Formulating PROTACS and other Targeted Protein Degraders

The larger the number of modalities that can deployed to treat diseases, the greater the chance of helping patients who have unmet clinical needs. An exciting new modality, targeted protein degradation, employs small molecules to induce the degradation of disease-causing proteins. Currently, most of these degrader compounds belong to the family of so-called bifunctional molecules, which consist of two moieties, a ubiquitin-ligase recruiter and a target recruiter, connected by a linker.

Many projects are in the early stages,  primarily focusing on the protein to be degraded and the optimisation of linkers and ligase recruiters. Ultimately, these molecules will need to be formulated for pre-clinical and clinical testing. Degraders that are currently in clinical trials are not limited to an intravenous route of administration, exemplified by a degrader in Phase I employing an oral route of administration. In addition to dealing with the large size of these ‘small molecules’, ignoring the solid form and formulation aspect of these molecules can lead to costly delays at best, or project failure at worst.

Formulation of targeted protein degraders – beyond the rule of five and the identification of optical isomers

Bifunctional degraders break several of the “rule of 5” guidelines for oral bioavailability. Early formulation strategies for the pre-clinical can enhance stability, solubility, bioavailability, permeation, and reduce efflux, offering valuable insights into the molecules’ effectiveness.

Thalidomide derivatives, known for their attractive physicochemical properties, are often used as ligase recruiters for degraders that progress to the clinic. However, this introduces a chiral centre, which is well documented to be unstable in solution. As a result, these molecules often don’t have any other stereocenters in the molecule. Removal of one of the carbonyl groups of the Thalidomide derivative renders that stereocenter stable, allowing the incorporation of stereocenters elsewhere in the molecule without fear of ending up with a complex mixture of diastereoisomers.  This would make the drug substance production more complex, slow and costly, with a knock-on effect on the cost of developing the drug product.

The regulatory authorities are becoming much less reluctant to allow mixtures of optical isomers to move forward.  However, the crystal landscape of such mixtures can quickly become more complex, as they could have different solubilities, dissolution profiles, and milling profiles. Any solid-state parameter could be different between these two diastereoisomers. Early solid form characterisation is recommended to identify any potential differences and mitigate any risk as you progress into formulation.

Formulations for pre-clinical and clinical evaluation; understanding your form is key

Formulating these molecules for pre-clinical species is often more challenging than that for humans. Although the use of some aggressive excipients is tolerated in the models, there may be a larger range in the amount of material required to be solubilised; this may be as little as 3 mg/ml or as high as 100 mg/ml. This is driven by the requirement of a formulation suitable for toxicology studies where dose linear PK is needed.

While most formulations for these models are in liquid form, the performance of the formation upon hitting the gastrointestinal fluids is often ignored by those unfamiliar with the nuance of drug delivery. One of the pitfalls is that the active pharmaceutical ingredient (API) can ‘dump’ out of the solution either in the gastric or intestinal fluid. Dissolution (the process of a substance forming a solution of a material) depends on various variables. These include:

• pH of the environment that the drug is in e.g., pH 1.5-2.0 in human gastric acid, pH in the duodenum
• pKa of any ionisable sites on the API
• the solid form(s) of the compound, is it amorphous or crystalline. Does it form a single crystal structure?
• Whether the API forms polymorphs or hydrates

Choosing the right formulation strategy for dissolution limited compounds

A solid form screen often identifies a salt form, hydrate or polymorph which has the optimum characteristics that you are screening for. If you are unable to resolve your dissolution issues by identifying a suitable solid form, there are tools in the formulation arsenal which can be used.

Under the Developability Classification System, Class IIa compounds are limited by dissolution. The main formulation technique for enhancing dissolution is to reduce particle size. This increases the surface area to volume ratio of a particle and increases its ability to dissolve. Particle reduction can be achieved by micronisation and nano-milling.

Formulation support at Sygnature Discovery

Sygnature Discovery is well-positioned to advance your molecule toward clinical success. We possess on-site instrumentation for a wide range of formulation techniques, including lipid-based SMEDDS/SNEDDs, spray drying, lyophilization for amorphous solid dispersions, as well as simple solvent-cosolvent systems and nano-milling. We can incorporate permeation enhancers, PGP inhibitors, or combinations thereof to give your degrader the best chance of success.

To find out more about how Sygnature Discovery can support the formulation development of degraders, as well as other formulation techniques that can progress your molecule to the clinic, meet Dr Rachel Hemsley, Business Development Director, and Dr Farhan Taherali, Lead Scientist in Formulation, at CPhI Barcelona.

Contact info@sygnaturediscovery.com or R.Hemsley@sygnaturediscovery.com to book a meeting.