Therapeutic Antibodies in Review

Recombinant therapeutic antibodies have remained a bright spot in a pharmaceutical industry beset with difficulties in recent years, bringing out a large number of blockbuster drugs. Indeed, during the past couple of years, at least four new antibodies were approved by FDA, namely, Benlysta (belimumab, HGS/Glaxo), Yervoy (ipilimumab, BMS), Adcetris (brentuximab vedotin, Seattle Genetics), and Perjeta (pertuzumab, Genentech/Roche), which have significant revenue potential. While the number of approvals did not set any records in terms of the number of market approvals, it is indeed a remarkable feat for the industry, at least by virtue of the wide array of the functional diversity of the approved antibodies. For example, Benlysta (approved in 2011) is a B-lymphocyte stimulator (BLyS)-specific inhibitor, and is the first FDA-approved immune medication specifically designed for the treatment of lupus. Yervoy (approved in 2011) blocks the modulation of T-cell activity carried out by CTLA-4. Adcetris (approved in 2011) is the first approved antibody-drug conjugate (ADC) from Seattle Genetics, and is also the latest approved ADC in the monoclonal antibody (mAb) therapeutic armamentarium since the market withdrawal by Pfizer of Mylotarg, the first approved mAb-chemotherapy conjugate. With the first regulatory approval of Perjeta in 2012 for use as a combination therapy with trastazumab and docetaxel, the path for future approval of other combination antibody therapeutic products is expected to be significantly eased.

(COURTESY: INGRAM PUBLISHING/GETTY IMAGES)

The commercial advancement of such a diverse array of antibody therapeutics has brought new enthusiasm and fueled an optimistic spirit within the industry concerning the discovery and commercialization of "next-generation" antibodies. This observation was borne out at the IBC Antibody Engineering and Antibody Therapeutics conference held in San Diego in December 2012. In addition to immunomodulatory antibodies and ADCs, bitargeted, bispecific, and multifunctional antibodies were prominently featured in the conference. Furthermore, combination antibody products in the presence or absence of chemotherapeutics were highlighted as one of the significant advancements towards the next generation of antibody therapeutics.

K. John Morrow, Jr., PhD

A few of the salient advances in the field are presented below, along with predictions on the future outcome in this crucial area of drug development.

ANTIBODY DRUG CONJUGATES

Early efforts at developing ADCs for cancer therapeutics were beset by failures (1). One of the most crucial challenges has been the instability of conjugates, whose breakdown may result in the release of a highly toxic, free drug molecule into the patient's circulation. Other serious side effects include binding of the antibody component to nontarget tissues, and concentration of ADCs in the liver and kidneys. The tremendous variability of tumors, allowing them to rapidly generate antigenic variants unresponsive to the ADC, is another vexing source of the loss of efficacy (2). This is, of course, an issue with all anticancer therapies, a consequence of the rampant genetic and epigenetic variability of tumor cells that allows them to rapidly develop resistance to chemotherapeutic intervention.

Rathin C. Das, PhD

There is no single route to the abrogation of these shortcomings, as investigators have determined that every case must be resolved individually. But, over the years, these issues have been addressed one-by-one, and a new generation of highly effective ADCs is being developed with the help of technological advances and a more profound understanding of their mechanism of action in three essential areas: better targets, better linkers, and better toxins (see Figure 1).

Figure 1: Primary mechanism of action of antibody-drug conjugates: targeted delivery of a potent cytotoxic agent to cause cell death.

Better targets

An ideal target would be the one which is expressed at a high copy number in diseased cells relative to normal, is rapidly internalized, and is not down regulated. For example, human epidermal growth factor receptor (HER2, target of Genentech/Roche's T-DM1, currently under review by FDA for approval) and CD-30, the target antigen of Adcetris, generally satisfy these criteria.

Better linkers

One of the primary reasons for the recent success of ADCs as a therapeutic modality is the technological advancement in linker technology, that is, the chemical group that forms a bridge between the toxin and the antibody. These linkers must combine the property of stability in the circulation with cleavability once the conjugate is internalized within the target cell.

Better toxins

A toxic payload must be sufficiently lethal to effectively kill the target cancer cells. Early studies with ADCs that employed doxorubicin were ineffective, and R&D teams focused on a search for more potent molecules such as DM-1 or the auristatins. Emtansine, or DM-1, developed by Immunogen is a derivative of the chemotherapeutic agent maytansine. Emtansine is 100-fold to 10,000-fold more potent than its parent compound. Auristatins are synthetic antineoplastic compounds developed by Seattle Genetics.

ADC developers

Seattle Genetics obtained FDA approval for its ADC product Adcetris (brentuximab, Brentuximabvedotin). Adcetris is composed of an anti-CD30 antibody linked to an auristatin derivative called vedotin, and is approved to treat Hodgkin lymphoma and a rare lymphoma, systemic anaplastic large cell lymphoma. Additionally, the company is conducting several other late-stage clinical studies in several lymphatic cancer indications.

Immunogen is another leading company in the ADC landscape. Genentech/Roche's Trastuzumab emtansine (T-DM1) is one of the leading representatives of promising ADCs, which uses Immunogen's DM-1 toxin system. Composed of the HER2-targeted antibody trastuzumab, a stable thioether linker, and the potent cytotoxic agent DM1, it is in phase III development for HER2-positive cancer. DM-1 possesses in vitro cytotoxicity that is up to 200 times greater than other tubulin inhibitors, such as the taxanes and vinca alkaloids. In the conjugate phase, however, it behaves as a prodrug and is not toxic. When it enters the cell via the endosomes, the thioether bond is broken, releasing the toxin into the cell where it can bring about the demise of the target cancer cell (see Figure 1).

Recently, Roche announced results from the Phase III EMILIA study, which clearly demonstrate that previously treated patients with HER2-positive metastatic breast cancer survived significantly longer when treated with T-DM1 compared with those who received the combination of lapatinib and Xeloda (capecitabine) (3).

NEW MAB APPROVALS

Lucentis (ranibizumab)

In August 2012, the earlier approval of the drug was expanded by FDA to include diabetic macular edema. This anti-angiogenic Fab fragment blocks VEGF-A in the eye, aimed at preventing vision loss caused by wet macular degeneration.

Perjeta (pertuzumab)

As a HER-2/neu receptor antagonist, Perjeta was approved for combined use with trastuzumab and docetaxel for patients suffering from metastatic breast cancer. When used as a first line treatment, the combination significantly prolonged progression-free survival (4).

Raxibacumab

The antibody was approved for the treatment of inhalational anthrax, a form of this infectious disease caused by breathing in the spores of the bacterium Bacillus anthracis. According to the FDA website, it is the first mAb approved under the Animal Efficacy Rule, which allows findings from well-controlled animal studies to support FDA approval when it is not possible to conduct trials in humans. Clearly, inhalational anthrax, being a rare and lethal disease, fits these criteria.

Poteligeo (mogamulizumab)

A glyco-engineered, defucosylated, humanized mAb, this innovative product was approved in Japan for the treatment of relapsed or refractory adult T-cell leukemia/lymphoma. It has also been licensed to Amgen for development as a therapy for asthma.

Adcetris (brentuximab vedotin)

Adcetris was approved by FDA in August 2011 and granted conditional marketing authorization by the European Commission in October 2012 for relapsed Hodgkin lymphoma and relapsed systemic anaplastic large-cell lymphoma.

BUSINESS AND MARKET

More than 30 approved antibody therapeutics are currently in the worldwide markets, as detailed in Figure 2. The antibodies generated in excess of $45 billion in sales in 2011. More than 40% of this revenue came from anticancer mAbs such as Rituxan, Erbitux, Herceptin, and Avastin. Remicade and Humira, two of the prominent mAbs for the treatment of inflammatory diseases such as rheumatoid arthritis and inflammatory bowel conditions, added another $14 billion, or 30% of the total sales.

Figure 2: List of FDA-approved antibody therapeutics.

Humira, the first fully human mAb, developed and marketed by Abbott Laboratories, accounted for $7.9 billion of 2011 sales, up from $6.5 billion in 2010 and $5.5 billion the year before, a remarkable increase of 44% in just two years. The drug is poised to become the top-selling therapeutic in the world within the next two years, because it has been approved for use of as many as seven inflammatory disease indications.

Some analysts predict that with the rate of current revenue growth and with the potential new approvals, the global market could reach $58 billion by 2016. But as always in the biotech sector, the situation is fluid and extremely difficult to forecast. Several of the antibodies that were approved during the past decades are still generating multibillion dollars in sales, while some have been withdrawn from the market due to either severe side effects (Raptiva approved in 2003, withdrawn in 2009) or lack of sales (Mylotarg approved in 2000, withdrawn in 2010).

Avastin and Lucentis, two anti-angiogenesis mAbs, generated more than $7 billion in sales in 2010. However, Avastin's performance was mixed. It does not extend lifespan and triggers side effects including high blood pressure and bleeding in metastatic breast cancer patients. While FDA revoked approval of Avastin for metastatic breast cancer, it still remains approved for colon, lung, kidney, and brain cancer.

Significant success has been achieved by several ADC companies, including Seattle Genetics and Immunogen, in building partnerships with pharmaceutical companies. And because of potential benefits expected from ADCs, not only pharmaceutical companies are generating business deals with new developers of ADCs but even venture investors are putting money into ADC startups. In March 2011, for instance, biotech investor Celtic Therapeutics committed $50 million to back ADC Therapeutics, a next-generation developer of the products. Additionally, Sutro Biopharma and Celgene struck a deal that could be worth as much as $500 million plus in milestones and royalties to Sutro.

In some circles, Sutro ADC technology is regarded as "next generation" compared with that of Seattle Genetics and Immunogen. This assessment is based on its ability to identify optimal sites in antibodies to design a site-specific insertion of non-natural amino acids for conjugation of linker and payload. Sutro has been on a roll, signing other partnerships with the likes of pharma giants, such as Pfizer. These as well as investments include investments from Lilly and Amgen venture groups among others.

The most recent deal completed between Seattle Genetics and Abbott Laboratories could be worth approximately $250 million when upfront, milestone payments, and royalties are taken into account. The list of Seattle Genetics' collaborators is legion: Genentech, Bayer, Celldex Therapeutics, Progenics Pharmaceuticals, AstellasPharma, Daiichi Sankyo, Millennium, GlaxoSmithKline, Genmab, Pfizer, and Abbott Laboratories. Their ADCs or "smart bombs" continue to be one of the hottest new technologies in the treatment of cancers, given their ability to destroy tumors while minimizing collateral damage to normal cells. The company has already generated nearly $150 million in sales in the US since Adcetris' approval in 2011.

Immunogen, not to be outdone, has established liaisons with Eli Lilly, Novartis, Amgen, Genentech, Biotest, Bayer, and Sanofi, among others. Roche/Immunogen's T-DM1 is expected to be approved in February of 2013 as the second ADC in the market. The drug is a potential blockbuster with projected sales of $5 billion, according to some of the industry analysts.

One of the largest deals involving ADC technology was consummated recently by Italy's Menarini Group. The company has embarked upon a $1 billion cancer ADC partnership with Oxford BioTherapeutics comprised of fivenew ADC programs.

Although, up till now, ADC programs have been geared to the treatment of various cancers, a recent deal between Merck and Ambrx focuses on other disease indications, including autoimmune conditions and diabetes. Merck will pay $15 million upfront with a great deal more (possibly as high as $288 million) in back-end incentives to partner with Ambrx on its ADC technology, based on proprietary binding methods for linking antibodies to toxins to better control the ratio of toxin molecules to antibodies.

FUTURE DEVELOPMENTS

Over the years, the technological focus of antibody engineering has shifted as new and better strategies were adopted throughout the antibody R&D community. Originally, mAbs were murine, constructed through a slow, laborious process of immunizing mice and fusing lymphocytes with myeloma cell lines. Thousands of such events were screened by hand, and the best representatives were cloned and recloned in order to develop the most specific antibodies. These antibodies were chimerized or humanized in order to build pharmacologically effective products. But today, newer approaches, such as phage display libraries and transgenic mice, have proven much more successful in generating fully human antibodies, and there is every indication that continuing advances in these protocols will make the process even more user-friendly (5).

Screening of large numbers of potential antibody producing clones is now performed with robotics technology. Antibody Solutions and Guava Technologies (acquired by Millipore) are among numerous companies that have developed automated work stations to identify unique antibody-producing clones. Using flow cytometry and the Guava EasyCyte cell analysis platform, the system can screen 10–20 96-well plates in a 24-hour period. Positive clones can be simultaneously tested for specificity using the ELISA method. Such automatic, robotic approaches have revolutionized labor-intensive antibody isolation protocols.

Deep sequencing technology is currently being applied to understand the diversity of antibody libraries and to improve the in vitro selection of antibodies using phage or yeast display. Also, significant information regarding the true diversity of expressed antibodies among different subsets of B-cells as well as the role of this diversity in disease processes such as lymphoid cancers, HIV infection, and autoimmune disease is being advanced by carrying out deep sequencing and companion algorithms.

Perhaps the most striking change in antibody strategy is the rise of ADCs. Effective ADCs could profoundly affect the demand for large quantities of antibodies, since they are effective at a fraction of the dose required by naked antibodies. While Adcetris has been approved and T-DM1 is expected to enter the market in 2013, both Seattle Genetics and Immunogen have several more ADCs in their clinical pipeline. Currently, around 100 ADCs are in active development, including 39 in clinical trials.

The pace of research pertaining to the identification of bispecific, bitargeted, and multifunctional antibodies and their clinical development should allow the introduction of more functionally versatile antibodies. This level of progress will provide better treatment possibilities for diseases beyond cancer and inflammation, such as those of the central nervous system and nosocomial infections. While the development of immunomodulatory antibodies, such as Yervoy has been one of the most significant advances in cancer therapy in the past decade, new and alternative approaches to creating immunomodulatory antibodies for the treatment of cancer and nonmalignant diseases, including rheumatoid arthritis and multiple sclerosis, will continue to be vigorously pursued.

The antibody R&D community is also tussling with a variety of challenges, the most pressing of which may be the concept of the rise of biosimilars and their potential market impact. One way to move beyond this issue is to develop, in some instances, polyethylene glycol-conjugated (PEGylated) antibodies, which may allow for extension of patent protection. There is a substantial literature establishing their extended halflife while at the same time retaining their potency and binding ability (6). UCB Group's Cimzia is a PEGylated anti-TNFα antibody that was approved in 2008 for the treatment of Crohn's disease and rheumatoid arthritis. The PEGylation method involves expressing antibody fragments in a bacterial system such as Escherichia coli, and then site-specifically PEGylating the fragment in a manner that avoids the loss of antigen-binding activity.

It should be noted that Amunix has developed an alternative technology to PEGylation, called XTENylation that utilizes a long, hydrophilic, and unstructured amino acid polymer (XTEN). When attached to molecules of interest, it greatly increases their effective size, thereby prolonging their presence in serum by slowing kidney clearance in a manner analogous to that of PEG. Because it can be recombinantly engineered into the organism producing an antibody of interest, thereby resulting exceptionally long half-lives and often monthly dosing of the therapeutic molecule, it offers an alternative to PEG.

Other systems that bear potential promise for generating high-potency products include ultrapotent antibodies produced through affinity maturation and replacement of crucial amino acids, bispecific antibodies and antibody fragments, glycoengineered antibodies, and innovative engineering of the Fc portion of the molecule, allowing expanded modifications and novel functions.

While it is always challenging to offer forecasts concerning the future, based upon the late-stage clinical trials of a large number of antibody therapeutic candidates as well as the recent deal-making activities involving newer antibodies, the authors believe that both the near- and long-term outlook for the antibody industry is quite positive. Consequently, the authors foresee the major contribution of antibody therapeutics to pharma revenues should continue.

K. John Morrow, Jr., PhD, is president of Newport Biotechnology Consultants, Newport, KY, and a member of BioPharm International's editorial advisory board, kjohnmorrowjr@insightbb.com, and Rathin C. Das, PhD, is chief executive officer of Synergys Biotherapeutics, Walnut Creek, CA, rathindas2009@gmail.com.

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