Peptide heterodimers for molecular imaging

Abstract

One main issue with peptide-based molecular imaging probes is their relatively low tumor affinity and short retention time. To improve peptide binding affinity, multivalency approach has been introduced. Traditionally, this approach involves the use of peptide homodimers or homomultimers in which peptide ligands of the same type are constructed with suitable linkers. Recently, a new approach using peptide heterodimers has emerged as a promising method for targeting multi-receptor over-expressed tumor cells. Significant affinity enhancements have been observed with peptide heterodimers compared with their parent peptide monomers. In a peptide heterodimer, two different peptide ligands capable of targeting two different receptors are covalently linked. The binding modes of peptide heterodimers can be monovalent or bivalent depending on whether simultaneous binding of two ligands can be achieved. Increased local ligand concentration and improved binding kinetics contribute to enhanced binding in both monovalent- and bivalent binding modes, while multivalency effect also plays an important role in bivalent binding mode. As many tumors overexpress multiple receptors, more peptide heterodimer-based molecular imaging probes are expected to be developed in future. This review article will discuss the peptide homodimers and heterodimers for molecular imaging with special emphasis on peptide heterodimers.

Fig. 1

Schematic illustration of receptor–peptide multimer interaction

Fig. 2

¹⁸F-labeled BBN-RGD heterodimer

Fig. 3

microPET images of mice bearing PC-3 tumor using ¹⁸F-labeled BBN-RGD, RGD and BBN peptides (Reprinted by permission of the Society of Nuclear Medicine from Li et al. 2008b)

Fig. 4

Binding mechanism for BBN-RGD heterodimer (Reprinted by permission of the Society of Nuclear Medicine from Li et al. 2008b)

Fig. 5

Schematic structure of ¹⁸F-FB-PEG₃-BBN-RGD

Fig. 6

DOTA and NOTA-BBN-RGD heterodimers

Fig. 7

Receptor–ligand interaction on cells under blocking and nonblocking conditions

Fig. 8

Receptor–ligand interaction on cells expressing one and two receptors

Fig. 9

Receptor–ligand interaction through two epitopes

Scheme 1

Synthesis of BBN-RGD heterodimer

Scheme 2

Synthesis of Delt-II-MSH(7) heterodimers

Scheme 3

Synthesis of CCK(6)-MSH(7) heterodimers

Scheme 4

Synthesis of EMP-ERP heterodimer

Scheme 5

Synthesis of heterodimers bearing functional groups