PubMed · 915722
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W D Walkling. 1977. Polymorphic changes caused by grinding.. https://pubmed.ncbi.nlm.nih.gov/915722/
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Pharmaceutical enantiomers often exhibit different pharmacodynamic and pharmacokinetic properties. Stereospecific chromatographic assays are available to separate these stereoisomers. Therapeutic agents often contain chemical functional groups (e.g. amino, hydroxyl, carbonyl, and carboxylic acid). These can be reacted with enantiomerically pure reagents to give diastereoisomers suitable for analysis on achiral gas chromatographic (GC) and high performance liquid chromatographic (HPLC) columns. Alternatively, derivatized or underivatized drugs may be resolved on chiral chromatographic phases. A wide variety of GC (e.g. amino acid, cyclodextrin, and metal-complex) and HPLC (mobile phase additive, crown ether, pi-pi interaction and related phases, protein, cyclodextrin, polysaccharide, methacrylate and amide polymer, and ligand exchange) columns are commercially available. This article reviews the chromatographic separation of enantiomers.
A model of a controlled-release drug delivery system was proposed and tested by mathematical simulation. The model consisted of a cylindrical drug reservoir coated with a permeable and erodible material. The release rate is relatively constant because, while the drug concentration is diminishing, the barrier is eroding. These factors may be balanced initially, but ultimately the release rate increases or decreases markedly depending on the permeability. Two kinds of permeability-dependent behaviors were observed: (1) for high-permeability coatings, the release rate is high and constant and then decreases as the drug is exhausted; and (2) for low-permeability coatings, the release rate is low and constant and then increases as the barrier becomes thin. The proposed design combines two cylinders whose coatings have different permeabilities and exhibit complementary release behavior. Complete release occurs before total erosion of either coating. Optimization of the design and the design parameters leads to a constant release rate (within 6%) until all of the drug is released from the device.