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i. ELEVATED PCFTM IN DIABETES
The results of this study provide evidence of
enhanced platelet activity in patients with diabetes. The PCFTM values found in diabetics with
CAD are the highest thus far reported. Elevated PCFTM in diabetics may correlate with
increased thrombotic risk in these patients.
ii. ENHANCED PLATELET FORCE DEVELOPMENT IN
BUERGER’S DISEASE
PCFTM
values in patients with Buerger’s disease were elevated despite the use
of anticoagulation drugs and documented suppression of ADP mediated
platelet aggregation.
B. Drug Monitoring
i. PCF(TM) AND GLYCOPROTEIN IIb/IIIa
BLOCKADE
The importance of glycoprotein IIb/IIIa (GP
IIb/IIIa), a constitutive platelet membrane protein, to platelet
contractile force (PCFTM
) is demonstrated below. In this figure, the decrements in PCFTM as functions of increasing
concentrations of various GP IIb/IIIa inhibitors are demonstrated. The
value of PCFTM in the
absence of inhibitors is taken to be 100% and the amount of force
measured in the presence of the inhibitors is normalized to the
baseline level (100%). The "Relative Dose" is simply
multiples of the indicated inhibitor dose.
GP IIb/IIIa is the receptor for multiple integrins,
and is the primary receptor for fibrinogen and a secondary receptor for
von Willebrand's factor. GP IIb/IIIa is also the receptor by which
platelets adhere to the fibrin network. The receptor can be blocked by
antibodies to GP IIb/IIIa or by peptides which contain the RGD
sequence. In the figure below, one anti-GP IIIa antibody (AP3), one
anti- GP IIb/IIIa Fab' fragment (7E3, marketed as Reopro) and three
synthetic peptides (RGDS, RGDW and CB68-22) all of which bind to GP
IIb/IIIa. Each of these molecules decreases PCFTM to different degrees. The
antibody and Fab' fragment bind tightly to the receptor and are
inhibitory in the 0.1 然 range. The Fab' fragment (Reopro), specific to
GP IIb/IIIa, totally inhibits PCFTM
at a concentration of only 0.08然. The AP3 antibody is specific to
GPIIIa and inhibits PCFTM
by 80% at 0.13 然. The inhibitory properties of the peptides, which
reversibly bind to GP IIb/IIIa, are more varied. RGDS produces 40%
inhibition at 234 然, RGDW produces virtually complete inhibition at
200 然 while CB68-22 product 55% inhibition at 0.9 然. This variability
reflects the marked differences in binding coefficients.
Monitoring PCFTM
inhibition measures GP IIb/IIIa in a clotting system. This represents a
significant advance over systems which measure GP IIb/IIIa in
anti-coagulated samples. Assay systems such as platelet aggregometry
will not work in the presence of clot formation, so thrombin formation
is prevented. Since additional GP IIb/IIIa is exposed when thrombin is
present, anti-coagulated systems will over estimate the potency of
blockade. In the PCFTM
system, thrombin is present so the maximum extent of GP IIb/IIIa
exposure on the platelet surface is achieved, and the ability of
inhibitors to block the system can be accurately measured.
ii. REVERSAL OF HEPARIN-INDUCED PCFTM INHIBITION BY APROTININ
Heparin blocks PCFTM
, and protamine sulfate reverses heparin's anticoagulant effects but
does not necessarily reverse its suppression of activity.
Baseline PCFTM
in the control (no heparin, protamine or aprotinin) was 5.6 kdynes. No
PCFTM was measurable in
plasma incubated with heparin at 1 U/mL. At 0.1 U/mL heparin, PCFTM was reduced 93%. By adding
aprotinin at 20 痢/mL to the plasma immediately before adding heparin,
no significant inhibition of PCFTM
was observed. However, when the heparin was added to the plasma prior
to the aprotinin, only partial recovery of PCFTM was observed.
iii. PCFTM AND NON-HEPARIN THROMBIN
ANTAGONISTS
In this figure,
the inhibition of PCFTM by various concentrations of anti-thrombins
(AT-III, Hirudin, Thrombomodulin, and PPACK) can be seen. The value of
PCFTM in the absence of anti-thrombins is taken to
be 100%. PCFTM activity is inhibited in a dose dependent
manner.
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