Targeting KRAS and the vitamin D receptor via microtubules

Plinabulin, an inhibitor of tubulin polymerization, targets KRAS signaling through disruption of endosomal recycling

Constitutive activation of Kirsten rat sarcoma viral oncogene homolog (KRAS) is the most common oncogenic event in certain types of human cancer and is associated with poor patient survival. Small molecule signaling inhibitors have improved the clinical outcomes of patients with various cancer types but attempts to target KRAS have been unsuccessful. Plinabulin represents a novel class of agents that inhibit tubulin polymerization with a favorable safety profile in clinical trials. In the present study, the potency of plinabulin to inhibit tubulin polymerization and growth of KRAS-driven cancer cells was characterized. In vivo efficacy of plinabulin was tested in two different mouse models; one being the RCAS/t-va gene transfer system and the other being a xenograft model. In vitro cell culture tubulin polymerization assays were used to complement the mouse models. There was improved survival in a KRAS-driven mouse gene transfer glioma model, but lack of benefit in a similar model, without constitutively active KRAS, which supports the notion of a KRAS-specific effect. This survival benefit was mediated, at least in part, by the ability of plinabulin to inhibit tubulin polymerization and disrupt endosomal recycling. It was proposed a mechanism of compromised endosomal recycling of displaced KRAS through targeting microtubules that yields inhibition of protein kinase B, but not extracellular signal regulated kinase (ERK) signaling, therefore lending rationale to combination treatments of tubulin- and ERK-targeting agents in KRAS-driven cancer.

A Novel Microtubule Inhibitor Overcomes Multidrug Resistance in Tumors

Microtubule inhibitors as chemotherapeutic drugs are widely used for cancer treatment. However, the development of multidrug resistance (MDR) in cancer is a major challenge for microtubule inhibitors in their clinical implementation. From a high-throughput drug screen using cells transformed by oncogenic RAS, we identify a lead heteroaryl amide compound that blocks cell proliferation. Analysis of the structure-activity relationship indicated that this series of scaffolds (exemplified by MP-HJ-1b) represents a potent inhibitor of tumor cell growth. MP-HJ-1b showed activities against a panel of more than 1,000 human cancer cell lines with a wide variety of tissue origins. This compound depolymerized microtubules and affected spindle formation. It also induced the spike-like conformation of microtubules in vitro and in vivo, which is different from typical microtubule modulators. Structural analysis revealed that this series of compounds bound the colchicine pocket at the intra-dimer interface, although mostly not overlapping with colchicine binding. MP-HJ-1b displayed favorable pharmacological properties for overcoming tumor MDR, both in vitro and in vivo Taken together, our data reveal a novel scaffold represented by MP-HJ-1b that can be developed as a cancer therapeutic against tumors with MDR.Significance: Paclitaxel is a widely used chemotherapeutic drug in patients with multiple types of cancer. However, resistance to paclitaxel is a challenge. This study describes a novel class of microtubule inhibitors with the ability to circumvent multidrug resistance across multiple tumor cell lines. Cancer Res; 78(20); 5949-57. ©2018 AACR.

Pancreatic Cancer: “A Riddle Wrapped in a Mystery inside an Enigma”

Pancreatic ductal adenocarcinoma (PDAC) is one of the most difficult-to-treat cancers. With an increasing incidence and inability to make major progress, it represents the very definition of unmet medical need. Progress has been made in understanding the basic biology—systematic genomic sequencing has led to the recognition that PDAC is not typically a heavily mutated tumor, although there are exceptions. The most consistently mutated genes are KRAS, CDKN2A, TP53, and SMAD4/DPC4. Study of familial PDAC has led to the recognition that a variety of defects in DNA repair genes can be associated with the emergence of pancreatic cancer. Recent studies suggest that epigenetics may play a larger role than previously recognized. A major new understanding is the recognition that PDAC should be considered a composite of tumor cells, as well as pancreatic stellate cells, immune cells, and extracellular matrix. The individual components contribute to metabolic aberration, immune dysfunction, and chemotherapy resistance, and therapeutic innovations may be needed to address them individually. It has also been recognized that metastatic seeding from PDAC occurs very early in the disease course—in an estimated 73% of cases, once the tumor reaches 2 cm. The implication of this is that therapies directed toward micrometastatic disease and increasing fractional cell kill are most needed. Neoadjuvant approaches have been taken to increase resectability and improve outcome. So much work remains, and most critical is the need to understand how this tumor originates and develops. Clin Cancer Res; 23(7); 1629–37. ©2017 AACR.

See all articles in this CCR Focus section, “Pancreatic Cancer: Challenge and Inspiration.”