Dihydropyrimidine dehydrogenase (DPD), also
referred to as dihydrouracil dehydrogenase, dihydrothymine
dehydrogenase, uracil reductase, and EC 220.127.116.11), is the initial,
rate-limiting enzymatic step in the catabolism of the naturally
occurring pyrimidines, uracil and thymine, and the widely used
antimetabolite cancer chemotherapy agent, 5-fluorouracil (5-FU).[1,2]
As shown in Figure 1, DPD
occupies an important position in the overall metabolism of 5-FU,
converting more than 85% of administered 5-FU to the inactive
metabolite 5-fluoro-5,6-dihydrouracil (5-FUH2) in an
enzymatic step that is essentially irreversible. Whereas anabolism
is clearly critical in the conversion of 5-FU to the
active nucleotides, 5-fluoro-2´-deoxyuridine
5´-monophosphate (FdUMP), 5-fluorouridine 5´-triphosphate
(FUTP), and 5-fluoro-2´-deoxy- uridine 5´-triphosphate
(FdUTP), catabolism controls the availability of 5-FU for anabolism,
and thus occupies a critical position in the overall metabolism of
5-FU. FdUMP, FUTP, and FdUTP are, in turn, responsible for inhibition
of cell replication through inhibition of thymidylate synthase or
through incorporation into ribonucleic acid (RNA) or deoxyribonucleic
acid (DNA), respectively.
The importance of DPD to the clinical pharmacology of 5-FU has been
further established by several recent observations (Table
1) that demonstrate that DPD can account for much of the
variability that has been noted in clinical studies with 5-FU. This
includes both intrapatient differences (circadian variation of drug
levels) and interpatient variability (in pharmacokinetics,
bioavailability, toxicity, and antitumor effectiveness) of 5-FUs
DPD has been shown to follow a circadian pattern in both animals and
humans.[4-6] This is thought to explain the variable plasma levels of
5-FU observed in patients receiving continuous 5-FU infusion by
automated pumps. Studies have, in fact, demonstrated a circadian
variation of tissue DPD levels associated with an inverse circadian
pattern in plasma 5-FU concentrations. This has led some
chemotherapists to propose time-modified 5-FU infusions to optimize
drug delivery during a 24-hour period. In Western Europe, some
oncologists have reported potential benefit with such regimens in the
treatment of certain human cancers.
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
sixfold variation from the lowest to the highest values.[8,9] This
wide variation in DPD activity is thought to be responsible for the
wide variation in the drugs half-life observed in patients in
population studies. In addition, it is now clear that a small
percentage (< 5%) of the population has, without apparent reason,
DPD activity significantly below the normal distribution that
characterizes most of the population.[11-13] These individuals are at
significant risk if they develop cancer and are administered 5-FU.
This is a true pharmacogenetic syndrome, the symptoms of which are
not recognized until the patient is exposed to the drug.
Variation in DPD activity has also been shown to be responsible for
the apparent variable bioavailability of 5-FU, which has historically
led to recommendations against oral 5-FU administration. This
understanding potentially explains the erratic bioavailability of a
small molecule with a pKa that should favor excellent
absorption and bioavailability. Experimental studies of DPD
inhibitors in animals reveal that inhibition of DPD following oral
5-FU administration yields a pharmacokinetic pattern essentially the
same as that produced by intravenous administration, thus implying
almost 100% bioavailability.
Tumors may also express variable DPD activity. This may explain
the inconsistent 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 relating inconsistent DPD activity to the observed
variability in 5-FU pharmacology make it attractive to consider
inhibiting DPD to eliminate the unpredictable variations. Inhibiting
DPD in 5-FUsusceptible host tissue, such as gastrointestinal
(GI) mucosa and bone marrow, should make dosing from patient to
patient less variable, avoiding the typical (with 5-FU and many other
cancer chemotherapeutic agents) dosing decisions based on observed
toxicity. Inhibition of DPD in tumor specimens is also attractive in
that most tumors probably become resistant through increased
intratumor DPD activity, leading to increased degradation and thus
less anabolism of 5-FU.
Over the years, there have been numerous attempts to synthesize
effective inhibitors of DPD, many of which turned out to be very
toxic. In the past several years, new fluoropyrimidine drugs using
DPD inhibitionagents referred to as dihydropyrimidine
dehydrogenaseinhibitory fluoropyrimidines (DIF)have been
introduced to the clinic.
There are currently four new DIF drugs (Table
2): UFT, ethynyluracil, S-1, and BOF-A2. These drugs differ both
in 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. All four of these drugs derive a therapeutic advantage from DPD
inhibition. Most impressive are 1) the capacity for oral delivery of
5-FU (bioavailability > 70%), and 2) the leveling of 5-FU
pharmacokinetic variability. In addition, inhibition of the catabolic
pathway allows more 5-FU to enter the anabolic pathway, potentially
increasing the antitumor effect. This may be particularly important
for resistant tumors with increased DPD expression. Finally, at least
some 5-FU toxicities (hand-foot syndrome, some forms of
neurotoxicity, and possibly cardiotoxicity) may be secondary to the
catabolic pathway, although this mechanism is not completely
understood. Inhibiting the catabolic pathway should decrease the
incidence of these toxicities.
UFT was the first of the DIF drugs to be synthesized and is therefore
the one with which we have the most experience. This new
fluoropyrimidine is a combination of the naturally occurring
pyrimidine, uracil, and the 5-FU prodrug, tegafur, 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 will not
be rapidly degraded and will remain present for a prolonged period.
Although this is not true inhibition of DPD, the competition between
5-FU and uracil for DPD produces an effect similar to what one
achieves with a true DPD inhibitor. In contrast to the effects of
true DPD inhibitors and inactivators, the effect of UFT on DPD is
more rapidly reversible, thereby possibly avoiding some of the
problems observed with the earlier DPD inhibitors. This alteration
may account for a more favorable toxicity profile compared with the
earlier DPD inhibitors, as well as with some of the newer DIF drugs.
Extensive data from Japan, as well as Europe, South America, and the
United States, are now demonstrating that orally administered UFT has
antitumor activity in several tumor types (particularly breast and
colon cancer), either as a single agent or combined with calcium
folinate.[20-22] It appears to be at least as effective as
intravenous infusion of 5-FU. Furthermore, the toxicity profile is
quite tolerable, with the typical fluoropyrimidine toxicities (eg,
diarrhea and nausea) seen at the maximum tolerated dose. Of note is
the near absence of other 5-FU toxicities, in particular hand-foot
syndrome, neurologic effects, and cardiotoxicity. Although not
proven, these toxicities, which are thought to be secondary to 5-FU
catabolites, may be eliminated by using a DIF drug such as UFT.
Several articles within this supplement provide evidence of the
efficacy and tolerable toxicity of UFT.
Recently, a new DPD inhibitor, ethynyluracil (Eniluracil, or
GW776C85), has been synthesized and demonstrated to be a potent
inactivator of DPD. This pyrimidine is structurally similar to
both uracil and 5-FU. In initial phase I clinical studies,
ethynyluracil administration led to rapid and complete DPD
inactivation, which was maintained for more than 1 day at clinical
doses.[25,26] At present, a number of phase II studies are underway
to evaluate the effectiveness of the coadministration of low-dose
5-FU and ethynyluracil in a number of different malignancies,
including colorectal and breast cancer.
In Japan, there have been several attempts to further develop this
concept. S-1 is a triple-drug combination consisting of the prodrug,
tegafur, together with a DPD inhibitor, chloro-2,4-dihydroxypyridine
(CDHP), plus potassium oxalate in a molar ratio of 1:0.4:1,
respectively. This combination not only provides sustained
release of 5-FU via use of the prodrug and DPD inhibitor, but also
utilizes potassium oxalate theoretically to lessen bothersome GI
toxicity (particularly diarrhea). In preclinical studies, potassium
oxalate has been shown to selectively inhibit 5-FU phosphorylation by
the enzyme, orotate phosphoribosyltransferase, particularly in the GI
tract, but not in a tumor. Preclinical study results have
demonstrated excellent antitumor activity. Clinical studies thus
far have shown S-1 to be quite tolerable.[30,31] United States
studies of this drug have been limited.
BOF-A2 represents another attempt to develop an improved
fluoropyrimidine. With this two-drug combination, the prodrug 1-ethoxymethyl
5-fluorouracil (EM-FU) is combined with the DPD inhibitor
3-cyano-2,6-dehydropyrimidine (CNDP) in a 1:1 molar ratio.[32,33]
EM-FU is relatively resistant to degradation and is metabolized to
5-FU by liver microsomes. Preclinical studies have confirmed
antitumor activity in several animal models and have demonstrated
sustained 5-FU levels resulting from the release of 5-FU by EM-FU.
Clinical studies have been undertaken in Japan and, more recently,
limited studies have been initiated in the United States. It is too
early to comment on the clinical effectiveness of this drug
combination. Thus far, BOF-A2 has demonstrated typical 5-FU
toxicities, with some patients experiencing more severe toxicity such
as mucositis and diarrhea. At present, the dose, schedule, and
possible combination with other modulators (eg, calcium folinate) are
It is now clear that DPD is a critical step in pyrimidine metabolism
and is responsible for much of the variability in pharmacokinetics,
bioavailability, toxicity, and efficacy following administration of
5-FU. Inhibition of DPD activity through the use of DPD inhibitors
should result in less variation in 5-FU pharmacokinetics and
bioavailability. At the same time, inhibition of the DPD pathway
should potentially improve the therapeutic effectiveness of 5-FU,
both by making toxicity more predictable and by overcoming the high
levels of DPD activity that may be a source of resistance in tumors.
The recent availability of DIF drugs provides a means by which 5-FU
may be administered orally at reduced doses, producing an effect
similar to continuous infusion of 5-FU without significant
intrapatient or interpatient variability in 5-FU pharmacokinetics.
Clinical studies thus far demonstrate tolerable toxicities. Clinical
trials are currently underway to evaluate the therapeutic
effectiveness of each of these drugs.
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