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

In this study, we reported a Tc-99m-labeled integrin alpha(6)-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 alpha(6)-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-99m-cKiE2 was evaluated for tumor uptake and in vivo pharmacokinetics properties in tumor models. We found that cKiE2 showed higher binding affinity to integrin alpha(6) than did monomeric RWY or cKiE peptide. The biodistribution results showed that the tumor uptake of Tc-99m-cKiE2 was twice higher than that of Tc-99m-RWY (3.20 +/- 0.12 vs 1.26 +/- 0.06 %ID/g, P < 0.001) at 0.5 h postinjection. The tumor to nontargeting tissue ratios were also enhanced in most normal organs. Specificity of Tc-99m-cKiE2 for integrin alpha(6) was demonstrated by competitive blocking of tumor uptake with excess cold peptide (3.20 +/- 0.24 to 1.38 +/- 0.23 %ID/g, P < 0.001). The integrin alpha(6)-positive tumors were clearly visualized by Tc-99m-cKiE2/SPECT with low background except with a relatively high kidney uptake. The tumor uptake of Tc-99m-cKiE2 correlates well with the tumor integrin a 6 expression levels in a linear fashion (R-2 = 0.9623). We also compared 99 mTc-cKiE2 with an integrin alpha(v)beta(3)-targeted radiotracer Tc-99m-3PRGD 2 in the orthotopic hepatocellular carcinoma tumor models. We found that the orthotopic tumor was clearly visualized with Tc-99m-cKiE2. Tc-99m-3PRGD 2 imaging did not show tumor contours in situ as clearly as Tc-99m-cKiE2. The tumor-to-liver ratios of 9(9m)Tc-cKiE2 and Tc-99m-3PRGD 2 were 2.20 +/- 0.17 and 0.85 +/- 0.20. In conclusion, Tc-99m-cKiE2 is an improved SPECT radiotracer for imaging integrin alpha(6)-positive tumors and has great potential for further clinical application.