Ion-pair liquid chromatography of amitriptyline and metabolites in plasma

Investigation of xenobiotic metabolism by CYP2D6 and CYP2C19: importance of enantioselective analytical methods

Investigations into the genetic polymorphism of drug metabolism have involved specific models to screen poor and extensive metabolisers of xenobiotics. Debrisoquine, sparteine, S-mephenytoin and dextromethorphan are particularly well known. They have been extensively described in the literature and are used to phenotype human subjects before performing investigations with new drugs which are believed to be under the control of a genetic polymorphism. Dextromethorphan, debrisoquine and sparteine are good substrates for CYP2D6, whereas the S-enantiomer of mephenytoin is a good substrate for CYP2C19, both being two isozymes of cytochrome P-450. In many drugs, the hepatic microsomal oxidative metabolism involving stereogenic centres congregates either with CYP2D6 or with CYP2C19 or, in certain cases, with both of them. The availability of both CYP2D6 from poor and extensive metabolisers and an enantioselective assay would allow genetic polymorphism in drug biotransformation to be investigated in vitro ex vivo at an early stage of drug development before the IND (investigational new drug). Single-dose investigations in vivo can also be performed when only minimal pre-clinical toxicological data are available and produce more reliable results than in vitro studies. This paper focuses on the problem of genetic polymorphism in drug development and specifically discusses some relevant knowledge gained in the last two decades on enantioselective bioassays. Specific examples are given.

Active hydroxymetabolites of antidepressants. Emphasis on E-10-hydroxy-nortriptyline

Hydroxymetabolites of the antidepressants nortriptyline and desipramine, like the parent drugs, inhibit neuronal uptake of noradrenaline (norepinephrine). In both plasma and cerebrospinal fluid (CSF), the concentrations of the 10-hydroxymetabolites of nortriptyline (10-OH-NT) are usually higher than those of the parent drugs, but there is a pronounced interindividual variation in the plasma concentrations. This shows that during treatment with nortriptyline, hydroxymetabolites exert, at least in some patients, major effects on brain noradrenaline neurons. Hydroxymetabolites of antidepressants are formed by the polymorphic cytochrome P450 enzyme CYP2D6. Nortriptyline is hydroxylated by this enzyme in a highly stereospecific way to the (-)-enantiomer of E-10-OH-NT. Among Caucasians, 7% are poor metabolisers of the CYP2D6 probe drug debrisoquine. These patients will form very little hydroxymetabolite. The affinity of E-10-OH-NT for muscarinic acetylcholine receptors in vitro was only one-eighteenth of the affinity of nortriptyline for these receptors. In healthy individuals, nortriptyline decreased saliva flow to a significantly greater extent than either E-10-OH-NT or placebo. In an ultrarapid hydroxylator of nortriptyline treated with very high doses of nortriptyline, the plasma concentration of unconjugated 10-OH-NT was very high without any sign of anticholinergic adverse effects. These results show that hydroxymetabolites of nortriptyline have much less anticholinergic effect than the parent drug. When racemic E-10-OH-NT per se was given to healthy individuals, the plasma concentration of the (-)-enantiomer was 5-fold higher than that of (+)-E-10-OH-NT. The 2 enantiomers were eliminated in parallel with an elimination half-life of 8 to 10 hours. A combined in vitro and in vivo investigation showed that a mean of 64% of (+)-E-10-OH-NT was glucuronidated in the liver and subsequently eliminated in urine. Of the administered (-)-enantiomer, a mean of 36% was eliminated as glucuronide formed in the intestine and 35% was actively secreted as unchanged form in urine. Plasma protein binding, determined by ultrafiltration, of the (+)- and (-)-enantiomers of E-10-OH-NT was 54 and 69%, respectively, which is less than that of nortriptyline (92%). The concentration of E-10-OH-NT in CSF was 50% of the concentration of unbound in plasma. There seems to be a stereoselective active transport of E-10-OH-NT from the CSF to blood. We administered racemic E-10-OH-NT to 5 patients during a major depressive episode.(ABSTRACT TRUNCATED AT 400 WORDS)

Development of a rapid extraction and high-performance liquid chromatographic separation for amitriptyline and six biological metabolites

The development of a rapid high-performance liquid chromatographic method for the determination of amitriptyline, amitriptyline-N-oxide, 10-hydroxyamitriptyline, 10-hydroxynortriptyline (E and Z isomers), nortriptyline and desmethylnortriptyline in plasma and liver tissue is described. A liquid--liquid extraction with hexane--butanol and back-extraction into phosphoric acid provides efficient extraction of amitriptyline-N-oxide along with amitriptyline and the other metabolites. A Supelcosil C8 reversed-phase column with 5-micron packing and a methanol--sodium phosphate buffer--amine modifier mobile phase was used. The combination of mobile phase pH and amine modifier concentration for the best separation within a reasonable analysis time for all seven solutes plus an internal standard was determined using a factorial design coupled with a multi-factor window diagram technique. Ultraviolet detection at 214 nm provided limits of detection of approximately 1 ng/ml.

Enzyme-linked immunosorbent assay for amitriptyline and other antidepressants using a monoclonal antibody

We describe and evaluate a method to measure amitriptyline and other tricyclic antidepressants by enzyme linked immunosorbent assay, using monoclonal antibody. In this assay, biological samples were first incubated with the antibody; in a second step, free remaining antibody was allowed to bind to lysozyme-nortriptyline coated immunotitration plates. The bound fraction of the monoclonal antibody was revealed with rabbit anti-mouse serum coupled to horseradish peroxidase. The optical density of the reaction product was measured with a colorimeter at 410 nm. Specificity of the antibody was investigated by means of a Farr test showing interferences in therapeutic ranges only for chlorpromazine and phenytoine. Means of intra- and inter-assay variations were 10 and 13%, respectively. The results when compared to those obtained by gas chromatography with a selective nitrogen detector gave a correlation coefficient of 0.897. Finally, the great reliability of the monoclonal antibody, the advantages of a decreased analysis time, low cost and high capacity of the procedure contribute to make this immunoassay most suitable for clinical monitoring and pharmacokinetic studies of tricyclic antidepressants.

Therapeutic monitoring of psychotherapeutic drugs: role of the clinical laboratory

Therapeutic monitoring of drugs is a well established clinical tool. However, the state of the art is somewhat less advanced for drugs used in psychiatry than it is for other classes of drugs, for several reasons. Most psychotherapeutic drugs have large volumes of distribution and achieve relatively low plasma concentrations following therapeutic doses. Many have one or more active metabolites. While psychotherapeutic drugs act through biochemical mechanisms, they are used to treat clinical syndromes which may be heterogeneous in their biochemical pathogenesis. As a consequence, the analytical methodologies are often complex and not always reliable; well-controlled clinical studies are difficult to perform; the therapeutic ranges have been difficult to establish. Despite these limitations, prudent and selective monitoring of serum drug concentrations, particularly of the tricyclic antidepressants, can be helpful in clinical management.

Analysis of tricyclic antidepressant drugs in plasma and serum by chromatographic techniques

A review of methods for the determination of tricyclic antidepressants in plasma or serum, based on the application of chromatographic techniques, is presented. A general discussion of the techniques in terms of their precision, accuracy, sensitivity and selectivity, with respect to parent drug and metabolites, is used to facilitate a comparison of methods. No one technique can be claimed as the method of choice for these drugs, although gas-liquid chromatography with nitrogen selective detection has some strong claims, viz. generally good sensitivity and reproducibility of assays and ready availability of equipment in most laboratories. The ultimate choice of a method for determining tricyclics will be determined more by the clinical application (routine monitoring versus pharmacokinetics) than by other factors.

Gas chromatographic--mass spectrometric determination of amitriptyline and its major metabolites in human serum

A gas chromatographic--electron-impact ionization mass spectrometric method has been developed for the determination of amitriptyline (AMT) and its metabolites, nortriptyline (NT), 10-hydroxyamitriptyline (10-OH-AMT) and 10-hydroxynortriptyline (10-OH-NT) in human serum. The lower limit of detection was 2 ng/ml for all compounds except 5 ng/ml for 10-OH-NT. The calibration curves for AMT and 10-OH-AMT were linear up to 100 ng/ml, and up to 200 ng/ml for NT and 10-OH-NT. The accuracy of the assay in terms of coefficient of variation was less than 7%. The extraction efficiency was almost quantitative for all compounds except 60% for 10-OH-NT. Using this method, human serum samples which had been collected after oral administration of a single 50-mg dose of AMT were analyzed. Ratios of the conjugation of each metabolite were estimated, including AMT.

Quantification of amitriptyline, nortriptyline, and 10-hydroxy metabolite isomers in plasma by capillary gas chromatography with nitrogen-sensitive detection

A selective, sensitive method for the determination of amitriptyline and its metabolites is described. This method involves liquid-liquid extraction and capillary gas chromatography with nitrogen-sensitive detection. The detection limits of amitriptyline, nortriptyline, 10-hydroxy(E)amitriptyline, 10-hydroxy(E)nortriptyline, and 10-hydroxy(Z)nortriptyline were slightly less than 0.5 ng/ml in 1.0-ml plasma samples. The coefficients of variation for within-run and between-run analyses of samples containing 100 ng/ml were less than 12% and 9%, respectively. The method offers rapid analysis of individual isomers, increased sensitivity over high-performance liquid chromatographic methodology and the conveniences of the gas chromatographic technique.

Biotransformation of amitriptyline in depressive patients: urinary excretion of seven metabolites

The urinary excretion of amitriptyline (AMT) and seven of its metabolites was studied by mass spectrometry in 10 depressive in-patients treated to steady-state condition with oral amitriptyline. An average of 68.3% of the dose was recovered in the urine, of which 68.6% was present as conjugates. Hydroxynortriptyline and its conjugate represented 54% of the total recovery. There was marked variation in metabolite pattern between patients. The variations were not due to concomitant medication with benzodiazepines. There was no correlation between the plasma and urine concentrations of AMT and its metabolites, except for amitriptyline conjugates. Two groups of patients could be distinguished - low and high excretors, who displayed alternative routes of metabolism. The disappearance rate of AMT from plasma was determined by the metabolic clearance of AMT to its metabolites. It varied considerably between patients.

Determination of amitriptyline-N-oxide, amitriptyline and nortriptyline in serum and plasma by high-performance liquid chromatography

A method for the determination of amitriptyline-N-oxide, amitriptyline and nortriptyline in serum and plasma has been developed. After extraction from serum or plasma the drugs were analysed by high-performance liquid chromatography. The detection limit was 10 ng/ml (2 ml serum or plasma actually used). The coefficient of variation for all three compounds was below 10%. Amitriptyline-N-oxide was found in rat plasma after an oral dose (10 mg/kg) of amitriptyline-N-oxide.

Neuropharmacological properties of amitriptyline, nortriptyline and their metabolites

Amitriptyline, nortriptyline and their metabolites, desmethylnortriptyline, cis and trans 10-hydroxyamitriptyline, cis and trans 10-hydroxynortriptyline and amitriptyline-N-oxide, have been tested for inhibitory effect on the uptake of serotonin (rabbit thrombocytes in vitro) and noradrenaline (mouse atria in vitro and mouse heart in vivo), for anticholinergic activity (guinea-pig ileum in vitro) and for antagonism against tetrabenazine induced inactivity as well as apomorphine and 5-hydroxytryptophan potentiating effect in mice. Amitriptyline inhibits serotonin and noradrenaline uptake equally, whereas nortriptyline is a more potent inhibitor of noradrenaline than of serotonin uptake. The metabolites resemble nortriptyline in this respect. The 10-hydroxylated metabolites are equipotent with amitriptyline as regards noradrenaline uptake inhibition. All the metabolites are less anticholinergic than amitriptyline and nortriptyline. The in vitro results are reflected in the in vivo behavioural tests, although some discrepancies are found, probably due to differences in absorption, distribution, metabolism and excretion. The importance of knowledge concerning pharmacological properties of the metabolites in comparison with amitriptyline and nortriptyline for correlating plasma levels of these and their metabolites to clinical outcome is discussed.

Displacement of nortriptyline and uptake of 14C-lidocaine in the lung after administration of 14C-lidocaine to nortriptyline intoxicated pigs

Six anaesthetized Swedish land-race pigs were intoxicated by an intravenous infusion of nortriptyline-HCl (NT) up to a concentration of 4.58 +/- 0.58 (mean +/- S.E.M.) microM in arterial whole blood. A rapid injection of 2 mg/kg b.wt. lidocaine-HCl in the right atrium was followed by a rise in arterial whole blood concentration of NT up to a maximum of 7.32 +/- 0.28 microM NT. Amount displaced NT from the cardio-pulmonary circulation after the 14C-lidocaine bolus, was calculated to be 0.66 +/- 0.03 mumol. Lung uptake of 14C-lidocaine during first-pass through the lung was not influenced to any statistically signficant degree compared to a control group. Thus, first pass uptake (FPU) was 30 +/- 8 (mean +/- S.E.M.)% and 39 +/- 5% respectively. The duration of the QRS-complex of the ECG was increased (P less than 0.01) during the infusion of NT from 0.07 +/- 0.01 (mean +/- S.E.M.) sec. to 0.14 +/- 0.02 sec. when 250 mg NT-HCl had been administered. The QRS-duration was decreased (P less than 0.01) after the injection of the 14C-lidocaine bolus to 0.09 +/- 0.01 sec. Mean arterial blood pressure and heartrate decreased slightly during the infusion of NT, but did not change immediately after the 14C-lidocaine bolus.