Combination chemotherapy with mitomycin C and methotrexate is active against metastatic HER2-negative breast cancer even after treatment with anthracycline, taxane, capecitabine, and vinorelbine

MM treatment yielded relatively good results as a late line of treatment for metastatic
breast cancer even after heavy treatment with ATCV.

MMC, which acts as an alkylating agent, has been used in combination chemotherapy
for various tumors since 1974. MMC binds to DNA to form an interstrand DNA–DNA cross-link,
which is believed to cause its cytotoxic effect. MTX inhibits dihydrofolate reductase,
which halts the cell cycle by decreasing the synthesis of thymidylate and purine nucleotide.

MTX is a folate analog designed to inhibit dihydrofolate reductase. Reduced folate
is the proximal single carbon donor in several reactions involved in the de novo synthetic
pathways for purine and pyrimidine precursors of DNA and RNA that are required for
cell proliferation.

While new drugs are currently being developed, several conventional drugs such as
MMC and MTX have become less often administered. But these useful drugs, well documented
for efficacy and safety, have the potential to control MBC even after aggressive treatment
with multiple regimens because MMC and MTX, respectively, have pharmacological mechanisms
that are different from previously used anti-cancer drugs. When considering the biological
function of anti-cancer action and cross-resistance, it has been speculated that drugs
with the same pharmacological mechanisms as previously used drugs would not have enough
power to control metastatic breast cancer after ATCV. Capecitabine and S-1 are known
as prodrugs of Fluorouracil (FU) and were selectively activated by tumor cells enough
to exert cytotoxic activity. Although they may seem to be newly administered drugs,
fluorouracil derivatives such as FU have already been used with anthracycline regimens
such as CAF (Cyclophosphamide, Adriamycin, and FU) or CEF (Cyclophosphamide, Epirubicin,
and FU) in most MBC patients. Therefore, it is supposed that in some cases, resistance
to FU might have been established before administration of capecitabine and S-1.

Vinorelbine binds to tubulin, inhibiting tubulin polymerization into microtubules.
Spindle formation leads to apoptosis of cancer cells. Vinorelbine’s mitotic microtubule-inhibiting
activity correlates with its antitumor efficacy (Toi et al. 2005]). Eribulin, which is a non-taxane inhibitor of microtubule used as monotherapy (Cortes
et al. 2011]), binds to the vinca domain of tubulin and inhibits polymerization of tubulin and
assembly of microtubules. In vinorelbine and eribulin treatment, inhibition of mitotic
spindle assembly leads to cell cycle arrest at the G2/M phase. For eribulin, although
the target points are different respectively, the target molecule itself is the same
as in taxane and vinorelbine (Saji 2013]; Andreopoulou and Sparano 2013]).

MM therapy has a mechanism potentially different from other drugs previously used.
In addition, combination chemotherapy has the potential to show clinical therapeutic
efficacy compared to single-drug therapy (Andreopoulou and Sparano 2013]).

As mentioned in the introduction, MM was effective for 24% of the MBC patients treated
with anthracycline and taxane. The median TTP was 4.8 months when MM was used as third-line
therapy (Tanabe et al. 2009]).

There are studies reporting the clinical efficacy of treatments including MMC and
MTX. In a combination of MMC, MTX, and VP-16 (VMM), the response rate was 31%, the
clinical benefit rate 47%, the median disease-stabilization duration was 9.1 months,
and the median PFS was 4.2 months (Aldabbagh 2009]). Combination treatment with mitoxantrone, MMC, and MTX (MMM) was reported to be
as effective as CMF. MMM achieved an objective response (51%) comparable to that with
CMF (60%). Overall median survival was 16 months for MMM and 12 months for CMF (Jodrell
et al. 1991]). However, when cardiac toxicity is considered, administration of mitoxantrone cannot
be recommended after treatment with anthracycline.

The clinical results in these studies support that MM shows strong potential as a
treatment for MBC. Moreover, MM might be more effective than monotherapies such as
capecitabine, vinorelbine, and eribulin (Andreopoulou and Sparano 2013]).

Treatments for primary breast cancer as well as MBC are selected depending on the
expression patterns of ER, PgR, and HER2. In luminal-type breast cancer, we have several
choices of endocrine therapy such as selective estrogen receptor modulators, selective
estrogen receptor down-regulators and aromatase inhibitors. In HER2 positive type
breast cancer, we usually use anti-HER2 target therapy such as trastuzumab, lapatinib,
pertuzumab with cytotoxic agents, or hormonal treatment. However, there is no promising
treatment that is effective for triple negative (TN) breast cancer, defined as ER
negative, PgR negative, and HER2 negative. Therefore, it is important to develop a
variety of treatments to increase the choices of chemotherapy with different molecular
mechanisms.

In our study, 10 patients were TN. Two out of the 10 had PR, 1 showed long SD, 2 had
SD, and the median TTF was 4.4 months. Although it was a small number, CBR was 30%.
On efficacy of eribulin as the third-line therapy in 22 TN patients, PFS was 1.8 months
and RR was 13.8% (Aogi et al. 2012]). Eribulin as 1st line therapy, 12 of TN patients were treated, RR was 16.7% CBR
was 25%, respectively (McIntyre et al. 2014]). Although our report was retrospective one, the results deserve attention in that
all patients were pretreated with ATCV and MM was administered later than the fifth
line. We found no report dealing with chemotherapy for patients with metastatic breast
cancer, all of whom had been pretreated with four regimens of ATCV. Based on our results,
MM is a potential choice of treatment as fifth-line therapy for MBC pretreated with
ATCV.

Jia et al. developed new drug delivery systems of MTX and MMC loaded PEGylated chitosan
nanoparticles (CS-NPs), which coordinate the early phase targeting effect with the
late-phase anticancer effect (Jia et al. 2014]). The (MTX + MMC)-PEG-CS-NPs exhibited concentration- and time-dependent cytotoxicity.
Jia et al. showed that codelivery of MTX and MMC suppresses tumor cell growth to a
greater extent than the delivery of either drug alone, indicating a synergistic effect.
Synergizing the therapeutic index might be effective for patients to maximize the
therapeutic effects of MMC while minimizing its toxicity. These encouraging data support
our results.