Guidance for Industry 

S1C(R2) Dose Selection for 

Carcinogenicity Studies 

U.S. Department of Health and Human Services 

Food and Drug Administration 

Center for Drug Evaluation and Research (CDER) 

Center for Biologics Evaluation and Research (CBER) 

September 2008 

ICH 

Revision 1 
 

 

Guidance for Industry 

S1C(R2) Dose Selection for 

Carcinogenicity Studies 

Additional copies are available from: 

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Division of Drug Information
 

Center for Drug Evaluation and Research
 

Food and Drug Administration 

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

Office of Communication, Training and
 

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Food and Drug Administration 

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(Tel) 1-800-835-4709 or 301-827-1800 

 http://www.fda.gov/cber/guidelines.htm. 

U.S. Department of Health and Human Services 

Food and Drug Administration 

Center for Drug Evaluation and Research (CDER) 

Center for Biologics Evaluation and Research (CBER) 

September 2008 

ICH 

Revision 1 
Contains Nonbinding Recommendations 

TABLE OF CONTENTS 

I. INTRODUCTION AND BACKGROUND (1)............................................................... 1
 

A. General Considerations for the Conduct of Dose-Ranging Studies (1.1)..................................3
 

B.	 Toxicity Endpoints in High Dose Selection (1.2).........................................................................3
 

C.	 Pharmacokinetic Endpoints in High Dose Selection (1.3)..........................................................4
 

D.	 Criteria for Comparisons of AUC in Animals and Man for Use in High Dose ........................4

 Selection (1.4) .................................................................................................................................4
 

E.	 Saturation of Absorption in High Dose Selection (1.5)...............................................................5
 

F.	 Pharmacodynamic Endpoints in High Dose Selection (1.6).......................................................5
 

G.	 Maximum Feasible Dose (1.7).......................................................................................................5
 

H.	 Limit Dose (1.8)..............................................................................................................................6
 

I.	 Additional Endpoints in High Dose Selection (1.9).....................................................................6
 

J.	 Selection of Middle and Low Doses in Carcinogenicity Studies (1.10)......................................6
 

II. SUMMARY (2).................................................................................................................. 6
 

III. NOTES (3) ......................................................................................................................... 7
 

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Contains Nonbinding Recommendations 

Guidance for Industry1 

S1C(R2) Dose Selection for Carcinogenicity Studies 

of Pharmaceuticals 

This guidance represents the Food and Drug Administration’s (FDA’s) current thinking on this topic. It 

does not create or confer any rights for or on any person and does not operate to bind FDA or the public. 

You can use an alternative approach if the approach satisfies the requirements of the applicable statutes 

and regulations. If you want to discuss an alternative approach, contact the FDA staff responsible for 

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

number listed on the title page of this guidance. 

I. INTRODUCTION AND BACKGROUND (1)2 

This guidance provides recommendations for dose selection for carcinogenicity studies of human 

pharmaceuticals. The guidance is intended to aid sponsors in selecting appropriate doses for 

rodent carcinogenicity studies. 

FDA first issued the ICH guidance S1C Dose Selection for Carcinogenicity Studies of 

Pharmaceuticals in October 1994 and the addendum to the parent guidance S1C(R) Addition of a 

Limit Dose and Related Notes in July 1997. In November 2005, ICH incorporated the addendum 

to the parent guidance (S1C(R1)). In March 2008, ICH further revised the guidance (S1C(R2)).3 

This revised guidance includes the changes made by ICH in November 2005 and March 2008. 

1 This guidance was developed within the Expert Working Group (Safety) of the International Conference on 

Harmonisation of Technical Requirements for Registration of Pharmaceuticals for Human Use (ICH) and has been 

subject to consultation by the regulatory parties, in accordance with the ICH process. This document has been 

endorsed by the ICH Steering Committee at Step 4 of the ICH process, March 2008. At Step 4 of the process, the 

final draft is recommended for adoption to the regulatory bodies of the European Union, Japan, and the United 

States. 

2

 Arabic numbers reflect the organizational breakdown of the document endorsed by the ICH Steering Committee at 

Step 4 of the ICH process, March 2008. 

3

 Specifically, note 2 of the parent guidance was deleted, the text referring to the notes was revised, and the 

addendum was integrated in the text. The pharmacokinetic endpoint of 25 was declared to be applicable also for 

pharmaceuticals with positive genotoxicity signals. This change has implications on “Refinement” (one of the 3Rs) 

in enhancing the welfare (i.e., reducing the pain or discomfort) of the animals at the maximally tolerated dose 

(MTD). Guidance terminology was revised to comply with FDA’s good guidance practices regulation (21 CFR 

10.115). 

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Contains Nonbinding Recommendations 

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. 

Traditionally, carcinogenicity studies for chemical agents have relied upon the maximally 

tolerated dose (MTD) as the standard method for high dose selection (Note 1). The MTD is 

generally chosen based on data derived from toxicity studies of 3 months’ duration. 

In the past, the criteria for high dose selection for carcinogenicity studies of human 

pharmaceuticals have not been uniform among international regulatory agencies. In Europe and 

Japan, dose selection based on toxicity endpoints or attaining high multiples of the maximum 

recommended human daily dose (>100x on a milligram (mg)/kilogram (kg) basis) has been 

accepted. However, in the United States, dose selection based on the MTD has traditionally been 

considered the only appropriate practice. All regions have used a maximum feasible dose as an 

appropriate endpoint. 

For pharmaceuticals with low rodent toxicity, use of the MTD can result in the administration of 

very large doses in carcinogenicity studies, often representing high multiples of the clinical dose. 

This has led to the concern that exposures in rodents greatly in excess of the intended human 

exposures might not be relevant to human risk; because they so greatly alter the physiology of 

the test species, the findings might not reflect what would occur following human exposure. 

Ideally, the doses selected for rodent bioassays for pharmaceuticals should provide an exposure 

to the agent that (1) allows an adequate margin of safety over the human therapeutic exposure, 

(2) is tolerated without significant chronic physiological dysfunction and is compatible with 

good survival, (3) is guided by a comprehensive set of animal and human data that focus broadly 

on the properties of the agent and the suitability of the animal, and (4) permits data interpretation 

in the context of clinical use. 

In order to achieve international harmonization of requirements for high dose selection for 

carcinogenicity studies of pharmaceuticals, and to establish a rational basis for high dose 

selection, the ICH Expert Working Group on Safety initiated a process to arrive at common, 

scientifically based criteria for high dose selection. Several features of pharmaceutical agents 

distinguish them from other environmental chemicals and can justify a guidance that might differ 

in some respects from other guidances. This should enhance the relevance of the carcinogenicity 

study for pharmaceuticals. Thus, much knowledge might be available on the pharmacology, 

pharmacokinetics, and metabolic disposition in humans. In addition, there will usually be 

information on the patient population, the expected use pattern, the range of exposure, and the 

toxicity and/or side effects that cannot be tolerated in humans. Diversity of the chemical and 

pharmacological nature of the substances developed as pharmaceuticals, and the diversity of 

mechanisms of carcinogenesis, call for a flexible approach to dose selection. This document 

proposes that any one of several approaches could be useful for dose selection, and should 

provide for a more rational approach to dose selection for carcinogenicity studies for 

pharmaceuticals. These include: (1) toxicity-based endpoints; (2) pharmacokinetic endpoints; (3) 

saturation of absorption; (4) pharmacodynamic endpoints; (5) maximum feasible dose; (6) limit 

dose; and (7) additional endpoints. 

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Consideration of all relevant animal data and integration with available human data is paramount 

in determining the most appropriate endpoint for selecting the high dose for the carcinogenicity 

study. Relevant pharmacokinetic, pharmacodynamic, and toxicity data should be considered in 

the selection of doses for the carcinogenicity study, regardless of the primary endpoint used for 

high dose selection. 

In the process of defining such a flexible approach, it is recognized that the fundamental 

mechanisms of carcinogenesis are only poorly understood at the present time. Further, it is also 

recognized that the use of the rodent to predict human carcinogenic risk has inherent limitations, 

although this approach is the best available option at this time. Thus, while the use of plasma 

levels of drug-derived substances represents an important attempt at improving the design of the 

rodent bioassay, progress in this field calls for continuing examination of the best method to 

detect human risk. This document is therefore intended to serve as guidance in this difficult and 

complex area, recognizing the importance of updating the specific provisions outlined below as 

new data become available. 

A. General Considerations for the Conduct of Dose-Ranging Studies (1.1) 

The considerations involved when undertaking dose-ranging studies to select the high dose for 

carcinogenicity studies are the same regardless of the final endpoint utilized. 

1.	 In practice, carcinogenicity studies are carried out in a limited number of rat and mouse 

strains for which there is reasonable information on spontaneous tumour incidence. 

Ideally, rodent species/strains with metabolic profiles as similar as possible to humans 

should be studied (Note 2). 

2.	 Dose-ranging studies should be conducted for both males and females for all strains and 

species to be tested in the carcinogenicity bioassay. 

3.	 Dose selection is generally determined from 90-day studies using the route and method of 

administration that will be used in the bioassay. 

4.	 Selection of an appropriate dosing schedule and regimen should be based on clinical use 

and exposure patterns, pharmacokinetics, and practical considerations. 

5.	 Ideally, both the toxicity profile and any dose-limiting toxicity should be characterized. 

Consideration should also be given to general toxicity, the occurrence of preneoplastic 

lesions and/or tissue-specific proliferative effects, and disturbances in endocrine 

homeostasis. 

6.	 Changes in metabolite profile or alterations in metabolizing enzyme activities (induction 

or inhibition) over time should be understood to allow for appropriate interpretation of 

studies. 

B. Toxicity Endpoints in High Dose Selection (1.2) 

ICH 1 agreed to evaluate endpoints other than the MTD for the selection of the high dose in 

carcinogenicity studies. These were to be based on the pharmacological properties and 

toxicological profile of the test compound. There is no scientific consensus on the use of toxicity 

endpoints other than the MTD. Therefore, the ICH Expert Working Group on Safety has agreed 

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to continue use of the MTD as a useful toxicity-based endpoint for high dose selection for 

carcinogenicity studies. 

The following definition of the MTD is considered consistent with those published previously by 

international regulatory authorities (Note 1): The top dose or maximum tolerated dose is that 

which is predicted to produce a minimum toxic effect over the course of the carcinogenicity 

study. Such an effect can be predicted from a 90-day dose range-finding study in which minimal 

toxicity is observed. Factors to consider are alterations in physiological function that would be 

predicted to alter the animal’s normal life span or interfere with interpretation of the study. Such 

factors include: no more than 10% decrease in body weight gain relative to controls; target organ 

toxicity; significant alterations in clinical pathological parameters. 

C. Pharmacokinetic Endpoints in High Dose Selection (1.3) 

A systemic exposure representing a large multiple of the human area under the exposure curve 

(AUC) (at the maximum recommended daily dose) can be an appropriate endpoint for dose 

selection for carcinogenicity studies for pharmaceuticals that have similar metabolic profiles in 

humans and rodents and low organ toxicity in rodents (i.e., high doses are well tolerated in 

rodents). The level of animal systemic exposure should be sufficiently great compared to 

exposure to provide reassurance of an adequate test of carcinogenicity. 

It is recognized that the doses administered to different species might not correspond to tissue 

concentrations because of different metabolic and excretory patterns. Comparability of systemic 

exposure is better assessed by blood concentrations of parent drug and metabolites than by 

administered dose. The unbound drug in plasma is thought to be the most relevant indirect 

measure of tissue concentrations of unbound drug. The AUC is considered the most 

comprehensive pharmacokinetic endpoint since it takes into account the plasma concentration of 

the compound and residence time in vivo. 

There is, as yet, no validated scientific basis for use of comparative drug plasma concentrations 

in animals and humans for the assessment of carcinogenic risk to humans. However, for the 

present, and based on an analysis of a database of carcinogenicity studies performed at the MTD, 

the selection of a high dose for carcinogenicity studies that represents a 25- to-1 exposure ratio of 

rodent to human plasma AUC of parent compound and/or metabolites is considered pragmatic 

(Note 3). 

D. Criteria for Comparisons of AUC in Animals and Man for Use in High Dose 

Selection (1.4) 

The following criteria are especially applicable for use in determining a pharmacokineticallydefined exposure for high dose selection. 

1.	 Rodent pharmacokinetic data are derived from the strains used for the carcinogenicity 

studies using the route of compound administration and dose ranges planned for the 

carcinogenicity study (Notes 4, 5 and 6). 

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2.	 Pharmacokinetic data are derived from studies of sufficient duration to take into account 

potential time-dependent changes in pharmacokinetic parameters that might occur during 

the dose ranging studies. 

3.	 Documentation is provided on the similarity of metabolism between rodents and humans 

(Note 7). 

4.	 In assessing exposure, scientific judgement is used to determine whether the AUC 

comparison is based on data for the parent, parent and metabolite(s), or metabolite(s). 

The justification for this decision is provided. 

5.	 Interspecies differences in protein binding are taken into consideration when estimating 

relative exposure (Note 8). 

6.	 Human pharmacokinetic data are derived from studies encompassing the maximum 

recommended human daily dose (Note 9). 

E. Saturation of Absorption in High Dose Selection (1.5) 

High dose selection based on saturation of absorption measured by systemic availability of drugrelated substances can be considered. The mid and low doses selected for the carcinogenicity 

study should take into account saturation of metabolic and elimination pathways. 

F. Pharmacodynamic Endpoints in High Dose Selection (1.6) 

The utility and safety of many pharmaceuticals depend on their pharmacodynamic receptor 

selectivity. Pharmacodynamic endpoints for high dose selection will be highly compoundspecific and can be considered for individual study designs based on scientific merits. The high 

dose selected should produce a pharmacodynamic response in dosed animals of such magnitude 

as would preclude further dose escalation. However, the dose should not produce disturbances of 

physiology or homeostasis that would compromise the validity of the study. Examples include 

hypotension and inhibition of blood clotting (because of the risk of spontaneous bleeding). 

G. Maximum Feasible Dose (1.7) 

Currently, the maximum feasible dose by dietary administration is considered to be 5% of diet. 

International regulatory authorities are re-evaluating this standard. It is believed that the use of 

pharmacokinetic endpoints (AUC ratio) for dose selection of low toxicity pharmaceuticals, 

discussed in this guidance, should significantly reduce the number of instances where high doses 

are selected based on feasibility criteria. 

When routes other than dietary administration are appropriate, the high dose can be limited based 

on considerations including practicality and local tolerance. 

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H. Limit Dose (1.8) 

In determining the high dose for carcinogenicity studies using the approaches outlined in this 

guidance it is appropriate to limit this dose to 1500 mg/kg/day (Note 10). This limit dose applies 

where the maximum recommended human dose does not exceed 500 mg/day (Note 11). 

Data should be provided comparing exposure of rodents and humans to drug and metabolites 

primarily to support dose selection for and interpretation of the carcinogenicity study. Based on 

such information, there might be cases where the limit of 1500 mg/kg/day is not applicable 

because it cannot be assured that animal exposure after 1500 mg/kg/day is sufficiently high 

compared to the exposure achieved in humans. The rodent systemic exposure at 1500 mg/kg/day 

should be greater by at least an order of magnitude than human exposure measured at the 

intended human therapeutic dose. (If this is not the case, efforts should be made to increase the 

rodent exposure or to reconsider the animal model in a case-by-case approach.) If the human 

dose exceeds 500 mg/day the high dose can be increased up to the maximum feasible dose. 

I. Additional Endpoints in High Dose Selection (1.9) 

It is recognized that there might be merit in the use of alternative endpoints not specifically 

defined in this guidance on high dose selection for rodent carcinogenicity studies. Use of these 

additional endpoints in individual study designs should be based on scientific rationale. Such 

designs should be evaluated based on their individual merits (Note 12). 

J. Selection of Middle and Low Doses in Carcinogenicity Studies (1.10) 

Regardless of the method used for the selection of the high dose, the selection of the mid and low 

doses for the carcinogenicity study should provide information to aid in assessing the relevance 

of study findings to humans. The doses should be selected following integration of rodent and 

human pharmacokinetic, pharmacodynamic, and toxicity data. The rationale for the selection of 

these doses should be provided. While not all-encompassing, the following points should be 

considered in selection of the middle and low doses for rodent carcinogenicity studies: 

1. Linearity of pharmacokinetics and saturation of metabolic pathways 

2. Human exposure and therapeutic dose 

3. Pharmacodynamic response in rodents 

4. Alterations in normal rodent physiology 

5. Mechanistic information and potential for threshold effects 

6. The unpredictability of the progression of toxicity observed in short-term studies 

II. SUMMARY (2) 

This guidance outlines six criteria for selection of the high dose for carcinogenicity studies of 

therapeutics: the maximum tolerated dose, 25-fold AUC ratio (rodent:human), dose-limiting 

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pharmacodynamic effects, saturation of absorption, maximum feasible dose, and limit dose. The 

use of other pharmacodynamic-, pharmacokinetic-, or toxicity-based endpoints in study design 

should be considered based on scientific rationale and individual merits. In all cases, appropriate 

dose ranging studies should be conducted. All relevant information should be considered for 

dose and species/strain selection for the carcinogenicity study. This information should include 

knowledge of human use, exposure patterns, and metabolism. The availability of multiple criteria 

for dose selection will provide greater flexibility in optimizing the design of carcinogenicity 

studies for therapeutic agents. 

III. NOTES (3) 

Note 1 

The following are considered equivalent definitions of the toxicity-based endpoint describing the 

maximum tolerated dose: 

The U.S. Interagency Staff Group on Carcinogens has defined the MTD as follows: 

The highest dose currently recommended is that which, when given for the duration of 

the chronic study, is just high enough to elicit signs of minimal toxicity without 

significantly altering the animal’s normal lifespan due to effects other than 

carcinogenicity. This dose, sometimes called the maximum tolerated dose (MTD), is 

determined in a subchronic study (usually of 90 days duration) primarily on the basis of 

mortality, toxicity, and pathology criteria. The MTD should not produce morphologic 

evidence of toxicity of a severity that would interfere with the interpretation of the study. 

Nor should it comprise so large a fraction of the animal’s diet that the nutritional 

composition of the diet is altered, leading to nutritional imbalance. 

The MTD was initially based on a weight gain decrement observed in the subchronic 

study; i.e., the highest dose that caused no more than a 10% weight gain decrement. More 

recent studies and the evaluation of many more bioassays indicate refinement of MTD 

selection on the basis of a broader range of biological information. Alterations in body 

and organ weight and clinically significant changes in hematologic, urinary, and clinical 

chemistry measurements can be useful in conjunction with the usually more definitive 

toxic, pathologic or histopathologic endpoints. 

(Environmental Health Perspectives, Vol. 67, pp. 201-281, 1986) 

The Ministry of Health and Welfare in Japan prescribes the following: 

The dose in the preliminary carcinogenicity study that inhibits body weight gain by less 

than 10% in comparison with the control and causes neither death due to toxic effects nor 

remarkable changes in the general signs and laboratory examination findings of the 

animals is the highest dose to be used in the full-scale carcinogenicity study. 

(Toxicity test guideline for pharmaceuticals, Chapter 5, pg. 127, 1985) 

The Committee on Proprietary Medicinal Products of the European Community prescribes the 

following: 

The top dose should produce a minimum toxic effect, for example a 10% weight loss or 

failure of growth, or minimal target organ toxicity. Target organ toxicity will be 

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demonstrated by failure of physiological functions and ultimately by pathological 

changes. 

(Rules Governing Medicinal Products in the European Community, Vol. III, 1987) 

Note 2 

This does not imply that all possible rodent strains should be surveyed for metabolic profile, but 

rather that standard strains used in carcinogenicity studies should be examined. 

Note 3 

In order to select a multiple of the human AUC that would serve as an useful endpoint for dose 

selection for carcinogenicity studies, a retrospective analysis was performed on data from 

carcinogenicity studies of therapeutics conducted at the MTD for which there was sufficient 

human and rodent pharmacokinetic data for comparison of AUC values. 

In 35 drug carcinogenicity studies carried out at the MTD for which there were adequate 

pharmacokinetic data in rats and humans, approximately 1/3 had a relative systemic exposure 

ratio less than or equal to 1, and another 1/3 had ratios between 1 and 10. 

An analysis of the correlation between the relative systemic exposure ratio, the relative dose ratio 

(rat mg/kg:human mg/kg MRD), and the dose ratio adjusted for body surface area (rat mg/M2 

MTD:human mg/M2 MRD), performed in conjunction with the above-described database 

analysis, indicates that the relative systemic exposure corresponds better with dose ratios 

expressed in terms of body surface area rather than body weight. When 123 compounds in the 

expanded FDA database were analysed by this approach, a similar distribution of relative 

systemic exposures was observed. In the selection of a relative systemic exposure ratio (AUC 

ratio) to apply in high dose selection, consideration was given to a ratio value that would 

represent an adequate margin of safety, would detect known or probable human carcinogens, and 

could be attained by a reasonable proportion of compounds. 

To address the issue of detection of known or probable human carcinogenic pharmaceuticals, an 

analysis of exposure and/or dose ratios was performed on International Agency for Research on 

Cancer (IARC) class 1 and 2A pharmaceuticals with positive rat findings. For phenacetin, 

sufficient rat and human pharmacokinetic data are available to estimate that a relative systemic 

exposure ratio of at least 15 was found to produce positive findings in a rat carcinogenicity 

study. For most of 14 IARC 1 and 2A drugs evaluated with positive carcinogenicity findings in 

rats, there is a lack of adequate pharmacokinetic data for analysis. For these compounds, the 

body surface area adjusted dose ratio was employed as a surrogate for the relative systemic 

exposure ratio. The results of this analysis indicated that using doses in the rodent corresponding 

to body surface area ratios of 10 or more would identify the carcinogenic potential of these 

pharmaceuticals. 

As a result of the evaluations described above, a minimum systemic exposure ratio of 25 can be 

considered as a useful pharmacokinetic endpoint for high dose selection. This value was attained 

by approximately 25% of compounds tested in the FDA database (see Note 10), is high enough 

to detect known or probable (IARC 1, 2A) human carcinogenic drugs, and represents an 

adequate margin of safety. Those pharmaceuticals tested using a 25-fold or greater AUC ratio for 

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the high dose will have exposure ratios greater than do 75% of pharmaceuticals tested previously 

in carcinogenicity studies performed at the MTD. 

Note 4 

The rodent AUCs and metabolite profiles can be determined from separate steady-state kinetic 

studies, as part of the subchronic toxicity studies, or dose-ranging studies. 

Note 5 

AUC values in rodents are usually obtainable using a small number of animals, depending on the 

route of administration and the availability of data on the pharmacokinetic characteristics of the 

test compound. 

Note 6 

Equivalent analytical methods of adequate sensitivity and precision should be used to determine 

plasma concentrations of pharmaceuticals in rodents and humans. 

Note 7 

It is recommended that in vivo metabolism be characterized in humans and rodents, if possible. 

However, in the absence of appropriate in vivo metabolism data, in vitro metabolism data (e.g., 

from liver slices, uninduced microsomal preparations) can provide appropriate support for the 

similarity of metabolism across species. 

Note 8 

While in vivo determinations of unbound drug might be the best approach, in vitro 

determinations of protein binding using parent and/or metabolites as appropriate (over the range 

of concentrations achieved in vivo in rodents and humans) might be used in the estimation of 

AUC unbound. When protein binding is low in both humans and rodents, or when protein 

binding is high and the unbound fraction of drug is greater in rodents than in humans, the 

comparison of total plasma concentration of drug is appropriate. When protein binding is high 

and the unbound fraction is greater in humans than in rodents, the ratio of the unbound 

concentrations should be used. 

Note 9 

Human systemic exposure data can be derived from pharmacokinetic monitoring in normal 

volunteers and/or patients. The possibility of extensive inter-individual variation in exposure 

should be taken into consideration. In the absence of knowledge of the maximum recommended 

human daily dose, at a minimum, doses producing the desired pharmacodynamic effect in 

humans should be used to derive the pharmacokinetic data. 

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Note 10 

Review of the FDA carcinogenicity database of nearly 900 carcinogenicity tests indicated that 

about 20 tests had been conducted that used doses of 1000 mg/kg or greater as the highest dose 

tested. About 10 of these tests were considered as having demonstrated a carcinogenic response. 

Seven of these were positive only at or above 1000 mg/kg. Regulatory action has resulted from 

some of these cases. Based on these results, the limit dose for carcinogenicity testing should be 

1500 mg/kg rather than 1000 mg/kg to eliminate the risk that a carcinogen will not be able to be 

identified as a result of adoption of a limit dose of 1000 mg/kg. 

Note 11 

It has been agreed that if a drug is only positive in rodents at doses above those producing a 25-

fold exposure over exposure in humans, such a finding would not be considered likely to reflect 

a relevant risk to humans. 

It has been shown that systemic exposure comparisons between rodents and humans are better 

estimated by a dose using mg/m2 than using mg/kg (see Note 3 above). Therefore, the human 

dose should be at least 25-fold lower on a mg/m2 basis than the high dose in the carcinogenicity 

study. The factor 6-7 (6.5) is used to convert rat doses from mg/kg to mg/m2 and the factor 40 is 

used to convert human doses from mg/kg to mg/m2

. Thus, the estimated systemic exposure ratio 

of 25-fold rodent/human is equal to about a 25-fold mg/m2 ratio or a 150-fold mg/kg ratio (150 ≈

25 x 40/6.5). Therefore a human dose below 10 mg/kg/day (about 500 mg/day or less) could be 

tested in rats at 1500 mg/kg as the high dose. 

Note 12 

Additional pharmaceutical-specific endpoints to select an appropriate high dose are currently 

under discussion (e.g., additional pharmacodynamic, pharmacokinetic, and toxicity endpoints, as 

well as alternatives to a maximum feasible dose). 

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