Introduction
Most clinically-used local anaesthetics are tertiary amine molecules that exist in an equilibrium between cationic (protonated) and neutral (unprotonated) forms. The neutral molecule can gain access to the key binding site in the channel pore (Ragsdale et al., 1996) by diffusing into the lipid membrane surrounding the channel and then entering the channel protein through “fenestrations” in the wall of the pore region (Payendah et al., 2011; Martin and Corry, 2014; Gamal El-Din et al., 2018; Nguyen et al., 2019), corresponding to the “hydrophobic pathway” proposed in a classic analysis of local anesthetic action (Hille, 1977). In a second “hydrophilic pathway” proposed in this analysis, the neutral form of the drug can diffuse across the membrane into the cytoplasm, where a fraction is re-protonated. From the cytoplasm, both cationic and neutral drug molecules can enter the channel to reach the binding site in the pore, but only when the channel is open. Consistent with this hypothesis, QX-314, a permanently charged quaternary analogue of lidocaine, cannot inhibit neuronal sodium channels when applied externally, but is effective when applied intracellularly (Frazier et al., 1970; Strichartz et al., 1973).
Although QX-314 cannot readily diffuse across the cell membrane, it can enter some neurons by permeating through certain cation-selective ion channels that have unusually large pores, notably TRPV1 and TRPA1 channels (Binshtok et al., 2007; Puopolo et al., 2013). Because these channels are selectively expressed in populations of nociceptor (pain-triggering) primary sensory neurons, application of QX-314 together with activators of TRPV1 or TRPA1 channels can produce selective inhibition/silencing of nociceptors with minimal inhibition of motor neurons or non-nociceptive sensory neurons, in contrast to the non-selective nerve block produced by lidocaine (Gerner et al., 2008; Binshtok et al., 2009a; Kim et al., 2010; Roberson et al., 2011; Brenneis et al., 2013; Zhou et al., 2014).
Pain associated with tissue inflammation is mediated in part by activation of TRPV1 and TRPA1 channels in nociceptors (reviewed by Bautista et al., 2013; Julius, 2013), raising the possibility that endogenous activation of these channels might be sufficient to enable entry of QX-314 or other cationic sodium channel blockers in inflamed tissue, without requiring co-application with exogenous TRP activators. An especially intriguing possible application of the strategy is in cough, which is mediated by airway sensory neurons, including a population of nociceptors expressing TRPV1 and TRPA1 channels (Bonvini et al., 2015; Canning et al., 2014; Mazzone and Undem, 2016). Inhaled lidocaine is used to inhibit reflexive laryngospasm and cough during bronchoscopy and is highly effective for acute suppression of cough in patients with upper respiratory tract infections (Peleg and Binyamin, 2002), COPD (Chong et al., 2005; Udezue, 2001), and asthma (Slaton et al., 2013; Udezue, 2001). However, lidocaine has a short duration of action (Chong et al., 2005) and produces potential cardiac and CNS side effects (Shirk et al., 2006) as a consequence of its high lipophilicity and ready diffusion into the bloodstream. Also, because lidocaine blocks activity in motor neurons as well as sensory neurons, it inhibits swallowing and the gag reflex (Noitasaeng et al., 2016), limiting its clinical utility.
We have designed and synthesized a novel cationic compound, BW-031, with improved potency for inhibiting sodium channels compared to QX-314. We find that BW-031 when applied alone to inflamed tissue produces long-lasting inhibition of inflammatory pain in several rat and mouse models and that inhaled BW-031 can effectively inhibit cough in a guinea pig model of allergic airway inflammation. (570 words)