Elements of an Antibody-Drug Conjugate (ADC)

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References: 1. Carter PJ et al. Cancer J. 2008;14(3):154-169. 2. Senter PD. Curr Opin Chem Biol. 2009;13(3):235-244. 3. Polson AG et al. Cancer Res. 2009;69(6):2358-2364.

Primary Mechanism of Action of ADCs:

Targeted Delivery of a Cytotoxic Agent

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Reference: Carter PJ et al. Cancer J. 2008;14(3):154-169.

Source: Antibody-drug conjugates (ADCs): empowering monoclonal antibodies to fight cancer. Seattle Genetics website. seagen.com. Published June 2011. Accessed May 29, 2012. Reprinted with permission.

Antibodies directed against tumor cell antigens or overexpressed proteins are currently the fastest-growing class of targeted cancer therapeutics. However, their success has been tempered by their limited efficacy as single agents. They are often combined with chemotherapeutic agents, which are much more effective at killing cells, but lack the selectivity of antibodies and, as a result, often have side effects as the drugs attack both normal and cancerous cells indiscriminately.

The concept of combining the two approaches in the hope of achieving the best of both worlds is not novel, but has proved more difficult than expected to put into practice. The first generation of so-called antibody-drug conjugates (ADCs) met with disappointing results, and the first FDA-approved agent, Mylotarg (gemtuzumab ozogamicin; Pfizer) was withdrawn from the market in mid-2010.

In recent years, we have come to appreciate the complexity of these molecules, and further research has led to the development of increasingly refined ADCs. These second- and even third-generation molecules are showing great promise. Last year, the FDA approved brentuximab vedotin (Adcetris; Seattle Genetics) for the treatment of Hodgkin lymphoma and systemic anaplastic large cell lymphoma. Positive phase III trial results for trastuzumab emtansine (T-DM1; Genentech) for the treatment of HER2-positive metastatic breast cancer were recently announced, and many other new agents are entering clinical trials. (Table Below)

What Are Antibody-Drug Conjugates?

ADCs are therapeutic agents designed to target the delivery of chemotherapy to tumor cells. ADCs link cytotoxic agents to monoclonal antibodies that bind to tumor cell-specific antigens or to antigens that are overexpressed on the surface of tumor cells. The antibody acts as a sort of GPS system, and the theory is that this should increase delivery of potent cell-killing drugs to the tumor, while reducing the exposure of normal cells.

ADCs are made up of three parts: a monoclonal antibody, a cytotoxic drug, and a linker that joins the two together. The antibody guides the ADC to target tumor cells, where it binds to cell surface antigens. The ADC is then taken into the cell and the cytotoxic drug is released to perform its cell-killing function.

The Antibody

An important factor in the successful development of ADCs is the selection of wellcharacterized antigens to serve as the target for the antibody. The full expression pattern of the antigen throughout the body and on both healthy and tumor cells must be taken into consideration in order to avoid unwanted toxic side effects. Likewise, the expression of tumor-specific antigens within the tumor itself may be heterogeneous, with some cells displaying the antigen and others not, and this can affect the efficacy of the ADC.

A number of tumor-associated antigens have been investigated as potential targets for the antibody component of ADCs. Blood cell cancers, including leukemia, lymphoma, and multiple myeloma are often chosen as cancer types in which to study ADCs, since malignant blood cells are more accessible to antibodies than solid tumors. As such, B- and T-cell surface proteins are frequently chosen as target antigens since they are widely expressed on the surface of malignant B and T cells in these types of cancer.

Targets include B-cell surface proteins such as CD20, CD22, CD40, and CD79, and T-cell surface proteins such as CD25 and CD30, as well as proteins that are overexpressed on carcinoma cells, including the human epidermal growth factor receptor 2 (HER2), epidermal growth factor receptor (EGFR), prostate-specific membrane antigen (PSMA), and Cryptic family protein 1B (Cripto).

Targeting the growth of new blood vessels (angiogenesis) and reducing the tumor blood supply is also a potentially promising option for ADCs, since angiogenesis is a rare process in healthy adults, but is a common hallmark of invasive cancers. The anti-vascular endothelial growth factor (VEGF) monoclonal antibody bevacizumab (Avastin; Genentech) is being investigated as a possible component of ADCs. Other targets include endothelial cell-linked antigens such as integrins or endoglin.

Research is also being directed to developing a new class of vascular-targeting ADCs that release their cytotoxin into the extracellular space. The advantage of this approach is that it offers more comprehensive tumor coverage and doesn’t depend on the expression of a cell-surface antigen.

Target Antigen Should Be Abundantly Expressed on Tumor Cells Compared to Healthy Cells

Expressed abundantly

on tumor cells^1,2

Limited or no expression on normal or vital tissues^1,2

Efficient internalization of target antigen increases drug delivery and enhances cell-killing^1,3

References: 1. Alley SC et al. Curr Opin Chem Biol. 2010;14(4):529-537. 2. Carter PJ et al.

Cancer J. 2008;14(3):154-169. 3. Polson AG et al. Expert Opin Investig Drugs. 2011;20(1):75-85.

Other Potential ADC Antitumor Activities

Some ADCs may retain mAb-mediated anticancer activities^1,2

Apoptosis through direct intracellular signaling³

Tumor lysis through host immune effector cells³

References: 1. Carter PJ et al. Cancer J. 2008;14(3):154-169. 2. Junttila TT et al [published online ahead of print August 21, 2010]. Breast Cancer Res Treat. doi:10.1007/s10549-010-1090-x. 3. Sharkey RM et al. CA Cancer J Clin. 2006;56(4):226-243.

Source: Antibody-drug conjugates (ADCs): empowering monoclonal antibodies to fight cancer. Seattle Genetics website. seagen.com. Published June 2011. Accessed May 29, 2012. Reprinted with permission.

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The Cytotoxic Agent

Three main types of drugs are used as the cytotoxic component of ADCs: calicheamicin, maytansinoids, and auristatins. They fall into two classes, according to the mechanism via which they lead to cell death, with the two latter types causing the unraveling of structural fibers in the cell, and calicheamycin causing irreparable DNA damage.

In order to maximize killing potential, the cytotoxic agent needs to be highly potent. The amount of ADC uptake has been found to be less than 0.1% of the total injected dose per gram of tumor. Therefore, using highly toxic drugs, such as maytansine derivatives, which are 100- to 10, 000-fold more potent than standard chemotherapy agents, ensures that what little gets into the cell has the maximum effect.

The Linker

The linker region is extremely important to ADC design, as a delicate balance must be struck between stability in the bloodstream (to ensure the toxin is not released prematurely) and efficient release of the payload once inside the cell.

Cleavable or releasable linkers that are broken down inside the cell, allowing drug release, have been heavily investigated. Initially, hydrazone linkers (which are broken down only in the acidic environment of the cell) were used. However, these often suffer from poor stability in the blood, meaning that the antibody can fall off, and the errant ADC ”missile” either fails to detonate or strikes multiple targets around the body, rendering it ineffective or causing toxic side effects.

Next-Generation ADCs: Designed to be Stable in Circulation and Release a Cytotoxic Agent in Target Cells

Newer ADC linker technology is intended to spare non-targeted cells and thus reduce many of the toxic effects of traditional chemotherapy while enhancing the antitumor activity^1,2,3

References: 1. Ducry L et al. Bioconjug Chem. 2010;21(1):5-13. 2. Alley SC et al. Curr Opin Chem Biol. 2010;14(4):529-537. 3. Teicher BA.Curr Cancer Drug Targets. 2009;9(8):982-1004.

Antibody-drug conjugates (ADCs). Seattle Genetics website. Reprinted with permission.

In recent years, many companies have begun to develop a range of linker chemistries that improve the stability of the ADC, while still allowing efficient drug release in the tumor cell. These include disulfide-based linkers (selectively broken down inside the tumor cell where the concentration of thiols is higher than in the blood) and peptide linkers (broken down by enzymes found inside the tumor cell).

Some companies have even begun to use noncleavable linkers, including thioether linkers. It was initially thought that the linker had to be cleaved in order for the drug to become activated; however, it was discovered somewhat by accident that the linker can be degraded by the cellular protein degradation machinery, leading to drug activation. Another linker type that was recently described is the PEG4Mal linker, which is designed to target drug-resistant tumor cells as it cannot be pumped through multidrug resistance channels on the cell surface.

No linker type is necessarily better than another; they may in fact be suited to different ADC designs and cancer types. However, cleavable linkers may be able to pass back out of the cell and attack surrounding cells, in a phenomenon dubbed “bystander killing.” This may be advantageous in certain situations but not in others.

New Approaches Take Shape

Mylotarg was the first ADC to be granted approval by the FDA, over a decade ago, for the treatment of relapsed CD33-positive acute myelogenous leukemia in older patients. However, it failed to demonstrate a clinical benefit in postmarketing studies and was withdrawn from the market in 2010.

Interest in ADCs has been reignited in recent years, a fact that is reflected in the number of companies investing in ADC research and new agents reaching late-stage development, with brentuximab approved in August 2011. Researchers believe that further improvements can still be made to ADC design, and a third generation of agents is already emerging.

ADC Therapeutics Sarl is developing ADCs based on a new type of cytotoxic agent, pyrrolobenzodiazepines, developed by Spirogen and researchers at University College, London. Mersana Therapeutics is focusing on developing ADCs using their Fleximer polymer backbone and customized linkers, designed to optimally link antibody and drug. Alternative targeting systems to antibodies are also in the early stages of evaluation. As companies pursue these and other efforts to combine optimized antibody, linker, and cytotoxin technology, the future looks extremely bright for these targeted agents.

Table. The ADC Landscape

Approved

Brentuximab vedotin

(Adcetris; Seattle Genetics)

Approved by the FDA for the treatment of Hodgkin lymphoma (HL) and systemic anaplastic large cell lymphoma (ALCL) in August 2011, brentuximab is the first new drug for the treatment of HL in more than 30 years. It consists of an anti-CD30 antibody conjugated to the antimitotic agent monomethyl auristatin E (MMAE). CD30 is a cell membrane protein of the tumor necrosis factor receptor family.

It was granted accelerated approval based on two single-arm multicenter clinical trials of patients with CD30-positive HL after failure of autologous stem cell transplant and patients with CD30-positive systemic ALCL who had previously received chemotherapy. The objective response rates (ORR) were 73% and 86%, respectively, while the complete remission (CR) rates were 32% and 57%, respectively, and the partial remission (PR) rates were 40% and 29%, respectively.

Late-Stage

Trastuzumab emtansine

(T-DM1; Genentech)

T-DM1 combines the anti-HER2 antibody trastuzumab (Herceptin) with a derivative of the cytotoxin maytansine, called DM1. It is designed to selectively bind to HER2 receptors that are often overexpressed on tumor cells, particularly in patients with breast cancer.

In the phase III EMILIA trial, T-DM1 extended median progression-free survival by 3.2 months in women with HER2-positive locally advanced or metastatic breast cancer compared with standard therapy of capecitabine plus lapatinib (9.6 months vs 6.4 months, respectively), according to findings presented at the American Society of Clinical Oncology (ASCO) annual meeting in June.

Genentech has announced its intention to submit a biologics license application to the FDA, and its European counterpart, Roche, plans to submit a marketing authorization application to the European Medicines Agency.

Inotuzumab ozogamicin

(CMC-544; Pfizer)

Despite disappointment over the withdrawal of its flagship ADC Mylotarg in 2010, Pfizer is trying again with inotuzumab ozogamicin, a monoclonal anti-CD22 antibody linked to an antitumor agent from the calicheamicin class. CD22 is a molecule found on the surface of mature B cells. This agent is currently in phase III trials in combination with the anti-CD20 antibody rituximab for patients with relapsed/refractory aggressive non-Hodgkin lymphoma (NHL). NCT01232556

Selected Earlier-Stage Agents

Lorvotuzumab mertansine

(IMGN901; ImmunoGen)

IMGN901 consists of an anti-CD56 antibody linked to DM-1. CD56 is a molecule expressed on the surface of neurons, glia, skeletal muscle, and natural killer cells, which has a role in cell-to-cell adhesion, among other things. ImmunoGen recently initiated a phase II trial, the NORTH trial, to assess the impact of adding this agent to standard care (carboplatin plus etoposide) for the first-line treatment of small-cell lung cancer (SCLC). It is also being assessed in phase I/II trials in patients with solid tumors. NCT01237678

Glembatumumab vedotin

(CDX-011; Celldex Therapeutics)

CDX-011 is an anti-GPNMB monoclonal antibody conjugated to monomethyl auristatin E (MMAE). Glycoprotein NMB (GPNMB) is a transmembrane protein that is overexpressed in many different types of cancer. It is currently being evaluated in the EMERGE trial, a phase II trial of patients with advanced GPNMB-expressing breast cancer. NCT01156753

SAR3419

(Sanofi)

Sanofi is developing SAR3419, consisting of an anti-CD19 monoclonal antibody (CD19 is an antigen found on the surface of B-cells) linked to maytansine. It is undergoing phase II trials for relapsed/refractory diffuse large B-cell lymphoma (DLBCL), acute lymphoblastic leukemia, and in combination with rituximab in patients with relapsed/refractory DLBCL. NCT01472887; NCT01440179; NCT01470456

SGN-75

(Seattle Genetics)

An anti-CD70 antibody conjugated to monomethyl auristatin F (MMAF), SGN-75 is currently undergoing phase I trials in patients with non-Hodgkin lymphoma or renal cell carcinoma. CD70 is a ligand for CD27, a receptor for tumor necrosis factor. NCT01015911

AGS-16M8F

(Astellas Pharma)

An anti-AGS-16 monoclonal antibody linked to MMAF, this agent is being studied in phase I trials in patients with advanced renal cell carcinoma. AGS-16 is a cell-surface molecule expressed in more than 95% of kidney cancers and approximately 40% of hepatocellular carcinomas. NCT01114230.

ASG-5ME

(Astellas Pharma)

Astellas is evaluating ASG-5ME for the treatment of prostate cancer in phase I trials. It consists of an anti-AGS-5 monoclonal antibody conjugated to monomethyl auristatin E (MMAE). AGS-5 is a transmembrane antigen highly expressed on epithelial tumors. NCT01228760

BIIB015

(Biogen Idec)

This agent, which is composed of an anti-Cripto monoclonal antibody linked to the maytansine derivative DM4, recently completed phase I trials in solid tumors. Cripto acts as a receptor for the TGF beta-signaling pathway. NCT00674947

BT062

(Biotest Pharmaceuticals)

An anti-CD138 monoclonal antibody linked to DM1 that is currently undergoing phase I/II trials in patients with relapsed/refractory multiple myeloma. CD138 is a transmembrane protein predominantly expressed on mature B cells. NCT01001442

PSMA ADC

(Progenics Pharmaceuticals)

An anti-prostate-specific membrane antigen antibody (PSMA) linked to MMAE, being evaluated in phase I trials in patients with prostate cancer. Preclinical data from the first in-human evaluation of PSMA ADC were presented at last year's ASCO meeting and the drug was shown to exhibit dose-proportional pharmacokinetics. NCT01414283

Key Research

Jane de Lartigue, PhD, is a freelance medical writer and editor based in the United Kingdom.

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