Discovery and evaluation of a novel 18F-labeled vasopressin 1a receptor PET ligand with peripheral binding specificity

Abstract

The arginine-vasopressin (AVP) hormone plays a pivotal role in regulating various physiological processes, such as hormone secretion, cardiovascular modulation, and social behavior. Recent studies have highlighted the V1a receptor as a promising therapeutic target. In-depth insights into V1a receptor-related pathologies, attained through in vivo imaging and quantification in both peripheral organs and the central nervous system (CNS), could significantly advance the development of effective V1a inhibitors. To address this need, we develop a novel V1a-targeted positron emission tomography (PET) ligand, [¹⁸F]V1A-2303 ([¹⁸F]8), which demonstrates favorable in vitro binding affinity and selectivity for the V1a receptor. Specific tracer binding in peripheral tissues was also confirmed through rigorous cell uptake studies, autoradiography, biodistribution assessments. Furthermore, [¹⁸F]8 was employed in PET imaging and arterial blood sampling studies in healthy rhesus monkeys to assess its brain permeability and specificity, whole-body distribution, and kinetic properties. Our research indicated [¹⁸F]8 as a valuable tool for noninvasively studying V1a receptors in peripheral organs, and as a foundational element for the development of next-generation, brain-penetrant ligands specifically designed for the CNS.

Keywords: Autism; Fluorine 18; Kinetic modeling; PET imaging; Positron emission tomography; Radiolabeling; V1a receptor; Vasopressin.

Graphical abstract

Figure 1

Representative antagonists (A) and radiolabeled ligands (B) for V1a receptor.

Scheme 1

Synthesis of target compound 8.

Scheme 2

Synthesis of radiolabeled V1a ligand [¹⁸F]V1A-2303.

Figure 2

In vitro cell uptake assay and autoradiography of [¹⁸F]8. (A) Normalized uptake ratio of [¹⁸F]V1A-2303 in CHO cell line that expresses the human V1a receptor, in the presence of non-radioactive V1A-2303, V1a receptor antagonists PF-184563, balovaptan or SRX246 as blocking agents (0.5 μmol/L); Representative images for baseline and blocking (10 μmol/L) by (B) ex vivo autoradiography in mice liver and (D) in vitro autoradiography in monkey adrenal; (C, E) Comparative analysis under baseline and blocking conditions to quantitatively assess the specificity of [¹⁸F]V1A-2303. All data are shown as mean ± SD (n ≥ 3) and analyzed by unpaired t-test. ∗∗∗P < 0.001, ∗∗∗∗P < 0.0001.

Figure 3

Ex vivo biodistribution of [¹⁸F]8 in mice. (A) Biodistribution of tracer [¹⁸F]8 at five different time points characterized by injected dose per gram of wet tissue (%ID/g) under baseline condition. The inset shows the brain uptake; (B) Standard uptake value (SUV) in the brain at five different time points; and comparison of tracer uptake in the liver (C) and kidney (D) under baseline and blocking conditions. V1a antagonist balovaptan (10 mg/kg) was used as a blocking agent. All data were shown as mean ± standard deviations (n = 4) and analyzed by paired t-test. ^nsP＞0.05, ∗P ≤ 0.05.

Figure 4

PET imaging studies of [¹⁸F]8 in the rhesus monkey. (A) Representative summed PET/MR images (0–90 min) of the brain under baseline conditions; (B) Time-activity curves (TACs) in representative brain regions under baseline conditions; (C) Whole body maximum intensity projection (MIP) images; (D) PET/CT static images (100–110 min) of liver; (E) Liver mean standard uptake value (SUV_mean) of monkey after 100 min injection under both baseline and blocking conditions. Blocking was pre-treated with balovaptan (3 mg/kg).

Figure 5

Plasma data, kinetic modeling, and time stability analysis in the brains of non-human primates (NHPs). (A) Individual metabolite-corrected [¹⁸F]V1A-2303 standard uptake value (SUV) time courses in plasma; (B) Individual fraction of unchanged parent compound over time; (C) Representative radio-chromatograms of plasma samples under baseline conditions; (D, E) Model fits of one-tissue compartment model (1TCM), two-tissue compartment model (2TCM), and Logan plot on the baseline scan of monkey striatum. (F) Estimation of total volume of distribution (V_T) calculated for 60–100 min scan windows in 10 min increments compared to the final 90 min V_T for the thalamus and striatum.