Particle replication in nonwetting templates nanoparticles with tumor selective alkyl silyl ether docetaxel prodrug reduces toxicity

Article abstract of DOI:10.1021/nl4046558

Delivery systems designed to have triggered release after passively targeting the tumor may improve small molecule chemotherapeutic delivery. Particle replication in nonwetting templates was used to prepare nanoparticles to passively target solid tumors i

Full text of DOI:10.1021/nl4046558

Letter
pubs.acs.org/NanoLett
Particle Replication in Nonwetting Templates Nanoparticles with
Tumor Selective Alkyl Silyl Ether Docetaxel Prodrug Reduces Toxicity
Kevin S. Chu,† Mathew C. Finniss,§ Allison N. Schorzman,† Jennifer L. Kuijer,† J. Christopher Luft,‡,§
Charles J. Bowerman,‡,§ Mary E. Napier,‡,§,∥ Zishan A. Haroon,¶,□ William C. Zamboni,†,§,⊥,¶,□
and Joseph M. DeSimone*,†,‡,§,¶,□,■,○,●,△
‡
§
∥
†Department of Pharmaceutical Sciences, Department of Chemistry, Lineberger Comprehensive Cancer Center, Department of
⊥
¶
Biochemistry and Biophysics, Institute for Pharmacogenomics and Individualized Therapy, Carolina Center of Cancer
Nanotechnology Excellence, □Department of Pharmacology, Eshelman School of Pharmacy, ■Institute for Nanomedicine, ○Institute
for Advanced Materials, University of North Carolina at Chapel Hill, Chapel Hill, North Carolina 27599, United States
●Department of Chemical and Biomolecular Engineering, North Carolina State University, Raleigh, North Carolina 27607, United
States
△Sloan-Kettering Institute for Cancer Research, Memorial Sloan-Kettering Cancer Center, New York, New York 10065, United
States
S
* Supporting Information
ABSTRACT: Delivery systems designed to have triggered release after passively targeting the tumor may improve small
molecule chemotherapeutic delivery. Particle replication in nonwetting templates was used to prepare nanoparticles to passively
target solid tumors in an A549 subcutaneous xenograft model. An acid labile prodrug was delivered to minimize systemic free
docetaxel concentrations and improve tolerability without compromising efficacy.
KEYWORDS: Soft-lithography, docetaxel, polylactic acid, silyl ether
he synthesis of prodrugs is a common approach to
more common in usage for prodrugs, do not offer fast
Tovercome drug delivery issues, including poor aqueous conversion within the tumor microenvironment.9 Amino acid
solubility1 or permeability,2 and to provide site-specific release.3
Nanotechnology can be a powerful tool to improve drug
linkers were found to cleave more rapidly at higher pH values,
but the tumor microenvironment is thought to be acidic.10,11
Thus, a linker with triggered release at basic conditions may not
be preferable for chemotherapeutic delivery. Utilization of the
endosomal enzyme cathepsin B has been used with success for
targeted delivery systems12 but has yielded mixed results for
non targeted polymer drug conjugates. Retrospective analysis of
phase III clinical trial results of a cathepsin B sensitive polymer
drug conjugate suggested that different levels of cathepsin B
activity may lead to differences in survival.13 For this work, we
selected a prodrug chemistry with established tunability to
ensure active drug release at the target site.14 There are no
known enzymes that degrade silyl ethers, thus the use of this
chemistry may ensure that the drug is only released within
more acidic environments. Chlorosilanes with different
delivery, but does so by altering the biodistribution of the
encapsulated small molecule.4,5 In this report, we combined the
merits of both approaches to improve the pharmacokinetics
and toxicity of the chemotherapeutic docetaxel by passively
targeting an encapsulated docetaxel prodrug to solid tumors,
where it could selectively release and convert to active
docetaxel.
Many chemistries have been utilized to prepare prodrugs of
taxanes6−9 or camptothecins.10,11 Hydrazone bonds cleave
quickly at acidic conditions, but derivatives of taxanes must first
be synthesized to accommodate this chemistry.8 Thus, free
paclitaxel or docetaxel is not released immediately after the
hydrazone bond cleaves. Many of these other linkers were too
stable in vivo and did not release the active therapeutic at high
concentrations.6,11 Insufficient prodrug conversion may hinder
therapeutic efficacy because the active compound never reaches
therapeutically relevant concentrations. Ester prodrugs, though
Received: December 16, 2013
Revised: February 18, 2014
Published: February 20, 2014
© 2014 American Chemical Society
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Letter
Scheme 1. Synthesis of Alkyl Silyl Ether Docetaxel Prodrugs
combinations of steric bulk and alkyl chain lengths are
commercially available to synthesize prodrugs with varying
rates of hydrolysis, water solubility, and affinity to the particle
matrix in order to control drug release rates. Alkyl silyl ether
prodrugs of docetaxel were synthesized for incorporation into
poly(lactide-co-glycolide) nanoparticles prepared by the
particle replication in nonwetting templates (PRINT) process.
The increased lipophilicity of the prodrug improved drug
retention in the particles, which delayed the release of the
prodrug until the particles passively targeted the solid tumor.
Prodrug Synthesis and Particle Fabrication. The silyl
ether docetaxel prodrugs, C2 (ethyldimethylsilyl ether docetax-
el) and C8 (octyldimethylsilyl ether docetaxel), were prepared
by a single step reaction of docetaxel with chlorodimethyle-
thylsilane or chloro(dimethyl)octylsilane, respectfully (Scheme
1). It has been well documented that the C2′ alcohol of taxanes
preferentially react with electrophiles, such as acid chlorides and
anhydrides.15,16 As expected, the C2′ monosubstituted silyl
ether prodrugs of docetaxel formed and were isolated in good
yield.
The rate of conversion of the prodrug to docetaxel is the
consequence of simple hydrolysis and can be tuned by altering
the substituents on the silicon atom. To achieve rapid prodrug
hydrolysis upon release from the NP, alkyl dimethyl silyl
chlorides were selected (Figure 1A,B). Hydrolysis of the
prodrugs was evaluated in aqueous solutions at different pH
conditions (pH 5 and pH 7) and 37 °C in the presence of
human serum albumin and measured by LC-MS/MS. At pH 5,
the C2 prodrug was quickly degraded; full conversion was
achieved at 6 h (Figure 1A), where as the C8 prodrug was not
fully converted until 24 h. The conversion of the prodrugs to
docetaxel was also studied in mouse plasma at physiological
conditions. The t1/2 of C2 and C8 in mouse plasma were
similar, 8 h for the C2 prodrug and 10 h for the C8 prodrug.
The majority of the C2 and C8 prodrugs are converted within
the first 24 h. The toxicity of C2 and C8 were compared to free
docetaxel in vitro in A549 cells (Figure 1C). With a 24 h
incubation time, the toxicity of the C2 and C8 prodrugs were
less than that of free docetaxel. Modification of the C2′ alcohol
of taxanes reduces its activity and requires conversion of the
prodrug to achieve efficacy.7
DTXL-NP (docetaxel nanoparticle), C2-NP, and C8-NP, were
evaluated at 37 °C in phosphate-buffered saline (Figure 1D).
Release kinetics were dependent upon the length of the alkyl
chain of the docetaxel prodrug; prodrugs with the longer alkyl
chain lengths had slower release from the particles. Unmodified
docetaxel was fully released after 24 h where as the C2 prodrug
was fully released after 4 days and C8 prodrug was not fully
released after 7 days.
In Vivo Pharmacokinetics of Prodrug NPs. The
pharmacokinetic parameters and profiles of DTXL-NP and
C2-NP, the two faster-releasing formulations, compared to free
docetaxel at equal-molar dosing are shown in Figure 2 and
Table 2. Mice with A549 flank tumors were administered one
dose at 10 mg/kg docetaxel via vein injection and subsequent
drug concentrations were measured in plasma, tumor and tissue
by LC-MS/MS. All NP formulations had greater sum total
plasma exposures (encapsulated docetaxel or prodrug +
released docetaxel) compared to free docetaxel when measured
by the area under the concentration − time curve (AUC). A
60-fold increase in AUC was realized for the DTXL-NP where
as the C2-NP formulations displayed a 182-fold in AUC. The
improved sum total plasma AUC of the C2-NP relative to the
DTXL-NP may be attributed to the slower release kinetics of
the C2 prodrug. The improved retention of the C2 prodrug in
the NPs most likely reduced the tissue distribution of C2 as
evidenced by the reduced Vd of sum total C2 compared to sum
total docetaxel (237 vs 4513 mL/kg). Comparison of the sum
total Cmax also suggests that improved NP retention of drug
reduces immediate tissue distribution. The order of Cmax from
least to highest for the NP formulations emulated the in vitro
release kinetics, docetaxel and C2 (23 359 4528 vs 78 952
6589, P = 0.0002). Furthermore, minimal conversion of C2
docetaxel was observed in plasma of the total drug (Figure 1A).
The percentage of released docetaxel from C2-NP was 2.3%.
This percentage was calculated by AUCC2/(AUCC2+AUCDTXL
)
× 100.
In vitro, the C2 had a short conversion half-life of 8 h; the
NP likely protects the prodrug in plasma to prevent conversion
before the prodrug reaches the target site of the tumor. This
design attribute has the potential to decrease systemic toxicity
of chemotherapeutics.
Cylindrical particles with diameter (d) = 80 nm and height
(h) = 320 nm were prepared using a poly(lactide-co-glycolide)
polymer. By dynamic light scattering (DLS), the hydrodynamic
radius was measured as ∼200 nm. The particle samples were
monodisperse with a polydispersity index (PDI) of less than 0.1
and as low as 0.05. PRINT NPs were loaded with drugs at
weight percents of 20−22%. NP formulations of docetaxel and
two docetaxel prodrugs, C2 and C8, were prepared. All
formulations were similar in particle size and drug loading
(Table 1). The release kinetics of the three formulations,
Though low C2 plasma conversion is preferred, the prodrug
must convert at its target site to be effective. 32.5% of C2 was
determined to be converted to docetaxel in the tumor, much
higher than the 2.3% observed within the plasma. This
percentage compares favorably to other polymeric prodrug
strategies that have entered clinical development. For a
camptothecin polymer drug conjugate, only 1.3% unconjugated
camptothecin was observed in xenograft tumors over 48 h.11
Only 4−17% of unconjugated paclitaxel from a paclitaxel
polymer drug conjugate was observed in a xenograft tumors
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Table 1. NP Characterization of DTXL-NP, C2-NP and C8-
NP Formulations
zeta potential
(mV)
weight percent
loading
formulation size (nm)
PDI
DTXL-NP
C2-NP
213
205
208
1
2
7
0.07 0.01
0.05 0.02
0.09 0.02
−2.81 0.23
−2.65 0.52
−3.78 0.36
21.2 0.5
20.7 0.4
22.3 1.9
C8-NP
Figure 2. Pharmacokinetic profiles of the following: red circle, free
docetaxel; gray square, DTXL-NP; and green diamond, C2-NP
formulations (dotted line indicates C2 prodrug and solid line indicates
converted DTXL). Each replicate is shown and lines are connected by
the means of three replicates at each time point. Mice bearing A549
flank tumors received one iv injection via tail vein at 10 mg/kg
docetaxel or docetaxel molar equivalents.
acid-labile.17 The hypothesized acidic tumor microenvironment
is thought to contribute to the site selective conversion.
In Vivo Efficacy and Tolerability. The efficacy of the C2-
NP formulation was compared to free docetaxel in an A549
subcutaneous xenograft mouse model. Mice were administered
weekly doses via tail vein injection over 6 weeks. Figure 3A
shows the tumor growth curves for free docetaxel at its
MTD18(20 mg/kg), C2-NP at 20 mg/kg, and C2-NP at its
MTD of 50 mg/kg. The final relative tumor volume of mice
receiving C2-NP was equal to free docetaxel when administered
at 20 mg/kg. However, at 50 mg/kg, mice receiving the C2-NP
had statistically lower mean relative tumor volume than mice
receiving free docetaxel at 20 mg/kg starting at day 10. Body
weights (Figure 3B) of mice receiving saline and equimolar
doses of free docetaxel and C2-NP remained stable over the
course of treatment, while some body weight loss was observed
in mice receiving the 50 mg/kg dose of C2-NP over the course
of 6 doses. On the basis of the pharmacokinetic profile of the
tumor, mice receiving C2-NP at equal molar dosing to free
docetaxel did not have increased sum total docetaxel
concentrations (Figure 1B). The similar tumor docetaxel levels
Figure 1. (A) Hydrolysis of prodrugs in pH 5 and pH 7 buffer at 37
°C. (B) Hydrolysis of prodrugs in mouse plasma at physiological
conditions. (C) Cytotoxicity of docetaxel, C2 and C8 on A549 cells in
vitro. (D) Release kinetics of docetaxel, C2 and C8 from PLGA NPs at
pH 7.4 and 37 °C.
over 144 h.6 In other tissues where the NPs distribute, the liver
and spleen (Supporting Information Figure 1 and Table 1) only
had 13.2 and 2.7% free docetaxel from C2. The silyl ether
prodrug has selective conversion at the target site of the tumor.
Silyl ethers are commonly used as protecting groups that are
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Table 2. Pharmacokinetic Parameters of Free Docetaxel, DTXL-NP, and C2-NP Formulations
formulation
taxotere
DTXL
DTXL-NP
DTXL
C2-NP
specimen
plasma
parameter
C2
227 735
DTXL
5381
AUC (ng/mL h)
Cmax (ng/mL)
CL (mL/h/kg)
Vd (mL/kg)
1227
79 192
2314 362
8150
23 359 4 528
126
78 952 6 589
49
1994 107
1858
10 508
4513
237
9535
tumor
AUC (ng/mL h)
Cmax (ng/mL)
73 222
60 858
26 799
12 897
453 225
476 73
946 743
288 139
docetaxel (Taxotere). Patients that experience dose limiting
toxicity due to neutropenia may benefit from different dosing
schedules, such as receiving doses every 2 weeks versus 3
weeks, but this may be inconvenient to the patient.19 Thus, a
formulation change to improve the toxicity profile of docetaxel
is an attractive option. Table 3 shows the mean white blood cell
counts of mice receiving the C2-NP (20 mg/kg or 50 mg/kg)
compared to free docetaxel (20 mg/kg). Blood was collected 4
days after injection and measured for complete blood counts.
Mice receiving the C2-NP formulation at an equal molar dose
to free docetaxel trended toward statistically higher WBC 4
days after the first dose (P = 0.12) and almost double the WBC
4 days after the sixth dose (P = 0.008). The pharmacokinetic
data demonstrates that only a fraction of C2 is converted to
docetaxel within the plasma, liver and spleen. The minimized
free docetaxel concentrations likely account for the improved
tolerability of the C2-NP. With improved tolerability, mice
could receive a 2.5 times higher docetaxel dose with the C2-NP
than free docetaxel, resulting tumor reduction in the efficacy
study.
Compared to poly(lactide) docetaxel NPs in clinical
development, release of docetaxel directly from NPs has not
shown improvement in tolerability relative to the clinical
control Taxotere.20,21 Nanoxel-PM is a micelle formulation of
docetaxel, consisting of PDLLA-mPEG. In preclinical models,
Nanoxel-PM had similar pharmacokinetics to Taxotere and a
similar hematological toxicity profile.20 Instability of the micelle
may be a potential reason why an improved toxicity profile was
not observed. However, BIND-014 is a stable targeted PLA-
PEG nanoparticle with controlled release of docetaxel and
differentiated pharmacokinetics and efficacy.21 Even with these
improved attributes, BIND-014 did not achieve a higher
maximum tolerated dose than Taxotere in a phase I clinical
trial.21 Thus, unlike reformulations of paclitaxel, where
removing cremophor EL as an excipient contributed to
increased tolerability,22 there has yet to be a formulation
change that improves the hematological toxicity of docetaxel in
a clinical setting. Our prior data that evaluated PLGA docetaxel
particles in vivo also did not improve the hematological toxicity
of docetaxel compared to Taxotere, even though the maximum
Figure 3. (A) Tumor growth inhibition curve (mean sem). Mice
bearing A549 flank tumors received 6 weekly doses via IV tail vein
injection. Black circle, saline; red square, free docetaxel (20 mg/kg);
gray triangle up, C2-NP (20 mg/kg dtxl equivalents); and blue triangle
down, C2 NP (50 mg/kg dtxl equivalents) Mice receiving C2 NP at 50
mg/kg had tumor volumes lower than mice receiving free docetaxel (P
< 0.05 starting on day 10). (B) Body weights (mean
std dev).
Twenty mg/kg weekly × 6 is the maximum tolerated dose for
docetaxel in this model.18
may explain the similar tumor growth rates for mice receiving
C2-NP and free docetaxel at equal molar doses.
However, the C2-NP formulation improved the tolerability
of docetaxel in mice. Neutropenia, characterized by reduced
white blood cell counts (WBC), is a common side effect of
Table 3. White Blood Cell Counts Measured 4 Days after Injection with Saline, Free Docetaxel or C2-NP at Two Dose Levels
time point
saline
free docetaxel (20 mg/kg)
C2-NP (20 mg/kg)
C2-NP (50 mg/kg)
WBC (103 cells/μL)
day of dose 1
2.07 0.21
2.81 0.91
3.43 1.13
1.34 0.36
0.84 0.65
1.53 0.52
1.67 0.54
1.54 0.35
c
c
ac
,
c
4 days after dose 1
4 days after dose 6
1.41 0.65
3.00 1.10
0.49 0.38
b
c
0.56 0.28
a
b
Indicates trend toward statistically significance compared to free docetaxel (P = 0.12). Indicates statistical significance compared to free docetaxel
c
(P = 0.008). Indicates statistical significance compared to saline (P < 0.006).
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tolerated dose was increased by 50%.18 Thus, the use of a
prodrug strategy appears to be key in improving the toxicity
profile of docetaxel by reducing the amount of docetaxel in
systemic circulation.
We have demonstrated that combining a labile prodrug of
docetaxel with NP delivery improves the tolerability of
docetaxel without decreasing efficacy by minimizing systemic
conversion of the prodrug but preferential converting within
the tumor. Further work is being conducted to more
thoroughly identify the appropriate dose level to balance
efficacy and toxicity.
(4) Chu, K. S.; Hasan, W.; Rawal, S.; Walsh, M. D.; Enlow, E. M.;
Luft, J. C.; Bridges, A. S.; Kuijer, J. L.; Napier, M. E.; Zamboni, W. C.;
DeSimone, J. M. Nanomedicine 2013, 9, 686−693.
(5) Tang, L.; Fan, T. M.; Borst, L. B.; Cheng, J. ACS Nano 2012, 6,
3954−3966.
(6) Li, C.; Newman, R. A.; Wu, Q. P.; Ke, S.; Chen, W.; Hutto, T.;
Kan, Z.; Brannan, M. D.; Charnsangavej, C.; Wallace, S. Cancer
Chemother. Pharmacol. 2000, 46, 416−422.
(7) Feng, L.; Wu, H.; Ma, P.; Mumper, R. J.; Benhabbour, S. R. Int. J.
Nanomed. 2011, 6, 2545−2556.
(8) Etrych, T.; Sirova, M.; Starovoytova, L.; Rihova, B.; Ulbrich, K.
Mol. Pharm. 2010, 7, 1015−1026.
(9) Ma, P.; Benhabbour, S. R.; Feng, L.; Mumer, R. J. Cancer Lett.
2013, 334, 253−262.
ASSOCIATED CONTENT
■
(10) Zhao, H.; Rubio, B.; Sapra, P.; Wu, D.; Reddy, P.; Sai, P.;
Martinez, A.; Gao, Y.; Lozanguiez, Y.; Longley, C.; Greenberger, L. M.;
Horak, I. D. Bioconjugate Chem. 2008, 19, 849−859.
(11) Schluep, T.; Cheng, J.; Khin, K. T.; Davis, M. E. Cancer
Chemother. Pharmacol. 2006, 57, 654−662.
S
* Supporting Information
Liver and spleen pharmacokinetics. Additional hematological
data. Materials and methods. This material is available free of
charge via the Internet at http://pubs.acs.org.
(12) Younes, A.; Gopal, A. K.; Smith, S. E.; Ansell, S. M.; Rosenblatt,
J. D.; Savage, K. J.; Ramchandren, R.; Bartlett, N. L.; et al. J. Clin.
Oncol. 2012, 30, 2183−2189.
AUTHOR INFORMATION
■
(13) Chipman, S. D.; Oldham, F. B.; Pezzoni, G.; Singer, J. W. Int. J.
Nanomed. 2006, 1, 375−383.
Corresponding Author
*E-mail: desimone@unc.edu.
(14) Parrott, M. C.; Finniss, M.; Luft, J. C.; Pandya, A.; Gullapalli, A.;
Napier, M. E.; DeSimone, J. M. J. Am. Chem. Soc. 2012, 134, 7978−
7982.
Present Address
Department of Chemistry, The University of North Carolina at
Chapel Hill, Campus Box #3290, 257 Caudill, Chapel Hill, NC
27599−3290.
(15) Ali, S.; Ahmad, I.; Peters, A.; Masters, G.; Minchey, S.; Janoff,
A.; Mayhew, E. Anticancer Drugs 2001, 12, 117−128.
(16) Huynh, L.; Leroux, J. C.; Allen, C. Org. Biomol. Chem. 2009, 7,
3437−3446.
(17) Wuts, P. G. M.; Greene, T. W. Greene’s Protective Groups in
Organic Synthesis, 4th ed.; Wiley: New York, 2007. doi:
(18) Chu, K. S.; Schorzman, A. N.; Finniss, M. C.; Bowerman, C. J.;
Peng, L.; Luft, J. C.; Madden, A. J.; Wang, A. Z.; et al. Biomaterials
2013, 34, 8424−8429.
(19) Kellokumpu-Lehtinen, P. L.; Harmenberg, U.; Joensuu, T.;
McDermott, R.; Hervonen, P.; Ginman, C.; Luukkaa, M.; Nyandoto,
P.; et al. Lancet Oncol. 2013, 14, 117−124.
Author Contributions
The manuscript was written through contributions of all
authors. All authors have given approval to the final version of
the manuscript.
K.S.C., M.C.F., and A.N.S. contributed equally.
Funding
Joseph DeSimone is a founder and maintains a financial interest
in Liquidia Technologies.
(20) Lee, S. W.; Yun, M. H.; Jeong, S. W.; In, C. H.; Kim, J. Y.; Seo,
M. H.; Pai, C. M.; Kim, S. O. J. Controlled Release 2011, 155, 262−271.
(21) Hrkach, J.; Von Hoff, D.; Mukkaram Ali, M.; Andrianova, E.;
Auer, J.; Campbell, T.; De Witt, D.; Figa, M.; et al. Sci. Transl. Med.
2012, 4, 128ra39.
(22) Gradishar, W. J.; Tiulandin, S.; Davidson, N.; Shaw, H.; Desai,
N.; Bhar, P.; Hawkins, M.; O’Shaughnessy, J. J. Clin. Oncol. 2005, 23,
7794−7803.
Notes
The authors declare the following competing financial
interest(s): Joseph DeSimone is a founder and maintains a
financial interest in Liquidia Technologies. PRINT is a
registered trademark of Liquidia Technologies, Inc.
ACKNOWLEDGMENTS
■
The authors acknowledge Charlene Ross of the Animal Studies
Core at the University of North Carolina at Chapel Hill. This
work was supported by the Carolina Center for Nano-
technology Excellence (U54-CA151652 and U54-CA119343),
University Cancer Research Fund, and Liquidia Technologies,
Inc.
ABBREVIATIONS
■
ethyldimethylsilyl ether docetaxel C2;; octyldimethylsilyl ether
docetaxel C8;; docetaxel nanoparticle DTXL-NP;
REFERENCES
■
(1) Myer-Losic, F.; Nicolazzi, C.; Quinonero, J.; Ribes, F.; Michel,
M.; Dubois, V.; de Coupade, C.; Boukaissi, M.; Chen
Tranchant, I.; Arranz, V.; Zoubaa, I.; Fruchart, J. S.; Ravel, D.; Kearsey,
J. Clin. Cancer Res. 2008, 14, 2145−2153.
́
e,
́
A. S.;
(2) Gupta, D.; Varghese Gupta, S.; Dahan, A.; Tsume, Y.; Hilfinger,
J.; Lee, K. D.; Amidon, G. L. Mol. Pharm. 2013, 10, 512−522.
(3) Lee, Y.; Kim, H.; Kim, W.; Yoon, J. H.; Jeong, S. H.; Jung, Y. J.
Drug Target 2012, 20, 524−534.
1476
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