Quantitative [18F]FMISO PET Imaging Shows Reduction of Hypoxia Following Trastuzumab in a Murine Model of HER2+ Breast Cancer

Abstract

Purpose: Evaluation of [¹⁸F]fluoromisonidazole ([¹⁸F]FMISO)-positron emission tomography (PET) imaging as a metric for evaluating early response to trastuzumab therapy with histological validation in a murine model of HER2+ breast cancer.

Procedures: Mice with BT474, HER2+ tumors, were imaged with [¹⁸F]FMISO-PET during trastuzumab therapy. Pimonidazole staining was used to confirm hypoxia from imaging.

Results: [¹⁸F]FMISO-PET indicated significant decreases in hypoxia beginning on day 3 (P < 0.01) prior to changes in tumor size. These results were confirmed with pimonidazole staining on day 7 (P < 0.01); additionally, there was a significant positive linear correlation between histology and PET imaging (r ² = 0.85).

Conclusions: [¹⁸F]FMISO-PET is a clinically relevant modality which provides the opportunity to (1) predict response to HER2+ therapy before changes in tumor size and (2) identify decreases in hypoxia which has the potential to guide subsequent therapy.

Keywords: BT474; FMISO; Herceptin; Misonidazole; Oxygenation; Pimonidazole; Vascular maturation.

Fig 1

Outline of tumor implantation, imaging and treatment schedule.

Fig 2

a Longitudinal [¹⁸F]-FMISO-PET central slice images of a representative control and treated animal (tumor denoted by dashed circle) over seven days. [¹⁸F]-FMISO retention increased longitudinally in the control mouse, in contrast to the observed decrease in the trastuzumab- treated mouse. b Graph revealing group mean of hotspot SUV in control versus treated tumors. [¹⁸F]-FMISO uptake in treated tumors is significantly lower than controls as early as day three (P = 0.008). c Graph of tumor volume changes in response to treatment over seven days. Day seven reveals a significant difference between the groups (P = 0.02).

Fig 3

Correlation of hypoxia imaging and histology is shown through central slice analysis of the treated and control tumors. a Graph revealing quantitative analysis of percent hypoxia staining from histology. Treated tumors reveal significantly decreased hypoxia compared to controls. b Representative histology central slices showing overall hypoxia (as stained with pimonidazole, brown) in the treated and control animals. c Day seven scatterplot of central slice SUV comparing treatment and controls. d Quantitative hypoxia imaging (SUV hotspot) and quantitative histology shows a positive linear correlation (r² = 0.85, P < 0.001).

Fig 4

Quantitative analysis comparing treated and control tumors and representative histology sections are shown for H&E and CD31. CD31 microvessel density (MVD) is shown for a control and b treated tumors; c quantitative analysis reveals no significant changes between the groups on day seven (P = 0.22). H&E cellularity is shown for d control and e treated tumors; f quantitative analysis reveals no significant difference in cellularity in treated tumors compared to controls on day seven (P = 0.22). H&E percent necrosis is shown for g control and h treated tumors; i quantitative analysis reveals no significant changes in necrosis in treated tumors compared to controls on day seven (P = 0.53). Squares in g and h demonstrate the representative regions displayed in tiles a,c and b,d, respectively.

Fig 5

Longitudinal immunofluorescence analysis of pimonidazole in separate cohorts of mice over 7 days confirm trends of decreased hypoxia markers in trastuzumab-treated tumors.