PurposeThe aim of the present study was to evaluate the utility of the relatively hydrophilic Δ9-tetrahydrocannabinol (THC) prodrugs, mono and di-valine esters (THC-Val and THC-Val-Val) and the amino acid (valine)-dicarboxylic acid (hemisuccinate) ester (THC-Val-HS), with respect to ocular penetration and intraocular pressure (IOP) lowering activity. THC, timolol, and pilocarpine eye drops were used as controls.MethodsTHC-Val, THC-Val-Val, and THC-Val-HS were synthesized and chemically characterized. Aqueous solubility and in vitro transcorneal permeability of THC and the prodrugs, in the presence of various surfactants and cyclodextrins, were determined. Two formulations were evaluated for therapeutic activity in the α-chymotrypsin induced rabbit glaucoma model, and the results were compared against controls comprising of THC emulsion and marketed timolol maleate and pilocarpine eye drops.ResultsTHC-Val-HS demonstrated markedly improved solubility (96-fold) and in vitro permeability compared to THC. Selected formulations containing THC-Val-HS effectively delivered THC to the anterior segment ocular tissues in the anesthetized rabbits: 62.1 ng/100 μL of aqueous humor (AH) and 51.4 ng/50 mg of iris ciliary bodies (IC) (total THC). The duration and extent of IOP lowering induced by THC-Val-HS was 1 hour longer and 10% greater, respectively, than that obtained with THC and was comparable with the pilocarpine eye drops. Timolol ophthalmic drops, however, exhibited a longer duration of activity. Both THC and THC-Val-HS were detected in the ocular tissues following multiple dosing of THC-Val-HS in conscious animals. The concentration of THC in the iris-ciliary bodies at the 60- and 120-minute time points (53 and 57.4 ng/50 mg) were significantly greater than that of THC-Val-HS (24.2 and 11.3 ng/50 mg). Moreover, at the two time points studied, the concentration of THC was observed to increase or stay relatively constant, whereas THC-Val-HS concentration decreased by at least 50%. A similar trend was observed in the retina-choroid tissues.ConclusionsA combination of prodrug derivatization and formulation development approaches significantly improved the penetration of THC into the anterior segment of the eye following topical application. Enhanced ocular penetration resulted in significantly improved IOP-lowering activity.

Purpose-The aim of this study was to evaluate the contribution of amino acid transporters in the transcorneal permeation of the aspartate (Asp) ester acyclovir (ACV) prodrug.Methods-Physicochemical characterization, solubility and stability of acyclovir L-aspartate (LAsp-ACV) and acyclovir D-aspartate (D-Asp-ACV) were studied. Transcorneal permeability was evaluated across excised rabbit cornea.Results-Solubility of L-Asp-ACV and D-Asp-ACV were about 2-fold higher than that of ACV. The prodrugs demonstrated greater stability under acidic conditions. Calculated pK a and logP values for both prodrugs were identical. Transcorneal permeability of L-Asp-ACV (12.1±1.48×10 −6 cm/s) was 4-fold higher than D-Asp-ACV (3.12±0.36×10 −6 cm/s) and ACV (3.25±0.56×10 −6 cm/s). ACV generation during the transport process was minimal. L-Asp-ACV transport was sodium and energy dependent but was not inhibited by glutamic acid. Addition of BCH, a specific B o,+ and L amino acid transporter inhibitor, decreased transcorneal L-Asp-ACV permeability to 2.66±0.21×10 −6 cm/ s. L-Asp-ACV and D-Asp-ACV did not demonstrate significant difference in stability in ocular tissue homogenates.Conclusion-The results demonstrate that enhanced transport of L-Asp-ACV is as a result of corneal transporter involvement (probably amino acid transporter B 0,+ ) and not as a result of changes in physicochemical properties due to prodrug derivatization (permeability of D-Asp-ACV and ACV were not significantly different).

The aim of the present study was to evaluate and improve the in vitro transcorneal permeability characteristics of Δ9-tetrahydrocannabinol (THC) through prodrug derivatization and formulation approaches. In vitro corneal permeability of THC and its hemisuccinate (THC-HS) and hemiglutarate (THC-HG) ester prodrugs and WIN 55-212-2 (WIN), a synthetic cannabinoid, was determined using isolated rabbit cornea. The formulations studied included hydroxypropyl beta cyclodextrin (HPβCD) or random methylated beta cyclodextrin (RMβCD), as well as prodrug/ion-pair complexes with l-arginine or tromethamine. Corneal permeability of WIN was found to be two-fold higher than THC in the presence of HPβCD. THC-HS and THC-HG exhibited pH dependent permeability. In the presence of HPβCD, at pH 5 (donor solution pH), both prodrugs exhibited six-fold higher permeability compared to THC. However, permeability of the prodrugs was about three-fold lower than that of THC at pH 7.4. RMβCD, at pH 7.4, led to a significant improvement in permeability. Formation of ion-pair complexes markedly improved the solubility and permeability of THC-HG (7-fold and 3-fold greater permeability compared to THC and WIN, respectively) at pH 7.4. The in vitro results demonstrate that the use of an ion-pair complex of THC-HG could be an effective strategy for topical delivery of THC.

In vitro transcorneal permeability studies are an important screening tool in drug development. The objective of this research is to examine the feasibility of using corneas isolated from preserved rabbit eyes as a model for permeability evaluation. Eyes from male New Zealand White rabbits were used immediately or were stored overnight in PBS or HBSS over wet ice. Integrity of isolated corneas was evaluated by measuring the TEER and by determining the permeability of paracellular and transcellular markers. Active transport was assessed by measuring transcorneal permeability of selected amino acids. Esterase activity was estimated using p-nitrophenyl assay. In all cases, corneas from freshly enucleated eyes were compared to those isolated from the day-old preserved eyes. Transcellular and paracellular passive diffusion was not affected by the storage medium and observed to be similar in the fresh and preserved eye models. However, amino acid transporters demonstrated lower functional activity in corneas excised from eyes preserved in PBS. Moreover, preserved eyes displayed almost 1.5-fold lower esterase activity in the corneal tissue. Thus, corneas isolated from day-old eyes, preserved in HBSS, closely mimics freshly excised rabbit corneas in terms of both active and passive transport characteristics but possesses slightly reduced enzymatic activity.

Purpose The goal of the present study is to develop a poly (ethylene oxide) N10 (PEO N10) based melt-cast matrix system for efficient and prolonged delivery of hesperetin (HT), a promising bioflavonoid, to the posterior segment of the eye through the topical route. Methods HT film was prepared by melt-cast method using PEO N10 and cut into 4 mm × 2 mm segments, each weighing 8 mg. This film was evaluated with respect to in vitro release rates and also transmembrane delivery across Spectra/Por® membrane (MWCO: 10000 Daltons) and isolated rabbit corneas. Ocular tissue concentrations were also determined post application of the film in ex vivo and in vivo models. Results HT release from the film was determined to be about 95.3 % within 2 h. In vitro transcorneal flux was observed to be 0.58 ± 0.05 μg/min/cm2 across the isolated rabbit cornea. High levels of HT were detected in the retina-choroid (RC) and vitreous humor (VH) in the ex vivo model following topical application of the film. Significant levels of HT were observed in both anterior and posterior segment ocular tissues 1h post topical application of the 10 and 20 %w/w HT films on the rabbit eye. Moreover, HT was detected in the VH and RC even after 6h following topical application of the film in vivo. Conclusion The results from this study suggest that the melt-cast films can serve as a viable platform for sustained topical delivery of bioflavonoids, and other therapeutic agents, into the back-of-the eye tissues.