HERBICIDES:

Diphenyl Ethers

And

Developmental Toxicity

DIPHENYL ETHER HERBICIDES• Contact herbicides

– Readily absorbed by roots, leaves– Limited translocation– Pre-emergence or early post-emergence

• Uses– Control broadleaf weeds, grasses

• Soybeans, legumes• Rice

– Post-emergence on• Soybeans, wheat, barley, sugar beets

Cl or other sub-stituents required

Mode of Action

• Inhibit photosynthesis– Inhibit electron transport – Inhibit coupled photophosphorylation?– Some auxin-like action– Inhibit protoporphyrinogen oxidase

• Last common enzyme in synthesis of chlorophyll and hemoglobin

– (some diphenyl ethers may affect hemoglobin production , as shown by gray color of treated mice)

Acifluorfen

• Uses• Water solubility• Persistence

– Not very great• Toxicology

– LD50 po in rats• EPA: 1,300 mg/kg• UI Extension: 3,300 mg/kg

Oxyfluorfen• Water solubility: 0.1 ppm

– Bioaccumulation quite probable under normal use

– Herbicidal activity– 2-4 months (in medium-textured IL soil)

• Pre- and post-emergent herbicide– Broad range of crops

• Most uses cancelled in 1982

Oxyfluorfen toxicology

• Acute– LD50, po, in rats: 5,000 mg/kg

• Delayed effects– Probable mutagen– Probable carcinogen

• Contaminated with perchloroethylene

– Probable hepatotoxicant– Probable thyrotoxicant– Develomental toxicity data inadequate

Bifenox• Uses:• Paddy rice• Pre-emergent

– Corn, soybeans• Toxicity

– LD50, po, in rats:– UI Extension: 1,630– EPA: 6,400 mg/kg

• Delayed toxicities– Carcinogen??

• Ecotoxicology– Benign

Bifenox

Nitrofen

The 2nd group on the nitrophenyl ring of bifenox acts as a degradophore

Brief History of Nitrofen

• 1966: First registered in U.S.• 1971: Ambrose et al: Neonatal mortality at 100 ppm in

maternal diet• 1974: Kimbrough et al, Arch. Environ. Health: Neonatal

mortality confirmed• 1981: Costlow and Manson: Heart and lung defects

identified• 1981: Withdrawn from all U.S. uses

Toxicity of Nitrofen in Rats• Adult toxicity

– LD50 > 1 g/kg– Adverse effect at LOAEL: liver enlargement

• Fetal toxicity– NOAEL: < 0.1 mg/kg/day– Adverse effect at LOAEL: diaphragmatic

hernias– Other: heart, lung, kidney defects; cleft palate.

Reproductive Cycle

Human Development: Weeks 3 to 8

3 4 5

6 7 8

Protocol for 2-Generation AssayF0: parental animals

1st mating

F1A

Necropsy at weaning

2nd mating

F1B

Select F1 parental animals

F2BF2A

Necropsy at weaning

Necropsy F2B at weaning; including complete histopathology

Continue feeding chemical to each group at the appropriate dosing level throughout the study (progeny, too!)

Necropsy parents [F0 and F1] aftertheir 2nd litter is weaned.

Advantages of the 2-Generation Assay

• A single assay identifies:

– acute or cumulative toxicity leading to• male or female infertility, • pre- and post-natal mortality,• pre- and post-natal growth retardation,• functional deficits in offspring

– transplacental carcinogenesis– infertility– behavioral anomalies

The 2-Generation Assay: Disadvantages

• Cost– well over $500,000 per species

• but still cheaper than the alternatives…

• Labor-intensive– Necropsies of all parents– Necropsies of offspring of all litters– Histopathology of offspring from 2nd litters

• Identifies the existence of a problem, but not necessarily its nature

Advantages of the 2-Generation Assay

• A single assay identifies:

– acute or cumulative toxicity leading to• male or female infertility, • pre- and post-natal mortality,• pre- and post-natal growth retardation,• functional deficits in offspring

– transplacental carcinogenesis– infertility– behavioral anomalies

The 2-Generation Assay: Disadvantages

• Cost– well over $500,000 per species

• but still cheaper than the alternatives…

• Labor-intensive– Necropsies of all parents– Necropsies of offspring of all litters– Histopathology of offspring from 2nd litters

• Identifies the existence of a problem, but not necessarily its nature

Chemically induced birth defetcs

• Vary with– Chemical– Genotype

• Of dam and of embryo• Within and between species

– Developmental stage at time of exposure– Dose

• Either severity of defects or probability of defect increases with increasing dose

“Karnofsky’s Law”

• Any chemical, given at the right time, and at the right dose, to the right species will cause malformations

• The fact that a pesticide causes malformations in one species - especially at high doses - is not necessarily enough reason to ban it.

• In the case of nitrofen, malformations occurred in both rats and mice, but not in rabbits.

Developmental Toxicity is a Threshold Phenomenon

• For agents other than mutagens, there is a minimum dose that will not affect the embryo– because it is metabolized by the dam and does

not reach the embryo, and/or– it does not significantly perturb embryonic

development, and/or– compensatory mechanisms result in repair of

the damage.

Nitrofen

• Causes malformations– In both rats and mice

• Heart• Kidneys• Diaphragm• Eyes

– At a fraction of the adult LD50 • NOAEL in rats estimated at 0.03 mg/kg/day