Dihydropyrimidine dehydrogenase (DPD), also
referred to as dihydrouracil dehydrogenase, dihydrothymine
dehydrogenase, uracil reductase, and EC 184.108.40.206), 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.
1. Daher GC, Harris BE, Diasio RB: Metabolism of pyrimidine analogues
and their nucleosides, in Metabolism and Reactions of Anticancer
Drugs. The International Encyclopedia of Pharmacology and
Therapeutics, vol 1, chap 2, pp 55-94. Oxford, Pergamon Press, 1994.
2. Diasio RB, Harris BE: Clinical pharmacology of 5-fluorouracil.
Clin Pharmcokinet 16:215-237, 1989.
3. Lu Z-H, Zhang R, Diasio RB: Purification and characterization of
dihydropyrimidine dehydrogenase from human liver. J Biol Chem
4. Harris BE, Song R, He Y, et al: Circadian rhythm of rat liver
dihydropyrimidine dehydrogenase: Possible relevance to
fluoropyrimidine chemotherapy. Biochem Pharmacol 37:4759-4763, 1988.
5. Harris BE, Song R, Soong S-J, et al: Circadian variation of
5-fluorouracil catabolism in isolated perfused rat liver. Cancer Res
6. Harris BE, Song R, Soong SJ, et al: Relationship of
dihydropyrimidine dehydrogenase activity and plasma 5-fluorouracil
levels: Evidence for circadian variation of 5-fluorouracil levels in
cancer patients receiving protracted continuous infusion. Cancer Res
7. Zhang R, Diasio RB: Pharmacologic basis for circadian
pharmacodynamics, in Hrushesky WJM (ed): The Scientific Basis for
Optimized Cancer Therapy, chap 4, pp 60-103. Boca Raton, Florida, CRC
8. Lu Z, Zhang R, Diasio RB: Dihydropyrimidine dehydrogenase activity
in human peripheral blood mononuclear cells and liver: Population
characteristics, newly identified patients, and clinical implication
in 5-fluorouracil chemotherapy. Cancer Res 53:5433-5438, 1993.
9. Lu Z, Zhang R, Diasio RB: Dihydropyrimidine dehydrogenase activity
in human liver: Population characteristics and clinical implication
in 5-FU chemotherapy. Clin Pharmacol Ther 58:512-522, 1995.
10. Heggie GD, Sommadossi JP, Cross DS, et al: Clinical
pharmacokinetics of 5-fluorouracil and its metabolites in plasma,
urine, and bile. Cancer Res 47:2203-2206, 1987.
11. Diasio RB, Beavers TL, Carpenter JT: Familial deficiency of
dihydropyrimidine dehydrogenase: Biochemical basis for familial
pyrimidinemia and severe 5-fluorouracil-induced toxicity. J Clin
Invest 81:47-51, 1988.
12. Harris BE, Carpenter JT, Diasio RB: Severe 5-fluorouracil
toxicity secondary to dihydropyrimidine dehydrogenase deficiency: A
potentially more common pharmacogenetic syndrome. Cancer 68:499-501, 1991.
13. Takimoto CH, Lu Z-H, Zhang R, et al: Severe neurotoxicity
following 5-fluorouracil-based chemotherapy in a patient with
dihydropyrimidine dehydrogenase deficiency. Clin Cancer Res
14. Lu Z, Diasio RB: Polymorphic drug metabolizing enzymes, in
Schilsky RL, Milano GA, Ratain MJ (eds): Principles of Antineoplastic
Drug Development and Pharmacology, pp 281-305. New York, Marcel
Dekker, Inc., 1996.
15. Baccanari DP, Davis ST, Knick VC, et al: 5-Ethynyluracil: Effects
on the pharmacokinetics and antitumor activity of 5-fluorouracil.
Proc Natl Acad Sci U S A 90:11064-11068, 1993.
16. Jiang W, Lu Z, He Y, et al: Dihydropyrimidine dehydrogenase
activity in hepatocellular carcinoma: Implication for 5-fluorouracil-based
chemotherapy. Clin Cancer Res 3:395-399, 1997.
17. Danenberg K, Salonga D, Park JM, et al: Dihydropyrimidine
dehydrogenase and thymidylate synthase gene expressions identify a
high percentage of colorectal tumors responding to 5-fluorouracil
(abstract 992). Proc Am Soc Clin Oncol 17:258a, 1998.
18. Naguib FNM, el Kouni MH, Cha S: Enzymes of uracil catabolism in
normal and neoplastic human tissues. Cancer Res 45:5405-5412, 1985.
19. Majima H: Phase I and preliminary phase II study of
coadministration of uracil and FT-207 (UFT therapy). Gan To Kagaku
Ryoho 7:1383-1387, 1980.
20. Takino T: Clinical studies on the chemotherapy of advanced cancer
with UFT (uracil plus futraful preparation). Gan To Kagaku Ryoho
21. Gonzalez Baron M, Colmenarejo A, Feliu J, et al: Preliminary
results of phase I clinical trial: UFT modulated by folinic acid (PO)
in the treatment of advanced colorectal cancer. Therapeutic Research
22. Pazdur R, Lassere Y, Diaz-Canton E, et al: Phase I trials of
uracil-tegafur (UFT) using 5- and 28-day administration schedules:
Demonstration of schedule-dependent toxicities. Anticancer Drugs
23. Pazdur R, Lassere Y, Diaz-Canton E, et al: Phase I trial of
uracil-tegafur (UFT) plus oral leucovorin: 14-day schedule. Invest
New Drugs 15:123-128, 1997.
24. Spector T, Porter DJT, Nelson DJ, et al: 5-Ethynyluracil
(776C85), a modulator of the therapeutic activity of 5-fluorouracil.
Drugs of the Future 19:566-571, 1994.
25. Baker SD, Khor SP, Adjei AA, et al: Pharmacokinetics, oral
bioavailability, and safety study of fluorouracil in patients treated
with 776C85, an inactivator of dihydropyrimidine dehydrogenase. J
Clin Oncol 14:3085-3096, 1996.
26. Schilsky RL, Burris H, Ratain M, et al: Phase I clinical and
pharmacologic study of 776C85 plus 5-fluorouracil in patients with
advanced cancer. J Clin Oncol 16:1450-1457, 1998.
27. Shirasaka T, Shimamato Y, Ohsimo H, et al: Development of a novel
form of oral 5-fluorouracil derivative (S-1) directed to the
potentiation of the tumor selective cytotoxicity of 5-fluorouracil by
two biochemical modulators. Anticancer Drugs 7:548-557, 1996.
28. Shirasaka T, Shimamato Y, Fukushima M: Inhibition by oxonic acid
of gastrointestinal toxicity of 5-fluorouracil without loss of its
antitumor activity in rats. Cancer Res 53:4004-4009, 1993.
29. Shirasaka T, Nakano K, Takechi T, et al: Antitumor activity of 1M
tegafur, 0.4M 5-chloro-2,4-dihydroxypyridine and 1M potassium oxonate
(S-1) against human colon carcinoma orthotopically implanted into
nude rats. Cancer Res 56:2602-2606, 1996.
30. Ohtsu A, Sakata Y, Horioshi N, et al: A phase II study of S-1 in
patients with advanced gastric cancer (abstract 1005). Proc Am Soc
Clin Oncol 17:262a, 1998.
31. Baba H, Ohtsu A, Sakata Y, et al: A late phase II study of S-1 in
patients with advanced colorectal cancer in Japan (abstract 1065).
Proc Am Soc Clin Oncol 17:277A, 1998.
32. Shirasaka T, Fujita F, Fujita M, et al: Antitumor activity and
metabolism of BOF-A2, a new 5-fluorouracil-derivative with human
cancers xenografted in nude mice. Gan To Kagaku Ryoho 17:1871-1876, 1990.
33. Sasaki T: New anti-cancer drugs for gastrointestinal cancers. Gan
To Kagaku Ryoho 24:1925-1931, 1997.