Intracellular doxorubicin kinetics in lymphoma cells and lymphocytes infiltrating the tumor area in vivo: a flow cytometric study.

Recently we have reported the development of a safe and effective chemoimmunotherapy protocol involving doxorubicin (Dox) in combination with interleukin 2 which, in C57BL/6 mice, boosts local T cell responses, and, in 50 to 80% of the cases, this resulted in the complete eradication of established syngeneic EL4 lymphoma or its Dox-resistant variant, EL4/A. Accumulation of host-derived leukocytes in the peritoneal cavity was increased up to 8-fold after tumor inoculation, but, in absolute numbers, did not increase further following Dox administration. The cellular pharmacokinetic studies undertaken to clarify the role of Dox following a single IV injection indicate that 4 h later, lymphocytes found in the peritoneal cavity have detectable levels of Dox; but the lymphoma cells (both EL4 and EL4/A) have, in proportion to their larger size, taken up more drug as judged by flow cytometry. The estimated drug "concentration" (i.e., intracellular amount divided by estimated cell size) at the 4-h time point, however, was found to be essentially equivalent in both the lymphoma cells and the lymphocytes. Thereafter, the drug content and intracellular "concentration" in the EL4/A cells rapidly declined while their numbers progressively increased. In contrast, the EL4 lymphoma cells and the lymphocytes found in the peritoneal cavity in the presence of either lymphoma consistently exhibited higher levels of drug 24-48 h than at 4 h. Splenic and tumor-infiltrating mature T (CD3+) cells were completely insensitive to Dox cytotoxicity and actually showed increased CTL activity when examined ex vivo. Although EL4 cells had identical Dox uptake patterns to those of CD3+ cells, they were sensitive to the drug and their numbers decreased, resulting in increased host/tumor cell ratios in these mice. The pharmacokinetic parameters of the drug and the insensitivity of the mature T cells to the drug determined in this study can explain, in part, the efficacy of a chemoimmunotherapy protocol boosting local T-cell responses.