Drug Physical Characterisation

Physical characterisation of the drug substance establishes its fundamental solid-state identity and is typically the earliest preformulation activity...

Physical characterisation of the drug substance establishes its fundamental solid-state identity and is typically the earliest preformulation activity undertaken. Differential Scanning Calorimetry, commonly abbreviated DSC, is the principal technique employed, measuring the heat flow associated with thermal transitions — melting, crystallisation, glass transition, and decomposition — as a sample is heated at a controlled rate. A sharp, well-defined melting endotherm is generally indicative of a pure, single crystalline form, whereas a broad or multiple endotherm suggests the presence of impurities, a polymorphic mixture, or an amorphous component. DSC is frequently complemented by hot-stage microscopy, which allows the direct visual observation of melting and recrystallisation events under polarised light, and by X-ray Powder Diffraction, which provides a definitive fingerprint of crystalline structure and is the gold-standard technique for detecting and quantifying polymorphism.

Polymorphism — the capacity of a single chemical entity to exist in more than one crystalline arrangement — is a phenomenon of enormous pharmaceutical significance, since different polymorphic forms of the same drug can exhibit markedly different solubility, dissolution rate, and even chemical stability, despite being chemically identical. A formulation developed using a metastable polymorph risks unpredictable conversion to a more thermodynamically stable, and often less soluble, form during manufacture or storage, with consequent loss of bioavailability. Preformulation scientists therefore invest considerable effort in polymorph screening, typically through controlled crystallisation from a range of solvents, to identify the most stable and pharmaceutically desirable form before formulation work begins.

Particle size and particle size distribution are further critical physical attributes, influencing dissolution rate, powder flow, content uniformity, and, for inhalation and parenteral products, the very feasibility of administration. Particle size is typically measured by laser diffraction for bulk powders or by microscopy and dynamic light scattering for finer suspensions and nanoparticulate systems. Related physical properties assessed during this stage include bulk and tapped density, which inform powder flow and compressibility behaviour relevant to tablet manufacture, and hygroscopicity, the tendency of a solid to absorb atmospheric moisture, which has direct implications for packaging selection and storage condition specification.

Collectively, these physical characterisation studies establish an unambiguous identity for the drug substance and flag, at the earliest possible stage, any solid-state risks — such as an unfavourable polymorphic landscape or excessive hygroscopicity — that must be actively managed throughout subsequent formulation development.