Human leukemia cell binding and killing by anti-CD5 radioimmunotoxins

A comparison of 131I-labeled monoclonal antibody 17-1A treatment to external beam irradiation on the growth of LS174T human colon carcinoma xenografts

Inhibition of growth of LS174T human colon cancer xenografts in athymic nude mice due to 131I-labeled MoAb 17-1A treatment was compared to inhibition due to different single doses of 60Co external radiation. From those data, conditions which produced equivalent radiobiological end points could be identified and compared to dose estimates calculated using a technique analogous to the Medical Internal Radiation Dose (MIRD) Committee formalism. The tumor growth rate in mice injected with a single intraperitoneal administration of 300 microCi of 131I-labeled MoAb was reduced relative to tumor growth in untreated control animals and in mice administered unlabeled MoAb and was found to be similar to the growth rate of tumors given a single 6 Gy dose of 60Co radiation. Furthermore, the growth rate of tumors in mice that received three injections of 300 microCi of 131I-labeled MoAb on days 9, 16 and 28 after tumor cell injection was similar to the growth rate of tumors given a single 60Co dose of 8 or 10 Gy. The biodistribution data for 125I-labeled 17-1A MoAb were used to calculate total doses for the tumor and various normal tissues in animals given a single administration of 131I-labeled 17-1A MoAb. The absorbed radiation dose in tumor was approximately five times higher than in normal tissues. The results of the present study indicate that the tumor growth inhibition produced by the administration of radiolabeled antibody can equal that produced by up to 10 Gy of external beam radiation. In addition, the MIRD calculations allow comparison of this form of low dose radiation to external photon irradiation.

The effects of aromatic and aliphatic maleimide crosslinkers on anti-CD5 ricin immunotoxins

The aromatic maleimide crosslinkers m-maleimidobenzoyl-N-hydroxysuccinimide ester (MBS), sulfosuccinimidyl 4-(p-maleimidophenyl) butyrate (S-SMPB) and m-maleimidobenzoylsulfosuccinimide ester (S-MBS) and the aliphatic crosslinker N-gamma-maleimidobutyryloxysuccinimide ester (GMBS) were used to make anti-CD5 intact ricin immunotoxins (IT). IT made with the various crosslinkers were compared under standard conjugation conditions for differences in yield, toxicity of the toxin moiety, binding of the antibody moiety, IT activity, and IT specificity. Our findings showed that IT yield was dramatically improved using crosslinkers with an aromatic, rather than an aliphatic configuration. Gel analysis showed that all IT were of similar, but not identical composition. Conjugation resulted in several IT species including antibody linked to one or two molecules of ricin. For MBS IT and S-SMPB IT, differences in amounts of IT in final fractions and IT in fractions after removal of IT species containing galactose binding sites showed that differences in yield may be attributable to the formation of IT species with obstructed galactose binding sites. All IT bound selectively by FACS analysis and blocking studies. The aliphatic GMBS crosslinker yielded the most toxic IT in cell-free translation assays as well as in shorter-term protein synthesis inhibition and mitogen assays. However, evaluation in the longer-term, more sensitive clonogenic assay showed that at 1000 ng/ml there were no differences in potency between any of the IT. We conclude that the yield of intact ricin IT can be improved using aromatic maleimide crosslinkers without sacrificing IT potency.