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This is an interdisciplinary project involving Physics and Chemical Engineering faculty. R. M. Suter (Physics) is the nominal PI; his role is to oversee the application of recently developed x-ray microscopy techniques to a new area of research. The x-ray techniques thus far have been applied only to polycrystalline metals; this project will open up a new field of application and will address an important need in the pharmaceutical field. Chemical Engineers T. Przybycien, J. W. Schneider, D. Sholl, and R. D. Tilton will develop appropriate samples and guide the experimental work to optimize its impact on the pharmaceutical industry. The group is structured to be able to apply complementary experimental and theoretical techniques to the problem and, thus, to be able to compete for future industrial and federal funding.
Many drugs are administered in crystalline form. The Food and Drug Administration approves such drugs only in a specific crystal structure or polymorph. Different polymorphs have different solubilities, different residence times in the body and different therapeutic values. This presents a significant problem to the pharmaceutical industry, as is made clear by the following [1]:
"A recent analysis by Bernstein and Dunitz [10] has highlighted this issue by documenting a number of so-called "disappearing polymorphs'', i.e., sudden appearances of new structures or the unexplained disappearances of existing ones. Such examples are almost certainly mirrored (although not documented) by industrial practice and one can only speculate at the disastrous consequences of a sudden unexplained switch of polymorph during the isolation of a high value specialty product."
A recent patent case, based on the discovery of new polymorphic forms of Zantac, illustrates another facet of the importance of understanding and controlling polymorphs. [1]
There are a number of examples in which polymorphic molecules change crystal structure under processing conditions while in contact with liquids or solid materials. In these environments, it is difficult to apply standard techniques to identify and predict the transformations. Furthermore, little is known about how to control polymorphic forms.
Common industry practice when formulating a new drug to manufacture in dosage form is to rely on formulations (composition of inert additives) that worked with other drugs in the past. When this procedure fails (as is often the case) the formulation is changed by trial-and-error. The challenge is exacerbated by the fact that the formulation must achieve numerous, possibly competing objectives, such as control of chemical stability, disintegration and dissolution rates, polymorphism, crystal habit, and dosage uniformity. Because these systems have such complex compositions, and interactions between two or more formulation components often lead to unexpected consequences for one or more of the design objectives; there are few reliable, rational formulation design rules. Some formulations preserve the correct polymorph, others do not, and the reasons are usually not understood. This is a severe barrier in the pharmaceutical industry. Our aim is to develop new tools to give fundamental insight into identifying and controlling polymorphism in industrial processes through a molecular-level understanding.
In this project we will demonstrate that the 3DXDM and related techniques are useful for studies of pharmaceutical crystals in solutions and in the presence of binder materials used in pills. It is in these environments that polymorphic changes occur and where other techniques are difficult to apply. The 3DXDM gives us the opportunity to "look inside" bulk material and isolate detailed information about crystallography and crystal geometry. We will perform proof-of-principle measurements that demonstrate the ability to watch changes take place in individual crystals and we will do measurements that show how, in a particular model system, polymorphic transformations are prevented.
It is apparent from the differences in therapeutic properties of polymorphs of the same molecule that the structures of crystalline surfaces are important in determining dissolution rates. Surface structures are also a key to controlling polymorphs by adsorption of molecules that stabilize one structure over another. [1] In fact, unwanted polymorphic transformations probably take place due to interactions of specific surfaces with liquids, solid binder molecules, and undetected impurities. We will use x-ray reflectivity techniques [4] to study surface morphologies in various chemical environments. In this work we will use both the Physics Department x-ray scattering facility and micro-focused synchrotron x-ray beams. In the latter case, we will also be able to use grazing incidence x-ray diffraction to study atomic-level surface structure and structural changes on adsorption of surface active molecules in solution.