Abstract LB-317: BRAF and PIK3CA mutations in addition to KRAS, improve prediction of resistance to cetuximab in a large panel of colon carcinoma xenografts
Authors: Pechanska Paulina , Becker Michael , Pauli Roland , Mantke René , Hertel Kay , Pertschy Jörg , Hellwig Karsten , Ridwelski Karsten , Radke Cornelia , Pross Matthias , Hinzmann Bernd , Adams Hans-Peter , Mayr Tobias , Rosenthal André , Hoffmann Jens , Fichtner Iduna 
Published: July 2012

Accurate prediction of response to therapy is a prerequisite for individualized approaches to colorectal cancer treatment. Testing of various cancer drugs on the individual patient tumor can support selection of effective therapies. To this end, we have established 149 stably passageable NOD/SCID mouse tumor xenografts from patients of all four Dukes stages within the framework of the prospective multicenter clinical study. All patients gave written informed consent prior to surgery.

In a pharmacological study, 75 xenograft tumor models underwent single-agent treatment with oxaliplatin (OX), cetuximab (CE), or bevacizumab (BE). Each of the 75 tumors was transplanted onto 20 mice (5 controls and 5 for each drug). Models with a treated-to-control tumor growth ratio of 20% or lower were defined as responders. This resulted in response rates of 7% for OX, 24% for CE, and 4% for BE.

Mutation analysis was conducted on 71 xenografts by allele-specific RT-PCR for the following hotspots: KRAS: 34G&gt;A, 34G&gt;T, 34G&gt;C, 35G&gt;A, 35G&gt;T, 35G&gt;C, 38 G&gt;A and 436 G&gt;A; BRAF: 1799 T&gt;A, and PIK3CA: 1624G&gt;A, 1633G&gt;A, 3140A&gt;G. KRAS mutations were found in 46%, BRAF in 10%, and PIK3CA in 15% of the xenografts. 59% of the xenografts carry at least one mutation. KRAS mutations were never observed together with BRAF-mutations while it combined with PIK3CA in 8 cases (11%). The corresponding primary tumors showed the same mutation profile.

We examined mutation status with response to therapy to CE and OX. 3/5 OX responders were KRAS wildtype (wt) (Sensitivity (S+) 66%) while 31/66 OX non-responders were KRAS mutated (Specificity (S-) 46%). If also mutations in BRAF and PIK3CA are considered 40/60 OX non-responders were identified (S-66%). Of the 18 CE responders, all are wt for BRAF and PIK3CA. Three show mutations of KRAS, two of them in codon 13, which seems to confirm a recently published clinical observation by De Roock et al. (JAMA 2010, 304:1812) that this mutation may be related to an improved response under CE treatment. Wt status in all three genes was observed in 83% of the responders. KRAS mutations were seen in 57% of the non-responders. If mutations in BRAF and PIK3CA are also considered, the fraction of recognized non-responders increases to 73%.

We also investigated how well response can be predicted from the mutation data. A machine learning approach was used to estimate unbiased prospective prediction rates for response to CE based on the mutation profile. In each of a large number of bootstrap loops, a new predictor was trained on a new randomly selected subset of the tumors and applied to the remaining tumors. KRAS, BRAF and PIK3CAmutations achieve a S+ of 84% (95% CI: 59% and 96%) and a S- of 68% (95% CI: 54% and 80%).

KRAS, BRAF, and PIK3CA mutation status combined has potential clinical utility higher than KRAS alone to individualize CE treatment, also in the adjuvant setting, for patients with Dukes B and Dukes C.

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 LB-317. doi:10.1158/1538-7445.AM2011-LB-317