In the Trivehexin molecule, three of these cyclopeptides are attached by covalent bonds to a single TRAP chelator core. Since TRAP possesses three equivalent carboxylic acids for conjugation of other molecular units via amide formation, Trivehexin is a C3-symmetrical molecule with its three peptide bioligands being fully equivalent. The peptides are attached to the chelator core via the terminal amine group of the side chains of N-methyllysine. Actually, the conjugation is not done by amide bonding directly, but involves prior functionalization of the peptide with a short molecular extension (a linker) bearing a terminal alkyne, and of TRAP with three linkers bearing terminal azides.[19] These components are assembled by means of copper(I)catalyzed alkyne-azide cycloaddition (CuAAC, also known as Huisgen reaction, a Click chemistry reaction), giving rise to the three 1,3-triazole linkages in the 68Ga-Trivehexin structure.[1]
68Ga radiolabeling
68Ga-Trivehexin is a radioactive drug. The radioactive atom, gallium-68 (68Ga), decays with a half-life of approximately 68 min to the stable isotopezinc-68 (68Zn), to 89% by β+ decay whereby a positron with a maximum kinetic energy of 1.9 MeV is emitted (the remaining 11% are EC decays). Due to the short half life, 68Ga-Trivehexin can not be manufactured long before use but the 68Ga has to be introduced into the molecule shortly before application. This process is referred to as radiolabeling, and is done by complexation of the trivalentcation68GaIII by the TRAP chelator in Trivehexin.
68GaIII is usually obtained from a dedicated mobile radionuclide source, a Gallium-68 generator, in form of a solution in dilute (0.04–0.1 M) hydrochloric acid (frequently and imprecisely referred to as "68Ga chloride solution in HCl" despite it contains no species with a Ga–Cl bond but [68Ga(H2O)6]3+ complex hydrate cations).[20] For radiolabeling, the pH of the 68Ga containing generator eluate has to be raised from its initial value (depending on HCl concentration, pH 1–1.5) to pH 2–3.5 [21] using suitable buffers, such as sodium acetate. Then, Trivehexin (5–10 nmol) is added to the buffered 68Ga-containing solution, and the mixture is briefly heated to 50–100 °C (usually 2–3 min) to finalize the complexation reaction.[1][22]
68Ga-Trivehexin has a high binding affinity to αvβ6-integrin (IC50 = 0.047 nM). Its affinity to other RGD-binding integrins is much lower (IC50 for αvβ3, αvβ8, and α5β1 are 2.7, 6.2, and 22 nM, respectively; note that for IC50, higher values mean lower affinity),[1] resulting in a high selectivity for αvβ6-integrin.
Imaging procedure
Since 68Ga is a positron emitter, 68Ga-Trivehexin is applicable for PET imaging. However, PET is rarely used as a standalone imaging technique these days. Most clinics use PET/CT or even PET/MRI systems that acquire morphological and functional images in a single workflow and thus, provide more detailed and useful medical information to the physician.
For clinical PET/CT diagnostics, an activity in the range of 80–150 MBq68Ga-Trivehexin is injectedintravenously (i.v.).[27][28] The tracer then distributes with the blood flow and moves into tissues by diffusion, where it specifically binds to its target αvβ6-integrin, while an excess is excreted via the kidneys and the urine. As a result, 68Ga-Trivehexin and, therefore, the positron-emitting radionuclide 68Ga, is preferably accumulated by αvβ6-integrin abundant tissues (for example, tumor tissue). Next, a PET/CT scanner is used to detect the gamma radiation which is generated by the annihilation of the positrons emitted by 68Ga (not the actual positrons, which do not leave the body but travel only a few millimetres through the tissue). The spatial distribution of the annihilation events is reconstructed from the raw detector data (referred to as listmode data), which eventually delivers a 3-dimensional data set of radioactivity distribution in the body. These data allow the visualization of αvβ6-integrin positive tissues as 2-dimensional tomographic images or 3-dimensional volume rendering. Typically, the PET/CT imaging is performed 45–60 minutes after the i.v. administration of 68Ga-Trivehexin.[28]
Like for other radioactive imaging agents in medicine, the applied amounts of radioactivity are so low that radiation-related adverse effects are very unlikely to occur, and have not been observed in practice. Consistent with the "tracer principle", the amount of pharmacologically active compound injected to a patient in the course of such an examination is extremely low. Adverse events, such as toxicity or allergic reactions, are thus highly improbable. No adverse or clinically detectable pharmacologic effects were observed following intravenous administration of 68Ga-Trivehexin when administered to cancer patients, and there were no significant changes in vital signs, laboratory study results, or electrocardiograms.[28] In a study involving healthy volunteers, researchers again reported no adverse or clinically detectable pharmacologic effects and no significant changes in vital signs.[21]
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^ abBaranyai Z, Reich D, Vágner A, Weineisen M, Tóth I, Wester HJ, et al. (June 2015). "A shortcut to high-affinity Ga-68 and Cu-64 radiopharmaceuticals: one-pot click chemistry trimerisation on the TRAP platform". Dalton Transactions. 44 (24): 11137–11146. doi:10.1039/C5DT00576K. PMID25999035.
^Breuss JM, Gallo J, DeLisser HM, Klimanskaya IV, Folkesson HG, Pittet JF, et al. (June 1995). "Expression of the beta 6 integrin subunit in development, neoplasia and tissue repair suggests a role in epithelial remodeling". Journal of Cell Science. 108 (Pt 6): 2241–51. doi:10.1242/jcs.108.6.2241. PMID7673344.
^ abDas SS, Ahlawat S, Thakral P, Malik D, Simecek J, Cb V, et al. (May 2024). "Potential Efficacy of 68Ga-Trivehexin PET/CT and Immunohistochemical Validation of αvβ6 Integrin Expression in Patients With Head and Neck Squamous Cell Carcinoma and Pancreatic Ductal Adenocarcinoma". Clinical Nuclear Medicine. 49 (8): 733–740. doi:10.1097/RLU.0000000000005278. PMID38768077.
^Kömek H, Güzel Y, Kaplan İ, Yilmaz EE, Can C (November 2024). "Superiority of 68 Ga-Trivehexin PET/CT Over 18 F-FDG PET/CT in the Evaluation of Lymph Nodes in Patients With Breast Cancer". Clinical Nuclear Medicine. doi:10.1097/RLU.0000000000005585. PMID39601487.