Background: Despite the recent increase in treatment options for patients with multiple myeloma (MM), the disease remains largely incurable. Both arsenic trioxide (ATO) and melphalan have shown clinical activity in MM. Recent in vitro and in vivo studies in our laboratory have shown that arsenic trioxide sensitizes chemoresistant MM cells to melphalan-induced cytotoxicity; the addition of ascorbic acid (AA) further improves this effect. We conducted a multi-center clinical trial to evaluate the safety and efficacy of this steroid-free combination, melphalan, ATO and vitamin C (MAC), for patients with relapsed/refractory MM.

Methods: MM pts who relapsed after responding to 1st-line therapy and/or were refractory to prior treatment were enrolled. During week 1 of each 6-week cycle, pts received ATO, 0.25 mg/kg IV, followed by ascorbic acid (AA), 1 g IV, days 1–4. ATO followed by AA 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. Pts received a maximum of 6 cycles followed by weekly maintenance treatment with ATO and AA. The primary objectives of this study were to determine response rate and safety and tolerability of MAC therapy.

Results: 65 patients have been enrolled and 51 are currently evaluable for response. 26 (1 CR, 10 PR, 15 MR) of the 51 evaluable patients (51%) had an objective response and an additional 14 patients achieved stable disease, resulting in a total of 40 patients (78%) with disease control. Among patients with elevated serum creatinine levels at baseline, renal function improved for those with responsive or stable disease. 20 of the 26 responding patients had failed ≥ 2 prior therapies: 19 pts had received prior thalidomide or lenalidomide therapy and 8 pts had received prior bortezomib. The regimen was well-tolerated with few significant side effects reported. Mild cytopenias occurred infrequently and were reversible.

Conclusions: The results from this large multi-center phase II trial show that the MAC regimen is active in a group of MM patients who had either relapsed or were refractory to standard and/or investigational MM treatments. The regimen was well-tolerated even in this heavily pre-treated patient population. These findings are consistent with preclinical studies that showed the efficacy of this combination from both in vitro and in vivo studies.

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