Effects of Telmisartan on Intraocular Pressure, Blood Pressure, and Ocular Perfusion Pressure in Normal and Glaucomatous Cats

Abstract

Purpose: To determine the effect of telmisartan on intraocular pressure (IOP), blood pressure (BP), and ocular perfusion pressure (OPP) in normal and glaucomatous cats.

Methods: A four-week study was conducted in six normal adult cats, followed by a longer six-month study performed in 37 cats with spontaneous glaucoma and 11 age-matched normal cats. Telmisartan (1 mg/kg/day) or placebo-vehicle were administered orally once daily. IOP was measured by rebound tonometry. BP readings were obtained by oscillometric method. OPP was calculated as mean arterial pressure (MAP) − IOP. IOP and BP were obtained three times a week for the first study and weekly for the second study.

Results: Baseline IOP was significantly higher, and OPP was significantly lower in glaucomatous cats than in normal cats (P < 0.0001). These differences between glaucomatous and normal cats persisted throughout the study, regardless of treatment (P < 0.001). No significant differences in IOP, BP, or OPP were detected between any study phases in the first, normal feline cohort or between telmisartan- and placebo-treated glaucomatous cats at any timepoint in the second study.

Conclusions: Oral telmisartan was well tolerated and did not have a detrimental effect on BP or OPP in cats but did not lower IOP or improve OPP in cats with glaucoma.

Translational Relevance: While showing telmisartan could not be used as a sole therapy for IOP lowering, our data affirmed a lack of detrimental effects of telmisartan on BP and OPP in a translationally-relevant, spontaneous, large animal glaucoma model.

Introduction

Glaucoma is a devastating cause of vision loss in animals and humans and is characterized by progressive degeneration of retinal ganglion cell (RGC) somas and axons.^1,2 Intraocular pressure (IOP) is the only modifiable, consistent risk factor for glaucoma in humans and animals,^3–5 whereas other risk factors include age,^6,7 genetics,⁸ and vascular factors such as low blood pressure (BP) and low ocular perfusion pressure (OPP).^4,9 Lowering IOP limits disease progression in glaucoma¹⁰ and remains the only established treatment strategy for glaucoma in humans and animals. However, currently available IOP lowering treatments cannot prevent progressive vision loss in all glaucoma patients.^3,5 Thus the development of neuroprotective therapies for glaucoma remains an unmet clinical goal. 

Spontaneous, large animal models of glaucoma can provide valuable opportunities to unravel the underlying pathology of glaucoma and represent an important step in the translational continuum as new therapies are proposed. The feline eye has many similarities to the human eye relative to size, function and structure including ocular microanatomy.^11,12 Although glaucomas in cats are relatively less common than in people, feline glaucoma may be underdiagnosed because of insidious onset and the relatively mild clinical signs of the disease.¹³ Recessively inherited primary congenital glaucoma has been identified in cats homozygous for LTBP2 mutation¹⁴ that present with gradual IOP elevation beginning by two months of age and varying degrees of progressive RGC and optic nerve degeneration identifiable by six months of age, which mirror many hallmarks of glaucoma in humans.^15–17 

Angiotensin 2 receptor type 1 (AT1R) is a primary mediator for the renin-angiotensin-aldosterone system and is known to regulate systemic BP. Among AT1R blockers (ARBs) available to date, telmisartan has been widely used in cats for reduction of proteinuria associated with chronic kidney disease at 1 mg/kg/day¹⁸ and, more recently, approved by the Food and Drug Administration as a treatment for systemic hypertension in cats at a higher dosage (3 mg/kg/day; 1.5 mg/kg twice daily).¹⁹ Telmisartan selectively blocks the AT1R with high affinity and displaces angiotensin II, while leaving the beneficial effects of AT2 receptor activation unaffected.²⁰ ARBs have been studied as a potential neuroprotective therapy for glaucomatous optic neuropathy using rodent models.^21–24 These studies suggest that ARBs have the potential to fulfill an unmet need for neuroprotection in glaucoma. The potential beneficial effect of telmisartan has been suggested to relate to suppression of transforming growth factor β (TGF-β) expression and canonical signaling,^1,2 although other direct mechanisms of neuroprotection could include peroxisome proliferator-activated receptor gamma activation and indirect mechanisms such as IOP reduction would also potentially be beneficial. Previous studies have demonstrated that orally administered ARBs lower IOP in humans²⁵ and mice.²⁶ However, ARBs also have the potential to induce systemic hypotension in otherwise normotensive patients because they were originally developed as treatments for systemic hypertension. The BP-lowering effects of ARBs, particularly in systemic mean arterial blood pressure (MAP), could be detrimental to ocular health by lowering OPP, estimated by the difference between MAP and IOP, in turn negatively impacting blood supply to ocular tissues including the optic nerve and retina.^27,28 A large body of literature has suggested that low OPP increases the vulnerability of the optic nerve and retina by diminishing ocular tissue perfusion, increasing the risk of glaucoma development and progression.^4,5,27,29,30 Importantly, conflicting evidence from human epidemiologic studies shows both reduced rates of glaucoma conversion and progression in human patients with ocular hypertension³¹ but a lack of association between ARB use, IOP, and glaucoma progression in recent, large meta-analyses.^32–34 Thus there is a lack of direct evidence supporting the use of ARBs in people or animals with naturally-occurring glaucoma and the potential for the BP-lowering effect of ARBs to negatively impact OPP and thus to have potential detrimental effects on disease progression in glaucoma must also be considered. With the long-term goal of addressing this need for direct evidence of potentially beneficial or detrimental effects of the ARB, telmisartan, in naturally-occurring glaucoma, we sought in this study to determine the effect of telmisartan on IOP, BP, and OPP in a spontaneous feline glaucoma model and in normal cats to lay a foundation for a larger clinical study of the neuroprotective effects of telmisartan in cats with glaucoma. 

Methods

Animals

Two feline cohorts were used in the current study. The first cohort used in this study was composed of six normal young adult cats (five females, one male; mean age [standard deviation {SD}] = 1.4 [0.3] years). The second cohort studied consisted of 37 cats (18 females, 19 males) with feline congenital glaucoma, homozygous for LTBP2 mutation, and 11 age-matched normal cats (three females; eight males). The mean age of the animals was 6.5 months (SD = 0.47, range 6.0–8.1 months) when the study was initiated. Animals were maintained under a consistent 12-hour light/dark cycle. All animal procedures were performed in accordance with protocols approved by the University of Wisconsin-Madison Institutional Animal Care and Use Committee and in compliance with the Association for Research in Vision and Ophthalmology statement for the use of animals in ophthalmic and vision research and the National Institutes of Health (NIH) Guide for the Care and Use of Laboratory Animals. 

Ophthalmic Examination

All animals were examined by a board-certified veterinary ophthalmologist (G.J.M.) before the study. Ophthalmic examinations included slit-lamp biomicroscopy (SL-15; Kowa Company, Ltd., Tokyo, Japan), fluorescein staining (Ful-Glo; Akorn, Inc., Buffalo Grove, IL, USA), and indirect ophthalmoscopy (Keeler Vantage Plus; Keeler Instruments Inc., Broom, PA, USA). Eyes with potentially confounding ocular abnormalities such as corneal edema or other corneal lesions, anterior lens luxation, as well as fractious animals in which tests could not be conducted with only gentle manual restraint, were excluded from the study. 

Tonometry

IOP was measured by rebound tonometry using TonoVet (ICare Finland Oy, Vantaa, Finland) as previously validated, in awake, gently restrained cats.³⁵ The animals were well acclimated to frequent tonometry, conducted at least weekly from eight weeks of age or under. Three acceptable (SD < 2.5 mm Hg) instrument-derived tonometer readings were acquired and averaged to provide a single IOP value for each cat at each timepoint. To minimize circadian variation, all IOP values were obtained between 8 AM and 10 AM.^36,37 

BP Measurement

BP including systolic, diastolic and mean arterial pressure readings (SBP, DBP and MAP, respectively) were obtained by oscillometric method (Cardell monitor; Midmark, Dayton, OH, USA) following the American College of Veterinary Internal Medicine consensus statement on the identification, evaluation, and management of systemic hypertension in dogs and cats.³⁸ A blood pressure cuff was placed on the forelimb or tail. The cuff size was selected so that the cuff width was approximately 40% of the circumference at the cuff site. All cats were well acclimated to BP measurements before the baseline measurements, from early in life. At least six BP measurements were recorded, with the last five measurements averaged to obtain single BP values at each time point. Baseline BP and IOP were obtained on at least four occasions before initiating treatment. 

Study Design

This prospective study comprised two distinct components and cohorts. The first component of the study was conducted in a normal feline cohort over a six-week period consisting of a pre-treatment baseline phase (no drug administration; Days −7 to 0); a treatment phase (Days 1–28) and then a final post-treatment/recovery phase (Days 29–36). Telmisartan oral solution (Semintra, 1 mg/kg/day; Boehringer Ingelheim, Ingelheim am Rhein, Germany) was administered daily at 7 AM during the treatment phase. The second, longer study, spanning six months of treatment, was conducted in both normal and glaucomatous cats. To determine the effects of telmisartan on IOP, BP, and OPP in glaucomatous cats, a cohort of 48 cats, which consisted of 37 glaucomatous and 11 age-matched normal cats, was used. The glaucomatous cats were randomly assigned to receive either daily telmisartan oral solution or placebo (same volume of oral vehicle) for the six-month treatment period. The normal cats were untreated and included to control for any effects of age on IOP, BP, and OPP. For the second study, investigators administering the treatments and collecting data were masked to the treatment randomly assigned to each cat. 

Liquid Chromatography-Mass Spectrometry

Serum samples were collected from venous blood obtained by venipuncture 30 minutes after initial oral telmisartan administration. Sample preparation and analyses were carried out at the University of Wisconsin–Madison Biotechnology Center Mass Spectrometry Core Facility. For sample preparation, 300 µL of 2:1 acetonitrile: methanol was added to 100 µL serum samples. The mixture was vortexed and spun at 16,000g for 10 minutes at room temperature. The supernatant was pulled and placed in a new tube. A 100 µL aliquot of 2:1 acetonitrile: methanol was added to the samples followed by vertexing and centrifugation at 16,000g for 10 minutes at room temperature. The supernatant was pooled with the previous supernatant. The samples were dried in a speed-vac and resolubilized in 100 µL 1:1 solvent A (10 mM ammonium acetate) and B (100% acetonitrile). Calibration standards of 0, 0.2, 1, 5 and 20 ng/mL telmisartan were prepared in 100 µL of blank feline serum. The calibration standard curve was linear with a correlation coefficient = 0.997. Sciex 5500 QTRAP triple quadrupole/ion trap mass spectrometer (AB Sciex, Framingham, MA, USA) connected to an Agilent 1100 Nano flow HPLC system with temperature-controlled autosampler was used for LC-MS. A Waters XBridge OST C18 2.5 µm particle, 2.1 × 50 mm analytical column, was used for chromatographic separations and maintained at 40°C, and the autosampler was held at 6°C. Telmisartan was eluted isocratically at 50% solvent B at a flow rate of 300 µL/min. Data collection and quantitation using Analyst ver 1.6.3 software. 

Statistics

Mean values were compared between groups or time points over the study by t-test or analysis of variance with Tukey-Kramer multiple comparisons post-test. Longitudinal data were analyzed by repeated measures analysis of variance. Normality of the data was assessed by the D'Agostino-Pearson omnibus test and by the Shapiro-Wilk test. P value < 0.05 was considered significant. 

Results

Effects of Telmisartan on IOP, BPs, and OPP in Normal Adult Cats

Baseline IOP, BPs (SBP, DBP, and MAP) and OPP values were acquired over the week before, during, and after the four-week oral telmisartan treatment. IOP and BPs were measured three times a week (at one- to three-day intervals) and averaged to obtain mean weekly values. Figure 1 shows IOP, BP, and OPP values for the baseline, treatment, and post-treatment phases. Because no significant differences between sexes were identified for these parameters, data were pooled for analyses. No significant differences in IOP, BPs, or OPP were detected between timepoints over the study period or between the treatment phases (Figs. 1A–C). No significant changes in BPs were noted at 0, 12, 24, and 48 hours after initial telmisartan administration (Table 1). 

Figure 1.

 

Effects of IOP, BPs and OPP by telmisartan in normal cats. (A) Mean IOP, (B) BP, and (C) OPP were not significantly different between the pretreatment, treatment, and post-treatment/recovery phases. SBP, DBP, and MAP (n = 6). Error bars: SD.

Table 1.

 

SBP, DBP, and MAP in Normal Cats 0, 12, 24 and 48 Hours After Initial Oral Telmisartan Administration

 

IOP Elevation and Lower OPP in Glaucomatous Cats

IOP and BPs were measured weekly in glaucomatous and age-matched normal cats for the second cohort study. Baseline IOP was significantly higher (P < 0.0001; Fig. 2A), and OPP was significantly lower (P < 0.001; Fig. 2B) in glaucomatous cats compared to normal cats whereas BP metrics (SBP, DBP, and MAP; Fig. 2C) were not different between the groups. 

Figure 2.

 

High IOP and low OPP in cats with glaucoma. At the pre-treatment baseline, glaucomatous cats had significantly higher IOP (A), and lower OPP (B) compared to age-matched normal cats (P < 0.001). BP metrics did not differ between the groups (C). Error bars: SD.

Effects of Telmisartan on IOP, BPs, and OPP in Glaucomatous Cats

IOP was significantly higher (P < 0.001), and OPP was significantly lower (P < 0.001) in glaucomatous cats than in age-matched normal controls at all time points in the study (Figs. 3A, B), while BPs (SBP, DBP and MAP) did not differ between groups over the study period (Table 2). There was no significant difference in IOP, BPs or OPP between telmisartan- and placebo-treated glaucomatous cats throughout the study period. No adverse effects, including overt clinical signs of hypotension such as ataxia, abnormal mentation and/or weakness, or inappetence or vomiting were observed during any phases of the study. 

Figure 3.

 

Telmisartan does not affect IOP and OPP in glaucomatous cats. IOP and OPP values were compared in glaucomatous cats orally administered telmisartan (TEL) or placebo (PLA) for six months and age-matched normal cats. Mean IOP was significantly higher (A), and OPP was lower (B) in glaucomatous cats relative to in age-matched normal cats at all time points during the study. *P < 0.05; **P < 0.01; ***P < 0.001; ****P < 0.0001. Error bars: SD.

Table 2.

 

SBP, DBP, and MAP in Glaucomatous Cats Treated With Either TEL or PLA, as Well as in Untreated Age-Matched Normal Cats

 

Telmisartan Serum Concentration

Telmisartan was detectable in serum samples from animals treated with oral telmisartan by mass spectrometry. Mean recovery rate (SD) of telmisartan in feline serum samples was 86.7% (1.53). Mean serum telmisartan concentration (SD) was 15.19 (7.88) ng/mL (n = 9) in these serum samples whereas no telmisartan was detectable in serum samples from placebo-vehicle treated or untreated animals (n = 5). 

Discussion

The current study was designed to determine whether orally-administered telmisartan affects IOP, BPs, or OPP in normal and glaucomatous cats. An initial longitudinal study in a normal feline cohort and a second, randomized, masked, placebo-controlled study in glaucomatous cats demonstrated that telmisartan does not have significant effects on IOP, BPs, or OPP in normal and glaucomatous cats. 

Previous studies in rodent glaucoma models treated with telmisartan and ARBs showed a profound reduction in SBP.^22,23,26 In contrast, our results demonstrated that telmisartan did not have potentially detrimental, hypotensive effects on well-acclimated, normotensive normal, or glaucomatous animal subjects. For the current study, we selected the clinically approved anti-fibrotic dosage of telmisartan (1 mg/kg/day), which has been widely used in cats for the reduction of proteinuria associated with chronic kidney disease.¹⁸ This dosage has been shown to effectively block pressor responses to escalating doses of exogenous angiotensin I administered in cats and to have modest effects on BP lowering (SBP reduction ∼13 mm Hg) in normotensive cats.³⁹ It should be noted that cats studied in the present study were acclimated to their environment and well trained within the first weeks of life to accept BP and IOP measurement with minimal, gentle restraint. This likely prevented stress-induced hypertension^40–42 and helped to maintain normal levels of physiological activity of the renin-angiotensin-aldosterone system.⁴³ 

Our data also demonstrated no significant differences in BPs between glaucomatous and age-matched normal cats at any time point. Thus the disease-causing LTBP2 mutation in glaucomatous cats does not affect systemic BPs. Our findings on BPs are corroborated by a recent study demonstrating no alterations in BPs in Ltbp2^−/− mice.⁴⁴ However, we observed significantly lower OPP in glaucomatous compared to normal cats, which was driven by consistently higher IOP in this feline glaucoma model. In the current study, IOP was consistently higher in glaucomatous cats compared to precisely age-matched normal cats at all time points, regardless of treatments. IOP elevation in glaucomatous cats further developed over the six months of the study period (which spanned six to 12 months of age), whereas no change in IOP with age was observed in normal cats over either study period. These findings were consistent with our previous findings that IOP reached adult values by three months of age in normal cats whereas IOP in glaucomatous cats progressively increases.¹⁵ 

Because significant or clinically meaningful IOP reduction was not observed in normal cats or cats with glaucoma after telmisartan administration, our results suggest that oral telmisartan treatment does not represent a promising IOP-lowering treatment for feline glaucomas at the dose tested. Although the LTBP2 mutant feline model of glaucoma used in the current study has been rigorously characterized,^{14–17,45–48} it may not be representative of the most common forms of feline glaucomas, because glaucoma secondary to uveitis is more frequently presented clinically.¹³ Nevertheless, our data suggest that the effects of oral telmisartan on IOP are likely to be minimal in other forms of feline glaucoma. Although IOP-lowering effects of orally-administered ARBs have previously been shown in humans and mice,^23,25,26 there is conflicting evidence demonstrating a lack of IOP-lowering effects by oral administration of ARBs in glaucoma models.^21,22 Additionally, epidemiological studies have shown that the use of ARBs is not associated with lower IOP in humans.^49–51 The results are not directly comparable between these studies because of differences in study design, including tonometry techniques, experimental design, measurement timeline, specific ARB drug and dose used, and species studied. Although it is conceivable that a higher dosage of oral telmisartan treatment might decrease IOP in cats, a higher dose carries a greater likelihood of systemic BP lowering. Notably, an initial telmisartan dose of 3 mg/kg/day (1.5 mg/kg twice daily) has been approved by the Food and Drug Administration for cats with systemic hypertension, but because the glaucomatous cats in our study were normotensive, any reduction in MAP would further reduce OPP, with the potential to intensify rather than mitigate glaucomatous damage. Importantly, although earlier studies in mice have reported that telmisartan significantly reduces IOP (∼13%), this was accompanied by profound BP lowering effects (∼40%).^23,26 Because lower OPP is a risk factor for glaucoma development and progression,^4,27 maintaining physiological BPs and OPP is important from a clinical perspective. A recent study conducted in large cohorts of human glaucoma patients also demonstrated that lower BPs were significantly associated with faster rates of retinal nerve fiber layer loss,⁵² thus systemic BP may be a significant factor in glaucoma progression. As an alternative approach, topical ocular application of ARBs may achieve higher drug concentration in the anterior segment and IOP reduction without altering BP, as previously reported in rabbits^53,54 and nonhuman primates. Although the high lipophilicity of telmisartan has afforded high tissue penetration in other studies, further studies will be needed to directly assess telmisartan concentrations in ocular tissues to ensure that local therapeutic concentrations are achieved after systemic administration. Whether topical ocular ARB application represents a potential IOP lowering strategy in other species also warrants further studies. 

The limitations of the current study include a reliance on indirect, oscillometric BP measurement, which may be less accurate than direct measurement and may underestimate SBP at higher values.^55,56 Although BP values are generally highly variable dependent on many factors including animals, acclimation, environments, and BP measurement techniques⁴² and thus difficult to compare results across studies, it should be noted that the BP values of cats in the current study were consistent with previous reports using the same BP measurement methods.^57,58 It is also conceivable that a larger number of cats could have enhanced power to detect very subtle differences in IOP, BP, and OPP after telmisartan administration. However, our post hoc analysis showed that the sample size used in this study provided adequate power to identify a clinically relevant alteration in IOP and BPs between telmisartan and placebo treated cats (at 85% power to detect 20% and 10% reduction, respectively, with α = 0.05). 

Importantly, ARBs have also been shown to have neuroprotective properties in animal models with glaucoma independent of lowering IOP. Candesartan preserved RGCs over 10 weeks of administration in a rat model of experimental glaucoma²¹ and high-dose losartan preserved RGC axons over six weeks in a mouse bead-injection glaucoma model.²² Together these published data support a neuroprotective mechanism by ARBs that are independent of IOP. Furthermore, there is accumulating evidence that the use of low-dose ARBs without systemic BP lowering effects provides neuroprotection in animal models of other neurodegenerative diseases such as Alzheimer's disease.^59,60 Therefore, despite a lack of IOP lowering effects, telmisartan may have a neuroprotective role in glaucoma. By establishing a lack of effect on IOP or OPP, the current study provides a foundation for ongoing research addressing the neuroprotective effects of telmisartan in cats with naturally occurring glaucoma. 

Conclusions

Telmisartan administration at a dosage previously shown to be anti-fibrotic in cats did not alter IOP in normal and glaucomatous cats and thus could not be used as a sole therapy for IOP control in cats with glaucoma. However, oral telmisartan (1 mg/kg every 24 hours) appears to be well tolerated in cats and was not associated with potential detrimental effects on BP or OPP. Ongoing studies will determine neuroprotective effects of telmisartan in cats with spontaneous glaucoma. 

Acknowledgments

The authors thank Rebecca L. Stepien, DVM, MS, DACVIM (Cardiology), for her helpful comments on the study design and methods for BP measurement. The authors extend their gratitude to Gregory Barrett-Wilt at the UW-Madison Mass Spectrometry Core Facility for telmisartan detection and quantification in serum samples. 

Supported in part by NIH R01 EY027396 and Core Grant for Vision Research, P30 EY0016665; Lions Eye Bank of Wisconsin Gift of Sight Discovery Fund; UW-Madison Companion Animal Fund, a National Glaucoma Research Grant from the BrightFocus Foundation, and an unrestricted grant to the Department of Ophthalmology and Visual Sciences from Research to Prevent Blindness. 

Disclosure: K. Oikawa, None; J.A. Kiland, None; V. Mathu, None; O. Torne, None; C. Wickland, None; S. Neufcourt, None; C. Mitro, None; R. Lopez, None; G.J. McLellan, None