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  • RP-3500: A Novel, Potent, and Selective ATR Inhibitor that is Effective in Preclinical Models as a Monotherapy and in Combination with PARP Inhibitors.

RP-3500: A Novel, Potent, and Selective ATR Inhibitor that is Effective in Preclinical Models as a Monotherapy and in Combination with PARP Inhibitors.

Molecular cancer therapeutics (2021-12-17)
Anne Roulston, Michal Zimmermann, Robert Papp, Alexander Skeldon, Charles Pellerin, Émilie Dumas-Bérube, Valerie Dumais, Stéphane Dorich, Lee D Fader, Sara Fournier, Li Li, Marie-Eve Leclaire, Shou Yun Yin, Amandine Chefson, Hunain Alam, William Yang, Chloe Fugère-Desjardins, Sabrina Vignini-Hammond, Kathryn Skorey, Amina Mulani, Victoria Rimkunas, Artur Veloso, Martine Hamel, Rino Stocco, Yael Mamane, Zuomei Li, Jordan T F Young, Michael Zinda, W Cameron Black
ABSTRACT

Ataxia telangiectasia and Rad3-related (ATR) kinase protects genome integrity during DNA replication. RP-3500 is a novel, orally bioavailable clinical-stage ATR kinase inhibitor (NCT04497116). RP-3500 is highly potent with IC50 values of 1.0 and 0.33 nmol/L in biochemical and cell-based assays, respectively. RP-3500 is highly selective for ATR with 30-fold selectivity over mammalian target of rapamycin (mTOR) and more than 2,000-fold selectivity over ataxia telangiectasia mutated (ATM), DNA-dependent protein kinase (DNA-PK), and phosphatidylinositol 3-kinase alpha (PI3Kα) kinases. In vivo, RP-3500 treatment results in potent single-agent efficacy and/or tumor regression in multiple xenograft models at minimum effective doses (MED) of 5 to 7 mg/kg once daily. Pharmacodynamic assessments validate target engagement, with dose-proportional tumor inhibition of phosphorylated checkpoint kinase 1 (pCHK1) (IC80 = 18.6 nmol/L) and induction of phosphorylated H2A.X variant histone (γH2AX), phosphorylated DNA-PK catalytic subunit (pDNA-PKcs), and phosphorylated KRAB-associated protein 1 (pKAP1). RP-3500 exposure at MED indicates that circulating free plasma levels above the in vivo tumor IC80 for 10 to 12 hours are sufficient for efficacy on a continuous schedule. However, short-duration intermittent (weekly 3 days on/4 days off) dosing schedules as monotherapy or given concomitantly with reduced doses of olaparib or niraparib, maximize tumor growth inhibition while minimizing the impact on red blood cell depletion, emphasizing the reversible nature of erythroid toxicity with RP-3500 and demonstrating superior efficacy compared with sequential treatment. These results provide a strong preclinical rationale to support ongoing clinical investigation of the novel ATR inhibitor, RP-3500, on an intermittent schedule as a monotherapy and in combination with PARP inhibitors as a potential means of maximizing clinical benefit.

MATERIALS
Product Number
Brand
Product Description

Sigma-Aldrich
Gemcitabine hydrochloride, ≥98% (HPLC)
Sigma-Aldrich
Anti-phospho-Histone H2A.X (Ser139) Antibody, clone JBW301, clone JBW301, Upstate®, from mouse
Sigma-Aldrich
Anti-DNA-PKcs Antibody, clone 3A9.1, clone 3A9.1, from mouse
Sigma-Aldrich
Bleomycin sulfate from Streptomyces verticillus, crystalline, 1.5-2.0 U/mg