Problem: Crystallization is the most common method used for final purification and isolation of the active pharmaceutical ingredient, or API, when synthesizing a drug at commercial scale. Molecules can and do adopt more than one type of crystal structure upon precipitation. These different crystal structures are known as polymorphs when they differ only in the relative orientation of one molecule to another, or pseudopolymorphs when there is a change in the chemical make-up of the unit cell, such as the inclusion of a different number of solvent molecules or a different counter ion.
While the formation of more than one polymorph is not a big concern when the only desire is to isolate a pure substance, the physical properties associated with two different crystal forms are usually different. This can cause serious issues in the pharmaceutical industry where changes in chemical stability, the proclivity to absorb water, the rate of dissolution, etc., are required to be measured, understood, and controlled for any marketed drug. The situation gets even worse when one considers that manufacturing processes such as milling, mixing, pressing and drying can cause a compound to change its polymorphic state. Just testing for the desired polymorph in the bulk drug substance used to start the manufacturing process isn’t good enough to control the polymorphic state of the final product.
Characterization of pharmaceutical polymorphs also plays an important role in the procedures of legal approval for novel drugs.
Solid-state NMR spectra for monohydrate and dehydrate pseudopolymorphs of a sample analyte Sample Courtesy - APL Research Centre, Hyderabad, India
Shown is a NMR probe (blue component) alongside a rotor (white oblong piece in foreground) containing the sample. Two capsules typical of the types of substances that would be tested this way are shown for perspective. The rotor is inserted into the probe, which is then lowered into the center of the solid-state system’s magnet for analysis.
Typical solid-state NMR equipment. Shown is the 600 MHz NMR system with Direct Drive 2 console, an Agilent solid-state NMR machine.
Solution: Because the packing energy and close contacts are unique for each different crystal form of the same compound, the electronic environment for each atom is also unique. Nuclear Magnetic Resonance (NMR) spectra are exquisitely sensitive to changes in the local electronic environment around every NMR active atom in a molecule. Changes in the NMR resonance frequency even as small as 1 part per billion are readily detected. If molecular crystal has more than one unique asymmetric position of the molecules in the crystal structure, then NMR lines are likely to split according to the number of magnetically non-equivalent molecules, providing a unique NMR spectrum for every polymorphic form. This sensitivity to local structure makes solid-state NMR the definitive tool for testing and measuring the polymorphic state of pharmaceutical compounds. Each polymorph yields a distinctive spectrum that can be used as a fingerprint for that specific crystal form. When combined with quantitative solid-state NMR experiments, this technique can be used to directly investigate drug substance or drug product materials, up to and including the final pills, for polymorphic integrity.
As seen in top graphic, the solid-state NMR spectra collected for the monohydrate and dihydrate pseudopolymorphs of the analyte are distinctly different. It is easy to distinguish the presence of both forms, a 90/10 mixture of the two and, as all NMR signals yield a unit response factor, quantification of such mixtures is robust and reliable.
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