#224 
 
G I  
UIDANCE FOR  NDUSTRY
 
B : B L  
IOEQUIVALENCE    LOOD  EVEL
B S  
IOEQUIVALENCE  TUDY
 
VICH GL52 
 
 
Submit comments on this guidance at any time.  Submit electronic comments on the guidance 
to http://www.regulations.gov.  Submit written comments to the Division of Dockets 
Management (HFA-305), Food and Drug Administration, 5630 Fishers Lane, Room 1061, 
Rockville, MD 20852.  Identify all comments with the docket number FDA-2014-D-1352. 
 
For further information regarding this document, contact AskCVM@fda.hhs.gov. 
 
Additional copies of this document may be requested from the Policy and Regulations Staff 
(HFV-6), Center for Veterinary Medicine, Food and Drug Administration, 7519 Standish 
Place, Rockville, MD 20855, and may be viewed on the Internet at either 
http://www.fda.gov/AnimalVeterinary/default.htm or http://www.regulations.gov. 
 
 
 
 
U.S. Department of Health and Human Services 
Food and Drug Administration 
Center for Veterinary Medicine 
December 2016 
    Contains Nonbinding Recommendations 
VICH GL 52 (BIOEQUIVALENCE) 
For implementation at Step 7 
 
 
 
 
 
 
 
 
 
 
 
 
BIOEQUIVALENCE: BLOOD LEVEL 
    
BIOEQUIVALENCE STUDY 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Adopted at Step 7 of the VICH Process 
by the VICH Steering Committee in August 2015 
for implementation by August 2016. 
 
 
This Guidance has been developed by the appropriate VICH Expert Working Group 
in accordance with the VICH Process.  At Step 7 of the Process the final draft is 
recommended for adoption to the regulatory bodies of the European Union, Japan and USA. 
 
   
2  Contains Nonbinding Recommendations 
Table of Contents 
 
I.  INTRODUCTION .......................................................................................................... 4 
A.  Objective: .................................................................................................................... 4 
B.  Background: ................................................................................................................ 4 
C.  Scope: .......................................................................................................................... 5 
II.  IN VIVO PROTOCOL DEVELOPMENT ...................................................................... 6 
A.  Product Selection: ....................................................................................................... 6 
B.  Dose Selection: ............................................................................................................ 7 
C.  Route of Administration Selection: ............................................................................. 8 
D.  Study Design Considerations: ..................................................................................... 8 
1.  Crossover versus parallel study design:................................................................... 8 
2.  Replicate study design: ............................................................................................ 9 
3.  Sequential study design: ........................................................................................ 10 
4.  Single dose versus multiple dose study design:..................................................... 10 
E.  Subject and Species Selection: .................................................................................. 11 
F.  Prandial State: ........................................................................................................... 11 
G.  Exclusion of Data from Analysis: ............................................................................. 11 
H.  Sample Size Determination: ...................................................................................... 12 
I.  Blood Sampling Schedule: ........................................................................................ 12 
J.  Blood Level BE Parameters: ..................................................................................... 13 
K.  Defining the Analyte: ................................................................................................ 14 
1.  Pro-drugs ............................................................................................................... 14 
2.  Enantiomers ........................................................................................................... 14 
L.  Bioanalytical Method Validation: ............................................................................. 14 
III.  STATISTICAL ANALYSIS: .................................................................................... 15 
A.  The Statistical Model: ............................................................................................... 15 
B.  Ln-Transformation: ................................................................................................... 15 
C.  Dose Normalization: ................................................................................................. 16 
D.  Confidence Interval Acceptance Criteria: ................................................................. 16 
E.  Statistical Report: ...................................................................................................... 16 
IV.  GLOSSARY .............................................................................................................. 17 
V.  APPENDIX ................................................................................................................... 21 
REFERENCE ....................................................................................................................... 23 
3  Contains Nonbinding Recommendations 
GUIDANCE FOR INDUSTRY 
 
BIOEQUIVALENCE:  BLOOD LEVEL BIOEQUIVALENCE STUDY 
 
This guidance represents the current thinking of the Food and Drug Administration (FDA or Agency) 
on this topic.  It does not establish any rights for any person and is not binding on FDA or the public.  
You can use an alternative approach if the approach satisfies the requirements of the applicable 
statutes and regulations.  To discuss an alternative approach, contact the FDA office responsible for 
this guidance as listed on the title page. 
 
I.  INTRODUCTION 
 
A. OBJECTIVE: 
 
This guidance is intended to harmonize the data recommendations associated with in vivo 
blood level bioequivalence (BE) for veterinary pharmaceutical products.  To meet this 
objective, the guidance addresses the following topics: 
 
  A harmonized definition of BE. 
  Factors/variables that should be considered when developing scientifically 
sound blood level BE study designs. 
  Information that should be included in a blood level BE study report. 
 
The International Cooperation on Harmonisation of Technical Requirements for Registration 
of Veterinary Medicinal Products (VICH) strives to eliminate repetitious and unnecessary 
testing through harmonization of regulatory recommendations for the registration of 
veterinary products, a goal that undoubtedly leads to a reduction in the number of animals 
used for product development and registration. 
 
In general, FDAs guidance documents do not establish legally enforceable responsibilities.  
Instead, guidances describe the Agencys current thinking on a topic and should be viewed 
only as recommendations, unless specific regulatory or statutory requirements are cited.  The 
use of the word should in Agency guidances means that something is suggested or 
recommended, but not required. 
 
B. BACKGROUND: 
 
Within the context of this guidance, BE is defined as the absence of a difference (within 
predefined acceptance criteria) in the bioavailability of the active pharmaceutical ingredient 
(API) or its metabolite(s) at the site of action when administered at the same molar dose 
under similar conditions in an appropriately designed study.  When using blood drug 
concentrations as a surrogate for demonstrating product BE, there is an underlying 
assumption that two products having an equivalent rate and extent of drug absorption, as 
measured in the blood, will be therapeutically indistinguishable and therefore interchangeable 
in a clinical setting. 
 
   
4  Contains Nonbinding Recommendations 
The determination of product BE in animal species can present numerous statistical, 
logistical, and regulatory challenges.  International differences in addressing these challenges 
and in the respective criteria for defining product BE can lead to barriers in data exchange 
and scientific confusion.  Therefore, the development of a harmonized guidance will unify the 
global veterinary community understanding of the basic pharmacokinetics (PK), study design 
considerations, and statistical principles upon which BE determinations are based.  By their 
nature, guidances address most, but not all possible eventualities.  Alternative approaches can 
be used if they satisfy the requirements of applicable statutes and regulations. 
 
C. SCOPE: 
 
This guidance focuses on the study designs and principles specific to the determination of in 
vivo blood level BE for veterinary drug products.  The following topics are outside the scope 
of this guidance: 
 
  Biowaivers 
  Biomass products 
  Therapeutic proteins or peptides 
  Medicated premixes 
  Pharmacological endpoint studies 
  Clinical endpoint studies 
  In vitro dissolution tests 
  Human food safety 
  Products where the blood concentrations may not be indicative of drug levels at the 
site of action.  Examples include topically active formulations, intramammary 
products, and intravenous administration of complex drug delivery systems that 
release the API directly at the site of action. 
  The potential need for supportive studies, such as palatability or licking studies (e.g. 
transdermal products, medicated blocks). 
  Animal species from which multiple blood sampling is difficult (e.g., fish, honeybee 
etc.). 
 
As appropriate, other guidance documents should be followed for addressing topics that are 
outside the scope of this BE guidance. 
 
BE is relevant not only for the comparison of generic (test) and reference products, but also in 
product development.  For example, BE or relative bioavailability assessments can be used to 
bridge between different formulations, pharmaceutical forms, routes of administration, and 
comparison of formulations used in pivotal versus early clinical trials. 
 
The glossary provides a definition of the various terms used in this guidance and provides 
some synonymous terms that can be applied in guidances available in local jurisdictions. 
 
An appendix is provided as additional clarification for the scientific and statistical concepts 
described in the guidance.  Other relevant VICH guidances should be consulted. 
 
   
5  Contains Nonbinding Recommendations 
A sample exercise describing sample size estimation BE data statistical analysis, and a 
sequential analysis is provided in a separate, supporting document titled:  Supplemental 
Examples For Illustrating Statistical Concepts Described in the VICH In Vivo Bioequivalence 
Guidance GL52.  This can be found at the following URL:  
http://www.fda.gov/downloads/AnimalVeterinary/GuidanceComplianceEnforcement/Guidan
ceforIndustry/UCM415701.pdf. 
 
Please note the examples provided in the supplemental material are intended solely for 
informational purposes. 
 
Throughout this guidance, the terms blood, plasma and serum can be used interchangeably. 
 
II. IN VIVO PROTOCOL DEVELOPMENT 
 
All BE studies should be conducted in a manner that assures the reliability of the data 
generated.  To be internationally acceptable,1 BE studies should be performed in conformity 
with the principles of Good Laboratory Practices (GLP). 
 
A. Product Selection: 
 
Whereas the product selection for BE or relative bioavailability studies conducted during 
reference product development is not defined, the following conditions generally apply for 
product selection in BE studies supporting approval of generic veterinary drug products: 
 
  BE studies should be performed on test and reference products that contain the same 
API. 
  The test product should be representative of the final formulation of the product to be 
marketed. 
  The reference product should be from a lot associated with a veterinary medicinal 
product that has been granted approval within the jurisdiction for which the generic 
product approval is being sought. 
  The API content of the test and reference products should be assayed prior to 
conducting the BE study.  To be internationally acceptable, it is recommended that the 
assay content of the batches from which test and reference products were obtained 
should differ by no more than 5% from each other. 
  For use in the in vivo BE study, the test product should originate from a batch of at 
least 1/10 of production scale, unless otherwise justified. 
  The characterization and specification of critical quality attributes of the API, such as 
dissolution, should be established from the test batch for which BE has been 
demonstrated. 
 
The study report should include the reference product name, strength (including assayed 
content), dosage form, batch number, expiry date (when available), and country of purchase.  
The test product name, strength (including assayed content), dosage form, composition, batch 
size, batch number, manufacturing date, and expiry date (where available) should be 
provided. 
 
                                                            
1 Where this phrase is used, it indicates that within some jurisdictions, requirements may be less stringent.  This 
difference may be a consideration if a study is to be submitted to support product marketing solely within a 
specific region. 
6  Contains Nonbinding Recommendations 
B. DOSE SELECTION: 
 
For blood level BE studies, it is recommended to dose animals according to the labeled dose 
as opposed to the assay content of the test and reference batches. 
 
The blood level BE study should generally be conducted at the highest labeled (e.g., mg/kg) 
dose approved for the reference product.  By using the highest approved dose, significant 
formulation differences are more easily detected in most cases.  However, if it can be 
substantiated that the reference product exhibits linear PK across the entire dose range, then 
any approved dose can be used if a scientific justification is provided as to why the highest 
dose cannot be used.  In exceptional cases where a batch of reference product with an assay 
content differing less than 5% from the test product cannot be found, the data could be dose 
normalized.  In such cases, the procedure for dose normalization should be pre-specified and 
justified by inclusion of the results from the assay of the test and reference products in the 
protocol. 
 
A BE study conducted at a higher than approved dose can be appropriate when a multiple of 
the highest approved dose is needed to achieve measurable blood levels.  In general, the 
maximum dose would be limited to 3x the highest dose approved for the reference product.  
The reference product should have an adequate margin of safety at the higher than approved 
dose level and should exhibit linear PK (i.e., there are no saturable absorption or elimination 
processes).  In this case, a scientific justification should accompany the choice of the dose. 
 
For reference products with less than proportional increase in AUC with an increase in dose 
(nonlinear kinetics) across the therapeutic range, the following should be considered: 
 
  When there is evidence indicating that the product absorption may be limited by 
saturable absorption processes, this can lead to two formulations appearing to be 
bioequivalent when administered at the highest labeled dose but fail to be 
bioequivalent when administered at lower approved doses.  To avoid this situation, 
use of a dose that is less than the highest approved dose is preferable.  In this case, a 
scientific justification should accompany the choice of the dose (showing that the dose 
is within the linear range). 
  If there is nonlinearity over the therapeutic range due to low solubility, then BE 
should be established at both the highest labeled dose and at the lowest labeled dose 
(or a dose in the linear range), i.e. in this situation, two BE studies may be 
recommended. 
 
In crossover studies, the same total dose should be administered to each animal in all study 
periods.  The use of dose adjustments in those rare situations where large weight changes are 
anticipated (e.g., studies conducted in rapidly growing animals where there is a risk of 
differences in drug absorption, distribution, metabolism, or elimination in period 1 vs 2 that 
could bias the within-subject comparison) should be considered on a case-by-case basis. 
 
   
7  Contains Nonbinding Recommendations 
Where relevant, doses should be rounded up based on the available strength of the solid oral 
dosage form, or to the nearest upper division on the dosing equipment. 
 
Solid oral dosage forms should not be manipulated in a way that could bias the study, e.g., by 
grinding or filing to achieve equal doses.  Breaking tablets along score lines can be 
appropriate if the uniformity of the scored sections can be supported by 
pharmaceutical/manufacturing data (e.g., content uniformity of the halves).  For reference 
products, in the absence of manufacturing or pharmaceutical data, the information included in 
the product labeling can be used as a guide for allowable tablet manipulation. 
 
The study report should include the labeled dose administered to each animal in each period 
of the study. 
 
C. ROUTE OF ADMINISTRATION SELECTION: 
 
Unless otherwise justified when conducting an in vivo BE study: 
 
  The same route and site of administration should be used for the test and reference 
products. 
  Separate BE studies should be submitted for each route of administration approved 
for the reference product. 
 
D. STUDY DESIGN CONSIDERATIONS: 
 
1. CROSSOVER VERSUS PARALLEL STUDY DESIGN: 
 
A two-period, two-sequence, crossover study is commonly used in blood level BE trials 
because it eliminates a major source of study variability: between subject differences in the 
rates of drug absorption, drug clearance, and the volume of drug distribution.  The study 
design is as follows: 
 
Period  Sequence A  Sequence B 
1  Test  Reference 
2  Reference  Test 
 
Note that to eliminate potential confounding by period effects, there should be two sequences 
included in the design of a two period crossover study. 
 
Due to the potential risk of invalidating the crossover design, the treatment administered in 
Period 1 should not affect the PK associated with the treatment administered during Period 2.  
For this reason, the duration of the washout interval should ensure that the drug and its 
metabolites are essentially cleared from the body, and there are no residual physiological 
effects that will alter how the drug administered in Period 2 is processed by the study 
subjects.  Therefore, in addition to proof of absence of pre-dose concentrations, to minimize 
the risk of carryover effects, it is recommended that the duration of the washout interval 
should be at least 5 times the blood terminal elimination half-life of the API and its 
metabolite(s) (when there is indication that the metabolites may affect pharmacokinetics of 
the parent compound in the second period). 
 
   
8  Contains Nonbinding Recommendations 
When dealing with endogenous substances, the presence of carry-over effects is very difficult 
to quantify.  Therefore, caution should be exercised to ensure that the washout period is of an 
adequate duration.  The length of the washout period should be addressed and justified a 
priori in the protocol.  For endogenous substances the pre-dose (baseline) drug concentrations 
for Period 1 should be comparable to the pre-dose concentrations for Period 2. 
 
A parallel study design may be preferable in the following situations: 
 
  The parent compound and/or its metabolites induce physiological changes in the 
animal (e.g., liver microsomal enzyme induction, altered blood flow) that can alter the 
bioavailability of the product administered in Period 2. 
  The parent compound and/or metabolites, or the drug product (e.g., flip flop kinetics) 
has a terminal elimination half-life so long that a risk is created of residual drug 
present in the blood at the time of Period 2 dosing (i.e., a wash-out period is not 
practical). 
  The duration of the washout for the two-period crossover study is so long as to result 
in significant physiological changes in the study subjects. 
  The total blood volume of the species precludes the capture of blood concentration- 
time profiles for more than one period. 
 
Alternative study designs can be considered.  For example: 
 
  Replicate study designs (See subsection II. D. 2) 
  Sequential study designs (See subsection II. D. 3) 
  To obtain approvals in multiple regions, a 3-treatment crossover or a multiple 
reference parallel study design can be considered when performing one study with 
two different reference products, depending on the products registered in the 
respective regions. 
 
Alternative designs and corresponding proposed method of statistical analysis can be 
discussed with the regulatory authority prior to conducting the BE study.  Pilot data or 
literature can be used in support of alternative study designs. 
 
Regardless of how the study will be conducted, the design should be described a priori in the 
protocol. 
 
2. REPLICATE STUDY DESIGN: 
 
A replicate study design is an investigation where at least one of the treatments is repeated. 
 
If it is estimated that a traditional crossover design would not be feasible without the 
inclusion of a very high number of animals, replicate study designs can be considered using 
three (partial replication where for example, the reference is replicated in all subjects) or four 
(full replication, where each subject receives the test and reference products twice) periods 
within each group.  In some jurisdictions, a replicate study design can also be used for 
applying a reference scaled in vivo bioequivalence approach.  Individuals wishing to consider 
the use of alternative statistical approaches should contact individual regulatory authorities 
for additional information on potential statistical considerations and conditions under which 
such alternative approaches are considered acceptable. 
 
9  Contains Nonbinding Recommendations 
3. SEQUENTIAL STUDY DESIGN: 
 
It is appropriate to use a sequential approach when attempting to demonstrate product BE.  
When employing a sequential study design, an initial group of subjects can be treated and 
their data analyzed.  If bioequivalence has not been demonstrated, an additional group can be 
recruited and the results from both groups combined in a final analysis. 
 
If this approach is adopted, appropriate steps should be taken to preserve the overall Type I 
error of the experiment and the stopping criteria should be clearly defined prior to initiating 
the study.  The analysis of the first stage data should be treated as an interim analysis and 
both analyses should be conducted at adjusted significance levels (with the confidence 
intervals corrected accordingly using an adjusted coverage probability that will exceed 90%).  
The plan to use a two-stage approach should be pre-specified in the protocol along with the 
number of animals to be included in each stage and the adjusted significance levels to be used 
for each of the analyses. 
 
4. SINGLE DOSE VERSUS MULTIPLE DOSE STUDY DESIGN: 
 
In most situations, a single dose BE study is recommended for both immediate- and 
modified- release drug products because single dose studies are generally the more sensitive 
approach for assessing differences in the release of the API from the drug product into the 
systemic circulation. 
 
For extended release formulations intended for repeated dosing, demonstration of BE should 
be based on multiple dose studies if there is accumulation between doses (i.e., if there will be 
at least a 2-fold increase in drug concentrations at steady state as compared to that observed 
after a single dose).  In such cases, the C  could be an important parameter to consider, in 
trough
addition to C  and the AUC.  It should be noted that C  can not be equal to C  in the 
max trough min
case of products with a lag time.  If there is no or negligible accumulation, single dose BE 
data could also be sufficient for extended release formulations intended for repeated dosing. 
 
Furthermore, a multiple dose study can also be appropriate when: 
 
  There are saturable elimination processes. 
  The assay sensitivity is inadequate to permit drug quantification that sufficiently 
characterizes the AUC after administration of a single dose (see section II. I. Blood 
Sampling Schedule). 
 
Both single and multiple dose studies can be conducted using a crossover study or parallel 
design.  Due to complications associated with studies of very long duration, the use of 
sequential and replicate study designs are generally not recommended for multiple dose 
studies. 
   
10  Contains Nonbinding Recommendations 
E. SUBJECT AND SPECIES SELECTION: 
 
The animals to be studied should be of the target species.  For each jurisdiction within which 
registration is sought, the BE studies should be performed on each of the major target animal 
species included on the approved reference product label.  Extrapolation of results from a 
major species in which BE has been established to minor species could be appropriate if valid 
scientific arguments are provided to support such extrapolation, taking into account species 
anatomy and physiology, and properties of the API and formulation. 
 
The experimental animals should be free of any drug residues prior to the in vivo phase of the 
BE study.  In some cases, the necessary drug-free period may need to exceed that associated 
with drug residues to account for potential physiological carryover effects that could 
influence the data generated in the BE trial. 
 
Studies should be conducted with healthy animals that are representative of the target 
population.  For parallel design studies, the animals/treatment groups should be homogeneous 
and comparable in all known and prognostic variables that can affect the PK of the API, e.g. 
age, body weight, gender, nutrition, physiological state, and level of production (if relevant). 
 
Animals should be randomized and an equal number of animals should be assigned to each 
sequence (crossover design) or each treatment (parallel study design). 
 
A complete description of the above information should be included in the study report. 
 
F. PRANDIAL STATE: 
 
For all species prandial state and exact timing of feeding should be consistent with animal 
welfare (e.g., ruminants would not be fasted) and the PK of the API. 
 
For canine and feline drug products administered via the oral route, studies should be 
conducted in fasted animals unless the approval for the reference product recommends 
administration in the fed state only, in which case the study should be conducted accordingly. 
Fasting should be a minimum of 8 hours prior to dosing and 4 hours after dosing. 
 
For orally administered modified release formulations intended for non-ruminants, BE 
normally should be established under both fed and fasted conditions unless adequately 
justified. 
 
The study protocol and study report should contain the rationale for conducting the BE study 
under fed or fasted conditions and should describe the diet and feeding regimen. 
 
G. EXCLUSION OF DATA FROM ANALYSIS: 
 
There are numerous situations that can occur that could necessitate removal of all or a portion 
of an animals data from the study.  When this occurs, adequate justification for removal 
should be provided in the study report, and decisions to eliminate data should be made prior 
to analysis of blood samples to avoid bias. 
 
There are situations that occur with sufficient frequency to require stipulation in the study 
protocol.  For example, because there is the risk of losing all or part of the administered dose 
for oral formulations, the criteria for removal of subject data from analysis due to vomiting 
11  Contains Nonbinding Recommendations 
are expected to be specified a priori in the study protocol.  Aspects to consider when defining 
such criteria are: 
 
  What is an acceptable time between drug administration and a vomiting event (taking 
into account e.g. the expected time for the drug to exit the stomach, the prandial state 
of the animal)? 
  What will be considered an allowable amount of material lost in the vomitus? 
 
In addition, when re-dosing after vomiting is considered to be an option in the study, the 
criteria for re-dosing should be specified a priori in the study protocol.  It is important that all 
available data be included in the statistical analysis.  If for example, an animal is excluded 
from Period 2, the data gathered from that animal in Period 1 should not be excluded from the 
statistical evaluation. 
 
To insure that all potential statistical concerns have been addressed, 
 
  descriptive statistics with 
Han.d S wAMithPoLuEt  SdIaZtEa  fDroEmTE aRnMimINalAsT eIxOcNlu:d ed from the BE evaluation should be provided.
 
Pilot studies are useful for estimating the appropriate sample size for the pivotal BE study. 
 
Sample size calculations assume that the estimates used (e.g., treatment differences and 
variances) will be realized in the future study.  Additionally, sample sizes are generally 
estimated as the minimum number needed to demonstrate BE if those estimates are 
realized.  A reference is provided that describes sample size calculations. 
 
Sample size for a BE study should be based upon the number of subjects needed to achieve 
BE for the PK parameter anticipated to have the greatest magnitude of variability and/or 
difference in treatment means (e.g., C ).  Equations and examples are provided in the 
max
Appendix. 
 
It should be noted that for a study to be internationally acceptable, a minimum of 12 
evaluable animals per treatment is necessary.  For a crossover trial, this implies that the 
minimum number of subjects per sequent (n) = 6 (and therefore, the total number of study 
animals in a two-period, two-sequence crossover study, N, should be equal to or greater than 
12).  For a parallel study design, there should be no less than 12 evaluable subjects per 
treatment group (and thus the total number of animals enrolled in the BE trial would be equal 
to or greater than 24). 
 
When the risk of subject loss is a concern, the sponsor can elect to design the study to include 
additional animals.  In this situation, if animals are removed as the study progresses (due to 
vomiting or dosing errors or death/injury), the additional animals placed on study can allow 
appropriate statistical power to be maintained. 
 
Sample size selection should be justified a priori in the study protocol. 
 
I.  BLOOD SAMPLING SCHEDULE: 
 
The sampling schedule should include frequent sampling around T  to provide a reliable 
max
estimate of C .  For routes of administration other than intravenous injection, the sampling 
max
schedule should avoid situations where the first sampling time corresponds with C .  The 
max
12  Contains Nonbinding Recommendations 
duration of blood sampling should provide a reliable estimate of the extent of exposure which 
is achieved if AUC  is at least 80% of AUC .  At least 3 samples are recommended 
0-Last 0-
during the terminal log-linear phase in order to reliably estimate k  and obtain an accurate 
e
estimation of AUC0-. 
 
For an API with a long terminal elimination half-life, BE can be based on AUC values that 
are less than 80% of total systemic exposure (in addition to C ) as long as the absorption 
max
phase has been completed during the applied sample collection period. 
 
In multiple dose studies, the pre-dose sample should be taken immediately before dosing and 
the last sample is recommended to be taken as close to the end of the dosing interval as 
possible to ensure an accurate determination of AUC .  Sampling should also be performed to 

show that steady state conditions are reached (i.e., trough concentrations should be sampled 
sequentially until Ctrough is stable). 
 
For endogenous compounds, the pre-dose sampling schedule should be consistent with the 
method of baseline correction (see section II. J. Blood Level BE Parameters). 
 
The planned and actual timing of blood sample collections for each individual should be 
included in the study report. 
 
J. BLOOD LEVEL BE PARAMETERS: 
 
The following parameters should be collected.  Some of these parameters will not be used for 
the statistical BE parameters (see section II. D. Study Design Considerations). 
 
In single dose studies, C , T , AUC , and AUC  should be determined. 
max max 0-Last 0-
 
In multiple dose studies, the AUC, steady state Cmax values (Cmax ss), steady state Ctrough 
values, and steady state T values (T ) should be determined.  In situations involving 
max  max ss
dosage forms associated with an intentional delayed release, a comparison of test and 
reference product Ctrough values may also be appropriate. 
 
If the API is an endogenous compound, the calculation of BE parameters should include a 
correction for baseline concentrations.  The method for baseline correction should be 
specified and justified a priori in the study protocol.  The recommended method of baseline 
correction is subtraction of the mean endogenous concentrations obtained from the pre-dose 
concentrations estimated at the same time on three consecutive days.  If diurnal variations in 
the concentrations of the endogenous compound are anticipated, profiles characterizing this 
variation can be appropriate. 
 
Additional parameters that may be relevant to report include k , terminal elimination half-life 
e
and T . 
lag
 
Non-compartmental methods should be used for the determination of PK parameters in BE 
studies. 
 
The study report should state the method used to derive the PK parameters from the raw data. 
 
   
13  Contains Nonbinding Recommendations 
K. DEFINING THE ANALYTE: 
 
In principle, BE evaluations should be based upon measured concentrations of the parent 
compound because the Cmax of a parent compound is usually more sensitive to differences 
between product absorption rates as compared to the Cmax of a metabolite.  In general, 
product BE will be determined on the basis of total (free plus protein bound) concentrations 
of the API. 
 
1. PRO-DRUGS 
 
BE demonstration should be based upon the parent compound unless the parent compound is 
a pro-drug and that pro-drug is associated with negligible blood concentrations.  In cases 
where there are negligible systemic concentrations of the pro-drug, the active metabolite (the 
compound formed upon absorption of the pro-drug) should be measured.  Sponsors should 
provide scientific rationale for the compound to be quantified. 
 
2. ENANTIOMERS 
 
Under most situations, use of an achiral assay will suffice for the assessment of product 
bioequivalence.  However, the use of an enantiomer-specific analytical method is 
recommended when all of the following conditions are met: 
 
  The enantiomers exhibit different PK. 
  The AUC ratio of the enantiomers is modified by a difference in their respective rates 
of absorption. 
  The enantiomers have different pharmacodynamic characteristics. 
 
If all three conditions are met, chiral (stereospecific) analytical methods are recommended.  
In addition, chiral methods may be important when the test or reference products include the 
use of a stereospecific (chiral) excipient that can selectively alter the absorption of one or 
both enantiomers.  It may also be important when a drug is a single enantiomer that 
undergoes in vivo chiral conversion. 
 
L. BIOANALYTICAL METHOD VALIDATION: 
 
The bioanalytical phase of the BE study should be based upon an appropriately validated 
bioanalytical method. 
 
The following aspects of bioanalytical method validation and performance should be 
summarized in the study report (or as otherwise deemed appropriate by the regulatory 
authority): 
 
  Concentration range and linearity 
  Matrix effects 
  Limit of quantitation (LOQ) 
  Specificity (selectivity) 
  Accuracy 
  Precision 
  Stability of analyte and internal standard 
   
14  Contains Nonbinding Recommendations 
The following data from quality control (QC) samples obtained during in-phase analytical 
runs containing incurred samples should be provided: 
 
  Precision 
  Accuracy 
 
Regulatory authorities should be contacted regarding the possible need to include incurred 
sample reanalysis (IRS) as a component of the method validation (where IRS is the repeat 
analysis of a subset of subject samples in separate analytical runs). 
 
III. STATISTICAL ANALYSIS: 
 
The statistical BE evaluation is best generated by the use of 90% confidence intervals (i.e., 
the two-sided confidence interval approach).  The two-sided confidence interval for the ratio 
of the treatment parameter means can be characterized as follows:  If an investigator 
repeatedly calculates these intervals from many independent and random samples, 90% of 
these intervals would correctly bracket the true population ratio. 
 
The confidence interval approach should be applied to the individual parameters of interest, 
typically AUC and Cmax (refer to section J).  The sponsor should use the natural logarithmic 
transformation (Ln-transformation) of the parameters prior to statistical analysis. 
 
A. THE STATISTICAL MODEL: 
 
The precise model to be used for the Analysis of Variance (ANOVA) should take into 
account sources of variation that can be reasonably assumed to have an effect on the response 
variable. 
 
For a two-period, two- sequence, two-treatment crossover study, model terms usually include 
(but are not limited to) sequence, animal within sequence, period and treatment.  Fixed 
effects, rather than random effects, should be used for testing of period and treatment effects.  
When using a parallel study design, the treatments are generally compared using a one-way 
ANOVA (i.e., treatment is the sole effect being tested by the statistical model).  Accordingly, 
the residual error (random effect) is the appropriate error for statistically comparing the test 
and reference products. 
 
Other statistical methods may be appropriate, depending upon study design. 
 
The statistical model and randomization process should be defined a priori in the study 
protocol. 
 
B. LN-TRANSFORMATION: 
 
Ln transformation should be used for BE evaluation because it generally improves our ability 
to meet the assumptions of the ANOVA.  Reasons for this include: 
 
  PK models are multiplicative rather than additive 
  Ln transformation stabilizes the variances 
  BE comparisons are generally expressed as ratios rather than differences 
 
Other types of data transformation will be difficult to interpret.   
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C. DOSE NORMALIZATION: 
 
Dose normalization is not appropriate when employing a crossover study design, except as 
described in section II. B. Dose Selection.  In rare instances involving BE trials designed as a 
parallel study and when the drugs are administered on a mg rather than on a mg/kg basis, 
between-animal differences in body weight could inflate the magnitude of the residual error 
to an extent that a prohibitively large increase in subject numbers would be necessary to 
maintain study power.  In these situations, the acceptability of dose normalization and the 
corresponding method of data analysis should be discussed with the regulatory authorities 
during protocol development. 
 
D. CONFIDENCE INTERVAL ACCEPTANCE CRITERIA: 
 
To be internationally acceptable: 
 
  The acceptance criteria for AUC and Cmax should be 0.80 to 1.25, and 
  In cases where multiple dose studies have been employed for extended release 
formulations and there is drug accumulation, these criteria will also be applied to 
Ctrough values. 
 
In cases where a sponsor intends to use an alternative study design to allow for an adjustment 
to the acceptance criteria based upon the variability of the reference product, the regional 
authorities could be consulted about appropriate statistical methods and study designs. 
 
E. STATISTICAL REPORT: 
 
At a minimum, the study report should include the individual subject concentration versus 
time data for each study period (indicating period and treatment associated with each blood 
level profile), subject allocation to sequence, individual parameter estimates, methods used 
for parameter estimation, summary statistics, and the statistical output (e.g., ANOVA).  This 
would enable regulatory authorities to perform PK and statistical analyses if necessary. 
   
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IV. GLOSSARY 
 
  Acceptance criteria (syn:  confidence bounds):  The upper and lower limits (boundary) 
of the 90% confidence interval that is used to define product BE. 
 
  Active pharmaceutical ingredient (API) (syn:  active substance):  A substance used in 
a finished pharmaceutical product, intended to furnish pharmacological activity or to 
otherwise have direct effects in the diagnosis, cure, mitigation, treatment or prevention of 
disease or to have direct effect in restoring, correcting or modifying physiological 
functions of the body. 
 
Note:  Due to international differences in the interpretations of what is considered to be 
the same API when considering, for example, different salts and esters, no agreed upon 
definition is provided.  Sponsors should consult with the local regulatory authority for that 
jurisdictions interpretation of what could be considered the same API. 
 
  Area under the curve (AUC):  Area under the plasma drug concentration versus time 
curve, which serves as a measure of drug exposure.  It includes several different types of 
AUC estimates: 
 
  AUC :  AUC to the last blood sampling time associated with quantifiable drug 
0-Last
concentrations.  The last quantifiable concentration (the limit of quantification, LOQ) 
is determined by the sensitivity of the analytical method.  The last quantifiable drug 
concentration may occur prior to the last blood sampling time. 
 
  AUC0-:  AUC0-Last with the addition of the extrapolated area from the last quantifiable 
drug concentration to time infinity.  The terminal area from the last quantifiable drug 
concentration to time infinity is estimated as C / , where C  is the last quantifiable 
last e last
drug concentration and   is the terminal slope of the Ln concentration-time profile. 
e
 
  AUC  (AUC ):  AUC over one steady state dosing interval.  Mathematically, the 
tau 
quantity equals AUC  of the first dose if there is linear (non-saturable) PK. 
0-
 
  Assay content:  The amount of the analyte in a sample. 
 
  Bioavailability:  The rate and extent to which the API or active metabolites enters the 
systemic circulation. 
 
  Bioequivalence:  The absence of a difference (within predefined acceptance criteria) in 
the bioavailability of the API or its metabolite(s) at the site of action when administered at 
the same molar dose under similar conditions in an appropriately designed study. 
 
  Biomass:  Crude products of fermentation, where the fermentation derived product is not 
extracted or purified; rather the resulting fermentation mixture, including the API and 
fermentation broth, is dried and used as is in the manufacture of medicated feeds or feed 
additives. 
 
  Biowaiver:  A waiver of the requirement to demonstrate in vivo BE between a test and 
reference drug product.   
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  Blood:  Within this guidance, the terms blood, plasma, and serum are used 
interchangeably. 
 
  Composition:  The ingredients as well as the absolute amounts of these ingredients 
included in the formulation. 
 
  C :  The maximum (or peak) concentration of API or its metabolite(s) in blood. 
max
 
  C :  The minimum concentration of the API or its metabolites in the blood at steady 
min
state.  In the absence of a measurable delay between drug administration and the first 
appearance of drug in the systemic circulation C equals C . 
min  trough
 
  C :  The concentration of API or its metabolite(s) in blood at steady state immediately 
trough
prior to the administration of a next dose. 
 
  Dosage form (syn: pharmaceutical form):  The physical form of a dose of a medication 
such as tablet, capsule, paste, solution, suspension, etc. 
 
Note:  Due to international differences, what is considered to be the same dosage form 
in some jurisdictions may be considered as different dosage forms in other jurisdictions.  
Drug sponsors should consult with the local regulatory authority for that jurisdictions 
interpretation of what could be considered the same dosage form. 
 
  Drug product (syn: medicinal product):  A finished dosage form that contains the API 
usually in association with one or more excipients. 
 
  Elimination rate constant (k ):  The first-order rate constant describing drug elimination 
e
from the body.  Although the amount of drug eliminated in a first-order process changes 
proportionally with concentration, the fraction of a drug eliminated remains constant.  The 
elimination rate constant is, then, a fraction of a drug that is removed from the body per 
unit of time. 
 
  Enantiomer:  A pair of chiral isomers (stereoisomers) that are direct, nonsuperimposable 
mirror images of each other.  Enantiospecificity in pharmacokinetics can arise because of 
enantioselectivity in one or more of the processes of drug absorption, distribution, 
metabolism and excretion. 
 
  Excipient (syn: inactive ingredient):  A substance other than the API that has been 
appropriately evaluated for safety and is included in a drug product to either aid in its 
manufacturing; protect, support or enhance stability, bioavailability, or target animal 
acceptability; assist in product identification; or enhance any other attribute of the overall 
safety and effectiveness of the drug product during storage or use. 
 
  Extended release formulation:  A dosage form that is deliberately modified to protract 
the release rate of the API compared to that observed for an immediate release dosage 
form.  This term is synonymous with prolonged or sustained release dosage forms. 
 
  Finished dosage form:  A dosage form of the API which is intended to be dispensed or 
administered to the animal and requires no further manufacturing or processing other than 
packaging and labelling.   
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  Good Laboratory Practice (GLP):  Quality standards for conducting non-clinical 
laboratory studies and field trials.  Regional standards/regulations are specified by each 
regulatory jurisdiction. 
 
  Highest labeled dose:  The highest approved dose of the reference product as indicated 
on the label (usually defined as strength per unit body weight, e.g., mg/kg).  If there is an 
approved dose range, the highest labeled dose would be the highest dose in that range. 
 
  Linear pharmacokinetics:  When the concentration of the API or its metabolite(s) in the 
blood increases proportionally with the increasing dose, and the rate of elimination is 
proportional to the concentration, the drug is said to exhibit linear pharmacokinetics.  The 
clearance and volume of distribution of these drugs are dose-independent. 
 
  Modified release formulation:  Drug products where the rate and/or place of release of 
the API(s) is different from that of an immediate release dosage form administered by the 
same route.  This deliberate modification is achieved by a special formulation design 
and/or manufacturing method. 
 
  Medicated Premix:  A veterinary medicinal product which has been granted marketing 
authorization and is intended for oral administration following its incorporation into 
animal feedstuffs.  The medicated premix frequently consists of the API, a carrier, and a 
diluent. 
 
  Nonlinear pharmacokinetics:  As opposed to linear pharmacokinetics, the concentration 
of the API or metabolites in the blood does not increase proportionally with the increasing 
dose.  The clearance and volume of distribution of these may vary depending on the 
administered dose.  Nonlinearity may be associated with any component of the 
absorption, distribution, and/or elimination processes. 
 
  Pharmacokinetics (PK):  The study of the absorption, distribution, metabolism, and 
excretion of an API and/or its metabolite(s). 
 
  Reference product:  The drug product to which the in vivo BE and, in some instances, 
the in vitro equivalence of the test drug product is compared. 
 
  Replicate study design:  An investigation where at least one of the treatments is 
repeated. 
 
  Relative bioavailability:  The bioavailability of a drug product when compared with 
another formulation of the same drug administered by an extravascular route. 
 
  Steady state (ss):  The condition where the API input rate is in dynamic equilibrium with 
its output (elimination) rate. 
 
  Stereoisomer:  Compounds differing only in the spatial arrangement of their atoms. 
 
  Strength:  The amount of API in a drug product expressed in specific unit of 
measurement (e.g., 10 mg/mL, 25 mg/tablet). 
 
  Test product:  The drug product used for BE comparison to the reference product.   
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  Tlag:  The duration of time between drug administration and the appearance of the API in 
the systemic circulation. 
 
  Tmax:  Time to the Cmax. 
 
  Transdermal product:  A dosage form designed to be applied to intact skin for the 
purpose of delivering the API for absorption through the skin and into the systemic 
circulation. 
   
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V. APPENDIX 
 
An example of the sample size needed to attain a power of 80% at  = 0.05 for a single 
variable in the case of the multiplicative model is provided in Table 1 below.  Since the BE 
assessment is based upon the two one-sided tests procedure, the sample size calculation is 
based upon  = 0.05 per tail, which translates into as a 90% confidence interval (2 = 0.10). 
The number of subjects provided in the table (N), is the total number of subjects required in a 
two-period crossover design (where N = 2n and n = the number of subjects per sequence) for 
a given ratio of the test/reference product. 
 
Table 1:  An example of sample size estimates based upon a given ratio of test and reference 
treatment means and within subject variability where the confidence bounds (acceptance 
criteria) are 0.80 to 1.25. 
 
  Ratio Test/Reference Products 
%CV  0.85  0.9  0.95  1  1.05  1.1  1.15  1.2 
12.5  56  16  10  8  10  14  30  118 
15  78  22  12  10  12  20  42  170 
17.5  106  30  16  14  16  26  58  230 
20  138  38  20  16  18  32  74  300 
22.5  172  48  24  20  24  40  92  378 
25  212  58  28  24  28  50  114  466 
27.5  256  70  34  28  34  60  138  >500 
30  306  82  40  34  40  70  162  >500 
35  414  112  54  44  52  96  220  >500 
40  >500  146  70  58  68  124  288  >500 
50  >500  226  108  88  104  192  446  >500 
 
The %CV reflects the residual error and includes any source of variability that is not 
accounted for in the statistical model.  The benefit derived from a crossover trial is that 
the residual error only includes sources of within-subject variability.  Typically, parallel 
study designs are associated with larger residual errors because the comparisons are 
generated between and not within subjects.  As such, the residual error includes both 
within and between subject sources of error. 
 
When considering a crossover study design, if a multiplicative model is used (where the 
within-subject %CV is 20 and the ratio of the test/reference products = 0.95), the equation 
results in an estimate of 20 subjects (10 in Sequence 1, 10 in Sequence 2).  However, when 
this equation is applied to a parallel study design, N = the number of subjects per treatment.  
Therefore, 2xN =total number of subjects = N (test) + N (reference). 
 
Sample Size Estimation 
 
Ln-Transformed Data (based upon Hauschke et al., 1992). 
 
In a crossover study, the number of subjects needed to achieve a 1- power at the  nominal 
level is termed N, and N = 2n, where n is the number of subjects required per sequence. For 
the multiplicative model, the number of subjects can be estimated as follows: 
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If = 1, then: n > [t(, 2n-2) + t(/2, 2n-2)]2 [CV/ ln1.25]2 
If 1< < 1.25, then: n > [t(, 2n-2) + t(, 2n-2)]2 [CV/ (ln 1.25  ln )]2 
If 0.8 < < 1, then: n > [t(, 2n-2) + t(, 2n-2)]2 [CV/ (ln 0.8  ln )]2 
 
Where: 
n = the number of subjects per sequence 
t(, 2n-2) = t-value associated with the estimated confidence interval 
 = Type 1 error = 0.05 for a one tailed test or 0.10 for a two-tailed test.  For example, for a 
two tailed test ( = 0.05 per tail) and with 10 degrees of freedom, the corresponding 
value from a T distribution table = 1.812 
2n-2 = the error degrees of freedom used to estimate the confidence interval 
 = Type II error (usually 0.20).  For example, with 10 degrees of freedom, the corresponding 
value from a T distribution table = 0.879.  Similarly, /2 (used when = 1) = 1.372. 
  = the expected population mean for the test product (log-transformed value) 
T
  = the expected population mean for the reference product (log-transformed value) 
R
 = (   ) 
T- R
CV = coefficient of variation.  This is calculated as the square root of the variance (i.e., the 
standard error) divided by the mean of all of the study observations. 
 
This same equation can be applied to situations when a parallel rather than a crossover study 
design is used.  However, when this equation is applied to a parallel study design, n = the 
number of subjects per treatment.  Therefore, N =total number of subjects = n (test) + n 
(reference). 
 
Note:  This is an iterative equation.  Because of the potential for greater differences and 
variances to occur when the pivotal study is performed, it may be prudent to repeat sample 
size estimates using both greater variability and both higher and lower estimates of ratios 
between treatment means.  Based upon this additional information, the number of animals can 
be selected that will provide the best chance for success using the resources available. 
 
   
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REFERENCE 
 
Hauschke D, Steinijans VW, Diletti E, and Burke M (1992). Sample size determination for 
bioequivalence assessment using a multiplicative model. J Pharmacokinet Biopharm.20:557-
561. 
 
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