Introduction

With an estimated 1.3 million new cancer cases and 700,000 deaths worldwide, colorectal cancer (CRC) is a major cause of cancer-related morbidity and mortality globally.¹ The incidence of CRC is higher in developed countries compared with developing countries, with the overall age-adjusted rate of 36.3 per 100,000 population.¹ Despite the decreasing incidence of CRC, a total of 132,700 new cancer cases and 49,700 deaths are estimated to occur in the United States in 2015, making it the second most common cause of cancer-related mortality.² About 20% of all CRC cases are metastatic at diagnosis, and approximately 30% of all stage II and III CRC cases will develop recurrent metastatic disease after initial treatment.^3,4 Medical oncologists frequently encounter patients with metastatic CRC, and understanding the nuances of treating this disease is vital for improved outcomes in these patients. This review summarizes the existing evidence underlying the paradigm for treatment of metastatic CRC in the first-line setting.

An armamentarium of therapies has accumulated over the past few decades in metastatic CRC, leading to an incremental increase in the survival of patients with this disease. However, the composite array of cytotoxic and targeted therapies has made treatment of metastatic CRC relatively complicated. Currently approved agents for treatment of metastatic CRC include cytotoxics: 5-fluorouracil (5-FU), capecitabine, oxaliplatin, and irinotecan; and targeted therapies: bevacizumab, cetuximab, panitumumab, regorafenib, ziv-aflibercept, and ramucirumab (Figure and Table).^5-15 Apart from regorafenib, which is approved for the refractory setting, and aflibercept and ramucirumab, which are approved for oxaliplatin-resistant patients, all other drugs can be used in the first-line setting.^13-15 Consequently, selecting the appropriate first-line therapy in patients with metastatic CRC is a complex and critical decision that medical oncologists need to make on a routine basis.

For the purpose of this review we have divided this decision into 2 main modules: the choice of chemotherapy backbone and the choice of biologic agent. The last section deals with management of metastatic CRC in elderly patients, due to the singular treatment issues in this population. The scope of this review is restricted to systemic management of metastatic CRC and does not include surgical or other locoregional management strategies. With appropriate patient selection and multidisciplinary care involving liver resection and systemic perioperative chemotherapy, a small subset of patients with limited metastatic liver disease can achieve long-term disease control and even cure.¹⁶ The 5-year survival in these select patients approaches 50%.¹⁶

Choice of Chemotherapy Backbone

The fluoropyrimidine, 5-FU, has been shown to have significant activity in metastatic CRC, with response rates ranging from 15% to 30% and overall survival (OS) ranging from 57 weeks to 62 weeks.¹⁷  Modulation of 5-FU by addition of leucovorin (LV) and by administering continuous infusion has been shown to increase tumor response rates (21% for 5-FU-LV vs 11% for 5-FU alone and 22% for infusional 5-FU vs 14% for bolus 5-FU, respectively) compared with bolus 5-FU alone.^5,18  Additionally, infusional  regimens  are associated with longer  median progression-free survival (PFS; 27.6 vs 22 weeks) and lower grade 3-4 toxicities (11.1% vs 23.9%), namely granulocytopenia, diarrhea, and mucositis.¹⁷

Systemic 5-FU-based doublet chemotherapy in combination with either irinotecan (FOLFIRI) or oxaliplatin (FOLFOX) has been the standard of care for metastatic CRC for many years, and has been shown in multiple trials to improve response rate (RR), PFS, and OS compared with 5-FU alone.^7,9 Randomized comparisons of FOLFOX and FOLFIRI regimens have shown similar efficacy with different toxicity profiles, namely neutropenia and neuropathy with oxaliplatin, and mucositis and diarrhea with irinotecan.^19,20 The choice of chemotherapy backbone between FOLFOX and FOLFIRI is therefore dictated by toxicity; grade 3-4 mucositis, nausea/vomiting, and diarrhea are more frequent with FOLFIRI, and grade 3-4 neutropenia and neurotoxicity are more frequent with FOLFOX.¹⁹ Furthermore, in the era when resection of liver metastases has been adopted in routine management of metastatic CRC, the differential hepatotoxic effects of these agents should also be deliberated if a patient is considered to be a surgical candidate.^21,22 In a retrospective analysis, oxaliplatin was associated with sinusoidal dilation, and irinotecan was associated with steatohepatitis.²² Patients with steatohepatitis had increased 90-day mortality.²² The choice and duration of chemotherapy in patients who may be surgical candidates should be discussed with surgeons as a part of a multidisciplinary approach.

Multiple trials have  also established the equivalence of capecitabine and 5-FU in combination with oxaliplatin.²³ Despite a meta-analysis that showed similar efficacy and tolerability of capecitabine and 5-FU in combination with irinotecan, toxicity of some concern has been reported in some trials with the use of capecitabine and irinotecan, specifically vomiting, diarrhea, and dehydration, compared with FOLFIRI.^24,25

Summary: The cytotoxic chemotherapy line systemic therapy for unresectable metastatic CRC should include an infusional regimen of 5-FU or either oxaliplatin or irinotecan.

Two of 3 studies have shown a benefit to using a 3-cytotoxic-drug upfront strategy compared with doublet regimen (FOLFIRI). In a phase III study comparing fluorouracil, leucovorin, oxaliplatin, and irinotecan (FOLFOXIRI) with FOLFIRI in highly selected patients (age <75 years and ECOG PS 0 or 1) with metastaticCRC, FOLFOXIRI improved RR, PFS, and OS, with increased toxicity in patients with metastatic CRC.²⁶

A similar study with less stringent inclusion criteria performed by the Hellenic Oncology Research Group failed to demonstrate superiority of FOLFOXIRI combination compared with FOLFIRI.²⁷ The third study compared bevacizumab plus FOLFOXIRI with bevacizumab plus FOLFIRI and showed improved PFS and RR and a statistically insignificant improvement in OS (31.0 months vs 25.8 months), again at the cost of increased toxicity.²⁸ The lack of molecular stratification and inability to adjust for second-line therapies limits the clinical applicability of this study. Therefore, although the use of 3 cytotoxics is a viable option in select patients with metastatic CRC with good performance status (PS), this benefit should be carefully weighed against the increased risk of neuropathy, diarrhea, and neutropenia.

Summary: A 3-drug cytotoxic regimen (FOLFOXIRI) is a reasonable option in younger patients with good PS, but is associated with significant toxicity.

Choice of Biologic Agent

Antiangiogenic therapy with bevacizumab has shown benefit in metastatic CRC in combination with chemotherapy.²⁹ Bevacizumab combined with irinotecan, bolus 5-FU, and leucovorin (IFL) was shown to improve survival (median OS, 20.3 months vs 15.6 months) compared with chemotherapy alone.¹⁰ Similarly, anti-EGFR therapy with cetuximab and panitumumab in combination with chemotherapy has been shown to improve PFS, RR, and OS in metastatic CRC with RAS wild-type tumors.^11,30 Initial analyses showed no benefit of anti-EGFR therapy in patients harboring KRAS mutations in exon 2.^11,12 More recent prospective-retrospective analyses have shown that other activating RAS mutations (KRAS exon 3 or 4 and NRAS exon 2, 3, or 4) are also negative predictive biomarkers for anti-EGFR therapy,³⁰ and therefore use of anti-EGFR therapy should be restricted to patients with RAS wild-type tumors. Furthermore, although BRAF^V600E mutation is a negative prognostic factor in metastatic CRC, current data are insufficient to restrict use of anti-EGFR agents in this subset of CRC.³⁰ Participation of these patients in clinical trials should be encouraged due to the poor prognosis associated with this molecular subtype.

Due to their similar efficacy, cetuximab and panitumumab can be used interchangeably with either FOLFOX or FOLFIRI regimens. Multiple trials have been performed to address the issue of the optimal biologic therapy in first-line management of metastatic CRC. The largest study, the US Intergroup 80405 trial,³¹ in which patients with KRAS exon 2 wild-type tumors were randomized to receive either first-line cetuximab or bevacizumab with either FOLFOX or FOLFIRI, showed similar OS for all regimens. However, 2 other trials comparing efficacy of anti-VEGF therapy with anti-EGFR therapy have shown contrasting results.^32,33 The FIRE-3 trial³² compared first-line FOLFIRI combined with either bevacizumab or cetuximab, and the PEAK trial³³ compared FOLFOX combined with either panitumumab or bevacizumab. Both studies showed an OS benefit without a PFS benefit with anti-EGFR therapy. However, lack of details regarding therapy administered beyond the first-line treatment limits the ideal interpretation of these studies. Although the general consensus is that the various permutations and combinations of biologic therapies and chemotherapy are equivalent in metastatic CRC, the randomized EPOC study³⁴ in metastatic CRC with resectable colorectal liver metastasis showed a shorter PFS with addition of cetuximab to pre-operative chemotherapy. The biological basis for such an interaction is unclear, and confirmatory trials are needed to address this specific question.³⁵

Efforts at intensifying therapy using dual antibody therapy (bevacizumab and cetuximab or panitumumab) with chemotherapy in untreated metastatic CRC were investigated in the Panitumumab Advanced Colorectal Cancer Evaluation (PACCE) and CAIRO 2 trials.^36,37 Both studies showed increased toxicity and a significantly shorter PFS with the dual-antibody strategy, and therefore this approach should not be used in management of metastatic CRC.

Summary: Targeted therapy with either bevacizumab or anti-EGFR agents (for patients with RAS wild-type tumors only) should be used as a part of first-line therapy in combination with cytotoxic therapy.

Treatment of Elderly Patients

A substantial proportion of patients with metastatic CRC are age 75 years or older.³⁸ The palliative intent of chemotherapy in this setting coupled with limited life expectancy, medical comorbidities, and inadequate participation in clinical trials makes management of these patients a challenging endeavor. Although studies have shown that compared with younger patients, elderly patients with metastatic CRC derive similar benefits from first-line chemotherapy with oxaliplatin-fluoropyrimidine combinations without much increase in toxicity, elderly patients are less likely to receive chemotherapy, and more specifically, combination chemotherapy.^39,40 Tools beyond PS that assess a more complete health status, such as the Comprehensive Geriatric Assessment (CGA), can be used to predict treatment-related toxicity in elderly patients.⁴¹

A pooled analysis of oxaliplatin-fluorouracil chemotherapy in elderly patients from 4 clinical trials showed that the relative benefit of FOLFOX did not differ by age.⁴² The MRC FOCUS 2 trial,⁴³ a randomized trial of capecitabine or 5-FU with or without dose-reduced oxaliplatin in patients considered unfit for full-dose chemotherapy, showed a trend toward longer PFS and OS with FOLFOX. The subgroup analysis of patients older than 65 years in a study comparing FOLFOXIRI with FOLFIRI as first-line treatment revealed no substantial benefit of the triple combination in terms of OS, and resulted in higher incidence of grade 3/4 adverse events compared with younger patients.⁴⁴ Dosage reductions and treatment delays were also more frequent in the FOLFOXIRI arm. Use of bevacizumab in elderly patients is supported by the AVEX trial,⁴⁵ a randomized study of capecitabine with or without bevacizumab, which showed improved PFS and a trend toward improved OS, but with an increased risk of thromboembolic events. Limited data exist regarding use of anti-EGFR therapy in elderly patients with metastatic CRC, but indicate similar efficacy compared with younger patients.

Although age alone should not be an exclusion criterion for the use of multi-agent chemotherapy, in light of the noninferiority of single-agent therapy in elderly patients and modest benefit with combination regimens, multi-agent treatment should be used with careful consideration for quality of life, toxicity profile, and patient preferences.

Summary: Combination chemotherapy should be used only in fit elderly patients with good PS. In frail elderly patients, sequential single-agent therapy should be used with caution.

Affiliations: Kanwal Raghav, MD, and Cathy Eng, MD, are from the Department of Gastrointestinal Medical Oncology, The University of Texas MD Anderson Cancer Center, Houston.
Disclosures: Drs Raghav and Eng report no relevant financial conflicts of interest to disclose.
Address correspondence to: Kanwal Raghav, MD, Assistant Professor, Department of Gastrointestinal Medical Oncology, The University of Texas MD Anderson Cancer Center, 1515 Holcombe Blvd, Unit 426, Houston, TX 77030. Phone: 713-792-2828; email: [email protected].

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