Abstract A034: PRKG1 hinders myogenic differentiation and predicts response to AKT inhibitor ipatasertib in Rhabdomyosarcoma

Abstract Rhabdomyosarcoma (RMS) is marked by a myogenesis differentiation blockade, and while the AKT/mTOR pathway is universally activated, its pharmacological inhibition has shown limited success. This study stems from an effort to understand the failure to translate preclinical findings to clinical application. We evaluated the activity of two pan-AKT inhibitors: ipatasertib, an ATP-competitive inhibitor, and miransertib, an allosteric AKT inhibitor. In vitro and in vivo efficacy assessment was conducted in RMS cell lines and fusion-positive/negative patient-derived xenografts (PDX). Unlike miransertib, ipatasertib showed significant antitumor activity against a subset of RMS. Besides AKT, another kinase target of ipatasertib, but not of miransertib, is PRKG1, a cGMP-dependent protein kinase that shares the ATP binding pocket with AKT. The role of PRKG1 in RMS was investigated in PRKG1-depleted RMS cells and in xenograft models by transcriptomic approaches. PRKG1 silencing in RMS cells reduced tumor formation in xenograft models and induced a differentiated myogenic transcriptome. RMS showed higher PRKG1 expression compared to any other developmental cancer, akin to fetal skeletal muscle. Importantly, PRKG1 expression in RMS correlates with mesodermal transcriptional signature and enhanced sensitivity to ipatasertib, regardless of the fusion oncogene status. The antitumor activity of ipatasertib is dose-dependent, reaching an effective intra-tumor concentration when administered at 25 mg/kg daily. This study unveils the role of PRKG1 in myogenesis and highlights the potential of PRKG1 as a clinical biomarker for ipatasertib therapy in RMS. Citation Format: Estela Prada, Pablo Taboas, Angel Montero, Alexandra Avgustinova, Inmaculada Hernandez, Jaume Mora. PRKG1 hinders myogenic differentiation and predicts response to AKT inhibitor ipatasertib in Rhabdomyosarcoma [abstract]. In: Proceedings of the AACR Special Conference in Cancer Research: Discovery and Innovation in Pediatric Cancer— From Biology to Breakthrough Therapies; 2025 Sep 25-28; Boston, MA. Philadelphia (PA): AACR; Cancer Res 2025;85(18_Suppl_2):Abstract nr A034.