Structure-Based Discovery of Proline-Derived Arginase Inhibitors with Improved Oral Bioavailability for Immuno-Oncology

Article abstract of DOI:10.1021/acsmedchemlett.1c00195

Recent data suggest that the inhibition of arginase (ARG) has therapeutic potential for the treatment of a number of indications ranging from pulmonary and vascular disease to cancer. Thus, high demand exists for selective small molecule ARG inhibitors wi

Full text of DOI:10.1021/acsmedchemlett.1c00195

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Structure-Based Discovery of Proline-Derived Arginase Inhibitors
with Improved Oral Bioavailability for Immuno-Oncology
Min Lu,* Hongjun Zhang, Derun Li, Matthew Childers, Qinglin Pu, Rachel L. Palte, Symon Gathiaka,
Thomas W. Lyons, Anandan Palani, Peter W. Fan, Peter Spacciapoli, J. Richard Miller, Hyelim Cho,
Mangeng Cheng, Kalyan Chakravarthy, Jennifer O’Neil, Padmanabhan Eangoor, Adam Beard,
Hai-Young Kim, Josep Saurí, Hakan Gunaydin, David L. Sloman, Phieng Siliphaivanh, Jared Cumming,
and Christian Fischer
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ABSTRACT: Recent data suggest that the inhibition of arginase (ARG) has therapeutic potential for the treatment of a number of
indications ranging from pulmonary and vascular disease to cancer. Thus, high demand exists for selective small molecule ARG
inhibitors with favorable druglike properties and good oral bioavailability. In light of the significant challenges associated with the
unique physicochemical properties of previously disclosed ARG inhibitors, we use structure-based drug design combined with a
focused optimization strategy to discover a class of boronic acids featuring a privileged proline scaffold with superior potency and
oral bioavailability. These compounds, exemplified by inhibitors 4a, 18, and 27, demonstrated a favorable overall profile, and 4a was
well tolerated following multiple days of dosing at concentrations that exceed those required for serum arginase inhibition and
concomitant arginine elevation in a syngeneic mouse carcinoma model.
KEYWORDS: Arginase inhibitor, structure-based drug design, oral bioavailability, cancer immunotherapy, proline
derived suppressor cells (MDSCs)7 in the tumor micro-
arnessing the innate immune system to cure diseases has
Hrevived and witnessed great progress in the past few
environment, has become increasingly recognized as an
important mechanism of tumor immune evasion by (1)
years, in particular, in the field of cancer immunotherapy.1
Some immune checkpoint inhibitors, such as mAbs for
programmed cell death protein 1 (PD-1), programmed death
ligand 1 (PD-L1), or cytotoxic T-lymphocyte antigen 4
(CTLA-4), have demonstrated unprecedented and durable
efficacy in a variety of cancers, including some tough-to-treat
subtypes.2,3 Despite these breakthroughs, the objective
response rate of these novel antitumor therapies remains low
in certain tumor types, thus limiting their potential to benefit a
much broader patient population.4 To address this issue,
different combination strategies are being intensively explored,
including chemotherapy, radiotherapy, as well as those
mechanisms capable of overcoming tumor-induced local
immunosuppression.5
depriving T cells of arginine and (2) increasing the
concentration of immunosuppressive metabolites.8,9 Elevated
levels of ARG1 have been detected in tumors of patients with
various types of cancer,10,11 and elevated plasma ARG1 and
reduced L-arginine levels are reported to correlate with
suppressed T-cell function and proliferation in patients with
different histologies.12,13 Therefore, pharmacological inhibition
Received: April 8, 2021
Accepted: July 7, 2021
Arginase (ARG) is an enzyme that metabolizes arginine to
ornithine and urea.6 The role of ARG1, secreted by myeloid
© XXXX The Authors. Published by
American Chemical Society
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A

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Figure 1. SBDD strategy: (A) Structure of 1 (pink) bound to hARG1 (PDB 6Q92)19 indicates underexplored space (white circle) between the
ligand and active site. (B) Structure of 2 (cyan) bound to hARG1 (PDB 4HWW)20 reveals proximity between the piperidine ring and Asp 183
(gold) and Asp181 (through water, gold) respectively, which contributes to >15× improvement in biochemical potency. (C) Two-stage approach
of introducing N-containing heterocyclic cores as well as H-bond interactions for entropic and enthalpic improvements in binding energies.
of ARG1 by small molecules and its combination with anti-PD-
1 or anti-PD-L1 agents have attracted great attention for
cancer immunotherapy.14,15 Here we describe the discovery
and optimization of a novel class of boronic acid ARG1
inhibitors16 guided by structural-based drug design (SBDD),
which features a privileged natural proline moiety. Extensive
studies were also carried out to evaluate and improve the oral
bioavailability of this class of inhibitors, which occupies a
unique property space that was proven to be challenging for
oral absorption and bioavailability.17,18
Scheme 1. Modular Strategy for Diverse Core SAR
Exploration.
As part of a comprehensive effort to discover novel small-
molecule inhibitors of ARG to enhance cancer immunother-
apy,21 we took inspiration from the reported human ARG1
(hARG1) cocrystal structures of (S)-2-amino-6-boronohex-
anoic acid (ABH, 1, Figure 1A).22 Our starting point, as
exemplified by 1 (hARG1 IC50 = 311 nM), has a moderate
enzymatic activity.23 Based on careful analysis of the binding
site, in particular, the underexplored space not effectively
occupied by 1, we envisioned that (1) a judicious cyclization of
the α-amino group to the carbon backbone might be tolerated
and fit better into the narrow pocket; and (2) the resulting N-
heterocyclic core could be leveraged as a template to better
engage in direct or water-mediated H-bonds with residues
Asp183 and/or Asp181, as observed by Van Zandt and co-
workers (2, Figure 1B)20 and ourselves previously.21 In an
attempt to identify the most appropriate cyclization approach,
we employed a modular synthetic strategy by investigating a
broad scope of N-heterocyclic cores with a variety of linker
sizes (Scheme 1). The strategy was enabled by readily available
building blocks and robust cross-coupling methods, while
subsequent hydroboration, hydrogenation, and global depro-
tection readily yielded a diverse set of core and linker
combinations.24
enzymatic assay23 between 1,2-cyclization [( )-3 and ( )-4]
and 1,3-cyclization approaches [( )-5 and ( )-6], presumably
due to the relatively small volume of the binding site to
accommodate nonideal conformations, as further evidenced by
the complete loss of activity in other combinations (data not
shown). Nevertheless, we were encouraged by the identi-
fication of pipecolic acid [( )-3] and proline [( )-4] cores
with comparable potencies as the starting point ABH (1), as
mixtures of isomers from the hydrogenation step. In addition,
as the individual isomers were resolved by chiral SFC
separation, the absolute stereochemistry was also found to be
crucial and (2S,3R)-4a was favored over (2S,3S)-4b by >25-
fold in hARG1 IC50, while the unnatural (2R) isomers were
completely inactive (data not shown). Interestingly, despite
almost identical crystal structure binding modes between 3a
and 4a (see overlay: Figure 2A), there was a remarkable 23-
fold improvement as the ring size was reduced. As the ring size
was further reduced to an L-azetidine-carboxylate, no further
improvement could be achieved by either a cis- (7a) or trans-
isomer (7b), presumably due to the most favorable conforma-
tional bias presented by the L-proline core. Introducing an
additional stereoelectronic effect, represented by the morpho-
line core in 8, slightly improved the biochemical IC50. Overall,
L-proline analogue 4a presented an optimal profile, in
particular with a remarkable ligand efficiency (LE = 0.76).26
Recognizing the significant challenges associated with the
As summarized in Table 1, while each core−linker
combination had comparable modeling overlay with ABH,40
significant differences of activities were observed in the hARG1
B
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abstraction (HAA) process (Scheme 2A).27 Alternatively, a
previously reported building block II28 was carefully selected as
the linchpin for regio- and stereoselective β-enolate chemistry
as well as sequential γ-reduction and displacement with a
variety of nucleophiles. (Scheme 2B).40
Table 1. Selected SAR of N-Heterocyclic Core Exploration:
General Design Strategy and Structures
a
Significant SAR exploration centered around various α-
substitutions of ABH has been disclosed previously,29−32 with
2-piperidinyl-ethyl being one of the most privileged side chains
(2, Figure 1);20 incorporation of the same moiety in our
scaffold, however, resulted in a 4-fold drop in enzymatic IC50
(9, Table 2). While a preliminary survey of α-side chains did
not further improve potency, modeling based on the cocrystal
structure of 4a with hARG1 indicated that small polar
substituents growing off β, γ, and δ-positions of the L-proline
core might be not only tolerated but also beneficial for
potency, presumably via polar interactions with residues
Asp181 and Asp183 (Figure 2B).
We started with the β-substitutions to expand SAR toward
the solvent region. Similar to the SAR at the α-substitutions,
although simple methylation resulted in a slight drop in
potency (10, Table 2), a hydroxymethyl group was found to be
neutral in terms of both activity and bioavailabilities (11). A
dramatic improvement in potency was observed when a basic
amine was introduced (12), albeit with a slightly diminished
LE when compared with the baseline 4a (0.76 → 0.71).
Encouraged by these results, we then shifted our attention
toward the δ-substitutions, since the HAA-formylation
chemistry turned out to be highly stereoselective, resulting in
the desired trans-configuration as a single isomer (Scheme
2A). As summarized in Table 2, the methyl analogue 13 turned
out to be one of the most potent, with a comparable LE to 4a
and only a slight decrease in oral bioavailability (43% → 31%).
Surprisingly, the SAR trend was somewhat reversed from β- to
δ-substitutions, as additional hydroxy- (14) or amino-
extension (15) did not further improve potency as designed
to engage Asp181, but rather led to lower LEs (0.69 and 0.65,
respectively). Further evaluation of the surface residues around
the γ-carbon led us to introduce the same hydroxy and amino
groups to directly engage the Asp181 residue and increase H-
bond interactions with the backbone of the protein.
Satisfyingly, both hydroxy and amino analogues (16 and 18)
proved to be beneficial to the potency, with good to moderate
bioavailabilities and achieving the best LEs (0.77 and 0.78) so
far. As expected, methylation of the γ-hydroxy led to >10-fold
loss of activity (17), presumably due to the disruption of a key
H-bond interaction between the hydroxy group and Asp181.
The highly polar nature of this structural class of Arginase
inhibitors led to lower than usual ALogP98 (−4.0 to −1.0)
values than the ranges generally preferred for good oral
absorption (1.0 ∼ 3.0).33 At first glance, it seemed surprising
for compound 4a to still show good oral bioavailabilities (43%
and 25%, mouse and rat). In addition to its low ALogP98
value,34 several other parameters including H-bond donor
(HBD) counts and PSA also fall outside of typically preferred
value ranges,33 which, unsurprisingly, led to significantly lower
activities against intracellular ARG in the HEK293 cell line
(Table 2), likely due to poor permeability.35 Since the elevated
ARG1 and reduced L-arginine levels detected in cancer patient
plasma are extracellular (vide supra), achieving high cellular
potency was not key to our optimization strategy; in fact, we
hypothesized it might ultimately lead to a widening of the
therapeutic index. Nevertheless, understanding key parameters
for obtaining good oral absorption would be key for further
a
NA = data not available.
Figure 2. (A) Comparison of crystal structure binding modes for
compounds 3a (cyan, PDB 7KLK) and 4a (white, PDB 7K4G) to
hARG1. (B) Surface view of cocrystal structure of 4a (white) bound
to hARG1 (PDB 7K4G).
nontraditional property space of known boronic acid ARG
inhibitors (ALogP98 < −2, HBD ≥ 5, PSA > 120),16 we
further evaluated these representative single isomers in rodent
pharmacokinetic studies to assess in particular their oral
bioavailability.21,25 Gratifyingly, these novel core modifications
turned out to be beneficial for bioavailabilities when dosed
orally (p.o.) at 10 mg/kg (mpk) in both C57BL/6 mice and
Wister Han rats (Table 1).
Next, our attention was shifted to explore the feasibility of
introducing additional substituents around the L-proline core.
To enable broad assessment of diverse substitution patterns,
we first envisioned the use of a key intermediate I on the route
to 4a24 for both stereoselective α-alkylation and δ-formylation,
with the latter enabled by a novel photoexcited hydrogen atom
C
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a
Scheme 2. Novel Synthetic Routes for Enabling Facile Sidechain Exploration
a
Reagents and conditions: (α) NaHMDS, THF, −40 °C; then allyl iodide or alkyl bromide, −40 °C to rt; (β) DBU (cat.), HCHO (37 w t% in
H2O), THF, 0 °C to rt; (γ) KHMDS, THF, −78 °C; then (+)-CSA, THF, −78 °C; then MeOH, NaBH4, −78 °C to rt; (δ) benzophenone,
phenylsulfonylmethanal O-benzyl oxime, CH3CN, 365 nM LED, rt.
a
Table 2. Selected SAR for Side Chain Optimization: General Design Strategy and Structures
a
NA = data not available.
optimization. As shown in Figure 3, we plotted binned
distributions of oral bioavailability in C57BL/6 mice against a
variety of molecular descriptors, and good oral bioavailability
(i.e., ≥20% F shown in green bands) tends to enrich in
compounds with MW lower than 230, which seems to indicate
the possibility of a paracellular transport mechanism.35
Furthermore, HBD counts also seem to have a significant
impact on achieving good oral bioavailability, with less than
five HBDs being seemingly optimal. These observations
prompted us to focus on the most ligand efficient compound
D
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a
Table 3. Optimizing for Potency and Oral Bioavailability
Figure 3. Percent distributions of mouse oral bioavailability (p.o., 10
mpk) vs (A) MW, and (B) HBD.
18 to allow further exploration of several strategies including
paracellular transport as well as groups that may allow for
active uptake mechanisms.15,36
As shown in Scheme 3, leveraging our access to a key
intermediate III, our strategy aimed at a general and robust
displacement strategy of a superior leaving group, chlorome-
thylsulfonate,37 that allowed the expeditious investigation of a
variety of substitution patterns of the γ-amino group. From the
outset, we looked to maintain or improve potency by further
extending toward surface residues (i.e., Asp181 and Asp183)
while focusing on (1) reducing HBD counts and (2)
introducing strategically placed amino acid groups to explore
whether the engagement of uptake transporters could
potentially improve bioavailability.38 Although the former
approach failed to yield any improvement in potency or
bioavailability (Table 3, 18−22), we were very encouraged by
the significant potency gain (>20-fold) when an L-alanine
moiety was installed (23), compared to the corresponding acyl
analogue (22). Interestingly, the peptide motif turned out to
have relatively poor in vitro and in vivo stability in mouse
hepatocytes and pharmacokinetic experiments, where rapid
a
Asterisk (*): Discernible exposures of 18 detected in plasma
samples. NA = data not available.
clearance of 23 was observed and hydrolysis product 18 was
detected as the predominant species. Interestingly, the
a
Scheme 3. Modular Strategy for Diverse Core SAR Exploration and Modular Synthesis for γ-Side Chain Optimization
a
Reagents and conditions: (1) 2,6-lutidine, ClCH2SO2Cl, CH2Cl2, 0 °C to rt; (2) CsOAc, 18-crown-6, toluene, 80 °C; (3) NaN3, DMSO, 80 °C;
(4) Pd/C, H2, EtOAc, rt; (5) R2CHO, AcOH, NaBH3CN, MeOH, 0 °C to rt; (6) R2CO2H, HATU, Et3N, DMF, rt; (7a) HBpin, [Ir(COD)Cl]2,
dppe, CH2Cl2, rt; (7b) (+)-pinanediolborane used instead of HBpin; (8a) 6N HCl, 80 °C; (8b) KOTMS, THF, rt; then 6N HCl, 35 to 60 °C; (9)
K2CO3, MeOH, rt; (10) MeI, NaH, DMF, rt.
E
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combined exposures of 18 and 23 when 23 was dosed orally
(10 mpk) surpassed that of dosing 18 (27% vs 14% F) itself,
indicating the potential contribution of an active uptake
mechanism via recognition of the L-Ala moiety. Modeling
indicated that the side chain of the amino acid motif would
likely extend toward the solvent region without detrimental
effects on binding affinity; therefore, a focused set of natural as
well as unnatural amino acids were evaluated. Growing from L-
Ala to L-Val dramatically improved potency (24a, IC50 = 2
nM), but good metabolic stability was not achieved.
Furthermore, the natural S-configuration was found to be
important for good potency, as evidenced by the >15-fold loss
of activity when D-Val was introduced (24b, IC50 = 34 nM).
Finally, a variety of unnatural amino acids as a follow-up of the
L-Val result were evaluated to further improve metabolic
stability. Interestingly, while moving the additional methyl
group to the amine (25) or α′-carbon (26) resulted in
suboptimal bioavailability and potency, respectively; the L-α-
tert-butyl glycine analogue 27 proved to be hitting the sweet
spot of potency, stability, and bioavailability. The superior
profile presented by 27 supported the potential of this novel
class of molecules.
To gain a detailed understanding of the binding mode of our
inhibitors, cocrystal structures of 18 and 24a with hARG1
protein were obtained (Figure 4). As the structural overlays
Figure 5. Pharmacokinetic−pharmacodynamic assessment of 4a
(p.o.) in EMT6 tumor-bearing BALB/c mice in (top) 2 day (10,
30, and 300 mpk, BID) and (bottom) 14 day (250 mpk, BID)
mDX400 (5 mpk, Q5D) experiments.
Figure 4. Overlay of cocrystal structures of (left) 4a (white, PDB
7K4G) and 18 (orange, PDB 7KLL); (right) 18 and 24a (green, PDB
7KLM)
4a showed comparable dose-dependent increases in compound
exposures, all of which surpassed the enzymatic arginase IC50
of 16 nM by >300-fold (Figure 5 top, blue and red lines,
respectively); accordingly, both serum and tumor arginine
levels were also substantially elevated in a dose/exposure
dependent manner, as compared to the control group (saline, n
= 5).38 Encouraged by these results, 4a was then dosed alone
(p.o., 250 mpk, BID) and in combination with a mouse anti-
PD-1 mAb (mDX400, i.p., 5 mpk, Q5D) in a 14 day repeated
dosing experiment (n = 10 for each dosing group).39 To our
delight, 4a was well tolerated with no discernible body weight
changes and demonstrated comparable pharmacodynamic
effects on serum and tumor arginine levels with or without
codosing of mDX400.
In conclusion, guided by SBDD, a novel class of ARG1
inhibitors featuring a proline core was identified. Further
optimization of the side chain to improve potency while
maintaining favorable pharmacokinetics was greatly accelerated
by harnessing a variety of stereoselective late-stage function-
alizations of late-stage building blocks. Compounds 4a and 27
were discovered and exhibited favorable overall profiles,
including excellent potency, selectivity, and oral bioavailability.
The binding model for this class of molecules was also
established unambiguously by the X-ray cocrystal structure of
compounds 4a, 18, and 24a with the hARG1 protein. Finally,
revealed, both 18 and 24a adopt a similar binding mode as that
of the starting point 4a and the boronate, alkyl chain, and
pyrrolidine overlay extremely well. As expected, the boronate,
pyrrolidine, and carboxylate groups make numerous inter-
actions within the active site. Interestingly, the additional
amine moiety on the γ carbon of the pyrrolidine ring of 18 is
hydrogen bonding to nearby Asp181, consistent with the
original design, while the valine side chain of 24a is oriented to
interact with the pocket entrance through favorable electro-
static and van der Waals interactions, as well as the water
molecules of the first solvation shell. These fruitful interactions,
therefore, allow the ligand to explore the area directly outside
of the active site, though it does not make any direct
interactions with Asp181 or Asp183.
Contemporaneous to the optimization efforts described in
the preceding paragraphs, 4a was selected as a fit-for-purpose
tool for further profiling in mouse pharmacokinetic−
pharmacodynamic experiments (Figure 5). In our first study,
4a was administered to EMT6 tumor-implanted BALB/c mice
at escalating doses of 30, 100, and 300 mpk twice daily (BID)
by oral gavage (n = 5 for each dosing group). Gratifyingly, at 1
h time points after the fourth dose, serum and tumor levels of
F
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4a demonstrated promising pharmacodynamic effects in a
mouse EMT6 tumor syngeneic model and was well tolerated.
These findings provide rationale for further investigations of
this novel series of hARG1 inhibitors as anticancer agents in
order to add to the armamentarium of agents that activate the
immune system to fight cancer, and future development will be
communicated in due course.
Adam Beard − Analytical Research and Development, Merck
& Co., Inc., Boston, Massachusetts 02115, United States
Hai-Young Kim − Analytical Research and Development,
Merck & Co., Inc., Boston, Massachusetts 02115, United
States
Josep Saurí − Analytical Research and Development, Merck &
Co., Inc., Boston, Massachusetts 02115, United States
Hakan Gunaydin − Computational and Structural Chemistry,
Merck & Co., Inc., Boston, Massachusetts 02115, United
States; orcid.org/0000-0003-1093-2787
David L. Sloman − Discovery Chemistry, Merck & Co., Inc.,
Boston, Massachusetts 02115, United States
Phieng Siliphaivanh − Discovery Chemistry, Merck & Co.,
Inc., Boston, Massachusetts 02115, United States
Jared Cumming − Discovery Chemistry, Merck & Co., Inc.,
Boston, Massachusetts 02115, United States
ASSOCIATED CONTENT
■
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* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acsmedchemlett.1c00195.
Experimental procedures for protein preparation; X-ray
crystallography; synthesis of all new compounds (PDF)
Christian Fischer − Discovery Chemistry, Merck & Co., Inc.,
Boston, Massachusetts 02115, United States
AUTHOR INFORMATION
■
Corresponding Author
Complete contact information is available at:
https://pubs.acs.org/10.1021/acsmedchemlett.1c00195
Min Lu − Discovery Chemistry, Merck & Co., Inc., Boston,
Massachusetts 02115, United States; orcid.org/0000-
0003-0378-9134; Email: min.lu1@merck.com
Author Contributions
All authors have given approval to the final version of the
manuscript.
Authors
Hongjun Zhang − Discovery Chemistry, Merck & Co., Inc.,
Boston, Massachusetts 02115, United States; orcid.org/
0000-0002-1136-6142
Derun Li − Discovery Chemistry, Merck & Co., Inc., Boston,
Massachusetts 02115, United States; orcid.org/0000-
0003-0512-6684
Funding
This work was funded entirely by Merck Sharp & Dohme
Corp., a subsidiary of Merck & Co., Inc., Kenilworth, NJ, USA.
Notes
The authors declare no competing financial interest.
Matthew Childers − Discovery Chemistry, Merck & Co., Inc.,
Boston, Massachusetts 02115, United States
○Peter Spacciapoli died April 15, 2019.
Biography
Qinglin Pu − Discovery Chemistry, Merck & Co., Inc., Boston,
Massachusetts 02115, United States; orcid.org/0000-
0002-7853-4542
Rachel L. Palte − Computational and Structural Chemistry,
Merck & Co., Inc., Boston, Massachusetts 02115, United
States
Symon Gathiaka − Computational and Structural Chemistry,
Merck & Co., Inc., Boston, Massachusetts 02115, United
States
Thomas W. Lyons − Discovery Process Chemistry, Merck &
Co., Inc., Boston, Massachusetts 02115, United States
Anandan Palani − Discovery Chemistry, Merck & Co., Inc.,
Boston, Massachusetts 02115, United States
Min Lu received his Ph.D. in Organic Chemistry from Nanyang
Technological University, Singapore, under the supervision of Prof.
Guofu Zhong. After his Post Doctoral Fellowships at The Scripps
Research Institute, CA, and Rice University, TX, in the laboratories of
Prof. K. C. Nicolaou, he joined the Discovery Chemistry team at
Merck & Co., Inc., Boston, MA, in 2015. In 2018 he was promoted to
Associate Principal Scientist working on projects in various drug
discovery phases. Recent publications include orally available non-
nucleotide STING agonists with antitumor activity.
ACKNOWLEDGMENTS
Peter W. Fan − Pharmacokinetics, Pharmacodynamics, and
Drug Metabolism, Merck & Co., Inc., Boston, Massachusetts
02115, United States
■
The authors gratefully acknowledge Abdelghani Achab,
Matthew Mitcheltree, and Theodore Martinot for contribu-
tions to the synthesis and biological profiling of compounds
described here. We also recognize Jialiang Wang, Qingxin Li,
Xiong Xiao, Peng Zhang, Wenqin Hu, Jiuyin Yi, Minfang
Zhang, Zhenxiang Xie, Yaheng Wang, and Yang Wang from
WuXi Apptec for their synthetic efforts. We thank Donovon
Adpressa for NMR structure-elucidation work, Foster Tenkor-
ang and Janet Diratsouian for high resolution mass
spectrometry support, and Brian Hall and Angie Sun for
providing purified protein for crystallography studies.
○Peter Spacciapoli − Quantitative Biosciences, Merck & Co.,
Inc., Boston, Massachusetts 02115, United States
J. Richard Miller − Quantitative Biosciences, Merck & Co.,
Inc., Boston, Massachusetts 02115, United States
Hyelim Cho − Quantitative Biosciences, Merck & Co., Inc.,
Boston, Massachusetts 02115, United States
Mangeng Cheng − Quantitative Biosciences, Merck & Co.,
Inc., Boston, Massachusetts 02115, United States
Kalyan Chakravarthy − Quantitative Biosciences, Merck &
Co., Inc., Boston, Massachusetts 02115, United States
Jennifer O’Neil − Discovery Oncology, Merck & Co., Inc.,
Boston, Massachusetts 02115, United States
Padmanabhan Eangoor − Pharmacokinetics,
Pharmacodynamics, and Drug Metabolism, Merck & Co.,
Inc., Boston, Massachusetts 02115, United States
DEDICATION
■
This paper is dedicated to the memory of our colleague, Peter
Spacciapoli.
G
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