BIOPHARMACEUTICS & DRUG DISPOSITION, VOL. 11, 351-363 (1990)

ANALYSIS AND STEREOSELECTIVE METABOLISM AFTER SEPARATE ORAL DOSES OF TOCAINIDE ENANTIOMERS TO HEALTHY VOLUNTEERS *

KURT-JURGEN HOFFMANN, LARS RENBERG AND OLLE GYLLENHAAL

AB Hassle Cardiovascular Research Laboratories, S-43183 Molndal, Sweden

ABSTRACT The potential of stereoselective metabolism of tocainide was studied in six healthy volunteers after separate oral administration of the pure enantiomers in solution. A method was developed to convert the N-carbamoylglucuronide of tocainide in plasma and urine by base treatment to a hydantoin derivative which after extraction and silylation was analysed by selected ion monitoring using a deuterated internal standard. Analytical problems concerning side-reactions during derivatization of the conjugate are discussed. The peak plasma levels of the enantiomers, observed at < 2 h after dosing, were similar but plasma clearances and terminal half-lives were different after oral administration of (R)-tocainide (195.5 f 20.1 ml min-' and 9.7 f 0.8 h) and (S)-tocainide (110.2 f 10.5 ml min-' and 14.5 f 1.7 h). Over 0-96 h the averaged urinary recovery of (R)-tocainide was 36 per cent and of (S)-tocainide 50 per cent. Stereoselective metabolism was a likely mechanism for the observed differences as the urinary recovery of the conjugate formed from (R)-tocainide differed substantially from that of (S)-tocainide (45 vs 1.2 per cent of given dose). Plasma t,,, of the (R)- and (S)-conjugate were 9.9 and 18.7 h, respectively, indicating formation rate limited kinetics of the metabolite. The renal clearances of the conjugates were not significantly different (13 1 vs 97 ml min- '). KEY WORDS

Tocainide Stereoselectivepharmacokinetics Metabolism GC-MS Humans Oral administration Glucuronide

INTRODUCTION Tocainide (2-amino-N-(2,6-dimethylphenyl)-propanamide HCI, Figure 1) is a primary amine analogue of lidocaine' and is classified as a 1B antiarrhythmic agent.* The racemic drug is given orally and exhibits a complete bioavaila b i l i t ~ .Differences ~,~ in the pharmacokinetics of drug enantiomers because of stereoselective absorption, distribution, metabolism, and elimination are known for numerous drugs.5 Several studies reported the stereoselective disposition of tocainide in healthy young volunteers,@ in patients with acute ventricular arrhythmiasg and in rats and mice.IOIn the studies with stereoselective analysis

* Addressee for correspondence. 0 142-2782/90/04035 1-13$06.50 0 1990by John Wiley & Sons, Ltd.

Received 30 January 1989 Revised 1 June 1989

352

K.-J. HOFFMANN, L. RENBERG A N D 0. GYLLENHAAL

of tocainide in plasma8J1a more rapid elimination of the (R)-enantiomer has been demonstrated. Conjugation of tocainide accounts for about 20 per cent of the given dose.I2 This metabolite, which after /3-glucuronidase treatment yields tocainide (see Figure l), is of quantitative importance in man only.13Based on urinary excretion data, the half-life of the racemic metabolite (10.3 h) was close to that of (R)-t~cainide,~ indicating that the metabolite is preferentially formed from the (R)-stereoisomer. This was also supported by the observed low urinary recovery of the (S)-tocainide conjugate. This report describes a sensitive GC-MS method to measure the plasma concentration of the tocainide conjugate where the conditions for derivatization were optimized. The procedure, however, does not separate the metabolite enantiomers. Consequently, pure enantiomers of tocainide were administered to healthy volunteers at different occasions to study the potential of stereoselective metabolism of tocainide using plasma-concentration data. The results should be compared with those from a previous study in which the urinary excretion of the enantiomers was followed after administration of a pseudoracemic m i ~ t u r e . ~ EXPERIMENTAL Study design

Six healthy male volunteers (average age 27 years and weight 79 kg) who had given their informed consent, participated in the study reviewed by the Swedish Board of Health and Welfare. Subjects arrived at the laboratory about 8 am after an overnight fast and received an oral dose of 300 mg (10 mg ml-' of water) of (R)- or (S)-tocainide HCI on a randomized, crossover basis. The interval between the two doses was at least 2 weeks. Standardized meals were served 3, 5, and 8 h after dosing. Blood samples were drawn at selected time-points via a vein flow catheter inserted into an antecubital arm vein. After separation of the plasma the samples were kept frozen until analysed. Urine was quantitatively collected during 3 days at the following intervals: 0-4, 4-8, 8-12, 12-24, 24-36, 3648, and 48-72 h. After weighing the urine, aliquots from each collection period were stored frozen before analysis. Chemicals

The (R)- and (S)-enantiomers of tocainide, the deuterated hydantoin derivative of tocainide (3-(2,6-xyly1)-5[C2H3]hydantoin),the p-methyl hydantoin derivative of tocainide (3-(2,4,6-trimethylphenyl)-S-rnethylhydantoin) and the trichloroethylcarbamate of dibenzylamine were synthesized at the Department of Organic Chemistry, AB Hassle. N , O-bis (trimethylsily1)-acetamide (BSA) was purchased from Machery-Nagel (Duren, FRG). All solvents and chemicals

353

STEREOSELECTIVETOCAINIDE METABOLISM

-

- ._ m 0) N

0

N

2

.

0)

E

:*

-: 8

... n

.I .. n 0 I

n I

m

- ..

I

.::- ..

I

.

- -

.,

.3

.

n

*

-

354

K.-J.HOFFMANN, L. RENBERG AND 0. GYLLENHAAL

for preparation of buffer solutions were of reagent grade and were used without further purification. Analysis of tocainide Unchanged drug in plasma and urine was extracted with dichloromethane, derivatized with heptafluorobutyric anhydride, and stereoselectively analysed by capillary gas chromatography and electron-capture detection. Analysis of tocainide conjugate The method for analysis of the metabolite is outlined schematically in Figure 1. Preparation of conjugate standard. Human urine collected after an oral dose of 400 mg tocainide tablet was the source for the conjugate of tocainide to be used for preparation of standard samples. The concentration of the metabolite was determined indirectly as follows: unchanged drug in urine as triplicates was extracted at pH 9 with five volumes of dichloromethane and the remaining aqueous phase was subjected to enzyme hydrolysis with 8-glucuronidase (E. coli K12: 20 units mg-l, Boehringer Mannheim GmbH, Mannheim, FRG). The sample was incubated for 16 h at pH 4.5 (37") and the released tocainide was extracted as described above. Free and released tocainide were analysed by liquid chr~matography'~ and the difference in concentrations was taken as a measure of metabolite concentration assuming that the enzyme hydrolysis was complete and that the carbamoyl-conjugate was the only metabolite to give tocainide upon hydrolysis. This metabolite standard (183 pmol 1-I) was used for evaluation of the analytical method. Optimization of metabolite hydrolysis. Blank plasma (0.9 ml), metabolite standard (0.1 ml), and internal standard (p-methyl hydantoin derivative of tocainide, 5 pg) were mixed. Sodium hydroxide was added at room temperature to give pH 12 (1 ml of 0.1 M NaOH), pH 12.6 (0.5 ml of 0.5 M NaOH), and pH 12.8 (1 ml of 1 M NaOH), respectively. Aliquots were taken at different time-points and the hydrolysis was stopped by adding 0.2 ml of 1 M sodium dihydrogen phosphate. The hydantoin derivative was extracted with 5 ml of diethylether/dichloromethane (3 :2 v/v) containing 1.2 pg of the trichloroethylcarbamate of dibenzylamine as a marker. The upper organic phase was separated and evaporated to dryness before reconstitution in 15 p1 of ethyl acetate. About 3 pl was injected into a Varian 3700 gas chromatograph equipped with a fused silica capillary column (20 m X 0.32 mm ID) coated with CP-Sil 8. The injection inlet pressure was 100 kPa using nitrogen as the carrier gas.

STEREOSELECTIVE TOCAINIDE METABOLISM

355

Injections were made in the splitless mode at 100" using a silanized liner and the temperature was increased to 250" at a rate of 20" min-'. The injector and the thermionic detector were operated at 250 and 300", respectively. Peak area ratios of the formed hydantoin derivative of tocainide with internal standard or marker were plotted versus time.

Determination of conjugate. The concentration of the tocainide conjugate in plasma was determined as follows. The sample (0.1-1.0 g) was mixed, if required, with drug-free plasma to give a final weight of 1.0 g. An aqueous solution of the deuterated hydantoin derivative of tocainide (100 p1, 10 pM) as internal standard and 0.5 ml of sodium hydroxide (0.5 M) were added followed by vigorous mixing for 30 s. An IKA-VIBRAX-VXR apparatus from Janke and Kunkel (Staufen, FRG) with an attachment for 36 vertical tubes was used. Caps were not required. After 15 min the reaction was stopped by the addition of 0.5 ml of 1 M sodium dihydrogen phosphate. The samples were extracted with 5 ml of diethylether/dichloromethane (3:2 by vol.) in the same apparatus for 15 min. After centrifugation the organic phase was isolated and evaporated to dryness by a stream of nitrogen. The residue was dissolved in 25 pl of BSA just before GC-MS analysis in the selected ion monitoring mode. A Finnigan MAT 44s quadrupole instrument, equipped with a Varian 3700 gas chromatograph, was used for analysis. The CP-Sil8 capillary column was connected to the ion source (temp 220") via an open split interface using helium as carrier gas. Samples were injected in splitless mode (1 min) at an oven temperature of loo", which was maintained for I min. The temperature of the oven was then increased at a rate of 20" min-' to 240". Mass spectra of the reference materials and the selected ion monitoring at m/z 290 and 293 were obtained using electron impact ionization (70 eV). Data were acquired and processed using a Finnigan MAT SS 300 data system. A set of calibration standards were prepared by addition of various amounts of metabolite standard to 1.0 ml of blank plasma (concentration range 25-960 nmol 1-') and by analysis of these samples according to the procedure given above. In the calibration curve, the peak area ratios of metabolite to deuterated internal standard were plotted against known concentrations of the metabolite. Urine was analysed by the same method except that deionized water was used instead of blank plasma. If required, the samples were diluted to 2 pmol 1-1 or less. Data analysis The elimination rate constant (K,) and the corresponding half-life were estimated by linear least squares regression of the logarithm of the plasma concentration with time. The area under the plasma concentration versus time curve (AUC) was calculated using the linear trapezoidal rule and the AUC

356

K.-J. HOFFMANN, L. RENBERG AND 0. GYLLENHAAL

was extrapolated to infinity by addition of the term C,l K, where C, was the last plasma concentration measured. The oral clearance, Cloral,was calculated by the equation Cloral= Doseoral/ AUCOraI.As the bioavailability of tocainide , ~ , ~can be set equal to systemic clearance (ClsYs). is virtually 100 per ~ e n t Clara, Renal clearance, Cl,,, of drug and metabolite was estimated by the equation Cl,, = X,/AUC where Xuis the amount of druglmetabolite found in 0 4 8 h urine. The AUC value was determined in plasma for the same time interval. Linear regression analysis of the logarithm of the urinary excretion rate in the midpoint of each collection interval versus time plot was used for determination of elimination rate constants of tocainide/metabolite and halflives were calculated according to t l R = 0.693 I K,.

RESULTS Analysis of metabolite The analytical procedures and mass spectrometric data are summarized in Figure 1. The hydantoin derivative formed upon base treatment of the tocainide metabolite in plasma and urine exhibited good gas chromatographic properties after silylation of the amide bond. Without silylation pronounced peak tailing occurred at low concentrations on the CP-Sil8 methyl silicone capillary column. Removal of the reagent by evaporation before GC-MS analysis followed by dissolution in, for example, ethyl acetate led to losses of the derivative which, however, was stable in neat BSA for days at 4". The EI mass spectra in Figure 1 showed molecular ions at d z 290 (analyte) and m/z 293 (internal standard) with a relative abundance of about 25 per cent. Despite the loss of ion current due to extensive fragmentation of the derivative under electron impact, the M+, ions proved to be favourable in terms of background interferences and usually very clean selected ion chromatograms for both plasma and urine samples were obtained (see Figure 1). The minimum determinable concentration of about 20 nmol l-' allowed for analysis of all samples collected in this study. The standard curve 0,= 1.43. 10-3x 6.3. was linear in the range of 25 to 960 nmol I-' with a correlation coefficient > 0.999. The precision of repeated analysis (n = 8) was 1.6 per cent (96 nmol 1-l) and 2 per cent (960 nmol l-l). The conditions for the formation of the hydantoin derivative from the tocainide conjugate were evaluated with respect to pH, reaction time, and solvent extraction, and these samples were analysed by GC with nitrogen selective detection. Three different pHs of the aqueous phase were studied and the results are given in Figure 2. The hydrolysis of formed hydantoin was prominent at pH 12-8with a tIl2of approximately 30 min. At pH 12, however, the formation of the hydantoin did not reach its maximum even after 45 min. In the final method pH 12.6 was selected where an optimum yield was found after 10 min. The area ratio to the p-methyl analogue standard remained constant at

+

357

STEREOSELECTIVE TOCAINIDE METABOLISM Are8 ratio

pH 12

Are8 ratio

pH 12.8

I .o

I .o

0.5

10

30 Area ratio

1 .o

c

0.5

-

min.

45

10

45

30 min.

pH 12.6

pH 12.6 for up to 1 h. A reaction time of 15 min was found suitable and the hydrolysis was stopped at that time by the addition of sodium dihydrogen phosphate. The hydantoin derivative in the neutralized aqueous phase was then quantitatively (>99 per cent) extracted with a combination of diethyl ether and dichloromethane. The organic phase becomes the upper phase thus facilitating further work up.

358

K.-J.HOFFMANN, L. RENBERG AND 0.GYLLENHAAL

10000

6-4

\ r(

0

E

1000

C

1004 0

I

10

I

20

30

40

1

50

HOURS

Figure 3. Mean plasma concentration vs time profile of (R)-tocainide (0)and (S)-tocainide (0) given as oral solutions on two separate occasions to six healthy volunteers

Pharmacokinetics in plasma The mean plasma concentration-time curves of (R)-and (S)-tocainide given separately are shown in Figure 3 and the one-compartment disposition characteristics are summarized in Tables 1 and 2. No inter-conversion of the pure enantiomers was found by the stereoselective analysis. In all subjects peak plasma concentrations were attained within 2 h and the mean plasma levels were comparable between the enantiomers ((R)-tocainide 8083 nmol 1-' and (S)-tocainide 7993 nmol 1-l) indicating similar absorption. Substantial differences between (R)- and (S)-tocainide kinetics were found in terms of mean plasma clearance (195.5 vs 110.2 ml min-I) and plasma half-lives (9.7 vs 14.5 h). Mean plasma concentration-time profiles of the enantiomeric tocainide conjugates are shown in Figure 4 and the pharmacokinetic parameters are given in Tables 1 and 2. Peak plasma levels were approximately 55-fold higher for the metabolite with (R)-configuration which had an average plasma elimination half-life of 9.9 h. This value was almost identical to that of (R)-tocainide (9.7 h) which suggested that the rate of metabolite elimination was limited by its formation. Mean plasma t1,2of 18.7 h for the (S)-metabolite was significantly (p < 0.05, paired t-test) longer than for (S)-tocainide (14.5 h).

359

STEREOSELECTIVE TOCAINIDE METABOLISM

Table 1. Pharmacokinetic parameters derived from plasma concentration and urinary excretion data of R-tocainide and its conjugate after an oral dose of 300mg to healthy volunteers R- tocainide Cloral Clrenal 0-48h Subject (ml min-I) (ml min-I)

1 2 3 4 5 6

-

M SD

R-conjuga te tl,

t!12

urine (h)

Clrenai 0-48h (mlmin-I)

plasma (h)

urine (h)

10.8 8.7 10.4 10.1 10.9 8-9

12.4 9.1 11.5 10.9 9.5 11.3

9.9 0.95

10.8 1.3

t112

t!12

plasma (h)

200 170 180 191 205 227

69 50 36 93 110 60

10.5 9.0 9.6 10.2 10.3 8.5

15.5 10.3 10.5 14.8 7.7 8.6

119 158 96 96 166 150

195.5 20.1

69.5 27.5

9.7 0.8

11.2 3.2

130.8 31.3

Table 2. Pharmacokinetic parameters derived from plasma concentration and urinary excretion data of S-tocainide and its conjugate after an oral dose of 300mg to healthy volunteers S-conjugate

S-tocainide tll2

101 127 115 108 98 112

34 62 69 67 54 61

16.5 14.7 12.6 16.1 14.2 12.6

urine (h) 21.0 17.4 11.3 19.2 15.8 16.3

110.2 10.5

57.8 12.8

14.5 1.7

16.8 3.3

plasma (h) 1 2 3 4 5 6

-

M SD

$12

Clrenal 0-48h (mlmin-I)

412

t!/2

plasma (h)

urine (h)

120 102 80 107 62 113

19.2 14.5 15.3 21.3 24.6 17.1

19.3 14.3 14.7 17.7 14.2 15-7

18.7 3.8

16.0 2.1

97.3 22.0

Pharmacokinetics in urine

The urinary recoveries of tocainide and its conjugate are shown in Table

3 and renal clearance values and elimination half-lives are presented in Tables 1 and 2. On average 36 per cent of given (R)-tocainide was cleared unchanged by the kidneys and about 45 per cent was excreted as its conjugate which means a dose recovery of more than 80 per cent. Interestingly, only 1.2 per cent of the (S)-metabolite was excreted in urine together with 50 per cent of unchanged (S)-tocainide. Renal clearance of (R)- and (S)-tocainide was essentially the same (69.7 vs 57.8 ml min-l). Despite pronounced difference in the

360

K.-J. HOFFMANN, L. RENBERG AND 0. GYLLENHAAL

IooooT

J.

100

10 0

10

30

20

40

50

HOURS Figure 4. Mean plasma concentration vs time profile of the carbamoyl conjugate of tocainide formed after separate oral administration of (R)-tocainide (0)and (S)-tocainide (0)to six healthy volunteers

urinary recovery of (R)-and (S)-metabolites (45 vs 1.2 per cent of dose) their renal clearances were not significantly different (p = 0.11, paired t-test). Taking the lower plasma concentration of (S)-metabolite into account these data might indicate an alternative metabolic pathway for (S)-tocainide. The estimates in Tables 1 and 2 on tl,2derived from urinary excretion rate plots of drug and metabolite were frequently different from those observed in the same individual using plasma concentration-time data. Despite numerous data points from all urine collection intervals, line fitting was often hampered by irregular excretion of the compounds. As reported earlier, markedly increased urinary pH by sodium bicarbonate loading caused a 75 per cent reduction of tocainide’s renal clearance3 and possibly variation of urinary pH in the present study might influence the renal elimination of both tocainide and its conjugate. Therefore these data are not further discussed.

DISCUSSION Analysis

This study represents an extension of our previous work which demonstrated stereoselective kinetics and metabolism of tocainide assessed by analysis of

36 1

STEREOSELECTIVE TOCAINIDE METABOLISM

Table 3. Urinary excretion of tocainide and its conjugate, per cent of given dose, in healthy volunteers over 0-72h after oral administration of 300mg of the R- and Senantiomer, respectively ~~~

Subject 1 2 3 4

5 6

R SD

~

R-tocainide given R-tocainide R-conjugate

34.8

S-tocainide given S-tocainide S-conjugate 1.32

29.6 20.0 50.7 53.0 26.2

34.3 55.8 38.5 38.8 50.9 50.9

35.2 42.9 53.3 61.0 53.5 56.5

0.92 1.16 1.58 0.80 1.56

35.1 13.4

44.9 8-7

50.4 9.5

1-22 0.32

urine.7 The report describes an improvement of the GC-MS method for the tocainide conjugate allowing for the determination of the metabolite in all plasma samples collected. The cyclic derivative was quantitatively extracted with a combination of diethylether and dichloromethane (< 1per cent remained in the neutralized aqueous phase). Sensitivity and precision were sufficient using capillary GC, a deuterated internal standard and silylation of the formed hydantoin derivative of tocainide. The use of a marker stable at pH > 12, the trichloroethylcarbamate of dibenzylamine, during the method development led to the observation that the formed hydantoin derivative was not stable under alkaline conditions (Figure 2). The concentration of the deuterated hydantoin standard decreased by 22 per cent during 15 min at pH 12.6 in plasma. No attempts were made to identify reaction products. Immediate extraction of the formed hydantoin in a two-phase system would be attractive to avoid hydrolysis. In such a system, however, the ratio of the tocainide hydantoin versus the 4-methyl analogue decreased after the maximum yield had been obtained. The reason for this difference in stability is not known but different partition ratios may be involved. Consequently, the pH and time for the reaction had to be optimized for complete and fast conversion of the conjugate and for negligible hydrolysis of the formed hydantoin, while the deuterated hydantoin as internal standard is present at time zero. The precision of the method below 2 per cent a 96 nmol l-l level demonstrated reproducible reaction conditions and the metabolite standard in urine stored at -20" was stable for at least 1 year. Due to recent advances in stereoselective assay methodology it is appropriate to examine the individual enantiomers in a racemic mixture. The separation of (R)-and (S)-tocainide on a glass capillary column coated with Chirasil-Val@" has been applied in various ~ t u d i e s .The ~ , ~same GC approach with the optically and only moderate active hydantoin derivative of tocainide was in~estigated'~ separation was accomplished (a 1.03). Formation of a diastereomeric

-

362

K.-J. HOFFMANN, L. RENBERG AND 0. GYLLENHAAL

derivative by extractive alkylation with (-)-myrtenyl bromide did improve separation slightly on a packed column but the hydantoin partly racemizes under the alkaline conditions used. l 5 By separate administration of the tocainide enantiomers, however, their stereospecific conjugation could be measured as no interconversion of the drug is taking place in vivo. Pharmacokinetics Racemic tocainide is used clinically and only recently stereoselectivity of pharmacodynamics in vivo and in vitro,16effects of the enantiomers on experimental arrhythmia^,'^ stereospecific interaction with cardiac sodium channel,'* and effects on guinea-pig monophasic action potential durationt9 have been reported. The kinetics of the enantiomers given separately in the present study are in good agreement with those reported after administration of the racemate to healthy subjects6 and patienkg These results may indicate that the enantiomers do not influence their kinetic behaviour at the dose level studied. Interestingly, if the individual plasma concentrations of (R)- and (S)-tocainide were added the mean value for tl12= 12.5 h correlates well with those reported without stereospecific analysis (13.5 h,4 11 h,3and 13-6h20). Renal clearance of racemic tocainide conjugate has been reported in patients with acute myocardial infarction.20Obtained values, 140 ml min-', were slightly higher compared to those reported in Tables 1 and 2. The plasma levels of the metabolite in the infarction patients were described by a monoexponential equation with a f I I 2 of 13 h.20 Linear regression analysis of the individually added plasma concentrations of the (R)- and (S)-metabolite vs time in the present study revealed a mean tII2= 10.1 h which correlates well with the mean The tl12= 10.3 h evaluated from urinary excretion data of racemic metab~lite.~ low urinary recovery of (S)-metabolite was virtually the same as in three volunteers receiving a tracer dose of S[3H]-tocainide.7Furthermore, if the overall recovery of (R)- and (S)-tocainide conjugate is taken collectively, the results, 23 per cent, are almost identical with data in the literature for the racemate.I2 Plasma concentration, tl12and urinary recovery of the (R)-metabolite indicated that the kinetics of the metabolite are formation rate limited and that conjugation is the major metabolic pathway for (R)-tocainide. Similarly,the comparable half-life of (S)-tocainide and its metabolite is consistent with formation ratelimited kinetics of this metabolite as well, while the lower urinary recovery indicates that (S)-tocainide is a poor substrate for the enzyme catalysing the formation of the conjugate. Similar results have been reported for mexiletine, a structure closely related to tocainide, which is conjugated with glucuronic acid. This reaction has been found to be stereoselective with (R)-mexiletine being more extensively conjugated than the corresponding (S)-enantiomer.21 The mean (R)/(S) enantiomeric ratio in human urine of the mexiletine conjugates was 9.65.22The corresponding value for tocainide was 36.8 (Table 3). Most likely a different metabolic reaction not identified so far is taking place

STEREOSELECTIVE TOCAINIDE METABOLISM

363

with (S)-tocainide. This process seems to be less efficient than the conjugation of (R)-tocainide because of the longer half-life of (S)-tocainide. The potential of sequential metabolism specific for the (S)-conjugatemay be ruled out because the polar compound is unlikely to be a good substrate for further metabolism. In conclusion, the formation of the carbamoyl conjugate was shown to be dependent on the stereochemistry of tocainide. This metabolic pathway, which may be unnoticed with other amine substrates due to instability of the intennediate carbamic acid, has recently been reported for the secondary amine sertraline.23Ethanolysis of this class of metabolite to give a stable ethyl carbamate derivative should prove convenient for the detection of these conjugate^.^^

REFERENCES 1. D. C. Harrison, Drugs, 31,93 (1986). 2. D. M. Roden and R. L. Woosley, New Engl. J. Med., 315,41(1986). 3. D. Lalka, M. B. Meyer, B. R. Duce and A. T. Elvin, Clin. Pharmacol. Ther., 19,757 (1976). 4. C. Graffner, T. B. Conradson, S . Hofvendahl and L. Ryden, Clin. Pharmacol. Ther., 27, 64 (1980). 5. K. Williams and E. Lee, Drugs, 30,333 (1985). 6. B. Edgar, A. Heggelund, L. Johansson, G. Nyberg and C. G. RegHrdh, Br.J.Clin.Pharmacol., 16,216P (1983). 7. K.-J. Hoffmann, L. Renberg and C . Baarnhielm, Eur. J. Drug Metab. Pharmacokinet., 9, 215 (1984). 8 . K. M. McErlane and G. K. Pillai, J.Chromatogr., 274, 129 (1983). 9. A. H. Thomson, G. Murdoch, A. Pottage, A. W. Kelman, B. Whiting and W. S . Hillis, Br. J.Clzn.Pharmacol.,21, 149 (1986). 10. J. Gal, T. A. French, T. Zysset and P. E. Haroldsen, Drug Metab. Disposit., 10,399 (1982). 11. A.-M. Antonsson, 0.Gyllenhaal, K. Kylberg-Hanssen, L. Johansson and J. Vessman, J. Chromatogr., 308,181 (1984). 12. A. T. Elvin, J. B. Keenaghan, E. W. Byrnes, P. A. Tenthorey, P. D. McMaster, B. H. Takman, D. Lalka, C. V. Manion, D. T. Baer, E. M. Wolshin, M. B. Meyer and R. A. Ronfeld, J. Pharm. Sci., 69,47 (1980). 13. K. J. Gipple, K. T. Chan, A. T. Elvin, D. Lalka and J. E. Axelson, J.Pharm.Sci., 71, 1011 (1982). 14. P.-0. Lagerstrom and B.-A. Person, J.Chrornatogr., 149,331 (1978). 15. 0.Gyllenhaal, B. L a m and J. Vessman, J.Chromatogr.,411,285 (1987). 16. A. J. Block, D. Merrilland E. R. Smith, J.Cardiovasc. Pharmacol., 11,216(1988). 17. A. C. G. Uprichard, D. J. Allen and D. W. G. Harron, J.Cardiovasc.Pharmacol., 11, 235 (1988). 18. R. S. Sheldon, N. J. Cannon, A. S. Nies and H. J. Duff, Molec.Pharmacol., 33,327 (1988). 19. 0. Almgren, G. Duker and J. Reid, Abstract Xth International Congress of Pharmacology, IUPHAR, Sydney, 1987. 20. R. A. Ronfeld, E. M. Wolshin and A. J. Block, Clin.Pharmacol.Ther.,31,384 (1982). 21. 0.Grech-BPlanger, J. Turgeon and M. Gilbert, J.Chromatogr., 337, 172 (1985). 22. 0.Grech-BBlanger, J. Turgeon and M. Gilbert, Br.J.Clin.Pharmacol., 21,481 (1986). 23. L. M. Tremaine, J. G. Stroh and R. A. Ronfeld, Drug Metab. Disposit., 17,58 (1989). 24. K. Straub, M. Davis and B. Hwang, Drug Metab. Disposit., 16,359 (1989).

Analysis and stereoselective metabolism after separate oral doses of tocainide enantiomers to healthy volunteers.

The potential of stereoselective metabolism of tocainide was studied in six healthy volunteers after separate oral administration of the pure enantiom...
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