Inhaled drugs are typically cleared very rapidly from the airway mucosa. Inhaled amiloride, accelerated mucociliary clearance in clinical studies, but with a half-life in the airway mucosa of <30 min, it was a short acting effect which did not translate into clinical efficacy. To achieve a long duration of action in the clinic with novel inhaled ENaC blockers, it will be necessary to maintain compound levels in the airway lumen at a concentration that will maintain target engagement for a sufficient period of time.
The aim of the present project, was to discover novel inhaled ENaC blocker compounds. A key element of the optimisation phase was to understand lung-specific pharmacokinetics, and how these related to in vivo efficacy. To this end, two key models were used. The first employed intra-tracheal delivery of test compound into the lungs of rats with sampling of compound levels in the airway, lung tissue and plasma. Compounds were then tested for duration of action and efficacy in the sheep model of mucociliary clearance (MCC).
Over the course of this project, we discovered numerous novel and potent ENaC blockers with a range of pharmacodynamic, pharmacokinetic and physicochemical properties. Of these, ETD001 was selected as a development candidate based upon its overall profile. Notably, ETD001 was not the most potent compound, with an in vitro potency (in nM) of: 40, 31 and 30 on human, sheep and rat ENaC respectively. ETD001 did however show a long residence in the airway lumen of the rat, which was accompanied by a potent and long lasting acceleration of MCC in the sheep following inhaled delivery. Other, more potent examples of novel ENaC blockers, showed significantly greater on-target potency (<5 nM) but failed to show efficacy in the sheep, which could be correlated with a diminished retention of compound in the airway lumen.
Based on the observations that ETD001 showed a long duration of action in vivo and that this was due to sustained compound levels in the airway lumen, we tested the hypothesis that repeat inhaled dosing would further increase the potency of the compound. After a single inhaled dose of ETD001, maximum efficacy in the sheep was observed at 13 µg/kg. In contrast, following twice daily dosing of ETD001 for 3.5 days, the maximal efficacious dose was reduced to 3 µg/kg, a 4-fold increase in potency.
In parallel and consistent with the sheep efficacy data, a 7 day repeat dose study in the rat revealed that lung levels of ETD001 were increased between days 1-7 in a dose-dependent manner. Furthermore, there was no change in blood potassium levels induced by ETD001 at any of these dose levels studied.
In summary, ETD001 is a novel inhaled ENaC blocker with a long duration of action in the airway lumen. Safety and efficacy data support a human tolerated dose that is 30-40 fold over the predicted minimum efficacious human dose. This is significant in view of the potential under-dosing of the Vertex ENaC blocker, VX-371, in a recent negative Phase 2 study.