This is a Phase I dose-escalation study of the intravenous administration of NK012 in patients with refractory solid tumors. Patients will receive NK012 as an intravenous infusion over 30 minutes on Day 1 followed by a 20-day observation period for a total of 21 days (3 weeks) per cycle. Two patient populations will be evaluated separately: patients with UGT1A1*28 genotype homozygous wild type (wt/wt) and heterozygous (wt/*28) variants as one group, and patients with UGT1A1*28 homozygous variant (*28/*28) as another group.