The following inclusion criteria were used for patient selection: (1) histologically confirmed solid tumors refractory to standard therapy or for which there is no standard therapy; (2) age ≥ 19 years; (3) the Eastern Cooperative Oncology Group (ECOG) performance status ≤ 2; (4) life expectancy of longer than three months; (5) adequate organ function (absolute neutrophil count ≥ 1,500/μL, platelet count ≥ 75,000 cells/μL, hemoglobin ≥ 10.0 g/dL, serum creatinine ≤ 1.5×upper limit of normal [ULN], serum bilirubin ≤ 2.5×ULN, aspartate aminotransferase, alanine transaminase, alkaline phosphatase ≤ 2.5×ULN); and (6) capable of swallowing OPB-31121 tablets. Written informed consent was obtained from all patients. The important exclusion criteria were (1) symptomatic central nervous system metastasis; (2) prior chemotherapy, radiation therapy, or surgery within 4 weeks prior to enrolling in the study; (3) uncontrolled concurrent illness, including active infection, heart failure, angina pectoris, and cardiac arrhythmia; and (4) use of CYP3A4 and CYP2C9 inducers, inhibitors, or substrates, and CYP2B6, CYP2C8, and CYP2D6 substrates.

OPB-31121 was administered orally to patients after breakfast once daily for 28 days followed by 2 weeks rest in each cycle of treatment. In the first cycle (cycle 1), OPB-31121 was administered on day 1, followed by a 2-day treatment-free interval for pharmacokinetics evaluation, and then administration resumed on day 4 and continued until day 28. The starting dose was 100 mg/day, and dose was escalated using 3+3 design. In each cohort, patients who were withdrawn for any reason other than DLT before completion of the 28-day administration period were replaced by other patients.

For management of nausea and vomiting, metoclopramide, 5-HT3 antagonist, NK1 receptor antagonist or dexamethasone were allowed.

Patients in whom antitumor effect was assessed as ‘stable disease’ (SD), ‘partial response’ (PR), or ‘complete response’ (CR) for overall response at the end of cycle 1 of treatment were allowed to receive cycle 2 of treatment after a 2-week rest period. Treatment was to be continued until the subject experienced disease progression or unacceptable toxicity, withdrew consent, or required treatment with another therapeutic modality.

DLT was defined as follows: (1) grade 4 neutropenia lasting for ≥ 7 days, grade 3 or 4 neutropenia with fever or infection; (2) grade 4 thrombocytopenia or grade 3 thrombocytopenia lasting for ≥ 7 days; (3) grade 3 or 4 nausea/vomiting or diarrhea despite optimal use of antiemetic drugs or antidiarrheal drugs; (4) other grade 3 non-hematological toxicity (except alopecia); (5) missed doses: more than three consecutive doses per cycle due to treatment related toxicity;and (6) delay of > 2 weeks in administration of the next cycle due to inadequate recovery from toxicity of cycle 1.

The lowest dose at which DLT was observed in 2 or more of the three or six patients in cycle 1 of treatment was judged to be the MTD.

Blood was collected before and 1, 2, 4, 6, 8, 12, 24 (days 2 and 29), 36 (days 2 and 29), 48 (days 3 and 30), 60 (days 3 and 30), and 72 (days 4 and 31) hours after dosing. Plasma concentrations of OPB-31121 were measured by liquid chromatography tandem mass spectrometry (LC-MS/MS) in accordance with the specified procedure of the bioanalytical laboratory.

The primary outcomes were safety and tolerability, which were measured by adverse events (AEs), vital sign, body weight, electrocardiogram, and laboratory tests. Safety was assessed according to the National Cancer Institute–Common Terminology Criteria for Adverse Events (NCI-CTCAE) ver. 3.0. The secondary outcomes were pharmacokinetics and efficacy. The following pharmacokinetic parameters were determined: maximum (peak) plasma concentration (Cmax), time to Cmax (tmax), area under concentration-time curve (AUC), terminal-phase elimination half-life (t1/2,z), CL/F and accumulation ratio. Efficacy was measured using Response Evaluation Criteria in Solid Tumor (RECIST) ver. 1.0. Tumor responses were assessed after cycle 1 or earlier in patients with suspected progression.

All safety analyses were performed on the intentionto-treat population. The objective response was assessed on the per-protocol population. The results are expressed as the mean±standard deviation or as ranges when appropriate. Follow-up started at the outset of treatment.

A total of 25 patients were enrolled in the study and received the investigational product. Baseline characteristics of patients are shown in Table 1. Median age was 53 years (range, 19 to 76 years). Sixteen patients (64.0%) were male. Six patients had ECOG performance status 0, 18 ECOG 1, and one ECOG 2. The most common tumors were colorectal cancer [8] and gastric cancer [6]. Others included lung cancer, esophageal cancer, melanoma, breast cancer, and pancreatic cancer, etc. All patients had been heavily pretreated with multiple lines of chemotherapy before enrollment.

The investigational products were administered at doses of up to 800 mg/day. Finally, 18 patients completed cycle 1 of the study, and seven patients (1 patient at 100 mg level, 1 patient at 600 mg level, and 5 patients at 800 mg level) discontinued during cycle 1 for various reasons (consent withdrawal, 4; disease progression, 1; other medical illness, 2). The reasons for consent withdrawal were aspiration pneumonia in one patient, dyspnea in one patient, and refusal of further treatment in two patients (Table 2).

DLT was reported in three patients. Two patients (1 patient at 600 mg, 1 patient at 800 mg) experienced grade 3 vomiting and one patient at 800 mg experienced grade 3 diarrhea. Thus, the MTD of OPB-31121 was determined as 800 mg/day once daily.

Most commonly reported AEs were nausea (84%), vomiting (72%), diarrhea (68%), and asthenia (56%). Most were grade 1/2 except DLT cases (Table 3). Grade 1/2 anorexia and asthenia were 40% and 56%. Grade 1/2 anemia, neutropenia, and thrombocytopenia were reported as 12%, 4%, and 4%, respectively. Death occurred in two patients, at 600 mg (progression of underlying disease, timing of death was 51 days after last medication of OPB-31121) and 800 mg (pulmonary embolism, on the last day of medication of OPB-31121), respectively.

The maximum plasma concentration of OPB-31121 was reached between 2.0 and 6.0 hours after single and multiple administrations at all dose levels (Table 4). After that, OPB-31121 was gradually eliminated from the plasma with t1/2, z of 17.92 to 44.30 hours on day 1, and 24.80 to 66.60 hours on day 28, and the plasma concentration of OPB-31121 could still be measured 72 hours after administration at all dose levels. Higher exposure was observed on day 28 compared to day 1 (Fig. 1). No dose-proportionality for OPB-31121 was confirmed.

Tumor response was assessed in 18 patients. There was no patient with CR or PR response. Overall responses were assessed as SD in eight patients and progressive disease in 10 patients (Table 5). Two patients obtained tumor shrinkage (1 colon cancer and 1 rectal cancer, –8.5% and –3.3%, respectively). One patient with gastric cancer at 400 mg continued the study treatment up to cycle 13 before disease progression.

One patient with gastric cancer at 400 mg continued the study treatment up to cycle 13 before disease progression. One patient with colon cancer at 600 mg continued the study up to cycle 4. One patient with non-small cell lung cancer at 100 mg continued the study up to cycle 3. Three patients (one gastric cancer, one hepatocellular carcinoma, and one rectal cancer) continued the treatment up to cycle 2.