Abstract 1523: Subcutaneous dosing of amatoxin-based ADCs increases the therapeutic index

Abstract Background: Amatoxin-based antibody-drug conjugates (so called ATACs) are a new class of ADCs that use the RNA polymerase II inhibitor amanitin as toxic payload. HDP-101 is the first ATAC in the clinic, currently being tested in a phase I/II clinical trial in multiple myeloma patients. HDP-101 is given to patients intravenously (i.v.) as this is the most common administration route for ADCs. I.v. is often selected as route of administration in a clinical setting since it results in a bioavailability of 100%. However, i.v. dosing has disadvantages such as very high Cmax levels which can trigger toxicity, requires hospitalization of the patient for administration, and bears the risk of vessel damage. In this study, we demonstrate that subcutaneous (s.c.) dosing is a promising alternative route of administration for ATACs which is well tolerated and able to improve the therapeutic index (TI) of ATACs. Material and Methods: ATACs: ADCs based on cysteine-reactive and site-specific amatoxin-linker constructs synthesized at Heidelberg Pharma. Monkey Studies: Cynomolgus monkeys, escalating doses of ATACs i.v or s.c. q21d Mouse Studies: CB17 Scid mice, single dose of ATACs i.v. or s.c.; if applicable bleeding at 8 - 12 time points between 5 min and 14 days after dosing; tumor studies in s.c. and i.v. tumor models Results: The impact of the dosing route on organ distribution of ATACs was tested in a PK study after a single i.v. or s.c. ATAC dose. S.c. dosing resulted in increased serum half-life of ATACs and reduced Cmax values as compared to i.v. dosing, while the AUC was comparable. Toxicity of ATACs is most probably driven by its Cmax level in serum, whereas the efficacy is mainly driven by AUC. Thus, we hypothesized that s.c. dosing might improve the TI of ATACs. The maximal tolerated dose (MTD) of ATACs in mice was compared between s.c. and i.v. dosing. S.c. dosing resulted in higher MTDs as compared to i.v. dosing independent of the antibody and amatoxin payload variant used. These findings were also confirmed in cynomolgus monkeys, where s.c. dosing of the ATAC HDP-103 resulted in a reduced Cmax, a comparable AUC, and an improved HNSTD of HDP-103. The impact of s.c. dosing on anti-tumor efficacy of ATACs was investigated in several tumor models in mice. ATACs with different antibodies and amatoxin payloads were applied as single i.v. or s.c. dose. In all models tested, the anti-tumor efficacy of ATACs was similar between groups receiving s.c. or i.v. dosing. Conclusions: The present study demonstrates that s.c. dosing is a promising route of administration for ATACs as it not only refines the pharmacokinetic distribution of ATACs but may improve the TI in patients. By reducing Cmax, s.c. dosing improves the tolerability of ATACs in mice and cynomolgus monkeys. At the same time the anti-tumor efficacy of ATACs is maintained as the AUC is not negatively impacted. Thus, s.c. dosing improves the TI of ATACs and may represent a promising route of administration also in humans. Citation Format: Kristin Decker, Marija Vranic, Marisa Schmitt, Irina Dranova, Anikó Pálfi, Torsten Hechler, Andreas Pahl, Michael Kulke. Subcutaneous dosing of amatoxin-based ADCs increases the therapeutic index [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 1523.