Influence of low doses of an oxazaphosphorine on natural killer activity of human lymphocytes.

The influence of cis-4-sulfoethylthio-cyclophosphamide (mafosfamide) on natural killer cell activity was examined in vitro in order further to elucidate the possible immunological mechanisms of tumor regressions following low-dose oxazaphosphorine therapy. It was observed that cytotoxicity of human blood lymphoid cells was unchanged or reduced when the lymphocytes were pretreated for 24 h with mafosfamide or when the drug was present during incubation with K562 cells. However, when lymphoid cells were preincubated with human leukocyte interferon plus mafosfamide, natural killer activity was enhanced above the level caused by interferon alone. This enhancement was noted at mafosfamide concentrations of 1 nM-1 microM and was only present when the lymphocyte preparation was contaminated with monocytes.