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

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 IC 50 values of 1.0 and 0.33 nM in biochemical and cell-based assays, respectively. RP-3500 is highly selective for ATR with 30-fold selectivity over mTOR and >2,000-fold selectivity over ATM, DNA-PK, and PI3K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 5-7 mg/kg once daily. Pharmacodynamic assessments validate target engagement, with dose-proportional tumor pCHK1 inhibition (IC 80 = 18.6 nM) and induction of -H2AX, pDNA-PKcs, and pKAP1. RP-3500 exposure at MED indicates that circulating free plasma levels above the in vivo tumor IC 80 for 10-12 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.

ATM Germline-Mutated Gastroesophageal Junction Adenocarcinomas: Clinical Descriptors, Molecular Characteristics, and Potential Therapeutic Implications

“…Though these observations need to be tested in additional cohorts, the findings here support the contention that g ATM -mut GEJ cancers may be sensitive to radiation ( 11 ), because only 10% of patients with resections post treatment had recurrent disease. Consistent with previous observations demonstrating that ATM and ATR loss of function are synthetically lethal ( 32 , 43 ), silencing of ATM and ATR in GEJ, esophageal, and gastric adenocarcinoma cell lines resulted in statistically significant loss of cell viability as demonstrated here and in Roulston et al ( 32 ) Alterations in DDR and homologous recombination (HR) DNA repair-related genes have received attention as potential therapeutic targets through synthetic lethality, with PARP inhibitors being FDA approved for the treatment breast, ovarian, and pancreatic cancers in patients with BRCA1/2 germline P/LP variants as well as prostate cancer patients with mutations affecting DDR- or HR-related genes ( 43 ). Sensitivity to PARP inhibitors, however, varies according to the DDR or HR genes affected in cancers.…”

Guiding ATR and PARP inhibitor combinations with chemogenomic screens

“…Another class of compounds targeting the DDR based on synthetic lethality are inhibitors of ataxia telangiectasia-and Rad3-related (ATR) kinase, a key regulator of the replication stress response (Bradbury et al, 2020;Lecona and Fernandez-Capetillo, 2018). Multiple ATR inhibitors (ATRis) are in clinical development as monotherapies and in combination with PARPis, including RP-3500, a novel, highly potent, selective, and orally bioavailable ATRi that shows robust preclinical efficacy in models deficient in BRCA1/2 or the ataxia telangiectasia mutated kinase (ATM) gene (Roulston et al, 2022).…”

Thymidine rescues ATR kinase inhibition induced deoxyuridine contamination in genomic DNA, cell death, and Type 1 interferon expression

“…For example, Olaparib (Lynparza), the poly (ADP-ribose) polymerase (PARP) inhibitor, is approved for the treatment of tumors harboring BRCA1 or BRCA2 mutations. More recently, four ATR kinase inhibitors (we use the abbreviation ATRi’s for these four inhibitors) have advanced to phase 1 and phase 2 trials: ceralasertib (AZD6738); berzosertib (M6620, VX-970); elimusertib (BAY 1895344); and RP-3500 (Foote et al, 2018; Hall et al, 2014; Roulston et al, 2021; Wengner et al, 2020). ATRi’s potentiate chemotherapies that target DNA replication forks, selectively kill tumor cells with inactivating mutations in ATM, ERCC1, RNASE H2, and XRCC1, and block the upregulation of PD-L1 after radiation (Hustedt et al, 2019; Mohni et al, 2015; Roulston et al ., 2021; Sato et al, 2017; Vendetti et al, 2018; Vendetti et al, 2015; Wang et al, 2019).…”

“…More recently, four ATR kinase inhibitors (we use the abbreviation ATRi’s for these four inhibitors) have advanced to phase 1 and phase 2 trials: ceralasertib (AZD6738); berzosertib (M6620, VX-970); elimusertib (BAY 1895344); and RP-3500 (Foote et al, 2018; Hall et al, 2014; Roulston et al, 2021; Wengner et al, 2020). ATRi’s potentiate chemotherapies that target DNA replication forks, selectively kill tumor cells with inactivating mutations in ATM, ERCC1, RNASE H2, and XRCC1, and block the upregulation of PD-L1 after radiation (Hustedt et al, 2019; Mohni et al, 2015; Roulston et al ., 2021; Sato et al, 2017; Vendetti et al, 2018; Vendetti et al, 2015; Wang et al, 2019). Phase I clinical trial data show that ATRi’s suppress circulating monocytes and proliferating T cells, and that these populations rebound after cessation of ATRi’s, as expected (Krebs et al, 2018).…”