Pharmacokinetics of intravenous caffeine: comparison of high-performance liquid chromatographic and gas chromatographic methods

Effects of paraxanthine and caffeine on sleep, locomotor activity, and body temperature in orexin/ataxin-3 transgenic narcoleptic mice

STUDY OBJECTIVE

Caffeine, an adenosine A1 and A2a receptor antagonist, is a widely consumed stimulant and also used for the treatment of hypersomnia; however, the wake-promoting potency of caffeine is often not strong enough, and high doses may induce side effects. Caffeine is metabolized to paraxanthine, theobromine, and theophylline. Paraxanthine is a central nervous stimulant and exhibits higher potency at A1 and A2 receptors, but has lower toxicity and lesser anxiogenic effects than caffeine.

DESIGN

We evaluated the wake-promoting efficacy of paraxanthine, caffeine, and a reference wake-promoting compound, modafinil, in a mice model of narcolepsy, a prototypical disease model of hypersomnia. Orexin/ataxin-3 transgenic (TG) and wild-type (WT) mice were subjected to oral administration (at ZT 2 and ZT14) of 3 doses of paraxanthine, caffeine, modafinil, or vehicle.

RESULTS

Paraxanthine, caffeine, and modafinil significantly promoted wakefulness in both WT and narcoleptic TG mice and proportionally reduced NREM and REM sleep in both genotypes. The wake-promoting potency of 100 mg/kg p.o. of paraxanthine during the light period administration roughly corresponds to that of 200 mg/kg p.o. of modafinil. The wake-promoting potency of paraxanthine is greater and longer lasting than that of the equimolar concentration of caffeine, when the drugs were administered during the light period. The wake-promotion by paraxanthine, caffeine, and modafinil are associated with an increase in locomotor activity and body temperature. However, the higher doses of caffeine and modafinil, but not paraxanthine, induced hypothermia and reduced locomotor activity, thereby confirming the lower toxicity of paraxanthine. Behavioral evaluations of anxiety levels in WT mice revealed that paraxanthine induced less anxiety than caffeine did.

CONCLUSIONS

Because it is also reported to provide neuroprotection, paraxanthine may be a better wake-promoting agent for hypersomnia associated with neurodegenerative diseases.

Emerging treatments for narcolepsy and its related disorders

IMPORTANCE OF THE FIELD

Narcolepsy is a chronic sleep disorder, characterized by excessive daytime sleepiness (EDS), cataplexy, hypnagogic hallucinations, sleep paralysis and nocturnal sleep disruption. Non-pharmacological treatments (i.e., behavioral modification) are often helpful for the clinical management of narcoleptic patients. As these symptoms are often disabling, most patients need life-long treatments. Over 90% of diagnosed narcoleptic patients are currently prescribed medications to control their symptoms; however, available treatments are merely symptomatic.

AREAS COVERED IN THIS REVIEW

This review presents a description of the clinical symptoms of narcolepsy, followed by a discussion of the state-of-the-art knowledge regarding the disorder and related emerging treatments. In preparing this review, an extensive literature search was conducted using Pubmed. Only selected references from 1970 to 2008 are cited.

WHAT THE READER WILL GAIN

This review focuses on emerging treatments for human narcolepsy, and the reader will gain significant knowledge of current and future treatment for this and related disorders. Traditionally, amphetamine-like stimulants (i.e., dopaminergic release enhancers) have been used for clinical management to improve EDS, and tricyclic antidepressants have been used as anticataplectics. However, treatments have recently evolved which utilize better tolerated compounds, such as modafinil (for EDS) and adrenergic/serotonergic selective reuptake inhibitors (as anticataplectics). In addition, night time administration of a short-acting sedative, gamma-hydroxybutyrate, has been used for the treatment for EDS and cataplexy. As a large majority of human narcolepsy is hypocretin peptide deficient, hypocretin replacement therapy may also be a new therapeutic option; yet, this option is still unavailable. In addition to the hypocretin-based therapy, a series of new treatments are currently being tested in animal and/or humans models. These potential options include novel stimulant and anticataplectic drugs as well as immunotherapy, based on current knowledge of the pathophysiology of narcolepsy with cataplexy.

TAKE HOME MESSAGE

We expect that more pathophysiology-based treatments, capable of curing and/or preventing narcolepsy and related diseases, will be available in near future. As cases of EDS, associated with other neurological conditions (i.e., symptomatic narcolepsy or narcolepsy due to medical conditions), are often linked with hypocretin deficiency, these novel therapeutic options may also be applied to treatment of these disabling conditions.

Pharmacokinetic and Pharmacodynamic Interactions Between Zolpidem and Caffeine

The kinetic and dynamic interaction of caffeine and zolpidem was evaluated in a double-blind, single-dose, six-way crossover study of 7.5 mg zolpidem (Z) or placebo (P) combined with low-dose caffeine (250 mg), high-dose caffeine (500 mg), or placebo. Caffeine coadministration modestly increased maximum plasma concentration (C(max)) and area under the plasma concentration-time curve of zolpidem by 30-40%, whereas zolpidem did not significantly affect the pharmacokinetics of caffeine or its metabolites. Compared to P+P, Z+P significantly increased sedation, impaired digit-symbol substitution test performance, slowed tapping speed and reaction time, increased EEG relative beta amplitude, and impaired delayed recall. Caffeine partially, but not completely, reversed most pharmacodynamic effects of zolpidem. Thus, caffeine only incompletely reverses zolpidem's sedative and performance-impairing effects, and cannot be considered as an antidote to benzodiazepine agonists.

Fluvoxamine impairs single-dose caffeine clearance without altering caffeine pharmacodynamics

BACKGROUND

Coadministration of fluvoxamine impairs the clearance of caffeine and prolongs its elimination half-life, which is attributable to inhibition of CYP1A2 by fluvoxamine. The clinical importance of this interaction is not established.

AIM

To evaluate the effects of fluvoxamine on the kinetics and dynamics of single doses of caffeine.

METHODS

Seven healthy subjects received single 250 mg doses of caffeine (or matching placebo) together with fluvoxamine (four doses of 100 mg over 2 days) or with matching placebo in a double-blind, four-way crossover study. For 24 h after caffeine or placebo administration, plasma caffeine and fluvoxamine concentrations were determined. Psychomotor performance, sedation, and electroencephalographic (EEG) "beta" frequency activity were also assessed.

RESULTS

Fluvoxamine significantly reduced apparent oral clearance of caffeine (105 vs. 9.1 mL min(-1), P < 0.01; mean difference: 95.7 mL min(-1), 95% CI: 54.9-135.6), and prolonged its elimination half-life (4.9 vs. 56 h, P < 0.01; mean difference: 51 h, 95% CI: 26-76). Caffeine produced CNS-stimulating effects compared with placebo. However, psychomotor performance, alertness, or EEG effects attributable to caffeine were not augmented by coadministration of fluvoxamine.

CONCLUSIONS

Fluvoxamine greatly impaired caffeine clearance, but without detectable changes in caffeine pharmacodynamics. However, this study does not rule out possible adverse effects due to extensive accumulation of caffeine with daily ingestion in fluvoxamine-treated individuals.

Caffeine Use in Sports, Pharmacokinetics in Man, and Cellular Mechanisms of Action

Caffeine is the most widely consumed psychoactive 'drug' in the world and probably one of the most commonly used stimulants in sports. This is not surprising, since it is one of the few ergogenic aids with documented efficiency and minimal side effects. Caffeine is rapidly and completely absorbed by the gastrointestinal tract and is readily distributed throughout all tissues of the body. Peak plasma concentrations after normal consumption are usually around 50 microM, and half-lives for elimination range between 2.5-10 h. The parent compound is extensively metabolized in the liver microsomes to more than 25 derivatives, while considerably less than 5% of the ingested dose is excreted unchanged in the urine. There is, however, considerable inter-individual variability in the handling of caffeine by the body, due to both environmental and genetic factors. Evidence from in vitro studies provides a wealth of different cellular actions that could potentially contribute to the observed effects of caffeine in humans in vivo. These include potentiation of muscle contractility via induction of sarcoplasmic reticulum calcium release, inhibition of phosphodiesterase isoenzymes and concomitant cyclic monophosphate accumulation, inhibition of glycogen phosphorylase enzymes in liver and muscle, non-selective adenosine receptor antagonism, stimulation of the cellular membrane sodium/potassium pump, impairment of phosphoinositide metabolism, as well as other, less thoroughly characterized actions. Not all, however, seem to account for the observed effects in vivo, although a variable degree of contribution cannot be readily discounted on the basis of experimental data. The most physiologically relevant mechanism of action is probably the blockade of adenosine receptors, but evidence suggests that, at least under certain conditions, other biochemical mechanisms may also be operational.

Dose-dependent pharmacokinetics and psychomotor effects of caffeine in humans

Twelve healthy volunteers received oral placebo, 250 mg of caffeine, and 500 mg of caffeine in a randomized, double-blind, single-dose crossover study. Caffeine kinetics were nonlinear, with clearance significantly reduced and elimination half-life prolonged at the 500-mg compared to the 250-mg dose. The lower dose of caffeine produced more favorable subjective effects than the higher dose (elation, peacefulness, pleasantness), whereas unpleasant effects (tension, nervousness, anxiety, excitement, irritability, nausea, palpitations, restlessness) following the 500-mg dose exceeded those of the 250-mg dose. The lower dose of caffeine enhanced performance on the digit symbol substitution test and a tapping speed test compared to placebo; high-dose caffeine produced less performance enhancement than the lower dose. The plasma concentration versus response relationship revealed concentration-dependent increases in anxiety and improvements in cognitive and motor performance at low to intermediate concentrations. Both caffeine doses reduced electroencephalographic amplitude over the 4 Hz to 30 Hz spectrum, as well as in the alpha (8-11 Hz) and beta (12-30 Hz) ranges; however, effects were not dose-dependent. While favorable subjective and performance-enhancing stimulant effects occur at low to intermediate caffeine doses, the unfavorable subjective and somatic effects, as well as performance disruption, from high doses of caffeine may intrinsically limit the doses of caffeine used in the general population.

Population pharmacokinetics of intravenous caffeine in neonates with apnea of prematurity

OBJECTIVE

The study the population pharmacokinetics of caffeine after intravenous administration to premature infants with apnea.

METHODS

A prospective, blinded parallel study in which daily caffeine citrate doses of 30, 15, and 3 mg/kg were administered over 7 days by intermittent intravenous infusion. Arterial blood samples (three to six per patient) were assayed for caffeine content by means of HPLC. Population pharmacokinetic modeling was performed with NONMEM.

RESULTS

Clearance (L/hr) = (0.00000399 . current weight [grams]) + (0.000128 . postnatal age [days]). For gestational age > 28 weeks, volume of distribution (L) = (0.000764 . weight [grams] + (0.0468 . postnatal age [days]); for gestational age < or = 28 weeks, volume of distribution (L) = (0.000755 . weight [grams]) + (0.0224. postnatal age [days]). Interpatient variability (coefficient of variation, in percent) was approximately 25% for clearance and approximately 11% for volume of distribution. Intrapatient error (standard deviation) was 3.9 mg/L. There was insignificant bias between observed and model-predicated serum caffeine concentrations in a separate group of 30 infants.

CONCLUSIONS

Caffeine was well tolerated at all doses. Clearance was markedly lower and volume of distribution was higher than the values reported previously for term infants and adults. Both parameters were significantly influenced by postnatal age and current body weight, whereas volume of distribution in infants > 28 weeks' gestational age was higher than that in more premature babies. The predictive performance and the clinical application of the derived population models was satisfactorily shown.

Usefulness of the pain-induced functional impairment model to relate plasma levels of analgesics to their efficacy in rats

In this work we show that the pain-induced functional impairment model (PIFIR) can be used with cannulated rats as a useful procedure for pharmacokinetic/pharmacodynamic modelling. This model evaluates analgesia by measuring motor impairment of the right limb after intra-articular administration of uric acid. Time of contact with a rotating cylinder is referred to the control limb. We studied the pharmacokinetic and pharmacodynamics of naproxen after six peroral doses to Wistar rats, and we examined the adjuvant action of caffeine with naproxen. Surgery and blood sampling did not produce any difference on functional impairment either in rats without uric acid or in the dysfunction produced by uric acid. The relation between naproxen plasma concentration and the analgesic effect was obtained with few rats. Caffeine alone did not produce any significant modification in functional impairment but the co-administration significantly increased the effect of naproxen. Plasma levels of naproxen did not change when caffeine was co-administered. The PIFIR model with blood sampling is a suitable method for pharmacokinetic/pharmacodynamic relationship studies and is specially useful to characterize drug-drug interactions.

Caffeine reversal of sleep deprivation effects on alertness and mood

This study assessed the ability of high doses of caffeine to reverse changes in alertness and mood produced by prolonged sleep deprivation. Fifty healthy, nonsmoking males between the ages of 18 and 32 served as volunteers. Following 49 h without sleep, caffeine (0, 150, 300, or 600 mg/70 kg, PO) was administered in a double-blind fashion. Measures of alertness were obtained with sleep onset tests, the Stanford Sleepiness Scale (SSS), and Visual Analog Scales (VAS). Sleep deprivation decreased onset to sleep from a rested average of 19.9 min to 7 min. Following the highest dose of caffeine tested, sleep onset averaged just over 10 min; sleep onset for the placebo group averaged 5 min. Scores on the SSS increased from a rested mean of 1.6-4.8 after sleep deprivation. Caffeine reduced this score to near rested values. Caffeine reversed sleep deprivation-induced changes in three subscales of the POMS (vigor, fatigue, and confusion) and produced values close to fully rested conditions on several VAS. Serum caffeine concentrations peaked 90 min after ingestion and remained elevated for 12 h. This study showed that caffeine was able to produce significant alerting and long-lasting beneficial mood effects in individuals deprived of sleep for 48 h.

High-performance liquid chromatographic separation of caffeine, theophylline, theobromine and paraxanthine in rat brain and serum

Caffeine and its metabolites theophylline, theobromine and paraxanthine have been determined in rat brain and serum samples by high-performance liquid chromatography with ultraviolet detection. The recovery, 85-103%, allowed quantification by external standard methods. The variability was found to be less than 3 and 7% for intra-day and inter-day assays, respectively. The detection limit, 1.57 ng of methylxanthines on column, allowed the determination of 62.5 ng/g or ml in biological material. Rats treated with 30 mg/kg caffeine (subcutaneously) were sacrificed at different times (1, 6, 12 and 24 h). Higher concentrations of methylxanthines (specially paraxanthine) were observed in the striatum than in the rest of the brain, and it was also observed that the clearance of methylxanthines was faster in serum than in brain structures.

Validity of self-reports of caffeine use

The relationship between self-reports of caffeine ingestion on two occasions and measured plasma concentrations of caffeine and its major metabolites was examined. A subject population [25 men and 25 women, age 20-45 years (mean: 28.7 yr)] that was enrolled in a benzodiazepine pharmacokinetic study underwent general medical screening on two occasions, each including detailed caffeine histories. Before beginning their scheduled study, plasma samples were obtained and evaluated by HPLC for caffeine, paraxanthine, theophylline, and theobromine. These values were compared with estimates of caffeine consumption in mg/day generated from both histories. There was no significant difference between plasma levels of caffeine, metabolites, or caffeine plus metabolites for categories corresponding to reports of low, intermediate or high caffeine use. A self-reported caffeine consumption of greater than 300 mg/day (high) did correlate, however, with a significant smoking history. The authors conclude that self-reports of caffeine ingestion do not accurately reflect acute exposure, and that if caffeine use is of importance in a given setting, reports should be confirmed by biochemical means.

Simultaneous determination of cocaine and its metabolites with caffeine in rat serum microsamples by high-performance liquid chromatography

A single, isocratic high-performance liquid chromatographic method is described for the determination of cocaine and three of its metabolites along with caffeine in serum microsamples (50 microliters). The small sample size permits the tracking of pharmacokinetic data over time in individual, small animals. The method also was used to demonstrate that cocaine, benzoylecgonine and norcocaine in rat serum samples were stable for at least a month without the presence of sodium fluoride.

Caffeine-induced behavioural stimulation is dose- and concentration-dependent

1. The relationship between plasma and brain caffeine and metabolite concentrations and behavioural stimulation was investigated over a 4 h time course. 2. CD-1 mice receiving single intraperitoneal doses of caffeine-sodium benzoate solution (caffeine doses: 0, 20, and 40 mg kg-1) were evaluated in an activity monitor, and their plasma and brain caffeine and metabolite concentrations were determined by high performance liquid chromatography (h.p.l.c.). 3. Kinetic variables for caffeine at low and high caffeine doses were: volume of distribution (Vd), 1.16 and 0.88 l kg-1; plasma elimination half-life (t1/2), 1.25 and 1.62 h; brain t1/2, 0.93 and 1.30 h; clearance, 0.64 and 0.38 l h-1 kg-1, respectively, with Vd and brain t1/2 differing significantly between the two caffeine doses. 4. Low-dose caffeine stimulated vertical behaviours significantly more than high-dose, during the first 150 min post-dosage; both doses stimulated vertical behaviours significantly more than vehicle treatment. 5. Low-dose and high-dose caffeine stimulated horizontal and stereotypic behaviours equivalently, during the first 150 min post-dosage; both doses stimulated these behaviours significantly more than vehicle. 6. Only later, 150 min post-dosage, did high-dose caffeine stimulate all behaviours significantly more than both low-dose and vehicle treatment; this occurred when caffeine concentrations had fallen to approximately 10 micrograms g-1 in the high-dose group. 7. The maximal stimulant effects of caffeine occurred in an intermediate concentration range, between 10-20 micrograms g-1, while lower and higher concentrations produced either no additional stimulation or decrements in activity.

Separate and combined effects of caffeine and alprazolam on motor activity and benzodiazepine receptor binding in vivo

CD-1 mice received single intraperitoneal (IP) doses of caffeine-sodium benzoate (caffeine doses: 0, 20 and 40 mg/kg) followed by injections of alprazolampropylene glycol (0, 0.05, and 2 mg/kg, IP) to determine brain concentrations, effects on in vivo receptor binding of a specific high-affinity benzodiazepine receptor ligand [3H]Ro15-1788, and effects on motor activity over a 1-h period. A behavioral monitoring device, using infrared sensors, measured horizontal and ambulatory activity. Caffeine produced significant increases in all motor activity measures as compared to vehicle treatment, with low dose caffeine (with brain concentrations of 13 micrograms/g) stimulating activity to a greater degree than the high dose (with brain concentrations of 30 micrograms/g). The overall effect of caffeine on benzodiazepine receptor binding was not significant. Alprazolam significantly diminished motor activity and altered benzodiazepine receptor binding. Low dose alprazolam increased binding, while the high dose diminished it. Caffeine and alprazolam antagonized each other's behavioral effects in this study, but did not alter each other's uptake into brain. Alprazolam's antagonism of caffeine-induced motor stimulation was associated with decreases in receptor binding, whereas caffeine's reversal of alprazolam-induced motor depression was not associated with any changes in binding. The lack of a clear association between drug effects on benzodiazepine binding and on motor activity suggests that behavioral effects of caffeine and alprazolam may be mediated by other sites in addition to the benzodiazepine receptor.