Irish endocrine society: 23rd annual meeting

C CLARKE, CG BRENNAN, K RODGERS, RM DWYER, PPA SMYTH ENDOCRINE LABORATORY, DEPARTMENT OF MEDICINE AND THERAPEUTICS, UNIVERSITY COLLEGE DUBLIN, IRELAND he demonstration in extrathyroidal human tissues of the sodium iodide symporter (NIS) has raised the possibility that 1311, commonly used as a systemic therapeutic ablative agent in hyperthyroidism and thyroid cancer, might be applied in the treatment of tumours in other NISexpressing tissues such as human breast cancer. Thyroidal transport of 1311 is known to be proportional to circulating stable I and the aim of this study was to determine how stable I (KI) would effect such transport in human breast cancer cell lines MDA-MB-231, MCF-7 and in FRTL-5 thyroid cells. All cells were incubated with KI (01 00mM) for 72 hours after which 1Th I was added . Incubation and uptake of 'l by cells was counted every four hours. Timed efflux of '^I was measured every five minutes. KI in the incubation medium blocked 1251 uptake in a dose-dependent manner in the E receptor positive MCF7 cell line. The effect was less marked in the E receptor negative MDA-MB-231 with significant uptake being maintained even at an I concentration of 50mM. A similar blockade was seen in the FRTL-5 cells with maximum uptake blockade of 25mM I. The rate of efflux of 15I was similar in both MCF-7 and MDA-MB-231 cell lines with a tin of 35 and 40 minutes respectively. In contrast, efflux from the FRTL-5 cells was faster (tire=15 mins). As the human breast has a much lower avidity for I than the thyroid, control of dietary intake would assume even greater importance in radioactive iodine treatment of breast tumours or their metastases.