HA22 (R490A) is a recombinant immunotoxin with increased antitumor activity without an increase in animal toxicity

Targeting CD22 for B-cell hematologic malignancies

CD19-targeted chimeric receptor antigen (CAR)-T cell therapy has shown remarkable clinical efficacy in the treatment of relapsed or refractory (R/R) B-cell malignancies. However, 30%-60% of patients eventually relapsed, with the CD19-negative relapse being an important hurdle to sustained remission. CD22 expression is independent of CD19 expression in malignant B cells. Consequently, CD22 is a potential alternative target for CD19 CAR-T cell-resistant patients. CD22-targeted therapies, mainly including the antibody-drug conjugates (ADCs) and CAR-T cells, have come into wide clinical use with acceptable toxicities and promising efficacy. In this review, we explore the molecular and physiological characteristics of CD22, development of CD22 ADCs and CAR-T cells, and the available clinical data on CD22 ADCs and CAR-T cell therapies. Furthermore, we propose some perspectives for overcoming tumor escape and enhancing the efficacy of CD22-targeted therapies.

Binding domain on CD22 molecules contributing to the biological activity of T cell-engaging bispecific antibodies

CD22, as the B-cell malignancies antigen, has been targeted for immunotherapies through CAR-T cells, antibody-drug conjugates (ADCs) and immunotoxins via interaction of antibodies with binding domains on the receptor. We hypothesized that avidity and binding domain of antibody to target cells may have significant impact on the biological function in tumor immunotherapy, and T cell-engaging bispecific antibody (TCB) targeting CD22 could be used in the therapy of hematologic malignancies. So, to address the question, we utilized the information of six previously reported CD22 mAbs to generate CD22-TCBs with different avidity to different domains on CD22 protein. We found that the avidity of CD22-TCBs to protein was not consistent with the avidity to target cells, indicating that TCBs had different binding mode to the protein and cells. In vitro results indicated that CD22-TCBs mediated cytotoxicity depended on the avidity of antibodies to target cells rather than to protein. Moreover, distal binding domain of the antigen contributed to the avidity and biological activity of IgG-[L]-scfv-like CD22-TCBs. The T cells' proliferation, activation, cytotoxicity as well as cytokine release were compared, and G5/44 BsAb was selected for further in vivo assessment in anti-tumor activity. In vivo results demonstrated that CD22-TCB (G5/44 BsAb) significantly inhibited the tumors growth in mice. All these data suggested that CD22-TCBs could be developed as a promising candidate for B-cell malignancies therapy through optimizing the design with avidity and binding domain to CD22 target in consideration.

Full-length recombinant antibodies from Escherichia coli: production, characterization, effector function (Fc) engineering, and clinical evaluation

Although several antibody fragments and antibody fragment-fusion proteins produced in Escherichia coli (E. coli) are approved as therapeutics for various human diseases, a full-length monoclonal or a bispecific antibody produced in E. coli has not yet been approved. The past decade witnessed substantial progress in expression of full-length antibodies in the E. coli cytoplasm and periplasm, as well as in cell-free expression systems. The equivalency of E. coli-produced aglycosylated antibodies and their mammalian cell-produced counterparts, with respect to biochemical and biophysical properties, including antigen binding, in vitro and in vivo serum stability, pharmacokinetics, and in vivo serum half-life, has been demonstrated. Extensive engineering of the Fc domain of aglycosylated antibodies enables recruitment of various effector functions, despite the lack of N-linked glycans. This review summarizes recent research, preclinical advancements, and clinical development of E. coli-produced aglycosylated therapeutic antibodies as monoclonal, bispecific, and antibody-drug conjugates for use in autoimmune, oncology, and immuno-oncology areas.Abbreviations: ADA Anti-drug antibody; ADCC Antibody-dependent cellular cytotoxicity; ADCP Antibody-dependent cellular phagocytosis; ADC Antibody-drug conjugate; aFc Aglycosylated Fc; AMD Age-related macular degeneration aTTP Acquired thrombotic thrombocytopenic purpura; BCMA B-cell maturation antigen; BLA Biologics license application; BsAb Bispecific antibody; C1q Complement protein C1q; CDC Complement-dependent cytotoxicity; CDCC Complement-dependent cellular cytotoxicity; CDCP Complement-dependent cellular phagocytosis; CEX Cation exchange chromatography; CFPS Cell-free protein expression; CHO Chinese Hamster Ovary; CH1-3 Constant heavy chain 1-3; CL Constant light chain; DLBCL Diffuse large B-cell lymphoma; DAR Drug antibody ratio; DC Dendritic cell; dsFv Disulfide-stabilized Fv; EU European Union; EGFR Epidermal growth factor receptor; E. coli Escherichia coli; EpCAM Epithelial cell adhesion molecule; Fab Fragment antigen binding; FACS Fluorescence activated cell sorting; Fc Fragment crystallizable; FcRn Neonatal Fc receptor; FcɣRs Fc gamma receptors; FDA Food and Drug Administration; FL-IgG Full-length immunoglobulin; Fv Fragment variable; FolRαa Folate receptor alpha; gFc Glycosylated Fc; GM-CSF Granulocyte macrophage-colony stimulating factor; GPx7 Human peroxidase 7; HCL Hairy cell leukemia; HIV Human immunodeficiency virusl; HER2 Human epidermal growth factor receptor 2; HGF Hepatocyte growth factor; HIC Hydrophobic interaction chromatography; HLA Human leukocyte antigen; IBs Inclusion bodies; IgG1-4 Immunoglobulin 1-4; IP Intraperitoneal; ITC Isothermal titration calorimetry; ITP Immune thrombocytopenia; IV Intravenous; kDa Kilodalton; KiH Knob-into-Hole; mAb Monoclonal antibody; MAC Membrane-attack complex; mCRC Metastatic colorectal cancer; MM Multipl myeloma; MOA Mechanism of action; MS Mass spectrometry; MUC1 Mucin 1; MG Myasthenia gravis; NB Nanobody; NK Natural killer; nsAA Nonstandard amino acid; NSCLC Non-small cell lung cancer; P. aeruginosa Pseudomonas aeruginosa; PD-1 Programmed cell death 1; PD-L1 Programmed cell death-ligand 1; PDI Protein disulfide isomerase; PECS Periplasmic expression cytometric screening; PK Pharmacokinetics; P. pastoris Pichia pastoris; PTM Post-translational modification; Rg Radius of gyration; RA Rheumatoid arthritis; RT-PCR Reverse transcription polymerase chain reaction; SAXS Small angle X-ray scattering; scF Single chain Fv; SCLC Small cell lung cancer; SDS-PAGE Sodium dodecyl sulfate-polyacrylamide gel electrophoresis; SEC Size exclusion chromatography; SEED Strand-exchange engineered domain; sRNA Small regulatory RNA; SRP Signal recognition particle; T1/2 Half-life; Tagg Aggregation temperature; TCR T cell receptor; TDB T cell-dependent bispecific; TF Tissue factor; TIR Translation initiation region; Tm Melting temperature; TNBC Triple-negative breast cancer; TNF Tumor necrosis factor; TPO Thrombopoietin; VEGF Vascular endothelial growth factor; vH Variable heavy chain; vL Variable light chain; vWF von Willebrand factor; WT Wild type.

Targeted delivery of cytotoxic proteins to prostate cancer via conjugation to small molecule urea-based PSMA inhibitors

Prostate cancer cells are characterized by a remarkably low proliferative rate and the production of high levels of prostate-specific proteases. Protein-based toxins are attractive candidates for prostate cancer therapy because they kill cells via proliferation-independent mechanisms. However, the non-specific cytotoxicity of these potent cytotoxins must be redirected to avoid toxicity to normal tissues. Prostate-Specific Membrane Antigen (PSMA) is membrane-bound carboxypeptidase that is highly expressed by prostate cancer cells. Potent dipeptide PSMA inhibitors have been developed that can selectively deliver and concentrate imaging agents within prostate cancer cells based on continuous PSMA internalization and endosomal cycling. On this basis, we conjugated a PSMA inhibitor to the apoptosis-inducing human protease Granzyme B and the potent Pseudomonas exotoxin protein toxin fragment, PE35. We assessed selective PSMA binding and entrance into tumor cell to induce cell death. We demonstrated these agents selectively bound to PSMA and became internalized. PSMA-targeted PE35 toxin was selectively toxic to PSMA producing cells in vitro. Intratumoral and intravenous administration of this toxin produced marked tumor killing of PSMA-producing xenografts with minimal host toxicity. These studies demonstrate that urea-based PSMA inhibitors represent a simpler, less expensive alternative to antibodies as a means to deliver cytotoxic proteins to prostate cancer cells.

Triton X-114 and Amine-Based Wash Strategy Reduces Lipopolysaccharides to FDA Limit and Achieves Purer, More Potent Recombinant Immunotoxin

Many proteins are still routinely expressed prokaryotically in Escherichia coli, some because they are toxic to eukaryotes. Immunotoxins, which are fusion proteins of a targeting moiety and a truncated Pseudomonas exotoxin A, kill target cells by arresting protein synthesis. Thus, immunotoxins must be expressed in E. coli. Proteins expressed in E. coli are contaminated by endotoxin (also called lipopolysaccharides (LPS)). LPS binds to toll-like receptors, inducing up to life-threatening systemic inflammation in mammals. Therefore, accepted LPS limits for therapeutics as well as for substances used in immunological studies in animals are very low. Here, we report the use of Triton X-114 and polyamine-based wash strategies, which only in combination achieved LPS-contamination well below FDA limits. Resulting LPS-reduced immunotoxins were purer and up to 2.4-fold more active in vitro. Increased activity was associated with a 2.4-fold increase in affinity on cell surface expressed target antigen. The combination method maintained enzymatic function, protein stability, and in vivo efficacy and was effective for Fab as well as dsFv formats. With some modifications, the principle of this novel combination may be applied to any chromatography-based purification process.

Engineered Anti-GPC3 Immunotoxin, HN3-ABD-T20, Produces Regression in Mouse Liver Cancer Xenografts Through Prolonged Serum Retention

BACKGROUND AND AIMS

Treatment of hepatocellular carcinomas using our glypican-3 (GPC3)-targeting human nanobody (HN3) immunotoxins causes potent tumor regression by blocking protein synthesis and down-regulating the Wnt signaling pathway. However, immunogenicity and a short serum half-life may limit the ability of immunotoxins to transition to the clinic.

APPROACH AND RESULTS

To address these concerns, we engineered HN3-based immunotoxins to contain various deimmunized Pseudomonas exotoxin (PE) domains. This included HN3-T20, which was modified to remove T-cell epitopes and contains a PE domain II truncation. We compared them to our previously reported B-cell deimmunized immunotoxin (HN3-mPE24) and our original HN3-immunotoxin with a wild-type PE domain (HN3-PE38). All of our immunotoxins displayed high affinity to human GPC3, with HN3-T20 having a KD value of 7.4 nM. HN3-T20 retained 73% enzymatic activity when compared with the wild-type immunotoxin in an adenosine diphosphate-ribosylation assay. Interestingly, a real-time cell growth inhibition assay demonstrated that a single dose of HN3-T20 at 62.5 ng/mL (1.6 nM) was capable of inhibiting nearly all cell proliferation during the 10-day experiment. To enhance HN3-T20's serum retention, we tested the effect of adding a streptococcal albumin-binding domain (ABD) and a llama single-domain antibody fragment specific for mouse and human serum albumin. For the detection of immunotoxin in mouse serum, we developed a highly sensitive enzyme-linked immunosorbent assay and found that HN3-ABD-T20 had a 45-fold higher serum half-life than HN3-T20 (326 minutes vs. 7.3 minutes); consequently, addition of an ABD resulted in HN3-ABD-T20-mediated tumor regression at 1 mg/kg.

CONCLUSION

These data indicate that ABD-containing deimmunized HN3-T20 immunotoxins are high-potency therapeutics ready to be evaluated in clinical trials for the treatment of liver cancer.

Antibody Structure and Function: The Basis for Engineering Therapeutics

Antibodies and antibody-derived macromolecules have established themselves as the mainstay in protein-based therapeutic molecules (biologics). Our knowledge of the structure–function relationships of antibodies provides a platform for protein engineering that has been exploited to generate a wide range of biologics for a host of therapeutic indications. In this review, our basic understanding of the antibody structure is described along with how that knowledge has leveraged the engineering of antibody and antibody-related therapeutics having the appropriate antigen affinity, effector function, and biophysical properties. The platforms examined include the development of antibodies, antibody fragments, bispecific antibody, and antibody fusion products, whose efficacy and manufacturability can be improved via humanization, affinity modulation, and stability enhancement. We also review the design and selection of binding arms, and avidity modulation. Different strategies of preparing bispecific and multispecific molecules for an array of therapeutic applications are included.

Simulation and Stability Assessment of Anti-EpCAM Immunotoxin for Cancer Therapy

Purpose: Epithelial cell adhesion molecule (EpCAM) is a dominant antigen in human colon carcinoma tissue. Topology features of this antigen are different in normal and malignant conditions; for instance, EpCAM is much less accessible to antibodies in normal cells than in cancerous tissues. Hence, EpCAM has been considered as a suitable candidate for cancer target therapy via immunotoxins (ITs) development. In this study, attention was focused on the stability assessment of anti-EpCAM-IT (anti-Ep-IT) to design a novel IT.

Methods: The 3D structures of the antibody template and the toxin segment of anti-Ep-IT were retrieved from PDB. Discovery Studio3.0 was used to separate the ligands and water molecules. The antibody (Ab) fragment of anti-Ep-IT was aligned using protein blast (BLAST-p), and SWISS-MODEL database was used for Ab modeling. IT modeling was accomplished using MODELLER 9.15. Also, GROMACS 5.07 was used for molecular dynamic (MD) simulation step. Moreover, ERRAT and RAMPAGE databases were used for quality assessment of the structures.

Results: BLAST-p results indicated that antibody moiety of IT has the highest E-value and query coverage scores to the monoclonal antibody (mAb) 4D5MOC-B. Modeling by SWISS-MODEL provided a reasonable template for Ab portion compared to MODELLER. The best modeled full-length IT with the lowest RMSD values was selected. Finally, RMSD plot for MD stage demonstrated constant values from 7000ps to 20000ps.

Conclusion: In general, both modeling results and their quality evaluations were satisfactory for designing IT. Moreover, RMSD plot revealed that IT stability was preserved during the simulation. Overall, our findings led to modeling and simulation of the anti-Ep-IT with more structural stability.

A new anti-mesothelin antibody targets selectively the membrane-associated form

Mesothelin is a glycosylphosphatidylinositol (GPI)-anchored membrane protein that shows promise as a target for antibody-directed cancer therapy. High levels of soluble forms of the antigen represent a barrier to directing therapy to cellular targets. The ability to develop antibodies that can selectively discriminate between membrane-bound and soluble conformations of a specific protein, and thus target only the membrane-associated antigen, is a substantive issue. We show that use of a tolerance protocol provides a route to such discrimination. Mice were tolerized with soluble mesothelin and a second round of immunizations was performed using mesothelin transfected P815 cells. RNA extracted from splenocytes was used in phage display to obtain mesothelin-specific antigen-binding fragments (Fabs) that were subsequently screened by flow cytometry and ELISA. This approach generated 147 different Fabs in 34 VH-CDR3 families. Utilizing competition assays with soluble protein and mesothelin-containing serum obtained from metastatic cancer patients, 10 of these 34 VH-CDR3 families were found to bind exclusively to the membrane-associated form of mesothelin. Epitope mapping performed for the 1H7 clone showed that it does not recognize GPI anchor. VH-CDR3 sequence analysis of all Fabs showed significant differences between Fabs selective for the membrane-associated form of the antigen and those that recognize both membrane bound and soluble forms. This work demonstrates the potential to generate an antibody specific to the membrane-bound form of mesothelin. 1H7 offers potential for therapeutic application against mesothelin-bearing tumors, which would be largely unaffected by the presence of the soluble antigen.

Novel PSCA targeting scFv-fusion proteins for diagnosis and immunotherapy of prostate cancer

PURPOSE

Despite great progress in the diagnosis and treatment of localized prostate cancer (PCa), there remains a need for new diagnostic markers that can accurately distinguish indolent and aggressive variants. One promising approach is the antibody-based targeting of prostate stem cell antigen (PSCA), which is frequently overexpressed in PCa. Here, we show the construction of a molecular imaging probe comprising a humanized scFv fragment recognizing PSCA genetically fused to an engineered version of the human DNA repair enzyme O6-alkylguanine-DNA alkyltransferase (AGT), the SNAP-tag, enabling specific covalent coupling to various fluorophores for diagnosis of PCa. Furthermore, the recombinant immunotoxin (IT) PSCA(scFv)-ETA' comprising the PSCA(scFv) and a truncated version of Pseudomonas exotoxin A (PE, ETA') was generated.

METHODS

We analyzed the specific binding and internalization behavior of the molecular imaging probe PSCA(scFv)-SNAP in vitro by flow cytometry and live cell imaging, compared to the corresponding IT PSCA(scFv)-ETA'. The cytotoxic activity of PSCA(scFv)-ETA' was tested using cell viability assays. Specific binding was confirmed on formalin-fixed paraffin-embedded tissue specimen of early and advanced PCa.

RESULTS

Alexa Fluor® 647 labeling of PSCA(scFv)-SNAP confirmed selective binding to PSCA, leading to rapid internalization into the target cells. The recombinant IT PSCA(scFv)-ETA' showed selective binding leading to internalization and efficient elimination of target cells.

CONCLUSIONS

Our data demonstrate, for the first time, the specific binding, internalization, and cytotoxicity of a scFv-based fusion protein targeting PSCA. Immunohistochemical staining confirmed the specific ex vivo binding to primary PCa material.

Ranking Differential Drug Activities from Dose-Response Synthetic Lethality Screens

Synthetic lethal screens are used to discover new combination treatments for cancer. In traditional high-throughput synthetic lethal screens, compounds are tested at a single dose, and hit selection is based on threshold activity values from the variance of the efficacy of the compounds tested. The limitation of the single-dose screening for synthetic lethal screens is that it does not allow for the robust detection of differential activities from compound collections with a broad range of potencies and efficacies. There is therefore a need to develop screening approaches that enable the identification of compounds with synthetic lethal effects based on changes in both potency and efficacy. Here we describe the implementation of a dose response-based synthetic lethal screen to find drugs that enhance or mitigate the cytotoxic effect of an immunotoxin protein (HA22). We developed a data analysis framework for the selection of compounds with enhancing or mitigating cytotoxic activities based on the use of dose-response parameters. The data analysis framework includes an ensemble ranking approach that allows the use of multiple dose-response parameters in a nonparametric fashion. Quantitative high-throughput screening (HTS) enables the identification of compounds with synthetic lethal activity not identified by single-dose HTS.

Intraocular Lymphoma Models

Primary vitreoretinal lymphoma (PVRL) is a subtype of primary central nervous system lymphoma (PCNSL), a high-grade, extranodal, non-Hodgkin's lymphoma, predominantly of B-cell origin. PVRL is an aggressive disease with a poor prognosis. Human studies are not ideally suited for the study of intraocular lymphoma pathogenesis or treatment strategies due to the rare nature of the disease, its variable presentation, limited volume of available ocular fluids, and fragility of sampled lymphoma cells. Animal models have been critical in making progress in understanding intraocular lymphoma pathogenesis and investigating potential therapeutic strategies. Early murine models for intraocular lymphoma used intraperitoneal injection of mouse T-cell lymphomas. This was followed by intravitreal T-cell murine models. More recent murine models have used B-cell lymphomas to more closely mimic human disease. The most current B-cell lymphoma models employ a combined approach of inoculating both the mouse vitreous cavity and brain. The challenge in murine models for intraocular lymphoma lies in recreating the clinical features, disease behavior, molecular profile, systemic immunity, and the microenvironment observed in human disease. In the future, animal models will continue to be central to furthering our understanding of the disease and in the investigation of potential treatment targets.

The novel multispecies Fc-specific Pseudomonas exotoxin A fusion protein α-Fc-ETA' enables screening of antibodies for immunotoxin development

Immunoconjugates that deliver cytotoxic payloads to cancer cells represent a promising class of therapeutic agents which are intensively investigated in various clinical applications. Prerequisites for the generation of effective immunoconjugates are antibodies which efficiently deliver the respective cytotoxic payload. To facilitate the selection of human or mouse antibodies that display favorable characteristics as immunotoxins, we developed a novel Pseudomonas exotoxin A (ETA)-based screening protein. The α-Fc-ETA' consists of a multispecies-specific Fc-binding domain antibody genetically fused to a truncated ETA version (ETA'). α-Fc-ETA' non-covalently bound to human and mouse antibodies but did not form immune complexes with bovine immunoglobulins. In combination with antibodies harboring human or mouse Fc domains α-Fc-ETA' inhibited proliferation of antigen-expressing tumor cells. The cytotoxic effects were strictly antibody dependent and were observed with low α-Fc-ETA' concentrations. Mouse antibodies directed against CD7 and CD317/HM1.24 that previously had been used for the generation of functional recombinant immunotoxins, also showed activity in combination with α-Fc-ETA' by inhibiting growth of antigen-positive myeloma and leukemia cell lines. In contrast, α-kappa-ETA', a similarly designed human kappa light chain-specific fusion protein, was only specifically active in combination with antibodies containing a human kappa light chain. Thus, the novel α-Fc-ETA' fusion protein is broadly applicable in screening antibodies and Fc-containing antibody derivatives from different species to select for candidates with favorable characteristics for immunotoxin development.

Lessons learned from a highly-active CD22-specific chimeric antigen receptor

CD22 is an attractive target for the development of immunotherapeutic approaches for the therapy of B-cell malignancies. In particular, an m971 antibody-derived, second generation chimeric antigen receptor (CAR) that targets CD22 holds significant therapeutic promise. The key aspect for the development of such a highly-active CAR was its ability to target a membrane-proximal epitope of CD22.

Development and scale-up of a commercial fed batch refolding process for an anti-CD22 two chain immunotoxin

We describe the development and scale-up of a novel two chain immunotoxin refolding process. This work provides a case study comparing a clinical manufacturing process and the commercial process developed to replace it. While the clinical process produced high quality material, it suffered from low yield and high yield variability. A systematic approach to process development and understanding led to a number of improvements that were implemented in the commercial process. These include a shorter inclusion body recovery process, limiting the formation of an undesired deamidated species and the implementation of fed batch dilution refolding for increased refold titers. The use of a combination of urea, arginine and DTT for capture column cleaning restored the binding capacity of the capture step column and resulted in consistent capture step yields compared to the clinical process. Scalability is shown with data from 250 L and 950 L scale refolding processes. Compared to the clinical process it replaces, the commercial process demonstrated a greater than fivefold improvement in volumetric productivity at the 950 L refolding scale. © 2014 American Institute of Chemical Engineers Biotechnol. Prog., 30:1380–1389, 2014

An improved recombinant Fab-immunotoxin targeting CD22 expressing malignancies

Moxetumomab pasudotox (HA22) is a recombinant immunotoxin, now in clinical trials, that combines an anti-CD22-Fv with a 38-kDa fragment of Pseudomonas exotoxin A. To produce a less immunogenic molecule without reducing the half-life in circulation, we constructed LMB11 combining an anti-CD22 Fab with a less immunogenic version of PE38. We found that LMB11 retains full activity toward CD22-expressing cells. In mice, the half-life of LMB11 is 29 min and the antitumor activity of LMB11 is better than that of HA22. Because it can be safely given at much higher doses, LMB11 produced complete tumor remissions in 7/7 mice.

Novel targeted drug therapies for the treatment of childhood acute leukemia

The cure rates for childhood acute leukemia have dramatically improved to approximately 70% overal, with treatments that include intensive cytotoxic chemotherapy and, in some cases, hematopoietic stem cell transplantation. However, many children still die of their disease or of treatment-related toxicities. Even in patients that are cured, there can be significant and, not uncommonly debilitating, acute and late complications of treatment. Improved understanding of the molecular and cellular biology of leukemia and the increasing availability of high-throughput genomic techniques have facilitated the development of molecularly targeted therapies that have the potential to be more effective and less toxic than the standard approaches. In this article, we review the progress to date with agents that are showing promise in the treatment of childhood acute leukemia, including monoclonal antibodies, inhibitors of kinases and other signaling molecules (e.g., BCR-ABL, FLT3, farnesyltransferase, mTOR and γ-secretase), agents that target epigenetic regulation of gene expression (DNA methyltransferase inhibitors and histone deacetylase inhibitors) and proteasome inhibitors. For the specific agents in each of these classes, we summarize the published preclinical data and the clinical trials that have been completed, are in progress or are being planned for children with acute leukemia. Finally, we discuss potential challenges to the success of molecularly targeted therapy, including proper target identification, adequate targeting of leukemia stem cells, developing synergistic and tolerable combinations of agents and designing adequately powered clinical trials to test efficacy in molecularly defined subsets of patients.

Linked-in: design and efficacy of antibody drug conjugates in oncology

The use of antibody drug conjugates (ADCs) as targeted chemotherapies has successfully entered clinical practice and holds great promise. ADCs consist of an antibody and toxin-drug combined together via a chemical linker. While the antibody and drug are of vital importance in the direct elimination of cancer cells, more advanced linker technology was instrumental in the delivery of more potent drugs with fewer side effects. Here, we discuss the preclinical experience as well as clinical trials, with a specific emphasis on the clinical outcomes and side effects, in addition to linker strategies for five different ADCs, in order to describe different approaches in the development of this new class of anticancer agents. Brentuximab vedotin is approved for use in Hodgkin’s lymphoma and Trastuzumab emtansine is approved for breast cancer. Combotox, Inotuzumab Ozogamicin, and Moxetumomab Pasudotox are in various stages of clinical development and are showing significant efficacy in lymphoid malignancies. These ADCs illustrate the promise and future potential of targeted therapy for presently incurable malignancies.

Anti-CD22-chimeric antigen receptors targeting B-cell precursor acute lymphoblastic leukemia

Immune targeting of B-cell malignancies using chimeric antigen receptors (CARs) is a promising new approach, but critical factors impacting CAR efficacy remain unclear. To test the suitability of targeting CD22 on precursor B-cell acute lymphoblastic leukemia (BCP-ALL), lymphoblasts from 111 patients with BCP-ALL were assayed for CD22 expression and all were found to be CD22-positive, with median CD22 expression levels of 3500 sites/cell. Three distinct binding domains targeting CD22 were fused to various TCR signaling domains ± an IgG heavy chain constant domain (CH2CH3) to create a series of vector constructs suitable to delineate optimal CAR configuration. CARs derived from the m971 anti-CD22 mAb, which targets a proximal CD22 epitope demonstrated superior antileukemic activity compared with those incorporating other binding domains, and addition of a 4-1BB signaling domain to CD28.CD3 constructs diminished potency, whereas increasing affinity of the anti-CD22 binding motif, and extending the CD22 binding domain away from the membrane via CH2CH3 had no effect. We conclude that second-generation m971 mAb-derived anti-CD22 CARs are promising novel therapeutics that should be tested in BCP-ALL.

B-cell prolymphocytic leukemia and hairy cell leukemia: new advances in biology and treatment

SUMMARY B-cell prolymphocytic leukemia and hairy cell leukemia are mature lymphoid neoplasms recognized by WHO classification as distinct entities. The diagnosis relies on a constellation of clinical and laboratory features. B-cell prolymphocytic leukemia is a rare disease that lacks a genetic signature and represents a challenge to clinicians, due to the limited information on its pathogenesis, the difficulty of setting up prospective clinical trials and its refractoriness to treatments used in other chronic lymphoid neoplasms. By contrast, major advances have taken place in the understanding of the pathogenesis of hairy cell leukemia, as well as in the therapeutic armamentarium available for this disease. In this review, data will be presented on the natural history, pathogenesis, diagnosis and current therapies for these two leukemias.

A recombinant immunotoxin against the tumor-associated antigen mesothelin reengineered for high activity, low off-target toxicity, and reduced antigenicity

SS1P is a recombinant immunotoxin (RIT) engineered for the targeted elimination of malignant cells that express the tumor-associated antigen mesothelin. It is composed of an antimesothelin antibody variable fragment (Fv) linked to a cytotoxic fragment of Pseudomonas exotoxin A (PE) that includes domains II and III of native PE. The clinical use of SS1P is limited by its propensity to induce neutralizing antibodies and to cause a dose-limiting capillary leak syndrome (CLS) in patients. In this article, we describe a reengineered SS1P with improved properties that overcome these deficits. The redesign of SS1P consists of (i) removing the bulk of PE domain II (residues 251-273 and 284-394 of native PE), leaving only an 11-residue furin cleavage site, (ii) adding a Gly-Gly-Ser peptide linker after the furin cleavage site, and (iii) replacing eight highly solvent-exposed residues in the catalytic domain of PE. The new molecule, SS1-LR/GGS/8M, has cytotoxic activity comparable with SS1P on several mesothelin-expressing cell lines and remarkably improved activity on primary cells from patients with mesothelioma. In a mouse xenograft tumor model, high doses of SS1-LR/GGS/8M elicit antitumor activity superior to the activity of SS1P at its maximum-tolerated dose. In addition, SS1-LR/GGS/8M has greatly decreased ability to cause CLS in a rat model and reduced antigenicity or reactivity with antibodies to the sera of patients previously treated with SS1P.

Process scale separation of an anti-CD22 immunotoxin charge variant

We describe the analytical characterization and process scale separation of a deamidated variant of an immunotoxin. The different charge variants of the immunotoxin were separated using analytical ion-exchange HPLC. These charge variants were analyzed by peptide mapping and LC-MS/MS to identify the site of modification, which was determined to reside in the toxin portion of the molecule. Using a cell-based bioassay it was also determined that deamidation led to reduced biological activity, requiring it be controlled during manufacturing. This was accomplished using process scale anion-exchange chromatography. The process was capable of reducing the deamidated form to a level low enough for the resulting product to maintain acceptable biological activity. Keys to the successful control of this impurity at process scale were a good understanding of structure-function relationship and the availability of an analytical HPLC assay to provide a surrogate for the cell-based bioassay.

Purification of clinical-grade disulfide stabilized antibody fragment variable--Pseudomonas exotoxin conjugate (dsFv-PE38) expressed in Escherichia coli

Immunotoxins are rationally designed cancer targeting and killing agents. Disulfide stabilized antibody Fv portion-toxin conjugates (dsFv-toxin) are third generation immunotoxins containing only the antibody fragment variable portions and a toxin fused to the V(H) or V(L). Pseudomonas exotoxin fragment (PE-38) is a commonly used toxin in immunotoxin clinical trials. dsFv-toxin purification was previously published, but the recovery was not satisfactory. This report describes the development of a cGMP production process of the dsFv-toxin that incorporated a novel purification method. The method has been successfully applied to the clinical manufacturing of two dsFv-PE38 immunotoxins, MR1-1 targeting EGFRvIII and HA22 targeting CD22. The two subunits, V(L) and V(H) PE-38 were expressed separately in Escherichia coli using recombinant technology. Following cell lysis, inclusion bodies were isolated from the biomass harvested from fermentation in animal source component-free media. The dsFv-toxin was formed after denaturation and refolding, and subsequently purified to homogeneity through ammonium sulfate precipitation, hydrophobic interaction and ion-exchange chromatography steps. It was shown, in a direct comparison experiment using MR1-1 as model protein, that the recovery from the new purification method was improved three times over that from previously published method. The improved recovery was also demonstrated during the clinical production of two dsFv-PE38 immunotoxins-MR1-1 and HA22.

Recombinant immunotoxin engineered for low immunogenicity and antigenicity by identifying and silencing human B-cell epitopes

Recombinant immunotoxins (RITs) are hybrid proteins used to treat cancer. These proteins are composed of an Fv that reacts with cancer cells joined to a portion of Pseudomonas exotoxin A, which kills the cell. Because the toxin is a foreign protein, it can induce neutralizing antibodies and thereby limit the number of doses a patient can receive. We previously identified seven major mouse B-cell epitopes in the toxin, and subsequently silenced them using point mutations that converted large hydrophilic amino acids to alanine, yet retained full antitumor activity. Here we present results in which we identify and silence human B-cell epitopes in the RIT HA22. We obtained B cells from patients with antibodies to RITs, isolated the corresponding variable fragments (Fvs), and constructed a phage-display library containing Fvs that bind to the RITs. We then used alanine scanning mutagenesis to locate the epitopes. We found that human and mouse epitopes frequently overlap but are not identical. Most mutations that remove mouse epitopes did not remove human epitopes. Using the epitope information, we constructed a variant immunotoxin, HA22-LR-LO10, which has low reactivity with human antisera, yet has high cytotoxic and antitumor activity and can be given to mice at high doses without excess toxicity. The toxin portion of this RIT (LR-LO10) can be used with Fvs targeting other cancer antigens and is suitable for clinical development.

Efficacy of an immunotoxin to folate receptor beta in the intra-articular treatment of antigen-induced arthritis

Introduction

We previously demonstrated that synovial sublining macrophages express folate receptor beta (FRβ). The aim of this study was to evaluate the efficacy of intra-articular administration of a recombinant immunotoxin to FRβ for treating rat antigen-induced arthritis.

Methods

A monoclonal antibody (mAb) to rat FRβ was produced by immunizing mice with B300-19 cells (murine pre-B cells) transfected with the rat FRβ gene. Recombinant immunotoxin was prepared by conjugating the Fv portion of the anti-rat FRβ mAb heavy chain with a truncated Pseudomonas exotoxin A and the Fv portion of the anti-rat FRβ mAb light chain. Antigen-induced arthritis was induced through intra-articular injection of methylated bovine serum albumin (mBSA) after two subcutaneous injections of mBSA and complete Freund's adjuvant. Immunotoxin was intra-articularly injected into the arthritis joint every other day for seven days after arthritis onset. Joint swelling was measured and histological scores of inflammation, synovial thickness, cartilage, and bone destruction were determined. Immunohistochemistry was performed to detect osteoclast and osteoclast precursor FRβ-expressing macrophages and cathepsin K-positive cells on day 21.

Results

Intra-articular administration of the immunotoxin attenuated joint swelling (61% suppression; P < 0.01 compared to the control on day 21) and improved histological findings, particularly cartilage and bone destruction (scores of rats treated with control versus the immunotoxin: 2.2 versus 0.5; P < 0.01), by reducing the number of FRβ-expressing macrophages and cathepsin K-positive cells.

Conclusions

Intra-articular administration of an immunotoxin to FRβ is effective for improving rat antigen-induced arthritis.

Pharmacokinetics of recombinant bifunctional fusion proteins

INTRODUCTION

The development of biotechnology has enabled the creation of various recombinant fusion proteins as a new class of biotherapeutics. The uniqueness of fusion proteins lies in their ability to fuse two or more protein domains, providing vast opportunities to generate novel combinations of functions. Pharmacokinetic (PK) studies, which are critical components in preclinical and clinical drug development, have not been fully explored for fusion proteins. The lack of general PK models and study guidelines has become a bottleneck for translation of fusion proteins from basic research to the clinic.

AREAS COVERED

This article reviews the current status of PK studies for fusion proteins, covering the processes that affect PK. According to their PK properties, a classification of fusion proteins is suggested along with examples from the clinic or under development. Current limitations and future perspectives for PK of fusion proteins are also discussed.

EXPERT OPINION

A PK model for bifunctional fusion proteins is presented to highlight the importance of mechanistic studies for a thorough understanding of the PK properties of fusion proteins. The model suggests investigating the receptor binding and subsequent intracellular disposition of individual domains, which can have dramatic impact on the PK of fusion proteins.

Single-chain antibody-based immunotoxins targeting Her2/neu: design optimization and impact of affinity on antitumor efficacy and off-target toxicity

Recombinant immunotoxins, consisting of single-chain variable fragments (scFv) genetically fused to polypeptide toxins, represent potentially effective candidates for cancer therapeutics. We evaluated the affinity of various anti-Her2/neu scFv fused to recombinant gelonin (rGel) and its effect on antitumor efficacy and off-target toxicity. A series of rGel-based immunotoxins were created from the human anti-Her2/neu scFv C6.5 and various affinity mutants (designated ML3-9, MH3-B1, and B1D3) with affinities ranging from 10(-8) to 10(-11) mol/L. Against Her2/neu-overexpressing tumor cells, immunotoxins with increasing affinity displayed improved internalization and enhanced autophagic cytotoxicity. Targeting indices were highest for the highest affinity B1D3/rGel construct. However, the addition of free Her2/neu extracellular domain (ECD) significantly reduced the cytotoxicity of B1D3/rGel because of immune complex formation. In contrast, ECD addition had little impact on the lower affinity constructs in vitro. In vivo studies against established BT474 M1 xenografts showed growth suppression by all immunotoxins. Surprisingly, therapy with the B1D3-rGel induced significant liver toxicity because of immune complex formation with shed Her2/neu antigen in circulation. The MH3-B1/rGel construct with intermediate affinity showed effective tumor growth inhibition without inducing hepatotoxicity or complex formation. These findings show that while high-affinity constructs can be potent antitumor agents, they may also be associated with mistargeting through the facile formation of complexes with soluble antigen leading to significant off-target toxicity. Constructs composed of intermediate-affinity antibodies are also potent agents that are more resistant to immune complex formation. Therefore, affinity is an exceptionally important consideration when evaluating the design and efficacy of targeted therapeutics.

Human kappa light chain targeted Pseudomonas exotoxin A--identifying human antibodies and Fab fragments with favorable characteristics for antibody-drug conjugate development

Antibody-drug conjugates (ADC) represent promising agents for targeted cancer therapy. To allow rational selection of human antibodies with favorable characteristics for ADC development a screening tool was designed obviating the need of preparing individual covalently linked conjugates. Therefore, α-kappa-ETA' was designed as a fusion protein consisting of a human kappa light chain binding antibody fragment and a truncated version of Pseudomonas exotoxin A. α-kappa-ETA' specifically bound to human kappa light chains of human or human-mouse chimeric antibodies and Fab fragments. Antibody-redirected α-kappa-ETA' specifically inhibited proliferation of antigen-expressing cell lines at low toxin and antibody concentrations. Selected antibodies that efficiently delivered α-kappa-ETA' in the novel assay system were used to generate scFv-based covalently linked immunotoxins. These molecules efficiently triggered apoptosis of target cells, indicating that antibodies identified in our assay system can be converted to functional immunoconjugates. Finally, a panel of human epidermal growth factor receptor (EGFR) antibodies was screened--demonstrating favorable characteristics with antibody 2F8. These data suggest that antibodies with potential for Pseudomonas exotoxin A-based ADC development can be identified using the novel α-kappa-ETA' conjugate.

Novel antibody-based proteins for cancer immunotherapy

The relative success of monoclonal antibodies in cancer immunotherapy and the vast manipulation potential of recombinant antibody technology have encouraged the development of novel antibody-based antitumor proteins. Many insightful reagents have been produced, mainly guided by studies on the mechanisms of action associated with complete and durable remissions, results from experimental animal models, and our current knowledge of the human immune system. Strikingly, only a small percent of these new reagents has demonstrated clinical value. Tumor burden, immune evasion, physiological resemblance, and cell plasticity are among the challenges that cancer therapy faces, and a number of antibody-based proteins are already available to deal with many of them. Some of these novel reagents have been shown to specifically increase apoptosis/cell death of tumor cells, recruit and activate immune effectors, and reveal synergistic effects not previously envisioned. In this review, we look into different approaches that have been followed during the past few years to produce these biologics and analyze their relative success, mainly in terms of their clinical performance. The use of antibody-based antitumor proteins, in combination with standard or novel therapies, is showing significant improvements in objective responses, suggesting that these reagents will become important components of the antineoplastic protocols of the future.

Recombinant immunotoxin against B-cell malignancies with no immunogenicity in mice by removal of B-cell epitopes

Many nonhuman proteins have useful pharmacological activities, but are infrequently effective in humans because of their high immunogenicity. A recombinant immunotoxin (HA22, CAT8015, moxetumomab pasudotox) composed of an anti-CD22 antibody variable fragment fused to PE38, a 38-kDa portion of Pseudomonas exotoxin A, has produced many complete remissions in drug-resistant hairy-cell leukemia when several cycles of the agent can be given, but has much less activity when antibodies develop. We have pursued a strategy to deimmunize recombinant immunotoxins by identifying and removing B-cell epitopes. We previously reported that we could eliminate most B-cell epitopes using a combination of point mutations and deletions. Here we show the location and amino acid composition of all of the B-cell epitopes in the remaining 25-kDa portion of Pseudomonas exotoxin. Using this information, we eliminated these epitopes to produce an immunotoxin (HA22-LR-8M) that is fully cytotoxic against malignant B-cell lines, has high cytotoxic activity against cells directly isolated from patients with chronic lymphocytic leukemia, and has excellent antitumor activity in mice. HA22-LR-8M does not induce antibody formation in mice when given repeatedly by intravenous injection and does not induce a secondary antibody response when given to mice previously exposed to HA22. HA22-LR-8M also has greatly reduced antigenicity when exposed to sera from patients who have produced antibodies to HA22. The properties of HA22-LR-8M make it an excellent candidate for further clinical development.

CD22 is a recycling receptor that can shuttle cargo between the cell surface and endosomal compartments of B cells

CD22 is a member of the sialic acid-binding Ig-like lectin (Siglec) family that is known to be a regulator of B cell signaling. Its B cell-specific expression makes it an attractive target for immunotoxin-mediated B cell depletion therapy for the treatment of B cell lymphomas and autoimmune diseases. Although CD22 is well documented to be an endocytic receptor, it is believed that after internalization, it is targeted for degradation. We show in this study that CD22 is instead constitutively recycled to the cell surface. We also find that glycan ligand-based cargo is released from CD22 and accumulates intracellularly as CD22 recycles between the cell surface and endosomal compartments. In contrast, Abs to CD22 do not accumulate but remain bound to CD22 and recycle to the cell surface. The results have implications for development of agents that target CD22 as an endocytic receptor for delivery of cytotoxic cargo to B cells.

Targeting paediatric acute lymphoblastic leukaemia: novel therapies currently in development

Modifications to the treatment of acute lymphoblastic leukaemia (ALL) in children have led to a dramatic increase in survival in the past 40 years. Despite this success, a significant subset of paediatric leukaemia patients either relapse or fail to ever achieve a complete remission. Additionally, some patients necessitate treatment with intensified chemotherapy regimens due to clinical or laboratory findings which identify them as high risk. These patients are unlikely to respond to further minor adjustments to the dosing or timing of administration of the same chemotherapy medications. Many novel targeted therapies for the treatment of childhood ALL provide potential mechanisms to further improve cure rates, and provide the possibility of minimizing toxicity to non-malignant cells, given their specificity to malignant cell phenotypes. This article explores many of the potential targeted therapies in varying stages of development, from those currently in clinical trials to those still being refined in the research laboratory.

Targeted immunotherapy for colorectal cancer: monoclonal antibodies and immunotoxins

Colorectal cancer (CRC) is a major health concern worldwide. It is the third most frequently diagnosed cancer and the second leading cause of cancer death. There currently are a number of treatment options for CRC, however many of them have failed to demonstrate desired therapeutic benefit. Therefore, significant efforts are being directed towards the development of new biological therapies with improved efficacy. Immunotherapy is an emerging treatment modality for a variety of cancers. Several promising treatments have already been approved by the US FDA and are being tested in clinical trials. Antibodies have been proved to be useful in cancer therapy due to their ability to recognize tumor-associated antigens expressed at higher density on malignant cells in comparison with those that are normal. Antibodies can be used as a single therapy or in combination with other therapies. A large variety of monoclonal antibodies have been developed. However, only a very few are able to kill a sufficient number of malignant cells and cause tumor regression. Hence, it is often necessary to arm the antibody with a cytotoxic agent to enhance the efficacy of the anti-tumor activity. This review provides a brief overview of some of the current agents being employed in targeted immunotherapy for CRC.

Preparation and Cytotoxicity Effect of Anti-Hepatocellular Carcinoma Scfv Immunoliposome on Hepatocarcinoma Cell in Vitro

The use of PE38 for cancer therapy has attracted considerable attention for a long time. However, the extensive use of PE38 is prohibited by its severe side effects. Even though immunotoxin PE38 has been researched for cancer therapy, it has displayed low antitumor activity. The aim of this study is to compare the killing efficacy on Hepatocellular carcinoma (HCC) SMMC-7721 cell of immunoliposome PE38, immunotoxin PE38 and liposome PE38. In this study, the sterically stabilized liposomal PE38 was prepared using soybean phosphatidylcholine, cholesterol, and Cholesterol-PEG-COOH. The humanized anti-hepatoma disulfide-stabilized Fv (hdsFv25) was coupled to sterically stabilized liposomes using the N-hydroxysuccinimide ester method. The immunoliposome PE38 was prepared in our lab using the above-mentioned single-chain antibody. The hdsFv25-immunoliposomes were immunoreactive as determined by ELISA assay. Immunoliposome PE38 can kill SMMC-7721 cells in vitro with higher efficiency than non-targeted liposomes. These results indicate that immunoliposome PE38 may be potential in the treatment of hepatocarcinoma.

A recombinant immunotoxin targeting CD22 with low immunogenicity, low nonspecific toxicity, and high antitumor activity in mice

Recombinant immunotoxins (RITs) are genetically engineered proteins designed to kill cancer cells. The RIT HA22 contains the Fv portion of an anti-CD22 antibody fused to a 38 kDa fragment of Pseudomonas exotoxin A (PE38). As PE38 is a bacterial protein, patients frequently produce antibodies that neutralize its activity, preventing retreatment. We have earlier shown in mice that PE38 contains 7 major B-cell epitopes located in domains II and III of the protein. Here we present a new mutant RIT, HA22-LR-6X, in which we removed most B-cell epitopes by deleting domain II and mutating 6 residues in domain III. HA22-LR-6X is cytotoxic to several lymphoma cell lines, has very low nonspecific toxicity, and retains potent antitumor activity in mice with CA46 lymphomas. To assess its immunogenicity, we immunized 3 MHC-divergent strains of mice with 5 microg doses of HA22-LR-6X, and found that HA22-LR-6X elicited significantly lower antibody responses than HA22 or other mutant RITs with fewer epitopes removed. Furthermore, large (50 microg) doses of HA22-LR-6X induced markedly lower antibody responses than 5 microg of HA22, indicating that high doses can be administered with low immunogenicity. Our experiments show that we have correctly identified and removed B-cell epitopes from PE38, producing a highly active immunotoxin with low immunogenicity and low animal toxicity. Future studies will determine if these properties carry over to humans with cancer.

The use of single chain Fv as targeting agents for immunoliposomes: an update on immunoliposomal drugs for cancer treatment

IMPORTANCE OF THE FIELD

Targeted liposomal drugs represent the next evolution of liposomal drug delivery in cancer treatment. In various preclinical cancer models, antibody-targeted PEGylated liposomal drugs have demonstrated superior therapeutic effects over their non-targeted counterparts. Single chain Fv (scFv) has gained popularity in recent years as the targeting agent of choice over traditional targeting agents such as monoclonal antibodies (mAb) and antibody fragments (e.g., Fab').

AREAS COVERED IN THIS REVIEW

This review is focused mainly on advances in scFv-targeted liposomal drug delivery for the treatment of cancers, based on a survey of the recent literature, and on experiments done in a murine model of human B-lymphoma, using anti-CD19 targeted liposomes targeted with whole mAb, Fab' fragments and scFv fragments.

WHAT THE READER WILL GAIN

This review examines the recent advances in PEGylated immunoliposomal drug delivery, focusing on scFv fragments as targeting agents, in comparison with Fab' and mAb.

TAKE HOME MESSAGE

For clinical development, scFv are potentially preferred targeting agents for PEGylated liposomes over mAb and Fab', owing to factors such as decreased immunogenicity, and pharmacokinetics/biodistribution profiles that are similar to non-targeted PEGylated (Stealth) liposomes.

Specific targeting to B cells by lipid-based nanoparticles conjugated with a novel CD22-ScFv

The CD22 antigen is a viable target for therapeutic intervention for B-cell lymphomas. Several therapeutic anti-CD22 antibodies as well as an anti-CD22-based immunotoxin (HA22) are currently under investigation in clinical settings. Coupling of anti-CD22 reagents with a nano-drug delivery vehicle is projected to significantly improve treatment efficacies. Therefore, we generated a mutant of the targeting segment of HA22 (a CD22 scFv) to increase its soluble expression (mut-HA22), and conjugated it to the surface of sonicated liposomes to generate immunoliposomes (mut-HA22-liposomes). We examined liposome binding and uptake by CD22(+) B-lymphocytes (BJAB) by using calcein and/or rhodamine PE-labeled liposomes. We also tested the effect of targeting on cellular toxicity with doxorubicin-loaded liposomes. We report that: (i) Binding of mut-HA22-liposomes to BJAB cells was significantly greater than liposomes not conjugated with mut-HA22 (control liposomes), and mut-HA22-liposomes bind to and are taken in by BJAB cells in a dose and temperature-dependent manner, respectively; (ii) This binding occurred via the interaction with the cellular CD22 as pre-incubation of the cells with mut-HA22 blocked subsequent liposome binding; (iii) Intracellular localization of mut-HA22-liposomes at 37 degrees C but not at 4 degrees C indicated that our targeted liposomes were taken up through an energy dependent process via receptor-mediated endocytosis; and (iv) Mut-HA22-liposomes loaded with doxorubicin exhibited at least 2-3 fold more accumulation of doxorubicin in BJAB cells as compared to control liposomes. Moreover, these liposomes showed at least a 2-4 fold enhanced killing of BJAB or Raji cells (CD22(+)), but not SUP-T1 cells (CD22(-)). Taken together these data suggest that these 2nd-generation liposomes may serve as promising carriers for targeted drug delivery to treat patients suffering from B-cell lymphoma.

Historical aspects and milestones in the development of effective treatment for hairy cell leukemia

Hairy cell leukemia (HCL) is a rare B-cell lymphoproliferative disorder, but is gratifying to treat for both physicians and patients. During the 50 years since its initial description as a clinical entity, hematologists have been fascinated by the bizarre appearance of the malignant cell with its hair-like projections. Therapeutic strategies have evolved from splenectomy to interfere to the purine analogues which have become the treatments of-choice and are very effective. Immunoconjugate Therapy with BL22 (anti-CD22 [corrected] antibody linked to truncated to Pseudomonas exotoxin) represents the newest milestone in the development of effective treatment for hairy cell leukemia.

Antibody-based therapy of leukaemia

Over the past decade, monoclonal antibodies have dramatically impacted the treatment of haematological malignancies, as evidenced by the effect of rituximab on the response rate and survival of patients with follicular and diffuse large B cell non-Hodgkin's lymphoma. Currently, only two monoclonal antibodies - the anti-CD33 immunotoxin gemtuzumab ozogamicin and the CD52-directed antibody alemtuzumab - are approved for treatment of relapsed acute myeloid leukaemia in older patients and B cell chronic lymphocytic leukaemia, respectively. Although not approved for such treatment, alemtuzumab is also active against T cell prolymphocytic leukaemia, cutaneous T cell lymphoma and Sézary syndrome, and adult T cell leukaemia and lymphoma. In addition, rituximab has demonstrated activity against B cell chronic lymphocytic and hairy cell leukaemia. Monoclonal antibodies targeting CD4, CD19, CD20, CD22, CD23, CD25, CD45, CD66 and CD122 are now being studied in the clinic for the treatment of leukaemia. Here, we discuss how these new antibodies have been engineered to reduce immunogenicity and improve antibody targeting and binding. Improved interactions with Fc receptors on immune effector cells can enhance destruction of target cells through antibody-dependent cellular cytotoxicity and complement-mediated cell lysis. The antibodies can also be armed with cellular toxins or radionuclides to enhance the destruction of leukaemia cells.

Identification and characterization of fully human anti-CD22 monoclonal antibodies

CD22 is a member of the B cell receptor family and is implicated in B cell function and development. It is expressed on multiple forms of B cell lymphoma and is an attractive cancer therapeutic target. We report here the identification of two fully human anti-CD22 antibodies using phage display methodology. Both antibodies exhibit specific binding to cell surface-associated CD22 in multiple B cell lines. Through ELISA using mammalian cell-expressed sub-domains of CD22 as binding antigen, we mapped the binding epitopes of the newly identified CD22 antibodies to be within the Ig-like domains 5 to 7 of CD22. Their epitopes do not overlap with those of several therapeutic antibodies currently in preclinical or clinical development. These antibodies have potential as cancer therapeutic candidates and research reagents.

A protease-resistant immunotoxin against CD22 with greatly increased activity against CLL and diminished animal toxicity

Immunotoxins based on Pseudomonas exotoxin A (PE) are promising anticancer agents that combine a variable fragment (Fv) from an antibody to a tumor-associated antigen with a 38-kDa fragment of PE (PE38). The intoxication pathway of PE immunotoxins involves receptor-mediated internalization and trafficking through endosomes/lysosomes, during which the immunotoxin undergoes important proteolytic processing steps but must otherwise remain intact for eventual transport to the cytosol. We have investigated the proteolytic susceptibility of PE38 immunotoxins to lysosomal proteases and found that cleavage clusters within a limited segment of PE38. We subsequently generated mutants containing deletions in this region using HA22, an anti-CD22 Fv-PE38 immunotoxin currently undergoing clinical trials for B-cell malignancies. One mutant, HA22-LR, lacks all identified cleavage sites, is resistant to lysosomal degradation, and retains excellent biologic activity. HA22-LR killed chronic lymphocytic leukemia cells more potently and uniformly than HA22, suggesting that lysosomal protease digestion may limit immunotoxin efficacy unless the susceptible domain is eliminated. Remarkably, mice tolerated doses of HA22-LR at least 10-fold higher than lethal doses of HA22, and these higher doses exhibited markedly enhanced antitumor activity. We conclude that HA22-LR advances the therapeutic efficacy of HA22 by using an approach that may be applicable to other PE-based immunotoxins.

New therapeutic strategies in acute lymphoblastic leukemia

While cure rates of over 80% are achieved in contemporary pediatric acute lymphoblastic leukemia (ALL) protocols, most adults with ALL succumb to their disease, and little progress has been made in the treatment of refractory and relapsed ALL. Moreover, the burden of therapy is high in a significant number of newly diagnosed patients, and in all those with relapse. Early response to therapy measured by minimal residual disease evaluation has proven the single most important prognostic factor and is increasingly used in risk stratification. However, as the benefit from intensification of frontline therapy becomes limiting, it becomes increasingly challenging to rescue patients who fail on contemporary risk-adapted protocols. New therapeutic strategies are needed, not only in salvage regimens but also in frontline protocols for patients who are at high risk of relapse. Current novel approaches include new formulations of existing chemotherapeutic agents, new antimetabolites and nucleoside analogs, monoclonal antibodies against leukemic-associated antigens, cellular immunotherapy, and molecular therapeutics. Some have already been adopted into standard regimens, while others remain in early stages of development. This review summarizes the current status of these novel therapies as they get integrated into ALL regimens.

CAT-8015: a second-generation pseudomonas exotoxin A-based immunotherapy targeting CD22-expressing hematologic malignancies

PURPOSE

To compare the in vitro and in vivo efficacy of CAT-8015, a second-generation recombinant immunotoxin composed of disulfide-linked affinity matured V(H) and V(L) chains of the mouse anti-CD22 monoclonal antibody RFB4 fused to PE38, to the parental compound CAT-3888.

EXPERIMENTAL DESIGN

The biological activity of CAT-8015 was examined in vitro using B-cell tumor lines and in vivo in a JD38-based s.c. tumor model in NCr athymic mice. Pharmacokinetics and interspecies scaling of CAT-8015 were evaluated in mice, rats, and cynomolgus monkeys. The potential toxicity of CAT-8015 was assessed in monkeys in a toxicologic study and compared with CAT-3888.

RESULTS

The IC50 values of CAT-8015 in vitro using the EHEB, MEC1, Daudi, CA46, and JD38 cell lines ranged from 0.3 to 8.6 ng/mL. Pharmacokinetic studies with CAT-8015 were conducted in mouse, rat, and cynomolgus monkey. The t1/2 was calculated to be 0.42, 0.61, and 0.79 hours and the Vss was 1.37, 5.57, and 140.3 mL in mouse, rat, and monkey, respectively. In vivo, when JD38 tumor-bearing animals were treated with CAT-8015 at doses > or =75 microg/kg at 48-hour intervals for a total of three doses, a rapid reduction in tumor volume and in some cases complete remission in tumor growth was observed. The comparative toxicologic study showed comparable clinical and anatomic pathology changes for CAT-8015 and CAT-3888.

CONCLUSIONS

CAT-8015 is a CD22-targeting immunotoxin that, in preclinical studies, has greatly improved efficacy compared with CAT-3888.

Recombinant immunotoxins for the treatment of chemoresistant hematologic malignancies

Recombinant immunotoxins are proteins composed of fragments of monoclonal antibodies fused to truncated protein toxins. No agents of this class are approved yet for medical use, although a related molecule, denileukin diftitox, composed of interleukin-2 fused to truncated diphtheria toxin, is approved for relapsed/refractory cutaneous T-cell lymphoma. Recombinant immunotoxins which have been tested in patients with chemotherapy-pretreated hematologic malignancies include LMB-2 (anti-CD25), BL22 (CAT-3888, anti-CD22) and HA22 (CAT-8015, anti-CD22), each containing an Fv fragment fused to truncated Pseudomonas exotoxin. Major responses were observed with LMB-2 in adult T-cell leukemia, chronic lymphocytic leukemia (CLL), cutaneous T-cell lymphoma, Hodgkin's disease, and hairy cell leukemia (HCL). BL22 resulted in a high complete remission rate in patients with HCL, particularly those without excessive tumor burden. HA22, an improved version of BL22 with higher affinity to CD22, is now undergoing phase I testing in HCL, CLL, non-Hodgkin's lymphoma, and pediatric acute lymphoblastic leukemia.

Soluble CD22 as a tumor marker for hairy cell leukemia

CD22 is an important immunotherapeutic target on B-cell malignancies, particularly hairy cell leukemia (HCL), but its soluble extracellular domain, sCD22, has not yet been reported in the blood. By immunoaffinity and enzyme-linked immunosorbent assay techniques using anti-CD22 monoclonal antibodies, we identified the 100-kDa extracellular domain of CD22 and an 80-kDa processed form in serum of patients with HCL. The median sCD22 level measured by enzyme-linked immunosorbent assay was 18 ng/mL for 93 patients with HCL. sCD22 levels varied from 2.1 to 163 ng/mL and were higher (P < .001) than 23 normal donors (median, 0.6 ng/mL). More than 95% of normal donors had sCD22 levels less than 1.9 ng/mL. sCD22 levels were proportional to concentrations of circulating HCL cells (P = .002), and HCL spleen size (P < .001). sCD22 levels normalized with complete but not partial response to treatment. sCD22 levels up to 300 ng/mL had less than a 2-fold effect on the cytotoxicity of the anti-CD22 recombinant immunotoxin BL22. sCD22 levels may be useful to follow in patients with HCL and may be more specific than sCD25 in patients with CD22(+)/CD25(-) disease. Trials are listed on www.cancer.gov as NCT00002765, NCT00021983, NCT00074048, NCT00085085, NCT00337311, and NCT00462189.

PE38KDEL-loaded anti-HER2 nanoparticles inhibit breast tumor progression with reduced toxicity and immunogenicity

The clinical use of Pseudomonas exotoxin A (PE)-based immunotoxins is limited by the toxicity and immunogenicity of PE. To overcome the limitations, we have developed PE38KDEL-loaded poly(lactic-co-glycolic acid) (PLGA) nanoparticles conjugated with Fab' fragments of a humanized anti-HER2 monoclonal antibody (rhuMAbHER2). The PE38KDEL-loaded nanoparticles-anti-HER2 Fab' bioconjugates (PE-NP-HER) were constructed modularly with Fab' fragments of rhuMAbHER2 covalently linked to PLGA nanoparticles containing PE38KDEL. Compared with nontargeted nanoparticles that lack anti-HER2 Fab', PE-NP-HER specifically bound to and were sequentially internalized into HER2 overexpressing breast cancer cells, which result in significant cytotoxicity in vitro. In HER2 overexpressing tumor xenograft model system, administration of PE-NP-HER showed a superior efficacy in inhibiting tumor growth compared with PE-HER referring to PE38KDEL conjugated directly to rhuMAbHER2. Moreover, PE-NP-HER was well tolerated in mice with a higher LD(50) (LD(50) of 6.86 +/- 0.47 mg/kg vs. 2.21 +/- 0.32 mg/kg for PE-NP-HER vs. PE-HER (mean +/- SD); n = 3), and had no influence on the plasma level of plasma alanine aminotransferase (ALT) of animals when injected at a dose of 1 mg/kg where PE-HER caused significant increase of serum ALT in the treated mice. Notably, PE-NP-HER was of low immunogenicity in development of anti-PE38KDEL neutralizing antibodies and was less susceptible to inactivation by anti-PE38KDEL antibodies compared with PE-HER. This novel bioconjugate, PE-NP-HER, may represent a useful strategy for cancer treatment.

A test of highly optimized tolerance reveals fragile cell-cycle mechanisms are molecular targets in clinical cancer trials

Robustness, a long-recognized property of living systems, allows function in the face of uncertainty while fragility, i.e., extreme sensitivity, can potentially lead to catastrophic failure following seemingly innocuous perturbations. Carlson and Doyle hypothesized that highly-evolved networks, e.g., those involved in cell-cycle regulation, can be resistant to some perturbations while highly sensitive to others. The “robust yet fragile” duality of networks has been termed Highly Optimized Tolerance (HOT) and has been the basis of new lines of inquiry in computational and experimental biology. In this study, we tested the working hypothesis that cell-cycle control architectures obey the HOT paradigm. Three cell-cycle models were analyzed using monte-carlo sensitivity analysis. Overall state sensitivity coefficients, which quantify the robustness or fragility of a given mechanism, were calculated using a monte-carlo strategy with three different numerical techniques along with multiple parameter perturbation strategies to control for possible numerical and sampling artifacts. Approximately 65% of the mechanisms in the G1/S restriction point were responsible for 95% of the sensitivity, conversely, the G2-DNA damage checkpoint showed a much stronger dependence on a few mechanisms; ∼32% or 13 of 40 mechanisms accounted for 95% of the sensitivity. Our analysis predicted that CDC25 and cyclin E mechanisms were strongly implicated in G1/S malfunctions, while fragility in the G2/M checkpoint was predicted to be associated with the regulation of the cyclin B-CDK1 complex. Analysis of a third model containing both G1/S and G2/M checkpoint logic, predicted in addition to mechanisms already mentioned, that translation and programmed proteolysis were also key fragile subsystems. Comparison of the predicted fragile mechanisms with literature and current preclinical and clinical trials suggested a strong correlation between efficacy and fragility. Thus, when taken together, these results support the working hypothesis that cell-cycle control architectures are HOT networks and establish the mathematical estimation and subsequent therapeutic exploitation of fragile mechanisms as a novel strategy for anti-cancer lead generation.