Evaluation of reconstruction algorithms in SPECT neuroimaging: II. Computation of deterministic and statistical error components

For the reconstruction of a series of computer simulations of statistically-independent noisy realizations of projection data, the total error of the ith reconstructed voxel in the rth realization, Er,i, is composed of the statistical error, Sr,i, and the (deterministic) inaccuracy in the presence of noise, Di+. Di+ is composed of the (deterministic) inaccuracy in the absence of noise, Di-, and the (deterministic) additional inaccuracy in the presence of noise, Di delta. E(Er,i), the theoretical expected value of Er,i, is given by E(Er,i) = E(Di+) + E(Sr,i). Similarly, E(Di+) = E(Di-) + E(Di delta). The corresponding theoretical variances are given by sigma 2(Er,i) = sigma 2(Di+)+2C(Di+, Sr,i)+ sigma 2(Sr,i) and sigma 2(Di+) = sigma 2(Di-)+2C(Di-, Di delta)+ sigma 2(Di delta), where C(.,.) is the covariance. We have utilized these relationships to evaluate three reconstruction algorithms: standard filtered back projection (FBP), an iterative reconstruction algorithm (IRA), and a version of the IRA which incorporates a linear transformation (TIRA). For simulated brain images in which the projection data (500,000 events detected) were degraded as the result of convolution of the true radioactivity distribution with a realistic distance-dependent detector response function, for FBP the major contribution to both E(Er,i) and sigma 2(Er,i) was Di-. For the IRA and TIRA, the major contributions to E(Er,i) were Di- and Di delta, and the major contribution to sigma 2(Er,i) was Sr,i, although in some cases Di delta was also a contributing factor. Furthermore, the errors due to sigma 2(Er,i) (that is, [sigma 2(Er,i)]0.5) were more severe than those due to E(Er,i). We conclude that, in contrast to FBP, the effects of statistical noise are an important limiting factor for the IRA and TIRA, and that the future development of tomographic devices with higher sensitivity would expand the quantitative potential of the IRA and TIRA.

Evaluation of reconstruction algorithms in SPECT neuroimaging: I. Comparison of statistical noise in SPECT neuroimages with 'naive' and 'realistic' predictions

In the presence of statistical noise, an iterative reconstruction algorithm (IRA) for the quantitative reconstruction of single-photon-emission computed tomographic (SPECT) brain images overcomes major limitations of applying the standard filtered back projection (FBP) reconstruction algorithm to projection data which have been degraded by convolution of the true radioactivity distribution with a finite-resolution distance-dependent detector response: (a) the non-uniformity within the grey (or white) matter voxels which results even though the true model is uniform within these voxels; (b) a significantly lower ratio of grey/white matter voxel values than in the true model; and (c) an inability to detect an altered radioactivity value within the grey (or white) matter voxels. It is normally expected that an algorithm which improves spatial resolution and quantitative accuracy might also increase the magnitude of the statistical noise in the reconstructed image. However, the noise properties in the IRA images are very similar to those in the FBP images. In fact, the noise magnitude in both the FBP and IRA images is only slightly greater than that computed by the 'naive prediction', which presumably is a lower limit to the amount of statistical noise in a reconstructed image. Thus, the IRA should provide the potential for quantitative SPECT imaging of normal physiological responses or diseases involving both the brain grey and white matter.

Kinetic analysis of rat parotid gland muscarinic receptors in vivo: comparison with brain and heart

(RR)- and (SS)-quinuclidinyl iodobenzilate enantiomers [(RR)- and (SS)-IQNB, active and inert, respectively] have been synthesized for quantitative evaluation of muscarinic acetylcholine receptor (mAChR) binding. Pharmacokinetic approaches have not been used previously to assess in vivo IQNB binding in nonexcitable tissues. We have applied this method to examine mAChRs in rat parotid gland in comparison to those in brain and heart. Short-term infusion studies in vivo showed that the "instantaneous" reversible binding of (RR)- and (SS)-IQNB was high in the parotid (greater nonspecific binding potential), intermediate in the heart, and lowest in cortex and cerebellum. Long-term bolus injection experiments showed that the parotid gland mAChRs possessed a binding potential for receptor specific sites (380), which was intermediate between that of parietal cortex (930) and cerebellum (10) and greater than that of heart (165). In vitro binding to plasma membranes was generally consistent with the in vivo findings. In aggregate, these studies show that mAChRs can be evaluated in vivo in a nonexcitable tissue with the use of stereospecific ligands and a pharmacokinetic approach. The data suggest that IQNB, a mAChR antagonist, can identify characteristics of specific binding sites, which may reflect tissue differences.

Synthesis and structure-activity relationship of some 5-[[[(dialkylamino)alkyl]-1-piperidinyl]acetyl]-10,11-dihydro-5H- benzo[b,e][1,4]diazepin-11-ones as M2-selective antimuscarinics

A series of 5-[[[(dialkylamino)alkyl]-1-piperidinyl]acetyl]- 10,11-dihydro-5H-dibenzo[b,e][1,4]-diazepin-11-ones were prepared as potential M2-selective ligands. The compounds were evaluated for their affinity and selectivity for the muscarinic cholinergic receptor. The best M2-selective antimuscarinic agent studied is 5-[[4-[4-diethylamino)butyl]-1- piperidinyl]acetyl]-10,11-dihydro-5H-dibenzo[b,e][1,4]diazepin-11- one, which is approximately 10 times more potent at M2 receptors than previously known compounds such as 11-[[4-[4-(diethylamino)butyl]- 1-piperidinyl]acetyl]-5,11-dihydro-6H- pyrido[2,3-b][1,4]benzodiazepin-6-one (AQ-RA 741).

An improved synthesis of [125I]N-(diethylaminoethyl)-4-iodobenzamide: a potential ligand for imaging malignant melanoma

To improve the radiolabeling yield and the specific activity of [125I]N-(2-diethylaminoethyl)-4-iodobenzamide (DAB), the aryltributyltin precursor was synthesized from the N-(2-diethylaminoethyl)-4-bromobenzamide derivative by palladium catalyzed stannylation using bis(tributyltin). The radiolabeled product, [125I]DAB, was obtained by an iododestannylation reaction in high radiochemical yields (85-94%, radiochemical purity, > 98%) using chloramine-T as an oxidizing agent. The specific activity was greater than 1600 Ci/mmol. The biodistribution studies in nude mice implanted with human malignant melanoma xenograft showed a good tumor uptake (6.14% ID/g at 1 h, 2.81% ID/g at 6 h and 0.42% ID/g at 24 h) of [125I]DAB. Unfortunately, a high uptake in the non-target organs, such as liver and lung, was found. At 1 h post-injection the activity level in liver and lung was 11.76 and 7.58% ID/g, respectively. A slow clearance of activity from liver and lung was observed at 6 h (3.43 and 0.49% ID/g). These results demonstrate that iodinated IDAB is a potential radiopharmaceutical for the management of patients with malignant melanoma.

Estimation of relative regional neuroreceptor concentration by PET or SPECT: Theoretical comparisons of using a single late image or a late plus early image

The potential for using a single (SPECT) single-photon-emission computed tomography or (PET) positron emission tomography image to estimate quantitatively the relative regional neuroreceptor concentration depends on acquiring the image at a time when changes in the regional radioactivity localization are much more sensitive to changes in receptor concentration than to changes in delivery. Using the binding of [(11)C]carfentanil to the opiate receptor as a model, the authors have applied a computer simulation approach to determine the joint and marginal probability distributions for the ipsilateral/contralateral ratio of receptor concentrations and delivery. They have found that the probability distributions depend on the sensitivities for both delivery and receptor. Incorporation of data at an early time point results in a significant sharpening of the probability distributions. There is an insignificant effect of subtraction of the radioactivity localization in a control region.

A novel muscarinic receptor ligand which penetrates the blood brain barrier and displays in vivo selectivity for the m2 subtype

Alzheimer's disease (AD) involves selective loss of muscarinic m2, but not m1, subtype neuroreceptors in the posterior parietal cortex of the human brain. Emission tomographic study of the loss of m2 receptors in AD is limited by the fact that there is currently no available m2-selective radioligand which can penetrate the blood-brain barrier. In our efforts to prepare such a radioligand, we have used competition studies against currently existing muscarinic receptor radioligands to infer the in vitro and in vivo properties of a novel muscarinic receptor ligand, 5-[[4-[4-(diisobutylamino)butyl]-1-phenyl]acetyl]-10,11-dihydro-5H - -dibenzo [b,e][1,4]diazepin-11-one (DIBD). In vitro competition studies against [3H](R)-3-quinuclidinylbenzilate ([3H]QNB) and [3H]N-methylscopolamine ([3H]NMS), using membranes derived from transfected cells expressing only m1, m2, m3, or m4 receptor subtypes, indicate that DIBD is selective for m2/m4 over m1/m3. In vivo competition studies against (R,R)-[125I]IQNB indicate that DIBD crosses the blood brain barrier (BBB). The relationship of the regional percentage decrease in (R,R)-[125I]IQNB versus the percentage of each of the receptor subtypes indicates that DIBD competes more effectively in those brain regions which are known to be enriched in the m2, relative to the m1, m3, and m4, receptor subtype; however, analysis of the data using a mathematical model shows that caution is required when interpreting the in vivo results. We conclude that a suitably radiolabeled derivative of DIBD may be of potential use in emission tomographic study of changes in m2 receptors in the central nervous system.

Use of ex vivo binding to measure the brain concentrations of putative radioligands

The development of radioligands capable of imaging brain receptors depends on, amongst other factors, the ability of such compounds to penetrate the blood-brain barrier. We describe an ex vivo binding technique for measuring the brain concentration of peripherally administered unlabeled compounds. This technique can be used early in the development of putative radioligands. The pharmacokinetics of brain penetration of three muscarinic antagonists are described: QNB, BrQNB and the 2-thienyl derivative of BrQNB and were found to compare favorably to previous studies using [3H]QNB. These studies demonstrate the effectiveness of ex vivo binding in assessing the brain concentration of peripherally administered unlabeled compounds.

Radiolabeled products in rat liver and serum after administration of antibody-amide-DTPA-indium-111

Anti-human serum albumin antibody (Ab) was used as a model antibody. Ab was conjugated with DTPA using cyclic DTPA dianhydride reaction and radiolabeled with 111In. The labeled Ab was purified by affinity chromatography. Size exclusion HPLC of this product showed 62% of 111In bound to monomeric Ab and 38% of the activity bound to antibody oligomers with molecular weights ranging from 300,000 to 450,000. The labeled antibody preparation was injected into the tail vein of rats. The radioactive substances in serum and the supernatant from liver homogenates were analyzed for molecular weight and immunoreactivity. Size exclusion HPLC of the serum samples indicated that the monomeric and dimeric Abs disappeared from the serum at a similar rate over a 48 h period. In addition, a new radioactive substance with an estimated molecular weight of 35,000 appeared in the serum. The immunoreactive fraction of the circulating 111In substances decreased slowly, somewhat proportional to the appearance of the metabolite. On the other hand, the immunoreactivity of the 111In substances in the supernatant from the liver homogenate decreased rapidly and no appreciable immunoreactivity was observed after 48 h. The labeled antibody was catabolized very rapidly in the liver and the major activity in the supernatant was associated with a small molecular weight metabolite which had a HPLC retention time identical to that of DTPA-111In. The second metabolite had an estimated molecular weight of 35,000. No radioactivity was associated with transferrin.

A novel m2-selective muscarinic antagonist: binding characteristics and autoradiographic distribution in rat brain

Although several m2-selective muscarinic antagonists have been described, they are not particularly potent. Thus, the development of potent m2-selective compounds remains an important goal. We now report that a bio-isoster of AQ-RA 741 is both one order of magnitude more potent and slightly more selective than previously described compounds. DIBA, a di-benzo derivative of AQ-RA 741, in which the pyridine of the tricycle is replaced with a benzene ring, had Ki values of 4, 0.3, 11 and 2 nM at m1 through m4 receptors, respectively. These values were determined in competition studies with [3H]N-methylscopolamine ([3H]NMS) in membranes from transfected A9 L cells (m1 and m3), rat heart (m2) and NG108-15 cells (m4). AQ-RA 741 had Ki values of 34, 4, 86 and 15 nM at each of these receptors. The autoradiographic distribution of DIBA binding sites was determined by competition studies of [3H]NMS in rat brain. At low concentration, DIBA reduced [3H]NMS binding most significantly from superior colliculi, thalamus, hypothalamus, pontine nucleus, and interpeduncular nucleus, and not appreciably from caudate nucleus, cerebral cortical regions, or hippocampus, consistent with its binding to m2 receptors. These data indicate that DIBA is the most potent, m2-selective muscarinic antagonist yet described. DIBA should therefore become a useful probe in future studies of muscarinic function.

Compensation for three-dimensional detector response, attenuation and scatter in SPECT grey matter imaging using an iterative reconstruction algorithm which incorporates a high-resolution anatomical image

The use of SPECT to diagnose physiological alterations in disease states depends on the potential of SPECT to provide a quantitatively accurate reconstructed image. However, the reconstructed values depend upon the shape and size of the brain region as strongly as they depend upon true radioactivity concentration. We report here the results of applying an iterative reconstruction algorithm (IRA) to compensate for shape- and size-dependence, as well as for attenuation and scatter. The IRA is designed only for the reconstruction of images for which the true radioactivity in the white matter within the actual brain is negligible compared with the true radioactivity in the grey matter within the actual brain. The IRA incorporates an accurate three-dimensional model of detector response and utilizes an MRI image which defines the anatomical features of the brain being imaged by segmenting the grey, white and ventricular regions. It is the assumption of radioactivity localization exclusively in the grey matter which permits the efficient incorporation of the MRI image. The IRA was validated by simulation studies that utilized a slice through the basal ganglia in the realistic Hoffman three-dimensional mathematical brain model. FBP images deviate significantly from true radioactivity distribution, whereas IRA images are nearly identical to true radioactivity distribution, except for random fluctuations due to the presence of statistical noise. These results indicate that the application of the IRA will permit SPECT to distinguish deficits due to true physiological changes from apparent deficits due to imaging/reconstruction artifacts.

Synthesis, characterization and biodistribution of a new hexadentate aminethiol ligand labeled with Tc-99m

A new hexadentate aminethiol ligand (TACNS) derived from triazacyclononane was synthesized and characterized for the development of technetium radiopharmaceuticals. The ligand formed a neutral, lipophilic and stable complex with [99mTc]pertechnetate in the presence of tin(II)tartarate as a reducing agent. The biodistribution of [99mTc]TACNS indicates slight uptake in brain (0.23% ID/organ at 5 min) with a washout at 30 min to 0.14% ID/organ. A small uptake in heart (0.48% ID at 5 min) was also observed. The characterization of [99mTc]TACNS complex using single crystal x-ray analysis and mass spectroscopy has shown that an Sn-N3S3 complex was formed in which tin is oxidized from Sn(II) to Sn(IV). Pertechnetate was incorporated into the complex as counter anion. The nature of the species formed with Tc-99 and "no-carrier-added" [99mTc]pertechnetate is different as confirmed by ratio TLC. From these results, it is demonstrated that sometimes it may be difficult to predict the structure of new technetium radiopharmaceuticals, especially when stannous ion is used as a reducing agent. Moreover, the nature of the chemical species may not be the same at millimolar and at nanomolar levels.

Synthesis and receptor affinities of new 3-quinuclidinyl alpha-heteroaryl-alpha-aryl-alpha-hydroxyacetates

Five analogues of 3-quinuclidinyl benzilate were prepared in which one phenyl ring was substituted by a heterocycle; a bromine was included on either the remaining phenyl or the heterocycle to provide information relating to the affinity of potential radiohalogenated derivatives. Their affinities for the muscarinic cholinergic receptor were determined. Replacing a phenyl ring with either the 2- or 3-furyl moiety or the 2- or 3-thienyl moiety did not significantly alter the affinity to the muscarinic receptor compared with 3-quinuclidinyl 4-bromobenzilate.

A comparison of FDG PET and IQNB SPECT in normal subjects and in patients with dementia

Prior studies of patients with dementia have found similar qualitative patterns of cerebral glucose utilization with [18F]2-fluoro-2-deoxyglucose (FDG) PET and of putative muscarinic receptor activity with [123I]3-quinuclidinyl-4-iodobenzilate (IQNB). This raised doubts about whether receptor binding determines IQNB distribution and whether clinical information in IQNB scans is unique. To compare the methods directly, 4 normal volunteers and 7 patients with dementia underwent FDG PET and high-resolution IQNB SPECT scans. In normal subjects, relative regional activity from the paired scans was only weakly correlated (r = 0.29). Some regions (e.g., thalamus, frontal cortex) showed a clear disassociation of activity. In demented patients, IQNB scans tended to show larger defects than FDG scans, although one focal defect appeared only with PET. Results suggest that IQNB SPECT data are not primarily related to general physiological activity or regional cerebral blood flow and are not explained by attenuation or volume-averaging artifacts. Further studies should investigate whether IQNB scanning is a more sensitive in vivo measure of the extent of Alzheimer's disease than is FDG PET.

Pharmacokinetic simulations of SPECT quantitation of the M2 muscarinic neuroreceptor subtype in disease states using radioiodinated (R,R)-4IQNB

Alzheimer's disease (AD) involves selective loss of muscarinic M2, but not M1, subtype neuroreceptors in the posterior parietal cortex of the human brain. Emission tomographic study of the loss of M2 receptors in AD is limited by the fact that there is currently no available M2-selective radioligand which can penetrate the blood-brain barrier. However, by taking advantage of the different pharmacokinetic properties of (R,R)-[123I]IQNB for the M1 and M2 subtypes, it may be possible to estimate losses in M2. It has previously been hypothesized that the difference between an early study and a late study should provide information on the M2 receptor population. In order to test this hypothesis, we present here the results of pharmacokinetic simulations of the in vivo localization of (R,R)-[123I]IQNB in brain regions containing various proportions of M1 and M2 subtypes. These results permit us to conclude that SPECT imaging of (R,R)-[123I]IQNB localization can potentially be used to quantitate changes in the M2 subtype in a disease state within a brain region for which the ratio M2/M1 is sufficiently high in normal individuals.

3-D SPECT simulations of a complex 3-D mathematical brain model: effects of 3-D geometric detector response, attenuation, scatter, and statistical noise

The quantitative imaging characteristics of ultrahigh-resolution parallel-hole SPECT, including 3-D geometric detector response, attenuation, scatter, and statistical noise, were investigated by simulations based on a complex digitized 3-D brain model of the gray and white matter distributions. The projection data resulting from a uniform distribution of gray and white matter radioactivity, in a ratio of 5:1, were simulated. The results demonstrate significant qualitative and quantitative artifacts in reconstructed human brain images. In the absence of attenuation, scatter, and noise, artifactual variation caused inaccuracies in regional radioactivity quantification. Inclusion of attenuation scatter, and noise in the simulation caused additional artifacts, and resulted in reconstructed images which qualitatively and quantitatively corresponded very closely to reconstructed images of the actual 3-D brain phantom which was constructed from the same set of data as the mathematical 3-D brain model. It is concluded that the major degrading factor in SPECT neuroimaging is the 3-D geometric detector response function.

The in vitro dissociation kinetics of (R,R)-[125I]4IQNB is reflected in the in vivo washout of the radioligand from rat brain

We have determined the kinetics of dissociation of (R)-3-Quinuclidinyl (R)-4-[125I]Iodobenzilate ((R,R)-[125I]4IQNB) from muscarinic acetylcholine receptor preparations from the cortex, hippocampus, caudate/putamen, thalamus, pons and colliculate bodies. The dissociation curves are well described by a biexponential function and are consistent with subtype selectivity favoring slow dissociation from the M1, M3, and M4 receptors with a 20-fold faster dissociation rate for the M2 receptor. Following intravenous injection, (R,R)-[125I]4IQNB binds to receptor in the rat brain in concentrations which reflect the receptor concentration present in a structure. We determined the extent of radioligand present at two times, 2 and 24 hrs, as an indication of the relative proportions of m-AChR which exhibits rapid vs. slow dissociation of (R,R)-[125I]4IQNB. A good correlation between in vitro and in vivo results suggests that the relative populations of receptor subtypes can be imaged using in vivo pharmacokinetics of (R,R)-[125I]4IQNB.

Three-dimensional SPECT simulations of a complex three-dimensional mathematical brain model and measurements of the three-dimensional physical brain phantom

We have developed a three-dimensional computer simulation of SPECT imaging. We have applied the simulation procedure to the realistic mathematical Hoffman three-dimensional brain model to generate the projection data (in the absence of attenuation, scatter, or noise) of both a parallel-hole and a multidetector SPECT system with point-focusing collimators. The simulated projection data were then reconstructed using standard software. The projection data resulting from the distribution of grey matter alone, or grey and white matter, were simulated. The results of these simulations indicate the existence of significant qualitative and quantitative artifacts in reconstructed human brain images. For example, the reconstructed values for grey matter along a cortical circumferential profile in a transverse slice through the basal ganglia varied by a factor of 2.40 (parallel-hole) and 2.99 (point-focusing), although the original grey matter values were identical in all cortical regions in the model. We have compared the simulated reconstructed images with those obtained by imaging the physical three-dimensional Hoffman brain phantom, which was constructed based upon the same set of data from which the mathematical three-dimensional Hoffman brain model was derived. Although the simulation did not include all of the degrading factors present in the physical imaging, the two images were in good agreement, indicating the applicability of the simulation to a realistic situation and the importance of the detector resolution effect.

Synthesis and muscarinic cholinergic receptor affinities of 3-quinuclidinyl alpha-(alkoxyalkyl)-alpha-aryl-alpha-hydroxyacetates

Seven analogues of 3-quinuclidinyl benzilate (QNB) in which one phenyl ring was replaced by an alkoxyalkyl moiety were synthesized and their affinities for the muscarinic cholinergic receptor determined. An oxygen in the beta-position of the moiety was not well-tolerated. By contrast, an oxygen in the gamma-position did not change the affinity for the muscarinic receptor. However, when a bromine was placed on the remaining phenyl ring, the affinity was significantly reduced in striking contrast to results obtained on halogenation of QNB.

Binding of radioiodinated SPECT ligands to transfected cell membranes expressing single muscarinic receptor subtypes

The equilibrium dissociation constant and the kinetic rate constants were determined for the binding of (R)-[3H]3-quinuclidinyl benzilate ([3H]QNB) and [125I]3-quinuclidinyl-4-iodobenzilate ((R,R)- and (R,S)-[125I]IQNB) to transfected cell membranes expressing one single muscarinic acetylcholine receptor (mAChR) subtype. The association and dissociation kinetics for the m2 subtype were more rapid than for the m1 and m3 subtypes. The differential kinetic properties may be useful for the single photon emission computed tomographic (SPECT) evaluation of regional mAChR subtype alterations in disease states.

Urinary fibrinopeptide A in evaluation of patients with suspected acute pulmonary embolism. A prospective pilot study

This pilot study assessed the urinary fibrinopeptide A (uFPA) levels and the combination of uFPA test plus ventilation/perfusion (V/Q) scan in the diagnostic evaluation of acute pulmonary embolism (PE). One hundred consecutive patients were studied prospectively. Twenty-nine patients fulfilled diagnostic criteria defined in this study (seven with and 22 without PE). The uFPA concentration was significantly higher in patients with than without PE (41.1 +/- 2.6 vs 4.8 +/- 2.5 ng/mg of creatinine, p less than 0.0001). In all patients with PE, the uFPA levels were higher than threshold value derived by adding 2 standard deviations to the mean uFPA concentration of patients without PE. In patients without PE, the V/Q scan was negative in 16, the uFPA test was negative in 18, and at least one of the tests was negative in 21. These preliminary data suggest that a negative uFPA test may be helpful in excluding PE and that uFPA in combination with V/Q lung scans may correctly exclude PE in more patients than either test alone. Further studies in a large unselected population are needed to confirm these results.

In vivo dissociation kinetics of [3H]quinuclidinyl benzilate: relationship to muscarinic receptor concentration and in vitro kinetics

The in vivo washout kinetics of [3H]quinuclidinyl benzilate ([3H]QNB) varies significantly in various structures in the rat brain. The slowest washout rates are from the hippocampus, corpus striatum, and cortex, intermediate rates are exhibited from the thalamus and colliculi, while the fastest washout rate is from the cerebellum. We have also demonstrated a difference in the in vitro dissociation rates (k-1) of [3H]QNB from various structures. The k-1 for the hippocampus, corpus striatum and cortex, is two-fold slower than that observed in the thalamus, colliculi, and cerebellum. The differences in the in vitro dissociation kinetics are not, however, sufficient to explain the differences in the in vivo washout kinetics. We have developed a theoretical formulation which describes conditions under which the washout kinetics are a function of the concentration of receptor in a structure. Furthermore, we present a graphical method in which a plot of the reciprocal of the observed washout rate constant, 1/k(obs), vs receptor concentration is linear. Analysis of the washout kinetics of [3H]QNB from various structures of the CNS of rat were well described by this theory when the differences in in vitro k-1 are included.

Three-dimensional simulations of multidetector point-focusing SPECT imaging

We have applied an efficient algorithm for mathematically simulating the three-dimensional (3-D) response of a SPECT imaging system with a depth-dependent 3-D point spread function (3-DPSF). The input object whose reconstructed image is to be simulated is restricted to a binary map; more complex objects may be treated as linear combinations of binary maps. The 3-D convolution reduces to a sequence of additions of a 3-D line spread function (3-DLSF), appropriately translated, to the 3-D response. We have simulated the projection data from a multidetector SPECT system with point-focusing collimators. The simulated projection data were then reconstructed using the manufacturer's software. The objects simulated included simple geometrical solids such as spheres and cylinders, as well as the distribution of muscarinic cholinergic receptors in a realistic brain slice. The results of these simulations indicate the existence of significant qualitative and quantitative artifacts in reconstructed human brain images.

The distribution of cerebral muscarinic acetylcholine receptors in vivo in patients with dementia. A controlled study with 123IQNB and single photon emission computed tomography

A high-affinity muscarinic receptor antagonist, 123IQNB (3-quinuclidinyl-4-iodobenzilate labeled with iodine 123), was used with single photon emission computed tomography to image muscarinic acetylcholine receptors in 14 patients with dementia and in 11 healthy controls. High-resolution single photon emission computed tomographic scanning was performed 21 hours after the intravenous administration of approximately 5 mCi of IQNB. In normal subjects, the images of retained ligand showed a consistent regional pattern that correlated with postmortem studies of the relative distribution of muscarinic receptors in the normal human brain, having high radioactivity counts in the basal ganglia, occipital cortex, and insular cortex, low counts in the thalamus, and virtually no counts in the cerebellum. Eight of 12 patients with a clinical diagnosis of Alzheimer's disease had obvious focal cortical defects in either frontal or posterior temporal cortex. Both patients with a clinical diagnosis of Pick's disease had obvious frontal and anterior temporal defects. A region of interest statistical analysis of relative regional activity revealed a significant reduction bilaterally in the posterior temporal cortex of the patients with Alzheimer's disease compared with controls. This study demonstrates the practicability of acetylcholine receptor imaging with 123IQNB and single photon emission computed tomography. The data suggest that focal abnormalities in muscarinic binding in vivo may characterize some patients with Alzheimer's disease and Pick's disease, but further studies are needed to address questions about partial volume artifacts and receptor quantification.

3-D computer simulations of resolution effects of multidetector point focusing SPECT imaging

The authors present an efficient algorithm and the results of its application in simulating the three-dimensional (3-D) projection data resulting from a 3-D distribution of radioactivity. The algorithm was applied to a series of geometrical mathematical phantoms and to a realistic mathematical brain phantom. The authors simulated the projection data from a multidetector single-photon emission computed tomography (SPECT) system with point focusing collimators. The simulated projection data were then reconstructed using the manufacturer's software. The objects simulated included simple geometrical solids such as spheres and sheets, as well as the distribution of muscarinic cholinergic receptors in a realistic brain slice. Spheres were chosen as a model for brain structures such as caudate nucleus, thalamus, and cerebellum; sheets were selected as representing lateral cortical gray matter regions. The results of these simulations indicate the existence of significant qualitative and quantitative artifacts in reconstructed human brain images.

Muscarinic receptor subtype selectivity of novel heterocyclic QNB analogues

In an effort at synthesizing centrally-active subtype-selective antimuscarinic agents, we derivatized QNB (quinuclidinyl benzilate), a potent muscarinic antagonist, by replacing one of the phenyl groups with less lipophilic heterocyclic moieties. The displacement of [3H]-N-methyl scopolamine binding by these novel compounds to membranes from cells expressing m1-m4 receptor subtypes was determined. Most of the novel 4-bromo-QNB analogues were potent and slightly selective for m1 receptors. The 2-thienyl derivative was the most potent, exhibiting a 2-fold greater potency than BrQNB at m1 receptors, and a 4-fold greater potency at m2 receptors. This compound was also considerably less lipophilic than BrQNB as determined from its retention time on C18 reverse phase HPLC. This compound may therefore be useful both for pharmacological studies and as a candidate for a radioiodinated SPECT imaging agent for ml muscarinic receptors in human brain.

Specific in vivo binding of [125I]-iodomelatonin to melatonin receptors in rat brain

The in vivo distribution of [125I]-2-iodomelatonin in rat brain was determined at different time intervals after intraarterial injection. After one hour, radioactivity in brain areas devoid of melatonin receptors had washed out to very low concentrations, but significant retention occurred in the medial basal hypothalamus (mbh) which contained the median eminence and in the anterior pituitary gland (ap), areas known to contain high concentrations of melatonin receptors. Coinjection of unlabelled melatonin reduced radioactivity concentrations in the ap and mbh by 44% and 75% respectively at one hour, whereas radiotracer concentrations in other regions remained unchanged. These results indicate the potential for the in vivo study of melatonin receptor concentration changes in human brain in disease states by means of single photon emission computed tomography.

Immunoreactivity affects the biodistribution and tumor targeting of radiolabeled anti-P97 Fab fragment

Hydroxylapatite high performance liquid chromatography was used to prepare two fractions from 125I- Fab 96.5. One fraction (peak 1) had relatively low immunoreactivity (25-38%) and the second fraction (peak 2) had high immunoreactivity (70-81%). Scatchard analysis showed similar affinity constants for the two preparations (2.9 x 10(9) M-1 for peak 1; 3.4 x 10(9) M-1 for peak 2). In biodistribution and imaging studies in athymic mice with human melanoma (FEMX-II) xenografts the high immunoreactivity preparation rapidly cleared from the blood and nontumor organs while retention of radioactivity in the tumor was prolonged. The low immunoreactivity preparation, had slower blood and nontumor organ clearance but faster tumor clearance than the high immunoreactivity fraction. Thus, in these studies highly immunoreactive antibody gave higher tumor to nontumor ratios and enhanced the target to nontarget image contrast.

Theoretical investigation of the estimation of relative regional neuroreceptor concentration from a single SPECT or PET image

The validity of estimating changes in regional neuroreceptor concentration based upon a single ECT (emission computed tomography) image is examined to determine whether an image can be acquired at a time when changes in the observed regional radioactivity are much more sensitive to changes in receptor concentration than to changes in radioligand delivery. These sensitivities are defined as the normalized partial derivatives of the regional radioactivity signal (S) with respect to the total receptor (R(t)) and with respect to radioligand delivery (k(1)). Using computer simulations, it is found that ( partial differentials/ partial differentialR(t))/( S/R(t)) can be less than unity (receptor hyposensitivity), approximately equal to unity, or significantly greater than unity (receptor hypersensitivity). It is possible to find classes of parameter sets under which the receptor sensitivity is close to unity and the delivery sensitivity is of much lower magnitude. The results indicate that if the parameters for a given radioligand-neuroreceptor system can be established as belonging to one of these classes, then changes in regional neuroreceptor concentration can be estimated based upon a single ECT image.

Quantification of the dopamine D2 receptor in the living human caudate nucleus by PET: comparison of in vivo and in vitro kinetic parameters

It is demonstrated that the set of kinetic parameters (including first-order rate constants for the dissociation of N-methylspiperone (NMSP) from, and second-order rate constants for the association of NMSP to, the dopamine D2 and serotonin S2 receptors of the caudate nucleus) which can be derived from previously reported human caudate PET (positron emission tomographic) data is not uniquely determined, but that multiple sets generate approximately equivalent curve fits. In particular, the set consisting of the in vitro values can generate the PET data. Thus, the in vitro rate constants may apply in vivo.

The use of indium-111 labeled platelet scanning for the detection of asymptomatic deep venous thrombosis in a high risk population

Five hundred indium-111 labeled platelet imaging studies (387 donor and 113 autologous) were performed postoperatively in 473 patients who had undergone total hip replacement, total knee replacement, or internal fixation of a hip fracture to detect occult deep venous thrombosis. All patients had been anticoagulated prophylactically with aspirin, warfarin sodium (Coumadin), or dextran. Thirty-four possible cases of proximal deep venous thrombosis were identified in 28 asymptomatic patients. To verify the scan results, 31 venograms were performed in 25 patients (three refused). In 21 of 31 cases, totally occlusive thrombi were detected; in 5 cases, partially occlusive thrombi were detected; in 5 cases, no thrombus was seen. No patient who had a negative scan nor any patient who had a verified positive scan (and received appropriate heparin therapy) subsequently developed symptoms or signs of pulmonary embolism. One hundred forty-one indium study patients also underwent Doppler ultrasonography/impedance plethysmography (Doppler/IPG) as a comparative non-invasive technique. In 137 cases, the results of the indium study and Doppler/IPG studies were congruent. The indium study had no false negative results that were detected by Doppler/IPG. No patient had any clinically evident toxicity. These results suggest that indium-111 labeled platelet scanning is a safe, noninvasive means for identifying DVT in high risk patients.

Preparation and properties of (R)-(-)-1-azabicyclo[2.2.2]oct-3-yl- (R)-(+)-alpha-hydroxy-alpha-(4-[125I]iodophenyl)-alpha-phenyl acetate and (R)-(-)-1-azabicyclo[2.2.2]oct-3-yl-(S)-(-)-alpha-hydroxy-alpha- (4-[125I]iodophenyl)-alpha-phenyl acetate as potential radiopharmaceuticals

rac-4-Nitrobenzilic acid was synthesized and resolved with quinidine and quinine to give the corresponding (R)- and (S)-salts. The resolved diastereomeric salts were converted to (R)- and (S)-4-nitrobenzilic acids and subsequent esterification gave their corresponding ethyl esters. Transesterification with (R)-(-)-3-quinuclidinol afforded (R)-(-)-1-azabicyclo[2.2.2]oct-3-yl-(R)-(+)-alpha-hydroxy-alpha- (4-nitrophenyl)-alpha-phenyl acetate and (R)-(-)-1-azabicyclo[2.2.2]oct-3-yl-(S)-(-)-alpha-hydroxy- alpha-(4-nitrophenyl)-alpha-phenyl acetate. After hydrogenation, the (R,R)- and (R,S)-amines were converted to the respective triazene derivatives. The triazene derivatives reacted with sodium [125I]iodide to give (R)-(-)-1-azabicyclo[2.2.2]oct-3-yl-(R)-(+)- alpha-hydroxy-alpha-(4-[125I]iodophenyl)-alpha-phenyl acetate and (R)-(-)-1-azabicyclo[2.2.2]oct-3-yl-(S)-(-)-alpha-hydroxy- alpha-(4-[125I]iodophenyl)-alpha-phenyl acetate. The evaluation of their affinities to muscarinic acetylcholine receptors (MAcChR) shows that (R)-(-)-1-azabicyclo[2.2.2]oct-3-yl-(S)-(-)-alpha-hydroxy-alpha-(4- [125I]iodophenyl)-alpha-phenyl acetate exhibits an affinity for the MAcChR from corpus striatum that is approximately threefold lower than that of (R)-(-)-1-azabicyclo[2.2.2]oct-3-yl-(R)-(+)-alpha-hydroxy-alpha-(4- [125I]iodophenyl)-alpha-phenyl acetate.

Reduction of background activities by introduction of a diester linkage between antibody and a chelate in radioimmunodetection of tumor

A diester linkage was added between monoclonal anti-melanoma antibody 96.5 and a diethylenetriaminepentaacetic acid derivative to test if a tumor-to-blood and -to-organ ratio of the injected antibody in nude mice with human melanoma FEM XII xenografts could be increased by the addition of the readily cleavable linkage. Compared to the 111In-labeled antibody DTPA with a peptide linkage, the diester conjugate cleared much faster from the blood and was retained much less in muscle and normal organs such as liver, spleen and kidney over a 48-hr period. On the other hand, the activity retained in the tumor was larger than or similar to that of the peptide conjugate for this time period. This resulted in a 2.5, 2.1, and 2.6 fold increase in a tumor to blood, to liver and to kidney ratio at 48 hr for the diester conjugate as compared to the peptide conjugate. The whole-body biologic half life of the antibody was 36 hr, three times shorter than the peptide conjugate. The external imaging demonstrated a clearly visible tumor at 4 hr and a lower pool activity at 72 hr for the diester conjugate. The peptide conjugate, however, showed a persistant blood-pool activity at 72 hr. The addition of the diester linkage, therefore may be beneficial for imaging tumors in patients at early time intervals after injection.

In vitro and in vivo characteristics of [iodine-125] 3-(R)-quinuclidinyl (S)-4-iodobenzilate

The radioiodinated muscarinic acetylcholine receptor antagonist, [125I] 3-quinuclidinyl 4-iodobenzilate, has two high affinity diastereomeric forms, the (R,R) and (R,S)-isomers. The (R,S)-diastereomer is only threefold lower in affinity than the (R,R)-isomer, but the kinetic properties are considerably different--the dissociation rate constant is 13-fold faster for the (R,S)-isomer and the association rate constant is two to threefold faster. The calculated affinity is therefore only fourfold lower. In vivo, the clearance of (R,S)-4IQNB from receptor-rich tissue is also more rapid than that of the (R,R)-isomer, that is a reflection of the more rapid in vitro kinetic properties since the physicochemical properties and the metabolic clearance of the diastereomers is the same.

Platelet survival and turnover: important factors in predicting response to splenectomy in immune thrombocytopenic purpura

Autologous indium-111 platelet sequestration and survival studies were performed on 59 immune thrombocytopenic purpura (ITP) patients, 21 of whom underwent splenectomy shortly thereafter. Sequestration patterns were primarily splenic in 46 patients, primarily hepatic in 6 patients, and both splenic and hepatic in 8 patients. The mean platelet survival ranged from 15 to 211 hr (normal, 180-220 hr), and mean platelet turnover (a measure of platelet production rate) varied from 99 platelets/microliters/hr to 7,585 platelets/microliters/hr (normal 1,200-1,600 platelets/microliters/hr). Among splenectomy patients, 13 had an excellent response, and 8 had a fair or poor response. Neither the pattern of platelet sequestration nor the quantity of platelet-associated IgG was useful in predicting response to splenectomy. There was, however, a striking correlation between platelet studies showing short survival/high turnover and subsequent excellent response to splenectomy. Conversely, patients with only moderately decreased survival and low turnover had an unpredictable response to splenectomy. This investigation demonstrates that ITP patients are a heterogeneous population and include a significant subset whose thrombocytopenia results primarily from decreased turnover. Platelet kinetic studies appear useful in predicting beneficial response to splenectomy.

Clinical and hemodynamic significance of left ventricular diastolic volume changes by exercise radionuclide ventriculography in coronary artery disease

Recent studies have suggested that left ventricular (LV) dilatation during exercise radionuclide ventriculography may identify coronary artery disease (CAD). Coronary anatomy and LV end-diastolic pressure at catheterization were compared with results of supine exercise radionuclide ventriculography in 66 patients evaluated for chest pain. Forty-six patients had significant CAD (greater than 75% diameter stenosis) and 20 patients were normal. Radionuclide ventriculography was performed within 18 hours of catheterization, at rest and at peak exercise. Relative LV end-diastolic volumes were extrapolated from end-diastolic counts. LV end-diastolic counts increased during exercise in 19 of 20 normal subjects. In patients with CAD, LV end-diastolic counts increased in 35 (group A) and decreased in 11 (group B). The percent change in LV end-diastolic counts from rest to exercise, rest ejection fraction, exercise ejection fraction and rest LV end-diastolic pressure for each group were 20 +/- 23%, 60 +/- 13%, 67 +/- 13% and 8 +/- 3 mm Hg in normal subjects; 20 +/- 20%, 50 +/- 12%, 47 +/- 13% and 12 +/- 4 mm Hg in group A; and -9 +/- 8%, 54 +/- 21%, 49 +/- 18% and 21 +/- 7 mm Hg in group B (mean +/- standard deviation). An increase in LV end-diastolic counts was unrelated to ejection fraction response or presence of underlying CAD but only correlated to rest LV end-diastolic pressure (p less than 0.01).(ABSTRACT TRUNCATED AT 250 WORDS)

Interposition of different chemical linkages between antibody and 111In-DTPA to accelerate clearance from non-target organs and blood

Two chemically labile linkages, disulfide and diester, and two stable linkages, thioether and hydrocarbon, were introduced between antibody and 111In-DTPA in order to modify their biodistributions. The biodistributions of the new linkages were evaluated in rats with target antigens localized in lungs. For a comparison purpose, the antibody-DTPA conjugate with a peptide linkage was used as a control conjugate. The antibody conjugates with the stable linkages produced the biodistributions similar to that of the peptide linked conjugate during a 48 h period. The disulfide and diester conjugates, however, cleared from blood much faster and are retained in normal organs much lower than the peptide conjugate. The disulfide and the diester conjugate amplified the lung (target) to blood ratio by 15 and 6 times, respectively at 48 h, as compared to the corresponding target to blood ratio of the control conjugate. Compared to the control conjugate, a 3 times higher target to liver ratio was also obtained by the disulfide conjugate and a 4 times higher target to kidney ratio was obtained by the diester conjugate at 48 h.

Gallbladder visualization during technetium-99m RBC blood pool imaging. Case report and literature review

Gallbladder visualization occurred after a Tc-99m red blood cell (RBC) cardiac gated blood pool scan. To date, seven cases of gallbladder visualization after the intravenous injection of Tc-99m RBCs have been reported. In the previous six patients the gallbladder was visualized incidentally during a search for gastrointestinal (GI) bleeding. All of the patients were anemic, six of seven had chronic renal failure, and five of seven had received multiple blood transfusions. When interpreting GI bleeding scans in patients with anemia and renal failure, awareness of the possibility of gallbladder visualization in the delayed images is important to avoid false-positive results.

Accuracy of in vivo neuroreceptor quantification by PET and review of steady-state, transient, double injection, and equilibrium models

The accuracy of in vivo dopamine D2 receptor quantification by positron emission tomography (PET) was determined for several models by means of singular-value decomposition, and some of the model assumptions were reviewed. These include steady-state, transient, double injection, and equilibrium approaches. All four modes are augmented by including a realistic kinetic interaction of the radioligand with the striatal serotonin S2 receptor. When a set of parameters derived specifically with reference to the equilibrium model was applied, it was found that a reversible radioligand used in conjunction with the equilibrium model permits accurate quantification. However, an assumption of the equilibrium model-that equilibrium is achieved during the time of measurement-is shown to be unsupported by the published experimental data. These results indicate that the equilibrium approach can provide an alternative to the kinetic approaches, but that additional experimental evidence is required to demonstrate the validity of the equilibrium assumption.

Synthesis and structure-activity relationships of new muscarinic antagonists

In an attempt to develop more selective muscarinic acetylcholine receptor (m-AcChR) antagonists, (R)-1-azabicyclo[2.2.2]oct-3-yl-thioxanthene-9-carboxylate, (R,S)-thiochromane-4-carboxylate, and (R,S)-chromane-4-carboxylate were synthesized. Evaluation of the binding affinities of these compounds to muscarinic receptors indicates that replacing the oxygen by sulfur in the xanthenyl and chromanyl moieties does not significantly change selectivity, but does reduce the affinity of 5 and enhance the affinity of 9a.

[125I]17-alpha-iodovinyl 11-beta-methoxyestradiol interaction in vivo with estrogen receptors in hormone-independent MCF-7 human breast cancer transfected with the v-rasH oncogene

[125I]17-alpha-Iodovinyl 11-beta-methoxyestradiol [( 125I]MIVE2) has been evaluated as a potential radiotracer for the diagnostic imaging of estrogen receptor (ER)-positive human breast cancer. In vivo distribution experiments with athymic ovariectomized nude mice bearing human breast tumors revealed an apparent correlation between uptake of 125I-labeled compound and estrogen receptor concentration in the tumors. At 4 h after i.v. injection of [125I]MIVE2, HS578T (ER negative), ZR75-B (intermediate ER), and MCF-7ras (high ER) tumors accumulated 0.320 +/- 0.186, 0.679 +/- 0.467, and 2.6163 +/- 1.0121% injected dose/g, respectively. With coinjection of unlabeled 17-beta-estradiol, levels of radioactivity in MCF-7ras tumors were decreased to 0.4859 +/- 0.1424% injected dose/g, indicating a receptor-mediated process. Peak activity of radioligand in MCF-7ras tumors and uteri was observed at 2 h and was retained for the 8-h time course. Blood and nontarget tissue, such as muscle, revealed a rapid clearance of 125I-labeled compound by 8 h. Eight hours after injection, uterus and tumor-to-blood ratios were calculated to be 225 and 21, respectively. Also, MCF-7ras tumors were shown to accumulate 6.5-fold more radioactivity than muscle. These data suggest that [125I]MIVE2 has the capability of interacting specifically and with high affinity with estrogen receptors in human breast tumors in nude mice and may possibly be used for imaging receptor-positive tumors in breast cancer patients with very low serum estrogen levels. Selective uptake of compound in MCF-7ras tumors emphasizes the usefulness of an estrogen receptor-positive tumor model which has a unique ability to grow in a host system without circulating estrogens.

Binding characteristics and biological activity of 17 alpha-[125I]iodovinyl-11 beta-methoxyestradiol, an estrogen receptor-binding radiopharmaceutical, in human breast cancer cells (MCF-7)

17 alpha-[125I]Iodovinyl-11 beta-methoxyestradiol ([125I]MIVE2), a gamma-emitting analogue of estradiol, previously shown to bind to rat uterine estradiol receptor, was studied to determine the binding characteristics and biological activity in human breast cancer cells. In vitro determination of receptor binding by dextran-coated charcoal assays indicates that [125I]-MIVE2 binds specifically and with a high affinity to cytosolic estrogen receptors in the human breast cancer cell line, MCF-7. [3H]Estradiol binds to the receptor with approximately four times the affinity of [125I]-MIVE2 (Kd = 2.55 X 10(-9) M for [125I]MIVE2; Kd = 6.4 X 10(-10) M for [3H]estradiol). Unlabeled MIVE2 produces estrogenic effects similar to those of estradiol such as progesterone receptor induction and increases in thymidine incorporation in MCF-7 cells in culture. Cytosolic progesterone receptor levels were elevated 2.8-fold over control levels by 6 X 10(-9) M MIVE2. Stimulation of thymidine incorporation (approximately 300% above control levels) was observed after exposure to 1 X 10(-9) M MIVE2. Preliminary data show receptor-mediated uptake by the uterus in biodistribution studies in athymic nude mice given injections of [125I] MIVE2 (32-34 microCi). At 4 h, uterus:blood ratios are 20.5 and target tissue:nontarget tissue ratios are 12.9. In light of the fact that this compound can be prepared with a high specific activity, [125I]MIVE2 may have potential as a radiotracer for imaging estrogen receptor-positive breast tumors or metastatic lesions in human breast cancer patients.

Labeling monoclonal antibodies and F(ab')2 fragments with (111In) indium using cyclic DTPA anhydride and their in vivo behavior in mice bearing human tumor xenografts

Monoclonal antibodies (MAb) and their F(ab')2 fragments to human colorectal carcinoma (CRC) and human melanoma-associated antigens were conjugated to diethylenetriaminepentaacetic acid (DTPA) via an acylation reaction using cyclic DTPA dianhydride. Relative immunoreactivity of the F(ab')2 fragments was as high as 70% when an average of only 0.7 DTPA molecules was conjugated per fragment, decreasing rapidly to less than 5% when 9.0 DTPA molecules were conjugated. The 111In-labeled whole MAb in mice bearing human tumor xenografts showed higher concentrations in tumor, liver, kidney, and spleen 7 days after injection of MAb when compared with the same MAb labeled with 131I. F(ab')2 labeled with 111In showed a marked persistence in the tumor-bearing mice with higher concentrations in all organs except blood, when compared with 131I-labeled F(ab')2. Radioactivity was particularly high in the kidneys. Although images of human tumor xenografts were easily visualized using 131I-labeled F(ab')2 3 days after injection, it was difficult to visualize tumor grafts with 111In-labeled F(ab')2 due to persistently high renal, liver, and background activity. Increased catabolism of the 131I-labeled MAb may be the cause of the difference; but antibodies with high immunological activity are a necessity for in vivo imaging studies before firm conclusions can be drawn.

Transchelation of 99mTc from low affinity sites to high affinity sites of antibody

An exclusive labeling of high affinity sites of IgG and its F(ab')2 fragments with 99mTc was accomplished. Antibody was first labeled in 0.1 M acetate buffer at pH 4.5, using stannous chloride as a reducing agent. Thus, high capacity, low affinity sites and low capacity, high affinity sites were both labeled. These 99mTc complexes were stable at pH 4.5 and 7.0; however, they became destabilized at pH 8.2 and 9.0. Transchelation of 99mTc to DTPA took place at the higher pH values and leveled off at 54% 99mTc-F(ab')2 and 73% 99mTc-IgG. These results indicate that the majority of 99mTc bound to the low affinity sites was transchelated to the high affinity sites rather than to DTPA since low affinity sites account for 84% of total F(ab')2 sites and 76% of IgG sites. Biodistribution data in mice at 2.5 h postinjection were consistent with this hypothesis in that tissue concentrations of 111In-DTPA-F(ab')2 were similar to the reequilibrated 99mTc-F(ab')2 but were much higher than that of the unequilibrated 99mTc-F(ab')2.

Muscarinic acetylcholine receptors in Alzheimer's disease. In vivo imaging with iodine 123-labeled 3-quinuclidinyl-4-iodobenzilate and emission tomography

In vivo imaging of muscarinic acetylcholine receptor binding function in a patient with Alzheimer's disease, using single-photon emission computed tomography and iodine 123-labeled 3-quinuclidinyl-4-iodobenzilate (123I-QNB), and perfusion imaging using 123I-N-isopropyl p-iodoamphetamine are described. A profound decrease in perfusion to the posterior temporal and parietal cortex and a more uniform uptake of 123I-QNB throughout the cerebral cortex were observed. The 123I-QNB activity ratio was reduced compared with that of a normal age-matched subject, suggesting a moderate impairment in muscarinic receptor binding function in Alzheimer's disease. Furthermore, and more importantly, our study demonstrates that images of in vivo receptor binding can be obtained easily and nontraumatically using 123I-QNB and single-photon emission computed tomography.

Use of 3-quinuclidinyl 4-iodobenzilate as a receptor binding radiotracer

3-Quinuclidinyl 4-iodobenzilate was shown to bind to the muscarinic acetylcholine receptor (mAChR) by testing the saturability and the stereoselectivity in the corpus striatum, cerebellum, and the heart. But the ratio of radioactivity in tissues containing different concentrations of mAChR was less than the ratio of mAChR concentrations determined by in vitro saturation assay. As a result, the sensitivity to change in receptor concentration by external imaging will be reduced for this receptor binding radiotracer.