SOME ASPECTS OF THE FATE AND RELATIONSHIP OF THE N-METHYL GROUP OF MORPHINE TO ITS PHARMACOLOGICAL ACTIVITY

Deuterium- and Tritium-Labelled Compounds: Applications in the Life Sciences

Hydrogen isotopes are unique tools for identifying and understanding biological and chemical processes. Hydrogen isotope labelling allows for the traceless and direct incorporation of an additional mass or radioactive tag into an organic molecule with almost no changes in its chemical structure, physical properties, or biological activity. Using deuterium-labelled isotopologues to study the unique mass-spectrometric patterns generated from mixtures of biologically relevant molecules drastically simplifies analysis. Such methods are now providing unprecedented levels of insight in a wide and continuously growing range of applications in the life sciences and beyond. Tritium (³ H), in particular, has seen an increase in utilization, especially in pharmaceutical drug discovery. The efforts and costs associated with the synthesis of labelled compounds are more than compensated for by the enhanced molecular sensitivity during analysis and the high reliability of the data obtained. In this Review, advances in the application of hydrogen isotopes in the life sciences are described.

Identification with liquid chromatography-ionspray mass spectrometry of the metabolites of the enantiomers N-methyl dextrorphan and N-methyl levorphanol after rat liver perfusion

To gather more information on stereochemical factors in the hepatic disposition of organic cations, mass spectrometry coupled to liquid chromatography was used to determine the identity of the metabolites excreted in bile after isolated rat liver perfusions with the quaternary ammonium derivatives of the enantiomeric drugs dextrorphan and levorphanol. Ionspray mass spectrometry was chosen for its soft ionization and absence of thermal degradation of labile compounds. The drugs were labelled with a stable (2H) isotope and mixed with unlabelled drugs to create an artificial isotope pattern in the mass spectrum and facilitate the recognition of unknown metabolites. In mass spectra that were recorded under normal conditions, fragmentation was absent and metabolites of N-methyl dextrorphan and N-methyl levorphanol were visible as parent-ion 'doublets'. Collision-induced fragmentation studies were performed to support the identification of the metabolites. For N-methyl dextrorphan the glucuronide, the glutathione conjugate and the glucuronide of the N-demethylated metabolite were found in bile. For N-methyl levorphanol the glucuronide, the glutathione conjugate, the sulphate conjugate and the glucuronide of a hydroxylated N-methyl levorphanol were excreted in bile. Thus a remarkable stereoselectivity occurs in the metabolism of these quaternary ammonium compounds in the rat liver.

Percentages of the deuterium retained after para-hydroxylation of (R)(+)4-2H-phenytoin and (S)(-) 4-2H-phenytoin in rat

(R)(+) and (S)(-) 4-2H-phenytoin have been used as substrates for the determination of the percentage of deuterium retention (NIH shift) after para-hydroxylation of the substrates in rat. By using GC-MS analyses, the percentages of deuterium retention were found to be 69% and 70% for the (R) and (S) phenyl rings, respectively. The results add additional evidence for the involvement of arene oxide in the oxidation of the pro (R) and pro (S) phenyls of phenytoin. The oxidation process of each ring could be mediated by independent enzyme systems, a rapid oxidative enzyme for the pro (S) phenyl and a slow oxidative enzyme for the pro (R) phenyl.

Isotope effects: definitions and consequences for pharmacologic studies

The use of stable isotope-labeled compounds for pharmacologic studies requires careful consideration of the nature of the stable isotope label (2H, 13C, 15N, 18O) and its position of incorporation in the molecule. When deuterium is used, improper positioning can lead to significant primary isotope effects. Primary isotope effects occur when the breaking of the bond to the heavy isotope is the rate-limiting step in a reaction (or metabolic transformation). A reaction will proceed slower for the molecule with the heavy isotope label because of the mass difference between the light and the heavy isotope. In addition to these primary isotope effects, smaller but nevertheless important secondary isotope effects, physicochemical isotope effects, active hydrogen/deuterium exchange, or isotope effects associated with either the enzyme-catalyzed biotransformation or the mass spectrometric ionization and fragmentation can be operative. In mechanistic studies, isotope effects are used to their advantage; however, in pharmacokinetic studies, the occurrence of isotope effects can lead to grossly misleading biologic and analytic results: the metabolism of the drug will differ when 'in vivo' isotope effects are operative, and isotope effects occurring during the analysis procedure will obscure the true metabolic profile of the drug.

Effect of deuteration of imipramine on its pharmacokinetic properties in the rat

Imipramine was specifically deuterated in either both aromatic rings or in the N-methyl group, or in both positions, and the pharmacokinetic properties of the products were determined in the rat and compared with those of the non-deuterated analogue. Deuteration of imipramine resulted in a small but significant isotope effect on N-demethylation while aromatic hydroxylation was unaffected. This isotope effect led to a slower rate of systemic clearance, a longer half-life and, when orally administered, enhanced bioavailability. Urinary excretion of didesmethylimipramine-d4, following oral administration of imipramine-d7, was significantly lower than the excretion of didesmethylimipramine following administration of unlabelled imipramine, indicating inhibited demethylation. Similarly, the urinary excretion of desmethylimipramine-d4, didesmethylimipramine-d4 and 2-hydroxydesmethylimipramine-d4 were lower than for the corresponding unlabelled or d7-analogues, indicating the stability of the N-CD3 group. Deuteration had no effect on the pharmacological properties of imipramine as determined in this study.

Studies on the metabolism of tripamide, a new antihypertensive agent. I. Characterization of metabolites in rats

1. Tripamide (N-(4-aza-endo-tricyclo[5.2.1.0(2,6)]decan-4-yl)-4-chloro-3-sulphamoyl [carbonyl-14C]benzamide) was metabolized in rat to yield five metabolites which were separated by chromatography, and characterized by mass spectrometry and reverse isotope dilution analysis. 2. The five metabolites were identified as follows; N-(3(or 5)-hydroxy-4-aza-endo-tricyclo[5.2.1.0(2,6)]decan-4-yl)-4-chloro-3-sulphamoylbenzamide (3-hydroxy-tripamide); N-8(or 9)-hydroxy-4-aza-endo-tricyclo[5.2.1.0(2,6)]decan-4-yl)-4-chloro-3-sulphamoylbenzamide (8-hydroxy-tripamide); 4-chloro-3-sulphamoylbenzamide; 4-chloro-3-sulphamoylbenzoic acid; and 4-chloro-3-sulphamoylbenzoic acid-(N'-acetyl)hydrazide. 3. Tripamide was metabolized rapidly and the major metabolite was 4-chloro-3-sulphamoylbenzoic acid in rat blood. 4. After intravenous injection, the excretion rate of the hydroxylated tripamide into bile was about 2-3 times faster than that into urine.

A multiple-scan high resolution mass spectrometer data system for the analysis of mixtures: the analysis of morphine in liver

A high resolution mass spectrometer operating with a laboratory data system in multiple scan mode allows the generation of high resolution evaporation profiles for any m/z value. The system allows fast qualitative and quantitative analysis of components of complex mixtures. System performance is evaluated by analysis of morphine in liver samples; a precision of 15% at the 0.1 microgram g-1 level is obtainable on post-mortem samples.

Determination of imipramine in plasma by high pressure liquid chromatography and field ionization mass spectrometry: increased sensitivity in comparison with gas chromatography mass spectrometry

A quantitative method is reported for the determination of imipramine in plasma samples in the low nanogram and subnanogram range. The sensitivity and precision of the technique, which involves high pressure liquid chromatography and direct probe field ionization mass spectrometry, are approximately an order of magnitude greater than are offered by gas chromatography mass spectrometry with selected ion monitoring using deuterated or other types of internal standards. [2H6]Imipramine, labeled in the ethylene bridge and in the aromatic rings, serves as the isotopic diluent. The method has been used for the determination of the comparative bioavailabilities of two different commercial preparations of imipramine. In these tests, subjects ingested a 25 mg tablet of one or the other drug preparation together with a solution containing an equivalent amount of imipramine deuterated in the ethylene bridge ([2H2]imipramine). The latter served as an internal check for intrasubject variability in absorption of the imipramine tablets. Typical results from one of the subjects are presented.

Warfarin. Stereochemical aspects of its metabolism and the interaction with phenylbutazone

An examination of the metabolic fate of the R and the S isomers of warfarin revealed that the two isomers were metabolized by different routes. R warfarin was oxidized to 6-hydroxywarfarin and was reduced to the (R,S) warfarin alcohol. In contrast, S warfarin was oxidized to 7-hydroxywarfarin and was reduced to the (S,S) warfarin alcohol. S warfarin was also oxidized to 6-hydroxywarfarin. These observations suggested that interactions between warfarin and other drugs might be manifest stereo-specifically, i.e., have a different effect on the isomers of warfarin, so a series of experiments were conducted with each isomer of warfarin, before and after phenylbutazone. The plasma clearance of S warfarin was slowed from 3.1 to 1.1% per h in one subject and from 2.3 to 1.6% per h in another. In contrast, the clearance of R warfarin was increased from 1.5 to 3.0% per h and from 0.9 to 1.6% per h in two subjects after phenylbutazone. The rate of clearance of racemic warfarin was unaffected by phenylbutazone; the depression of the rate of clearance of the S isomer masked the stimulation of the clearance of the R isomer. Since S warfarin is five times more potent an anticoagulant than R warfarin, it is concluded that inhibition of the metabolism of S warfarin provides one mechanism for the augmented anticoagulation which follows phenylbutazone.