Phase I Study of Bortezomib, (BTZ) Followed by High-Dose Melphalan, (HD Mel) and BTZ as Conditioning Regimen for Tandem Peripheral Blood Stem Cell Transplants (TanPSCT) in Patients with Primary Refractory Multiple Myeloma (MM) and Plasma Cell Leukemia (PCL).

Background: Approximately 25% of patients with newly diagnosed (MM) fail to respond and progress while receiving conventional induction. As a result, these patients have primary refractory disease. Even though these patients still benefit from HD chemotherapy, their PFS ranges from 4–8 months, while OS is approximately 12 months. PCL accounts for approx. 2% of newly diagnosed MM patients and represents the most aggressive form of disase. These patients often do not respond to standard systemic cytotoxic therapies and their median survival is less than 12 months. Therefore, novel treatment approaches are paramount for patients with primary refractory MM and PCL. The proteasome inhibitor BTZ has been shown to sensitize myeloma cells to melphalan both in vitro and in vivo, but the mechanism is not well understood. In this study we examine the effects of BTZ, followed by BTZ and HDMel as conditioning regimen for TanPSCT in this poor-risk group.

Methods: Patients with primary refractory MM or PCL received 2 cycles of BTZ at 1.3 mg/m2. followed by HD Mel (200 mg/m²) and one dose of BTZ at 0.7 mg/m2, 1.0 mg/m2 or 1.3 mg/m2, as a conditioning regimen prior to TanPSCT. The dose of BTZ was given immediately after the last dose of HD Mel. Bone marrow(BM) samples were collected at baseline, on day 4 and after 2 cycles of BTZ, and at 3 months after TanPSCT, for GEP and Fanconi anemia(FA) pathway genes assessment.

Results: To date, 17 patients have been enrolled and treated, and 11 patients are evaluable for response. Median age is 59 years (46–70) with the following myeloma distribution: 55% IgA and 45% IgG. FISH analysis showed the following: del 13q (44%), t (4; 14) (11%), t (11; 14) (11%), trisomy 11 (11%), and polyploid (11%); standard karyotype was normal in 88% of patients and complex karyotype in 12%. Median time to WBC engraftment (days) was 13 and 12 after the first and second transplant, respectively. Median time to plt engraftment (days) was 20 and 17, after the first and second transplant, respectively. There were no dose limiting toxicities. Observed grade 3 toxicities were related to the conditioning regimen and similar to those observed with HDMel alone. One patient developed diffuse alveolar hemorrhage after the first cycle of BTZ, which resolved, but was removed from study. After 2 cycles of BTZ, 45% of patients achieved PR, and 55% had stable disease. Overall response rate at 3 months from the second transplant increased to 90%(CR=36%,VGPR=27% andPR=27%). Only 1 patient developed progressive disease after the first transplant. Evaluations of BM samples by GEP and FA pathway gene expression are underway.

Conclusions: Single agent BTZ induced responses in 45% and the combination of HD Mel and BTZ as conditioning regimen for TanPSCT was well tolerated and improved response rates to 90%. These early results suggest that this regimen is very active in this poor-risk group. Evaluation of collected BM samples for GEP and FA pathway genes may provide important insight into the mechanisms associated with the observed synergy between BTZ and HD Mel. A Phase II study is underway.