Radioimmunotherapy of malignancy using antibody targeted radionuclides

Immunofluorescence imaging as a tool for studying the pharmacokinetics of a human monoclonal single chain fragment antibody

We have used immunofluorescence imaging to study binding of a monoclonal antibody fragment to subcutaneously implanted human melanoma cells in nude mice. The data acquired using this nontraditional approach was then analyzed using standard pharmacokinetic methods to produce estimates of k(e) (0.06h(-1)), t 1/2 (16 h), mean residency time (23.4 h) and percent exposure of the antibody to the tumor (40%). To our knowledge this is the first time standard pharmacokinetic analyzes have been conducted on immunofluorescence imaging data. The combination of this novel imaging technique and standard pharmacokinetic analytical methods should prove to be a useful tool for comparing the properties of antibody fragments in animal models.

Advancements in cancer therapy with alpha-emitters: a review

PURPOSE

This synopsis attempts to shed light on the progresses made in the field of cancer therapy using alpha-particles.

HURDLES AND PROGRESSES

The rationale of selection of radionuclides focusing on comparison of alpha- and beta-emitters, the hurdles and their solutions, and recent developments are addressed. The efforts made in the field of alpha-radioimmunotherapy of hematologic malignancies are emphasized. A good deal of progress has been achieved in the past decade, and preclinical studies with a variety of radioimmunoconjugates with astatine and bismuth radioisotopes (At-211, Bi-212, and Bi-213) have generated encouraging results, providing an impetus for future clinical trials.

CONCLUSION

The onset of early clinical trials with alpha-emitters will hopefully enable the cancer researchers to come up with extremely effective and highly specific "smart bombs" to target cancer cells.

Status of radioimmunotherapy in the new millennium

This synopsis attempts to summarize progress made in radioimmunotherapy (RIT) by the end of the 20th century addressing the problems, possible solutions, and recent developments. The reduction of minimal residual disease in an adjuvant setting appears to be a feasible goal for RIT utilizing short-range alpha-emitters. RIT has been more successful in the radiosensitive hematologic malignancies, for example lymphomas and leukemias as compared with small solid tumors. Several radiopharmaceuticals seem near approval for RIT in patients with non-Hodgkin's lymphoma (NHL) as therapeutic responses, including complete responses, are common. Obstacles to successful RIT have been recognized and strategies to overcome these hurdles and to improve efficacy are continuously being developed resulting in encouraging outcome particularly with locoregional routes of administration in solid tumors. Systemic RIT for solid tumors will need manipulating the tumor-host to improve the tumor uptake and retention of radioimmunoconjugates (RICs). The utilization of radiometals, stable chelators, biodegradable linkers and bone marrow transplantation should be able to deliver the radiation dose required for successful treatment. In conjunction with additional synergistic agents, RIT is likely to have a great impact on the treatment of solid tumors. The ability to generate new constructs, such as bivalent antibodies or fusion proteins incorporating two different functional proteins opens exciting opportunities for new therapeutic modalities. These developments will hopefully offset the impediments to the successful use of RIT.

Cancer therapy with radiolabeled antibodies. An overview

Considerable progress has been achieved during the last two decades in the use of radiolabeled tumor-selective monoclonal antibodies in the diagnosis and therapy of cancer. The concept of localizing the cytotoxic radionuclide to the cancer cell is an important supplement to conventional forms of radiotherapy. In theory the intimate contract between a radioactive antibody conjugate and a target cell enables the absorbed radiation dose to be concentrated at the site of abnormality with minimal injury to the normal surrounding cells and tissues. A variety of approaches and combinations of this strategy are now being pursued. This synopsis attempts to summarize the theoretical and biological basis for radio-immuno-therapy (RIT), and to review present efforts to further develop this treatment. Some of the critical issues in RIT are highlighted, and novel ways of improving the therapeutic indices of these radiopharmaceuticals are outlined. The attention is focused on the results obtained in clinical trials employing RIT. Encouraging complete response rates have recently been reported in patients with non-Hodgkin's lymphoma resistant to combination chemotherapy. More modest results have been obtained in patients with solid cancers. The promises and hurdles in creating tumor-selective radiolabeled antibodies for cancer therapy are discussed, and prospects for further improvements are presented.

Radio-immunotherapy dosimetry with special emphasis on SPECT quantification and extracorporeal immuno-adsorption

Results from therapeutic trials with radiolabelled monoclonal antibodies are difficult to compare, because of lack of accurate macroscopic and microscopic dosimetry for both tumours and normal tissues. Requirements for such a dosimetry are covered in the paper. Accurate in vivo dosimetric measurement techniques for verification of calculated absorbed doses are also needed to verify treatment planning. In the review, important topics related to dosimetry in therapeutic trials in RIT are covered, such as, absorbed-dose calculations and activity-quantification techniques for planar imaging and SPECT. The latter is particularly discussed, including a summary of different correction techniques. Absorbed-dose calculations and treatment-planning techniques are also discussed. Possible ways of enhancing the therapeutic ratio are reviewed, especially the novel technique with extracorporeal immuno-adsorption. The review could form the basis of the development of future treatment-planning protocols and for dosimetry calculations in radio-immunotherapy, considering some of the most important parameters for approaching an accurate in vivo dosimetry.

A general extracorporeal immunoadsorption method to increase tumor-to-tissue ratio

The idea of applying extracorporeal immunoadsorption (ECIA) in radioimmunodiagnosis and radioimmunotherapy has been proposed previously. The authors here report on the development of new concept using a general method for ECIA based on biotinylated MoAb adsorbed on an avidin column. Athymic rats heterotransplanted with either human melanomas or human lung carcinoma were injected with iodine-125-labeled biotinylated 96.5 or L6 MoAb, respectively. At 24 or 48 hours after the injection, ECIA was performed by pumping blood through a hollow-fiber plasma filter. The separated plasma then was passed through an absorbent (avidin-agarose) column. The whole ECIA procedure lasted for 3 hours. By this ECIA method, the tumor-to-normal tissue ratios were increased in various tissues (i.e., radiosensitive and blood rich organs) by a factor of four to five.

Intralesional radioimmunotherapy of malignant gliomas. An effective treatment in recurrent tumors

BACKGROUND

Intralesional radioimmunotherapy (RAIT) may improve the management of malignant gliomas whose prognosis is, at present, very poor. Current treatment modalities (e.g., surgery, radiotherapy, and chemotherapy) may prolong survival by a few months but cannot prevent tumor recurrence.

METHODS

Following one or more surgical operations, radiotherapy, and chemotherapy, 24 patients with recurrent malignant gliomas (23 brain and 1 spinal cord) underwent RAIT with 2 murine monoclonal antibodies (MoAb), BC-2 and BC-4, raised against tenascin (TN). This antigen is expressed in large amounts in the stroma of glial tumors but not normal brain tissue. The isotope used was iodine-131 (131I). The radiolabelled antibodies were injected directly into the tumor by means of a removable catheter or an indwelling catheter placed in the site of disease at the time of craniotomy. The patients were admitted to the protocol if histochemical analysis of their tumors demonstrated the presence of TN in high abundance. Biodistribution and dosimetry of an intralesional tracer dose (1 mg MoAb and 37 MBq 131I) were studied. RAIT was performed by the administration of escalating doses of radioiodine, ranging from 15 mCi to 57 mCi. In many cases, RAIT was was repeated two, three, or four times (on 8, 3 and 4 patients, respectively).

RESULTS

Pharmacokinetic data resulted, on average, as follows: the 24-hour tumor/background ratio was 16.6; the percentage of injected dose concentrated per gram of tumor at 24 hours was 2.4%; and the effective half-life of the MoAb at the tumor was 74.5 hours. The mean radiation dose to the tumor was 36.48 cGy per MBq of 131I injected. Both systemic and brain toxicities were absent, while human anti-mouse antibody production after MoAb administration occurred in only a few cases. At present, 17 patients are assessable, with a median survival time of 16 months. Objective responses consisted of 5 tumor stabilizations (median time, 9 months), 3 partial remissions (11 months), and 3 complete remissions (15 months).

Radioimmunotherapy dosimetry--a review

Results from therapeutic trials in systemic radiation therapy with radiolabelled monoclonal antibodies are difficult to compare, because of lack of accurate dosimetry. This applies macroscopically as well as microscopically for both tumours and normal tissues. For treatment planning in radioimmunotherapy both the macroscopic and the microscopic absorbed dose distribution must be known. The former is based on a proper knowledge of parameters, such as activity quantitation techniques in both planar and SPECT imaging, different correction techniques, and high activity measurements. Absorbed dose calculations and treatment planning techniques are based on analytical or Monte Carlo calculations. The PET technique with higher resolution is also suggested for radioimmunotherapy planning. Accurate in vivo absorbed dose measurement techniques to verify the calculated absorbed doses are needed in treatment planning. Monitoring the absorbed rate is desirable to assess radiobiological effect. Several ways of enhancing the therapeutic ratio are suggested, especially novel technique with extracorporeal immunoadsorption. An important topic is small scale dosimetry, which is based on techniques for detailed imaging of activity distributions to calculate the absorbed dose distribution.

Radioiodinated anti-hepatocellular carcinoma (HCC) ferritin. Targeting therapy, tumor imaging and anti-antibody response in HCC patients with hepatic arterial infusion

Radioimmunoimaging and radioimmunotherapy with radioiodinated anti-(hepatocellular carcinoma ferritin) antibody (131I- or 125I-FtAb) have been applied in patients with primary liver cancer. A total of 41 patients with surgically unresectable hepatocellular carcinoma (HCC) and receiving hepatic artery ligation and cannulation during exploratory laparotomy were treated with this regimen by intrahepatic arterial infusion. Compared with the control group, a decline of serum alpha-fetoprotein (65.7% versus 42.9%) and shrinkage of tumor (68.3% versus 33.9%) were observed in the treated group, and a higher second-look resection rate (31.7% versus 5.1%) and longer survival (1-year: 61.0% versus 37.3%, 3-year: 25.0% versus 6.9%) resulted. The administration of antibody through a hepatic arterial catheter (n = 16) was compared with intravenous injection (n = 17) in terms of the tumor-imaging sensitivity in 33 patients with liver cancer. The results indicated that hepatic arterial infusion was superior to intravenous injection. The sensitivity 7 days after the administration was 100% in the i.a. group and 76.5% in the i.v. group, the uptake ratio of tumor to liver being 1.74 +/- 0.57 in the former and 1.34 +/- 0.29 in the latter. Furthermore, intrahepatic arterial infusion revealed a lower anti-antibody detection rate than intravenous injection (0/14 versus 4/11).

Quantitative measurement of uptake of radiolabeled monoclonal antibody by means of planar data

Data obtained from planar images were used to measure the uptake of monoclonal antibody in organs and tumors. Background counts included in the region of interest were eliminated, and the attenuation of the photons emitted by the radionuclide through the intervening tissues was compensated for. The background counts and the intensity of the attenuation were determined from the results of phantom studies and numerical integration with a personal computer. The quantitative uptake of 111In labeled anti-melanoma Monoclonal Antibody (ZME-018) in a melanoma lesion, the liver, and the bone marrow of a patient was measured by the planar method which we developed.

The effect of the alpha-particle emitter astatine-211 in the mouse at the minimum toxic dose

The radioactive halogen astatine-211 was injected into mice in an amount producing minimal toxicity. Histopathological examination of tissues at intervals between 3 d and 16 weeks showed the following changes: 1. Radiation-induced necrosis and progressive fibrosis of the thyroid gland. The gland was reduced to 25% of its original mass with only a few relatively normal follicles persisting. 2. A small, temporary, reduction in peripheral blood lymphocytes, platelets and red cells and a significant persistant increase in polymorphs. 3. Severe reduction in reproductive cells in the testis with some signs of recovery at 16 weeks.

The radiobiology of targeted radiotherapy

Targeted radiotherapy consists of biologically selective irradiation of malignant cells by means of radionuclides attached to tumour-seeking molecules. A variety of clinical strategies for targeted radiotherapy may be used, for which different normal tissues will be critical. A large number of radionuclides exist, emitting nuclear particles with a range of path lengths from nanometres to millimetres. An important feature of normal-tissue radiobiology is the dose-rate effect, which is especially marked for late-responding tissues. Radiobiological calculations imply that tolerance dose for targeted radiotherapy using low-LET emitters will depend strongly on the effective half-life of the radionuclide, which will be affected by pharmacokinetics and may vary between patients. Some strategies designed to improve the therapeutic radio (e.g. accelerated clearance of radionuclide) may have modulating effects on the tolerance dose. Tumour response will be governed by the 'four Rs' (repair, repopulation, reoxygenation, redistribution) as well as by mechanisms peculiar to targeted radiotherapy. Analysis based on the extended linear quadratic model predicts that dose-rate effects will be of major importance for only a minority of tumours. Most of the radiation dose to tumour will usually be delivered over a time-scale of a few days. This might give insufficient time for tumour reoxygenation, making the use of hypoxic sensitizers appropriate. A special feature of targeted radiotherapy is the complex relationship between tumour curability and tumour size for different radionuclides. For long-range beta-emitters, microscopic tumours may be operationally resistant because of inefficient absorption of radionuclide disintegration energy in small volumes. Short-range emitters will be more efficient in sterilization of micrometastases but sterilization of larger tumours may require an unattainable degree of homogeneity of radionuclide distribution. Optimal use of targeted radiotherapy may require it to be combined with external-beam irradiation or chemotherapy. Experimental studies will be necessary to investigate those features of targeted radiotherapy which differ from external-beam irradiation. Future directions may include targeted radiotherapy of minimal numbers of tumour cells detected by use of molecular probes. Such applications call for use of short-range alpha-emitters and Auger emitters whose radiobiology will become increasingly important.

Antibody mediated targeting of radioisotopes, drugs and toxins in diagnosis and treatment

The recent resurgence of interest in site specific delivery of radioisotopes, chemotherapeutic drugs and toxins for the diagnosis and treatment of cancer, and for the selective manipulation of the immune system, can be directly related to the need for improved diagnosis and the fact that for many cancers, for example lung, colon and gastric, the conventional treatments of surgery, radiotherapy and chemotherapy have reached a plateau in terms of the number of patients cured. To date, because of their specificity, the major emphasis has been on the use of antibodies as carriers and extensive in vitro, in vivo preclinical and clinical evaluation is underway. The aim of this article is to review recent progress, highlight avenues being explored to overcome limitations and to indicate new approaches that are evolving in antibody mediated targeting.

A new calculational method to assess the therapeutic potential of Auger electron emission

This paper discusses a new computer code to estimate the efficacy of Auger electron sources in cancer therapy. Auger electron emission accompanies the decay of many radionuclides already commonly used in nuclear medicine, for example; 99mTc and 201Tl. The range of these electrons is in general sub-cellular, therefore, the toxicity of the source depends on the site of decay relative to the genetic material of the cell. Electron track structure methods have been used which enable the study of energy deposition from Auger sources down to the Angstrom level. A figure for the minimum energy required per single strand break is obtained by fitting our energy deposition calculations for 125I decays in a model of the DNA to experimental data on break lengths from 125I labeled plasmid fragments. This method is used to investigate the efficiency of double strand break production by other Auger sources which have potential value for therapy. The high RBE of Auger sources depends critically on the distance between the source and target material. The application of Auger emitters for therapy may necessitate a carrier molecule that can append the source to the DNA. Many DNA localizing agents are known in the field of chemotherapy, some of which could be carrier molecules for Auger sources; the halogenated thymidine precursors are under scrutiny in this field. The activation of Auger cascades in situ by high energy, collimated X ray and neutron beams is also assessed.

Effect of different linkages between chelates and monoclonal antibodies on levels of radioactivity in the liver

After injection of radiometal labeled antibodies, the radionuclide accumulates in the liver. This might be altered by a metabolizable linkage between metal chelate and antibody. Four benzyl-EDTA chelating agents were synthesized and conjugated to mouse monoclonal antibody Lym-1. Liver uptake of 111In in nontumored mice 72 h after injection was 2.2, 13.4, 7.6 and 20% for disulfide, thioether, thiourea or peptide linkages, respectively. 111In excreted in the urine was still in the benzyl-EDTA chelate form, as shown by binding to a specific anti-chelate antibody.

Intratumour factors influencing the access of antibody to tumour cells

The histological structure and biochemical composition of human tumours is very varied, as is the structure of the microvascular network. It can be expected, therefore, that the extravasation, diffusion and convection of macromolecules will vary between tumours - and also between areas in the same tumour. The major factors influencing the intratumour distribution of injected antibody are reviewed and an attempt made to identify the tumour types in which therapeutic antibody, with or without a cytotoxin, will distribute most effectively.

Toxicity of astatine-211 in the mouse

The toxicity of the alpha particle emitting halogen astatine-211 was examined in male and female mice. Pathological changes were seen in mice killed at 14 days and/or at 56 days following a single injection of 61 kBq211 At per g body weight. The tissues affected, in order of severity were: spleen, lymph nodes, bone marrow, gonads, thyroid, salivary glands and stomach.

Strategies for systemic radiotherapy of micrometastases using antibody-targeted 131I

A simple analysis is developed to evaluate the likely effectiveness of treatment of micrometastases by antibody-targeted 131I. Account is taken of the low levels of tumour uptake of antibody-conjugated 131I presently achievable and of the "energy wastage" in targeting microscopic tumours with a radionuclide whose disintegration energy is widely dissipated. The analysis shows that only modest doses can be delivered to micrometastases when total body dose is restricted to levels which allow recovery of bone marrow. Much higher doses could be delivered to micrometastases when bone marrow rescue is used. A rationale is presented for targeted systemic radiotherapy used in combination with external beam total body irradiation (TBI) and bone marrow rescue. This has some practical advantages. The effect of the targeted component is to impose a biological non-uniformity on the total body dose distribution with regions of high tumour cell density receiving higher doses. Where targeting results in high doses to particular normal organs (e.g. liver, kidney) the total dose to these organs could be kept within tolerable limits by appropriate shielding of the external beam radiation component of the treatment. Greater levels of tumour cell kill should be achievable by the combination regime without any increase in normal tissue damage over that inflicted by conventional TBI. The predicted superiority of the combination regime is especially marked for tumours just below the threshold for detectability (e.g. approximately 1 mm-1 cm diameter). This approach has the advantage that targeted radiotherapy provides only a proportion of the total body dose, most of which is given by a familiar technique. The proportion of dose given by the targeted component could be increased as experience is gained. The predicted superiority of the combination strategy should be experimentally testable using laboratory animals. Clinical applications should be cautiously approached, with due regard to the limitations of the theoretical analysis.

Human monoclonal antibodies and monoclonal antibody multispecificity

The majority of human anti-tumour monoclonal antibodies (Mabs) isolated to date have been disappointing. Firstly, they react or cross react with intracellular cytoskeletal proteins or nuclear antigens and therefore are of limited value as blood borne agents. They are also generally of the IgM isotype and show relatively low intrinsic affinity for the primary epitope. Secondly, such Mabs can be generated from normal, non tumour bearing subjects at a frequency comparable to their production from tumour patients. This latter observation is true also for common autoantigens such as DNA and IgG since Mabs to these can also be generated from normal subjects in addition to autoimmune individuals. This article rationalises these observations in the context of the requirement for clinical use for human Mabs. It discusses the evidence that there is a potentially useful B cell response to be immortalised, and examines the consequences of the newly recognised phenomenon of monoclonal antibody multispecificity both on the methodology of their generation and on their subsequent use as imaging and therapeutic tools.