Two recent publications highlight the challenges inherent in the targeting of voltage-gated and ion-gated ion channels. It is only in relatively recent times that technological advances (e.g. higher throughput patch-clamping) have opened up many of these targets to drug discovery and both papers are interesting in that they illustrate how these challenges are now being met with respect to two different channels of current interest:
1) N-type Calcium Channel Antagonists
The N-type calcium channel (Cav2.2) is thought to play a key role in pain pathways and the snail derived ω-conotoxin (Ziconotide), a potent Cav2.2 blocker, is known to be efficacious in the management of severe, intractable pain. However, the use of Ziconotide is limited to severe cases and requires intrathecal administration. Small-molecule, orally administered agents are hence of considerable interest to the pharmaceutical industry.
This paper from the Johnson & Johnson labs describes the screening cascade that they put in place to best identify those compounds with the requisite pharmacology. Ion channel assays, and voltage-gated channels in particular, are often difficult to configure in a manner which provides sufficiently discriminating data for the purposes of lead identification and optimisation. The combined challenge of generating robust inhibition curves (in tonic and/or use-dependent modes), confirming activity by electrophysiological means, and ensuring that the assay systems are providing physiologically relevant data is not to be underestimated. This paper stands out in that, in addition to calcium fluorescence and automated e-phys assays, they describe their use of an ex-vivo, and more therapeutically relevant, spinal cord neurotransmitter (CGRP) release assay. They describe how the data from each assay was utilised and look at the relationship between the data from the different assays. Compounds with good activity in all the assay formats were prioritised for in vivo studies. The paper does not describe any of their inhibitors, or data from their in vivo studies, so it will be interesting to follow subsequent publications from this group to ascertain the success of their approach.
2) ASIC3 Inhibitors
Acid Sensing Ion Channel 3 (ASIC3), another channel implicated in the treatment of pain, has proved to be a challenging target for which to find tractable chemical leads. In this second paper scientists at Merck describe how a lack of success in using an HTS approach led them to explore fragment screening as an alternative lead discovery method. Whilst unprecedented for this type of target, this exercise then led to the identification of novel inhibitors with high ligand efficiencies. These could not be progressed to highly potent (<100nM) lead series, but as the authors point out, this is perhaps not surprising given the lack of structural data that is usually required for the successful prosecution of fragment-based approaches. However, that aside, the paper is of considerable interest in that it demonstrates that an electrophysiology-based assay can be configured to screen a fragment library at high concentrations (of compound and DMSO) and that meaningful data can be obtained. The outcome of this particular study may also be more a reflection of the nature of ASIC3 as a target. It may be that a similar approach would be more successful applied to other ion channel targets. Again, it’ll be interesting to see how others fare using similar approaches in the future.