Background: Over-expression of mdm2 is a major block to p53 activation in soft tissue sarcoma (STS). AMG-232 specifically inhibits human MDM2-p53 interactions in vitro and in vivo, thereby activating p53. Resistance mechanisms to MDM2 inhibition by AMG-232 is through accumulation of MDM2 and MDMX. However, DNA damaging agents, such as radiotherapy (RT), promote MDMX degradation and lead to sustained p53 activation, which is critically important for radiation-induced tumor cell killing in STS. Indeed, preclinical studies reveal that AMG-232 synergizes with RT in vitro and in vivo when treating STS with wild type (WT) P53 gene. Since a majority of STS harbor WT TP53 gene and RT is part of the standard of care in treating STS; it is the next rational step to test the safety and efficacy of combination of AMG-232 and RT followed by surgery. In this clinical trial, we hypothesize that preoperative AMG-232 plus RT is safe and well tolerated in 2 separate cohorts: (A) extremity and body wall; (B) abdomen/pelvis/retroperitoneum. Methods: A key feature of this trial is to incorporate TP53 NGS sequencing result as one of the eligibility criteria. Adult patients with pathologically proven grade 2-3 STS with size ≥ 5 cm are eligible to enroll if there is a planned definitive surgical resection of the primary tumor. There must be sufficient tissue to submit to central laboratory for NGS sequencing of the TP53 gene, which is an integral biomarker. Only those with tumors harboring WT p53 gene will be allowed to continue protocol treatment. The primary objective is to evaluate the safety and tolerability of AMG-232 in combination with standard-dose RT in STS in two separate cohort; as well as to determine the maximum tolerated dose (MTD) and recommended phase II dose (RP2D) of AMG-232 in combination with RT. We define DLT as grade 4-5 toxicities attributable to AMG-232 for up to 4 weeks after the completion of combination treatment (prior to surgery) as well as any AMG-232 related grade 3 toxicity that should lead to > 2 weeks treatment delay or ≥ two dose reductions. There are 3 dose levels for AMG-232: dose level 1 is 120 mg daily for 3 days per week; dose level 2 is 120 mg daily 4 days per week and dose level 5 is 120 mg daily 5 days a week. If patients did not tolerate dose level 1, AMG-232 will be de-escalated to a lower level, which is 120 mg daily 2 days a week. The trial was open since 11/2017; 3 patients have been enrolled to cohort A and 2 patients in cohort B at dose level 1. The first patient has a tumor with WT p53 gene and has completed protocol treatment safely. Clinical trial information: NCT03217266