PhD Candidate Duke University Durham, North Carolina, United States
Introduction:: Amphiphilic block copolymers can self-assemble into nano to mesoscale structures that can be used to enhance the delivery of drugs by enhancing their in vivo residence time, enhancing stability by reducing their premature degradation, and minimizing toxic side effects by reducing their off-target accumulation. Polyethylene glycol (PEG) is the most common hydrophilic block in self-assembled block copolymer nanoparticles, as the PEG corona provides “stealth” behavior imparting a long plasma residence time to the nanoparticles. PEG is also non-toxic and has a long history of relatively safe use in humans, as many approved biologic drugs are conjugated to PEG, and PEG is present as an excipient in many drug formulations. However, the prevalence of pre-existing anti-PEG antibodies in more than 60% of the human population raises several concerns, including opsonization of the PEG nanoparticles, leading to their premature clearance and the incidence of rare—but potentially fatal—anaphylactoid reactions. Furthermore, the core-forming block in these nanoparticles are composed of another more hydrophobic monomer, some which are poorly degradable, or are potentially toxic.
We have previously reported a next-generation stealth polymer—poly[oligo (ethylene glycol) methyl ether methacrylate] (POEGMA)— that present short oligomers of ethylene glycol (EG)n as side chains and that when the n is ≤3, the polymer does not bind pre-existing human PEG antibodies nor does it elicit a significant IgM or IgG response. Here, we leverage these findings to design non-immunogenic nanoparticles that are composed of only POEGMA to address the limitations of PEG and—that at the same time—preserve their stealth behavior.
Materials and Methods:: We synthesized a set of AB type di-block and ABA type tri-block copolymers of OEGMA that present short oligomers of EGn (n = 1-3) as a sidechain. Since the longer EG imparts more hydrophilicity, we assigned the EG3 monomer as the hydrophilic block and EG2 or EG1 monomer as the hydrophobic block and synthesized a library of block copolymers with varied ratios of the monomers and varied degree of polymerization (DP) using RAFT. The rate of polymerization reactions was monitored by NMR and the molecular weights —Mn, Mw—and polydispersity (PDI) of all the polymers was determined by GPC. The thermal phase behavior of block POEGMA was characterized by absorbance measurements at 600 nm and the size of self-assembling block polymers was determined by dynamic light scattering (DLS). The aggregation number and the radius of gyration for these nanoparticles was determined by static light scattering (SLS). Furthermore, the nanoparticles were frozen in aqueous media and imaged in their native state via cryo-EM.
Results, Conclusions, and Discussions:: We successfully synthesized a library of 15 block polymers with varied DP and monomer ratio including AB type diblocks and ABA type triblocks that were screened to establish rules for self-assembling block constructs. We discovered that even the difference of a single EG unit in each block imparts sufficient amphiphilicity to a subset of these block copolymers to drive their self-assembly into nanoparticles that are spherical micelles. We also discovered that the higher DP of the hydrophobic block results into larger aggregates that physically can’t be spherical micelles and further characterizations are being performed to determine the shape and nature of these aggregates. Another unique attribute of POEGMA that we have explored in this study is that POGMA exhibits lower critical solution temperature (LCST) phase behavior, wherein POEGMA phase separates from a single soluble phase in an aqueous solvent into two immiscible phases above a critical transition temperature (Tt). The phase diagram and hence Tt can be tuned by DP and the mole fraction of the different (EG)n monomers, with longer DP and monomers with shorter EG units exhibiting a lower Tt. We found that the Tt of non-self-assembling block POEGMA or random coPOEGMA remain concentration dependent, whereas the Tt of block POEGMA becomes concentration independent when polymer self-assembles into micelles— highlighting a shift in thermodynamic parameters. Additionally, the Tt of non-self-assembling block POEGMA or random coPOEGMA is positively correlated with the total hydrophilic monomer weight fraction whereas the Tt of micelle forming block POEGMA tend to lose that positive correlation. Current work is focused on characterizing—and controlling—the size, shape, critical micelle temperature and critical aggregation concentration of these nano to mesoscale particles. Future work will focus on demonstrating their utility in different drug delivery applications.
