James W. Brodack

NCA 16α-[18F]fluoroestradiol-17β: The effect of reaction vessel on fluorine-18 resolubilization, product yield, and effective specific activity

Abstract Although the reported synthesis of the title compound resulted in a high radiochemical yield (43% based on resolubilized 18F), the effective specific activity at EOS was low (166 Ci/mmol). Reduction in the amount of carrier fluoride in the target water improved the effective specific activity of the product, but with a concommitant decrease in the resolubilized yield of the fluoroestradiol (12.7%). A re-examination of the labeling parameters was performed to determine the conditions that would increase the yield of the fluoroestrogen and maintain a high effective activity for the product. Since the amount of resolubilized 18F in THF is important in obtaining high specific activity compounds in this type of synthesis, several types of vessels were investigated to determine their effect on the evaporation of the [18O]H2O target water and subsequent resolubilization of 18F into THF. Of these vessels (Pt crucible, borosilicate glass, siliconized borosilicate glass, VacutainerR), the Vacutainer afforded the highest resolubilization of 18F into THF (90%), resulting in an improved resolubilized yield for the fluoroestradiol (28%) and an increased effective specific activity at EOS for the product (1600–3939 Ci/mmol).

Synthesis of no-carrier-added N-([18F]fluoroalkyl)spiperone derivatives

3-N-([18F]fluoroalkyl)spiperone derivatives (2,3) can be prepared by N-alkylation of spiperone (1) with fluoroalkyl halides. The fluoroalkylating species 2-[18F]fluoroethyl bromide (7), 3-[18F]fluoropropyl bromide (8) and 4-[18F]fluorobutyl bromide (9) were prepared by [18F]fluoride ion displacement of the corresponding trifluoromethanesulfonates (triflates 4,5,6). By this method, the 2-[18F]fluoroethyl-, 3-[18F]fluoropropyl-, and 4-[18F]fluorobutyl spiperone derivatives (2a-c) can be prepared and purified rapidly and conveniently, within 40 min, in yields of 30-40% (end of synthesis, EOS). An alternative approach, suitable for the preparation of 2-[18F]fluoroalkyl (ethyl, propyl, butyl, pentyl and hexyl) spiperone derivatives (3a-d), involves iodo[18F]fluorination of terminal olefins, followed by N-alkylation of spiperone. This sequence is less convenient and proceeds in lower overall yields (less than 5%).

Characterization of the uptake of 16α-([18F]fluoro)-17β-estradiol in DMBA-induced mammary tumors☆

Abstract In order to investigate possible correlations between the uptake of 16α-([18F]fluoro)-17β-estradiol (18F-ES) by 7,12-dimethylbenz(a) anthracene (DMBA)-induced tumors in rats and the estrogen receptor (ER) content of these tumors, a comprehensive study was performed in which the tissue distribution of 18F-ES was measured in tumor-bearing rats, together with simultaneous measurements of blood volume (by technetium-labeled red blood cells) and blood flow (by iodoantipyrine infusion). In addition, the time course of 18F-ES metabolism and the tissue distribution of the metabolites was studied. Metabolism of 18F-ES is very rapid, and after 2 h, most of the activity in blood and nontarget tissues is due to metabolites; target tissue activity, however, is due mainly to unmetabolized compound. Most of the circulating activity, both 18F-ES and its metabolites, is strongly associated with macromolecules or cells, and while the metabolites are not taken up selectively by target tissues, they do enter nontarget tissues. Tumor blood volume and blood flow vary widely, but not in a way that appears related to tumor necrosis. The uptake of 18F-ES by the uterus and DMBA-induced mammary tumors of adult rats reaches maximum levels (ca 0.35 and 0.10% I.D./g·kg, respectively) at early times (0–1 h), and drops slowly thereafter. The uterus to nontarget or tumour to nontarget tissue ratios, however, start low and continue to increase, reaching maximum levels (ca 20 and 15, respectively) at 2–3 h. There does not, however, appear to be a simple relationship between tumor uptake (either as % I.D./g·kg or tumor to nontarget ratio) measured at a single 3 h time point and tumor ER content, even considering differences in tumor blood flow. This suggests that an estimation of tumor ER content will require the application of more complex pharmacodynamic models that involve the measurement of the complete profile of receptor lignad uptake, retention, and washout from target to nontarget areas. The application of such models will be assisted by the development of estrogen receptor binding ligands that are not converted to circulating metabolites.

Titration of the in vivo uptake of 16α-[18F]fluoroestradiol by target tissues in the rat: competition by tamoxifen, and implications for quantitating estrogen receptors in vivo and the use of animal models in receptor-binding radiopharmaceutical development

We have measured in vivo the uptake of 16 alpha-[18F]estradiol (FES) by target tissues in the immature rat at increasing dose levels (obtained by dilution of [18F]FES with unlabeled estradiol). This was done to examine the binding capacity of target tissues in vivo and to determine whether the uptake in receptor-rich tissues was flow limited, as this has implications concerning the appropriateness of using receptor-rich tissues in experimental animals as models for FES uptake by receptor-poor breast tumors in humans. We also wanted to establish the dose level of the anti-estrogen tamoxifen required to block target tissue uptake of FES. We found that in untreated rats, specific uptake in the uterus saturated at c. 180 pmol/g, in the ovary at c. 54 pmol/g and in the muscle at c. 2 pmol/g. At an intermediate dose of tamoxifen (570 micrograms/kg), uptake saturated at somewhat lower levels, and at a high tamoxifen dose (1710 micrograms/kg), yet lower specific uptake was evident. In the FES titrations at low dose levels of FES, both the uterus and the ovaries, but not the muscle, showed characteristics of flow-limited uptake, i.e. the uptake-to-dose ratio reached a maximum level. This flow limitation suggests that only when receptor levels are sufficiently low will the FES uptake be related to receptor concentration. While receptor-rich tissues such as the rat uterus will show this flow limitation, the receptor concentration in most primary and metastatic human breast tumors is sufficiently low, so that the uptake should parallel receptor content. In in vivo distribution studies, target tissues (or tumors) with low receptor content will be more fully saturated and ligand more readily displaced. Also, uptake by secondary target tissues (i.e. those with a lower content of estrogen receptor, such as muscle, thymus and kidney) may be better models for assessing the effectiveness of new breast tumor imaging agents than uptake by receptor-rich tissues.

N-(3-[18F]fluoropropyl)-spiperone: the preferred 18F labeled spiperone analog for positron emission tomographic studies of the dopamine receptor.

The ligands currently used for PET studies of the dopamine receptor are fluorine-18-labeled spiperone (FSp) and carbon-11 or fluorine-18-labeled N-methyl-spiperone. All three of these ligands have drawbacks in either their chemical preparation or their biological behavior. We have previously prepared a series of N-fluoroalkyl-spiperone derivatives which are simple to prepare in high radiochemical yield. N-[18F]fluoropropyl-spiperone (3-F-Pr-Sp) and N-[18F]fluoroethyl-spiperone (2-F-Et-Sp) were the most promising ligands. In vitro competitive binding studies showed affinities for the dopamine receptor of 3-F-Pr-Sp greater than FSp greater than 2-F-Et-Sp. Brain extraction studies in a primate model showed that FSp, 2-F-Et-Sp, and 3-F-Pr-Sp were not completely extracted by the brain. High bone uptake and kidney clearance was observed with 3-F-Pr-Sp, while 2-F-Et-Sp cleared through the intestine in rats. This is in contrast to FSp where clearance is through the kidney. Studies to evaluate the extraction of metabolites in the brain were carried out by administering large doses (10 mCi) of FSp, 2-F-Et-Sp and 3-F-Pr-Sp to rats and reinjecting the metabolites in blood into other rats. These experiments showed that less than 0.02% of the metabolites from FSp and 3-F-Pr-Sp entered the brain, while 0.5% of the metabolites from 2-F-Et-Sp entered the brain. The majority of the activity present in the cerebellum after the administration of 2-F-Et-Sp is metabolites; therefore 2-F-Et-Sp is unsuitable for PET imaging studies. PET imaging studies in baboons and in one normal human volunteer with 3-F-Pr-Sp showed a high striatum-to-cerebellum ratio, showing that 3-F-Pr-Sp can replace ligands currently in use to study dopamine receptors.

Elimination of contaminant kryptofix 2.2.2 in the routine production of 2-[18F]fluoro-2-deoxy-d-glucose

Radiopharmaceutical solutions of 2-[18F]fluoro-2-deoxy-D-glucose (2-[18F]FDG) prepared via an aminopolyether-supported nucleophilic-substitution mechanism were analyzed for contaminant 4,7,13,16,21,24-hexaoxa-1,10-diazabicyclo-[8,8,8]-hexacosane (Kryptofix 2.2.2). Washing the C18-immobilized [18F]fluoro-tetraacetylated intermediate with 10 mL 0.1 M HCl was found to remove impurity Kryptofix 2.2.2 from the final product. Inclusion of this synthetic step allowed the robotic production of drug-quality 2-[18F]FDG in 52-56% radiochemical yield within 75 min. A thin-layer chromatographic system for the clinical screening of the radiochemical and chemical purity of this radiopharmaceutical is described.

Robotic production of 2-deoxy-2-[18F]fluoro-D-glucose: a routine method of synthesis using tetrabutylammonium [18F]fluoride.

Using existing robotic hardware and software programs developed for the synthesis of several positron-emitting radiopharmaceuticals for PET imaging [Brodack et al. (1988) Appl. Radiat. Isot. 39, 689], the additional automated synthesis of 2-deoxy-2-[18F]fluoro-D-glucose (2-[18F]FDG) has been incorporated into our Zymate Laboratory Automation System. The robotic synthesis of 2-[18F]FDG took less than one week to implement, including the organization of software subroutines and construction of an additional heating station. The end of synthesis yield (12-17%) and radiochemical purity (96-99%) for the robotic preparation of 2-[18F]FDG is similar to that of the manual synthesis. This automated method uses anhydrous tetrabutylammonium [18F]fluoride as the reactive fluoride source in the labeling step. The procedure is a modification of the synthesis reported by Hamacher et al. [Hamacher et al. (1986) J. Nucl. Med. 27, 235].

Automated production of several positron-emitting radiopharmaceuticals using a single laboratory robot

A Zymate Laboratory Automation System (Zymark Corp.), previously set up for the automated synthesis of 16 alpha-[18F]fluoroestradiol-17 beta [Brodack et al. (1986a) J. Nucl. Med. 27, 714] has been modified for the production of several short-lived radiopharmaceuticals in a single hot cell. All manipulations and apparatus normally used in the syntheses of carbon-11 and fluorine-18-labeled radiopharmaceuticals have been incorporated into the robot system. This achievement permits facile modifications of existing procedures used by the robot in addition to the incorporation of new routines in a minimal amount of time. Currently, the Zymate robot is programmed for the routine production of 16 alpha-[18F]fluoroestradiol-17 beta (7-11% EOS in 80 min), N-(3-[18F]fluoropropyl)spiperone (15-18% EOS in 70 min), and [1-11C]butanol (11-15% EOS in 25 min). A fourth compound, 2-deoxy-2-[18F]fluoro-D-glucose, is also synthesized by the robot. The yields and synthesis times of the robot-produced compounds are comparable to those obtained during manual syntheses. This method of automation represents a flexible and versatile alternative for the routine production of radiopharmaceuticals used in PET studies.

A remote system for the synthesis of copper-62 labeled Cu(PTSM)

Abstract The construction and performance of a remote system is described for the synthesis of 62 Cu-labeled pyruvaldehyde bis( N 4 -methylthiosemicarbazonato)copper(II), [ 62 Cu]Cu(PTSM), a positron-emitting radiopharmaceutical for perfusion imaging. The remote system allows the shielded synthesis and purification of tracer using the 2 M HCl eluate of high-level (10–18 GBq) 62 Zn/ 62 Cu radionuclide generator system. The remote system allows the [ 62 Cu]Cu(PTSM) synthesis to be performed in under 8 min with ca 40% E.O.S. radiochemical yield and >98% radiochemical purity.

Biodistribution of N-alkyl and N-fluoroalkyl derivatives of spiroperidol; Radiopharmaceuticals for PET studies of dopamine receptors

There is great interest in the application of positron labeled ligands to map the dopamine receptor in vivo. A series of fluorine-18-labeled N-alkyl and N-fluoroalkyl spiroperidol (SP) derivatives N-methyl[18F]SP; N-ethyl[18F]SP; N-(2-[18F]fluoroethyl)SP; N-propyl[18F]fluoropropyl) SP; N-(3-fluoropropyl) [18F]SP; N-(2-[18F]fluoropropyl)SP; N-(2-[18F]fluorobutyl)SP; N-(2-[18F]fluoropentyl)SP; and N-(2-[18F]fluorohexyl)SP were synthesized. The lipophilicity of these ligands (log octanol/water partition coefficient) varies from 2.67 to 5.56 and the initial brain uptake in rats, measured at 2 min, was greatest with the methyl, ethyl, propyl, fluoroethyl, and fluoropropyl derivatives. The highest striatum/cerebellum values 1 h after administration were obtained with the N-methyl, N-propyl, and N-3-fluoropropyl derivatives, while that of N-2-fluoroethyl showed the greatest uptake of total activity in the brain at this time. The uptake of all these ligands in the striatum could be blocked by cold SP showing the striatal uptake to be by the dopamine receptors.