Milbemycin A3

Milbemycin A3	Last updated: 28/08/2025
(Also known as: antibiotic B-41A4; milbemectin A3)

GENERAL INFORMATION

Description

An active component of milbemectin with acaricidal activity and is also used as an antibiotic drug.

Examples of veterinary uses

Examples of species treated

Approval status

VMR 2013/2033 approval status (GB/UK)

EU Regulatory approval status

Chemical structure

Isomerism

Milbemycin A3 is one of the main active components of the milbemycin complex. It exhibits complex stereoisomerism, with ten distinct chiral centres and multiple double bonds that contribute to its intricate three-dimensional structure. These chiral centres give rise to numerous stereoisomers, but only specific configurations, such as those found in naturally occurring milbemycin A3, possess potent biological activity. Additionally, the molecule contains geometric (E/Z) isomerism across several double bonds.

Chemical formula

C₃₁H₄₄O₇

Canonical SMILES

CC1CCC2(CC3CC(O2)CC=C(CC(C=CC=C4COC5C4(C(C=C(C5O)C)C(=O)O3)O)C)C)OC1C

Isomeric SMILES

C[C@H]1CC[C@]2(C[C@@H]3C[C@H](O2)C/C=C(/C[C@H](/C=C/C=C/4\CO[C@H]5[C@@]4([C@@H](C=C([C@H]5O)C)C(=O)O3)O)C)\C)O[C@@H]1C

International Chemical Identifier key (InChIKey)

ZLBGSRMUSVULIE-GSMJGMFJSA-N

International Chemical Identifier (InChI)

InChI=1S/C31H44O7/c1-18-7-6-8-23-17-35-28-27(32)21(4)14-26(31(23,28)34)29(33)36-25-15-24(10-9-19(2)13-18)38-30(16-25)12-11-20(3)22(5)37-30/h6-9,14,18,20,22,24-28,32,34H,10-13,15-17H2,1-5H3/b7-6+,19-9+,23-8+/t18-,20-,22+,24+,25-,26-,27+,28+,30-,31+/m0/s1

2D structure diagram/image available?

Yes

General status

Veterinary substance type

Substance groups

Micro-organism derived substance

Minimum active substance purity

237.5 g A3/kg milbemectin

Known relevant impurities

EU 2017 Dossier: None declared

Substance origin

Natural

Mode of action

Not applicable

Molecular targets

[Glycine receptor subunit alpha-3, Agonist], [Gamma-aminobutyric-acid (GABA) receptor subunit beta-3, Agonist]

CAS RN

51596-10-2

EC number

CIPAC number

660

US EPA chemical code

Not applicable

PubChem CID

Therapeutic Class

ATCvet Code

Controlled Drug?

Regulation 37/2010 MRL Classification

Molecular mass

528.7

PIN (Preferred Identification Name)

IUPAC name

(10E,14E,16E,22Z)-(1R,4S,5S,6R,6R,8R,13R,20R,21R,24S)-21,24-dihydroxy-5,6,11,13,22-pentamethyl-3,7,19-trioxatetracyclo[15.6.1.14.8.020,24]pentacosa-10,14,16,22-tetraene-6-spiro-2-tetrahydropyran-2-one

CAS name

(6R,25R)-5-O-demethyl-28-deoxy-6,28-epoxy-25-methylmilbemycin B

Forever chemical

Other status information

Relevant Environmental Water Quality Standards

Physical state

White crystalline solid

Commercial

Property

Product

Manufacturer

Authorisation Route

Legal Class (GB/UK)

Availability status

Current

Introduction & key dates

1990, introduced Japan

Example products (past and present) using this active

Formulation and application details

ENVIRONMENTAL FATE

Property

Value

Source; quality score; and other information

Interpretation

Solubility - In water at 20 °C (mg l⁻¹)

2.68

Low

Solubility - In organic solvents at 20 °C (mg l⁻¹)

203000

Acetone for milbemectin

3450

n-Octanol for milbemectin

340

n-Heptane for milbemectin

251000

Methanol for milbemectin

Melting point (°C)

236

Boiling point (°C)

Degrades before boiling

Degradation point (°C)

280

Flashpoint (°C)

Octanol-water partition coefficient at pH 7, 20 °C

3.47 X 10⁰⁶

Calculated

Log P

6.54

at 25 °C

High

Fat solubility of residues

Solubility

Data type

Density (g ml⁻¹)

1.2

Dissociation constant pKa) at 25 °C

Not applicable

No dissociation

Vapour pressure at 20 °C (mPa)

1.3 X 10^-02

Low volatility

Henry's law constant at 25 °C (Pa m³ mol⁻¹)

2.68

Moderately volatile

Volatilisation as max % of applied dose lost

From plant surface

From soil surface

Maximum UV-vis absorption L mol⁻¹ cm⁻¹

Neutral pH: 238nm=31300, 244nm=34300, 252nm=22100
Acid pH: 238nm=26200, 244nm=28400, 252nm=18300
Basic pH: 238nm=28500, 244nm=31200, 252nm=20200

as milbemectin

Surface tension (mN m⁻¹)

49.4

Refractive Index

Environmental release

Degradation

Property

Value

Source; quality score; and other information

Interpretation

Soil degradation (days) (aerobic)

DT₅₀ (typical)

8.5

Non-persistent

DT₅₀ (lab at 20 °C)

DT₅₀ (field)

8.5

Non-persistent

DT₉₀ (lab at 20 °C)

DT₉₀ (field)

Note

EU 2002 dossier data USA DT₅₀ field studies range 8.1-8.8 days, soils =2

Manure DT₅₀ (days)

Aqueous photolysis DT₅₀ (days) at pH 7

Value

2.6

Moderately fast

Note

Aqueous hydrolysis DT₅₀ (days) at 20 °C and pH 7

Value

260

at 25 °C

Persistent

Note

pH sensitive. DT₅₀ at 25 °C at pH 5 = 12days, at pH 9 = 226 days

Water-sediment DT₅₀ (days)

Water phase only DT₅₀ (days)

Sediment phase only DT₅₀ (days)

Air degradation

As this parameter is not normally measured directly, a surrogate measure is used: ‘Photochemical oxidative DT₅₀’. Where data is available, this can be found in the Fate Indices section below.

Decay in stored produce DT₅₀

Soil adsorption and mobility

Property

Value

Source; quality score; and other information

Interpretation

Linear

K_d (mL g⁻¹)

K_oc (mL g⁻¹)

Notes and range

Freundlich

K_f (mL g⁻¹)

63.3

Slightly mobile

K_foc (mL g⁻¹)

2843

¹/_n

0.98

Notes and range

EU 2002 dossier K_f range 12-153 g/Kg, K_foc range 1550-4400 g/Kg, soils =4

pH sensitivity

Fate indices

Property

Value

Source; quality score; and other information

Interpretation

GUS leaching potential index

0.51

Calculated

Low leachability

Bio-concentration factor

BCF (l kg⁻¹)

Whole fish

Low potential

CT₅₀ (days)

Not available

Known metabolites

None

ECOTOXICOLOGY

Terrestrial ecotoxicology

Property

Value

Source; quality score; and other information

Interpretation

Mammals - Acute oral LD₅₀ (mg kg⁻¹)

456

Rat as milbemectin

Moderate

Mammals - Short term dietary NOEL

(mg kg⁻¹)

(ppm diet)

Mammals - Chronic 21d NOAEL (mg kg⁻¹ bw d⁻¹)

Birds - Acute LD₅₀ (mg kg⁻¹)

347

Anas platyrhynchos as milbemectin

Moderate

Birds - Short term dietary (LC₅₀/LD₅₀)

1922 mg kg feed⁻¹

Anas platyrhynchos as milbemectin

Birds - Chronic 21d NOEL (mg kg⁻¹ bw d⁻¹)

Earthworms - Acute 14 day LC₅₀ (mg kg⁻¹)

28.5

Moderate

Earthworms - Chronic NOEC, reproduction (mg kg⁻¹)

0.109

Moderate

Soil micro-organisms

Nitrogen mineralisation: No significant adverse effect
Carbon mineralisation: No significant adverse effect

Dose: 0.1 mg kg⁻¹ soil Milbemectin

Collembola

Acute LC₅₀ (mg kg⁻¹)

Chronic NOEC (mg kg⁻¹)

Non-target plants

Vegetative vigour ER₅₀ (g ha⁻¹)

Seedling emergence ER₅₀ (g ha⁻¹)

Honeybees (Apis spp.)

Contact acute LD₅₀ (worst case from 24, 48 and 72 hour values - μg bee⁻¹)

0.025

Apis mellifera

High

Oral acute LD₅₀ (worst case from 24, 48 and 72 hour values - μg bee⁻¹)

0.026

Apis mellifera

High

Unknown mode acute LD₅₀ (worst case from 24, 48 and 72 hour values - μg bee⁻¹)

Chronic

Notes

Bumblebees (Bombus spp.)

Contact acute LD₅₀ (worst case from 24, 48 and 72 hour values - μg bee⁻¹)

> 9.7

Bombus terrestris

Moderate

Oral acute LD₅₀ (worst case from 24, 48 and 72 hour values - μg bee⁻¹)

> 10

Bombus terrestris

Moderate

Mason bees (Osmia spp.)

Contact acute LD₅₀ (worst case from 24, 48 and 72 hour values - μg bee⁻¹)

Oral acute LD₅₀ (worst case from 24, 48 and 72 hour values - μg bee⁻¹)

Other bee species (1)

Acute LD₅₀ (worst case from 24, 48 and 72 hour values - μg insect⁻¹)

Mode of exposure

Other bee species (2)

Acute LD₅₀ (worst case from 24, 48 and 72 hour values - μg insect⁻¹)

Mode of exposure

Beneficial insects (Ladybirds)

Beneficial insects (Lacewings)

Beneficial insects (Parasitic wasps)

Beneficial insects (Predatory mites)

Beneficial insects (Ground beetles)

Aquatic ecotoxicology

Property

Value

Source; quality score; and other information

Interpretation

Temperate Freshwater Fish - Acute 96 hour LC₅₀ (mg l⁻¹)

0.0044

Oncorhynchus mykiss as milbemectin

High

Temperate Freshwater Fish - Chronic 21 day NOEC (mg l⁻¹)

0.00065

Oncorhynchus mykiss as milbemectin 88 day

High

Tropical Freshwater Fish - Acute 96 hour LC₅₀ (mg l⁻¹)

0.07

Danio rerio EC₅₀ Embryo

High

Temperate Freshwater Aquatic invertebrates - Acute 48 hour EC₅₀ (mg l⁻¹)

0.011

Daphnia magna as milbemectin

High

Temperate Freshwater Aquatic invertebrates - Chronic 21 day NOEC (mg l⁻¹)

0.00012

Daphnia magna as milbemectin

High

Tropical Freshwater Aquatic invertebrates - Acute 48 hour EC₅₀ (mg l⁻¹)

Aquatic crustaceans - Acute 96 hour LC₅₀ (mg l⁻¹)

Sediment dwelling organisms - Acute 96 hour LC₅₀ (mg l⁻¹)

Sediment dwelling organisms - Chronic 28 day NOEC, static, water (mg l⁻¹)

> 2.0

Chironomus riparius

Moderate

Sediment dwelling organisms - Chronic 28 day NOEC, sediment (mg kg⁻¹)

Aquatic Plants (free-floating, fonds growth, fresh) - 7 day (mg l⁻¹)

Aquatic plants (rooted, growth rate, fresh) - 14 day (mg l⁻¹)

Algae - Acute (growth rate, fresh; mg l⁻¹) (120 hr ErC₅₀, Static)

2.0

Raphidocelis subcapitata as milbemectin

Moderate

Algae - Chronic (growth rate, fresh; mg l⁻¹)

Mesocosm study data

NOEAEC mg l⁻¹

Marine bivalves

HUMAN HEALTH AND PROTECTION

General

Property

Value

Source; quality score; and other information

Interpretation

Threshold of Toxicological Concern (Cramer Class)

High (class III)

Mammals - Acute oral LD₅₀ (mg kg⁻¹)

456

Rat as milbemectin

Moderate

Mammals - Dermal LD₅₀ (mg kg⁻¹ body weight)

2000

Rat

Mammals - Inhalation LC₅₀ (mg l⁻¹)

1.90

Rat 4 hr (whole body)

Other Mammal toxicity endpoints

ADI - Acceptable Daily Intake (mg kg⁻¹ bw day⁻¹)

0.07

Rat SF=100 as milbemectin

ARfD - Acute Reference Dose (mg kg⁻¹ bw day⁻¹)

0.03

Dog SF=100 as milbemectin

AAOEL - Acute Acceptable Operator Exposure Level (mg kg⁻¹ bw day⁻¹)

0.014

Dog SF=100 as milbemectin

AOEL - Acceptable Operator Exposure Level - Systemic (mg kg⁻¹ bw day⁻¹)

0.014

Dog SF=100 as milbemectin

Dermal penetration studies (%)

2-19

concentration dependent as milbemectin

Dangerous Substances Directive 76/464

Exposure Routes

Public

No unacceptable risks identified

Occupational

No unacceptable risks identified when using PPC/PPE

Mammalian dose elimination route and rate

Extensively metabolised and elimination is mainly faecal (85-100%) with some urinary: (5-9) % in 24 hrs

Health issues

Specific human health issues

Carcinogen

Genotoxic

Endocrine disruptor

; ; ; ;

No data found

Reproduction / development effects

Acetyl cholinesterase inhibitor

Neurotoxicant

Respiratory tract irritant

Skin irritant

Skin sensitiser

Eye irritant

Phototoxicant

No data found

General human health issues

Possible blood, liver, kidney, adrenals and uterus toxicant especially in vulnerable groups
Possible CNS toxicant
CLP data - suspected carcinogen

Handling issues

Property

Value and interpretation

General

Not explosive or oxidising
Not expected to auto-ignite; Not highly flammable

CLP classification 2013

Health: H302, H332, H351, H361d, H373
Environment: H400, H410

WHO Classification

Not listed (Not listed)

UN Number

Waste disposal & packaging

Shelf-life, storage, stability and reactivity

TRANSLATIONS

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Name

English

milbemycin A3

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Record last updated:	28/08/2025
Contact:	aeru@herts.ac.uk
Please cite as:	Lewis, K.A., Tzilivakis, J., Warner, D. and Green, A. (2016) An international database for pesticide risk assessments and management. Human and Ecological Risk Assessment: An International Journal, 22(4), 1050-1064. DOI: 10.1080/10807039.2015.1133242