Surface charge-modification prevents sequestration and enhances tumor-cell specificity of a recombinant granzyme B-TGFα fusion protein

Modeling antibody drug conjugate potential using a granzyme B antibody fusion protein

BACKGROUND

Antibody drug conjugates (ADCs) constitute a promising class of targeted anti-tumor therapeutics that harness the selectivity of monoclonal antibodies with the potency of cytotoxic drugs. ADC development is best suited to initially screening antibody candidates for desired properties that potentiate target cell cytotoxicity. However, validating and producing an optimally designed ADC requires expertise and resources not readily available to certain laboratories.

RESULTS

In this study, we propose a novel approach to help streamline the identification of potential ADC candidates by utilizing a granzyme B (GrB)-based antibody fusion protein (AFP) for preliminary screening. GrB is a non-immunogenic serine protease expressed by immune effector cells such as CD8 + T cells that induces apoptotic activity and can be leveraged for targeted cell killing.

CONCLUSIONS

Our innovative model allows critical antibody parameters (including target cell binding, internalization, and cytotoxic potential) to be more reliably evaluated in vitro through the creation of an ADC surrogate. Successful incorporation of this AFP could also significantly expand and enhance ADC development pre-clinically, ultimately leading to the accelerated translation of ADC therapies for patients.

Critical Issues in the Development of Immunotoxins for Anticancer Therapy

Immunotoxins (ITs) are attractive anticancer modalities aimed at cancer-specific delivery of highly potent cytotoxic protein toxins. An IT consists of a targeting domain (an antibody, cytokine, or another cell-binding protein) chemically conjugated or recombinantly fused to a highly cytotoxic payload (a bacterial and plant toxin or human cytotoxic protein). The mode of action of ITs is killing designated cancer cells through the effector function of toxins in the cytosol after cellular internalization via the targeted cell-specific receptor-mediated endocytosis. Although numerous ITs of diverse structures have been tested in the past decades, only 3 ITs-denileukin diftitox, tagraxofusp, and moxetumomab pasudotox-have been clinically approved for treating hematological cancers. No ITs against solid tumors have been approved for clinical use. In this review, we discuss critical research and development issues associated with ITs that limit their clinical success as well as strategies to overcome these obstacles. The issues include off-target and on-target toxicities, immunogenicity, human cytotoxic proteins, antigen target selection, cytosolic delivery efficacy, solid-tumor targeting, and developability. To realize the therapeutic promise of ITs, novel strategies for safe and effective cytosolic delivery into designated tumors, including solid tumors, are urgently needed.

Targeted human cytolytic fusion proteins at the cutting edge: harnessing the apoptosis-inducing properties of human enzymes for the selective elimination of tumor cells

Patient-specific targeted therapy represents the holy grail of anti-cancer therapeutics, allowing potent tumor depletion without detrimental off-target toxicities. Disease-specific monoclonal antibodies have been employed to bind to oncogenic cell-surface receptors, representing the earliest form of immunotherapy. Targeted drug delivery was first achieved by means of antibody-drug conjugates, which exploit the differential expression of tumor-associated antigens as a guiding mechanism for the specific delivery of chemically-conjugated chemotherapeutic agents to diseased target cells. Biotechnological advances have expanded the repertoire of immunology-based tumor-targeting strategies, also paving the way for the next intuitive step in targeted drug delivery: the construction of recombinant protein drugs consisting of an antibody-based targeting domain genetically fused with a cytotoxic peptide, known as an immunotoxin. However, the most potent protein toxins have typically been derived from bacterial or plant virulence factors and commonly feature both off-target toxicity and immunogenicity in human patients. Further refinement of immunotoxin technology thus led to the replacement of monoclonal antibodies with humanized antibody derivatives, including the substitution of non-human toxic peptides with human cytolytic proteins. Preclinically tested human cytolytic fusion proteins (hCFPs) have proven promising as non-immunogenic combinatory anti-cancer agents, however they still require further enhancement to achieve convincing candidacy as a single-mode therapeutic. To date, a portfolio of highly potent human toxins has been established; ranging from microtubule-associated protein tau (MAP tau), RNases, granzyme B (GrB) and death-associated protein kinase (DAPk). In this review, we discuss the most recent findings on the use of these apoptosis-inducing hCFPs for the treatment of various cancers.

Human Granzyme B Based Targeted Cytolytic Fusion Proteins

Cancer immunotherapy aims to selectively target and kill tumor cells whilst limiting the damage to healthy tissues. Controlled delivery of plant, bacterial and human toxins or enzymes has been shown to promote the induction of apoptosis in cancerous cells. The 4th generation of targeted effectors are being designed to be as humanized as possible—a solution to the problem of immunogenicity encountered with existing generations. Granzymes are serine proteases which naturally function in humans as integral cytolytic effectors during the programmed cell death of cancerous and pathogen-infected cells. Secreted predominantly by cytotoxic T lymphocytes and natural killer cells, granzymes function mechanistically by caspase-dependent or caspase-independent pathways. These natural characteristics make granzymes one of the most promising human enzymes for use in the development of fusion protein-based targeted therapeutic strategies for various cancers. In this review, we explore research involving the use of granzymes as cytolytic effectors fused to antibody fragments as selective binding domains.

Phase Separation Behavior of Supercharged Proteins and Polyelectrolytes

Membraneless organelles, like membrane-bound organelles, are essential to cell homeostasis and provide discrete cellular subcompartments. Unlike classical organelles, membraneless organelles possess no physical barrier but rather arise by phase separation of the organelle components from the surrounding cytoplasm or nucleoplasm. Complex coacervation, the liquid-liquid phase separation of oppositely charged polyelectrolytes, is one of several phenomena that are hypothesized to drive the formation and regulation of some membraneless organelles. Studies of the molecular properties of globular proteins that drive complex coacervation are limited as many proteins do not form complexes with oppositely charged macromolecules at neutral pH and moderate ionic strengths. Protein supercharging overcomes this problem and drives complexation with oppositely charged macromolecules. In this work, several distinct cationic supercharged green fluorescent protein (GFP) variants were designed to examine the phase behavior with oppositely charged polyanionic macromolecules. Cationic GFP variants phase separated with oppositely charged macromolecules at various mixing ratios, salt concentrations, and pH values. Efficient protein incorporation in the macromolecule rich phase occurred over a range of protein and polymer mass fractions, but the protein encapsulation efficiency was highest at the midpoint of the phase separation regime. More positively charged proteins phase separated over broader pH and salt ranges than those of proteins with a lower charge density. Interestingly, each GFP variant phase separated at higher salt concentrations with anionic synthetic macromolecules compared to anionic biological macromolecules. Optical microscopy revealed that most variants, depending on solution conditions, formed liquid-liquid phase separations, except for GFP/DNA pairs that formed solid aggregates under all tested conditions.

Generation of an artificial human B cell line test system using Transpo-mAbTM technology to evaluate the therapeutic efficacy of novel antigen-specific fusion proteins

The antigen-specific targeting of autoreactive B cells via their unique B cell receptors (BCRs) is a novel and promising alternative to the systemic suppression of humoral immunity. We generated and characterized cytolytic fusion proteins based on an existing immunotoxin comprising tetanus toxoid fragment C (TTC) as the targeting component and the modified Pseudomonas aeruginosa exotoxin A (ETA') as the cytotoxic component. The immunotoxin was reconfigured to replace ETA' with either the granzyme B mutant R201K or MAPTau as human effector domains. The novel cytolytic fusion proteins were characterized with a recombinant human lymphocytic cell line developed using Transpo-mAb™ technology. Genes encoding a chimeric TTC-reactive immunoglobulin G were successfully integrated into the genome of the precursor B cell line REH so that the cells could present TTC-reactive BCRs on their surface. These cells were used to investigate the specific cytotoxicity of GrB(R201K)-TTC and TTC-MAPTau, revealing that the serpin proteinase inhibitor 9-resistant granzyme B R201K mutant induced apoptosis specifically in the lymphocytic cell line. Our data confirm that antigen-based fusion proteins containing granzyme B (R201K) are suitable candidates for the depletion of autoreactive B cells.

A novel fully-human cytolytic fusion protein based on granzyme B shows in vitro cytotoxicity and ex vivo binding to solid tumors overexpressing the epidermal growth factor receptor

Human cytolytic fusion proteins (hCFPs) offer a promising immunotherapeutic approach for the treatment of solid tumors, avoiding the immunogenicity and undesirable side-effects caused by immunotoxins derived from plants or bacteria. The well-characterized human serine protease granzyme B has already been used as a therapeutic pro-apoptotic effector domain. We therefore developed a novel recombinant hCFP (GbR201K-scFv1711) consisting of an epidermal growth factor receptor-specific human antibody fragment and a granzyme B point mutant (R201K) that is insensitive to serpin B9 (PI9), a natural inhibitor of wild-type granzyme B that is often expressed in solid tumors. We found that GbR201K-scFv1711 selectively bound to epidermoid cancer and rhabdomyosarcoma cells and was rapidly internalized by them. Nanomolar concentrations of GbR201K-scFv1711 achieved the specific killing of epidermoid cancer cells by inducing apoptosis, and similar effects were observed in rhabdomyosarcoma cells when GbR201K-scFv1711 was combined with the endosomolytic substance chloroquine. The novel hCFP was stable in serum and bound to human rhabdomyosarcoma tissue ex vivo. These data confirm that GbR201K-scFv1711 is a promising therapeutic candidate suitable for further clinical investigation.

The functionalized human serine protease granzyme B/VEGF₁₂₁ targets tumor vasculature and ablates tumor growth

The serine protease granzyme B (GrB) induces apoptosis through both caspase-dependent and -independent multiple-cascade mechanisms. VEGF₁₂₁ binds to both VEGF receptor (VEGFR)-1 and VEGFR-2 receptors. We engineered a unique GrB/VEGF₁₂₁ fusion protein and characterized its properties in vitro and in vivo. Endothelial and tumor cell lines showed varying levels of sensitivity to GrB/VEGF₁₂₁ that correlated closely to total VEGFR-2 expression. GrB/VEGF₁₂₁ localized efficiently into VEGFR-2-expressing cells, whereas the internalization into VEGFR-1-expressing cells was significantly reduced. Treatment of VEGFR-2(+) cells caused mitochondrial depolarization in 48% of cells by 48 hours. Exposure to GrB/VEGF₁₂₁ induced apoptosis in VEGFR-2(+), but not in VEGFR-1(+), cells and rapid caspase activation was observed that could not be inhibited by treatment with a pan-caspase inhibitor. In vivo, GrB/VEGF₁₂₁ localized in perivascular tumor areas adjacent to microvessels and in other areas in the tumor less well vascularized, whereas free GrB did not specifically localize to tumor tissue. Administration (intravenous) of GrB/VEGF₁₂₁ to mice at doses up to 40 mg/kg showed no toxicity. Treatment of mice bearing established PC-3 tumor xenografts with GrB/VEGF₁₂₁ showed significant antitumor effect versus treatment with GrB or saline. Treatment with GrB/VEGF₁₂₁ at 27 mg/kg resulted in the regression of four of five tumors in this group. Tumors showed a two-fold lower Ki-67-labeling index compared with controls. Our results show that targeted delivery of GrB to tumor vascular endothelial cells or to tumor cells activates apoptotic cascades and this completely human construct may have significant therapeutic potential.

EGFR-targeted granzyme B expressed in NK cells enhances natural cytotoxicity and mediates specific killing of tumor cells

Natural killer (NK) cells are highly specialized effectors of the innate immune system that hold promise for adoptive cancer immunotherapy. Their cell killing activity is primarily mediated by the pro-apoptotic serine protease granzyme B (GrB), which enters targets cells with the help of the pore-forming protein perforin. We investigated expression of a chimeric GrB fusion protein in NK cells as a means to augment their antitumoral activity. For selective targeting to tumor cells, we fused the epidermal growth factor receptor (EGFR) peptide ligand transforming growth factor α (TGFα) to human pre-pro-GrB. Established human NKL natural killer cells transduced with a lentiviral vector expressed this GrB-TGFα (GrB-T) molecule in amounts comparable to endogenous wildtype GrB. Activation of the genetically modified NK cells by cognate target cells resulted in the release of GrB-T together with endogenous granzymes and perforin, which augmented the effector cells' natural cytotoxicity against NK-sensitive tumor cells. Likewise, GrB-T was released into the extracellular space upon induction of degranulation with PMA and ionomycin. Secreted GrB-T fusion protein displayed specific binding to EGFR-overexpressing tumor cells, enzymatic activity, and selective target cell killing in the presence of an endosomolytic activity. Our data demonstrate that ectopic expression of a targeted GrB fusion protein in NK cells is feasible and can enhance antitumoral activity of the effector cells.