Radiopharmaceuticals

In this laboratory, you will prepare the radioactive pharmaceutical, Technetium-99m Sulfur Colloid Injection (^99mTc-SC). This product is used in nuclear medicine to help diagnose diseases of the liver, spleen, and bone marrow which comprise the major organs of the reticuloendothelial system (RES). These organs contain arterial and venous sinuses lined with phagocytic cells that are responsible for removing foreign particles from the blood, including radioactive colloids. Thus, intravenous administration of ^99mTc-SC will result in localization of radioactivity in these organs, providing a means to evaluate their morphologic and functional status by radiation detection methods.

The radioactive colloid is produced by reacting Tc-99m sodium pertechnetate (Na^99mTcO₄) with an acidified solution of sodium thiosulfate in the presence of gelatin. It is believed that during the reaction a small amount of hydrogen sulfide is produced which reacts with the trace quantity of pertechnetate to yield insoluble technetium heptasulfide. The technetium heptasulfide then co-precipitates with the elemental sulfur particles released by the acid hydrolysis of thiosulfate. The following reactions are proposed to occur:

Correct organ localization of radioactivity is predicated on the Tc-99m being bound to the colloidal particles. If incomplete radiolabeling occurs, some of the Tc-99m will remain unreacted in solution as Tc-99m pertechnetate ion (^99mTcO^–₄). This represents a radiochemical impurity which is undesirable since its biologic distribution is different from Tc-99m bound to sulfur colloid. Significant free pertechnetate ion (> 5%) will localize in the thyroid gland and stomach creating unnecessary radiation dose to the patient. Stomach localization of radioactivity is also a problem because it interferes with the evaluation of liver radioactivity due to the close proximity of these organs in the body. The radiopharmacist checks for Tc-99m pertechnetate impurity using radiochromatography. A simple and accurate technique involves spotting a sample of ^99mTc-SC on a strip of chromatography medium (paper or thin layer) which is then developed in a solvent, dried and cut in half for analysis. The ^99mTc-SC remains at the origin (bottom half) because of its insolubility, but the soluble pertechnetate ion migrates to the solvent front (top half). Each half is counted in a gamma scintillation counter and the percent activity bound to sulfur colloid determined. The USP limit is not less than 92% activity as ^99mTc-SC.

Another potential problem with ^99mTc-SC is the precipitation of the colloid caused by an excess of aluminum ion in the Tc-99m pertechnetate solution. The radiopharmacist can test for the presence of aluminum ion in the Tc-99m pertechnetate solution by a spot test method. With this method a piece of filter paper which is impregnated with aurintricarboxylic acid (aluminon) is spotted side by side with a sample of standard aluminum ion solution containing 10 µg Al³⁺ per ml and a sample of Tc-99m pertechnetate. Aluminum ion reacts with aluminon to form a pink colored lake with the intensity of color directly proportional to the amount of A1³⁺. Since the official limit is not more than 10 µg A1³⁺ per ml of Tc-99m pertechnetate solution, a comparison of the two spots provides evidence for a pass/fail type test.

Since ^99mTc-SC is to be injected intravenously, the product must be sterile and pyrogen-free. Therefore, the radiopharmacist must exercise good aseptic technique during the radiolabeling procedure to minimize contamination. This primarily means that rubber closures on vials must be cleansed with 70% alcohol prior to transfer of sterile solutions from one vial to another. Of course, sterile needles and syringes must be used. If unprotected needles drop or touch non-sterile surfaces they must be discarded. If opened vials of sterile solutions are used, then transfers should ideally be made in a properly functioning laminar air flow hood. For radioactive substances the hood should be of the vertical flow exhausting type.

Frequently, parenteral solutions, including radiopharmaceuticals, are sterilized by membrane filtration, i.e., filtering the non-sterile solution through a sterile 0.22 µm membrane. This method should NOT be used for sterilizing parenterals that contain dispersed particles such as colloidal dispersions. The particle size of colloids is frequently larger than the pore size of the membrane and the active ingredient will be removed by the filter. ^99mTc-SC is an example of a radiopharmaceutical that should NOT be sterilized by membrane filtration. For this reason the product is prepared aseptically using presterilized reagents supplied as a kit.

Tc-99m decays by isomeric transition with the emission of 140 keV gamma rays. No primary particulate radiation, such as beta particles is emitted. Since gamma rays readily penetrate matter, personal protection from Tc-99m sources is necessary. The 3 primary modes of protection are: (1) minimizing time of exposure, (2) maximizing the distance from the source, and (3) use of shielding material such as lead. The potential hazard from a radioactive source increases directly with the amount of radioactivity present. The maximum quantity of Tc-99m activity that you will be working with in this laboratory (about 20 µCi) is 2 to 3 thousand times less than the usual amount used to prepare ^99mTc-SC and will not present a significant biological hazard to you. Nevertheless, you will be required to use standard safety precautions as one would routinely use in handling large amounts of radioactivity. That is, lead shields and vial pigs will be used, disposable gloves will be worn, adsorbent plastic backed paper will cover the work bench and radiation monitors (Geiger-Mueller meters) will be available. All solutions will be marked with radioactive labels stating the radionuclide, amount, time and date. You will be required to clean your work space and monitor its surface for radioactive contamination before leaving. Special receptacles will be provided and used for any radioactive waste.

The pertechnetate ion (^99mTcO^–₄) is created in a commercial generator. The radioactivity is generated, and then decays over time. This isotope has a half-life of 6 hours. You can calculate the expected radioactivity for any time after generation using a physical decay chart.

For example: The generation activity and time for a sample of (^99mTcO^–₄) obtained from a generator was 25 µCi at 9:00 a.m. You take your sample at 2:30 p.m. and find the assayed activity is 13.2 µCi. The expected activity can be calculated as:

Obviously, the assayed activity and the calculated expected activity should be similar.

Radiopharmaceuticals are labeled to contain a certain radioconcentration, e.g., µCi/ml, at a given time stated on the label. This is the calibration time. The radiopharmacist must use this information and the half-life of the particular radionuclide to obtain doses for all other times when doses are requested. The physical decay chart can also be used to determine the fraction of activity left in the product at various times after calibration.

Example: The calibration activity and time of your product was found to be 3.3 µCi/ml at 3:00 p.m. You want to inject 6 µCi of activity into a patient at 6:00 p.m.  What volume of your product must be used to have 6 µCi at 6:00 p.m.?

    1. Calculate the activity your product will have at 6:00 p.m. (use physical decay chart).

     2. Calculate the number of ml of your product needed to contain 6 µCi.

So, you would dispense 2.56 ml of your product in a syringe labeled with the “stringed tag” found in the sulfur colloid kit.