#279 
 
Demonstrating Bioequivalence for Type A Medicated 
Articles Containing Active Pharmaceutical 
Ingredient(s) Considered to be Poorly Soluble in 
Aqueous Media, That Exhibit Little to No Systemic 
Bioavailability, and Are Locally Acting 
 
Draft Guidance 
 
This guidance document is being distributed for comment purposes only. 
 
Submit comments on this draft guidance by the dated provided in the Federal Register notice 
announcing the availability of the draft guidance.  Submit electronic comments to 
https://www.regulations.gov.  Submit written comments to the Dockets Management Staff 
(HFA-305), Food and Drug Administration, 5630 Fishers Lane, Rm. 1061, Rockville, MD  
20852.  All comments should be identified with docket number FDA-2023-D-1275. 
 
For further information regarding this document, contact the following individuals:  
 
Generic Drug Approval Requirements: Ian S. Hendricks, Center for Veterinary Medicine 
(HFV-172), Food and Drug Administration, 7500 Standish Place, Rockville, MD 20855, 240- 
402-0853, email: Ian.Hendricks@fda.hhs.gov. 
 
Biopharmaceutics and Pharmacokinetics: Marilyn Martinez, Center for Veterinary Medicine 
(HFV-100), Food and Drug Administration, 7500 Standish Place, Rockville, MD 20855, 240- 
402-0635, e-mail: Marilyn.Martinez@fda.hhs.gov. 
 
Chemistry, Manufacturing, and Controls: Catherine Finnegan, Center for Veterinary Medicine 
(HFV-140), Food and Drug Administration, 7500 Standish Place, Rockville, MD 20855, 240-
402-0650, email: Catherine.Finnegan@fda.hhs.gov.  
 
Additional copies of this draft guidance document may be requested from the Policy and 
Regulations Staff (HFV-6), Center for Veterinary Medicine, Food and Drug Administration, 
7500 Standish Place, Rockville MD 20855, and may be viewed on the Internet at 
https://www.fda.gov/animal-veterinary, https://www.fda.gov/regulatory-information/search-fda-
guidance-documents, or http://www.regulations.gov. 
 
U.S. Department of Health and Human Services 
Food and Drug Administration 
Center for Veterinary Medicine (CVM) 
June 2023
 
  Contains Nonbinding Recommendations 
Draft  Not for Implementation 
 
Table of Contents 
 
I.  Introduction .......................................................................................................................... 1 
II.  Background .......................................................................................................................... 1 
III.  Bioequivalence Approach for Poorly Soluble, Locally Acting Type A Medicated 
Articles. ................................................................................................................................. 3 
A. Data to Support a Determination of Bioequivalence .................................................. 4 
1.  Q1, Q2, and Q3 Comparison to the RLNAD ......................................................... 4 
2.  In Vivo Bioequivalence Studies. .............................................................................. 5 
3.  Comparative In Vitro Dissolution Testing. ............................................................ 5 
IV.  Logistical Considerations .................................................................................................... 8 
V.  Post-approval Changes ........................................................................................................ 8 
A. Change to API source .................................................................................................... 8 
B.  Change in Manufacturing Process ............................................................................... 9 
VI.  Human Food Safety (HFS) Considerations ....................................................................... 9 
 
 
i  Contains Nonbinding Recommendations 
Draft  Not for Implementation 
 
Demonstrating Bioequivalence for Type A Medicated Articles 
Containing Active Pharmaceutical Ingredient(s) Considered to be 
Poorly Soluble in Aqueous Media, That Exhibit Little to No 
Systemic Bioavailability, and Are Locally Acting 
Draft Guidance for Industry 
 
This draft guidance, when finalized, will represent 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 it satisfies the 
requirements of the applicable statutes and regulations.  To discuss an alternative approach, 
contact the FDA staff responsible for this guidance as listed on the title page. 
 
I.  Introduction 
This guidance describes an alternative approach for satisfying the requirements for the 
completion of the Bioequivalence technical section for generic Type A medicated articles 
(TAMAs) containing poorly soluble, locally acting, active pharmaceutical ingredients (APIs) that 
have little to no systemic absorption, and for which blood level studies are not considered 
appropriate to demonstrate product bioequivalence.  For the purpose of this guidance, poorly 
soluble refers to TAMAs containing APIs that do not meet the criteria of fully soluble as defined 
in Guidance for Industry (GFI) #171.1  This guidance does not cover TAMAs containing poorly 
soluble APIs that are systemically absorbed and for which blood level bioequivalence studies are 
appropriate.    
This guidance is applicable to generic investigational new animal drug (JINAD) files and to 
abbreviated new animal drug applications (ANADAs).  Although the recommendations in this 
guidance are discussed relative to generic new animal drug applications, the general principles 
described to demonstrate product bioequivalence may also be applicable to pioneer new animal 
drug applications (NADAs), investigational new animal drug (INAD) files, and supplemental 
ANADAs and NADAs. 
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. 
II.  Background 
 
1 GFI #171, Demonstrating Bioequivalence for Soluble Powder Oral Dosage Form Products, and Type A 
Medicated Articles Manufactured from Active Pharmaceutical Ingredients Considered to be Soluble in Aqueous 
Media (https://www.fda.gov/media/131173/download) 
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Section 512(c)(2)(A)(vi) of the Federal Food, Drug, and Cosmetic Act (FD&C Act) requires that 
generic new animal drug products be shown to be bioequivalent to the reference listed new 
animal drug (RLNAD), and section 512(n)(1)(E) of the FD&C Act requires that the sponsor 
provide information to show that the proposed product is bioequivalent to the RLNAD.  Two 
products are considered to be bioequivalent when they are equal in the rate and extent to which 
the active pharmaceutical ingredient (API) becomes available at the site(s) of drug action 
(section 512(c)(2)(H)(i) of the FD&C Act). FDAs Center for Veterinary Medicine (CVM) has 
issued guidance on demonstrating bioequivalence through in vivo studies, and guidance on 
product types that may be eligible for a waiver from the requirement to perform in vivo 
bioequivalence studies (biowaivers), including oral solutions and other solubilized forms, 
parenteral solutions, some topically applied dosage forms2 and TAMAs with APIs that are 
considered to be water soluble.3   
These guidance documents do not specifically address the unique challenges associated with 
demonstrating bioequivalence of TAMAs containing poorly soluble, locally acting APIs.  As 
stated in GFI #35, the preferred hierarchy of bioequivalence studies (in descending order of 
sensitivity) is the blood level study, pharmacologic end-point study, and clinical end-point study. 
Because TAMAs containing poorly soluble, locally acting APIs are not amenable to 
demonstrating bioequivalence through in vivo studies or pharmacologic end-point studies, a 
demonstration of bioequivelance would normally involve submission of clinical end-point 
bioequivalence study data for each indication, but this approach does not account for the unique 
characteristics of these products and typically would require multiple clinical end-point studies 
using hundreds of animals.  While the use of multiple clinical end-point studies is still a valid 
scientific approach, CVM believes that the approach as described in this guidance document 
provides an alternative path to obtaining the required data for the demonstration of 
bioequivalence, while preserving the scientific foundations of such a determination.  This 
alternate approach refines and reduces the use of animals in the pre-approval setting and 
addresses the obstacles associated with multiple clinical end-point studies, thereby reducing the 
number of clinical end-point studies needed to support bioequivalence of these products.  In so 
doing, this approach can enhance the availability of generic drug products that have 
characteristics that do not align well with the recommendations in previously published 
guidance.  
The approach to demonstrating bioequivalence as described in this guidance incorporates several 
established concepts that are described in bioequivalence guidance(s) for other dosage forms, 
using a combination of in vitro and in vivo data to support a determination of bioequivalence.  
The use of comparative physicochemical evaluations in concert with in vitro drug release are 
well-documented approaches to the demonstration of bioequivalence for a variety of human drug 
products.  Also, for tablet products that come in multiple strengths, the bioequivalence 
requirements for both human and veterinary products are typically satisfied using a combination 
of a single in vivo study and comparative in vitro dissolution data. 
 
2 Guidance for Industry #35, Bioequivalence Guidance. 
3 See footnote 1 on page 1. 
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While the application of this approach to TAMAs containing poorly soluble, locally acting APIs 
has not previously been described, the underlying principles are consistent with how CVM has 
approached the determination of bioequivalence in several other dosage forms.  Consistent with 
previous guidance, an in vivo clinical end-point study remains an essential component of the 
evidence supporting a determination of bioequivalence for TAMAs containing poorly soluble, 
locally acting APIs.   
Based on the Agencys experience using both in vivo and in vitro data to support a determination 
of bioequivalence, the alternate approach described herein should provide sufficient scientific 
evidence to demonstrate bioequivalence of TAMAs containing poorly soluble, locally acting 
APIs. 
III.  Bioequivalence Approach for Poorly Soluble, Locally Acting Type A Medicated 
Articles. 
For TAMAs containing APIs that fail to meet the biowaiver criteria as defined in CVMs GFI 
#171, and for which blood level studies are not considered appropriate for demonstrating product 
bioequivalence, the rate-limiting factor influencing the equivalence of the proposed generic and 
reference products is their respective in vivo dissolution characteristics.  Such a products 
formulation and physicochemical characteristics are key determinants of in vivo dissolution 
performance.  Based on an understanding of the critical factors influencing the rate and extent of 
API availability at the site of activity for these products, the determination of product 
bioequivalence can be based upon a combination of in vitro and in vivo bioequivalence 
assessments typically consisting of the following: 
 
a.  Qualitative (Q1), Quantitative (Q2), and Physicochemical (Q3) equivalence of the 
generic and RLNAD TAMA.  In this context, equivalence is defined as the absence of 
any differences that would have an impact on the rate and extent that the API reaches its 
active site. 
 
b.  Equivalence of the in vivo performance of the generic and RLNAD TAMAs when used 
as indicated for one indication in each major species on the RLNAD product label. 
 
c.  Comparative in vitro dissolution testing of the generic and RLNAD TAMA formulations 
under varying conditions.  
 
An assessment of bioequivalence will then be made by evaluating the data from the three 
determinations (a, b, and c) in its entirety.  Therefore, if one determination in this approach fails 
to indicate sameness, then the product would be determined not to be bioequivalent.  This 
approach leverages an understanding that the purpose of a clinical end-point bioequivalence 
study is not to repeat an effectiveness study to determine the dose-effect relationship of a 
particular drug, but rather it is to detect any formulation difference between the generic product 
and the RLNAD that may affect product bioavailability.  As such, a separate clinical end-point 
study for multiple indications is unnecessary if the rate and extent of API availability at the site 
of action is not dependent on the indication and the products are sufficiently similar that any 
differences are not expected to impact the in vivo dissolution behavior.  Thus, a determination of 
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bioequivalence can be made based on the combined acceptable in vitro dissolution and 
physicochemical comparisons, along with a successful clinical end-point bioequivalence 
assessment. 
 
A.  Data to Support a Determination of Bioequivalence 
1.  Q1, Q2, and Q3 Comparison to the RLNAD  
Q1, Q2, and Q3 data from the generic TAMA should demonstrate equivalence to the 
RLNAD.  CVM recommends testing multiple lots of the generic product (minimum of 
three) in parallel with multiple lots (minimum of five) of the RLNAD. 
 
a.  Q1 and Q2 (Qualitative and Quantitative Assessment) 
Q1 and Q2 equivalence requires that the same API at the same concentration is used 
for the proposed generic and RLNAD products and that there are no inactive 
ingredients that may significantly affect the bioavailability of the API(s).  If there 
are known differences in the proposed formulation relative to that of the RLNAD, a 
justification should be provided as to why the proposed change will have no effect 
on the equivalence of the two products.  If the API is biomass derived (i.e., where 
the drug substances may contain the active molecule(s), inactivated microorganisms 
used for production, other metabolites produced by the microorganisms used for 
production, and media components), the biomass characterization will be 
considered in the Q1 assessment.  
 
b.  Q3 (Physicochemical Assessment) 
A meeting with CVMs Office of New Animal Drug Evaluations (ONADE) 
Division of Manufacturing Technologies (HFV-140) should be requested to discuss 
the Q3 attribute characterizations needed to support a demonstration of product 
comparability.  The physicochemical tests should reflect the critical quality 
attributes (CQAs) of the RLNAD, which are determined by formulation and 
manufacturing process.  Testing should include all compendial tests, as well as 
impurity analysis and particle size distribution.  If a United States Pharmacopeia 
(USP) monograph is not published for the RLNAD, then the attributes for 
evaluation should include assay and impurity analysis, along with a particle size 
distribution comparison of the generic and RLNAD products.  
 
CVM considers the particle size distribution of the API to be a critical attribute 
impacting the bioequivalence of locally acting TAMAs but recognizes there is no 
practical way to measure the API particle size distribution when comparing the 
generic and RLNAD TAMAs.  In lieu of directly comparing the API particle size 
distributions, the particle size distribution of the generic and RLNAD TAMAs 
should be assessed with the awareness that these data will include particle size 
information from both the API and excipients, making a meaningful comparison 
more difficult.  The particle size distribution range of the generic TAMA should 
match or be within that of the RLNAD. 
4  Contains Nonbinding Recommendations 
Draft  Not for Implementation 
 
 
2.  In Vivo Bioequivalence Studies 
Sponsors should submit data demonstrating that their proposed product is bioequivalent 
to that of the RLNAD in an in vivo clinical bioequivalence study for one of the 
indications, for each major species on the RLNAD labeling.  Typically, the indication 
being evaluated should have its site of action within the portion of the GI tract closest 
to the site of in vivo dissolution (i.e., where there are multiple indications, then sponsors 
should choose the one which is closest to the site where the majority of the in vivo 
dissolution is occurring).  Sponsors proposing to demonstrate bioequivalence of an 
indication associated with a site not meeting this description, such as when local sites 
of action are situated throughout the gastrointestinal tract and the chosen indications 
site of action is not the one closest to where the majority of the in vivo dissolution is 
occurring, should contact CVM to discuss the acceptability of the proposal prior to 
submission of a protocol or conducting an in vivo study.  CVM highly recommends the 
submission of a protocol prior to initiating pivotal studies.  If one of the indications 
requires systemic drug absorption for activity, a blood level bioequivalence study will 
be necessary to demonstrate bioequivalence for that indication in lieu of, or in addition 
to, a clinical end-point study, even if another indication is associated solely with local 
drug concentrations. If a blood level study is needed, then a meeting should be arranged 
with CVM to determine the best path forward to demonstrate product bioequivalence. 
 
3.  Comparative In Vitro Dissolution Testing 
The pivotal dissolution data should include at least three sets of dissolution curves, with 
each set using a different set of dissolution conditions (e.g., pH, paddle speed, buffer 
and/or surfactants).  Prior to starting the pivotal studies, CVM recommends that 
sponsors submit preliminary data for five (5) dissolution conditions, using the RLNAD, 
so that CVM can concur upon the use of three of these conditions to support the pivotal 
generic-reference product comparison.  For at least one of the pivotal dissolution 
conditions, the average percent dissolved for the RLNAD should be at least 85% and be 
demonstrably discriminatory (for example, the ability to detect changes in the rate and 
extent of API release due to modifications in product CQAs, or the ability to 
distinguish between marketed lots of the RLNAD). 
 
a.  Sample Considerations 
  Across all in vitro test conditions, all proposed generic and RLNAD samples 
should contain the same amount (typically expressed as weight) of the 
TAMA. 
 
  For any test condition, the volume of medium should be sufficient to ensure 
that the rate and extent of dissolution is determined by formulation rather 
than API solubility limitations.  Sink conditions are desirable but may not be 
possible across all three test conditions.  Optimally, the same volume of 
medium will be used across all dissolution procedures. The amount (weight) 
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of the TAMA sample used across all test conditions should be selected such 
that the in vitro dissolution is sensitive to potential formulation effects.  
 
b.  Generating Comparative Dissolution Data 
The pivotal test should compare the in vitro dissolution profiles (12 vessels per lot) 
obtained from a minimum of five lots of the RLNAD and three lots of the proposed 
generic product.  
 
  Typically, USP apparatus II (paddle) should be employed.  If a sponsor 
determines that some other USP apparatus is preferable, the sponsor should 
provide their rationale for this decision to CVM. 
 
  When the paddle is employed, rotational speeds should not exceed 100 
RPM.  Issues such as foaming and fluid hydrodynamics should be 
considered when selecting a paddle speed. 
 
  The temperature of the dissolution media should be consistent with that 
found in the digestive tract of the indicated species.4 
  The pH range covered by the dissolution media should be consistent with 
that found across the digestive tract of the indicated species, as documented 
in GFI #171.  If it is observed that the RLNAD degrades at a particular pH, 
the sponsor should provide data confirming this degradation.  That 
information should be provided to CVM as justification for excluding that 
specific condition from the media used in the bioequivalence assessment. 
 
  The buffers used in the media should be consistent with those documented 
in the USP Reagents and Reference Tables for Buffer Solutions. 
 
  At least one condition should achieve  85% dissolution of the RLNAD 
formulation, and all conditions should achieve at least 60% dissolution. 
 
  The use of surfactants should not exceed 2.0%.  Justification for surfactant 
selection should be provided.   
 
c.  Dissolution Curves 
  Dissolution methods should be discriminative.  If dissolution is such that 
CVM questions the dissolution conditions ability to detect inter-product 
 
4 The use of a single set of in vitro conditions is generally considered appropriate for confirmation of product 
bioequivalence within a species.  However, where there are multiple species involved there can be clinically relevant 
inter species differences that may impact the dissolution profile of the API and thereby affect the bioavailability of 
the API.  Therefore, the dissolution conditions used to evaluate bioequivalence in one species may not be 
appropriate or acceptable for the evaluation of bioequivalence in animals of different species. 
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differences, then CVM will request more information to demonstrate the 
discriminative nature of the selected condition. 
 
  The selection of dissolution timepoints should be such that it characterizes 
the beginning, middle, and terminal portions of the in vitro dissolution 
versus time profiles.  It is acceptable for some modification in timepoint 
selection across the dissolution conditions if there are substantial differences 
in profile shape between the separate dissolution conditions.  However, it is 
highly advised that if such modifications are needed, they be justified on the 
basis of the pilot study data generated with the RLNAD.  These results 
should be discussed with CVM prior to initiating the pivotal dissolution 
study.  
 
  Dissolution curves should show a gradual increase in percent dissolved over 
time (i.e., no less than 4 time points prior to reaching maximum dissolution). 
The first time point should be taken no later than 15 minutes after the 
initiation of testing.  Product dissolution should proceed gradually over 
time.  Samples should be taken to adequately define the entirety of the 
profile. 
 
d.  Criteria for Demonstrating Profile Comparability5 
Given that the dissolution comparisons will include both within and between lot 
variability, the recommended method for generic versus RLNAD profile 
comparisons is the tolerance limit approach.6  Alternatives to the use of this 
approach can be proposed by sponsors and will be considered on a case-by-case 
basis.  
 
e.  Data Acceptability 
Because these TAMAs contain drugs that do not meet the solubility criteria 
described in CVM GFI #171, the resulting product dissolution is expected to 
proceed slowly (i.e., a test duration of at least 1 hour prior to achieving 100% 
dissolved).  Conditions that result in profiles that rapidly achieve a maximum 
percent dissolved, that fail to adequately describe early, middle, and late phases of 
product in vitro behavior, or that fail to discriminate between batches that differ in 
the CQAs will affect CVMs final decision.  What constitutes an appropriate 
demonstration of the discriminatory nature of the test is product specific and 
 
5 In this context comparability means that there are no changes or differences that would alter the rate and extent at 
which the API interacts with its active site. 
6 Martinez MN and Zhao X. A Simple Approach for Comparing the In-vitro Dissolution Profiles of Highly Variable 
Drug Products: A Proposal. The AAPS Journal (2018) 20:78. 
 
 
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therefore should be discussed with CVM prior to proceeding with the pivotal in 
vitro dissolution study. 
IV.  Logistical Considerations 
To maximize process efficiency for demonstrating the bioequivalence of TAMAs containing 
poorly soluble APIs (Figure 1), the sponsor should consider the following: 
 
1.  Meetings should be arranged with CVM early in the product development process to 
obtain recommendations on preliminary studies, data to be collected, and the most 
expeditious way to submit pilot and pivotal data for CVM review. 
 
2.  Protocols for all studies should be submitted for CVM concurrence prior to initiating 
pivotal studies. 
 
3.  To be considered bioequivalent to an RLNAD TAMA, all three components (Q1, Q2, and 
Q3 equivalence; in vitro dissolution comparisons; and in vivo bioequivalence assessment) 
should be evaluated by CVM and found to meet the criteria for declaring the proposed 
generic and reference product bioequivalent.  
 
4.  It is recommended that pivotal in vivo studies be initiated after the Q1, Q2, and Q3, and 
dissolution studies are completed. 
V.  Post-approval Changes  
In addition to the typical chemistry, manufacturing, and controls (CMC) expectations for any 
post-approval change, the post-approval requirements for TAMAs with poorly soluble APIs may 
have additional considerations to address in order to demonstrate that bioequivalence has not 
been affected.  These include but are not limited to:  
 
A.  Change to API Source 
1.  For a change in API supplier for a TAMA containing a poorly soluble API, a Q3 
assessment of the drug product manufactured with the approved and proposed 
API source should be provided.  At a minimum, testing should include assay 
(including potency, as applicable) and impurities testing, and should comply with 
all compendial requirements.  In addition, a direct comparison of the particle size 
distributions of the API from the approved and proposed supplier(s) should be 
made.  If particle size analysis is performed using sieving, at least five individual 
sieves within the range should be used; if a particle size analyzer is used, then 
D10, D50, and D90 should be reported.  If the particle size distribution of the 
proposed API source changes relative to that of the approved API source, CVM 
may request additional data to support the approval if the nature of the change 
leads to an uncertainty as to whether it has the potential to impact in vivo product 
performance. 
 
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2.  For a new supplier of a biomass API, or a change in production microbial strain 
for an existing supplier, full characterization of the biomass API, including 
impurities, should be provided in addition to a comparison of particle size 
distributions of the proposed versus approved API. 
 
B.  Change in Manufacturing Process 
For a change in the manufacturing process, a Q3 assessment of the TAMA manufactured 
before and after the change should be provided.  If the process used to manufacture the 
TAMA changes such that it has the potential to impact in vivo product performance (as 
defined by changes identified in the Q3 assessment), CVM may request additional data to 
demonstrate that the change in manufacturing process does not impact the bioavailability of 
the API. 
VI.  Human Food Safety (HFS) Considerations 
The submission of acceptable data to satisfy the requirements of a bioequivalence technical 
section does not imply that the drug product has satisfied the HFS technical section requirements 
for an (A)NADA.  For drug product approval, all applicable legal requirements must be met, 
which includes addressing the tissue residue portion of the HFS technical section of the 
application.  Sponsors should contact ONADEs Division of Human Food Safety (HFV-150) to 
discuss what, if any, additional information may be needed to satisfy the HFS requirements for 
an approval. 
     
   
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Figure 1. Decision tree for the applicable use of GFI #279 
          
       
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