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Optimised GMP Production of [¹⁸F]DPA-714: Advancing TSPO PET Imaging for Multi-Centre Neuroinflammation Studies
Neuroinflammation is a key pathological feature of numerous neurological disorders and is commonly associated with activation of microglia in the central nervous system. The 18 kDa translocator protein (TSPO) is highly upregulated in activated microglia and has therefore become an established biomarker for imaging neuroinflammatory processes using positron emission tomography (PET). Among second-generation TSPO radiotracers, [¹⁸F]DPA-714 has attracted considerable interest due to its high affinity, favourable pharmacokinetics, and suitability for clinical imaging. However, widespread clinical adoption of this tracer requires a robust, high-yielding, and fully GMP-compliant production process.
This article describes the rationale, challenges, and impact of an optimised automated radiosynthesis of [¹⁸F]DPA-714 implemented on the Trasis AllinOne synthesiser. By refining reaction conditions, reducing precursor loading, and simplifying reagent composition, the developed process delivers consistently high radiochemical yields and high molar activity while maintaining operational simplicity and regulatory compliance. The cassette-based workflow supports reliable batch-to-batch reproducibility and enables production scales sufficient for multi-patient and multi-centre PET studies. Collectively, these advances strengthen the translational potential of [¹⁸F]DPA-714 and support its broader use as a quantitative imaging biomarker for neuroinflammation in clinical research and future routine practice.
Neuroinflammation Imaging and the Role of TSPO PET Tracers
Neuroinflammation is increasingly recognised as a central pathological mechanism underlying a wide range of neurological and neurodegenerative disorders. Conditions such as Alzheimer’s disease, multiple sclerosis, stroke, traumatic brain injury, and brain tumours share a common feature: the activation of resident immune cells in the central nervous system (CNS), particularly microglia. While acute inflammatory responses may be protective, chronic microglial activation contributes to neuronal dysfunction, synaptic loss, and disease progression. Consequently, the ability to visualise neuroinflammation in vivo has become a major objective in both clinical neurology and translational neuroscience.
Among the molecular targets investigated for imaging neuroinflammation, the 18 kDa translocator protein (TSPO) has emerged as one of the most extensively validated biomarkers. TSPO is a mitochondrial membrane protein with low baseline expression in the healthy brain. Under pathological conditions, however, TSPO expression is markedly upregulated in activated microglia and, to a lesser extent, astrocytes. This strong association between TSPO density and neuroinflammatory activity makes the protein particularly suitable for non-invasive imaging using positron emission tomography (PET).
Fig 1. TSPO PET Imaging of Brain Inflammation
PET imaging offers high sensitivity and quantitative capabilities, enabling the detection of subtle molecular changes before structural alterations become apparent on conventional imaging modalities. The first TSPO-targeting PET tracer, (R)-[¹¹C]PK11195, was widely used to investigate microglial activation in vivo and played a pivotal role in establishing TSPO imaging as a viable approach. Nevertheless, its clinical utility is limited by high non-specific binding, poor signal-to-noise ratios, and complex kinetic behaviour, all of which hinder accurate quantification.
These limitations have driven the development of second-generation TSPO tracers with improved binding characteristics and pharmacokinetic profiles. In parallel, increasing emphasis has been placed on the use of fluorine-18–labelled radiotracers. Compared with carbon-11, fluorine-18 offers a longer half-life (109.8 minutes), enabling regional distribution, multi-centre clinical studies, and higher image resolution due to its favourable decay properties. Collectively, these advances have positioned TSPO PET imaging as a powerful tool for probing neuroinflammatory processes in vivo and have laid the foundation for the clinical translation of high-affinity fluorine-18–labelled TSPO ligands, such as [¹⁸F]DPA-714.
[¹⁸F]DPA-714: A Second-Generation TSPO Radiotracer with Clinical Potential
The development of second-generation TSPO radiotracers has significantly advanced the field of neuroinflammation imaging by addressing the limitations of earlier ligands. Among these, [¹⁸F]DPA-714 has emerged as a particularly promising positron emission tomography (PET) tracer due to its high affinity for TSPO, favourable pharmacokinetics, and suitability for clinical imaging. Structurally, DPA-714 is a pyrazolopyrimidine derivative designed to bind selectively to TSPO expressed on the outer mitochondrial membrane of activated microglia. When labelled with fluorine-18, the compound enables sensitive and quantitative visualisation of neuroinflammatory processes in vivo.
One of the defining advantages of [¹⁸F]DPA-714 is its strong specific binding to TSPO combined with low non-specific uptake in healthy brain tissue. This translates into improved contrast and signal-to-noise ratios compared with first-generation tracers such as (R)-[¹¹C]PK11195. Importantly, [¹⁸F]DPA-714 exhibits efficient blood–brain barrier penetration, allowing rapid brain uptake and favourable washout kinetics, which are critical parameters for robust PET image quantification. These properties facilitate more accurate assessment of microglial activation across different brain regions.
From a practical perspective, the use of fluorine-18 confers clear logistical and technical benefits. The longer half-life of fluorine-18 compared with carbon-11 enables centralised production and regional distribution, making [¹⁸F]DPA-714 well suited for multi-centre clinical studies. In addition, fluorine-18 provides higher spatial resolution in PET imaging, enhancing the detection of subtle neuroinflammatory changes that may occur in early disease stages.
The clinical relevance of [¹⁸F]DPA-714 has been demonstrated in a range of preclinical and human studies. Biodistribution and blocking experiments have confirmed its TSPO specificity, while clinical investigations in patients with Alzheimer’s disease and multiple sclerosis have shown increased tracer uptake in regions associated with neuroinflammation. These studies highlight the potential of [¹⁸F]DPA-714 not only as a research tool but also as a candidate imaging biomarker for disease progression, patient stratification, and therapeutic monitoring.
As interest in TSPO imaging continues to grow, the availability of a high-affinity, fluorine-18–labelled tracer such as [¹⁸F]DPA-714 underscores the importance of developing robust, scalable, and GMP-compliant production methods. Such advances are essential to support broader clinical adoption and to fully realise the tracer’s potential in neuroinflammatory disease research.
Challenges in Radiosynthesis of [¹⁸F]DPA-714: Lessons from the Literature
Despite the strong clinical potential of [¹⁸F]DPA-714 as a TSPO PET tracer, its radiosynthesis presents several technical and practical challenges that have driven ongoing optimisation efforts. At the core of most reported synthetic routes is a nucleophilic aliphatic substitution, in which [¹⁸F]fluoride displaces a suitable leaving group—most commonly a tosylate—on the DPA-714 precursor. While this reaction is well established in radiochemistry, achieving high radiochemical yield, reproducibility, and compliance with Good Manufacturing Practice (GMP) requirements simultaneously has proven non-trivial.
Early reports demonstrated that [¹⁸F]DPA-714 could be synthesised using conventional Kryptofix-222/potassium carbonate activation of [¹⁸F]fluoride in acetonitrile. However, these conditions often required relatively high precursor loadings and delivered only modest radiochemical yields. In addition, reactions performed at reflux temperatures in low-boiling solvents such as acetonitrile were susceptible to variability, particularly when translated to automated synthesis modules. Such inconsistencies pose a significant barrier for routine clinical production, where batch-to-batch robustness is critical.
Subsequent studies explored alternative solvents and reaction conditions to address these limitations. High-boiling polar aprotic solvents such as dimethyl sulfoxide (DMSO) enabled reactions at elevated temperatures, resulting in improved incorporation of [¹⁸F]fluoride and higher radiochemical yields. However, the use of DMSO introduces its own challenges, including more demanding purification steps and stricter control of residual solvent levels to meet regulatory specifications. Balancing reaction efficiency with downstream formulation and quality control thus became a central consideration.
Another key challenge lies in optimising molar activity. High molar activity is essential to avoid receptor saturation and to ensure accurate quantification of TSPO expression, particularly given the relatively low density of TSPO in healthy brain tissue. Excess precursor or competing non-radioactive fluoride sources can significantly reduce molar activity, necessitating careful control over reagent purity, precursor mass, and synthesis timing.
Automation further complicates the process. Legacy synthesis platforms often require extensive modification to accommodate optimised reaction conditions, and complex sequences can increase the risk of operator error or system failure. As a result, translating literature protocols into GMP-compliant, cassette-based systems suitable for multi-centre clinical trials has remained a persistent bottleneck.
Collectively, these challenges underscore the need for simplified, robust, and high-yielding radiosynthesis protocols for [¹⁸F]DPA-714. Lessons learned from earlier approaches have directly informed the development of more efficient automated processes, paving the way for reliable clinical-scale production.
Optimised GMP-Compliant Production of [¹⁸F]DPA-714 on the Trasis AllinOne Module
The growing clinical interest in [¹⁸F]DPA-714 has created a clear demand for a radiosynthesis process that is not only efficient but also robust, reproducible, and fully compliant with Good Manufacturing Practice (GMP) requirements. To meet these criteria, an optimised production strategy was developed using the Trasis AllinOne (AIO) synthesiser, a cassette-based automated platform widely adopted in clinical radiopharmacies. Its modular design, straightforward operation, and compatibility with GMP workflows make it particularly well suited for routine clinical production and rapid personnel training.
The synthesis of [¹⁸F]DPA-714 on the Trasis AllinOne relies on a classic nucleophilic aliphatic substitution, in which [¹⁸F]fluoride displaces a tosylate leaving group on the DPA-714 precursor. In contrast to earlier approaches, the optimised process employs tetraethylammonium bicarbonate as the phase-transfer agent, avoiding the use of Kryptofix-222 and potassium carbonate. This modification simplifies the synthesis, reduces the number of critical reagents, and streamlines quality control by eliminating the need to test for residual Kryptofix.
The reaction is performed in acetonitrile at 100 °C under fully automated conditions using an in-house–designed single-use cassette and synthesis sequence. Notably, the precursor loading was reduced to approximately 4 mg without compromising performance. This optimisation resulted in consistently high non–decay-corrected radiochemical yields in the range of 55–71%, alongside high molar activities at the end of synthesis. Such performance represents a substantial improvement over many previously reported protocols and demonstrates excellent batch-to-batch reproducibility.
From a GMP perspective, the cassette-based approach offers significant advantages. Single-use fluid paths minimise the risk of cross-contamination, while automated execution reduces operator-dependent variability. The simplified sequence shortens synthesis time and lowers the probability of technical failure, both of which are critical for high-throughput clinical environments. Importantly, the achieved activity yields allow a single production batch to supply multiple patient doses or support multi-centre PET studies, enhancing operational efficiency.
Overall, the implementation of this optimised [¹⁸F]DPA-714 synthesis on the Trasis AllinOne module illustrates how thoughtful process design and automation can overcome long-standing challenges in radiotracer production. By combining high yield, high molar activity, and full GMP compliance, this approach provides a reliable foundation for the broader clinical deployment of [¹⁸F]DPA-714 in neuroinflammation imaging.
Enabling Multi-Centre PET Studies: Impact and Future Perspectives
The availability of a robust, high-yield, and GMP-compliant production process for [¹⁸F]DPA-714 has important implications for both clinical research and the broader adoption of TSPO PET imaging. One of the most significant outcomes of the optimised synthesis is the ability to generate sufficient radioactivity from a single production batch to support multiple patient scans or distribution to external imaging centres. This capability is essential for multi-centre clinical trials, where standardisation of tracer quality and imaging protocols is critical for reliable data comparison and regulatory acceptance.
