Selecting the Right Polymer Excipient for Drug Delivery

Historically polymer excipients have been described as inactive formulation components. In today's drug delivery systems, however, that definition can´t be more inaccurate.

The system doesn´t matter; long-acting injectables, polymer-drug conjugates, nanoparticles or advanced implantable systems, but the polymer often determines far more than the physical characteristics of a formulation. Most of the times the polymer directly influence drug stability, release kinetics, manufacturability, storage conditions and even the regulatory pathway. In many cases, the excipient becomes a functional component of the therapy rather than simply a vehicle for delivering the active pharmaceutical ingredient (API).

As drug delivery continue to evolve, choosing a polymer is no longer a matter of choosing a biocompatible material. Formulation scientists must balance a complex combination of chemical and biological considerations while ensuring the final product remains scalable and manufacturable.

This is precisely why polymer selection has become one of the most challenging, and most influential, decisions in modern drug delivery development.

1.The Evolving Role of Polymer Excipients in Drug Delivery

For many years, polymer excipients were selected aiming to improve formulation stability or facilitate drug administration. While these remains important, advances in pharmaceutical science have significantly expanded their role.

Today, polymers are expected to perform far more complex functions. They can protect fragile molecules from degradation, control drug release over extended periods, form nanoparticles, provide functional groups for conjugation or even influence biodistribution. In many advanced drug delivery systems, the polymer is no longer simply supporting the formulation, it is actively contributing to its therapeutic performance.

This evolution changed the way materials are selected. Rather than asking whether a polymer is compatible with a formulation, developers must consider how its molecular architecture, physicochemical properties and manufacturing characteristics will influence the success of the final product. As a result, polymer selection has become a strategic decision that impacts every stage of development, from early formulation studies to large-scale GMP manufacturing.

2.Factors to keep in mind while selecting a polymer

Selecting a polymer excipient starts by finding a material that is compatible with the active pharmaceutical ingredient (API). In practice there are also many other factors to consider, formulation scientists face a combination of interconnected variables, each of which can influence the performance, manufacturability and long-term viability of the final drug product.

Polymers actively participate in the behaviour of many advanced drug delivery systems. Their molecular weight, chemical composition and drug compatibility (DP) can all affect how the formulation is manufactured, how the drug is released and even how the therapy performs in vivo. Because these factors are closely linked, improving one characteristic often requires compromising another.

For example, increasing the molecular weight of a polymer may extend drug release by slowing diffusion or degradation. However, the same modification can increase solution viscosity, making the formulation more difficult to manufacture or administer. Similarly, introducing functional groups may improve conjugation efficiency or nanoparticle stability while also altering degradation behaviour or increasing process complexity.

Rather than searching for an ideal polymer, formulation scientists are typically looking for the best balance between multiple design requirements, and also favoring wich parameter is more important for each drug.

A useful way to understand this process is to consider the main questions that guide material selection during formulation development.

Design consideration	Why it matters
Drug compatibility	Determines whether the polymer can maintain API stability without unwanted interactions.
Release profile	Influences whether the formulation achieves immediate, sustained or controlled drug release.
Biocompatibility	Ensures the material and its degradation products are appropriate for the intended route of administration.
Manufacturability	Affects process robustness, scalability and reproducibility during production.
Stability	Contributes to shelf life by influencing moisture sensitivity, hydrolysis and physical stability.
Regulatory strategy	Determines the amount of safety, quality and documentation required for clinical and commercial use.

Each of these considerations influences the others. For instance a polymer with an established regulatory history may not provide the functionality required for an innovative delivery system. Consequently, polymer selection becomes a complicated excercise that requires collaboration of formulation scientists, analytical specialists, process engineers and regulatory experts even at the earliest stages of development .

For this reasons, selecting a polymer over another is rarely based on a single reason. Instead, it depends on understanding both polymer and API and choose the combination that better supports the overall objectives of the drug product.

3.Finding the balance : Performance, Manufacturability and Quality

Developing a successful formulation is not simply about identifying a polymer that performs well during R&D work. A material that seems to deliver excellent propierties during experimental conditions may introduce significant challenges once the project moves towards process development, scale-up or GMP manufacturing.

This is why polymer must support the entire product lifecycle, from early feasibility studies to commercial production.

As we have seen in our previous point the polymer properties are interconnected and usually boosting one property tends to have some negative effects in other.

The relationship between polymer properties and formulation performance illustrates this multidimensional decision-making process.

Polymer property	Potential benefit	Potential trade-off
Higher molecular weight	Extended circulation time or sustained drug release.	Increased viscosity, more challenging processing and injectability.
Greater hydrophilicity	Improved solubility and colloidal stability.	Lower compatibility with highly hydrophobic APIs.
Cationic functionality	Efficient nucleic acid complexation and conjugation.	Increased cytotoxicity and non-specific biological interactions.
Biodegradability	Reduced long-term accumulation and controlled degradation.	More complex control of degradation kinetics
Reactive functional groups	Greater flexibility for conjugation and functionalisation.	Increased analytical and manufacturing complexity.

These trade-offs become even clearer when considering commonly used polymer excipients. Materials that have become well established in pharmaceutical development each offer distinct advantages while presenting their own formulation challenges.

Polymer	Typical advantages	Important to consider
Polyethylene glycol (PEG)	Excellent hydrophilicity, prolonged circulation time, reduced protein adsorption and extensive regulatory experience.	Growing interest in PEG alternatives due to anti-PEG antibodies, accelerated blood clearance (ABC) phenomena and the need for more versatile shielding materials in certain applications.
Polysarcosine (pSar)	Promising PEG alternative with stealth behaviour, excellent hydrophilicity and low protein adsorption.	Fewer clinically approved products and more limited long-term regulatory experience than PEG.
PLGA	Biodegradable and biocompatible polymer widely used in long-acting injectables, microspheres and implants, with well-established regulatory acceptance.	Drug release can be difficult to predict due to autocatalytic degradation, and acidic degradation products may affect sensitive APIs.
Poly(L-glutamic acid) (PGA)	Biodegradable polymer with multiple carboxyl groups for conjugation, drug delivery and nanoparticle engineering.	Performance depends strongly on molecular weight, polymer architecture and degradation behaviour.
Poly(L-lysine) (PLL)	Highly functional polymer for conjugation strategies and electrostatic complexation of nucleic acids.	High positive charge may increase cytotoxicity and non-specific interactions if not carefully controlled.
Chitosan	Natural, biodegradable and mucoadhesive polymer with excellent potential for mucosal and nucleic acid delivery.	Variable molecular weight and degree of deacetylation can affect reproducibility, while limited solubility at physiological pH may restrict certain applications.
Hyaluronic acid (HA)	Naturally occurring polymer with excellent biocompatibility and intrinsic affinity for CD44 receptors, making it attractive for targeted drug delivery and hydrogels.	Rapid enzymatic degradation and relatively poor mechanical stability often require chemical modification or crosslinking.
Alginate	Mild gelation conditions and excellent biocompatibility make it suitable for encapsulation and cell delivery applications.	Mechanical strength, degradation behaviour and batch-to-batch variability depend heavily on composition and crosslinking conditions.

These examples proves that selecting a polymer is based on a sum of pros and cons instead of looking for the “best” polymer. For instance, PEG remains one of the most widely used shielding polymers because of its proven ability to improve circulation time and formulation stability. At the same time, the emergence of alternatives such as polysarcosine highlights how evolving therapeutic modalities continue to reshape material selection criteria.

Similarly, polymers such as poly(L-glutamic acid) and poly(L-lysine) are valued not simply because they are biodegradable or cationic, but because their chemical structures enable conjugation, nanoparticle engineering and controlled interactions with therapeutic payloads. However, these same properties also require careful optimisation to ensure reproducible manufacturing, appropriate degradation profiles and acceptable biocompatibility.

As development progresses, considerations beyond formulation performance become also quite important. Batch-to-batch consistency, impurity control, scalability and regulatory documentation influence whether a polymer is suitable for clinical translation and commercial manufacturing. Material selection therefore extends well beyond the laboratory and becomes a strategic decision that impacts the entire development pathway.

4. Today polymer selection matters more than ever

As therapies evolve and grow in complexity, the way innovators approach the design is also different. Nowadays the well established polymer may not be enough, and researchers are looking for polymers that adapt better to a specific need for each application.

Not only innovative polymers are important, also improving how existing materials are synthesised, characterised and manufactured. Greater control over parameters such as molecular weight, polydispersity and end-group functionality allows to increase performance, reduce variability an ensure better scale up.

As a result, polymer selection is changing from a catalogue choice to designing the excipient with the best qualities for the intended therapeutic outcome. This shift is expected to play an increasingly important role as advanced modalities continue to expand, requiring excipients that combine functionality, quality and manufacturability.

Whether selecting an established excipient or developing a custom polymer, having the right partner that understand these trade-offs early in development can help reduce technical risk and accelerate the path from research to manufacturing.