Background: Aurora kinase B (AURKB) represents a potential target for therapy in solid and hematological malignancies. AURKB inhibitor AZD1152 (barasertib) was previously investigated in solid tumor pts in a phase I setting. AZD2811-nanoparticle (np) is a novel, encapsulated slow release AURKB inhibitor offering several advantages over AZD1152 (Ashton S et al., Sci Transl Med 2016). We report the completed dose-escalation safety, pharmacokinetics (PK), preliminary activity and defined maximum tolerated dose (MTD) of AZD2811-np in pts with advanced solid tumors (NCT02579226). Methods: Adult pts with advanced solid tumors received AZD2811-np IV on Day 1 (D1) and 4 (D4) Q4 week (wk) in six cohorts 15-200 mg/infusion without the use of g-csf in cycle 1. D1 Q4wk and Q3wk schedules were investigated up to 600 mg/infusion (including cohorts with mandatory g-csf prophylaxis on day 8). A standard 3+3 design was used. PK was assessed in cycle 1. Results: 50 pts were recruited into 12 cohorts. D1, D4 Q4wk schedule: 24 pts (15, 25,38, 50, 100 mg/infusion (n=3/cohort), 200 mg/infusion (n=9)). All cohorts were tolerated. Transient grade 4 neutropenia was observed in 7/9 pts at 200 mg/infusion, including 1 DLT (gr4 > 7 days) D1 Q4wk: 200 mg(n=3) was tolerated. D1 Q3wk: 23 pts were evaluated (200/400 mg (n=3,7), and 400/600/500 mg with mandatory g-csf (n=3/5/6)). 400 mg without g-csf was not tolerated (1 gr3 mucosal inflammation & 1 gr4 neutropenia > 7 days). 600 mg with g-csf was not tolerated (gr3 febrile neutropenia & gr3 fatigue). 25/50 pts experienced AE ≥gr 3 (21 considered AZD2811-np-related, 19 neutropenia-related, no deaths within-DLT period). AZD2811-np caused transient gr1/2 fatigue, nausea, diarrhoea and mucosal inflammation. AZD2811 total blood PK appears dose proportional with a t_1/2 of 30-50 hours irrespective of schedule. Released AZD2811 concentrations ~1% of total. 14 pts (28%) had disease stabilisation. 1 prostate ca. pt had a confirmed partial response (PR) (continued tx to 451 days). Conclusions: The MTD for AZD2811-np is 500 mg D1 Q3wk. AZD2811-np is now being investigated in a small cell lung cancer expansion. Clinical trial information: NCT02579226