In vivo evaluation of melanoma targeted multivalent α-melanocyte-stimulating hormone analogs using microPET studies

Objectives Multivalent peptides have been explored for their advantages of improving binding affinities and even activity. Herein multivalent Melanocyte-Stimulating Hormone (α-MSH) analogs and B16F10 melanoma-bearing mice was employed as a model system, to systematically investigate the influence of multivalent effect on α-MSH analogs’ binding affinities and in vivo melanoma targeted profiles using microPET imaging.

Methods Three α-MSH analogs named as MSH1, MSH2 and MSH4 were designed and synthesized, which contained one, two or four valency of α-MSH core sequences, His-d, Phe-Arg-Trp, respectively. α -MSH analogs were designed using the multiple antigenic peptide (MAP) scaffold. They were labeled with ⁶⁴Cu and tested for their in vitro receptor binding affinities, in vivo biodistribution and microPET studies.

Results In the receptor competition studies, DOTA-MSH4 showed the highest binding affinity. However, in vivo study demonstrated poor performance of MSH4 as an imaging agent due to its low tumor uptake and high kidney accumulation. In comparison, DOTA-MSH2 displayed medium affinity, yet highest tumor uptake and lower kidney accumulation. Further blocking study of DOTA-MSH2 confirmed its specificity in vivo.

Conclusions Despite the tetramer showed the highest binding affinities in vitro, it was not mirrored by its in vivo behavior. DOTA-MSH2 was identified as the best melanoma imaging agent.