An Integrin Alpha 6-Targeted Radiotracer with Improved Receptor Binding Affinity and Tumor Uptake.

In this study, we reported a Tc-labeled integrin α-targeted peptide as the molecular imaging probe for tumor imaging by single-photon emission computed tomography (SPECT). We found that replacing Cys-Cys cyclized RWY peptide (sequence: cCRWYDENAC) with lactam-bridged cyclic cKiE peptide (sequence: cKRWYDENAisoE) did not sacrifice the integrin α-binding affinity and specificity of cKiE radiotracer. To further improve the radiotracer's tumor targeting capability, the dimerized cKiE peptide (termed cKiE2) was designed, and the corresponding radiotracer Tc-cKiE2 was evaluated for tumor uptake and in vivo pharmacokinetics properties in tumor models. We found that cKiE2 showed higher binding affinity to integrin α than did monomeric RWY or cKiE peptide. The biodistribution results showed that the tumor uptake of Tc-cKiE2 was twice higher than that of Tc-RWY (3.20 ± 0.12 vs 1.26 ± 0.06 %ID/g, < 0.001) at 0.5 h postinjection. The tumor to nontargeting tissue ratios were also enhanced in most normal organs. Specificity of Tc-cKiE2 for integrin α was demonstrated by competitive blocking of tumor uptake with excess cold peptide (3.20 ± 0.24 to 1.38 ± 0.23 %ID/g, < 0.001). The integrin α-positive tumors were clearly visualized by Tc-cKiE2/SPECT with low background except with a relatively high kidney uptake. The tumor uptake of Tc-cKiE2 correlates well with the tumor integrin α expression levels in a linear fashion (R = 0.9623). We also compared Tc-cKiE2 with an integrin αβ-targeted radiotracer Tc-3PRGD in the orthotopic hepatocellular carcinoma tumor models. We found that the orthotopic tumor was clearly visualized with Tc-cKiE2. Tc-3PRGD imaging did not show tumor contours in situ as clearly as Tc-cKiE2. The tumor-to-liver ratios of Tc-cKiE2 and Tc-3PRGD were 2.20 ± 0.17 and 0.85 ± 0.20. In conclusion, Tc-cKiE2 is an improved SPECT radiotracer for imaging integrin α-positive tumors and has great potential for further clinical application.