Efficacy and Safety of Trifluridine/Tipiracil-Containing Combinations in Colorectal Cancer and Other Advanced Solid Tumors: A Systematic Review

We performed a systematic literature review to identify and summarize data from studies reporting clinical efficacy and safety outcomes for trifluridine/tipiracil (FTD/TPI) combined with other antineoplastic agents in advanced cancers, including metastatic colorectal cancer (mCRC). We conducted a systematic search on May 29, 2021, for studies reporting one or more efficacy or safety outcome with FTD/TPI-containing combinations. Our search yielded 1378 publications, with 38 records meeting selection criteria: 35 studies of FTD/TPI-containing combinations in mCRC (31 studies second line or later) and 3 studies in other tumor types. FTD/TPI plus bevacizumab was extensively studied, including 19 studies in chemorefractory mCRC. Median overall survival ranged 8.6-14.4 months and median progression-free survival 3.7-6.8 months with FTD/TPI plus bevacizumab in refractory mCRC. Based on one randomized and several retrospective studies, FTD/TPI plus bevacizumab was associated with improved outcomes compared with FTD/TPI monotherapy. FTD/TPI combinations with chemotherapy or other targeted agents were reported in small early-phase studies; preliminary data indicated higher antitumor activity for certain combinations. Overall, no safety concerns existed with FTD/TPI combinations; most common grade ≥ 3 adverse event was neutropenia, ranging 5%-100% across all studies. In studies comparing FTD/TPI combinations with monotherapy, grade ≥ 3 neutropenia appeared more frequently with combinations (29%-67%) vs. monotherapy (5%-41%). Discontinuation rates due to adverse events ranged 0%-11% for FTD/TPI plus bevacizumab and 0%-17% with other combinations. This systematic review supports feasibility and safety of FTD/TPI plus bevacizumab in refractory mCRC. Data on non-bevacizumab FTD/TPI combinations remain preliminary and need further validation.

Combination of ganciclovir and trifluridine prevents drug-resistance emergence in HSV-1

Ocular herpes simplex virus 1 (HSV-1) infections can lead to visual impairment. Long-term acyclovir (ACV) prophylaxis reduces the frequency of recurrences but is associated with drug resistance. Novel therapies are needed to treat drug-resistant HSV-1 infections. Here, we describe the effects of trifluridine (TFT) in combination with ACV or ganciclovir (GCV) on HSV-1 replication and drug-resistance emergence. Wild-type HSV-1 was grown under increasing doses of one antiviral (ACV, GCV, or TFT) or combinations thereof (ACV + TFT or GCV + TFT). Virus cultures were analyzed by Sanger sequencing and deep sequencing of the UL23 [thymidine kinase (TK)] and UL30 [DNA polymerase (DP)] genes. The phenotypes of novel mutations were determined by cytopathic effect reduction assays. TFT showed overall additive anti-HSV-1 activity with ACV and GCV. Five passages under ACV, GCV, or TFT drug pressure gave rise to resistance mutations, primarily in the TK. ACV + TFT and GCV + TFT combinatory pressure induced mutations in the TK and DP. The DP mutations were mainly located in terminal regions, outside segments that typically carry resistance mutations. TK mutations (R163H, A167T, and M231I) conferring resistance to all three nucleoside analogs (ACV, TFT, and GCV) emerged under ACV, TFT, ACV + TFT pressure and under GCV + TFT pressure initiated from suboptimal drug concentrations. However, higher doses of GCV and TFT prevented drug resistance in the resistance selection experiments. In summary, we identified novel mutations conferring resistance to nucleoside analogs, including TFT, and proposed that GCV + TFT combination therapy may be an effective strategy to prevent the development of drug resistance.

Efficacy of Regorafenib and Trifluridine/Tipiracil According to Extended RAS Evaluation in Advanced Metastatic Colorectal Cancer Patients: A Multicenter Retrospective Analysis

BACKGROUND

There are few molecular markers driving treatment selection in later lines of treatment for advanced colorectal cancer patients. The vast majority of patients who progress after first- and second-line therapy undergo chemotherapy regardless of molecular data.

OBJECTIVE

We aimed to assess the prognostic and predictive effects of specific RAS mutations on overall survival of patients receiving regorafenib (rego), trifluridine/tipiracil (TFD/TPI), or both.

PATIENTS AND METHODS

This was a retrospective observational study based on data from a previous study of our research network, involving nine Italian institutions over a 10-year timeframe (2012-2022). Extended RAS analysis, involving KRAS exon 2-4 and NRAS exon 2-4, and BRAF were the main criteria for inclusion in this retrospective evaluation. Patients with BRAF mutation were excluded. Patients were classified according to treatment (rego- or TFD/TPI-treated) and RAS mutational status (wild-type [WT], KRAS codon 12 mutations, KRAS codon 13 mutations, KRAS rare mutations and NRAS mutations, KRAS G12C mutation and KRAS G12D mutation).

RESULTS

Overall, 582 patients were included in the present analysis. Overall survival did not significantly differ in rego-treated patients according to RAS extended analysis, although a trend toward a better median survival in patients carrying G12D mutation (12.0 months), Codon 13 mutation (8.0 months), and Codon 12 mutation (7.0 months) has been observed, when compared with WT patients (6.0 months). Overall survival did not significantly differ in TFD/TPI-treated patients according to RAS extended analysis, although a trend toward a better median survival in WT patients had been observed (9.0 months) in comparison with the entire population (7.0 months). Patients receiving both drugs displayed a longer survival when compared with the population of patients receiving rego alone (p = 0.005) as well as the population receiving TFD/TPI alone (p < 0.001), suggesting a group enriched for favorable prognostic factors. However, when each group was analyzed separately, the addition of TFD/TPI therapy to the rego-treated group improved survival only in all-RAS WT patients (p = 0.003). Differently, the addition of rego therapy to TFD/TPI-treated patients significantly improved OS in the Codon 12 group (p = 0.0004), G12D group (p = 0.003), and the rare mutations group (p = 0.02), in addition to all-RAS WT patients (p = 0.002). The rego-TFD/TPI sequence, compared with the reverse sequence, significantly improved OS only in the KRAS codon 12 group (p = 0.003).

CONCLUSIONS

Our data demonstrate that RAS mutations do not affect outcome in rego-treated patients as well as TFD/TPI-treated patients. Nevertheless, a trend toward a higher efficacy of rego in RAS-mutated (in particular codon 12, rare RAS mutations, and G12D) patients has been recorded. The rego-TFD/TPI sequence seems to be superior to the reverse sequence in patients carrying an RAS codon 12 mutation, although the impact of other factors as disease burden or performance status cannot be excluded.

A Phase II Study of FOLFIRI Plus Ziv-Aflibercept After Trifluridine/Tipiracil Plus Bevacizumab in Patients with Metastatic Colorectal Cancer: WJOG 11018G

BACKGROUND

Non-inferiority of trifluridine/tipiracil (FTD/TPI) plus bevacizumab (BEV) to irinotecan/fluoropyrimidine plus BEV in metastatic colorectal cancer was investigated in the phase III TRUSTY study, and we conducted a phase II study of FOLFIRI (5-FU+leucovorin+irinotecan) plus zib-aflibercept (AFL) after FTD/TPI plus BEV. However, the TRUSTY study failed during the recruitment of our patients.

OBJECTIVE

We present the findings of a phase II study on the efficacy of FOLFIRI plus zib-aflibercept (AFL) after FTD/TPI plus BEV, including clinical results with plasma biomarker analyses.

METHODS

This was a multicenter, single-arm, phase II study in patients with metastatic colorectal cancer refractory or intolerant to oxaliplatin, fluoropyrimidine, BEV, and FTD/TPI. The primary endpoint was progression-free survival. Fifteen plasma angiogenesis-associated biomarkers were analyzed using a Luminex® multiplex assay U-kit.

RESULTS

Between January 2020 and May 2022, 26 patients (median age, 68 years) from 15 sites were enrolled. The median progression-free survival was 4.9 months (85% confidence interval, 3.4 month-not estimated). The overall response and disease control rates were 8% and 62%, respectively. The median levels of vascular endothelial growth factor-A and placental growth factor, both targets of AFL, were below the measurable limit of 30 pg/mL and 16 pg/mL, respectively. Patients were divided into two groups at the median levels of baseline biomarkers. The progression-free survival did not differ between high and low expressers of placental growth factor (p = 0.7), while it tended to be shorter in those with high levels of osteopontin (p = 0.05), angiopoietin-2 (p = 0.07), and tissue inhibitor of matrix metalloproteinases-1 (p = 0.1).

CONCLUSIONS

This study did not meet the primary endpoint. Hence, FOLFIRI plus AFL should not be used after FTD/TPI plus BEV for metastatic colorectal cancer. Further studies are needed to determine factors not targeted by AFL that may affect the efficacy of the treatment.

CLINICAL TRIAL REGISTRATION

jRCTs041190100.

Trifluridine for treatment of mpox infection in drug combinations in ophthalmic cell models

The Mpox virus can cause severe disease in the susceptible population with dermatologic and systemic manifestations. Furthermore, ophthalmic manifestations of mpox infection are well documented. Topical trifluridine (TFT) eye drops have been used for therapy of ophthalmic mpox infection in patients, however, its efficacy against mpox virus infection in this scenario has not been previously shown. In the present study, we have established ophthalmic cell models suitable for the infection with mpox virus. We show, that TFT is effective against a broad range of mpox isolates in conjunctival epithelial cells and keratocytes. Further, TFT remained effective against a tecovirimat-resistant virus strain. In the context of drug combinations, a nearly additive effect was observed for TFT combinations with brincidofovir and tecovirimat in conjunctival epithelial cells, while a slight antagonism was observed for both combinations in keratocytes. Altogether, our findings demonstrate TFT as a promising drug for treatment of ophthalmic mpox infection able to overcome tecovirimat resistance. However, conflicting results regarding the effect of drug combinations with approved compounds warrant close monitoring of such use in patients.

Exploratory Biomarker Analysis Using Plasma Angiogenesis-Related Factors and Cell-Free DNA in the TRUSTY Study: A Randomized, Phase II/III Study of Trifluridine/Tipiracil Plus Bevacizumab as Second-Line Treatment for Metastatic Colorectal Cancer

BACKGROUND

The TRUSTY study evaluated the efficacy of second-line trifluridine/tipiracil (FTD/TPI) plus bevacizumab in metastatic colorectal cancer (mCRC).

OBJECTIVE

This exploratory biomarker analysis of TRUSTY investigated the relationship between baseline plasma concentrations of angiogenesis-related factors and cell-free DNA (cfDNA), and the efficacy of FTD/TPI plus bevacizumab in patients with mCRC.

PATIENTS AND METHODS

The disease control rate (DCR) and progression-free survival (PFS) were compared between baseline plasma samples of patients with high and low plasma concentrations (based on the median value) of angiogenesis-related factors. Correlations between cfDNA concentrations and PFS were assessed.

RESULTS

Baseline characteristics (n = 65) were as follows: male/female, 35/30; median age, 64 (range 25-84) years; and RAS status wild-type/mutant, 29/36. Patients in the hepatocyte growth factor (HGF)-low and interleukin (IL)-8-low groups had a significantly higher DCR (risk ratio [95% confidence intervals {CIs}]) than patients in the HGF-high (1.83 [1.12-2.98]) and IL-8-high (1.70 [1.02-2.82]) groups. PFS (hazard ratio {HR} [95% CI]) was significantly longer in patients in the HGF-low (0.33 [0.14-0.79]), IL-8-low (0.31 [0.14-0.70]), IL-6-low (0.19 [0.07-0.50]), osteopontin-low (0.39 [0.17-0.88]), thrombospondin-2-low (0.42 [0.18-0.98]), and tissue inhibitor of metalloproteinase-1-low (0.26 [0.10-0.67]) groups versus those having corresponding high plasma concentrations of these angiogenesis-related factors. No correlation was observed between cfDNA concentration and PFS.

CONCLUSION

Low baseline plasma concentrations of HGF and IL-8 may predict better DCR and PFS in patients with mCRC receiving FTD/TPI plus bevacizumab, however further studies are warranted.

CLINICAL TRIAL REGISTRATION NUMBER

jRCTs031180122.

Combination therapy with WEE1 inhibition and trifluridine/tipiracil against esophageal squamous cell carcinoma

Despite advanced therapeutics, esophageal squamous cell carcinoma (ESCC) remains one of the deadliest cancers. Here, we propose a novel therapeutic strategy based on synthetic lethality combining trifluridine/tipiracil and MK1775 (WEE1 inhibitor) as a treatment for ESCC. This study demonstrates that trifluridine induces single-strand DNA damage in ESCC cells, as evidenced by phosphorylated replication protein 32. The DNA damage response includes cyclin-dependent kinase 1 (CDK1) (Tyr15) phosphorylation as CDK1 inhibition and a decrease of the proportion of phospho-histone H3 (p-hH3)-positive cells, indicating cell cycle arrest at the G2 phase before mitosis entry. The WEE1 inhibitor remarkedly suppressed CDK1 phosphorylation (Try15) and reactivated CDK1, and also increased the proportion of p-hH3-positive cells, which indicates an increase of the number of cells into mitosis. Trifluridine combined with a WEE1 inhibitor increased trifluridine-mediated DNA damage, namely DNA double-strand breaks, as shown by increased γ-H2AX expression. Moreover, the combination treatment with trifluridine/tipiracil and a WEE1 inhibitor significantly suppressed tumor growth of ESCC-derived xenograft models. Hence, our novel combination treatment with trifluridine/tipiracil and a WEE1 inhibitor is considered a candidate treatment strategy for ESCC.

Bevacizumab mitigates codon-specific effects of trifluridine/tipiracil on efficacy outcome parameters in metastatic colorectal cancer

BACKGROUND

Molecular informed therapy changed treatment patterns of metastatic colorectal cancer (mCRC). Recently KRAS G12, the most prevalent RAS mutation in mCRC, was investigated to be a negative predictive marker for the efficacy of trifluridine/tipiracil (FTD/TPI). Whether this proposed selectivity remains when FTD/TPI is combined with bevacizumab remains elusive. We aimed to describe the efficacy of FTD/TPI + bevacizumab depending on the RAS mutational status in a real-world population.

PATIENTS AND METHODS

Patients from five different cancer centers in Austria who received FTD/TPI + bevacizumab in any treatment line having available information on their molecular profile were eligible. Data were retrospectively collected by chart review. Survival data were compared using log-rank test. Multivariate Cox regression models included several established covariates.

RESULTS

One hundred and twenty-three patients with mCRC were included in this study. Median overall survival (OS) was highly similar in the RAS wild type (WT) [9.63 months (95% confidence interval [CI] 8.055-13.775 months)] and the RAS mutant cohorts [8.78 months (95% CI 8.055-11.014 months)], which was confirmed in a multivariable model adjusting for potential confounders; hazard ratio (HR): 1.05 (95% CI 0.618-1.785; P = 0.857). In addition, no effect of KRAS G12 status on patient outcome was observed. In detail, OS was 8.88 months (95% CI 7.332-12.921 months) in patients with KRAS G12 mutation, compared to 9.47 months (95% CI 8.088-11.375 months) in patients with RAS WT/no-KRAS G12 disease [HR: 0.822 (95% CI 0.527-1.282; P = 0.387)].

CONCLUSION

This real-world study indicates that the efficacy of FTD/TPI + bevacizumab is independent of RAS mutational status and that bevacizumab may therefore mitigate the potentially limited efficacy of FTD/TPI monotherapy in the KRAS G12-mutated population.

Phase II Trial of Trifluridine/Tipiracil Plus Irinotecan in Patients with Advanced, Refractory Biliary Tract Carcinoma

BACKGROUND

We sought to determine the safety and efficacy of trifluridine/tipiracil in combination with irinotecan in a phase II trial setting for refractory, advanced unresectable biliary tract carcinoma (BTC).

METHODS

A total of 28 patients (27 were evaluable) with advanced BTCs who progressed on at least one prior systemic therapy were enrolled and were treated with trifluridine/tipiracil 25 mg/m2 (days 1-5 of 14-day cycle) and irinotecan 180 mg/m2 (day 1 of the 14-day cycle). The primary endpoint for the study was 16-week progression-free survival (PFS16) rate. Overall survival (OS), progression-free survival (PFS), objective response rate (ORR), disease control rate (DCR), and safety were pre-specified secondary endpoints.

RESULTS

Of 27 patients, PFS16 rate was 37% (10/27; 95% CI: 19%-58%), thereby meeting the criteria for success for the primary endpoint. The median PFS and OS of the entire cohort were 3.9 months (95% CI: 2.5-7.4) and 9.1 months (95% CI: 8.0-14.3), respectively. In the patients evaluable for tumor response (n = 20), the ORR and DCR were 10% and 50%, respectively. Twenty patients (74.1%) had at least one grade 3 or worse adverse event (AE), and 4 patients (14.8%) had grade 4 AEs. A total of 37% (n = 10/27) and 51.9% (n = 14/27) experienced dose reductions in trifluridine/tipiracil and irinotecan, respectively. Delay in therapy was noted in 56% of the patients while 1 patient discontinued the therapy, primarily due to hematologic AEs.

CONCLUSION

The combination of trifluridine/tipiracil plus irinotecan is a potential treatment option for patients with advanced, refractory BTCs with good functional status and no targetable mutations. A larger randomized trial is needed to confirm these results. (ClinicalTrials.gov Identifier: NCT04072445).

Effect of KRAS codon 12 or 13 mutations on survival with trifluridine/tipiracil in pretreated metastatic colorectal cancer: a meta-analysis

Background

KRAS gene mutations can predict prognosis and treatment response in patients with metastatic colorectal cancer (mCRC).

Methods

We undertook a meta-analysis of three randomized, placebo-controlled trials (RECOURSE, TERRA and J003) to investigate the impact of KRAS mutations in codons 12 or 13 on overall survival (OS) and progression-free survival in patients receiving trifluridine/tipiracil (FTD/TPI) for refractory mCRC.

Results

A total of 1375 patients were included, of whom 478 had a KRAS codon 12 mutation and 130 had a KRAS codon 13 mutation. In univariate analyses, the absence of a KRAS codon 12 mutation was found to significantly increase the OS benefit of FTD/TPI relative to placebo compared with the presence of the mutation {hazard ratio (HR), 0.62 [95% confidence interval (CI): 0.53-0.72] versus 0.86 (0.70-1.05), respectively; interaction P = 0.0206}. Multivariate analyses showed that taking confounding factors into account reduced the difference in treatment effect between the presence and the absence of KRAS codon 12 mutations, confirming that treatment benefit was maintained in patients with [HR, 0.73 (95% CI: 0.59-0.89)] and without [HR, 0.63 (95% CI: 0.54-0.74)] codon 12 mutations (interaction P = 0.2939). KRAS mutations in codon 13 did not reduce the OS benefit of FTD/TPI relative to placebo, and, furthermore, KRAS mutations at either codon 12 or codon 13 did not affect the progression-free survival benefit.

Conclusions

Treatment with FTD/TPI produced a survival benefit, relative to placebo, regardless of KRAS codon 12 or 13 mutation status in patients with previously treated mCRC.

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KRAS mutations are associated with negative outcomes in patients with mCRC; codon 12 and 13 mutations are the most common.

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FTD/TPI was associated with longer median overall survival vs placebo both in patients with wild-type KRAS and mutant KRAS.

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FTD/TPI produced a survival benefit, relative to placebo, regardless of KRAS codon 12 or 13 mutation status in this patient group.

The effect of trifluridine/tipiracil for patients with heavily pretreated metastatic gastric cancer: A protocol for systematic review and meta-analysis

BACKGROUND

Gastric cancer is a global health problem with high incidence rate and mortality rate. Due to the limitations of traditional chemotherapy drugs, such as patient intolerance, low efficacies and serious adverse effects, trifluridine/tipiracil has been considered to be a promising treatment for patients with heavily pretreated metastatic gastric cancer. However, the relevant systematic review has not been occurring. The presentation of this protocol is to scientifically evaluate the efficacy and safety of trifluridine/tipiracil in patients with highly pretreated metastatic gastric cancer.

METHODS

The protocol followed Preferred Reporting Items for Systematic Reviews and Meta-Analyses Protocols. We will systematically search MEDLINE, PubMed, Embase, Cochrane Library, Web of Science, China National Knowledge Infrastructure Database (CNKI), VIP Chinese Science and Technology Periodical Database (VIP), Wan Fang Database up to November 1, 2020 to identify published articles. Using the Cochrane risk assessment tool to assess the methodological quality of the RCTs, and all included studies will be analyzed according to the criteria in the Cochrane Handbook. Review Manager 5.3 software will be used for literature quality evaluation and data analysis.

RESULTS

Objective to evaluate the efficacy and safety of trifluridine/tipiracil in patients with heavily pretreated metastatic gastric cancer by analyzing the eligible data extracted under limited conditions.

CONCLUSION

This study provides clear evidence to evaluate the effectiveness and safety of trifluridine/tipiracil for patients with highly pretreated metastatic gastric cancer, and the findings will also be published in a peer-reviewed journal.

ETHICS AND DISSEMINATION

No ethical statement will be required for this study because there is no direct involvement of human. This review will be published in a peer-reviewed journal as a conference report or an article.

REGISTRATION

OSF registration number: DOI 10.17605/OSF.IO/6MF5U.

Detection of trifluridine in tumors of patients with metastatic colorectal cancer treated with trifluridine/tipiracil

PURPOSE

Trifluridine (FTD) is the active component of the nucleoside chemotherapeutic drug trifluridine/tipiracil (FTD/TPI), which is approved worldwide for the treatment of patients with metastatic gastrointestinal cancer. FTD exerts cytotoxic effects via its incorporation into DNA, but FTD has not been detected in the tumor specimens of patients. The purpose of this study was to detect FTD in tumors resected from metastatic colorectal cancer (mCRC) patients who were administered FTD/TPI. Another purpose was to investigate the turnover rate of FTD in tumors and bone marrow in a mouse model.

METHODS

Tumors and normal tissue specimens were obtained from mCRC patients who were administered FTD/TPI or placebo at Kyushu University Hospital. Tumors and bone marrow were resected from mice with peritoneal dissemination treated with FTD/TPI. To detect and quantitate FTD incorporated into DNA, immunohistochemical staining of paraffin-embedded specimens (IHC-p staining) and slot-blot analysis of DNA purified from these tissues were performed using an anti-BrdU antibody. IHC-p staining of proliferation and apoptosis markers was also performed.

RESULTS

FTD was detected in metastatic tumors obtained from mCRC patients who were administered FTD/TPI, but who had discontinued the treatment several weeks before surgery. In a peritoneal dissemination mouse model, FTD was still detected in tumors 13 days after the cessation of FTD/TPI treatment, but had disappeared from bone marrow within 6 days.

CONCLUSION

These results indicate that FTD persists longer in tumors than in bone marrow, which may cause a sustained antitumor effect with tolerable hematotoxicity.

Is There an Optimal Choice in Refractory Colorectal Cancer? A Network Meta-Analysis

BACKGROUND

In the absence of head-to-head comparison studies, the present network meta-analysis evaluated and compared the efficacy of 4 therapeutic alternatives for refractory colorectal cancer.

MATERIALS AND METHODS

The search focused on results from phase III randomized controlled trials. Separate (subgroup) network meta-analyses were conducted to obtain drug comparisons stratified by various patient characteristics. The principal outcome of interest was overall survival (OS).

RESULTS

No difference in OS was found between regorafenib and TAS-102. For a rectal primary location, TAS-102 conferred benefit versus placebo (hazard ratio [HR], 0.671), but regorafenib did not (HR, 0.950). For patients aged > 65 years, TAS-102 showed benefit versus placebo (HR, 0.579) but regorafenib did not (HR, 0.816). For patients with an Eastern Cooperative Oncology Group (ECOG) performance status (PS) of 0 in the indirect comparison, regorafenib showed benefit versus placebo (HR, 0.687), as did TAS-102 (HR, 0.756) but with a lower advantage. For patients with RAS wild type not previously treated with anti-EGFR antibodies, panitumumab was the optimal choice for OS.

CONCLUSIONS

No differences in OS were found between regorafenib and TAS-102. Possible greater efficacy was found for TAS-102 compared with regorafenib for patients with a rectal primary location, ECOG PS > 0, and age > 65 years. In contrast, regorafenib showed possible greater effectiveness for patients with ECOG PS 0 and age < 65 years. In the RAS WT population, the anti-EGFR drug showed superiority with respect to TAS-102 and regorafenib. These results should be viewed as only exploratory, and further prospective studies are warranted to validate these data.

pH-induced variations in the TK1 gene model

A physiological decrease in extracellular pH (pHe) alters the efficiency of DNA repair and increases formation of DNA double-strand breaks (DSBs). Whether this could translate into genetic instability and variations, was investigated using the TK6 cell model, in which positive selection of the TK1 gene loss-of-function mutations can be achieved from resistance to trifluorothymidine. Cell exposure to suboptimal pH (down to 6.9) for 3 weeks resulted in the 100 % frequency of a stronger frameshift mutation that has spread to both TK1 alleles, whereas weaker frameshift mutations within the 3'exon were eliminated during the selection. Suboptimal pHe values were also found to alter the proportion of the TK1 splicing variant expressed as percent spliced in index values and promote selection of truncated exons as well as intron retention. Although recovery at pH 7.4 did not reverse the selected frameshift mutation, reversal of splice variants and exon truncation towards control values were observed. Hence, suboptimal pHe can induce a combination of mutational events and splicing alterations within the same gene in the resistant clones. This model of positive selection for loss-of-function clearly demonstrates that suboptimal pHe may confer a similar growth advantage when such instability occurs within tumor suppressor genes.

Real-World Adherence in Patients with Metastatic Colorectal Cancer Treated with Trifluridine plus Tipiracil or Regorafenib

BACKGROUND

Trifluridine and tipiracil (FTD + TPI) and regorafenib (REG) are approved treatments for the treatment of refractory metastatic colorectal cancer (mCRC). This study assesses adherence and duration of therapy with FTD + TPI versus REG and explores the effect of sequencing on adherence.

MATERIALS AND METHODS

Adults diagnosed with mCRC were identified in the IQVIA Real-World Data Adjudicated Claims: U.S. database (October 2014-July 2017). The observation period spanned from the index date (first dispensing of FTD + TPI or REG) to the earliest of a switch to another mCRC agent, the end of continuous enrollment, or the end of data availability. Medication possession ratio (MPR), proportion of days covered (PDC), and persistence and time to discontinuation (gap ≥45 days) were compared between FTD + TPI and REG users and among switchers (FTD + TPI-to-REG vs. REG-to-FTD + TPI).

RESULTS

A total of 469 FTD + TPI and 311 REG users were identified. FTD + TPI users had higher compliance with an MPR ≥80% (odds ratio [OR], 2.47; p < .001) and PDC ≥80% (OR, 2.77; p < .001). FTD + TPI users had better persistence (82.8% vs. 68.0%; p < .001) and lower risk of discontinuation (hazard ratio [HR], 0.76; p = .006). Among switchers (96 FTD + TPI-to-REG; 83 REG-to-FTD + TPI), those switching from FTD + TPI to REG were more likely to have an MPR ≥80% (OR, 2.91; p < .001) and PDC ≥80% (OR, 4.60; p < .001) compared with REG-to-FTD + TPI switchers while treated with these drugs. Additionally, FTD + TPI-to-REG switchers had a lower risk of first treatment discontinuation (HR, 0.66; p = .009).

CONCLUSION

FTD + TPI users had significantly higher adherence and persistence, and patients who were treated with FTD + TPI before switching to REG also had higher adherence and persistence outcomes.

IMPLICATIONS FOR PRACTICE

Trifluridine plus tipiracil (FTD + TPI) and regorafenib (REG) prolong survival in refractory metastatic colorectal cancer (mCRC) but have different tolerability profiles. This study assessed real-world adherence to treatment with FTD + TPI versus REG and compared outcomes among patients who switched from FTD + TPI to REG and vice versa. FTD + TPI was associated with significantly higher medication adherence and longer time to discontinuation than REG. Patients treated with FTD + TPI prior to switching to REG also showed higher adherence outcomes. Findings could help inform decision making regarding the choice and sequencing of treatment with FTD + TPI versus REG in patients with mCRC.

Trifluridine/Tipiracil (TAS-102) in Refractory Metastatic Colorectal Cancer: A Multicenter Register in the Frame of the Italian Compassionate Use Program

BACKGROUND

TAS-102 is indicated for patients with metastatic colorectal cancer (mCRC) previously treated with, or not considered candidates for, available therapies. Given the complete inefficacy in half of patients, the lack of predictive factors, the palliative setting, and the financial and clinical toxicity, optimizing the cost-benefit ratio is crucial. The "ColonLife" nomogram allows an estimate of the 12-week life expectancy of patients with refractory mCRC.

MATERIALS AND METHODS

We collected data from patients treated at eight Italian centers in the compassionate use program. Baseline characteristics of patients who were or were not progression free at 6 months were compared. The discriminative ability of the ColonLife nomogram was assessed. Among patients who received both TAS-102 and regorafenib, clinical outcomes of the two sequences were compared.

RESULTS

This study included 341 patients. Six (2%) and 93 (27%) patients achieved response and disease stabilization, respectively. The median progression-free survival (PFS) was 2.4 months with an estimated 6-month PFS rate of 19%; the median overall survival (OS) was 6.2 months. An Eastern Cooperative Oncology Group performance status (ECOG PS) of 0, normal lactate dehydrogenase (LDH), and a time from the diagnosis of metastatic disease of >18 months were independently associated with higher chances of a patient being progression free at 6 months. The discriminative ability of ColonLife was confirmed. Among 121 patients who received both regorafenib and TAS-102, no differences in first or second PFS or OS were reported between the two sequences.

CONCLUSION

One out of five patients achieves clinical benefit with TAS-102. ECOG PS, LDH, and time from diagnosis of metastatic disease may help to identify these patients. Excluding patients with very short life expectancy appears a reasonable approach.

IMPLICATIONS FOR PRACTICE

Improving the cost-efficacy ratio of TAS-102 in metastatic colorectal cancer is needed to spare useless toxicities in a definitely palliative setting. Eastern Cooperative Oncology Group performance status, lactate dehydrogenase levels, and time from the diagnosis of metastatic disease may help to identify patients more likely to achieve benefit. Properly designed prognostic tools (i.e., the "ColonLife" nomogram) may enable excluding from further treatments patients with very limited life expectancy.

Effective Sequential Combined Chemotherapy with Trifluridine/Tipiracil and Regorafenib in Human Colorectal Cancer Cells

Salvage chemotherapy for refractory metastatic colorectal cancer using trifluridine/tipiracil (FTD/TPI) and regorafenib has shown survival benefits. We evaluated the antitumor effects of FTD or FTD/TPI combined with regorafenib in vitro and in vivo. SW620, HCT 116, and HT-29 human colorectal cancer cell lines were treated with FTD and regorafenib simultaneously and sequentially. Cell death, incorporation of FTD into DNA, and molecules related to FTD and regorafenib-associated cell death were investigated. The antitumor effects of FTD combined with regorafenib in SW620 and COLO205 xenografts were also evaluated. Cell death was greater after sequential treatment with FTD followed by regorafenib in SW620 cells, but not in HCT 116 and HT-29 cells, than after treatment with FTD alone, which was attributable to thymidylate synthase reduction with the induction of apoptosis. In contrast, simultaneous and sequential exposure to regorafenib followed by FTD, but not FTD alone, attenuated the cell death effect. Furthermore, combined FTD/TPI treatment followed by regorafenib had greater antitumor activity than either monotherapy in SW620 and COLO205 xenograft models. Treatment results following regorafenib administration subsequent to FTD or FTD/TPI suggest that sequential therapy with FTD/TPI prior to regorafenib may be effective in a clinical setting.

Thymidine Kinase 1 Loss Confers Trifluridine Resistance without Affecting 5-Fluorouracil Metabolism and Cytotoxicity

Acquired resistance to therapeutic drugs is a serious problem for patients with cancer receiving systemic treatment. Experimentally, drug resistance is established in cell lines in vitro by repeated, continuous exposure to escalating concentrations of the drug; however, the precise mechanism underlying the acquired resistance is not always known. Here, it is demonstrated that the human colorectal cancer cell line DLD1 with acquired resistance to trifluridine (FTD), a key component of the novel, orally administered nucleoside analogue-type chemotherapeutic drug trifluridine/tipiracil, lacks functional thymidine kinase 1 (TK1) expression because of one nonsense mutation in the coding exon. Targeted disruption of the TK1 gene also conferred severe FTD resistance, indicating that the loss of TK1 protein expression is the primary cause of FTD resistance. Both FTD-resistant DLD1 cells and DLD1-TK1 ^-/- cells exhibited similar 5-fluorouracil (5-FU) sensitivity to that of the parental DLD1 line. The quantity of cellular pyrimidine nucleotides in these cells and the kinetics of thymidylate synthase ternary complex formation in 5-FU-treated cells is similar to DLD1 cells, indicating that 5-FU metabolism and cytotoxicity were unaffected. The current data provide molecular-based evidence that acquired resistance to FTD does not confer 5-FU resistance, implying that 5-FU-based chemotherapy would be effective even in tumors that become refractory to FTD during trifluridine/tipiracil treatment. Mol Cancer Res; 16(10); 1483-90. ©2018 AACR.

The Role of Fluoropirimidines in Gastrointestinal Tumours: from the Bench to the Bed

PURPOSE

Gastrointestinal tumours are one of the most common types of cancer. Therapeutic options include surgery, radiotherapy, local ablation techniques, targeted agents, and chemotherapy. Fluoroprimidines are one of the most active drug families in digestive tumours and remains the cornerstone of the most commonly used chemotherapy schemes.

METHODS

We review the molecular basis of thymidylate synthase inhibition and the mechanisms of action of 5-fluorouracil, next generation oral fluoropyrimidines (capecitabine, tegafur and the latest S-1 and TAS-102) and antifolates.

RESULTS

In addition, mechanisms and biomarkers of resistance and toxicity are explored. Finally, new fluoropyrimidines development and clinical trials ongoing in digestive tumours are reviewed.

CONCLUSIONS

Further research is necessary to avoid resistance mechanisms, improve clinical outcomes and continue reducing toxicities. Until new drugs become available, the optimization of current therapies should be a priority.

A novel antimetabolite: TAS-102 for metastatic colorectal cancer

TAS-102 is a new oral anti-tumor drug, composed of a thymidine-based nucleoside analog (trifluridine: FTD) and a thymidine phosphorylase inhibitor (tipiracil hydrochloride: TPI). TAS-102 has been shown to significantly improve overall survival and progression-free survival in patients with refractory metastatic colorectal cancer (mCRC) in placebo-controlled randomized phase II and III trials. The current review summarizes mechanisms of action, pharmacokinetics/dynamics and preclinical and clinical data of TAS-102 in colorectal cancer. TAS-102 is a new salvage-line treatment option for patients with mCRC. TAS-102 is well tolerated and has great potential in future clinical drug combination therapies.

TAS-102, a novel antitumor agent: a review of the mechanism of action

Inhibition of nucleoside metabolism is an important principle in cancer therapy as evidenced by the role of fluoropyrimidines, such as 5-fluorouracil (5-FU), and antifolates in the treatment of many cancers. TAS-102 is an oral combination therapy consisting of trifluridine (FTD), a thymidine-based nucleoside analog, plus tipiracil hydrochloride (TPI), a novel thymidine phosphorylase inhibitor that improves the bioavailability of FTD. TAS-102 has demonstrated efficacy in 5-FU-refractory patients based on a different mechanism of action and has been approved for the treatment of metastatic colorectal cancer in Japan. This review describes the mechanism of action of TAS-102, highlighting key differences between TAS-102 and 5-FU-based therapies. While both FTD and 5-FU inhibit thymidylate synthase (TS), a central enzyme in DNA synthesis, sufficient TS inhibition by FTD requires continuous infusion; therefore, it is not considered a clinically relevant mechanism with oral dosing. Instead, the primary cytotoxic mechanism with twice-daily oral dosing, the schedule used in TAS-102 clinical development, is DNA incorporation. FTD incorporation into DNA induces DNA dysfunction, including DNA strand breaks. Uracil-based analogs such as 5-FU may also be incorporated into DNA; however, they are immediately cleaved off by uracil-DNA glycosylases, reducing their ability to damage DNA. Moreover, the TPI component may enhance the durability of response to FTD. With its distinct mechanism of action and metabolism, TAS-102 is a promising treatment option for patients resistant to or intolerant of 5-FU-based fluoropyrimidines.

Efficacy of Combination Chemotherapy Using a Novel Oral Chemotherapeutic Agent, TAS-102, with Oxaliplatin on Human Colorectal and Gastric Cancer Xenografts

TAS-102 is a novel oral nucleoside antitumor agent consisting of trifluridine (FTD) and the thymidine phosphorylase inhibitor tipiracil hydrochloride (at a molar ratio of 1:0.5) that was approved in Japan in 2014 for the treatment of unresectable advanced or recurrent colorectal cancer. In the present study, the enhancement of therapeutic efficacy using a combination of TAS-102 and oxaliplatin was evaluated in a xenograft-bearing nude mouse model of colorectal and gastric cancer. TAS-102 was orally administered twice-a-day from day 1 to 14, and oxaliplatin was administered intravenously on days 1 and 8. The in vivo growth-inhibitory activity was evaluated based on the tumor volume and the growth-delay period, was estimated based on the period required to reach a tumor volume five-times greater than the initial volume (RTV5). The tumor growth-inhibitory activity and RTV5 in mice administered TAS-102 with oxaliplatin were significantly superior to those associated with either monotherapy in mice with colorectal (HCT 116, SW-48; p<0.001) and gastric cancer (SC-2, MKN74; p<0.001). MKN74/5FU, a 5-fluorouracil-resistant MKN74 sub-line, was sensitive to both FTD and oxaliplatin in vitro. In vivo, TAS-102 alone was effective in MKN74/5FU, and its anti-tumor activity was significantly enhanced in combination with oxaliplatin (p<0.001). No significant decrease in body weight or toxicity was observed compared to either monotherapy. The present pre-clinical findings indicate that combination of TAS-102 and oxaliplatin is a promising treatment option for colorectal or gastric cancer, and can be utilized in both chemo-naïve tumors and recurrent tumors after 5-fluorouracil treatment.

Positron emission tomography imaging of human colon cancer xenografts in mice with [18F]fluorothymidine after TAS-102 treatment

PURPOSE

TAS-102 is an orally administered anticancer agent composed of α,α,α-trifluorothymidine (FTD) and thymidine phosphorylase inhibitor (TPI). This study assessed 3'-deoxy-3'-[(18)F]fluorothymidine ([(18)F]FLT) uptake after TAS-102 administration.

METHODS

The human colorectal carcinoma cell lines HCT116, HT29, HCT8 and SW620 were exposed to FTD for 2 h, further incubated for 0, 2 and 24 h, and assayed for [(3)H]FLT uptake, nucleoside transport, thymidine kinase 1 (TK1) expression and TK1 activity. Static and 2-h dynamic [(18)F]FLT positron emission tomography (PET) was performed in mice bearing HT29 or SW620 tumours orally administered with vehicle or TAS-102.

RESULTS

FTD decreased the viability of all cell lines, whereas increased [(3)H]FLT uptake (P < 0.05). Increased nucleoside transport and/or TK1 expression were observed 24 h after FTD, but not in 0-2 h. Static [(18)F]FLT PET in mice bearing HT29 tumours showed accumulation of [(18)F]FLT in tumours 1 h (day 1) after TAS-102. Two-hour dynamic PET in mice bearing SW620 tumours showed increased influx constant and volume of distribution of phosphorylated [(18)F]FLT on days 1 and 8 (P < 0.05) after TAS-102 with decreased dephosphorylation on day 1 (P < 0.001). Ex vivo studies showed that SW620 tumours after TAS-102 had higher TK1 expression than those with vehicle on days 8 and 15.

CONCLUSION

TAS-102 administration induces an increase in [(18)F]FLT uptake. Mechanisms may involve decreased dephosphorylation of [(18)F]FLT phosphate early after TAS-102 administration. Increased TK1 expression and/or nucleoside transporter may be related to increased [(18)F]FLT uptake at a later time. [(18)F]FLT PET has a potential to assess the pharmacodynamics of TAS-102 in cancer patients.

Trifluorothymidine resistance is associated with decreased thymidine kinase and equilibrative nucleoside transporter expression or increased secretory phospholipase A2

Trifluorothymidine (TFT) is part of the novel oral formulation TAS-102, which is currently evaluated in phase II studies. Drug resistance is an important limitation of cancer therapy. The aim of the present study was to induce resistance to TFT in H630 colon cancer cells using two different schedules and to analyze the resistance mechanism. Cells were exposed either continuously or intermittently to TFT, resulting in H630-cTFT and H630-4TFT, respectively. Cells were analyzed for cross-resistance, cell cycle, protein expression, and activity of thymidine phosphorylase (TP), thymidine kinase (TK), thymidylate synthase (TS), equilibrative nucleoside transporter (hENT), gene expression (microarray), and genomic alterations. Both cell lines were cross-resistant to 2'-deoxy-5-fluorouridine (>170-fold). Exposure to IC(75)-TFT increased the S/G(2)-M phase of H630 cells, whereas in the resistant variants, no change was observed. The two main target enzymes TS and TP remained unchanged in both TFT-resistant variants. In H630-4TFT cells, TK protein expression and activity were decreased, resulting in less activated TFT and was most likely the mechanism of TFT resistance. In H630-cTFT cells, hENT mRNA expression was decreased 2- to 3-fold, resulting in a 5- to 10-fold decreased TFT-nucleotide accumulation. Surprisingly, microarray-mRNA analysis revealed a strong increase of secretory phospholipase-A2 (sPLA2; 47-fold), which was also found by reverse transcription-PCR (RT-PCR; 211-fold). sPLA2 inhibition reversed TFT resistance partially. H630-cTFT had many chromosomal aberrations, but the exact role of sPLA2 in TFT resistance remains unclear. Altogether, resistance induction to TFT can lead to different mechanisms of resistance, including decreased TK protein expression and enzyme activity, decreased hENT expression, as well as (phospho)lipid metabolism. Mol Cancer Ther; 9(4); 1047-57. (c)2010 AACR.

Efficacy of antiviral agents in feline herpetic keratitis: Results of anin vitrostudy

PURPOSE

To determine, by a plaque reduction assay, the in vitro efficacy of novel antiviral agents in the treatment of feline herpes virus 1 (FHV-1) keratitis in the domestic cat (Felis felis).

MATERIALS AND METHODS

A standard plaque reduction assay was performed using a laboratory strain of FHV-1 and embryo-derived feline kidney cells to determine the in vitro efficacy of the antiviral drugs penciclovir (PCV), bromovinyldeoxyuridine (BVdU), and (S)-9-(3-hydroxy-2-phosphonylmethoxypropyl) adenine (HPMPA) and to compare these with the drugs acyclovir (ACV) and trifluorothymidine (TFT). Efficacy was assessed by determining the dose of drug at which 50% plaque reduction was noted (ED(50)).

RESULTS

HPMPA was found to have greatest antiviral activity (ED(50) 0.07 microg/ml). ACV was least active (ED(50) 24 microg/ml), while TFT was active with an ED(50) of 5.7 microg/ml. PCV and BVdU had intermediate activity (ED(50) 1.6 and 1.7 microg/ml, respectively).

CONCLUSIONS

This study suggests that the efficacy of HPMPA, BVdU, and penciclovir in cats with herpesviral keratitis should be determined in vivo as their efficacy in vitro was substantially greater than that of acyclovir, already shown to have demonstrable but limited clinical antiviral activity.

Therapeutic potential of the dual-targeted TAS-102 formulation in the treatment of gastrointestinal malignancies

Current treatment modalities for cancer combine cytotoxic drugs against DNA and novel targeted drugs affecting signal transduction pathways, which are required for growth progression and metastasizing tumors. Classical chemotherapeutic regimens for gastro-intestinal tumors include antimetabolites based on 5-fluorouracil (5FU), the platinum analog oxaliplatin and the topoisomerase inhibitor irinotecan. The thymidine analog trifluorothymidine (TFT) has been shown to bypass resistance pathways for 5FU derivatives (S-1, UFT, Xeloda) in model systems, while concurrent application with a thymidine phosphorylase inhibitor (TPI) increases the bioavailability of TFT, thereby potentiating the in vivo efficacy of TFT. The formulation TAS-102 is given orally in a 1.0:0.5 molar ratio (TFT:TPI). The formulation is dual-targeted due to the cytotoxic effect of TFT, which is enhanced by TPI, while TPI also exerts antiangiogenic effects by inhibiting thymidine phosphorylase (TP), also known as platelet-derived endothelial cell growth factor. Evidence is accumulating from in vitro and in vivo preclinical studies that these properties favor further combinations with other cytotoxic agents currently being used in the treatment of gastro-intestinal tumors. Also treatment with targeted agents will synergistically down-regulate signal transduction pathways responsible for growth and progression of tumors. In this review, we summarize the available information on (clinical) pharmacology, mechanisms of action, pharmacodynamic and pharmacokinetic properties, early clinical trials and future directions of the new potent combination drug TAS-102.

Mechanism of trifluorothymidine potentiation of oxaliplatin-induced cytotoxicity to colorectal cancer cells

Oxaliplatin (OHP) is an anticancer agent that acts by formation of Platinum-DNA (Pt-DNA) adducts resulting in DNA-strand breaks and is used for the treatment of colorectal cancer. The pyrimidine analog trifluorothymidine (TFT) forms together with a thymidine phosphorylase inhibitor (TPI) the anticancer drug formulation TAS-102, in which TPI enhances the bioavailability of TFT in vivo. In this in vitro study the combined cytotoxic effects of OHP with TFT were investigated in human colorectal cancer cells as a model for TAS-102 combinations. In a panel of five colon cancer cell lines (WiDr, H630, Colo320, SNU-C4 and SW1116) we evaluated the OHP-TFT drug combinations using the multiple drug–effect analysis with CalcuSyn software, in which the combination index (CI) indicates synergism (CI<0.9), additivity (CI=0.9–1.1) or antagonism (CI>1.1). Drug target analysis was used for WiDr, H630 and SW1116 to investigate whether there was an increase in Pt-DNA adduct formation, DNA damage induction, cell cycle delay and apoptosis. Trifluorothymidine combined with OHP resulted in synergism for all cell lines (all CI<0.9). This was irrespective of schedule in which either one of the drugs was kept at a constant concentration (using variable drug ratio) or when the two drugs were added in a 1 : 1 IC₅₀-based molar ratio. Synergism could be increased for WiDr using sequential drug treatment schedules. Trifluorothymidine increased Pt-DNA adduct formation significantly in H630 and SW1116 (14.4 and 99.1%, respectively; P<0.05). Platinum-DNA adducts were retained best in SW1116 in the presence of TFT. More DNA-strand breaks were induced in SW1116 and the combination increased DNA damage induction (>20%) compared with OHP alone. Exposure to the drugs induced a clear cell-cycle S-phase arrest, but was dose schedule and cell line dependent. Trifluorothymidine (TFT) and OHP both induced apoptosis, which increased significantly for WiDr and SW1116 after TFT–OHP exposure (18.8 and 20.6% respectively; P<0.05). The basal protein levels of ERCC1 DNA repair enzyme were not related to the DNA damage that was induced in the cell lines. In conclusion, the combination of TFT with the DNA synthesis inhibitor OHP induces synergism in colorectal cancer cells, but is dependent on the dose and treatment schedule used.

Irinotecan-induced cytotoxicity to colon cancer cells in vitro is stimulated by pre-incubation with trifluorothymidine

SN38 is the active metabolite of the anti-cancer agent irinotecan (CPT-11) and is a potent inhibitor of topoisomerase-I (topo-I), leading to DNA strand breaks and eventually cell death. The pyrimidine analog trifluorothymidine (TFT) is part of the anti-cancer drug formulation TAS-102, which was developed to enhance the bioavailability of TFT in vivo, and is currently being evaluated as an oral chemotherapeutic agent in phase I clinical studies. In this study, the combined cytotoxic effects of dual-targeted TFT with SN38 were investigated in a panel of human colon cancer cell lines (WiDr, H630, Colo320, SNU-C4, SW1116). We used different drug combination treatment schedules of SN38 with TFT, and possible synergism was evaluated using median drug effect analysis resulting in combination indexes (CI), in which CI<0.9 indicates synergism, CI=0.9-1.1 indicates additivity and CI>1.1 indicates antagonism. Drug target analysis was performed to investigate the effect of TFT on SN38-induced DNA damage, cell cycle delay and apoptosis. Simultaneous exposure to SN38 in combination with TFT was not more than additive, whereas pre-incubation with TFT resulted in synergism with SN38 (CI=0.3-0.6). Only for Colo320 synergism could be induced for both simultaneous and sequential drug combinations. SN38 and TFT induced most DNA damage in H630 and Colo320 cells, which was increased in combination. TFT pre-incubation further enhanced SN38-induced DNA strand breaks in H630 and Colo320 (>20%), which was most pronounced in H630 cells (p<0.01). Exposure to SN38 alone induced a clear cell cycle G2M-phase arrest and pre-incubation with TFT enhanced this effect in WiDr and H630 (p<0.05). Both drugs induced significant apoptosis; SN38-induced apoptosis increased significantly in the presence of TFT (p<0.01), either when added simultaneously (about 3-fold) or at pre-incubation (about 2-fold). Topo-I protein levels varied among the cell lines and TFT hardly affected these. In conclusion, TFT pre-incubation can enhance SN38-induced cytotoxicity to colon cancer cells resulting in synergism between the drugs, thereby increasing DNA damage and apoptosis induction.

Low folate conditions may enhance the interaction of trifluorothymidine with antifolates in colon cancer cells

PURPOSE

Trifluorothymidine (TFT) is a fluoropyrimidine that is part of the novel combination metabolite TAS-102, in which TFT is combined with a potent thymidine phosphorylase inhibitor (TPI). TAS-102 is currently tested as an orally chemotherapeutic agent in different schedules in a phase I study. In its monophosphate form, TFT can inhibit thymidylate synthase (TS) activity after binding to the TS-nucleotide binding site leading to dTTP depletion, and in its triphosphate form TFT is incorporated into DNA, eventually leading to DNA damage. In this in vitro study, we investigated whether TFT could potentiate cytotoxicity of the antifolate-based TS inhibitors AG337 (Nolatrexed), ZD1694 (Raltitrexed) and GW1843; and whether increased TS inhibition or DNA damage would be related to this result.

METHODS

The drug combinations were studied in colon cancer cell lines either grown at low or high folate conditions. Multiple drug effect analysis was performed after measuring growth inhibition when the drugs were combined (MTT Assay) and expressed as Combination Index (CI), where CI<0.9 indicates synergism, CI=0.9-1.1 indicates additivity and CI>1.1 indicates antagonism. Drug target analysis was performed using the TS in situ inhibition assay and the FADU DNA-damage assay. Cells were exposed to either the drugs alone or in combination to determine the effect on TS activity and DNA damage induction, respectively.

RESULTS

Three experimental procedures were used to test the interaction of the drugs: either one of the drugs was kept at a constant concentration (IC25) or two drugs were added in a 1:1 IC50-based molar ratio. The combinations of TFT with one of the antifolates in which one of the drugs was kept at a constant concentration were synergistic for all antifolates in WiDr/F cells, which grow in low folate medium (CI=0.6-0.8), but only additive to antagonistic for the cell lines growing in high folate medium: TFT-AG337: CI=0.9-2.3; TFT-ZD1694: CI=0.9-1.3; TFT-GW1843: CI=0.8-1.7. The procedure in which the two drugs were added in a 1:1 IC50-based molar ratio showed antagonism for all three combinations in all cell lines (CI>2.7). TS inhibition (14.3%) and DNA damage (8%) were more pronounced than expected (P<0.05) when TFT was combined with GW1843 in WiDr/F cells, in contrast to AG337 and ZD1694, which showed inhibiting effects as expected (additive).

CONCLUSIONS

The combination of TFT with the antifolates AG337, ZD1694 and GW1843 is mainly additive when the drugs are given simultaneously and this is mediated by an additive TS inhibition and DNA damage. The drug interaction may partly be dependent on the folate homeostasis since WiDr/F cells growing at low folate conditions show pronounced synergism in growth inhibition, two-sided TS inhibition and DNA damage, especially when TFT is combined with the tight-binding TS inhibitor GW1843.

Determinants of trifluorothymidine sensitivity and metabolism in colon and lung cancer cells

Trifluorothymidine (TFT) is a fluorinated thymidine analog that after conversion to its monophosphate derivative can inhibit thymidylate synthase (TS) and be incorporated into DNA. TFT is a good substrate for thymidine phosphorylase (TP), and the combination of TFT and a TP inhibitor (TPI), called TAS-102, has been developed to enhance the bioavailability of TFT in vivo, and is currently being studied in a phase I study. We aimed to determine the limiting factor(s) in the cytotoxicity of TFT with or without TPI to cancer cells. Colon cancer and lung cancer cell lines with either an overexpression or deficiency of one of the enzymes involved in TFT metabolism were used to study the effect of TPI on TFT sensitivity and the role of TS inhibition. The synthesis of radioactive TFT metabolites was studied using thin-layer chromatography together with the incorporation of TFT into DNA. We found that despite a high rate of TFT phosphorolysis, cells with high TP expression are not more resistant to TFT, while TPI did not increase TFT sensitivity. High TS-expressing cells were shown to be cross-resistant to a 72-h exposure to TFT compared to 5-fluorouracil (5-FU), although this was more pronounced at a 4-h exposure (3.4-fold or more for TFT and 1.4-fold or more for 5-FU). Despite a moderate inhibition of TS activity in cells expressing high TS, these cells were more sensitive to TFT than 5-FU (3.8-fold or more). Only in Colo320TP1 cells expressing high TP, inhibition of TFT phosphorolysis by TPI increased formation of active TFT metabolites 1.8-fold, although this was not related to an increase in TFT incorporation into DNA. These studies show that uptake of TFT and subsequent phosphorylation of TFT by cancer cells is very rapid. Despite a high rate of degradation, the activation pathways are still saturated and sufficient to inhibit TS and enable incorporation into DNA, although the contribution of each effect is exposure time dependent.

Intracellular thymidylate synthase inhibition by trifluorothymidine in FM3A cells

Trifluorothymidine (TFT) can be phosphorylated by thymidine kinase (TK) to TFTMP which can inhibit thymidylate synthase (TS), resulting in depletion of thymidine nucleotides. TFT can be degraded by thymidine phosphorylase (TP) which can be inhibited by thymidine phosphorylase inhibitor (TPI). Using the TS in situ Inhibition Assay (TSIA) FM3A breast cancer cells were exposed 4 h or 24 h to TFT and 5-Fluorouracil (5FU). TS activity reduced to 9% (0.1 microM TFT) and 58% (1 microM 5FU) after 4 h exposure and to 6% (TFT) and 21% (5FU) after 24 h exposure. TPI did not affect TS inhibition by TFT. FM3A cells lacking TK or TS activity (FM3A/TK-) were far less sensitive to TFT compared to FM3A cells.

CONCLUSION

TFT can be taken up and activated very rapidly by FM3A cancer cells, probably due to favourable TK enzyme properties, and TPI did not influence this.

A novel combination antimetabolite, TAS-102, exhibits antitumor activity in FU-resistant human cancer cells through a mechanism involving FTD incorporation in DNA

TAS-102 is a new antimetabolite agent composed of a alpha, alpha, alpha-trifluorothymidine (FTD; 1 M) and thymidine phosphorylase inhibitor (TPI; 0.5 M). Here, we investigated the antitumor effect and mechanism of TAS-102 against 5-FU, or FdUrd, resistant human cancer cell lines. The respective tumor growth inhibition rate of orally administered FTD against 5-FU-resistant NUGC-3 was about 70% at a dose level of 200 mg/kg/day; this value was comparable to that against the parental NUGC-3. On the other hand, the tumor inhibition rates of 5-FU, FdUrd, and TS-1 against 5-FU-resistant NUGC-3 were lower than those against parental NUGC-3. Similar observations were made in an FdUrd-resistant human colorectal cancer cell line (DLD-1). TAS-102 was also effective in 5-FU-less sensitive human pancreatic cancer cell lines (PAN-12 and BxPC-3) and human esophagus cancer (T.T.) when compared with 5-FU or UFT. Our hypothesis was that a relatively short and high dosage of TAS-102 results in an additional mechanism of FTD incorporation into DNA other than thymidylate synthase (TS) inhibition. We then examined the effects of FTD on DNA at the cellular level. After treatment with FTD or FdUrd, the DNA fragmentation pattern was examined using filter elution and in situ nick translation. Treatment with FTD for 2 h resulted in marked DNA fragmentation. When the tumor cells were treated with FTD for 72 h or with FdUrd for 2 or 72 h, only a small amount of DNA fragmentation was observed, and the appearance of the tumor cells did not differ markedly from that of untreated cells. Moreover, the DNA fragmentation rate in the TAS-102 treatment group was significantly higher than that in the control group in vivo. These results suggest that when tumor cells are exposed to high concentrations of FTD for short periods of time, FTD manifests its antitumor activity primarily through the induction of DNA fragmentation after FTD incorporation into the DNA. We conclude that TAS-102 is expected to manifest antitumor effects against 5-FU-resistant tumors that are similar to those exerted in 5-FU-sensitive tumors.

A novel antimetabolite, TAS-102 retains its effect on FU-related resistant cancer cells

TAS-102 is a new oral anti-tumor drug preparation, composed of a 1:0.5 mixture (on a molar basis) of alpha,alpha,alpha-tri-fluorothymidine (FTD) and thymidine phosphorylase inhibitor (TPI). TAS-102 is currently undergoing clinical trials, and has been demonstrated to have at least 2 mechanisms; inhibition of thymidylate synthase (TS) and incorporation into DNA. 5-FU is widely used in the treatment of solid tumor, but the inherent or acquired resistance of certain tumors to 5-FU therapy is a major clinical problem. In the present study, we investigated FTD in vitro and in vivo comparing with 5-FU and using FU-resistant cells. There was no relationship between FTD and 5-FU growth inhibition effect in vitro. A different sensitivity pattern was observed by the log-mean graph. We next investigated the anti-tumor activity of TAS-102 in a FU-resistant xenograft model. Comparative efficacy was observed between FU-resistant cell and its parent cell. We also studied the influence of TAS-102 on liver metastasis in a mouse model of human colorectal cancer, because liver metastasis of colorectal cancer is associated with patient survival. Human cancer DNA was detected by PCR, and TAS-102 markedly inhibited the number of liver metastasis. A novel angiogenic factor, platelet-derived endothelial cell growth factor (PD-ECGF), was shown to be identical to a previously characterized intracellular enzyme, thymidine phosphorylase, TAS-102 can be expected to have not only anti-tumor cytocidal effects but also antiangiogenesis activity and may inhibit liver metastasis. Our findings suggested that TAS-102 is a promising candidate for clinical use and can be expected to decrease minimal residual disease.

An optimal dosing schedule for a novel combination antimetabolite, TAS-102, based on its intracellular metabolism and its incorporation into DNA

TAS-102 is a combination drug consisting of alpha,alpha,alpha-trifluorothymidine (FTD) and thymidine phosphorylase inhibitor (TPI). FTD is converted to F3TMP by thymidine kinase and inhibits the thymidylate synthetase (TS) activity by binding to TS. In addition, FTD triphosphate form, F3TTP is incorporated into DNA, which leads to cytocidal effects. Therefore, the incorporation of FTD into DNA is expected to be an important factor, discriminating it from 5-FU showing TS inhibitory activity as their main mechanism of action. To assess a clinically more effective regimen protocol, the intracellular metabolism and the incorporation of FTD into DNA were investigated using human cancer cell lines in vitro and in vivo. FTD was incorporated into DNA in a time-dependent manner, but not in a concentration-dependent manner. FTD was the most efficiently incorporated into DNA after treatment with a several-micro molar level of FTD for around 8 h. The intracellular F3TTP was rapidly eliminated from tumor cells, as soon as FTD was washed out from the culture medium, whereas the FTD incorporated into DNA was retained by 80% or more even at 24 h after a washing-out procedure. When TAS-102 was administered into tumor-bearing mice once daily or three times daily at 3-h intervals at a dose of 150 mg/kg/day for one or 3 consecutive days, incorporation of FTD into tumor DNA by divided dosing significantly higher than that of single dosing. Based on our findings, the antitumor effects of TAS-102 against 3 different human cancer cell xenografts into mice were examined. The divided daily dosing resulted in enhancement of the antitumor effects of TAS-102 without any additional side effects. It is concluded that multiple daily dosing may result in better clinical benefits of TAS-102, when compared with single daily dosing and TAS-102 is a promising candidate for not only FU-sensitive but also FU-resistant cancer patients.

Role of platelet-derived endothelial cell growth factor/thymidine phosphorylase in fluoropyrimidine sensitivity

Platelet-derived endothelial cell growth factor (PD-ECGF)/thymidine phosphorylase (TP) catalyses the reversible phosphorolysis of thymidine to thymine and 2-deoxyribose-1-phosphate and is involved in the metabolism of fluoropyrimidines. It can also activate 5'-deoxyfluorouridine (5'DFUR) and possibly 5-fluorouracil (5FU) and Ftorafur (Ft), but inactivates trifluorothymidine (TFT). We studied the contribution of TP activity to the sensitivity for these fluoropyrimidines by modulating its activity and/or expression level in colon and lung cancer cells using a specific inhibitor of TP (TPI) or by overproduction of TP via stable transfection of human TP. Expression was analysed using competitive template-RT-PCR (CT-RT-PCR), Western blot and an activity assay. TP activity ranged from nondetectable to 70678 pmol h(-1) 10(-6) cells, in Colo320 and a TP overexpressing clone Colo320TP1, respectively. We found a good correlation between TP activity and mRNA expression (r=0.964, P&<0.01) in our cell panel. To determine the role of TP in the sensitivity to 5FU, 5'DFUR, Ft and TFT, cells were cultured with the various fluoropyrimidines with or without TPI and differences in IC(50)'s were established. TPI modified 5'DFUR, increasing the IC(50)'s 2.5- to 1396-fold in WiDR and Colo320TP1, respectively. 5-Fluorouracil could be modified by inhibiting TP but to a lesser extent than 5'DFUR: IC(50)'s increased 1.9- to 14.7-fold for WiDR and Colo320TP1, respectively. There was no effect on TFT or Ft. There appears to be a threshold level of TP activity to influence the 5'DFUR and 5FU sensitivity, which is higher for 5FU. Even high levels of TP overexpression only had a moderate effect on 5FU sensitivity.

Effect of famciclovir on herpes simplex virus type 1 corneal disease and establishment of latency in rabbits

Famciclovir (FCV) is efficacious in the treatment of acute herpes zoster and recurrent genital infections but has not been used to treat ocular herpes simplex virus (HSV) infections. We evaluated the efficacy of orally administered FCV in treating HSV-1 epithelial keratitis and determined its effects on the establishment of latency and subsequent reactivation. Rabbits were inoculated with HSV-1 strain 17 syn+ and treated twice daily with increasing concentrations of FCV (60 to 500 mg/kg of body weight). This resulted in a significant, dose-dependent improvement in keratitis scores, as well as prolonged survival. Regardless of the dose of drug used, all groups exhibited the high rates of spontaneous and induced reactivation characteristic of 17syn+. The efficacy of 250 mg of FCV per kg was also compared to topical treatment with 1% trifluorothymidine (TFT). Although TFT treatment was more effective at reducing eye disease, FCV-treated rabbits had a better survival rate. Real-time quantitative PCR analysis of rabbit trigeminal ganglia (TG) demonstrated that FCV significantly reduced the HSV-1 copy number compared to that after treatment with TFT or the placebo but not in a dose-dependent manner. In summary, oral FCV treatment significantly reduces the severity of corneal lesions, reduces the number of HSV-1 genomes in the TG, improves survival, and therefore may be beneficial in reducing the morbidity of HSV keratitis in the clinic.

In vitro effects of antiviral agents on human keratocytes

PURPOSE

To study the effects of antiviral agents on human keratocytes in vitro.

METHODS

Cultured human keratocytes were incubated with either ganciclovir, idoxuridine, trifluridine, or cidofovir at concentrations from 0.0001 to 10 mg/mL. Phase-contrast microscopy and XTT (sodium [2,3-bis [2-methoxy-4-nitro-5-sulphophenyl]-2h-tetrazolium-5-carboxanilide, inner salt) colorimetric assay were performed after 24, 48, and 72 hours of incubation.

RESULTS

When adjustments were made for time of incubation and concentration, trifluridine reduced cell viability significantly more than ganciclovir, idoxuridine, and cidofovir (p<0.001, three-way analysis of variance). There was significant time-and dose-dependent reduction of cell viability (p<0.001) with trifluridine and cidofovir. After a 72-hour incubation with ganciclovir or idoxuridine, cell viability was reduced as compared with 24- and 48-hour incubation (p<0.001); only the effects of the highest concentration tested (1.0 mg/mL) were significantly different from those of the lower concentrations (p<0.002). At a concentration of 1.0 mg/mL, trifluridine and cidofovir produced moderate to severe signs of cytotoxicity, whereas ganciclovir and idoxuridine displayed much less severe morphologic signs.

CONCLUSIONS

Our results indicate that antiviral agents may have both time- and concentration-related toxic effects on stromal keratocytes. These findings may impact the selection of the most appropriate antiviral drug when it is needed to treat infections involving the corneal stroma.

Efficacy of cobalt chelates in the rabbit eye model for epithelial herpetic keratitis

PURPOSE

A new class of antiviral agent, cobalt chelates (the CTC series), was evaluated for treating epithelial herpetic keratitis, consequent stromal disease being the major infectious cause of blindness in industrial nations.

METHODS

Effects of CTC complexes were monitored in cell cultures and in a rabbit eye model, either infected with herpes simplex virus type 1 (HSV-1) or uninfected. Several antiviral concentrations of CTC complexes nontoxic to Vero cells were administered to rabbit eyes with HSV-1-induced keratitis. Corneal surface virus titers were measured, and corneal lesions of epithelial keratitis were monitored by slit-lamp microscopy and scored. Recovery rates and incidence were compared in eyes treated with CTC complexes, placebo, or clinically formulated trifluorothymidine (Viroptic), using nonparametric statistics.

RESULTS

All CTC complexes inhibited HSV-1 replication in vitro, CTC-96 being best. CTC-96, CTC-23, and CTC-67 eliminated (<1 plaque-forming unit[pfu]) corneal surface HSV-1 (otherwise >10(5) pfu) in order of descending potency, but CTC-82 was ineffective. CTC-96 (either 5 microg/ml six times daily or 10 microg/ml five times daily) accelerated herpetic dendritic keratitis recovery better than or the same as trifluorothymidine (10 mg/ml nine times daily). CTC complexes were nontoxic to Vero cells continuously exposed to < or =25 microg/ml; 50 microg/ml of CTC 96 nine times daily did not irritate uninfected rabbit eyes.

CONCLUSION

Topical CTC-96 applications were at least as effective as Viroptic in diminishing disease signs and corneal surface virus at concentrations less than one-thousandth that of Viroptic.

p53, mutations, and apoptosis in genistein-exposed human lymphoblastoid cells

The phytoestrogen, genistein, is a naturally occurring isoflavone found in soy products. On a biochemical basis, genistein is a competitive inhibitor of tyrosine kinases and the DNA synthesis-related enzyme, topoisomerase-II (topo-II). Exposure of mammalian cells to genistein results in DNA damage that is similar to that induced by the topo-II inhibitor and chromosomal mutagen, m-amsa. In order to determine the potential genotoxicity of genistein, human lymphoblastoid cells which differ in the functional status of the tumor suppressor gene, p53, were exposed to genistein and the induction of micronuclei quantified by microscopic analysis. In addition, the mutant fraction at the thymidine kinase (tk) locus (both the normal-growth and slow-growth phenotypes) was determined by resistance to trifluorothymidine (TFT) and at the hypoxanthine phosphoribosyl transferase (hprt) locus by resistance to 6-thioguanine (6-TG). Flow cytometric analysis of the percentage of viable, apoptotic and degenerating cells was utilized to determine the rate and kinetics of cell death after genistein exposure. The detection of micronuclei in both cell lines indicated that genistein-induced damage had occurred in both AHH-1 tk+/- and L3. Linear regression analysis detected a significant increase in the number of 6-TG-resistant clones in both AHH-1 tk+/- (p53+/-) and L3 (p53+/+). A comparison of slopes revealed no difference between the lines. In contrast, a significant, concentration-dependent increase in the number of TFT-resistant clones with the slow-growth phenotype was detected in AHH-1 tk+/- (mutant p53), but not in L3 (wild-type p53). Cell death occurred primarily by apoptosis in both cell lines; however, a concentration-dependent decrease in the percentage of viable cells was detected immediately after exposure in L3, but not until 32 h after exposure in AHH-1 tk+/-. A comparison of the slopes of the concentration-response curves for the percentage of viable cells revealed no difference between the cell lines in the effect of genistein on cell viability. Our results may be interpreted that genistein is a chromosomal mutagen and that p53 functional status affects the recovery of chromosomal mutants, possibly by signalling cells into the apoptosis pathways.

Chromosome painting analysis of spontaneous and methyl methanesulfonate-induced trifluorothymidine-resistant L5178Y cell colonies

Spontaneous and methyl methanesulfonate-induced trifluorothymidine-resistant mutants in mouse lymphoma L5178Y cells were analyzed using fluorescence in situ hybridization with mouse probes specific for chromosome 11, on which the tk gene is located, and chromosome 3, as the control. 76.5% (13/17) of small-colony mutants (thought to be the result of chromosomal mutation) and 28.6% (4/14) of large-colony mutants (thought to be the result of gene mutation) showed rearranged chromosome 11. Of the mutants with abnormal chromosome 11 painting pattern, 5 small- and 2 large-colony mutants carried clonal aberrations, while the remaining 8 small- and 2 large-colony mutants showed mosaic aberrations. Most abnormalities in the small-colony mutants involved the distal region of one painted chromosome 11, where the tk+ gene maps. An increase, rather than a decrease, in chromosome 11 material was found in a majority of abnormally painted mutants. On the contrary, no rearrangements involving chromosome 3 were found in any small- and large-colony mutants analyzed except one large-colony mutant, which showed chromosome rearrangements involving both chromosome 11 and 3. The present study confirms that the majority of small-colony mutants in L5178Y cells have chromosome 11 rearrangements that can be detected by chromosome painting and that the majority of the chromosomal abnormalities in TFT-resistant mutants involved complex rearrangements.

In vitro sensitivity of macropodid herpesvirus 2 to selected anti-herpetic compounds

We tested the in vitro sensitivity of Macropodid Herpesvirus 2 to eight commonly used anti-herpetic compounds using plaque reduction tests, March and April, 1995. The virus was most susceptible to inhibition by (E)-5-(2'-bromovinyl)-2'-deoxyuridine and adenine 9-beta-D-arabino-furanoside. Both compounds have been used for anti-herpetic therapy in humans and may prove useful in the treatment of macropodoids in captivity.

Development of a novel mouse tk+/- embryonic stem cell line for use in mutagenicity studies

A tk+/- mouse embryonic stem (ES) cell line, designated 1G2, has been created in which one allele of the thymidine kinase (tk) gene was inactivated by targeted homologous recombination. This line is an analog of the mouse lymphoma tk+/- L5178Y cell line, which is used widely to assess the mutagenicity of chemical agents. Treatment of 1G2 cells with the alkylating agent N-ethyl-N-nitrosourea (ENU) resulted in a dose-related increase in trifluorothymidine-resistant colonies. Mutant frequencies of 152 and 296 per 10(6) cells were determined for 0.1 and 0.3 mg/ml doses of ENU, compared with a spontaneous mutant frequency of 15 per 10(6) cells. The data indicate that tk+/- 1G2 ES cells may be useful for the creation of a transgenic mouse model for assessing in vivo mutation using an endogenous autosomal gene.

rTS gene expression is associated with altered cell sensitivity to thymidylate synthase inhibitors

rTS is a recently discovered gene, phylogenetically conserved and found to be expressed in a wide variety of cell lines. rTS has also been found to be overexpressed in two cell lines resistant to FU and to MTX. The MTX-resistant cell line was found to have a high degree of cross resistance to several TS inhibitors. An apparent paradox to this correlation of rTS overexpression and resistance to TS inhibitors is the observation that expression of transfected rTS alpha results in enhanced sensitivity of cells to the TS inhibitor prodrug TFT and a modest increase in resistance to FUdR. Since immunoprecipitation of TS leads to the co-immunoprecipitation of two proteins within the expected molecular weight range of the two rTS proteins, it may be that both proteins bind to TS in vivo and modify its activity. Preliminary data substantiate this conclusion. It is conceivable that the ratio of the two rTS proteins associated with TS in vivo may differentially alter TS activity depending upon their stoichiometry or possibly posttranslational modification. Thus it may be possible for rTS to confer greater sensitivity to one pyrimidine analog (e.g., TFT) which is a product analog but to increase resistance or have a minor effect on a substrate analog (e.g., FdUMP) by stabilizing different conformations of TS. The structure of the rTS proteins suggests they are expected to have catalytic activity which involves proton abstraction from an alpha-carbon of a carboxyl group. Whether this enzyme activity is functional and related to pyrimidine metabolism awaits further study.

The use of L5178Y mouse lymphoma cells to assess the mutagenic, clastogenic and aneugenic properties of chemicals

Guidelines have been proposed to assess the potential of chemicals to affect human health. Written into these guidelines is the requirement that information be submitted on mutagenic activity. Although regulatory agencies accept mutagenicity data from both the hprt and tk loci in mammalian cells, many studies suggest that the L5178Y mouse lymphoma assay at the thymidine kinase locus is likely to detect a greater spectrum of mutagenic lesions. Thus, there is increasing emphasis being placed on this assay in many proposed and published guidelines. The L5178Y mouse lymphoma suspension protocol produces both small and large colonies which are the products of mutants growing at different rates. There is a reduction in the proportion of slowly growing mutants with respect to the total population of cells when expression is carried out in suspension. This potentially leads to quantitatively inaccurate assessments of the mutagenic activity of chemicals. Therefore an in situ procedure was developed that more accurately assesses the mutagenic activity of chemicals by maximizing the detection of small colonies. Many guidelines recommend tests that assess the clastogenic activity of chemicals. Some regulatory agencies accept data from the mouse lymphoma mutation assay to detect clastogens if the protocol is optimized for the detection of small colonies or if colony sizing data are submitted. The conventional suspension assay protocol is not sufficiently validated for this purpose. The in situ protocol has greater potential to meet these requirements.(ABSTRACT TRUNCATED AT 250 WORDS)

HPMPC, a broad-spectrum topical antiviral agent, inhibits herpes simplex virus type 1 replication and promotes healing of dendritic keratitis in the New Zealand rabbit ocular model

Previously, we demonstrated that HPMPC, a new, broad-spectrum antiviral agent, inhibited adenovirus type 5 in the New Zealand (NZ) rabbit ocular model (Cornea 1992; 11:529-33). Historically, no antiviral agent has been demonstrated to be effective against both herpes simplex virus type 1 (HSV-1) and adenovirus eye infections in an experimental animal model. In this study, we compared topical 0.2% HPMPC to 1% trifluridine and vehicle control in the NZ rabbit HSV-1 keratitis model. Using a double-masked, two-eye design, NZ rabbits were inoculated in both eyes with HSV-1 W strain (10(5) pfu/eye), and dendritic keratitis and HSV-1 ocular titers were measured serially. Compared with the control group, both topical 0.2% HPMPC and 1% trifluridine significantly reduced healing time of HSV-1 dendritic keratitis, lowered HSV-1 ocular titers on days 3 through 11, and shortened duration of HSV-1 shedding in the tear film. For all outcome parameters measured, topical 0.2% HPMPC was as effective as 1% trifluridine. A new concept of a broad-spectrum topical antiviral agent was shown to be effective against HSV-1 in an NZ rabbit keratitis model, and further development toward clinical application appears desirable.

Is micronucleus induction by aneugens an early event leading to mutagenesis?

This study was designed to investigate a previously unidentified potential mechanism for mutation induction as well as to clarify a biological consequence of micronuclei with mutation induction as measured by trifluorothymidine (TFT) resistance in mouse L5178Y cells using four aneugens: colcemid, diethylstilbestrol, griseofulvin and vinblastine. All four compounds induced micronuclei which appeared in the first cell cycle after treatment. More than 85% of the micronuclei induced by each compound stained positive for the presence of kinetochores implying that the micronuclei contained whole chromosomes. However, these same compounds were unable to induce TFT resistance under three different treatment regimes. We concluded that these compounds, under conditions where they induce primarily kinetochore positive micronuclei, were not able to induce mutations. Thus, the induction of micronuclei containing whole chromosomes harboring a selectable gene is not an early event leading to mutations in these cells.

Effect of genetically determined host factors on the efficacy of vidarabine, acyclovir and 5-trifluorothymidine in herpes simplex virus type 1 infection

The susceptibility of Herpes simplex virus (HSV) to acyclovir (ACV), 5-trifluorothymidine (TFT) and vidarabine (Ara-A) in HSV-infected embryo fibroblasts from BALB/c and C57BL/6 mice as well as Vero cells were measured. Ara-A and TFT (at its highest concentration) were more effective in Vero cells and BALB/c mouse embryo fibroblasts (MEF) than in C57BL/6 MEF. In contrast, ACV was more effective in C57BL/6 MEF than BALB/c MEF and Vero cells. These data suggest that genetically determined differences in the ability of host cells to support the replication of HSV influence the activity of antiviral drugs.