Letter pubs.acs.org/acsmedchemlett
Sulfonamides as Selective NaV1.7 Inhibitors: Optimizing Potency and Pharmacokinetics to Enable in Vivo Target Engagement Isaac E. Marx,*,† Thomas A. Dineen,†,# Jessica Able,∥ Christiane Bode,† Howard Bregman,† Margaret Chu-Moyer,† Erin F. DiMauro,† Bingfan Du,†,∇ Robert S. Foti,§ Robert T. Fremeau, Jr.,∥,○ Hua Gao,‡ Hakan Gunaydin,‡,◆ Brian E. Hall,⊥ Liyue Huang,§ Thomas Kornecook,∥ Charles R. Kreiman,† Daniel S. La,† Joseph Ligutti,∥ Min-Hwa Jasmine Lin,§ Dong Liu,∥ Jeff S. McDermott,∥,¶ Bryan D. Moyer,∥ Emily A. Peterson,†,+ Jonathan T. Roberts,§ Paul Rose,⊥ Jean Wang,∥ Beth D. Youngblood,∥ Violeta Yu,∥ and Matthew M. Weiss† †
Department of Medicinal Chemistry, ‡Department of Molecular Engineering, §Department of Pharmacokinetics and Drug Metabolism, ∥Department of Neuroscience, and ⊥Department of Biologics, Amgen, Inc., 360 Binney Street, Cambridge, Massachusetts 02142, and One Amgen Center Drive, Thousand Oaks, California 91320, United States S Supporting Information *
ABSTRACT: Human genetic evidence has identified the voltage-gated sodium channel NaV1.7 as an attractive target for the treatment of pain. We initially identified naphthalene sulfonamide 3 as a potent and selective inhibitor of NaV1.7. Optimization to reduce biliary clearance by balancing hydrophilicity and hydrophobicity (Log D) while maintaining NaV1.7 potency led to the identification of quinazoline 16 (AM-2099). Compound 16 demonstrated a favorable pharmacokinetic profile in rat and dog and demonstrated dose-dependent reduction of histamine-induced scratching bouts in a mouse behavioral model following oral dosing. KEYWORDS: Sodium channel, NaV1.7, NaV1.5, pain, histamine scratching model
H
clearance in rodents. We believed constraining the linker within a bicyclic core such as indole 2 or naphthalene 3 would afford a similar conformation and potentially help address the pharmacokinetic liabilities of this class of compounds. An overlay of global minima conformations of compounds 1, 2, and 3 supported this hypothesis (Figure 1B),10 and we were pleased to find that 2 and 3 were potent NaV1.7 inhibitors and showed greater than 200-fold selectivity over NaV1.5 (Figure 1C).11 In general, analogues in the naphthalene series demonstrated superior NaV1.7 inhibition compared to the corresponding indole analogues, thus the naphthalene scaffold was chosen for further optimization. Additional profiling showed 3 also suffered from high clearance; however, we believed that 3 represented a promising starting point for further optimization that would ultimately deliver tool compounds to evaluate the pharmacology of an orally bioavailable, isoform selective, peripherally restricted NaV1.7 inhibitor. The disparity observed between the low in vitro metabolic turnover and high rat iv clearance of our initial set of compounds (see Supporting Information) led us to focus on
uman genetics has implicated the voltage-gated sodium channel NaV1.7, which is expressed in nociceptive sensory neurons in dorsal root ganglia (DRG),1 as a compelling target for pain.2−4 The primary challenge associated with the development of NaV1.7 inhibitors has historically been achieving selectivity over the other eight NaV isoforms. These isoforms are differentially expressed throughout the body, but inhibition of NaV1.5, which is expressed in cardiac tissue, is of particular concern as it has been shown to prolong the cardiac QRS wave in humans.5,6 Previous efforts, including our own, have met with limited success.4 Here we report the characterization, structure−activity relationship (SAR) and optimization of a series of sulfonamide-derived NaV1.7 inhibitors. These efforts delivered an isoform-selective compound that was effective in a histamine-induced scratching model that is representative of NaV1.7 target engagement. Recently Pfizer and Icagen described a series of heteroarylsulfonamide NaV1.7 inhibitors with high levels of selectivity over NaV1.5.7−9 These results were reproduced by our group and are exemplified by compound 1 (Figure 1A). The lack of NaV1.5 activity was noteworthy, and we envisioned this as a good starting point for our own lead optimization efforts, which would initially be aimed at addressing some of the liabilities and shortcomings associated with this class of compounds. Namely, this series suffered from low passive permeability and high © 2016 American Chemical Society
Received: June 23, 2016 Accepted: September 21, 2016 Published: September 21, 2016 1062
DOI: 10.1021/acsmedchemlett.6b00243 ACS Med. Chem. Lett. 2016, 7, 1062−1067
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Figure 1. Design of bicyclic sulfonamide scaffolds. (A) Representative example of a NaV1.7 selective heteroarylsulfonamide. (B) Overlay of biaryl ether 1 (blue), indole 2 (green), and naphthalene 3 (orange) (pyrazole rings omitted for clarity). (C) Profile of initial indole 2 and naphthalene 3.
Table 1. Naphthalene Sulfonamide SAR
Rat liver microsomal (RLM) and human liver microsomal (HLM) clearance. Compound concentration = 1 μM. Microsomal protein concentration = 250 μg/mL. bApparent permeability measured in parental MDCK cells. cCompounds were dosed intravenously at 0.5 mg/kg as a solution in 100% DMSO to male rats. n = 2 animals per group. CLu = unbound clearance. dSee ref 18. eRecovery 30 >30 >30 >30 >30
12 34 31 36 3.2 12
0.001 0.012 0.016 0.014 0.013
0.049 (49) 0.18 (15) 0.54 (34) 1.1 (81) 2.4 (160)
a
cLogD 3.4 2.6 2.1 1.9 0.95 2.1
See notes in Table 1. bpKa = 6.5.
although impact on clearance was minimal. Turning to the ortho-pyrazole moiety, it was found that basic heterocycles afforded very potent NaV1.7 inhibition (4); however, clearance for these highly polar compounds, which have cLogD values in the 0−1 range, remained high.17 Attempts to replace the pyrazole with a smaller polar group such as phenol 5 were poorly tolerated, but bromide 6 demonstrated only a modest loss of NaV1.7 potency while improving passive permeability and iv clearance relative to compounds 2−4. Methyl analogue 7 was the first compound with a truncated ortho-substituent to achieve sub-100 nM potency, demonstrating that potent NaV1.7 inhibition as well as high levels of NaV1.5 selectivity could be achieved without the use of a large polar heterocycle. Furthermore, methoxy analogue 8 showed equivalent levels of NaV1.7 inhibition and greatly improved passive permeability compared to previous compounds. As the ortho-methoxy moiety afforded the best overall potency and selectivity profile of the substituents examined, this substituent was kept constant while optimizing the rest of the molecule to further improve the potency and pharmacokinetic profile. As shown in Table 1, the heteroarylsulfonamide moiety had a profound effect on potency and pharmacokinetics. Interestingly, upon intravenous dosing to rats, compound 8 was found to have high clearance. This result was attributed to the significant drop in cLogD afforded by the ortho-methoxy moiety relative to the ortho-bromo substituent of 6, and we thus focused on modifying the polarity of the heteroarylsulfonamide as a means of attaining compounds in the desired Log D range. Given the acidity of the 5-amino-1,2,4-thiadiazole sulfonamide moiety (see entry 8 above),18 it was reasoned that modulating the pKa of the sulfonamide could significantly impact Log D and provide another handle to help modulate the rate of biliary clearance. Indeed, compound 9, which exhibited a 10-fold loss of NaV1.7 potency, demonstrated a 100-fold improvement in total clearance, a manifestation that was attributed to the pKa difference between 8 and 9 and the corresponding increase in cLogD. Apart from 2-aminothiazole (9) and 4-aminopyrimidine (10), a dramatic loss in potency was observed with aminoheterocycle modifications, representatives of which are shown in Table 1 (entries 11−14). Despite the reduction in NaV1.7 potency afforded by the 2aminothiazole sulfonamide (9), our efforts to this point
better understanding the clearance mechanism of this class of compounds. Toward that end, a series of experiments were undertaken that suggested hepatic transporters play a significant role in the elimination of this class of compounds. First, compound 2 was rapidly removed from the medium when incubated with attached rat hepatocytes and showed much higher apparent uptake intrinsic clearance (CLint) compared to metabolic CLint from suspended hepatocytes.12 A metabolite profiling study with a compound closely related to compound 2, wherein the para-CF3 was replaced with a para-Cl, showed that the parent compound was the major observed peak in the bile following intravenous administration to bile duct cannulated rats; no parent or metabolites were detected in the urine (see Supporting Information). Furthermore, CL and volume of distribution (Vdss) of a structurally related heteroarylsulfonamide were reduced by approximately 80% in rats pretreated with a single dose of rifampicin (50 mg/kg, iv), a known inhibitor of hepatic uptake transporters OATP1B1 and OATP1B3. These results together suggested that hepatic uptake and efflux transporters play a significant role in the elimination of this class of heteroarylsulfonamides. It is known that Log D and pKa both play a important role in determining the extent of biliary excretion of compounds into bile.13,14 With an eye toward occupying drug-like chemical space, a strategy was thus adopted that focused on optimizing potency while targeting a cLogD range of 2−3.15 Compounds were evaluated using a PatchXpress (PX) platform and, in an effort to identify and evaluate compounds that exhibited state-dependent block, a voltage protocol that led to inactivation of approximately 20% of the NaV1.7 channels was employed. As a first step toward exploring the SAR of the naphthalene series, we turned our attention to the substitution of the southern ring (Table 1). Early SAR found this aryl ring was required and that a para-trifluoromethyl group provided superior potency. We were pleased to find that NaV1.5 selectivity remained high and, interestingly, despite the high homology between human, rat, and mouse NaV1.7, compounds from this series were generally equipotent on human and mouse NaV1.7 but much less potent against rat NaV1.7.16 Various lipophilic groups were examined on the southern ring. Such groups proved well tolerated in the para position, 1064
DOI: 10.1021/acsmedchemlett.6b00243 ACS Med. Chem. Lett. 2016, 7, 1062−1067
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Table 3. Cross Species NaV1.7 Activity of 16 mouse IC50 (μM)a
rat IC50 (μM)a
dog IC50 (μM)a
monkey IC50 (μM)a
mouse DRG IC50 (μM)b
0.18 ± 0.14
3.5 ± 1.3
0.18 ± 0.14
0.16 ± 0.091
0.13 ± 0.049
a
Generated using automated PatchXpress at a holding potential that gives 20% inactivation. bGenerated using manual electrophysiology at a holding potential that gives 20% inactivation.
Table 4. Selectivity Profile of 16 hNaV1.1 (μM)a
hNaV1.2 (μM)a
hNaV1.3 (μM)a
hNaV1.4 (μM)a
hNaV1.5 (μM)a
hNaV1.6 (μM)a
hNaV1.7 (μM)a
hNaV1.8 (μM)a
hERG Ki (μM)
7.3 ± 0.91
2.1 ± 0.092
21 ± 0.21
17 ± 1.6
16 ± 2.5
3.7 ± 0.021
0.14 ± 0.021
>30
>30
a
Generated using manual electrophysiology at a holding potential that gives 20% inactivation.
(Table 4). Compound 16 was >100-fold selective over NaV1.3, NaV1.4, NaV1.5, and NaV1.8, while lower levels of selectivity were observed against NaV1.1, NaV1.2, and NaV1.6. The diminished levels of selectivity over NaV1.1 and NaV1.2 were not of particular concern given that these isoforms are predominantly expressed in the central nervous system and this class of sulfonamides is peripherally restricted (see below). Compound 16 demonstrated low affinity for hERG (>30 μM) and did not show greater than 50% inhibition against a panel of 100 kinases (1 μM) and a broad CEREP panel (10 μM). The pharmacokinetic profile of compound 16 was explored in more detail (Table 5). In rats, 16 showed low total clearance
suggested that consistent challenges were associated with designing a 5-amino-1,2,4-thiadiazole sulfonamide with an acceptable pharmacokinetic profile; thus, we proceeded to further optimize NaV1.7 potency with the 2-aminothiazole sulfonamide in place by focusing on the central bicyclic core (Table 2). In general, the addition of polarity in the lower half of the core was well-tolerated from a potency perspective, and permeability was greatly improved relative to the parent naphthalene. Additionally, as expected, compounds with lower cLogD values generally showed increased clearance, further strengthening the association between cLogD and clearance. Interestingly, isoquinoline 15 demonstrated improved NaV1.7 potency and unbound clearance relative to 9 despite a decrease in cLogD. Adding additional polarity in the form of quinazoline 16 led to further improvements in NaV1.7 potency, although at the cost of a slight increase in unbound clearance. Compounds 17 and 18, which demonstrated lower cLogDs, also demonstrated higher unbound clearance, while shifting polarity to the upper ring of the [6,6] system led to loss of NaV1.7 potency (e.g., naphthyridine 19). Ultimately, quinazoline 16 afforded the best combination of NaV1.7 potency, permeability, and unbound clearance. Overall, this series of bicyclic sulfonamides demonstrated much improved pharmacokinetics in preclinical species, enabling the identification of a tool compound to evaluate the pharmacology of the sulfonamide series after oral dosing. On the basis of its overall balance of NaV1.7 potency, selectivity, and favorable rat pharmacokinetics, compound 16 (AM-2099) was further profiled. To understand the statedependence of this series, 16 was evaluated using electrophysiology protocols comparing channels in closed (fully noninactivated) and inactivated states. While an IC50 of 7.2 μM was determined when inhibition was measured at a holding potential of −140 mV, when cells were depolarized to a voltage yielding 20% channel inactivation, the NaV1.7 IC50 shifted to 0.14 μM, and when cells were depolarized to a voltage yielding ∼90% channel inactivation, the IC50 shifted to 0.009 μM, demonstrating the steep state-dependent block exhibited by 16. In heterologous cells, comparable inhibition was observed across human, mouse, dog, and cynomolgus monkey NaV1.7 with reduced activity against rat NaV1.7 (Table 3); as such, it was decided to evaluate the pharmacological activity of 16 in mice. The activity of 16 on TTX-sensitive sodium channels recorded from the cell bodies of sensory neurons dissociated from mouse dorsal root ganglia was in good agreement with heterologously expressed mouse NaV1.7 in stably transfected HEK293 cells. To evaluate the selectivity of 16 across the NaV isoforms, whole cell manual patch clamp experiments on partially inactivated NaV channels (∼20% inactivated) were conducted
Table 5. Pharmacokinetic Profiles and Plasma Protein Binding of 16 iva
pob
species
CL (CLu, L/h/kg)
Vdss (L)
t1/2 (h)
Cmax (μM)
tmax (h)
%F
PPB (Fu)
rat dog
0.54 (34) 0.013 (1)
1.5 0.31
1.9 18
3.4 8.0
1.7 2.3
100 53
0.016 0.014
a
0.5 mg/kg as a solution in 100% DMSO (rat), 30% HPBCD/70% water/KOH at pH = 10 (dog). CLu = unbound clearance. b10 mg/kg (rat) and 1.7 mg/kg (dog) oral dose as a solution in 30% HPBCD/ 70% water/KOH at pH = 10.
and moderate Vdss and half-life. In contrast, when dosed in dogs 16 showed very low clearance, a low Vdss, and long half-life (18 h). Metabolic CLint in hepatocytes was comparable between rats and dogs; further investigation is required to understand mechanisms for the disparity in rates of iv clearance across species. Bioavailability was high in rats and moderate in dogs when dosed orally, and absorption was rapid in both species. Compound 16 was administered intravenously to bile duct catheterized rats (vide supra) to understand its clearance mechanisms. Biliary excretion as parent accounted for 4.7% of the dose,19 demonstrating that our strategy to reduce biliary clearance was successful. In light of the emergence of literature implicating NaV1.7 in the itch pathway, we opted to use a histamine-induced scratching model to measure NaV1.7 target engagement in vivo. It has been shown that NaV1.7 knockout mice do not scratch following a histamine challenge, demonstrating that histamine-induced scratching is a NaV1.7-dependent behavior,3 and that gain-of-function NaV1.7 variants produce increased itch in humans.20 The ease and simplicity with which this model can be translated in human studies makes it an appealing option for measuring target engagement in vivo. 16 was dosed orally to C57BL/6 male mice 120 min prior to intradermal administration of histamine. Instances of scratching behavior 1065
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Figure 2. (A) Reduction of scratching behavior in a mouse histamine-induced scratching model with vehicle, 16, and DPH (diphenhydramine, 30 mg/kg po dosing). **, p < 0.01, ***, p < 0.0001 versus vehicle group (one-way ANOVA followed by Dunnet’s tests). (B) Total and unbound plasma exposure levels of 16 30 min post histamine administration.
Present Addresses
were then measured over a 30 min time period. 16 demonstrated a dose-dependent increase in plasma exposure with a concomitant dose-dependent reduction in scratching bouts compared to vehicle-treated animals, with a statistically significant reduction observed at the 60 mg/kg dose (Figure 2). It should be noted that these effects occurred at doses and exposures that did not manifest in any reduction in locomotor activity in a mouse open-field movement assay. Compound 16 demonstrated a brain-to-plasma ratio of 0.008 in mice, indicating that the compound was peripherally restricted and that the observed effect was a result of NaV1.7 inhibition in the peripheral nervous system. The unbound concentration of 16 required to observe a significant reduction of scratching behavior is noteworthy and warrants further investigation. In conclusion, utilizing compound 1 as a starting point, a novel series of bicyclic sulfonamide NaV1.7 inhibitors with high levels of NaV1.5 selectivity was designed. On the basis of pharmacokinetic experiments that suggested hepatic transporters as the primary route of elimination, key strategies for reducing clearance were identified such as targeting an optimal cLogD range by modulating overall hydrophobicity and the pKa of the sulfonamide moiety. NaV1.7 potency was optimized through modification of the bicyclic core topology and the polarity of the phenyl and sulfonamide substituents. Ultimately, optimally distributing polarity in the central core led to the identification of quinazoline 16, which demonstrated a favorable pharmacokinetic profile, good selectivity over a range of NaV isoforms, and dose-dependent reduction of scratching behavior in a histamine-induced scratching model. Further efforts to improve the potency, pharmacokinetics, and efficacy of the bicyclic sulfonamide series will be reported in due course.
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For T.A.D.: Novartis Institutes for Biomedical Research, 250 Massachusetts Avenue, Cambridge, Massachusetts 02139, United States. ∇ For B.D.: Mersana Therapeutics, 840 Memorial Drive, Cambridge, Massachusetts 02139, United States. ○ For R.T.F.: NeuroRx Consulting, 854 Massachusetts Avenue, Unit 3, Cambridge, Massachusetts 02139, United States. ◆ For H.G.: Merck Research Laboratories, 33 Avenue Louis Pasteur, Boston, Massachusetts 02115, United States. ¶ For J.S.M.: Neuroscience Discovery, Lilly Research Laboratories, 307 East Merrill Street, Indianapolis, Indiana 46285, United States. + For E.A.P.: Biogen Inc., 225 Binney Street, Cambridge, Massachusetts 02142, United States. Author Contributions
The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We gratefully acknowledge Grace Bi and Larry Miller for purification support, Paul Krolikowski and Steve Hollis for analytical support, Yohannes Teffera for PK support, Christopher Ilch and Kimberley Nye for in vivo support, Roman Shimanovich and Melanie Cooke for formulations support, and Benjamin Milgram for proofreading the manuscript.
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ABBREVIATIONS CLint, intrinsic clearance; CLu, unbound clearance; DRG, dorsal root ganglion; hERG, human ether-a-go-go-related gene; HPBCD, hydroxypropyl β-cyclodextrin; Papp, apparent permeability; PPB, plasma protein binding; SAR, structure−activity relationship; TTX, tetrodotoxin; Vdss, volume of distribution
ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsmedchemlett.6b00243. Synthetic procedures of all final compounds, description of in vitro and in vivo assays, and additional experimental details for determining clearance mechanism (PDF)
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REFERENCES
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AUTHOR INFORMATION
Corresponding Author
*Phone: 617-914-6086. Fax: 617-914-4569. E-mail: isaac.
[email protected]. Address: Biogen Inc., 225 Binney Street, Cambridge, Massachusetts 02142, United States. 1066
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(3) Gingras, J.; Smith, S.; Matson, D. J.; Johnson, D.; Nye, K.; Couture, L.; Feric, E.; Yin, R.; Moyer, B. D.; Peterson, M. L.; Rottman, J. B.; Beiler, R. J.; Malmberg, A. B.; McDonough, S. I. Global NaV1.7 Knockout Mice Recapitulate the Phenotype of Human Congenital Indifference to Pain. PLoS One 2014, 9, e105895. (4) For recent review: de Lera Ruiz, M.; Kraus, R. L. Voltage-Gated Sodium Channels: Structure, Function, Pharmacology, and Clinical Indications. J. Med. Chem. 2015, 58, 7093−7118. (5) Wood, J. N.; Boorman, J. P.; Okuse, K.; Baker, M. D. VoltageGated Sodium Channels and Pain Pathways. J. Neurobiol. 2004, 61, 55−71. (6) Erdemli, G.; Kim, A. M.; Ju, H.; Springer, C.; Penland, R. C.; Hoffmann, P. K. Cardiac Safety Implications of hNaV1.5 Blockade and a Framework for Pre-Clinical Evaluation. Front. Pharmacol. 2012, 3, 6. (7) Beaudoin, S.; Laufer-Sweiler, M.; Markworth, C. J.; Marron, B. E.; Millan, D. S.; Rawson, D. J.; Reister, S. M.; Sasaki, K.; Storer, R. I.; Stupple, P. A.; Swain, N. A.; West, C. W.; Zhou, S. Sulfonamide Derivatives. WO2010/079443A1, July 15, 2010. (8) For additional examples see: Focken, T.; Liu, S.; Chahal, N.; Dauphinais, M.; Grimwood, M. E.; Chowdhury, S.; Hemeon, I.; Bichler, P.; Bogucki, D.; Waldbrook, M.; Bankar, G.; Sojo, L. E.; Young, C.; Lin, S.; Shuart, N.; Kwan, R.; Pang, J.; Chang, J. H.; Safina, B. S.; Sutherlin, D. P.; Johnson, J. P., Jr.; Dehnhardt, C. M.; Mansour, T. S.; Oballa, R. M.; Cohen, C. J.; Robinette, C. L. Discovery of Aryl Sulfonamides as Isoform-Selective Inhibitors of NaV1.7 with Efficacy in Rodent Pain Models. ACS Med. Chem. Lett. 2016, 7, 277−282. (9) Sun, S.; Jia, Q.; Zenova, A. Y.; Chafeev, M.; Zhang, Z.; Lin, S.; Kwan, R.; Grimwood, M. E.; Chowdhury, S.; Young, C.; Cohen, C. J.; Oballa, R. M. The Discovery of Benzenesulfonamide-Based Potent and Selective Inhibitors of Voltage-Gated Sodium Channel NaV1.7. Bioorg. Med. Chem. Lett. 2014, 24, 4397−4401. (10) Conformational analyses were performed using Chemical Computing Group’s Molecular Operating Environment (MOE) software. (11) For synthetic routes and procedures for all compounds, see Supporting Information. (12) Huang, L.; Chen, A.; Roberts, J.; Janosky, B.; Be, X.; Berry, L.; Lin, M.-H. J. Use of Uptake Intrinsic Clearance from Attached Rat Hepatocytes to Predict Hepatic Clearance for Poorly Permeable Compounds. Xenobiotica 2012, 42, 830−840. (13) Chen, Y.; Cameron, K.; Guzman-Perez, A.; Perry, D.; Li, D.; Gao, H. Structure−Pharmacokinetic Relationship of In Vivo Rat Biliary Excretion. Biopharm. Drug Dispos. 2010, 31, 82−90. (14) Tu, M.; Mathiowetz, A. M.; Pfefferkorn, J. A.; Cameron, K. O.; Dow, R. L.; Litchfield, J.; Di, L.; Feng, B.; Liras, S. Medicinal Chemistry Design Principles for Liver Targeting Through OATP Transporters. Curr. Top. Med. Chem. 2013, 13, 857−866. (15) Waring, M. J. Lipophilicity in Drug Discovery. Expert Opin. Drug Discovery 2010, 5, 235−248. (16) This disparity has previously been observed with related chemical matter (see ref 8), suggesting a common binding site for this class of structurally related sulfonamide-based NaV1.7 inhibitors. Studies to further understand this disparity will be reported elsewhere. (17) cLogD(7.4) was calculated using daylight cLogP and an internal pKa calculator. (18) pKa values were calculated from capillary electrophoresis data obtained with pKa PRO Analyzer (AATI, Ames, IA) in the pH range of 1.7 to 11.2. (19) 0.02% and 0.60% of the dose was detected as parent in the urine and feces, respectively. (20) Devigili, G.; Eleopra, R.; Pierro, T.; Lombardi, R.; Rinaldo, S.; Lettieri, C.; Faber, C. G.; Merkies, I. S. J.; Waxman, S. G.; Lauria, G. Paroxysmal Itch Caused by Gain-of-Function NaV1.7 Mutation. Pain 2014, 155, 1702−1707.
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