Ligand binding to a humanized anti-cocaine mAb measured by dye absorption spectroscopy

Novel partial reduction of the humanized anti-cocaine mAb h2E2 for selective cysteine labeling

Monoclonal antibodies are utilized for treating many diseases and disorders, as well as for basic research and development. Covalent labeling of mAbs is important for various antibody applications and creating antibody drug conjugates. Labeling at reactive lysine residues using lysine selective reagents is useful, but is non-selective and can interfere with antigen binding and interactions of the Fc antibody region. In this work, using an anti-cocaine mAb (h2E2), we utilized triphenylphosphine-3,3',3″-trisulfonic acid (TPPTS), and demonstrated for the first time reduction of disulfides in an antibody by TPPTS. More importantly, this reduction was very reproducible, limited, and selective, and permitted selective labeling of the antibody with a cysteine reactive fluorescent reagent, resulting in labeling of a few specific cysteines. Similar results were obtained using TCEP-agarose reduction. We demonstrated that both of these selective partial reduction methods gave rise to approximately two labels per mAb, mostly by selective reduction of the heavy chain to light chain disulfide bond, as demonstrated by non-reducing SDS-PAGE protein band analysis. Thus, convenient, reproducible, and selective mAb disulfide reduction was achieved under mild conditions. These labeled, partially reduced mAbs were characterized by differential scanning fluorimetry (DSF), detecting the incorporated fluorescein instead of an exogenously added dye, and for antigen (cocaine) binding by isothermal titration calorimetry (ITC). Both the structure and antigen binding of the mAb was maintained. This novel selective reduction and labeling is generally relevant to modification of antibodies and to future development of conjugated mAbs for experimental and therapeutic purposes.

Characterization and optimization of fluorescein isothiocyanate labeling of humanized h2E2 anti-cocaine mAb

Fluorescein isothiocyanate (FITC) is widely used to fluorescently label reactive lysine residues on proteins, including antibodies. The rate and extent of labeling varies with reaction conditions, concentration of label, and the concentration and nature of the protein. Fluorescently labeled proteins are very useful, and one use for FITC labeled mAbs is development of assays to measure anti-mAb antibodies produced in vivo during treatment with antibody therapeutics. Our laboratory has developed a humanized anti-cocaine mAb (h2E2) intended for the treatment of cocaine use disorders. Thus, a well characterized FITC labeled h2E2 mAb is needed to quantitate possible anti-mAb antibodies. The time course of labeling and the relative incorporation of FITC into the heavy and light chains, as well as into the Fab and Fc portions of the mAb, was assessed. A novel use of differential scanning fluorimetry in the absence of any extrinsic fluorophore was developed and demonstrated to be capable of measuring antigen (cocaine) binding. In addition, the effect of increasing degrees of labeling by FITC on the thermodynamic parameters driving the binding of cocaine to the mAb was assessed via isothermal titration calorimetry (ITC). This binding technique, unlike others developed recently to measure cocaine binding, is not dependent on, or subject to interference by, the absorbance or fluorescence of the incorporated FITC label. The methods and results reported herein guide the optimization of FITC labeling needed for anti-mAb assays and other assays important for the development of therapeutic mAbs, which are some of the most specific and clinically useful drugs available.

Isothermal titration calorimetry determination of thermodynamics of binding of cocaine and its metabolites to humanized h2E2 anti-cocaine mAb

We analyzed the thermodynamics of binding of cocaine and several cocaine metabolites to a humanized anti-cocaine mAb (h2E2), which is under development for the treatment of cocaine use disorders, using isothermal titration calorimetry. The calculated equilibrium dissociation (binding) constants were consistent with previous findings using other methods. All three ligands that display high affinity (nM) binding to the mAb (cocaine, cocaethylene, and benzoylecgonine) displayed similar enthalpically driven binding with substantial enthalpy-entropy compensation. The increased affinity of the cocaethylene metabolite compared to cocaine and benzoylecgonine is mostly attributable to a substantially less negative entropic binding component for cocaethylene, resulting in a more favorable binding energy, and thus, a higher affinity. The much lower affinity cocaine metabolites, norcocaine and ecgonine methyl ester, have much lower binding enthalpies than the high affinity ligands, and in contrast to the three high affinity ligands, have favorable (positive) entropic thermodynamic components of binding. Surprisingly, approximately 3.7 molecules of norcocaine are bound per mAb Fab site, as determined by isothermal titration calorimetry. This is in contrast to the three high affinity ligands, which bound with the expected stoichiometry of one drug molecule bound per one mAb Fab site. The results are discussed in relation to the previously published Fab:benzoylecgonine crystal structure for this h2E2 mAb, and compared to the isothermal titration calorimetry results published previously using an unrelated anti-cocaine mAb, mAb08.

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ITC was used to measure the thermodynamics of ligand binding to anti-cocaine h2E2 mAb.

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Binding of high and low affinity cocaine metabolites were compared to cocaine binding.

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Low affinity metabolites have favorable/positive entropic binding components.

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h2E2 mAb ITC results differ from those published for mAb08 anti-cocaine mAb.

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Surprisingly, ≈3.7 molecules of low affinity norcocaine are bound per mAb Fab site.

Tyrosine nitration of a humanized anti-cocaine mAb differentially affects ligand binding of cocaine and its metabolites

Tetranitromethane was used to selectively modify tyrosine residues of a humanized anti-cocaine mAb (h2E2), under development for the treatment of cocaine use disorders. The effect of mild tyrosine nitration on the affinity of cocaine and two high affinity cocaine metabolites, cocaethylene and benzoylecgonine, was assessed using differential scanning fluorimetry to measure ligand affinities via ligand-induced thermal stabilization of the mAb antigen binding region. Nitrated tyrosine residues were identified by mass spectral analysis of thermolysin peptides. One objective was to understand the binding affinity differences observed for these three ligands, which are not explained by the published crystal structure of the h2E2 mAb Fab fragment co-crystalized with benzoylecgonine, since the carboxylic acid of benzoylecgonine that is esterified to form cocaine and cocaethylene is not in contact with the mAb. Importantly, the binding affinity of the cocaine metabolite benzoylecgonine was not decreased by mild nitration, whereas the binding affinities of cocaine and cocaethylene were decreased about two-fold. These ligands differ only in the substituent attached to the carboxylate moiety of the compound, with benzoylecgonine having an unesterified carboxylate, and cocaine and cocaethylene having methyl and ethyl esters, respectively, at this position. The results are consistent with nitration of light chain tyrosine residue 34, resulting in a less favorable interaction with cocaine and cocaethylene carboxylate esters, while not affecting binding of benzoylecgonine. Thus, light chain Tyr34 residue may have molecular interactions with cocaine and cocaethylene not present for benzoylecgonine, leading to the observed affinity differences for these three ligands.

Oxidation of specific tryptophan residues inhibits high affinity binding of cocaine and its metabolites to a humanized anti-cocaine mAb

Cocaine addiction remains a serious problem lacking an effective pharmacological treatment. Thus, we have developed a high-affinity anti-cocaine monoclonal antibody (mAb), h2E2, for the treatment of cocaine use disorders. We show that selective tryptophan (Trp) oxidation by 2,2′-azobis(2-amidinopropane) dihydrochloride (AAPH) resulted in a loss of high-affinity binding of cocaine to this mAb. The newly developed use of excess methionine (Met) to protect mAb met residues from AAPH oxidation did not substantially attenuate the effects of oxidation on cocaine binding but greatly decreased the modification of met residues in the mAb. Similar large decreases in ligand affinity (5000–10,000-fold) upon oxidation were observed using cocaine and two cocaine metabolites, cocaethylene and benzoylecgonine, which also bind with nanomolar affinity to this h2E2 mAb. The decrease in binding affinity was accompanied by a decrease of approximately 50% in Trp fluorescence, and increases in mAb 310 to 370 nm absorbance were consistent with the presence of oxidized forms of Trp. Finally, mass spectral analysis of peptides derived from control and AAPH-oxidized mAb indicated that excess free met did effectively protect mAb met residues from oxidation, and that AAPH-oxidized mAb heavy-chain Trp33 and light-chain Trp91 residues are important for cocaine binding, consistent with a recently derived h2E2 Fab fragment crystal structure containing bound benzoylecgonine. Thus, protection of the anti-cocaine h2E2 mAb from Trp oxidation prior to its clinical administration is critical for its proposed therapeutic use in the treatment of cocaine use disorders.

Cocaine binding to the Fab fragment of a humanized anti-cocaine mAb quantitated by dye absorption and fluorescence spectroscopy

In this work, we establish that cocaine binding to the Fab fragment of a recombinant humanized anti-cocaine mAb (h2E2) can be directly and easily quantitated using simple and inexpensive absorption and fluorescence measurements, employing dyes typically used for differential scanning fluorimetry, DASPMI and SYPRO Orange. For concentrated samples of the Fab fragment, absorbance spectroscopy employing these dyes reveals the number of cocaine sites present, using either DASPMI (by measuring the increase in dye absorbance) or SYPRO Orange (by measuring the change in dye maximal absorbance wavelength). Interestingly, we observed that cocaine binding to the Fab fragment had a much different effect on the SYPRO Orange dye absorbance than previously reported for the intact h2E2 mAb, resulting in a large decrease in the total dye absorbance for the Fab fragment, in contrast to previous results with the intact h2E2 mAb. For dilute samples of Fab fragment, a dye fluorescence emission spectroscopy assay was developed to quantitate the number of cocaine (and other high affinity cocaine metabolites) binding sites via the ligand-induced decrease in fluorescence emission of both of these extrinsic dyes. The difference in the cocaine titrations for the high affinity (Kd < 30 nM) ligands, cocaine, cocaethylene and benzoylecgonine and the low affinity (Kd > 30 μM) ligands, norcocaine, ecgonine methyl ester, and ecgonine were obvious using this assay. These simple, direct, and inexpensive techniques should prove useful for evaluation of other small molecule antigen binding Fab fragments, enabling quantitation and rapid biochemical assessments necessary for determining Fab fragment suitability for in vivo uses and other assays and experiments.