Cosmetics, Medicines, Shower Gels: Why the Omnipresence of Polyethylene Glycol Raises Questions and What Are the POx That Could Replace It?

Polyethylene glycols (PEGs) are ubiquitous in modern life, found in an astonishing array of everyday products. From the refreshing lather of shower gels and shampoos to the lubricating action of personal lubricants, the sanitizing efficacy of hand sanitizers, the life-saving power of vaccines and medications, the soothing touch of moisturizers, and even in the vibrant hues of paints – PEGs are an integral component of our daily routines. Their widespread adoption stems from a potent combination of desirable properties: their exceptional ability to enhance the solubility of various active substances, their cost-effectiveness, and the relative simplicity of their production processes. This versatility has cemented their position across a diverse spectrum of industries, including pharmaceuticals, chemicals, detergents, paints, and inks.

For a considerable period, PEGs were largely regarded as inert substances, their presence in formulations deemed benign. However, a growing body of research and increasing public awareness have begun to cast a different light on these workhorse polymers. Concerns are mounting regarding their petrochemical origins, their inherent resistance to biodegradation, and a rising tide of questions surrounding their long-term effects on human health and the environment. These emerging concerns have catalyzed a significant shift in scientific and industrial focus, intensifying the search for viable alternatives. Among the polymer families being rigorously investigated, poly(2-oxazolines) (POx) are now garnering substantial attention. Their unique properties position them as promising candidates to potentially replace PEGs in numerous applications.

Understanding Polyethylene Glycol (PEG): A Petrochemical Polymer

Polyethylene glycols, derived from petrochemical sources, constitute a broad class of compounds widely employed in the cosmetic and pharmaceutical sectors. At their core, PEGs are polymers formed by the repetitive linkage of a fundamental building block: ethylene oxide. The specific characteristics and applications of different PEG variants are primarily determined by their molecular weight, which is a direct consequence of the number of ethylene oxide units repeated in their polymer chains. This molecular weight, typically expressed in grams per mole (g/mol), is often appended to their nomenclature, as seen in designations like PEG-40 or PEG-400. Notably, high molecular weight PEGs, such as PEG-3350 or PEG-4000, are the active ingredients in certain laxatives, like macrogol.

The molecular weight of a PEG profoundly influences its physical properties. PEGs with molecular weights below approximately 500 g/mol are generally liquid at room temperature and are frequently incorporated into cosmetic formulations. As their molecular weight increases, their viscosity rises, transitioning them into more oily or waxy substances. This property is particularly sought after in specific pharmaceutical applications and for certain industrial additives.

The Multifaceted Role of PEGs: Industrial Versatility

Often lauded as the "Swiss Army knives" of modern cosmetics, PEGs are prized for their low cost and remarkable versatility, enabling them to perform multiple functions within product formulations.

• Humectant Properties: Being hydrophilic, PEGs possess an inherent ability to attract and retain water. This makes them valuable in skincare products, where they help hydrate the skin by forming a thin film that minimizes trans-epidermal water loss.
• Emulsifying Agents: PEGs also act as effective emulsifiers, a crucial role in stabilizing mixtures of oil and water, which is essential for the texture and consistency of creams and lotions.
• Cleansing Aids: In washing products, PEGs significantly facilitate the removal of greasy and oily impurities, contributing to enhanced cleansing efficacy.

These multifaceted properties explain not only their widespread use in cosmetics but also their critical function as excipients in pharmaceuticals. In medicinal formulations, PEGs are instrumental in improving the solubility and stability of active pharmaceutical ingredients (APIs), thereby enhancing drug delivery and efficacy. Beyond personal care and medicine, certain PEG derivatives are also approved as technological agents in the food industry and household cleaning products, albeit under more stringent regulatory frameworks.

PEGs in mRNA Vaccines: A Critical Component

The recent global health landscape has highlighted the indispensable role of PEGs in the development of messenger RNA (mRNA) vaccines, most notably those deployed against SARS-CoV-2, the virus responsible for the COVID-19 pandemic. The inherent fragility of mRNA necessitates a protective delivery system to ensure it reaches target cells intact without premature degradation. This protective function is fulfilled by microscopic vesicles known as "lipid nanoparticles."

These lipid nanoparticles are complex structures composed of various lipids and surfactants, substances characterized by both hydrophobic (water-repelling) and hydrophilic (water-attracting) components. Crucially, a specific type of lipid, a "PEGylated lipid," which bears a PEG chain, is a key constituent. This PEGylated lipid plays a pivotal role in stabilizing the lipid nanoparticles that encapsulate mRNA. It achieves this by preventing the aggregation of these nanoparticles and meticulously controlling their interactions with the biological environment. Furthermore, the PEG chain forms a hydrophilic coating on the surface of the nanoparticles, rendering them less visible to the immune system. This "stealth" effect prolongs their circulation time within the body, thereby significantly enhancing the efficiency of mRNA delivery.

The Emerging Concerns and Limitations of PEGs

Despite their widespread industrial success and critical applications, PEGs are not without their drawbacks and emerging concerns.

• Manufacturing Byproducts: The synthesis of PEGs can inadvertently leave trace amounts of byproducts, notably ethylene oxide and 1,4-dioxane. Both of these substances are classified as toxic and potentially carcinogenic. Consequently, their presence in finished products is strictly regulated and ideally absent. Regulatory bodies worldwide, such as the U.S. Food and Drug Administration (FDA) and the European Chemicals Agency (ECHA), have established guidelines and limits for these impurities to safeguard public health. For instance, the FDA has issued guidance on acceptable levels of 1,4-dioxane in cosmetic products.
• Immunological Responses: In a subset of individuals, exposure to PEGs has been associated with the development of immune responses, characterized by the production of anti-PEG antibodies. While rare, these reactions can, in some instances, lead to allergic reactions or even anaphylaxis, particularly following the administration of certain medications or vaccines. However, it is crucial to emphasize that such events remain exceedingly infrequent when considering the vast number of PEG-containing doses administered globally. Scientific studies published in journals like the Journal of Allergy and Clinical Immunology have investigated the prevalence and mechanisms of anti-PEG antibody formation.
• Environmental Persistence: The biodegradability of PEGs is a growing environmental concern. While low molecular weight PEGs are largely eliminated through renal excretion, higher molecular weight variants can persist in aquatic environments. The long-term ecotoxicological effects of this persistence are not yet fully understood, prompting further research into their environmental fate and impact. Studies in environmental science journals have explored the detection and persistence of PEGs in wastewater and surface waters.

The Rise of Poly(2-oxazolines) (POx): A Promising Alternative

In light of these limitations, the scientific community is increasingly turning its attention to alternative polymers, with poly(2-oxazolines) (POx) emerging as a particularly interesting class.

A Rediscovery of Older Polymers: The Potential of POx

Known since the 1960s, POx polymers were historically underutilized, largely due to less standardized and industrialized synthesis processes compared to those for PEGs. However, significant advancements in polymer chemistry in recent years have enabled greater control over their synthesis and manufacturing, reigniting interest in these materials.

A key advantage of POx over PEGs lies in their enhanced "functionalizability." This refers to the capacity to chemically modify their extremities or their polymer backbone, allowing for the grafting of other molecular units, such as fluorescent probes or biological ligands. This fine-tuned control over their architecture and modularity offers unprecedented flexibility in designing sophisticated therapeutic systems. Consequently, POx exhibit several properties that are highly attractive to researchers in the field of nanomedicine.

Intriguing Properties for Nanomedicine Applications

Research indicates that POx possess low immunogenicity, meaning they are less likely to trigger a significant immune system response. This is a major advantage, especially in applications like vaccination and cancer therapy. Their reduced potential to elicit an immune reaction makes them suitable for repeated administration, even in individuals who may have developed sensitivities to other polymers, including PEGs.

Moreover, the biodegradability of POx can be precisely modulated through chemical engineering. This allows for the design of structures that remain stable for the duration of their therapeutic action before degrading in a controlled manner within the body. This degradation can be influenced by biological stimuli, such as changes in pH, the presence of specific enzymes, or unique chemical environments. This controllable degradation opens avenues for personalized medicine, particularly for long-term treatments or high-dose therapies.

Furthermore, the behavior of POx can be temperature-dependent, a property known as thermosensitivity. This characteristic is especially valuable in pharmaceutical formulation (the science of transforming an active ingredient into an administrable drug). For example, it enables the creation of injectable gels that are liquid at room temperature but solidify upon entering the body, or vice versa. This ability to "adapt" offers exciting prospects for controlled drug release and targeted delivery systems.

Applications and Limitations of POx

The unique properties of POx position them as strong contenders for drug delivery, particularly in the fight against cancer. Research is actively exploring systems to improve the delivery of chemotherapies, such as taxanes (derived from yew trees), including paclitaxel. These experimental efforts are currently in the preclinical or early clinical trial stages. Applications are also being investigated in the fields of vaccines and cosmetics, though these remain largely experimental.

A Developing Alternative, Not an Immediate Replacement

As of now, POx do not represent an established alternative to PEGs. While industrialization is underway, their production can be more complex and less standardized than that of PEGs. Furthermore, long-term toxicological data and their environmental fate are still subjects of ongoing research, necessitating a cautious approach.

Currently, POx are not supplanting PEGs but are being explored as a complementary avenue. They are being investigated for specific applications where chemical modularity and immunological tolerance are paramount. Their large-scale development will hinge on scientific advancements and their capacity to be produced reproducibly, safely, and economically. The journey from promising research to widespread industrial adoption is often a lengthy one, requiring rigorous testing, regulatory approval, and market acceptance. As scientific understanding deepens and manufacturing processes mature, poly(2-oxazolines) may well carve out a significant niche in the landscape of advanced materials, offering a compelling complement, and potentially, in specific contexts, a superior alternative to the long-standing utility of polyethylene glycols.

Author: Oksana Krupka, Professor, University of Angers.
This article is republished from The Conversation under a Creative Commons license. Read the original article here.