Abstract 2688: SGN-CD228A: A novel humanized anti-CD228 antibody-drug conjugate for the treatment of solid tumors

Abstract Melanotransferrin (CD228/MFI2/MELTF) is a cell-surfaced glycosylphosphatidylinoitol (GPI)-anchored glycoprotein that belongs to the transferrin family of iron-binding proteins. CD228 was first described as an oncofetal protein highly expressed on malignant melanoma cells. Data from The Cancer Genome Atlas (TCGA) suggests that CD228 has broad expression across many types of carcinomas and it was recently described as a potential biomarker of invasive colorectal carcinoma. In this study, we characterize protein expression of CD228 using an immunohistochemical (IHC) assay and describe pre-clinical antitumor activity of SGN-CD228A, a potent CD228-targeting antibody-drug conjugate (ADC). We found that in addition to melanoma, CD228 is highly expressed in mesothelioma, non-small cell lung (NSCL), breast, colorectal, and pancreatic carcinomas. Monoclonal antibodies (mAbs) specific for human CD228 were evaluated and a lead antibody was selected based on binding characteristics, internalization properties, and cytotoxic activity as an ADC. SGN-CD228A is a humanized anti-CD228 mAb to which eight molecules of MMAE, a potent microtubule disrupting cytotoxic drug, have been conjugated via a β-glucuronidase-cleavable linker, which incorporates a PEG side chain and self-stabilizing maleimide to achieve homogenous conjugation with decreased plasma clearance and increased preclinical antitumor activity. Interestingly, when evaluating drug linkers, we found that changing the linker from a di-peptide to β-glucuronidase resulted in a striking improvement in the cytotoxicity of MMAE, likely due to unique trafficking and recycling of CD228. We examined 50 carcinoma cell lines and found 41 had >10,000 CD228 receptors per cell of which 60% had EC50 values <10ng/ml, and 30% had EC50 values between 10-100ng/ml when treated with SGN-CD228A in vitro. We also evaluated antitumor activity of SGN-CD228A in melanoma and NSCLC xenograft and PDX models. In the Colo-853 and Sk-Mel-5 melanoma models, a single dose of 0.33 mg/kg caused tumor delay whereas 1.0 mg/kg resulted in 5/8 and 4/8 durable complete responses (CRs), respectively. Similarly, in the squamous NSCLC model, Calu-1, a single dose of 1.0 mg/kg produced 6/6 durable responses (DRs). Similar results were obtained in NSCLC PDX models with 3 out of 4 models achieving tumor delay or DRs when dosed with 1.0 mg/kg and durable (CRs) at 3.0 mg/kg. Additionally, in a mouse TNBC PDX clinical trial (n=22), we found that SGN-CD228A achieved durable CRs even in low expressing PDX models. In summary, CD228 is a highly expressed carcinoma target and the novel glucuronide-MMAE ADC, SGN-CD228A, shows potent antitumor activity in vitro and in vivo. Citation Format: Sharsti L. Sandall, Marsha Mason, Devra Olson, Rebecca Mazahreh, Disha Sahetya, Lori Westendorf, Chris Leiske, Brian Schimpf, Liem Nguyen, Madhu Katepalli, Esther Trueblood, Christopher Hale, Albina Nesterova, Jason Wall, Timothy S. Lewis. SGN-CD228A: A novel humanized anti-CD228 antibody-drug conjugate for the treatment of solid tumors [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2019; 2019 Mar 29-Apr 3; Atlanta, GA. Philadelphia (PA): AACR; Cancer Res 2019;79(13 Suppl):Abstract nr 2688.