USEOFFLOWCYTOMETRYTOSTUDY DRUG TARGET INHIBITION IN LABORATORY ANIMALS AND IN EARLY-PHASE CLINICAL TRIALS - Flow Cytometry in Drug Discovery and Development - page 155

(a)

PMA
SCF

4

3

2

1

0 5 10 15 20 25

Days

(b)

1.7

r 2 =0.73
p =0.0073

1.6

1.5

1.4

1.3

1.2

1.1

1.0

0.9

012345678

Plasma sorafenib (µg/mL)

FIGURE 7.2 (a) Pharmacodynamic effects of sorafenib 400mg given twice daily to a cohort
of patients with acute myeloid leukemia. Whole blood samples were obtained at the indicated
time points, and activated ex vivo using either PMA or stem cell factor prior to fixation. P-ERK
was measured in the blast cells and the results plotted as the mean fluorescence intensities of the
stimulated/unstimulated. Note that almost 100% inhibition is seen when the blasts are activated
using SCF, whereas there is no significant inhibition using PMA. This is explained by the fact
that sorafenib inhibits the SCF receptor c-Kit at the drug levels achieved, but not the Raf kinase.
(b) Inhibition of P-ERK activation by SCF versus mean plasma sorafenib concentrations in the
phase I clinical trial of sorafenib in acute myeloid leukemia. Each data point is the mean value
for a patient cohort.

with some showing almost complete target inhibition, as we previously noted using
sorafenib, and others showing no detectable pharmacodynamic effect. Furthermore,
patients who showed a larger pharmacodynamic effect were significantly more likely
to respond to chemotherapy, which is consistent with the hypothesis that increased
signaling through c-Kit contributes to drug resistance in AML patients.

7.2.4.3 High-Complexity Cell Signaling Protocols There are many potential
signaling targets expressed in leukemia, and there is increasing evidence for
considerable heterogeneity in signaling activity between individual patients. It is