Synthesis and preliminary PET imaging of 11C and 18F isotopologues of the ROS1/ALK inhibitor lorlatinib

Development and preclinical evaluation of a gallium-68 labeled novel diagnostic tracer for visualizing ALK expression in tumor

Anaplastic lymphoma kinase (ALK) is prominently expressed in numerous malignant tumors, which lead to aberrant tumor proliferation, invasion and metastasis. Ceritinib (LDK378), as second-generation targeted drugs, has been used to treat advanced ALK-positive non-small cell lung cancer (NSCLC). Herein, we sought to develop a novel ALK-positron emission tomography/magnetic resonance (PET/MR) tracer 68Ga-DOTA-CTB (68Ga labeled ceritinib) based on ceritinib scaffold to monitor the ALK expression levels during targeted therapy with ceritinib. The 68Ga-DOTA-CTB radiotracer, obtained via a simple labeling procedure, exhibits favorable radiochemical purity, stability, and pharmacokinetic properties. Subsequently, cellular uptake experiments have demonstrated that 68Ga-DOTA-CTB could be accumulated in H2228 cells. Imaging and biodistribution experiments have revealed significant uptake of the radiotracer in the tumors of the experimental group, while tumors in the blocking group, which were saturated with an excess of precursor, exhibited a markedly reduced level of radioactivity. These empirical findings suggest that 68Ga-DOTA-CTB holds substantial potential as a novel PET/MR imaging tracer for ALK-positive tumors.

PEARL: A Multicenter Phase 2 Study of Lorlatinib in Patients with ALK-Rearranged NSCLC and Central Nervous System Disease

BACKGROUND

Patients with ALK-rearranged non-small cell lung cancer (ALK+ NSCLC) with symptomatic brain (BM) and leptomeningeal (LM) metastases are underrepresented in clinical trials due to poor performance status. Additionally, the need for improved and validated assessment criteria for evaluating central nervous system (CNS) response remains critical. Lorlatinib has demonstrated systemic activity in patients with ALK+ NSCLC. This ongoing phase II study aims to evaluate the efficacy and safety of lorlatinib in ALK+ NSCLC patients with progressive CNS metastases.

PATIENTS AND METHODS

This study is a multicenter, open-label, single-arm, prospective trial. Fifty eligible subjects will be divided into 2 cohorts: BM (progressive or new brain parenchymal; n = 30) and LM (positive cerebrospinal fluid cytology and/or MRI ± brain parenchymal metastasis; n = 20). Key inclusion criteria include ALK status confirmed using FDA-approved tests, at least 1 CNS lesion, with or without CNS-related symptoms, and an ECOG performance status of 0-2 for the BM cohort and 0-3 for the LM cohort. Primary endpoint is intracranial objective response rate based on the modified version of response evaluation criteria in solid tumors v1.1 for BM and modified RANO-LM criteria for LM. Key secondary endpoints include intracranial progression-free survival, overall progression-free survival, objective response rate, overall survival, safety and quality of life. Biomarker analysis of paired pretreatment tumor, blood and optional cerebrospinal fluid will be performed as preplanned exploratory analyses.

CONCLUSIONS

The PEARL study (CTONG2303) will evaluate efficacy of lorlatinib in CNS metastases in ALK+ NSCLC using refined CNS response evaluation criteria, with biomarker analyses providing insights into response and resistance mechanisms.

Macrocycles and macrocyclization in anticancer drug discovery: Important pieces of the puzzle

Increasing disease-related proteins have been identified as novel therapeutic targets. Macrocycles are emerging as potential solutions, bridging the gap between conventional small molecules and biomacromolecules in drug discovery. Inspired by successful macrocyclic drugs of natural origins, macrocycles are attracting more attention for enhanced binding affinity and target selectivity. Due to the conformation constraint and structure preorganization, macrocycles can reach bioactive conformations more easily than parent acyclic compounds. Also, rational macrocyclization combined with sequent structural modification will help improve oral bioavailability and combat drug resistance. This review introduces various strategies to enhance membrane permeability in macrocyclization and subsequent modification, such as N-methylation, intramolecular hydrogen bonding modulation, isomerization, and reversible bicyclization. Several case studies highlight macrocyclic inhibitors targeting kinases, HDAC, and protein-protein interactions. Finally, some macrocyclic agents targeting tumor microenvironments are illustrated.

The Riddle of the Sphinx: Progress in Leptomeningeal Metastasis of Non-Small Cell Lung Cancer

Leptomeningeal metastasis (LM) is a serious complication of advanced non-small cell lung cancer (NSCLC), and the incidence of LM has been increasing yearly in recent times. There is no consensus on the best treatment modality for LM, which underscores a difficult problem in the management of advanced NSCLC patients. The existing treatments include molecular targeted therapy, systemic chemotherapy, local radiotherapy, antivascular tumor therapy, intrathecal chemotherapy, and immunotherapy, but their efficacy is not satisfactory. In this article, we briefly describe the clinical manifestations, diagnosis, and treatment of NSCLC-LM and discuss progress regarding evaluation of the efficacy of LM treatment to better provide a necessary reference for clinical practice and clinical trial evaluation.

State of the art procedures towards reactive [18F]fluoride in PET tracer synthesis

BACKGROUND

Positron emission tomography (PET) is a powerful, non-invasive preclinical and clinical nuclear imaging technique used in disease diagnosis and therapy assessment. Fluorine-18 is the predominant radionuclide used for PET tracer synthesis. An impressive variety of new 'late-stage' radiolabeling methodologies for the preparation of 18F-labeled tracers has appeared in order to improve the efficiency of the labeling reaction.

MAIN BODY

Despite these developments, one outstanding challenge into the early key steps of the process remains: the preparation of reactive [18F]fluoride from oxygen-18 enriched water ([18O]H2O). In the last decade, significant changes into the trapping, elution and drying stages have been introduced. This review provides an overview of the strategies and recent developments in the production of reactive [18F]fluoride and its use for radiolabeling.

CONCLUSION

Improved, modified or even completely new fluorine-18 work-up procedures have been developed in the last decade with widespread use in base-sensitive nucleophilic 18F-fluorination reactions. The many promising developments may lead to a few standardized drying methodologies for the routine production of a broad scale of PET tracers.

Case Report: A rare case of non-small cell lung cancer with STRN-ALK fusion in a patient in very poor condition treated with first-line ensartinib

Several cases of STRN-ALK fusion have been reported, and some anaplastic lymphoma kinase (ALK) inhibitors have been shown to be effective for treatment. Nevertheless, no cases of COVID-19 leading to heart failure and respiratory failure have been reported in people older than 70 years treated with ALK inhibitors. The present case report describes a 70-year-old patient with usual chronic obstructive pulmonary disease, diabetes, depression, and carotid plaque disease. Next-generation sequencing of tissue obtained by puncture biopsy revealed a STRN-ALK mutation accompanied by a TP53 mutation. The patient was treated with ensartinib and developed COVID-19 leading to heart failure and respiratory failure; nevertheless, he had a good clinical outcome and exhibited high treatment tolerability.

Spirocyclic Iodonium Ylides for Fluorine-18 Radiolabeling of Non-Activated Arenes: From Concept to Clinical Research

Positron emission tomography (PET) is a powerful imaging tool for drug discovery, clinical diagnosis, and monitoring of disease progression. Fluorine-18 is the most common radionuclide used for PET, but advances in radiotracer development have been limited by the historical lack of methodologies and precursors amenable to radiolabeling with fluorine-18. Radiolabeling of electron-rich (hetero)aromatic rings remains a long-standing challenge in the production of PET radiopharmaceuticals. In this personal account, we discuss the history of spirocyclic iodonium ylide precursors, from inception to applications in clinical research, for the incorporation of fluorine-18 into complex non-activated (hetero)aromatic rings.

ALK Amplification and Rearrangements Are Recurrent Targetable Events in Congenital and Adult Glioblastoma

PURPOSE

Anaplastic lymphoma kinase (ALK) aberrations have been identified in pediatric-type infant gliomas, but their occurrence across age groups, functional effects, and treatment response has not been broadly established.

EXPERIMENTAL DESIGN

We performed a comprehensive analysis of ALK expression and genomic aberrations in both newly generated and retrospective data from 371 glioblastomas (156 adult, 205 infant/pediatric, and 10 congenital) with in vitro and in vivo validation of aberrations.

RESULTS

ALK aberrations at the protein or genomic level were detected in 12% of gliomas (45/371) in a wide age range (0-80 years). Recurrent as well as novel ALK fusions (LRRFIP1-ALK, DCTN1-ALK, PRKD3-ALK) were present in 50% (5/10) of congenital/infant, 1.4% (3/205) of pediatric, and 1.9% (3/156) of adult GBMs. ALK fusions were present as the only candidate driver in congenital/infant GBMs and were sometimes focally amplified. In contrast, adult ALK fusions co-occurred with other oncogenic drivers. No activating ALK mutations were identified in any age group. Novel and recurrent ALK rearrangements promoted STAT3 and ERK1/2 pathways and transformation in vitro and in vivo. ALK-fused GBM cellular and mouse models were responsive to ALK inhibitors, including in patient cells derived from a congenital GBM. Relevant to the treatment of infant gliomas, we showed that ALK protein appears minimally expressed in the forebrain at perinatal stages, and no gross effects on perinatal brain development were seen in pregnant mice treated with the ALK inhibitor ceritinib.

CONCLUSIONS

These findings support use of brain-penetrant ALK inhibitors in clinical trials across infant, pediatric, and adult GBMs. See related commentary by Mack and Bertrand, p. 2567.

Radiochemistry for positron emission tomography

Positron emission tomography (PET) constitutes a functional imaging technique that is harnessed to probe biological processes in vivo. PET imaging has been used to diagnose and monitor the progression of diseases, as well as to facilitate drug development efforts at both preclinical and clinical stages. The wide applications and rapid development of PET have ultimately led to an increasing demand for new methods in radiochemistry, with the aim to expand the scope of synthons amenable for radiolabeling. In this work, we provide an overview of commonly used chemical transformations for the syntheses of PET tracers in all aspects of radiochemistry, thereby highlighting recent breakthrough discoveries and contemporary challenges in the field. We discuss the use of biologicals for PET imaging and highlight general examples of successful probe discoveries for molecular imaging with PET - with a particular focus on translational and scalable radiochemistry concepts that have been entered to clinical use.

Heck Macrocyclization in Forging Non-Natural Large Rings including Macrocyclic Drugs

The intramolecular Heck reaction is a well-established strategy for natural product total synthesis. When constructing large rings, this reaction is also referred to as Heck macrocyclization, which has proved a viable avenue to access diverse naturally occurring macrocycles. Less noticed but likewise valuable, it has created novel macrocycles of non-natural origin that neither serve as nor derive from natural products. This review presents a systematic account of the title reaction in forging this non-natural subset of large rings, thereby addressing a topic rarely covered in the literature. Walking through two complementary sections, namely (1) drug discovery research and (2) synthetic methodology development, it demonstrates that beyond the well-known domain of natural product synthesis, Heck macrocyclization also plays a remarkable role in forming synthetic macrocycles, in particular macrocyclic drugs.

Nitrogen Containing Heterocycles as Anticancer Agents: A Medicinal Chemistry Perspective

Cancer is one of the major healthcare challenges across the globe. Several anticancer drugs are available on the market but they either lack specificity or have poor safety, severe side effects, and suffer from resistance. So, there is a dire need to develop safer and target-specific anticancer drugs. More than 85% of all physiologically active pharmaceuticals are heterocycles or contain at least one heteroatom. Nitrogen heterocycles constituting the most common heterocyclic framework. In this study, we have compiled the FDA approved heterocyclic drugs with nitrogen atoms and their pharmacological properties. Moreover, we have reported nitrogen containing heterocycles, including pyrimidine, quinolone, carbazole, pyridine, imidazole, benzimidazole, triazole, β-lactam, indole, pyrazole, quinazoline, quinoxaline, isatin, pyrrolo-benzodiazepines, and pyrido[2,3-d]pyrimidines, which are used in the treatment of different types of cancer, concurrently covering the biochemical mechanisms of action and cellular targets.

Imaging Leucine-Rich Repeat Kinase 2 In Vivo with 18F-Labeled Positron Emission Tomography Ligand

Leucine-rich repeat kinase 2 (LRRK2) has been demonstrated to be closely involved in the pathogenesis of Parkinson's disease (PD), and pharmacological blockade of LRRK2 represents a new opportunity for therapeutical treatment of PD and other related neurodegenerative conditions. The development of an LRRK2-specific positron emission tomography (PET) ligand would enable a target occupancy study in vivo and greatly facilitate LRRK2 drug discovery and clinical translation as well as provide a molecular imaging tool for studying physiopathological changes in neurodegenerative diseases. In this work, we present the design and development of compound 8 (PF-06455943) as a promising PET radioligand through a PET-specific structure-activity relationship optimization, followed by comprehensive pharmacology and ADME/neuroPK characterization. Following an efficient 18F-labeling method, we have confirmed high brain penetration of [18F]8 in nonhuman primates (NHPs) and validated its specific binding in vitro by autoradiography in postmortem NHP brain tissues and in vivo by PET imaging studies.

Leveraging Molecular and Immune-Based Therapies in Leptomeningeal Metastases

Leptomeningeal metastases represent an aggressive stage of cancer with few durable treatment options. Improved understanding of cancer biology, neoplastic reliance on oncogenic driver mutations, and complex immune system interactions have resulted in an explosion in cancer-directed therapy in the last two decades to include small molecule inhibitors and immune checkpoint inhibitors. Most of these therapeutics are underexplored in patients with leptomeningeal metastases, limiting extrapolation of extracranial and even intracranial efficacy outcomes to the unique leptomeningeal space. Further confounding our interpretation of drug activity in the leptomeninges is an incomplete understanding of drug penetration through the blood-cerebrospinal fluid barrier of the choroid plexus. Nevertheless, a number of retrospective studies and promising prospective trials provide evidence of leptomeningeal activity of several small molecule and immune checkpoint inhibitors and underscore potential areas of further therapeutic development for patients harboring leptomeningeal disease.

Molecular pathways, resistance mechanisms and targeted interventions in non-small-cell lung cancer

Lung cancer is the leading cause of cancer-related mortality worldwide. The discovery of tyrosine kinase inhibitors effectively targeting EGFR mutations in lung cancer patients in 2004 represented the beginning of the precision medicine era for this refractory disease. This great progress benefits from the identification of driver gene mutations, and after that, conventional and new technologies such as NGS further illustrated part of the complex molecular pathways of NSCLC. More targetable driver gene mutation identification in NSCLC patients greatly promoted the development of targeted therapy and provided great help for patient outcomes including significantly improved survival time and quality of life. Herein, we review the literature and ongoing clinical trials of NSCLC targeted therapy to address the molecular pathways and targeted intervention progress in NSCLC. In addition, the mutations in EGFR gene, ALK rearrangements, and KRAS mutations in the main sections, and the less common molecular alterations in MET, HER2, BRAF, ROS1, RET, and NTRK are discussed. The main resistance mechanisms of each targeted oncogene are highlighted to demonstrate the current dilemma of targeted therapy in NSCLC. Moreover, we discuss potential therapies to overcome the challenges of drug resistance. In this review, we manage to display the current landscape of targetable therapeutic patterns in NSCLC in this era of precision medicine.

Affinity probes based on small-molecule inhibitors for tumor imaging

Methods for molecular imaging of target areas, including optical imaging, radionuclide imaging, magnetic resonance imaging and other imaging technologies, are helpful for the early diagnosis and precise treatment of cancers. In addition to cancer management, small-molecule inhibitors are also used for developing cancer target probes since they act as the tight-binding ligands of overexpressed proteins in cancer cells. This review aims to summarize the structural designs of affinity probes based on small-molecule inhibitors from the aspects of the inhibitor, linker, dye and radionuclide, and discusses the influence of the modification of these structures on affinity and pharmacokinetics. We also present examples of inhibitor affinity probes in clinical applications, and these summaries will provide insights for future research and clinical translations.

Hyperpolarized [1-13C]Pyruvate Magnetic Resonance Spectroscopic Imaging for Evaluation of Early Response to Tyrosine Kinase Inhibition Therapy in Gastric Cancer

PURPOSE

To evaluate the use of hyperpolarized [1-13C]pyruvate magnetic resonance spectroscopic imaging (HP-13C MRSI) for quantitative measurement of early changes in glycolytic metabolism and its ability to predict response to pan-tyrosine kinase inhibitor (Pan-TKI) therapy in gastric cancer (GCa).

PROCEDURES

Pan-TKI afatinib-sensitive NCI-N87 and resistant SNU16 human GCa cells were assessed for GLUT1, hexokinase-II (HKII), lactate dehydrogenase (LDHA), phosphorylated AKT (pAKT), and phosphorylated MAPK (pMAPK) at 0-72 h of treatment with 0.1 μM afatinib. Subcutaneous NCI-N87 tumor-bearing nude mice underwent [18F]FDG PET/MRI and HP-13C MRSI at baseline and 4 days after treatment with afatinib 10 mg/kg/day or vehicle (n = 10/group). Changes in PET and HP-13C MRSI metabolic parameters were compared between the two groups. Imaging findings were correlated with tumor growth and histopathology over 3 weeks of treatment.

RESULTS

In vitro analysis showed a continuous decrease in LDHA, pAKT, and pMAPK in NCI-N87 compared to SNU16 cells within 72 h of treatment with afatinib, without a significant change in GLUT1 and HKII in either cell type. [18F]FDG PET of NCI-N87 tumors showed no significant change in PET measures at baseline and day 4 of treatment in either treatment group (SUVmean day 4/day 0: 2.7 ± 0.42/2.34 ± 0.38, p = 0.57 in the treated group vs. 1.73 ± 0.66/2.24 ± 0.43, p = 0.4 in the control group). HP-13C MRSI demonstrated significantly decreased lactate-to-pyruvate ratio (L/P) in treated tumors (L/P day 4/day 0: 0.83 ± 0.30/1.10 ± 0.20, p = 0.012 vs. 0.94 ± 0.20/0.98 ± 0.30, p = 0.75, in the treated vs. control group, respectively). Response to afatinib was confirmed with decreased tumor size over 3 weeks (11.10 ± 16.50 vs. 293.00 ± 79.30 mm3, p < 0.001, treated group vs. control group, respectively) and histopathologic evaluation.

CONCLUSIONS

HP-13C MRSI is a more representative biomarker of early metabolic changes in response to pan-TKI in GCa than [18F]FDG PET and could be used for early prediction of response to targeted therapies.

Silibinin Suppresses the Hyperlipidemic Effects of the ALK-Tyrosine Kinase Inhibitor Lorlatinib in Hepatic Cells

The third-generation anaplastic lymphoma tyrosine kinase inhibitor (ALK-TKI) lorlatinib has a unique side effect profile that includes hypercholesteremia and hypertriglyceridemia in >80% of lung cancer patients. Here, we tested the hypothesis that lorlatinib might directly promote the accumulation of cholesterol and/or triglycerides in human hepatic cells. We investigated the capacity of the hepatoprotectant silibinin to modify the lipid-modifying activity of lorlatinib. To predict clinically relevant drug−drug interactions if silibinin were used to clinically manage lorlatinib-induced hyperlipidemic effects in hepatic cells, we also explored the capacity of silibinin to interact with and block CYP3A4 activity using in silico computational descriptions and in vitro biochemical assays. A semi-targeted ultrahigh pressure liquid chromatography accurate mass quadrupole time-of-flight mass spectrometry with electrospray ionization (UHPLC-ESI-QTOF-MS/MS)-based lipidomic approach revealed that short-term treatment of hepatic cells with lorlatinib promotes the accumulation of numerous molecular species of cholesteryl esters and triglycerides. Silibinin treatment significantly protected the steady-state lipidome of hepatocytes against the hyperlipidemic actions of lorlatinib. Lipid staining confirmed the ability of lorlatinib to promote neutral lipid overload in hepatocytes upon long-term exposure, which was prevented by co-treatment with silibinin. Computational analyses and cell-free biochemical assays predicted a weak to moderate inhibitory activity of clinically relevant concentrations of silibinin against CYP3A4 when compared with recommended (rosuvastatin) and non-recommended (simvastatin) statins for lorlatinib-associated dyslipidemia. The elevated plasma cholesterol and triglyceride levels in lorlatinib-treated lung cancer patients might involve primary alterations in the hepatic accumulation of lipid intermediates. Silibinin could be clinically explored to reduce the undesirable hyperlipidemic activity of lorlatinib in lung cancer patients.

Lorlatinib as a treatment for ALK-positive lung cancer

Lorlatinib, a third-generation ALK tyrosine kinase inhibitor, has been approved as a treatment for ALK-positive lung cancer. This review provides information regarding the pharmacology and clinical features of lorlatinib, including its efficacy and associated adverse events. Pivotal clinical trials are discussed along with the current status of lorlatinib as a treatment for ALK-positive lung cancer and future therapeutic challenges.

Evaluation of Lorlatinib Cerebrospinal Fluid Concentrations in Relation to Target Concentrations for ALK Inhibition

Lorlatinib is a third‐generation, brain‐penetrant anaplastic lymphoma kinase (ALK) and c‐ros oncogene 1 (ROS1) tyrosine kinase inhibitor (TKI) with robust intracranial activity in patients with ALK‐ or ROS1‐positive non‐small cell lung cancer (NSCLC). Data from the ongoing open‐label, single‐arm, multicenter, phase‐1/2 study of lorlatinib in patients with metastatic ALK‐ or ROS1‐positive NSCLC were used to further investigate the potential brain penetration of lorlatinib. Patients received escalating lorlatinib doses (10–200 mg once daily or 35–100 mg twice daily) or the approved dosing (100 mg daily). Plasma was collected from all patients, and cerebrospinal fluid (CSF) was collected at baseline and during the study from 5 patients with suspected or confirmed leptomeningeal carcinomatosis or carcinomatous meningitis. For those 5 patients, lorlatinib concentrations ranged from 2.64 to 125 ng/mL in the CSF and from 12.7 to 457 ng/mL in the plasma; free plasma concentrations ranged from 4.318 to 155.385 ng/mL. The CSF/free plasma ratio was 0.77 (R² = 0.96 and P < .001). Using a post‐hoc population pharmacokinetic model, the average steady‐state unbound plasma concentration of lorlatinib was derived and the CSF concentration was estimated for all patients. Known minimum efficacy concentrations (C_eff) for wild‐type and mutated (L1196M and G1202R) ALK were used to derive central nervous system (CNS) C_eff. Estimated CNS concentrations exceeded the derived CNS C_eff values in all patients for wild‐type ALK and the ALK L1196M mutation, and in 35.8% of patients for the ALK G1202R mutation. Projected lorlatinib CNS concentrations were consistent with the high intracranial response rates reported in clinical trials and provide further evidence of the potent CNS penetration of lorlatinib.

Automated light-induced synthesis of 89Zr-radiolabeled antibodies for immuno-positron emission tomography

Clinical production of 89Zr-radiolabeled antibodies (89Zr-mAbs) for positron emission tomography imaging relies on the pre-conjugation of desferrioxamine B (DFO) to the purified protein, followed by isolation and characterization of the functionalized intermediate, and then manual radiosynthesis. Although highly successful, this route exposes radiochemists to a potentially large radiation dose and entails several technological and economic hurdles that limit access of 89Zr-mAbs to just a specialist few Nuclear Medicine facilities worldwide. Here, we introduce a fully automated synthesis box that can produce individual doses of 89Zr-mAbs formulated in sterile solution in < 25 min starting from [89Zr(C2O4)4]4- (89Zr-oxalate), our good laboratory practice-compliant photoactivatable desferrioxamine-based chelate (DFO-PEG3-ArN3), and clinical-grade antibodies without the need for pre-purification of protein. The automated steps include neutralization of the 89Zr-oxalate stock, chelate radiolabeling, and light-induced protein conjugation, followed by 89Zr-mAb purification, formulation, and sterile filtration. As proof-of-principle, 89ZrDFO-PEG3-azepin-trastuzumab was synthesized directly from Herceptin in < 25 min with an overall decay-corrected radiochemical yield of 20.1 ± 2.4% (n = 3), a radiochemical purity > 99%, and chemical purity > 99%. The synthesis unit can also produce 89Zr-mAbs via the conventional radiolabeling routes from pre-functionalized DFO-mAbs that are currently used in the clinic. This automated method will improve access to state-of-the-art 89Zr-mAbs at the many Nuclear Medicine and research institutions that require automated devices for radiotracer production.

Recent progress in the application of iodonium ylides in organic synthesis

This review summarizes the recent advances in the synthetic application of iodonium ylides covering 2017 to 2022.

Radiofluorination of non-activated aromatic prosthetic groups for synthesis and evaluation of fluorine-18 labelled ghrelin(1-8) analogues

The growth hormone secretagogue receptor 1a (GHSR) is differentially expressed in various disease states compared to healthy tissues and thus is a target for molecular imaging. The endogenous ligand for the GHSR is ghrelin, a 28 amino acid peptide with a unique octanoyl group on the serine-3 residue. A recently reported ghrelin analogue revealed the successful use of fluorine-containing, polycyclic aromatic groups in place of the octanoyl side chain, thereby providing potential access to new 18F-PET imaging probes. The peptide [Inp1,Dpr3(6-FN),1Nal4,Thr8]ghrelin(1-8) amide (1) showed sub-nanomolar receptor affinity (IC50 = 0.11 nM) toward the GHSR making it the strongest affinity ghrelin analogue reported to date. However, attempts to label such non-activated aromatic groups with fluoride-18 through conventional substitution methods resulted in low radiochemical yields, impractical for use in vivo. Since larger, non-activated aromatic groups appear to be of value for incorporating fluorine into ghrelin(1-8) analogues, an additional peptide bearing a 4'-fluorobiphenyl-4-carboxyl (4'-FBC) group in place of the octanoyl side chain was also of interest. Herein, we describe the radiosynthesis of [Inp1,Dpr3([18F]6-FN),1Nal4,Thr8]ghrelin(1-8) amide ([18F]1) and [Inp1,Dpr3([18F]4'-FBC),1Nal4,Thr8]ghrelin(1-8) amide ([18F]2) using a prosthetic group approach from iodonium ylide precursors as well as initial in vitro and in vivo evaluation of [18F]1 as a potential PET tracer for targeted imaging of the GHSR.

A Comprehensive Analysis of Metabolomics and Transcriptomics Reveals Novel Biomarkers and Mechanistic Insights on Lorlatinib Crosses the Blood-Brain Barrier

Of late, lorlatinib has played an increasingly pivotal role in the treatment of brain metastasis from non-small cell lung cancer. However, its pharmacokinetics in the brain and the mechanism of entry are still controversial. The purpose of this study was to explore the mechanisms of brain penetration by lorlatinib and identify potential biomarkers for the prediction of lorlatinib concentration in the brain. Detection of lorlatinib in lorlatinib-administered mice and control mice was performed using liquid chromatography and mass spectrometry. Metabolomics and transcriptomics were combined to investigate the pathway and relationships between metabolites and genes. Multilayer perceptron was applied to construct an artificial neural network model for prediction of the distribution of lorlatinib in the brain. Nine biomarkers related to lorlatinib concentration in the brain were identified. A metabolite-reaction-enzyme-gene interaction network was built to reveal the mechanism of lorlatinib. A multilayer perceptron model based on the identified biomarkers provides a prediction accuracy rate of greater than 85%. The identified biomarkers and the neural network constructed with these metabolites will be valuable for predicting the concentration of drugs in the brain. The model provides a lorlatinib to treat tumor brain metastases in the clinic.

Translation of 11C-labeled tracer synthesis to a CGMP environment as exemplified by [11C]ER176 for PET imaging of human TSPO

Radiotracers labeled with carbon-11 (t_1/2 = 20.4 min) are widely used with positron emission tomography for biomedical research. Radiotracers must be produced for positron emission tomography studies in humans according to prescribed time schedules while also meeting current good manufacturing practice. Translation of an experimental radiosynthesis to a current good manufacturing practice environment is challenging. Here we exemplify such translation with a protocol for the production of an emerging radiotracer for imaging brain translocator protein 18 kDa, namely [¹¹C]ER176. This radiotracer is produced by rapid conversion of cyclotron-produced [¹¹C]carbon dioxide into [¹¹C]iodomethane, which is then used to treat N-desmethyl-ER176 in the presence of base (^tBuOK) at room temperature for 5 min. [¹¹C]ER176 is separated in high purity by reversed-phase HPLC and formulated for intravenous injection in sterile ethanol-saline. The radiosynthesis is reliable and takes 50 min. Quality control takes another 20 min. All aspects of the protocol, including quality control, are discussed.

An update of new small-molecule anticancer drugs approved from 2015 to 2020

A high incidence of cancer has given rise to the development of more anti-tumor drugs. From 2015 to 2020, fifty-six new small-molecule anticancer drugs, divided into ten categories according to their anti-tumor target activities, have been approved. These include TKIs (30 drugs), MAPK inhibitors (3 drugs), CDK inhibitors (3 drugs), PARP inhibitors (3 drugs), PI3K inhibitors (3 drugs), SMO receptor antagonists (2 drugs), AR antagonists (2 drugs), SSTR inhibitors (2 drugs), IDH inhibitors (2 drugs) and others (6 drugs). Among them, PTK inhibitors (30/56) have led to a paradigm shift in cancer treatment with less toxicity and more potency. Each of their structures, approval statuses, applications, SAR analyses, and original research synthesis routes have been summarized, giving us a more comprehensive map for further efforts to design more specific targeted agents for reducing cancer in the future. We believe this review will help further research of potential antitumor agents in clinical usage.

Brain Penetration of Lorlatinib: Cumulative Incidences of CNS and Non-CNS Progression with Lorlatinib in Patients with Previously Treated ALK-Positive Non-Small-Cell Lung Cancer

Background

Lorlatinib is a potent, third-generation ALK/ROS1 tyrosine kinase inhibitor (TKI) designed to penetrate the blood–brain barrier.

Objective

We report the cumulative incidence of central nervous system (CNS) and non-CNS progression with lorlatinib in patients with ALK-positive non-small-cell lung cancer (NSCLC) previously treated with ALK TKIs.

Patients and methods

In an ongoing phase II study (NCT01970865), 198 patients with ALK-positive NSCLC with ≥ 1 prior ALK TKI were enrolled into expansion cohorts (EXP) based on treatment history. Patients received lorlatinib 100 mg once daily. Patients were analyzed for progressive disease, categorized as CNS or non-CNS progression, by independent central review. Cumulative incidence probabilities were calculated adopting a competing risks approach.

Results

Fifty-nine patients received crizotinib as their only prior ALK TKI (EXP2–3A); cumulative incidence rates (CIRs) of CNS and non-CNS progression were both 22% at 12 months in patients with baseline CNS metastases (n = 37), and CIR of non-CNS progression at 12 months was higher versus that for CNS progression in patients without baseline CNS metastases [43% vs. 9% (n = 22)]. In patients who received ≥ 1 prior second-generation ALK TKI [EXP3B–5 (n = 139)], CIR of non-CNS progression at 12 months was higher versus that for CNS progression in patients both with and without baseline CNS metastases (35% vs. 23% (n = 94) and 55% vs. 12% (n = 45), respectively).

Conclusions

Lorlatinib showed substantial intracranial activity in patients with pretreated ALK-positive NSCLC, with or without baseline CNS metastases, whose disease progressed on crizotinib or second-generation ALK TKIs.

ClinicalTrials.gov identifier

NCT01970865.

Electronic supplementary material

The online version of this article (10.1007/s11523-020-00702-4) contains supplementary material, which is available to authorized users.

Brigatinib and lorlatinib: their effect on ALK inhibitors in NSCLC focusing on resistant mutations and central nervous system metastases

Major issues in anaplastic lymphoma kinase-positive non-small cell lung carcinoma are acquired resistance against anaplastic lymphoma kinase inhibitors and control of central nervous system metastasis. The development of these inhibitors has changed therapeutic strategy in patients with advanced anaplastic lymphoma kinase-positive non-small cell lung carcinoma. Brigatinib and lorlatinib were designed to penetrate the blood-brain barrier and to inhibit resistant mutations against anaplastic lymphoma kinase inhibitors. We review the clinical data supporting treatment of advanced anaplastic lymphoma kinase-positive non-small cell lung carcinoma with brigatinib and lorlatinib. Brigatinib has shown promising antitumour activity, including substantial activity against central nervous system metastases, in crizotinib-treated (ALTA trial) patients and crizotinib-naïve (ALTA-1L trial) patients with anaplastic lymphoma kinase-positive non-small cell lung carcinoma. In addition, brigatinib improved progression-free survival compared with crizotinib in anaplastic lymphoma kinase inhibitor-naïve patients with anaplastic lymphoma kinase-positive non-small cell lung carcinoma. Lorlatinib has demonstrated clinical antitumour activity against both intracranial and extracranial lesions in patients with anaplastic lymphoma kinase- or c-ros oncogene 1 (ROS1)-positive non-small cell lung carcinoma. Ongoing trials and further studies of these agents' biological and clinical properties would provide insight into the optimal therapeutic strategy for administering them to achieve the best survival benefit.

The Unique Pharmacometrics of Small Molecule Therapeutic Drug Tracer Imaging for Clinical Oncology

Simple Summary

New clinical radiology scans using trace amounts of therapeutic cancer drugs labeled with radioisotope injected into patients can provide oncologists with fundamentally unique insights about drug delivery to tumors. This new application of radiology aims to improve how cancer drugs are used, towards improving patient outcomes. The article reviews published clinical research in this important new field.

Abstract

Translational development of radiolabeled analogues or isotopologues of small molecule therapeutic drugs as clinical imaging biomarkers for optimizing patient outcomes in targeted cancer therapy aims to address an urgent and recurring clinical need in therapeutic cancer drug development: drug- and target-specific biomarker assays that can optimize patient selection, dosing strategy, and response assessment. Imaging the in vivo tumor pharmacokinetics and biomolecular pharmacodynamics of small molecule cancer drugs offers patient- and tumor-specific data which are not available from other pharmacometric modalities. This review article examines clinical research with a growing pharmacopoeia of investigational small molecule cancer drug tracers.

Macrocyclization via C-H functionalization: a new paradigm in macrocycle synthesis

The growing emphasis on macrocycles in engaging difficult therapeutic targets such as protein-protein interactions and GPCRs via preferential adaptation of bioactive and cell penetrating conformations has provided impetus to the search for de novo macrocyclization strategies that are efficient, chemically robust and amenable to diversity creation. An emerging macrocyclization paradigm based on the C-H activation logic, of particular promise in the macrocyclization of complex peptides, has added a new dimension to this pursuit, enabling efficacious access to macrocycles of various sizes and topologies with high atom and step economy. Significant achievements in macrocyclization methodologies and their applications in the synthesis of bioactive natural products and drug-like molecules, employing strategic variations of C-H activation are captured in this review. It is expected that this timely account will foster interest in newer ways of macrocycle construction among practitioners of organic synthesis and chemical biology to advance the field.

A user's guide to lorlatinib

Rearrangements of the ALK gene are found in approximately 5% of non-small-cell lung cancer. It is of particular importance to test for this rearrangement in patients with metastatic lung adenocarcinoma because these tumors are highly sensitive to therapy with ALK-targeted inhibitors. Lorlatinib is a reversible potent third generation tyrosine kinase inhibitor that is highly selective and targets ALK and ROS1. It was developed to target resistant ALK mutants including the most common G1202R. Lorlatinib has excellent central nervous system (CNS) penetration and its efficacy has also been demonstrated even in patients with intracranial metastases after progression on second generation ALK inhibitors. Potential toxicities include neurocognitive effects and hyperlipidemia. "A User's Guide to Lorlatinib" reviews the mechanism of action, pharmacology and clinical trial data. Also covering the management of adverse events, this "guide" has been prepared to be a practical reference tool to both clinicians and basic researchers.

Lorlatinib for the treatment of ALK-positive metastatic non-small cell lung cancer

Introduction: The treatment of lung cancer has changed dramatically with the development of tyrosine kinase inhibitors (TKIs) that target sensitizing somatic (gene) activations including anaplastic lymphoma kinase (ALK)-rearrangements. Despite remarkable initial responses, patients develop progressive disease via various resistance mechanisms, some of which are ALK dependent. Various next-generation ALK TKIs have been developed to improve on central nervous system (CNS) activity and also target the multitude of acquired resistance mechanisms. Of these, lorlatinib has the greatest spectrum of clinical activity against multiple ALK resistance mutations and has also demonstrated promising efficacy in patients with known brain metastases.Areas covered: We discuss the structure, pharmacology and efficacy of lorlatinib and also provide future perspectives in the management of patients with ALK-rearranged non-small cell lung cancer (NSCLC).Expert opinion: Patients invariably develop resistance during treatment with lorlatinib. Unique combinations of ALK resistance mutations may confer sensitivity to alternate ALK TKIs. There is a move toward individualized biomarker-driven treatment strategies to identify the select group of candidates that can benefit from existing therapies.

New generation anaplastic lymphoma kinase inhibitors

Anaplastic lymphoma kinase (ALK) gene translocations are pro-tumoral driver alterations that encompass 3-7% of non-squamous non-small cell lung cancer (NSCLC) with specific, clinic and histologic features. The therapeutic strategy depends on anti-ALK tyrosine kinase inhibitors (TKIs) of which crizotinib was the first approved for clinical use. Despite its use improved significantly progression-free survival, overall response rate and duration of response of this illness, after a median period of 10.9 months all patients progress due to the development of acquired resistance mutations in the ALK tyrosine kinase domain in approximately one third of patients. Moreover, 60-90% of patients treated with crizotinib has a progression in the central nervous system (CNS) in absence of extracranial worsening of the disease. This is primarily attributed to poor CNS penetration by crizotinib as many pre-clinical and clinical models suggest. For instance, in order to overtake acquired resistance to crizotinib, prolong the control of the disease and manage CNS localizations, several II and III generation TKIs have been developed. Some of them were approved after the failure of crizotinib (ceritinib, alectinib, brigatinib and lorlatinib) and in first line setting (ceritinib, alectinib and brigatinib) while others are still under evaluation for TKI-naive patients such as lorlatinib, ensartinib and entrectinib. In this review we will discuss the most recent results of new TKIs in order to describe a fast growing therapeutic landscape in this setting.

Novel derivatives of anaplastic lymphoma kinase inhibitors: Synthesis, radiolabeling, and preliminary biological studies of fluoroethyl analogues of crizotinib, alectinib, and ceritinib

Anaplastic lymphoma kinase (ALK), an oncogenic receptor tyrosine kinase, is a therapeutic target in various cancers, including non-small cell lung cancer. Although several ALK inhibitors, including crizotinib, ceritinib, and alectinib, are approved for cancer treatment, their long-term benefit is often limited by the cancer's acquisition of resistance owing to secondary point mutations in ALK. Importantly, some ALK inhibitors cannot cross the blood-brain barrier (BBB) and thus have little or no efficacy against brain metastases. The introduction of a lipophilic moiety, such as a fluoroethyl group may improve the drug's BBB penetration. Herein, we report the synthesis of fluoroethyl analogues of crizotinib 1, alectinib 4, and ceritinib 9, and their radiolabeling with ¹⁸F for pharmacokinetic studies. The fluoroethyl derivatives and their radioactive analogues were obtained in good yields with high purity and good molar activity. A cytotoxicity screen in ALK-expressing H2228 lung cancer cells showed that the analogues had up to nanomolar potency and the addition of the fluorinated moiety had minimal impact overall on the potency of the original drugs. Positron emission tomography in healthy mice showed that the analogues had enhanced BBB penetration, suggesting that they have therapeutic potential against central nervous system metastases.

Facile 18F labeling of non-activated arenes via a spirocyclic iodonium(III) ylide method and its application in the synthesis of the mGluR5 PET radiopharmaceutical [18F]FPEB

Non-activated (electron-rich and/or sterically hindered) arenes are prevalent chemical scaffolds in pharmaceuticals and positron emission tomography (PET) diagnostics. Despite substantial efforts to develop a general method to introduce ¹⁸F into these moieties for molecular imaging by PET, there is an urgent and unmet need for novel radiofluorination strategies that result in sufficiently labeled tracers to enable human imaging. Herein, we describe an efficient method that relies on spirocyclic iodonium ylide (SCIDY) precursors for one-step and regioselective radiofluorination, as well as proof-of-concept translation to the radiosynthesis of a clinically useful PET tracer, 3-[¹⁸F]fluoro-5-[(pyridin-3-yl)ethynyl] benzonitrile ([¹⁸F]FPEB). The protocol begins with the preparation of a SCIDY precursor for FPEB, followed by radiosynthesis of [¹⁸F]FPEB, by either manual operation or an automated synthesis module. [¹⁸F]FPEB can be obtained in quantities >7.4 GBq (200 mCi), ready for injection (20 ± 5%, non-decay corrected), and has excellent chemical and radiochemical purity (>98%) as well as high molar activity (666 ± 51.8 GBq/μmol; 18 ± 1.4 Ci/μmol). The total time for the synthesis and purification of the corresponding labeling SCIDY precursor is 10 h. The subsequent radionuclide production, experimental setup, ¹⁸F labeling, and formulation of a product that is ready for injection require 2 h.

Chemistry for Positron Emission Tomography: Recent Advances in 11 C-, 18 F-, 13 N-, and 15 O-Labeling Reactions

Positron emission tomography (PET) is a molecular imaging technology that provides quantitative information about function and metabolism in biological processes in vivo for disease diagnosis and therapy assessment. The broad application and rapid advances of PET has led to an increased demand for new radiochemical methods to synthesize highly specific molecules bearing positron-emitting radionuclides. This Review provides an overview of commonly used labeling reactions through examples of clinically relevant PET tracers and highlights the most recent developments and breakthroughs over the past decade, with a focus on 11 C, 18 F, 13 N, and 15 O.

Brigatinib for ALK-positive metastatic non-small-cell lung cancer: design, development and place in therapy

Despite the benefits of first and second generation anaplastic lymphoma kinase (ALK) inhibitors in the management of ALK-rearranged advanced non-small-cell lung cancer (NSCLC), the development of acquired resistance poses an ongoing dilemma. Brigatinib has demonstrated a wider spectrum of preclinical activity against crizotinib-resistant ALK mutant advanced NSCLC. The current review narrates a brief history of tyrosine kinases, the development and clinical background of brigatinib (including its pharmacology and molecular structure) and its use in ALK-positive NSCLC.

Spotlight on lorlatinib and its potential in the treatment of NSCLC: the evidence to date

The identification of anaplastic lymphoma kinase (ALK), an oncogenetic driver mutation, in lung cancer has paved the way for a new era in the treatment of non-small cell lung cancer (NSCLC). Targeting ALK using tyrosine kinase inhibitors (TKI) has dramatically improved the prognosis of patients with ALK-rearranged NSCLC. However, most patients relapse on ALK-TKI therapy within a few years because of acquired resistance. One mechanism of acquiring resistance is a second mutation on the ALK gene, and the representative mutation is L1996M in the gatekeeper residue. In particular, the solvent-front ALK G1202R mutation is the common cause of resistance against first- and second-generation ALK-TKIs. Another major concern regarding ALK-TKI is metastasis to the central nervous system, commonly observed in patients relapsing after ALK-TKI therapy. The next-generation ALK inhibitor lorlatinib (PF-06463922) has therefore been developed to inhibit resistant ALK mutations, including ALK G1202R, and to penetrate the blood–brain barrier. In a Phase I/II trial, the safety and efficacy of lorlatinib were demonstrated in patients with advanced ALK-positive NSCLC, most of whom had central nervous system metastases and had previous ALK-TKI treatment. In this review, we discuss the structure, pharmacodynamics, and pharmacokinetics of lorlatinib and compare its characteristics with those of other ALK inhibitors. Furthermore, clinical trials for lorlatinib are summarized, and future perspectives in the management of patients with ALK-rearranged NSCLC are discussed.

Brain Penetration of the ROS1/ALK Inhibitor Lorlatinib Confirmed by PET

The Massachusetts General Hospital Radiochemistry Program, in collaboration with Pfizer, has developed unique ¹¹C and ¹⁸F-labeling strategies to synthesize isotopologs of lorlatinib (PF-06463922) which is undergoing phase III clinical trial investigations for treatment of non-small-cell lung cancers with specific molecular alterations. A major goal in cancer therapeutics is to measure the concentrations of this drug in the brain metastases of patients with lung cancer, and penetration of the blood–brain barrier is important for optimal therapeutic outcomes. Our recent publication in Nature Communications employed radiolabeled lorlatinib and positron emission tomography (PET) studies in preclinical models including nonhuman primates (NHPs) that demonstrated high brain permeability of this compound. Our future work with radiolabeled lorlatinib will include advanced PET evaluations in rodent tumor models and normal NHPs with the goal of clinical translation.

First-line alectinib for ALK-positive lung cancer: is there room for further improvement?

In the present editorial we describe the therapeutic achievements in the treatment of patients with anaplastic lymphoma kinase (ALK)-positive advanced non-small cell lung cancer (NSCLC). We focus on the major breakthroughs we have been witnessing in this context, from the introduction of crizotinib as the first approved targeted drug, to the meaningful improvement in terms of clinical benefit that alectinib, a second generation ALK-inhibitor, has recently provided over crizotinib. Finally, we address major trends of clinical research in this setting, and whether this might translate into further clinical improvement in the near future.

Systemic Therapy of Lung Cancer CNS Metastases Using Molecularly Targeted Agents and Immune Checkpoint Inhibitors

Central nervous system (CNS) metastases most commonly arise from lung cancer, with the majority of patients affected during their disease course. The prognosis for patients with untreated brain metastases is poor, with surgical resection and/or radiotherapy as classic therapeutic options. However, the value of systemic therapy in the management of CNS metastases from lung cancer is growing. Novel targeted agents for the treatment of non-small cell lung cancer (NSCLC) have demonstrated activity in treating patients with CNS involvement, and are potential alternatives to radiation and surgery. These agents include anaplastic lymphoma kinase (ALK) inhibitors such as alectinib, crizotinib, ceritinib, lorlatinib, and others; epidermal growth factor receptor (EGFR) tyrosine kinase inhibitors, including the recently developed third-generation inhibitor osimertinib, and even immune checkpoint inhibitors such as nivolumab, pembrolizumab, and atezolizumab. This review summarizes current activity of systemic agents in the management of CNS metastases from NSCLC, as well as potential mechanisms of action of these small and large molecules.

Imaging techniques to study drug transporter function in vivo

Transporter systems involved in the permeation of drugs and solutes across biological membranes are recognized as key determinants of pharmacokinetics. Typically, the action of membrane transporters on drug exposure to tissues in living organisms is inferred from invasive procedures, which cannot be applied in humans. In recent years, imaging methods have greatly progressed in terms of instruments, synthesis of novel imaging probes as well as tools for data analysis. Imaging allows pharmacokinetic parameters in different tissues and organs to be obtained in a non-invasive or minimally invasive way. The aim of this overview is to summarize the current status in the field of molecular imaging of drug transporters. The overview is focused on human studies, both for the characterization of transport systems for imaging agents as well as for the determination of drug pharmacokinetics, and makes reference to animal studies where necessary. We conclude that despite certain methodological limitations, imaging has a great potential to study transporters at work in humans and that imaging will become an important tool, not only in drug development but also in medicine. Imaging allows the mechanistic aspects of transport proteins to be studied, as well as elucidating the influence of genetic background, pathophysiological states and drug-drug interactions on the function of transporters involved in the disposition of drugs.

Hypervalent aryliodine compounds as precursors for radiofluorination

Over the last 2 decades or so, hypervalent iodine compounds, such as diaryliodonium salts and aryliodonium ylides, have emerged as useful precursors for labeling homoarenes and heteroarenes with no-carrier-added cyclotron-produced [¹⁸ F]fluoride ion (t_1/2  = 109.8 min). They permit rapid and effective radiofluorination at electron-rich as well as electron-deficient aryl rings, and often with unrestricted choice of ring position. Consequently, hypervalent aryliodine compounds have found special utility as precursors to various small-molecule ¹⁸ F-labeling synthons and to many radiotracers for biomedical imaging with positron emission tomography. This review summarizes this advance in radiofluorination chemistry, with emphasis on precursor synthesis, radiofluorination mechanism, method scope, and method application.

Recent Advances in 18F Radiochemistry: A Focus on B-18F, Si-18F, Al-18F, and C-18F Radiofluorination via Spirocyclic Iodonium Ylides

Straightforward radiosynthesis protocols for ¹⁸F-labeled radiopharmaceuticals are an indispensable but often overlooked prerequisite to successfully perform molecular imaging studies in vivo by PET. In recent years, thanks to the expansion of the ¹⁸F chemical toolbox, structurally diverse and novel clinically relevant radiopharmaceuticals have been synthesized with both high efficiency and ready implementation. This article provides an overview of recent ¹⁸F-labeling methodologies, specifically for B-¹⁸F, Si-¹⁸F, Al-¹⁸F, and iodine (III)-mediated radiofluorination via the spirocyclic iodonium ylide technology.

Discovery of PET radiopharmaceuticals at the academia-industry interface

Project-specific collaborations between academia and pharmaceutical partners are a growing phenomenon within molecular imaging and in particular in the positron emission tomography (PET) radiopharmaceutical community. This cultural shift can be attributed in part to decreased public funding in academia in conjunction with the increased reliance on outsourcing of chemistry, radiochemistry, pharmacology and molecular imaging studies by the pharmaceutical industry. This account highlights some of our personal experiences working with industrial partners to develop new PET radiochemistry methodologies for drug discovery and neuro-PET research studies. These symbiotic academic-industrial partnerships have not only led to novel radiotracers for new targets but also to the application of new carbon-11 and fluorine-18 labeling methodologies and technologies to label previously unprecedented compounds for in vivo evaluations.