CHAPTER ELEVEN

Tramadol HydrochlorideRobert Smyj, Xiao-Ping Wang, Feixue HanApotex Inc., Toronto, Ontario, Canada

Contents

1.

ProfISShttp

General Information

iles of Drug Substances, Excipients, and Related Methodology, Volume 38 # 2013 Elsevier Inc.N 1871-5125 All rights reserved.://dx.doi.org/10.1016/B978-0-12-407691-4.00011-3

464

1.1 Nomenclature 464 1.2 Formulae 464 1.3 Elemental analysis 465 1.4 Appearance 465

2.

Physical Characteristics 465 2.1 Ionization constant 465 2.2 Solubility characteristics 465 2.3 Partition coefficient 466 2.4 Optical activity 466 2.5 Crystallographic properties 466 2.6 Hygroscopicity 467 2.7 Thermal methods of analysis 468 2.8 Spectroscopy 469 2.9 Mass spectrometry 471

3.

Stability 475 3.1 Solid-state stability 475 3.2 Solution-phase stability 475

4.

Methods of Analysis 477 4.1 Known impurities of tramadol 477 4.2 Compendial methods of analysis 479 4.3 Thin-layer chromatography 481 4.4 High-performance liquid chromatography with UV detection 482 4.5 High-performance liquid chromatography with fluorescence detection 482 4.6 High-performance liquid chromatography–mass spectrometry 483 4.7 Electrochemical analysis 484 4.8 Spectrophotometric analysis 484 4.9 Gas chromatography with flame ionization or mass spectrometry detection 485 4.10 Capillary electrophoresis analysis 485 4.11 Potentiometric titration 485

5.

Pharmacokinetics and Metabolism 486 5.1 Absorption and bioavailability 486 5.2 Distribution 487

463

464 Robert Smyj et al.

5.3

Metabolism 487 5.4 Elimination 488 5.5 Pharmacokinetics in special population 488

6.

Pharmacological Effects 488 6.1 Mechanism of action 488 6.2 Adverse reactions 488 6.3 Drug interactions 489

7.

Method of Chemical Synthesis 489 Acknowledgments 490 References 490

1. GENERAL INFORMATION

1.1. Nomenclature

1.1.1 Systematic chemical names

(1RS,2RS)-2-[(Dimethylamino)methyl]-1-(3-methoxyphenyl)

cyclohexanol hydrochloride [1].

(�)-cis-2-[(Dimethylamino)methyl]-1-(3-methoxyphenyl)

cyclohexanol hydrochloride [2–4].

(�)-cis-2-[(Dimethylamino)methyl]-1-(m-methoxyphenyl)

cyclohexanol hydrochloride [2,3].

1.1.2 Nonproprietary names

Tramadol hydrochloride (USAN, JAN) [3].

Tramadol (INN, BAN) [3].

1.1.3 Proprietary namesAmadol [5,6], Contramal [5], Contramol [6], Conzip [7], Dromadol [8],

Crispin [5,6], Fortradol [6], Rybix ODT [7], Ryzolt [7], TRADOL-

PUREN [6] Tradonal [5], Trama [6], Trama AbZ [6], Trama beta [6],

Tramadol [6], Tramadura [6], Tramagetic [6], Tramagit [6], Tramake

[6,8], Tramal [5,6,8], Trama-Sanorania [6], Tramdolar [6], Tramedphano

[6], Tramundin [6], Topalgic [6], Ultram [3–8], Ultram ER [7], Zamadol

[5,8], Zamadol SR [6], Zydol [5,8], Zyndol SR [6].

465Tramadol Hydrochloride

1.2. Formulae1.2.1 Empirical formula, molecular weight, CAS number

Tramadol

C16H25NO2 263.38 27203-92-5

Tramadol hydrochloride

C16H25NO2�HCl 299.84 36282-47-0

1.2.2 Structural formula

N

OH

OCH3

H3C

H3CH

HCland enantiomer .

1.3. Elemental analysis

Free base: %C: 72.96, %H: 9.57, %N: 5.32, %O: 12.15.

Hydrochloride salt: %C: 64.09, %H: 8.74, %Cl: 11.82, %N: 4.67, %O:

10.67.

1.4. Appearance

White or almost white, crystalline powder [1].

White crystals [5].

White, crystalline powder [9].

2. PHYSICAL CHARACTERISTICS

2.1. Ionization constant

Tramadol is known to have a pKa of 9.41 [4].

2.2. Solubility characteristicsThe EP [1] and USP [9] both indicate that tramadol hydrochloride is freely

soluble in water and in methanol and very slightly soluble in acetone. The

drug substance is also described as being readily soluble in water and ethanol

[4]. In our laboratory, the aqueous solubility of tramadol hydrochloride in

the 1.2–7.5 pH range has been found to be >20 mg/mL (see Table 11.1).

Table 11.1 Aqueous solubility of tramadol hydrochloride at different pHSolvent pH Solubility (mg/mL)

0.1N HCl 1.2 >20

SGF 1.3 >20

0.01N HCl 2.0 >20

0.05 M phosphate buffer 2.5 >20

0.05 M phosphate buffer 3.5 >20

0.05 M phosphate buffer 4.5 >20

0.05 M phosphate buffer 5.5 >20

0.05 M phosphate buffer 6.0 >20

0.05 M phosphate buffer 6.8 >20

0.05 M phosphate buffer 7.2 >20

0.05 M phosphate buffer 7.5 >20

SGF, simulated gastric fluid.

466 Robert Smyj et al.

2.3. Partition coefficient

Th

chl

e n-octanol/water log partition coefficient (log P) for tramadol hydro-

oride is known to be 1.35 at pH 7 [4].

2.4. Optical activityTramadol hydrochloride consists of a racemic mixture of the 1R,2R and

1S,2S isomers, thus lacks any optical activity. Methods for separating the

racemate of tramadol have been reported in Refs. [10,11]. The (1R,2R)

isomer as the hydrochloride salt is described to have a specific rotation:

[a]DRT¼þ29.6� (c¼1.00; methanol) with a melting point of 171–172 �C

[11]. While the (1S,2S) hydrochloride salt is reported to have a specific

rotation: [a]DRT¼�29.6� (c¼1.00; methanol) and a melting point of

172–173 �C [11].

2.5. Crystallographic propertiesTramadol hydrochloride is known to exist in crystalline and amorphous

forms [12–14]. Tramadol base has been reported to exist as a crystalline

monohydrate [15]. Tramadol base in anhydrous form at room temperature

is described as an oil [15].

(Counts)8000

7000

6000

5000

4000

3000

2000

1000

00 10 20 30 (�2q) 40

Figure 11.1 X-ray powder diffraction pattern of crystalline tramadol hydrochloride.

467Tramadol Hydrochloride

The X-ray powder diffraction pattern of crystalline tramadol hydrochlo-

ride was obtained in our laboratory with a Phillips PW3710 X-ray diffrac-

tometer using Cu Ka irradiation and is displayed in Figure 11.1. The most

intense peaks observed in the X-ray powder diffraction pattern (Figure 11.1)

have 2y angles of 10.3�, 13.0�, 15.3�, 16.7�, 18.5�, 20.5�, 20.8�, 21.5�,24.4�, 26.1�, and 30.7�.

2.6. HygroscopicityIn our laboratory, tramadol hydrochloride was analyzed using dynamic

vapor sorption. For the analysis, a VTI SGA 100 Symmetric Vapor Sorption

Analyzer was used. The drug substance was first dried to a constant weight at

40 C, then the adsorption/desorption experiment was performed at 25 �C.Adsorption analysis occurred from 5% RH to 95% RH in 10% increments,

while desorption was monitored from 95%RH back to 5%RH, also in 10%

increments. The resulting adsorption/desorption isotherm that was obtained

is displayed in Figure 11.2. The experimental results indicate that tramadol

hydrochloride is nonhygroscopic below 75%RH. Above 85%RH, the drug

substance absorbs water readily resulting in an approximate 16% increase in

weight at the end of the absorption phase. During the desorption phase of

the experiment, some water is lost during the 85–75% RH interval; how-

ever, a water content of about 10% is retained throughout the 75–5% RH

desorption phase.

Adsorption

20,000

18,000

16,000

14,000

12,000

10,000

8,000

6,000

4,000

2,000

0,0000 10 20 30 40 50

% RH

60 70 80 90 100

Desorption

Weig

ht

(% c

han

ge

)

Figure 11.2 Adsorption/desorption isotherm of tramadol hydrochloride.

468 Robert Smyj et al.

2.7. Thermal methods of analysis2.7.1 Melting behaviorMelting points of 180–184 �C [1] and 180–181 �C [5,8] have been reported

for tramadol hydrochloride.

2.7.2 Differential scanning calorimetryDSC analysis of tramadol hydrochloride has been performed in our labora-

tory on a TA 2920 DSC unit with Universal Thermal Solutions V2.5H

Software. The sample was weighed directly into an aluminum holder with

the lid placed on top (uncrimped). After an initial equilibration at 25 �C, thesample was heated to 200 �C at a rate of 10 �C/min. All activities were car-

ried out under a N2 purge (50 cc/min). One endothermic event was

observed for the sample with melting onset and peak maximum tempera-

tures of 180.53 and 182.38 �C, respectively. The thermogram for tramadol

hydrochloride is presented in Figure 11.3.

2.7.3 Thermogravimetric analysisTGA analysis was performed in our laboratory on a TA Instruments Q500 Q

Series TGA unit using a dynamic high-resolution mode. The sample was

heated from ambient temperature to 220 �C at a rate of 10 �C/min. All activ-

ities were carried out under a helium purge (balance purge: 10 mL/min, sam-

ple purge: 60 mL/min). The thermogravimetric thermogram of tramadol

hydrochloride (Figure 11.4) indicates a weight loss of about 0.34% up to

Temperature (�C)

Wei

ght (

%)

Der

iv. w

eigh

t (%

/�C

)

0.3435%(0.03862 mg)

150.07 �C

Universal V3.5B TA instruments

0

0

20

40

60

80

100

120

50−20

100 150 200 250

0

2

4

6

8

10

−2

Figure 11.4 Thermogravimetric thermogram of tramadol hydrochloride.

0

20 40 60 80 100 120 140 160 180 200

Hea

t flo

w (

W/g

)

Exo up Temperature (�C)

182.38 °C

180.53 °C115.6 J/g

Universal V3.5B TA instruments

−2

−4

−6

−8

Figure 11.3 Differential scanning calorimetry thermogram of tramadol hydrochloride.

469Tramadol Hydrochloride

150 �C.Rapidweight loss of the drug substance starts to occur near its melting

point (ca. 180�185 �C), followed by complete decomposition by �205 �C.

2.8. Spectroscopy2.8.1 UV–vis spectroscopyThe UV spectrum of tramadol hydrochloride was obtained on a Perkin-

Elmer Lambda 2 UV/vis spectrometer. The drug substance was dissolved

1.0

0.8

0.6

0.4

0.2

0.00200 220 240 260 280 300

nm

A

320 340 360 380 400

Figure 11.5 Ultraviolet absorption spectrum of tramadol hydrochloride.

470 Robert Smyj et al.

in methanol at a concentration of 10.58 mg/L and scanned from 200 to

400 nm. The UV spectrum (Figure 11.5) shows absorption maxima at

217 nm (e¼7.2�103) and 272 nm (e¼2.0�103).

2.8.2 Vibrational spectroscopyThe FT-IR spectrum of tramadol hydrochloride was obtained using a

Perkin-Elmer Paragon 16PC FT-IR spectrometer. The spectrum was

recorded for a potassium bromide pellet containing 1.7 mg of the drug sub-

stance and 178 mg of KBr. The resulting spectrum is displayed in

Figure 11.6. A summary of the functional group assignments for the char-

acteristic absorption bands that were observed is provided in Table 11.2.

2.8.3 Nuclear magnetic resonance spectroscopyBoth the 1H NMR (including a D2O exchange experiment) and 13C

NMR spectra of tramadol hydrochloride were obtained in a Bruker

AV-400 spectrometer, operating at 400.133 MHz (1H NMR) or at

100.623 MHz (13C NMR). Spectra were recorded for a solution of

tramadol hydrochloride in DMSO-d6. Chemical shifts are reported in

ppm relative to TMS.

2.8.3.1 1H NMR spectrumThe 1H NMR spectra of tramadol hydrochloride are shown in Figures 11.7

and 11.8 (D2O exchange spectrum), and the resonance signal assignments

are provided in Table 11.3.

Table 11.2 Band assignments for the infrared absorption spectrum of tramadolhydrochlorideBand energy (cm�1) Assignment

3307 Alcohol (OdH) stretch

3018 Aromatic (CdH) stretch

2930, 2861 Aliphatic (CdH) stretch

2632, 2514, 2482 Ammonium (NþdH) stretch

1607, 1579, 1481 Aromatic ring skeleton stretch

1289, 1243 Alcohol (CdO) stretch, ether asymmetric (CdOdC)

stretch

1045 Ether symmetric (CdOdC) stretch

777, 703 Aromatic (CdH) out of plane bend

4400

24.1

30

35

40

45

50

55

60

65

70

75

80

4000 3000 2000

cm-1

%T

1500 1000 600

Figure 11.6 Infrared absorption spectrum of tramadol hydrochloride.

471Tramadol Hydrochloride

2.8.3.2 13C NMR spectrumThe 13C NMR and DEPT-135 13C NMR spectra of Tramadol hydrochlo-

ride are shown in Figures 11.9 and 11.10, respectively. The resonance signal

assignments are provided in Table 11.4.

2.9. Mass spectrometryAn electrospray ionization mass spectrometry study of tramadol hydrochlo-

ride was carried out on a Perkin-Elmer/Sciex API-300 triple quadrupole

0.91

43

ppm 10 8 6 4 2 0

0.97

861.

9111

0.96

57

0.94

14

2.88

05

0.52

06

0.94

41

7.15

85

2.00

00

6.81

34

0.02

21

Inte

gra

l

Figure 11.7 1H NMR spectrum of tramadol hydrochloride.

Inte

gra

l

ppm 10 8 6 4 2 0

0.05

86

0.95

08

0.08

32

0.94

35

6.92

82

0.95

14

0.02

28

6.67

48

1.00

00

2.27

232.

8112

0.92

781.

8721

Figure 11.8 1H NMR (D2O exchange) spectrum of tramadol hydrochloride.

Table 11.3 1H NMR spectral data for tramadol hydrochloride

1

2

8

14

9

N+

15

OH7

10

613

3

O17

12

11

CH3

H3C

H3C16

19

18

5

4

HH

Cl−and enantiomer .

Chemical shift(ppm)

Multiplicitya; couplingconstant (Hz) Integration Assignmentb

10.33c br 1H H15

7.28–7.24 app t 1H H12

7.09–7.07 m 2H H9d, H11, H13

6.80–6.78 m 1H

5.12c s 1H H7

3.76 s 3H H18

2.81 dd; J¼10.5, 12.8 1H H14A

2.55e, 2.41f Each br 7H H14B, H16, H19

2.27–2.22 app t 2H H2, H3, H4, H5, H6

2.15–2.12 app d

1.80–1.38 m 7H

aapp, apparent; br, broad; d, doublet; dd, doublet of doublets; m, multiplet; s, singlet; t, triplet.bAssignments containing the letters “A” and “B” denote geminal chemical shift nonequivalent protons.cThese signals nearly disappear in the D2O exchange spectrum.dSignal originating from H9 can be assigned to the 7.09–7.07 ppm region.eThis signal partially overlaps with the solvent residual peak originating from DMSO-d6.fThis signal consists of an overlap with the signal originating from either H16 or H19 and H14B.

473Tramadol Hydrochloride

mass spectrometer. The sample was dissolved inmethanol and injected into a

5-mL sample loop of the mass spectrometer and carried into the ionization

source by the mobile phase (1:1 mixture of methanol and 0.1% aqueous

acetic acid) at a flow rate of 100 mL/min. The electrospray ionization mass

spectrum of tramadol hydrochloride is shown in Figure 11.11. The spectrum

displays the protonated tramadol molecular ion peak [MþH]þ at m/z 264.

TheMS/MS spectrum of this ion is shown in Figure 11.12. One major frag-

ment ion having an m/z of 58 is observed and is proposed to originate from

the protonated molecular ion of tramadol as shown in Scheme 11.1.

160 140 120 100 80 60 40 20180ppm

Figure 11.9 13C NMR spectrum of tramadol hydrochloride.

ppm 160 140 120 100 80 60 40 20

Figure 11.10 DEPT-135 13C NMR spectrum of tramadol hydrochloride.

474 Robert Smyj et al.

Table 11.4 13C NMR spectral data for tramadol hydrochloride

1

2

8

14

9

N+

15

OH7

10

613

3

O17

12

11

CH3

H3C

H3C16

19

18

5

4

HH

Cl−

and enantiomer .

Chemical shift (ppm) DEPT Assignment

159.12 C C10

150.00 C C8

129.06 CH C12

117.22 CH C13

111.52, 111.12 Each CH C9, C11

73.87 C C1

59.33 CH2 C14

54.96 CH3 C18

44.77, 40.60 Each CH3 C16, C19

40.39 CH2 C6

40.21 CH C2

26.16, 24.47, 21.16 Each CH2 C3, C4, C5

475Tramadol Hydrochloride

3. STABILITY

3.1. Solid-state stability

From solid-state stress studies, tramadol hydrochloride has been observed to

be a stable compound. Subjecting the drug substance to thermal stress

(60��C, 14 days), heat/high humidity stress (40��C/75% RH, 14 days) and

light stress (380–770 nm, 1.9�106 lux-h) did not result in any degradation.

3.2. Solution-phase stabilityThe stability of tramadol hydrochloride has also been investigated from solu-

tion stress conditions. Tramadol hydrochloride was stable from basic (0.1N

50

58.365.3

125.4

208.7 246.2286.1

264.2

302.2312.1

416.4 876.0816.2777.3

713.4678.3

664.0641.5

563.4

527.7

400.0

114.1

97.7

5.0e5

1.0e6

1.5e6

2.0e6

2.5e6

Inte

nsity

(cp

s)

3.0e6

3.5e6

4.0e6

4.5e6

5.0e6

5.5e6

6.0e6

6.5e6

6.9e6

100 150 200 250 300 350 400 450 500 550

m/z, a.m.u.

600 650 700 750 800 850 900 950 1000

Figure 11.11 Electrospray ionization mass spectrum of tramadol hydrochloride.

8.4e6

8.0e6

7.5e6

7.0e6

6.5e6

6.0e6

5.5e6

5.0e6

Inte

nsity

(cp

s)

4.5e6

4.0e6

3.5e6

3.0e6

2.5e6

2.0e6

1.5e6

1.0e6

5.0e5

30 40 50 60 70

58.1

80 90 100 110 120 130 140 150 160

m/z, a.m.u.

170 180 190 200 210 220 230 240 250 260 270 280

Figure 11.12 MS/MS spectrum of the protonated tramadol ion peak [MþH]þ atm/z 264.

476 Robert Smyj et al.

NaOH, 100 �C, 4 h), thermal (water, 100 �C, 4 h), and light (380–770 nm,

1.2�106 lux-h) stress. The drug substance was observed to degrade slightly

from acidic (0.1N HCl, 100 �C, 4 h) and oxidative (3% H2O2, room tem-

perature, 24 h) stress. A degradation pathway for tramadol occurring from

acidic and oxidative solution stress involves conversion into the (1RS,2SR)

stereoisomer (Scheme 11.2). Demethylation of the aryl ether methyl group

of tramadol was also observed to occur from oxidative solution stress

N

OH

OCH

3

CH2

H3C

H3C

H3C

H3C

H

and enantiomer

H+

m/z 264 [M + H]+

N+

m/z 58

Scheme 11.1 Proposed fragmentation observed in the MS/MS spectrum of the proton-ated tramadol ion peak.

N

OH

OCH3

CH3

H3C H3C

H3CH3CH

and enantiomer

Tramadol

H2O

HCl or H2O2

N

OH

O

H

and enantiomer

(1RS,2SR)-2-[(dimethylamino)methyl]-

1-(3-methoxyphenyl)cyclohexanol

Scheme 11.2 Epimerization of tramadol from acidic and oxidative solution stressconditions.

N

OH

OCH3

H3C

H3C

H3C

H3CH

and enantiomer

Tramadol

H2O2

H2O

N

OH

OH

H

and enantiomer

(1RS,2RS)-2-[(dimethylamino)methyl]-

1-(3-hydroxyphenyl)cyclohexanol

Scheme 11.3 Demethylation of tramadol from oxidative solution stress conditions.

477Tramadol Hydrochloride

(Scheme 11.3). The aryl ether demethylated derivative of tramadol is also

known to be a metabolite (see Section 5.3).

4. METHODS OF ANALYSIS

4.1. Known impurities of tramadol

Anumber of impurities have been identified as related compounds of tramadol

[1,2,16,17]. The structures, chemical names, and classificationof the impurities

are shown in the table below. Impurities A and E are specified impurities listed

in both USP and EPmonographs of tramadol drug substance. Impurities B–D

are listed in the EP monograph as “other detectable impurities.” All of the

compendial listed impurities are potential manufacturing process-related

impurities and degradation products. Impurity D is also a human metabolite.

Impurities of tramadol

Structure Chemical name Classification

N

OH

O

H3C

CH3

H3C

and enantiomer

Impurity A

(1RS,2SR)-2-

[(dimethylamino)methyl]-1-

(3-methoxyphenyl)

cyclohexanol

EP impurity A

USP RCA

Synthetic impurity/

degradation product

NH3C

H3C

OCH3

Impurity B

[2-(3-methoxyphenyl)-

cyclohex-1-enyl]-N,N-

dimethylmethanamine

EP impurity B

Synthetic impurity/

degradation product

N

OCH3

H3C

H3C

and enantiomer

Impurity C

(1RS)-[2-(3-methoxyphenyl)

cyclohex-2-enyl]-N,N-

dimethylmethanamine

EP impurity C

Synthetic impurity/

degradation product

N

OH

H3C

H3C

OH and enantiomer

Impurity D

(1RS,2RS)-2-

[(dimethylamino)methyl]-1-

(3-hydroxyphenyl)

cyclohexanol (O-

desmethyltramadol)

EP impurity D

Synthetic impurity/

degradation product

Metabolite

N

H3C

H3C

O and enantiomer

Impurity E

(2RS)-2-[(dimethylamino)

methyl]cyclohexanone

EP impurity E

USP RCB

Synthetic impurity/

degradation product

478 Robert Smyj et al.

479Tramadol Hydrochloride

4.2. Compendial methods of analysis4.2.1 USP methods of analysisThe USP [2] prescribes the following tests for tramadol drug substance:

• Identification A: infrared absorption h197 Ki.• Identification B: chloride h191i: an aqueous solution (1 in 100) meets the

requirements.

• Residue on ignition h281i: NMT 0.1%.

• Heavy metals h231i method I: NMT 20 ppm.

• Content of chloride: 11.6–12.1% of chloride is found.

• Water determination h921i method 1a: NMT 0.5%.

• Acidity: NMT 0.4 mL of 0.01N sodium hydroxide is required to produce

an yellow color.

• Assay by isocratic HPLC: 98.0–102.0% calculated on the anhydrous basis.

• Organic impurities

1. Procedure 1 by TLC:

Me

Ap

Tim

Qu

Impurity E (USP tramadol related compound B): NMT 0.2%

2. Procedure 2 by HPLC:

Impurity A (USP tramadol related compound A): NMT 0.2%

Any individual impurity: NMT 0.1% each

Total impurities: NMT 0.4%

USP pharmaceutical preparations include immediate release oral solid dos-

age Tramadol Hydrochloride Tablets and Tramadol Hydrochloride

Extended-Release Tablets. The following tests are prescribed for USP

Tramadol Tablets [16]:

• Identification A: infrared absorption h197 Ki.• Identification B: the retention time of the major peak of the sample solu-

tion corresponds to that of the standard solution as obtained in the Assay.

• Assay by isocratic HPLC: 90.0–110.0% of the labeled amount of

tramadol hydrochloride.

• Organic Impurities by isocratic HPLC:

Impurities A: NMT 0.2%.

Impurities B and C: NMT 0.2% each.

Any unspecified impurity: NMT 0.2%.

Total impurities: NMT 0.7%.

• Dissolution h711i

dium: 0.1N HCl, 900 mL.

paratus 1: 100 rpm.

e: 30 min.

antitation: isocratic HPLC method as directed in the Assay.

Lim

Im

Im

Im

un

An

Me

Ap

Sam

Qu

Lim

dis

Lim

480 Robert Smyj et al.

it: NLT 80% (Q) of the labeled amount of tramadol hydrochloride is

solved in 30 min.

dis

• Uniformity of dosage units h905i: quantitative method by isocratic

HPLC method as directed for the Assay; meets the requirements of

h905i.The following tests are prescribed for USP Tramadol Hydrochloride

Extended-Release Tablets [17]:

• Identification: the retention time of the major peak in the chromatogram

of the sample solution corresponds to that in the chromatogram of the

standard solution as obtained in the Assay.

• Ultraviolet Absorption h197Ui: the UV absorption spectrum of the sam-

ple solution exhibits maximum and minima at the same wavelength as

that of a similar solution of the standard solution.

• Assay by isocratic HPLC: 90.0–110.0% of the labeled amount of

tramadol hydrochloride.

• Organic impurities by isocratic HPLC:

purities A: NMT 0.2%.

purities D: NMT 0.1%.

purity B and Impurity C: NMT 0.1% each (as an individual

specified impurity).

y unspecified impurity: NMT 0.1%.

tal impurities: NMT 0.5%.

To

• Dissolution h711i

dium: 0.1N HCl, 900 mL.

paratus 1: 75 rpm.

ple time: 2, 4, 8, 10, and 16 h.

antitative method: UV at 271 nm.

it: NLT 80% (Q) of the labeled amount of tramadol hydrochloride is

solved in 30 min.

its: 2 h: NMT 15%; 4 h: 10–40%; 8 h: 50–85%; 10 h: 65–95%; 16 h:

T 80%.

NL

• Uniformity of dosage units h905i: meets the requirements.

4.2.2 EP/BP methods of analysisThe EP/BP harmonized monograph prescribes the following tests for

tramadol hydrochloride drug substance [1]:

• Identification A: melting point (2.2.14) 180–184 �C.• Identification B: infrared absorption (2.2.24).

481Tramadol Hydrochloride

• Identification C: TLC (2.2.27); the principal spot in the chromatogram

obtained with test solution for impurity E is similar in position and size to

the principal spot in the chromatogram obtained with reference solution.

• Identification D: it gives reaction of chloride (2.3.1).

• Appearance of solution: the solution is clear (2.2.1) and colorless (2.2.2,

method II).

• Acidity: NMT 0.4 mL of 0.01 M sodium hydroxide is required to

change the color of the indicator to yellow.

• Optical rotation (2.2.7): �0.10� to þ0.10� (1 g/20 mL, water).

• Heavy metals (2.4.8): maximum 20 ppm.

• Water (2.5.12): maximum 0.5% determined on 1.000 g.

• Sulfated ash (2.4.14): maximum 0.1% determined on 1.0 g.

• Assay by potentiometric titration (2.2.20): 99.0–101.0% (anhydrous

substance).

• Impurity E by thin-layer chromatography (2.2.7): NMT 0.2%

• Related substances by HPLC isocratic method:

Impurity A: NMT 0.2%.

Any unspecified impurity: NMT 0.10% each.

Total impurities: NMT 0.4%.

Disregard limit: 0.02%.

4.3. Thin-layer chromatographyKrzek et al. [18] reported a thin-layer chromatography and densitometric

procedure for quantitative determination of tramadol and its major impuri-

ties in pharmaceutical preparations. The separation was performed on silica

gel-coated chromatographic plate using two mobile phases: (i)

chloroform–methanol–glacial acetic acid (9:2:0.1, v/v/v) and (ii) chloro-

form–toluene–ethanol (9:8:1, v/v/v). The UV densitometry was carried

out at l¼270 nm. The developed method is of high sensitivity and low

detection and determination limits ranging from 0.044 to 0.35 mg. For indi-vidual constituents, the recovery ranges from 93.23% to 99.66%. In addi-

tion, the stability of tramadol in solution was investigated, including an

effect of solution pH, temperature, and incubation time. It was found that

tramadol decomposes in various ways in acidic and basic environments pro-

ducing Impurity C and Impurity A as major degradation products in acidic

conditions and Impurity A as a major degradation product in basic condi-

tions, respectively. The levels of these impurities depend on solution pH

and temperature. The thin-layer chromatography and densitometric

482 Robert Smyj et al.

method can be used for impurity control of medicines containing tramadol

hydrochloride.

Meyyanathan et al. [19] reported a simple, precise, rapid, and selectivehigh-

performance thin-layer chromatographymethod for the analysis of tramadol in

pharmaceutical formulations. The method uses chlorzoxazone as an internal

standard. The stationary phasewas silica gel 60 F254 prewashedwithmethanol.

Ethyl acetate–methanol–ammonia solution (7:1:0.5, v/v/v)wasused asmobile

phase. Detection and quantitation were performed densitometrically at

l¼275 nm. The linear range of the analysis was 1.0–2.5 mg and percentage

recovery was 100.8–108.4%. This high-performance thin-layer chromatogra-

phy and densitometric procedure for determination of tramadol in solid dosage

forms is accurate, precise, rapid, and selective. It can, therefore, be easily and

conveniently adopted for routine quality control analysis.

4.4. High-performance liquid chromatography withUV detection

Several researchgroupshave reportedquantitative determinationof tramadol in

drug substance, drug dosage forms, human plasma, serum, and blood samples

using high-performance liquid chromatography with UV detection (HPLC-

UV) [20–25]. For example, Zecevic et al. [20] reported a novel, rapid

HPLC-UV method for the determination of tramadol and its major related

impurities and degradation products. The separation was carried out on a C18

XTerra™ (150�4.6 mm, 5 m) column using acetonitrile–0.015 MNa2HPO4

buffer (2:8, v/v) as mobile phase (pH 3.0) at a flow rate 1.0 mL/min, temper-

ature of the column 20 �C, and UV detection at 218 nm. The method was

found to be linear (r>0.995) in the range of 0.15–2.4 mg/mL. The low

RSDvalues indicate goodprecision.Thehigh recovery values indicate excel-

lent accuracy of the method. Rajendraprasad et al. [21] developed a simple

reverse-phaseHPLCmethod for assay of tramadol in bulk and capsule dosage

form. The assay was carried out on Phenomonex Gemini C-18

(250�4.6 mm, 5 m) column using a mobile phase consisting of potassium

dihydrogen phosphate buffer–methanol–acetonitrile (40:40:20). The eluent

was monitored at 280 nm. The method was validated and recovery studies

confirmed the accuracy of the assay method.

4.5. High-performance liquid chromatography withfluorescence detection

Although tramadol molecule contains a benzene ring, UV detection is

unsuitable for its analysis in low concentration and in urine and plasma

483Tramadol Hydrochloride

samples due to lack of sensitivity and selectivity. Several research groups

have reported quantitative determination of tramadol and its major metab-

olites using high-performance liquid chromatography with fluorescence

detection (HPLC-fluorescence detection) [26–36]. For example, Mehvar

et al. [26] reported a stereospecific HPLC-fluorescence detection method

for simultaneous quantitation of the enantiomers of tramadol and its active

metabolites O-demethyl tramadol and O-demethyl-N-demethyl tramadol

in human plasma. The separation was achieved using a Chiralpak AD col-

umn with a mobile phase of hexane:ethanol:diethylamine (94:6:0.2) and a

flow rate of 1 mL/min. The fluorescence of analytes was then detected at

excitation and emission wavelengths of 275 and 300 nm, respectively.

The method was validated in the plasma concentration range of

2.5–250 ng/mL with a lower limit of quantitation of 2.5 ng/mL. Bahrami

et al. [27] developed a HPLC method with enhancement of fluorescence

intensity of tramadol and its main metabolites using precolumn derivatiza-

tion with 9-fluorenylmethyl chloroformate as labeling agent. The analytical

method was linear over the concentration range of 1.0–1280 ng/mL of the

parent drug and its metabolites and limit of quantitation of 1.0 ng/mL was

obtained for analytes using 10 mL injection.

4.6. High-performance liquid chromatography–massspectrometry

Several high-performance liquid chromatography–mass spectrometry

(LC–MS) methods for the analysis of tramadol, its degradation products,

and metabolites have been published in the literature [37–40]. Godoy

et al. [37] recently reported a simultaneous analysis of tramadol and major

degradation products and metabolites using an HPLC–tandem mass spec-

trometry method (LC–MS–MS). Tramadol is available as a racemic mixture

of (þ)-trans-tramadol and (�)-trans-tramadol. This method was claimed to

be the first study using tandem mass spectrometry as a detection system for

the simultaneous analysis of two trans-tramadol enantiomers and their major

metabolites. The best chromatographic resolution was obtained on a

Chiralpak AD column, which was operated under normal-phase conditions

using a mixture of hexane–ethanol (95.5:4.5, v/v) plus 0.1% diethylamine as

mobile phase. Under these conditions, recovery of 80–90% was obtained.

Quantitation limit of 0.5, 0.5, and 0.1 ng/mL was obtained for each

trans-tramadol, O-desmethyltramadol, and N-desmethyltramadol enantio-

mers, respectively. The method was validated to be precise and accurate.

484 Robert Smyj et al.

4.7. Electrochemical analysisThere are a few examples of the determination of tramadol hydrochloride in

pharmaceutical dosage forms by an electrochemical analysis method [41,42].

Garrido et al. [41] reported a square-wave voltammetric (SWV) method and

a flow injection analysis system. Amperometric detection was developed for

the determination of tramadol hydrochloride in pharmaceutical dosage

forms. The SWV method enables the determination of tramadol over the

concentration range of 15–75 mM with a detection limit of 2.2 mM.

Tramadol could be determined concentrations between 9 and 50 mM at a

sampling rate of 90 samples/h with a detection limit of 1.7 mM using the

flow injections system. The electrochemical methods developed were suc-

cessfully applied to the determination of tramadol in pharmaceutical dosage

forms without any pretreatment of the samples. Recovery values were

between 97–102%.

4.8. Spectrophotometric analysisSeveral spectrophotometric analysis methods for tramadol and its related

impurities were reported in literature [43–45]. Rajasekhar et al. [43] devel-

oped a spectrophotometric assay determination method of tramadol in bulk

as well as capsule dosage forms. Tramadol obeyed Beer’s law in a concen-

tration of 10–150 mg/mL exhibiting maximum absorption at 270 nm. The

results have been validated statistically and recovery studies confirmed that

the accuracy of the assay method. Abdellatef et al. [44] reported two simple

and sensitive kinetic spectrophotometric analysis methods for the determi-

nation of tramadol hydrochloride. The first method is based upon a kinetic

investigation of the oxidation reaction of the drug with alkaline potassium

permanganate at room temperature for a fixed time at 20 min. The absor-

bance of the colored manganate ions was measured at 610 nm. The second

method is based on the reaction of tramadol hydrochloride with 4-chloro-7-

nitrobenzofurazan (NBD-Cl) in the presence of 0.1 M sodium bicarbonate.

The spectrophotometric measurements were recorded by measuring the

absorbance at 467 nm at fixed time at 25 min on thermostated water bath

at 90�1 �C. The absorbance concentration plots in both methods were lin-

ear over the range of 5–25 and 50–250 mg/mL, for the first and second

methods, respectively. The two methods have been applied successfully

to commercial capsules and other dosage forms. Vinay et al. [45] used

two sulfonthalein dyes in the extraction-free spectrophotometric assay of

tramadol in dosage forms and in spiked human urine based on the ion-pair

485Tramadol Hydrochloride

reaction followed by spectrophotometric analysis. Themethods are based on

the formation of yellow ion-pairs between tramadol and two sulfonthalein

dyes. Under the optimum conditions, tramadol could be assayed in the con-

centration ranges, 1–15 and 1–16 mg/mL with correlation coefficient

greater than 0.999.

4.9. Gas chromatography with flame ionization or massspectrometry detection

Several gas chromatographymethods were reported for the determination of

tramadol and its impurities with either flame ionization detection or mass

spectrometry detection [46–48]. Ho et al. [46] reported a rapid, sensitive,

precise, and accurate GCmethod with flame ionization detection. It is com-

prised of only a one-step extraction procedure with dichloromethane. The

recovery of tramadol was greater than 88%. Calibration graphs were linear

over the concentration range of 12.5–10,000 ng/mL with a coefficient of

variation, both within-day and between-day, of less than 10% at any level.

The limit of detection was 8 ng/mL based on signal-to-noise ratio of 3.

Chytil et al. [47] developed a GC–MS enantiomeric assay determination

method of tramadol and O-desmethyltramadol by GC–MS. Chromatogra-

phy was performed on an Rt-bDEXcst column containing alkylated

b-cyclodextrins as a chiral selector. Nefopam was used as an internal stan-

dard. The method involves a simple solid-phase extraction with chiral anal-

ysis by gas chromatography–electron ionization mass spectrometry. This

methodwas successfully used to determine the concentration of enantiomers

of tramadol and its metabolites.

4.10. Capillary electrophoresis analysisSeveral research groups have reported separation of tramadol enantiomers by

capillary electrophoresis (CE) [49–51]. For instance, Rudaz et al. [49] suc-

cessfully applied cyclodextrins to the enantiomeric resolution of racemic

tramadol in drug substance, drug dosage forms. The CE method has been

demonstrated to be selective, linear, accurate, and precise, and the method

was able to detect 0.3% and to quantitate 1% of the minor enantiomer in the

presence of the major one at the target value.

4.11. Potentiometric titrationThe EP/BP harmonized monograph prescribes an assay method by poten-

tiometric titration [1]. Bodiroga et al. [52] also reported a potentiometric

486 Robert Smyj et al.

titration method for the determination of tramadol. The titration is per-

formed in nonaqueous media with perchloric acid as titrant. The titration

with perchloric acid resulted in potentiometric and conductometric cur-

ves with a clear defined peak thus suitable for the determination of the

titration endpoint providing accurate and reproducible results. Each mil-

liliter of 0.1 mol/L HClO4 is equivalent to 29.98 mg of tramadol hydro-

chloride. The method is fast, accurate, reproducible, and convenient for

the quality control of tramadol hydrochloride in pure state and its dosage

forms.

5. PHARMACOKINETICS AND METABOLISM

The pharmacokinetics of tramadol has been well documented in sev-

eral reviews [53,54].

5.1. Absorption and bioavailabilityAfter oral administration, tramadol is rapidly and almost completely absorbed.

The plasma concentrations are detectable within approximately 0.5 h [55].

Times to reach peak plasma concentration (Tmax) are within 1.2 h after oral

administration of drops [56] and within 2 h after oral administration of solid

dose [55]. Peak plasma concentrations (Cmax) and area under the curve (AUC)

increase linearly over the dose range of 50–400 mg [53,56]. The extent of oral

absorption of tramadol is almost 100%. Due to the first-pass metabolism, the

absolute bioavailability is 70% [53,56].

Following multiple oral administration of tramadol 100 mg four times

daily,Cmax and AUC are 16% and 36% higher, respectively, than after a sin-

gle 100-mg dose. The increased bioavailability is likely due to the saturated

first-pass metabolism [53,57].

Oral administration of tramadol 100 mg following a high-fat food results

in a 17% higherCmax and a 10% higher AUC than the corresponding values

in fasted subjects. This increase of the bioavailability by food is not consid-

ered clinically relevant [53].

The absorption of tramadol after rectal administration of 100 mg

suppositories began within a few minutes. Cmax was reached within

3.3 h. The absolute bioavailability is slightly higher than that after oral

administration, probably due to a reduced first-pass metabolism after rectal

administration [58].

487Tramadol Hydrochloride

5.2. DistributionTramadol is rapidly distributed in the body after intravenous administration,

with a distribution half-life in the initial phase of 6 min, followed by a slower

distribution phase with a half-life of 1.7 h [53,54]. Volumes of distribution

following oral and intravenous administration to young healthy subjects

were 306 and 203 L, respectively, indicating that tramadol has a high tissue

affinity. Plasma protein binding is low (20%) [54,55].

Tramadol passes placental barrier with umbilical venous plasma concen-

trations being 80% of maternal concentration. Very small amount of

tramadol and its active metabolite are excreted in breast milk [53].

5.3. MetabolismTramadol is extensively metabolized in the liver by cytochrome P450 2D6

(CYP2D6) [59–61]. Tramadol undergoes biotransformation to form theN-

and O-demethylated compounds (phase 1 reactions) displayed in

Figure 11.13 [60]. The O-demethylated metabolites (M1, M4, and M5)

are further conjugated to glucuronides and sulfates (phase 2 reactions).

The main metabolites areO-demethyl tramadol (M1) and its conjugates,

di-N,O-demethyl tramadol (M5) and its conjugates and N-demethyl

tramadol (M2). N,N-Didemethyl tramadol (M3) and O-demethyl-N,N-

didemethyl tramodal (M4) and its conjugates are only formed in minor

quantities. Among them, M1 is pharmacologically active and is mainly

responsible for the analgesic efficacy of tramadol [62].

N

OH

O H2N

H2N

CH3

CH3

CH3

H3C

H3C H3C

H3C

H3C

H3C

H

and enantiomer

Tramadol

N

OH

OHO-demethylation

O-demethylation

O-demethylation

N-demethylation

N-demethylation

N-demethylation

N-demethylation

H

and enantiomer

NH

OH

O

H

and enantiomer

M1

M2

NH

OH

OH

H

and enantiomer

M5

OH

O

H

and enantiomer

M3

OH

OH

H

and enantiomer

M4

Figure 11.13 Metabolic pathways of tramadol in phase 1 reactions.

488 Robert Smyj et al.

5.4. EliminationTramadol is mainly excreted via the kidneys. Following oral administration

of 14C-labeled tramadol to humans, approximately 90% is excreted in urines

and 10% in feces [59]. 25–32% of an oral dose is excreted as unchanged

tramadol. The mean elimination half-life is about 5–6 h. The mean total

clearance of tramadol was 467 and 742 mL/min following intravenous

and oral administration.

5.5. Pharmacokinetics in special populationThe metabolism and analgesic effect of tramadol was higher in “extensive

metabolizers” of CYP2D6 than in “poor metabolisers.” M1 production

in microsomes prepared from the liver of a poor metabolizer was markedly

reduced [63]. Biotransformation of tramadol appears to be significantly

reduced in African subjects. In 10 Nigerian volunteers, about 96% of

tramadol was excreted unchanged in the urine after oral administration [64].

The terminal elimination half-life of tramadol is prolonged in patients

with impaired renal and hepatic function. Doses and intervals may be

adjusted [53,54].

6. PHARMACOLOGICAL EFFECTS

6.1. Mechanism of action

Tramadol produces its analgesia in humans by a multimodal mechanism

[53].

(a) (þ)-M1 enantiomer acts as a m opioid agonist. Its affinity for the m opi-

oid receptor is about 700-fold more than that of parent drug (�)-

tramadol [65]. (�)-M5 also has higher affinity than (�)-tramadol

[65]. However, since M5 does not penetrate the blood–brain barrier

due to its high polarity, its responsibility for the m opioid derived anal-

gesic effect is very limited [53].

(b) Tramadol inhibits serotonin reuptake of serotonin (5-HT). (þ)Enan-

tiomer is about fourfold more potent than the (�) Enantiomer [66].

(c) Tramadol inhibits norepinephrine reuptake. (�)Tramadol is a more

potent blocker than its (þ) counterpart or M1 [67].

6.2. Adverse reactionsThe most frequent adverse events of tramadol are nausea, dizziness, drows-

iness, fatigue, sweating, vomiting, and dry mouth [68]. Tramadol has less

489Tramadol Hydrochloride

respiratory depressant potential than other opioids, such as morphine and

oxycodone [69,70]. Tramadol also has a low abuse potential and does not

precipitate a withdrawal syndrome [71,72]. Overdose of tramadol is associ-

ated with neurological toxicity. Cardiovascular toxicity has not been

reported. The most common symptoms of tramadol overdose are lethargy,

nausea, tachycardia, agitation, seizures, coma, hypertension, and respiratory

depression [73].

6.3. Drug interactionsCoadministration of Cimetidine, a typical enzyme inhibitor, results in an

increase of tramadol AUC and elimination half-life. When coadministrated

with carbamazepine, a typical enzyme inducer, tramadol Cmax, AUC, and

elimination half-life is reduced by 51%, 26%, and 54%, respectively [53].

7. METHOD OF CHEMICAL SYNTHESIS

The synthesis of tramadol hydrochloride (Scheme 11.4) is known to

begin with a Mannich reaction between cyclohexanone (1), paraformalde-

hyde (2) (or formaldehyde [74,75]), and dimethylamine hydrochloride (3)

(or monodimethylamine sulfate [75]) to form 2-dimethylaminomethyl-

cyclohexanone hydrochloride (4) [15,74–77]. After converting 4 to the

free base (5), the addition of m-methoxyphenyl magnesium bromide (or

m-methoxyphenyl lithium [76,78]) to 5 occurs to produce tramadol base

O

X

N

O

N

OH

O

H

and enantiomer

Tramadol

+

X = Li or MgBr

O

+ (CH3)2NH·HCI(CH2O)n H3C

H3C

CH3

CH3

H3C

H3C

H3CH3C

H3C

CH3

H3C

N

OH

O

H

and enantiomer

Tramadol hydrochloride

NaOH· HCI

· HCI

HCI

1

2 3

N

O

4

5

Scheme 11.4 Synthesis of tramadol.

490 Robert Smyj et al.

[6,10,15,75–84], which is then converted to the HCl salt

[10,76,79,80,83,85]. In the addition reaction, the 1RS,2SR stereoisomer

of tramadol is produced as a by-product [10,74,75,77–84]. There are several

strategies given in the literature for the removal of the 1RS,2SR isomer from

tramadol [15,74,75,77,80–82,86] or tramadol hydrochloride [10,79,87].

Modifications to the addition reaction such as the use of additives [85] or

transition metal salts [78] are reported to increase stereoselectivity in the for-

mation of tramadol. Recently, a synthetic route for the asymmetric synthesis

of one enantiomer of tramadol has been described [88]. Continuous flow

conditions have also been used to produce tramadol [83,84].

ACKNOWLEDGMENTSThe authors are indebted to Dr. Yan Alsmeyer for encouragement and management support.

We also wish to express our appreciation to Dr. Yuri Goldberg for his guidance and to

Ms. Janet Mensah for her assistance in retrieving the cited literature and lastly many

thanks go to our colleagues in the laboratories who contributed material needed for the

preparation of this chapter.

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