Abstract 1284: Analysis of circulating CD70+ lymphocytes and CD70 transcript expression in a phase 1 clinical trial of the CD70-targeting antibody-drug conjugate SGN-75
Authors: Law Che-Leung , Gordon Kristine A. , Klussman Kerry , Whiting Nancy C. , McEarchern Julie A. 
Published: June 2012

Abstract
Aberrant expression of CD70 has been reported in various cancers, e.g., non-Hodgkin lymphoma (NHL), renal cell carcinoma (RCC), and pancreatic carcinoma. An auristatin-based anti-CD70 drug conjugate, SGN-75 (h1F6-mcMMAF), has demonstrated antitumor activity in preclinical experiments modeling these malignancies and is currently being evaluated in a phase 1 clinical trial in relapsed/refractory NHL or metastatic RCC. Tumor expression aside, CD70 is expressed on a low and consistently detectable percentage of circulating normal T and B cells. Hence, we explored the utility of monitoring CD70+ lymphocytes in patients treated with SGN-75 as a potential pharmacodynamic marker for SGN-75. A flow cytometry assay utilizing an anti-CD70 monoclonal antibody whose binding to CD70+ cells is minimally affected in the presence of SGN-75 was developed. Applying this method, 8 of 8 patients (4 NHL and 4 RCC) with pre- and post-treatment blood samples demonstrated decreases in the percentage of CD70+ T and B cells post treatment with SGN-75. Conversely, there were no changes in the total lymphocytes or the activated CD25+ T lymphocytes, demonstrating target specificity of SGN-75. To complement the flow cytometry assay, a real-time quantitative PCR (qPCR) assay to measure CD70 mRNA in peripheral blood was developed. Amplification of CD70 mRNA was performed by a single-step reverse transcriptase PCR reaction with 97% amplification efficiency. A correlation between CD70 mRNA and protein expression was then established. CD70 mRNA expression in 13 tumor cell lines with CD70 receptor copy numbers ranging from 2,000 to 189,000 per cell showed significant correlation (ρ=0.8517, p=0.0002) between CD70 mRNA and cell surface protein expression. Similarly, correlation (ρ=0.8125, p=0.0004) was seen between CD70 mRNA expression levels and the percentage of CD70+ lymphocytes in healthy blood donor samples ranging from 1 to 10% CD70+ cells. Analysis of peripheral blood spiked with increasing numbers of the CD70+ 786-O or Caki-1 RCC cells showed corresponding increases in qPCR signals for CD70 mRNA. A reduction of CD70 mRNA was detected in 5 (2 NHL and 3 RCC) of 7 (2 NHL and 5 RCC) patients post SGN-75 treatment. Of the 5 patients showing reduction of CD70 mRNA 3 (2 NHL, 1 RCC) also had available blood samples for flow cytometric analysis; all 3 demonstrated a corresponding decline in the frequency of CD70+ circulating lymphocytes post SGN-75 treatment. Our data suggest that changes in the frequency of circulating CD70+ cells assayed by flow cytometry and CD70 mRNA level assay by qPCR can be potentially used as pharmacodynamic biomarkers for SGN-75 in clinical trials. Ongoing and future studies include correlating these biomarker changes with the pharamcokinetics of SGN-75 and clinical antitumor activity to assist in selecting the optimal dose and schedule for future development.
Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 102nd Annual Meeting of the American Association for Cancer Research; 2011 Apr 2-6; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2011;71(8 Suppl):Abstract nr 1284. doi:10.1158/1538-7445.AM2011-1284