211At labeled substance P (5–11) as potential radiopharmaceutical for glioma treatment

Introduction The purposes of the present work were to label substance P (5–11) with 211At using a rhodium(III) complex with a bifunctional ligand–2-(1,5,9,13-tetrathiacyclohexadecan-3-yloxy)acetic acid ([16aneS4]-COOH) and to assess the in vitro stability and toxicity of the obtained radiobioconjugate. Methods Two approaches were evaluated to obtain 131I/211At-Rh[16aneS4]-SP5–11 radiobioconjugates, based on 2-step and 1-step syntheses. In the first method 131I/211At-Rh[16aneS4]-COOH complexes were obtained that required further coupling to a biomolecule. In the second approach, the bioconjugate [16aneS4]-SP5–11 was synthesized and further labeled with 131I and 211At through the utilization of a Rh(III) metal cation bridge. The synthesized compounds were analyzed by HPLC, TLC and paper electrophoresis. Results The 131I/211At-Rh[16aneS4]-COOH complexes were obtained in high yield and possessed good stability in PBS and CSF. Preliminary studies on coupling of 131I-Rh[16aneS4]-COOH to substance P (5–11) in 2-step synthesis showed that this procedure was too long with respect to 211At half-life, prompting us to improve it by finally using a 1-step synthesis. This strategy not only shortened the labeling time, but also increased final yield of 131I/211At-Rh[16aneS4]-SP5–11 radiobioconjugates. The stability of both compounds in PBS and CSF was high. Toxicity studies with the 211At-Rh[16aneS4]-SP5–11 demonstrated that radiobioconjugate significantly reduced T98G cell viability in a dose dependent manner reaching 20% of survival at the highest radioactivity 1200 kBq/mL. Conclusions The radiobioconjugate 211At-Rh[16aneS4]-SP5–11 revealed its potential in killing glioma T98G cells during in vitro studies; therefore further animal studies to are required to determine its in vivo stability and treatment potential in normal and xenografted mice.