Background: Poly (ADP-ribose) polymerase (PARP) enzymes play critical roles in DNA damage detection and repair. IMP4297 is a novel, potent PARP1/2 inhibitor (IC₅₀ 6.27nM/1.57nM) and has demonstrated to be 20-fold more potent than Olaparib in anticancer animal models. Two phase I studies were performed to evaluate and characterize the tolerability and safety, pharmacokinetics, and antitumor activity of single agent IMP4297 in Chinese and Australian patients with advanced ovarian, breast, prostate and other solid tumors. Methods: Dose escalation used a 3+3 design with a modified Fibonacci escalation. Dose cohort expansion was planned after efficacy was observed at the lowest dose level. Patients received IMP4297 monotherapy orally once a day until disease progression or unacceptable toxicity. Results: As of Jan 12, 2019, 56 patients, including 23 BRCA mutation carriers (BRCA+), had been enrolled at 2-100 mg dose level. No DLT was observed. In these two studies, the most frequent treatment-related adverse events (TRAEs) were leukopenia (20%), followed by anemia (18%), nausea (18%) and thrombocytopenia (14%). The majority of TRAEs were grade 1 or 2. Grade 3 TRAEs occurred in five patients (anemia, n=2; vomiting, n=1; thrombocytopenia, n=1; elevated AST, n=1). Only one patient had a dose reduction due to grade 3 thrombocytopenia. No serious TRAEs were observed. In 15 BRCA+ patients who had measurable lesions, the ORR was 33% and the DCR was 80%. There were 4 BRCA+, platinum-sensitive ovarian cancer patients with an ORR of 75% and a DCR of 100%. One patient with somatic BRCA mutated urothelial carcinoma showed a 76% decrease in tumor size. Conclusions: IMP4297 has been well-tolerated with significant anti-tumor activity. The 100 mg daily dose was selected as the RP2D based on safety, pharmacokinetics and clinical activity, and will be further characterized in dose expansion and phase II studies. Tumor response to treatment (RECIST 1.1) in patients with measurable lesions. Clinical trial information: NCT03508011 and NCT03507543