TG20, a Transgenically-Derived Anti-CD20 Monoclonal Antibody, Exhibits Enhanced Cytotoxicity Against Cells with Low Levels of CD20

Abstract 2730

CD20 is a cell-surface glycoprotein that is highly expressed on most B-cells, tightly restricted to the B-cell lineage, and not expressed on either precursor lymphoid cells or the majority of plasma cells. These characteristics make CD20 an appealing target for mAb therapy of B-cell malignancies and B-cell dependent autoimmune conditions, because antibody production is maintained during therapy and B-cell regeneration post-CD20 mAb treatment is facilitated. Among several marketed anti-CD20 mAbs, rituximab is the prototype; a chimeric human–mouse type-1 antibody that has proven efficacy in a wide variety of mature B-cell malignancies, including CD20⁺ B-cell lymphoma, and has also been approved for the treatment of refractory rheumatoid arthritis. However, patients do not always respond to this therapy, and it has been reported that close to 60% of follicular lymphoma patients previously treated with chemotherapy, while initially responsive, become resistant upon repeat treatment with rituximab monotherapy. Furthermore, the high cost of treatment with rituximab and other anti-CD20 mAbs severely curtails their availability to patients in emerging economies, as well as their use post treatment for maintenance therapy.

Transgenic production offers an easily scalable system for the cost-effective manufacturing of large amounts of complex therapeutic proteins. The regulatory approvals of ATryn® (recombinant antithrombin), first by the EMA (August 2006) and by the FDA (February 2009) have provided a strong regulatory validation of this production platform. Produced in the milk of transgenic goats, TG20, a novel chimeric IgG1 strongly binding a specific discontinuous epitope on CD20 (K_D = 1–2 × 10⁻⁸ M), is being developed as a second-generation anti-CD20 mAb with enhanced cytotoxic activity. Using regulatory sequences from the caprine beta-casein gene to target expression of the TG20 heavy- and light-chains to the lactating mammary gland, 7 independent lines of transgenic goats were generated by somatic cell nuclear transfer. Induced lactation studies indicated that these lines expressed TG20 in their milk at levels varying from 2 g/liter up to ∼ 10 g/liter. Following genetic characterization (Southern blotting, FISH analysis) and expression analysis (protein and peptide mapping, glycosylation profiling, amino-acid sequence analysis), one line was selected for expansion and further development.

In-vitro studies comparing TG20 to rituximab showed a significantly increased ability to activate effector cells expressing the FcgRIIIa receptor (CD16a). Not surprisingly, this translates to a significantly increased ability to activate NK cells and to induce target lysis. This increased ADCC has been demonstrated both with CD20+ Raji cells and with primary B-cells isolated from patients with B-CLL (TG20 EC50 = 3.6 ng/ml vs. rituximab EC50 = 59.4 ng/ml). The ability of TG20 to activate the complement system was assessed using human serum as complement and WIL2-S cells as target exhibiting a > 50% increase in CDC activity vs. rituximab (TG20 = 170% CDC activity vs. rituximab = 100%). Finally, studies conducted in Cynomolgus monkeys were performed to assess pharmacokinetics and in-vivo depletion activities of TG20.

In conclusion, TG20 is a highly active anti-CD20 mAb showing interesting characteristics in terms of cytotoxicity that makes it a promising agent for indications in which rituximab is poorly active. Furthermore, the use of the cost-efficient transgenic production platform for the large-scale production of this antibody drug candidate may allow expanded access of anti-CD20 therapy to those who currently cannot afford these expensive treatments, especially in emerging market countries.