Abstract 6228: APR-246, which restores p53 function, is highly active against alternative lengthening of telomere (ALT) cell lines and PDXs

Abstract Introduction: Most cancers proliferate by activating telomerase (TA+) while 10% of cancers utilize alternate lengthening of telomeres (ALT). ALT has been associated with resistance to DNA damaging agents, p53 loss-of-function (p53LOF), ATRX mutations, and very poor survival. ATM kinase, which activates functional p53, is constitutively activated in ALT cancers (Science Translational Medicine 18:eabd5750, 2021). We hypothesized that the constitutive activation of ATM kinase in ALT cancers would confer high sensitivity to pharmacological reactivation of p53 function. Methods: We used patient-derived ALT (telomeric DNA C-circle positive) and TA+ neuroblastoma, sarcoma, colorectal, and breast cancer cell lines and xenografts treated with the clinical-stage p53 reactivator eprenetapopt (APR-246) and irinotecan. In vitro cytotoxicity was assayed by DIMSCAN digital imaging microscopy, ATM activation by immunofluorescence microscopy, and protein expression by western blotting. Results: ALT p53LOF cell lines were significantly (p<0.05) less sensitive to DNA-damaging agents relative to TA+ p53LOF comparators. Constitutive phosphorylation (activation) of ATM kinase and the DNA damage marker 53BP1 were observed at telomeres in ALT but not TA+ cell lines. APR-246 induced p53 targets p21 and NOXA and was significantly more cytotoxic (p<0.001) for ALT relative to TA+ cell lines, regardless of TP53 status. Overexpression of p53 in a TP53-null ALT cell line increased sensitivity (p<0.0001) to APR-246. Knockdown of p53 or ATM kinase in ALT TP53 mutated and wild-type cell lines antagonized (p<0.0001) APR-246 activity. Induction of telomere dysfunction in a TA+ p53LOF neuroblastoma cell line using dominant-negative TRF2 (a shelterin protein that blocks ATM activation at telomeres) activated ATM and sensitized cells to APR-246 (p<0.01). APR-246 enhanced the cytotoxicity of irinotecan (as SN38) in ALT cell lines in vitro to a higher degree than in TA+ p53LOF cell lines (p<0.05). Single-agent APR-246 significantly (p<0.0001) increased event-free survival (EFS) of mice with ALT xenografts relative to controls. APR-246 enhanced (p<0.0001) the activity of irinotecan in 3 neuroblastoma, 2 rhabdomyosarcoma, 1 colorectal, and 1 triple-negative breast ALT xenograft models with most mice (47/56) achieving complete responses and an EFS of >100 days (45/56) compared to no complete responses (median EFS ~ 34 days) in mice treated with only irinotecan (p<0.0001). APR-246 + irinotecan had no significant effect relative to irinotecan alone (p=0.08) on EFS of mice with TA+ p53LOF xenografts (median EFS ~ 40 days). Conclusion: ALT cancers of a variety of histologies are highly resistant to DNA damaging agents, have p53LOF, and constitutive activation of ATM kinase, which confers high sensitivity to p53 reactivation by APR-246. APR-246 warrants clinical testing in patients with ALT cancers. Citation Format: Shawn J. Macha, Balakrishna Koneru, Trevor Burrow, Charles Zhu, Dzmitry Savitski, Jonas Nance, Kristyn McCoy, Cody Eslinger, C Patrick Reynolds. APR-246, which restores p53 function, is highly active against alternative lengthening of telomere (ALT) cell lines and PDXs [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2022; 2022 Apr 8-13. Philadelphia (PA): AACR; Cancer Res 2022;82(12_Suppl):Abstract nr 6228.