Cellular cytotoxicity of antiglaucoma drugs in cultured corneal endothelial cells

In this study, the various antiglaucoma drugs including betaxolol, timolol, levobunolol, carteolol, brimonidine, dipivefrin, dorzolamide, brinzolamide, latanoprost, unoprostone, and pilocarpine were used to investigate the effects of cellular cytotoxicity in cultured bovine corneal endothelial cells. After exposure to the drugs in three dilutions, 1/100, 1/1,000, and 1/10,000, for 100 minutes, cells were estimated based on the release assay of lactate dehydrogenase (LDH) enzyme. It was found that cellular LDH was significantly released in the medium only at 1/100th dilution of betaxolol, brimonidine, dorzolamide, dipivefrin, latanoprost and unoprostone to 130%, 123%, 145%, 157%, 128% and 237%, respectively, compared with controls upon exposure to drugs for 100 minutes. Moreover, benzalkonium chloride preservative at the concentrations ranging from 0.001 to 0.00001 mg/mL did not affect cellular LDH release in bovine corneal endothelial cells. These results indicate that high concentrations of antiglaucoma drugs may induce cytotoxicity in corneal endothelial cells.

Évaluation in vivo par échographie à très haute fréquence des modifications épithéliales cornéennes induites par un bêta-bloquant avec 0,01 % de chlorure de benzalkonium

PURPOSE

To assess in vivo the corneal epithelial damage caused by a topical toxic medication using a 60-MHz ultrasound device.

MATERIAL

and methods: A solution of timolol with 0.01% benzalkonium chloride (BAC) was applied twice a day in the test eyes of ten rabbits, and a BAC-free solution of timolol in the control eyes, for 56 days. We used a 60-MHz ultrasound device to evaluate the epithelial damage in BAC-exposed eyes, compared to control eyes. The clinical and ultrasound examinations were performed every week, and the histological analysis at the end of the experiment.

RESULTS

The clinical findings were conjunctival redness, corneal staining and instability of the tear film. In vivo VHF ultrasound revealed a thinning of the epithelium of test eyes (from 40.9+/-1,6 microm at D0 to 31.8+/-3.4 microm at D56; p=0.0006 for D0 vs D56), while the epithelium of control eyes remained unchanged. Ultrasound epithelial thickness was correlated with corneal staining (at D34 and D56; p=0.0025 and 0.0377, respectively) and histological epithelial pachymetry (p=0.0176 for control and 0.0505 for tested epithelium). Moreover, we report qualitative VHF ultrasound imaging of early epithelial damage.

CONCLUSION

This new device could be very useful in ocular toxicity evaluation as a reproducible and reliable tool for multicentric clinical research.

[A comparative study of the ocular tolerance of 3 timolol-based preparations: the influence of preservatives on ocular tolerance]

PURPOSE

Ophthalmic preparations can cause toxic ocular reactions, often associated with the use of preservatives. The aim of this study was to compare the ocular tolerance of three ophthalmic preparations based on timolol: a preservative free ophthalmic preparation (Timabak) and two other commercially available preserved preparations (Timoptol) and Timoptol LP).

METHODS

The effect of repeatedly instilling eye drops for 28 days on rabbit eyes was assessed in vivo by mean of a confocal laser scanning ophthalmoscope. The corneal microlesions were selectively marked by fluorescein.

RESULTS AND CONCLUSION

The overall results show the good ocular tolerance of the three tested products. However, a closer comparison between the products brought out differences in the extent of lesions among the tested products depending on their composition. Indeed the preservative free eye drops appeared better tolerated than the two preserved preparations.

[Toxicity of preserved and unpreserved beta-blocker eyedrops in an in vitro model of human conjunctival cells]

PURPOSE

To compare the toxicity of a short-time application of timolol with benzalkonium chloride (timolol-BAC+) and unpreserved timolol (timolol-BAC-) in an in vitro model of human conjunctival cells.

METHODS

Chang's conjunctival cell line (ATCC CCL 20.2) was treated for 15min. with 0.1%, 0.25% or 0.4% timolol-BAC(+) or BAC(-) and then examined immediately or 24h later. Cell viability, chromatin condensation, mitochondrial mass and activity, free radicals production were studied by microplate cold light cytometry. Moreover, relative cell number was evaluated by crystal violet colorimetric test. In addition, cell size and the expression of an apoptotic marker Apo2.7 were studied by flow cytometry.

RESULTS

Timolol-BAC(+) induced a rapid decrease in cell viability ranging from 40% immediately after treatment to 85% 24h later. A small, significantly less important decrease in cell viability was also observed with all tested concentrations of timolol-BAC(-). 24h after treatment with 0.25% timolol-BAC(+), the relative cell number was reduced by 55% whereas it did not vary after 0.25% timolol -BAC(-) treatment. Only timolol-BAC(+) induced chromatin condensation, decrease in mitochondrial membrane potential and cell size reduction. Moreover, cells treated with timolol-BAC(+) overexpressed the apoptotic marker Apo2.7. Also reactive oxygen species (ROS) production was significantly more important after cell exposure to timolol-BAC(+).

CONCLUSION

In our model of conjunctival cells in vitro, timolol-BAC(+) induced irreversible cytotoxic damage with some characteristics of apoptosis. The active compound of timolol-BAC(-) could be responsible for ROS production and for cell viability variations. Oxidative stress could also play a role in timolol-BAC(+)-induced toxicity. In vitro toxic effects of antiglaucoma drugs could, in part, explain some ocular surface disorders in long-term treated patients.

Comparison of the effects of preserved and unpreserved formulations of timolol on the ocular surface of albino rabbits

Thirty-six albino rabbits, randomly divided into six groups, were used to study their ocular tolerance to (a) 0.25 and 0.50% Timoptol preserved with 0.01% benzalkonium chloride, (b) 0.25 and 0.50% Timoptol-LP, a gel-forming solution preserved with 0. 012% benzododecinium bromide, and (c) 0.25 and 0.50% Timabak unpreserved in the ABAK eyedrops dispenser. All eyedrops were applied in the right eye for 60 days. A clinical follow-up with slitlamp examination and break-up time evaluation was performed for 2 months. At the end of the experimentation, the animals were sacrificed and their eyes enucleated for histological analyses of the conjunctiva and cornea. There was no significant difference in the clinical examination between each group, except for the break-up time evaluation between Timoptol and Timabak at each concentration which was better with the unpreserved timolol. Histological results showed a significant difference in the corneal stroma edema between preserved and unpreserved timolol. This study confirms that using unpreserved timolol may be beneficial for the long-term treatment of glaucomatous patients as it increases tear film stability and decreases epithelial permeability and stromal aggression of the cornea.

Relationship between amount of beta-blockers permeating through the stratum corneum and skin irritation after application of beta-blocker adhesive patches to guinea pig skin

We evaluated the relationship between the cumulative amounts of 5 kinds of beta-blockers (alprenolol, oxprenolol, timolol, acebutolol and atenolol) permeating through the stratum corneum and a* values obtained by measuring the formation of erythema, a skin irritation reaction, with a chromameter after transdermal application of adhesive patches containing 2 beta-blocker to the skin of guinea pigs. The cumulative amount of beta-blocker released from each adhesive patch to the skin increased with the increase in application time. The contents of alprenolol, oxprenolol and timolol in the stratum corneum and in the stripped skin increased markedly up to 4 h after application and thereafter were maintained at high levels up to 24 h. The contents of acebutolol and atenolol, on the other hand, increased up to 24 h, but these values were low. a* values of all adhesive patches 24 h after application were higher than those before application. The correlation coefficients between the cumulative amounts of alprenolol, oxprenolol, timolol, acebutolol or atenolol permeating through the stratum corneum and (delta a* -delta a*Placebo) values were 0.739, 0.717, 0.722, 0.551 and 0.633, respectively. The correlation coefficient calculated by averaging the cumulative amounts of 6 kinds of beta-blockers permeating through the stratum corneum [including propranolol which was reported previously (Kobayashi I., et al., Biol. Pharm. Bull., 19, 839-844 (1996))] was 0.731, higher than the correlation coefficient between contents of these beta-blockers in the stripped skin and (delta a* -delta a*Placebo) values (r = 0.552). This suggests that there was a high correlation between the cumulative amounts of beta-blockers permeating through the stratum corneum and (delta a* -delta a*Placebo) values.

[The action of amphiphilic drugs on human fibroblasts in vitro. Their possible use in antiviral and antitumor chemotherapy]

The LDH cytotoxicity test is able to emphasize the stabilisation of human embryo fibroblast membranes, in vitro, by two amphiphilic drugs: the metomidate and the thymolol. The membrane stabilisation is emphasized by the reduced LDH externalization as compared with the untreated cells. The use of the LDH test for the selection of some natural complexes or synthetic drugs with membrane stabilising and potential antiviral activity is proposed. The inhibition of LDH by metomidate was recorded, as decreasing of the enzyme activity with increasing metomidate concentration. This fact explains the known lactic acid accumulation, under metomidate treatment of human subjects. The use of metomidate in classical cancer treatment potentiation is proposed instead of lactate externalization inhibitors, already used for this aim.

Ultrastructural effects of topical betoptic, betagan, and timoptic on the rabbit corneal endothelium

Scanning and transmission electron microscopy was used to evaluate the cytotoxic effect of three commercially available beta adrenergic blockers (Betoptic 0.5%, Betagan 0.5% and Timoptic 0.5%), and preservative (benzalkonium chloride 0.01%) on rabbit corneal endothelium. Evaluation was performed on groups including intact corneas, de-epithelialized corneas, full-thickness corneal grafts with intact epithelium, and de-epithelialized corneal grafts. Both Betoptic and Betagan produced minor damage to endothelial microvilli in the intact epithelial group. The de-epithelialized and the grafted-cornea groups had less damage induced by Betoptic than Betagan. Timoptic produced more endothelial damage than Betoptic and Betagan in all groups. Benzalkonium chloride produced minimal endothelial damage. Results indicate that Betoptic produces less endothelial toxicity than the two non-specific beta blockers when corneal epithelium is damaged and after penetrating keratoplasty.

Microemulsions as topical delivery vehicles: ocular administration of timolol

The topical administration of timolol as an ion-pair with octanoate was achieved by use of an oil-in-water microemulsion containing lecithin as a surfactant. The microemulsion, a solution of the ion-pair and a solution of timolol alone were instilled in the conjunctival sac of rabbits. A rapid method for the separation and determination of timolol in aqueous humour by HPTLC was used. The bioavailability of timolol from the microemulsion and the ion-pair solution was higher than that obtained from timolol alone. The areas under the curve for timolol in aqueous humour after administration of the microemulsion and the ion-pair solution were 3.5 and 4.2 times higher, respectively, than that observed after the administration of timolol alone.

Toxic effects of topically administered Betagan, Betoptic, and Timoptic on regenerating corneal epithelium

Re-epithelialization of 40 iatrogenic cornea ulcers produced in rabbits with iodine vapour burn were studied after topical administration of Betagan, Betoptic or Timoptic. At 24 hours there was no statistical difference in the wound healing rates among all the treated animal groups. At 48 hours, all the drug-treated groups were significantly different from the control group (p less than 0.05). There was no difference between the Betagan and Betoptic treated groups but the Timoptic-treated group was slower (p less than 0.05) than the Betoptic treated group, while the difference between the Timoptic and Betagan treated groups is not significant. At 72 hours the re-epithelialization rate of the Timoptic treated group was significantly slower than both the Betagan and Betoptic treated groups (p less than 0.05). By the 16th day following ulceration SEM examination revealed the Timoptic treated group had extensive epithelial cell desquamation, as well as hole formation and plasma membrane disruption. The Betagan treated corneas had mild superficial epithelial changes with loss of microvilli or slight desquamation. The Betoptic treated corneas were completely healed. In conclusion, Betagan is less toxic than Timoptic. Betoptic is the least toxic of the three drugs to regenerating rabbit corneal epithelium.

A toxicological review of beta-adrenergic blockers

The use of various beta-adrenergic blockers has become extensive as they have been found to be efficacious in the treatment of a number of cardiovascular problems including cardiac arrhythmias, angina pectoris, and hypertension. The widespread and chronic use of these drugs has generated a concern for their potential chronic toxicity. Eighteen beta-adrenergic blockers have been reviewed and the available literature pertaining to their potential carcinogenicity, mutagenicity, and teratogenicity has been summarized and compared.

Different toxicological profiles for various beta-blocking agents on cardiac function in isolated rat hearts

Propranolol, timolol and sotalol were compared regarding their cardiotoxic properties in isolated, perfused and catecholamine depleted rat hearts. Catecholamine depletion was performed in order to exclude interference of the drugs with beta-adrenergic receptors. The results demonstrate that both in spontaneously beating and atrial- stimulated hearts propranolol (3 - 30 micrograms/ml) and timolol (30 - 300 micrograms/ml) induced a dose dependent decrease in myocardial contractility, stimulus formation and stimulus conduction. Lacking local anesthetic properties as evidenced by effects on coronary flow and threshold voltage in the heart it can be deduced that the negative inotropic effect and an impaired stimulus conduction due to timolol can neither attributed to beta-adrenoceptor antagonism nor membrane stabilising activity. In addition, both propranolol (5 micrograms/ml) and timolol (200 micrograms/ml) reduced myocardial contractility to the same extent in ventricular-paced hearts. Therefore, a direct myocardial depressive effect rather than an indirect effect due to a reduced heart rate must be responsible for the negative inotropy. The hydrophilic beta-blocker sotalol demonstrated a slight cardiodepressant activity either in the spontaneously beating and atrial-stimulated hearts (30 - 300 micrograms/ml) or ventricular-paced hearts (300 micrograms/ml). It is concluded that the toxicological profile of various beta-blocking drugs might be determined by an yet unknown pharmacological property apart from beta-adrenoceptor blockade or membrane stabilising activity. Furthermore, the degree of lipophilicity of the drug might be an important determinant for the cardiotoxic profile of this class of drugs.

The beta-adrenergic blocking agents and the treatment of glaucoma

The autonomic nervous system is divided into the parasympathetic and sympathetic systems, with three types of adrenergic receptors: alpha (smooth muscle contraction), beta1 (cardiac acceleration and fatty acid mobilization) and beta2 (smooth muscle relaxation). Substances affecting the function of the adrenergic system are the agonists or stimulators, which mimic the effects of endogenous norepinephrine or epinephrine, and antagonists or blockers, which block the receptors and prevent stimulation by the agonists. Autonomic stimulation in the eye mediates various changes which apparently affect outflow facility and rate of formation of aqueous humor. Alteration of either or both of these factors by autonomic agonists or antagonists may have a direct or an indirect effect on intraocular pressure. Beta-adrenergic blocking substances have been used to treat a variety of diseases. Some of the effects of these drugs are attributable to properties other than beta blockade, such as intrinsic sympathomimetic activity and local anaesthetic activity. Side effects of this class of drugs require caution in cases of congestive heart failure and in asthmatics. Autonomic agents used in the treatment of ocular hypertension and glaucoma include pilocarpine, a chilinergic agonist, epinephrine, an adrenergic agonist, and various beta adrenergic blockers or antagonists including propranolol, atenolol and timolol. The physico-chemical properties and pharmacokinetics of timolol are reviewed. Data showing a significant reduction in intraocular pressure as a result of ocular instillation of timolol are presented. Reduction of the rate of aqueous formation appears to be the mechanism of action. A low incidence of non-serious side effects is reported.