Synthesis of [11C]HG-10-102-01 as a new potential PET agent for imaging of LRRK2 enzyme in Parkinson’s disease

Preclinical Evaluation of Novel Positron Emission Tomography (PET) Probes for Imaging Leucine-Rich Repeat Kinase 2 (LRRK2)

Parkinson's disease (PD) is one of the most highly debilitating neurodegenerative disorders, which affects millions of people worldwide, and leucine-rich repeat kinase 2 (LRRK2) mutations have been involved in the pathogenesis of PD. Developing a potent LRRK2 positron emission tomography (PET) tracer would allow for in vivo visualization of LRRK2 distribution and expression in PD patients. In this work, we present the facile synthesis of two potent and selective LRRK2 radioligands [11C]3 ([11C]PF-06447475) and [18F]4 ([18F]PF-06455943). Both radioligands exhibited favorable brain uptake and specific bindings in rodent autoradiography and PET imaging studies. More importantly, [18F]4 demonstrated significantly higher brain uptake in the transgenic LRRK2-G2019S mutant and lipopolysaccharide (LPS)-injected mouse models. This work may serve as a roadmap for the future design of potent LRRK2 PET tracers.

Exploring natural compound, Panicutine as leucine-rich repeat kinase 2 inhibitor against Parkinson's disease: a structure-guided approach

Leucine-rich repeat kinase 2 (LRRK2) is a promising drug target for the therapeutic management of Parkinson's disease (PD) and other neurodegenerative disorders. LRRK2 inhibitors have the potential to modulate neuroinflammation, reduce alpha-synuclein aggregation and improve motor symptoms in PD patients. Although LRRK2 inhibitors are still in the early stages of clinical development, the identification of potent and selective inhibitors through structure-guided approaches provides a promising avenue for the development of effective therapies for PD and other neurodegenerative disorders. In this study, natural compounds from the IMPPAT database were screened using a state-of-the-art computational virtual screening approach to identify potential inhibitors of LRRK2. We carried out a docking screening on a library of natural compounds and identified a few compounds with strong binding affinity, docking score and specificity towards LRRK2 as the top hits. These hits were then subjected to further analysis based on multiple parameters for the Pan-assay interference compounds and their physicochemical and pharmacokinetics evaluation followed by a detailed interaction analysis. After careful evaluation, one natural compound, Panicutine, was identified as a promising candidate for LRRK2 due to its significant affinity and specificity towards the LRRK2 binding pocket. Additionally, it exhibited drug-like properties with blood-brain barrier permeability as determined by ADMET properties. To gain a deeper understanding of the stability and conformational changes of the LRRK2-ligand complex, MD simulations were conducted for 100 nanoseconds under explicit solvent conditions followed by principal component analysis and free energy dynamics. The simulation results demonstrated that the LRRK2-Panicutine complex remained stable throughout the simulation trajectories. Based on these findings, it is concluded that Panicutine has the potential to act as a LRRK2 inhibitor against PD and other neurodegenerative disorders.Communicated by Ramaswamy H. Sarma.

PET imaging in animal models of Parkinson's disease

Alpha-synucleinopathies, such as Parkinson's disease, dementia with Lewy bodies and multiple system atrophy, are characterized by aberrant accumulation of alpha-synuclein and synaptic dysfunction leading to motor and cognitive deficits. Animal models of alpha-synucleinopathy have greatly facilitated the mechanistic understanding of the disease and the development of therapeutics. Various transgenic, alpha-synuclein fibril-injected, and toxin-injected animal models of Parkinson's disease and multiple system atrophy that recapitulate the disease pathology have been developed and widely used. Recent advances in positron emission tomography have allowed the noninvasive visualization of molecular alterations, underpinning behavioral dysfunctions in the brains of animal models and the longitudinal monitoring of treatment effects. Imaging studies in these disease animal models have employed multi-tracer PET designs to reveal dopaminergic deficits together with other molecular alterations. This review focuses on the development of new positron emission tomography tracers and studies of alpha-synuclein, synaptic vesicle glycoprotein 2A neurotransmitter receptor deficits such as dopaminergic receptor, dopaminergic transporter, serotonergic receptor, vesicular monoamine transporter 2, hypometabolism, neuroinflammation, mitochondrial dysfunction and leucine rich repeat kinase 2 in animal models of Parkinson's disease. The outstanding challenges and emerging applications are outlined, such as investigating the gut-brain-axis by using positron emission tomography in animal models, and provide a future outlook.

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.

Evaluation of [18F]PF-06455943 as a Potential LRRK2 PET Imaging Agent in the Brain of Nonhuman Primates

Mutations in the leucine-rich repeat kinase 2 (LRRK2) gene are the common causes of inherited Parkinson's disease (PD) and emerged as a causative PD gene. Particularly, LRRK2-Gly2019Ser mutation was reported to alter the early phase of neuronal differentiation, increasing cell death. Selective inhibitors of LRRK2 kinase activity were considered as a promising therapeutic target for PD treatment. However, the development of effective brain-penetrant LRRK2 inhibitors remains challenging. Recently, we have developed a novel positron emission tomography (PET) radioligand for LRRK2 imaging and demonstrated preferable tracer properties in rodents. Herein, we evaluate [18F]PF-06455943 quantification methods in the nonhuman primate (NHP) brain using full kinetic modeling with radiometabolite-corrected arterial blood samples, and homologous blocking with two doses (0.1 and 0.3 mg/kg). Kinetic analysis results demonstrated that a two-tissue compartmental model and a Logan graphical analysis are appropriate for [18F]PF-06455943 PET quantification. In addition, we observed that total distribution volume (VT) values can be reliably estimated with as short as a 30 min scan duration. Homologous blocking studies confirmed the specific binding of [18F]PF-06455943 and revealed that the nonradioactive mass of PF-06455943 achieved 45-55% of VT displacement in the whole brain. This work supports the translation of [18F]PF-06455943 PET imaging for the human brain and target occupancy studies.

The role of neuroimaging in Parkinson's disease

Parkinson's disease (PD) is a neurodegenerative disorder that affects millions of people worldwide. Two hallmarks of PD are the accumulation of alpha-synuclein and the loss of dopaminergic neurons in the brain. There is no cure for PD, and all existing treatments focus on alleviating the symptoms. PD diagnosis is also based on the symptoms, such as abnormalities of movement, mood, and cognition observed in the patients. Molecular imaging methods such as magnetic resonance imaging (MRI), single-photon emission computed tomography (SPECT), and positron emission tomography (PET) can detect objective alterations in the neurochemical machinery of the brain and help diagnose and study neurodegenerative diseases. This review addresses the application of functional MRI, PET, and SPECT in PD patients. We provide an overview of the imaging targets, discuss the rationale behind target selection, the agents (tracers) with which the imaging can be performed, and the main findings regarding each target's state in PD. Molecular imaging has proven itself effective in supporting clinical diagnosis of PD and has helped reveal that PD is a heterogeneous disorder, which has important implications for the development of future therapies. However, the application of molecular imaging for early diagnosis of PD or for differentiation between PD and atypical parkinsonisms has remained challenging. The final section of the review is dedicated to new imaging targets with which one can detect the PD-related pathological changes upstream from dopaminergic degeneration. The foremost of those targets is alpha-synuclein. We discuss the progress of tracer development achieved so far and challenges on the path toward alpha-synuclein imaging in humans.

18F-Labelled pyrrolopyrimidines reveal brain leucine-rich repeat kinase 2 expression implicated in Parkinson's disease

¹⁸F-Labelled pyrrolopyrimidines were synthesized and evaluated as positron emission tomography (PET) probes to determine leucine-rich repeat kinase 2 (LRRK2) expression in the brain. With pyrrolopyrimidine derivative PF-06447475 as the lead compound, two in vivo-stable ¹⁸F-labelled pyrrolopyrimidines ([¹⁸F]1 and [¹⁸F]2) were synthesized automatically at radiochemical yields 8-10% (non-decay-corrected) with molar activities of 0.95 and 0.5 GBq/μmol, respectively. The measured K_d of 6.90 nM for 1 and 14.27 nM for 2 demonstrated high affinities for LRRK2. The LRRK2 G2019S mice had higher uptakes (P < 0.01) of [¹⁸F]1 in the olfactory bulb, striatum, and hippocampus than WT mice during microPET/CT imaging, consistent with immunohistology results of LRRK2 distribution. [¹¹C]CFT microPET/CT imaging demonstrated a lower expression of dopamine transporter in LRRK2 G2019S mice. Parkinson's disease-like deficits in dopamine transporter synthesis and cognitive declines were noticed along with LRRK2 expression increase in the olfactory bulb, striatum, and hippocampus. Therefore, ¹⁸F-labelled pyrrolopyrimidines can reflect real-time LRRK2 expression changes implicated in Parkinson's disease, which paves the way for LRRK2-related neurodegenerative precise therapy.

The Current State-of-the Art of LRRK2-Based Biomarker Assay Development in Parkinson's Disease

Evidence is mounting that LRRK2 function, particularly its kinase activity, is elevated in multiple forms of Parkinson's disease, both idiopathic as well as familial forms linked to mutations in the LRRK2 gene. However, sensitive quantitative markers of LRRK2 activation in clinical samples remain at the early stages of development. There are several measures of LRRK2 activity that could potentially be used in longitudinal studies of disease progression, as inclusion/exclusion criteria for clinical trials, to predict response to therapy, or as markers of target engagement. Among these are levels of LRRK2, phosphorylation of LRRK2 itself, either by other kinases or via auto-phosphorylation, its in vitro kinase activity, or phosphorylation of downstream substrates. This is advantageous on many levels, in that multiple indices of elevated kinase activity clearly strengthen the rationale for targeting this kinase with novel therapeutic candidates, and provide alternate markers of activation in certain tissues or biofluids for which specific measures are not detectable. However, this can also complicate interpretation of findings from different studies using disparate measures. In this review we discuss the current state of LRRK2-focused biomarkers, the advantages and disadvantages of the current pallet of outcome measures, the gaps that need to be addressed, and the priorities that the field has defined.

Radiosynthesis and evaluation of 4-(6-[18F]Fluoro-4-(5-isopropoxy-1H-indazol-3-yl)pyridin-2-yl)morpholine as a novel radiotracer candidate targeting leucine-rich repeat kinase 2

Mutations that increase leucine-rich repeat kinase 2 (LRRK2) activity in the brain are associated with Parkinson's disease. Here, we synthesized a novel compound 4-(6-fluoro-4-(5-isopropoxy-1H-indazol-3-yl)pyridin-2-yl)morpholine (FIPM) and labeled it with fluorine-18 (¹⁸F), to develop a positron emission tomography (PET) tracer for in vivo visualization of LRRK2 in the brain. FIPM showed high in vitro binding affinity for LRRK2 (IC₅₀ = 8.0 nM). [¹⁸F]FIPM was prepared in 5% radiochemical yield (n = 5), by inserting ¹⁸F into a pyridine ring, followed by removal of the protecting group. After HPLC separation and formulation, [¹⁸F]FIPM was acquired with >97% radiochemical purity and 103-300 GBq μmol^-1 of molar activity at the end of radiosynthesis. Biodistribution and small-animal PET studies in mice indicated a low in vivo specific binding of [¹⁸F]FIPM. While [¹⁸F]FIPM presented limited potential as an in vivo PET tracer for LRRK2, we suggested that it can be used as a lead compound for developing new radiotracers with improved in vivo brain properties.

LRRK2 in Parkinson disease: challenges of clinical trials

One of the most common monogenic forms of Parkinson disease (PD) is caused by mutations in the LRRK2 gene that encodes leucine-rich repeat kinase 2 (LRRK2). LRRK2 mutations, and particularly the most common mutation Gly2019Ser, are observed in patients with autosomal dominant PD and in those with apparent sporadic PD, who are clinically indistinguishable from those with idiopathic PD. The discoveries that pathogenic mutations in the LRRK2 gene increase LRRK2 kinase activity and that small-molecule LRRK2 kinase inhibitors can be neuroprotective in preclinical models of PD have placed LRRK2 at the centre of disease modification efforts in PD. Recent investigations also suggest that LRRK2 has a role in the pathogenesis of idiopathic PD and that LRRK2 therapies might, therefore, be beneficial in this common subtype of PD. In this Review, we describe the characteristics of LRRK2-associated PD that are most relevant to the development of LRRK2-targeted therapies and the design and implementation of clinical trials. We highlight strategies for correcting the effects of mutations in the LRRK2 gene, focusing on how to identify which patients are the optimal candidates and how to decide on the timing of such trials. In addition, we discuss challenges in implementing trials of disease-modifying treatment in people who carry LRRK2 mutations.

Synthesis and Preliminary Evaluation of [11 C]GNE-1023 as a Potent PET Probe for Imaging Leucine-Rich Repeat Kinase 2 (LRRK2) in Parkinson's Disease

Leucine-rich repeat kinase 2 (LRRK2) is a large protein involved in the pathogenesis of Parkinson's disease (PD). It has been demonstrated that PD is mainly conferred by LRRK2 mutations that bring about increased kinase activity. As a consequence, selective inhibition of LRRK2 may help to recover the normal functions of LRRK2, thereby serving as a promising alternative therapeutic target for PD treatment. The mapping of LRRK2 by positron emission tomography (PET) studies allows a thorough understanding of PD and other LRRK2-related disorders; it also helps to validate and translate novel LRRK2 inhibitors. However, no LRRK2 PET probes have yet been reported in the primary literature. Herein we present a facile synthesis and preliminary evaluation of [¹¹ C]GNE-1023 as a novel potent PET probe for LRRK2 imaging in PD. [¹¹ C]GNE-1023 was synthesized in good radiochemical yield (10 % non-decay-corrected RCY), excellent radiochemical purity (>99 %), and high molar activity (>37 GBq μmol^-1 ). Excellent in vitro binding specificity of [¹¹ C]GNE-1023 toward LRRK2 was demonstrated in cross-species studies, including rat and nonhuman primate brain tissues by autoradiography experiments. Subsequent whole-body biodistribution studies indicated limited brain uptake and urinary and hepatobiliary elimination of this radioligand. This study may pave the way for further development of a new generation of LRRK2 PET probes.

Synthesis of carbon-11-labeled CK1 inhibitors as new potential PET radiotracers for imaging of Alzheimer's disease

The reference standards methyl 3-((2,2-difluoro-5H-[1,3]dioxolo[4',5':4,5]benzo[1,2-d]imidazol-6-yl)carbamoyl)benzoate (5a) and N-(2,2-difluoro-5H-[1,3]dioxolo[4',5':4,5]benzo[1,2-d]imidazol-6-yl)-3-methoxybenzamide (5c), and their corresponding desmethylated precursors 3-((2,2-difluoro-5H-[1,3]dioxolo[4',5':4,5]benzo[1,2-d]imidazol-6-yl)carbamoyl)benzoic acid (6a) and N-(2,2-difluoro-5H-[1,3]dioxolo[4',5':4,5]benzo[1,2-d]imidazol-6-yl)-3-hydroxybenzamide (6b), were synthesized from 5-amino-2,2-difluoro-1,3-benzodioxole and 3-substituted benzoic acids in 5 and 6 steps with 33% and 11%, 30% and 7% overall chemical yield, respectively. Carbon-11-labeled casein kinase 1 (CK1) inhibitors, [¹¹C]methyl 3-((2,2-difluoro-5H-[1,3]dioxolo[4',5':4,5]benzo[1,2-d]imidazol-6-yl)carbamoyl)benzoate ([¹¹C]5a) and N-(2,2-difluoro-5H-[1,3]dioxolo[4',5':4,5]benzo[1,2-d]imidazol-6-yl)-3-[¹¹C]methoxybenzamide ([¹¹C]5c), were prepared from their O-desmethylated precursor 6a or 6b with [¹¹C]CH₃OTf through O-[¹¹C]methylation and isolated by HPLC combined with SPE in 40-45% radiochemical yield, based on [¹¹C]CO₂ and decay corrected to end of bombardment (EOB). The radiochemical purity was >99%, and the molar activity (MA) at EOB was 370-740 GBq/μmol with a total synthesis time of ∼40-min from EOB.

Synthesis and preliminary biological evaluation of radiolabeled 5-BDBD analogs as new candidate PET radioligands for P2X4 receptor

P2X4 receptor has become an interesting molecular target for treatment and PET imaging of neuroinflammation and associated brain diseases such as Alzheimer's disease. This study reports the first design, synthesis, radiolabeling and biological evaluation of new candidate PET P2X4 receptor radioligands using 5-BDBD, a specific P2X4 receptor antagonist, as a scaffold. 5-(3-Hydroxyphenyl)-1-[¹¹C]methyl-1,3-dihydro-2H-benzofuro[3,2-e][1,4]diazepin-2-one (N-[¹¹C]Me-5-BDBD analog, [¹¹C]9) and 5-(3-Bromophenyl)-1-[¹¹C]methyl-1,3-dihydro-2H-benzofuro[3,2-e][1,4]diazepin-2-one (N-[¹¹C]Me-5-BDBD, [¹¹C]8c) were prepared from their corresponding desmethylated precursors with [¹¹C]CH₃OTf through N-[¹¹C]methylation and isolated by HPLC combined with SPE in 30-50% decay corrected radiochemical yields with 370-1110GBq/µmol specific activity at EOB. 5-(3-[¹⁸F]Fluorophenyl)-1,3-dihydro-2H-benzofuro[3,2-e][1,4]diazepin-2-one ([¹⁸F]F-5-BDBD, [¹⁸F]5a) and 5-(3-(2-[¹⁸F]fluoroethoxy)phenyl)-1,3-dihydro-2H-benzofuro[3,2-e][1,4]diazepin-2-one ([¹⁸F]FE-5-BDBD, [¹⁸F]11) were prepared from their corresponding nitro- and tosylated precursors by nucleophilic substitution with K[¹⁸F]F/Kryptofix 2.2.2 and isolated by HPLC-SPE in 5-25% decay corrected radiochemical yields with 111-740GBq/µmol specific activity at EOB. The preliminary biological evaluation of radiolabeled 5-BDBD analogs indicated these new radioligands have similar biological activity with their parent compound 5-BDBD.

Metal complexes for multimodal imaging of misfolded protein-related diseases

Aggregation of misfolded proteins and progressive polymerization of otherwise soluble proteins is a common hallmark of several highly debilitating and increasingly prevalent diseases, including amyotrophic lateral sclerosis, cerebral amyloid angiopathy, type II diabetes and Parkinson's, Huntington's and Alzheimer's diseases.