CADD-57. THE EFFICACY OF THERAPY WITH ABT-414, AN EGFR-TARGETING ADC, IS POTENTIALLY ALTERED BY HETEROZYGOUS DELETION OF THE ENDOCYTIC TRAFFICKING REGULATOR RBSN

Therapy with ABT-414 (depatux-m), a humanized monoclonal antibody drug conjugate (ADC) directed toward EGFRvIII and activated wild-type EGFR linked to MMAF, has demonstrated promising clinical activity in glioblastoma (GBM) patients with amplified EGFR and is currently being evaluated in clinical trials in first-line GBM disease settings. However, not all patients with amplified EGFR respond to ABT-414 and the reason is unclear. Cancer stem-like cells (CSLCs) have been reported to be responsible for drug and radiation resistance, and we have shown previously that internalization and endocytic trafficking of the humanized mAb bevacizumab affected the survival of CSLCs. Thus, we examined the endocytic trafficking of ABT-414 in EGFRvIII+ CSLCs, and the effect of downregulating a regulator of endocytic trafficking, RBSN (rabenosyn-5). In CSLCs treated with 75µg/ml ABT-414, a fraction of ABT-414 was trafficked to a Rab4+ fast recycling compartment (40% at 5 and 15 min), and a fraction was trafficked to the Lamp1+ lysosome (20% at 30 min and 3 hr). Similarly, in a PDX model of GBM with amplified EGFR treated with ABT-414, a fraction of ABT-414 was detected in a Rab4+ fast recycling compartment and a fraction was detected in the Lamp1+ lysosome. Downregulation of RBSN with specific-siRNA resulted in increased targeting of ABT-414 to the Lamp1+ lysosome at 30 min and a trend-toward-an-increase at 180 min. This could potentially suggest an increased release of the drug conjugate MMAF; lysosomal trafficking is necessary for release of MMAF. Expression of RBSN in a GBM tissue microarray showed very weak/absent expression in 9% of GBM biopsies, consistent with the heterozygous deletion of RBSN in 8% of GBM tumors (TCGA database) and the corresponding significant decrease in RBSN mRNA in these tumors. These data suggest that alterations in endocytic trafficking genes, such as RBSN, could affect the efficacy of ABT-414 therapy in patients.