4 Synthesis and Fabrication of Nanoparticles for
1.4 Self-assembly of drug conjugates
Besides CPPs and CTPs self-assembling peptide, drug conjugates have also emerged as a potential subset of PepDCs. This novel approach utilized the drug as a building block of a supramolecular architecture for its self-delivery. This kind of PepDCs have gained significant attention for the past few years due to its unique features like (i) a fixed and higher drug loading content, (ii) avoidance of premature degradation and rapid clearance, (iii) retaining of inherent activity of the drug upon release from the conjugate. Several research groups have designed such PepDCs by conjugating a hydrophilic peptide to a hydrophobic drug and vice versa. Such a strategy was followed by Cui and co-workers at the time of developing a supramolecular filament comprised of camptothecin (CPT) (Figure 1.12), a widely used anticancer drug.70 They have conjugated CPT, a hydrophobic drug to a hydrophilic and β sheet forming peptide sequence derived from Tau protein through a reduction cleavable disulfylbutyrate (buSS) linker. By using the two amino group of lysine a branching point was created inside the molecule so that one, two even four CPT molecule can be attached with the whole system. All the molecules formed filamentous structure in water. They have also showed the controlled and steady release of bioactive CPT in glutathione responsive manner.
Figure 1.12 Design of a camptothecin peptide conjugate and its self-assembly mechanism.
As described earlier nanomaterials gained profound importance in the field of drug delivery since the emergence of this concept. Initial attempts were made by physically encapsulating the drug inside or on the surface of the nanocarriers. However later on the origination of covalent conjugation of drug with the carrier was able to tackle the drawbacks associated with physical encapsulation. Lately, researchers have combined the advantage of both nanosystems, drug conjugation by designing discrete amphiphiles with the conjugation of drugs with polymers, peptides, oligonucleotides, targeting agent, and sometimes even with another drugs. The basic idea of designing this kind of amphiphiles was to use the drug as a building block to generate diverse supramolecular structures. In this concept, instead of attaching or physically encapsulating the drug with the nanoparticles, it was used to create molecular building block, which can self-assemble by non-covalent forces, generating the nanostructure for its self-delivery. For example, Cui and his group conjugated the hydrophobic anticancer agent paclitaxel (PTX) with a β-sheet forming peptide to prepare the drug amphiphile which can self-assemble into filamentous nanostructures71 (Figure 1.13). 4-(pyridin-2-yl-disulfanyl)butyrate (buSS) was used as a connector between the drug and the peptide owing to its degradability in presence of glutathione (GSH), an enzyme known to be overexpressed in many cancer cells. This system provided quite a high 41% fixed loading of PTX, markedly improving it’s loading efficiency of the previously reported delivery systems. In addition, the conjugate was shown to have gained more stability due to its self-assembly into supramolecular architecture. In this report, it was also shown that the cytotoxic effect of the filaments remained comparable with the free PTX against a variety of cancer cell lines.
Figure 1.13 (A) Chemical structure of PTX-Tau drug amphiphile, (B) schematic presentation of its self-assembly into filamentous nanostructures.
Similarly, Agarwal and her group has also demonstrated a photo controlled biocompatible DDS based on coumarin functionalized block copolymers.72 A micelle forming drug conjugate was made by a covalent attachment of 5-Fu with the coumarin functionalized block co-polymers by photoirradiation at ˃ 310nm (Figure 1.14). The controlled release of 5-Fu from the micelle was shown by in vitro drug release experiment under UV irradiation of 254 nm.
Figure 1.14 Schematic illustration of photo-controlled release of 5-Fu from the micelle comprised of the polymer 5-Fu conjugate.
Apart from polymers peptides drug amphiphiles were also designed by conjugation of oligonucleotides with it. Zhang and his group designed a camptothecin (CPT) oligonucleotide amphiphile by connecting three CPT molecules with DNA strands via a self-immolative and a photolabile o-nitrobenzyl linker.73 Depending on the DNA size and assembly condition, the synthesized amphiphiles were able to form micellar structures with different morphology (Figure 1.15).
Figure 1.15 Graphical presentation of nanostructure formed by the self-assembly of DNA-CPT conjugate and photo-induced release of free CPT from it.
The DNA-CPT conjugate was shown to have light controlled localized cytotoxicity.
Drug amphiphiles were not only designed by conjugating drug with polymers, peptides or oligonucleotides. Depending upon the inherent hydrophilic or hydrophobic nature of the drug itself, two of them was also combined to originate amphiphilic drug-drug conjugate (ADDC) (Figure 1.16). This novel and highly promising strategy was very recently pioneered by Yan and his group aiming at the development of carrier-free delivery system.74
Figure 1.16 Schematic presentation of the amphiphilic drug-drug conjugate (ADDC) and its self-assembly into nanoparticles.
They have attached irinotecan (Ir) and Chlorumbucil (Cb), two renowned anticancer drugs by an ester linkage.74 The hydrophilic nature of Ir and the hydrophobic nature of Cb provided the whole molecule an amphiphilic structure. This ADDC was shown to self-assemble into nanoparticles (Figure 1.8) in water providing longer retention time, which facilitates their accumulation inside tumor cells. Both the drugs exhibited synergetic cytotoxicity to the cancer cells leading to higher apoptosis compared to the free drugs. Lately, a number of ADDCs were developed by this group with the
combination of various anticancer drugs.75-76