Synthesis of the first radiolabeled 188Re N-heterocyclic carbene complex and initial studies on its potential use in radiopharmaceutical applications

A Primer on Radiopharmaceutical Therapy

The goal of this article is to serve as a primer for the United States-based radiation oncologist who may be interested in learning more about radiopharmaceutical therapy (RPT). Specifically, we define RPT, review the data behind its current and anticipated indications, and discuss important regulatory considerations for incorporating it into clinical practice. RPT represents an opportunity for radiation oncologists to leverage 2 key areas of expertise, namely therapeutic radiation therapy and oncology, and apply them in a distinct context in collaboration with nuclear medicine and medical oncology colleagues. Although not every radiation oncologist will incorporate RPT into their day-to-day practice, it is important to understand the role for this modality and how it can be appropriately used in select patients.

Direct labeling of a somatostatin receptor antagonist via peptide cyclization with Re, 99mTc and 186Re metal centers: Radiochemistry and in vitro evaluation

INTRODUCTION

With the long-term goal of developing a diagnostic (99mTc) and therapeutic (186Re) agent pair for targeting somatostatin receptor (SSTR)-positive neuroendocrine tumors (NETs), we developed novel metal-cyclized peptides through direct labeling of the potent SSTR2 antagonist Ac-4-NO2-Phe-c(DCys-Tyr-DTrp-Lys-Thr-Cys)-DTyr-NH2 (1) with Re (in Re-1), 99mTc (in [99mTc]Tc-1) and 186Re (in [186Re]Re-1).

METHODS

Re-1 was characterized by LC-ESI-MS and HR-ESI-MS and was tested for receptor affinity in SSTR-expressing cells (AR42J). Radiolabeling of the peptide was achieved via ligand exchange from 99mTc-labeled glucoheptonate or [186Re]ReOCl3(PPh3)2, yielding [99mTc]Tc-1 or [186Re]Re-1, respectively. In vitro stability of [99mTc]Tc-1/[186Re]Re-1 in PBS (10 mM) at pH 7.4 and 37 °C was determined by HPLC analysis. Moreover, [99mTc]Tc-1 stability was tested in cysteine (1 mM) and rat serum under the same conditions.

RESULTS

Re-1 consisted of two isomers, confirmed by LC-ESI-MS, with good SSTR2 affinity (IC50 = 43 ± 6 nM). Optimization of the 99mTc labeling through varying reaction parameters such as pH, reaction time, and Sn2+ and ligand concentrations resulted in high radiochemical yield (RCY ≥92%). Similarly, [186Re]Re-1 was prepared in reasonable RCY (≥50%). Both 99mTc/186Re-tracers consisted of two product isomers as identified by HPLC co-injection with Re-1. While [99mTc]Tc-1 was sufficiently stable in vitro (≥71% intact through 4 h in PBS, cysteine and rat serum), [186Re]Re-1 exhibited more moderate in vitro stability (58% intact after 1 h in PBS).

CONCLUSIONS

Novel 99mTc/186Re-cyclized SSTR2 antagonist peptides were synthesized and characterized using the Re-cyclized analogue as a reference. Due to the nanomolar SSTR2 affinity of Re-1 and good in vitro stability of [99mTc]Tc-1, the latter shows early promise for development as a radiodiagnostic agent for SSTR-expressing NETs.

ADVANCES IN KNOWLEDGE AND IMPLICATIONS FOR PATIENT CARE

The 99mTc-cyclized complex showed promising in vitro properties, and future in vivo studies will determine the potential for translating such a design into the human clinic.

Current Status of Radiopharmaceutical Therapy

In radiopharmaceutical therapy (RPT), a radionuclide is systemically or locally delivered with the goal of targeting and delivering radiation to cancer cells while minimizing radiation exposure to untargeted cells. Examples of current RPTs include thyroid ablation with the administration of ¹³¹I, treatment of liver cancer with ⁹⁰Y microspheres, the treatment of bony metastases with ²²³Ra, and the treatment of neuroendocrine tumors with ¹⁷⁷Lu-DOTATATE. New RPTs are being developed where radionuclides are incorporated into systemic targeted therapies. To assure that RPT is appropriately implemented, advances in targeting need to be matched with advances in quantitative imaging and dosimetry methods. Currently, radiopharmaceutical therapy is administered by intravenous or locoregional injection, and the treatment planning has typically been implemented like chemotherapy, where the activity administered is either fixed or based on a patient's body weight or body surface area. RPT pharmacokinetics are measurable by quantitative imaging and are known to vary across patients, both in tumors and normal tissues. Therefore, fixed or weight-based activity prescriptions are not currently optimized to deliver a cytotoxic dose to targets while remaining within the tolerance dose of organs at risk. Methods that provide dose estimates to individual patients rather than to reference geometries are needed to assess and adjust the injected RPT dose. Accurate doses to targets and organs at risk will benefit the individual patients and decrease uncertainties in clinical trials. Imaging can be used to measure activity distribution in vivo, and this information can be used to determine patient-specific treatment plans where the dose to the targets and organs at risk can be calculated. The development and adoption of imaging-based dosimetry methods is particularly beneficial in early clinical trials. In this work we discuss dosimetric accuracy needs in modern radiation oncology, uncertainties in the dosimetry in RPT, and best approaches for imaging and dosimetry of internal radionuclide therapy.

Cyclopalladation in the Periphery of a NHC Ligand as the Crucial Step in the Synthesis of Highly Active Suzuki-Miyaura Cross-Coupling Catalysts

Starting from 2,4-dichloropyrimidine, 4-(2-dialkylamino)pyrimidinyl functionalized mesitylimidazolium chlorides are accessible in a five-step reaction sequence. Two routes leading to palladium NHC complexes derived from these ligands have been worked out: By transmetalation with the corresponding NHC-AgCl complexes, C,N-coordinated palladium(II) complexes can be obtained. Treatment of palladium dichloride with the imidazolium salts in pyridine and in the presence of K₂ CO₃ gives cyclometalated and thus C,C-coordinated compounds. The reactivities of all these compounds were investigated in detail as well as their performance in the catalytic Suzuki-Miyaura cross-coupling reaction. It turned out that the C,C-coordinated derivatives exhibit high catalytic activities in the coupling of arylboronic acids with aryl chlorides, which is consistent with the generally accepted mechanistic ideas on substrate activation.

Cationic rhenium complexes ligated with N-heterocyclic carbenes – an overview

This review provides an overview of the currently known cationic rhenium NHC complexes.

Rhenium complexes of bidentate, bis-bidentate and tridentate N-heterocyclic carbene ligands

Rhenium(i) tricarbonyl complexes of a range of bidentate, bis-bidentate and tridentate NHC ligands have been prepared. These NHC ligands are of interest for possible applications in the development of Tc-99m or Re-186/188 radiopharmaceuticals and the stability of two complexes were evaluated in ligand challenge experiments using the metal binding amino acids l-histidine or l-cysteine.