Cardiac glycosides with selectivity for the α2 isoform of NaK‐ ATPase (974.1)

Abstract

Cardiac glycosides (CG) are specific inhibitors of Na,K‐ATPase. Na,K‐ATPase consists of, α and β subunits and an FXYD regulatory protein. There are four isoforms of α and three isoforms of β, which are expressed in a tissue‐specific fashion. α1 is the general purpose isoform, α2 is expressed in heart, and plays a key role in cardiac contraction and thought to be an important target of CG’s. Inhibitors of α2 could induce cardiac contraction (positive inotropy) with minimal toxic (arhythmogenic) effects. α2 is also expressed in the eye, in non‐pigmented cells of the ciliary epithelium, and is the motor for production of aqueous humour. The aim of this project was to develop CG’s with selectivity for human α2 isoforms of NaK‐ATPase that could effectively control raised intra‐ocular pressure (IOP), and induce cardiac contraction with minimal toxic effects.We combined techniques from organic synthesis, biochemistry‐protein expression, purification, binding assays and ATPase inhibition measurements. We used recombinant purified human α1β1, α2β1 isoforms to screen a large No.of known CG’s, and found that the sugar moiety determines isoform selectivity. Based on these finding we synthesized a series of perhydro‐1,4‐oxazepine derivatives of digoxin, modified in the third digitoxose moiety. Several derivatives show improved α2/α1 selectivity, close to 8‐fold. The ability of these derivatives to prevent an acute pharmacologically‐induced increase in IOP in rabbit eyes has been studied. Two α2‐selective derivatives efficiently prevent the ocular hypertension, by comparison with known CG's. These selective inhibitors of α2 isoform can turn into safer treatment of heart failure and high ocular pressure, the main risk factor in glaucoma.