(T1230-06-32) Intentionally Made Amorphous Substance to Quantify Unintentionally Produced Amorphous Materials: Decades of Hands-On Experience

Purpose: There has been extensive work carried out for amorphous content determination using various techniques that has added invaluable insights to fundamental research. However, hands-on experience of handling numerous inhaled APIs and their corresponding products motivated this author to highlight practical approaches required to develop a reliable method(s) for this challenging subject. Research in this field clearly demonstrates that given a right skills, knowledge and experience, a suitable method can be developed which ensures reproducible data consistently. The scope of this study limits within, arguably, three (1) solution calorimetry, 2) gas perfusion microcalorimetry and 3) dynamic vapour sorption) most sensitive techniques which are successfully used to detect ~1% amorphous content. The aim of this study is to share knowledge and experience from practical aspects so that one can attempt to address this challenging issue very early stage of their DPI product development activity in an open and pragmatic manner to reduce time and cost in future.

Methods: Solution Calorimetry is the determination of heat of solution when a solid is dissolved in a liquid, or two liquids are mixed upon breaking of a glass ampoule whereby sealed glass ampoule holds solute in question. During the solvation process, higher energy is required to break the crystal lattice resulting in an endothermic heat of solution for a crystalline material. Conversely, as there is no crystal lattice arrangement in amorphous material, the process is exothermic. The difference in heat of solution for crystalline and amorphous substances is used to generate a linear response between heat of solution (J/g) and % amorphous content (w/w). Review the data by checking the difference between heat of solutions of both crystalline and amorphous forms and if the difference is big enough (e.g. ≥ 20J/g) then investigate 1%w/w amorphous material using the same test conditions. Gas Perfusion Microcalorimetry is the measurement of heat as the sample adsorbs, absorbs, crystallizes and then expels any solvent during a plasticization process. A robust method can be easily developed that functions well with various solvent systems (e.g. DI water, ethanol) over a period of 24 hours or more, thereby ensuring adequate collection of enough data during the wetting and drying processes, as well as observing that the baseline is at a plateau upon completion of each measurement. This author has worked in collaboration to develop numerous methods using this technique for inhaled APIs (e.g. formoterol). However, this technique presents challenges especially if the sample is either a hydrate/solvate. Dynamic Vapour Sorption (DVS) is the determination of change in sample weight of the solid particulate sample when exposed to a vapour (e.g. DI water, ethanol, n-Octane, acetone with dry nitrogen mixed), the weight changes commonly being expressed in mass-percentage terms. An increase represents vapour adsorption/absorption whilst a decrease represents recrystallisation, for example upon exposure of the sample to increased water vapour at a constant temperature. A robust method can be developed easily that functions with various solvents (e.g. deionised water, ethanol, n-octanol, hexane, acetone etc.). This author has developed numerous methods using this technique for inhaled APIs (e.g. formoterol, fluticasone). DVS can be very useful to identify that the standard is crystalline, since there should be no weight loss for such a sample upon exposure to 90%RH or 90%RVP.

Results: Important aspects of data review considering all three techniques. A calibration plot is then generated by using the best sensitive method between 1 and 10% w/w amorphous material in the crystalline bulk. Each determination should be measured in triplicate. The fully crystalline sample should be determined at least six times and a fully amorphous sample requires to be run in triplicate. There are two approaches to calculate % of amorphous content in the processed sample, i) assume linearity in the calibration relationship and determine amorphous content from the relationship: y = mx + c ii) use the pharmacopoeia method. For an example, y = heat of the solution (J/g) for the test sample, x = % of amorphous content to be determined using m (slope) and c (constant) values from calibration equation. In author’s experience, both approaches often work well and there is a good agreement between them.  However, it is more reasonable to use the calibration equation approach as the concern is with low levels of amorphous material. The amorphous content should not vary by more than 1% between the two approaches of calculation.

Conclusion: The practical knowledge shared in this article clearly demonstrates that it is essential to understand the process of detecting small level of amorphous material and then having a control over this amorphous content is critical. That goal can only be achieved by understanding data in depth which would support the hypothesis. Obtaining reproducible data and understanding the processes for these challenging systems (crystalline, amorphous, hydrate, solvate, milled samples etc.) and their responses to each technique requires in-depth knowledge, skills and experience.

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Acknowledgments: No conflict of interests

Figure 1: Heat of solution data as a power (y axis) vs Time (x-axis) – a) the break, solid-liquid interaction and dissolvation processes have been completed; b) processes were not completed and requires more time and c) showed some tailing effect. The tailing effect could be due to the heterogenous material.



Figure 2: Gas perfusion microcalorimetric data as a power (y axis) vs Time (x-axis) – a) the complex response using DI water b) using ethanol. These complex responses are similar to the reported data for salbutamol sulphate.
Figure 3: DVS data as a % of weight change (y axis) vs Time (x-axis) – a) the 6%w/w weight loss from a fully amorphous sample at 30%RVP using ethanol b) the 0.02%w/w weight loss from ca. 2% amorphous sample at 90%RVP using ethanol.



Table 1: A summary with Pros and Cons of each technique.