Background: Trophoblast antigen 2 (Trop2) or Tumor-Associated Calcium Signal Transducer 2 (TACSTD2) is a transmembrane glycoprotein that is overexpressed in several solid tumor cancers. Antibody drug conjugate (ADC) targeting of Trop2 has been clinically validated. The Trop2 ADC Trodelvy® (also known as Sacituzumab govitecan/IMMU-132) is approved for therapy of TNBC and urothelial cancer, and Datopotamab DXd (DS-1062) is currently being tested in NSCLC.

Peak Bio’s Thailanstatin suite of immunomodulatory linker-toxins (L-Ts) is a set of 7 related molecules with distinct ADC features that have been extensively characterized in vitro and in vivo as Her2 ADCs and is being used as a platform to generate a pipeline of potentially differentiated ADCs (AACR; Cancer Res 2021;81(13_Suppl):Abstract nr 1832).

Results: M2.8 is a fully humanized Peak Bio antibody (Ab) that targets human Trop2 (hTrop2) with high binding affinity [Kd (M)= 8.97-09]. M2.8 was optimized for binding affinity and humanization relative to its parent M2.1 [Kd (M)= 1.78-08], and, both M2.1 and M2.8, display species-specificity for the human and cynomolgus monkey Trop2 proteins. When bound to hTrop2-positive cell lines, M2.1 and M2.8 can rapidly internalize and deliver an ADC payload.

To identify the optimal release for therapeutic efficacy, we conjugated our family of L-Ts with hRS7 at a comparable drug-to-antibody ratio (DAR) of 4. Maximal tumor growth inhibition (TGI) was associated with the Trop2 ADCs that were conjugated via amine coupling to lysine residues using non-cleavable L-T L22, later renamed PH1.

To determine the effect of antibody on TGI efficacy, different doses, and DARs of M2.1 PH1 ADC were compared to PH1 ADCs made from other Trop2 Abs. At low dose levels, M2.1 PH1 ADCs exhibited comparable TGI with hRS7 PH1, TINA PH1, and T6-16 PH1 ADCs, but exhibited significantly greater TGI than hRS7 SN38 (IMMU-132), suggesting that payload potency, rather than epitope, was important in targeting Trop2. The results also suggested that a PH1 DAR of 4 was sufficient to obtain 90% TGI with multiple anti-Trop2 Abs.

M2.1 PH1 ADC was safely tolerated by cynomolgus monkeys at 3 and 6 mg/kg Q3W x 3 doses with mild-to-moderate changes in liver enzymes and platelets that reversed to baseline within 7-10 days of administration of each dose.

The optimized M2.8 PH1 DAR4 ADCs killed Trop2-positive cell lines with single-digit nanomolar potency, while exhibiting low off-target cytotoxicity vs normal human skin fibroblasts and Trop2-negative cell lines in vitro. M2.8 PH1 ADC regressed all 200mm3 sized NCI-N87 tumors and 50% of these tumors exhibited long-term regression for 5 months.

Conclusion: The target validation studies suggest that Trop2 PH1 ADCs were sufficiently differentiated in areas of preclinical efficacy and well tolerated in a toxicologically relevant host.

Citation Format: Satyajit K. Mitra, William Monteith, Mary Do, Greg Tuffy, Scott Savage, Jeffrey Kang, Mastewal Abuhay, Teodora Losic, Sanjeevani Ghone, William E. Haskins, Vasu Jammalamadaka, Sanjeev Satyal. Rationale for the development of a differentiated Trop2 ADC [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2023; Part 1 (Regular and Invited Abstracts); 2023 Apr 14-19; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2023;83(7_Suppl):Abstract nr 6297.