Anticoagulant Rodenticides - Organic Pollutants: An Ecotoxicological Perspective - page 228

11.6.2 p o p u l a T i o n g e n e T i c s
Resistance to warfarin has developed in populations of rats after repeated expo-
sure to the rodenticide (Thijssen 1995). One resistant strain discovered in Wales
was found to have a much reduced capacity to bind warfarin to liver microsomes,
in comparison to susceptible rats. Resistance was due to a gene that encoded for a
form of vitamin K epoxide reductase that was far less sensitive to warfarin inhibition
than the form in susceptible rats. Another resistant strain, which arose in the area of
Glasgow, was found to differ from the resistant Welsh strain. Resistance was again
due to an altered form of vitamin K epoxide reductase. However, the strain from the
Glasgow area contained a form of the enzyme that bound warfarin just as strongly
as that from susceptible rats. The difference was that the binding was readily revers-
ible (Thijssen 1995). Recent work has established that a number of mutations of the
VKORC1 of vitamin K epoxide reductase are associated with resistance in rodents
(Pelz et al. 2005). Different mutations of VKOCR1 could explain the contrasting
warfarin-binding properties in the two resistant strains mentioned earlier.
Much of the known resistance to warfarin and related ARs in rodents has been
attributed to mutant forms of vitamin K epoxide reductase, but a few strains show evi-
dence of a metabolic mechanism. For example, a resistant strain of rats from the area
of Andover, United Kingdom, appears to owe its resistance to enhanced detoxication
by a P450-based monooxygenase. It is known that CYP2C9, the principal cytochrome
P450 form concerned with the hydroxylation of warfarin, exists in different forms
with differing catalytic properties. Indeed, there is evidence that it is differences in
these forms that underlie corresponding differences between individual humans in
their sensitivity to this anticoagulant (Aithal et al. 1999). Thus, it seems clear that
some warfarin resistance is due to relatively rapid hydroxylation by monooxygenase.
There is also evidence from a Danish study suggesting that cytochrome-P450-me-
diated detoxication may contribute to bromodiolone resistance in one strain of rats. In
comparison with susceptible rats, resistant ones showed overexpression of the genes
CYP2e1, CYP2c13, CYP3a2, and CYP3a3 (Markussen et al. 2008). Although these
results strongly suggest the involvement of cytochrome P450 forms in conferring resis-
tance, the role of these forms in metabolism of bromodiolone has not been clarified.
11.7 Summ ary
Warfarin and the second-generation superwarfarins are ARs that have a structural
resemblance to dicoumarol and vitamin K. They act as vitamin K antagonists,
thereby retarding or stopping the carboxylation of clotting proteins in the hepatic
endoplasmic reticulum. The buildup of nonfunctional, undercarboxylated clotting
proteins in the blood leads eventually to death by hemorrhaging.
Brodifacoum, difenacoum, flocoumafen, and other superwarfarins bind strongly
to proteins of the hepatic endoplasmic reticulum and consequently have long half-
lives in vertebrates, often exceeding 100 days. Thus, they present a hazard to preda-
tors and scavengers that feed on rodents which have been exposed to superwarfarins.
A number of species of predatory and scavenging birds have died as a consequence