technical report rationale and strategy for in vitro toxicity … · 2019-09-30 · ivt-225-ctr in...

In Vitro Toxicity Testing Sub-Group

Technical Report

Rationale and Strategy for In Vitro

Toxicity Testing of Combustible Tobacco Products

September 2019

Authors:

Kristen Jordan (RAI), U.S.A.

Roman Wieczorek (Imperial), Germany

Oliver Moennikes (PMI), Switzerland

Julie Clements (Covance), U.K.

Ian Crooks (BAT), U.K.

Tsuneo Hashizume (JT), Japan

Jacqueline Miller (JTI), Switzerland

Elisabeth Weber (JTI Oekolab), Austria

Kei Yoshino (JT), Japan

Project Leader:

Monica Lee (ALCS), U.S.A.

Table of Contents

1. INTRODUCTION .............................................................................................................. 3

2. RECOMMENDATIONS ................................................................................................... 3

3. REGULATORY BACKGROUND.................................................................................... 4

4. RATIONALES FOR BATTERY TESTING ..................................................................... 5

5. OVERVIEW OF TEST METHODS .................................................................................. 6

5.1 Cytotoxicity Assays ................................................................................................. 6

5.2 Genotoxicity Assays ................................................................................................ 7

5.2.1 Bacterial – Ames Mutagenicity Assay ........................................................ 8

5.2.2 Mammalian – In vitro Micronucleus (MN) Assay ...................................... 9

5.2.3 Mammalian – Mouse Lymphoma Assay (MLA) ........................................ 9

5.2.4 Mammalian – Chromosomal Aberration (CA) Assay ................................. 9

6. TEST MATERIALS ........................................................................................................ 10

6.1 Total Particulate Matter (TPM) ............................................................................. 10

6.2 Gas-Vapor Phase (GVP) ....................................................................................... 11

6.3 Whole Smoke (WS) ............................................................................................... 11

6.4 Other Test Materials .............................................................................................. 12

7. TEST METHOD SUMMARY ......................................................................................... 12

7.1 Neutral Red Uptake (NRU) Cytotoxicity Assay ................................................... 12

7.1.1 Selected References ................................................................................... 12

7.1.2 Points of Interest (Examples) .................................................................... 12

7.2 Ames Bacterial Mutagenicity Assay ..................................................................... 13



7.3 In vitro Micronucleus (MN) Assay ....................................................................... 14



7.4 Mouse Lymphoma Assay (MLA) ......................................................................... 15



7.5 Chromosomal Aberration (CA) Assay .................................................................. 16



8. APPENDIX ...................................................................................................................... 17

8.1 Test Materials ........................................................................................................ 17

9. REFERENCES ................................................................................................................. 18

IVT-225-CTR In Vitro Test Battery – September 2019 3/25

1. INTRODUCTION

In January 2002, the Cooperation Centre for Scientific Research Relative to Tobacco

(CORESTA) Scientific Commission requested the formation of a Task Force concerning the in

vitro Toxicity (IVT) Testing of Tobacco Smoke. The mandate of the Task Force was twofold:

1. To prepare a report covering the rationale and strategy for conducting in vitro toxicity

testing of tobacco smoke.

2. To identify key procedures based upon internationally recognized guidelines, adapted to

accommodate the nature and unique properties of tobacco smoke.

The Task Force achieved the objectives with the issuance of the first guideline report in 2004

(CORESTA, 2004). The IVT Task Force became the IVT Sub-Group (IVT SG) in 2015 and,

based on these guidelines, has performed regular proficiency testing of representative assays.

Since the original recommendations in the 2004 report, the field of in vitro science and related

inhalation toxicology has continued to develop, and many new alternative and supplementary

methods have been introduced (examples in the Applied In Vitro Toxicology, ([Vol 4, Issue 2]

2018 Special Issue, Inhalation Toxicity Testing). However, many novel and mechanistic assays

are still being developed (proof-of-concept) and are not fully standardized or validated via

international regulatory guidelines, nor are harmonized guidelines for testing available at this

time.

At the 2018 CORESTA Sub-Group meeting, considering the time passed and recognizing the

expansion of novel in vitro testing methods, the IVT SG has decided to review the 2004

guideline in order to: 1) re-evaluate the relevance of the initial rationale and strategy of in vitro

testing of combustible tobacco products, 2) identify recent and comparable regulatory testing

guidelines and examples in publications, and 3) provide a pragmatic summary of key features

of each recommended assay. The review effort revealed the continued usage and reference of

the CORESTA in vitro test battery, especially where standardized and validated testing is

required, upholding that the overall strategy and rationale remains valid and relevant.

Sometimes these standardized testing results were supplemented with newer and exploratory in

vitro assays (e.g., air-liquid-interface testing with fresh whole smoke). However, the in vitro

tests recommended in 2004 continue to be used in a comparative product testing, such as to

evaluate the biological impact of changes in ingredients or product designs against reference

tobacco products as part of weight-of-evidence toxicity evaluation.

Consistent with the original 2004 guidelines, the IVT SG provides the recommendations to

enable meaningful and reproducible data from validated in vitro assays, fit for potential

regulatory submission. It is stressed however that interpretation of the biological significance

of the in vitro data can only be done on a case-by-case basis, considering all available chemical

and biological data, exposure/dosimetry assessment, and placing the data in the context of the

overall product risk assessment. Hence, interpretation of results should be attributed to a

combination of statistical significance and biological relevance.

2. RECOMMENDATIONS

In vitro toxicology tests are widely accepted by regulatory agencies around the world for

evaluating consumer products and can be conducted rapidly and relatively inexpensively.

Furthermore, many in vitro assays have been harmonized and are available as Organization for

Economic Co-operation and Development (OECD) Test Guidelines or in validation reports.


For tobacco product testing, they have also been applied to evaluate the effect of tobacco

ingredients or product design on smoke toxicity.

The wide variety of potential toxic responses produced by tobacco smoke requires the use of

an in vitro test “battery”, rather than reliance on any individual assay, addressing mutagenic,

genotoxic and cytotoxic potential. Cytotoxicity assays measure general cellular viability and

growth rates, while mutagenicity and genotoxicity assays evaluate DNA damage and

chromosomal changes.

For studies that may be submitted to regulatory agencies and where in vitro toxicity

testing is deemed appropriate, the CORESTA In Vitro Toxicity Testing Sub-Group (IVT

SG) recommends a test battery composed of the following standardized assays:

1. Neutral Red Uptake (NRU) assay to assess cytotoxicity with an appropriate

mammalian cell line.

2. Ames test for bacterial mutagenicity using Salmonella typhimurium strains.

3. A mammalian cell assay for cytogenetics/mutation: the in vitro micronucleus (MN)

assay, the mouse lymphoma assay (MLA), or the chromosome aberration (CA) assay.

In addition to the above standardized assays, other in vitro assays may provide a scientifically

sound mechanistic understanding of biological activities of tobacco smoke. These

supplementary assays, when properly conducted, can contribute to additional support for

product assessment and regulatory submission.

3. REGULATORY BACKGROUND

In vitro toxicity tests have been performed as part of standard industry practice for conventional

(combustible) tobacco product assessment for many years (Baker et al., 2004; Bombick et al.,

1997; Carmines, 2002; Dempsey et al., 2011; Doolittle et al., 1990; Roemer et al., 2002; Zenzen

et al., 2012; Simms et al., 2019; Stabbert et al., 2019). The in vitro testing framework is based

on scientific standards, using existing toxicological assays recognized by bodies such as the

OECD, World Health Organization (WHO), and the German Institute for Standardization

(Deutsches Institut für Normung [DIN]). These assays also feature among those reviewed by

the Life Sciences Research Office (LSRO) in its evaluation of the feasibility of testing

ingredients added to cigarettes (LSRO, 2004) where the tests are used to verify if an ingredient

or technology does not increase the toxicological properties compared to a tobacco product

without this ingredient or technology (comparative approach).

For tobacco ingredients, several studies have been used to assess mixtures of ingredients applied

to experimental cigarettes (Carmines, 2002; Baker et al., 2004; Renne et al., 2006), while others

have focused on single ingredients (Heck et al., 2002; Lemus et al., 2007; Stavanja et al., 2008;

Coggins et al., 2011a-h; Gaworski et al., 2011; Stabbert et al., 2019). The same approach has

also been used to successfully evaluate the effect of different cigarette design features (Coggins

and Gaworski, 2008; Rickert et al., 2007; Coggins et al., 2013; Oldham et al., 2013; Smith et

al. 2013; Werley et al., 2013). In the European Union (EU), the Directive 2001/37/EC (TPD1,

Article 5, “Reporting of ingredients and emissions”) states that “The (ingredient) list… be

accompanied by the relevant toxicological data regarding the ingredients in burnt or unburnt

form, as appropriate…” (EU Directive, 2001). Although not specifically mandated by the

Directive, tobacco companies submitted in vitro studies as part of the obligation to provide

toxicological data on the burnt form in order to allow Member States to assess the toxicity of


ingredients added to tobacco products. A similar approach was continued particularly for the

reporting of data for priority additive ingredients under Directive 2014/40/EU, (TPD2) (EU

Directive, 2014), which repealed Directive 2001/37/EC, (TPD1) (EU Directive, 2001).

Mandatory use of in vitro testing on tobacco products first occurred in 2005 when the Canadian

health authority (Health Canada) introduced toxicological testing regulations that required three

toxicity tests to be performed annually on each commercialized product. The selected in vitro

tests were as follows:

• Ames bacterial mutagenicity test (Health Canada, 2017a)

• Neutral red uptake (NRU) cytotoxicity test (Health Canada, 2017b)

• In vitro Micronucleus (MN) test (Health Canada, 2017c)

In 2011, the US Food and Drug Administration (FDA) required manufacturers to submit

“toxicological data comparing the toxicity of the new tobacco product to a predicate tobacco

product and (if applicable) to a grandfathered tobacco product” as part of the claim of

“substantial equivalence”: “Comparisons between the new tobacco product and the predicate

tobacco product could be assessed by a battery of studies, including non-clinical studies such

as in vitro and in vivo mutagenicity and clastogenicity studies, general toxicology studies…”

(US Department of Health and Human Services [DHHS], 2011). While the consideration of in

vivo assays for tobacco toxicity testing is beyond the scope of this document, the IVT SG

recommends to implement alternative in vitro methods to animal testing where possible

according to the “3R principle” (National Centre for 3Rs, 2017).

4. RATIONALES FOR BATTERY TESTING

Currently there is no single validated in vitro assay available that can provide comprehensive

information on toxicity or biological activity. For tobacco product testing, the IVT SG

recommends a test “battery”, composed of cytotoxicity, mutagenicity, and genotoxicity assays.

Regulatory agencies worldwide accept the results of in vitro test battery approaches for

evaluating consumer products. Expert advisory groups continue to work with regulatory

agencies to harmonize assay protocols and test battery recommendations (Committee on

Mutagenicity of Chemicals in Food, Consumer Products and the Environment [COM], 2011;

International Conference on Harmonisation [ICH], 2011). The components of a test battery

should be carefully selected, depending on biological activities of interest and the availability

of validated methods. A well-designed test battery will result in an “economical use of human,

animal, and material resources” (ICH, 2011), while providing complementary evaluation of

primary mechanisms underlying measured responses (e.g., mutation, chromosomal damage).

A test battery has clear advantages in the assessment of tobacco products. Cigarette smoke is a

complex mixture, containing more than 7,000 identified chemical compounds (Dube and

Green, 1982; US DHHS, 2010) and can induce a wide variety of biological responses, including

both genetic and cytotoxic responses. In Canada, Health Canada requires cigarette

manufacturers to issue an annual report for each cigarette brand produced and sold. Since 2005,

the requirements also call for in vitro toxicity data that are collected following prescribed

standard guidelines: cytotoxicity (NRU assay [Health Canada, 2017b]), bacterial mutagenicity

(Ames assay [Health Canada, 2017a]) and in vitro MN assay [Health Canada, 2017c]).

The US FDA issued guidance related to added ingredients (FDA, 2009). In the case of design

changes, including changes in the ingredient addition, cigarette manufacturers are required to

demonstrate the ‘‘substantial equivalence’’ of a new product with the predicate product. The

comparisons could be performed using analytical (harmful and potentially harmful chemicals)


and biological studies, including ‘‘…in vitro and in vivo mutagenicity and clastogenicity

studies…” (US DHHS, 2011). However, further details on the specific types of in vitro assays

and guidance regarding whether all types of assays must be performed in each case are not

provided.

In 2014, the DIN issued guidance for toxicological assessment of additives for tobacco

products, also including an in vitro test battery. The following tests were regarded as mandatory:

1) Bacterial mutagenicity; 2) Cytotoxicity in mammalian cells (preferably the NRU assay); and

3) Genotoxicity in mammalian cells (CA assay or in vitro MN assay) (DIN, 2014).

5. OVERVIEW OF TEST METHODS

Two major endpoints are investigated with the in vitro toxicity test battery: cytotoxicity and

genotoxicity.

5.1 Cytotoxicity Assays

Cytotoxicity is regarded as a potential early step in disease processes and the use of in vitro

cytotoxicity assays could minimize the use of animals and support the estimation of starting in

vivo doses for acute toxicity testing of test items (TG129; OECD, 2010). Cytotoxicity tests are

also complementary and performed as part of other in vitro assays, such as the selection of

appropriate treatment doses for genotoxicity assays. The major endpoints evaluated include the

effect of a test agent on cell viability (e.g., a stand-alone NRU cytotoxicity assay) and the effect

of a test agent on cellular growth rates (e.g., population doubling used to select treatment doses

for genotoxicity assays). Decreases in viability and/or growth rates are widely interpreted as

evidence of cytotoxicity.

The value for in vitro cytotoxicity assays, as part of the toxicological assessment of a chemical

or chemical mixtures, has been an active area of consideration. Several large research groups

have investigated the role of cytotoxicity assays in toxicology and have made recommendations

to industrial and government bodies. In the US, the Center for Alternatives to Animal Testing

(CAAT) at Johns Hopkins University has recommended several cytotoxicity tests for the safety

assessment of consumer products, such as cosmetics, detergents, and other personal hygiene

products (Balls et al, 1990). A large multi-national effort entitled Multi-center Evaluation of In

vitro Cytotoxicity (MEIC) has examined a large number of cytotoxicity endpoints in a variety

of cell types using a standardized list of 50 diverse chemicals with varying mechanisms of

action. Overall, the MEIC studies have indicated a good correlation with in vitro cytotoxicity

and acute lethal potency as well as irritant potential (Ekwall et al., 1990; Clothier et al., 1987;

Clemedson and Ekwall, 1999), and efforts are being made to further improve the predictability

(e.g., considering potential confounders such as protein binding and partitioning) (Clemedson

et al., 2003; Romert et al., 1994). Another large research program, Fund for the Replacement

of Animals for Medical Experimentation (FRAME), has identified several cytotoxicity assays

including the NRU test for the toxicological assessment of chemicals and potential

pharmaceuticals (Fautrel et al., 1991; Knox et al., 1986; Rowan et al., 1980). Cytotoxicity

assays are now considered to replace the Draize eye test and the OECD guideline no. 405 for

acute eye irritation/corrosion has been modified for this accommodation (US National

Toxicology Program [US NTP], 2010). With respect to cosmetics, Directive 76/768/EEC,

article 4 has prohibited the marketing of products that have been tested on animals as of January

1998, therefore requiring in vitro tests, including cytotoxicity assays (EU Directive, 2013).

Considerable effort has been made to validate the use of in vitro cytotoxicity assays in lieu of

potential in vivo toxicity assessment (Balls et al, 1990; Frazier, J.M., 1992; FRAME, 1991). In


2000, the Interagency Coordinating Committee on the Validation of Alternative Methods

(ICCVAM) and the US NTP Interagency Center for the Evaluation of Alternative Toxicological

Methods (NICEATM) sponsored a workshop to review the status of in vitro methods for

assessing acute systemic toxicity and to make recommendations for further efforts. This

workshop also produced a “Guidance Document on Using In vitro Data to Estimate In Vivo

Starting Doses for Acute Toxicity”, containing a Standard Operating Procedure for the BALB/c

3T3 Neutral Red Uptake Cytotoxicity Test (ICCVAM, 2006). This was subsequently adapted

as the OECD TG129 (OECD, 2010; US NTP, 2006).

When used to test commodity chemicals and pharmaceuticals, the NRU assay may be used to

assess if the test material is cytotoxic or non-cytotoxic (e.g., viability > 70 % of the

negative/vehicle control; ISO 10993-5, 2009). For tobacco product testing, test materials such

as cigarette smoke (total particulate matter [TPM]; see section 6) have already been

demonstrated as cytotoxic in the NRU assay. Therefore, the main objective for tobacco product

testing is to compare the responses between two (already cytotoxic) test materials. For example,

one could determine if a product change of interest has resulted in meaningful modification in

cytotoxic responses compared with the product without the change. The resulting differences

could be a “positive” effect (increased cytotoxicity), a neutral effect (no change), or lowered

effect (reduced cytotoxicity) of the modified product compared to the background or reference

product. The assessment could be conducted by comparing the concentration reducing 50 % of

viability derived from the dose-response curve (IC50) of the reference and modified products

(for example, Renne et al., 2006; Gaworski et al., 2011; Schaller et al., 2016; Thorne et al.,

2017).

5.2 Genotoxicity Assays

Genotoxicity testing includes the measurement of DNA damage that can be repaired and is

therefore reversible, as well as the detection of stable and irreversible damage that is

transmissible to the next generation when it occurs in germ cells. Other genotoxicity testing

evaluates the perturbation in mechanisms involved in the preservation of genome integrity. For

an adequate assessment of genotoxicity, three major endpoints need to be evaluated (gene

mutation, structural chromosome aberrations, and numerical chromosome aberrations). Each of

these events has been implicated in carcinogenesis and heritable diseases (Corvi and Madia,

2017). Because there is no single validated assay available that can provide comprehensive

information on all three types of genetic damage, a test “battery” is recommended (COM, 2011;

Corvi and Madia, 2017). A paired in vitro genotoxicity test, for example, bacterial Ames test

and mammalian in vitro MN assay (or MLA), improves the specificity of genotoxicity screening

based on the tendency of high false positive in vitro results compared to in vivo rodent

carcinogenicity testing (Kirkland et al., 2005).

Much discussion and many meetings on the topic of genotoxicity assessment have resulted in

a general international consensus on the appropriate assays (as well as assay methodology) to

be included in a test battery for genotoxicity. ICH guidelines call for a test for gene mutation in

bacteria, an in vitro cytogenetics assay or the mouse lymphoma tk assay, and an in vivo test for

chromosomal damage using rodent hematopoietic cells (ICH, 2011; US DHHS, 2012). In 2011,

UKCOM revised their earlier strategy of using three tests for “Stage 1” genotoxic assessment

(Ames assay, in vitro gene mutation in mammalian cells using the mouse lymphoma assay, and

either the in vitro CA or in vitro MN assay) and now recommends only two tests (Ames and in

vitro MN). Their rationale for this reduction in the number of tests is two-fold: 1) the Ames and

in vitro MN assays allow for the efficient identification of mutagenic end-points, and 2) by

reducing the number of mammalian cell tests from that recommended by the UKCOM in 2000

(UKCOM, 2000), the risk of misleading positive results is decreased (UKCOM, 2011).


When used to test compounds such as commodity chemicals and pharmaceuticals, the Ames

and in vitro MN assays may be used to assess if the test materials are genotoxic or non-

genotoxic, which yields a yes/no response. For tobacco product testing, test materials such as

TPM are already positive (genotoxic) in both in vitro assays. Therefore, similar to the above

cytotoxicity testing, the main objective is to perform a comparative assessment, such that one

could determine if a product change of interest has resulted in meaningful modification of the

genotoxicity responses in vitro compared with the product without specific change (for

example, Renne et al., 2006; Gaworski et al., 2011).

5.2.1 Bacterial – Ames Mutagenicity Assay

The bacterial mutagenicity (Ames) assay has been used in research laboratories and by

regulatory agencies for over three decades (Josephy et al., 1997; Claxton et al., 2010). The assay

is highly sensitive for mutagens and carried out with relative technical ease, rapidity, and

economy (Sawyer, 1994). It is also a powerful tool for research and development, and is

generally the first step in a standard in vitro toxicity test battery. The Ames assay is regarded

as the most widely used genetic toxicology test worldwide.

The Ames assay measures the ability of a test substance to produce a mutation in specific strains

of Salmonella typhimurium that have been modified to be sensitive to mutagens. The Ames

assay measures “point mutations”; each tester strain has specific sensitivities to different types

of point mutations. The Ames assay is a reverse mutation assay such that in the presence of a

mutagen, the bacteria revert to their normal ability to synthesize histidine (i.e., wild type) and

therefore grow on an agar plate which contains only minimal nutrients (Maron and Ames,

1983). An increase in revertants is a measure of the mutagenicity of a test substance. Response

in the Ames assay is measured by scoring revertant bacterial colonies on agar plates that cover

a range of dose concentrations. Because bacteria lack many of the metabolic capabilities of

mammals, an exogenous metabolic activation system was developed to enable conversion of

promutagens to mutagens in the Ames assay. This metabolic activation system typically

consists of an induced rat liver homogenate and cofactor mix; the tissue homogenate used most

often is a 9000 x g supernatant (S9) of rat liver (Maron and Ames, 1983; Claxton et al., 1987).

The Ames test is usually conducted under two conditions: with and without the addition of S9

metabolic activation. At least 5 strains of S. typhimurium are recommended for use and those

that are most commonly used are TA98, TA100, TA1537, TA1535, and TA102.

The first reports of Ames assays of cigarette smoke were published in 1974 (Kier et al., 1974)

and 1975 (Hutton and Hackney, 1975). The TPM of mainstream smoke is reported to be

mutagenic in several of the Ames tester strains in the presence of S9 metabolic activation,

particularly in TA98, which is a strain that measures frame shift mutations (Steele et al., 1995).

The mutagenicity of TPM is reported to be influenced by many factors: tobacco type (for

example, burley vs. flue-cured), stalk position, age, sugar and nitrogen content of tobacco leaves

(DeMarini, 1983). Mutagenicity may also be affected by the “draw resistance” of the product

which is related to product use, filters, and other physical characteristics (Sato et al., 1977).

The Ames assay can also be used as a convenient and non-invasive way of evaluating recent

human exposure to mutagens, including tobacco smoke. The vast majority of studies to evaluate

human exposure to mutagens have been conducted on urine specimens (Rynard, 1990; Lupi et

al., 2007). Urinary mutagenicity studies have been used to assess environmental, dietary, and

occupational exposures; for example, methods of food preparation can significantly affect

urinary mutagenicity (Doolittle et al., 1989a). Urinary mutagenicity studies have also been used

to compare cigarettes which burn or primarily heat tobacco (Doolittle et al., 1989b; Smith et al,

1996; Schorp et al., 2012).


5.2.2 Mammalian – In vitro Micronucleus (MN) Assay

Chromosome defects induced by chemical insults are recognized as the basis of a number of

human genetic diseases (Mitelman, 2000). Assays for the detection of chromosome damage in

mammalian cells cultured are recommended in regulatory guidelines as a complement to Ames

tests in a genotoxicity test battery. The in vitro MN test is a genotoxicity test that measures

micronuclei in the cytoplasm of interphase cells. Micronuclei are produced from whole

chromosomes or acentric fragments that are unable to attach to the spindle at mitosis and appear

as micronuclei near the main daughter nucleus and represent aneugenic or clastogenic effects.

As the formation of micronuclei is dependent on cell division, the MN can be measured via the

cytokinesis‐block micronucleus (CBMN) assay in which the frequency of MN in binucleated

(BN) cells is quantified following the addition of Cytochalasin‐B (Cyto‐B) to halt cellular

division. The use of Cyto‐B limits scoring of MN to once‐divided BN cells and avoids the

scoring of spurious MN that may appear from confounding factors such as less than optimal

cell culture conditions and altered cell division kinetics. Non-cytokinesis blocked versions of

the assay are also available and acceptable for use, provided that it can be demonstrated that

there is evidence that the cell population analysed has undergone mitosis. A wide variety of

different cell types have been used from rodent and human sources and they are typically

exposed to the test item for short exposure in the presence and absence of S9 and for an extended

period in the absence of S9.

5.2.3 Mammalian – Mouse Lymphoma Assay (MLA)

The MLA is a mammalian cell assay, and its purpose is to evaluate the potential of test

substances to induce forward mutation at the thymidine kinase (tk) gene. The tk locus is

autosomal and the cell lines are heterozygous (tk+/-), producing the enzyme thymidine kinase.

This enzyme is a salvage enzyme for nucleic acid breakdown products but if a toxic base

analogue (5-trifluorothymidine, TFT) is present in the medium, the enzyme will incorporate the

analogue into the cells. Mutants (tk-/-) are unable to use the toxic analogue whilst non mutated

cells will incorporate the toxic analogue into the cells.

The assay is conducted in the L5178Y TK+/- -3.7.2C mouse lymphoma cell line (Lorge et al.,

2016). The MLA can detect a wide range of mutations from gene mutations to various sizes of

chromosome deletions; these are typically observed as large colonies (normal-growing) and

small (slow-growing) colonies. Molecular analysis has indicated that the large colonies tend to

represent events within the gene (base-pair substitutions and deletions) whereas small colony

mutants often involve large genetic changes frequently visible as chromosome aberrations (Guo

et al. 2016; Schramke et al., 2006). The tk system has a high spontaneous mutant frequency and

because of the high numbers of cells that can be treated and sampled, it could be the most

satisfactory mammalian cell mutation assay from the statistical point of view. The MLA is

recommended as one of the in vitro mammalian assays by UKCOM guidelines (2011) and

WHO/IPCS harmonized scheme (Eastmond, 2009).

5.2.4 Mammalian – Chromosomal Aberration (CA) Assay

The in vitro CA assay identifies the test substances that cause structural chromosomal

aberrations in cultured mammalian cells (Evans, 1976; Ishidate and Sofuni, 1985; Galloway et

al., 1987). Structural aberrations may be of two types, chromosome or chromatid. A

chromosomal aberration occurs when the structure or number of chromosomes is abnormal or

irregular (e.g. missing, extra or irregular portion of chromosomal DNA).

In the CA assay (performed in cells in metaphase), the asymmetrical structural and/or numerical

chromosome aberrations are scored. Tests conducted in vitro generally require the use of an

exogenous source of metabolic activation unless the cells are metabolically competent with


respect to the test substances. An ideal cell line for use in genotoxicity testing is defined by

certain characteristics (Tweats et al., 2007), many of which are exemplified in primary human

peripheral lymphocytes (Scott et al., 1990; Evans, 1984; Freshney, 2006).

The use of human peripheral blood lymphocytes is recommended because the cells are only

used in short-term culture and maintain a stable karyotype. The use of lymphocytes in the CA

assay also often results in fewer false positive outcomes than rodent cell lines; however, other

human cell systems (e.g., HepG2, TK6, and MCL-5 cells, as well as 3D skin models based on

primary human keratinocytes), also show some promise (Kirkland et al., 2007). When using

human lymphocytes in the CA assay, it is important to recognize that human peripheral blood

lymphocytes do not normally divide in vivo, but can be stimulated to do so in vitro by the

addition of a mitogen such as phytohaemagglutinin (PHA) (Nowel, 1960; Evans and

O’Riordan, 1975). The study timings rely on estimating the cell cycle time (average generation

time, AGT) (OECD TG473, 2016c; US EPA, 1998).

6. TEST MATERIALS

The in vitro tests of combustible tobacco products are conducted using various cigarette smoke-

derived test materials, including:

1) cigarette smoke “total particulate matter (TPM)” or “Particulate Phase (PP)”; sometimes

also called “crude smoke condensates” or “cigarette smoke condensates (CSC)”,

2) gas/vapor phase (GVP), and

3) a combination of particulate and vapor phases (TPM+GVP); also referred as “whole

smoke (WS)”.

There is no widely agreed consensus on the definitions of test materials used in tobacco smoke

in vitro testing. Therefore, it is important to report the actual collection methods used and

analytical characterization of the test materials.

Representative cigarette smoke materials tested in in vitro tests are briefly explained below and

schematically shown in Figure 1 (Appendix).

6.1 Total Particulate Matter (TPM)

The conditioning and smoking of cigarettes (including conditioning of glass filter pads, also

referred as Cambridge filter pads) should be carried out as described in the International

Organisation for Standardisation (ISO) (ISO 3308 [2012], ISO 3402 [1999], and ISO 4387

[2017]) unless alternative regimes are requested, such as those specified by Health Canada.

Multiple cigarettes per TPM sample should be used to accommodate any individual product

variation and provide a minimum of 180 mg TPM per 92 mm filter pad. At the same time,

breakthrough of TPM should not occur and the TPM limits (Maximum of 600 mg TPM per 92

mm filter pad) are not to be exceeded (Health Canada, 2017b; ISO 4387, 2000).

When TPM is tested in the selected in vitro assays, concurrently smoked samples should be

used for chemical analysis of nicotine and water content, as well as possibly other constituents,

in order to confirm the acceptable functioning of the smoking machine and for consistency

within smoking samples. In addition, once collected, any excess TPM samples should be stored

frozen (below -70 °C).

Experiments using freshly prepared versus stored TPM samples have been carried out for the

Ames test, in vitro MN assay, MLA, and NRU assay. Crooks et al. (2013) demonstrated that

the tested in vitro activity of the stored TPM does not significantly change in comparison to the


fresh TPM if Cambridge filter pads with TPM or solvent extracts (including dimethyl

sulphoxide [DMSO]) are stored below -70 °C for up to 2 years.

The IVT SG makes the following recommendations regarding TPM sample preparation and

storage:

• Cambridge filter pads should be kept at room temperature for the minimum time possible

after smoking, which ideally is one hour or less. Pads should be extracted immediately or

stored at below -70 °C as soon as possible after smoking.

• If TPM extracts are not used immediately, they should be stored at below -70 °C within

1 hour of preparation.

• Pads and TPM extracts can be stored at below -70 °C for up to 2 years (Crooks et al.,

2013).

• Pads and TPM extracts should not be re-frozen once removed from storage and thawed.

The IVT SG recommends that TPM extracts be aliquoted into individual vials prior to

freezing if multiple assays are to be conducted on the same TPM extract.

• The TPM extract should be analysed for nicotine (and any additional constituents of

interest) to confirm extract concentration and/or stability.

6.2 Gas-Vapor Phase (GVP)

The conditioning and smoking of cigarettes (including conditioning of glass filter pads) should

be carried out the same as for TPM samples. Collection of GVP should be carried out as per

Health Canada method T-502 (Health Canada, 2017b).

The IVT SG recommends the generation of GVP concurrently with TPM, so that the combined

“whole smoke” (TPM+GVP) can be prepared at the same time. However, if conducted as a

separate generation, concurrent smoking or similar methodology should be used and provide

concurrent analytical data on nicotine and water content if TPM is used in in vitro assays.

GVP should be administered to test systems within 1 hour of the completion of sample

generation (Health Canada, 2017b). The IVT SG recommends the following:

• Calcium and magnesium-free Phosphate Buffered Saline (CMF-PBS) is the preferred

solvent and should be added to an impinger or equivalent vessel.

• TPM is filtered out by passing smoke through a glass (Cambridge) filter pad.

• Remaining GVP is trapped via gas “washing” through ice-cold CMF-PBS.

• The filter pads are retained to determine the deposited mass, in order to calculate

equivalent deposited particulate matter.

6.3 Whole Smoke (WS)

Whole Smoke (WS), also known as “TPM and GVP combined (TPM+GVP)”, is commonly

generated as per Health Canada method T-502 (Health Canada, 2017b). WS should be

administered to test systems within 1 hour of the completion of the GVP component of sample

generation. The IVT SG recommends the following:

• TPM concentration and GVP (as equivalent TPM concentration) should be evaluated and

be of equal concentration.

• Equal amounts of TPM and GVP should be mixed together such that the combined

preparation contains equal amounts of particulate matter or equivalent from the TPM and

GVP.


6.4 Other Test Materials

The Health Canada method (2017b) is regarded as the primary method used to collect

combustible cigarette test materials (TPM) for in vitro assays. There are other methods reported,

including collecting whole smoke using bubbler, sometimes called “Cigarette Smoke Extract

(CSE)” (e.g., Kim et al., 2018). In addition recently, investigators have employed methods to

generate “fresh” cigarette smoke and directly expose to cellular systems using collectively

called “Air-Liquid-Interface (ALI) in vitro exposure systems” (reviewed in Thorne and

Adamson, 2013). The ALI exposure systems are gaining acceptance as part of weight-of-

evidence toxicity testing of aerosols, for instance comparing biological activities of combustible

cigarettes and non-combustible products (e.g., e-cigarette or heat-not-burn tobacco products:

Schaller et al., 2016, Thorne et al., 2017). However, at this time there is no consensus or

standardized performance criteria widely accepted by regulatory and research communities on

CSE and fresh smoke in vitro testing. The IVT SG will continuously monitor, and if deemed

appropriate, update this document accordingly.

7. TEST METHOD SUMMARY

7.1 Neutral Red Uptake (NRU) Cytotoxicity Assay

7.1.1 Selected References:

1. OECD TG129 (2010)

2. Health Canada, T-502 (2017b)

3. DIN (2014)

4. Borenfreund and Puerner (1985)

7.1.2 Points of Interest (Examples):

Selected points of interest Examples*

Cell lines BALB/c3T3, A549, CHO, BEAS-2B, HepG2

Cell density for seeding 3,000 – 10,000 cells/well

Medium volume 100 - 200 µL

Pre-culture 20 - 24 hours

Exposure time 24 to 65-70 hours

Concentration of serum - Serum free medium (e.g., Promocell C-21060 + C-39165)*

- Up to 10 % calf serum in medium

Concentration of DMSO 0.45 - 2 %

Concentration of NR dye 25 - 66 mg/L

Duration for NR incorporation 3 hours

Duration for NR extraction 10 – 60 minutes

Positive control Sodium dodecyl sulfate (SDS); nicotine

Endpoints >20-30 % cytotoxicity b Max. dose (ISO 10993-5, 2009)

EC50 (50 % reduction in viability) & dose-response curve fit

*Technical Report: NRU Assay Proficiency Study CORESTA IVT SG, (2015); Wieczorek et al. (2018)


7.2 Ames Bacterial Mutagenicity Assay


1. OECD TG471 (1997)

2. Health Canada, T-501 (2017a)

3. Maron and Ames (1983)



Strain TA98, TA100; TA1537, TA1535, TA102

Cell volume 109 cells/plate

Metabolic activation Absence and presence of S9

Pre-incubation period (if direct plate incorporation is not used)

20 - 60 minutes

Solvent control DMSO; ethanol; PBS

Maximum solvent control Up to 4 % for DMSO (HC T-501, 2004a); 10 % for PBS

Maximun concentration of TPM to be tested

Up to 5,000 µg/plate

Replicates/concentration Minimum of 3

Positive control (+S9)

One of: 9,10-Dimethylanthracene (CAS No. 781-43-1); 7,12-Dimethylbenzanthracene (CAS No, 57-97-6); Congo Red (CAS No. 573-58-0); Benzo(a)pyrene (CAS No. 50-32-8); Cyclophosphamide (CAS No. 50-18-0, 6055-19-2); 2-Aminoanthracene (CAS No. 613-13-8)

Positive control (-S9)

TA1535 and TA100: Sodium azide (CAS No. 26628-22-8)

TA98: 2-Nitrofluorene (CAS No. 607-57-8)

TA1537: 9-Aminoacridine (CAS No. 90-45-9) or ICR191 (CAS No. 17070-45-0)

TA102: Cumene hydroperoxide (CAS No. 80-15-9) or Mitomycin C (CAS No. 50-07-7)

Criteria for positive response

The positive controls induce a statistically significant increase in the number of revertants relative to the vehicle control; and the strain-specific fold-increase is acceptable. It is recommended that laboratories maintain the historical control ranges for each positive control in that strain to demonstrate proficiency.

A concentration-related increase over the range tested and/or a reproducible increase compared to the vehicle control at one or more concentrations tested in at least one strain in the presence and absence of S9. Statistical methods (e.g. Dunnett’s test) may be used to aid evaluating the test results; however, statistical significance should not be the only determining factor for a positive response.

*Technical Report: Ames Assay Proficiency Study CORESTA IVT SG (2016a); Roemer et al. (2002)


7.3 In vitro Micronucleus (MN) Assay


1. OECD TG487 (2016a)

2. Health Canada, T-503 (2017c)

3. Fenech (2000)



Cell line CHO, V79, CHL, L5178Y, TK6, blood lymphocytes

Cell density Cultures that are sub-confluent should be used

Exposure time/condition Short term (3-6 hours) ±S9, long term (20-27 hours) -S9;

With or without cytokinesis blocker (Cyto-B) (Optional)

Concentration of DMSO Up to 1 % of the culture volume (according to historical control)

Cytotoxicity Not to exceed 60 %

Number of cultures per concentration

Minimum of 2

MN staining Acridine orange, Giesma or other DNA-specific dyes or DNA probes

Number of cells to be scored Minimum of 2,000 per concentration

Number of concentrations (for MN reading)

Minimum of 3

Positive controls

Methyl methanesulphonate (CAS No. 66-27-3); Mitomycin C (CAS No. 50-07-7); 4-Nitroquinoline-N-oxide (CAS No. 56-57-5); Cytosine arabinoside (CAS No. 147-94-4); Benzo(a)pyrene (CAS No. 50-32-8); Cyclophosphamide (CAS No. 50-18-0); Colchicine (CAS No. 64-86-8); Vinblastine (CAS No. 143-67-9)

Criteria for positive response

Positive control responses are compatible with the laboratory historical positive control database and produce a statistically significant increase compared to the concurrent negative control.

At least one of the test concentrations shows a statistically significant increase compared to the concurrent negative control; the increase is dose-related in at least one experimental condition when evaluated with an appropriate trend test; any of the results are outside the distribution of the historical negative control data (e.g., Poisson based 95 % control limits; Sobol et al. (2012)).

*Technical Report: In vitro MN Assay Proficiency Study 2014 CORESTA IVT SG (2016b)


7.4 Mouse Lymphoma Assay (MLA)


1. OECD TG490 (2016b)

2. Moore et al. (2007)



Cell line L5178Y tk +/- (3.7.2C)

Culture medium RPMI 1640 / Horse serum (heat-inactivated)

Culture preparation At least 1 x 107 cells (3 hours) or 4 x 106 (24 hours) for treatment

Metabolic activation Co-factor supplemented S9 fraction

Final concentration 1-2 % in culture medium

Treatment period 3 and 24 hours -S9, 3 hours +S9

Concentration of solvent 1 % (v/v) organic, 10 % (v/v) aqueous

Concentration selection Minimum 4 concentrations

Expression period 48 hours

Selective agent Triflurothymidine (TFT)

Incubation period 10 – 14 days

Positive control MMS, 4-NQO (-S9); B[a]P, CP, DMBA (+S9)

Criteria for positive control Absolute increase in mutant frequency of at least 300 x 10-6 (minimum 40 % small colony formation) or an increase in small colony mutant frequency of at least 150 x 10-6 compared to the vehicle control.

Mutagenic response Mutant frequency of test concentration exceeds the vehicle control plus the global evaluation factor (GEF) (126 x 10-6).

*Technical Report: in vitro MLA Assay Proficiency Study (Pending) CORESTA IVT SG (2019)


7.5 Chromosomal Aberration (CA) Assay


1. OECD TG473 (2016c)

2. EC Commission Directive 2000/32/EC (2000). Annex 4A B.10


Selected points Examples

Cell line Chinese Hamster Ovary (CHO), Chinese Hamster Lung (V79), Chinese Hamster Lung (CHL)/IU, TK6 or primary cell cultures, including human or other mammalian peripheral blood lymphocytes

Medium and culture conditions

Appropriate culture medium and incubation conditions dependent on cell line

Metabolic activation Co-factor supplemented S9 fraction. Typically used at concentrations ranging from 1 to 2 % (v/v) but may be increased to 10 % (v/v) in the final test medium

Treatment period A short-term treatment (3-6 hours) with and without metabolic activation and long-term treatment without metabolic activation

Sampling time 1.5 normal cell cycle lengths after the beginning of treatment

Concentration of solvent 1 % (v/v) organic, 10 % (v/v) aqueous

Concentration selection At least 3 concentrations. If the maximum concentration is based on cytotoxicity, the highest concentration should aim to achieve 55 ± 5 % cytotoxicity using the recommended cytotoxicity parameters

Number of cells scored At least 300 well-spread metaphases per concentration and control

Positive controls Methyl methanesulphonate or 4-Nitroquinoline-N-oxide (-S9), Benzo(a)pyrene or Cyclophosphamide (+S9), Mitomycin C (-S9)

Criteria for positive control

Concurrent positive controls should induce responses that are comparable with those generated in the historical positive control data base and produce a statistically significant increase compared with the concurrent negative control.

Clastogenic response

If in any of the experimental conditions:

a) at least one of the test concentrations exhibits a statistically significant increase compared with the concurrent negative control, b) the increase is dose-related when evaluated with an appropriate trend test, c) any of the results are outside the distribution of the historical negative control data (e.g., Poisson based 95 % control limits).


8. APPENDIX

8.1 Test Materials

There is no widely agreed consensus and only limited regulatory guidelines provide the

definitions of test materials used in tobacco smoke testing (ISO 4387, 2000; Health Canada,

2017b). Often the terms are used interchangeably or informally defined by investigators; see

for example, the terms TPM and CSC commonly used to represent the same particulate test

materials tested in in vitro testing. It is important to report the actual collection methods used

and analytical characterization of the test materials.

• TPM (Total Particulate Matter): The particulate matter of mainstream cigarette smoke

captured in a filter, and then extracted by an organic solvent. The term is commonly used

interchangeably with Cigarette Smoke Condensate (CSC): smoke samples condensed in

glass beads on dry ice (Johnson et al., 2009).

• GVP (Gas Vapor Phase): Gas and vapor phase of cigarette smoke; composed of

permanent gases and volatilized compounds.

• WS (Whole Smoke): The combination of TPM and GVP (term used interchangeably with

“TPM+GVP”).

Figure 1. Representative Test Materials


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