Poor solubility remains a major challenge in formulation development. As the number of new chemical entities that are poorly soluble keeps increasing, formulation scientists are faced with the important task of addressing these solubility issues so that more compounds can be translated into clinically useful medicines. A popular approach to this problem is to use polymers as solubilizing excipients. Pharmaceutical Technology brought together a panel of industry experts for a special forum to discuss solubilizing polymers and the related formulation strategies for poorly soluble drugs. Participants include Michael Morgen, PhD, director of new technology at Bend Research; Brian Koblinski, strategic marketing manager at Dow Wolff Cellulosics; and Firouz Asgarzadeh, PhD, director of technical services, North America, pharma polymers & services at Evonik Corporation.
Chemical classes and physicochemical properties
Solubilizing polymers are used in many types of formulations, depending on the route of administration, dose, API properties, and specific delivery challenges associated with the particular development program. For oral delivery, solid amorphous dispersions can be an enabling formulation approach for improving the bioavailability of Biopharmaceutics Classification System (BCS) Class II and IV compounds. Two common processes for making such dispersions are spray-drying and hot-melt extrusion (HME). Spray-drying can be used to form a monolithic dispersion particle or alternatively, the dispersion can be deposited onto a solid support to increase surface area and dissolution rate. Two-phase physical mixtures of polymer and crystalline API in a solid dosage form can be used to solubilize the API. For early scoping studies, liquid solutions of polymer (e.g., PEO) and the API may be valuable. In each case, the choice of formulation type and the manufacturing process will depend on the requirements of the specific drug-development program.
Koblinski (Dow): Cellulose-based polymers, such as hypromellose (HPMC) and hydroxypropyl methyl cellulose acetate succinate (HPMCAS, also known as hypromellose acetate succinate) have high utility in solid dispersions. Their safety and low drug reactivity make them an ideal candidate for most drugs. Spray-dried dispersions (SDDs) and HME are the most common technologies for solubilization used with cellulosic polymers. Cellulosic polymers maintain stable solid dispersions, inhibit API crystallization, and can promote supersaturation of the drug. Options in viscosity and substitution levels both with HPMC and HPMCAS help a formulator to optimize performance in solubility enhancement as well as in material processibility, which includes postprocessing SDDs and HME formulations into traditional dosage forms such as tablets and capsules.
Asgarzadeh (Evonik): The chemical structure and physicochemical properties of APIs and polymers play a major role in the formation of polymer-API solid solutions. Poly(meth)acrylates, cellulosic polymers and PVP are extensively used in solid dispersion formulation development. Each class of polymers contains specific chemical bonds that can interact with specific groups on APIs. HME, SDD, and coprecipitation techniques transform poorly-soluble crystalline drugs into an amorphous, metastable, higher-state-of-energy structure dispersed in a polymer matrix that is more easily solubilized, leading to enhanced solubility. Strong hydrogen bonding and ionic interactions between the amorphous API and the polymer would inhibit or retard the recrystallization of the API even with polymers that have low glass-transition temperatures (Tg's). Because there is a myriad of API structures with different physicochemical characteristics, there is not one class of polymers that would be considered better solubilizers than others. It is essential to select appropriate polymers for screening studies based on a good understanding of both the polymer and drug structures and the possible interactions they may have.