Chronopharmacokinetics and cardiovascular effects of nifedipine

Dosing time optimization of antihypertensive medications by including the circadian rhythm in pharmacokinetic-pharmacodynamic models

Blood pressure (BP) follows a circadian variation, increasing during active hours, showing a small postprandial valley and a deeper decrease during sleep. Nighttime reduction of 10–20% relative to daytime BP is defined as a dipper pattern, and a reduction of less than 10%, as a non-dipper pattern. Despite this BP variability, hypertension’s diagnostic criteria and therapeutic objectives are usually based on BP average values. Indeed, studies have shown that chrono-pharmacological optimization significantly reduces long-term cardiovascular risk if a BP dipper pattern is maintained. Changes in the effect of antihypertensive medications can be explained by circadian variations in their pharmacokinetics (PK) and pharmacodynamics (PD). Nevertheless, BP circadian variation has been scarcely included in PK-PD models of antihypertensive medications to date. In this work, we developed PK-PD models that include circadian rhythm to find the optimal dosing time (Ta) of first-line antihypertensive medications for dipper and non-dipper patterns. The parameters of the PK-PD models were estimated using global optimization, and models were selected according to the lowest corrected Akaike information criterion value. Simultaneously, sensitivity and identifiability analysis were performed to determine the relevance of the parameters and establish those that can be estimated. Subsequently, Ta parameters were optimized to maximize the effect on BP average, BP peaks, and sleep-time dip. As a result, all selected models included at least one circadian PK component, and circadian parameters had the highest sensitivity. Furthermore, Ta with which BP>130/80 mmHg and a dip of 10–20% are achieved were proposed when possible. We show that the optimal Ta depends on the therapeutic objective, the medication, and the BP profile. Therefore, our results suggest making chrono-pharmacological recommendations in a personalized way.

Circadian regulation of transporter expression and implications for drug disposition

Introduction: Solute Carrier (SLC) and ATP-binding cassette (ABC) transporters expressed in the intestine, liver, and kidney determine the absorption, distribution, and excretion of drugs. In addition, most molecular and cellular processes show circadian rhythmicity controlled by circadian clocks that leads to diurnal variations in the pharmacokinetics and pharmacodynamics of many drugs and affects their therapeutic efficacy and toxicity.Area covered: This review provides an overview of the current knowledge on the circadian rhythmicity of drug transporters and the molecular mechanisms of their circadian control. Evidence for coupling drug transporters to circadian oscillators and the plausible candidates conveying circadian clock signals to target drug transporters, particularly transcription factors operating as the output of clock genes, is discussed.Expert opinion: The circadian machinery has been demonstrated to interact with the uptake and efflux of various drug transporters. The evidence supports the concept that diurnal changes that affect drug transporters may influence the pharmacokinetics of the drugs. However, more systematic studies are required to better define the timing of pharmacologically important drug transporter regulation and determine tissue- and sex-dependent differences. Finally, the transfer of knowledge based on the results and conclusions obtained primarily from animal models will require careful validation before it is applied to humans.

Chronopharmacology of high blood pressure—a critical review of clinical evidence

Physiological functions of cardiovascular system (CVS) are exhibiting circadian patterns regulated by complex system of endogenous factors. Preserving this rhythmicity is important for its normal function, whereas disturbing the synchronization with natural day–night cycle can increase the risk of cardiovascular damage. Cardiovascular pathophysiology also follows cyclic variation; time susceptibility and period with maximum risk associated with elevated blood pressure (BP) can be predicted. Given this rhythmic nature, significant changes in efficacy between morning and evening administration of the drug may occur; appropriate timing of pharmacological intervention in therapy of hypertension may affect the efficacy of the treatment.

Chronotherapy for Hypertension

PURPOSE OF REVIEW

Given the emerging knowledge that circadian rhythmicity exists in every cell and all organ systems, there is increasing interest in the possible benefits of chronotherapy for many diseases. There is a well-documented 24-h pattern of blood pressure with a morning surge that may contribute to the observed morning increase in adverse cardiovascular events. Historically, antihypertensive therapy involves morning doses, usually aimed at reducing daytime blood pressure surges, but an absence of nocturnal dipping blood pressure is also associated with increased cardiovascular risk.

RECENT FINDINGS

To more effectively reduce nocturnal blood pressure and still counteract the morning surge in blood pressure, a number of studies have examined moving one or more antihypertensives from morning to bedtime dosing. More recently, such studies of chronotherapy have studied comorbid populations including obstructive sleep apnea, chronic kidney disease, or diabetes. Here, we summarize major findings from recent research in this area (2013-2017). In general, nighttime administration of antihypertensives improved overall 24-h blood pressure profiles regardless of disease comorbidity. However, inconsistencies between studies suggest a need for more prospective randomized controlled trials with sufficient statistical power. In addition, experimental studies to ascertain mechanisms by which chronotherapy is beneficial could aid drug design and guidelines for timed administration.

Chronotherapy: Intuitive, Sound, Founded…But Not Broadly Applied

Circadian rhythms are a collection of endogenously driven biochemical, physiological, and behavioral processes that oscillate in a 24-h cycle and can be entrained by external cues. Circadian clock molecules are responsible for the expression of regulatory components that modulate, among others, the cell’s metabolism and energy consumption. In clinical practice, the regulation of clock mechanisms is relevant to biotransformation of therapeutics. Accordingly, xenobiotic metabolism and detoxification, the two processes that directly influence drug effectiveness and toxicity, are direct manifestations of the daily oscillations of the cellular and biochemical processes taking place within the gastrointestinal, hepatic/biliary, and renal/urologic systems. Consequently, the impact of circadian timing should be factored in when developing therapeutic regimens aimed at achieving maximum efficacy, minimum toxicity, and decreased adverse effects in a patient. However, and despite a strong mechanistic foundation, only 0.16 % of ongoing clinical trials worldwide exploit the concept of ‘time-of-day’ administration to develop safer and more effective therapies. In this article, we (1) emphasize points of control at which circadian biology intersects critical processes governing treatment interventions; (2) explore the extent to which chronotherapeutics are incorporated into clinical trials; (3) recognize roadblocks; and (4) recommend approaches to precipitate the integration of chronobiological concepts into clinical practice.

Blood pressure variability: assessment, predictive value, and potential as a therapeutic target

A large body of evidence has consistently supported the relationship between blood pressure (BP) levels and the risk of cardiovascular complications. In recent years, several independent studies have also indicated that this risk may not only depend on the magnitude of the blood pressure elevation per se but also on the presence of other associated conditions such as increased blood pressure variability. This concept has been supported by a series of reports, most of which post hoc analyses of clinical trials in hypertension, showing that increasing values of BP variability (BPV) (either in the short term, in the midterm, or in the long term) may predict development, progression, and severity of cardiac, vascular, and renal organ damage, as well as cardiovascular events and mortality. Remarkably, studies conducted in populations at high cardiovascular risk have shown increasing values of BPV in the individual subjects (so-called intra- or within-individual BPV) to be strong predictors of cardiovascular morbidity and mortality, even to a larger extent than average BP values. However, in subjects at low to moderate cardiovascular risk, the contribution of BPV to cardiovascular risk prediction over and beyond average BP values has been shown to be only moderate. The aim of this paper is to critically review the evidence addressing the prognostic relevance of different components of BPV addressing a yet open question, i.e., whether routine assessment of BPV in clinical practice should be regarded as an additional target of antihypertensive treatment to improve cardiovascular protection.

Population Pharmacokinetic Model Characterizing 24-Hour Variation in the Pharmacokinetics of Oral and Intravenous Midazolam in Healthy Volunteers

Daily rhythms in physiology may affect the pharmacokinetics of a drug. The aim of this study was to evaluate 24-hour variation in the pharmacokinetics of the CYP3A substrate midazolam. Oral (2 mg) and intravenous (1 mg) midazolam was administered at six timepoints throughout the 24-hour period in 12 healthy volunteers. Oral bioavailability (population mean value [RSE%] of 0.28 (7.1%)) showed 24-hour variation that was best parameterized as a cosine function with an amplitude of 0.04 (17.3%) and a peak at 12:14 in the afternoon. The absorption rate constant was 1.41 (4.7%) times increased after drug administration at 14:00. Clearance (0.38 L/min (4.8%)) showed a minor 24-hour variation with an amplitude of 0.03 (14.8%) L/min and a peak at 18:50. Simulations show that dosing time minimally affects the concentration time profiles after intravenous administration, while concentrations are higher during the day compared to the night after oral dosing, reflecting considerable variation in intestinal processes.

Calcineurin inhibitors differentially alter the circadian rhythm of T-cell functionality in transplant recipients

Background

Graft survival in transplant recipients depends on pharmacokinetics and on individual susceptibility towards immunosuppressive drugs. Nevertheless, pharmacodynamic changes in T-cell functionality in response to drugs and in relation to pharmacokinetics are poorly characterized. We therefore investigated the immunosuppressive effect of calcineurin inhibitors and steroids on general T-cell functionality after polyclonal stimulation of whole blood samples.

Methods

General T-cell functionality in the absence or presence of immunosuppressive drugs was determined in vitro directly from whole blood based on cytokine induction after stimulation with the polyclonal stimulus Staphylococcus aureus enterotoxin B. In addition, diurnal changes in leukocyte and lymphocyte subsets, and on T-cell function after intake of immunosuppressive drugs were analyzed in 19 patients during one day and compared to respective kinetics in six immunocompetent controls. Statistical analysis was performed using non-parametric and parametric tests.

Results

Susceptibility towards calcineurin inhibitors showed interindividual differences. When combined with steroids, tacrolimus led to more pronounced increase in the inhibitory activity as compared to cyclosporine A. While circadian alterations in leukocyte subpopulations and T-cell function in controls were related to endogenous cortisol levels, T-cell functionality in transplant recipients decreased after intake of the morning medication, which was more pronounced in patients with higher drug-dosages. Interestingly, calcineurin inhibitors differentially affected circadian rhythm of T-cell function, as patients on cyclosporine A showed a biphasic decrease in T-cell reactivity after drug-intake in the morning and evening, whereas T-cell reactivity in patients on tacrolimus remained rather stable.

Conclusions

The whole blood assay allows assessment of the inhibitory activity of immunosuppressive drugs in clinically relevant concentrations. Circadian alterations in T-cell function are determined by dose and type of immunosuppressive drugs and show distinct differences between cyclosporine A and tacrolimus. In future these findings may have practical implications to estimate the net immunosuppressive effect of a given drug-regimen that daily acts in an individual patient, and may contribute to individualize immunosuppression.

Population pharmacokinetic analysis of circadian rhythms in hepatic CYP3A activity using midazolam

Diurnal changes in the activity of drug metabolizing enzymes may contribute to the variability in drug disposition and drug effects. The aim of this study was to quantify the circadian rhythmicity exhibited by hepatic CYP3A. A 10 μg/kg intravenous bolus dose, followed by a 30-hour 4 μg/kg/h intravenous infusion of midazolam, used as a probe substrate for hepatic CYP3A activity, was administered to 16 healthy volunteers (8 males and 8 females). Blood samples were drawn hourly for 24 hours after achieving steady state, and plasma concentrations of midazolam and its main metabolite 1-OH midazolam were determined. Population pharmacokinetic analysis was performed using nonlinear mixed effects modeling. One-compartment pharmacokinetic models best described midazolam and 1-OH midazolam pharmacokinetic disposition. An unequivocal but minor diurnal pattern was identified in the midazolam plasma concentration profiles, which was described using a cosine function with a 24-hours period. The fluctuation in the relative CYP3A activity ranged between 10% above average around 15:00, and 10% below average around 03:00. None of the covariates tested had a significant impact on the parameters estimated. Although a diurnal pattern in hepatic CYP3A activity was identified, its magnitude suggests that it is small and without clinical significance for drug therapy.

In vitro-in vivo extrapolation of drug-induced proarrhythmia predictions at the population level

Drug cardiotoxicity is a serious issue for patients, regulators, pharmaceutical companies and health service payers because they are all affected by its consequences. Despite the wide range of data they generate, existing approaches for cardiac safety testing might not be adequate and sufficiently cost-effective, probably as a result of the complexity of the problem. For this reason, translational tools (based on biophysically detailed, mathematical models) allowing for in vitro-in vivo extrapolation are gaining increasing interest. This current review describes approaches that can be used for cardiac safety assessment at the population level, by accounting for various sources of variability including kinetics of the compound of interest.

Molecular clocks in pharmacology

Circadian rhythms regulate a vast array of biological processes and play a fundamental role in mammalian physiology. As a result, considerable diurnal variation in the pharmacokinetics, efficacy, and side effect profiles of many therapeutics has been described. This variation has subsequently been tied to diurnal rhythms in absorption, distribution, metabolism, and excretion, as well as in pharmacodynamic variables, such as target expression. More recently, the molecular basis of circadian rhythmicity has been elucidated with the identification of clock genes, which oscillate in a circadian manner in most cells and tissues and regulate transcription of large sets of genes. Ongoing research efforts are beginning to reveal the critical role of circadian clock genes in the regulation of pharmacologic parameters, as well as the reciprocal impact of drugs on circadian clock function. This chapter will review the role of circadian clocks in the pharmacokinetics and pharmacodynamics of drug response and provide several examples of the complex regulation of pharmacologic systems by components of the molecular circadian clock.

Administration-time differences in effects of hypertension medications on ambulatory blood pressure regulation

Specific features of the 24-h blood pressure (BP) pattern are linked to progressive injury of target tissues and risk of cardiovascular disease (CVD) events. Several studies have consistently shown an association between blunted asleep BP decline and risk of fatal and nonfatal CVD events. Thus, there is growing focus on ways to properly control BP during nighttime sleep as well as during daytime activity. One strategy, termed chronotherapy, entails the timing of hypertension medications to endogenous circadian rhythm determinants of the 24-h BP pattern. Significant and clinically meaningful treatment-time differences in the beneficial and/or adverse effects of at least six different classes of hypertension medications, and their combinations, are now known. Generally, calcium channel blockers (CCBs) are more effective with bedtime than morning dosing, and for dihydropyridine derivatives bedtime dosing significantly reduces risk of peripheral edema. The renin-angiotensin-aldosterone system is highly circadian rhythmic and activates during nighttime sleep. Accordingly, evening/bedtime ingestion of the angiotensin-converting enzyme inhibitors (ACEIs) benazepril, captopril, enalapril, lisinopril, perindopril, quinapril, ramipril, spirapril, trandolapril, and zofenopril exerts more marked effect on the asleep than awake systolic (SBP) and diastolic (DBP) BP means. Likewise, the bedtime, in comparison with morning, ingestion schedule of the angiotensin-II receptor blockers (ARBs irbesartan, olmesartan, telmisartan, and valsartan exerts greater therapeutic effect on asleep BP, plus significant increase in the sleep-time relative BP decline, with the additional benefit, independent of drug terminal half-life, of converting the 24-h BP profile into a more normal dipping pattern. This is the case also for the bedtime versus upon-awakening regimen of combination ARB-CCB, ACEI-CCB, and ARB-diuretic medications. The chronotherapy of conventional hypertension medications constitutes a new and cost-effective strategy for enhancing the control of daytime and nighttime SBP and DBP levels, normalizing the dipping status of their 24-h patterning, and potentially reducing the risk of CVD events and end-organ injury, for example, of the blood vessels and tissues of the heart, brain, kidney, and retina.

The role of clock genes in pharmacology

The physiology of a wide variety of organisms is organized according to periodic environmental changes imposed by the earth's rotation. This way, a large number of physiological processes present diurnal rhythms regulated by an internal timing system called the circadian clock. As part of the rhythmicity in physiology, drug efficacy and toxicity can vary with time. Studies over the past four decades present diurnal oscillations in drug absorption, distribution, metabolism, and excretion. On the other hand, diurnal variations in the availability and sensitivity of drug targets have been correlated with time-dependent changes in drug effectiveness. In this review, we provide evidence supporting the regulation of drug kinetics and dynamics by the circadian clock. We also use the examples of hypertension and cancer to show current achievements and challenges in chronopharmacology.

Administration-Time-Dependent Effects of Doxazosin GITS on Ambulatory Blood Pressure of Hypertensive Subjects

Previous studies have shown that a single nighttime dose of standard doxazosin, an alpha-adrenergic antagonist, reduces blood pressure (BP) throughout the 24 h. We investigated the administration-time-dependent effects of the new doxazosin gastrointestinal therapeutic system (GITS) formulation. We studied 91 subjects (49 men and 42 women), 56.7+/-11.2 (mean+/-SD) yrs of age with grade 1-2 essential hypertension; 39 patients had been previously untreated, and the remaining 52 had been treated with two antihypertensive medications with inadequate control of their hypertension. The subjects of the two groups, the monotherapy and polytherapy groups, respectively, were randomly assigned to receive the single daily dose of doxazosin GITS (4 mg/day) either upon awakening or at bedtime. BP was measured by ambulatory monitoring every 20 min during the day and every 30 min at night for 48 consecutive hours just before and after 3 months of treatment. After 3 months of doxazosin GITS therapy upon awakening, there was a small and nonstatistically significant reduction in BP (1.8 and 3.2mm Hg in the 24 h mean of systolic and diastolic BP in monotherapy; 2.2 and 1.9mm Hg in polytherapy), mainly because of absence of any effect on nocturnal BP. The 24 h mean BP reduction was larger and statistically significant (6.9 and 5.9 mm for systolic and diastolic BP, respectively, in monotherapy; 5.3 and 4.5 mm Hg in polytherapy) when doxazosin GITS was scheduled at bedtime. This BP-lowering effect was similar during both the day and nighttime hours. Doxazosin GITS ingested daily on awakening failed to provide full 24h therapeutic coverage. Bedtime dosing with doxazosin GITS, however, significantly reduced BP throughout the 24h both when used as a monotherapy and when used in combination with other antihypertensive pharmacotherapy. Knowledge of the chronopharmacology of doxazosin GITS is key to optimizing the efficiency of its BP-lowering effect, and this must be taken into consideration when prescribing this medication to patients.

Dose‐ And Administration Time‐Dependent Effects Of Nifedipine Gits On Ambulatory Blood Pressure In Hypertensive Subjects

Previous chronotherapy studies have shown that the circadian pattern of blood pressure (BP) remains unchanged after either morning or evening dosing of several calcium channel blockers (CCB), including amlodipine, isradipine, verapamil, nitrendipine, and cilnidipine. This trial investigated the antihypertensive efficacy and safety profile of the slow-release, once-a-day nifedipine gastrointestinal therapeutic system (GITS) formulation administered at different times with reference to the rest-activity cycle of each participant. We studied 80 diurnally active subjects (36 men and 44 women), 52.1+/-10.7 yrs of age, with grade 1-2 essential hypertension, who were randomly assigned to receive nifedipine GITS (30 mg/day) as a monotherapy for eight weeks, either upon awakening in the morning or at bedtime at night. Patients with uncontrolled BP were up-titrated to a higher dose, 60 mg/day nifedipine GITS, for an additional eight weeks. BP was measured by ambulatory monitoring every 20 min during the day and every 30 min at night for 48 consecutive hours before and after therapy with either dose. The BP reduction after eight weeks of therapy with the lower dose of 30 mg/day was slightly, but not significantly, larger with bedtime dosing. The efficacy of 60 mg/day nifedipine GITS in non-responders to the initial 30 mg/day dose was twice as great with bedtime as compared to morning dosing. Moreover, bedtime administration of nifedipine GITS reduced the incidence of edema as an adverse event by 91%, and the total number of all adverse events by 74% as compared to morning dosing (p=0.026). Independent of the time of day of administration, a single daily dose of 30 mg/day of nifedipine GITS provides full 24 h therapeutic coverage. The dose-dependent increased efficacy and the markedly improved safety profile of bedtime as compared to morning administration of nifedipine GITS should be taken into account when prescribing this CCB in the treatment of essential hypertension.

The influence of circadian rhythms on the kinetics of drugs in humans

In clinical practice, it is important to consider circadian rhythms in pharmacokinetics and cell responses to therapy in order to design proper protocols for drug administration. Scientists have arrived at this conclusion after several experiments in animals and in humans have clearly demonstrated that all organisms are highly organised according to circadian rhythms. These temporal cycles influence different physiological functions and, consequently, can influence the pharmacokinetic phases of drugs. A drug's pharmacokinetics can be modified according to the time of drug administration. In fact, the circadian changes of > 100 different compounds have been documented. The results obtained have led several scientific societies to provide guidelines concerning the timing of drug dosing for anticancer, cardiovascular, respiratory, anti-ulcer, anti-inflammatory, immunosuppressive and antiepileptic drugs. Absorption may be influenced by circadian rhythms and most lipophilic drugs seem to be absorbed faster when the drug is taken in the morning compared with the evening; for water-soluble compounds, no circadian variation in the absorption of drugs has been found. Concerning drug distribution, the higher the blood flow fraction an organ receives, the higher the rate constant for transferring drugs out of the capillaries. This drug pharmacokinetic phase may be influenced by circadian variations in the protein binding of acidic and basic drugs. Drug metabolism may be influenced by daily modifications of blood flow. For drugs with a high extraction ratio, metabolism depends on hepatic blood flow, while that of drugs with a low extraction ratio depends on liver enzyme activity. Hepatic blood flow has been shown to be greatest at 8 am and metabolism seems to be reduced during the night. Finally, concerning drug elimination, the clearance of 'flow-limited' drugs that present a high extraction rate is affected by the blood flow delivered to the organ, independent of the cardiac output fraction supplied. Chronopharmacokinetics can explain individual differences in drug levels revealed by therapeutic drug monitoring and can be used to optimise the management of patients receiving drug therapy.

Clinical chronopharmacology: the importance of time in drug treatment

Nearly all functions of the body, including those influencing pharmacokinetic parameters such as drug absorption and distribution, drug metabolism and renal elimination, show significant daily variations: these include liver metabolism, hepatic blood flow and the first-pass effect; glomerular filtration, renal plasma flow and urine volume and pH; blood pressure, heart rate and organ perfusion rates; acid secretion in the gastro-intestinal tract and gastric emptying time. The onset and symptoms of diseases such as asthma attacks, coronary infarction, angina pectoris, stroke and ventricular tachycardia are circadian phase dependent. In humans, variations during the 24 h day in pharmacokinetics (chrono-pharmacokinetics) have been shown for cardiovascularly active drugs (propranolol, nifedipine, verapamil, enalapril, isosorbide 5-mononitrate and digoxin), anti-asthmatics (theophylline and terbutaline), anticancer drugs, psychotropics, analgesics, local anaesthetics and antibiotics, to mention but a few. Even more drugs have been shown to display significant variations in their effects throughout the day (chronopharmacodynamics and chronotoxicology) even after chronic application or constant infusion. Moreover, there is clear evidence that even dose/concentration-response relationships can be significantly modified by the time of day. Thus, circadian time has to be taken into account as an important variable influencing a drug's pharmacokinetics and its effects or side-effects.

Chronobiology and chronotherapy of ischemic heart disease

The occurrence of the clinical manifestations of ischemic heart disease (IHD)--myocardial ischemia and angina pectoris, acute myocardial infarction, and sudden cardiac death--is unevenly distributed during the 24 h with greater than expected events during the initial hours of the daily activity span and in the late afternoon or early evening. Such temporal patterns result from circadian rhythms in pathophysiological mechanisms plus cyclic environmental stressors that trigger ischemic events. Both the pharmacokinetics (PK) and pharmacodynamics (PD) of many, though not all, anti-ischemic oral nitrate, calcium channel blocker, and beta-adrenoceptor antagonist medications have been shown to be influenced by the circadian time of their administration. The requirement for preventive and therapeutic interventions varies predictably during the 24 h, and thus therapeutic strategies should also be tailored accordingly to optimize outcomes. During the past decade, two first generation calcium channel blocker chronotherapies have been developed, trialed, and marketed in North America for the improved treatment of IHD. Nonetheless, there has been relatively little investigation of the administration-time (circadian rhythm) dependencies of the PK and PD of conventional anti-ischemic medications, and there has been little progress in the development of new generation IHD chronotherapies. Available epidemiologic, pharmacologic, and clinico-therapeutic evidence demonstrates how the chronobiologic approach to IHD can contribute new insight and opportunities to improve drug design and drug delivery to enhance therapeutic outcomes.

Chronotherapy of hypertension: administration-time-dependent effects of treatment on the circadian pattern of blood pressure

Some specific features of the 24-hour blood pressure (BP) pattern are linked to the progressive injury of target tissues and the triggering of cardiac and cerebrovascular events. Thus, there is growing interest in how to best tailor the treatment of hypertensive patients according to the circadian BP pattern of each individual. Significant administration-time differences in the kinetics (i.e., chronokinetics) plus beneficial and adverse effects (i.e., chronodynamics) of antihypertensive medications are well known. Thus, bedtime dosing with nifedipine GITS is more effective than morning dosing, while also significantly reducing adverse effects. The dose-response curve, therapeutic coverage, and efficacy of doxazosin GITS are all markedly dependent on the circadian time of drug administration. Moreover, valsartan administration at bedtime, as opposed to upon wakening, results in an improved diurnal/nocturnal BP ratio, increased percentage of controlled patients, and significant reduction in urinary albumin excretion in hypertensive patients. Chronotherapy provides a means of individualizing the treatment of hypertension according to the circadian BP profile of each patient, and constitutes a new option to optimize BP control and to reduce the risk of cardiovascular disease (myocardial infarction and stroke) and of end-organ injury of the blood vessels and tissue of the heart, brain, kidney, eye, and other organs.

Chronotherapy in hypertensive patients: administration-time dependent effects of treatment on blood pressure regulation

Ambulatory blood pressure measurements (ABPM) correlate more closely with target organ damage and cardiovascular events than clinical cuff measurements. ABPM reveals the significant circadian variation in BP, which in most individuals presents a morning increase, small post-prandial decline, and more extensive lowering during nocturnal rest. However, under certain pathophysiological conditions, the nocturnal BP decline may be reduced (nondipper pattern) or even reversed (riser pattern). This is clinically relevant since the nondipper and riser circadian BP patterns constitute a risk factor for left ventricular hypertrophy, microalbuminuria, cerebrovascular disease, congestive heart failure, vascular dementia and myocardial infarction. Hence, there is growing interest in how to best tailor and individualize the treatment of hypertension according to the circadian BP pattern of each patient. Significant administration-time differences in the kinetics and in the beneficial and adverse effects of antihypertensive medications are well known. Thus, bedtime dosing with nifedipine gastrointestinal therapeutic system (GITS) is more effective than morning dosing, while also significantly reducing adverse effects. The therapeutic coverage and efficacy of doxazosin GITS are dependent on the circadian time of drug administration. Moreover, valsartan administration at bedtime, as opposed to upon wakening, results in an improved diurnal/nocturnal BP ratio, increased percentage of controlled patients, and significant reduction in urinary albumin excretion in hypertensive patients. Chronotherapy provides a means of individualizing the treatment of hypertension according to the circadian BP profile of each patient, and constitutes a new option to optimize BP control and reduce the risk of cardiovascular disease.

Optimal timing for antihypertensive dosing: focus on valsartan

Some specific features of the 24 h blood pressure (BP) pattern are linked to the progressive injury of target tissues and the triggering of cardiac and cerebrovascular events. In particular, many studies show the extent of the nocturnal BP decline relative to the diurnal BP mean (the diurnal/nocturnal ratio, an index of BP dipping) is deterministic of cardiovascular injury and risk. Normalization of the circadian BP pattern is considered to be an important clinical goal of pharmacotherapy because it may slow the advance of renal injury and avert end-stage renal failure. The chronotherapy of hypertension takes into account the epidemiology of the BP pattern, plus potential administration-time determinants of the pharmacokinetics and dynamics of antihypertensive medications, as a means of enhancing beneficial outcomes and/or attenuating or averting adverse effects. Thus, bedtime dosing with nifedipine gastrointestinal therapeutic system (GITS) is more effective than morning dosing, while also reducing significantly secondary effects. The dose-response curve, therapeutic coverage, and efficacy of doxazosin GITS are all markedly dependent on the circadian time of drug administration. Moreover, valsartan administration at bedtime as opposed to upon wakening results in improved diurnal/nocturnal ratio, a significant increase in the percentage of patients with controlled BP after treatment, and significant reductions in urinary albumin excretion and plasma fibrinogen. Chronotherapy provides a means of individualizing treatment of hypertension according to the circadian BP profile of each patient, and constitutes a new option to optimize BP control and reduce risk.

Chronopharmacokinetics of ciclosporin and tacrolimus

The correct use of immunosuppressive drugs has a considerable influence on the prognosis of patients with organ transplants. The appropriate utilisation of the drugs involves the administration of an adequate dosage to reach the blood concentrations that will suppress the alloimmune response, while avoiding secondary toxicities. However, transplanted patients exhibit heterogeneous immunological responses and high inter- and intraindividual pharmacokinetic variabilities. One cause of these variabilities that is rarely considered is circadian rhythms. In vitro and in vivo experiments have clearly demonstrated that all organisms are highly organised according to an internal biological clock that influences various physiological functions. Considering that the absorption, distribution, metabolism and elimination of drugs is influenced by the physiological functions of the body, it is not surprising that the pharmacokinetic, and consequently the pharmacodynamic, profiles of drugs can be influenced by circadian rhythms. Ciclosporin, a mainstay immunosuppressive drug used following organ transplantation, displays minimum blood concentration (C(min)), maximum blood concentration (C(max)) and area under the blood concentration-time curve (AUC) in the morning that are generally higher than the corresponding parameters in the evening. These observations are supported by the ciclosporin total body clearance and elimination half-life in the morning, which are, on average, higher and shorter, respectively, than those in the evening. In addition, the disposition of tacrolimus is determined by the time of administration. The tacrolimus C(max) and AUC after the morning dose are significantly higher than those after the evening dose. Finally, the results reported in this review suggest considering more carefully the chronopharmacokinetics of tacrolimus and ciclosporin in order to obtain better results with fewer adverse effects. Significantly, the morning appears to be the best time for therapeutic monitoring using the C(min), C(max), concentration at 2 hours after dosing and AUC to modify dosages of tacrolimus and ciclosporin. Less certain are any conclusions about whether, in order to obtain better immunosuppressive control, higher doses must be administered when these drugs are given in the evening to compensate for the higher levels of interleukin-2.

Circadian variations in the pharmacokinetics, tissue distribution and urinary excretion of nifedipine after a single oral administration to rats

Circadian variations in the pharmacokinetics, tissue distribution and urinary excretion of nifedipine were examined in fasted rats after administering a single oral dose at three different dosing times (08:00 am, 16:00 pm, 00:00 am). The plasma concentrations, the areas under the plasma concentration-time curve from zero to 6 h (AUC(0-6 h)) and the peak plasma concentration (C(max)) were significantly higher in the rats dosed at 08:00 am (immediately inactive), and was lower at 16:00 pm (most inactive) and 00:00 am (most active). The time to reach the C(max) (T(max)) was the shortest in the rats dosed at 08:00 am. It was very interesting to observe the double peak phenomena in the plasma concentration profiles, showing a larger peak followed by a smaller peak. There was a dosing time dependency on the tissue distribution 30 min after administration, showing a similar tendency to the pharmacokinetic behavior. However, there was no distinct dosing time dependency observed at 2 h after administration due to the extensive disposition. The cumulative urine excretion of nifedipine in the rats dosed at 08:00 am was significantly higher (about two-fold) than in those dosed at 16:00 pm and 00:00 am. The pharmacokinetics of nifedipine in the rats was consistent with that observed in human subjects in terms of the day-night clock time but the biological time was the opposite, as marked by the rest-activity cycles. These results may help to explain the circadian time-dependency of nifedipine pharmacokinetics.

Chronopharmacology and controlled drug release

Circadian rhythms in the body are well established and are an important factor to consider when administering drugs. Many diseases display symptoms and onset characteristics that are not randomly distributed within 24 h (e.g., coronary infarction, angina pectoris, asthmatic attacks and peptic ulcer perforations); therefore, it is not surprising that the effects and/or pharmacokinetics of drugs can display significant daily variations. Recent data, primarily concerned with the chronopharmacokinetics of antiasthmatics, histamine H2-blockers and cardiovascular active drugs (e.g., propanolol, organic nitrate and nifedipine) are described as representative examples in this review. The data demonstrate that biological rhythms should have been taken into account when evaluating drug delivery systems, galenic formulations and pharmacokinetics as a basis for drug treatment.

Administration-time-dependent effects of antihypertensive treatment on the circadian pattern of blood pressure

PURPOSE OF REVIEW

Many studies show that the extent of the nocturnal blood-pressure decline is deterministic of cardiovascular injury and risk. Accordingly, there is growing interest in how to tailor the treatment of hypertensive patients according to their circadian blood-pressure pattern.

RECENT FINDINGS

Differences in efficacy depending on the time of day of drug administration lead to differences in effects of antihypertensive drugs on the nocturnal decline relative to the diurnal mean of blood pressure. Thus, bedtime dosing with nifedipine gastrointestinal therapeutic system (GITS) is more effective than morning dosing, while also reducing significantly secondary effects. Bedtime administration of trandolapril results in a safe and effective means of controlling morning blood pressure without inducing excessive reduction nocturnally. The dose-response curve, therapeutic coverage, and efficacy of doxazosin GITS are all markedly dependent on the circadian time of drug administration. Moreover, valsartan administration at bedtime as opposed to upon wakening results in improved day/night blood-pressure ratio, a significant increase in the percentage of controlled patients after treatment, and a significant reduction in urinary albumin excretion.

SUMMARY

Nocturnal hypertension increases one's risk of cardiovascular and cerebrovascular events, nephrosclerosis, and progression to end-stage kidney failure in renal patients. Normalization of the circadian blood-pressure pattern is considered an important clinical goal of pharmacotherapy because it may slow the advance of renal injury. Chronotherapy provides a means of individualizing treatment of hypertension according to the circadian blood-pressure profile of each patient, and constitutes a new option in optimizing blood-pressure control and reducing risk.

Absence of circadian variation in the pharmacokinetics of lopinavir/ritonavir given as a once daily dosing regimen in HIV-1-infected patients

AIMS

To compare the pharmacokinetics of lopinavir/ritonavir (LPV/r) 800/200 mg administered once daily in the morning compared with the evening.

METHODS

This was a randomized, two-way, cross-over study in HIV+ subjects. In each subject the pharmacokinetics of each drug were characterized after 2 weeks of LPV/r 800/200 mg administered once daily at 08.00 h and 19.00 h. On study days, LPV/r was taken with a standardized meal (800 kCal, 25% from fat) after fasting for at least 5 h. LPV/r concentrations were measured by LC-MS/MS, and the data were analyzed by noncompartmental pharmacokinetic analysis.

RESULTS

Fourteen subjects completed the study (all men, mean age/weight 44 year/81 kg). The median (interquartile range) LPV AUC(0,24 h), maximum plasma concentration (C(max)) and concentration at the end of the dosing interval (C(24 h)) after am and pm dosing was, respectively, 143 (116-214) mg l(-1) h, 12.8 (10.3-17.2) mg l(-1), 1.34 (0.58-3.25) mg l(-1), and 171 (120-232) mg l(-1) h, 12.9 (8.22-16.3) mg l(-1), 1.15 (0.59-1.98) mg l(-1). The geometric mean ratio (GMR, am : pm) and 95% CI of the LPV AUC(0,24 h), C(max), and C(24 h) was 0.91 (0.79, 1.06), 1.11 (0.94, 1.32), and 1.19 (0.72, 1.96), respectively. The median ritonavir C(max) after am and pm dosing was 1.05 and 0.90 mg l(-1), respectively. The GMR (95% CI) of the RTV AUC(0,24 h), C(max), and C(24 h) was 0.93 (0.80, 1.08), 1.27 (1.00, 1.63), and 1.04 (0.68, 1.60), respectively. Administration of LPV/r in a once-daily regimen was generally well tolerated.

CONCLUSIONS

No differences were observed in the pharmacokinetics of LPV/r after am or pm dosing with food, which suggests that this once daily combination, can be taken in the morning or evening. Such flexibility in dosing may improve adherence.

Chronotherapy of hypertension

PURPOSE OF REVIEW

Blood pressure displays appreciable predictable-in-time circadian variation. The chronotherapy of hypertension takes into account the clinically relevant features of the 24-h pattern of blood pressure, e.g. the accelerated morning rise and nighttime decline during sleep, plus potential administration circadian time determinants of the pharmacokinetics and dynamics of antihypertensive medications.

RECENT FINDINGS

Significant administration-time differences in the kinetics (i.e. chronokinetics) plus the beneficial and adverse effects (termed chronodynamics) of antihypertensive drugs are well known. Thus, bedtime, but not morning, dosing with cilnidipine significantly reduces nocturnal blood pressure. In addition, the dose-response curve, therapeutic coverage, and efficacy of the doxazosin gastrointestinal therapeutic system are all markedly dependent on the circadian time of drug administration. Moreover, valsartan administration at bedtime as opposed to upon awakening results in improved diurnal/nocturnal blood pressure ratio, such that the dosing time of valsartan can be chosen in relation to the dipper status of any given patient to improve therapeutic benefit and reduce cardiovascular risk.

SUMMARY

Nocturnal hypertension, which is characterized by the loss or even reversal of the expected 10-20% sleep-time blood-pressure decline, increases the risk of cardiac and cerebrovascular events. Chronotherapy provides a means of individualizing treatment of hypertension according to the circadian profile of blood pressure of each patient. The chronotherapeutic strategy constitutes a new option to optimize blood-pressure control and to reduce risk.

Administration time-dependent effects of valsartan on ambulatory blood pressure in hypertensive subjects

This study investigated the administration time-dependent antihypertensive efficacy of valsartan, an angiotensin II receptor blocker. We studied 90 subjects (30 men and 60 women), 49.0+/-14.3 (mean+/-SD) years of age with stage 1 to 2 essential hypertension; they were randomly assigned to receive valsartan (160 mg/d) as a monotherapy either on awakening or at bedtime. Blood pressure was measured by ambulatory monitoring every 20 minutes during the day and every 30 minutes at night for 48 consecutive hours before and after 3 months of treatment. Physical activity was simultaneously monitored every minute by wrist actigraphy to accurately calculate the diurnal and nocturnal means of blood pressure on a per-subject basis. The highly significant blood pressure reduction after 3 months of treatment with valsartan (P<0.001) was similar for both treatment times (17.0 and 11.3 mm Hg reduction in the 24-hour mean of systolic and diastolic blood pressure with morning administration and 14.6 and 11.4 mm Hg reduction with bedtime administration; P>0.174 for treatment time effect). Valsartan administration at bedtime as opposed to on wakening resulted in a highly significant average increase by 6% (P<0.001) in the diurnal-nocturnal ratio of blood pressure; this corresponded to a 73% relative reduction in the number of nondipper patients. The findings confirm that valsartan efficiently reduces blood pressure throughout the entire 24 hours, independent of treatment time. They also suggest that time of treatment can be chosen according to the dipper status of a patient to optimize the effect of antihypertensive therapy, an issue that deserves further investigation.

Ambulatory blood pressure monitoring to assess the comparative efficacy and duration of action of a novel new angiotensin II receptor blocker--telmisartan

Although a wide range of antihypertensive agents is available, adequate blood pressure control is achieved in only about 25% of hypertensive patients. Poor control rates are often due to inadequate patient compliance and unacceptable side-effects. The importance of once-daily dosing is now fully acknowledged, but it is crucial that therapy when given once a day must be effective at the end of the dosing interval to order to minimize the likelihood of sudden cardiac death, myocardial infarction and stroke. Ambulatory blood pressure monitoring (ABPM) provides a thorough assessment of the blood pressure-lowering characteristics of an antihypertensive agent throughout the dosing interval and can more accurately evaluate differences in the duration of the antihypertensive effect of different agents. Telmisartan, a new angiotensin II receptor antagonist, has been extensively studied in clinical trials using ABPM. When compared with dihydropyridine calcium antagonists, beta-adrenergic blockers, angiotensin-converting enzyme inhibitors and other angiotensin II receptor antagonists, telmisartan has proved superior in diminishing ambulatory blood pressure throughout the 24-h period between doses. Telmisartan also has excellent tolerability and in clinical trials demonstrates no increase, irrespective of dose level, over placebo in the incidence of adverse events. Thus, telmisartan given once daily provides high efficacy and tolerability, and will hopefully assist in improving both blood pressure control rates and cardiovascular outcomes in the future.

Morning-evening administration time differences in digoxin kinetics in healthy young subjects

Digoxin, frequently used in the treatment of congestive heart failure, has a very narrow therapeutic index. We studied the differences in digoxin pharmacokinetics when ingested in the morning versus evening. A single digoxin (0.25 mg) dose was given orally to the same group of 10 diurnally active healthy (6 male and 4 female) volunteers in the morning at 08:00 and evening at 20:00 in separate experiments scheduled 2 weeks apart. Blood samples were collected at specific times for 48h after each timed dose; digoxin was determined by radioimmunoassay (RIA). Maximum plasma concentration Cmax; Tmax, the time to reach Cmax; area under plasma concentration curve AUC; and elimination half-time T1/2 of digoxin were determined. Tmax was statistically significantly shorter (54 min) following 08:00 dosing com pared to 20:00 dosing (96 min). Although the Cmax was higher after morning than evening dosing, it was not significantly so. No other parameter of digoxin pharmacokinetics except Tmax exhibited administration time dependency.

Relevance for chronopharmacology in practical medicine

Nearly all functions of the body, including those influencing pharmacokinetic parameters, such as drug absorption and distribution, drug metabolism, and renal elimination display significant daily variations. Also, the onset and symptoms of diseases such as asthma attacks, coronary infarction, angina pectoris, stroke, and ventricular tachycardia are circadian-phase dependent. Asthma attacks predominantly occur around 4 o'clock at night. Blood pressure and heart rate in normotensives and essential (primary) hypertensive patients display highest values during daytime followed by a nightly drop and an early morning rise. In about 70% of forms of secondary hypertension, however, this rhythmic pattern is abolished or even reversed exhibiting nightly peaks in blood pressure. Similar findings were obtained in children. This form of hypertension is accompanied by increased end organ damages. These observations call for a circadian time-specified drug treatment. In nocturnal asthma unequal dosing of antiasthmatic drugs with a higher/single evening dose is recommended. In secondary hypertension not only the elevated blood pressure must be reduced but the disturbed blood pressure profile should be normalized, too, possibly best achieved by evening dosing. Pharmacokinetics may also not be constant within 24 hours of a day as shown for cardiovascular active drugs, antiasthmatics, anticancer drugs, psychotropics, analgesics and local anesthetics, antibiotics to mention but a few. Far more drugs were shown to display significant daily variations in their effects even after chronic application or constant infusion. Because circadian rhythms undergo maturation with development, drug therapy in children can/may also be modified by circadian time of drug dosing as shown for anticancer drugs. In conclusion, there is clear evidence that the dose/concentration-response relationship of drugs can be significantly dependent on the time of day. Thus, circadian time has to be taken into account as an important variable influencing a drug's pharmacokinetics and/or its effects or side effects.

Chronopharmacology of intravenous and oral modified release verapamil

AIMS

Using a stable isotope technique which allows simultaneous and differential measuring of orally and intravenously administered drugs we compared the pharmacokinetics and pharmacodynamics of unlabelled modified release verapamil p.o. (steady state) and deuterated verapamil i.v. (single dose) following morning and evening administration.

METHODS

Twelve female and 12 male healthy volunteers were studied in a randomized, crossover design. During the last day of each treatment period (day 6 and day 10) pharmacokinetics and pharmacodynamics (PR interval) of verapamil were assessed; 1 h before ingestion of a new R/S-verapamil 240 mg modified release formulation (08.00 h vs 20.00 h) a single dose of 10 mg d7-R/S-verapamil was administered intravenously. Serum levels of unlabelled and labelled R/S-verapamil were measured by gas chromatography/mass spectrometry. In selected samples of serum which were chosen at tmin,po and tmax,po the enantiomers were separated by chiral high-performance liquid chromatography in order to calculate R- to S-verapamil serum concentration ratios.

RESULTS

We observed no significant differences in pharmacokinetics (AUCpo, Cmax, tmax, CLo, F and R/S enantiomer ratio) between morning and evening treatment with modified release verapamil and there was no influence of time of dosing on mean prolongation of PR interval. AUCiv, CL, Vss and d7-R/d7-S enantiomer ratio following verapamil i.v. did not show circadian variation. t1/2 was slightly but statistically significantly increased after the morning infusion. PR-prolongation was significantly greater after verapamil i.v. in the morning than in the evening. The 90% confidence intervals of the differences between morning and evening administration in AUCpo, Cmax and AUCiv were within the equivalence range of 0.8-1.25.

CONCLUSIONS

Time of dosing has no significant influence on pharmacokinetics and pharmacodynamics of this new modified release formulation of verapamil. Circadian variation in presystemic metabolism of verapamil was not observed.

Chronopharmacological dependence of antihypertensive effects of the imidazoline-like drugs in stroke-prone spontaneously hypertensive rats

In the present study we investigated the chronopharmacological dependence of dose-dependent hypotensive and cardiochronotropic effects of the imidazoline-like drugs (clonidine, rilmenidine and moxonidine) in stroke-prone spontaneously hypertensive rats (SHR-SP), using radio-telemetric system (Data Sciences, USA). The 24-h blood pressure, heart rate and locomotor activity profiles showed peak values during the rats' active phase during the night period. The degree of hypotensive and bradycardic effects of all drugs were most evident at this time and occurred in the absence of a change in locomotor activity. These studies show that clonidine, rilmenidine and moxonidine decrease blood pressure and heart rate in a time-dependent manner in SHR-SP. It was demonstrated that the degree and duration of hypotensive action of imidazoline-like drugs vary with the time of drug administration.

24-hour blood pressure monitoring to evaluate the effects of nifedipine in pre-eclampsia and in chronic hypertension in pregnancy

OBJECTIVE

To investigate the effect of 7 to 14 days of therapy with nifedipine (sustained-release preparation) on the 24-hour blood pressure patterns of pregnant women with pre-eclampsia or chronic hypertension, and to test the utility of blood pressure monitoring in modulating the timing and dosage of the drug.

DESIGN

24-hour automatic blood pressure monitoring of pregnant women with pre-eclampsia or chronic hypertension before and after nifedipine treatment.

SETTING

Centre for Prevention, Diagnosis and Treatment of Hypertension in Pregnancy, University of Turin, Italy.

POPULATION

Sixteen pregnant women with pre-eclampsia and 17 with chronic hypertension.

METHODS

24-hour blood pressure monitoring was performed before the beginning of the therapy and after 7 to 14 days of treatment with sustained-release nifedipine.

MAIN OUTCOME MEASURES

Chronobiological analysis of systolic and diastolic blood pressure values was performed; MESOR, amplitude, acrophase, hyperbaric index, percent time elevation and significance of rhythm were calculated before and after treatment.

RESULTS

6336 blood pressure measurements were analysed. Systolic and diastolic MESOR values were significantly decreased after nifedipine treatment both in pre-eclampsia and in chronic hypertension. However, the antihypertensive effect of nifedipine in pre-eclampsia was especially pronounced during evening and night, while in chronic hypertension it was more constant during the 24-hour period. 24-hour blood pressure monitoring allowed adjustment, when necessary, to the timing and dosage of nifedipine in accordance with the blood pressure patterns of each patient, using the hyperbaric index and percent time elevation as objective parameters for the evaluation of treatment efficacy.

CONCLUSIONS

24-hour blood pressure monitoring is a good method to optimise treatment, and confirms that nifedipine is useful for the control of maternal blood pressure in pregnancy.

Drug effects on BP rhythm in secondary hypertension

Antihypertensive drug treatment is necessary in most patients with secondary hypertension. Only a small percentage of cases can be cured by operation or angioplasty. Because blunted or reversed nocturnal blood pressure fall is frequently found in patients with secondary hypertension, time of day of drug application is of special interest in these patients. The time structure of blood pressure over 24 hours or even longer can be obtained using ambulatory blood pressure monitoring. This is a prerequisite for individualization of antihypertensive therapy and for achievement of optimal drug effects. Chronopharmacologic aspects are playing an increasingly important role in the treatment of secondary hypertension, especially in renal forms. Recently, the first studies in this field were completed, isradipine and doxazosin in chronic renal failure and trandolapril in hypertensive diabetic subjects. Results show that restoration of normal nocturnal blood pressure fall and constitution of normal circadian rhythm are possible in patients with mild to moderate renal hypertension after evening dosing as compared to morning dosing with monotherapy. In more severe hypertension combination therapy including multiple dosing with special attention to evening application is necessary. In conclusion, optimal drug effects and individualization of antihypertensive treatment in patients with secondary forms of hypertension can be achieved using a stepped care program including chronopharmacologic regimens to restore normal circadian rhythm on a normotensive level.

Chronokinetics of liposomal encapsulated ampicillin in fasting and nonfasting rats

The chronobiological effect on the pharmacokinetics of liposomal encapsulated ampicillin (LEA) was compared at noon (1200) and midnight (2400) after intravenous injection of 50 mg/kg of LEA in rats. The effects of fasting on the circadian rhythms of LEA were also investigated. Serial blood samples were collected for 2 h. Bile and urine were collected during the entire study. Plasma data was analyzed by noncompartmental methods. Dosing of LEA at 2400, when the animals were active, resulted in a 50% decrease in mean residence time (MRT) and a 20% increase in systemic clearance (Cltotal). The increase in Cltotal was reflected by increases in biliary (Clbile) and renal (Clrenal) clearance at 2400. In addition, the steady-state volume of distribution (Vss) at 2400 was also decreased by about 50% as compared to dosing at 1200. Interestingly, no difference in the pharmacokinetic parameters were observed in fasting animals at 1200 and 2400. Since rats consume very little food during their sleep cycle, restriction of food intake did not have any affect on the pharmacokinetics of LEA at 1200. However, fasting rats had an approximately 36% decrease in Cltotal as compared to nonfasting rats at 2400. This decrease in systemic clearance was paralleled by a 60% and 24% decrease in Clbile and Clrenal, respectively. These variations could be attributed to changes in bile composition and/or lipoprotein concentrations in the plasma as a result of "forced" fasting at 2400 when the animals are generally more active and food intake is high.

Chronopharmacology of amlodipine in rats

The present study was undertaken to examine whether plasma concentrations of amlodipine, a calcium antagonist, and its diuretic effects vary with the time of dosage. Pharmacokinetic study; 20 mg/kg of amlodipine was given orally to rats at 10 am (day trial) or 10 pm (night trial), and blood samples were obtained during a 24-hour period. Pharmacodynamic study; two doses (10 and 20 mg/kg) of amlodipine were given orally at 10 am or 10 pm by a cross-over design, and urine was collected for 12 hours after dosage. Rats were maintained under condition of light from 7 am to 7 pm. The following results were obtained; The tmax of amlodipine was shorter and the Cmax was greater in the night trial than in the day trial. Its diuretic effects were greater in the night trial. These results suggest that the pharmacokinetic and pharmacodynamic profiles of amlodipine vary with its time of dosage.