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Correlation between in Vivo Toxicity and Preclinical in Vitro Parameters for the Immunotoxin Anti-B4-blocked Ricin¹

Division of Tumor Immunology [M. L. G., L. M. N.], Dana-Farber Cancer Institute and Department of Medicine [M. L. G., L. M. N.], Harvard Medical School, Boston 02115, and ImmunoGen Inc., Cambridge [J. M. L., V. S. G., W. A. B.], Massachusetts 02139

Anti-B4-blocked ricin (Anti-B4-bR) is an immunotoxin comprised of the anti-B4 monoclonal antibody and the protein toxin, "blocked ricin." In blocked ricin, the galactose-binding sites of the ricin B-chain which mediate nonspecific binding to cells are blocked by covalently linked affinity ligands prepared from N-linked oligosaccharides of fetuin. Blocked ricin consists of two species, one with two covalently attached ligands and one with three covalently attached ligands. In a Phase I dose escalation clinical trial, Anti-B4-bR was administered to patients with relapsed and refractory B-cell neoplasms by 7-day continuous infusion. Although several different lots of Anti-B4-bR had similar IC₃₇ values as determined by in vitro cytotoxicity testing on cultured human cell lines, these lots differed in their in vivo toxicity when administered to patients. Thus, IC₃₇ values alone were not sufficient to predict in vivo toxicity. We report that the degree of cell kill at concentrations of drug that saturate the B4 antigen and murine 50% lethal dose values provide additional parameters that may be predictive of in vivo cytotoxicity. Furthermore, we performed detailed cytotoxicity studies of the ricin species containing two and three covalently attached ligands, respectively. In vitro cytotoxicity testing using these samples revealed that Anti-B4-bR made with blocked ricin containing two covalently attached ligands is capable of depleting five logs of target cells in an in vitro cytotoxicity assay, while Anti-B4-bR comprised of blocked ricin with three ligands can deplete only one log of cells. Log cell kill at antigen saturating concentration, murine 50% lethal dose and biochemical analysis of the composition of blocked ricin are therefore important considerations for establishing the potential efficacy and safety of Anti-B4-bR.

¹ Supported by NIH Grant CA34183.

² To whom requests for reprints should be addressed, at Division of Tumor Immunology, Dana-Farber Cancer Institute, 44 Binney Street, Boston, MA 02115.

Received 2/28/92. Accepted 5/13/92.

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