Molecular imaging of the pulmonary circulation in health and disease

SPECT and PET imaging of adrenomedullin receptors: a promising strategy for studying pulmonary vascular diseases

Circulating adrenomedullin (AM) levels are elevated in several cardiovascular diseases, including pulmonary vascular diseases causing pulmonary hypertension. To date the perfusion agent ^99mTc-albumin macroaggregates (MAA) is the only approved radiopharmaceutical used for imaging of pulmonary circulation. Unlike ^99mTc-MAA, imaging the AM receptors involves a molecular process dependent on the density of the receptors and the affinity of specific radioligands. The AM receptors are abundantly distributed in lung capillaries and its integrity provides protection in the development of pulmonary vascular diseases. This review summarizes the development and characterization of radioligands for in vivo imaging of AM receptors as an early predictor of the onset of a pulmonary vascular disease.

PulmoBind Imaging Measures Reduction of Vascular Adrenomedullin Receptor Activity with Lack of effect of Sildenafil in Pulmonary Hypertension

Endothelial dysfunction is a core pathophysiologic process in pulmonary arterial hypertension (PAH). We developed PulmoBind (PB), a novel imaging biomarker of the pulmonary vascular endothelium. ^99mTechnetium (^99mTc)-labelled PB binds to adrenomedullin receptors (AM₁) densely expressed in the endothelium of alveolar capillaries. We evaluated the effect of sildenafil on AM₁ receptors activity using ^99mTc-PB. PAH was induced in rats using the Sugen/hypoxia model and after 3 weeks, animals were allocated to sildenafil (25 or 100 mg/kg/day) for 4 weeks. ^99mTc-PB uptake kinetics was assessed by single-photon emission computed tomography. PAH caused right ventricular (RV) hypertrophy that was decreased by low and high sildenafil doses. Sildenafil low and high dose also improved RV function measured from the tricuspid annulus plane systolic excursion. Mean integrated pulmonary uptake of ^99mTc-PB was reduced in PAH (508% · min ± 37, p < 0.05) compared to controls (630% · min ± 30), but unchanged by sildenafil at low and high doses. Lung tissue expressions of the AM₁ receptor components were reduced in PAH and also unaffected by sildenafil. In experimental angio-proliferative PAH, sildenafil improves RV dysfunction and remodeling, but does not modify pulmonary vascular endothelium dysfunction assessed by the adrenomedullin receptor ligand ^99mTc-PB.

Oxymatrine prevents the development of monocrotaline-induced pulmonary hypertension via regulation of the NG, NG-dimethyl-L-arginine metabolism pathways in rats

The purpose of this study was to investigate the potential effect of oxymatrine in monocrotaline-induced pulmonary hypertension and its possible influence on the N^G,N^G-dimethyl-L-arginine (ADMA) metabolism pathway. Pulmonary hypertension was induced in rats by a single-dose injection of monocrotaline (60 mg/kg). Daily oral administration of oxymatrine (25, 50 and 100 mg/kg) was started on the day following the monocrotaline injection for 28 days. Oxymatrine (50 and 100 mg/kg) significantly attenuated monocrotaline-induced lung and right ventricular hypertrophy, right ventricular systolic pressure elevation, and right ventricular dysfunction. Oxymatrine also reduced the thickening of monocrotaline-induced pulmonary arterial medial wall. Meanwhile, oxymatrine normalized the level of pulmonary asymmetric ADMA and attenuated the upregulated expression of protein arginine methyltransferase 1 (PRMT1). Oxymatrine had no effect on the expression of protein arginine methyltransferase 2 (PRMT2) and N^G,N^G-Dimethylarginine dimethylaminohydrolase 1 (DDAH1), which were upregulated in monocrotaline-induced pulmonary arterial hypertensive rats. However, the expression of the protein N^G,N^G-Dimethylarginine dimethylaminohydrolase 2 (DDAH2) did not differ among all groups (all P﹥0.05). These results suggest that oxymatrine may offer protective effects on the development of pulmonary hypertension by ameliorating pulmonary remodeling and modulating the ADMA metabolism pathway.

Al[18F]F-complexation of DFH17, a NOTA-conjugated adrenomedullin analog, for PET imaging of pulmonary circulation

INTRODUCTION

Adrenomedullin receptors are highly expressed in human alveolar capillaries and provide a molecular target for imaging the integrity of pulmonary microcirculation. In this work, we aimed to develop a NOTA-derivatized adrenomedullin analog (DFH17), radiolabeled with [¹⁸F]AlF, for PET imaging of pulmonary microcirculation.

METHODS

Highly concentrated [¹⁸F](AlF)²⁺ (15 μL) was produced from purified fluorine-18 in NaCl 0.9%. Various complexation experiments were carried out at Al-to-NOTA molar ratios ranging from 1:1 to 1:40 to assess optimal radiolabeling conditions before using the peptide. DFH17 peptide (2 mM, pH 4) was radiolabeled with [¹⁸F](AlF)²⁺ for 15 min at 100 °C in a total volume of 60 μL. As part of the radiolabeling process, parameters such as fluorine-18 activity (~37 and 1480 MBq), concentration of AlCl₃ (0.75, 2, 3, 6 or 10 mM) and the effects of hydrophilic organic solvent (aqueous vs ethanol 50%) were studied. The final formulation was tested for purity, identity and stability in saline. Initial in vivo evaluation of [¹⁸F]AlF-DFH17 was performed in normal rats by PET/CT.

RESULTS

The scaled-up production of [¹⁸F]AlF-DFH17 was performed in high radiochemical and chemical purities in an overall radiochemical yield of 22-38% (at end-of-synthesis) within 60 min. The final formulation was stable in saline at different radioactive concentrations for 8 h. PET evaluation in rats revealed high lung-to-background ratios and no defluorination in vivo up to 1 h post-injection.

CONCLUSION

The novel radioconjugate [¹⁸F]AlF-DFH17 appears to be a promising PET ligand for pulmonary microcirculation imaging.

Molecular imaging of the human pulmonary vascular endothelium in pulmonary hypertension: a phase II safety and proof of principle trial

PURPOSE

The adrenomedullin receptor is densely expressed in the pulmonary vascular endothelium. PulmoBind, an adrenomedullin receptor ligand, was developed for molecular diagnosis of pulmonary vascular disease. We evaluated the safety of PulmoBind SPECT imaging and its capacity to detect pulmonary vascular disease associated with pulmonary hypertension (PH) in a human phase II study.

METHODS

Thirty patients with pulmonary arterial hypertension (PAH, n = 23) or chronic thromboembolic PH (CTEPH, n = 7) in WHO functional class II (n = 26) or III (n = 4) were compared to 15 healthy controls. Lung SPECT was performed after injection of 15 mCi ^99mTc-PulmoBind in supine position. Qualitative and semi-quantitative analyses of lung uptake were performed. Reproducibility of repeated testing was evaluated in controls after 1 month.

RESULTS

PulmoBind injection was well tolerated without any serious adverse event. Imaging was markedly abnormal in PH with ∼50% of subjects showing moderate to severe heterogeneity of moderate to severe extent. The abnormalities were unevenly distributed between the right and left lungs as well as within each lung. Segmental defects compatible with pulmonary embolism were present in 7/7 subjects with CTEPH and in 2/23 subjects with PAH. There were no segmental defects in controls. The PulmoBind activity distribution index, a parameter indicative of heterogeneity, was elevated in PH (65% ± 28%) vs. controls (41% ± 13%, p = 0.0003). In the only subject with vasodilator-responsive idiopathic PAH, PulmoBind lung SPECT was completely normal. Repeated testing 1 month later in healthy controls was well tolerated and showed no significant variability of PulmoBind distribution.

CONCLUSIONS

In this phase II study, molecular SPECT imaging of the pulmonary vascular endothelium using ^99mTc-PulmoBind was safe. PulmoBind showed potential to detect both pulmonary embolism and abnormalities indicative of pulmonary vascular disease in PAH. Phase III studies with this novel tracer and direct comparisons to lung perfusion agents such as labeled macro-aggregates of albumin are needed.

CLINICAL TRIAL

ClinicalTrials.gov, NCT02216279.

Molecular imaging of the human pulmonary vascular endothelium using an adrenomedullin receptor ligand

This phase I study (NCT01539889) evaluated the safety, efficacy, and dosing of PulmoBind for molecular imaging of pulmonary circulation. PulmoBind is a ligand of the adrenomedullin receptor abundantly distributed in lung capillaries. Labeled with 99mTc, it allows single-photon emission computed tomographic (SPECT) imaging of lung perfusion. In preclinical studies, PulmoBind scans enabled detection of lung perfusion defects and quantification of microcirculatory occlusion caused by pulmonary hypertension. Healthy humans (N  =  20) were included into escalating groups of 5 mCi (n  =  5), 10 mCi (n  =  5), or 15 mCi (n  =  10) 99mTc-PulmoBind. SPECT imaging was serially performed, and 99mTc-PulmoBind dosimetric analysis was accomplished. The radiochemical purity of 99mTc-PulmoBind was greater than 95%. There were no safety concerns at the three dosages studied. Imaging revealed predominant and prolonged lung uptake with a mean peak extraction of 58% ± 7%. PulmoBind was well tolerated, with no clinically significant adverse event related to the study drug. The highest dose of 15 mCi provided a favorable dosimetric profile and excellent imaging. The postural lung perfusion gradient was detectable. 99mTc-PulmoBind is safe and provides good quality lung perfusion imaging. The safety/efficacy of this agent can be tested in disorders of pulmonary circulation such as pulmonary arterial hypertension.