Addition of Bortezomib to High Dose Melphalan As Conditioning Regimen for Autologous Stem Cell Transplantation Improves the Response Rate in Newly Diagnosed Multiple Myeloma Patients

High-dose Melphalan (HDM) is the most commonly used conditioning regimen for autologous stem cell transplantation (ASCT) in newly diagnosed transplant-eligible Multiple Myeloma (MM) patients. Recent evidence suggests that the depth of response after induction therapy influences progression free survival (PFS) and, in most studies, overall survival (OS). Actually the effect of the inclusion of new drugs in the conditioning regimen for ASCT is still unknown. Phase I-II clinical study showed that the addition of Bortezomib (Bor) to HDM is safe and a promising conditioning regimen. Thus, in this study we analyzed a single center experience on the use of Bor in combination with HDM (Bor-HDM) as conditioning regimen before ASCT as frontline treatment in MM patients in order to evaluate the safety and the response rates of this combination regimen. We analyzed a total cohort of 48 newly diagnosed MM patients (25 females and 23 males; median age 60 years, range 42-70) admitted consecutively to our Bone Marrow Transplantation Center from 2008 to 2014. Regarding prognostic stratification, 25% of patients were classified as stage III according to International Staging System (ISS), and 12% exhibited high-risk cytogenetic features (defined by presence of del(17p) and/or t(4;14) and/or t(14;16)). All patients with the exception of three underwent to 3-drugs Bor-based induction therapy including Bor (median cumulative dose: 28.9 mg/m²), Thalidomide and Dexamethasone (VTD). After induction therapy and stem cell collection 28 out of 48 patients were treated with Bor-HDM, whereas 20 patients with HDM alone as conditioning regimen before ASCT. HDM was administered on day -2 (median dose: 200 mg/m²), and a single-dose Bortezomib at the dosage of 1.3 mg/m2 was administered on day -1 in the Bor-HDM group. Stem cells were reinfused on day 0 (median number of CD34⁺ infused: 3.3x10⁶/Kg, range 1.86-6x10⁶/Kg). Responses were evaluated at day 100 after ASCT, and definitions of response were used according to the International Myeloma Working Group (IMWG) Uniform Response Criteria for MM. Quantitative variables were compared by non-parametric Kruskal-Wallis and Mann-Whitney tests as appropriate and categorical variables were analyzed by Chi-square and Fisher's exact test. pvalue of <0.05 was considered significant.

Any significant difference was not observed between Bor-HDM group vs HDM group, regarding age at diagnosis (p=0.77), sex (p=0.24), cumulative induction dose of Bortezomib (p=0.42), dose of Melphalan used for conditioning (p=0.16) and the median number of CD34⁺ cells infused (p=0.12). Distribution of ISS stage was similar in the two groups (p=0.14), as well as that of high-risk cytogenetic (p=0.14). Moreover the response rates after the induction therapy was not statistically different in the two groups of patients analyzed (sCR + CR: 19% in Bor-HDM group vs 26% in HDM group; VGPR: 33% vs 48%; PR: 48% vs 26%, p=0.22). Any significant difference on hematopoietic recovery rates was not observed in Bor-HDM as compared to HDM alone with a mean time to neutrophil recovery of 12 days (range 9-18) and to platelet recovery of 12 days (range 9-21) in both groups. Bor-HDM conditioning was well tolerated, without increase of neuropathy occurrence.

Then we analyzed the response rate after ASCT, showing that the overall response rate (ORR) was significantly higher in Bor-HDM group as compared to HDM (sCR+CR: 56% in Bor-HDM vs 17% in HDM; VGPR 30% vs 28%; PR: 15% vs 56%, p=0.0072) with a higher number of "good quality" response (sCR + CR + VGPR: 86% vs 45%; p=0.0075). The number of sCR was also significantly higher in Bor-HDM as compared to HDM alone (23% vs 0%, p=0.067). Moreover an improvement of the response rate after ABMT was seen more frequently in the Bor-HDM group as compared to the HDM group (p=0.0063). We then observed that the number of patients that underwent tandem ASCT was higher in the HDM group (66% vs 41%), even not reaching a statistical significance. In conclusion, this retrospective analysis suggests that BOR-HDM is safe as conditioning regimen with a higher response rate after ASCT as compared to the standard HDM regimen giving the rational design for randomized studies needed to assess whether this conditioning regimen is superior to HDM alone.