Cytochrome P450 Activity in Ex Vivo Cornea Models and a Human Cornea Construct

Challenges and Opportunities in P450 Research on the Eye

Of the 57 cytochrome P450 enzymes found in humans, at least 30 have ocular tissues as an expression site. Yet knowledge of the roles of these P450s in the eye is limited, in part because only very few P450 laboratories expanded their research interests to studies of the eye. Hence the goal of this review is to bring attention of the P450 community to the eye and encourage more ocular studies. This review is also intended to be educational for eye researchers and encourage their collaborations with P450 experts. The review starts with a description of the eye, a fascinating sensory organ, and is followed by sections on ocular P450 localizations, specifics of drug delivery to the eye, and individual P450s, which are grouped and presented based on their substrate preferences. In sections describing individual P450s, available eye-relevant information is summarized and concluded by the suggestions on the opportunities in ocular studies of the discussed enzymes. Potential challenges are addressed as well. The conclusion section outlines several practical suggestions on how to initiate eye-related research. SIGNIFICANCE STATEMENT: This review focuses on the cytochrome P450 enzymes in the eye to encourage their ocular investigations and collaborations between P450 and eye researchers.

Development of In Vitro Methodologies to Investigate Binding by Sodium Hyaluronate in Eye Drops to Corneal Surfaces

PURPOSE

To develop in vitro methods to assess binding by sodium hyaluronate in eye drops to corneal surfaces.

METHODS

Two different, complementary corneal binding set-ups were developed. In a dynamic in vitro model, confluent corneal epithelial cells (HCE-T) were assembled in chamber slides and a declining channel. A static model was constructed with ex vivo porcine corneas clamped in Franz cells. To test the predictive capacity of models, four different eye drops containing sodium hyaluronate were spiked with tritium-labeled sodium hyaluronate to standardize quantification. In both settings, eye drops were applied for 5 min and physiological conditions were mimicked by flushing with artificial tear fluid. Spreading experiments on HCE-T next to synthetic membranes were used for further characterization.

RESULTS

Binding was more pronounced in dynamic HCE-T model. Three of the four eye drops demonstrated sigmoidal elution of sodium hyaluronate, suggesting pronounced binding. One solution eluted distinctly faster, likewise the buffer control. The static method produced a similar ranking but at lower levels. When eye drops in which phosphate buffer was replaced by citrate buffer (i.e., to prevent calcification) were used, binding was not influenced. All eye drops spread immediately when placed on HCE-T and at the same order of magnitude on glass and polyethylene terephthalate surfaces.

CONCLUSION

Dynamic and static models performed on different corneal sources were used to determine sodium hyaluronate binding kinetics in solutions under physiological conditions. These methodologies resulted in a ranking of the capacity of sodium hyaluronate to bind in vitro to corneal surfaces.

Biopharmaceutical evaluation of surface active ophthalmic excipients using in vitro and ex vivo corneal models

The objective of this study was to systematically investigate the effects of surface active ophthalmic excipients on the corneal permeation of ophthalmic drugs using in vitro (HCE-T cell-based model) and ex vivo (freshly excised porcine cornea) models. The permeation of four ophthalmic drugs (i.e., timolol maleate, chloramphenicol, diclofenac sodium and dexamethasone) across in vitro and ex vivo corneal models was evaluated in the absence and presence of four commonly used surface active ophthalmic excipients (i.e., Polysorbate 80, Tyloxapol, Cremophor® EL and Pluronic® F68). The concentration and self-aggregation-dependent effects of surface active ophthalmic excipients on ophthalmic drug permeability were studied from the concentration region where only dissolved monomer molecules of surface active ophthalmic excipients exist, as well as the concentration region in which aggregates of variable size and dispersion are spontaneously formed. Neither the surface active ophthalmic excipients nor the ophthalmic drugs at all concentrations that were tested significantly affected the barrier properties of both corneal models, as assessed by transepithelial electrical resistance (TEER) monitoring during the permeability experiments. The lowest concentration of all investigated surface active ophthalmic excipients did not significantly affect the ophthalmic drug permeability across both of the corneal models that were used. For three ophthalmic drugs (i.e., chloramphenicol, diclofenac sodium and dexamethasone), depressed in vitro and ex vivo permeability were observed in the concentration range of either Polysorbate 80, Tyloxapol, Cremophor® EL or Pluronic® F68, at which self-aggregation is detected. The effect was the most pronounced for Cremophor® EL (1 and 2%, w/V) and was the least pronounced for Pluronic® F68 (1%, w/V). However, all surface active ophthalmic excipients over the entire concentration range that was tested did not significantly affect the in vitro and ex vivo permeability of timolol maleate, which is the most hydrophilic ophthalmic drug that was investigated. The results of the dynamic light scattering measurements point to the association of ophthalmic drugs with self-aggregates of surface active ophthalmic excipients as the potential mechanism of the observed permeability-depressing effect of surface active ophthalmic excipients. A strong and statistically significant correlation was observed between in vitro and ex vivo permeability of ophthalmic drugs in the presence of surface active ophthalmic excipients, which indicates that the observed permeability-altering effects of surface active ophthalmic excipients were comparable and were mediated by the same mechanism in both corneal models.

Improved in vitro models for preclinical drug and formulation screening focusing on 2D and 3D skin and cornea constructs

The present overview deals with current approaches for the improvement of in vitro models for preclinical drug and formulation screening which were elaborated in a joint project at the Center of Pharmaceutical Engineering of the TU Braunschweig. Within this project a special focus was laid on the enhancement of skin and cornea models. For this reason, first, a computation-based approach for in silico modeling of dermal cell proliferation and differentiation was developed. The simulation should for example enhance the understanding of the performed 2D in vitro tests on the antiproliferative effect of hyperforin. A second approach aimed at establishing in vivo-like dynamic conditions in in vitro drug absorption studies in contrast to the commonly used static conditions. The reported Dynamic Micro Tissue Engineering System (DynaMiTES) combines the advantages of in vitro cell culture models and microfluidic systems for the emulation of dynamic drug absorption at different physiological barriers and, later, for the investigation of dynamic culture conditions. Finally, cryopreserved shipping was investigated for a human hemicornea construct. As the implementation of a tissue-engineering laboratory is time-consuming and cost-intensive, commercial availability of advanced 3D human tissue is preferred from a variety of companies. However, for shipping purposes cryopreservation is a challenge to maintain the same quality and performance of the tissue in the laboratory of both, the provider and the customer.

DynaMiTES - A dynamic cell culture platform for in vitro drug testing PART 2 - Ocular DynaMiTES for drug absorption studies of the anterior eye

In the present study, a formerly designed Dynamic Micro Tissue Engineering System (DynaMiTES) was applied with our prevalidated human hemicornea (HC) construct to obtain a test platform for improved absorption studies of the anterior eye (Ocular DynaMiTES). First, the cultivation procedure of the classic HC was slightly adapted to the novel DynaMiTES design. The obtained inverted HC was then compared to classic HC regarding cell morphology using light and scanning electron microscopy, cell viability using MTT dye reaction and epithelial barrier properties observing transepithelial electrical resistance and apparent permeation coefficient of sodium fluorescein. These tested cell criteria were similar. In addition, the effects of four different flow rates on the same cell characteristics were investigated using the DynaMiTES. Because no harmful potential of flow was found, dynamic absorption studies of sodium fluorescein with and without 0.005%, 0.01% and 0.02% benzalkonium chloride were performed compared to the common static test procedure. In this proof-of-concept study, the dynamic test conditions showed different results than the static test conditions with a better prediction of in vivo data. Thus, we propose that our DynaMiTES platform provides great opportunities for the improvement of common in vitro drug testing procedures.