Background: APG-115 is a potent and orally active small-molecule MDM2 protein inhibitor. Binding to MDM2 protein, APG-115 restores p53 tumor suppressive function via induction of apoptosis in tumor cells retaining wild-type p53. In addition, enhanced antitumor activity was demonstrated in the syngeneic tumor models after APG-115 combined with PD-1 blockade. Methods: This Phase I study (APG-115-US-001) was designed to enroll the patients with advanced solid tumors in US (NCT02935907). Study objectives included to assess safety, dose limited toxicity (DLT), pharmacokinetics (PK), pharmacodynamics (PD), and antitumor activity (assessed every 8 weeks per RECIST v1.1). The patients received APG-115 orally every other day (QOD) at the designated dose (ranging from 10 to 300 mg) for first 21 days of a 28-day cycle, until disease progression. Results: Up until Jan 4 2019, total 28 patients were treated with APG-115 at various doses (one patient at 10mg, 20mg and 50mg, respectively; 14 patients at 100mg, 6 patients at 200mg, and 5 patients at 300mg). The median number of prior systemic anticancer therapies was 4 (range 0-15). The DLTs were observed during cycle 1, including one grade 2 thrombocytopenia at 200mg, one grade 3 thrombocytopenia at 300mg, and one grade 3 fatigue at 100mg and 300mg respectively. The most common AEs (reported in ≥10% of pts) included: fatigue, nausea, vomiting, diarrhea, decreased appetite, dehydration, neutrophil count decreased, white blood cell count decreased, pain in extremity, thrombocytopenia. The most common Grade 3 or 4 treatment related AEs were fatigue (10.7%), and thrombocytopenia (10.7%). Six patients had stable disease (SD) after two cycle treatments, two of them are continuing in this study. PK analyses indicated that exposure (Cmax and AUC) generally increases with the increase of dose level from 20 mg to 300 mg. Conclusions: APG-115 was well tolerated and had manageable adverse events. The MTD/RP2D of APG-115 monotherapy with oral administration, QOD for 21 days of a 28-day cycle for treatment of patients with advanced solid tumors was determined as 100 mg. Further evaluation of APG-115 in combination with pembrolizumab in patients with advanced solid tumors is ongoing. Clinical trial information: NCT02935907