Influence of lipophilicity on drug partitioning into sclera, choroid-retinal pigment epithelium, retina, trabecular meshwork, and optic nerve

Abstract

In vitro bovine eye tissue/phosphate-buffered saline, pH 7.4, partition coefficients (Kt:b), in vitro binding to natural melanin, and in vivo delivery at 1 h after posterior subconjunctival injection in Brown Norway rats were determined for eight beta-blockers. The Kt:b was in the order intact tissue, dry weight method >or= intact tissue, wet weight method corrected for tissue water and drug in tissue water >> intact tissue, wet weight method > homogenized tissue. In intact tissue methods, Kt:b followed the order choroid-retinal pigment epithelium (RPE) > trabecular meshwork > retina > sclera approximately optic nerve; propranolol > betaxolol > pindolol approximately timolol approximately metoprolol > sotalol approximately atenolol approximately nadolol. Intact tissue, wet weight log (Kt:b) correlated positively with log D for all tissues (R(2) of 0.7-0.9). Log (melanin binding capacity) correlated positively with choroid-RPE log (Kt:b) (R(2) of 0.5). With an increase in concentration, Kt:b decreased in trabecular meshwork for all beta-blockers and for some lipophilic beta-blockers in choroid-RPE and sclera. With an increase in drug lipophilicity, in vivo tissue distribution increased in choroid-RPE, iris-ciliary body, sclera, and cornea but exhibited a declining trend in retina, vitreous, and lens. In vitro bovine intact tissue, wet weight Kt:b correlated positively with rat in vivo tissue/vitreous humor distribution for sclera, choroid-RPE, and retina (R(2) of 0.985-0.993). In vitro tissue partition coefficients might be useful in predicting in vivo drug distribution after trans-scleral delivery. Less lipophilic solutes exhibiting limited nonproductive binding in choroid-RPE might exhibit greater trans-scleral delivery to the retina and vitreous.

Fig. 1.

Comparison of various bovine eye tissues for β-blocker partition coefficients estimated using different methods. A, intact tissue, wet weight method. B, intact tissue, dry weight method (tissues were dried post incubation for 12 h at 90°C and dry tissue weight were used for calculating partition coefficients). C, intact tissue, wet weight, water and drug in water corrected method (measured tissue water content was subtracted from tissue weights and the drug amount in tissue water was subtracted as well). D, homogenized tissue, wet weight method. Tissue partitioning ratios are calculated as ratio of concentration of β-blockers in tissue (micrograms per gram of tissue) to concentration in buffer (micrograms per milliliter). Data for 5 μg/ml β-blockers cocktail solution in PBS has been plotted. The data are expressed as mean ± S.D. for n = 5. ∗, P ≤ 0.01 compared with sclera, retina, optic nerve, and trabecular meshwork. +, P ≤ 0.05 compared with sclera, retina, and optic nerve.

Fig. 2.

Correlation of log in vitro tissue/buffer partition coefficients of β-blockers with estimated log D (distribution coefficients). A, intact tissue, wet weight method. B, intact tissue, dry weight method (tissues were dried post incubation for 12 h at 90°C and dry tissue weight were used for calculating partition coefficients. C, intact tissue, wet weight, water and drug in water corrected method (measured tissue water content was subtracted from tissue weights and the drug amount in tissue water was subtracted as well). D, homogenized tissue, wet weight method. Data for 5 μg/ml β-blockers cocktail solution in PBS have been plotted. Correlation coefficients (R²) for linear regression analysis are given in front of respective tissue legends. Results for tissue partitioning ratio are expressed as the mean for n = 5.

Fig. 3.

Comparison of β-blocker partition coefficient estimates in each bovine eye tissue. A, sclera. B, choroid-RPE. C, retina. D, optic nerve. E, trabecular meshwork. Data for 5 μg/ml β-blockers cocktail solution in phosphate buffer have been plotted. The data are expressed as mean ± S.D. for n = 5. ∗, P ≤ 0.005 compared with dry and water content subtracted tissue. +, P ≤ 0.05 compared with intact tissue.

Fig. 4.

Influence of β-blocker concentrations on tissue partition coefficients in sclera (A), choroid-RPE (B), retina (C), optic nerve (D), and trabecular meshwork (E). Data for 0.5, 5, and 25 μg/ml β-blockers cocktail solution in PBS using intact, wet weight method have been plotted. The data are expressed as mean ± S.D. for n = 5. ∗, P ≤ 0.001 and +, P ≤ 0.05 compared with 25 μg/ml; ±, P ≤ 0.01 compared with 5 μg/ml.

Fig. 5.

Correlation of in vitro melanin (S. officinalis) binding of β-blockers with tissue partition coefficients in intact choroid-RPE (A and B), homogenized choroid-RPE (C and D), and solute lipophilicity (E and F). Melanin binding capacities correlated better than binding affinities. Results are expressed as the mean of n = 5 for tissue partitioning and mean of n = 3 for melanin binding.

Fig. 6.

Ocular tissue concentrations of β-blockers at the end of 1 h in BN rats after periocular administration of a solution of eight β-blockers in PBS. The data are expressed as mean ± S.D. for n = 4.

Fig. 7.

Correlation of in vivo tissue distribution of β-blockers in BN rat eye tissue with estimated log D (distribution coefficients). Correlation coefficients (R²) for linear regression analysis are indicated in the insets. Results for tissue distribution are expressed as the mean for n = 4.

Fig. 8.

Correlation of in vivo tissue/vitreous distribution ratio of β-blockers in BN rat at 1 h after dosing with bovine in vitro tissue/PBS partition coefficients for sclera (A), choroid-RPE (CRPE; B, and retina (C). Data for 5 μg/ml β-blockers cocktail solution in PBS have been used for in vitro partition coefficients. Results are expressed as mean ± S.D. (n = 5) for in vitro tissue partitioning data and mean ± S.D. (n = 4) for in vivo data.