Metronomic cyclophosphamide regimen electively depletes CD4+ CD25+ regulatory T cells in patients with advanced solid tumors
2561 Background: CD4+CD25+ regulatory T cells (Treg) are involved in the prevention of autoimmune diseases and in tumor-induced tolerance. We previously demonstrated in tumor-bearing rodents that low doses of cyclophosphamide (CPM) could significantly decrease both number and suppressive functions of Treg, facilitating vaccine-induced tumor rejection. Repeated low doses of cyclophosphamide, referred to as metronomic treatment aiming at reducing tumor angiogenesis, are used in patients with advanced, chemoresistant tumors. Methods: Six patients with metastatic solid tumors were treated after informed consent with metronomic CPM - 50 mg orally twice a day, with one week on, and one week off, until progression or limiting toxicity. Peripheral blood mononucleated cells (PBMC) were sampled before and after 4 weeks of treatment. Occurrence of blood leucocyte subpopulation and capacity of Treg to inhibit NK and T cells patients functions were measured. Results: metronomic CPM induced a profound and selective reduction of circulating Treg, both in percentage (7.7 ± 0.8 % before versus 3.3 ± 0.8 % after, p < 0.005) and absolute number (25.2 ± 7.5 cells/mm3 before versus 6.1 ± 3.3 cells/mm3 after, p < 0.0006). Inhibitory functions of Treg on conventional T cells and NK cells lead to a restoration of peripheral T cell proliferation - i.e. PBMC from cancer patients exhibited 17 ± 3% T cells proliferation capacity after CD3 CD28 stimulation versus 44 ± 5% (p < 0.01) 1 month after the beginning of the treatment, and versus 45 ± 6% in healthy volunteers- and innate NK killing activities - i.e. PBMC from cancer patients exhibited 9 ± 1% NK killing capacity against K562 target versus 26 ± 5% (p < 0.005) after 4 week treatment. Conclusions: Metronomic CPM has not only effect on tumor angiogenesis, but also strongly curtail immunosuppressive Treg, which could favor a better control of tumor progression. No significant financial relationships to disclose.