Summary: | Serum albumin binding moieties (ABMs) such as the Evans blue (EB) dye fragment and the 4-(<i>p</i>-iodophenyl)butyryl (IP) have been used to improve the pharmacokinetic profile of many radiopharmaceuticals. The goal of this work was to directly compare these two ABMs when conjugated to an integrin α<sub>v</sub>β<sub>6</sub> binding peptide (α<sub>v</sub>β<sub>6</sub>-BP); a peptide that is currently being used for positron emission tomography (PET) imaging in patients with metastatic cancer. The ABM-modified α<sub>v</sub>β<sub>6</sub>-BP peptides were synthesized with a 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetracetic acid (DOTA) chelator for radiolabeling with copper-64 to yield [<sup>64</sup>Cu]Cu DOTA-EB-α<sub>v</sub>β<sub>6</sub>-BP ([<sup>64</sup>Cu]<b>1</b>) and [<sup>64</sup>Cu]Cu DOTA-IP-α<sub>v</sub>β<sub>6</sub>-BP ([<sup>64</sup>Cu]<b>2</b>). Both peptides were evaluated in vitro for serum albumin binding, serum stability, and cell binding and internalization in the paired engineered melanoma cells DX3puroβ6 (α<sub>v</sub>β<sub>6</sub> +) and DX3puro (α<sub>v</sub>β<sub>6</sub> −), and pancreatic BxPC-3 (α<sub>v</sub>β<sub>6</sub> +) cells and in vivo in a BxPC-3 xenograft mouse model. Serum albumin binding for [<sup>64</sup>Cu]<b>1</b> and [<sup>64</sup>Cu]<b>2</b> was 53–63% and 42–44%, respectively, with good human serum stability (24 h: [<sup>64</sup>Cu]<b>1</b> 76%, [<sup>64</sup>Cu]<b>2</b> 90%). Selective α<sub>v</sub>β<sub>6</sub> cell binding was observed for both [<sup>64</sup>Cu]<b>1</b> and [<sup>64</sup>Cu]<b>2</b> (α<sub>v</sub>β<sub>6</sub> (+) cells: 30.3–55.8% and 48.5–60.2%, respectively, vs. α<sub>v</sub>β<sub>6</sub> (−) cells <3.1% for both). In vivo BxPC-3 tumor uptake for both peptides at 4 h was 5.29 ± 0.59 and 7.60 ± 0.43% ID/g ([<sup>64</sup>Cu]<b>1</b> and [<sup>64</sup>Cu]<b>2</b>, respectively), and remained at 3.32 ± 0.46 and 4.91 ± 1.19% ID/g, respectively, at 72 h, representing a >3-fold improvement over the non-ABM parent peptide and thereby providing improved PET images. Comparing [<sup>64</sup>Cu]<b>1</b> and [<sup>64</sup>Cu]<b>2</b>, the IP-ABM-α<sub>v</sub>β<sub>6</sub>-BP [<sup>64</sup>Cu]<b>2</b> displayed higher serum stability, higher tumor accumulation, and lower kidney and liver accumulation, resulting in better tumor-to-organ ratios for high contrast visualization of the α<sub>v</sub>β<sub>6</sub> (+) tumor by PET imaging.
|