Lipophilic, acid-stable, adenosine deaminase-activated anti-HIV prodrugs for central nervous system delivery. 3. 6-amino prodrugs of 2′-β-fluoro-2′,3′-dideoxyinosine

Article abstract of DOI:10.1021/jm9509197

A series of 6-substituted amino analogs of 9-(2,3-dideoxy-2-fluoro-β-D-threo-pentofuranosyl) purines (F-ddN) has been synthesized and characterized with the objective of finding compounds which might be superior to existing drugs for the treatment of HIV

Full text of DOI:10.1021/jm9509197

J . Med. Chem. 1996, 39, 1619-1625
1619
Lip op h ilic, Acid -Sta ble, Ad en osin e Dea m in a se-Activa ted An ti-HIV P r od r u gs for
Cen tr a l Ner vou s System Deliver y. 3. 6-Am in o P r od r u gs of
2′-â-F lu or o-2′,3′-d id eoxyin osin e
J ohn S. Driscoll,*,† Maqbool A. Siddiqui,† Harry Ford, J r.,† J ames A. Kelley,† J eri S. Roth,† Hiroaki Mitsuya,‡
Masatoshi Tanaka,‡ and Victor E. Marquez†
Laboratory of Medicinal Chemistry, Division of Basic Sciences, and Experimental Retrovirology Section, Medicine Branch,
Division of Clinical Sciences, National Cancer Institute, National Institutes of Health, Bethesda, Maryland 20892
Received December 14, 1995X
A series of 6-substituted amino analogs of 9-(2,3-dideoxy-2-fluoro-â-D-threo-pentofuranosyl)
purines (F-ddN) has been synthesized and characterized with the objective of finding compounds
which might be superior to existing drugs for the treatment of HIV in the central nervous
system. These compounds are intended to be more lipophilic than the currently approved anti-
HIV drugs for better blood-brain barrier penetration. Subsequent adenosine deaminase (ADA)-
catalyzed hydrolysis of these prodrugs in the brain is expected to produce the anti-HIV agent,
9-(2,3-dideoxy-2-fluoro-â-D-threo-pentofuranosyl)hypoxanthine (F-ddI). The new compounds,
synthesized from the corresponding 6-chloro analog, include F-ddN which contain methylamino,
ethylamino, dimethylamino, hydroxylamino, methoxyamino, benzyloxyamino, hydrazino, and
nitro substituents in the 6-position. The 6-nitro analog was isolated as an unexpected product
during the preparation of the 6-chloro derivative. Among the analogs with anti-HIV activity,
the ethylamino and dimethylamino compounds are ca. 100 times more lipophilic than ddI or
F-ddI. As expected, 2′-fluoro substitution protects the compounds from acid-catalyzed glycosylic
cleavage. Only the hydroxylamino and nitro analogs underwent any nonenzymatic hydrolysis
at pH 1.0 or 7.4. This reaction, however, results in hydrolysis of the group in the 6-position
rather than glycosylic bond cleavage. ADA catalyzes the hydrolysis of the 6-substituents at
rates which vary from slightly slower (NO2, 1.7×) to much slower (NHEt, 5000×) than F-ddA.
The 6-dimethylamino analog is the only compound which possesses anti-HIV activity (ED50 18
µM) without ADA hydrolysis. With the exception of the two inactive alkoxyamino compounds,
the other prodrugs exhibited cellular protection in the HIV-1/PHA-PBM system with IC50
potencies of 7-40 µM.
In tr od u ction
catabolic inactivation by purine nucleoside phosphory-
lase (PNP).9
HIV infection of the central nervous system (CNS)1
and the attendant AIDS-related dementia remains a
major problem for people with AIDS.2 Although sig-
nificant progress has been made in the discovery of new
therapeutic approaches to the systemic treatment of
HIV infection,3 a continuing effort is needed for the
specific problem of CNS HIV infection.4a Among the
enzymes used in the enzyme-activated prodrug ap-
proach to this problem are xanthine oxidase4b and
adenosine deaminase (ADA).5 The detailed rationale for
our ADA approach has been described previously.5 In
general, this involves the design of molecules which
might penetrate the blood-brain barrier (BBB) better
than the drugs currently available. Specifically, it
utilizes fluorinated dideoxypurine nucleoside (F-ddN)
analogs with increased lipophilicity for better BBB
diffusion, in anticipation that these compounds will be
enzymatically activated by adenosine deaminase (ADA)
in the CNS to 2′-â-fluoro-2′,3′-dideoxyinosine (F-ddI,
1).5,6 F-ddI is an anti-HIV-active relative of F-ddA
(2),6,7a a compound scheduled for clinical trial in AIDS
patients in the near future.8 F-ddI possesses several
favorable qualities as an anti-HIV agent such as acid
stability for ease of oral formulation6 and a lack of
Ch em istr y
Syn th esis. Since it is the 2′-fluoro group which
confers acid stability on the normally acid-unstable 2′,3′-
dideoxypurine nucleosides,5-8,10 this functionality is
included in all the prodrugs synthesized. Fluorine in
the 2′-â-configuration (threo) is utilized, since the ini-
tially prepared 2′-R-fluoro (erythro) compounds, while
acid-stable,7a are inactive as anti-HIV agents.5,7 Al-
though the prodrugs themselves could possess anti-HIV
activity, earlier compounds evaluated in this series
indicated that conversion to 1 was necessary before
activity was observed.5b
The amino analogs 4-6 are prepared from the 5′-
benzoyl-protected 6-chloro compound, 3a , by straight-
forward nucleophilic displacement reactions with vari-
ous amine derivatives (Scheme 1). We prepared 4 by a
different procedure earlier.5b The syntheses of 7-10
proceed from the deprotected compound, 3b, an analog
† Laboratory of Medicinal Chemistry.
‡ Experimental Retrovirology Section.
X Abstract published in Advance ACS Abstracts, March 15, 1996.
0022-2623/96/1839-1619$12.00/0 © 1996 American Chemical Society

1620 J ournal of Medicinal Chemistry, 1996, Vol. 39, No. 8
Driscoll et al.
Sch em e 1
Sch em e 2
we described previously in this series.5a Since the
ultimate fate of these prodrugs involves ADA-catalyzed
hydrolysis of the 6-substituent (Scheme 1), and because
it is known that larger groups are more resistant to this
reaction,11 the 6-substituents were chosen in an attempt
to keep the variations simple in order to see how minor
structural changes, e.g. 4 vs 5 and 5 vs 6, influence the
ADA-catalyzed reaction. Hydroxylamino functionality
(7) was included since this group has been shown to be
a good substrate for ADA in the adenosine series.11,12
In addition, if hydrolyzed at a rate which produced
concentrations of F-ddI adequate for anti-HIV activity,
alkoxyamino substitution could provide an entry to
progressively more lipophilic nucleosides through longer
alkyl groups. The methoxyamino (8) and benzyloxyami-
no (9) analogs were synthesized in an attempt to exploit
this approach. Although the 6-hydrazino riboside is
reported to be unstable in aqueous solution,13a the
dideoxy analog, 10, was prepared and evaluated because
the riboside was shown in earlier studies to be a
substrate for ADA.13b In addition, N-alkylhydrazino
compounds would provide an entry to longer chain, more
lipophilic analogs if the parent compound proved active.
The 6-dimethylamino analog, 5, was prepared in good
yield as described above. This compound also had been
observed earlier as a minor side product during the
preparation of the 6-fluoro prodrug from 3b via a
6-trimethylammonium intermediate.5a
Ch a r t 1
The 6-chloro analogs, 3a and 3b, are critical inter-
mediates in the preparation of the prodrugs reported
here (Scheme 1). If the deblocked chloro compound (3b)
is required, 3a cannot be used as an intermediate since
removal of the 5′-benzoyl protecting group with metha-
nolic ammonia produces a mixture of F-ddA (1b) and
the corresponding 6-methoxy compound.5a However,
changing 5′-protection to tert-butyldimethylsilyl (11)
prior to photochlorination provides an intermediate (12)
(Scheme 2) which can be deblocked with fluoride ion5a
to give 3b as the major product. Upon repeating this
reaction, we were able to isolate a minor product (10%),
the blocked 6-nitro analog (13). This nitro group proved
to be quite reactive. Attempted 5′-deblocking of 13 with
fluoride ion produced the previously described5a 6-fluoro
analog, 14. Alternatively, removal of the 5′-TBDMS
group with aqueous acetic acid provides the deblocked
nitro compound, 15 (Scheme 2). While this material
proved somewhat difficult to characterize since it is
hydrolytically unstable and partially converted to F-ddI
(1) during the deprotection procedure, the use of ap-
propriate chromatographic conditions allowed the isola-
tion of 15. In addition to the expected NMR and mass
spectral data, 15 was further characterized by reduction
to the known 6-amino compound, F-ddA (2).
The proton NMR spectra of the O-alkylhydroxylamino
derivatives 8 and 9 in DMSO-d6 indicates an equilib-
rium between the alkoxyamino and alkoxyimino tau-
tomers (Chart 1). The alkoxyimino tautomer appears
to be favored, possibly because of a stabilizing intramo-
lecular hydrogen bond formed between the imino proton
and the oxygen atom. This is evidenced by the observed
splitting of the H-2 signal (J 2,NH ca. 3.6 Hz) in both 8
and 9, indicating the presence of the imino tautomer.
After deuterium exchange, the minor (alkoxyamino)
tautomers are no longer detectable.
Lip op h ilicity. Octanol/buffer (pH 7.4) partition
coefficients, P, were obtained for the prodrugs prepared
in this study. They were determined by a convenient
microscale shake-flask method utilizing HPLC/UV quan-
titation.14 By convention, they are reported as log P
values in Table 1. The (benzyloxy)amino compound, 9,
is over 170 times more lipophilic than the hydroxyl-
amino prodrug, 7, the most hydrophilic of the new
compounds. Compounds 5 and 6, the most lipophilic of
the compounds with anti-HIV activity, are 100 times
more lipophilic than either ddI or F-ddI. Compound 9,
while 290 times more lipophilic than ddI, is inactive
with an IC50 > 80 µM under our experimental condi-

Adenosine Deaminase-Activated Anti-HIV Prodrugs
J ournal of Medicinal Chemistry, 1996, Vol. 39, No. 8 1621
Ta ble 1. Octanol-Water Partition Coefficients and
Chromatographic Properties of Some Purine
2′-â-Fluoro-2′,3′-dideoxynucleosides
HPLC
measureda
mobile isocraticc λmax
d
compd 6-substituent
log P
phaseb
k′
(nm)
1
2
4
5
6
7
8
9
15
OH (F-ddI)e -1.210 ( 0.017
NH2 (F-ddA) -0.183 ( 0.004
A
B
C
D
D
F
A
E
C
0.5
0.8
1.7
6.1
247
259
265
275
266
265
267
268
305
NHCH3
N(CH3)2
NHC2H5
NHOH
0.273 ( 0.008
0.797 ( 0.007
0.779 ( 0.010
-0.983 ( 0.019
-0.414 ( 0.007
1.255 ( 0.010
-0.395 ( 0.006
3.8
f
-
NHOCH3
NHOC7H7
NO2
1.1
27.1
2.6
a
Mean ( standard deviation of three independent determina-
b
tions. The following mobile phases were used at 1.0 mL/min with
F igu r e 1. Hydrolysis kinetics of the 6-NHOH analog (7) in
pH 7.4 phosphate buffered saline at 37 °C and an initial 50
µM concentration. The curve for the hydrolysis of 7 (2) is the
best nonlinear regression fit (r2 ) 0.993) for monoexponential
decay. The appearance curves for F-ddI, 1 (b), and F-ddA, 2
(9), are plotted point-to-point. The peak area for F-ddI has
been adjusted to reflect the fact that the HPLC-UV detector
molar response at 260 nm is 0.48× that at the λmax of 247 nm.
a 4.6 × 250 mm 5-µm Ultrasphere ODS column to determine log
P: (A) 10%, (B) 12%, (C) 15%, (D) 20%, or (E) 25% CH3CN in pH
7.0, 0.01 M phosphate buffer; (F) 12.5% CH3CN in pH 3.0, 0.005
M heptanesulfonic acid. c Average capacity factor (k′ ) (tr - t0)/t0)
for isocratic elution with 15% CH3CN (mobile phase C) in pH 7.0,
d
0.010 M phosphate buffer. Wavelengths determined on-the-fly
in HPLC mobile phase. e Reference log P values for F-ddI, ddI
(-1.242) and AZT (0.052) are from ref 5a. f Could not be chro-
matographed using mobile phase C.
Ta ble 2. Relative Adenosine Deaminase Hydrolysis Ratesa
disappearance
of substrateb
appearance
of F-ddI
tions. The experimental partition coefficient values
were internally consistent, giving good agreement with
calculated values using the standard π value of +0.5
for methyl and methylene groups (2, 4, 5, 6 and 7, 8).15
We were not able to discover chromatographic condi-
tions suitable for determining the P value for the
hydrazino compound, 10.
Sta bility a t p H 1.0 a n d 7.4. Normally, ddN are
quite unstable to the acidic conditions encountered in
the stomach,16 producing inactive products through
glycosylic cleavage, and making oral formulations dif-
ficult. Because 2′-fluoro substitution had previously
conferred acid stability on ddN,5-8 stability at pH 1.0
was evaluated for several of the new compounds (4, 6,
7). No decomposition of any type was observed by
HPLC analysis for 4 or 6 over a 24 h period. The
hydroxylamino analog, 7, however, decomposes at pH
1.0 with a half-life of 103 h to produce unidentified
products which did not include either 1 or 2, compounds
which are stable under these conditions.5-7
Several compounds (5, 6, 7, 15) were evaluated at the
pH of the biological test system (7.4) in an attempt to
determine whether there were any nonenzymatic, hy-
drolytic effects in addition to those observed in the
presence of ADA (see below). While 5 and 6, as well as
2, were unaffected over a 24 h period, 7 and 15 showed
slow changes at pH 7.4. The hydroxylamino analog 7
produced both 1 and 2 with t1/2 ) 105 h (Figure 1). The
mechanism for the production of 2 remains to be
studied. The nitro compound (15) is hydrolyzed to F-ddI
(1) with a half-life of 27.4 h. This observation is
consistent with the difficulty encountered in isolating
15.
compd
6-substituent
2 (F-ddA)
NH2
Cl
100
2.0
100
1.6
3bc
4
NHCH3
N(CH3)2
NHC2H5
NHOH
NHOCH3
NHOC7H7
NHNH2
F
0.9
0.74
NR
5
6
7
8
NRd
0.02
9.0
e
-
8.2
e
0.02
NR
NDf
202
58
-
9
NR
3.2
233
58
10
14c
15
NO2
a
Standard conditions: 50 µM substrate; pH 7.4, 0.01
M
b
phosphate buffer; 37 °C. Disappearance rate per unit ADA
d
normalized to that of F-ddA. c Reference 5a. No reaction; decrease
in substrate or conversion to F-ddI not detectable after 24 h using
1 unit/mL ADA. e Conversion to F-ddI detectable, but not sufficient
to calculate rate of appearance. f Not determined because of
inability to measure parent compound by HPLC.
hydrolysis with commercially available, purified enzyme
(1.0 unit/mL) at prodrug concentrations (50 µM) which
produced in vitro anti-HIV activity for previous mem-
bers of this series.5a Table 2 shows the rates of this
reaction relative to that of the 6-amino compound,
F-ddA (2). Relative rates at 1.0 unit/mL ADA are given
for compound comparison because the amount of ADA
in the anti-HIV test system used (PHA/PBM cells and
media) is not known. The analytical HPLC method
measures not only the disappearance of prodrug, but
also the resulting production of F-ddI, allowing hydroly-
sis quantitation in two ways. The time course of this
reaction is shown in Figure 2 for the 6-hydroxylamino
analog 7. ADA-catalyzed hydrolysis was not detected
for either the 6-dimethylamino (5) or the (benzyloxy)-
amino (9) analog. The addition of a methyl group (4)
to the 6-amino function of F-ddA (2) reduced its hy-
drolysis rate about 100-fold. Ethyl group addition (6)
gave a significantly greater reduction (5000×), produc-
ing a rate similar to that found for the methoxyamino
compound (8). The hydroxyamino (7) hydrolysis rate
is only about 10 times slower than that for the amino
compound, consistent with data reported for the ad-
enosine analog.11,12 The fastest ADA-catalyzed hydroly-
Biology
En zym a tic Hyd r olysis by Ad en osin e Dea m in a se.
The prodrugs in this study have been designed with the
possibility of ADA-catalyzed hydrolysis to the anti-HIV
agent, F-ddI, 1 (Scheme 1). It is well-known that, in
addition to the amino group, adenosine deaminase
(ADA) catalyzes the hydrolysis of other 6-position
functionality in purine nucleosides.11,12,13b,17 The com-
pounds in this study were subjected to ADA-catalyzed

1622 J ournal of Medicinal Chemistry, 1996, Vol. 39, No. 8
Driscoll et al.
compound, 7, decomposes nonenzymatically in pH 7.4
buffer to give F-ddA as well as F-ddI with a t1/2 ) 105
h (Figure 1). This might explain the somewhat higher
than expected potency of this compound (based only on
the ADA rate) in our 7-day anti-HIV assay. A somewhat
similar effect is operational in the case of the 6-NO2
analog, 15, which produces F-ddI at relatively rapid
rates by both pH 7.4 solvolysis (t1/2 ) 27.4 h) and ADA-
catalyzed hydrolysis (Table 2). Although the inactive
compounds in this study (alkoxyamino analogs 8 and
9) are at the low end of the hydrolysis rates as might
be expected, the alkylamino compounds, 4 and 6, also
have slow ADA rates (Table 2), but are protective (Table
3). The dimethylamino analog 5 is an anomaly in this
data set. No ADA-catalyzed hydrolysis is observed with
this compound, yet an 18 µM anti-HIV IC50 value is
obtained. This compares well with a 17.3 µM value
found in a separate PHA-PBM experiment, as well as a
149 µM IC50 value in the standard NCI CEM/HIV-1
screening system20 (data not shown). Earlier work has
shown that the activity of 6-substituted dideoxypurine
prodrugs is dependent upon conversion to their corre-
sponding inosine analogs.5b,19 It is possible that this
might not be the case with 5 which may have intrinsic
activity of its own.
F igu r e 2. Hydrolysis kinetics of 50 µM 6-NHOH analog (7)
by 1.0 unit/mL of adenosine deaminase at pH 7.4 and 37 °C.
A unit of adenosine deaminase is the amount of enzyme which
hydrolyzes a standard substrate (e.g. adenosine) at a rate of
1.0 µmol/min at 25 °C and pH 7.5. The curve depicting the
enzymatic hydrolysis of 7 (2) is the best nonlinear regression
fit (r2 ) 0.998) for monoexponential decay. The appearance
curve for the resulting F-ddI, 1 (b), product represents the
best nonlinear regression fit (r2 ) 0.996) to the equation for
monoexponential association (y ) A(1 - e-bt ). See Experimen-
tal Section for hydrolysis conditions and kinetic calculations.
Ta ble 3. Anti-HIV Activity in the PHA/PBM System
IC50
IC50
Discu ssion
compd
6-substituent (µM) compd 6-substituent (µM)
A useful prodrug in this series must be optimized for
a number of unrelated, but structurally dependent
properties (lipophilicity, ADA hydrolysis rate, acid-
stability, systemic metabolism and elimination). One
critical event for prodrug activation is the necessity for
an adequate concentration of ADA in the CNS. Recent
reports indicate that this is probable, and that the level
of this enzyme may increase in both the CSF and serum
during HIV infection.21,22 In addition, a significant
amount of work supporting the CNS prodrug concept
has been reported for the nonfluorinated, 6-halo-2′,3′-
dideoxypurine nucleosides, both in vitro19 and in animal
models.23-27 While these compounds and their active
metabolite, ddI, are both acid-sensitive and catabolized
at rates higher than those for the 2′-fluoro derivatives,
a similarly enhanced CNS penetration should also be
realized with the fluorinated prodrugs.
1 (F-ddI)
2 (F-ddA)
3b
4
5
OH
NH2
Cl
NHMe
NMe2
NHEt
NHOH
6.3
3.3
8
9
10
14
15
d d I
NHOMe
>80
NHOCH2Ph >80
NHNH2
F
5.9a
13.1
18.0
11.1
6.8
36.3
<5a
9.6
0.3
NO2
6
7
a
Reference 5a.
sis rate among the new compounds belongs to the nitro
analog (15) which is only fractionally slower than F-ddA.
An ti-HIV Activity. The HIV-infected, phytohemag-
glutinin-stimulated peripheral blood mononuclear cell
(PHA-PBM) test system employed in this study uses a
decrease in HIV-1 p24 Gag protein relative to an
untreated control as the measure of HIV-exposed cell
protection and compound activity.18,19 Compounds 1-10,
14, and 15, along with ddI as a positive control, were
evaluated in this system (Table 3). These data are
representative of duplicate evaluations and are from an
experiment where all the new compounds were tested
in direct comparison. No significant toxicity to unin-
fected PBM cells was noted at the 10-80 µM drug
concentrations used, and good dose response data were
obtained for the active compounds (i.e. IC50 < 80 µM).
While no quantitative correlation exists between IC50
values (Table 3) and ADA hydrolysis rates (Table 2), a
trend can be discerned. Compounds with the faster
F-ddI conversion rates are generally among the more
potent prodrugs (2, 15, 7, 3b,5a and 14.5a F-ddA (2) is
seen to be about twice as potent as F-ddI (1), its
hydrolysis product. While this would be abnormal for
the other compounds in this study whose anti-HIV
activity depends on the metabolic generation of F-ddI,
it is the usual situation for F-ddA.6 F-ddA is directly
metabolized to F-ddATP by deoxycytidine kinase9 in
addition to being catabolized to F-ddI and utilizing the
5′-nucleotidase pathway for conversion to F-ddATP (the
anti-HIV-active metabolite for all the compounds in this
series). Among the other analogs, the hydroxylamino
With the addition of the compounds described previ-
ously,5 a varied set of 14 6-substituted-2′-fluoro-2′,3′-
dideoxypurine derivatives is available to test the hy-
pothesis that an acid-stable, lipophilic, ADA-activated
prodrug might provide a useful CNS-directed anti-HIV
drug. The groups introduced into the 6-position include
various substituted-amino, halogen, and alkoxy groups.
Active prodrugs have been prepared with ADA hydroly-
sis rates that vary by a factor of >25000 (e.g. F vs OEt).
The partition coefficients for the complete series span
a 180-fold range (NHOH to OBn), with several of the
active prodrugs 100 times more lipophilic than ddI. At
this point it is not possible to determine which com-
pound in the series might have the optimum combina-
tion of properties for CNS treatment. One major
variable bearing on this question is the rate of human
whole-body ADA drug hydrolysis, which will affect
prodrug ability to reach the brain. This question should
be answered in a pharmacokinetic phase I study sched-
uled as part of an upcoming clinical trial with F-ddA
(2). Meanwhile, a comparison of the CNS penetration

Adenosine Deaminase-Activated Anti-HIV Prodrugs
J ournal of Medicinal Chemistry, 1996, Vol. 39, No. 8 1623
4-7 and 15 was evaluated at 37 °C in pH 7.4, 0.1 M phosphate-
buffered saline solution (PBS, Biofluids). For each compound,
1.0 mL PBS was added to a 1.5 mL polypropylene microcen-
trifuge tube (Eppendorf) and prewarmed to 37 °C in an
Eppendorf Model 5436 Thermomixer before being spiked to a
concentration of approximately 50 µM. After mixing, a 50 µL
aliquot was immediately removed for analysis and the remain-
ing solution incubated at 37 °C. This initial sample aliquot
was mixed with 50 µL of the appropriate chilled (4 °C) mobile
phase, and a 20 µL aliquot was analyzed by HPLC. Subse-
quent 50 µL samples were taken at predetermined times and
treated in the same manner as the first.
Acid Sta bility (p H 1.0) of Su ga r -F lu or in a ted Did eoxy-
n u cleosid es. The pH 1.0 acid stability of compounds 4-7 and
15 was evaluated at 37 °C as previously described.5a The only
modification made was for compound 7 where the 50 µL
sample aliquots were diluted with 0.45 mL of chilled (4 °C)
pH 3.0, 0.005 M heptanesulfonic acid ion pair cocktail prior
to HPLC analysis instead of pH 7.0 phosphate buffer. The
concentration of dideoxynucleoside in each sample was deter-
mined by HPLC analysis of a 50 µL aliquot as described above.
En zym a tic Hyd r olysis by Ad en osin e Dea m in a se. Ad-
enosine deaminase (ADA, adenosine aminohydrolase, EC
3.5.4.4) from calf intestinal mucosa was prepared as previously
described5b to give a stock solution of 1 mg/mL (250-280 units/
mL) in pH 7.4, 0.01 M phosphate buffer. The adenosine
deaminase inhibitor, 2′-deoxycoformycin (dCF), was obtained
from the Pharmaceutical Resources Branch, NCI, and used
as a 2.6 mM solution in distilled water. Relative ADA
hydrolysis rates for selected nucleosides and characterization
of products were carried out as previously described.5
properties of the 6-chloro prodrug 3b, ddA, ddI, F-ddA,
and F-ddI is underway in rats.
Exp er im en ta l Section
Column chromatography was performed on silica gel 50 (E.
Merck, 230-400 mesh), and analytical TLC was performed
on Analtech Uniplates silica gel GF with the solvents indi-
cated. Detection of compounds by TLC was accomplished
either by UV light or by 10% methanol-sulfuric acid spray,
followed by heating on a hot plate. Proton NMR spectra were
recorded in the solvents indicated at 250 MHz on a Bruker
AC-250 instrument. Chemical shifts are expressed as δ values
with reference to Me4Si. Positive-ion fast atom bombardment
mass spectra were obtained on a VG 7070E mass spectrometer
operated at an accelerating voltage of 6 kV and a resolution
of 1500. Glycerol or 3-nitrobenzyl alcohol28 were used as
sample matrices, and ionization was effected by a beam of
xenon atoms derived by charge-exchange neutralization of a
1.0-1.2 mA beam of xenon ions accelerated through 8.4-8.9
kV. Spectra were acquired under the control of a VG 11/250
J
+ data system at a scan speed of 10 s/decade, and the matrix
background was automatically subtracted. Accurate mass
measurements of protonated molecular ion (MH+) peaks were
carried out in separate experiments at a dynamic resolution
of 2500 using a voltage scan over a limited mass range and
signal acquisition in a multichannel analysis mode. Software-
based peak matching was then employed using selected
glycerol peaks within the mass range as internal references.
Elemental analyses were performed by Atlantic Microlab, Inc.,
Atlanta, GA, or by Galbraith Laboratories, Inc., Knoxville, TN.
HP LC An alysis of Dideoxyn u cleosides. Except as noted,
solutions and diluted enzymatic reaction mixtures of dideoxy-
nucleosides were analyzed on a reversed phase, high-perfor-
mance liquid chromatography (RP-HPLC) system that has
been previously described.5a This system contained a Thermo
Separations Products Model AS3000 automatic, refrigerated,
sample injector that was maintained at 5 °C. Dideoxynucleo-
side retention time was adjusted to be in the range 5-22 min
by changing the percentage of mobile phase organic modifier
(Table 1). Analysis of the 6-NHOH analog, 7, however,
required the use of ion pairing for separation on the standard
4.6 × 250 mm 5-µm Beckman Ultrasphere C18 column that
was used for all dideoxynucleoside separations. This reversed
phase, ion pairing (RP-IP) HPLC was conducted on the above
column without using a precolumn and by employing a mobile
phase consisting of 12.5% CH3CN in aqueous 0.005 M hep-
tanesulfonic acid ion pair cocktail (Regis Chemical Co.) that
had been adjusted to pH 3.0 with 1.0 N sodium hydroxide.
Before mixing and use, all mobile phase components were
filtered and then degassed by sonication and/or sparging with
ultra-high-purity helium (Matheson Gas Products). Where
possible, enzymatic and hydrolysis reaction products were
identified by coincidence of retention time with standards and
by comparison of full-scan (210-360 nm) on-the-fly UV spectra
obtained with the diode array detector. Peak areas were
measured by a Spectra-Physics 4400 Chromjet integrator
which was interfaced to a Thermo Separations Products WOW
multichannel chromatography data system running on a Zeos
486/DX2 66 MHz personal computer. For kinetic studies,
these data were plotted as a function of time and curve fit to
either a first order exponential decay (y ) Ae-kt ) or exponential
association [y ) A(1 - e-kt )] using Prism (Version 1.0,
GraphPad Software), a PC-based curve-fitting program.
Mea su r em en t of Oct a n ol-Wa t er P a r t it ion Coeffi-
cien ts. 1-Octanol/pH 7.4 (0.01 M potassium phosphate) buffer
partition coefficients (P) were determined for individual com-
pounds using a microscale shake-flask method employing
HPLC analysis of both the buffer and 1-octanol phases14 (Table
1). Determinations were conducted in triplicate. Because of
chromatographic constraints, the P for the 6-NHOH compound
7 was determined by difference after measurement of com-
pound concentration in the aqueous phase both before and
after partitioning with 1-octanol. Satisfactory chromato-
graphic conditions were not found for the determination of the
P value for the hydrazino compound, 10.
6-Ch lor o-9-(5-O-ben zoyl-2,3-d id eoxy-2-flu or o-â-D-th r eo-
p en t ofu r a n osyl)-9H -p u r in e (3a ) a n d 6-Ch lor o-9-(2,3-
d id eoxy-2-flu or o-â-D-th r eo-p en t ofu r a n osyl)-9H -p u r in e
(3b). These compounds were synthesized as described in refs
10a and 5b, respectively.
6-(Me t h yla m in o)-9-(2,3-d id e oxy-2-flu or o-â-D -t h r eo-
p en tofu r a n osyl)-9H-p u r in e (4). This compound has previ-
ously been synthesized by a different route.5b A suspension
of 3a (1.44 g, 3.82 mmol) in EtOH (8 mL) was treated with a
40% solution of aqueous methylamine (22 mL) and stirred at
40 °C for 30 min under a nitrogen atmosphere. The solution
was cooled and concentrated under reduced pressure to give
an oily residue. In order to ensure complete neutralization of
the product due to the volatile nature of methylamine, the
residue was dissolved in MeOH and treated with a few drops
of 10% NaOH. The solution was again reduced to dryness,
and the crude product was purified by flash column chroma-
tography using silica gel and a 0-10% gradient of MeOH in
CH2Cl2 as eluant. Collection of the proper fractions produced
0.878 g (86%) of 4 as a foam: 1H NMR (D2O) δ 2.20 (m, 1 H,
H-3′a), 2.60 (m, 1 H, H-3′b), 2.85 (br s, 3 H, NHCH3), 3.68 (m,
2 H, H-5′a,b), 4.30 (m, 1 H, H-4′), 5.35 (dm, J 2′,F ) 53.6 Hz, 1
H, H-2′), 6.08 (dd, J 1′,F ) 18.6, J 1′,2′ ) 3.2 Hz, 1 H, H-1′), 7.84
(s, 1 H, H-2), 8.05 (d, J 8,F ) 2.4 Hz, 1 H, H-8); FAB MS m/ z
(relative intensity) 268 (MH+, 100), 150 (bH2+, 70). Anal.
(C11H14FN5O2‚0.5H2O) C, H, N.
6-(Dim et h yla m in o)-9-(2,3-d id eoxy-2-flu or o-â-D-th r eo-
p en tofu r a n osyl)-9H-p u r in e (5). A suspension of 3a (0.215
g, 0.57 mmol) in EtOH (5 mL) was treated with a 40% solution
of aqueous dimethylamine (10 mL) and heated at reflux for 4
h. The solution was cooled and concentrated under reduced
pressure. The residue was purified by preparative TLC
(Analtech; silica gel, 2000 µm) using CH2Cl2:MeOH (9:1) as
eluant to give 0.125 g (78%) of 5 as a solid: mp 141-143 °C
(CH2Cl2:ether); 1H NMR (D2O) δ 2.15 (m, 1 H, H-3′a), 2.55 (m,
1 H, H-3′b), 3.10 (br s, 6 H, N(CH3)2), 3.65 (m, 2 H, H-5′a,b),
4.25 (m, 1 H, H-4′), 5.25 (dm, J 2′,F ) 53.5 Hz, 1 H, H-2′), 6.05
(dd, J 1′,F ) 18.6, J 1′,2′ ) 3.0 Hz, 1 H, H-1′), 7.80 (s, 1 H, H-2),
8.00 (d, J 8,F ) 2.3 Hz, 1 H, H-8); FAB MS m/ z (relative
intensity) 282 (MH+, 100), 164 (b + 2 H, 34). Anal. (C12H16
FN5O2) C, H, N.
-
6-(Eth ylam in o)-9-(2,3-dideoxy-2-flu or o-â-D-th r eo-pen to-
fu r a n osyl)-9H-p u r in e (6). A suspension of 3a (0.260 g, 0.69
mmol) in excess aqueous ethylamine (70%, 30 mL) was heated
at reflux for 20 h. The solution was cooled and concentrated
under reduced pressure, and the residue was purified by flash
H yd r olyt ic St a b ilit y (p H 7.4) of Su ga r -F lu or in a t ed
Did eoxyn u cleosid es. The hydrolytic stability of compounds

1624 J ournal of Medicinal Chemistry, 1996, Vol. 39, No. 8
Driscoll et al.
column chromatography, in the same manner as for compound
4, to give 0.180 g (93%) of 6 as a foam: 1H NMR (D2O) δ 1.10
(t, 3 H, CH2CH3), 2.15 (m, 1 H, H-3′a), 2.55 (m, 1 H, H-3′b),
3.30 (br q, 2 H, CH2CH3), 3.65 (m, 2 H, H-5′a,b), 4.25 (m, 1 H,
(relative intensity) 360 (MH+, 100), 242 (b + 2 H, 24), 91
(PhCH2+, 44). Anal. (C17H18FN5O3‚1.5H2O) C, H, N.
6-H yd r a zin o-9-(2,3-d id eoxy-2-flu or o-â-D-th r eo-p en t o-
fu r a n osyl)-9H-p u r in e, Hyd r och lor id e (10). Compound 3b
(0.128 g, 0.47 mmol) was treated with anhydrous hydrazine
(2 mL), and the resulting solution was stirred at room
temperature for 30 min. After the addition of 2-propanol (5
mL), the reaction mixture was reduced to dryness under
vacuum. The residue was purified by flash column chroma-
tography on silica gel using a 0-20% gradient of MeOH in
CH2Cl2 as eluant to give 0.113 g (79%) of 10 as a hygroscopic
hydrochloride salt: 1H NMR (MeOH-d4) δ 2.40 (m, 1 H, H-3′a),
2.60 (m, 1 H, H-3′b), 3.75 (m, 2H, H-5′a,b), 4.30 (m, 1 H, H-4′),
5.37 (dm, J 2′,F ) 54.2 Hz, 1 H, H-2′), 6.35 (dd, J 1′,F ) 16.4, J 1′,2′
) 3.6 Hz, 1 H, H-1′), 8.32 (s, 1 H, H-2), 8.36 (d, J 8,F ) 2.2 Hz,
1 H, H-8); FAB MS m/ z (relative intensity) 269 (MH+, 100),
151 (b + 2 H, 62). Anal. (C10H13FN6O2‚HCl‚1.75H2O) C, H,
N.
6-Nitr o-9-[5-O-(ter t-bu tyld im eth ylsilyl)-2,3-d id eoxy-2-
flu or o-â-D-th r eo-p en tofu r a n osyl]-9H-p u r in e (13). This
compound was obtained as a minor byproduct during the
diazotization/radical chlorination reaction of 6-amino-9-[5-O-
(tert-butyldimethylsilyl)-2,3-dideoxy-2-fluoro-â-D-threo-pento-
furanosyl]-9H-purine (11) with tert-butyl nitrite and CCl4.5a
A suspension of 11 (1.90 g, 5.17 mmol) in freshly distilled tert-
butyl nitrite (24 mL, 210.7 mmol) was heated to 80 °C and
treated with freshly distilled CCl4 (250 mL) while being
maintained under a nitrogen atmosphere. Stirring and heat-
ing (80 °C) continued under direct incandescent illumination
(200 W bulb) for 3 h. The reaction mixure was then cooled,
concentrated under reduced pressure, and purified by flash
column chromatography using silica gel and hexane:EtOAc
(10:0 to 7:3) to give 0.79 g (39%) of 6-chloro-9-[5-O-(tert-
butyldimethylsilyl)-2,3-dideoxy-2-fluoro-â-D-threo-pentofurano-
syl]-9H-purine (12).5a This was followed by 13 (0.20 g, 10%),
which was used in the next step without additional purifica-
tion: 1H NMR (CDCl3) δ 0.15 (s, 6 H, Me2Si), 0.95 (s, 9 H,
Me3C), 2.45 (m, 1 H, H-3′a), 2.60 (m, 1 H, H-3′b), 3.90 (m, 2
H, H-5′a,b), 4.35 (m, 1 H, H-4′), 5.40 (dm, J 2′,F ) 54.0 Hz, 1 H,
H-2′), 6.50 (dd, J 1′,F ) 15.5, J 1′,2′ ) 3.6 Hz, 1 H, H-1′), 8.75 (d,
J 8,F ) 2.2 Hz, 1 H, H-8), 9.00 (s, 1 H, H-2).
Dep r otection of 6-Nitr o-9-[5-O-(ter t-bu tyld im eth ylsi-
lyl)-2,3-d id eoxy-2-flu or o-â-D-th r eo-p en t ofu r a n osyl]-9H -
p u r in e (13) To Give 14 or 15. A. With Tetr a -n -bu tyla m -
m on iu m F lu or id e. A solution of 13 (0.010 g, 0.025 mmol)
in THF (4 mL) was treated with a 1 M solution of tetra-n-
butylammonium fluoride in THF (TBAF, 0.2 mL) and stirred
at room temperature for 1 h. The solution was directly applied
to a preparative TLC (Analtech; silica gel, 1500 µm) which was
developed with EtOAc as eluant to give 0.0032 g (50%) of
6-fluoro-9-(2,3-dideoxy-2-fluoro-â-D-threo-pentofuranosyl)-9H-
purine (14) as a foam.5a
B. With AcOH. A solution of 13 (0.10 g, 0.251 mmol) in
THF (4 mL) was treated with a 4:1:1 mixture of AcOH:THF:
water (6 mL) and stirred at room temperature for 16 h. The
solution was reduced to dryness under reduced pressure and
the crude product was purified by flash column chromatogra-
phy on silica gel using EtOAc as eluant to give 0.0382 g (54%)
of 6-nitro-9-(2,3-dideoxy-2-fluoro-â-D-threo-pentofuranosyl)-9H-
purine (15) as a solid, mp 148-150 °C. This sample was >96%
pure by HPLC analysis with <4% F-ddI identified as the
impurity: 1H NMR (CDCl3) δ 2.55 (m, 1 H, H-3′a,b), 3.95 (m,
2 H, H-5′a,b), 4.45 (m, 1 H, H-4′), 5.45 (dm, J 2′,F ) 54.0 Hz, 1
H, H-2′), 6.52 (dd, J 1′,F ) 15.2, J 1′,2′ ) 3.7 Hz, 1 H, H-1′), 8.85
(d, J 8,F ) 1.6 Hz, 1 H, H-8), 9.05 (s, 1 H, H-2); FAB MS m/ z
(relative intensity) 284 (MH+, 100), 166 (b + 2 H, 62); accurate
mass positive ion FAB MS m/ z 284.1092 (MH+, calcd 284.0795).
Compound 15 could not be purified completely because of facile
hydrolytic conversion to F-ddI. Changing the flash chromato-
graphic eluant from EtOAc to 10% MeOH in CH2Cl2 also
permited the isolation of 0.012 g (19%) of F-ddI which was
identified by comparison with an authentic sample.6
H-4′), 5.22 (dm, J 2′,F ) 53.5 Hz, 1 H, H-2′), 6.03 (dd, J 1′,F
)
18.6, J 1′,2′ ) 3.2 Hz, 1 H, H-1′), 7.85 (s, 1 H, H-2), 8.03 (d, J 8,F
) 2.4 Hz, 1 H, H-8); FAB MS m/ z (relative intensity) 282
(MH+, 100), 164 (b + 2 H, 40). Anal. (C12H16FN5O2‚0.75H2O)
C, H, N.
6-(H yd r oxya m in o)-9-(2,3-d id eoxy-2-flu or o-â-D-th r eo-
p en tofu r a n osyl)-9H-p u r in e, Hyd r och lor id e (7). A solu-
tion of 3b (0.095 g, 0.35 mmol) in dry THF (5 mL) was treated
with excess of O-(trimethylsilyl)hydroxylamine (2.5 mL) and
heated at reflux for 20 h. The solution was cooled, quenched
with MeOH (5 mL), and concentrated under reduced pressure.
The crude product was purified by flash column chromatog-
raphy using silica gel and a 0-20% gradient of MeOH in CH2-
Cl2 as eluant to give 0.067 g (63%) of 7 as a foam. An
analytical sample was obtained after purification by reversed
phase C-18 chromatography using a 0-10% gradient of MeOH
in water as eluant. Lyophilization of the product-containing
fractions afforded pure 7 as an amorphous white solid: 1H
NMR (D2O) δ 2.18 (m, 1 H, H-3′a), 2.54 (m, 1 H, H-3′b), 3.68
(m, 2 H, H-5′a,b), 4.30 (m, 1 H, H-4′), 5.30 (dm, J 2′,F ) 53.5 Hz,
1 H, H-2′), 6.15 (dd, J 1′,F ) 18.4, J 1′,2′ ) 3.0 Hz, 1 H, H-1′), 7.88
(s, 1 H, H-2), 8.09 (d, J 8,F ) 1.8 Hz, 1 H, H-8); FAB MS m/ z
(relative intensity) 270 (MH+, 100), 152 (b + 2 H, 33). Anal.
(C10H12FN5O3‚HCl) C, H, N.
6-(Met h oxya m in o)-9-(2,3-d id eoxy-2-flu or o-â-D-th r eo-
p en tofu r a n osyl)-9H-p u r in e (8). A solution of 3b (0.150 g,
0.55 mmol) in dry THF (15 mL) was treated with excess of
O-methylhydroxylamine (8.50 g). O-Methylhydroxylamine
was obtained as a clear liquid by distillation (bp 48-50 °C) of
a KOH-neutralized aqueous solution of O-methylhydroxyl-
amine hydrochloride. The reaction mixture was heated at
reflux for 20 h, cooled, and concentrated under reduced
pressure. The crude product was purified by preparative TLC
(Analtech; silica gel, 2000 µm) using CH2Cl2:MeOH (9:1) as
eluant to give 0.145 g (93%) of 8 as a solid: mp 200-202 °C
dec (EtOH:ether); 1H NMR (Me2SO-d6) δ 2.20 (m, 1 H, H-3′a),
2.55 (m, 1 H, H-3′b), 3.60 (m, 2 H, H-5′a,b), 3.65 and 3.66
(singlets, 3 H, CH3O), 4.15 (m, 1 H, H-4′), 5.01 (m, 1 H, OH),
5.40 (dm, J 2′,F ) 54.3 Hz, 0.8 H, H-2′), 5.45 (dm, J 2′,F ) 54.3
Hz, 0.2 H, H-2′), 6.15 (dd, J 1′,F ) 15.3, J 1′,2′ ) 3.8 Hz, 0.8 H,
H-1′), 6.38 (dd, J 1′,F ) 15.3, J 1′,2′ ) 3.8 Hz, 0.2 H, H-1′), 7.56
(d, J 2,NH ) 3.6 Hz, 0.8 H, H-2), 8.00 (d, J 8,F ) 1.9 Hz, 0.8 H,
H-8), 8.30 (br s, 0.2 H, H-2), 8.36 (br s, 0.2 H, H-8), 11.00 (br
1
s, 0.2 H, NH), 11.30 (br s, 0.8 H, NH); H NMR (Me2SO-d6
+
D2O) δ 2.20 (m, 1 H, H-3′a), 2.55 (m, 1 H, H-3′b), 3.60 (m, 2 H,
H-5′a,b), 4.15 (m, 1 H, H-4′), 5.40 (dm, J 2′,F ) 54.3 Hz, 1 H,
H-2′), 6.15 (dd, J 1′,F ) 15.3, J 1′,2′ ) 3.8 Hz, 1 H, H-1′), 7.75 (br
s, 1 H, H-2), 8.10 (br s, 1 H, H-8); FAB MS m/ z (relative
intensity) 284 (MH+, 100), 166 (b + 2 H, 27). Anal. (C11H14
FN5O3‚0.5H2O) C, H, N.
-
6-[(Ben zyloxy)am in o]-9-(2,3-dideoxy-2-flu or o-â-D-th r eo-
p en tofu r a n osyl)-9H-p u r in e (9). A solution of 3b (0.10 g,
0.36 mmol) in dry THF (10 mL) was treated with excess of
O-benzylhydroxylamine29 (4 g) and triethylamine (50 µL). The
reaction mixture was heated at reflux for 36 h, cooled, and
concentrated under reduced pressure. The crude product was
purified by preparative TLC (Analtech; silica gel, 1500 µm)
using CH2Cl2:MeOH (9:1) as eluant to give 0.040 g (30%) of 9
as a foam [some 3b (0.028 g, 28%) was also isolated]: 1H NMR
(Me2SO-d6) δ 2.20 (m, 1 H, H-3′a), 2.50 (m, 1 H, H-3′b), 3.60
(m, 2 H, H-5′a,b), 4.15 (m, 1 H, H-4′), 5.01 (m, 3 H, PhCH2O,
OH), 5.35 (dm, J 2′,F ) 54.3 Hz, 0.8 H, H-2′), 5.45 (dm, J 2′,F
)
54.3 Hz, 0.2 H, H-2′), 6.15 (dd, J 1′,F ) 15.2, J 1′,2′ ) 3.9 Hz, 0.8
H, H-1′), 6.38 (dd, J 1′,F ) 15.2, J 1′,2′ ) 3.9 Hz, 0.2 H, H-1′), 7.20-
7.50 (m, 5 H, Ph), 7.60 (d, J 2,NH ) 3.7 Hz, 0.8 H, H-2), 8.00 (d,
J 8,F ) 2.1 Hz, 0.8 H, H-8), 8.36 (br s, 0.2 H, H-2), 8.40 (br s,
0.2 H, H-8), 11.05 (br s, 0.2 H, NH), 11.30 (d, J 2,NH ) 3.7 Hz,
0.8 H, NH); 1H NMR (Me2SO-d6 + D2O) δ 2.20 (m, 1 H, H-3′a),
2.50 (m, 1 H, H-3′b), 3.60 (m, 2 H, H-5′a,b), 4.15 (m, 1 H, H-4′),
5.00 (s, 2 H, PhCH2), 5.35 (dm, J 2′,F ) 54.3 Hz, 1 H, H-2′), 6.15
(dd, J 1′,F ) 15.2, J 1′,2′ ) 3.8 Hz, 1 H, H-1′), 7.20-7.50 (m, 5 H,
Ph), 7.70 (br s, 1 H, H-2), 8.05 (br s, 1 H, H-8); FAB MS m/ z
Red u ction of 6-Nitr o-9-(2,3-d id eoxy-2-flu or o-â-D-th r eo-
p en tofu r a n osyl)-9H-p u r in e (15) to th e 6-Am in o An a log
2. A suspension of 15 (0.022 g, 0.07 mmol) in EtOH (5 mL)
was treated with ammonium formate (0.020 g, 0.31 mmol) and
10% Pd/C (0.020 g), and the mixture was stirred at room

Adenosine Deaminase-Activated Anti-HIV Prodrugs
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Refer en ces
(1) Abbreviations: ADA, adenosine deaminase; AIDS, acquired
immunodeficiency syndrome; BBB, blood-brain barrier; CNS,
central nervous system; dCF, 2′-deoxycoformycin; ddA, 2′,3′-
dideoxyadenosine; ddI, 2′,3′-dideoxyinosine; ddN, 2′,3′-dideoxy-
nucleosides; F-ddA, 9-(2,3-dideoxy-2-fluoro-â-D-threo-pentofu-
ranosyl)adenine; F-ddI, 9-(2,3-dideoxy-2-fluoro-â-D-threo-pento-
furanosyl)hypoxanthine; F-ddN, 2′-â-fluoro-2′,3′-dideoxynucleo-
sides; HIV, human immunodeficiency virus type 1; IC50, com-
pound concentration inhibiting HIV cytopathogenic effects by
50%; NCI, National Cancer Institute; P, octanol/pH 7.4 buffer
partition coefficient; PHA-PBM, phytohemagglutinin-stimulated
peripheral blood mononuclear cells; PNP, purine nucleoside
phosphorylase.
(2) (a) Geleziunas, R.; Schipper, H. M.; Wainberg, M. A. Pathogen-
esis and Therapy of HIV-1 Infection of the Central Nervous
System. AIDS 1992, 6, 1411-1426. (b) Atwood, W. J .; Berger,
J . R.; Kaderman, R.; Tornatore, C. S.; Major, E. O. Human
Immunodeficiency Virus Type 1 Infection of the Brain. Clin.
Microbiol. Rev. 1993, 6, 339-366.
(3) De Clercq, E. Antiviral Therapy for Human Immunodeficiency
Virus Infections. Clin. Microbiol. Rev. 1995, 8, 200-239.
(4) (a) Yarchoan, Y.; Mitsuya, H.; Broder, S. Challenges in the
therapy of HIV Infection. Immunol. Today 1993, 14, 303-309.
(b) Shanmuganathan, K.; Koudriakova, T.; Nampalli, S.; Du, J .;
Gallo, J . M.; Schinazi, R. F.; Chu, C. K. Enhanced Brain Delivery
of an Anti-HIV Nucleoside 2′-F-ara-ddI by Xanthine Oxidase
Mediated Biotransformation. J . Med. Chem. 1994, 37, 821-827.
(5) (a) Ford, H.; Siddiqui, M. A.; Driscoll, J . S.; Marquez, V. E.;
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