This is a single-centre retrospective study of consecutive MM patients who were seen by the BMT program for HSPC collection for a first-line consolidative aHSCT between January 1991 and September 2015. Patients were excluded if their first HSPC collection was attempted outside TOH or occurred following salvage therapy. The details of patient selection are shown in supplemental figure S1. The study was approved by the local research ethics board.

The data was retrospectively extracted from office charts, hospital electronic medical records and the BMT database. Information collected included patients’ demographic data and their disease description comprising baseline characteristics, therapy and treatment outcomes. Cytogenetics and FISH were not routinely performed during the study period and therefore were not collected.

The myeloma subtype was classified according to the secreted paraprotein heavy chain (IgG, IgA, IgD, IgM, or IgE) and light chain (kappa or lambda), or their absence (non-secretory). Patients were staged by the Salmon–Durie (SD) criteria and by the International Staging System (ISS).

Responses were defined by the International Myeloma Working Group (IMWG) criteria for serum (SPEP) and urine (UPEP) electrophoresis and immunofixation (IFE). We did not have access to serum free light chain assays until 2016 and therefore response was defined by SPEP, UPEP and IFE alone. Changes in the paraprotein were compared between the diagnosis and the last measurement before mobilization chemotherapy (induction response) and between the diagnosis and the last measurement before aHSCT (mobilization response). Responses were categorized as follows: complete response (CR) (patients with negative serum and urine IFE); very good partial response (VGPR) (detectable abnormal bands only by IFE or 90–99% monoclonal peak reduction in SPEP and <0.1g/24h of urinary paraprotein); partial response (PR) (50–89% paraprotein reduction in SPEP and ≥90% drop in UPEP); stable disease (SD) (<50% paraprotein reduction in SPEP and <25% paraprotein increase in SPEP or <90% paraprotein reduction in UPEP), and; progressive disease (PD) (≥25% paraprotein increase in SPEP or UPEP).

Toxicity was defined as an unscheduled medical visit to an emergency room or medical day care unit during the follow-up period after mobilization had been initiated. The reason for an unscheduled medical visit, need for admission, length of admission, and number of days spent in the intensive care unit (ICU) were collected. Infections included febrile neutropenia, non-neutropenic fever, skin infections or pneumonia after mobilization.

Neutrophil engraftment was defined as the first of 3 consecutive days with absolute neutrophil count ≥0.5×109/L and platelet engraftment was the first of 3 consecutive days with platelet count ≥20×109/L in the absence of platelet transfusion in the previous 7 days.

The most common regimens administered, VAD, BD (bortezomib and dexamethasone) and CyBorD (CTX, bortezomib, and dexamethasone) were analyzed individually, whereas smaller groups (single agent dexamethasone, melphalan, thalidomide, and lenalidomide) and patients who received two different induction regimens were grouped together. The details of VAD, BD, and CyBorD regimens are outlined in supplemental Table S1.

Patients mobilized with GCSF alone often received 5μg/kg/day of filgrastim subcutaneously for 4 days and proceeded to apheresis on day 5 if peripheral CD34+ assessment showed adequate HSPC mobilization. In CTX-GCSF mobilization, 1.5–4.5g/m2 of CTX was given, followed by filgrastim two days later until the last day of apheresis, as needed.

The target for HSPC collection was ≥2×106CD34+cells/kg, but the decision to proceed with aHSCT with <2×106CD34+cells/kg was made individually. Patients who did not mobilize sufficient HSPC were offered salvage plerixafor (0.24mg/kg) beginning in 2012.

We looked at the total CD34+ cells collected, and how many aphereses were required for a successful collection. The number of CD34+ cells re-infused was used when the number of cells collected was missing (n=56) because the patient had been treated before 2004, in an era when storing CD34+ cells for a second transplant was not routine practice in our centre.

Conditioning regimens varied during the study period and are described in supplemental Table 1.

Descriptive statistics were used to report demographic, disease and treatment characteristics. Non-normal data was presented as medians with the range. Categorical data was compared using Pearson's chi-square test or Fisher's exact test, as appropriate. The analysis of variance (ANOVA) and t-tests were used to compare continuous variables and statistical significance was assigned when p<0.05. Statistical analyses were performed using the GraphPad Prism version 6.00.

Five hundred and ten consecutive MM patients were referred for a first-line HSPC mobilization. Thirty-eight patients were excluded, as their HSPC mobilization had not been part of the initial treatment (n=20) or because they had received a first-line allogeneic HSCT instead (n=18), leaving 472 patients in the study. Patient and disease baseline characteristics were similar among patients who had received a different induction therapy, except for the increased age seen in CyBorD patients (Table 1).

The overall response rate after induction was 77% (n=292) among the patients whose response data was known (n=379): 54 (14.2%) patients achieved CR, 65 (17.2%) achieved VGPR and 173 (45.7%) had a PR. The other patients were in SD (75, 20%) or PD (12, 3%) when they were seen for HSPC mobilization.

The ISS stage was not available for many patients diagnosed before 2005 (n=112) or who were referred from other centres (n=26), while the DS stage was unavailable in the medical records of early-diagnosed patients (n=15) and for some patients referred from outside TOH (n=12). The overall survival from the time of diagnosis and the time of aHSCT is presented in supplement Figure S2.

The VAD, BD, and CyBorD regimens were the 3 most frequently used induction therapies for newly-diagnosed MM patients (Table 2) and reflected the evolving practice of the Myeloma Program at TOH throughout the study period. A minority of patients received other induction regimens (see Table 1). Information on the induction therapy was unavailable for 10 patients.

The VAD regimen was first used in March 1994 and remained the predominant induction treatment until 2009. Bortezomib was used in selected patients as first-line therapy beginning in March 2008. Beginning in February 2010, all patients received either BD or CyBorD as induction therapy, based on their physicians’ preference, while CyBorD has been the standard induction regimen since August 2013.

The CTX-GCSF was the most frequent mobilization regimen used throughout the study period, with CTX doses ranging from 1.5g/m2 to 4.5g/m2 (Table 2). Plerixafor was first given in March 2012, and 11 of the 12 patients given plerixafor had received induction therapy with CyBorD.

Toxic events, severe enough to warrant an unscheduled medical visit to the clinic or emergency room after mobilization, were not different between groups of patients who had received different induction regimens (p=0.33), but fewer hospital admissions were required by those who received BD (Table 3 and Table S2).

Infections were the most common adverse events after HSPC mobilization (Table 3) and trended towards a lower frequency in the cohort that had previously received BD induction. On the other hand, a greater proportion of patients experienced bone pain during the mobilization period, for those who had previously received BD and CyBorD (p=0.01).

Among those admitted to hospital, the median length of hospital stay was 3 days, irrespective of the remission induction given (p=0.86). Only one patient required ICU care after mobilization due to hyponatremia. Eleven patients mobilized with CTX-GCSF failed to collect sufficient HPSC to proceed with one aHSCT (Table 3), thus remaining 461 patients, who proceeded to a first-line aHSCT.

The increase in the proportion of patients achieving VGPR or better after mobilization with cyclophosphamide did not seem significant (31.4% following induction and 36.3% following mobilization, p=0.18).

Of the 461 MM patients who successfully collected HSPC, 55 underwent aHSCT out of TOH and they were excluded from the engraftment analyses. The conditioning regimens of the analyzed patients were melphalan 140–200 (Mel140–200) (87.4%), melphalan, VP-16 and total-body irradiation (MelVPTBI) (11.6%), busulfan, cyclophosphamide and total-body irradiation (BuCyTBI) (0.7%), and busulfan and melphalan hydrochloride (BuMel) (0.3%).

Six (1.4%) of the 406 transplanted patients died before engraftment. Neutrophil engraftment occurred in a median of 12 days after aHSCT (range 8–65 days) for all cohorts (p=0.52). Platelet engraftment occurred in a median of 12 days after aHSCT (range 7–151 days) and it likewise did not differ between cohorts (p=0.71). The platelet count of 17 (4%) patients never dropped below 20×109/L, and these patients did not require platelet transfusion support.

The neutrophil or platelet counts were abnormal in 12% of the whole cohort at the first anniversary of the aHSCT. The 3 patients that experienced severe neutropenia or thrombocytopenia in the 1-year follow-up of the aHSCT had relapsed myeloma and were on salvage therapy (able 4). Moderate cytopenias at 1 year after the aHSCT were felt to be due to relapsed myeloma (n=14), infection (n=5), lenalidomide maintenance therapy (n=2), amegakaryocytic thrombocytopenia (n=1) and heparin-induced thrombocytopenia (n=1). The cause of cytopenias was unexplained in 23 (5.7%) patients.