Guidance for Industry 
 
Interpreting Sameness of 
Monoclonal Antibody Products 
Under the Orphan Drug 
Regulations  
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
U.S. Department of Health and Human Services 
Food and Drug Administration 
Center for Drug Evaluation and Research (CDER) 
Office of Orphan Products Development (OOPD) 
 
April 2014 
CMC 
   
Guidance for Industry 
Interpreting Sameness of 
Monoclonal Antibody Products 
Under the Orphan Drug 
Regulations 
 
 
Additional copies are available from: 
Office of Communications 
Division of Drug Information, WO51, Room 2201 
Center for Drug Evaluation and Research 
Food and Drug Administration 
10903 New Hampshire Ave., Silver Spring, MD 20993  
Phone: 301-796-3400; Fax: 301-847-8714 
druginfo@fda.hhs.gov  
http://www.fda.gov/Drugs/GuidanceComplianceRegulatoryInformation/Guidances/default.htm 
http://www.fda.gov/AnimalVeterinary/GuidanceComplianceEnforcement/GuidanceforIndustry/default.htm 
 
or 
 
Office of Orphan Products Development 
Office of Special Medical Programs 
Food and Drug Administration 
10903 New Hampshire Ave, Silver Spring, MD 20993 
Phone: 301-796-8660; Fax:301-847-8621 
orphan@fda.hhs.gov 
http://www.fda.gov/ForIndustry/DevelopingProductsforRareDiseasesConditions/default.htm 
 
 
 
 
 
 
 
U.S. Department of Health and Human Services 
Food and Drug Administration 
Center for Drug Evaluation and Research (CDER)  
Office of Orphan Products Development (OOPD) 
 
April 2014 
CMC
 
  Contains Nonbinding Recommendations 
 
TABLE OF CONTENTS 
 
 
 
I.  INTRODUCTION............................................................................................................. 1 
II.  BACKGROUND ............................................................................................................... 1 
III.  SCOPE ............................................................................................................................... 3 
IV.   INTERPRETING SAMENESS OF MONOCLONAL ANTIBODY PRODUCTS .... 3 
A.   Structural Features of Antibodies ................................................................................................ 3 
B.   Sameness of Unmodified Monoclonal Antibody Products ......................................................... 4 
C.   Sameness of Antibody Conjugates, Fusion Proteins, and Bispecific and Multispecific 
Antibodies ....................................................................................................................................... 4 
V.   CHANGES IN ANTIBODY STRUCTURE THAT DO NOT NECESSARILY 
CONSTITUTE DIFFERENCES BETWEEN TWO MONOCLONAL ANTIBODY 
PRODUCTS WITH THE SAME COMPLEMENTARITY DETERMINING 
REGIONS .......................................................................................................................... 5 
A.   Framework Region ........................................................................................................................ 5 
B.  Constant Region ............................................................................................................................. 5 
C.  Antibody Fragments ...................................................................................................................... 6 
i 
  Contains Nonbinding Recommendations 
 
1
Guidance for Industry  
Interpreting Sameness of Monoclonal Antibody Products Under the 
Orphan Drug Regulations  
 
 
This guidance represents the Food and Drug Administrations (FDAs) current thinking on this topic.  It 
does not create or confer any rights for or on any person and does not operate to bind FDA or the public.  
You can use an alternative approach if the approach satisfies the requirements of the applicable statutes 
and regulations.  If you want to discuss an alternative approach, contact the FDA staff responsible for 
implementing this guidance.  If you cannot identify the appropriate FDA staff, call the appropriate 
number listed on the title page of this guidance.  
 
 
 
I.  INTRODUCTION  
 
The purpose of this guidance is to describe the Agencys current thinking on the criteria by 
which two monoclonal antibody products would be considered the same under the Orphan Drug 
Act and its implementing regulations.   
 
FDAs guidance documents, including this guidance, 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 
 
The regulations implementing the Orphan Drug Act are codified in 21 CFR Part 316.  The 
Agency published the Proposed Rule for these regulations on January 29, 1991,2 and the Final 
Rule on December 29, 1992.3  More recently, the Agency finalized certain amendments to Part 
316 in order to clarify regulatory provisions and make minor improvements to address issues that 
have arisen since 1992.4   
 
One of the incentives for orphan drug development is the exclusive approval of a product for a 
period of 7 years.  During this 7-year period, no approval will be given to a subsequent sponsors 
marketing application for the same drug for the same indication unless the sponsor demonstrates 
that the subsequent product is clinically superior and therefore does not contain the same drug.5   
 
                                                 
1 This guidance has been prepared by the Office of Pharmaceutical Science in the Center for Drug Evaluation and 
Research (CDER) and FDAs Office of Orphan Product Development (OOPD).  
2 FDA, Orphan Drug Regulations Proposed Rule, 56 FR 3338 (Jan. 29, 1991). 
3 FDA, Orphan Drug Regulations Final Rule, 57 FR 62076 (Dec. 12, 1992). 
4 FDA, Orphan Drug Regulations Final Rule, 78 FR 35117 (June 12, 2013). 
5 See 21 CFR 316.3 (b)(3) & (b)(12). 
  1  Contains Nonbinding Recommendations 
 
In determining whether or not two drugs would be considered to be the same under the orphan 
drug regulations, we recognized that different criteria were necessary for macromolecules versus 
small molecules.6  Macromolecules include a variety of molecules, such as proteins, nucleic 
acids, carbohydrates, as well as closely related, complex, partly definable drugs such as live viral 
vaccines.    
 
A drug containing macromolecules is the same as another, previously approved drug if it 
contains the same principal molecular structural features (but not necessarily all of the same 
structural features) and is intended for the same use as a previously approved drug . . . .7  Two 
protein drugs would be considered the same if the only differences in structure between them 
were due to posttranslational events or infidelity of translation or transcription or were minor 
differences in amino acid sequence . . . .8  For monoclonal antibody drugs, these definitions lay 
the groundwork for the determination of sameness, but, because of the unique considerations 
applicable to antibody molecules, additional guidance as to how two monoclonal antibody drugs 
would be considered to be the same drug under the Orphan Drug regulations is needed. 
 
An antibody molecule is composed of four polypeptide chains:  two identical heavy (H) chains 
and two identical light (L) chains.  Both heavy and light chains are divided into variable (V) and 
constant (C) regions.  The V -V  pairs confer specificity for antigen, while the constant region 
H L
of the heavy chain is responsible for effector functions such as complement fixation and antibody 
dependent cellular cytotoxicity.  The variable and constant regions were so named because 
amino acid sequence data showed that the amino terminal regions of heavy and light chains from 
different antibodies had different sequences (the variable region), while the carboxy terminal 
region amino acid sequences were the same within a given isotype (class or subclass) (the 
constant region).  Subsequent analysis of variable region amino acid sequences defined three 
hypervariable regions (also known as complementarity determining regions (CDRs)) each in the 
V  and V regions, which form the antigen binding site of the molecule.9 
H L 
 
Antibody diversity is created by the use of multiple germline genes encoding variable regions 
and a variety of somatic events.  The somatic events include recombination of variable gene 
segments with diversity (D) and joining (J) gene segments to make a complete V  region and the 
H
recombination of variable and joining gene segments to make a complete V  region.  The 
L
recombination process itself is imprecise, resulting in the loss or addition of amino acids at the 
V(D)J junctions.  These mechanisms of diversity occur in the developing B cell prior to antigen 
exposure.  After antigenic stimulation, the expressed antibody genes in B cells undergo somatic 
mutation.  Based on the estimated number of germline gene segments, the random recombination 
of these segments, and random VH-VL pairing, up to 1.6 x 107 different antibodies could be 
produced.10   When other processes that contribute to antibody diversity (such as somatic 
mutation) are taken into account, it is thought that upwards of 1 x 1010 different antibodies could 
                                                 
6 See 21 CFR 316.3(b)(14). 
7 21 CFR Part 316.3(b)(14)(ii). 
8 21 CFR Part 316.3(b)(14)(ii)(A). 
9 Kabat, E.A., 1978, The Structural Basis of Antibody Complementarity, Advances in Protein Chemistry, 3, pp. 1-
75. 
10 Max, E.E., 1993, Immunoglobulins:  Molecular Genetics, In Fundamental Immunology, Third Edition (Ed. 
W.E. Paul), pp. 315-382. 
  2  Contains Nonbinding Recommendations 
 
be generated.11  Because of the many processes involved in generating antibody diversity, it is 
unlikely that independently derived monoclonal antibodies with the same antigen specificity will 
have identical amino acid sequences. 
 
III.  SCOPE 
 
For the purpose of this guidance, a monoclonal antibody is a clonal product defined as any intact 
antibody, antibody fragment, conjugate, fusion protein, bispecific, or multi-specific antibody that 
contains a V  -V pair, single V domain, or combinations of single V domains where the CDRs 
H L 
form the antigen binding site.  Antibody fragments or fusion proteins containing only constant 
region domains are not within the purview of this guidance. 
 
The mechanisms generating antibody diversity are the same for all antibodies whether they are 
immortalized as monoclonal antibodies or purified from serum as polyclonal antibodies.  The 
recommendations described in this guidance, however, apply only to monoclonal antibody 
products. 
 
Diversity of the T-cell receptor also is generated by multiple T-cell receptor specific germline 
genes and somatic events similar to those described for antibodies.  The T-cell receptor is 
membrane bound in its native functional form.  We anticipate the development of soluble T-cell-
receptor-products for therapeutic use.  The considerations underlying the interpretation of 
sameness of monoclonal antibody products in this guidance should apply to soluble T-cell-
receptor-products. 
 
The Biologics Price Competition and Innovation Act of 2009 (BPCI Act) amended the Public 
Health Service (PHS) Act and other statutes to create an abbreviated licensure pathway in section 
351(k) of the PHS Act for biological products shown to be biosimilar to, or interchangeable with, 
an FDA-licensed biological reference product.12  The interpretation of sameness of monoclonal 
antibody products in this guidance is intended only for the purposes of determining sameness 
under the Orphan Drug Act, and the Agency does not intend to apply the considerations 
discussed in this guidance to determinations under the BPCI Act. 
 
IV.   INTERPRETING SAMENESS OF MONOCLONAL ANTIBODY PRODUCTS 
 
  A.   Structural Features of Antibodies 
 
As described in Section II, antibodies have two functional regions:  the variable region, which is 
responsible for antigen-specific binding, and the constant region, which carries out effector 
functions.  The variable region is divided into complementarity-determining regions (CDR1, 
CDR2, and CDR3) and framework regions (FR1, FR2, FR3, and FR4).  Using the Kabat system, 
CDRs 1, 2, and 3 are delineated by amino acid positions 31-35, 50-65, and 95-102, respectively, 
for heavy chains and amino acid positions 24-34, 50-56, and 89-97, respectively, for light 
                                                 
11 Burrows, P.D., Schroeder, H.W., and Cooper, M.D., 1995, B-Cell Differentiation in Humans, Immunoglobulin 
Genes, Second Edition (Ed. T. Honjo and F.W. Alt), pp. 3-32.  
12 See sections 7001 through 7003 of the Patient Protection and Affordable Care Act (Public Law 111-148). 
  3  Contains Nonbinding Recommendations 
 
chains.13  Alternatively, the International Immunogenetics (IMGT) information system defines 
the positions of both heavy and light chain CDRs 1, 2, and 3 as amino acid positions 27  38, 56 
 65, and 105  117, respectively.14  While these amino acid positions define each CDRs 
boundaries, the CDR lengths can vary.15  CDRs create the antigen-binding pocket of the 
molecule through the interaction between heavy and light chain variable regions, while the 
framework regions provide the scaffolding on which the antigen binding pocket sits.  The 
constant region is responsible for antibody effector functions, but has little influence on antibody 
specificity or affinity. 
 
  B.   Sameness of Unmodified Monoclonal Antibody Products 
 
The sameness of a macromolecule for purposes of the Orphan Drug Act and its implementing 
regulations is based on its principal molecular structures.16  For the purpose of determining 
sameness of unmodified monoclonal antibodies under the Orphan Drug Act and its implementing 
regulations, the Agency will consider the CDRs of the heavy and light chain variable regions to 
be the principal molecular structural features of a monoclonal antibody product.  The residues 
comprising the CDRs will be those defined by either the Kabat or IMGT systems as stated in 
Section IV.A. above.17  
 
FDA intends to interpret the applicable regulatory provisions such that two monoclonal antibody 
drugs would be considered to be the same drug if the CDRs amino acid sequences were the 
same or if there were only minor amino acid differences between them.  Other potentially 
important amino acid differences outside the CDRs, or differences due to glycosylation patterns 
or posttranslational modifications, would not necessarily cause the products to be considered to 
be different. 
 
FDA intends to make determinations of this nature on a case-by-case basis.  The types of 
information that would be useful in making such a determination include the sequence of the 
heavy and light chain variable regions of the product, any modifications in antibody sequence, 
and whether any particular residues have been established to be important for antigen binding. 
 
  C.   Sameness of Antibody Conjugates, Fusion Proteins, and Bispecific and Multispecific 
Antibodies 
 
Monoclonal antibody products can be conjugated by chemical methods with radionuclides, drugs, 
macromolecules, or other agents or can be made as fusion proteins.  A monoclonal antibody 
fusion protein contains a V -V  pair, where one of these chains (usually V or C ) and another 
H L H   H
protein are synthesized as a single amino-acid chain.  These types of products differ from 
unmodified monoclonal antibodies in that they generally have an important additional functional 
                                                 
13 National Institutes of Health, U.S. Department of Health and Human Services, 1991, Sequences of Proteins of 
Immunological Interest, Kabat, E.A., Wu, T.T., Perry, H.M., Gottesman, K.S., and Foeller, C. (Eds.). 
14 Lefranc, M.P., Pommie, C., Ruiz, M., Giudicelli, V., Foulquier, E., Truong, L., Thouvenin-Cntet, V., and Lefranc, 
G. 2003.  IMGT unique numbering for immunoglobulin and T cell receptor variable domains and Ig superfamily V-
like domains.  Developmental and Comparative Immunology, 27, pp 55-77. 
15 See footnotes 13 and 14.16 21 CFR 316.3(b)(14). 
16 21 CFR 316.3(b)(14). 
17 See footnotes 13 and 14. 
  4  Contains Nonbinding Recommendations 
 
element:  the active moiety of a small molecule or the principal molecular structural feature(s) of 
the conjugated or fused macromolecule. 
 
FDA intends to interpret the applicable regulatory provisions such that the determination of 
sameness of such monoclonal antibodies will be based on a determination of sameness of the 
monoclonal antibody element and sameness of the functional element of the conjugated 
molecule.18  A difference in any one of these elements can result in a determination that the 
molecules are different.  Conversely, two monoclonal antibody conjugates or fusion proteins 
would be determined to be the same drug if both the CDR sequences of the antibody and the 
functional element of the conjugated molecule were the same. 
 
Many different platforms produce bispecific antibodies.  In general, bispecific antibodies are 
generated by combining a heavy-light chain pair from a monoclonal antibody of one specificity 
with a heavy-light chain pair from a monoclonal antibody of a different specificity and, therefore, 
have two different sets of CDRs.  In some cases, a bispecific antibody can be composed of two 
single V domain antibodies with different specificities.  A multispecific antibody generally 
contains multiple single V domain antibodies or V -V  pairs, each with unique specificities.  Two 
H L
bispecific or multispecific antibodies will be considered to be the same drug if all sets of CDRs 
are the same or had only minor amino acid differences between them. 
 
V.   CHANGES IN ANTIBODY STRUCTURE THAT DO NOT NECESSARILY 
CONSTITUTE DIFFERENCES BETWEEN TWO MONOCLONAL ANTIBODY 
PRODUCTS WITH THE SAME COMPLEMENTARITY DETERMINING 
REGIONS  
 
Listed below are certain potential changes that sponsors may make in areas outside the CDRs in 
monoclonal antibody products.  FDA intends to consider such changes as not necessarily causing 
two monoclonal antibody products with the same CDRs to be different drugs for the purpose of 
determining sameness of monoclonal antibodies under the Orphan Drug Act and its 
implementing regulations.   
 
  A.   Framework Region 
 
Framework region changes include, humanizing a nonhuman-derived monoclonal antibody or 
engineering certain framework residues that are important for antigen contact or stabilizing the 
binding site.   
 
  B.  Constant Region 
 
Constant region differences include changing the class or subclass of the constant region, 
changing specific amino acid residues that might alter an effector function such as Fc receptor 
binding, or changing the species from which the constant region is derived. 
 
                                                 
18 See 21 CFR 316.3(b)(13)(i) regarding small molecules and 21 CFR 316.3(b)(13)(ii) regarding macromolecules. 
  5  Contains Nonbinding Recommendations 
 
  C.  Antibody Fragments  
 
Intact monoclonal antibodies and antibody fragments with the same CDR sequences or with only 
minor amino acid differences between them, will not be considered to be different drugs.  This is 
consistent with the Agencys policy regarding peptides and whole proteins as explained in the 
Orphan Drug Regulations Final Rule, which states that ...in order for a peptide that resembles a 
portion of a protein product to be considered a different drug, FDA will require a clear 
demonstration that the peptide is clinically superior to the entire protein.19 
                                                 
19 Final Rule, supra note 3, at 62078. 
  6