Synthesis and evaluation of radioiodinated phenoxyquinazoline and benzylaminoquinazoline derivatives as new EGF receptor tyrosine kinase imaging ligands for tumor diagnosis using SPECT

Synthesis Principle and Practice with Radioactive Iodines and Astatine: Advances Made So Far

Radioactive iodines and astatine, possessing distinct exploitable nuclear properties, play indispensable roles in the realms of nuclear imaging and therapy. Their analogous chemical characteristics shape the design, preparation, and substrate range for tracers labeled with these radiohalogens through interconnected radiosynthetic chemistry. This perspective systematically explores the labeling methods by types of halogenating reagents─nucleophilic and electrophilic─underpinning the rational design of such compounds. It delves into the rapidly evolving synthetic strategies and reactions in radioiodination and radioastatination over the past decade, comparing their intrinsic relationships and highlighting variations. This comparative analysis illuminates potential radiosynthetic methods for exploration. Moreover, stability concerns related to compounds labeled with radioactive iodines and astatine are addressed, offering valuable insights for radiochemists and physicians alike.

Design of Radioiodinated Pharmaceuticals: Structural Features Affecting Metabolic Stability towards in Vivo Deiodination

Radioiodinated pharmaceuticals are convenient tracers for clinical and research investigations because of the relatively long half-lives of radioactive iodine isotopes (i.e., 123I, 124I, and 131I) and the ease of their chemical insertion. Their application in radionuclide imaging and therapy may, however, be hampered by poor in vivo stability of the C-I bond. After an overview of the use of iodine in biology and nuclear medicine, we present here a survey of the catabolic pathways for iodinated xenobiotics, including their biodistribution, accumulation, and biostability. We summarize successful rational improvements in the biostability and conclude with general guidelines for the design of stable radioiodinated pharmaceuticals. It appears to be necessary to consider the whole molecule, rather than the radioiodinated fragment alone. Iodine radionuclides are generally retained in vivo on sp2 carbon atoms in iodoarenes and iodovinyl moieties, but not in iodinated heterocycles or on sp3 carbon atoms. Iodoarene substituents also have an influence, with increased in vivo deiodination in the cases of iodophenols and iodoanilines, whereas methoxylation and difluorination improve biostability.

Development of [18F]afatinib as new TKI-PET tracer for EGFR positive tumors

INTRODUCTION

Afatinib is an irreversible ErbB family blocker that was approved for the treatment of EGFR mutated non-small cell lung cancer in 2013. Positron emission tomography (PET) with fluorine-18 labeled afatinib provides a means to obtain improved understanding of afatinib tumor disposition in vivo. PET imaging with [(18)F]afatinib may also provide a method to select treatment responsive patients. The aim of this study was to label afatinib with fluorine-18 and evaluate its potential as TKI-PET tracer in tumor bearing mice.

METHODS

A radiochemically novel coupling, using peptide coupling reagent BOP, was explored and optimized to synthesize [(18)F]afatinib, followed by a metabolite analysis and biodistribution studies in two clinically relevant lung cancer cell lines, xenografted in nude mice.

RESULTS

A reliable [(18)F]afatinib radiosynthesis was developed and the tracer could be produced in yields of 17.0 ± 2.5% calculated from [(18)F]F(-) and >98% purity. The identity of the product was confirmed by co-injection on HPLC with non-labeled afatinib. Metabolite analysis revealed a moderate rate of metabolism, with >80% intact tracer in plasma at 45 min p.i. Biodistribution studies revealed rapid tumor accumulation and good retention for a period of at least 2 hours, while background tissues showed rapid clearance of the tracer.

CONCLUSION

We have developed a method to synthesize [(18)F]afatinib and related fluorine-18 labeled 4-anilinoquinazolines. [(18)F]Afatinib showed good stability in vivo, justifying further evaluation as a TKI-PET tracer.

A useful EGFR-TK ligand for tumor diagnosis with SPECT: development of radioiodinated 6-(3-morpholinopropoxy)-7-ethoxy-4-(3'-iodophenoxy)quinazoline

OBJECTIVE

Epidermal growth factor receptor tyrosine kinase (EGFR-TK) represents an attractive target for tumor diagnosis agents. Previously, radioiodinated 4-(3-iodophenoxy)-6,7-diethoxyquinazoline (PHY) was reported to possess good characteristics as a tumor imaging agent. We have explored the feasibility of developing tumor diagnosis ligands superior to radioiodinated PHY.

METHODS

New phenoxyquinazoline derivatives were designed with various side chains introduced to the 6th position of PHY. The IC50 values of the new derivatives to interrupt EGFR-TK phosphorylation were evaluated and compared to well-known EGFR-TK inhibitors. Tumor uptake studies of the new (125)I-labeled derivatives were conducted with A431 tumor-bearing mice. Selectivity and binding characteristics were analyzed by in vitro blocking studies and a binding assay. Furthermore, SPECT/CT scans were performed using A431 tumor-bearing mice.

RESULTS

Six quinazoline derivatives were designed and synthesized, and among these, 6a-d were found to have relatively high EGFR-TK inhibitory potency. In tumor uptake studies, [(125)I]6a ([(125)I]PYK) was found to have the highest tumor uptake and longest retention in tumors. In contrast, [(125)I]PYK was rapidly cleared from peripheral tissues, resulting in a high tumor-to-tissue ratio 24 h after injection. Moreover, the EGFR-TK selectivity of [(125)I]PYK was confirmed by pretreatment experiments with specific EGFR-TK inhibitors. Furthermore, [(125)I]PYK provided clear SPECT images of tumors.

CONCLUSIONS

Radioiodinated PYK, one of the newly synthesized quinazoline derivatives, was found to be a desirable ligand for EGFR-TK SPECT imaging. [(125)I]PYK showed high tumor accumulation and selective EGFR-TK binding and also succeeded in delivering high contrast imaging of tumors. These favorable characteristics of [(125)I]PYK suggest that the (123)I-labeled counterpart, [(123)I]PYK, would have great potential for diagnostic SPECT tumor imaging.