National 

Toxicology

Program
Table of Contents

I Overview of the National Toxicology Program

II Current Directions

III Evolving Strategies

IV NTP Centers

V Report on Carcinogens

VI Communication and Public Outreach

National Toxicology Program 1

U.S. Department of 

Health and Human Services

Public Health Service

National Toxicology Program

NTP

Current Directions and Evolving Strategies
2 National Toxicology Program

I

Overview

Mission and Goals

More than 80,000 chemicals are registered for use in

commerce in the United States, and an estimated 2,000

new ones are introduced annually for use in everyday

items such as foods, personal care products, prescription

drugs, household cleaners, and lawn care products. The

effects of many of these chemicals on human health are

unknown, yet people and our environment may be

exposed to them during their manufacture, distribution,

use, and disposal or as pollutants in our air, water, or soil.

Although relatively few chemicals are thought to pose a

significant risk to human health, safeguarding public

health depends on identifying the effects of these

chemicals and the levels of exposure at which they may

become hazardous to humans. 

The National Toxicology Program (NTP) was

established by the Department of Health and Human

Services (DHHS) in 1978 to coordinate toxicological

testing programs within the Department; strengthen the

science base in toxicology; develop and validate improved

testing methods; and provide information about potentially

toxic chemicals to health regulatory and research agencies,

the scientific and medical communities, and the public.

The NTP is an interagency program whose mission is to

evaluate agents of public health concern by developing

and applying tools of modern toxicology and molecular

biology. The Program maintains an objective, science-based

approach in dealing with critical issues in toxicology and

is committed to using the best science available to

prioritize, design, conduct, and interpret its studies. To 

that end, the NTP is continually evolving to remain at the

cutting edge of scientific research and development and the

application of technology.

Focusing on the Future

The NTP provides information that improves the

nation’s ability to evaluate human health effects from

chemical and physical exposures. The NTP maintains a

number of complex interrelated research and testing

programs that provide unique information and

knowledge needed by health, regulatory, and research

agencies to protect public health. All of these activities are

executed in the public arena, including communication

with all stakeholders. The NTP has always drawn

strength and direction from the commitment of its

scientists to open information exchange, adherence to
impartiality, and rigorous scientific peer review. This

will remain a central priority of the program now and

in the years to come.

The NTP seeks to maintain a balanced research and

testing program that provides data addressing a wide

variety of issues of importance to public health. In

particular, assistance is sought for the nomination of

studies that permit the testing of hypotheses to enhance

the predictive ability of future NTP studies, to address

mechanisms of toxicity, or to fill significant gaps in the

knowledge of the toxicity of chemicals or classes of

chemicals. Currently the NTP is focusing on several

areas that have received inadequate attention in the past,

i.e., photoactive chemicals, contaminants of finished

drinking water, endocrine-disrupting agents, and certain

occupational exposures and addressing potential safety

issues associated with herbal medicines or DNA-based

therapies. In general, these initiatives are broad-based

and include various health-related end points.

The NTP continues to work to develop and validate

alternative testing methods that will improve the

identification of chemical hazards using fewer animals.

This effort includes development of more efficient,

mechanism-based testing strategies such as transgenic

models for toxicology testing and implementation of

microchip array-based technologies for genomics. Future

initiatives in mechanism-based toxicology research 

will involve the integration of information from

traditional and gene-expression-based studies to guide

the development of new testing strategies for identification

and study of environmental toxicants. These studies hold

the promise of providing a true mechanistic basis for

hazard identification through the use of short-term

assays that can be practically applied over the broad range

of agents to which humans are exposed. 

The NTP also evaluates the potential for human

exposures to result in adverse effects on reproduction;

development; and the immune, respiratory, and central

nervous systems. The NTP is expanding its effort to

include routine investigations of changes in the

competence of the immune system and functioning of

the nervous system from exposures occurring during

fetal development and early life. End points evaluated in

reproductive studies are being refined to address emerging

knowledge concerning the subtle effects of low doses of

endocrine-active chemicals. 

The NTP continues to expand activities designed 

to place research and testing results from animals into a

more relevant human health perspective. This encompasses

such efforts as human exposure assessment, toxicokinetics,

mechanism-based pharmacokinetic modeling, and the

interpretation of results in molecular epidemiology for

use in human hazard identification (e.g., Report on

Carcinogens and NTP Center for the Evaluation of

Risks to Human Reproduction). The NTP is also 

coordinating an effort to obtain “real world” information

about worker practices, complex occupational exposures,

and potentially related adverse health effects. Such

information is needed to identify areas for research 

and to design better laboratory studies on the health

effects of chemicals, complex mixtures, and exposure

circumstances encountered in the workplace. 

National Toxicology Program 3

Good

Science

for

Good

Decisions

Dr. Kenneth Olden, Director NTP
Role in Shaping Public Health Policy 

Over the past two decades, the NTP has developed an

increasingly interactive relationship with regulatory

agencies. Through this relationship, the NTP plays an

important, although indirect, role in shaping public

health policy. Federal and State government agencies

rely on the science base provided by the NTP to make

credible decisions that protect public health without

unnecessarily increasing the regulatory burden on

industry. The NTP plays a critical role in providing

needed scientific data, interpretations, and guidance

concerning the appropriate uses of these data to

regulatory agencies and other groups involved in

health-related research. The NTP also plays an

important role in fostering interagency collaborations

in research and exposure assessment, providing

regulatory agencies information about alternative

methods for use in toxicity screening, and exploring

new technologies that can be used to strengthen the

knowledge base of how chemicals cause disease. 

Organizational Structure 

and Oversight 

Three agencies form the core of the NTP: the 

National Institute of Environmental Health Sciences of

the National Institutes of Health (NIEHS/NIH), the

National Institute for Occupational Safety and Health 

of the Centers for Disease Control and Prevention

(NIOSH/CDC), and the National Center for Toxicological

Research of the Food and Drug Administration

(NCTR/FDA) (Figure 1). Each voluntarily provides

resources in support of NTP research, testing, centers,

and outreach. The NTP is administratively located at the

NIEHS/NIH. The Director of the NIEHS/NIH, Dr.

Kenneth Olden, serves as Director of the NTP and

reports to the Secretary, DHHS. The National Cancer

Institute of the National Institutes of Health (NCI/NIH)

was a charter agency for the NTP and continues to serve

on the NTP Executive Committee. The NTP Executive

Committee (Figure 1) provides oversight to the NTP for

policy issues. This committee is composed of the heads

of Federal research and regulatory agencies.

External advisory groups provide review and advice 

to the NTP (Figure 1). The NTP Board of Scientific

Counselors (“the Board”), its subcommittees, and the

Advisory Committee on Alternative Toxicological Methods

(ACATM) assure regular scientific and public peer review

and input to the NTP about its activities and priorities.

Members of the Board and ACATM are scientists from

the public and private sectors selected by the Secretary,

DHHS. These bodies each meet once or twice annually,

and all meetings are open to the public. 

The NTP Board of Scientific Counselors provides

primary scientific oversight to the Director regarding the

NTP and the NTP Center for the Evaluation of Risks to

Human Reproduction and evaluates the scientific merit

4 National Toxicology Program

Executive Committee

ATSDR

CPSC

EPA

FDA

NCEH/CDC

NCI/NIH

NIEHS/NIH

NIH

NIOSH/CDC

OSHA

Policy Oversight

NTP Board of Scientific Counselors

Technical Reports Review Subcommittee

Report on Carcinogens Subcommittee

Advisory Committee on

Alternative Toxicological Methods

External Science Oversight

and Peer Review

NCTR/FDA NIEHS/NIH NIOSH/CDC

Director

ATSDR, Agency for Toxic Substances and Disease Registry; CPSC, U.S. Consumer Product Safety Commission; EPA, U.S. Environmental Protection Agency; FDA, Food and Drug Administration;

NCEH/CDC, National Center for Environmental Health of the Centers for Disease Control and Prevention; NCI/NIH, National Cancer Institute of the National Institutes of Health; NCTR/FDA, National

Center for Toxicological Research of the FDA; NIEHS/NIH, National Institute of Environmental Health Sciences of the National Institutes of Health; NIH, National Institutes of Health; NIOSH/CDC,

National Institute for Occupational Safety and Health of the Centers for Disease Control and Prevention; OSHA, Occupational Safety and Health Administration.

Overview

Figure 1 National Toxicology Program
of intramural and collaborative programs of the NTP.

The Technical Reports Review Subcommittee of the

Board provides peer review of NTP long-term toxicology

and carcinogenesis studies and short-term toxicity study

reports. The Report on Carcinogens Subcommittee of

the Board provides external scientific evaluation of

substances nominated for listing in or delisting from the

Report on Carcinogens. 

ACATM specifically advises the NTP on the NTP

Interagency Center for the Evaluation of Alternative

Toxicological Methods and the Interagency Coordinating

Committee on the Validation of Alternative Methods 

on their priorities, opportunities for alternative test

methods, development of more efficient processes 

for determining the scientific validity and acceptability 

of new test methods, and the fostering of partnerships

with stakeholders.

Toxicology and Carcinogenesis

Evaluation Process

The NTP has a broad mandate to provide toxicological

characterizations for chemicals and agents of public health

concern. The NTP continually solicits and reviews

nominations for toxicology studies; specific categories are

given in Table 1. The nomination process is open to all

interested individuals and groups. Information about

nominating a substance for testing by the NTP is available

on the NTP web site (http://ntp-server.niehs.nih.gov) or

by contacting Dr. Scott Masten, Office of Chemical

Nominations and Selection, NIEHS/NTP, P.O. Box

12233, MD B3-10, Research Triangle Park, NC 27709;

ntpnomin@niehs.nih.gov.

Nominations undergo several levels of review before

agents are selected for study and toxicological studies are

designed and implemented (Figure 2). Representatives

from Federal agencies on the Interagency Committee for

Chemical Evaluation and Coordination (ICCEC) and

the NTP Board of Scientific Counselors, an external

scientific advisory body, all participate. In addition, 

the NTP solicits and considers public comment on

nominations throughout the process. At the final step 

National Toxicology Program 5

Nomination Principles 

for NTP Studies

• Chemicals found in the environment not closely associated 

with a single commercial organization

• Biological or physical agents that may not be adequately 

evaluated without Federal involvement

• Commercial chemicals with significant exposure that were first

marketed prior to current testing requirements or those that

generate too little revenue to support further evaluations

• Potential substitutes for existing chemicals or drugs that might

not be developed without Federal involvement

• Substances that occur as mixtures for which evaluations 

cannot be required of industry

• Chemicals or agents that will aid the understanding of chemical

toxicities or an understanding of the use of test systems to 

evaluate potential toxicities

• Chemicals that should be evaluated to improve the scientific

understanding of structure-activity relationships, and thereby help

limit the number of chemicals requiring extensive evaluations

• Emergencies or other events that warrant immediate 

government evaluation of a chemical or agent

Table 1

Federal and

State Agencies

Public

International

Organizations

Labor

Industry

Other

Organizations

Academia

Public Review

NTP Board of Scientific Counselors Review

(Public Meeting)

NTP Executive Committee Review and Selection

NTP designs and initiates studies based on

resources, priorities, and knowledge gaps

ICCEC Review

Chemical

Nominations

Figure 2 NTP Chemical Nomination

and Selection Process
of this formal process, the NTP Executive Committee

reviews and evaluates public comments and the testing

recommendations and makes its own recommendations

to the NTP: to test, to not test at this time, or to defer

testing until additional information is received and

considered. This multi-step process helps ensure that the

NTP’s testing program addresses toxicological concerns

pertinent to all areas of public health and that there is

balance among the types of substances being evaluated. 

The Executive Committee’s recommendation that a

substance be selected for study does not automatically

commit the NTP to its evaluation. The NTP strives to

balance the selection of chemical sources (e.g.,occupational,

environmental, food additives, agricultural chemicals, and

pharmaceuticals), and studies are initiated as time and

resources permit. When selecting chemicals for study, 

the NTP operates under the principle that industry 

will evaluate chemicals or other agents for health and

environmental impacts as mandated by Congress under

legislative authorities. At any time during the selection

process, a chemical or study may be withdrawn if suitable

data become available, higher priority studies are identified,

or a study proves impractical.

Substances may be studied for a variety of health-related

effects, including but not limited to reproductive and

developmental toxicity, genotoxicity, immunotoxicity,

neurotoxicity, metabolism, disposition, and carcinogenicity.

Rodent models are generally used for NTP studies. 

Pre-chronic, short-term studies generally are performed

for two or thirteen weeks. Chronic, long-term bioassays

last up to two years. Each agent studied is assigned 

an NIEHS/NIH study scientist who designs a 

comprehensive testing strategy (design, methods,

hypothesis, etc.). A project review committee evaluates

the testing strategy and proposes a vehicle for execution

(grant, contract, etc.).

The results of short-term rodent toxicology studies

are published as a Toxicology Study Report. Longer-term

studies, generally two-year rodent studies, are published

as NTP Technical Reports and may also be published in

peer-reviewed scientific journals. Both types of studies

undergo peer review prior to publication. The NTP

Technical Reports Review Subcommittee (Figure 1)

evaluates the Technical Reports from carcinogenicity and

toxicity studies in open public meetings. Candidates for

peer review in 2001 are listed in Table 2. 

6 National Toxicology Program

Overview

Candidate Chemicals for Peer

Review in 2001

May 2001 October 2001

Acrylonitrile 2,4-Hexadienal

Citral Riddelliine

Methacrylonitrile Urethane + Ethanol

o-Nitrotoluene Vanadium pentoxide

p-Nitrotoluene

Table 2

8 National Toxicology Program

he NTP conducts research on a broad range of

high priority agents and issues of public health

concern. The following are brief overviews of some

current NTP initiatives.

Safe Drinking Water Program 

It is estimated that more than 200 million Americans

use municipally treated drinking water, so the availability

of safe drinking water is of enormous public health

significance. Although chlorination is considered one 

of the major public health advances of the twentieth

century, chemical disinfection by-products (DBPs) of

the chlorination or other disinfection process may cause

health problems such as cancer. In addition, there are

agents found naturally in water or that are present by

contamination of public water systems that pose a

threat to public health.

To provide scientific data for setting sound water

quality standards, the NTP is collaborating with 

the EPA on a research program to assess potential 

risks from human exposure to DBPs. This program 

includes a systematic, mechanism-based, toxicological 

evaluation of DBPs focusing on reproductive toxicity, 

immunotoxicity, neurotoxicity, and carcinogenicity.

Selection of DBPs for study is based on their presence

in drinking water, occurrence with different disinfection

processes, chemical structures, and representation of the

different DPB families: trihalomethanes, haloacetic

acids, and haloacetonitriles. DBPs under study by the

NTP are listed in Table 3.

Besides DBPs, a complex array of agents occur

naturally (e.g., arsenic, aluminum), as a result of

contamination (e.g., methyl tertiary butyl ether,

pesticides, organotins), or with environmental changes

(e.g., algal blooms resulting in mycotoxins in surface

waters). Several of these have been nominated to the

NTP for evaluation, including aluminum complexes,

organotins, and the two most common blue-green 

algal toxins, microcystin-LR and cylindrospermopsin. 

Phototoxicology

A new phototoxicology research and testing laboratory,

a joint endeavor between the NIEHS/NIH and the

FDA, has been established: the NTP Center for

Phototoxicology. Currently a study is underway to test

whether creams containing alpha-hydroxy acids

(glycolic acid) or beta-hydroxy acid (salicylic acid)

affect the carcinogenicity of ultraviolet (UV)-containing

light on SKH-1 mouse skin.These chemicals are found

in over-the-counter cosmetics as dermatological 

T

IICurrent Directions
chemoexfoliants, and the impact of their continuous

use for risk of solar light (e.g., UV)-induced skin cancer

is not known. Protocols are in development to study

possible phototoxic effects of products derived from the

aloe vera plant.

Endocrine-Disrupting Agents

Endocrine disruptors are naturally occurring or 

man-made substances that may mimic or interfere with

natural hormones in the body. Endocrine disruptors

may turn on, shut off, or modify signals that hormones

carry and thus affect the normal functions of tissues

and organs. The NTP is involved in several efforts that

help to strengthen the science base within this field.

The NIEHS/NIH and the National Center for

Environmental Health of the Centers for Disease

Control and Prevention are collaborating on a pilot

project to quantify approximately 70 chemicals found

in human blood or urine that are considered

endocrine-disrupting agents, including phthalates and

phytoestrogens. The biological samples are collected

under the National Health and Nutrition Examination

Survey, which includes men and women from a range

of age, socioeconomic, and ethnic groups. These data

will provide estimates of human exposures to

endocrine-disrupting agents within the U.S. population

and will help identify agents of public health concern. This

study complements the recently completed scientific peer

review by the NTP Center for the Evaluation of Risks

to Human Reproduction of the potential reproductive

and developmental toxicity of phthalate esters. 

Endocrine-disrupting chemicals are of interest to

the FDA, and through an interagency agreement 

the NIEHS/NIH supports toxicology studies being

conducted at the NCTR/FDA. Chemicals under study

include the phytoestrogen genistein, the pesticides

methoxychlor and vinclozolin, and the plastic additive

nonylphenol. These studies assess effects on reproduction,

development of hormone-sensitive organs, and cancer

end points in rodents over multiple generations.

Behavioral and immunological end points are also

being evaluated.

As required by the 1996 Food Quality Protection Act,

the EPA is in the process of choosing appropriate assays

to screen endocrine-active agents and develop standardized,

validated protocols for those assays. One concern is the

adequacy of current guidelines for assessing the reproductive

and developmental toxicities of chemicals at environmental

levels. On behalf of the EPA, the NTP organized a

scientific peer review to evaluate reported effects and

dose-response relationships for endocrine-disrupting

chemicals studied using dosing paradigms lower than

those normally recommended under the EPA’s standard

toxicity testing guidelines. The panel examined data from

major selected studies (excluding studies on dioxin and

dioxin-like compounds), discerning the presence or

absence of low-dose effects in laboratory animals that

could be relevant for human health assessments. The

panel also evaluated the dose-response curve for

endocrine-active substances in the low-dose region. The

report from this peer-review panel is anticipated in 2001. 

The NTP Interagency Center for the Evaluation of

Alternative Toxicological Methods is involved in preparing

background documents for the EPA on current in vitro

methods for assessing androgenic and estrogenic activities

of chemicals. A peer review of these methods is tentatively

planned for 2002.

National Toxicology Program 9

Water Disinfection By-Products 

under Study

Bromochloroacetic acid Dibromoacetonitrile

Bromodichloroacetic acid Dibromochloroacetic acid

Bromodichloromethane Dichloroacetic acid

Chloramine MX (a furanone)

Chloroform Sodium bromate

Dibromoacetic acid Sodium chlorate

Sodium chlorite

Table 3
Herbal Medicines

Medicinal herbs are among our oldest medicines, and

their increasing use in recent years is evidence of a

public interest in alternatives to conventional medicine.

Approximately one-third of the U.S. population is

believed to use some form of alternative medicine,

including herbal remedies. The use of herbal medicines

and other dietary supplements has increased substantially

since passage of the 1994 Dietary Supplement Health

and Education Act. However, although approximately

1,500 botanicals are sold as dietary supplements or

ethnic traditional medicines, herbal formulations are

not subjected to FDA pre-market toxicity testing to

ensure their safety or efficacy. 

The NTP is planning or conducting research 

(Table 4) on several medicinal herbs and compounds

found in herbs that focus on carcinogenicity, reproductive

toxicity, neurotoxicity, immunotoxicity, and effects 

associated with acute exposures to high doses and chronic

exposures to low doses.

DNA-Based Products

Although DNA-based therapies are being developed for

the treatment of a wide range of human diseases, by

their very nature they pose a risk of interacting with the

host genome or disrupting normal cellular processes in

unexpected and unpredictable ways with potentially

adverse consequences. Examples of DNA-based products

include plasmid DNA encoding one or more antigenic

proteins for vaccines against viral and bacterial pathogens,

triplex forming synthetic oligonucleotides to modulate

gene expression, and viral vectors for gene therapy.

Presently the NTP is collaborating with the FDA to

study the safety of DNA-based products and address

life-long risks presented by their use, potential for

reproductive toxicities, transmission of altered genetic

material to subsequent generations, and the potential

for DNA-based products to cause autoimmune disease

or immune dysfunction. 

Occupational Exposures

The NTP is coordinating an effort between NIEHS/NIH

and NIOSH/CDC to better characterize worker

exposures, educate workers, and identify occupational

health research gaps. Current efforts are addressing

worker exposure to asphalt fumes and 1-bromopropane. 

Asphalt fumes generated during road paving have

been linked to acute irritation of mucous membranes

and skin, but to date no cancer risk has been established.

Using a system designed to produce asphalt fumes

similar to those found in the field, NIOSH/CDC has

developed methods for characterizing these fumes and

for monitoring asphalt fume exposure in inhalation

toxicity studies. Laboratory inhalation studies are

underway to evaluate the effects of exposure to asphalt

fumes in cells and animals.

An industry consortium has petitioned the EPA to

list 1-bromopropane as an alternative for ozone-depleting

10 National Toxicology Program

Herbs and Herbal Ingredients under Study by the NTP

Aloe vera gel Seventh most widely used herb; used as both a dietary supplement and component of cosmetics.

Black walnut extract Found in hair dye formulations and walnut oil stain. Juglone is a major constituent.

Echinacea purpurea extract Most commonly used medicinal herb in the United States.

Ginkgo biloba extract Among the five or six most frequently used medicinal herbs.

Ginseng and ginsenosides Fourth most widely used medicinal herb. Gensenosides are thought to be the active ingredients. 

Ginseng has been associated with various adverse health effects.

Goldenseal Second or third most popular medicinal herb used in this country.

Kava kava Reported to be the fifth most widely used medicinal herb. Has psychoactive properties. 

Sold as a calmative and antidepressant.

Milk thistle extract Used to treat depression and several liver conditions and to increase breast milk production.

Pulegone A toxic compound found in pennyroyal.

Thujone A toxic compound of wormwood.

Table 4

Current Directions
solvents. If this occurs, there is the potential for a vast

increase in the exposure of workers and the public to

this compound. To obtain information on exposures 

to this chemical, NIOSH/CDC is conducting an

industry-wide exposure assessment. The target population

is a variety of industrial sectors: chemical, aerosol, and

adhesive manufacturers; adhesive users; and the metal

degreasing and electronics industries. Study sites will 

be selected on the basis of quantity and manner of 

1-bromopropane use, number of workers exposed, type

of manufacturing process, and representativeness of the

industry. Exposure will be characterized using inhalation,

exhaled breath, and biological measures. Following

completion of this survey and identification of the major

sites for occupational exposure to 1-bromopropane, a

multifaceted health and exposure evaluation of workers

at those sites will take place. 

NIOSH/CDC is planning to conduct a national,

cross-sectional, on-site survey of establishments and

workers. The survey will include all industry sectors

covered by OSHA and the Mine Safety and Health

Administration and will gather nationally representative

data on chemical, physical, and biological agents to

which workers could be exposed, as well as data on

exposure controls and health and safety practices.

Information from this initiative will be used to educate

workers, identify occupational health knowledge gaps,

and help target areas where research is likely to reduce

workplace illness.

Children’s Health

The NTP continues to be a leader in issues related to

children’s health through research and the NTP Center

for the Evaluation of Risks to Human Reproduction. The

NTP has ongoing efforts to evaluate effects of various

agents on developing immune and nervous systems

through laboratory studies of pesticides, water DBPs, and

endocrine-disrupting agents. The Program is expanding

these efforts establishing study protocols where animals

will be dosed routinely in perinatal exposure scenarios to

examine developmental immunotoxicology, neurotoxicology,

and reproductive end points. Toxicokinetic data collected

from dams, fetuses, and neonates will be used to develop

physiologically based pharmacokinetic models of risks to

humans during perinatal developmental periods from

environmental toxicants. 

National Toxicology Program 11
onsidering the large number of chemicals in

commercial use, the NTP continually must set

priorities and develop strategies that best use its available

resources. Implementing new strategies that provide

additional or more accurate information can strengthen

the science base on which regulatory matters are decided.

Development and validation of a wide range of testing

methods are ongoing at the NTP’s core agencies, and 

in addition, university-based researchers are involved

through NIEHS/NIH extramural grants.

Many testing strategies focus on providing more

rapid screening, developing alternative or complementary

in vivo tests to use with rodent bioassays, and decreasing

dependence on two-year rodent bioassays for determining

toxicities. Targeted systems include molecular screening

methods, non-mammalian test species, transgenic animal

models, genetically engineered in vitro cell systems,

microchip array technology, and computer-based

predictive toxicology models. Such strategies can

provide insight into the molecular and biological events

associated with a chemical’s toxic effect and provide

mechanistic information that can be used to assess

human risk. These models can be used to clarify

dose-response relationships, make species comparisons,

and identify sources of inter-individual variability. The

following are brief overviews of some current emerging

NTP strategies.

Toxicogenomics

With the advent of novel molecular technologies, the

NTP is moving into the arena of toxicogenomics, a new

scientific field that examines how the entire genome is

involved in biological responses of organisms exposed to

environmental toxicants. Toxicogenomics applies genetic

knowledge to environmental medicine by studying the

effect of toxicants on gene activity and specific proteins

produced by genes in response to these toxicants. It

combines information from studies of genomic-scale

mRNA profiling (by microarray analysis), cell-wide or

tissue-wide protein profiling (proteomics), genetic

susceptibility, and computational models to illustrate

the roles of gene-environment interactions in disease.

This field could have a revolutionary impact on

environmental health, drug safety, and risk assessment. 

In an effort toward centralizing activities in 

toxicogenomics, the NIEHS/NIH has established the

National Center for Toxicogenomics (NCT). The NCT

will coordinate toxicogenomic research, create a public

database of gene responses to specific toxicants and the

proteins synthesized by those genes, and develop computer

C

12 National Toxicology Program

III Evolving Strategies
software to manage the growing toxicogenomics database.

The NCT will accomplish its goals by supporting

scientists at academic and other research institutions

and in-house research and collaborations with public

agencies, private healthcare, and biotechnology companies.

cDNA microarray technology is a research tool that

will enable the NCT to assess the genetic impact of

toxicants. The NIEHS/NIH has developed a human

cDNA chip, ToxChip, that contains clones of about

2,000 human genes and a human Discovery Chip

containing 12,000 clones. Microarrays are currently in

use at the NIEHS/NIH from common test animals 

and organisms including mice, rats, and yeast. At

NIOSH/CDC a hepatic microarray has been compiled.

The NIEHS/NIH and NIOSH/CDC are evaluating

their arrays against known toxicants and building a

database of expression information in order to determine

the typical genetic changes or “signature” profiles they

produce. Identification of such changes in gene expression

on a genome-wide basis could provide a global perspective

on how an organism responds to a specific stress, drug,

or toxicant. As this technology continues to be used and

improved, it will assist NTP scientists in evaluating and

comparing compounds under study. Such information

could define cellular networks of response genes, identify

target molecules of toxicity, provide future biomarkers

and alternative testing procedures, and identify individuals

who are sensitive to drugs or environmental agents. 

Transgenic Animals

For more than three decades, the conventional rodent

bioassay has been used to identify carcinogens thought

to pose risks to human health. With the advent of

genomic technologies, transgenic models are being

evaluated as alternative or complementary to the rodent

bioassay. Genetically altered or transgenic mouse models

carry activated oncogenes or inactivated tumor suppressor

genes known to be involved in neoplastic processes in

both humans and rodents. This trait may allow them to

respond to carcinogens more quickly than conventional

rodent strains. In addition, the neoplastic effects of

carcinogenic agents can be observed in the transgenic

models within a time frame in which few, if any,

spontaneous tumors would arise. The use of target or

reporter genes also allows for direct molecular and cellular

analysis of a chemical’s effects and can provide additional

mechanistic information about its mode of action.

The NTP is evaluating the usefulness of a number

of transgenic rodent models for studies of carcinogenesis

[p16INK4a, p53def (p53 +/–), Tg.AC (v-Ha-ras), and

Tg.NK (MMTV/c-Neu)] and mutagenesis [Tg.LAC1/

C57Bl6 (Big Blue)]. The p53def and Tg.AC models

appear to have the capacity to identify a selected group of

known human carcinogens. The p53def has demonstrated

preferential identification of genotoxic/mutagenic

carcinogens, and the Tg.AC has responded to both

genotoxic and nongenotoxic carcinogens. As the NTP’s

understanding of the complex signaling pathways

activated or repressed during carcinogenesis progresses,

the Program will be able to select transgenic animal

models that best mimic human tissue processes, providing

a firmer foundation for cross-species extrapolations of

hazards. Efforts are also underway to develop transgenic

cell strains and to evaluate the usefulness of transgenic

fish as alternate models to mice or cultured cells.

Risk Assessment Methodology

Risk assessment involves synthesis of large amounts of

diverse data to decide the plausibility and magnitude of

hazards posed by environmental agents. Physiologically

based pharmacokinetic (PBPK) models are mathematical

models that quantify processes of absorption, distribution,

metabolism, and elimination of an agent resulting from

exposure in animals or humans. The development of

these PBPK models, in combination with physiologically

based pharmacodynamic models, is an iterative process

National Toxicology Program 13
that builds on data related to the biological behavior of

the agent in tissue samples, animal models, or humans

and on the rapidly expanding knowledge of the

physiological, cellular, and molecular events contributing

to various disease processes. They may be used to define

dose-response relationships, clarify these relationships

in the “low-dose” region, make species comparisons,

assess inter-individual variability, and create science-based

models for specific subpopulations (e.g., age, gender,

genetic predisposition, ethnicity). NTP initiatives in

human exposure assessment and new toxicogenomic

technologies should provide human and animal

mechanistic data to develop and improve these models. 

PBPK models have been developed or are under

development at the NIEHS/NIH to assess carcinogenicity

relationships between exposure, target tissue dosimetry,

and tissue response. As such, they can provide a

mechanistic basis for dose-response relationships for

carcinogenicities and developmental and reproductive

toxicities (Table 5). Inclusion of PBPK models in the

NTP Technical Reports is becoming routine, and this

information should aid regulatory agencies to extrapolate

data across species. 

As toxicogenomic data related to gene expression,

protein levels, receptor binding and interaction, and

cellular protein changes become available, biochemical

models (more complex than absorption, distribution,

metabolism, and elimination models) are being

developed. Such mechanistic models have characterized

Ah receptor-dependent transcriptional activation of

dioxin-responsive genes, enzyme induction in 2,3,7,

8-tetrachloro-dibenzo-p-dioxin-treated rats, and the

physiological and biochemical processes involved in renal

accumulation of α2μ-globulin and in the deposition of

α2μ-globulin-binding ligands in male rats.

14 National Toxicology Program

Evolving Strategies

Physiologically Based 

Toxicokinetic Modeling

Chemical Route of Exposure 

Anthraquinone Oral in feed

Butadiene Inhalation

p-p ́-Dichlorodiphenylsulfone Oral in feed

Isoprene Inhalation

Melatonin Endogenous

Mercury (pregnant rat) Inhalation

Methyleugenol Oral by gavage

Naphthalene Inhalation

Polychlorinated biphenyls Multiple routes

(209 congeners)

Primidone Oral in feed

Sodium nitrite Oral in drinking water

2,3,7,8-Tetrachloro-dibenzo- Oral and dermal

p-dioxin 

Table 5

16 National Toxicology Program

IVCenters

NTP Center for the Evaluation of

Risks to Human Reproduction

The NTP Center for the Evaluation of Risks to Human

Reproduction (CERHR) serves as an environmental

health resource to the public and to regulatory and health

agencies for scientifically based, uniform assessments of

the potential for adverse effects on reproduction and

development caused by agents to which humans are

exposed. The Center was established in 1998. Its 

assessments of individual chemicals are carried out

through rigorous evaluations by independent, scientific

panels and are intended to

• interpret for and provide information to the public

about the strength of the scientific evidence that a

given exposure or exposure circumstance poses a

hazard to reproduction or the health and welfare

of children;

• provide regulatory agencies with objective and

scientifically sound assessments of data related to

the reproductive/developmental health effects

associated with exposure to specific chemicals or

classes of chemicals, including descriptions of any

uncertainties associated with these assessments; and 

• identify knowledge gaps to help establish research

and testing priorities.

The NTP Board of Scientific Counselors advises the

Center on its processes, priorities, and direction. The

Center follows a formal, open process for nomination,

selection, and review of chemicals; public input is

encouraged. Chemicals for review are selected based on

the evaluation of several factors, including production

volume, extent of human exposures, public concern

about the chemical hazard, published evidence of

reproductive or developmental toxicities, and 

other information. 

The Center’s first expert panel was formed in the

summer of 1999 to evaluate evidence that seven selected

phthalate esters (butyl benzyl phthalate, di(2-ethylhexyl)

phthalate, di-isodecyl phthalate, di-isononyl phthalate,

di-n-butyl phthalate, di-n-hexyl phthalate, and 

di-n-octyl phthalate) may pose a reproductive and/or

developmental risk for exposed humans. The Expert

Panel completed its review in 2000, and its reports are

available on the Center’s homepage, http://cerhr.niehs.

nih.gov. The NTP is preparing its Center Report and

anticipates its transmittal to Federal and State agencies,

interested stakeholders, and the public in 2001. 

The Center will conduct its next Expert Panel

evaluation for potential reproductive and developmental

toxicity on methanol. Methanol is a commercially
important, high-production volume chemical with

potential for occupational, consumer, and environmental

exposure. It is used in chemical syntheses and as an

industrial solvent. It is also found in a variety of consumer

products such as paints, antifreeze, cleaning solutions, and

adhesives and is a by-product of sewage treatment, 

fermentation, and paper production. Methanol is currently

used in race car fuels and there is potential for expanded

use in the future as a vehicle fuel or fuel additive.

Ethylene glycol, 1-bromopropane, and 2-bromopropane

are being considered for future evaluations.

The Center’s web site has information on various

environmental exposures and their potential to affect

pregnancies and child development, and it has links to

other resources. The Center welcomes the nomination

of chemicals for review or scientists for its expert registry.

Information about the Center and the nomination

process can be obtained from its homepage or by 

contacting Dr. Michael Shelby, Director, CERHR,

NIEHS/NTP, P.O. Box 12233 MD EC-32, Research

Triangle Park, NC 27709; Telephone: (919) 541-3455;

Fax: (919) 316-4511; shelby@niehs.nih.gov.

NTP Center for Phototoxicology

The NTP Center of Phototoxicology (NCP) was created

in 2000 to conduct mechanistic-based research and 

photocarcinogenesis studies on compounds of regulatory

importance to the FDA and to address NTP nominations

that require phototoxicology testing. Research conducted

through this center should have a significant impact on

public health by generating scientific data that address

potential phototoxicities or photocarcinogenicities of the

combination of therapeutics, cosmetics, devices, or food

additives with sunlight.

The Center’s state-of-the-art laboratory is designed

for testing the effects of UV radiation or simulated

solar light-induced toxicity and cancer on drugs, 

chemicals used in cosmetic preparations, and other

agents (e.g., sun block additives, tanning enhancers,

skin colorants, and tattoo inks). The light simulated

closely matches the spectrum of terrestrial solar light

and can emulate the conditions to which humans are

exposed. The facility is also capable of performing

studies using different types of fluorescent tube-generated

light, such as those used in fluorescent lamps and 

suntan-bed lamps. With the public’s increasing 

exposure to UV radiation or sunlight through more

frequent use of tanning booths and leisure time spent

in outdoor activities, research in this area is becoming

increasingly important. 

Chemicals selected for testing are nominated

directly from the FDA and from outside submissions to

the NTP. An FDA committee, the Phototoxicology

Chemical Selection Working Group, prioritizes

nominations and forwards them to the Interagency

Committee for Chemical Evaluation and Coordination

for entry into the NTP nomination and selection process.

The standing Toxicology Study Selection and Review

Committee reviews the experimental design and the

progress of studies. Current research initiatives are

described on page 8 under “Phototoxicology.”

Information and questions can be directed to Dr. Paul

C. Howard, Director, NCP, NCTR/FDA, HFT-110,

3900 NCTR Road, Jefferson, AR 72079;

phoward@nctr.fda.gov.

NTP Interagency Center for the

Evaluation of Alternative

Toxicological Methods

Toxicity testing is absolutely necessary to assess the

hazards and safety of substances in our food, air, and

water, in the workplace and at home. Development,

validation, acceptance, and harmonization of new and

revised toxicological test methods are coordinated in

the Federal government through the Interagency

Coordinating Committee on the Validation of

National Toxicology Program 17
18 National Toxicology Program

Alternative Methods (ICCVAM). ICCVAM was first

established in 1997. The NTP Interagency Center for

the Evaluation of Alternative Toxicological Methods

(NICEATM) was established in 1998 to collaborate with

the ICCVAM to facilitate the development, scientific

review, and validation of novel toxicological methods that

predict human health risks while reducing, refining, and/

or replacing animal tests and to promote communication

with stakeholders.

In December of 2000, Congress enacted the

ICCVAM Authorization Act of 2000 (Public Law 

106-545) and established ICCVAM as a permanent

NIEHS/NIH committee under the NICEATM.

ICCVAM consists of the heads of 15 Federal agencies

or their designees (Table 6). 

The purposes of ICCVAM are to:

• increase the efficiency and effectiveness of Federal

agency test method review;

• eliminate unnecessary duplicative efforts and share

experiences between Federal regulatory agencies;

• optimize utilization of scientific expertise outside

the Federal government;

• ensure that new and revised test methods are

validated to meet the needs of Federal agencies; and

• reduce, refine, or replace the use of animals in

testing, where feasible.

Public Law 106-545 establishes a Scientific Advisory

Committee and requires agencies to determine that 

alternative test methods are valid. Currently the Advisory

Committee on Alternative Toxicological Methods

provides ICCVAM and NICEATM advice on activities

and priorities.

NICEATM and ICCVAM convene workshops to

evaluate the adequacy of existing methods, identify

areas needing alternative methods, and evaluate

proposed validation studies. ICCVAM and NICEATM

recently organized an international workshop to review

and evaluate in vitro methods for assessing acute

systemic toxicity and a peer review meeting on the 

Up-and-Down Procedure (UDP) for assessing acute

oral toxicity. The UDP is a proposed replacement for the

conventional LD50 test. ICCVAM and the Health and

Environmental Sciences Institute of the International

Life Sciences Institute held a training workshop on 

the local lymph node assay (LLNA) in January 2001.

LLNA, a method for assessing allergic contact dermatitis

of chemicals, was the first method to be evaluated under

the ICCVAM peer-review process. The independent

peer-review panel concluded that the LLNA is a valid

substitute for currently accepted guinea pig test methods.

EPA, OSHA, and FDA announced their acceptance 

of the LLNA as an alternative method in October

1999. A future peer review meeting is being planned 

to assess the validation status of several in vitro assays

proposed for use in the EPA’s Endocrine Disruptor

Screening Program. 

Additional information about ICCVAM, NICEATM,

meeting schedules, meeting reports/minutes, and

information on the nomination process of alternative

toxicological methods can be obtained from the

ICCVAM/NICEATM homepage (http://iccvam.

niehs.nih.gov) or by contacting Dr. William S. Stokes,

Director, NICEATM, NIEHS/NTP, P.O. Box 12233,

MD EC-17, Research Triangle Park, NC 27709;

Telephone: (919) 541-2384; Fax: (919) 541-0947;

iccvam@niehs.nih.gov.

Centers

ICCVAM

Agency for Toxic Substances and Disease Registry

Food and Drug Administration

National Institute for Occupational Safety and Health/

Centers for Disease Control and Prevention 

National Institutes of Health

National Cancer Institute/National Institutes of Health 

National Institute of Environmental Health Sciences/National

Institutes of Health 

National Library of Medicine/National Institutes of Health 

Occupational Safety and Health Administration

U.S. Consumer Product Safety Commission

U.S. Department of Agriculture

U.S. Department of Defense

U.S. Department of Energy

U.S. Department of the Interior

U.S. Department of Transportation

U.S. Environmental Protection Agency

Table 6

20 National Toxicology Program

VRoC

Report on Carcinogens

The Report on Carcinogens (RoC) is prepared biennially

in response to Section 301 of the Public Health Service

Act as amended. The RoC contains a list of all substances

(i) that either are known to be human carcinogens or

may reasonably be anticipated to be human carcinogens;

and (ii) to which a significant number of persons residing

in the United States are exposed. The Secretary, DHHS

has delegated responsibility for preparation of the RoC

to the NTP Director. The listing of a substance in 

the RoC is descriptive in nature and represents an 

initial step in hazard identification, which is generally

considered the first step in the analytical process known

as risk assessment. 

The NTP solicits and encourages broad participation

from interested parties in nominating agents, substances,

mixtures, or exposure circumstances for listing in or

delisting from the RoC. Nominations should be 

submitted to Dr. C.W. Jameson, Head, Report on

Carcinogens, NIEHS/NIH, P.O. Box 12233, 

MD EC-14, Research Triangle Park, NC 27709;

Telephone: (919) 541-4096; Fax: (919) 541-0144; 

jameson@niehs.nih.gov. Information about the RoC 

is also available through the NTP homepage 

(http://ntp-server.niehs.nih.gov).

Specific criteria are used to assess whether a nomination

should be listed in the RoC. The review of nominations

for listing in or delisting from the RoC involves a

multi-phased, peer-review process with participation by

representatives from Federal agencies (NIEHS/NIH

Review Group, NTP Executive Committee Interagency

Working Group, and the NTP Executive Committee)

and the NTP Board of Scientific Counselors Report on

Carcinogens Subcommittee (Figure 3). The NTP

Director evaluates all review group recommendations,

public comments, and other information in developing

his recommendation to the Secretary, DHHS. 

The 9th RoC was released May 15, 2000 and is

available from the Environmental Health Information

Service (see page 22 under “Communication and

Public Outreach”). Table 7 lists the nominations under

consideration for the 10th RoC. The preparation and

review process for the RoC extends over approximately

a two-year period; publication of the 10th Edition is

anticipated for completion in 2002. 
National Toxicology Program 21

Summary of the Agents, Substances, Mixtures, or Exposure Circumstances Nominated 

for Possible Listing in or Delisting from the 10th Report on Carcinogens

Reviewed in January 2000 Reviewed in December 2000

Table 7

Nominations from

Federal and State

Agencies, Industry,

Public, Labor,

Academia

Solicit Public

Comment

Secretary,

DHHS

Director,

NTP

NIEHS/NIH Review Group

NTP Executive Committee

Interagency Working Group

Solicit Public Comment

NTP Board of Scientific Counselors

RoC Subcommittee

(External Peer Review, Public Meeting)

Solicit Public Comment

(NCI/NIH, NIEHS/NIH, NCTR/FDA, NCEH/CDC,

NIOSH/CDC, ATSDR, CPSC, EPA, OSHA)

NTP Executive

Committee

(NCI/NIH, NIEHS/NIH, NIH,

NCEH/CDC, FDA, NIOSH/CDC,

ATSDR, CPSC, EPA, OSHA)

Figure 3 Report on Carcinogens Review Process

2-Amino-3-methylimidazo[4,5-f]quinoline 

Beryllium and beryllium compounds

2,2-bis-(Bromomethyl)-1,3-propanediol

2,3-Dibromo-1-propanol

Dyes metabolized to 3,3 ́-dimethoxybenzidine (dimethoxybenzidine dyes as a class)

Dyes metabolized to 3,3 ́-dimethylbenzidine (dimethylbenzidine dyes as a class)

Styrene-7,8-oxide

Vinyl bromide

Vinyl fluoride

Broad-spectrum UV radiation and UVA and

UVB and UVC

Chloramphenicol

Estrogens, Steriodal

Metallic nickel and certain nickel alloys

Methyleugenol

Talc Asbestiform and Non-Asbestiform

Trichloroethylene

Wood dust

ATSDR, Agency for Toxic Substances and Disease Registry; CPSC, U.S. Consumer Product Safety Commission; EPA, U.S. Environmental Protection Agency; FDA, Food and Drug Administration;

NCEH/CDC, National Center for Environmental Health of the Centers for Disease Control and Prevention; NCI/NIH, National Cancer Institute of the National Institutes of Health; NCTR/FDA, National

Center for Toxicological Research of the FDA; NIEHS/NIH, National Institute of Environmental Health Sciences of the National Institutes of Health; NIH, National Institutes of Health; NIOSH/CDC,

National Institute for Occupational Safety and Health of the Centers for Disease Control and Prevention; OSHA, Occupational Safety and Health Administration.
Communication and 

Public Outreach

Open communication with Federal and State agencies,

industry, stakeholders, academia, and the public is crucial

for the success of NTP projects. Partnerships with sister

Federal agencies are increasing, and the NTP continues

to collaborate with the private sector. NTP conferences

and workshops give researchers, regulators, policy makers,

and the public the chance to examine issues together,

exchange information, and reach agreement on future

directions for toxicology and risk assessment. 

The NTP is interested in stakeholder input into its

programs and priorities. Nominations, inquiries, and

comments from the public and other interested parties are

welcome at any time. The NTP Liaison and Scientific

Review Office serves as the focal point for receiving input

to the Program and oversees the distribution of information

about programs, workshops, initiatives, etc. 

NTP testing and research results, program plans, and

other publications are distributed through mailings,

Federal Register announcements, and the NTP homepage,

http://ntp-server.niehs.nih.gov. In addition, individuals

can subscribe free of charge to the NTP listserver by

registering online through the homepage or by sending

e-mail to ntpmail-request@list.niehs.nih.gov with

“subscribe” as the message. The NTP homepage also

offers access to information about the NTP, and links are

available that detail and highlight ongoing and future

initiatives and NTP centers.

The Central Data Management (CDM) Office oversees

distribution (on request) of specific chemical study 

information and NTP documents, including the NTP

Annual Plan, NTP Study Status Reports, pre-peer-review

copies of draft NTP Technical Reports, background

documents for chemicals nominated to the NTP for study,

and minutes from meetings of the NTP Board of

Scientific Counselors and its subcommittees. To request

any of these documents, contact CDM, NIEHS/NIH,

P.O. Box 12233 MD E1-02, 111 T.W. Alexander Drive,

Research Triangle Park, NC 27709; Telephone: (919)

541-3419; Fax: (919) 541-3687; cdm@niehs.nih.gov.

Online, searchable access and printed copies of NTP

publications, including the Report on Carcinogens, NTP

Technical Reports, and NTP Toxicology Reports, are

available through the Environmental Health Information

Service (EHIS). Subscription packages to EHIS include

access to NTP publications as well as Environmental

Health Perspectives (primary issues and supplements), the

Rodent Historical Controls Database, and the Chemical

Health and Safety Database. For additional information,

contact EHIS, NIEHS/NIH, P.O. Box 12233 MD EC-15,

79 T.W. Alexander Drive, Research Triangle Park, NC

27709; Telephone: (800) 315-3010; Fax: (919) 541-0763;

ehis@niehs.nih.gov.

22 National Toxicology Program

VI Outreach
U.S. Department of Health and Human Services

headquartered at the

National Institutes of Health

National Institute of Environmental Health Sciences

P.O. Box 12233

Research Triangle Park, NC 27709

Website: http://ntp-server.niehs.nih.gov/

Dr. Kenneth Olden

Director, NTP and Director, NIEHS/NIH

Dr. Christopher J. Portier

Acting Associate Director, NTP and

Acting Director, Environmental Toxicology

Program, NIEHS/NIH

Contact:

NTP Liaison and Scientific Review Office

NIEHS/NIH

P.O. Box 12233, MD A3-01

111 T.W. Alexander Dr.

Research Triangle Park, NC 27709

Telephone: (919) 541-0530

Fax: (919) 541-0295

E-mail: liaison@starbase.niehs.nih.gov

U.S. Department of 

Health and Human Services

Public Health Service

National Toxicology Program

National Toxicology Program

NTP