Uptake in melanoma cells of N-(2-diethylaminoethyl)-2-iodobenzamide (BZA2), an imaging agent for melanoma staging: relation to pigmentation

N-(2-diethylaminoethyl)-2-iodobenzamide (BZA2) has been singled out as the most efficacious melanoma scintigraphy imaging agent. Our work was designed to assess the mechanisms of the specific affinity of the radioiodinated iodobenzamide for melanoma tissue. We studied the cellular uptake and retention of [125I]-BZA2 on various cell lines. In vitro, cellular [125I]-BZA2 uptake was related to the pigmentation status of the cells: higher in pigmented melanoma cell lines (M4 Beu, IPC 227, B 16) than in a nonpigmented one (M3 Dau) and nonmelanoma cell lines (MCF 7 and L 929). Two mechanisms were assessed: binding of the tracer to melanin or to sigma receptors of melanoma cells. First, the uptake of [125I]-BZA2 after melanogenesis stimulation by α-melanocyte-stimulating hormone and l-tyrosine increased in the B 16 melanoma cell line both in vitro and in vivo according to melanin concentration. Moreover, the binding of [125I]-BZA2 to synthetic melanin was dependent on melanin concentration and could be saturated. Second, no competition was evidenced on M4 Beu cells between [125I]-BZA2 and haloperidol, a sigma ligand, at concentrations ≤10−6 M. We show that the specificity and sensibility of BZA2 as a melanoma scintigraphic imaging agent are mostly due to interactions with melanic pigments.