Abstract 4420: Factors potentially contributing to sensitivities of CD22-targeting agents in B-cell malignancies

In order to understand factors which may contribute to in vitro sensitivity of B-cell malignancies to CD22-targeting agents, we evaluated the mRNA expression levels of genes involved with diphthamide biosynthesis, multi-drug resistance, and CD22 (including the splice variant 3 with exon12 deletion) in untreated and 72-hour CD22-targeting agent-treated DLBCL and CLL cell lines. Cell line sensitivity (EC50) to CD22-targeting agent treatment was evaluated by 72 hour exposureto moxetumomab pasudotox (MP).Cell lines were characterized as insensitive if the EC509s were greater than100 ng/mL. The sensitivity to CD22 -targeting agent treatment was found to be significantly associated with baseline CD22 mRNA expression level, as determined by regression analysis. The highly resistant DLBCL cell line RCK8 and the resistant CLL cell line Z138 demonstrated low CD22 expression, corresponding to their elevated EC50′s. However, baseline mRNA was not significantly lower in some other less sensitive cell lines, suggesting that total mRNA CD22 levels may not be the only factor contributing to reduced sensitivity to anti-CD22 exposure. Given the critical roles of dipthamides in CD22 targeting, we evaluated baseline DPH4 levels and their methylation status. There was no correlation between CD22-targeting agent sensitivity and baseline levels of DPH4 expression/promoter methylation. We further explored how CD22, DPH4, WDR85 and MRP1 mRNA expression changed following a 72 hour exposure to MP. Following MP exposure, both mRNA and methylation levels of DPH4 were not significantly changed. However, the mRNA levels of both the diphthamide biosynthesis protein WDR85 and multidrug resistance-associated protein 1 (MRP1) were up-regulated in half of the cell lines following MP exposure. As expression of truncated CD22 has been reported in ALL, we also evaluated CD22 variant expression before and following CD22-targeting agent exposure. All CD22 variants were co-expressed in most cell lines, with significantly increased V3 observed following treatment in some cell lines with lower EC50′s. In conclusion, our results show sensitivity to CD22-targeting agent in the majority of the cell lines tested, and suggest that multiple factors may contribute to sensitivity including CD22 expression. Additionally, our results suggest that factors which may affect continued response to CD22 targeting agents may change with treatment, including increases in CD22 variant 3, MRP1 and WDR85. Ongoing research will evaluate whether these changes are relevant to CD22-targeting agent sensitivity in vivo. Citation Format: Xin Yao, Patricia Burke, Joyce O. Obidi, Xiaoru Chen, Haifeng Bao, Yihong Yao, Jiaqi Huang. Factors potentially contributing to sensitivities of CD22-targeting agents in B-cell malignancies. [abstract]. In: Proceedings of the 106th Annual Meeting of the American Association for Cancer Research; 2015 Apr 18-22; Philadelphia, PA. Philadelphia (PA): AACR; Cancer Res 2015;75(15 Suppl):Abstract nr 4420. doi:10.1158/1538-7445.AM2015-4420