Poly(2-oxazoline)s: The Emerging Frontiers of Biomedical Polymer Engineering

The field of biomedical applications has been significantly enriched with the inclusion of polymers, which have forged new paths in drug delivery, tissue engineering, implant fabrication, and biosensing. Polymers fused with pharmaceuticals can innovatively address numerous unmet medical needs, such as sustained drug release or targeted delivery to specific sites within the body.

Poly(ethylene glycol) (PEG), also referred to as poly(ethylene oxide) (PEO), has traditionally been the polymer of choice in biomedicine, widely recognized for its capability to enhance the half-life and reduce the immunogenicity of proteins. While PEG's biocompatibility, low dispersity, and evasion of the immune system have established it as a standard in biomedicine, it is not without its shortcomings. Notably, the prevalence of anti-PEG antibodies in patients poses a potential hindrance, as does the vulnerability of its polyether backbone to oxidative degradation.

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This is where Poly(2-alkyl/aryl-2-oxazoline)s, abbreviated as PAOx, POx, or POZ, step in, offering a promising alternative with higher stability and tunability than PEG. PAOx polymers maintain the critical properties required for biomedical use, such as biocompatibility and "stealth" behavior, but with enhanced functionalization options. The synthesis of PAOx through cationic ring-opening polymerization (CROP) yields polymers with a tertiary amide backbone, which interacts minimally with proteins and is largely ignored by the immune system.

PAOx can be tailored at the molecular level to adjust the hydrophilic-hydrophobic balance and control the lower critical solution temperature (LCST), enabling a fine-tuning of physical properties for a range of applications. Notably, some PAOx variants exhibit thermoresponsive behavior, becoming more hydrophobic at higher temperatures—a property leveraged in innovative material design for applications like diagnostics and triggered drug release.

The similarity of PAOx structures to natural polypeptides accounts for their biocompatibility and stealth behavior. Studies demonstrate that PAOx-based pharmaceuticals exhibit rapid clearance from the bloodstream and minimal accumulation in the reticuloendothelial system, suggesting a favorable toxicity profile and a promising future in human clinical applications.

The extensive applications of PAOx in drug delivery are diverse and inventive. PAOx can significantly improve drug solubility and bioavailability as excipients in drug formulations. PAOx-based micellar systems take advantage of the polymer's amphiphilic nature to facilitate high drug loading, which is especially beneficial for cancer therapeutics with low solubility in water. Additionally, PAOx-based hydrogels provide versatile platforms for drug delivery and tissue engineering, with the potential for injectable applications and customization through the functionalization of the polymer chain.

The conjugation of drugs and proteins with PAOx—referred to as PAOxylation—has yielded conjugates that often outperform their PEGylated equivalents. For example, PAOx-protein conjugates have demonstrated prolonged efficacy compared to non-conjugated forms, as well as increased cellular uptake.

Advances in PAOx research are continuous, and applications are expanding to include the functionalization of nanoparticles. These PAOx-functionalized nanoparticles possess unique characteristics that are beneficial for imaging and drug delivery, and they are "smart" materials that respond to external stimuli.

Given the breadth of possibilities that PAOx polymers offer, the biomedical field stands on the cusp of a new era where drug delivery and patient care may be significantly enhanced through the use of these versatile materials. With ongoing research and clinical trials, the potential for PAOx to become a leading platform in precision medicine and beyond is becoming increasingly apparent.