Reductive metabolism of aliphatic tertiary amine n-oxides.
Date
1999
Authors
Tien, Pamela
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DOI
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Publisher
De Montfort University
Peer reviewed
Abstract
This study is based on a proposal concerning the feasibility of using aliphatic tertiary
amine N-oxides as antiarrhythmic agent prodrugs. Lignocaine was selected as a
candidate for prodrug development, because the N-oxide is a non-active, polar
derivative of lignocaine and the drug of choice for ventricular arrhythmia, a symptom
associated with ischaemic episodes leading to regions of transiently hypoxic heart
tissue. An HPLC analytical method was developed to study the metabolism of
lignocaine N-oxide. The rapid and sensitive analysis of lignocaine and its metabolites
was demonstrated with good reproducibility, stability and high recovery. In this
study, it was identified that lignocaine N-oxide can be reduced to its active parent
compound, lignocaine with no other metabolites detected in the absence of oxygen.
Under anaerobic conditions, no further metabolism of lignocaine was demonstrated in
rat liver microsomes and heart S9 fractions suggesting no secondary metabolites were
formed. The reduction of lignocaine N-oxide has been shown to be both enzymic and
non-enzymic, NADPH dependent, oxygen sensitive and can be suppressed by CO,
CN- and protein denaturation. Under anaerobic conditions, in vitro lignocaine N-oxide
reduction was found to occur in NADPH supplemented rat liver homogenates,
microsomal suspensions; rat heart homogenates, cytosolic solutions; human
phenotyped cytochrome P450 isoforms; purified enzymes- cytochrome P450
reductase, xanthine oxidase, deoxymyoglobin and NADPHI ascorbate reduced
protohaem (haemin). This reaction can be suppressed through the chemically
mediated decrease ofP450 and bs levels in rat liver microsomes. Previous studies
demonstrated that lignocaine N-oxide was non-active in aerobic rat heart in vivo and
was potent under ischaemic conditions. In this study, high recovery of lignocaine
N-oxide was found in the urine of normal rats suggesting low metabolism of the
prodrug in oxic tissues. However, in hypoxic isolated rat hearts, lignocaine N-oxide
was found to be reduced to lignocaine. The data presented suggested that the
bioactivation of lignocaine N-oxide could be regulated by the prevailing oxygen
tension in the ischaemic arrhythmic heart. Essentially the pro drug activation of
lignocaine N-oxide may be triggered by the ischaemic state of the heart and
terminated as the oxygen content in the heart returns to a more normal level. A
controlled release and site-specific active drug delivery of lignocaine N-oxide, a
hypoxia-mediated antiarrhythmic agent, may thus be achieved.