Reliable routine method for the determination of plasma amitriptyline and nortriptyline by gas chromatography

Population pharmacokinetics of clomipramine, desmethylclomipramine, and hydroxylated metabolites in patients with depression receiving chronic treatment: model evaluation

Because metabolites play a major role in the clinical response to clomipramine, the objective of the current study was to develop a population model and evaluate its performance to describe the pharmacokinetic profiles of clomipramine (C) and its active metabolites desmethylclomipramine (DC), 8-hydroxy-clomipramine (OHC) and 8-hydroxy-desmethylclomipramine (OHDC). A first sample of 14 patients served for development of a 2-molecule C and DC model, which was shown to provide reasonable estimates of AUC-based clearances, as well as precise estimation of interindividual variability. Simulated data, generated to mimic a semi-rich sampling design and chronic treatment with clomipramine, indicated that clearance estimation was feasible under routine treatment conditions. A second sample of 30 patients, recruited prospectively and followed for a median 4-week period, was used to extend the 2-molecule model to a 4-molecule model. Goodness-of-fit assessment revealed that model-predicted concentrations were reasonably close to observed concentrations for a majority of patients. Interindividual variability was 50% to 60% for hydroxylation and desmethylation clearances, and residual variability was 30%. The proposed model incorporates much of what is known about the metabolism of clomipramine and may valuably integrate the influence of genetic and environmental factors on each metabolic pathway.

Inhibition of head twitch response to quipazine in rats by chronic amitriptyline but not fluvoxamine or citalopram

Chronic (twice daily/14 days), but not acute, treatment with 10 mg/kg PO amitriptyline reduced the number of quipazine (5 mg/kg)-induced head twitches in rats, measured 2 h (but not 72 h) after the last administration of the drug. Similar treatment with fluvoxamine or citalopram, which are more potent and much more specific serotonin uptake inhibitors than amitriptyline, did not affect the quipazine-induced response. In acute experiments, fluvoxamine (10 mg/kg PO) and citalopram (10 mg/kg PO) potentiated the head twitch reaction induced by L-5-hydroxytryptophan (50 mg/kg IP) given together with Ro 4-4602 (25 mg/kg IP), a peripheral decarboxylase inhibitor. Amitriptyline (10 mg/kg PO) slightly decreased the number of L-5-hydroxytryptophan (5-HTP)-induced head twitches. Higher doses of amitriptyline (20-40 mg/kg PO) also inhibited the quipazine-induced head twitch reaction. The brain level of amitriptyline measured 0.5-24 h after the last oral administration of the chronic dose of 10 mg/kg was always much higher than that observed at the same time intervals after an acute oral dose of 20 or 40 mg/kg. The results obtained indicate that a postsynaptic rather then presynaptic mechanism is responsible for the development of subsensitivity of the central serotonin receptors in the course of chronic treatment with amitriptyline.

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.

Continuation therapy with lithium and amitriptyline in unipolar depressive illness: a randomized, double-blind, controlled trial

A detailed analysis of the results of a multi-centre clinical trial shows that, while the relapse rate following recovery from an operationally defined depressive illness was smaller among patients subsequently treated with either amitryptiline or lithium than with a placebo, there was no clinically significant difference between the prophylactic efficacy of the 2 antidepressants. An account is given of the relative adverse effects of the treatments, and the implications of the findings are discussed.

The demethylation of amitriptyline: a cross-over study of steady-state plasma levels of amitriptyline and nortriptyline in depressed patients

Five elderly depressed patients were treated with amitriptyline (AT) and nortriptyline (NT) in turn, in a cross-over design. Steady-state plasma drug levels were compared with those calculated for eight healthy subjects of previous single-dose studies. Plasma clearances were on average about 2.5 times lower in the patients than the healthy subjects, but the ratios of the different reaction rates did not differ significantly between the two groups. The interpretation of the ratio of NT levels during AT treatment and NT treatment, in terms of the fraction of AT that is metabolised by demethylation, is discussed. The ratio of NT level to AT level during AT treatment was particularly variable between individuals. This was apparently due to independent variation in (a) the ratio of plasma clearances of AT and NT and (b) the fraction of AT metabolised to NT.

The demethylation of amitriptyline administered by oral and intramuscular routes

A single intramuscular (IM) dose of 50 mg amitriptyline (AT) was given to each of two healthy subjects who had previously taken three oral doses of AT tablets. The concentration-time curves of plasma nortriptyline (NT) arising from IM AT were fitted by computer to an equation based on the assumptions that NT is produced at a rate proportional to plasma AT concentration and is eliminated at a rate proportional to plasma NT concentration. The total amount of systemic NT produced was approximately equal for IM and oral AT doses, suggesting that the hepatic elimination of a given quantity of AT produces a constant amount of systemic NT, whether the AT is metabolised in the first pass or from the systemic circulation. This conclusion is supported by analysis of the time course of plasma NT concentration arising from oral AT doses. A large early production had previously been demonstrated and attributed to first-pass metabolism. The proportion of each oral AT dose eliminated in the first pass could now be estimated. Values ranged from 51% to 70% and were approximately equal to the proportion of the total NT apparently formed in the first pass.

Isocratic high-performance liquid chromatographic method for the determination of tricyclic antidepressants and metabolites in plasma

An isocratic high-performance liquid chromatographic method for the determination of six tricyclic antidepressants and their major metabolites is presented. Hexane containing 0.5% diethylamine was used as an extraction solvent to minimize adsorption onto glass. A reversed-phase cyanopropylsilane column was used with a mobile phase consisting of 70% acetonitrile and 30% 0.03 M acetate buffer, pH 7.0. Good identification and quantitation were obtained by the use of both UV detection at 245 nm and spectrofluorometric detection with an excitation wavelength of 276 nm and an emission filter with a 370-nm cutoff. A minimum detectable limit of less than 5 ng/ml of plasma is possible with this system. The reproducibility and precision of the method are shown from the analysis of samples containing 20-400 ng/ml in plasma.

Time course of plasma drug levels during once-daily oral administration of clomipramine

Fourteen depressed in-patients were treated with 150 mg clomipramine (CLO) daily, given as one oral dose. Using a gas-chromatographic method, concentrations of CLO and desmethyl clomipramine (DMCLO) were determined in plasma samples taken at frequent intervals during 24 h. The plasma level of each compound 12 h after the dose correlated well with the average value in the same patient, calculated over the whole 24-h period. Levels at other times gave poorer correlations, and at 24 h it was particularly poor. Plasma DMCLO concentrations were usually maximum 4-6 h after the dose. The ratios of maximum to minimum levels averaged only 1.31 +/- 0.15 SD. Peak CLO levels occurred 3 or 4 h after the dose. Maximum; minimum ratios averaged 2.72 +/- 0.73 SD, contrasting with the much smaller fluctuations of plasma nortriptyline (NT) levels observed in patients given this drug once daily. The difference is not due to a shorter half-life of CLO, but to the absorption and/or distribution behaviour of the two drugs. Although not fully understood, this difference between tertiary and secondary amines appears to hold generally among the tricyclic antidepressants.

Continuation therapy with lithium and amitriptyline in unipolar depressive illness: a controlled clinical trial. A report by the Medical Research Council Drug Trials Subcommittee

An account is given of a multi-centre, controlled clinical trial of the comparative efficacy of lithium, amitriptyline and placebo in terms of the comparative relapse-rates over a three-year period among patients who had recovered from a depressive illness. The results indicate that while both the pharmacodynamically active drugs are more effective than placebo the differences between them are minimal.

Rapid isothermal gas--liquid chromatographic determination of tricyclic antidepressants in serum with use of a nitrogen-selective detector

A rapid and simple gas--liquid chromatographic method suitable for routine use is described for quantitation of tricyclic antidepressants. The method can be used for quantitation of amitriptyline, nortriptyline, imipramine, desmethylimipramine, doxepin, desmethyldoxepin, clomipramine, desmethylclomipramine, trimipramine, desmethyltrimipramine and dibenzepine. The method involves one extraction with hexane--isopropyl alcohol, no derivative formation of the metabolites and chromatography using an OV-25 liquid phase which permits isothermal quantitation of the different drugs within 5--10 min. Standard curves for the different tricyclic antidepressants were linear over the concentration range tested (10--1500 micrograms/l) and the maximum coefficient of variation was 7.7%.

Amitriptyline pharmacokinetics. A crossover study with single doses of amitriptyline and nortriptyline

Six healthy volunteers were given single doses of amitriptyline (AT) and of nortriptyline (NT) separated by at least 10 days. Plasma concentrations of both compounds were measured at intervals for 48 or 72 h. The total areas under the concentration-time curves for the ingested drug were greater for NT, but AT concentrations showed much higher peak values and took more than 12 h to reach the terminal beta phase of elimination. Doses of 50 mg AT produced areas averaging slightly less than half those for 100 mg AT in the same subject, suggesting some saturation of the elimination process. The consumption of a large, fatty meal just before taking the AT tablets had little effect on the plasma drug concentration curves. NT half-lives, measured after ingestion of NT tablets, were used in analysing the production of NT from doses of AT in the same subject. There was a rapid early production, amounting to 30-67% of the total and presumably resulting from the first pass of AT through the liver. NT was then formed continuously at a rate always proportional to the simultaneous rate of AT elimination. The total amount of NT entering the systemic circulation was about one-quarter of the AT dose.

Binding of amitriptyline and nortriptyline in plasma determined from their equilibrium distributions between red cells and plasma, and between red cells and buffer solution

In a new method for the measurement of plasma binding, amitriptyline (AT) and nortriptyline (NT) were allowed to reach equilibrium distribution between the cells and plasma of whole blood. A separate sample of the cells was equilibrated with a buffer solution containing the drugs. From the two distribution ratios for each drug, the fraction free in the plasma was calculated. Equilibria were achieved rapidly, avoiding denaturation of binding proteins. The pH was adequately controlled and the composition of the plasma was not altered by the experimental procedures. Large volumes of buffer solution gave amounts of free drug readily measurable by gas chromatography. Duplicate determinations showed coefficients of variation of 6% and 4.8% respectively for the free fractions of AT and NT in a given plasma sample. In 51 subjects the mean percentage of AT free in plasma was 4.31 +/- 0.59 SD and, of NT, 8.59 +/- 0.86. Binding was independent of drug concentration in the therapeutic range and did not differ between males and females nor between patients and normal subjects. It increased slightly with age. It was not affected by chylomicrons in the blood.

Amitriptyline pharmacokinetics. Single doses of Lentizol compared with ordinary amitriptyline tablets

Two separate single doses of Lentizol (W. R. Warner, Pontypool, U.K.), a sustained-release preparation of amitriptyline (AT) were taken by each of six healthy subjects. Plasma concentrations of AT and of nortriptyline (NT) were determined at intervals over a period of 48 or 72 h. Faeces were collected and their drug content measured. Results were compared with those obtained when the same subjects took ordinary AT tablets. AT was found in the faeces after the ingestion of Lentizol or of ordinary AT tablets. However, after NT tablets negligible amounts of NT appeared in the faeces. AT was sometimes absorbed slowly from Lentizol, but on other occasions it was absorbed as rapidly as from ordinary tablets. Plasma levels of AT 24 h after the dose were usually not higher after Lentizol than after an equal dose of ordinary tablets. The systemic bioavailability of Lentizol as judged by areas under the plasma concentration-time curves, both for AT and for the NT formed metabolically, was on average lower than that of the ordinary tablets. However, the amounts of AT found in the faeces were not large enough to account for the AT area reduction by simple failure of absorption. Possible explanations of the discrepancy are discussed.

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.

Technical considerations in the gas chromatographic analysis of desipramine

Procedures are presented for minimizing variability in and interferences with the gas chromatographic determination of desipramine in plasms. Careful consideration of procedures for sample collection and storage, drug separation from matrix components, and chromatography appears to be a prerequisite for avoiding inaccurate and imprecise determinations of this antidepressant, especially at levels below 20 micrograms/l. Numerous pitfalls are examined and optimal conditions for obviating them are presented.

Simplified method for determination of the tetracyclic antidepressant mianserin in human plasma using gas chromatography with nitrogen detection

A simplified gas chromatographic method for determination of the antidepressant drug mianserin in human plasma is described. Application of a nitrogen-sensitive detector reduces the assay procedure to extraction, concentration and gas chromatographic determination. The method is suitable to determine mianserin in human plasma at the 1 ng/ml level on a routine basis. At the 20 ng/ml level the deviation of the mean from the true value and the relative standard deviation amount to 1.0% and 6.8%, respectively.