The emergence of antibody-drug conjugates (ADC), such as brentuximab vedotin and ado-trastuzumab emtansine, has led to increased efforts to identify new payloads and develop improved drug-linker technologies. Most antibody payloads impart significant hydrophobicity to the ADC, resulting in accelerated plasma clearance and suboptimal in vivo activity, particularly for conjugates with high drug-to-antibody ratios (DAR). We recently reported on the incorporation of a discrete PEG 24 polymer as a side chain in a b-glucuronidase-cleavable monomethylauristatin E (MMAE) linker to provide homogeneous DAR 8 conjugates with decreased plasma clearance and increased antitumor activity in xenograft models relative to a non-PEGylated control. In this work, we optimized the drug-linker by minimizing the size of the PEG side chain and incorporating a self-stabilizing maleimide to prevent payload de-conjugation in vivo. Multiple PEG-glucuronide-MMAE linkers were prepared with PEG size up to 24 ethylene oxide units, and homogeneous DAR 8 ADCs were evaluated. A clear relationship was observed between PEG length and conjugate pharmacology when tested in vivo. Longer PEG chains resulted in slower clearance, with a threshold length of PEG 8 beyond which clearance was not impacted. Conjugates bearing PEG of sufficient length to minimize plasma clearance provided a wider therapeutic window relative to faster clearing conjugates bearing shorter PEGs. A lead PEGylated glucuronide-MMAE linker was identified incorporating a self-stabilizing maleimide and a PEG 12 side chain emerged from these efforts, enabling highly potent, homogeneous DAR 8 conjugates and is under consideration for future ADC programs.

A quaternary ammonium-based drug-linker has been developed to expand the scope of antibody-drug conjugate (ADC) payloads to include tertiary amines, a functional group commonly present in biologically active compounds. The linker strategy was exemplified with a b-glucuronidase-cleavable auristatin E construct. The drug-linker was found to efficiently release free auristatin E (AE) in the presence of b-glucuronidase and provide ADCs that were highly stable in plasma. Anti-CD30 conjugates comprised of the glucuronide-AE linker were potent and immunologically specific in vitro and in vivo, displaying pharmacologic properties comparable with a carbamate-linked glucuronide-monomethylauristatin E control. The quaternary ammonium linker was then applied to a tubulysin antimitotic drug that contained an N-terminal tertiary amine that was important for activity. A glucuronide-tubulysin quaternary ammonium linker was synthesized and evaluated as an ADC payload, in which the resulting conjugates were found to be potent and immunologically specific in vitro, and displayed a high level of activity in a Hodgkin lymphoma xenograft. Furthermore, the results were superior to those obtained with a related tubulysin derivative containing a secondary amine N-terminus for conjugation using previously known linker technology. The quaternary ammonium linker represents a significant advance in linker technology, enabling stable conjugation of payloads with tertiary amine residues.Mol Cancer Ther; 15(5); 938-45. Ó2016 AACR.

Although antibody-drug conjugates (ADCs) find increasing applications in cancer treatment, 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) and loss of cognate antigen expression. New technologies that circumvent these resistance mechanisms may serve to extend the utility of next-generation 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 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 phenotype. The ADCs also demonstrated potent bystander activity in a coculture model 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 phenotype and robust activity in models of tumor heterogeneity in a structure-dependent manner. .

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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.