Molecularly targeted
radionuclide therapies (TRTs) struggle
with
balancing efficacy and safety, as current strategies to increase tumor
absorption often alter drug pharmacokinetics to prolong circulation
and normal tissue irradiation. Here we report the first covalent protein
TRT, which, through reacting with the target irreversibly, increases
radioactive dose to the tumor without altering the drug’s pharmacokinetic
profile or normal tissue biodistribution. Through genetic code expansion,
we engineered a latent bioreactive amino acid into a nanobody, which
binds to its target protein and forms a covalent linkage via the proximity-enabled
reactivity, cross-linking the target irreversiblyin vitro, on cancer cells, and on tumorsin vivo. The radiolabeled
covalent nanobody markedly increases radioisotope levels in tumors
and extends tumor residence time while maintaining rapid systemic
clearance. Furthermore, the covalent nanobody conjugated to the α-emitter
actinium-225 inhibits tumor growth more effectively than the noncovalent
nanobody without causing tissue toxicity. Shifting the protein-based
TRT from noncovalent to covalent mode, this chemical strategy improves
tumor responses to TRTs and can be readily scaled to diverse protein
radiopharmaceuticals engaging broad tumor targets.