[Federal Register Volume 59, Number 40 (Tuesday, March 1, 1994)]
[Unknown Section]
[Page 0]
From the Federal Register Online via the Government Publishing Office [www.gpo.gov]
[FR Doc No: 94-4569]


[[Page Unknown]]

[Federal Register: March 1, 1994]


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DEPARTMENT OF HEALTH AND HUMAN SERVICES
[Docket No. 94D-0015]

 

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

AGENCY: Food and Drug Administration, HHS.

ACTION: Notice.

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SUMMARY: The Food and Drug Administration (FDA) is publishing a draft 
guideline entitled, ``Toxicokinetics: A Guidance on the Assessment of 
Systemic Exposure in Toxicity Studies.'' This guideline was prepared by 
the Safety Expert Working Group of the International Conference on 
Harmonisation of Technical Requirements for Registration of 
Pharmaceuticals for Human Use (ICH). This draft 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: Written comments by May 16, 1994.

ADDRESSES: Submit written comments on the draft guideline to the 
Dockets Management Branch (HFA-305), Food and Drug Administration, rm. 
1-23, 12420 Parklawn Dr., Rockville, MD 20857.

FOR FURTHER INFORMATION CONTACT:
    Regarding the draft guideline: Alan S. Taylor, Center for Drug 
Evaluation and Research (HFD-502), Food and Drug Administration, 5600 
Fishers Lane, Rockville, MD 20857, 301-443-2544.
    Regarding the ICH: Janet Showalter, Office of Health Affairs (HFY-
50), 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 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.
    ICH was organized to provide an opportunity for tripartite 
harmonization initiatives to be developed with technical input from 
both regulatory and industry representatives. FDA also seeks input from 
consumer representatives and other interested parties. Through notices 
such as this, FDA invites public comment on ICH initiatives that have 
reached the draft guideline stage. 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, FDA, and the U.S. Pharmaceutical 
Manufacturers Association. The ICH Secretariat, which coordinates the 
preparation of documentation, is provided by the International 
Federation of Pharmaceutical Manufacturers Associations (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.
    At a meeting held on October 27 through 29, 1993, the ICH Steering 
Committee agreed that the draft tripartite guideline entitled ``The 
Assessment of Systemic Exposure in Toxicity Studies'' should be made 
available for public comment. The draft guideline will be made 
available for comment by the European Commission and Japanese Ministry 
of Health and Welfare, as well as by FDA, in accordance with their 
respective consultation procedures. After analyzing the comments and 
revising the guideline, if appropriate, FDA will determine whether it 
will adopt and issue the guideline. The draft 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.
    Guidelines are generally issued under Secs. 10.85(d) and 10.90(b) 
(21 CFR 10.85(d) and 10.90(b)), which provide for the use of guidelines 
to establish procedures or standards of general applicability that are 
not legal requirements but that are acceptable to FDA. The agency is 
now in the process of considering whether to revise Secs. 10.85(d) and 
10.90(b). Therefore, if the agency issues this guideline in final form, 
the guideline would not be issued under the authority of Secs. 10.85(d) 
and 10.90(b), and would not create or confer any rights, privileges, or 
benefits for or on any person, nor would it operate to bind FDA in any 
way.
    Interested persons may, on or before May 16, 1994, submit written 
comments on the draft guideline to the Dockets Management Branch 
(address above). Two copies of any comments are to be submitted, except 
that individuals may submit single copies. Comments are to be 
identified with the docket number found in brackets in the heading of 
this document. The draft 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 draft guideline follows:

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

1. Introduction

    This Note for Guidance concerns toxicokinetics only with respect 
to the development of pharmaceutical products intended for use in 
human subjects1.
    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.
    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 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 programme may be of value in contributing to the 
interpretation of toxicology findings. However, the toxicokinetic 
data focuses 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 programme; 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 feed-back 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 
judgement 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 decision making 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:
     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:
     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;
     to support (Note 1) the choice of species and treatment 
regimen in nonclinical toxicity studies;
     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, Cmax, and C(time) (Note 2) are the most 
commonly used parameters in assessing exposure in toxicokinetic 
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 in a toxicity study, 
in representative subgroups, or in satellite groups (see 3.5 and 
Note 3).
    Toxicity studies which may be usefully supported by 
toxicokinetic information include single and repeated dose toxicity 
studies, and 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 should also conform to GLP2,3. 
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.

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 or toxicodynamic effects might also give 
supporting evidence of exposure or even replace pharmacokinetic 
parameters in some circumstances.
    Toxicokinetic monitoring or profiling of the 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 
non-linear dose related changes in exposure (Note 4) which may have 
occurred. Toxicokinetic information may allow better interspecies 
comparisons than simple dose/body-weight (or surface area) 
comparisons4.

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 5). 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

3.4.1 Introduction

    The setting of dose levels in repeat dose 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.

3.4.2 Low dose levels

    At the low dose level, preferably a no-toxic-effect dose level 
(Note 6), the exposure in toxicity studies (of all kinds) should 
normally exceed that expected or known to be attained in humans at 
steady state following therapeutic dose levels. There are, however, 
cases where this objective may not be achieved even with the maximum 
dose which can be administered.

3.4.3 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.

3.4.4 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 7), 
the lowest dose level of the substance producing the maximum 
exposure should be accepted as the top dose level to be used 
(particularly when no other dose-limiting constraint applies, Note 
8).
    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 non-linear kinetics (Note 4). However, 
non-linear 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 dosed groups (Note 9) to provide a 
basis for risk assessment.
    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, 3 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 utilised 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 profiles should be considered. For 
example, it may be more appropriate for some 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 its relevant metabolite(s) (plasma AUC and Cmax) 
in humans generated by the existing and proposed routes of 
administration. If the new route results in increased AUC and/or 
Cmax, or a change in metabolic route, 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:
     When the administered compound acts as a `pro-drug' and 
the delivered metabolite is acknowledged to be the primary active 
entity.
     When the compound is metabolised to a pharmacologically 
or toxicologically active metabolite which would make a significant 
contribution to the pharmacological or toxicological response, in 
addition to the compound itself (Note 10).
     When the administered compound is very extensively 
metabolised 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 11).

3.9 Statistical evaluation of data

    The data should allow a representative assessment of the 
exposure. However, because large intra- and interindividual 
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 possible or 
required. Consideration should be given to the calculation of mean 
or median values and estimates of variability, but in some cases the 
data for individual animals may be more important than a refined 
statistical analysis of group data.

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 precision6. 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 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 nonclinical and clinical studies they should all 
be suitably validated6.

3.11 Reporting

    A rationale for the toxicokinetic policy adopted should be 
reported either in the toxicity study report or in a separate 
report. 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.

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 individual animals on 
a study 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; appropriate stability data for the analyte in the 
matrix sampled would then be needed.
    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 12) 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 1) at appropriate dose levels at the start and 
towards the end of the treatment period of the first repeat dose 
study (Note 13). 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 
or reduced, or modified for specific compounds where problems have 
arisen in the interpretation of earlier toxicity studies.

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 
tissue7.

4.5 Carcinogenicity (Oncogenicity) studies

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.
    Toxicokinetic data may assist in the selection of dose levels in 
the light of information about clinical exposure and in the event 
that non-linear kinetics (Note 4) may complicate the interpretation 
of the study. Particular attention should be paid to the 
establishment of appropriate toxicokinetic data when administration 
is to be in the diet (Note 14).
    It is recommended 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.
    In practice, the `Maximum Tolerated Dose' (MTD) has been used, 
whenever possible, as the top dose level in these studies. However, 
it has been suggested8 that it may be acceptable to select a 
high dose level based on consideration of the kinetics in humans and 
in the test species.
    For nongenotoxic compounds of comparatively low general 
toxicity, in addition to a toxicity-based endpoint (MTD) which 
remains acceptable, it has been proposed9 reasonable to define 
a level of animal exposure that would be considered sufficiently 
great, compared to human exposure, to provide reassurance of an 
adequate test of carcinogenicity. It is considered important to 
compare exposure rather than administered dose because the latter 
does not take into account inter-species differences in 
pharmacokinetics9.

4.5.2 The main studies

    The treatment regimen and 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 are conducted in the rat and mouse. 
Reassurance should be sought from the toxicokinetic data that the 
exposure level in the chosen species is consistent with the results 
from the dose ranging studies.
    Concomitant toxicokinetics may be confined to monitoring 
exposure at appropriate dose levels at a number of stages in the 
study. Appropriate stages may be early in the study, and after 
prolonged treatment, for example at one year. It is not considered 
necessary to monitor exposure beyond one year in these studies. The 
design for each test should be selected on a compound by compound 
basis utilizing data gathered from earlier studies (see 4.5.1).

4.6 Reproductive toxicity studies

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 animals10. At the time of 
study evaluation, further information on pharmacokinetics in 
pregnant or lactating animals may be necessary depending on the 
results obtained10.
    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 necessary for all compounds.
    Where appropriate, toxicokinetic principles should be applied to 
determine the exposures achieved in the different stages of the 
reproduction toxicity studies. 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.
    Toxicokinetics may involve exposure assessment of dams, embryos, 
fetuses, or newborn at specified days (Note 15). Secretion in milk 
may be assessed to define its role in the exposure of newborn. 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 possibility that 
pharmacokinetics may differ in pregnant and non-pregnant animals.
    Consideration should be given to the interpretation of 
reproductive toxicity tests in species in which placental transfer 
of the substance cannot be demonstrated (Note 16).

5. Supplementary Notes

    Note 1 Definitions of expressions appearing in this ``Note for 
Guidance'':
    Analyte: the chemical entity assayed in biological samples.
    Concomitant toxicokinetics: toxicokinetic measurements performed 
in the toxicity study animals, either in all 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 plasma 
level concentration-time curve (AUC) and/or the measurement of 
plasma concentrations at the expected peak-concentration time Cmax, 
or at some other selected time C(time), are the most commonly 
used parameters. Others might be more appropriate in particular 
cases.
    Monitor: to take a small number of blood samples (say 1-3) 
during a dosing interval to estimate C(time) or Cmax.
    Profile: to take (say) 4-8 blood samples during a dosing 
interval to make an estimate of Cmax and/or C(time) and area 
under the plasma concentration-time curve (AUC).
    Satellite groups: groups of animals included in the design and 
conduct of the toxicity study and housed with 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 b) 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 
quantified6.
    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:
    Cmax - Maximum (peak) plasma concentration
    C(time) - Plasma concentration at a specified time after 
administration of a given dose
    tmax - Time to reach peak or maximum concentration following 
administration
    AUC(0-t) - Area under concentration-time curve from zero to 
time t. It should be noted that AUC(0-infinity) is a special 
case of AUC(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 Satellite groups (Note 1) to toxicity studies should be 
housed in conditions identical to those provided for the main test 
animals and be subject to the same dosing procedures and animal 
husbandry procedures.
    Note 4 Increases in exposure may arise unexpectedly as a result 
of non-linear kinetics11 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 plasma Cmax 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 
metabolic enzymes.
    Note 5 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 6 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 7 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 by metabolism.
    Note 8 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 9 It is often considered unnecessary to assay samples from 
control groups, but 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 10 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 metabolites5.
    Note 11 It is recognized that measurement of metabolite(s) as a 
part of toxicokinetic evaluation serves only to assess exposure and 
cannot account for possible reactive intermediate 
metabolites12.
    Note 12 Treatment regimen encompasses dosage, formulation, route 
of administration, and dosing frequency.
    Note 13 The first repeat dose study incorporating toxicokinetic 
data for each species is normally of 14 days' duration or longer.
    Note 14 Additional studies may be necessary in order to compare 
exposure to the compound administered in diet and by gavage or by 
routes different from the intended clinical route.
    Note 15 Separate pharmacokinetic studies may be needed in order 
to establish the pharmacokinetic profile in species and strains 
selected for reproductive toxicity studies which have not been 
previously selected for general toxicity studies. 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 and expressed as 
`placental transfer'.
    Note 16 For practical reasons, it is normally accepted that 
placental transfer has not been demonstrated if the concentration in 
the whole fetus does not exceed 1% of the maternal plasma 
concentration.

7. References

    1 Design of Toxicokinetic Studies, Smith D. A., Humphrey M. J., 
and Charuel, Xenobiotica, 1990, Vol. 20, No. 11. 1187-1199.
    2 Food and Drug Administration, Department of Health and Human 
Services, Statement dated June 9th 1993.
    3 Commission of the European Communities, Statement on 
Applicability of Good Laboratory Practice (III/3824/92).
    4 Opportunities for Integration of Pharmacokinetics, 
Pharmacodynamics, and Toxicokinetics in Rational Drug Development, 
Peck C. C. et al., Pharmaceutical Research, 1992, Vol. 9, No. 6, 
826-833.
    5 Proceedings of The First International Conference on 
Harmonisation, Brussels 1991. Ed: D'Arcy, P. F. and Harron, D. W. G. 
(1992), page 188.
    6 Analytical methods validation: Bioavailability, Bioequivalence 
and Pharmacokinetic Studies, Shah, V. P. et al., European Journal of 
Drug Metabolism and Pharmacokinetics, 1991, Vol. 16, No. 4, 249-255.
    7 ICH Joint Position Paper: Genotoxicity, 1993.
    8 Proceedings of The First International Conference on 
Harmonisation, Brussels 1991. Ed: D'Arcy, P. F. and Harron, D. W. G. 
(1992), pages 185 and 331.
    9 ICH Position Paper: `High Dose Selection for Carcinogenicity 
Studies', 1993.
    10 ICH Tripartite Guideline: `Guideline on Detection of Toxicity 
to Reproduction for Medicinal Products', 1993.
    11 Gibaldi M. and Perrier D., `Pharmacokinetics' Second Edition, 
Chapter 7, Marcel Dekker Inc., New York (1982).
    12 What is an appropriate measure of exposure when testing drugs 
for carcinogenicity in rodents? Monro, A., Toxicology and Applied 
Pharmacology, 1992, 112, 171-181.

    Dated: February 23, 1994.
Michael R. Taylor,
Deputy Commissioner for Policy.
[FR Doc. 94-4569 Filed 2-24-94; 1:35 pm]
BILLING CODE 4160-01-F