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Medical matters: Exploring the benefits of co-crystals

Unlike salts, co-crystals do not rely on ionic interactions and can therefore be used for non-ionizable drugs.

Neuroscience lab to study of the nervous system. — Photo: © AFP
Neuroscience lab to study of the nervous system. — Photo: © AFP

Co-crystallization is an alternative to traditional methods of crystallization for improving the physiochemical properties of medicinal products, like solubility and even taste, without affecting their pharmacology.

The topic has been raised to Digital Journal by Dr. Thomas Kendall, application specialist at crystallization firm Technobis Crystallization Systems. Kendall has discussed the benefits of co-crystals and how they can be screened in drug formulation.

Kendall  explains hat: “When developing a drug product, solid formulations like tablets and capsules are often selected because of the benefits offered to patients. These benefits include high patient adherence, low variability between dosages and ease of taking the drug. However, solid formulations often have poor solubility, which can affect the absorption rate in the body.”

Kendall adds that there are several possible solid forms that can be investigated as part of the drug development process, each offering different physicochemical properties. He states: “A common strategy to increase the solubility of an ionizable drug molecule is salt formation. However, this approach is not suitable for non-ionizable materials, meaning scientists may look at alternative methods for solid state optimization.”

Image by Zherebetskyy – Using Vesta visualisation software plotted the developed model similar to the Science paper, CC BY-SA 3.0

Co-crystals for solubility

Continuing with the advantages, Kendall states: “An alternative strategy for improving solid state properties is co-crystallization.”

The U.S. Food and Drug Administration (FDA) defines co-crystals as “crystalline materials composed of two or more different molecules, typically API and co-formers, in the same crystal lattice”. Unlike salts, co-crystals do not rely on ionic interactions and can therefore be used for non-ionizable drugs.

What does this mean in practice? Kendall cites: “An example of a drug that incorporates co-crystals is Abilify®, otherwise known as Aripiprazole, an antipsychotic used in the treatment of schizophrenia. The drug exhibits poor aqueous solubility because of its twelve known anhydrous polymorphs. To improve the solubility and thermal stability of the drug, scientists used a co-crystal with fumaric acid. This ensured the drug would dissolve at the correct critical time.”

As a further example, Kendall references: “Co-crystals can also be used if the solubility of a compound is too high and is absorbed too quickly in the body, modulating solubility for a sustained, slower release. One example of where this has been used is Ribavirin, an antiviral medication used to treat RSV infection and hepatitis C with barbituric acid.”

Improving taste

As a different application, Kendall looks at flavours, noting: “Taste has been shown to be an important factor in the acceptability and willingness of a patient to take oral drug products, especially in children, adolescents and the elderly. Co-crystals can be formed with sweeteners and taste maskers as co-formers, which leads to an improvement in the taste of a drug. This can be relatively straightforward, as many sweeteners and taste maskers are already FDA approved for application in food.”

Drawing on an example, Kendall references: “Theophylline, for example, is a drug used in therapy for respiratory diseases such as asthma and chronic obstructive pulmonary disease (COPD). Scientists formed a co-crystal with saccharin, an artificial sweetener, which was found to improve the overall taste of the drug and mask bitterness.”

The co-crystal screening process

In terms of pharmaceutical applications, Kendall  lists: “For scientists looking to use co-crystals to optimize their API during drug development, co-crystal screening is needed to identify candidates for characterization. Co-crystal screening can be achieved using a number of techniques, including liquid assisted grinding or decreasing the temperature and driving supersaturation. Certain co-crystals will only be formed using certain techniques so it is important to use a wide range of screening methods. To avoid missing co-crystals, scientists can employ a systematic approach, which will also aid further development in the scale-up phase.”

In terms of practical application, Kendall says: “One popular tool for co-crystal screening is the Crystal16, which can be used to design an extensive co-crystal screening program by measuring the clear points of a series of co-formers and the API using 16 parallel reactors. By measuring the saturation temperatures of the mixtures, the possibility for co-crystallization can be easily assessed.”

Kendall’s company, Technobis Crystallization Systems develops several proprietary technologies that aid in the research for crystallization and solid-form. It also releases monthly webinars on topics like solubility and crystallization, sharing how products like Crystal16 can facilitate the crystallization process.

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Written By

Dr. Tim Sandle is Digital Journal's Editor-at-Large for science news. Tim specializes in science, technology, environmental, and health journalism. He is additionally a practising microbiologist; and an author. He is also interested in history, politics and current affairs.

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