Specific Tumor Targeting and Potent Bystander Killing with SGN-35, an Anti-CD30 Antibody Drug Conjugate.

CD30 is expressed on the surface of Reed Sternberg cells in Hodgkin’s Lymphoma (HL) and on anaplastic large cell lymphoma (ALCL) cells. The anti-mitotic agent monomethyl auristatin E (MMAE) has been conjugated to cAC10, the anti-CD30 monoclonal antibody SGN-30, to yield the antibody-drug conjugate SGN-35. SGN-35 is internalized into lysosomes of CD30-positive cells and MMAE is released intracellularly when the protease-sensitive linker is cleaved by lysosomal proteases such as cathepsin B. In mouse xenograft models of HL and ALCL, complete remissions have been observed at well tolerated doses of SGN-35. To characterize further the mode of action in vivo, mice with established CD30-positive xenografts were treated with SGN-35, and the concentration of released MMAE was measured in the tumor and serum. The tumor concentrations of released MMAE achieved were several hundred-fold above the IC₅₀ for MMAE in tumor cell cultures. Peak tumor to serum ratios for released MMAE reached more than 1000:1. In tumors and serum, the peak released MMAE concentrations were achieved 3 days and 1 hour post dose, respectively. As a considerable portion of the tumor volume in HL consists of a CD30-negative cellular infiltrate, the ability of SGN-35 to kill CD30-negative cells in mixed cell populations was investigated. First, cell growth medium from the SGN-35-treated HL cell line L540cy (CD30 positive) was shown to be cytotoxic to the SGN-35-insensitive Ramos cells (CD30 negative). Measurements with mass spectrometry confirmed that MMAE was released into the growth medium of SGN-35-treated L540cy cells, and that the observed concentration of MMAE was consistent with the cytotoxic effect on the Ramos cells. Second, a mixed culture of Karpas 299 cells (ALCL, CD30 positive) and Ramos cells was treated with SGN-35 and examined by FACS. The viability of both cell populations was reduced or eliminated, in contrast to proliferation observed in single Ramos cultures treated with SGN-35 or untreated mixed cultures. These results demonstrate that SGN-35 effectively targets the potent cytotoxic agent MMAE to CD30-positive tumor cells, while free MMAE released by these tumor cells is also available for killing CD30-negative infiltrating bystander cells. SGN-35 is a promising treatment for tumors with homogeneous (ALCL) and heterogeneous (HL) expression of CD30. Clinical trials with SGN-35 are expected to begin by the end of 2006.