Patent Publication Number: US-2023138852-A1

Title: Systems, materials, and methods for reversed-phase high performance liquid chromatography (rp-hplc) identification of multi-specific molecules

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of U.S. Provisional Application Ser. Nos. 63/010,907, 63/010,912, 63/010,920, and 63/010,926 filed 16 Apr. 2020. The entire content of the aforementioned applications is incorporated herein by reference in its entirety. 
    
    
     FIELD OF THE INVENTION 
     This invention relates to a high-pressure liquid chromatography (HPLC) method for detecting multi-specific molecule formation after a multi-specific molecule manufacturing or development process. The HPLC method provides fast, quantitative, real-time processing of multi-specific molecule formation. 
     BACKGROUND OF THE INVENTION 
     During the manufacturing or process research and development (R&amp;D) of a multi-specific molecule (e.g., a multi-specific antibody), the potential for incomplete formation of the multi-specific molecule (e.g., a multi-specific antibody) remains regardless of the manufacturing process. Non-reduced capillary sodium dodecyl sulfate (NR cSDS) has previously been employed to monitor incomplete multi-specific antibody formation; however, the technique is not practical for delivering fast, quantitative, real-time results that are needed when studying multi-specific formation in a process manufacturing environment or during multi-specific molecule (e.g., multi-specific antibody) development. Thus, a fast, quantitative, real-time process monitoring multi-specific molecule (e.g., multi-specific antibody) formation is needed. 
     BRIEF SUMMARY OF THE INVENTION 
     Provided herein are methods of detecting formation of a multi-specific molecule. The methods comprise (a) obtaining a multi-specific molecule sample; (b) obtaining a reversed-phase high performance liquid chromatography (RP-HPLC) column; (c) contacting the multi-specific molecule sample in a polar aqueous mobile phase A with the RP-HPLC column, wherein the polar aqueous mobile phase comprises an ion-pairing agent; (d) contacting an organic non-polar mobile phase B with the RP-HPLC column, wherein the organic non-polar phase comprises an ion-pairing agent; (e) eluting the multi-specific molecule sample; and (f) monitoring the amount of multi-specific molecule formation in the eluted multi-specific molecule sample. In certain embodiments, the multi-specific molecule is a multi-specific antibody. 
     Also provided herein are methods of detecting formation of a multi-specific antibody while performing a manufacturing process to produce a multi-specific antibody. The methods comprise (a) performing a manufacturing process to produce a multi-specific antibody, wherein the manufacturing process results in a multi-specific antibody sample comprising an amount of a multi-specific antibody; (b) setting up a reversed-phase high performance liquid chromatography (RP-HPLC) column; (c) injecting the multi-specific antibody sample in a polar aqueous mobile phase A in the RP-HPLC column, wherein the polar aqueous mobile phase comprises an ion-pairing agent; (d) applying an organic non-polar mobile phase B to the RP-HPLC column to form a gradient elution, wherein the organic non-polar mobile phase comprises an ion-pairing agent; (e) eluting the multi-specific antibody sample; and (f) monitoring the amount of multi-specific antibody formation in the eluted sample. 
     Also provided herein are methods of detecting formation of a bispecific antibody while performing a controlled FAB arm exchange (cFAE) to produce a bispecific antibody. The methods comprise (a) performing a controlled FAB arm exchange (cFAE) to produce a cFAE sample, wherein the cFAE sample comprises an amount of the bispecific antibody; (b) setting up a reversed-phase high performance liquid chromatography (RP-HPLC) column; (c) injecting the cFAE sample in a polar aqueous mobile phase A in the RP-HPLC column, wherein the polar aqueous mobile phase comprises an ion-pairing agent; (d) applying an organic non-polar mobile phase B to the RP-HPLC column to form a gradient elution, wherein the organic non-polar mobile phase comprises an ion-pairing agent; (e) eluting the cFAE sample; and (f) monitoring the amount of the bispecific antibody formation in the eluted sample. 
     Also provided are methods of designing a stable multi-specific antibody. The methods comprise (a) performing a manufacturing process to produce a multi-specific antibody, wherein the manufacturing process results in a multi-specific antibody sample comprising an amount of a multi-specific antibody; (b) setting up a reversed-phase high performance liquid chromatography (RP-HPLC) column; (c) injecting the multi-specific antibody sample in a polar aqueous mobile phase A in the RP-HPLC column, wherein the polar aqueous mobile phase comprises an ion-pairing agent; (d) applying an organic non-polar mobile phase B to the RP-HPLC column to form a gradient elution, wherein the organic non-polar mobile phase comprises an ion-pairing agent; (e) eluting the multi-specific antibody sample; and (f) monitoring the amount of multi-specific antibody formation in the eluted sample, wherein an increased amount of multi-specific antibody formation as compared to a control indicates the design of a stable multi-specific antibody. 
     Also provided are methods of obtaining a stability profile of a multi-specific antibody during a manufacturing process to produce a multi-specific antibody. The methods comprise (a) performing a manufacturing process to produce a multi-specific antibody, wherein an amount of a multi-specific antibody sample is obtained at different timepoints during the manufacturing process; (b) setting up a reversed-phase high performance liquid chromatography (RP-HPLC) column; (c) injecting the multi-specific antibody sample from each timepoint in a polar aqueous mobile phase A in the RP-HPLC column, wherein the polar aqueous mobile phase comprises an ion-pairing agent; (d) applying an organic non-polar mobile phase B to the RP-HPLC column to form a gradient elution, wherein the organic non-polar mobile phase comprises an ion-pairing agent; (e) eluting the multi-specific antibody sample; and (f) monitoring the amount of multi-specific antibody formation in the eluted sample, wherein the amount of multi-specific antibody formation at each time point provides a stability profile of the multi-specific antibody. 
     Also provided are methods of obtaining a degraded fragment species profile of a multi-specific antibody during a manufacturing process to produce a multi-specific antibody. The methods comprise (a) performing a manufacturing process to produce a multi-specific antibody, wherein an amount of a multi-specific antibody sample is obtained at different timepoints during the manufacturing process; (b) setting up a reversed-phase high performance liquid chromatography (RP-HPLC) column; (c) injecting the multi-specific antibody sample from each timepoint in a polar aqueous mobile phase A in the RP-HPLC column, wherein the polar aqueous mobile phase comprises an ion-pairing agent; (d) applying an organic non-polar mobile phase B to the RP-HPLC column to form a gradient elution, wherein the organic non-polar mobile phase comprises an ion-pairing agent; (e) eluting the multi-specific antibody sample; and (f) monitoring the amount of degraded fragment species in the eluted sample, wherein the amount of degraded fragment species at each time point provides a degraded fragment species profile of the multi-specific antibody manufacturing process. 
     Also provided are systems comprising a reversed-phase high performance liquid chromatography (RP-HPLC) member for detecting formation of a multi-specific molecule. The systems comprise (a) a multi-specific molecule sample in a polar aqueous mobile phase A, wherein the polar aqueous mobile phase comprises an ion pairing agent; (b) a RP-HPLC column; and (c) an organic non-polar mobile phase B, wherein the organic non-polar mobile phase comprises an ion pairing agent. The RP-HPLC column is configured to be contacted with the multi-specific molecule sample in polar aqueous mobile phase A, and the RP-HPLC is further configured to be contacted with the organic non-polar mobile phase B to elute a multi-specific molecule sample. The amount of multi-specific molecule formation can be identified in the eluted multi-specific molecule sample. 
     Also provided are means for reversed-phase high performance liquid chromatography (RP-HPLC) detection of multi-specific molecule formation comprising (a) a multi-specific molecule sample in a polar aqueous mobile phase A, wherein the polar aqueous mobile phase comprises an ion pairing agent; (b) a RP-HPLC column; and (c) an organic non-polar mobile phase B, wherein the organic non-polar mobile phase comprises an ion-pairing agent. The RP-HPLC column is configured to be contacted with the multi-specific molecule sample in polar aqueous mobile phase A, and the RP-HPLC column is further configured to be contacted with the organic non-polar mobile phase B to elute a multi-specific molecule sample. The amount of multi-specific molecule formation can be identified in the eluted multi-specific molecule sample. 
     In certain embodiments, the polar aqueous mobile phase A is a solution comprising about 1% to about 5% isopropanol. The solution can, for example, comprise about 2% isopropanol. 
     In certain embodiments, the ion-pairing agent in polar aqueous mobile phase A is selected from the group consisting of trifluoroacetic acid (TFA), difluoroacetic acid (DFA), and formic acid (FA). In certain embodiments, the ion-pairing agent is TFA. The TFA can, for example, be present in the polar aqueous mobile phase A at about 0.1% to about 2%. In certain embodiments, the TFA is present in the polar aqueous mobile phase A at about 0.1%. 
     In certain embodiments, the organic non-polar mobile phase is a solution comprising about 60% to about 80% isopropanol and about 15% to about 25% acetonitrile. The solution can, for example, comprise about 70% isopropanol. The solution can, for example, comprise about 20% acetonitrile. 
     In certain embodiments, the ion-pairing agent in organic non-polar mobile phase B is selected from the group consisting of trifluoroacetic acid (TFA), difluoroacetic acid (DFA), and formic acid (FA). In certain embodiments, the ion-pairing agent is TFA. The TFA can, for example, be present in the organic non-polar mobile phase B at about 0.1% to about 2%. In certain embodiments, the TFA is present in the organic non-polar mobile phase B at about 0.1%. 
     In certain embodiments, the gradient elution comprises a solution comprising about 75% of polar mobile phase A solution and about 25% of organic non-polar mobile phase B solution at the moment the multi-specific (e.g., bispecific) antibody sample is injected in the RP-HPLC column. The gradient elution can, for example, comprise a solution comprising about 66% of polar mobile phase A solution and about 34% of organic non-polar mobile phase B solution at about 16 minutes after the multi-specific antibody sample is injected in the RP-HPLC column. The gradient elution can, for example, comprise a solution comprising about 5% of polar mobile phase A solution and about 95% of organic non-polar mobile phase B solution at about 20 minutes after the multi-specific antibody sample is injected in the RP-HPLC column. The gradient elution can, for example, comprise a solution comprising about 75% of polar mobile phase A solution and about 25% of organic non-polar mobile phase B solution at about 21 minutes after the multi-specific antibody sample is injected in the RP-HPLC column. 
     In certain embodiments, the manufacturing process to produce a multi-specific antibody is selected from the group consisting of a knob in hole process, a strand exchange engineered domain process, a chemical linked bispecific antibody (BsAb) process, an immunoglobulin domain crossover process, and a controlled Fab arm exchange (cFAE) and dual variable domain process. 
     In certain embodiments, the multi-specific antibody is a bispecific antibody. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The foregoing summary, as well as the following detailed description of preferred embodiments of the present application, will be better understood when read in conjunction with the appended drawings. It should be understood, however, that the application is not limited to the precise embodiments shown in the drawings. 
         FIG.  1    shows a schematic of the reversed-phase high performance liquid chromatography (RP-HPLC) single stack configuration. 
         FIG.  2    shows a graph demonstrating a representative blank chromatogram. 
         FIG.  3    shows a graph demonstrating a representative chromatogram of reference material (fully oxidized) for anti-GPRCSD/anti-CD3 bispecific antibody. 
         FIG.  4    shows a graph demonstrating a representative chromatogram of partially oxidized anti-GPRCSD/anti-CD3 bispecific antibody. 
         FIG.  5    shows a graph demonstrating a representative chromatograms of reference material (fully oxidized) for anti-CD33/anti-CD3 bispecific antibody. 
         FIG.  6    shows a graph demonstrating a representative chromatogram of partially oxidized anti-CD33/anti-CD3 bispecific antibody. 
         FIG.  7    shows a graph demonstrating a representative chromatogram of reference material (fully oxidized) for anti-TMEFF2/anti-CD3 bispecific antibody. 
         FIG.  8    shows a graph demonstrating a representative chromatogram of partially oxidized anti-TMEFF2/anti-CD3 bispecific antibody. 
         FIG.  9    shows a graph demonstrating a representative chromatogram of reference material (fully oxidized) for anti-CD123/anti-CD3 bispecific antibody. 
         FIG.  10    shows a graph demonstrating a representative chromatogram of partially oxidized anti-CD123/anti-CD3 bispecific antibody. 
         FIG.  11    shows a graph demonstrating representative chromatograms of reference material (fully oxidized) for anti-BCMA/anti-CD3 bispecific antibody. 
         FIG.  12    shows a graph demonstrating a representative chromatogram of partially oxidized anti-BCMA/anti-CD3 bispecific antibody. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Various publications, articles and patents are cited or described in the background and throughout the specification; each of these references is herein incorporated by reference in its entirety. Discussion of documents, acts, materials, devices, articles or the like which has been included in the present specification is for the purpose of providing context for the invention. Such discussion is not an admission that any or all of these matters form part of the prior art with respect to any inventions disclosed or claimed. 
     Unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood to one of ordinary skill in the art to which this invention pertains. Otherwise, certain terms used herein have the meanings as set forth in the specification. 
     It must be noted that as used herein and in the appended claims, the singular forms “a,” “an,” and “the” include plural reference unless the context clearly dictates otherwise. 
     Unless otherwise stated, any numerical values, such as a concentration or a concentration range described herein, are to be understood as being modified in all instances by the term “about.” Thus, a numerical value typically includes ±10% of the recited value. For example, a concentration of 1 mg/mL includes 0.9 mg/mL to 1.1 mg/mL. Likewise, a concentration range of 1% to 10% (w/v) includes 0.9% (w/v) to 11% (w/v). As used herein, the use of a numerical range expressly includes all possible subranges, all individual numerical values within that range, including integers within such ranges and fractions of the values unless the context clearly indicates otherwise. 
     Unless otherwise indicated, the term “at least” preceding a series of elements is to be understood to refer to every element in the series. Those skilled in the art will recognize or be able to ascertain using no more than routine experimentation, many equivalents to the specific embodiments of the invention described herein. Such equivalents are intended to be encompassed by the invention. 
     As used herein, the terms “comprises,” “comprising,” “includes,” “including,” “has,” “having,” “contains” or “containing,” or any other variation thereof, will be understood to imply the inclusion of a stated integer or group of integers but not the exclusion of any other integer or group of integers and are intended to be non-exclusive or open-ended. For example, a composition, a mixture, a process, a method, an article, or an apparatus that comprises a list of elements is not necessarily limited to only those elements but can include other elements not expressly listed or inherent to such composition, mixture, process, method, article, or apparatus. Further, unless expressly stated to the contrary, “or” refers to an inclusive or and not to an exclusive or. For example, a condition A or B is satisfied by any one of the following: A is true (or present) and B is false (or not present), A is false (or not present) and B is true (or present), and both A and B are true (or present). 
     As used herein, the conjunctive term “and/or” between multiple recited elements is understood as encompassing both individual and combined options. For instance, where two elements are conjoined by “and/or,” a first option refers to the applicability of the first element without the second. A second option refers to the applicability of the second element without the first. A third option refers to the applicability of the first and second elements together. Any one of these options is understood to fall within the meaning, and therefore satisfy the requirement of the term “and/or” as used herein. Concurrent applicability of more than one of the options is also understood to fall within the meaning, and therefore satisfy the requirement of the term “and/or.” 
     As used herein, the term “consists of,” or variations such as “consist of” or “consisting of,” as used throughout the specification and claims, indicate the inclusion of any recited integer or group of integers, but that no additional integer or group of integers can be added to the specified method, structure, or composition. 
     As used herein, the term “consists essentially of,” or variations such as “consist essentially of” or “consisting essentially of,” as used throughout the specification and claims, indicate the inclusion of any recited integer or group of integers, and the optional inclusion of any recited integer or group of integers that do not materially change the basic or novel properties of the specified method, structure or composition. See M.P.E.P. § 2111.03. 
     It should also be understood that the terms “about,” “approximately,” “generally,” “substantially,” and like terms, used herein when referring to a dimension or characteristic of a component of the preferred invention, indicate that the described dimension/characteristic is not a strict boundary or parameter and does not exclude minor variations therefrom that are functionally the same or similar, as would be understood by one having ordinary skill in the art. At a minimum, such references that include a numerical parameter would include variations that, using mathematical and industrial principles accepted in the art (e.g., rounding, measurement or other systematic errors, manufacturing tolerances, etc.), would not vary the least significant digit. 
     As used herein, the term “polynucleotide,” synonymously referred to as “nucleic acid molecule,” “nucleotides” or “nucleic acids,” refers to any polyribonucleotide or polydeoxyribonucleotide, which can be unmodified RNA or DNA or modified RNA or DNA. “Polynucleotides” include, without limitation single- and double-stranded DNA, DNA that is a mixture of single- and double-stranded regions, single- and double-stranded RNA, and RNA that is mixture of single- and double-stranded regions, hybrid molecules comprising DNA and RNA that can be single-stranded or, more typically, double-stranded or a mixture of single- and double-stranded regions. In addition, “polynucleotide” refers to triple-stranded regions comprising RNA or DNA or both RNA and DNA. The term polynucleotide also includes DNAs or RNAs containing one or more modified bases and DNAs or RNAs with backbones modified for stability or for other reasons. “Modified” bases include, for example, tritylated bases and unusual bases such as inosine. A variety of modifications can be made to DNA and RNA; thus, “polynucleotide” embraces chemically, enzymatically or metabolically modified forms of polynucleotides as typically found in nature, as well as the chemical forms of DNA and RNA characteristic of viruses and cells. “Polynucleotide” also embraces relatively short nucleic acid chains, often referred to as oligonucleotides. 
     As used herein, the term “vector” is a replicon in which another nucleic acid segment can be operably inserted so as to bring about the replication or expression of the segment. 
     As used herein, the term “host cell” refers to a cell comprising a nucleic acid molecule of the invention. The “host cell” can be any type of cell, e.g., a primary cell, a cell in culture, or a cell from a cell line. In one embodiment, a “host cell” is a cell transfected with a nucleic acid molecule of the invention. In another embodiment, a “host cell” is a progeny or potential progeny of such a transfected cell. A progeny of a cell may or may not be identical to the parent cell, e.g., due to mutations or environmental influences that can occur in succeeding generations or integration of the nucleic acid molecule into the host cell genome. 
     The term “expression” as used herein, refers to the biosynthesis of a gene product. The term encompasses the transcription of a gene into RNA. The term also encompasses translation of RNA into one or more polypeptides, and further encompasses all naturally occurring post-transcriptional and post-translational modifications. The expressed bispecific antibody can be within the cytoplasm of a host cell, into the extracellular milieu such as the growth medium of a cell culture or anchored to the cell membrane. 
     As used herein, the terms “peptide,” “polypeptide,” or “protein” can refer to a molecule comprised of amino acids and can be recognized as a protein by those of skill in the art. The conventional one-letter or three-letter code for amino acid residues is used herein. The terms “peptide,” “polypeptide,” and “protein” can be used interchangeably herein to refer to polymers of amino acids of any length. The polymer can be linear or branched, it can comprise modified amino acids, and it can be interrupted by non-amino acids. The terms also encompass an amino acid polymer that has been modified naturally or by intervention; for example, disulfide bond formation, glycosylation, lipidation, acetylation, phosphorylation, or any other manipulation or modification, such as conjugation with a labeling component. Also included within the definition are, for example, polypeptides containing one or more analogs of an amino acid (including, for example, unnatural amino acids, etc.), as well as other modifications known in the art. 
     The terms “detecting,” “detect,” or variants thereof, as used herein in the broadest sense include both qualitative and quantitative measurements of a target molecule (e.g., a multi-specific antibody). Detecting includes identifying the mere presence of the target molecule in a sample, as well as determining whether the target molecule is present in the sample at detectable levels. 
     The term “sample,” as used herein, refers to a small portion of a larger quantity of material. Generally, testing according to the methods described herein is performed on a sample. The sample is typically obtained from a multi-specific molecule preparation (e.g., a multi-specific antibody preparation) obtained, for example, from a manufacturing process to produce a multi-specific molecule, as described below. 
     The terms “inject” or “injecting,” as used herein, can refer to loading of sample in an HPLC column in an aqueous phase. The sample can be equilibrated with an equilibration buffer prior to injection and/or loading of the composition to be detected and/or purified. 
     The terms “elute,” “eluting,” and/or “elution,” as used herein can refer to the removal of a product (e.g., a multi-specific antibody) from a chromatography material. Elution buffer is the buffer used to elute the multi-specific antibody from a chromatography material. 
     Systems, Materials, and Methods for Detecting Multi-Specific Molecule Formation 
     Provided herein are methods of detecting formation of a multi-specific molecule. The methods comprise (a) obtaining a multi-specific molecule sample; (b) obtaining a reversed-phase high performance liquid chromatography (RP-HPLC) column; (c) contacting the multi-specific molecule sample in a polar aqueous mobile phase A with the RP-HPLC column, wherein the polar aqueous mobile phase comprises an ion-pairing agent; (d) contacting an organic non-polar mobile phase B with the RP-HPLC column, wherein the organic non-polar phase comprises an ion-pairing agent; (e) eluting the multi-specific molecule sample; and (f) identifying the amount of multi-specific molecule formation in the eluted multi-specific molecule sample. In certain embodiments, the multi-specific molecule is a multi-specific antibody. In certain embodiments, the multi-specific antibody is a bispecific antibody. 
     Provided herein are methods of detecting formation of a multi-specific antibody while performing a manufacturing process to produce a multi-specific antibody. The methods comprise (a) performing a manufacturing process to produce a multi-specific antibody, wherein the manufacturing process results in a multi-specific antibody sample comprising an amount of a multi-specific antibody; (b) setting up a reversed-phase high performance liquid chromatography (RP-HPLC) column; (c) injecting the multi-specific antibody sample in a polar aqueous mobile phase A in the RP-HPLC column, wherein the polar aqueous mobile phase comprises an ion-pairing agent; (d) applying an organic non-polar mobile phase B to the RP-HPLC column to form a gradient elution, wherein the organic non-polar mobile phase comprises an ion-pairing agent; (e) eluting the multi-specific antibody sample; and (f) monitoring the amount of multi-specific antibody formation in the eluted sample. 
     Also provided herein are methods of detecting formation of a bispecific antibody during a controlled FAB arm exchange (cFAE) to produce a bispecific antibody. The methods comprise (a) performing a controlled FAB arm exchange (cFAE) to produce a cFAE sample, wherein the cFAE sample comprises an amount of a bispecific antibody; (b) setting up a reversed-phase high performance liquid chromatography (RP-HPLC) column; (c) injecting the cFAE sample in a polar aqueous mobile phase A in the RP-HPLC column, wherein the polar aqueous mobile phase comprises an ion-pairing agent; (d) applying an organic non-polar mobile phase B to the RP-HPLC column to form a gradient elution, wherein the organic non-polar mobile phase comprises an ion-pairing agent; (e) eluting the cFAE sample; and (f) monitoring the amount of the bispecific antibody formation in the eluted sample. 
     In certain embodiments, the amount of the multi-specific antibody formed can be determined by preparing and testing a reference sample. The amount of partially formed multi-specific antibody can be determined by comparing to the reference sample (fully formed). The reference sample can comprise a sample with 80-100% of fully formed multi-specific antibody. Comparing the process manufacturing samples to the reference samples can provide a determination of the efficiency and effectiveness of the manufacturing process to produce the multi-specific antibody by being able to determine both the amount of the fully formed multi-specific antibody and its partially formed antibody fragments. 
     Also provided are methods of utilizing this assay in process development kinetic studies to develop a multi-specific antibody. The methods comprise (a) performing a research and development process to produce a multi-specific antibody, wherein the research and development process results in a multi-specific antibody sample comprising an amount of a multi-specific antibody; (b) setting up a reversed-phase high performance liquid chromatography (RP-HPLC) column; (c) injecting the multi-specific antibody sample in a polar aqueous mobile phase A in the RP-HPLC column, wherein the polar aqueous mobile phase comprises an ion-pairing agent; (d) applying an organic non-polar mobile phase B to the RP-HPLC column to form a gradient elution, wherein the organic non-polar mobile phase comprises an ion-pairing agent; (e) eluting the multi-specific antibody sample; and (f) monitoring the amount of multi-specific antibody formation in the eluted sample, wherein an increased amount of multi-specific antibody formation as compared to a reference sample indicates the successful design of a multi-specific antibody. 
     Also provided are methods of obtaining a stability profile of a multi-specific antibody during a manufacturing process to produce a multi-specific antibody. The methods comprise (a) performing a manufacturing process to produce a multi-specific antibody, wherein an amount of a multi-specific antibody sample is obtained at different timepoints during the manufacturing process; (b) setting up a reversed-phase high performance liquid chromatography (RP-HPLC) column; (c) injecting the multi-specific antibody sample from each timepoint in a polar aqueous mobile phase A in the RP-HPLC column, wherein the polar aqueous mobile phase comprises an ion-pairing agent; (d) applying an organic non-polar mobile phase B to the RP-HPLC column to form a gradient elution, wherein the organic non-polar mobile phase comprises an ion-pairing agent; (e) eluting the multi-specific antibody sample; and (f) monitoring the amount of multi-specific antibody formation in the eluted sample, wherein the amount of multi-specific antibody formation at each time point provides a stability profile of the multi-specific antibody. 
     Comparing the process manufacturing samples to the reference samples can provide a determination of the efficiency and effectiveness of the manufacturing process to produce the multi-specific antibody by being able to determine both the amount of the fully formed multi-specific antibody and its partially formed antibody fragments. An increase in the amount of fully formed multi-specific antibody as compared to the reference samples can indicate that the design of the multi-specific antibody is favorable to produce a multi-specific antibody that does not comprise unformed and/or partially formed multi-specific antibody fragments. 
     Also provided are systems comprising a reversed-phase high performance liquid chromatography (RP-HPLC) member for detecting formation of a multi-specific molecule. The systems comprise (a) a multi-specific molecule sample in a polar aqueous mobile phase A, wherein the polar aqueous mobile phase comprises an ion pairing agent; (b) a RP-HPLC column; and (c) an organic non-polar mobile phase B, wherein the organic non-polar mobile phase comprises an ion pairing agent. The RP-HPLC column is configured to be contacted with the multi-specific molecule sample in polar aqueous mobile phase A, and the RP-HPLC is further configured to be contacted with the organic non-polar mobile phase B to elute a multi-specific molecule sample. The amount of multi-specific molecule formation can be identified in the eluted multi-specific molecule sample. 
     Also provided are means for reversed-phase high performance liquid chromatography (RP-HPLC) detection of multi-specific molecule formation comprising (a) a multi-specific molecule sample in a polar aqueous mobile phase A, wherein the polar aqueous mobile phase comprises an ion pairing agent; (b) a RP-HPLC column; and (c) an organic non-polar mobile phase B, wherein the organic non-polar mobile phase comprises an ion-pairing agent. The RP-HPLC column is configured to be contacted with the multi-specific molecule sample in polar aqueous mobile phase A, and the RP-HPLC column is further configured to be contacted with the organic non-polar mobile phase B to elute a multi-specific molecule sample. The amount of multi-specific molecule formation can be identified in the eluted multi-specific molecule sample. 
     As disclosed herein, reversed-phase high pressure liquid chromatography (RP-HPLC) is a chromatography technique using a hydrophobic stationary phase (e.g., a stationary phase comprising alkyl chains covalently bonded to the stationary phase particles). The use of the hydrophobic stationary phase is the reverse of normal phase chromatography, as the polarity of the mobile and stationary phases are inverted. RP-HPLC employs a polar aqueous mobile phase. As a result, hydrophobic molecules in the polar mobile phase tend to adsorb to the hydrophobic stationary phase, and hydrophilic molecules in the mobile phase will pass through the column and are eluted first. Hydrophobic molecules can be eluted from the column by decreasing the polarity of the mobile phase using an organic (non-polar) solvent, which reduces the hydrophobic interactions. The more hydrophobic the molecule, the more strongly it binds the stationary phase, and the higher the concentration of the organic solvent that is required to elute the molecule. 
     In certain embodiments, the polar aqueous mobile phase is a solution comprising about 1% to about 5% isopropanol. The solution can, for example, comprise isopropanol at about 1%, about 2%, about 3%, about 4%, about 5%, or any value in between. In preferred embodiments, the polar aqueous mobile phase comprises about 2% isopropanol. 
     In certain embodiments, the polar aqueous mobile phase is a solution comprising at least one component selected from the group consisting of acetonitrile, propanol, and tetrahydrofuran (THF). The solution can, for example, comprise acetonitrile, propanol, and/or tetrahydrofuran (THF) at about 1%, about 2%, about 3%, about 4%, about 5%, or any value in between. In preferred embodiments, the polar aqueous mobile phase comprises about 2% acetonitrile, propanol, and/or tetrahydrofuran (THF). 
     In certain embodiments, the ion-pairing agent in mobile phase A is selected from the group consisting of trifluoroacetic acid (TFA), difluoroacetic acid (DFA), and formic acid (FA). 
     In certain embodiments, the ion-pairing agent is TFA. The TFA can, for example, be present in the polar aqueous mobile phase at about 0.1% to about 2%. The TFA can, for example, be present in the polar aqueous mobile phase at about 0.1%, about 0.2%, about 0.3%, about 0.4%, about 0.5%, about 0.6%, about 0.7%, about 0.8%, about 0.9%, about 1.0%, about 1.1%, about 1.2%, about 1.3%, about 1.4%, about 1.5%, about 1.6%, about 1.7%, about 1.8%, about 1.9%, about 2.0% or any value in between. In preferred embodiments, the TFA is present in the polar aqueous mobile phase at about 0.1%. 
     In certain embodiments, the ion-pairing agent is DFA. The DFA can, for example, be present in the polar aqueous mobile phase at about 0.1% to about 2%. The DFA can, for example, be present in the polar aqueous mobile phase at about 0.1%, about 0.2%, about 0.3%, about 0.4%, about 0.5%, about 0.6%, about 0.7%, about 0.8%, about 0.9%, about 1.0%, about 1.1%, about 1.2%, about 1.3%, about 1.4%, about 1.5%, about 1.6%, about 1.7%, about 1.8%, about 1.9%, about 2.0% or any value in between. In preferred embodiments, the DFA is present in the polar aqueous mobile phase at about 0.1%. 
     In certain embodiments, the ion-pairing agent is FA. The FA can, for example, be present in the polar aqueous mobile phase at about 0.5% to about 5%. The FA can, for example, be present in the polar aqueous mobile phase at about 0.5%, about 0.6%, about 0.7%, about 0.8%, about 0.9%, about 1.0%, about 1.1%, about 1.2%, about 1.3%, about 1.4%, about 1.5%, about 1.6%, about 1.7%, about 1.8%, about 1.9%, about 2.0%, about 2.1%, about 2.2%, about 2.3%, about 2.4%, about 2.5%, about 2.6%, about 2.7%, about 2.8%, about 2.9%, about 3.0%, about 3.1%, about 3.2%, about 3.3%, about 3.4%, about 3.5%, about 3.6%, about 3.7%, about 3.8%, about 3.9%, about 4.0%, about 4.1%, about 4.2%, about 4.3%, about 4.4%, about 4.5%, about 4.6%, about 4.7%, about 4.8%, about 4.9%, about 5.0%, or any value in between. 
     In certain embodiments, the organic non-polar mobile phase is a solution comprising about 60% to about 100% isopropanol and about 0% to about 25% acetonitrile. The organic non-polar mobile phase solution can, for example, comprise isopropanol at about 60%, about 65%, about 70%, about 75%, about 80%, about 90%, about 100%, or any value in between. In preferred embodiments, the organic non-polar mobile phase solution comprises about 70% isopropanol. The organic non-polar mobile phase solution can, for example, comprise acetonitrile at about 0%, about 5%, about 10%, about 15%, about 16%, about 17%, about 18%, about 19%, about 20%, about 21%, about 22%, about 23%, about 24%, about 25%, or any value in between. In preferred embodiments, the organic non-polar mobile phase comprises about 20% acetonitrile. 
     In certain embodiments, the organic non-polar mobile phase is a solution comprising at least one component selected from the group consisting of acetonitrile, isopropanol, and propanol (e.g., normal propanol (n-propanol)). The solution can, for example, comprise acetonitrile, isopropanol, and/or propanol at about 0%, about 10%, about 20%, about 30%, about 40%, about 50%, about 60%, about 70%, about 80%, about 90%, about 100%, or any value in between. In certain embodiments, the organic non-polar mobile phase comprises about 70% isopropanol, about 20% acetonitrile, and about 10% water; about 70% n-propanol, about 20% acetonitrile, and about 10% water; about 70% acetonitrile, about 20% isopropanol, and about 10% water; about 80% isopropanol and about 20% acetonitrile; about 80% acetonitrile and about 20% isopropanol; about 75% isopropanol and about 25% acetonitrile; about 25% isopropanol and about 75% acetonitrile; and about 50% isopropanol and about 50% acetonitrile. 
     In certain embodiments, the ion-pairing agent in mobile phase B is selected from the group consisting of trifluoroacetic acid (TFA), difluoroacetic acid (DFA), and formic acid (FA). 
     In certain embodiments, the ion-pairing agent is TFA. The TFA can, for example, be present in the polar aqueous mobile phase at about 0.1% to about 2%. The TFA can, for example, be present in the polar aqueous mobile phase at about 0.1%, about 0.2%, about 0.3%, about 0.4%, about 0.5%, about 0.6%, about 0.7%, about 0.8%, about 0.9%, about 1.0%, about 1.1%, about 1.2%, about 1.3%, about 1.4%, about 1.5%, about 1.6%, about 1.7%, about 1.8%, about 1.9%, about 2.0% or any value in between. In preferred embodiments, the TFA is present in the polar aqueous mobile phase at about 0.1%. 
     In certain embodiments, the ion-pairing agent is DFA. The DFA can, for example, be present in the polar aqueous mobile phase at about 0.1% to about 2%. The DFA can, for example, be present in the polar aqueous mobile phase at about 0.1%, about 0.2%, about 0.3%, about 0.4%, about 0.5%, about 0.6%, about 0.7%, about 0.8%, about 0.9%, about 1.0%, about 1.1%, about 1.2%, about 1.3%, about 1.4%, about 1.5%, about 1.6%, about 1.7%, about 1.8%, about 1.9%, about 2.0% or any value in between. In preferred embodiments, the DFA is present in the polar aqueous mobile phase at about 0.1%. 
     In certain embodiments, the ion-pairing agent is FA. The FA can, for example, be present in the polar aqueous mobile phase at about 1.0% to about 5%. The FA can, for example, be present in the polar aqueous mobile phase at about 1.0%, about 1.1%, about 1.2%, about 1.3%, about 1.4%, about 1.5%, about 1.6%, about 1.7%, about 1.8%, about 1.9%, about 2.0%, about 2.1%, about 2.2%, about 2.3%, about 2.4%, about 2.5%, about 2.6%, about 2.7%, about 2.8%, about 2.9%, about 3.0%, about 3.1%, about 3.2%, about 3.3%, about 3.4%, about 3.5%, about 3.6%, about 3.7%, about 3.8%, about 3.9%, about 4.0%, about 4.1%, about 4.2%, about 4.3%, about 4.4%, about 4.5%, about 4.6%, about 4.7%, about 4.8%, about 4.9%, about 5.0%, or any value in between. 
     In certain embodiments, the gradient elution comprises a solution comprising about 75% of polar mobile phase A solution and about 25% of organic non-polar mobile phase B solution at the moment the multi-specific antibody sample is injected in the RP-HPLC column. The gradient elution can, for example, comprise a solution comprising about 66% of polar mobile phase A solution and about 34% of organic non-polar mobile phase B solution at about 16 minutes after the multi-specific antibody sample is injected in the RP-HPLC column. The gradient elution can, for example, comprise a solution comprising about 5% of polar mobile phase A solution and about 95% of organic non-polar mobile phase B solution at about 20 minutes after the multi-specific antibody sample is injected in the RP-HPLC column. The gradient elution can, for example, comprise a solution comprising about 75% of polar mobile phase A solution and about 25% of organic non-polar mobile phase B solution at about 21 minutes after the multi-specific antibody sample is injected in the RP-HPLC column. 
     Antibodies 
     Provided herein are methods of detecting multi-specific antibody formation while performing a manufacturing process to produce a multi-specific antibody, methods of designing a stable multi-specific antibody, and methods of obtaining a stability profile or obtaining a degraded fragment species profile during a manufacturing process to produce a multi-specific antibody. In certain embodiments, the multi-specific antibody is a bispecific antibody. 
     As used herein, the term “antibody” is used in a broad sense and includes immunoglobulin or antibody molecules including human, humanized, composite and chimeric antibodies and antibody fragments that are monoclonal or polyclonal. In general, antibodies are proteins or peptide chains that exhibit binding specificity to a specific antigen. Antibody structures are well known. Immunoglobulins can be assigned to five major classes (i.e., IgA, IgD, IgE, IgG and IgM), depending on the heavy chain constant domain amino acid sequence. IgA and IgG are further sub-classified as the isotypes IgA1, IgA2, IgG1, IgG2, IgG3 and IgG4. Accordingly, the antibodies of the invention can be of any of the five major classes or corresponding sub-classes. Preferably, the antibodies of the invention are IgG1, IgG2, IgG3 or IgG4. Antibody light chains of vertebrate species can be assigned to one of two clearly distinct types, namely kappa and lambda, based on the amino acid sequences of their constant domains. Accordingly, the antibodies of the invention can contain a kappa or lambda light chain constant domain. According to particular embodiments, the antibodies of the invention include heavy and/or light chain constant regions from rat or human antibodies. In addition to the heavy and light constant domains, antibodies contain an antigen-binding region that is made up of a light chain variable region and a heavy chain variable region, each of which contains three domains (i.e., complementarity determining regions 1-3; CDR1, CDR2, and CDR3). The light chain variable region domains are alternatively referred to as LCDR1, LCDR2, and LCDR3, and the heavy chain variable region domains are alternatively referred to as HCDR1, HCDR2, and HCDR3. 
     As used herein, the term an “isolated antibody” refers to an antibody which is substantially free of other antibodies having different antigenic specificities (e.g., an isolated multi-specific antibody that specifically bind two or more antigens is substantially free of multi-specific antibodies that do not bind the two or more antigens). In addition, an isolated antibody is substantially free of other cellular material and/or chemicals. 
     As used herein, the term “monoclonal antibody” refers to an antibody obtained from a population of substantially homogeneous antibodies, i.e., the individual antibodies comprising the population are identical except for possible naturally occurring mutations that may be present in minor amounts. The monoclonal antibodies of the invention can be made by the hybridoma method, phage display technology, single lymphocyte gene cloning technology, or by recombinant DNA methods. For example, the monoclonal antibodies can be produced by a hybridoma which includes a B cell obtained from a transgenic nonhuman animal, such as a transgenic mouse or rat, having a genome comprising a human heavy chain transgene and a light chain transgene. 
     As used herein, the term “antigen-binding fragment” refers to an antibody fragment such as, for example, a diabody, a Fab, a Fab′, a F(ab′)2, an Fv fragment, a disulfide stabilized Fv fragment (dsFv), a (dsFv) 2 , a bispecific dsFv (dsFv-dsFv′), a disulfide stabilized diabody (ds diabody), a single-chain antibody molecule (scFv), a single domain antibody (sdab) an scFv dimer (bivalent diabody), a multi-specific antibody formed from a portion of an antibody comprising one or more CDRs, a camelized single domain antibody, a nanobody, a domain antibody, a bivalent domain antibody, or any other antibody fragment that binds to an antigen but does not comprise a complete antibody structure. An antigen-binding fragment is capable of binding to the same antigen to which the parent antibody or a parent antibody fragment binds. According to particular embodiments, the antigen-binding fragment comprises a light chain variable region, a light chain constant region, and an Fd segment of the heavy chain. According to other particular embodiments, the antigen-binding fragment comprises Fab and F(ab′). 
     As used herein, the term “single-chain antibody” refers to a conventional single-chain antibody in the field, which comprises a heavy chain variable region and a light chain variable region connected by a short peptide of about 15 to about 20 amino acids. As used herein, the term “single domain antibody” refers to a conventional single domain antibody in the field, which comprises a heavy chain variable region and a heavy chain constant region or which comprises only a heavy chain variable region. 
     As used herein, the term “human antibody” refers to an antibody produced by a human or an antibody having an amino acid sequence corresponding to an antibody produced by a human made using any technique known in the art. This definition of a human antibody includes intact or full-length antibodies, fragments thereof, and/or antibodies comprising at least one human heavy and/or light chain polypeptide. 
     As used herein, the term “humanized antibody” refers to a non-human antibody that is modified to increase the sequence homology to that of a human antibody, such that the antigen-binding properties of the antibody are retained, but its antigenicity in the human body is reduced. 
     As used herein, the term “chimeric antibody” refers to an antibody wherein the amino acid sequence of the immunoglobulin molecule is derived from two or more species. The variable region of both the light and heavy chains often corresponds to the variable region of an antibody derived from one species of mammal (e.g., mouse, rat, rabbit, etc.) having the desired specificity, affinity, and capability, while the constant regions correspond to the sequences of an antibody derived from another species of mammal (e.g., human) to avoid eliciting an immune response in that species. 
     As used herein, the term “multi-specific antibody” refers to an antibody that comprises a plurality of immunoglobulin variable domain sequences, wherein a first immunoglobulin variable domain sequence of the plurality has binding specificity for a first epitope and a second immunoglobulin variable domain sequence of the plurality has binding specificity for a second epitope. In an embodiment, the first and second epitopes are on the same antigen, e.g., the same protein (or subunit of a multimeric protein). In an embodiment, the first and second epitopes overlap or substantially overlap. In an embodiment, the first and second epitopes do not overlap or do not substantially overlap. In an embodiment, the first and second epitopes are on different antigens, e.g., the different proteins (or different subunits of a multimeric protein). In an embodiment, a multispecific antibody comprises a third, fourth, or fifth immunoglobulin variable domain. In an embodiment, a multispecific antibody is a bispecific antibody molecule, a trispecific antibody molecule, or a tetraspecific antibody molecule. 
     As used herein, the term “bispecifc antibody” refers to a multi-specific antibody that binds no more than two epitopes or two antigens. A bispecific antibody is characterized by a first immunoglobulin variable domain sequence which has binding specificity for a first epitope and a second immunoglobulin variable domain sequence that has binding specificity for a second epitope. In an embodiment, the first and second epitopes are on the same antigen, e.g., the same protein (or subunit of a multimeric protein). In an embodiment, the first and second epitopes overlap or substantially overlap. In an embodiment, the first and second epitopes are on different antigens, e.g., the different proteins (or different subunits of a multimeric protein). In an embodiment, a bispecific antibody comprises a heavy chain variable domain sequence and a light chain variable domain sequence which have binding specificity for a first epitope and a heavy chain variable domain sequence and a light chain variable domain sequence which have binding specificity for a second epitope. In an embodiment, a bispecific antibody comprises a half antibody, or fragment thereof, having binding specificity for a first epitope and a half antibody, or fragment thereof, having binding specificity for a second epitope. In an embodiment, a bispecific antibody comprises a scFv, or fragment thereof, having binding specificity for a first epitope, and a scFv, or fragment thereof, having binding specificity for a second epitope. 
     Provided herein are methods of detecting multi-specific antibody formation after performing a manufacturing process to produce a multi-specific antibody. Also provided are methods of obtaining a stability profile of a multi-specific antibody and methods of obtaining a degraded fragment species profile during a manufacturing process to produce a multi-specific antibody. In certain embodiments, the manufacturing process to produce a multi-specific antibody is selected from the group consisting of a controlled arm exchange (cFAE) process, a knob in hole process, a heavy chain heterodimerization process, and an in vitro disulfide bond isomerization process. 
     Full length bispecific antibodies of the invention can be generated for example using Fab arm exchange (or half molecule exchange) between two mono specific bivalent antibodies by introducing substitutions at the heavy chain CH3 interface in each half molecule to favor heterodimer formation of two antibody half molecules having distinct specificity either in vitro in cell-free environment or using co-expression. The Fab arm exchange reaction is the result of a disulfide-bond isomerization reaction and dissociation-association of CH3 domains. The heavy-chain disulfide bonds in the hinge regions of the parent mono specific antibodies are reduced. The resulting free cysteines of one of the parent monospecific antibodies form an inter heavy-chain disulfide bond with cysteine residues of a second parent monospecific antibody molecule and simultaneously CH3 domains of the parent antibodies release and reform by dissociation-association. The CH3 domains of the Fab arms can be engineered to favor heterodimerization over homodimerization. The resulting product is a bispecific antibody having two Fab arms or half molecules which each binding a distinct epitope, i.e. an epitope on a first antigen and an epitope on a second antigen. 
     “Homodimerization” as used herein refers to an interaction of two heavy chains having identical CH3 amino acid sequences. “Homodimer” as used herein refers to an antibody having two heavy chains with identical CH3 amino acid sequences. 
     “Heterodimerization” as used herein refers to an interaction of two heavy chains having non-identical CH3 amino acid sequences. “Heterodimer” as used herein refers to an antibody having two heavy chains with non-identical CH3 amino acid sequences. 
     The “knob-in-hole” strategy (see, e.g., PCT Publ. No. WO2006/028936) can be used to generate full length bispecific antibodies. Briefly, selected amino acids forming the interface of the CH3 domains in human IgG can be mutated at positions affecting CH3 domain interactions to promote heterodimer formation. An amino acid with a small side chain (hole) is introduced into a heavy chain of an antibody specifically binding a first antigen and an amino acid with a large side chain (knob) is introduced into a heavy chain of an antibody specifically binding a second antigen. After co-expression of the two antibodies, a heterodimer is formed as a result of the preferential interaction of the heavy chain with a “hole” with the heavy chain with a “knob.” Exemplary CH3 substitution pairs forming a knob and a hole are (expressed as modified position in the first CH3 domain of the first heavy chain/modified position in the second CH3 domain of the second heavy chain): T366Y/F405A, T366W/F405W, F405W/Y407A, T394W/Y407T, T394S/Y407A, T366W/T394S, F405W/T394S and T366W/T366S_L368A_Y407V. 
     Other strategies such as promoting heavy chain heterodimerization using electrostatic interactions by substituting positively charged residues at one CH3 surface and negatively charged residues at a second CH3 surface can be used, as described in US Pat. Publ. No. US2010/0015133; US Pat. Publ. No. US2009/0182127; US Pat. Publ. No. US2010/028637; or US Pat. Publ. No. US2011/0123532. In other strategies, heterodimerization can be promoted by the following substitutions (expressed as modified position in the first CH3 domain of the first heavy chain/modified position in the second CH3 domain of the second heavy chain): L351Y_F405AY407V/T394W, T366I_K392M_T394W/F405A_Y407V, T366L_K392M_T394W/F405A_Y407V, L351Y_Y407A/T366A_K409F, L351Y_Y407A/T366V K409F Y407A/T366A_K409F, or T350V_L351Y_F405A Y407V/T350V_T366L_K392L_T394W as described in U.S. Pat. Publ. No. US2012/0149876 or U.S. Pat. Publ. No. US2013/0195849. 
     In addition to methods described above, bispecific antibodies of the invention can be generated in vitro in a cell-free environment by introducing asymmetrical mutations in the CH3 regions of two mono specific homodimeric antibodies and forming the bispecific heterodimeric antibody from two parent monospecific homodimeric antibodies in reducing conditions to allow disulfide bond isomerization according to methods described in PCT Pat. Publ. No. WO2011/131746. In the methods, the first monospecific bivalent antibody (e.g., anti-CD33 antibody) and the second monospecific bivalent antibody (e.g., anti-CD3 antibody) are engineered to have certain substitutions at the CH3 domain that promotes heterodimer stability; the antibodies are incubated together under reducing conditions sufficient to allow the cysteines in the hinge region to undergo disulfide bond isomerization; thereby generating the bispecific antibody by Fab arm exchange. The incubation conditions can optionally be restored to non-reducing conditions. Exemplary reducing agents that can be used are 2-mercaptoethylamine (2-MEA), dithiothreitol (DTT), dithioerythritol (DTE), glutathione, tris (2-carboxyethyl) phosphine (TCEP), L-cysteine and beta-mercaptoethanol, preferably a reducing agent selected from the group consisting of: 2-mercaptoethylamine, dithiothreitol and tris (2-carboxyethyl) phosphine. For example, incubation for at least 90 min at a temperature of at least 20° C. in the presence of at least 25 mM 2-MEA or in the presence of at least 0.5 mM dithiothreitol at a pH from 5-8, for example at pH of 7.0 or at pH of 7.4 can be used. 
     Embodiments 
     This invention provides the following non-limiting embodiments. 
     Embodiment 1 is a method of detecting formation of a multi-specific molecule, the method comprising:
         a. obtaining a multi-specific molecule sample;   b. obtaining a reversed-phase high performance liquid chromatography (RP-HPLC) column;   c. contacting the multi-specific molecule sample in a polar aqueous mobile phase A with the RP-HPLC column, wherein the polar aqueous mobile phase comprises an ion-pairing agent;   d. contacting an organic non-polar mobile phase B with the RP-HPLC column, wherein the organic non-polar phase comprises an ion-pairing agent;   e. eluting the multi-specific molecule sample; and   f. monitoring the amount of multi-specific molecule formation in the eluted multi-specific molecule sample.       

     Embodiment 2 is the method of embodiment 1, wherein the polar aqueous mobile phase A is a solution comprising about 1% to about 5% isopropanol. 
     Embodiment 3 is the method of embodiment 2, wherein the solution comprises about 2% isopropanol. 
     Embodiment 4 is the method of any one of embodiments 1 to 3, wherein the ion-pairing agent in the polar aqueous mobile phase A is selected from the group consisting of trifluoroacetic acid (TFA), difluoroacetic acid (DFA), and formic acid (FA). 
     Embodiment 5 is the method of embodiment 4, wherein the TFA is present in the polar aqueous mobile phase A at about 0.1% to about 2%. 
     Embodiment 6 is the method of embodiment 5, wherein the TFA is present in the polar aqueous mobile phase A at about 0.1%. 
     Embodiment 7 is the method of embodiment 1, wherein the organic non-polar mobile phase B is a solution comprising about 60% to about 80% isopropanol and about 15% to about 25% acetonitrile. 
     Embodiment 8 is the method of embodiment 7, wherein the solution comprises about 70% isopropanol. 
     Embodiment 9 is the method of embodiment 7 or 8, wherein the solution comprises about 20% acetonitrile. 
     Embodiment 10 is the method of any one of embodiments 7 to 9, wherein the ion-pairing agent in the organic non-polar mobile phase B is selected from the group consisting of trifluoroacetic acid (TFA), difluoroacetic acid (DFA), and formic acid (FA). 
     Embodiment 11 is the method of embodiment 10, wherein the TFA is present in the organic non-polar aqueous mobile phase B at about 0.1% to about 2%. 
     Embodiment 12 is the method of embodiment 11, wherein the TFA is present in the organic non-polar aqueous mobile phase B at about 0.1%. 
     Embodiment 13 is the method of any one of embodiments 1-12, wherein the multi-specific molecule is a multi-specific antibody, preferably wherein the multi-specific antibody is a bispecific antibody. 
     Embodiment 14 is a method of detecting formation of a multi-specific antibody while performing a manufacturing process to produce a multi-specific antibody, the method comprising:
         a. performing a manufacturing process to produce a multi-specific antibody, wherein the manufacturing process results in a multi-specific antibody sample comprising an amount of a multi-specific antibody;   b. setting up a reversed-phase high performance liquid chromatography (RP-HPLC) column;   c. injecting the multi-specific antibody sample in a polar aqueous mobile phase A in the RP-HPLC column, wherein the polar aqueous mobile phase comprises an ion-pairing agent;   d. applying an organic non-polar mobile phase B to the RP-HPLC column to form a gradient elution, wherein the organic non-polar mobile phase comprises an ion-pairing agent;   e. eluting the multi-specific antibody sample; and   f. monitoring the amount of multi-specific antibody formation in the eluted sample.       

     Embodiment 15 is the method of embodiment 14, wherein the polar aqueous mobile phase A is a solution comprising about 1% to about 5% isopropanol. 
     Embodiment 16 is the method of embodiment 15, wherein the solution comprises about 2% isopropanol. 
     Embodiment 17 is the method of any one of embodiments 14 to 16, wherein the ion-pairing agent in the polar aqueous mobile phase A is selected from the group consisting of trifluoroacetic acid (TFA), difluoroacetic acid (DFA), and formic acid (FA). 
     Embodiment 18 is the method of embodiment 17, wherein the TFA is present in the polar aqueous mobile phase A at about 0.1% to about 2%. 
     Embodiment 19 is the method of embodiment 18, wherein the TFA is present in the polar aqueous mobile phase A at about 0.1%. 
     Embodiment 20 is the method of embodiment 14, wherein the organic non-polar mobile phase is a solution comprising about 60% to about 80% isopropanol and about 15% to about 25% acetonitrile. 
     Embodiment 21 is the method of embodiment 20, wherein the solution comprises about 70% isopropanol. 
     Embodiment 22 is the method of embodiment 20 or 21, wherein the solution comprises about 20% acetonitrile. 
     Embodiment 23 is the method of any one of embodiments 20 to 22, wherein the ion-pairing agent in the organic non-polar mobile phase B is selected from the group consisting of trifluoroacetic acid (TFA), difluoroacetic acid (DFA), and formic acid (FA). 
     Embodiment 24 is the method of embodiment 23, wherein the TFA is present in the non-polar aqueous mobile phase at about 0.1% to about 2%. 
     Embodiment 25 is the method of embodiment 24, wherein the TFA is present in the non-polar aqueous mobile phase at about 0.1%. 
     Embodiment 26 is the method of any one of embodiments 14 to 25, wherein the gradient elution comprises a solution comprising about 75% of polar mobile phase A solution and about 25% of organic non-polar mobile phase B solution at the moment the multi-specific antibody sample is injected in the RP-HPLC column. 
     Embodiment 27 is the method of embodiment 26, wherein the gradient elution comprises a solution comprising about 66% of polar mobile phase A solution and about 34% of organic non-polar mobile phase B solution at about 16 minutes after the multi-specific antibody sample is injected in the RP-HPLC column. 
     Embodiment 28 is the method of embodiment 26, wherein the gradient elution comprises a solution comprising about 5% of polar mobile phase A solution and about 95% of organic non-polar mobile phase B solution at about 20 minutes after the multi-specific antibody sample is injected in the RP-HPLC column. 
     Embodiment 29 is the method of embodiment 26, wherein the gradient elution comprises a solution comprising about 75% of polar mobile phase A solution and about 25% of organic non-polar mobile phase B solution at about 21 minutes after the multi-specific antibody sample is injected in the RP-HPLC column. 
     Embodiment 30 is the method of any one of embodiments 14 to 29, wherein the manufacturing process to produce a multi-specific antibody is selected from the group consisting of a knob in hole process, a strand exchange engineered domain process, a chemical linked bispecific antibody (BsAb) process, an immunoglobulin domain crossover process, and a controlled Fab arm exchange (cFAE) and dual variable domain process. 
     Embodiment 31 is the method of any one of embodiments 14 to 30, wherein the multi-specific antibody is a bispecific antibody. 
     Embodiment 32 is a method of detecting formation of a bispecific antibody while performing a controlled FAB arm exchange (cFAE) to produce a bispecific antibody, the method comprising:
         a. performing a controlled FAB arm exchange (cFAE) to produce a cFAE sample, wherein the cFAE sample comprises an amount of a bispecific antibody;   b. setting up a reversed-phase high performance liquid chromatography (RP-HPLC) column;   c. injecting the cFAE sample in a polar aqueous mobile phase A in the RP-HPLC column, wherein the polar aqueous mobile phase comprises an ion-pairing agent;   d. applying an organic non-polar mobile phase B to the RP-HPLC column to form a gradient elution, wherein the organic non-polar mobile phase comprises an ion-pairing agent;   e. eluting the cFAE sample; and   f. monitoring the amount of multi-specific antibody formation in the eluted sample.       

     Embodiment 33 is the method of embodiment 32, wherein the polar aqueous mobile phase A is a solution comprising about 1% to about 5% isopropanol. 
     Embodiment 34 is the method of embodiment 33, wherein the solution comprises about 2% isopropanol. 
     Embodiment 35 is the method of any one of embodiments 32 to 34, wherein the ion-pairing agent in the polar aqueous mobile phase A is selected from the group consisting of trifluoroacetic acid (TFA), difluoroacetic acid (DFA), and formic acid (FA). 
     Embodiment 36 is the method of embodiment 35, wherein the TFA is present in the polar aqueous mobile phase A at about 0.1% to about 2%. 
     Embodiment 37 is the method of embodiment 36, wherein the TFA is present in the polar aqueous mobile phase A at about 0.1%. 
     Embodiment 38 is the method of embodiment 32, wherein the organic non-polar mobile phase is a solution comprising about 60% to about 80% isopropanol and about 15% to about 25% acetonitrile. 
     Embodiment 39 is the method of embodiment 38, wherein the solution comprises about 70% isopropanol. 
     Embodiment 40 is the method of embodiment 38 or 39, wherein the solution comprises about 20% acetonitrile. 
     Embodiment 41 is the method of any one of embodiments 38 to 40, wherein the ion-pairing agent in the organic non-polar mobile phase B is selected from the group consisting of trifluoroacetic acid (TFA), difluoroacetic acid (DFA), and formic acid (FA). 
     Embodiment 42 is the method of embodiment 41, wherein the TFA is present in the non-polar aqueous mobile phase at about 0.1% to about 2%. 
     Embodiment 43 is the method of embodiment 42, wherein the TFA is present in the non-polar aqueous mobile phase at about 0.1%. 
     Embodiment 44 is a system comprising a reversed-phase high performance liquid chromatography (RP-HPLC) member for detecting formation of a multi-specific molecule, the system comprising:
         a. a multi-specific molecule sample in a polar aqueous mobile phase A, wherein the polar aqueous mobile phase comprises an ion pairing agent;   b. a RP-HPLC column;   c. an organic non-polar mobile phase B, wherein the organic non-polar mobile phase comprises an ion pairing agent;
 
wherein the RP-HPLC column is configured to be contacted with the multi-specific molecule sample in polar aqueous mobile phase A, and
 
wherein the RP-HPLC is further configured to be contacted with the organic non-polar mobile phase B to elute a multi-specific molecule sample, wherein an amount of multi-specific molecule formation can be identified in the eluted multi-specific molecule sample.
       

     Embodiment 45 is the system of embodiment 44, wherein the polar aqueous mobile phase A is a solution comprising about 1% to about 5% isopropanol. 
     Embodiment 46 is the system of embodiment 45, wherein the solution comprises about 2% isopropanol. 
     Embodiment 47 is the system of any one of embodiments 44 to 46, wherein the ion-pairing agent in the polar aqueous mobile phase A is selected from the group consisting of trifluoroacetic acid (TFA), difluoroacetic acid (DFA), and formic acid (FA). 
     Embodiment 48 is the system of embodiment 47, wherein the TFA is present in the polar aqueous mobile phase A at about 0.1% to about 2%. 
     Embodiment 49 is the system of embodiment 48, wherein the TFA is present in the polar aqueous mobile phase A at about 0.1%. 
     Embodiment 50 is the system of embodiment 44, wherein the organic non-polar mobile phase B is a solution comprising about 60% to about 80% isopropanol and about 15% to about 25% acetonitrile. 
     Embodiment 51 is the system of embodiment 50, wherein the solution comprises about 70% isopropanol. 
     Embodiment 52 is the system of embodiment 50 or 51, wherein the solution comprises about 20% acetonitrile. 
     Embodiment 53 is the system of any one of embodiments 50 to 52, wherein the ion-pairing agent in the organic non-polar mobile phase B is selected from the group consisting of trifluoroacetic acid (TFA), difluoroacetic acid (DFA), and formic acid (FA). 
     Embodiment 54 is the system of embodiment 53, wherein the TFA is present in the organic non-polar aqueous mobile phase B at about 0.1% to about 2%. 
     Embodiment 55 is the system of embodiment 54, wherein the TFA is present in the organic non-polar aqueous mobile phase B at about 0.1%. 
     Embodiment 56 is the system of any one of embodiments 44 to 55, wherein the multi-specific molecule is a multi-specific antibody, preferably wherein the multi-specific antibody is a bispecific antibody. 
     Embodiment 57 is a means for reversed-phase high performance liquid chromatography (RP-HPLC) detection of multi-specific molecule formation comprising:
         a. a multi-specific molecule sample in a polar aqueous mobile phase A, wherein the polar aqueous mobile phase comprises an ion pairing agent;   b. a RP-HPLC column;   c. an organic non-polar mobile phase B, wherein the organic non-polar mobile phase comprises an ion-pairing agent;
 
wherein the RP-HPLC column is configured to be contacted with the multi-specific molecule sample in polar aqueous mobile phase A, and
 
wherein the RP-HPLC column is further configured to be contacted with the organic non-polar mobile phase B to elute a multi-specific molecule sample, wherein an amount of multi-specific molecule formation can be identified in the eluted multi-specific molecule sample.
       

     Embodiment 58 is the means for reversed-phase high performance liquid chromatography (RP-HPLC) detection of multi-specific molecule formation of embodiment 57, wherein the polar aqueous mobile phase A is a solution comprising about 1% to about 5% isopropanol. 
     Embodiment 59 is the means for reversed-phase high performance liquid chromatography (RP-HPLC) detection of multi-specific molecule formation of embodiment 58, wherein the solution comprises about 2% isopropanol. 
     Embodiment 60 is the means for reversed-phase high performance liquid chromatography (RP-HPLC) detection of multi-specific molecule formation of any one of embodiments 57 to 59, wherein the ion-pairing agent in the polar aqueous mobile phase A is selected from the group consisting of trifluoroacetic acid (TFA), difluoroacetic acid (DFA), and formic acid (FA). 
     Embodiment 61 is the means for reversed-phase high performance liquid chromatography (RP-HPLC) detection of multi-specific molecule formation of embodiment 60, wherein the TFA is present in the polar aqueous mobile phase A at about 0.1% to about 2%. 
     Embodiment 62 is the means for reversed-phase high performance liquid chromatography (RP-HPLC) detection of multi-specific molecule formation of embodiment 61, wherein the TFA is present in the polar aqueous mobile phase A at about 0.1%. 
     Embodiment 63 is the means for reversed-phase high performance liquid chromatography (RP-HPLC) detection of multi-specific molecule formation of embodiment 57, wherein the organic non-polar mobile phase B is a solution comprising about 60% to about 80% isopropanol and about 15% to about 25% acetonitrile. 
     Embodiment 64 is the means for reversed-phase high performance liquid chromatography (RP-HPLC) detection of multi-specific molecule formation of embodiment 63, wherein the solution comprises about 70% isopropanol. 
     Embodiment 65 is the means for reversed-phase high performance liquid chromatography (RP-HPLC) detection of multi-specific molecule formation of embodiment 63 or 64, wherein the solution comprises about 20% acetonitrile. 
     Embodiment 66 is the means for reversed-phase high performance liquid chromatography (RP-HPLC) detection of multi-specific molecule formation of any one of embodiments 63 to 65, wherein the ion-pairing agent in the organic non-polar mobile phase B is selected from the group consisting of trifluoroacetic acid (TFA), difluoroacetic acid (DFA), and formic acid (FA). 
     Embodiment 67 is the means for reversed-phase high performance liquid chromatography (RP-HPLC) detection of multi-specific molecule formation of embodiment 66, wherein the TFA is present in the organic non-polar aqueous mobile phase B at about 0.1% to about 2%. 
     Embodiment 68 is the means for reversed-phase high performance liquid chromatography (RP-HPLC) detection of multi-specific molecule formation of embodiment 67, wherein the TFA is present in the organic non-polar aqueous mobile phase B at about 0.1%. 
     Embodiment 69 is the means for reversed-phase high performance liquid chromatography (RP-HPLC) detection of multi-specific molecule formation of any one of embodiments 57 to 68, wherein the multi-specific molecule is a multi-specific antibody, preferably wherein the multi-specific antibody is a bispecific antibody. 
     Embodiment 70 is a method of designing a stable multi-specific antibody, the method comprising:
         a. performing a manufacturing process to produce a multi-specific antibody, wherein the manufacturing process results in a multi-specific antibody sample comprising an amount of a multi-specific antibody;   b. setting up a reversed-phase high performance liquid chromatography (RP-HPLC) column;   c. injecting the multi-specific antibody sample in a polar aqueous mobile phase A in the RP-HPLC column, wherein the polar aqueous mobile phase comprises an ion-pairing agent;   d. applying an organic non-polar mobile phase B to the RP-HPLC column to form a gradient elution, wherein the organic non-polar mobile phase comprises an ion-pairing agent;   e. eluting the multi-specific antibody sample; and   f. monitoring the amount of multi-specific antibody formation in the eluted sample,
 
wherein an increased amount of multi-specific antibody formation as compared to a control indicates the design of a stable multi-specific antibody.
       

     Embodiment 71 is the method of embodiment 70, wherein the polar aqueous mobile phase A is a solution comprising about 1% to about 5% isopropanol. 
     Embodiment 72 is the method of embodiment 71, wherein the solution comprises about 2% isopropanol. 
     Embodiment 73 is the method of any one of embodiments 70 to 72, wherein the ion-pairing agent in the polar aqueous mobile phase A is selected from the group consisting of trifluoroacetic acid (TFA), difluoroacetic acid (DFA), and formic acid (FA). 
     Embodiment 74 is the method of embodiment 73, wherein the TFA is present in the polar aqueous mobile phase A at about 0.1% to about 2%. 
     Embodiment 75 is the method of embodiment 74, wherein the TFA is present in the polar aqueous mobile phase A at about 0.1%. 
     Embodiment 76 is the method of embodiment 70, wherein the organic non-polar mobile phase is a solution comprising about 60% to about 80% isopropanol and about 15% to about 25% acetonitrile. 
     Embodiment 77 is the method of embodiment 76, wherein the solution comprises about 70% isopropanol. 
     Embodiment 78 is the method of embodiment 76 or 77, wherein the solution comprises about 20% acetonitrile. 
     Embodiment 79 is the method of any one of embodiments 76 to 78, wherein the ion-pairing agent in the organic non-polar mobile phase B is selected from the group consisting of trifluoroacetic acid (TFA), difluoroacetic acid (DFA), and formic acid (FA). 
     Embodiment 80 is the method of embodiment 79, wherein the TFA is present in the non-polar aqueous mobile phase at about 0.1% to about 2%. 
     Embodiment 81 is the method of embodiment 80, wherein the TFA is present in the non-polar aqueous mobile phase at about 0.1%. 
     Embodiment 82 is the method of any one of embodiments 70 to 81, wherein the manufacturing process to produce a multi-specific antibody is selected from the group consisting of a knob in hole process, a strand exchange engineered domain process, a chemical linked bispecific antibody (BsAb) process, an immunoglobulin domain crossover process, and a controlled Fab arm exchange (cFAE) and dual variable domain process. 
     Embodiment 83 is the method of any one of embodiments 70 to 82, wherein the multi-specific antibody is a bispecific antibody. 
     Embodiment 84 is a method of obtaining a stability profile of a multi-specific antibody during a manufacturing process to produce a multi-specific antibody, the method comprising:
         a. performing a manufacturing process to produce a multi-specific antibody, wherein an amount of a multi-specific antibody sample is obtained at different timepoints during the manufacturing process;   b. setting up a reversed-phase high performance liquid chromatography (RP-HPLC) column;   c. injecting the multi-specific antibody sample from each timepoint in a polar aqueous mobile phase A in the RP-HPLC column, wherein the polar aqueous mobile phase comprises an ion-pairing agent;   d. applying an organic non-polar mobile phase B to the RP-HPLC column to form a gradient elution, wherein the organic non-polar mobile phase comprises an ion-pairing agent;   e. eluting the multi-specific antibody sample; and   f. monitoring the amount of multi-specific antibody formation in the eluted sample,
 
wherein the amount of multi-specific antibody formation at each time point provides a stability profile of the multi-specific antibody.
       

     Embodiment 85 is the method of embodiment 84, wherein the polar aqueous mobile phase A is a solution comprising about 1% to about 5% isopropanol. 
     Embodiment 86 is the method of embodiment 85, wherein the solution comprises about 2% isopropanol. 
     Embodiment 87 is the method of any one of embodiments 84 to 86, wherein the ion-pairing agent in the polar aqueous mobile phase A is selected from the group consisting of trifluoroacetic acid (TFA), difluoroacetic acid (DFA), and formic acid (FA). 
     Embodiment 88 is the method of embodiment 87, wherein the TFA is present in the polar aqueous mobile phase A at about 0.1% to about 2%. 
     Embodiment 89 is the method of embodiment 88, wherein the TFA is present in the polar aqueous mobile phase A at about 0.1%. 
     Embodiment 90 is the method of embodiment 84, wherein the organic non-polar mobile phase is a solution comprising about 60% to about 80% isopropanol and about 15% to about 25% acetonitrile. 
     Embodiment 91 is the method of embodiment 90, wherein the solution comprises about 70% isopropanol. 
     Embodiment 92 is the method of embodiment 90 or 91, wherein the solution comprises about 20% acetonitrile. 
     Embodiment 93 is the method of any one of embodiments 90 to 92, wherein the ion-pairing agent in the organic non-polar mobile phase B is selected from the group consisting of trifluoroacetic acid (TFA), difluoroacetic acid (DFA), and formic acid (FA). 
     Embodiment 94 is the method of embodiment 93, wherein the TFA is present in the non-polar aqueous mobile phase at about 0.1% to about 2%. 
     Embodiment 95 is the method of embodiment 94, wherein the TFA is present in the non-polar aqueous mobile phase at about 0.1%. 
     Embodiment 96 is the method of any one of embodiments 84 to 95, wherein the manufacturing process to produce a multi-specific antibody is selected from the group consisting of a knob in hole process, a strand exchange engineered domain process, a chemical linked bispecific antibody (BsAb) process, an immunoglobulin domain crossover process, and a controlled Fab arm exchange (cFAE) and dual variable domain process. 
     Embodiment 97 is the method of any one of embodiments 84 to 96, wherein the multi-specific antibody is a bispecific antibody. 
     Embodiment 98 is a method of obtaining a degraded fragment species profile of a multi-specific antibody during a manufacturing process to produce a multi-specific antibody, the method comprising:
         a. performing a manufacturing process to produce a multi-specific antibody, wherein an amount of a multi-specific antibody sample is obtained at different timepoints during the manufacturing process;   b. setting up a reversed-phase high performance liquid chromatography (RP-HPLC) column;   c. injecting the multi-specific antibody sample from each timepoint in a polar aqueous mobile phase A in the RP-HPLC column, wherein the polar aqueous mobile phase comprises an ion-pairing agent;   d. applying an organic non-polar mobile phase B to the RP-HPLC column to form a gradient elution, wherein the organic non-polar mobile phase comprises an ion-pairing agent;   e. eluting the multi-specific antibody sample; and   f. monitoring the amount of degraded fragment species in the eluted sample,
 
wherein the amount of degraded fragment species at each time point provides a degraded fragment species profile of the multi-specific antibody manufacturing process.
       

     Embodiment 99 is the method of embodiment 98, wherein the polar aqueous mobile phase A is a solution comprising about 1% to about 5% isopropanol. 
     Embodiment 100 is the method of embodiment 99, wherein the solution comprises about 2% isopropanol. 
     Embodiment 101 is the method of any one of embodiments 98 to 100, wherein the ion-pairing agent in the polar aqueous mobile phase A is selected from the group consisting of trifluoroacetic acid (TFA), difluoroacetic acid (DFA), and formic acid (FA). 
     Embodiment 102 is the method of embodiment 101, wherein the TFA is present in the polar aqueous mobile phase A at about 0.1% to about 2%. 
     Embodiment 103 is the method of embodiment 102, wherein the TFA is present in the polar aqueous mobile phase A at about 0.1%. 
     Embodiment 104 is the method of embodiment 98, wherein the organic non-polar mobile phase is a solution comprising about 60% to about 80% isopropanol and about 15% to about 25% acetonitrile. 
     Embodiment 105 is the method of embodiment 104, wherein the solution comprises about 70% isopropanol. 
     Embodiment 106 is the method of embodiment 104 or 105, wherein the solution comprises about 20% acetonitrile. 
     Embodiment 107 is the method of any one of embodiments 104 to 106, wherein the ion-pairing agent in the organic non-polar mobile phase B is selected from the group consisting of trifluoroacetic acid (TFA), difluoroacetic acid (DFA), and formic acid (FA). 
     Embodiment 108 is the method of embodiment 107, wherein the TFA is present in the non-polar aqueous mobile phase at about 0.1% to about 2%. 
     Embodiment 109 is the method of embodiment 108, wherein the TFA is present in the non-polar aqueous mobile phase at about 0.1%. 
     Embodiment 110 is the method of any one of embodiments 98 to 109, wherein the manufacturing process to produce a multi-specific antibody is selected from the group consisting of a knob in hole process, a strand exchange engineered domain process, a chemical linked bispecific antibody (BsAb) process, an immunoglobulin domain crossover process, and a controlled Fab arm exchange (cFAE) and dual variable domain process. 
     Embodiment 111 is the method of any one of embodiments 98 to 110, wherein the multi-specific antibody is a bispecific antibody. 
     EXAMPLES 
     Example 1 
     Reversed-Phase High Performance Liquid Chromatography for Determining Bispecific Antibody Formation after Controlled FAB Arm Exchange (FAE). 
     Reversed-phase high performance liquid chromatography (RP-HPLC) separates molecules based on a hydrophobic absorption/desorption process across a hydrophobic solid support. Using an increasing polar mobile phase, the hydrophobic binding interaction between the protein and the solid support resulted in desorption of the analyte from the column. In this particular method, where antibody formation is established through reoxidation of parental mAbs, RP-HPLC was employed to quantitate intact, fully oxidized bispecific DuoBody® (anti-GPRC5D/anti-CD3, anti-CD33/anti-CD3, anti-TMEFF2/anti-CD3, anti-7959/anti-CD3, anti-BCMA/anti-CD3) from partially reoxidized fragments. Test articles were sampled after controlled FAB arm exchange (FAE) and injected onto the column in a highly polar aqueous mobile phase containing an ion pairing agent (trifluoroacetic acid) used for improving peak shape and molecule retention. An increasing organic non-polar mobile phase, also containing trifluoracetic acid, was then applied to the column in a gradient elution to elute the molecules based on their hydrophobic properties. The column eluate was monitored continuously at 280 nm and the relative amount of each IgG was measured by comparing peak area count to the sub fragment IgG species. 
     Example 2 
     Preparation of Reagents/Solutions for Bispecific Antibodies 
     Note A: Different total volumes of mobile phase A and mobile phase B can be prepared, but the ratios of each component must be maintained. 
     Note B: Trifluoroacetic acid (TFA) has a low boiling point and high volatility. It is recommended to store TFA at +2-8° C. for dispensing accurately by pipetting. 
     
       
         
           
               
             
               
                 TABLE 1 
               
             
            
               
                   
               
               
                 Mobile Phase A (98% Water/2% Isopropanol (IPA)/0.1% TFA) 
               
            
           
           
               
               
               
            
               
                   
                 Component 
                 Amount 
               
               
                   
                   
               
               
                   
                 Deionized Water 
                 979 mL  
               
               
                   
                 Isopropanol 
                 20 mL 
               
               
                   
                 TFA 
                  1 mL 
               
            
           
           
               
            
               
                 Expiration (Room Temperature) 
               
               
                 1 week 
               
               
                   
               
            
           
         
       
     
     To prepare mobile phase A, in a fume hood, approximately 800 mL of deionized water was added to a 1 liter beaker. In a 50 mL graduated cylinder, 20 mL of isopropanol was added. The 20 mL of isopropanol was added to the 1 liter beaker and the deionized water and isopropanol was mixed with a stir bar for 3 minutes. 1 mL of refrigerated TFA was carefully pipetted and added to the 1 liter beaker containing isopropanol and deionized water. The TFA was delivered below the solution surface, and the mixture was stirred for an additional 3 minutes. (Note: delivery of the TFA below the solution surface was critical for assay retention times due to the high volitalization of TFA). The solution from the 1 liter beaker was transferred to a 1 liter graduated cylinder and was filled to the 1 liter mark with deionized water. The solution was then transferred to mobile phase A bottle. 
     
       
         
           
               
             
               
                 TABLE 2 
               
             
            
               
                   
               
               
                 Mobile Phase B (70% IPA, 20% acetonitrile, 
               
               
                 9.9% water, 0.1% TFA) 
               
            
           
           
               
               
               
            
               
                   
                 Component 
                 Amount 
               
               
                   
                   
               
               
                   
                 Isopropanol 
                 700 mL 
               
               
                   
                 Acetonitrile 
                 200 mL 
               
               
                   
                 Deionized Water 
                  99 mL 
               
               
                   
                 TFA 
                  1 mL 
               
            
           
           
               
            
               
                 Expiration (Room Temperature) 
               
               
                 1 week 
               
               
                   
               
            
           
         
       
     
     To prepare mobile phase B, in a fume hood, approximately 99 mL of deionized water was added to a 1 liter beaker followed by 200 mL of acetonitrile. The solution was mixed together for 3 minutes with a stir bar. 1 mL of refrigerated TFA was carefully pipetted and added to the 1 liter beaker containing acetonitrile and deionized water. The TFA was delivered below the solution surface, and the mixture was stirred for an additional 3 minutes. (Note: delivery of the TFA below the solution surface was critical for assay retention times due to the high volitalization of TFA). The solution was transferred from the 1 liter beaker to a 1 liter graduated cylinder and filled to the 1 liter mark with isopropanol. The solution was then transferred to mobile phase B bottle. 
     Example 3 
     Sample and Standard Preparations 
     The bispecific antibody is made from reducing the parental homodimer bispecifics and reoxidizing these reactants via a cFAE to produce the heterodimer bispecific antibody. To prepare a post cFAE bispecific antibody process sample for testing, the process sample is diluted from the cFAE UF/DF concentration, determined by 
     A 280 , to a final testing concentration of 2 mg/mL (diluted in mobile phase A) and according to Table 3. 
     
       
         
           
               
             
               
                 TABLE 3 
               
             
            
               
                   
               
               
                 Example dilution for anti-CPRC5D/anti-CD3 
               
               
                 bispecific antibody sample (2 mg/mL) 
               
            
           
           
               
               
               
               
               
            
               
                 Stock 
                 Sample 
                 Diluent 
                 Total 
                 Target 
               
               
                 Conc. 
                 Volume 
                 Volume 
                 Volume 
                 Conc. 
               
               
                 (mg/mL) 
                 (μL) 
                 (μL) 
                 (μL) 
                 (mg/mL) 
               
               
                   
               
               
                 35.0 
                 50.0 
                 825 
                 875 
                 2.0 
               
               
                   
               
            
           
         
       
     
     To prepare a reference material sample at 2 mg/mL, a stock concentration of the bispecific antibody was diluted in mobile phase A as the sample diluent according to Table 4. 
     
       
         
           
               
             
               
                 TABLE 4 
               
             
            
               
                   
               
               
                 Example of a reference sample dilution for 
               
               
                 anti-CPRC5D/anti-CD3 bispecific antibody 
               
            
           
           
               
               
               
               
               
            
               
                 Stock 
                 Sample 
                 Diluent 
                 Total 
                 Target 
               
               
                 Conc. 
                 Volume 
                 Volume 
                 Volume 
                 Conc. 
               
               
                 (mg/mL) 
                 (μL) 
                 (μL) 
                 (μL) 
                 (mg/mL) 
               
               
                   
               
               
                 10.2 
                 78.4 
                 321.6 
                 400 
                 2.0 
               
               
                   
               
            
           
         
       
     
     Example 4 
     HPLC Instrument Setup 
     The instrument parameters are provided in Table 5 and Table 6. The column module thermostat was controlled at 80° C. to heat both the right and left side compartments. The connecting thermostat capillary tubing was plumbed to the left side (3 μL) heating element. To avoid over-pressurization of the LC system and damage to the column, the thermostat temperature of 80° C. was reached before any flow was started to the system. 
     
       
         
           
               
             
               
                 TABLE 5 
               
             
            
               
                   
               
               
                 Bispecific instrument parameters (Universal) 
               
            
           
           
               
               
               
            
               
                   
                 Parameter 
                 Value 
               
               
                   
                   
               
            
           
           
               
               
               
               
            
               
                   
                 Flow rate 
                 0.5 
                 mL/min 
               
               
                   
                 Injection Volume 
                 5 
                 μL 
               
               
                   
                 Column Temperature 
                 80° 
                 C. 
               
               
                   
                 Detector Wavelength 
                 280 
                 nm 
               
               
                   
                 Run Time 
                 28 
                 minutes 
               
            
           
           
               
               
               
            
               
                   
                 Post Run Time 
                 None 
               
            
           
           
               
               
               
               
            
               
                   
                 Auto-sample temperature 
                 4° 
                 C. 
               
               
                   
                 Maximum Pressure 
                 400 
                 Bar 
               
               
                   
                   
               
            
           
         
       
     
     
       
         
           
               
             
               
                 TABLE 6 
               
             
            
               
                   
               
               
                 Instrument parameters (Agilent 1200 or 1260) 
               
            
           
           
               
               
               
            
               
                   
                 Parameters 
                 Agilent 1200-1260 
               
               
                   
                   
               
               
                   
                 Bandwidth 
                 4 
               
            
           
           
               
               
               
               
            
               
                   
                 Slit 
                 4 
                 nm 
               
               
                   
                 Reference wavelength, Bandwidth 
                 360, 100 
                 nm 
               
               
                   
                 Peak Width 
                 &gt;0.1 
                 min 
               
               
                   
                   
               
            
           
         
       
     
     
       
         
           
               
             
               
                 TABLE 7 
               
             
            
               
                   
               
               
                 Example gradient for anti-CPRC5D/anti-CD3 bispecific antibody 
               
            
           
           
               
               
               
            
               
                 Time (min) 
                 % A 
                 % B 
               
               
                   
               
            
           
           
               
               
               
            
               
                 0.0 
                 75 
                 25 
               
               
                 2 
                 70 
                 30 
               
               
                 16 
                 66 
                 34 
               
               
                 17 
                 66 
                 34 
               
               
                 17.1 
                 5 
                 95 
               
               
                 20 
                 5 
                 95 
               
               
                 21 
                 75 
                 25 
               
               
                 28 
                 75 
                 25 
               
               
                   
               
            
           
         
       
     
     
       
         
           
               
             
               
                 TABLE 8 
               
             
            
               
                   
               
               
                 Example sample sequence for anti-CPRC5D/anti- 
               
               
                 CD3 bispecific antibody 
               
            
           
           
               
               
               
            
               
                   
                 Injection Type 
                 No. of injections 
               
               
                   
                   
               
               
                   
                 Column condition (mobile phase A) 
                 1 
               
               
                   
                 Anti-GPRC5D/anti-CD3 reference 
                 2 
               
               
                   
                 material 
               
               
                   
                 FAE in-process sample 
                 1-10 
               
               
                   
                 Anti-GPRC5D/anti-CD3 reference 
                 2 
               
               
                   
                 material 
               
               
                   
                 Blank (mobile phase A) 
                 1 
               
               
                   
                   
               
               
                   
                 Note: 
               
               
                   
                 The sequence can be extended for multiple samples. 
               
            
           
         
       
     
     Example 5 
     Integrating Results 
     Note: Manual integration was used and integrated with a processing method. 
     Peak Integrations for Reference Material. 
     Integration of main component: the reference material main component was comprised of 2 peaks, which included a main component peak and main component shoulder peak ( FIGS.  3 ,  5 ,  7 ,  9 , and  11   ). These 2 peaks were integrated together between the valley of the last pre-peak and main peak and ending between the valley of the shoulder main peak and the first post-peak ( FIGS.  3 ,  5 ,  7 ,  9 , and  11   ). The pre-peak(s) and post-peak(s) to the main peak and main shoulder peak were comprised of unoxidized fragments and/or residual parental homodimer bispecific antibody that were contained in the reference material (control). 
     Integration of the pre-peak in the reference material (RM) was integrated as 1 peak. Integration of the post-peak in the reference material (RM) was integrated as 1 peak. 
     Peak Integration of Post cFAE Test Sample 
     Integration of the post cFAE test article main component: the post cFAE test bispecific antibody was comprised of 2 peaks, which included the main component peak and main component shoulder peak ( FIGS.  4 ,  6 ,  8 ,  10 , and  12   ). These 2 peaks were integrated together between the valley of the last pre-peak and main peak and ending between the valley of the shoulder main peak and the first post-peak ( FIGS.  4 ,  6 ,  8 ,  10   , and  12 ). 
     Non baseline resolved pre-peak clusters prior to the main component were integrated together as a single pre-peak and individual baseline resolved peaks were integrated separately. All post-peaks after the main component shoulder peak were integrated as 1 peak. The pre- and post-peaks represent the unoxidized bispecific species ( FIGS.  4 ,  6 ,  8 ,  10 , and  12   ). 
     Peak identification for reference material: The 2 peaks which correspond to the bispecific antibody reference material were labeled as main component and main component shoulder peak. The multiple cluster peaks prior to the bispecific antibody were integrated together ( FIGS.  3 - 12   ) and labeled as pre-peaks. The post-peaks to main component and main component shoulder peak were integrated together ( FIGS.  3 - 12   ) and labeled as post-peaks. 
     It will be appreciated by those skilled in the art that changes could be made to the embodiments described above without departing from the broad inventive concept thereof. It is understood, therefore, that this invention is not limited to the particular embodiments disclosed, but it is intended to cover modifications within the spirit and scope of the present invention as defined by the present description.