Hans-Peter Gerber

Investigational antibody drug conjugates for solid tumors

Introduction: Despite the progress made in the past 20 years in understanding the molecular events leading to the formation of cancer, the success of targeted antitumor agents in solid tumors has lagged behind the scientific discoveries. The most difficult to treat patient segments are those with refractory solid tumors, resistant to standard chemotherapy, and novel therapeutic compounds with improved therapeutic indexes are needed. Antibody drug conjugates (ADCs) are poised to become an important class of cancer therapeutics, as evidenced by the promising objective response rates when administered as single agents to chemorefractory cancer patients. Areas covered: The basic concept for ADCs is to combine the strengths of the two most successful classes of therapeutic compounds developed in oncology, the high selectivity of antibodies with the unrivaled potency of small molecules, with the goal to improve the therapeutic index. Currently, approximately 60 ADCs are being developed in oncology. Among them, about 20 are undergoing clinical testing, the majority of which are tubulin inhibitor-based immunoconjugates. Herein, we review ADCs targeting solid tumors, with the focus on 11 programs currently undergoing clinical development. Expert opinion: Key challenges the ADC field is facing, including potency and safety, can be addressed effectively by introducing novel research concepts with transformational potential for ADC development.

Advances in bispecific biotherapeutics for the treatment of cancer.

Conventional monoclonal antibody (mAb) therapeutics interfering with cellular signaling of their respective target antigens are frequently limited in their ability to induce significant anti-tumor activities when administered as single agents in patients with solid tumors. To overcome these limitations, several new technologies are being developed to empower biotherapeutics and to improve their anti-tumor activities, while maintaining their high tumor selectivity and superior safety profiles. The various efficacy enhancement technologies developed for mAbs can be divided broadly into two categories: First, technologies that improve the intrinsic anti-tumor activities of conventional immunoglobulin mAb formats, including the enhancement of effector cell functions and modulations of target binding properties, including interference with multiple signaling pathways. The second category of empowered biologics combines complementary anti-tumor modalities independent of the IgG format, including antibody drug conjugates (ADCs). In addition, bispecific compounds designed to recruit different subsets of inflammatory cells to the tumor environment, also belong to the mechanistic complementation strategy. This approach termed redirected immune cell killing, belongs to one the most promising new biotherapeutic platforms developed in oncology. Over 20 bispecific compounds are currently being developed pre-clinically, and several compounds are undergoing early stage clinical trials. In this report, we review the progress made in the development of bispecific biotherapeutics in the context of ADCs, redirected T- and B-cell killing and targeting of multiple signaling pathways. We also discuss the status of the clinical development of this class of compounds in oncology and the promises and challenges this field is currently facing.

Long-term Tumor Regression Induced by an Antibody–Drug Conjugate That Targets 5T4, an Oncofetal Antigen Expressed on Tumor-Initiating Cells

Antibody–drug conjugates (ADC) represent a promising therapeutic modality for the clinical management of cancer. We sought to develop a novel ADC that targets 5T4, an oncofetal antigen expressed on tumor-initiating cells (TIC), which comprise the most aggressive cell population in the tumor. We optimized an anti-5T4 ADC (A1mcMMAF) by sulfydryl-based conjugation of the humanized A1 antibody to the tubulin inhibitor monomethylauristatin F (MMAF) via a maleimidocaproyl linker. A1mcMMAF exhibited potent in vivo antitumor activity in a variety of tumor models and induced long-term regressions for up to 100 days after the last dose. Strikingly, animals showed pathologic complete response in each model with doses as low as 3 mg antibody/kg dosed every 4 days. In a non–small cell lung cancer patient-derived xenograft model, in which 5T4 is preferentially expressed on the less differentiated tumor cells, A1mcMMAF treatment resulted in sustained tumor regressions and reduced TIC frequency. These results highlight the potential of ADCs that target the most aggressive cell populations within tumors, such as TICs. In exploratory safety studies, A1mcMMAF exhibited no overt toxicities when administered to cynomolgus monkeys at doses up to 10 mg antibody/kg/cycle × 2 and displayed a half-life of 5 days. The preclinical efficacy and safety data established a promising therapeutic index that supports clinical testing of A1mcMMAF. Mol Cancer Ther; 12(1); 38–47. ©2012 AACR.

Advances in patient-derived tumor xenografts: From target identification to predicting clinical response rates in oncology

Most oncology compounds entering clinical development have passed stringent preclinical pharmacology evaluation criteria. However, only a small fraction of experimental agents induce meaningful antitumor activities in the clinic. Low predictability of conventional preclinical pharmacology models is frequently cited as a main reason for the unusually high clinical attrition rates of therapeutic compounds in oncology. Therefore, improvement in the predictive values of preclinical efficacy models for clinical outcome holds great promise to reduce the clinical attrition rates of experimental compounds. Recent reports suggest that pharmacology studies conducted with patient derived xenograft (PDX) tumors are more predictive for clinical outcome compared to conventional, cell line derived xenograft (CDX) models, in particular when therapeutic compounds were tested at clinically relevant doses (CRDs). Moreover, the study of the most malignant cell types within tumors, the tumor initiating cells (TICs), relies on the availability of preclinical models that mimic the lineage hierarchy of cells within tumors. PDX models were shown to more closely recapitulate the heterogeneity of patient tumors and maintain the molecular, genetic, and histological complexity of human tumors during early stages of sequential passaging in mice, rendering them ideal tools to study the responses of TICs, tumor- and stromal cells to therapeutic intervention. In this commentary, we review the progress made in the development of PDX models in key areas of oncology research, including target identification and validation, tumor indication search and the development of a biomarker hypothesis that can be tested in the clinic to identify patients that will benefit most from therapeutic intervention.

Preclinical and clinical development of inotuzumab-ozogamicin in hematological malignancies☆

Calicheamicin is a DNA-damaging agent that, following intracellular activation, binds to DNA in the minor groove and introduces double-strand DNA breaks, leading to G2/M arrest and subsequent cell death. Importantly, the mechanism of action of calicheamicin is fundamentally different from the tubulin-binding class of cytotoxics targeting the mitotic spindle, which represent the most common class of payloads for antibody-drug conjugates (ADCs) currently undergoing clinical development. Spindle poisons that target tubulin, including auristatins and maytansines, are most effective against rapidly proliferating cells. In contrast, calicheamicin induces DNA double-strand breaks and apoptosis independent of cell cycle progression. Such properties may be advantageous when targeting malignant cells that are not markedly different in their proliferation status compared to normal cells. Here we review calicheamicin conjugates, with a particular focus on the preclinical- and clinical development of inotuzumab ozogamicin, targeting the CD22 antigen expressed on a large variety of hematologic malignancies. In pre-clinical experiments, inotuzumab ozogamicin potently induced tumor regressions in models of non-Hodgkins lymphoma (NHL), either alone or in combination with the anti-CD20 antibody Rituximab. Promising anti-tumor responses were observed in early stage clinical trials, where inotuzumab ozogamicin was administered either as single agent or in combination with Rituximab. Consistent with the cell cycle independent mechanism of action of the calicheamicin payload, high rates of complete responses were observed in less aggressive forms of lymphomas, including follicular lymphoma (FL) and relapsed, diffuse large B-cell lymphoma (DLBCL). Inotuzumab ozogamicin is currently being tested in phase III clinical trials in acute lymphocytic leukemia (ALL). Particular focus is dedicated to reviewing the pre-clinical and clinical data generated with this compound in NHL and to outline future focus areas for pre-clinical- and clinical research of inotuzumab ozogamicin, and the calicheamicin class of antibody-drug conjugates more generally.

Tumor Cells Chronically Treated with a Trastuzumab–Maytansinoid Antibody–Drug Conjugate Develop Varied Resistance Mechanisms but Respond to Alternate Treatments

Antibody–drug conjugates (ADC) are emerging as clinically effective therapy. We hypothesized that cancers treated with ADCs would acquire resistance mechanisms unique to immunoconjugate therapy and that changing ADC components may overcome resistance. Breast cancer cell lines were exposed to multiple cycles of anti-Her2 trastuzumab–maytansinoid ADC (TM-ADC) at IC80 concentrations followed by recovery. The resistant cells, 361-TM and JIMT1-TM, were characterized by cytotoxicity, proteomic, transcriptional, and other profiling. Approximately 250-fold resistance to TM-ADC developed in 361-TM cells, and cross-resistance was observed to other non–cleavable-linked ADCs. Strikingly, these 361-TM cells retained sensitivity to ADCs containing cleavable mcValCitPABC-linked auristatins. In JIMT1-TM cells, 16-fold resistance to TM-ADC developed, with cross-resistance to other trastuzumab-ADCs. Both 361-TM and JIMT1-TM cells showed minimal resistance to unconjugated mertansine (DM1) and other chemotherapeutics. Proteomics and immunoblots detected increased ABCC1 (MRP1) drug efflux protein in 361-TM cells, and decreased Her2 (ErbB2) in JIMT1-TM cells. Proteomics also showed alterations in various pathways upon chronic exposure to the drug in both cell models. Tumors derived from 361-TM cells grew in mice and were refractory to TM-ADC compared with parental cells. Hence, acquired resistance to trastuzumab–maytansinoid ADC was generated in cultured cancer cells by chronic drug treatment, and either increased ABCC1 protein or reduced Her2 antigen were primary mediators of resistance. These ADC-resistant cell models retain sensitivity to other ADCs or standard-of-care chemotherapeutics, suggesting that alternate therapies may overcome acquired ADC resistance. Mol Cancer Ther; 14(4); 952–63. ©2015 AACR.

Combining antibody–drug conjugates and immune-mediated cancer therapy: What to expect?

Blockade of immune-checkpoints has emerged as one of the most promising approaches to improve the durability of anti-tumor responses in cancer patients. However, the fraction of patients experiencing durable responses to single agent immune checkpoint inhibitor treatment remains limited. Recent clinical reports suggest that patients responding best to checkpoint blockade therapies display higher levels of CD8(+) T-cells in the tumor prior to treatment. Therefore, combination treatments of immune-checkpoint inhibitors with compounds that increase the number of tumor infiltrating CD8(+) T cells may expand the therapeutic benefit of immuno-oncology (IO) drugs. Immunogenic cell death (ICD) of tumor cells is induced by certain classes of cytotoxic compounds and represents a potent stimulator of effector T-cell recruitment to tumors. In addition, several cytotoxics directly stimulate dendritic cell activation and maturation, resulting in improved anti-tumor immune responses when combined with IO compounds. Among them, several cytotoxic agents are currently utilized as payloads for antibody-drug conjugates (ADCs). Therefore, identification of optimal combination regimens between ADC- and IO compounds holds strong promise to overcome the current limitations of immune checkpoint inhibitors, by increasing the recruitment of CD8(+) effector T-cells to the tumor core. Here we review the emerging field of ADC/IO combination research, with a focus on how to optimally combine both modalities. The answer to this question may have a broader impact on oncology drug development, as synergistic activities between IO compounds and ADCs may increase the formation of tumor specific immunological memory, ultimately leading to durable responses in a larger fraction of cancer patients.

Anti-EFNA4 Calicheamicin Conjugates Effectively Target Triple-Negative Breast and Ovarian Tumor-Initiating Cells To Result In Sustained Tumor Regressions

Purpose: Triple-negative breast cancer (TNBC) and ovarian cancer each comprise heterogeneous tumors, for which current therapies have little clinical benefit. Novel therapies that target and eradicate tumor-initiating cells (TIC) are needed to significantly improve survival. Experimental Design: A panel of well-annotated patient-derived xenografts (PDX) was established, and surface markers that enriched for TIC in specific tumor subtypes were empirically determined. The TICs were queried for overexpressed antigens, one of which was selected to be the target of an antibody–drug conjugate (ADC). The efficacy of the ADC was evaluated in 15 PDX models to generate hypotheses for patient stratification. Results: We herein identified E-cadherin (CD324) as a surface antigen able to reproducibly enrich for TIC in well-annotated, low-passage TNBC and ovarian cancer PDXs. Gene expression analysis of TIC led to the identification of Ephrin-A4 (EFNA4) as a prospective therapeutic target. An ADC comprising a humanized anti-EFNA4 monoclonal antibody conjugated to the DNA-damaging agent calicheamicin achieved sustained tumor regressions in both TNBC and ovarian cancer PDX in vivo. Non-claudin low TNBC tumors exhibited higher expression and more robust responses than other breast cancer subtypes, suggesting a specific translational application for tumor subclassification. Conclusions: These findings demonstrate the potential of PF-06647263 (anti–EFNA4-ADC) as a first-in-class compound designed to eradicate TIC. The use of well-annotated PDX for drug discovery enabled the identification of a novel TIC target, pharmacologic evaluation of the compound, and translational studies to inform clinical development. Clin Cancer Res; 21(18); 4165–73. ©2015 AACR.

Emerging immunotherapies targeting CD30 in Hodgkin's lymphoma

The immunotherapy of Hodgkins lymphoma (HL) has been particularly challenging because of the unique features of tumor intrinsic and host mediated factors, interfering with the antitumor activities of therapeutic antibodies. Despite a wide array of compounds tested successfully in preclinical studies, immunotherapy in HL patients resulted in only limited success when compared to the significant improvements in patient survival provided by chemotherapeutic agents. Antibody-drug conjugates (ADCs) may surmount the restrictions posed by the unique pathobiology of HL tumors as they combine the selective tumor targeting of monoclonal antibodies with the potent anti-neoplastic activities of cytotoxic drugs. In early clinical trials, this class of compounds induced robust antitumor effects in patients with relapsed or refractory lymphoproliferative diseases, in the absence of overt toxicities, while naked antibodies failed to induce therapeutic benefit. Here we review some of the unique features of HL tumor biology and the key advantages of ADC-based lymphoma therapies, which may ultimately account for the improved therapeutic benefit provided by ADCs compared to first generation immunotherapeutics tested in HL patients.

Mechanisms of Resistance to Antibody-Drug Conjugates.

Drug resistance limits the effectiveness of cancer therapies. Despite attempts to develop curative anticancer treatments, tumors evolve evasive mechanisms limiting durable responses. Hence, diverse therapies are used to attack cancer, including cytotoxic and targeted agents. Antibody–drug conjugates (ADC) are biotherapeutics designed to deliver potent cytotoxins to cancer cells via tumor-specific antigens. Little is known about the clinical manifestations of drug resistance to this class of therapy; however, recent preclinical studies reveal potential mechanisms of resistance. Because ADCs are a combination of antibody and small molecule cytotoxin, multifactorial modes of resistance are emerging that are inherent to the structure and function of the ADC. Decreased cell-surface antigen reduces antibody binding, whereas elevated drug transporters such as MDR1 and MRP1 reduce effectiveness of the payload. Inherent to the uniqueness of the ADC, other novel resistance mechanisms are emerging, including altered antibody trafficking, ADC processing, and intracellular drug release. Most importantly, the modular nature of the ADC allows components to be switched and replaced, enabling development of second-generation ADCs that overcome acquired resistance. This review is intended to highlight recent progress in our understanding of ADC resistance, including approaches to create preclinical ADC-refractory models and to characterize their emerging mechanisms of resistance. Mol Cancer Ther; 15(12); 2825–34. ©2016 AACR.