Injectable Hydrogel: Amplifying the pH Sensitivity of a Triblock Copolypeptide by Conjugating the N-Termini via Dynamic Covalent Bonding

Abstract

We explore the self-assembly behavior of aqueous solutions of an amphiphilic, pH-sensitive poly(l-alanine)-b-poly(l-glutamic acid)-b-poly(l-alanine), (A5E11A5) triblock copolypeptide, end-capped by benzaldehyde through Schiff base reaction. At elevated concentrations and under physiological pH (7.4) and ionic strength (0.15M), the bare copolypeptide aqueous solutions underwent a sol-gel transition after heating and slow cooling thermal treatment, forming opaque stiff gels due to a hierarchical self-assembly that led to the formation of β-sheet-based twisted super fibers (Popescu et al. Soft Matter 2015, 11, 331-342). The conjugation of the N-termini with benzaldehyde (Bz) through a Schiff base reaction amplifies the copolypeptide pH-sensitivity within a narrow pH window relevant for in vivo applications. Specifically, the dynamic character of the imine bond allowed coupling/decoupling of the Bz upon switching pH. The presence of Bz conjugates to the N-termini of the copolypeptide resulted in enhanced packing of the elementary superfibers into thick and short piles, which inhibited the ability of the system for gelation. However, partial cleavage of Bz upon lowering pH to 6.5 prompted recovery of the hydrogel. The sol-gel transition triggered by pH was reversible, due to the coupling/decoupling of the benzoic-imine dynamic covalent bonding, endowing thus the gelling system with injectability. Undesirably, the gelation temperature window was significantly reduced, which however can be regulated at physiological temperatures by using a suitable mixture of the bare and the Bz-conjugated coplypeptide. This triblock copolypeptide gelator was investigated as a scaffold for the encapsulation of polymersome nanocarriers, loaded with a hydrophilic model drug, calcein. The polymersome/polypeptide complex system showed prolonged probe release in pH 6.5, which is relevant to extracellular tumor environment, rendering the system potentially useful for sustained delivery of anticancer drugs locally in the tumor.

Hydrogels Based on Dynamic Covalent and Non Covalent Bonds: A Chemistry Perspective

“…The same concept of covalent reversible bonds can be used for the hydrogel formation and disruption. Examples of interactions used in this case are (Figure 2) electrostatic ones [19,20], cycloadditions [8,21], redox reactions [22,23,24], and other ones such as imine [25,26,27,28] and enamine formation [29], acylhydrazone [30,31,32,33], and borax acid reaction with hydroxyls [13,34,35,36].…”

“…Indeed, synthetic polymers are still widely used for the preparation of hydrogels, with polyacrylamide representing the most popular choice [45]. On the other hand, hydrogels based on natural products (e.g., chitosan [4,17], proteins [11,31,36], modified alginate [46], peptides [28,47]) represent a very convenient choice in view of clear advantages related to biological applications, but also to general sustainability principles. Indeed, attention is being paid also to novel synthetic pathways in agreement with green chemistry principles [48].…”

Mechanochemical Drug Conjugation via pH-Responsive Imine Linkage for Polyether Prodrug Micelles

“…For that purpose, pH-responsive polymeric systems are particularly appealing to selectively release therapeutics by exploiting the various pH gradients that exist in specific cellular compartments. − The application of these polymers can further be expanded to other biological applications, including tumor-targeted delivery of drugs, intracellular delivery of nucleic acids or proteins, treatment of inflammatory diseases, and oral administration. , For example, imines, hydrazides, hydrazones, orthoesters, and acetal linkages degrade selectively in response to pH changes. , Among them, the imine linkage is of particular interest owing to its sensitivity under different acidic conditions, facilitating its widespread utility as a stimuli-responsive linkage. , As a notable example, Xu and co-workers conjugated doxorubicin to polymers through imine bond and demonstrated its pH-controlled drug release in acidic environments . In addition, Banerjee and co-workers developed pH-responsive glyconanoparticles conjugated to drugs through imine linkages …”

A pH-/thermo-responsive hydrogel formed from N,N′-dibenzoyl-l-cystine: properties, self-assembly structure and release behavior of SA

“…Among the various external stimuli, the pH/temperature responsiveness of hydrogels has been most extensively investigated in sensors and biological applications due to the controllability both under in vitro and in vivo conditions. [23][24][25] For example, Gao et al 26 reported a kind of sowood kra lignin pH-responsive hydrogels which were self-assembled through strong intermolecular hydrogen bonds in aqueous solution. However, most of them are formed from polymers.…”