Definition of Bioavailability as per CFR 320.1
Bioavailability is defined in CFR 320.1 as: The rate and extent to which the active ingredient or active moiety is absorbed from a drug product and becomes available at the site of action. For drug products that are not intended to be absorbed into the bloodstream, bioavailability may be assessed by measurements intended to reflect the rate and extent to which the active ingredient or active moiety becomes available at the site of action.
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Bioavailability (BA)data for a given formulation provide an estimate of the relative fraction of the orally administered dose that is absorbed into the systemic circulation when compared to the bioavailability (BA)data for a solution, suspension, or intravenous dosage form (21 CFR 320.25(d)(2) and (3)). In addition, bioavailability (BA)studies provide other useful pharmacokinetic information related to distribution, elimination, the effects of nutrients on absorption of the drug, dose proportionality, linearity in pharmacokinetics of the active moieties and, where appropriate, inactive moieties. bioavailability (BA)data can also provide information indirectly about the properties of a drug substance before entry into the systemic circulation, such as permeability and the ifluence of presystemic enzymes and/or transporters (e.g., p-glycoprotein).
Bioavailability (BA) for orally administered drug products can be documented by developing a systemic exposure profile. A profile can be obtained by measuring the concentration of active ingredients and/or active moieties and, when appropriate, its active metabolites over time in samples collected from the systemic circulation. Systemic exposure patterns reflect both release of the drug substance from the drug product and a series of possible presystemic/systemic actions on the drug substance after its release from the drug product. US FDA recommend that additional comparative studies be performed to understand the relative contribution of these processes to the systemic exposure pattern.
One regulatory objective is to assess, through appropriately designed bioavailability (BA)studies, the performance of the formulations used in the clinical trials that provide evidence of safety and efficacy (21 CFR 320.25(d)(1)). Before marketing a drug product, the performance of the clinical trial dosage form can be optimized, in the context of demonstrating safety and efficacy. The systemic exposure profiles of clinical trial material can be used as a benchmark for subsequent formulation changes and can be useful as a reference for future BE studies.
Although bioavailability (BA)studies have many pharmacokinetic objectives beyond formulation performance as described above, FDA note that subsequent sections of this guidance focus on using relative bioavailability (BA)(referred to as product quality bioavailability (BA)) and, in particular, BE studies as a means to document product quality. In vivo performance, in terms of bioavailability (BA)/BE, can be considered to be one aspect of product quality that provides a link to the performance of the drug product used in clinical trials and to the bioavailability (BA) data containing evidence of safety and efficacy.
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