Loxistatin is a possible therapeutic agent of
muscular dystrophy. A single
oral administration of
loxistatin to male rats caused focal
necrosis of the liver with inflammatory cell infiltration. The severity of the lesions was dose-dependent up to 200 mg/kg and also manifest by an increase in serum
alanine aminotransferase and
aspartate aminotransferase activities. Hepatic
glutathione (GSH) levels decreased with a maximum 20% depletion within 5 hr after the
oral administration of
loxistatin. Pretreatment with
diethyl maleate did not potentiate the
loxistatin-induced hepatic injury. On the other hand, the hepatoprotective effect of
cysteamine was observed when
cysteamine was administered 24 hr before
loxistatin dosing, but the effect was not observed when the
antidote was administered concomitantly with
loxistatin. Pretreatment of rats with
phenobarbital or
trans-stilbene oxide provided partial protection against the hepatotoxic effect of
loxistatin. Pretreatment with
SKF-525A resulted in increased hepatic injury, while pretreatment with
piperonyl butoxide,
cimetidine, or
3-methylcholanthrene had no effect on hepatic damage by
loxistatin. Five hours after [14C]
loxistatin administration to rats, the covalent binding of the radioactivity to
proteins was greatest in the liver, followed by the kidney, then muscle and blood to a lesser extent. [14C]
Loxistatin acid, the pharmacologically active form of
loxistatin, irreversibly bound to rat liver microsomal
proteins; more binding occurred when the
NADPH-generating system was omitted and when the microsomes were boiled first. GSH did not alter the extent of irreversible binding, whereas
N-ethylmaleimide decreased the binding of [14C]
loxistatin acid to rat liver microsomal
proteins by 75%. Unlike the rat, administration of
loxistatin to hamsters caused neither hepatic injury nor hepatic GSH depletion even at a high dose (500 mg/kg). Both the distribution and covalent binding of radioactivity in the hamster liver were one-third of those in rats following [14C]
loxistatin dosing. These results suggest that
loxistatin causes species-specific hepatotoxicity and that, at least in part, some of the toxic effects of
loxistatin are mediated by the nonenzymatic covalent binding of
loxistatin acid to
thiol residues on cellular macromolecules.