Discovery of an Orally Bioavailable Inhibitor of Defective in Cullin Neddylation 1 (DCN1)-Mediated Cullin Neddylation

Article abstract of DOI:10.1021/acs.jmedchem.7b01282

We previously reported the discovery, validation, and structure-activity relationships of a series of piperidinyl ureas that potently inhibit the DCN1-UBE2M interaction. We demonstrated that compound 7 inhibits both the DCN1-UBE2M protein-protein interaction and DCN1-mediated cullin neddylation in biochemical assays and reduces levels of steady-state cullin neddylation in a squamous carcinoma cell line harboring DCN1 amplification. Although compound 7 exhibits good solubility and permeability, it is rapidly metabolized in microsomal models (CL_int = 170 mL/min/kg). This work lays out the discovery of an orally bioavailable analogue, NAcM-OPT (67). Compound 67 retains the favorable biochemical and cellular activity of compound 7 but is significantly more stable both in vitro and in vivo. Compound 67 is orally bioavailable, well tolerated in mice, and currently used to study the effects of acute pharmacologic inhibition of the DCN1-UBE2M interaction on the NEDD8/CUL pathway.

Full text of DOI:10.1021/acs.jmedchem.7b01282

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Discovery of an Orally Bioavailable Inhibitor of Defective in
Cullin Neddylation 1 (DCN1)-mediated Cullin Neddylation
Jared T. Hammill, Deepak Bhasin, Daniel C. Scott, Jaeki Min, Yizhe Chen, Yan Lu,
Lei Yang, Ho Shin Kim, Michele C. Connelly, Courtney Hammill, Gloria Holbrook,
Cynthia Jeffries, Bhuvanesh Singh, Brenda A. Schulman, and R. Kiplin Guy
J. Med. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.jmedchem.7b01282 • Publication Date (Web): 16 Mar 2018
Downloaded from http://pubs.acs.org on March 17, 2018
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Discovery of an Orally Bioavailable Inhibitor of Defective in Cullin Neddylation 1 (DCN1)-
mediated Cullin Neddylation
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Jared T. Hammill1,6; Deepak Bhasin1; Daniel C. Scott2,3; Jaeki Min1; Yizhe Chen1,6; Yan Lu1; Lei
Yang1; Ho Shin Kim1,6; Michele C. Connelly1; Courtney Hammill1; Gloria Holbrook1; Cynthia
Jeffries1; Bhuvanesh Singh4; Brenda A. Schulman2,3,5; R. Kiplin Guy1,6
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1Department of Chemical Biology and Theraputics, St. Jude Children’s Research Hospital,
Memphis, Tennessee, 38105 USA
2Howard Hughes Medical Institute, St. Jude Children’s Research Hospital, Memphis, Tennessee,
38105 USA
3Department of Structural Biology, St. Jude Children’s Research Hospital, Memphis, Tennessee,
38105 USA
4Department of Surgery, Laboratory of Epithelial Cancer Biology, Memorial Sloan Kettering
Cancer Center, New York, New York, 10065 USA
5Present address: Department of Molecular Machines and Signaling, Max Planck Institute of
Biochemistry, Martinsried, Germany
6Present address: Department of Pharmaceutical Sciences, University of Kentucky, Lexington,
KY 40508
*Correspondence: kip.guy@uky.edu
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Abstract:
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We previously reported the discovery, validation, and structure-activity relationships of a
series of piperidinyl ureas that potently inhibit the DCN1-UBE2M interaction. We demonstrated
that compound 7 inhibits both the DCN1-UBE2M protein-protein interaction and DCN1-mediated
cullin neddylation in biochemical assays and reduces levels of steady-state cullin neddylation in
a squamous carcinoma cell line harboring DCN1 amplification. Although compound 7 exhibits
good solubility and permeability, it is rapidly metabolized in microsomal models (CLint = 170
mL/min/kg). This work lays out the discovery of an orally bioavailable analog, NAcM-OPT (67).
Compound 67 retains the favorable biochemical and cellular activity of compound 7 but is
significantly more stable both in vitro and in vivo. Compound 67 is orally bioavailable, well
tolerated in mice, and currently used to study the effects of acute pharmacologic inhibition of the
DCN1-UBE2M interaction on the NEDD8/CUL pathway.
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Introduction
Ubiquitin-like protein (UBL) modification pathways have emerged as important targets for
oncology drug discovery. Successful examples include clinical proteasome inhibitors (e.g.,
Bortezomib or Carfilzomib)2, E3 inhibitors3, and the NEDD8 E1 inhibitor (Pevonedistat)4-6. Here
we present the discovery of an orally bioavailable small molecule inhibitor of the UBL NEDD8 co-
E3 enzyme, defective in cullin neddylation 1 (DCN1). DCN1 is also known as DCUN1D1, DCNL1,
or SCCRO (squamous cell carcinoma-related oncogene); we use “DCN1” hereafter. DCN1 is an
oncogenic driver gene contained within a recurrent 3q26.3 chromosomal amplification that is
common in squamous cell carcinomas (SCC) of the lung, head, and neck.7-13 DCN1 amplification
is associated with malignant transformation, metastasis, and poorer patient survival rates.7-13
DCN1 is part of a hierarchical E1-E2-E3 enzymatic cascade that covalently attaches the
UBL NEDD8 to target proteins14. The first step of this cascade is the ATP-dependent formation of
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an E1-NEDD8 thioester intermediate. Next, the NEDD8 is transferred from the E1 to a catalytic
Cys on the E2. E3s then join the E2-NEDD8 complex and recruit the target protein. This complex
consequently catalyzes the formation of an isopeptide bond connecting NEDD8’s C-terminus to
the ɣ-amino group of a lysine side-chain on the target protein. DCN1 is a co-E3 that stimulates
the neddylation reaction.15-16 Cullins, the best-characterized substrates of the NEDD8 pathway,
are a family of ubiquitin E3 ligases that are activated by neddylation17-19. Cullins serve as the
catalytic domain for approximately 300 human multi-protein CUL-RING E3 ligases (CRLs) that
regulate ubiquitin (UB) conjugation.18, 20
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The CRLs control diverse biological processes and their dysfunction is implicated in
human diseases, including cancer.21-23 Thus, drug discovery efforts targeting the CRLs continues
to grow.24-27 One approach to inhibiting CRL activity is to block cullin neddylation.28-31 Pevonedistat
(MLN4924) is the most well characterized inhibitor of the NEDD8 pathway and is currently in
phase II clinical trials for the treatment of cancer.6, 32-33 Pevonedistat is an adenosine sulfamate
analogue that covalently inhibits the NEDD8 E1, thus inhibiting all neddylation.34 The success of
Pevonedistat has stimulated discovery of other inhibitors of the neddylation pathway. Virtual
screening has yielded both reversible and covalent small molecule inhibitors of the NEDD8 E1.35-
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Several rhodium(III) complexes also inhibit the NEDD8 E1, offering an alternative to small
molecules.39-40 Recently, an inhibitor of the COP9 signalosome, responsible for de-neddylation
of the CRLs, has been reported and also displays anti-tumor activity.41 We, and others, have
recently reported the discovery of inhibitors of DCN1.42-44
While complete ablation of neddylation appears to be clinically efficacious,29 it is also toxic.
In an effort to maintain efficacy and reduce toxicity, we sought more specific ways to interrogate
the neddylation pathway.42 DCN1 plays a role in tumor formation, maintenance, and regulation
of the neddylation pathway.7-17, 45-46 Therefore, we sought to develop DCN1 inhibitors that may
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offer advantages and/or alternative specificities compared to pan-neddylation or de-neddylation
inhibitors.1, 42-43
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We recently identified a class of piperidinyl ureas that inhibit the DCN1-UBE2M protein-
protein interaction and thereby reduce steady-state
neddylation of specific cullins.42 Subsequent
iterative structure-enabled optimization of our high
throughput screening hit (compound 2, Table 1, IC50
= 7.6 µM) yielded compound 7 (IC50 = 0.060 µM)1
that was 100-fold more potent than our initial hit.1, 42
Our optimization targeting five sub-pockets within
the DCN1-UBE2M binding site is reported in the
companion manuscript.1 The first three sub-pockets
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are the Ile, N-acetyl, and Leu, named for the
UBE2M amino acid side chains that occupy these
pockets. The last two sub-pockets are the urea and
hinge pockets, named for the moieties from the
chemotype that occupy them (Figure 1).42
Figure 1. Top: Chemical structure of 1.
Bottom: Overlay of 1(teal): DCN1
(electrostatic potential surface) (PDB:
6BG5)1 and UBE2MNAc (magenta): DCN1
(PDB: 3TDU) X-ray co-crystal structures,
highlighting regions targeted for
optimization1.
Examination of several X-ray co-crystal
structures led us to identify four key drivers of
potency. This finding is further exemplified by the 1:DCN1 X-ray co-structure shown in Figure 1
(PDB: 6BG5)1. The four potency drivers are: (1) the 4-aminopiperidinyl urea core that positions
the hydrophobic alky and aryl ends of the molecule into the Ile and hinge pockets; (2) a key H-
bond between the urea aryl N-H and the backbone amide of Gln114 in the urea pocket; (3) the
induced hinge pocket, which affords tight steric packing of hydrophobic residues around the m-
CF3 phenyl ring; and (4) the hydrophobic interactions of the methylbenzodioxole urea substituent
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within the Leu pocket (Figure 1).42 Compound 7 biochemically inhibits the DCN1-UBE2M protein-
protein interaction and cullin neddylation.1 This compound also suppresses steady-state levels of
cullin neddylation in HCC95 cells, a squamous carcinoma (SCC) cell line highly expressing
DCN1.1 However, compound 7 is rapidly metabolized in microsomal and murine models. This
report describes our continuing optimization efforts aimed at improving the clearance of
compound 7. Our optimization strategy focused on 1) understanding the physical drivers for
bioavailability (SPR); and 2) evaluating metabolic and toxic liabilities associated with the scaffold.
This strategy has yielded a potent, orally bioavailable, and significantly more stable chemical
probe, NAcM-OPT (67)42.
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Results
Given the clear effect of compound 7 in cells1, we examined whether this compound
possessed appropriate absorption, distribution, metabolism, excretion, and toxicology (ADME/T)
characteristics to block DCN1 activity using in vivo models. Compound 7 was moderately soluble
(57 ± 1.4 µM) and highly cell permeable (1400 ± 39 x 10-6 cm/s)1. However, compound 7 was
unstable in mouse microsomes (CLint = 170 mL/min/kg, Table 1). We tested several structurally
similar analogs and found that they were also rapidly metabolized (CLint > 150 mL/min/kg, Table
1). The most stable analog was the initial screening hit 2 (CLint = 90 mL/min/kg). Therefore, it
seems as though the structural modifications made to improve potency also reduced metabolic
stability. In particular, the benzyl urea substitution that was critical for potency and specificity due
to its interactions within the Leu pocket1 (Figure 1) appears to increase rates of oxidative
metabolism.
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Table 1. Rapid metabolism of early lead DCN1-UBE2M inhibitors. Values for the half-life and
intrinsic clearance were determined at a concentration of 0.8 µM using mouse microsomes and
are represented as the means plus or minus standard deviation from three replicates. Compound
preparation and potency was previously reported.1
In order to ensure that the microsomal models accurately predict in vivo clearance and
reveal any systemic toxicology exhibited by the series, we tested compound 7 in a single-dose
intravenous (IV) pharmacokinetic (PK) study using three doses: 0.5, 1.0, and 1.5 mg/kg (Figure
2A). We did not observe any adverse reactions or compound-related side effects within 48 h after
dosing (Supplementary Table 1). Compound 7 was rapidly cleared (t1/2 = 0.14 h, Clplasma = 42
L/h/kg) at all doses, suggesting that the microsomal models correctly predict clearance of
compound 7 in vivo. Therefore, we sought to mitigate the scaffold’s inherent metabolic liabilities
using microsomal models as a predictor of in vivo clearance.
We utilzed a recursive process of analog design, synthesis, testing, and design refinement
to optimize the series. We assessed biochemical potency using our previously reported TR-FRET
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assay,42 which measures the binding of full-length DCN1 to a constrained peptide corresponding
to N-terminally acetylated UBE2M15-16, 42. We used structurally directed modifications, as well as
more empircal investigation of the moeity fitting into the induced fit hinge pocket to probe the steric
and electronic requirements of binding47-51. We assessed the metabolic stability of all analogs
exhibiting reasonable potency in mouse microsomes. Analogs that retained potency and
posessed significantly improved microsomal stability were profiled in murine models to determine
in vivo toxicity, clearance, and bioavailability.
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To further guide the design of metabolically stable analogs, we conducted metabolite
identification (Met-ID) studies using a close structural analog of compound 7, compound 5 (Figure
2B). This study identified N-dealkylation (11) of the piperidine ring in the Ile targeting substituent
and oxidation of the aromatic ring of the urea (12), targeting the hinge region, as the primary
metabolic transformations (Figure 2B; Supplementary Table 2). Time-course metabolite formation
studies suggest that these modifications take place sequentially; oxidation of the aromatic ring
occurs first, followed by N-dealkylation (Supplementary Table 2). Although we were unable to
identify the specific oxidized carbon on the aromatic ring, we hypothesize that the carbons ortho-
or para- to the amino group are the most likely the sites of oxidation. This hypothesis is based on
the electron donating nature of the amino substituent and on previous reports52-53. Therefore, we
targeted optimization of the Ile and hinge moieties. We used both a rational, structure-based,
design and an empirical parallel chemistry approach to understand the key structural elements
that drive the structure-activity relationship (SAR) and structure-property relationship (SPR) that
define metabolic liabilities.
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Figure 2. Characterization of compound metabolism. A) Compound 7 plasma concentrations
over 24 h following IV administration 0.5, 1.0, and 1.5 mg/kg doses. Plasma concentrations from
time points taken after 2 h were below the limit of quantitation and are thus excluded from the
graph. B) Summary of metabolite identifications studies using compound 5. For details, see
Supplementary Table 2.
Following these findings, we sought to maintain potency and improve metabolic stability
by leaving invariant the benzyl group (targeting the Leu pocket) of compound 7 and varying the
alkyl side chain (targeting the Ile pocket) and aniline (targeting the hinge pocket) (Figure 1). We
devised two synthetic routes that permitted late stage introduction of either substituent (Scheme
1; routes i, ii). Both routes were amenable to parallel synthesis.
Synthesis of aniline analogs began by alkylation of 4-piperidone (13) with 2-iodopentane
(Scheme 1; routes i)42 to generate ketone 14. Reductive amination of 14 with benzylamine
furnished the secondary amine 1542. 15 was then either coupled directly with isocyanates or with
anilines using cabonyldiimidazole (CDI) as a coupling agent (21 – 46, Tables 2 & 3)54-57. Given
the wider availability of anilines, we used the CDI-mediated reaction for our parallel chemistry.
For this reaction, it was critical to allow CDI to react with the aniline prior to addition of the
secondary amine 15. The optimized procedure required no work-up and direct purification by
HPLC chromatography afforded moderate yields (30-55%) of the desired products. The scalability
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and ease of this synthetic route allowed us to prepare roughly 140 analogs variant in the aniline
substituent.
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Synthesis of analogs targeting variation of the alkyl sidechain started with reductive
amination of 1-N-boc-4-piperidone (16) with benzylamine to give 17. A limited range of anilines
with electron-withdrawing substitutions provided appropriate activity and the potential for
increased stability. Thus, the secondary amine 17 was coupled with 3-fluoro-5-
(trifluoromethyl)phenyl isocyanate to give 18. After BOC de-protection, Lewis acid-mediated
reductive amination58 of 18 with either ketones or aldehydes, afforded the targeted analogs (47 –
61, Table 4) in moderate to good yields (40-75%). We used this route to prepare roughly 45
analogs variant in the alkyl side chain (Supplementary Dataset 1). Compounds prepared by
parallel synthesis were characterized for purity (≥ 95% as characterized by UV/ELSD) and identity
using UPLC-LRMS (Supplementary Dataset 1). Compounds that proved critical to our SAR
analysis were further characterized using 1H NMR and HRMS (Supplementary Information).
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Our studies revealed that the n-butyl alkyl sidechain was as potent but more metabolically
stable than the 2-pentyl substitution (Table 5). Thus, we synthesized a final set of analogs starting
from commercially available n-butyl piperidinone 19 (Scheme 1; route iii)42. This two-step
procedure is readily scalable and has been used to prepare more than five grams of our top
compound 67 with > 98% purity as characterized by UV/ELSD and NMR analysis42.
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Scheme 1. General synthetic routes for generating analogs. (a) potassium carbonate, 2-
iodopentane, acetonitrile, 80 ºC, 16 h, 30-48% (b) benzylamine, sodium triacetoxyborohydride,
acetic acid, dichloromethane, rt, 16 h, 90-100%; (c) R1-isocyanate, N,N-diisopropylethylamine,
dichloromethane, rt, 2 h, 40-80%; (d) R1-amine, 1,1’-carbonyldiimidazole, tetrahydrofuran, 16 h,
30-55%; (e) 3-Fluoro-5-(trifluoromethyl)phenyl isocyanate, N,N-diisopropylethylamine,
dichloromethane, rt, 2 h, 80-99%; (f) trifluoroacetic acid, dichloromethane, 0 ºC - rt, 16 h, 95%;
(g) R2 (aldehyde or ketone), dibutyltin (IV) chloride, phenylsilane, tetrahydrofuran, rt, 16 h, 40-
75%.
First, we explored the SAR/SPR of the aniline urea substituent targeting the hinge pocket.
Our previous SAR studies and X-ray co-structures revealed that to permit deeper compound
binding, the hinge pocket undergoes a structural rearrangement to produce a hydrophobic cavity
composed of alkyl and aromatic residues (Ile86, Phe89, Val102, Ile105, Ala106, Phe109, Ala111,
Phe117, Phe122, Phe164)1, 42. Throughout all of our investigations, an aromatic ring with a small
hydrophobic substituent proved optimal for filling this sub-pocket. Compound 7 (IC50 = 0.060 µM),
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the most potent analog, utilizes a meta-trifluoromethylphenyl aniline that fits perfectly into the
hinge sub-pocket1. Previous work suggests that although meta- and para- substitutions were
tolerated, modestly increased steric bulk or ortho-substitution eliminate binding1. Thus, we used
the parallel chemistry route to rapidly produce diverse analogs while paying particular attention to
roughly isosteric para- and meta- substituted aniline rings (Table 2).
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Initial exploration of mono-substituted phenyl ureas (Table 2, 21 – 30) revealed that only
the para-bromo (26) and para-iodo (27) analogs were as potent as compound 7, with half maximal
inhibitory concentrations (IC50’s) of roughly 0.060 µM. All of the mono-substituted phenyl ureas
were rapidly cleared in the microsomal assay (CLint > 120 mL/min/kg). Halogen substituents were
generally more potent than the corresponding alkyl substituents, suggesting that while
hydrophobicity is necessary, the presence of an electron-withdrawing group, and potentially a
hydrogen bond acceptor, is important for potency (25 – 27). This result suggested that we pursue
the common strategy of decreasing the rate of metabolic oxidation of aromatic rings by decreasing
their electron density. Indeed, such analogs were slightly more stable in microsomal models than
compound 7 (CLint = 170 mL/min/kg vs. between 120 and 160 mL/min/kg, Table 2).
In concordance with previous studies,1 larger substitutions such as i-Pr (23, 29) and t-Bu
(30) significantly reduced analog potency (Table 2, Supplementary Dataset 1). Similar to the para-
series, the meta-substituted compounds’ potency appeared to improve with hydrophobic electron-
withdrawing substituents. The meta-trifluoromethylphenyl (7, IC50 = 0.060 µM) was more potent
than the meta-chlorophenyl (22, IC50 = 0.300 µM), a result that stands in stark contrast to the
para-substituted analogs (p-CF3 (24) IC50 = 0.300 µM; p-Cl (25), IC50 = 0.100 µM). The meta-i-Pr
analog (23, IC50 = 0.350 µM) was more potent than the para-i-Pr analog (29, IC50 = 0.650 µM).
The meta-fluoro analog (21) was less potent (IC50 = 1.70 µM), likely due to its inability to fill the
available pocket. Together, these results suggest that the meta-position affords slightly more
room for increased steric bulk compared to the para-position. The results also highlight that subtle
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variations in size and electronic properties can significantly affect potency when there is induced
fit in the binding site. The para-iodo analog (27, IC50 = 0.050 µM) was the only compound that
maintained the potency of compound 7 and increased stability (CLint = 120 mL/min/kg). However,
the observed increase in stability (CLint = 170 mL/min/kg vs. 120 mL/min/kg) was not sufficient to
render the compound suitable for in vivo studies. We next investigated the di-substituted phenyl
ureas with the intent to identify a substituent with improved potency and stability.
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Table 2. Optimization of the phenyl urea occupying the hinge pocket. IC50 values were
generated using our TR-FRET binding assay and are represented as the mean of three replicates
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with errors reported as the 95% confidence interval. 95% confidence intervals were generated
based on non-linear regression using the R drc package with the four-parameter log-logistic
function (LL2.4)59. Values for the half-life and intrinsic clearance were determined at a
concentration of 0.8 µM using mouse microsomes and are represented as the means plus or
minus standard deviation from three replicates. ND = data not determined.
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The targeted di-substituted phenyl ureas (Table 3) were designed to solidify the trends
observed for the mono-substituted analogs. That is, the analogs should incorporate small
hydrophobic and electron-withdrawing substituents at either the ortho- or para- position. These
substitutions appear to electronically and sterically block oxidation of the aromatic ring without
negatively affecting potency. We first tested meta- and para- di-substituted analogs (31 – 35).
These analogs retained moderate potency with the meta-nitro, para-chloro analog (31) exhibiting
an IC50 value below 0.100 µM. However, due to the potential toxicology associated with the nitro
substituent60, this compound was deprioritized. These compounds were also subject to rapid
metabolism. The meta-trifluoromethyl, para-nitrile derivative 34 (IC50 = 0.260 µM) was the only
compound that was significantly more stable in the microsomal model than compound 7 (CLint =
60 mL/min/kg vs. 120 mL/min/kg). Among the phenyl ureas with bis-meta substitution patterns,
only the meta-trifluoromethyl, meta-fluoro analog 36 was equipotent to compound 7 (IC50 = 0.060
µM). The corresponding meta-chloro, meta-fluoro analog 37 was significantly less potent, further
emphasizing the tightly constrained nature of the SAR. All of the bis-meta analogs were rapidly
metabolized.
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Table 3. Second round of optimization of the phenyl urea occupying the hinge pocket. IC50
values were generated using our TR-FRET binding assay and are represented as the mean of
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three replicates with errors reported as the 95% confidence interval. 95% confidence intervals
were generated based on non-linear regression using the R drc package with the four-parameter
log-logistic function (LL2.4)59. Values for the half-life and intrinsic clearance were determined at a
concentration of 0.8 µM using mouse microsomes and are represented as the means plus or
minus standard deviation from three replicates. ND = data not determined.
We next examined ortho-, meta- di-substituted phenyl ureas. We observed reduced
potency even in the presence of small ortho- substituents (Table 3, 38 – 41), which is consistent
with previous SAR data1. This series led to the emergence of the pyridine analog 41 - our first
compound to exhibit good metabolic stability (CLint < 13 mL/min/kg). Unfortunately, this
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compound, like other pyridine-containing analogs, exhibited significantly reduced potency (IC50 =
13 µM, Supplementary Dataset 1)1, 42. Examination of fused bicyclic aryl ureas revealed a general
trend that the hydrophobic carbocyclic compounds were more potent than their heterocyclic
counterparts (42 – 46). The 2-napthyl analog (46) afforded our first significant gain in potency
(IC50 = 0.020 µM). However, all of the fused bicyclic analogs were rapidly metabolized in the
microsomal model (Table 3). This result illustrates a common phenomenon in medicinal
chemistry: as one parameter is optimized, another in compromised. Based on these analogs, we
hypothesized that reducing the electron density of the aniline ring would mitigate the oxidation of
the aromatic ring, but the scope of the SAR would be narrow.
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In parallel to our investigations of aniline analogs, we sought to address N-dealkylation of
the alkyl-piperidine by exploring various substitutions occupying the Ile pocket. Initially, we fixed
the aniline substituent as the meta-CF3, meta-F, based on inhibitory potency. Using this base,
we screened a range of analogs replacing the 2-pentyl piperidine substitution that targets the Ile
sub-pocket (Figure 1). Our previous studies closely explored alkyl substituents at this position and
revealed that an n-butyl group was preferred and branched alkyl chains improved potency1. We
have also previously prepared simple benzyl and aryl analogs. To expand our SAR, we explored
several other alkyl, aryl, and heteroaryl substituents (Table 4, 47 – 61). Unfortunately, with the
exception of the n-butyl analog 47 (IC50 = 0.260 µM), all of the substituents led to significantly
reduced potency (Table 4, Supplementary Dataset 1). Thus, we selected only compound 47 for
further evaluation in the microsomal model.
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Table 4. Optimization of the alkyl piperidine. IC50 values were generated using our TR-FRET
binding assay and are represented as the mean of three replicates with errors reported as the
a
95% confidence interval. 95% confidence intervals were generated based on non-linear
regression using the R drc package with the four-parameter log-logistic function (LL2.4)59.
Microsomal modeling revealed that compound 47 was significantly more stable than
compound 7 (CLint = 22 mL/min/kg vs. 170 mL/min/kg), with only a five-fold reduction in
biochemical potency (IC50 = 0.260 µM vs. 0.060 µM). Comparison of 47 to the 2-pentyl derivative
36 revealed that replacing the 2-pentyl substitution with an n-butyl substitution markedly increased
stability (CLint = 22 mL/min/kg vs. 290 mL/min/kg). Assuming this was a general trend, we
examined other aryl ureas containing the n-butylpiperidine (Table 5). The di-substituted analogs
were more stable than the corresponding mono-substituted aryl urea analogs. The di-substituted
para-fluoro, meta-trifluoromethyl (70) or meta-fluoro, meta-trifluoromethyl (47) were more stable
than the ortho-fluoro, meta-trifluoromethyl (69). This result supports our hypothesis that the site
of oxidation is most likely the carbon para- to the amino substituent. Compound 67, which contains
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a meta-chloro, para-chloro phenyl urea afforded the best balance of potency and stability.
Compound 67 (IC50 = 0.050 µM) retains the biochemical potency of our optimized hit compound
7 but is significantly more stable in microsomal models with a calculated intrinsic clearance of 25
mL/min/kg.
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In order to determine whether we had altered the landscape of primary metabolites or
simply decreased their rate of formation, we conducted metabolite identification studies with 67
(Supplementary Table 3). These studies revealed that replacing the meta-trifluoromethyl group
of the aniline with a meta-, para-dichloro group effectively decreased the rate of microsomal
oxidation. However, oxidation of the aniline ring remained a minor metabolic pathway. N-
dealkylation was now the predominant metabolism event. Additionally, we observed oxidation of
the benzyl urea substituent targeting the Leu pocket. These results indicate that further
optimization may yield even more stable compounds. We believed we had successfully increased
simulated oxidative stability without negatively affecting potency, and therefore sought to
characterize the toxicology and pharmacokinetic characteristics of compound 67 in murine
models.
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Table 5. Final optimization of n-butylpiperidines. IC50 values were generated using our TR-
FRET binding assay and are represented as the mean of three replicates with errors reported as
a
the 95% confidence interval. 95% confidence intervals were generated based on non-linear
regression using the R drc package with the four-parameter log-logistic function (LL2.4)59. Values
for the half-life and intrinsic clearance were determined at a concentration of 0.8 µM using mouse
microsomes and are represented as the means plus or minus standard deviation from three
replicates. ND = data not determined.
We first conducted a low-dose IV PK experiment to understand the gross toxicity and
clearance of compound 67 (Figure 3A). Similar to compound 7, 67 failed to induce any
measureable adverse reactions during the 48 h monitoring period following IV administration
(Supplementary Table 4, 5). The observed Cmax of compound 7 (Figure 2A) and 67 (Figure 3A)
shows a non-linear increase from 0.5 to 1.5 mg/kg. Both compounds exhibit similar and large
volumes of distribution (87.3 – 176 L/kg). The IV half-life of 67 is 3-4 hours, which is significantly
longer than the half-life of compound 7 (t1/2 = 0.14-0.2 h). The increased metabolic stability of
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compound 67 correlates with the results from the microsomal model, which showed that this
compound has a six-fold lower clearance in mouse microsomes (Supplementary Table 6). Due
to the comparatively shorter half-life and lower Cmax, the AUClast of compound 7 is much smaller
than that of compound 67. Therefore, compound 67 is the preferred early lead for preliminary in
vivo studies. Based on these results, we determined that it was unethical to conduct an oral dosing
study for compound 7. However, we did conduct an oral PK experiment with compound 67 (Figure
3B).
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Figure 3. Preliminary pharmacokinetic profiling of compound 67. A) Time course of the drug
plasma concentration over 24 h resulting from single dose intravenous administration of 67 at
doses of 0.5, 1.0, and 1.5 mg/kg. B) Time course of the drug plasma concentration over 24 h
resulting from single oral doses of 67 at 50, 100, and 200 mg/kg. C) Repeated dose-exposure
studies to determine toxicological responses. Subjects were dosed orally with 200 mg/kg either
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once a day (QD) or twice a day (BID) and drug plasma concentration was measured 1 h after
drug administration. D) Table comparing the in vitro and in vivo clearance data. Results support
our hypothesis that the microsomal models predict in vivo clearance. Additional PK parameters
are in Supplementary Table 6.
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When dosed orally as a suspension in EtOH/PG/PEG/PBS (10/10/40/39; v/v/v/v; pH = 7.4)
and 1% (w/v) 2-hydroxy-β-cyclodextrin (MW~1396 Da), compound 67 displayed a half-life of 3-5
h. This observation concurs with the IV half-life of 3-4 h and suggests that 1) the total exposure
is driven by clearance and 2) levels of absorption are optimal. The observed Cmax and AUClast
show a dose-dependent increase from 1.5 to 3 µM and 3.4 to 17.1 µM*h, respectively. The
apparent volume of distribution and clearance are similar to those of IV. Based on these data,
compound 67 has an oral bioavailability of 88% (calculated using IV dose at 1.5 mg/kg and PO
dose at 50 mg/kg). This apparent bioavailability (Fin vivo) is close to what we predicted from in vitro
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clearance (Fin vitro = 78%), calculated by 1 - CL/Qh . Compound 67 did not cause a significant
adverse reaction at any dose tested during the 48 h monitoring period following oral administration
(Supplementary Table 7, 8).
In order to enable efficacy modeling in mice, we investigated the toxicology and
pharmacokinetics of compound 67 during repeat-dose experiments over a period of 14 days using
either once (QD) or twice (BID) daily schedule with a fixed oral dose of 200 mg/kg. Throughout
the 14-day dosing period, no adverse reactions or compound-related side effects were observed
(Supplementary Table 9, 10). Dosed animals in both groups experienced minor weight loss (3-
5%) during the first 3 days, but resumed normal growth within 5 days (Supplementary Figure 1).
In order to determine if there were significant changes in metabolism and clearance during the
study period, drug plasma concentrations were monitored one hour post administration after each
oral dose (Figure 3C). C1h was 2.4 ± 0.93 µM (QD) and 3.3 ± 1.5 (BID), and was consistent with
earlier data from our single oral dose study (C1h = 3 µM at 200 mg/kg). Therefore, compound 67
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is well tolerated, possesses good oral bioavailability, and is suitable for use in animal efficacy
models.
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The details of the biochemical selectivity and cellular activity of compound 67 (NAcM-
OPT), along with an irreversible inhibitor (NAcM-COV), and a structurally related negative control
(NAcM-NEG), have recently been disclosed.42 Briefly, compound 67 maintains the high level of
selectivity for inhibition of DCN1/2 over the highly homologous remaining human DCN isoforms.42
Compound 67 effectively disrupts the DCN1-UBE2M interaction and UBE2M interactome in
cells.42 In HCC95, a squamous carcinoma cell line highly expressing DCN1, either treatment with
compound 67 or knockdown of DCN1 with shRNA selectively reduces steady-state levels of
neddylated CUL1 and CUL3 while CUL2, CUL4A, and CUL5 are relatively unaffected.42
Compound 67 inhibits steady-state levels of neddylated CUL1 and CUL3 in a dose-dependent
manner (Figure 4A). Consistent with our previous studies,42 treatment of HCC95 cells with
compound 67 does not elicit gross substrate stabilization (Figure 4B). However, at longer
exposures, which oversaturate MLN4924 treated protein levels making direct comparison no
longer possible, compounds 67 elicits a dose-dependent increase in the known CUL1/CUL3
substrates NRF2 and p27 (Figure 4B) but not other known CUL1/CUL3 substrates Cylin E or p21
(Supplementary Figure 2).
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Figure 4. Western blot for inhibition of cellular neddylation and increased levels of substrates at
48 hours by DMSO, MLN4924 (positive control, 1 µM, single dose at T = 0), and compound 67
(indicated concentration, dosing at T = 0, and T = 24 hr). A) Dose-dependent inhibition of steady-
state levels of CUL1 and CUL3 neddylation. B) Dose-dependent effects of 67 on steady-state
levels of NRF2 and p27.
Discussion and Conclusions
In the present study, we employed both a rational, structure-based, design as well as an
empirical chemistry approach to uncover the key structural elements critical for binding to DCN1
and driving compound metabolism. Our studies yielded compound 67, which is equipotent to
earlier leads like compound 7 but is significantly more bioavailable. By developing short, scalable,
synthetic routes, we rapidly prepared over 200 analogs (Supplementary Dataset 1) to select
compounds with better PK profiles. A combinatory approach of microsomal modeling and in vivo
PK allowed us to confidently conclude that the microsomal models correctly predict in vivo
clearance. Metabolite identification studies focused our attention on the most rapidly metabolized
moieties of the inhibitor series: the aniline and alkyl side chain of the piperidine. The structure-
based design incorporated our previously described X-ray co-structures and SAR data1, 42 and
allowed us to narrow our focus to the substituents that were most likely to yield potent molecules.
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In total, 38 of the newly synthesized analogs possessed potency values less than or equal to 360
nM in our TR-FRET binding assay (Supplementary Dataset 1). In parallel to the rational design
approach, we employed an empirical approach to discover unexpectedly potent analogs, such as
the bicyclic compound 45, which appears to bypass the stringent steric restrictions in the hinge
sub-pocket, as suggested by our previous data. In the face of a known induced fit binding event,
we believe it is imperative to keep testing the limits of the design model. The ease and scalability
of the synthetic routes allowed us to produce ample material for both the PK and extended dosing
studies. The synthetic routes will also allow us to produce sufficient quantities for future in vivo
studies.
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We observed two general SAR/SPR trends. First, the incorporation of small hydrophobic
and electronegative substituents at either or both of the meta- and para- positions of the aniline
ring electronically and sterically blocked oxidation of the aromatic ring without negatively affecting
potency. Second, the SAR of the alkyl piperidine substituent was highly restricted only tolerating
small hydrophobic substituents.1 Subsequent metabolite identification studies using the optimized
compound 67 revealed that although incorporating the di-chlorophenyl ring reduced the oxidation
of the aniline substituent. N-dealkylation of the 4-aminopiperidine and oxidation of the benzyl
urea substituent targeting the Leu pocket were still prevalent. Future synthetic efforts will focus
on these regions as well as on accessing the currently unoccupied N-acetyl pocket, which likely
affords the best opportunity for potency gains.16
We are currently using compound 67 and others to dissect the effects of acute
pharmacologic inhibition of the DCN1-UBE2M interaction on the NEDD8/CUL pathway in both
cellular and in vivo models. The data suggest that inhibiting this NEDD8 E2-E3 interaction has
significantly different biological consequences from inhibition of the upstream NEDD8 E1 by
Pevonedistat or other E1 inhibitors. Pevonedistat acts by irreversibly inhibiting the E1 enzyme
that initiates the NEDD8 tri-enzyme cascade, completely blocking all neddylation. Compound 67
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selectively reduces the steady-state neddylation of specific cullins. The most striking phenotypic
difference is that unlike E1 inhibition, compound 67 does not grossly stabilize canonical Cullin-
RING ligase substrates.42 However, compound 67 does elicit a minor and selective increase in
the levels of two known CRL substrates, NRF2 and p27. Compound 67 also inhibits the
anchorage-independent growth of HCC95 cells, a phenotypic consequence associated with
DCN1-amplification and hallmark of cancer in this squamous cell carcinoma line.42 Future
experiments, outside the scope of this work, will focus on understanding the mechanisms driving
the observed selectivity and phenotypic consequences associated with acute DCN inhibition in
cells and murine models. These results suggest that selective regulation of the neddylation
pathway still influences tumor progression, at least in cells harboring DCN1 amplification.
However, we are clear to point out that selective inhibition of the DCN1-UBE2M interaction is not
grossly toxic to the proliferation of monolayer cultures. Given the high levels of DCN1
amplification exhibited in many human cancers,7, 11, 13 selective inhibition of DCN1-mediated
neddylation could provide a more targeted therapy relative to the complete ablation of neddylation
afforded by Pevonedistat or other NEDD8 E1 inhibitors. Recent reports of a second class of
DCN1 inhibitors demonstrates a similar spectrum of activity.43-44
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In conclusion, the development of compound 67 permits a more selective and acute
pharmacologic regulation of this highly complex and dynamic pathway that is already helping to
discover previously unappreciated biological roles of DCN1-mediated neddylation,42, 62 and
hopefully will provide further insight into tumor biology in areas with significant unmet clinical need.
Experimental Section
TR-FRET Assay1, 42
The TR-FRET assay was carried out in black 384-well microtiter plates at a final volume
of 20 μL per well. To screen library compounds, the assay cocktail was prepared as a mixture of
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50 nM Biotin-DCN1, 20 nM Ac-UBE2M12-AlexaFluor488, 2.5 nM Tb-Streptavidin (ThermoFisher)
in assay buffer (25 mM HEPES, 100 mM NaCl, 0.1% Triton X-100, 0.5 mM DTT, pH 7.5). The
assay cocktail was then incubated for 1 h at room temperature and distributed using a WellMate
instrument (Matrix). Compounds to be screened were added to assay plates from DMSO stock
solutions by pin transfer using 50SS pins (V&P Scientific). The assay mixture was incubated for
1 h at room temperature prior to measuring the TR-FRET signal with a PHERAstar FS plate reader
(BMG Labtech) equipped with excitation modules at 337 nm and emissions at 490 and 520 nm.
We set the integration start to 100 µs and the integration time to 200 µs. The number of flashes
was set to 100. The ratio of 520:490 was used as TR-FRET signal in calculations. Assay
endpoints were normalized from 0% (DMSO only) to 100% inhibition (unlabeled competitor
peptide) for hit selection and curve fitting. All compounds were tested in triplicate or more.
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Liver Microsomal Stability
We mixed 0.633 mL of mouse liver microsome (20 mg/mL, female CD9 mice, Fisher
Scientific, #NC9567486) with 0.051 ml of 0.5 M EDTA solution and 19.316 ml potassium
phosphate buffer (0.1 M, pH 7.4, 37°C) to produce 20 ml of liver microsome solution. One part 10
mM DMSO compound stock was mixed with 4 parts acetonitrile to make 2 mM diluted compound
stock in DMSO and acetonitrile. We added 29.1 µL diluted compound stock to 2.3 mL liver
microsomal solution and ran the mixture through a vortex to make final microsomal solution with
0.8 µM compound. We dispensed 180 µL of the mixed solutions into a 96-well storage plate (pION
Inc., MA, #110323). Five time points (0-4 hr) were taken for each microsome assay. For the 0 h
time point, 450 µL pre-cooled (4 ºC) internal standard (5 µM caffeine in methanol) was added to
the deep wells before the reaction started. For other time points, 1.25 ml of microsome assay
solution A (Fisher Scientific, #NC9255727) was combined with 0.25 ml of solution B (Fisher
Scientific, #NC9016235) in 3.5 ml of potassium phosphate buffer (0.1 M, pH 7.4). We added 45
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µL of this A+B solution to each of the 96 wells in the storage or reaction plate. The reaction plate
was then sealed and incubated at 37 ºC and shaken at a speed of 60 rpm. At 0.5, 1, 2, and 4 h,
we added 450 µL of pre-cooled internal standard (4 µg/ml caffeine in methanol) to the respective
wells in the reaction plate. The liquid was then transferred to a quenched plate. The quenched
plate was centrifuged (Eppendorf, model 5810R) at 4000 rpm for 20 minutes at 4 ºC. We
transferred 200 µL of supernatant to a 96-well plate and analyzed using UPLC-MS (Waters Inc.).
Selected ion recording (SIR) was used to detect the compounds and internal standard. The log
peak area ratio (compound peak area / internal standard peak area) was plotted against time and
the slope was used to calculate the elimination rate constant [k = (-2.303) * slope]. The half-life
(h) was calculated as t (1/2) = 0.693 / k. All compounds were tested in triplicate.
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Characterization of key compounds and intermediates
All moisture-sensitive reactions were performed using syringe-septum techniques under
an atmosphere of dry N2 unless otherwise noted. Prior to use, all glassware was dried in an oven
at 120° C for at least 6 h or flame-dried under an atmosphere of dry nitrogen. Methylene chloride
and acetonitrile were dried using an aluminum oxide column. Deuterated chloroform was stored
over anhydrous potassium carbonate. Reactions were monitored by TLC analysis (pre-coated
silica gel 60 F254 plates, 250 μm layer thickness) and visualized using an ultraviolet lamp (254 nm)
or by staining with either Vaughn’s reagent (4.8 g of (NH4)6Mo7O24·4H2O and 0.2 g of Ce(SO4)2 in
100 mL of a 3.5 N H2SO4) or a potassium permanganate solution (1.5 g of KMnO4 and 1.5 g of
K2CO3 in 100 mL of a 0.1% NaOH solution). Unless otherwise specified, commercially available
reagents were used as received. Flash column chromatography was performed using a Biotage
Isolera One and Biotage KP-SIL SNAP cartridges. Melting points were acquired on Buchi Melting
Point B-545. All NMR data was collected at room temperature in CDCl3 or (CD3)2SO on a 400 or
500 MHz Bruker instrument. Chemical shifts (δ) are reported in parts per million (ppm) with
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internal CHCl3 (δ 7.26 ppm for 1H and 77.00 ppm for 13C), internal DMSO (δ 2.50 ppm for 1H and
39.52 ppm for 13C), or internal TMS (δ 0.0 ppm for 1H and 0.0 ppm for 13C) as the references. 1H
NMR data are reported as follows: chemical shift; multiplicity (s = singlet, bs = broad singlet, d =
doublet, t = triplet, q = quartet, p = pentet, sext = sextet, sep = septet, m= multiplet, dd = doublet
of doublets, dt = doublet of triplets, td = triplet of doublets, qd = quartet of doublets); coupling
constant(s) (J) in Hertz (Hz); and integration. Purity was assessed by LC/MS/UV/ELSD using a
Waters Acquity UPLC-MS with the purity being assigned as the average determined by UV/ELSD.
All compounds were confirmed to ≥ 95% purity. Compounds that proved critical to our SAR
analysis were further characterized using 1H NMR and HRMS.
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N-benzyl-1-(pentan-2-yl)piperidin-4-amine (15).
We added acetic acid (2.11 mL, 36.9 mmol, 1.2 equiv) to a stirred solution of 1442 (5.20 g,
30.7 mmol, 1 equiv) and benzylamine (3.36 ml, 30.7 mmol, 1 equiv) in CH2Cl2 (50 mL). The
resulting cloudy mixture was stirred at room temperature for 1 h and sodium triacetoxyborohydride
(8.46 g, 39.9 mmol, 1.3 equiv) was added portion-wise over 10 min. The resulting heterogeneous
mixture was stirred at room temperature overnight (ca. 16 h), quenched with a saturated aqueous
solution of 1M sodium hydroxide, extracted with EtOAc (x2), dried (MgSO4), filtered, and
concentrated under reduced pressure to give 8.0 g (100% yield) of the desired product (15) as a
pale yellow oil. 1H NMR (500 MHz, Chloroform-d) δ 7.36 – 7.28 (m, 4H), 7.26 – 7.17 (m, 1H), 3.81
(s, 2H), 2.81 – 2.76 (m, 2H), 2.59 (sext, J = 6.6 Hz, 1H), 2.51 – 2.46 (m, J = 1H), 2.32 (t, J = 11.2
Hz, 1H), 2.19 (t, J = 11.3 Hz, 1H), 1.98 – 1.83 (m, 2H), 1.70 – 1.22 (m, 6H), 0.96 (d, J = 6.5 Hz,
3H), 0.90 (t, J = 7.1 Hz, 3H). 13C NMR (126 MHz, CDCl3) δ 140.8, 128.4, 128.0, 126.8, 59.0, 54.4,
50.7, 48.5, 45.7, 35.7, 33.1, 32.9, 20.2, 14.3, 14.1. HRMS (ESI+) m/z calcd for C17H29N2 [M+H]+,
261.2331, found 261.2327.
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tert-butyl 4-(benzylamino)piperidine-1-carboxylate (17).
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We added acetic acid (2.76 mL, 48.2 mmol, 1.2 equiv) to a stirred solution of 1-Boc-4-
piperidone (8.00 g, 40.2 mmol, 1 equiv) and benzylamine (4.39 mL, 40.2 mmol, 1 equiv) in dry
CH2Cl2 (150 mL). The resulting cloudy mixture was stirred at room temperature for 1 h and sodium
triacetoxyborohydride (11.4 g, 60.3 mmol, 1.5 equiv) was added portion-wise over 30 minutes.
The resulting heterogeneous mixture was stirred at room temperature overnight (ca. 16 h),
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quenched with a saturated aqueous solution of NaHCO , extracted with EtOAc (x2), dried
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(MgSO4), filtered, and concentrated under reduced pressure. The crude mixture was purified by
chromatography on SiO2 (MeOH/CH2Cl2, 2% to 15%) to give 11.4 g (98% yield) of the desired
product of a colorless oil. 1H NMR (500 MHz, Chloroform-d) δ 7.37 – 7.32 (m, 4H), 7.30 – 7.23
(m, 1H), 4.20 – 3.90 (m, 3H), 3.85 (s, 2H), 2.85 – 2.75 (m, 2H), 2.72 – 2.67 (m, 1H), 1.89 (d, J =
12.8 Hz, 2H), 1.47 (s, 9H), 1.40 – 1.32 (m, 2H).13C NMR (126 MHz, CDCl3) δ 154.7, 139.3, 128.5,
128.2, 127.2, 79.4, 54.1, 50.4, 42.6, 42.1, 31.9, 28.4. HRMS (ESI+) m/z calcd for C17H27N2O2
[M+H]+, 291.2073, found 291.2067
tert-butyl 4-(1-benzyl-3-(3-fluoro-5-(trifluoromethyl)phenyl)ureido)piperidine-1-carboxylate (18).
To a stirred solution of 17 (2.90 g, 9.99 mmol, 1 equiv) in CH2Cl2 (50 mL), we added 1-
isocyanato-3-(trifluoromethyl)benzene (1.44 mL, 9.99 mmol, 1.1 equiv). The reaction mixture was
stirred at room temperature overnight (ca. 16 h), diluted with EtOAc, washed with a saturated
aqueous solution of NaHCO3, brine, dried (MgSO4), filtered, and concentrated under reduced
pressure to give a pale yellow liquid. The crude mixture was purified by chromatography on SiO2
(MeOH/CH2Cl2, 1% to 10%) to give 4.8 g (97% yield) of the desired product 18 as a white solid.
1H NMR (400 MHz, Chloroform-d) δ 7.49 – 7.40 (m, 2H), 7.39 – 7.21 (m, 4H), 7.11 (s, 1H), 6.90
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(d, J = 8.1 Hz, 1H), 6.62 (s, 1H), 4.60 – 4.52 (m, 1H), 4.48 (s, 2H), 4.36 – 4.01 (m, 2H), 2.90 –
2.65 (m, 2H), 1.79 (d, J = 12.3 Hz, 2H), 1.66 – 1.50 (q, J = 12.3 Hz, 2H), 1.44 (s, 9H). 13C NMR
(101 MHz, Chloroform-d) δ 162.6 (d, J = 246.4 Hz), 155.0, 154.5, 141.4 (d, J = 11.3 Hz), 132.2
(qd, J = 33.5, 9.4 Hz), 129.4, 128.2, 126.0, 123.1 (q, J = 273.5 Hz), 111.7, 109.9 (d, J = 26.2 Hz),
106.7 (d, J = 25.0 Hz), 79.8, 53.0, 46.2, 43.2, 30.0, 28.4. HRMS (ESI-) m/z calcd for C25H28F4N3O3
[M-H]-, 494.2067, found 494.2058.
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General reductive amination procedure.58 To a stirred solution of 1-benzyl-3-(3-fluoro-5-
(trifluoromethyl)phenyl)-1-(piperidin-4-yl)urea (0.040 g, 0.101 mmol, 1 equiv) in tetrahydrofuran (1
ml), we added the aldehyde or ketone (0.202 mmol, 2 equiv) and dibutyltin (IV) chloride (1.53 mg,
0.005 mmol, 0.05 equiv). After stirring the heterogeneous mixture for 5 minutes at room
temperature, we added phenylsilane (0.025 ml, 0.202 mmol, 2 equiv). The resulting solution was
stirred at room temperature overnight (ca. 16 h). For some more sterically encumbered ketones,
we observed less than 20% conversion after overnight reaction. For these ketones, we added an
extra 3-4 equiv of ketone and 2 equiv of phenylsilane and the reaction was stirred at room
temperature for an additional 24 h. After the starting material was completely consumed, the
crude mixture was diluted with DMSO, filtered through a 0.45 micron filter, and subjected to HPLC
chromatography.
General isocyanate addition. To a stirred solution of 1-benzylpiperidin-4-amine (0.250 g,
1.314 mmol, 1.2 equiv) in CH2Cl2 (6 mL), we added 1-isocyanato-3-(trifluoromethyl)benzene
(1.53 mL, 1.10 mmol, 1 equiv) and N,N-diisopropylethylamine (0.286 mL, 1.64 mmol, 1.5 equiv).
The resulting mixture was stirred at room temperature overnight (ca. 16 h), concentrated under
reduced pressure, and purified by chromatography on SiO2.
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