Graft and statistical copolymers are considered special cases of block copolymers. In the first case, B blocks are attached to the A backbone either regularly or randomly.
In the second case, the polymer is a linear chain whose alternating A and B blocks are short and have random lengths. These polymers often do not from micelles or other self-assembled systems spontaneously but can be used to create nanoparticles for biomedical purposes by different physicochemical procedures such as emulsifi- cation [63], complexation [64], and nanoprecipitation [65]. The latter technique has become quite popular in the past few years, since it allows a controlled preparation of well-defined nanoparticles.
k k
Ligand
Bilayer
Hydrophilic chain
Hydrophobic
chain Hydrophilic drug
Hydrophobic drug
Figure 8.6 Structure of a polymersome with both a hydrophobic and hydrophilic drug incor- porated [59]. (See color insert for color representation of this figure).
Polymer nanoparticles prepared by self-assembling thermoresponsive poly(N- isopropylacrylamide)-graft-poly[N-(2-hydroxypropyl)methacrylamide] copolymers with hydrolytically degradableN-glycosylamine groups between the polymer blocks were prepared [66] for delivery of diagnostic and therapeutic radionuclides into solid tumors. The nanoparticles are formed by fast heating an aqueous solution of the copolymer to 37.8∘C. They have a hydrodynamic diameter of 128 nm measured by dynamic light scattering, and they slowly degrade during incubation in aqueous buffer at pH = 7.4. Labeling with both 131I and 90Y proceeds with high yields (>85%). The unlabeled polymers are not cytotoxic for any of the tested murine and human cell lines.
Statistical graft copolymers of poly[N-(2-hydroxypropyl)methacrylamide and a different amount of cholesterol moieties attached to the backbone by a noncleavable spacer have been studied previously [67, 68]. Additionally, some of the copolymers were modified with the anticancer drug doxorubicin (Dox), which was bound via the pH-sensitive hydrazone bond. Dilute solutions of conjugates in phosphate buffers at pH=5.0 and 7.2 were examined by dynamic and static light scattering (DLS/SLS), small-angle X-ray, and neutron scattering—SAXS (Figure 8.7a) and SANS. The pres- ence of any amount of cholesterol proved to result in the formation of anisotropic nanoparticles. The obtained results show that the size and anisotropy of the nanopar- ticles grow with increasing amounts of cholesterol moieties (Figure 8.7b).
The SAXS and SANS experiments led to the determination of the 3D structure of the nanoparticles as composed of poly[N-(2-hydroxypropyl)methacrylamide-graft-
k k
(a)102
(d)
(c) (b)
Dox Cholesterol
0.000 1 0 0 0 0 0
γ, nm pc(I)
7 6 6
6 10
0.005 0.010 0.015 0.020 0.025 0.030 0.035 0.040
IIγ=1.4 mol%
γ=2.7 mol%
γ=2.8 mol%
γ=3.0 mol%
V VI VII 3.0 mol %
1.4 mol % 0 mol %
10–2
10–1 100
10 10 –1 100 101 10 2 10 3
2.0%
1.0%
0.5%
0.2%
10 4 5 106
I II IV V 101 VII
100 Is(q) Is(q), arbitrary units
10–1
10–1 100
0.0 0
0 5 0
r, nm Dmax increases 15 20 25 30 35 40 45 0.1
0.2 0.3 0.4
PDDF(r)
0.5 0.6 0.7
0.8 I pH=7.2
II pH=5.0 II pH=7.2 IV pH=5.0 IV pH=7.2 V pH=5.0 V pH=7.2 VI pH=5.0 VI pH=7.2 VII pH=5.0
q,nm–1
q, nm–1
Figure 8.7 (a) SAXS intensity profiles for conjugates with different amounts of cholesterol: I (0 mol%)—red, II (1.4 mol%)—cyan; IV (2.3 mol%)—green, V (2.7 mol%)—blue, and VII (3.0 mol%)—black. (b) Pair-distance-distribution function for some conjugates. (c) Cross-sectional pair-distance distribution functions, pc(r), for some conjugates. (d) 3D structure of HPMA-Dox-cholesterol NP with bound Dox [69]. (See color insert for color representation of this figure).
k k cholesterol conjugates. Fromab initio calculations and the shape of pair-distance
distribution function (Figure 8.7b), together with the shape of cross-sectional pair-distance distribution function (Figure 8.7c), it was concluded that the most probable structure of nanoparticles is the pearl-necklace type whereby the ellipsoidal pearls composed of cholesterol are mainly connected by bridges composed of hydrophilic HPMA copolymer chains (Figure 8.7d).
Self-assembled polymeric chelate nanoparticles were prepared [69, 70] as poten- tial theranostic agents for improving cancer diagnostics and therapy. The graft copoly- mers consist of a backbone containing 8-hydroxyquinoline-5-sulfonic acid-chelating groups and poly(ethylene oxide) hydrophilic grafts. The polymers assemble and form nanoparticles after the addition of a multivalent metal cation, such as Fe3+or Cu2+
(Figure 8.8).
The obtained nanoparticles exhibit a hydrodynamic diameter of around 25 nm and a stability of at least several hours, which are counted as essential parameters for biomedical purposes. Their biodegradability was confirmed by degradation with deferoxamine. Convenient usage of these nanoparticles for nuclear medicine pur- poses was demonstrated by their high radiolabeling efficiency with64Cu.
Thermoresponsive statistical polyoxazolines, poly[(2-butyl-2-oxazoline)-stat- (2-isopropyl-2-oxazoline)] [pBuOx-co-piPrOx] with different hydrophobic moi- eties were combined with the FDA-approved triblock copolymer Pluronic F127 (PEO-PPO-PEO) as a template system for the creation of thermosensitive nanopar- ticles [71]. It was shown that the presence of the thermosensitive F127 triblock copolymer in solution reduces the nanoparticle size and polydispersity. When the temperature is raised above the CPT, nanoparticles composed of polyoxazolines and F127 are dominant in the solution. The molecular weight and hydrophobicity of the polymer do not influence the outer radius and only slightly change the inner radius of the nanoparticles. Poly(2-oxazoline) molecules were fully incorporated inside of F127 micelles, and this result is very promising for the successful application of such systems in the delivery of radionuclides and bioactive compounds. Pluronic F127 and some similar polyamphiphiles are also known to be inhibitors of ABC cassette
O O
O OH O
OH H
H O
O
O CH3 CH3
CH3
CH3
OH OH O
H H
Figure 8.8 Scheme of reaction of the graft-polymer (PMMHA–PEO) with metal cation (Fe3+
or Cu2+) [71].
k k transporters such as P-glycoprotein, whose overexpression causes multidrug resis-
tance of tumor cells. Therefore such copolymers are able to restore the sensitivity of the cancer cells toward chemotherapy.
An interesting group of thermoresponsive graft copolymers was prepared by Hrubyet al. [72] and is based on glycogen-graft-poly(2-alkyl-2-oxazolines). These nanostructured hybrid dendrimeric stimuli-responsive polymers combine the body’s own biodegradable polysaccharidic dendrimer glycogen with the widely tunable thermoresponsive poly(2-alkyl-2-oxazoline)s, the latter being polypeptide analogues and are known to be biocompatible. Glycogen-graft-poly(2-alkyl-2-oxazoline)s were prepared by a simple one-pot, two-step procedure involving cationic ring-opening polymerization of 2-alkyl-2-oxazolines followed by termination of the living cationic ends with sodium glycogenate. As confirmed by light and X-ray scattering, as well as by cryotransmission electron microscopy, the grafted dendrimer structure allows easy adjustment of the cloud point temperature and the nanostructure of the self-assembled phase separated polymer by the graft composition, length, and the grafting density.
Aliphatic biodegradable copolyesters are a very promising group of copolymers for biologically applicable nanoparticles. Surfactant-free, narrowly distributed, nano- sized spherical particles (Rh <60 nm) have been produced from the poly(butylene succinate-co-butylene dilinoleate) (PBS/PBDL) by a single-step nanoprecipitation method [73]. Combined SLS and DLS measurements suggested that the nanoparticles comprise a porous core (Figure 8.9). The nanoparticles loaded with 6–7% paclitaxel had a pronounced stability and relatively rapid degradation. Cell viability experiments
Figure 8.9 Porosity of PBS/PBDL nanoparticles. Schematic structure reconstructed from SAXS by DAMMIF procedure [74].
k k have demonstrated that the nanoparticles are fully biocompatible and nontoxic, mak-
ing them useful for biomedical applications.
Their porosity enables water to be entrapped, which is responsible for their pro- nounced stability and relatively fast degradation as followed by size exclusion chro- matography (SEC). The polymeric nanoparticles could be loaded with the hydropho- bic model drug paclitaxel (PTX) with an encapsulation efficiency of∼95% and drug loading content of 6–7%. The drug encapsulation and release modifies the inner structure of the nanoparticles, which holds a large amount of entrapped water in the drug-free condition. PTX encapsulation leads to replacement of the entrapped water by the hydrophobic model drug and to shrinking of the nanoparticles, due to favorable drug–polymer hydrophobic interactions. Cell viability experiments demonstrated that the nanoparticles are biocompatible and nontoxic, making them potentially useful for applications in nanomedicine.