Effect of XlogP and Hansen Solubility Parameters on Small Molecule Modified Paclitaxel Anticancer Drug Conjugates Self-Assembled into Nanoparticles

Article abstract of DOI:10.1021/acs.bioconjchem.7b00767

Small molecule modified anticancer drug conjugates (SMMDCs) can self-assemble into nanoparticles (NPs) as therapeutic NP platforms for cancer treatment. Here we demonstrate that the XlogP and Hansen solubility parameters of paclitaxel (PTX) SMMDCs is essential for SMMDCs self-assembling into NPs. The amorphous state of PTX SMMDCs will also affect SMMDCs self-assembling into NPs. However, the antitumor activity of these PTX SMMDCs NPs decreased along with their XlogP values, indicating that a suitable XlogP value for designing the SMMDCs is important for self-assembling into NPs and for possessing antitumor activity. For higher level XlogP SMMDCs, a degradable linker should be considered in the design of SMMDCs to overcome the problem of lower antitumor activity. It is preferable that the hydrophilic groups in the SMMDCs should be present on the surface of self-assembling NPs.

Full text of DOI:10.1021/acs.bioconjchem.7b00767

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Article
The effect of XlogP and Hansen solubility parameters
on small molecule modified paclitaxel anti-cancer
drug conjugates self-assembled into nanoparticles
Xuan Zhang, Ting Zhong, Yan-Li Hao, Xin Yao, Shuang Zhang, Xiao-Chuan
Duan, Yi-Fan Yin, Mei-Qi Xu, Yang Guo, Zhan-Tao Li, Xiu-Chai Zheng, and Hui Li
Bioconjugate Chem., Just Accepted Manuscript • DOI: 10.1021/acs.bioconjchem.7b00767 • Publication Date (Web): 04 Jan 2018
Downloaded from http://pubs.acs.org on January 7, 2018
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The effect of XlogP and Hansen solubility parameters on small molecule
modified paclitaxel anti-cancer drug conjugates self-assembled into
nanoparticles
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Ting Zhonga,b, YanꢀLi Haoa,b, Xin Yaoa,b, Shuang Zhanga,b, XiaoꢀChuan Duana,b, YiꢀFan Yina,
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MeiꢀQi Xua,b, Yang Guoab, ZhanꢀTao Lia, XiuꢀChai Zhengb, Hui Lia, Xuan Zhang a,b
*
a Beijing Key Laboratory of Molecular Pharmaceutics and New Drug Delivery Systems, School
of Pharmaceutical Sciences, Peking University, Beijing 100191, China
b Department of Pharmaceutics, School of Pharmaceutical Sciences, Peking University, Beijing
100191, China
* Corresponding author:
Dr. Xuan Zhang
Professor
xuanzhang@bjmu.edu.cn
Department of Pharmaceutics,
School of Pharmaceutical Sciences,
Peking University,
Xueyuan Road 38,
Beijing 100191, China
Tel: +86ꢀ10ꢀ82805928
Fax: +86ꢀ10ꢀ82805928
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Abstract:
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Small molecule modified antiꢀcancer drug conjugates (SMMDCs) can selfꢀassemble
into nanoparticles (NPs) as therapeutic NP platforms for cancer treatment. Here we
demonstrate that the XlogP and Hansen solubility parameters of paclitaxel (PTX)
SMMDCs is essential for SMMDCs selfꢀassembling into NPs. The amorphous state
of PTX SMMDCs will also affect SMMDCs selfꢀassembling into NPs. However, the
antiꢀtumor activity of these PTX SMMDCs NPs decreased along with their XlogP
values, indicating that a suitable XlogP value for designing the SMMDCs is important
for selfꢀassembling into NPs and for possessing antiꢀtumor activity. For higher level
XlogP SMMDCs, a degradable linker should be considered in the design of SMMDCs
to overcome the problem of lower antiꢀtumor activity. It is preferable that the
hydrophilic groups in the SMMDCs should be present on the surface of
selfꢀassembling NPs.
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Keywords
Small molecule modified antiꢀcancer drug conjugates (SMMDCs); Paclitaxel (PTX);
Selfꢀassemble; Nanoparticles (NPs); XlogP; Hansen solubility parameters.
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Introduction:
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Several therapeutic nanoparticle (NP) platforms, such as liposomes, lipid NPs,
albumin NPs and polymeric micelles, have been widely used as delivery vehicles for
antiꢀcancer drugs.1ꢀ6 However, the ratio of drug to carrier material in these NPs is
relatively low (below 1:10 or less).
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Although many polymerꢀdrug conjugates have been reported to selfꢀassemble forming
nanomedicines by themselves, the drug occupancy rate of these polymerꢀdrug
conjugates is not very high since the molecular weight of the polymer is higher than
that of the antiꢀcancer drug.7, 8
In order to increase the drug occupancy rate, small molecule modified antiꢀcancer
drug conjugates (SMMDCs), which have a higher drug occupancy, have been
synthesized. Interestingly, these SMMDCs can selfꢀassemble into NPs in water and
the antiꢀtumor activity of these SMMDCs was confirmed in vitro and in vivo.
Therefore, many SMMDCs have been designed and prepared for selfꢀassembling into
NPs.9ꢀ17 However, to some extent, the selection of small molecular weight agents and
antiꢀcancer drugs for preparing SMMDCs has been seemingly random. The
supporting principle for the selection of small molecular weight agents and
antiꢀcancer drugs has been unclear. The intrinsic characteristic change of SMMDCs
compared with that of small molecular weight agents and antiꢀcancer drugs has also
been unclear.
It is wellꢀknown that many antiꢀcancer drugs have a high lipophilicity and low
waterꢀsolubility and so we wanted to know whether the lipophilicity and solubility of
the SMMDCs were changed compared with that of small molecular weight agents and
antiꢀcancer drugs. Also, it is important to know whether these changes are key factors
for SMMDCs selfꢀassembling into NPs.
Here we selected paclitaxel as an antiꢀcancer drug to prepare a series of small
molecule modified antiꢀcancer drug conjugates (SMMDCs) and used the XlogP (the
theoretical nꢀoctanolꢀwater partition coefficient calculated by the XLOGP3 method,
representing the lipophilicity) and Hansen solubility parameters (HSPs, representing
the waterꢀsolubility) as significant factors to investigate the relationship between these
parameters for selfꢀassembly into NPs.
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Results & Discussion
The effect of XlogP and Hansen solubility parameters on SA-PTX, OA-PTX,
CLA-PTX and ALA-PTX self-assembled into NPs
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Recently, it has been reported that oleic acidꢀSꢀPTX (OAꢀSꢀPTX) can selfꢀassemble
into NPs.12 The reported mechanism may be due to the intermolecular π−π stacking
facilitated by the unsaturated alkyl chains in oleic acid with the planar structures of
PTX.12 Considering the results of our previous research, we know that the conjugated
linoleic acidꢀpaclitaxel conjugate (CLAꢀPTX) can selfꢀassemble into NPs in water.18
Therefore, we considered whether the conjugated double bond in CLA was also
essential for CLAꢀPTX selfꢀassembling into NPs. So, we selected stearic acid (SA),
oleic acid (OA) and alphaꢀlinolenic acid (ALA) to synthesize the SAꢀPTX, OAꢀPTX
and ALAꢀPTX, respectively. SA, OA, CLA and ALA are all straightꢀchain fatty acids
with a chain length of 18 carbon atoms, but containing different unsaturated bonds (no
unsaturated bond (SA), a double bond (OA), two double bonds (CLA) or three
separate double bonds (ALA).
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We investigated whether SAꢀPTX, OAꢀPTX or ALAꢀPTX could also selfꢀassemble
into NPs. Our results indicated that, like CLAꢀPTX, SAꢀPTX, OAꢀPTX and
ALAꢀPTX all spontaneously formed NPs in water with a particle size in the range
80ꢀ170 nm, as shown in Fig. 1A, 1B and S1, indicating that the unsaturated bond
contained in the fatty acids was not a key factor for fatty acidꢀPTX conjugates
selfꢀassembling into NPs.
We determined the retention time of SAꢀPTX, OAꢀPTX, CLAꢀPTX and ALAꢀPTX in
order to determine the hydrophobicity of SAꢀPTX, OAꢀPTX, CLAꢀPTX and
ALAꢀPTX compared with PTX. Our results indicated that the retention times of
SAꢀPTX, OAꢀPTX, CLAꢀPTX and ALAꢀPTX were all longer than that of PTX (Fig.
1C), showing that the hydrophobicity of SAꢀPTX, OAꢀPTX, CLAꢀPTX and
ALAꢀPTX was increased compared with that of PTX due to the introduction of fatty
acid.
It is well known that the nꢀoctanolꢀwater partition coefficient is an important
parameter for evaluating hydrophobicity and lipophilicity. Therefore, the theoretical
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nꢀoctanolꢀwater partition coefficient (expressed by XlogP) of PTX, SAꢀPTX,
OAꢀPTX, CLAꢀPTX and ALAꢀPTX was calculated using the XLOP3 method.19 As
shown in Fig. 1D, the calculated XlogP values of SAꢀPTX, OAꢀPTX, CLAꢀPTX and
ALAꢀPTX were all significantly higher than that of PTX.
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According to theoretical concepts,20 the total cohesion can be divided into three
individual components, which are the group contribution to the dispersion forces, the
group contribution to the polar forces, and the group contribution to the
hydrogenꢀbonding energy. The corresponding solubility parameters were divided into
three parts: dispersion solubility parameters (δd), polar solubility parameters (δp) and
hydrogenꢀbonding solubility parameters (δh). The total solubility parameter (dt) is also
called the threeꢀdimensional solubility parameter. These four solubility parameters
(also called Hansen solubility parameters) can be calculated by using the group
contribution method (GCM) proposed by Beerbower.21 In the present research, the
Hansen solubility parameters (δd, δp, δh and δt) of PTX, SAꢀPTX, OAꢀPTX,
CLAꢀPTX and ALAꢀPTX were calculated by using the GCM. As shown in Fig. 1E,
the results indicated that the Hansen solubility parameters of SAꢀPTX, OAꢀPTX,
CLAꢀPTX and ALAꢀPTX were all significantly lower than that of PTX.
According to these findings, we speculated that the increased XlogP and the decreased
Hansen solubility parameters of SAꢀPTX, OAꢀPTX, CLAꢀPTX and ALAꢀPTX, owing
to the introduction of a fatty acid, might be one of the key factors for SAꢀPTX,
OAꢀPTX, CLAꢀPTX and ALAꢀPTX selfꢀassembling into NPs. Considering the results
of Xꢀray di raction (Fig.1F), SAꢀPTX, OAꢀPTX, CLAꢀPTX and ALAꢀPTX, unlike
PTX, were in an amorphous state. We believe that a compound with a higher
lipophilicity and lower water solubility could easily precipitate in water. If its
crystallinity decreased, then this compound would precipitate as spheres forming
nanoparticles in water.
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Fig.1. The effect of XlogP and Hansen solubility parameters on SA-PTX, OA-PTX,
CLA-PTX and ALA-PTX self-assembling NPs.
A: The structures of PTX, SAꢀPTX, OAꢀPTX, CLAꢀPTX and ALAꢀPTX and TEM images of
PTX, SAꢀPTX, OAꢀPTX, CLAꢀPTX and ALAꢀPTX NPs.
B: Particle size of PTX, SAꢀPTX, OAꢀPTX, CLAꢀPTX and ALAꢀPTX NPs.
C: The retention time of PTX, SAꢀPTX, OAꢀPTX, CLAꢀPTX and ALAꢀPTX determined by high
performance liquid chromatography (HPLC).
D: Theoretical nꢀoctanolꢀwater partition coefficients (XlogP) of PTX, SAꢀPTX, OAꢀPTX,
CLAꢀPTX and ALAꢀPTX calculated using XLOGP3.
E: Hansen solubility parameters (HSPs) of PTX, SAꢀPTX, OAꢀPTX, CLAꢀPTX and ALAꢀPTX
according to Beerbower, including dispersion (δd), polar (δp) and hydrogen bonding (δh,) and total
solubility parameter (δt).
F: The Xꢀray diffraction (XRD) spectra of PTX, SAꢀPTX, OAꢀPTX, CLAꢀPTX and ALAꢀPTX.
The effect of XlogP and Hansen solubility parameters on straight-chain fatty
acid-PTX self-assembled into NPs
In order to confirm the importance of XlogP and Hansen solubility parameters for
selfꢀassembling NPs, we selected a series of straightꢀchain fatty acids (acetic acid (Ac,
2C), butyric acid (BA, 4C), hexanoic acid (HA, 6C), octanoic acid (OcA, 8C),
decanoic acid (DA, 10C), lauric acid (LA, 12C), myristic acid (MA, 14C), palmitic
acid (PA, 16C), and arachidic acid (AA, 20C)) and then we designed and synthesized
a series of straightꢀchain fatty acidꢀPTX conjugates (AcꢀPTX, BAꢀPTX, HAꢀPTX,
OcAꢀPTX, DAꢀPTX, LAꢀPTX, MAꢀPTX, PAꢀPTX and AAꢀPTX).
The calculated XlogP values of these fatty acidꢀPTX conjugates was all higher than
that of PTX and increased with the chain length of the fatty acids (Fig.2A). Their
Hansen solubility parameters were lower than that of PTX and decreased with the
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chain length of the fatty acids (Fig.2B). The selfꢀassemble forming NPs of these fatty
acidꢀPTX conjugates were investigated. As shown in Fig.2C, all these fatty acidꢀPTX
conjugates spontaneously formed NPs in water with a particle size in the range
90ꢀ160 nm (Fig.2D and S1), indicating that the increased XlogP and the decreased
Hansen solubility parameters might be key factors for straightꢀchain fatty acidꢀPTX
conjugates selfꢀassembling into NPs.
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The retention time and Xꢀray diffraction of these fatty acidꢀPTX conjugates were also
determined. The retention times of these fatty acidꢀPTX conjugates were all longer
than that of PTX and increased with the chain length of the fatty acid (Fig.2E). All
these fatty acidꢀPTX conjugates were in an amorphous state (Fig.2F).
A
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Fig.2. The effect of XlogP and Hansen solubility parameters on straight-chain fatty
acid-PTX self-assembling NPs
A: Theoretical nꢀoctanolꢀwater partition coefficients (XlogP) of PTX and straightꢀchain fatty
acidꢀPTX calculated by XLOGP3.
B: Hansen solubility parameters (HSPs) of PTX and straightꢀchain fatty acidꢀPTX according to
Beerbower, including the dispersion (δd), polar (δp) and hydrogen bonding (δh) and total solubility
parameter (δt).
C: The structures of PTX and straightꢀchain fatty acidꢀPTX and TEM images of PTX, and
straightꢀchain fatty acidꢀPTX NPs.
D: Particle size of PTX and straightꢀchain fatty acidꢀPTX NPs.
E: The retention time of PTX and straightꢀchain fatty acidꢀPTX determined by high performance
liquid chromatography (HPLC).
F: The Xꢀray diffraction (XRD) spectra of PTX and straightꢀchain fatty acidꢀPTX.
The effect of XlogP and Hansen solubility parameters on B-G-PTX, BeA-PTX
and SaA-PTX self-assembled into NPs
In order to further confirm the importance of the XlogP and Hansen solubility
parameters for selfꢀassembling NPs, we designed and synthesized the
Nꢀtertꢀbutoxycarbonyl glycineꢀpaclitaxel conjugate (BꢀGꢀPTX), in which the
Nꢀ(tertꢀbutoxycarbonyl) glycine had a branched structure and heteroatomꢀcontaining
small molecule, the benzoic acidꢀpaclitaxel conjugate (BeAꢀPTX), in which the
benzoic acid was the simplest aromatic acid, and the salicylic acidꢀpaclitaxel
conjugate (SaAꢀPTX).
The calculated XlogP values of BꢀGꢀPTX, BeAꢀPTX and SaAꢀPTX was all higher
than that of PTX (Fig.3A). Most of the Hansen solubility parameters BꢀGꢀPTX,
BeAꢀPTX and SaAꢀPTX were lower than that of PTX (Fig.3B). We found that
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BꢀGꢀPTX, BeAꢀPTX and SaAꢀPTX spontaneously formed NPs in water with a
particle size in the range of 100ꢀ130 nm (Fig.3C, 3D and S1). These results indicated
that the increased XlogP and decreased Hansen solubility parameters might be also
key factors for nonꢀstraightꢀchain fatty acidꢀPTX conjugates selfꢀassembling into
NPs.
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As shown in Fig.3E, the retention times of BꢀGꢀPTX, BeAꢀPTX and SaAꢀPTX were
all longer than that of PTX. In addition, BꢀGꢀPTX, BeAꢀPTX and SaAꢀPTX were all
in an amorphous state (Fig.3F).
The effect of XlogP and Hansen solubility parameters on reported PTX
SMMDCs self-assembled into NPs
It has been reported that several PTX SMMDCs can selfꢀassemble into NPs, and these
PTX SMMDCs include SQꢀPTX,9 SQꢀsuccꢀPTX,9 SQꢀdiglꢀPTX,9 SQꢀPEG3ꢀPTX,9
SQꢀPEG11ꢀPTX,9 VEꢀSꢀSꢀPTX,10 OAꢀSꢀSꢀPTX,11 OAꢀSꢀPTX,12 C18ꢀSꢀSꢀPTX13 and
PTXꢀSꢀSꢀPTX.14 The XlogP and Hansen solubility parameters of these PTX
SMMDCs were calculated. As shown in Fig.3A and 3B, all the XlogP values were
increased compared with PTX and all the Hansen solubility parameters were
decreased compared with PTX.
Considering the XlogP values of these reported PTX SMMDCs, we found that the
XlogP was increased 1.75ꢀ to 3.25ꢀfold compared with PTX (Fig. 3G). For our
synthetic PTX SMMDCs, such as AcꢀPTX, BAꢀPTX, HAꢀPTX, BꢀGꢀPTX, BeAꢀPTX
and SaAꢀPTX, we found that the increases in their XlogP were all below 1.0ꢀfold
compared with PTX (Fig. 3G). In addition, the selfꢀassembling NPs of SaAꢀPTX and
BꢀGꢀPTX were found to become unstable if kept for two days. Although the increased
XlogP values of SaAꢀPTX and BꢀGꢀPTX were not the lowest among AcꢀPTX,
BAꢀPTX, HAꢀPTX, BꢀGꢀPTX, BeAꢀPTX and SaAꢀPTX, we found that the Hansen
solubility parameters of SaAꢀPTX and BꢀGꢀPTX were more similar to that of PTX,
especially the calculated δp and δh values which were only decreased about 9.6% and
3.8% (SaAꢀPTX), 2.6% and 8.6% (BꢀGꢀPTX) (Fig. 3H), respectively.
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Fig.3. The effect of XlogP and Hansen solubility parameters on B-G-PTX, BeA-PTX and
SaA-PTX self-assembling NPs and reported PTX SMMDC NPs
A: Theoretical nꢀoctanolꢀwater partition coefficients (XlogP) of PTX and BꢀGꢀPTX, BeAꢀPTX,
SaAꢀPTX and reported PTX SMMDCs calculated by XLOGP3.
B: Hansen solubility parameters (HSPs) of PTX and BꢀGꢀPTX, BeAꢀPTX, SaAꢀPTX and PTX
SMMDCs according to Beerbower, including dispersion (δd), polar (δp) and hydrogen bonding (δh)
and total solubility parameter (δt).
C: The structures of PTX and BꢀGꢀPTX, BeAꢀPTX and SaAꢀPTX and TEM images of PTX and
BꢀGꢀPTX, BeAꢀPTX and SaAꢀPTX NPs.
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D: Particle size of PTX and BꢀGꢀPTX, BeAꢀPTX and SaAꢀPTX NPs.
E: The retention time of PTX and BꢀGꢀPTX, BeAꢀPTX and SaAꢀPTX determined by high
performance liquid chromatography (HPLC).
F: The Xꢀray diffraction (XRD) spectra of PTX and BꢀGꢀPTX, BeAꢀPTX and SaAꢀPTX.
G: The ꢁXlogP values of our synthetic PTX SMMDCs and reported PTX SMMDCs
(ꢁXlogP=(XlogP value of PTX SMMDCs ꢀ XlogP value of PTX)/ XlogP value of PTX).
H: The ꢁHansen solubility parameters values of our synthetic PTX SMMDCs and reported PTX
SMMDCs (ꢁHansen solubility parameters=(Hansen solubility parameters value of PTXꢀHansen
solubility parameters value of PTX SMMDCs)*100%/ Hansen solubility parameters value of
PTX).
Considering the results of the XlogP and Hansen solubility parameters of our
synthetic PTX SMMDCs and reported PTX SMMDCs, we suggested that for
designing selfꢀassembling PTX SMMDC NPs, the XlogP value of PTX SMMDCs
should increase more than 1.0ꢀfold compared with that of the parent drug PTX,
otherwise, their Hansen solubility parameters of δp and δh, especially the δh value,
would decrease more than 10% compared with the parent drug PTX.
The effect of XlogP and Hansen solubility parameters on SMMDCs
self-assembled into NPs
The lipophilicity of compounds can be correlated with the pharmacodynamic drug
property. Increasing lipophilicity will reduce solubility22 and, so, if a compound has a
higher lipophilicity, it will tend to precipitate in water. In addition, the crystallinity of
organic compounds is influenced by strong hydrogen bonding and the dipole force
between molecules. Compounds exhibiting strong hydrogen bonding and a dipole
force tend to form oneꢀdimensional nanostructures, while compounds with weak
hydrogen bonding and a weak dipole force readily form nanoparticles.23 Here, we
selected XlogP as the parameter for evaluating the compound lipophilicity and the
polar solubility (δp) and hydrogen bonding solubility (δh) as parameters for evaluating
the compound crystallinity.
Considering the XlogP results of our synthetic PTX SMMDCs (16 compounds) and
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reported PTX SMMDCs (10 compounds), we suggested that for designing
selfꢀassembling SMMDC NPs, the XlogP value of SMMDCs should increase more
than 1.0ꢀfold compared with that of the parent drug, otherwise the Hansen solubility
of the polar solubility parameter (δp) and hydrogen bond solubility (δh), especially the
δh value, should decrease more than 10% compared with that of the parent drug. The
amorphous state of SMMDCs also had an impact on SMMDCs selfꢀassembling into
NPs.
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The effect of XlogP on the in vitro anti-tumor activity of SMMDCs self-assembled
into NPs
Lipophilicity is also
a
very commonly used parameter in Quantitative
StructureꢀActivity Relationship (QSAR) analysis for predicting the biological activity
of a compound. Here, we examined the effect of the Xlogp of PTX SMMDCs on the
antiꢀtumor activity of these PTX SMMDC NPs. The in vitro antiꢀtumor activity of our
synthetic PTX SMMDC NPs was investigated in MCFꢀ7 cells and MCFꢀ7/A cells. We
selected the IC50 value as a parameter which was defined as the drug concentration
which inhibited the tumor cell growth by 50%. A higher IC50 value means lower in
vitro antiꢀtumor activity. As shown in Fig. 4A and 4B, the IC50 value of PTX SMMDC
NPs was increased along with the XlogP values (Fig. 4A vs Fig. 2A, Fig. 4B vs Fig.
2A), indicating that the in vitro antiꢀtumor activity of SMMDC NPs was reduced
along with the increased XlogP values. For designing SMMDCs with potential ability
to selfꢀassemble into NPs, their XlogP should increase at a relatively higher level
compared with that of the parent drug, however, the in vitro antiꢀtumor activity of
these SMMDCs NPs decreased along with the XlogP, indicating that a suitable XlogP
value for designing SMMDCs is important for selfꢀassembling into NPs and for
possessing antiꢀtumor activity.
In addition, we also investigated the in vitro antiꢀtumor activity of these PTX
SMMDCs in MCFꢀ7 cells. As shown in Supporting Information, the IC50 value of
PTX SMMDCs was increased along with the XlogP values (Fig. S2), similar with that
of PTX SMMDC NPs.
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Considering to the reported PTX SMMDCs, their XlogP values were relative high,
however, some PTX SMMDCs contained disulfide bonds as a linker, such as
VEꢀSꢀSꢀPTX,10 OAꢀSꢀSꢀPTX,11 OAꢀSꢀPTX,12 C18ꢀSꢀSꢀPTX13 and PTXꢀSꢀSꢀPTX.14
These disulfide bonds could respond to the tumor microenvironment to release the
parent drug PTX. For SQꢀPTX,9 SQꢀsuccꢀPTX,9 SQꢀdiglꢀPTX,9 SQꢀPEG3ꢀPTX9 and
SQꢀPEG11ꢀPTX,9 the in vitro antiꢀtumor activity of SQꢀsuccꢀPTX and SQꢀdiglꢀPTX
was relatively higher than that of the others being more susceptible to hydrolysis to
form PTX. The in vitro antiꢀtumor activity of SQꢀPTX, with a higher XlogP value,
was less potent than free PTX. Therefore, for a higher XlogP, the degradable linker
should be considered in the design of SMMDCs for overcoming the problem of lower
antiꢀtumor activity.
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Fig.4. The effect of XlogP on the in vitro anti-tumor activity of PTX SMMDCs
self-assembling NPs.
The in vitro antiꢀtumor activity of our synthetic PTX SMMDC NPs was investigated in MCFꢀ7
cells (A) and MCFꢀ7/A cells (B).
Probable structure of SMMDCs self-assembling NPs
It is well known that amphiphilic block copolymers are spontaneously formed when
polymer micelles composed of a hydrophilic shell and a lipophilic core are dispersed
in water. For our synthetic selfꢀassembling PTX SMMDCs, the small molecular
weight compounds and PTX were all hydrophobic compounds, however, the
hydrophilic groups, such as hydroxyl, is still present in PTX. The hydrophobic groups
preferred to come together due to hydrophobic forces, while the hydrophilic groups
tended to extend into the water, resulting in the formation of a hydrated layer.
Therefore, the hydrophobic groups of small molecular weight compounds
preferentially exist in the inner region of selfꢀassembling NPs, while, the antiꢀtumor
drug, containing hydrophilic group, preferentially are present on the surface of
selfꢀassembling NPs (Fig. 5). In order to confirm this hypothesis, we investigated the
zeta potential of our synthetic PTX SMMDC selfꢀassembling NPs. As shown in Fig. 6,
the zeta potentials of PTX NPs were all negative. Also, a negative zeta potential has
also been reported in PTX SMMDC selfꢀassembling NPs, such as SQꢀPTX NP
(ꢀ36.88 mV),9 SQꢀsuccꢀSQ NP (ꢀ44.33 mV),9 SQꢀdiglꢀPTX NP (ꢀ38.03 mV),9
SQꢀPEG3ꢀPTX NP (ꢀ41.42 mV),9 SQꢀPEG11ꢀPTX NP (ꢀ40.88 mV),9 VEꢀSꢀSꢀPTX NP
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(−29.2 mV),10 C18ꢀSꢀSꢀPTX NP (ꢀ26.2 ± 0.64 mV)13 and PTXꢀSꢀSꢀPTX NP (−33.9 ±
0.777 mV).14 While, a positive zeta potential has been reported irinotecan (Ir)
SMMDC selfꢀassembling NPs (CbꢀIr NP, +3.4 mV).17 We suggested that there was a
hydroxyl group in PTX, while, there was a piperidyl group in Ir. These results
confirmed that hydrophilic groups are preferentially located on the surface of
selfꢀassembling NPs.
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Fig.5. The schematic diagram of SMMDC self-assembling into NPs
The hydrophobic groups in small molecular weight agents preferred to gather together due to
hydrophobic forces, while the hydrophilic groups in the antiꢀtumor drug tended to stretch into the
water, resulting in the formation of a hydrated layer owing to hydration. Therefore, the
hydrophobic group of small molecular weight agents preferred to exist in the inner part of
selfꢀassembling NPs, while, the antiꢀtumor drug, containing hydrophilic groups, preferred to be
located on the surface of selfꢀassembling NPs.
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Fig.6. The zeta potential of our synthetic PTX SMMDC NPs.
Conclusion
We suggest that for designing selfꢀassembling SMMDC NPs, the XlogP value of the
SMMDCs should increase more than 1.0ꢀfold compared with that of the parent drug,
otherwise, their Hansen solubility of the polar solubility parameter (δp) and hydrogen
bond solubility (δh), especially the δh value, should decrease more than 10% compared
with the parent drug. The amorphous state of SMMDCs should also have an effect for
SMMDCs selfꢀassembling into NPs. However, the antiꢀtumor activity of these
SMMDCs NPs decreased along with the XlogP, indicating that a suitable XlogP value
for designing SMMDCs is important for selfꢀassembling into NPs and for exhibiting
antiꢀtumor activity. For higher level XlogP SMMDCs, a degradable linker should be
considered in the design SMMDCs to overcome the problem of lower antiꢀtumor
activity. The hydrophilic groups in the SMMDCs, should preferentially be located on
the surface of selfꢀassembling NPs.
Experimental Procedures
Materials
Paclitaxel (PTX) was obtained from Ouhe Co., Ltd. (Beijing, China). Other materials
are shown in Supporting Information.
Synthesis of PTX SMMDCs
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The detailed processes for PTX SMMDCs are shown in Supporting Information. In
brief, PTX SMMDCs were synthesized through a direct esterifying reaction on the
C2’ꢀhydroxyl of PTX catalyzed by DCC/DMAP.24
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Characterization of SMMDCs
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Crystallinity The crystallinity of all samples was examined by powder Xꢀray
diffraction (XRD) using a D/MAX 2000 rotating anode Xꢀray diffractometer (Rigaku
Co., Japan) equipped with a CuꢀKα Xꢀray source (λ = 1.541 nm, 40 kV /100 mA),18 as
shown in Supporting Information.
Polarity The polarity of all samples was evaluated by high performance liquid
chromatography (HPLC), involving an LCꢀ20AT liquid chromatograph (SHIMADZU,
Japan) and SPDꢀM20A diode array detector (SHIMADZU, Japan), as shown in
Supporting Information.
Theoretical partition coefficient: XlogP value
The theoretical partition coefficient of each compound was calculated using the
XLOGP3 developed by the Wang group.19 Threeꢀdimensional structural models of
each compound were constructed with the ChemBio3D Ultra 2010 software. All final
models were saved in Tripos Mol2 format as input data for the calculation.
Hansen solubility parameters
Based on the concept of dividing the total cohesive energy into individual components,
the solubility parameters are constituted of three parts: dispersion (
δd), polar (δp) and
hydrogen bonding ( t), also called the
δ
h).20 The total solubility parameter (
δ
threeꢀdimensional solubility parameter, can be defined as follows:
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δt=(δd +δp +δh )2
Only requiring knowledge of the compound’s chemical structure, the group
contribution method (GCM) is a commonly used theoretical method to calculate the
HSPs. In this method, the cohesive energy of a molecule is considered to be an
additive property and is the sum of the contributions from individual groups contained
in the molecule. Using the GCM proposed by Beerbower,21 the disperse solubility
parameter (δd), polar solubility parameter (δp) and hydrogen bonding solubility
parameter (δh) and the total solubility parameter (δt) were calculated as follows:
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∑
i Fd
i
δd=
∑
i Vi
ꢀ
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∑
(
i Fp )
i
δp=
δh=
∑
i Vi
∑
i Fh
i
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ꢀ
ꢁ
∑
(
i Vi
)
where i is the structural group within the molecule, Fd is the group contribution to
i
the dispersion forces, Fp is the group contribution to the polar forces, Fh is the
i
i
group contribution to the hydrogenꢀbonding energy, and Vi is the group contribution
to the molar volume of drug compounds or conjugates. The detail calculation is
shown in Supporting Information.
Preparation and characterization of self-assembled NPs
All the selfꢀassembled NPs in the present study were prepared by nanoꢀprecipitation,
as shown in our previous report.18 The particle size, particle size distribution and zeta
potential were measured by dynamic light scattering (DLS). A transmission electron
microscope (TEM) (JEMꢀ1400Plus, JEOL Ltd. Tokyo, Japan) was used to examine
the morphology of the NPs.
In vitro cytotoxicity studies
The cytotoxicity of selfꢀassembling NPs was evaluated against MCFꢀ7 cells and
MCFꢀ7/ADR cells using a sulfonyl rhodamine B (SRB) colorimetric assay, as shown
in our previous report.18, 25
Associated Content
Supporting Information
The Supporting Information is available free of charge on the ACS Publications
website at DOI:
Additional details on materials, synthesis and characterization of PTX SMMDDCs, in
vitro cytotoxicity studies, structural characteristics of PTX and synthetic PTX
SMMDCs, calculation of HSPs for PTX according to Beerbower, XlogP and HSPs of
PTX and PTX SMMDCs, DLS of PTX SMMDCs selfꢀassembled nanoparticles, in
vitro antiꢀtumor activity of PTX SMMDCs solution (PDF).
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Author Information
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Corresponding Authors
*Eꢀmail: xuanzhang@bjmu.edu.cn
ORCID
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Xuan Zhang
Acknowledgments
The authors gratefully acknowledge the financial support from the National Natural
Science Foundation of China (No. 81573360), the National Key Research and
Development Program of China (2017YFA0205603), the National Basic Research
Program of China (973 Program 2013CB932501) and National Natural Science
Foundation of China (number 81172992).
Conflict of Interest statement:
The authors declare no competing financial interest.
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