Pre-Clinical Activity of Adct-301, a Novel Pyrrolobenzodiazepine (PBD) Dimer-Containing Antibody Drug Conjugate (ADC) Targeting CD25-Expressing Hematological Malignancies
Authors: Flynn Michael J , van Berkel Patrick , Zammarchi Francesca , Levy Jean-Noel , Tiberghien Arnaud , Masterson Luke A , D'Hooge François , Adams Lauren , Williams David G , Howard Philip W , Hartley John A 
Published: October 2019

Abstract
Interleukin-2 receptor-α (IL2R-α, CD25), one of a heterotrimer that makes up the IL2R, plays a key role in signal transduction pathways involved in the pathogenesis of autoimmunity and graft rejection (Burchill et al Immunol Lett 2007). In addition, the preponderance of CD25+ cells in hematological malignancies (Srivastava et al Leuk Lymphoma 1994) and the relationship between increased CD25 expression and poor prognosis (Yoshida et al PLoS One 2013) raise the possibility of using an anti-CD25 antibody to deliver a cytotoxin to these cells in patients. Clinical proof of concept for treatment of CD25-positive malignancies has previously been established using radio-immunoconjugates (Dancey et al Clin Cancer Res 2009) and immunotoxins (Kreitman et al J Clin Oncol 2000) utilising antibodies basiliximab and daclizumab.
ADCT-301 is an ADC composed of a recombinant human IgG1, HuMax®-TAC against human CD25 attached to a PBD warhead. The drug-antibody ratio is 2.3 ± 0.3. ADCT-301 was potently cytotoxic against CD25-expressing anaplastic large cell lymphoma lines SUDHL1 (341,000 CD25 copies/cell, GI50 0.7 ng/ml) and Karpas 299 (112,000 copies/cell, GI50 3.9 ng/ml) and Hodgkin's lymphoma line L540 (91,000 copies/cell, GI50 3.9 ng/ml). In contrast, CD25-negative Burkitt's lymphoma line, Daudi, gave a GI50 >> 1 mg/ml. The released PBD dimer warhead induces highly cytotoxic interstrand cross-links in the DNA minor groove. Unique to this class of ADCs, the single cell gel electrophoresis (comet) assay can therefore be used as a pharmacodynamic endpoint. For ADCT-301, cross-link formation was dose dependent and the peak of cross-linking occurred 16 to 24 hours after a 2 hour exposure of Karpas 299 cells.
In vivo, ADCT-301 demonstrated dose-dependent antitumor activity against SUDHL1 and Karpas 299 xenograft and disseminated models. For example, ADCT-301 at a single dose of 0.2 mg/kg significantly delayed Karpas 299 tumor growth compared to vehicle-treated and isotype control ADC-treated mice, and at 0.4 and 0.6 mg/kg gave 3/10 and 10/10 tumor-free survivors, respectively (Figure A). 10/10 tumor-free survivors were also observed at a single dose of 0.5 mg/kg, whereas Adcetris gave only a modest delay in mean tumor growth at a single dose of 0.5 mg/kg despite this tumor expressing three times the level of Adcetris target CD30 antigen compared to CD25 (Figure B). ADCT-301 was well tolerated with no signs of toxicity at 6 mg/kg, currently the highest dose tested.
Together, these data clearly demonstrate the potent antitumor activity of ADCT-301 against CD25-expressing hematological tumors and warrants the rapid development of this agent into the clinic.
Figure 1A B Figure 1A B.

Disclosures
Flynn: Medimmune: Employment. van Berkel:ADC Therapeutics Sarl: Employment, Equity Ownership, Patents & Royalties. Zammarchi:ADC Therapeutics Sarl: Employment. Levy:Medimmune: Employment. Tiberghien:Medimmune: Employment, Patents & Royalties. Masterson:Medimmune: Employment. D'Hooge:Medimmune: Employment. Adams:Medimmune: Employment. Williams:Medimmune: Employment. Howard:ADC Therapeutics Sarl: Equity Ownership, Patents & Royalties. Hartley:Medimmune: Employment, Equity Ownership, Patents & Royalties; ADC Therapeutics Sarl: Equity Ownership, Patents & Royalties.