Radioiodinated N-(2-diethylaminoethyl)benzamide derivatives with high melanoma uptake: Structure-affinity relationships, metabolic fate, and intracellular localization

Several radioiodinated N-(dialkylaminoalkyl)benzamides have been used for p lanar scintigraphy and single-photon emission computed tomography (SPECT) o f melanoma metastases. In a quest for improved melanoma uptake and tissue s electivity, structure-activity studies for N-(2-diethylaminoethyl)benzamide s with variation of phenyl substituents were performed using C57Bl/6 mice b earing B16 melanoma. Compounds 2 (4-amino-5-bromo-N-(2-diethylaminoethyl)-3 -[I-131]iodo-2-methoxybenzamide) and 6 (4-acetamido-N-(2-diethylaminoethyl) -5-[I-131]iodo-2-methoxybenzamide) showed at 6 h post iv injection, for exa mple, melanoma uptake of 16.6 and 23.2% ID/g, respectively (mean values, n = 3). Uptake was 3-5 times higher (P < 0.01) than observed with benzamides known from the literature and was probably facilitated by the relatively sl ow urinary excretion of 2 or 6. In contrast, analogues lacking either the M eO, Ac, AcNH, or Br substituents exhibited reduced tumor uptake and high ur inary excretion of radioactivity in various benzamide metabolites. Uptake o f radioiodinated benzamides in B16 melanoma is not mediated by a specific m echanism such as <sigma>-receptor binding. 2 and 6 exhibited similar melano ma uptake values but quite different sigma (1)-receptor affinities of K-i = 0.278 +/- 0.018 and 5.19 +/- 0.40 muM, respectively. Uptake studies with I MBA (N-(2-diethylaminoethyl)-3-[I-131]-iodo-4-methoxybenzamide) or BZA (N-( 2-diethylaminoethyl)-4-[I-131]iodobenzamide) showed that with increasing do se of unlabeled compound the measured uptake of label was unchanged (IMBA) or even enhanced (BZA) while receptor binding of label decreased. Different ial and equilibrium density-gradient centrifugation revealed that most of t he radioactivity from labeled IMBA was associated with fractions containing melanin granules. Thus, structure-activity studies indicate that blood cle arance rates and metabolic stability are the main determinants for benzamid e uptake in melanoma. The high uptake and slow clearance of 6 offer conside rable potential for melanoma imaging in patients, and this compound may als o prove to be useful for radionuclide therapy.