RT Journal Article SR Electronic T1 Micro-SPECT/CT with 111In-DTPA-Pertuzumab Sensitively Detects Trastuzumab-Mediated HER2 Downregulation and Tumor Response in Athymic Mice Bearing MDA-MB-361 Human Breast Cancer Xenografts JF Journal of Nuclear Medicine JO J Nucl Med FD Society of Nuclear Medicine SP 1340 OP 1348 DO 10.2967/jnumed.109.062224 VO 50 IS 8 A1 Kristin McLarty A1 Bart Cornelissen A1 Zhongli Cai A1 Deborah A. Scollard A1 Danny L. Costantini A1 Susan J. Done A1 Raymond M. Reilly YR 2009 UL http://jnm.snmjournals.org/content/50/8/1340.abstract AB Pertuzumab is a HER2 dimerization inhibitor that binds to an epitope unique from that of trastuzumab. Our objective was to determine whether SPECT with 111In-diethylenetriaminepentaacetic acid–pertuzumab (111In-DTPA-pertuzumab) could sensitively detect an early molecular response to trastuzumab manifested by HER2 downregulation and a later tumor response revealed by a decreased number of HER2-positive viable tumor cells. Methods: Changes in HER2 density in SKBr-3 and MDA-MB-361 BC cells exposed to trastuzumab (14 μg/mL) in vitro were measured by saturation binding assays using 111In-DTPA-pertuzumab and by confocal immunofluorescence microscopy and flow cytometry with fluorescein isothiocyanate–labeled HER2/neu antibodies. Imaging of HER2 downregulation was studied in vivo in athymic mice with subcutaneous MDA-MB-361 tumors treated for 3 d with trastuzumab (4 mg/kg) or nonspecific human IgG (hIgG) or phosphate-buffered saline (PBS). Imaging of tumor response to trastuzumab was studied in mice bearing subcutaneous MDA-MB-361 xenografts treated with trastuzumab (4 mg/kg), followed by weekly doses of nonspecific hIgG or rituximab or PBS (2 mg/kg). Mice were imaged on a micro-SPECT/CT system at 72 h after injection of 111In-DTPA-pertuzumab. Tumor and normal-tissue biodistribution was determined. Results: 111In-DTPA-pertuzumab saturation binding to SKBr-3 and MDA-MB-361 cells was significantly decreased at 72 h after exposure in vitro to trastuzumab (14 μg/mL), compared with untreated controls (62% ± 2%, P < 0.0001; 32% ± 9%, P < 0.0002, respectively). After 3 d of trastuzumab, in vivo tumor uptake of 111In-DTPA-pertuzumab decreased 2-fold in trastuzumab- versus PBS-treated mice (13.5 ± 2.6 percentage injected dose per gram [%ID/g] vs. 28.5 ± 9.1 %ID/g, respectively; P < 0.05). There was also a 2-fold decreased tumor uptake in trastuzumab- versus PBS-treated mice by image volume-of-interest analysis (P = 0.05), suggesting trastuzumab-mediated HER2 downregulation. After 3 wk of trastuzumab, tumor uptake of 111In-DTPA-pertuzumab decreased 4.5-fold, compared with PBS-treated mice (7.6 ± 0.4 vs. 34.6 ± 9.9 %ID/g, respectively; P < 0.001); this decrease was associated with an almost-completed eradication of HER2-positive tumor cells determined immunohistochemically. Conclusion: 111In-DTPA-pertuzumab sensitively imaged HER2 downregulation after 3 d of treatment with trastuzumab and detected a reduction in viable HER2-positive tumor cells after 3 wk of therapy in MDA-MB-361 human breast cancer xenografts.