Lactone Stabilization is Not a Necessary Feature for Antibody Conjugates of Camptothecins

Camptothecins exist in a pH-dependent equilibrium between the active, closed lactone and the inactive open-carboxylate forms. Several previous reports underscore the need for lactone stabilization in generating improved camptothecins, and indeed, such designs have been incorporated into antibody–drug conjugates containing this drug. Here, we demonstrate that lactone stabilization is not necessary for camptothecin-based ADC efficacy. We synthesized and evaluated camptothecin SN-38 drug linkers that differed with respect to lactone stability and released SN-38 or the hydrolyzed open-lactone form upon cleavage from the antibody carrier. An α-hydroxy lactone-linked SN-38 drug linker preserved the closed-lactone ring structure, while the phenol-linked version allowed conversion between the closed-lactone and open-carboxylate structures. The in vitro cytotoxicity, pharmacokinetic properties, and in vivo efficacy in the L540cy Hodgkin's lymphoma model of the corresponding ADCs were found to be indistinguishable, leading us to conclude that camptothecin-based antibody–drug conjugates possess pronounced activity regardless of the lactone state of the bound drug. This is most likely a result of ADC processing within acidic intracellular vesicles, delivering camptothecin in its active closed-lactone form.