Autotaxin-Specific PET/CT Imaging in a Preclinical Model of Pulmonary Fibrosis

Introduction: Idiopathic pulmonary fibrosis (IPF) is a fatal disease with unpredictable progression and limited therapeutic options. Current diagnosis relies on high resolution computed tomography (HRCT) which may not adequately capture early signs of deterioration. Hence, it is imperative to develop accurate assessment methods capable of early-stage detection and facilitate continuous monitoring of IPF progression. The enzyme Autotaxin (ATX) emerges as a prominently expressed extracellular secretory enzyme in the lungs of patients afflicted with IPF, making it a pivotal therapeutic target for intervention. Thus, it stands as a promising target for the development of positron emission tomography (PET) tracers associated with the progression of pulmonary fibrosis. The objective of this study was to evaluate the effectiveness of ¹⁸F-labeled ATX-targeted tracer [¹⁸F]ATX-1905, in comparison with [¹⁸F]FDG, for the early fibrosis diagnosis, monitoring of disease evolution, and treatment efficacy in a preclinical model of bleomycin-induced pulmonary fibrosis.

Methods: A mouse model of bleomycin-induced pulmonary fibrosis (BPF) was established by administering a single dose of bleomycin via intratracheal administration. To assess treatment efficacy, mice received oral administration of two commonly used drugs for IPF, pirfenidone or nintedanib, from D9 (Day 9) to D23 (Day 23) post-bleomycin administration. Lung tissues from the mice were collected to evaluate pulmonary inflammation using hematoxylin-eosin (HE) staining and CD3 immunohistochemistry (IHC), and to assess the degree of fibrosis using Masson staining. ATX expression in lungs of BPF mice was examined through immunohistochemistry (IHC) and enzyme-linked immunosorbent assay (ELISA). A lung uptake assay of [¹⁸F]ATX-1905 was performed and uptake value was measured via ex vivo gamma counting. PET imaging with [¹⁸F]FDG and [¹⁸F]ATX-1905 on the mice was conducted at various disease stages or treatment phases. The target binding specificity of [¹⁸F]ATX-1905 was investigated through blocking experiments utilizing an ATX inhibitor, PF-8380.

Results: The extent of pulmonary fibrosis in the BPF mouse model correlated with fluctuations in ATX expression levels. For PET imaging at any stage, especially the early stage (D9), the uptake of [¹⁸F]ATX-1905 in the lungs of BPF mice exceeded that in the control group and remained high throughout the study (Fig. B), aligning with the sustained high degree of pulmonary fibrosis. This elevated lung uptake could be suppressed by pre-administration of the ATX inhibitor, PF-8380 (Fig. C). In contrast, the lung uptake of [¹⁸F]FDG significantly increased and peaked at D15 (mid-term), and dropped later (Fig. A), possibly due to diminished inflammation levels. A two-week treatment regimen using either pirfenidone or nintedanib in BPF mice resulted in notable reductions of ATX expression and fibrosis within lung tissues compared to untreated mice, based on ELISA and Masson staining, as evidenced by PET imaging with [¹⁸F]ATX-1905 (Fig. B). Additionally, [¹⁸F]FDG uptakes also decreased following the treatment period (Fig. A).

Conclusions: The PET tracer [¹⁸F]ATX-1905 exhibited excellent ability in early fibrosis detection, monitoring fibrosis progression, and assessing treatment outcomes of lungs in PET imaging of the mouse model with pulmonary fibrosis. Additionally, non-invasive PET imaging with [¹⁸F]ATX-1905 showed a high specificity in identifying alterations in ATX expression within the mouse lungs. All these findings suggest the promising potential of [¹⁸F]ATX-1905 as a tracer for future applications in monitoring IPF patients with varying ATX expression in lungs.

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