Cyclic peptides and peptidomimetics have gained enormous attention as therapeutics in the past decade, with numerous natural and non-natural macrocycles being identified for their promising pharmacological activities. Extensive research efforts have been dedicated to developing efficient synthetic strategies for accessing cyclopeptides and cyclopeptidomimetics.¹ Nucleophilic aromatic substitution (S_NAr) reaction is an effective cyclisation method orthogonal to conventional peptide chemistries. While the assembly of alkyl-aryl or biaryl ether bridged peptide macrocycles utilising S_NAr reaction is well documented, the synthetic viability of S_NAr macrocyclisation has focused primarily on nitro activated substrates.²

We have recently demonstrated the expansion of S_NAr reactivity to include substrates featuring nitro-, cyano-, ethynyl-fluoroarenes and fluoroheteroarenes, and established a library of alkylthioaryl-bridged melanocortin peptide ligands possessing high agonist activity and different receptor subtype selectivity.³ The orthogonality of this cyclisation method, coupled with the presence of functional moieties, has introduced new avenues for additional conjugations.

In pursuit to further broaden the utility of S_NAr macrocyclisation, we attempted to harness the potential of the chemical handles, namely the nitro, cyano, ethynyl and amino group, for bioconjugation with various chemical entities, and subsequently created a diverse collection of melanocortin peptide conjugates. The resultant peptide conjugates bearing DOTA-based chelators and fluorescent labels exhibited profound agonist activity at the melanocortin 1 receptor (MC1R) with varying degrees of receptor subtype selectivity. This underscores their potential for applications in biomolecular imaging and theranostics in melanoma, where the overexpression of MC1R serves as a promising biomarker.