Xenobiotica the fate of foreign compounds in biological systems
ISSN: 0049-8254 (Print) 1366-5928 (Online) Journal homepage: http://www.tandfonline.com/loi/ixen20
Metabolism of Methocarbamol (Robaxin) in the Isolated Perfused Rat Liver and Identification of Glueuronides Richard M. Thompson, Nicholas Gerber & Richard A. Seibert To cite this article: Richard M. Thompson, Nicholas Gerber & Richard A. Seibert (1975) Metabolism of Methocarbamol (Robaxin) in the Isolated Perfused Rat Liver and Identification of Glueuronides, Xenobiotica, 5:3, 145-153 To link to this article: http://dx.doi.org/10.3109/00498257509056100
Published online: 22 Sep 2008.
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Full Terms & Conditions of access and use can be found at http://www.tandfonline.com/action/journalInformation?journalCode=ixen20 Download by: [NUS National University of Singapore]
Date: 01 February 2016, At: 09:32
XENOBIOTICA,
1975, VOL. 5,
NO.
3, 145-153
Metabolism of Methocarbamol (Robaxin) in the Isolated Perfused Rat Liver and Identification of Glucuronides RICHARD M. T H O M P S O N Downloaded by [NUS National University of Singapore] at 09:32 01 February 2016
Department of Pediatrics, Indiana University School of Medicine, Indianapolis, Indiana 46202, U.S.A.
NICHOLAS GERBER Department of Pharmacology, University of Oregon Medical School, Portland, Oregon 97201,U.S.A.
and RICHARD A. SEIBERT Department of Pharmacology, Baylor College of Medicine, Houston, Texas 77025, U.S.A. (Received 5 June 1974)
1. Permethylation and g.1.c.-mass spectrometric analysis of bile from an isolated rat liver perfusion to which methocarbamol was added showed seven components not present in control bile : methocarbamol, glucuronides of methocarbamol and desmethyl-methocarbamol,and four glucuronides of hydroxylated methocarbamol metabolites. 2. An intersting rearrangement of a methyl group has been found in the mass the permethylation spectrum of 3-(2-methoxyphenyloxy)-l,2-dimethoxypropane, product from methocarbamol.
Introduction T h e metabolism of methocarbamol (Robaxin) (see Fig. l), a non-toxic muscle relaxant, has been examined previously in the rat (Bruce, Turnbull & Newman, 1971), dog (Campbell et al., 1961 ; Bruce et al., 1971), and human (Campbell et al., 1961 ; Bruce et al., 1971). Campbell et al. (1961) concluded that the carbamate ester group of the drug was not hydrolysed in vivo and that small amounts of unconjugated drug and at least three glucuronides, one of which was derived from unchanged methocarbamol, were present in urine. More recently, Bruce et al. (1971) identified two metabolites of radioactive methocarbamol, both of which were excreted as sulphate or glucuronide conjugates, namely 3-(2-hydroxyphenoxy)-1,2-propanediol-l-carbamateand 3-(4-hydroxy2-methoxyphenoxy)-l,2-propanediol-l-carbarnate. These two metabolites and unchanged methocarbamol were the three major radioactive components in the urine. No hydrolysis of the carbamate ester was observed. Previous metabolism studies in the isolated perfused rat liver indicated that bile was an excellent source of glucuronide metabolites of drugs and that permethylation followed by gas chromatographic and mass spectrometric analysis provided a rapid method for the separation and characterization of glucuronides and other metabolites of drugs (Gerber, Seibert & Thompson, 1973 ; Thompson
R . M . Thompson et al.
146
Downloaded by [NUS National University of Singapore] at 09:32 01 February 2016
et al., 1972 ; Thompson et al., 1973). We now describe the use of these methods to determine the structures of the glucuronides of methocarbamol and its metabolites.
Materials and methods Methocarbamol (Robaxin) was a gift from the A. H. Robins Company, Richmond, Virginia. T h e drug was permethylated by the method of Leclercq and Desiderio (1971). Mass spectra of methocarbamol and the permethylated product, 3-(2-methoxyphenoxy)-1,2-dimethoxypropane (see Fig. 2), were obtained with an LKB-9000-S GC-MS instrument with the direct insertion probe. T h e accelerating and ionizing potentials were 3500 kV and 70 eV respectively, the trap current was 60 PA, and the source temperature 270". T h e isolated perfused rat liver was set up as previously described (Gerber et al., 1971). An aqueous solution of methocarbamol (50 mg in 10 ml saline) was added after about 0.2 ml of control bile was collected. T h e perfusion was allowed to run for about 1 h, yielding a sample bile of about 0-5 ml. Aliquots of bile were evaporated to dryness, permethylated, and analysed by g.1.c.-mass spectrometry as previously described (Thompson et al., 1973), except that a 1.83 m 1% SE-30 column was used. Results Figure 1 shows the mass spectrum of methocarbamol, with the molecular ion at m/e 241 (3.5%). T h e principal cleavage under electron impact is alpha to the substituted phenoxy group to yield the ions at m/e 118 (42%) and 124 (lOO~o), t
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ISSN: 0049-8254 (Print) 1366-5928 (Online) Journal homepage: http://www.tandfonline.com/loi/ixen20
Metabolism of Methocarbamol (Robaxin) in the Isolated Perfused Rat Liver and Identification of Glueuronides Richard M. Thompson, Nicholas Gerber & Richard A. Seibert To cite this article: Richard M. Thompson, Nicholas Gerber & Richard A. Seibert (1975) Metabolism of Methocarbamol (Robaxin) in the Isolated Perfused Rat Liver and Identification of Glueuronides, Xenobiotica, 5:3, 145-153 To link to this article: http://dx.doi.org/10.3109/00498257509056100
Published online: 22 Sep 2008.
Submit your article to this journal
Article views: 17
View related articles
Full Terms & Conditions of access and use can be found at http://www.tandfonline.com/action/journalInformation?journalCode=ixen20 Download by: [NUS National University of Singapore]
Date: 01 February 2016, At: 09:32
XENOBIOTICA,
1975, VOL. 5,
NO.
3, 145-153
Metabolism of Methocarbamol (Robaxin) in the Isolated Perfused Rat Liver and Identification of Glucuronides RICHARD M. T H O M P S O N Downloaded by [NUS National University of Singapore] at 09:32 01 February 2016
Department of Pediatrics, Indiana University School of Medicine, Indianapolis, Indiana 46202, U.S.A.
NICHOLAS GERBER Department of Pharmacology, University of Oregon Medical School, Portland, Oregon 97201,U.S.A.
and RICHARD A. SEIBERT Department of Pharmacology, Baylor College of Medicine, Houston, Texas 77025, U.S.A. (Received 5 June 1974)
1. Permethylation and g.1.c.-mass spectrometric analysis of bile from an isolated rat liver perfusion to which methocarbamol was added showed seven components not present in control bile : methocarbamol, glucuronides of methocarbamol and desmethyl-methocarbamol,and four glucuronides of hydroxylated methocarbamol metabolites. 2. An intersting rearrangement of a methyl group has been found in the mass the permethylation spectrum of 3-(2-methoxyphenyloxy)-l,2-dimethoxypropane, product from methocarbamol.
Introduction T h e metabolism of methocarbamol (Robaxin) (see Fig. l), a non-toxic muscle relaxant, has been examined previously in the rat (Bruce, Turnbull & Newman, 1971), dog (Campbell et al., 1961 ; Bruce et al., 1971), and human (Campbell et al., 1961 ; Bruce et al., 1971). Campbell et al. (1961) concluded that the carbamate ester group of the drug was not hydrolysed in vivo and that small amounts of unconjugated drug and at least three glucuronides, one of which was derived from unchanged methocarbamol, were present in urine. More recently, Bruce et al. (1971) identified two metabolites of radioactive methocarbamol, both of which were excreted as sulphate or glucuronide conjugates, namely 3-(2-hydroxyphenoxy)-1,2-propanediol-l-carbamateand 3-(4-hydroxy2-methoxyphenoxy)-l,2-propanediol-l-carbarnate. These two metabolites and unchanged methocarbamol were the three major radioactive components in the urine. No hydrolysis of the carbamate ester was observed. Previous metabolism studies in the isolated perfused rat liver indicated that bile was an excellent source of glucuronide metabolites of drugs and that permethylation followed by gas chromatographic and mass spectrometric analysis provided a rapid method for the separation and characterization of glucuronides and other metabolites of drugs (Gerber, Seibert & Thompson, 1973 ; Thompson
R . M . Thompson et al.
146
Downloaded by [NUS National University of Singapore] at 09:32 01 February 2016
et al., 1972 ; Thompson et al., 1973). We now describe the use of these methods to determine the structures of the glucuronides of methocarbamol and its metabolites.
Materials and methods Methocarbamol (Robaxin) was a gift from the A. H. Robins Company, Richmond, Virginia. T h e drug was permethylated by the method of Leclercq and Desiderio (1971). Mass spectra of methocarbamol and the permethylated product, 3-(2-methoxyphenoxy)-1,2-dimethoxypropane (see Fig. 2), were obtained with an LKB-9000-S GC-MS instrument with the direct insertion probe. T h e accelerating and ionizing potentials were 3500 kV and 70 eV respectively, the trap current was 60 PA, and the source temperature 270". T h e isolated perfused rat liver was set up as previously described (Gerber et al., 1971). An aqueous solution of methocarbamol (50 mg in 10 ml saline) was added after about 0.2 ml of control bile was collected. T h e perfusion was allowed to run for about 1 h, yielding a sample bile of about 0-5 ml. Aliquots of bile were evaporated to dryness, permethylated, and analysed by g.1.c.-mass spectrometry as previously described (Thompson et al., 1973), except that a 1.83 m 1% SE-30 column was used. Results Figure 1 shows the mass spectrum of methocarbamol, with the molecular ion at m/e 241 (3.5%). T h e principal cleavage under electron impact is alpha to the substituted phenoxy group to yield the ions at m/e 118 (42%) and 124 (lOO~o), t
NY
-24.2
100 -
-
A
+ j, al +
75-
-
