Efficient Elimination of B-Lineage Lymphomas by Anti-CD20–Auristatin Conjugates

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Abstract
The anti-CD20 antibody rituximab is useful in the treatment of certain B-cell malignancies, most notably non-Hodgkin's lymphoma. Its efficacy has been increased when used in combination with chemotherapy, yet anti-CD20 monoclonal antibodies (mAbs) directly conjugated with drugs such as doxorubicin (Dox) have failed to deliver drug or to demonstrate antitumor activity. We have produced anti-CD20 antibody-drug conjugates that possess potent antitumor activity by using the anti-mitotic agent, monomethyl auristatin E (MMAE), linked via the lysosomally cleavable dipeptide, valine-citrulline (vc). Two anti-CD20 conjugates, rituximab-vcMMAE and 1F5-vcMMAE, were selectively cytotoxic against CD20(+) B-lymphoma cell lines, with IC(50) values ranging from 50 ng/mL to 1 microg/mL. Unlike rituximab, which showed diffuse surface localization, rituximab-vcMMAE capped and was internalized within 4 hours after binding to CD20(+) B cells. Internalization of rituximab-vcMMAE was followed by rapid G(2)-M phase arrest and onset of apoptosis. Anti-CD20 antibody-drug conjugates prepared with Dox were internalized and localized as with rituximab-vcMMAE, yet these were not effective for drug delivery (IC(50) > 50 microg/mL). Consistent with in vitro activity, rituximab-vcMMAE showed antitumor efficacy in xenograft models of CD20-positive lymphoma at doses where rituximab or rituximab-Dox conjugates were ineffective. These data indicate that anti-CD20-based antibody-drug conjugates are effective antitumor agents when prepared with a stable, enzyme-cleavable peptide linkage to highly potent cytotoxic agents such as MMAE.