Abstract 4043: SYD985 synergy with PARP inhibitors (PARPi) in human tumor cell lines and patient-derived xenografts (PDXs)
Authors: Dokter Wim H. , Dijcks Fred A. , Groothuis Patrick G. , Mattaar Ellen , Loosveld Eline , Jacobs Daniëlle , van der Vleuten Monique , Schellens Jan H. , van der Lee Miranda 
Published: September 2020

Abstract
SYD985 is a HER2-targeting antibody-drug conjugate comprised of a cleavable linker and the prodrug seco-DUBA which, upon release, spontaneously rearranges to form the active toxin DUBA. DUBA covalently binds adenine in the DNA resulting in DNA damage, cell cycle perturbation and associated cytotoxicity. SYD985 induces dose-dependent cytotoxicity of human tumor cell lines and PDXs expressing HER2 on the outer cell membrane. The cytotoxicity of PARPi depends on PARP trapping, i.e. the formation of non-covalent protein-DNA adducts composed of inhibited PARP1 bound to DNA lesions of clear1 and unclear origins2,3. We investigated potential synergy in cytotoxic activity of SYD985 and the PARPi niraparib and olaparib towards HER2-expressing tumor cells in vitro and in vivo. Tumor cell lines were from breast (BT-474, AU-565, UACC-893) or ovarian (TOV-112D, NIH:OVCAR-3) origin. Cytotoxicity was established by measuring DNA content and metabolic activity of living cells. For both PARPi the highest non-cytotoxic concentration for each cell line was established. This concentration was used to measure the potential potency shift of the SYD985 concentration-response curve by comparing SYD985 IC50 values in the absence and presence of PARPi. PARPi-induced fold change in IC50 for SYD985 was established in two independent experiments (N=2, n=3). Niraparib, dosed at a non-cytotoxic concentration, induced a 1.5 to 136-fold potentiation of SYD985-induced tumor cell killing. In BT-474 cells, niraparib co-treatment unveils a biphasic concentration-response curve to SYD985. This is in line with the fact that BT-474 cells express high levels of HER2 and are known to respond to naked trastuzumab at concentrations slightly above those of SYD985. Olaparib, dosed at a non-cytotoxic concentration, induced a 0.4 to 7.5-fold potentiation of SYD985-induced tumor cell killing. In a breast cancer PDX mouse clinical trial in immunodeficient mice that lack carboxylesterase 1c (Ces1c) we studied the combination of SYD985 (1 mg/kg, IV, SD) and niraparib or olaparib (50 mg/kg Q1DX21, IP). In order to detect potential additive and/or synergistic effects of SYD985 and PARPi, a suboptimal dose of SYD985 (1 mg/kg) was selected. PARPi doses were chosen known to inhibit growth of tumors with a defective DNA repair mechanism4. Data clearly show that the combination of SYD985 and PARPi are more effective in tumor volume reduction than either monotherapy alone.
In conclusion, niraparib and olaparib act in synergy with SYD985 in killing human HER2-expressing tumor cells in vitro, both in high HER2 (BT-474, AU-565, UACC-893) and low HER2 (NIH:OVCAR-3; TOV-112D) expressing cells. In vivo such synergistic activity appears to translate in improved tumor growth inhibition of various breast cancer PDX models.
1Zimmermann M, Nature 559, 2018; 2Pommier Y, Sci Transl Med 8, 2016; 3Lord C, Science 355, 2017; 4Henneman L, PNAS 112, 2015
Citation Format: Wim H. Dokter, Fred A. Dijcks, Patrick G. Groothuis, Ellen Mattaar, Eline Loosveld, Daniëlle Jacobs, Monique van der Vleuten, Jan H. Schellens, Miranda van der Lee. SYD985 synergy with PARP inhibitors (PARPi) in human tumor cell lines and patient-derived xenografts (PDXs) [abstract]. In: Proceedings of the Annual Meeting of the American Association for Cancer Research 2020; 2020 Apr 27-28 and Jun 22-24. Philadelphia (PA): AACR; Cancer Res 2020;80(16 Suppl):Abstract nr 4043.