Status of Radiolabeled Monoclonal Antibodies for Diagnosis and Therapy of Cancer
Status of Radiolabeled Monoclonal Antibodies for Diagnosis and Therapy of Cancer
Monoclonal antibodies (MoAbs) have the property of reactive specificity
to tumor-associated antigens. In addition, their mammalian origin
makes them nontoxic. They are, therefore, attractive candidates
as therapeutic agents, either by themselves or conjugated to radionuclides,
cytotoxic drugs, or toxins. The same specificity is exploited
in radioimmunodetection, wherein the antibody is labeled with
a suitable radionuclide that can be detected using current gamma
Radiolabeled antibodies have probably been studied in more clinical
trials than any other form of immunoconjugate. The largest number
of clinical trials with radiolabeled MoAbs, both diagnostic and
therapeutic, have been carried out in patients with metastatic
colorectal cancer . Fewer trials have been conducted in other
solid tumors, notably, ovarian carcinoma , melanoma , lung
carcinoma , renal cell carcinoma , breast cancer , carcinoma
of the prostate , head and neck carcinoma , and central
nervous system tumors. Immunotherapy with radioimmunoconjugates
as well as immunotoxins has shown promise, especially in non-Hodgkin's
lymphoma [9,10] and myelogenous leukemia . Phase I and II
trials with radiolabeled antibodies in solid tumors have provided
tantalizing responses, albeit none major [12-15].
All trials with murine antibodies, with the exception of those
in B-cell lymphomas and pediatric neuroblastoma , have demonstrated
that the probability of significant responses is hampered by the
limitation of single use caused by their invariable immunogenicity.
This has led to the development of nonimmunogenic forms, including
chimeric  and humanized  antibodies, which will potentially
permit multiple administrations.
Initial clinical trials of MoAbs focused on antibodies against
carcinoembryonic antigen (CEA) [19,20], an antigen secreted by
most differentiated adenocarcinomas. These trials, with iodine-131
as a radioconjugate permitting external detection (scintigraphy),
revealed targeting of radioantibody to sites of known disease.
They further demonstrated the safety of antibody administration.
This led to the exploration of multiple "tumor-associated"
antigens--secreted, cell-surface, and intracellular--in most tumor
These trials have reached several conclusions:
- Monoclonal antibodies can be safely administered, with toxicity
limited to rare allergic reactions.
- Toxicity of immunoconjugates is limited to the toxicity of
the nuclide or toxin attached to the antibody. For radioimmunotherapy,
this means that the critical organ is the bone marrow, with thrombocytopenia
being the dose-limiting toxicity.
- Murine monoclonal antibodies usually evoke an immune response
in humans, which precludes repeat administration, except in B-cell
lymphoma, and in pediatric patients with neuroblastoma who have
been heavily pretreated--both conditions in which immune compromise
For radiolabeled antibodies, there are several additional considerations:
- Indium-111, used in radioimmunodetection, is limited in its
ability to specifically target hepatic metastases owing to considerable
nonspecific hepatic uptake of radioactivity, probably caused by
- Technetium-99m, another radionuclide used for detection, will,
owing to its short physical half-life of 6 hours, probably be
more effective conjugated to antibody fragments, although rapidly
targeting intact immunoglobulins are also likely to be useful.
- Radioimmunotherapy, especially for solid tumors, may require
either large doses with marrow support or carefully planned fractionations.
- Beta-emitters, such as yttrium-90, iodine-131, and rhenium-186
are nuclides of choice for established disease; their major toxicity
is hematopoietic. The low ionizing potential of these radionuclides
probably limits their utility in adjuvant treatment, and their
major role is probably in the treatment of residual small-volume
Antibodies are members of a family of immunoglobulins produced
in mammals by B-lymphocytes. The development of hybridoma technology
by Köhler and Milstein in 1975 made possible the production
of large quantities of MoAbs that recognize human tumor-associated
antigens . Most currently used antibodies belong to the immunoglobulin
G (IgG) subclass. The IgG molecule, with a molecular weight of
between 150 and 180 kD, consists of two heavy and two light polypeptide
chains connected by disulfide bonds. Pepsin splits part of the
constant region (Fc) to produce an F(ab')2 fragment (~110-kD molecular
weight). The enzyme papain splits the antibody molecule into one
Fc fragment and two Fab' (~ 50 kD each) fragments. Figure 1 depicts
the basic structure of the molecule, as well as the sites of action
of pepsin and papain.
Radioimmunodetection studies have been carried out not only with
intact immunoglobulin but with MoAb fragments as well. These have
included F(ab')2 and, more frequently, Fab' fragments. The smaller
the molecule, the faster its clearance from serum . However,
Fab' fragments tend to be retained in the kidneys for extended
periods, limiting their usefulness in the detection of disease
around the kidneys. Moreover, fragments show less affinity for
target antigens than intact immunoglobulin, and they are more
difficult to produce and purify to clinical standard.
The radionuclide with energy emission characteristics most suitable
for use with current nuclear medicine cameras is technetium-99m,
which has gamma emissions of 140 KeV. This transitional element
has no particulate emission; its short half-life further reduces
radiation dose to the patient, permitting use of relatively large
amounts of radioactivity. However, studies with technetium-99m-labeled
MoAbs have been few . This is because fragment production
and antibody (both immunoglobulin and Fab') labeling with technetium-99m
are difficult procedures.
Rhenium is another transitional element with chemical characteristics
comparable to those of technetium-99m. Rhenium-186 and -188 have
been used in therapy. Both emit beta-minus radiation with concurrent
gamma emissions, permitting imaging. Stability of binding of rhenium
to antibody (usually carried out by an indirect linker) is an
important consideration in the development of suitable rhenium-labeled
antibodies in immunotherapy.
Chelation of MoAbs with radiometals, particularly indium-111,
is difficult, with hepatic uptake of radioindium (presuMoAbly
via trans-chelation in vivo to transferrin) limiting their utility
in the detection of hepatic metastatic disease.24 The physical
characteristics of indium-111 (half-life of 3 days, photons of
187 and 245 KeV, no particulate emission) make it an attractive
nuclide for use in radioimmunodetection with intact immunoglobulin,
despite its cost. Perhaps for this reason, the first radiolabeled
MoAb to obtain FDA approval as a cancer-imaging agent was an indium-111-labeled
Yttrium-90, copper-67, and lutetium-177 are beta-emitting metals
with therapeutic potential. Chelates do not bind radiometals to
antibodies comparably. Lutetium-177 permits external imaging;
yttrium-90 does not. Stability in vivo of therapeutic radiometals
has not been optimal; advances in radiochemistry are encouraging,
Radioiodination of proteins, including MoAbs, is a relatively
simple process, and thus, the greatest number of radioimmunodetection
trials have been carried out with radioiodinated MoAbs. Iodine-123
has a short half-life (13 hours) and ideal emission characteristics
(no particulate emission, 159-KeV photon emission). Its exorbitant
cost has precluded its more widespread use, however, and its short
half-life has deterred investigators from studying immunoglobulins
labeled with iodine-123 .
Iodine-125 has low-energy photons that preclude external imaging
but make it ideal for use in intraoperative radioimmunodetection
(radioimmunoguided surgery) . Moreover, its long half-life
of about 60 days permits its detection up to 6 weeks after administration,
at which point normal tissue background levels are relatively
low, permitting the detection of extremely small foci of abnormal
antibody uptake. Consequently, most surgical radioimmunodetection
studies have used iodine-125-labeled MoAbs.
Iodine-131 has a long half-life of 8 days and a complex decay
scheme that includes beta-minus emission, precluding use of large
amounts of radioactivity. This constraint, in addition to its
high energy (364-KeV) gamma emission makes it less than optimal
for gamma camera imaging, necessitating special collimation for
current gamma cameras. Despite these limitations, however, the
majority of radioimmunodetection trials have been carried out
with iodine-131-labeled MoAbs . There are several reasons for
this: (1) Iodine-131 is easily available and relatively inexpensive.
(2) Protein radioiodination is relatively easy to carry out. (3)
Radioiodinated MoAbs are relatively stable in vivo. (4) Persistence
of free radioiodine in the body can be obviated by saturation
of physiologic iodine stores with nonradioactive iodine (Lugol's
solution or a saturated solution of potassium iodide), permitting
prompt clearance of free radioiodine by the kidneys without significant
thyroid or stomach uptake; thus, nonspecific uptake of iodine-131-labeled
MoAbs is not a problem. Moreover, the 8-day half-life of iodine-131
makes it possible to study the kinetics of antibody uptake by
target tissue over time.
Successful clinical imaging of cancer was first reported using
iodine-131 labeled to anti-CEA antibodies . It is particularly
useful for the detection of neoplastic disease in the abdomen
due to its low nonspecific uptake in the liver, spleen, and bone
marrow. Iodine-131 has also been used in most radioimmunotherapy
trials to date.
Regardless of the antibody used, dose-limiting toxicity has been
myelosuppression. The high gamma emission entails hospitalization
until radiation dose from the patient is sufficiently low (5 mR/h
or less at 1 meter in the United States). Therapeutic trials with
iodine-125 have been limited to MoAbs that internalize into the
cell following interaction with antigen.