Kinetic analysis of 3-quinuclidinyl 4-[125I]iodobenzilate transport and specific binding to muscarinic acetylcholine receptor in rat brain in vivo: implications for human studies

Stereoselective neuroimaging in vivo

Stereoselectivity is a basic property of many neuronal processes due to the spatial features of molecules involved in neurotransmission. Today, neuroimaging procedures are available for studying stereoselectivity in the living brain. Mirror-image radiotracers are the molecular tools that are used, together with single photon emission tomography (SPECT) and positron emission tomography (PET), for studying stereoselective neuronal mechanisms. This review presents the findings obtained in those studies of cholinergic, noradrenergic, dopaminergic, serotonergic, glutamatergic, opioid, cannabinoid, and second messenger neurotransmission.

Neurochemical imaging of dementias

Neurochemical imaging is one of the most established "molecular" imaging techniques. There have been tremendous efforts expended to develop radioligands specific to each neurochemical system. Investigational applications of neurochemical imaging in dementing disorders are extensive. Cholinergic, dopaminergic, and serotonergic systems, as well as benzodiazepine receptors, opioid receptors, and glutamatergic receptors have been imaged in Alzheimer disease and other dementing disorders. These investigations have provided important insights into disease processes in living human patients. The clinical diagnostic use of neurochemical imaging for dementing disorders is currently limited, but this technique is used to help develop therapeutic drugs at multiple levels.

Stereoselective synthesis, in vitro, and initial in vivo evaluation of 1-methylpiperidin-4-yl alpha-hydroxy-alpha-(1-iodo-1-propen-3-yl)-alpha-phenylacetate (IPIP): a novel radioiodinated molecular probe with high affinity for the muscarinic receptor

1-Methylpiperidin-4-yl alpha-hydroxy-alpha-(1-iodo-1-propen-3-yl)-alpha-phenylacetate (IPIP, Fig. 1) was investigated as a potential radioiodinated molecular probe targeted to the muscarinic receptor complex. The IPIP stereoisomers were synthesized via a chiral intermediate in >95% enantiomeric excess. The R-isomers demonstrated a M(1) to M(2) subtype selectivity of approximately 3 to 1 and the S-isomers demonstrated non-subtype selective binding in vitro. IPIP was radiolabeled with iodide-125 with an average radiochemical yield of 74.4% (+/-14.8, n = 5), specific activities >800 mCi/micromol, and radiochemical purities >97%. In vivo the Z-isomers demonstrated high uniform cerebral uptake suggesting non-subtype selective binding. In contrast, E-R-IPIP, after allowing a low uptake in M(2) rich areas to clear, demonstrated a retention of activity in M(1) and M(4) rich cerebral regions. In addition, the cerebral uptake of E-R-IPIP and Z-S-IPIP were inhibited by 70-90% via pretreatment with R-QNB, an established muscarinic antagonist. An ex vivo metabolism study demonstrated Z-S-IPIP was stable at the receptor site with an absence of radiolabeled metabolites.

Assessment of muscarinic receptor concentrations in aging and Alzheimer disease with [11C]NMPB and PET

Cerebral cholinergic deficits have been described in Alzheimer disease (AD) and as a result of normal aging. At the present time, there are very limited options for the quantification of cholinergic receptors with in vivo imaging techniques such as PET. In the present study, we examined the feasibility of utilizing [11C]N-methyl-4-piperidyl benzilate (NMPB), a nonselective muscarinic receptor ligand, in the study of aging and neurodegenerative processes associated with cholinergic dysfunction. Based on prior data describing the accuracy of various kinetic methods, we examined the concentration of muscarinic receptors with [11C]NMPB and PET using two- and three-compartment kinetic models. Eighteen healthy subjects and six patients diagnosed with probable AD were studied. Pixel-by-pixel two-compartment model fits showed acceptable precision in the study of normal aging, with comparable results to those obtained with a more complex and less precise three-compartment model. Normal aging was associated with a reduction in muscarinic receptor binding in neocortical regions and thalamus. In AD patients, the three-compartment model appeared capable of dissociating changes in tracer transport from changes in receptor binding, but suffered from statistical uncertainty, requiring normalization to a reference region, and therefore limiting its potential use in the study of neurodegenerative processes. After normalization, no regional changes in muscarinic receptor concentrations were observed in AD.

Sensitivity of binding of high-affinity dopamine receptor radioligands to increased synaptic dopamine

PET and SPECT studies have documented that D2 radioligands of moderate affinity, but not radioligands of high affinity, are sensitive to pharmacological challenges that alter synaptic dopamine levels. The objective of this work was to determine whether the brain kinetics of high-affinity radioligands for dopamine D1 ([(3)H]SCH 23390) and D2 ([(123)I]epidepride) receptors were altered by a prolonged elevation of synaptic dopamine induced by the potent cocaine analog RTI-55. Mice were injected intravenously with radioligands either 30 min after or 4 h before intraperitoneal administration of RTI-55 (2 mg/kg). In separate experiments, the pharmacological effects of RTI-55 were assessed biochemically by measuring uptake of dopamine in synaptosomes prepared from RTI-treated mice and behaviorally by monitoring locomotor activity. Consistent with the expected elevation of synaptic dopamine, RTI-55 induced a long-lasting decrement in dopamine uptake measured ex vivo, and a prolonged increase in locomotor activity. RTI-55 injected prior to the radioligands induced a significant (P < 0.05) increase in striatal concentration of [(123)I]epidepride at 15 min, relative to saline-treated controls, but there were no differences between the two groups at later time-points. For [(3)H]SCH 23390, both initial striatal uptake and subsequent clearance were slightly increased by preadministration of RTI-55. Administration of RTI-55 4 h after the radioligands (i.e., when it was presumed that a state of near equilibrium binding of the radioligands had been reached), was associated with a significant reduction of striatal radioactivity for both radiotracers. Our results are consistent with increased competition between dopamine and radioligand for binding to both D1 and D2 receptors after treatment with RTI-55. We suggest that the magnitude of the competition is reduced by failure of the receptor binding of high-affinity radioligands to rapidly attain equilibrium.

Assessment of extrastriatal vesicular monoamine transporter binding site density using stereoisomers of [11C]dihydrotetrabenazine

Previous studies have demonstrated the utility of [11C]dihydrotetrabenazine ([11C]DTBZ) as a ligand for in vivo imaging of the vesicular monoamine transporter system. The (+)-isomer has a high affinity (approximately 1 nmol/L) for the vesicular monoamine transporter (VMAT2) binding site, whereas the (-)-isomer has an extremely low affinity (approximately 2 micromol/L). Efforts to model dynamic (+)-[11C]DTBZ data demonstrate the difficulty in separating the specific binding component from the free plus nonspecific component of the total positron emission tomography (PET) measure. The authors' previous PET work, as well as in vitro studies, indicate that there is little specific VMAT2 binding in neocortical regions. However, precise determination of in vivo binding levels have not been made, leaving important questions unanswered. At one extreme, is there sufficient specific binding in cortex or other extrastriate regions to be estimated reliably with PET? At the other extreme, is there sufficiently little binding in cortex so that it can be used as a reference region representing nonsaturable tracer uptake? The authors address these questions using paired studies with both active (+) and inactive (-) stereoisomers of [11C]DTBZ. Six normal control subjects were scanned twice, 2 hours apart, after injections of 16 mCi of (+)- and (-)-[11C]DTBZ (order counter-balanced). Three-dimensional PET acquisition consisted of 15 frames over 60 minutes for each scan. Arterial samples were acquired throughout, plasma counted, and corrected for radiolabeled metabolites. Analysis of specific binding was assessed by comparison of total distribution volume measures from the (+)- and (-)-[11C]DTBZ scans. The authors' findings indicate that only approximately 5% of the cortical signal in (+)-[11C]DTBZ scans results from binding to VMAT2 sites. The strongest extrastriatal signal comes from the midbrain regions where approximately 30% of the PET measure results from specific binding. The authors conclude that (1) the density of VMAT2 binding sites in cortical regions is not high enough to be quantified reliably with DTBZ PET, and (2) binding does appear to be low enough so that cortex can be used as a free plus nonspecific reference region for striatum.

Pharmacokinetic computer simulations of the relationship between in vivo and in vitro neuroreceptor subtype selectivity of radioligands

Pharmacokinetic computer simulations reveal a discrepancy between the in vivo and in vitro neuroreceptor subtype selectivity of radioligands. For radioligands with an in vitro neuroreceptor subtype selectivity between 0.1 and 10.0, the in vivo neuroreceptor subtype selectivity appears to be constrained to be between 0.1 and 10.0, but, in general, is not equal to the in vitro selectivity. For example, if the in vitro selectivity is 1.0 (that is, the radioligand is nonselective in vitro) the in vivo selectivity may be thought of as a random variable having a significant nonzero probability for values as low as 0.1 or as high as 10.0, with a moderate peak at a value of 1.0. For a radioligand whose in vitro subtype selectivity is greater than 10.0, the in vivo selectivity is bounded above by the in vitro subtype selectivity, but may be several orders of magnitude lower than the in vitro subtype selectivity. Thus, in spite of the discrepancy between the in vivo and in vitro neuroreceptor subtype selectivity of radioligands, there are two useful inferences about the in vivo selectivity that might be drawn from knowledge of the in vitro selectivity: (1) If the in vitro selectivity is between 0.1 and 10.0, then, at best, the in vivo selectivity might be as high as 10.0. (2) If the in vitro selectivity is greater than 10.0, then, at best, the in vivo selectivity might be as high as the in vitro selectivity.

Using single photon emission tomography (SPECT) and positron emission tomography (PET) to trace the distribution of muscarinic acetylcholine receptor (MACHR) binding radioligands

Two [18F] labeled ligands for the mAChR were prepared and evaluated in rodents and nonhuman primates. The properties of both compounds, one an agonist and the other an antagonist, were consistent with M2 subtype specificity.

Iodo-QNB cortical binding and brain perfusion: effects of a cholinergic basal forebrain lesion in the rat

This study deals with the question of whether in vivo application of [125I]iodo-quinuclidinyl-benzilate (QNB) is able to demonstrate changes in cortical muscarinic receptor density induced by a cholinergic immunolesion of the rat basal forebrain cholinergic system, and whether the potential effects on IQNB distribution in vivo are also associated with effects on regional cerebral perfusion. Immunolesioned and control animals were injected with (R,S) [125]iodo-QNB and with [99mTc]-d,l-hexamethylpropyleneamine oxime (HMPAO). The cerebral distribution of both tracers was imaged using double tracer autoradiography. Impaired cholinergic transmission was paralleled by a 10-15% increase of [125I]iodo-QNB binding in the regions of cortex and hippocampus. The local cerebral blood flow remained unchanged after cholinergic lesion.

Quantification of muscarinic cholinergic receptors with [11C]NMPB and positron emission tomography: method development and differentiation of tracer delivery from receptor binding

Quantification of human brain muscarinic cholinergic receptors was investigated with the use of [11C]N-methyl-4-piperidyl benzylate (NMPB) and positron emission tomography (PET). Whole-brain uptake of NMPB at 90 to 110 minutes after intravenous injection was approximately 10% of the administered dose. The initial cerebral distribution of NMPB corresponded to the pattern of cerebral perfusion; however, at progressively longer postinjection intervals, regional distinctions consistent with muscarinic receptor binding were evident: activity at 90 to 110 minutes postinjection was highest in the striatum and cerebral cortex, intermediate in the thalamus and pons, and lowest in the cerebellum. After the development of a chromatographic system for isolation of authentic [11C]NMPB in plasma, tracer kinetic modeling was used to estimate receptor binding from the cerebral and arterial plasma tracer time-courses. Ligand transport rate and receptor-binding estimates were obtained with the use of compartmental models and analytical methods of varying complexity, including a two-parameter pixel-by-pixel-weighted integral approach and regional least-squares curve-fitting analyses employing both two- and three-compartment model configurations. In test-retest experiments, precision of the methods and their abilities to distinguish altered ligand delivery from binding in occipital cortex during an audiovisual presentation were evaluated. Visual stimulation increased the occipital blood-to-brain NMPB transport rate by 25% to 46% in estimates arising from the various approaches. Weighted integral analyses resulted in lowest apparent transport changes and in a concomitant trend toward apparent binding increases during visual activation. The regional least-squares procedures were superior to the pixel-by-pixel method in isolating the effects of altered tracer delivery from receptor-binding estimates, indicating larger transport effects and unaltered binding. Precision was best (less than 10% test-retest differences) for the weighted integral analyses and was somewhat lower in the least-squares analyses (10-25% differences). The authors conclude that pixel-by-pixel-weighted integral analyses of NMPB distribution introduce transport biases into receptor-binding estimates. Similar confounding effects also are predicted in noncompartmental analyses of delayed radiotracer distribution. The use of regional nonlinear least-squares fitting to two- and three-compartment models, although more labor intensive, provides accurate distinction of receptor-binding estimates from tracer delivery with acceptable precision in both intra- and intersubject comparisons.

Evaluation and metabolite studies of 125I- and 123I-labelled E-(R,R)-IQNP: potential radioligands for visualization of M1 muscarinic acetylcholine receptors in brain

A new ligand for the M1 muscarinic receptor subtype, E-(R,R)-1-azabicyclo[2.2.2]oct-3-yl alpha-hydroxy-alpha-(1-iodo-1-propen-3-yl)-alpha-phenylacetate (E-IQNP), was labelled with 125I and 123I for autoradiographic studies on human whole-brain cryosections and SPET studies, respectively, in Cynomolgus monkey. Autoradiography demonstrated E-[125I]IQNP binding in M1 receptor-rich regions such as the neocortex and the striatum. The binding was displaceable by the selective M1 antagonist biperiden. In vivo single photon emission tomography (SPET) studies with E-[123I]IQNP demonstrated a high accumulation of radioactivity in the monkey neocortex. Rapid hydrolysis of the quinuclidinyl ester to the free acid was found to be a major biotransformation route for E-[123I]IQNP. The free acid of E-[123I]IQNP does not pass the blood-brain barrier, but the plasma concentration was high as compared to the total radioactivity in brain. It is thus necessary to correct for the high concentration of radioactive metabolites in parenchymal blood (CBV) to obtain accurate values for E-[123I]IQNP binding in brain.

Linear least squares compartmental-model-independent parameter identification in PET

A simplified approach involving linear-regression straight-line parameter fitting of dynamic scan data is developed for both specific and nonspecific models. Where compartmental-model topologies apply, the measured activity may be expressed in terms of: its integrals, plasma activity and plasma integrals--all in a linear expression with macroparameters as coefficients. Multiple linear regression, as in spreadsheet software, determines parameters for best data fits. Positron emission tomography (PET)-acquired gray-matter images in a dynamic scan are analyzed: both by this method and by traditional iterative nonlinear least squares. Both patient and simulated data were used. Regression and traditional methods are in expected agreement. Monte-Carlo simulations evaluate parameter standard deviations, due to data noise, and much smaller noise-induced biases. Unique straight-line graphical displays permit visualizing data influences on various macroparameters as changes in slopes. Advantages of regression fitting are: simplicity, speed, ease of implementation in spreadsheet software, avoiding risks of convergence failures or false solutions in iterative least squares, and providing various visualizations of the uptake process by straight line graphical displays. Multiparameter model-independent analyses on lesser understood systems is also made possible.

Kinetic evaluation of positron-emitting muscarinic receptor ligands employing direct intracarotid injection

The development and characterization of new receptor ligands for in vivo binding assays are often both lengthy and expensive. It is therefore desirable to predict the suitability of a ligand early in the process of its evaluation. In the present study, compartmental analysis following intracarotid ligand injection in the monkey is used to evaluate the in vivo kinetics of the muscarinic cholinergic receptor antagonists [11C]tropanyl benzilate ([11C]TRB) and [11C]N-methylpiperidyl benzilate ([11C]NMPB). Animals were implanted with chronic subcutaneous access ports and indwelling catheters with tips located in the common carotid artery, just proximal to its bifurcation. The external carotid artery was ligated to ensure selective tracer delivery through the internal carotid artery to the brain. Positron emission tomography was used to measure brain tissue time-activity curves following tracer injections. CBF was estimated from the clearance of [15O]H2O, and receptor ligand distributions were analyzed according to a physiologic model consisting of an intravascular compartment and nonspecific plus free and receptor-bound tissue ligand compartments. In [11C]TRB studies, marked reductions in the forward ligand-receptor binding rate and in both the total and the specific binding tissue-to-plasma volumes of ligand distribution were observed after scopolamine receptor blockade or with low administered specific activity. Conversely, neither the distribution volume of the nonspecific plus free ligand compartment nor the rate of ligand dissociation from receptor sites was affected. In [11C]NMPB studies, tissue compartments describing specific binding and nonsaturable components could not be reliably separated. The receptor-related term in this case, the total tissue-to-plasma distribution volume, demonstrated reduction after low specific activity ligand injection. Comparison of the two ligands suggests that NMPB interacts more rapidly with the receptors and has a lower apparent volume of distribution than does TRB. Thus, NMPB may be the more suitable ligand if accurate estimates of binding dissociation rate are limited by temporal constraints or if simplified, one-tissue-compartment analyses are used. The carotid injection method appears promising for the initial evaluation of ligand kinetics, permitting physiologic compartmental analyses without measurement of input functions or chromatography of blood samples.

Autoradiographic evidence that (R)-3-quinuclidinyl (S)-4-fluoromethylbenzilate ((R,S)-FMeQNB) displays in vivo selectivity for the muscarinic m2 subtype

Alzheimer's disease (AD) involves selective loss of muscarinic m2, but not m1, subtype neuroreceptors in cortical and hippocampal regions of the human brain. Until recently, emission tomographic study of the loss of m2 receptors in AD has been limited by the absence of available m2-selective radioligands that can penetrate the blood-brain barrier. We now demonstrate the in vivo m2 selectivity of a fluorinated derivative of QNB, (R)-3-quinuclidinyl (S)-4-fluoromethylbenzilate ((R,S)-FMeQNB), by studying autoradiographically the in vivo inhibition of radioiodinated (R)-3-quinuclidinyl (S)-4-iodobenzilate ((R,S)-[125I]IQNB) binding by unlabelled (R,S)-FMeQNB. In the absence of (R,S)-FMeQNB, (R,S)-[125I]IQNB labels brain regions in proportion to the total muscarinic receptor concentration; in the presence of 75 nmol of (R,S)-FMeQNB, (R,S)-[125I]IQNB labelling in those brain regions containing predominantly m2 subtype is reduced to background levels. We conclude that (R,S)-FMeQNB is m2-selective in vivo, and that (R,S)-[18F]FMeQNB may be of potential use in positron emission tomographic (PET) study of the loss of m2 receptors in AD.

Specific binding component of the "inactive" stereoisomer (S,S)-[125I] IQNB to rat brain muscarinic receptors in vivo

In vivo nonspecific binding can be estimated using the inactive stereoisomer of a receptor radioligand. However, the binding of the inactive stereoisomer may be partially specific. Specific binding of the inactive (S,S)-[125I]IQNB was estimated from the inhibition induced by a competing nonradioactive ligand. This technique differed from the usual approach, since it was used to study the inactive rather than the active stereoisomer. The results indicate that there is substantial specific binding for (S,S)-[125I]IQNB.

Differential cholinergic regulation in Alzheimer's patients compared to controls following chronic blockade with scopolamine: a SPECT study

The effects of low-dose chronic scopolamine on measures of cerebral perfusion and muscarinic receptors were tested in eight Alzheimer's disease (AD) subjects and eight elderly controls. Single photon emission computed tomography (SPECT) scans using technetium-labelled hexamethypropylene amine oxide (99mTc-HMPAO) to measure cerebral perfusion before and after chronic scopolamine revealed a significant 12% increase in the normal controls (P < 0.01) while the AD subjects showed no significant change. In contrast, the controls showed decreased muscarinic binding as evidenced by 123I-quinuclidinyl-4-iodobenzilate (123I-QNB) labelling after chronic drug (-10%, P < 0.01) whereas the AD subjects showed increased 123I-QNB labelling (+8%, P < 0.05). The difference between AD and control subjects was even more marked when the ratio of I-QNB to HMPAO uptake was compared, pointing to a double dissociation in the SPECT results. These data cannot be explained by group differences in cerebral perfusion alone and suggest a differential sensitivity between AD and elderly controls to chronic cholinergic blockade.

Evaluation of stereoisomers of 4-fluoroalkyl analogues of 3-quinuclidinyl benzilate in in vivo competition studies for the M1, M2, and M3 muscarinic receptor subtypes in brain

To develop a subtype selective muscarinic acetylcholine receptor (mAChR) antagonist for PET, fluorine-19 labeled alkyl analogues of quinuclidinyl benzilate (QNB) were synthesized by stereoselective reactions. To investigate these analogues for tissue subtype specificity, in vivo competitive binding studies were performed in rat brain using (R)-3-quinuclidinyl (R)-4-[125I]iodobenzilate (IQNB). Five, fifty, or five-hundred nmol of the non-radioactive ligands were coinjected intravenously with 8 pmol of the radioligand, Cold (R,R)-IQNB blocked (R,R)-[125I]IQNB in a dose-dependent manner, without showing regional specificity. For the (R,S)-fluoromethyl, -fluoroethyl and -fluoropropyl derivatives, a higher percent blockade was seen at 5 and 50 mmol levels in M2 predominant tissues (medulla, pons, and cerebellum) than in M1 predominant tissues (cortex, striatum and hippocampus). The blockade pattern of the radioligand also correlated qualitatively with the percentage of M2 receptors in the region. The S-quinuclidinyl analogues showed M2 selectivity but less efficient blockade of the radioligand, indicating lower affinities. Radioligand bound to the medulla was inversely correlated to the M2 relative binding affinity of the fluoroalkyl analogues. These results indicate that the nonradioactive ligand blocks the radioligand based on the affinity of the nonradioactive ligand for a particular receptor subtype compared to the affinity of the radioligand for the same receptor subtype. Of the seven compounds evaluated, (R,S)-fluoromethyl-QNB appears to show the most selectivity for the M2 subtypes in competition studies in vivo.

Cholinergic neurotransmission studied in vivo using positron emission tomography or single photon emission computerized tomography

During the past decade, considerable efforts have been made in the development of radiopharmaceuticals for the in vivo study of the cholinergic neurotransmission using positron emission tomography or single photon emission computerized tomography. The main cholinergic radioligands, labelled with positron- or gamma-photon-emitting radionuclides, are reviewed with respect to use as in vivo markers of either acetylcholinesterase, vesicular acetylcholine transporter, brain and heart muscarinic receptors, or cholinergic nicotinic receptors. The main results obtained in the in vivo study of the physiology, pharmacology or pathology of the different steps of the cholinergic neurotransmission using single photon emission computerized tomography and positron emission tomography are discussed.

Effects of blood flow on [11C]raclopride binding in the brain: model simulations and kinetic analysis of PET data

To assess the stability of different measures of receptor occupancy from [11C]raclopride (a D2 antagonist) studies with positron emission tomography, we analyze data from five test/retest studies in normal volunteers in terms of individual model parameters from a three-compartment model, the distribution volume (DV) and the ratio of DVs from a receptor-containing region of interest to a non-receptor-containing region. Large variations were found in the individual model parameters, limiting their usefulness as an indicator of change in receptor systems. The DV ratio showed the smallest variation. Individual differences were reflected in the greater intersubject variation in DV than intrasubject variation. The potential effects of blood flow on these measurements were addressed both experimentally and by simulation studies using three models that explicitly incorporate blood flow into a compartmental model that also includes receptor-ligand binding. None of the models showed any variation in the DV with changes in blood flow as long as flow was held constant during the simulation. Experimentally, blood flow was significantly reduced by hyperventilation in a human subject. The DV was found to be reduced relative to baseline in the hyperventilation study, but the DV ratio remained unchanged. The effect of elevated and reduced flow was also tested in two baboon experiments in which PCO2 was varied. Some variability in the DV ratio was observed but was not correlated with changes in blood flow. This raises the possibility that other factors indirectly related to changes in blood flow (or PCO2) may cause changes in DV, and these effects need to be considered when evaluating experimental results.

Interaction of iodinated quinuclidinyl benzilate enantiomers with M3 muscarinic receptors

We examined the interaction of 3-quinuclidinyl-4-iodobenzilate enantiomers, (RR)- and (SS)-IQNB, relatively receptor-active and -inactive, respectively, with M3-muscarinic receptors (mAChRs) in rat parotid acinar cells in vitro. This stereospecific antagonist pair has often been used for in vivo studies of mAChRs. There was a 16-fold difference in the ability of (RR)- and (SS)-IQNB to bind in vitro to mAChRs; Ki values estimated by competition with N-methylscopolamine were 5.3 and 84.2 nM, respectively. However, the ability of these antagonists to inhibit carbachol-stimulated inositol trisphosphate formation (Ki values determined by Schild analyses) was more similar, 16.3 and 47.7 nM, respectively for (RR)- and (SS)-IQNB. These data suggest that while it may be useful to employ this antagonist pair to evaluate some mAChR subtypes in vivo, it is difficult to use them in studies of M3-mAChRs.

Rat salivary gland blood flow and blood-to-tissue partition coefficients following X-irradiation

Effects of a single high dose of X-irradiation (15 Gy) to the head and neck region in adult male rats were examined. Three days after radiation, rats were infused with [14C]-iodoantipyrine (IAP) via the femoral vein. Animals were killed at designated times thereafter and the three major salivary glands removed. The uptake of IAP by these tissues was analysed using a pharmacokinetic model. Radiation had relatively modest effects on all measured variables when compared with results from the non-irradiated control and pair-fed groups.

[11C]tropanyl benzilate-binding to muscarinic cholinergic receptors: methodology and kinetic modeling alternatives

Quantitative estimation of cerebral muscarinic receptors was investigated with the use of the antagonist [11C]tropanyl benzilate ([11C]TRB) and positron emission tomography (PET). Kinetic modeling alternatives were examined with the goal of identifying an analysis method providing stable receptor measures, yet avoiding biases from inappropriate reductions in model complexity. Dynamic PET scans were performed on six young normal volunteers. Several modeling approaches yielding relative receptor density measures were evaluated: (a) a single "late" scan using relative tracer concentration values; (b) a slope estimate from graphic analysis (Patlak plot); (c) a two-compartment, two-parameter model (transport and total ligand distribution volume); (d) a three-compartment, two-parameter model using the free+nonspecific distribution volume, DV', fixed to the cerebellar value; (e) an early scan for transport, a fixed value for DV', and a single late scan for the binding rate constant; and (f) a three-compartment, three-parameter model. Both computer simulations and PET scan results indicate all methods provide receptor density index measures with the same rank order as in vitro measures. Oversimplified approaches (methods 1 and 2) yield a more highly nonlinear relation between the estimated receptor density index and the known receptor density than do methods retaining greater model complexity (methods 3-6). However, noise propagation into the receptor measure is greater for the more complex methods. Reliable receptor density information can be obtained from kinetic [11C]TRB PET studies, with methods 3-5 providing the most appropriate levels of model complexity for estimates of relative muscarinic receptor density.

[3H]QNB 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. [3H](R)-3-quinuclidinylbenzilate ([3H]QNB) is commonly used for performing in vitro studies of the muscarinic acetylcholine receptor (mAChR), either with membrane homogenates or with autoradiographic slices, in which [3H]QNB is nonsubtype-selective. We report here the results of in vivo studies, using both carrier-free and low specific activity [3H]QNB, which show that [3H]QNB exhibits a substantial in vivo m2-selectivity. Previously reported in vivo (R)-3-quinuclidinyl (R)-4-iodobenzilate ((R,R)-[125I]IQNB) binding appears to be nonsubtype-selective. Apparently the bulky iodine substitution in the 4 position reduces the subtype selectivity of QNB. It is possible that a less bulky fluorine substitution might permit retention of the selectivity exhibited by QNB itself. We conclude that a suitably radiolabeled derivative of QNB, possibly labeled with 18F, may be of potential use in positron emission tomographic (PET) study of the loss of m2 receptors in AD.

New approaches for determining the site of salivary fluid secretory disorders

Considerable progress has been made in understanding the events which underlie salivary fluid secretion. We have utilized novel approaches to evaluate the functional status of three distinct sites in the secretory cascade. First, salivary sympathetic nerve function studies used isotopic dopamine to measure catecholamine uptake and metabolism in vivo. Second, salivary muscarinic-cholinergic receptors were characterized in vivo by use of stereo-specific ligands and pharmacokinetic analyses. Finally, microfluorometric methods were used to study intracellular Ca2+ signaling in dispersed cells prepared from biopsied tissue. We conclude that it is possible to determine the functional status of key steps in salivary fluid generation with isotopic and fluorometric imaging techniques.

Measurement of benzodiazepine receptor number and affinity in humans using tracer kinetic modeling, positron emission tomography, and [11C]flumazenil

Kinetic methods were used to obtain regional estimates of benzodiazepine receptor concentration (Bmax) and equilibrium dissociation constant (Kd) from high and low specific activity (SA) [11C]flumazenil ([11C] Ro 15-1788) positron emission tomography studies of five normal volunteers. The high and low SA data were simultaneously fit to linear and nonlinear three-compartment models, respectively. An additional inhibition study (pretreatment with 0.15 mg/kg of flumazenil) was performed on one of the volunteers, which resulted in an average gray matter K1/k2 estimate of 0.68 +/- 0.08 ml/ml (linear three-compartment model, nine brain regions). The free fraction of flumazenil in plasma (f1) was determined for each study (high SA f1: 0.50 +/- 0.03; low SA f1: 0.48 +/- 0.05). The free fraction in brain (f2) was calculated using the inhibition K1/k2 ratio and each volunteer's mean f1 value (f2 across volunteers = 0.72 +/- 0.03 ml/ml). Three methods (Methods I-III) were examined. Method I determined five kinetic parameters simultaneously [K1, k2, k3 (= konf2Bmax), k4, and konf2/SA] with no priori constraints. An average kon value of 0.030 +/- 0.003 nM-1 min-1 was estimated for receptor-rich regions using Method I. In Methods II and III, the konf2/SA parameter was specifically constrained using the Method I value of kon and the volunteer's values of f2 and low SA (Ci/mumol). Four parameters were determined simultaneously using Method II. In Method III, K1/k2 was fixed to the inhibition value and only three parameters were estimated. Method I provided the most variable results and convergence problems for regions with low receptor binding. Method II provided results that were less variable but very similar to the Method I results, without convergence problems. However, the K1/k2 ratios obtained by Method II ranged from 1.07 in the occipital cortex to 0.61 in the thalamus. Fixing the K1/k2 ratio in Method III provided a method that was physiologically consistent with the fixed value of f2 and resulted in parameters with considerably lower variability. The average Bmax values obtained using Method III were 100 +/- 25 nM in the occipital cortex, 64 +/- 18 nM in the cerebellum, and 38 +/- 5.5 nM in the thalamus; the average Kd was 8.9 +/- 1.0 nM (five brain regions).

Imaging muscarinic cholinergic receptors in human brain in vivo with Spect, [123I]4-iododexetimide, and [123I]4-iodolevetimide

A method to image muscarinic acetylcholine receptors (muscarinic receptors) noninvasively in human brain in vivo was developed using [123I]4-iododexetimide ([123I]IDex), [123I]4-iodolevetimide ([123I]ILev), and single photon emission computed tomography (SPECT). [123I]IDex is a high-affinity muscarinic receptor antagonist. [123I]ILev is its pharmacologically inactive enantiomer and measures nonspecific binding of [123I]IDex in vitro. Regional brain activity after tracer injection was measured in four young normal volunteers for 24 h. Regional [123I]IDex and [123I]ILev activities were correlated early after injection, but not after 1.5 h. [123I]IDex activity increased over 7-12 h in neocortex, neostriatum, and thalamus, but decreased immediately after the injection peak in cerebellum. [123I]IDex activity was highest in neostriatum, followed in rank order by neocortex, thalamus, and cerebellum. [123I]IDex activity correlated with muscarinic receptor concentrations in matching brain regions. In contrast, [123I]ILev activity decreased immediately after the injection peak in all brain regions and did not correspond to muscarinic receptor concentrations. [123I]IDex activity in neocortex and neostriatum during equilibrium was six to seven times higher than [123I]ILev activity. The data demonstrate that [123I]IDex binds specifically to muscarinic receptors in vivo, whereas [123I]ILev represents the nonspecific part of [123I]IDex binding. Subtraction of [123I]ILev from [123I]IDex images on a pixel-by-pixel basis therefore reflects specific [123I]IDex binding to muscarinic receptors. Owing to its high specific binding, [123I]IDex has the potential to measure small changes in muscarinic receptor characteristics in vivo with SPECT. The use of stereoisomerism directly to measure nonspecific binding of [123I]IDex in vivo may reduce complexity in modeling approaches to muscarinic acetylcholine receptors in human brain.

In vivo muscarinic cholinergic receptor imaging in human brain with [11C]scopolamine and positron emission tomography

Cerebral muscarinic cholinergic receptors were imaged and regionally quantified in vivo in humans with the use of [11C]scopolamine and positron emission tomography. Previous studies in experimental animals have suggested the utility of radiolabeled scopolamine for in vivo measurements, on the bases of its maintained pharmacologic specificity following systemic administration and the exclusion of labeled metabolites from the brain. The present studies describe the cerebral distribution kinetics of [11C]scopolamine in normal subjects following intravenous injection. Scopolamine is initially delivered to brain in a perfusion-directed pattern. After 30 to 60 min, activity is lost preferentially from cerebral structures with low muscarinic receptor density including the cerebellum and thalamus. Activity continues to accumulate throughout a 2 h postinjection period in receptor-rich areas including cerebral cortex and the basal ganglia. The late regional concentration of [11C]scopolamine does not, however, accurately parallel known differences in muscarinic receptor numbers in these receptor-rich areas. Tracer kinetic analysis of the data, performed on the basis of a three-compartment model, provides receptor binding estimates in good agreement with prior in vitro measurements. Kinetic analysis confirms significant contributions of ligand delivery and extraction to the late distribution of [11C]scopolamine, reconciling the discrepancy between receptor levels and tracer concentration. Finally, a novel dual-isotope method for rapid chromatographic processing of arterial blood samples in radiotracer studies is presented. The combination of rapid chromatography and compartmental analysis of tracer distribution should have broad utility in future in vivo studies with short-lived radioligands.

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.

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.

Differentiation of radioligand delivery and binding in the brain: validation of a two-compartment model for [11C]flumazenil

We recently developed a two-compartment, two-parameter tracer kinetic model to estimate the in vivo ligand transport rate (K1) and distribution volume (DV) for the benzodiazepine antagonist [11C]flumazenil (FMZ) as measured by positron emission tomography (PET). The aim of the present study was to validate that this simplified model provides a stable measure of regional benzodiazepine receptor availability even when ligand delivery is altered. Six young normal volunteers underwent two PET studies subsequent to intravenous injections of [11C]FMZ. Each FMZ study was immediately preceded by measurements of CBF following injection of [15O]water. One set of scans (water/FMZ) was acquired under resting conditions and the other set during audiovisual stimulation. Six additional volunteers underwent two FMZ studies under identical resting conditions. Parametric images were analyzed and a comparison of test-retest studies in the stimulation group revealed a significant increase of CBF and K1 of FMZ in the occipital cortex evoked by visual activation, whereas no regional changes were noted for the DV of FMZ. No significant changes were noted for either K1 or DV of FMZ when comparing studies in the rest-rest setting. The results indicate that the use of a simple two-compartment model for the tracer kinetic analysis of [11C]FMZ makes it possible to separate high-affinity binding from altered radio-ligand delivery to the human brain.

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.

Regional brain measurement of Bmax and KD with the opiate antagonist cyclofoxy: equilibrium studies in the conscious rat

Brain distribution of the opiate receptor antagonist, cyclofoxy (CF), was evaluated at equilibrium in rats. A combination of i.v. injection and constant i.v. infusion was used to administer CF over a wide dose range (2.4-450 nmol/rat). Kinetic simulations and experimental results showed that this administration schedule accomplishes "true" tissue-blood equilibrium of CF within 60 min. To estimate the receptor-ligand binding parameters, we assumed that the CF concentration at the receptor site is identical to that in plasma water at equilibrium, and can be calculated from measured blood data after corrections for radiolabeled metabolites and plasma protein binding. This assumption was supported by CSF and plasma water measurements at equilibrium. Regional KD, Bmax, and a nonspecific tissue binding equilibrium constant (Keq) were estimated by fitting the tissue and plasma water concentrations to a single receptor model; the estimated values were 1.4-2.9 nM, 15-74 pmol/g of tissue, and 5.2-8.0, respectively. They are in good agreement with previous in vitro measurements (Rothman and McLean, 1988) as well as in vivo estimates from i.v. injection experiments (Sawada et al., 1990c). The conventional method to estimate the receptor-ligand binding parameters using data from cerebellum to approximate nonspecific tissue binding was found to be unacceptable. Although cerebellum is a brain region with no opiate receptors in rats, small differences in nonspecific tissue binding in different brain regions resulted in significant overestimations of KD and Bmax with this method. Receptor-active and -inactive enantiomers [[18F](-)-CF and [3H](+)-CF)] were simultaneously administered to the same animal and the receptor-bound CF concentration could be accurately measured; this method was used to estimate Bmax from a single study in a single animal and has potential for direct application in human studies using positron emission tomography.

Kinetic analysis of transport and opioid receptor binding of [3H](-)-cyclofoxy in rat brain in vivo: implications for human studies

[3H]Cyclofoxy (CF: 17-cyclopropylmethyl-3,14-dihydroxy-4,5-alpha-epoxy-6-beta-fluoromorp hinan) is an opioid antagonist with affinity to both mu and kappa subtypes that was synthesized for quantitative evaluation of opioid receptor binding in vivo. Two sets of experiments in rats were analyzed. The first involved determining the metabolite-corrected blood concentration and tissue distribution of CF in brain 1 to 60 min after i.v. bolus injection. The second involved measuring brain washout for 15 to 120 s following intracarotid artery injection of CF. A physiologically based model (Sawada et al., 1990a) and a classical compartmental pharmacokinetic model (Wong et al., 1986a) were compared. The models included different assumptions for transport across the blood-brain barrier (BBB); estimates of nonspecific tissue binding and specific binding to a single opiate receptor site were found to be essentially the same with both models. The nonspecific binding equilibrium constant varied modestly in different brain structures (Keq = 3-9), whereas the binding potential (BP) varied over a much broader range (BP = 0.6-32). In vivo estimates of the opioid receptor dissociation constant were similar for different brain structures (KD = 2.1-5.2 nM), whereas the apparent receptor density (Bmax) varied between 1 (cerebellum) and 78 (thalamus) pmol/g of brain. The receptor dissociation rate constants in cerebrum (k4 = 0.08-0.16 min-1; koff = 0.16-0.23 min-1) and brain vascular permeability (PS = 1.3-3.4 ml/min/g) are sufficiently high to achieve equilibrium conditions within a reasonable period of time. Graphical analysis (Patlak and Blasberg, 1985) of the data is inappropriate due to the high tissue-loss rate constant (kb = 0.03-0.07 min-1) for CF in brain. From these findings, CF should be a very useful opioid receptor ligand for the estimation of the receptor binding parameters in human subjects using [18F]CF and positron emission tomography.