Europ.J.clin.Pharmacol. 8, 353-357 (1975) © by Springer-Verlag 1975
Pharmacokinetics of Chlormethiazole in Humans R.G. Moore, E.J. Triggs, C.A. Shanks and J. Thomas Department of Pharmacy, University of Sydney, and Royal Prince Alfred Hospital, Sydney, Australia Received: August,
12, 1974,
accepted: December 15, 1974
Summary. The pharmacokinetics of chlormethiazole have been studied in six healthy volunteers following an intravenous infusion of the drug~ The log. plasma concentration-time curve of chlormethiazole after cessation of the infusion was found to be curvilinear and was fitted therefore, by a hi-exponential equation computed by non-linear least squares regression analysis. Half-lives for the inital a-phase (0.54 ± 0.05 h) and the terminal B-phase (4.05 ± 0.60 h) were calculated together with other pharmacokinetic parameters of the two compartment open model. An explanation for the discrepancy between the presently reported plasma half-lives and those appearing in the literature has been presented. The pharmacokinetic treatment of the plasma concentration-time data obtained following intravenous infusion also enabled the prediction that the maximal systemic availability of an orally administered dose of chlormethiazole would be of the order of 15%.
Key words: Chlormethiazole, pharmacokinetics, man, plasma levels, gas-liquid chromatography.
Chlormethiazole (Fig. I) is an anti-convulsant drug with sedative and hypnotic action. It has been used extensively in the treatment of conditions characterized by anxiety and agitation, withdrawal symptoms of alcoholism and, especially, delerium tremens. More recently chlormethiazole has been found effective in the treatment of preeclamptic toxaemia of pregnancy (1). Chlormethiazole is administered both orally and by intravenous infusion. The intravenous route has been found more suitable in the treatment of severe agitation, when a more rapid onset of action, or better control of dosage is required. Chlormethiazole plasma concentrations in man have been measured after the ingestion of several oral formulations (2,3) and a plasma half-life for the drug of approximately 53 min has been reported (3). There appears to be no information available in the literature with regard to plasma concentrations of chlormethiazole in man following intravenous administration, and consequently no meaningful elucidation of the pharmacokinetics in
N--C
II
HC
\S /
CH 3
II
C
NCH2CH2C1
Fig. i. Structural formula of chlormethiazole (4-methyl-5(2'-chloroethyl)thiazole)
man has been attempted. In the present study, the disposition kinetics of chlormethiazole have been studied in six healthy adults following intravenous infusion of the drug, and have been used to examine drug bioavailability and possible "first-pass" effect (4) after oral administration.
Materials and Methods Subjects and Dosing Three healthy volunteers of each sex received a constant-rate intravenous infusion of an 0.8% (w/v) solution of chlormethiazole ethanedisulphonate (Hemineurin ®) via the cephalic vein. The weight, sex, age, dose, and infusion rate of the drug for each subject are recorded in Table I. The dose and infusion rate were metered with a paediatric microdrip set (Metriset, McGaw). All subjects were supine during the infusion and for at least 30 min thereafter. In a separate study, subject G.M. received, on an empty stomach, an oral dose of 384 mg of chlormethiazole base in two gelatine capsules containing an equal amount of arachis oil. Blood samples (iO ml) were drawn through a catheter inserted in the cephalic vein of the opposite arm to that receiving the infusion and collected in heparinized plastic tubes. For subjects B.M., J.G. and P.D., blood samples were
354 Table ]. Subject details and pharmacokinetic parameters describing the disposition of chlormethiazole following intravenous infusion (~, ~, B', B ± standard deviation of the estimate) Subject Sex Dose Infusion Weight age (g) rate kg (yr) mg/min
~
~g/ml B.M. J.G. P.D. G.M. R.N. D.M.
F ] . 2 0 17.7 20 F 1 . 2 0 ]4.6 21 F 1 . 2 0 11.9 21 M 2 . 2 5 23.9 27 M 2 . 2 5 25.0 24 M 2 . 2 5 24.5 23
~
h-I
B'
B
~g/ml
h-I
tl~
h
tl~B k12
h
h-I
k21
h-I
kel
clear- EAUCiV] Plasma
h-I
ance ml/min/ (mgxh)/1 kg l/kg i/kg
Vl
V2
55 4.56±0.36
.32±0.15 0.84±0.14 0.17±0.03 0.53 4.13 0.50 0.35 0.64 2 4 . 0
;5.15
2.25 3.21
67 2.24±0.25
.36±0.20 0.83±0.15 0.17±0.04 0.51 4.05 0.56 0.49 0.47 2 7 . 8
10.73
3.55 4.06
50 7.13±0.59
.15±O.O9 I.O9±O.|5 O.|7±0.02 0.60 4.08 0.37 0.30 0.65 2 0 . 2
19.77
1.87 2.31
75 8.43±O.49
.13±O.O5 |.58±0.07 0.14±0.O1 0.6] 4.95 0.44 0.30 0.53 ]6.3
30.75
1.84 2.70
75 6.45±0.57
.36±O.15 1.46±0.17 0.23±0.O1 O.51 3.07 0.44 0.43 O.71 2 4 . 6
20.32
2.08 2.13
75 5.02±0.35
.38±O.|O 1.30±0.16 O.17±O.O1 0.50 4.03 0.57 0.42 0.56 24.9
20.09
2.65 3.60
Mean
0.54 4.05 0.48 0.38 0.59 22.97
2.37 3.00
Standard deviation
0.05 0.60 0.08 0.08 0.09 4.07
0.65 0.76
Standard deviation x IOO mean
taken before commencement of the infusion, following infusion of approximately one third and two thirds of the dose, at the cessation of infusion and then at approximately 0.25, 0.5, 0.75, I, 1.5, 2, 2.5, 3, 4, 5, 6, 7, 8, I0 and 12 h post-infusion. Subjects G.M., R.N. and D.M. had further blood samples taken at 14 and 16 h post-infusion. After oral administration of chlormethiazole to subject G.M. blood samples were taken at about 0.25, 0.5, 0.75, i, 1.33, 1.66, 2, 2.5 and 3 h after ingestion of the dose. All blood samples were centrifuged immediately and the plasma stored at -4oc prior to analysis. Urine was collected for 24 h after intravenous infusion to subjects G.M., R.N. and D.M. and stored as above. Chlormethiazole Assay I. Extraction. To plasma or urine (1-5 ml) was added internal standard (i ml, 3 ~g/ml solution of 2-methylquinoline in 0 . 1 M HCI) and NaOH solution (0.5 ml, 5 M). Freshly distilled anaesthetic grade ether (5 ml) was added to the combined solution which was then mixed on a vortex mixer for 1 min and the phases separated by centrifugation. The organic phase was transferred to another glass tube containing HCI (2 ml, 0. I M), then shaken and centrifuged as previously described. The organic phase was discarded, the aqueous phase basified with NaOH solution (I ml, 5 M) and extracted as above with freshly distilled anaesthetic grade ether. The etherial phase was transferred to an evaporation tube with an elongated capillary at the base and the ether evaporated by warming the contents of the tube on a water bath at 40°C. The internal walls of the tube were washed down with ether condensed by immersion of the stoppered tube in an ice bath towards the end of the evaporation procedure. When the extract had been concentrated to approximately 20 ~i, carbon
9.26 14.82 16.67 21.05 15.25 17.7
27.4 25.3
disulphide (15 ~i) was added and the majority of the remaining ether was then evaporated. The total remaining extract was injected into the gas chromatograph. Gas Chromatography A Hewlett Packard Model 5755 research gas chromatograph equipped with a flame ion±sat±on detector was used. A glass column (1.5 m, 2mm I.D.) silylated with 3% dimethyl-chlorosilane in toluene and packed with 3% OV-17 on Gaschrom Q (100/120 mesh) was used. The temperature of the injection port, oven and detector were maintained at 230oc, lO0°C and 250oc respectively. The nitrogen carrier gas and the hydrogen flow rates were maintained at 40 ml/min and the air flow rate at 400 ml/min. Under these conditions chlormethiazole had a retention time of 8 min and 2-methylquinoline of 10.6 min. A calibration curve was obtained by assaying plasma containing known amounts of chlormethiazole ethanedisulphonateandplotting the ratio of peak heights (chlormethiazole:2~methylquinoline) against amount of chlormethiazole ethanedisulphonate added. A linear relationship was obtained over the range 0.05 ~g to I0 ~g of chlormethiazole ethanedisulphonate. The peak height ratio of chlormethiazole to 2-methylquinoline was calculated for each sample and the amount of chlormethiazole ethanedisulphonate determined by reference to the calibration curve, Pharmacokinetics For all subjects, the disposition kinetics of chlormethiazole in the body after cessation of the intravenous infusion could be described by a two compartment open model (Fig. 2) (5). The timecourse of chlormethiazole concentration in plasma is given by the general solution (Eq. I) of the
355 1
kl 2
•
central
The volume of the central compartment VI, was calculated (7) by rearranging the following relationship: Dose Plasma Clearance ~ kelV 1 IAUC IV] 0
2 peripheral
I
k21
kel
Fig. 2. Two compartment open model describing the disposition kinetics of chlormethiazole in man. k12 and k21 are first order rate constants of distribution, kel is the first order rate constant of elimination from the central compartment.
to g i v e
Dose -
VI
[I]
where CPt is the concentration of chlormethiazole in plasma, ~ and ~ are constants l, a and ~ are the rapid and slow hybrid rate constants, respectively (5). Estimation of parameters of Eq. 1 for each subject's post-infusion plasma concentration-time data was obtained by the use of SAAM, a nonlinear regression analysis program (6), on a CDC 6600 computer. The microscopic rate constants peculiar to this model were calculated from the following equations (7):
[8]
kel
k12 v2
x vI
[9]
k21 Gibaldi et al. (4) have reported equations which can be used to predict, from plasma leveltime curves following intravenous or oral administration, the approximate maximal bioavailability of an orally administered drug. The following equations 10-12 were developed to calculate the systemic availability (f) which is the fraction of drug administered orally which reaches the systemic circulation.
f =
+ A~I3)
A' ~ + B'a A'+ }~
k2] -
[AUCIVI0
~UCoral]~
/ oral dose
[I~
[2]
A' B' (~-a) 2
k12 = (A' + B') (aB'
x - -
The volume of the peripheral compartment V 2 was calculated as in Eq. 9.
system of differential equations resulting from the two-compartment open model: CP t = ~e -~t + Be-fit
1 ~
~UCIv] ~0
/IV dose
[3] IV dose f=
~
[4]
+ Bh
0.693
t! a
Ill] [Ar aV]t0 e x U flOw C l
aft (~J + B)
kel =
l-
[5]
f =
flow rate
~
2
flow rate + (oral dose/ tl
[AUCoral]o
)
0.693 -
-
F 6 ].I L
-
The plasma clearance for chlormethiazole was calculated from Eq. 7. Plasma Clearance =
Dose
[7]
where, ~ U C I ~ ~ is the area under the plasma leveltime curve, rnis area was calculated by summing the area under the curve from drug administration to infusion cessation (measured by the trapezoidal rule) and from infusion cessation to infinite time (computed as ~/a +B]B).
where, UCora .0 is the area under the plasma concentrat~on-t~me curve from t = o to t = after oral drug administration, FAUCIv]0 is the area under the plasma concentratlon-tlme curve from t = O to t = oo after intravenous drug administration, flow rate is the hepatic blood flow rate taken to be 1.7 I/min (8). The value of f provided by these equations is a maximal estimate of the fraction of an equivalent oral dose which would be systemically available, since it does not allow for incomplete release of the drug from the dosage form, incomplete absorption, or extrahepatic metabolism of the drug.
Results = A/a(1_ex p (-at)) and B' = B/B(l_exp(_~t) ) where t = time of infusion cessation.
! A'
All chlormethiazole quantities are expressed in terms of the ethanedisulphonate salt.
356 Analytical Method
Discussion
The analytical method was found to be applicable to the determination of chlormethiazole concentrations in both plasma and urine. No peaks with retention times corresponding to chlormethiazole or 2-methylquinoline could be detected when urine and plasma samples collected before drug administration were used. The low detector response to carbon disulphide enabled quantitation of amounts of chlormethiazole as low as 50 ng. No difference was found in the calibration curves when chlormethiazole was assayed from plasma or urine. The calibration curve was linear (r = 0.999, n = 9) over the concentration range 0.05 to i0 vg/ml and passed through the origin. Assays of i Pg of chlormethiazole ethanedisulphonate from i, 2, 3 and 4 ml plasma samples had a coefficient of variance of 1.72%.
The suitability of the two compartment open model for describing the disposition kinetics of chlormethiazole was supported by the low standard deviation estimates for the computer generated values of ~, B', a and B (see Table i). The lack of any trend away from the computed curve by the experimentally obtained data for all subjects also indicated the adequacy of this model. The output from the SAAM program also included a correlation matrix which gave a measure of the dependence of A', B~ a and ~ terms for each other. The correlation matrices for each of the six subjects showed a low degree of interactive dependence for these terms and hence supported the choice of a two rather than a one compartment model for the data analysis. An example of a semi-logarithmic plot showing the computer calculated and experimentally determined plasma concentration-time data following cessation of infusion in one typical subject is presented in Fig. 3. Other workers (2,3) who have followed the concentration of chlormethiazole in plasma after oral dosing of healthy subjects found no such biphasic decay. However, the limited sensitivity of their analytical method (2) and the low doses administered orally, allowed the plasma concentration to be followed for only 3 h. The data plotted in Fig. 3 shows that the disposition of chlormethiazole in the 3 h after dosing will be significantly influenced by the distribution phase. The plasma half-lives of about 53 min reported by Fischler e# al. (3) were more probably a reflection of the distribution rather than the elimination of chlormethiazole from the central compartment. The mean volume of distribution for the central compartment, V 1 (2.37 I/kg) being much higher than the plasma volume (approx. 0.05 I/kg) (9a) and the mean total volume of distribution, V I + V 2 (5.37 I/kg) also being much greater than the volume of total body water (approx. 0.6 I/kg) (9b) implies a high degree of tissue distribution for chlormethiazole.
Chlormethiazole
Disposition Kinetics
The individual plasma concentration-time curves after cessation of the intravenous infusion were fitted satisfactorily in each case by a relationship of the type described by Eq. I. The computer generated values of f, ~, a and B with their respective estimated standard deviations are presented in Table I. These results yield a mean half-life among the suhjects of 0.54 h for the initial phase (p-phase) and a mean half-life of 4.05 h for the terminal B-phase. The rate constants k12, k21, and kel , the volume constants V I and V2, and the plasma clearance relevant to the model are also presented in Table I. The systemic availability (f) was calculated for the six subjects using Eq. II and the results are summarised below. On the average only 15.2% (f = O.152) of an orally administered equivalent dose will he systemically available. The individual values of (f) were B.M. (0.223); J.G. (0); P.D. (0.405); G.M. (0.282); R.N. (0) D.M. (0). Plasma concentrations of chlormethiazole could be followed for only 1.66 h after oral administration of ehlormethiazole to subject G.M,; the maximum concentration of 0.4 vg/ml being reached after i h. The area under the plasma concentrationtime curve ~UCora~ ~ was calculated to be 1.223 mg x h/ml using~'the tZapezoidal rule and an extrapolation to infinity correction of CPt/B where CPt is the last plasma concentration of chlormethiazole determined and B assumed to equal the mean value reported in Table I, A systemic availability of 15% was obtained when this value of AUCora~ ~ and the appropriate data for subject M. from Table i were substituted into Eq. IO. Using Equations Ii and 12 and the data for subject G.M. the predicted systemic availability was 28% and 17% respectively. The amount of unchanged chlormethiazole was measured in urine collected during 24 h after dosing to subjects G.M., R.N. andD.M. Less than 5% of the dose could be accounted for as unchanged chlormethiazole in the urine from each subject.
~g/m[ 10 6 4 2 1.0
o Experimentelty determined point + Computer calcutated point x Point where the above coincide
6xo +6 q
0.6 vr~
x
•
o ÷
0.2
×
Fig. 3. Plasma concentrations of chlormethiazole following cessation of the intravenous infusion to subject P.D.
Q
357 The highest renal clearance values of approximately 650 ml/min have been obtained after intravenous administration of iodine compounds, paraamminohippuric acid and phenol red, and may be considered as the normal plasma-kidney flow rate (10). Since the plasma clearance of chlormethiazole for the average 70 kg man would be of the order of 1610 ml/min, then metabolism is indicated as a major route of elimination. This proposition is strengthened by the negligible quantities of unchanged chlormethiazole recovered from urine. After intraperitoneal injection of chlormethiazole into rat, small amounts of unchanged drug in urine have been reported (11). Since it is highly probable that hepatic metabolism is the main process of elimination of chlormethiazole from plasma, it is therefore likely that the systemic availability (f) of orally administered chlormethiazole will be low because of appreciable metabolism on the drug's initial passage through the liver. Gibaldi et aS. (4) have developed equations to predict the maximum fraction of the dose of an orally administered drug reaching the systemic circulation from plasma concentration-time data. The usefulness of these equations has been demonstrated by Perrier and Gibaldi (12) for propoxyphene, and Perrier et al. (13) for alprenolol. A significant "first-pass" effect for orally administered chlormethiazole has been predicted from the plasma concentrationtime data from the six healthy subjects following intravenous dosing. Treatment of this data indicated that less than 15% of an orally administered dose would be systemically available. In order to gauge the accuracy of this prediction chlormethiazole was administered orally to subject G.M. who had also participated in the intravenous infusion study. The experimentally derived value of f for this subject indicated that the systemic availability of a 610 mg dose of chlormethiazole ethanedisulphonate would be 15% which agrees well with the value of 17% predicted from the oral dosing study. A low value for f was further confirmed for this subject when Eq. ll was used giving a systemic availability of 28%. From these observations it appears that a substantial fraction of an orally administered dose of chlormethiazole will be metabolized during its initial passage through the liver. Prior knowledge of the plasma clearance of chlormethiazole would enable the design of intravenous dosage regimens in order to maintain specified plasma concentrations of the drug. Although the variance of this parameter is relatively low among the six healthy volunteers, the plasma clearance of chlormethiazole is likely to be more variable in patients suffering from severe toxaemia of pregnancy or liver cirrhosis, it is planned to report in a subsequent paper the results of a current investigation of the influence of these disease states on the disposition of chlormethiazole.
References I.
2.
3.
4.
5.
6.
7.
8.
9.
i0.
ii.
12.
Tunstall, M., in: Official transation II. Chlormethiazole (Hemineurin, Astra), (Ingerslev, M., Resen Steenstrup, O., Eds.), pp. 20-27. Aarhus: Universitetsforlaget, 17th Congr. Obstet. Gynec. 1972 Frisch, E.P., Ortengren, B.: Plasma concentration of chlormethiazole following oral intake of tablets and capsules. Acta. psychiatr. scand., 42 (Suppl. 192), 35-40 (1966) Pischler, M., Frisch, P., Ortengren, B.: Plasma concentrations after oral administration of different pharmaceutical preparations of chlormethiazole. Acta Pharm. Suecica 10, 483-492 (1973) Gibaldi, M., Boyes, R.N., Feldman, S.: Influence of first-pass effect on availability of drugs on oral administration. J. pharm. Sci. 60, 1338-1340 (1971) Riegelman, S., Loo, J.C.K., Rowland, M.: Shortcomings in pharmacokinetic analysis by conceiving the body to exhibit properties of a single compartment. J. pharm. Sci. 57, 117123 (1968) Berman, M., Weiss, M.F.: Users manual for SAAM. National Institute for Arthritis and Metabolic Diseases, Bethesda, Maryland ]968 Portman, G.A., in: Pharmacokinetics. Current concepts in the pharmaceutical sciences: biopharmaceutics, pp. 6-13 (Swarbrick, J., Ed.). Philadelphia: Lea and Febiger 1970 Price, H.L., Kovnat, P.J., Safer, J.N., Conner, E.M., Price, M.L.: The uptake of thiopental by body tissues and its relation to the duration of narcosis. Clin. Pharmacol. Ther. I~ 16-22 (1960) Scientific Tables, 7th ed.(a) p. 555 (b) p.517 (Diem, K., Lentner, C., Eds.). Basle: CibaGeigy Limited 1971 Goldstein, A., Aronow, L., Kalman, S., in: Principles of drug action, p. 194. New York: Harper and Row 1969 Herbertz, G., Reinauer, H.: Stoffwechsel yon Chlormethiazol in der Ratte. Naunyn-Schmiedebergs Arch. Pharmak. 270, 192-202 (1971) Perrier, D., Gibaldi, M.: Influence of firstpass effect on the systemic availability of propoxyphene. J. clin. Pharmaeol. 12, 449-452
(1972) 13. Perrier, D., Gibaldi, M., Boyes, R.N.: Predication of systemic availability from plasma-level data after oral drug administration. J. Pharm. Pharmacol. 25, 256-257 (1973) Dr. J. Thomas Dept. of Pharmacy Univ. of Sydney Sydney, N.S.W. 2006 Australia
Pharmacokinetics of Chlormethiazole in Humans R.G. Moore, E.J. Triggs, C.A. Shanks and J. Thomas Department of Pharmacy, University of Sydney, and Royal Prince Alfred Hospital, Sydney, Australia Received: August,
12, 1974,
accepted: December 15, 1974
Summary. The pharmacokinetics of chlormethiazole have been studied in six healthy volunteers following an intravenous infusion of the drug~ The log. plasma concentration-time curve of chlormethiazole after cessation of the infusion was found to be curvilinear and was fitted therefore, by a hi-exponential equation computed by non-linear least squares regression analysis. Half-lives for the inital a-phase (0.54 ± 0.05 h) and the terminal B-phase (4.05 ± 0.60 h) were calculated together with other pharmacokinetic parameters of the two compartment open model. An explanation for the discrepancy between the presently reported plasma half-lives and those appearing in the literature has been presented. The pharmacokinetic treatment of the plasma concentration-time data obtained following intravenous infusion also enabled the prediction that the maximal systemic availability of an orally administered dose of chlormethiazole would be of the order of 15%.
Key words: Chlormethiazole, pharmacokinetics, man, plasma levels, gas-liquid chromatography.
Chlormethiazole (Fig. I) is an anti-convulsant drug with sedative and hypnotic action. It has been used extensively in the treatment of conditions characterized by anxiety and agitation, withdrawal symptoms of alcoholism and, especially, delerium tremens. More recently chlormethiazole has been found effective in the treatment of preeclamptic toxaemia of pregnancy (1). Chlormethiazole is administered both orally and by intravenous infusion. The intravenous route has been found more suitable in the treatment of severe agitation, when a more rapid onset of action, or better control of dosage is required. Chlormethiazole plasma concentrations in man have been measured after the ingestion of several oral formulations (2,3) and a plasma half-life for the drug of approximately 53 min has been reported (3). There appears to be no information available in the literature with regard to plasma concentrations of chlormethiazole in man following intravenous administration, and consequently no meaningful elucidation of the pharmacokinetics in
N--C
II
HC
\S /
CH 3
II
C
NCH2CH2C1
Fig. i. Structural formula of chlormethiazole (4-methyl-5(2'-chloroethyl)thiazole)
man has been attempted. In the present study, the disposition kinetics of chlormethiazole have been studied in six healthy adults following intravenous infusion of the drug, and have been used to examine drug bioavailability and possible "first-pass" effect (4) after oral administration.
Materials and Methods Subjects and Dosing Three healthy volunteers of each sex received a constant-rate intravenous infusion of an 0.8% (w/v) solution of chlormethiazole ethanedisulphonate (Hemineurin ®) via the cephalic vein. The weight, sex, age, dose, and infusion rate of the drug for each subject are recorded in Table I. The dose and infusion rate were metered with a paediatric microdrip set (Metriset, McGaw). All subjects were supine during the infusion and for at least 30 min thereafter. In a separate study, subject G.M. received, on an empty stomach, an oral dose of 384 mg of chlormethiazole base in two gelatine capsules containing an equal amount of arachis oil. Blood samples (iO ml) were drawn through a catheter inserted in the cephalic vein of the opposite arm to that receiving the infusion and collected in heparinized plastic tubes. For subjects B.M., J.G. and P.D., blood samples were
354 Table ]. Subject details and pharmacokinetic parameters describing the disposition of chlormethiazole following intravenous infusion (~, ~, B', B ± standard deviation of the estimate) Subject Sex Dose Infusion Weight age (g) rate kg (yr) mg/min
~
~g/ml B.M. J.G. P.D. G.M. R.N. D.M.
F ] . 2 0 17.7 20 F 1 . 2 0 ]4.6 21 F 1 . 2 0 11.9 21 M 2 . 2 5 23.9 27 M 2 . 2 5 25.0 24 M 2 . 2 5 24.5 23
~
h-I
B'
B
~g/ml
h-I
tl~
h
tl~B k12
h
h-I
k21
h-I
kel
clear- EAUCiV] Plasma
h-I
ance ml/min/ (mgxh)/1 kg l/kg i/kg
Vl
V2
55 4.56±0.36
.32±0.15 0.84±0.14 0.17±0.03 0.53 4.13 0.50 0.35 0.64 2 4 . 0
;5.15
2.25 3.21
67 2.24±0.25
.36±0.20 0.83±0.15 0.17±0.04 0.51 4.05 0.56 0.49 0.47 2 7 . 8
10.73
3.55 4.06
50 7.13±0.59
.15±O.O9 I.O9±O.|5 O.|7±0.02 0.60 4.08 0.37 0.30 0.65 2 0 . 2
19.77
1.87 2.31
75 8.43±O.49
.13±O.O5 |.58±0.07 0.14±0.O1 0.6] 4.95 0.44 0.30 0.53 ]6.3
30.75
1.84 2.70
75 6.45±0.57
.36±O.15 1.46±0.17 0.23±0.O1 O.51 3.07 0.44 0.43 O.71 2 4 . 6
20.32
2.08 2.13
75 5.02±0.35
.38±O.|O 1.30±0.16 O.17±O.O1 0.50 4.03 0.57 0.42 0.56 24.9
20.09
2.65 3.60
Mean
0.54 4.05 0.48 0.38 0.59 22.97
2.37 3.00
Standard deviation
0.05 0.60 0.08 0.08 0.09 4.07
0.65 0.76
Standard deviation x IOO mean
taken before commencement of the infusion, following infusion of approximately one third and two thirds of the dose, at the cessation of infusion and then at approximately 0.25, 0.5, 0.75, I, 1.5, 2, 2.5, 3, 4, 5, 6, 7, 8, I0 and 12 h post-infusion. Subjects G.M., R.N. and D.M. had further blood samples taken at 14 and 16 h post-infusion. After oral administration of chlormethiazole to subject G.M. blood samples were taken at about 0.25, 0.5, 0.75, i, 1.33, 1.66, 2, 2.5 and 3 h after ingestion of the dose. All blood samples were centrifuged immediately and the plasma stored at -4oc prior to analysis. Urine was collected for 24 h after intravenous infusion to subjects G.M., R.N. and D.M. and stored as above. Chlormethiazole Assay I. Extraction. To plasma or urine (1-5 ml) was added internal standard (i ml, 3 ~g/ml solution of 2-methylquinoline in 0 . 1 M HCI) and NaOH solution (0.5 ml, 5 M). Freshly distilled anaesthetic grade ether (5 ml) was added to the combined solution which was then mixed on a vortex mixer for 1 min and the phases separated by centrifugation. The organic phase was transferred to another glass tube containing HCI (2 ml, 0. I M), then shaken and centrifuged as previously described. The organic phase was discarded, the aqueous phase basified with NaOH solution (I ml, 5 M) and extracted as above with freshly distilled anaesthetic grade ether. The etherial phase was transferred to an evaporation tube with an elongated capillary at the base and the ether evaporated by warming the contents of the tube on a water bath at 40°C. The internal walls of the tube were washed down with ether condensed by immersion of the stoppered tube in an ice bath towards the end of the evaporation procedure. When the extract had been concentrated to approximately 20 ~i, carbon
9.26 14.82 16.67 21.05 15.25 17.7
27.4 25.3
disulphide (15 ~i) was added and the majority of the remaining ether was then evaporated. The total remaining extract was injected into the gas chromatograph. Gas Chromatography A Hewlett Packard Model 5755 research gas chromatograph equipped with a flame ion±sat±on detector was used. A glass column (1.5 m, 2mm I.D.) silylated with 3% dimethyl-chlorosilane in toluene and packed with 3% OV-17 on Gaschrom Q (100/120 mesh) was used. The temperature of the injection port, oven and detector were maintained at 230oc, lO0°C and 250oc respectively. The nitrogen carrier gas and the hydrogen flow rates were maintained at 40 ml/min and the air flow rate at 400 ml/min. Under these conditions chlormethiazole had a retention time of 8 min and 2-methylquinoline of 10.6 min. A calibration curve was obtained by assaying plasma containing known amounts of chlormethiazole ethanedisulphonateandplotting the ratio of peak heights (chlormethiazole:2~methylquinoline) against amount of chlormethiazole ethanedisulphonate added. A linear relationship was obtained over the range 0.05 ~g to I0 ~g of chlormethiazole ethanedisulphonate. The peak height ratio of chlormethiazole to 2-methylquinoline was calculated for each sample and the amount of chlormethiazole ethanedisulphonate determined by reference to the calibration curve, Pharmacokinetics For all subjects, the disposition kinetics of chlormethiazole in the body after cessation of the intravenous infusion could be described by a two compartment open model (Fig. 2) (5). The timecourse of chlormethiazole concentration in plasma is given by the general solution (Eq. I) of the
355 1
kl 2
•
central
The volume of the central compartment VI, was calculated (7) by rearranging the following relationship: Dose Plasma Clearance ~ kelV 1 IAUC IV] 0
2 peripheral
I
k21
kel
Fig. 2. Two compartment open model describing the disposition kinetics of chlormethiazole in man. k12 and k21 are first order rate constants of distribution, kel is the first order rate constant of elimination from the central compartment.
to g i v e
Dose -
VI
[I]
where CPt is the concentration of chlormethiazole in plasma, ~ and ~ are constants l, a and ~ are the rapid and slow hybrid rate constants, respectively (5). Estimation of parameters of Eq. 1 for each subject's post-infusion plasma concentration-time data was obtained by the use of SAAM, a nonlinear regression analysis program (6), on a CDC 6600 computer. The microscopic rate constants peculiar to this model were calculated from the following equations (7):
[8]
kel
k12 v2
x vI
[9]
k21 Gibaldi et al. (4) have reported equations which can be used to predict, from plasma leveltime curves following intravenous or oral administration, the approximate maximal bioavailability of an orally administered drug. The following equations 10-12 were developed to calculate the systemic availability (f) which is the fraction of drug administered orally which reaches the systemic circulation.
f =
+ A~I3)
A' ~ + B'a A'+ }~
k2] -
[AUCIVI0
~UCoral]~
/ oral dose
[I~
[2]
A' B' (~-a) 2
k12 = (A' + B') (aB'
x - -
The volume of the peripheral compartment V 2 was calculated as in Eq. 9.
system of differential equations resulting from the two-compartment open model: CP t = ~e -~t + Be-fit
1 ~
~UCIv] ~0
/IV dose
[3] IV dose f=
~
[4]
+ Bh
0.693
t! a
Ill] [Ar aV]t0 e x U flOw C l
aft (~J + B)
kel =
l-
[5]
f =
flow rate
~
2
flow rate + (oral dose/ tl
[AUCoral]o
)
0.693 -
-
F 6 ].I L
-
The plasma clearance for chlormethiazole was calculated from Eq. 7. Plasma Clearance =
Dose
[7]
where, ~ U C I ~ ~ is the area under the plasma leveltime curve, rnis area was calculated by summing the area under the curve from drug administration to infusion cessation (measured by the trapezoidal rule) and from infusion cessation to infinite time (computed as ~/a +B]B).
where, UCora .0 is the area under the plasma concentrat~on-t~me curve from t = o to t = after oral drug administration, FAUCIv]0 is the area under the plasma concentratlon-tlme curve from t = O to t = oo after intravenous drug administration, flow rate is the hepatic blood flow rate taken to be 1.7 I/min (8). The value of f provided by these equations is a maximal estimate of the fraction of an equivalent oral dose which would be systemically available, since it does not allow for incomplete release of the drug from the dosage form, incomplete absorption, or extrahepatic metabolism of the drug.
Results = A/a(1_ex p (-at)) and B' = B/B(l_exp(_~t) ) where t = time of infusion cessation.
! A'
All chlormethiazole quantities are expressed in terms of the ethanedisulphonate salt.
356 Analytical Method
Discussion
The analytical method was found to be applicable to the determination of chlormethiazole concentrations in both plasma and urine. No peaks with retention times corresponding to chlormethiazole or 2-methylquinoline could be detected when urine and plasma samples collected before drug administration were used. The low detector response to carbon disulphide enabled quantitation of amounts of chlormethiazole as low as 50 ng. No difference was found in the calibration curves when chlormethiazole was assayed from plasma or urine. The calibration curve was linear (r = 0.999, n = 9) over the concentration range 0.05 to i0 vg/ml and passed through the origin. Assays of i Pg of chlormethiazole ethanedisulphonate from i, 2, 3 and 4 ml plasma samples had a coefficient of variance of 1.72%.
The suitability of the two compartment open model for describing the disposition kinetics of chlormethiazole was supported by the low standard deviation estimates for the computer generated values of ~, B', a and B (see Table i). The lack of any trend away from the computed curve by the experimentally obtained data for all subjects also indicated the adequacy of this model. The output from the SAAM program also included a correlation matrix which gave a measure of the dependence of A', B~ a and ~ terms for each other. The correlation matrices for each of the six subjects showed a low degree of interactive dependence for these terms and hence supported the choice of a two rather than a one compartment model for the data analysis. An example of a semi-logarithmic plot showing the computer calculated and experimentally determined plasma concentration-time data following cessation of infusion in one typical subject is presented in Fig. 3. Other workers (2,3) who have followed the concentration of chlormethiazole in plasma after oral dosing of healthy subjects found no such biphasic decay. However, the limited sensitivity of their analytical method (2) and the low doses administered orally, allowed the plasma concentration to be followed for only 3 h. The data plotted in Fig. 3 shows that the disposition of chlormethiazole in the 3 h after dosing will be significantly influenced by the distribution phase. The plasma half-lives of about 53 min reported by Fischler e# al. (3) were more probably a reflection of the distribution rather than the elimination of chlormethiazole from the central compartment. The mean volume of distribution for the central compartment, V 1 (2.37 I/kg) being much higher than the plasma volume (approx. 0.05 I/kg) (9a) and the mean total volume of distribution, V I + V 2 (5.37 I/kg) also being much greater than the volume of total body water (approx. 0.6 I/kg) (9b) implies a high degree of tissue distribution for chlormethiazole.
Chlormethiazole
Disposition Kinetics
The individual plasma concentration-time curves after cessation of the intravenous infusion were fitted satisfactorily in each case by a relationship of the type described by Eq. I. The computer generated values of f, ~, a and B with their respective estimated standard deviations are presented in Table I. These results yield a mean half-life among the suhjects of 0.54 h for the initial phase (p-phase) and a mean half-life of 4.05 h for the terminal B-phase. The rate constants k12, k21, and kel , the volume constants V I and V2, and the plasma clearance relevant to the model are also presented in Table I. The systemic availability (f) was calculated for the six subjects using Eq. II and the results are summarised below. On the average only 15.2% (f = O.152) of an orally administered equivalent dose will he systemically available. The individual values of (f) were B.M. (0.223); J.G. (0); P.D. (0.405); G.M. (0.282); R.N. (0) D.M. (0). Plasma concentrations of chlormethiazole could be followed for only 1.66 h after oral administration of ehlormethiazole to subject G.M,; the maximum concentration of 0.4 vg/ml being reached after i h. The area under the plasma concentrationtime curve ~UCora~ ~ was calculated to be 1.223 mg x h/ml using~'the tZapezoidal rule and an extrapolation to infinity correction of CPt/B where CPt is the last plasma concentration of chlormethiazole determined and B assumed to equal the mean value reported in Table I, A systemic availability of 15% was obtained when this value of AUCora~ ~ and the appropriate data for subject M. from Table i were substituted into Eq. IO. Using Equations Ii and 12 and the data for subject G.M. the predicted systemic availability was 28% and 17% respectively. The amount of unchanged chlormethiazole was measured in urine collected during 24 h after dosing to subjects G.M., R.N. andD.M. Less than 5% of the dose could be accounted for as unchanged chlormethiazole in the urine from each subject.
~g/m[ 10 6 4 2 1.0
o Experimentelty determined point + Computer calcutated point x Point where the above coincide
6xo +6 q
0.6 vr~
x
•
o ÷
0.2
×
Fig. 3. Plasma concentrations of chlormethiazole following cessation of the intravenous infusion to subject P.D.
Q
357 The highest renal clearance values of approximately 650 ml/min have been obtained after intravenous administration of iodine compounds, paraamminohippuric acid and phenol red, and may be considered as the normal plasma-kidney flow rate (10). Since the plasma clearance of chlormethiazole for the average 70 kg man would be of the order of 1610 ml/min, then metabolism is indicated as a major route of elimination. This proposition is strengthened by the negligible quantities of unchanged chlormethiazole recovered from urine. After intraperitoneal injection of chlormethiazole into rat, small amounts of unchanged drug in urine have been reported (11). Since it is highly probable that hepatic metabolism is the main process of elimination of chlormethiazole from plasma, it is therefore likely that the systemic availability (f) of orally administered chlormethiazole will be low because of appreciable metabolism on the drug's initial passage through the liver. Gibaldi et aS. (4) have developed equations to predict the maximum fraction of the dose of an orally administered drug reaching the systemic circulation from plasma concentration-time data. The usefulness of these equations has been demonstrated by Perrier and Gibaldi (12) for propoxyphene, and Perrier et al. (13) for alprenolol. A significant "first-pass" effect for orally administered chlormethiazole has been predicted from the plasma concentrationtime data from the six healthy subjects following intravenous dosing. Treatment of this data indicated that less than 15% of an orally administered dose would be systemically available. In order to gauge the accuracy of this prediction chlormethiazole was administered orally to subject G.M. who had also participated in the intravenous infusion study. The experimentally derived value of f for this subject indicated that the systemic availability of a 610 mg dose of chlormethiazole ethanedisulphonate would be 15% which agrees well with the value of 17% predicted from the oral dosing study. A low value for f was further confirmed for this subject when Eq. ll was used giving a systemic availability of 28%. From these observations it appears that a substantial fraction of an orally administered dose of chlormethiazole will be metabolized during its initial passage through the liver. Prior knowledge of the plasma clearance of chlormethiazole would enable the design of intravenous dosage regimens in order to maintain specified plasma concentrations of the drug. Although the variance of this parameter is relatively low among the six healthy volunteers, the plasma clearance of chlormethiazole is likely to be more variable in patients suffering from severe toxaemia of pregnancy or liver cirrhosis, it is planned to report in a subsequent paper the results of a current investigation of the influence of these disease states on the disposition of chlormethiazole.
References I.
2.
3.
4.
5.
6.
7.
8.
9.
i0.
ii.
12.
Tunstall, M., in: Official transation II. Chlormethiazole (Hemineurin, Astra), (Ingerslev, M., Resen Steenstrup, O., Eds.), pp. 20-27. Aarhus: Universitetsforlaget, 17th Congr. Obstet. Gynec. 1972 Frisch, E.P., Ortengren, B.: Plasma concentration of chlormethiazole following oral intake of tablets and capsules. Acta. psychiatr. scand., 42 (Suppl. 192), 35-40 (1966) Pischler, M., Frisch, P., Ortengren, B.: Plasma concentrations after oral administration of different pharmaceutical preparations of chlormethiazole. Acta Pharm. Suecica 10, 483-492 (1973) Gibaldi, M., Boyes, R.N., Feldman, S.: Influence of first-pass effect on availability of drugs on oral administration. J. pharm. Sci. 60, 1338-1340 (1971) Riegelman, S., Loo, J.C.K., Rowland, M.: Shortcomings in pharmacokinetic analysis by conceiving the body to exhibit properties of a single compartment. J. pharm. Sci. 57, 117123 (1968) Berman, M., Weiss, M.F.: Users manual for SAAM. National Institute for Arthritis and Metabolic Diseases, Bethesda, Maryland ]968 Portman, G.A., in: Pharmacokinetics. Current concepts in the pharmaceutical sciences: biopharmaceutics, pp. 6-13 (Swarbrick, J., Ed.). Philadelphia: Lea and Febiger 1970 Price, H.L., Kovnat, P.J., Safer, J.N., Conner, E.M., Price, M.L.: The uptake of thiopental by body tissues and its relation to the duration of narcosis. Clin. Pharmacol. Ther. I~ 16-22 (1960) Scientific Tables, 7th ed.(a) p. 555 (b) p.517 (Diem, K., Lentner, C., Eds.). Basle: CibaGeigy Limited 1971 Goldstein, A., Aronow, L., Kalman, S., in: Principles of drug action, p. 194. New York: Harper and Row 1969 Herbertz, G., Reinauer, H.: Stoffwechsel yon Chlormethiazol in der Ratte. Naunyn-Schmiedebergs Arch. Pharmak. 270, 192-202 (1971) Perrier, D., Gibaldi, M.: Influence of firstpass effect on the systemic availability of propoxyphene. J. clin. Pharmaeol. 12, 449-452
(1972) 13. Perrier, D., Gibaldi, M., Boyes, R.N.: Predication of systemic availability from plasma-level data after oral drug administration. J. Pharm. Pharmacol. 25, 256-257 (1973) Dr. J. Thomas Dept. of Pharmacy Univ. of Sydney Sydney, N.S.W. 2006 Australia
