Toward a Template for Synthetic Actin-Targeting Macrolide Analogues That Inhibit Cancer Cell Invasiveness

Actin barbed end-binding macrolides have been shown to inhibit cancer cell motility and invasion of extracellular matrix (ECM), evoking their potential utility as therapies for metastatic cancers. Unfortunately, the direct use of these compounds in clinical settings is impeded by their limited natural abundance, challenging total synthesis, and detrimental effects on normal tissues. To develop potent analogues of these compounds that are simpler to synthesize and compatible with cell-specific targeting systems, such as antibodies, we designed over 20 analogues of the acyclic side chain (tail) of the macrolide Mycalolide B. These analogues probed the contributions of four distinct regions of the tail towards the inhibition of actin polymerization and ECM invasion by human lung cancer A549 cells. We observed that two of these regions tolerate considerable substituent variability, and we identified a specific combination of substituents that leads to the optimal inhibition of the ECM invasion activity of A549 cells.

Truncated Actin-Targeting Macrolide Derivative Blocks Cancer Cell Motility and Invasion of Extracellular Matrix

Cancer metastasis is a complex process involving highly motile tumor cells that breach tissue barriers, enter the bloodstream and lymphatic system, and disseminate throughout the body as circulating tumor cells. The primary cellular mechanism contributing to these critical events is the reorganization of the actin cytoskeleton. Mycalolide B (MycB) is an actin-targeting marine macrolide that can suppress proliferation, migration, and invasion of breast and ovarian cancer cells at low nanomolar doses. Through structure-activity relationship studies focused on the actin-binding tail region (C24-C35) of MycB, we identified a potent truncated derivative that inhibits polymerization of G-actin and severs F-actin by binding to actin's barbed end cleft. Biological analyses of this miniature MycB derivative demonstrate that it causes a rapid collapse of the actin cytoskeleton in ovarian cancer cells and impairs cancer cell motility and invasion of the extracellular matrix (ECM) by inhibiting invadopodia-mediated ECM degradation. These studies provide essential proof-of-principle for developing actin-targeting therapeutic agents to block cancer metastasis and establish a synthetically tractable barbed end-binding pharmacophore that can be further improved by adding targeting groups for precision drug design.

Phenotypic Screen Identifies a Small Molecule Modulating ERK2 and Promoting Stem Cell Proliferation

Stem cells display a fundamentally different mechanism of proliferation control when compared to somatic cells. Uncovering these mechanisms would maximize the impact in drug discovery with a higher translational applicability. The unbiased approach used in phenotype-based drug discovery (PDD) programs can offer a unique opportunity to identify such novel biological phenomenon. Here, we describe an integrated phenotypic screening approach, employing a combination of in vitro and in vivo PDD models to identify a small molecule increasing stem cell proliferation. We demonstrate that a combination of both in vitro and in vivo screening models improves hit identification and reproducibility of effects across various PDD models. Using cell viability and colony size phenotype measurement we characterize the structure activity relationship of the lead molecule, and identify that the small molecule inhibits phosphorylation of ERK2 and promotes stem cell proliferation. This study demonstrates a PDD approach that employs combinatorial models to identify compounds promoting stem cell proliferation.