Exposing the molecular heterogeneity of glycosylated biotherapeutics.

The heterogeneity inherent in today's biotherapeutics, especially as a result of heavy glycosylation, can affect a molecule's safety and efficacy. Characterizing this heterogeneity is crucial for drug development and quality assessment, but existing methods are limited in their ability to analyze intact glycoproteins or other heterogeneous biotherapeutics. Here, we present an approach to the molecular assessment of biotherapeutics that uses proton-transfer charge-reduction with gas-phase fractionation to analyze intact heterogeneous and/or glycosylated proteins by mass spectrometry. The method provides a detailed landscape of the intact molecular weights present in biotherapeutic protein preparations in a single experiment. For glycoproteins in particular, the method may offer insights into glycan composition when coupled with a suitable bioinformatic strategy. We tested the approach on various biotherapeutic molecules, including Fc-fusion, VHH-fusion, and peptide-bound MHC class II complexes to demonstrate efficacy in measuring the proteoform-level diversity of biotherapeutics. Notably, we inferred the glycoform distribution for hundreds of molecular weights for the eight-times glycosylated fusion drug IL22-Fc, enabling correlations between glycoform sub-populations and the drug's pharmacological properties. Our method is broadly applicable and provides a powerful tool to assess the molecular heterogeneity of emerging biotherapeutics.

Application of N-Terminal Site-Specific Biotin and Digoxigenin Conjugates to Clinical Anti-drug Antibody Assay Development.

Biotin- and digoxigenin (DIG)-conjugated therapeutic drugs are critical reagents used for the development of anti-drug antibody (ADA) assays for the assessment of immunogenicity. The current practice of generating biotin and DIG conjugates is to label a therapeutic antibody with biotin or DIG via primary amine groups on lysine or N-terminal residues. This approach modifies lysine residues nonselectively, which can impact the ability of an ADA assay to detect those ADAs that recognize epitopes located at or near the modified lysine residue(s). The impact of the lysine modification is considered greater for therapeutic antibodies that have a limited number of lysine residues, such as the variable heavy domain of heavy chain (VHH) antibodies. In this paper, for the first time, we report the application of site-specifically conjugated biotin- and DIG-VHH reagents to clinical ADA assay development using a model molecule, VHHA. The site-specific conjugation of biotin or DIG to VHHA was achieved by using an optimized reductive alkylation approach, which enabled the majority of VHHA molecules labeled with biotin or DIG at the desirable N-terminus, thereby minimizing modification of the protein after labeling and reducing the possibility of missing detection of ADAs. Head-to-head comparison of biophysical characterization data revealed that the site-specific biotin and DIG conjugates demonstrated overall superior quality to biotin- and DIG-VHHA prepared using the conventional amine coupling method, and the performance of the ADA assay developed using site-specific biotin and DIG conjugates met all acceptance criteria. The approach described here can be applied to the production of other therapeutic-protein- or antibody-based critical reagents that are used to support ligand binding assays.

UniDec Processing Pipeline for Rapid Analysis of Biotherapeutic Mass Spectrometry Data.

Recent advances in native mass spectrometry (MS) and denatured intact protein MS have made these techniques essential for biotherapeutic characterization. As MS analysis has increased in throughput and scale, new data analysis workflows are needed to provide rapid quantitation from large datasets. Here, we describe the UniDec processing pipeline (UPP) for the analysis of batched biotherapeutic intact MS data. UPP is built into the UniDec software package, which provides fast processing, deconvolution, and peak detection. The user and programming interfaces for UPP read a spreadsheet that contains the data file names, deconvolution parameters, and quantitation settings. After iterating through the spreadsheet and analyzing each file, it returns a spreadsheet of results and HTML reports. We demonstrate the use of UPP to measure the correct pairing percentage on a set of bispecific antibody data and to measure drug-to-antibody ratios from antibody-drug conjugates. Moreover, because the software is free and open-source, users can easily build on this platform to create customized workflows and calculations. Thus, UPP provides a flexible workflow that can be deployed in diverse settings and for a wide range of biotherapeutic applications.

Insights into ultra-low affinity lipase-antibody noncovalent complex binding mechanisms.

Detection of host cell protein (HCP) impurities is critical to ensuring that recombinant drug products, including monoclonal antibodies (mAbs), are safe. Mechanistic characterization as to how HCPs persist in drug products is important to refining downstream processing. It has been hypothesized that weak lipase-mAb interactions enable HCP lipases to evade drug purification processes. Here, we apply state-of-the-art methods to establish lipase-mAb binding mechanisms. First, the mass spectrometry (MS) approach of fast photochemical oxidation of proteins was used to elucidate putative binding regions. The CH1 domain was identified as a conserved interaction site for IgG1 and IgG4 mAbs against the HCPs phospholipase B-like protein (PLBL2) and lysosomal phospholipase A2 (LPLA2). Rationally designed mutations in the CH1 domain of the IgG4 mAb caused a 3- to 70-fold KD reduction against PLBL2 by surface plasmon resonance (SPR). LPLA2-IgG4 mutant complexes, undetected by SPR and studied using native MS collisional dissociation experiments, also showed significant complex disruption, from 16% to 100%. Native MS and ion mobility (IM) determined complex stoichiometries for four lipase-IgG4 complexes and directly interrogated the enrichment of specific lipase glycoforms. Confirmed with time-course and exoglycosidase experiments, deglycosylated lipases prevented binding, and low-molecular-weight glycoforms promoted binding, to mAbs. This work demonstrates the value of integrated biophysical approaches to characterize micromolar affinity complexes. It is the first in-depth structural report of lipase-mAb binding, finding roles for the CH1 domain and lipase glycosylation in mediating binding. The structural insights gained offer new approaches for the bioengineering of cells or mAbs to reduce HCP impurity levels.Abbreviations: CAN, Acetonitrile; AMAC, Ammonium acetate; BFGS, Broyden-Fletcher-Goldfarb-Shanno; CHO, Chinese Hamster Ovary; KD, Dissociation constant; DTT, Dithiothreitol; ELISA, Enzyme-linked immunosorbent assay; FPOP, Fast photochemical oxidation of proteins; FA, Formic acid; F(ab'), Fragment antibodies; HCP, Host cell protein; IgG, Immunoglobulin; IM, Ion mobility; LOD, Lower limit of detection; LPLA2, Lysosomal phospholipase A2; Man, Mannose; MS, Mass spectrometry; MeOH, Methanol; MST, Microscale thermophoresis; mAbs, Monoclonal antibodies; PPT1, Palmitoyl protein thioesterase; ppm, Parts per million; PLBL2, Phospholipase B-like protein; PLD3, Phospholipase D3; PS-20, Polysorbate-20; SP, Sphingomyelin phosphodiesterase; SPR, Surface plasmon resonance; TFA, Trifluoroacetic acid.

High-Throughput, Quantitative Analysis of Peptide-Exchanged MHCI Complexes by Native Mass Spectrometry.

Immune monitoring in cancer immunotherapy involves screening CD8+ T-cell responses against neoantigens, the tumor-specific peptides presented by Major histocompatibility complex Class I (MHCI) on the cell surface. High-throughput immune monitoring requires methods to produce and characterize small quantities of thousands of MHCI-peptide complexes that may be tested for a patient's T-cell response. MHCI synthesis has been achieved using a photocleavable peptide that is exchanged by the neoantigen; however, assays that measure peptide exchange currently disassemble the complex prior to analysis─precluding direct molecular characterization. Here, we use native mass spectrometry (MS) to profile intact recombinant MHCI complexes and directly measure peptide exchange. Coupled with size-exclusion chromatography or capillary-zone electrophoresis, the assay identified all tested human leukocyte antigen (HLA)/peptide combinations in the nanomole to picomole range with minimal run time, reconciling the synthetic and analytical requirements of MHCI-peptide screening with the downstream T-cell assays. We further show that the assay can be "multiplexed" by measuring exchange of multiple peptides simultaneously and also enables calculation of Vc50, a measure of gas-phase stability. Additionally, MHCI complexes were fragmented by top-down sequencing, demonstrating that the intact complex, peptide sequence, and their binding affinity can be determined in a single analysis. This screening tool for MHCI-neoantigen complexes represents a step toward the application of state-of-the-art MS technology in translational settings. Not only is this assay already informing on the viability of immunotherapy in practice, the platform also holds promise to inspire novel MS readouts for increasingly complex biomolecules used in the diagnosis and treatment of disease.

Structural basis for SHOC2 modulation of RAS signalling.

The RAS-RAF pathway is one of the most commonly dysregulated in human cancers1-3. Despite decades of study, understanding of the molecular mechanisms underlying dimerization and activation4 of the kinase RAF remains limited. Recent structures of inactive RAF monomer5 and active RAF dimer5-8 bound to 14-3-39,10 have revealed the mechanisms by which 14-3-3 stabilizes both RAF conformations via specific phosphoserine residues. Prior to RAF dimerization, the protein phosphatase 1 catalytic subunit (PP1C) must dephosphorylate the N-terminal phosphoserine (NTpS) of RAF11 to relieve inhibition by 14-3-3, although PP1C in isolation lacks intrinsic substrate selectivity. SHOC2 is as an essential scaffolding protein that engages both PP1C and RAS to dephosphorylate RAF NTpS11-13, but the structure of SHOC2 and the architecture of the presumptive SHOC2-PP1C-RAS complex remain unknown. Here we present a cryo-electron microscopy structure of the SHOC2-PP1C-MRAS complex to an overall resolution of 3 Å, revealing a tripartite molecular architecture in which a crescent-shaped SHOC2 acts as a cradle and brings together PP1C and MRAS. Our work demonstrates the GTP dependence of multiple RAS isoforms for complex formation, delineates the RAS-isoform preference for complex assembly, and uncovers how the SHOC2 scaffold and RAS collectively drive specificity of PP1C for RAF NTpS. Our data indicate that disease-relevant mutations affect complex assembly, reveal the simultaneous requirement of two RAS molecules for RAF activation, and establish rational avenues for discovery of new classes of inhibitors to target this pathway.

Loss of the intracellular enzyme QPCTL limits chemokine function and reshapes myeloid infiltration to augment tumor immunity.

Tumor-associated macrophages are composed of distinct populations arising from monocytes or tissue macrophages, with a poorly understood link to disease pathogenesis. Here, we demonstrate that mouse monocyte migration was supported by glutaminyl-peptide cyclotransferase-like (QPCTL), an intracellular enzyme that mediates N-terminal modification of several substrates, including the monocyte chemoattractants CCL2 and CCL7, protecting them from proteolytic inactivation. Knockout of Qpctl disrupted monocyte homeostasis, attenuated tumor growth and reshaped myeloid cell infiltration, with loss of monocyte-derived populations with immunosuppressive and pro-angiogenic profiles. Antibody targeting of the receptor CSF1R, which more broadly eliminates tumor-associated macrophages, reversed tumor growth inhibition in Qpctl-/- mice and prevented lymphocyte infiltration. Modulation of QPCTL synergized with anti-PD-L1 to expand CD8+ T cells and limit tumor growth. QPCTL inhibition constitutes an effective approach for myeloid cell-targeted cancer immunotherapy.

High-Throughput Kinetic Characterization of Irreversible Covalent Inhibitors of KRASG12C by Intact Protein MS and Targeted MRM.

With recent advances and success in several drugs designed to treat acute and chronic diseases, targeted covalent inhibitors show a resurgence in drug discovery. As covalent inhibition is time-dependent, the preferred quantitative potency metric of irreversible inhibitors is the second-order rate constant kinact/Ki, rather than IC50. Here, we present the development of a mass spectrometry-based platform for rapid kinetic analysis of irreversible covalent inhibitors. Using a simple liquid handling robot for automated sample preparation and a solid-phase extraction-based RapidFire-MS system for rapid MS analysis, kinetic characterization of covalent inhibitors was performed in high throughput both by intact protein analysis and targeted multiple reaction monitoring (MRM). In addition, a bimolecular reaction model was applied to extract kinact/Ki in data fitting, providing tremendous flexibility in the experimental design to characterize covalent inhibitors with various properties. Using KRASG12C inhibitors as a test case, the platform was demonstrated to be effective for studying covalent inhibitors with a wide range of kinact/Ki values from single digit to 3 × 105 M-1 s-1.

Primary Amine Tethered Small Molecules Promote the Degradation of X-Linked Inhibitor of Apoptosis Protein.

We hypothesized that the proximity-driven ubiquitylation of E3-interacting small molecules could affect the degradation of E3 ubiquitin ligases. A series of XIAP BIR2 domain-binding small molecules was modified to append a nucleophilic primary amine. This modification transforms XIAP binders into inducers of XIAP degradation. The degradation of XIAP is E1- and proteasome-dependent, dependent on the ligase function of XIAP, and is rescued by subtle modifications of the small molecule that would obviate ubiquitylation. We demonstrate in vitro ubiquitylation of the small molecule that is dependent on its interaction with XIAP. Taken together, these results demonstrate the designed ubiquitylation of an engineered small molecule and a novel approach for the degradation of E3 ubiquitin ligases.

Fc galactosylation follows consecutive reaction kinetics and enhances immunoglobulin G hexamerization for complement activation.

Fc galactosylation is a critical quality attribute for anti-tumor recombinant immunoglobulin G (IgG)-based monoclonal antibody (mAb) therapeutics with complement-dependent cytotoxicity (CDC) as the mechanism of action. Although the correlation between galactosylation and CDC has been known, the underlying structure-function relationship is unclear. Heterogeneity of the Fc N-glycosylation produced by Chinese hamster ovary (CHO) cell culture biomanufacturing process leads to variable CDC potency. Here, we derived a kinetic model of galactose transfer reaction in the Golgi apparatus and used this model to determine the correlation between differently galactosylated species from CHO cell culture process. The model was validated by a retrospective data analysis of more than 800 historical samples from small-scale and large-scale CHO cell cultures. Furthermore, using various analytical technologies, we discovered the molecular basis for Fc glycan terminal galactosylation changing the three-dimensional conformation of the Fc, which facilitates the IgG1 hexamerization, thus enhancing C1q avidity and subsequent complement activation. Our study offers insight into the formation of galactosylated species, as well as a novel three-dimensional understanding of the structure-function relationship of terminal galactose to complement activation in mAb therapeutics.

Activation of the IRE1 RNase through remodeling of the kinase front pocket by ATP-competitive ligands.

Inositol-Requiring Enzyme 1 (IRE1) is an essential component of the Unfolded Protein Response. IRE1 spans the endoplasmic reticulum membrane, comprising a sensory lumenal domain, and tandem kinase and endoribonuclease (RNase) cytoplasmic domains. Excess unfolded proteins in the ER lumen induce dimerization and oligomerization of IRE1, triggering kinase trans-autophosphorylation and RNase activation. Known ATP-competitive small-molecule IRE1 kinase inhibitors either allosterically disrupt or stabilize the active dimeric unit, accordingly inhibiting or stimulating RNase activity. Previous allosteric RNase activators display poor selectivity and/or weak cellular activity. In this study, we describe a class of ATP-competitive RNase activators possessing high selectivity and strong cellular activity. This class of activators binds IRE1 in the kinase front pocket, leading to a distinct conformation of the activation loop. Our findings reveal exquisitely precise interdomain regulation within IRE1, advancing the mechanistic understanding of this important enzyme and its investigation as a potential small-molecule therapeutic target.

Targeted IgMs agonize ocular targets with extended vitreal exposure.

Treatment of ocular disease is hindered by the presence of the blood-retinal barrier, which restricts access of systemic drugs to the eye. Intravitreal injections bypass this barrier, delivering high concentrations of drug to the targeted tissue. However, the recommended dosing interval for approved biologics is typically 6-12 weeks, and frequent travel to the physician's office poses a substantial burden for elderly patients with poor vision. Real-world data suggest that many patients are under-treated. Here, we investigate IgMs as a novel platform for treating ocular disease. We show that IgMs are well-suited to ocular administration due to moderate viscosity, long ocular exposure, and rapid systemic clearance. The complement-dependent cytotoxicity of IgMs can be readily removed with a P436G mutation, reducing safety liabilities. Furthermore, dodecavalent binding of IgM hexamers can potently activate pathways implicated in the treatment of progressive blindness, including the Tie2 receptor tyrosine kinase signaling pathway for the treatment of diabetic macular edema, or the death receptor 4 tumor necrosis family receptor pathway for the treatment of wet age-related macular degeneration. Collectively, these data demonstrate the promise of IgMs as therapeutic agonists for treating progressive blindness.

Dimerization Induced by C-Terminal 14-3-3 Binding Is Sufficient for BRAF Kinase Activation.

The Ras-RAF-MEK-ERK signaling axis, commonly mutated in human cancers, is highly regulated to prevent aberrant signaling in healthy cells. One of the pathway modulators, 14-3-3, a constitutive dimer, induces RAF dimerization and activation by binding to a phosphorylated motif C-terminal to the RAF kinase domain. Recent work has suggested that a C-terminal "DTS" region in BRAF is necessary for this 14-3-3-mediated activation. We show that the catalytic activity and ATP binding affinity of the BRAF:14-3-3 complex is insensitive to the presence or absence of the DTS, while the ATP sites of both BRAF molecules are identical and available for binding. We also present a crystal structure of the apo BRAF:14-3-3 complex showing that the DTS is not required to attain the catalytically active conformation of BRAF. Rather, BRAF dimerization induced by 14-3-3 is the key step in activation, allowing the active BRAF:14-3-3 tetramer to achieve catalytic activity comparable to the constitutively active oncogenic BRAF V600E mutant.

Data on charge separation of bispecific and mispaired IgGs using native charge-variant mass spectrometry.

The data supplied in this work are related to the research article entitled "Characterization of Bispecific and Mispaired IgGs by Native Charge-Variant Mass Spectrometry" (Phung et al., 2019). This data article describes a powerful analytical platform using native weak cation exchange chromatography coupled to a high-resolution mass spectrometer, charge variant mass spectrometry (CV-MS), to characterize bispecific and mispaired antibody species. Elution order is investigated through analytical methods and molecular modeling in an effort to understand the intrinsic charge, size and shape differences of these molecules.

Negative regulation of RAF kinase activity by ATP is overcome by 14-3-3-induced dimerization.

The RAS-RAF-MEK-ERK signaling axis is frequently activated in human cancers. Physiological concentrations of ATP prevent formation of RAF kinase-domain (RAFKD) dimers that are critical for activity. Here we present a 2.9-Å-resolution crystal structure of human BRAFKD in complex with MEK and the ATP analog AMP-PCP, revealing interactions between BRAF and ATP that induce an inactive, monomeric conformation of BRAFKD. We also determine how 14-3-3 relieves the negative regulatory effect of ATP through a 2.5-Å-resolution crystal structure of the BRAFKD-14-3-3 complex, in which dimeric 14-3-3 enforces a dimeric BRAFKD assembly to increase BRAF activity. Our data suggest that most oncogenic BRAF mutations alter interactions with ATP and counteract the negative effects of ATP binding by lowering the threshold for RAF dimerization and pathway activation. Our study establishes a framework for rationalizing oncogenic BRAF mutations and provides new avenues for improved RAF-inhibitor discovery.

Denisovan, modern human and mouse TNFAIP3 alleles tune A20 phosphorylation and immunity.

Resisting and tolerating microbes are alternative strategies to survive infection, but little is known about the evolutionary mechanisms controlling this balance. Here genomic analyses of anatomically modern humans, extinct Denisovan hominins and mice revealed a TNFAIP3 allelic series with alterations in the encoded immune response inhibitor A20. Each TNFAIP3 allele encoded substitutions at non-catalytic residues of the ubiquitin protease OTU domain that diminished IκB kinase-dependent phosphorylation and activation of A20. Two TNFAIP3 alleles encoding A20 proteins with partial phosphorylation deficits seemed to be beneficial by increasing immunity without causing spontaneous inflammatory disease: A20 T108A;I207L, originating in Denisovans and introgressed in modern humans throughout Oceania, and A20 I325N, from an N-ethyl-N-nitrosourea (ENU)-mutagenized mouse strain. By contrast, a rare human TNFAIP3 allele encoding an A20 protein with 95% loss of phosphorylation, C243Y, caused spontaneous inflammatory disease in humans and mice. Analysis of the partial-phosphorylation A20 I325N allele in mice revealed diminished tolerance of bacterial lipopolysaccharide and poxvirus inoculation as tradeoffs for enhanced immunity.

Elucidating heavy/light chain pairing preferences to facilitate the assembly of bispecific IgG in single cells.

Multiple strategies have been developed to facilitate the efficient production of bispecific IgG (BsIgG) in single host cells. For example, we previously demonstrated near quantitative (≥90%) formation of BsIgG of different species and isotypes by combining 'knob-into-hole' mutations for heavy chain heterodimerization with engineered antigen-binding fragments (Fabs) for preferential cognate heavy/light chain pairing. Surprisingly, in this study we found high yield (>65%) of BsIgG1without Fab engineering to be a common occurrence, i.e., observed for 33 of the 99 different antibody pairs evaluated. Installing charge mutations at both CH1/CL interfaces was sufficient for near quantitative yield (>90%) of BsIgG1 for most (9 of 11) antibody pairs tested with this inherent cognate chain pairing preference. Mechanistically, we demonstrate that a strong cognate pairing preference in one Fab arm can be sufficient for high BsIgG1 yield. These observed chain pairing preferences are apparently driven by variable domain sequences and can result from a few specific residues in the complementarity-determining region (CDR) L3 and H3. Transfer of these CDR residues into other antibodies increased BsIgG1 yield in most cases. Mutational analysis revealed that the disulfide bond between heavy and light chains did not affect the yield of BsIgG1. This study provides some mechanistic understanding of factors contributing to antibody heavy/light chain pairing preference and subsequently contributes to the efficient production of BsIgG in single host cells.

Production, characterization, and in vivo half-life extension of polymeric IgA molecules in mice.

IgA antibodies have broad potential as a novel therapeutic platform based on their superior receptor-mediated cytotoxic activity, potent neutralization of pathogens, and ability to transcytose across mucosal barriers via polymeric immunoglobulin receptor (pIgR)-mediated transport, compared to traditional IgG-based drugs. However, the transition of IgA into clinical development has been challenged by complex expression and characterization, as well as rapid serum clearance that is thought to be mediated by glycan receptor scavenging of recombinantly produced IgA monomer bearing incompletely sialylated N-linked glycans. Here, we present a comprehensive biochemical, biophysical, and structural characterization of recombinantly produced monomeric, dimeric and polymeric human IgA. We further explore two strategies to overcome the rapid serum clearance of polymeric IgA: removal of all N-linked glycosylation sites creating an aglycosylated polymeric IgA and engineering in FcRn binding with the generation of a polymeric IgG-IgA Fc fusion. While previous reports and the results presented in this study indicate that glycan-mediated clearance plays a major role for monomeric IgA, systemic clearance of polymeric IgA in mice is predominantly controlled by mechanisms other than glycan receptor clearance, such as pIgR-mediated transcytosis. The developed IgA platform now provides the potential to specifically target pIgR expressing tissues, while maintaining low systemic exposure.

Inhibition of the dipeptidyl peptidase DPP4 (CD26) reveals IL-33-dependent eosinophil-mediated control of tumor growth.

Post-translational modification of chemokines mediated by the dipeptidyl peptidase DPP4 (CD26) has been shown to negatively regulate lymphocyte trafficking, and its inhibition enhances T cell migration and tumor immunity by preserving functional chemokine CXCL10. By extending those initial findings to pre-clinical models of hepatocellular carcinoma and breast cancer, we discovered a distinct mechanism by which inhibition of DPP4 improves anti-tumor responses. Administration of the DPP4 inhibitor sitagliptin resulted in higher concentrations of the chemokine CCL11 and increased migration of eosinophils into solid tumors. Enhanced tumor control was preserved in mice lacking lymphocytes and was ablated after depletion of eosinophils or treatment with degranulation inhibitors. We further demonstrated that tumor-cell expression of the alarmin IL-33 was necessary and sufficient for eosinophil-mediated anti-tumor responses and that this mechanism contributed to the efficacy of checkpoint-inhibitor therapy. These findings provide insight into IL-33- and eosinophil-mediated tumor control, revealed when endogenous mechanisms of DPP4 immunoregulation are inhibited.

Quantitative Determination of Protein-Ligand Affinity by Size Exclusion Chromatography Directly Coupled to High-Resolution Native Mass Spectrometry.

High throughput protein-ligand interaction screening assays employing mass spectrometric detection are widely used in early stage drug discovery. Mass spectrometry-based screening approaches employ a target protein added to a pool of small-molecule compounds, and binding is assessed by measuring ligands denatured from the complexes. Direct analysis of protein-ligand interactions using native mass spectrometry has been demonstrated but is not widely used due to the detection limit on protein size, the requirement of volatile buffers, and the necessity for specialized instrumentation to preserve weak interactions under native conditions. Here we present a robust, quantitative, and automated online size-exclusion chromatography-native mass spectrometry (SEC-nMS) platform for measuring affinities of noncovalent protein-small-molecule interactions on an Orbitrap mass spectrometer. Indoleamine 2,3-dioxygenase 1, a catabolic enzyme, and inhibitory ligands were employed as a demonstration of the method. Efficient separation and elution enabled preservation of protein-ligand complexes and increased throughput. The high sensitivity and intra charge state resolution at high m/ z offered by the Exactive Plus EMR Orbitrap allowed for protein ligand affinity quantitation and resolved individual compounds close in mass. Vc50 values determined via collision-induced dissociation experiments enabled the evaluation of complex stability in the gas phase and were found to be independent of the extent of complex formation. For the first time, Vc50 determinations were achieved on an inline SEC-nMS platform. Systematic comparison of our method with optimized chip-based nanoelectrospray infusion served as a reference for ligand screening and affinity quantitation and further revealed the advantages of SEC-MS.