Antigen-binding site protection during radiolabeling leads to a higher immunoreactive fraction

It is generally accepted that the immunointegrity of an antibody (Ab) depends on the preservation of its antigen-binding sites. Our goal was to radiolabel an antibody at several iodine:antibody molar ratios under conditions protecting its combining site and to compare its immunoreactive fraction (IRF) and electrophoretic mobility with those of the same antibody radiolabeled without protection. The data indicate that an antibody radiolabeled while its antigen-binding site is occupied by its antigen had the same IRF, regardless of the number of iodine atoms per antibody molecule. On the other hand, even at an I:Ab ratio of 1:1, the IRF of the same antibody radiolabeled without protection was lower than that of a protected one and decreased with increasing I:Ab ratios. In addition, the iodination of these Ab changes their electrophoretic mobility; however, when the Ab is labeled in the protected state, the degree of change is less. The binding of an antibody to its antigen prior to radiolabeling, therefore, enhances its immuno-integrity and prevents major conformational changes as reflected by electrophoresis.

Specific uptake of the auger electron-emitting thymidine analogue 5-[123I/125I]iodo-2'-deoxyuridine in rat brain tumors: diagnostic and therapeutic implications in humans

Glial neoplasms of the human central nervous system are malignancies that have defied treatment. Part of the problem lies in the limitations of current diagnostic techniques which are unable to identify small collections of neoplastic glia within normal parenchyma and in the difficulty of sterilizing these tumors because of limited selectivity of the cytotoxic agents available. The thymidine analogue 5-iodo-2'-deoxyuridine (IdUrd) radiolabeled with 123I and 125I was injected directly into an intracerebral rat 9L gliosarcoma and found to be a sensitive and specific agent for the detection of this neoplasm in rats. External gamma camera imaging (123I) visualized tumors as small as 0.5 mm in diameter. Autoradiography (125I) indicated that IdUrd was incorporated into the DNA of neoplastic glia only. Since 123I emits gamma-photons suitable for scintigraphy, [123I]IdUrd holds promise for the diagnosis of brain tumors in humans as well. Furthermore, since 123I and 125I are Auger electron emitters that have demonstrated antineoplastic effects, direct administration of [123I]IdUrd or [125I]IdUrd into tumors may also have potential for the treatment of central nervous system malignancies.

Cellular radiation dosimetry and its implications for estimation of radiation risks. Illustrative results with technetium 99m-labeled microspheres and macroaggregates

Radiation absorbed doses at the cellular level were calculated for routine, human lung perfusion examinations after the intravenous injection of technetium 99m-labeled microspheres or macroaggregated albumin. In such studies, more than 90% of these particles are trapped in the precapillary arterioles of the lung, resulting in an extremely inhomogeneous distribution of radionuclide. We used a computer program that accounted for the inhomogeneity of radiopharmaceutical distribution and calculated the dose to individual lung cells. Absorbed doses to individual lung cells were found to vary by a factor of about 30,000. We believe that such findings call for a reevaluation of the justification for dosimetry at the organ level and an examination of the implications of absorbed doses calculated at the cellular level for the estimation of radiation risks.

Inhomogeneous deposition of radiopharmaceuticals at the cellular level: experimental evidence and dosimetric implications

We have undertaken an experimental examination of the conventional internal dosimetry assumptions of homogeneity of radionuclide deposition in tissues. The distribution of radiolabeled Microlite has been quantitated in mouse liver at the millimeter (multicellular) and the micrometer (cellular) levels. Measurements of radioactivity in 1-mm3 tissue samples indicate homogeneous radionuclide distribution; those derived from autoradiographs of 0.5-micron tissue sections show that, relative to other cells, the colloid was concentrated 200- to 1000-fold in liver macrophages. The dosimetric implications of such inhomogeneous radionuclide distribution in human liver, where similar radionuclide distribution is expected, are discussed on the basis of a recently developed model for calculating the dose at the cellular level, and the estimates are compared to conventional internal dosimetry predictions. It is demonstrated that during routine diagnostic examinations with 99mTc-Microlite, conventional dosimetry underestimates the dose to labeled human liver cells by factors of 8-30.

Mutation induction by 125iodoacetylproflavine, a DNA-intercalating agent, in human cells

Survival and the induction of mutations at the hprt and tk loci were measured in TK6 human lymphoblastoid cells following treatment with the DNA-intercalating agent 125iodoacetylproflavine (125IAP). 125IAP was readily taken up into the cells, was localized to the nucleus, and was released rapidly following resuspension of the cells in fresh medium. Treatment with 125IAP for 24 h yielded a D0 of 110 decays/cell and an induced mutant fraction of 0.13 x 10(-6) per decay at the hprt locus and 0.4 x 10(-6) per decay at the tk locus. Molecular analyses of 125IAP-induced hprt mutants by Southern blot revealed a high proportion of large-scale changes at this locus. When these results are compared with those observed with 125IdUrd, 125IAP shows a reduced effectiveness per decay, related perhaps to the non-covalent nature of intercalator binding, resulting in reduced energy deposition in the DNA.

Preadsorption of radiolabeled monoclonal antibodies to liver and spleen tissues leads to higher tumor-to-normal-tissue ratios

This study addresses the impact of background activity on the use of radioimmunoconjugates for radioimmunodiagnosis and radioimmunotherapy. Since the liver and the spleen represent organs with preferential nonspecific uptake, we exposed radiolabeled (iodinated and Indium-111 labeled) preparations of monoclonal antibodies to a suspension of fresh liver and spleen cells at physiological temperature and compared their immunoreactivity, in vivo biodistribution, and tumor targeting to those of the same radiolabeled proteins without prior adsorption to this suspension. The biodistribution studies were performed under conditions of high background activity, i.e., shortly after the injection (1 hour) and using a high dose of the protein. Preadsorption of radiolabeled monoclonal antibodies results in a significant decreased uptake in certain normal tissues, i.e., greater contrast between normal and tumor tissues, as demonstrated by the quotient of the two target-to-nontarget ratios (exposed/unexposed antibody) which was greater than one for most of the tissues examined.

Synthesis and biological activity of the intercalating agent 3-acetamido-5-[123/125I]iodo-6-aminoacridine

3-Acetamido-5-iodo-6-aminoacridine (3), a derivative of the known intercalating agent proflavine (3,6-diaminoacridine) (1) was synthesized, and no-carrier-added 123I and 125I labeled compounds prepared. Compound 3 was taken up by live cells and localized in the nucleus. The intracellular concentration of [125I]3 was 7-fold greater in human prostate carcinoma (PC-3) cells than in normal Chinese hamster lung fibroblast (V-79) cells.

5-iodo-2'-deoxyuridine-protein conjugates: synthesis and enzymatic degradation

Several halogenated analogs of thymidine and cytidine possess antineoplastic and antiviral activity. They are also powerful sensitizers of bacterial and mammalian cells to lethal effects of x-irradiation. An important factor limiting the effectiveness of these agents in therapy is their extremely short half-life in circulation due to rapid hepatic dehalogenation. An approach to this problem is to deliver the drug directly to its target using monoclonal antibodies. This study evaluates the lysosomotropic delivery systems of halogenated pyrimidines using 5-iodo-2'-deoxyuridine [IUdR] as a model. IUdR, derivatized and activated at either the 3'- or the 5'-position forms covalent adducts with the epsilon-amino groups of the lysine residues in proteins (bovine serum albumin [BSA], and immunoglobulins [IgG]). Two methods suitable for conjugation of IUdR to proteins involving either the formation of acyl-imidazoles in the reaction of IUdR succinates with N,N'-carbonyldiimidazole or the preparation of N-succinimidyl esters of IUdR succinates were established. Both derivatives express comparable reactivity toward proteins. The degree of IUdR incorporation is easily controlled by the ratio of reagents. The succinate "arm" linking IUdR to protein is susceptible to lysosomal hydrolysis in vitro releasing intact IUdR. The half-life of the IUdR-IgG conjugate in the presence of the lysosomal enzymes was shown to be approximately twice that of the IUdR-BSA conjugate.

Limitations of conventional internal dosimetry at the cellular level

A theoretic examination of the validity at the cellular level of assumptions used in classic internal dosimetry has been undertaken. An alternate dosimetric model accounting for the consequences of selective uptake of a radiolabeled compound by specific cells in a multicellular cluster of hexagonal geometry has been developed. At the cellular level, derived dose estimates for electrons have been compared to dose estimates obtained employing the assumptions of conventional internal dosimetry. The study has been performed for all electron energies and then applied specifically to electrons emitted by 99mTc, 201Tl, 111In, and 123I. The dosimetric consequences of altering (a) the intracellular-to-extracellular radionuclide concentration, (b) the labeled cell density, and (c) the cell size have been examined for the labeled and nonlabeled cells in a cell cluster, and the conditions in which conventional dosimetry underestimates or overestimates the dose to individual cells have been indicated. It is shown that when selective intracellular uptake of a radiolabeled compound occurs in specific cells within a cell cluster, conventional dosimetry underestimates the radiation dose delivered to the labeled cells by twofold to more than 25-fold if the emitted electrons have ranges of a few micrometers or less, i.e., energies smaller than approximately 10 keV. Under the same conditions, conventional dosimetry overestimates slightly (20% to 50%) the electron radiation dose to the nonlabeled cells of the cell cluster. It is shown that inclusion of photons in the calculation of the total dose to individual cells does not alter significantly the conclusions of the present investigation.

Absence of preferential uptake of [125I]iododihydrorhodamine 123 by four human tumor xenografts

The biodistribution of [125I]iododihydrorhodamine 123 has been studied over a 96-h period in four human tumor xenograft models: HT-29 colon adenocarcinoma, PC-3 prostate carcinoma, HT-1080 fibrosarcoma, and PaCa-2 pancreatic carcinoma. Elimination of radioactivity in the tumor-bearing nude mice was rapid during the first 24 h and slow thereafter. The lack of uptake in the thyroid indicated there was little, if any, deiodination of the molecule. Activity was found mainly in the liver and spleen. Accumulation of radioactivity was low in all four tumors examined. At 4 h postinjection, as well as at 24 and 48 h, however, the total radioactive content in each of the four tumors was directly proportional to the weight of the tumor sample. This correlation was independent of tumor type, route of injection (i.v./i.p.) or dose (1.2-6 microCi/mouse). This was not true for any of the normal tissues, suggesting that this accumulation may be governed by certain intrinsic characteristics of the cancers tested.

Radiotoxicity of 5-[123I]iodo-2'-deoxyuridine in V79 cells: a comparison with 5-[125I]iodo-2'-deoxyuridine

The toxic effects of the short-lived (T 1/2 = 13.2 h) Auger-electron-emitting isotope 123I, incorporated in the form of 123IUdR into the DNA of V79 cells in vitro, have been investigated and compared to those of 125IUdR. For the concentrations tested, the rate of incorporation of 123IUdR at any time is proportional to the concentration of extracellular radioactivity. The curve for survival of clonogenic cells decreases exponentially and exhibits no shoulder at low doses. The mean lethal dose (D37) to the nucleus is 79 +/- 9 cGy and is about the same as that obtained previously with 125IUdR. However, the total number of decays needed to produce this D37 with 123IUdR is about twice that required with 125IUdR, approximately equal to the ratio of the energy deposited in microscopic volumes by 125I and 123I, respectively. This correlation suggests that nuclear recoil, electronic excitation, and chemical transmutation are probably of minor importance to the observed biological toxicity with either isotope. The results also indicate that there are no saturation effects in the decay of 125IUdR in the DNA of V79 cells (i.e., all of the emitted energy is biologically effective) and that each of the two steps involved in the 125I decay is equally effective in causing biological damage.

Radiotoxicity of an 125I-labeled DNA intercalator in mammalian cells

To explore the effect of the Auger electron emitter 125I attached to a DNA intercalator, we have synthesized 125I- and 127I-labeled 3-acetamido-5-iodoproflavine (AIP) and have examined the uptake, intracellular distribution, and radiotoxicity of A125IP in Chinese hamster V79 cells. After incubation with AIP, the nuclei of V79 cells become fluorescent. Uptake of A125IP is directly proportional to its extracellular radioactive concentration and reaches a plateau at about 10 h. Of the cell-associated radioactivity, 60% is retained by the cells after extensive washing. When the survival of V79 cells is plotted as a function of radioactive cell content, the curve has no shoulder with a mean lethal dose (DN) of about 1.3 Gy to the cell nucleus. Because the DN of these cells when irradiated with 250 kVp X rays is 5.8 Gy, the relative biological effectiveness (RBE) of A125IP is about 4.5. The dependence of the RBE values on the localization of the Auger emitter is discussed on the basis of our extended studies on the same cell line.

The physician's responsibility toward hopelessly ill patients. A second look

Physicians have a specific responsibility toward patients who are hopelessly ill, dying, or in the end stages of an incurable disease. In a summary of current practices affecting the care of dying patients, we give particular emphasis to changes that have become commonplace since the early 1980s. Implementation of accepted policies has been deficient in certain areas, including the initiation of timely discussions with patients about dying, the solicitation and execution in advance of their directives for terminal care, the education of medical students and residents, and the formulation of institutional guidelines. The appropriate and, if necessary, aggressive use of pain-relieving substances is recommended, even when such use may result in shortened life. We emphasize the value of a sensitive approach to care--one that is adjusted continually to suit the changing needs of the patient as death approaches. Possible settings for death are reviewed, including the home, the hospital, the intensive care unit, and the nursing home. Finally, we consider the physician's response to the dying patient who is rational and desires suicide or euthanasia.

Monoclonal antibody internalization by tumor cells: an experimental model for potential radioimmunotherapy applications

The monoclonal IgM 3G5, which reacts with the surface membranes of rat insulinoma cells RINm5F, was purified by HPLC and labeled with 125I using Protag-125; bovine IgG (bIgG) was similarly radiolabeled, and used as a control. 125I-3G5 was incubated with RINm5F cells either at 4 degrees C or at 37 degrees C. 125I-3G5 bound onto RINm5F cells growing in Petri dishes remained approx. constant over 44 h when incubated at 4 degrees C, whereas at 37 degrees C radioactivity was released back in the medium starting at 3 h (plateau at approx. 20 h). At the end of incubation at 37 degrees C, activity in the medium included a high percentage of free 125I (15.69 vs 2.62% for bIgG, and 1% for 3G5 at 4 degrees C). In a cell suspension experiment, cell-bound 125I-3G5 also remained constant over a 24 h incubation at 4 degrees C, whereas at 37 degrees C it decreased to 37.5% of its initial value (64.1% at 4 h). Concomitant microautoradiography showed diffuse radioactive deposits within the RINm5F cells following incubation with 125I-3G5 (but not 125I-bIgG) at 37 degrees C. These results indicate that 3G5-IgM reacts with a surface antigen on the RINm5F cells, but is rapidly internalized by the cells: within the cells, this antibody undergoes some metabolic processing which results in the release of free 125I outside the cells.

Radioimmunotherapy with alpha-particle-emitting immunoconjugates

Alpha particles are energetic short-range ions whose higher linear energy transfer produces extreme cytotoxicity. An alpha-particle-emitting radioimmunoconjugate consisting of a bismuth-212-labeled monoclonal immunoglobulin M specific for the murine T cell/neuroectodermal surface antigen Thy 1.2 was prepared. Analysis in vitro showed that the radioimmunoconjugate was selectively cytotoxic to a Thy 1.2+ EL-4 murine tumor cell line. Approximately three bismuth-212-labeled immunoconjugates per target cell reduced the uptake of [3H]thymidine by the EL-4 target cells to background levels. Mice inoculated intraperitoneally with EL-4 cells were cured of their ascites after intraperitoneal injection of 150 microcuries of the antigen-specific radioimmunoconjugate, suggesting a possible role for such conjugates in intracavitary cancer therapy.

Synthesis and biological studies of iodinated (127/125I) derivatives of rhodamine 123

Rhodamine 123, a mitochondrial stain that preferentially accumulates in certain cancer cells, has been reduced and iodinated by using NaI in the presence of N-chlorosuccinimide. The various mono-, di-, and triiodo derivatives have been isolated and characterized. These nonfluorescent compounds are taken up by mammalian cells, become fluorescent within the cytoplasm (presumably following oxidation), and show the same pattern of localization as the parent compound. Iodination with no-carrier-added Na125I yields the same mixture of compounds. All 125I derivatives accumulate preferentially in PC3 adenocarcinoma cells compared with V79 lung fibroblasts, with the differential being greatest for the monoiodo compound, followed by the di- and triiodo derivatives.

Uncertainty and relative risks of radiation exposure

Exposure to ionizing radiation in a nonmilitary emergency is likely to alarm those persons exposed and their families. Not only is the outcome uncertain, but the risks are difficult to convey. This communication discusses uncertainties in short-term outcome and long-term effects, as well as means of conveying the magnitude of risks to patients and their families. It emphasizes that the perception of radiation hazards is contextual. It offers three ways of presenting the prognostic long-term risks of accidental radiation exposure by comparing them with the incidence of cancer and genetic abnormalities in unexposed persons, the hazards of everyday living, and the level of natural and medical exposures to ionizing radiation.

Radiotoxicity of 125I in mammalian cells

The radiotoxicity of 125I in Chinese hamster V79 lung fibroblasts has been studied following extracellular (Na125I), cytoplasmic [125I]iododihydrorhodamine (125I-DR), and nuclear (125IUdR) localization of the radionuclide. Exposure of the cells for 18 h to Na125I (less than or equal to 7.4 MBq/ml) had no effect on survival. A similar exposure to 125I-DR produced a survival curve with a distinct shoulder and with a mean lethal dose (D37) of 4.62 Gy to the nucleus. While this value compares well with the 5.80 Gy X-ray D37 dose, it is in contrast to the survival curve obtained with DNA-bound 125IUdR which is of the high LET type and has a D37 of 0.80 Gy to the nucleus. Furthermore, when the uptake of 125I into DNA is reduced by the addition of nonradioactive IUdR or TdR to the medium and the survival fraction is determined as a function of 125I contained in the DNA, a corresponding increase in survival is observed. This work demonstrates the relative inefficiency of the Auger electron emitter 125I when located in the cytoplasm or outside the cell. It indicates that a high dose deposited within the cytoplasm contributes minimally to radiation-induced cell death and that radiotoxicity depends not upon the specific activity of IUdR but upon the absolute amount of 125I that is associated with nuclear DNA.

Radiation damage to histone H2A by the primary aqueous radicals

Histone H2A has been examined for radiation-induced changes in structure and in amino acid composition. The effects of the individual radical intermediates--the hydroxyl radical, solvated electron, and superoxide radical--have been determined by irradiating in dilute aqueous solution under controlled environmental conditions. Amino acid analysis of irradiated histone H2A shows a selective attack on a few residues; only the aromatic residues, phenylalanine and tyrosine, and the heterocyclic residue, histidine, are significantly decreased. A significant increase in aspartic acid is also observed. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis shows that the hydroxyl radical is the effective radical for promoting changes in protein structure.

HPLC analysis of the dissociation and recombination of rabbit immunoglobulin G

Rabbit immunoglobulin G (RIgG) was reduced with dithioerythritol and analyzed by high performance liquid chromatography. A quantitative method for determining the percentage of reduced half-molecules in the mixture was developed. An acetic acid concentration-dependent rate of dissociation of reduced half-molecules was observed. The specific optical absorptivity was determined for whole molecules and half-molecules and found to be significantly greater for the half-molecules. Purified half-molecules were reconstituted into RIgG with a yield greater than 90% following a 16 h incubation at pH 8.0 and room temperature.

Radiobiologic implications of the microscopic distribution of energy from radionuclides

A significant increase in our understanding of the dosimetry and therapeutic potential of various types of radioactive decay has augmented the possibility of utilizing specific radiolabeled carriers in tumor therapy. At least three types of radioactive decay lend themselves to this approach. Most fastidious in their distribution requirements are the Auger electron emitters whose radiotoxicity can only be realized with their intranuclear localization. Alpha emitters irradiate regions of several cell diameters. Beta emitters are less stringent as far as proximity to the total tumor mass, but doses to normal tissues can be high. The advantages and the limitations of each type of decay are presented.