(89g) Pairwise Agonist Screening and Microfluidics Reveal Platelet Function Phenotype

Pairwise agonist screening
and microfluidics reveal platelet function phenotype

T. Colace, M. Chatterjee, and S.L. Diamond

In this study we consider the relationship between a
high-throughput approach designed to investigate platelet activation via
calcium mobilization and a microfluidic model used to study platelet function. Chatterjee
et al. (2010) have demonstrated that calcium responses to a variety of platelet
agonists can serve as a platelet activation phenotype in an assay called
Pairwise Agonist Scanning (PAS), but did not illustrate a connection to
platelet function. Using PAS data collected for 3 healthy donors we attempted
to predict changes in platelet function in the presence of the anti-platelet
agent Indomethacin, a non-selective COX inhibitor, and to confirm them in a
microfluidic model of platelet aggregation on a collagen surface. The
combination of these techniques represents a link between the complex and
unique activating environment experienced by a platelet under flow conditions
and the traditional methods of measuring platelet activation in a static, well
mixed setting.

Whole blood, anticoagulated with PPACK (100 μM), was treated
with Indomethacin (140 μM), Iloprost (a stable prostacyclin analog that
globally inhibits platelet activation, 1 μM), both agents together, or was
untreated (control). Each condition was loaded in duplicate into a separate
inlet of an 8 channel microfluidic device with a single outlet [Figure 1A].
The samples were perfused over a fibrillar collagen surface at a shear rate of
200 s^-1 for 8 min [Figure 1B]. Platelet accumulation at the
collagen strip was measured in each channel simultaneously in 15 sec intervals.
To perform the PAS study platelet rich plasma from the same blood draw was loaded
with a calcium sensitive dye and Indomethacin (to prevent autocrine activation
via TXA₂). Samples were treated with all pairwise combinations of 4
concentrations (zero, low, medium, high) of the platelet agonists convulxin (a
collagen mimetic), ADP, and U46619 (a stable TXA₂ mimetic), in the
presence or absence of Iloprost. Platelet calcium mobilization was measured via
fluorescence for 200 sec.

The platelet collagen response was recorded for 4 trials per donor
on two separate days, with corresponding PAS data. While Iloprost was effective
for all donors (mean aggregation inhibition of 70%), Indomethacin showed
effectiveness only for donors 1 and 2 (35% and 20%, respectively), and caused a
significant increase in platelet accumulation for donor 3 (20%) [Figure 1C].
Platelet calcium mobilization in response to U46619 alone illustrated a reduced
sensitivity to the TXA₂ analog for donor 2 as compared to donor 1 (~44%
reduction in Ca²⁺ mobilization) and for donor 3 as compared to donor
2 (~80% reduction) [Figure 1D]. In order to confirm an activating role
for high doses of Indomethacin, the study was repeated at 14 μM
Indomethacin. At this concentration donor 2 showed a 34% reduction in platelet
adhesion while donor 3 showed at 13% reduction (donor 1 has yet to be investigated).
PAS data was not affected by the change in Indomethacin concentration. Furthermore,
we have observed a 2-fold increase in platelet aggregation for donor 1 as
compared to donors 2 and 3. We believe this result is reflective of a universal
increase in Ca²⁺ mobilization as measured under all conditions in
the PAS output.

These data reveal that donor sensitivity to anti-platelet agents
can be predicted using PAS and confirmed in a microfluidic model of platelet
adhesion. The combination of these techniques bridges the gap between traditional
measures of platelet activation (Ca²⁺ mobilization) and studies of
platelet function under physiologic flow conditions. We believe these methods
hold promise as a novel diagnostic tool for evaluating platelet function and
efficacy of anti-platelet therapy.