Michael J. Curcio

Impact of the high tyrosine fraction in complementarity determining regions: measured and predicted effects of radioiodination on IgG immunoreactivity

Although iodine-131 is the most widely used radionuclide for radioimmunotherapy, direct radiolabeling methods yield decreased immunoreactivity of the antibody as a function of increased iodine incorporation. We have studied the amino acid sequences of a therapeutic IgG (HuM195), and in particular its complementarity determining regions (CDR), in order to correlate the iodination of tyrosine residues in the antigen binding site with changes in immunoreactivity. The CDR contained an overabundance of tyrosines relative to an expected random distribution of amino acids. In contrast, lysine residues that can be used for ligand attachment were evenly distributed throughout the IgG. HuM195 was first trace labeled with 111In and then labeled with stable 127I at various specific activities. The immunoreactivity of each product was determined using the 111In tracer. The immunoreactivity measured after varying levels of iodination fit a theoretical curve that was generated based on the assumption that a single iodine incorporation anywhere on a tyrosine residue in a CDR destroys the immunoreactivity of the antibody. Similar theoretical curves for antibody fragments (Fab, Fv) suggest an even faster decrease in immunoreactivity with increasing iodination. A review of the sequences of other therapeutic IgG shows that a similar overabundance of tyrosine residues is found in the CDRs. Using enzyme digestion, the distribution of iodine on different parts of the antibody was also studied. The iodinated residues were distributed non uniformly throughout the IgG, with the heavy chain variable region tyrosines having a higher propensity for iodine incorporation than tyrosines in the other regions of the IgG. The common abundance of tyrosine in the CDR of IgG and its correlation with loss of function have important implications for therapeutic use of high specific activity radioiodinated monoclonal antibodies or fragments.

Targeting the Intracellular WT1 Oncogene Product with a Therapeutic Human Antibody

A therapeutic monoclonal antibody specific for the intracellular oncoprotein Wilms tumor 1 treats human leukemias in mice. Destroying from Within Anticancer antibody-based drugs have largely targeted proteins on the surface of cancer cells. But, arguably the most important, tumor-specific proteins are on the inside—safely tucked away within the cell. Wilms tumor 1 (WT1) is one of these intracellular oncoproteins. Despite its insider status, degraded WT1 fragments are presented on the surface of leukemia cells and many other cancer tissues, including ovarian. To kill leukemia, Dao and colleagues hypothesized that intracellular WT1 was the perfect target. Dao et al. engineered a monoclonal antibody, named “ESK1,” that recognizes a peptide fragment of WT1, called RMF, complexed with human leukocyte antigen (HLA)–A0201. After demonstrating that ESK1 bound to several WT1+ cell lines in vitro and leukemia patient cells ex vivo, the authors tested their new antibody in two mouse models of human acute lymphoblastic leukemia. They delivered ESK1 alone or along with human “effector” cells (peripheral blood natural killer cells) and saw that the combination therapy killed nearly all leukemia in comparison to control groups, allowing all of the treated mice to have prolonged or even leukemia-free survival. Treating animals with cancers that lacked either HLA-A0201 or WT1 had no effect. With a defined mechanism and no toxicity in mice, this ESK1 antibody is poised for testing in human trials. The authors point out that more than 1 million patients in the world may have a WT1+ cancer, with many of these being HLA-A02+. In this case, ESK1—with its ability to target a cancer protein inside the cell—could help treat many patients that have not responded to antibody-based therapies focused on the cell surface. The Wilms tumor 1 (WT1) oncoprotein is an intracellular, oncogenic transcription factor that is overexpressed in a wide range of leukemias and solid cancers. RMFPNAPYL (RMF), a WT1-derived CD8+ T cell human leukocyte antigen (HLA)–A0201 epitope, is a validated target for T cell–based immunotherapy. Using phage display technology, we discovered a fully human “T cell receptor–like” monoclonal antibody (mAb), ESK1, specific for the WT1 RMF peptide/HLA-A0201 complex. ESK1 bound to several leukemia and solid tumor cell lines and primary leukemia cells, in a WT1- and HLA-A0201–restricted manner, with high avidity [dissociation constant (Kd) = 0.1 nM]. ESK1 mediated antibody-dependent human effector cell cytotoxicity in vitro. Low doses of naked ESK1 antibody cleared established, disseminated, human acute lymphocytic leukemia and Philadelphia chromosome–positive leukemia in nonobese diabetic/severe combined immunodeficient γc−/− (NSG) mouse models. At therapeutic doses, no toxicity was seen in HLA-A0201 transgenic mice. ESK1 is a potential therapeutic agent for a wide range of cancers overexpressing the WT1 oncoprotein. This finding also provides preclinical validation for the strategy of developing therapeutic mAbs targeting intracellular oncogenic proteins.

A rapid, single vessel method for preparation of clinical grade ligand conjugated monoclonal antibodies

A rapid, single vessel method for the preparation of clinical grade chelate conjugated monoclonal antibodies has been developed. By use of an Amicon concentrator with reservoir, each of the steps necessary for the preparation of the conjugated drug may be performed in a single vessel. Advantages include reduced risk of metal, pyrogen and bacterial contamination; buffer exchanges are achieved rapidly and efficiently using a continuous dilution method. The radiolabeling efficiency, the radiochemical purity, the total immunoreactivity and the affinity of the final product have been evaluated in the production of CHXA-DTPA-chelate conjugated HuM195. The characteristics compare favorably to those achieved using our conventional synthetic methods.

Therapeutic bispecific T-cell engager antibody targeting the intracellular oncoprotein WT1.

Intracellular tumor antigens presented on the cell surface in the context of human leukocyte antigen (HLA) molecules have been targeted by T cell–based therapies, but there has been little progress in developing small-molecule drugs or antibodies directed to these antigens. Here we describe a bispecific T-cell engager (BiTE) antibody derived from a T-cell receptor (TCR)-mimic monoclonal antibody (mAb) ESK1, which binds a peptide derived from the intracellular oncoprotein WT1 presented on HLA-A*02:01. Despite the very low density of the complexes at the cell surface, ESK1-BiTE selectively activated and induced proliferation of cytolytic human T cells that killed cells from multiple leukemias and solid tumors in vitro and in mice. We also discovered that in an autologous in vitro setting, ESK1-BiTE induced a robust secondary CD8 T-cell response specific for tumor-associated antigens other than WT1. Our study provides an approach that targets tumor-specific intracellular antigens without using cell therapy and suggests that epitope spreading could contribute to the therapeutic efficacy of this BiTE.

Influence of the linker on the biodistribution and catabolism of actinium-225 self-immolative tumor-targeted isotope generators.

Current limitations to applications of monoclonal antibody (mAb) targeted isotope generators in radioimmunotherapy include the low mAb labeling yields and the nonspecific radiation of normal tissues by nontargeted radioimmunoconjugates (RIC). Radiotoxicity occurs in normal organs that metabolize radiolabeled proteins and peptides, primarily liver and kidneys, or in radiosensitive organs with prolonged exposure to the isotope from the blood, such as the bone marrow. Actinium-225 nanogenerators also have the problem of released agar-emitting daughters. We developed two new bifunctional chelating agents (BCA) in order to address these issues. Thiol-maleimide conjugation chemistry was employed to increase the efficiency of the mAb radiolabelings by up to 8-fold. In addition, one bifunctional chelating agent incorporated a cleavable linker to alter the catabolism of the alpha-particle-emitting mAb conjugate. This linker was designed to be sensitive to cathepsins to allow release and clearance of the chelated radiometal after internalization of the radioimmunoconjugate into the cell. We compared the properties of the cleavable conjugate (mAb-DOTA-G3FC) to noncleavable constructs (mAb-DOTA-NCS and mAb-DOTA-SH). The cleavable RIC was able to release 80% of its radioactive payload when incubated with purified cathepsin B. The catabolism of the constructs mAb-DOTA-G3FC and mAb-DOTA-NCS was investigated in vitro and in vivo. RIC integrity was retained at 85% over a period of 136 h in mouse serum in vivo. Both conjugates were degraded over time inside HL-60 cells after internalization and in mouse liver in vivo. While we found that the rates of degradation of the two RICs in those conditions were similar, the amounts of the radiolabeled product residues were different. The cleavable mAb-DOTA-G3FC conjugate yielded a larger proportion of fragments below 6kDa in size in mouse liver in vivo after 12 h than the DOTA-NCS conjugate. Biodistribution studies in mice showed that the mAb-DOTA-G3FC construct yielded a higher liver dose and prolonged liver retention of radioactivity compared to the mAb-DOTA-NCS conjugate. The accumulation in the liver seemed to be in part caused by the maleimide functionalization of the antibody, since the noncleavable mAb-DOTA-SH maleimide-functionalized control conjugate displayed the same biodistribution pattern. These results provide an insight into the catabolism of RICs, by demonstrating that the release of the radioisotope from a RIC is not a sufficient condition to allow the radioactive moiety to clear from the body. The excretion mechanisms of radiolabeled fragments seem to constitute a major limiting step in the chain of events leading to their clearance.

A therapeutic T cell receptor mimic antibody targets tumor-associated PRAME peptide/HLA-I antigens

Preferentially expressed antigen in melanoma (PRAME) is a cancer-testis antigen that is expressed in many cancers and leukemias. In healthy tissue, PRAME expression is limited to the testes and ovaries, making it a highly attractive cancer target. PRAME is an intracellular protein that cannot currently be drugged. After proteasomal processing, the PRAME300–309 peptide ALYVDSLFFL (ALY) is presented in the context of human leukocyte antigen HLA-A*02:01 molecules for recognition by the T cell receptor (TCR) of cytotoxic T cells. Here, we have described Pr20, a TCR mimic (TCRm) human IgG1 antibody that recognizes the cell-surface ALY peptide/HLA-A2 complex. Pr20 is an immunological tool and potential therapeutic agent. Pr20 bound to PRAME+HLA-A2+ cancers. An afucosylated Fc form (Pr20M) directed antibody-dependent cellular cytotoxicity against PRAME+HLA-A2+ leukemia cells and was therapeutically effective against mouse xenograft models of human leukemia. In some tumors, Pr20 binding markedly increased upon IFN-&ggr; treatment, mediated by induction of the immunoproteasome catalytic subunit &bgr;5i. The immunoproteasome reduced internal destructive cleavages within the ALY epitope compared with the constitutive proteasome. The data provide rationale for developing TCRm antibodies as therapeutic agents for cancer, offer mechanistic insight on proteasomal regulation of tumor-associated peptide/HLA antigen complexes, and yield possible therapeutic solutions to target antigens with ultra-low surface presentation.

Abstract A055: Potent therapeutic and immunological effects of the first T-BiTE derived from a TCR-mimic antibody targeting intracellular oncoprotein WT1

Bi-specific T cell engager antibody (BiTE) therapy has recently emerged as an effective immunotherapy by redirecting polyclonal T cell cytotoxicity against cell surface protein on tumor cells. We generated the first BiTE construct derived from a TCR-mimic monoclonal antibody (mAb), ESK1, specific for a peptide from an intracellular oncoprotein, WT1, in the context of HLA-A*02:01 molecules. Despite the low density peptide/HLA-A2 complex on the cell surface, ESK-BiTE was able to selectively activate and induce proliferation of cytolytic human T cells to kill multiple leukemias and cancers in vitro and in mice. Surprisingly, we also discovered that in an autologous setting, ESK-BiTE induced a robust secondary CD8 T cell response specific for antigens other than WT1, including HLA-A2-restricted her2-neu-derived peptide 369-377, in patients with her2- positive ovarian cancer. Therefore, the study demonstrated a new vaccinal mechanism for BiTE mAb action that could contribute to more effective long-term therapeutic activity of BiTE9s and further broaden their reach to other tumor antigens not previously known or originally targeted. Citation Format: Tao Dao, Dmitry Pankov, Andrew Scott, Tatyana Korontsvit, Victoriya Zakhaleva, Manuel Direito de Morais Guerrerio, Yiyang Xu, Jingyi Xiang, Su Yan, Nicholas Veomett, Nicholas Veomett, Leonid Dubrovsky, Michael Curcio, Ekaterina Doubrovina, Cheng Liu, Richard J. O9Reilly, David A. Scheinberg. Potent therapeutic and immunological effects of the first T-BiTE derived from a TCR-mimic antibody targeting intracellular oncoprotein WT1. [abstract]. In: Proceedings of the CRI-CIMT-EATI-AACR Inaugural International Cancer Immunotherapy Conference: Translating Science into Survival; September 16-19, 2015; New York, NY. Philadelphia (PA): AACR; Cancer Immunol Res 2016;4(1 Suppl):Abstract nr A055.