Dofetilide, a class III antidysrhythmic agent, undergoes both renal and metabolic clearance. Characterization of the metabolism in vitro allows explanation of species differences, whereas identification of the human enzymes involved permits assessment of potential drug interaction. In liver microsomes, the rate of oxidative metabolism of dofetilide is in the order: male rat > female rat > dog > humans, which correlates with the metabolic clearance seen in vivo. In vitro products of oxidative metabolism, formed by N-dealkylation, are the same as those formed in vivo, with the N-desmethyl being the major product. This route of dofetilide metabolism is mediated by cytochrome P450 (CYP). In humans, N-demethylation has a high KM of 657 +/- 116 uM, indicating low affinity for the enzyme's active site. In a number of human liver microsomal preparations, this rate correlated (r = 0.903) with the activity of CYP3A4. There was no correlation with the activities of other isozymes. Specific isozyme inhibitors also indicated the involvement of CYP3A4, with partial inhibition being observed with ketoconazole and troleandeomycin, whereas the activator, alpha-naphthaflavone, caused increased turnover. No inhibition was observed with specific inhibitors or competing substrates for other isozymes. Dofetilide did not significantly inhibit CYP2C9, CYP2D6, or CYP3A4 at concentrations up to 100 microM in vitro. In contrast, amiodarone (IC50, 25 uM) and flecainide (49 microM) inhibited CYP2C9 and quinidine (0.26 uM), and flecainide (0.44 uM) inhibited CYP2D6. Many antidysrhythmic drugs have active, circulating metabolites, complicating the relationship of dose and clinical response. In vitro pharmacology studies allow assessment of the potential contribution to the pharmacological profile by metabolites. Potency of dofetilide and metabolites has been compared for class III (K+ channel blockade) and class I (Na+ channel blockade) antidysrhythmic activities. Three of the metabolites of dofetilide displayed class III activity but at concentrations at least 20-fold higher than dofetilide. Dofetilide N-oxide showed class I activity, but only at high concentration. Neither resting membrane potential or action potential amplitude were affected by any metabolite. This lack of biologically relevant activity is in accord with the close correlation between plasma concentrations of dofetilide and pharmacological response.
Approximately 80% of a single dose of dofetilide is excreted in urine, of which approximately 80% is excreted as unchanged dofetilide with the remaining 20% consisting of inactive or minimally active metabolites. ... In vitro studies with human liver microsomes show that dofetilide can be metabolized by CYP3A4, but it has a low affinity for this isoenzyme. Metabolites are formed by N-dealkylation and N-oxidation. There are no quantifiable metabolites circulating in plasma, but 5 metabolites have been identified in urine
In clinical trials, serum aminotransferase and alkaline phosphatase elevations were no more common during dofetilide than placebo therapy. Some degree of ALT elevation was reported in 15% of dofetilide but a similar proportion of placebo recipients; these elevations were above 3 times the upper limit of normal in 1.5% vs 2.0%. Thus, the background rate of serum ALT elevations in patients with atrial fibrillation eligible for dofetilide treatment appears to be high. Despite this, dofetilide has not been linked to instances of clinically apparent liver injury with symptoms or jaundice. The product label for dofetilide does not mention hepatotoxicity and does not specifically recommend monitoring of liver tests.
参考文献:M Chen, V Vijay, Q Shi, Z Liu, H Fang, W Tong. 用于研究药物诱导肝损伤的FDA批准药物标签,药物发现今日,16(15-16):697-703, 2011. PMID:21624500 DOI:10.1016/j.drudis.2011.05.007
M Chen, A Suzuki, S Thakkar, K Yu, C Hu, W Tong. DILIrank:根据人类发生药物诱导肝损伤风险排名的最大参考药物清单。药物发现今日2016, 21(4): 648-653. PMID:26948801 DOI:10.1016/j.drudis.2016.02.015
References:M Chen, V Vijay, Q Shi, Z Liu, H Fang, W Tong. FDA-Approved Drug Labeling for the Study of Drug-Induced Liver Injury, Drug Discovery Today, 16(15-16):697-703, 2011. PMID:21624500 DOI:10.1016/j.drudis.2011.05.007
M Chen, A Suzuki, S Thakkar, K Yu, C Hu, W Tong. DILIrank: the largest reference drug list ranked by the risk for developing drug-induced liver injury in humans. Drug Discov Today 2016, 21(4): 648-653. PMID:26948801 DOI:10.1016/j.drudis.2016.02.015
Approximately 80% of a single dose of dofetilide is excreted in urine, of which approximately 80% is excreted as unchanged dofetilide with the remaining 20% consisting of inactive or minimally active metabolites. Renal elimination involves both glomerular filtration and active tubular secretion (via the cation transport system, a process that can be inhibited by cimetidine, trimethoprim, prochlorperazine, megestrol and ketoconazole). ...
The oral bioavailability of dofetilide is >90%, with maximal plasma concentrations occurring at about 2-3 hours in the fasted state. Oral bioavailability is unaffected by food or antacid. The terminal half life of Tikosyn is approximately 10 hours; steady state plasma concentrations are attained within 2-3 days, with an accumulation index of 1.5 to 2.0. Plasma concentrations are dose proportional. Plasma protein binding of dofetilide is 60-70%, is independent of plasma concentration, and is unaffected by renal impairment. Volume of distribution is 3 L/kg.
1. Pharmacokinetics of dofetilide were studied in man, dog, rat and mouse after single IV and oral doses of dofetilide or (14)C-dofetilide. 2. Dofetilide was absorbed completely in all species. Low metabolic clearance in man resulted in complete bioavailability following oral administration. Higher metabolic clearance in rodents, and to a lesser extent dogs, resulted in decreased bioavailability because of first-pass metabolism. 3. Following IV administration, the volume of distribution showed only moderate variation in all species (2.8-6.3 l/kg). High plasma clearance in rodents resulted in short half-life values (mouse 0.32, male rat 0.5 and female rat 1.2 hr), while lower clearance in dog and man gave longer terminal elimination half-lives (4.6 and 7.6 hr respectively). 4. After single IV doses of (14)C-dofetilide, unchanged drug was the major component excreted in urine of all species with several metabolites also present. 5. Metabolites identified in urine from all species were formed by N-oxidation or N-dealkylation of the tertiary nitrogen atom of dofetilide. 6. After oral and IV administration of (14)C-dofetilide to man, parent compound was the only detectable component present in plasma and represented 75% of plasma radioactivity. No single metabolite accounted for greater than 5% of plasma radioactivity.