Preclinical investigation of the antitumour effects of anti-CD19-idarubicin immunoconjugates

Payload diversification: a key step in the development of antibody-drug conjugates

Antibody-drug conjugates (ADCs) is a fast moving class of targeted biotherapeutics that currently combines the selectivity of monoclonal antibodies with the potency of a payload consisting of cytotoxic agents. For many years microtubule targeting and DNA-intercalating agents were at the forefront of ADC development. The recent approval and clinical success of trastuzumab deruxtecan (Enhertu^®) and sacituzumab govitecan (Trodelvy^®), two topoisomerase 1 inhibitor-based ADCs, has shown the potential of conjugating unconventional payloads with differentiated mechanisms of action. Among future developments in the ADC field, payload diversification is expected to play a key role as illustrated by a growing number of preclinical and clinical stage unconventional payload-conjugated ADCs. This review presents a comprehensive overview of validated, forgotten and newly developed payloads with different mechanisms of action.

Potential drugs used in the antibody-drug conjugate (ADC) architecture for cancer therapy

Cytotoxic small-molecule drugs have a major influence on the fate of antibody-drug conjugates (ADCs). An ideal cytotoxic agent should be highly potent, remain stable while linked to ADCs, kill the targeted tumor cell upon internalization and release from the ADCs, and maintain its activity in multidrug-resistant tumor cells. Lessons learned from successful and failed experiences in ADC development resulted in remarkable progress in the discovery and development of novel highly potent small molecules. A better understanding of such small-molecule drugs is important for development of effective ADCs. The present review discusses requirements making a payload appropriate for antitumor ADCs and focuses on the main characteristics of commonly-used cytotoxic payloads that showed acceptable results in clinical trials. In addition, the present study represents emerging trends and recent advances of payloads used in ADCs currently under clinical trials.

Antibody-drug conjugates (ADCs) for cancer therapy: Strategies, challenges, and successes

Targeted delivery of therapeutic molecules into cancer cells is considered as a promising strategy to tackle cancer. Antibody-drug conjugates (ADCs), in which a monoclonal antibody (mAb) is conjugated to biologically active drugs through chemical linkers, have emerged as a promising class of anticancer treatment agents, being one of the fastest growing fields in cancer therapy. The failure of early ADCs led researchers to explore strategies to develop more effective and improved ADCs with lower levels of unconjugated mAbs and more-stable linkers between the drug and the antibody, which show improved pharmacokinetic properties, therapeutic indexes, and safety profiles. Such improvements resulted in the US Food and Drug Administration approvals of brentuximab vedotin, trastuzumab emtansine, and, more recently, inotuzumab ozogamicin. In addition, recent clinical outcomes have sparked additional interest, which leads to the dramatically increased number of ADCs in clinical development. The present review explores ADCs, their main characteristics, and new research developments, as well as discusses strategies for the selection of the most appropriate target antigens, mAbs, cytotoxic drugs, linkers, and conjugation chemistries.

Linker technologies for antibody-drug conjugates

Antibody-drug conjugates (ADCs), which combine the specificity, favorable pharmacokinetics, and biodistribution of a monoclonal antibody (mAb) with the cytotoxic potency of a drug, are promising new therapies for cancer. Along with the development of monoclonal antibodies (mAbs) and cytotoxic drugs, the design of the linker is of essential importance, because it impacts the efficacy and tolerability of ADCs. The linker needs to provide sufficient stability during systemic circulation but allow for the rapid and efficient release of the cytotoxic drug in an active form inside the tumor cells. This review provides an overview of linker technologies currently used for ADCs and advances that have resulted in linkers with improved properties. Also provided is a brief summary of some considerations for the conjugation of antibody and drug linker such as drug-to-antibody ratio and site of conjugation.

Immunotoxins and anticancer drug conjugate assemblies: the role of the linkage between components

Immunotoxins and antibody-drug conjugates are protein-based drugs combining a target-specific binding domain with a cytotoxic domain. Such compounds are potentially therapeutic against diseases including cancer, and several clinical trials have shown encouraging results. Although the targeted elimination of malignant cells is an elegant concept, there are numerous practical challenges that limit conjugates’ therapeutic use, including inefficient cellular uptake, low cytotoxicity, and off-target effects. During the preparation of immunoconjugates by chemical synthesis, the choice of the hinge component joining the two building blocks is of paramount importance: the conjugate must remain stable in vivo but must afford efficient release of the toxic moiety when the target is reached. Vast efforts have been made, and the present article reviews strategies employed in developing immunoconjugates, focusing on the evolution of chemical linkers.

A nonfucosylated human antibody to CD19 with potent B-cell depletive activity for therapy of B-cell malignancies

A human anti-CD19 antibody was expressed in fucosyltransferase-deficient CHO cells to generate nonfucosylated MDX-1342. Binding of MDX-1342 to human CD19-expressing cells was similar to its fucosylated parental antibody. However, MDX-1342 exhibited increased affinity for FcγRIIIa-Phe158 and FcγRIIIa-Val158 receptors as well as enhanced effector cell function, as demonstrated by increased potency and efficacy in antibody-dependent cellular cytotoxicity (ADCC) and phagocytosis assays. MDX-1342 showed dose-dependent improvement in survival using a murine B-cell lymphoma model in which Ramos cells were administered systemically. In addition, low nanomolar binding to cynomolgus monkey CD19 and increased affinity for cynomolgus monkey FcγRIIIa was observed. In vivo administration of MDX-1342 in cynomolgus monkeys revealed potent B-cell depletion, suggesting its potential utility as a B-lymphocyte depletive therapy for malignancies and autoimmune indications.

Potent in vitro and in vivo activity of an Fc-engineered anti-CD19 monoclonal antibody against lymphoma and leukemia

CD19 is a pan B-cell surface receptor expressed from pro-B-cell development until its down-regulation during terminal differentiation into plasma cells. CD19 represents an attractive immunotherapy target for cancers of lymphoid origin due to its high expression levels on the vast majority of non-Hodgkin's lymphomas and some leukemias. A humanized anti-CD19 antibody with an engineered Fc domain (XmAb5574) was generated to increase binding to Fcgamma receptors on immune cells and thus increase Fc-mediated effector functions. In vitro, XmAb5574 enhanced antibody-dependent cell-mediated cytotoxicity 100-fold to 1,000-fold relative to an anti-CD19 IgG1 analogue against a broad range of B-lymphoma and leukemia cell lines. Furthermore, XmAb5574 conferred antibody-dependent cell-mediated cytotoxicity against patient-derived acute lymphoblastic leukemia and mantle cell lymphoma cells, whereas the IgG1 analogue was inactive. XmAb5574 also increased antibody-dependent cellular phagocytosis and apoptosis. In vivo, XmAb5574 significantly inhibited lymphoma growth in prophylactic and established mouse xenograft models, and showed more potent antitumor activity than its IgG1 analogue. Comparisons with a variant incapable of Fcgamma receptor binding showed that engagement of these receptors is critical for optimal antitumor efficacy. These results suggest that XmAb5574 exhibits potent tumor cytotoxicity via direct and indirect effector functions and thus warrants clinical evaluation as an immunotherapeutic for CD19(+) hematologic malignancies.

Targeted drug conjugates: principles and progress

Reports of targeting drugs using antibodies have appeared in the literature since 1958, but exciting clinical results in this field have only been reported in the last few years. Progress in this field has occurred largely through an understanding how drug-immunoconjugates work. The objective of this review is to draw together the fundamental principles on which this field of work is based, to examine the evidence supporting those principles, and the effectiveness and selectivity of targeted drug conjugates. The activity of many drug-immunoconjugates can now largely be accounted for by the underlying principles. Excellent development work, both with conventional anti-cancer agents and very potent drugs have led to a number of interesting clinical trials. In the best Phase I and II trials, good evidence of effectiveness have been reported, which suggest that drug-immunoconjugates may now be heralding a new era for chemotherapy.

Receptor-mediated and enzyme-dependent targeting of cytotoxic anticancer drugs

This review is a survey of various approaches to targeting cytotoxic anticancer drugs to tumors primarily through biomolecules expressed by cancer cells or associated vasculature and stroma. These include monoclonal antibody immunoconjugates; enzyme prodrug therapies, such as antibody-directed enzyme prodrug therapy, gene-directed enzyme prodrug therapy, and bacterial-directed enzyme prodrug therapy; and metabolism-based therapies that seek to exploit increased tumor expression of, e.g., proteases, low-density lipoprotein receptors, hormones, and adhesion molecules. Following a discussion of factors that positively and negatively affect drug delivery to solid tumors, we concentrate on a mechanistic understanding of selective drug release or generation at the tumor site.

Tumour eradication with high-dose idarubicin-anti-Ly-2.1 with murine tumour necrosis factor-alpha in mice

The antitumour efficacy of Ida-anti-Ly-2.1 immunoconjugate in combination with murine TNF-alpha (mTNF-alpha) has been evaluated in a disseminated murine thymic lymphoma model E3. This tumour model shows characteristics similar to lymphoma in humans with widely disseminated tumour in major organs. In vitro both idarubicin (Ida) and Ida-anti-Ly-2.1 showed additive and synergistic cytotoxic effects, respectively, with mTNF-alpha when tested using the isobolagram method. Similar synergistic effects were also seen in vivo. Tumour-bearing mice were treated with various doses of mTNF-alpha and Ida alone, however, neither showed a therapeutic response in mice. When mice were treated with Ida-anti-Ly-2.1 'early' the median survival time (MST) was increased by 8 days, however, 'late' treatment was ineffective. Combination therapy of mTNF-alpha and Ida-anti-Ly-2.1 conjugate (42 micrograms) given 'early' was capable of curing 50% of mice. However, when tumour-bearing mice were given a high dose (234 micrograms) of Ida-anti-Ly-2.1 together with mTNF-alpha 100% of mice survived disease free. Such an effect was not observed when free mTNF-alpha, Ida-anti-Ly-2.1, anti-Ly-2.1 or a mixture of anti-Ly-2.1+mTNF-alpha was given at the same dose. These results show that a combination of very toxic drug immunoconjugates and TNF could lead to the eradication of disseminated tumour in mice and may be relevant for the treatment of minimal residual disease in cancer patients.

CD19 antigen in leukemia and lymphoma diagnosis and immunotherapy

The CD19 antigen plays an important role in clinical oncology. In normal cells, it is the most ubiquitously expressed protein in the B lymphocyte lineage. CD19 expression is induced at the point of B lineage commitment during the differentiation of the hematopoietic stem cell, and its expression continues through preB and mature B cell differentiation until it is finally down-regulated during terminal differentiation into plasma cells. CD19 expression is maintained in B-lineage cells that have undergone neoplastic transformation, and therefore CD19 is useful in diagnosis of leukemias and lymphomas using monoclonal antibodies (mAbs) and flow cytometry. Interestingly, CD19 is also expressed in a subset of acute myelogenous leukemias (AMLs) indicating the close relationship between the lymphoid and myeloid lineages. Because B lineage leukemias and lymphomas rarely lose CD19 expression, and because it is not expressed in the pluripotent stem cell, it has become the target for a variety of immunotherapeutic agents, including immunotoxins. Treatment of non-Hodgkin's lymphoma (NHL) and acute lymphocytic leukemia (ALL) with anti-CD19 mAbs coupled to biological toxins has proven to be effective in vitro and in animal models, and has shown some promising results in Phase I clinical trials. Recently, the analysis of anti-CD19 effects on lymphoma cell growth has highlighted a novel mechanism of immunotherapy. Engagement of cell surface receptors like CD19 by mAbs can have anti-tumor effects by the activation of signal transduction pathways which control cell cycle progression and programmed cell death (apoptosis).

In vitro and in vivo antitumor activity of a chimeric anti-CD19 antibody

Mouse monoclonal antibodies to CD19 detect an antigenic determinant expressed exclusively on the surface of B lymphocytes, and have previously been shown to be potentially useful therapeutic reagents for human B cell lymphoma. We report the production and characterization of a mouse/human chimeric antibody, cCD19, with potent in vivo antitumour activity. The genes encoding the variable domains for heavy (VH) and light (VL) chains were subcloned into eukaryotic expression vectors containing human constant region genes (IgG1 and kappa), and co-transfected into non-secreting Sp2/0 mouse myeloma cells. Intraperitoneal administration of cCD19 produced inhibition of growth of subcutaneous CD19+ Sultan human B lymphoma tumours in scid/scid mice. When the antibody was administered 18 and 20 days after subcutaneous tumour inoculation, an approximately 30% reduction in tumour size was noted by day 29. cCD19 faithfully mimicked the in vitro binding characteristics of mCD19 as (a) the chimeric antibody was shown by flow cytometry to bind exclusively to cell lines that expressed CD19, (b) cCD19 was able to inhibit the binding of mCD19 on CD19+ cells completely and (c) the affinity of binding of the two antibodies was not significantly different [Ka = (2.03 +/- 1.5) x 10(8)]. In bio-distribution studies, up to 14.8% of the total injected antibody dose per gram of tissue was localized in CD19+ Sultan tumours at 24 h approximately, 14.4% was present in the tumors at 48 h, and about 13.7% at 72 h. These levels were comparable to mCD19 administered in the same fashion. cCD19 conjugated to idarubicin was specifically and strongly cytotoxic to CD19+ cells cultured in vitro, and demonstrated an IC50 of 0.17 microM, similar to that of mCD19 (0.32 microM) and approximately 14-fold greater than the IC50 of free idarubicin. The specific cytotoxic capacity of cCD19 and its likely reduced immunogenicity suggest that it may potentially be of use in the treatment of refractory B cell lymphoma in humans.

Antibody-targeted drugs for the therapy of cancer

The advent of monoclonal antibodies has revitalised the concept of magic bullets and various agents (eg. drugs, toxins and isotopes) have been conjugated to monoclonal antibodies for selective delivery to tumours. Preclinical studies in mouse tumour models have been impressive and have lead to several clinical trials. These phase I trials have been less impressive. However, keeping in mind the aim of Phase I trials, the safety of using these conjugates in humans have been established. Several, major problems still remain to be overcome before these agents may be useful for the treatment of cancer. These problems stem from the nature of tumour vasculature, cytotoxic activity of the moiety linked to antibody and the targeted tumour antigen expressed on the cell surface. This review will deal with these various aspects described above and possible approaches to overcome these obstacles with a definite bias towards drug-monoclonal antibody conjugates. However, these concepts are equally applicable for improved targeting of other agents.