[Federal Register Volume 60, Number 40 (Wednesday, March 1, 1995)]
[Notices]
[Pages 11264-11268]
From the Federal Register Online via the Government Publishing Office [www.gpo.gov]
[FR Doc No: 95-4957]




[[Page 11263]]

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Part IX





Department of Health and Human Services





_______________________________________________________________________



Food and Drug Administration



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International Conference on Harmonisation; Guideline on the Assessment 
of Systemic Exposure in Toxicity Studies; Notice

Federal Register / Vol. 60, No. 40 / Wednesday, March 1, 1995 / 
Notices 
[[Page 11264]] 

DEPARTMENT OF HEALTH AND HUMAN SERVICES

Food and Drug Administration
[Docket No. 94D-0015]


International Conference on Harmonisation; Guideline on the 
Assessment of Systemic Exposure in Toxicity Studies; Availability

AGENCY: Food and Drug Administration, HHS.

ACTION: Notice.

-----------------------------------------------------------------------

SUMMARY: The Food and Drug Administration (FDA) is publishing a final 
guideline entitled ``Toxicokinetics: Guidance on the Assessment of 
Systemic Exposure in Toxicity Studies.'' This guideline was prepared 
under the auspices of the International Conference on Harmonisation of 
Technical Requirements for Registration of Pharmaceuticals for Human 
Use (ICH). The guideline is intended to help ensure that the assessment 
of systemic exposure in toxicity studies to support drug registration 
is carried out according to sound scientific principles.

DATES: Effective on March 1, 1995. Submit written comments at any time.

ADDRESSES: Submit written comments on the guideline to the Dockets 
Management Branch (HFA-305), Food and Drug Administration, rm. 1-23, 
12420 Parklawn Dr., Rockville, MD 20857. Copies of the guideline are 
available from CDER Executive Secretariat Staff (HFD-8), Center for 
Drug Evaluation and Research, Food and Drug Administration, 7500 
Standish Pl., Rockville, MD 20855.
FOR FURTHER INFORMATION CONTACT: 
    Regarding the guideline: Roger L. Williams, Center for Drug 
Evaluation and Research (HFD-4), Food and Drug Administration, 5600 
Fishers Lane, Rockville, MD 20857, 301-594-6740.
    Regarding ICH: Janet J. Showalter, Office of Health Affairs (HFY-
20), Food and Drug Administration, 5600 Fishers Lane, Rockville, MD 
20857, 301-443-1382.
SUPPLEMENTARY INFORMATION: In recent years, many important initiatives 
have been undertaken by regulatory authorities and industry 
associations to promote international harmonization of regulatory 
requirements. FDA has participated in many meetings designed to enhance 
harmonization and is committed to seeking scientifically based 
harmonized technical procedures for pharmaceutical development. One of 
the goals of harmonization is to identify and then reduce differences 
in technical requirements for drug development among regulatory 
agencies.
    ICH was organized to provide an opportunity for tripartite 
harmonization initiatives to be developed with input from both 
regulatory and industry representatives. FDA also seeks input from 
consumer representatives and others. ICH is concerned with 
harmonization of technical requirements for the registration of 
pharmaceutical products among three regions: The European Union, Japan, 
and the United States. The six ICH sponsors are the European 
Commission; the European Federation of Pharmaceutical Industry 
Associations; the Japanese Ministry of Health and Welfare; the Japanese 
Pharmaceutical Manufacturers Association; the Centers for Drug 
Evaluation and Research and Biologics Evaluation and Research, FDA; and 
the Pharmaceutical Research and Manufacturers of America. The ICH 
Secretariat, which coordinates the preparation of documentation, is 
provided by the International Federation of Pharmaceutical 
Manufacturers Association (IFPMA).
    The ICH Steering Committee includes representatives from each of 
the ICH sponsors and IFPMA, as well as observers from the World Health 
Organization, the Canadian Health Protection Branch, and the European 
Free Trade Area.
    Harmonization of the assessment of systemic exposure in toxicity 
studies was selected as a priority topic during the early stages of the 
ICH initiative. In the Federal Register of March 1, 1994 (59 FR 9755), 
FDA published a draft tripartite guideline entitled, ``Toxicokinetics: 
A Guidance on the Assessment of Systemic Exposure in Toxicity 
Studies.'' The notice gave interested persons an opportunity to submit 
comments by May 16, 1994.
    After consideration of the comments received and revisions to the 
guideline, a final draft of the guideline was submitted to the ICH 
Steering Committee and endorsed by the three participating regulatory 
agencies at the ICH meeting held in October 1994.
    The guideline discusses toxicokinetics, which is the generation of 
pharmacokinetic data in nonclinical toxicity studies or ancillary 
studies to assess exposure. The objectives of toxicokinetics are: (1) 
To describe the systemic exposure achieved in animals, its relationship 
to dose level, and the time course of the toxicity study; (2) to relate 
the exposure achieved in toxicity studies to toxicological findings; 
(3) to support the choice of species and treatment regimen in 
nonclinical toxicity studies; and (4) to supply information which, 
along with the toxicity findings, will contribute to developing 
additional nonclinical toxicity studies.
    In the past, guidelines have generally been issued under 
Sec. 10.90(b) (21 CFR 10.90(b)), which provides for the use of 
guidelines to state procedures or standards of general applicability 
that are not legal requirements but are acceptable to FDA. The agency 
is now in the process of revising Sec. 10.90(b). Therefore, this 
guideline is not being issued under the authority of Sec. 10.90(b), and 
it does not create or confer any rights, privileges, or benefits for or 
on any person, nor does it operate to bind FDA in any way.
    As with all of FDA's guidelines, the public is encouraged to submit 
written comments with new data or other new information pertinent to 
this guideline. The comments in the docket will be periodically 
reviewed, and, where appropriate, the guideline will be amended. The 
public will be notified of any such amendments through a notice in the 
Federal Register.
    Interested persons may, at any time, submit written comments on the 
guideline to the Dockets Management Branch (address above). Two copies 
of any comments are to be submitted, except that individuals may submit 
one copy. Comments are to be identified with the docket number found in 
brackets in the heading of this document. The guideline and received 
comments may be seen in the office above between 9 a.m. and 4 p.m., 
Monday through Friday.
    The text of the guideline follows:

Toxicokinetics: Guidance on the Assessment of Systemic Exposure in 
Toxicity Studies

l. Introduction

    This Note for Guidance concerns toxicokinetics only with respect 
to the development of pharmaceutical products intended for use in 
human subjects.
    In this context, toxicokinetics is defined as the generation of 
pharmacokinetic data, either as an integral component in the conduct 
of nonclinical toxicity studies or in specially designed supportive 
studies, in order to assess systemic exposure. These data may be 
used in the interpretation of toxicology findings and their 
relevance to clinical safety issues (see Note 1 for definitions of 
other terms used in this document).
    The Note for Guidance has been developed in order to provide an 
understanding of the meaning and application of toxicokinetics and 
to provide guidance on developing test strategies in toxicokinetics. 
The guidance highlights the need to integrate pharmacokinetics into 
toxicity testing, which should aid in the interpretation of the 
toxicology findings and promote rational study design 
development. [[Page 11265]] 
    Toxicokinetic measurements are normally integrated within the 
toxicity studies and as such are described in this document as 
``concomitant toxicokinetics'' (Note 1). Alternatively, data may be 
generated in other supportive studies conducted by mimicking the 
conditions of the toxicity studies.
    Toxicokinetic procedures may provide a means of obtaining 
multiple dose pharmacokinetic data in the test species, if 
appropriate parameters are monitored, thus avoiding duplication of 
such studies; optimum design in gathering the data will reduce the 
number of animals required.
    Various components of the total nonclinical pharmacokinetics and 
metabolism program may be of value in contributing to the 
interpretation of toxicology findings. However, the toxicokinetic 
data focus on the kinetics of a new therapeutic agent under the 
conditions of the toxicity studies themselves.
    Toxicokinetics is thus an integral part of the nonclinical 
testing program; it should enhance the value of the toxicological 
data generated, both in terms of understanding the toxicity tests 
and in comparison with clinical data as part of the assessment of 
risk and safety in humans. Due to its integration into toxicity 
testing and its bridging character between nonclinical and clinical 
studies, the focus is primarily on the interpretation of toxicity 
tests and not on characterizing the basic pharmacokinetic parameters 
of the substance studied.
    As the development of a pharmaceutical product is a dynamic 
process which involves continuous feedback between nonclinical and 
clinical studies, no rigid detailed procedures for the application 
of toxicokinetics are recommended. It may not be necessary for 
toxicokinetic data to be collected in all studies and scientific 
judgment should dictate when such data may be useful. The need for 
toxicokinetic data and the extent of exposure assessment in 
individual toxicity studies should be based on a flexible step-by-
step approach and a case-by-case decisionmaking process to provide 
sufficient information for a risk and safety assessment.

2. The Objectives of Toxicokinetics and the Parameters Which May Be 
Determined

    The primary objective of toxicokinetics is:
    <bullet> To describe the systemic exposure achieved in animals 
and its relationship to dose level and the time course of the 
toxicity study.
    Secondary objectives are:
    <bullet> To relate the exposure achieved in toxicity studies to 
toxicological findings and contribute to the assessment of the 
relevance of these findings to clinical safety.
    <bullet> To support (Note 1) the choice of species and treatment 
regimen in nonclinical toxicity studies.
    <bullet> To provide information which, in conjunction with the 
toxicity findings, contributes to the design of subsequent 
nonclinical toxicity studies.
    These objectives may be achieved by the derivation of one or 
more pharmacokinetic parameters (Note 2) from measurements made at 
appropriate time points during the course of the individual studies. 
These measurements usually consist of plasma (or whole blood or 
serum) concentrations for the parent compound and/or metabolite(s) 
and should be selected on a case-by-case basis. Plasma (or whole 
blood or serum) AUC, C<INF>max, and C<INF>(time)  (Note 2) are the 
most commonly used parameters in assessing exposure in 
toxicokinetics studies. For some compounds it will be more 
appropriate to calculate exposure based on the (plasma protein) 
unbound concentration.
    These data may be obtained from all animals on a toxicity study, 
in representative subgroups, in satellite groups (see 3.5 and Note 
1) or in separate studies.
    Toxicity studies which may be usefully supported by 
toxicokinetic information include single and repeated dose toxicity 
studies, reproductive, genotoxicity, and carcinogenicity studies. 
Toxicokinetic information may also be of value in assessing the 
implications of a proposed change in the clinical route of 
administration.

3. General Principles to be Considered

3.1 Introduction

    In the following paragraphs some general principles are set out 
which should be taken into consideration in the design of individual 
studies.
    It should be noted that for those toxicity studies whose 
performance is subject to Good Laboratory Practice (GLP) the 
concomitant toxicokinetics must also conform to GLP. Toxicokinetic 
studies retrospectively designed to generate specific sets of data 
under conditions which closely mimic those of the toxicity studies 
should also conform to GLP when they are necessary for the 
evaluation of safety.

3.2 Quantification of exposure

    The quantification of systemic exposure provides an assessment 
of the burden on the test species and assists in the interpretation 
of similarities and differences in toxicity across species, dose 
groups, and sexes. The exposure might be represented by plasma 
(serum or blood) concentrations or the AUC's of parent compound and/
or metabolite(s). In some circumstances, studies may be designed to 
investigate tissue concentrations. When designing the toxicity 
studies, the exposure and dose-dependence in humans at therapeutic 
dose levels (either expected or established) should be considered in 
order to achieve relevant exposure at various dose levels in the 
animal toxicity studies. The possibility that there may be species 
differences in the pharmacodynamics of the substance (either 
qualitative or quantitative) should also be taken into 
consideration.
    Pharmacodynamic effects or toxicity might also give supporting 
evidence of exposure or even replace pharmacokinetic parameters in 
some circumstances.
    Toxicokinetic monitoring or profiling of toxicity studies should 
establish what level of exposure has been achieved during the course 
of the study and may also serve to alert the toxicologist to 
nonlinear, dose-related changes in exposure (Note 3) that may have 
occurred. Toxicokinetic information may allow better interspecies 
comparisons than simple dose/body weight (or surface area) 
comparisons.

3.3 Justification of time points for sampling

    The time points for collecting body fluids in concomitant 
toxicokinetic studies should be as frequent as is necessary, but not 
so frequent as to interfere with the normal conduct of the study or 
to cause undue physiological stress to the animals (Note 4). In each 
study, the number of time points should be justified on the basis 
that they are adequate to estimate exposure (see 3.2). The 
justification should be based on kinetic data gathered from earlier 
toxicity studies, from pilot or dose range-finding studies, from 
separate studies in the same animal model, or in other models 
allowing reliable extrapolation.

3.4 Contribution to the setting of dose levels in order to produce 
adequate exposure

    The setting of dose levels in toxicity studies is largely 
governed by the toxicology findings and the pharmacodynamic 
responses of the test species. However, the following toxicokinetic 
principles may contribute to the setting of the dose levels.

3.4.1 Low dose levels

    At the low dose, preferably a no-toxic-effect dose level (Note 
5), the exposure in the animals of any toxicity study should ideally 
equal or just exceed the maximum expected (or known to be attained) 
in patients. It is recognized that this ideal is not always 
achievable and that low doses will often need to be determined by 
considerations of toxicology; nevertheless, systemic exposure should 
be determined.

3.4.2 Intermediate dose levels

    Exposure at intermediate dose levels should normally represent 
an appropriate multiple (or fraction) of the exposure at lower (or 
higher) dose levels dependent upon the objectives of the toxicity 
study.

3.4.3 High dose levels

    The high dose levels in toxicity studies will normally be 
determined by toxicological considerations. However, the exposure 
achieved at the dose levels used should be assessed.
    Where toxicokinetic data indicate that absorption of a compound 
limits exposure to parent compound and/or metabolite(s) (Note 6), 
the lowest dose level of the substance producing the maximum 
exposure should be accepted as the top dose level to be used (when 
no other dose-limiting constraint applies, Note 7).
    Very careful attention should be paid to the interpretation of 
toxicological findings in toxicity studies (of all kinds) when the 
dose levels chosen result in nonlinear kinetics (Note 3). However, 
nonlinear kinetics should not necessarily result in dose limitations 
in toxicity studies or invalidate the findings; toxicokinetics can 
be very helpful in assessing the relationship between dose and 
exposure in this situation.

3.5 Extent of exposure assessment in toxicity studies

    In toxicity studies, systemic exposure should be estimated in an 
appropriate number of animals and dose groups (Note 8) to provide a 
basis for risk assessment. [[Page 11266]] 
    Concomitant toxicokinetics may be performed either in all or a 
representative proportion of the animals used in the main study or 
in special satellite groups (Notes 1 and 5). Normally, samples for 
the generation of toxicokinetic data may be collected from main 
study animals, where large animals are involved, but satellite 
groups may be required for the smaller (rodent) species.
    The number of animals to be used should be the minimum 
consistent with generating adequate toxicokinetic data. Where both 
male and female animals are utilized in the main study it is normal 
to estimate exposure in animals of both sexes unless some 
justification can be made for not so doing.
    Toxicokinetic data are not necessarily required from studies of 
different duration if the dosing regimen is essentially unchanged 
(see also 4.3).

3.6 Complicating factors in exposure interpretation

    Although estimating exposure as described above may aid in the 
interpretation of toxicity studies and in the comparison with human 
exposure, a few caveats should be noted.
    Species differences in protein binding, tissue uptake, receptor 
properties, and metabolic profile should be considered. For example, 
it may be more appropriate for highly protein bound compounds to 
have exposure expressed as the free (unbound) concentrations. In 
addition, the pharmacological activity of metabolites, the 
toxicology of metabolites, and antigenicity of biotechnology 
products may be complicating factors. Furthermore, it should be 
noted that even at relatively low plasma concentrations, high levels 
of the administered compound and/or metabolite(s) may occur in 
specific organs or tissues.

3.7 Route of administration

    The toxicokinetic strategy to be adopted for the use of 
alternative routes of administration, for example, by inhalation, 
topical, or parenteral delivery, should be based on the 
pharmacokinetic properties of the substance administered by the 
intended route.
    It sometimes happens that a proposal is made to adopt a new 
clinical route of administration for a pharmaceutical product; for 
example, a product initially developed as an oral formulation may 
subsequently be developed for intravenous administration. In this 
context, it will be necessary to ascertain whether changing the 
clinical route will significantly reduce the safety margin.
    This process may include a comparison of the systemic exposure 
to the compound and/or its relevant metabolite(s) (AUC and 
C<INF>max) in humans generated by the existing and proposed routes 
of administration. If the new route results in increased AUC and or 
C<INF>max, or a change in metabolic profile, the continuing 
assurance of safety from animal toxicology and kinetics should be 
reconsidered. If exposure is not substantially greater, or 
different, by the proposed new route compared to that for the 
existing route(s) then additional nonclinical toxicity studies may 
focus on local toxicity.

3.8 Determination of metabolites

    A primary objective of toxicokinetics is to describe the 
systemic exposure to the administered compound achieved in the 
toxicology species. However, there may be circumstances when 
measurement of metabolite concentrations in plasma or other body 
fluids is especially important in the conduct of toxicokinetics 
(Note 9).
    <bullet> When the administered compound acts as a ``pro-drug'' 
and the delivered metabolite is acknowledged to be the primary 
active entity.
    <bullet> When the compound is metabolized to one or more 
pharmacologically or toxicologically active metabolites which could 
make a significant contribution to tissue/organ responses.
    <bullet> When the administered compound is very extensively 
metabolized and the measurement of plasma or tissue concentrations 
of a major metabolite is the only practical means of estimating 
exposure following administration of the compound in toxicity 
studies (Note 10).

3.9 Statistical evaluation of data

    The data should allow a representative assessment of the 
exposure. However, because large intra- and inter-individual 
variation of kinetic parameters may occur and small numbers of 
animals are involved in generating toxicokinetic data, a high level 
of precision in terms of statistics is not normally needed. 
Consideration should be given to the calculation of mean or median 
values and estimates of variability, but, in some cases, the data of 
individual animals may be more important than a refined statistical 
analysis of group data.
    If data transformation (e.g., logarithmic) is performed, a 
rationale should be provided.

3.10 Analytical methods

    Integration of pharmacokinetics into toxicity testing implies 
early development of analytical methods for which the choice of 
analytes and matrices should be continually reviewed as information 
is gathered on metabolism and species differences.
    The analytical methods to be used in toxicokinetic studies 
should be specific for the entity to be measured and of an adequate 
accuracy and precision. The limit of quantification should be 
adequate for the measurement of the range of concentrations 
anticipated to occur in the generation of the toxicokinetic data.
    The choice of analyte and the matrix to be assayed (biological 
fluids or tissue) should be stated and possible interference by 
endogenous components in each type of sample (from each species) 
should be investigated. Plasma, serum, or whole blood are normally 
the matrices of choice for toxicokinetic studies.
    If the drug substance is a racemate or some other mixture of 
enantiomers, additional justification should be made for the choice 
of the analyte (racemate or enantiomer(s)).
    The analyte and matrix assayed in nonclinical studies should 
ideally be the same as in clinical studies. If different assay 
methods are used in non-clinical and clinical studies they should 
all be suitably validated.

3.11 Reporting

    A comprehensive account of the toxicokinetic data generated, 
together with an evaluation of the results and of the implications 
for the interpretation of the toxicology findings, should be given.
    An outline of the analytical method should be reported or 
referenced. In addition, a rationale for the choice of the matrix 
analysed and the analyte measured (see 3.8 and 3.10) should be 
given.
    The positioning of the report within the application will depend 
upon whether the data are specific to any one toxicity study or is 
supportive of all toxicity testing.

4. Toxicokinetics in the Various Areas of Toxicity Testing--Specific 
Aspects

4.1 Introduction

    Based on the principles of toxicokinetics outlined above, the 
following specific considerations refer to individual areas of 
toxicity testing. The frequency of exposure monitoring or profiling 
may be extended or reduced where necessary.
    It may be appropriate to take samples from some individual 
animals only, where this may help in the interpretation of the 
toxicology findings for these animals.

4.2 Single dose toxicity studies

    These studies are often performed in a very early phase of 
development before a bioanalytical method has been developed and 
toxicokinetic monitoring of these studies is therefore not normally 
possible. Plasma samples may be taken in such studies and stored for 
later analysis, if necessary; appropriate stability data for the 
analyte in the matrix sampled would then be required.
    Alternatively, additional toxicokinetic studies may be carried 
out after completion of a single dose toxicity study in order to 
respond to specific questions which may arise from the study.
    Results from single dose kinetic studies may help in the choice 
of formulation and in the prediction of rate and duration of 
exposure during a dosing interval. This may assist in the selection 
of appropriate dose levels for use in later studies.

4.3 Repeated dose toxicity studies

    The treatment regimen (Note 11) and species should be selected 
whenever possible with regard to pharmacodynamic and pharmacokinetic 
principles. This may not be achievable for the very first studies, 
at a time when neither animal nor human pharmacokinetic data are 
normally available.
    Toxicokinetics should be incorporated appropriately into the 
design of the studies. It may consist of exposure profiling or 
monitoring (Note l) at appropriate dose levels at the start and 
towards the end of the treatment period of the first repeat dose 
study (Note 12). The procedure adopted for later studies will depend 
on the results from the first study and on any changes in the 
proposed treatment regimen. Monitoring or profiling may be extended, 
reduced, or modified for specific compounds where problems have 
arisen in the interpretation of earlier toxicity 
studies. [[Page 11267]] 

4.4 Genotoxicity studies

    For negative results of in vivo genotoxicity studies, it may be 
appropriate to have demonstrated systemic exposure in the species 
used or to have characterized exposure in the indicator tissue.

4.5 Carcinogenicity (Oncogenicity) studies<SUP>1

4.5.1 Sighting or dose-ranging studies

    Appropriate monitoring or profiling of these studies should be 
undertaken in order to generate toxicokinetic data which may assist 
in the design of the main studies (see 4.5.2.). Particular attention 
should be paid to species and strains which have not been included 
in earlier toxicity studies and to the use of routes or methods of 
administration which are being used for the first time.
    Particular attention should be paid to the establishment of 
appropriate toxicokinetic data when administration is to be in the 
diet (Note 13).
    Toxicokinetic data may assist in the selection of dose levels in 
the light of information about clinical exposure and in the event 
that nonlinear kinetics (Note 3) may complicate the interpretation 
of the study.
    In principle, the ideal study design would ensure that dose 
levels in oncogenicity studies generate a range of systemic exposure 
values that exceed the maximum therapeutic exposure for humans by 
varying multiples. However, it is recognized that this idealized 
selection of dose levels may be confounded by unavoidable species-
specific problems. Thus, the emphasis of this guidance is on the 
need to estimate systemic exposure, to parent compound and/or 
metabolite(s) at appropriate dose levels and at various stages of an 
oncogenicity study, so that the findings of the study may be 
considered in the perspective of comparative exposure for the animal 
model and humans.
    A highest dose based on knowledge of probable systemic exposure 
in the test species and in humans may be an acceptable end-point in 
testing for carcinogenic potential. Historically, a toxicity end-
point<SUP>1 has been often used to select the top dose level.

4.5.2 The main studies

    The treatment regimen, species, and strain selection should, as 
far as is feasible, be determined with regard to the available 
pharmacokinetic and toxicokinetic information. In practice, the vast 
majority of these studies is conducted in the rat and mouse.
    As mentioned in the ``Introduction'' to this section, it is 
recommended that reassurance be sought from monitoring that the 
exposure in the main study is consistent with profiles of kinetics 
established in free-standing or specific dose-ranging studies. Such 
monitoring will be appropriate on a few occasions during the study, 
but it is not considered essential to continue beyond 6 months.

4.6 Reproductive toxicity studies<SUP>2

4.6.1 Introduction

    It is preferable to have some information on pharmacokinetics 
before initiating reproduction studies since this may suggest the 
need to adjust the choice of species, study design, and dosing 
schedules. At this time, the information need not be sophisticated 
or derived from pregnant or lactating animals. At the time of study 
evaluation, further information on pharmacokinetics in pregnant or 
lactating animals may be required depending on the results obtained.
    The limitation of exposure in reproductive toxicity is usually 
governed by maternal toxicity. Thus, while toxicokinetic monitoring 
in reproductive toxicity studies may be valuable in some instances, 
especially with compounds with low toxicity, such data are not 
generally needed for all compounds.
    Where adequate systemic exposure might be questioned because of 
absence of pharmacological response or toxic effects, toxicokinetic 
principles could usefully be applied to determine the exposures 
achieved by dosing at different stages of the reproductive process.
    A satellite group of female animals may be used to collect the 
toxicokinetic data.

4.6.2 Fertility studies

    The general principles for repeated dose toxicity studies apply 
(see 4.3). The need to monitor these studies will depend on the 
dosing regimen used and the information already available from 
earlier studies in the selected species.

4.6.3 Studies in pregnant and lactating animals

    The treatment regimen during the exposure period should be 
selected on the basis of the toxicological findings and on 
pharmacokinetic and toxicokinetic principles.
    Consideration should be given to the possibility that the 
kinetics will differ in pregnant and nonpregnant animals.
    Toxicokinetics may involve exposure assessment of dams, embryos, 
fetuses, or newborn at specified days (Note 14). Secretion in milk 
may be assessed to define its role in the exposure of newborns. In 
some situations, additional studies may be necessary or appropriate 
in order to study embryo/fetal transfer and secretion in milk.
    Consideration should be given to the interpretation of 
reproductive toxicity tests in species in which placental transfer 
of the substance cannot be demonstrated.

5. Supplementary Notes

    Note 1: Definitions of expressions appearing in this ``Note for 
Guidance:''
    Analyte: The chemical entity assayed in biological samples.
    Matrix: Blood, plasma, urine, serum, or other fluid or tissue 
selected for assay.
    Concomitant toxicokinetics: Toxicokinetic measurements performed 
in the toxicity study, either in all animals or in representative 
subgroups or in satellite groups.
    Exposure: Exposure is represented by pharmacokinetic parameters 
demonstrating the local and systemic burden on the test species with 
the test compound and/or its metabolites. The area under the matrix 
level concentration-time curve (AUC) and/or the measurement of 
matrix concentrations at the expected peak-concentration time 
C<INF>max, or at some other selected time C<INF>(time) are the most 
commonly used parameters. Other parameters might be more appropriate 
in particular cases.
    Monitor: To take a small number of matrix samples (e.g., 1 to 3) 
during a dosing interval to estimate C<INF>(time) or C<INF>max.
    Profile: To take (e.g., 4 to 8) matrix samples during a dosing 
interval to make an estimate of C<INF>max and/or C<INF>(time) and 
area under the matrix concentration-time curve (AUC).
    Satellite: Groups of animals included in the design and groups: 
conduct of a toxicity study, treated and housed under conditions 
identical to those of the main study animals, but used primarily for 
toxicokinetics.
    Support: In the context of a toxicity study--to ratify or 
confirm the design of a toxicity study with respect to 
pharmacokinetic and metabolic principles. This process may include 
two separate steps:
    (a) Confirmation using toxicokinetic principles that the animals 
on a study were exposed to appropriate systemic levels of the 
administered compound (see 3.4) and/or its metabolite(s).
    (b) Confirmation that the metabolic profile in the species used 
was acceptable; data to support this will normally be derived from 
metabolism studies in animals and in humans.
    Validate: In the context of an analytical method--to establish 
the accuracy, precision, reproducibility, response function, and the 
specificity of the analytical method with reference to the 
biological matrix to be examined and the analyte to be quantified.
    Note 2: Symbols and definitions according to ``Manual of 
Symbols, Equations and Definitions in Pharmacokinetics,'' Committee 
for Pharmacokinetic Nomenclature of the American College of Clinical 
Pharmacology, Philadelphia, PA, May 1982:
    C<INF>max-Maximum (peak) concentration.
    C<INF>(time)-Maximum concentration at a specified time after 
administration of a given dose.
    t<INF>max-Time to reach peak or maximum concentration following 
administration.
    AUC<INF>(0-t)-Area under concentration-time curve from zero to 
time t. It should be noted that AUC<INF>(O-infinity) is a special 
case of AUC<INF>(0-t).
    Other measurements, for example, urinary excretion, may be more 
appropriate for some compounds. Other derived parameters, for 
example, bioavailability, half-life, fraction of unbound drug, and 
volume of distribution, may be of value in interpreting 
toxicokinetic data. Thus, the selection of parameters and time 
points has to be made on a case-by-case basis considering the 
general principles as outlined in Section 3.
    Note 3: Increases in exposure may arise unexpectedly as a result 
of nonlinear kinetics due to saturation of a clearance process. 
Increasing exposure may also occur during the course of a study for 
those compounds which have a particularly long plasma half-life. 
Careful attention should also be paid to compounds which achieve 
high C<INF>max values over comparatively short time periods within 
the dosing interval. Conversely, unexpectedly low exposure may occur 
during a study as a result of auto-induction of metabolizing 
enzymes. [[Page 11268]] 
    Note 4: If samples are taken from main study animals it should 
be considered whether samples should be taken from all the dosed 
animals and the controls in order to treat all animals on the study 
in the same way, or whether samples should be taken from 
representative subgroups of the same size.
    Note 5: In this context, a ``no-toxic-effect dose level'' 
(deemed to be the same as ``no-observed-adverse-effect dose level'') 
is defined as a dose level at which some pharmacological response 
may be observed, but at which no adverse effect is found.
    Note 6: In these circumstances it should be established that 
absorption is the rate limiting step and that limitations in 
exposure to the administered substance are not due to an increased 
clearance.
    Note 7: The limits placed on acceptable volumes which can be 
administered orally to animals may constrain the dose levels 
achievable for comparatively non-toxic compounds administered as 
solutions or suspensions.
    Note 8: It is often considered unnecessary to assay samples from 
control groups. Samples may be collected and then assayed if it is 
deemed that this may help in the interpretation of the toxicity 
findings, or in the validation of the assay method.
    Note 9: Measurement of metabolite concentrations may be 
especially important when documentation of exposure to human 
metabolite(s) is needed in the nonclinical toxicity studies in order 
to demonstrate adequate toxicity testing of these metabolites.
    Note 10: It is recognised that measurement of metabolite(s) as a 
part of toxicokinetic evaluation serves only to assess exposure and 
cannot account for possible reactive intermediate metabolites.
    Note 11: Treatment regimen encompasses dosage, formulation, 
route of administration, and dosing frequency.
    Note 12: The first repeat dose study incorporating toxicokinetic 
data for each species is normally of 14 day's duration or longer.
    Note 13: Additional studies may be required in order to compare 
exposure to the compound administered in diet and by gavage or by 
routes different from the intended clinical route.
    Note 14: It should be noted that while it is important to 
consider the transfer of substances entering the embryo-fetal 
compartment, fetal exposure is the parameter which is most often 
assessed in practice in separate studies and expressed as 
``placental transfer.''

6. References (other ICH Guidance)

    1. Code SlC ``Carcinogenicity: Guidance for Dose Selection Dose 
Selection for Carcinogenicity Studies of Therapeutics.''
    2. Code S5A ``Detection of Toxicity to Reproduction for 
Medicinal Products.''

    Dated: February 23, 1995.
William B. Schultz,
Deputy Commissioner for Policy.
[FR Doc. 95-4957 Filed 2-28-95; 8:45 am]
BILLING CODE 4160-01-F