Uptake of radiolabelled tyrosine and iodo-methyl tyrosine in experimental rat tumours: influence of blood flow and tumour growth rate.

Radiolabelled amino acids combined with Positron Emission Tomography (PET) may be useful for delineation of the extent of viable tumour and may also provide a rapid and sensitive indicator of response to therapy. Promising early clinical reports led us to investigate the potential use of the amino acid analogue L-3-iodo-alpha-methyl tyrosine (IMT), which may be radioiodinated with isotopes suitable for PET or conventional single photon imaging. We have studied the biodistribution and kinetics of [125I]IMT using two transplantable tumour systems in hooded rats, and have compared the findings with those using the natural amino acid L-tyrosine (TYR) radiolabelled with tritium. Similar levels of IMT and TYR uptake were found in HSN and OES.HR1 tumours during tumour growth. Following arrest of OES.HR1 tumour growth by oestrogen ablation, reduced IMT and TYR uptake was found to be closely matched by a fall in tumour blood flow. Unlike IMT, a substantial proportion of TYR uptake in tumours was found to be protein incorporated, even following tumour growth arrest. Quantitative autoradiography revealed sharp delineation of tumour boundary using either radiotracer. We conclude that IMT and TYR kinetics are strongly influenced by blood flow and diffusion, and that tumour growth status may not be closely associated with amino acid uptake.