^99mTc-sestamibi imaging in the assessment of toremifene as a modulator of multidrug resistance in patients with breast cancer

Multidrug resistance (MDR) due to expression of a membrane-associated permeability glycoprotein (P-glycoprotein [Pgp]) prevents successful cytotoxic chemotherapy for breast cancer. Identification of MDR would facilitate selection of chemotherapy regimens and MDR modulators. This study aimed to evaluate ^99mTc-sestamibi imaging for predicting overexpression of Pgp in primary breast cancer and to measure the efficacy of toremifene, the MDR modulator, in vivo. Methods: Twenty patients with untreated breast cancer had ^99mTc-sestamibi imaging 20 and 120 min after tracer injection before and after a 3-d course of toremifene (780 mg/d). Tumor samples were obtained during surgery for correlation of imaging and Pgp immunohistochemistry. Results: Sixteen of 20 tumors were visualized with sestamibi. Before toremifene, there was a significant inverse correlation (Spearman rank correlation coefficient [Rs]) between staining intensity, based on the anti-Pgp monoclonal antibodies C494 and C219, and the tumor-to-background ratio (T/B) at 120 min (R_s = -0.85; P < 0.001 and R_s = -0.71; P < 0.001, respectively). However, the correlation between the T/B and immunohistochemistry at 20 min was significant only for C494 (R_s = -0.57; P < 0.01). Similarly, before toremifene, there was an inverse correlation between staining intensity and the change in the T/B between 20 and 120 min (R_s = -0.77; P < 0.001 and -0.75; P < 0.001 for C494 and C219). After toremifene, an inverse correlation between staining intensity and the T/B was seen only at 120 min and only with C494 (R_s = -0.68; P < 0.01). However, the change in the T/B between 20 and 120 min correlated significantly with staining intensity for C494 and C219 (R_s = -0.68; P < 0.01 and -0.7; P < 0.01 for C494 and C219, respectively). Toremifene did not significantly alter the overall T/B at either 20 or 120 min when data were compared before and after toremifene. Nevertheless, at 120 min, 8 of 8 tumors with low Pgp expression showed reduced uptake after toremifene, whereas 5 of 6 tumors with strong expression showed increased uptake (P < 0.003). Moreover, there was a significant correlation between the change in the T/B and staining intensity with C494 (R_s = 0.59; P < 0.05) and C219 (R_s = 0.56; P < 0.05) at 120 min but not at 20 min. Conclusion: ^99mTc-Sestamibi accumulation in breast cancer correlates with Pgp expression. Toremifene has a dual effect on this accumulation, increasing it through an inhibitory effect on Pgp while at the same time reducing it by a direct competition with sestamibi. The latter implies that in response to Pgp modulation the efflux of various agents may be affected differently.