Though clinical results for radioimmunoconjugate therapy of most common epithelial tumors have been disappointing, dramatic responses have been observed repeatedly in the treatment of high- and low-grade malignant lymphomas. This high clinical responsiveness after radioimmunoconjugate therapy sometimes appears to be out of proportion to the calculated radiation dose absorbed by the lymphoma tissue. Here we describe some key aspects of the kinetics, dosimetry, and cellular radiobiology of murine lymphoma cells exposed to <tex-math>${}^{212}{\rm Bi}\text{-radiolabeled}$</tex-math> α-particle-emitting immunoconjugates specific for the differentiation antigen Thy 1.2. Approximately 25 cell-bound α-particle-emitting immunoconjugates per target cell were required to reduce clonogenic survival by 90% (the radiobiological <tex-math>$D_{10}$</tex-math>). Serial kinetic analyses of the antibody and radioisotope components of the immunoconjugates revealed significant levels of dechelation and up to 7.5% cellular internalization of the isotope. Cellular radiation dosimetry performed by Monte Carlo computer simulation of α-particle energy deposition patterns based on the observed radiopharmacokinetics showed that the <tex-math>$D_{10}$</tex-math> resulted from approximately four α-particle traversals through the nucleus, corresponding to an absorbed radiation dose of approximately 0.95 Gy to the cell nucleus. Electron micrographs and DNA gel studies of murine lymphoma cells undergoing radioimmunoconjugate therapy in vivo and in vitro demonstrated bizarre blebbing patterns, condensation of chromosomal material, and internucleosomal DNA fragmentation patterns characteristic of programmed cell death (apoptosis). We conjecture that the efficacy of radioimmunoconjugates against responsive cell types may be the result of passive DNA damage by ionizing radiation and the initiation of apoptosis in response to radioimmunotherapy.

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