Digoxin derivatives with selectivity for the α2ß3 isoform of Na,K-ATPase potently reduce intraocular pressure.

The ciliary epithelium in the eye consists of pigmented epithelial cells that express the α1ß1 isoform of Na,K-ATPase and nonpigmented epithelial cells that express mainly the α2ß3 isoform. In principle, a Na,K-ATPase inhibitor with selectivity for α2ß3 that penetrates the cornea could effectively reduce intraocular pressure, with minimal systemic or local toxicity. We have recently synthesized perhydro-1,4-oxazepine derivatives of digoxin by NaIO4 oxidation of the third digitoxose and reductive amination with various R-NH2 substituents and identified derivatives with significant selectivity for human α2ß1 over α1ß1 (up to 7.5-fold). When applied topically, the most α2-selective derivatives effectively prevented or reversed pharmacologically raised intraocular pressure in rabbits. A recent structure of Na,K-ATPase, with bound digoxin, shows the third digitoxose approaching one residue in the ß1 subunit, Gln84, suggesting a role for ß in digoxin binding. Gln84 in ß1 is replaced by Val88 in ß3. Assuming that alkyl substituents might interact with ß3Val88, we synthesized perhydro-1,4-oxazepine derivatives of digoxin with diverse alkyl substituents. The methylcyclopropyl and cyclobutyl derivatives are strongly selective for α2ß3 over α1ß1 (22-33-fold respectively), as determined either with purified human isoform proteins or intact bovine nonpigmented epithelium cells. When applied topically on rabbit eyes, these derivatives potently reduce both pharmacologically raised and basal intraocular pressure. The cyclobutyl derivative is more efficient than Latanoprost, the most widely used glaucoma drug. Thus, the conclusion is that α2ß3-selective digoxin derivatives effectively penetrate the cornea and inhibit the Na,K-ATPase, hence reducing aqueous humor production. The new digoxin derivatives may have potential for glaucoma drug therapy.

Digoxin derivatives with enhanced selectivity for the α2 isoform of Na,K-ATPase: effects on intraocular pressure in rabbits.

In the ciliary epithelium of the eye, the pigmented cells express the α1ß1 isoform of Na,K-ATPase, whereas the non-pigmented cells express mainly the α2ß3 isoform of Na,K-ATPase. In principle, a Na,K-ATPase inhibitor with selectivity for α2 could effectively reduce intraocular pressure with only minimal local and systemic toxicity. Such an inhibitor could be applied topically provided it was sufficiently permeable via the cornea. Previous experiments with recombinant human α1ß1, α2ß1, and α3ß1 isoforms showed that the classical cardiac glycoside, digoxin, is partially α2-selective and also that the trisdigitoxose moiety is responsible for isoform selectivity. This led to a prediction that modification of the third digitoxose might increase α2 selectivity. A series of perhydro-1,4-oxazepine derivatives of digoxin have been synthesized by periodate oxidation and reductive amination using a variety of R-NH2 substituents. Several derivatives show enhanced selectivity for α2 over α1, close to 8-fold in the best case. Effects of topically applied cardiac glycosides on intraocular pressure in rabbits have been assessed by their ability to either prevent or reverse acute intraocular pressure increases induced by 4-aminopyridine or a selective agonist of the A3 adenosine receptor. Two relatively α2-selective digoxin derivatives efficiently normalize the ocular hypertension, by comparison with digoxin, digoxigenin, or ouabain. This observation is consistent with a major role of α2 in aqueous humor production and suggests that, potentially, α2-selective digoxin derivatives could be of interest as novel drugs for control of intraocular pressure.