CHIE KOJIMA
4.2 Temperature-responsive Dendrimers
Hyperthermia therapy (thermotherapy) involves the killing of cancer cells by exposure to high temperatures. This therapy is performed using clinically approved radio frequency ablation as a local heating system.29Temperature- responsive DDSs can be applied in conjunction with thermotherapy.
Temperature-sensitive polymers, of which poly(N-isopropylacrylamide) (PNIPAAm) is a representative, exhibit a phase transition at which their solubility drastically decreases. The ‘‘cloud point’’ or lower critical solution
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temperature (LCST) of PNIPAAm is 321C.1–6 The cloud point is heavily influenced by the balance between the hydrophobicity and hydrophilicity of the polymer and hence it can be tuned by its chemical composition.
4.2.1 Dendrimers Containing Thermo-sensitive Polymers
Temperature-sensitive polymers can be attached to the core or the terminal groups of dendrimers (Figure 4.2A and 4.2B). Kimuraet al.30reported the first temperature-responsive dendrimer, which was prepared by polymerization of NIPAAm from the termini of a polypropyleneimine (PPI) dendrimer with terminal thiol groups (Figure 4.2A). The core of the dendrimer was a cobalt complex, which catalyzed the temperature-dependent oxidation of the thiol compounds.30 The PNIPAAm-based copolymers, PNIPAAm- b-poly(dimethylaminoethyl methacrylate) and polycaprolactone-b-PNIPAAm, were also conjugated to the dendrimer termini.31,32 It was reported that the latter dendrimer acted as a temperature-dependent nanocapsule of daidzein, a traditional Chinese medicine. In this dendrimer, polymer layers of polycapro- lactone and PNIPAAm acted as drug reservoir and temperature sensor, respectively.32 For in vivo application, modification of polyethylene glycol (PEG) is indispensable. Zhaoet al.33 recently reported that both PNIPAAm and PEG-conjugated dendrimers could release indomethacin in a temperature- dependent manner.
Figure 4.2 Design of temperature-sensitive dendritic polymers. (A) Dendrimer with temperature-sensitive polymers attached at the surface, (B) dendritic polymer assembly with a temperature-sensitive polymer and (C) dendrimer modified with a temperature-sensitive moiety. Grey single balls and linked balls indicate temperature-sensitive moieties and temperature-sensitive polymers, respectively. (D) Temperature dependency of collagen-mimic dendrimer.
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Temperature-sensitive polymers can also be attached to the dendrimer core (Figure 4.2B). NIPAAm was polymerized from the core of polyol dendrons, to produce a temperature-dependent dendritic polymer. These polymers underwent self-assembly as a function of temperature.34,35 Stover et al.36reported the controlled release of ceramide, a pro-apoptotic drug, using a dendritic polymer composed of poly(L-lactide)-co-NIPAAm and a poly
L-lysine dendrimer. The core block polymer associated with ceramide in a temperature-dependent manner. In addition, cellular uptake of the dendritic polymer was sensitive to temperature. The drug action induced by this nano- particle was of a similar magnitude to that induced by the free drug or the liposomal drug formulation at 371C.36
4.2.2 Dendrimers Containing Thermo-sensitive Moieties
Incorporation of temperature-sensitive components to dendritic polymers may impart temperature-responsiveness to the dendrimer. Kono’s group reported that temperature-sensitive dendrimers could be prepared by modification with only one temperature-sensitive unit at the surface (Figure 4.2C).37–39They first reported the temperature-sensitivity of PAMAM and PPI dendrimers after modification with isobutyric acid to provide the isobutylamide (IBAM) group on the dendrimer surface, which is mimetic to the temperature-dependent poly(N-vinylisobutyramide).37 The temperature-sensitivity was largely dependent on the generation number or the molecular weight, different from thermo-sensitive linear polymers. In addition, these dendrimers were also influenced by pH because of their inner tertiary amine.37 Dendrimers with NIPAAm at the surface were also synthesized as dendritic analogs of PNIPAAm by reacting isopropylamine with succinylated PAMAM dendrimers.38 Linear PNIPAAm has an endothermal peak near the cloud point, which contributes to dehydration of the polymer. In contrast, temperature-sensitive dendrimers have an extremely small endothermal peak. Therefore, these dendrimers are much different from the linear structured temperature-sensitive polymers.38PAMAM dendrimers were also modified with phenylalanine (Phe) instead of NIPAAm and IBAM.39 This modification also induced thermo-sensitivity under physi- ological pH. Conversely, leucine and isoleucine were not able to communicate such responsiveness. Tuning of the phase transition temperature to values close to body temperature is crucial for the application of these dendrimers to DDSs.
For example, the Phe/dendrimer ratio determines the cloud point.39One of the advantages of dendrimers is their ability to encapsulate small molecules. The guest molecule (e.g. rose bengal, RB) can also influence the temperature- sensitivity of the dendrimers.40 We recently synthesized dendritic lipids with IBAM groups. They were assembled into vesicles and tubular micelles in aqueous solution, whose size and morphology were dependent on temperature.41 Asthmanikandan et al.42 and Chang and Dai43 reported oligo(ethylene glycol)-bound dendritic compounds that exhibited temperature-dependent phase transitions. These dendrimers bear hydrophobic and hydrophilic regions and form dendritic micelles, whose morphology changes as a function of
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temperature. Thayumanavan’s group reported that Rhodamine 6G could be encapsulated by these dendritic micelles.42Liet al.44reported the temperature- responsiveness of oligo(ethylene glycol)-containing dendrimers, which is dependent on both the generation number and the terminal structure.
The cloud point of the ethoxy-terminated dendrimers was lower than that of the methoxy-terminated ones, due to the greater hydrophobicity of the former.
Differently from the decrease in cloud point observed for larger linear polymers, Li’s dendrimers of higher molecular weight exhibited a higher cloud point.44Therefore, such dendrimers are unique temperature-sensitive polymers.
Taken together, oligo(ethylene glycol)-bound dendritic polymers are another class of candidate materials for temperature-sensitive DDSs.
4.2.3 Collagen-mimic Dendrimers
Collagen is the most abundant protein in mammals and is composed of glycine-proline-(hydroxy)proline (Gly-Pro-Pro (Hyp)) repeats, which form a triple helix in a temperature-dependent manner.45–47 The triple helical structure is formed at low temperature, but it dissociates above the melting point. Thermal denaturation of collagen at high temperature results in the formation of gelatin. The temperature-dependent behavior of collagen is different from that of temperature-sensitive synthetic polymers that have an LCST. In the collagen, the temperature-dependent behavior is induced by alteration of its higher order structure, while in the synthetic polymers it is due to dehydration. Therefore, collagen-related materials provide an alternative temperature-dependent material. Unfortunately, it is difficult for short collagen peptides to form a triple helix, which limits the development of artificial collagen materials. However, a covalent knot of collagen peptides can induce triple helix formation.45 We reported a collagen model peptide (Pro-Pro-Gly)5-attached dendrimer, in which the peptides knotted at the surface of the dendrimer formed a collagen-like triple helix.48 Interestingly, unlike in natural collagens, helix formation by this dendrimer was thermally reversible. The collagen-mimic dendrimer could encapsulate a model drug, RB, and release it faster at high temperature. This thermo-sensitivity was based on temperature-dependent helix formation and not on a phase transition.
The formation of a collagen-like triple helix at lower temperature may improve the binding properties of RB to the dendrimer owing to enhanced hydrophobic interactions and/or suppressed permeability of RB at the surface (Figure 4.2D).48Because the temperature-dependency was insufficient for DDS applications, we also prepared different types of collagen-mimic dendrimers with (Pro-Hyp-Gly)n. Even though the temperature dependency of the release profiles from these dendrimers was much improved, further optimization is still required.49It was also reported that the (Pro-Pro-Gly)10and (Pro-Hyp-Gly)10 collagen-mimic dendrimers form temperature-dependent hydrogels, which dissolve above 401C and below 251C, respectively.50,51 These hydrogels are also useful as smart drug containers.
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