Fundamentals of radioimmunotherapy

NW/. Med. EioL Vol. 18, No. I, pp. 105-108,

1991

ht. J. Radiat. Appl. hstrum. Part B

Copyright

0

0883~2897/91 $3.00 + 0.00 1991 Pergamon Press plc

Printed in Great Britain. All rights reserved

Fundamentals

of Radioimmunotherapy JANET F. EARY*

University of Washington Medical Center, Seattle, WA 98195, U.S.A. Design of a radioimmunotherapy trial is a complex process. This involves antibody selection, choice of radioisotope, and labeling method. Observation of the behavior of the new radiopharmaceutical involves determination of normal organ and tumor uptake, as well as residence time. Calculation of external radiation absorbed dose must also be performed, and criteria set for therapy. Once treatment has taken place, mechanisms for patient follow-up must be implemented. This contribution discusses important considerations in the design and implementation of a clinical trial in radioimmunotherapy.

Introduction In the past decade, we have seen advances of radiolabeled monoclonal from ideas to implementation

antibodies

Antibody and Radioisotope Selection in the uses (MoAbs)

in techniques for imaging and treatment of malignancy. Trace-labeled antibodies have appropriate sensitivity and specificity to show clinical usefulness as tools for staging and follow-up of disease (Eary et al., 1989; Carrasquillo et al., 1985; Epenetos et al., 1985a). Most of these antibodies have proven amenable to high specific activity labeling to be useful for treatment (Horowitz et al., 1988; Rosen et al., 1987; Eary et al., 1990). Because of the wide variety of antibodies with several isotopes available for use, many groups have entered into phase I and phase II trials for evaluation of clinical utility and toxicity of radiolabeled MoAbs for a variety of therapeutic protocols. With these new achievements, the design of clinical protocols for evaluating these new radiopharmaceuticals for clinical use has become extremely complex. Particular attention must be paid to various aspects of a radioimmunotherapy clinical trial design in order to produce meaningful results. First of all, in preparing to start a radioimmunotherapy trial, the group of investigators must decide on specific goals. Although the ultimate goal is to diagnose and cure cancer, there are many decision steps before arriving at the point of commencing the trial. In this article, specific considerations and fundamentals for design and implementation of clinical trials for radioimmunotherapy will be discussed. This discussion will provide guidelines for the process of designing and conducting clinical trials in radioimmunotherapy. *All correspondence should be addressed to: Janet F. Eary, M.D., Division of Nuclear Medicine, RC-70, University of Washington Medical Center, 1959 N.E. Pacific, Seattle, WA 98195, U.S.A.

Each radiolabeled antibody or its fragment is a unique radiopharmaceutical. The antibody selected must have adequate specificity for the tumor site without significant cross-reactivity with normal tissues. Although pre-clinical evaluations of tissue cross-reactivity of antibodies using immunoperoxidase staining are essential, only trace-labeled doses administered to carefully selected patients will provide a true evaluation of the specificity of the labeled antibody product. Also, trace-labeled antibody studies will be informative about whether clearance time from normal tissues is appropriate for therapy, or advantages would be gained with the use of antibody with slower or faster clearance. Smaller molecular weight antibody fragments might also be considered for more rapid alterations in clearance. Selection of the isotope must be tailored in several respects to the clearance of the antibody, which may include specific antibody metabolism (Press et al., 1990). The isotope must have stable attachment to the antibody. If labeled antibody metabolism is a feature of the non-specific biodistribution, then the distribution of the free isotope must have acceptable normal tissue biodistribution and kinetics. In particular, the radioisotope, or labeled small metabolic fragments of antibody should clear rapidly and not be translocated to a non-tumor site, causing normal tissue irradiation. This is a particularly critical issue with radiometals where the free isotope may localize in marrow or bone. The half-life of the isotope must also be compatible with the biokinetics of the antibody. Most whole antibodies require 48-72 h for maximum tumor deposition, so the half-life of the isotope must be at least as long to selectively irradiate tumor compared to normal organs. Shorter lived isotopes might be more appropriate for antibody fragments which have faster clearance times. After 105

JANETF. EARY

106

the appropriate radioisotope has been decided upon, the new radiopharmaceutical must have a set of easy to perform, quality control tests that consistently yield reliable reproducible results (Eary et al., 1990). An example is shown in Table 1. Isotopes under consideration for radioimmunotherapy currently fall into categories of /3 emitters, and a emitters (Table 2). Beta emitters include the standard 13’1,with the introduction of “Y, and ls6Re. Proponents of the use of /3 emitters believe that they are most appropriate for radioimmunotherapy because the energetic emissions have a long path length in tissue. In most instances, not all tumor cells are saturated with infused antibody. The longer path length of the /3 particle would insure a more homogeneous dose distribution than the antibody deposition pattern in tumor alone would indicate. The longer tissue penetration of the b particle also would cause irradiation of nearby antigen negative tumor cells. Proponents of the c1particle (“‘At, *‘*Bi, etc.) for the therapeutic agent hold that the higher LET of the CIparticle would ensure a higher proportion of cell kills per decay by direct hits into the cell (Humm and Cobb, 1990).

Biodistribution and Estimation of Internal Absorbed Dose In order to understand the pharmacokinetics of the radiolabeled antibody, it is most important to acquire biodistribution data. Within this topic, several issues need to be addressed. In particular, the amount of antibody to be administered is critical. Different amounts of antibody have varying biodistributions based on the antigen burden, the nonspecific binding, and metabolism in the tumor, or in non-specific sites. Associated with this is the issue of dosing schedule; whether small amounts should be given frequently, or large doses should be given in one single administration. Another consideration is how the antibody is going to be administered. Although many antibody trials are constructed with the use of i.v. administration of antibodies, numerous other trials are currently in place that are very creative (Fig. l), where antibodies are instilled into a body cavity such as the subarachnoid space, the peritoneum for ovarian cancers (Hammersmith Oncology Group, 1984) or direct arterial injections (Epenetos et al., 1985b). Other considerations for lymphatic-based disease are interlymphatic injections or S.C.injections (Epenetos, 1985; Nelp et al., 1986). All of these routes of injection are possibilities, and the clinical protocol Table

I. Quality

Test Radiochemical purity Apyrogenicity Sterility lmmunoreactivity

control

Table 2. Isotopes Isotope 0.34 0.61 2.29 0.93 1.07 1.97 2. I3 5.87 7.45 6.05 6.09

‘31, my ‘86Re ‘88Re *“At ZZRi

considered

for radioimmunotherapy

Emission

Half-life

Photons

MeV B (I 3%) MeV /5’(86%) MeV b (100%) MeV p (22%) MeV fi (70%) MeV B (24%) MeV fl (74%) MeV OL(42%) MeV a (58%) MeV a (25%) MeVa ilo%j

8.1 d

364 keV (82%)

6.4 h 90.6 h

none 137 keV (9.2%)

6.9 h

155 keV (15%)

7.2 h

none

60.6 min

none

route of administration, dosing schedule, and amount of antibody, may be combined to address the specific issues of the malignancy, and stage of disease. Once pharmacokinetic studies have been designed for trace labeled antibody studies, radiation dosimetry of the antibody must be determined if it is truly going to be useful as a radioactive therapeutic agent. Estimation of internal radiation absorbed dose using trace doses of radiolabeled antibody as a predictor of the radiation dose delivered by a high specific activity therapeutic dose is essential to determine specific antibody localization. The objective is to deliver increased radiation absorbed dose per gram of tumor compared to normal organs. If there is no such advantage, then there is little potential effectiveness in this alternative route of radiation therapy compared to total body radiation. Therefore, it is essential to establish a mechanism to calculate estimated internal radiation dose to tumors and susceptible normal organs. Usually considered in dose estimations for potential toxicity, are the liver, lungs, kidneys, and Introthecol

or

Arterial

(1 Introvenous

Introperitoneol

1

d

Introlymphotic

J

of the final product

Method Electrophoresis Limulus lysate Culture Cell binding assay (Scatchard)

ROUTES

Result >99% protein Negative Negative >80% normal

bound

levels

OF ADMINISTRATION

proposed for radioimmunotherapy. Localized administration of antibody may provide a means of directing antibody localization to malignancy confined to a specific site. Intravenous administration may be more appropriate for disseminated disease. Fig.

1. Routes of administration

107

Fundamentals of radioimmunotherapy bone marrow. Time-activity curves and quantitative uptakes for these normal organs can be determined, providing input data for calculation of MIRD dosimetry estimates (Eary et al., 1990; Loevinger and Berman, 1976). Although the MIRD schema can estimate internal radiation absorbed dose for the intestines, it is difficult for most investigators to determine quantitative uptake and time-activity information for the intestines in routine trace labeled antibody imaging studies. Input data into MIRD schema for estimation of internal radiation absorbed dose requires residence time of the isotope, and quantitative uptake, which is expressed as PCi-hours. Appropriate S values from the MIRD table are used. Dose is estimated based on rads delivered to the organ for 1 mCi of radioactivity. These can be adjusted for individual patient organ volumes and tumor volumes, if data are available from another quantitative imaging modality such as CT or MR scanning (Heyersfeld et al., 1979; Moss et al., 1981). Although quantitative imaging techniques can be applied to imaging patient tumors, additional information on tumor uptake is best obtained by biopsy and counting the radioactive content of the tumor directly (Eary et al., 1990). In addition, blood clearance, urinary clearance, and whole body counting using an independent counting device, can provide other information for input into MIRD schema for estimating whole body radiation dose and estimates of antibody serum clearance. After performance of an absorbed internal radiation dose estimate, toxicity of treatment by radioimmunotherapy must be evaluated. Toxicity from this type of treatment falls into two categories; that related to infusion of the antibody itself, and toxicity associated with radiation delivered by the antibody (Table 3). Immediate toxicity is associated with infusion of the antibody, and can involve fever, chills, hypotension, hives, and pancytopenia. These symptoms are related to serum sickness or acute anaphylactoid reactions. Intermediate types of toxocity are those which result within a week or so after antibody administration. They include bone marrow suppression, and formation of human anti-mouse antibodies (HAMA), which can become very important. If the patient has an acquired immune reaction to the labeled antibodies, there may be a higher frequency with which anaphylactoid reactions occur after infu-

Table (I)

3. Toxicity

sion. Altered biodistribution of the antibody may occur with subsequent antibody administrations because of antibody complexation with the therapeutic antibody. Long term toxicities are dependent upon the type of radiopharmaceutical that is used, however those commonly seen are those related to free iodine deposition in the thyroid, resulting in hypothyroidism in spite of thyroid blocking with cold iodine solutions. Accurate documentation of all toxicities related to antibody infusion are critical to evaluating the overall usefulness and therapeutic potential of the radiopharmaceutical.

Designing

Post-treatment

Table

Fever (I)

(2)

Thrombocytopenia (2)

Bone

Marrow

Human (3)

Long

Term

Thyroid

(3)

Suppression

Anti-mouse

Antibody

(HAMA)

Radiation

disposal

Transport (a)

Intermediate

4.

Radiolabeling, (a)

Hypotension Hives

Follow-up

Although the goal of a phase I trail is toxicity, most investigators have an interest in efficacy of new therapies as well. All patients coming in to the radioimmunotherapy trial must have adequate documentation of disease sites, with specific locations and measurement of the disease sites as well as documentation of pre-existing conditions for evaluation of future response and toxicities. The most accurate documentation of patient’s disease is from complete physical exam in conjunction with whole body CT scanning, or MR scanning. In addition, assessment of marrow-based disease can be accomplished by STIR

Immediate Chills

a Trial

Once the new radiopharmaceutical has been designated for study, with an appropriate set of criteria for dose infusion and administration, there are a new set of considerations. Are radioimmunotherapy doses going to be delivered that will cause a severe degree of bone marrow suppression? If this is the case, is there a need for bone marrow cryopreservation, and possible re-administration to the patient? What will the antibody dosing schedule for radioimmunotherapy be? Is it going to be such that bone marrow toxicity can be avoided? Because of the radioactive body burden, must the patient be hospitalized for radiommunotherapy? If so, then institution of appropriate radiation safety procedures for labeling, handling the dose, protecting personnel, taking care of the potentially ill patient, and radiation waste and room clean-up must be implemented (Table 4). Becoming familiar with hospital safety regulations, and local and state radiation safety regulations is also critical.

safety

handling

of high specific of therapy

handling

concerns

of therapy activity

patient

(a)

monitoring

(b)

monitoring

(c)

handling

(d)

length

room health waste

of stay

waste

dose to the patient

the dose

Radioactive

dose

care personnel

JANETF. EARY

108

sequence magnetic resonance imaging (Shields et al., 1987) and bone marrow biopsy. The clinical protocol must include regular follow-ups for evaluation of disease remission or progression, and have regularly scheduled blood samples for evaluation of blood chemistries and hematologic parameters. In addition, duration of responses or toxicities must be accurately documented. Serial blood sampling for development of human anti-mouse antibodies is of interest for assessment of immunogenicity of the radiolabeled antibody product.

Conclusion Although the list of considerations presented in this article may seem extensive, they all need to be addressed critically for development of a radioimmunotherapy trial that will produce meaningful Interpretable results. Omission of any of the items mentioned, can lead to confusing or ambiguous results, and potentially misleading information regarding the efficacy of radioimmunotherapy in general. Radioimmunotherapy is one of the most promising new approaches for treatment of malignancy currently available. But because of the comnlexitv of the issues involved in performing a trial which involves Stan ard oncologic practice as well as radiochemistry an iI nuclear medicine techniques, 1

-

there are a number of issues that must be addressed by a multi-disciplinary group. It is hoped that in this presentation guidelines are presented to assist investigators in designing trials through which the new potential for radioimmunotherapy in treatment of malignancy will be revealed.

References Carrasquillo, J. A.; Colcher, D.; Sugarbaker, P.; Reynolds, J. R.; Keenan, A. M.; Simpson S.; Bryant, G.; Thor, A.; Perentesis, P.; Esteban, J.; Horowitz, M.; Schlom, J.; Larson, S. M. Radiolocalization of colon cancer with I-13 1 872.3 monoclonal antibody. J. Nucl. Med. 16: 15; 1985. Eary, J. F.; Schroff, R. W.; Abrams, P. G.; Fritzberg, A. R.; Morgan, A. C.; Kasina, S.; Reno, J. M.; Srinivasan, A.; Woodhouse, C. S.; Wilbur, D. S.; Natale, R. B.; Collins, C.; Stehlin, J. S.; Mitchell, M.; Nelp, W. B. Successful imaging of malignant melanoma with Tc-99m labeled monoclonal antibodies. J. Nucl. Med. 30(l): 25-32: 1989.

Eary, J. F.; Press, 0. W.; Badger, C. C.; Durack, L. D.: Richter, K. Y.; Addison, S. J.; Krohn, K. A.; Fisher, D. R.; Porter, B. A.; Williams, D. L.; Martin, P. J.; Appelbaum, F. R.; Brown, S.; Miller, R. A.; Nelp, W. B.; Berstein, I. D. Imaging and treatment of B-cell lymphoma. J. Nucl. Med. 31: 1257-1268; 1990. Epenetos, A. A. Antibody guided lymphangiography in the staging of cervical cancer. Br. J. Cancer. 51: 805; 1985. Epenetos, A. A.; Shepherd, J.; Britton, K. E.; Mather, S.; Taylor-Papadimitriou, J.; Granowska, M.; Durbin. H.; Nimmon, C. C.; Hawkins, L.; Malpas, J.; Bodmer. W. F. 1231 radioiodinated antibody imaging of occult ovarian cancer. Cancer. 55(5): 984; 1985a. Epenetos, A. A.; Courtenay-Luck, N.; Pickering, D.; Hooker, G.; Durbin, H.; Lavender, J. P.; McKenzie, C. C. Antibody guided irradiation of brain glioma by arterial infusion of radioactive monoclonal antibody against epidermal growth factor receptor and blood group A antigen Br. Med. J. 290: 1463; 1985b. Hammersmith Oncology Group and the Imperial Cancer Research Fund. Antibody-guided irradiation of malignant lesions: three cases illustrating a new method of treatment. Lancer I: 1441; 1984. Heyersfeld, S. B.; Fulenweider, T.; Nordlinger, B. et al. Accurate measurement of liver, kidney and spleen volume and mass by computerized axial tomography. Ann. Inr. Med. 90: 185-187; 1979. Howowitz, J. A.; Goldenberg, D. M.; DeJager, R. et al. Phase I/II trial of radioimmunotherapy (RAIT) with I-131 labeled anti-CEA and anti-AFP monoclonal antibodies J. Nucl. Med. 29: 846; 1988. Humm. J. L.: Cobb, L. M. Non-uniformity of tumor dose in radioimmunotherapy. J. Nucl. Med. 31: 75-83; 1990. Loevinger, R.; Berman, M. A revised schema for calculating the absorbed dose from biologically distributed radionuclides. MIRD Pamphlet No. 1. New York: Society for Nuclear Medicine; 1976. Moss, A. A.; Friedman, M. A.; Brito, A. C. Determination of liver, kidney and spleen volumes by computed tomography: an experimental study in dogs. J. Compur. Assist. Tomogr. 5: 1; 1981. Nelp, W. B.; Eary, J. F.; Jones, R. F.; Hellstrom, K. E.; Hellstrom, I.; Beaumier, P. L.; Krohn, K. A. Preliminary studies of monoclonal antibody lymphoscintigraphy in malienant melanoma. J. Nucl. Med. 28: 3441: 1986. Press, 6. W.; DeSantes, K.; Anderson, S. K.; Geissler, F. Inhibition of catabolism of radiolabeled antibodies by tumor cells using lysosomotropic amine and carboxylic ionophores. Cancer Res. 50: 1243-1250; 1990. Rosen, S. A.; Zimmer, A.; Goldman-Leiken, R. ef al. Radioimmuno-detection and radioimmunotherapy of cutaneous T-cell lymphomas using an I. 131 labeled monoclonal antibody; an Illinois Cancer Council study. J. Clin. Oncol. 5: 562-573; 1987. Shields, A. F.; Porter, B. A.; Churchley, S.; Olson, D. A.; Appelbaum, F. R.; Thomas, E. D. The detection of bone marrow involvement of lymphoma using magnetic resonance imaging. J. Clin. Oncol. 5(2): 225-230; 1987.