Objectives Recent studies have demonstrated that inhibition of epidermal growth factor receptor (EGFR) kinase activity rapidly reduces tumor glucose metabolism in sensitive cell lines. This suggests that imaging of tumor metabolism with positron emission tomography (PET) with the glucose analog [18F]fluorodeoxyglucose (FDG) could be used clinically to monitor treatment with EGFR kinase inhibitors. In this study, I investigated whether EGFR kinase inhibition specifically affects glucose metabolism or whether it also affects lipid and amino acid metabolism. In doing so, I am attempting to determine the best PET tracers to identify malignancies responding to EGFR kinase inhibitors in the clinics.
Methods The lung cancer cell lines H3255 and A549 served as models for tumors that are sensitive and resistant to EGFR kinase inhibition, respectively. Glucose, amino acid, and lipid metabolism were studied by measuring the uptake of radiolabeled analogs of glucose, tyrosine, and choline. Uptake of these tracers was assayed in vitro after incubating cells with increasing doses of the EGFR kinase inhibitor erlotinib and in vivo by PET imaging of tumor bearing mice before and after erlotinib treatment. In vitro blocking studies with specific reagents were performed to elucidate the mechanisms of tyrosine and choline uptake.
Results In sensitive H3255 cells, erlotinib decreased glucose uptake 54 ± 2% within 4 hours, whereas tyrosine and choline uptake were unaffected. In resistant A549 cells, no reduction of tracer uptake was observed for any of the studied tracers. These in vitro findings were confirmed in H3255 xenografted mice, which demonstrated a selective reduction of glucose uptake by 50 ± 5% after two doses of erlotinib. In A549 xenografts, erlotinib had no measurable metabolic effects. In H3255 and A549 cells, 2-amino-2-norbornanecarboxylic acid, a specific inhibitor of l-type amino acid transport, reduced tyrosine uptake by 83 ± 1% and 75 ± 1%, and cold choline inhibited choline uptake with IC50s of 5 μM and 6 μM, respectively.
Conclusion These observations suggest that efficacious erlotinib treatment specifically modulates glucose, but not amino acid or lipid metabolism, as EGFR kinase inhibition may not affect the l-type amino acid transport system or the high-affinity choline transport system. Thus, the glucose analog FDG is the most promising tracer for monitoring treatment with EGFR kinase inhibitors in the clinics.
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