Neue Konzepte für druckgesteuerte Glaukomimplantate

Comparison of two prototypes of a magnetically adjustable glaucoma implant in rabbits

Glaucoma drainage devices are used in surgical glaucoma therapy. Success of controlling the intraocular pressure is limited due to fibrous implant encapsulation and fibrin coating on the implant which lead to drainage obstructions. An innovative implant with a magnetically adjustable valve was developed. The valve opening of the implant should eliminate inflammatory products from the outflow area and affect fibrous tissue formation to achieve a sufficient long-term aqueous humour outflow. Lifting of this valve should disturb cell adhesion by exerting mechanical forces. Before testing this hypothesis, the flow characteristics of glaucoma drainage devices, especially the outflow resistance by regular IOP, should be considered in a pilot study, as they are important in preventing too low postoperative intraocular pressure known as ocular hypotony. Therefore, two prototypes of the innovative implant differing in their valve area design were examined regarding their flow characteristics in a limited animal experiment lasting two weeks. Ten healthy New Zealand White rabbits were divided into two groups (A & B) with different implanted prototypes. Daily, tonometry and direct ophthalmoscopy were performed to assess the intraocular pressure and the inflammatory reaction of the eye. After two weeks, the rabbits were euthanised to evaluate the initially histological inflammatory reaction to the implant. In group A, one case of hypotony emerged. When considering the entire observation period, a highly statistically significant difference between the intraocular pressure in the operated eye and that in the control eye was detected in group A (p < 0.0001) in contrast to group B (p = 0.0063). The postoperative inflammatory signs decreased within two weeks. Histologically, a typical but low level foreign body reaction with macrophages and lymphocytes as well as mild to moderate fibrosis was seen after the short experimental period. Based on our tonometric results, prototype B seems to be the system of choice for further research assessing its long-term function and biocompatibility.

Adopting oculopressure tonometry as a transient in vivo rabbit glaucoma model

Glaucoma represents a group of eye disorders partly related to raised intraocular pressure (IOP) leading to progressive optic nerve damage resulting in impaired vision and possibly blindness. To assess the suitability of new IOP lowering therapeutic strategies, such as the implantation of glaucoma drainage devices, appropriate animal models have to be used. Currently, a number of rodent glaucoma models are available [1], however, especially for surgical interventions rodent eyes are too small. Rabbits are much more suitable with respect to dimension. Unfortunately, rabbit glaucoma model systems described in literature are difficult to reproduce or fail totally, associated with a high level of discomfort and pain for treated animals. Therefore, development of an in vivo rabbit glaucoma model is one of the most important goals in glaucoma research. Here, we describe the adaptation of the oculopressure tonometry, an existing method to quantify the outflow of aqueous humor in humans, to generate a transient glaucoma model in rabbits. The existing suction-cup oculopressor (SCOP) is extended with newly designed suction-cups, which are adjusted to the anatomy of the rabbit eye. The modification of the oculopressure tonometry method facilitates an increase in IOP over a time frame of 9 minutes by vacuum induced deformation of the rabbit eye. This method can be used to test functionality of fistulating glaucoma surgeries or implanted drainage devices in a long term follow-up without any side effects and suffering of the animals.

Development of an Experimental Drug Eluting Suprachoroidal Microstent as Glaucoma Drainage Device

PURPOSE

A novel glaucoma drainage device (GDD) with local drug delivery (LDD) system was created and characterized for safety and effectiveness after implantation into the suprachoroidal space (SCS) of rabbit eyes.

METHODS

Thin films of two different polymers, Poly(3-hydroxybutyrate) (P(3HB)) and Poly(4-hydroxybutyrate) (P(4HB)), containing the drugs mitomycin C (MitC) or paclitaxel (PTX) were attached to silicone-tubes to create LDD devices. The release kinetics of these drugs were explored in vitro using high performance liquid chromatography (HPLC).  Twenty-four New Zealand white rabbits, randomly divided into eight groups, were implanted with different kinds of microstents into SCS. The intraocular pressure (IOP) was monitored noninvasively. After 6 weeks, rabbits were sacrificed and enucleated eyes were used for anterior segment optical coherence tomography (OCT), micro magnetic resonance imaging (MRI), and histology.

RESULTS

In vitro, faster drug release from both polymers was observed for MitC compared to PTX. Comparing polymers, the release from P(3HB) matrix was slower for both drugs. MRI and OCT showed all implants maintained a proper location. An effective IOP reduction was observed for up to 6 weeks in eyes with microstents combined with a drug-releasing LDD system. Overall, the surrounding tissue revealed mild-to-moderate inflammation. No pronounced fibrosis was observed in any of the groups. However, both drugs caused damage to the neighboring retina.

CONCLUSIONS

The suprachoroidal microstent reduced IOP with mild inflammation in rabbit eyes. To avoid negative effects on the retina, it is necessary to identify novel drugs with less cytotoxicity. Future studies are needed to explore the fibrotic process over the long-term.

TRANSLATIONAL RELEVANCE

The presented data serve as a proof of principle study for the concept of a suprachoroidal drug eluting microstent. Future device improvements will be focused on the design of LDD systems and the use of specific anti-inflammatory or antifibrotic agents with less cytotoxicity compared to MitC or PTX. Long-term animal studies using a reliable glaucoma model will be a further step towards clinical application and improvement of surgical glaucoma therapy.