Guidance for Industry and
Review Staff
Recommended Approaches to Integration of Genetic Toxicology Study
Results
(PDF of
this document)
U.S. Department of
Health and Human Services
Food and Drug Administration
Center for Drug Evaluation and Research (CDER)
January 2006
Pharmacology and
Toxicology
Guidance for
Industry and Review Staff
Recommended Approaches to Integration
of Genetic Toxicology Study Results
Additional copies
are available from:
Office of Training and Communications
Division of Drug Information, HFD-240
Center for Drug Evaluation and Research
Food and Drug Administration
5600 Fishers Lane
Rockville, MD 20857
(Tel) 301-827-4573
http://www.fda.gov/cder/guidance/index.htm
U.S. Department of
Health and Human Services
Food and Drug Administration
Center for Drug Evaluation and Research (CDER)
January 2006
Pharmacology and
Toxicology
TABLE OF CONTENTS
I.
INTRODUCTION
II. BACKGROUND
III.
INTEGRATION OF GENETIC TOXICOLOGY STUDY RESULTS
REFERENCES
Guidance for
Industry and Review Staff1
Recommended Approaches to Integration of
Genetic Toxicology Study Results
This guidance
represents the Food and Drug Administration's (FDA's) current
thinking on this topic. It does not create or confer any
rights for or on any person and does not operate to bind FDA
or the public. You can use an alternative approach if the
approach satisfies the requirements of the applicable statutes
and regulations. If you want to discuss an alternative
approach, contact the FDA staff responsible for implementing
this guidance. If you cannot identify the appropriate FDA
staff, call the appropriate number listed on the title page of
this guidance. |
I.
INTRODUCTION
The purpose of this guidance is to
inform industry and the review staff in the Center for Drug
Evaluation and Research (CDER) on how CDER views positive findings
in genetic toxicology assays during drug development. The guidance
provides recommendations on how to proceed with clinical studies
while ensuring the safety of study participants when results in
genotoxicity studies suggest a potential cancer or genetic hazard.
This guidance pertains to pharmaceuticals administered through oral,
intravenous, topical, and other routes, as appropriate.
FDA's guidance documents, including
this guidance, do not establish legally enforceable
responsibilities. Instead, guidances describe the Agency's current
thinking on a topic and should be viewed only as recommendations,
unless specific regulatory or statutory requirements are cited. The
use of the word should in Agency guidances means that
something is suggested or recommended, but not required.
II.
BACKGROUND
The timing and conduct of genetic
toxicology studies have been described in the ICH guidelines M3,
S2A, and S2B.2 We recommend that
these guidances be consulted and that this document be considered an
adjunct guidance.
Risk for carcinogenesis is usually
determined in rodent assays, either 2-year studies or shorter-term
studies using alternative models.3
A core battery of genetic toxicology studies has been accepted by
industry and regulators through the International Conference on
Harmonisation (ICH) consultative process. These studies, which are
designed to identify genotoxic hazard, include:
- A test for gene mutation in
bacteria3B
- An in vitro assessment of
chromosomal damage using mammalian cells or an in vitro mouse
lymphoma tk+/- assay3B and
- An in vivo test for chromosomal
damage using rodent hematopoietic cells.
The following discussion is based on
current guidance documents.4 We
recommend that results from in vitro genetic toxicology studies be
available before the initiation of phase 1 trials.
III.
INTEGRATION OF GENETIC TOXICOLOGY STUDY RESULTS
The Agency takes into account the
totality of safety data when considering whether it is safe to
proceed with a clinical trial when there are positive genetic
toxicology study results. This consideration includes a thorough
evaluation of all the genetic toxicology data and the nature of the
proposed trial. If the results of the genetic toxicology tests
indicate a lack of genotoxic potential, then clinical trials can
generally be undertaken in healthy subjects or patient populations
with the proposed medical indication.
Pharmaceuticals that give positive
results in genetic toxicology assays but do not directly interact
with DNA do not always present a significant in vivo risk. In such
cases, we recommend providing evidence of the mechanism of
genotoxicity and relevance of the mechanism to anticipated in vivo
exposure. Alternatively, it is also appropriate to rule out
mechanisms involving direct interaction with DNA (e.g.,
demonstration that a drug does not cause DNA alkylation or DNA
strand breakage).
Drugs known to directly damage DNA
may be permitted to be used in patients with debilitating or
life-threatening diseases, such as cancer, but should not be
administered to healthy subjects.5
If any of the three assays in the
ICH genotoxicity standard battery are positive, then we recommend
completing the fourth test in the ICH battery. Equivocal studies
should be repeated to determine the reproducibility of the results.
If a positive response is seen in one or more assays, sponsors
should consider choosing from one or more of the following options.
In some instances, after evaluation
of all available data, the weight of evidence (WOE) suggests a lack
of genotoxic hazard. For example, a positive response is observed in
one exposure regimen of an in vitro cytogenetics assay. The positive
result is seen only at the high dose, and the increase is within or
just outside the range for historical control values for the solvent
and cell line employed. The WOE approach could indicate that
although a small increase in the frequency of chromosomal
aberrations is statistically significant, it lacks biological
relevance. Contributing considerations could include (1) the level
of cytotoxicity at which the response was seen, and (2)
corroborating data from the same or complementary assays. For
example, a positive response seen with a short-term exposure without
metabolic activation but not corroborated with the longer exposure
at comparable levels of cytotoxicities would argue against the
biological significance of the positive result. Similarly, such a
positive finding in an in vitro chromosomal aberration assay that is
not corroborated by the matching exposure regimen of the mouse
lymphoma assay could also call into question the significance of the
positive finding. If the WOE approach indicates a lack of genotoxic
hazard, clinical studies could proceed provided the positive
response is described in the investigator's brochure and the
informed consent form.
Positive results are sometimes
satisfactorily explained by knowledge of the mechanism of action.
For example, it has been demonstrated that in vitro clastogenic
effects can result from excessively high osmolarity or low pH.
Positive responses elicited under such nonphysiologic exposure
conditions are not relevant to human risk. In addition, certain
genotoxic responses are thought to have thresholds below which a
hazard does not exist. Agents that induce effects by indirect
mechanisms (e.g., interference with metabolism of nucleotides and
their precursors, damage to spindle proteins, inhibition of DNA
synthesis, or inhibition of topoisomerase) can have thresholds for
genotoxic effects. In such cases, we recommend presenting evidence
of the existence of a threshold that would not be attained during
the proposed clinical exposure or presenting evidence of a mechanism
not expected to be operative in vivo. Positive responses that are
satisfactorily explained by an MOA may allow clinical studies in
normal volunteers or in patients to proceed without additional
studies.
On occasion, results from in vitro
studies demonstrate a reproducible positive dose-response. Results
from bone marrow cytogenetic studies are frequently negative, even
for those compounds giving positive results in in vitro genetic
toxicology assays. This discrepancy can result from a number of
differences between cultured cells and intact animals: differing
metabolic pathways occurring in vitro and in vivo, metabolic
inactivation in the intact animal, failure of the parent compound or
active metabolite to reach the target cell, or simply, an inability
to achieve plasma levels in vivo comparable to concentrations that
generated positive responses in the in vitro assays.
Additional in vivo assays can be
useful in clarifying in vitro positive results. For example,
peripheral blood smears from repeat-dose toxicity studies in mice
can be evaluated for micronucleus induction, and peripheral blood
lymphocytes from repeat-dose studies in rats or monkeys can be
cultured and assessed for chromosome damage in metaphase spreads.
DNA damage can be assessed in potential target tissues (e.g., DNA
adducts or DNA strand breakage using the Comet or alkaline elution
assay), or transgenic rats or mice can be used to assess
mutagenicity in potential target tissues.6
The Syrian hamster embryo cell (SHE)
transformation assay has been suggested as a follow-up assay in the
face of positive in vitro genotoxicity results. Data in the
literature suggest that the SHE assay correlates well with rodent
carcinogenicity results for chemicals in general (Isfort et al.
1996). Results from an International Life Sciences Institute (ILSI)
validation effort on human pharmaceuticals, although smaller in
scope, suggest that the SHE assay is less predictive for human
carcinogenic risk (Mauthe et al. 2001). With respect to human
pharmaceuticals, the ILSI study found that the SHE assay had high
sensitivity (83 percent) for detection of human carcinogens.
However, its low specificity (15 percent) for prediction of
putative human noncarcinogens led to a poor overall concordance
of 37 percent. Although transformation assays measure endpoints more
akin to the health effect of concern (cancer) and can be useful in
making a WOE judgment, they also have inherent limitations. Many
pharmaceuticals that give positive responses in 2-year rodent
carcinogenicity studies do so through exaggerated pharmacological
effects, immune suppression, or hormonal disequilibrium. It is
unclear how an in vitro assay could be responsive to these
mechanisms.
In the last several years, a number
of transgenic mouse strains have become available for use in
short-term carcinogenicity studies. The p53 haplo insufficient mouse
has been found to be useful in the identification of mutagenic
carcinogens (MacDonald et al. 2004). Negative results in a p53
carcinogenicity study are considered evidence that a genotoxic agent
does not present a carcinogenic hazard to humans through a
p53-mediated mechanism.
Supportive studies contribute to the
WOE determination as to whether a drug giving a positive response in
one of the ICH-specified assays presents a risk of genetic damage to
subjects involved in clinical trials. The decision as to whether
early assessment of oncogenic potential will be needed will, out of
necessity, be on a case-by-case basis. Factors influencing the
decision include target population, disease indication, duration of
exposure, and safety profile of other drugs in the class or other
drugs serving the same medical need.
REFERENCES
ICH guidance for industry S1A The
Need for Long-Term Rodent Carcinogenicity Studies of Pharmaceuticals.
(http://www.fda.gov/cder/guidance/index.htm)
Isfort, RJ, GA Kerckaert, and RA
LeBoeuf, 1996, Comparison of the Standard and Reduced pH Syrian
Hamster Embryo (SHE) Cell Transformation Assays in Predicting the
Carcinogenic Potential of Chemicals, Mutat. Res. 356:11-63.
MacDonald, J, JE French, RJ Gerson,
J Goodman, T Inoue et al., 2004, The Utility of Transgenic Mouse
Assays for Identifying Human Carcinogens - A Basic Understanding and
Path Forward, Toxicol. Sci. 77(2):188-194.
Mauthe, RJ, DP Gibson, RT Bunch, and
L Custer, 2001, The Syrian Hamster Embryo (SHE) Cell Transformation
Assay: Review of Methods and Results, Toxicologic Pathology 29
(Supplement): 138-146.
1 This guidance has been prepared by
the Pharmacology Toxicology Coordinating Committee (PTCC) in the
Office of New Drugs (OND) in the Center for Drug Evaluation and
Research (CDER) of the Food and Drug Administration.
2 ICH guidance for industry M3
Nonclinical Safety Studies for the Conduct of Human Clinical Trials
for Pharmaceuticals, ICH guidance for industry S2A Specific
Aspects of Regulatory Genotoxicity Tests for Pharmaceuticals,
and ICH guidance for industry S2B Genotoxicity: A Standard
Battery for Genotoxicity Testing of Pharmaceuticals. (http://www.fda.gov/cder/guidance/index.htm)
3 ICH guidance for industry S1B
Testing for Carcinogenicity of Pharmaceuticals. (http://www.fda.gov/cder/guidance/index.htm)
4 We update guidances periodically.
To make sure you have the most recent version of a guidance, check
the CDER guidance Web page at
http://www.fda.gov/cder/guidance/index.htm.
5 ICH guidance for industry S2A
Specific Aspects of Regulatory Genotoxicity Tests for
Pharmaceuticals. (http://www.fda.gov/cder/guidance/index.htm)
6 ICH guidance for industry S2B
Genotoxicity: A Standard Battery for Genotoxicity Testing of
Pharmaceuticals. (http://www.fda.gov/cder/guidance/index.htm)
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Date created: January 3, 2006 |