Introduction:
Drugs frequently exist in the solid state. It is important to get the physico-chemical properties such as solubility and bioavailability of the active pharmaceutical ingredient (API) right as well as any excipients which may also contribute to the characterisation of the drug, this is due to high costs and demand of time needed for drug development. Toxicity due to chronic exposure, a small therapeutic range or a poor drug release profile are also major problems along with the administration problem of new drugs such as proteins and DNA possibly leading to higher doses due to poor absorption. Around 33% of drugs usually exhibit drug safety risks post market approval, this may be due to inability to replicate the complex multi organ system of humans in pre-clinical studies using animals along with the failure of seeing the effect of metabolites of the drug. Insufficient knowledge of underlying biological mechanisms is also a problem as this results in unexpected effects from the drug. Pharmaceutical companies need to ensure quick, efficient and cost effective development and production of drugs whilst maintaining safety and efficacy. However, increasing regulation is leading to higher costs and longer development times which consequently gives less time until the patent expires. Along with higher costs of drug development are cost pressures on the national health service leading to a downward pressure on prices of new drugs, this is due to inadequate collaboration between academia, industry and the government. (Torjesen, 2015)
Drugs in solid form:
Many APIs which are present in final medicines can exist in a variety of solid forms, this is because different crystalline conditions such as the solvent or temperature used will govern what is produced; a combination of thermodynamics and kinetics can result in the production of a wide range of solid forms such as crystals, polymorphs, salts or solvates. Crystallisation is a key process in medicine manufacture as it is used to purify and separate API’s from other components such as the solvent, unreacted material, by products and contaminants so the correct solid form can be produced. Crystallisation is a two-stage process, nucleation and crystal growth; crystals nucleate from the joining together of molecules, most APIs are small organic molecules which can easily form hydrogen bonds and this hydrogen bond pairing is known as synthon. Crystal growth involves molecules and other growth units making interactions with molecules in the crystal surface to join together. However, crystals are anisotropic, so different surfaces have different chemical compositions leading to different growth rates; crystal growth rates govern the morphology of the crystal, but macroscopic and bulk properties may also be affected. (Mullin & J.W, 2001)
An amorphous solid has molecules arranged randomly, if a substance is polymorphic it can also exist as more than one three-dimensional structure. Polymorphic material is less preferred as the different structural relationships results in different physico-chemical properties making polymorphs slightly unpredictable. Solvates which can also be polymorphic, are made when a solvent molecule is incorporated into the extended arrangement of molecules usually in stoichiometric quantities. Solubility of solvates can vary as they can be strongly bound via hydrogen bonds or loosely bound allowing them to be easily desolvated. Another solid form is the salt form; salts are usually formed when acid hydrogens on the API molecule react with common salt formers. Physico-chemical properties change when a salt is formed so different salts of the same molecule are considered different APIs. (Aitipamula, 2012)
A crystalline form contains molecules in a regular, repeated arrangement extending in all three spatial dimensions forming an extended structure such as a crystal lattice; crystalline forms are more preferred as they are easy to prepare and more stable. A co-crystal is a solid material formed via a hydrogen bond pairing with another molecule from the same crystal lattice, which is known as the co-crystal former; new bonds and association to molecules alters the physicochemical properties in comparison to the parent material allowing development for desired properties. Co-crystals can be formed from lots of different methods such as solid grinding, which is when the two materials are ground together in the dry state, directly bonding to form the material without needing solvent; solvent drop grinding which is the same as drop grinding just uses a small amount of solvent. Solution crystallisation can also be done which is when the two materials are dissolved in a solvent in the correct ratio. (Yadav, 2009)
Benefits of crystals:
In the current climate, co-crystals are getting a lot of attention in the pharmaceutical network, this is due to their use in the design of new crystalline structures because of the nature of their intermolecular interactions and their new behaviours during manufacture, storage and transport. New co-crystals being formed could lead to more patents and therefore improved pharmaceutical products; economical benefits would be seen based on improved properties of co-crystals such as enhanced solubility and dissolution rate, improved stability. Ability to form salts, susceptibility to a supramolecular design and improved bioavailability of an API without replacing the API can lead to displacement of current commercial solid drug forms. There is a difficulty in establishing in-vitro to in-vivo correlation for salts and polymorphs but co-crystals are easier to predict along with improved hygroscopicity, chemical stability, purification and flowability. Co-crystals also have limitations, development can be challenging along with difficult translation into viable drug products. Further concerns have been raised regarding the safety of conformers due to their unpredictable performance during dissolution and solubility. (Almarsson, 2004)
Aim:
The aim of this practical was to make a co-crystal of caffeine (figure 1) by solvent drop grinding and solution crystallisation using maleic acid (figure 2) and acetone. The ternary phase diagram shown in figure 3 shows a hatched region, which approximates the zone in which the metastable 2:1 co-crystal would appear, region 3 shows an area with crystals of pure caffeine as well as the 1:1 co-crystal and region 4 shows the stability zone of the 1:1 co-crystal. The data points around the area which represents the 2:1 co-crystal have used drops of acetone showing it was formed via the dry or drop grinding method however when adding a larger amount of acetone which is done in solution crystallisation the crystals from region 3 appeared. This allowed a prediction for the results of the experiment.
Co-crystal characterisation:
The co-crystals formed were then characterised by differential scanning calorimetry and powder X-ray diffraction to verify that the preparation was successful.
Differential scanning calorimetry includes a sample and reference substance; these two substances reside in separate chambers. The reference chamber contains a solvent and the sample chamber contains the same amount of solvent as well as the substance of interest. The change in enthalpy associated with the heat of reaction is measured; the same type and amount of solvent is present in both chambers so any enthalpy change would be due to the presence of the substance of interest. Changes such as melting point, crystallisation, polymorphic transformation and loss of solvent can also be indicated. Comparison to reference material would allow identification of the substance of interest.
Powder x-ray diffraction is an x-ray technique useful for crystalline samples only because amorphous materials wouldn’t give useful patterns. The crystals diffract the x-rays produced by electrifying an x-ray tube with a metal filament such as copper (which is the most common) or iron; wavelength would change with the metal used, the wavelength of x-rays should be like the spaces in the crystal structure so the x-rays can be diffracted to form a diffraction pattern. As crystals have planes of molecules within the structure, the spacing between the planes act as a diffraction grating; in a 3D structure, there are many sets of planes which would diffract but the relative orientation of the crystal and the x-ray must be varied to collect all possible diffraction. Powder x-ray diffraction also works better when the sample is ground with no solvent and a reasonable quantity is needed for good analysis. The powder x-ray diffraction pattern of a substance would be like its fingerprint, this is because each crystalline substance would give a unique pattern which is why powder x-ray diffraction is a very reliable technique. Again, comparing the sample data to a reference material would allow identification of a substance, in this case the co-crystal of caffeine. (Elbagerma, 2010)