Colorectal clinical trials: what is on the horizon?

Abstract

Substantial progress has been made in the treatment of colorectal cancer, where more effective therapies have led to improved outcomes in patients with advanced disease. However, the 5-year overall survival rate remains poor. Genomic sequencing has allowed us to understand that colorectal cancer is a heterogeneous disease, where tumor-specific variants affect the prognosis and outcomes in patients. This has shaped the future directions of treatment and the development of clinical trials, including the incorporation of novel targeted therapies and investigations into the role of immunotherapy in colorectal cancer.

Colorectal cancer (CRC) is the fourth most common cancer among men and women (when skin cancer is included) and is the second leading cause of cancer-related deaths in the USA [1–3]. Proactive measures, including screening colonoscopies and fecal occult blood testing, have allowed the early identification of premalignant polyps and CRC in its early and curable stages. A diagnosis of stage I–III CRC allows more patients to undergo treatment with a curative intent. However, a great proportion of patients are diagnosed with metastatic disease. Significant progress has been made over the past two decades in the treatment of metastatic CRC with the duration of the median overall survival increasing from approximately 12 months to nearly 30 months [4–6]. This has been in large part due to a greater understanding that CRC is driven by a variety of genetically heterogeneous mutations. This realization is leading to an improvement in patient selection for treatment with selected targeted treatment regimens. However, the 5-year survival rate of metastatic (mCRC) remains <12.5%, highlighting the need for the development of new agents and new approaches to treatment [2]. With an increased understanding, clinical trials are now being developed to assign therapy based upon specific tumor features identified principally by analysis of the individual tumor's genomic changes and targeting these mutations with novel therapeutic agents. Herein, we will discuss future directions in the treatment for advanced or metastatic CRC.

Immunotherapeutic approaches in mCRC

• The role of checkpoint inhibitors in the treatment of colorectal cancer

With the exception of the recent development of specific treatments for certain solid tumor malignancies (renal cell carcinoma, melanoma, prostate cancer and non-small-cell lung cancer), immunotherapy has been extensively tested yet it has remained experimental. Tumors evade a host immune response by dampening the response of tumor-specific T cells and by expressing ligands that bind to inhibitory receptors or ‘checkpoints’ that decrease the innate antitumor immune response [7,8]. Using pharmacologic interventions to disable these checkpoints can enhance pre-existing anticancer immune responses. Immunotherapies targeting negative regulatory molecules on activated T cells, including CTLA-4, PD-1 and its binding ligand PD-L1 have shown promising antitumor activity in several malignancies. While initial studies in CRC with these immunotherapeutic agents have been disappointing [9,10], findings suggest that selected patients with CRC may benefit from immunotherapy. A Phase I study conducted by Brahmer et al. demonstrated a sustained complete response (>21 months) in one patient with mCRC treated with MDX-1106, a PD-1 inhibitor [10]. The patient's tumor was found to lack expression of mismatch repair proteins and thus was classified as MSI-H or having a high degree of microsatellite instability. Previous studies demonstrated that mismatch repair-deficient cancers contain prominent Crohn's disease-like lymphocyte infiltrates suggesting that these tumors elicit an innate immune response [11–14]. In addition, initial studies indicated antitumor activity with immunotherapeutic agents in tumors with high rates of somatic mutations. Because of these findings, a single-arm Phase II study was conducted to investigate the clinical activity of pembrolizumab, a PD-1 inhibitor, in patients with progressive metastatic carcinoma with or without mismatch-repair deficiency [15]. Patients with mismatch repair-deficient CRC demonstrated an objective response rate of 40% and immune-related progression-free survival of 78% at 20 weeks. Interestingly, no immune-related objective response was seen in mismatch repair-proficient CRC patients, confirming that immunotherapy may be beneficial in only certain subsets of CRC unless additional strategies can be developed to render those tumors to be more immunogenic [15]. Based on this promising clinical activity, several ongoing studies are investigating various immunotherapeutic agents in the treatment of CRC, including trials in patients with microsatellite instability high tumors and those with high levels of PD-1 expression (Table 1). While ongoing studies will assess and confirm its clinical utility in a subset of mCRC patients, an understanding in mechanisms of resistance, duration of required therapy and predictive biomarkers of response are needed.

Table 1. . A highlight of ongoing immunotherapy trials for colorectal cancer.

Agent	Class of agent	Trial number	Phase	Comment
MK-3475	Anti-PD-1	NCT01876511	II	MSI-high tumors
MEDI4736	Anti-PD-L1	NCT01693562	I/II
Nivolumab ± ipiliumumab	Anti-PD-1/anti-CTLA-4	NCT02060188	I/II	Recurrent and metastatic CRC
MK-3475 + mFOLFOX6	Anti-PD-1	NCT02375672	II
Tremelimumab + MEDI4736	Anti-CTLA-4 + anti-PD-L1	NCT01975831	I

CRC: Colorectal cancer; MSI: Microsatellite instability.

• Cancer vaccine therapies

Cancer vaccine therapies are an attractive potential therapeutic approach as they have the potential to trigger the immune system to respond to tumor-specific antigens and attack cancer cells. Several types of vaccinations are under investigation against CRC and include DNA, viral, peptide and tumor cell vaccines.

• GVAX

GVAX is an irradiated whole-cell-modified vaccine composed of autologous irradiated colon cancer cell lines engineered to express granulocyte-macrophage colony stimulating factor. Granulocyte-macrophage colony stimulating factor plays a vital role in stimulating the immune system response by inducing dendritic cell differentiation. Several studies investigating the immunologic effects of GVAX have demonstrated its ability to create an inflammatory reaction causing an upregulation of PD-L1. This finding suggests the potential utility of combining this vaccine with immune checkpoint inhibitors [16,17]. GVAX is currently being investigated with the combination of SGI-110, a DNA hypomethylating agent and cyclophosphamide in mCRC (NCT01966289).

• Peptide vaccines

Peptide vaccines employ an eight to 11 amino acid epitope of an antigen that is recognized by effector T cells. This approach is based on the identification and synthesis of epitopes, which can induce tumor antigen-specific immune responses. Since these agents are derived from tumor-specific antigens, they have a decreased risk of inducing autoimmunity. Several peptide vaccines for CRC have reached Phase I trials, demonstrating promising signs of clinical activity [18,19]. With HER2 overexpression present in a proportion of CRC [20,21], HER2 peptide vaccines and their potential roles as a therapeutic agent in CRC are currently being investigated (NCT01376505).

• Oncolytic viral therapy

Given their tumor selectivity and ability to induce cancer cell lysis, oncolytic viral therapy represents an area of interest in cancer treatment. Through alterations induced in their genetic structure, these viruses target and lead to the destruction of cancer cells, and through additional alterations, prevent the binding and replication of the virus in normal, healthy cells. Reovirus is a family of naturally occurring, nonenveloped human virus whose replication is dependent upon the cellular activity of RAS. Specifically, it is cytopathic in transformed cells possessing an activated RAS signaling pathway [22–25]. Given the prevalence of K-RAS and N-RAS mutations in CRC, the use of reovirus has represented a promising and attractive candidate as an oncolytic virus in this disease. It is currently being investigated in combination with FOLFIRI and bevacizumab in K-RAS mutant metastatic colorectal cancer (NCT01274624).

Targeting relevant downstream signaling pathways in mCRC

Targeting signaling pathways remains an attractive therapeutic strategy in CRC. Given the high presence of mutations in the oncogene RAS (KRAS and N-RAS) in CRC and its role on cell survival and proliferation, targeting RAS represents a promising strategy. While its role as a predictive biomarker in anti-EGFR therapy has been established, its relevance as a therapeutic target remains undefined. Targeting RAS mutations directly has remained a challenge. An alternative approach has been to inhibit downstream effector pathways of the MAPK pathway (e.g., BRAF, MEK). The clinical activity is often short-lived, due to compensatory mechanisms that include cross-talk between parallel downstream signaling pathways, downstream activation and negative loop feedback inhibition, and the development of treatment resistance [26,27].

Alternative strategies against RAS-mutant mCRC include combining agents against multiple signals of the MAPK pathway to cause sufficient inhibition of RAS activity, where preliminary findings have demonstrated promising clinical activity [28]. The combination of MEK and EGFR inhibitors have demonstrated the reversal of acquired anti-EGFR resistance when MEK inhibition is added to therapy [29,30], which has prompted the development of clinical trials investigating combination of signaling pathway inhibitors as a primary therapeutic option and as salvage therapies in the refractory disease setting (Table 2). Additionally, targeting multiple signaling pathways may be an effective treatment strategy to overcome resistance of secondary activation of parallel signaling pathways, including studies investigating the concurrent inhibition of the PI3K and MAPK pathway [31].

Table 2. . A highlight of ongoing signaling pathway inhibitor trials for colorectal cancer.

Agent	Class of agent	Trial number	Phase	Comment
MEK162 + panitumumab	MEK tyrosine kinase inhibitor, anti-EGFR mAb	NCT01927341	Ib/II	mCRC with mutant or wild-type RAS tumors
Dabrafenib + trametinib + panitumumab + 5-fluorouracil	BRAF tyrosine kinase inhibitor, MEK tyrosine kinase inhibitor, anti-EGFR mAb	NCT01750918	I/II	BRAF-mutation V600E + and in patients with secondary resistance to anti-EGFR mAb
LGX818 + cetuximab ± BYL719	BRAF tyrosine kinase inhibitor, anti-EGFR mAb, PI3K tyrosine kinase inhibitor	NCT01719380	I/II	BRAF-mutant mCRC
Irinotecan + cetuximab ± vemurafenib	anti-EGFR mAb, BRAF tyrosine kinase inhibitor	NCT02164916	II	BRAF-mutant mCRC
Neratinib + cetuximab	HER2 tyrosine kinase inhibitor, anti-EGFR mAb	NCT01960023	I/II	KRAS, NRAS, BRAF, PIK3CA wild-type

mAb: Monoclonal antibody; mCRC: Metastatic colorectal cancer.

Mutations of the oncogene BRAF are present in approximately 5–10% of mCRC [32,33]. Patients with mCRC whose tumors harbor BRAF V600 mutations generally respond poorly to conventional systemic therapies and are associated with poor outcomes [34–39]. BRAF inhibition with BRAF small molecule inhibitors (vemurafenib or dabrafenib) has led to improve outcomes in progression-free survival and overall survival in patients with BRAF-mutated melanoma [40–43]. However, in contrast to melanoma, mCRC with BRAF V600 mutations have not shown similar efficacy, with a lack of sensitivity to BRAF inhibitor monotherapy [42,44]. One rationale for the lack of clinical activity in BRAF-mutant CRC may be due to insufficient blockade of the MAPK pathway due to a compensatory feedback loop mechanism, leading to reactivation of the pathway (Figure 1) [27,45]. The combination of multiple inhibitors of the MAPK pathway has demonstrated significant improvement in patient outcomes in metastatic BRAF V600-mutated melanoma [46]. Based on these findings, a recent Phase II study by Corcoran et al. investigated the clinical efficacy of combined BRAF and MEK inhibition with dabrafenib and trametinib in patients with BRAF-mutant mCRC [47]. The findings overall were disappointing, where only 12% of patients experienced a partial or complete response and 56% had stable disease. Correlative studies demonstrated MAPK signaling inhibition but to a lesser degree that was observed in BRAF-mutant melanoma treated with dabrafenib [43]. One rationale for the lack of activity may be due to inadequate MAPK signaling inhibition. Preclinical studies have suggested that EGFR may contribute to overcoming BRAF inhibition, leading to reactivation of the MAPK and other key signaling pathways [26]. Ongoing clinical trials are evaluating the combination of EGFR monoclonal antibodies with BRAF inhibitors [48–51] in BRAF-mutant mCRC.

Figure 1. . RAS/RAF/MEK/ERK pathway.

In patients whose tumors do not express mutations in RAS, antiangiogenic and targeted agents against EGFR have demonstrated a clinical benefit in mCRC [52–61]. Ongoing studies in this patient population include strategies targeting both VEGF and EGFR that include combining cetuximab and bevacizumab with chemotherapy (ClinicalTrials.gov NCT00265850) and cabozantinib, a multi-target (VEGFR2, MET) small molecule inhibitor with panitumumab (CaboMab trial, ClinicalTrials.gov NCT02008383).

• Molecular profiling, heterogeneity & personalized therapies with targeted agents against signaling pathways in CRC

Through the efforts by the Cancer Genome Atlas Network, we have a better understanding of the genomic alterations present in CRC which has allowed us to identify potential therapeutic targets in CRC [62]. A total of 224 CRCs underwent comprehensive molecular characterization, where several mutated genes were considered relevant targets for treatment. HER-2, ROS-1, ALK fusion and c-MET overexpression were among the identified mutations in a small proportion of CRC [63,64]. This increased understanding of the genomic alterations in CRC in addition to the availability of next-generation sequencing has allowed development of ‘personalized’ therapies through clinical trials investigating genomic mutations of interest.

Conclusion & future perspective

With the incorporation of combination cytotoxic chemotherapy and targeted therapies into the treatment for mCRC, patient outcomes have been progressively improving over the past two decades. However, the prospect for long-term survival and the prognosis remains poor, with a subset of patients surviving less than 1 year. Advancements in genomic sequencing have led to a new understanding that CRC is a heterogeneous disease, where tumor-specific variants significantly affect the prognosis and outcomes in patients. Incorporation of molecular profiling can direct the development of clinical trials, allowing treatment arms to be tailored to individual tumor-specific genomic alterations.

EXECUTIVE SUMMARY.

• The role for immunotherapy in colorectal cancer (CRC) remains undefined but specific interventions appear to benefit subsets of patients.

• Immunotherapy may be beneficial in selected patients with CRC, notably those with somatic mutations, including microsatellite instability high tumors that are hypermutated and thus present more antigens for potential targets.

• Confirmatory studies are investigating the role of immunotherapy in selected CRC and attempting to identify predictive biomarkers for response.

• Vaccine therapies remain a promising but experimental therapeutic approach in the treatment of CRC.

• Antitumor activity from signaling pathway inhibition is short-lived due to multiple mechanisms for resistance.

• Ongoing strategies are investigating the role of multiple pathway inhibition.

• Next-generation sequencing has demonstrated that a small proportion of CRC have genomic alterations that are of therapeutic interest.