Tubulysin M

Tubulysins have emerged in recent years as a compelling drug class for delivery to tumor cells via antibodies. The ability of this drug class to exert bystander activity while retaining potency against multidrug-resistant cell lines differentiates them from other microtubule-disrupting agents. Tubulysin M, a synthetic analogue, has proven to be active and well tolerated as an antibody-drug conjugate (ADC) payload, but has the liability of being susceptible to acetate hydrolysis at the C11 position, leading to attenuated potency. In this work, we examine the ability of the drug-linker and conjugation site to preserve acetate stability. Our findings show that, in contrast to a more conventional protease-cleavable dipeptide linker, the β-glucuronidase-cleavable glucuronide linker protects against acetate hydrolysis and improves ADC activity in vivo. In addition, site-specific conjugation can positively impact both acetate stability and in vivo activity. Together, these findings provide the basis for a highly optimized delivery strategy for tubulysin M.

Molecular design, synthesis, and biological evaluation of tubulysin analogues, linker-drugs, and antibody-drug conjugates are described. Among the new discoveries reported is the identification of new potent analogues within the tubulysin family that carry a C11 alkyl ether substituent, rather than the usual ester structural motif at that position, a fact that endows the former with higher plasma stability than that of the latter. Also described herein are X-ray crystallographic analysis studies of two tubulin-tubulysin complexes formed within the α/β interface between two tubulin heterodimers and two highly potent tubulysin analogues, one of which exhibited a different binding mode to the one previously reported for tubulysin M. The X-ray crystallographic analysis-derived new insights into the binding modes of these tubulysin analogues explain their potencies and provide inspiration for further design, synthesis, and biological investigations within this class of antitumor agents. A number of these analogues were conjugated as payloads with appropriate linkers at different sites allowing their attachment onto targeting antibodies for cancer therapies. A number of such antibody-drug conjugates were constructed and tested, both in vivo and in vitro, leading to the identification of at least one promising ADC (Herceptin-LD3), warranting further investigations.

Microtubule-targeting agents (MTAs) are the most commonly used anti-cancer drugs. At least fourteen microtubule inhibitors and ten antibody drug conjugates (ADCs) linking MTAs are approved by FDA for clinical use in cancer therapy. In current research, we determined the crystal structure of tubulysin analogue TGL in complex with tubulin at a high resolution (2.65 Å). In addition, we summarized all of the previously published high-resolution crystal structures of ligands in the vinca site to provide structural insights for the rational design of the new vinca-site ligands. Moreover, based on the aligned results of the vinca site ligands, we provided three possible routes for designing new tubulysin analogues, namely macrocyclization between the N-14 side chain and the N-9 side chain, the hybird of tubulysin M and phomopsin A, and growing new aryl group at C-21. These designed structures will inspire the development of new MTAs or payloads in cancer therapy.