Drug-initiated, controlled ring-opening polymerization for the synthesis of polymer-drug conjugates

Article abstract of DOI:10.1021/ma202581d

Paclitaxel, a polyol chemotherapeutic agent, was covalently conjugated through its 2′-OH to polylactide with 100% regioselectivity via controlled polymerization of lactide mediated by paclitaxel/(BDI-II)ZnN(TMS)₂ (BDI-II = 2-((2,6-diisopropylph

Full text of DOI:10.1021/ma202581d

Article
pubs.acs.org/Macromolecules
Drug-Initiated, Controlled Ring-Opening Polymerization for the
Synthesis of Polymer−Drug Conjugates
†
Rong Tong and Jianjun Cheng*
Department of Materials Science and Engineering, University of Illinois at Urbana−Champaign, Urbana, Illinois 61801, United States
*
S Supporting Information
ABSTRACT: Paclitaxel, a polyol chemotherapeutic agent,
was covalently conjugated through its 2′-OH to polylactide
with 100% regioselectivity via controlled polymerization of
lactide mediated by paclitaxel/(BDI-II)ZnN(TMS) (BDI-II =
2
2
-((2,6-diisopropylphenyl)amino)-4-((2,6-diisopropylphenyl)-
imino)-2-pentene). The steric bulk of the substituents on the
N-aryl groups of the BDI ligand drastically affected the
regiochemistry of coordination of the metal catalysts to
paclitaxel and the subsequent ring-opening polymerization of
lactide. The drug-initiated, controlled polymerization of lactide was extended, again with 100% regioselectivity, to docetaxel,
a chemotherapeutic agent that is even more structurally complex than paclitaxel. Regioselective incorporation of paclitaxel
(
or docetaxel) to other biopolymers (i.e., poly(δ-valerolactone), poly(trimethylene carbonate), and poly(ε-caprolactone)) was
also achieved through drug/(BDI-II)ZnN(TMS) -mediated controlled polymerization. These drug−polylactide conjugates with
precisely controlled structures are expected to be excellent building blocks for drug delivery, coating, and controlled-release
applications.
2
INTRODUCTION
Research on the use of drug-containing polymeric nanoparticles
for improved cancer treatments is an emerging field.
groups on the therapeutic agents by means of protection/
deprotection chemistry.
■
10
1
We have been interested in controlling the conjugation of
drugs to polymers with reduced heterogeneity via a controlled
ring-opening polymerization (ROP) strategy. In our initial
attempt, we used a complex of paclitaxel (Ptxl), a mitotic
inhibitor with three hydroxyl groups, and a Zn catalyst to
initiate the ROP of lactide (LA), which results in efficient
conjugation of Ptxl molecules to the termini of polylactide
Chemotherapeutic agents incorporated in polymeric nano-
particulate delivery vehicles typically have improved water
2
3
solubility, prolonged retention in circulation, and reduced
4
drug resistance relative to conventional chemotherapeutics.
Therefore, polymeric nanomedicines are expected to have
reduced toxicity and improved efficacy. A handful of polymeric
nanomedicines have been developed for clinical cancer
treatment in the past 2−3 decades; among these, polymer−
drug conjugates have been one of the most intensively
investigated delivery platforms.
The preparation of polymer−drug conjugates with well-
defined structures and compositions is extremely important
for their in vivo applications, but this task can be difficult.
Polymer−drug conjugates usually exhibit heterogeneous
structures. Heterogeneities may result from (1) molecular
weight distributions (MWDs) of the polymers (Scheme 1A),
5
11
(
PLA) chains. In ROP studies conducted by the chemistry
community, the focus is on control of the molecular weights
MWs), MWDs, and structures of the polymer products (that
6
(
12
7
is, the chain propagation step), whereas the focus of our
studies on drug-initiated ROP has been more on the initiation
step. We are particularly interested in controlling the
regioselective coordination of a structurally complex drug
molecule (the initiator) bearing multiple hydroxyl groups with
the metal catalyst. In the Ptxl/Zn polymerization system
specifically, investigation of Zn catalysts that interact with Ptxl
to afford regioselective initiation and controlled polymerization
is imperative. Exploring whether this chemistry is widely
applicable for the synthesis of drug−polymer conjugates with
various hydroxyl-containing therapeutics and with various poly-
mer backbones is also important.
(
2) lack of control of the drug conjugation site on the polymer
backbone (Scheme 1B), and (3) lack of regioselectivity with
regard to the conjugation site on drugs with multiple
conjugation-amenable functional groups. These heterogeneities,
which are not easily addressed, may present bottlenecks in the
clinical translation of polymer−drug conjugates. In the past
several decades, there have been numerous efforts to minimize
these heterogeneities, including the development of polymers
6d
Here, we report the rationale design of metal catalysts that
allow for regioselective conjugation of PLA to Ptxl via its 2′-OH
8
with low MWDs (e.g., dendrimer), the conjugation of
Received: November 27, 2011
Revised: February 3, 2012
Published: February 27, 2012
therapeutic agents to specific sites along the polymer backbone
2,9
(
e.g., the termini), and the activation of specific functional
©
2012 American Chemical Society
2225
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Scheme 1. (A) Polymer−Drug Conjugates with a Broad
Distribution of Polymer Chain Lengths; (B) Polymer−Drug
Conjugates in Which the Conjugation Site on the Polymer
Backbone Is Uncontrolled; (C) Polymer−Drug Conjugates
in Which a Multifunctional Therapeutic Agent Is Conjugated
without Regioselectivity; (D) Drug-Initiated, Controlled
Polymerization for the Synthesis of Polymer−Drug
found that this chemistry could be extended to the ROP of
cyclic esters and carbonates to prepare a large variety of drug−
polyester and drug−polycarbonate conjugates. Docetaxel−PLA
conjugates were also prepared with remarkably controlled MWs
and low MWDs. These materials are expected to be excellent
building blocks for the design of systems for drug delivery,
coating, and controlled release applications.
a
Conjugates with Low MWDs
RESULTS AND DISCUSSION
■(
BDI-EE)Zn/Ptxl-Mediated LA Polymerization. Metal
oxides (M-ORs) are well-known initiators for living ROP of
12c
cyclic esters such as LA (Figure 1). By judicious design, labile
M-ORs prepared in situ by mixing a hydroxyl-containing
12a
compound with an active metal complex can initiate the
controlled living ROP of LA, resulting in quantitative
incorporation of the initiating alcohol group at the PLA
12c
terminals with nearly 100% monomer conversion. The use of
various hydroxyl-containing cholesterol derivatives coordinated
with an Al(III) or a Sn(II) complex as co-initiators for cyclic
13
ester polymerization has been reported previously. Because
Ptxl has hydroxyl groups, we attempted to use it to initiate the
ROP of LA in the presence of (BDI-X)ZnN(TMS) (BDI = 2-
2
(
(2,6-dialkylphenyl)amino)-4-((2,6-dialkylphenyl)imino)-2-
11
pentene, Table 1), a class of catalysts developed by Coates
and co-workers for controlled ROP of LA. (BDI-EE)ZnN-
(
TMS) (Table 1) was synthesized and mixed with Ptxl,
2
instantly resulting in (BDI-EE)Zn/Ptxl; this active complex
subsequently initiated LA polymerization at a LA/Ptxl/(BDI-
a
The drug molecules are conjugated regioselectively to the polymer chain
termini.
EE)ZnN(TMS) ratio of 200:1:1.01. Ptxl was quantitatively
2
conjugated to PLA to afford Ptxl-LA (in this paper, PLA is
n
1
to afford Ptxl−PLA conjugates with precisely controlled MWs,
denoted as LA , where n = M/I) as indicated by HPLC and H
n
predefined drug loading, and narrow MWDs (Scheme 1D). We
NMR analysis (entry 1, Table 2). The drug loadings could be
Figure 1. Preparation of PEGylated Ptxl−PLA nanoconjugates (NCs) via Ptxl-initiated LA polymerization in the presence of (BDI-X)ZnN(TMS)
2
1
1,19
(
X = II, EE, EI, IICN; see Table 1), followed by nanoprecipitation and noncovalent surface modification
with poly(lactide-co-glycolide)-methoxy
poly(ethylene glycol) (PLGA-mPEG) or poly(lactide)-methoxy poly(ethylene glycol) (PLA-mPEG).
2
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a
Table 1. Synthesis of (BDI-X)ZnN(TMS)2
BDI-X
BDI-II
R1
iPr
R2
iPr
R3
H
BDI-EE
BDI-EI
Et
Et
H
i
Et
iPr
Pr
iPr
H
BDI-IICN
CN
a
i
Abbreviations: Pr, isopropyl; Et, ethyl.
precisely controlled by adjusting the LA/Ptxl ratio. The M
n
3
value was 30.2 × 10 g/mol, which is in excellent agreement
3
with the expected value (29.7 × 10 g/mol). However, the
MWD was fairly broad (M /M = 1.30).
w
n
Use of Reductive Reaction To Determine Regiose-
lectivity of Ptxl-LA Formation. Ptxl has three hydroxyl
5
groups, at the C-2′, C-1, and C-7 positions (Figure 1). All three can
potentially initiate LA polymerization, resulting in Ptxl-PLA con-
jugates with one to three PLA chains attached to Ptxl. The tertiary
14
1
-OH is least accessible and is typically inactive, but both the
2
′-OH and 7-OH are active, with the 2′-OH being sterically more
15
accessible. We next evaluated the regioselectivity of poly-
merization initiation in the presence of (BDI-EE)ZnN(TMS)2.
Tetrabutylammonium borohydride (Bu NBH ) has been
4
4
used for selectively and quantitatively reducing the C13-ester
bond of Ptxl to afford baccatin III (BAC) and (1S,2R)-N-1-(1-
16
phenyl-2,3-dihydroxypropyl)benzamide (PDB; Figure 2A).
We attempted to use this reductive reaction to disassemble
Ptxl-LA obtained from (BDI-EE)ZnN(TMS) /Ptxl-mediated
5
2
polymerization and then determine whether PLA was attached
only to the 2′-OH or to both the 2′-OH and 7-OH (Figure 2B).
If the initiation occurred regioselectively at the 2′-OH, no BAC-
PLA should have been observed. However, analysis of the
fragments obtained from the reductive cleavage of the C13-
Figure 2. (A) Bu NBH -induced site-specific degradation of Ptxl to
4
4
form PDB and BAC. (B) Reductive degradation of C13-ester of Ptxl-
PLA. (C) HPLC traces for (i) Ptxl, (ii) Ptxl treated with Bu NBH
iii) Ptxl-LA , and (iv−vii) Ptxl-LA prepared using (BDI-X)ZnN-
,
4
4
(
(
5
5
TMS) (X = EE, EI, II, and IICN; see Table 1) and subsequent
2
treatment with Bu NBH . *The PDB and partial BAC detected in
4
4
ester bond of Ptxl-LA clearly showed the presence of both
5
(iv−vii) were produced from free Ptxl not completely consumed
PDB−PLA and BAC−PLA, indicating that the (BDI-EE)ZnN-
during the polymerization at low [LA]/[Ptxl] (i.e., ≤5). Separation of
Ptxl was not attempted. Comparison of the patterns of BAC and PDB
in (vi) and (ii) clearly indicates that the BAC in (vi) was produced
partially by degradation of unreacted Ptxl and partially by reduction of
Ptxl-PLA with the PLA being attached to the 2′-OH of Ptxl. **Peaks
were verified by mass spectrometry.
(
TMS) /Ptxl-mediated polymerization exhibited poor regiose-
2
lectivity (Figure 2C), which may explain the relatively broad
MWD of the Ptxl-LA200 synthesized with (BDI-EE)ZnN-
(
TMS) /Ptxl (entry 1, Table 2).
2
The substituents on the N-aryl groups (R and R ) and at
1
2
the β-position (R ) of the BDI ligand drastically affect its
3
12a,17
ability to control the ROP of LA.
By varying the steric bulk
of the N-aryl substituents and the electronic properties of R3,
we studied the effects of the BDI substituents on the initia-
tion regioselectivity and the LA polymerization mediated by
a
Table 2. Effect of Ligand Substituents on LA ROP Mediated by Ptxl/(BDI-X)ZnN(TMS)2
3
3
entry
ligand (BDI-X)
[LA]/[Ptxl]
LA conv (%)
incorp eff (%)
Mexp (×10 g/mol)
M (×10 g/mol)
MWD (M /M )
w n
n
1
2
3
4
5
6
BDI-EE
BDI-EI
BDI-II
200
200
200
200
50
>99
>99
>99
>99
>99
>99
>99
>99
>99
>99
>99
>99
29.7
29.7
29.7
29.7
8.1
30.2
31.4
28.1
27.4
7.8
1.30
1.17
1.02
1.04
1.04
1.03
BDI-IICN
BDI-II
BDI-II
100
15.3
12.7
a
All reactions were performed with [Ptxl]/[(BDI-X)ZnN(TMS) ] = 1:1.01 in anhydrous THF with [LA] = 0.69 M at room temperature for 12 h.
2
0
Abbreviations: LA, lactide; Ptxl, paclitaxel; conv, conversion; incorp eff, incorporation efficiency; Mexp: expected MW; MWD, molecular weight
−1
distribution. LA conversion was measured by monitoring the disappearance of the LA peak (1772 cm ) in the FT-IR spectrum. Incorporation
efficiency was determined by reversed-phase HPLC analysis of unincorporated Ptxl. M and MWD were measured by GPC.
n
2
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BDI-X)Zn(TMS) /Ptxl (Table 1). When one of the ethyl
Article
18
(
2
groups in BDI-EE was replaced with the more bulky isopropyl
i
(
Pr) group (BDI-EI), the polymerization reaction afforded
Ptxl-LA200 with a much narrower MWD (M /M = 1.17,
w
n
LA/Ptxl/(BDI-EI)ZnN(TMS) = 200:1:1.01, entry 2, Table 2).
2
i
When both the ethyl groups were replaced with Pr groups,
remarkably well controlled polymerization was observed: Ptxl-
3
LA200 was obtained with the expected MW (M = 28.1 × 10 g/
n
mol) and an extremely narrow MWD (M /M = 1.02, entry 3).
w
n
Interestingly, the regioselectivities obtained with these
catalysts followed a similar trend as the polymerization results.
Analysis of the fragments of the reductive cleavage of Ptxl-LA5
prepared from (BDI-EI)ZnN(TMS) /Ptxl showed reduced
2
amounts of BAC-PLA relative to the amount observed with
(
(
BDI-EE)ZnN(TMS) /Ptxl. When Ptxl-LA prepared from
2
5
BDI-II)ZnN(TMS) /Ptxl was treated with Bu NBH and
2
4
4
analyzed similarly, no BAC-PLA peaks were identified, indicating
that (BDI-II)ZnN(TMS) /Ptxl-mediated polymerization was
2
highly regioselective (Figure 2). The steric bulk of the substituents
at the 2- and 6-positions of the N-aryl group clearly had a strong
effect on the regioselectivity and controlled polymerization. The
i
bulky Pr groups on the BDI ligand may have hindered metal
complexation with the 7-OH of Ptxl, thereby minimizing undesired
chain initiation and propagation via this hydroxyl group. To
determine whether R also affected initiation regioselectivity and
3
polymerization, we prepared BDI-IICN, an analogue of BDI-II with
an additional CN group on R . (BDI-IICN)ZnN(TMS) behaved
3
2
similarly to (BDI-II)ZnN(TMS) (entry 4).
2
Controlled polymerizations of PLA were observed when
(
BDI-II)ZnN(TMS) /Ptxl was used at various [LA]/[Ptxl]
2
ratios (50:1−300:1; Figure 3A): the obtained MWs were in
excellent agreement with the expected MWs (Table S1); the
MWDs were in a range of 1.02−1.09. Monomodal gel
permeation chromatography (GPC) curves were observed in
all cases (Figure 3B).
Figure 3. (A) Plots of M and MWD versus [LA]/[I] (I = Ptxl or
n
Dtxl) for LA polymerization initiated by Ptxl/(BDI-II)ZnN(TMS)
and Dtxl/(BDI-II)ZnN(TMS) . (B) Overlay of GPC chromatograms
for Ptxl-LA200, Ptxl-LA100, and Ptxl-LA50 obtained by (BDI-II)ZnN-
TMS) -mediated polymerization.
2
2
(
2
Regioselectivity of Ptxl/Anhydride Reaction. We next
investigated whether the regioselectivity observed at the initiation
step for the formation of Ptxl-LA could be achieved with other
n
Scheme 2. (BDI-II)ZnN(TMS) -Mediated Ring-Opening of
(A) LA and (B) SA via a Coordination−Insertion
Mechanism
2
substrates besides LA. Previous studies showed that (BDI-II)-
i
ZnOPr (or other Zn-alkoxides) can mediate controlled ROP of
LA via a coordination−insertion mechanism (Scheme 2A). In
addition, the same catalyst can also mediate ROP of anhydrides
20
and epoxides by a similar mechanism. These results suggested
that succinic anhydride (SA) could be used as a model monomer
and that ROP reactions mediated by (BDI-X)ZnN(TMS) /Ptxl
2
could terminate after the initiation step (O-acylation, Scheme 2B).
Such reactions would afford not a polymer but the small molecule
Ptxl-succinic acid (Ptxl-SA), whose structure could be easily and
precisely determined by routine characterization methods.
To perform the acylation, we prepared various (BDI-X)ZnN-
(
TMS) /Ptxl complexes in situ and mixed them with SA for 4 h at
2
40 °C at a Ptxl/SA ratio of 1:1.1 (Table 3). The reaction mixtures
were quenched and analyzed by HPLC. The structures of the
acylation products were determined by NMR after separation by
1
TLC or HPLC ( H NMR shown in Figure 4). The chemical shift
of the C2′-H of Ptxl-2′-SA was shifted downfield relative to that of
Ptxl (from 4.78 to 5.53 ppm), whereas the chemical shifts of C7-H
were nearly the same (δ = 4.40 ppm, Figure 4). For Ptxl-2′,7-diSA,
both C2′-H and C7-H were shifted downfield relative to the
corresponding protons of Ptxl (δ (C2′-H) from 4.78 to 5.53 ppm;
δ(C7-H) from 4.40 to 5.64 ppm). By measuring the yields of Ptxl-
2′-SA and Ptxl-2′,7-diSA (Table 3), we found that the 2′-OH
regioselectivity obtained with the various catalysts followed exactly
2
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Table 3. Ring-Opening Reactions of SA Mediated by Ptxl (or
Dtxl)/(BDI-X)ZnN(TMS)2
Scheme 3. Regioselective Acylation of (A) Ptxl and (B) Dtxl
with SA
a
total
catalyst
ligand
(BDI-X)
yield of yield of R-2′,7- yield of regioselec-
initiator
entry (R)
R-2′-SA diSA and other R-SA
tivity for
(%)
R-SA (%)
(%) 2′-OH (%)
1
2
3
4
5
Ptxl
Ptxl
Ptxl
Ptxl
Dtxl
BDI-EE
BDI-EI
BDI-II
29.6
45
55.4
13.4
0
85
34.8
77.1
58.4
39.7
53.6
71.5
39.7
53.6
71.5
100
BDI-IICN
BDI-II
0
100
100
0
a
All reactions were performed with [Ptxl]/[(BDI-X)ZnN(TMS) ]/
2
[
SA] = 1/1.01/1.1 in anhydrous THF with [SA] = 0.011 M at 40 °C
0
for 4 h. Abbreviations: SA, succinic anhydride; Ptxl, paclitaxel; Dtxl,
docetaxel. Yields were determined by reversed-phase HPLC analysis.
SA derivatives were confirmed by NMR and MS.
1
Figure 4. H NMR spectrum of Ptxl, Ptxl-2′-SA, and Ptxl-2′,7-diSA.
the same trend observed for the reactions with LA, as determined
by Bu NBH -mediated reductive cleavage (Figure 2). That is, the
regioselectivity of the Ptxl/SA reaction increased as the sizes of R1
LA and for ring-opening acylation of SA with a more
structurally complex therapeutic agent. We chose docetaxel
(Dtxl), which is an important clinical therapeutic for the
treatment of prostate cancer and which has four hydroxyl
groups, at C-2′, C-1, C-7, and C-10 (Figure 1). When
4
4
and R2 increased; (BDI-II)ZnN(TMS)2 showed the best
regioselectivity and (BDI-EE)ZnN(TMS) the worst (Scheme 3A).
2
Specifically, the (BDI-EE)ZnN(TMS) /Ptxl-mediated ring-
complexed with (BDI-II)ZnN(TMS) , Dtxl reacted with SA
2
2
opening acylation with SA showed poor regioselectivity (34.8%,
entry 1, Table 3). Only 29.6% of the Ptxl was converted to Ptxl-
to yield Dtxl-2′-SA with 100% regioselectivity and 71.5% yield
(entry 5, Table 3; Scheme 3B). In addition, (BDI-II)ZnN-
2′-SA), whereas 55.4% was converted to Ptxl-2′,7-diSA, the
(TMS) /Dtxl showed excellent control for the ROP of LA,
2
undesired byproduct. In contrast, with (BDI-EI)ZnN(TMS)2/
Ptxl, the regioselectivity of the acylation was 77.1% (entry 2).
affording Dtxl-PLA with predictable MWs and very narrow
MWDs (Figure 3A and Supporting Information Table S2).
Ptxl-Initiated Controlled Polymerization of δ-Valer-
olactone, Trimethylene Carbonate, and ε-Caprolactone.
Poly(δ-valerolactone) (PVL), poly(trimethylene carbonate)
(PTMC), and poly(ε-caprolactone) (PCL) have been exten-
sively used as alternatives to PLA in suturing, drug delivery, and
tissue engineering applications. We next studied whether the
Ptxl (or Dtxl)/(BDI-II)ZnN(TMS)2-mediated controlled ROP
of LA could be extended to the synthesis of Ptxl-PCL, Ptxl-
PVL, and Ptxl-PTMC conjugates from the corresponding
When Ptxl/(BDI-II)ZnN(TMS) was used, the regioselectivity
2
of the reaction was 100%: Ptxl-2′-SA was the only acylation
product (entry 3). Although the reactions of Ptxl with SA and
LA showed similar regioselectivity trends, the overall reactivity
of (BDI-X)ZnN(TMS) was inversely correlated with the sizes
2
of R and R , as indicated by the overall yields of the Ptxl/SA
1
2
reactions (entries 1−3). Changing R from −H (BDI-II) to the
3
electron-withdrawing −CN group (BDI-IICN) did not change
the regioselectivity of the Ptxl/SA reaction. However, additional
of the CN group enhanced the reactivity of the resulting
monomers. Ptxl/(BDI-II)ZnN(TMS) showed excellent con-
2
catalyst ((BDI-IICN)ZnN(TMS) ), leading to a higher yield of
Ptxl-2′-SA (entry 4).
trol over the polymerization of δ-valerolactone (VL), trimethyl-
ene carbonate (TMC), and ε-caprolactone (CL) (Table 4; for
comprehensive polymerization results, see Supporting Informa-
tion Table S3). All the polymerization reactions gave drug−
polymer conjugates with the expected MWs and narrow
MWDs (M /M < 1.2); Ptxl (or Dtxl) was quantitatively
2
Regioselective (BDI-II)ZnN(TMS) /Docetaxel-Initiated
2
Ring-Opening of LA and SA. After demonstrating the
regioselectivity of the Ptxl ring-opening reactions with LA and
SA in the presence of (BDI-II)ZnN(TMS) , we determined
2
w
n
whether this catalyst could be used for regioselective ROP of
conjugated to the termini of the polymers via ester bonds. The
2
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a
Table 4. Ptxl (or Dtxl)/(BDI-II)ZnN(TMS) -Mediated ROP of VL, TMC, and CL
2
3
entry
initiator (R)
monomer
[M]/[R]
time (h)
temp (°C)
conv (%)
incorp eff (%)
M /M ( × 10 g/mol)
MWD (M /M )
n
exp
w
n
1
2
3
4
5
6
Ptxl
Ptxl
Ptxl
Ptxl
Dtxl
Ptxl
VL
100
200
300
200
100
100
12
12
12
10
10
6
rt
rt
rt
rt
rt
50
>99
>99
>99
>99
>99
>99
>99
>99
>99
>99
>99
>99
15.1/10.8
1.17
VL
23.1/20.8
31.2/30.8
20.3/23.7
11.2/12.2
14.7/11.1
1.18
1.18
1.07
1.05
1.10
VL
CL
CL
TMC
a
All reactions were performed in anhydrous THF with [monomer] = 0.69 M. Abbreviations: conv = conversion; Dtxl = docetaxel; incorp eff = incorporation
0
efficiency; M , expected MW; MWD = molecular weight distribution; Ptxl = paclitaxel; rt = room temperature. Monomer conversion was measured by
exp
FT-IR. Incorporation efficiency was determined by reversed-phase HPLC analysis of unreacted Ptxl (or Dtxl). M and MWD were determined by GPC.
n
polymerizations of VL and CL proceeded at room temperature,
and the monomer conversions were quantitative (entries 1−5,
Table 4). However, the polymerization of TMC required a
slightly elevated reaction temperature; quantitative TMC
conversation took place in 6 h at 50 °C (entry 6).
Santa Clara, CA), a DAWN HELEOS 18-angle laser light scattering
detector (MALLS), and an Optilab rEX refractive index detector
(Wyatt Technology, Santa Barbara, CA). The wavelength of the
HELEOS detector was set at 658 nm. Size-exclusion columns used for
the separation of PLA or drug−PLA conjugates were connected in
3
4
series to the GPC system (Phenogel columns 100, 500, 10 , and 10 Å,
5
μm, 300 × 7.8 mm, Phenomenex, Torrance, CA). THF (HPLC grade)
was used as the mobile phase for PLA. DMF containing 0.1 M LiBr at 60
C was used as the mobile phase for PVL, PCL, and PTMC. Low-
CONCLUSION
■
°
When complexed with hydroxyl-containing therapeutics, Zn
catalysts mediated the polymerization of LA and afforded PLA
chains with drugs incorporated at the chain termini through
resolution electrospray ionization mass spectrometry (LR-ESI-MS)
experiments were conducted on a Waters Quattro II mass spectrometer.
HPLC analysis was performed on a System Gold system (Beckman
Coulter, Fullerton, CA) equipped with a 126P solvent module, a System
Gold 128 UV detector, and an analytical pentafluorophenyl column
6
a,11,21
ester linkages.
Our previous work showed that the
resulting drug−polymer conjugates could be nanoprecipitated
to formulate nanoparticles with high drug loadings, nearly
quantitative loading efficiencies, controlled release profiles
without burst release effects, and narrow particle size
(Curosil-PFP or Luna C18(2), both 250 × 4.6 mm, 5 μ, Phenomenex,
Torrance, CA). The UV wavelengths for Ptxl and Dtxl were set at 227 nm;
the mobile phase was acetonitrile/water containing 0.1 wt % trifluoroacetic
acid. NMR analyses were conducted on a Varian U500, VXR500, or
UI500NB (500 MHz).
1
1,19
distributions.
We systemically investigated the effects of
BDI ligand substituents on the initiation and chain propagation
during LA polymerization. The steric bulk of the N-aryl sub-
stituents of the BDI ligand drastically affected the regiose-
lectivity of the coordination of the metal catalysts with the
drugs. (BDI-II)ZnN(TMS) , which bears bulky Pr groups at
the 2- and 6-positions of the N-aryl rings, formed coordination
complexes with Ptxl only via the 2′-OH of Ptxl and yielded Ptxl-
′-PLA and Ptxl-2′-SA with 100% regioselectivity. This strategy
for the controlled polymerization of LA (and acylation with
SA) was extended to Dtxl, which has a more complex structure,
with 100% regioselectivity. We also demonstrated that the
Monomer. DL-Lactide (LA) was purchased from TCI America
(
Portland, OR). It was recrystallized three times from toluene and
stored in a glovebox. ε-Caprolactone (CL) was purchased from Sigma-
Aldrich, dried over CaH for 48 h, and fractionally distilled under
i
2
2
reduced pressure. δ-Valerolactone (VL) was purchased from Sigma-
Aldrich, dried over CaH overnight, and fractionally distilled under
reduced pressure. The whole process was repeated. Trimethyl
carbonate (TMC) was synthesized according to published proce-
dures. The monomer was recrystallized from ether twice, dried, and
stored in a glovebox. All monomers were stored in the freezer of a
glovebox at −30 °C before use.
Ptxl-Mediated Ring-Opening Polymerization of LA in the
Presence of a Zn Catalyst. In a glovebox, Ptxl (8.5 mg, 0.01 mmol)
2
2
2
3
scope of the Ptxl (or Dtxl)/(BDI-II)ZnN(TMS) -mediated
2
controlled polymerization could be expanded to three other
biopolymers, poly(δ-valerolactone, poly(trimethylene carbo-
nate), and poly(ε-caprolactone). Importantly, the heterogeneity
of the resulting polyester−drug conjugates was substantially
reduced by means of this unprecedented controlled chemistry.
Particularly noteworthy is that the extension of the reaction to
various monomers may have benefits for drug delivery: drugs
could be entrapped within separate populations of particles
with different release kinetics because different polyesters
12a
was dissolved in anhydrous THF (2 mL). (BDI-II)ZnN(TMS)2
(
2
6.4 mg, 0.01 mmol) was added and allowed to react with Ptxl for 15−
0 min. LA (144.0 mg, 1.0 mmol) in THF (1 mL) was added dropwise
to the mixture of Ptxl and (BDI-II)ZnN(TMS) with vigorous stirring.
The polymerization was monitored using FT-IR by following the
2
−1
1
lactone band at 1772 cm or using H NMR by checking the methine
(−CH−) peak of LA around 5.2−5.0 ppm. After the polymerization
was complete, an aliquot of the polymerization solution was analyzed
using HPLC to quantify the unreacted Ptxl to determine the efficiency
of Ptxl incorporation into Ptxl-LA100. One drop of water was added to
the polymerization solution to hydrolyze the Zn-Ptxl oxide. The
resulting Ptxl-LA100 was precipitated from ethyl ether (10 mL), and the
precipitate was washed with ether and toluene to remove the BDI
ligand, dried under vacuum, and characterized by GPC. Complete
removal of BDI from Ptxl-PLA was verified by TLC.
22
degrade at different rates.
EXPERIMENTAL SECTION
Materials and Methods. General. BDI ligands and the
corresponding metal catalysts (BDI-X)ZnN(TMS) were prepared
■
2
12a
by following the published procedure and were stored at −30 °C in
a glovebox. All anhydrous solvents were purified through alumina
columns and kept anhydrous over molecular sieves. Paclitaxel (Ptxl)
and docetaxel (Dtxl) were purchased from LC Laboratories (Woburn,
MA) and used as received. All other chemicals were purchased from
Sigma-Aldrich (St. Louis, MO) and used as received unless otherwise
noted. The molecular weights of polymer−drug conjugates were
determined by gel permeation chromatography (GPC) on a system
equipped with an isocratic pump (Model 1100, Agilent Technology,
Reductive Degradation of Ptxl-PLA Using Bu NBH . The
4
4
reductive degradation of Ptxl at C-13 was achieved by following the
literature reported procedure. Ptxl (5 mg, 5.8 mmol) in anhydrous
dichloromethane (1 mL) was allowed to react with Bu NBH (2.5 mg,
10 mmol) for 1.5 h in a glovebox. Acetic acid (∼100 μL) was added to
the reaction mixture to terminate the reaction. The reaction solution
was stirred for 20 min, and then the solvent was completely removed.
The residual solid was redissolved in acetonitrile. An aliquot of the
solution was analyzed on an HPLC equipped with a Curosil RP
1
6
4
4
2
230
dx.doi.org/10.1021/ma202581d | Macromolecules 2012, 45, 2225−2232

Macromolecules
Article
1
column (250 × 4.6 mm, 5 μm; Phenomenex, Torrance, CA). The
desired fractions were collected and analyzed by LR-ESI-MS. The Ptxl-
PLA conjugate was reductively degraded and analyzed similarly.
Reaction of Ptxl with SA in the Presence of a Zn Catalyst. In
a glovebox, Ptxl (8.5 mg, 0.01 mmol) was dissolved in anhydrous THF
procedures were used for Ptxl-PCL synthesis. The H NMR spectra
of Ptxl-VL100 and Ptxl-CL100 are shown in Figures S4 and S5.
Synthesis of Ptxl-TMC Polymer. In a glovebox, Ptxl (8.6 mg,
0.010 mmol) and (BDI-II)ZnN(TMS) (6.6 mg, 0.011 mmol) were
mixed in 300 μL of THF. The mixture stirred was stirred for 20 min.
TMC (102 mg, 1.0 mmol) in toluene (300 μL) was added into the
mixture. The reaction vessel was tightly sealed and heated at 50 °C for
5 h out of the box. The reaction solution was quenched with 5 mL of
n
2
(
400 μL). (BDI-II)ZnN(TMS) (6.5 mg, 0.01 mmol) was added and
2
allowed to react with Ptxl for 15−20 min. SA (1.1 mg, 0.011 mmol) in
THF (600 μL) was added dropwise to the mixture of Ptxl and (BDI-
1
II)ZnN(TMS) with vigorous stirring ([SA] = 0.011 M). The reaction vial
ice-cold methanol. The conversion of TMC was determined by H
2
was tightly sealed and heated at 40 °C outside the glovebox. The reaction
was allowed to proceed for 4 h and quenched with 1 mL of ice-cold
methanol. The resulting solution was immediately analyzed by HPLC.
Pure Ptxl-2′-SA (or Dtxl-2′-SA) for NMR analysis was obtained by
preparative TLC on a silica gel matrix (UV254, 1500 μm thickness,
Sigma-Aldrich). Silica gel containing the desired compounds was scraped
from the glass plate and extracted with methanol (2 × 30 mL). The
NMR. The polymer was precipitated with ethyl ether (10 mL), and
the precipitate was washed with ether and toluene to remove the BDI
ligand, dried under vacuum, and characterized by GPC. The H NMR
1
spectrum of Ptxl-TMC100 is shown in Figure S6.
ASSOCIATED CONTENT
Supporting Information
■
*
S
resulting solution was evaporated under vacuum for NMR analysis.
1
Polymerization data; characterization of polymer−drug con-
jugates. This material is available free of charge via the Internet
at http://pubs.acs.org.
Ptxl-2′-SA. H NMR (500 MHz, CDCl ): δ 8.13 (d, J = 7.28 Hz,
3
2
6
5
5
4
H), 7.77 (d, J = 7.36 Hz, 2H), 7.61−7.36 (m, 15H), 7.26−7.24 (m,
H), 7.03 (d, J = 9.16 Hz, 1H), 6.25 (s, 1H), 6.22 (d, J = 8.9 Hz, 1H),
.97, 5.95 (dd, J = 2.84 Hz, 2.76 Hz, 1H), 5.67 (d, J = 7.08 Hz, 1H),
.56 (d, 1H), 5.47 (d, J = 2.96 Hz, 1H), 4.96 (d, J = 9.36 Hz, 1H),
.85−4.79 (m, 3H), 4.65 (d, J = 11.9 Hz, 1H), 4.41 (s, 1H), 4.31
AUTHOR INFORMATION
Corresponding Author
E-mail: jianjunc@illinois.edu.
■
−
4.27 (m, 2H), 4.19 (d, J = 8.44 Hz, 1H), 3.99 (t, J = 9.20 Hz, 1H),
*
3
4
6
.81−3.73 (m,3H), 3.64 (t, J = 9.20 Hz, 1H), 3.31 (s, 3H), 2.72−2.51 (m,
H), 2.42 (s, 3H), 2.40−2.30 (m, 1H), 2.21 (s, 3H), 1.89 (s, 3H), 1.66 (s,
H), 1.64 (s, 3H), 1.23 (s, 3H), 1.20 (s, 3H), 1.11 (s, 3H). 13C NMR
Present Address
†
Department of Chemical Engineering, Massachusetts Institute of
(
125 MHz, CDCl ): δ 203.8, 171.5, 171.2, 171.1, 169.8, 167.9, 167.3,
3
Technology, 77 Massachusetts Avenue, Cambridge, MA 02139;
Laboratory for Biomaterials and Drug Delivery, Department of
Anesthesiology, Division of Critical Care Medicine, Children’s
Hospital Boston, Harvard Medical School, 300 Longwood Avenue,
Boston, MA 02115.
1
1
9
4
1
67.0, 142.6, 138.4,137.2, 136.9, 133.7, 133.6, 132.8, 132.0, 130.2, 129.2,
29.1, 129.0, 128.7, 128.5, 128.3, 127.7, 127.6, 127.2, 126.6, 126.0, 101.4,
7.5, 84.4, 82.1, 81.0, 79.0, 76.4, 75.6, 75.1, 74.3, 73.1,72.1, 71.9, 58.5, 52.8,
9.1, 45.6, 43.2,35.6, 33.9, 29.7, 29.0, 28.9, 26.8, 25.6, 24.9, 22.7, 22.1, 20.8,
4.8, 9.6. ESI-MS (low resolution, positive mode): calculated for
+
+
C H NO , m/z 954.3 [M + H] ; found 954.8 [M + H] .
Notes
52
57
16
1
Ptxl-2′-7-diSA. H NMR (500 MHz, CDCl ): δ 8.13 (d, 2H), 7.77
The authors declare no competing financial interest.
3
(d, 2H), 7.61−7.36 (m, 15H), 7.26−7.24 (m, 6H), 6.21(s, 1H), 6.16
(s, 1H), 6.09 (d, 1H), 5.95 (dd, 1H), 5.86 (dd, 1H), 5.64 (m, 1H),
ACKNOWLEDGMENTS
■
5
1
3
8
.58 (d, 1H), 4.91 (d, 1H), 4.85−4.79 (m, 3H), 4.65 (d, J = 11.9 Hz,
H), 4.41 (s, 1H), 4.31 −4.27 (m, 2H), 4.17 (d, 1H), 3.99 (t, 1H),
.86 (m, 3H), 3.64 (t, J = 9.20 Hz, 1H), 3.31 (s, 3H), 2.8−2.5 (m,
H), 2.39 (s, 3H), 2.40−2.30 (m, 1H), 2.13 (s, 3H), 1.88 (s, 3H), 1.79
J.C. acknowledges support from the NSF (DMR-0748834), the
NIH (NIH Director’s New Innovator Award 1DP2OD007246-
01), and the Midwest Cancer Nanotechnology Training
Center, University of Illinois at Urbana−Champaign (UIUC).
R.T. acknowledges a student fellowship (2007−2010) from the
Siteman Center for Cancer Nanotechnology Excellence
(
s, 6H), 1.64 (s, 3H), 1.19 (s, 3H), 1.15 (s, 3H), 1.09 (s, 3H). 13
C
NMR (125 MHz, CDCl ): δ 203.8, 171.6, 169.8, 169.1, 168.3, 167.5,
3
1
1
1
6
2
66.8, 162.7, 141.2, 138.0, 136.8, 133.7, 133.5, 132.4, 132.0, 130.2,
29.1, 129.0, 128.7, 128.5, 128.2, 127.2, 126.7, 127.2, 126.6, 103.6,
00.2, 99.3, 83.9, 80.8, 78.5, 76.4, 75.8, 75.3, 74.4, 74.1, 71.7, 71.9,
5.8, 55.9, 53.1, 46.9, 43.2, 36.6, 35.2, 33.1, 31.5, 29.0, 28.8, 27.3, 26.4,
(
SCCNE, Washington University)−Center for Nanoscale
Science and Technology (CNST, UIUC).
4.7, 22.6, 21.1, 20.7, 15.2, 14.5, 10.8. ESI-MS (low resolution, positive
+
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1
■
55
59
20
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1
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4
7
57
17
+
+
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n
n
(
8.6 mg, 0.010 mmol) and (BDI-II)ZnN(TMS) (6.5 mg, 0.01 mmol)
2
were dissolved in 300 μL of THF. The mixture was stirred for 20 min.
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2
−1
polymerization was monitored by FT-IR; the 1739 cm peak of the
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−1
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the precipitate was washed with ether and toluene to remove BDI
ligand, dried under vacuum, and characterized by GPC. Complete
removal of BDI from Ptxl-PVL was verified by TLC. Similar
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