Investigation of the Multi-Step Dehydration Reaction of Theophylline Monohydrate Using 2-Dimensional Powder X-ray Diffractometry
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- Nunes, C., Mahendrasingam, A. & Suryanarayanan, R. Pharm Res (2006) 23: 2393. doi:10.1007/s11095-006-9022-5
(i) To study the dehydration kinetics of theophylline monohydrate using 2-dimensional (2D) powder X-ray diffractometry (XRD), and (ii) to investigate the effect of polyvinylpyrrolidone (PVP) on the dehydration pathway and kinetics.
Theophylline monohydrate (C7H8N4O2·H2O; M) was recrystallized from aqueous PVP solutions of different concentrations. Dehydration kinetics was studied isothermally, at several temperatures, from 35 to 130°C. The experimental set-up comprised of a high intensity X-ray source (synchrotron radiation or 8 kW rotating anode), a heating chamber, and a 2D area detector. Diffraction patterns were collected continuously, with a time resolution ranging from 40 ms to 30 s, over the angular range of 3 to 27°2θ.
Dehydration of M resulted in either the stable (C7H8N4O2; A), or the metastable anhydrate (A*), with the latter having a tendency to transform to A. The XRD technique allowed simultaneous quantification of M, A* and A during the dehydration reaction. The rate constants for individual reaction steps (M→A*; M→A and A*→A) were determined by fitting the data to solid-state reaction models. In presence of PVP, there was a decrease in the magnitude of the rate constant associated with the M→A transition, resulting in an increased build-up of A* in the product. The inhibitory effect of PVP on M→A transition was more pronounced at lower dehydration temperatures, and was proportional to the concentration of PVP.
Two dimensional powder X-ray diffractometry, using a high intensity source, is a powerful technique to study kinetics of rapid solid-state reactions. The inhibitory effect of excipients can have profound effect on phases formed during pharmaceutical processing.