Molecular Imaging of Monocrotaline-Induced Pulmonary Vascular Disease with Radiolabeled Linear Adrenomedullin

Abstract

No test currently exists for molecular imaging of pulmonary arterial hypertension (PAH). Adrenomedullin is a vasodilator peptide predominantly cleared by pulmonary endothelial receptors. We developed a linear adrenomedullin derivative radiolabeled with 99m Tc ( 99m Tc-AM-L) for imaging of pulmonary circulation and tested its capacity to detect anomalies of pulmonary circulation caused by PAH. Methods: PAH was induced by monocrotaline in rats and compared with controls. After 5 wk, 99m Tc-AM-L was injected intravenously. Plasma kinetics were measured, lung activity was determined in vivo after 30 min using a nuclear camera, and lung activity was determined ex vivo in explanted lungs. Expression of adrenomedullin receptors was measured in lung homogenates. Results: The plasma levels of 99m Tc-AM-L significantly increased in PAH by approximately 2-fold. Uptake by the lungs was homogeneous but greatly reduced in PAH by about 70%. In vivo retention was 14% 6 1% (mean 6 SD) of the injected dose in controls and 4% 6 1% in PAH (P , 0.0001). A similar reduction was measured ex vivo (6.0 6 1.6 percentage injected dose per gram [%ID/g] vs. 0.95 6 0.21 %ID/g, P , 0.0001). The expression of the heterodimeric component of the adrenomedullin receptor, receptor activity modifying protein 2, was also greatly reduced in PAH lungs (P , 0.001). Interestingly, right ventricular uptake of 99m Tc-AM-L was increased by PAH (P 5 0.02) and correlated with the degree of right ventricular hypertrophy (r 5 0.83, P 5 0.001). Conclusion: Pulmonary uptake of 99m Tc-AM-L is greatly reduced in monocrotaline-induced PAH. This novel molecular imaging agent may be useful in the diagnosis and follow-up of pulmonary vascular disorders. No clinically available test currently exists that can provide both anatomic and functional information on the status of pulmonary circulation. Pulmonary arterial hypertension (PAH) is a disorder characterized by endothelial dysfunction, with intimal and vascular smooth muscle proliferation leading to gradual obliteration of pulmonary arterioles (1). PAH is screened by transthoracic Doppler echocardiography, with the estimation of pulmonary artery systolic pressure obtained using the tricuspid valve regurgitant jet. Although this approach correlates with hemodynamically measured pulmonary pressure, it does not provide direct information on the biology of pulmonary circulation and may miss the early presence of pulmonary vascular disease (1). The recent availability of oral therapies for PAH such as endothelin receptor antagonists and phophodiesterase inhibitors advocates for earlier diagnosis of this condition and treatment of subjects in functional class II (1). There is, therefore, a clinical need for novel diagnostic approaches toward pulmonary vascular disease that could provide an earlier and more precise diagnosis.Adrenomedullin is a vasodilator peptide produced by the vascular endothelium. When injected intravenously, adrenomedullin is predominantly cleared by pulmonary circulation by specific adrenomedullin rec...

PulmoBind, an Adrenomedullin-Based Molecular Lung Imaging Tool

“…Finally, we demonstrated that radiolabeled PulmoBind could detect a lung microcirculatory perfusion defect associated with pulmonary arterial hypertension using the monocrotaline model. This model causes progressive obliteration of pulmonary vessels, and we demonstrated that it was associated with reduced lung expression of the specific heterodimeric component of the AM receptor, RAMP2 (4). Evidence for the abundant distribution of AM receptors in the pulmonary microcirculation was also obtained previously.…”

“…The Fmoc-D-(polyethylene glycol) 4 -COOH acid used as a spacer in the peptide synthesis was purchased from Quanta Biodesign Ltd. Disulfide bond formation to yield cyclic derivatives was performed by overnight air oxidation after dissolution of the linear peptide in an aqueous solution (pH 8.8) at a concentration of 0.1 mg/mL. Peptides were purified by reversed-phase high-performance liquid chromatography (HPLC).…”

“…A simple tetrapeptide having 99m Tc chelating properties, Gly-Gly-D-Ala-Gly (19), was used. Thus, compound 1 was created by complexing the antagonist AM to this tetrapeptide 99m Tc chelating moiety through a molecular linker, the heterobifunctional polyethylene glycol derivative Fmoc-D-(polyethylene glycol) 4 -COOH. Two cysteine residues were introduced into the AM analog construction to generate cyclic compound 2.…”

“…We recently showed that adrenomedullin (AM) is a promising molecular lung imaging tool for the diagnosis of pathologies such as pulmonary embolism and pulmonary hypertension (4,5). AM, a peptide from the calcitonin family, allows specific lung imaging through binding with its specific receptor, AM1 (CRLR/RAMP2), which is expressed at a high density in pulmonary vessels (6,7).…”

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

“…Specific AM receptors are present in the pulmonary vascular endothelium with dense expression in capillaries [2, 3]. The lungs are a major site for circulating AM clearance [4] and pulmonary clearance of AM is reduced in animal models of pulmonary arterial hypertension (PAH) with reduced AM receptor expression [5–7]. Based on these premises, we developed AM derivatives that can be labeled with 99m Tc for molecular SPECT imaging of the pulmonary circulation [7].…”