Weekly Cyclophosphamide and Alternate Day Prednisone: An Effective, Convenient and Well Tolerated Treatment for Relapsed Multiple Myeloma Following Autologous Stem Cell Transplant.
Authors: Stewart A. Keith , Trieu Young , Trudel Suzanne , Pond Greg , Mikhael Joseph , Reece Donna , Chen Christine 
Published: April 2020

Abstract
Alkylating agents remain among the most potent therapies available for treatment of Multiple Myeloma (MM). Their use prior to, or following, autologous stem cell transplant (ASCT) is, however, compromised by concerns about stem cell quality and by myelosuppression limiting effective dose delivered. To address this concern we have studied a combination of cyclophosphamide 500 mg p.o. once weekly and prednisone 100 mg p.o. on alternate days in 66 patients requiring salvage therapy post-ASCT. Dose reductions were allowed for toxicity beginning at cycle 2. On an intent to treat basis, 66 patients received this regimen, however, 7 of these patients were not fully evaluable for response due to non-secretory disease. Of the 59 patients evaluable for response, the median time from transplant to treatment was 26.4 months (range, 6.0 to 66.6). The median time from post-transplant relapse to start of cyclophosphamide and prednisone (C/P) therapy was 1.4 months. The median number of therapies from time of diagnosis to C/P initiation was 2 (range, 1.0 to 5.0). At the date of analysis, treatment with C/P is ongoing in 12 (20.3%) patients, with a median duration of 3.6 months (range, 1.9 to 11.6). The 47 patients who have completed C/P therapy were treated for a median time of 5.5 months (range, 0.5 to 21.7). The reason for discontinuation among these 47 patients included disease progression (42.6% of patients discontinued), plateau disease (21.3%), receiving a second transplant (17.0%), toxicity (10.6%), or switched to another regimen (8.5%). A partial response (&gt;50% protein reduction) was obtained in 37.3% of patients, 18.6% attained minimal response (25–50% protein reduction), 33.8% patients stable disease, while 10.2% patients had progressed on treatment. The estimated median (95% CI) months of progression-free survival after start of C/P treatment is 14.9 (8.7, 21.7). Twenty-three (38.9%) of patients have relapsed after C/P treatment, a median (range) of 8.7 (0.5–65.7) months after start of C/P treatment. At 6 months 74.3% (95% C.I. 61.9% – 89.1%) of patients were progression-free with 28% (95% CI: 16.1–49.2%) progression free at two years. At time of analysis, 44 (74.6%) patients are still alive, with a median follow up of 10.6 months (range, 1.2 to 65.7) since the start of C/P therapy. Fifteen patients have died at a median 13.0 months (range, 1.4 to 61.7) since the time of C/P initiation. The median overall survival (95% C.I.) is estimated to be 35.9 months (24.2, NA). These results demonstrate that the combination of oral cyclophosphamide and prednisone is an effective (56% MR or PR), very well tolerated (10% discontinued due to toxicity) and convenient treatment as salvage MM therapy post-ASCT. The relative lack of myelosuppression allows for re-collection of stem cells and salvage transplant while retaining other active second line agents for later relapse. This regimen thus compares favorably with recent salvage therapeutics introduced in MM and is now being studied in combination with these newer active agents and in induction therapy.