Synthesis and evaluation of highly releasable and structurally stable antibody-SN-38-conjugates

Camptothecins, traditional chemotherapy drugs, have been clinically used in antibody-drug conjugates (ADCs), which refreshes the recognition that ADCs preferably incorporate highly potent payloads. However, SN-38, active metabolite of irinotecan from camptothecins, tended to be incorporated into ADCs with an unstable acid sensitive bond, not with the widely used Cathepsin B (CTSB) sensitive bond, which may pose the risk of off-target. Herein, we reported a novel strategy to construct highly releasable and structurally stable SN-38-conjugates, in which CTSB linkers directly connected to the 10-OH group through ether bond, not to the common 20-OH group of lactones of SN-38. In this paper, rapid release of SN-38 was skillfully demonstrated by utilizing the fluorescence properties of SN-38. The SN-38-ether-ADC displayed highly stable serum stability with the half-life over 10 days. Moreover, the drug-antibody-ratio (DAR) of ADC could be elevated to 7.1 through the introduction of polyethylene glycol (PEG) moieties without aggregation. The optimized ADC exhibited potent in vitro activities up to 5.5 nM, comparable to SN-38. Moreover, this ADC group significantly delayed tumor growth in vivo. In conclusion, the novel strategy has the potential to promote the development of SN38-ADCs and enrich the conjugation approaches for hydroxyl-bearing payloads.