Targeting dopamine receptors subtype 2 (D2DR) in pheochromocytomas: head-to-head comparison between in vitro and in vivo findings.

Context: Dopamine subtype 2 receptors (D2DRs) are overexpressed in pheochromocytomas (PHEOs). D2DR-expressing tumors can be visualized by iodine-123 labeled iodobenzamide (¹²³I-IBZM) single-photon emission computed tomography (SPECT).
Objective: The hypothesis of this study was that D2DR high expression in PHEOs would allow in vivo visualization through ¹²³I-IBZM SPECT. The present prospective pilot study aims to evaluate the performance of ¹²³I-IBZM SPECT in PHEOs and to correlate the tumor uptake with D2DR expression in tumor samples after surgery. SETTING, MATERIALS, AND METHODS: Ten unrelated patients with PHEOs were evaluated, prior to adrenalectomy, with ¹²³I-IBZM SPECT (whole body scan at 4 and 24 h after the injection; and SPECT centered on the abdomen at 24 h). D2DR mRNA and protein expressions were evaluated in all tumors by quantitative real-time RT-PCR and immunohistochemistry, respectively.
Main Outcome Measure: Intensity of tumoral uptake of ¹²³I-IBZM was measured.
Results: All PHEOs express D2DR mRNA (ranging from 2.1 to 14.7 copy/copy β-glucuronidase) and protein (immunostaining score: moderate or strong in 9 of 10 cases). However, none of the patients (0%) showed an increased tumor uptake of ¹²³I-IBZM.
Conclusions: These results suggest that ¹²³I-IBZM is not a useful radiopharmaceutical in the detection and characterization of PHEOs despite D2DR expression. Our findings and data from the related literature may support different hypotheses to explain the failure of D2DR targeting by ¹²³I-IBZM.