1. Overview

Bioavailability (BA) and bioequivalence (BE) are important indicators for evaluating the quality of innovative drug formulations. BA studies are one of the important bases for selecting appropriate administration routes and determining medication plans (such as dosage and dosing intervals) during the research process of innovative drugs. BE studies are comparative studies conducted based on predetermined equivalence standards, which ensure the consistency of in vivo behavior of different formulations containing the same active pharmaceutical ingredient and whether the two formulations can be interchangeable.

During the clinical trials of innovative drugs and after they are marketed, as clinical trial data and clinical medication experience continue to accumulate, the understanding of the drug's biopharmaceutical characteristics, safety, and efficacy deepens. Changes may occur in the drug's raw materials, formulations, and administration plans, which may affect the drug's pharmacokinetic behavior and, in turn, impact safety and efficacy. Therefore, it is necessary to conduct studies, including BA or BE, to evaluate the aforementioned changes when necessary.

This guideline mainly applies to oral formulations of chemical innovative drugs that can use systemic exposure indicators to evaluate BA and BE. Non-oral formulations (such as transdermal absorption formulations, some rectal administration, and nasal administration formulations, etc.) can also refer to this guideline.

(1) Bioavailability

Bioavailability refers to the extent and rate at which the active ingredient of a drug is released from the formulation and absorbed into the systemic circulation. It is generally divided into absolute bioavailability and relative bioavailability.

1. Absolute Bioavailability

Absolute bioavailability is the relative amount of the drug's active ingredient absorbed into the systemic circulation obtained by using intravenous formulations (which are generally considered to have a bioavailability of 100%) as a reference formulation.

2. Relative Bioavailability

Relative bioavailability is the relative amount of the drug's active ingredient absorbed into the systemic circulation obtained by using other non-intravenous administration formulations (such as tablets and oral solutions) as a reference formulation.

BA studies are generally used during the development of innovative drugs to evaluate the changes between two formulations before and after changes, and pharmacokinetic information of the new formulation can also be obtained through BA studies. Generally, pharmacokinetic parameters, blood concentration-time curves, and the geometric mean ratios of major pharmacokinetic parameters AUC and Cmax, along with their 90% confidence intervals, should be provided for the two formulations before and after the changes.

Differences in pharmacokinetic properties such as absorption rate or extent between the formulations before and after changes may affect the evaluation of benefits and risks of the new formulation or new administration method. For example, if the bioavailability of the formulation after the change is significantly higher or lower than that before the change, researchers need to consider adjusting the dosage based on the degree of change; when the variability of pharmacokinetic parameters of the formulation after the change is significantly higher than that before the change, it may affect the drug's safety and efficacy, indicating that the two formulations before and after the change are not comparable.

In some cases, conclusions of BA similarity based on peak concentration (Cmax) and exposure (AUC) may not be sufficient to prove that there are no differences in safety or efficacy between the two formulations before and after the change. For example, differences in time to peak concentration (Tmax) and blood concentration-time curves often indicate different clinical responses between the formulation before the change and the formulation after the change. In such cases, additional data analysis (such as partial AUCs), exposure-effect evaluations, or clinical studies may be required to evaluate the BA of the two formulations.

(2) Bioequivalence

Bioequivalence (with PK as the endpoint) refers to the situation where, under similar experimental conditions, the differences in the absorption rate and extent of the drug in the test formulation compared to the reference formulation after administering the same dose of the test drug once or multiple times are within an acceptable range. Generally, the equivalence standard for BE studies is that the 90% confidence interval of the geometric mean ratio of the main pharmacokinetic parameters (AUC and Cmax) of the test formulation and the reference formulation falls within the range of 80.00% to 125.00%.

When the test formulation and the reference formulation are not bioequivalent, the applicant should demonstrate based on existing dose-effect or concentration-effect data that the differences in absorption rate and extent will not have a significant impact on the drug's safety and efficacy. When there is insufficient evidence, consideration should be given to adjusting the formulation, changing the production process, or supplementing new safety and efficacy data.

(3) Application of BA/BE Studies at Different Stages

1. Early Clinical Trials

In the early stages of clinical trials for innovative drugs, BA studies can be selected to reflect the quality of the formulation, clarify the absolute bioavailability of the drug, and provide reference for formulating the next dosing plan. The reference formulation chosen for such studies is usually an intravenous formulation.

2. Changes Before Market Approval

During the clinical trials of the drug, when changes occur, BA/BE studies should be conducted based on the 'Technical Guideline for Pharmaceutical Changes During Clinical Trials of Innovative Drugs (Chemical Drugs)' and other guidelines, while considering the potential impacts on the drug's pharmacokinetic characteristics, safety, and efficacy, and comprehensively assessing whether BA/BE studies are needed, such as:

(1) When the formulations in early and late clinical trials are different;

(2) When the specifications are different;

(3) When the formulation intended for market approval is different from the key clinical trial formulation (BE studies should be conducted).

3. Changes After Market Approval

When changes occur after the drug is approved for marketing, whether BE studies are needed should refer to the 'Technical Guideline for Pharmaceutical Change Studies of Marketed Chemical Drugs' and other guidelines, and a comprehensive assessment should be conducted based on the actual situation of the drug.

2. Methods for Human BA/BE Studies

Generally, the recommended methods for BA/BE studies include both in vivo and in vitro methods. According to the evaluation effectiveness of the research methods, the priority order is pharmacokinetic (PK) studies, pharmacodynamic (PD) studies, clinical studies, and in vitro studies.

(1) Pharmacokinetic Studies

By measuring the concentration of the drug in biological matrices (such as blood, plasma, serum, etc.), PK parameters are obtained to reflect the speed and extent of the drug being released from the formulation and absorbed into the circulatory system. PK endpoint indicators Cmax and AUC are usually used for evaluation.

The overall design, sample size, subject selection, choice of single-dose/multiple-dose (steady-state) studies, biological sample analysis, PK parameters for evaluation, specific requirements for trial implementation processes and data statistical analysis involved in innovative drug BA/BE studies can refer to relevant guidelines such as "Technical Guidance Principles for Human Pharmacokinetic Studies of Chemical Innovative Drugs", "Technical Guidance Principles for Human Bioequivalence Studies of Chemical Generic Drugs with Pharmacokinetic Parameters as the Endpoint Evaluation Index", "Statistical Guidance Principles for Bioequivalence Studies", "Technical Guidance Principles for Bioequivalence Studies of Highly Variable Drugs", and "Technical Guidance Principles for Bioequivalence Studies of Narrow Therapeutic Index Drugs". This guidance mainly elaborates on the special considerations for innovative drug BA/BE studies.

1. Pre-trial/Formal Trial Applicants may conduct a pre-trial in a small number of subjects before conducting formal BA/BE studies. The purpose of the pre-trial is to:

(1) Initially evaluate the variability of PK parameters;

(2) Determine the sample size that can achieve sufficient confidence for conducting formal BA/BE studies;

(3) Optimize sample collection times;

(4) Determine the washout period between periods.

In certain cases, if the design and implementation of the pre-trial are scientifically sound, and a sufficient number of subjects are included to obtain evaluable PK data, the results of the pre-trial may serve as the basis for confirming the BA/BE of the formulation. This situation should be clearly stated in the study protocol.

2. Study Design

It is generally recommended to use a non-repeated crossover study design. For highly variable drugs and narrow therapeutic index drugs, a repeated study design may be used; for long half-life drugs (e.g., elimination half-life ≥ 24h), a parallel study design may be adopted. Applicants may also use other study designs for innovative drug BA/BE studies and provide sufficient scientific justification.

3. Administration under Fasting or Postprandial Conditions

For innovative drug BA/BE studies, it is usually conducted under fasting conditions, as this is the most sensitive condition for evaluating potential differences between formulations. When the proposed labeling clearly states that the drug can only be taken on an empty stomach, or can be taken either on an empty stomach or with food, a study under fasting conditions should be conducted; when the proposed labeling clearly states that the drug can only be taken with food or after meals, studies should be conducted comparing the reference formulation administered after meals with the test formulation administered under both fasting and postprandial conditions; if there are serious safety risks under fasting conditions, studies may only be conducted under postprandial conditions.

In addition, it should be assessed whether a food effect study is needed, specifically referring to the "Technical Guidance Principles for Food Effect Studies in New Drug Development".

4. Dosage

In general, the highest specification formulation should be administered as one unit (e.g., a single tablet or capsule). If the highest specification formulation poses safety risks to healthy subjects, patients may be included as subjects, or a lower specification formulation may be used.

When the specifications of the test formulation and the reference formulation are inconsistent, it is recommended to administer the same or similar doses (not exceeding the maximum clinical dose intended). If similar doses are administered and the dosing is within the linear range of PK characteristics, bioavailability can be calculated using dose correction methods.

5. Analytes

It is generally recommended to measure the parent drug, as the pharmacokinetic profile of the parent drug can more sensitively reflect differences between formulations than active metabolites. In cases where active metabolites primarily arise before entering systemic circulation (e.g., from first-pass effects or intestinal metabolism) and affect efficacy or safety, both the parent drug and active metabolites should be measured.

If the concentration of the parent drug is too low to be reliably measured in biological matrices, active metabolites may be measured.

(2) Other Methods Supporting BA/BE

When pharmacokinetic methods are indeed not feasible, PD studies, clinical studies, and in vitro studies may also be considered, but it must be sufficiently demonstrated that the methods used are scientific and reasonable.

3. Common Dosage Forms of BA/BE Studies

(1) Oral Solutions

For oral solutions, syrups, and other liquid dosage forms, BA/BE studies may be waived if the following conditions are met:

(1) The formulation is in a true solution state;

(2) The solution does not contain excipients that affect drug absorption (e.g., sorbitol, vitamin E, etc.).

When oral solutions may precipitate when exposed to gastric contents or when diluted (e.g., if the formulation contains potential solvents, buffers, etc.), changes to the formulation should consider conducting in vivo studies.

(2) Immediate Release Formulations

Immediate release formulations include tablets, capsules, lozenges, chewable tablets, orally disintegrating tablets, and sublingual tablets. It is generally recommended to conduct single-dose fasting BA studies and food effect studies. In certain cases, multiple-dose BA studies may also be required.

Unconventional dosage forms (e.g., lozenges, chewable tablets, orally disintegrating tablets, and sublingual tablets) should be administered according to the proposed labeling. Additionally, complete swallowing BA studies may be needed for such products to evaluate the impact of accidentally swallowing the entire drug.

During the new drug development process, changes in specifications often occur. If the following conditions are met, BA/BE studies may be waived:

(1) The drug dosage form is the same, but the specifications are different;

(2) The formulation ratios of each specification are similar;

(3) The in vitro dissolution profiles of each specification are similar in different pH media;

In addition, new specifications should be considered within the linear range of PK characteristics.

(3) Oral Suspensions

The research technical requirements for oral suspensions are the same as those for immediate release formulations.

(4) Modified Release Formulations

Modified release formulations include delayed release formulations and sustained release formulations. The relevant requirements for BA/BE studies of modified release formulations refer to the "Guidance Principles for Clinical Pharmacokinetic Studies of Modified Release Formulations of New Drugs".

4. References

1. FDA. Bioavailability Studies Submitted in NDAs or INDs — General Considerations (Draft Guidance). 2019

2. National Medical Products Administration. "Technical Guiding Principles for the Study of Human Bioavailability and Bioequivalence of Chemical Drug Preparations". 2005

3. National Medical Products Administration. "Technical Guiding Principles for the Study of Human Bioequivalence of Generic Chemical Drugs with Pharmacokinetic Parameters as Endpoints". 2016 4. National Medical Products Administration. "Statistical Guiding Principles for Bioequivalence Studies". 2018

5. National Medical Products Administration. "Technical Guiding Principles for Bioequivalence Studies of Highly Variable Drugs". 2018

6. National Medical Products Administration. "Technical Guiding Principles for Bioequivalence Studies of Narrow Therapeutic Index Drugs". 2020

7. National Medical Products Administration. "Technical Guiding Principles for Food Effect Studies in New Drug Development". 2021

8. National Medical Products Administration. "Guiding Principles for Clinical Pharmacokinetic Studies of Modified New Drug Release Formulations". 2021