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.
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. Finally, 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).
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.