Effects of 0.5% Timolol Maleate Ophthalmic Solution on Heart Rate and Selected Echocardiographic Indices in Apparently Healthy Cats

Unmet needs in glaucoma therapy: The potential role of hydrogen sulfide and its delivery strategies

Glaucoma is an optic neuropathy disorder marked by progressive degeneration of the retinal ganglion cells (RGC). It is a leading cause of blindness worldwide, prevailing in around 2.2% of the global population. The hallmark of glaucoma, intraocular pressure (IOP), is governed by the aqueous humor dynamics which plays a crucial role in the pathophysiology of the diesease. Glaucomatous eye has an IOP of more than 22 mmHg as compared to normotensive pressure of 10-21 mmHg. Currently used treatments focus on reducing the elevated IOP through use of classes of drugs that either increase aqueous humor outflow and/or decrease its production. However, effective treatments should not only reduce IOP, but also offer neuroprotection and regeneration of RGCs. Hydrogen Sulfide (H₂S), a gasotransmitter with several endogenous functions in mammalian tissues, is being investigated for its potential application in glaucoma. In addition to decreasing IOP by increasing aqueous humor outflow, it scavenges reactive oxygen species, upregulates the cellular antioxidant glutathione and protects RGCs from excitotoxicity. Despite the potential of H₂S in glaucoma, its delivery to anterior and posterior regions of the eye is a challenge due to its unique physicochemical properties. Firstly, development of any delivery system should not require an aqueous environment since many H₂S donors are susceptible to burst release of the gas in contact with water, causing potential toxicity and adverse effects owing to its inherent toxicity at higher concentrations. Secondly, the release of the gas from the donor needs to be sustained for a prolonged period of time to reduce dosing frequency as per the requirements of regulatory bodies. Lastly, the delivery system should provide adequate bioavailability throughout its period of application. Hence, an ideal delivery system should aim to tackle all the above challenges related to barriers of ocular delivery and physicochemical properties of H₂S itself. This review discusses the therapeutic potential of H₂S, its delivery challenges and strategies to overcome the associated chalenges.

Timolol 0.5% ophthalmic solution influences cardiac function in healthy cats

OBJECTIVES

The aim of this study was to ascertain the effect of a drop of timolol 0.5% ophthalmic solution on the systolic function of the left ventricle (LV) and left atrium (LA), and to confirm if timolol helped appraisal of diastolic function by reducing heart rate (HR) and separating the transmitral outflow waves from tissue Doppler imaging (TDI).

METHODS

A total of 41 client-owned healthy cats underwent two echocardiograms 20 mins apart. The timolol group (33 cats) received a drop of timolol solution after the first examination. Standard and speckle-tracking echocardiography evaluated the LV and LA function of both groups at the two time points evaluated.

RESULTS

Timolol reduced HR (19%), and fractional shortening from LV (20.3%) and LA (16.6%). Septal S' decreased by 51% (from 7.7 to 5.2 cm/s) and lateral S' dropped by 43.1% (7.3 to 5.1 cm/s). Most longitudinal techniques did not change after timolol, including the mitral annular plane systolic excursion from the interventricular annulus, tricuspid annular plane systolic excursion, LV longitudinal strain and LV tissue motion annular displacement. The isovolumic relaxation time increased by 15.2% (from 54 to 64.6 ms), with most cats presenting this variable above the reference (>60 ms). Timolol did not support diastolic assessment, enabling evaluation in only 2/11 cats when using lateral TDI and 1/9 cats using septal TDI. Regarding side effects, miosis occurred in 18 cats (54.5%).

CONCLUSIONS AND RELEVANCE

Timolol reduced systolic function, decreasing standard echocardiographic variables. Regarding diastolic evaluation, although timolol decreased HR, it did not separate the mitral diastolic waves, as expected.

Effects of timolol maleate eye drops on experimentally dilated cardiomyopathy and healthy rabbits

Background

Dilated cardiomyopathy (DCM) is the most frequently diagnosed cardiomyopathy in rabbits. Timolol maleate (nonselective beta-blockers), which reduces the heart rate (HR) and blood pressure, has been used for glaucoma. However, this effect has not been assessed in a rabbit DCM model.

Aim

This study aimed to evaluate the circulatory effects of 0.5% timolol maleate eye drops on rabbit DCM models.

Methods

Rabbits were randomly divided into healthy (n = 8) and DCM rabbits (n = 8). Rabbit DCM models were established through intravenous administration of daunorubicin (4 mg/kg/week), an anthracycline anticancer drug, for 6 weeks. We measured the HR, mean arterial pressure (MAP), and echocardiography before (pre) and at 10, 30, 60, and 120 minutes after timolol maleate administration.

Results

Timolol maleate 0.5% eye drops significantly reduced the HR at 10, 30, and 60 minutes in DCM models and 10 minutes in healthy rabbits; however, there were no alterations in MAP. There was a significant fraction shortening elevation at 10 and 120 minutes in DCM models; moreover, there were marked ejection fraction elevations at all measurement points in only DCM models.

Conclusion

Timolol maleate eye drops exert several effects on the circulatory system in rabbit DCM models and healthy rabbits.

The Feline Cardiomyopathies: 3. Cardiomyopathies other than HCM

Practical relevance:

Although feline hypertrophic cardiomyopathy (HCM) occurs more commonly, dilated cardiomyopathy (DCM), restrictive cardiomyopathy (RCM), arrhythmogenic right ventricular cardiomyopathy (ARVC), left ventricular noncompaction (LVNC) and cardiomyopathy – nonspecific phenotype (NCM; formerly unclassified cardiomyopathy) are all recognized in domestic cats.

Patient group:

Any adult domestic cat, of either sex and of any breed, can be affected.

Diagnostics:

The non-HCM cardiomyopathies are rarely suspected in subclinically affected cats, so most are first identified when a cat presents with signs of heart failure or systemic thromboembolic disease. The definitive clinical confirmatory test for these other feline cardiomyopathies is echocardiography.

Key findings:

‘Cardiomyopathy – nonspecific phenotype’ is a catch-all term that groups hearts with myocardial changes that either do not meet the criteria for any one type of cardiomyopathy (HCM, RCM, DCM, ARVC, LVNC) or meet the echocardiography criteria for more than one type. RCM is characterized by diastolic dysfunction due to fibrosis that results in a restrictive transmitral flow pattern on Doppler echocardiography and usually marked left or biatrial enlargement. DCM is characterized by decreased myocardial contractility and is rare in cats. When it occurs, it is seldom due to taurine deficiency. However, since taurine-deficient DCM is usually reversible, a diet history should be obtained, whole blood and plasma taurine levels should be measured and taurine should be supplemented in the diet if the diet is not commercially manufactured. ARVC should be suspected in adult cats with severe right heart enlargement and right heart failure (ascites and/or pleural effusion), especially if arrhythmia is present. Feline LVNC is rare; its significance continues to be explored. Treatment of the consequences of these cardiomyopathies (management of heart failure, thromboprophylaxis, treatment of systemic arterial thromboembolism) is the same as for HCM.

Conclusions:

While these other cardiomyopathies are less prevalent than HCM in cats, their clinical and radiographic presentation is often indistinguishable from HCM. Echocardiography is usually the only ante-mortem method to determine which type of cardiomyopathy is present. However, since treatment and prognosis are often similar for the feline cardiomyopathies, distinguishing among the cardiomyopathies is often not essential for determining appropriate therapy.

Areas of uncertainty:

The feline cardiomyopathies do not always fit into one distinct category. Interrelationships among cardiomyopathies in cats may exist and understanding these relationships in the future might provide critical insights regarding treatment and prognosis.

Assessment of diastolic function using mitral flow propagation velocity in cats

The objectives of this study were to investigate the usefulness of mitral flow propagation velocity (Vp) in cats by evaluating the effect of the flow pattern summation and evaluation of Vp variables in cats with hypertrophic cardiomyopathy (HCM). Healthy cats were categorized into summation (Sum) and separation (Sepa) groups to evaluate the effects of the flow pattern summation on Vp. Cats with HCM were categorized into HCM left atrial (LA) (-), LA (+), and LA (++) groups according to the degree of LA enlargement to investigate the feasibility of Vp. There were no significant differences noted in Vp between the Sum and Sepa groups and no significant correlation between Vp and heart rate. Decline of Vp was associated with the degree of LA enlargement. Mitral flow propagation velocity appeared to be clinically feasible in cats and could possibly be useful in the detection of diastolic dysfunctions in cats with HCM.

The effects of topical dorzolamide 2% and brinzolamide 1%, either alone or combined with timolol 0.5%, on intraocular pressure, pupil diameter, and heart rate in healthy cats

Objective

To investigate the effects of topical dorzolamide 2% q8h and brinzolamide 1% q8h, administered either alone (A and B, respectively) or in combination with topical timolol 0.5% q12h (C and D, respectively), on the circadian pattern of intraocular pressure (IOP), the pupil size, and heart rate in healthy cats.

Methods

In this prospective, randomized, double‐blinded study, 10 healthy, adult cats were randomly assigned to one of four groups and the eye to be medicated was randomly assigned. IOP, pupil diameter, and heart rate were measured at 3‐hour intervals. A 5 days’ adjustment period was followed by a 5 days’ placebo (baseline) period. Then, all groups of cats received all four treatments (A‐D) according to a Latin square‐based rotating schedule. Five days’ medication periods were alternated with 3 days’ washout periods.

Results

Mean baseline IOP was 13.6 ± 2.7 mm Hg. All treatments resulted in a statistically significant decrease in mean IOP in the treated eye: A: −2.33 mm Hg (95% CI: −2.71, −1.94), B: −1.91 mm Hg (95% CI: −2.30, −1.53), C: −2.36 mm Hg (95% CI: −2.74, −1.97), and D: −2.37 mm Hg (95% CI: −2.76, −1.98) and the nontreated eye: A: −0.19 mm Hg (95% CI: −0.28, −0.11), B: −0.18 mm Hg (95% CI: −0.27, −0.10), C −0.31 mm Hg (95% CI: −0.40, −0.23), and D: −0.24 mm Hg (95% CI: −0.32, −0.15). Timolol resulted in an additional, significant decrease in IOP of 4% and 5%, respectively, compared to A and B, and in mild bradycardia and miosis.

Conclusions

Topical administration of dorzolamide 2% and brinzolamide 1% q8h significantly decreased IOP in healthy cats. Supplemental timolol 0.5% eye drops q12h resulted in an additional, statistically significant reduction of IOP.