Positron emission tomography (PET) can be used to confirm how well estrogen-blocking and estrogen-depleting therapies are working in a patient with metastatic breast cancer, a new study demonstrates.

Investigators measured regional estrogen receptor (ER)-blocking and binding by performing PET scans with a special estrogen-containing isotope, 18F-fluoroestradiol (FES), on 30 patients with metastatic breast cancer. The scans were obtained before and during the patients’ treatment with aromatase inhibitors, tamoxifen, or fulvestrant.

Imaging revealed significant differences between agents, including differences in the efficacy of blockade by different ER antagonists: As expected, tumor FES uptake declined more markedly with the ER blockers tamoxifen and fulvestrant (average decline 54%), compared with a less than 15% average decline with estrogen-depleting aromatase inhibitors. The rate of complete tumor blockade was highest following use of tamoxifen (5 of 5 patients) vs fulvestrant (4 of 11 patients).

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“What we’re suggesting in this paper—that we couldn’t fully test for before—is if estrogen is incompletely blocked, you’re not getting a good outcome for the patient,” explained breast oncologist Hannah Linden, MD, in a statement describing her group’s results.

Linden, of the Fred Hutchinson Cancer Research Center in Seattle, Washington, and colleagues concluded in their report for Clinical Cancer Research that “Our findings support the ability of FES PET to visualize the in vivo activity of endocrine therapy.”

The researchers noted that this technology could be used early in drug development to measure effectiveness at the intended therapeutic targets, and to help refine selection and dosing for agents to move forward in development. Pharmacodynamic imaging also could provide clinicians with a promising tool for selecting ER-targeted therapy, and for predicting and evaluating a patient’s response to that treatment.