Comparative Efficacy Of Ocular Hypotensive Agents In Microbead-induced Ocular Hypertensive Mouse Model

Objective:As the microbead-induced ocular hypertensive (OHT) mouse model is becoming widely accepted, we evaluated the hypotensive and neuroprotective effects of intraocular pressure (IOP) lowering agents in these mice and investigated the potential of using this model for preclinical testing of ocular hypotensive drugs. We also investigated the use of non-invasive spectral domain-optical coherence tomography in the OHT mouse model.Materials and Methods:Adult C57BL/6J mice were anesthetized by intraperitoneal injection of a mixture of ketamine (120mg/kg) and xylazine (12mg/kg). Ocular hypertension was induced by unilateral injection of2μL polystyrene microbeads (5.0X106beads/ml) into the anterior chamber. The effectiveness of most commonly used ocular hypotensive agents (0.5%timolol,0.15%brimonidine tartrate,1%brinzolamide,0.005%latanoprost, and2%pilocarpine hydrochloride) was compared. Short-term and long-term effects on mouse intraocular pressure (IOP) were monitored with a TonoLab rebound tonometer. Glaucomatous neural damage was quantitatively evaluated by counting retinal ganglion cells (RGCs) and their axon. In addition, thickness of ganglion cell complex (GCC, retinal nerve fiber layer, ganglion cell layer and inner plexiform layer) was assessed with spectral domain-optic coherence tomography (SD-OCT) coupled with a custom image analyzing software. The effects of the most commonly used ocular hypotensive drugs were evaluated by SD-OCT through comparing the GCC thickness reduction among groups. The concordance of RGC loss with GCC thickness reduction was studied by correlation analysis. P<0.05was considered statistically significant.Results:Immediately following the injection, microbeads were seen to distribute throughout the anterior chamber, but gradually accumulated at the angle of the anterior chamber that resulted in blockade of the aqueous humor outflow. Elevation of IOP reached the peak at day8after injection and lasted for more than3weeks. Numbers of RGC and axon in the microbeads injected eyes were reduced significantly compared with the contralateral (untreated) control eyes. Ocular hypertensive mouse model was successfully established by unilateral injection of microbeads into the anterior chamber. Ocular hypotensive drugs (0.5%timolol,0.15%brimonidine tartrate and1%brinzolamide) that suppress aqueous production reduced the IOP levels effectively and improved neuronal and axon survival as compared to vehicle-treated control group. SD-OCT coupled with the custom software could calculate the GCC thickness successfully. Accordingly, SD-OCT detected significantly less reduction of GCC thickness in mice treated with aqueous suppressants. In contrast, ocular hypotensive drugs (0.005%latanoprost and2%pilocarpine hydrochloride) that act on the outflow system failed to decrease IOP in the microbead-induced ocular hypertensive mouse model. In addition, quantification of the GCC thickness showed a strong correlation between its reduction and RGC loss from histological analysisConclusions:Anterior chamber injection of microbeads effectively induced IOP elevation and glaucomatous optic neuropathy in mice. Three aqueous production suppressants showed IOP-lowering and neuroprotective effect in OHT mouse model. Microbead-induced OHT mouse model offers a unique model system that display dysfunction of the outflow facility of the aqueous humor. Thus, it may allow selective screening of aqueous production suppressant drugs. Our data shows that GCC thickness assessed by SD-OCT provides a reliable imaging biomarker for non-invasive tracking of glaucomatous neurodegeneration and medical neuroprotection in mice.