Discovery of SCH 900188: A potent hepatitis C virus NS5B polymerase inhibitor prodrug as a development candidate

Article abstract of DOI:10.1021/ml400192w

Starting from indole-based hepatitis C virus (HCV) NS5B polymerase inhibitor lead compound 1, structure modifications were performed at multiple indole substituents to improve potency and pharmacokinetic (PK) properties. Bicyclic quinazolinone was found to be the best substituent at indole nitrogen, while 4,5-furanylindole was identified as the best core. Compound 11 demonstrated excellent potency. Its C2 N,N-dimethylaminoethyl ester prodrug 12 (SCH 900188) demonstrated significant improvement in PK and was selected as the development candidate.

Full text of DOI:10.1021/ml400192w

Letter
pubs.acs.org/acsmedchemlett
Discovery of SCH 900188: A Potent Hepatitis C Virus NS5B
Polymerase Inhibitor Prodrug As a Development Candidate
Kevin X. Chen,* Srikanth Venkatraman, Gopinadhan N. Anilkumar, Qingbei Zeng, Charles A. Lesburg,
Bancha Vibulbhan, Francisco Velazquez, Tin-Yau Chan, Frank Bennet, Yueheng Jiang, Patrick Pinto,
Yuhua Huang, Oleg Selyutin, Sony Agrawal, Hsueh-Cheng Huang, Cheng Li, Kuo-Chi Cheng,
Neng-Yang Shih, Joseph A. Kozlowski, Stuart B. Rosenblum, and F. George Njoroge
Merck Research Laboratories, 2015 Galloping Hill Road, Kenilworth, New Jersey 07033, United States
S
* Supporting Information
ABSTRACT: Starting from indole-based hepatitis C virus (HCV)
NS5B polymerase inhibitor lead compound 1, structure modifications
were performed at multiple indole substituents to improve potency and
pharmacokinetic (PK) properties. Bicyclic quinazolinone was found to
be the best substituent at indole nitrogen, while 4,5-furanylindole was
identified as the best core. Compound 11 demonstrated excellent
potency. Its C2 N,N-dimethylaminoethyl ester prodrug 12 (SCH
900188) demonstrated significant improvement in PK and was selected
as the development candidate.
KEYWORDS: HCV, hepatitis C virus, NS5B polymerase, polymerase inhibitor, prodrug
have been developed: nucleoside active site inhibitors (NIs)
and non-nucleoside allosteric inhibitors (NNIs).12,13 NIs have
similar potency against all genotypes and a high genetic barrier
to resistance due to the highly conserved NS5B active site.13
Several NIs are at the advanced stage of clinical trials.14 NNIs,
however, achieve NS5B inhibition by binding to one of the
allosteric sites. A number of NNI candidates have been
progressed to phase II clinical trials.12,13 Rapid development of
resistant mutants has been observed with NNIs because they
bind to the sites that are away from the active site of NS5B
enzyme, but mutations at the NNI binding site may not
necessarily lead to complete impairment of the enzyme
function.13
Our early research in HCV NS5B polymerase inhibitor
program led to inhibitor 1 (Scheme 1).15 It has good potency
in both genotype 1b NS5B RdRp enzyme assay16 (IC50 = 3
nM) and cell-based replicon assay17 (EC50 = 40 nM). X-ray
structure of a similar compound in complex with NS5B enzyme
indicated that it bound to an allosteric palm site apoprotein
cavity adjacent to the active site.15 The C3 pyridone carbonyl
and N−H formed a pair of key hydrogen bonds with the
backbone. The N1, C2, and C5 substituents had hydrophobic
interactions with the protein surface. Further optimization was
needed to improve cellular potency and PK properties.
Structural modifications of compound 1 could either involve
the change of indole core to deliver better scaffold or replace
t is estimated that over 170 million people have been
1
Iinfected by hepatitis C virus (HCV). In most patients, the
disease progresses to a chronic state that persists for decades,
but eventually leads to cirrhosis, liver failure, or liver cancer.2
With no anti-HCV vaccines available, the current standard of
care (SOC) involves the combination of a protease inhibitor
with pegylated-interferon and ribavirin.3 The sustained
virologic response (SVR) rate of the triple combination therapy
is about 70% for patients with the most-difficult to treat
genotype-1 HCV.3 However, there are still large unmet medical
needs for safer and more effective HCV treatments due to the
side effects associated with α-interferon and ribavirin.
HCV was identified more than two decades ago as a positive-
sense single-stranded RNA virus.4 The viral genome encodes a
polyprotein, which can be divided into structural and
nonstructural (NS) precursor regions. The nonstructural
proteins NS2, NS3, NS4A, NS4B, NS5A, and NS5B are
essential for the replication of HCV virus; thus, intensive
research has been focused on finding drugs directly targeting
these proteins.5,6 Recent approval of the two first-in-class HCV
NS3 protease inhibitors, boceprevir7 and telaprevir,8 has
generated great interest and created a race in the search for
new treatments of HCV infection aiming at all-oral therapies
with less side effects, higher SVR rate, broader genotype
coverage, and shorter treatment period.5
The NS5B gene encodes an RNA-dependent RNA polymer-
ase (RdRp), which is critical to the replication of the viral RNA
genome.9,10 X-ray crystal structure of the catalytic domain of
N5SB reveals that the NS5B polymerase11 has a well-conserved
active site and multiple allosteric inhibitor binding sites.12,13
Two distinctive classes of HCV NS5B polymerase inhibitors
Special Issue: HCV Therapies
Received: May 17, 2013
Accepted: August 12, 2013
© XXXX American Chemical Society
A
dx.doi.org/10.1021/ml400192w | ACS Med. Chem. Lett. XXXX, XXX, XXX−XXX

ACS Medicinal Chemistry Letters
Letter
a
structure−activity relationship (SAR) of the indole nitrogen
substituent was investigated. To identify a group that was
capable of establishing additional interactions with protein
backbone residues, moieties with different C5′ functionalities
were examined (Table 1, 13−17). The C5′ cyano analogue 13
Scheme 1
Table 1. Optimization of R4, R5, R6, and R5′ Substituents
a
Reaction conditions: (a) NIS, acetone; (b) 2-methoxy-3-pyridinebor-
onic acid, PdCl2(dppf)2, K2CO3, 1,2-dimethoxyethane; (c) ArCH2Br
(or ArCH2Cl), Cs2CO3, DMF; (d) LiOH, THF/H2O; (e) 4 M HCl,
dioxane; (f) EDC, Et3N, DMF; (g) ArCH2Br (or ArCH2Cl), Cs2CO3,
DMF; or ArCH2OH, DIAD, PPh3, THF; (h) CDI, RSO2NH2, DBU,
THF; (i) LiOH, THF/H2O; (j) RSO2NH2, DBU, DMF; (k)
(Me)2NCH2CH2OH, n-BuLi, THF, then HCl.
the substituents at various positions of the indole ring to
enhance existing or create additional interactions. Herein, we
report our progress toward this endeavor and the discovery of
an orally bioavailable clinical candidate.
offered no potency improvement in both enzyme (IC50 = 7
nM) and cellular assays (EC50 = 80 nM) compared to 1. When
C5′ was substituted with a primary amide (14), EC50 improved
slightly to 30 nM. Changing amide to a methyl sulfone or a
primary sulfonamide as in 15 and 16 did not result in similar
enhancement in biological activity since both IC50 and EC50
were higher than those of compound 1. The C5′-hydroxy
analogue 17 was also not as potent as 1. Keeping the C5′
primary amide substituent, the SAR of C4, C5, and C6
substitution at indole core was investigated (Table 1, 18−23).
Since our early research indicated that C4 could not tolerate
any sizable substituent,15 most modifications were made at C5
and C6 positions. When C5 ethyl group was changed to a
smaller methyl or trifluoromethyl group, the resulting inhibitors
18 and 19 gained some activity in enzyme assay, but lost
cellular potency compared to 14. The addition of a C6-fluoro
substituent (20−22) maintained good IC50 (4−8 nM) but gave
no improvement in EC50 (50−150 nM). Besides investigation
on individual C4, C5, and C6 substituents, the SAR of C4,C5-
cyclized analogues was also studied in parallel. Among various
tricyclic indoles studied, the 4,5-furan-6-F indole was found to
be the best core.20 When this tricyclic indole was incorporated
into the current series, the resulting inhibitor 23 demonstrated
Syntheses of the inhibitors are outlined in Schemes 1.15 Ethyl
indole-2-carboxylate 2 was iodinated at C3 and subsequently
converted to 3 through a Suzuki coupling.18 After alkylation at
N1, the C2 ethyl ester was hydrolyzed to give a carboxylic acid
4, which was elaborated to an acylsulfonamide. Then the 2-
methoxypyridine at C3 was demethylated to the pyridone final
product 9. Alternatively, compound 3 could be converted to a
pyridone, which was followed by hydrolysis of the ester to give
a carboxylic acid 5. Intramolecular cyclization of the pyridone
to the carboxylic acid afforded the tetracyclic lactone 6.
Alkylation at the indole nitrogen and opening of the lactone
with a sulfonamide gave rise to the acylsulfonamide product 9.
Compound 7 could also be hydrolyzed to the carboxylic acid
inhibitor 8. The quinazolinone series of inhibitors were
synthesized in similar reaction sequences. The final lactone
intermediate 10 was either hydrolyzed to the carboxylic acid
derivative 11 or converted to the N,N-dimethylaminoethyl ester
(12).
Several attempted modifications of the indole core to
different bicyclic heterocycles such as 7-azaindole failed to
improve potency.19 Efforts were then focused on the
modification of various indole substituents. First, the
B
dx.doi.org/10.1021/ml400192w | ACS Med. Chem. Lett. XXXX, XXX, XXX−XXX

ACS Medicinal Chemistry Letters
Letter
superior potency in replicon assay (EC50 = 10 nM), which
represented a 3-fold improvement over compound 14.
Continued investigation of bicyclic heterocycles led to the
discovery of quinazolinone moiety (11). Compound 11 (SCH
900782) had excellent potencies in both enzyme (IC50 = 5 nM)
and replicon assays (EC50 = 6 nM). It also had good AUC of
4.3 μM·h in rat PK. Several related bicyclic ring systems were
also examined (29−33) in attempts to further improve potency
and PK. Methylation at C2′ (29) resulted in higher EC50 and a
lower AUC. The quinazolinedione derivative 30 had excellent
IC50, EC50, and rat PK AUC. Unfortunately, it was not very
stable and could be hydrolyzed under basic conditions. The
C2′-amino quinazolinone compound 31 also demonstrated
impressive IC50 and EC50, but it had a negligible AUC.
Methylation at N1′ of 30 or N3′ of 11 did not affect IC50 (32
and 33), but their EC50s were surprisingly much worse than
other quinazoline analogues.
Although compound 23 had very good potency, its PK was
poor with an area-under-the curve (AUC) of only 0.21 μM·h
when dosed orally in rats at 10 mg/kg. The acylsulfonamide
and primary amide were potential liabilities. Indeed, when the
C2 acylsulfonamide was replaced with a carboxylic acid, the
resulting compound 24 (Table 2) demonstrated a significant
Table 2. Optimization of Indole N1 Aromatic Moiety
Although compound 11 had a very good overall profile, to
achieve adequate exposure necessary for the ultimate goal of
once-daily (QD) dosing, further improvement in PK properties
was necessary. Prodrug is an attractive approach to enhance the
exposure level of very polar and less soluble molecules.21 The
pyridone or quinazolinone nitrogens and the C2 carboxylic acid
were potential sites for prodrug development. The methylene
pivalate prodrug 34 (Table 3) maintained most of the potency,
while analogue 35 lost significant activity, probably due to the
loss of a hydrogen bond at pyridone. No rat PK AUC was
observed for both compounds suggesting inefficient conversion
to the active compound 11. The ethyl ester prodrug (36) did
a
Table 3. Prodrugs of Compound 11
a
Oral dosing of 10 mg/kg for 0−6 h data.
AUC improvement to 1.7 μM·h in rat PK although it was
slightly less potent (EC50 = 15 nM) than 23. Thus, further SAR
studies were conducted with C2 carboxylic acids. Various
mono- or bicyclic N1 benzyl substituents were designed to
replace primary amide moiety (Table 2). The C5′-carboxylic
acid derivative 25 was equally active in enzyme assay but lost
much of the potency in replicon assay. The C5′-acetyl analogue
26 was 5-fold less potent than 24 in both enzyme and cellular
assays, although its rat PK AUC was good (8.8 μM·h).
Surprisingly, the C4′,C5′-bicyclic benzoxazolidinedione ana-
logue 27 lost all activities in both assays, while the C4′,C5′-
bicyclic benzimidazolidinedione derivative 28 had an impressive
EC50 of 10 nM along with a good AUC of 4.0 μM·h.
a
Oral dosing of 10 mg/kg for 0−24 h data. AUC was for active parent
b
compound 11. CH2Piv = −CH2OCO-t-Bu. Oral dosing of 10 mg/kg
for 0−6 h data.
C
dx.doi.org/10.1021/ml400192w | ACS Med. Chem. Lett. XXXX, XXX, XXX−XXX

ACS Medicinal Chemistry Letters
Letter
Table 4. Profile of Compounds 11 and 12
compound
11
12
dog
animal species
rat
dog
monkey
rat
monkey
a
b
b
a
b
b
AUC (PO) (μM·h)
Cmax (μM)
10
0.4
0.1
3.2
4
8
22
2.4
0.8
144
4.1
1.4
9.4
0.7
0.2
0.8
2.1
b
c
c
b
c
c
t1/2 (IV) (h)
8.9
11
4
22
bioavailability (%) (F)
clearance (mL/min/kg)
Mini Ames test
5
2
29
63
11
0.6
2.4
0.8
negative
negative
negative
negative
in vitro micronucleus
a
b
c
At 10 mg/kg. At 3 mg/kg. At 1 mg/kg. All AUCs were 0−24 h data.
not provide much AUC either. So was the methylene pivalate
and ester dual prodrug 37. Inefficient conversion to active
compound 11 and poor solubility of prodrugs 34−37
presumably contributed to their poor PK performance. One
way to improve solubility was to incorporate a tertiary amine
into the C2 ester so that a salt could be made. Indeed, the
hydrochloride salt of N,N-dimethylamino-ethyl ester (12, SCH
900188) was highly soluble (0.9 mM at pH 3.0). Its excellent
potency in replicon assay (EC50 = 5 nM) implied efficient
conversion of the prodrug 12 to its parent 11. Most
importantly, when the prodrug 12 was dosed orally in rats at
10 mg/kg, it delivered excellent AUC (22 μM·h) of parent
compound 11. Two other prodrugs with different chain lengths
(38 and 39) were found to produce much lower AUCs of
parent 11 in rat PK. The concentration of the active parent 11
in the liver was also measured for compounds with reasonable
PK. When prodrug 12 was dosed in rats, at 6 h after dosing, the
liver/plasma concentration ratio is 0.5 for parent 11. The liver/
plasma ratio was 0.4 when compound 38 was dosed. Clearly,
compound 12 was the best among the prodrugs studied.
In enzyme assays, compound 11 was equally potent against
genotype 1a and 1b NS5B enzymes (IC50 = 7 and 5 nM,
respectively). Significant loss in potency was observed with
genotype 2a, 3a, and 4a enzymes (IC50 > 5 μM). Compounds
11 and 12 were cross-resistant to several major mutations (e.g.,
C316Y, M414T, and G554D) against known palm site
inhibitors but were fully active on resistant mutants in the
active site and other allosteric sites. The frequency of
emergence of resistance was significantly reduced when used
in combination with a protease inhibitor. The PK profile and in
vitro assay results of compounds 11 and 12 are summarized in
Table 4. The oral bioavailability of carboxylic acid 11 in rats,
dogs, and monkeys were 0.7%, 4%, and 5%, respectively. When
prodrug 12 was dosed, the oral bioavailability of active
metabolite 11 increased substantially to 2%, 29%, and 63%,
respectively. The low bioavailability in rats was a result of
extremely high AUC from IV dosing. Following IV
administration of 12, low clearance of 2.4 and 0.8 mL/min/
kg and long half-life of 11 and 22 hours were observed in rats
and monkeys, respectively. Compounds 11 and 12 were highly
selective for the NS5B polymerase and showed no cell toxicity
at up to 10 μM in a variety of cell lines. Both 11 and 12 were
negative in the Ames mutagenicity and in vitro micronucleus
assays. On the basis of the excellent overall profile, compound
12 was recommended as a preclinical candidate for further
development.
Figure 1. X-ray structure of compound 11 bound to the palm site of
HCV NS5B polymerase.
of Met-414 and Tyr-448. The C3 pyridone group forms
hydrogen bonds both with the backbone carbonyl oxygen of
Gln-446 and with the backbone amide nitrogen of Tyr-448.
The pyridone ring rests atop the side chain of Met-414. The C2
carboxylate moiety interacts with the backbone amide nitrogen
of Gly-449 via a bridging water molecule. The N1 substituent
packs atop the side chain of Cys-366 for hydrophobic
interactions. It projects into a hydrophilic zone where it may
form some hydrogen bonds through adjacent water molecules.
In summary, SAR studies of the indole substituents were
performed to optimize potency and PK properties. The initial
good potency of the primary amide group at the indole
nitrogen substituent led to the eventual discovery of the novel
quinazolinone series of highly potent inhibitors. Optimization
of C4, C5, and C6 substituents led to the identification of 4,5-
furanylindole as the best core. The C2 carboxylic acids
demonstrated better PK than corresponding acylsulfonamides.
After all, compound 11 was identified as the lead with very
good potency and rat oral PK AUC. Its N,N-dimethylami-
noethyl ester prodrug 12 achieved significant improvement in
PK properties and was selected as a preclinical development
candidate.
ASSOCIATED CONTENT
■
S
* Supporting Information
The X-ray crystal structure of 11 in complex with HCV
NS5B protein has been solved (Figure 1).22 The compound is
bound at the base of the active site cavity at the palm allosteric
site, with the furanylindole core packed against the side chains
Experimental procedures and characterization data. This
material is available free of charge via the Internet at http://
pubs.acs.org.
D
dx.doi.org/10.1021/ml400192w | ACS Med. Chem. Lett. XXXX, XXX, XXX−XXX

ACS Medicinal Chemistry Letters
Letter
Huang, H. C.; Li, C.; Cheng, K. C.; Shih, N. Y.; Kozlowski, J. A.;
Rosenblum, S. B.; Njoroge, F. G. Structure−activity-relationship
(SAR) development and discovery of potent indole-based inhibitors of
the hepatitis C virus (HCV) NS5B polymerase. J. Med. Chem. 2012,
55, 754−765.
AUTHOR INFORMATION
Corresponding Author
*(K.X.C.) E-mail: kxcn@yahoo.com. Tel: (732) 902-9385.
Notes
■
(16) Ferrari, E.; He, Z.; Palermo, R. E.; Huang, H. C. Hepatitis C
virus NS5B polymerase exhibits distinct nucleotide requirements for
initiation and elongation. J. Biol. Chem. 2008, 283, 33893−33901.
The authors declare no competing financial interest.
ABBREVIATIONS
■
(17) Lohmann, V.; Korner, F.; Koch, J.-O.; Herian, U.; Theilmann,
̈
HCV, hepatitis C virus; PK, pharmacokinetics; SAR, structure−
activity relationship; NS, nonstructural; AUC, area-under-the-
curve; SOC, standard of care; SVR, sustained virologic
response; RdRp, RNA-dependent RNA polymerase; NIs,
nucleoside inhibitors; NNIs, non-nucleoside inhibitors
L.; Bartenschlager, R. Replication of subgenomic hepatitis C virus
RNAs in a hepatoma cell line. Science 1999, 285, 110−113.
(18) Ishiyama, T.; Murata, M.; Miyaura, N. Palladium(0)-catalyzed
cross-coupling reaction of alkoxydiboron with haloarenes: A direct
procedure for arylboronic esters. J. Org. Chem. 1995, 60, 7508−7510.
(19) Unpublished results at Merck Research Laboratories.
(20) The SAR and development of 4,5-cyclzed indole series of
inhibitors and their synthesis are being reported in a separate
publication.
REFERENCES
■
(1) Cohen, J. The scientific challenge of hepatitis C. Science 1999,
285, 26−30.
(2) Shepard, C. W.; Finelli, L.; Alter, M. J. Global epidemiology of
hepatitis C virus infection. Lancet Infect. Dis. 2005, 5, 558−567.
(3) Sheridan, C. New Merck and Vertex drugs raise standard of care
in hepatitis C. Nat. Biotechnol. 2011, 29, 553−554.
(4) Choo, Q. L.; Kuo, G.; Weiner, A. J.; Weiner, A. J.; Overby, L. R.;
Bradley, D. W.; Houghton, M. Isolation of a cDNA clone derived from
a blood-borne non-A, non-B viral hepatitis genome. Science 1989, 244,
359−362.
(5) Chen, K. X.; Njoroge, F. G. A review of HCV protease inhibitors.
Curr. Opin. Invest. Drugs 2009, 10, 821−837.
(21) Rautio, J.; Kumpulainen, H.; Heimbach, T.; Oliyai, R.; Oh, D.;
Jarvinen, T.; Savolainen, J. Prodrugs: design and clinical applications.
̈
Nature Rev. Drug Discovery 2008, 7, 225−270.
(22) X-ray crystal structure data of HCV NS5B in complex with 11
has been deposited in the Protein Data Bank with access code 2YOJ.
(6) Watkins, W. J.; Ray, A. S.; Chong, L. S. HCV NS5B polymerase
inhibitors. Curr. Opin. Drug Discovery Dev. 2010, 13, 441−465.
(7) Njoroge, F. G.; Chen, K. X.; Shih, N.-Y.; Piwinski, J. J. Challenges
in modern drug discovery: A case study of boceprevir, an HCV
protease inhibitor for the treatment of hepatitis C virus infection. Acc.
Chem. Res. 2008, 41, 50−59.
(8) Perni, R. B.; Almquist, S. J.; Byrn, R. A.; Chandorkar, G.;
Chaturvedi, P. R.; Courtney, L. F.; Decker, C. J.; Dinehart, K.; Gates,
C. A.; Harbeson, S. L.; Heiser, A.; Kalkeri, G.; Kolaczkowski, E.; Lin,
K.; Luong, Y.-P.; Rao, B. G.; Taylor, W. P.; Thomson, J. A.; Tung, R.
D.; Wei, Y.; Kwong, A. D.; Lin, C. Preclinical profile of VX-950, a
potent, selective, and orally, bioavailable inhibitor of hepatitis C virus
NS3-4A serine protease. Antimicrob. Agents Chemother. 2006, 50, 899−
909.
(9) Behrens, S. E.; Tomei, L.; De Francesco, R. Identification and
properties of the RNA-dependent RNA polymerase of hepatitis C
virus. EMBO J. 1996, 15, 12−22.
(10) Quinkert, D.; Bartenschlager, R.; Lohmann, V. Quantitative
analysis of the hepatitis C virus replication complex. J. Virol. 2005, 79,
13594−13605.
(11) Lesburg, C. A.; Cable, M. B.; Ferrari, E.; Hong, Z.; Mannarino,
A. F.; Weber, P. C. Crystal structure of the RNA-dependent RNA
polymerase from hepatitis C virus reveals a fully encircled active site.
Nat. Struct. Biol. 1999, 6, 937−943.
(12) Liu, Y.; He, Y.; Molla, A. Hepatitis C Virus Polymerase as a
Target for Antiviral Drug Intervention: Non-Nucleoside Inhibitors. In
Antiviral Research: Strategies in Antiviral Drug Discovery; LaFemina, R.
L., Ed.; ASM Press: Washington, DC., 2009; pp 137−151.
(13) Watkins, W. J.; Ray, A. S.; Chong, L. S. HCV NS5B polymerase
inhibitors. Curr. Opin. Drug Discovery Dev. 2010, 13, 441−465.
(14) Kneteman, N. M.; Howe, A. Y.; Gao, T.; Lewis, J.; Pevear, D.;
Lund, G.; Douglas, D.; Mercer, D. F.; Tyrrell, D. L.; Immermann, F.;
Chaudhary, I.; Speth, J.; Villano, S. A.; O’Connell, J.; Collett, M.
HCV796: A selective nonstructural protein 5B polymerase inhibitor
with potent anti-hepatitis C virus activity in vitro, in mice with
chimeric human livers, and in humans infected with hepatitis C virus.
Hepatology 2009, 49, 745−52.
(15) Chen, K. X.; Vibulbhan, B.; Yang, W.; Sannigrahi, M.;
Velazquez, F.; Chan, T. Y.; Venkatraman, S.; Anilkumar, G. N.;
Zeng, Q.; Bennet, F.; Jiang, Y.; Lesburg, C. A.; Duca, J.; Pinto, P.;
Gavalas, S.; Huang, Y.; Wu, W.; Selyutin, O.; Agrawal, S.; Feld, B.;
E
dx.doi.org/10.1021/ml400192w | ACS Med. Chem. Lett. XXXX, XXX, XXX−XXX