UNITED STATES ENVIRONMENTAL PROTECTION AGENCY 

WASHINGTON, D.C. 20460 

OFFICE OF CHEMICAL SAFETY 

AND POLLUTION PREVENTION 

MEMORANDUM 

DATE: March 16, 2016 

SUBJECT: Difen acoum. Revised Human Health Assessment Scoping Document in Support 

of Registration Review. 

PC Code: I 1990 l 

Decision No.: ..f/>070 

Petition No.: NA 

DP Barcode: 0 43229 1 

Registration No.: NA 

Regulatory Action: Registration Review 

Risk Assessment Type: Single ChemicaVe Case No.: 7630 

TXRNo.: NA CAS No.: 56073-07-5 

MRID No.: NA 40 CFR: NA 

FROM: 

THROUGH: 

T O: 

Margarita C. ollantes, M.S., Biologist ~?~i/t-· !°d'fc»iL!:z..-.-

Jaim

Risk 

e 

Assessme

D"Agostino

nt Branch 

, Ph.D., 

II 

Toxicologist 

(RAB2) 

~ 

Health Effects Division (7509P) 

Christina Swartz, Branch Chief c:S:l~--

Jes

Ris

s 

k 

Rowland, 

Assessment 

Deputy 

Branch 

Division 

II (RAB2) 

Director <:]---

\ 

::;;. ~ 

~ 

Jeff Dawson, Acting Deputy Division Director 

Health Effects Division (7509P) 

Office of Pesticide Programs 

Ricardo Jones, Chemical Review Manager 

Risk Management and Implementation Branch I (RMIB l) 

Pesticide Reevaluation Division (PRO) (7508P) 

As part of Registration Review, the Pesticide Reevaluation Division (PRO) has requested that the 

Health Effects Division (HED) provide a human health risk assessment scoping document for the 

rodenticide agent difenacoum to support Registration Review. This document was revised to 

include the correct date of the Hazard Assessment and Science Policy Council (HASPOC) 

decision memo as well as the current methodology used to determine the dermal absorption for 

cl ifenacoum. The most recent risk assessment for difenacoum was conducted in July, 20, 2007 

(K. Schumacher, 0 330185). 

Page I of24 
Executive Summary 

HED has prepared a scoping document to support Registration Review for di fenacoum (3-(3-

biphenyl-4-yl-1,2,3,4-tetrahydro- 1-naphthy l)-4-hydroxycoumarin). Difenacoum is a second 

generation anticoagulant rodenticide. Difenacoum is registered to control house mice, a variety 

of rats as well as subterranean control of rats and voles in burrows inside and within 50 to 100 

feet of agricultural buildings, man-made agricultural structures, residential homes, processing 

facilities, industrial and commercial buildings, trash receptacles, public buildings, transport 

vehicles (ships, trains and aircraft), docks, ports and terminal buildings. There are currently six 

difenacoum end-use products comprised of 0.005 % active ingredient (ai) and formulated as 

pellets, blocks, and ready-to-use place packs. Second-generation anti-coagulant rodenticides, 

consumer-use products (i.e., products '.S I lb) containing difethialone are prohibited. Professional/ 

commercial uses of second-generation anticoagulants, require bait stations for all outdoors and 

above-ground placements and/or indoor placements where children, wildlife or domestic animals 

may be exposed. Furthermore, commercial labels prohibit broadcast application of pellet baits. 

Products that are intended primarily for use in and around agricultural structures are packaged in 

individual containers holding no less than 8 pounds (lbs) of bait. Products intended for 

commercial/professional use are packaged in individual containers holding no less than 16 

pounds (lbs) of bait. 

Difenacoum is a racemic mixture; both the cis- and trans- isomers are pesticidally active. 

To evaluate the scope of work necessary to support Registration Review, HED has considered 

recent risk assessments for difenacoum, updates to its toxicity, exposure, and usage databases, 

and the latest Agency science policy and risk assessment methodologies. 

The toxicology database for difenacoum is considered adequate for risk assessment except for the 

lack of an inhalation and a dermal study (0057384, U. Habiba, 3/1 /20 16) which are required at this 

time. An alternative to conducting an inhalation study would be to add additional PPE (i.e., use 

of respirators) to registered labels which would address risks of concern resulting from inhalation 

exposures to a point where the margins of exposure (MO Es) based on the Hazard Assessment and 

Science Policy Council (HASPOC) screening criteria would no longer be of concern. Based on 

the low MOEs calculated for dermal exposure, label changes are not an option instead of 

conducting a dermal study. HED also determined that a 2-generation reproduction study, an acute 

neurotoxicity battery, a subchronic neurotoxicity battery, and an immunotoxicity study are not 

required for difenacoum (0057384, U. Habiba, 3/ 1/2016). During Registration Review, the 

Agency may update PODs and toxicological endpoints based on updated policies for endpoint 

selection. 

Difenacoum inhibits the regeneration of vitamin K, which decreases the synthesis of blood 

coagulation factors and can lead to hemon-haging and death. Evidence of these anticoagulant 

effects, consisting of clinical signs, effects on clotting time, hemorrhaging, and subsequent death 

were observed in all species tested in the available mammalian studies. Since difenacoum affects 

the synthesis of new clotting factors, rather than directly affecting the circulating factors, a latency 

period typically exists between the time of exposure and the time of effect that is dependent upon 

the half-lives of the circulating clotting factors. No other types of adverse effects were observed 

in the database. including neurotoxicity. immunotoxicity, or developmental effects. The acute 

Page 2 of24 
Page 3 of 24

toxicity of difenacoum is severe via the oral, dermal, and inhalation routes of exposure (Category 

I). Difenacoum is not an eye or dermal irritant, nor is it a dermal sensitizer.

In the previous 2007 assessment dietary (food and drinking water) exposures were not quantified 

because of the limited use pattern (strictly non-food rodenticide use with no dietary exposure). A 

quantitative assessment was performed for commercial and residential exposure only. Therefore 

an aggregate risk assessment was not conducted. 

A short term (ST) dermal and inhalation occupational and commercial handler exposure 

assessment will be required during Registration Review based on the potential exposure to 

handlers (loaders, applicator, etc.) during the use of difenacoum. Applicator/handler exposure to 

products formulated as blocks, and ready-to-use placement packs is considered to be negligible. 

However, applicator/handler assessments will be conducted for the loose pellet formulation only. 

Occupational post-application exposure is not expected due to the expectation of no significant 

surface residue from solid formulation applications nor worker contact with surfaces containing 

residues in areas treated with difenacoum products. Consumer use products may not contain 

second generation anticoagulants. Therefore, residential applicator/handler exposure will not be 

assessed. Residential post-application exposure is not anticipated as products must be placed in

tamper resistant packaging if used above ground or where children, may be exposed. There have 

been recent changes in occupational and commercial assessment assumptions since the previous 

assessments that will need to be incorporated during Registration Review along with any 

changes to reevaluate the points of departure, toxicity endpoints, and uncertainty factors used for 

risk assessment purposes.

Exposure from spray drift and volatilization is not anticipated for the current difenacoum use 

pattern.

Introduction

Difenacoum (3-(3-biphenyl-4-yl-1,2,3,4-tetrahydro-1-naphthyl)-4-hydroxycoumarin) is a second 

generation rodent control agent registered for the control of house mice, a variety of rats as well 

subterranean burrows control of voles in and around agricultural , man-made agricultural 

structures, residential homes, processing facilities, industrial and commercial buildings, trash 

receptacles, public buildings, transport vehicles (ships, trains and aircraft), docks, ports and 

terminal buildings. It is a warfarin-like compound that acts as an anticoagulant and is formulated 

as paraffin pellets, bait ready-to-use place packs, and paraffin blocks. All products contain 0.005 

percent active ingredient. The most recent human health risk assessment was finalized in July

2007 (K. Schumacher; D330185).

In 2008, the Agency issued a risk mitigation decision for ten rodenticides, of which difenacoum

was included. The mitigation decision set restrictions in order to minimize exposure to children, 

wildlife and domestic animals. The risk mitigation decision does not apply to rodenticide field 

(non-commensal rodents) uses or tracking powder products. However, it should be noted that 

difenacoum has no tracking powder end use products. A brief summary is below. The details of 

the RMD can be found in docket number EPA-HQ-OPP-2006-0955-0764 at 

www.regulations.gov. 
Page 4 of 24

Consumer-use products (≤ 1 lb.)

 Second-generation anticoagulants are prohibited as well as loose treated wholegrain/pellets/meal as a form for indoors and above ground outdoor uses.

Professional-use products

 Must be sold in quantities of ≥ 4 lbs. (first-generation anticoagulants/non-anticoagulants), 

≥ 8 lbs. (second-generation anticoagulants; agricultural building areas) and ≥ 16 lbs. 

(second generation anticoagulants; above ground outdoors) 

 May not be offered for sale in consumer stores (hardware, grocery, etc.).

 Must also be placed within 100 feet of a structure (fence and perimeter baiting beyond 

100’ of a man-made structure are prohibited) when used outdoors and above ground, see 

50 foot Document Clarification Letter to Bell (03.20.12) (signed) in docket number EPAHQ-OPP-2006-0955-0820 found at www.regulations.gov. 

 For uses of first-generation anticoagulants or non-anticoagulants, product labels require 

bait stations if children, wildlife or domestic animals may be exposed. 

 For uses of second-generation anticoagulants, labels require bait stations for all outdoors 

and above-ground placements and/or indoor placements where children, wildlife or 

domestic animals may be exposed. 

The majority of difenacoum products are applied using tamper resistant bait stations, which are 

required for all rodenticides where children and pets may be present per 2008 anticoagulant 

rodenticide mitigation. However, tamper resistant bait stations are not required for belowground burrows. If below-ground application of bait is performed properly according to product 

labels, no bait should be available above ground for exposure to children or non-target animals. 

See use profile table in the appendix (attachment 3) for more information on application rate and

application methods of difenacoum products.

Hazard Identification/Toxicology

Difenacoum is a vitamin K antagonist. Vitamin K is essential for the blood to clot, which 

prevents the loss of significant amounts of blood resulting from internal bleeding that occurs 

under normal circumstances, such as with breached blood vessels, or as a result of injury.

Difenacoum and the other anticoagulant rodenticides inhibit vitamin K epoxide reductase,

blocking the regeneration of vitamin K to its active form. Because the amount of vitamin K in 

the body is finite, blocking the regeneration of vitamin K leads to depletion of the clotting 

factors, which causes the coagulation process to become progressively less effective and can lead 

to massive internal hemorrhaging and, ultimately, death.

The toxicology database for difenacoum includes: the acute six–pack of studies, a non-guideline

6-week oral study in dogs, a developmental study in both rats and rabbits, a mutagenicity battery 

of studies, and a metabolism study in rats. Based on a weight of evidence approach considering 

all the available hazard and exposure information, HED determined that the reproduction, acute 

and subchronic neurotoxicity, and immunotoxicity studies are not required at this time (0057384, 

U. Habiba, 3/1/2016). Using the same approach, HED determined that a subchronic dermal 
Page 5 of 24

study and a subchronic inhalation study are required at this time for difenacoum (0057384, U. 

Habiba, 3/1/2016). An alternative to conducting an inhalation study would be to add additional 

PPE (i.e., use of respirators) to registered labels which would address risks of concern resulting 

from inhalation exposures to a point where the margins of exposure (MOEs) based on the 

HASPOC screening criteria would no longer be of concern. Based on the low MOEs calculated 

for dermal exposure, label changes are not an option instead of conducting a dermal study.

In the previous risk assessment, HED recommended that an in vivo dermal absorption study in rats 

be required as a condition of registration. The study has not been submitted; however, HED has 

determined that the study is not required at this time because the dermal absorption can be 

determined using a weight of the evidence approach considering the currently available data. The 

physical chemical properties of difenacoum (molecular weight of 444.5 g/mol and log Kow of 6.1)

support a low dermal absorption. In addition, the European Food and Safety Authority (EFSA), as 

part of their evaluation of difenacoum in 2008, used a dermal absorption factor of 3% based on an 

in vitro study conducted with human skin. Furthermore, using the National Institute for 

Occupational Health and Safety (NIOSH) Finite Dose Skin Permeation Calculator 

(http://www.cdc.gov/niosh/topics/skin/finiteSkinPermCalc.html) and the physical chemical 

properties of difenacoum (see appendix 7 for a full list), a predicted dermal absorption factor of 

0.5% was obtained over an 8 hour period with a dermal load of 0.52 μg/cm2. The dermal load of 

0.52 μg/cm2 was derived by taking the expected daily exposure (0.572 mg) and dividing by the 

entire surface area of the hands (~1100 cm2). The Finite Dose Skin Permeation Calculator was 

developed through support of a Center of Disease Control/National Institute for Occupational 

Safety and Health (CDC/NIOSH) grant, and provides an estimation of fluxes, skin concentrations, 

and amounts absorbed from any size dose applied to partially or fully hydrated skin, using the 

physicochemical properties of the test compound and defined exposure parameters (Kasting, G.B. 

2006; Wang, T.F. 2007; Kasting, G.B. 2008; Miller, M.A. 2010). Currently, OPP does not rely on 

(Q)SAR modeling alone to derive a Dermal Absorption Factor (DAF) for risk 

assessment. However, estimates from the Finite Dose Skin Permeation Calculator may be used 

with read across data to support a weight of the evidence evaluation. Based on a weight of the 

evidence approach, the dermal absorption of difenacoum is likely less than 4%; however, HED 

chose a dermal absorption value of 4% as a conservative estimate based on the available data.

Difenacoum inhibits the regeneration of vitamin K, which decreases the synthesis of blood 

coagulation factors and can lead to hemorrhaging and death. Evidence of these anticoagulant 

effects, consisting of clinical signs (pallor, lethargy, decreased activity, subdued appearance, and 

piloerection, effects on clotting time (increased prothrombin and Kaolin-cephalin times), 

hemorrhaging, and subsequent death were observed in all species tested (dogs, rats, mice, and 

rabbits) in the available mammalian studies. Since difenacoum affects the synthesis of new 

clotting factors, rather than directly affecting the circulating factors, a latency period typically 

exists between the time of exposure and the time of effect that is dependent upon the half-lives of 

the circulating clotting factors. For example, death and clinical signs were not observed until 5-8 

days following an acute oral exposure. Similarly, effects in repeat dosing studies did not occur 

until late in the studies; effects on clotting time were first seen at around day 30 in the 6-week dog 

study. The delayed effects of the chemical were to some extent related to dose; in the 6-week dog 

study the effects on clotting time were seen earlier in dogs that received higher doses. No other 

types of adverse effects were observed in the database, including neurotoxicity, immunotoxicity, 
Page 6 of 24

or developmental effects.

The acute toxicity of difenacoum is severe via the oral, dermal, and inhalation routes of exposure 

(Category I). Difenacoum is not an eye or dermal irritant, nor is it a dermal sensitizer.

At the time of Registration Review or the next risk assessment, the Agency may update PODs 

and toxicological endpoints based on updated policies for endpoint selection. Additionally, the 

uncertainty factors will be reassessed in the context of the updated PODs and endpoints. Please 

refer to Attachment 4 for a summary of the toxicity endpoints and points of departure (PODs)

from the previous risk assessment and Attachment 6 for the complete toxicity profile table.

Conclusions: The toxicity database is adequate for the purpose of registration review, except for 

the lack of an inhalation study and a dermal study which are required at this time. An alternative 

to conducting an inhalation study would be to add additional PPE (i.e., use of respirators) to 

registered labels which would address risks of concern resulting from inhalation exposures to a 

point where the margins of exposure (MOEs) based on the Hazard Assessment and Science 

Policy Council screening criteria would no longer be of concern. Based on the low MOEs 

calculated for dermal exposure, label changes are not an option instead of conducting a dermal 

study. At the time of Registration Review, the Agency may update PODs and toxicological 

endpoints based on updated policies for endpoint selection.

Residue Chemistry and Dietary Exposure

There are no agricultural field crop uses of difenacoum, nor are there any tolerances established 

or needed. Furthermore, based on difenacoum’s physicochemical properties, no measurable 

concentrations are expected in drinking water.

Conclusions: A new dietary (food and water) exposure assessment is not anticipated to be 

needed during Registration Review.

Residential Exposure

Residential Handlers 

Consumer use products may not contain second generation anticoagulants. Therefore, all 

Sublabel A Consumer Use labels expired June 4, 2011 and are thus no longer actively registered.

Residential Post-Application 

Post application exposure to products placed in tamper resistant bait stations or below ground are 

not anticipated. Tamper resistant bait stations are mandatory for any bait placed above ground 

where children and/or pets may be present. 

Non Occupational Spray Drift or Volatilization

Since difenacoum is applied as a solid formulation by hand, largely in tamper-resistant bait 

stations, neither exposure via drift or volatilization is expected.
Page 7 of 24

Conclusions: Neither a residential handler nor post-application assessment will be required 

during Registration Review. Additionally, neither exposure via drift or volatilization is 

expected. 

Aggregate Risk Assessment

In accordance with the Food Quality Protection Act, HED must consider and aggregate pesticide 

exposures and risks from three major sources: food, drinking water, and residential exposures. 

Based on the registered uses, HED does not anticipate dietary (food or drinking water) or 

residential exposure due to the lack of consumer product use and requirement for bait stations 

where children are present. Therefore, an aggregate risk assessment is not anticipated for 

children or adults. 

Conclusions: An aggregate risk assessment is not required for children or adults during 

Registration Review based on the current difenacoum use pattern and formulations. 

Occupational Exposure

Occupational Handler Exposure

In the previous 2007 assessment, difenacoum pellets and place packs were formulated as readyto-use; therefore, an assessment of mixer/loader exposure was not conducted. HED used PHED 

(Granular Bait Dispersed by Hand) to assess occupational applicator exposure. Applicator 

exposure was assessed for the maximum application rate (i.e., 7 oz of product) and 4 treatment 

sites. The level of concern (LOC) is equal to a margin of exposure (MOE) of 100 or greater. The 

risk estimates for all applicator scenarios result in MOEs equal to or greater than 870 and were not 

of concern. 

Occupational handlers are anticipated to have short-term dermal and inhalation exposures from 

applying difenacoum products. Occupational dermal and inhalation exposure to bait blocks, and 

disposable placement packs is considered to be negligible. Intermediate- and long-term 

occupational exposure is not expected. The registered end use difenacoum products require 

occupational handlers to wear long sleeved shirts, long pants, shoes, socks, and in some cases 

water proof gloves. Currently registered labels for pelleted products do not provide sufficient 

guidance regarding a method or use of container for measuring specified amount of bait per 

refillable station, nor a maximum number of refillable bait stations within the 100 foot perimeter 

of agricultural and commercial buildings. An updated occupational handler exposure assessment 

will be conducted during Registration Review. The revised assessment will include updated

policies and body weight assumptions as well as any additional information provided by the 

registrant regarding methods for applying bait to refillable stations and maximum number of bait 

stations per 100 foot perimeter. 

Occupational Post-Application Exposure

There is no anticipated occupational post-application exposure to the current use pattern of 

difenacoum. If new policies or procedures are put into place, the Agency may revisit the need 

for a quantitative occupational post-application exposure assessment for difenacoum.
Page 8 of 24

Occupational Handler and Post-Application Exposure Conclusions: An updated occupational 

handler exposure assessments will be conducted during Registration Review. Additional 

information regarding the maximum number of refillable trap/bait stations within the 100 foot 

perimeter of agricultural buildings could be beneficial in estimating potential handler exposure 

and risk. Occupational post-application exposure is not anticipated for Registration Review.

Public Health and Pesticide Epidemiology Data

As part of the Rodenticide Risk Mitigation Decision (RMD), and the subsequent Notice of Intent 

to Cancel (NOIC), the Environmental Protection Agency’s Office of Pesticide Programs’ 

Pesticide Re-evaluation Division requested an analysis of human incidents involving exposure to 

rodenticides in 2011 (S. Recore, 10/28/2011, D395567) and updated in 2015 (S. Recore,

D430176, 11/17/2015). For this evaluation, the Agency consulted the OPP Incident Data System 

(IDS), the National Pesticide Information Center (NPIC), Centers for Disease Control and 

Prevention/National Institute for Occupational Safety and Health (CDC/NIOSH), Sentinel Event 

Notification System for Occupational Risk-Pesticides (SENSOR), and Agricultural Health Study 

(AHS) databases for pesticide incident data on the active ingredient difenacoum. 

For the Main IDS, which contains incidents resulting in higher severity outcomes (i.e., death, 

major and moderate incidents) from January 1, 2010 to May 27, 2015, there were no incidents 

reported involving difenacoum. In Aggregate IDS, which contains incidents resulting in less 

severe human incidents (minor, unknown, or no effects outcomes), there was one incident 

reported involving difenacoum. The incident was classified as having no or unknown effects. 

Overall, there are few incidents reported to IDS involving difenacoum. A query of SENSORPesticides from 1998-2011, identified no cases involving difenacoum. Difenacoum is not 

included in the AHS; therefore, that study does not provide information for this document.

The Agency will continue to monitor the incident information and provide an updated review of 

incident, considering pre- and post-mitigation incidents separately, when the information 

becomes available.

Tolerance Assessment and International Harmonization

There are no tolerances for difenacoum.

Environmental Justice

Potential areas of environmental justice concerns, to the extent possible, were considered in this 

human health risk assessment, in accordance with U.S. Executive Order 12898, "Federal Actions 

to Address Environmental Justice in Minority Populations and Low-Income Populations," 

http://www.epa.gov/compliance/environmentaljustice/resources/policy/exec_order_12898.pdf. 

As a part of every pesticide risk assessment, OPP considers a large variety of consumer 

subgroups according to well-established procedures. In line with OPP policy, HED estimates 

risks to pesticide exposures that are based on activities in and around the home that involve 

pesticide use in a residential setting. Whenever appropriate, these non-dietary exposures are 

calculated based on home use of pesticide products and associated risks for adult applicators and 
Page 9 of 24

for toddlers, youths, and adults entering or playing on treated areas post-application are 

evaluated.

Cumulative

Unlike other pesticides for which EPA has followed a cumulative risk approach based on a 

common mechanism of toxicity, EPA has not made a common mechanism of toxicity finding as 

to difenacoum and any other substances and difenacoum does not appear to produce a toxic 

metabolite produced by other substances. For information regarding EPA’s efforts to determine 

which chemicals have a common mechanism of toxicity and to evaluate the cumulative effects of 

such chemicals, see the policy statements released by EPA’s Office of Pesticide Programs 

concerning common mechanism determinations and procedures for cumulating effects from 

substances found to have a common mechanism on EPA’s website at 

http://www.epa.gov/pesticides/cumulative/.

Human Studies

The risk assessments which have been previously conducted for difenacoum relied in part on 

data from studies in which adult human subjects were intentionally exposed to a pesticide to 

determine their dermal and inhalation exposure. Many such studies, involving exposure to many 

different pesticides, comprise generic pesticide exposure databases such as the Pesticide 

Handlers Exposure Database (PHED), and the Outdoor Residential Exposure Task Force 

(ORETF). EPA has reviewed all the studies supporting these multi-pesticide generic exposure 

databases, and has found no clear and convincing evidence that the conduct of any of them was 

either fundamentally unethical or significantly deficient relative to the ethical standards 

prevailing at the time the research was conducted. All applicable requirements of EPA’s Rule 

for the Protection of Human Subjects of Research (40 CFR Part 26) have been satisfied, and 

there is no regulatory barrier to continued reliance on these studies.

Data Deficiencies and Label Revisions

The following studies are required to support Registration Review of difenacoum:

870.3200 21/28-day dermal toxicity

870.3465 90-day inhalation study

Due to the expected short-term nature of occupational exposure, a shorter term inhalation study 

(21/28-day inhalation study) is appropriate.

No residue chemistry, or occupational and residential exposure data are needed to support the 

registered uses of difenacoum at this time. HED recommends that PRD ensure that all expired 

difenacoum Sublabel A Consumer Use labels be removed from the Pesticide Product Label 

System (PPLS). Furthermore, HED recommends that bait pellets are packaged with a measuring 

cone container and instructions for refillable bait stations.
Page 10 of 24

References

Recent Memoranda Relevant to Registration Review of Difenacoum

Author DP/TXR # Date Title

Risk Assessment

K. Schumacher D330185 July 20, 2007

Difenacoum: Human Health Risk Assessment for 

Proposed Uses on Commensal Rodents.

Not available May 28, 2008

(revised June 24, 2008) Risk Mitigation Decision for Ten Rodenticides

Occupational and Residential Exposure Assessment

M. Collantes D338316 June 2007

OCCUPATIONAL AND RESIDENTIAL 

EXPOSURE ASSESSMENT for Section 3 

Registration Use of Difenacoum

Incident Report

J. Blondell

EPA-HQOPP-2006-

0955-0010

June 3, 1999 Updated Review of Rodenticides Incident Reports

Primarily Concerning Children

M. Hawkins D332563 January 9, 2006 Rodenticide: Tier I (Scoping) Review of Human 

S. Recore, D430176, November 17, 2015 Incidents and Epidemiology Assessment

Attachments

1. Chemical Identity Table

2. Physicochemical Properties of Technical Grade Difenacoum Table

3. Use Profile Table for Difenacoum

4. Endpoint Selection Tables

5. Toxicology Data Requirements

6. Toxicity Profile Tables

7. Physical and Chemical Properties for Assessing Dermal Absorption
Page 11 of 24

Attachment 1

Chemical Identity Table

Table A.1. Difenacoum Nomenclature.

Chemical Structure

OH

O O

Common Name Difenacoum, diphenacoum

Molecular Formula C31H24O3

Molecular Weight 444.5

IUPAC Name 3-(3-biphenyl-4-yl-1,2,3,4-tetrahydro-1-naphthyl)-4-hydroxycoumarin

CAS Name 3-[3-(1,1′-biphenyl)-4-yl-1,2,3,4-tetrahydro-1-naphthalenyl]-4-hydroxy-2H-1-

benzopyran-2-one

CAS Registry Number 56073-07-5

End-use Products Difenacoum Rat and Mouse Block IV (0.005% w/w), Difenacoum Rat and Mouse Place 

Packs IV (0.005% w/w), Difenacoum Rat and Mouse Pellets IV (0.005% w/w), 

Difenacoum Rat and Mouse Pellets (0.005% w/w), Difenacoum Rat and Mouse Place 

Packs (0.005% w/w), Difenacoum Rat and Mouse Blocks (0.005% w/w)

Chemical Class Anticoagulant Rodenticide
Page 12 of 24

Attachment 2

Physicochemical Properties of Technical Grade Difenacoum Table

. 

Table 2.2. Physicochemical Properties of Technical Grade Difenacoum.

Parameter Value Reference

Melting Point/Range 211.0 – 215.0oC (98.7% w/w) MRID 46750965

pH Not available

Relative Density (20.5oC) 1.27 (98.7% w/w)

Water Solubility (20oC) <0.05 mg/L at pH 4

1.7 mg/L at pH 7

61 mg/L at pH 9

Solvent Solubility (g/L at 20oC) Acetone 7.6

Propan-2-ol 1.5

Ethyl acetate 3.7

Toluene 1.2

Methanol 1.2

Hexane 0.01

Dichloromethane 19.6

Vapor Pressure (25oC) 6.7 x 10-9 – 5.4 x 10-14 Pa

Dissociation Constant (pKa) 4.5

Octanol/Water Partition Coefficient (Log[POW]) 7.6

UV/Visible Absorption Spectrum

Wavelength of peak (nm) 310.6 259.4

Absorbance 0.267 0.664

Molar Extinction Coefficient 17,100 46,600
Page 13 of 24

Attachment 3. Preliminary Use Profile Table for Difenacoum

Table A.3. Preliminary Use Profile Table for Difenacoum (PC 119901)

Company

Formulation 

[EPA Reg. 

No.]

% ai Formulation 

Type

Net 

Contents

Packaging

Target 

Pest

Tamper 

Resistant 

Bait 

Stations 

Application Rate PPE RUP Use Directions and Limitations

Woodstream 

Corporation 

Difenacoum 

Rat and Mouse 

Block IV

Registration 

# 36488-63

Sub-label A: 

Agriculture 

Use

0.005 Bait Block

8 lb/128oz 

(128 x 1.0 

oz block)

to

20 lb/320 oz 

(320 oz - 1.0 

oz block) 

Rats, 

house 

mice and 

meadow 

voles.

Yes, for all 

placements 

where 

children, 

pets nontarget 

mammal or 

birds may 

access bait.

However, 

for all 

outdoor 

placements

, product is 

to be 

placed in 

bait 

stations 

RATS: 3 to 10 

blocks/placement 

using bait stations, 

placed at intervals of 

15 to 30 ft 

(0.0625 lbs x 0.005% 

ai = 0.000003126 lb 

ai/block

0.0000031 lb ai x 10 

blocks = 0.00003126

lb ai/10 blocks

MICE: 1 block 

placement using bait 

station placed at 8 to 

12 ft intervals

SL/G Yes

Apply within 100 feet of agricultural 

buildings and man-made agricultural 

structures. Burrow bating with this 

product is prohibited. Fence and 

perimeter baiting beyond 100 feet from 

structure is prohibited. .Do Not Use in 

Homes or Other Residential Settings.

Do Not Use in sewers. Do not broadcast 

bait. Do not sell this product in 

individual containers holding less than 

8 pounds of bait. 

Difenacoum 

Rat and Mouse 

Block IV

Registration 

# 36488-63

Sub-label B: 

Profession Use

16 lb/256 oz

(128 x 1 oz 

block)

To 

20 lb/320 oz 

(320 x 1 oz 

block)

Use in and within 100 feet of man-made 

structures such as homes, foodprocessing facilities, industrial and

commercial buildings, trash receptacles, 

agricultural, and public buildings, 

transport vehicles, docks and port of 

terminal buildings. Do not sell less than 

16 pounds of bait. Burrow baiting is 

prohibited. Do not use in sewer. Do not 

broadcast. Fence and perimeter baiting 

beyond 100 feet from structure is 

prohibited.

Woodstream 

Corporation

Difenacoum 

Rat and Mouse 

Place Packs IV

Registration 

# 36488-64

Sub-label A: 

Agriculture 

Use

0.005 Place Packs

8.06 

lb/129oz (86 

x 1.5 oz 

place pack, 

bait pack or 

bait bags)

to

51.56 lb/825 

oz (550 x 

1.5 oz place 

pack, bait 

pack or bait 

bags) 

Rats, 

house 

mice, 

and 

meadow 

voles 

Yes

RATS: 3 to 10 place 

packs/placement using 

bait stations, placed at 

intervals of 15 to 30 ft 

(0.09375 lbs x 0.005% 

ai = 0.00000468 lb 

ai/place pack

0.0.00000468 lb ai x 

10 place pack = 

0.0000468 lb ai/10 

place pack

Apply within 100 feet of agricultural 

buildings and man-made agricultural 

structures. Burrow bating with this 

product is prohibited. Fence and 

perimeter baiting beyond 100 feet from 

structure is prohibited. .Do Not Use in 

Homes or Other Residential Settings.

Do Not Use in sewers. Do not broadcast 

bait. Do not sell this product in 

individual containers holding less than 

8 pounds of bait. 
Page 14 of 24

Table A.3. Preliminary Use Profile Table for Difenacoum (PC 119901)

Company

Formulation 

[EPA Reg. 

No.]

% ai Formulation 

Type

Net 

Contents

Packaging

Target 

Pest

Tamper 

Resistant 

Bait 

Stations 

Application Rate PPE RUP Use Directions and Limitations

Difenacoum 

Rat and Mouse 

Place Packs IV

Registration 

# 36488-64

Sub-label B: 

Profession Use

16.03 

lb/256.5 oz

(171 x 1.5 

oz place 

pack, bait 

pack or bait 

bags)

To 

51.56 lb/825 

oz (550 x 

1.5 oz place 

pack, bait 

pack or bait 

bags) 

MICE: 1⁄4 to 1⁄2 oz of 

bait per placement 

using bait stations 

placed at intervals of 8 

to 12 feet.

(0.03125 lb x 0.00005 

ai = 0.0000015 lb 

ai/placement

Use in and within 100 feet of man-made 

structures such as homes, foodprocessing facilities, industrial and

commercial buildings, trash receptacles, 

agricultural, and public buildings, 

transport vehicles, docks and port of 

terminal buildings. Do not sell less than 

16 pounds of bait. Burrow baiting is 

prohibited. Do not use in sewer. Do not 

broadcast. Fence and perimeter baiting 

beyond 100 feet from structure is 

prohibited.

Woodstream 

Corporation

Difenacoum 

Rat and Mouse 

Pellets IV

Registration 

# 36488-65

Sub-label A: 

Agriculture

Use

0.005 Pelleted

8 lb/128oz 

(3628.74g) 

to 50 lb/800 

oz 

(22670.62g) Rats, 

house 

mice, 

and 

meadow 

voles

Yes

RATS: 4 to 16 oz of 

pellets/placement 

using bait stations 

placed at intervals of 

15 to 30 ft

(1 lb x 0.00005 ai = 

0.00005 lb 

ai/placement)

MICE: 1⁄4 to 1⁄2 oz of 

bait per placement 

using bait stations 

placed at intervals of 8 

to 12 feet.

(0.03125 lb x 0.00005 

ai = 0.0000015 lb 

ai/placement

SL/G

Yes

Apply within 100 feet of agricultural 

buildings and man-made agricultural 

structures. Burrow bating with this 

product is prohibited. Fence and 

perimeter baiting beyond 100 feet from 

structure is prohibited. .Do Not Use in 

Homes or Other Residential Settings.

Do Not Use in sewers. Do not broadcast 

bait. Do not sell this product in 

individual containers holding less than 

8 pounds of bait. 

Difenacoum 

Rat and Mouse 

Pellets IV

Registration 

# 36488-65

Sub-label B: 

Profession Use

Yes

Use in and within 100 feet of man-made 

structures such as homes, foodprocessing facilities, industrial and

commercial buildings, trash receptacles, 

agricultural, and public buildings, 

transport vehicles, docks and port of 

terminal buildings. Do not sell less than 

16 pounds of bait. Burrow baiting is 

prohibited. Do not use in sewer. Do not 

broadcast. Fence and perimeter baiting 
Page 15 of 24

Table A.3. Preliminary Use Profile Table for Difenacoum (PC 119901)

Company

Formulation 

[EPA Reg. 

No.]

% ai Formulation 

Type

Net 

Contents

Packaging

Target 

Pest

Tamper 

Resistant 

Bait 

Stations 

Application Rate PPE RUP Use Directions and Limitations

beyond 100 feet from structure is 

prohibited.

Woodstream 

Corporation

Difenacoum 

Rat and Mouse 

Pellets

Registration 

# 47629-14

Sub-label B: 

Agriculture 

Use 0.005 

% Pelleted

8 lbs to 50 

lbs.

Rat and 

Mouse Yes

RATS: 4 to 16 oz of 

pellets/placement 

using bait stations 

placed at intervals of 

15 to 30 ft

(1 lb x 0.00005 ai = 

0.00005 lb 

ai/placement)

MICE: 1⁄4 to 1⁄2 oz of 

bait per placement 

using bait stations 

placed at intervals of 8 

to 12 feet.

(0.03125 lb x 0.00005 

ai = 0.0000015 lb 

ai/placement

Yes

Apply within 50 feet of agricultural 

buildings and man-made agricultural 

structures. Do Not Use in Homes or 

Other Residential Settings. Do Not Use 

in sewers. Do not broadcast bait. Do 

not sell this product in individual 

containers holding less than 8 pounds of 

bait

Difenacoum 

Rat and Mouse 

Pellets

Registration 

# 47629-14

Sub-label C: 

Professional 

Use

16 lbs to 50 

lbs.

Use in and within 50 feet of man-made 

structures such as homes, foodprocessing facilities, industrial and

commercial buildings, trash receptacles, 

agricultural, and public buildings, 

transport vehicles, port or terminal 

buildings. Do not sell less than 16

pounds of bait.. Do not use in sewer. 

Do not broadcast. 

Woodstream 

Corporation

Difenacoum 

Rat and Mouse 

Block

Registration 

# 47629-16

Sub-label B: 

Agriculture 

Use

0.005 

%

Block

8 to 20 lbs

Rat and 

Mouse Yes

RATS: 3 to 10 

blocks/placement 

using bait stations, 

placed at intervals of 

15 to 30 ft 

(0.0625 lbs x 0.005% 

ai = 0.000003126 lb 

ai/block

0.0000031 lb ai x 10 

blocks = 0.00003126 

lb ai/10 blocks

MICE: 1 block 

placement using bait 

SL/G

Yes

Apply within 50 feet of agricultural 

buildings and man-made agricultural 

structures. Do Not Use in Homes or 

Other Residential Settings. Do Not Use 

in sewers. Do not broadcast bait. Do 

not sell this product in individual 

containers holding less than 8 pounds of 

bait

Difenacoum 

Rat and Mouse 

Block

Registration 

# 47629-16

16 to 20 lbs SL/G
Page 16 of 24

Table A.3. Preliminary Use Profile Table for Difenacoum (PC 119901)

Company

Formulation 

[EPA Reg. 

No.]

% ai Formulation 

Type

Net 

Contents

Packaging

Target 

Pest

Tamper 

Resistant 

Bait 

Stations 

Application Rate PPE RUP Use Directions and Limitations

Sub-label C: 

Professional 

Use

station placed at 8 to 

12 ft intervals

Woodstream 

Corporation

Difenacoum 

Rat and Mouse 

Place Packs

Registration 

# 47629-17

Sub-label B: 

Agriculture 

Use

0.005 

%

Place Packs

8.06 lbs to 

51.56 lbs

Rat and 

Mouse Yes

RATS: 3 to 10 place 

packs/placement using 

bait stations, placed at 

intervals of 15 to 30 ft 

(0.09375 lbs x 0.005% 

ai = 0.00000468 lb 

ai/place pack

0.0.00000468 lb ai x 

10 place pack = 

0.0000468 lb ai/10 

place pack

MICE: 1⁄4 to 1⁄2 oz of 

bait per placement

using bait stations 

placed at intervals of 8 

to 12 feet.

(0.03125 lb x 0.00005 

ai = 0.0000015 lb 

ai/placement

SL/G NO

This product may only be used inside 

and within 100 feet of buildings or 

inside of transport vehicles (ships, 

trains, or aircraft). This Product May 

Not Be Sold in Packaging that Holds 

Less Than 4 Pounds of Bait. Each bait 

pack is 2 ounces. Burrow baiting is 

prohibited

Difenacoum 

Rat and Mouse 

Place Packs

Registration 

# 47629-17

Sub-label C: 

Professional 

Use

16.03 lb to 

51.56 lbs
Page 17 of 24

Attachment 4. Endpoint Selection Table

Table A.4. Toxicological Doses and Endpoints for Difenacoum for Use in Occupational Human Health 

Risk Assessments.

Exposure/

Scenario

Point of 

Departure

Uncertainty 

Factors

Level of Concern 

for Risk 

Assessment

Study and Toxicological Effects

Acute 

Dietary (All 

Populations)

Dietary endpoints were not selected because difenacoum is strictly a non-food use chemical.

Chronic 

Dietary (All 

Populations)

Dietary endpoints were not selected because difenacoum is strictly a non-food use chemical.

Dermal 

(Short-Term)

NOAEL= 

0.005 

mg/kg/day

Dermal 

absorption rate 

= 4%

UFA= 10x

UFH = 10x

Occupational LOC 

for MOE = 100

Developmental toxicity study in 

rabbits

LOAEL = 0.015 mg/kg/day based on 

changes in blood coagulation 

(increased prothrombin and kaolincephalin times).

Inhalation

(Short-Term)

NOAEL= 

0.005 

mg/kg/day

Toxicity by 

the inhalation 

route is 

considered to 

be equivalent 

to the 

estimated 

toxicity by the 

oral route of 

exposure. 

UFA= 10x

UFH = 10x

Occupational LOC 

for MOE = 100

Developmental toxicity study in 

rabbits

LOAEL = 0.015 mg/kg/day based on 

changes in blood coagulation 

(increased prothrombin and kaolincephalin times).

Cancer Classification: N/A

Point of Departure (POD) = A data point or an estimated point that is derived from observed dose-response data and 

used to mark the beginning of extrapolation to determine risk associated with lower environmentally relevant human 

exposures. NOAEL = no observed adverse effect level. LOAEL = lowest observed adverse effect level. UF = 

uncertainty factor. UFA = extrapolation from animal to human (interspecies). UFH = potential variation in sensitivity 

among members of the human population (intraspecies). MOE = margin of exposure. LOC = level of concern. N/A 

= not applicable.
Page 18 of 24

Attachment 5. Toxicology Data Requirements

The requirements (40 CFR 158.500) for non-food uses are in the table below. Use of the new 

guideline numbers does not imply that the new (1998) guideline protocols were used.

Table A.5: Toxicology Data Requirements for Difenacoum

Guideline Study Type Required Submitted Satisfied

870.1100 Acute Oral Toxicity Yes Yes Yes

870.1200 Acute Dermal Toxicity Yes Yes Yes

870.1300 Acute Inhalation Toxicity Yes Yes Yes

870.2400 Acute Eye Irritation Yes Yes Yes

870.2500 Acute Dermal Irritation Yes Yes Yes

870.2600 Skin Sensitization Yes Yes Yes

870.3100 90-Day Oral Toxicity in Rodents CR Yes Yes

870.3150 90-Day Oral Toxicity in Non-Rodents CR No NA

870.3200 21/28-Day Dermal Toxicity NR No NA

870.3250 90-Day Dermal Toxicity Yes Yes Yes

870.3465 90-Day Inhalation Toxicity CR No Yesb

870.3700

Prenatal Developmental Toxicity Study (Rat) Yes Yes Yes

Prenatal Developmental Toxicity Study (Rabbit) Yes Yes Yesb

870.3800 Reproduction and Fertility Effects Yes No Yesb

870.4100 Chronic Toxicity (Rodent) CR No NA

870.4200

Carcinogenicity (Rat) CR No NA

Carcinogenicity (Mouse) CR No NA

870.5100 Bacterial Reverse Mutation Test Yes Yes Yes

870.5300

870.5375

in vitro Mammalian Cell Gene Mutation Test

in vitro Mammalian Chromosome Aberration Test Yesa YesYes Yes

870.5385

870.5395

Mammalian Bone Marrow Chromosomal Aberration Test

Mammalian Erythrocyte Micronucleus Test Yesa NoYes Yes

870.6100

Delayed Neurotoxicity of Organophosphorus Substances (Acute, Hen) CR No NA

Delayed Neurotoxicity of Organophosphorus Substances (28-Day, Hen) CR No NA

870.6200

Neurotoxicity Screening Battery (Acute, Rat) Yes No Yesb

Neurotoxicity Screening Battery (90-Day, Rat) Yes No Yesb

870.6300 Developmental Neurotoxicity Study CR No NA

870.7200 Companion Animal Safety CR No NA

870.7485 Metabolism and Pharmacokinetics Yes Yes Yes

870.7600 Dermal Penetration CR No NA

870.7800 Immunotoxicity Yes No Yesb

CR = Conditionally Required. NA = Not Applicable.

a Either guideline study type may be used to satisfy the data requirement (e.g. 870.5300 or 870.5375).

b Study requirement waived (TXR 0057384).
Page 19 of 24

Attachment 6. Toxicity Profile for Difenacoum

Table A.6.1 Acute Toxicity Profile – Difenacoum Technical

Guideline 

No. Study Type MRID(s) Results Toxicity Category

870.1100 Acute oral – rat 46750935

LD50 = 1.8 mg/kg (♂)

(95% C.I.= 1.5-2.1 mg/kg) I

870.1100 Acute oral – rat 46750936 LD50 = 2.6 mg/kg (♀)

(95% C.I.= 2.1-3.7 mg/kg) I

870.1100 Acute oral – rat and mouse 46766206

LD50 (cis)

Male Rats = 1.17 mg/kg, 

Female Rats = 1.625 mg/kg 

Male Mice = 0.45 mg/kg

Female Mice = 1.0 mg/kg

LD50 (trans)

Male Rats = 7.33 mg/kg, 

Female Rats = 6.0 mg/kg

Male Mice = 1.181 mg/kg

Female Mice = 2.75 mg/kg

I

870.1200 Acute dermal – rat 46750938 LD50 = 63 mg/kg (♀)

(95% C.I. = 34-85 mg/kg) I

870.1300 Acute inhalation – rat 46750939

LC50 =>0.003646 mg/L 

(♂+♀) I

870.2400 Acute eye irritation – rabbit 46750940

1/3 conjunctival irritation 

immediately after. No 

positive irritation at 24-

hours.

IV

870.2500 Acute dermal irritation – rabbit 46750941

No irritation at the 24-hour 

observation. IV

870.2600 Skin sensitization – guinea pig 46750942 Not a sensitizer N/A
Page 20 of 24

Table A.6.2 Subchronic, Chronic and Other Toxicity Profile – Difenacoum

Guideline 

No. 

Study Type MRID No. (year)/ 

Classification /Doses

Results

Nonguideline6-Week oral 

toxicity (dog)

46750944 (1994)

Acceptable/nonguideline

0, 0.01, 0.025, 0.05, 0.1, 0.2 

mg/kg/day

NOAEL = Not determined

LOAEL = 0.01 mg/kg/day based on changes in 

blood coagulation, particularly increased 

prothrombin time beginning on day 30.

All animals exposed to ≥ 0.025 mg/kg/day were 

sacrificed early due to increased coagulation 

times and/or clinical evidence of hemorrhage.

870.3100 90-Day oral 

toxicity (rat)

46766207 (1991)

Unacceptable/guideline

0, 0.01, 0.02/0.09, 0.03 

mg/kg/day

NOAEL = Cannot be determined 

LOAEL = Cannot be determined 

No effect on coagulation at 0.01 mg/kg/day

This study was classified unacceptable due to 

inadequate number of animals/sex/dose. There 

were no surviving males after day 78. The dose 

level for the 0.02 mg/kg/day animals was 

increased to 0.09 mg/kg/day on Day 65 (F) or 66 

(M). Hematology data are not available for the 

low dose group.

870.3700a Prenatal 

developmental in 

rodents (rat)

46766208 (1994)

Acceptable/guideline

0, 0.01, 0.03, 0.09 

mg/kg/day

Maternal NOAEL = 0.03 mg/kg/day

Maternal LOAEL = 0.09 mg/kg/day based on 

vaginal bleeding and increased kaolin-cephalin 

time.

Developmental NOAEL = 0.09 mg/kg/day

Developmental LOAEL = Not observed

870.3700b Prenatal 

developmental in 

non-rodents 

(rabbit)

46750947 (1994)

Acceptable/guideline

0, 0.001, 0.005, 0.015 

mg/kg/day

Maternal NOAEL = 0.005 mg/kg/day

Maternal LOAEL = 0.015 mg/kg/day based on 

changes in blood coagulation (increased 

prothrombin and kaolin-cephalin times).

All animals in the 0.015 mg/kg/day group were 

sacrificed to prevent or minimize fatal maternal 

hemorrhage.

Developmental NOAEL = 0.015 mg/kg/day

Developmental LOAEL = Not observed

870.5100 Gene mutation –

bacterial reverse 

mutation assay

46750948 (1995)

Acceptable/guideline

Negative.

870.5100 Gene mutation –

bacterial reverse 

mutation assay

46766209 (1986)

Acceptable/nonguideline

Negative.

870.5300 Gene mutation –

in vitro

mammalian cell 

gene forward 

mutation assay

46750952 (1995)

Acceptable/guideline

Negative.
Page 21 of 24

Table A.6.2 Subchronic, Chronic and Other Toxicity Profile – Difenacoum

Guideline 

No. 

Study Type MRID No. (year)/ 

Classification /Doses

Results

870.5375 Cytogenetics - in 

vitro mammalian 

cell chromosome 

aberration test

46750950 (1995)

Acceptable/guideline

Positive for chromosomal aberrations in cultured 

human lymphocytes with and without S9 

activation, with a variable dose-response.

870.5375 Cytogenetics - in 

vitro mammalian 

cell chromosome 

aberration test

46766210 (1986)

Acceptable/guideline

Positive for chromosomal aberrations in Chinese 

hamster lung cells with and without S9 activation, 

with a variable dose-response.

870.5395 Cytogenetics –

mammalian 

erythrocyte 

micronucleus test 

(rat)

46766211 (1996)

Acceptable/guideline

Negative.

870.5395 Cytogenetics –

mammalian 

erythrocyte 

micronucleus test 

(mouse)

46766212 (1987)

Acceptable/guideline

Negative.

870.5550 Other effects –

unscheduled 

DNA synthesis

in mammalian 

cells in culture

46773601 (1996)

Acceptable/guideline

Negative.

870.5550 Other effects –

unscheduled 

DNA synthesis 

in mammalian 

cells in culture

46766214 (1990)

Acceptable/guideline

Negative.

870.7485 Metabolism and 

pharmacokinetics

(rat)

46766216 (1996)

Acceptable/guideline

0.051 or 0.54 mg/kg

Groups of 4 female SD rats were given a single 

oral gavage dose of radiolabled difenacoum. 

Absorption: Rapid; reached mean peak blood 

level for each dose group 4 hours after dosing. 

Distribution: Radioactivity was present in all 

tissues examined. At necropsy (168 hours), the 

majority of the unexcreted dose was present in 

the liver (37.6%/20.6% mean radioactivity from 

administered low/high dose), with smaller 

amounts in the carcass (17.2%/13.5%; low/high 

dose), the GI tract (4.0%/2.7%; low/high dose), 

and pancreas (1.1%/1.2%; low/high dose). 

Metabolism: Absorbed difenacoum is 

extensively metabolized, with both parent 

compound and up to 4 metabolites present in the 

liver at 7 days after dosing. In the liver, 4 

metabolites were found in low dose animals and 

2 metabolites were found in high dose animals. 

Both the parent compound and two of the 

hepatic metabolites are eliminated in the faeces 
Page 22 of 24

Table A.6.2 Subchronic, Chronic and Other Toxicity Profile – Difenacoum

Guideline 

No. 

Study Type MRID No. (year)/ 

Classification /Doses

Results

within the first 24 hours, along with a polar 

metabolite(s). 

Elimination: At 24 hours, 17.7%/19.8% 

(feces), 1.4%/1.2% (urine), and 0.36%/0.06% 

(expired CO2) of the radioactivity from the 

administered low/high dose, respectively, was 

recovered. By 168 hours, 39.9%/50.9% (feces) 

and 1.9%/1.44% (urine) of the radioactivity from 

the administered low/high dose, respectively, 

was recovered.

870.7485 Metabolism and 

pharmacokinetics

(rat)

46766215 (2002)

Acceptable/nonguideline

0.57 mg/kg (cis isomer) or 

0.55 mg/kg (trans isomer)

Groups of 4 female SD rats were given a single 

oral gavage dose of radiolabled cis or transdifenacoum. 

Absorption: Reached plateau in plasma 8-24 

hours after dosing, followed by slow elimination 

phase. Peak blood level of the cis-isomer (0.21 

g equiv/ml) was approximately 2.5 times 

greater than the trans-isomer (0.08 g equiv/ml) 

at 8 hours. 

Distribution: Radioactivity was present in all 

tissues examined. At 96 hrs, the highest levels 

were found in the liver (19-22% of the 

administered dose of cis-isomer; 22-29% trans), 

carcass (13.9-16.3% cis; 11.9-13.5% trans), 

pancreas (1.2-1.8% cis; 1.3-1.4% trans), and GI 

tract, including stomach, intestines, cecum, and 

content (4-4.2% cis; 3.1-4.3% trans).

Metabolism: Not investigated in this study.

Elimination: Within 96 hours, the radioactivity 

was recovered mostly in the feces (41% of the 

administered dose of cis-isomer; 44.5% trans), 

with small amounts in urine (0.5% cis; 0.6% 

trans).
Page 23 of 24

Table A.6.2 Subchronic, Chronic and Other Toxicity Profile – Difenacoum

Guideline 

No. 

Study Type MRID No. (year)/ 

Classification /Doses

Results

870.7485 Metabolism and 

pharmacokinetics

(rat)

46750957 (1987)

Acceptable/nonguideline

1.2 mg/kg

32 male Alpk:AP rats were given a single oral 

gavage dose of radiolabled difenacoum and were 

sacrificed in groups of three at 1, 4 and 8 days, 

and 2, 4, 8, 12 and 26 weeks after dosing. 

Absorption: The concentration of radioactivity 

in the blood and tissues was highest 24 hours 

after dosing, which was the earliest 

measurement.

Distribution: The highest concentration of 

radioactivity, expressed as % of the dose, was 

found in the liver (42%), followed by the 

pancreas (0.48%), kidney (0.22%), and salivary 

glands (0.09%), with lower concentrations in the 

blood and fat. Whole body autoradiography 

showed that radioactivity levels were similarly 

distributed and not significantly reduced (except 

for the GI tract) at 5 days after dosing.

Metabolism: The cis:trans ratio of difenacoum 

in the liver at 24 hours was 64:36, while the ratio 

of administered difenacoum was 44:56, which 

suggests that the tran-isomer was being 

preferentially metabolized or eliminated. Five 

metabolites were found in the liver, accounting 

for 53% of the total radioactivity in the liver 

extract, but these were not identified.

Elimination: Elimination from the liver was 

biphasic, with a rapid phase during days 1-8 

accompanied by a reduction in clotting factor 

synthesis (t 1⁄2 = 3 days), followed by a slower 

terminal phase during days 28-182 (t 1⁄2 = 118 

days). Similar biphasic elimination was found in 

the kidney and salivary glands, but not the 

pancreas. The relative rate of elimination was 

liver, fat, and blood > kidney > salivary glands > 

pancreas.
Page 24 of 24

Attachment 7. Physical and Chemical Properties for Assessing Dermal Absorption

Input Value Reference

Log Kow 6.1, 20oC, pH 6.51http://pubchem.ncbi.nlm.nih.gov/

Melting Point 216oC http://pubchem.ncbi.nlm.nih.gov/

Boiling Point 290oC http://pubchem.ncbi.nlm.nih.gov/

Vapor pressure 5.35 x 10-11 mm Hg, 

25oC

http://pubchem.ncbi.nlm.nih.gov/

Simulation time 8 hours N/A

1 A value of 5.5 was used in the National Institute for Occupational Health and Safety finite dose 

skin permeation calculator because it was the highest acceptable value the program would allow.