Glucuronide-Linked Antibody-Tubulysin Conjugates Display Activity in MDR+ thorn and Heterogeneous Tumor Models

Although antibody-drug conjugates (ADCs) find increasing applications in cancer treatment, de novo or treatment-emergent resistance mechanisms may impair clinical benefit. Two resistance mechanisms that emerge under prolonged exposure include upregulation of transporter proteins that confer multidrug resistance (MDR thorn) and loss of cognate antigen expression. New technologies that circumvent these resistance mechanisms may serve to extend the utility of nextgeneration ADCs. Recently, we developed the quaternary ammonium linker system to expand the scope of conjugatable payloads to include tertiary amines and applied the linker to tubulysins, a highly potent class of tubulin binders that maintain activity in MDR thorn cell lines. In this work, tubulysin M, which contains an unstable acetate susceptible to enzymatic hydrolysis, and two stabilized tubulysin analogues were prepared as quaternary ammonium-linked glucuronide- linkers and assessed as ADC payloads in preclinical models. The conjugates were potent across a panel of cancer cell lines and active in tumor xenografts, including those displaying the MDR thorn phenotype. The ADCs also demonstrated potent bystander activity in a coculturemodel comprised of a mixture of antigen-positive and -negative cell lines, and in an antigen-heterogeneous tumor model. Thus, the glucuronide-tubulysin drug-linkers represent a promising ADC payload class, combining conjugate potency in the presence of the MDR thorn phenotype and robust activity in models of tumor heterogeneity in a structure-dependent manner. (C) 2018 AACR.