William C. Eckelman

[125I] 3-quinuclidinyl 4-iodobenzilate: A high affinity, high specific activity radioligand for the M1 and M2-acetylcholine receptors

We have prepared a radioiodinated ligand which binds with high affinity to the muscarinic acetylcholine receptor (m-AChR). A derivative of 3-quinuclidinyl benzilate, [125I] labeled (R) 1-aza-bicyclo(2.2.2)oct-3-yl (R,S)-alpha-hydroxy-alpha-(4-[125I]iodophenyl)phenyl acetate (4- IQNB ) exhibits an affinity for the m-AChR from corpus striatum higher than that of (R) [3H] QNB. Additionally, [125I] 4- IQNB exhibits receptor selectivity for the M1 receptor since the affinity for the receptor from dog and rat heart is lower than that using dog or rat corpus striatum.

Differences in affinities of muscarinic acetylcholine receptor antagonists for brain and heart receptors.

The affinities of atropine, scopolamine, 3-quinuclidinyl benzilate and twelve analogues of 3-quinuclidinyl benzilate were determined for the muscarinic acetylcholine receptor (m-AChR) using membrane preparations from caudate/putamen. The affinity constants thus obtained were compared with affinities previously reported for the m-AChR obtained from ventricular muscle. The affinities differed significantly for six of the compounds, the largest difference being 16-fold. Neither solubilization nor variation of physiologically significant salts led to a significant change in the affinity of that compound. These results are interpreted as supporting the subclassification of the muscarinic acetylcholine receptor.

[77Br]-17-α-Bromoethynylestradiol: In vivo and In vitro characterization of an estrogen receptor radiotracer

Abstract [ 77 Br]17-α-Bromoethynylestradiol has been prepared by direct, no-carrier-added bromination of 17-α-iodoethynylestradiol. The product thus obtained has an effective specific activity up to 185 Ci/mmol and an association constant( K A ) of 7.1 × 10 9 M −1 . In vivo distribution studies indicate accumulation in rat uterus to the same levels as [ 3 H]E 2 (10% dose/g wet weight) and a target to blood ratio of 5–6.

Calculation of binding isotherms when ligand and receptor are in different volumes of distribution

Abstract An equation for the determination of the concentration of ligand bound in a system where the receptor and ligand occupy different volumes of distribution (nonhomogeneous) is derived. The validity of the equation is demonstrated using equilibrium dialysis as the model experimental system. The nonhomogeneous equation and the equation describing the homogeneous system are contrasted and the results discussed in terms of application to in vivo steady-state systems.

The use of tritium labeled compounds to develop gamma-emitting receptor-binding radiotracers.

In an effort to evaluate receptor binding drugs for their potential as gamma labeled radiopharmaceuticals suitable for clinical heart scanning, in vivo data were compared with the results obtained from a theoretical model. The distribution of selected tritium-labeled, receptor-binding radiotracers was studied in animals to determine if the heart to blood ratios agree with those obtained using a theoretical model of receptor binding. In general, the in vivo studies agree with the theoretical model when the concentration of the radiotracer in the heart is due to specific receptor binding. The use of the theoretical model for a first approximation followed by in vivo biodistribution studies is an efficient strategy to select those few from among the large number of receptor binding compounds that will ultimately yield an efficacious radiopharmaceutical to study receptor changes in the intact human heart.

In vivo and in vitro characteristics of the N-Methyl derivative of [125I]3-Quinuclidinyl 4-Iodobenzilate

Abstract Radiolabeled analogues of 3-quinuclidinyl benzilate (QNB) localize in the myocardium by virtue of ligand-receptor interactions. The radioiodinated derivative exhibits less favorable non-receptor interactions than tritiated QNB in the myocardium, probably due to increased lipophilicity. In order to prevent intracellular localization and partitioning into lipid membranes, we prepared the N-methyl quaternary analogue of the iodinated derivative [ 125 I]3-quinuclidinyl 4-iodobenzilate. In vitro and in vivo results indicate that the quaternization reduces the affinity of the analogue to a value too low for advantageous receptor-mediated localization.

Receptor-selective localization in pancreas

We examined the distribution of three tritiated ligands and two radioiodinated ligands for their ability to localize in the pancreas of rat and rabbit. The ligands examined are selective for the alpha- and beta-adrenoceptors and the muscarinic acetylcholine receptor. Of the ligands examined, the results indicate that only (R) 3H-3-quinuclidinyl benzilate (QNB) localized in the pancreas by the receptor-mediated mechanism. The % dose/g tissue, the pancreas-to-blood and pancreas-to-liver ratios are such that a 18F-labeled derivative of QNB should provide images of the pancreas.

Electrophilic iodination of aromatic rings

Abstract Five iodinating reagents were compared to determine the percentage iodination of benzene under similar conditions. Both ICI and chloramine-T in a 1:1 mixture of trifluoroacetic acid and trifluoroacetic anhydride gave high iodination yields using 0.2 M benzene as substrate. Chloramine-T in a 1:1 mixture of trifluoroacetic acid and trifluoroacetic acid anhydride is the system of choice of those studied for no-carrier-added iodination.

THE DEVELOPMENT OF MUSCARINE CHOLINERGIC RECEPTOR-BINDING RADIOTRACERS

Abstract The goal of this work is to develop muscarinic-cholinergic, receptor-binding, high-specific-activity radiotracers containing a gamma-emitting radionuclide. These radiotracers will be used to determine noninvasively the presence or change in concentration of muscarinic cholinergic receptors as a function of disease.

Important Chemical Variables for Labeling Antibodies with Indium-111 and Technetium-99m

The use of radiolabeled antibodies for radioimmunodetection of malignant tumors involves research in several disciplines such as chemistry, immunology and pharmacokinetics. Each of these areas has to be investigated systematically to guarantee a successful radioimmunodetection technology. This paper focuses its discussion primarily on chemical and immunochemical aspects associated with radiolabeling of antibodies with In-111 and Tc-99m. A bifunctional chelate approach using a cyclic DTPA dianhydride is used as a model reaction for the indirect labeling of antibody and antihuman serum albumin antibody is used as a model antibody. The optimization of important chemical parameters is discussed for the conjugation and labeling reactions. The relationship between the number of DTPA groups and the antibody immunoreactivity is also discussed.