Feasibility of Autologous Tumor Cell-TLR9 Agonist Vaccination for Metastatic Colorectal Cancer

As the third most common cancer in incidence and second in mortality, colorectal cancer (CRC) significantly impacts the lives of many Americans.1 In 2008, it is estimated that 148,810 cases will be diagnosed and 49,960 patients will die from this disease. Approximately 20% of patients present with metastatic disease at diagnosis. The introduction of more effective chemotherapy regimens and biologically targeted agents over the last few years has led to considerable improvement in treatment options for metastatic CRC yet median survival approximates only 2 years. Resection of the primary tumor when clinically indicated followed by combinations of oxaliplatin or irinotecan with intravenous or oral 5-FU, leucovorin, and bevacizumab for first-line therapy of metastatic CRC is standard of care.

In 2004, Goldberg and colleagues established FOLFOX4 as the standard of care chemotherapy regimen in metastatic CRC when they demonstrated its superiority over two older regimens, IFL (bolus 5-FU/leucovorin/irinotecan) and IROX (irinotecan/oxaliplatin), in terms of prolonging median overall survival (OS), progression-free survival (PFS), and increased response.2 FOLFOX4 increased median survival time to 19.5 months compared to 15 months and 17.4 months for IFL and IROX respectively (p= .0001; HR .66, 95% CI 0.54-0.82). Time to progression was also significantly increased to 8.7 months compared to 6.9 and 6.5 months (p=.0014). Furthermore FOLFOX-4 effected a 45% overall response rate compared to 31% (p=.002) and 35% (p=.03) for IFL and IROX respectively. It also induced significantly less associated grade; 3 nausea, vomiting, diarrhea, febrile neutropenia, and dehydration than the other two regimens.

Hoping to improve this regimen further, capecitabine, an oral pro-drug of 5-FU, was introduced. It has significant advantages over infusional 5-FU including ease of administration with its oral formulation, lack of infusion-related toxicities, and decreased duration of hospitalization and clinic time. Multiple trials have pitted capecitabine-based therapies against infusional 5-FU regimens and have shown comparable efficacy.3-8 Overall toxicity profiles are also comparable between the two regimens with the exception of less myelosuppression and more hand-foot syndrome with capecitabine compared to the infusional-5-FU-based regimens. Thus, in clinical practice, CAPOX (capecitabine-oxaliplatin) is largely considered to be a comparable regimen to FOLFOX, with significantly more convenient administration.

The addition of targeted therapies that inhibit vascular endothelial growth factor (VEGF) and endothelial growth factor receptor (EGFR) to the 5-FU/LV regimens have further increased survival. Bevacizumab, a monoclonal antibody against VEGF, was approved for metastatic CRC in 2004 after the pivotal phase III trial, AVF2107g, showed a significant improvement in OS from 15.6 months to 20.3 months (HR for death, 0.66, p <0.001) with the addition of bevacizumab to IFL.9 PFS and response were also significantly increased from 6.2 months to 10.6 months (p<0.001) and 34.8% to 44.8% (p= 0.004) respectively. The first phase III trial to evaluate the combination of bevacizumab with oxaliplatin-based chemotherapy (FOLFOX-4 or CAPOX), NO16966, demonstrated that the addition of bevacizumab improved PFS by 1.4 months (9.4 vs. 8.0 months, HR 0.83, p=.0023) but overall response rates were similar.10 Median OS also increased from 19.9 months in the placebo group to 21.3 months in the bevacizumab arm but was not statistically significant (HR 0.89, p=.077).

Cetuximab, a mouse/human chimeric monoclonal antibody to EGFR, has also shown promise in metastatic CRC.11, 12 The BOND trial, a multicenter randomized phase II trial showed a significant doubling of response rate and a 2.6 month increase in PFS with the combination of irinotecan-cetuximab over cetuximab alone in the second line setting, but no difference in median OS.11 These results led the FDA to approve cetuximab in February 2004 for second-line treatment either as a monotherapy in those who cannot tolerate irinotecan or in combination with irinotecan in those who do not have a response to irinotecan alone. The CRYSTAL trial, a phase III multicenter randomized trial also demonstrated that cetuximab holds promise in the first line setting.13 In this trial (n=1217), the addition of cetuximab to FOLFIRI significantly increased response rate by 6% (46.9% vs. 38.7%, p=0.005) and PFS by 1.9 months (p=0.036). Trials evaluating the first line use of cetuximab with oxaliplatin-based regimens appear promising and are ongoing.14, 15 When using cetuximab, the presence of K-RAS mutations must be considered. Activating mutations in the K-RAS gene are present in 40-45% of colorectal cancer patients.16 The presence of these mutations correlates with a worse outcome and a lack of response to cetuximab in patients with advanced chemotherapy-refractory CRC.17, 18 Even with these new agents and improved combinations, median survival for metastatic CRC patients remains less than 2 years with less than 5%surviving to 5 years.19 Furthermore, one can expect 20% of patients to progress within 4-6 months.10 Better regimens and treatments are greatly needed to impact this pervasive and fatal disease.