Bioequivalence and Bioavailability Studies

Bioavailability (BA) refers to the fraction of an active pharmaceutical ingredient absorbed from a drug product into the systemic circulation. It is typically measured by comparing the exposure following extravascular with intravenous doses. Bioequivalence (BE) is a measure of comparability between two dosage forms of the same drug and is used to determine whether these drugs can be used interchangeably. For oral drugs, bioequivalence is determined by comparing the relative oral bioavailability of the brand name versus generic drug. Bioavailability and bioequivalence studies are performed for reformulated drugs or new generics to demonstrate comparability with marketed drug products.

The purpose of bioequivalence (BE) studies is to identify pharmaceutical equivalents or alternatives to be used interchangeably for the same therapeutic effect as a previously marketed drug. In general, we perform bioequivalence testing by comparing the bioavailability of the test product with the reference product. These days bioequivalence trials are receiving close attention because of the increased availability and utilization of generic drugs in today’s pharmaceutical market. Bioequivalent drug products must demonstrate comparability of AUC and Cmax between the two products within the bioequivalence range of typically 80-125%.

Bioavailability study is a PK study that demonstrates the rate and extent to which the active ingredient is absorbed from a drug product and becomes available to the site of action. In this context, absolute bioavailability represents the total fraction of drug absorbed by comparing the intended clinical dose route to direct IV systemic administration. On the other hand, relative bioavailability is determined by comparing the plasma concentration-time curves after administering two different formulations of the same compound by the same dosing route. Relative bioavailability is used for a direct comparison of an active pharmaceutical ingredient’s absorption between two products to determine bioequivalence. The clinical significance of bioavailability and bioequivalence is to demonstrate the comparability between reformulated or generic vs. marketed drugs.

Drug bioavailability is calculated during pharmacokinetic (PK) studies following IV and oral (or extravascular) doses. It is measured as the ratio of the dose-normalized area under the plasma time concentration curve (AUC) from the oral (or another extravascular route) administration to the AUC of an intravenous (IV) administration. In this setting, IV dosage is considered 100% available as it is administered directly into the systemic circulation. The importance of bioavailability studies lies in testing whether an active pharmaceutical ingredient can reach systemic circulation and its intended site of action while minimizing undesired off-target side effects. Typically, high bioavailability is desirable, but in some cases, low bioavailability is beneficial if the drug is administered directly to the site of action, or there are concerns about systemic toxicity.

Bioequivalence requirements from the US FDA and other agencies entail the comparability of new formulations and generics to products approved already to maintain existing standards of safety and efficacy. Bioequivalence is most often measured directly by comparing pharmacokinetic parameters following dosing (AUC and Cmax). If drug levels cannot be measured, a pharmacodynamic approach may be employed, comparing therapeutic responses and side effect profiles in patients at equivalent doses. Relative bioavailability, AUC, and Cmax are the pivotal PK parameters in a bioequivalence test study. A new product is considered bioequivalent if the relative AUC and Cmax levels following a dose are comparable to the approved product within an acceptable range, typically 80-125%, with a 90% confidence interval.

Bioequivalence studies are part of clinical research, where the systemic exposure of test and reference compounds are compared. Drug development is an expensive process with a high failure rate. Reformulated or generic drugs can be manufactured and marketed through bioequivalence study design, without the need to demonstrate therapeutic equivalence in clinical trials, given they are benchmarked against an approved product. The objective of BA/BE studies in clinical trials is to evaluate the rate and extent of absorption of a drug from a test formulation. Bioequivalence and bioavailability studies are essential in early (Pilot Bioequivalence) and late (Pivotal Bioequivalence) clinical development of drug candidates. Generally, BE studies are performed if there is a change in dose formulation, salt forms, or manufacturing process.

For filing abbreviated new drug application (ANDA), an applicant generally must demonstrate that their drug candidate is reasonably bioequivalent to the reference listed drug (RLD). To this end, FDA regulations require an applicant to use the most accurate, sensitive, and reproducible approach available among those outlined in the FDA guidance on bioequivalence (21CFR 320.24(b), Procedures for Determining Bioavailability and Bioequivalence). As per these guidelines, the descending order of determining bioequivalence (BE) includes pharmacokinetic, pharmacodynamic, clinical, and then in vitro studies. Here, the establishment of BE based on pharmacokinetics relies on endpoints such as Cmax and AUC for a direct comparison between the two products. Applicants may conduct a pilot study before full BE study to validate the analytical methodology, optimize sample collection time intervals, and flesh out protocol details.