Structural and functional characterization of IgG- and non-IgG-based T-cell-engaging bispecific antibodies.

Bispecific T-cell-engaging antibodies are a growing class of therapeutics with numerous molecules being tested in clinical trials and, currently, seven of them have received market approval. They are structurally complex and function as adaptors to redirect the cytotoxicity of T cells to kill tumor cells. T-cell-engaging bispecific antibodies can be generally divided into two categories: IgG/IgG-like and non-IgG-like formats. Different formats may have different intrinsic potencies and physiochemical properties, and comprehensive studies are needed to gain a better understanding of how the differences in formats impact on structural and functional characteristics. In this study, we designed and generated bispecific T-cell-engaging antibodies with IgG-like (DVD-Ig) and non-IgG (BiTE) formats. Both target the same pair of antigens (EGFR and CD3) to minimize the possible influence of targets on functional characterization. We performed a side-by-side comparison to assess differences in the physiochemical and biological properties of these two bispecific T-cell-engaging antibodies using a variety of breast and ovarian cancer cell-based functional assays to delineate the structural-functional relationships and anti-tumor activities/potency. We found that the Fc portion of T-cell-engaging bispecific antibodies can significantly impact antigen binding activity, potency, and stability in addition to eliciting different mechanisms of action that contribute the killing of cancer cells.

Total Synthesis of Rameswaralide Utilizing a Pharmacophore-Directed Retrosynthetic Strategy.

A pharmacophore-directed retrosynthetic strategy was applied to the first total synthesis of the cembranoid rameswaralide in order to simultaneously achieve a total synthesis while also developing a structure-activity relationship profile throughout the synthetic effort. The synthesis utilized a Diels-Alder lactonization process, including a rare kinetic resolution to demonstrate the potential of this strategy for an enantioselective synthesis providing both the 5,5,6- and, through a ring expansion, 5,5,7-tricyclic ring systems present in several Sinularia soft coral cembranoids. A pivotal synthetic intermediate, a tricyclic epoxy α-bromo cycloheptenone, displayed high cytotoxicity with interesting selectivity toward the HCT-116 colon cancer cell line. This intermediate enabled the pursuit of three unique D-ring annulation strategies including a photocatalyzed intramolecular Giese-type radical cyclization and a diastereoselective, intramolecular enamine-mediated Michael addition, with the latter annulation constructing the final D-ring to deliver rameswaralide. The serendipitous discovery of an oxidation state transposition of the tricyclic epoxy cycloheptenone proceeding through a presumed doubly vinylogous, E1-type elimination enabled the facile introduction of the required α-methylene butyrolactone. Preliminary biological tests of rameswaralide and precursors demonstrated weak cytotoxicity; however, the comparable cytotoxicity of a simple 6,7-bicyclic ß-keto ester, corresponding to the CD-ring system of rameswaralide, to that of the natural product itself suggests that such bicyclic ß-ketoesters may constitute an interesting pharmacophore that warrants further exploration.

Pharmacophore-Directed Retrosynthesis Applied to Rameswaralide: Synthesis and Bioactivity of Sinularia Natural Product Tricyclic Cores.

A pharmacophore-directed retrosynthesis strategy applied to rameswaralide provided simplified precursors bearing the common 5,5,6 (red) and 5,5,7 (blue) skeleton present in several cembranoid and norcembranoids from Sinularia soft corals. Key steps include a Diels-Alder lactonization organocascade delivering the common 5,5,6 core and a subsequent ring expansion affording a 5,5,7 core serviceable for the synthesis of rameswaralide. Initial structure-activity relationships of intermediates en route to the natural product have revealed interesting differential and selective cytotoxicity.

Inhibition of Eukaryotic Translation by the Antitumor Natural Product Agelastatin A.

Protein synthesis plays an essential role in cell proliferation, differentiation, and survival. Inhibitors of eukaryotic translation have entered the clinic, establishing the translation machinery as a promising target for chemotherapy. A recently discovered, structurally unique marine sponge-derived brominated alkaloid, (-)-agelastatin A (AglA), possesses potent antitumor activity. Its underlying mechanism of action, however, has remained unknown. Using a systematic top-down approach, we show that AglA selectively inhibits protein synthesis. Using a high-throughput chemical footprinting method, we mapped the AglA-binding site to the ribosomal A site. A 3.5 Å crystal structure of the 80S eukaryotic ribosome from S. cerevisiae in complex with AglA was obtained, revealing multiple conformational changes of the nucleotide bases in the ribosome accompanying the binding of AglA. Together, these results have unraveled the mechanism of inhibition of eukaryotic translation by AglA at atomic level, paving the way for future structural modifications to develop AglA analogs into novel anticancer agents.