Initial Studies with 11C-Vorozole PET Detect Overexpression of Intratumoral Aromatase in Breast Cancer

Abstract

Aromatase inhibitors are the mainstay of hormonal therapy in estrogen receptor-positive breast cancer, although the response rate is just over 50% and in vitro studies suggest that only two thirds of postmenopausal breast tumors overexpress aromatase. The goal of the present study was to validate and optimize PET with ¹¹C-vorozole for measuring aromatase expression in postmenopausal breast cancer in vivo. Methods: Ten newly diagnosed postmenopausal women with biopsy-confirmed breast cancer were administered ¹¹C-vorozole intravenously, and PET emission data were collected between 40 and 90 min after injection. Tracer injection and scanning were repeated 2 h after ingestion of 2.5 mg of letrozole. Mean and maximal SUVs and ratios to nontumor tissue in the contralateral breast were determined at baseline and after letrozole. Biopsy specimens from the same tumors were stained for aromatase using immunohistochemistry and evaluated for stain intensity and the percentage of immune-positive cells. Results: Seven of the 10 women (70%) demonstrated increased mean focal uptake of tracer (SUV ratio > 1.1) coinciding with the mammographic location of the lesion, whereas the other 3 women (30%) did not (SUV ratio ≤ 1.0). All patients with an SUV ratio above 1.1 had mean SUVs above 2.4, and there was no overlap (SUV ratio ≤ 1; SUV_mean, 0.8-1.8). The SUV ratio relative to breast around tumor was indistinguishable from the ratio to contralateral breast. Pretreatment with letrozole reduced tracer uptake in most subjects, although the percentage of blocking varied across and within tumors. Tumors with a high SUV in vivo also showed a high immunohistochemical staining intensity. Conclusion: PET with ¹¹C-vorozole is a useful technique for measuring aromatase expression in individual breast lesions, enabling noninvasive quantitative measurement of baseline and posttreatment aromatase availability in primary tumors and metastatic lesions.

Keywords: PET; aromatase; breast cancer; cyp19A; vorozole.

FIGURE 1.

Aromatase availability imaged by ¹¹C-vorozole in women with breast cancer. (A) SUV map from patient with single lesion on mammography (arrow) before and after letrozole administration (blocking). Focal uptake is seen in tumor area at baseline, and decreased uptake is seen in tumor and heart. This pattern was observed in 6 of 10 patients. (B) SUV map from patient with single lesion on mammography (arrow), which did not show increased tracer uptake. This pattern was seen in 3 women. (C) CT (left) and PET overlaid on CT (right) from patient with stage IV metastatic cancer. Arrow indicates location of primary tumor on patient’s mammogram. (D) SUV maps of same patient. Node 1 arrow points to location and size identified on mammography. Multiple bone metastases and high-uptake area adjacent to primary tumor (node 2) can be seen with no apparent blocking.

FIGURE 2.

¹¹C-vorozole uptake in breast tissue contralateral to aromatase-overexpressing and non–aromatase-overexpressing tumors and breasts of healthy women. Bars depict mean and SEM for each group. One-way ANOVA demonstrated significant main effect of group (F_2,14 = 5.94, P < 0.02). Fisher post hoc least significant difference test demonstrated highly significant difference (**P = 0.005) between healthy breasts (healthy, n = 5) and breasts contralateral to aromatase-overexpressing (positive, n = 7) lesion. Intermediate values, not significantly different from either group, were observed in breast tissue contralateral to aromatase-negative (negative, n = 3) lesions.

FIGURE 3.

Mean tracer uptake in healthy breast and tumor over various intervals. Bars represent mean and SEM for 10 subjects (baseline scan). Emission acquisition began 40 min after tracer injection and continued for 50 min. SUV was averaged over entire acquisition (40–90 min) and several shorter intervals. There was no statistically significant difference (1-way ANOVA) among the various intervals.

FIGURE 4.

Immunohistochemical staining for aromatase in biopsy material from breast cancer patients imaged with ¹¹C-vorozole in vivo. (A and D) Patient with low-intensity labeling and low ¹¹C-vorozole uptake. (B and E) Patient with moderately high staining intensity and moderately high ¹¹C-vorozole uptake. (C and F) Patient with high staining intensity and high ¹¹C-vorozole uptake. Top row is ×200; bottom row is ×400.

FIGURE 5.

Quantitative immunohistochemical analysis of aromatase expression in patients with high and low vorozole uptake. (Left) Mean percentage of cells positively stained for aromatase among patients with high (above median, n = 5) and low (below median, n = 5) SUV. Difference was not statistically significant (P = 0.2). (Right) Mean staining intensity (density, 255 minus gray level) for aromatase among patients with high (above median, n = 5) and low (below median, n = 5) SUV. *P < 0.02, Student t test, 2-tailed.