Bioanalytical methods, bioavailability

During the 1990 Washington Conference on Analytical Methods Validation Bioavailability, Bioequivalence and Pharmacokinetic Studies [1], parameters that should be used for method validation were defined. The final report of this conference is considered the most comprehensive document on the validation of bioanalytical methods. Many multinational pharmaceutical companies and contract research organizations contributed to its final draft. This scientific meeting was sponsored by the American Association of Pharmaceutical Scientists (AAPS), the Association of Official Analytical Chemists (AOAC), and the U.S. Food and Drug Administration (FDA). The conference report has been used as a reference by bioanalytical laboratories and regulatory agencies worldwide. 

Robust and rugged LC-MS/MS methods are essential in support of drug discovery, toxicology studies, and clinical trials, for the data generated from these bioanalytical methods is used to evaluate the bioavailability, bioequivalence, toxicokinetic, and pharmacokinetic parameters of drug candidates. Thus, it is critical to invest significant thought and effort in the method development process [25-27], Fast sample... 

Almost all of the above mentioned publications referred to bioanalytical methods for bioavailability, bioequivalence or pharmacokinetic studies. This field is of course very closely related to forensic and clinical toxicology, especially if only routine methods are considered. Therefore, it seems reasonable to base the discussion concerning method validation in toxicological analysis on the experiences and consensus described above and not to start the whole discussion anew. In the following, possible implications for forensic and clinical toxicology will be discussed. 

Stereoisomer Assays. There are many drugs that are administered as racemic mixtures. They may undergo stereoselective metabolism and/or elimination, and one isomer may be more active than the other. Therefore, there is the need to develop and validate bioanalytical assays for stereoselective determination in bioavailability/bioequivalence studies. All methods used for measurement of stereoisomer should be validated (with emphasis on stereospecificity). For bioequivalence studies of an existing racemic product, a stereospecific assay is not required if the rate and extent of profiles are superimposable (within the usual statistical boundaries) [3,23]. 

Although the lower limit of quantitation is established during assay validation and prior to microdosing, assay sensitivity remains an uncertainty until the actual analysis of the microdose samples as well. There is always the danger that plasma exposures from the microdose are lower than predicted and as a result plasma concentrations from some or all of the time points cannot be detected by the LC-MS/MS method. Reduction of this risk is achieved by collaborative communication between the bioanalytical chemist and the project team. Conservative estimates on bioavailability and clearance can be used to establish the necessary limit of detection needed to determine plasma concentrations for all time points. Updates on the progress of the assay development allow the team to decide if the achievable limit of detection will enable the determination of plasma concentrations from enough time points to make a go-no go decision. Of course, sensitivity is not an issue with AMS, which practically ensures that plasma concentrations will be determined, possibly for several days, enabling the observation of complex PK and clearance from deep compartments. 

A Clinical Summary is to start with a subsection on Biopharmaceutical Studies, and Associated Analytical and Bioanalytical Chemistry Methods conducted during the clinical development of a drug candidate. The background of the formulation development process is to be briefly provided and is to include information on in vitro and in vivo dosage form performance and the general approach and rationale for developing the bioavailability (BA), comparative BA, bioequivalence (BE), and in vitro dissolution profile database. Also to be included is a summary of the analytical and bioanalytical chemistry methods and the validation characteristics of these methods. 

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