Influence of the bifunctional chelate on the biological behavior of (99m)Tc-labeled chemotactic peptide conjugates

99mTc(CO)3+ labeled domain I/II-specific anti-EGFR (scFv)2 antibody fragment for imaging EGFR expression

Bifunctional chelators (BFCs) are covalently linked to biologically active targeting molecules and radiolabeled with radiometals. Technetium-99 m (^99mTc) is the most widely used isotope in nuclear medicine because of its excellent physical properties. The objective of this study was to synthesize and characterize a novel BFC that allows for the labeling of antibodies and antibody fragments using the ^99mTc(CO)₃⁺ core which forms a very stable complex with ^99mTc in the +1 oxidation sate. This study reports the synthesis of a BFC 1-pyrrolidinyl-2,5-dione-11-(bis((1-(carboxymethyl)-1H-imidazol-2-yl)methyl)amino)undecanoic acid (SAAC-CIM NHS ester), and the in vitro and in vivo evaluation of ^99mTc(CO)₃-SAAC-CIM-DLO6-(scFv)₂ (^99mTc(CO)₃-DLO6-(scFv)₂), a domain I/II-specific anti-epidermal growth factor receptor I (anti-EGFR) antibody fragment. The chelator allowed radiolabeling the (scFv)₂ antibody fragment in very mild conditions with no significant decrease in binding to EGFR. Radiochemical yields of >50% (radiochemical purity > 95%) of the resulting anti-EGFR (scFv)₂ immunoconjugate ^99mTc(CO)₃-DLO6-(scFv)₂ was obtained. The radioimmunoconjugate was stable in histidine challenge experiments with less than 20% transchelation at 24 h after challenge in the presence of a 1500-fold excess of histidine. In vivo biodistribution of ^99mTc(CO)₃-DLO6-(scFv)₂ indicates that the tracer was mainly cleared via renal excretion and to a lesser extent via the hepatobiliary pathway. The microSPECT imaging studies performed in mice confirmed the in vitro affinity results. The ^99mTc(CO)₃-DLO6-(scFv)₂ shows some promising properties and warrants further investigation for imaging EGFR.

(99m)Tc-maEEE-Z(HER2:342), an Affibody molecule-based tracer for the detection of HER2 expression in malignant tumors

Detection of HER2-overexpression in tumors and metastases is important for the selection of patients who will benefit from trastuzumab treatment. Earlier investigations showed successful imaging of HER2-positive tumors in patients using indium- or gallium-labeled Affibody molecules. The goal of this study was to evaluate the use of (99m)Tc-labeled Affibody molecules for the detection of HER2 expression. The Affibody molecule Z(HER2:342) with the chelator sequences mercaptoacetyl-Gly-Glu-Gly (maGEG) and mercaptoacetyl-Glu-Glu-Glu (maEEE) was synthesized by peptide synthesis and labeled with technetium-99m. Binding specificity, cellular retention, and in vitro stability were investigated. The biodistribution of (99m)Tc-maGEG-Z(HER2:342) and (99m)Tc-maEEE-Z(HER2:342) was compared with (99m)Tc-maGGG-Z(HER2:342) in normal mice, and the tumor targeting properties of (99m)Tc-maEEE-Z(HER2:342) were determined in SKOV-3 xenografted nude mice. The results showed that the Affibody molecules were efficiently labeled with technetium-99m. The labeled conjugates were highly stable in vitro with preserved HER2-binding capacity. The use of glutamic acid in the chelator sequences for (99m)Tc-labeling of Z(HER2:342) reduced the hepatobiliary excretion 3-fold with a single Gly-to-Glu substitution and 10-fold with three Gly-to-Glu substitutions. (99m)Tc-maEEE-Z(HER2:342) showed a receptor-specific tumor uptake of 7.9 +/- 1.0 %IA/g and a tumor-to-blood ratio of 38 at 4 h pi. Gamma-camera imaging with (99m)Tc-maEEE-Z(HER2:342) could detect HER2-expressing tumors in xenografts already at 1 h pi. It was concluded that peptide synthesis for the coupling of chelator sequences to Affibody molecules for (99m)Tc labeling is an efficient way to modify the in vivo kinetics. Increased hydrophilicity, combined with improved stability of the mercaptoacetyl-triglutamyl chelator, resulted in favorable biodistribution, making (99m)Tc-maEEE-Z(HER2:342) a promising tracer for clinical imaging of HER2 overexpression in tumors.

Influence of prosthetic radioiodination on the chemical and biological behavior of chemotactic peptides labeled at high specific activity

The influence of radioiodination made through prosthetic group N-succinimidyl-3-[131I]iodo-benzoate ([131I]SIB) on the behavior of small peptides was investigated using as model the chemotactic hexapeptide Nalpha-for-Nle-Leu-Phe-Nle-Tyr-Lys. No carrier added labeled peptide was isolated by reverse-phase HPLC (RP-HPLC) with coupling efficiencies up to 59-75%. Biodistribution in normal and infected C57 mice showed mainly a hepatobiliary clearance, a very low thyroid uptake and the highest uptake at the infection site was within 1h of injection. Superoxide production and competitive binding assays studies in human polymorphonuclear leukocytes showed a preserved biological activity and high-affinity specific binding. However, the results indicated that the changes observed in the receptor-binding properties with an IC50 almost twice than the unlabeled peptide and the increasing in the hepatobiliary excretion could be the consequence of the increased lipophicity observed due to the presence of the prosthetic group together with a strong influence of the radioisotope per se.

New chelation strategy allows for quick and clean 99mTc-labeling of synthetic peptides

Analogues of bombesin have been synthesized in which a N2S2 (bis-mercaptoacetyl functionalized diaminopropionic acid) or a N3S (mercaptoacetyl-Gly-Gly-Gly) radiometal-chelating center has been incorporated that allows radiolabeling of these peptides with 99mTc without the need for conjugation or harsh reaction conditions. A mild radiolabeling is possible by using an acetyl-moiety as sulfur protecting group, which can be removed by mild hydroxylamine-treatment at room temperature before radiolabeling. Retained receptor binding is demonstrated in competitive binding experiments with 99mTc-radiolabeled peptides and PC-3 cells with bombesin receptors.

Preparation and preliminary evaluation of 99mTc-EC-For-MLFK

For-Met-Leu-Phe-Lys (For-MLFK), a chemotactic peptide that binds with high affinity to granulocytes and monocytes, was labeled with 99mTc using ethylene dicysteine (EC) as the metal chelating system. EC was selected because of the rapid renal excretion of its 99mTc-complex and therefore, was expected to enhance the degree of urinary elimination of the peptide-conjugate. 99mTc-EC-For-MLFK was prepared using a preformed chelate approach. After incubation of 99mTc-EC-For-MLFK with total blood, 68.1% of the labeled peptide was associated with WBC and 86% of this cell-associated activity was bound to granulocytes. Biodistribution studies in normal mice revealed a very fast blood clearance (4.1% and 0.6% of I.D. in blood at respectively 5 and 60 min p.i.). However, elimination of the labeled peptide proceeds mainly via the hepatobiliary system (24.5% of I.D. in liver and 48.8% of I.D. in intestines at 60 min p.i.) and to a much lower degree via the kidneys (17.9% in renal system at 60 min p.i.). From these results, it is concluded that 99mTc-EC-For-MLFK is not suited to image infections, despite its high binding to granulocytes, since it leads to high, non-specific, abdominal activity.