Secondary ion mass spectrometry as a tool for investigating radiopharmaceutical distribution at the cellular level: the example of I-BZA and (14)C-I-BZA.

Unlabelled: Further development of nuclear medicine for imaging and internal radiotherapy demands a precise knowledge of the tissue and cellular distribution of radiopharmaceuticals. Ion microscopy (secondary ion mass spectrometry [SIMS]) may be particularly useful in this respect. We used SIMS to study the biodistribution of the melanoma-targeting molecule N-(2-diethylaminoethyl)-4-iodobenzamide (I-BZA), both in its native state and radiolabeled with (14)C.

Methods: C57BL6/J1/co mice bearing pulmonary colonies of B16 melanoma cells were injected with I-BZA or (14)C-I-BZA. Appropriate tissues were fixed and included in epoxy embedding resin for SIMS studies. The distribution of unlabeled I-BZA was studied by detecting its stable iodine atom ((127)I). (14)C-I-BZA distribution was studied by dual detection of (127)I and (14)C. The time course of I-BZA concentrations at sites of tissue fixation was studied by measuring the signal ratio of (14)C and the naturally occurring isotope (13)C.

Results: SIMS showed that I-BZA concentrated in the cytoplasm of tumoral melanocytes (melanoma cells) and in the cytoplasm of tumor-infiltrating macrophages (melanophages). I-BZA was also detected in the cytoplasm of normal melanocytes in the pigmented structures of skin and eye. Interpretation of I-BZA distribution by using electron micrographs of adjacent sections showed that the intracytoplasmic melanin-rich organelles (melanosomes) were responsible for I-BZA retention. The distributions of (127)I and (14)C after (14)C-I-BZA injection were identical, even when I-BZA was separately labeled with (14)C at 2 different positions, indicating the stability of the amide bond of I-BZA. The time course of the (14)C/(13)C ratio in the melanosomes of melanoma cells suggested a retention half-life of about 38 h.

Conclusion: Contrary to previous suggestions that I-BZA fixes principally to sigma-1 membrane receptors, our results strongly indicate that I-BZA associates with intracytoplasmic melanin pigments. Early I-BZA accumulation, in both melanocytes and melanophages, suggests that this compound fixes to preformed melanin rather than being incorporated during de novo melanin synthesis. These quantitative and qualitative data obtained with I-BZA illustrate the excellent potential of SIMS for studying the biologic fate of radiopharmaceuticals.