Discovery and Optimization of Selective Na_v1.8 Modulator Series That Demonstrate Efficacy in Preclinical Models of Pain

Voltage-gated sodium channels, in particular Na_v1.8, can be targeted for the treatment of neuropathic and inflammatory pain. Herein, we described the optimization of Na_v1.8 modulator series to deliver subtype selective, state, and use-dependent chemical matter that is efficacious in preclinical models of neuropathic and inflammatory pain

Subtype-Selective Small Molecule Inhibitors Reveal a Fundamental Role for Nav1.7 in Nociceptor Electrogenesis, Axonal Conduction and Presynaptic Release

<div><p>Human genetic studies show that the voltage gated sodium channel 1.7 (Na_v1.7) is a key molecular determinant of pain sensation. However, defining the Na_v1.7 contribution to nociceptive signalling has been hampered by a lack of selective inhibitors. Here we report two potent and selective arylsulfonamide Na_v1.7 inhibitors; PF-05198007 and PF-05089771, which we have used to directly interrogate Na_v1.7’s role in nociceptor physiology. We report that Na_v1.7 is the predominant functional TTX-sensitive Na_v in mouse and human nociceptors and contributes to the initiation and the upstroke phase of the nociceptor action potential. Moreover, we confirm a role for Na_v1.7 in influencing synaptic transmission in the dorsal horn of the spinal cord as well as peripheral neuropeptide release in the skin. These findings demonstrate multiple contributions of Na_v1.7 to nociceptor signalling and shed new light on the relative functional contribution of this channel to peripheral and central noxious signal transmission.</p></div

PF-05198007 reduces the capsaicin flare response in WT, but not Na_v1.7^Nav1.8Cre mice.

<p>A, B.Time-course plots showing the effects of PF-05198007 on skin blood flow measured before and after topical capsaicin application for WT (A) and Nav1.7^Nav1.8Cre (B) mice (for each genotype, n = 8 per group). C, D. Corresponding summary bar graphs showing flare response measured as area under the curve for WT (C) and Nav1.7^Nav1.8Cre (D) mice before and after PF-05198007 treatment. 1 mg/kg and 10 mg/kg PF-05198007 significantly reduced capsaicin-induced flare in WT mice (C, both 1 mg/kg and 10 mg/kg, p < 0.01, ANOVA) but had no effect in Na_v1.7^Nav1.8Cre mice (D, both 1 mg/kg and 10 mg/kg, p > 0.05, ANOVA).</p

Evidence for functional Na_v1.7 in human DRG neurons.

<p>A. Representative TTX-S current traces (recorded in the presence of 1 μM A-803467 and following graded voltage steps from -110 mV to 10 mV. B. Voltage dependence of activation (red, n = 4 for each voltage) generated from the protocol described in A and steady state fast inactivation (blue) generated by conditioning 500 msec prepulses to voltages between -110 mV and +10 mV followed by a test pulse to 0 mV from a holding potential of -110 mV (n = 4 for each voltage). Both datasets are fitted with Boltzmann functions. C. Representative timecourse relationship for peak TTX-S current following the application of 100 nM PF-05089771 and 500 nM TTX. D. Concentration-response relationship for PF-05089771 block of TTX-S current (IC₅₀, slope: 8.4 nM, 1.1; n = 3–6 per concentration) E. Example voltage traces from a current clamp recording. Single action potentials were evoked by a 20 ms suprathreshold current step at 0.1 Hz. The scale bar refers to the voltage traces whereas the start-to-start interval is 10 s. F. Summary pie charts showing that the application of 30 and 100 nM PF-05089771 resulted in action potential block in 3/7 and 5/8 DRG neurons respectively.</p

Potency of PF-05089771 across hNa_v1.7 splice variants.

<p>Potency of PF-05089771 across hNa_v1.7 splice variants.</p

PF-05089771 is a potent, state-dependent and selective inhibitor of Na_v1.7.

<p>A. Structure of PF-05089771 (4-(2-(3-amino-1H-pyrazol-4-yl)-4-chlorophenoxy)-5-chloro-2-fluoro-N-(thiazol-4-yl)benzenesulfonamide) B. Representative PatchXpress current recordings illustrating the near-complete block following 300 nM PF-05089771 application to half-inactivated WT hNa_v1.7 channels (97% ± 3%, n = 10) which was partially reversed following a 5 min washout duration. In contrast there was minimal block following application of 300 nM PF-05089771 to resting WT hNa_v1.7 channels (5% ± 3%, n = 4). Inset: PatchXpress voltage protocols for half-inactivation (upper) and resting state (lower). For a full description of the voltage protocols see <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0152405#sec002" target="_blank">Methods</a>. C. Block of half-inactivated WT hNa_v1.7 channels (n = 6–22 per concentration) was nearly 1000-fold more potent than resting channels (n = 4–11 per concentration) (11 nM <i>vs</i> 10 μM). D. Potency of PF-05089771 was similar across hNa_v1.7 splice variants. IC₅₀ values and Hill slopes are provided in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0152405#pone.0152405.t001" target="_blank">Table 1</a>. Data points represent n = 2–9 observations per concentration. E. PF-05089771 activity is impacted by mutation of a novel interaction site and not by local anaesthetic or toxin binding sites. Data points represent n = 3–6 observations per concentration except for hNa_v1.7 where n = 6–22 observations per concentration. F. Potency of PF-05089771 was assessed on orthologous channels cloned from common preclinical species. IC₅₀ values and Hill slopes are provided in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0152405#pone.0152405.t002" target="_blank">Table 2</a>. Data points represent n = 2–28 observations per concentration. G. PF-05089771 is a selective Na_v1.7 subtype-selective inhibitor. Selectivity was assessed across a collection of heterologously expressed human Na_vs on PatchXpress at the unique half inactivation voltage for each channel. Hill slopes and IC₅₀ values are provided in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0152405#pone.0152405.t003" target="_blank">Table 3</a>. Data points represent n = 3–12 observations per concentration except for hNa_v1.7 where n = 6–22 observations per concentration. Selectivity over the TTX-R Na_v1.5 and Na_v1.8 channels was greater than 1000-fold.</p

PF-05198007 increases action potential rheobase in small diameter mDRG neurons.

<p>A. Overlayed representative voltage traces in response to graded current step injections before (blue) and after PF-05198007 application (red). Current step stimulations are shown below. B. Example timecourse of change in rheobase following PF-05198007 application and washout. C. Summary bar graph, n = 8 neurons, ** p < 0.01, ANOVA. Data are shown ±SEM.</p

Potency of PF-05089771 assessed at orthologous channels from selected species.

<p>Potency of PF-05089771 assessed at orthologous channels from selected species.</p

Na_v1.7 is the major TTX-sensitive Na_v channel in small diameter mDRG neurons.

<p>A. RNASeq analysis of Na_v channel mRNA from pooled small diameter mouse DRG neurons. B. Structure of PF-05198007 (4-(2-(3-amino-1H-pyrazol-4-yl)-4-(trifluoromethyl)phenoxy)-5-chloro-2-fluoro-N-(thiazol-4-yl)benzenesulfonamide C. Patch clamp data showing concentration-response relationship for PF-05198007 assessed against recombinantly expressed mouse Na_v1.7, Na_v1.6 and Na_v1.1 (IC₅₀, Slope: 5.2 nM, 1.1; 149 nM, 1.5; 174 nM, 0.7 respectively; n = 3–4 per concentration). D. Representative patch clamp current traces of peak sodium current from small diameter mouse DRG neurons in the presence of A-803467 and following concurrent application of PF-05198007 and TTX. E. Representative peak TTX-S current <i>vs</i> time plot before and after 30 nM PF-05198007 and 500 nM TTX. G. Scatter plot of cell capacitance <i>vs</i> Na_v1.7/TTX-S ratio (n = 35). Note that in every cell tested, Na_v1.7 provided the predominant TTX-S sodium conductance.</p

PF-05198007 acts peripherally and centrally to influence neurotransmitter release.

<p>A. Upper: representative evoked EPSCs during control (blue) and after 30 mins PF-05198007 application (red). Lower: representative synaptically evoked action potential trace (blue) recorded in SG neurons of the dorsal horn following dorsal root stimulation. PF-05198007 (20 mins) abolished synaptically evoked action potentials (red). B. Example time course of EPSC block following PF-05198007 application to the whole preparation. C. Action potentials induced <i>via</i> current injection steps in SG neurons were not abolished by PF-05198007 (30 nM). Representative voltage traces are shown following current injection steps of -20, 0 and 50 pA before (blue traces) and after (red traces) PF-05198007 application. Line chart shows change in firing frequency (Hz) during control and after application of PF-05198007 for all neurons tested (n = 5, p > 0.05, paired t-test). D. Example time course of EPSC block following PF-05198007 application to the dorsal root alone. E. Representative EPSC traces and summary bar graph showing that the application of PF-05198007 (30 nM) to the dorsal root alone inhibited C-fibre mediated EPSCs and resulted in a significant conduction delay (n = 7, * p < 0.05; ANOVA on Ranks). F. PF-05198007 (30 nM; n = 15: 100 nM; n = 19) reduced veratridine evoked CGRP release in spinal cord synaptosomes. Reduction was compared with mexilitine (100 μM; n = 19), Ca²⁺ free conditions (n = 8) and TTX (500 nM; n = 6) (Data are shown ±SEM; * p < 0.05; ANOVA on Ranks).</p