Timolol Concentrations Research Articles

Timolol Metabolism in Human Liver Microsomes Is Mediated Principally by CYP2D6

Timolol has mainly been used topically for the treatment of glaucoma. It has been suggested that the drug is metabolized by cytochrome P450 CYP2D6. The matter has not, however, been extensively studied. The aim here was to tentatively identify timolol metabolites and to determine the P450-associated metabolic and interaction properties of timolol in vitro. Four metabolites were identified, the most abundant being a hydroxy metabolite, M1. The K(m) value for the formation of M1 was 23.8 microM in human liver microsomes. Metabolism of timolol with recombinant P450s and correlation analysis have confirmed the conception that the drug is metabolized principally by CYP2D6, CYP2C19 being only a minor contributor (<10%) to the intrinsic microsomal clearance. The CYP2D6 inhibitor quinidine proved a potent competitive inhibitor of timolol metabolism, with an in vitro K(i) value of 0.08 microM. Fluvoxamine, an inhibitor of CYP2C19, inhibited timolol metabolism to a lesser extent, confirming its minor contribution. Timolol itself did not inhibit CYP2D6-catalyzed dextromethorphan O-demethylation. Judging from the disappearance of timolol in human liver homogenate, the in vivo half-life was extrapolated to be about 3 h, an estimate close to the half-life of about 2 to 5 h observed in vivo. In conclusion, the inhibition of timolol metabolism by quinidine should be taken into account when patients are treated with timolol. However, when plasma timolol concentrations in patients remain low (< or = 0.2 microg/l), it is suggested that such interaction is of minor clinical relevance.

Improved systemic safety and risk–benefit ratio of topical 0.1% timolol hydrogel compared with 0.5% timolol aqueous solution in the treatment of glaucoma

The purpose of the study was to compare the systemic safety and risk-benefit ratio of 0.1% timolol hydrogel and 0.5% aqueous timolol eye drops in the treatment of glaucoma. An 8-week randomised, double-blind, cross-over, multicentre study. A total of 25 patients with primary open-angle glaucoma, exfoliation glaucoma, or ocular hypertension was enrolled. After completing a wash-out period, patients were randomly chosen to receive either 0.1% timolol hydrogel once daily or 0.5% aqueous timolol eye drops twice daily. Intraocular pressure and heart rate during rest and exercise, head-up tilt test results, spirometry readings, and plasma concentrations of timolol were recorded. The risk-benefit ratio was determined by calculating the ratio between several heart rate endpoints and the change in intraocular pressure (IOP). The mean drug-induced change in the peak heart rate during exercise was -13.5 beats/min (SD 7.6) in the 0.5% aqueous timolol group and -5.1 beats/min (SD 6.7) in the 0.1% timolol hydrogel group (P<0.001; 95% CI 4.06-12.18). There was no significant difference in the IOP-reducing efficacy between these compounds. The risk-benefit ratio was significantly improved when 0.1% timolol hydrogel was used, compared with 0.5% aqueous timolol in the exercise test. In the head-up tilt test the risk-benefit ratio was significantly improved at rest (P<0.05), at 1 min (P<0.05) and at 5 min (P<0.001) after patients had received 0.1% timolol hydrogel. There were, however, no differences in spirometry readings. After patients had been treated with 0.1% timolol hydrogel, plasma concentrations of timolol were 1/6 (at peak) and 1/50 (at trough) of those of 0.5% aqueous timolol. Drug-induced changes in the peak heart rate, and head-up tilt test results as well as plasma concentrations of timolol, were significantly more pronounced after treatment with 0.5% aqueous timolol than with 0.1% timolol hydrogel. Because of the statistically similar IOP-reducing efficacy of these formulations the risk-benefit ratio was significantly improved when patients used 0.1% timolol hydrogel instead of 0.5% aqueous timolol.

Efficacy and systemic side‐effects of topical 0.5% timolol aqueous solution and 0.1% timolol hydrogel

The objective of this randomized, double-blind, controlled crossover trial was to compare 0.1% timolol hydrogel formulation eyedrops with 0.5% timolol aqueous solution in terms of systemic effects, hypotensive efficacy and pharmacodynamics. Twenty-four healthy subjects underwent careful ocular, cardiovascular and pulmonary function evaluation before and after 2 weeks of topical treatment with 0.1% timolol hydrogel or 0.5% aqueous timolol maleate. Intraocular pressure (IOP), heart rate, blood pressure, forced expiratory volume and plasma levels of timolol were measured. There was a statistically significant difference in the systemic absorption of timolol between these two ophthalmic timolol solutions. The peak concentration and mean area under the plasma drug concentration-time curve (AUC) were about 10-fold higher after 0.5% timolol aqueous solution. The mean peak heart rate during exercise was reduced by 19 bpm (SD 6.4 bpm) after 0.5% timolol aqueous solution and by only 4.6 bpm (SD 3.8 bpm) after 0.1% timolol hydrogel (p < 0.0001). There was no difference between the two formulations in efficacy in reducing IOP. No differences between treatments were found in respect of pulmonary function. The lower timolol concentration in the hydrogel vehicle and its better bioavailability resulted in reduced systemic absorption and side-effects without loss of efficacy.

Polymorphisms of genes CYP2D6, ADRB1 and GNAS1 in pharmacokinetics and systemic effects of ophthalmic timolol. A pilot study

To test the hypotheses that (1) CYP2D6 genotype is associated with pharmacokinetics of ophthalmic timolol and (2) variation in genotypes of ADRB1 (beta(1)-adrenoceptor) and GNAS1 (alpha-subunit of G-protein) modulate heart rate (HR), and systolic (SAP) and diastolic (DAP) arterial pressure responses to timolol. Nineteen glaucoma patients and eighteen healthy volunteers were treated with 0.5% aqueous and 0.1% hydrogel formulations of ophthalmic timolol using a randomised cross-over design. The participants conducted head-up tilt and maximum exercise test at four visits. Plasma concentration of timolol was measured twice for glaucoma patients and ten times for healthy volunteers on each visit. Also, the genotypes for CYP2D6, ADRB1 and GNAS1 were determined. Among healthy volunteers using aqueous timolol, poor metabolisers (PMs, n=2) of CYP2D6 had higher maximum plasma concentrations (C(max), values 2.63 and 2.94 ng/ml), longer elimination half-lives ( T(1/2), 5.49 and 6.75 h), and higher area-under-curve (AUC, 19.54 and 23.25 ng.h/ml) than intermediate [IMs, n=6, mean+/-SD 1.73+/-0.59 ng/ml (not significant), 3.30+/-0.48 h, 11.32+/-3.72 ng.h/ml], extensive (EMs, n=8, 1.60+/-0.72 ng/ml, 3.24+/-1.24 h, 8.52+/-6.12 ng.h/ml) and ultra-rapid (UMs, n=2, values 1.23 and 1.67 ng/ml, 2.22 and 2.52 h, 6.16 and 6.94 ng.h/ml) metabolisers. The IMs, EMs and UMs did not differ from each other for any of the kinetic variables. Also, the elevation of HR from rest to maximum level tended to differ between PMs and IMs, and between PMs and UMs. The pharmacokinetics and pharmacodynamics between the CYP2D6 groups did not differ with statistical significance when hydrogel timolol was used. Upon head-up tilt, the Ser49 homozygotes (n=26) had higher SAP (P=0.03) and DAP (P<0.01) than the Gly carriers (n=11). The change in DAP from rest to maximum during exercise was lower (P<0.01) in subjects with CC alleles of GNAS1 (n=13) than those with at least one T allele (n=24). The CYP2D6 poor metabolisers may be more prone to systemic adverse events with aqueous timolol than extensive metabolisers. Since CYP2D6 genotyping is not routine clinical practice, using 0.1% timolol hydrogel instead of 0.5% aqueous preparation will increase patient safety.

Association between low plasma levels of ophthalmic timolol and haemodynamics in glaucoma patients

The aims of the study were to assess the correlation between the plasma concentration of ophthalmic timolol and cardiovascular parameters, and the influence of timolol on advanced haemodynamic variables, such as stroke (SI), cardiac (CI) and systemic vascular resistance (SVRI) indices and arterial pulse wave velocity (PWV). Twenty-five glaucoma or ocular hypertensive patients were treated with 0.5% aqueous and 0.1% hydrogel formulations of timolol using a randomised, double-masked, crossover, multicentre design. All the patients were subjected to passive head-up tilt, electrocardiography, exercise test and measurement of plasma concentration of timolol. In the analysis, the data on the two treatments were combined, and the Spearman correlation coefficients between the plasma level of timolol and physiological effects were calculated. During the head-up tilt test before rising the bed up, the resting heart rate (HR; R=-0.52, P=0.001) and PWV (R=-0.34, P=0.04) were inversely correlated with timolol level. In the upright position, ophthalmic timolol effectively suppressed the rise in HR (R=-0.36, P=0.03). The SI did not change with timolol concentration, while CI diminished as timolol concentration rose (R=-0.39, P=0.02). The SVRI correlated with timolol concentration (R=0.38, P=0.02). In the exercise test, correlation between HR and plasma level of timolol steadily grew stronger as the load increased, reaching R=-0.60 (P<0.0001) at the maximum load. Systolic and diastolic arterial pressures were not associated with the timolol concentration. The plasma concentration of ophthalmic timolol correlates with several haemodynamic effects. As HR decreases, SVRI increases and blood pressure is kept unchanged.

Change in Corneal Permeability to Timolol After Laser In Situ Keratomileusis and Photorefractive Keratectomy in Rabbit

To evaluate the change in corneal permeability to timolol 1 month after laser in situ keratomileusis (LASIK) or photorefractive keratectomy (PRK) in rabbits. LASIK or PRK was performed on one eye of each of 20 rabbits. One month later, 20 microl of 0.5% timolol was instilled into the 10 LASIK eyes, 10 PRK eyes, and the 20 control contralateral eyes. One hour later, 150 microl of aqueous humor was collected from all eyes, and the concentration of timolol in the aqueous humor was measured by high-performance liquid chromatography. The effect of LASIK and PRK on the corneal permeability to timolol was analyzed. The average timolol concentrations in the aqueous humor of the ten eyes 1 month after LASIK and in the ten contralateral eyes were 1.16 +/- 0.58 microg/ml and 1.07 +/- 0.39 microg/ml (average +/- SD, P = 0.678), respectively. The average timolol concentrations in the aqueous humor of the ten eyes 1 month after PRK and in the ten contralateral eyes were 1.24 +/- 0.43 microg/ml and 1.12 +/- 0.30 microg/ml (P = 0.483), respectively. The average ratio of timolol concentration in the aqueous humor of the LASIK or PRK eyes 1 month after surgery to that of the contralateral normal eyes was 1.18 +/- 0.58 or 1.19 +/- 0.67 (P = 0.972), respectively. LASIK and PRK do not affect corneal permeability to timolol 1 month after surgery.

Pharmacokinetic and Pharmacodynamic Analysis of Systemic Effect of Topically Applied Timolol Maleate Ophthalmic Gelling Vehicle (Rysmon® TG)

Purpose: The objective of this study was to evaluate the degree of systemic absorption and the systemic side effect after instillation of timolol maleate ophthalmic gelling vehicle in human. Methods: A volunteer study was employed, and a randomized crossover design with the two phases was used. In one phase, the volunteers instilled a single drop of the 0.5% timolol maleate ophthalmic gelling vehicle; in the other phase, the volunteers instilled a single drop of the 0.5% timolol maleate ophthalmic solution. The plasma concentration of timolol and the heart rates were studied during the following 120 min and 60 min, respectively. Results: The area under the blood concentration time curve (AUC) in timolol maleate ophthalmic gelling vehicle was lower than that in timolol maleate ophthalmic solution (p < 0.05). No differences were observed in heart rates between ophthalmic gelling vehicle and ophthalmic solution. The correlation between the calculated occupancy of beta-adrenergic receptors and the systemic side effects after instillation could be successfully analyzed with a pharmacokinetic and pharmacodynamic model, showing the predictability of the model for the systemic side effects of timolol. Conclusions: The result of our analysis clearly shows that timolol maleate ophthalmic gelling vehicle reduced the systemic absorption below that of ophthalmic solution, but the degree in difference of systemic effects was negligible.

Pharmacokinetics of timolol in aqueous humor sampled by microdialysis after topical administration of thermosetting gels

In order to develop a thermosetting gel-based formulation, the ocular pharmacokinetics of timolol was studied utilizing microdialysis sampling technique after topical administration. A linear microdialysis probe was characterized and implanted in the anterior chamber of a rabbit. Dialysate samples collected from the aqueous humor (AH) were directly injected into the HPLC system without any pre-treatment and no interference was observed in the blank sample. The measured in vitro recovery of the probe was 57.67%; however, the in vivo recovery significantly decreased to 16.78% when assessed by the retrodialysis method, which was used to calculate the timolol concentration in AH. Although in the initial 15 min the drug concentrations in AH were comparable to that of the timolol solution, increased Cmax and significantly improved ocular bioavailability were obtained for the gel. When sodium deoxycholate (DC) was incorporated in the gel as a penetration enhancer, a 2-fold increment in the ocular bioavailability was achieved with an increased Cmax and significantly suspended Tmax. The results demonstrated that microdialysis coupled to HPLC is a powerful tool to investigate the ocular pharmacokinetic, and hence facilitates the design of ophthalmic formulations.

Inhibition of endothelin-1 and KCL-induced increase of [CA2+]i by antiglaucoma drugs in cultured A7r5 vascular smooth-muscle cells.

Over contraction of vascular smooth muscle may result in ischemia to ocular neuronal cells and deteriorate the glaucoma. The purpose of this study was to investigate the inhibitory effects of various commercial antiglaucoma drugs including brimonidine, dipivefrin, betaxolol, timolol, levobunolol, carteolol, brinzolamide, dorzolamide, unoprostone, latanoprost, pilocarpine, and preservative benzalkonium chloride on endothelin-1(ET-1) and KCl-induced increase of intracellular free Ca2+ ([Ca2+]i) in cultured rat A7r5 vascular smooth muscle cells. These drugs were diluted from original concentrations to 1/100, 1/1000, and 1/10000. [Ca2+]i mobility was analyzed by spectrofluorometry after loading with fura-2-AM. Betaxolol, timolol, levobunolol, and carteolol were found to inhibit KCl-induced release of [Ca2+]i in a dose-dependent manner. High concentrations of betaxolol, timolol, levobunolol, carteolol, and unoprostone also inhibited ET-1-induced increase of [Ca2+]i in A7r5 cells. However, ET-1- and KCl-induced increase of [Ca2+]i was not diminished by other drugs including brimonidine, dipivefrin, brinzolamide, dorzolamide, latanoprost, pilocarpine, and benzalkonium chloride. These results indicate that high concentrations of unoprostone and beta-adrenergic blocking agents including betaxolol, timolol, levobunolol, and carteolol may inhibit ET-1-induced increase of [Ca2+]i. The mechanism may be mediated by inhibition of extracellular calcium influx via blocking of L-type voltage-dependent Ca2+ channel in A7r5 cells.

Ocular release of timolol from molecularly imprinted soft contact lenses

The aim of this study was to evaluate “in vivo” the usefulness of molecular imprinting technology to obtain therapeutic soft contact lenses capable of prolonging the permanence of timolol in the precorneal area, compared to conventional contact lenses and eyedrops. Soft contact lenses (diameter 14 mm, center thickness 0.08 mm) consisted of N, N-diethylacrylamide (DEAA; main component of the matrix), methacrylic acid (MAA; functional monomer) and ethylene glycol dimethacrylate (EGDMA; cross-linker) were prepared by the conventional methodology (non-imprinted) or by applying a molecular imprinting technique using timolol as the template (imprinted ones). After washing and reloading, timolol release studies carried out in rabbits showed that the soft contact lenses made by the molecular imprinting method (34 μg dose) provided measurable timolol concentrations in the tear fluid for 2.0- and 3.0-fold longer than the non-imprinted contact lenses (21 μg dose) and eyedrops (doses of 34 and 125 μg), respectively. Furthermore, the area under the timolol concentration–time curve (AUC) was 3.3- and 8.7-fold greater for imprinted contact lenses than non-imprinted contact lenses and eyedrops, respectively. The timolol concentration of the eyedrops did not affect the precorneal residence time of drug significantly. On the other hand, timolol loading capacity of the contact lenses was improved by the molecular imprinting method; the sustaining of the drug levels in the tear fluid being proportional to the loading capacity of the contact lenses. These results indicate that imprinted soft contact lenses are promising drug devices able to provide greater and more sustained drug concentrations in tear fluid with lower doses than conventional eyedrops.

In vitro and in vivo characterization of high specific activity S-(−)[ 11C]CGP-12177, a radioligand for β-adrenoreceptor, in rats

Background: S-(−)[ 11C]CGP-12177 is reported to be a useful PET ligand for β-adrenoreceptor in the heart and lung. Its pharmacokinetic properties, however, have not been fully examined due to difficulty in S-(−)[ 11C]CGP-12177 production. Recently, we have developed a simple and improved synthesis of S-(−)[ 11C]CGP-12177 with high yields and high specific activity. The aim of this study is to determine pharmacokinetic properties of high specific activity S-(−)[ 11C]CGP-12177 in vitro and in vivo using rats. Methods: In vitro studies: S-(−)[ 11C]CGP-12177 (final concentration, 1 nM) with various concentrations of (±)CGP-12177, (±)propranolol, or (−)timolol was added in vitro to rat myocardial slices ( n=4 in each condition). Incubation was performed at 30 °C for 90 min. The radioactivity in the myocardial slices was determined at the end of incubation. In vivo studies: Control and (±)propranolol-pretreated (7 μmol/kg, i.v., 5 min prior to S-(−)[ 11C]CGP-12177 injection) rats ( n=4 in each group) were given S-(−)[ 11C]CGP-12177 (0.4 nmol/kg) intravenously. Radioactivity in tissues was measured at 20 min after injection of S-(−)[ 11C]CGP-12177. Results: In vitro studies: Uptake of S-(−)[ 11C]CGP-12177 in the myocardial slices was significantly decreased, depending on the concentration of the β-adrenoreceptor antagonists. S-(−)[ 11C]CGP-12177 uptake was 26%, 31%, and 36% of the control value, respectively, at 5×10 −6 M (±)CGP-12177, (±)propranolol, and (−)timolol. In vivo studies: In the control group, uptake of S-(−)[ 11C]CGP-12177 was highest in the lung (10.3±0.5% ID/g), followed in decreasing order by the heart (2.4±0.1 %ID/g), kidneys, and liver. Pretreatment with (±)propranolol significantly reduced S-(−)[ 11C]CGP-12177 uptake in the lung and heart, to 11% and 17% of the control value, respectively ( p<0.0001 for both tissues). Conclusions: The accumulation of S-(−)[ 11C]CGP-12177 in the heart and lung can be largely ascribed to the specific binding of the compound to β-adrenoreceptors, providing further evidence for the feasibility of high specific activity S-(−)[ 11C]CGP-12177 as an imaging agent for β-adrenoreceptors.

Improvement of the ocular bioavailability of timolol by sorbic acid

The ocular bioavailability of timolol increased in sorbic acid solution due to ion pair formation. Its octanol/water partition coefficient also increased, suggesting the formation of a more lipophilic complex. The concentration of timolol in rabbit aqueous humor was determined after instillation of timolol ophthalmic solution containing sorbic acid. When the molar ratio of sorbic acid to timolol was two or higher, the concentration of timolol in the aqueous humor was higher than with timolol alone. In the presence of sorbic acid the maximal aqueous humor concentration and the area under the curve were more than two-fold higher than those of Timoptol ®, a timolol maleate ophthalmic solution, and similar in value to TIMOPTIC-XE ®, a gel-forming ophthalmic solution. To investigate the transcorneal absorption mechanism, in vitro permeation profiles across the intact and de-epithelialyzed cornea were analyzed on the basis of the bilayer diffusion model. The partition coefficient in the epithelium was about twice as high in the presence of sorbic acid than with timolol alone, although the diffusion coefficient in the epithelium did not change. We conclude that the improved ocular bioavailability in the presence of sorbic acid is due to increased partitioning of timolol in the corneal epithelium.

Characterization of Intrinsic Sympathomimetic Activity of Carteolol in Rat Cardiovascular Preparations

We evaluated in vitro, in myocardial and vascular preparations isolated from reserpine-treated rats, the intrinsic sympathomimetic activity (ISA) of carteolol, a β1/β2-adrenoceptor blocking agent used in cardiovascular and non-cardiovascular diseases. In spontaneously beating atria, carteolol, at low concentrations (0.01 and 0.1 μM), antagonized the positive inotropic effect of isoprenaline, whereas at higher concentrations (1 μM to 1 mM), it caused an increase in the force of contraction (EC50: 4.6 ± 0.1 μM, Emax: 17.1 ± 1.1%, with respect to the maximum isoprenaline response) and a slight increase (7.8 ± 1.9% over basal values) in the heart rate. The positive inotropic effect of carteolol was abolished by concentrations of propranolol or timolol (10 μM) much higher than those blocking isoprenaline effects in the same preparations. A similar positive inotropic effect was also observed in electrically driven left atrium and in Langendorff perfused hearts. Functional and biochemical evidences supported the involvement of cAMP in the cardiac action of carteolol. In peripheral arteries (femoral and tail) pre-contracted with phenylephrine, carteolol exerted ISA-related relaxing effects, independent of the presence of endothelium and sensitive to high concentrations (10 μM) of conventional β-blockers. On the basis of these results, we propose to categorize carteolol as a non-conventional partial agonist of both cardiac and vascular β-adrenoceptors.

Timolol concentrations in rat ocular tissues and plasma after topical and intraperitoneal dosing.

Topical beta-blockers, such as timolol, have been used extensively in the medical treatment of glaucoma to lower intraocular pressure (IOP). Recently, these drugs have been shown to have effects on the retinal and optic nerve circulation as well as potential neuroprotective properties. In the current study, the concentration of timolol attained in the cornea, iris-ciliary body, retina, vitreous, and plasma was measured after topical or intraperitoneal administration in rats to determine the relative contributions of each route to intraocular timolol concentrations. One group of rats received one drop of commercially available 0.5% timolol in the right eye and two drops in the left eye for 3 to 12 days. Another group of rats received one drop of 0.5% timolol in one eye only and concentrations were studied in the ocular tissues at 15, 30, 60, 120, and 240 minutes after instillation. The final group of rats received a single intraperitoneal injection of timolol ranging in concentration from 5 to 75 mg/kg after which tissue and plasma concentrations were measured 30 minutes after injection. All tissue and plasma concentrations were measured by high performance liquid chromatography. Rats that received topical timolol daily for 3 to 12 consecutive days accumulated timolol concentrations of 2.3 to 4.4 microg/g in cornea, 198 to 326 microg/g in iris, 0.05 to 0.11 microg/ml in vitreous, and 0.17 to 0.77 microg/g in retina. In rats that received a single drop of timolol in one eye, the tissue concentrations were higher in the treated eye than in the untreated eye in all cases except for vitreous. In these experiments, timolol levels in plasma were either low or not detectable. Increasing timolol doses administered intraperitoneally resulted in corresponding increased tissue and plasma concentrations. Absorption of drug into the systemic circulation plays a significant role in delivering timolol to the retina and vitreous in addition to a local ocular route. A clear dose-response relationship exists in all ocular tissues studied after an intraperitoneal dose of timolol. High doses of timolol were required to achieve measurable concentrations of drug in the ocular tissues via our high performance liquid chromatography assay suggesting that a significant hepatic first-pass effect may be involved after an intraperitoneal injection of timolol.

Efficacy and tolerability of timolol maleate ophthalmic gel-forming solution versus timolol ophthalmic solution in adults with open-angle glaucoma or ocular hypertension: a six-month, double-masked, multicenter study

Background: Timolol has been formulated in a highly purified gellan gum to improve its duration of action. The efficacy of this formulation in short-term studies using once-daily dosing has been reported. Objective: The purpose of this study was to evaluate the efficacy and tolerability of 0.5% timolol maleate ophthalmic gel-forming solution (timolol GS) given once daily versus 0.5% timolol solution given twice daily in a long-term trial. Methods: This was a multicenter, double-masked, 6-month trial. After a washout of ocular hypotensive medication, 286 patients with open-angle glaucoma or ocular hypertension were randomly assigned in a 2:1 ratio to receive 0.5% timolol GS in both eyes once daily or 0.5% timolol solution in both eyes twice daily. All patients received a morning (9 am) and evening (9 pm) dose. For patients in the timolol GS group, the evening doses consisted of a vehicle only, whereas for patients in the timolol solution group, both doses consisted of active drug. Intraocular pressure (IOP) was measured at trough (before morning instillation) and peak (2 hours after instillation) at follow-up examinations at weeks 2, 4, 8, 12, and 24. Adverse events were monitored using patient reports. Results: Of the 286 patients randomized, 191 received timolol GS and 95 received timolol solution. Ninety-three percent of patients ( 265 286 ) completed the study. At the end of the treatment interval (week 24), the mean decrease in IOP at trough ranged from 5.6 to 5.9 mm Hg in the timolol GS group and from 6.3 to 6.6 mm Hg in the timolol solution group. Similar efficacy was observed at 11 am (peak). At week 24, the difference in mean IOP between treatments was −0.61 mm Hg (95% CI −1.44 to 0.22) at trough and −0.79 mm Hg (95% CI −1.77 to 0.20) at peak, indicating no significant difference between the 2 timolol formulations. The number of reports of blurred vision and tearing was significantly higher in the timolol GS group than in the timolol solution group ( P = 0.04), whereas burning/stinging was reported more frequently in the timolol solution group than in the timolol GS group ( P = 0.04). At week 12, the decrease in mean heart rate at trough (hour 0) was significantly less for patients in the timolol GS group than for those in the timolol solution group (−1.1 vs −4.2 bpm; P = 0.024). At week 24 (hour 0), the decrease in mean heart rate was less for patients treated with timolol GS by 2.5 bpm ( P = 0.051). The heart rate data at peak (hour 2) was similar to that observed at trough at week 12 (−2.7 vs −5.7 bpm; P = 0.006) and week 24 (−3.1 vs −4.7 bpm; P = 0.063). The mean change in blood pressure was not significantly different between treatments. There were no clinically significant differences between the groups in visual acuity, biomicroscopy and ophthalmoscopy results, or visual fields. Conclusions: Timolol 0.5% GS administered once daily was shown to be as effective in lowering IOP as the equivalent concentration of timolol 0.5% solution administered twice daily in patients with ocular hypertension or open-angle glaucoma.

Enhanced ocular hypotensive response to timolol in rabbits with prior dexamethasone treatment.

The purpose of this study was to determine glucocorticoid modulation of ocular pressure to timolol applied topically to rabbit eyes that were pretreated with dexamethasone. Rabbits were pretreated with five applications of topical 0.007% of dexamethasone (0.01% dexamethasone phosphate) or saline drops, administered at ten min intervals. The eyes were then treated with timolol maleate drops at concentrations of free base of 0.00007%, 0.0007%, 0.007% and 0.07%. An additional group of rabbits received dexamethasone pre-treatment only. Intraocular pressure was measured for the next four hr. Enhanced lowering of intraocular pressure was observed with dexamethasone pretreatment. Rabbits receiving the smaller dose of timolol had the least decrease in pressure. The most significant decreases in pressures occurred at 45 min after the administration of timolol with an average difference of 4.8 mm Hg between the timolol- and dexamethasone/timolol-treated eyes for the three largest concentrations of timolol. This synergism between glucocorticoids and beta-adrenergic blockers in lowering IOP may be potentially useful in the therapy of ocular hypertension and glaucoma.

Xyloglucan as a novel vehicle for timolol: pharmacokinetics and pressure lowering activity in rabbits.

This study was aimed at verifying the performances of a mucoadhesive polysaccharide from tamarind seed (xyloglucan or TSP, tamarind seed polysaccharide) as an adjuvant for ophthalmic vehicles containing timolol. Three formulations (one experimental vehicle based on TSP and two reference commercial eye drops) containing 5 mg/ml timolol base equivalents were administered to the eyes of pigmented rabbits. Drug concentrations in tear fluid, cornea, iris-ciliary body, aqueous humor and plasma were determined, as well as intraocular pressure. The polymer under investigation, in spite of a comparatively low viscosity, produced high timolol concentrations in the ocular tissues and a low systemic absorption. The performances of the TSP vehicle were comparable to those of a reference "in situ" gelling formulation (Timoptic XE). The results point to TSP as a potentially useful adjuvant for ophthalmic delivery systems.

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 ® 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.

Electroanalytical study and square wave voltammetric techniques for the determination of β-blocker timolol at the mercury electrode

An electroanalytical study of timolol in Britton–Robinson buffer is described. The reduction process on the hanging mercury drop electrode gives rise to two adjacent peaks above −1.0 V (versus Ag/AgCl), the latter on the background discharge, within the entire pH range studied (1.8–10.5), the polarographic p K a value obtained was 6.75. The results proved that the reduction of timolol is irreversible and the limiting current is adsorption controlled. The dependence of the peak current on concentration was studied by means of different polarographic and voltammetric techniques. Using adsorptive stripping square wave voltammetry (AdS-SWV), the height for the first peak was proportional to the concentration of timolol in two linear ranges: 1.0×10 −9 to 1.2×10 −8 M and 1.2×10 −8 to 1.1×10 −7 M, the 3 σ detection limit being 6.6×10 −10 mol l −1. For square wave polarography (SWP) the linear range and the detection limit were 4.0×10 −8 to 3.0×10 −6 M with two different slopes and 2.5×10 −8 M, respectively. A mechanism is proposed for the reduction of timolol. Two procedures, based on SWP and AdS-SWV, were developed for the determination of timolol in ophthalmologic drops.

Characterization of ocular pharmacokinetics of beta-blockers using a diffusion model after instillation.

To characterize the ocular pharmacokinetics of beta-blockers (timolol and tilisolol) after instillation in the albino rabbit using a mathematical model that includes a diffusion process. The disposition of fluorescein isothiocyanate-dextran (FITC-dextran, molecular weight 4400), timolol, and tilisolol was determined in tear fluid and aqueous humor after instillation or ocular injection in rabbits. The in vivo penetration parameters were estimated by fitting the concentration-time profiles to the Laplace equations based on a diffusion model using MULTI(FILT) program. The in vitro permeability of drugs was measured across cornea using a two-chamber diffusion cell. Concentration-time profiles of drugs in the tear fluid after instillation showed a monoexponential curve. Although a monoexponential curve was observed in the aqueous humor concentration of FITC-dextran after injection into the aqueous chamber, timolol and tilisolol showed a biexponential curve. On the basis of these results, an in vivo pharmacokinetic model was developed for estimation of penetration parameters. The in vitro partition parameters were higher than those of the in vivo parameters. The ocular absorption of timolol and tilisolol was characterized using an in vivo pharmacokinetic model and in vivo penetration parameters.