Biology Reference
In-Depth Information
to two parallel phase II trials that evaluated the efficacy
of clofarabine in pediatric patients with relapsed or
refractory ALL and another in pediatric patients with
relapsed or refractory AML. Clofarabine showed an
impressive activity in the ALL setting as 12 of 61 treated
patients presented with an overall positive response
rate.
215
Surprisingly, similar results were not observed
in the AML trial. The most common side effects in
both studies were febrile neutropenia, fever, and liver
dysfunction. Despite encouraging results in pediatric
leukemia, clofarabine has yet to be approved for the
treatment of ALL in adults.
Pharmacokinetic Properties of Pyrimidine
Nucleosides
The transport of pyrimidine analogs across the cell
membrane occurs through the action of multiple nucle-
oside transporters (
Figure 5.12
). Gemcitabine is
a substrate for five of the human nucleoside trans-
porters. These include two of the equilibrative tranport-
ers (hENT1, hENT2) and three of the concentrative
transporters (hCNT1, hCNT2, hCNT3).
216
However,
the majority of gemcitabine uptake appears to be cata-
lyzed by hENT1 as cells that are deficient in this trans-
porter are highly resistant to its cytotoxic effects.
216
This preclinical data suggests that nucleoside transport
deficiency is an important predictive factor for gemcita-
bine response in clinical settings.
After entry into the cell, gemcitabine is phosphory-
lated by dCK to produce dFdCMP and this represents
the rate-limiting step in intracellular accumulation
of the drug.
In vitro
mechanistic studies have shown
that the K
m
for gemcitabine for dCK is 4.6
m
M while
the K
m
for deoxycytidine is only three-fold lower at
1.5
m
M.
217
In addition, dCK has a two-fold higher affinity
for gemcitabine (K
m
¼
CLINICAL UTILITY OF PYRIMIDINE
NUCLEOSIDE ANALOGS
Cytarabine (1-
b
-D-arabinofuranosylcytosine (Ara-C))
is a structural analog of deoxycytidine (
Figure 5.10
)
that is used primarily in the treatment of acute leuke-
mias and lymphomas. Ara-C differs from deoxycyti-
dine by the presence of a hydroxyl group in the
b
-configuration at the 2'-position of the sugar moiety.
Gemcitabine (2', 2'-difluorodeoxycytidine (dFdC)) is
another synthetic nucleoside analog that differs from
deoxycytidine by the addition of two fluorine atoms
in the “geminal configuration” at the 2'-position of
the carbohydrate. Gemcitabine is arguably the most
important nucleoside analog to be developed over
the past 10 years as it displays distinctive pharmaco-
logical properties and a wide spectrum of anticancer
activities against both hematological disorders and
solid tumors.
4.6
m
M) compared to Ara-C (K
m
¼
8.8
m
M).
218
Gemcitabine can also be phosphorylated by
thymidine kinase 2 (TK2), a mitochondrial enzyme that
catalyzes the conversion of natural nucleosides into their
monophosphate form. However, the specificity of this
enzyme for gemcitabine is rather low and only 5
10%
of that for the natural substrate, deoxycytidine.
219
Regardless of how the monophosphate of gemcitabine
is formed, it is effectively converted into its active
diphosphate and triphosphate metabolites, dFdCDP
and dFdCTP, by pyrimidine nucleoside kinases.
e
FIGURE 5.12
Pharmacokinetic features associ-
ated with the metabolism of gemcitabine. Step 1 is
transport by nucleoside transporters. Step 2 is
phosphorylation by dCK. Steps 3 and 4 are phos-
phorylation by pyrimidine kinases. Step 5 is incor-
poration of the triphosphate into DNA while step 6
is incorporation of the triphosphate into RNA.
Step 7 reflects inhibition of ribonucleotide reductase
(RR) as a mechanism for self-potentiation. Step 8 is
deamination of dFdC to dFdU. Step 9 is dephos-
phorylation of dFdCMP by 5'-nucleotidase to dFdC.
Step 10 is deamination of dFdCMP to dFdUMP
which then acts as an inhibitor of thymidylate syn-
thase (TS) (step 11).
Gemcitabine
(dFdC)
1
2
3
dFdC
dFdCMP
dFdCDP
7
9
8
10
Ribonucleotide
reductase
(RnR)
4
dFdU
dFdUMP
dFdCTP
11
5
6
Thymidylate
synthase
(TS)
RNA-dFdCMP
DNA-dFdCMP
