Rational Design of Azastatin as a Potential ADC Payload with Reduced Bystander Killing

ChemMedChem. 2020 Dec 15;15(24):2500-2512. doi: 10.1002/cmdc.202000497. Epub 2020 Oct 16.


Auristatins are a class of ultrapotent microtubule inhibitors, whose growing clinical popularity in oncology is based upon their use as payloads in antibody-drug conjugates (ADCs). The most widely utilized auristatin, MMAE, has however been shown to cause apoptosis in non-pathological cells proximal to the tumour ("bystander killing"). Herein, we introduce azastatins, a new class of auristatin derivatives encompassing a side chain amine for antibody conjugation. The synthesis of Cbz-azastatin methyl ester, which included the C2-elongation and diastereoselective reduction of two proteinogenic amino acids as key transformations, was accomplished in 22 steps and 0.76 % overall yield. While Cbz-protected azastatin methyl ester (0.13-3.0 nM) inhibited proliferation more potently than MMAE (0.47-6.5 nM), removal of the Cbz-group yielded dramatically increased IC50 -values (9.8-170 nM). We attribute the reduced apparent cytotoxicity of the deprotected azastatin methyl esters to a lack of membrane permeability. These results clearly establish the azastatins as a novel class of cytotoxic payloads ideally suited for use in next-generation ADC development.

Keywords: Antibodies; Cytotoxicity; Diastereoselectivity; Medicinal chemistry; Total synthesis.

MeSH terms

  • Antineoplastic Agents / chemical synthesis
  • Antineoplastic Agents / pharmacology*
  • Cell Line, Tumor
  • Cell Proliferation / drug effects
  • Drug Design
  • Drug Screening Assays, Antitumor
  • Humans
  • Oligopeptides / chemical synthesis
  • Oligopeptides / pharmacology*
  • Tubulin Modulators / chemical synthesis
  • Tubulin Modulators / pharmacology*


  • Antineoplastic Agents
  • Oligopeptides
  • Tubulin Modulators