Acidosis plus melphalan induces nitric oxide-mediated tumor regression in an isolated limb perfusion human melanoma xenograft model

BACKGROUND: Isolated limb perfusion (ILP) with melphalan is an accepted treatment for intransit melanoma of the extremities. Using an ILP human melanoma xenograft model, we tested the hypothesis that acidosis augments the antitumor effect of melphalan and that nitric oxide (NO) induction mediates tumor regression. METHODS: NIH1286 human melanoma tumor bearing athymic nude rats underwent a 10-minute ILP. Group C was perfused at physiologic pH without acid or melphalan, group M received melphalan at physiologic pH (7.2), group A received 0.2 N of HCl at pH 6.8, and group A/M received melphalan and HCl at pH 6.8. Groups 1400W + A and 1400W + A/M were injected with 1400W, a specific inhibitor of inducible NO synthase, 1 hour pre-ILP. Tumor response was followed for up to 60 days in all survival experiments. In 4 to 6 animals from groups C, M, A, and A/M, tumor NO was measured pre- and post-ILP, and tumor and thigh muscle from 2 additional animals in each group were collected at 20 minutes and 24 hours post-ILP and processed for terminal deoxynucleotidyl transferase dUTP nick end labeling staining. RESULTS: Maximum mean reduction in tumor size after ILP in the different groups was as follows: C = 0%, M = 55%, A = 99.6% (3 of 4 complete responses), A/M = 100% (all complete responses), 1400W + A = 0%, and 1400W + A/M = 25%. Median tumor NO was 0.87 +/- 0.74 (SD) micromol/L before ILP and increased significantly (Mann-Whitney rank sum test, P <.001) after ILP (C = +6.9%, n = 4; M = +7.5%, n = 5; A = +66.0%, n = 6; A/M = +35.9%, n = 6). Also, minimal apoptotic cell death was seen in C and M, whereas A and A/M showed evidence of widespread apoptosis. CONCLUSIONS: Acidosis enhances the antitumor effect of melphalan. NO induction appears to play a role in tumor regression.