Macromolecular prodrug that provides the irinotecan (CPT-11) active-metabolite SN-38 with ultralong half-life, low C max, and low glucuronide formation

Article abstract of DOI:10.1021/jm401644v

We have recently reported a chemical approach for half-life extension that utilizes β-eliminative linkers to attach amine-containing drugs or prodrugs to macromolecules. The linkers release free drug or prodrug over periods ranging from a few hours to over 1 year. We adapted these linkers for use with phenol-containing drugs. Here, we prepared PEG conjugates of the irinotecan (CPT-11) active metabolite SN-38 via a phenyl ether that release the drug with predictable long half-lives. Pharmacokinetic studies in the rat indicate that, in contrast to other SN-38 prodrugs, the slowly released SN-38 shows a very low C_max, is kept above target concentrations for extended periods, and forms very little SN-38 glucuronide (the precursor of enterotoxic SN-38). The low SN-38 glucuronide is attributed to low hepatic uptake of SN-38. These macromolecular prodrugs have unique pharmacokinetic profiles that may translate to less intestinal toxicity and interpatient variability than the SN-38 prodrugs thus far studied.

Full text of DOI:10.1021/jm401644v

Article
pubs.acs.org/jmc
Macromolecular Prodrug That Provides the Irinotecan (CPT-11)
Active-Metabolite SN-38 with Ultralong Half-Life, Low Cmax , and Low
Glucuronide Formation
Daniel V. Santi,* Eric L. Schneider, and Gary W. Ashley
ProLynx, 455 Mission Bay Boulevard South, Suite 145, San Francisco, California 94158, United States
S
* Supporting Information
ABSTRACT: We have recently reported a chemical approach for half-life extension
that utilizes β-eliminative linkers to attach amine-containing drugs or prodrugs to
macromolecules. The linkers release free drug or prodrug over periods ranging from
a few hours to over 1 year. We adapted these linkers for use with phenol-containing
drugs. Here, we prepared PEG conjugates of the irinotecan (CPT-11) active
metabolite SN-38 via a phenyl ether that release the drug with predictable long half-
lives. Pharmacokinetic studies in the rat indicate that, in contrast to other SN-38
prodrugs, the slowly released SN-38 shows a very low Cmax, is kept above target
concentrations for extended periods, and forms very little SN-38 glucuronide (the
precursor of enterotoxic SN-38). The low SN-38 glucuronide is attributed to low
hepatic uptake of SN-38. These macromolecular prodrugs have unique
pharmacokinetic profiles that may translate to less intestinal toxicity and interpatient
variability than the SN-38 prodrugs thus far studied.
electron-withdrawing effects of the modulators and, unlike ester
bonds commonly used in releasable linkers,2 the β-elimination
reaction was not catalyzed by general bases or serum enzymes.
With releasable conjugates of this type, the terminal half-life
(t1/2,β) of a rapidly cleared released drug is transformed into
that of the conjugate and has a longer t1/2,β and diminished Cmax
compared to repeated bolus doses of a short-lived drug.
INTRODUCTION
■
Conjugation of drugs to macromolecular carriers is an
established strategy for improving pharmacokinetics. In one
approach, the drug is covalently attached to a long-lived
circulating macromolecule, such as poly(ethylene glycol)
(PEG), through a linker that is slowly cleaved to release the
native drug.1,2 Most of these conjugates use ester-containing
linkers that cleave either by spontaneous or esterase-catalyzed
reactions and have several limitations: (a) the rate of cleavage
can be faster, but not slower than the spontaneous hydrolytic
rate, and PEG−ester conjugates have half-lives of about 12−30
h at pH 7.4,1−5 (b) they can show intra- and interspecies
variability of in vivo cleavage rates because of different esterase
levels, and (c) cleavage rates are not generally predictable or
easily adjustable.
We have recently reported conjugation linkers that self-cleave
by a nonenzymatic β-elimination reaction in a highly
predictable manner and with a large range of half-lives of
cleavage spanning hours to over a year at physiological pH.6 In
this approach, a macromolecular carrier is attached to a linker
that is attached to a drug via a carbamate group (1; Scheme 1);
the β-carbon has an acidic carbon−hydrogen bond (C−H) and
also contains an electron-withdrawing modulator (Mod) that
controls the pKa of that C−H bond. Upon hydroxide-ion-
catalyzed proton removal (2), a rapid β-elimination occurs to
cleave the linker−carbamate bond and to release the free drug
or prodrug and a substituted alkene 3. The rate of drug release
is proportional to the acidity of the proton, which is controlled
by the chemical nature of the modulator; thus, the rate of drug
release is controlled by the modulator. It was shown that both
in vitro and in vivo cleavages were linearly correlated with
Recently, we described an approach to adapt such β-
eliminative linkers for use with phenol-containing drugs
(Scheme 2, 4).7 In addition to the pKa modulator, the modified
linkers include a methylene adaptor that connects the
carbamate nitrogen to the oxygen atom of the drug and an
electron-withdrawing N-aryl stabilizer that inhibits undesirable
cleavage of the adaptor−oxygen bond. Upon β-elimination
(Scheme 2, path A), a carbamic acid is released, which rapidly
disassembles to the free phenol-containing drug. Spontaneous
SN1 and acid-catalyzed solvolytic reactions were also charac-
terized (Scheme 2, paths B and C); here, an iminium ion of the
carbamate (9) is formed to assist prodrug expulsion that
ultimately loses formaldehyde to give an N-aryl carbamate.
We were interested in applying this technology to produce
conjugates of SN-38 with very long half-lives. SN-38 is the
active metabolite of the anticancer agent irinotecan (CPT-11),
primarily used to treat colon cancer, and is one of the most
potent known inhibitors of topoisomerase 1 (topo 1). CPT-11
is extensively metabolized and shows large interpatient
variability in its pharmacokinetics, pharmacodynamics, and
toxicities;8 it can be reasoned that direct administration of SN-
Received: October 23, 2013
Published: February 4, 2014
© 2014 American Chemical Society
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Scheme 1
38 might escape some of the metabolism of CPT-11 and thus
reduce some of the variability. However, SN-38 is too insoluble
to administer directly and has a short half-life and thus requires
the use of soluble prodrug depots, such as CPT-11 or
conjugates of long-lived macromolecules.
Scheme 2
SN-38 prodrugs are usually administered as an i.v. bolus
every 1 or 3 weeks. However, topo 1 inhibitors such as SN-38
display time-dependent cytotoxicity because prolonged ex-
posure is needed to inhibit topo 1 during DNA replication.
Hence, continuous, protracted administration of low-dose topo
1 inhibitors may be advantageous over the commonly used
intensive bolus delivery. It has been proposed that a minimum
threshold of exposure to a topo 1 inhibitor (i.e., continuous
inhibition) must be achieved for optimal antitumor activity and
that a higher dose intensity enhances toxicity but not tumor
regression.9−11 To improve delivery of SN-38, numerous PEG
conjugates of CPT-11 or SN-38 with cleavable ester-containing
linkers have been developed.12 Although these are somewhat
longer acting than CPT-11, they still achieve a time-over-target
concentration for only a small fraction of the dosing schedules
used. Indeed, none of the current conjugates maintains the
concentration of free SN-38 over the target threshold for
protracted periods, nor are they able to because, unless
somehow protected, they are limited by the relatively short half-
lives of 12 to 30 h for hydrolysis of ester linkages.1−4 In
addition, all of the current SN-38 prodrugs show high Cmax
values of SN-38 that likely contribute to toxicities.
In the present work, we describe the synthesis, properties,
and pharmacokinetics of a series of PEG−SN-38 conjugates
that use β-eliminative linkers to connect PEG to the 10-
phenolic group of SN-38 via a stable ether linkage. Using
appropriate pKa modulators, the in vitro and in vivo half-life of
drug release can be controlled from several to ∼400 h, and
conjugates were obtained that achieve time-over-threshold
concentrations of SN-38 for long periods of time with low Cmax
and very low SN-38 glucuronide (SN-38G) formation. Taken
together, these features suggest that the conjugates described
here may show high efficacy with lower toxicity and interpatient
variability.
RESULTS
■
Synthesis. On the basis of procedures described for
conjugation of phenols to β-eliminative linkers,7 we prepared
PEG−SN-38 conjugates 14A−E and 16D,E (Scheme 3). For
the carbamate stabilizer, we used 4-NH2PhCON(Et)2 (pKa 3.5)
because of its suppression of SN1 solvolysis of analogous
compounds7 and its relationship to amides of p-aminobenzoic
acid that are generally safe. Azidoalcohols 10A−E were
converted to N-aryl carbamates 11A−E via chloroformate
reaction with 4-NH2PhCON(Et)2. The carbamates were
treated with TMSCl/paraformaldehyde to provide the reactive
N-chloromethyl carbamates 12A−E. Reaction of 12A−E with
the SN-38 10-oxyanion (10-OH, pKa 8.5613) gave the
corresponding C-10 ethers 13A−E, which were connected to
4-arm PEG40kDa-(DBCO)4 by Cu-free click chemistry to form
PEG conjugates 14A−E in near quantitative yield.14 To
synthesize a less hydrophobic conjugate that was more
economical to prepare, we developed a method to attach the
linker to PEG via an amide bond. Here, azido linker−SN-38
13D,E were reduced with Me3P to the corresponding amine
linker−SN-38 analogues 15D,E, which were reacted with 4-arm
PEG40kDa−N-hydroxysuccinimido ester to give 16D,E.
Solvolysis Products of N-Aryl-N-SN-38−CH2-carba-
mates. The solvolysis products of N-aryl-N-SN-38−CH2-
carbamates 13 and 14 identify the pathways of SN-38 release.
β-Eliminative cleavage (Scheme 2, path A) should give SN-38,
the aryl amine, and an alkene (ModCHCHR), whereas
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Scheme 3
spontaneous (path B) or acid-catalyzed (path C) reactions
should both provide SN-38 and N-aryl carbamate 11.7 Over
most of the pH range studied, the insolubility of the SN-38
product required use of such low concentrations (≤10 μM)
that aryl amine could not be detected by HPLC−UV. However,
SN-38 is ionized at low and high pH (pKa 1.4 and 8.613) and is
significantly more soluble. At pH 0, the azido linker−carbamate
13C (∼40 μM) disappeared with concomitant production of
SN-38 (t1/2 ∼ 12 min); an intermediate presumed to be N-
HOCH2−11C initially appeared that converted to final product
11C. The analogous PEG−SN-38 conjugate 14C released the
expected SN-38, leaving the aryl amine coupled to PEG as the
acid-stable N-aryl carbamate. At pH 10.5, the azide linker−
carbamate 13C produced stoichiometric quantities of SN-38
and 4-NH2PhCON(Et)2 at the same rate (t1/2 ∼ 6 min). Stable
analogue 13E produced SN-38 and carbamate 11E in acid or
basic media.
In Vitro Kinetics of SN-38 Release. The kinetics for
solvolysis of the SN-38-containing carbamate without a pKa
modulator (14E), described by eq 1, shows an acid-catalyzed
rate between pH 0 and 5.5 with a slope of −1.0 in a log kobsd
versus pH plot; at higher pH, the rate was pH-independent up
to at least pH 9.4. Determinations of spontaneous rates were
limited by difficulties of measuring the extremely slow reaction.
Conjugates containing pKa modulators (14A−D and 16D)
show an additional hydroxide-catalyzed rate, as in eq 2.
+
+
kobs = kH [H3O ] + k
H2O
(1)
(2)
+
−
+
−
kobs = kH [H3O ] + k
+ kOH [OH ]
H2O
Solvolysis of 14A−D possessing pKa modulators were
likewise first order in hydronium ion between pH 0 to ∼4.5
(Figure 1 and Table 1) and first order in hydroxide ion at pH
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There is a spontaneous release of the leaving group with
assistance of an iminium ion intermediate (Scheme 2, path B).
The acid-catalyzed solvolysis is first order in hydronium ion
from pH 0 to ∼4.5, indicating preprotonation of a site with pKa
< 0 prior to cleavage. The quinoline nitrogen of SN-38 has a
pKa of 1.4,13 but its protonation is not apparent from the pH−
log kobsd profile (Figure 1). Thus, the kinetically relevant
protonation must occur at a site remote from and unaffected by
protonation of quinoline nitrogen. Because it is unlikely that
protonation of the ether linkage of 14 would be unaffected by
protonation of SN-38, the site of preprotonation may be the
carbonyl oxygen, as proposed for analogous aryl ethers.7
As with other β-eliminative carbamates6,7 conjugates with a
pKa modulator, 14A−D showed release rates of SN-38 that
were first order in hydroxide ion (Table 1). In the absence of an
ionizable proton at the β-position of the carbamate (i.e., 14E),
SN-38 release was extremely slow and did not exhibit
hydroxide-ion catalysis. Thus, as shown for related ethers7
and depicted in Scheme 2, path A, the base-catalyzed reaction
involves β-eliminative cleavage of the O-alkyl bond of the
carbamate to form carbamic acid 5; subsequently, there is
decarboxylation, which, from related models,17 is acid-catalyzed,
followed by spontaneous collapse18 of the neutral Mannich
base 6 to give CH2O, the aryl amine, and SN-38.
We desired to determine whether the rate of decarboxylation
of the carbamic acid 5 (k2[H3O+], Scheme 2) or collapse of the
Mannich base intermediate 6 (k3, Scheme 2) was slow enough
to allow accumulation of intermediates at physiologic pH. For
this, we used a previously described7 kinetic approach that
allows an estimate of the lower limit of the rate of collapse of
these intermediates at pH 7.4. The rationale is that over the pH
range where the hydroxide-catalyzed β-elimination is slow
compared to subsequent rates the pH−log rate profile has a
slope of +1. If the hydroxide-catalyzed rate becomes faster than
subsequent reactions, then the pH−log rate profile will have a
slope of −1 or 0, depending on the new rate-determining step.
Thus, the rate at the highest pH where SN-38 release remains
first order in hydroxide allows assignment of a lower limit on
the rate of reaction of either intermediate at that pH. Because
the rate of reaction either remains the same (spontaneous
Mannich base collapse18) or decreases (acid-catalyzed decar-
boxylation17) at lower pH, this rate also corresponds to a lower
limit of the overall rate of SN-38 release at pH 7.4.
Figure 1. pH−log kobsd profile for solvolysis of 14A,C. For 14A
2
■
(−− −−), the slope is +1.0 (r = 0.993) between pH 8.7 and 11.5.
●
The solvolysis of 14C ( ) was fit to eq 2 from pH 0 to 11.5
using the kH O rate of 14E; the slope from pH 0 to 4 is −1.0 (r2 =
2
0.989) and from 6 to 11.5 is +1.0 (r2 = 0.999).
≥6. The rate of SN1 solvolysis of 14C, as estimated by the SN1
reaction of 14E, was in agreement with the nadir of the pH−log
k
obsd. Also, the SN1 reaction of 14E was ∼7-fold slower than the
corresponding p-nitrophenyl ether, which is in accord with the
finding that rates of such reactions are inversely related to the
pKa of the leaving group.15 Of primary relevance are the base-
catalyzed rates dictated by the pKa modulators and the
difference between these rates and the spontaneous solvolysis
rate at physiological pH. The acid-catalyzed rates, which are
independent of the pKa modulator, primarily provide guidance
in handling of samples in pharmacokinetic studies to prevent ex
vivo hydrolysis and to assist design of long-term storage
conditions.
Table 1 shows that the rates of base-catalyzed β-elimination
are directly related to the electron-withdrawing ability of the
modulator, and at pH 7.4, they have half-lives ranging from ∼9
to 450 h. The rate of 14A is only slightly slower than that of the
analogous p-nitrophenyl ether, indicating little dependence of
the β-elimination reaction on the leaving group pKa of the
ether.7 They are, however, 6- to 8-fold faster than reported
simple primary carbamates with the same modulators6 because
of the higher acidity of the initially formed carbamic acids.7
Importantly, β-elimination rates dominate the significantly
slower spontaneous SN1 hydrolysis at physiological pH. It is
noteworthy that for 14D and 16D the solvolysis rates are
relatively unaffected by the spatially remote DBCO or amide
connectors to the PEG macromolecule.
Above pH 8.5, we could directly observe formation of the 10-
phenolate ion of SN-38 by continuous spectrophotometric
assay, which monitors the relevant C−O bond cleavage step.
The log kobsd versus pH profile for SN-38 formation from
14A,C show a slope of +1 to at least pH 11.5. Here, t1/2 for 14A
was ∼3 s, so neither intermediate 5 or 6 in Scheme 2, path A
have a t1/2 > 3 s at that or lower pH. Thus, we conclude that
Mechanism. The pH-independent spontaneous hydrolysis
of 14E has precedence in the analogous reactions of N-
substituted N-aryloxy-7,15 and N-acyloxy-methyl16 carbamates.
Table 1. Rates of SN-38 Cleavage from PEG−SN-38 Conjugates
a
a
b
‑1
kH O , M h−1
kOH_, M−1 h−1 × 10−3
t1/2, h, pH 7.4
t1/2, h, in vivo
+
R
3
c
14E
14A
14B
14C
14D
16D
H-
9.5 3.0
1.2 0.5
1.6 0.7
2.4 0.4
1.4 0.6
2.2 0.6
NA
3600
NA
12
PhSO2CH2-
301 60
34 4.0
25 3.8
6.5 0.6
6.1 0.1
9.2
MeSO2CH2-
82
99
O(CH2CH2)NSO2CH2-
NCCH2- (DBCO)
NCCH2- (amide)
110
420
450
138
342
362
a
b
c
Values are from best fits to first-order acid- or base-catalyzed reactions SD. From Table 2. Calculated from kH O = 0.19 × 10−3 h−1 at pH 6.0−
2
9.5.
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both intermediates decompose rapidly and would not
significantly accumulate at pH 7.4.
that there is no significant carbamate hydrolysis that might arise
from the action of some unidentified protease or esterase.
We have shown6 that the steady-state rate for loss of a
cleavable conjugate is kβ = k1 + k3, where k1 is the rate constant
for β-elimination and k3 is the renal elimination rate of the
conjugate. The k3 values were estimated for the DBCO- and
amide-linked conjugates using the elimination rate of the
corresponding stable conjugates, 14E or 16E, as surrogates that
can only undergo clearance. Examples of the method to
determine in vivo cleavage rates, k1, are given in Figure 3A,B,
with complete data for all conjugates presented in Table 2.
Figure 3C,D shows the C versus t profiles of PEG−SN-38
conjugates, SN-38, and SN-38G with the highest and lowest k1
values, respectively. As shown, the PEG−SN-38 conjugates give
very low levels of SN-38G, with AUCSN‑38G/AUCSN‑38 ratios
ranging from only 0.08 to 0.2, and the ratios track the rate of
linker cleavage, k1.
Pharmacokinetics. For pharmacokinetic studies of releas-
able PEG conjugates with slow cleavage rates, the longer t1/2 of
PEG in the rat (ca. 30−40 h) compared to mouse (12−24 h)
makes it the preferred rodent model. PEG−SN-38 conjugates
14A−E and 16D,E were administered i.v. to rats, blood samples
were removed at various times and immediately quenched in
acidified citrate to avoid ex vivo linker cleavage, and plasma was
obtained by centrifugation; samples were analyzed by HPLC
using UV and fluorescence detection.
Pharmacokinetic data for PEG−SN-38 conjugates, SN-38,
and SN-38G in the rat were fit to a two-compartment model
depicted in Figure 2 (see Supporting Information for
DISCUSSION
■
Rationale for Slow-Releasing PEG−SN-38 Conjugates.
Topo 1 inhibitors, such as SN-38, bind and stabilize the
enzyme−DNA covalent complex to prevent DNA religation
during S phase and thus cause time-dependent DNA damage
and apoptosis. Because binding is reversible, constant exposure
to the inhibitor is needed to maintain the complex until
irreversible events occur that lead to cell death.21,22 Inhibitors
of topo 1 also show a time-dependent increase of in vitro
cytotoxicity.23,24
Figure 2. Two-compartment model for releasable PEG−SN-38
conjugates and released SN-38. Rate constant k1 is for SN-38 release
from the conjugate, k2 and k3 are for elimination of the SN-38 and
intact conjugate, respectively, from the central compartment, and k12
and k21 are for distribution of the conjugate between the central and
peripheral compartments; Kdist describes partitioning of the free drug
between central and peripheral compartments. Derivations are given in
the Supporting Information.
Preclinical and clinical studies have also suggested that low-
dose, prolonged exposure of topo 1 to inhibitors may be less
toxic and more efficacious than conventional high-intensity
treatments. It has been convincingly demonstrated that
continuous infusion of CPT-11 over 4 days is significantly
less toxic than equivalent four daily bolus doses.19 Houghton et
al.9−11 showed that the efficacy of CPT-11 in human tumor
xenografts and in humans is highly schedule-dependent: a given
dose administered on a protracted schedule over 21 days was
more efficacious than dose-intensive schedules. They proposed
that a minimum threshold of the drug must be maintained over
long periods for optimal antitumor activity; higher dose-
intensity did not provide further benefit, but it enhanced
toxicity. Several phase 1 clinical trials of low-dose continuous
infusions, including 1 to over 9 weeks of CPT-11, yielded
steady-state levels of SN-38 of ∼3 to 15 nM, confirming the
tolerability and feasibility of low, protracted dosing.25−28
Similar phase 1 trials of continuous infusions of the topo 1
inhibitor topotecan also showed activity in several tumors,29−32
again suggesting that efficacy may be achieved by time-over-
target inhibition. Thus, topo 1 inhibitors may display exposure-
time-dependent rather than concentration-dependent cytotox-
icity: achieving a high inhibitor concentration for a short period
does not necessarily produce the same effect as maintaining a
low concentration for a prolonged period, even if the total AUC
per treatment period are equal.
derivations); relevant data with selected values normalized to
constant PEG−SN-38 concentrations are summarized in Table
2, and all original data is provided in the Supporting
Information. In a separate experiment, Cmax and AUC were
linear over a range of at least 1 to 4 μmol of 16D. For
comparison, reported pharmacokinetic parameters after a 60
mg kg−1 bolus dose of CPT-11 in rats that gave a SN-38 Cmax
similar to that from 16D (0.38 nM) were Cmax,CPT‑11 47 nM and
t1/2,CPT‑11 2.8 h, Cmax,SN‑38 0.43 nM and t1/2,SN‑38 8 h, Cmax,SN‑38G
0.81 nM and t1/2,SN‑38G, 9.2 h, and AUCSN‑38G/AUCSN‑38 5.5.19
Preliminary pharmacokinetic experiments of 16D, 16E, and
CPT-11 were also performed in the African green monkey.
Overall, the AUC of SN-38 derived from CPT-11 over the first
4 h was only ∼2% of CPT-11, but SN-38G was formed in large
amounts such that AUCSN‑38G/AUCCPT‑11 was 0.32 and
AUCSN‑38G/AUCSN‑38 was 15. In view of the high SN-38 +
SN-38G derived from CPT-11 (33%), it is likely that, as
suggested for the rhesus monkey,20 the low levels of SN-38 are
a consequence of high glucuronidation rather than decreased
production. In striking contrast, 16D gave similar SN-38 levels
as in rats and a very low AUCSN‑38G/AUCSN‑38 of 0.23.
Interestingly, the linker cleavage t1/2 in the monkey (360 h) was
almost identical to that in the rat (362 h), indicating that the
SAR of in vitro versus in vivo cleavage6 likely holds in animals
other than rodents.
Although promising, none of the phase 1 trials of protracted
continuous infusion of topo 1 inhibitors have been taken
forward in adequately powered clinical trials; a major
impediment is the impractical requirement for continuous
infusions to a large number of patients. A possible solution to
this problem is to use long-lived macromolecular drug
conjugates that slowly release the drug at low levels over long
periods. Toward that end, numerous macromolecular con-
jugates of CPT analogues have been prepared and stud-
Finally, we saw no free SN-38 in plasma of rats or monkeys
treated with stable conjugates 14E and 16E that cannot
undergo β-eliminative release; because we can detect SN-38 at
levels <0.1% of conjugate (e.g., 16D, Table 2), we conclude
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Table 2. Pharmacokinetic Parameters for PEG−SN-38 Conjugates 14 and 16, Released SN-38 and SN-38G in the Rat
conjugate
R-
14E
H-
16E
H-
14A
14B
14C
O(CH2)2NSO2CH2-
DBCO
14D
16D
NCCH2-
amide
PhSO2CH2-
DBCO
MeSO2CH2-
DBCO
NCCH2-
DBCO
PEG connector
DBCO
amide
PEG−SN-38
μmol SN-38
0.22
86
0.54
36
2.3
1.0
3.8
3.1
4.9
C
max, μM
379
0.025
9
116
0.032
28
579
387
623
Vd, L kg−1
0.011
0.055
58
0.025
30
0.029
35
0.029
50
t1/2,β, h
39
a
t1/2 cleavage (ln 2/k1), h
AUC0−∞, μM h
12
99
138
342
362
2132
47 485
32
1768
16 043
19.6
3632
7738
177
377
3076
15 072
51
12 440
16 041
247
7588
11 994
118
16 664
16 664
199
b
N−AUC0−∞, μM h
c
C0,PEG‑SN‑38
N−C0,PEG,SN‑38
713
178
250
319
187
199
SN-38
C
max, μM
1.4
0.18
0.18
0.28
38
0.38
0.38
58
N−Cmax, μM
t1/2,β, h
2.98
9
34
AUC0‑∞, μM h
N−AUC0−∞, μM h
C0,SN‑38
13.1
27.9
1.27
2.71
6
4.2
10.9
10.9
0.137
0.14
7.7
6.6
0.154
0.20
0.065
0.10
N−C0,SN‑38
SN-38G
C
max, μM
0.277
0.590
2.7
0.019
0.017
0.027
0.45
0.026
0.026
0.86
N−Cmax, μM
AUC0−∞, μM h
N−AUC0−∞, μM h
Ratios
0.83
5.75
1.07
0.71
0.86
AUCSN‑38G/AUCSN‑38
0.21
0.14
0.11
0.079
a
b
Not applicable or not determined. The prefix N- with an italicized entry designates that values have been normalized to a constant dose of 4.9
c
μmol of SN-38 content in PEG−SN-38 conjugate 16D. C0 is the concentration at t = 0 extrapolated from t1/2,β
.
ied;12,33,34 in the forefront are three PEGylated conjugates with
cleavable ester linkers that have yielded promising results in
clinical trials: NKTR-102,35,36 a PEGylated CPT-11 with a Gly-
ester linkage to the C-20 alcohol, ENZ-2208, an analogous
PEGylated SN-38,3,37,38 and NK-012, a soluble PEG−Glun
micelle linked via a Glu ester to the 10-phenol of SN-38.4,39
The presumed advantages of these macromolecular conjugates
are that they provide a slow-release depot of the attached drug
and they also accumulate in tumors by the enhanced
permeability and retention (EPR) effect4,37,40 to give rise to
high intratumoral concentrations of the released drug.
However, these conjugates all use an ester linkage to attach
the drug to the carrier and probably involve serum esterase(s)
in drug release. Hence, in addition to variations in metabolism
and transport, drug exposure may be subject to variability in
serum esterases. Moreover, unless there is some form of
protection against hydrolysis in vivo, the rate of spontaneous,
noncatalyzed ester cleavage places an upper limit on the
cleavage t1/2 of a conjugate and hence the t1/2 of the released
drug. At pH 7.4, the in vitro t1/2 for cleavage of a Gly-C20 ester,
as in Enz-2208 and NKTR-102, is ∼12 h,3 and for cleavage of
the C10-phenyl ester of NK-012, is 20 h.4 Indeed,
pharmacokinetic data for NKTR-102, Enz-2208, and NK-012
in humans show that >99.5% of the conjugates are lost with t1/2
of ∼12−30 h.36,38,39 The extraordinarily long half-lives of SN-
38 released from such conjugates sometimes cited41 likely
include or refer to the prolonged terminal disposition phase of
very low, perhaps insignificant subnanomolar levels of SN-38.42
As an illustrative example, Figure 4 shows the human C
versus t plot for free SN-38 and SN-38G released from the
PEG(Glu)n−SN-38 conjugate, NK-012, as constructed from
reported parameters at a recommended schedule of q3w
cycles.39 Here, the t1/2 for SN-38 release is 20 h, and ∼95% of
the SN-38 has cleared by ∼4 days; the time over a presumed 5
nM threshold is only a small fraction of the interval between
doses. The exposure, or AUC, is dominated by the early release
of SN-38, but the drug remains over target for only ∼2 days;
thus, much of the exposure is “wasted AUC”. Also shown is a C
versus t simulation in humans for SN-38 released from a PEG−
SN-38 conjugate, 16D, that uses a β-eliminative linker with a
much longer cleavage t1/2 of 362 h . By dosing q3wk, SN-38 can
be kept above a target threshold of 5 nM (or higher with
increased dose) for the duration of treatment with a very low
Cmax and peak-to-trough ratio. Here, unlike the conjugates
linked by esters, t1/2 is limited by clearance of the PEG−SN-38
conjugate and not the cleavage t1/2 of the linker. Importantly,
although the estimated AUC of the SN-38 released by this
conjugate is similar to that of NK-012, the exposure displays
time dependence rather than concentration dependence. On
the basis of the above, we undertook the current studies to
develop PEG−SN-38 conjugates that would release continuous
low levels of SN-38 for protracted periods, mimicking a
continuous infusion insofar as possible with a single injection.
Chemistry of PEG−SN-38 Conjugates. In the present
work, we used β-eliminative linkers of carbamates adapted for
use in conjugates containing an ether linkage. The approaches
for synthesis and mechanistic evaluation follow directly from
previous studies.7 Thus, PEG−SN-38 conjugates 14A−E
containing a DBCO-derived triazole connector between the
drug and macromolecule were prepared by a four-step
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sequence used for analogous ethers of p-nitrophenol (Scheme
3). Conjugates 16D,E containing a less hydrophobic amide
connector were prepared by diverting intermediates 13D,E to
prepare amines 15D,E used for acylation with a 4-armed
PEG40 kDa-HSE.
Solvolysis of conjugates with stable linkers (14E, 16E) at all
pH values and those containing modulators (14A−D and 16D)
in acidic media gave SN-38 and the N-aryl carbamate as final
products, indicating initial cleavage of the methylene adaptor−
oxygen bond. In contrast, solvolysis of the conjugate containing
pKa modulators at pH > 6 yielded SN-38, the PEG-
alkenylsulfone, and 4-(Et)2NCOPhNH2 as products, indicating
initial β-eliminative O-alkyl cleavage of the carbamate bond.
Kinetic studies of solvolysis of PEG−SN-38 conjugates
showed spontaneous, hydronium-ion-, and hydroxide-ion-
catalyzed reactions. The acid-catalyzed reaction involves pre-
equilibrium protonation of the reactant followed by rate-
determining cleavage to SN-38 and the unstable carbamate
iminium ion (Scheme 2); the latter would rapidly hydrate to
the N-hydroxymethyl carbamate and lose formaldehyde in
acidic media. Because of several kinetically equivalent
mechanisms, it was impossible to assign the exact site of initial
protonation from the available data. The precise contribution of
the uncatalyzed SN1 pathway cannot be ascertained with linkers
containing a pKa modulator because of competing, more rapid
hydroxide-catalyzed β-elimination, but it could be estimated
from the nadir of the pH rate profile and the spontaneous
solvolysis of stable carbamate 16E. In the present case, the t1/2
of SN1 solvolysis of 16E, and hence 14A−D and 16D, at pH 7.4
is estimated to be ∼3600 h.
The specific base-catalyzed β-eliminative cleavage is driven
by the electron-withdrawing effect of the pKa modulator on the
adjacent C−H bond. Depending on the modulator used, the
t1/2 values at pH 7.4 ranged from ∼10 to 450 h, all significantly
faster than the ∼3600 h t1/2 for SN1 cleavage. The initial step is
the cleavage of the O-alkyl bond of the carbamate to give a
carbamic acid, which undergoes decarboxylation to the
Mannich base17 and subsequent collapse to the aryl amine,
formaldehyde, and free drug (Scheme 2, path A). Using a
kinetic approach previously described,7 we could estimate that
at pH 7.4 decarboxylation of carbamic acid 8 and collapse of
the Mannich base 9 both occur with t1/2 < 3 s at physiologic pH
and hence would not accumulate to any significant extent in
vivo.
Figure 3. Pharmacokinetic profiles for PEG−SN-38 and SN-38 in the
rat. (A) C vs t profile in rat of selected PEG−SN-38 conjugates with
■
◆
,
▲
different pKa modulators: 14A ( , green), 14B ( , blue), 14C (
●
red), and 14E ( , black). Note that the data is from a different
experimental set than reported in Table 2. (B) C vs t plot of ratio of
cleavable to stable PEG−SN-38 concentrations vs time. First-order
rate constants for linker cleavage (k1) are obtained as slopes of the
lines; for derivation, see the Supporting Information. (C) C vs t plot of
■
●
PEG−SN-38 conjugate 14A ( , red), SN-38 ( , blue), and SN-38G
◆
(
●
, green). (D) C vs t plot of PEG−SN-38 conjugate 16D ( , red),
■
◆
SN-38 ( , blue), and SN-38G ( , green).
Pharmacokinetic Studies. Accurate determination of SN-
38 blood levels in the presence of high levels of cleavable
PEG−SN-38 conjugates is challenging. In addition to the high
sensitivity needed for SN-38 and SN-38G analysis, precautions
are needed to minimize ex vivo cleavage of the relatively high
amount of PEG−SN-38 present (up to ∼1800:1 in the present
work). A low level of linker cleavage of the conjugate at pH 7.4,
occurring after only a few minutes, can result in artifactual
increases in free SN-38. Thus, drawn blood samples were
rapidly quenched with cold, acidified citrate to pH ∼5.5, and
plasma samples were never subjected to conditions (e.g., strong
acid) shown to catalyze cleavage. PEG−SN-38 and free SN-38
are easily separated from interfering substances on HPLC and
quantitated by UV-fluorescence; however, the SN-38G peak co-
eluted with an interfering fluorescent substance from plasma
that prevented high-sensitivity analysis. Hence, independent
measurements of SN-38G were made after acid treatment to
convert it to its lactone form that eluted in a region free of
interfering substances.
Figure 4. C vs t curves in humans reported for SN-38 and SN-38G
◆
derived from NK-012 and from simulations of 16D. SN-38 (-- --,
■
blue) and SN-38G (−− −−, black) in patient plasma after 28 mg
m
−2 q3wk SN-38 equivalents of NK-012; cleavage t1/2 20 h, AUCSN‑38
5.4 μM h, and AUCSN‑38G 4.1 μM h from Figure 2A in ref 39. SN-38
(, red) from simulation of PEG−SN-38 conjugate 16D with −CN
pKa modulator having cleavage t1/2 of 362 h after 33 mg m−2 qwk × 3
SN-38 equivalents targeting Cmin 5 nM; inset shows expanded C vs t
plot of target 5 nM Cmin (−−−) and released SN-38 (, red).
Parameters used for simulation were SN-38 VSS 100 L, t1/2 2 h; PEG
conjugate t1/2,β 168 h. AUCSN‑38 is 1.4 μM h per injection or 4.3 μM h
for 3 weeks.
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The cost of prolonged release of SN-38 from the conjugates
is a decreased efficiency of drug utilization; that is, when
t1/2,cleavage ≫ t1/2,β, the efficiency of SN-38 delivery [k1/(k1 +
k3)] is limited by the residence time of PEG. For example, 14A
has a relatively high rate of drug release (t1/2 12 h) compared to
renal elimination (t1/2 39 h) but delivers ∼76% of its SN-38
cargo; in contrast, 16D, with a very slow cleavage (t1/2 362 h)
delivers only 12% of its SN-38 in the rat, whereas the remainder
is excreted unchanged. In humans, the t1/2,β of 16D is estimated
to be ∼7 days (Figure 4), so the efficiency of SN-38 utilization
is increased to ∼32%. Thus, an appropriate balance must be
reached between the duration of action desired and the
economics of SN-38 utilization.
Clearly, it would be desirable to use conjugates with the
longest circulating residence times possible. In the cleavable
PEG−SN-38 conjugates with a −CN pKa modulator (14D and
16D) and their stable counterparts (14E and 16E), both
DBCO and a simple amide were tested as connecting groups to
PEG in the rat. Interestingly, although the cleavage rates were
the same, conjugates with simple amide connections to PEG
(16D and 16E) showed ∼50% longer t1/2,β values and over 2-
fold increased exposure than corresponding conjugates with a
hydrophobic DBCO connection (14D and 14E); the t1/2,β
values are likewise ∼50% longer than the 40 h t1/2 of the
unsubstituted 4-arm PEG carrier.35 The SN-38 released from
amide-linked 16D also showed ∼1.6-fold increase in AUC
compared to the DBCO-linked 14D. It appears that SN-38
increases t1/2,β of the conjugates, whereas the concurrent
presence of the hydrophobic DBCO remnant decreases it. It is
likely that the SN-38 moiety on the conjugate prolongs t1/2,β
through its high-affinity binding (∼99%) to plasma proteins
and blood cells.43 Because the circulating lifetime of conjugates
with slow cleavage rates are limited by renal elimination, an
extension of the residence time, as in 16D, makes the conjugate
a more efficient source of SN-38 that would require less
frequent administration.
It is of interest to compare the plasma SN-38 levels in rats
treated with PEG−SN-38 conjugates to those treated with
CPT-11. In a study of the toxicity of CPT-11 as a function of
dosing schedule, rats were administered a very high 60 mg (102
μmol) CPT-11 kg−1 d−1 × 4 as daily boluses or as a continuous
infusion.19 Serious acute and delayed-onset diarrhea and
myelosuppresion observed in the bolus injection group were
alleviated in the continuous infusion group, illustrating the
lower toxicity that can be achieved by time- versus dose-
dependent exposure. Nevertheless, SN-38 exposure increased
from 1.9 μM h in the bolus dosing to 3.0 μM h in the 4 day
infusion, whereas Cmax dropped from 0.43 to 0.15 μM. After
normalizing the 20 μmol kg−1 dose of 16D to 5.5 μmol, 3.6-fold
lower than in Table 2, the exposure of SN-38 derived from a
single dose of the PEG−SN-38 conjugate is estimated to be 3
μM h, whereas Cmax is 0.17 μM, closely matching parameters of
the SN-38 formed during continuous infusion of CPT-11.
Conjugate 16D also showed an AUCSN‑38G/AUCSN‑38 of 0.08,
which is dramatically lower than the 3.6 ratio observed in the 4
day infusion of CPT-11. Thus, with exception of the 45-fold
lowering of the glucuronidation ratio, the SN-38 released from
a single dose of 16D closely mimics the SN-38 formed in the 4
day CPT-11 continuous infusion.
The current study has implications for issues of toxicity and
interpatient variability frequently observed with other SN-38
prodrugs, namely, CPT-11 and esterase-cleavable PEG−SN-38
and CPT-11 conjugates. CPT-11 is the most widely used
prodrug of SN-38; its metabolism is well-understood45 and
explains the late-onset severe diarrhea caused by toxicity to
intestinal cells. As illustrated in Figure 5A, CPT-11 is converted
Primary objectives of this work were to develop cleavable
PEG−SN-38 conjugates that release SN-38 (a) with predictable
long half-lives, (b) with low Cmax values, and (c) that can be
kept above a target concentration for topo 1 inhibition for very
long periods. In vivo t1/2 values for cleavage of PEG−SN-38
conjugates with various pKa modulators vary from ∼12 to 360 h
and agree reasonably well with in vitro t1/2 values at pH 7.4
(Table 1). Most are considerably longer than the 12−20 h t1/2
values reported for PEG−SN-38 conjugates that require ester
cleavage. Whereas the t1/2,β of SN-38 in the rat is only about
∼22 min,44 the t1/2,β of the SN-38 released from the conjugates
vary from 8 to 50 h, up to a 135-fold half-life extension. In
rodents, the rapid renal elimination of 4-arm PEG40kDa (t1/2 ∼1
day in mice, 2 days in rat) does not illustrate the full potential
of slowly cleaved linkers in humans, where the renal elimination
of PEG is much slower (t1/2 ∼ 5 to 7 days). Because the rate of
linker cleavage is species-independent, knowledge of the t1/2 of
PEG and pharmacokinetics of the drug in species of interest
allows cross-species modeling.6 Thus, we simulated the C
versus t for free SN-38 from our lead conjugate 16D in humans,
which was presented in Figure 4. As shown, SN-38 can be kept
above a target concentration of 5 nM for at least 1 week with a
very low Cmax and peak-to-trough ratio. Indeed, the Cmax of SN-
38 in the simulation is ∼20- to 30-fold lower than reported for
NK-012 and Enz-2208, two ester-linked PEG−SN-38 con-
jugates in clinical trials.38,39
Figure 5. Hepatic uptake of (A) CPT-11 or (B) SN-38 using
OATP1B1 transporter and conversion of SN-38 to SN-38G followed
by deconjugation by intestinal bacterial β-glucuronidase back to SN-
38.
into SN-38 by hepatic carboxyesterase (CE), which is then
metabolized by hepatic UGT1A to its 10-glucuronide, SN-38G.
Glucuronidation facilitates biliary excretion, and bacterial β-
glucuronidase causes reconversion of SN-38G to SN-38 in the
cecum and colon. Unless intestinal UGT1A converts the drug
back to the inert SN-38G,46,47 the SN-38 exerts toxic effects on
the intestine. Thus, as a precursor of SN-38, SN-38G is a source
of the intestinal toxin, and as an inert product of SN-38
glucuronidation, SN-38G serves as a protection against
intestinal toxicity.
Surprisingly, with the slow-releasing conjugates tested here,
plasma SN-38G was barely detectable, with exposure (AUC)
representing only ∼8% of that of the SN-38 released in our
longest-acting conjugate, 16D. The levels of SN-38G from the
other conjugates described here track the linker cleavage rate
but are all very low (≤21% of SN-38). In contrast,
administration of CPT-1119,39 by bolus or continuous infusion,
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bolus SN-38,44 or esterase-cleavable PEG−SN-38 or CPT-11
conjugates36,38,39 results in plasma AUCSN‑38G/AUCSN‑38 ratios
that can approach 10- to over 100-fold higher than we observe
with 16D. Preliminary results on the pharmacokinetics of 16D
and CPT-11 in the African green monkey track those observed
in the rat; that is, the AUCSN‑38G/AUCSN‑38 ratio is extremely
high for CPT-11 and extremely low for 16D. Thus, the low SN-
38G levels observed for our PEG conjugates in the rat are not
due to peculiarities specific to rodents. Although the results
may at first seem paradoxical, there is a congruent mechanism
that explains the uniquely low SN-38G levels observed here and
suggests its relevance to chemotherapy.
In the case of CPT-11, the source of hepatic SN-38 is CE
metabolism in the liver; because high blood levels of CPT-11
drive its hepatic uptake, CPT-11 acts as a large and efficient
trojan horse for a continuous source of hepatic SN-38. In
contrast, as in Figure 5B, SN-38 generated by PEG−SN-38
conjugates in the systemic circulation enters the liver primarily
via the organic anion transporter OATP1B1, which does not
transport CPT-11 or SN-38G.48,49 Thus, differential hepatic
uptake can explain why CPT-11 results in higher levels of
hepatic and plasma SN-38G and hence higher intestinal SN-38
than the very low levels of SN-38 delivered to the liver by the
conjugates described here. It appears that the high systemic SN-
38G observed after CPT-11 treatment reflects its high hepatic
uptake and the detoxification effects of liver and intestinal
UGT1A glucuronidation against intestinal exposure to SN-
38.46,47 However, if SN-38G is the source of enterotoxic SN-38,
then it is evident that an effective way to avoid the toxic agent is
simply to reduce levels of SN-38G in the intestine. Because the
low SN-38G formed from analogues described here likely
reflects low hepatic uptake of SN-38, it may be surmised that
intestinal exposure to SN-38 would likewise be low.
mechanisms of SN-38 release, and their pharmacokinetic
properties in rats. Results show that such conjugates provide
a unique pharmacokinetic profile to the released SN-38 that
differs dramatically from those reported for other SN-38
prodrugs, including CPT-11. First, the presence of SN-38 on
the conjugates greatly increases its circulatory residence time,
providing a benefit in the duration of action and efficiency of
drug delivery. Second, using linkers with very long half-lives for
cleavage, SN-38 is delivered with a low Cmax yet at
concentrations that keep the drug over presumed target levels
for sustained periods. Third, very low levels of SN-38G are
formed, suggesting that toxicities and other pharmacodynamic
variabilities seen with other SN-38 prodrugs may be decreased.
Last, as well established with other PEG−SN-38 conjugates, we
expect accumulation of our conjugates in tumors by the EPR
effect4,37,40 and consequent high intratumoral concentrations of
the released drug. We are hopeful that these properties will
faithfully translate to humans.
EXPERIMENTAL PROCEDURES
■
General. Materials purchased were of the purest grade
commercially available. HPLC analyses were reverse-phase and used
UV or fluorescence detection. Purity of the tested conjugates was all
≥95% as assessed by HPLC, and SN-38/PEG ≥ 3.98. Animal studies
were performed at Invitek (rat) and RxGen (monkey). Details are
provided in the Supporting Information.
Synthesis. O-[7-Azido-1-cyano-2-heptyl]-N-[4′-
(diethylcarbamoyl)phenyl)]carbamate (11D). A solution of 7-
azido-1-cyano-2-heptanol 10D (4.6 g, 25.0 mmol) in 200 mL of
anhydrous THF was treated with triphosgene (12.5 g, 42.0 mmol) and
pyridine (1.6 mL, 50.0 mmol) for 10 min at ambient temperature. The
solution was filtered and evaporated to yield the crude chloroformate,
which was dissolved in 200 mL of anhydrous THF and treated with 4-
(N,N-diethylcarboxamido)aniline (4.80 g, 25.0 mmol) and triethyl-
amine (7.5 mL, 53.8 mmol) for 1 h. The mixture was diluted with
EtOAc and washed successively with water, 1 N HCl, sat. aq.
NaHCO3, and brine, dried with MgSO4, filtered, and evaporated. The
crude material was chromatographed on SiO2 using a step gradient of
hexane, 1:1 hexane/EtOAc, and 1:2 hexane/EtOAc . The product
crystallized on standing and was recrystallized from 1:1 EtOAc/hexane
Bolus SN-3844 or esterase-cleavable PEG−SN-3838,39 con-
jugates also produce SN-38G, albeit not at the high levels seen
with CPT-11 or the low levels observed with SN-38 conjugates
described here; for example, the AUCSN‑38G/AUCSN‑38 of NK-
012 is ∼1.1 (Figure 4). Because liver SN-38 is associated with
intestinal toxicity and blood SN-38, with myelosuppression, it
may not be coincidental that the DLT of many PEG−SN-38
prodrugs is neutropenia rather than diarrhea, which can be
alleviated by treatment with G-CSF.38 The long-lived PEG−
SN-38 conjugates described here, such as 16D, show even
lower SN-38G than bolus SN-38 or esterase-cleavable PEG−
SN-38. This may be explained by the comparatively high Cmax
of free SN-38 that occurs with bolus injection or with the
shorter-lived esterase-cleavable conjugates. With these, SN-38
should enter the liver via OATP1B1 in proportion to plasma
levels (i.e., Cmax effect) because SN-38 levels never exceed the
∼20 μM Km of the transporter.49
A prodrug of SN-38 such as 16D that does not produce
significant SN-38G might also result in less interpatient
variability than CPT-11 because major sources of variability
with CPT-11, namely, CE and Cyp3A, are not involved and
UGT1A is not as important. Thus, if the low SN-38G levels
formed from long-acting PEG−SN-38 conjugates in the rat
translate to humans, then we may achieve therapeutic SN-38
levels without enteric injury and with lower interpatient
variability than currently used SN-38 prodrugs.
1
to provide product 11D (4.4 g, 11.0 mmol, 44%). H NMR (400
MHz, CDCl3): δ 7.40 (2H, d, J = 8 Hz; H3′), 7.33 (2H, d, J = 8 Hz;
H2′), 7.07 (1H, br s; NH), 4.98 (1H, m; H2), 3.49 (4H, br s; NCH2),
3.27 (2H, t, J = 6.8 Hz; H7), 2.82 (1H, dd, J = 17, 5.3 Hz; H1a), 2.65
(1H, dd, J = 17, 4.4 Hz; H1b), 1.85 (1H, m; H3a), 1.73 (1H, m; H3b),
1.60 (2H, m; H4), 1.43 (4H, m; H5 + H6), 1.16 (6H, br s; CH2CH3).
LC−MS m/z: [M + H]+ calcd for C20H29N6O3, 401.2; found, 401.2.
N-Aryl carbamates 11A−C,E were prepared in a similar fashion and
are described in the Supporting Information.
O-[7-Azido-1-cyano-2-heptyl]-N-(SN-38-CH2 )-N-[4′-
(diethylcarbamoyl)phenyl)]carbamate (13D). A mixture of 11D
(400 mg, 1.0 mmol), paraformaldehyde (35 mg, 1.5 mmol),
chlorotrimethylsilane (0.5 mL, 4.0 mmol), and 5 mL of anhydrous
toluene was heated at 50 °C in a sealed 20 mL vial for 24 h, at which
time a clear solution containing a small amount of precipitate was
obtained. HPLC analysis of a 5 μL aliquot diluted into 1.0 mL of 4
mM N,N-diisopropylethylamine in ethanol indicated complete
consumption of the starting carbamate (λmax 243 nm) and formation
of a slightly later-eluting peak (λmax 231 nm), consistent with the N-
(ethoxymethyl)-carbamate of 11D. The solution was cooled to
ambient temperature, filtered, and evaporated. The residue was
dissolved in 5 mL of dry toluene, filtered, and evaporated to provide
458 mg of an unstable moisture-sensitive yellow oil containing 89% by
weight of N-(chloromethyl)carbamate 12D as determined by the
HPLC assay. A 1.0 M solution of KOtBu in THF (250 μL, 0.25 mmol)
was added to a solution of SN-38 (100 mg, 0.255 mmol) in 10 mL of
1:1 THF/DMF cooled on ice under nitrogen, forming an initial dark
green color that changed to a bright gold suspension. After 15 min, a
CONCLUSIONS
We have described the synthesis of PEG−SN-38 conjugates
linked by a series of cleavable β-eliminative linkers, their
■
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solution of the crude N-(chloromethyl)carbamate (190 mg, 0.36
mmol) in 1 mL of THF was added. After 15 min, the pale yellow
solution was diluted with EtOAc, washed sequentially with 5% citric
acid, water, and brine, then dried over MgSO4, filtered, and
evaporated. Excess DMF was removed by trituration of the oily
residue with water, and the residue was dissolved in DCM, dried, and
then chromatographed on SiO2 using a step gradient of hexane, 20, 40,
60, and 80% acetone in hexane, providing purified azido linker−SN-38
was determined spectrophotometrically on a 0.45 mg mL−1 solution in
ACN using ε363 nm = 22 500 M−1 cm−1 to be SN-38/PEG = 3.9.
Treatment in basic solution released all SN-38.
PEGylated SN-38 16E was prepared in a similar fashion and is
described in the Supporting Information. An alternative large-scale
preparation of 16D is described in the Supporting Information.
Kinetic Studies. Kinetic studies were performed by HPLC or
spectrophotometric determination of SN-38 anion at pH > 8.5 as
described for analogous p-nitrophenyl ethers.7
1
13D as a pale yellow foam (150 mg, 0.186 mmol, 73%). H NMR
Pharmacokinetics. Pharmacokinetic studies were performed on
female Sprague−Dawley rats (n = 3 for each conjugate), assayed by
HPLC, and analyzed by a two-compartment model, as detailed in
Figure 2 and in the Supporting Information.
(400 MHz, CDCl3): δ 8.15 (1H, d, J = 9.2 Hz), 7.60 (1H, s), 7.48
(1H, dd, J = 2, 9 Hz), 7.40 (4H, m), 7.25 (1H, d, J = 2), 5.75 (2H, br),
5.73 (1H, d, J = 16 Hz), 5.28 (1H, d, J = 16 Hz), 5.22 (2H, s), 4.99
(1H, m), 3.84 (1H, s), 3.53 (2H, br), 3.53 (2H, br), 3.17 (2H, t, J = 7
Hz), 3.12 (2H, q, J = 7 Hz), 2.74 (1H, dd, J = 1, 17 Hz), 2.54 (1H, dd,
J = 5, 17 Hz), 1.86 (2H, m), 1.6 (1H, m), 1.46 (1H, m), 1.37 (3H, t, J
= 7 Hz), 1.25 (6 H, m), 1.12 (4 H, m), 1.02 (3H, t, J = 7.3 Hz). LC−
MS m/z: [M + H]+ calcd for C44H51N8O8, 805.3; found, 805.3.
Compounds 13A−C,E were prepared in a similar fashion and are
described in the Supporting Information. An alternative large-scale
preparation of 12D and 13D is described in the Supporting
Information.
ASSOCIATED CONTENT
■
S
* Supporting Information
Detailed synthetic procedures and analytical and spectroscopic
data for compounds as well as kinetic and pharmacokinetic
methods. This material is available free of charge via the
Internet at http://pubs.acs.org.
O-[7-Amino-1-cyano-2-heptyl]-N-(SN-38-CH2 )-N-[4′-
(diethylcarbamoyl)phenyl)]carbamate Acetate (15D). A 1 M
solution of trimethylphosphine in THF (2.9 mL, 2.9 mmol) was
added to a solution of 13D (1.13 g, 1.4 mmol) and acetic acid (0.19
mL, 3.3 mmol) in 10 mL of THF. Gas was slowly evolved. After
stirring for 2 h, water (1.0 mL) was added, and the mixture was stirred
for an additional 1 h. The residue was partitioned between ether and
water. The water phase was washed with EtOAc, and the clear yellow
aqueous phase was evaporated to provide 800 mg of yellow foam. This
was dissolved in THF, filtered, and quantitated by UV absorbance to
provide a solution containing 1.2 μmol (86%) of product. C18 HPLC
showed a single peak. LC−MS m/z: [M + H]+ calcd for C43H51N6O8,
779.4; found, 779.3.
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: Daniel.V.Santi@prolynxllc.com. Phone: 415 552 5306.
Notes
The authors declare the following competing financial
interest(s): D.V.S, G.W.A., and E.L.S. are inventors on patent
applications protecting the compounds and technologies
disclosed in this article.
ACKNOWLEDGMENTS
■
Compounds 15A−C,E were prepared in a similar fashion and are
described in the Supporting Information. An alternative large-scale
preparation synthetic method of 15D is described in the Supporting
Information.
We thank Philip Smith, Pieter Timmermans, and Joseph
Dougherty for their comments on this work.
O-[7-PEG-triazole-1-cyano-2-heptyl]-N-(SN-38-CH2)]-N-[4′-
(diethylcarbamoyl)phenyl)]carbamate (14D). A 30 mM solution of
13D in THF (400 uL, 12 μmol) was added to a gently stirred solution
of 4-arm PEG40kDa-(DBCO)4 (100 mg, 2.5 μmol PEG, 10 μmol
DBCO) in 1.0 mL of THF. After 20 h at ambient temperature, the
solvent was evaporated, and the residue was dissolved in methanol and
dialyzed (SpectraPor 2 membrane, 12 kDa cutoff) against methanol to
remove excess azide linker−SN-38. After evaporation of the methanol,
the product was taken up in 1 mL of THF and precipitated with 10
mL of MTBE to provide 98 mg of conjugate (2.2 μmol, 88%). Size-
exclusion HPLC indicated a single peak corresponding to the
conjugate. Complete loss of the DBCO absorbance at 308 nm was
noted. Unconjugated SN-38 was 0.6 mol % by reversed-phase HPLC.
SN-38 content was determined spectrophotometrically on a 0.45 mg
mL−1 solution in ACN using ε363 nm = 22 500 M−1 cm−1 to be SN-38/
PEG = 3.9. Treatment in basic solution released all SN-38.
ABBREVIATIONS USED
■
CPT-11, irinotecan; CPT, camptothecin; DBCO, dibenzoaza-
cyclooctyne; EPR, enhanced retention and permeability; HSE,
hydroxysuccimide ester; PEG, poly(ethylene glycol); SN-38G,
SN-38 10-glucuronide; SN-38, 7-ethyl-10-hydroxy-camptothe-
cin; topo 1, topoisomerase 1
REFERENCES
■
(1) Greenwald, R. B.; Yang, K.; Zhao, H.; Conover, C. D.; Lee, S.;
Filpula, D. Controlled release of proteins from their poly(ethylene
glycol) conjugates: Drug delivery systems employing 1,6-elimination.
Bioconjugate Chem. 2003, 14, 395−403.
(2) Filpula, D.; Zhao, H. Releasable PEGylation of proteins with
customized linkers. Adv. Drug Delivery Rev. 2008, 60, 29−49.
(3) 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. Novel prodrugs of SN38 using multiarm poly(ethylene
glycol) linkers. Bioconjugate Chem. 2008, 19, 849−859.
(4) Koizumi, F.; Kitagawa, M.; Negishi, T.; Onda, T.; Matsumoto, S.;
Hamaguchi, T.; Matsumura, Y. Novel SN-38-incorporating polymeric
micelles, NK012, eradicate vascular endothelial growth factor-secreting
bulky tumors. Cancer Res. 2006, 66, 10048−10056.
(5) Fox, M. E.; Guillaudeu, S.; Frechet, J. M.; Jerger, K.; Macaraeg,
N.; Szoka, F. C. Synthesis and in vivo antitumor efficacy of PEGylated
poly(l-lysine) dendrimer-camptothecin conjugates. Mol. Pharmaceutics
2009, 6, 1562−1572.
PEGylated SN-38 analogues 14A−C,E were prepared in a similar
fashion and are described in the Supporting Information.
O-[7-PEG-carboxamido-1-cyano-2-heptyl]-N-(SN-38-CH2)]-N-[4′-
(diethylcarbamoyl)phenyl)]carbamate (16D). A 50 mM solution of
15D in ACN (1.2 mL, 60 μmol) was added to a gently stirred solution
of 4-arm PEG40kDa-(NHS)4 (500 mg, 12.5 μmol PEG, 50 μmol NHS)
in 2.0 mL of ACN followed by addition of N,N-diisopropylethylamine
(20 μL, 150 μmol). After 1 h at ambient temperature, 5 mL of 0.1 M
KPi, pH 7.0, was added, and the mixture was dialyzed (SpectraPor 2
membrane, 12 kDa cutoff) sequentially against 50:50 methanol/water
and methanol to remove excess azide linker−SN-38. After evaporation
of methanol, the product was dissolved in 2 mL of THF and
precipitated with 10 mL of MTBE to provide 490 mg of conjugate
(10.9 μmol conjugate, 87%). Size-exclusion HPLC indicated a single
peak corresponding to the conjugate. Unconjugated SN-38 was
determined to be 0.6 mol % by reversed-phase HPLC. SN-38 content
(6) Santi, D. V.; Schneider, E. L.; Reid, R.; Robinson, L.; Ashley, G.
W. Predictable and tunable half-life extension of therapeutic agents by
controlled chemical release from macromolecular conjugates. Proc.
Natl. Acad. Sci. U.S.A. 2012, 109, 6211−6216.
2312
dx.doi.org/10.1021/jm401644v | J. Med. Chem. 2014, 57, 2303−2314

Journal of Medicinal Chemistry
Article
(7) Schneider, E.; Robinson, L.; Reid, R.; Ashley, G. W.; Santi, D. V.
β-Eliminative releasable linkers adapted for bio-conjugation of
macromolecules to phenols. Bioconjugate Chem. 2013, 24, 1990−1997.
(8) Mathijssen, R. H.; Verweij, J.; Loos, W. J.; de Bruijn, P.; Nooter,
K.; Sparreboom, A. Irinotecan pharmacokinetics-pharmacodynamics:
The clinical relevance of prolonged exposure to SN-38. Br. J. Cancer
2002, 87, 144−150.
(9) Furman, W. L.; Stewart, C. F.; Poquette, C. A.; Pratt, C. B.;
Santana, V. M.; Zamboni, W. C.; Bowman, L. C.; Ma, M. K.; Hoffer, F.
A.; Meyer, W. H.; Pappo, A. S.; Walter, A. W.; Houghton, P. J. Direct
translation of a protracted irinotecan schedule from a xenograft model
to a phase I trial in children. J. Clin. Oncol. 1999, 17, 1815−1824.
(10) Ma, M. K.; Zamboni, W. C.; Radomski, K. M.; Furman, W. L.;
Santana, V. M.; Houghton, P. J.; Hanna, S. K.; Smith, A. K.; Stewart, C.
F. Pharmacokinetics of irinotecan and its metabolites SN-38 and APC
in children with recurrent solid tumors after protracted low-dose
irinotecan. Clin. Cancer Res. 2000, 6, 813−819.
(11) Santana, V. M.; Zamboni, W. C.; Kirstein, M. N.; Tan, M.; Liu,
T.; Gajjar, A.; Houghton, P. J.; Stewart, C. F. A pilot study of
protracted topotecan dosing using a pharmacokinetically guided
dosing approach in children with solid tumors. Clin. Cancer Res.
2003, 9, 633−640.
(12) Venditto, V. J.; Simanek, E. E. Cancer therapies utilizing the
camptothecins: a review of the in vivo literature. Mol. Pharmaceutics
2010, 7, 307−349.
(13) Fassberg, J.; Stella, V. J. A kinetic and mechanistic study of the
hydrolysis of camptothecin and some analogues. J. Pharm. Sci. 1992,
81, 676−684.
(14) Debets, M. F.; van Berkel, S. S.; Schoffelen, S.; Rutjes, F. P.; van
Hest, J. C.; van Delft, F. L. Aza-dibenzocyclooctynes for fast and
efficient enzyme PEGylation via copper-free (3 + 2) cycloaddition.
Chem. Commun. 2010, 46, 97−99.
(15) Majumdar, S.; Sloan, K. B. Synthesis, hydrolyses and dermal
delivery of N-alkyl-N-alkyloxycarbonylaminomethyl (NANAOCAM)
derivatives of phenol, imide and thiol containing drugs. Bioorg. Med.
Chem. Lett. 2006, 16, 3590−3594.
(16) Iley, J.; Moreira, R.; Calheiros, T.; Mendes, E. Acyloxymethyl as
a drug protecting group: Part 4. The hydrolysis of tertiary
amidomethyl ester prodrugs of carboxylic acid agents. Pharm. Res.
1997, 14, 1634−1639.
(17) Johnson, S. L.; Morrison, D. L. Kinetics and mechanism of
decarboxylation of N-arylcarbamates. Evidence for kinetically
important zwitterionic carbamic acid species of short lifetime. J. Am.
Chem. Soc. 1972, 94, 1323−1334.
(18) Bundgaard, H.; Johansen, M. Prodrugs as drug delivery systems.
XIX. Bioreversible derivitization of aromatic amines by formation of
N-Mannich bases with succinimide. Int. J. Pharm. 1981, 8, 183−192.
(19) Kurita, A.; Kado, S.; Kaneda, N.; Onoue, M.; Hashimoto, S.;
Yokokura, T. Alleviation of side effects induced by irinotecan
hydrochloride (CPT-11) in rats by intravenous infusion. Cancer
Chemother. Pharmacol. 2003, 52, 349−360.
(20) Inaba, M.; Ohnishi, Y.; Ishii, H.; Tanioka, Y.; Yoshida, Y.;
Sudoh, K.; Hakusui, H.; Mizuno, N.; Ito, K.; Sugiyama, Y.
Pharmacokinetics of CPT-11 in rhesus monkeys. Cancer Chemother.
Pharmacol. 1998, 41, 103−108.
(21) Pommier, Y. Topoisomerase I inhibitors: Camptothecins and
beyond. Nat. Rev. Cancer 2006, 6, 789−802.
(25) Herben, V. M.; Schellens, J. H.; Swart, M.; Gruia, G.; Vernillet,
L.; Beijnen, J. H.; ten Bokkel Huinink, W. W. Phase I and
pharmacokinetic study of irinotecan administered as a low-dose,
continuous intravenous infusion over 14 days in patients with
malignant solid tumors. J. Clin. Oncol. 1999, 17, 1897−1905.
(26) Masi, G.; Falcone, A.; Di Paolo, A.; Allegrini, G.; Danesi, R.;
Barbara, C.; Cupini, S.; Del Tacca, M. A phase I and pharmacokinetic
study of irinotecan given as a 7-day continuous infusion in metastatic
colorectal cancer patients pretreated with 5-fluorouracil or raltitrexed.
Clin. Cancer Res. 2004, 10, 1657−1663.
(27) Allegrini, G.; Falcone, A.; Fioravanti, A.; Barletta, M. T.;
Orlandi, P.; Loupakis, F.; Cerri, E.; Masi, G.; Di Paolo, A.; Kerbel, R.
S.; Danesi, R.; Del Tacca, M.; Bocci, G. A pharmacokinetic and
pharmacodynamic study on metronomic irinotecan in metastatic
colorectal cancer patients. Br. J. Cancer 2008, 98, 1312−1319.
(28) Takimoto, C. H.; Morrison, G.; Harold, N.; Quinn, M.;
Monahan, B. P.; Band, R. A.; Cottrell, J.; Guemei, A.; Llorens, V.;
Hehman, H.; Ismail, A. S.; Flemming, D.; Gosky, D. M.; Hirota, H.;
Berger, S. J.; Berger, N. A.; Chen, A. P.; Shapiro, J. D.; Arbuck, S. G.;
Wright, J.; Hamilton, J. M.; Allegra, C. J.; Grem, J. L. Phase I and
pharmacologic study of irinotecan administered as a 96-h infusion
weekly to adult cancer patients. J. Clin. Oncol. 2000, 18, 659−667.
(29) Hochster, H.; Liebes, L.; Speyer, J.; Sorich, J.; Taubes, B.; Oratz,
R.; Wernz, J.; Chachoua, A.; Raphael, B.; Vinci, R. Z.; et al. Phase I trial
of low-dose continuous topotecan infusion in patients with cancer: An
active and well-tolerated regimen. J. Clin. Oncol. 1994, 12, 553−559.
(30) Hochster, H.; Wadler, S.; Runowicz, C.; Liebes, L.; Cohen, H.;
Wallach, R.; Sorich, J.; Taubes, B.; Speyer, J. Activity and
pharmacodynamics of 21-day topotecan infusion in patients with
ovarian cancer previously treated with platinum-based chemotherapy.
New York Gynecologic Oncology Group. J. Clin. Oncol. 1999, 17,
2553−2561.
(31) Wiernik, P. H.; Li, H.; Weller, E.; Hochster, H. S.; Horning, S. J.;
Nazeer, T.; Gordon, L. I.; Habermann, T. M.; Minniti, C. J.; Shapiro,
G. R.; Cassileth, P. A. Activity of topotecan 21-day infusion in patients
with previously treated large cell lymphoma: long-term follow-up of an
Eastern Cooperative Oncology Group study (E5493). Leuk.
Lymphoma 2012, 53, 1137−1142.
(32) Muggia, F.; Kosloff, R.; Liebes, L.; Hochster, H. Topotecan
continuous infusion: CA-125 responses including patients pretreated
with other schedules of topotecan. Oncologist 2006, 11, 529−531.
(33) Duncan, R.; Ringsdorf, H.; Satchi-Fainaro, R. Polymer
therapeutics–polymers as drugs, drug and protein conjugates and
gene delivery systems: past, present and future opportunities. J. Drug
Targeting 2006, 14, 337−341.
(34) Haag, R.; Kratz, F. Polymer therapeutics: concepts and
applications. Angew. Chem., Int. Ed. 2006, 45, 1198−2015.
(35) Eldon, M. A.; Antonian, L.; Burton, K.; Goodin, R.; Bentley, M.
NKTR-102, a novel PEGylated-irinotecan conjugate, demonstrates
improved pharmacokinetics with sustained exposure of irinotecan and
its active metabolite, poster P-0727, 14th European Cancer Confer-
ence (ECCO 14), Barcelona, Spain, Sept 23−27, 2007.
(36) Eldon, M. A.; Hoch, U. Population pharmacokinetics of NKTR-
102, a topoisomerase I inhibitor-polymer conjugate in patients with
advanced solid tumors, abstract 2598, poster 8E, ASCO 2011,
Developmental Therapeutics − Clinical Pharmacology and Immuno-
therapy, Chicago, IL, June 3−7, 2011.
(37) Sapra, P.; Zhao, H.; Mehlig, M.; Malaby, J.; Kraft, P.; Longley,
C.; Greenberger, L. M.; Horak, I. D. Novel delivery of SN38 markedly
inhibits tumor growth in xenografts, including a camptothecin-11-
refractory model. Clin. Cancer Res. 2008, 14, 1888−1896.
(22) Tanizawa, A.; Fujimori, A.; Fujimori, Y.; Pommier, Y.
Comparison of topoisomerase I inhibition, DNA damage, and
cytotoxicity of camptothecin derivatives presently in clinical trials. J.
Natl. Cancer Inst. 1994, 86, 836−842.
(23) Levasseur, L. M.; Slocum, H. K.; Rustum, Y. M.; Greco, W. R.
Modeling of the time-dependency of in vitro drug cytotoxicity and
resistance. Cancer Res. 1998, 58, 5749−5761.
(24) Hassan, S. B.; Jonsson, E.; Larsson, R.; Karlsson, M. O. Model
for time dependency of cytotoxic effect of CHS 828 in vitro suggests
two different mechanisms of action. J. Pharmacol. Exp. Ther. 2001, 299,
1140−1147.
(38) Kurzrock, R.; Goel, S.; Wheler, J.; Hong, D.; Fu, S.; Rezai, K.;
Morgan-Linnell, S. K.; Urien, S.; Mani, S.; Chaudhary, I.; Ghalib, M.
H.; Buchbinder, A.; Lokiec, F.; Mulcahy, M. Safety, pharmacokinetics,
and activity of EZN-2208, a novel conjugate of polyethylene glycol and
SN38, in patients with advanced malignancies. Cancer 2012, 118,
6144−6151.
(39) Hamaguchi, T.; Doi, T.; Eguchi-Nakajima, T.; Kato, K.; Yamada,
Y.; Shimada, Y.; Fuse, N.; Ohtsu, A.; Matsumoto, S.; Takanashi, M.;
2313
dx.doi.org/10.1021/jm401644v | J. Med. Chem. 2014, 57, 2303−2314

Journal of Medicinal Chemistry
Article
Matsumura, Y. Phase I study of NK012, a novel SN-38-incorporating
micellar nanoparticle, in adult patients with solid tumors. Clin. Cancer
Res. 2010, 16, 5058−5066.
(40) Hoch, U.; Eldon, M. A. Activity of NKTR-102 in nonclinical
models of gastrointestinal cancers, abstract P-0025, ESMO: 12th
World Congress on Gastrointestinal Cancer, Barcelona, Spain, June
30−July 3, 2010.
(41) Azim, H. A., Jr.; Awada, A. Clinical development of new
formulations of cytotoxics in solid tumors. Curr. Opin. Oncol. 2012, 24,
325−331.
(42) Kehrer, D. F.; Yamamoto, W.; Verweij, J.; de Jonge, M. J.; de
Bruijn, P.; Sparreboom, A. Factors involved in prolongation of the
terminal disposition phase of SN-38: Clinical and experimental studies.
Clin. Cancer Res. 2000, 6, 3451−3458.
(43) Combes, O.; Barre, J.; Duche, J. C.; Vernillet, L.; Archimbaud,
Y.; Marietta, M. P.; Tillement, J. P.; Urien, S. In vitro binding and
partitioning of irinotecan (CPT-11) and its metabolite, SN-38, in
human blood. Invest. New Drugs 2000, 18, 1−5.
(44) Kato, A.; Ueyama, J.; Abe, F.; Hotta, K.; Tsukiyama, I.; Oshima,
T.; Kondo, F.; Saito, H.; Hasegawa, T. Panipenem does not alter the
pharmacokinetics of the active metabolite of irinotecan SN-38 and
inactive metabolite SN-38 glucuronide (SN-38G) in rats. Anticancer
Res. 2011, 31, 2915−2922.
(45) Mathijssen, R. H.; van Alphen, R. J.; Verweij, J.; Loos, W. J.;
Nooter, K.; Stoter, G.; Sparreboom, A. Clinical pharmacokinetics and
metabolism of irinotecan (CPT-11). Clin. Cancer Res. 2001, 7, 2182−
2194.
(46) Tallman, M. N.; Miles, K. K.; Kessler, F. K.; Nielsen, J. N.; Tian,
X.; Ritter, J. K.; Smith, P. C. The contribution of intestinal UDP-
glucuronosyltransferases in modulating 7-ethyl-10-hydroxy-camptothe-
cin (SN-38)-induced gastrointestinal toxicity in rats. J. Pharmacol. Exp.
Ther. 2007, 320, 29−37.
(47) Chen, S.; Yueh, M. F.; Bigo, C.; Barbier, O.; Wang, K.; Karin,
M.; Nguyen, N.; Tukey, R. H. Intestinal glucuronidation protects
against chemotherapy-induced toxicity by irinotecan (CPT-11). Proc.
Natl. Acad. Sci. U.S.A. 2013, 110, 19143−19148.
(48) Nozawa, T.; Minami, H.; Sugiura, S.; Tsuji, A.; Tamai, I. Role of
organic anion transporter OATP1B1 (OATP-C) in hepatic uptake of
irinotecan and its active metabolite, 7-ethyl-10-hydroxycamptothecin:
In vitro evidence and effect of single nucleotide polymorphisms. Drug
Metab. Dispos. 2005, 33, 434−439.
(49) Fujita, K. I.; Sugiura, T.; Okumura, H.; Umeda, S.; Nakamichi,
N.; Watanabe, Y.; Suzuki, H.; Sunakawa, Y.; Shimada, K.; Kawara, K.;
Sasaki, Y.; Kato, Y. Direct inhibition and down-regulation by uremic
plasma components of hepatic uptake transporter for SN-38, an active
metabolite of irinotecan, in humans. Pharm. Res. 2013, 31, 204−215.
2314
dx.doi.org/10.1021/jm401644v | J. Med. Chem. 2014, 57, 2303−2314