The standardization of 177Lu by 4pibeta liquid scintillation spectrometry with 3H-standard efficiency tracing.

Solutions containing the potential radiotherapy radionuclide 177Lu have been standardized at the National Institute of Standards and Technology (NIST) by 4pibeta liquid scintillation (LS) counting with 3H-standard efficiency tracing using the CIEMAT/NIST method. Confirmatory measurements were made with 4pi NaI(TI) gamma-ray spectrometry. Activity determinations were made on 4 solutions over the course of 10 months with an expanded (k = 2) uncertainty on the activity of 0.8%. Half-life measurements were carried out using the NIST "4pi" gamma ionization chamber (IC) and LS counting and gave a new value of 6.65+/-0.01 d, which is shorter than the current ENSDF-recommended value by 1.3%. Impurity analyses were performed by high-purity germanium (HPGe) gamma-ray spectrometry and indicated only the presence of 177mLu at a level of 0.02% that of the 177Lu as of the respective reference times for the four solutions. Calibration factors for the NIST IC and Vinten 671 ionization chambers were developed, as were dial settings for the NIST-maintained Capintec CRC-12.

Radiation synovectomy revisited.

Radiation synovectomy is a potential weapon in the therapeutic armamentarium of nuclear medicine. It is an attractive alternative to surgical or chemical synovectomy for the treatment of rheumatoid arthritis. In this article the clinical results obtained with radiation synovectomy from the 1950s through 1992 are summarized and reviewed. Even after taking into account the paucity of well-controlled trials and rigorous clinical follow-up, it is clear that radiation synovectomy is efficacious in controlling the symptoms of rheumatoid arthritis. However, the procedure is not widely used because of concerns about leakage of radioactivity from the treated joint, and the resulting high doses that can be delivered to nontarget organs. New approaches to the preparation of radiolabeled particles for use in radiation synovectomy promise to minimize this leakage and thus allow the full potential of this important radiotherapy to be realized.

Chemistry and biological behavior of samarium-153 and rhenium-186-labeled hydroxyapatite particles: potential radiopharmaceuticals for radiation synovectomy.

Hydroxyapatite (HA), a natural constituent of bone, was studied as a particulate carrier for beta-emitting radionuclides in radiation synovectomy. Particles were radiolabeled with 153Sm or 186Re and their in vivo safety was investigated following intra-articular injection into knees of normal rabbits and rabbits with antigen-induced arthritis (AIA). Radiolabeling efficiency was greater than 95%; in vitro studies showed minimal (< or = 1%) loss of activity from particles over a 6-day period with 153Sm-labeled HA and about 5% loss of activity over a 5-day period with 186Re-labeled HA. The total cumulative extra-articular leakage of 153Sm over 6 days was 0.28% in normal rabbits and 0.09% in AIA rabbits. Leakage of 186Re from the joint was 3.05% over a 4-day period with 80% of extra-articular activity found in the urine. Histopathological evaluation of treated knees showed that HA particles are distributed throughout the synovium, embedded in the synovial fat pad. The ease and efficiency with which this HA carrier is labeled, coupled with observed extremely low leakage rates from the joint, make radiolabeled HA particles an attractive candidate as a radiation synovectomy agent for evaluation in rheumatoid arthritis patients.

Titration of the in vivo uptake of 16 alpha-[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.

Positron tomographic assessment of 16 alpha-[18F] fluoro-17 beta-estradiol uptake in metastatic breast carcinoma.

The positron-emitting estrogenic steroid 16 alpha-[18F]fluoro-17 beta-estradiol (FES) has been shown to exhibit selective uptake in primary breast carcinomas; the uptake of tracer by positron emission tomography (PET) is strongly correlated with the tumor estrogen-receptor concentration. We have now extended the use of this radiopharmaceutical for imaging of metastases of breast carcinoma by PET in 16 patients with clinical or radiographic evidence of metastatic disease. Increased uptake of FES was identified on PET images in 53 of 57 metastatic lesions (93%); only two apparent false-positive foci of FES uptake were seen. In seven of the patients, evaluable PET studies were obtained both before and after initiation of antiestrogen therapy. In all cases, there was a decrease in FES uptake in the tumor deposits after initiation of antiestrogen therapy, and the mean (+/- standard deviation) uptake decreased from 2.22 (+/- 1.23) to 0.80 (+/- 0.42) x 10(3)+ dose/ml. These results indicate that PET with FES has high sensitivity and specificity for detecting metastatic breast carcinoma and provide additional confirmatory evidence that the tumor uptake of this ligand is a receptor-mediated process.

Binding of 16 alpha-[18F]fluoro-17 beta-estradiol to alphafetoprotein in Sprague-Dawley female rats affects blood levels.

To examine the relationship between blood levels of 16 alpha-[18F]fluoro-17 beta-estradiol(18F-ES) and serum alphafetoprotein (AFP) concentration, we undertook a study in which serum from various aged (20-33 days old) Sprague-Dawley female rats injected with 18F-ES was analyzed for both blood activity levels and AFP. There is a strong positive correlation between serum AFP concentration and 18F-ES blood levels (r = 0.914, P less than 0.001), suggesting that the binding of 18F-ES by AFP has a significant effect on blood activity levels. The AFP concentration and ultimately the AFP-18F-ES binding is dependent on the age and weight of the rat: younger, as well as low weight rats exhibited high AFP concentrations and consequently increased 18F-ES blood activity. The rats most suitable for comparative studying of labeled estrogens are 25-28 days of age and weigh a minimum of 50-55 g. Thus, the use of the immature rat model to compare labeled estrogens requires a careful consideration of possible interference from blood binding proteins (i.e. AFP), as well as potential receptor binding competition from endogenous estrogens produced during the estrous cycle. Comparable consideration of blood binding proteins (sex steroid binding protein, SBP) and endogenous estrogens must be made in human studies, as well.

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.

Breast cancer: PET imaging of estrogen receptors.

Thirteen patients with primary breast masses were studied with positron emission tomography (PET) and 16 alpha-[fluorine-18]-fluoroestradiol-17 beta. PET images demonstrated uptake of the labeled estrogen analog at sites of primary carcinomas and in several foci of axillary lymph node metastases, as well as in one distant metastatic site. There was excellent correlation between uptake within the primary tumor measured on the PET images and the tumor estrogen-receptor concentration measured in vitro after excision (r = .96). This technique may provide an in vivo method of assessing estrogen receptors in primary and metastatic breast cancers and thus guide management of this disease with antiestrogen chemotherapy.

21-[18F]fluoro-16 alpha-ethyl-19-norprogesterone: synthesis and target tissue selective uptake of a progestin receptor based radiotracer for positron emission tomography.

We have synthesized 21-[18F]fluoro-16 alpha-ethyl-19-norprogesterone (FENP), a high affinity ligand for the progesterone receptor, labeled with the positron-emitting radionuclide fluorine-18 (t1/2 = 110 min). The synthesis proceeds in two steps from 21-hydroxy-16 alpha-ethyl-19-norprogesterone and involves [18F]fluoride ion displacement of the 21-trifluoromethanesulfonate (21-triflate). This material is purified by HPLC and is obtained in 4-30% overall yield (decay corrected) within 40 min after the end of bombardment to produce [18F]fluoride ion. The effective specific activity, determined by competitive radioreceptor binding assays, is 700-1400 Ci/mmol. In vivo, [18F]FENP demonstrates highly selective, receptor-mediated uptake by the uterus of estrogen-primed rats; the uterus to blood and uterus to muscle ratios were respectively 26 and 16 at 1 h and 71 and 41 at 3 h after injection. The high target tissue selectivity of this uptake suggests that this compound may be useful for the in vivo imaging of progestin target tissues and receptor-rich tumors (such as human breast tumors) by positron emission tomography.

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.

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.

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].

Characterization of the uptake of 16 alpha-([18F]fluoro)-17 beta-estradiol in DMBA-induced mammary tumors.

In order to investigate possible correlations between the uptake of 16 alpha-([18F]fluoro)-17 beta-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 X 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 X 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.

Application of robotics to radiopharmaceutical preparation: controlled synthesis of fluorine-18 16 alpha-fluoroestradiol-17 beta.

A commercially available robot system, the Zymark Zymate Laboratory Automation System, has been utilized for the preparation of a positron-emitting radiopharmaceutical, 16 alpha-[18F] fluoroestradiol-17 beta. This radiopharmaceutical is prepared in a three-step synthesis (preparation of [18F]-tetrabutylammonium fluoride, SN2 displacement of a triflate, and ketone reduction) and is purified by high performance liquid chromatography (HPLC). All steps in the synthesis and HPLC purification are controlled by the robot system with no manual intervention. This represents a new approach to the complete automation of radiopharmaceutical production.

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.

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%).

NCA 16 alpha-[18F]fluoroestradiol-17 beta: the effect of reaction vessel on fluorine-18 resolubilization, product yield, and effective specific activity.

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 the vessels (Pt crucible, borosilicate glass, siliconized borosilicate glass, Vacutainer), 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).

Biological studies of a new class of technetium complexes: the hexakis(alkylisonitrile)technetium(I) cations.

This study describes the preparation and synthesis of a new class of cationic technetium compounds, the hexakis(alkylisonitrile)technetium(+ 1) complexes, at both carrier added and no carrier added concentrations in aqueous media from pertechnetate. Biological distribution and imaging data in animals indicate that certain members of this class may be effective for cardiac imaging in man. The usefulness of these lipophilic water-soluble species for labeling mammalian cells is also reported.

The investigation of radiopharmaceutical components by fast atom bombardment mass spectrometry: the identification of Tc-HIDA and the epimers of Tc-CO2DADS.

The nature of two technetium-labeled radiopharmaceutical components has been established by means of fast-atom-bombardment mass spectrometry (FABMS) in combination with carrier-added (CA) and no-carrier-added (NCA) reversed-phase high-pressure liquid chromatography (HPLC). Negative-ion FABMS was used to determine that the epimers of Tc-CO2DADS are the oxo[N,N'-(1-carboxyethylene)-bis-(2-mercaptoacetimido)]technetate(V) ions; positive-ion FABMS showed that Tc-HIDA is bis[N-(2,6-dimethylphenyl-carbamoylmethyliminodiaceto]technetate(III).