We studied the FCGR2A and the FCGR3A polymorphisms in a large cohort of mCRC patients treated with conventional chemotherapy with and without cetuximab in an effort to explore potential associations between these polymorphisms and cetuximab effect. Our results show that the addition of cetuximab to Nordic FLOX lead to a statistically significant increase in response rate in patients with the FCGR2A R/R genotype. Subgroup analysis of patients with KRAS mutated tumors and the FCGR2A R/R genotype showed an even larger increase in response after the addition of cetuximab.
Previous studies exploring the relation between the FCGR polymorphisms and cetuximab efficacy in mCRC have demonstrated conflicting or negative results and have been mostly low-powered studies with small sample sizes. Our study is one of the largest reported so far and unlike most of the other studies we included a control group where patients did not receive cetuximab.
Even though their results were not statistically significant, the FCGR2A R/R genotype had a better response rate compared to the H/R or the H/H genotypes in KRAS wild-type patients treated with cetuximab or panitumumab as monotherapy or in combination with chemotherapy in a study of 104 refractory mCRC patients . Furthermore, a pooled analysis including 217 mCRC patients treated with cetuximab alone or with chemotherapy showed that patients with the FCGR2A R/R or H/R alleles had a statistically significant longer median PFS than the H/H genotype . Moreover, a study by Negri et al., where most of the 86 mCRC patients enrolled in the study were treated with cetuximab and irinotecan, demonstrated a higher OS in mCRC patients with the FCGR2A R/R polymorphism . However, the authors concluded that the polymorphism was not predictive of cetuximab effect since no relation to response or time to progression (TTP) was demonstrated .
Conversely, a study which included 69 mCRC patients reported the FCGR2A H/H alone or in combination with FCGR3A V/V to be associated with longer PFS in irinotecan-refractory mCRC patients with KRAS wild-type and KRAS mutated tumors treated with cetuximab plus irinotecan . The difference remained significant for KRAS mutated patients. Similar results were demonstrated by Rodriguez et al., who reported that patients with any FCGR2A H and/or FCGR3A V allele were more likely to show a response or have stable disease . Rodriguez et al. explored if the FCGR genotypes would predict which patients with a KRAS, or other downstream mutations, would respond to cetuximab. They included 47 mCRC patients treated with cetuximab and standard chemotherapy with a KRAS, BRAF, NRAS, or PI3K mutation in the FCGR genotype analysis. Two other studies including 52 and 49 mCRC patients, respectively, reported only the FCGR3A V/V genotype to be associated with a better response to cetuximab [25, 26].
In contrast, three other studies including 65, 58, and 122 mCRC patients, respectively, have reported the FCGR3A F/F allele to be associated with a better clinical outcome [27–29]. The former study demonstrated that patients enrolled in the BOND-2 study with the FCGR3A F/F allele had a significantly better response to cetuximab in combination with bevacizumab in irinotecan-refractory mCRC patients . There was shorter survival in patients with the FCGR3A V/V genotype as compared to V/F or F/F in the study of 58 mCRC patients who received irinotecan in combination with cetuximab . This was shown in the whole study population and in a subgroup analysis of patients with KRAS wild-type tumors. Moreover, the latter study by Pander et al., found mCRC patients in the CAIRO2 study with the FCGR3A F/F allele to be associated with longer PFS in KRAS wild-type patients treated with cetuximab as first-line treatment in combination with capecitabine, oxaliplatin and bevacizumab . A smaller study including only 39 mCRC patients reported the FCGR2A, any H allele, and FCGR3A, any F allele, to be associated with longer PFS in mCRC patients who were treated with single-agent cetuximab . These results could though not be replicated when the sample size was increased to a total of 130 patients . In addition to the study by Lurje et al., four other studies with a higher number of patients have reported lack of significant associations of the FCGR2A or FCGR3A polymorphisms and cetuximab efficacy in mCRC [20, 32–34].
Our study show that patients with KRAS mutated tumors and the FCGR2A R/R genotype responded poorly when treated with chemotherapy only and experienced the most benefit of the addition of cetuximab in terms of response rate. In line with this, Correale et al. demonstrated that activating KRAS mutations in colon cancer cell lines may correlate with a higher susceptibility to cetuximab-mediated ADCC . Another study by Schlaeth et al. found that KRAS mutated tumor cells could be effectively killed by ADCC, indicating that mutated KRAS is not enough to confer resistance to antibody-mediated cell killing .
The conflicting findings in the different studies demonstrate the importance of sample size when studying the effect of polymorphisms in relation to clinical outcome. Moreover, the heterogeneity among the different studies, such as study design, ethnicity, previous and concomitant treatment, and the distribution of genotypes may also partly explain the discordance. Furthermore, the retrospective nature of most of the studies and the use of different endpoints may also contribute to the conflicting results. Additionally, Clynes et al. found the IgG1 antibodies trastuzumab and rituximab to engage in both activatory (FCGR3A) and inhibitory receptors (FCGR2B) and the in vivo activity of the antibodies may be more predictable by the ratio of FCGR3A to FCGR2B (A/I ratio)  which has not been investigated in the reported studies. Furthermore, all the studies have only tested two polymorphisms in only two genes involved in the ADCC mechanism. Also, other effector mechanisms of cetuximab may play a more important role, such as complement-dependent cytotoxicity, apoptosis and phagocytosis.
More importantly, ADCC may not play a correspondingly important role in metastatic cancer patients as demonstrated in in vitro models. ADCC has been shown to be markedly impaired with natural killer cell dysfunction in cancer patients with metastatic disease . Moreover, the immune function in cancer patients may be impaired by the myeloablative effects of chemotherapy which may impair ADCC .
Primary tumors in the NORDIC VII study were screened for KRAS exon 2 (codons 12 and 13) mutations. Recent studies have though demonstrated that the selection of patients for anti-EGFR therapy may improve by considering RAS mutations other than KRAS exon 2 mutations (NRAS exons 2, 3, and 4 and KRAS exons 3 and 4) [16–18]. It is expected to find up to 17% mutations in the KRAS exon 2 wild-type population in the NORDIC VII cohort. We do not expect that the contribution of the additional mutations will considerably alter the outcome of the FCGR polymorphisms. Lack of this data is however a limitation of the present study.