Abstract 3652: Pegylated arginine deiminase (ADI-PEG20) as a potential therapeutic agent for rarer variants of bladder cancer that are deficient for argininosuccinate synthetase

Background: A previous study had identified microsatellite instability in the argininosuccinate synthetase (ASS1) gene locus in urothelial cancers (UCC), although the significance of this finding was unclear. ASS1 plays a critical role in the biosynthesis of arginine from citrulline and a deficiency of ASS1 leads to arginine auxotrophy. This can be further exacerbated by administration of the pegylated form of arginine deiminase (ADI-PEG20), which has been shown to carry low toxicity in phase I/II clinical trials. Thus, reduced levels of ASS1 can serve as an indicator for ADI-PEG20-responsive tumors. We therefore evaluated the expression of ASS1 in bladder cancer variants to explore the possibility of expanding therapeutic avenues for this population. Design: Formalin fixed, paraffin embedded samples of normal urothelium (n=19), conventional UCC (n=148), and other variants including the micropapillary variant of UCC (n=17), small cell carcinoma (n=19), squamous cell carcinoma (n=39) and adenocarcinoma (n=19), were used in the construction of tissue microarrays (TMAs). TMAs were immunostained for ASS1 and scored based on the intensity of stain (0-3+; high staining was considered 2-3+). The results were then correlated with various parameters such as age at diagnosis, sex, race and pathologic staging. Also, these results were correlated with ASS1 expression as determined by western blotting in multiple bladder cancer derived cell-lines. Results: Compared to normal urothelium where 68.4% of the cases showed high expression of ASS, conventional UCCs demonstrated lower levels of staining (41% of the non-metastatic subset; n=61 and 52.8% of the metastatic subset; n=87). Surprisingly, the paired lymph node metastases from the latter group showed high staining in only 31.5% (n=38) of the patients. High levels of ASS1 immunoreactivity were maintained in the micropapillary variant of UCC (76.4% of cases) and pure bladder adenocarcinoma (84.2%). In contrast, both pure small cell and squamous cell carcinoma of the bladder demonstrated marked reduction in ASS1 expression: only 26.3% of small cell carcinoma specimens and 10.2% of the squamous cell carcinoma specimens had high expression of ASS1, suggesting these two subtypes of bladder cancer could potentially respond to ADI-PEG20 therapy. We have also identified 6 bladder cancer cell-lines (including one derived from a squamous cell carcinoma) that are deficient for ASS1, and a primary cell-line which has high levels of ASS1 expression. These can serve as excellent tools for further in vitro studies. Conclusion: ADI-PEG20 represents a potentially exciting new therapeutic agent for metastatic conventional UCC, small cell carcinoma and squamous cell carcinoma. This is especially relevant as limited therapies are currently available for these less common forms of bladder cancer. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 103rd Annual Meeting of the American Association for Cancer Research; 2012 Mar 31-Apr 4; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2012;72(8 Suppl):Abstract nr 3652. doi:1538-7445.AM2012-3652