Aziridines are highly reactive alkylating compounds used in
cancer treatment. Salsola tuberculatiformis Botsch., which causes prolonged gestation in sheep and
contraception in rats, contains a very labile hydroxy-phenylaziridine or its precursor. A less labile analogue,
2-(4-acetoxyphenyl)-2-chloro-N-methyl-ethylammonium chloride (Compound I), was synthesized and has been shown to be
contraceptive in rats and to be stabilized by
corticosteroid-binding globulin (
CBG). The current study compared the binding parameters of rat and ovine
CBG and evaluated the effect of the
aziridine precursor, Compound I, on these parameters. Kd and Bmax values of 0.646 and 578 nM for
corticosterone binding to rat
CBG and 0.577 and 19.8 nM for
cortisol binding to sheep
CBG, respectively, were measured. In competitive binding studies with rat plasma, Ki values of 3.48 nM, 0.856 nM, 22.2 nM, 722 microM, and > 1,000,000 microM for
cortisol,
corticosterone,
progesterone, Compound I, and
synephrine (Compound II), respectively, were found, while in sheep plasma the values were 0.409 nM, 1.78 nM, 5.28 nM, 594 microM, and > 1,000,000 microM, respectively. Concentrations of Compound I equivalent to an effective pharmacological dose resulted in a significant (P < 0.01) decrease in
CBG bound
corticosterone and a significant (P < 0.01) increase in free
corticosterone in rat plasma. In sheep, a similar effect was observed with
cortisol.
Progesterone binding, however, did not appear to be affected significantly by Compound I in either rat or sheep plasma. Compound I was found to be a competitive inhibitor of
glucocorticoid binding to
CBG. These results suggest that binding of Compound I to
CBG with concomitant displacement of endogenous
glucocorticoids, but not
progesterone, may be part of the mechanism of action of these phenylaziridine compounds.