Quantification of [11C]GB67 binding to cardiac α1-adrenoceptors with positron emission tomography: validation in pigs
An increase in human cardiac α1-adrenoceptor (α1-AR) density is associated with various diseases such as myocardial ischemia, congestive heart failure, hypertrophic cardiomyopathy and hypertension. Positron emission tomography (PET) with an appropriate radioligand offers the possibility of imaging receptor function in the normal and diseased heart. [11C]GB67, an analogue of prazosin, has been shown in rats to have potential as a PET ligand with high selectivity to α1-AR. However, α1-AR density is up to ten times higher in rat heart compared to that in man. The aim of the present preclinical study was to extend the previous evaluation to a large mammal heart, where the α1-AR density is comparable to man, and to validate a method for quantification before PET studies in man.
Seven [11C]GB67 PET studies, with weight-adjusted target dose of either 5.29 MBq kg−1 (pilot, test–retest and baseline–predose studies) or 8.22 MBq kg−1 (baseline–displacement studies), were performed in four anaesthetised pigs (39.5 ± 3.9 kg). Total myocardial volume of distribution (V T) was estimated under different pharmacological conditions using compartmental analysis with a radiolabelled metabolite-corrected arterial plasma input function. A maximum possible blocking dose of 0.12 μmol kg−1 of unlabeled GB67 was given 20 min before [11C]GB67 administration in the predose study and 45 min after administration of [11C]GB67 in the displacement study. In addition, [15O]CO (3,000 MBq) and [15O]H2O, with weight adjusted target dose of 10.57 MBq kg−1, were also administered for estimation of blood volume recovery (RC) of the left ventricular cavity and myocardial perfusion (MBF), respectively.
[11C]GB67 V T values (in ml cm−3) were estimated to be 24.2 ± 5.5 (range, 17.3–31.3), 10.1 (predose) and 11.6 (displacement). MBF did not differ within each pig, including between baseline and predose conditions. Predose and displacement studies showed that specific binding of [11C]GB67 to myocardial α1-ARs accounts for approximately 50% of V T.
The present study offers a methodology for using [11C]GB67 as a radioligand to quantify human myocardial α1-ARs in clinical PET studies.
Keywordsα1-Adrenoceptors [11C]GB67 Myocardium Pig Positron emission tomography (PET)
The authors are grateful to the other members of Hammersmith Imanet for their interest and support, in particular Mr. Andy Blyth and Ms. Andreanna Williams for their help in PET data acquisition, Ms. Safiye Osman and her team for the blood analyses and Dr. Christopher Rhodes, Dr. Terence Spinks and Dr. Kris Thielemans for valuable discussions.
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