Immunotherapy in Colorectal Cancer: Present and Future

Although it affects 2 distinct anatomical sites, colorectal cancer is often regarded as a single entity, and treatment plans for colon cancer and rectal cancer are quite similar.^1,2 Cases of colorectal cancer comprise more than 8% of newly diagnosed cancer cases and nearly 9% of all cancer deaths in the United States, making it the fourth most common type of cancer in the country.³ By the close of 2014, an estimated 136,830 new cases will be diagnosed, and there will be approximately 50,300 deaths associated with this disease.³ Nearly 1 in 20 Americans will be diagnosed with either colon or rectal cancer at some point in their lives.³ The proportion of patients with colo­rectal cancer who survive 5 years is <65% overall (data calculated for the period 2004 to 2010). For those with advanced or metastasized disease (20% of cases grouped by stage), the 5-year survival rate falls to <13%.³

In general, colorectal cancer is more common in older populations. More than 60% of cases are diagnosed in individuals between the ages of 55 years and 84 years, with the most cases diagnosed among those aged 65 to 74 years. The median age at diagnosis is 68 years, and the median age at death is 74 years.³ Black males have the highest number of new cases (62.3 of every 100,000 cases) compared with males and females of all ethnicities (50.6 of every 100,000 cases and 38.2 of every 100,000 cases, respectively).³ Aside from race and sex, other risk factors include history of familial polyposis, hereditary nonpolyposis colon cancer or Lynch syndrome variants I and II, personal history of ulcerative colitis or Crohn’s disease, first-degree family history of colorectal cancer, and personal history of ovarian, endometrial, or breast cancer.^4-7 From a histological perspective, adenocarcinomas are the predominant cellular type seen in colorectal cancers.^4,5

Because the 5-year survival rate for advanced disease is <13%, current research has focused on novel therapies for this patient segment. This review will discuss 3 new immunotherapeutic modalities presented at the 2014 annual meeting of the American Society of Clinical Oncology (ASCO).

Current Immunotherapy for Colorectal Cancer

Current guidelines from the National Comprehensive Cancer Network (NCCN) for advanced colon cancer recommend FOLFOX (leucovorin, 5-fluorouracil, oxali­platin) alone or with either bevacizumab (Avastin) or panitumumab (Vectibix) initially. At the first disease progression, the regimen changes to FOLFIRI (leucovorin, 5-fluorouracil, irinotecan) plus bevacizumab or ziv-aflibercept (Zaltrap), or irinotecan plus cetuximab (Erbitux) or panitumumab.¹ For advanced rectal cancer, NCCN guidelines recommend starting with FOLFIRI plus either bevacizumab or panitumumab, changing to FOLFOX or CAPEOX (capecitabine, oxaliplatin) plus bevacizumab at first progression. Cetuximab or panitumumab with irinotecan may be used as an alternative.² Regimens like those in the NCCN guidelines that include antibody-targeted therapy plus a fluoropyrimidine as standard first-line treatment for advanced colorectal cancer have been widely adopted,⁸ and antibody-targeted therapy may have value beyond first-line treatment. For example, continuing bevacizumab as second-line treatment improved overall survival.⁹ Bevacizumab is well known for its effects in neutralizing vascular endothelial growth factor (VEGF)-A and thus inhibiting angiogenesis. Yet, bevacizumab and other targeted antibodies (ie, panitumumab and cetuximab) used in colorectal cancer have immune-modulating effects, which enhance their antitumor activities.

Through their effects on signaling pathways, targeting antibodies promote dendritic cell maturation and T-cell priming.¹⁰ In mouse models, anti-VEGF increased T-lymphocyte infiltration into tumors, which correlated with antitumor effects.¹¹ In patients with myeloma, dysfunctional dendritic cells regained much of their normal function when patients were treated with anti-VEGF antibodies.¹²

Cetuximab has complex immune-modulating actions. On one hand, it enhances or activates immune responses, such as antibody-dependent cell-mediated cytotoxicity (ADCC), complement fixation, expression of class I and II major histocompatibility markers, and T-cell priming by dendritic cells.^13-18 On the other hand, cetuximab may inhibit immune responses by activating M2 macrophages.¹⁹

The immune-modulating effects of panitumumab are less clear. It belongs to a noncomplement fixing class of immunoglobin (Ig) G2, yet these Ig classes can either guide differentiation of cancer cells or target them for destruction through other pathways. This was observed with early anti–epidermal growth factor receptor (EGFR) antibodies in mouse models.²⁰ Immunohistochemical examination of tumors revealed the extent to which anti-EGFR antibody treatment enhanced host antitumor cell infiltration, and also showed necrotic areas in the tumors consistent with ADCC.²⁰ Indeed, activation of ADCC may be a key feature of anti-EGFR antibodies.²¹

Although not an antibody, ziv-aflibercept may have some immune-modulating activities. It may share the immune-modulating properties of other antiangiogenic therapies.²² More intriguing is the effect it may have on suppressing regulatory T cells through its efficient trapping of VEGF. VEGF is induced by hypoxia inducible factor (HIF)-1 alfa so that when oxygen is low, increased blood vessel formation commences and progresses. HIF-1 alfa is stabilized by a succession of reactive-oxygen intermediates, which are induced by hypoxic conditions.²³ Under similar conditions, HIF-1 coordinates the expression of VEGF, which impacts HIF-1 stabilization in a feed-forward loop. Thus, when VEGF is inhibited, so is HIF-1 alfa to some extent, potentially leading to suppression of regulatory T-cell development.^24,25

Several immune-modulating agents are currently in development for colorectal cancer. Three novel agents in development discussed at the recent ASCO meeting are presented below.

IMMU-130: Antibody-Drug Conjugate of Irinotecan Targeting CEACAM5 on Colorectal Cancer Cells

Antibodies are attractive vehicles for bringing drugs to tumor sites, thereby reducing bystander effects when cytotoxic agents are required. A novel antibody-drug conjugate (ADC), IMMU-130 consists of the humanized monoclonal antibody to the carcinoembryonic antigen cell adhesion molecule 5 (CEACAM5; labetuzu­mab²⁶) coupled to the active metabolite of irinotecan, SN-38.²⁷ On its own, SN-38 is an effective antitumor drug, approximately 3 orders of magnitude more potent than the prodrug irinotecan, but it is poorly soluble in water and pharmaceutical solvents.²⁸ Irinotecan as a prodrug is approved for colorectal cancer; however, its conversion to SN-38 varies from patient to patient. Thus, coupling a cancer-targeted antibody to the active metabolite of irinotecan is an attractive and novel method to treat cancer. Many solid tumors express CEACAM5 on their surfaces, and carcinoembryonic antigens taken as a family of proteins may be the best biomarker for specific targeting of colorectal cancer.^29-31

In a pilot study using cell cultures and xenograft models, IMMU-130 showed significant activity at doses much lower than the irinotecan comparator.³² The fact that tumor specificity of IMMU-130 is due to targeting CEACAM5 was demonstrated when IMMU-130 failed to inhibit the growth of a CEACAM5-negative lymphoma, but the lymphoma regressed when the appropriately targeted antibody conjugated to SN-38 was given to study mice.³²

To test the activity of IMMU-130 in the clinic, 13 patients with metastatic colorectal cancer enrolled for twice-weekly treatment, at doses ranging from 4 mg/kg to 12 mg/kg for 2 weeks. IMMU-130 was well tolerated, with most common adverse events being manageable nausea and vomiting, fatigue, and diarrhea.³³ In this phase 1, dose-escalation study, 2 of 6 patients receiving 6 mg/kg showed improvement; in 1 patient, the tumor was reduced by 28% after 38 doses.³³ The other patient’s tumor was reduced by 63% after 27 doses. Both tumor reductions were seen when the dose was reduced from 6 mg/kg to 3 mg/kg due to toxicity.³³ The authors noted that some participants failed previous irinotecan therapy, and it remains to be seen if the ADC containing the active drug now causes tumor regression in these patients. Plans for a phase 2 trial are under way,³⁴ and the current trial information can be found as NCT01605318.³⁵ Final enrollment is planned for 104 patients.³⁵

Adoptive Cell Transfer: Tumor-Specific Cytolytic T Cells

Reprogramming cells of a patient’s immune system and then infusing them back is a form of adoptive cell therapy (ACT) with proven success. Sipuleucel-T, the first FDA-approved immunotherapy for advanced prostate cancer, consists of activating peripheral blood cells in vitro. The mononuclear cells (a mixture of antigen-presenting cells, such as T cells, B cells, and natural killer cells) are recovered by leukapheresis approximately 72 hours preceding the planned infusion.³⁶ Integrated analysis of 2 randomized phase 3 trials revealed that sipuleucel-T reduced the risk of death by 33%³⁷; in 1 of the trials the risk of death was reduced by 41%.³⁸ Adverse events were mostly limited to grades 1/2, with durations lasting 1 to 2 days. These included fever, chills, headache, asthenia, dyspnea, vomiting, and tremor.³⁷ Based on these results, the FDA approved sipuleucel-T in April 2010. It should be noted that the exact mechanism by which sipuleucel-T reduced risk of death in the trials has not been fully elucidated, and that a contrary age-related effect, in which those aged >65 years seemed to benefit disproportionally more than those aged <65 years, remains unexplained.³⁹ These questions demonstrate the continued need for research in the area of ACT as a type of cancer therapy.

ACT for solid tumors is an active area of research.⁴⁰ It is under investigation for several types of solid tumors such as melanoma, and for malignancies where viral transformation is a pivotal event, such as Epstein-Barr virus–positive nasopharyngeal carcinoma.^41,42 A few different ACT modalities are currently under exploration for treatment in solid tumors. Cytokine-induced killer (CIK) cells are an outgrowth of cytokine-activated killer cells, an approach that was abandoned due to the adverse events observed in the clinic. CIK cells are polyclonal cells expanded in vitro. These have been shown to have antitumor activity in vitro, in mouse models, and in limited clinical trials.⁴³ Expansion of tumor-infiltrating lymphocytes has so far been limited to metastatic melanoma at a single center, with the patients showing durable responses.⁴⁴ Tumor-specific cytolytic T cells (CTLs) used in a phase 1 trial of patients with metastatic melanoma demonstrated success among enrolled patients.⁴⁵

Several lines of evidence support the use of CTLs in colorectal cancer especially. First, it is possible to generate CTLs that recognize human colon cancer stem-like cells and inhibit tumor growth.⁴⁶ Next, CTLs and conventional chemotherapy may be synergized for treating colorectal cancer.⁴⁷ Finally, it may be possible to monitor the success of CTL therapy by measuring the CTL/regulatory T-cell ratio during treatment.^48,49

Generating large numbers of CTLs from patients with colorectal cancer is feasible. In 1 study, tumor cell lines were prepared from tumor biopsies of enrolled patients. Dendritic cells were grown from peripheral blood mononuclear cells. The dendritic cells and the tumor cell lines were used to elicit the tumor-specific CTLs in culture. Cytotoxic activity of the CTLs was monitored by interferon gamma secretion.⁵⁰

To see if tumor-specific CTLs generated by the method described above had clinical activity, 78 patients with colorectal cancer were enrolled in a study. Twenty primary tumor lines were established from these patients. Monocyte-derived dendritic cells were generated from 26 of the 78 patients. These were used to create antitumor CTLs as described.⁵¹ There were 6 matched sets of dendritic cells, CTLs, and tumor lines. Cocultures of matched sets showed interferon gamma secretion to various degrees after repeated stimulation. These results demonstrate that it is possible to establish cultures of tumor-specific CTLs, and that future refinements may make this adoptive cell transfer approach more widely accessible.⁵¹ As for clinical trials, a search at www.clini caltrials.gov revealed 1 phase 1/2 trial for treating metastatic colorectal cancer using anti-VEGF–engineered CTLs. Recruitment is ongoing, and the estimated completion date is October 2018.⁵²

NEO-102: Antimucin Targeting for Colorectal Cancer Specificity

Mucins are glycoproteins of high molecular weight found in epithelial mucosal cells, particularly goblet cells.^53,54 Mucins are highly glycosylated, having long sugar side chains attached to the protein backbone.⁵⁴ Colorectal cancer adenocarcinomas overexpress a mucin called MUC5AC, whereas MUC5AC is absent from normal colorectal tissue.⁵³ One study found that MUC5AC was expressed in particular by neoplasias in the proximal colon of granular type.⁵⁵ Overexpression of mucins such as MUC5AC is postulated to correlate with increased genetic instability⁵⁶ and also a higher KRAS-induced mutation rate.⁵⁵ In addition, MUC5AC expression appears to correlate with decreased p53 expression.⁵⁷ Moreover, a mucinous histology in metastatic colorectal cancer is associated with worse prognosis⁵⁸ and increased liver metastases.⁵⁹ MUC5AC is expressed by other cancer types besides colorectal cancer; thus, an agent targeting this molecule could have potential use to treat pancreatic ductal adenocarcinomas and cholangiocarcinomas.⁶⁰ MUC5AC expressed by colorectal cancer differs structurally in its core carbohydrates and its peripheral carbohydrates⁵³; such differences can be exploited for diagnostic and treatment purposes.⁵³

To target MUC5AC for treating colorectal cancer, investigators developed a novel monoclonal antibody that discriminates between MUC5AC from normal tissue and that from cancerous colorectal tissue. The chimeric antibody NEO-102 (ensituximab), an IgG1 developed using antigens from a tumor vaccine trial,⁶¹ recognizes an epitope called NPC-1C.⁶² Flow cytometry revealed that 52% to 94% of cells from xenografts bound to NEO-102. Immunohistochemistry demonstrated that while 43% of colorectal cancers and 48% of pancreatic cancers stained with NEO-102, little to no staining was observed in normal tissue.⁶¹ In vitro studies demonstrated that NEO-102 mediated tumor-cell killing; in vivo experiments demonstrated that the antibody induced 2-fold to 3-fold regression in colorectal and pancreatic cancer xenograft models, compared with saline and normal human IgG1.⁶¹ About 45% to 55% of examined pancreatic and colorectal cell lines express the NPC-1C epitope.⁶²

Based on these preclinical findings, NEO-102 is currently in a phase 1b/2a trial to treat patients with pancreatic or colorectal cancer.⁶³ Early results show that at 3 mg/kg, no patient exhibited dose-limiting toxicities, and safety of the antibody seems to be acceptable.^64,65 After nearly 2 months of treatment, 8 patients showed signs that their disease had stabilized; these were heavily pretreated patients with both colorectal and pancreatic cancers.^64,65 Patients are now being enrolled for an increased dose of 4 mg/kg.^64,65 The trial is registered at www.clinicaltrials.gov as NCT0104000.⁶³

Conclusions

As the research presented at the 2014 ASCO annual meeting reveals, novel immunotherapy for advanced colorectal cancer represents the cutting edge of future therapy for this disease. More data are expected as the trials progress, and updates are eagerly anticipated from upcoming cancer research meetings.

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