Guideline for Industry
Pharmacokinetics: 
Guidance for Repeated Dose
Tissue Distribution
Studies
ICH-S3B
March 1995 TABLE OF CONTENTS
I.INTRODUCTION1
II.CIRCUMSTANCES UNDER WHICH REPEATED DOSE TISSUE DISTRIBUTION
STUDIES SHOULD BE CONSIDERED2
III.DESIGN AND CONDUCT OF REPEATED DOSE TISSUE DISTRIBUTION
STUDIES2
IV.SUMMARY3
V.REFERENCE3
i GUIDELINE FOR INDUSTRY 1
PHARMACOKINETICS:
GUIDANCE FOR REPEATED DOSE TISSUE DISTRIBUTION
STUDIES
I.INTRODUCTION
A comprehensive knowledge of the absorption, distribution, metabolism and elimination
of a compound is important for the interpretation of pharmacology and toxicology
studies.  Tissue distribution studies are essential in providing information on distribution
and accumulation of the compound and/or metabolites, especially in relation to
potential sites of action; this information may be useful for designing toxicology and
pharmacology studies and for interpreting the results of these experiments.
In the European Union, United States and Japan, there has been a general agreement
on the need to conduct single dose tissue distribution studies as part of the non-clinical
program.  These studies often provide sufficient information about tissue distribution.
There has been no consistent requirement for repeated dose tissue distribution
studies.  However, there may be circumstances when assessments after repeated
1This guideline has been developed within the Expert Working Group (Safety) of
the International Conference on Harmonisation of Technical Requirements for
Registration of Pharmaceuticals for Human Use (ICH) and has been subject to
consultation by the regulatory parties, in accordance with the ICH process.  This
document has been endorsed by the ICH Steering Committee at  Step 4 of the ICH
process, October 27, 1994.  At  Step 4 of the process, the final draft is recommended
for adoption to the regulatory bodies of the European Union, Japan and the USA.  In
the past,  guidelines have generally been issued under  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 that are acceptable to FDA.  The
agency is now in the process of revising 10.90(b).  Therefore, this guideline is not
being issued under the authority of 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.  For additional copies of this guideline, contact the Consumer Affairs Branch
(formerly the Executive Secretariat Staff), HFD-210, Center for Drug Evaluation and
Research, 7500 Standish Place, Rockville, MD  20855, 301-594-1012.  An electronic
version of this guideline is also available via Internet by connecting to the CDER file
transfer protocol (FTP) server (CDVS2.CDER.FDA.GOV). dosing may yield important information.
This paper provides guidance on circumstances when repeated dose tissue
distribution studies should be considered and on the conduct of such studies.
II.CIRCUMSTANCES UNDER WHICH REPEATED DOSE TISSUE
DISTRIBUTION STUDIES SHOULD BE CONSIDERED
A.When single dose tissue distribution studies suggest that the apparent half-life of the
test compound (and/or metabolites) in organs or tissues significantly exceeds the
apparent half-life of the elimination phase in plasma and is also more than twice the
dosing interval in the toxicity studies, repeated dose tissue distribution studies may be
appropriate.
B.When steady-state levels of a compound/metabolite in the circulation, determined in
repeated dose pharmacokinetic or toxicokinetic studies, are markedly higher than
those predicted from single dose kinetic studies, then repeated dose tissue distribution
studies should be considered.
C.When histopathological changes, critical for the safety evaluation of the test
substances, are observed that would not be predicted from short-term toxicity studies,
single dose tissue distribution studies and pharmacological studies, repeated dose
tissue distribution studies may aid in the interpretation of these findings.  Those organs
or tissues which were the site of the lesions should be the focus of such studies.
D.When the pharmaceutical is being developed for site-specific targeted delivery,
repeated dose tissue distribution studies may be appropriate.
III.DESIGN AND CONDUCT OF REPEATED DOSE TISSUE DISTRIBUTION
STUDIES
The objectives of these studies may be achieved using radiolabelled compounds or
alternative methods of sufficient sensitivity and specificity.
Dose level(s) and species should be chosen to address the problem that led to the
consideration of the repeated dose tissue distribution study.
Information from previous pharmacokinetic and toxicokinetic studies should be used in
selecting the duration of dosing in repeated dose tissue distribution studies.  One week
of dosing is normally considered to be a minimum period.  A longer duration should be
selected when the blood/plasma concentration of the compound and/or its metabolites
does not reach steady state.  It is normally considered unnecessary to dose for longer
than three weeks.
2 Consideration should be given to measuring unchanged compound and/or metabolites
in organs and tissues in which extensive accumulation occurs or if it is believed that
such data may clarify mechanisms of organ toxicity.
IV.SUMMARY
Tissue distribution studies are an important component in the non-clinical kinetics
program.  For most compounds, it is expected that single dose tissue distribution
studies with sufficient sensitivity and specificity will provide an adequate assessment of
tissue distribution and the potential for accumulation.  Thus, repeated dose tissue
distribution studies should not be required uniformly for all compounds and should only
be conducted when appropriate data cannot be derived from other sources.  Repeated
dose studies may be appropriate under certain circumstances based on the data from
single dose tissue distribution studies, toxicity and toxicokinetic studies.  The studies
may be most appropriate for compounds which have an apparently long half-life,
incomplete elimination or unanticipated organ toxicity.  The design and timing of
repeated dose tissue distribution studies should be determined on a case-by-case
basis.
V.REFERENCE
Federal Register, Vol. 60, No. 40, Wednesday, March 1, 1995, pages 11274-11275.
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