Screening and characterization
Screening protocols involve recrystallization from a diverse array of solvents under thermodynamically and kinetically controlled
conditions, explains Steven Byard, PhD, head of physical and molecular characterization at Alnwick, United Kingdom, at Covance.
The transformation of metastable states and extended solvent-based studies conducted under a range of crystallization conditions,
such different concentrations, temperatures, cooling rates, and stirring rates, are examined. The effect of different impurity
profiles and seeding experiments are further considered.
"Of course, one should never forget that one of the principal criteria for recrystallization may be to obtain a desired target
purity, with control over both the impurity profile and residual solvents, in addition to obtaining a defined physical quality.
Under some circumstances, we apply controlled stress to the API in a variety of carefully chosen matrices to afford physical
forms not normally readily obtained by other means," says Byard.
As development progresses to Phases II and III, there is an increased emphasis on the most appropriate crystal morphology,
crystal quality, and crystal-size distribution for the commercial formulation and also definition of specifications for the
physical quality of API, explains Byard. Verification that the correct polymorph will be retained in later stages of drug-product
development is completed by performing a new polymorph screening with a batch of API from the final commercial synthesis route.
The critical manufacturing process variables and their ranges are determined and controlled to produce a robust API process
that meets established quality attributes.
Screening for physical forms takes into account properties of the molecular structure and explores the effects of solvents,
temperature, concentration and various other parameters that can influence crystallization, explains Byard. Many solid forms
are generated by the different crystallization approaches, based on the effect of the interfacial energy between the nucleus
and the crystallization media, supersaturation (a driving force of crystallization), and temperature. The crystallization
methods can be broadly classified into four groups: crystallization by sublimation, melt crystallization, crystallization
by spray drying, and crystallization from solution, which is the most commonly used method because it provides data for the
crystallization process development. High-throughput screening methods can be used to cover a wide range of conditions to
help ensure that all different forms are recognized.
Many complementary techniques exist for characterizing solid forms, such as single crystal X-ray diffraction, X-ray powder
diffraction, solid-state nuclear magnetic resonance (SSNMR), infrared spectroscopy, Raman spectroscopy, Terahertz spectroscopy,
hot-stage optical microscopy, and thermal analyses. These methods are routinely used and provide the platforms for incremental
improvements.
Recent advances
SSNMR probes samples directly at the molecular level to provide information about structure and mobility. Consequently, the
physical form of constituents in either physical or chemical mixtures can be examined with relative ease. "This makes SSNMR
a technique of choice for studying drug products, where the physical form of the active can be determined in a complex matrix,
even if multiple components are amorphous," says Srinivasan. For example, the presence of physical impurities can be determined
when lattice modifications are not altered significantly and, by implication, not readily detected by X-ray powder diffraction
(1). In another example, researchers at GlaxoSmithKline reported on SSNMR experiments based on dipolar correlation, spin diffusion,
and relaxation measurements to characterize the structure of amorphous solid dispersions (2).
NMR crystallography, which incorporates density functional theory calculations, is used to provide molecular-level information
about structure and dynamics of drug substances, including solvate characteristics (3, 4). Recently, X-ray photoelectron spectroscopy
(XPS), in conjunction with SSNMR and density functional theory prediction, was used to determine co-crystal formation (5,
6). "This too is a promising approach to understanding exactly what is happening at the molecular level and, by implication,
enabling a sound basis for making decisions about formulation processes," says Srinivasan.
Surface properties and the related methods for characterization also are important considerations. Researchers at the University
of Manchester and Sanofi recently reported on using a surface-sensitive technique, XPS, in detecting the free-base surface
enrichment of a pharmaceutical fumarate salt . They reported that a yellow discoloration was observed at the surface of normally
white crystals of the fumarate salt, which was preliminary attributed to the presence of trace amounts of free base. The samples
with yellow surfaces could not be successfully milled, which was an important part of the production process for providing
material of the required physical quality for product formulation. Because no conventional bulk analytical technique could
readily provide an explanation for the yellow color, the researchers used XPS to characterize the salt. The identification
of residual free base at the surface of the crystalline material by XPS was significant for optimizing the crystallization
process to yield material of required quality for milling at the plant scale (7).
Crystal-structure prediction is an active area of research. "Ab initio crystal structure prediction is another promising area
as is elucidating hydrogen bonding potential to provide information about the possibility of discovering additional polymorphic
forms of a drug substance," says Srinivasan.
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