Breast cancer is the leading cause of cancer-related death in women(1). It is a heterogeneous disease comprising different subtypes according to estrogen, progesterone and human epidermal growth factor 2 receptors expression which varies in terms of morphology, biology and prognostic(2).
International adopted classification, according to immunohistochemical staining of estrogen, progesterone and human epidermal growth factor 2 receptors and KI67 proliferation marker distinguishes four principal entities of breast cancer, of which two expressing hormone receptors (luminal A, luminal B), one only HER2 (HER2 positive) and one highly proliferative, negative for all specific receptors (triple negative)(3).
Triple negative breast cancer (TNBC) accounts for 10-20% of all invasive breast cancers(4) and is defined by American Society of Clinical Oncology/College of American Pathology (ASCO/CAP) panel as less than 1 percent staining of tumor cells by IHC for hormone receptors either 0-1+ or 2+ score and fluorescence in situ hybridization (FISH)-negative for HER 2(5). It represents a surrogate for basal-like subtype defined as microarray expression of genes usually found in myoepithelial cells of the breast, including those encoding basal cytokeratin (CK) 5/6, 14 and 17 which are normally found in the basal layer of stratified epithelium, representing about 75% of triple negative(1,6).
General characteristics of TNBC are the prevalence in young age, large tumor size, high histological and proliferation rate, pushing border of invasion and high rate of advanced stages at diagnostic(7).
Prognosis is poor even in early stages: 5-year relapse rate in stage I-III patients, that underwent adjuvant chemotherapy, is about 30%, the median time to metastatic recurrence is less than 3 years and the 5-year survival rate after the metastatic event is less than 30%(8).The risk to develop visceral metastases is three times higher than in other types of breast cancer, frequently in lung and brain(9). In very early stages, T1N0-1, 5-year disease free survival (DFS) and cancer-specific survival are approximately 85%(10).
Highly proliferative tumors, such as TNBC, have an enhanced angiogenesis that supports rapid growth and early metastasis, being correlated with levels of vascular endothelial growth factor(11). Inside TNBC by gene expressing analysis, it has been identified a mesenchymal stem-like (MSL) subtype enriched in genes involved in angiogenesis and one immunomodulatory (IM) highly enriched in gene involved in immune cell processes, markers and signaling, the last one being found in 20% of cases(12).
In localized disease, in the absence of validated maintenance adjuvant treatment, the cytotoxic chemotherapy is still the mainstay for TNBC, despite the promise of new targeted and biologic agents(13).
Metronomic chemotherapy (MT) refers to the chronic administration of low doses of cytotoxic drugs at close and regular intervals with no prolonged drug-free interruptions(14). Low-doses schedule was found to affect not only tumor cells, but also their microenvironment and repairing process of endothelial cells, leading to an anti-angiogenic effect(15).
In vitro studies showed that low dose of cyclophosphamide results in cytoskeleton alteration, enhancement of antioagiogenic factors expression (such as TSP-1) or induction of pro-apoptotic signal (NF-kB)(16).
In addition, MT exerts an immunological action through the restoration of the anticancer effect of immune system(17).
MT efficacy has shown promising activity in metastatic setting in several solid tumors including breast cancer(18). By general low profile toxicity, this schedule showed to be feasible for long term administration including in adjuvant setting(19).
Thus, the setting of metronomic chemotherapy in operable TNBC seems to be adjuvant either after neoadjuvant chemotherapy (CHT) especially in non pCR, either in continuation of standard adjuvant treatment.
A general review about the role that MT might have as adjuvant treatment in operable triple negative breast cancer was herein conducted.
The role of metronomic chemotherapy
Most studied anticancer agents as metronomic schedule, in breast cancer, belong to alkylating agents class that interact with DNA molecules (cyclophosphamide), antimetabolites, interfering with DNA and RNA synthesis (methotrexate, capecitabine) and vinca alcaloids that blocks microtubules formations, cancer cells accumulation in mitosis and deaths by apoptosis(20).
In vitro exposure to 4-hydroxi-cyclophospmide showed that high doses have a direct cytotoxic activity, whereas the lowest doses inhibit the endothelial proliferation(21). In a prospective observational study (n=63) serum VEGF level drop down below to median baseline value was significantly correlated with clinical benefit of low-dose oral methotrexate (MTX) and cyclophosphamide (CTX) in patients with metastatic breast cancer(22).
Low doses of CTX have also been shown to modulate tumor microenvironment cells, including pericytes and tumor infiltrating lymphoid cells(23). Furthermore, several immunological effects have been reported. In mice models, metronomic cytotoxic administration induces a decrease in level of several immunosupresive cytokines, such as TGF-b, IL10, and IL2(24). Dendritic cells maturation is also stimulated, resulting in prolonged memory T cells survival(25). Low doses of CTX showed to stimulate Galactin-1 expression, a down regulator of cells proliferation and dendritic cells differentiator(26).
Generally used metronomic regimens
In breast cancer, metronomic regimens were firstly used and related ongoing studies are mainly conducted in metastatic setting.
Cyclophosphamide is the first cytotoxic drug studied in MT administration in pretreated metastatic breast cancer (MBC) patients, by Colleoni et al., in 2002. The dose was fifty milligrams daily associating methotrexate, 2.5 mg twice daily, 2 days per week. Clinical benefit initially reported was of 24 weeks with no grade 3, 4 adverse reactions(27). A subsequent report showed a median time to progression in patients with prolonged clinical benefit of 21 months, ranging from 12 to 37 months(28).
Other anticancer agents tested in low doses continuous schedules, either in monotherapy or in combination, are capecitabine and vinorelbine.
Capecitabine in low doses, continuously 650 mg/m2 - 800 mg/m2 bid, seems to be similar in terms of efficacy to standard regimen either in pretreated or in first line MBC(29).
In sixty heavily treated patients, 1500 mg capecitabine single daily dose led to a clinical benefit (CB) of 62% and an overall survival (OS) of 17 months. Grade 3 toxicities, hand and foot syndrome and neutropenia were described in less than 10% of cases(30).
The pharmacokinetic profile permitted vinorelbine oral formulation to be administrated in metronomic schedules. In monotherapy in non-treated elderly patients (n=34) a weekly dose of 70 mg/m2, fractionated on days 1, 3, and 5, for 3 weeks of 4, secured a 38% overall response rate (ORR), a median progression free survival (PFS) and median OS of 7.7 and 15.9 months respectively(31).
Due to the observed in vitro synergetic effect of aforementioned cytotoxic agents, combinations in metronomic schedule have been proposed. In addition to clinical benefit reported by Calleoni and colleagues, a phase II study testing cyclophosphamide - metotrexat association found a good tolerability even in pretreated patients(32).
Dose limiting toxicity (DLT) for capecitabine - vinorelbine association was reached at 70 mg for oral metronomic vinorelbine and at 1250 mg/m2 for capecitabine(33). CB reported for a regimen of fixed dose of capecitabine (500 mg three times a day) and vinorelbine (40 mg three times a week) was of 58.1% in 31 pretreated MBC(34).
Even though the chemotherapy - hormonal combination isn’t feasible because of contradictory antitumor mechanism, metronomic schedules acting mainly through antiangiogenic mechanism and less through cytotoxic activity might allow the concomitant administration of the two classes of anticancer agents(35). In a phase II study, Schwartzberg et al. explored the activity of fulvestrant and low-dose metronomic capecitabine in 41 MBC patients with positive hormone receptor and negative HER2. The median duration of therapy was 11 months, with a median PFS of 14.98 months, and a median time to progression (TTP) of 26.94 months, and the tolerance was good(36). Safety and antitumor activity of a cyclophosphamide - megestrol acetate combination was evaluated in twenty-nine pretreated post-menopausal MBC patients. The treatment consisted in cyclophosphamide 50 mg daily, days 1-21 of 28 and fractionated megestrol acetate 80 mg twice a day. The overall ORR was 31% and the mean OS was 13.4 months(37).
Antiangiogenic effect of MC might be synergetic with targeted approved agents. In HER2 overexpressed or amplified small sample population (n=22), the combination of trastuzumab and metronomic CM in trastuzumab pretreated MBC patients, the CB was 46% and the median time to progression was 6 months(38).
Regarding bevacizumab, added to metronomic cyclophosphamide containing regimen, in pretreated population, allowed a 63% clinical benefit (CB), with a mild toxicity(39). In non-pretreated triple negative breast cancer, a phase II study, bevacizumab combined with erlotinib along with cyclophosphamide and capecitabine combination reported (n=24) an ORR of 62% and a CB of 75%. Grade 3 toxicity, including diarrhea, thrombosis and hypertension, was reported in 16% of cases(40).
In 2014, at the ASCO meeting, the results of a randomized phase III trial of bevacizumab therapy with either weekly paclitaxel or daily oral metronomic capecitabine and cyclophosphamide as first-line treatment in 147 HER2-negative MBC patients showed no difference in term of ORR (58% vs. 50%, p=0.45) or incidence of grade 3-5 adverse events (25% vs. 24%). Nevertheless, a statistically difference has been reported regarding the quality of life in favor of metronomic regimen(41).
The metronomic chemotherapy in non-metastatic TNBC
In operable triple negative breast cancer, due to very low level of expression of estrogen, progesterone and HER2 receptors, cytotoxic chemotherapy remains the mainstay and there is no validated maintenance adjuvant treatment(42).
Several metronomic regimens have been evaluated for safety and benefit in adjuvant setting, in early stages of TNBC. Two years of uracil tegafur (UFT), an oral fluropirimidine approved in metastatic colorectal cancer, assessed as adjuvant treatment, was found as effective as 6 cycles of classic CMF in high risk breast cancer, node negative population, primary treated by surgery: OS 96.2% vs 96%, HR for RFS 0.98 (p=0.92). Stratification along hormonal receptor status was preplanned and most of the patients had invasive ductal histology (>90%), T2 stage, in both arms. In ER-PgR- population, hazard ratio was in favor of CMF regimen (HR: 1, 1 95% CI [0,6-2,2](43).
The two-year analysis of a phase III study, early closed due to inadequate accrual, showed that in older cancer patients with operable ER-PgR - breast cancer, 16-week adjuvant metronomic cyclophosphamide and methotrexate was associated with a better quality of life and cognitive function as compared to pegylated liposomal doxorubicin (PLD). At a median follow-up of 42 months, 19% of the 77 enrolled patients experienced a breast cancer related recurrence. The 3-year Kaplan-Meier estimate of breast cancer free interval was similar for the PLD and non-PLD group of 78%(44).
Always referring to elderly patients, when single-agent capecitabine adjuvant treatment was compared to standard chemotherapy (classical CMF or AC) in the CALGB49907 trial, which included women aged 65 years or older with early-stage breast cancer, it was found inferior in terms of survival. Among patients with hormone-receptor-negative tumors who received capecitabine, the risk of relapse was more than quadrupled (HR 4.39, 95% CI [2.9 - 6.7], p<0.001) and the risk of death was more than tripled (HR 3.76, 95% CI [2.23 - 6.34] p<0.001), as compared with patients in all other study groups combined(45).
Based on the synergism observed in xenograft models between cyclophosphamide, docetaxel and capecitabine, the FINXX trial questioned the benefit of adding fluoropirimidine to three cycles of docetaxel (TX) followed by three cycles of cyclophosphamide, epirubicin and capecitabine (CEX) in axillary node-positive or high-risk node-negative breast cancer patients(46). Capecitabine schedule was 900 mg/m2 given twice daily on day 1 to day 15 every 3 weeks. In a post hoc exploratory analysis, TX/CEX sequence was more effective than T/CEF (HR= 0.48; 95% CI [0.26-0.88], p=0.018) in triple negative subgroup. Regarding the regimen’s safety, four of six deaths reported in experimental arm were treatment-related. Treatment discontinuation was more often recorded in patients assigned to receive capecitabine: 24% (178 patients) versus 3% (23 patients), p=0.001(47).
At SABC 2010, there have been presented the first safety data from a large randomize trial (n=816) assessing the survival benefit of capecitabine maintenance therapy after standard treatment in early stages of TNBC. The preliminary report in 400 patients who have received 8 cycles of capecitabine 1000 mg/m2 twice daily, for fourteen days, every three weeks, found 25.7% of experimental related grade 3-4 adverse events of which 17.4% grade 3 hand-foot syndrome. The planned dose was administrated in 77.3% of cases(48).
Two Egyptian phase II studies explored the safety of adjuvant extended administration of capecitabine in adjuvant setting in specific population. In one study conducted between June 2010 and December 2013, in 41 early TNBC, capecitabine was administrated continuously for six months, 500 mg PO twice daily, following standard adjuvant treatment (six cycles of adjuvant FEC100 ± postoperative radiotherapy). The tolerance was good, the adverse events registered being only of grade I. Survival estimations were also appealing, mean DFS being of 42.4 months (95% CI [39.02 - 45.79]) and mean OS of 44.34 months (95% [CI 41.9 - 46.9])(49). The aim of the other study was the efficacy of 1 year of oral metronomic capecitabine (650 mg/m2, twice every day), after completion of standard adjuvant treatment. In nineteen TNBC patients, the median DFS was 41.7 months ± 2.7 (95% CI [36.5 - 46.9]), the actuarial rate of DFS was 88.8% and 82.05% at 2 and 3 years respectively. Related to safety, there was reported only one case of grade 3 hand-foot syndrome and one case of diarrhea. All patients completed the proposed treatment without dose reduction(50).
Recently, the results of a phase III trial have been published, that evaluated the efficacy of 12 months of maintenance of cyclophosphamide (50 mg/day) and methotrexate (2.5 mg/twice a day orally, days 1 and 2 of every week) in early breast cancer patients displaying less than 10% staining for estrogen and progesterone receptor in tumor cells. After a median follow-up of 6.9 months, DFS wasn’t found significant either in the experimental group - HR=0.84 (95% CI [0.66 -1.06], p =0.14) - or in TN patients, HR=0.72 (95% CI [0.49 - 1.05]. Grade 3 or 4 treatment-related adverse events were reported in 14% (64 patients) of cases, of which elevated serum transaminases being the most frequently reported (7%), followed by leukopenia (2%)(51).
Several ongoing phase II-III studies are testing perioperative MT, specific in operable TN population, beyond standard adjuvant treatment, mostly comprising CTX. The results of a pilot study, assessing one year of adjuvant bevacizumab with or without cyclophosphamide and methotrexate in women with operable breast cancer, are pending (Harold J. Burstein, MD, PhD NCT00121134).
Metronomic maintenance chemotherapy is feasible in terms of toxicities to be administrated in adjuvant setting in operable triple negative breast cancer, following standard treatment.
There is an important need of adjuvant maintenance treatment in non-metastatic hormone-negative, HER2-negative breast cancer.
Survival benefit of this approach is still under investigation and remains to be proven.
In the era of immunotherapy, metronomic chemotherapy, through its immunomodulatory mechanism, might be an affordable surrogate for this type of treatment. n
Conflict of interest: The author declares no conflict of interest.