Lysosomal disruption preferentially targets acute myeloid leukemia cells and progenitors

Despite efforts to understand and treat acute myeloid leukemia (AML), there remains a need for more comprehensive therapies to prevent AML-associated relapses. To identify new therapeutic strategies for AML, we screened a library of on- and off-patent drugs and identified the antimalarial agent mefloquine as a compound that selectively kills AML cells and AML stem cells in a panel of leukemia cell lines and in mice. Using a yeast genome-wide functional screen for mefloquine sensitizers, we identified genes associated with the yeast vacuole, the homolog of the mammalian lysosome. Consistent with this, we determined that mefloquine disrupts lysosomes, directly permeabilizes the lysosome membrane, and releases cathepsins into the cytosol. Knockdown of the lysosomal membrane proteins LAMP1 and LAMP2 resulted in decreased cell viability, as did treatment of AML cells with known lysosome disrupters. Highlighting a potential therapeutic rationale for this strategy, leukemic cells had significantly larger lysosomes compared with normal cells, and leukemia-initiating cells overexpressed lysosomal biogenesis genes. These results demonstrate that lysosomal disruption preferentially targets AML cells and AML progenitor cells, providing a rationale for testing lysosomal disruption as a novel therapeutic strategy for AML.

Abstract C170: The combination of statins and dipyridamole is effective preclinically in AML, MM, and breast cancer

Statins are drugs that have been utilized for years to treat hyperlipidemia through inhibition of the rate-limiting enzyme of the mevalonate (MVA) pathway, 3-hydroxy-3-methylglutaryl coenzyme A reductase (HMGCR). Preclinical evidence has demonstrated statins to possess anti-cancer properties against a wide range of tumors but not normal cells. Through the use of a chemical library screen, we hypothesize that the identification of compounds which potentiate the anti-cancer effects of statins will uncover novel molecular pathways and/or targets that can be exploited in combination with the MVA pathway to maximize tumor cell death.

A pilot 100-compound library, composed of off-patent pharmacologically active drugs clinically used for a wide spectrum of diseases was screened in the multiple myeloma (MM) KMS11 cell line. Dipyridamole (DP), a commonly prescribed anti-platelet agent potentiated the anti-cancer effects of atorvastatin. The DP-statin combination was synergistic and capable of inducing apoptosis in a variety of acute myelogenous leukemia (AML), MM and breast cancer cell lines. The DP-statin combination also induced apoptosis in primary AML patient samples, but was not toxic to normal PBSCs. In an in vivo AML tumor model, the DP-statin combination was found to be effective at inhibiting tumor growth.

DP is known to elicit numerous effects, amongst them, phosphodiesterase (PDE) inhibition. In AML cell lines, activators of the PKA pathway including other PDE inhibitors, also induced apoptosis in combination with statins similar to DP. Interestingly, the DP-statin combination prevented the increase of HMGCR, which occurs following statin treatment as part of a classic feedback response. Further mechanistic investigations to determine how DP potentiates statin-induced apoptosis are underway. As both statins and DP are pre-approved for use in humans, off-patent, and readily available, they have the potential to directly impact patient care.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the AACR-NCI-EORTC International Conference: Molecular Targets and Cancer Therapeutics; 2011 Nov 12-16; San Francisco, CA. Philadelphia (PA): AACR; Mol Cancer Ther 2011;10(11 Suppl):Abstract nr C170.

Antagonism of the Stat3-Stat3 protein dimer with salicylic acid based small molecules

More than 50 new inhibitors of the oncogenic Stat3 protein were identified through a structure-activity relationship (SAR) study based on the previously identified inhibitor S3I-201 (IC₅₀ =86 μM, K(i) >300 μM). A key structural feature of these inhibitors is a salicylic acid moiety, which, by acting as a phosphotyrosine mimetic, is believed to facilitate binding to the Stat3 SH2 domain. Several of the analogues exhibit higher potency than the lead compound in inhibiting Stat3 DNA binding activity, with an in vitro IC₅₀ range of 18.7-51.9 μM, and disruption of Stat3-pTyr peptide interactions with K(i) values in the 15.5-41 μM range. One agent in particular exhibited potent inhibition of Stat3 phosphorylation in both breast and multiple myeloma tumor cells, suppressed the expression of Stat3 target genes, and induced antitumor effects in tumor cells harboring activated Stat3 protein.

Design, synthesis, and in vitro characterization of novel hybrid peptidomimetic inhibitors of STAT3 protein

Aberrant activation of oncogenic signal transducer and activator of transcription 3 (STAT3) protein signaling pathways has been extensively implicated in human cancers. Given STAT3's prominent dysregulatory role in malignant transformation and tumorigenesis, there has been a significant effort to discover STAT3-specific inhibitors as chemical probes for defining the aberrant STAT3-mediated molecular events that support the malignant phenotype. To identify novel, STAT3-selective inhibitors suitable for interrogating STAT3 signaling in tumor cells, we explored the design of hybrid molecules by conjugating a known STAT3 inhibitory peptidomimetic, ISS610 to the high-affinity STAT3-binding peptide motif derived from the ILR/gp-130. Several hybrid molecules were examined in in vitro biophysical and biochemical studies for inhibitory potency against STAT3. Lead inhibitor 14aa was shown to strongly bind to STAT3 (K(D)=900 nM), disrupt STAT3:phosphopeptide complexes (K(i)=5 μM) and suppress STAT3 activity in in vitro DNA binding activity/electrophoretic mobility shift assay (EMSA). Moreover, lead STAT3 inhibitor 14aa induced a time-dependent inhibition of constitutive STAT3 activation in v-Src transformed mouse fibroblasts (NIH3T3/v-Src), with 80% suppression of constitutively-active STAT3 at 6h following treatment of NIH3T3/v-Src. However, STAT3 activity recovered at 24h after treatment of cells, suggesting potential degradation of the compound. Results further showed a suppression of aberrant STAT3 activity in NIH3T3/v-Src by the treatment with compound 14aa-OH, which is the non-pTyr version of compound 14aa. The effect of compounds 14aa and 14aa-OH are accompanied by a moderate loss of cell viability.

Abstract 3684: Developing STAT3 protein inhibitors as adjuvant therapeutics: Promising synergistic effects in human cancers

Signal Transducer and Activator of Transcription 3 (Stat3) protein is a cytosolic transcription factor that relays signals from receptors in the plasma membrane directly to the nucleus, and is routinely hyper-activated in many human cancers and diseases. STAT3 induces anti-apoptotic gene expression programs (e.g. Bcl-xL) and the over-expression of cell cycle regulators (e.g. cyclin D1) that contribute significantly to the resistance of cancer to current chemotherapeutic strategies. Since most cancer drugs aim to initiate apoptosis, tumor cells containing activated STAT3 have an intrinsic resistance to current treatment strategies. It has therefore been postulated that STAT3 inhibitors could play a significant role in the future of adjuvant cancer therapies by sensitizing human tumors to traditional chemotherapy. By examining the protein-protein interaction interface and employing computational modeling, we have thus developed a series of small molecule inhibitors of the transcriptionally active STAT3-STAT3 homo-dimer complex. Lead inhibitors showed potent anti-STAT3 activity in vitro and in tumor cell lines, as well as in malignant cells taken from leukemia patients. More specifically, these compounds displayed single digit micromolar activity against breast, prostate, pancreatic and leukemia cell lines and showed negligible cytotoxic effects on healthy cells treated with high µM concentrations of compounds. Preliminary adjuvant studies with a series of clinically relevant therapeutics have shown impressive synergistic effects in leukemia cell lines, as well as in patient tumor cells.

Note: This abstract was not presented at the AACR 101st Annual Meeting 2010 because the presenter was unable to attend.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 101st Annual Meeting of the American Association for Cancer Research; 2010 Apr 17-21; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2010;70(8 Suppl):Abstract nr 3684.

Coordination complex SH2 domain proteomimetics: an alternative approach to disrupting oncogenic protein-protein interactions

We report the first application of coordination complexes as functional proteomimetics of the Src homology 2 (SH2) phosphopeptide-binding domain. As a proof-of-concept, functionalized bis-dipicolylamine (BDPA) copper(ii) complexes are shown to disrupt oncogenic Stat3-Stat3 protein complexes and elicit promising anti-tumour activity.