Monitoring therapeutic response to tamoxifen in NMU-induced rat mammary tumours by 31P MRS.

Tamoxifen injections were given once a week for 4 weeks to 19 rats bearing N-methyl-N-nitrosourea (NMU)-induced mammary carcinomas. NMR spectra were collected on days 2, 7, 14, 21 and 28. Only 42% of the tumours responded to the tamoxifen in that they regressed significantly; another 21% did not change in size and 37% grew significantly. In the ones that did subsequently regress there were significant changes in the NTP/Pi ratio as early as 2 days after treatment, before any detectable change in volume was recorded, and continuing up to 21 days. The significance of these findings and the possible mechanisms underlying the changes are discussed.