Final Report of Safety and Efficacy From a Novel Conditioning Regimen, Individualized Once-Daily Intravenous Busulfan with Bortezomib, in Relapsed Multiple Myeloma Patients Undergoing a Second Autologous Hematopoietic Stem Cell Transplantation.
Authors: Stadtmauer Edward A. , Ratanatharathorn Voravit , Yeh Rosa F. , Freytes Cesar O. , Toro Juan J. , Akpek Gorgun , Sahovic Entezam A. , Tricot Guido J. , Shaughnessy Paul J. , White Darrell J. , Rodriguez Tulio E. , Solomon Scott R. , Yu Louie H. , Patil Shiva , Sun Yiping , Armstrong Elizabeth , Smith Angela , Elekes Agnes , Kato Kazunobu , Reece Donna 
Published: October 2019

Abstract
Abstract 3080

Background:
Salvage therapeutic options are limited for multiple myeloma (MM) patients who relapse after autologous hematopoietic stem cell transplantation (ASCT). A second ASCT using a different conditioning regimen may provide long-term disease control. We report efficacy and safety of daily intravenous busulfan (IV Bu) conditioning given with bortezomib for second ASCT.

Materials and Methods:
In this prospective, multicenter, Phase IIa study, thirty MM patients who relapsed ≥ 1 year after initial ASCT and were candidates for second ASCT were enrolled at eleven centers in the US and Canada. Patients received a test dose of IV Bu (0.8 mg/kg) over 2 hours between Days -12 and -9 prior to ASCT. Pharmacokinetic (PK) analysis from test dose determined Bu exposure as area under the concentration-time curve (AUC). This analysis was used to determine individualized Bu PK-directed dosing for the conditioning regimen in order to achieve a total regimen AUC of 20,000 mM*min. IV Bu was administered over 3 hours once daily from Day -5 to Day -2. Confirmatory PK analysis was conducted in all patients on Day -5. Bu doses were adjusted on Days -3 and -2, if needed. Bortezomib (1.3 mg/m2 QD) was administered as an IV bolus injection on Day -1. Disease response was evaluated prior to the ASCT and at 3 and 6 months post-transplant, based on the International Myeloma Working Group uniform response criteria in 2006.

Results:

Patient Demographics:
Median age at second ASCT was 59 years (range: 48–73). All patients had previously been treated with bortezomib (86.7%), thalidomide (46.7%), and/or lenalidomide (66.7%). All subjects underwent first ASCT with high-dose melphalan. Median time from first ASCT to second ASCT was 28.0 months (range: 12–119). The disease status at second ASCT was seven very good partial response (VGPR; 23.3%), twelve partial response (PR; 40.0%), two stable disease (SD; 6.7%); and nine progressive disease (PD; 30.0%).

Safety:
The most common grade 3 or 4 adverse event (CTCAE v3.0) was febrile neutropenia in 15 patients (50.0%), followed by stomatitis in 13 patients (43.3%), nausea in four (13.3%) and hypokalemia in three (10.0%). One transplant-related death due to pulmonary complications was reported for a patient with Parkinsonism on post-transplant Day 20. There was no instance of seizure, worsening neuropathy, or hepatic veno-occlusive disease (VOD) meeting the Baltimore criteria.

Efficacy:
28 patients had evaluable disease response at least at one time point after second ASCT. Disease response at 3 months were two complete responses (CR; 6.7%), five VGPR (16.7%), four PR (13.3%), eight SD (26.7%), nine PD (30.0%), and two cases without evaluable assessment (6.7%). Two patients who achieved CR at 3 months had PD and VGPR prior to ASCT, respectively. Disease response at 6 months were one stringent CR (sCR; 3.3%), one CR (3.3%), four VGPR (13.3%), seven SD (23.3%), fourteen PD (46.7%), and three cases without evaluable assessment (10.0%). Median progression-free survival was 191 days, while median overall survival has not been reached yet.

PK:
40.0% (n=12/30) of patients had AUC outside the expected range from pre-transplant test dose, 0.8 mg/kg of IV Bu: eleven cases with AUC <1,000 μM*min and one case with AUC >1,500 μM*min. If only weight was used (e.g. 3.2 mg/kg daily) to determine the dose without considering difference in individual busulfan metabolism, this 40% would have been dosed outside the total target AUC range. Based on test PK, IV Bu dosing for conditioning was individualized ranging between 1.99 and 4.73 mg/kg, which resulted in 93.3% of patients (n=28/30) falling between 16,000 and 24,000 μM*min as a total target AUC without any further dose alteration during conditioning. Only 2 patients (6.7%) needed dose reduction on Days -3 and -2. Mean Bu clearance for test dose and on Day -5 were comparable, 3.00 and 2.92 ml/min/kg, respectively.

Conclusions:

Disclosures:
Stadtmauer: Millenium: Consultancy, Speakers Bureau; Celgene: Consultancy, Speakers Bureau. Off Label Use: IV busulfan and bortezomib-based conditioning regimen prior to transplant for myeloma. Freytes:Otsuka Pharmaceuticals: Research Funding. Shaughnessy:Otsuka: Honoraria, Speakers Bureau. White:Otsuka: Honoraria, Research Funding. Rodriguez:Otsuka: Consultancy, Research Funding, Speakers Bureau; Millennium: Research Funding, Speakers Bureau; Celgene: Consultancy, Speakers Bureau; SOBI: Consultancy, Speakers Bureau. Sun:Otsuka Pharmaceutical Development & Commercialization, Inc.: Employment. Armstrong:Otsuka: Employment. Smith:Otsuka Pharmaceutical Development & Commercialization, Inc: Consultancy. Elekes:Otsuka Pharmaceutical Development & Commercialisation., Inc.: Employment. Kato:Otsuka Pharmaceutical Development & Commercialization, Inc.: Employment. Reece:Otsuka: Honoraria, Research Funding; Janssen: Consultancy, Honoraria, Research Funding; Celgene: Consultancy, Honoraria, Research Funding; Bristol Myers Squibb: Honoraria, Research Funding; Novartis: Honoraria, Research Funding; Millinneum Pharmaceuticals: Research Funding; Merck: Consultancy, Honoraria, Research Funding.