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When the body faces an injury, the immune system triggers inflammation to initiate tissue repair. However, dysregulation of this process can lead to chronic inflammation, driving persistent scar formation and resulting in fibrosis. Fibrosis, characterised by pathological scar tissue accumulation, impairs organ function which could ultimately lead to death. Despite its clinical significance, treatment options remain limited. Positron Emission Tomography (PET) imaging, a highly sensitive and quantitative technique, offers significant potential for the non-invasive assessment of inflammatory and fibrotic processes.

Papers I and II investigate the Affibody molecule Z09591, which targets platelet-derived growth factor receptor β (PDGFR β), as a PET tracer for assessing liver fibrogenesis in a murine model of toxin-induced fibrosis (the CCl₄ model). PDGFRβ, expressed on fibrogenic cells such as activated hepatic stellate cells, is absent in quiescent cells. Two radiolabeling techniques were compared: the TCO-TZ conjugation method ([¹⁸F]TZ-Z09591, Paper I) and the Al¹⁸F-RESCA method ([¹⁸F]AlF-RESCA-Z09591, Paper II). Both tracers demonstrated specific uptake in fibrotic regions with low liver background, highlighting their potential for non-invasive assessment of fibrogenic activity. These findings have supported the initiation of a first-in-human clinical trial evaluating a Z09591-based PET tracer.

Papers III and IV focus on two PET tracers, [¹¹C]NES and [⁶⁸Ga]Ga-FAPI-46, targeting neutrophil elastase (NE) and fibroblast activation protein (FAP), respectively, in pulmonary fibrosis. NE is a protease released by activated neutrophils, while FAP is expressed on activated fibroblasts. Sequential PET scans were performed in patients with long COVID-19 (Paper III) and interstitial lung disease (Paper IV), with [¹¹C]NES assessing neutrophil-mediated inflammation and [⁶⁸Ga]Ga-FAPI-46 imaging tissue remodeling activity. Tracer uptake correlated with lung abnormalities seen on computed tomography scans, underscoring their potential in imaging inflammation and tissue remodeling activity processes.

Given the complex pathogenesis of fibrosis and the lack of curative treatments, PET tracers that enable earlier diagnosis and disease monitoring may improve patient management and support the development of anti-fibrotic therapies.

The present thesis is a collection of studies focused on the establishment and optimization of radiolabeling methods for novel carbon-11 and fluorine-18 PET radiotracers.

Paper I investigated the flexibility of the Al[18F]F-RESCA labeling method by testing the chelation reaction between Al[18F]F and two RESCA-conjugated Affibody molecules, Z09591 and Z0185, at different temperatures ranging from room temperature to at 60°C. The resulting optimized reaction showed significant and reproducible improvements in radiochemical yield at temperatures as low as 37°C, retaining biological function post-purification and displaying promising in vitro and in vivo binding. 

In paper II, Al[18F]F-RESCA-Z09591 was evaluated as a potential tracer assessment of fibrogenic activity using U-87 xenograft and CCl4 murine models. The tracer showed high selectivity for PDGFRβ in U-87 tumor-bearing mice and demonstrated strong uptake in fibrotic liver tissue, while also showing decreased uptake during recovery in CCl4 models, indicating specificity for fibrogenic cells.

Papers III and IV compare the viability of three tetrazine compounds, the amide TzAm and esters TzE and TzE2, as prosthetic groups for indirect fluorine-18 labeling of Affibody molecule and antibody-based PET tracers via IEDDA reaction (using so called “click chemistry”) with TCO-conjugated biomolecules. [18F]TzAm, [18F]TzE and [18F]TzE2 were all synthesized using an automated, cartridge-based method and all successfully clicked to TCO-Z09591. [18F]TzAm-Z09591 and [18F]TzE-Z09591 both presented very distinct advantages and disadvantages: the former displayed excellent in vivo and in vitro properties, but required a long and complex multi-step synthesis, while the latter could be readily labeled in one step but was highly unstable in plasma; [18F]TzE2-Z09591 was designed to bridge the gap between the two previously established tracers, and retained the optimal labeling conditions of [18F]TzE-Z09591 while showing encouraging improvements in terms of stability.

In paper V, a first-in-class carbon-11 small molecule GLP-1R radiotracer, [11C]Methyl-V-0219, a carbon-11 methyl analogue of the GLP-1R positive allosteric modulator V-0219, was synthesized via Pd(0)-catalyzed Suzuki-Miyaura cross-coupling. The automated synthesis strategy offered high radiochemical yield and purity. In vitro autoradiography assays showed clear binding in GLP-1R-expressing tissues; moreover, self-blocking could be performed successfully, but a certain degree of observed cross-reactivity puts in question the selectivity of the tracer towards GLP-1R