Ocular pharmacokinetics of intravitreally administered brimonidine and dexamethasone in animal models with and without blood-retinal barrier breakdown.

PURPOSE: We compared ocular and systemic pharmacokinetics of brimonidine and dexamethasone following a single intravitreal dose in animals with blood-retinal barrier (BRB) breakdown and in healthy controls. METHODS: We induced BRB breakdown in rabbits by intravitreal injection of recombinant human VEGF165 and choroidal neovascularization (CNV) in monkeys with laser. Control and disease animals then received single intravitreal injections of brimonidine alone, dexamethasone alone, or brimonidine in combination with dexamethasone. Ocular tissues and plasma were collected and quantified for drug concentration using LC-MS/MS assays. Statistical analysis was performed to compare the pharmacokinetic parameters between the control and disease animal models. RESULTS: In rabbits, brimonidine and dexamethasone exposure, as assessed by area under the drug concentration-time curve (AUC) in aqueous humor, retina, and choroid, was lower in disease than control animals, with a greater difference observed for dexamethasone than brimonidine. In monkeys, dexamethasone exposure was lower in disease than control animals for the central retina/choroid and peripheral choroid, whereas brimonidine exposure was lower in disease animals only in the central retina/choroid. Plasma exposure to both drugs was comparable between control and disease animals in both species. CONCLUSIONS: In animal models with a breakdown of the blood-retina barrier, drug clearance could be increased, resulting in lower drug concentration in ocular tissues compared to normal animals. However, the extent of difference may be compound- and disease model-specific. Therefore, extrapolation of ocular pharmacokinetic data obtained in normal animals to disease models for the purpose of pharmacokinetic/pharmacodynamic data analysis should be performed with caution.

Assessment of the differences in pharmacokinetics and pharmacodynamics between four distinct formulations of triamcinolone acetonide.

PURPOSE: To compare the durability of Kenalog, Trivaris, Triesence, and compounding pharmacy preservative-free triamcinolone acetonide in pigmented rabbits with syneretic vitreous using direct visualization, pharmacodynamics, and pharmacokinetics. METHODS: Twenty-five Dutch-belted rabbits were used. Pharmacokinetic experiment: Rabbits were intravitreally injected with one of four 4-mg triamcinolone acetonide formulations. Wide-field imaging was serially performed to document residual drug mass. Pharmacodynamics experiment: Four triamcinolone acetonide groups and one control group received intravitreal recombinant human vascular endothelial growth factor 165 every 2 weeks and were followed with fluorescein angiography to assess vascular endothelial growth factor retinal vasculopathy as a measure of residual steroid effect. Particle size of the formulations was measured with Mastersizer 2000. RESULTS: Remaining triamcinolone acetonide mass after 19 weeks: 12,091 ± 2,512 pixels for the Kenalog group, 1,307.36 ± 695.57 for Trivaris, 5577 ± 1477 for Triesence, and 1,535 ± 329 for compounded preservative-free triamcinolone acetonide. Kenalog suppressed recombinant human vascular endothelial growth factor-induced retinopathy more effectively than the other triamcinolone acetonide groups at Week 39, the final time point assessed. Particle size (90th percentile) was 47 µm for Kenalog, 26 µm for Triesence, and 22 µm for both compounded preservative-free triamcinolone acetonide and Trivaris. CONCLUSION: Triamcinolone acetonide formulations do not have the same pharmacokinetics/pharmacodynamics. Kenalog has the longest vitreous visibility and durability. Particle size appears to correlate with efficacy and durability.

Pharmacokinetics of a sustained-release dexamethasone intravitreal implant in vitrectomized and nonvitrectomized eyes.

PURPOSE: To evaluate dexamethasone pharmacokinetics after implantation of a sustained-release dexamethasone (DEX) intravitreal implant in nonvitrectomized and vitrectomized eyes. METHODS: The right eyes of 25 rabbits underwent vitrectomy; contralateral eyes served as nonvitrectomy controls. The 0.7-mg DEX implant was injected into both eyes, and drug concentrations were determined in the vitreous humor and retina for 31 days (on days 2, 8, 15, 22, and 31). RESULTS: DEX was present in nonvitrectomized and vitrectomized eyes for at least 31 days. There were no statistically significant differences in DEX concentration between nonvitrectomized and vitrectomized eyes at any time point (P > 0.05). The maximum concentration of DEX in nonvitrectomized versus vitrectomized eyes for vitreous humor was 791 ng/mL (day 22) versus 731 ng/mL (day 22), respectively, and for retina it was 4110 ng/mL (day 15) versus 3670 ng/mL (day 22), respectively. Mean absorption (AUC(0-tlast)) of dexamethasone in nonvitrectomized and vitrectomized eyes was not different for both the vitreous humor (13,600 vs. 15,000 ng/day/mL; P = 0.73) and retina (67,600 vs. 50,200 ng/day/mL; P = 0.47). CONCLUSIONS: The vitreoretinal pharmacokinetic profiles were similar between nonvitrectomized and vitrectomized eyes. These observations are consistent with clinical findings of the DEX implant in patients who have undergone vitrectomy and should reduce concerns about the use of the DEX implant in eyes that have undergone vitrectomy.

Treatment of experimental anterior and intermediate uveitis by a dexamethasone intravitreal implant.

PURPOSE: To assess the efficacy of a dexamethasone (DEX) intravitreal implant in a rabbit model of anterior and intermediate uveitis. METHODS: Experimental anterior and intermediate uveitis was induced by a unilateral intracameral injection of Mycobacterium tuberculosis H37Ra antigen in preimmunized rabbits. Four days after uveitis induction, rabbits received DEX implant or underwent a sham procedure (no implant). Clinical and histopathologic signs of uveitis were assessed for 13 days, and levels of inflammatory markers in the iris/ciliary body were measured after 21 days. RESULTS: All signs of anterior and intermediate uveitis were reduced by the DEX implant compared with sham procedure. At day 13, mean anterior chamber cell scores ± SD for the DEX implant versus the sham procedure were, respectively, 1.9 ± 1.3 versus 4.0 ± 0.0 (P = 0.04), and mean total histologic inflammatory scores were 3.9 ± 2.5 versus 15.4 ± 6.0 (P = 0.026). Similarly, at day 13, mean vitreous haze severity scores (SD) for the DEX implant versus the sham procedure were, respectively, 0.1 ± 0.2 versus 2.7 ± 1.5 (P = 0.026), and mean vitreous inflammatory cell infiltration scores were 0.0 ± 0.0 versus 1.5 ± 1.3. Treatment with the DEX intravitreal implant also significantly reduced the proinflammatory immune response, as measured by cytokine levels in iris/ciliary body. CONCLUSIONS: A single administration of DEX implant significantly reduced inflammation in an animal model of anterior and intermediate uveitis.

Anterior segment growth and peripheral neural pathways in chick.

PURPOSE: To learn if peripheral nerve pathways are necessary for corneal expansion and anterior segment growth under a 12-hr light:dark cycle or for the inhibition of corneal expansion under constant light rearing. METHODS: Recently hatched White Leghorn chicks under anesthesia received unilateral ciliary ganglionectomy (CGx), cranial cervical ganglionectomy (Sx), or section of the ophthalmic nerve (TGx), along with sham-operated and/or never-operated control cohorts. Chicks were reared postoperatively under either a 12-hr light:dark cycle or under constant light. After 2 weeks and with the chicks under anesthesia, corneal radii of curvature and diameters were obtained with a photokeratoscope, refractometry and A-scan ultrasonography were performed, and the axial and equatorial dimensions of enucleated eyes were measured with digital calipers. Corneal areas were calculated from corneal curvatures and diameters. RESULTS: Despite the rich peripheral innervation to the eye, the selective denervations performed here exerted remarkably limited effects on corneal expansion and anterior segment development in chicks reared under either lighting condition. Ophthalmic nerve section did reverse in large part the inhibition of equatorial expansion of the vitreous chamber occurring under constant light rearing. CONCLUSIONS: The ciliary, sympathetic, or ophthalmic peripheral nerve pathways to the eye are not required either for corneal expansion and anterior segment development under a 12-hr light:dark cycle or for the inhibition of corneal expansion under constant light rearing. The ocular sensory innervation may be a means for regulating vitreous cavity shape.

Local patterns of image degradation differentially affect refraction and eye shape in chick.

PURPOSE: To evaluate visual blur as a mechanism for modulating eye shape. METHODS: Chicks wore a unilateral full goggle or one of several goggles modified with apertures. After 2 weeks, eyes were measured with refractometry, ultrasound, and calipers, and three retinal regions were assayed for dopamine and DOPAC (3,4-dihydroxyphenylacetic acid). RESULTS: Goggled eyes were diffusely enlarged or enlarged predominantly along the axial dimension, depending on the goggle. Myopia developed under goggle types inducing primarily axial growth and under some of the goggles inducing diffuse eye expansion. Enlarged eyes remained emmetropic beneath other goggles that caused diffuse eye expansion. Reductions in retinal dopamine and DOPAC were proportional to the eye growth and refraction effects. CONCLUSIONS: Localized image degradation can cause myopia with predominantly axial expansion, myopia with more diffuse vitreous chamber expansion, or eye expansion without myopia. Robust expansion of the equatorial diameter alone was not observed. The associated alterations in retinal dopamine metabolism are consistent with a hypothesized role of dopaminergic amacrine cells in the visual regulation of eye growth. Besides refraction and overall size, visual blur can affect eye shape; but the goggle responses do not correspond to a simple summation of blur signals across the retina. Therefore, other mechanisms seemingly are needed to account for the full range of refractions and ocular shapes seen in chicks and, by analogy, in humans.

Ocular changes after photodynamic therapy.

PURPOSE: The aim of this study was to identify the changes in the primate visual system after a single session of photodynamic therapy (PDT) in an intact nonhuman primate retina. METHODS: As part of a larger study, PDT (wavelength 689 nm, 50 J/cm2, 600 mW/cm2, 83 seconds, 4-mm spot size) with verteporfin (6 mg/m2 intravenous infusion) was performed in one eye each of two cynomolgus monkeys. Fundus photography, fluorescein angiography (FA), indocyanine green angiography (ICG), optical coherence tomography (OCT), and multifocal electroretinography (mfERG) were performed at baseline and 12 time points (1-283 days) after PDT. In addition, retinal histopathologic findings were evaluated at 9 months. RESULTS: Various morphologic changes, including whitening of the treated area, RPE proliferation, closure of the choroidal vasculature, and subretinal edema (followed by foveolar thinning) were observed. Most of the changes persisted and were detectable in histopathologic evaluation at 9 months. Reductions of the mfERG amplitude, followed by varying degrees of recovery from the treated and the border regions, were observed. This was accompanied by progressive delay of P1 peak time up to 3 months after treatment, followed by complete recovery at 9 months. In addition, the nontreated area showed amplitude and timing mfERG deficits, which underwent gradual (but not complete) recovery. CONCLUSIONS: In a primate model, under standard clinical parameters, a single PDT treatment resulted in various dynamic morphologic and functional retinal changes detectable for up to 9 months after treatment. The significance of the observed changes and possible ways of pharmacologic interference with PDT adverse effects are discussed.