A Phase 1 Study of Total Marrow Irradiation Combined with High-Dose Melphalan for Patients with Relapsed/ Refractory Multiple Myeloma

High dose melphalan at 200mg/m²(Mel200) followed by autologous stem cell transplant (ASCT) prolongs the survival of patients with multiple myeloma (MM) although it does not prevent relapse. Enhancing the anti-myeloma effect of pre-transplant conditioning without increasing toxicity is an important goal. To this purpose, intensity modulated radiation therapy (IMRT) can be used to deliver radiation to the marrow (total marrow irradiation, TMI) while sparing other organs. Here we tested the safety of combining linear accelerator based TMI to Mel200 in a phase 1, 3+3 trial.

Twelve patients with MM who relapsed after at least one line of therapy were enrolled in 3 dose cohorts (3Gy, 6Gy and 9Gy). Prior ASCT was permitted. All patients received Mel200 over 2 days. In addition, 1.5Gy TMI was administered twice daily for 1, 2 or 3 days depending on dose cohort. Dose-limiting toxicity was defined as the occurrence of any NCI-CTCAE grade 4/5 non-hematologic toxicity or failure to engraft prior to day 30 after ASCT. Quality of life (QoL) was assessed using the FACT-BMT scale at baseline and 90-100 days after ASCT. Three groups of patients were enrolled and received 3Gy (n=3), 6Gy (n=3) or 9Gy (n=6). Median age at time of transplant was 66 years (range 40-71). Three patients had high risk FISH/ karyotype as defined by IMWG criteria. Median lines of prior therapy was 2 (range 1-4). Five patients (42%) had undergone prior autologous transplant. Of eleven patients (92%) who received prior lenalidomide, 7 (58%) were considered lenalidomide refractory. Similarly, of 11 (92%) patients previously treated with bortezomib, 6 (50%) were considered refractory. Eleven patients had a pre-transplant PET scan performed with 8 (73%) having skeletal PET avidity.

All patients received TMI as scheduled. The mean reduction in dose to organs at risk (lens, oral cavity, kidneys, liver, bowels, lung) ranged from 25-63%. Median time to neutrophil (greater than 0.5x10⁹/L) and platelet (greater than 20x10⁹/L) engraftment were 10 (range 9-15) and 13 (range 9-17) days respectively. There were no dose limiting toxicities. Five patients experienced a total of 7 NCI CTCAE grade 3 toxicities including: diarrhea, n=2; mucositis, n=3; and nausea, n=2. Four of 6 patients who received 9Gy did not experience any toxicity greater than grade 2. Using the FACT-BMT scale, we observed that there was no significant difference in QoL between baseline and day 90 assessments. At day 100 overall response rate was 82% with 5 patients (45%) achieving a complete response. Four of 6 patients in the 9Gy cohort achieved at least a very good partial response. With a median follow up of 314 days, all patients were alive and only 4 patients (33%) relapsed.

In this phase 1 trial we showed that TMI at 9Gy can be safely added to Mel200 without an increase in transplant related toxicities. Initial promising clinical results, even in high risk MM patients, will be further tested in a phase 2 study.