Oct. 15, 2016
Elevated intraocular pressure (IOP) is the most prevalent and the only treatable risk factor for glaucoma. Unfortunately, current therapeutics used to treat elevated IOP have significant and sometimes irreversible side effects.
Michael P. Fautsch, Ph.D., Ophthalmology, and a team at Mayo Clinic's campus in Rochester, Minnesota, work to develop novel compounds for the treatment of glaucoma. In a study published in PLOS One in 2015, the team evaluated the IOP lowering ability of ATP-sensitive potassium (KATP) channel opener cromakalim in human and murine models. Results indicate that cromakalim can significantly lower IOP in human and animal model systems without observable toxic side effects.
Combination treatment with latanoprost free acid, a commonly used ocular hypotensive drug, produced a significantly greater reduction in IOP compared with treatment with either drug alone in mice.
"Compatibility with existing glaucoma drugs also is important," says Dr. Fautsch. "The use of multiple agents working through different pathways to lower IOP will provide better opportunities for IOP reduction in patients with glaucoma to slow disease progression."
KATP and IOP
KATP channels are hetero-octameric proteins that are affected by changes in micromolar concentrations of intracellular ATP. These channels connect the metabolic and energetic state of cells and are involved in the regulation of several vital cellular functions. Previous studies from Dr. Fautsch's laboratory identified KATP channel openers diazoxide and nicorandil as IOP reducing agents.
In contrast, cromakalim, another KATP channel opener, had been reported to increase IOP modestly in rabbits. "Because this was the opposite result the team had found with KATP channel openers diazoxide and nicorandil, we evaluated the effect of cromakalim on IOP in human anterior segment organ cultures and murine experimental model systems," says Dr. Fautsch.
In the human anterior segment model, 17 donor eyes — from 12 males and five females, ages 40-93 — were bisected at the equator and the front of the eyes were clamped in modified petri dishes and perfused with Dulbecco's modified Eagle's media containing 1 percent antibiotic-antimycotic solution at the normal human aqueous humor flow rate (2.5 µl/min). Pressure inside the anterior chamber was monitored in real time through a secondary access cannula built into the modified petri dishes. Following attainment of stable baseline pressure, different concentrations of cromakalim were added to one eye while the contralateral eye received the vehicle in the same proportion.
Cultured human anterior segments, when treated with 2 μM cromakalim, showed a decrease in pressure (19.3 ± 2.8 mm Hg at 0 hours to 13.2 ± 2.6 mm Hg at 24 hours; P < 0.001) when compared with vehicle-treated controls (15.9 ± 5.3 mm Hg at 0 hours to 15.6 ± 4.9 mm Hg at 24 hours; P = 0.9).
In the murine model, IOP was measured in live, conscious wild-type C57BL/6 and Kir6.2(-/-) mice using a hand-held rebound tonometer. IOP was measured daily for three days before treatment to obtain baseline IOP. For treatment, a 5µl drop of cromakalim (5-mM concentration) was added to one eye while the contralateral eye received a vehicle (DMSO and Cremophor EL) in equivalent proportions to the treated eye. Once-daily treatment continued for five consecutive days with IOP measured at one, four and 23 hours after each treatment. Post-treatment IOP was recorded for three consecutive days at similar time points.
In wild-type C57BL/6 mice, cromakalim reduced IOP by 18.8 ± 2.2 percent compared with vehicle-treated contralateral eyes (17.0 ± 0.3 mm Hg at 0 hours to 13.8 ± 0.4 mm Hg at 24 hours; n = 10, P = 0.002). Cessation of treatment resulted in IOP returning to baseline levels within 48 hours.
To examine KATP channel subunit specificity, mice lacking one of the functional KATP channel subunits (Kir6.2) were treated with cromakalim. In contrast to wild-type mice, no change in IOP was noted.
In the last set of experiments, cromakalim was added separately and in combination with latanoprost to Kir6.2(-/-) mice. Results showed that the combination of cromakalim and latanoprost lowered IOP greater than either drug alone, indicating the drugs have an additive effect on IOP reduction.
"Treatments targeting the underlying physiology of the outflow pathways are beneficial in slowing disease progression in patients with glaucoma and advancing our knowledge and understanding of glaucoma," says Dr. Fautsch. "This study suggests that cromakalim is an excellent candidate for therapeutic development." Results confirm:
- Cromakalim is a potent ocular hypotensive agent that lowers IOP via activation of Kir6.2 containing KATP channels
- Its effect is additive when used in combination with the commonly used glaucoma drug latanoprost
For more information
Chowdhury UR, et al. Ocular hypotensive effects of the ATP-sensitive potassium channel opener cromakalim in human and murine experimental model systems. PLOS One. 2015;10:e0141783. (Erratum in PLOS One. 2015;10:e0144791.)