Chapter          2      

 

PHARMACOKINETICS OF METHADONE AND ITS PRIMARY METABOLITE IN TWENTY OPIATE ADDICTS

 

J.W. de Vos1, P.J. Geerlings2, W. van den Brink3, J.G.R. Ufkes1, H. van Wilgenburg1

 

1 Department of Pharmacology, Academic Medical Centre, University of Amsterdam, Meibergdreef 15, 1105 AZ  Amsterdam, The Netherlands.

2 Jellinek Centrum, Jacob Obrechtstraat 92, 1017 KR  Amsterdam, The Netherlands.

3 Amsterdam Institute for Addiction Research, Jacob Obrechtstraat 92, 1017 KR Amsterdam, The Netherlands.

 

printed in:

 

European Journal of Clinical Pharmacology (1995) 48: 361-366.


Since methadone maintenance treatment (MMT) was introduced (Dole and Nyswander, 1965), many studies have been conducted to establish the pharmacokinetics of methadone. Unfortunately they did not lead to a general consensus with regard to well-defined dosage schedules related to clinical and therapeutic efficacy. This might be explained in part by the very divergent results of most of the studies performed so far. For example, plasma half-lifes of methadone in tolerant subjects under MMT have varied from 19 h (Nilsson et al., 1983) to even 75 h (Ånggård et al., 1979). Both studies were performed in closed metabolic wards. The same variability has applied for bioavailability, which has ranged from 36% to 106 % (mean 87 %) among 12 tolerant opiate addicts (Nilsson et al., 1982), also in a closed metabolic ward. Another study revealed a mean bioavailability of 79 % (range 41-99 %)(Meresaar et al., 1981). Furthermore, there has been no consensus as to whether the disposition of methadone follows a one-(Olsen et al., 1981; Denson et al., 1990; Wolff et al., 1993) or a two-(Nilsson et al., 1982; Meresaar et al., 1981; Plummer et al., 1988) compartment model. However, most of these studies were performed under divergent, hardly comparable circumstances. The pharmacokinetics in non-tolerant ("naive") subjects might differ from those in tolerant subjects (Verebely et al., 1975; Ånggård et al., 1975). Also short-term methadone treatment might give different results from long-term (steady-state) treatment (Såwe et al., 1981; Horns et al., 1975). However, it seems that determinations of plasma methadone in outdoor addicts show wider scatters in elimination half-lifes than those performed in closed metabolic wards: 17.8-63.8 h (Wolff et al., 1993) and 18.9-43 h (Nilsson et al., 1983), respectively.

The metabolism of methadone has been studied intensively. Many metabolites have been traced and identified in human urine (Pohland et al., 1971; Beckett et al., 1968). Nevertheless essential knowledge about the formation and disposition of the major metabolite of methadone, EDDP (1,5-dimethyl-3,3-diphenyl-2-ethylidene-pyrrolidine) in plasma is still lacking.

The aim of our study was to establish the steady-state pharmacokinetics of methadone and its major metabolite in 20 opiate addicts under well-controlled conditions. For this reason we developed a high-performance liquid chromatography (HPLC) method to monitor methadone and EDDP concentrations in plasma simultaneously. The design of this study enabled us to supplement incomplete or conflicting data from earlier studies.

 

 

Methods

 

Subjects

 


Twenty long-term opiate addicts, each on MMT for at least 4 months, joined our study on a voluntary basis after their written informed consent. The study, performed in a closed metabolic ward (Jellinek Centrum, Amsterdam), which completely excluded illicit methadone or other drug supplementation, involved 4 days during which a 24-h period was used for plasma sampling. Standardized meals were supplied at fixed times. All participants were habitual smokers (20-30 cigarettes per day). The participants were subjected to physical examination, including clinical blood and urine analysis. Subject characteristics are shown in Table 1. Approval was obtained from the Medical Ethics Committee (Academic Medical Centre, University of Amsterdam) before the study was started.

 

 

Table 1 Subject characteristics

 

 

sub.

 

sex

 

age

 

weight

 

dose

 

blood analysis

 

urine

 

comedication

 

no.

 

m/f

 

y

 

kg

 

mg/day

 

(1)

 

(2)

 

(3)

 

(4)

 

(5)

 

(6)

 

(7)

 

pH

 

+/-

 

1

 

m

 

29

 

69

 

70

 

9.2

 

95

 

18

 

18

 

71

 

-

 

6

 

7.5

 

+a

 

2

 

f

 

31

 

62

 

40

 

9.4

 

90

 

8

 

6

 

88

 

13

 

23

 

5

 

+b

 

3

 

m

 

32

 

70

 

55

 

10.2

 

80

 

9

 

10

 

85

 

-

 

6

 

6

 

-

 

4

 

m

 

24

 

53

 

40

 

7.6

 

62

 

21

 

40

 

76

 

-

 

29

 

7

 

-

 

5

 

m

 

23

 

58

 

60

 

9.3

 

79

 

27

 

21

 

92

 

51

 

64

 

6

 

+c

 

6

 

f

 

26

 

49

 

30

 

8.1

 

75

 

17

 

25

 

59

 

10

 

2

 

5

 

-

 

7

 

f

 

32

 

60

 

50

 

8.1

 

83

 

18

 

22

 

43

 

40

 

-

 

5.5

 

+d

 

8

 

m

 

28

 

78

 

30

 

8.8

 

95

 

10

 

8

 

53

 

14

 

4

 

5

 

-

 

9

 

f

 

39

 

52

 

50

 

8.8

 

88

 

71

 

64

 

66

 

31

 

34

 

5.5

 

-

 

10

 

m

 

39

 

69

 

65

 

9.1

 

91

 

19

 

15

 

66

 

11

 

2

 

6.5

 

-

 

11

 

m

 

24

 

68

 

70

 

8.4

 

91

 

34

 

46

 

43

 

-

 

5

 

5

 

-

 

12

 

f

 

21

 

68

 

30

 

7.5

 

76

 

12

 

11

 

87

 

17

 

20

 

5

 

-

 

13

 

f

 

31

 

75

 

60

 

7.8

 

54

 

8

 

7

 

45

 

9

 

8

 

8

 

-

 

14

 

f

 

28

 

52

 

20

 

8.6

 

73

 

14

 

16

 

57

 

14

 

6

 

7

 

-

 

15

 

m

 

30

 

50

 

10

 

9.6

 

81

 

8

 

14

 

57

 

15

 

3

 

5

 

-

 

16

 

m

 

34

 

83

 

60

 

9.1

 

103

 

18

 

26

 

66

 

20

 

1

 

7

 

-

 

17

 

f

 

31

 

72

 

225

 

9.3

 

79

 

16

 

29

 

80

 

26

 

14

 

5.5

 

+d

 

18

 

f

 

28

 

75

 

70

 

8.9

 

84

 

68

 

132

 

78

 

41

 

8

 

5

 

+e

 

19

 

m

 

27

 

67

 

70

 

8.7

 

89

 

24

 

18

 

82

 

24

 

6

 

6

 

+d

 

20

 

m

 

28

 

72

 

90

 

8.5

 

73

 

15

 

11

 

51

 

13

 

2

 

5

 

-

 

Blood analysis: (1) haemoglobin (mmol·l-1); (2) creatinine (μmol·l-1); (3) aspartate aminotransferase (U·l-1); (4) alanine aminotransferase (U·l-1); (5) alkaline phosphatase (U·l-1); (6) γ-glutamyltransferase (U·l-1); (7) erythrocyte sedimentation rate (mm·h-1)

Comedication: aMesterolon/chlordiazepoxide; bFloctafenine; cIsoniazid/rifampicin/azidothymidine; dChlordiazepoxide; eDoxepine.

 

 

Drug Administration and Blood Sampling

 


The 20 subjects in the study were prescribed various daily amounts of d,l-methadone-HCl (Brocades, Leiderdorp, The Netherlands), prepared as a methadone linctus of 5 mg·ml-1, depending on the previous amount of opiate use. They received the daily oral dose (average 60 mg, range 10-225 mg methadone-HCl, see Table 1) at about 0920 hours just after the standardized breakfast. at 0, 0.5, 1,2, 4, 6, 8, 12 and 24 h thereafter blood samples (10 ml) were taken by venipuncture and placed in heparinized tubes. The blood samples were immediately centrifuged for 10 min at 1500 g. The supernatant plasma was stored frozen at -25oC until required for analysis. Prior to analysis all samples were preventatively HIV-deactivated by incubation at 56oC for 30 min.

 

Sample Preparation

 

Subject plasma (0.5 ml) was added with 50 μl methanol, 100 μl trihexyphenidyl-HCl (2 μg·ml-1) as internal standard (IS) to a stoppered glass tube and alkalized with 0.5 ml 2M K2CO3. This mixture was extracted into 3.5 ml n-hexane by gently agitating for 90 min at room temperature. After centrifugation (1500 g for 5 min) the tube contents were chilled to -25oC, which enabled us to separate the unfrozen upper layer (n-hexane layer) from the frozen lower layer. The n-hexane layer was evaporated to dryness under a gentle stream of dry nitrogen. The dry residue was redissolved in 100 μl KH2phosphate buffer (25 mM, pH 2.5). In order to prepare calibration curves the same procedure was performed using plasma from healthy, non-drug-using volunteers, which was spiked with methadone-HCl and EDDP-HClO4 (Sigma, St.Louis, USA) over a concentration range of 5 to 800 ng·ml-1.

 

Analytical Equipment

 

The HPLC system consisted of an HP 1050 Series Quarternary Pump and Variable Wavelength Detector (Hewlett Packard, USA), a Model 7125 Sample Injector (Rheodyne, USA) fitted with a 50-μl loop and a HP 3395 Integrator (Hewlett Packard, USA) in combination with a BD41 Kipp Recorder (Kipp & Zonen, Delft, The Netherlands). Separation was performed on a Supelcosil LC-ABZ column (50x4.6 mm ID) packed with 5-μm-diameter particles and protected by a 20-mm Supelguard column (Supelco, USA). The mobile phase consisted of KH2phosphate buffer (25 mM, pH 2.5) mixed with acetonitrile (78.5:21.5, v/v). The flow rate was set at 1.5 ml·min-1, whit UV detection at 206 nm. The analytical procedure was performed in an airconditioned room at about 20oC.

 

Calculations

 


The methadone and EDDP levels in the plasma of our subjects were calculated by comparing the UV-absorption values of the extracted subject samples with those of the extracted spiked plasma samples (calibration curves) using the internal standard method. Recovery values were calculated by comparing the UV absorption values of the extracted spiked plasma samples with those of the unextracted standard solutions of methadone HCl and EDDP HClO4 in KH2phosphate buffer in the concentration range 5-800 ng·ml-1. The area under the plasma concentration - time curve during the dosing interval, AUC(0-24h), for each subject at steady-state was calculated by the trapezoid rule (Gibaldi and Perrier, 1982). Time to peak concentration (tmax) of methadone and EDDP for each subject was the period (h:min) between the methadone administration at time 0 (0920 hours) and the peak concentration detected of methadone and EDDP, respectively. The steady-state concentration (Css) was calculated as the mean of the plasma concentration just before methadone administration and the plasma concentration 24 h later, just before the next methadone administration. By means of the method of residuals (Gibaldi and Perrier, 1982), values of ka, α, ß, A en B were calculated. Using these values in MW/Pharm, an integrated computerprogram with a nonlinear curve-fitting module for the assesment of pharmacokinetic parameters (Proost and Meijer, 1992), the data from plasma concentration measurements were fitted according to the two- or three-exponential equation to calculate the correlation coefficient (r) between observed and predicted values. Additionally the secondary rate constants, i.e. k10, k12, and k21 as well as the volumes of the central compartment (VC), the volumes of distribution during the post-distributive phase (Vβ) and the body clearances (CL) were calculated by the computer using the appropriate formula (Gibaldi and Perrier, 1982). The steady-state level in each curve was thus simulated using an imaginary loading dose calculated as: D·F·B / B-Css , where D is the individual daily dose (in milligrams) and F is the bioavailability which is assumed to be 0.90 (Inturissi et al., 1987).

 

Statistics

 

Data were expressed as the mean (SD) or the 95 % confidence interval as appropriate. The paired t-test was used to compare tmax values for methadone with those for EDDP. Student's t-test was used to compare methadone elimination rate constants (β) of the male subjects with those of the female subjects. Correlation analysis was performed using Spearman's rank-order correlation method.

 

 

Results

 

HPLC-assay validation

 

The determination was found to be both sensitive and specific for both methadone and its metabolite EDDP. Retention times for EDDP, IS and methadone were 2.5, 3.9 and 5.5 min respectively. Detection limits in plasma (signal-to-noise ratio at least 3) appeared to be about 4 ng·ml-1 for EDDP and 6 ng·ml-1 for methadone. The calibration curves for both EDDP and methadone in plasma showed linearity (r³0.998) in the concentration ranges 5-400 ng·ml-1 and 10-800 ng·ml-1, respectively. The calculated recovery values were 60.1 % (9.4), n = 95 for EDDP, 77.5 % (7.5), n = 97 for methadone and 80.6 % (6.2), n = 88 for IS. The day-to-day assay coefficient of variation was 6.7 % for EDDP, 7.7 % for methadone and 3.1 % for IS (n = 104).

 

Pharmacokinetics

 


Figure 1 shows a representative plasma concentration - time curve for methadone and EDDP (subject 13) after the oral ingestion of 60 mg methadone-HCl at time 0 (0920 hours). As can be seen the shape of the EDDP-curve paralleled that of the methadone curve, but at a substantially lower level. Another phenomenon observed in 19 out of the 20 methadone curves, is the rapid decrease after the peak concentration followed by a considerably slower disposition, indicating pharmacokinetics according to a two-compartment model. This was confirmed using the curve-fitting computerprogram , which preferred a two-compartment model to a one-compartment model in all these cases. However, in one case (subject 1) the pharmacokinetics of methadone was best described using a one-compartment model.

 

 

Fig 1     Plasma concentration-time curve for methadone and EDDP in subject                      No. 13 after the oral ingestion of 60 mg methadone HCl at time 0

 

The 20 data sets, including steady-state concentration (Css), peak concentration (Cmax), time to peak concentration (tmax) and AUC(0-24h) for methadone and EDDP, are shown in Table 2. The mean values of tmax for methadone (0220 hours, range 0106-0408 hours) differed significantly (P = 0.010, paired t-test) from those of EDDP (0149 hours, range 0057-0404 hours). The calculated ratios between AUC(0-24h) for methadone and AUC(0-24h) for EDDP varied from 5.9 - 44.6.

 

The pharmacokinetic parameters for methadone calculated by means of the residual method are shown in Table 3. The computer calculated secondary rate constants (k10, k12 and k21), the distribution volumes (VC and Vβ) and the body clearances (CL) are also given together with the correlation coefficient (r) between observed and predicted values. The mean elimination rate constant (β) was 0.026 h-1 (0.011) and the mean plasma half-life t½β as calculated from β, was 31.2 h (12.4) with a 95 % confidence interval from 25.6 to 37.0 h. Differences were found between the elimination rate constants (β) of the male subjects [0.030·h-1 (0.012), n = 11] and those of the female subjects [0.021·h-1 (0.057), n = 9], significance level: P = 0.067 (Student's t-test).

 

 

 

 

 

 


Table 2 Analytical data sets for the 20 subjects

 

 

 

 

 

 

Methadone determinations

 

EDDP determinations

 

Ratios

 

sub.

 

dose

 

Css

 

Cmax

 

tmax

 

AUC(0-24h)

 

Css

 

Cmax

 

tmax

 

AUC(0-24h)

 

AUC(0-24h)methadone /

 

no.

 

mg·kg-1

 

ng·ml-1

 

ng·ml-1

 

h:min

 

mg·h·l-1

 

ng·ml-1

 

ng·ml-1

 

h:min

 

mg·h· l-1

 

AUC(0-24h)EDDP

 

1

 

1.01

 

487

 

699

 

2:32

 

13.08

 

33

 

53

 

1:27

 

0.81

 

16.2

 

2

 

0.65

 

305

 

668

 

1:53

 

11.20

 

15

 

27

 

0:58

 

0.47

 

23.8

 

3

 

0.79

 

137

 

324

 

2:19

 

5.05

 

13

 

42

 

2:19

 

0.45

 

11.2

 

4

 

0.75

 

305

 

651

 

2:30

 

10.03

 

8

 

23

 

1:00

 

0.23

 

44.6

 

5

 

1.03

 

115

 

323

 

3:02

 

3.60

 

12

 

50

 

1:26

 

0.61

 

5.9

 

6

 

0.61

 

287

 

639

 

2:00

 

9.44

 

15

 

40

 

0:57

 

0.42

 

22.6

 

7

 

0.83

 

279

 

531

 

2:28

 

8.96

 

26

 

65

 

1:40

 

0.86

 

10.5

 

8

 

0.38

 

65

 

180

 

2:09

 

2.61

 

5

 

10

 

2:09

 

0.16

 

16.0

 

9

 

0.96

 

451

 

755

 

2:36

 

13.69

 

16

 

34

 

4:02

 

0.49

 

27.8

 

10

 

0.94

 

282

 

586

 

1:59

 

8.95

 

25

 

81

 

1:03

 

0.89

 

10.4

 

11

 

1.03

 

254

 

570

 

4:04

 

9.52

 

17

 

39

 

4:04

 

0.54

 

17.6

 

12

 

0.44

 

191

 

400

 

1:47

 

6.25

 

11

 

20

 

1:47

 

0.28

 

22.1

 

13

 

0.80

 

114

 

279

 

2:02

 

4.07

 

7

 

37

 

1:00

 

0.37

 

11.1

 

14

 

0.38

 

88

 

160

 

2:18

 

2.59

 

9

 

16

 

2:18

 

0.24

 

10.6

 

15

 

0.20

 

69

 

124

 

2:08

 

2.03

 

9

 

15

 

2:08

 

0.20

 

10.4

 

16

 

0.72

 

224

 

519

 

2:28

 

7.58

 

11

 

31

 

2:28

 

0.31

 

24.8

 

17

 

3.13

 

630

 

1255

 

1:06

 

18.45

 

55

 

301

 

1:06

 

2.55

 

7.2

 

18

 

0.93

 

426

 

1108

 

1:20

 

14.28

 

32

 

89

 

1:20

 

1.08

 

13.2

 

19

 

1.04

 

278

 

477

 

4:08

 

8.93

 

17

 

42

 

2:03

 

0.51

 

17.6

 

20

 

1.25

 

276

 

703

 

1:58

 

10.54

 

18

 

68

 

1:58

 

0.82

 

12.8

 

mean

 

0.89

 

263

 

548

 

2:20

 

8.54

 

17.7

 

54.15

 

1:51

 

0.61

 

16.82

 

 

Table 4 shows the calculated correlation coefficients between the daily dose (in milligrams per kilogram body weight), the steady-state concentration (Css), Css normalized to a dose of 1 mg·kg-1 (Css-norm.), the AUC(0-24h) and AUC(0-24h) normalized to a dose of 1 mg·kg-1 (AUC(0-24h)-norm.), slow disposition rate constant (β), elimination rate constant (k10) and the body clearance (CL).

 

 

 

 

 

 

 

 

 

 

 

 

 

 


Table 3 Pharmacokinetics including the graphically calculated parameters (A, B, Css, ka, α, β) and the computer-calculated parameters (k10, k12, k21, VC, Vβ, CL) with an assumed bioavailibility of 0.90; r is the correlation coefficient between observed and predicted values

 

 

sub.

 

A

 

B

 

Css

 

ka

 

α

 

β

 

k10

 

k12

 

k21

 

VC

 

Vβ

 

CL

 

r

 

no.

 

ng·ml-1

 

ng·ml-1

 

ng·ml-1

 

h‑1

 

h‑1

 

h‑1

 

h‑1

 

h‑1

 

h‑1

 

l·kg‑1

 

l·kg‑1

 

ml·min‑1·kg‑1

 

 

 

1

 

674

 

674

 

487

 

0.67

 

-

 

0.016

 

0.016

 

-

 

-

 

6.29

 

6.29

 

1.69

 

0.9100

 

2

 

230

 

570

 

305

 

1.31

 

0.32

 

0.026

 

0.035

 

0.08

 

0.24

 

1.39

 

1.87

 

0.81

 

0.981

 

3

 

230

 

340

 

137

 

1.01

 

0.58

 

0.044

 

0.070

 

0.19

 

0.36

 

1.85

 

2.95

 

2.17

 

0.998

 

4

 

410

 

608

 

305

 

0.95

 

0.81

 

0.038

 

0.062

 

0.29

 

0.49

 

1.22

 

2.00

 

1.27

 

0.976

 

5

 

700

 

254

 

115

 

0.72

 

0.62

 

0.053

 

0.161

 

0.31

 

0.20

 

1.59

 

4.82

 

4.26

 

0.955

 

6

 

800

 

420

 

287

 

1.01

 

0.42

 

0.013

 

0.036

 

0.25

 

0.15

 

1.27

 

3.50

 

0.76

 

0.993

 

7

 

820

 

405

 

279

 

0.58

 

0.35

 

0.015

 

0.042

 

0.19

 

0.12

 

1.75

 

4.90

 

1.23

 

0.994

 

8

 

320

 

125

 

65

 

0.80

 

0.55

 

0.015

 

0.050

 

0.34

 

0.16

 

1.44

 

4.78

 

1.20

 

0.967

 

9

 

390

 

690

 

451

 

0.56

 

0.27

 

0.020

 

0.030

 

0.08

 

0.18

 

2.04

 

3.06

 

1.02

 

0.828

 

10

 

520

 

370

 

282

 

0.58

 

0.22

 

0.013

 

0.029

 

0.11

 

0.10

 

3.57

 

7.95

 

1.72

 

0.963

 

11

 

181

 

568

 

254

 

0.52

 

0.24

 

0.030

 

0.038

 

0.04

 

0.19

 

2.00

 

2.53

 

1.27

 

0.896

 

12

 

380

 

340

 

191

 

0.66

 

0.48

 

0.025

 

0.050

 

0.22

 

0.24

 

1.13

 

2.27

 

0.94

 

0.963

 

13

 

255

 

215

 

114

 

0.71

 

0.32

 

0.029

 

0.057

 

0.13

 

0.16

 

2.91

 

5.71

 

2.76

 

0.995

 

14

 

305

 

125

 

88

 

0.76

 

0.59

 

0.017

 

0.055

 

0.37

 

0.18

 

2.42

 

7.84

 

2.22

 

0.970

 

15

 

297

 

106

 

69

 

0.31

 

0.21

 

0.026

 

0.073

 

0.09

 

0.07

 

1.27

 

3.54

 

1.53

 

0.960

 

16

 

540

 

440

 

224

 

0.58

 

0.40

 

0.033

 

0.067

 

0.17

 

0.20

 

1.20

 

2.45

 

1.35

 

0.899

 

17

 

523

 

1060

 

630

 

2.50

 

0.77

 

0.026

 

0.039

 

0.24

 

0.52

 

3.90

 

5.85

 

2.54

 

0.962

 

18

 

1212

 

680

 

426

 

1.95

 

0.76

 

0.018

 

0.048

 

0.44

 

0.28

 

1.05

 

2.81

 

0.84

 

0.999

 

19

 

123

 

520

 

278

 

0.49

 

0.28

 

0.027

 

0.033

 

0.04

 

0.23

 

3.11

 

3.79

 

1.71

 

0.981

 

20

 

492

 

635

 

276

 

1.15

 

0.77

 

0.032

 

0.055

 

0.30

 

0.45

 

1.57

 

2.70

 

1.44

 

0.89

 

mean

 

 

 

 

 

263

 

0.89

 

0.47

 

0.026

 

0.052

 

0.20

 

0.24

 

2.15

 

4.03

 

1.64

 

 

 

95 % confidence intervals

 

0.65-1.13

 

0.37-0.57

 

0.021-0.031

 

0.038-0.066

 

0.15-0.26

 

0.18-0.30

 

1.55-2.74

 

3.23-4.94

 

1.25-2.02

 

 

 

* the pharmacokinetics of subject 1 are described at best using a one compartment model


Table 4 Relevant correlation coefficients between the dose (mg·kg-1), steady-state concentration Css and Css-norm. (Css normalized to a dose of 1 mg·kg-1), AUC(0-24h) and AUC(0-24h)-norm. (AUC(0-24h) normalized to a dose of 1 mg·kg-1), slow disposition rate constant (β), elimination rate constant (k10) and the body clearance (CL)

 

 

 

 

Css

 

AUC(0-24h)

 

β

 

k10

 

CL

 

dose

 

0.502a

 

0.569a

 

 

 

 

 

 

 

Css-norm.

 

 

 

 

 

-0.436b

 

-0.527a

 

-0.697c

 

AUC(0-24h)-norm.

 

 

 

 

 

-0.398

 

-0.450b

 

-0.869c

 

a P £ 0.025

b P £ 0.05

c P = 0.000

 

 

Discussion

 

In the present study an analytical procedure to investigate the pharmacokinetics of methadone and its primary metabolite EDDP in 20 long-term opiate addicts was developed. The reverse-phase HPLC method used appeared to be highly sensitive and suitable for the determination of methadone and EDDP levels in plasma simultaneously. Apart from some incidental data in the literature (Jacob et al., 1981; Kintz et al., 1990) this is the first study which systematically measured EDDP concentrations in relation to methadone concentrations in plasma from opiate addicts during a 24-h period. Earlier studies had focused exclusively on EDDP levels in human urine (Kreek et al., 1980) and rat plasma (Pierce et al., 1992). The present study indicates that the plasma concentration-time curves of EDDP parallel those of methadone in each subject but at substantially lower levels. The calculated ratios between the AUC(0-24h) for methadone and the AUC(0-24) for EDDP varied from 5.9 to 44.6 in our subjects. These considerable interindividual variations indicate a rather variable metabolic avtivity. The peak plasma concentrations of EDDP occurred in most cases ahead of the methadone peak: 0149 hours and 0220 hours respectively. This might be explained partly by a first-pass effect, but other explanations are also possible.

Although in early studies using animal models no analgesic activity for EDDP has been found (Kreek et al., 1980; Sullivan and Due, 1973), it might be premature to exclude any psychopharmacological activity for EDDP. Considering the continuous presence of EDDP in long-term methadone addicts and the close chemical resemblance with methadone, it would be worthwhile to submit EDDP to further pharmacodynamic investigations.

Our setup subjecting the opiate addicts during their 4-day stay in our closed metabolic ward to the procedure of blood collection over 24 h, had several important advantages over studies using outdoor subjects. The main one was of course that the daily methadone administration was strictly controlled, which completely excluded illicit methadone or other drug supplementation.


As can be seen from the plasma concentration-time curve in Fig. 1, which is representative of those obtained from all other subjects, and also from the calculations and fittings summarized in Table 3, we established that the steady-state pharmacokinetics of methadone are best described with a two-compartment model. This has been suggested previously by Nillson et al. (1982), but was contradicted in a recent study by Wolff et al. (1993). However, the latter study using outdoor subjects was not performed under circumstances comparable with our study.

Much less distinctness was observed with regard to the correlation between the daily dose and the plasma levels of methadone. In contrast with other studies (Wolff et al., 1991; Loimer and Schmid, 1992) poor correlations were found between dose (in milligrams per kilogram body weight) and the Css (r = 0.502) or AUC(0-24h) (r = 0.569); see Table 4. In trying to explain this phenomenon we considered the general pharmacokinetic parameters determining the steady-state level best represented using the normalized Css or the normalized AUC(0-24h). As can be seen in Table 4, the latter parameters showed a rather poor correlation with the elimination rate represented by ß or k10, but a much stronger correlation with the body clearance (CL): r = -0.697 and r = -0.869, respectively. Considering the other terms in the steady-state equation, it was obvious that the bioavailability could not be established in our study. In our calculations we assumed a bioavailability of 0.90 which was based on data in the literature (Inturissi et al., 1987). However, this is highly questionable, since in other studies very divergent values have also been established (Nilsson et al., 1982;Meresaar et al., 1981). Possibly the variable values of time to peak concentration (tmax, see Table 2), ranging from 0106 to 0408 hours, and the highly variable interindividual ratios between the AUC(0-24h) for methadone and the AUC(0-24h) for EDDP might be other relevant indications of doubt about the constant value of the bioavailability used in this study. Considering the oral methadone dose was administered to each single subject at about the same time (0920 hours) after a standard breakfast, food interactions or diurnal fluctuations might be excluded. Further investigations with regard to the bioavailability of methadone in long-term opiate addicts under strictly controlled circumstances need to be given a high priority.


The elimination rate constants (β or k10) showed a wide range of variation (see Table 3). Converted into plasma half-lifes (t½β) a range of 13-53 h with a mean of 31.2 h was found, which is in conformity with other studies (Nilsson et al., 1983; Wolff et al., 1993). The variation in the plasma half-lifes between individuals has been suggested to be greatly influenced by urinary pH (Bellward et al., 1977; Nilsson et al., 1982). Patients with alkaline urine have plasma half-lifes twice those with acidic urines (Nilsson et al., 1982). Our findings did not support this view. A more important factor influencing methadone kinetics might be the concomitant administration of enzyme inducing drugs, whereas the smoking habits of all participants were about equal. In our data set subject 5 was on long-term treatment with a combination of isoniazid, rifampicin and azidothymidine (see Table 1). The plasma half-life found was 13 h, the shortest one of the whole data set by far. Also the lowest ratio between the AUC(0-24h) for methadone and the AUC(0-24h) for EDDP (5.9, see Table 2) was found in subject 5. In addition, we found differences between the elimination rate constants (β) of the male subjects (0.030·h-1) and those of the female subjects (0.021·h-1). Converted into plasma half-lifes (t½β) these values are 27.9 h and 35.4 h respectively. In our analysis we found no indications that these differences were due to differences in body weight, comedication, volume of distribution or other factors. Although the level of significance was rather low (P = 0.067), it might be worthwhile submitting these gender differences with regard to the pharmacokinetics of methadone to further investigation.