Dosage adjustment from simple nortriptyline spot level predictor tests in depressed patients

The role of plasma level monitoring of tricyclic antidepressant drugs as an aid to treatment

Use of the tricyclic antidepressant drugs is the most common pharmacotherapeutic approach to the treatment of depression. It is a common clinical experience to find that a fair proportion of patients fail to show a satisfactory response and that others complain of side-effects. Important factors which influence this situation are the diagnostic criteria used to select patients for drug treatment, the dosage prescribed and individual compliance. Measurement of plasma drug concentrations has been proposed as a more rational way of increasing the efficacy of antidepressant medication and avoiding toxicity. A number of studies have shown a correlation between plasma concentrations of these drugs and clinical effects, but the relationship is far from simple. With amitriptyline and nortriptyline there is good evidence for a 'therapeutic window' within which maximum antidepressant action is obtained. Many patients being treated with these drugs have plasma levels outside recommended therapeutic ranges. Experience in our department has shown that there are a number of clinical situations where routine plasma level monitoring of selected antidepressants is of value: (1) inadequate clinical response; (2) side-effects/toxicity; (3) complicating medical conditions; (4) suspected poor compliance; and (5) long-term therapy. Such measurements are a relatively simple and inexpensive way of maximizing the benefits of drug therapy and a means of tailoring drug dosages to suit individual requirements.

Therapeutic drug monitoring of psychotropic medications

Therapeutic drug monitoring (TDM) of a number of psychotropic medications has proven to be of value, enabling minimization of the limitations of considerable genetic variability in their metabolism and the high rates of poor compliance with many psychiatric disorders. Therapeutic ranges have been established for lithium, some of the tricyclic antidepressants, and clozapine. TDM has also been shown to be useful in avoiding toxicity (as many psychotropics have narrow therapeutic indices), particularly that due to interactions with other compounds.

Prediction of haloperidol steady-state levels in plasma after a single test dose

Because of large interindividual variabilities in the pharmacokinetics of haloperidol (HPDL), empirically adjusting the dose to achieve steady-state levels in plasma (Css) is a time-consuming process. We report a method to individualize dose to achieve a desired Css from an observed drug level 24 hours after a single 15-mg test dose of HPDL. Drug-free schizophrenic and schizo-affective patients were blindly and randomly assigned to achieve a low (< 5 ng/ml), medium (10-18 ng/ml), or high (> 25 ng/ml) Css range of HPDL. On day 1 of the study, each patient received an oral "test" dose of HPDL (15 mg), and blood was drawn 24 hours later to determine drug levels in plasma (C24h). The first 34 patients (group I) were then maintained empirically on 2, 5 to 8, or 10 to 15 mg twice daily of oral HPDL concentrate for 5 days to achieve a low, medium, or high Css range, respectively. For the next 58 patients (group II), the dose of HPDL to achieve the assigned Css range was computed by using C24h in a prediction formula. Application of the C24h correctly predicted the maintenance dose required to achieve the Css in 73.2% of the cases. Further, there was a highly significant correlation (R2 = 0.877, p < 0.0001) between the predicted dose and the actual dose required to achieve the targeted Css range. On the basis of these results, we have formulated a nomogram to help predict the maintenance dose required to achieve low, medium, or high HPDL targeted ranges at various C24h values.

Sources of prediction error when using a Bayesian method to evaluate nortriptyline serum concentrations

A Bayesian method was used to evaluate nortriptyline (NTP) serum concentrations (Cps) and predict future Cps in two populations: five simulated groups (n = 20 each) with known clearance (CL) and volume of distribution (Vd), and an actual inpatient group (n = 20). The effects of weight, CL, Vd, and magnitude of Cps on absolute prediction error (APE) were evaluated. In simulated groups, Cps after two doses of NTP and for steady-state were calculated for normal, increased, and decreased Vd and CL. In the actual patient group, Cps were measured in the first few days after starting NTP administration and again during maintenance therapy. The first Cps were used in the Bayesian program to estimate CL and Vd to predict the second Cps. In the simulated group, PE and APE differed significantly between normal and decreased values of CL. A large Vd resulted in less of a change in PE or APE in these subjects, but when combined with low CL led to the largest errors. In the actual patient group, PE was -5.9 +/- 19.2 ng/mL and APE was 15.4 +/- 12.6 ng/mL. In these patients, only body weight was correlated with the percent APE (r = 0.607, P = 0.005). The Bayesian method performs well clinically, but increased Vd and decreased CL can lead to higher PE. Clinically, the only factor that predicted higher APE was obesity. This may reflect an effect on Vd, and in these patients, a high APE may occur.

Simultaneous determination of antidepressant drugs and metabolites by HPLC. Design and validation of a simple and reliable analytical procedure

An analytical procedure suitable for routine use has been developed and validated for the simultaneous separation and quantification of most of the antidepressant drugs (bicyclic, tricyclic and tetracyclic) and their metabolites in human serum by reversed-phase high performance liquid chromatography with ultraviolet detection. The method has good sensitivity and specificity, without the need for long and complex extraction procedures. The results obtained with three different analytical columns were compared and an improvement in specificity by selection of column and detection wavelength used was achieved.

Plasma concentrations of mianserin after single dose and at steady-state in depressed elderly patients

Plasma mianserin and desmethylmianserin concentrations were measured in 17 clinically depressed elderly patients after a single 30 mg dose of mianserin. The patients then received mianserin 30 mg daily for up to 6 weeks and the plasma concentrations were measured at weekly intervals. The relationship between concentrations of mianserin and desmethylmianserin at steady-state and at 16 and 24 hours after the single test dose was not good enough to be used for prediction of dosage requirements. Reasons for this finding are discussed.

Maintenance-dose prediction based on a single determination of concentration: general applicability to two-compartment drugs with reference to lithium

A general approach to the selection of the maintenance dose (Dm) required to give a desired steady-state concentration of drug based on a single determination of concentration after a test dose (C*) is extended to drugs with two-compartment pharmacokinetic characteristics. Using the equation developed, the value of the proportionality factor relating 1/Dm to C was found to be within 3.2% of the value calculated from a published nomogram for lithium. The inherent error is shown to be a function of the value of the hybrid rate constants alpha and beta, as well as the value of an intercompartmental transfer rate constant, k21, in an individual.

Maintenance-dose prediction based on a single determination of concentration: dose of parent drug required to give a desired steady-state concentration of metabolite

A method for predicting the maintenance dose of parent drug required to give a desired steady-state concentration of metabolite, using a single determination of metabolite concentration in serum following the first dose of parent drug, is described. Clinical evidence that such a method is feasible for the drug-metabolite pair imipramine-desipramine has been reported. The error inherent in an estimation of maintenance dose based on a single determination of metabolite concentration is a function of sampling time and the first-order elimination rate constants for parent drug and metabolite (K and km, respectively). The method is applicable to drug-metabolite pairs in general by selecting the sampling time (t*) to give minimum error: t* equal 1/km + 1.3/K, when km less than or equal to K, and t* equal 1/K + 1.3/km, when km greater than K (bars denote population mean value). The error expected to be encountered in the application of the method to specific drug-metabolite pairs can be analyzed by the graphical methods described.

Clinical pharmacokinetics of dothiepin. Single-dose kinetics in patients and prediction of steady-state concentrations

The pharmacokinetics of dothiepin were evaluated in 9 depressed patients following a single oral dose of 75 mg. Blood and plasma concentrations of dothiepin and 2 major metabolites, northiaden and dothiepin S-oxide, were measured by gas chromatography/mass fragmentography. The mean (+/-SD) peak plasma concentrations of dothiepin were 49 +/- 27 micrograms/L at 3 +/- 1.2h. Mean (+/-SD) estimates of other parameters were as follows: absorption half-life 1.1 +/- 1.1h; distribution half-life 2.2 +/- 0.8 h; elimination half-life 25 +/- 7h; apparent volume of distribution 70 +/- 62 L/kg; and oral clearance 2.1 +/- 1.6 L/kg/h. The mean (+/-SD) peak plasma concentration of dothiepin S-oxide was 125 +/- 43 micrograms/L at 3.5 +/- 1.3h with an elimination half-life of 22 +/- 12 h. The mean peak plasma concentration of northiaden was 6 +/- 3 micrograms/L at 4.5 +/- 1.1h, with an elimination half-life of 31 +/- 12 h. No significant differences were found in pharmacokinetic parameters compared with a previous study in 7 healthy volunteers. When data for the patients and healthy volunteers were combined (n = 16), pharmacokinetic parameters were not found to be affected by age. However, a significant difference was found between males and females for the elimination half-lives of dothiepin and northiaden, and for the apparent volume of distribution of dothiepin. The 24-hour blood/plasma concentrations of dothiepin and dothiepin S-oxide accurately predicted the steady-state concentrations obtained following 4 weeks' treatment with dothiepin 150 mg nocte.

Single-point prediction methods: a critical review

This article presents a critical review of a proposed method of dose prediction. Previously published experiences with this method are reviewed. New applications with drugs that have short elimination half-lives and drugs that demonstrate nonlinearity of clearance are discussed. Potential sources of error are demonstrated, using computer simulations.

The single-point method of dosage prediction: pharmacokinetic basis and method optimization

Based on the principle of superposition an expression has been established relating a drug concentration at steady-state to a concentration after a single dose. This relationship applies for drugs with linear pharmacokinetics given at equal dosage intervals and it is independent of the route of administration. The relationship provides theoretical justification for the single-point method of dosage prediction reported in the literature. The test conditions in the method can therefore be optimized and the limits of the method defined. The expression can also be used for individualized prediction of maintenance dose after estimation of drug half-life in the patient with no limit to half-life values to which it can be applied.

Amitriptyline and nortriptyline response profiles in unipolar depressed patients

The relationship between steady-state plasma concentration and clinical response was studied in 22 hospitalized unipolar depressed patients. In a double-blind format the patients were randomly assigned to receive amitriptyline or nortriptyline. Dosage was adjusted based on plasma level with the aim of achieving a concentration of 60-180 ng/ml. By week 4 of treatment, 83% of amitriptyline patients and all nortriptyline patients were within the targeted plasma range. Based on final ratings of clinical state, the drug level adjustment improved the outcome for nortriptyline-treated patients, but not amitriptyline-treated patients. Nortriptyline patients with plasma levels of 60-230 ng/ml had lower Hamilton Rating Scale depression scores than patients outside that range. By contrast, amitriptyline plasma levels were not associated with depression ratings. After 1 week, patients treated with nortriptyline had a significantly greater mean reduction in Hamilton depression score, i.e., 55% compared to 25% for amitriptyline patients.

Pharmacokinetic linearity of desipramine hydrochloride

The pharmacokinetic linearity of two single oral doses of desipramine hydrochloride was examined in a parallel study involving 30 subjects. Fourteen subjects received 75 mg (3 x 25 mg) of desipramine hydrochloride, and 16 subjects received 150 mg (1 x 150 mg). An open one-compartment model with a lag time to the start of absorption was used to examine the pharmacokinetic linearity. The results of the study suggest that the kinetics are linear in the dose range studied.

The pharmacokinetics of nortriptyline in patients with chronic renal failure

1 The pharmacokinetics of single oral doses of nortriptyline were studied in twenty patients with chronic renal failure, eight of whom were receiving treatment with haemodialysis. 2 The median nortriptyline half-life was 25.2 h (range 14.5-140.0 h) and the median nortriptyline clearance was 32.3 l/h (range 8.1-122.0 l/h). 3 No differences were observed between the dialysed and non-dialysed groups. 4 Comparisons of nortriptyline half-life and clearance between the patients and groups of physically healthy subjects revealed no significant differences. 5 There was no significant linear correlation between age and either of these measurements. In the twelve patients not receiving haemodialysis there was no correlation between nortriptyline clearance and glomerular filtration rate. 6 Chronic renal failure is not associated with a significant alteration in nortriptyline metabolism as measured by its half-life or clearance, but the drug should nonetheless be used with caution, and monitored whenever possible. However, the marked inter-individual differences observed in nortriptyline half-life and clearance in patients with chronic renal failure may not be solely responsible for their unpredictable response to tricyclic antidepressant therapy, and other possible contributory factors are discussed.

A double blind comparison of zimelidine and amitriptyline in endogenous depression

In a randomized double-blind group comparison study of 40 patients with endogenous depression zimelidine appeared to be as effective an antidepressant as amitriptyline at 4 and 6 weeks using the Hamilton Rating Scale (HRS) and the Montgomery and Asberg Depression Rating Scale (MADRS). At 2 weeks there was a significantly better response (P less than 0.05) on zimelidine compared to amitriptyline on the clinician's global scale and 4 out of 10 items on the MADRS suggesting an early onset of action. A significant better response to zimelidine was seen on the item somatic anxiety (HRS) while the effect on sleep and appetite was better in the amitriptyline group. There were significantly more side effects, raw and corrected, in the amitriptyline-treated group. High steady state plasma concentrations of norzimelidine (greater than 800 nmol/l) which were significantly correlated with age (r = 0.8) were associated with a significantly poorer response suggesting that a lower dose than 200 mg in older patients may be appropriate.

Cardiovascular, sedative and anticholinergic effects of amitriptyline and zimelidine in young and elderly volunteers

Single doses of amitriptyline (50 mg) and zimelidine (100 mg) were administered orally to 4 young and 4 elderly healthy volunteers (aged 21--26 and 68--75 respectively) in a comparative study of the two drugs. Systolic time intervals, supine and standing blood pressure, salivary flow and indices of sedation were measured at regular intervals up to 6 hours post dose. No conclusive changes in the QS2 interval, left ventricular ejection time (LVETc) LVET index, pre-ejection period (PEPc), resting heart rate, or blood pressures were observed with either drug. Salivary flow was reduced by about 40% at 3.5 hours and 50% at 5.5 hours after ingestion of amitriptyline. No change in salivary flow was observed after zimelidine (significant difference between treatments; P less than 0.01 at 3.5 and 5.5 hours). Subjective drowsiness as measured by visual analogue ratings was significantly greater for amitriptyline at 3.5 hours (P less than 0.01). Postural sway was also increased by amitriptyline but not zimelidine (difference between treatments significant at 3.5 and 5.5 hours; P less than 0.01 and 0.05 respectively). Auditory reaction time increased with amitriptyline, the difference between treatments achieving significance at 5.5 hours. No differences in response to either drug between young and elderly volunteers were demonstrable in these small groups, although variability was greater in the older group. Possible implications of these findings for the treatment of depression in the elderly are discussed.

Prediction of steady state plasma and saliva levels of desmethylimipramine using a single dose, single time point procedure

In normal volunteer study (20 subjects) where each ingested 75 mg desmethylimipramine (DMI), blood and saliva samples were collected at 1, 2, 3, 4, 5, 7, 12, 24, and 32 h post dose. Each subject then was given DMI 25 mg b.i.d. for 15 days. Blood and saliva samples were collected on days 3, 4, 12, 13, 14, and 15. All samples were analyzed for total DMI content. Strong correlations were found between the blood samples collected 12, 24, and 32 h post dose (r = 0.93, 0.96, 0.95) and saliva samples collected 24 and 32 h post single dose (r = 0.91, 0.84) and the subjects' respective steady states. Although the correlation between blood and saliva levels was weaker (r = 0.7) because of considerable interindividual variation in the saliva/plasma DMI ratio (16-fold variation), this ratio in individual subjects was stable. These data suggest that, as has been shown for other psychotropic drugs, single blood measures at 24 h post ingestion of 75 mg DMI can be used to predict optimal dosage in individual patients. Acceptable predictions of steady state plasma levels were obtained when this technique was applied to patient data available in the literature. It is also suggested that if the saliva/plasma ratio is established for each individual patient, their drug level monitoring may be possible using this noninvasive approach.