Background: Irinotecan is a prodrug of the potent topoisomerase inhibitor SN-38. In animals, oral administration of irinotecan with the selective minimally absorbed P-gp inhibitor HM30181A increased the bioavailability of irinotecan. Oral administration of irinotecan may also increase the conversion to SN-38. Objectives: To determine the MTD and DLT of orally administered irinotecan in combination with HM30181A 15 mg on day 1 of a 21-day cycle. Additional objectives include determining the recommended phase 2 dose and the PK of irinotecan and SN-38. Methods: This was a phase 1 dose escalation study enrolling cohorts of 3-6 patients with advanced malignancies. Patients had Hb ≥9 gm/dL, ANC ≥ 1.5x10⁹/L, platelets ≥ 100x10⁹/L, adequate hepatic and renal function, ECOG 0-1 and were not homozygous for UGT1A1*28. Patients were administered HM30181A 15 mg and oral irinotecan 20, 40, 80, 120, 160, 200, 240, 280 and 320mg/m². Results: Thirty male and female patients, mean age 60.9 (range 33-78) were enrolled into this ongoing study. The most common cancers were ovarian (6), colorectal (4), breast (4), endometrial (3), and pancreatic (3). The median number of cycles administered was 3 (range 1-9). Treatment-related Grade 3-4 AEs were experienced by 12 (40%) subjects. The most common were nausea 7 (23%), vomiting 6 (20%) and abdominal pain 3 (10%). Treatment-related SAEs were experienced by 6 (20%) patients (nausea or vomiting in 4 subjects). DLTs occurred in 2 patients at the 320 mg/m² dose level (neutropenia and C. Difficile diarrhea) and additional patients are being enrolled at the 280mg/m² dose level to define the MTD. Acute cholinergic diarrhea has not been observed. The best response was stable disease in 9/21 evaluable patients. PK at the three highest dose levels is summarized below. Conclusions: Oral administration of HM30181A in combination with irinotecan tablets results in pharmacologically active concentrations of SN-38. Confirmation of the MTD when dosed on a 21-day cycle is ongoing. Phase 2 studies are being planned. Clinical trial information: NTC02250157.