Summary: Investigation of Age-Dependent 

Pharmacodynamic Sensitivity to 

Pyrethroids

Larry Sheets, Bayer CropScience

on behalf of

The Council for the Advancement of Pyrethroid 

Human Risk Assessment (CAPHRA) 
1

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Pyrethroid Human Risk Assessment, LLC (CAPHRA)
In Vivo Rat: Acoustic Startle Response and 

Detailed Clinical Observations

• Working hypothesis

– If there is no age-related PD difference in sensitivity in rats, we would 

expect to see comparable effects (ASR and neurological signs) at the 

same internal dose (PYR concentration in the brain).

• Results: Support hypothesis & previous results

– ASR: Comparable effect in PND 15 and adult rats at “low” doses

– DCO: PND 15 more sensitive than adults to higher doses (>2-4 mg/kg)

– Sensitivity of PND 15 rats associated with higher brain tissue levels

– Permethrin: ASR is a less sensitive assay in PND 15 than adult rats; 

Sensitivity to tremor is the same at both ages

• Assessment: In vivo relevance but relatively nonspecific

– Requires considerably more animals and time to investigate, with less 

certainty regarding the neurobiologic basis for comparison (AOP)

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In Vivo Rat: Acoustic Startle Response and 

Detailed Clinical Observations

• AOP: In vivo relevance but least specific

– Functional assessment of CNS & PNS

– PK and PD elements

– Complex of channels involved: Na+, Ca2+, Cl-

• PD sensitivity based on neurobehavior

– Results consistent with previous findings

• No difference in age-dependent sensitivity at “low” dose

• Acute neurotoxic signs in young at higher doses (2-4 mg/kg)

– NOAEL for PND 15 (~1 mg/kg) comparable to adult (1 or <2 mg/kg)

– Brain tissue levels of deltamethrin are higher in PND 15 rat than adult 

at given effect level

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1 3 5 7 9 11 15 19 23 27 31 35 39 43 47 51 55 59 63 0

200

400

600

800

1000

1200

TIME (msec)

A/D Units

Peak

Onset
Adult Male SD Rats PND 15 SD Rats (Vorhees)

Dose (mg/kg)* ASR FOB / DCO / MA Brain Conc. 

(ng/ml)

Dose 

(mg/kg) ASR FOB / DCO Brain Conc. (ng/ml)

1 (Wolansky 2006) Threshold effect for MA (0.99 mg/kg) 1 Equivocal No effect 27 (4 h)

2 -41% (2 h)

-6% (4 h) No effect 9 (2 h)14 (4 h) 2 -28% Impaired mobility (87%), tremor (17%) 48 (4 h)

-- -- -- 3 -37%

Impaired mobility (93%), tremor 

(27%), salivation (33%),

15% mortality

--

4 -44% (2 h)

-31% (4 h) No effect

12 (2 h)

21 (4 h)

28 (6 h)

4 -59%

Impaired mobility (100%), tremor 

(100%), salivation (73%),

6.7% mortality

110 (4 h)

5 (WIL/Vorhees) -- No effect (FOB or MA; ASR) 28 (2 h)

20 (4 h) 5 -78%

Impaired mobility (100%), tremor 

(100%), salivation (93%)

15% mortality

--

6 --

Impaired mobility (100%),tremor (-), 

salivation (100%)

73% mortality

--

8 -48% (2 h)

-31% (4 h) No effect 18 (2 h)

33 (6 h) 8 -- 100% mortality --

12.5 (PWG) -- Salivation (10%) & splayed hind limbs 

(20%) --

15 (WIL) -- Salivation, impaired mobility, 

decreased MA --

25 (PWG) -- Salivation (50%), flattened posture 

(10%), splayed hind limbs (30%) --

25 (Vorhees) -- Impaired mobility (75%), tremor 

(25%), salivation (75%); decreased ASR

44 (2 h)

153 (4 h)

35 (PWG) --

Convulsions (30%), salivation (10%), 

flattened posture (60%), splayed hind 

limbs (70%)

--

35 (Vorhees) -- Impaired mobility (100%), tremor 

(12%), salivation (88%), --

50 (WIL) -- Convulsions, writhing, salivation, 

impaired mobility & 1 death --

55-75 (Vorhees) -- 1 of 8 (12.5%) mortality

95 (Aventis) -- LD50 --

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4

Overview: In Vivo Rat (Deltamethrin)

Wolansky et al., 2006 (MA relative potency)

WIL = Acute Neurotoxicity Study (OPPTS 870.6200)

PWG = FOB comparison 12 PYR (Weiner et al., 2009)

Vorhees = CAPHRA-sponsored research ASR & DCO

Aventis = Acute oral LD50 (adult male rats)

NOAEL ~1 mg/kg in PND15 & adult

PND 15 rat has higher brain levels of 

DLM than adult at a given effect level
Isolated Human VGSCs

Working Hypothesis 

• If there is no age-dependent PD sensitivity, VGSCs expressed in children will 

not express greater sensitivity than in adults.

Results (7 core actives)

• The VGSC most prevalent as early as 1-2 y/o (Nav 1.1) is less sensitive to PYR 

than the VGSC most prevalent at birth (Nav 1.3);

• These results confirm reports hVGSCs are less sensitive than rat VGSCs & the 

response of Nav 1.1 vs. Nav 1.3 favors children being less sensitive than adults.

Assessment

• Uncertain how the response of single cloned channels compares to in vivo 

sensitivity (lacking elements that could impact their response). 

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Rat Neurolemma

• Working Hypothesis

– If there is no age-dependent PD sensitivity, VGSCs from PND 15 rat 

neurolemma will not express greater sensitivity than VGSCs from adults

• Results (deltamethrin and permethrin)

– The VGSCs expressed in PND 15 rats are not more sensitive to 

deltamethrin or permethrin than those expressed in adult rats

• Assessment

– This system lends itself well to evaluate age-dependent sensitivity of 

VGSCs at defined stages of development

– We propose to use this system to evaluate the remaining five core 

actives for age-dependent pharmacodynamic sensitivity.

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Overview: Pharmacodynamic Sensitivity

• In Vivo Rat (DLM & PRM): Acoustic Startle Response & DCOs

– Adult: Confirmed differential Type I and Type II effect on the ASR and 

sensitivity as a measure of acute neurotoxicity;

– PND 15: Overlap of clinical signs with effects on ASR

• DLM: Combination ASR & DCO suited to compare PND 15 and adult; 

• PRM: Only modest increase ASR at 120 mg/kg (tremor PND 15 & 90)

– Adult & PND 15 (DLM): NOAEL ~1 mg/kg at both ages, with higher brain 

conc. at a given effect level in the PND 15 rat

• Isolated Human VGSC (7 core actives)

– Human channels are less sensitive than rat VGSC;

– The channel most prevalent in juveniles aged 1-2 years (and most likely, 

adults) is less sensitive than the isoform more prevalent at birth

• Rat Neurolemma (DLM & PRM)

– Relative sensitivity of VGSCs from PND 15 and 90 rats is best suited for 

comparison of relative sensitivity at defined stages of development.

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Conclusions: Pharmacodynamic Sensitivity

• Results to date: Consistent with Sheets et al. (1998; 2000) & 

Meacham et al. (2008), which indicate:

– The age-dependent increased sensitivity to Type II PYR observed in 

young rats is likely due to PK factors; and

– Children >6 mo would not be more sensitive than adults to PYR due to 

PD factors.

• Future Plans (proposed):

– In vivo rat: Finish the studies with DLM and PRM (PND 15 & adult rats)

– Isolated human channels (7 core actives): No additional work

– Neurolemma (DLM and PRM): Evaluate the remaining five core actives

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Pyrethroid Human Risk Assessment, LLC (CAPHRA) 8