Possible metabolic activation of the
dopamine receptor agonist DK-118 (5-hydroxy-6-methyl-N,N-di-n-propyl-2-aminotetralin) was investigated in cats.
Metyrapone, an inhibitor of oxidative
drug metabolism, was given to cats before
DK-118 and the pharmacologic effects of the
dopamine agonist were compared to those observed in nonpretreated animals. A sensitive high-performance liquid chromatography assay using electrochemical detection was developed to monitor urine and plasma concentrations of
DK-118 in
metyrapone-pretreated and control animals. The DK-118-mediated inhibition of cardioaccelerator nerve stimulation-induced
tachycardia was reduced markedly in cats pretreated with
metyrapone but the pretreatment had no effect on the
hypotension or
bradycardia produced by
DK-118. In a separate group of cats, the
tachycardia inhibitory effect of a nonbioactivated
dopamine agonist, dipropyldopamine, was unaffected by
metyrapone pretreatment, confirming that the inhibitor of
drug metabolism does not interfere with this
dopamine receptor-mediated effect. Pretreatment with
metyrapone before a 0.14-mumol/kg i.v. dose of
DK-118 increased the half-life, reduced total drug clearance and increased urinary excretion of unchanged
DK-118. All of the changes are consistent with a
metyrapone-related inhibition of
DK-118 metabolism. The results of this study show that inhibition of
DK-118 metabolism reduces certain of its
pharmacologic actions, indicating that one or more of the metabolites of the
drug may contribute to its effects.