Gemcitabine and Pemetrexed Disodium in Treating Breast Cancer
Gemcitabine and Pemetrexed Disodium in Treating Breast Cancer
Gemcitabine (Gemzar) is a pyrimidine antimetabolite that is
anabolized sequentially to the nucleoside mono-, di-, and triphosphate
intracellularly. Difluorodeoxycytidine triphosphate (dFdCTP) is incorporated
into DNA, resulting in chain termination. In addition, gemcitabine inhibits
ribonucleotide reductase, an enzyme that catalyzes the formation of
deoxynucleosides required for DNA synthesis. Gemcitabine has broad activity
in a variety of solid tumors, including breast cancer, and is approved for
the treatment of pancreatic and non-small-cell lung cancers. The clinical
pharmacology of gemcitabine has been reviewed recently.
Pemetrexed disodium (Alimta, LY231514) is a novel antimetabolite
that targets at least three enzymes involved in the synthesis of purines and
pyrimidines. A primary enzyme target is thymidylate synthase (TS),[4-6] a
folate-dependent enzyme, whose inhibition results in decreased thymidine
necessary for DNA synthesis.
The other known enzyme targets of pemetrexed disodium are
dihydrofolate reductase (DHFR) and glycinamide ribonucleotide formyl transferase
(GARFT). The dependence of the cytotoxicity of pemetrexed disodium on both
DHFR and GARFT is supported by the fact that thymidine and hypoxanthine are both
required to rescue cells from the cytotoxic effects of this agent in vitro.
Key Enzyme Targets
The three enzyme targets for pemetrexed disodium are shown in
Figure 1. As illustrated, pemetrexed disodium is not only similar to
methotrexate (a DHFR inhibitor), fluorouracil (5-FU), and raltitrexed (Tomudex,
a TS inhibitor), but also inhibits GARFT, which currently has no clinically
relevant inhibitor. Because of its ability to inhibit multiple enzymes,
pemetrexed disodium may prove to have superior clinical activity in comparison
to other antifolates and TS inhibitors.
Pemetrexed is transported into cells via the reduced folate
carrier, and is polyglutamated in a reaction that is catalyzed by
folylpolyglutamate synthase (FPGS). The predominant intracellular glutamated
form of pemetrexed disodium is the pentaglutamate, which is greater than 60-fold
more potent in its inhibition of TS than the monoglutamate. The pharmacology
and clinical activity of pemetrexed disodium has been comprehensively
Folate Status and Toxicity
Folic acid and its derivatives have been traditionally utilized
to reverse the toxicity of antifolate agents. However, there has not been a
consistent correlation between cellular or serum folate levels of patients and
the incidence of toxicity from antifolate chemotherapeutic agents. Recent data
have provided some insight into this apparent contradiction by indicating that
plasma homocysteine is a much more sensitive measure of the functional folate
status of patients than the traditionally used measures of red blood cell counts
or serum folate levels. In view of these data, serum homocysteine and
vitamin metabolite levels have been evaluated as predictors of pemetrexed
disodium toxicity in phase I and phase II studies.
The s-adenosylmethionine (SAM) cycle responsible for critical
single-carbon transfer reactions (through methyl groups) in mammalian systems is
illustrated in Figure 2. The transfer of a methyl group from N5-methyltetrahydrofolate
(CH3FH4) to homocysteine generates methionine, which
in turn generates SAM. When humans are folate-deficient, the lack of CH3FH4
leads to an increase in the levels of plasma homocysteine, which is an early,
sensitive, and reliable indicator of folate deprivation.
Vitamins B12 and B6
Cobalamin (vitamin B12) and pyridoxine (vitamin
can also lead to an increase in plasma homocysteine levels. Cobalamin is a
cofactor for two synthetic processes in mammals. The first process is the
synthesis of methionine from homocysteine and CH3FH4, catalyzed by methionine
synthase. The second process is the formation of succinyl co-enzyme A from
L-methylmalonic acid co-enzyme A. Serum methylmalonic acid levels are therefore
elevated in cobalamin deficiency, but not in folate deficiency, and are useful
in the differential diagnosis of cobalamin and folate deficiency in the setting
of elevated plasma homocysteine levels.
Pyridoxine is involved in the conversion of homocysteine to
cystathionine, and the subsequent conversion of cystathionine to cysteine and
alphaketoglutaric acid. Cystathionine levels are elevated to a much greater extent in
vitamin B6 deficiency, compared to folate and B12 deficiency.[15,16] Isolated
vitamin B6 deficiency, however, occurs very rarely.
Phase I and Phase II Studies
With the emerging information described above, plasma
homocysteine, cystathionine, and methylmalonic acid levels have been measured
and included in a multivariate analysis of potential prognostic factors that
predict serious toxicity in patients treated with pemetrexed disodium. In
phase II trials, 139 patients with a variety of solid tumors were treated with
pemetrexed disodium at doses of 600 mg/m2 every 3 weeks.
Plasma homocysteine, other vitamin deficiency markers, serum
albumin, and hepatic enzymes were measured at baseline and once each cycle
thereafter. Preliminary data find baseline plasma homocysteine concentrations to
be the only statistically significant prognostic factors for serious toxicities,
predominantly myelosuppression. Mucositis and diarrhea were also correlated with
plasma concentrations of methylmalonic acid. A threshold baseline homocysteine
value of 10 µmol/L was used to differentiate between high- and low-risk
Plasma concentrations > 10 µmol/L predicted a greatly
increased rate of toxicities, even though a continuum existed for various serum
levels. These studies support the notion that individuals who are
folate-deficient are at subclinically or clinically increased risk of
severe toxicity when treated with standard doses of pemetrexed disodium.
In an ongoing phase I trial, pemetrexed disodium is administered with high-dose intermittent folic acid
supplementation (5 mg orally, days -2 to +2). Available results suggest that
this folic acid supplementation schedule permits marked dose escalation of pemetrexed disodium with minimal toxicity. Minimally pretreated patients have
tolerated pemetrexed disodium doses of up to 925 mg/m2.
A strict interpretation of these findings would suggest that
patients with reduced folate pools should receive folic acid supplementation
before receiving pemetrexed disodium. Since plasma homocysteine is a continuous
predictor for severe toxicities, it may be assumed that folic acid
supplementation significantly improves the tolerability of pemetrexed disodium in all patients. Early evidence that supplementation with
low-dose daily oral folic acid and quarterly IM vitamin B12 significantly
reduced toxicities lends support to this idea.
Pemetrexed disodium has been investigated as a single agent in
advanced breast cancer without folic acid and vitamin B12 support. Preliminary
results from two completed phase II studies in Europe have been reported.
In one study, most patients (33 out of 38) had been previously
treated with chemotherapy. Of these, 26 patients had previously received
anthracyclines, while 4 had been treated with taxanes; 16 patients received
therapy in the adjuvant setting. Out of 36 evaluable patients, 10 achieved a
partial response, and 1 patient achieved a complete response, for an overall
response rate of 31% (95% confidence interval [CI]: 16%-46%). Responses were
seen in three out of the four patients previously treated with taxanes and in
three out of the 26 patients previously treated with anthracyclines. The median
survival was 13 months, and median time to disease progression was 5 months. The
median response duration was > 9 months.
In the second study, all enrolled patients had been previously
treated with anthracyclines. In all, 26 patients had documented disease
progression ≤ 30 days after anthracycline treatment (anthracycline refractory)
and 43 patients developed progressive disease > 30 days after stopping
treatment (anthracycline failures); 29 patients had also received a taxane. The
overall response rate in 69 evaluable patients was 23% in anthracycline failures
and 19% in anthracycline refractory patients.
In the cohort of patients who received both anthracyclines and
taxanes, objective responses were documented in 8 out of a total of 29 patients
for an overall response rate of 28%. The median response duration was 6 months,
the median time to progression was 4 months, and the 1-year survival was
These early results indicate that
pemetrexed disodium may not demonstrate complete cross resistance with taxanes
or anthracyclines. To test this hypothesis, a phase II study of pemetrexed disodium in patients with stage IV breast cancer who have previously
received anthracyclines and taxanes is underway in the United States and
Europe. This study enrolled 40 patients prior to a protocol amendment that
added folic acid and vitamin B12 supplementation to the remaining patients
enrolled. Preliminary data report a 19% objective response rate with grade 3/4
neutropenia as the most frequent toxicity. Final reports of the analysis from
this study are awaited.