Antibody Drug Conjugates: A Marriage of Biologics and Small Molecules

Targeted therapies such as monocolonal antibodies are an important part of anticancer drug development. Humanized antibodies may be used either alone in unlabeled or naked form or conjugated with radioactive isotopes, chemotherapeutics, or toxins to create highly targeted agents. Antibody drug conjugates (ADCs), or monoclonal antibodies linked to a cytotoxic small molecule, provide a niche opportunity for biopharmaceutical companies and contract manufacturers.

Patricia Van Arnum

Monclonal antibodies target cancer

Six engineered monoclonal antibodies ["Rituxan" (rituximab), "Herceptin" (trastuzumab), "Campath" (alemtuzumab), "Avastin" (bevacizumab), "Erbitux" (cetuximab), and "Vectibix" (panitumumab)], two radionuclide-conjugated monoclonal antibodies ["Zevalin" (ibritumomab tiuxetan) and "Bexxar" (tositumomab; iodine I 131 tositumomab)], and one ADC ["Mylotarg" (gemtuzumab ozogamicin)] have been approved to treat cancer (1).

Wyeth's (Madison, NJ) Mylotarg is the only ADC approved by the US Food and Drug Administration. The drug was approved in 2000. Mylotarg consists of a recombinant humanized IgG4 kappa antibody conjugated with a cytotoxic antitumor antibiotic, calicheamicin, isolated from fermentation of a bacterium, Micromonospora echinospora subspecies calichensis (see Figure 1). The antibody portion of Mylotarg binds specifically to the CD33 antigen, a sialic acid-dependent adhesion protein found on the surface of leukemic blasts and immature normal cells of myelomonocytic lineage, but not on normal hematopoietic stem cells. The anti-CD33 hP67.6 antibody is produced by mammalian cell suspension culture using a myeloma NS0 cell line and is purified under conditions which remove or inactivate viruses. Three separate and independent steps in the hP67.6 antibody purification process achieves retrovirus inactivation and removal. These include low pH treatment, diethylaminoethyl (DEAE)-"Sepharose" chromatography, and viral filtration. Mylotarg's amino-acid sequences are 98.3% of human origin. The constant and framework regions contain human sequences. The complementarity-determining regions are derived from a murine antibody (p67.6) that binds CD33. This antibody is linked to N-acetyl-gamma calicheamicin via a bifunctional linker. Gemtuzumab ozogamicin has approximately 50% of the antibody loaded with four to six moles calicheamicin per mole of antibody. The remaining 50% of the antibody is not linked to the calicheamicin derivative (2).

Figure 1: "Mylotarg" (gemtuzumab ozogamicin) is an antibody drug conjugate. It is composed of a recombinant humanized IgG4 kappa antibody conjugated with a cytotoxic antitumor antibiotic, calicheamicin, isolated from fermentation of a bacterium, Micromonospora echinospora subspecies calichensis.

Specialists in ADCs

In addition to Wyeth, Seattle Genetics (Bothell, WA), ImmunoGen (Waltham, MA), and Medarex (Princeton, NJ) are developing ADCs and related technology. Seattle Genetics uses its proprietary ADC technology in its collaborations with Genentech (South San Francisco, CA), Bayer Schering Pharma (Berlin, Germany), Curagen (Branford, CT), Progenics Pharmaceuticals (Tarrytown, NY), AstraZeneca's MedImmune (Gaithersburg, MD), and Astellas Pharma's Agensys (Santa Monica, CA). AstraZeneca (London) acquired MedImmune for $15.2 billion in 2007, and Astellas Pharma (Tokyo) acquired Agensys in 2007 for $387 million.

ADCs are monoclonal antibodies that link highly potent cyotoxics to the antibody, which delivers and releases the drug inside the tumor cell. A key component of Seattle Genetics's ADC technology is the linker that attaches the drug to the monoclonal antibody until internalized within the target cell where the drug is released. The company uses auristatin derivatives, which are synthetically produced and readily scaleable (1).

The company has three drug candidates using its auristatin-ADC technology. "SGN-35" is in Phase I testing for treating Hodgkin lymphoma and CD30-positive T-cell lymphomas. SGN-35 uses the same antigen candidate as Wyeth's Mylotarg. "SGN-75" is designed to treat CD70-positive hematologic malignancies and solid tumors. Seattle Genetics is planning to file an investigational new drug (IND) application for SGN-75 in 2009. An anti-CD19 ADC is being developed as a future IND candidate for treating D19-positive hematologic malignancies (1).

Select examples of antibody drug conjugates under development.

Seattle Genetics uses contract manufacturers to support is antibody and ADC development. For example, Laureate Pharma (Princeton, NJ) mamufactures the antibody component of SGN-75. For its ADC technology, several contract manufacturers, including Albany Molecular Research (Albany, NY) and SAFC (St. Louis, MO) perform drug-linker manufacturing, and several other contract manufacturers, including NPIL Pharma (Mumbai, India), perform conjugation of the drug linker to the antibody (1).

Several drug candidates from Seattle Genetics's collaborations are also advancing. In 2005, Seattle Genetics formed an ADC pact with PSMA Development Company, now a subsidiary of Progenics. Progenics plans to begin clinical trials for "PSMA-ADC" during 2008. PSMA ADC is a fully human monoclonal antibody to a prostate-specific membrane antigen linked to a derivative of auristatin, according to Progenics (1,3).

CuraGen also is developing ADCs. "CR011-vcMMAE" is an ADC composed of a fully-human monoclonal antibody against the glycoprotein NMB (GPNMB) attached to a potent, synthetic drug monomethyl auristatin E (MMAE), using Seattle Genetics's ADC technology. CR011-vcMMAE contains an enzyme-cleavable linker designed to be stable in the bloodstream and to release MMAE after internalization into tumor cells that express GPNMB, resulting in cancer-cell death. CR011-vcMMAE was advanced into a PhaseI cllinical trial in April 2008, according to a CuraGen press release.

Cytokine-antibody conjugates using the dock-and-lock method.

In a strategic move to broaden its pipeline beyond naked monoclonal antibodies, Agensys (now part of Astellas Pharma) formed a broad collaboration with Seattle Genetics in 2007 to jointly research, develop, and commercialize four ADCs using Agensys's proprietary targets and fully human monoclonal antibodies with Seattle Genetics' linker and toxin technology. Under the pact, two products will be developed on a 50-50 basis, and two products will be developed solely by Agensys, according to an Agensys 2007 release. Work has begun on the first product, "AGS-5 ADC," for treating solid tumors.

ImmunoGen has pacts with Sanofi-Aventis (Paris) and Genentech for its ADC technology, "TAP" (tumor activated prodrug). All of ImmunoGen's TAP technology involves attaching potent small-molecule cytotoxics, either DM1 or DM4. DM1 and DM4 are proprietary derivatives of a naturally occurring substance, maytansine.

Genentech's trastuzumab-DM1 is an ADC consisting of ImmunoGen's DM1 agent linked to Genentech's anti-HER2 antibody, trastuzumab. Genentech began Phase II testing of the drug in July 2007 and plans to make a Phase III "go/no" decision in 2008. Genentech licensed the right to use ImmunoGen's TAP technology with its antibodies for three other targets and has certain rights to further test the technology.

Sanofi-Aventis and ImmunoGen are collaborating on ADCs. "AVE9633" is in Phase I testing for treating acute myeloid leukemia, and "SAR3419" is in Phase I testing for treating non-Hodgkin's lymphoma. A third TAP compound, "SAR566658," is in preclinical development; the drug targets the DS-6 epitope, which is found on breast and ovarian cancers and on other solid tumors.

Biogen Idec (Cambridge, MA) submitted an IND for "BIIB015" in early 2008. This TAP compound consists of Biogen Idec's Cripto-targeting antibody and ImmunoGen's DM4 agent. Biotest (Dreieich, Germany) submitted an IND for "BT-062" in early 2008. This TAP compound consists of Biotest's antibody to a target found on multiple myeloma and other cancers and ImmunoGen's DM4.

ImmunoGen has its own TAP compounds in human testing. "IMGN242" in Phase II testing for treating CanAg-expressing gastric cancer. "IMGN901" is in Phase I testing for treating multiple myeloma and in Phase I and Phase II testing for treating solid tumors that express CD56. The company is on track to submit an IND for "IMGN388," a TAP compound, during the second quarter of 2008. IMGN388 consists of ImmunoGen's DM4 and an integrin-binding antibody developed by Centocor (Horsham, PA), a Johnson and Johnson (New Brunswick, NJ) subsidiary.

Medarex, which specializes in monoclonal antibody development and manufacturing, has several ADCs in development. Its ADC platform includes a class of DNA-alklyating agents, which have been designed to overcome multidrug resistance. "We are encouraged by the single-dose efficacy observed in animal models from our antibody-drug conjugate technology platform, and remain on track to prepare for our first antibody-drug conjugate candidate IND filing this year," said Howard H. Pien, president and CEO of Medarex, in an April 2008 release.

CMOs expand

Several contract manufacturers are expanding their ADC capacity. Lonza (Basel, Switzerland) will bring on line a commercial-scale plant for ADC production in Visp, Switzerland, in the second half of 2008. The plant will initially be capable of producing over 100 kg of ADCs per year, and future expansion plans are built into the design. Lonza operates laboratory-scale production and brought on small-scale pilot facilities on stream in 2007.

In October 2007, SAFC announced plans to construct a new CGMP potent active pharmaceutical ingredients (APIs) conjugation suite in St. Louis, Missouri. The 800-ft² conjugation suite is now operational and accommodates early-stage clinical supplies of potent conjugated APIs, with capabilities to expand production into commercial-scale for multikilogram quantities of conjugated high-potency APIs per batch.

NPIL Pharma commissioned its sixth high-potency substance production suite at its Grangemouth, Scotland, facility. The suite came on stream during the first quarter of 2008 at an investment of $270,000. The new facility is optimized for GMP manufacture of ADCs. The facility will run batch sizes from 500–1000 g and have annual production of up to 50 kg.

Patricia Van Arnum is a senior editor at Pharmaceutical Technology, 485 Route One South, Bldg F, First Floor, Iselin, NJ 08830 tel. 732.346.3072, pvanarnum@advanstar.com

References

1. US Securities and Exchange Commission, "Seattle Genetics Form 10K Annual Report," (Washington, DC, 2007).

2. FDA, "Efficacy Supplement with Clinical Data to Support," NDA 021174 (Rockville, MD, 2006).

3. D. Ma et al., "Potent Antitumor Activity of an Auristatin-Conjugated, Fully Human Monoclonal Antibody to Prostate-Specific Membrane Antigen," Clin. Cancer Res. 12 (8), 2591–2596 (2006).