Enhanced binding and inertness to dehalogenation of alpha-melanotropic peptides labeled using N-succinimidyl 3-iodobenzoate

Two peptides of potential utility for targeting melanoma cells, alpha-melanocyte-stimulating hormone (alpha-MSH) and its more potent analogue [Nle(4),D-Phe(7)]-alpha-MSH, were radioiodinated in 45-65% yield using N-succinimidyl 3-[I-125]iodobenzoate (SIB). To determine whether this labeling method resulted in improved in vitro and in vivo characteristics, these peptides also were labeled with I-131 by direct iodination with the iodogen method. For alpha-MSH, the rapid tissue clearance of both radionuclides in mice was consistent with rapid degradation of the peptide; however, significantly lower levels of I-125 were observed in thyroid and stomach, reflecting a greater inertness to deiodination; More extensive comparisons were performed with [Nle(4),D-Phe(7)]-alpha-MSH. The in vitro binding of [Nle(4),D-Phe(7),Lys(11-) (I-125)IBA]-alpha-MSH (prepared using SIB) to the murine B-16 melanoma cell line, 34.1 +/- 4.7%, was more than twice as high as that for [Tyr(2)(I-131),Nle(4),D-Phe(7)]-alpha-MSH (15.0 +/- 0.1%), and its K-D was more than 10-fold lower than that for conventionally labeled peptide (10 +/- 5 versus 140 +/- 14 pM). The normal tissue clearance of [Nle(4),D-Phe(7),Lys(11-)(I-125)IBA]-alpha-MSH in mice was faster than that of [Tyr(2)(I-131),- Nle(4),D-Phe(7)]-alpha-MSH. The 19-40-fold lower activity concentrations of [Nle(4),D-Phe(7),Lys(11-)(I-125)IBA]-alpha-MSH in tissues accumulating free iodide (thyroid and stomach) suggest a greater inertness of this peptide to deiodination. The primary urinary catabolite of [Nle(4),D-Phe(7),Lys(11-)(I-125)IBA]-alpha-MSH was the lysine conjugate of iodobenzoic acid, whereas radioiodide was the chief catabolite generated from [Tyr(2)(I-131),Nle(4),D-Phe(7)]-alpha-MSH. We conclude that further evaluation of [Nle(4),D-Phe(7),Lys(11-)(I-125)IBA]-alpha-MSH for targeting alpha-MSH receptors is warranted and that SIB may be a useful method for the radioiodination of peptides.