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

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:

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

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

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 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).

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 metabolized to one or more

pharmacologically or toxicologically active metabolites which could

make a significant contribution to tissue/organ responses.

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) studies1

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-

point1 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 studies2

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

Cmax, or at some other selected time C(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(time) or Cmax.

Profile: To take (e.g., 4 to 8) matrix samples during a dosing

interval to make an estimate of Cmax and/or C(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:

Cmax-Maximum (peak) concentration.

C(time)-Maximum 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(O-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: 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 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 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