A Phase I/II Multicenter, Safety and Efficacy Study of Combination Treatment with Melphalan, Arsenic Trioxide and Vitamin C (MAC) in Patients with Relapsed or Refractory Multiple Myeloma.

Background: An urgent need exists for new treatments to overcome chemoresistance in MM patients. Recent in vitro and In vivo studies in our laboratory show that arsenic trioxide (ATO) can sensitize chemoresistant MM cells to melphalan-induced cytotoxic effects. Pre-clinical studies also show the most profound anti-MM effects when ATO, ascorbic acid and melphalan are used in combination compared with the effects observed when the drugs are used alone or combinations of any two of these agents. Based on encouraging results from a pilot study¹, a larger, multicenter trial was recently started.

Methods: MM patients who showed relapse after responding to first-line chemotherapy and/or having proved to be refractory to chemotherapy are enrolled. Patients received a loading dose of ATO at 0.25 mg/kg IV followed by ascorbic acid 1 g IV days 1–4 of week 1 of each six-week cycle. ATO/ascorbic acid was given twice-weekly for the next 4 weeks of each cycle. Low-dose melphalan (0.10 mg/kg) was administered orally for the first 4 days of each cycle. Patients received a maximum of 6 cycles followed by weekly maintenance treatment with weekly ATO followed by ascorbic acid. The primary objectives of this study are to determine response rate and safety and tolerability of MAC therapy.

Results: Twenty patients have received at least one treatment cycle. Preliminary data show that eight (4 PR, 4 MR) of the 14 evaluable patients (57%) had an objective response, an additional three patients achieved stable disease, resulting in a total of 11 patients (79%) with disease control. Since responses were seen after 2 to 5 treatment cycles, it is possible that some patients with stable disease may experience additional disease response. Seven of the eight responding patients had failed two or more treatments: five patients had received prior thalidomide therapy, two had received melphalan and bortezomib, and two patients had undergone autologous peripheral stem cell transplantation. Of the six patients who have now completed the maximal numbers of cycles, four achieved PR, one MR, and one SD. The regimen was well tolerated with few significant side effects reported; mild cytopenias were reported as reversible.

Conclusions: These preliminary results in this treatment group of heavily pre-treated MM patients who had either relapsed or were refractory to standard and/or investigational multiple myeloma treatments suggests that the MAC treatment regimen (1) shows efficacy using a low dose of melphalan supporting the preclinical evidence that ATO can sensitize tumors to chemotherapy; (2) is well tolerated; (3) may require multiple cycles before response.