Perifusion of rat pancreatic tissue in vitro: substrate modification of theophylline-induced biphasic insulin release.

The immunoreactive insulin (IRI) release patterns produced by continuous theophylline stimulation of rat pancreas have been defined, using an in vitro perfusion system. In the presence of glucose, citrate, and pyruvate at concentrations which were nonstimulatory by themselves, continuous stimulation with theophylline produced a biphasic IRI release profile. In the absence of substrate, continuous theophylline stimulation produced only an abrupt and limited primary response. Of the substrates tested, only glucose significantly enhanced this primary response. With increasing theophylline concentrations, whether in the presence or absence of substrate, significant increases were noted in the primary response as estimated by either the maximum rate of IRI release attained or by the total amount of IRI released during this time. Similarly, the secondary responses to theophylline increased with theophylline concentration in the presence of either citrate or pyruvate. With glucose as substrate, however, increasing theophylline concentrations from 2.5 to 5, then 10 mM produced a progressive reduction in both indices of the secondary response, which was inversely related to the primary response. These findings suggest that cyclic AMP not only mediates IRI release in quantitative terms but is also implicated in the qualitative nature of the response pattern. They also indicate a possible metabolic basis for biphasic IRI release, the acute or primary response being dependent upon the basal state of the cell and the availability of endogenous energy sources, the secondary response upon the availability of exogenous substrate.

Insulin and insulin inhibitor on amino acid transport in rat diaphragm in vivo.

Insulin stimulated the accumulation of alpha-aminoisobutyric acid by the rat diaphragm in vivo. This stimulation, but not the accumulation in the absence of insulin, was reduced by 30-40% by insulin inhibitor present in extracts of liver.

Density of guanine nucleotide-binding proteins in platelets of patients with major depression: increased abundance of the G alpha i2 subunit and down-regulation by antidepressant drug treatment.

The aim of this study was to quantitate the density of guanine nucleotide-binding (G) protein subunits (inhibitory G alpha i, stimulatory G alpha s, G alpha q/11, and G beta) in platelets of unipolar depressed patients to assess the status of these signal transduction proteins in depression and the effects of antidepressant drug treatment. Blood platelets were collected from 22 drug-free depressed patients and 22 age- and sex-matched healthy controls. The levels of the various G protein subunits were assessed by immunoblotting techniques. The immunoreactivity of G alpha 12 was increased (41%) and that of G alpha i3 decreased (25%) in platelets of depressed patients. The levels of other G protein subunits (G alpha s, G alpha q/11, G beta) did not change significantly with respect to those of control subjects. Chronic administration of cyclic antidepressant drugs (citalopram, clomipramine, imipramine) decreased the immunoreactivity of the up-regulated G alpha i2 protein (31%). Since platelet G alpha i2 is in line with the existence of supersensitivity of these receptors in major depression.

Renal failure, drug pharmacokinetics and drug action.

Patients with renal insufficiency often react abnormally to a number of drugs. Small doses that are safe under normal conditions may cause severe and even fatal side-effects. As a consequence, modification of the usual drug dosage of these drugs in required in renal insufficiency. Since the risk of retention concerns only those drugs which are mainly excreted by the kidney, it is possible to establish a mathematical relationship between glomerular filtration rate and the rate of drug elimination. These relationships serve as a basis for the determination of the proper dosage regimen for the individual patient. Such dosage adaptation for intermitten drug administration can be obtained by two methods and a series of compromises between them: (1) increase of the dosage interval without changing the dose, and (2) reduction of the does without changing the frequency of administration. One must however, not only consider inadequate drug elimination but also a number of other factors. Some of these modify the behaviour of the drug, such as hypoalbumineamia, which causes an increase of the unbound portion of the drug; anomalies of the volume of distribution, as found in patients with oedema; metabolic disturbance; alteration of absorption from the gastro-intestinal tract, etc. Other factors are related only indirectly to the pharmacokinetic behaviour of the drug. Frequently, there is an increased sensitivity to the undesirable side-effects of certain drugs in patients with renal insufficiency, causing the level of tolerance to be lowered compared with normal patients. Such an effect probably involves functional or morphological modifications of the drug receptors, or interaction with substance retained in renal insufficiency. Furthermore, drugs may accentuate the consequences of the nephropathy or have increased nephrotoxicity for those with diseased kidneys. It is with these important reservations that a critical analysis of the proposed methods of adapting drug dosage in renal insufficiency is presented. An appendix tabulates the effects of renal insufficiency on the behaviour of 117 drugs. Irrespective of the method used to calculate drug dosage, all patients with renal disease must be monitored closely, particularly for signs of unexpected drug toxicity.

Clinical pharmacokinetics of the third generation cephalosporins.

At the present time, the third generation cephalosporins that are already on the market or close to this point include cefsulodin, cefotaxime, cefoperazone, latamoxef, ceftriaxone, ceftazidime, ceftizoxime and cefotetan. Other newer compounds are also under development but have not been included in this review. None of the third generation compounds is suitable for oral administration and, accordingly, their pharmacokinetics have been studied only after intravenous and intramuscular administration. Microbiological assays and HPLC methods have been used for the measurement of plasma/serum, urine, bile and cerebrospinal fluid (CSF) concentrations. As found with cefotaxime, microbiological assays should only be used when the full metabolite spectrum of a particular drug is known, as otherwise, the presence of microbiologically active metabolites may lead to erroneous conclusions. Under normal conditions, the major route of elimination is via the kidneys for cefsulodin, latamoxef, ceftazidime, ceftizoxime and cefotetan. In contrast, cefoperazone is mainly eliminated in the bile, whereas cefotaxime and ceftriaxone depend both on the liver and the kidneys for their elimination. With the exception of ceftriaxone, which has a longer elimination half-life (i.e. around 8 hours), all the other third generation cephalosporins have a t1/2 ranging between 1.5 and 2.5 hours. Plasma protein binding is variable from one compound to another. However, the clinical relevance of this parameter is not clearly established since tissue penetration also depends on the relative affinity of the drug for tissue components. Third generation cephalosporins seem to penetrate adequately into the CSF and, thus pharmacokinetically appear to be appropriate agents for the treatment of meningitis. The degree of modification of pharmacokinetic parameters by renal insufficiency or hepatic diseases depends, as for other drugs, on the extent to which the compound is excreted via the kidneys or the liver. The third generation cephalosporins have been extensively studied under these conditions and recommendations for dosage modification in special circumstances are available for most of them. The pharmacokinetics of some third generation cephalosporins may be modified in neonates and elderly patients. Accordingly, their use at the extremes of age must be accompanied by a closer than usual clinical monitoring of the patient. From a clinical point of view, the third generation cephalosporins possess reliable pharmacokinetic properties.(ABSTRACT TRUNCATED AT 400 WORDS)

Antipsychotic drugs. Clinical pharmacokinetics of potential candidates for plasma concentration monitoring.

Antipsychotic drugs (neuroleptics) are candidates for plasma concentration monitoring, but not all agents have the same potential in this respect. The present review analyses the available data on the kinetics and metabolism of fluphenazine, perphenazine, thiothixene, flupenthixol, clopenthixol, haloperidol, pimozide, penfluridol, sulpiride and clozapine. Although some of the drugs described in this review have been in use for many years, knowledge of their pharmacokinetics is still only approximate. This is primarily because determination in biological fluids is not always feasible. Accordingly, analytical methods useful for pharmacokinetic studies or plasma concentration monitoring of these antipsychotic drugs are discussed. With the exception of sulpiride, all the neuroleptics reviewed share some basic pharmacokinetic properties: good gastrointestinal absorption but reduced systemic availability because of hepatic first-pass metabolism, high hepatic clearance and a large apparent volume of distribution leading to an apparent elimination half-life of about 24 hours for most of these compounds. The renal elimination is negligible and it seems that these drugs do not possess active metabolites. The pharmacokinetic properties of antipsychotic drugs are important for the inclusion of a set of drugs in a psychiatric institution where there is a possibility of drug concentration monitoring. In addition, the availability of a depot preparation is of importance. These factors are discussed in view of the experience made during the last years in the University Psychiatric Institutions of Geneva.

Clinical pharmacokinetics of clomipramine.

Clomipramine is a tricyclic antidepressant medication widely used in Western Europe. Its pharmacokinetics have been studied essentially in healthy volunteers. By combining published information obtained during observational studies, it has been possible to derive a fairly precise picture of the behaviour of both parent compound and main metabolite (demethyl-clomipramine) in humans. Clomipramine can be compared with amitriptyline or imipramine so far as its physicochemical properties are concerned. As a consequence, its pharmacokinetic profile is also similar to that observed for these 2 drugs. Clomipramine is well absorbed from the gastrointestinal tract, but undergoes an important first-pass metabolism to demethyl-clomipramine which is pharmacologically active and participates in both therapeutic and unwanted effects. Protein binding is high, and the apparent volume of distribution is very large (i.e. greater than 1000L). After reaching the systemic circulation, clomipramine is further biotransformed into demethyl-clomipramine, and both active principles are hydroxylated to metabolites which are further conjugated before being excreted in urine. Hydroxylation of parent drug and metabolite is under polymorphic genetic control by the same cytochrome P450 as debrisoquine and sparteine. The apparent elimination half-life of clomipramine is about 24h and that of demethyl-clomipramine, 96h. Accordingly, the time to reach steady-state for both active moieties is in general around 3 weeks. Various pathological or environmental factors influence the behaviour of clomipramine and demethyl-clomipramine. Patients genetically deficient in hydroxylation accumulate demethyl-clomipramine at high concentrations that can produce serious side effects and/or nonresponse. The same is true for the coadministration of neuroleptics, in particular phenothiazines. Smoking induces demethylation, whereas long term alcohol intake appears to reduce this metabolic pathway. Finally, age usually diminishes both demethylation and hydroxylation, leading to a lower daily dose of clomipramine in most elderly patients. Studies relating blood concentrations of clomipramine and demethyl-clomipramine are conflicting. However, analysis of the available information indicates that blood concentrations lower than 150 micrograms/L are usually associated with nonresponse, whereas those above 450 micrograms/L seldom lead to an improvement in the efficacy of therapy. As a consequence clomipramine, like the other tricyclics, is an antidepressant with a fairly narrow therapeutic range. This property, combined with a high interindividual variability, makes this class of drugs ideal candidates for blood concentration monitoring.

Pharmacokinetic optimisation of the treatment of psychosis.

Psychosis is a generic term covering (for the purposes of the present article)) schizophrenia, brief reactive psychoses and manic episodes. Traditionally, research has focused on the effect of antipsychotic agents on positive or productive symptoms such as hallucinations or delusions. More recently, attention has been focused on negative symptoms such as emotional withdrawal or impairment of social participation. Typical antipsychotic medications such as phenothiazines have little effect on these clinical manifestations. This has raised interest in atypical antipsychotics such as clozapine. Acute psychotic episodes are less difficult to treat than long term schizophrenic manifestations. Current research indicates that antipsychotics are effective only if a threshold concentration is reached, but that above a certain level, dose escalation is of no benefit to the patient. This implies the existence of an optimal therapeutic concentration range. Due to interindividual variability caused by age, genetic and interethnic factors or drug-drug interactions, antipsychotic plasma concentrations show a wide range of values for the same dosage regimen. This is why clinical pharmacokinetic principles and therapeutic drug monitoring are essential tools for dosage individualization. Clinical pharmacokinetics in therapeutics implies that the pharmacokinetic parameters of the medication under scrutiny are known. This is, however, not always the case with antipsychotics since, due to the difficulties encountered in conducting phase I studies in healthy volunteers with these substances, published data are not always complete.

Differences in kinetic properties of drugs: implications as to the selection of a particular drug for use in patients with renal failure with special emphasis on antibiotics and beta-adrenoceptor blocking agents.

Adjustment of dosage of a renally excreted drug (or active metabolite) for patients with severe renal failure still causes some difficulties. It is therefore helpful to select, within a given therapeutic group of drugs, a compound that is particularly safe and easy to use and, if possible, does not require adjustment of dosage. This is 'the drug of choice for renal patients'. Such a drug would ideally meet the following pharmacokinetic criteria: normal urinary excretion less than 30% of the administered dose, predominant biliary and intestinal removal, disposition essentially unaffected by parameters likely to be modified in renal failure (e.g. changes in serum proteins or fluid compartments: receptor sensitivity, etc), and pharmacokinetics not complicated by the formation of active or toxic metabolites that depend on urinary elimination. Above all, the drug should have a wide therapeutic margin and must be free of nephrotoxicity. Examples of drugs of choice for patients with impaired renal function are given for some important therapeutic groups and special emphasis is placed on antibiotics and beta-adrenoceptor blocking agents.

The use of population pharmacokinetics in drug development.

Currently, there is an increasing focus on the implementation of pharmacokinetic-pharmacodynamic (PK-PD) studies and modelling as essential tools for drug development. Strategies involving specifically the population approach, which are based on relatively recent statistical methodology (e.g. nonlinear mixed effects modelling, NONMEM) have been advocated for investigating pharmacokinetic and pharmacodynamic variability as well as dose-concentration-effect relationships. The present article outlines this approach, and discusses how it can be implemented within the framework of the studies currently performed as part of the clinical phases of new drug development. It also considers study design and performance, based on real-life experiences. Population approaches, if designed carefully and early, as part of the planning of the drug development programme, are expected to play a significant role at every phase of the programme and to contribute to providing information that is valuable for registration purposes. Statistical methodology and software are now widely available. However, practical issues such as integration of the population approach within existing protocols, quality control of the data, timing of laboratory and statistical analyses, as well as resource allocation, remain legitimate concerns to be considered in prospective studies.

Clomipramine metabolism. Model-based analysis of variability factors from drug monitoring data.

A steady-state model is here developed as a framework for the analysis of blood concentrations of clomipramine, obtained during routine drug monitoring. A model is proposed to account for its major metabolic pathways, hydroxylation and demethylation, including first-pass effect. Impaired hydroxylation capacity is shown to lead to a dramatic increase in the concentration of demethyl-clomipramine, with a concomitant moderate increase in that of the parent drug. Deficient demethylation capacity is associated with a reduced ratio of demethyl metabolite to parent drug. A nomogram is provided to allow easy determination of hydroxylation and demethylation capacities from routinely measured blood concentrations. Data from 150 patients are analysed in order to identify interindividual variability factors. Average pseudo-clearances, calculated from trough blood concentrations at steady-state, are 17 L/h for hydroxylation, 23 L/h for demethylation and 40 L/h for elimination of hydroxylated metabolites. Maximum to minimum ratios are 8, 27 and 11, respectively. The metabolising capacity through either process significantly decreases with increasing age, clearance estimates being 40 to 50% lower for patients 75 years or older than for those 40 years or younger. Tobacco smoking and chronic alcohol consumption induce and reduce the demethylation clearance, respectively. Inhibition of hydroxylation in the presence of phenothiazine comedication is also shown. Finally, small but significant differences according to sex are observed. Potential implications of the proposed model-based approach include adaptation of the dosage regimen to individual characteristics at the very beginning of antidepressant therapy, and early detection of patients with impaired metabolising capacities.

Therapeutic drug monitoring databases for postmarketing surveillance of drug-drug interactions.

Drug-drug interactions can be associated with patient morbidity due to either increased toxicity or a potentially ineffective concentration. Because interactions cannot always be anticipated during drug development and actual patients receiving a drug for therapeutic use often differ from those included in clinical trials, postmarketing surveillance is essential. Therapeutic drug monitoring (TDM) databases offer a unique opportunity in this respect. Prerequisites for TDM databases to provide valid information in a pharmacoepidemiological perspective include the following: precise description of exposure to the potentially interacting drugs; measurement of parent compound and active metabolites through accurate and precise analytical techniques; documentation of relevant patient characteristics that may act as confounding factors (e.g. gender, age, smoking habits); repeated assessments over time if possible; and sound pharmacokinetic framework for data selection, analysis and interpretation. The contribution of TDM to the documentation of drug-drug interactions takes advantage of different possible study designs, discussed on the basis of recently published studies. The single case report plays an important role as an alert signal. It is illustrated for a patient on long-term treatment, who displayed an unexpectedly high clozapine concentration after the introduction of ciprofloxacin comedication. The prospective on and off comedication panel study shows advantages in terms of carefully selected inclusion criteria and control of treatment modalities. A study of the thioridazine-fluvoxamine interaction is presented, with patients followed on thioridazine monotherapy, after introduction of fluvoxamine and after its discontinuation. The main advantage of the retrospective large-scale TDM database screen is representativeness of patients actually treated, whereas drawbacks are related to quality of data and suitability for valid interpretation. Such an approach is illustrated by a review of data collected over 10 years of routine TDM that allowed documenting induction of nortriptyline metabolism by carbamazepine and inhibition by several phenothiazines. Finally, population pharmacokinetics is well suited to observational data collected for TDM purpose, provided quality is ascertained. Focus is placed on interindividual variability and relationship between pharmacokinetic parameters and patient characteristics, including comedication. The population approach is discussed with respect to a study that documented a 32% increase of haloperidol clearance associated with anticonvulsant comedication, in addition to effects of age and bodyweight. Among factors to consider for improved effectiveness in the use of TDM databases for postmarketing surveillance of drug-drug interactions, integration of efficacy and safety data in future studies and communication of expert recommendations to prescribing physicians are essential.

Pharmacokinetics in the 1980s.

This article summarizes the achievements of pharmacokinetics and focuses on some areas of current interest. The aims, advantages, and drawbacks of different pharmacokinetic models are discussed. The differences and similarities of compartmental and clearance models, as well as those of other noncompartmental systems, are presented. "Variabilities" are currently the subject of major investigations. A new approach to the study of the behavior of drugs in renal and hepatic diseases has emerged, as has an interest in the combined influences of genetic and environmental factors on the kinetics of drugs. New pharmacokinetic concepts are being developed to analyze the numerous factors responsible for observed variability in drug kinetics and dynamics. These concepts include "population kinetics" and "integrated kinetic-dynamic" models. It appears that pharmacokinetics is now fully accepted as a scientific discipline necessary for the development of new and better drugs for the improved treatment of the individual patient.

Optimized release of dexamethasone and gentamicin from a soluble ocular insert for the treatment of external ophthalmic infections.

In the case of external ophthalmic infections, repeated instillations of antibiotics are required to reach therapeutic level, above the minimal inhibitory concentration (MIC). An additional administration of a corticosteroid is often needed, in order to limit the precorneal damages caused by the infection. However, repeated administration of a corticosteroid can increase intraocular pressure and thus lead to glaucoma. To overcome the disadvantages of separated and repeated instillations of two products and to avoid the side effects of dexamethasone, a soluble insert containing gentamicin sulfate and dexamethasone phosphate was developed. The new system ensures the concomitant release of the two drugs during the first 10 h of treatment, followed by an adequate concentration of gentamicin sulfate, above the MIC of 4.0 microgram ml-1, during 50 h, due to a combination of gentamicin sulfate with cellulose acetate phthalate, which reduces the solubility of gentamicin.

Influence of genetic variation in alcohol and aldehyde dehydrogenase on serotonin metabolism.

The influence of genetic variation in alcohol dehydrogenase (ADH; EC 1.1.1.1) and aldehyde dehydrogenase (ALDH; EC 1.2.1.3) on the metabolic pattern of serotonin (5-hydroxytryptamine, 5-HT) in humans was examined from the relative urinary concentrations of the end products 5-hydroxyindole-3-acetic acid (5-HIAA) and 5-hydroxytryptophol (5-HTOL). Healthy Caucasian (Swedish) and Oriental (Chinese) subjects were genotyped for ADH2, ADH3 and ALDH2 by a PCR/SSCP technique. The 5-HTOL/5-HIAA ratios ranged between 0.9-9.4 pmol/nmol (4.4 +/- 1.8, mean +/- SD, n = 143). No significant difference in the 5-HT metabolic pattern was observed between Caucasians and Orientals (4.3 +/- 1.8 and 4.4 +/- 1.8 pmol/nmol, respectively), nor between any of the ADH2, ADH3 and ALDH2 genotypes. Despite the modulatory effects of genetic variation of these enzymes on ethanol metabolism, the present results indicate that the individual isozyme composition of ADH2, ADH3 and ALDH2 is not important for the metabolic pattern of 5-HT.

Enantioselective analysis of methadone in saliva by liquid chromatography-mass spectrometry.

Saliva was tested and evaluated as a biological matrix for methadone (Mtd) monitoring. Conventional method using a narrow bore C18 column, and an enantioselective method using a narrow bore alpha1-acid glycoprotein column, were developed using liquid chromatography coupled with a mass spectromeric (MS) detector. After optimisation of MS conditions by flow injection analysis, selected ion monitoring detection was used to enhance sensitivity. The total Mtd concentration and the enantiomeric ratio in saliva were validated using an experimental design. The methods were applied to samples provided by heroin addicts undergoing a Mtd treatment. Results on total Mtd determination showed a very poor correlation between saliva and serum, whereas the enantiomeric ratios of Mtd gave a very good one.

Modelling during drug development.

With the advancement of both biological and computer sciences, new drug development faces the challenge to integrate a huge amount of knowledge accumulated from the very early quantitative structure-activity relationship investigations of the candidate molecule to the large scale clinical trials in patients. Whereas pharmacokinetics and pharmacodynamics are fields in which modelling has long demonstrated its value, its potential in many other areas of drug development has recently been the object of intensive scientific activity. The present review places emphasis on these newer applications; it includes the opinion of many experts in often highly specialised areas such as in vitro to in vivo extrapolation, toxicokinetics, non-continuous response models, population approaches and computer assisted simulation of clinical trials. It is most probable that in the near future many of these areas of research will be the objects of intensive and interesting developments. This will undoubtedly lead to improve developmental strategies for new drugs as well as more individualised pharmacological strategies for patients.

Cultural differences: implications on drug therapy and global drug development.

INTRODUCTORY REMARKS: Discussing the "inter-ethnicity" of kinetics and actions of drugs is fraught with terminology problems. It is, however, generally accepted that "ethnicity" covers the influences of factors genetically and culturally transmitted. The study of inter-ethnic variability in drug response addresses the problem of distinguishing variability factors that are common to one particular group of individuals from those which are not specifically shared. DIFFERENCES IN DAILY DOSES BETWEEN DIFFERENT GEOGRAPHIC REGIONS: It is well known that for a number of drugs, the daily dose prescribed in Japan is lower than in the US and Europe. Presently, independent surveys strongly indicate that for a majority of drugs dose differences are not the result of pharmacokinetic differences. In addition, they indicate that inter-ethnic differences do not seem to be larger than intra-ethnic variations. The differences observed for daily doses must thus be found elsewhere than in pharmacogenetic traits. DIFFERENCES IN DIAGNOSES: The most important impediments encountered in the evaluation of minority patients include differences with respect to language, communication style, cultural belief. The same problems arise if studies performed in different geographic areas are compared, socio-economic aspects play then an even greater role. Language problems arise differently if minorities are evaluated and compared to a majority of patients living in the same country or if clinical studies are performed in different regions. Communication styles also differ markedly between cultures. As an example, certain gestures may be considered as disrespectful or insulting by some ethnic groups and constitute normal behavior in others. RATING SCALES: Ethnicity clearly plays a role on the cross-cultural use of rating scales. Sophisticated rating scales established and validated in Western culture must undergo culturally sensitive revision and rigorous evaluation before their use in non-Western culture. EFFICACY-SAFETY ASSESSMENT: As an example, the assessment of risk and benefit is different in Japan, Europe and the United States. In Japan, safety is given a greater weighting relative to efficacy than in the two other regions. PLACEBO/NOCEBO EFFECTS: Placebo and nocebo effects are difficult to study, even in the absence of any cultural difference. They are even more so if ethnicity is concerned. PATIENT COMPLIANCE: Clinicians treating cross-cultural patients must carefully explore the beliefs held by their patient regarding illness causality and treatment expectations. CONCLUDING REMARKS: There are many unanswered questions in the field of inter-ethnic variability in drug response. The present overview will not pretend to have given specific answers, but it is hoped that it will point to some areas where more research is needed, in particular in the area of methodologies to take inter-ethnicity into account during drug development.

Alcohol flushing, patch test, and ADH and ALDH genotypes in Brazilian ethnic groups.

Self reports of flushing reaction after drinking, cutaneous sensitivity to alcohol (patch test), and genotypic determination of ADH2, ADH3, and ALDH2 were studied in 53 Brazilian volunteers of different ethnic groups. Genotypes were determined using single-strand conformation polymorphism in discontinuous buffer electrophoresis. Analysis of the results indicated several cases of a reported flushing reaction among ALDH2 1/1 individuals, while all but 2 cases of ALDH2 heterozygotes reported a flushing reaction. The latter subjects also had a negative result in the patch test. These preliminary results indicate that variability in the facial flushing reaction to alcohol seems to be a phenomenon resulting not only from the presence of a deficient ALDH2*2 allele, but also from other polymorphisms of alcohol-metabolizing enzymes.

Basis for dosing time-dependent changes in the ocular and systemic absorption of topically applied timolol.

The objective of the present study was to determine the basis for dosing time-dependent changes in the ocular and systemic absorption of topically applied timolol in pigmented rabbits. The gamma scintigraphic technique was used to monitor the changes in precorneal solution retention following instillation. Changes in timolol concentration in the plasma over 120 min and in various anterior segment eye tissues at 30 min following the topical instillation of 25 microliters of 0.65% timolol maleate solutions at various dosing times were monitored using reversed phase HPLC. Corneal and conjunctival permeability at various dosing times was evaluated in the modified Ussing chamber. The results indicated that precorneal solution drainage was slowest at noon. Suppressing tear production by anesthesia led to an increase in ocular timolol absorption at 6 a.m. but not at other dosing times, in spite of the lowest corneal permeability then. There was no statistically significant dosing time influence on systemic timolol absorption following nasal or conjunctival dosing. In conclusion, possible diurnal changes in precorneal solution clearance may be the main factor underlying the diurnal changes in ocular as well as systemic timolol absorption in rabbits. In addition, diurnal changes in corneal permeability may also contribute to diurnal changes in ocular timolol absorption.