Circularly permuted TRAIL (CPT), a recombinant mutant of human Apo2L/TRAIL, is a novel antitumor candidate for multiple myeloma (MM) and other hematologic malignancies. In this phase II study, the safety and efficacy of CPT plus thalidomide was investigated in thalidomide-resistant MM patients. A total of 43 patients were recruited into three CPT plus thalidomide cohorts based on CPT dosage in sequence: 5 mg/kg (n=11), 8 mg/kg (n=17), and 10 mg/kg (n=15). CPT was administered via intravenous infusion on days 1-5, and thalidomide was given orally at 100 mg once daily in each 21-day cycle. The overall response rate (ORR) of 41 efficacy-evaluable patients was 22.0% (2 complete response, 3 near complete response, and 4 partial response). No significant difference in the ORR was observed among the three dose cohorts; however, the ORR tended to be higher with the higher-dose regimen. Median progression-free survival and median duration of response were 6.6 months and 6.1 months, respectively. The most common treatment-related adverse events (TRAEs) were neutropenia (46.5%), leukopenia (41.9%), fever (37.2%), elevated AST (32.6%), and elevated ALT (20.9%). TRAEs of Grade 3-4 were mainly neutropenia (18.6%), anemia (9.3%), elevated AST (7.0%), and leukopenia (4.7%). No significant differences were found in the incidence and severity of TRAEs among the three cohorts. In conclusion, CPT plus thalidomide was well tolerated with no occurrence of dose-limiting toxicities and demonstrated promising antitumor activity in RRMM patients. CPT at 10 mg/kg for 5 days in combination with thalidomide and dexamethason will be studied in the next clinical trial. Am. J. Hematol. 89:1037-1042, 2014. (c) 2014 Wiley Periodicals, Inc.