Chem. Res. Toxicol. 2006, 19, 1561-1563
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A Regulatory Perspective on Issues and Approaches in Characterizing Human Metabolites† Karen L. Davis-Bruno* and Aisar Atrakchi U.S. Food and Drug Administration, Center for Drug EValuation and Research, Office of New Drugs, SilVer Spring, Maryland 20993 ReceiVed August 22, 2006
This document captures the current thinking within FDA/CDER on the non-clinical safety assessment of human drug metabolites in new drug products. Examples are provided, which define a scientific based approach to the safety evaluation of human metabolites in new drug candidates. A discussion of the need for, and the adequacy of, the assessment of human drug metabolites with specific regard to their potential as mediators of toxicity is presented from a regulatory perspective. Contents 1. Introduction 2. Magnitude of the Problem 3. Approaches for the Non-Clinical Safety Assessment of Major Human Metabolites 4. Approaches on How to Deal with Major Human Drug Metabolites 4.1. Example 1 4.2. Example 2
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1. Introduction During the drug development process, a series of non-clinical studies are performed that address efficacy and safety in a relevant rodent and non-rodent animal species (1). The nonclinical evaluation of safety, typically consists of standard toxicology studies that commonly incorporate an assessment of drug exposure. Plasma levels of the parent drug are monitored in non-clinical and clinical studies, thereby providing an index of systemic exposure, serving as part of the risk assessment for clinical studies. This particular testing paradigm is sufficient when the metabolic profile in at least one of the relevant animal species is similar to that in humans. However, there are cases when significant qualitative and/or quantitative differences in the metabolic profile exist between the animal test species and humans. In such cases, the typical non-clinical toxicity testing battery may be inadequate to characterize organ toxicity because animals may not have been sufficiently exposed to these particular human metabolites during toxicity testing because some metabolites might form at disproportionately higher levels in humans than in the animal species used in non-clinical testing. The role of metabolites as mediators of drug toxicity has not always been considered in safety assessment, although examples of toxic metabolites, such as acetaminophen, mephenytoin, and so forth, have been known for decades. Most often, the parent drug plasma concentration is measured and used as an index of † Disclaimer: The views and opinions expressed in this document are solely those of the authors and are not the official policy of the USFDA. * To whom correspondence should be addressed. Fax: (301) 796-9712. E-mail:
[email protected].
systemic exposure and safety assessment in humans. This lack of appreciation related to the potential toxicity of metabolites is partly due to the inadequacy of analytical methods, which allow the detection and characterization of these compounds in addition to the parent drug molecule. However, technological advances during the past decade have greatly improved analytical capabilities to detect, identify, and characterize metabolites at previously unattainable levels. A renewed interest in potential toxicity associated with drug metabolites has been fostered by several publications, workshops, and a recent publication of the draft FDA/CDER Guidance to Industry: Safety Testing of Drug Metabolites (2). Prior to this FDA/CDER regulatory guidance, scientists from the pharmaceutical industry, academia, and regulatory agencies organized a forum to discuss the contributions of metabolites to drug toxicity and published these deliberations (3, 4). Several other workshops followed after this publication, which further emphasized the potential impact of drug metabolites in drug development. The objectives of the FDA/CDER draft guidance are to address situations in which metabolites of drugs may need further safety characterization as part of an investigational new drug application (IND) or new drug marketing application (NDA) in the United States. The purpose of this current publication is to discuss from a regulatory perspective what constitutes a safety concern as it relates to a human drug metabolite, how to address these concerns, and what type and duration of non-clinical studies are needed to adequately characterize the safety of the metabolite. The views expressed in this publication do not necessarily reflect those of the FDA. A decision to conduct direct safety testing of a metabolite should be based on a comprehensive evaluation of all studies conducted with the parent drug and all of the parameters involved in the safety assessment. On many occasions, a caseby-case approach is adopted because of the diverse nature and individuality of each drug molecule.
2. Magnitude of the Problem Studies in laboratory animals generally form the basis of clinical risk assessment for potential drug toxicity. A measurement of the circulating levels of the parent drug has been used as an index of systemic exposure and is the primary focus of toxicokinetic evaluations. However, it is important to consider the combined exposure of parent and active metabolites in safety
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assessment. This is because a metabolite might be more active (codeine to morphine), have equal activity (aspirin to salicylic acid), have less activity (glucuronide conjugates of the parent drug), or possess different activity (phenylbutazone to oxyphenbutazone) compared to that of the parent molecule. An active metabolite may not only bind to the therapeutic target receptors but also cause non-receptor mediated effects. This is particularly important when such a metabolite is produced in humans and not in animals. The incidence of a metabolite formed only in humans and not in any animal test species (hence referred to as unique metabolite) is a rare occurrence, on the basis of information available in the CDER databases. A more frequent situation is the formation of a metabolite at disproportionately higher levels in humans than in the animal species used in safety testing of the parent drug. This disproportionality stems from the typical qualitative and/or quantitative differences in metabolic profiles between humans and animals. If at least one animal test species forms this human metabolite and some assessment of exposure was done during toxicology testing of the parent, it is assumed that the metabolite’s contribution to overall toxicity has been established. Individual drug metabolites do not commonly undergo systematic evaluation in routine safety assessment across animal species, and therefore, their specific contribution to the overall toxicity of the drug is unknown. Nevertheless, when a human metabolite is formed at disproportionally higher levels compared to those of any of the animal test species, for example, >10% of administered dose, such a product may be considered a major metabolite. The contribution of this major metabolite to clinical risk is usually unknown because its potential toxicity in animals has not been adequately investigated. In these cases, this major human metabolite would need further evaluation in non-clinical studies to determine its toxicologic potential. Conjugated metabolites are generally excluded from such testing, unless there is a reason to believe that they are chemically reactive, as in the case of acyl glucuronides. Moreover, chemically reactive intermediates are rarely detectable because of their short half-lives. However, stable products of these intermediates, for example, glutathione conjugates, can be measured and may provide some indication of exposure to these short-lived reactive metabolites. This can eliminate the need for additional qualification of the reactive metabolites.
3. Approaches for the Non-Clinical Safety Assessment of Major Human Metabolites One approach to qualify a major human metabolite would be to find alternative animal species that form the metabolite in ViVo. A second approach, if alternative animal species cannot be identified, would be the direct dosing of the major metabolite to the appropriate animal species. This second approach would require synthesis of the major metabolite and development of analytical methods capable of identifying and measuring the metabolite in non-clinical toxicity studies. The type and duration of the non-clinical studies needed to qualify the metabolite would be similar to those applied to the parent drug. However, it should be noted that a case-by-case approach applies, and in some cases, not all non-clinical studies need to be conducted. This second approach is consistent with other FDA guidelines, whereby an impurity or a degradant should be qualified in a drug product if present at >0.1% of drug substance (5, 6). Without addressing specific issues of proper methods for qualification under ICH Q3A and the inherent differences between a contaminant or degradant and human metabolite, when there are concerns for human safety, major metabolites
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should be evaluated and risk to humans determined. These guidelines on qualification are consistent with those of other government agencies for similar situations (7). The typical nonclinical studies needed to qualify a major human metabolite include, but are not limited to, general toxicity, genotoxicity, embryo-fetal development, and carcinogenicity studies. As stated earlier, not all of these studies may be needed to qualify a major metabolite. A comprehensive review and evaluation of all available data for the parent drug and metabolite can be considered along with other factors to determine the type and duration of non-clinical studies needed with the major metabolite. These factors include, but are not limited to, duration of clinical use, the patient population, metabolite solubility and stability in stomach pH (and its other physical-chemical properties), phase 1 versus phase 2 derived metabolites, and so forth. Modified drug development plans may be more appropriate for drugs indicated for life-threatening diseases, such as amyotrophic lateral sclerosis (ALS) and stroke, or oncology treatments in order to optimize and expedite their development. It would be prudent to distinguish, as early as possible, between metabolites that are major versus minor, those that are pharmacologically/toxicologically active versus inactive, those that are oxidative or conjugated, or formed from reactive intermediates. As indicated earlier, conjugated metabolites are generally excluded from safety evaluation unless there is a reason to believe that they are chemically reactive (e.g., acyl glucuronides). Chemically reactive intermediates are rarely detectable because of their short half-lives, but if they form stable intermediates (e.g., glutathione conjugates), this may provide some indication of exposure to these potentially toxic species and eliminate the need for further qualification. In general, the timing for the conduct of the non-clinical studies with the major human metabolite should precede large scale clinical trials. Studies may need to be conducted earlier if the metabolite belongs to a chemical class with known toxicity or has a positive structural alert for toxicity or is potentially associated with a novel toxicity not observed with the parent drug.
4. Approaches on How to Deal with Major Human Drug Metabolites Early identification and evaluation of major human drug metabolites is encouraged to prevent delays in drug development. This is not a new concept because in Vitro metabolism studies using humans and several animal test species are typically performed early in drug development to screen for qualitative similarities (or differences) in the metabolism between humans and animal species. This initial screen would allow the selection of the most appropriate animal species possessing the metabolic profile most similar to humans for use in future toxicity studies. Although there are instances in which the in Vitro profile does not necessarily represent in ViVo metabolism, this would be an initial step that could be taken prior to subsequent in ViVo metabolism studies in humans. There have been cases in which the clinical development of a drug was halted and/or delayed because sponsors did not conduct in ViVo metabolism studies until later in phase 3 of clinical development, at which time a major human metabolite was identified. The difficulties associated with synthesizing a specific metabolite as well as the inherent complexities that accompanies its direct administration are acknowledged. Direct dosing of a metabolite in animals may lead to subsequent metabolism that may not reflect the clinical situation and complicate the toxicity
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evaluation. Moreover, it is possible that new and different toxicities may arise from metabolite administration that were not observed with the parent drug. Another confounding factor may be the presence of impurities associated with the major metabolite that by themselves may compromise the safety evaluation of direct dosing of the major metabolite. However, notwithstanding these possible complications, the potential toxicity of a major human metabolite should be investigated in order to ensure adequate clinical safety. Below are proprietary examples of two cases where major human metabolites were identified. In the first case based on the available information from non-clinical toxicity studies with the parent, toxicity studies with the major metabolite were not needed because the metabolite was adequately characterized. In the second case, the major metabolite had to be qualified, and toxicity studies with direct administration of the metabolite were conducted.
4.1. Example 1 • A metabolite represents 1-2% of total radioactive circulating drug-related material in rat plasma, 5% in dogs, and 20% in humans and is, therefore, considered a major human metabolite. • Because toxicology testing in rats and dogs typically involves the use of doses higher than those in human clinical trials, the exposure of the metabolite in the animal test species, in this case, is much greater than the clinical exposure of the metabolite in humans at the maximum recommended clinical therapeutic dose. Therefore, the available general toxicity studies adequately characterize the toxicity of the metabolite in rats and dogs. • The concentrations of the major human metabolite in the in ViVo genotoxicity and the rat embryo-fetal developmental and carcinogenicity studies provided adequate exposure and characterization of the metabolite. Therefore, additional testing with the metabolite was not needed.
Table 1. Steady-State Exposure at the Maximum Dose steady-state exposure (AUC0-24) parent M4
human
monkey
rat
1800 7700
15,000 5000
12,500 135
• Rat embryo-fetal development study; • Genotoxicity testing in Vitro: M4 was positive for point mutations and chromosomal aberrations, and the parent drug was negative. • Because of the positive genotoxicity testing with M4, a carcinogenicity assessment was recommended. It is important to understand that it is not the general practice of the FDA/CDER to request a complete toxicity profile once a major human metabolite has been identified. The decisions that determine the types of non-clinical toxicity studies that need to be performed must be based on scientific justification and comprehensive evaluation of the available data on both the parent drug and its major metabolite(s). In some cases, more extensive toxicological characterization of a major human metabolite may be necessary, whereas in other circumstances, safety testing of metabolites may not be necessary (e.g., cytotoxic agents for an oncology treatment indication). Scientific justification can be used in a weight-of-evidence approach to address the minimal safety concern for metabolites that are similar to the parent drug, present at clinically low exposures, or have a negative SAR for a particular toxicity. Sponsors are encouraged to contact the appropriate FDA division and discuss their findings along with proposed study protocols and duration of dosing. With the availability of more sophisticated analytical technologies, it is highly desirable and advantageous to sponsors to investigate parent drug metabolism in humans and animals early during drug development. Failure to do so could lead to significant delays in clinical development programs, should major differences in human and animal drug metabolism be identified, especially if these studies are delayed until later phases of drug development.
4.2. Example 2 Two primary hydroxylated metabolites, M1 and M2, underwent further oxidative metabolism to form secondary metabolites M3 and M4 as observed using hepatic microsomes and hepatocytes from humans, monkeys, rats, dogs, rabbits, and mice. The results showed the following. • M1 and M4 were the predominant metabolites in human, monkey, and dog microsomes, whereas M2 and M3 were formed in rat, mouse, and rabbit microsomes. • M4 is formed in humans at levels 4-fold higher than that of the parent drug, but M4 is not formed in rodents. M4 is formed in monkeys but at much lower levels than those in humans. At the highest doses tested in toxicity studies in monkeys, M4 concentrations were ∼60% of the human maximum therapeutic exposures. Severe drug related and novel target organ toxicities were observed in monkeys. • M4 is not pharmacologically active at the drug target receptor. • Additional testing was performed with M4: • Subchronic toxicity study (3-months) in rats;
References (1) U.S. Food and Drug Administraton/CDER (1995) Guidance for Industry: Content and Format of INDs for Phase 1 Studies of Drugs, CFR 21, section 31. (2) U.S. Food and Drug Administration (2005) Draft Guidance to Industry Safety Testing of Drug Metabolites. www.fda.gov/cder/guidance. (3) Baillie, T. A., M. N., Cayen, H., Fouda, R. H., Gerson, J. D., and Green et al. (2002) Drug metabolites in safety testing. Toxicol. Appl. Pharmacol. 182, 188-196. (4) Hastings, K. L., El-Hage, J., Jacobs, A., Leighton, J., and Morse, D., Osterberg, R. (2003) Drug metabolites in safety testing. Toxicol. Appl. Pharmacol. 190, 91-92. (5) U.S. Food and Drug Administration (2002) Guideline for Metabolism Studies and for Selection of Residues for Toxicological Testing, U.S. Food and Drug Administration, Center for Veterinary Medicine. www.fda.gov/cvm/guidance/guideline3pt1.html. (6) U.S. Food and Drug Administration (2003) Guidance to Industry: Impurities in New Drug Substances; ICH Q3A. www.fda.gov/cder/ guidance. (7) U.S. Environmental Protection Agency (1998) Health Effects Test Guidelines, OPPTS 870.7485, Metabolism and Pharmacokinetics. www.epa.gov/epahome/research.htm.
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