Background: Recent data has shown that single-agent CFZ can produce responses among MM pts refractory to previous treatment regimens including those containing BTZ. We conducted an intrapatient phase I/II trial investigating the safety and efficacy of CFZ as a replacement for BTZ in BTZEcontaining regimens to which pts have progressed. Methods: Eligible pts progressed while receiving their most recent BTZEcontaining regimen after at least 4 doses of BTZ at ≥1.0 mg/m² in ≤4 weeks per cycle. CFZ replaced BTZ in each regimen via intravenous administration over 30 min on days 1, 2, 8, 9, 15, and 16 of each cycle. Treatment continued using the same dose(s) and schedule(s) of each drug administered in the BTZEcontaining regimen. CFZ doses were escalated on each of the first 4 cycles from 20 to 27, 36, and 45 mg/m² or until a maximum tolerated dose (MTD) was reached for that regimen. Results: Of 37 enrolled pts, 33 were evaluable for efficacy and 37 for safety. Pts received a median of 4 prior treatments (range, 1-23) and 2 different BTZ-containing regimens (range, 1-13). Pts were treated with CFZ and a variety of different combinations of agents, including: bendamustine, clarithromycin, cyclophosphamide, dexamethasone, melphalan, methylprednisolone, pegylated liposomal doxorubicin, thalidomide, lenalidomide, and ascorbic acid. Pts have completed a median of 3 cycles with 12 pts going on to maintenance. One of 14 combinations, CFZ + ascorbic acid + cyclophosphamide, has reached a MTD at 45 mg/m². Clinical benefit was seen in 23 (70%) evaluable pts (complete response = 6%; very good partial response = 18%; partial response = 21%; minor response = 24%) with another 18% showing stable disease. The median time to progression is 8.8 mo. (95% CI: 6.4-11.1 mo.) with 21 pts progressing overall and 9 progressing on study treatment. The most common ≥ G3 adverse events were lymphopenia (35% of pts), thrombocytopenia (19%), neutropenia (11%), and anemia (8%). Conclusions: These results suggest that replacement of BTZ with CFZ in a BTZ-containing combination regimen to which a MM patient is failing often leads to responses and is well-tolerated. Clinical trial information: NCT01365559.