Introduction to Pharmaceutical Co-amorphous Systems Using a Green Co-milling Technique

The concept of co-amorphous systems is introduced in an integrated laboratory experiment, designed for advanced chemistry students, using solvent-free, environmentally friendly mechanochemistry. The dual-drug naproxen-cimetidine co-amorphous system (NPX-CIM) is investigated as an example of the emergent field of medicinal mechanochemistry. Students are trained in solid-state characterization techniques including X-ray powder diffraction, Fourier-transform infrared spectroscopy, and thermal analysis by differential scanning calorimetry. This lab experiment also provides an opportunity to discuss the relevance of different solid forms of pharmaceutics, emphasizing particular properties of disordered materials. This experiment can easily fit the curriculum of any Chemistry or Pharmacy master level degree in courses dealing with instrumental analysis, solid state chemistry, or green chemistry, for classes of 6 to 18 students, in a 5-h lab session. Suggestions to adapt it to the use of a single characterization technique are provided.


Introduction
The aqueous solubility of active pharmaceutical ingredients (APIs) is one of the most important parameters for achieving the systemic concentration necessary for the intended biological effect. However, the vast majority of APIs and API candidates exhibit low aqueous solubility, resulting in reduced absorption after oral administration. Thus, one of the major challenges in the pharmaceutical industry is the development of strategies to improve the aqueous solubility of these compounds, thereby incrementing their oral bioavailability. Several techniques are available to improve the aqueous solubility of poorly soluble drugs, including physical modification, such as particle size reduction, or the search for new solid forms (polymorphs, solvates, salts, co-crystals, co-amorphous phases).
The development of amorphous solid pharmaceuticals has been shown to be an effective approach to increase the aqueous solubility of APIs. Amorphous solids differ from crystalline materials by the absence of long-range structural order, keeping only short-range order. On thermodynamic grounds, amorphous solids are unstable or metastable, being in a higher energy state relatively to the crystalline form. Therefore, they may exhibit low physical stability due to their tendency to relax, crystallizing after some time. Thus, to circumvent the low stability inherent to the amorphous state, several techniques have been proposed for kinetic stabilization of amorphous compounds, namely the formation of co-amorphous materials. These are multicomponent amorphous solids that result from the combination of an API with a pharmaceutically acceptable low molecular weight coformer, or with another API in a dual-drug perspective. Therefore, co-amorphous formulations aim at increasing, not only the aqueous solubility, but also the physical stability of the disordered solid. The increased stability of co-amorphous solids is due to mixing effects and, mainly, to the establishment of intermolecular interactions between the drug and the co-former.
Among the various techniques available for preparing co-amorphous systems (rapid cooling of melts, rapid solvent evaporation from a solution of the two components, freeze-drying...), mechanochemistry has proven to be an effective technique, a green alternative to classical processes: no organic solvents or high temperatures are required, also reducing the amount of chemical waste. Consequently, mechanochemical processes have some advantages that include lower chemical degradation and higher recovery than other methods of amorphous preparation, which may involve high energy consumption and/or high consumption of organic solvents. The biopharmaceutical classification system, BCS, groups APIs into four groups according to their aqueous solubility and permeability. According to this classification, class II compounds have low solubility and high permeability and class IV compounds present low solubility and low permeability. These APIs are preferential targets for co-amorphous phase investigation.

Objectives
-To produce, using green mechanochemical methods, an equimolar co-amorphous binary system of cimetidine and naproxen, Figure S1, both classified as BCS class II.
-To characterize the starting solids and those submitted to the milling process, by several physicochemical techniques: differential scanning calorimetry, infrared spectroscopy and X-ray powder diffraction.

General procedure
Mechanochemistry -To synthesize the co-amorphous system, weigh a total of 100 mg cimetidine and (S)-naproxen in an equimolar ratio. Grind the mixture during 60 min at 30 Hz in the oscillatory ball-mill. The individual components should be submitted to the same process to account for the effects of grinding in the solid phase. Notice that the milling jar and the milling balls must be made of the same material; the two milling stations should be always loaded symmetrically, with the two mixing jars carrying approximately the same mass, and properly secured by screw clamps. If you need to run only one experiment, an empty jar with no balls must be used in the other milling station. Take special care to not heat the jars in your hands and avoid water vapor condensation on the materials that can lower the glass transition temperature.
Differential Scanning Calorimetry -These experiments are useful to characterize the thermal behavior of the samples, including the melting temperature for crystalline solids and the glass transition for amorphous materials. Prepare DSC pans and submit them to a heating run from 0 to 165 °C at 10 °C/min. Repeat the procedure for all starting materials and ground solids.
Infrared Spectroscopy (ATR-FTIR) and X-Ray Powder Diffraction -Collect infrared spectra and powder diffractograms for all samples.