Phase 2 trial of intravenously administered plerixafor for stem cell mobilization in patients with multiple myeloma following lenalidomide-based initial therapy.

Initial therapy of multiple myeloma with lenalidomide-based regimens can compromise stem cell collection, which can be overcome with the addition of plerixafor. Plerixafor is typically given subcutaneously (SQ), with collection ∼11 h later for maximum yield. Intravenous administration may allow more rapid and predictable mobilization. This trial was designed to assess the efficacy and feasibility of IV plerixafor in patients receiving initial therapy with a lenalidomide-based regimen. Patients received G-CSF at 10 μg/kg/day for 4 days followed by IV plerixafor at 0.24 mg/kg/dose starting on day 5; plerixafor was administered early in the morning with apheresis 4-5 h later. Thirty-eight (97%) patients collected at least 3 × 10(6) CD34+ cells/kg within 2 days of apheresis. The median CD34+ cells/kg after 1 day of collection was 3.9 × 10(6) (range: 0.7-9.2) and after 2 days of collection was 6.99 × 10(6) (range: 1.1-16.5). There were no grade 3 or 4 non-hematological adverse events, and one patient experienced grade 4 thrombocytopenia. The most common adverse events were nausea, diarrhea and abdominal bloating. IV plerixafor is an effective strategy for mobilization with low failure rate and is well tolerated. It offers flexibility with a schedule of early-morning infusion followed by apheresis later in the day.

INTRODUCTION

Autologous stem cell transplantation (ASCT) remains an integral part of current management of multiple myeloma in transplant eligible patients.[1-3] Traditionally, patients undergo 4–6 months of initial therapy with one of several commonly used regimens followed by peripheral blood stem cell mobilization. Following a successful stem cell harvest, patients either proceed to an immediate ASCT or continue with the initial therapy and use ASCT at the time of relapse.[4] One of the critical steps in this process remains the ability to collect adequate number of stem cells for a successful ASCT. Nearly 10% of patients may fail to collect the minimum number of stem cells required for the ASCT, depending on the mobilization process utilized.[5-7] In addition, the initial therapy employed for myeloma management also has significant impact on the success of stem cell mobilization.[8-12] While alkylating agents that can impair stem cell mobilization are rarely used currently as part of initial therapy in transplant eligible patients, newer drugs such as lenalidomide can also impair the collection process.[11-14] The most common approaches to stem cell mobilization until recently have been the use of G-CSF alone or G-CSF following pulse dose chemotherapy.[5] The chemotherapy approach has lower failure rates, but is associated with increased risk of neutropenic fever and consequent complications. More recently, the introduction of plerixafor, a CXCR4 antagonist, has radically changed stem cell mobilization, considerably reducing the rate of mobilization failures when used in conjunction with G-CSF.[15-19] Risk adapted strategies for the use of plerixafor based on circulating CD34+ cell numbers or apheresis yields have allowed us to successfully mobilize and collect stem cells in nearly all patients and provide the opportunity to proceed with a stem cell transplant when recommended.[18, 20–25] However, the current schedule for plerixafor administration late in the evening prior to collection and the relatively narrow window for collecting the stem cells introduce logistical difficulties.[26] While the majority of the studies have used plerixafor by the subcutaneous route, and the current label indicates SQ route, intravenous administration has been studied in a limited fashion. Following SQ administration, plerixafor is absorbed rapidly with 70–80% bioavailability in healthy volunteer studies. Estimates of Cmax and AUC were higher following IV administration compared with SC dosing, while terminal half-lives were comparable between the two routes. In the healthy volunteer studies, the peak peripheral blood CD34+ cell counts were seen 10–14 hours after administration of plerixafor, leading to the current recommendations of injection and apheresis schedules. We designed this trial with two objectives: (1) to determine the risk of failure of stem cell mobilization with plerixafor and G-CSF among patients receiving a lenalidomide based induction therapy for myeloma and, (2) to determine the safety and efficacy of intravenously administered plerixafor in the setting of patients with myeloma undergoing peripheral blood stem cell mobilization.

SUBJECTS AND METHODS

Patients

Patients with a diagnosis of symptomatic multiple myeloma receiving initial treatment with a lenalidomide based treatment regimen started ≤12 months prior to registration were enrolled. Patients should have received at least 2 cycles of treatment with the lenalidomide regimen with the last dose of lenalidomide > 2 weeks prior to registration and patients should be eligible for and be considered for stem cell transplant. The trial was approved by the Mayo Foundation Institutional Review Board and was carried out in accordance with the Helsinki Principle. The clinical trial was registered at www.clinicaltrials.gov as NCT00998049.

The objectives of the trial were to determine the proportion of patients reaching a stem cell yield of 3 × 106 CD34+ cells/kg by the second day of apheresis with intravenously administered plerixafor, the safety and tolerability of intravenously administered plerixafor, and the overall rate of failure to mobilize minimum required number of stem cells for an ASCT (< 2.5 × 106 CD34+ cells/kg). Toxicities were graded using CTCAE v 4.0. Adverse event assessment was performed daily during the study.

Treatment

Patients received G-CSF (10 ug/kg), daily subcutaneous injection beginning Day 1, once they completed the required pre-transplant evaluation. On the morning of day 5 of G-CSF administration, plerixafor was administered at a dose of 240 mcg/kg (160 mcg/kg if CrCl < 30 ml/min) intravenously. The same formulation as that used for subcutaneous administration was used, but diluted in a larger volume. The dose to be administered was added to 50 ml of normal saline. The drug was administered using standard infusion tubing via slow infusion over 30 minutes. At the end of the infusion, the line was flushed with 10 ml of normal saline. Patients then proceeded to large volume leukapheresis on the Fenwal Amicus (Fenwal Inc., Lake Zurich IL, USA) utilizing version 2.5 software. The collection method has previously been described.[27] In brief, patients underwent leukapheresis for five hours with patients with white blood cell counts of less than 35×109/L processed at a blood flow rate of 90 ml/min utilizing a cycle volume of 1,400 ml and those with white blood cell counts greater than 35×109/L processed at a blood flow rate of 65 ml/min utilizing a cycle volume of 1,000 ml.[28] Anticoagulant consisted of a mixture of ACD-A (Baxter Healthcare Corp., Deerfield, IL, USA), normal saline, and heparin. The citrate infusion rate was 2.50 mg/kg/min and anticoagulant ratio was 13:1. MNC offset was 1.5 ml and RBC offset was 5 ml and adjusted during the procedure as necessary. [27] Patients began collection approximately 4 hours after the completion of the plerixafor infusion. Patients continued to receive daily G-CSF and IV plerixafor each morning of apheresis for a maximum of four doses or until collection goal met. Patients were allowed to undergo additional apheresis collections beyond the fourth collection at the discretion of the treating physician, but only four doses of plerixafor could be administered.

Patients were typically conditioned with melphalan 200 mg/m2, with dose reduction to 140 mg/m2 for patients with reduced renal function or patients over 70 years. Post transplant GCSF was not routinely used for any of the patients. Engraftment kinetics was examined in the subgroup of patients who proceeded to a stem cell transplant. Neutrophil engraftment was defined as neutrophil count more than or equal to 0.5 × 109/L for 3 days or more than or equal to 1.0 × 109/L for 1 day. Platelet engraftment was defined as platelet count more than or equal to 20 × 109/L without a transfusion for the preceding 7 days.

Statistical analysis

For primary endpoint, success was defined as collection of 3 × 106 CD34+ cells/kg after two days of apheresis. The largest success proportion where the proposed treatment regimen would be considered ineffective in this population was 60%, and the smallest success proportion supporting future studies in this patient population was 80%. This design required 36 evaluable patients, where at least 26 successes were required to conclude that further studies be recommended. This design has 91% power and a 9% Type I error rate.

RESULTS

Forty patients were accrued between December 2009 – October 2011, and 39 were eligible for analysis. The baseline characteristics of the patients as well as other myeloma related details are provided in Table 1. The patients had received a median of 4 cycles with a lenalidomide-based regimen, mostly lenalidomide and dexamethasone. Nearly a fourth of the patients received a bortezomib and lenalidomide combination. The majority of patients had remained on full dose lenalidomide prior to proceeding with study registration and stem cell collection.

Table 1

Baseline Characteristics

	Total (N=39)
Age
Median	60.0
Range	(28.0–73.0)
Gender: Male	25 (64.1%)
ECOG Performance Score
0	27 (69.2%)
1	12 (30.8%)
Months from initial myeloma therapy to registration
Median	4.9
Range	(2.6–11.1)
Days from first dose of lenalidomide to registration
Median	141.0
Range	(78.0–311.0)
Days from last dose of lenalidomide to registration
Median	24.0
Range	(14–110.0)
Ending lenalidomide Dose
10 mg	1 (2.6%)
15 mg	7 (18.4%)
25 mg	30 (78.9%)
Total number of cycles of lenalidomide
Median	4.0
Range	(3.0–11.0)
Other drugs used in combination with lenalidomide	39 (100.0%)
Dexamethasone	39
Cyclophosphamide	1
Velcade	10
Other	2

In terms of the primary endpoint, thirty-eight (97%) of the patients achieved at least 3 × 106 CD34+ cells/kg, adequate to proceed to one stem cell transplant, within 2 days of apheresis (Table 2). The median CD34+ cells/kg after 1 day of collection was 3.9 × 106 (range; 0.7 to 9.2) and after two days of collection was 6.99 × 106 (range: 1.1–16.5). The median number of cells collected on each apheresis day is shown in figure 1. We then examined the time taken to reach 4 × 106 and 8 × 106 CD34+ cells/kg, given that these are typically considered the ideal numbers required for 1 and 2 transplants respectively. As shown in figure 2, 38 patients were able to reach the 4 × 106 threshold after 4 apheresis sessions and 25 of the patients in whom the target was to collect for more than one transplant, achieved 8 × 106 target after 4 sessions. The sole patient who failed to reach the primary goal of 3 × 106 was a 61-year-old male who had received 4 cycles of previous lenalidomide at 25 mg (with dexamethasone). The total CD34+ cell yield for this patient, over the course of 3 days, was 1.42 × 106 cells/kg. The kinetics of the peripheral blood CD34+ cell counts are shown in figure 3.

Table 2

Stem cell mobilization and harvest outcomes

Rate of achieving 3 × 106 CD34+ cells/kg after 2 days of apheresis1	97% (95%CI: 86–99)
Number of patients	38
Median CD34+cell yield Day 1	3.87 × 106 cells/kg (range: 0.67–9.16)
Median CD34+cell yield Day 2	3.55 × 106 cells/kg (range: 0.47–9.17)
Median number of days of apheresis	4 (range: 2–5)
Median time (from first GCSF dose) to reach 6 × 106 CD34+ cells/kg 2	5 days (95%CI: 5–6)
Rate of failure to mobilize (never achieve 2.5 × 106 cells/kg) 1	3% (95%CI: 0.006–13)
Number of patients	1

CI: confidence interval

Binomial distribution

Kaplan Meier

Figure 1

Figure shows the median number of CD34+cells collected (/kg body weight) on each day of apheresis. X-axis shows the day of apheresis and the Y-axis show the median CD34+ cells (× 106)/kg. The error bars denote the interquartile range.

Figure 2

Figure shows the number of days to reach specific targets (4 × 106/kg and 8 × 106/kg from start of GCSF administration. X-axis shows the number of patients. The number of days from start of GCSF administration (Day 5 is the first day of plerixafor) is denoted by the color of the shaded portion.

Figure 3

Figure shows the kinetics of peripheral blood CD34+ cell counts. Data is presented from before the administration of plerixafor and from one, two and three days after the initiation of plerixafor. The error bars show interquartile range. Day 5 is the first day of plerixafor.

The IV administration was well tolerated with no grade 3 or higher adverse events (Table 3). The most common grade 1 or 2 adverse events seen were gastrointestinal, namely nausea, diarrhea and abdominal pain or bloating. Grade 1 dizziness was reported in 8 patients. Infusion site reactions were observed in one patient.

Table 3

Maximum Severity of Toxicities1 (N=39)

Toxicity2	Grade 1	Grade 2	Grade 3 or higher	Total
Anemia	0	1	0	1
Abdominal Pain	5	2	0	7
Bloating	0	1	0	1
Diarrhea	9	1	0	10
Nausea	12	1	0	13
Injection Site Reaction	1	0	0	1
Thrombocytopenia	0	1	0	1
Dizziness	4	0	0	4
Headache	4	1	0	5

Possibly, probably or definitely related

Common Terminology Criteria for Adverse Events version 3.0.

We then performed additional analysis to identify factors potentially contributing to slower collection. Given that all but one patient achieved the primary goal of 3 × 106 CD34+ cells/kg in two days, we compared the baseline clinical and laboratory characteristics between patients achieving 6 × 106 in 2 days (N=25) vs. those who did not. Specifically, we examined if age, time from diagnosis to registration, lenalidomide dose at start of therapy and at end of therapy, days between stopping lenalidomide and start of mobilization, duration of lenalidomide therapy, and blood counts, serum creatinine, serum albumin, bone marrow plasma cell percentage, plasma cell labeling index (PCLI) and beta2 microglobulin from study registration influenced the ability to collect stem cells. Presence of active myeloma, as reflected in a higher percentage of (bone marrow) plasma cells, higher beta 2 microglobulin, and higher plasma cell labeling index, was the only factor affecting the ability to mobilize and the rate of collection.

At the time of data analysis, 34 (87%) patients had received autologous stem cell transplantation. The median time to ANC engraftment was 14 days (range; 11–21) and to platelet engraftment was 15.5 days (range; 12–38).

DISCUSSION

The results of the current trial highlights two aspects of plerixafor and GCSF based stem cell mobilization; the ability to administer the drug intravenously in a safe and effective manner and the ability of plerixafor based mobilization to overcome the adverse impact of lenalidomide based initial therapy. The current study represents the first trial specifically designed to evaluate the feasibility of intravenous administration of plerixafor for stem cell mobilization. The results of the current trial should be interpreted in the context of the previous trials evaluating the subcutaneous administration.[16, 29] In the randomized trial comparing plerixafor and G-CSF to G-CSF alone in patients with myeloma undergoing peripheral blood stem cell mobilization, a similar treatment schedule and dosing was utilized.[16] The proportion of patients collecting 6×106 CD34+ cells/kg in 2 days in this study was 67% (26 of 39), similar to the 71.6% seen with use of SQ plerixafor in the randomized trial. With respect to the minimal collection, 97% of the patients collected at least 3 × 106 CD34+ cells/kg, which is comparable to the 95.3% who collected at least 2 × 106 CD34+ cells/kg in 4 apheresis sessions in the randomized trial. However, the peripheral blood CD34+ cell counts were lower with IV plerixafor than those seen in the plerixafor arm of the randomized trial. Overall, the results are comparable with what has been observed previously in this patient population with the use of SQ plerixafor. In terms of toxicity, the types of toxicity and the severity were comparable with the SQ administration with gastrointestinal symptoms being the most common.

It is difficult to directly compare the results of the current study with those from the randomized trials given that less than 5% of the patients in the plerixafor arm had received lenalidomide. We have previously shown that lenalidomide therapy can adversely affect the ability to mobilize stem cells in the context of G-CSF based mobilization.[11] In that study, patients who had received lenalidomide therapy for induction had a significantly lower total CD34+ cell yield, lower average daily CD34+ cell and lower CD34+ cell collection on first day, first 2 days and first 3 days and a greater number of collections compared with those receiving VAD (Vincristine, adriamycin, dexamethasone) or dexamethasone alone for induction therapy. Overall, 7% of patients failed mobilization and stem cell collection in the context of prior therapy with lenalidomide. The median CD34+ cell collection in the lenalidomide treated patients on days 1 and 2 of apheresis were 1.7 × 106 CD34+ cells/kg each, compared with 3.55 and 3.7×106 CD34+ cells/kg in the current trial, respectively. Other studies have also reported higher failure rates with G-CSF based mobilization among lenalidomide treated patients. Popat et al reported a mobilization failure (<2 × 106 CD34+ cells/kg) rate of 25% with G-CSF alone in patients who had previously received lenalidomide.[14] In a case series, mobilization with G-CSF (10 mg/kg/day) alone or G-CSF (7.5 mg/kg/day) plus GM-CSF (7.5 mg/kg/day) resulted in a failure rate of 43%.[13] In the series by Paripati et al, 45% of patients failed to reach the target of at least 2 × 106 CD34+ cells/kg with G-CSF (10 mg/kg/day).[12] Given the difficulty seen across multiple reports, several studies have looked at the utility of plerixafor in patients receiving initial therapy with lenalidomide. In a study examining the efficacy of plerixafor among lenalidomide treated patients from across multiple studies, the overall median number of CD34+ cells collected was 5.6 × 106/kg (range, 0.45×106–37.2×106/kg).[30] Of 60 patients, 52 (86.7%) had the minimum number of 2×106 CD34+ cells/kg collected, and 38 (63.3%) had 5×106/kg CD34+ cells collected. In the European compassionate use program, thirty-five patients previously treated with lenalidomide were given plerixafor plus G-CSF for remobilization.[31] The overall median number of CD34+ cells collected was 3.4 × 106/kg (range: 1.1–14.8). The minimum required number of CD34+ cells (>/=2.0 × 106/kg) was collected from 69% of patients in a median of 2 days. More recently, risk adapted strategies for use of plerixafor have substantially reduced the failure rate in these patients.

Introduction of plerixafor clearly has increased the options for stem cell mobilization, enabling a substantial number of patients who otherwise would not have been able to proceed to stem cell transplantation due to failure to collect adequate stem cells, to receive the benefit of this therapy. However, this has to be viewed in the context of the cost associated with the agent, which is substantial. The high cost of the drug has led to evaluation and development of several risk-adapted therapy models, all aimed at selectively using the drug in the patients who are most likely to fail with GCSF alone. In addition, the duration of therapy with plerixafor is an important determinant of the cost and randomized controlled trials have shown the maximum benefit during the initial 2–4 days of therapy with diminishing returns beyond that time point.

In conclusion, intravenous administration is a safe and effective approach to plerixafor administration. The intravenous administration offers flexibility in patient scheduling with a schedule of early morning infusion followed by apheresis later in the day. However, prospective randomized controlled trials will have to be performed for a more accurate comparison of the pros and cons of the two routes of administration. Use of plerixafor clearly allows for effective stem cell mobilization in patients previously treated with lenalidomide, an important finding given the common use of lenalidomide for initial therapy of myeloma.