Estimations of intra- and extracellular volume and pH by ³¹P magnetic resonance spectroscopy: Effect of therapy on RIF-1 tumours

Quantification of metabolite or drug concentrations in living tissues requires determination of intra- and extracellular volumes. This study demonstrates how this can be achieved non-invasively by ³¹P magnetic resonance spectroscopy (MRS) employing dimethyl methylphosphonate (DMMP) as a marker of total water space, 3-aminopropylphosphonate (3-APP) as a marker of extracellular space and P_i and 3-APP as markers of intracellular pH (pH_i) and extracellular pH (pH_e) respectively. The MRS measurements of the tumour volumes were validated by classic radiolabelling methods using ³H₂O and [¹⁴C]inulin as markers of total and extracellular space respectively. The extracellular volume fraction measured by radiolabelling of RIF-1 tumours was 23 ± 0.83% (mean ± s.e.m. n = 9), not significantly different (P > 0.1) from that found by MRS (27 ± 2.9%, n = 9, London, and 35 ± 6.7, n = 14, Baltimore). In untreated RIF-1 tumours, pH_i was about 0.2 units higher than pH_e (P < 0.01). 5-Fluorouracil (5FU) treatment (165 mg kg^-1) caused no significant changes in either pH_e or per cent extracellular volume. However significant increases in pH_i 48 h after treatment (P < 0.01) correlated with decreased tumour size and improved bioenergetic status [NTP/inorganic phosphate (P_i) ratio]. This study shows the feasibility of an MR method (verified by a 'gold standard') for studying the effects of drug treatment on intra- and extracellular spaces and pH in solid tumours in vivo.