MONOCLONAL ANTIBODY Imaging

Frank P. Dawry

Antibodies and Antimatter: The Resurgenceof Immuno-PET

THE JOURNAL OF NUCLEAR MEDICINE • Vol. 50 • No. 1 • January 2009

Antibodies offer the promise of highly selective binding and recognition of molecular targets

They have propelled the identification and analysis of specific proteins in research and in vitro diagnostic laboratories.

Problems with PET

• Because of the short half-life of positron emitters, hospitals must install cyclotrons to take advantage of the range of PET isotopes– only 20%–25% of medical institutions in the United

States own a cyclotron - worse in in developing countries.

• Radiopharmaceuticals based on C-11, O-15, or N-13 will never reach a patient in a hospital at a distance of >20 miles from the site of the cyclotron.

Arguments for SPECT

• Many companies are strongly invested in improvements in SPECT technology.

• The availability of longer lived SPECT-friendly radionuclides allows worldwide distribution of SPECT radiopharmaceuticals.

• The practical and economic aspects of SPECT instrumentation place this modality within the reach of smaller clinics and health care centers.

• Patient costs are also lower.

However

The development of antibodies as clinical imaging agents has had a checkered history, with a handful of antibody tracers for nuclear medicine imaging gaining approval in the 1990s– (satumomab pendetide [Oncoscint; Cytogen Corp.– capromab pendetide [Prostascint; Cytogen Corp.– arcitumomab [CEA-Scan; immunomedics]– …and others

Disappointment

• As a group, antibody-based imaging agents have yet to make a significant impact in the clinical or commercial setting

Antibodies vs. Small Molecules

• Antibodies offer unmatched utility in their ability to recognize essentially any target of interest.

• In contrast, small molecules and peptides often require the presence of an active site, pocket, or groove for high-affinity interaction.

Limitation

• One potential limitation of antibody-based molecular imaging is that the approach is restricted to the assessment of cell surface markers; intracellular targets are not accessible

Targets

The surface-expression patterns of – receptors, – ligands, – adhesion molecules, – proteases, – differentiation and activation markers

provide a rich source of information on the state of a cell; these proteins represent potential targets for antibody-based molecular imaging

Basic Structure of antibodiesproteins that seek out antigens and help destroy them.

Constant fragments – determines antibody class (e.g. IgG, IgA …)

Variable region – Site of antigen antibody reaction

Frank P. Dawry

NATURE OF ANTIGEN-ANTIBODY REACTIONS

• A. Lock and Key Concept - our concept of Ag-Ab reactions is one of a key (i.e. the Ag) which fits into a lock (i.e. the Ab).

– The combining site of an antibody is located in the Fab portion of the molecule

• B. Non-covalent Bonds - The bonds that hold the Ag in the antibody combining site are all non-covalent in nature. These include hydrogen bonds, electrostatic bonds, Van der Waals forces and hydrophobic bonds. Multiple bonding between the Ag and the Ab ensures that the Ag will be bound tightly to the Ab.

• C. Reversible - Since Ag-Ab reactions occur via non-covalent bonds, they are by their nature reversible.

ANTIBODIEStake advantage of the body's infection-fighting (immune) system.

• If a foreign substance (an antigen) is injected into a vertebrate such as a mouse or a human, some of the immune system's B-cells will turn into plasma cells and start to produce antibodies that recognize that antigen.

• Each B-cell produces only one kind of antibody, but different B-cells will produce structurally different antibodies that bind to different parts ("epitopes") of the antigen.

Frank P. Dawry

Possibilities

• attaching a radioactive tag to the antibody, we can see where it goes.

• treatment of cancer by using powerful radioactive tags on the antibodies.– large doses of radiation could be delivered only to the

tumors.

Frank P. Dawry

MONOCLONAL ANTIBODIES

Monoclonal antibodies (mAb) are antibodies that are identical because they were produced by one type of immune cell and are all clones of a single parent cell.

Frank P. Dawry

monoclonal antibody production

• In monoclonal antibody production technology, tumor cells that can replicate endlessly are fused with mammalian cells that produce antibodies.

• Each monoclonal antibody (MAb) recognizes different proteins on specific cancer cells.

• MAbs are approved as biologics by the U.S. Food and Drug Administration (FDA) and are used in any of the following ways: – alone (once bound to their targets, they trigger the body's normal

effector mechanisms) – coupled with fluorescent molecules or radioactive materials to

aid in imaging the targets – coupled with drugs, toxins or radioactive materials to aid in killing

the target directly

Process by which large quantities of

antibodies can be produced. • A mouse is immunized by injection of an

antigen X to stimulate the production of antibodies targeted against X.

• The antibody forming cells are isolated from the mouse's spleen.

• Monoclonal antibodies are produced by fusing single antibody-forming cells to tumor cells grown in culture.

• The resulting cell is called a hybridoma.

Process by which large quantities of

antibodies can be produced. • Each hybridoma produces relatively large

quantities of identical antibody molecules. • These antibodies are called "monoclonal

antibodies" because they are produced by the identical offspring of a single, cloned antibody producing cell.

• Once a monoclonal antibody is made, it can be used as a specific probe to track down and purify the specific protein (antigen) that induced its formation.

Monoclonal Antibody Production

INDICATIONS: • Imaging

– Detection of primary and/or recurrent colorectal cancer. • 5 mCi of In-111 B72.3 MoAb (OncoScint CR/OV)• 25-30 mCi Tc-99m Anti-CEA (CEA Scan)

– Detection of primary and/or recurrent ovarian cancer. • 5 mCi of In-111 B72.3 MoAb (OncoScint CR/OV)

– Detection of Vascular Thrombus• 20 mCi Technetium-99m Acutect • attaches to receptors found on activated platelets

– Detection of Prostate Cancer• 5.0 millicuries In-111 Prostascint

• Therapy – Patients with relapsed or refractory low-grade, follicular, or transformed B-cell

non-Hodgkin’s lymphoma (NHL).• 0.4 mCi/kg of Y-90 ZEVALIN• Iodine131 Bexxar (tositumomab) [both a gamma and beta emitter ]

Frank P. Dawry

OncoScint colorectal cancer

Myoscint™

• Myoscint™ is manufactured by Centocor B.V. (Leiden, The Netherlands).

• Indium-111 labeled imciromab pentetate is FDA-approved for use as a cardiac imaging agent to detect the presence and/or identify the location of myocardial injury in patients with suspected myocardial infarction.

• Specific for myocardial necrosis, the agent binds irreversibly to exposed myosin filaments of damaged myosites.

• The antibody targets the heavy chain of human cardiac myosin. • The anticipated use of Myoscint™ is in situations where

electrocardiography and cardiac enzymes are nondiagnostic.

• Although Myoscint™ is FDA-approved, – the published, peer-reviewed scientific literature contains insufficient

evidence that it is equal or superior to other diagnostic modalities.

ProstaScint®

• ProstaScint® is manufactured by Cytogen Corporation (Princeton, NJ). • Capromab pendetide labeled with indium-111 is a MAb that recognizes

prostate-specific membrane antigen (PSMA), a substance found only in normal and cancerous prostate cells.

• It is FDA-approved as an imaging agent for use in – newly diagnosed patients with biopsy-proven prostate cancer that standard

diagnostic evaluation indicates is clinically localized and who are at high risk for pelvic lymph node metastases,

– as well as in post-prostatectomy patients with rising prostate-specific antigen (PSA) levels and negative or equivocal standard metastatic evaluations in whom there is a high clinical suspicion of occult metastatic disease.

• The advantage of this test is that it has the potential to detect the spread of prostate cancer to bone, lymph nodes, and other organs and can distinguish prostate cancer from other cancers and from benign disorders.

Verluma®

• Verluma® is manufactured by Dr. Karl Thomae GmbH (Biberach/Riss, Germany).

• Nofetumomab is a fragment of an MAb that, when tagged with the radioisotope technetium, can detect a protein found on the surface of most small-cell lung cancers (SCLCs).

• Nofetumomab is FDA-approved for use in patients who have been diagnosed with SCLC through a confirmed biopsy but who have not yet received treatment.

NeutroSpec

• NeutroSpec imaging was, rapid, and sensitive for diagnosis of appendicitis. – removed from use due to questions about safety.– was used for off-label purposes (infection imaging)

• The potential advantages of NeutroSpec over available radio-pharmaceuticals for infection imaging (e.g. Ceretec) were – ease of preparation, – absence of blood handling, – excellent image quality, – ability to perform high-quality SPECT,– rapid diagnostic uptake.