Lee C. Washburn

Preparation and preliminary tissue studies of optically active 11C-d- and l-phenylalanine

Abstract A rapid method for the preparation of l -phenylalanine-1-11C and d -phenylalanine-1-11C is described. dl -Phenylalanine-1-11C (375 mCi) was synthesized from 11C-cyanide (2.4 Ci) by a modified Bucherer- Strecker reaction with a chemical yield of 65% in 40 min (including purification). The resolution of the d - and l -isomers was accomplished in 35 min (including purification) by oxidative deamination using immobilized l - and d -amino acid oxidase, respectively; the yields for d - and l -phenylalanine-1-11C were 19 mCi and 27 mCi. Preliminary tissue distribution studies of these labeled isomers in the rat showed that the pancreas-to-liver ratio for the l -isomer increased throughout the first hour of observation following i.v. administration while that of the d -isomer decreased after 30 min. Whole body retention data revealed that the loss of radioactivity from dl - or d -phenylalanine-1-11C was less than 2% during the first hour of observation after i.v. administration.

Blood-Brain Barrier Transport of 1-Aminocyclohexanecarboxylic Acid, a Nonmetabolizable Amino Acid for In Vivo Studies of Brain Transport

Abstract: Regional transport of 1‐aminocyclohexanecarboxylic acid (ACHC), a nonmetabolizable amino acid, across the blood‐brain barrier was studied in pentobarbital‐anesthetized rats using an in situ brain perfusion technique. The concentration dependence of influx was best described by a model with a saturable and a nonsaturable component. Best‐fit values for the kinetic constants of the frontal cortex equaled 9.7 × 10‐−4μmol/s/g for Vmax, 0.054 μmol/ml for Km, and 1.0 × 10‐−4 ml/s/g for KD in the absence of competing amino acids. Saturable influx could be reduced by >85% by either l‐phenylalanine or 2‐aminobicyclo[2.2.1]heptane‐2‐carboxylic acid, consistent with transport by the cerebrovascular neutral amino acid transport system. The transport Km for ACHC was one‐fifth that for the more commonly used homologue, I‐aminocyclopentanecarboxylic acid, and was similar to values for several natural amino acids, such as l‐methionine, l‐isoleucine, and l‐tyrosine. The results indicate that ACHC may be a useful probe for in vivo studies of amino acid transport into brain.

90Y-labeled monoclonal antibodies for cancer therapy

Monoclonal antibody 17-1A, which has specificity for colorectal carcinoma, was labeled with 90Y (10-20% radiolabeling yield). Tissue distribution studies in tumor-bearing nude mice were carried out. 90Y-labeled 17-1A showed good uptake in the SW 948 colon carcinoma cell line. However, 90Y-labeled A5C3, a monoclonal antihepatitis virus antibody studied as a control, showed similar uptake in this tumor. Neither antibody was taken up well by a WM-9 melanoma. It is believed that the loss of specificity observed is due to the low specific activity of the 90Y-labeled monoclonal antibody preparations used. This hypothesis is supported by radioimmunoassay data.

Comparison of five bifunctional chelate techniques for 90Y-labeled monoclonal antibody CO17-1 A☆

Monoclonal antibody CO17-1A was radiolabeled with 90Y by five bifunctional chelate techniques. Radiation absorbed dose estimates for normal organs and tumor were calculated for each preparation based on timed tissue distribution studies in nude mice bearing SW 948 human colorectal carcinoma xenografts. The cyclic DTPA anhydride technique was inferior to the four other techniques studied. Data for SCN-Bz-DTPA and SCN-Bz-Mx-DTPA, which were conjugated to epsilon-lysyl amino groups, were similar to those for NH2-Bz-DTPA and NH2-Bz-Mx-DTPA, which were conjugated site specifically to oligosaccharides.

Liposomal encapsulated Zn-DTPA for removing intracellular 169Yb.

Abstract— Multilamellar liposomes possessing neutral, positive or negative charges were tested for their capacity to encapsulate sodium ethylenediaminetetraacetate (EDTA) and for their selectivity in depositing in specific tissues after being injected into rats. Negative‐charged liposomes had the greatest trapping efficiency over a wide range of lipid‐to‐aqueous phase ratios. In contrast, except for lung, liposomal charge had no significant effect on the tissue distribution of encapsulated EDTA; liver and spleen exhibited the highest uptake with all preparations. The proportion of encapsulated EDTA taken up by the liver decreased as the amount of injected liposomes was increased. Free zinc diethylenetriaminepentaacetate (Zn‐DTPA) and multilamellar liposomes containing entrapped Zn‐DTPA were administered to rats that had been injected with 169Yb‐citrate 24 hr earlier. At doses of 14 mg Zn‐DTPA per kg body weight, both free Zn‐DTPA and the liposomal‐bound Zn‐DTPA caused increased removal of 169Yb from the animals. However, treatment with the liposomal Zn‐DTPA caused significantly more of the 169Yb to be removed than did the free Zn‐DTPA treatment by itself. Our data indicate that lipophilic forms of chelators can effectively increase the removal rates of heavy metal contamination in tissues.

Liposomal preparations of calcium- or zinc-DTPA have a high efficacy for removing colloidal ytterbium-169 from rat tissues

Results from this study demonstrate that liposomes increase the effectiveness of chelating agents in removing heavy metals from contaminated tissues in vivo. We compared the ability of free and liposomal preparations of Ca- and Zn-diethylenetriaminepentaacetic acid (DTPA) to remove 169Yb from tissues of rats previously injected intravenously with either soluble or colloidal forms of 169Yb . Although single injections of liposomal Zn-DTPA were better than the free chelator in reducing the body burden of 169Yb administered in a soluble form (citrate), the liposomal preparations of Zn-DTPA or Ca-DTPA were even more effective in removing the 169Yb that had been injected as the colloidal form. However, a second injection of liposomal Zn-DTPA given 8 days after the initial treatment was not as effective in removing 169Yb as a second injection of the free chelator. Whether injected in the free or liposomal forms, Ca-DTPA was more effective than Zn-DTPA in removing the colloidal 169Yb . Significantly lower amounts of colloidal 169Yb remained in the liver, kidney, muscle, bone, and blood of rats after injection of the liposomal preparations of Ca- or Zn-DTPA than in the corresponding organs of the controls (P less than 0.01). The liposomal preparations were also more efficient in reducing the retention of colloidal 169Yb in bone and blood than the free chelators (P less than 0.01).

RAPID AND MILD SYNTHESES OF RADIOIODINATED ESTROGEN DERIVATIVES VIA ORGANOBORANE TECHNOLOGY

Abstract Radioiodinated estrogen derivatives of potential use as receptor-binding agents have been synthesized via the reaction of organoboranes with 125 ICl, The utility of organoboranes as intermediates in radiopharmaceutical syntheses is discussed.

Preclinical assessment of 90Y-labeled monoclonal antibody Co17-1A, a potential agent for radioimmunotherapy of colorectal carcinoma☆

Monoclonal antibody CO17-1A was radiolabeled with 90Y using the cyclic DTPA anhydride technique and administered intravenously to athymic nude mice bearing SW 948 human colorectal carcinomas. The tumor specificity of 90Y-CO17-1A was improved by coadministration of 100 micrograms of the unlabeled antibody per animal and by purification using HPLC instead of column gel filtration chromatography. Absorbed radiation dose estimates for 90Y-CO17-1A were calculated. The radiation dose to the bone marrow will limit the amount of 90Y-CO17-1A that can be administered for cancer therapy.

Comparison of 68Ga-EDTA, [l-11C]α-aminoisobutyric acid, and [99mTC]sodium pertechnetate in an experimental blood-brain barrier lesion

Unilateral opening of the blood-brain barrier (BBB) in rats, induced by infusion of a hyperosmotic solution of mannitol into the left internal carotid artery, was applied to compare 68Ga-EDTA, [1-11C]alpha-aminoisobutyric acid (using the long-lived 14C-labeled analog), and [9mTc]sodium pertechnetate as agents for diagnosis of BBB disruption. Of the three agents and two time intervals studied, 68Ga-EDTA at 30 min postinjection gave the highest target-to-nontarget ratio. In addition, 68Ga-EDTA, unlike commonly used [99mTc]sodium pertechnetate, can be used in conjunction with positron emission tomography, which could make possible earlier and better assessment of BBB defects using 68Ga-EDTA.

Radiochemistry of Monoclonal Antibody Labeling

Monoclonal antibodies (MoAbs) to tumor-associated antigens are useful for targeting radionuclides to tumors for both diagnostic and therapeutic applications. MoAbs have been radiolabeled with numerous gamma-, beta-, alpha-, positron-, and Auger electron-emitting radionuclides. Key radionuclide examples for each type of emission are shown in Table 1. Some radionuclides, e.g., iodine-131 (131I), exhibit both beta and gamma emissions and, therefore, may be used for both radioimmunodiagnosis (RID) and radioimmunotherapy (RIT). The chemical properties of the radionuclides listed in Table 1 are very different, and consequently different radiolabeling methods must be used. The purposes of this paper are to review the radiochemistry involved in labeling MoAbs with several of the radionuclides commonly used in nuclear medicine and to examine some special techniques aimed at increasing the tumor-to-nontumor concentration ratios obtained with radiolabeled MoAbs.