SPHINGOLIPIDS AS TUMOR SUPPRESSOR LIPIDS
The natural product, phytosphingosine, protects yeast from stress by triggering the endocytosis of transporters for amino acids and uracil, starving yeast into adaptive quiescence under unfavorable growth conditions. This phytosphingosine-sensitive pathway is conserved in mammalian cells and could be activated to starve cancer cells to death. Cancer cells are nutrient addicts – oncogenic mutations constitutively drive growth, sensitizing transformed cells to an attack on the supply wagons. Normal cells, in contrast, respond to nutrient stress by becoming quiescent and catabolic until nutrients are restored. This differential response is the basis of the therapeutic index of SH-BC-893 (893), an orally bioavailable phytosphingosine mimic that selectively starves cancer cells to death by limiting access to amino acids, glucose, and low density lipoprotein particles in vitro and in vivo. 893 is an excellent tool compound to dissect the mechanism by which natural sphingolipids control cell growth. Natural sphingolipids are metabolized into many derivatives that have diverse, often opposing, functions. Constraining the conformationally flexible aminodiol portion of FTY720 has generated a stable molecule that is not phosphorylated and is resistant to metabolism both in vitro or in vivo. More importantly, 893 is an efficacious pre-clinical drug candidate with outstanding pharmacological properties and an impressive safety profile. In a rigorous, genetically-engineered mouse model for aggressive prostate cancer, oral dosing with 893 reduces tumor growth by 82%. In a related subcutaneous isograft model, 893 produces tumor regressions in >50% of mice. 893 is also effective against patient-derived prostate tumor organoids and xenografts. Using the dosing protocol that inhibits tumor growth, 893 does not disrupt organ function or the proliferation of normal bone marrow or intestinal crypt cells even after 11 wk of treatment. The resistance of normal cells is likely attributable to oscillating blood levels with once a day dosing and the accumulation of 893 in tumors relative to plasma. Taken together, these studies clearly demonstrate the outstanding therapeutic potential of this novel approach to targeting cancer metabolism. Our published work establishes that 893 activates the tumor suppressor protein phosphatase 2A (PP2A) and that PP2A activation is necessary for 893 to limit nutrient access. Current research investigates how 893 activates PP2A and whether this activity is sufficient to explain 893’s anti-cancer effects.
STRUCTURE-ACTIVITY RELATIONSHIP (SAR) STUDIES
Through a long-term collaboration with Stephen Hanessian’s chemistry group, we have dissected the structural features of 893 that are critical for its ability to disrupt intracellular trafficking and kill cancer cells. A series of analogs have been developed that are differentially active, producing some, but not all, of the 893-associated phenotypes. These compounds are powerful tools to dissect the pleiotropic actions of sphingolipids. Working with Pierre Thibault’s proteomics group at the University of Montreal, we have identified the protein targets of 893 and several of these analogs. We are now using biochemical assays, molecular modeling, and crystal structures to map the specific binding sites of 893 and related natural sphingolipids on these protein targets. These SAR studies coupled with chemical biology and structural biology efforts currently underway will lead to new analogs with increased potency and selectivity for different 893 target proteins.
MACROPINOCYTOSIS AND NUTRIENT SCAVENGING
Cancer cells require a steady stream of nutrients to support their unchecked growth. However, the blood vessels that supply these nutrients are often inadequate, tortuous, and leaky. Desmoplasia can also lead to elevated interstitial pressure that collapses tumor blood vessels. Cancer cells overcome these limitations on nutrient delivery by scavenging macromolecules from the microenvironment. One scavenging strategy employed by tumors is macropinocytosis, a process by which extracellular material is non-specifically engulfed. Oncogenic mutations in KRAS and loss of function mutations in PTEN drive macropinocytosis in pancreatic and prostate cancer, respectively. In both tumor classes, macropinocytosis confers the ability to proliferate in nutrient-limiting conditions. Blocking macropinocytosis with EIPA, an inhibitor of the Na+/H+ exchangers, significantly inhibits tumor growth in both model systems, even causing tumor regressions. We have identified AMP-activated protein kinase (AMPK) as a critical regulator of macropinocytosis downstream from oncogene activation. In addition, we have shown that macropinocytic tumor cells can feed off the corpses of their dead neighbors through a process we termed “necrocytosis.” The conditions present in virtually all solid tumors will favor nutrient scavenging and, because both KRAS activation and PI3K pathway activation can drive macropinocytosis, macropinocytosis inhibitors have the potential to make a large therapeutic impact across cancer classes. On-going research in the Edinger lab address key open questions in the field: How pervasive is macropinocytosis across different tumor types? How will blocking macropinocytosis affect tumor initiation and progression? How do agents that interfere with macropinocytosis affect the tumor microenvironment? Answering these questions will provide novel insights into tumor cell biology and could provide a rationale for new therapeutic strategies.
ANTISENSE OLIGONUCLEOTIDE POTENTIATION
Multiple antisense oligonucleotide (ASO) therapies are now FDA-approved for chronic diseases demonstrating the feasibility and safety of this strategy for reducing the level of cellular RNAs. ASO are the ultimate platform technology. In principle, any RNA, coding or non-coding, can be targeted by ASO making the “undruggable” targetable and the applications nearly limitless. Medicinal chemistry optimization over the last three decades has solved historical problems with stability and rapid clearance. However, a barrier to the widespread application of ASO therapeutics remains: inefficient delivery to the intracellular RNA targets. The majority of the systemically delivered ASO is not taken up by cells, and much of what is endocytosed ends up sequestered in the lysosome where ASO is stable, but inactive. ASO therapeutics have great potential as cancer therapies. In pre-clinical studies, ASO can limit primary tumor growth, slow metastasis, and even enhance the anti-tumor immune response. Four oncology ASO developed by Ionis Pharmaceuticals, the world leader in therapeutic antisense technology, are in clinical trials, and many more are in the pipeline. Our goal is to use our knowledge of endolysosomal trafficking to identify small molecules that enhance ASO activity in tumors. Enhancing ASO activity could convert tumor growth inhibition into tumor regression leading to significant gains in overall survival. If these compounds also increase ASO activity in normal cells, they could be transformative, dramatically expanding the number of diseases that could be treated with antisense therapies.