Leukemia stem cells contribute to drug-resistance and relapse in
chronic myeloid leukemia (CML) and BCR-ABL1 inhibitor monotherapy fails to eliminate these cells, thereby necessitating alternate therapeutic strategies for patients CML. The
peroxisome proliferator-activated receptor-γ (PPARγ) agonist
pioglitazone downregulates
signal transducer and activator of transcription 5 (STAT5) and in combination with
imatinib induces complete molecular response in
imatinib-refractory patients by eroding
leukemia stem cells.
Thiazolidinediones such as
pioglitazone are, however, associated with severe side effects. To identify alternate therapeutic strategies for CML we screened Food and Drug Administration-approved drugs in K562 cells and identified the
leprosy drug
clofazimine as an inhibitor of viability of these cells. Here we show that
clofazimine induced apoptosis of blood mononuclear cells derived from patients with CML, with a particularly robust effect in
imatinib-resistant cells.
Clofazimine also induced apoptosis of CD34+38- progenitors and quiescent CD34+ cells from CML patients but not of hematopoietic progenitor cells from healthy donors. Mechanistic evaluation revealed that
clofazimine, via physical interaction with PPARγ, induced nuclear factor kB-p65 proteasomal degradation, which led to sequential myeloblastoma
oncoprotein and
peroxiredoxin 1 downregulation and concomitant induction of
reactive oxygen species-mediated apoptosis.
Clofazimine also suppressed STAT5 expression and consequently downregulated stem cell maintenance factors
hypoxia-inducible factor-1α and -2α and Cbp/P300 interacting
transactivator with
Glu/Asp-rich carboxy-terminal domain 2 (CITED2). Combining
imatinib with
clofazimine caused a far superior synergy than that with
pioglitazone, with
clofazimine reducing the half maximal inhibitory concentration (IC50) of
imatinib by >4 logs and remarkably eroding quiescent CD34+ cells. In a K562 xenograft study
clofazimine and
imatinib co-treatment showed more robust efficacy than the individual treatments. We propose clinical evaluation of
clofazimine in
imatinib-refractory CML.