Metabolism of Epidermal Growth Factor Receptor Targeting Probe [11C]PD153035: Impact on Biodistribution and Tumor Uptake in Rats

Several tracers have been evaluated as probes for noninvasive epidermal growth factor receptor (EGFR) quantification with PET. One of the most promising candidates is the 11C-labeled analog of the EGFR tyrosine kinase inhibitor PD153035. However, previous in vitro studies indicated extensive metabolism of the tracer, which could be disadvantageous for the assessment of receptor density in vivo. The aim of this study was to investigate the in vivo metabolism of [11C]PD153035 to determine whether alterations in metabolite formation are accompanied by changes in biodistribution and tumor uptake. Methods: EGFR-overexpressing human epidermoid carcinoma xenografts in rats were used in all examinations of tumor uptake. Cytochrome P450 enzymes of subfamilies CYP2D and CYP3A were inhibited before intravenous injection of [11C]PD153035 into healthy and tumor-bearing male rats. Samples were taken from arterial blood and urine, and the occurrence of radioactive metabolites was assessed with radio–high-performance liquid chromatography. Dynamic PET examinations of healthy and tumor-bearing animals were performed. In 1 rat, the effect of local intraarterial administration was examined. Results: [11C]PD153035 labeled at position 6 was metabolized extensively in vivo in male rats, resulting in very low levels of the intact tracer in plasma only minutes after injection. The major identified radiolabeled metabolites found were the N-oxide and metabolites arising from O demethylation at position 7. They were reduced by inhibition of CYP2D and CYP3A enzymes. PET revealed enzyme activity–dependent changes in the radioactivity distribution in the liver and tumors. Local administration of [11C]PD153035 greatly increased radioactivity levels in the adjacent tumor compared with levels typically found after intravenous administration. The highest tumor-to-muscle ratio at 60 min after intravenous injection was found in the untreated animals, whereas the overall highest ratio was found in the tumor near the intraarterial administration site. Conclusion: We suggest that the metabolism of [11C]PD153035 should be taken into consideration when this tracer is used to quantify EGFR expression, as our results indicated that the distribution of radioactivity to EGFR-overexpressing tumors was affected by the rate of metabolism and the route of administration.