Guidance for Industry
S1B Testing for Carcinogenicity of
Pharmaceuticals
July 1997
ICH Guidance for Industry
S1B Testing for Carcinogenicity of
Pharmaceuticals
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July 1997
ICH Table of Contents
I. OBJECTIVE (1) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
II. BACKGROUND (2) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
III. SCOPE OF THE GUIDANCE (3) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2
IV. THE GUIDANCE (4) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2
A. Preamble (4.1) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2
B. Experimental Approaches to Testing for Carcinogenic Potential (4.2) . . . . . . . . . 3
V. MECHANISTIC STUDIES (5) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
A. Cellular Changes (5.1) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
B. Biochemical Measurements (5.2) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
C. Considerations for Additional Genotoxicity Testing (See ICH Guidances S2A and
S2B) (5.3) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
D. Modified Protocols (5.4) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
VI. GENERAL CONSIDERATIONS IN THE CHOICE OF AN APPROPRIATE SPECIES
FOR LONG-TERM CARCINOGENICITY TESTING (6) . . . . . . . . . . . . . . . . . . . . . . 5
A. Information from Surveys on Pharmaceuticals (6.1) . . . . . . . . . . . . . . . . . . . . . . 5
B. Potential to Study Mechanisms (6.2) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
C. Metabolic Disposition (6.3) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
D. Practicality (6.4) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
E. Testing in More than One Species (6.5) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
F. Exceptions (6.6) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
VII. EVALUATION OF CARCINOGENIC POTENTIAL (7) . . . . . . . . . . . . . . . . . . . . . . . 7
NOTES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
OTHER ICH GUIDANCES CITED . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
i GUIDANCE FOR INDUSTRY1
S1B Testing for Carcinogenicity of Pharmaceuticals
I. OBJECTIVE (1)
This document provides guidance on approaches for evaluating the carcinogenic potential of
pharmaceuticals.
II. BACKGROUND (2)
Historically, the regulatory requirements for the assessment of the carcinogenic potential of
pharmaceuticals in the three regions (the European Union (EU), Japan, the United States)
provided for the conduct of long-term carcinogenicity studies in two rodent species, usually the
rat and the mouse.  Given the cost of these studies and their extensive use of animals, it is in
keeping with the mission of ICH to examine whether this practice requiring long-term
carcinogenicity studies in two species could be reduced without compromising human safety.
This guidance should be read in conjunction with other guidances, especially:
S1A The Need for Carcinogenicity Studies of Pharmaceuticals
S1C Dose Selection for Carcinogenicity Studies of Pharmaceuticals
Long-term rodent carcinogenicity studies for assessing the carcinogenic potential of chemicals
(including pharmaceuticals) to humans are currently receiving critical examination.  Since the
early 1970's, many investigations have shown that it is possible to provoke a carcinogenic
response in rodents by a diversity of experimental procedures, some of which are now considered
to have little or no relevance for human risk assessment.  This guidance outlines experimental
approaches to the evaluation of carcinogenic potential that may obviate the necessity for the
routine conduct of two long-term rodent carcinogenicity studies for those pharmaceuticals that
need such evaluation. The relative individual contribution of rat and mouse carcinogenicity studies
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, July 1997.  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.  This guidance
was published in the Federal Register on February 23, 1998 (63 FR 8983), and is applicable to drug and biological
products.  This guidance represents the Agencys current thinking on methods for evaluating the carcinogenic activity of
pharmaceuticals.  It does not create or confer any rights for or on any person and does not operate to bind FDA or the
public.  An alternative approach may be used if such approach satisfies the requirements of the applicable statute,
regulations, or both. and whether the use of rats or mice alone would result in a significant loss of information on
carcinogenicity relevant to human risk assessment has been addressed by six surveys of the data
for human pharmaceuticals.  The surveys were those of the International Agency for Research on
Cancer (IARC), the U.S. Food and Drug Administration (FDA), the U.S. Physicians Desk
Reference (PDR), the Japanese Pharmaceutical Manufacturers Association (JPMA), the EU
Committee for Proprietary Medicinal Products (CPMP), and the UK Centre for Medicines
Research (CMR).  The dimensions of these surveys and the principal conclusions of the analyses
can be found in the Proceedings of the Third International Conference (1995) on Harmonisation.
Positive results in long-term carcinogenicity studies that are not relevant to the therapeutic use of
a pharmaceutical present a dilemma to all parties:  Regulatory reviewers, companies developing
drugs, and the public at large.  The conduct of one long-term carcinogenicity study (rather than
two long-term studies) would, in part, allow resources to be diverted to other approaches to
uncover potential carcinogenicity relevant to humans. A weight of evidence approach, that is use
of scientific judgment in evaluation of the totality of the data derived from one long-term
carcinogenicity study along with other appropriate experimental investigations, enhances the
assessment of carcinogenic risk to humans.
III. SCOPE OF THE GUIDANCE (3)
The guidance embraces all pharmaceutical agents that need carcinogenicity testing as indicated in
ICH guidance S1A. For biotechnology-derived pharmaceuticals, refer to ICH guidance S6
Preclinical Safety Evaluation of Biotechnology-Derived Pharmaceuticals.
IV. THE GUIDANCE (4)
A. Preamble (4.1)
The strategy for testing the carcinogenic potential of a pharmaceutical is developed only
after the acquisition of certain key units of information, including the results of genetic
toxicology (ICH guidances S2A Guidance on Specific Aspects of Regulatory Genotoxicity
Tests for Pharmaceuticals and S2B Genotoxicity:  A Standard Battery for Genotoxicity
Testing of Pharmaceuticals), intended patient population, clinical dosage regimen (ICH
guidance S1A), pharmacodynamics in animals and in humans (selectivity, dose-response)
(ICH guidance S1C), and repeated-dose toxicology studies.  Repeated-dose toxicology
studies in any species (including nonrodents) may indicate that the test compound
possesses immunosuppressant properties, hormonal activity, or other activity considered
to be a risk factor for humans, and this information should be considered in the design of
any further studies for the assessment of carcinogenic potential (see also Note 1).
2 B. Experimental Approaches to Testing for Carcinogenic Potential (4.2)
Flexibility and judgment should be exercised in the choice of an approach, which should be
influenced by the information cited in the above preamble.  Given the complexity of the
process of carcinogenesis, no single experimental approach can be expected to predict the
carcinogenic potential of all pharmaceuticals for humans.
The basic scheme comprises one long-term rodent carcinogenicity study, plus one other
study of the type mentioned in section IV.B.2 that supplements the long-term
carcinogenicity study and provides additional information that is not readily available from
the long-term assay.
1. Choice of Species for a Long-term Carcinogenicity Study (4.2.1)
The species selected should be appropriate, based on considerations that include
the following:
(a)  Pharmacology.
(b)  Repeated-dose toxicology. 
(c)  Metabolism (see also ICH guidances S1C and S3A Toxicokinetics:  The
Assessment of Systemic Exposure in Toxicity Studies).
(d)  Toxicokinetics (see also ICH guidances S1C, S3A, and S3B).
(e)  Route of administration (e.g., less common routes such as dermal and
inhalation).
In the absence of clear evidence favoring one species, it is recommended that the
rat be selected.  This view is based on the factors discussed in section VI.
2. Additional in vivo tests for carcinogenicity (4.2.2)
Additional tests may be either (a) or (b) (see Note 2).
(a)  Short- or medium-term in vivo rodent test systems.
Possibilities should focus on the use of in vivo models providing insight into
carcinogenic endpoints.  These may include models of initiation-promotion in
rodents or models of carcinogenesis using transgenic or neonatal rodents (Note 3).
(b)  A long-term carcinogenicity study in a second rodent species is still considered
acceptable (see section IV.A.1 for considerations). 
3. Considerations in the Choice of Short- or Medium-term Tests for
Carcinogenicity (4.2.3)
3 Emphasis should be placed on selection of a test method that can contribute
information valuable to the overall weight of evidence for the assessment of
carcinogenic potential. The rationale for this choice should be documented and
based on information available at the time of method selection about the
pharmaceutical, such as pharmacodynamics and exposure compared to human or
any other information that may be relevant. This rationale should include a
scientific discussion of the strengths and weaknesses of the method selected for the
pharmaceutical (see Note 4).
V. MECHANISTIC STUDIES (5)
Mechanistic studies are often useful for the interpretation of tumor findings in a carcinogenicity
study and can provide a perspective on their relevance to human risk assessment.  The need for or
the design of an investigative study will be dictated by the particular properties of the drug and/or
the specific results from the carcinogenicity testing.  Dose dependency and the relationship to
carcinogenicity study conditions should be evaluated in these investigational studies.  Suggestions
include:
A. Cellular Changes (5.1)
Relevant tissues may be examined for changes at the cellular level using morphological,
histochemical, or functional criteria.  As appropriate, attention may be directed to such
changes as the dose-relationships for apoptosis, cell proliferation, liver foci of cellular
alteration, or changes in intercellular communication.
B. Biochemical Measurements (5.2)
Depending on the putative mode of tumorigenic action, investigations could involve
measurements of:
!  plasma hormone levels, e.g. T3/T4, TSH, prolactin;
!  growth factors;
!  binding to proteins such as "2-globulin;
!  tissue enzyme activity, etc.
In some situations, it may be possible to test a hypothesis of, for example, a hormone
imbalance with another study in which the imbalance has been, at least in part,
compensated.
4 C. Considerations for Additional Genotoxicity Testing (See ICH Guidances S2A
and S2B) (5.3)
Additional genotoxicity testing in appropriate models may be invoked for compounds that
were negative in the standard test battery but that have shown effects in a carcinogenicity
test with no clear evidence for an epigenetic mechanism.  Additional testing can include
modified conditions for metabolic activation in in vitro tests or can include in vivo tests
measuring genotoxic damage in target organs of tumor induction (e.g., DNA damage and
repair tests, 32P-postlabeling, mutation induction in transgenes).
D. Modified Protocols (5.4)
Modified protocols may be helpful to clarify the mode of tumorigenic action of the test
substance.  Such protocols might include groups of animals to explore, for example, the
consequence of interrupted dosage regimens, or the reversibility of cellular changes after
cessation of dosing.
VI. GENERAL CONSIDERATIONS IN THE CHOICE OF AN APPROPRIATE
SPECIES FOR LONG-TERM CARCINOGENICITY TESTING (6)
There are several general considerations that, in the absence of other clear indications, suggest
that the rat will normally be the species of choice for a long-term carcinogenicity study.
A. Information from Surveys on Pharmaceuticals (6.1)
In the six analyses, attention was given to data on genetic toxicology, tumor incidence,
strain of animal, route and dosage regimen, pharmacological or therapeutic activity,
development and/or regulatory status, and, if relevant, reason for termination of
development.  Inevitably, there was considerable overlap of the data, but that is not
necessarily an impediment to drawing valid conclusions.
The main overall conclusions from the analysis were:
1.  Although very few instances have been identified of mouse tumors being the sole
reason for regulatory action concerning a pharmaceutical, data from this species may have
contributed to a weight of evidence decision and to identifying agents that caused tumors
in two rodent species.
2.  Of the compounds displaying carcinogenic activity in only one species, the number of
rat-only compounds was about double the number of mouse-only compounds, implying in
a simplistic sense that the rat is more sensitive than the mouse.
5 3.  As with other surveys accessible in the literature, the data for pharmaceuticals were
dominated by the high incidence of rodent liver tumors.  The high susceptibility of mouse
liver to nongenotoxic chemicals has been the subject of many symposia and workshops. 
These have concluded that these tumors may not always have relevance to carcinogenic
risk in humans and can potentially be misleading.
B. Potential to Study Mechanisms (6.2)
The carcinogenic activity of nongenotoxic chemicals in rodents is characterized by a high
degree of species, strain, and target organ specificity and by the existence of thresholds in
the dose-response relationship.  Mechanistic studies in recent years have permitted the
distinction between effects that are specific to the rodent model and those that are likely to
have relevance for humans.  Progress has often been associated with increased
understanding of species and tissue specificity. For example, receptor-mediated
carcinogenesis is being recognized as of growing importance.  Most of these advances are
being made in the rat, and only rarely in the mouse.
C. Metabolic Disposition (6.3)
Neither rats nor mice would seem, on metabolic grounds, to be a priori generally more
suitable for the conduct of long-term carcinogenicity studies.  However, much attention is
now being given to pharmacokinetic-pharmacodynamic relationships and rapid progress is
occurring in knowledge of the P-450 isozymes that mediate the biotransformation of
drugs.  Most of this research activity is confined to rats and humans.  Therefore, in the
near future at least, where specific information on the P-450 isozymes involved in
biotransformation is critical for the evaluation, it appears that mice would be less likely to
provide this mechanistic information.
D. Practicality (6.4)
Pertinent to the above two topics is the question of feasibility of investigative studies. 
Size considerations alone put the mouse at a severe disadvantage when it comes to the
taking of serial blood samples, microsurgery/catheterization, and the weighing of organs. 
Blood sampling often requires the sacrifice of the animals, with the result that many extra
animals may be needed when mice are subject to such investigations.
E. Testing in More than One Species (6.5)
Most of the currently available short- and medium-term in vivo models for carcinogenicity
testing involve the use of mice.  In order to allow testing in more than one species for
carcinogenic potential when this is considered important and appropriate, the rat will often
be used in the long-term carcinogenicity study.   
6 F. Exceptions (6.6)
Despite the above considerations, there may be circumstances under which the mouse or
another rodent species could be justified on mechanistic, metabolic, or other grounds as
being a more appropriate species for the long-term carcinogenicity study for human risk
assessment (cf. section IV.B.1). Under such circumstances, it may still be acceptable to
use the mouse as the short-term or medium-term model.
VII. EVALUATION OF CARCINOGENIC POTENTIAL (7)
Evidence of tumorigenic effects of the drug in rodent models should be evaluated in light of the
tumor incidence and latency, the pharmacokinetics of the drug in the rodent models as compared
to humans, and data from any ancillary or mechanistic studies that are informative with respect to
the relevance of the observed effects to humans.
The results from any tests cited above should be considered as part of the overall weight of
evidence, taking into account the scientific status of the test systems.
7 NOTES
Note 1.  Data from in vitro assays, such as a cell transformation assay, can be useful at the 
compound selection stage. 
Note 2.  If the findings of a short- or long-term carcinogenicity study and of genotoxicity tests
and other data indicate that a pharmaceutical clearly poses a carcinogenic hazard to humans, a
second carcinogenicity study would not usually be useful.
Note 3.  Several experimental methods are under investigation to assess their utility in
carcinogenicity assessment. Generally, the methods should be based on mechanisms of
carcinogenesis that are believed relevant to humans and applicable to human risk assessment.
Such studies should supplement the long-term carcinogenicity study and provide additional
information that is not readily available from the long-term assay.  There should also be
consideration given animal numbers, welfare, and the overall economy of the carcinogenic
evaluation process.  The following is a representative list of some approaches that may meet these
criteria and is likely to be revised in the light of further information.
(a)  The initiation-promotion model in rodent. One initiation-promotion model for the
detection of hepatocarcinogens (and modifiers of hepatocarcinogenicity) employs an
initiator, followed by several weeks of exposure to the test substance.  Another multi-
organ carcinogenesis model employs up to five initiators followed by several months of
exposure to the test substance.
(b)  Several transgenic mouse assays, including the p53+/- deficient model, the Tg.AC
model, the TgHras2 model, the XPA deficient model, etc.
(c)  The neonatal rodent tumorigenicity model.  
Note 4.  While there may be a number of approaches that will in general meet the criteria
described in Note 3 for use as the additional in vivo study, not all may be equally suitable for a
particular pharmaceutical. The following are examples of factors that should be considered and
addressed in the rationale:
1.  Can results from the model provide new information not expected to be available from
the long-term study that is informative with respect to hazard identification and/or risk
assessment?
2.  Can results from the model address concerns related to the carcinogenic process arising
from prior knowledge of the pharmaceutical or compounds with similar structures and/or
mechanisms of action?  These concerns may include genotoxic, mitogenic, promotional, or
receptor-mediated effects, etc.
8 3.  Does the metabolism of the pharmaceutical shown in the animal model affect the
evaluation of carcinogenic risk for humans?
4.  Is adequate systemic or local exposure attained in relation to human exposure?
5.  How extensively has the model been evaluated for its intended use? Prior to using any
new in vivo methods in testing the carcinogenic potential of pharmaceuticals for humans,
it is critical that the method be evaluated for its ability to contribute to the weight of
evidence assessment. Many experimental studies are in progress (1997) to evaluate the
new short or medium tests for carcinogenic potential.  These include selected
pharmaceuticals with known potencies and known mechanism of carcinogenic activity in
rodents and also putative human noncarcinogens.  When the results of these studies
become available, it may be possible to offer more specific guidance on which of these
tests have the most relevance for cancer assessment in humans.   
9 OTHER ICH GUIDANCES CITED
S1A The Need for Carcinogenicity Studies of Pharmaceuticals
S1C Dose Selection for Carcinogenicity Studies of Pharmaceuticals
S2A Guidance on Specific Aspects of Regulatory Genotoxicity Tests for Pharmaceuticals
S2B Genotoxicity:  A Standard Battery for Genotoxicity Testing of Pharmaceuticals
S3A Toxicokinetics:  The Assessment of Systemic Exposure in Toxicity Studies
S3B Pharmacokinetics:  Guidance for Repeated Dose Tissue Distribution Studies
S6 Preclinical Safety Evaluation of Biotechnology-Derived Pharmaceuticals
10