Application of tris-allyl-DOTA in the preparation of DOTA–peptide conjugates

PSMA-D4 Radioligand for Targeted Therapy of Prostate Cancer: Synthesis, Characteristics and Preliminary Assessment of Biological Properties

A new PSMA ligand (PSMA-D4) containing the Glu-CO-Lys pharmacophore connected with a new linker system (L-Trp-4-Amc) and chelator DOTA was developed for radiolabeling with therapeutic radionuclides. Herein we describe the synthesis, radiolabeling, and preliminary biological evaluation of the novel PSMA-D4 ligand. Synthesized PSMA-D4 was characterized using TOF-ESI-MS, NMR, and HPLC methods. The novel compound was subject to molecular modeling with GCP-II to compare its binding mode to analogous reference compounds. The radiolabeling efficiency of PSMA-D4 with 177Lu, 90Y, 47Sc, and 225Ac was chromatographically tested. In vitro studies were carried out in PSMA-positive LNCaP tumor cells membranes. The ex vivo tissue distribution profile of the radioligands and Cerenkov luminescence imaging (CLI) was studied in LNCaP tumor-bearing mice. PSMA-D4 was synthesized in 24% yield and purity >97%. The radio complexes were obtained with high yields (>97%) and molar activity ranging from 0.11 to 17.2 GBq mcmol-1, depending on the radionuclide. In vitro assays confirmed high specific binding and affinity for all radiocomplexes. Biodistribution and imaging studies revealed high accumulation in LNCaP tumor xenografts and rapid clearance of radiocomplexes from blood and non-target tissues. These render PSMA-D4 a promising ligand for targeted therapy of prostate cancer (PCa) metastases.

Synthesis of peptide radiopharmaceuticals for the therapy and diagnosis of tumor diseases

Despite the advances in molecular biology and biochemistry, the prognosis of patients suffering from tumor diseases remains poor. The limited therapeutic success can be explained by the insufficient performance of the common chemotherapeutic drugs that lack the ability to specifically target tumor tissues. Recently peptide radiopharmaceuticals have been developed that enable the concurrent imaging and therapy of tumors expressing a specific target. Here, with a special emphasis on the synthesis of the building blocks required for the complexation of metallic radioisotopes, the requirements to the design and synthesis of radiolabeled peptides for clinical applications are described.

(111)In- and (203)Pb-Labeled Cyclic RGD Peptide Conjugate as an α(v)β(3) Integrin-Binding Radiotracer

Methodology for site-specific modification and chelate conjugation of a cyclic RGD (cRGD) peptide for the preparation of a radiotracer molecular imaging agent suitable for detecting α(v)β(3) integrin is described. The method involves functionalizing the peptide with an aldehyde moiety and conjugation to a 1,4,7,10-tetraazacyclododecane-N,N',N″,N‴-tetraacetic acid (DOTA) derivative that possesses an aldehyde reactive aminooxy group. The binding assay of the (111)In-labeled peptide conjugate with α(v)β(3) integrin showed 60% bound when four equivalents of the integrin was added, a reasonable binding affinity for a mono-valent modified RGD peptide.

Solid-supported NOTA and DOTA chelators useful for the synthesis of 3'-radiometalated oligonucleotides

Esterified precursors of 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid (DOTA; 18) and 1,4,7-triazacyclononane-1,4,7-trisacetic acid (NOTA; 17,19) ligands bearing a dimethoxytritylated hydroxyl side arm were prepared and immobilized via an ester linkage to long chain alkyl amine derivatized controlled pore glass (LCAA-CPG). Oligonucleotide chains were then assembled on the hydroxyl function and conjugates were released and deprotected by a two-step cleavage with aqueous alkali and ammonia. The 3'-DOTA and 3'-NOTA conjugated oligonucleotides were converted to (68)Ga chelates by a brief treatment with [(68)Ga]Cl(3) at elevated temperature. Applicability of the conjugates for in vivo imaging with positron emission tomography (PET) was verified.

Multimerization of cRGD peptides by click chemistry: synthetic strategies, chemical limitations, and influence on biological properties

Integrin α(ν)β(3) is overexpressed on endothelial cells of growing vessels as well as on several tumor types, and so integrin-binding radiolabeled cyclic RGD pentapeptides have attracted increasing interest for in vivo imaging of α(ν)β(3) integrin expression by positron emission tomography (PET). Of the cRGD derivatives available for imaging applications, systems comprising multiple cRGD moieties have recently been shown to exhibit highly favorable properties in relation to monomers. To assess the synthetic limits of the cRGD-multimerization approach and thus the maximum multimer size achievable by using different efficient conjugation reactions, we prepared a variety of multimers that were further investigated in vitro with regard to their avidities to integrin α(ν)β(3.) The synthesized peptide multimers containing increasing numbers of cRGD moieties on PAMAM dendrimer scaffolds were prepared by different click chemistry coupling strategies. A cRGD hexadecimer was the largest construct that could be synthesized under optimized reaction conditions, thus identifying the current synthetic limitations for cRGD multimerization. The obtained multimeric systems were conjugated to a new DOTA-based chelator developed for the derivatization of sterically demanding structures and successfully labeled with (68)Ga for a potential in vivo application. The evaluated multimers showed very high avidities-increasing with the number of cRGD moieties-in in vitro studies on immobilized α(ν)β(3) integrin and U87MG cells, of up to 131- and 124-fold, respectively, relative to the underivatized monomer.

DOTA derivatives for site-specific biomolecule-modification via click chemistry: synthesis and comparison of reaction characteristics

Due to the high stability of its complexes with many M(2+) and M(3+)-ions, DOTA (1,4,7,10-tetraazacyclododecane-N,N',N″,N‴-tetraacetic acid) is the most commonly used chelator for the derivatization and radiolabeling of bioactive molecules. Most of the currently used DOTA derivatives comprise amine-reactive functionalities, limiting their application to the derivatization of fully protected molecules or otherwise resulting in randomly distributed conjugation sites of undefined number. Click chemistry reactions are a valuable alternative to this unspecific conjugation as they proceed efficiently and chemoselectively under mild conditions allowing a site-specific derivatization of unprotected biomolecules. In this work, we describe straightforward syntheses of DOTA derivatives containing thiol, maleimide, aminooxy, aldehyde, alkyne, and azide functionalities, amenable to the currently most often used click chemistry reactions. Furthermore, the efficiency of the respective click reactions introducing DOTA into bioactive molecules was investigated. For each of the synthesized DOTA synthons, the site-specific and efficient conjugation to Tyr(3)-octreotate could be shown. Among these, the addition and oxime formation reactions proceeded fast and without side reactions, giving the products in high yields of 64-83% after purification. The copper-catalyzed triazole formation reactions produced some side-products, giving the desired products in lower, but still reasonable overall yields of 19-25%. All synthesized peptide-DOTA-conjugates were labeled with (68)Ga in high radiochemical yields of 96-99% and high specific activities providing compounds of high purity, demonstrating the applicability of all synthons for biomolecule modification and subsequent radiolabeling.

Synthesis of a DOTA--biotin conjugate for radionuclide chelation via Cu-free click chemistry

A strain-induced copper-free click reaction mediated by a new and easily prepared cyclooctyne derivative was used to efficiently assemble a DOTA-biotin adduct capable of radionuclide ((68)Ga) uptake. This synthetic strategy offers a potentially general and convenient means of preparing targeted radiolabeling and radiotherapeutic agents.

Antibody-dendrimer conjugates: the number, not the size of the dendrimers, determines the immunoreactivity

Radioimmunotherapy using antibodies with favorable tumor targeting properties and high binding affinity is increasingly applied in cancer therapy. The potential of this valuable cancer treatment modality could be further improved by increasing the specific activity of the labeled proteins. This can be done either by coupling a large number of chelators which leads to a decreased immunoreactivity or by conjugating a small number of multimeric chelators. In order to systematically investigate the influence of conjugations on immunoreactivity with respect to size and number of the conjugates, the anti-EGFR antibody hMAb425 was reacted with PAMAM dendrimers of different size containing up to 128 chelating agents per conjugation site. An improved dendrimer synthesis protocol was established to obtain compounds of high homogeneity suitable for the formation of defined protein conjugates. The quantitative derivatization of the PAMAM dendrimers with DOTA moieties and the characterization of the products by isotopic dilution titration using (111)In/(nat)In are shown. The DOTA-containing dendrimers were conjugated with high efficiency to hMAb425 by applying Sulfo-SMCC as cross-linking agent and a 10- to 25-fold excess of the thiol-containing dendrimers. The determination of the immunoreactivities of the antibody-dendrimer conjugates by FACS analysis revealed a median retained immunoreactivity of 62.3% for 1.7 derivatization sites per antibody molecule, 55.4% for 2.8, 27.9% for 5.3, and 17.1% for 10.0 derivatization sites per antibody but no significant differences in immunoreactivity for different dendrimer sizes. These results show that the dendrimer size does not influence the immunoreactivity of the derivatized antibody significantly over a wide molecular weight range, whereas the number of derivatization sites has a crucial effect.

PET (positron emission tomography) imaging of biomolecules using metal-DOTA complexes: a new collaborative challenge by chemists, biologists, and physicians for future diagnostics and exploration of in vivo dynamics

Recently, PET has been paid a great deal of attention as a non-invasive imaging method. In this review, the recent advances of PET using biomolecules, such as peptides, monoclonal antibodies, proteins, oligonucleotides, and glycoproteins will be described. So far, PET of biomolecules has been mainly used for diagnosis of cancers. The biomolecules have been conjugated with the DOTA ligand, labeled with radiometals as the beta+ emitter, and targeted to specific tumors, where they have enabled visualization of even small metastatic lesions, due to the high sensitivity of the PET scanners. Some of the biomolecules have been used not only for PET diagnosis, but also for radiotherapeutic treatments by simply changing the radiometals to beta(-) emitters. Collaborative work between chemists, biologists, and physicians will be important for the future of biomolecule-based targeting and diagnosis.

Synthesis of p-(di-tert-butyl[(18)F]fluorosilyl)benzaldehyde ([(18)F]SiFA-A) with high specific activity by isotopic exchange: a convenient labeling synthon for the (18)F-labeling of N-amino-oxy derivatized peptides

The syntheses of different (18)F-labeled peptides using the highly effective labeling synthon p-(di- tert-butylfluorosilyl) benzaldehyde ([ (18)F]SiFA-A) for the development of (18)F-radiopharmaceuticals for oncological positron emission tomography (PET) is reported. The novel and mild labeling technique for the radiosynthesis of [ (18)F]SiFA-A, based on an unexpectedly efficient isotopic (19)F- (18)F exchange, yielded the (18)F-synthon [ (18)F]SiFA-A in almost quantitative yields in high specific activities between 225 and 680 GBq/micromol (6081-18 378 Ci/mmol) without applying HPLC purification. The [ (18)F]SiFA-A was finally used to label the N-terminal amino-oxy (N-AO) derivatized peptides AO-Tyr (3)-octreotate (AO-TATE), cyclo(fK(AO-N)RGD and N-AO-PEG 2-[D-Tyr-Gln-Trp-Ala-Val-betaAla-His-Thi-Nle-NH 2] (AO-BZH3, a bombesin derivative) in high radiochemical yields. Density functional theory (DFT) calculations confirmed high efficiency of the isotopic exchange, which is predicted to proceed via a pentacoordinate siliconate intermediate dissociating immediately to form the radiolabeled [ (18)F]SiFA-A.