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Understanding blood clots
How monoclonal antibodies first produced decades ago are helping scientists
develop drugs for cardiovascular disease
BY LYNN LOVE
Fine wine and single malt scotch are well
known to improve with age, but rarely do scientific discoveries
have much staying power, usually being supplanted by the next big
thing in weeks to months. But two monoclonal antibodies produced by
Barry Coller 25 years ago are still providing valuable new
information.
A pair of recently published studies from
Coller’s Rockefeller lab have yielded new knowledge about
deadly blood clots that trigger heart attacks and strokes, with
potential new implications on how an entire family of receptors
change shape with activation.
“Somebody told me about monoclonal
antibodies over lunch one day while I was at Stony Brook University
in the late 1970s, and I knew immediately what I wanted to
do,” Coller recalls. Normally, antibodies are manufactured in
breathtaking variety by the body’s immune system, and
function to help the body fight diseases. Unlike their naturally
occurring counterparts, however, monoclonal antibodies are derived
in the lab, from just one mouse spleen cell making an antibody to a
molecule of interest. Thus, they usually bind to just one site on a
protein. If the site is important in receptor function, they can be
highly useful tools due to their specific ability to block the
receptor’s normal function. And they can be used to study
proteins anywhere in the body, not just those of the immune system.
The only problem with developing monoclonal
antibodies is their sheer randomness: “Researchers easily can
create hundreds of monoclonal antibodies. But systematically
testing each one to determine if it binds to a target protein you
are studying is painstaking work. Many researchers frankly
don’t have the infinite patience to stay with this
task,” says Sarah Schlesinger, associate research professor
with appointments to both David Ho and Ralph Steinman’s
Rockefeller labs. (Schlesinger should know; her first project as a
summer intern in Steinman’s laboratory in 1977 was to assist
in creating a monoclonal antibody that could bind to dendritic
cells.)
Coller, who is today Rockefeller’s
physician-in-chief and David Rockefeller Professor, created and
tried hundreds of monoclonal antibodies in the 1970s and 1980s
before striking gold. In 1994 he successfully converted one to
medical use as the basis for the drug abciximab, which has been
administered to more than 2 million people worldwide. Abciximab, known
in the marketplace as ReoPro, keeps coronary arteries open in patients
who’ve undergone angioplasty and stent replacement to clear
blockages from their arteries, often as part of the treatment for a
heart attack. By connecting to and blocking the platelet integrin
receptor,  , which normally binds
fibrinogen, abciximab prevents platelets from aggregating one with
another, thus preventing unwanted clotting.
Yet despite this success, the integrin
receptor has continued to vex scientists. Now in two recent
publications, one in The Proceedings of
the National Academy of Sciences and
one in Nature, two of Coller’s early monoclonal antibodies, known
as 7E3 and 10E5, are helping explain the secret of how the
fibrinogen receptor functions, and are hinting at how even more
effective drugs to treat cardiovascular disease could be designed.
Coller and his colleagues in the Laboratory of
Blood and Vascular Biology localized the binding site for 7E3 to a
region on one of the two subunits of the receptor. This site
undergoes a major change in conformation when platelets are
activated and detailed studies of the binding of 7E3 report this
change. These new data help to confirm crystallography.
Many years ago Coller used another one of his
antibodies, 10E5, to hold the two subunits of the receptor
together. More recently, in collaboration with Timothy Springer and
his group at Harvard Medical School, they used 10E5 to hold the
subunits of a fragment of the receptor together during purification
for crystallography. This led to the first crystallographic
structure of the receptor — published in Nature — and
defined precisely where 10E5 binds on the other subunit of the
receptor. They also produced the first crystal structures of the
receptor in complex with several existing drugs used to treat
cardiovascular disease. In addition, the data localized sites of
conformational changes associated with ligand binding and thus open
the door for more precise drug tailoring for life-threatening
vascular events.
“Many people make monoclonal antibodies,
but they don’t always study the details of their binding
properties,” says Coller. “When we made ours, we made
the decision to characterize them in great detail. And it paid off.
It helped to obtain this aesthetically beautiful crystal structure,
which has generated exciting new information.”
May 13, 2005
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