Ew. Vogel et al., DNA-DAMAGE AND REPAIR IN MUTAGENESIS AND CARCINOGENESIS - IMPLICATIONS OF STRUCTURE-ACTIVITY-RELATIONSHIPS FOR CROSS-SPECIES EXTRAPOLATION, Mutation research, 353(1-2), 1996, pp. 177-218
Previous studies on structure-activity relationships (SARs) between ty
pes of DNA modifications and tumour incidence revealed linear positive
relationships between the log TD50 estimates and s-values for a serie
s of mostly monofunctional alkylating agents. The overall objective of
this STEP project was to further elucidate the mechanistic principles
underlying these correlations, because detailed knowledge on mechanis
ms underlying the formation of genotoxic damage is an absolute necessi
ty for establishing guidance values for exposures to genotoxic agents.
The analysis included: (1) the re-calculation and further extension o
f TD50 values in mmol/kg body weight for chemicals carcinogenic in rod
ents. This part further included the checking up data for Swain-Scott
s-values and the use of the covalent binding index (CBI); (2) the elab
oration of genetic toxicity including an analysis of induced mutation
spectra in specific genes at the DNA level, i.e., the vermilion gene o
f Drosophila, a plasmid system (pX2 assay) and the HPRT gene in cultur
ed mammalian cells (CHO-9); and (3) the measurement of specific DNA al
kylation adducts in animal models (mouse, rat, hamster) and mammalian
cells in culture. The analysis of mechanisms controlling the expressio
n of mammalian DNA repair genes (alkyltransferases, glycosylases) as a
function of the cell type, differentiation stage, and cellular microe
nvironment in mammalian cells. The 3 classes of genotoxic carcinogens
selected for the project were: (1) chemicals forming monoalkyl adducts
upon interaction with DNA; (2) genotoxins capable of forming DNA ethe
no-adducts; and (3) N-substituted aryl compounds forming covalent addu
cts at the C8 position of guanine in DNA. In general, clear SARs and A
ARs (activity-activity relationships) between physiochemical parameter
s (s-values, O-6/N7-alkylguanine ratios, CBI), carcinogenic potency in
rodents and several descriptors of genotoxic activity in germ cells (
mouse, Drosophila) became apparent when the following descriptors were
used: TD50 estimates (lifetime doses expressed in mg/kg b.wt. or mmol
/kg b.wt.) from cancer bioassays in rodents; the degree of germ-cell s
pecificity, i.e., the ability of a genotoxic agent to induce mutations
in practically all cell stages of the male germ-cell cycle of Drosoph
ila (this project) and the mouse (literature search), as opposed to a
more specific response in postmeiotic stages of both species; the M(ex
r-)/M(exr+) hypermutability ratio, determined in a repair assay utiliz
ing Drosophila germ cells; mutation spectra induced at single loci (th
e 7 loci used in the specific-locus test of the mouse (published data)
, and the vermilion gene of Drosophila); and doubling doses (DD) in mg
/kg (mmol/kg) for specific locus test results on mice. By and large, t
he TD50 values, the inverse of which can be considered as measures of
carcinogenic potency, were shown to be predictable from knowledge of t
he in vivo doses associated with the absorbed amounts of the investiga
ted alkylators and with the second-order constant, k(c), reaction at a
critical nucleophilic strength, n(c). For alkylating agents k(c) can
be expressed as the second-order rate constant for hydrolysis, k(H2O),
and the substrate constant s:k(H2O)TD(50) is a function of a certain
accumulated degree of alkylation, here given as the (average) daily in
crement a(c) for 2 years exposure of the rodents. The T-50 in mmol/kg
X day) could then be written: T-fsD(50) = a(c) X lambda/k(H2O) X 10(
n)c . s</SUP> This expression would be valid for monofunctional alkyla
tors provided the reactive species are uncharged. This is the case for
most S(N)2 reagents. Although it appears possible to predict carcinog
enic potency from measured in vivo doses and from detailed knowledge o
f reaction-kinetic parameter values, it is at present not possible to
quantify the uncertainty of such predictions. One main reason for this
is the complication due to uneven distribution in the body, with effe
cts on the dose in target tissues. The estimation can be improved by c
onsidering the distribution of dose in the body. In the present analys
is, direct-acting alkylators were studied. It is evident that dosimetr
y and reaction-kinetic characterization of genotoxic metabolites would
render it possible to predict the potency of precarcinogens as well.