The Novel Small Molecule Inhibitor KS99 Targets AML and Inhibits Stemness Markers STAT3 and ALDH

Introduction: Signal transducer and activator of transcription 3 (STAT3) has been shown to be activated in leukemic stem cells (LSCs), especially in Pre-LSCs, and activation has been associated with poor prognosis in multiple cancers including AML. In AML, sorted CD34⁺/CD38^- and aldehyde dehydrogenase-positive (ALDH⁺) cells have significantly phosphorylated STAT3 when compared to healthy controls. The Isatin analog, KS99 has shown selective anti-cancer activity against Multiple Myeloma (MM) which may, in part, be mediated by inhibition of BTK activation. Here we demonstrate that KS99 selectively targets AML CD34⁺/CD38^- and ALDH⁺ cells and inhibits STAT3 phosphorylation while sparing normal hematopoietic stem and progenitor cells (HSPCs).

Experimental Design: The pro-apoptotic activity of KS99 for primary human AML cells (n=21) and human AML cell lines (n=9) was evaluated using Annexin V and cell proliferation (MTS) assays. Colony forming assays were carried out to demonstrate the specific LSC-targeted effect of KS99. Flow cytometry was used to detect STAT3 phosphorylation and apoptosis in CD34⁺/CD38^- LSCs and ALDH activity in a series of human AMLs. Next, to assess the preclinical efficacy, the human AML cell line U937 was injected intravenously (IV) into NSG mice. Mice with an established leukemic burden were randomized into treatment groups and treated through an intraperitoneal (IP) injection with either vehicle control or KS99 (2.5mg/kg) every other day x8. Two days after the final treatment, leukemic burden (hCD45⁺) was quantitated in the bone marrow.

Results: AML cell lines exhibited dose-dependent sensitivity to KS99 in the nanomolar range (IC₅₀: 150-400nM) as did most primary AML cells (Figure: A). Interestingly, poor prognosis AML subsets, AML with MDS related changes (MDS-RCs), were more sensitive than De Novo AML cases (p= 0.0077) with or without a NMP1 mutation (p= 0.012, p=0.045, respectively). Within De Novo AML, NMP1 wild-type cases were more sensitive than NMP1 mutant cases (p= 0.02) (Figure: B). Furthermore, KS99 treatment selectively reduced the clonogenicity of human AML patient cells vs. normal cord blood mononuclear cells (p<0.001) (Figure: C). KS99 induced apoptosis in the CD34⁺/CD38^- gated cell population of primary AML, leading to reduced blast-like and stem-like cells, but not other cells. It also showed a reduction in ALDH⁺ cells in an Aldefluor assay. Likewise, KS99 treatment showed reduced pSTAT3 in viable CD34⁺/CD38^- cells (Figure: D). Inhibition of pBTK, BCL-2, and pSTAT3 were seen in Western blot experiments. Animal studies showed 76 % reduction of hCD45⁺ U937 cells in bone marrow of KS99-treated mice when compared to bone marrow isolated from vehicle-treated animals (p<0.05) (Figure: E).

Conclusion: KS99 selectively targets LSCs from poor prognosis AML patients while sparing normal HPSCs. This effect may be mediated via targeting of STAT3 signaling and ALDH, which are critical for proliferating LSCs, but not normal HPSCs. Further studies will aim to demonstrate mechanisms of activity and preclinical efficacy in patient-derived AML xenograft mouse models.

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