Stereoselective analysis of flecainide enantiomers using reversed-phase liquid chromatography for assessing CYP2D6 activity.

Stereoselective analyses of flecainide enantiomers were performed using reversed-phase high-performance liquid chromatography (HPLC) equipped with a polysaccharide-based chiral column (Chiralpak AS-RH) and fluorescence detector. Excitation and emission wavelengths were set at 300 and 370 nm, respectively. Flecainide enantiomers in serum and urine were extracted using diethyl ether. The mobile phase solution, comprising 0.1 m potassium hexafluorophosphate and acetonitrile (65:35, v/v), was pumped at a flow rate of 0.5 mL/min. The recoveries of flecainide enantiomers were greater than 94%, with the coefficients of variation (CVs) <6%. The calibration curves of flecainide enantiomers in serum and urine were linear in the concentration range 5-500 ng/mL and 0.75-15 µg/mL (r > 0.999), respectively. CVs in intra-day and inter-day assays were 1.8-5.8 and 3.4-7.5%, respectively. In a pharmacokinetic study, the ratios of (S)- to (R)-flecainide (S/R ratio) in the area under the curve and the amount of flecainide enantiomers excreted in urine were lower in a subject carrying CYP2D6*10/*10 than in subjects carrying CYP2D6*1/*2. The S/R ratio of trough serum flecainide concentration ranged from 0.79 to 1.16 in patients receiving oral flecainide. The present HPLC method can be used to assess hepatic flecainide metabolism in a pharmacokinetic study and therapeutic drug monitoring.

Fatal flecainide intoxication.

A fatal case attributed to flecainide acetate (Tambocor), a class Ic antiarrythmic drug, is presented. Flecainide was detected by GC/MS in gastric contents, blood and liver as well. The urine analysis revealed the presence of its dealkylated metabolite. Body fluids and tissue concentrations determined by GC/ECD were 7.7 mg/kg in femoral blood, 0.26 mg/kg in bile, 18 mg/kg in liver, 0.17 mg/kg in cerebrospinal fluid, 0.22 mg/kg in brain cortex and 28.9 mg/kg in urine. The total amount of flecainide in gastric contents was about 43 mg. Even taking into account the postmortem redistribution of flecainide, its blood level still remains in the toxic range.

Formation of formaldehyde adducts from various drugs by use of methanol in a toxicological screening procedure with gas chromatography-mass spectrometry.

Use of methanol as a solvent in a toxicological screening procedure with gas chromatography-mass spectrometry may be associated with artifact formation. Artifacts with a molecular ion of [M + 12]+ are formed from various drugs, such as amphetamine, propafenone, flecainide, beta-blockers and prilocaine. The mechanism of artifact formation was studied by mass spectral techniques, labelling and nuclear magnetic resonance spectroscopy. It was shown that the artifacts were generated by the addition of formaldehyde and subsequent loss of water. Formaldehyde is probably formed by thermal dehydrogenation of methanol in the injection port of the gas chromatograph.

High-performance liquid chromatographic determination of the enantiomers of flecainide in human plasma and urine.

A stereospecific high-performance liquid chromatographic method for the determination of (R,S)-flecainide acetate [(R,S)-N-(2-piperidylmethyl)-2,5-bis-(2,2,2-trifluoroethoxy)benzam ide acetate] in human plasma and urine is described. After addition of the internal standard [IS; (R,S)-N-(2-piperidylmethyl)-2,3-bis(2,2,2-trifluoroethoxy)- benzamide hydrochloride], a single-step extraction of alkalinized samples was performed with distilled diethyl ether. The organic layer was evaporated and the drug and IS were derivatized with 1-[(4-nitrophenyl)sulfonyl]-L-propyl chloride at 80 degrees C for 2 h. The diastereomeric derivatives of flecainide and IS were chromatographed on a C18 reversed-phase column with a mobile phase consisting of acetonitrile: water:triethylamine (45:55:0.2) at a flow rate of 1 mL/min. Flecainide diastereomers were separated with a resolution factor of 1.25 and detected by UV spectroscopy at a wavelength of 280 nm. An excellent linearity was observed between the peak area ratios (flecainide derivatives:IS) and plasma concentrations, and the intra- and interday coefficients of variation were always less than 9.8%. The lowest quantifiable concentration was set at 50 ng/mL for each enantiomer (CV of 4.9 and 4.4%), while the lowest limit of detection (signal:noise, 3:1) was on the order of a few nanograms. The assay was used to study the pharmacokinetics of flecainide enantiomers in a patient receiving (R,S)-flecainide therapy. The steady-state plasma time courses for the enantiomers were found to be parallel, but the difference between (R)- and (S)-flecainide concentrations was significant. The urinary excretion data were consistent with the plasma results. The method is suitable for therapeutic monitoring of flecainide enantiomers and for stereoselective pharmacokinetic studies in humans.

Reversed-phase LC resolutions of chiral antiarrhythmic agents via derivatization with homochiral isothiocyanates.

The search for new antiarrhythmic agents has been intense, because the established drugs for the treatment of cardiac arrhythmias are neither uniformly effective nor well-tolerated. Among the recently introduced new antiarrhythmic agents are tocainide (TOC), mexiletine (MEX), flecainide (FLE), and propafenone (PRO). Each of these drugs is a chiral amine used clinically as the racemic mixture. We have examined the high-performance liquid chromatographic chiral resolution of the above four drugs via derivatization with homochiral derivatizing agents (HDAs). The amino functionality of the drugs was reacted with four homochiral isothiocyanates, 2,3,4,6-tetra-O-acetyl-beta-D-glucopyranosyl isothiocyanate (TAGIT), (R)-alpha-methylbenzyl isothiocyanate (RAMBI), (S)-1-(1-naphthyl)ethyl isothiocyanate (SNEIT), and (R)-1-(2-naphthyl)ethyl isothiocyanate (RBEIT). Complete separation of the two peaks (resolution factor R = 1.5) was achieved with all four HDAs for TOC, with TAGIT, RBEIT, and RAMBI for MEX, with TAGIT and SNEIT for PRO, and only with TAGIT for FLE. SNEIT was used to develop analytical procedures for the determination of the enantiomeric composition of TOC in human urine and blood serum. The four HDAs offer several advantages over many other HDAs and should be useful in studies of enantioselective drug action and disposition.

The influence of the sparteine/debrisoquin phenotype on the disposition of flecainide.

The pharmacokinetics and urinary excretion of flecainide (50 mg administered orally) were investigated in five extensive metabolizers (EMs) and five poor metabolizers (PMs) of the sparteine/debrisoquin type of polymorphism under conditions of controlled urinary pH. Flecainide disposition was altered in the PMs. The AUC was higher (1462 +/- 407 versus 860 +/- 256 hr ng/ml), the elimination half-life prolonged (11.8 versus 6.8 hours), and the amount excreted in the urine was higher (26.7 +/- 7.2 versus 15.4 +/- 1.3 mg) in PMs compared with EMs (p less than 0.05). Oral clearance of flecainide was reduced (p less than 0.019) in PMs (600 +/- 139 versus 1041 +/- 307 ml/min in EMs). The renal clearance was similar (p greater than 0.05) in PMs (308 +/- 70 ml/min) and EMs (315 +/- 69 ml/min) and, consequently, PMs had a lower (p less than 0.008) metabolic clearance of flecainide (292 +/- 136 versus 726 +/- 240 ml/min in EMs). Under conditions of uncontrolled urinary flow and pH, renal excretion of flecainide will be reduced and the difference in disposition will be greater. In PMs with renal impairment, accumulation of flecainide to very high serum concentrations may be anticipated, and this may result in proarrhythmic effects.

Pharmacokinetics of flecainide in patients with cirrhosis of the liver.

The pharmacokinetics of flecainide were studied in six patients with cirrhosis of the liver and in six healthy subjects after a single 2 mg/kg intravenous dose. Hepatic biotransformation capability before flecainide dosing was assessed by antipyrine challenge. The mean plasma antipyrine t1/2 for patients (42.2 hours) was longer (p less than 0.01) than that for subjects (11.7 hours). For control subjects, the plasma t1/2 of flecainide (9.5 hours) was shorter (p less than 0.01), plasma clearance (9.1 ml/min/kg) was faster (p less than 0.01), and volume of distribution (7.5 L/kg) was smaller (p less than 0.05) compared with corresponding values in patients. Renal clearance did not differ (p greater than 0.05) between the two groups. The mean ratio of renal clearance to plasma clearance for subjects (0.4) was smaller (p less than 0.05) than that for patients. The slower rate of flecainide elimination from plasma in patients is likely due to reduced hepatic biotransformation. In patients with cirrhosis, plasma levels of flecainide may accumulate to unacceptably high levels with usual dosage regimens.

Pharmacokinetics of flecainide acetate in patients with severe renal impairment.

We studied the effect of renal disease on the pharmacokinetics of flecainide after single intravenous, single oral, and multiple oral doses to patients with severe renal disease (creatinine clearances less than 12 ml/min/m2). The absorption and volume of distribution of flecainide were not altered by renal impairment. The average plasma half-life was prolonged by about twofold that of healthy subjects but most patients were within the range of values for healthy subjects. Total body clearance was reduced. With multiple oral doses of 50 mg b.i.d. or 50 mg daily, steady-state plasma levels were reached by 6 days and no further accumulation in plasma was observed. In patients with severe renal disease, therapy with flecainide should be initiated at 100 mg daily (or 50 mg b.i.d.). If necessary, dosage increases should be made cautiously at intervals of more than 4 days when plasma levels have plateaued as demonstrated by plasma level monitoring.

Oral flecainide pharmacokinetics in patients with impaired renal function.

The pharmacokinetics of flecainide acetate were studied in 20 patients with varying degrees of renal impairment following a single oral dose. The patients were divided into two groups, on the basis of renal creatinine clearance (CLCR), for statistical and kinetic analysis. Patients with a CLCR between 4 and 41 mL/min/m2 were designated group 1 and those below 4 mL/min/m2 or unmeasurable because of lack of urine output were designated group 2. In both groups peak plasma flecainide concentrations, time to peak concentrations, and apparent volume of distribution (Vd) were similar to those reported in healthy subjects with normal renal function. The mean flecainide plasma elimination half-lives from both groups 1 and 2 were longer than those previously reported by several investigators in normal subjects. Nine patients in group 1 and seven patients in group 2 had half-lives within the range reported in healthy subjects. Therefore, CLCR alone is not a good predictor of plasma elimination half-life following a single oral dose of flecainide. Although renal clearance of flecainide is significantly reduced in end-stage renal disease (ESRD), total plasma clearance of flecainide (CLflec) was not reduced to the same degree, although there was a significant, modest correlation with CLCR. Less than 1% of the administered oral dose of flecainide was removed during hemodialysis. The relationship between dosage and plasma elimination half-life in patients with ESRD needs further study to evaluate possible dose-dependent kinetics.

Pharmacokinetics of flecainide in patients with mild and moderate renal failure compared with patients with normal renal function.

We have studied the pharmacokinetics of flecainide after the oral administration of 100 mg to 8 patients without renal impairment and 8 patients with mild to moderate renal failure. Both groups gave comparable results with respect to the peak plasma concentrations and the time to peak. There was a significant correlation between renal flecainide clearance and endogenous creatinine clearance. The elimination half-time in the patients with impaired renal function was significantly longer (19.9, SD 9.9 h) than that in the patients with normal renal function (11.5, SD 4.2 h), but the variability in the elimination half-time in renal failure could not be explained on the basis of the available results.