Sm. Cohen et Lb. Ellwein, USE OF CELL-PROLIFERATION DATA IN MODELING URINARY-BLADDER CARCINOGENESIS, Environmental health perspectives, 101, 1993, pp. 111-113
A multistage, probabilistic, biologically based model of carcinogenesi
s has been developed involving qualitative and quantitative aspects of
the process. A chemical can affect the risk of cancer by directly dam
aging DNA and/or increasing the number of cell divisions during which
errors in DNA replication can occur. Based on this model, carcinogens
are classified as genotoxic versus nongenotoxic; nongenotoxic chemical
s are further divided on the basis of whether or not they act through
a specific cell receptor. Nongenotoxic compounds, particularly those a
cting through a nonreceptor mechanism, are likely to have dose and/or
species-specific thresholds. This classification also implies the exis
tence of chemicals that will be carcinogenic at high doses in animal m
odels, but because of dose and/or mechanistic considerations, will not
be carcinogenic to humans at levels of exposure. N-[4-(5-nitro-2-fury
l)-2-thiazolyl] formamide (FANFT) and 2-acetylaminofluorene (AAF) are
classical genotoxic bladder carcinogens that also cause proliferative
effects at higher doses. Although there is an apparent no-effect level
for the urinary bladder carcinogenicity of these two compounds at low
doses, in reality, DNA adducts form at these low levels, and it is li
kely that there is a cancer effect (no threshold), but it is below the
level of detection of the bioassay. These conclusions are based on st
udies involving multiple doses and time points in rodents, including r
esults from the ED(01). Pellets implanted directly into the rodent bla
dder lumen or calculi formed in the urine as a result of an administer
ed chemical cause abrasion of the urothelium, and a marked increase in
cell proliferation and cell number, and ultimately tumors. A threshol
d is readily definable based on physiologic, chemical, and pharmacokin
etic considerations. Sodium saccharin also produces bladder cancer at
high doses in rats, particularly males, if it is administered beginnin
g at birth or earlier. The mechanism appears to be related to the form
ation of a silicate precipitate and/or crystals formed in the rat urin
e, which act as abrasives or cytotoxic materials, leading to increased
cell proliferation and ultimately tumors. Numerous other sodium salts
have similar effects. This effect is not observed in the mouse, hamst
er, or monkey, and epidemiological evidence suggests that it does not
occur in humans. Thus, for sodium saccharin, even in the susceptible s
pecies, the rat, there appears to be a dose threshold, and extrapolati
on to humans appears inappropriate based on mechanistic considerations
.