CLINICAL NEWS
On Location
Spliceosome Inhibition Is a
Potential Therapeutic Option for
Patients with ALL and AML
The novel therapeutic strategy of
spliceosome targeting is a promising
option for patients with acute lymphocytic leukemia (ALL) and acute
myeloid leukemia (AML), including
those with acquired resistance to other
anti-leukemic drugs, according to a
study presented by Anna Wojtuszkiewicz, from the VU Medical Center in
Amsterdam, Netherlands, at the AACR
Annual Meeting.
“Aberrant splicing of genes involved
in apoptosis regulation and drug
metabolism was shown to confer
chemoresistance in various tumor cells,
including acute leukemia,” the authors
explained. “Moreover, increased expression of abnormal splice variants caused
reduced sensitivity of leukemic cells to
crucial components of current treatment protocols, such as glucocorticoids
or methotrexate.” Targeting the spliceosome, they reasoned, could potentially
modulate the drug resistance–related
splicing and eradicate cells that do not
respond to conventional therapy.
Both ALL and “notoriously
apoptosis-resistant” AML
cell lines responded to subnanomolar concentrations of
the MAMB spliceosome inhibitor.
Study Examines Novel SecondGeneration Selective Inhibitor of
Nuclear Export for ALL
KPT-8602, a second-generation
selective inhibitor of nuclear export
(SINE), demonstrated similar antileukemic activity and better tolerability than the first-generation SINE
selinexor, according to early results
from an in vitro study presented at
the AACR Annual Meeting. The
novel agent also showed high specificity for its target, the exportin-1
(XPO1) gene.
“XPO1 is the key nuclearcytoplasmic transport protein that
exports a wide variety of different
cargo proteins, including tumor suppressors, out of the cell’s nucleus,”
the authors, led by Dirk Daelemans,
from the Rega Institute for Medical
Research at the University of Leuven
in Belgium, explained. “Inhibition
of XPO1 function consequently
restores nuclear localization of these
proteins and is a promising therapeutic strategy for cancer.”
In phase II/IIb clinical trials,
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ASH Clinical News
selinexor, the oral first-generation
SINE, led to remissions in patients
with acute lymphocytic leukemia
(ALL), both as a single agent and in
combination with other therapies.
While selinexor was well tolerated
when dosed one to three times a
week (administered every other
day when dosed), KPT-8602 has an
improved safety profile, the authors
noted, and may allow for more frequent dosing.
Researchers first assessed the
anti-XPO1 activity of KPT-8602,
finding that the SINE “potently
inhibited” XPO1-mediated nuclear
protein export at nanomolar concentrations and blocked the interaction
of XPO1 with cargo protein.
“KPT-8602 also induced potent
cytotoxicity on a panel of T-ALL
and B-ALL cell lines,” Mr. Daelemans and colleagues noted. This
cytotoxicity correlated with the
induction of caspase-dependent
Using in vitro studies, Ms. Wojtuszkiewicz and colleagues assessed
the sensitivity of ALL and AML cells
to spliceosome inhibitors, including
meayamycin B (MAMB), pladienolide
B (PB), and spliceostatin A (SSA).
First, they determined the growth
inhibitory activity of MAMB using a
72-hour MTT assay in a panel of ALL
and AML cell lines, including sub-lines
with acquired resistance to conventional chemotherapies. Next, the researchers assessed the effect of MAMB, PB,
and SSA on splicing profiles, cell cycle
distribution, and apoptosis induction
in a time-course experiment. Fina lly,
they compared MAMB sensitivity
among 10 primary ALL specimens, 10
AML specimens, and six healthy bone
marrow specimens.
Both ALL and “notoriously apoptosis-resistant” AML cell lines responded
to sub-nanomolar concentrations of
MAMB, the authors reported, with
IC50 values (the concentration of a drug
required for 50% growth inhibition in
the MTT assay) ranging from 0.07 to
0.16 nM. MAMB also retained full sensitivity toward leukemic sub-lines that
were resistant to conventional therapy,
including methotrexate, dexamethasone, bortezomib, and imatinib.
The growth inhibition induced by
MAMB, PB, and SSA was associated
with time- and dose-dependent alterations in splicing profiles of:
• Selected apoptosis-related genes
(Mcl-1, Bcl-X, FAS, and Casp2)
• Concomitant cell cycle arrest (G1
and G2/M phase)
• Apoptosis induction (up to 40%
after 24 hours of exposure to 1nM
MAMB)
“Consistent with cell-line observations,
both primary ALL and AML specimens
showed remarkable response to MAMB,”
Ms. Wojtuszkiewicz and colleagues
concluded, for a mean LC50 (lethal concentration causing 50% cell kill) of 0.42
nM (range = 0.26–0.69 nM) in ALL and
0.43 nM (range = 0.33–0.44 nM) in ALL.
In addition, significantly lower sensitivity to MAMB was observed in healthy
bone marrow samples (mean LC50=0.57;
range = 0.39–1.13 nM; p=0.02).
REFERENCE
Wojtuszkiewicz A, Sciarrillo R, Jansen G, et al. Spliceosome
inhibition as a novel therapeutic option in acute leukemia.
Abstract 4336. Presented at the AACR Annual Meeting, April 19,
2016; New Orleans, LA.
apoptosis and the nuclear
accumulation of p53, as
well as the subsequent induction of p53 response.
The researchers then
applied CRISPR/Cas9
technology to further
characterize KPT-8602’s
Plenary presentations at the AACR Annual Meeting.
mechanism of action; after
introducing a Cys528Ser
mutation in the XPO1 gene of four
leukemia cell numbers in the blood,
different leukemia cell lines, they
without affecting normal erythropoiobserved that mutant cells were
esis, and resulted in longer survival.
>100 times more resistant to KPT“KPT-8602 displays better toler8602. Drug-target interaction was
ability, [compared with] the firstalso confirmed via a pull-down asgeneration SINE selinexor, allowing
say of wild-type XPO1 protein.
for daily dosing and resulting in
“These results illustrate the
effective anti-ALL activity in in vivo
highly specific interaction of the
patient-derived tumor graft models,”
drug for its target and prove that the
the researchers concluded, “[thus]
anti-leukemic activity of KPT-8602
warranting further evaluation of this
is caused by inhibition of XPO1,” the new drug in patients.” A phase I/II
researchers noted.
study is underway. ●
Mr. Daelemans and researchers
REFERENCE
also examined KPT-8602’s antiDaelemans D, Neggers JE, De Bie J, et al. KPT-8602 is a secondleukemic activity in vivo, in mice
generation XPO1 inhibitor with improved in vivo tolerability that
engrafted with patient-derived
demonstrates potent acute lymphoblastic leukemia activity.
Abstract LB-210. Presented at the AACR Annual Meeting, April 18,
T-cell ALL. The mice received either
2016; New Orleans, LA.
KPT-8602 or placebo daily for
three weeks, and blood counts were
measured weekly. Compared with
placebo-treated animals, KPT-8602
led to a substantial reduction in
June 2016