Concern (TTC) derived from animal carcinogenicity data using multiple worst case assumptions to estimate a daily dose (1.5 microg/day) associated with a lifetime cancer risk of 1 in 100,000, a risk level considered acceptable for genotoxic impurities in human medicines. Based on these assumptions, presentation of the TTC as a single figure infers an unwarranted level of precision and supports the adoption of a more flexible approach by regulatory authorities when evaluating new drug products; a range within fivefold of the TTC limit would seem sensible. Furthermore, the limit is based on 70 years continuous daily exposure, a scenario that is uncommon for most medicines and irrelevant to the preregistration clinical development phase. To address this latter point, a staged TTC has been developed that proposes limits based on shorter durations of treatment, e.g., up to 1 year. Based on recent history, this approach has been acceptable to some authorities but not to others, and it is imperative that steps are taken to reach a common agreement between the pharmaceutical industry and regulatory authorities globally in order that new medicines can continue to be developed and delivered to benefit patients in a safe and timely manner.     

Quantitative assessment of cumulative carcinogenic risk for multiple genotoxic impurities in a new drug substance

In pharmaceutical development, significant effort is made to minimize the carcinogenic potential of new drug substances (NDS). This involves appropriate genotoxicity and carcinogenicity testing of the NDS, and understanding the genotoxic potential of its impurities. Current available guidance recommends the use of the threshold of toxicological concern (TTC) for a single impurity where mutagenicity but no carcinogenicity information exists. Despite best efforts, the presence of more than one genotoxic impurity in an NDS may occur at trace levels. This paper repeats the analysis performed by others for a single genotoxic compound, but also uses statistical simulations to assess the impact on cancer risk for a mixture of genotoxic compounds. In summary, with the addition of multiple impurities all controlled to the TTC, an increase in cancer risk was observed. This increase is relatively small when considering the conservative assumptions of the TTC. If structurally similar compounds had an assumed strong correlation (+/-10-fold from the first randomly selected impurity) in cancer potency, the resulting cancer risk was not negatively impacted. Findings based on probabilistic analysis here can be very useful in making appropriate decisions about risk management of multiple genotoxic impurities measured in the final drug substance.     

药物杂质的毒理学评价要求及进展

 药物原料或制剂中的杂质可能引起临床不良反应。杂质毒理学评价是药物研究的重要内容。ICH关于药物及制剂杂质方面指导原则规定了杂质的报告、鉴定和质控限度,含量超过质控限度的杂质应进行毒理学评价。但指导原则对于研发阶段的药物杂质和遗传毒性杂质的限度未作明确要求。EMEA对于遗传毒性杂质制定了专门的指导原则,引入了毒理学担忧阈值（TTC）的概念对遗传毒性杂质限度进行控制,遗传毒性杂质每日接触量应小于1.5 μg。FDA也推荐采用TTC原则控制遗传毒性和致癌性杂质。本文结合ICH,EMEA及美国FDA等指导原则,对药物杂质毒理学评价的要求及其进展进行了综述。     

Regulating impurities in pharmaceutical products: a tolerability of risk approach?

This paper explores the implications of the EMEA guideline EMEA/CHMP/QWP/251344/2006 for pharmaceutical risk decisions. The guidelines propose to consider the intake of 1.5 µg/day of a genotoxic impurity as a ‘threshold of toxicological concern’, and to treat this level as an acceptable risk (excess cancer risk of <10^-5 over a lifetime). The guidance document also introduces a specific decision-tree to assess the acceptability of genotoxic impurities. According to this decision-tree, when the presence of genotoxic impurities is unavoidable, their levels should be reduced ‘as low as reasonably practicable’ (ALARP). In the UK, the Health and Safety Executive has developed a ‘tolerability of risk’ (ToR) model to support ALARP requirements. The paper compares the EMEA risk-reduction requirements and the ToR model. EMEA/CHMP/QWP/251344/2006 introduces a risk-avoidance principle based on a controversial interpretation of ‘pollution control’. The paper supports the view that this model is not optimal from a risk-management point of view. Using a ToR model could bring improvements to pharmaceutical risk decisions and would support a more practical and consensual approach to meeting the ALARP requirements.     

药品中遗传毒性杂质的评估和控制

目的 综述药品遗传毒性杂质控制相关指南和法规,为制药企业执行国际标准和准则提供一些建议和思路。方法 通过查找数据库如Pubmed、Medline及欧洲药品管理局(European Medicines Agency,EMA)、美国食品药品监督管理局(US Food and Drug Administratio,U.S.FDA)、人用药品注册技术要求国际协调会议(ICH)等网站,比较各指南法规关于遗传毒性控制限度和控制措施的异同点,为遗传毒性杂质的控制提供一个可行性步骤。结果 通过比较发现,EMA、U.S.FDA和即将出版的ICH M7指南在关键原则的应用方面如毒理学关注阈值(threshold of toxicological concern,TTC)、风险评估步骤、杂质5分类法等基本相同,但现行EMA和U.S.FDA法规存在分歧,不利于其有效执行,而ICH M7将为遗传毒性杂质的控制提供一个可行框架。结论 目前还缺乏完善有效的遗传毒性控制指南,ICH M7将解决U.S.FDA 和EMA 指南间分歧,更好地指导制药企业遗传毒性杂质的控制。     

Guidelines and pharmacopoeial standards for pharmaceutical impurities: Overview and critical assessment

ICH/regional guidances and agency scrutiny provide the regulatory framework for safety assessment and control of impurities in small-molecule drug substances and drug products. We provide a critical assessment of the principal impurity guidances and, in particular, focus on deficiencies in the derivation of the threshold of toxicological concern (TTC) as applied to genotoxic impurities and the many toxicological anomalies generated by following the current guidelines on impurities. In terms of pharmacopoeial standards, we aim to highlight the fact that strictly controlling numerous impurities, especially those that are minor structural variants of the active substance, is likely to produce minimal improvements in drug safety. It is believed that, wherever possible, there is a need to simplify and rebalance the current impurity paradigm, moving away from standards derived largely from batch analytical data towards structure-based qualification thresholds and risk assessments using readily available safety data. Such changes should also lead to a minimization of in vivo testing for toxicological qualification purposes. Recent improvements in analytical techniques and performance have enabled the detection of ever smaller amounts of impurities with increased confidence. The temptation to translate this information directly to the regulatory sphere without any kind of safety evaluation should be resisted.     

Strategies for the identification, control and determination of genotoxic impurities in drug substances: a pharmaceutical industry perspective

Regulations alarmed the control of genotoxic impurities in drug substances at lower level based on the threshold of toxicological concern and daily dose. This review explores the details of various regulations and guidances, toxicology assessment, identification of structural alerts, synthetic origins, different synthetic approaches for elimination or control, various analytical determination strategies and pharmaceutical industry concern towards genotoxic impurities.     

An impact analysis of the application of the threshold of toxicological concern concept to pharmaceuticals

The recent application of the threshold of toxicological concern (TTC) concept to the regulation of pharmaceuticals in the European Union is analyzed. The derivation of TTC and the threshold of regulation that followed it were originally intended to provide makers of food contact materials greater flexibility with their products, while allowing the CFSAN branch of FDA to conserve its resources for more important issues. A reanalysis of the scientific data employed by EMEA regulators to rationalize its 1.5 mcg default genotoxic impurity limit is presented to demonstrate (a) that direct translation of conclusions relevant to food consumption are unduly influenced by many classes of potent carcinogens of historic concern which would be impossible to generate unknowingly as pharmaceutical impurities, and (b) that the majority of reactive chemicals that would be useful to synthetic chemists are among the least potent carcinogens in the underpinning supportive analyses. Evidence is further presented to show that implementation and acceptance of a 1.5 mcg TTC-based total limit on such impurities can be expected to impede pharmaceutical research and development efficiency while providing an insignificant cancer risk-avoidance benefit to patients who require pharmaceutical treatments. The conclusion drawn is that a significantly higher default limit can readily be defended that would be both in keeping with TTC principles and the best interest of patients.     

Evaluation of inhalation TTC values with the database RepDose

The thresholds of toxicological concern (TTCs) define limit values for substances of unknown toxicity below which dietary intake is considered to be of no concern to human health. The TTC concept has already been used for risk assessment of e.g. food contaminants or flavoring substances and is in discussion to be applied to other classes of compounds such as cosmetic ingredients, household products, non-relevant metabolites in drinking water, and impurities in pharmaceuticals. The present publication aimed to evaluate whether the current TTC concept can also be applied to define limit values for inhalation exposure, using a data set of 203 industrial chemicals from the database RepDose.     

Tolerability of risk: A commentary on the nitrosamine contamination issue

For decades, regulators have grappled with different approaches to address the issue of control of impurities. Safety-based limits, such as permissible daily exposure (PDE), acceptable intake (AI), threshold of toxicological concern (TTC) and less than lifetime limits (LTL) have all been used. For many years these safety-based limits have been recognized as virtually safe doses (VSDs). Recently, however, many regulatory agencies are seeking to impose limits for N-nitrosamine impurities, which are significantly below the VSD. This commentary will discuss the evolution of safety-based limits for impurities, provide an overview of the valsartan N-nitrosamine contamination issue and review the toxicology of N-nitrosamines. The outcome of a lessons-learned exercise on sartan medications undertaken by the European Medicines Agency (EMA) will also be discussed. The review will also highlight the many analytical challenges inherent with controlling impurities to ppb-based limits. The use of highly sensitive, low ppb limits, methods may lead to future issues of batch rejection, based on false positives. Regulators initially viewed the N-nitrosamine risk as being insufficient to prompt immediate product discontinuation and patients were specifically advised to continue using their affected medication. Patients were also informed that exposure to N-nitrosamines is extremely common via food and drinking water.     

In silico approaches to predicting cancer potency for risk assessment of genotoxic impurities in drug substances

The current risk assessment approach for addressing the safety of very small concentrations of genotoxic impurities (GTIs) in drug substances is the threshold of toxicological concern (TTC). The TTC is based on several conservative assumptions because of the uncertainty associated with deriving an excess cancer risk when no carcinogenicity data are available for the impurity. It is a default approach derived from a distribution of carcinogens and does not take into account the properties of a specific chemical. The purpose of the study was to use in silico tools to predict the cancer potency (TD₅₀) of a compound based on its structure. Structure activity relationship (SAR) models (classification/regression) were developed from the carcinogenicity potency database using MultiCASE and VISDOM. The MultiCASE classification models allowed the prediction of carcinogenic potency class, while the VISDOM regression models predicted a numerical TD₅₀. A step-wise approach is proposed to calculate predicted numerical TD₅₀ values for compounds categorized as not potent. This approach for non-potent compounds can be used to establish safe levels greater than the TTC for GTIs in a drug substance.     

Structure-based thresholds of toxicological concern (TTC): guidance for application to substances present at low levels in the diet.

The threshold of toxicological concern (TTC) is a pragmatic risk assessment tool that is based on the principle of establishing a human exposure threshold value for all chemicals, below which there is a very low probability of an appreciable risk to human health. The concept that there are levels of exposure that do not cause adverse effects is inherent in setting acceptable daily intakes (ADIs) for chemicals with known toxicological profiles. The TTC principle extends this concept by proposing that a de minimis value can be identified for many chemicals, in the absence of a full toxicity database, based on their chemical structures and the known toxicity of chemicals which share similar structural characteristics. The establishment and application of widely accepted TTC values would benefit consumers, industry and regulators. By avoiding unnecessary toxicity testing and safety evaluations when human intakes are below such a threshold, application of the TTC approach would focus limited resources of time, cost, animal use and expertise on the testing and evaluation of substances with the greatest potential to pose risks to human health and thereby contribute to a reduction in the use of animals. An Expert Group of the European branch of the International Life Sciences Institute-ILSI Europe-has examined the TTC principle for its wider applicability in food safety evaluation. The Expert Group examined metabolism and accumulation, structural alerts, endocrine disrupting chemicals and specific endpoints, such as neurotoxicity, teratogenicity, developmental toxicity, allergenicity and immunotoxicity, and determined whether such properties or endpoints had to be taken into consideration specifically in a step-wise approach. The Expert Group concluded that the TTC principle can be applied for low concentrations in food of chemicals that lack toxicity data, provided that there is a sound intake estimate. The use of a decision tree to apply the TTC principle is proposed, and this paper describes the step-wise process in detail. Proteins, heavy metals and polyhalogenated-dibenzodioxins and related compounds were excluded from this approach. When assessing a chemical, a review of prior knowledge and context of use should always precede the use of the TTC decision tree. The initial step is the identification and evaluation of possible genotoxic and/or high potency carcinogens. Following this step, non-genotoxic substances are evaluated in a sequence of steps related to the concerns that would be associated with increasing intakes. For organophosphates a TTC of 18microg per person per day (0.3 microg/kg bw/day) is proposed, and when the compound is not an OP, the TTC values for the Cramer structural classes III, II and I, with their respective TTC levels (e.g. 1800, 540 and 90 microg per person per day; or 30, 9 and 1.5 microg/kg bw /day), would be applied sequentially. All other endpoints or properties were shown to have a distribution of no observed effect levels (NOELs) similar to the distribution of NOELs for general toxicity endpoints in Cramer classes I, II and III. The document was discussed with a wider audience during a workshop held in March 2003 (see list of workshop participants).     

Application of the threshold of toxicological concern (TTC) to the safety evaluation of cosmetic ingredients

The threshold of toxicological concern (TTC) has been used for the safety assessment of packaging migrants and flavouring agents that occur in food. The approach compares the estimated oral intake with a TTC value derived from chronic oral toxicity data for structurally-related compounds. Application of the TTC approach to cosmetic ingredients and impurities requires consideration of whether route-dependent differences in first-pass metabolism could affect the applicability of TTC values derived from oral data to the topical route. The physicochemical characteristics of the chemical and the pattern of cosmetic use would affect the long-term average internal dose that is compared with the relevant TTC value. Analysis has shown that the oral TTC values are valid for topical exposures and that the relationship between the external topical dose and the internal dose can be taken into account by conservative default adjustment factors. The TTC approach relates to systemic effects, and use of the proposed procedure would not provide an assessment of any local effects at the site of application. Overall the TTC approach provides a useful additional tool for the safety evaluation of cosmetic ingredients and impurities of known chemical structure in the absence of chemical-specific toxicology data.     

Improved in silico prediction of carcinogenic potency (TD50) and the risk specific dose (RSD) adjusted Threshold of Toxicological Concern (TTC) for genotoxic chemicals and pharmaceutical impurities

The Threshold of Toxicological Concern (TTC) is a level of exposure to a genotoxic impurity that is considered to represent a negligible risk to humans. The TTC was derived from the results of rodent carcinogenicity TD50 values that are a measure of carcinogenic potency. The TTC currently sets a default limit of 1.5 μg/day in food contact substances and pharmaceuticals for all genotoxic impurities without carcinogenicity data. Bercu et al. (2010) used the QSAR predicted TD50 to calculate a risk specific dose (RSD) which is a carcinogenic potency adjusted TTC for genotoxic impurities. This promising approach is currently limited by the software used, a combination of MC4PC (www.multicase.com) and a Lilly Inc. in-house software (VISDOM) that is not available to the public. In this report the TD50 and RSD were predicted using a commercially available software, SciQSAR (formally MDL-QSAR, www.scimatics.com) employing the same TD50 training data set and external validation test set that was used by Bercu et al. (2010). The results demonstrate the general applicability of QSAR predicted TD50 values to determine the RSDs for genotoxic impurities and the improved performance of SciQSAR for predicting TD50 values.     

Guidance on the establishment of acceptable daily exposure limits (ADE) to support Risk-Based Manufacture of Pharmaceutical Products

Health-based limits for active pharmaceutical ingredients (API) referred to as acceptable daily exposures (ADEs) are necessary to the pharmaceutical industry and used to derive acceptance limits for cleaning validation purposes and evaluating cross-carryover. ADEs represent a dose of an API unlikely to cause adverse effects if an individual is exposed, by any route, at or below this dose every day over a lifetime. Derivations of ADEs need to be consistent with ICH Q9 as well as other scientific approaches for the derivation of health-based limits that help to manage risks to both product quality and operator safety during the manufacture of pharmaceutical products. Previous methods for the establishment of acceptance limits in cleaning validation programs are considered arbitrary and have largely ignored the available clinical and toxicological data available for a drug substance. Since the ADE utilizes all available pharmaceutical data and applies scientifically acceptable risk assessment methodology it is more holistic and consistent with other quantitative risk assessments purposes such derivation of occupational exposure limits. Processes for hazard identification, dose response assessment, uncertainty factor analysis and documentation are reviewed.     

A perspective on testing of existing pharmaceutical excipients for genotoxic impurities

Guidance recommendations by the Committee for Medicinal Products for Human Use (CHMP) and Pharmaceutical Research and Manufacturers of America (PhRMA) acknowledge the presence of potential toxic impurities in some pharmaceutical ingredients and have proposed setting limits on impurities with genotoxic activity as a means to protect patients in clinical trials and for marketing of the approved products. Recently, there have been suggestions that drug excipients, including existing products, should also be subjected to the same testing procedures and intake limits as proposed for active ingredients. This report is an attempt to put such recommendations into the proper perspective regarding the likelihood of protecting or improving public health.     

Best Practices in Stability Indicating Method Development and Validation for Non-clinical Dose Formulations

Non-clinical dose formulations (also known as pre-clinical or GLP formulations) play a key role in early drug development. These formulations are used to introduce active pharmaceutical ingredients (APIs) into test organisms for both pharmacokinetic and toxicological studies. Since these studies are ultimately used to support dose and safety ranges in human studies, it is important to understand not only the concentration and PK/PD of the active ingredient but also to generate safety data for likely process impurities and degradation products of the active ingredient. As such, many in the industry have chosen to develop and validate methods which can accurately detect and quantify the active ingredient along with impurities and degradation products. Such methods often provide trendable results which are predictive of stability, thus leading to the name; stability indicating methods. This document provides an overview of best practices for those choosing to include development and validation of such methods as part of their non-clinical drug development program. This document is intended to support teams who are either new to stability indicating method development and validation or who are less familiar with the requirements of validation due to their position within the product development life cycle.     

Analytical control of genotoxic impurities in the pazopanib hydrochloride manufacturing process

Pharmaceutical regulatory agencies are increasingly concerned with trace-level genotoxic impurities in drug substances, requiring manufacturers to deliver innovative approaches for their analysis and control. The need to control most genotoxic impurities in the low ppm level relative to the active pharmaceutical ingredient (API), combined with the often reactive and labile nature of genotoxic impurities, poses significant analytical challenges. Therefore, sophisticated analytical methodologies are often developed to test and control genotoxic impurities in drug substances. From a quality-by-design perspective, product quality (genotoxic impurity levels in this case) should be built into the manufacturing process. This necessitates a practical analysis and control strategy derived on the premise of in-depth process understanding. General guidance on how to develop strategies for the analysis and control of genotoxic impurities is currently lacking in the pharmaceutical industry. In this work, we demonstrate practical examples for the analytical control of five genotoxic impurities in the manufacturing process of pazopanib hydrochloride, an anticancer drug currently in Phase III clinical development, which may serve as a model for the other products in development. Through detailed process understanding, we implemented an analysis and control strategy that enables the control of the five genotoxic impurities upstream in the manufacturing process at the starting materials or intermediates rather than at the final API. This allows the control limits to be set at percent levels rather than ppm levels, thereby simplifying the analytical testing and the analytical toolkits to be used in quality control laboratories.     

N-亚硝胺类基因毒性杂质的研究进展

自“缬沙坦事件”之后，N-亚硝胺类基因毒性杂质引起了业界的广泛关注。本文概述了药物中N-亚硝胺类基因毒性杂质和相关检测方法的研究进展，以及近20年来国内外有关药物中基因毒性杂质监管指南的完善历程。N-亚硝胺类基因毒性杂质作为一类高反应活性的基因毒性杂质，主要来源于药物合成过程中发生的副反应，以及药物在储存或者运输过程中发生的氧化或还原等反应。所有的动物实验表明，N-亚硝胺类具有很强的致癌性。在理论上，所有药物都存在N-亚硝胺类杂质或被N-亚硝胺类杂质污染的风险，由于该类化合物在药物中常以痕量形式存在，在分析检测过程中药物基质干扰大，因此建立便捷、高效的分析方法是非常有必要的。     

An evaluation of the sensitivity of the Ames assay to discern low-level mutagenic impurities

Low level impurities often reside in active pharmaceutical ingredients (API). Some of these impurities are potentially genotoxic since reactive intermediates are used in the synthetic route for the production of API. Routine mutagenicity testing is conducted in support of clinical trials with the intent to identify genotoxic hazards associated with API. Depending on the amount of impurity present in the API tested, the potency of the impurities and the relative sensitivity of the Ames assay, it is possible that mutagenicity associated with the presence of genotoxic impurities could also be detected while testing API. Therefore, we evaluated published data and generated new information to understand the sensitivity of the Ames assay. Based on a literature survey of approximately 450 mutagens, it was estimated that 85% of mutagens are detected at concentrations of 250 microg/plate or less. Based on this estimate, most mutagens should be detected in an Ames assay testing API concentrations up to 5000 microg/plate if present at a 5% or greater concentration. Data from experiments where several direct and indirect-acting mutagens were spiked into representative API further support the literature-based evaluation. Some limitations of this approach, including toxicity of API and competing metabolism are discussed.