Guidance for Industry 

Genotoxic and Carcinogenic 

Impurities in Drug Substances and 

Products: Recommended Approaches 

DRAFT GUIDANCE

This guidance document is being distributed for comment purposes only. 

Comments and suggestions regarding this draft document should be submitted within 60 days of 

publication in the Federal Register of the notice announcing the availability of the draft 

guidance. Submit comments to the Division of Dockets Management (HFA-305), Food and 

Drug Administration, 5630 Fishers Lane, rm. 1061, Rockville, MD 20852. All comments 

should be identified with the docket number listed in the notice of availability that publishes in 

the Federal Register. 

For questions regarding this draft document contact David Jacobson-Kram at 301-796-0175. 

U.S. Department of Health and Human Services 

Food and Drug Administration 

Center for Drug Evaluation and Research (CDER) 

December 2008 

Pharmacology and Toxicology 

I:\7834dft.doc 

12/03/08 
Guidance for Industry 

Genotoxic and Carcinogenic 

Impurities in Drug Substances and 

Products: Recommended Approaches 

Additional copies are available from: 

Office of Communications 

Division of Drug Information 

Center for Drug Evaluation and Research 

Food and Drug Administration 

10903 New Hampshire Ave., Bldg. 51, rm. 2201 

Silver Spring, MD 20993-0002 

E-mail: druginfo@fda.hhs.gov 

Fax: 301-847-8714 

(Tel) 301-796-3400 

http://www.fda.gov/cder/guidance/index.htm 

U.S. Department of Health and Human Services 

Food and Drug Administration 

Center for Drug Evaluation and Research (CDER) 

December 2008 

Pharmacology and Toxicology 
TABLE OF CONTENTS 

I. INTRODUCTION............................................................................................................. 1

II. BACKGROUND ............................................................................................................... 2

A. ICH Guidances for Industry Relating to Drug Impurities and Residual Solvents ..................3

B. EMEA Proposed Guideline on Limits of Genotoxic Impurities ................................................4

III. RECOMMENDED APPROACHES FOR INITIAL ASSESSMENT OF 

GENOTOXIC POTENTIAL OF IMPURITIES ........................................................... 6

IV. RECOMMENDED APPROACHES FOR HANDLING GENOTOXIC AND 

CARCINOGENIC IMPURITIES ................................................................................... 7

A. Prevention of Genotoxic and Carcinogenic Impurity Formation..............................................7

B. Reduction of Genotoxic and Carcinogenic Impurity Levels......................................................7

1. Acceptable Levels to Support Marketing Applications ....................................................................7

2. Acceptable Levels during Clinical Development ...........................................................................10

C. Additional Characterization of Genotoxic and Carcinogenic Risk.........................................11

D. Considerations for Flexibility in Approach...............................................................................12

APPENDIX A: DECISION TREE FLOW DIAGRAM......................................................... 13
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Guidance for Industry1 1 

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Genotoxic and Carcinogenic Impurities in Drug 

Substances and Products: Recommended Approaches 

7 

8 This draft guidance, when finalized, will represent the Food and Drug Administration’s (FDA’s) current 

9 thinking on this topic. It does not create or confer any rights for or on any person and does not operate to 

10 bind FDA or the public. You can use an alternative approach if the approach satisfies the requirements of 

11 the applicable statutes and regulations. If you want to discuss an alternative approach, contact the FDA 

12 staff responsible for implementing this guidance. If you cannot identify the appropriate FDA staff, call 

13 the appropriate number listed on the title page of this guidance. 

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I. INTRODUCTION 

This guidance is intended to inform pharmaceutical manufacturers of the Food and Drug 

Administration’s (FDA’s) current thinking regarding genotoxic and carcinogenic impurities in 

drug substances and drug products, including biologic products that are regulated by the Center 

for Drug Evaluation and Research (CDER). This guidance provides recommendations on how to 

evaluate the safety of these impurities during clinical development (investigational new drug 

applications (INDs)) and for marketing applications (new drug applications (NDAs), biologics 

license applications (BLAs), and abbreviated new drug applications (ANDAs)). This guidance 

provides recommended exposure thresholds on the clinical exposure to genotoxic or 

carcinogenic impurities. Also provided are additional testing and exposure threshold 

recommendations for situations where there are known or theoretical safety concerns based on 

available data, structural alerts, and/or assessment of the synthetic pathway. 

This guidance is intended as an adjunct to the ICH guidances for industry Q3A(R2) Impurities in 

New Drug Substances, Q3B(R2) Impurities in New Drug Products, and Q3C(R3) Impurities: 

Residual Solvents that deal with the topic of impurities in a more general fashion.2

 This 

guidance provides specific recommendations regarding the safety qualification of impurities with 

known or suspected genotoxic or carcinogenic potential while the ICH guidances provide only 

general direction. This guidance addresses synthetic impurities and degradants in drug 

substances, but does not otherwise address the genotoxicity or carcinogenicity of actual drug 

substances or intended drug product ingredients. This guidance also applies to known starting 

materials or anticipated reaction products. 

1

 This guidance has been prepared by the Office of New Drugs in the Center for Drug Evaluation and Research 

(CDER) at the Food and Drug Administration. 

2

 See http://www.fda.gov/cder/guidance/index.htm. The FDA has incorporated revision 3 (R3) of ICH Q3C into the 

guidance for industry Q3C — Tables and List, which is posted on the CDER guidance Web site. 

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This guidance describes a variety of ways to characterize and reduce the potential lifetime cancer 

risk associated with patient exposure to genotoxic and carcinogenic impurities both during 

clinical development and after approval. These approaches include: 

• Changing the synthetic and/or purification routes to minimize the formation and/or 

maximize the removal of the relevant impurity. 

• Allowing a maximum daily exposure target of 1.5 μg per day for the relevant impurity as 

a general target for marketed products, though higher levels may be acceptable during 

clinical development. Certain impurities with structural alerts suggesting particularly 

high genotoxic and carcinogenic potential would not be appropriate for this general 

threshold approach and would need to be evaluated on a case-by-case basis. 

• Further characterizing the genotoxic and carcinogenic risk via mechanism of action or 

weight-of-evidence approaches, or through additional studies to better support 

appropriate impurity specifications. 

This guidance also applies to drug products approved before the issuance of this guidance, but 

only in the presence of a specific safety signal that suggests the potential for an increased 

carcinogenic risk associated with the presence of an impurity or degradant, or with regard to a 

supplemental application for a previously approved drug product that proposes a significant 

change in the drug product’s approved labeling that suggests the potential for an increased 

carcinogenic risk associated with the presence of an impurity or degradant (e.g., new indication, 

new dosage regimen, longer duration of use). Applicants also should take these 

recommendations into consideration when preparing supplemental manufacturing submissions to 

NDAs, BLAs, and ANDAs, such as submissions proposing new formulations or new synthetic 

routes. Although this guidance applies to impurities present in biologic products regulated by 

CDER, it is noted that, in most cases, the genotoxicity assays conducted for small molecule 

pharmaceuticals are not applicable to biopharmaceuticals. Likewise, the standard assessment of 

the genotoxic potential of impurities in biopharmaceuticals may not be appropriate in many cases 

since they may include residual host cell proteins and nucleic material, fermentation components, 

and bacterial and viral components and do not include organic chemicals typically found in small 

molecule manufacturing. 

FDA’s guidance documents, including this guidance, do not establish legally enforceable 

responsibilities. Instead, guidances describe the Agency’s current thinking on a topic and should 

be viewed only as recommendations, unless specific regulatory or statutory requirements are 

cited. The use of the word should in Agency guidances means that something is suggested or 

recommended, but not required. 

II. BACKGROUND 

Compounds that have been demonstrated to induce genetic mutations, chromosomal breaks, 

and/or chromosomal rearrangements are considered genotoxic and have the potential to cause 

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cancer in humans. Exposures to even low levels of these impurities may be of significant 

concern. Therefore, the identification limits provided in ICH Q3A(R2) and ICH Q3B(R2) may 

not be acceptable for genotoxic or carcinogenic impurities. For instance, under some scenarios 

the limits in these ICH guidances would allow a genotoxic or carcinogenic impurity to be present 

in a drug product at a level resulting in exposures up to 3,000 μg per day without needing 

identification. Although genotoxic and carcinogenic properties can be acceptable for some 

active pharmaceutical ingredients (APIs) depending on clinical circumstances (e.g., cancer 

chemotherapies), impurities in drug substances and drug products generally do not have 

beneficial effects and may impose a risk without associated benefit. Therefore, manufacturers 

should strive to achieve the lowest levels of genotoxic or carcinogenic impurities that are 

technically feasible and/or levels that convey no significant cancer risk. 

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Currently available guidances that address issues related to impurities and residual solvents 

include ICH Q3A(R2), ICH Q3B(R2), and ICH Q3C(R3). In addition, the European Medicines 

Agency’s (EMEA) Committee for Medicinal Products for Human Use (CHMP) published a 

guideline regarding limits of genotoxic impurities.3

 These documents are discussed below to 

provide a background to this guidance, but the inclusion of the EMEA guideline in this 

background discussion should not be interpreted as an FDA endorsement of that document. 

A. ICH Guidances for Industry Relating to Drug Impurities and Residual 

Solvents 

ICH Q3A(R2) and ICH Q3B(R2) address the issue of impurities in drug substances and drug 

products, respectively. ICH Q3A(R2) addresses the identification and qualification of impurities 

in drug substances approved after the issuance of the guidance, and ICH Q3B(R2) addresses only 

those impurities in drug products approved after the issuance of the guidance that are classified 

as degradation products of the drug substance or reaction products of the drug substance with an 

excipient and/or immediate container closure system. These guidances define an impurity as any 

component of the drug substance or drug product other than the chemical entity that makes up 

the drug substance or an excipient in the drug product. Depending on the quantity of drug 

substance or drug product to which a patient is exposed, these guidances recommend thresholds 

for the identification, reporting, and qualification of impurities. Qualification, as defined by the 

two guidances, is the process of acquiring and evaluating data that establishes the biological 

safety of an individual impurity (or degradation product) or a given impurity (or degradation) 

profile at the level(s) specified.4

 Higher or lower thresholds for qualification can be considered 

appropriate based on scientific rationale and level of concern.5

These guidances recommend when, after consideration of factors such as the patient population 

and duration of use, qualification studies of an impurity are appropriate. Part of the battery of 

tests used to qualify an impurity could include assays to determine whether the impurity is 

3

 Guideline on the Limits of Genotoxic Impurities (EMEA guideline), June 2006 (http://www.emea.europa.eu). 

4

 See the Glossary sections in ICH Q3A(R2) and ICH Q3B(R2). 

5

 See ICH Q3A(R2), section VII, and ICH Q3B(R2), section VI. 

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genotoxic.6 These guidances also recommend that, when considered appropriate, assays to 

assess genotoxic potential include the “minimum screen” of in vitro assays: a gene mutation 

assay and a chromosomal aberration assay.

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7

 ICH Q3A(R2) indicates that “such studies can be 

conducted on the new drug substance containing the impurities to be controlled, although studies 

using isolated impurities can sometimes be appropriate.”8

 A similar recommendation is included 

in ICH Q3B(R2). 

It should be noted, however, that allowing genotoxicity assessment of the impurity as it is 

present with the drug substance, rather than in isolation, renders the genotoxicity assessments 

much less sensitive. For example, the potent mutagens that are typically used as positive 

controls in the bacterial mutation assay, such as 9-aminoanthracene and methyl 

methanesulfonate, when present with a noncytotoxic drug substance at the minimal level for 

qualification, would not be detected by these genotoxicity assays because the maximum 

concentration of the impurity at the limit concentration of the drug substance would not be 

sufficient to produce a genotoxic response in the assays. If the drug substance is cytotoxic, this 

approach of assessing the impurity as it is present with the drug substance would be even more 

insensitive, since the drug’s toxicity would further limit the level at which the impurity could be 

tested. 

Although the ICH guidances provide some recommendations on the types of tests that should be 

conducted, the guidances do not provide specific recommendations on how to proceed if one or 

both of the genetic toxicology tests are positive; they simply state that additional testing, removal 

of the impurity, or lowering the level of the impurity should be considered. 

ICH Q3C(R3) recommends acceptable concentration limits or permissible daily exposures for 

various classes of solvents, which are one type of impurity. The guidance does not, however, 

include a recommendation on limiting exposure based upon concerns for genotoxic potential. 

The guidance recommends only that mathematical models be used for setting exposure limits in 

cases where reliable carcinogenicity data are available. 

The ICH guidances on impurities and residual solvents do not apply to drug substances or drug 

products used during the clinical research stages of development. 

B. EMEA Proposed Guideline on Limits of Genotoxic Impurities 

In June 2006, the EMEA’s CHMP published a guideline on the limits of genotoxic impurities in 

support of a marketing application.9

 A subsequent CHMP safety working party published a 

6

 See ICH Q3A(R2), section VII and Attachment 3, and ICH Q3B(R2), section VI and Attachment 3. 

7

 Ibid. 

8

 See ICH Q3A(R2), section VII. 

9

 EMEA guideline (http://www.emea.europa.eu) 

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question and answers document to provide clarification on the 2006 guideline.10 This guideline 

recommends dichotomizing genotoxic impurities into those for which there is “sufficient 

(experimental) evidence for a threshold-related mechanism” and those “without sufficient 

(experimental) evidence for a threshold-related mechanism.” The genotoxic impurities with 

sufficient evidence for a threshold-related mechanism would be addressed using methods 

outlined in ICH Q3C(R3) for Class 2 solvents. This approach calculates a “permitted daily 

exposure,” which is derived using the “no observed effect level” or, alternatively, the “lowest 

observed effect level” from the most relevant animal study and incorporating a variety of 

uncertainty factors. Examples of genotoxic compounds that might fall into this category include 

compounds that induce aneuploidy by interfering with the mitotic spindle, compounds that 

interfere with the activity of topoisomerase, and/or compounds that inhibit DNA synthesis. 

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For genotoxic impurities without sufficient evidence for a threshold-related mechanism, the 

guideline proposes a policy of controlling levels to “as low as reasonably practicable” (called the 

ALARP principle). The ALARP approach specifies that every effort should be made to prevent 

the formation of such impurities during drug substance synthesis and, if that is not possible, 

technical effort should be made post-synthesis to reduce impurities (e.g., purification steps). 

Compounds that fall into this category are those that interact with DNA either directly or 

indirectly, such as alkylating agents, intercalating agents, or agents that can generate free 

radicals. Since any exposure to these agents can convey some level of carcinogenic risk, and 

since complete elimination of genotoxic impurities from drug substances is often unachievable, 

the presence of a concerning impurity requires the implementation of a concept of an acceptable 

risk level. Methods for the derivation of acceptable risk levels are discussed in ICH Q3C(R3), 

Appendix 3, in reference to Class 1 carcinogenic solvents. 

Although the approach described above is acceptable, in most instances mechanistic data 

sufficient to allow for an assessment of whether there is a threshold mechanism are lacking. 

Furthermore, it is relatively uncommon for there to be sufficient data to allow for a quantitative 

risk assessment. The EMEA guideline recognizes these limitations and, therefore, proposes the 

use of a “threshold of toxicological concern” (TTC) for genotoxic impurities. The TTC refers to 

a threshold exposure level to compounds that does not pose a significant risk for carcinogenicity 

or other toxic effects. The EMEA guideline recommends a TTC of 1.5 μg per day for all but a 

highly potent subset of compounds. This threshold corresponds to an incremental 10-5 lifetime 

risk of cancer, a risk level that the EMEA considers justified because of the benefits derived 

from pharmaceuticals. The guideline indicates that a TTC value higher than 1.5 μg per day may 

be acceptable based on a weight-of-evidence approach to the profile of genotoxicity results, in 

situations where the anticipated human exposure will be short-term, for the treatment of lifethreatening conditions, when life expectancy is less than 5 years, or where the impurity is a 

known substance and human exposure will be much greater from other sources. The derivation 

of the TTC is discussed in more detail in section IV.B.1. 

The approach taken in the EMEA guideline for setting an exposure limit for genotoxic or 

carcinogenic impurities in drug products in support of a marketing application is reasonable. 

However, issues regarding the presence of genotoxic or carcinogenic impurities often occur 

10 Question & Answers on the CHMP Guideline on the Limits of Genotoxic Impurities, June 2008 

(http://www.emea.europa.eu) 

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during the clinical development stages. Therefore, this guidance provides recommendations for 

acceptable exposure thresholds during clinical development as well as for marketing 

applications. 

III. RECOMMENDED APPROACHES FOR INITIAL ASSESSMENT OF 

GENOTOXIC POTENTIAL OF IMPURITIES

If adequate data characterizing genotoxic and carcinogenic potential are not already available, 

impurities identified in drug substances or drug products at levels exceeding the stated 

qualification thresholds in the relevant ICH guidances should be assessed for genotoxic potential 

in an initial minimal screen. Assays conducted with the impurity in isolation are recommended. 

However, studies with the drug substance containing, or spiked with, the impurity can be 

considered in cases where it can be demonstrated that synthesizing sufficient amounts of the 

impurity is infeasible. 

As mentioned, the ICH guidances on impurities do not apply to drug substances or drug products 

for use in clinical trials. However, in cases where the presence of an impurity with genotoxic or 

carcinogenic potential is identified or where such an impurity may be expected based on the 

synthetic pathway, steps should be taken during the clinical development stage to address safety 

concerns associated with these impurities. 

If an impurity that is present at levels below the ICH qualification thresholds is identified, the 

impurity should be evaluated for genotoxicity and carcinogenicity based on structural activity 

relationship (SAR) assessments (i.e., whether there is a structural alert). This evaluation can be 

conducted via a review of the available literature or through a computational toxicology 

assessment; commonly used software includes MDL-QSAR, MC4PC, and Derek for Windows. 

The conduct of an in vitro mutation assay (i.e., bacterial reverse mutation assay) generally would 

be an acceptable initial screen for impurities with an identified alert, since positive signals in 

computational toxicology programs are often derived from the results of bacterial mutation 

assays and mutagenic carcinogens are considered to operate through nonthreshold-related 

mechanisms. An assessment in a mammalian cell assay may be needed for impurities with 

specific structural groups, such as carbamates, that are not well characterized in bacterial assays, 

or for compounds that are toxic to E. coli and Salmonella, such as antibiotics. 

If the initial evaluation of the genotoxic potential of an impurity is negative, no further 

genotoxicity studies are recommended and the impurity should be considered to be adequately 

qualified regarding its genotoxic potential. It should be noted that in cases where it is necessary 

from a feasibility standpoint to conduct the assays with the drug substance containing, or spiked 

with, the impurity, the proposed acceptance criterion should be commensurate with the level of 

impurity observed in clinical, stability, and/or production batches, taking into consideration the 

manufacturing and analytical variability. This acceptance criterion should not exceed the level 

present in the drug batch used in the genotoxicity assay and should be supported by the relevant 

qualification thresholds discussed in the ICH guidances or supporting general toxicity 

information. 

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In some cases, the structure of an impurity leading to the structural alert is shared with the API. 

The genotoxic potential of such an impurity can be evaluated through the standard testing of the 

API if the chemical environment for the alerting structure of the compounds is deemed 

comparable for the reactivity potential. 

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IV. RECOMMENDED APPROACHES FOR HANDLING GENOTOXIC AND 

CARCINOGENIC IMPURITIES 

Positive results in one or more genotoxicity assays or other information indicating a carcinogenic 

potential, such as positive data from a carcinogenicity study with the impurity, should be 

addressed further. Recommended approaches for handling genotoxic or carcinogenic impurities 

are described in this section and are summarized in Table 2 at the end of section IV.C. A 

decision tree is also included in Appendix A. 

A. Prevention of Genotoxic and Carcinogenic Impurity Formation 

Since drug-related impurities presumably provide limited, if any, therapeutic benefits and 

because of their potential to cause cancer in humans, every feasible technical effort should be 

made to prevent the formation of genotoxic or carcinogenic compounds during drug substance 

synthesis or drug product manufacturing. However, we recognize that completely preventing the 

formation of or removing an impurity of concern may not be possible in many cases. 

B. Reduction of Genotoxic and Carcinogenic Impurity Levels 

In lieu of completely preventing the formation of a genotoxic or carcinogenic impurity, steps to 

reduce the level of impurity present in the drug substance or drug product should be considered. 

The following sections discuss acceptable thresholds to support safety during clinical 

development and for a marketing application. Analytical methodologies should be used that can 

adequately identify impurities of concern at levels associated with the relevant qualification 

thresholds. This threshold approach should be applied only in the absence of adequate 

qualification data (data that establish the biological safety of an impurity at the level specified) 

for the given impurity. 

1. Acceptable Levels to Support Marketing Applications 

In general, an exposure level of 1.5 μg per person per day for each impurity can be considered an 

acceptable qualification threshold for supporting a marketing application. Any impurity found at 

a level below this threshold generally should not need further safety qualification for 

genotoxicity and carcinogenicity concerns. The threshold is an estimate of daily exposure 

expected to result in an upper bound lifetime risk of cancer of less than 10-6 (one in a million), a 

risk level that is thought to pose negligible safety concerns. The threshold was based on an 

analysis of the carcinogenic potencies of 477 chemicals and was derived from the probability 

distribution of carcinogenic potencies of those compounds.11 Subsequent analyses of an 

11 Fiori, JM and RD Meyerhoff, 2002, Extending the Threshold of Regulation Concept: De Minimis Limits for 

Carcinogens and Mutagens, Reg Toxicol Pharmacol, 35, 209-216. 

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expanded carcinogenic potency database of more than 700 carcinogens further confirmed the 

threshold.

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12 An additional analysis of subsets of highly potent carcinogens suggested that a 

threshold of 0.15 μg per day, corresponding to a 10-6 lifetime risk of cancer, may be more 

appropriate for chemicals with structural alerts for potential genotoxicity.13 However, there are 

some compounds containing certain structural groups (aflatoxin-like-, N-nitroso-, and azoxystructures) that have extremely high carcinogenic potency and are excluded from the threshold 

approach. 

Federal regulatory agencies in the United States, such as the Environmental Protection Agency 

(EPA) (in the context of ambient water quality criteria), typically use a 10-6 lifetime risk of 

cancer to determine negligible risk from chemical exposures.14 This approach supports an 

acceptable threshold level for genotoxic or carcinogenic impurities of 0.15 μg per day. 

However, other regulatory bodies have proposed a 10-5 level as an acceptable cancer risk.15,16 

Given that there is an overriding expected benefit of an approved drug product, a daily exposure 

level of 1.5 μg per day, associated with a 10-5 lifetime risk of cancer, can be acceptable for most 

genotoxic or carcinogenic impurities for a marketing application. This level of exposure is 

expected to produce a negligible increase in carcinogenic risk based on the existing background 

rate of human cancer and the conservative nature of cancer risk assessments. Additionally, this 

threshold is considered to be low enough to ensure that the presence of a compound with an 

uncharacterized genotoxic or carcinogenic potential would not significantly alter the risk-benefit 

ratio of a drug product, even if the impurity is later shown to be a carcinogen. 

The database from which the exposure threshold for genotoxic or carcinogenic impurities is 

derived includes studies that primarily use oral administration, though a smaller number use the 

inhalation route. Although the recommended threshold approach applies to all drug products 

regardless of the intended route of administration, the qualification threshold of 1.5 μg per day 

may not be appropriate for some routes (e.g., dermal, ophthalmic) because of the lack of a 

relevant database from which an exposure threshold can be derived. Applicants should contact 

specific drug review divisions regarding acceptable approaches in these cases. 

As part of this threshold approach, applicants can conduct and provide to the FDA an SAR 

assessment to identify structural similarities to known carcinogens. In cases where significant 

structural similarities to a known carcinogen are identified, an estimate of the potential human 

12 Ibid. 

13 Kroes, R, AG Renwick, M Cheeseman, J Kleiner, I Mangelsdorf, A Piersma, B Schilter, J Schlatter, F Schothorst, 

JG Vos, and G Würtzen, 2004, Structure-Based Threshold of Toxicological Concern (TTC): Guidance for 

Application to Substances Present at Low Levels in the Diet, Food Chem Toxicol, 42, 65-83. 

14 U.S. Environmental Protection Agency, Office of Water and Office of Science and Technology, 2000, 

Methodology for Deriving Ambient Water Quality Criteria for the Protection of Human Health, document number 

EPA-822-B-00-004, section 1.5.3 (http://www.epa.gov/waterscience/humanhealth/method/complete.pdf). 

15 See EMEA guideline, section 5.2.3. 

16 World Health Organization Guidelines for Drinking-Water Quality, 2nd ed., Vol. 2, 1996, Health Criteria and 

Other Supporting Information, Geneva, World Health Organization, section 12.4.2 

(http://www.who.int/water_sanitation_health/dwq/gdwq2v1/en/index1.html). 

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cancer risk can be calculated based on the available information for the confirmed carcinogen. 

This assessment can result in an increase in the acceptable exposure threshold for impurities that 

are highly similar to carcinogens with relatively low potency, or a reduction in the limit for 

impurities that are highly similar to relatively potent carcinogens. 

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The EPA guidance Supplemental Guidance for Assessing Susceptibility from Early-Life 

Exposure to Carcinogens (EPA/630/R-03/003F) regarding cancer susceptibility in pediatric 

populations indicates that children exposed to mutagenic carcinogens between age 0 (birth) and 

16 have an increased cancer risk over a 70-year lifetime when compared to adults.17 EPA 

concludes that cancer risks generally are higher from early-life exposure than from similar 

exposure durations later in life and recommends the application of adjustment factors to risk 

calculations to account for this observation. EPA recommends an adjustment factor of 10 for 

exposures before 2 years of age (i.e., spanning a 2-year time interval from the first day after birth 

up until a child’s second birthday), which represents an approximation of the weighted geometric 

mean tumor incidence ratio from juvenile or adult exposures in repeated dosing studies. In the 

absence of data to calculate a specific dose-response adjustment factor for exposures between 2 

and less than 16 years of age, EPA recommends an adjustment factor of 3, which represents an 

intermediate level of adjustment and reflects a midpoint between the 10-fold adjustment for the 

first two years of life and no adjustment (i.e., 1-fold) for adult exposures. However, the EPA 

guidance acknowledges that the resultant increases in cancer risk are relatively small for 

exposures that continue with fair uniformity over a lifetime. We recommend that this increase in 

susceptibility to carcinogens in pediatric populations be considered when determining the 

acceptable impurity level for a given drug product. 

The threshold approach for genotoxic or carcinogenic impurities limits the likelihood that any 

individual impurity in a given drug product will present more than a 10-5 excess cancer risk, but 

the approach is not intended to ensure an aggregate excess cancer risk of less than 10-5. This 

means the threshold approach to individual impurities is not intended to limit the overall excess 

cancer risk to 10-5 from all impurities in a single drug product or from multiple drug products 

concomitantly administered. As discussed above, this approach is consistent with approaches 

taken by various regulatory bodies such as EPA, World Health Organization, and EMEA in 

implementing threshold levels for carcinogenic risk when no benefit from the expected exposure 

is perceived. However, in cases where a class or family of structurally similar impurities is 

identified and is expected to have similar mechanisms resulting in their genotoxic or 

carcinogenic potential, the total daily exposure to the related compounds should be evaluated 

relative to the recommended threshold exposure. 

We recognize that drug products are often indicated for short-term use. However, for most 

drugs, these threshold considerations still apply since a drug may be used multiple times by the 

same individual or may be used outside of its approved indication. A detailed rationale should 

be provided to the FDA to support limits higher than generally considered appropriate for a 

marketing application. 

17 See http://cfpub.epa.gov/ncea/index.cfm. 

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374 2. Acceptable Levels during Clinical Development

375 

376 

377 

378 

379 

380 

381 

382 

383 

384 

385 

386 

387 

388 

389 

390 

391 

392 ssed the 

393 

394 

395 

396 

397 

398 

399 

400 

401 

402 

403 

404 

405 

406 

407 

408 

409 

410 

411 

412 

413 

414 

415 

 

The previous section describes the qualification threshold for genotoxic or carcinogenic 

impurities in support of a marketing application. Issues related to genotoxic impurities also can 

arise during a drug product’s clinical development period and can affect the assessment of safety 

for conducting the program. Some flexibility in the previously described threshold level can be 

applied during the investigational stages, since clinical trials vary widely in duration from shortterm (single dose to 4 weeks) to years and the qualification threshold for a marketing application 

is based on lifetime risk estimates. On the other hand, it should be recognized that during early 

clinical development, a benefit of the drug cannot be assumed. We recognize that the ability to 

identify and control drug-related impurities during early developmental stages is limited because 

of issues related to scale and maturity of production processes. Taking all these considerations 

into account, higher daily levels of exposure to potentially genotoxic impurities may be 

acceptable during the clinical development of the drug product compared to what is appropriate 

for a marketed drug product. 

Bos et al. reviewed the derived cancer risk from short-term, high-dose exposure to a genotoxic 

carcinogen relative to the same cumulative dose distributed over a lifetime (virtually safe 

dose).18 Briefly, the authors state that only a limited number of animal studies have asse

comparative tumor incidence from short-term versus long-term exposures with similar 

cumulative doses. From those studies that do exist, dose rate correction factors (factors by which 

a specific dose of a chemical carcinogen at long-term, low-dose rates should be multiplied to 

derive the expected tumor incidence from short-term, high-dose rates) ranged from unity to 8.3. 

The authors conclude that the most pragmatic approach to calculate acceptable short-term 

exposures to known genotoxic carcinogens is to linearly extrapolate the short-term exposure 

from the acceptable lifetime exposure or virtually safe dose. 

Acceptable daily intakes of genotoxic impurities during clinical development are presented in 

Table 1, based on the linear extrapolation approach described by Bos et al. The impurity 

threshold exposures for exposure durations of up to 12 months are based on a 10-6 cancer risk 

level (0.15 μg per day for a lifetime exposure), since these trials often include healthy subjects 

for whom there is no expected health benefit and the efficacy of the drug may still be uncertain. 

The values are derived from a linear extrapolation from the qualification threshold using the 

maximum duration of dosing for each time period specified in Table 1. In addition, these values 

incorporate an uncertainty factor of 2 to allow for deviations from the linear extrapolation model. 

For trials greater than 1-year duration, the threshold value is identical to the threshold for a 

marketing application and is based on a 10-5 cancer risk level (1.5 μg per day derived from 

lifetime exposures); subjects in these trials generally have the condition or disease being studied 

and are more certain to derive benefit from the treatment than subjects in early trials. When 

determining the acceptable impurity threshold exposure, the specifics of the patient population in 

the clinical trial should be evaluated. 

18 Bos, PMJ, B Baars, TM Marcel, and MTM van Raaij, 2004, Risk Assessment of Peak Exposure to Genotoxic 

Carcinogens: A Pragmatic Approach, Toxicol Letters, 151:43-50. 

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Table 1: Acceptable Qualification Thresholds for Genotoxic and Carcinogenic 

Impurities 

416 

417 

Duration of Clinical Trial Exposure 

< 14 days 14 days to 

1 mo 

1 mo to 

3 mos 

3 mos to 

6 mos 

6 mos to 

12 mos 

> 12 mos 

Genotoxic and 

carcinogenic 

impurity threshold 

(μg/day) 

120 60 20 10 5 1.5 

418 

419 

420 

421 

422 

423 

424 

425 

426 

427 

428 

429 

430 

431 

432 

433 

434 

435 

436 

437 

438 

439 

 

C. Additional Characterization of Genotoxic and Carcinogenic Risk 

In cases where attempts to prevent the formation of an impurity of concern and/or to reduce the 

amount of the impurity to an acceptable level as per Table 1 are not possible, further 

characterization of the genotoxic and carcinogenic potential should be conducted. The guidance 

for industry and review staff Recommended Approaches to Integration of Genetic Toxicology 

Study Results describes the FDA’s current thinking regarding appropriate additional evaluations 

that can be conducted.19 Briefly, these concepts include the consideration of the mechanism of 

action, weight of evidence, or the conduct of additional supportive studies. These concepts also 

can be considered relevant for genotoxic impurities. 

In addition to the above considerations, the conduct of an SAR evaluation of an impurity may 

provide useful information. When a significant structural similarity to a known carcinogen is 

identified, the drug substance and drug product acceptance criteria (typically in units of parts per 

million or percent) can be set at a level that is commensurate with the risk assessment specific to 

that of the known compound. As noted previously, the proposed factors should be considered in 

light of manufacturing batch data. 

Table 2 summarizes the recommended approaches for characterizing the presence and addressing 

the safety of genotoxic and carcinogenic impurities depending on the clinical development stage. 

19 We update guidances periodically. To make sure you have the most recent version of a guidance, check the 

CDER guidance Web page at http://www.fda.gov/cder/guidance/index.htm. 

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12

440 Table 2: Recommended Approaches Based on Development Stage 

Clinical Development 

Stage 

Recommended Approach 

IND • Evaluate identified impurities for genotoxic and carcinogenic risk via 

SAR assessment 

• Conduct assay for the presence of anticipated genotoxic and carcinogenic 

impurities 

• If impurity with genotoxic and carcinogenic potential is identified: 

− Modify synthetic pathway to eliminate the impurity, if possible 

OR 

− Conduct genotoxicity assays to characterize the genotoxic potential 

if not already known 

AND/OR 

− Set specification to that associated with a potential daily impurity 

exposure supported by compound-specific risk assessment or 

relevant qualification threshold (see Table 1) 

Marketing application 

(NDA, BLA, or ANDA) 

• Evaluate identified impurities for genotoxic and carcinogenic risk via 

SAR assessment 

• If impurity with genotoxic and carcinogenic potential is identified: 

− Conduct genotoxicity assays to characterize the genotoxic potential 

if not already known 

AND/OR 

− Set specification to that associated with a potential daily impurity 

exposure supported by compound-specific risk assessment or 1.5 μg 

per day threshold 

441 

442 

443 

444 

445 

446 

447 

448 

449 

450 

451 

452 

453 

454 

455 

456 

457 

D. Considerations for Flexibility in Approach 

The previous sections are intended to be general recommendations to consider when developing 

a drug product in which a potentially genotoxic or carcinogenic impurity is identified. We 

recognize that these approaches may not necessarily apply to every development program, and 

flexibility in the application of these recommendations may be appropriate. When applying the 

recommendations, consideration should be given to the drug product’s clinical development 

stage, the maximum duration of drug administration at that stage, the proposed indication (e.g., 

treatment of a life-threatening condition versus a less serious condition), the patient population 

(e.g., adults versus children), and the structural similarity of an impurity to a compound of 

known carcinogenic potency. In some of these cases, acceptance criteria higher than the 

recommended thresholds can be supported in the presence of a potential pharmacological benefit 

to patients. In rare cases, such as in the presence of highly potent carcinogens, decreases in the 

threshold also may be warranted. The appropriateness of a flexible approach should be informed 

by the feasibility of controlling impurity levels and the capabilities of the current process. 
Contains Nonbinding Recommendations 

Draft — Not for Implementation 

458 APPENDIX A: DECISION TREE FLOW DIAGRAM 

459 

460 

Observed level exceeds relevant ICH qualification 

threshold or is less than ICH qualification threshold 

but displays a structural alert?

Conduct appropriate 

genotoxicity assays

No 

Adequate evidence for threshold 

mechanism? 

Set specification based 

on calculated 

permitted daily 

exposure 

Yes

Yes

Consider alternate 

synthetic pathway 

No

Reduce the level of impurity to that 

associated with a daily exposure ≤

qualification threshold for genotoxic 

and carcinogenic impurity* (1.5 μg 

per day or see Table 1)

Impurity considered genotoxic 

based on assay results/weight 

of evidence?

Yes

No

Consider restricting or rejecting 

proposed use based on risk-benefit ratio 

*Safety threshold approach for genotoxic and carcinogenic impurities is not applicable to compounds with 

adequate data to derive compound-specific risk assessment or for those with SARs to high potency carcinogens. 

In addition, the approach may not be appropriate for some routes of administration (e.g., dermal, ophthalmic) 

because of the lack of a relevant database from which a threshold limit can be derived. 

Able to prevent formation of impurity?

Yes

No

No further 

action 

Identify impurity

13