Background
The cell membranes in our body are composed of components called phospholipids, and the outer membrane is primarily made from phosphatidylcholine, as shown in Figure 1. For drugs or nanoparticles for drug delivery system (DDS), the balance between hydrophilic and hydrophobic properties would be crucial to internalize into cells via interaction with the cell membrane.
In recent years, zwitterionic polymers with side chains possessing a molecular structure similar to phosphatidylcholine have attracted attention as biocompatible polymers. Since they avoid protein adsorption and recognition by immune cells even when retained in the body, artificial joints and contact lenses coated with zwitterionic polymers are already commercially available.
However, while the biocompatibility of zwitterionic polymers would suppress the interaction with blood components and cells, introducing hydrophobic moieties into the polymer might be necessary to enable selective interaction with cancer cells after reaching the tumor site. However, synthesizing zwitterionic polymers with hydrophobic moieties would be challenging, as reported by Roth et al. in 2014. (Roth et al., Macromolecules, 2014, 47, 750-762) Copolymerization of highly hydrophilic zwitterionic monomers and hydrophobic monomers is difficult due to their significantly different solubilities. Even if the copolymerization proceeds, the resulting polymer would precipitate.
Our research group then performed solid-state copolymerization of a zwitterionic monomer (sulfo-betaine methacrylate, SBMA) and a hydrophobic monomer (N-benzyl methacrylamide, BnMA) using a completely dry process, as shown in Figure 2, without using any organic solvents or catalysts.
This copolymerization resulted in the conversion of more than 91% of each monomer into polymer within 60 min, yielding a monodisperse polymer with a molecular weight of 9,200 g mol⁻¹ (Mw/Mn = 1.10). Furthermore, HPLC measurement and preparative chromatography of the products obtained in this study revealed that the main resulting polymer was a block copolymer structure composed of 65 mol% PSBMA and 35 mol% PBnMA, as confirmed by 1H/DOSYNMR
and DSC measurements. Furthermore, after clarifying the solid-state polymerization activity of each monomer using quantum chemical calculations, analysis of the products suggested that this copolymerization would proceed according to the mechanism shown in Figure 3.
The amphiphilic polymer was stable for over one month in an aqueous solution. Dynamic light scattering (DLS) measurements revealed that interactions between the amphiphilic polymer and serum proteins (albumin and fibrinogen) were suppressed, and the particle size did not increase due to the antifouling effects of the amphiphilic polymer.
Furthermore, we demonstrated that the amphiphilic polymers internalized into cancer cells within 5 min after administration, whereas intact sulfobetaine polymer without a hydrophobic moiety could not internalize into cancer cells even after 4 h, as shown in Figure 4.
On the other hand, sulfobetaine polymers with naphthyl or pyrene groups as hydrophobic chains other than benzyl groups exhibited significantly reduced water solubility. This result suggests that the formation of a hydrophobic hydration layer via the benzyl groups in the amphiphilic polymer would be crucial for interaction with cancer cells. Therefore, this study demonstrated that the introduction of even a small hydrophobic moiety is essential for drug delivery using sulfobetaine polymers.
Based on the above findings, the development of amphiphilic sulfobetaine polymers synthesized by solid-state copolymerization without the use of organic solvents or catalysts was successfully achieved. The resulting polymers can internalize into cancer cells within an extremely short time. This research was conducted by associate professor Naoki Doi and professor Shinichi Kondo from the Lab. of Pharmaceutical Physical Chemistry, Department of Drug Delivery Science and Technology at Gifu Pharmaceutical University. This article was published in the American Chemical Society journal, Macromolecules (H-Index=348, 2-year Impact Factor=5.2).
We have demonstrated that sulfobetaine polymers conjugated with anticancer drugs exhibited anticancer activity through internalizing into cancer cells and releasing the drugs with high efficiency for cancer treatment.
Highlights in this study
- We have solved the problem of copolymerization between zwitterionic monomers and hydrophobic monomers--a longstanding issue in conventional liquid-phase polymerization--using solid-state copolymerization technology. This has enabled the development of a novel block copolymer incorporating hydrophobic segments into zwitterionic polymers.
- It was demonstrated that the mechanical energy of solid-state copolymerization under specific conditions would yield block copolymers with a predetermined composition of hydrophilic and hydrophobic chains, regardless of the monomer preparation ratio.
- The amphiphilic sulfobetaine polymers would internalize into cancer cells within 5 min after administration.
Article information
Literature name : Macromolecules
Article title : Amphiphilic Sulfobetaine Copolymer for Boosting Internalization into Cancer Cells: Synthesis by Green Innovative Solid-State Copolymerization and Characterization
Author list : Naoki Doi, Yukinori Yamauchi, Yasushi Sasai, Tsukasa Ide, Hiroya Ishizuka, Taiyo Inagaki, Miyu Sato, Masayuki Kuzuya, Shin-ichi Kondo
DOI: https://doi.org/10.1021/acs.macromol.5c01571
Lab name
Laboratory of Pharmaceutical Physical Chemistry
https://www.gifu-pu.ac.jp/lab/bukka/