Abstract 2600: Novel dual TOP1i ADC inducing superior tumour growth inhibition at low-drug load vs. trastuzumab deruxtecan

Abstract The Araris’ site-specific and one-step conjugation technology aims at generating stable, safe and highly potent ADCs without the need for antibody engineering prior to payload conjugation. Here, we generated an anti-HER2 ADC using two different Topoisomerase-1 inhibitors (TOP1i) as payloads that shows superior anti-tumor efficacy compared to trastuzumab deruxtecan (T-DXd) in head-to-head in vivo studies. For proof-of concept and comparison to T-DXd, trastuzumab was used as the targeting antibody. The Araris ADC was designed to combine two features in one ADC to maximize tumor-specific activity by using two different exatecan-based payloads: one that is able to exert bystander activity to address tumor heterogeneity and low-target expression and one that accumulates in cancer cells (no bystander activity) to achieve greater potency. We found that the ADC was well-defined with a drug to antibody ratio (DAR) of 4 (2+2), as expected and no signs of aggregation under stressed conditions (40°C) during a period of 14 days. In in-vitro assays on target positive cell-lines, the ADC demonstrated taget-specific cell-cytotoxicity in the low nM-range similarly to T-DXd. Importantly, the non-bystander exatecan showed an increased intracellular concentration (up to 4x) and the bystander capable exatecan demonstrated high bystander activity in co-cultured, target-negative cell lines. In mouse pharmacokinetic studies, the ADC showed excellent stability in circulation with no signs of payload loss or linker-cleavage with a PK profile comparable to the unconjugated trastuzumab antibody which is key for a maximal and tumor-specific payload delivery. Finally, a head-to-head study vs T-DXd was done in a challenging, medium HER2-expressing breast cancer model (JIMT-1), known to be resistant against the FDA-approved ADC T-DM1 (trastuzumab emtansine) and for T-DXd only showing limited activity (Ogitani et al., 2016). Impressively, it was found that the dual-TOP1i DAR4 Araris ADC showed superior anti-tumor efficacy compared to the payload-dose-adjusted T-DXd (DAR8). Using published doses for T-DXd, the Araris ADC was administered at ADC doses of 2 × 10mg/kg on days 1 and 8, resulting in superior anti-tumor activity and tumor eradication compared to T-DXd lasting for more than 47 days. T-DXd dosed at 2 × 5mg/kg (same payload dose) on days 1 and 8, only showed a very limited tumor growth inhibition with tumor regrowth occurring in all animals already at around 21 days. We here show first encouraging results on a novel concept of combining TOP1i payloads that have two different features in one ADC to maximize efficacy. We believe that this concept in combination with a stable payload attachment at low DAR and an excellent exposure may help to develop ADCs with an improved therapeutic index for various solid tumor indications. Citation Format: Philipp Spycher, Rachael Fay, Romain Bertrand, Philipp Probst, Ramona Stark, Roger Santimaria, Patrick Maurhofer, Lia Kallenberger, Emma Renard, Bernd Schlereth, Dragan Grabulovski, Isabella Attinger-Toller. Novel dual TOP1i ADC inducing superior tumour growth inhibition at low-drug load vs. trastuzumab deruxtecan [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2024; Part 1 (Regular Abstracts); 2024 Apr 5-10; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2024;84(6_Suppl):Abstract nr 2600.