A novel affinity system for a two-phase delivery of
radionuclides to
tumor cells has been developed. In the first phase, a nontoxic bivalent
monoclonal antibody conjugated to an
enzyme is targeted to the
tumor cells. In the second phase, a
radionuclide-derivatized
enzyme inhibitor, specific for the
enzyme conjugated to the antibody, is administered. The model system selected for this study is the recombinant human
enzyme dihydrofolate reductase (rhDHFR) and its high-affinity competitive inhibitor
methotrexate (MTX). MTX was labeled with a
radionuclide by covalent attachment of diethylenetriaminepentaacetic
acid (
DTPA) complexed with 111In. Using the gamma-carboxyl residue of MTX for the attachment of
DTPA, binding of the inhibitor to rhDHFR was not affected. The inhibitory activities of nonderivatized MTX and
DTPA-MTX were indistinguishable. Human K562
erythroleukemia cells were used to evaluate under in vitro conditions the DHFR-MTX affinity system for the delivery of 111In-labeled
DTPA-MTX to pretargeted alpha-
transferrin receptor antibody-rhDHFR conjugates (alpha-TFR-DHFR). The data demonstrate that the delivery of 111In is dose dependent and highly specific. Under saturating conditions, binding of 111In-DTPA-MTX to alpha-TFR-DHFR-treated cells was 14-fold higher than to cells treated with nonconjugated alpha-TFR antibody. Further experiments indicated that the low level of nonspecific binding of 111In-DTPA-MTX was comparable to that of 111In-DTPA, known for its complete extracellular distribution and rapid clearance through the kidneys. Based on the data of this study, antibody-conjugated rhDHFR and
radionuclide-labeled
DTPA-MTX complexes provide components for an alternative radioimmunotherapeutic approach that can be expected to result in improved
tumor tissue ratios of both the targeting moiety and the
radionuclide-labeled derivative as compared to current approaches.