College of American Pathologists
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  Antibodies and kits for KIT


cap today

September 2004
Cover Story

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William Check, PhD

Accurately qualifying GIST patients for Gleevec therapy demands an antibody that provides strong consistent staining for KIT and that has good specificity, says Kenneth Bloom, MD. He notes that virtually all GI specimens contain internal positive and negative controls. "You want to make sure that normal structures—the mast cells that are almost always present—are staining strongly and cleanly and that smooth muscle cells in tissue are not staining," Dr. Bloom says. "Crush and edge artifacts are possible," he cautions, "especially within small GI biopsies." Fortunately, the pathologist is almost always looking at resected tissue, which contains a portion of the bowel wall.

In addition to being sensitive and specific, antibodies need to be robust across a wide range of conditions of tissue sampling, fixation, processing, and antigen retrieval, since these vary among institutions.

It can also be helpful to recognize different staining patterns within the two GIST differentiation lines—the more common spindle cells and epithelioid cells. Spindle-cell GISTs stain more intensely with typically strong cytoplasmic staining with accentuation along the membrane, Dr. Bloom says. In another pattern, instead of diffuse cytoplas mic and membrane staining, a perinuclear Golgi-like pattern predominates and this pattern tends to be seen more often in epithelioid cells. "So when you look at low power, epithelioid predominant-type GISTs might not appear to be stained as strongly and uniformly as you would expect," he says.

Dr. Bloom has experience with several antibodies and kits. He and his colleagues have used different clones that have ranged from a poor experience with one product to a very good experience with the Dako kit, which is the main kit being used at USLabs. "We also just evaluated the new Ventana rabbit monoclonal kit, which also looks superb," he says. "It is my opinion that both kits provide very good and consistent staining." And both appear to be robust.

Some pathologists want to move beyond comparing commercial products to greater standardization of IHC staining overall. "CAP helped sponsor a meeting at FDA last year to discuss this," says Christopher Fletcher, MD, FRCPath, of Brigham and Women’s Hospital. "Patented commercial kits are held up as the gold standard, whereas in real life they are no more reproducible than any other IHC stain." He agrees with Dr. Bloom that the key variables in accurate staining are how the tissue was processed, what it was fixed in and for how long, the pH of the water, and many other steps. "Having commercial patented antibodies does not get you over those hazards," Dr. Fletcher says. Antibody standardization is going to be difficult, he says. "Since we can’t control how people all over the country process specimens," he says, "one way forward is federally provided known positive controls to which you can calibrate your local staining." IHC accuracy will become even more important as it is increasingly used to detect biologically significant oncoproteins that have direct therapeutic applications.

William Check is a medical writer in Wilmette, Ill.