Digoxin derivatives with selectivity for the α2β3 isoform of Na,K-ATPase potently reduce intraocular pressure

The ciliary epithelium in the eye consists of pigmented epithelial cells that express the alpha 1 beta 1 isoform of Na,K-ATPase and nonpigmented epithelial cells that express mainly the alpha 2 beta 3 isoform. In principle, a Na,K-ATPase inhibitor with selectivity for alpha 2 beta 3 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 alpha 2 beta 1 over alpha 1 beta 1 (up to 7.5-fold). When applied topically, the most alpha 2-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 beta 1 subunit, Gln84, suggesting a role for beta in digoxin binding. Gln84 in beta 1 is replaced by Val88 in beta 3. Assuming that alkyl substituents might interact with beta 3Val88, we synthesized perhydro-1,4-oxazepine derivatives of digoxin with diverse alkyl substituents. The methylcyclopropyl and cyclobutyl derivatives are strongly selective for alpha 2 beta 3 over alpha 1 beta 1 (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 alpha 2 beta 3-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.