Chemical Category Formation: Toxicology and REACH Dr Steven Enoch Liverpool John Moores University 14 th May 2009.

Chemical Category Formation: Toxicology and REACH

Dr Steven Enoch

Liverpool John Moores University

14th May 2009

Is Regulatory Toxicology Important?

• Number of stories about the toxicity of chemicals

Is Regulatory Toxicology Important?

• Number of stories about the toxicity of chemicals

• Many chemicals have little or no toxicological data

• Concerns about the potential toxicity of chemicals

• New REACH legislation regarding chemical safety

• Applies to excipients, intermediates etc

• Cosmetics directive prohibits animal testing

REACH and Intelligent Testing Strategies

Risk Assessment

In vivo

In silico In chemico

In vitro

In-silico Category Formation

• Structural

• Mechanistic

• Toxicological

• Qualitative and quantitative predictions

OHOH

ON

OH

OH

OH

OH

In-silico Category Formation

• Structural

• Mechanistic

• Toxicological

• Qualitative and quantitative predictions

OHOH

ON

OH

OH

OH

OH

Mechanistic Category Formation – Skin Sensitisation

• Six key chemical reactions have been defined for protein reactivity1

• All known skin sensitising chemicals can be assigned to one of these mechanisms

• SMARTS based rules have been developed2

1Aptula AO and Roberts DW (2006) Chem Res Toxicol 19; 1097-11052Enoch SJ et al (2008) SAR QSAR Environ Sci 19; 555-578

Electrophilic Reaction Chemistry

X XNu

ProteinProteinNu

-carbon

-carbon

X = electron withdrawing substituent e.g. CO, CHO, NO2, CO2R.

Mechanism for Michael Addition

O

NO2

O

O

O O

O

O

O

N

O

Br

O

OMe

I

O

Mechanistic Category Formation

• Qualitative read-across using only mechanistic assignment

• Quantitative read-across using the electrophilicity index () to model protein reactivity within a category3

• Electrophilic index calculated from HOMO and LUMO using DFT

3Enoch SJ et al (2008) Chem Res Toxicol 21; 513-520

Read-Across within a Mechanistic Category

Cl

Cl O

O N

O O

O

O

Increasing electrophilicity ()

Increasing skin sensitising potential (pEC3)

pEC3 = NC, = 1.10 pEC3 = 0.55, = 1.49 pEC3 = 1.82, = 1.55

pEC3 = 1.25, = 1.61 pEC3 = 1.64, = 2.10 pEC3 = 4.04, = 3.90

Quantitative Electrophilicity () Ranking

O Chemical A:

= 1.61, EC3 = 5.5, pEC3 = 1.25

O

Chemical B:

= 1.80, EC3 = 7.5, pEC3 = 1.30

O

O

Chemical X:

= 1.73 Pred. pEC3 = 1.29 (1.31)

Pred. EC3 = 9.87 (9.30)

Quantitative Read-Across Predictions

Toxicological Category Formation - Developmental Toxicity

• Mechanistic read-across requires a priori mechanistic knowledge

• What about category formation when we don’t know about the mechanism of action?

• Can we use chemical similarity to form categories?4

Read-Across within a Toxicological Category

4Enoch SJ et al (2009) QSAR Comb Sci in-press

OO

OO

ethynodiol diacetate

Read-across prediction (atom environment similarity): D / XActual classification: D

Qualitative Read-AcrossOH

OH

ethynodiol (D)

OH

lynestrenol (D)

OH

O

norethindrone (X)

Read-across prediction (fingerprint similarity): BActual classification: B

Qualitative Read-Across

ON

O

N S

O

O

NO

O O

S N

N

cefotaxime

ON

O

N S

NO

O O

S N

N

ceftizoxime (B)

ON

O

N S

O N

O

NO

O O

Ocefuroxime (B)

O

ON

SN

O

S

N

OO

Ocephapirin (B)

Regulatory QSAR Tools

4http://www.oecd.org/document/23/0,3343,en_2649_34377_33957015_1_1_1_1,00.html5http://ecb.jrc.ec.europa.eu/qsar/qsar-tools/

• OECD QSAR Application Toolbox4

• Chemical category formation

• Read-across and trend analysis

• Regulatory reporting for ECHA

• Toxmatch and Toxtree5

• Similarity based category formation

• Rule based category formation

Conclusions

• REACH envisages intelligent testing of chemicals

• In silico developed chemical categories play a central role

• Qualitative and quantitative predictions of toxicity used to fill data gaps

• In silico methods must be transparent and simple in order for regulatory acceptance from EChA

The Future – Intelligent Testing Strategies

Risk Assessment

In vivo

In silico In chemico

In vitro

?

Acknowledgements

• The funding of the European Chemicals Agency (EChA) Service Contract No. ECHA/2008/20/ECA.203 is gratefully acknowledged