The Efficacy of Immune Checkpoint Inhibitors in Microsatellite Stable Colorectal Cancer: A Systematic Review

The use of immune checkpoint inhibitors (ICIs) has revolutionized cancer care, particularly in immune-inflamed tumors and tumors with a high mutational burden, like microsatellite instable colorectal cancer (CRC). However, their effectiveness in microsatellite stable (MSS) CRC is limited. This systematic review aims to evaluate the efficacy of ICIs in MSS CRC and explore promising combination strategies. A comprehensive search from the Web of Science, Medline, and Embase databases, for studies published until 14 November 2022, identified 53 clinical trials included in the review. ICI monotherapy or ICI-ICI combinations demonstrated limited clinical activity for patients with MSS CRC, with overall response rates below (ORR) 10% in most studies. The ICI and tyrosine kinase inhibitor (TKI) garnered ORRs ranging from 10% to 40% and indicated a higher benefit for patients, particularly those without active liver metastases. The combination of ICIs with anti-VEGF agents showed modest ORRs, especially in the earlier treatment lines and in combination with chemotherapy. While these combinations could lead to modest improvements, well-defined biomarkers for long-term benefit are yet to be delineated. Combinations involving BRAF inhibitors with ICIs were studied, showing promising responses with combination approaches in molecularly defined subgroups. In conclusion, while ICI monotherapy has limited efficacy in MSS CRC, combination strategies hold promise to enhance survival outcomes. Further research is necessary to identify optimal combination approaches, predictive biomarkers for treatment response, as well as enrollment according to tumor molecular characteristics.

Safety and antitumor activity of dostarlimab in patients with advanced or recurrent DNA mismatch repair deficient/microsatellite instability-high (dMMR/MSI-H) or proficient/stable (MMRp/MSS) endometrial cancer: interim results from GARNET—a phase I, single-arm study

Background

Dostarlimab is a humanized monoclonal antibody that binds with high affinity to PD-1, resulting in inhibition of binding to PD-L1 and PD-L2. We report interim data from patients with endometrial cancer (EC) participating in a phase I trial of single-agent dostarlimab.

Methods

GARNET, an ongoing, single-arm, open-label, phase I trial of intravenous dostarlimab in advanced solid tumors, is being undertaken at 123 sites. Two cohorts of patients with EC were recruited: those with dMMR/MSI-H disease (cohort A1) and those with proficient/stable (MMRp/MSS) disease (cohort A2). Patients received dostarlimab 500 mg every 3 weeks for 4 cycles, then dostarlimab 1000 mg every 6 weeks until disease progression. The primary endpoints were objective response rate (ORR) and duration of response (DOR) per RECIST V.1.1, as assessed by blinded independent central review.

Results

Screening began on April 10, 2017, and 129 and 161 patients with advanced EC were enrolled in cohorts A1 and A2, respectively. The median follow-up duration was 16.3 months (IQR 9.5–22.1) for cohort A1 and 11.5 months (IQR 11.0–25.1) for cohort A2. In cohort A1, ORR was 43.5% (95% CI 34.0% to 53.4%) with 11 complete responses and 36 partial responses. In cohort A2, ORR was 14.1% (95% CI 9.1% to 20.6%) with three complete responses and 19 partial responses. Median DOR was not reached in either cohort. In the combined cohorts, the majority of treatment-related adverse events (TRAEs) were grade 1–2 (75.5%), most commonly fatigue (17.6%), diarrhea (13.8%), and nausea (13.8%). Grade≥3 TRAEs occurred in 16.6% of patients, and 5.5% discontinued dostarlimab because of TRAEs. No deaths were attributable to dostarlimab.

Conclusion

Dostarlimab demonstrated durable antitumor activity in both dMMR/MSI-H (ORR 43.5%) and MMRp/MSS EC (ORR 14.1%) with a manageable safety profile.

Trial registration number

NCT02715284.

Nomogram to predict the outcomes of patients with microsatellite instability-high metastatic colorectal cancer receiving immune checkpoint inhibitors

BACKGROUND

The efficacy of immune checkpoint inhibitors (ICIs) in patients with microsatellite instability (MSI)-high metastatic colorectal cancer (mCRC) is unprecedented. A relevant proportion of subjects achieving durable disease control may be considered potentially 'cured', as opposed to patients experiencing primary ICI refractoriness or short-term clinical benefit. We developed and externally validated a nomogram to estimate the progression-free survival (PFS) and the time-independent event-free probability (EFP) in patients with MSI-high mCRC receiving ICIs.

METHODS

The PFS and EFP were estimated using a cure model fitted on a developing set of 163 patients and validated on a set of 146 patients with MSI-high mCRC receiving anti-programmed death (ligand)1 (PD-(L)1) ± anticytotoxic T-lymphocyte antigen 4 (CTLA-4) agents. A total of 23 putative prognostic factors were chosen and then selected using a random survival forest (RSF). The model performance in estimating PFS probability was evaluated by assessing calibration (internally-developing set and externally-validating set) and quantifying the discriminative ability (Harrell C index).

RESULTS

RFS selected five variables: ICI type (anti-PD-(L)1 monotherapy vs anti-CTLA-4 combo), ECOG PS (0 vs >0), neutrophil-to-lymphocyte ratio (≤3 vs >3), platelet count, and prior treatment lines. As both in the developing and validation series most PFS events occurred within 12 months, this was chosen as cut-point for PFS prediction. The combination of the selected variables allowed estimation of the 12-month PFS (focused on patients with low chance of being cured) and the EFP (focused on patients likely to be event-free at a certain point of their follow-up). ICI type was significantly associated with disease control, as patients receiving the anti-CTLA-4-combination experienced the best outcomes. The calibration of PFS predictions was good both in the developing and validating sets. The median value of the EFP (46%) allowed segregation of two prognostic groups in both the developing (PFS HR=3.73, 95% CI 2.25 to 6.18; p<0.0001) and validating (PFS HR=1.86, 95% CI 1.07 to 3.23; p=0.0269) sets.

CONCLUSIONS

A nomogram based on five easily assessable variables including ICI treatment was built to estimate the outcomes of patients with MSI-high mCRC, with the potential to assist clinicians in their clinical practice. The web-based system 'MSI mCRC Cure' was released.

Immune Checkpoint Inhibition in Colorectal Cancer: Microsatellite Instability and Beyond

Immune checkpoints inhibitors (ICIs) have been a breakthrough, with unique response and survival patterns compared with chemotherapy for patients with advanced Mismatch Repair-deficient/Microsatellite instable (dMMR/MSI) colorectal cancer, but have shown disappointing results in Mismatch Repair-proficient/Microsatellite stable (pMMR/MSS) colorectal cancer. As up to 50% of patients harboring dMMR/MSI advanced cancers will ultimately progress after PD-1 blockade, biomarkers are needed to predict response/resistance to immunotherapy and to select patients for immunomodulating combination therapies. Patients with pMMR/MSS colorectal cancer present with distinct immune profiles compared to dMMR/MSI tumors, giving evidence of different immune escape mechanisms, which could be overcome through individualized immunotherapeutic strategies. In this review we discuss the latest developments in the field of immunotherapy for dMMR/MSI and pMMR/MSS colorectal cancers, and unresolved questions and considerations concerning the use of ICI therapies in this population. Future immunomodulation strategies based on biomarker selection (tumor mutational burden, Immunoscore^®, mutational profile) are discussed.

RECIST and iRECIST criteria for the evaluation of nivolumab plus ipilimumab in patients with microsatellite instability-high/mismatch repair-deficient metastatic colorectal cancer: the GERCOR NIPICOL phase II study

BACKGROUND

Immune checkpoint inhibitors (ICIs) are highly effective in patients with microsatellite instability/mismatch repair-deficient (MSI/dMMR) metastatic colorectal cancer (mCRC). Response Evaluation Criteria in Solid Tumors (RECIST) 1.1 criteria may underestimate response to ICIs due to the pseudoprogression phenomenon. The GERCOR NIPICOL phase II study aimed to evaluate the frequency of pseudoprogressions in patients with MSI/dMMR mCRC treated with nivolumab and ipilimumab.

METHODS

Patients with MSI/dMMR mCRC previously treated with fluoropyrimidines, oxaliplatin, and irinotecan with/without targeted therapies received nivolumab 3 mg/kg plus ipilimumab 1 mg/kg every 3 weeks for four cycles then nivolumab 3 mg/kg every 2 weeks until progression or a maximum of 20 cycles. Computed tomography scan tumor assessments were done every 6 weeks for 24 weeks and then every 12 weeks. The primary endpoint was disease control rate at 12 weeks according to RECIST 1.1 and iRECIST by central review.

RESULTS

Of 57 patients included between December 2017 and November 2018, 48.0% received ≥3 prior lines of chemotherapy, 18.0% had BRAFV600E mutation, and 56.0% had Lynch syndrome-related cancer. Seven patients (12.0%) discontinued treatment due to adverse events; one died due to a treatment-related adverse event. The disease control rate (DCR) at 12 weeks was 86.0% with RECIST 1.1% and 87.7% with iRECIST. Two pseudoprogressions (3.5%) were observed, at week 6 and at week 36, representing 18% of patients with disease progression per RECIST 1.1 criteria. With a median follow-up of 18.4 months, median progression-free survival (PFS) and overall survival (OS) were not reached. The 12-month PFS rate was 72.9% with RECIST 1.1% and 76.5% with iRECIST. The 12-month OS rate was 84%. Overall response rate was 59.7% with both criteria. RAS/BRAF status, sidedness, Lynch syndrome, and other baseline parameters were not associated with PFS.

CONCLUSION

Pseudoprogression is rare in patients with MSI/dMMR mCRC treated with nivolumab and ipilimumab. This combined ICI therapy confirms impressive DCR and survival outcomes in these patients.

TRIAL REGISTRATION NUMBER

NCT03350126.

Microsatellite Instability-high Intrahepatic Cholangiocarcinoma with Portal Vein Tumor Thrombosis Successfully Treated with Pembrolizumab

A 60-year-old man presented with postoperative recurrence of intrahepatic cholangiocarcinoma with right portal vein tumor thrombosis (PVTT). After failure of standard chemotherapy, a liver biopsy showed that his microsatellite instability (MSI) status was high. Treatment with the immune checkpoint inhibitor (ICI) pembrolizumab was commenced, which resulted in a partial response and resolution of the PVTT. There were no significant immune-related adverse events. According to recently published reports, the frequency of MSI-high biliary tract cancer (BTC) is about 0-2.1%, which is extremely rare. However, ICIs may be effective in patients with MSI-high BTC, such as the present patient.

Immune checkpoint inhibitors for the treatment of MSI-H/MMR-D colorectal cancer and a perspective on resistance mechanisms

Metastatic colorectal cancer (CRC) with a mismatch repair-deficiency (MMR-D)/microsatellite instability-high (MSI-H) phenotype carries unique characteristics such as increased tumour mutational burden and tumour-infiltrating lymphocytes. Studies have shown a sustained clinical response to immune checkpoint inhibitors with dramatic clinical improvement in patients with MSI-H/MMR-D CRC. However, the observed response rates range between 30% and 50% suggesting the existence of intrinsic resistance mechanisms. Moreover, disease progression after an initial positive response to immune checkpoint inhibitor treatment points to acquired resistance mechanisms. In this review article, we discuss the clinical trials that established the efficacy of immune checkpoint inhibitors in patients with MSI-H/MMR-D CRC, consider biomarkers of the immune response and elaborate on potential mechanisms related to intrinsic and acquired resistance. We also provide a perspective on possible future therapeutic approaches that might improve clinical outcomes, particularly in patients with actionable resistance mechanisms.

Proteomic Analysis of miR-195 and miR-497 Replacement Reveals Potential Candidates that Increase Sensitivity to Oxaliplatin in MSI/P53wt Colorectal Cancer Cells

Most patients with advanced colorectal cancer (CRC) eventually develop resistance to systemic combination therapy. miR-195-5p and miR-497-5p are downregulated in CRC tissues and associated with drug resistance. Sensitization to 5-FU, oxaliplatin, and irinotecan by transfection with miR-195-5p and miR-497-5p mimics was studied using cell viability and clonogenic assays in cell lines HCT116, RKO, DLD-1, and SW480. In addition, proteomic analysis of transfected cells was implemented to identify potential targets. Significantly altered proteins were subjected to STRING (protein-protein interaction networks) database analysis to study the potential mechanisms of drug resistance. Cell viability analysis of transfected cells revealed increased sensitivity to oxaliplatin in microsatellite instable (MSI)/P53 wild-type HCT116 and RKO cells. HCT116 transfected cells formed significantly fewer colonies when treated with oxaliplatin. In sensitized cells, proteomic analysis showed 158 and 202 proteins with significantly altered expression after transfection with miR-195-5p and miR-497-5p mimics respectively, of which CHUK and LUZP1 proved to be coinciding downregulated proteins. Resistance mechanisms of these proteins may be associated with nuclear factor kappa-B signaling and G1 cell-cycle arrest. In conclusion, miR-195-5p and miR-497-5p replacement enhanced sensitivity to oxaliplatin in treatment naïve MSI/P53 wild-type CRC cells. Proteomic analysis revealed potential miRNA targets associated with the cell-cycle which possibly bare a relation with chemotherapy sensitivity.

Prognosis of microsatellite instability and/or mismatch repair deficiency stage III colon cancer patients after disease recurrence following adjuvant treatment: results of an ACCENT pooled analysis of seven studies

BACKGROUND

Microsatellite instable/deficient mismatch repair (MSI/dMMR) metastatic colorectal cancers have been reported to have a poor prognosis. Frequent co-occurrence of MSI/dMMR and BRAFV600E complicates the association.

PATIENTS AND METHODS

Patients with resected stage III colon cancer (CC) from seven adjuvant studies with available data for disease recurrence and MMR and BRAFV600E status were analyzed. The primary end point was survival after recurrence (SAR). Associations of markers with SAR were analyzed using Cox proportional hazards models adjusted for age, gender, performance status, T stage, N stage, primary tumor location, grade, KRAS status, and timing of recurrence.

RESULTS

Among 2630 patients with cancer recurrence (1491 men [56.7%], mean age, 58.5 [19-85] years), multivariable analysis revealed that patients with MSI/dMMR tumors had significantly longer SAR than did patients with microsatellite stable/proficient MMR tumors (MSS/pMMR) (adjusted hazard ratio [aHR], 0.82; 95% CI [confidence interval], 0.69-0.98; P = 0.029). This finding remained when looking at patients treated with standard oxaliplatin-based adjuvant chemotherapy regimens only (aHR, 0.76; 95% CI, 0.58-1.00; P = 0.048). Same trends for SAR were observed when analyzing MSI/dMMR versus MSS/pMMR tumor subgroups lacking BRAFV600E (aHR, 0.84; P = 0.10) or those harboring BRAFV600E (aHR, 0.88; P = 0.43), without reaching statistical significance. Furthermore, SAR was significantly shorter in tumors with BRAFV600E versus those lacking this mutation (aHR, 2.06; 95% CI, 1.73-2.46; P < 0.0001), even in the subgroup of MSI/dMMR tumors (aHR, 2.65; 95% CI, 1.67-4.21; P < 0.0001). Other factors associated with a shorter SAR were as follows: older age, male gender, T4/N2, proximal primary tumor location, poorly differentiated adenocarcinoma, and early recurrence.

CONCLUSIONS

In stage III CC patients recurring after adjuvant chemotherapy, and before the era of immunotherapy, the MSI/dMMR phenotype was associated with a better SAR compared with MSS/pMMR. BRAFV600E mutation was a poor prognostic factor for both MSI/dMMR and MSS/pMMR patients.

TRIAL IDENTIFICATION NUMBERS

NCT00079274, NCT00265811, NCT00004931, NCT00004931, NCT00026273, NCT00096278, NCT00112918.

Novel Targets and Precision Medicine for Prostate Cancer-Part 2: Tumor Profiling and Personalized Therapy in Patients With Castration-Resistant Prostate Cancer

Despite advances in the treatment of castration-resistant prostate cancer (CRPC), options remain limited and non-curative; thus, prostate cancer remains one of the deadliest cancers in men. The discovery of novel therapeutic targets is needed to improve outcomes for men with metastatic CRPC. Precision/personalized medicine creates new opportunities to discover these targets. With an increase in the use of next-generation sequencing and tumor profiling, potentially clinically relevant tumor mutations are being identified. Here, we review the current use of and future direction for genetic testing and tumor profiling in patients with metastatic CRPC.

Current treatment landscape and emerging therapies for pancreatic cancer

Pancreatic cancer remains a disease that is difficult to treat due to a typically late presentation, relatively high resistance to chemotherapy, and lack of effective targeted therapies. The standard of care relies on cytotoxic chemotherapy, primarily FOLFIRINOX and gemcitabine-based regimens. Dose modifications and/or the use of alternative combinations can reduce adverse effects, but these regimens remain highly toxic. As a result, long-term survival is low for patients with advanced or metastatic disease. There is a great need for novel anticancer agents that provide efficacy with minimal toxicity. Currently, inhibitors of immune tolerance and immune checkpoint inhibitors; PARP inhibitors; novel cytotoxic chemotherapies, such as trifluridine/tipiracil; and modifiers of the tumor microenvironment, such as pegylated hyaluronidase, are in clinical trials for the treatment of pancreatic cancer. This activity will review the current treatment landscape and preview emerging therapies for the treatment of advanced pancreatic cancer.

Managing the economic impact of advanced pancreatic cancer

Pancreatic cancer is typically diagnosed in the late stage of the disease, making it the fourth leading cause of cancer-related death in the United States. It is also one of the few cancers with an increasing incidence, particularly in the younger population. By 2030, it is expected to become the second leading cause of cancer-related death. Patients with pancreatic cancer encounter monthly medical costs 15 times higher than those without, with costs highest in the later stages of the disease. Treatments for pancreatic cancer include surgery (available to fewer than 20% of newly diagnosed patients) and, for advanced disease, chemotherapy with gemcitabine with nab-paclitaxel or FOLFIRINOX, which can increase overall survival (OS) by a few months. Economic and outcome analyses of clinical data find no significant difference in OS between the 2 regimens, although FOLFIRINOX carries a much higher rate of serious adverse effects, limiting its use to patients with good performance status. In 2017, the FDA approved immunotherapy for patients with microsatellite instability-high or mismatch repair-deficient solid tumors, which occurs in approximately 1% of pancreatic cancer diagnoses. Several immunotherapies and targeted therapies are currently in clinical trials and may significantly alter the trajectory of the disease. However, they typically cost more than $100,000 per year, putting significant strain on payers. Thus, it is important that payers plan now for the potential arsenal of new treatments and identify opportunities to manage their utilization as well as patients with the disease to contain costs.

Drug resistance and new therapies in colorectal cancer

Colorectal cancer (CRC) is often diagnosed at an advanced stage when tumor cell dissemination has taken place. Chemo- and targeted therapies provide only a limited increase of overall survival for these patients. The major reason for clinical outcome finds its origin in therapy resistance. Escape mechanisms to both chemo- and targeted therapy remain the main culprits. Here, we evaluate major resistant mechanisms and elaborate on potential new therapies. Amongst promising therapies is α-amanitin antibody-drug conjugate targeting hemizygous p53 loss. It becomes clear that a dynamic interaction with the tumor microenvironment exists and that this dictates therapeutic outcome. In addition, CRC displays a limited response to checkpoint inhibitors, as only a minority of patients with microsatellite instable high tumors is susceptible. In this review, we highlight new developments with clinical potentials to augment responses to checkpoint inhibitors.

Linking arsenite- and cadmium-generated oxidative stress to microsatellite instability in vitro and in vivo

Mismatch repair (MMR) corrects replicative errors and minimizes DNA damage that occurs frequently in microsatellites. MMR deficiency is manifested as microsatellite instability (MSI), which contributes to hypermutability and cancer pathogenesis. Genomic instability, including MSI and chromosomal instability, appears to be responsible for the carcinogenesis of arsenic and cadmium, common contaminants in our environment. However, few studies have addressed arsenic- or cadmium-induced MSI, especially its potential link with arsenic- or cadmium-generated oxidative stress, due to the lack of quantifiable MSI assays and cost-effective animal models. Here, using a dual-fluorescent reporter, we demonstrate that sub-lethal doses of cadmium or arsenite, but not arsenate, increased the MSI frequency in human colorectal cancer cells. Arsenite- and cadmium-induced MSI occurred concomitantly with increased levels of reactive species and oxidative DNA damage, and with decreased levels of MMR proteins. However, N-acetyl-l-cysteine (NAC) suppressed arsenite- and cadmium-induced MSI and oxidative stress while restoring the levels of MMR proteins in the cells. Similarly, MSI was induced separately by arsenite and cadmium, and suppressed by NAC, in zebrafish in a fluorescinated PCR-based assay with newly-developed microsatellite markers and inter-segmental comparisons. Of five selected antioxidants examined, differential effects were exerted on the MSI induction and cytotoxicity of both arsenite and cadmium. Compared to MMR-proficient cells, MMR-deficient cells were more resistant to arsenic-mediated and cadmium-mediated cytotoxicity. Our findings demonstrate a novel linkage between arsenite-generated and cadmium-generated oxidative stress and MSI induction. Our findings also caution that antioxidants must be individually validated before being used for preventing arsenite- and cadmium-induced MSI that is associated with cancer development.

Small molecule inhibitors uncover synthetic genetic interactions of human flap endonuclease 1 (FEN1) with DNA damage response genes

Flap endonuclease 1 (FEN1) is a structure selective endonuclease required for proficient DNA replication and the repair of DNA damage. Cellularly active inhibitors of this enzyme have previously been shown to induce a DNA damage response and, ultimately, cell death. High-throughput screens of human cancer cell-lines identify colorectal and gastric cell-lines with microsatellite instability (MSI) as enriched for cellular sensitivity to N-hydroxyurea series inhibitors of FEN1, but not the PARP inhibitor olaparib or other inhibitors of the DNA damage response. This sensitivity is due to a synthetic lethal interaction between FEN1 and MRE11A, which is often mutated in MSI cancers through instabilities at a poly(T) microsatellite repeat. Disruption of ATM is similarly synthetic lethal with FEN1 inhibition, suggesting that disruption of FEN1 function leads to the accumulation of DNA double-strand breaks. These are likely a result of the accumulation of aberrant replication forks, that accumulate as a consequence of a failure in Okazaki fragment maturation, as inhibition of FEN1 is toxic in cells disrupted for the Fanconi anemia pathway and post-replication repair. Furthermore, RAD51 foci accumulate as a consequence of FEN1 inhibition and the toxicity of FEN1 inhibitors increases in cells disrupted for the homologous recombination pathway, suggesting a role for homologous recombination in the resolution of damage induced by FEN1 inhibition. Finally, FEN1 appears to be required for the repair of damage induced by olaparib and cisplatin within the Fanconi anemia pathway, and may play a role in the repair of damage associated with its own disruption.

[Role of immunotherapy in the management of colorectal cancer]

Immunotherapy proved to be effective in various forms of cancer but it is in its infancy in colorectal cancer, although the Immunoscore was developed to classify this tumor immunologically. Various forms of immunotherapy were tested in early clinical trials but anti-PD-1 antibodies seem the most promising so far. These studies also revealed that one particular molecular subgroup of colorectal cancer, the microsatellite instable variant, is extremely sensitive for such modality.

First Tissue-Agnostic Drug Approval Issued

The PD-1 inhibitor pembrolizumab received accelerated approval for adult and pediatric patients with solid tumors that are mismatch repair-deficient or microsatellite instability-high. This is the first time the FDA has greenlighted a drug based not on tumor type, but on a common biomarker.

Cadmium-induced genomic instability in Arabidopsis: Molecular toxicological biomarkers for early diagnosis of cadmium stress

Microsatellite instability (MSI) analysis, random-amplified polymorphic DNA (RAPD), and methylation-sensitive arbitrarily primed PCR (MSAP-PCR) are methods to evaluate the toxicity of environmental pollutants in stress-treated plants and human cancer cells. Here, we evaluate these techniques to screen for genetic and epigenetic alterations of Arabidopsis plantlets exposed to 0-5.0 mg L(-1) cadmium (Cd) for 15 d. There was a substantial increase in RAPD polymorphism of 24.5, and in genomic methylation polymorphism of 30.5-34.5 at CpG and of 14.5-20 at CHG sites under Cd stress of 5.0 mg L(-1) by RAPD and of 0.25-5.0 mg L(-1) by MSAP-PCR, respectively. However, only a tiny increase of 1.5 loci by RAPD occurred under Cd stress of 4.0 mg L(-1), and an additional high dose (8.0 mg L(-1)) resulted in one repeat by MSI analysis. MSAP-PCR detected the most significant epigenetic modifications in plantlets exposed to Cd stress, and the patterns of hypermethylation and polymorphisms were consistent with inverted U-shaped dose responses. The presence of genomic methylation polymorphism in Cd-treated seedlings, prior to the onset of RAPD polymorphism, MSI and obvious growth effects, suggests that these altered DNA methylation loci are the most sensitive biomarkers for early diagnosis and risk assessment of genotoxic effects of Cd pollution in ecotoxicology.

Phase II Study of Olaparib (AZD-2281) After Standard Systemic Therapies for Disseminated Colorectal Cancer

BACKGROUND

Effective new agents for patients with colorectal cancer (CRC) with disease progression during standard therapy regimens are needed. We hypothesized that poly ADP ribose polymerase (PARP) inhibitor therapy in patients with CRC and inefficient tumor DNA repair mechanisms, such as those with high-level microsatellite instability (MSI-H), would result in synthetic lethality.

METHODS

This was an open-label phase II trial testing olaparib 400 mg p.o. b.i.d. for patients with disseminated, measurable CRC failing standard therapies with centrally confirmed tumor MSI status. The primary endpoint was the tumor response, assessed by RECIST, version 1.0. The secondary endpoints were safety/toxicity, progression-free survival (PFS), and overall survival (OS).

RESULTS

Thirty-three patients (20 microsatellite stable [MSS], 13 MSI-H) were enrolled. The median age for all patients was 57 years and for MSS and MSI-H patients was 51 and 61 years, respectively. All patients received at least one 28-day cycle of olaparib. No patient had a complete or partial response. Nausea (48%), fatigue (36%), and vomiting (33%) were the most commonly reported treatment-related adverse events. The median PFS for all patients was 1.84 months. No statistically significant differences were found in the median PFS or OS for the MSS group compared with the MSI-H group.

CONCLUSION

Single-agent olaparib delivered after failure of standard systemic therapy did not demonstrate activity for CRC patients, regardless of microsatellite status. Future trials, testing PARP inhibitors in patients with CRC should focus on the use of DNA-damaging chemotherapy and/or radiation therapy, combined with PARP inhibitors, remembering the toxicity reported in the present study.

IMPLICATIONS FOR PRACTICE

Microsatellite instability (MSI-H) colorectal tumors exhibit hypermethylation in tumor mismatch repair genes, or have mutations in one or more of these genes resulting from a germ-line defect (Lynch syndrome). PARP inhibitors such as olaparib are most effective in tumors associated with inability to repair DNA damage. However, in this trial, single agent olaparib failed to elicit responses in patients with MSI-H colorectal tumors, and in those with microsatellite-stable tumors. It is possible that by adding olaparib to radiation therapy, or to a systemic DNA damaging agent, tumor lethality could be obtained. However, the price would be increased toxicity.

Microsatellite instability detected in tumor-related genes in C57BL/6J mice with thymic lymphoma induced by N-methyl-N-nitrosourea

Microsatellite instability (MSI) has been observed within tumors and found to be closely associated with the degree of malignancy and prognosis in tumors. However, whether MSI in tumor-related genes can be induced by a chemical and whether a connection exists between MSI and tumors remain unclear. In the present study, we detected MSI in the tissues of N-methyl-N-nitrosourea (MNU) treated mice by targeting to 5, 29, 30 microsatellite loci in 3 mismatch repair (MMR) genes, 1 DNA repair gene, and 5 tumor suppressor (TS) genes, respectively. Among 26 mice survived in the MNU-group, 18 (69%) mice presented thymic lymphomas. Moreover, 61% (11/18) of the tumors metastasized to the other organs, including the liver, spleen, and kidney. We examined 104 tissues from MNU-treated mice using the 64 loci, and found 8 MSI events involved 4 loci in 4 tissues types. The MSI incidence in MMR, DNA repair, and TS genes was 67% (2/3), 0% (0/1) and 40% (2/5), respectively. MSI occurrence in tumor and non-tumor tissues was 5.6% (1/18) and 0% (0/8) and that in metastasis and non-metastasis tissues was 7.1% (1/14) and 9.4% (6/64), showing no significant difference. MSI loci in intronic regions of Atm, Msh6 and p21 and MSI in the 3'UTR of Pms2 were detected in MNU-treated mice. Specifically, we found a loss of heterozygosity in intron of Atm (ATM-8) in one metastasis mouse. Four similar events occurred in p21 gene intron (P21-1) of another non-metastasis mouse. Another MSI was a heterozygous mutation existed in an Msh6 allele (MSH6-2) in metastasis mouse. We also found a homozygous 2-bp insertion in the 3'UTR of Pms2 in two non-metastasis mice. These results imply that MNU can induce MSI in MMR and TS genes in C57BL/6J mice. MSI frequency does not seem to be associated with tumorigenesis or metastasis.

Cadmium-induced microsatellite instability in the kidneys and leukocytes of C57BL/6J mice

Cadmium is a cytotoxic, carcinogenic, and mutagenic industrial product or byproduct. The correlation between metal exposure and microsatellite instability (MSI) has been reported by several groups. In the present study, 50 C57BL/6J mice at 6 weeks of age were divided into five groups and intraperitoneally injected with 0, 0.25, 0.5, 1, or 2 mg/kg cadmium chloride quaque die alterna for 4 weeks. Then, the liver, kidney, testis, leukocytes, bone marrow, and small intestine were collected from the treated mice and weighed. Portions of these tissues were fixed for further histological analysis, and the remaining tissues were subjected to genomic DNA extraction for the analysis of a panel of 42 microsatellite markers. The liver and testis weight coefficients were significantly changed in the 1 and 2 mg/kg cadmium chloride-treated groups compared with the control group. Simultaneously, severe histopathologic changes in the liver and kidneys, along with a complete disorganization of testicular structure and obvious severe necrosis in the testes were observed in the cadmium-treated group. The cadmium accumulated in the liver and kidneys of the mice in all cadmium-treated groups; the tissue cadmium concentrations were significantly higher than those in the control group. After STR scanning, MSI was found at three loci (D15Mit5, D10Mit266, and DxMit172) in the kidneys and leukocytes of mice in the lower dose groups (0.25 and 0.5 mg/kg). In summary, we have successfully established a sub-chronic cadmium exposure model and confirmed that cadmium exposure can induce MSI in mice. We also identified two loci that could be regarded as "hotspots" of microsatellite mutation in mice.

Systematic review of the predictive effect of MSI status in colorectal cancer patients undergoing 5FU-based chemotherapy

BACKGROUND

We systematically reviewed the evidence for the interaction of microsatellite instability status (MSI) and treatment with 5FU in colorectal cancer to determine how well MSI status predicts health outcomes in patients undergoing 5FU-based chemotherapy.

METHODS

We conducted a search of four electronic databases through June 2013. We considered studies that included both colorectal cancer patients treated with 5FU-based chemotherapy and untreated patients with survival outcomes presented by MSI status.

RESULTS

We identified 16 studies for qualitative analysis (9,212 patients) with 14 studies eligible for meta-analysis. The microsatellite stable (MSS) group showed an effect of 5FU treatment on disease-free survival (HR of 0.62 [95% CI: 0.54, 0.71]) and overall survival (HR of 0.65 [95% CI: 0.54, 0.79]), indicating that MSS patients who received 5FU treatment had longer survival than MSS patients who were untreated. The effect of 5FU treatment was not statistically significant for microsatellite high (MSI-H) patients for disease-free survival (HR of 0.84 [95% CI: 0.53, 1.32]) or overall survival (HR 0.66 [95% CI: 0.43, 1.03]). However, the summarized point estimates of the effects of 5FU treatment for the MSS and MSI-H groups were not different at a statistically significant level.

CONCLUSIONS

Our analyses indicate that treatment with 5FU-based chemotherapy improves disease-free and overall survival in CRC patients, but that there is no difference in the effect of treatment based on MSI status. Therefore, the use of MSI status to guide treatment decisions about the use of 5FU treatment for CRC has no significant benefits for patients.

4-hydroxy estrogen induces DNA damage on codon 130/131 of PTEN in endometrial carcinoma cells

Catechol estrogens, such as 4-hydroxyestradiol (4-OHE2), are estrogen metabolites that form DNA adducts and may induce mutations and subsequent cell transformation in mammary cells; however, little is known about their roles in endometrial carcinogenesis. Furthermore, it remains unclear whether 4-OHE2 is able to induce DNA damage on specific genes involved in carcinogenesis or a 'pro'-mutation status such as microsatellite instability (MSI). Therefore, we modified terminal transferase-dependent PCR by the application of a capillary sequencer to detect DNA damage at the single base level. Using this method, we demonstrated that 4-OHE2 directly induced DNA damage on codon 130/131 in exon 5 of PTEN, which is a mutation hot spot for PTEN in endometrial carcinoma. Whereas, both estradiol and 4-OHE2 treatment did not affect MSI status in immortalized endometrial glandular cells. 4-OHE2 might contribute to endometrial carcinogenesis by inducing PTEN mutation on codon 130/131.

[Inhibitory effect of Xiaotan Sanjie Recipe on the microsatellite instability of orthotopic transplantation tumor in MKN-45 human gastric cancer nude mice]

OBJECTIVE

To study the inhibitory effect of Xiaotan Sanjie Recipe (XSR) on the microsatellite instability of orthotopic transplantation tumor in MKN-45 human gastric cancer nude mice.

METHODS

The 3rd passage subcutaneous transplantation tumor was taken as the origin of the model by using MKN-45 human gastric cancer cell lines. MKN-45 human gastric cancer nude mouse model was established using OB glue adhesive method. Then 30 nude mice were divided into the model group, the XSR group, and the chemotherapy group. Mice in the XSR group were intragastrically given XSR at the daily dose of 0.4 mL. Mice in the chemotherapy group were intragastrically given Fluorouracil at the daily dose of 0.4 mL. No intervention was given to mice in the model group. After 6 weeks of medication, the tumor weight was measured, and the tumor inhibition rate calculated. The size, the peak height, and the peak area of 5 microsatellite instability sites were detected.

RESULTS

The tumor inhibition rate was 40. 84% in the XSR group. The tumor weight was significantly lower in the XSR group than in the model group (P < 0.01), showing no statistical difference when compared with the chemotherapy group (P >0.05). The incidence of high microsatellite instability (MSI-H) in the model group was 70%, and the incidence of low microsatellite instability (MSI-L) was 30%. Microsatellite stable site tended be stable after 6 weeks of XSR treatment.

CONCLUSION

XSR showed inhibition on microsatellite instable orthotopic transplantation tumor in MKN-45 human gastric cancer nude mice.

A dual-fluorescent reporter facilitates identification of thiol compounds that suppress microsatellite instability induced by oxidative stress

The DNA mismatch-repair (MMR) system corrects replicative errors and minimizes mutations that occur at a high rate in microsatellites. Patients with chronic inflammation or inflammation-associated cancer display microsatellite instability (MSI), indicating a possible MMR inactivation. In fact, H2O2-generated oxidative stress inactivates the MMR function and increases mutation accumulation in a reporter microsatellite. However, it remains unclear whether MSI induced by oxidative stress is preventable because of the lack of a sufficiently sensitive detection assay. Here, we developed and characterized a dual-fluorescent system, utilizing DsRed harboring the (CA)13 microsatellite as a reporter and GFP for normalization, in near-isogenic human colorectal cancer cell lines. Via flow cytometry, this reporter sensitively detected H2O2-generated oxidative microsatellite mutations in a dose-dependent manner. The reporter further revealed that glutathione or N-acetylcysteine was better than aspirin and ascorbic acid for suppressing oxidative microsatellite mutations. These two thiol compounds also partially suppressed oxidative frameshift mutations in the coding microsatellites of the hMSH6 and CHK1 genes based on a fluoresceinated PCR-based assay. MSI suppression by N-acetylcysteine appears to be mediated through reduction of oxidative frameshift mutations in the coding microsatellite of hMSH6 and protection of hMSH6 and other MMR protein levels from being decreased by H2O2. Our findings suggest a linkage between oxidative damage, MMR deficiency, and MSI. The two thiol compounds are potentially valuable for preventing inflammation-associated MSI. The dual-fluorescent reporter with improved features will facilitate identification of additional compounds that modulate MSI, which is relevant to cancer initiation and progression.

An inducible, isogenic cancer cell line system for targeting the state of mismatch repair deficiency

The DNA mismatch repair system (MMR) maintains genome stability through recognition and repair of single-base mismatches and small insertion-deletion loops. Inactivation of the MMR pathway causes microsatellite instability and the accumulation of genomic mutations that can cause or contribute to cancer. In fact, 10-20% of certain solid and hematologic cancers are MMR-deficient. MMR-deficient cancers do not respond to some standard of care chemotherapeutics because of presumed increased tolerance of DNA damage, highlighting the need for novel therapeutic drugs. Toward this goal, we generated isogenic cancer cell lines for direct comparison of MMR-proficient and MMR-deficient cells. We engineered NCI-H23 lung adenocarcinoma cells to contain a doxycycline-inducible shRNA designed to suppress the expression of the mismatch repair gene MLH1, and compared single cell subclones that were uninduced (MLH1-proficient) versus induced for the MLH1 shRNA (MLH1-deficient). Here we present the characterization of these MMR-inducible cell lines and validate a novel class of rhodium metalloinsertor compounds that differentially inhibit the proliferation of MMR-deficient cancer cells.

Exposure to Cr(VI) induces organ dependent MSI in two loci related with photophosphorylation and with glutamine metabolism

Chromium (Cr), as a mutagenic agent in plants, has received less attention than other metal pollutants. To understand if Cr induces microsatellite instability (MSI), Pisum sativum seedlings were exposed for 28 days to different concentrations of Cr(VI) up to 2000mgL(-1), and the genetic instability of ten microsatellites (SSRs) was analyzed. In plants exposed to Cr(VI) up to 1000mg L(-1), MSI was never observed. However, roots exposed to 2000mgL(-1) displayed MSI in two of the loci analyzed, corresponding to a mutation rate of 8.3%. SSR2 (inserted in the locus for plastid photosystem I 24kDa light harvesting protein) and SSR6 (inserted in the locus for P. sativum glutamine synthetase) from Cr(VI)-treated roots presented alleles with, respectively, less 6bp and more 3bp than the corresponding controls. This report demonstrates that: (a) SSRs technique is sensitive to detect Cr-induced mutagenicity in plants, being Cr-induced-MSI dose and organ dependent (roots are more sensitive); (b) two Cr-sensitive loci are related with thylakoid photophosphorylation and with glutamine synthetase, respectively; (c) despite MSI is induced by Cr(VI), it only occurs in plants exposed to concentrations higher than 1000mgL(-1) (values rarely found in real scenarios). Considering these data, we also discuss the known functional changes induced by Cr(VI) in photosynthesis and in glutamine synthetase activity.

Pb2+ exposure induced microsatellite instability in Pisum sativum in a locus related with glutamine metabolism

Lead (Pb) is a toxic element, but its putative mutagenic effects in plant cells, using molecular markers, remain to unveil. To evaluate if Pb induces mutagenicity, Pisum sativum L. seedlings were exposed to Pb(2+) (up to 2000 mg L(-1)) for 28 days and the instability of microsatellites (or Simple Sequence Repeats, SSR) was analyzed in leaves and roots. The analysis of eight selected microsatellites (SSR1-SSR8) demonstrated that only at the highest dosage microsatellite instability (MSI) occurred, at a frequency of 4.2%. Changes were detected in one microsatellite (SSR6) that is inserted in the locus for glutamine synthetase. SSR6 products of roots exposed to the highest concentration of Pb were 3 bp larger than those of the control. Our data demonstrate that: (a) SSR technique is sensitive to detect Pb-induced mutagenicity in plants. MSI instability is Pb dose dependent and organ dependent (roots are more sensitive); (b) the Pb-sensitive SSR6 is inserted in the glutamine synthetase locus, with still unknown relation with functional changes of this enzyme; (c) Pb levels inducing MSI are much above the maximum admitted levels in some European Union countries for agricultural purpose waters. In conclusion, we propose here the potential use of SSR to evaluate Pb(2+)-induced mutagenicity, in combination with other genetic markers.

Clinical significance of microsatellite instability for stage II or III colorectal cancer following adjuvant therapy with doxifluridine

Microsatellite instability (MSI) is a molecular marker that can provide valuable prognostic information for colorectal cancer (CRC). However, the predictive role of the MSI status remains less clear than its role in prognostication due to mixed results from previous studies. Therefore, this study investigated the usefulness of the MSI status as a predictive factor for stage II or III CRC patients who received adjuvant doxifluridine therapy. Among 3030 patients with CRC who underwent surgical resection between 1997 and 2006, 564 patients were diagnosed with stage II or III, and adjuvant doxifluridine therapy was administered to 394 patients (70.0%). The MSI status was assessed using the markers BAT25 and BAT26, and samples with instability at both markers were scored as exhibiting high-frequency MSI (MSI-H). Among the 564 patients, 290 patients (51.4%) had stage II, and MSI-H was found in 41 patients (7.3%). With a median follow-up duration of 35.1 months (range, 0.5-135.2), the 5-year overall survival (OS) rate and relapse-free survival (RFS) rate were 87.5 and 76.2%, respectively. MSI-H showed a favorable survival trend for OS (P = 0.098) and significant survival benefit for RFS (P = 0.037) in all patients. In a univariate analysis, the doxifluridine-treated patients with MSI-H showed improved RFS compared to those with low or stable MSI (MSI-L/S) (P = 0.036), while the MSI status was not significantly associated with OS (P = 0.107). In a multivariate analysis, MSI-H was not significantly associated with RFS (Hazard ratio = 2.467, P = 0.125). In conclusion, this study confirmed the positive prognostic role of MSI-H. However, MSI-H patients with stage II or III CRC did not seem to benefit from doxifluridine adjuvant therapy.

Microsatellite instability in Lactuca sativa chronically exposed to cadmium

Cadmium (Cd) is a cytotoxic, mutagenic and carcinogenic metal. Mutagenesis is indicative of genetic instability and can be assayed by use of microsatellite markers (simple sequence repeats, SSRs). These are tandem-repeated tracts of DNA composed of units that are 1-6 base pairs (bp) long, spread throughout the genome and highly polymorphic. SSRs can be used in the detection of genomic DNA damage and/or mutational events (e.g. deletions, insertions, point mutations). In order to study chronic exposure to cadmium, Lactuca sativa L. seeds were germinated in distilled water and grown on modified Hoagland's medium, both supplemented with 0, 10 and 100microM Cd(NO(3))(2). After 28 days of exposure, the plants were harvested to assess shoot and root length and accumulation of cadmium. DNA was extracted from young and expanded leaves and roots in order to analyse microsatellite instability (MSI). Mutagenic effects of cadmium were evaluated on nine microsatellite loci. No MSI was found in leaves, but a 2-bp deletion in one lettuce root SSR was detected among the SSRs that were analysed. Thus, SSR analyses may provide a complementary tool in the assessment of different genotoxic effects of compounds on plants.

[Methylnitrosourea as challenge mutagen in assessment of the DNA mismatch repair (MMR) activity: association with some types of cancer]

Total repair capability is a widely used phenotypic marker of predisposition to cancer. Evaluation of this parameter implies using a challenge mutagen in an in vitro system to unmask latent genetic instability and repair insufficiency in the target cells. Traditionally, these investigations involve two tests, evaluation of mutagenic susceptibility (chromosomal aberrations) and genotoxic effect (DNA comet assay). The present study was focused on analysis of the effect of methylnitrosourea (MNU) on resting and PHA-stimulated lymphocytes from healthy donors and patients with gynecological cancer. Cytotoxic effect of MNU (apoptotic lymphocyte death) was estimated using two parameters, interaction of the cells with the annexin V-FITC complex, and morphological changes of the nuclei after their staining with the mixture of two DNA tropic dyes. The genotoxic effect of MNU, namely, secondary double-strand DNA breaks, was scored using the neutral comet assay, modified for the calculation of the comets produced exclusively by BrUdr-labeled proliferating lymphocytes. The proportion of these comets was represented as the proliferative cell index. It was shown that resting lymphocytes were resistant to genotoxic and cytotoxic effects of MNU. The response of proliferating cells to the action of MNU was expressed as the development of secondary DNA breaks (P <0.01), along with the increased frequency of apoptosis (P <0.05). The genotoxic effect of MNU on stimulated lymphocytes of gynecological cancer patients was fourfold lower compared to healthy donor lymphocytes. In response to the MNU action, patient lymphocytes did not change their proliferative index, while in healthy donor lymphocytes proliferative index was two times decreased in response to the MNU action. The data obtained pointed to the association between the cytotoxic response of the lymphocytes to the action of MNU and gynecological cancer. Since only proliferating lymphocytes response to the genotoxic effect of MNU, and the effect is revealed a day after the mutagen action, it is suggested that this phenomenon is associated with postreplicative repair, MMR, the substrate of which is O6-methylguanin. The MMR deficiency in patient lymphocytes determines their tolerance to the action of MNU. Genotoxic effect of lymphocytes to the action of MNU can serve as a marker of MMR, as well as of the MMR deficiency-associated gynecological cancer.

The role of chemotherapy in microsatellite unstable (MSI-H) colorectal cancer

INTRODUCTION

High-frequency microsatellite instability (MSI-H) is an alternate pathway of colorectal carcinogenesis, which accounts for 15% of all sporadic colorectal cancers. These tumours arise from mutations in the DNA mismatch repair system and thus have different responses to chemotherapeutic agents compared to microsatellite stable (MSS) cancers.

OBJECTIVE

This review aims to summarise the available literature on the responses to chemotherapy in MSI-H colorectal cancer (CRC).

RESULTS AND DISCUSSION

5 Fluorouracil (5FU) is commonly used as a chemotherapeutic agent in colon cancer and in vitro evidence shows reduced response to 5FU in MSI-H CRC. The clinical evidence is conflicting but favours a reduced response to 5FU in MSI-H CRC. Several newer agents such as COX-2 inhibitors and irinotecan are also reviewed.

CONCLUSION

Available evidence suggests that MSI-H CRC have different behaviour patterns and response to chemotherapy compared with MSS CRC.

Predictive Value of MSH2 Gene Expression in Colorectal Cancer Treated with Capecitabine

PURPOSE

The objective of the present study was to evaluate the gene expression of the DNA mismatch repair gene MSH2 as a predictive marker in advanced colorectal cancer (CRC) treated with first-line capecitabine.

PATIENTS AND METHODS

Microdissection of paraffin-embedded tumor tissue, RNA extraction, and quantitative polymerase chain reaction were performed on tumors obtained from 37 patients with advanced CRC.

RESULTS

The median relative gene expression of MSH2 was 0.65 (quartiles 0.5-0.8) in nonresponders and 1.25 (quartiles 0.92-1.38) for responders (P = 0.038). High expression of MSH2 was associated with a hazard ratio of 0.5 (95% confidence interval, 0.23-1.11; P = 0.083) in survival analysis.

CONCLUSION

The higher gene expression of MSH2 in responders and the trend for predicting overall survival indicates a predictive value of this marker in the treatment of advanced CRC with capecitabine.

[Influence of neoadjuvant chemotherapy on microsatellite instability in gastric carcinoma]

BACKGROUND & OBJECTIVE

Most results from clinical trials of neoadjuvant chemotherapy for gastric carcinoma are certain. Meanwhile, microsatellite instability (MSI) is thought to be a new mechanism of cell canceration. This study was to investigate the influence of neoadjuvant chemotherapy on MSI in gastric carcinoma.

METHODS

Two microsatellite loci D2S123 and D3S1298 in tumor samples from 48 patients with gastric carcinoma, received preoperative chemotherapy of 5-fluorouracil (5-FU) and leucovorin (CF), were analyzed by polymerase chain reaction (PCR), electrophoresis on 8% denatured polyacrylamide gel, and silver staining. The microsatellite statuses of preoperative and postoperative specimens were compared. Chi-square test was used to analyze the influence of neoadjuvant chemotherapy on MSI in gastric carcinoma.

RESULTS

The detection rate of MSI was significantly higher in preoperative samples than in postoperative samples (31.3% vs. 12.5%, P=0.026). After neoadjuvant chemotherapy, 9 cases changed from MSI-positive to MSI-negative, while no MSI-negative cases became MSI-positive.

CONCLUSION

The neoadjuvant chemotherapy regimen of 5-FU/CF can induce some MSI-positive gastric carcinoma change to MSI-negative, but has no effect on MSI-negative cases.

Chemotherapeutic implications in microsatellite unstable colorectal cancer

Chemotherapy for colorectal cancer is currently offered to patients based on the stage of their cancer, and there is evidence to show an overall survival benefit with 5-fluorouracil-based (5-FU) therapy for patients with lymph node metastasis who receive it. The pathogenesis of colorectal cancer involves genomic instability, with about 15% of tumors demonstrating a form of genomic instability called high-frequency microsatellite instability (MSI-H) and due to loss of DNA mismatch repair function, and the remainder of colorectal tumors lacking MSI-H with retained DNA mismatch repair function and called microsatellite stable (MSS), with a large proportion of these tumors demonstrating another form of genomic instability called chromosomal instability. There is now evidence to show that the form of genomic instability that is present in a patient's colorectal cancer may predict a survival benefit from 5-FU. In particular, patients whose colorectal tumors have MSI-H do not gain a survival benefit with 5-FU as compared to patients with MSS tumors. In vitro evidence supports these findings, as MSI-H colon cancer cell lines are more resistant to 5-FU compared to MSS cell lines. More specifically, components of the DNA mismatch repair system have been shown to recognize and bind to 5-FU that becomes incorporated into DNA and which could be a trigger to induce cell death. The binding and subsequent cell death events would be absent in colorectal tumors with MSI-H, which have lost intact DNA mismatch repair function. These findings suggest that: (a) tumor cytotoxicity of 5-FU is mediated by DNA mechanisms in addition to well-known RNA mechanisms, and (b) patients whose tumors demonstrate MSI-H may not benefit from 5-FU therapy. Future studies should include a better understanding of the cellular mechanisms of the DNA recognition of 5-FU, multi-centered prospective trials investigating the survival benefit of 5-FU based on genomic instability, and the investigation of alternative chemotherapeutic regimens for patients with MSI-H tumors to improve survival.