PK-Directed Intravenous Busulfan In Combination With High-Dose Melphalan and Bortezomib As Conditioning Regimen For Patients With Multiple Myeloma

High dose therapy followed by autologous hematopoietic stem cell transplantation (ASCT) has an established role in the treatment of patients with multiple myeloma (MM). The most commonly used conditioning regimen in this setting is high-dose melphalan (200mg/m^2; Mel200), which has been shown to result in improved progression free (PFS) and overall survival (OS). Achievement of a complete response (CR) following ASCT is an indicator for freedom from relapse, as well as PFS and OS. The CR rate observed after Mel200 followed by ASCT is between 10-35%. There is evidence that the combination of busulfan (Bu) and melphalan (Mel) results in longer PFS and OS compared to Mel alone. Additionally, the use of bortezomib (Btz) during conditioning with either high dose Mel alone or the combination of Mel and intravenous (i.v.) Bu has shown to be both safe and to have promising efficacy. The objective of our trial is to assess whether the combination of PK-directed Bu, Mel and Btz (BuMelBtz) during conditioning for a first ASCT in MM patients is both safe and efficacious.

Patients aged 18-72 with multiple myeloma, who had 1) measurable disease, 2) received less than one year of prior myeloma-directed therapy, 3) adequate organ function and performance status, and 4) an indication for ASCT were eligible. Exclusion criteria were >/= grade 2 neuropathy, prior stem cell transplant, uncontrolled intercurrent illnesses or comorbidities, unresolved >/= grade 2 toxicities from prior therapies, and prior malignancies except non-melanoma skin cancer. Treatment consisted of PK-directed i.v. Bu (4 daily 3-hour infusions from day (D) -6 to -3 to target a total AUC of 20,000 μMxmin), i.v. Mel 140mg/m2 on D-2, and i.v. Btz 1.4mg/m2 on D-6, -4, +1 and +4. The individual daily doses for Bu on D-6 and D-5 were determined by PK measures following a test dose (0.8mg/kg) 5-9 days prior to first Bu treatment dose; the last 2 doses (day -4 and -3) could be adjusted following another PK measure after the first full treatment dose on D -6. Stem cells were infused on D0. Subsequent consolidative or maintenance therapy was left to investigator choice. Primary outcome was CR rate assessed on D +100 post ASCT as per IMWG criteria. Secondary outcomes were overall response rate (ORR), toxicities, PFS and OS. The trial is registered at clinicaltrials.gov (NCT01605032).

To date, 13 patients have been treated. The median age was 63 years (range 44-70), 62% (n=8) were male, 23% had ISS stage 3 (3/13), no patient had high risk cytogenetic features. The median number of regimens prior to ASCT was 1 (range 1-3) and included bortezomib in 92% (n=12). Prior to BuMelBtz the best treatment response had been stable disease (SD) in 3 patients, partial response (PR) in 8; only 1 patient each had achieved a very good partial response (VGPR) or CR. Following BuMelBtz/ASCT, median days to ANC >/=0.5 x 10⁹/L and platelet count >/=30 x 10⁹/L were 11 (range 10-13) and 17 (11-29), respectively. The most common non-hematological toxicities were alopecia (100%), oral mucositis (62% G3), dysphagia (85% G3, but no patient required TPN or enteral feeding), as well as electrolyte abnormalities (62% G3/4). Other common toxicities were nausea (92%, all G1/2), diarrhea (84% G1/2, 8% G3), while 77% of patients developed fully reversible transaminitis (15% G3). Less common G3 toxicities included delirium (8%), colitis (8%), skin infection (zoster, 8%), other infections (23%), and delirium (8%). One patient developed GI symptoms suggestive of acute GVHD on a gastric biopsy 8 weeks after ASCT. No patient developed sinusoidal obstruction syndrome of the liver. At 100 days post BuMelBtz/ASCT, response assessment was available for 8 patients: 1 achieved a stringent CR (12.5%), 4 VGPR (50%), and 3 PR (37.5%), resulting in a 100% ORR. One patient improved from a VGPR to a stringent CR during follow up. After a median follow up of 5 months (range 1-15) all patients are alive and no patient has relapsed. The trial is ongoing.

PK directed i.v. Bu in combination with Mel and Btz (BuMelBtz) is an effective and safe conditioning regimen for patients with multiple myeloma. Further evaluation is warranted.

Barta: Otsuka: Research Funding; Onyx: Honoraria, Speakers Bureau; Janssen: Speakers Bureau; Seattle Genetics: Honoraria. Off Label Use: IV Busulfan for the treatment of multiple myeloma.