A pharmacokinetic and pharmacodynamic study in vivo of human HT29 tumours using 19F and 31P magnetic resonance spectroscopy

F-19-MRS (magnetic resonance spectroscopy) was used to study the pharmacokinetics of 5-fluorouracil (5-FU) in human (HT29) tumour xenografts, with and without pretreatment of the mice using either thymidine (40 min) or interferon-alpha (2 and 24h). A 200 mg/kg i.p. bolus dose of 5-FU was eliminated from control tumours with a t(1/2) of 25.4 +/- 2 min (mean +/- SEM, n = 11), while both thymidine (500 mg/kg) and interferon (50 000 IU/mouse) significantly increased t(1/2) to 36.5 +/- 6.1 (n = 5) and 48.1 +/- 13.6 min (n = 4), respectively (P = 0.04, Gabriel's ANOVA). Thymidine increased 5-FU anabolism to cytotoxic 5-fluoronucleotides, and decreased the amount of tumour catabolites; the latter probably recirculated from liver since isolated HT29 cells did not catabolise 5-FU. These in vivo observations were confirmed by F-19-MRS quantification of tumour extracts. Interferon did not significantly affect 5-FU metabolism in the tumour or liver, nor the 5-FU t(1/2) in liver. Treatment of tumours with 5-FU or interferon had no effect on tumour growth, whereas the combination strongly inhibited growth. P-31-MRS of HT29 tumours showed that 2 and 24 h after i.p. injections of interferon there was a significant increase in the pH(int), of 0.3 +/- 0.04 units (P = 0.002), while pH(ext) and the tumour NTP/Pi ratio were unchanged. The large increase in the negative pH gradient (-Delta pH) across the tumour plasma membrane caused by interferon suggests the a pH may be a factor in tumour retention of 5-FU, as recently shown in isolated tumour cells. (C) 1997 Elsevier Science Ltd.