The Result of a Phase 1 Study of Selinexor in Combination with High-Dose Melphalan and Autologous Hematopoietic Cell Transplantation for Multiple Myeloma

Background:

High-dose melphalan and autologous hematopoietic cell transplantation (HCT) remains a crucial treatment modality for patients with multiple myeloma (MM). Strategies to improve the conditioning regimen have been explored with addition of novel targeted therapies previously with limited success. Selinexor, an orally available selective inhibitor of nuclear export (SINE), targeting Exportin-1 (XPO-1), was recently approved by the Food and Drug Administration (FDA) for relapsed/refractory MM. Our pre-clinical data show synergy between selinexor and bifunctional alkylating agents in several MM models. Therefore, we hypothesized that the addition of selinexor to high-dose melaphalan would be safe and improve outcomes of autologous HCT in MM.

Methods:

We designed a single institution, standard 3+3 dose escalation phase 1 study to evaluate the combination of selinexor (given at 40 mg po (dose level 1); 60 mg (dose level 2); and 80 mg (dose level 3) on days -3 and -2 before melphalan) and high-dose melphalan (100 mg/m² IV on days -3 and -2) as a conditioning regimen for autologous HCT in patients with MM achieving either partial response (PR) or very good partial response (VGPR) after less than 4 lines of systemic anti-myeloma chemotherapy (NCT02780609). The primary objective was to establish a maximum tolerated dose (MTD) and identify a recommended phase 2 dose (RP2D).

Results:

From 08/2017 to 03/2019, a total of 12 MM patients (pts) received autologous HCT under the phase 1 protocol at Moffitt Cancer Center. Baseline characteristics included: a median age of 57 (range, 43-69); M:F = 7:5; IgG subtype=9, light chain type=2, IgD subtype=1; 92% with Durie-Salmon stage 3; 22% with high-risk disease based on del17p/t(4;14) (2/9, n=3 with unknown risk); a median number of induction=1 (range, 1-2); all received bortezomib-based induction, 83% received immunomodulatory agent, 17% received daratumumab. Pre-HCT responses were PR=4; VGPR=8. Pts received a median of 4.16 (range, 2.16-5.73) million CD34+ cells/kg. Neutrophil engraftment occurred with a median of 11 (range 11-12) days, and a platelet engraftment with a median of 15 (range, 10-36) days. Three pts each entered in dose level 1 and 2; and 6 pts at dose level 3. One pt in dose level 2 did not receive dexamethasone on day -1 due to grade (G) 3 hyperglycemia. One pt in dose level 3 (80 mg selinexor) did not receive day -2 dose of selinexor due to liver function test (LFT) abnormality (ALT > 2x ULN) which was considered as dose-limiting toxicity (DLT) as second dose of selinexor was not given. LFTs normalized after HCT. Dose level 3 was expanded to 3 additional pts and no additional DLTs were observed. Treatment-related serious adverse events (SAEs) included: G3 febrile neutropenia=3, G3 diarrhea=1, G3 nausea=1, G3 small bowel obstruction=1, G3 acute kidney injury=1, G3 lung infection=1. Post-HCT responses at day +90 were complete response (CR)=2, VGPR=6, PR=3, and progression=1. CR conversion rate was 16.7% though phase 1 portion of the study was not powered to evaluate the CR rate. Therefore, RP2D was established as selinexor 80 mg on days -3 and -2. The study is proceeding to the phase 2 portion to assess the efficacy of this combination.

Conclusions:

The combination with selinexor 80 mg po with high-dose melphalan at 100 mg/m² on days -3 and -2 (dose level 3) was well tolerated and engraftment kinetics were not altered. A phase 2 study of selinexor 80 mg with high-dose melphalan and autologous HCT is ongoing (NCT02780609).