*2.2.1 Skin toxicity*

Dermatological toxicities such as papulopustular rash (acneiform eruption), erythema, and skin fissures are common side effects of treatment with anti-EGFR antibodies, as EGFR is involved in the normal development and physiology of the epidermis [54]. Both undifferentiated and proliferating keratinocytes in the basal and suprabasal layers of the epidermis express EGFR, and keratinocytes depend on EGFR to regulate proliferation, differentiation, migration, and survival [55]. The emergence of skin toxicity has therefore been investigated as an on-target marker for anti-EGFR therapy efficacy in patients with mCRC.

Subset analyses of outcomes by skin toxicity severity suggest that improvements in outcome are associated with a higher grade of severity for patients treated with either panitumumab or cetuximab. For example, in the CRYSTAL trial of cetuximab as a first-line therapy, PFS was 11.3 months compared with 5.4 months in patients with G3 and G0-1 skin reactions, respectively [56]. Similarly, the randomized phase III EPIC study of cetuximab plus irinotecan *vs* irinotecan after fluoropyrimidine and oxaliplatin failure in patients with EGFR-expressing mCRC observed a median PFS of 15.6 months for patients who developed G3-4 rash compared to 5.8 months for those with no rash [57]. In the PRIME study of panitumumab plus FOLFOX4 (first line) and 20050181 study of panitumumab plus FOLFIRI (second line) the addition of a targeted agent even appeared detrimental for outcomes in patients with G0-1 skin toxicity as compared to the control arms [58, 59]. Better outcomes in patients with higher-grade skin toxicity were further noted for both arms in a randomized trial of cetuximab monotherapy and cetuximab plus irinotecan in irinotecan-refractory metastatic colorectal cancer [60]. A meta-analysis by Petrelli *et al* of 14 studies including a total of 3833 patients, found that the occurrence of skin toxicity was a predictive factor for survival (HR 0.51; 95% CI 0.40–0.64) and progression (HR 0.58; 95% CI 0.49–0.68). However, 12 of the studies included patients with either *KRAS* wild-type or mutated tumors, and data on skin toxicity by *KRAS* mutation status remains limited.

Analysis of skin toxicity in the randomized phase III ASPECCT study of panitumumab *vs* cetuximab in chemorefractory wild-type *KRAS* exon 2 mCRC observed improved outcomes in patients with higher grade of severity for both antibodies, although patients with higher-grade skin toxicity had longer median duration of treatment [61]. Two retrospective trial analyses (PRIME and AIO CRC-0104 [cetuximab with CAPOX or CAPIRI, first-line]) suggest that the relationship between skin toxicity and outcome may not only apply to patients with wild-type *RAS* tumors, but perhaps also to patients mutant *RAS* tumors [62, 63]. A recent meta-analysis of skin toxicity identified seven and five studies that reported information on PFS and OS stratified by *KRAS* mutation status, respectively [64]. Improved clinical outcome in the presence of higher grade severity was observed for both patients with wildtype *KRAS* tumors and those with mutant *KRAS* tumors (PFS for wild-type *KRAS*, HR = 0.60, 95% CI (0.51, 0.70); mutant *KRAS*, HR = 0.60, 95% CI (0.45, 0.80), OS for wild-type *KRAS*, HR = 0.54, 95% CI (0.46, 0.65), mutant *KRAS*, HR = 0.64, 95% CI (0.50, 0.81), P < 0.001]. However, only mCRC patients with wild-type *KRAS* tumors who suffered grade 2+ skin toxicity derived absolute benefit from anti-EGFR treatment additional to best BSC or chemotherapy (PFS HR = 0.58, 95% CI (0.41, 0.82), OS HR = 0.73, 95% CI (0.61, 0.88)).

These data raise the question whether wild-type *RAS* patients receiving anti-EGFR therapy who do not develop skin toxicity should receive a dose escalation to induce skin toxicity or whether treatment should be discontinued. Further prospective data are needed to establish the clinical value of skin toxicity as a predictive biomarker.

**115**

*2.2.4 BRAF mutation*

*Predictive Biomarkers for Monoclonal Antibody Therapies Targeting EGFR (Cetuximab…*

*2.2.3 Amphiregulin (AREG) and epiregulin (EREG) expression*

these ligands is related to efficacy of anti-EGFR therapy [71, 73–79].

of these ligands in prospective controlled trials appears warranted.

*EGFR* is localized on chromosome 7p11.2 which exhibits DNA copy number gain in approximately 35% of colorectal cancers [65]. Based on this observation, *EGFR* gene copy number has been investigated as a predictive biomarker for anti-EGFR therapy in multiple *post hoc* analyses. Study results have been aggregated in three meta-analyses [66-68], which broadly concurred in identifying gain of *EGFR* gene copy number as associated with improved outcomes among patients receiving cetuximab or panitumumab treatment. This association was found to be retained in subgroup analyses for patients with *KRAS* wild-type tumors, with one metaanalysis suggesting that this difference was not present in patients with *KRAS* mutated tumors [69]. However, the methodologies and criteria used for scoring increased EGFR gene copy number were highly inconsistent across different studies, and more research is required to clarify the predictive potential of this biomarker.

The EGFR ligands AREG and EREG are overexpressed in colorectal cancer at both the mRNA and protein levels [70, 71], and suppression of *AREG* or *EREG* gene expression reduces the therapeutic efficacy of cetuximab in tumor cell lines [72]. Accordingly, multiple studies have found evidence that the extent of expression of

For example, in the randomized phase III PICCOLO study of panitumumab and irinotecan *vs* irinotecan alone in fluorouracil-resistant mCRC, high messenger RNA (mRNA) expression of *EREG* or *AREG* (defined as either *EREG* or *AREG* in the top tertile for mRNA level) was a predictive marker for anti-EGFR therapy benefit in patients with wild-type *RAS* tumors. In contrast, patients with mutant *RAS* tumors gained no panitumumab therapy benefit regardless of ligand status [80]. Similarly, in the CO.17 study of cetuximab in chemotherapy-refractory mCRC, wild-type *KRAS* patients with high *EREG* gene expression obtained benefit from cetuximab therapy, while no benefit was observed in patients with low *EREG* expression; patients with mutant *KRAS* tumors showed no improvement on anti-EGFR

therapy irrespective of *EREG* expression levels [76]. A retrospective analysis of the single-arm phase II NCT 00508404 study of first-line panitumumab plus FOLFIRI similarly also found a higher overall response rate for patients with wild-type *RAS* tumors and high *vs* low *AREG* expression [81]. A meta-analysis including eight studies that used anti-EGFR therapy alone or in combination with chemotherapy reported that *AREG*/*EREG* mRNA overexpression was associated with longer overall survival in patients with wild-type *RAS* tumors who received cetuximab or panitumumab treatment; *AREG* overexpression was further associated with longer PFS. In contrast, *AREG* and *EREG* was found not to have predictive value in patients with mutant *KRAS* tumors [82]. Given these encouraging data, further examination

The *BRAF* gene encodes a serine-threonine protein kinase that is an integral member of the RAS/MAPK signaling pathway. Approximately 10% of colorectal cancers harbor activating mutations in *BRAF*, with a valine (V) to a glutamic acid (E) substitution at codon 600 (V600E) accounting for more than 95% of alterations [16]. Mutations in *BRAF* are mutually exclusive with *KRAS* mutations in CRC [83]. Patients with mCRC who possess a *BRAF* mutation have significantly poorer prognosis as measured by PFS and OS, and mutational analysis is recommended

*DOI: http://dx.doi.org/10.5772/intechopen.80690*

*2.2.2 EGFR gene copy number*

*Predictive Biomarkers for Monoclonal Antibody Therapies Targeting EGFR (Cetuximab… DOI: http://dx.doi.org/10.5772/intechopen.80690*
