Effect of fluoxetine on carvedilol pharmacokinetics, CYP2D6 activity, and autonomic balance in heart failure patients.

The objective of this study was to examine the pharmacokinetic and pharmacodynamic consequences of concomitant administration of fluoxetine and carvedilol in heart failure patients. Fluoxetine (20 mg) or matching placebo was administered in a randomized, double-blind, two-period crossover study to 10 patients previously identified as extensive metabolizers of CYP2D6 substrates. Patients were maintained on a carvedilol dose of 25 or 50 mg bid and given fluoxetine/placebo for a minimum of 28 days. Plasma was collected over the 12-hour carvedilol dosing interval, and the concentrations of the R(+) and S(-) enantiomers of carvedilol were measured. CYP2D6 phenotype was assessed during each study period using dextromethorphan (30 mg). Changes in autonomic modulation between study periods were measured by heart rate variability in the time and frequency domains using ambulatory electrocardiographic monitoring. Compared to placebo, fluoxetine coadministration resulted in a 77% increase in mean (+/- SD) R(+) enantiomer AUC0-12 (522 +/- 413 vs. 927 +/- 506 ng.h/mL, p = 0.01) and a nonsignificant increase in S(-) enantiomer AUC (244 +/- 185 vs. 330 +/- 179 ng.h/mL, p = 0.17). Mean apparent oral clearance for both enantiomers decreased significantly with fluoxetine administration (R(+): 10.3 +/- 7.2 vs. 4.5 +/- 2.2 mL/min/kg; S(-): 22.5 +/- 12.3 vs. 12.6 +/- 7.4 mL/min/kg; p = 0.004 and 0.03, respectively). No differences in adverse effects, blood pressure, or heart rate were noted between treatment groups, and there were no consistent changes in heart rate variability parameters. In conclusion, fluoxetine administration resulted in a stereospecific inhibition of carvedilol metabolism, with the R(+) enantiomer increasing to a greater extent than the S(-) enantiomer. However, this interaction was of little clinical significance in our sample population.

A pharmacokinetic drug-drug interaction study of venlafaxine and indinavir.

Depression is a common occurrence in the human immunodeficiency virus (HIV)-infected population. Complications in treating depressed HIV-infected individuals include the use of multiple medications, additive side effects, and potentially significant drug-drug interactions. Based on the pharmacologic characteristics of venlafaxine and indinavir, we hypothesized that significant pharmacokinetic drug-drug interactions would not occur when these drugs where taken concurrently. Nine healthy adult subjects were given a single 800 mg oral dose of indinavir and serial blood samples were collected for measurement of plasma drug concentrations. Over the next 9 days, venlafaxine was administered at a dosage of 50 mg every 8 hours following a brief titration. A venlafaxine trough plasma concentration and serial concentrations following venlafaxine administration were obtained on day 10. On day 11, venlafaxine and indinavir were administered together and serial blood sampling was repeated. Indinavir had no effect on venlafaxine plasma concentrations but resulted in a 7% decrease in plasma concentrations of O-desmethyl-venlafaxine (ODV)(P = 0.028). This effect is unlikely to be clinically significant. Venlafaxine coadministration resulted in a 28% decrease in the area under the concentration time curve (AUC) of plasma indinavir (P = 0.016) and a 36% decrease in its maximum plasma concentration (Cmax; P = 0.038). As the plasma concentration of protease inhibitors is a critical factor in maintaining efficacy and minimizing the potential for viral resistance, the decrease in both AUC and Cmax of indinavir from coadministration of venlafaxine is of concern. The clinical significance of these results obtained from a small number of healthy volunteers is unknown. Further studies are needed to substantiate or refute this apparent drug-drug interaction. Until such time, venlafaxine should be used cautiously in patients receiving indinavir.

Single-dose pharmacokinetics of methylphenidate in CYP2D6 extensive and poor metabolizers.

Six adults phenotyped as either extensive (N = 4) or poor (N = 2) metabolizers for cytochrome P450 (CYP) 2D6 were given a 10-mg oral dose of methylphenidate (MPH) on two separate occasions with and without quinidine, a potent CYP2D6 inhibitor. Quinidine had no significant effect on the pharmacokinetics of either MPH or ritalinic acid, its major metabolite, in either group of CYP2D6 metabolizers. These data suggest a lack of involvement of CYP2D6 in the metabolism of MPH. Drugs that are inhibitors of CYP2D6 when taken concurrently with MPH should not affect its plasma concentration.

Evaluation of the influence of diabetes mellitus on antipyrine metabolism and CYP1A2 and CYP2D6 activity.

STUDY OBJECTIVE: To evaluate the metabolism of antipyrine, a general metabolic probe, caffeine, a probe for cytochrome P450 (CYP) 1A2 and N-acetyltransferase activity, and dextromethorphan, a specific probe for CYP2D6 activity in patients with type 1 or 2 diabetes mellitus. DESIGN: Prospective, controlled study. SETTING: Research facility. Patients. Fifteen patients with type 1 and 16 with type 2 diabetes, and 16 healthy controls. INTERVENTION: Each subject simultaneously received antipyrine 10 mg/kg, caffeine 100 mg, and dextromethorphan 30 mg. MEASUREMENTS AND MAIN RESULTS: The pharmacokinetics of antipyrine and its primary metabolites were determined from saliva and urine samples. Type 1 diabetes had marked effects on antipyrine metabolism whereas type 2 disease did not alter the metabolism of any of the probe drugs. The apparent oral clearance of antipyrine was increased 72% in patients with type 1 disease compared with controls (p=0.0001). In addition, formation clearances of 4-hydroxyantipyrine and 3-hydroxymethylantipyrine were increased by 74% and 137% in those patients relative to controls. The caffeine metabolic index (paraxanthine/caffeine) was increased 34% (p=0.11), and N-acetylation and CYP2D6 phenotype were not altered. CONCLUSION: The metabolism of antipyrine is increased in patients with type 1 diabetes. Based on in vitro reports of antipyrine metabolism and current caffeine metabolic index data, the predominant effect of type 1 diabetes appears to be an increase in CYP1A2 activity. Assessment of the effect of the disease on other specific CYP metabolic pathways is warranted.

Effect of St. John's wort (Hypericum perforatum) on cytochrome P-450 2D6 and 3A4 activity in healthy volunteers.

The effects of the herb St. John's wort (Hypericum perforatum), a purported antidepressant, on the activity of cytochrome P-450 (CYP) 2D6 and 3A4 was assessed in seven normal volunteers. Probe substrates dextromethorphan (2D6 activity) and alprazolam (3A4 activity) were administered orally with and without the co-administration of St. John's wort. Urinary concentrations of dextromethorphan and dextrorphan were quantified and dextromethorphan metabolic ratios (DMRs) determined. Plasma samples were collected (0-60 hrs) for alprazolam pharmacokinetic analysis sufficient to estimate tmax, Cmax, t 1/2, and AUC. Validated HPLC methods were used to quantify all compounds of interest. No statistically significant differences were found in any estimated pharmacokinetic parameter for alprazolam or DMRs. These results suggest that St. John's wort, when taken at recommended doses for depression, is unlikely to inhibit CYP 2D6 or CYP 3A4 activity.

The effects of serotonin3 receptor blockade on the psychobiological response to intravenous clomipramine in healthy human subjects.

BACKGROUND: The purpose of this study was to determine the role that serotonin (5-HT)3 receptors play in the prolactin and nausea responses to clomipramine challenge. METHODS: Twenty healthy subjects were randomly assigned to pretreatment with either the selective 5-HT3 receptor antagonist ondansetron, or placebo, prior to intravenous infusion with clomipramine. RESULTS: Ondansetron pretreatment had no effect on the prolactin response to clomipramine challenge. There was a trend toward decreased nausea with ondansetron pre-treatment. CONCLUSIONS: These findings are consistent with other data suggesting that 5-HT3 receptors do not play a major role in the prolactin response to 5-HT challenge in human subjects, but may mediate nausea associated with enhanced 5-HT neurotransmission.

Effects of obesity on the cytochrome P450 enzyme system.

OBJECTIVE: This review evaluates the effect of obesity on the various isozymes of the cytochrome P450 enzyme system. MATERIALS AND METHOD: A Medline search of the international literature on drug metabolism and obesity from 1966 to early 1998 was conducted. All English language studies in humans that compared the pharmacokinetics of a given medication between obese and non-obese subjects were evaluated. Any study in which the substrate examined is excreted unchanged, not metabolized primarily by a single cytochrome P450 isozyme or which is considered a high extraction compound was excluded from this review. RESULTS: Despite the prevalence of obesity, the known health consequences of obesity and the increase of medication usage in the obese population, very few studies have attempted to establish the effect of this condition on the cytochrome P450 enzyme system. Numerous studies, however, have compared pharmacokinetic parameters between obese and non-obese subjects for individual drugs. By examining those trials in which the agents studied all undergo biotransformation via the same CYP450 isozyme, a preliminary appreciation of the effect of obesity on the activity of that particular isozyme can be attained. CONCLUSIONS: The effect of obesity on CYP450 appears to be isozyme-specific with the activity of cytochrome P450 3A4 decreasing and that of cytochrome P450 2E1 increasing. The effect of obesity on the cytochrome P450 1A2, 2C9, 2C19, and 2D6 isozymes is inconclusive.

Parenteral clomipramine challenge in depressed adolescents: mood and neuroendocrine response.

BACKGROUND: Major depressive disorder (MDD) in the adolescent demonstrates a unique clinical profile, and pathogenic serotonergic dysregulation is hypothesized. Parenteral clomipramine (CMI) is known to distinguish adult MDD from control, but neurochallenge data are lacking in adolescent MDD. METHODS: Thirteen drug-free outpatient adolescents who met DSM-III-R criteria for MDD were compared to adolescent controls by acute neuroendocrine and mood response to 12.5 mg of parenteral CMI. RESULTS: Repeated measures analysis revealed significant changes from baseline for sadness (p < .01) between groups, with normal controls increasing sadness rating after CMI. Prolactin (PRL) maximum change score from baseline was decreased in MDD relative to controls (p < .05). Gender effects on PRL were evident in controls but not in MDD. CONCLUSIONS: The findings of PRL blunting in adolescent MDD mirrors previous work in adults. A unique finding is the induction of sadness in normal adolescent controls after CMI infusion.

Intravenous clomipramine challenge in obsessive-compulsive disorder: predicting response to oral therapy at eight weeks.

BACKGROUND: Challenge with intravenous clomipramine (CMI) is serotonin selective and has been reported to transiently exacerbate symptoms in obsessive-compulsive disorder (OCD) patients, and to predict subsequent response to oral CMI therapy. METHODS: We administered CMI (12.5 mg, i.v.) to medication free OCD patients (N = 29) and normal controls (N = 22) to characterize neurohormonal response. A subset of OCD patients (26/29), was then treated with either pulse load i.v. or oral CMI followed by 8 weeks of oral CMI therapy. RESULTS: In response to CMI challenge, OCD patients exhibit blunted cortisol and exaggerated growth hormone response relative to normal controls. OCD patients differ from controls in "sadness" ratings, with control exhibiting increased dysphoria in response to CMI. Growth hormone response to CMI challenge predicts treatment response (> or = 25% decreases YBOCS from baseline) to oral CMI at 8 weeks. CONCLUSIONS: Growth hormone abnormalities associated with OCD in response to CMI challenge differentiates nonresponders after 8 weeks of oral CMI treatment from responders.

Differential effects of chronic antidepressant treatment on swim stress- and fluoxetine-induced secretion of corticosterone and progesterone.

Hypersecretion of cortisol occurs in numerous patients with major depression and normalizes with clinical recovery during the course of chronic antidepressant treatment. These clinical data suggest that investigation of the effects of antidepressant treatments on the regulation of the brain-pituitary-adrenal axis may assist in elucidating the therapeutic basis of antidepressant actions. In the present investigation, both swim stress and acute fluoxetine challenge increased release of corticosterone and progesterone to reflect an activation of the brain pituitary-adrenal axis. The effects of chronic antidepressant treatment (21 days) on corticosterone and progesterone secretion induced by these challenges were investigated. Chronic fluoxetine treatment (5 mg/kg/day) completely blocked the increased secretion of corticosterone and progesterone in response to the acute fluoxetine challenge. Chronic treatment with desipramine, imipramine or amytriptyline (15 mg/kg/day) also markedly attenuated fluoxetine-induced corticosterone and progesterone secretion. However, chronic treatment with the monoamine oxidase inhibitors, phenelzine (5 mg/kg) and tranylcypromine (5 mg/kg), did not affect this hormonal response to acute fluoxetine challenge. Plasma levels of fluoxetine after acute challenge were not significantly different for the various chronic antidepressant treatment conditions from the chronic saline controls; therefore, an increase in the metabolism of fluoxetine can not explain the antagonism of the fluoxetine-induced hormonal response after chronic antidepressant treatment. In contrast to the effects of selected antidepressants on acute fluoxetine-induced steroid release, chronic treatment with imipramine (20 mg/kg/day), fluoxetine (5 mg/kg/day) or phenelzine (5 mg/kg) did not significantly alter this swim stress-induced corticosterone or progesterone secretion. Because chronic fluoxetine and tricyclic antidepressant drugs blocked the acute action of fluoxetine to increase adrenal cortical secretion, but did not alter swim stress-induced secretion of these steroids, we propose that distinct neurochemical mechanisms control fluoxetine and swim stress-induced steroid release. We speculate that the substantial adaptive response to those chronic antidepressant treatments, which minimize the effect of acute fluoxetine challenge to increase in corticosterone and progesterone secretion, may be relevant to the therapeutic actions of these drugs.

Clinical and economic impact of replacing divalproex sodium with valproic acid.

The impact of replacing divalproex sodium with valproic acid on patient outcomes and direct drug costs was studied. Before-and-after medical chart review was performed in a state-supported facility for mentally retarded adults in which a pharmacy and therapeutics (P&T) committee recommended replacement of divalproex with valproic acid. Patients were studied if they had received divalproex for at least three months and if their antiepileptic drug was changed from divalproex to valproic acid between October 1993 and June 1994. Clinical, economic, and prescribing-pattern data were recorded for the periods extending 12 months before and 18 months after the change in therapy. Data for 46 patients were analyzed. Replacing divalproex with valproic acid was effective in 41 (89%) of the patients. There was no significant difference between the divalproex and valproic acid periods in seizure rate or frequency of new drug therapy for GI disorders. Between fiscal year 1992-93 and fiscal year 1995-96 there was a 56% decrease in total direct divalproex plus valproic acid costs, including drug products and packaging materials and labor. The rate of valproic acid prescriptions increased steadily after the replacement was recommended, and then plateaued. Replacing divalproex sodium with valproic acid in a group of institutionalized mentally retarded adults with epilepsy was clinically effective and economically advantageous.

Pulse intravenous clomipramine for depressed adolescents: double-blind, controlled trial.

OBJECTIVE: Major depressive disorder in adolescents is characterized as treatment resistant, but a previous open-label trial of pulse intravenous clomipramine demonstrated rapid relief of depressive symptoms. In the present study a single intravenous dose of clomipramine (200 mg) was compared with saline placebo in a randomized controlled trial for depressed adolescents. The hypothesis was that adolescents who were treated with pulse clomipramine would exhibit lower scores on the Hamilton Depression Rating Scale at endpoint than would adolescents who received saline and that clomipramine would be superior to saline in terms of antidepressant response. METHOD: Sixteen nonsuicidal outpatient adolescents (mean age = 16.2 years, SD = 1.0) who met the DSM-III-R criteria for major depression (score on 21-item Hamilton scale, > or = 18) were randomly assigned to receive either clomipramine (200 mg i.v., N = 8) or saline (N = 8). Assessments of depression severity were completed 36 hours and 6 days thereafter. RESULTS: The adolescents who received pulse clomipramine treatment demonstrated significant decreases in Hamilton depression scores from baseline at 6 days but not at 36 hours. A similar decrease from baseline was found in Clinical Global Impression severity at 6 days but not 36 hours. Seven of the clomipramine-treated patients and three of the saline-treated patients had drops of 50% or more from baseline in Hamilton depression score. CONCLUSIONS: Pulse clomipramine (200 mg i.v.) is associated with dramatic reduction in depressive symptoms at day 6 after infusion, which is significantly different from the effect of placebo.

Effect of a single dose of diazepam on balance measures in older people.

OBJECTIVE: This study examines the effect of a single dose of diazepam on a spectrum of balance measures in healthy older subjects. The measures include static (postural sway), dynamic (anterior tibialis muscle activation latency), and a complex self-initiated task of balance (functional reach) in addition to neuropsychological tests of attention. DESIGN: A double-blind, two-period, cross-over trial. SETTING: The community surrounding a university hospital. PARTICIPANTS: Community-dwelling, nonsmoking volunteers 65 years of age and older. MEASUREMENTS: Measures included response to unexpected perturbation (platform/EMG muscle latency), self-initiated perturbation (functional reach), and a static measure of balance (postural sway). Neurocognitive tests (digital symbol substitution test, card sorting) were included to document the cognitive effect. RESULTS: Twelve nonsmoking healthy subjects (average age = 70.4 years (66-76 years)) participated. The anterior tibalis muscle latency increased in response to a sudden perturbation with diazepam compared with placebo (TA latency 149 ms vs 142 ms, P < .001). Neurocognitive tests were adversely affected for 90 minutes after diazepam administration compared with placebo (P < .05). Other measures did not demonstrate significant effect of diazepam. CONCLUSIONS: This is among the first reports showing that benzodiazepines affect neuromuscular processing related to balance control. Increased muscle latency to sudden perturbations may represent an effect of diazepam upon the oligosynaptic spinal reflex distinct from the sedation. Surface electromyography may be a valuable noninvasive tool for future studies of drug effect on balance and falls risk among older people.

Clomipramine challenge responses covary with Tridimensional Personality Questionnaire scores in healthy subjects.

Cloninger's Unified Biosocial Theory of Personality postulates a relationship between the relative functional activity of central serotonergic, dopaminergic, and noradrenergic neurotransmitter systems, and the strength of three elemental dimensions of personality. These dimensions are Harm Avoidance, Novelty Seeking, and Reward Dependence, respectively. Accordingly, we predicted that neuroendocrine responses to serotonergic challenge would correlate with Harm Avoidance scores, but not with Novelty Seeking or Reward Dependence scores. We examined the relationship between the prolactin and cortisol responses to a 12.5-mg intravenous clomipramine challenge and these personality dimensions as measured by Cloninger's Tridimensional Personality Questionnaire in 32 healthy subjects. The cortisol response correlated only with Harm Avoidance scores, as predicted; however, prolactin response did not correlate with Harm Avoidance scores. Instead, it demonstrated an inverse relationship with Novelty Seeking scores. There was a positive relationship of baseline prolactin with Harm Avoidance in a post hoc analysis. Cortisol response to serotonergic challenge may be a better indicator for responsivity of serotonergic systems as they relate to the personality dimension of Harm Avoidance than is prolactin. Prolactin responses may be overly affected by dopaminergic influences; however, baseline prolactin may still be a valid indicator of serotonergic tone.

The effects of age, gender, and season on serotonergic function in healthy subjects.

1. Several factors, including age, gender, and season of the year, have been reported to affect physiologic indices of central serotonergic function, although some of these findings have not been consistent across groups of subjects and types of serotonergic measures. 2. The authors investigated the role that each of these variables might play in the neuroendocrine response to acute intravenous challenge with the serotonin reuptake inhibitor clomipramine (CMI) in healthy volunteers. 3. Thirty seven healthy subjects (17 women and 20 men), with an age range of 19 to 50 years, received 12.5 mg of CMI intravenously under standardized conditions. 4. The maximum change from baseline in plasma prolactin concentrations ("delta-max") was significantly related to age, after controlling for gender and season. 5. In contrast, neither gender nor season was significantly related to prolactin delta-max, after controlling for the other two variables. 6. Although the age range and sample size are relatively limited, the results from this study suggest that age, but not gender or season, may influence serotonergic function, as measured by the prolactin response to CMI challenge.

Pharmacokinetics of ondansetron in patients with hepatic insufficiency.

Ondansetron is primarily eliminated via hepatic metabolism; thus, liver disease may affect its clearance. The pharmacokinetics of ondansetron in patients with different degrees of hepatic insufficiency (N = 12 with hepatic impairment, as categorized by Pugh's classification method) were assessed and the results compared with results for age- and gender-matched control subjects with normal liver function (n = 12). A secondary objective was to correlate the Pugh method of assessing hepatic impairment and quantitative metabolic markers used to assess hepatic function (antipyrine clearance and indocyanine green clearance) with changes in the pharmacokinetics of ondansetron. This was an open-label study in which 8 mg ondansetron was given orally and intravenously, following a randomized crossover design. Clearance of ondansetron was lower among patients with hepatic impairment that control subjects. After a single, oral dose of ondansetron, mean absolute bioavailability increased markedly with increased hepatic insufficiency (approaching 100% in the group with severe hepatic impairment versus 66% for control subjects). These data suggest that there is a reduced first-pass effect in patients with liver disease resulting in a higher AUC0-infinity. A correlation existed between clearance of ondansetron and decreased antipyrine clearance; a smaller correlation existed between ondansetron clearance and indocyanine green clearance. Mean percent of ondansetron bound to plasma proteins was significantly lower in patients with liver disease than in control subjects. None of the patients experienced any severe adverse reactions attributed to ondansetron. A reduction in the clearance of ondansetron is associated with increasing degrees of hepatic insufficiency; therefore, patients with severe hepatic impairment (Pugh score of > 9) should have their daily dose of ondansetron limited to 8 mg (or 0.15 mg/kg).

Pharmacokinetic and pharmacodynamic drug interactions with polypharmacotherapy of treatment-resistant affective and obsessive-compulsive disorders.

The concomitant use of multiple therapeutic agents in the treatment of resistant affective and obsessive-compulsive disorders increases the likelihood of a patient experiencing a drug-drug interaction at either the pharmacokinetic or pharmacodynamic level. Recent developments in molecular biology and pharmacogenetics have allowed the identification of which psychotropic agents are substrates, inducers, or inhibitors of specific hepatic cytochrome P450 isozymes. This increases the predictability of which drug combinations may lead to kinetic interactions. However the individual variability in kinetic disposition and pharmacodynamic response still limits the predictability of a drug-drug interaction in an individual patient treated with polytherapy.

Comparison of quantitative methods to assess hepatic function: Pugh's classification, indocyanine green, antipyrine, and dextromethorphan.

STUDY OBJECTIVES: To compare three quantitative metabolic markers used to assess hepatic function, indocyanine green (ICG), a high-extraction marker; antipyrine, a low-extraction marker; and dextromethorphan, a P-450IID6 marker, with the clinically used Pugh's classification. DESIGN: Comparison of 12 healthy controls with 12 age- and sex-matched patients with different degrees of liver disease. SETTING: Research center in a university-affiliated teaching hospital. PATIENTS: The 12 patients had different degrees of liver disease: 4 mild (Pugh's score 6 or 7); 4 moderate (Pugh's score 8 or 9); and 4 severe (Pugh's score > or = 10). Each level had an equal number of men and women subjects. MEASUREMENTS AND MAIN RESULTS: Clearance of ICG detected mild alterations in hepatic function as efficiently as it did for moderate and severe impairment, but it lacked the specificity to distinguish among the classification groups. In contrast, antipyrine was effective in identifying moderate and severe hepatic impairment; however, its clearance was not reduced in mild liver disease. Pugh's classification appears to be a clinically useful method of assessing the global degree of hepatic impairment in patients with chronic disease, and there was a significant correlation between it and antipyrine clearance (r = 0.67, p = 0.0003) and ICG clearance (r = 0.86, p = 0.0001). Four of eight patients with a Pugh's score greater than 8 had a dextromethorphan metabolic ratio expression reflective of a poor metabolizer phenotype based on 0- to 4-hour urine collection, but only two of those eight patients were classified as poor metabolizers based on 4- to 12-hour urine collection. These percentages of poor metabolizers are substantially higher than for historical controls (8.5-10.4%) and most likely reflect a decrease in the P-450IID6 functional ability with progression of liver disease. However, due to small sample size and lack of knowledge of the patients' genotypes, these data are only suggestive. CONCLUSION: Pugh's classification appears to be a reliable indicator of the degree of chronic liver disease and could be employed as a drug development research classification tool; however, it does not replace quantitative metabolic markers, especially isozyme-specific markers.

An improved micromethod for vancomycin determination by high-performance liquid chromatography.

A micromethod using reversed-phase high-performance liquid chromatography for the analysis of vancomycin in human serum or plasma was developed. Ristocetin was used as the internal standard. Chromatographic conditions included an amino propyl column, a mobile phase with 62% acetonitrile and 38% sodium phosphate buffer (pH 7.0), a total run time of 10 min, and ultraviolet absorbance detection at 225 nm. Multilevel calibration was found to be linear between 1.0 and 100 micrograms/ml with correlation coefficients of the calibration line slope consistently > 0.999. Recovery of vancomycin from serum was nearly complete, and no interference from commonly used drugs was observed. This procedure is simple, sensitive, rapid, precise, selective, and requires only 50 microliters of serum or plasma for completion.