The Resistance Mechanisms To Cetuximab In Metastatic Colorectal Cancer

Colorectal cancer(CRC)is the world’s third common malignant tumor.Its incidence associated with environmental factors and genetic factors,such as increased genetic instability,methylation phenotype and mutations of KRAS,BRAF and other genes.In recent years,the incidence of CRC showed a rising trend,and most CRC patients have distant metastases before diagnosis.Although surgical resection of metastases can obtain certain prominent curative effect,but due to the number and size of liver metastases,the anatomic location and other reasons,only a small part of colorectal cancer patients with liver metastases could accept the surgical treatment.Therefore,the drugs treatment of metastatic colorectal cancer(mCRC)is particularly important.Although the chemotherapy including irinotecan,oxaliplatin and 5-fluorouracil for CRC can improve the efficacy,but the median survival of patients is still less than 2 years.In the 21 st century,monoclonal antibodies have shown clinical benefit in the treatment of numerous cancers including mCRC,and epidermal growth factor receptor(EGFR)-targeted monoclonal antibodies(cetuximab and panitumumab)are a great breakthrough in the treatment of mCRC.Cetuximab,a human-mouse chimeric IgG1 monoclonal antibody and panitumumab,a total human monoclonal antibody,both have a very high affinity to EGFR.As EGFR ligands including epidermal growth factor(EGF)can bind the extracellular domain of EGFR,induce receptor dimerization and activate downstream signaling pathways that are crucial for cell survival and proliferation,cetuximab could competitively prevent ligands binding to EGFR,thus blocking EGFR signaling.Thus,cetuximab may inhibit tumor cell survival,proliferation,invasion and metastasis.However,the prices of both cetuximab and panitumumab are quite expensive and these therapies have had only a modest impact on m CRC.Compared with the conventional chemotherapy,the efficacy of the combination therapy of EGFR-targeted monoclonal antibody and FOLFOX or FOLFIRI,the main chemotherapies to m CRC,only increased by 8%-16%.Based on these two aspects,enhance or improve the efficacy of anti-EGFR monoclonal antibody has become an urgent problem to be solved.A key reason for the limited success of cetuximab or panitumumab on mCRC is the severe primary(de novo)and secondary(acquired)resistance to EGFR-targeted therapies.Therefore,it warrants further investigations to further explore the resistance mechanisms of monoclonal antibodies and find out solutions to evaluate their effectiveness in mCRC patients.Currently,the researches of primary resistance mechanism for anti-EGFR monoclonal antibodies are mainly concentrated in the genes of EGFR signaling pathway.Through screening the mutations in KRAS,BRAF,NRAS,PIK3 CA and PTEN,many patients with primary resistance could be filtered out.However,for the metastatic colorectal cancer patients whose primary tumors has been carried out after the surgical tumor resection,due to the size and remote localization of the metastatic tumor,obtaining metastatic tissue for further analysis is a tricky task.It is costly,difficult and harmful for patients to get the metastatic tumor tissues.Furthermore,there are great tumor heterogeneity in tumor tissues.Further research to explore new resistance mechanisms of cetuximab is very necessary.In this study,we found out that the expression level of PRSS1 is significantly higher in the cetuximab resistant cell lines than that in the cetuximab sensitive cell lines.PRSS1 encodes pancreatic serine proteinases,also named trypsin-1(cationic trypsinogen),a major pancreatic digestive enzyme that also catalyzes activation of other pancreatic zymogens to active enzymes.It is also expressed in many tumor tissues and may be involved in tumor invasion and metastasis.We showed how the expression level of PRSS1 was highly correlated with the sensitivity of colorectal cancer cells to cetuximab,and the response of patients with colorectal cancer to cetuximab therapy in this work.The expression level of PRSS1 in the cetuximab resistant cell lines was significantly higher than that in the cetuximab sensitive cell lines,and changing the expression level of PRSS1 in cells could affect the resistance of the cells to cetuximab.Through detecting the expression of PRSS1 in tumor specimens of 49 colorectal cancer patients,we found that in the tumor specimens of 19 patients resistant to cetuximab,16 tumor specimens were PRSS1 positive while only 3 tumor specimens were PRSS1 negative,and in the tumor specimens of 30 patients sensitive to cetuximab,only 5 tumor specimens were PRSS1 positive while 25 tumor specimens were PRSS1 negative.These results indicated that the expression level of PRSS1 in the patients resistant to cetuximab treatment was significantly higher than that in the patients sensitive to cetuximab treatment.Further studies showed that PRSS1 could degrade monoclonal antibodies such as cetuximab bevacizumab and trastuzumab,weaken the function and efficacy of these monoclonal antibodies and lead to drug resistance.We also identified and preliminarily validated that in serum/plasma of patients with colorectal cancer,the PRSS1 level was significantly associated with lack of response to cetuximab therapy.Furthermore,through systematically characterized clinical case data from multiple cohorts of patients with colorectal cancer,we were able to subdivide patients with colorectal cancer into two major subgroups that were characterized by PRSS1 level in serum of mCRC patients,which showed significant differences in disease free survival.PRSS1 can recognize specific sites of a monoclonal antibody degrade the monoclonal antibodies.In other words,the high expression level of PRSS1 in cancer patients could decrease the efficacy of monoclonal antibodies by degrading them and eventully lead to the resistance of the monoclonal antibodies.Furthermore,the degradation of PRSS1 to monoclonal antibodies might have a sequence specificity,and mutating some of the amino acids in the potential recognition region based on the structural modeling and computational design could avoid the monoclonal antibody being hydrolyzed by PRSS1.Specially,the combination of PRSS1 inhibitor and monoclonal antibodies may be a viable option in cancer patients,whose PRSS1 level were relatively high.Together our findings illustrated that PRSS1 played an essential role in resulting in patients with mCRC resistant to cetuximab.It could be an useful candidate-secreted biomarker in predicting the response to monoclonal antibodies including cetuximab,bevacizumab and trastuzumab and identifying whether patients with mCRC were resistant to these monoclonal antibodies.This work provides a strong rationale for evaluating the status of PRSS1 as potential prognostic and predictive markers in future studies.All these results might improve the personalized cancer therapy by providing more practical guidance for different treatments.