title: Photostability of crystalline versus amorphous nifedipine and nimodipine.
abstract: True solid-state photostability of the drugs nifedipine and nimodipine was investigated during exposure to UV-visible radiation. Photostability was studied on a small scale as thin films of approximately 1 mg drug, which contained either amorphous or re-crystallised stable phases. High-performance liquid chromatography analysis revealed a greater rate and extent of decomposition for the amorphous phases. Photoexposed amorphous nifedipine exhibited approximately 1.8-fold larger first-order decomposition rate constant (k) relative to its crystalline phase. The increase in k was more significant for photoexposed amorphous nimodipine at approximately sixfold relative to its crystalline phase. Photodecomposition in scaled-up samples of the stable crystalline phases for both drugs was monitored with X-ray diffraction in Bragg-Brentano geometry. The similarities in the calculated photodecomposition extents to results from small scale validated the specificity of the X-ray analysis technique to the photodecomposition region. The considerably faster decomposition rates in small-scale studies were attributed to a maximised surface area (A) for quantity (m0 ) of exposed drug. Kinetic interpretations of true solid-state stability should consider the sample solid dimensions in terms of the direct exposed A and m0 in the photodecomposition region, that is, outer layers in solid.
authors.affiliation: Department of Chemistry, Nelson Mandela Metropolitan University, Port Elizabeth 6031, South Africa. Electronic address: dgrooff@nmmu.ac.za.
authors.forename: Driekus
authors.initials: D
authors.lastname: Grooff
authors.affiliation: Department of Chemistry, Nelson Mandela Metropolitan University, Port Elizabeth 6031, South Africa.
authors.forename: Farzaana
authors.initials: F
authors.lastname: Francis
authors.affiliation: School of Pharmacy, University of Wisconsin, Madison, Wisconsin 53705.
authors.forename: Melgardt M
authors.initials: MM
authors.lastname: De Villiers
authors.affiliation: Department of Chemistry, Nelson Mandela Metropolitan University, Port Elizabeth 6031, South Africa.
authors.forename: Ernst
authors.initials: E
authors.lastname: Ferg
mesh_terms: D003460:Crystallization; D004355:Drug Stability; D007700:Kinetics; D009543:Nifedipine / Q000737:chemistry*; D009553:Nimodipine / Q000737:chemistry*; D010782:Photolysis; D014665:Vasodilator Agents / Q000737:chemistry*; D014961:X-Ray Diffraction
chemical_list: D014665:Vasodilator Agents; D009553:Nimodipine; D009543:Nifedipine