Radiolabeling of Antibodies1

The radiolabeling of antibodies is considered in terms of the choice of radionuclide, the method of conjugation, and the effect of conjugation on plasma clearance, lodination tech niques are reviewed, but the major emphasis is placed on the methods of conjugating metallic radionuclides using bifunctional chelating agents. The technique of producing clinically useful radiolabeled antibodies by balancing altered substrate specificity caused by radiolabeling against accelerated plasma clearance is discussed. lodinated proteins have been used since the production of radioisotopes of iodine in the early 1940's. The first use of 131Ilabeled proteins of high specific activity was in the insulin radioassay by Berson ef al. (6). Procedures for labeling pro teins with other radionuclides were introduced at a much later date. Radiolabeled proteins can be divided into 2 groups according to use. Those that have been used as in vivo imaging agents are relatively unsophisticated radiochemicals. For in stance, many 99mTc-HSA2 preparations are clinically useful vascular pool agents, yet in fact they contain large percentages of radiolabeled small colloid (44). Since the radiolabeled colloid is cleared from the blood relatively slowly in relation to the duration of the clinical observation, this impure radiopharmaceutical serves the practical purpose of the blood pool agent. In addition, some radiopharmaceuticals have as their mecha nism of localization the clearance of foreign substances from the body and therefore do not have strict structural require ments (16). These 2 factors have minimized the requirement for biological, biochemical, physiological, and immunological integrity. Much of the current use of in vivo radiotracers has been through the pragmatic exploitation of easily produced and readily available combinations of radionuclide and sub strate. On the other hand, the in vitro use of radiotracers for radioimmunoassay and radioreceptor assay demands a critical control of the biological and immunological properties of the radiotracer. The recent interest to study pharmacology, biochemistry, and immunology in vivo has created a difficult challenge in new drug design. The biological and pharmacological properties must be as rigidly maintained as those required for the in vitro uses. Additionally, the target-to-nontarget distribution must be sufficient to allow identification of the target structure by the use of external detection. In conventional drug design, "the magic bullet" has been the goal, but in fact high blood levels 1Presented at the UICC Workshop on Radioimmunodetection of Cancer. July 19 to 21, 1979, Lexington. Ky. Supported in part by Grant CA 18675 awarded by the National Cancer Institute and Grant HL 19127 awarded by the Heart. Lung and Blood Institute. ' The abbreviations used are: HSA. human serum albumin; DTPA, diethylenetriaminepentaacetic acid; IDA, iminodiacetic acid; DTTA. diethylenetriaminetetraacetic acid. of drug have been the most expedient method for producing high concentration in the target structure. In the design of diagnostic radiopharmaceuticals, this latter approach is not possible. Choice of Radionuclide The choice of radionuclide is dependent on its nuclear prop erties, including its physical half-life, its production factors, the available imaging device, and the effective half-life of the la beled radiopharmaceutical (54). Most commercially available scintillation cameras are designed to maximize the detection efficiency and resolution of y-ray energy with a range of 100 to 250 keV. y-Rays in this energy range have satisfactory tissue penetration [the whole-body absorbed fraction for radiation distributed in the whole body is 37% for 100-keV and 34% for 250-keV y-rays (50)], but the energy is low enough to be easily collimated. A 0.5-inch-thick sodium iodide crystal can absorb more than 90% of 140-keV y-rays incident on the crystal surface but less than 20% of 500-keV y-rays (3). The most widely used radionuclides and those with the highest useful photon yield per absorbed radiation dose are 99mTc, 123I,131I, 111In, and 67Ga. Depending on the effective half-life in blood and the time of the function to be measured, in some instances, the physical half-life of the radionuclides 99mTc(6 hr) and 123I (13 hr) may be too short to be used. A major consideration for the use of any radiolabeled com pound in humans is the radiation dose to the patient. Most of the radionuclides are chosen because the equilibrium absorbed dose to the subject is small compared to the number of useful y-rays. Because of this, many 13tl radiopharmaceuticals have been replaced by a radiotechnetium compound. The absorbed dose constant (A,) is given for 5 radionuclides: 67Ga, 99mTc, 11'In, 123I,and 131I(Table 1) (12). The absorbed dose constants differ by approximately a factor of 6. However, depending on the effective half-life, this difference could be enlarged consid erably. The upper limit of radioactivity that can be injected will depend on the equilibrium absorbed dose constant to some extent but probably to a greater degree on the effective halflife. If the biological half-life is longer than the physical half-life, then the short-lived radionuclides 67Ga, 99mTc, 123I, and '"In would have a definite advantage. Radioiodide, the ultimate metabolite of iodinated proteins, does have a long biological half-life (Table 2) (5). Ideal Properties of the Radiolabeled Substrate The ideal properties of a radiolabeled substrate are not universally agreed upon. The expression "tracer" is used often, but this term should be reserved for those instances when the biochemical and immunological properties of the radiolabeled substrate are identical to the parent compound. In this circumstance, the biochemical and immunological prop erties of the radiopharmaceutical will be identical to the parent compound. 3036 CANCER RESEARCH VOL. 40 on June 6, 2017. © 1980 American Association for Cancer Research. cancerres.aacrjournals.org Downloaded from Radiolabeling of Antibodies