Precision control for target protein degraders

Target protein degraders (TPDs) offer a fundamentally different approach to drug discovery. Rather than simply inhibiting a disease-relevant protein, they are designed to recruit the body’s own degradation machinery to remove it. That mechanism has created major therapeutic interest, but it also creates a demanding drug product challenge.

Many TPD candidates are highly potent, poorly soluble and intended for administration at very low oral doses. Developing them successfully requires more than a conventional oral solid dosage approach. Content uniformity, particle engineering, enabling formulation technology, containment, analytical sensitivity and scalable manufacturing must be considered together from the earliest stages.

A mechanism that raises the formulation bar

Most TPDs act through the ubiquitin–proteasome system, in which E3 ubiquitin ligases help identify proteins for tagging and subsequent degradation by the proteasome. The two most prominent categories are molecular glues and proteolysis-targeting chimeras, or PROTACs.

Molecular glues are typically small molecules that promote or stabilise an interaction between an E3 ligase and a target protein. PROTACs are bifunctional molecules: one end binds the target protein and the other binds an E3 ligase, bringing the two into proximity so that the target can be ubiquitinated and destroyed.

Unlike a conventional inhibitor, a degrader molecule is not necessarily consumed when it drives a degradation event. It may dissociate and engage another target molecule. This catalytic behaviour helps explain why TPD programmes can involve very low drug loads and administered doses.

It also makes precision indispensable. When an active pharmaceutical ingredient (API) represents only a minute proportion of a tablet or capsule, small variations in its distribution can have an outsized effect on dose consistency, product performance and analytical control. The core development question is therefore clear: how can a highly potent, low-dose product be formulated and manufactured consistently from first-in-human supply through commercial scale?

Overcoming solubility and absorption barriers

Many TPDs, particularly PROTACs, sit outside the conventional physicochemical space associated with orally bioavailable small molecules. They can have high molecular weights, flexible structures, high melting points, poor aqueous solubility and, in some cases, limited permeability. In practical terms, many behave as “brick dust” compounds: they are difficult to dissolve in gastrointestinal fluid and may be difficult to absorb.

A successful strategy begins with identifying the primary barrier rather than applying a standard formulation route.”

These characteristics frequently place candidates within Biopharmaceutics Classification System Class II or Class IV, where absorption may be limited by solubility, dissolution rate, permeability or a combination of all three. A successful strategy begins with identifying the primary barrier rather than applying a standard formulation route.

The Developability Classification System can help distinguish between compounds limited primarily by dissolution rate and those limited by intrinsic solubility. For DCS Class IIa compounds, which dissolve slowly but can ultimately reach sufficient solubility, particle-size reduction may be effective. Micronization and nanomilling increase the API surface area exposed to gastrointestinal fluid, helping improve dissolution. For DCS Class IIb compounds, particle-size reduction alone may not be enough. These molecules have intrinsically low solubility, often linked to high crystallinity. In these cases, amorphous solid dispersion technologies (including spray-dried dispersion and hot-melt extrusion) may be needed. By converting the API into a higher-energy amorphous form, these approaches can improve apparent solubility and support absorption.

However, enabling technology is not simply a technical add-on. It becomes part of the overall product strategy. The resulting drug substance must deliver acceptable bioperformance while remaining stable, processable and compatible with downstream oral solid dosage manufacturing.

Content uniformity at low drug loads

Low API concentration is among the most consequential challenges in TPD formulation. Uniform distribution is essential for dose consistency, predictable pharmacokinetics and reliable product performance. Yet some products may require milligram or low-gram quantities of API to be distributed across kilograms of excipients.

In these circumstances, conventional blending can produce areas of excessive concentration, often described as hot spots, alongside areas containing too little API. While formulations containing more than roughly 10% w/w API can often be managed with more traditional approaches, many TPD products may contain API at 1–5% or below. Their processes must therefore be specifically engineered for uniformity.

Geometric mixing is one valuable approach. The API is first blended with an equal amount of excipient to form a small concentrate. That concentrate is then blended with a further portion of excipient, and the process is repeated in stages until the API has been evenly distributed throughout the full batch. Gradual dilution helps avoid the distribution problems that can occur when a small quantity of API is added directly to a large powder blend. Trituration may also support initial dispersion. In some cases, a carrier material such as colloidal silica can help bind hydrophobic API particles and distribute them more consistently. A conical mill may be used to bring API and carrier into closer contact before additional geometric mixing.

The best manufacturing route depends on the molecule and the dosage form. Roller compaction, wet granulation and other strategies may be appropriate, depending on API solubility, particle size, morphology, flow properties and environmental sensitivity.

Particle engineering and excipient design

Particle size and morphology affect much more than dissolution. At low concentrations, they can influence blend uniformity, powder flow, segregation risk, transfer behaviour and downstream manufacturability.

Micronization may improve dissolution, but it can also make an API harder to manage in a blend. Very fine particles can behave differently from larger excipient particles, increasing the potential for agglomeration, adhesion or uneven distribution. Developers may therefore need to consider the particle-size profile of the excipient system as carefully as that of the API.

In certain situations, particle-size engineering of commercial excipients may improve compatibility. However, modifying an excipient can introduce regulatory uncertainty, including the possibility that the altered material is regarded as a novel excipient. The potential benefit must be carefully balanced against this risk.

Morphology matters too. Needle-shaped crystals, for example, may align, bridge, segregate or flow differently from more regular particles. Where feasible, dissolving and recrystallising the API into a more manageable physical form may improve processing. Granulation can also help attach the API to excipients or carrier materials, reducing unpredictable movement through the blend.

The objective is not simply to change the particle: it is to create an API–excipient system that can be processed, scaled and controlled reproducibly.”

Roller compaction may be useful for certain problematic crystal structures, creating a more processable intermediate. Yet any particle-engineering intervention must be evaluated against the API’s sensitivity to heat, moisture, mechanical stress and other processing conditions. The objective is not simply to change the particle: it is to create an API–excipient system that can be processed, scaled and controlled reproducibly.

For low-dose TPD products, excipients are therefore not passive ingredients. Their grade, particle-size distribution, flow behaviour, binding capacity, chemical compatibility and suitability for the intended process all contribute to the control strategy. Early excipient and process gap analyses can identify whether the selected materials are capable of supporting the target dose and dosage form before the programme advances too far.

Designing for manufacture, containment and scale-up

Manufacturability cannot be treated as a late-stage consideration. A formulation that performs well in a small development batch but needs fundamental redesign for GMP manufacture can introduce avoidable delay and risk.

For low-dose TPDs, the formulation should be developed with the eventual equipment train, batch sizes, clinical supply plan, containment requirements and commercial manufacturing route in mind. Equipment similarity is particularly important because apparently minor differences in blending geometry, powder movement, transfer conditions or granulation behaviour can affect content uniformity.

Where development and production equipment are geometrically comparable, scale-up is more predictable. Intermediate bulk containers with consistent blending angles across different volumes, for example, can help preserve comparable powder movement. Geometrically scaled high-shear mixers can similarly support translation from development to larger GMP batches.

Process development may use one-factor-at-a-time studies or multi-parameter design-of-experiments approaches, according to programme stage, API availability, timelines and objectives. Either way, low-dose TPD products require close scrutiny of mixing time, speed, transfer steps, granulation conditions and powder handling, since small changes can influence uniformity and performance.

The final dosage form must also remain appropriate for the patient. Low-dose products may permit flexibility in tablet or capsule presentation, but dose size, frequency, ease of administration and patient compliance should remain central to formulation decisions.

Potency and analytics shape the strategy

Many novel TPDs are classified as highly potent or ultra-high potency before a complete toxicology package is available. Early assessments may rely on partial information regarding mechanism of action, therapeutic indication, pharmacokinetics, anticipated clinical dose and safety margin. Conservative assumptions are often necessary.

Low-concentration, highly potent products must achieve exceptional content uniformity while protecting operators, product, facility and environment.”

This potency profile has immediate manufacturing implications. Low-concentration, highly potent products must achieve exceptional content uniformity while protecting operators, product, facility and environment. Repeated geometric mixing, contained transfers and specialised cleaning can be operationally demanding within rigid or flexible isolators, where space and glove mobility are limited.

Potency therefore affects more than containment classification. It influences equipment selection, workflow, processing time, cleaning requirements and the realistic development timeline.

Analytical methods must be equally capable. High-performance liquid chromatography and ultra-performance liquid chromatography may provide suitable sensitivity for many low-dose products. As concentrations become lower, however, liquid chromatography–mass spectrometry may be required for accurate API quantification and cleaning verification.

LC-MS provides greater sensitivity, but it brings additional complexity. Beyond standard chromatographic factors—such as column chemistry, mobile phase, gradients and sample preparation—developers must optimise ionisation mode, source conditions, mass transitions, detector response and matrix effects. Recovering very small quantities of API from excipient matrices or equipment surfaces can further complicate sample preparation.

Analytical development should therefore be aligned early with formulation, containment and manufacturing strategy. The product must not only be manufacturable: it must also be measurable, releasable and cleanable with confidence.

A molecule-led route to commercial readiness

As TPD discovery advances, candidates may become even more potent and effective at lower doses. While therapeutically promising, this trajectory may also push formulation and manufacturing towards practical limits. An extremely low-dose tablet – for example, one containing only 0.005 µg of API – may be exceptionally difficult to manufacture consistently using conventional methods.

A molecule can be biologically elegant yet still present significant hurdles if its dose, solubility, physical form, morphology or stability make it difficult to formulate at scale.”

Early communication among discovery chemistry, drug substance development and drug product teams is essential. A molecule can be biologically elegant yet still present significant hurdles if its dose, solubility, physical form, morphology or stability make it difficult to formulate at scale.

Future solutions may include alternative drug-substance presentations, approaches that make low-dose material easier to handle and distribute, high-speed API-in-capsule technologies, and greater use of automation or robotics in synthesis, formulation, containment and filling operations.

For TPD developers, the most effective path is a molecule-led one. It integrates solubility enhancement, particle engineering, excipient selection, low-dose blending, high-potency containment, analytical development and scalable process design from the outset. That integrated strategy gives promising degraders the strongest possible chance of progressing from a compelling biological concept to a robust, manufacturable and commercially viable medicine.