Near-infrared Dye800CW–conjugated disulfide-based cyclic RGD peptide c(CRGDKGPDC) and its DOTA analog

Integrins are multifunctional cell-surface receptors that are implicated in the regulation of cell–cell interactions, angiogenesis, tumor growth, and cancer metastasis (1, 2). There are 24 known integrin receptors, and each receptor is made up by the combination of a single α subunit (with 18 subtypes) and a single β subunit (with 8 subtypes) (3). A characteristic feature of the integrin receptors is that they can be activated either by cytoplasmic events ("inside-out" activation) or by extracellular ligand binding ("outside-in" activation) as illustrated by Margadant et al. (3). Integrins such as αβ, αβ, and αβ are overexpressed in certain malignant tumors and are targeted with imaging agents for the detection, diagnosis, and therapy of cancers or to evaluate the efficacy of new anticancer agents (1). Peptides containing an arginine-glycine-aspartic acid (RGD) motif (RGD peptides) are known to recognize and bind to the integrin receptors, and a variety of radiolabeled cyclic analogs of the RGD peptides (e.g., c(RGDfk)) have been used for the molecular imaging and therapy of neoplastic tumors in the clinic (4). A disulfide-based cyclic RGD peptide, c(CRGDKGPDC) (iRGD), which interacts with the integrin and neurophilin-1 receptors, was shown to facilitate deeper penetration of imaging agents (5) and anticancer drugs (6) compared with the original RGD peptide. The complete mechanism of action of iRGD has been described and illustrated by Sugahara et al. (5). On the basis of this information, two cysteine-containing analogs of iRGD were conjugated to IRDye800CW, a near-infrared dye (Cys(IRDye800CW)iRGD), to generate probes that could potentially be used to detect tumors that overexpress integrins with optical imaging (1). The first peptide was an acylated form of the peptide (Ac-Cys(IRDye800CW)iRGD), and the second peptide was its 1,4,7,10-tetra-azacyclodecane-,','','''-tetraacetic acid (DOTA) analog (DOTA-Cys(IRDye800CW)iRGD) that could be used for dual labeling of the peptide. The purpose of introducing a DOTA moiety in the peptide was to generate an iRGD-based peptide that can be used for the imaging of tumors with optical and/or positron emission tomography (PET) techniques because DOTA can conjugate radionuclides such as Cu that emits positrons that can be detected with PET. The two peptides were evaluated for the optical imaging of MDA-MB-435 cell xenograft tumors that overexpress integrins in nude mice (1).