Pharmacokinetics of levosimendan and its active metabolite OR-1896 in rapid and slow acetylators

Abstract

Objective

The purpose of this study was to investigate the pharmacokinetics of levosimendan and to determine the primary pharmacokinetic parameters of the pharmacologically active metabolite OR-1896 in rapid and slow acetylators.

Methods

Levosimendan was administered as a constant rate (0.1 μg/(kg min)) i.v. infusion for 24 h in six rapid and six slow acetylators based on N-acetyltransferase 2 genotyping. At the end of the infusion, a small amount (2.5 μg/kg) of ¹³C-labeled OR-1896 was administered by i.v. infusion for 10 min. Blood samples were taken at predefined sampling points 14 days post-infusion and levosimendan and its metabolite concentrations were determined by LC–MS/MS.

Results

Steady-state concentrations of levosimendan were achieved within 4–8 h and no differences were found in the pharmacokinetics of the parent compound between the rapid and slow acetylators. The maximum concentrations of amino phenylpyridazinone metabolite OR-1855 and N-acetylated conjugate OR-1896 were observed approximately 24 h after terminating the infusion. AUC of OR-1896 was approximately 3.5 times higher in the rapid acetylators compared to the slow acetylators (P = 0.002, 95% confidence interval for group ratio from 2.0 to 8.2). The mean ± S.D. fraction of levosimendan metabolized to OR-1896 was 6.8 ± 2.8% in the rapid and 4.3 ± 2.4% in the slow acetylators (P = 0.12). ¹³C-OR-1855 concentrations were detected in plasma after administration of ¹³C-OR-1896 indicating deacetylation from OR-1896 to OR-1855.

Conclusions

Plasma OR-1896 levels during and after levosimendan treatment are dependent on the acetylation status of the subject—rapid acetylators having 3.5 times higher concentrations than slow acetylators.

Introduction

Levosimendan is developed for the treatment of congestive heart failure. It is a novel calcium sensitizer, which exerts positive dose- and time-dependent inotropic effects in the myocardium and induces coronary vasodilatation (Hasenfuss et al., 1998). These effects are related to several distinct pharmacological properties. Levosimendan enhances the sensitivity of the cardiac myofilaments to calcium by binding to troponin C in a calcium-dependent way, which leads to increase in the cardiac contractile force (Haikala et al., 1995a, Haikala et al., 1995b). It also acts as an agonist of ATP-dependent potassium channels in myocytes and in blood vessels, thus inducing vasodilation (Yokoshiki et al., 1997, Pataricza et al., 2000). In patients with moderate to severe heart failure (NYHA III-IV) levosimendan infusion increases cardiac output and decreases both afterload and preload with only a modest increase in heart rate at therapeutic doses (Nieminen et al., 2000, Slawsky et al., 2000). These effects last several days after stopping the infusion suggesting an active role of a metabolite for the therapeutic effects of the drug (Kivikko et al., 2003).

The pharmacokinetics of levosimendan is best described by an open two compartment model. The terminal half-life is about 1 h and total clearance is approximately 200–360 mL/min (Antila et al., 1997, Sundberg et al., 1998). About 97–98% of the parent compound is bound to plasma proteins (Sandell et al., 1995, Antila et al., 2000). After i.v. and oral administration, two metabolites of levosimendan have been detected and characterized in human plasma. Levosimendan seems to be excreted into small intestine and reduced mainly by intestinal bacteria to amino phenylpyridazinone metabolite (OR-1855), which is further metabolized by acetylation to N-acetylated conjugate OR-1896 (Antila et al., 1999). The circulating metabolite OR-1896 has been shown to have similar pharmacologic actions as the parent drug levosimendan (Takahashi et al., 2000).

The enzyme responsible for the acetylation of OR-1855 to OR-1896 is suspected to be N-acetyltransferase (NAT2) (Antila et al., 1999). This enzyme is polymorphically distributed in the population (Meyer and Zanger, 1997) suggesting that the acetylation capacity of the individual (rapid or slow) could determine the plasma levels of OR-1896 during treatment with levosimendan.

The objective of this study was to investigate the pharmacokinetics of a 24 h i.v. infusion of levosimendan and to determine the primary pharmacokinetic parameters of OR-1896 in both rapid and slow acetylators. Because the acetylated metabolite OR-1896 has hemodynamic activity, it was considered important to characterize the pharmacokinetic properties of this metabolite in man. Levosimendan was given as a 24 h infusion, since the metabolite OR-1896 cannot be detected after bolus or short term infusions. Furthermore, the subjects received OR-1896 over 10 min, since the primary pharmacokinetic parameters of the metabolite OR-1896 (clearance, volume of distribution) can be determined only after administration of metabolite OR-1896 itself. Labelling the benzene ring of the metabolite with ¹³C-isotope enabled simultaneous administration of levosimendan and metabolite OR-1896. The dual-isotope technique enabled us to differentiate plasma concentrations of OR-1896 and OR-1855 formed from levosimendan from those due to administration of ¹³C-OR-1896.