N-succinimidyl 4-methyl-3-(tri-n-butylstannyl)benzoate: synthesis and potential utility for the radioiodination of monoclonal antibodies

Radiolabelled Peptides for Positron Emission Tomography and Endoradiotherapy in Oncology

This review deals with the development of peptide-based radiopharmaceuticals for the use with positron emission tomography and peptide receptor radiotherapy. It discusses the pros and cons of this class of radiopharmaceuticals as well as the different labelling strategies, and summarises approaches to optimise metabolic stability. Additionally, it presents different target structures and addresses corresponding tracers, which are already used in clinical routine or are being investigated in clinical trials.

A facile method for post-conjugation prosthetic radioiodination of “mini-peptides”

Prosthetic radioiodination methods were developed to facilitate the labeling of proteins devoid of tyrosine(s) or when these moieties are crucial for biological activity. This method involves the use of the so-called Bolton-Hunter-type reagents. However, the in vivo instability of the label prompted the search for more stable groups. Although these second generation reagents have worked well with proteins and peptides, the current reaction scheme takes a long time to perform. A simplified method may be more appropriate especially from radiation safety point of view. More importantly, for short-lived halogens, advantage may be gained utilizing a shorter reaction time. Recently we reported on the radioiodination of interleukin-8 (IL-8) using the pyridine carboxylate-derived activated ester. We have successfully conjugated this prosthetic group to tri- and tetrapeptides harboring the somatostatin (SST) receptor recognition units and characterized by HPLC and MS. The radioiodination was accomplished using the Iodogen method in a reasonable yield (mean=60%). The total synthesis time was approximately 60 min, which was 3-4 times shorter than the classical two-step method. Preliminary biodistribution of the radiolabeled peptide showed uptake in some of the organs known to express SST receptors. Injection of a low specific activity tracer significantly decreased the retention of radioactivity in these organs.

Preparation and in vivo evaluation of a novel stabilized linker for 211At labeling of protein

Significant improvement of in vivo stability of 211At-labeled radioimmunoconjugates achieved upon employment of a recently reported new linker, succinimidyl N-2-(4-[211At]astatophenethyl)succinamate (SAPS), prompted additional studies of its chemistry. The 211At radiolabeling of succinimidyl N-2-(4-tributylstannylphenethyl)succinamate (1) was noted to decline after storage at -15 degrees C for greater than 6 months. Compound 1 was found to degrade via a ring closure reaction with the formation of N-2-(4-tributylstannylphenethyl)succinimide (3), and a modified procedure for the preparation of 1 was developed. The N-methyl structural analog of 1, succinimidyl N-2-(4-tributylstannylphenethyl)-N-methyl succinamate (SPEMS), was synthesized to investigate the possibility of improving the stability of reagent-protein linkage chemistry. Radiolabeling of SPEMS with 211At generates succinimidyl N-2-(4-[211At]astatophenethyl)-N-methyl succinamate (Methyl-SAPS), with yields being consistent for greater than 1 year. Radiolabelings of 1 and SPEMS with 125I generated succinimidyl N-2-(4-[125I]iodophenethyl)succinamate (SIPS) and succinimidyl N-2-(4-[125I]iodophenethyl)-N-methyl succinamate (Methyl-SIPS), respectively, and showed no decline in yields. Methyl-SAPS, SAPS, Methyl-SIPS and SIPS were conjugated to Herceptin for a comparative assessment in LS-174T xenograft-bearing mice. The conjugates of Herceptin with Methyl-SAPS or Methyl-SIPS demonstrated immunoreactivity equivalent to if not superior to the SAPS and SIPS paired analogs. The in vivo studies also revealed that the N-methyl modification resulted in a superior statinated product.

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.

Preparation and in vivo evaluation of novel linkers for 211At labeling of proteins

The syntheses, radiolabeling, antibody conjugation and in vivo evaluation of new linkers for (211)At labeling of monoclonal antibodies are described. Syntheses of the N-succinimidyl esters and labeling with (211)At to form succinimidyl 4-methoxymethyl-3-[(211)At]astatobenzoate (9) and succinimidyl 4-methylthiomethyl-3-[(211)At]astatobenzoate (11) from the corresponding bromo-aryl esters is reported. Previously reported succinimidyl N-{4-[(211)At]astatophenethyl}succinamate (SAPS) is employed as a standard of in vivo stability. Each agent is conjugated with Herceptin in parallel with their respective (125)I analogue, succinimidyl 4-methoxymethyl-3-[(125)I]iodobenzoate (10), succinimidyl 4-methylthiomethyl-3-[(125)I]iodobenzoate (12) and succinimidyl N-{4-[(125)I]iodophenethyl}succinamate (SIPS), respectively, for comparative assessment in LS-174T xenograft-bearing mice. With 9 and 11, inclusion of an electron pair donor in the ortho position does not appear to provide in vivo stability comparable to SAPS. Variables in radiolabeling chemistry of these three agents with (211)At are notable. Sequential elimination of acetic acid and oxidizing agent, N-chlorosuccinimide (NCS), from the (211)At radiolabeling protocol for forming SAPS improves yield, product purity and consistency. NCS appears to be critical for the radiolabeling of 6 with (211)At. Formation of 11, however, is found to require the absence of NCS. Elimination of acetic acid is found to have no effect on radiolabeling efficiency or yield for either of these reactions.

Polyhedral Boron Compounds as Potential Linkers for Attachment of Radiohalogens to Targeting Proteins and Peptides. A Review

Polyhedral boron clusters (PBC) are three-dimensional inorganic aromatic systems. Some of them can easily be halogenated, and the halogen-boron bond in such systems is very strong. We consider the use of PBC as linkers for attachment of radioactive halogen isotopes to tumor-targeting proteins and peptides. In this review the major preconditions for such applications, such as biological considerations, knowledge concerning coupling chemistry and radiolabeling of PBC, are described. A review with 90 references.

Preparation and in vivo evaluation of linkers for 211At labeling of humanized anti-Tac

The syntheses, radiolabeling, antibody conjugation, and in vivo evaluation of new linkers for 211At labeling of humanized anti-Tac (Hu-anti-Tac), an antibody to the alpha-chain of the IL-2 receptor (IL-2Ralpha) shown to be a useful target for radioimmunotherapy are described. Synthesis of the organometallic linker precursors is accomplished by reaction of the corresponding bromo- or iodoaryl esters with bis(tributyltin) in the presence of a palladium catalyst. Subsequent conversion to the corresponding N-succinimidyl ester and labeling with 211At of two new linkers, N-succinimidyl 4-[211At]astato-3-methylbenzoate and N-succinimidyl N-(4-[211At]astatophenethyl)succinamate (SAPS), together with the previously reported N-succinimidyl 4-[211At]astatobenzoate and N-succinimidyl 3-[211At]astato-4-methylbenzoate, are each conjugated to Hu-anti-Tac. The plasma survival times of these conjugates are compared to those of directly iodinated (125I) Hu-anti-Tac. The N-succinimidyl N-(4-[211At]astatophenethyl)succinamate compound (SAPS) emerged from this assay as the most viable candidate for 211At-labeling of Hu-anti-Tac. SAPS, along with the directly analogous radio-iodinated reagent, N-succinimidyl N-(4-[125I]astatophenethyl)succinamate (SIPS), are evaluated in a biodistribution study along with directly iodinated (125I) Hu-anti-Tac. Blood clearance and biological accretion results indicate that SAPS is a viable candidate for further evaluation for radioimmunotherapy of cancer.

Cellular processing of (125)I- and (111)in-labeled epidermal growth factor (EGF) bound to cultured A431 tumor cells

Low molecular weight of epidermal growth factor (EGF) enables better intratumoral penetration in comparison with larger targeting proteins, but the cellular retention of EGF-associated radioactivity is poor for directly iodinated EGF. An attempt was made to improve intracellular retention by the use of metal-diethylenetriaminepentaacetic acid or nonphenolic linker (N-succinimidyl-para-iodobenzoate) as labeling agents. The use of nonphenolic linker did not improve retention of the radioactivity in A431 carcinoma cell line. The use of the radiometal label provided an appreciable prolongation of radioactivity residence inside the cell.

Chloramine-T in high-specific-activity radioiodination of antibodies using N-succinimidyl-3-(trimethylstannyl)benzoate as an intermediate

Monoclonal antibodies C215 and MOv18 have been radiohalogenated, using a single-batch method employing N-succinimidyl-3-(trimethylstannyl)benzoate, m-MeATE. Labelling to the stannyl ester was optimized using chloramine-T as oxidizing agent. The results show that the stannyl ester is effectively labelled with short reaction times giving reproducible yields from 85% to 95%. Subsequent antibody conjugation, 10 to 80 microg MAb, resulted in biologically active, labelled antibodies with overall radiochemical yields of 50% to 80%, with corresponding specific activities of 490-50 kBq(125I)/microg.

Preparation of 211At-labeled humanized anti-Tac using 211At produced in disposable internal and external bismuth targets

These studies describe the production and purification of 211At as well as the procedure for labeling humanized anti-Tac, the antibody to the alpha-chain of the IL-2 receptor (IL-2R alpha), which has been shown to be a useful target for immunotherapy. The optimized protocol combines the advantages of the two-stage dry distillation procedure with the astatination of trialkylstannyl substances as labeling compounds for proteins. The 211At was produced by bombarding either an external or a recently developed disposable internal bismuth target with alpha-particles from a Cyclotron Corporation CS-30 cyclotron. The 211At was found to contain less than 0.01% 210At. The production rate for the external target was 0.15 mCi +/- 0.056 microA(-1) h(-1) (n = 9) (5.55 MBq mcroA[-1] h[-1]). The production rate for the internal target was 0.44 +/- 0.14 mCi microA(-1) h(-1) (n = 16) (16.28 MBq mcroA[-1] h[-1]).

Immunoreactivity, stability, pharmacokinetics and biodistribution of a monoclonal antibody to human leukemic B cells after three different methods of radioiodination

Dal B02, a murine monoclonal antibody against human chronic lymphocytic leukemia (CLL) was radioiodinated using chloramine T (Chl.T), Bolton-Hunter (B-H) or N-succinimidyl-p-iodobenzoate (PIB). The preparations had comparable radiochemical purity (> 97%) and immunoreactive fraction (65-80%) but the Chl.T-based product was most susceptible to deiodination and loss of immunoreactivity. After i.v. injection into CLL-xenografted nude mice, the preparations had identical patterns of clearance from the blood but the PIB-based product led to more radioactivity in liver and spleen and less in the thyroid compared to the other preparations. The Chl.T-based product showed loss of immunoreactivity in circulation and less tumor-localized radioactivity 168 h after administration. The differences between the B-H-based and PIB-based products were less impressive than between PIB-based and Chl.T-based products.

Monoclonal antibody F(ab')2 fragment labeled with N-succinimidyl 2,4-dimethoxy-3-halobenzoates: in vivo comparison of iodinated and astatinated fragments

N-Succinimidyl 3-[211At]astato-2, 4-dimethoxybenzoate (SADMB) was prepared from a trialkylstannyl precursor in about 70-75% yield. With either trimethyl or tri-n-butylstannyl precursor, no temporal effect was found in the astatodestannylation yield. However, the methyl analog gave slightly better yield which was found to be not statistically significant. A monoclonal antibody (MAb) fragment, Mel-14 F(ab')2, could be labeled using SADMB in 28% coupling efficiency. The specific binding of this labeled fragment to tumor homogenates in vitro was 61.0 +/- 0.5% (62.8 +/- 0.9% for the 131I labeled fragment). Paired-label tissue distribution in normal mice showed similar uptake of 131I and 211 At in many tissues. However, by 14.5 h selectivity of spleen, lungs and stomach for Mel-14 F(ab')2 labeled with 211At compared to 131I was 4.1, 3.8 and 6.4, respectively.