Development of a New Benzophenone–Diketopiperazine-Type Potent Antimicrotubule Agent Possessing a 2‑Pyridine Structure

A new benzophenone–diketopiperazine-type potent antimicrotubule agent was developed by modifying the structure of the clinical candidate plinabulin (<b>1</b>). Although the right-hand imidazole ring with a branched alkyl chain at the 5-position in <b>1</b> was critical for the potency of the antimicrotubule activity, we successfully substituted this moiety with a simpler 2-pyridyl structure by converting the left-hand ring from a phenyl to a benzophenone structure without decreasing the potency. The resultant compound <b>6b</b> (KPU-300) exhibited a potent cytotoxicity, with an IC₅₀ value of 7.0 nM against HT-29 cells, by strongly binding to tubulin (<i>K</i>_d = 1.3 μM) and inducing microtubule depolymerization

Development of Potent Myostatin Inhibitory Peptides through Hydrophobic Residue-Directed Structural Modification

Myostatin, a negative regulator of skeletal muscle growth, is a promising target for treating muscle atrophic disorders. Recently, we discovered a minimal myostatin inhibitor <b>1</b> (WRQN­TRYS­RIEAIK­IQILS­KLRL-amide) derived from positions 21–43 of the mouse myostatin prodomain. We previously identified key residues (N-terminal Trp²¹, rodent-specific Tyr²⁷, and all aliphatic amino acids) required for effective inhibition through structure–activity relationship (SAR) studies based on <b>1</b> and characterized a 3-fold more potent inhibitor <b>2</b> bearing a 2-naphthyloxyacetyl group at position 21. Herein, we performed <b>1</b>-based SAR studies focused on all aliphatic residues and Ala³², discovering that the incorporations of Trp and Ile at positions 32 and 38, respectively, enhanced the inhibitory activity. Combining these findings with <b>2</b>, a novel peptide <b>3d</b> displayed an IC₅₀ value of 0.32 μM, which is 11 times more potent than <b>1</b>. The peptide <b>3d</b> would have the potential to be a promising drug lead to develop better peptidomimetics

Novel Hybrid Compound of a Plinabulin Prodrug with an IgG Binding Peptide for Generating a Tumor Selective Noncovalent-Type Antibody–Drug Conjugate

Although several approaches for making antibody–drug conjugates (ADC) have been developed, it has yet to be reported that an antibody binding peptide such as Z33 from protein A is utilized as the pivotal unit to generate the noncovalent-type ADC (NC-ADC). Herein we aim to establish a novel probe for NC-ADC by synthesizing the Z33-conjugated antitumor agent, plinabulin. Due to the different solubility of two components, including hydrophobic plinabulin and hydrophilic Z33, an innovative method with a solid-supported disulfide coupling reagent is required for the synthesis of the target compounds with prominent efficiency (29% isolated yield). We demonstrate that the synthesized hybrid exhibits a binding affinity against the anti-HER2 antibody (Herceptin) and the anti-CD71 antibody (6E1) (<i>K</i>_d = 46.6 ± 0.5 nM and 4.5 ± 0.56 μM, respectively) in the surface plasmon resonance (SPR) assay. In the cell-based assays, the hybrid provides a significant cytotoxicity in the presence of Herceptin against HER2 overexpressing SKBR-3 cells, but not against HER2 low-expressing MCF-7 cells. Further, it is noteworthy that the hybrid in combination with Herceptin induces cytotoxicity against Herceptin-resistant SKBR-3 (SKBR-3HR) cells. Similar results are obtained with the 6E1 antibody, suggesting that the synthesized hybrid can be widely applicable for NC-ADC using the antibody of interest. In summary, a series of evidence presented here strongly indicate that NC-ADCs have high potential for the next generation of antitumor agents