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Particle Size Analysis of 99Mtc-Labeled and Unlabeled Antimony Trisulfide and Rhenium Sulfide Colloids Intended for Lymphoscintigraphic Application

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Particle Size Analysis of 99Mtc-Labeled and Unlabeled Antimony Trisulfide and Rhenium Sulfide Colloids Intended for Lymphoscintigraphic Application Particle Size Analysis of 99mTc-Labeled and Unlabeled Antimony Trisulfide and Rhenium Sulfide Colloids Intended for Lymphoscintigraphic Application Chris Tsopelas RAH Radiopharmacy, Nuclear Medicine Department, Royal Adelaide Hospital, Adelaide, Australia nature allows them to be retained by the lymph nodes (8,9) Colloidal particle size is an important characteristic to consider by a phagocytic mechanism, yet the rate of colloid transport when choosing a radiopharmaceutical for mapping sentinel through the lymphatic channels is directly related to their nodes in lymphoscintigraphy. Methods: Photon correlation particle size (10). Particles smaller than 4–5 nm have been spectroscopy (PCS) and transmission electron microscopy reported to penetrate capillary membranes and therefore (TEM) were used to determine the particle size of antimony may be unavailable to migrate through the lymphatic chan- trisulfide and rhenium sulfide colloids, and membrane filtration (MF) was used to determine the radioactive particle size distri- nel. In contrast, larger particles (ϳ500 nm) clear very bution of the corresponding 99mTc-labeled colloids. Results: slowly from the interstitial space and accumulate poorly in Antimony trisulfide was found to have a diameter of 9.3 Ϯ 3.6 the lymph nodes (11) or are trapped in the first node of a nm by TEM and 18.7 Ϯ 0.2 nm by PCS. Rhenium sulfide colloid lymphatic chain so that the extent of nodal drainage in was found to exist as an essentially trimodal sample with a contiguous areas cannot always be discerned (5). After dv(max1) of 40.3 nm, a dv(max2) of 438.6 nm, and a dv of 650–2200 injection, the radiocolloid travels to the sentinel node 99m nm, where dv is volume diameter. Tc-antimony trisulfide by through afferent subcapsular lymphatics, where they are MF showed that more than 96% of radioactive particles were retained by the sinusoids and phagocytosed by the reticu- smaller than 100 nm; however, 99mTc-rhenium sulfide showed that more than 21% were smaller than 100 nm. These radioac- loendothelial macrophages of the node (2). Particles smaller tive colloids were used with seven different membrane compo- than 0.1 ␮m show the most rapid disappearance from the sitions and found not to adsorb significantly to any of them. interstitial space into the lymphatic channels, with good Conclusion: MF was validated as a simple and reliable tech- retention in the lymph node (12). Most larger particles do nique to estimate the percentage radioactive particle size dis- not move easily beyond the first node site encountered. The tribution. permeability of colloids in lymph is maximal for particles Key Words: antimony trisulfide colloid; lymphoscintigraphy; smaller than 50 nm (2). An ideal radiopharmaceutical col- sulfur colloid; particle size; radioactive particle size distribution loid should therefore comprise particles near, but not less J Nucl Med 2001; 42:460–466 than, 4–5 nm in diameter to avoid capillary penetration, reduce retention of activity at the injection site, and achieve high retention in the subsequent lymph nodes. An early radiopharmaceutical used to image the lym- Since its description in 1965, 99mTc-antimony sulfide phatic system was 198Au-colloid, having a uniform particle colloid has been used for bone marrow imaging (1), size of 3–5 ␮m in diameter. However, this agent is not lymphedema assessment (2), and, more recently, scinti- readily available today because of clinical limitations from graphic mapping of lymphatic channels and sentinel nodes its isotope being a ␤-emitter and emitting an unfavorable (3,4) in melanoma (5) and breast cancer (6,7). In lympho- ␥-ray for scintillation camera imaging (13) and because of scintigraphy, a radiopharmaceutical should move at a rate the difficulties its manufacture represents (M. Jenson, writ- similar to that of the physiologic flow of lymph and should ten communication, July 1998). Other widely used techne- be sufficiently retained by intervening nodes. Colloids are tium-bound radiopharmaceuticals that comprise particles highly desirable for such studies because their particulate approaching the size of the 198Au-colloid include 99mTc- rhenium sulfide colloid and 99mTc-nanocolloid. 99mTc-sulfur colloid is a commonly used agent in the United States (12) Received May 30, 2000; revision accepted Nov. 14, 2000. for liver imaging, for oral gastric emptying studies (14), as For correspondence or reprints contact: Chris Tsopelas, PhD, RAH Radio- pharmacy, Nuclear Medicine Department, Royal Adelaide Hospital, North a filtered (0.1 ␮m) preparation for lymphoscintigraphy (15), Terrace, Adelaide, Australia, 5000. or as 99mTc-rhenium sulfide colloid (16). 99mTc-nanocolloid 460 THE JOURNAL OF NUCLEAR MEDICINE • Vol. 42 • No. 3 • March 2001 is used extensively in Europe, and similar preparations are TABLE 1 available (NANOCOLL and ALBU-Res; Nycomed-Amer- Particle Size Diameter of Reported Colloids sham Sorin, Milan, Italy). These products result in good Particle size lymph uptake and images of flow (17), but poor retention Colloidal agent (nm) Percentage Reference limits their use in delayed images. 99mTc-antimony trisulfide colloid is currently commer- Microaggregated albumin (nanocolloid) Ͻ80 95 12 cially unavailable in the United States (11) but is an ap- Rhenium sulfide 4–12 — 16 proved therapeutic drug in Australia and New Zealand for Sulfur colloid 100–300 — 16 routine clinical use as a lymphoscintigraphic agent. (The 305–340 — 15 Royal Adelaide Hospital Radiopharmacy, Adelaide, Austra- Lyocoll* Ͻ200 74.4 200–1,000 22.4 29 lia, is the only current manufacturer of the lymphoscinti- † 99m TCK-1 Ͻ200 58.2 graphic agent Lymph-Flo [kit for the preparation of Tc- 200–1,000 24.0 Colloidal Antimony Sulphide Injection], a product that has 1,000–3,000 7.8 been commercially available in Australia and New Zealand Ͼ3,000 10.0 29 for more than 10 y.) Antimony trisulfide colloid products 0.1-␮m filtered 10 — 89–173 Ͻ0.1 15 have been previously commercially available (5,18,19). The ‡ 99m Ͼ200 56.8 Tc-antimony trisulfide colloidal particles have been re- 30–200 7.3‡ ported as ranging from 3 to 30 nm, an optimum size for Ͻ30 36.2‡ 12 imaging lymphatic channels in lymphoscintigraphy (20). Antimony trisulfide 38.1 Ϯ 3.0 — 37 The most widely used measurement technique is transmis- 3–15 or 10–25 — 18 sion electron microscopy (TEM), which represents the pro- 3–18 — 24 ϳ9— 38 jected area diameter (da) of a particle in stable orientation, Ͻ30 — 25 but other techniques that measure and describe particle size, 4–10 — 39 such as surface diameter and volume diameter (dv), are also used. (21). *Mallinckrodt Diagnostica, Petten, Holland. Particle size is a primary consideration when a radiocol- †Oris-Industrie, Cedex, France. loid is chosen for clinical mapping of lymphatic channels ‡48.3% were Ͼ0.2 ␮m, 5.4% were 0.2–0.03 ␮m, and 46.7% and nodes (Table 1). Based on this concept, three sizing were Ͻ0.03 ␮m with old 99mTc-pertechnetate milked; 83.2% were techniques were investigated with two colloids, including Ͼ0.2 ␮m, 2.2% were 0.2–0.3 ␮m, and 14.7% were Ͻ0.03 ␮m with 99m photon correlation spectroscopy, TEM, and membrane fil- prolonged heating and freshly milked Tc-pertechnetate. tration (MF) for the 99mTc-colloids. In particular, an aim of this study was to validate the simple MF technique for 99m 99m Tc-rhenium sulfide colloid versus Tc-antimony trisul- mL water for injection], 1.0 mol/L HCl [8.0 mL], and basic fide colloid with different membrane compositions. phosphate buffer [280.0 mg sodium dihydrogen phosphate dihy- drate in 8.0 mL of 1.0 mol/L sodium hydroxide]). The standard MATERIALS AND METHODS procedure for preparing 99mTc-rhenium sulfide colloid included adding 99mTc-pertechnetate (0.5–1.0 GBq/0.5 mL saline), HCl (1.0 Radiolabeling Procedures mol/L; 0.5 mL), and then thiosulfate solution (0.5 mL) to one vial The rhenium sulfur colloid kit and Lymph-Flo were manufac- containing 10% gelatin. This reaction vial was heated in a boiling tured in this radiopharmacy, where current batches were used for water bath for 3–5 min, allowed to cool to room temperature, and 99m all experiments. Tc-pertechnetate was obtained from the daily then adjusted to pH 5.0–6.5 with basic phosphate buffer (0.5 mL) 99 99m milking of a wet-bed Mo/ Tc generator (TC9M1; Australian before MF. Radioisotopes; Sydney; Australia). Normal saline (0.9%) for in- jection was used for dilutions and washes. Radiochemical Analyses Preparation of 99mTc-Colloidal Antimony Sulphide Injection. Radiochemical purity (% RCP) was determined (Յ6 h of prep- Lymph-Flo consists of five separate nonradioactive components aration for 99mTc-antimony trisulfide and Ͻ1 h for 99mTc-rhenium (antimony trisulfide, phosphate buffer, HCl, an empty sterile sulfide) by ascending instant thin-layer chromatography using sil- 10-mL vial, and a sterile 0.2-␮m filter). The standard procedure for ica gel–impregnated glass fiber sheets (Gelman Sciences, Ann preparing this radiocolloid included adding 99mTc-pertechnetate Arbor, MI). Strips 1 cm wide ϫ 20 cm long were marked from the (0.8–0.9 GBq/1.0 mL saline) and then HCl (1.0 mol/L; 0.1 mL) to origin every 1 cm for 10 cm and then were spotted with sample and the vial containing antimony trisulfide. This reaction vial was developed in the solvent (0.9% saline). Radiocolloid remained at heated in a boiling water bath (100°C) for 30 min, allowed to cool the origin, and free 99mTc-pertechnetate migrated with the solvent to room temperature, and then adjusted to pH 5.5–6.5 with phos- front. The strips were cut into 1-cm sections and counted in a ␥ phate buffer (0.5 mL) before MF. counter (Packard Auto-Gamma 5650; Hewlett Packard, Palo Alto, Preparation of 99mTc-Rhenium Sulfide Colloid.
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