It has now been more than four decades since fluorouracil (5-FU) was
first synthesized and introduced as a clinical chemotherapeutic
agent. Designed as an analog of the naturally occurring pyrimidine
uracil in which a fluorine atom was substituted for a hydrogen atom
in the fifth position of the pyrimidine ring, 5-FU has been shown to
be a true antimetabolite. As such, it is capable of being taken up
into the cell like uracil or thymine and then metabolized by the
pyrimidine anabolic and catabolic pathways (see Figure
1).[1,2] The 5-FU nucleotide metabolites that are formed from
anabolism, including 5-fluorodeoxyuridine 5´-monophosphate
(FdUMP), 5-fluorouridine 5´-triphosphate (FUTP), and
5-formyl-2´-deoxyuridine, 5´-triphosphate (FdUTP) can
affect critical sites responsible for blocking cell replication. This
includes inhibition of thymidylate synthase or incorporation into RNA
or DNA, resulting in cytotoxicity and, in turn, anticancer activity.
While anabolism is clearly important, over the past several years
there has been an increasing appreciation for the important role of
catabolism in 5-FU pharmacology by regulating the amount of 5-FU
available for anabolism. The first enzyme in catabolism,
dihydropyrimidine dehydrogenase (also known as DPD, dihydrouracil
dehydrogenase, dihydrothymine dehydrogenase, uracil reductase, EC
184.108.40.206) is the critical rate-limiting step in the regulation of 5-FU
metabolism. DPD is responsible for converting more than 85% of
clinically administered 5-FU to 5-FUH2 (an inactive metabolite) in an
essentially irreversible enzymatic step.
The variability in DPD activity, in some cases within individual
patients and in others from patient to patient, has been shown to be
responsible for much of the variability noted in clinical studies
with 5-FU. The affect of variable DPD activity on the clinical
pharmacology of 5-FU is summarized in Table
Variation in DPD activity within patients is thought to be
responsible for the variable blood levels of 5-FU noted over a 24-hour
period. Studies have demonstrated that 5-FU levels can vary with a
peak and trough following a cosine wave within patients receiving
continuous infusion of 5-FU by automated pumps over a 24-hour
period.[4-6] The DPD levels in these patients have been observed to
follow a circadian pattern inverse to the circadian pattern observed
with 5-FU levels in this cohort. This finding has prompted some
chemotherapists to propose the use of time-modified 5-FU infusions to
optimize drug delivery during a 24-hour period as a potential benefit
in the treatment of certain human cancers.
In studies in different patients who otherwise have similar liver
function, demonstrated variation in DPD activity is now thought to be
responsible for the large variation in 5-FU pharmacokinetics.
Following administration of an intravenous (IV) bolus of 5-FU, the t ½ß
was shown to vary almost five-fold among patients studied. DPD
enzyme activity in normal tissues (peripheral blood mononuclear cells
and liver) has also been shown to vary from individual to individual
in a normal distribution pattern, with as much as a six-fold
variation from the lowest to the highest values.[9,10] This wide
variation in DPD activity is thought to be responsible for the wide
variation in the t ½ß observed in patients in
While most individuals have DPD activity within the normal
distribution, a small percentage (< 5%) of the population has DPD
activity significantly below the normal distribution.[11-13] These
individuals are at significant risk if they develop cancer and are
subsequently treated with 5-FU. In this setting, the normal
degradation of approximately 85% of 5-FU does not occur, resulting in
more available 5-FU for anabolism and patients effectively receiving
a drug overdose. This is a true pharmacogenetic syndrome, with
symptoms not being recognized until affected individuals are exposed
to the drug.
Variation in DPD activity from individual to individual has also
recently been shown to be responsible for the apparent variable
bioavailability of 5-FU. As a result, there has been a recommendation
that 5-FU not be administered orally. The erratic bioavailability has
not been well understood, particularly since 5-FU is a small molecule
with a pKa that should predict excellent absorption and
bioavailability. As noted below, the role of DPD in 5-FU
bioavailability was not appreciated until pharmacokinetic studies
with DPD inhibitors demonstrated that the area under the
concentration-time curve (AUC) from oral administration of 5-FU was
essentially the same as that produced by IV administration,
suggesting almost 100% bioavailability.
Tumors may also express variable levels of DPD activity. This may
explain the observed varied tumor response to 5-FU. A recent study of
interest demonstrated increased DPD expression in tumors from
patients who were resistant to 5-FU, even when thymidylate synthase
expression was low.
The above studies demonstrate the variability in DPD levels in both
normal and tumor tissues that may explain the observed variability in
5-FU pharmacology. It has become attractive to consider inhibiting
DPD in order to eradicate the variability in 5-FU pharmacology. DPD
inhibition in 5-FUsusceptible host tissue, such as
gastrointestinal mucosa and bone marrow, should decrease dosing
variability from patient to patient. This would be an improvement
over the current situation with 5-FU and many other cancer
chemotherapeutic agents in which dosing decisions are typically based
on observed toxicity. Inhibition of DPD in tumor specimens is also
attractive, particularly since many tumors may be 5-FU-resistant
based on increased DPD activity within the tumor, resulting in
increased degradation and thus less anabolism of 5-FU.
Over the years, many attempts have been made to synthesize more
effective fluoropyrimidine drugs, of which three generations
currently exist (Table 2). 5-FU
and its deoxyribonucleoside derivative 5-fluoro-2-deoxyuridine
(FdUrd) represent the first generation. While 5-FU continues to be
used as an IV bolus, as part of a continuous infusion regimen, or as
a protracted ambulatory infusion, FdUrd is used mainly as part of
hepatic arterial infusion regimens. The second-generation
fluoropyrimidines, including 5´dFUrd and tegafur, were developed
with the hope of permitting oral administration of 5-FU. Due to the
many undesirable side effects of these agents, they were never
approved in the United States; they are approved in other parts of
the world. The third-generation agents include several agents that
were developed from the second generation, as well as two separate
subclasses: the enzymatically activated prodrug capecitabine (Xeloda)
and the DPD inhibitory fluoropyrimidines, or DIF drugs.
Four DIF drugs have undergone clinical evaluation, including UFT,
eniluracil, S-1, and BOF-A2. These drugs differ both in the type
of DPD inhibition and the degree of inhibition produced.
The rationale for using DIF drugs is shown in Table
3. Basically, 5-FU, derived either from 5-FU itself or from a
prodrug converted to 5-FU, is administered together with
another drug that interferes with (or inhibits) the otherwise rapid
catabolism of 5-FU. This effect will lead to increased exposure to 5-FU
All four of the new fluoropyrimidine drugs derive a therapeutic
advantage from DPD inhibition. This inhibition permits oral delivery
of 5-FU (bioavailability > 70%) and results in less variability in
the pharmacokinetics of the fluoropyrimidines. In addition, by
inhibiting the catabolic pathway, more 5-FU can enter the anabolic
pathway and potentially increase the antitumor effect, which is
theoretically particularly important for tumors that have become
resistant secondary to an increase of intratumoral DPD.
Finally, although not completely understood, it appears that at least
several 5-FU toxicities (hand-foot syndrome, some forms of
neurotoxicity, and possibly cardiotoxicity) are secondary to the
catabolic pathway. Inhibiting the catabolic pathway should decrease
UFT was the first of the DIF drugs to be synthesized and is the one
for which we have the most experience. This drug is actually a
combination of the naturally occurring pyrimidine uracil with the
fluoropyrimidine tegafur (ftorafur) in a 4:1 molar ratio. The
presence of uracil in excess of 5-FU results in
competition at the level of DPD such that 5-FU formed
from tegafur will not be degraded rapidly and will remain present for
a longer time. While not true inhibition of DPD, the
competition between 5-FU and uracil for DPD produces an effect
similar to that which is achieved with a true DPD inhibitor.
In contrast to the true DPD inhibitors and inactivators (see below),
the affect on DPD by UFT is more rapidly reversible. The rapidly
reversible inhibition may avoid some of the problems observed with
the earlier DPD inhibitors and may account for a more favorable
toxicity profile compared to some of the earlier DPD inhibitors 
as well as some of the newer DIF drugs.
Extensive data from Japan, as well as from Europe, South America, and
the United States, demonstrate that orally administered UFT has
antitumor activity in several tumor types (particularly breast and
colon cancer) either as a single agent or combined with oral
leucovorin (a combination being developed under the trade name
Orzel).[21-23] Studies conducted thus far have shown that it is at
least as effective as intravenously infused 5-FU. Furthermore, the
toxicity profile has proven quite tolerable with the typical
fluoropyrimidine toxicities (eg, diarrhea and nausea) seen at the
maximum tolerated dose.
Notably, other toxicities , particularly hand-foot syndrome and
neurologic and cardiologic toxicities, are absent. Although not
well understood, these toxicities may be secondary to 5-FU
catabolites. 5-FU catabolites are less likely to form from UFT,
therefore, these toxicities are not typically observed. Within this
issue, several articles provide evidence of UFT efficacy and
Recently, a new DPD inhibitor, ethynyluracil (eniluracil or GW776),
has been synthesized and demonstrated to be a potent inactivator of
DPD. This compound is a pyrimidine possessing a structure similar
to both uracil and 5-FU.
Initial phase I and II clinical studies with ethynyluracil
demonstrated that DPD was rapidly and completely inactivated, with
inhibition maintained for more than 1 day at clinically used
doses.[26,27] Phase III studies are close to accrual and await
investigators survival data to evaluate the effectiveness of
coadministering low-dose 5-FU and ethynyluracil in a number of
different malignancies, particularly colorectal cancer and breast cancer.
In Japan, several attempts have been made to further develop this
concept. S-1 is a triple-drug combination, consisting of the prodrug,
tegafur, together with a DPD inhibitor 5-chloro-2,4-dihydroxypyridine
(CDHP) and potassium oxalate in a molar ratio of 1:0.4:1,
respectively. This combination not only provides the sustained
5-FU release from the use of the prodrug and DPD inhibitor, but also
utilizes potassium oxalate to theoretically lessen the chance of
bothersome GI toxicity, particularly diarrhea. Potassium oxalate has
been shown in preclinical studies to selectively inhibit 5-FU
phosphorylation by the enzyme orotate phosphoribosyltransferase,
especially in the GI tract but not in the tumor. Preclinical
studies have been encouraging, demonstrating excellent antitumor
activity. Clinical studies, thus far, have demonstrated S-1 to be
Based on its clinical efficacy in gastric cancer, S-1 has been
approved in Japan for treatment of this condition, where the agent
has been observed to be associated with less diarrhea. Unfortunately,
early clinical studies in Western Europe and the United States have
been notable because diarrhea continues to be the dose-limiting
toxicity. The basis for this is unclear, but may be secondary to
genetic differences in drug metabolism in the different populations.
BOF-A2 represents another attempt to develop an improved
fluoropyrimidine drug. In this two-drug combination, the prodrug
1-ethoxymethyl 5-fluorouracil (EM-FU) is combined with the DPD
inhibitor 3-cyano-2,6-dihydroxypyridine (CNDP) in a 1:1 molar
ratio.[33,34] EM-FU is relatively resistant to degradation and is
metabolized to 5-FU by the liver microsomes. Preclinical studies
demonstrated antitumor activity in several animal models, with
sustained 5-FU levels resulting from the release of 5-FU from EM-FU.
Clinical studies have been undertaken in both Japan and, more
recently, in the United States. While some clinical responses were
noted in early phase I and II studies, further research has been
placed on hold in these countries due to severe
fluoropyrimidine-type toxicities that occurred in several patients.
The availability of the DPD-inhibiting fluoropyrimidines, or DIF
drugs, provides a new strategy in which 5-FU may be administered
orally at reduced doses. This schedule produces an effect somewhat
similar to continuous infusion of 5-FU with potentially less
intrapatient or interpatient variability in 5-FU pharmacokinetics.
Thus far, clinical studies with several of these drugs demonstrate
tolerable toxicities. Clinical trials with at least one of these
drugs, single-agent UFT or combination UFT and leucovorin (described
elsewhere in this issue), demonstrate that 5-FU can be administered
orally and achieve similar therapeutic efficacy as that obtained with
the more standardized IV regimen of 5-FU.
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