Improving Antibody-Tubulysin Conjugates through Linker Chemistry and Site-Specific Conjugation

Improved Access to Potent Anticancer Tubulysins and Linker-Functionalized Payloads Via an All-On-Resin Strategy

Tubulysins are among the most recent antimitotic compounds to enter into antibody/peptide-drug conjugate (ADC/PDC) development. Thus far, the design of the most promising tubulysin payloads relied on simplifying their structures, e. g., by using small tertiary amide N-substituents (Me, Et, Pr) on the tubuvaline residue. Cumbersome solution-phase approaches are typically used for both syntheses and functionalization with cleavable linkers. p-Aminobenzyl quaternary ammonium (PABQ) linkers were a remarkable advancement for targeted delivery, but the procedures to incorporate them into tubulysins are only of moderate efficiency. Here we describe a novel all-on-resin strategy permitting a loss-free resin linkage and an improved access to super potent tubulysin analogs showing close resemblance to the natural compounds. For the first time, a protocol enables the integration of on-resin tubulysin derivatization with, e. g., a maleimido-Val-Cit-PABQ linker, which is a notable progress for the payload-PABQ-linker technology. The strategy also allows tubulysin diversification of the internal amide N-substituent, thus enabling to screen a tubulysin library for the discovery of new potent analogs. This work provides ADC/PDC developers with new tools for both rapid access to new derivatives and easier linker-attachment and functionalization.

Exploring the next generation of antibody-drug conjugates

Antibody-drug conjugates (ADCs) are a promising cancer treatment modality that enables the selective delivery of highly cytotoxic payloads to tumours. However, realizing the full potential of this platform necessitates innovative molecular designs to tackle several clinical challenges such as drug resistance, tumour heterogeneity and treatment-related adverse effects. Several emerging ADC formats exist, including bispecific ADCs, conditionally active ADCs (also known as probody-drug conjugates), immune-stimulating ADCs, protein-degrader ADCs and dual-drug ADCs, and each offers unique capabilities for tackling these various challenges. For example, probody-drug conjugates can enhance tumour specificity, whereas bispecific ADCs and dual-drug ADCs can address resistance and heterogeneity with enhanced activity. The incorporation of immune-stimulating and protein-degrader ADCs, which have distinct mechanisms of action, into existing treatment strategies could enable multimodal cancer treatment. Despite the promising outlook, the importance of patient stratification and biomarker identification cannot be overstated for these emerging ADCs, as these factors are crucial to identify patients who are most likely to derive benefit. As we continue to deepen our understanding of tumour biology and refine ADC design, we will edge closer to developing truly effective and safe ADCs for patients with treatment-refractory cancers. In this Review, we highlight advances in each ADC component (the monoclonal antibody, payload, linker and conjugation chemistry) and provide more-detailed discussions on selected examples of emerging novel ADCs of each format, enabled by engineering of one or more of these components.

Antibody Drug Conjugates in Bladder Cancer: Current Milestones and Future Perspectives

OPINION STATEMENT

Over the last several years, the treatment landscape of urothelial carcinoma has witnessed an unprecedented expansion of therapeutic options including checkpoint inhibitors, tyrosine kinase inhibitors, and antibody drug conjugates (ADC). Early trial data has shown that ADCs are safer and potentially effective treatment options in advanced bladder cancer as well as in the early disease. In particular, enfortumab-vedotin (EV) has shown promising results with a recent cohort of a clinical trial demonstrating that EV is effective as neoadjuvant monotherapy as well as in combination with pembrolizumab in metastatic setting. Similar promising results have been shown by other classes of ADC in other trials including sacituzumab-govitecan (SG) and oportuzumab monatox (OM). ADCs are likely to become a mainstay treatment option in the urothelial carcinoma playbook as either a monotherapy or combination therapy. The cost of the drug presents a real challenge, but further trial data may justify the use of the drug as mainstay treatment.

Evaluation of an ester-linked immunosuppressive payload: A case study in understanding the stability and cleavability of ester-containing ADC linkers

In spite of their value in prodrug applications, the use of esters in antibody-drug-conjugate (ADC) payloads and linkers has generally been avoided due to the ubiquitous and promiscuous nature of human esterases. ADCs generally have a long circulating half life (3-7 days) that makes them susceptible to esterase-mediated metabolism. Moreover, it is largely unclear whether lysosomal and cytosolic esterases cleave ester-containing linkers upon ADC internalization. Due to our interest in the targeted delivery of immune-modulators, our team has recently prepared a series of ester-linked dexamethasone ADCs. Herein, we report our studies of the functional activity of these ADCs, with a particular focus on their catabolism in various biological milieu. We found that esters are selectively but inefficiently cleaved upon cellular uptake, likely by cytosolic esterases. Lysosomal catabolism studies indicate that, in spite of the strong proteolytic activity, very little cleavage of ester-containing linkers occurs in the lysosome. However, ADCs bearing the ester-linked payloads are active in various immune-suppressive assays, suggesting that cytosolic cleavage is taking place. This was confirmed through LCMS quantitation of the payload following cell lysis. Finally, the stability of the ester linkage was evaluated in mouse and human plasma. We found, similar to other reports, there is a significant site-dependence on the cleavage. Esters attached at highly exposed sites, such as 443C, were rapidly cleaved in plasma while esters at more hindered sites, such at 334C, were not. Together, these results help to unravel the complexities of ester-incorporation into ADC linkers and pave a path forward for their utility in ADC applications.

Glucuronides: From biological waste to bio-nanomedical applications

Long considered as no more than biological waste meant to be eliminated in urine, glucuronides have recently contributed to tremendous developments in the biomedical field, particularly against cancer. While glucuronide prodrugs monotherapy and antibody-directed enzyme prodrug therapy have been around for some time, new facets have emerged that combine the unique properties of glucuronides notably in the fields of antibody-drug conjugates and nanomedicine. In both cases, glucuronides are utilized as a vector to improve pharmacokinetics and confer localized activation of potent drugs at tumor sites while also decreasing systemic toxicity. Here we will discuss some of the most promising strategies using glucuronides to promote successful anti-tumor therapeutic treatments.

Advanced Pyrrolidine-Carbamate Self-Immolative Spacer with Tertiary Amine Handle Induces Superfast Cyclative Drug Release

Amine-carbamate self-immolative (SI) spacers represent practical and versatile tools in targeted prodrugs, but their slow degradation mechanism limits drug activation at the site of disease. We engineered a pyrrolidine-carbamate SI spacer with a tertiary amine handle which strongly accelerates the spacer cyclization to give a bicyclic urea and the free hydroxy groups of either cytotoxic (Camptothecin) or immunostimulatory (Resiquimod) drugs. In silico conformational analysis and p K a calculations suggest a plausible mechanism for the superior efficacy of the advanced SI spacer compared to state-of-art analogues.

A camptothecin-based, albumin-binding prodrug enhances efficacy and safety in vivo

The albumin-based drug delivery system is an effective drug delivery strategy for traditional chemotherapeutic drugs that can improve their antitumour efficacies and reduce systemic toxicities. The camptothecin derivative CPTS0001 has excellent antitumour activity in vitro, but it shows toxicity and side effects in vivo. In this study, we report the synthesis and biological evaluation of the β-glucuronidase-reactive albumin-binding prodrug Mal-glu-CPTS0001 based on quaternary ammonium. After intravenous administration, the compound covalently binds to plasma albumin through Michael addition, enabling it to accumulate in tumours, where tumour-associated β-glucuronidase triggers the selective release of CPTS0001. This prodrug significantly reduced the toxicity of the parent drug, and the maximum tolerated dose was increased by 2.5 times. At the same time, this prodrug enhanced the selectivity in vivo and improved the preferential accumulation of prodrug in tumours. Notably, this prodrug exhibited excellent in vivo antitumour effects in a murine breast cancer xenograft model without visible pathological toxicity.