Eric A. Dubois

Demonstration of a reduction in muscarinic receptor binding in early Alzheimer's disease using iodine-123 dexetimide single-photon emission tomography

Decreased muscarinic receptor binding has been suggested in single-photon emission tomography (SPET) studies of Alzheimers disease. However, it remains unclear whether these changes are present in mildly demented patients, and the role of cortical atrophy in receptor binding assessment has not been investigated. We studied muscarinic receptor binding normalized to neostriatum with SPET using [123I]4-iododexetimide in five mildly affected patients with probable Alzheimers disease and in five age-matched control subjects. Region of interest (ROI) analysis was performed in a consensus procedure blind to clinical diagnosis using matched magnetic resonance (MRI) images. Cortical atrophy was assessed by calculating percentages of cerebrospinal fluid in each ROI. An observer study with three observers was conducted to validate this method. Alzheimer patients showed statistically significantly less [123I]4-iododexetimide binding in left temporal and right temporo-parietal cortex compared with controls, independent of age, sex and cortical atrophy. Mean intea-observer variability was 3.6% and inter-observer results showed consistent differences in [123I]4-iododexetimide binding between observers. However, differences between patients and controls were comparable among observers and statistically significant in the same regions as in the consensus procedure. Using an MRI-SPET matching technique, we conclude that [123I]4-iododexetimide binding is reduced in patients with mild probable. Alzheimers disease in areas of temporal and temporoparietal cortex.

Cardiac iodine-123 metaiodobenzylguanidine uptake in animals with diabetes mellitus and/or hypertension

The aim of the present study was to evaluate the use of the noradrenaline analogue iodine-123 metaiodobenzylguanidine ([123I]MIBG) for the assessment of cardiac sympathetic activity in the presence of diabetes mellitus and/or hypertension in animal models. One model used Wistar-Kyoto rats (WKY) and spontaneously hypertensive rats (SHR) rendered diabetic at 12 weeks of age by an intravenous injection of streptozotocin (STZ). The other model used lean and obese Zucker rats. In all groups basic haemodynamic values were established and animals received an intravenous injection of 50 μCi [123I]MIBG. Initial myocardial uptake and washout rates of [123I]MIBG were measured scintigraphically during 4 h. After sacrifice, plasma noradrenaline and left cardiac ventricular β-adrenoceptor density was determined. The diabetic state, both in STZ-treated rats (direct induction) and in obese Zucker rats (genetic induction), appeared to induce a lower cardiac density of β-adrenoceptors, indicative of increased sympathetic activity. Cardiac [123I]MIBG then showed increased washouts, thereby confirming enhanced noradrenergic activity. This parallism of results led to the conclusion that [123I]MIBG wash-out measurements could provide an excellent tool to assess cardiac sympathetic activity non-invasively. However, in hypertension (WKY vs SHR), both parameters failed to show parallelism: no changes in β-adrenoceptor density were found, whereas [123I]MIBG wash-out rate was increased. Thus, either [123I]MIBG washout or ß-adrenoceptor density may not be a reliable parameter under all circumstances to detect changes in the release of noradrenaline. Changes in the initial uptake of [123I]MIBG were observed as well. This may be a good marker for the disappearance of cardiac innervation, but it seems not to be a good parameter for distinguishing between loss of sympathetic innervation and enhanced uptake of noradrenaline in pathological conditions.

Cardiac sympathetic neuronal function in left ventricular volume and pressure overload

OBJECTIVES In heart failure cardiac sympathetic neuronal function and activity appear to be altered. Although these changes are widely accepted, controversy exists concerning the neurohormonal changes occurring in pressure and volume overloaded hearts. The present study in rabbits was performed to assess the effects of mechanical overload on cardiac sympathetic neuronal function and beta-adrenoceptor density, in relation to left ventricular function. METHODS In nine rabbits the aortic valve was perforated to induce left ventricular volume overload. Pressure overload was induced by suprarenal banding of the aorta abdominalis (group 1). Five animals were sham operated (group 2). Subanalysis of group 1 was performed for non-failing (n = 5) and failing (n = 4) hearts. Heart failure was defined as any reduction in left ventricular fractional shortening 2 weeks after the second operation compared to baseline. RESULTS In animals with cardiac overload, left ventricular weight was higher compared with the control animals, 7.99 +/- 1.13 vs. 6.16 +/- 0.86 g (P < 0.02). Left ventricular end diastolic diameter increased from 1.35 +/- 0.16 to 1.57 +/- 0.15 cm (P < 0.005) after surgically induced overload. Left ventricular end systolic diameter and fractional shortening did not change significantly. Myocardial noradrenaline (NA) concentration and beta-adrenoceptor density were significantly lower in group 1 than in group 2, 1005 +/- 393 vs. 1643 +/- 109 ng/g (P < 0.02) and 167 +/- 36 vs. 224 +/- 36 fmol/mg protein (P < 0.03), respectively. Myocardial [123I]-MIBG uptake did not significantly differ between group 1 and 2, 2.1 +/- 0.58 vs. 1.8 +/- 0.44 (%ID/g x kg). A significant positive correlation between myocardial NA concentration and beta-adrenoceptor density was found (r = 0.66, P < 0.02). Myocardial NA concentration was inversely related to left ventricular weight (r =-0.75, P < 0.003). CONCLUSION The present data indicate that in a condition of cardiac volume and pressure overload, sympathetic activity is enhanced as shown by myocardial noradrenaline depletion and beta-adrenoceptor downregulation. In contrast, no cardiac neuronal dysfunction is observed, even in the stage of early heart failure.

Quantitation of myocardial iodine-123 MIBG uptake in SPET studies : a new approach using the left ventricular cavity and a blood sample as a reference

In patients with chronic heart failure increased sympathetic activity is related to unfavourable prognosis. Since myocardial iodine-123 metaiodobenzylguanidine ([123I]MIBG) uptake is related to myocardial noradrenaline content, i.e. cardiac sympathetic activity, measurement of myocardial [123I]MIBG uptake may be of clinical use in determining prognosis or the effect of pharmacological intervention in these patients. The aim of the present study was to evaluate a new method to quantitate myocardial [123I]MIBG uptake with respect to reproducibility and accuracy. Eighteen [123I]MIBG planar and single-photon emission tomography (SPET) studies of patients with chronic heart failure were evaluated. Myocardial uptake was calculated from the myocardial (MYO) to left ventricular cavity (C) count density ratio and the123I activity in a blood sample. This was performed employing planar LAO images, a single-slice SPET method using the midventricular myocardial short-axis slice, and finally a multi-slice SPET method analysing semi-automatically drawn volumes of interest (VOIs). The accuracy of the multi-slice SPET method was verified using a cardiac phantom. The planar method was found to be reproducible [intra- and interobserver coefficients of variation (IACV and IRCV) were 0.025 and 0.012 respectively] but the mean MYO/C count density ratio was only 1.31±0.16 as a consequence of overprojection. For the single-slice SPET method IACV was 0.2 and IRCV was 0.13, representing poor reproducibility. For the multi-slice SPET method IACV was 0.051, IRCV was 0.047 and the mean MYO/C count density ratio was 5.4±2.42. Accuracy was 81% at a true MYO/C count density ratio of 6 in the phantom. It is concluded that the multi-slice SPET method using the left ventricular cavity VOI and a blood sample as a reference is a reproducible and accurate method for assessing myocardial [123I]MIBG uptake.

Pharmacologic characterization in vitro and in vivo of iodine 123-labeled derivatives of the beta-adrenoceptor antagonist CGP12177, designed for the imaging of cardiac beta-receptors.

BackgroundPotential new radioligands for the noninvasive imaging of cardiac β-adrenoceptors with single-photon emission computed tomography were investigated.Methods and ResultsTwo iodinated derivatives of CGP12177 para (S-CYBL2B) and ortho (CYBL2A) substituted CGP12177 and an iodinated form of nadolol (CYBL1) were synthesized. Their affinity was tested in vitro (left ventricular homogenates). The biodistribution of [123I]S-CYBL2B was evaluated in rabbits. Specific binding was assessed by pretreatment of the animals with 0.1 μmol propranolol. The inhibition constant values (in nanomolars, means±SEM; n=3 to 5) were determined at 1.17±0.42, 28800±9260, 11.1±2.1, 53.0±19.9, and 1790±700 for CGP12177, CYBL2A, S-CYBL2B, nadolol, and CYBL1. Myocardial uptake of [123I]S-CYBL2B was not inhibited by pretreatment of the animals with propranolol, but uptake by lung tissue could be blocked by propranolol (0.63%±0.09% vs 0.33%±0.02% % injected dose/g×kg; p<0.05). In isolated right atria, preincubation with S-CYBL2B induced a parallel rightward shift of the concentration-response curve with isoprenaline.ConclusionsS-CYBL2B shows high affinity for cardiac β-adrenoceptors, but binding proved nonspecific in vivo, whereas binding in lung tissue was specific. These results suggest that S-CYBL2B is probably not a suitable radioligand for receptor imaging.