This document discusses rodent tumors that are of questionable relevance to human cancer risk. It notes several types of tumors seen in rodents that are due to species-specific modes of action not relevant to humans, such as renal tumors from α-2u-globulin nephropathy in male rats or mesothelial tumors from β2 receptor agonists in rats. It also lists tumors found in unique rodent tissues like the Harderian gland that are absent in humans. Many of these rodent-specific tumors found in 2-year cancer studies were associated with genotoxic chemicals or irritants and are not predictive of human cancer risk.

1. Xenobiotic-induced Rodent
Tumors of Questionable
Relevance to Human Cancer Risk
R. R. Maronpot
Maronpot@me.com
Photomicrographs courtesy of the National Toxicology Program (http://ntp.niehs.nih.gov)

2. Rodent Tumors of Questionable
Relevance
Rodent-Specific Mode of Action
• Renal tumors secondary to α-2u-
globulin nephropathy
• Urinary bladder tumors
secondary to irritation
• Mesovarian leiomyomas
secondary to β2 receptor agonists
• Carcinoids secondary to H2
antagonists
• Thyroid follicular tumors
• Uterine endometrial carcinoma
secondary to dopamine agonists
• Leydig cell tumors in rats
secondary to hormonal
perturbations
Tumors in Unique Rodent Tissues
• Harderian Gland
• Preputial Gland
• Clitoral Gland
• Zymbal’s Gland
• Forestomach

3. Α-2u globulin nephropathy
Proposed key events
Chemical exposure and binding to α-2u globulin

Accumulation of the bound α-2u in renal tubular
epithelial cells

Cytotoxicity to renal tubular epithelial cells

Compensatory renal tubular cell proliferation

Development of renal tumors

4. Α-2u globulin nephropathy

5. Renal Tubular Adenoma

6. Α-2u globulin nephropathy
• Animals affected
– Mature male rat under influence of testosterone produces
large amount of α-2u globulin
• Xenobiotics associated with α-2u globulin nephropathy
– JP-4 & other jet fuels hexachloroethane
– Isophorone d-limonene
– Parachlorobenzene Pentachloroethane
– Unleaded gasoline Nitrotoluene
• Unaffected mammals
– Female rats Male and female mice Humans
– Male NCI-Black-Reiter rat

7. Urinary Bladder Tumors
• Secondary to irritation
– Mechanical irritation from foreign bodies or
calculi
• Affects rats and mice
– Threshold effect – only seen at doses of chemical
that produce calculi
• Agents associated with urinary bladder
tumors
– Uracil Cyclamate
– Chronic irritation

8. Urinary Bladder Papilloma

9. Mesovarian Leiomyomas in Rats
• Seen following administration of β2-receptor
adrenergic stimulants
– Proliferation of mesovarian smooth muscle
– Propanolol (adrenergic blocker) prevents formation of
this tumor
• Xenobiotics causing this effect
Salbutamol Terbutaline
• Specific to the rat
– Not seen in mice or hamsters
– Not seen in humans with long history of using β2-
stimulants

10. Carcinoids (Neuroendocrine Tumors)
in the Rodent Stomach
• Associated with prolonged blockage of acid secretion
– Seen for powerful and long-lasting H2-receptor antagonists
& proton-pump inhibitors given at high doses
– SK&F93479 Omeprazole Butachlor Methyleugenol
• Leads to increased gastrin levels
– Stimulation of neuroendocrine cells leading to hyperplasia
and neoplasia
– Slight hypergastrinemia in humans not associated with
hyperplasia or neoplasia of neuroendocrine cells
• Seen at high doses in rodents
– Humans have low serum gastrin levels (margin of safety)

11. Neuroendocrine Tumor
(Carcinoid)

12. Thyroid Follicular Cell Neoplasms
• Caused by altered feedback mechanism resulting in
prolonged stimulation of the thyroid by TSH
• Rats vs. humans lack high-affinity thyroxin-binding globulin,
are more sensitive to inhibition of thyroid peroxidase, are
more sensitive to hepatic enzyme induction
• Categories of altered feedback mechanism
– Interference with iodine uptake & thyroid hormone synthesis or
secretion
• sulfonamides
– Interference with metabolism of T4 or T3
• FD&C Red No. 3 (erythrosine)
– Increased metabolism & excretion of thyroid hormones
• Phenobarbital Polychlorinated biphenyls

13. Thyroid Follicular Adenoma

14. Uterine Endometrial Carcinomas in
Rats
• Seen in older rats associated with estrogen
dominance
• Permanent estrus and endometrial
stimulation
– Leads to endometrial carcinoma
• Seen in rats treated with dopamine agonists
– Bromocriptine
• Similar endocrine imbalance does not occur in
women

15. Uterine Endometrial Carcinoma

16. Leydig Cell Tumors in Rats
• Interference with Leydig cell control mechanisms
(7 different types of interference)
• All involve the hypothalamic-pituitary-testis axis
• Xenobiotics associated with rat Leydig cell tumors
– Cimetidine Finasteride Flutamide
– Gembibrozil Indomethacin DEHP
• Rats vs humans
– Rats have 14 times more LH receptors
– LHRH receptors unique to the rat

17. Leydig Cell Tumor

19. Rodent Tumors of Questionable
Relevance
Rodent-Specific Mode of Action
• Renal tumors secondary to α-2u-
globulin nephropathy
• Urinary bladder tumors
secondary to irritation
• Mesovarian leiomyomas
secondary to β2 receptor agonists
• Carcinoids secondary to H2
antagonists
• Thyroid follicular tumors
• Uterine endometrial carcinoma
secondary to dopamine agonists
• Leydig cell tumors in rats
secondary to hormonal
perturbations
Tumors in Unique Rodent Tissues
• Harderian Gland
• Preputial Gland
• Clitoral Gland
• Zymbal’s Gland
• Forestomach

20. 2-Year Oncogenicity Studies Evaluated
Pesticide database
(www.epa.gov/ncct/toxrefdb/)
257 Rat
246 Mouse
281 Total Rodent
1006 Sex/Species
National Toxicology Program database
(http://ntp.niehs.nih.gov/)
552 Total Rodent
~2200 Sex/Species

21. Pesticide Database
One study (1 of 281) has an increased incidence of
Harderian gland tumors in female mice and
forestomach tumors in male and female mice.
No other sex/species exposure groups in the 280
other 2-year oncogenicity studies had increased
incidences of these rodent only tumors.

22. NTP database
90 of the 552 studies available for review had
increased incidence of at least one of the list
of 5 rodent unique tissue tumors in at least
one sex/species.

23. NTP Database
Forestomach
Female rat 20; Male rat 24; Female mouse 34; Male mouse 29
Zymbal’s gland
Female rat 19; Male rat 22; Female mouse 4; Male mouse 3
Harderian gland
Male rat 1; Female mouse 15; Male mouse 16
Preputial gland Clitoral gland
Male rat 13; Male mouse 5 Female rat 16

24. NTP Database
In 11 cases there was an increased incidence of a single
rodent specific tumor in a single sex/species as the only
positive neoplastic finding.
Female rat: Clitoral gland 2; Zymbal’s gland 1
Male rat: Forestomach 1; Preputial gland 1
Female mouse: Forestomach 4
Male mouse: Forestomach 1
(90/552 positive studies)

25. Harderian gland
Described by Johann Jacob Harder in 1694
Present in terrestrial vertebrates – amphibians to
mammals
In mammals – not present in cattle, horses, carnivores,
higher primates including humans
Functional activities include (depending on species) –
lubrication, immune response, pheromone production,
saliva, osmoregulation, photoreception,
thermoregulation, elaboration of growth factors.
Has been shown to possess metabolic capability
– Cytochrome P450, lipid metabolism,
and porphyrin metabolism

26. Female mice
15 positive studies
In one case the only tumor in study.
9/15 involved genotoxic xenobiotics including the study with only
positive tumor site
Male mice
16 positive studies
In one case the only tumor in study
9/16 involved genotoxic xenobiotics including the study with only
positive tumor site
In the only tumor site cases in mice, both sexes had Harderian gland
tumors.
Male rat
Single study with Harderian gland tumor, multi-site genotoxic
carcinogen.
Harderian Gland Tumor

27. Harderian Gland Adenoma

28. Effect of Diet on Spontaneous
Harderian Gland Tumors
NIH-07
overall
incidence
NIH-2000
overall
incidence
Female
mouse
129/3286
3.9%
183/1448
12.6%
Male
mouse
175/3286
5.3%
223/1399
15.9%
Male
rat
1/3245
0.03%
1/1399
0.07%

29. Harderian Gland
• Multifunctional gland
• Metabolically active
• Spontaneous incidence influenced by diet
• Tumors typically seen in studies where other
tumors are associated with treatment
• Humans do not have Harderian glands

30. Clitoral glandPreputial gland
Paired Specialized sebaceous glands

31. Preputial Gland Tumors in NTP Studies
Male mice
5 positive studies
All are multisite carcinogens
4/5 are genotoxic
3/5 also had Zymbal’s gland tumors
4/5 had skin and/or Zymbal’s gland tumors
Male rat
13 positive studies
2 studies the only tumor in male rat, 1 case only tumor in study
5/13 are genotoxic
5/11 also had Zymbal’s gland tumors
4/11 had skin tumors
5/11 had skin and/or Zymbal’s gland tumors

32. Female rat
16 positive studies in NTP database
11/16 were genotoxic
4 studies only tumor in female rat,
2 cases only tumor in study
9/12 also had Zymbal’s gland tumors
5/12 also had skin tumors
10/12 had skin and/or Zymbal’s gland tumors
2/4 were positive in male rat preputial gland
Clitoral Gland Tumors in NTP Studies

33. Clitoral glandPreputial gland
NIH-07
overall
incidence
NIH-2000
overall
incidence
Male
mouse
0/3529
0%
2/1394
0.1%
Male
rat
274/3348
8.2%
70/1395
5.0%
NIH-07
overall
incidence
NIH-2000
overall
incidence
Female
rat
359/3259
11.0%
161/1347
12.0%

34. Preputial and Clitoral Glands
• Specialized sebaceous glands
• Typically associated with other cutaneous tumors
• In 7 studies both rat preputial (7/13) and clitoral
(7/16) gland tumors were present.
• Spontaneous tumor rates not affected by diet.
• Not present in humans.

35. Zymbal’s gland
Modified sebaceous glands along the base of the ear.
Possess cytochrome P450 metabolizing enzymes.
May be deficient in some phase II metabolizing enzymes
http://www.item.fraunhofer.de/reni

36. Mouse Zymbal’s gland Mouse skin sebaceous gland

37. Zymbal’s Gland Tumors
NTP Mouse Studies
Female mouse
4 positive studies all multisite carcinogens
2/4 were genotoxic carcinogens
Male mouse
3 positive studies all multisite carcinogens
All positive for preputial gland
2/3 were genotoxic carcinogens

38. Zymbal’s Gland Tumors
NTP Rat Studies
Female rat
19 studies positive
14/19 were genotoxic carcinogens
3 were the only female rat tumor in study
6/16 also had skin tumors
9/16 also had clitoral gland tumors
10/16 had clitoral gland and/or skin tumors
Male rat
22 studies positive
16/22 were genotoxic carcinogens
2 were the only male rat tumor in study
16/20 also had skin tumors
5/16 also had preputial gland tumors
16/16 had preputial gland and/or skin tumors

39. Zymbal’s Gland Carcinoma

40. Incidence of Zymbal’s gland tumors in Control Rodents
Fed NIH-07 Diet
NTP database
Derm
acetone
Derm
ethanol
Derm Gavage
corn oil
Gavage
methyl
cellulose
Gavage
water
Inhalation Feed Water Total and
%
Female
Rat
1/100 2/301 0/51 1/401 0/100 0/50 6/1052 6/1001 5/330 21/3386
0.6%
Male
Rat
1/100 4/302 0/50 2/402 1/101 0/50 15/1055 10/1004 6/331 39/3395
1.2%
Female
Mouse
0/150 0/302 nd 26/463
5.6%
0/100 0/51 1/1077 0/953 0/340 26/3436
0.7%
Male
Mouse
0/150 0/299 nd 23/464
5.0%
0/100 0/50 0/1074 0/952 0/340 24/3429
0.7%

41. Incidence of Zymbal’s gland tumors in Control Rodents
Fed NIH-2000 Diet
NTP database
Derm
ethanol
Gavage
corn oil
Gavage
methyl
cellulose
Gavage
water
Inhalation Feed Water Total and
%
Female
Rat
0/150 1/150 0/100 0/150 4/350 1/250 2/200 8/1350
Male
Rat
1/150 2/150 0/100 0/150 2/349 4/300 2/200 11/1399
Female
Mouse
0/150 0/148 0/100 0/150 0/400 1/300 0/200 1/1448
Male
Mouse
0/150 0/150 0/100 0/150 0/400 0/250 0/199 0/1399
FM
5.6%
MM
5.0%

42. Zymbal’s Gland Tumors
• Modified sebaceous gland
• Typically associated with other cutaneous tumors
• Metabolically active site
• Majority associated with genotoxic carcinogens
• Decreased diet-related spontaneous tumor rates in
mice

43. Forestomach Papilloma

44. Forestomach Tumors in NTP Mouse Studies
Female mouse
35 studies positive (from 90/552 studies)
22/35 genetox positive
17/35 irritants
28/35 genotoxic or irritant
16 only female mouse tumor
13/16 genetox positive or irritant
Male mouse
30 studies positive
20/30 genetox positive
16/30 irritants
25/30 genotoxic or irritant
8 only male mouse tumor
8/8 genetox positive or irritant

45. Female rat
20 studies positive (from 90/552 studies)
15/20 genetox positive
10/20 irritants
16/20 genotoxic and/or irritant
9 only female rat tumor
8/9 genotoxic or irritant
Male rat
24 studies positive
18/24 genetox positive
11/24 irritants
21/24 genotoxic and/or irritant
9 only male rat tumor
8/9 genotoxic or irritant
Forestomach Tumors in NTP Rat Studies

46. Forestomach Tumors by Route of Administration
Female mouse
21/35 Gavage studies
18/21 corn oil
3/21 water
7/35 Feed
Male mouse
19/30 Gavage studies
15/19 corn oil
3/19 water
1/19 methyl cellulose
5/30 Feed
Female rat
15/20 Gavage studies
10/15 corn oil
4/15 water
1/15 methyl cellulose
4/20 Feed
Male rat
15/24 Gavage studies
11/15 corn oil
4/15 water
8/24 Feed

47. Incidence of Forestomach tumors in Control Rodents
Fed NIH-07 Diet
NTP database
Derm
acetone
Derm
ethanol
Derm Gavage
corn oil
Gavage
methyl
cellulose
Gavage
water
Inhalation Feed Water Total and
%
Female
Rat
0/100 0/301 0/51 2/401 0/100 0/50 1/1052 2/1001 1/330 6/3386
0.2%
Male
Rat
1/100 0/302 2/50 2/402 0/101 0/50 1/1055 0/1004 0/331 6/3395
0.2%
Female
Mouse
2/150
1.3%
2/299
0.6%
nd 19/463
4.1%
1/100
1.0%
0/51
0%
10/1077
0.9%
16/953
1.7%
5/340
1.5%
55/3433
1.6%
Male
Mouse
2/150
1.3%
6/302
2.0%
nd 22/464
4.7%
0/100
0%
0/50
0%
7/1074
0.7%
14/952
1.5%
4/340
1.2%
55/3432
1.6%

48. Incidence of Forestomach tumors in Control Rodents
Fed 2000 Diet
NTP database
Derm
ethanol
Gavage
corn oil
Gavage
methyl
cellulose
Gavage
water
Inhalation Feed Water Total and
%
Female
Rat
0/150 1/150 1/100 1/150 0/350 1/250 1/200 5/1350
0.4%
Male
Rat
2/150 0/150 0/100 0/150 0/349 0/300 1/200 3/1399
0.2%
Female
Mouse
3/150
2.0%
4/148
2.7%
1/100
1.0%
3/150
2.0%
6/400
1.5%
5/300
1.7%
3/200
1.5%
25/1448
1.7%
Male
Mouse
1/150 1/150
0.7%
2/100 3/150 4/400 4/250 3/199 18/1399
1.3%
FM
4.1%
MM
4.7%

49. Forestomach Tumors
• Susceptible to local combination of chronic wounding (irritation)
and mutagenicity
• More likely to develop from direct exposure; more likely in gavage
studies
• Metabolically active site
• Decreased diet-related spontaneous tumor rate in mice
• In 8/9 and 9/9 cases of only tumor in male or female rats,
respectively, also tumor in opposite sex.
• In 7/8 and 12/16 cases of only tumor in male or female mice,
respectively, also tumor in opposite sex.

50. Site Concordance
Target organ concordance need not be a prerequisite for evaluating the
implications of animal study results for humans.
There is evidence that growth control mechanisms at the level of the cell
are homologous among mammals, but these mechanisms are not
necessarily site concordant.

51. Site Concordance
Target organ concordance need not be a prerequisite for evaluating the
implications of animal study results for humans.
There is evidence that growth control mechanisms at the level of the cell
are homologous among mammals, but these mechanisms are not
necessarily site concordant.
Tumors in Unique Rodent Tissues
• Harderian Gland
• Preputial Gland
• Clitoral Gland
• Zymbal’s Gland
• Forestomach
Concerns about the significance
of these to human cancer risk
can be addressed using the
mode of action/human relevance
framework approach.
<

52. Rodent Cancer Study Databases
Pesticide database
(www.epa.gov/ncct/toxrefdb/)
National Toxicology Program database
(http://ntp.niehs.nih.gov/)