[F]FLT–PET Imaging Does Not Always "Light Up" Proliferating Tumor Cells

Purpose: [F]FLT (30-Fluoro-30 deoxythymidine)–PET imaging was proposed as a tool for measuring in vivo tumor cell proliferation. The aim of this article was to validate the use of [F]FLT–PET imaging for measuring xenograft proliferation and subsequent monitoring of targeted therapy. Experimental Design: In exponentially growing xenografts, factors that could impact the outcome of [F]FLT–PET imaging, such as nucleoside transporters, thymidine kinase 1, the relative contribution of DNA salvage pathway, and the ratio of FLT to thymidine, were evaluated. The [F]FLT tracer avidity was compared with other proliferation markers. Results: In a panel of proliferating xenografts, [F]FLTor [H]thymidine tracer avidity failed to reflect the tumor growth rate across different tumor types, despite the high expressions of Ki67 and TK1.When FLTwas injected at the same dose level as used in the preclinical [F]FLT–PET imaging, the plasma exposure ratio of FLT to thymidine was approximately 1:200. Thymidine levels in different tumor types seemed to be variable and exhibited an inverse relationship with the FLT tracer avidity. In contrast, high-dose administration of bromdeoxyuridine (BrdUrd; 50 mg/kg) yielded a plasma exposure of more than 4-fold higher than thymidine and leads to a strong correlation between the BrdUrd uptake and the tumor proliferation rate. In FLT tracer-avid models, [F]FLT–PET imaging as a surrogate biomarker predicted the therapeutic response of CDK4/6 inhibitor PD-0332991. Conclusions: Tumor thymidine level is one of the factors that impact the correlation between [F]FLT uptake and tumor cell proliferation. With careful validation, [F]FLT–PET imaging can be used to monitor antiproliferative therapies in tracer-avid malignancies. Clin Cancer Res; 18(5); 1–10. 2011 AACR. Introduction In 1998, Shields and colleagues introduced [F]FLT (30Fluoro-30 deoxythymidine)–PET imaging as a noninvasive tool for visualizing tumor cell proliferation (1). Since then, the technology has attracted substantial attention for its various applications in oncology. The principalmechanism in [F]FLT–PET imaging (2) is the uptake of the tracer by proliferating cells in the pyrimidine salvage pathway, mostly during S-phase. The tracer is then phosphorylated by thymidine kinase 1 (TK1), at which point it becomes trapped in the cells. As a pyrimidine analog, FLT primarily relies on the pyrimidine transporters, including equilibrative nucleoside transporters 1 and 2 (ENT1 and 2) and concentrative nucleoside transporters 1 and 3 (CNT1 and 3), to enter cells prior to being phosphorylated (3, 4). Using a cell-based assay, FLT retention in cells was found to be strongly associated with TK1 expression (5). In some cases, tumor cells predominately synthesize the nucleosides that are needed for cell growth de novo (6), which results in low [F]FLT tracer avidity in the proliferating tumor, leading to false negative results (7, 8). Clearly, in order for [F]FLT– PET imaging to "light up" proliferating cells, a number of factors must come into play. This may explain why clinical [F]FLT–PET imaging exhibited lower overall uptake and less specificity compared with [F] FDG–PET imaging, despite being a more favorable tool for visualizing malignant tissues. Therefore, [F]FLT–PET imaging is less appropriate for clinical tumor staging (9). Because antiproliferation is often a primary or secondary phenotypic change caused by cytotoxic and targeted therapy, [F]FLT–PET imaging seemed to be a comparatively straight-forward approach for tracking therapeuticAuthors' Affiliations: Oncology Research Unit, Bio-Imaging Unit, La Jolla Laboratories, Pfizer Global Research and Development, San Diego, California; Pharmacokinetics, Dynamics & Metabolism, Molecular Medicine, Groton Laboratories, Pfizer Global Research and Development, Groton, Connecticut; Drug Safety Research & Development, Pfizer Global Research and Development, New York, New York; and Centre for Molecular Imaging and Translational Research Laboratory, Peter MacCallum Cancer Centre, East Melbourne, Victoria, Australia Note: Supplementary data for this article are available at Clinical Cancer Research Online (http://clincancerres.aacrjournals.org/). Corresponding Author: Cathy C. Zhang, Oncology Research Unit, Pfizer Global Research and Development, 10724 Science Center Road, San Diego, CA 92121. Phone: 858-622-3125; Fax: 858-622-5999; E-mail: [email protected] doi: 10.1158/1078-0432.CCR-11-1433 2011 American Association for Cancer Research. Clinical Cancer Research www.aacrjournals.org OF1 Research. on August 29, 2017. © 2011 American Association for Cancer clincancerres.aacrjournals.org Downloaded from Published OnlineFirst December 14, 2011; DOI: 10.1158/1078-0432.CCR-11-1433