Novel melanin‐binding carbonic anhydrase II inhibitor for long‐acting topical glaucoma treatment

Aims/Purpose: We are developing new eye drop treatment for glaucoma with dosing frequency once a week by utilizing drug binding to ocular melanin. Carbonic anhydrase (CA) II enzyme inhibitors decrease intraocular pressure (IOP) and thus halt the progression of retinal damage by reducing the secretion of aqueous humour in the ciliary body. Since ciliary body is a melanin‐rich tissue, we hypothesize that long‐lasting CAII inhibition and IOP‐lowering effect may be achieved due to gradual drug release from melanin‐bound drug depot. Methods: Melanin binding affinity for about 30 novel CAII inhibitors was studied with microscale thermophoresis, and three structurally rather similar and equipotent molecules with different melanin binding properties were selected for in vivo efficacy studies. IOP‐lowering effect of these new molecules was studied with albino and pigmented rabbits with iCare® PRO rebound tonometer. For pharmacokinetic studies, the high melanin binding new CAI (molecule 184) was labelled with tritium, and the study was conducted with albino and pigmented rats with conventional methods. Bound drug was analysed from iris‐ciliary body pellet and unbound fraction from supernatants. Results: Melanin‐binding affinity of new CAII inhibitors varies from low or non‐binders (Kd > 650 μM) to high melanin binders (Kd < 65 μM). In vivo efficacy results show that the molecules decrease IOP in both albino and pigmented eyes at the level comparable with dorzolamide (Trusopt®). In pigmented eyes, melanin binding prolongs the effect of compound 184 even up to 2 weeks after single eye drop. In the pharmacokinetic rat study, molecule 184 shows remarkably prolonged tissue binding and high iris‐ciliary body exposure in pigmented eyes. Conclusions: After topical administration IOP‐lowering effect of high melanin‐binding CA II inhibitor is significantly prolonged in pigmented eyes.