Separation, characterisation and biological evaluation of the individual isomers of the rat selective toxicant norbormide – isolated using a chemical derivatization strategy

Abstract

Norbormide[5-(α-hydroxy-α-2-pyridylbenzyl)-7-(α-2-pyridylbenzylidene)-5-norbornene-2,3-dicarboximide] (NRB, 1), an existing but infrequently used rodenticide, is known to be uniquely toxic to rats but relatively harmless to other rodents/mammals. However, as an acute vasoactive, NRB has a rapid onset of action, often leading to sub-lethal uptake/bait shyness. Recently, it was brought to our attention that baits containing two independently sourced batches of NRB (which differed noticeably in their stereochemical composition) displayed markedly different palatability/efficacy profiles in rats. Accordingly, with a view to independently evaluating the individual isomers of NRB in rats by means of a palatability and efficacy bait trial, this research describes the isolation of the individual isomers of endo-NRB (Y, V, W and U) from the parent mixture, by means of a chemical derivatization strategy.

The Enigma of Norbormide, a Rattus-Selective Toxicant

“…Several pieces of evidence support the hypothesis that NRBs' lethal effect in rats is a consequence of diffuse vasoconstriction of the peripheral arteries and veins [2,26]. For example, (1) NRB does not contract the blood vessels of non-Rattus species [19]; (2) the symptoms of NRB intoxication are consistent with such a mechanism [2]; (3) among NRB derivatives, only those capable of stimulating rat-isolated arteries in vitro are lethal [3,22]; and (4) among the 8 isomers of NRB, only those endowed with vasoconstrictor activity (i.e., the four endo isomers but not the four exo isomers) are toxic [3].…”

“…A review of literature synthetic protocols describing the preparation of NRB unequivocally revealed a two-step process-proceeding via a key Diels-Alder cycloaddition reaction (Figure 2A)-which gave rise to an endo-rich mixture of products consisting of up to eight stereoisomers (isomers thereafter), each of which exists as a racemate [20][21][22].…”

“…Despite these very low recoveries, sufficient samples were obtained to evaluate the toxicities of the individual isomers in rats by i.v., and, in some instances, by p.o. More recently, chemical derivatisation/chromatography strategies have allowed for the isolation of endo isomers U, V, W, and Y in higher yields [22].…”

“…; 2.1 mg/kg p.o.) (Figure 2B and Table 1) [3,22,23]. It was also noted that the stereochemistry at the carbinol carbon (C-5) of NRB had a pronounced effect on toxicity: endo isomers bearing a threo configuration at this centre (V and Y) were found to be approximately 10-fold more potent than their erythro counterparts (U and W) (Figure 2B and In vivo toxicity of NRB has been investigated by Russell [1] and Roszkowski [2] in more than 30 species, comprising both avians and mammals (including humans).…”

“…The activation of the relaxing mechanism can easily be evidenced in rat arteries which are poorly or not contracted by NRB (e.g., rat aorta) or even in non-rat arteries, where precontraction of these vessels by high KCl concentrations is readily reversed by NRB [19,26]; in addition, electrophysiological data have shown that the relaxant mechanism can also take place during NRB-induced contraction [35]. Finally, in NRB-contracted arteries, relaxation can be induced by both the endo (contractile) and the exo (non-contractile) isomers of NRB, suggesting that different binding sites may be involved in regulating relaxation versus contraction [22].…”