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First published online June 12, 2009, 10.2967/jnumed.109.062208
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Journal of Nuclear Medicine Vol. 50 No. 7 1102-1109
© 2009 by Society of Nuclear Medicine
doi: 10.2967/jnumed.109.062208

Basic Science Investigation

18F-FDG and 18F-FLT Uptake Early After Cyclophosphamide and mTOR Inhibition in an Experimental Lymphoma Model

Lieselot Brepoels1,2, Sigrid Stroobants1,2, Gregor Verhoef3, Tjibbe De Groot2,4, Luc Mortelmans1,2 and Christiane De Wolf-Peeters5

1 Department of Nuclear Medicine, University Hospital Gasthuisberg Leuven, Leuven, Belgium; 2 Molecular Small Animal Imaging Centre, University Hospital Gasthuisberg Leuven, Leuven, Belgium; 3 Department of Hematology, University Hospital Gasthuisberg Leuven, Leuven, Belgium; 4 Laboratory for Radiopharmacy, University Hospital Gasthuisberg Leuven, Leuven, Belgium; and 5 Department of Pathology, University Hospital Gasthuisberg Leuven, Leuven, Belgium

Correspondence: For correspondence or reprints contact: Lieselot Brepoels, Department of Nuclear Medicine, University Hospital Gasthuisberg, Leuven Herestraat 49, 3000 Leuven, Belgium. E-mail: lieselot.brepoels{at}uz.kuleuven.be

To be a reliable predictor of response, tracer uptake should reflect changes in the amount of active tumor cells. However, uptake of 18F-FDG, the most commonly used PET tracer, is disturbed by the inflammatory cells that appear early after cytotoxic therapy. The first aim of this study was to investigate whether 3'-18F-fluoro-3'-deoxy-L-thymidine (18F-FLT), a marker of cellular proliferation, is a better tracer for response assessment early after cytotoxic therapy. A second objective of this study was to investigate whether 18F-FDG and 18F-FLT responses were comparable early after mammalian target of rapamycin (mTOR) inhibition, as an example of proliferation-targeting therapies. Methods: Severe combined immunodeficient mice were subcutaneously inoculated with Granta-519 cells, a human cell line derived from a leukemic mantle cell lymphoma. Half the mice were treated with cyclophosphamide and the other half with mTOR inhibition. 18F-FDG and 18F-FLT uptake was evaluated by small-animal PET on day 0 (D0; before treatment), D+1, D+2, D+4, D+7, D+9, D+11, and D+14. At each time point, 2 mice of each treatment condition were sacrificed, and tumors were excised for histopathology. Results: After cyclophosphamide, 18F-FDG and 18F-FLT uptake decreased, with a maximum reduction of –29% for 18F-FDG and –25% for 18F-FLT uptake at D+2, compared with baseline. Although 18F-FDG uptake increased from D+4 on, with a maximum on D+7, 18F-FLT uptake remained virtually stable. Histology showed an increase in apoptotic or necrotic tumor fraction, followed by an influx of inflammatory cells. In mTOR-inhibited mice, 18F-FDG uptake dropped until D+2 after therapy (–43%) but increased at D+4 (–27%) to form a plateau on D+7 and D+9 (–14% and –16%, respectively). Concurrently, 18F-FLT uptake decreased to –31% on D+2, followed by an increase with a peak value of +12% on D+7, after which 18F-FLT uptake decreased again. Cyclin D1 expression dropped from D+1 until D+4 and returned to baseline at D+7. Conclusion: Because 18F-FLT uptake is not significantly influenced by the temporary rise in inflammatory cells early after cyclophosphamide, it more accurately reflects tumor response. However, a formerly unknown temporary rise in 18F-FLT uptake a few days after the administration of mTOR inhibition was defined, which makes it clear that drug-specific responses have to be considered when using PET for early treatment monitoring.

Key Words: therapy response • 18F-FLT • 18F-FDG • PET • lymphoma

COPYRIGHT © 2009 by the Society of Nuclear Medicine, Inc.


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