Assessment of 21-[¹⁸F]fluoro-16α-ethyl-19-norprogesterone as a positron-emitting radiopharmaceutical for the detection of progestin receptors in human breast carcinomas

We have used 21-[¹⁸F]fluoro-16α-ethyl-19-norprogesterone (FENP) for imaging progestin receptors by PET in patients with primary carcinoma of the breast. In vitro binding and in vivo tissue distribution studies in rats have shown that FENP has high specific activity, high affinity for progestin receptors, and receptor-mediated uptake in target tissues. Eight patients with primary breast carcinoma were studied. Breast carcinoma was identified correctly in 50% of the patients with progestin-receptor-positive tumors; however, the FENP uptake was not correlated with progestin-receptor levels. We noted a low target-to-background ratio in humans, with high relative activity in the spine, blood pool, and normal breast tissue. Our findings indicate that FENP is not a suitable agent for imaging progestin receptors in humans.