Poster Presentation 30th Lorne Cancer Conference 2018

Mechanisms of resistance of BRAFV600E colorectal cancer to BRAF inhibitors (#139)

Fiona Chionh 1 , Analia Lesmana 1 , Janson WT Tse 1 , Hoanh Tran 1 , Ian Luk 1 , Amardeep S Dhillon 1 , Andrew M Scott 1 , Jayesh Desai 2 , Niall C Tebbutt 1 , John M Mariadason 1
  1. Olivia Newton-John Cancer Research Institute, Heidelberg, VIC, Australia
  2. Peter MacCallum Cancer Centre, Melbourne, VIC, Australia

BRAF inhibitors improve response rates and survival outcomes in BRAFV600E metastatic malignant melanoma, however their clinical activity in BRAFV600E  metastatic colorectal cancer (CRC) is modest. We investigated potential mechanisms of inherent and rapid acquired resistance of BRAFV600E CRC to BRAF inhibitors.

Screening a panel of 11 BRAFV600E CRC cell lines by MTS assays identified lines which are relatively sensitive and resistant to vemurafenib. PIK3CA mutation/PIK3R1 deletion/PTEN null and microsatellite instability were not associated with vemurafenib response (p = 0.24 and p = 0.13, respectively), while CpG island methylator phenotype (CIMP) low status (2/11 cell lines) was associated with sensitivity (p = 0.03).  Importantly, we observed a negative correlation between basal pEGFR expression and resistance to vemurafenib (r = -0.85, p = 0.0018).

To investigate mechanisms of rapid acquired resistance, vemurafenib timecourse experiments were performed in 3 sensitive and 3 resistant CRC cell lines. Following initial suppression, rapid reactivation of pERK, and subsequently downstream targets (pRSK1, pFRA1), were observed in resistant but not sensitive cell lines.  Notably, pERK reactivation was not associated with increased pEGFR expression in resistant cell lines.

Vemurafenib-resistant CRC cell lines were similarly resistant to another BRAF inhibitor (Dabrafenib), a paradox breaker (PLX8394), a MEK inhibitor (Trametinib), and combination PLX4032 + Trametinib treatment. Resistance to these agents was also associated with pERK feedback.  In comparison, the ERK inhibitor SCH772984 suppressed pERK expression similarly in vemurafenib-sensitive and resistant cell lines, and the difference in growth inhibition between vemurafenib-resistant and sensitive cell lines was less pronounced.

Our findings demonstrate that the differential sensitivity of BRAFV600E  CRC cell lines to BRAF inhibition is underpinned by a differential capacity to reactivate MAPK signalling, which is also evident following MEK inhibition. This difference can be partially overcome by ERK inhibition.