Targeting and therapy of carcinoembryonic antigen-expressing tumors in transgenic mice with an antibody-interleukin 2 fusion protein
2000
The purpose of this study was to engineer a bivalent single-chain
anticarcinoembryonic antigen (CEA) antibody and an interleukin 2 (IL-2)
fusion protein derivative for selective tumor targeting of cytokines.
The variable domains of a high affinity anti-CEA antibody, T84.66, were
used to form a single-gene-encoded antibody [single-chain variable
fragment joined to the crystallizable fragment, Fc (scFvFc)]. The
fusion protein (scFvFc.IL-2) consisted of mouse IL-2-fused to the
COOH-terminal end of the scFvFc. The engineered proteins were assembled
as complete molecules and were similar to the intact anti-CEA
monoclonal antibody (Mab) in antigen-binding properties. Based on IL-2
content of the fusion protein, its ability to support proliferation of
CTLL-2 cells was identical with that of IL-2. Despite a molecular size
similar to that of the intact Mab, the blood clearance of the fusion
protein was markedly faster than that of the intact Mab or scFvFc.
Incubation of radiolabeled scFvFc.IL-2 but not the intact or scFvFc
antibodies in mouse serum was accompanied by the appearance of
complexes, suggesting that the latter may contribute to the accelerated
clearance of the fusion protein. Biodistribution and tumor targeting
studies were carried out in CEA-transgenic mice bearing CEA-positive
murine tumors as well as the antigen-negative parental tumor. The
bivalent anti-CEA scFvFc had tumor localization properties similar to
those of the intact Mab. Although fusion of IL-2 to the COOH-terminal
end of the bivalent scFvFc altered its pharmacokinetic properties, the
fusion antibody was able to target tumors specifically. Maximum uptake
of the intact Mab, scFvFc, and scFvFc.IL-2 in CEA-positive tumors was
29.3 ± 5.0, 19.5 ± 2.1, and
6.6 ± 0.9% injected dose/g, respectively. Maximum
tumor localization ratios (CEA-positive/CEA-negative tumor) were
similar for all three antibody types (4.6–6.0), demonstrating the
antigen specificity of the tumor targeting. Significant
antigen-specific targeting to CEA-positive normal tissues of transgenic
mice was not observed. Although the tumor-targeting properties of the
fusion protein were low, the growth of CEA-expressing
( P = 0.01) but not antigen-irrelevant
( P = 0.22) syngeneic tumor cells was
inhibited after treatment of transgenic mice with the anti-CEA-IL-2
antibody. Therapy of CEA-expressing tumors was improved after i.v.
administration of the fusion protein ( P = 0.0001). These studies indicate that anti-CEA antibody-directed
cytokine targeting may offer an effective treatment for CEA-expressing
carcinomas. The availability of an immunocompetent CEA transgenic mouse
model will also help to determine the immunotherapeutic properties of
these fusion proteins.
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