[11C-carbonyl]CEP-32496: Radiosynthesis, biodistribution and PET study of brain uptake in P-gp/BCRP knockout mice

[11C]phosgene: Synthesis and application for development of PET radiotracers

Carbon-11-labeled phosgene ([11C]phosgene, [11C]COCl2) is a useful labeling agent that connects two heteroatoms by inserting [11C]carbonyl (11C=O) function in carbamates, ureas, and carbonates, which are components of biologically important heterocyclic compounds and functional groups in drugs as a linker of fragments with in vivo stability. Development of 11C-labeled PET tracers has been performed using [11C]phosgene as a labeling agent. However, [11C]phosgene has not been frequently used for 11C-labeling because preparation of [11C]phosgene required dedicated synthesis apparatus (not commercially available) and had problems in reproducibility and reliability. In our laboratory, an improved method for synthesizing [11C]phosgene using a carbon tetrachloride detection tube kit in environmental air analysis and the automated synthesis system for preparing [11C]phosgene have been developed in 2009. This apparatus has been used for routine synthesis of 11C-labeled tracers 1-4 times/week. Using [11C]phosgene we have developed and produced many PET radiotracers containing [11C]urea and [11C]carbamate moieties. In this review, we report the performance of our method for preparing [11C]phosgene, including automated synthesis apparatus developed in house, and the application of [11C]phosgene for development and production of 11C-labeled PET tracers.

Synthesis of a 11C-Isotopologue of the B-Raf-Selective Inhibitor Encorafenib Using In-Loop [11C]CO2 Fixation

The serine/threonine kinase B-Raf is an essential regulator of cellular growth, differentiation, and survival. B-Raf protein expression is elevated throughout melanoma progression, making it an attractive target for noninvasive imaging using positron-emission tomography. Encorafenib is a potent and highly selective inhibitor of B-Raf used in the clinical management of melanoma. In this study, the radiosynthesis of a ¹¹C-isotopologue of encorafenib was developed using an in-loop [¹¹C]CO₂ fixation reaction. Optimization of reaction conditions reduced the formation of a radiolabeled side product and improved the isolated yields of [¹¹C]encorafenib (14.5 ± 2.4% radiochemical yield). The process was fully automated using a commercial radiosynthesizer for the production of 6845 ± 888 MBq of [¹¹C]encorafenib in high molar activity (177 ± 5 GBq μmol^-1), in high radiochemical purity (99%), and in a formulation suitable for animal injection. An in vitro cellular binding experiment demonstrated saturable binding of the radiotracer to A375 melanoma cells.

Pharmacokinetic Evaluation of [11C]CEP-32496 in Nude Mice Bearing BRAFV600E Mutation-Induced Melanomas

CEP-32496, also known as RXDX-105 or Agerafenib, is a new orally active inhibitor for the mutated v-raf murine sarcoma viral oncogene homolog B1 (BRAF^V600E), which has attracted considerable attention in clinical trials for the treatment of human cancers. Here, we used carbon-11-labeled CEP-32496 ([¹¹C]CEP-32496) as a positron emission tomography (PET) radiotracer to evaluate its pharmacokinetic properties and explore its potential for in vivo imaging. Following radiotracer synthesis, we performed in vitro binding assays and autoradiography of [¹¹C]CEP-32496 in the A375 melanoma cell line and on tumor tissue sections from mice harboring the BRAF^V600E mutation. These were followed by PET scans and biodistribution studies on nude mice bearing subcutaneous A375 cell-induced melanoma. [¹¹C]CEP-32496 showed high binding affinity for BRAF^V600E-positive A375 melanoma cells and densely accumulated in the respective tissue sections; this could be blocked by the BRAF^V600E selective antagonist sorafenib and by unlabeled CEP-32496. The PET and biodistribution results revealed that [¹¹C]CEP-32496 accumulated continuously but slowly into the tumor within a period of 0 to 60 minutes postinjection in A375-melanoma-bearing nude mice. Metabolite analysis showed high in vivo stability of [¹¹C]CEP-32496 in plasma. Our results indicate that [¹¹C]CEP-32496 has excellent specificity and affinity for the BRAF^V600E mutation in vitro, while its noninvasive personalized diagnostic role needs to be studied further.

Recent Advances in the Development and Application of Radiolabeled Kinase Inhibitors for PET Imaging

Over the last 20 years, intensive investigation and multiple clinical successes targeting protein kinases, mostly for cancer treatment, have identified small molecule kinase inhibitors as a prominent therapeutic class. In the course of those investigations, radiolabeled kinase inhibitors for positron emission tomography (PET) imaging have been synthesized and evaluated as diagnostic imaging probes for cancer characterization. Given that inhibitor coverage of the kinome is continuously expanding, in vivo PET imaging will likely find increasing applications for therapy monitoring and receptor density studies both in- and outside of oncological conditions. Early investigated radiolabeled inhibitors, which are mostly based on clinically approved tyrosine kinase inhibitor (TKI) isotopologues, have now entered clinical trials. Novel radioligands for cancer and PET neuroimaging originating from novel but relevant target kinases are currently being explored in preclinical studies. This article reviews the literature involving radiotracer design, radiochemistry approaches, biological tracer evaluation and nuclear imaging results of radiolabeled kinase inhibitors for PET reported between 2010 and mid-2015. Aspects regarding the usefulness of pursuing selective vs. promiscuous inhibitor scaffolds and the inherent challenges associated with intracellular enzyme imaging will be discussed.