Biowaiver requests

#171
Demonstrating Bioequivalence for Soluble
Powder Oral Dosage Form Products and
Type A Medicated Articles Containing Active
Pharmaceutical Ingredients Considered to Be
Soluble in Aqueous Media

Guidance for Industry
This guidance document makes revisions to the guidance published in May 2021. This revision updates
the recommendations for using the dose adjusted approach for solubility determinations in poultry, as
well as updates the appropriate buffers for use in solubility determinations involving cattle.

Submit comments on this guidance at any time. 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 the docket number FDA-2019-D-3764.
For further information regarding this document, contact AskCVM@fda.hhs.gov.
Additional copies of this 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/animalveterinary, 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

Table of Contents
I.

Introduction .......................................................................................................................... 1

II.

Background .......................................................................................................................... 2

III. Human Food Safety Considerations .................................................................................. 3
IV. Biowaivers for Soluble Powder Oral Dosage Form Products.......................................... 3
A. Qualifying for a waiver from the requirements for performing an in vivo
bioequivalence study (biowaiver) .............................................................................. 3
B. Data to support a waiver from the requirement to perform an in vivo
bioequivalence study (biowaiver) request.................................................................... 3
1. Information about the API(s): ................................................................................ 4
2. Information about the formulation: ....................................................................... 4
V.

Biowaivers for Type A Medicated Articles Manufactured from Water-Soluble Active
Pharmaceutical Ingredients ................................................................................................ 4
A. Qualifying for a waiver from the requirements for conducting an in vivo
bioequivalence study (biowaiver) ................................................................................. 4
B. Data to support a waiver from the requirement to perform an in vivo
bioequivalence study (biowaiver) request.................................................................... 5
1. Information about the API(s): ...................................Error! Bookmark not defined.
2. Information about the formulation: ....................................................................... 6
3. Solubility Data:......................................................................................................... 6
C. Manufacturing process .................................................................................................. 6
D. CVM evaluation of solubility data................................................................................ 6
1. Calculation based on the Maximum Daily Dose: .................................................. 7
2. Calculations based on the Dosage Adjusted Approach:....................................... 7
E. Solubility determination ................................................................................................ 8
F. Technical parameters to consider include: .................................................................. 9
1. Use of the Shaker flask: ........................................................................................... 9
2. Media composition and pH considerations: .......................................................... 9
3. Other considerations: ............................................................................................ 10

VI. GLOSSARY........................................................................................................................ 10

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Demonstrating Bioequivalence for Soluble Powder Oral Dosage
Form Products and Type A Medicated Articles Containing Active
Pharmaceutical Ingredients Considered to Be Soluble in Aqueous
Media
Guidance for Industry
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 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 document describes how the Center for Veterinary Medicine (CVM) intends to evaluate
requests for waiving the requirement for performing in vivo bioequivalence studies (biowaivers)
for animal drugs administered orally as soluble powders or as Type A medicated articles
manufactured from active pharmaceutical ingredients (APIs) considered to be soluble in aqueous
media (water-soluble APIs). This document expands upon CVM’s Guidance for Industry (GFI)
#35, “Bioequivalence Guidance,” 1 to include biowaivers for soluble powder oral dosage form
products as well as Type A medicated articles manufactured from APIs considered to be soluble
in aqueous media, and it offers particular focus on criteria for the waiver of the requirements for
submitting in vivo bioequivalence study data. This guidance does not address Type A medicated
articles manufactured from APIs considered to be insoluble in aqueous media.
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 refer to ANADAs, the general principles described may also be applicable to new
animal drug applications (NADAs), supplemental NADAs, and investigational new animal drug
(INAD) files.
The recommendations in this guidance are premised on the assumption that a sponsor will be
bridging between identical dosage forms (e.g., their Type A medicated article for use in complete
feed to the approved reference Type A medicated article for use in complete feed). Therefore,
you should not use the recommendations in this guidance to compare the solubility of two drug
products where the API will be administered in differing manners (e.g., drinking water versus
complete feed; complete feed for administration throughout the day versus top dress). CVM
encourages sponsors to contact the Center to discuss that type of comparison.
The granting of a waiver from the requirement to perform an in vivo bioequivalence study does
not imply that a drug product is approvable. For drug product approval, all applicable legal
1

https://www.fda.gov/media/70115/download

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requirements must be met (see, e.g., sections 512(c)(2) and 512(n) of the Federal Food, Drug,
and Cosmetic Act (FD&C Act)).
In general, FDA’s guidance documents do not establish legally enforceable responsibilities.
Instead, guidances describe the Agency’s 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 guidance means that something is suggested or recommended, but
not required.
II.

Background

An ANADA must include information to show that the proposed generic new animal drug and
reference listed new animal drug (RLNAD) are bioequivalent. 2 This requirement is patterned
very closely on the approval requirements for human generic drugs. 3
The Center for Drug Evaluation and Research’s (CDER) regulations implementing the
bioequivalence requirement for human generic drugs can be found in 21 CFR part 320. In most
cases, there must be an in vivo demonstration of no significant differences in the rate and extent
of drug availability associated with the proposed generic and reference drug products when
administered at the same molar dose under similar conditions. 4 In certain circumstances,
however, the demonstration of bioequivalence does not need to be established on the basis of in
vivo studies. 5 For several categories of human drug products, including oral solutions,
bioequivalence is considered self-evident under specified conditions. 6 In other circumstances, a
large body of research on human intestinal physiology has been used to support a determination
of product bioequivalence of solid oral dosage forms based on the use of in vitro approaches. In
this regard, the human Biopharmaceutics Classification System (BCS) criteria have been applied
to support the use of an in vitro approach to document product bioequivalence for highly soluble,
rapidly dissolving, and orally administered drug products. 7 However, because of the
physiological differences between the gastrointestinal (GI) tract of humans and that of veterinary
species, and because of the additional complexities associated with drugs that are administered as
medicated feeds, the human BCS criteria cannot be applied to support the use of an in vitro
approach for demonstrating product bioequivalence for orally administered veterinary drug
products.
CVM has issued guidance on in vivo bioequivalence studies, which includes a list of some of the
product categories, including oral solutions and other solubilized forms, that may be eligible for
a waiver from the requirement to perform in vivo bioequivalence studies. 8 This guidance
2

Section 512(n)(1)(E) of the FD&C Act

3

Section 505(j)(2)(A)(iv) of the FD&C Act

4

21 CFR 320.1(e) and 320.21(b)

5

21 CFR 320.21(b) and (f), and 320.22

6

21 CFR 320.22(b)(3)

See CDER Guidance for Industry, “Waiver of In Vivo Bioavailability and Bioequivalence Studies for ImmediateRelease Solid Oral Dosage Forms Based on a Biopharmaceutics Classification System,” December 2017, pages 2
and 11-12. (https://www.fda.gov/media/70963/download)

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8

See footnote 2.

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Contains Nonbinding Recommendations
provides additional information and recommendations regarding bioequivalence waivers for
soluble powder oral dosage form products intended for use in animal drinking water, and Type A
medicated articles manufactured from APIs considered to be soluble in aqueous media and
intended for use in animal feed.
III. Human Food Safety Considerations
The granting of a waiver from the requirement to perform an in vivo bioequivalence study
(biowaiver) does not imply that a drug product is approvable. For drug product approval, all
applicable legal requirements must be met, which includes addressing the tissue residue portion
of the Human Food Safety (HFS) technical section of the application (sections
512(c)(2)(A)(viii), 512(c)(2)(B), and 512(n)(1)(A) of the FD&C Act). For products that receive
a biowaiver using the concepts of this guidance, the sponsor should contact the Division of
Human Food Safety (HFV-150) directly to discuss what, if any, additional information may be
needed to satisfy the requirements for approval.
IV. Biowaivers for Soluble Powder Oral Dosage Form Products
A. Qualifying for a waiver from the requirements for performing an in vivo
bioequivalence study (biowaiver)
CVM believes it is appropriate to grant biowaivers for oral dosage forms known as
“soluble powders” that meet the solubility requirements discussed in this guidance. Such
products are intended for administration to animals via the drinking water that is provided
on an ad libitum basis under most husbandry systems.
The conceptual basis for granting biowaivers for “soluble powders” is that if an API is in
solution before administration, the drug product’s formulation will usually not influence
the bioavailability of the active ingredient. If the test and reference formulations contain
essentially the same inactive ingredients, then from a mechanistic perspective, the ratelimiting step in systemic API absorption will be either: (a) the rate of gastric transit; or (b)
the permeability of the API across the gastrointestinal (GI) mucosal membranes.
Typically, both these variables are formulation independent, relying solely on the API and
the characteristics of the GI tract of that animal species. Similarly, because the ratelimiting step is the API movement down the GI tract and its lateral diffusion across the
viscous intestinal contents, if an API acts locally within the GI tract (i.e., not systemically
absorbed), the local exposure to the dissolved API in the proposed and reference drug
product formulations will be equivalent if the API is already in solution. The only
exceptions of which CVM is aware are when the formulation of the drug product contains
substances other than the API that could cause a direct pharmacologic effect (e.g., altered
GI transit time, membrane permeability, or drug metabolism) or when there is inactivation
of the API.
B. Data to support a waiver from the requirement to perform an in vivo
bioequivalence study (biowaiver) request
For soluble powder oral dosage form products, CVM recommends that justification of
requests for biowaivers be made by a demonstration of solubility. CVM recommends that
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sponsors make these biowaiver requests before submitting an ANADA. The following
should be provided in the biowaiver request:
1. Information about the API(s)
CVM recommends that the API(s) used to support a biowaiver request for soluble
powder oral dosage forms be provided from the same supplier of the API(s) that will be
used to formulate the proposed drug product during production. CVM recommends
that the applicant identify the source of the API(s) and provide a certificate of analysis
(COA) for the API(s). If the API has a United States Pharmacopeia (USP) monograph,
the API should at a minimum comply with the monograph specifications. The
acceptability of the API(s) will be determined upon review of the Chemistry,
Manufacturing, and Controls (CMC) Technical Section.
2. Information about the formulation
The formulation of the proposed drug product should be submitted. The formulation
should be such that there are no differences between the RLNAD and the proposed
drug product that would have an effect on the bioavailability of the API. There should
be no inactive ingredients in the proposed drug product’s formulation likely to cause
adverse pharmacologic effects. The API should be qualitatively (Q1) and
quantitatively (Q2) the same as the API used in the RLNAD. 9 The solubility of the
drug product should be determined under the range of physical conditions that a user of
the product would typically encounter when reconstituting the soluble powder with
animal drinking water (i.e., well or municipal water) in the field. This study is the same
as the reconstitution study outlined in GFI #5, “Drug Stability Guidelines,” section
IV.F. Soluble Powders and Drinking Water. 10 The reconstitution data for soluble
powder oral dosage form products should be submitted in the Chemistry,
Manufacturing, and Controls (CMC) Technical Section.
V.

Biowaivers for Type A Medicated Articles Manufactured from Water-Soluble Active
Pharmaceutical Ingredients
A. Qualifying for a waiver from the requirements for conducting an in vivo
bioequivalence study (biowaiver)
Type A medicated articles that contain APIs that are not classified as water soluble are not
the subject of this guidance. The determination of the water solubility of APIs is discussed
in section V.E. Solubility Determination.
With respect to eligibility for a biowaiver, CVM believes there is no reasonable basis for
drawing a distinction between APIs intended for administration to animals via drinking
water and APIs intended to be administered via feed, provided these APIs have similar
solubility characteristics. A water-soluble API present in a Type A medicated article and

Current Standards & Emerging Issues of Pharmaceutical Equivalence. AAPS/EUFEPS/FIP Workshop. Mei-Ling
Chen, Ph.D. November 13–14, 2010.

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https://www.fda.gov/media/69957/download

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Contains Nonbinding Recommendations
mixed into a feed matrix rapidly dissolves when exposed to the fluids of the GI tract. If
such an API readily goes into solution across the range of physiological pH values, it will
also readily go into solution when exposed to the fluids in the GI tract. Accordingly, such
medicated feeds will effectively behave as oral solutions immediately after consumption by
the animal. Therefore, CVM intends to review biowaiver requests for Type A medicated
articles containing water soluble APIs based on a demonstration of solubility of the API, as
well as an evaluation of the drug product formulation, to ensure that there are no
ingredients in the proposed formulation likely to cause pharmacologic or
pharmacodynamic differences from the RLNAD.
The criteria detailed in this guidance assume that the in vivo dissolution of the test and
reference products represents the only factor influencing the relative bioavailability of both
products. However, for compounds that are fully soluble and systemically absorbed, there
is the potential for certain excipients to alter drug absorption and or pre-systemic
metabolism (liver or intestinal), and hence affect the relative bioavailability of the products.
Therefore, for Type A medicated articles containing water-soluble APIs that are
systemically absorbed, the formulation of the proposed product should be such that there
are no differences in the formulations of the proposed drug product and the RLNAD that
would have an effect on the bioavailability of the API. Further, there should be no inactive
ingredients in the proposed drug product’s formulation likely to cause adverse
pharmacologic effects.
Type A medicated articles may contain biomass drug substances, which are APIs produced
through fermentation that are not subjected to extensive post-fermentation purification.
Biomass drug substances may contain the active molecule(s), microorganisms used for
production, other metabolites produced by the microorganisms used for production, and
media components. Biomass drug substances are only used as the drug substance in Type
A medicated article formulations. Because certain biomass drug substances are wellaccepted and routinely used components of Type A medicated articles, CVM will consider
the potential for the biomass drug substance component of a Type A medicated article to
cause adverse pharmacologic effects or effects on API bioavailability in the same manner
that it considers these effects with respect to other excipients.
B. Data to support a waiver from the requirement to perform an in vivo
bioequivalence study (biowaiver) request
For Type A medicated articles containing water-soluble APIs, CVM recommends that
requests for biowaivers be made by a demonstration of solubility. CVM recommends that
sponsors make these biowaiver requests before submitting an ANADA. The following
should be provided in the biowaiver request:
1. Information about the API(s)
CVM recommends that the API(s) used to support a biowaiver request be provided
from the same supplier of the API(s) that will be used to formulate the proposed drug
product during production. CVM recommends that the applicant identify the source of
the API(s), indicate if the API(s) is a biomass drug substance (if applicable), and
provide a COA for the API(s). If the API has a USP monograph, the API should at a
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minimum comply with the monograph specifications. The acceptability of the API(s)
will be determined upon review of the Chemistry, Manufacturing, and Controls (CMC)
Technical Section.
2. Information about the formulation
The formulation of the proposed drug product should be submitted. The formulation
should be such that there are no differences between the RLNAD and the proposed
drug product that would have an effect on the bioavailability of the API. There should
be no inactive ingredients in the proposed drug product’s formulation likely to cause
adverse pharmacologic effects. The API should be qualitatively (Q1) and
quantitatively (Q2) the same as the API used in the RLNAD. Sponsors should address
the inactive ingredients in their formulation and any potential impact they may have on
the bioavailability of the API(s) or the likelihood to cause adverse pharmacologic
effects. If supporting data or information regarding any potential impact of the inactive
ingredients on the bioavailability of the API(s) or their likelihood to cause adverse
pharmacologic effects is available, it should be submitted.
3. Solubility Data
Solubility data including method validations, and chromatograms as appropriate, should
be submitted with each biowaiver request for Type A medicated articles. The solubility
of the API should be determined in aqueous media. The inherent ability of the API to
dissolve in aqueous media is critical to using the solubility data in support of any
request for granting a biowaiver. Solubility data should represent a pH range that is
inclusive of all the pH values expected in the gastric/ruminal environment for all the
major species on the RLNAD label.
CVM recommends that the Shaker flask method be used for solubility determinations,
employing an appropriate dissolution time and temperature, relevant to the labeled
species. CVM-recommended times and temperatures for major species are documented
in Table 1 of section V.E. Solubility Determination.
C. Manufacturing process
If the process used to manufacture the Type A medicated article has the potential to alter
the solubility of the API, thereby potentially impacting bioequivalence of the generic
product to the RLNAD, then CVM may request additional data regarding solubility of the
API in the final formulation.
D. CVM evaluation of solubility data
Sponsors should submit solubility data, analytical method validations, and additional
documentation such as chromatograms in support of their biowaiver request. CVM will
evaluate the acceptability of the solubility data for the purposes of a biowaiver based on the
criteria as documented below. If a sponsor wishes to propose an alternative to meeting the
bioequivalence requirements, the sponsor may request a meeting with CVM to discuss their
approach prior to requesting a biowaiver. In general, CVM will evaluate data submitted
with a biowaiver request using the following criteria:
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1. Calculation based on the Maximum Daily Dose
In this scenario, the entire daily dose is administered in a single feeding event.
Therefore, a demonstration that the maximum administered dose, as indicated on the
RLNAD label, is soluble in the minimum fluid volume, in the specified time, for the
indicated species (Table 1) will suffice as evidence for granting a biowaiver.
2. Calculations based on the Dosage Adjusted Approach
In this scenario, the dose is administered as multiple feeding events through the day.
The number of feeding events by which the dose is divided is species dependent (Table
1). Therefore, the aqueous solubility is to be evaluated based on the highest expected
mg of API per mL of gastric fluid at any point in time. This assessment is determined
based on the API concentration in the feed and characteristics of the gastric physiology
of the target animal species.
a. API concentration
The API concentration should be calculated by using the amount of API in the Type
A medicated article likely to be consumed per feeding event, e.g., dividing the daily
dose into the number of feeding events that the target animal species typically takes
to consume their daily ration (Table 1). This scenario is typically applicable to
situations where the animals do not consume their rations on a continual basis
through the day. An exception to this approach is seen in poultry, where
differences in feeding practices can influence the amounts of Type A Medicated
article consumed per bird over time. Therefore, in poultry, a demonstration of
product bioequivalence using the dosage adjusted approach will be evaluated on a
case-by-case basis to determine the appropriate quantity of the API to use in the
associated solubility studies. Accordingly, for Type A medicated articles intended
for administration to poultry, sponsors should contact CVM to discuss the
conditions most suitable for demonstrating product bioequivalence.
b. Gastric physiology
The animal physiology is critical as it determines the gastric residence time (how
long it takes for the consumed medicated feed to exit the stomach or rumen), the
gastric fluid volume of the target animal species (Table 1), the pH range over which
solubility measurements must be made, and the temperature at which solubility
should be determined.
The solubility of the API, when tested based on the API concentration, is defined by
the highest expected daily dose (mg of API) that will go into solution when tested in
a manner consistent with the most conservative conditions associated with the
approved product label (e.g., the largest dose to fluid volume ratio, minimum time
of dissolution, and the broadest pH range over which solubility is demonstrated).
When this level of solubility of the API is demonstrated, CVM will consider the
API to be eligible for a biowaiver.
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If there are multiple species on the labeling, solubility should be based on the most
conservative mg/mL scenario (i.e., use the species that produces the highest dose to
gastric fluid volume ratio).
This method of defining API solubility is similar to that described for categorizing
compounds when using the BCS and to the BCS-based approach described in
CDER guidance. 11 In this case, the appropriate fluid volume for testing API
solubility depends on the target animal species/class for which the medicated feed is
intended. For example, a conservative estimate of ruminal fluid volume (fluid
volume available for drug solubilization) for steers is 47 L. The sponsor should
provide the estimated daily drug intake (mg/kg body weight) based on the labeled
drug concentration (grams of drug per ton) in the feed administered to the animal
(e.g., the Type C medicated feed) and the highest amount of medicated feed
(kg/day) expected to be consumed by an individual animal. When using this
approach, CVM recommends using the species-specific animal data estimates
summarized in Table 1.
E. Solubility determination
For the purposes of granting a biowaiver under this guidance, CVM is interested in the
physiologically relevant solubility of the API in aqueous solutions. As such, the addition
of compounds, such as surfactants used to increase the solubility of the API in aqueous
media, is not considered acceptable. If the use of surfactants is required to achieve
acceptable solubility data, then the API is not considered to be fully soluble in water, and
therefore is no longer covered under the scope of this guidance. If the use of a buffer
results in any effect that may increase the solubility of the API or drug product, that waiver
request will also be denied.
The preferred method for determination of solubility is by use of the Shaker flask method 12
containing an aqueous media for the indicated time and temperature relevant to the labeled
species (Table 1). Solubility should be confirmed using a validated assay procedure that
can distinguish the API from its degradation products. The media used for testing should
represent a pH range that is inclusive of all the pH values expected in the gastric
environment for all the major species on the RLNAD label and as described in section
V.F.2. Media composition and pH considerations below. When the conditions within the
GI tract are markedly different across labeled species, appropriate media composition for
testing API solubility should be discussed with CVM.

11

See CDER Guidance for Industry, “Waiver of In Vivo Bioavailability and Bioequivalence Studies for Immediate-Release
Solid Oral Dosage Forms Based on a Biopharmaceutics Classification System,” December 2017.

12

USP 1236

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Table 1. Standard Conditions for Use in Determining Experimental Conditions
Species *

Gastric Fluid
Volume in Liters

Gastric resident
times (hours)

Temperature
(⁰C)

Feeding events per
day

Cattle 1, 3
Swine 3
Horse 2, 3
Chicken 3
Turkey 4

47 #
0.5
1.5
0.01 ‡
0.04 ‡

8
1
0.25
2
2

36.7 – 39.3
38.7 – 39.8
37.2 – 38.2
40.6 – 43.0
40.6 – 41.5

2
2
2
Variable 5
Variable 5

* CVM acknowledges that the estimates for the indicated species are very conservative. If alternate data points are
used, that use must be adequately justified. CVM notes that in certain instances the appropriate variable may be less
than that indicated in the table, as is the case when dealing with younger animals.
# Fluid volume of the rumen.
‡ Includes the fluid volumes of both the proventriculus and the ventriculus.
1 Martinez, M. N., Apley, M. D. Drug solubility classification in the bovine. J. vet. Pharmacol. Therap. 35 (Suppl. 1),
93–97, 2012.
2 The Equine Hospital Manual first edition, ed. Kevin Corley and Jennifer Stephen, Chapter 5, page 282. Wiley
Blackwell, 2008.
3 The Merck Veterinary Manual: http://www.merckvetmanual.com/appendixes/reference-guides/normal-rectaltemperature-ranges (accessed June 8, 2023).
4 Some Factors Affecting Body Temperature of Turkeys: Poultry Science, Volume 34, Issue 2, 1 March 1955, Pages
369–371, https://doi.org/10.3382/ps.0340369 (accessed June 8, 2023.)
5 Feeding events per day will be determined on a case-by-case basis due to the variability in feeding practices for
poultry.

F. Technical parameters to consider include:
1. Use of the Shaker flask
•

The temperature of the solution within the flask should be within the
physiological range of the healthy animal for the major indicated species on the
label. Temperatures should be maintained within ± 0.5°C of the indicated
temperature throughout the study to ensure solubility is not affected by variation
in temperature.

•

After the appropriate period of shaking (based on Gastric residence time as
described in Table 1), prior to sampling, there should be an excess of API
apparent on the bottom of the flask.

•

The supernatant (liquid above the solid) should be filtered to remove
undissolved particles not apparent to the naked eye.

•

Samples for analysis should be taken from the filtered supernatant. The
supernatant solution may need to be diluted before analysis to be within the
linear range of the analytical method and to avoid possible precipitation.

2. Media composition and pH considerations
•

For monogastric animals: pH 1.2, 4.6 (acetate buffer), and 7.5 phosphate
buffer.

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•

For ruminants: For the purposes of generating data for a biowaiver request, the
media should be buffered at a pH of 4.5 (acetate buffer) and 7.5 (phosphate
buffer).

•

For poultry: The sponsor should justify the sets of conditions under which
solubility will be tested. Without agreed upon justification, the default
conditions would be the same conditions as those described for monogastric
animals.

•

Solution pH should be verified after addition of the API to the buffer solution.
If the pH changes significantly after addition of the API, we would conclude
that the buffers selected are inadequate to control fluctuations in pH.

•

The pH of the solution should be verified to be consistent throughout the
assessment of the API solubility. At a minimum the pH should be verified at
the time the sample is taken for analysis.

Note that the buffers used may need to be modified if there are concerns that the buffer
itself may alter the inherent solubility of the API or drug product (e.g., common ion
effect). However, the inclusion of organic compound-containing buffer systems is not
considered to be acceptable.
3. Other considerations
•

CVM recommends a minimum of three replicate determinations of solubility
under each pH condition. Depending on study variability, additional replication
may be necessary to provide a reliable estimate of solubility.

•

A visual determination of solubility alone is insufficient. For the determination
of solubility, the concentration of the API in the selected media must be
determined using a validated quantitative assay method. 13

VI. GLOSSARY
For purposes of this guidance document, the following definitions apply:
Active Pharmaceutical Ingredient (API): 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.
Bioavailability: The rate and extent to which the active ingredient or active moiety is absorbed
from a drug product and becomes available at the site of action. For drug products that are not
intended to be absorbed into the bloodstream, bioavailability may be assessed by measurements
intended to reflect the rate and extent to which the active ingredient or active moiety becomes
available at the site of action.
13

21 CFR 514.1(b)(5)(vii)(a)

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Bioequivalence: The absence of a significant difference in the rate and extent to which the
active ingredient or active moiety in pharmaceutical equivalents or pharmaceutical alternatives
becomes available at the site of drug action when administered at the same molar dose under
similar conditions in an appropriately designed study.
Biomass Drug Substance: An active ingredient, which is an unpurified fermentation product
derived from the cultivation of microorganisms, that is intended to furnish pharmacological
activity or other direct effects in the diagnosis, cure, mitigation, treatment, or prevention of
disease or to affect the structure or any function of the animal.
Biowaiver: A waiver from the requirement to perform an in vivo bioequivalence study (21 USC
360b(n)(1)(E)).
Qualitative (Q1), Quantitative (Q2) and Structural Similarity (Q3): Q1 means qualitative
similarity between generic and reference listed products, while Q2 represents quantitative
similarity of composition. Q3 describes structural similarity and refers to the arrangement of
matter and state of aggregation of the product.

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