A sensitive radioligand binding assay for timolol in plasma

Trimetazidine, a Metabolic Modulator, Has Cardiac and Extracardiac Benefits in Idiopathic Dilated Cardiomyopathy

Background— The anti-ischemic agent trimetazidine improves ejection fraction in heart failure that is hypothetically linked to inhibitory effects on cardiac free fatty acid (FFA) oxidation. However, FFA oxidation remains unmeasured in humans. We investigated the effects of trimetazidine on cardiac perfusion, efficiency of work, and FFA oxidation in idiopathic dilated cardiomyopathy.

Methods and Results— Nineteen nondiabetic patients with idiopathic dilated cardiomyopathy on standard medication were randomized to single-blind trimetazidine (n=12) or placebo (n=7) for 3 months. Myocardial perfusion, FFA, and total oxidative metabolism were measured using positron emission tomography with [ 15 O]H 2 O, [ 11 C]acetate, and [ 11 C]palmitate. Cardiac function was assessed echocardiographically; insulin sensitivity was assessed by the homeostasis model assessment index. Trimetazidine increased ejection fraction from 30.9±8.5% to 34.8±12% ( P =0.027 versus placebo). Myocardial FFA uptake was unchanged, and β-oxidation rate constant decreased only 10%. Myocardial perfusion, oxidative metabolism, and work efficiency remained unchanged. Trimetazidine decreased insulin resistance (glucose: 5.9±0.7 versus 5.5±0.6 mmol/L, P =0.047; insulin: 10±6.9 versus 7.6±3.6 mU/L, P =0.031; homeostasis model assessment index: 2.75±2.28 versus 1.89±1.06, P =0.027). The degree of β-blockade and trimetazidine interacted positively on ejection fraction. Plasma high-density lipoprotein concentrations increased 11% ( P <0.001).

Conclusions— In idiopathic dilated cardiomyopathy with heart failure, trimetazidine increased cardiac function and had both cardiac and extracardiac metabolic effects. Cardiac FFA oxidation modestly decreased and myocardial oxidative rate was unchanged, implying increased oxidation of glucose. Trimetazidine improved whole-body insulin sensitivity and glucose control in these insulin-resistant idiopathic dilated cardiomyopathy patients, thus hypothetically countering the myocardial damage of insulin resistance. Additionally, the trimetazidine-induced increase in ejection fraction was associated with greater β1-adrenoceptor occupancy, suggesting a synergistic mechanism.

Decreased Myocardial Free Fatty Acid Uptake in Patients With Idiopathic Dilated Cardiomyopathy: Evidence of Relationship With Insulin Resistance and Left Ventricular Dysfunction

BACKGROUND

Results on myocardial substrate metabolism in the failing heart have been contradictory. Insulin resistance, a common comorbidity in heart failure patients, and medical therapy may modify myocardial metabolism in complex fashions. Therefore, we characterized myocardial oxidative and free fatty acid (FFA) metabolism in patients with idiopathic dilated cardiomyopathy (IDCM) and investigated the contributions of insulin resistance and beta-blocker therapy.

METHODS AND RESULTS

Nineteen patients with IDCM (age 58 +/- 8 years, ejection fraction 33 +/- 8.8%) and 15 healthy controls underwent examination of myocardial blood perfusion, oxidative and FFA metabolism using positron emission tomography and [(15)O]H(2)O, [(11)C]acetate and [(11)C]palmitate, respectively. Echocardiography was used to assess myocardial function, work, and efficiency of forward work. Insulin resistance was calculated using the homeostasis model assessment index (HOMA index) and the degree of beta-blockade was estimated with a beta-adrenoceptor occupancy test. IDCM patients were characterized by decreased cardiac efficiency (35 +/- 2 versus 57 +/- 12 mm Hg.L.g(-1), P < .0001) and reduced myocardial FFA uptake (5.5 +/- 2.0 versus 6.4 +/- 1.2 mumol.100 g(-1).min(-1), P < .05), but the FFA beta-oxidation rate constant was not changed. In the patients, myocardial FFA uptake was inversely associated with left ventricular (LV) ejection fraction (r = -0.63, P < .01), indicating that further depression of LV function induces an opposite switch to greater FFA uptake. The FFA beta-oxidation rate constant correlated positively with the HOMA index (r = 0.53, P < .05). In patients on beta-1 selective beta-blockers, beta-1 adrenoceptor occupancy correlated inversely with LV work, oxidative metabolism, and FFA uptake; similar relationships were not found in patients on nonselective beta-blocker.

CONCLUSIONS

Myocardial FFA metabolism is reduced in patients with IDCM. However, when LV function is further depressed and insulin resistance manifested, myocardial FFA uptake and oxidation are, in turn, upregulated. These findings may partly explain the discrepancies between previous studies about cardiac metabolism in heart failure.

Bioavailability in the human eye of a fixed combination of latanoprost and timolol compared to monotherapy

The ocular pharmacokinetics of a single topical administration of a fixed combination (FC) of latanoprost 0.005% and timolol 0.5% was compared to the monotherapies of latanoprost and timolol in cataract surgery patients. The absorption rate of latanoprost and timolol into the aqueous humor was similar after administration of the FC compared to the two drugs given separately. The aqueous humor concentration of the acid of latanoprost tended to be higher 1-4 hours after administration of FC compared to latanoprost monotherapy. This resulted in an increased AUC. The Tmax and elimination half-life of both latanoprost and timolol were similar after administration of either FC or the two drugs given as monotherapy. Latanoprost did not have any influence on the ocular pharmacokinetics of timolol. The bioavailability of latanoprost and timolol into human aqueous humor, after FC, was at least as good as for the two drugs administered separately.

Plasma concentration of topically applied betaxolol in elderly glaucoma patients

Our aim was to study the concentration of betaxolol in plasma after its topical ocular use during the normal 12 hr dosing interval. Twenty microliters of betaxolol 0.5% solution were applied into both eyes of nine glaucoma patients, and the plasma concentrations of the drug were measured 12 hr thereafter using a radioreceptor assay. The same amount of betaxolol was then applied ocularly, and its concentration in plasma was measured at 5, 10, 15, 30 min and 1, 2, 4 and 8 hr thereafter. The mean (SD) concentration of betaxolol in plasma twelve hr after the first dose was 0.4 (0.2) ng/ml. After the second dose, the patients showed a biphasic concentration vs. time curve, the first peak occurring at 8 (4) min, and the second peak at 210 (132) min; the mean (SD) peak concentrations being 1.1 (0.3) and 2.0 (1.1) ng/ml, respectively. The area under the concentration vs. time curve showed a 4-fold variation among our patients. Topically applied betaxolol was rapidly absorbed into systemic circulation, and concentrations were detectable even at 12 hr. The interindividual variation in the systemic absorption of betaxolol was large.

Water-activated, pH-controlled patch in transdermal administration of timolol. II. Drug absorption and skin irritation

The feasibility of the water-activated, pH-controlled silicone reservoir devices for transdermal administration was investigated using timolol maleate as a model drug. Timolol patches were applied to the arm of 12 volunteers for 81 h, two patches per subject. Timolol absorption from patches was compared to that from a peroral timolol tablet formulation (Blocanol((R)) 10 mg). Furthermore, in vivo plasma levels of timolol were compared with those predicted by kinetic simulations. Skin irritation induced by timolol patches was assessed by visual scoring and color reflectance measurements. With water-activated, pH-controlled patches both steady-state concentrations of timolol in plasma and its duration could be controlled. However, a considerable, inter-individual variability in the transdermal absorption of timolol was observed. This is due to the high fractional skin control in timolol delivery. Timolol patches were well tolerated by subjects. Skin irritation induced by the combination of timolol with long-term occlusion was mild, and after removal of the patches, skin changes were practically reversed in 24 h. Simulation model was useful in prediction of timolol levels in plasma after transdermal administration.

Pharmacokinetics of topical beta-adrenergic antagonists in rabbit aqueous humor evaluated with the microdialysis method

The microdialysis method was used to evaluate the pharmacokinetics of the beta-adrenergic antagonists carteolol and timolol and the new ophthalmic solution WP-934 in rabbit aqueous humor, following instillation. A probe with a microdialysis membrane (length, 5 mm; diameter, 0.2 mm) was implanted in the anterior chamber of the pigmented rabbit and perfused with Ringer's solution. Twenty microliters of 0.5% timolol maleate (0.5% Timoptol(R)), 2% carteolol hydrochloride (2% Mikelan(R)), or a novel preparation of 0.5% timolol maleate (WP-934) that gels after instillation were then instilled. The concentrations of these drugs in dialysates were measured using high-performance liquid chromatography and an electrochemical detection system. In vitro relative recovery of the membrane with timolol and carteolol was approximately 17.5% and 21. 6%, respectively. Timolol and carteolol levels in aqueous humor increased rapidly after instillation of Timoptol and Mikelan and reached maximal levels (Cmax) within 60 minutes. The Cmax of carteolol (4.25 microg ml-1) was lower than that of timolol (5.52 microg ml-1), suggesting that the corneal permeability of timolol is higher than that of carteolol. After instillation of WP-934, the Cmax of timolol (12.32 microg ml-1) was 2.2-fold higher than that after instillation of Timoptol. However, t1/2 values of beta-adrenergic antagonists after instillation of the three preparations were not significantly different. These data suggest that the microdialysis technique is useful for continuous monitoring of aqueous levels of beta-blockers and for analysis of their pharmacokinetic parameters while requiring much fewer animals than conventional sampling with paracentesis.

Systemic side effects of topical beta-adrenergic blockers

Glaucoma, a disease that affects between 1 and 3% of the population above the age of 60, is most commonly treated by topical beta-adrenergic blockers. Although effective in lowering intraocular pressure and helping to preserve sight, beta blockers also may have adverse influences on the cardiac, pulmonary, and central nervous systems, and on endocrine functions. Clinicians' awareness that their patients may be treated with topical beta blockers will help them to elicit this information and the history, prescribe the medicine correctly, and be cognizant of a possible role this medicine may have in any deterioration of a patient's systemic clinical status.

Determination of alpha 1-adrenoceptor antagonists in plasma by radioreceptor assay

A simple, rapid and sensitive radioreceptor assay (RRA) for the quantification of alpha 1-adrenoceptor antagonists such as prazosin in plasma is described. The method involves the use of an RRA based on [3H]prazosin displacement in rat cerebral cortical membranes. The method is reliable, with intra-assay and inter-assay RSDs ranging from 5.9 to 9.2%. The limit of detection is 0.2 (prazosin hydrochloride), 0.05 (tamsulosin hydrochloride) and 0.3 (bunazosin hydrochloride) pmol per assay. Using this method the plasma levels of prazosin hydrochloride were determined in beagle dogs administered orally 2.39 mumol kg-1 of this drug. The plasma levels of prazosin in beagle dogs are in good agreement with those obtained using a high-performance liquid chromatography (HPLC). This RRA proved to be applicable to the monitoring of plasma prazosin levels in patients with essential hypertension and/or benign prostatic hypertrophy receiving therapy with this drug with the therapeutic dosage schedule. Thus, the concentrations of alpha 1-adrenoceptor antagonists in plasma can be adequately monitored by RRA as well as by HPLC.

Plasma kinetics and antagonist activity of topical ocular timolol in elderly patients

BACKGROUND

Systemic adverse effects of ocular timolol therapy are due to absorption of the drug from the eye into the systemic circulation. Elderly patients are frequently more susceptible to side effects than younger patients. This study was conducted to evaluate the plasma kinetics and antagonist activity of ocular timolol in elderly patients.

METHODS

Plasma kinetics and antagonist activity of timolol were studied in 12 patients scheduled for extracapsular cataract extraction and intraocular lens implantation. The patients received 40 microliters of 0.25% timolol into the lower cul-de-sacs of each eye. Blood samples were collected over a period of 12 h and plasma concentrations of timolol were analyzed using a radioreceptor assay. The corresponding ex vivo beta 1- and beta 2-receptor occupancies were calculated using radioligand binding techniques.

RESULTS

Timolol was absorbed rapidly into the systemic circulation and occupied on average up to 68% of beta 1-receptors and up to 87% of beta 2-receptors. The beta 1- and beta 2-receptor occupancy decreased slowly and was on average 38% and 64%, respectively, 12 h after the single dose. The calculated mean area under concentration-time curve of timolol in plasma was 10.28 ng/ml per hour and the mean half-life was 4.8 h. Both values were about twice as high as those found in healthy young volunteers following an intravenous 0.25-mg dose of timolol.

CONCLUSIONS

In elderly patients the beta-receptor antagonist effect of ocular timolol after a single dose is strong and long-lasting. This finding may explain the frequent reported systemic side effects of ophthalmic timolol.

Controlled ocular timolol delivery: systemic absorption and intraocular pressure effects in humans

Timolol eyedrops may cause systemic side-effects in glaucoma patients due to absorption of the drug into systemic circulation. In a previous study, timolol concentrations in plasma were reduced if timolol was administered in ocular inserts instead of eyedrops. We compared the intraocular pressure lowering effect and systemic absorption of timolol inserts to those of 0.5% timolol eyedrops in humans. Inserts of silicone tubing released 90.3 +/- 13.9 micrograms of timolol in 24 hours in vivo. Timolol inserts afforded similar decreases in intraocular pressure in open-angle glaucoma patients as did b.i.d. eyedrops, but produced lower peak timolol concentrations in plasma, 0.70 +/- 0.10 ng/ml and 0.24 +/- 0.05 ng/ml, respectively. After eyedrops, peak concentrations were achieved at 15.0 +/- 2.2 min, while application of an insert resulted in a delayed peak (tmax = 623 +/- 195 min). The insert resulted in a higher systemically absorbed fraction of the timolol dose than the eyedrop, but the peak timolol concentration and daily absorbed amount of timolol were decreased. The release rate of timolol from the inserts in vivo was only slightly less than that in vitro. Silicone devices are useful for clinical testing of controlled delivery properties of ocular drugs.

Receptor binding assays in analysing the bioavailability and pharmacodynamic bioequivalence of active drug moieties. A study of metoprolol

The bioavailability and pharmacodynamic bioequivalence of a conventional and an experimental sustained-release formulation of 100 mg metoprolol tartrate were studied in a randomised cross-over study in seven healthy volunteers by assessing over 24 h the plasma kinetics of R,S-metoprolol, its beta 1-adrenoceptor binding component, and by determining the extent to which the active drug moiety in plasma occupied rabbit lung beta 1- and rat reticulocyte beta 2-adrenoceptors. The formulations differed markedly in their kinetic characteristics: the peak plasma concentration (Cmax) of R,S-metoprolol after administration of the conventional formulation was 140 ng.ml-1, (n = 7) and it was approximately one-third of that after the sustained-release formulation, 49 ng.ml-1, (n = 6); the AUC0-24 h-values for the formulations were 700 and 310 ng.h.ml-1, respectively. The Cmax for the beta 1-adrenoceptor binding component of metoprolol was 180 ng.ml-1 (n = 7) after administration of the conventional, and 74 ng.ml-1 after administration of the sustained-release formulation. The corresponding AUC0-24 h-values for the receptor binding component were 920 and 470 ng.h.ml-1 (n = 7). Thus, the kinetic differences between R,S-metoprolol and the beta 1-receptor binding component were considerable and they were affected by the type of formulation. In general, after administration of the sustained-release formulation, the percentage beta 1- and beta 2-adrenoceptor occupancy of metoprolol in plasma was 5-15% less than after administration of the conventional formulation.(ABSTRACT TRUNCATED AT 250 WORDS)

Extent of beta 1- and beta 2-receptor occupancy in plasma assesses the antagonist activity of metoprolol, pindolol, and propranolol in the elderly

We estimated antagonist activity of metoprolol, pindolol, and propranolol in elderly cardiovascular patients by determining the extent to which the drugs occupied rabbit lung beta 1- and rat reticulocyte beta 2-adrenoceptors in plasma samples during drug treatment. The randomized, double-blind, crossover study was carried out by administering twice daily 100 mg metoprolol, 5 mg pindolol, and 80 mg propranolol for 7 days to 20 hypertensive subjects with a mean age of about 70 years. A 2-week interval was kept between administration of the different regimens. Receptor occupancy was measured at 1 hour before and 2 hours after administration of the last dose of each regimen by adding rabbit lung beta 1- and rat reticulocyte beta 2-receptors to plasma samples and by labeling the receptors with a radiolabeled beta-antagonist, (-)-[3H]CGP-12177. The results and conclusions were the following: (a) The extent to which metoprolol, pindolol, and propranolol occupied rabbit lung beta 1- and rat reticulocyte beta 2-adrenoceptors in plasma samples estimated accurately the intensity of beta-receptor antagonism in the patients who did not tolerate physiological and pharmacological tests measuring the degree of beta 1- and beta 2-adrenoceptor blockade. (b) The mean beta 1- and beta 2-receptor occupancy of pindolol and propranolol varied between 76% and 99% during the treatments. The mean beta 1-receptor occupancy of the metoprolol regimen varied between 54% and 92%, and its beta 2-receptor occupancy varied between 6% and 38%. Thus the antagonist activity of the metoprolol regimen differed significantly from that of the other regimens (ANOVA for repeated measures, p < 0.05 and 0.001, for the beta 1- and beta 2-occupancy, respectively). (c) The extent of beta 1- and beta 2-receptor occupancy in plasma samples was in conformity with the literature on the intensity, selectivity, and duration of beta-blockade after similar drug doses. (d) The data on the receptor occupancy of beta-blocking drugs in plasma samples appear to be valuable in analyzing their effects, and it may be a method for optimizing drug therapy for aged cardiovascular patients.

Absorption of ocular timolol: drug concentrations and beta-receptor binding activity in the aqueous humour of the treated and contralateral eye

We studied the ocular and systemic absorption of 40 microliters of topical 0.5% timolol in 57 patients using radioligand binding techniques. The mean concentration of timolol in aqueous humour of the treated eye was 1.9 +/- 0.8 micrograms/ml 74 minutes after instillation of the drug. About 18 h after drug instillation the aqueous humour concentration of timolol was 105.5 +/- 60.9 ng/ml. Timolol was found in 15 (42%) contralateral eyes. Concentration of timolol in the contralateral eye increased from 0.04 +/- 0.08 ng/ml at 50 min to 0.3 +/- 0.2 ng/ml at 134 min and was 0.2 +/- 0.4 ng/ml at 18 h after instillation. Timolol concentrations in the aqueous humour of the treated eye appeared to be high enough to occupy beta 1- and beta 2-receptors completely (100%) at 74 min and at 18 h after drug instillation. Timolol concentrations in the contralateral eye were high enough to occupy up to 33.0 +/- 24.7% of the beta 2-receptors and up to 51.7 +/- 35.1% of beta 2-receptors. High drug concentrations and complete beta-receptor occupancy in the aqueous humour of the treated eye after topical timolol are in agreement with the long-lasting ocular hypotensive effects. The low drug concentrations and partial receptor occupancy in the contralateral eye may also be of some clinical significance.

Beta 1- and beta 2-antagonist activity of topically applied betaxolol and timolol in the systemic circulation

The beta 1- and beta 2-antagonist activity of betaxolol and timolol in the systemic circulation was studied ex-vivo after their ocular administration in thirty patients during cataract surgery. The patients received 40 microliters of 0.5% betaxolol or 0.25% timolol into the lower cul-de-sacs of both eyes. Blood samples were collected up to four h after instillation of the doses. Plasma concentrations of betaxolol and timolol were analyzed using a radioreceptor assay. The ex-vivo-beta 1-and beta 2-receptor occupancies corresponding drug plasma levels were calculated using radioligand binding techniques. The extent of beta 1-receptor occupancy of betaxolol in the systemic circulation was less than 20% and its beta 2-receptor occupancy was negligible. The extent of beta 1-receptor occupancy of timolol was about 65% and its beta 2-receptor occupancy about 80%. Because receptor occupancy is the basis of antagonist activity of beta-blocking agents, this study shows that the beta 1-antagonist activity of betaxolol in the systemic circulation is much less than that of timolol, and that its beta 2-antagonist activity is negligible. The study suggests that the reported side effects of betaxolol in patients with obstructive pulmonary diseases are not mediated via its beta 2-receptor blocking properties.

Concentrations and antagonist activity of topically applied betaxolol in aqueous humour

Aqueous humour concentrations and antagonist activity of betaxolol were studied after ocular administration in forty-five patients scheduled for cataract surgery. The patients were randomly divided into five groups and received 40 microliters of 0.5% betaxolol into the lower cul-de-sac of one eye. In groups I, II, III and IV the drug was instilled 5-6, 12, 24 and 48 h, respectively, before surgery, into the eye to be operated, and in group V 4 h before surgery into the contralateral eye. Aqueous humour samples were aspirated at the beginning of the operation. Aqueous humour concentrations of betaxolol were analyzed using a radioreceptor assay, and the ex-vivo-beta 1- and beta 2-receptor occupancies of betaxolol were calculated. The highest concentration of betaxolol in aqueous humour was found 5-6 hours after instillation of the drug. Topical betaxolol was found to stay in aqueous humour for 48 h, a much longer time than the recommended interval of dosage. Betaxolol beta 1-receptor occupancy was 99-95% during the study, but also beta 2-receptor occupancy was significant (52%) 24 h after instillation of the drug. Because receptor occupancy is the basis of antagonist activity, the role of beta 2-receptor blocking effect of betaxolol in lowering intraocular pressure cannot be excluded.

Selectivity of acebutolol, atenolol, and metoprolol in healthy volunteers estimated by the extent the drugs occupy beta 2-receptors in the circulating plasma

The selectivity of acebutolol, atenolol, and metoprolol in healthy volunteers was estimated by determining the extent to which the drugs occupied beta 1-receptors of rabbit lung and beta 2-receptors of rat reticulocytes in the circulating plasma after drug intake. This ex vivo method had the advantage of including all drug components contributing to the drug-receptor equilibrium in vivo and of excluding the factors regulating organ sensitivity to catecholamine stimulation. The oral doses of 400 mg acebutolol, 100 mg atenolol, and 100 mg metoprolol were administered to six healthy male volunteers using a double-blind, randomized, and cross-over study design. The three drugs occupied beta 1-receptors to a similar extent at 2 hours after drug intake. The receptor fraction occupied by metoprolol at 3 to 8 hours after drug intake was usually smaller, however (analysis of variance for repeated measures, P < .05) than that of the other drugs. Acebutolol occupied significantly larger fractions of beta 2-receptors (analysis of variance for repeated measures, P < .05) than did atenolol and metoprolol. Therefore, at an identical beta 1-receptor occupancy, the beta 2-receptor occupancy of acebutolol was larger than that of the other agents. Apparently, active metabolites decreased markedly the selectivity of acebutolol, but not that of metoprolol. The receptor occupancy of the agents was well in agreement with the literature concerning the selectivity, intensity, and time-course of drug actions after identical doses.(ABSTRACT TRUNCATED AT 250 WORDS)

Receptor occupancy in lumbar CSF as a measure of the antagonist activity of atenolol, metoprolol and propranolol in the CNS

1. The antagonist activity of atenolol, metoprolol and propranolol in the CNS was estimated by determining the extent to which the drugs occupy animal beta 1- and beta 2-receptors in CSF ex vivo at the time of lumbar puncture. 2. Five CSF and plasma samples were obtained 4 h after drug intake from subjects treated for hypertension with atenolol, 100 mg once daily and five from subjects treated with metoprolol, 50 mg three times daily. Twenty-four samples were obtained 1, 2, 4 or 12 h after drug intake from subjects receiving a single 40 mg dose of propranolol. 3. The receptor occupancy in the samples was determined by adding beta 1-receptors of rabbit lung and beta 2-receptors of rat reticulocytes into the samples and labeling the receptors with a nonselective beta-adrenoceptor antagonist, (-)-[3H]-CGP-12177. 4. Atenolol and metoprolol occupied, as expected, larger fractions of beta 1- than beta 2-receptors in CSF and plasma samples. The receptor fraction occupied by atenolol in CSF was significantly (P < 0.05) lower than that occupied by metoprolol. The differences in occupancy between the drugs in plasma, however, were not statistically significant. 5. Propranolol occupied larger fractions of beta 2- than beta 1-receptors in the samples. Although propranolol concentrations in CSF were only 1/20-1/40 of those in plasma, the receptor occupancy of propranolol in CSF was similar to that in plasma.(ABSTRACT TRUNCATED AT 250 WORDS)

Plasma and aqueous humour concentrations and systemic effects of topical betaxolol and timolol in man

Plasma and aqueous humour concentrations and systemic effects of timolol and betaxolol were studied after ocular administration in 45 patients scheduled for extracapsular cataract extraction and intraocular lens implantation. The patients were divided randomly into three groups and received 40 microliters of either 0.5% betaxolol, 0.25% timolol or placebo into the lower cul-de-sacs of both eyes. Blood samples were collected over a period of 4 h and blood pressure and heart rate were monitored during the study. Aqueous humour samples were aspirated at the beginning of the operation. Plasma and aqueous humour concentrations of timolol and betaxolol were analyzed using a sensitive radioreceptor assay. The mean plasma concentrations of betaxolol were lower than those of timolol. The concentration of betaxolol in the aqueous humour was twice as high as the concentration of timolol. Both drugs produced a significant decrease in heart rate. In the timolol group a decrease in heart rate was found 15 min after drug administration, and in the betaxolol group after one hour.

Determination of the beta-adrenergic blocker timolol in plasma by liquid chromatography-atmospheric pressure chemical ionization mass spectrometry

A method based on LC-MS-MS has been developed for the determination of timolol in plasma using the (CD3)3-labelled species as the internal standard. Timolol is isolated from plasma by a simple solid-phase extraction and converted to its oxazolidin-2-one prior to analysis on a 50 x 4.6 mm reversed-phase high-performance liquid chromatography column packed with SynChropak, C18, 5 microns. The column eluate is passed by means of a heated nebulizer interface into a corona discharge atmospheric pressure chemical ionization source where the analyte and its internal standard are detected using multiple reaction monitoring (MRM). The very high specificity of this technique permits chromatographic run times of less than 2 min. The method has a lower quantifiable limit of 0.5 ng ml-1, with intra- and inter-day relative standard deviations less than 10%, and enables the determination of timolol in plasma after ocular administration to volunteers.