We offer multiple XRPD methods for characterization of active pharmaceutical ingridients (APIs), excipients, intermediates and finished drug products, including method development and validation.
The XRPD method is described in chapter <941> of USP and chapter <2.9.33> of Ph.Eur.. Basic information about X-Ray Powder Diffraction (XRD or XRPD) may be found at www.xrpd.eu.
The investigation of polymorphs (chemically identical substances in different crystallographic forms) is a key step in drug product development. Since polymorphic forms differ in their solid-state structure, they can have different aqueous solubilities and dissolution rates. The solid-state properties of an API can therefore significantly influence the stability and bioavailability of the drug product. Polymorphs can also exhibit different physical and mechanical properties, such as hygroscopicity, flowability, and compactibility, which in turn may affect the processing or manufacturing of a drug substance. XRPD patterns are directly related to the crystalline structure of a material. Therefore, XRPD is considered the “gold standard” for fingerprinting of different crystalline phases, polymorphs, hydrates or solvates according to their unique diffraction patterns at different stages of drug development. Minor polymorphic impurities can have a significant impact on the stability or bioavailability of a drug compound. It is therefore crucial to be able to detect and quantify polymorphic impurities down to very low concentration levels throughout all development and production steps.
At DANNALAB we can detect and quantify polymorphic impurities down to very low limits of quantification (LOQs), in some cases as low as 0.1% wt of pure API. Each quantitation method is developed and validated following good manufacturing practice (GMP) and ICH guidelines.
Due to its direct structure-related selectivity, XRPD allows for the analysis of actual percentages of an API in formulation and in the final dosage form of a drug. XRPD therefore serves as a powerful tool for detecting the polymorph transformations of compounds in a pharmaceutical formulation. XRPD quantitation of active pharmaceutical substances is also used during stability studies and batch quality control (QC) testing.
By using proprietary techniques we are usually able to validate quantitation at LOQs as low as 0.5% wt from the total API weight in dosage form.
Since the physico-chemical properties of pharmaceuticals are influenced by their solid-state forms, the crystallinity of an active ingredient has a profound impact on both processing behavior (compressibility, compactibility and hygroscopicity) and the bioavailability of the active ingredient in the finished product. Due to its better stability, the desired solid-state form for an active pharmaceutical ingredient (API) is usually crystalline. However, the amorphous state is sometimes required to achieve sufficient efficacy for low soluble active ingredients. During the production or processing of pharmaceutical solids, certain procedures, such as milling, spray drying or lyophilization, can disrupt the crystalline structure and lead to the formation of amorphous regions. On the other hand, undesirable recrystallization can take place within amorphous formulations, driven by thermodynamic or physico-chemical factors. To establish the integrity of the finished product it is important, therefore, to be able to determine the existence and amount of amorphous material within a crystalline matrix, and crystalline material in an amorphous matrix. XRPD is successfully used for the determination of crystallinity, both for small crystalline contents in an amorphous matrix and vice versa. An approximate LOQ of 1% wt is achievable for the quantitation of crystalline material in amorphous matrices. Note that, in some cases, the combination of XRPD with additional SAXS characterization may reveal important information about the mesostructure of an API, such as the value of a specific surface, with direct implications for its dissolution profile.
Although a complete crystal structure with atomic positions may generally be determined from a XRPD pattern, in most cases it is sufficient to identify the lattice type and dimensions of a crystallographic unit cell for unknown material. The method based on determination of crystallographic cell symmetry and parameters from an XRPD pattern and subsequent matching of each experimental peak with peaks recalculated as the function of these parameters is called "indexing". Our instrumentation can be used to achieve a bandwidth of as low as 0.02 degrees between the theoretically predicted and actual peak positions. Once this is achieved for all peaks in the pattern, it serves as solid proof confirming the purity of a substance and ensuring that it is not a mixture of polymorphic forms. Once all the peaks in the pattern calculated from the cell parameters of a single crystalline form have been explained, a drug substance can be uniquely fingerprinted and considered to be a pure crystalline form. This form of crystallographic analysis is particularly useful for the characterization of polymorphs. An indexed XRPD pattern or crystallographic cell parameters are often of critical importance in securing a patent.
The Rietveld method is a full-pattern fitting method in which a calculated profile is fitted to a measured pattern by varying a range of crystallographic parameters. It is used for atomic structure refinement and for precise determination of the characteristics of a unit cell. The method derives crystallographic parameters, such as lattice constants, atom coordinates within the unit cell, site occupancy and preferred orientation. In addition, Rietveld refinement can be used to reveal the quantitative composition of multiphase mixtures.
The XRPD pattern of a crystalline phase is directly related to its crystalline structure, and an XRPD pattern of a compound mixture is generally the sum of the XRPD patterns of the single compounds. XRPD pattern decomposition often serves as the preliminary step before the subsequent identification of unknown compounds in a mixture by comparison with the available databases. Phase analysis using XRPD is ideal for the analysis of polymorphic mixtures, investigations of phase transformations in compatibility studies, and optimization and quality control of the final formulation. In addition, XRPD full-pattern analysis allows for the (semi-)quantitative analysis of amorphous compounds in the formulation.
