Drug disposition study methods

Mass spectrometry (MS) has become one of the most important analytical tools employed in the analysis of pharmaceuticals. This can most likely be attributed to the availability of new instrumentation and ionization techniques that can be used to help solve difficult bioanalytical problems associated with this field (1-8). Perhaps the best illustration of this occurrence is the development of electrospray (ESI) and related atmospheric-pressure ionization (API) techniques, ion-spray (nebulizer-assisted API), turbo ionspray (thermally assisted API), and atmospheric pressure chemical ionization (APCI nebulization coupled with corona discharge), for use in drug disposition studies. The terms ESI and ionspray tend to be used interchangeably in the literature. For the purpose of this review, the term API will be used to describe both ESI and ionspray. In recent years there has been an unprecedented explosion in the use of instrumentation dedicated to API/MS (4,6,8-14). API-based ionization techniques have now become the method of choice for the analysis of pharmaceuticals and their metabolites. This has made thermospray (TSP), the predominant LC/MS technique during the 1980s, obsolete (15). Numerous reports describing the utility of API/MS for pharmaceutical analysis have appeared in the literature over the last decade (7). The... 

Findlay, J.W. A. (1987) Applications of immunoassay methods to drug disposition studies. Drug Metabolism Reviews, 18, 83 129. 

The advantages of using non-compartmental methods for calculating pharmacokinetic parameters, such as systemic clearance (CZg), volume of distribution (Vd(area))/ systemic availability (F) and mean residence time (MRT), are that they can be applied to any route of administration and do not entail the selection of a compartmental pharmacokinetic model. The important assumption made, however, is that the absorption and disposition processes for the drug being studied obey first-order (linear) pharmacokinetic behaviour. The first-order elimination rate constant (and half-life) of the drug can be calculated by regression analysis of the terminal four to six measured plasma... 

These preclinical drug metabolism studies may also include metabolite profiling in plasma, selected tissues, urine, and bile to assess the distribution and disposition of potentially important metabolites, such as those having a level 5% or greater relative to the parent compound. Metabolite profiling requires a technique to separate the parent compound from metabolites and other endogenous compounds. For small organic molecules, HPLC is usually the method of choice. For macromolecules, gel or capillary electrophoresis techniques can be defined with sufficient resolution capability to separate the... 

The fate of ondansetron has been investigated in laboratory animals and man during studies undertaken as part of its development [81]. Radiola-belled ondansetron (42) has been used to determine the disposition of total drug-related material and sensitive chromatographic methods [82, 83] have been developed for the specific determination of unchanged ondansetron in biological fluids. 

In general, bioequivalence is demonstrated if the mean difference between two products is within 20% at the 95% confidence level. This is a statistical requirement, which may require a large number of samples (e.g. volunteers), if the drug exhibits variable absorption and disposition pharmacokinetics. For drugs for which there is a small therapeutic window or low therapeutic index, the 20% limit may be reduced. The preferred test method is an in vivo crossover study and, since this occurs in the development phase, necessitates the emplo)unent of volim-teers. These studies are, therefore, expensive and animal experiments may be substituted, or in vitro experiments if they have been correlated with in vivo studies. 

To form the amide derivatives, the acid and amine are condensed in the presence of such agents as N,N -carbonyldiimidazole (18) or a carbodiimide and 1-hydroxybenzotriazole (17). The amides can also be formed via the add chlorides (200,201). Bjorkman (199) described a novel approach to the formation of diastereomeric amides The NSAID indoprofen was coupled by means of ethyl chloroformate to L-leucinamide in a reaction that is complete in 3 min. The derivatives were separated by RP LC, and the procedure was used to study the disposition of the drug in surgical patients (199). Others have adopted this derivatization scheme (209). The chloroformate activation method has also been used with (R)-[42] for resolution of several acids (210). It was found that when hydroxyacids were derivatized, not only did the reaction produce the desired amide moiety at the carboxyl group, but the hydroxyl group was converted to the carbonate derivative of the chloroformate (210). 

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