Lepimectin

Lepimectin	Last updated: 12/09/2025
(Not known by any other names)

GENERAL INFORMATION

Description

A veterinary antiparasitic agent that targets biting and sucking parasites

Examples of veterinary uses

Mites; Fleas; Ticks

Examples of species treated

Cattle; Sheep; Goats; Horses; Dogs; Cats

Approval status

VMR 2013/2033 approval status (GB/UK)

Approved - usually supplied as a prescription only medicine to be authorised by a veterinarian (POM-V)

EU Regulatory approval status

Approved

Chemical structure

Isomerism

Lepimectin exhibits structural isomerism through its existence as a mixture of two closely related compounds: Lepimectin A3 and Lepimectin A4. These isomers differ subtly in their methyl and ethyl substitutions on the milbemycin backbone, which affects their molecular weight and biological activity. Both isomers share the same core macrocyclic lactone structure vary in the side chain configuration, particularly at the C-25 position, where one has a methyl group and the other an ethyl group. This difference classifies them as constitutional isomers, not stereoisomers, because the connectivity of atoms changes rather than their spatial arrangement. The mixture is typically composed of 80–100% Lepimectin A3 and 0–20% Lepimectin A4.

Chemical formula

C₈₁H₁₀₄N₂O₂₀

Canonical SMILES

Isomeric SMILES

International Chemical Identifier key (InChIKey)

International Chemical Identifier (InChI)

2D structure diagram/image available?

Yes

General status

Veterinary substance type

Anti-parasitic

Substance groups

Milbemycin (mixture of ethyl & methyl derivatives)

Minimum active substance purity

Known relevant impurities

Substance origin

Natural

Mode of action

Glutamate-gated chloride channel (GluCl) allosteric modulator.

Molecular targets

[Glutamate-gated chloride channel (GluCl); Allosteric modulator]

CAS RN

171249-05-1

EC number

CIPAC number

US EPA chemical code

PubChem CID

Therapeutic Class

Antiparasitic agents

ATCvet Code

None allocated

Controlled Drug?

Regulation 37/2010 MRL Classification

Molecular mass

1425.69

PIN (Preferred Identification Name)

mixture of 80 to 100% (2aE,4E,8E)-(5'S,6S,6'R,7R,11R,13R,15S,17aR,20R,20aR,20bS)-6'-ethyl-3',4',5',6,6',7,10,11,14,15,17a,20,20a,20b-tetradecahydro-20,20b-dihydroxy-5',6,8,19-tetramethyl-17-oxospiro[11,15-methano-2H,13H,17H-furo[4,3,2-pq][2,6]benzodioxacyclooctadecin-13,2'-[2H]pyran]-7-yl (Z)-2-methoxyimino-2-phenylacetate and 20–0% (2aE,4E,8E)-(5'S,6S,6'R,7R,11R,13R,15S,17aR,20R,20aR,20bS)-3',4',5',6,6',7,10,11,14,15,17a,20,20a,20b-tetradecahydro-20,20b-dihydroxy-5',6,6',8,19-pentamethyl-17-oxospiro(11,15-methano-2H13H,17H-furo(4,3,2-pq)(2,6)benzodioxacyclooctadecin-13,2'-(2H)pyran)-7-yl (Z)-2-methoxyimino-2-phenylacetate

IUPAC name

mixture of 80 to 100% (2aE,4E,8E)-(5'S,6S,6'R,7R,11R,13R,15S,17aR,20R,20aR,20bS)-6'-ethyl-3',4',5',6,6',7,10,11,14,15,17a,20,20a,20b-tetradecahydro-20,20b-dihydroxy-5',6,8,19-tetramethyl-17-oxospiro[11,15-methano-2H,13H,17H-furo[4,3,2-pq][2,6]benzodioxacyclooctadecin-13,2'-[2H]pyran]-7-yl (Z)-2-methoxyimino-2-phenylacetate and 20–0% (2aE,4E,8E)-(5'S,6S,6'R,7R,11R,13R,15S,17aR,20R,20aR,20bS)-3',4',5',6,6',7,10,11,14,15,17a,20,20a,20b-tetradecahydro-20,20b-dihydroxy-5',6,6',8,19-pentamethyl-17-oxospiro[11,15-methano-2H,13H,17H-furo(4,3,2-pq)(2,6)benzodioxacyclooctadecin-13,2'-(2H)pyran)-7-yl (Z)-2-methoxyimino-2-phenylacetate

CAS name

(6R,13R,25R)-5-O-demethyl-28-deoxy-6,28-epoxy-13-((Z)-((methoxyimino)phenylacetyl)oxy)-25-methylmilbemycin B mixture with (6R,13R,25R)-5-O-demethyl-28-deoxy-6,28-epoxy-25-ethyl-13-((Z)-((methoxyimino)phenylacetyl)oxy)milbemycin B

Forever chemical

Other status information

Relevant Environmental Water Quality Standards

Physical state

Off-white coloured solid

Commercial

Property

Value

Availability status

Current

Introduction & key dates

1972, first isolated

Example manufacturers & suppliers of products using this active now or historically

Mitsui Chemicals, Inc

Example products using this active

Aniki

Formulation and application details

Usually formulated as an emulsifiable concentrate

Commercial production

Lepimectin is commercially produced through a semi-synthetic process that begins with fermentation of Streptomyces species to yield milbemycin derivatives, specifically 15-hydroxy-5-ketomilbemycin A3 and A4. These intermediates are then chemically modified by coupling with methoxyiminophenylacetic acid, followed by reduction using agents like sodium borohydride to produce lepimectin A3 and A4, the active components of the final product

Impact on climate of production and use

As microbial-based products tend to use fermentation-based production processes rather than chemical synthesis, they typically have a lower fossil fuel input in formulation and active ingredient creation, and also have reduced downstream emissions due to biodegradability and minimal soil disruption, their life-cycle GHG emissions are expected to be low. Whilst hard and precise data is not available, broad estimates suggest that typically emissions are likely to be below 5 kg CO₂e/kg.

ENVIRONMENTAL FATE

Property

Value

Source; quality score; and other information

Interpretation

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

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

Melting point (°C)

Boiling point (°C)

Degradation point (°C)

Flashpoint (°C)

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

Log P

Fat solubility of residues

Solubility

Data type

Density (g ml⁻¹)

Dissociation constant pKa) at 25 °C

Vapour pressure at 20 °C (mPa)

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

Volatilisation as max % of applied dose lost

From plant surface

From soil surface

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

Surface tension (mN m⁻¹)

Refractive Index

Environmental release

Degradation

Property

Value

Source; quality score; and other information

Interpretation

Soil degradation (days) (aerobic)

DT₅₀ (typical)

DT₅₀ (lab at 20 °C)

DT₅₀ (field)

DT₉₀ (lab at 20 °C)

DT₉₀ (field)

Note

Manure DT₅₀ (days)

Aqueous photolysis DT₅₀ (days) at pH 7

Value

Note

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

Value

Note

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)

K_foc (mL g⁻¹)

¹/_n

Notes and range

pH sensitivity

Fate indices

Property

Value

Source; quality score; and other information

Interpretation

GUS leaching potential index

Bio-concentration factor

BCF (l kg⁻¹)

CT₅₀ (days)

Known metabolites

None

ECOTOXICOLOGY

Terrestrial ecotoxicology

Property

Value

Source; quality score; and other information

Interpretation

Mammals - Acute oral LD₅₀ (mg kg⁻¹)

> 2000

Rat

Low

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)

Birds - Short term dietary (LC₅₀/LD₅₀)

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

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

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

Soil micro-organisms

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)

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

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)

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

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)

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

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

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

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

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)

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)

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)

> 2000

Rat

Low

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

Mammals - Inhalation LC₅₀ (mg l⁻¹)

Other Mammal toxicity endpoints

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

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

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

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

Dermal penetration studies (%)

Dangerous Substances Directive 76/464

Exposure Routes

Public

Occupational

Mammalian dose elimination route and rate

Usually eliminated via hepatic metabolism, followed by biliary and fecal excretion

Health issues

Specific human health issues (hazard-based)

Carcinogen

Genotoxic

Endocrine disruptor

No data found

; ; ; ;

No data found

Reproduction / development effects

Acetyl cholinesterase inhibitor

Neurotoxicant

No data found

Respiratory tract irritant

Skin irritant

Skin sensitiser

No data found

Eye irritant

Phototoxicant

No data found

General human health issues

No information available

Handling issues

Property

Value and interpretation

General

No information available

CLP classification 2013

WHO Classification

Not listed (Not listed)

UN Number

Waste disposal & packaging

Shelf-life, storage, stability and reactivity

TRANSLATIONS

Language

Name

English

lepimectin

French

lepimectine

German

Danish

Italian

Spanish

Greek

Polish

Swedish

Hungarian

Dutch

Norwegian

Record last updated:	12/09/2025
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