Reductive metabolism of aliphatic tertiary amine n-oxides
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.