Just like PEG, PPG is an attractive candidate for a variety of nanoformulations. It can be utilized for the coating of nanoparticles to ensure their low immunogenic activity, to increase solubility and biocompatibility and consequently prevent opsonization by blood proteins (2,4-6). PPG is also an attractive additive for formulations of poorly soluble compounds, such as chemotherapy drugs. PPG can assist in solubilizing a hydrophobic drug whilst increasing chemical affinity towards the therapeutic agent (6).

There are several established applications of PPG in drug delivery systems, such as surface self-assembling polymers. This particularly refers to copolymers where PPG acts as a hydrophobic unit, which leads to micellar self-organization in aqueous solution and formation of nanogels. Due to the temperature sensitivity of the PPG block, a change in polymer chain conformation can occur. This feature allows for better control of drug release kinetics and eliminating burst release as it is observed in simple physical encapsulation of drugs. This approach does not require any covalent bonding between the drug and the polymer. The release of the therapeutic cargo is determined by diffusion and the degradation rate of the copolymer structure (4-6).

PPG is fully soluble in water below 400 g/mol and with increasing molecular weight exhibits useful LCST* between 400-3000 g/mol. In the solution above LCST, PPG undergoes phase transition from a loose coil to a compact globule. This is the property which makes PPG a desirable compound for thermoresponsive polymers. PPG can serve as hydrophobic block in multiblock polymers which will then undergo micellar self-assembly in aqueous solution. In drug delivery systems, this change of polymer chain conformation allows for better control of drug release kinetics and eliminates burst release as it is observed in simple physical encapsulation of drugs. Additionally, this approach doesn’t require any chemical linkage between components followed by breakage of the bond to trigger the release of the therapeutic cargo (5, 7). The use of PPG as a hydrophobic block is most abundant in poloxamers (triblock copolymers of PEG-PPG-PEG) but several different combinations are explored in literature, e.g., PPG-PCL** or PPG-PS*** and some of them commercialized, e.g., reversed poloxamers or poloxamines (1, 8, 9) . 

Currently raising attention is brought to the development of nanogels for drug delivery. Here PPG can serve as a building block for these self-assembling supramolecular polymers. Some of these solutions have already been demonstrated in literature. Cheng et al. obtained an attractive system for encapsulation of doxorubicin. Through click chemistry (Michael addition) they produced a nanogel from PPG-800 diacrylate and cytosine with 97 % yield. The PPG end-capped with cytosine formed a stable nanogel with a specific LCST at 45 °C at concentration of 1 mg/mL. This doxorubicin nanogel showed 4 times lower IC50 than free doxorubicin in vitro towards cancer cell lines (H146, H1688) (7).

The special properties of PPG can be deemed attractive when considering it as and additive for formulations of poorly soluble compounds, such as chemotherapy drugs. In this scenario, PPG can assist with solubility of the hydrophobic drug just as PEG, but at the same time offer higher chemical affinity to the hydrophobic drug of choice. It has been shown that coating of mesoporous silica nanoparticles loaded with doxorubicin with PPG-2000 showed higher encapsulation efficiency when compared to PEG-2000. For PPG coated NPs the EE was 95 %, whereas for PEG coated NPs EE was 79,5 %. This result can be explained by hydrophobic character of PPG due to extra methyl groups. It can aid interactions with highly hydrophobic doxorubicin which leads to enhanced penetration through silica pores and increases entrapment efficiency (10).

*LCST – lower critical solution temperature. Temperature of the polymer aqueous solution above which a reversible change in the polymer confirmation occurs. This results in insolubility of the polymer (cloudy solution). Lowering of the temperature restores the original confirmation and the polymer becomes soluble again (transparent solution).

**PCL – polycaprolactone.

***PS – polystyrene.

Commercially, PPG is synthesized through conventional ROP of propylene glycol. PPG is widely used in the polyurethane industry for production of foams. It is available in a variety of molecular weights ranging from 400 g/mol to a maximum of 6000 g/mol. The typical synthesis results in a high degree of unsaturation with increasing molecular weights. This stems from the elimination reaction of the activated monomer in highly basic conditions with high temperature. There have been some attempts to minimize the percentage of unsaturated PPG through using different catalysts, milder reaction conditions or post-synthetic treatments (extractions with ethylene glycol or glycerol, acidic washes). However, none of these approaches deliver PPG completely deprived from allyl alcohols. The higher the molecular weight, the higher percentage of unsaturation of the final products (1,3). According to our estimations, it varies from 1-5 % in PPG-425 to 15-20 % in PPG-2000. A meaningful decrease in quality can also be observed when comparing small-scale laboratory grade PPG to industrial-scale PPG purchased in a multikilo scale. The presence of unsaturated impurities in PPG are responsible for worsening the solubility of the high molecular weight PPG, lower concentration cloud point, as well as the unpleasant odor (3).

As for all polymers, changing molecular weight affects the physical bulk properties. In PPGs, the increasing molecular weight has a visible effect on the solubility and water affinity. If compared to PEG, PPG remain an oil for MW over 5000, where PEG become solids already around MW 1000. Outside from physical state, different solubility dynamics in water can be observed. In general, increasing molecular weight means lower temperature cloud point and lower solubility.

We use of PPG in the biomedical industry has not yet been widely popularized due to inconsistent composition and presence of unsaturated impurities. These stem from the polymerization process of the commercially available products. Approaches to minimize the degree of unsaturation are not fully effective and render PPG of high polydispersity (1). Thus, working with high molecular weight PPG is challenging. Despite this complexity, several examples of PPG use in bioformulations can be found in literature. Therefore, we see an increasing need for high purity PPG derivatives delivered in multi-kilogram scale. This is particularly important to enable reproducible drug manufacturing and support effective bench-to-bedside translation of preclinical solutions developed by the pharmaceutical industry. 

At Polypure, we have expanded our established SDC purification technology from PEG to PPG (7). Our technology allows us to utilize commercial PPG as a raw material and we can efficiently purify it to over 98 % oligomer purity. Our purification process relies exclusively on green solvents with continuous recycling to minimize overall consumption. 

Twenty years of experience with PEG chemistry paves the way to transfer this knowledge to derivatization of PPG. This has led us to develop a new library of PPG compounds with high purity, increased reactivity, and defined chemical composition. These include maleimide, thiol or amino functionalized PPG of varying lengths. Our dedication is in ensuring the highest reproducibility and control over chemistry of the PPG derivatives for pharma and biotech industries.

• Herzberger, J., Niederer, K., Pohlit, H., Seiwert, J., Worm, M., Wurm, F. R., & Frey, H. (2016). Polymerization of ethylene oxide, propylene oxide, and other alkylene oxides: synthesis, novel polymer architectures, and bioconjugation. Chemical reviews, 116(4), 2170-2243.
• Ajiro, H., & Akashi, M. (2010). Cell proliferation on stereoregular isotactic-poly (propylene oxide) as a bulk substrate. Biomacromolecules, 11(11), 2840-2844.
• Lambert, T. L. (1997). U.S. Patent No. 5,698,746. Washington, DC: U.S. Patent and Trademark Office.
• Bauduin, P. (2005). Characterization of short polypropylene glycol monoalkylethers and design of enzymatic reaction media. PhD, Universität Regensburg.
• Bordat, A., Boissenot, T., Nicolas, J., & Tsapis, N. (2019). Thermoresponsive polymer nanocarriers for biomedical applications. Advanced drug delivery reviews, 138, 167-192.
• Al-Nadaf, A. H., Dahabiyeh, L. A., Bardaweel, S., Mahmoud, N. N., & Jawarneh, S. (2020). Functionalized mesoporous silica nanoparticles by lactose and hydrophilic polymer as a hepatocellular carcinoma drug delivery system. Journal of Drug Delivery Science and Technology, 56, 101504.
• Cheng, C. C., Liang, M. C., Liao, Z. S., Huang, J. J., & Lee, D. J. (2017). Self‐Assembled Supramolecular Nanogels as a Safe and Effective Drug Delivery Vector for Cancer Therapy. Macromolecular Bioscience, 17(5), 1600370.
• Brewer, K., Gundsambuu, B., Facal Marina, P., Barry, S. C., & Blencowe, A. (2020). Thermoresponsive poly (ε-caprolactone)-poly (ethylene/propylene glycol) copolymers as injectable hydrogels for cell therapies. Polymers, 12(2), 367.
• Alli, A., Hazer, B., Menceloğlu, Y., & Süzer, Ş. (2006). Synthesis, characterization and surface properties of amphiphilic polystyrene-b-polypropylene glycol block copolymers. European Polymer Journal, 42(4), 740-750.
• Al-Nadaf, A. H., Dahabiyeh, L. A., Bardaweel, S., Mahmoud, N. N., & Jawarneh, S. (2020). Functionalized mesoporous silica nanoparticles by lactose and hydrophilic polymer as a hepatocellular carcinoma drug delivery system. Journal of Drug Delivery Science and Technology, 56, 101504.
• Agner, E. (2003). U.S. Patent No. 6,576,134. Washington, DC: U.S. Patent and Trademark Office.

In the world of therapeutics, several challenges have hindered development and overall success of drugs and vaccines. One pressing issue is the short half-life of therapeutics, with drugs rapidly breaking down and being eliminated from the body. This leads to poor drug efficacy and consequently having to introduce frequent dosing, and potential issues with patient compliance. Additionally, some drugs face problems with solubility, making it difficult for poorly soluble compounds to be effectively absorbed. Maintaining drug stability during storage and administration is also crucial as many compounds, in particular biologics tend to degrade or undergo chemical changes. Addressing these problems is essential for optimal therapeutic outcomes and clinical approval. One strategy for increasing the stability of drugs before and after administration, as well as increasing water solubility, is to chemically incorporate inert polyethylene glycol (PEG) into the drugs. This process has been critical in new drug discovery as well as in generating new therapeutics from previously known drugs.

PEGylation is a bioconjugation technique, which involves attaching PEG chains to therapeutic agents, such as drugs or vaccines. This process enhances the pharmacokinetic and pharmacodynamic properties of the molecules, leading to prolonged circulation time in the body, better solubility, and improved stability. PEGylation has been a successful strategy for new drug development in many instances but until now it has been performed with commercially available off-the-shelf chemicals that are composed of polymers with non-uniform chain length, known as polydisperse PEGs.

Peptide-based cancer vaccines are made up of sequences of amino acids derived from tumor antigens (Figure 1). These peptide sequences are typically lengthy and comprise blocks of hydrophobic amino acids, making the synthesis difficult and requiring rigorous purification to obtain a high-quality product. The contribution of Polypure is to assist in PEGylating peptides to improve solubility and simplify downstream processing of vaccines. We have shown that incorporation of PEG chains in peptide sequences reduces synthesis, purification, and solubility problems frequently encountered with hydrophobic peptides.

Another area of research that Polypure has been exploring is the development of synthesis methods for acquiring monodisperse PEG lipids utilized in LNPs. Incorporating PEG lipids in vaccine LNPs, extends circulation time and thereby improves vaccine efficacy. Commonly polydisperse DMG-PEG-2000 has been used in the rapid development of new vaccines. We intend to provide similar structures but with defined chain lengths for reliable reproducibility and better outcomes. 

The technology of producing uniform PEGs utilized at Polypure has been previously employed on a commercial scale to provide multi-kilogram quantities. This will be particularly important when incorporating monodisperse PEG in LNPs manufacturing.