Diaryl ester prodrugs of FR900098 with improved in vivo antimalarial activity

Article abstract of DOI:10.1016/S0960-894X(01)00075-0

The fosmidomycin derivative FR900098 represents an inhibitor of the 1-deoxy-D-xylulose 5-phosphate (DOXP) reductoisomerase with potent antimalarial activity. Prodrugs of FR900098 with increased activity after oral administration were obtained by chemical modification of the phosphonate moiety to yield phosphodiaryl esters. One diaryl ester prodrug demonstrated efficacy in mice infected with the rodent malaria parasite Plasmodium vinckei comparable to ip drug administration.

Full text of DOI:10.1016/S0960-894X(01)00075-0

Bioorganic & Medicinal Chemistry Letters 11 (2001) 833–835
Diaryl Ester Prodrugs of FR900098 with Improved In Vivo
Antimalarial Activity
Armin Reichenberg,a Jochen Wiesner,a,b Claus Weidemeyer,a Erhard Dreiseidler,a
Silke Sanderbrand,a,b Boran Altincicek,a,b Ewald Beck,b Martin Schlitzerc
and Hassan Jomaaa,*
aJomaa Pharmaka GmbH, Frankfurter Straße 50, D-35392 Gießen, Germany
bBiochemisches Institut der Universitatsklinik Gießen, Friedrichstraße 24, D-35392 Gießen, Germany
¨
cInstitut fur Pharmazeutische Chemie, Philipps-Universitat Marburg, Marbacher Weg 6, D-35032 Marburg, Germany
¨
¨
Received 27 November 2000; revised 5 January 2001; accepted 31 January 2001
Abstract—The fosmidomycin derivative FR900098 represents an inhibitor of the 1-deoxy-d-xylulose 5-phosphate (DOXP) reduc-
toisomerase with potent antimalarial activity. Prodrugs of FR900098 with increased activity after oral administration were obtained
by chemical modification of the phosphonate moiety to yield phosphodiaryl esters. One diaryl ester prodrug demonstrated efficacy
in mice infected with the rodent malaria parasite Plasmodium vinckei comparable to ip drug administration. # 2001 Elsevier
Science Ltd. All rights reserved.
Malaria is one of the most serious tropical diseases
causing between 1.5 and 2.7 million fatal cases per year.
Nearly all fatal cases are caused by Plasmodium
falciparum, the causative agent of Malaria tropica. This is
largely due to the widespread emergence of P. falciparum
strains that are resistant to presently available drugs. As
a result, there is an urgent need for new efficient anti-
malarial agents.1
multidrug resistant P. falciparum strains in vitro.2 In a
murine malaria model, mice infected with P. vinckei
were cured by both drugs administrated orally (po) or
by intraperitoneal (ip) injection.2 However, the efficacy
was lower upon oral administration presumably due to
a poor rate of absorption.5 To increase bioavailability
via oral administration, the charged phosphonate moi-
ety of FR900098 was transformed into phosphodiaryl
esters expected to be hydrolysed by non-specific plasma
esterases.6 FR900098 was chosen as the lead molecule
instead of fosmidomycin since it possesses approxi-
mately 2-fold greater antimalarial activity.2
In previous studies we have demonstrated that inhibi-
tors of 1-deoxy-d-xylulose 5-phosphate (DOXP) reduc-
toisomerase exhibit considerable antimalarial activity.2
The DOXP reductoisomerase converts DOXP into 2-C-
methyl-d-erythritol 5-phosphate (MEP) and is an
essential enzyme of the MEP pathway providing iso-
pentenyl diphosphate (IPP) for isoprenoid biosynthesis.
In addition to malaria parasites, the MEP pathway is
present in algae, higher plants, and most bacteria.3 In
mammals, however, IPP is synthesised by the mevalonate
pathway while the MEP pathway is absent.
Synthesis of the prodrugs 7a–c was accomplished fol-
lowing the sequence shown in Scheme 1.7 First, Arbusov
reaction of triethylphosphite with 1,3-dibromopropane
Fosmidomycin and its derivative FR900098 (Fig. 1) are
strong and selective inhibitors of the DOXP reducto-
isomerase4 and were found to inhibit the growth of
*Corresponding author. Tel.: +49-641-9947528; fax: +49-641-
9947529; e-mail: kontakt@jomaa-pharmaka.de
Figure 1. Structures of fosmidomycin and FR900098.
0960-894X/01/$ - see front matter # 2001 Elsevier Science Ltd. All rights reserved.
PII: S0960-894X(01)00075-0

834
A. Reichenberg et al. / Bioorg. Med. Chem. Lett. 11 (2001) 833–835
Scheme 1. Reagents and conditions: (i) 0.2 equiv triethylphosphite, 30 min, 160 ꢁC; (ii) trimethylbromosilane, 30 min, 0 ꢁC and overnight, rt; (iii)
H2O; (iv) PCl5, CHCl3, 2.5 h, 70 ꢁC; (v) phenols (see table), pyridin, 1 h, 0 ꢁC and overnight, rt; (vi) BocONHBoc, NaH, DMF, overnight, rt; (vii)
trifluoroacetic acid, CH2Cl2, 6 h, 0 ꢁC; (viii) triethylamine, acetylchloride, ether, 30 min, 0 ꢁC and 2 h, rt.
provided diethylphosphonate 1, which was cleaved with
trimethylbromosilane to the corresponding phosphonic
acid 2. Chlorination of 2 with PCl5 and quenching of
(tert-Boc)-hydroxylamine gave the fully protected pre-
cursers 5a–c. Deprotection with trifluoroacetic acid to
trifluoroacetates 6a–c and acylation with acetylchloride
led to the desired prodrugs 7a–c.
the intermediate phosphonodichloride
3 with the
appropriate phenol resulted in formation of the diphe-
nylphosphonoesters 4a–c. Subsequently, condensation
of the bromides 4a–c with the sodium salt of N,O-bis-
The antimalarial activity of the prodrugs was tested in
P. vinckei-infected mice. On day 0, mice were inoculated
by ip injection of 5ꢀ107 infected erythrocytes from a
donor mouse. On days 1 and 2, the mice were treated
orally with sub-therapeutic dosages (0.16 mmol/kg) of
FR900098 and diaryl ester prodrugs 7a–c.8 For com-
parison, one group of mice was treated with an equiva-
lent dosage of FR900098 by ip injection. On the
following days, parasitemia was monitored to assess the
efficacy of the compounds. In all treated animals, the
parasitemia was markedly reduced compared to
untreated control animals (Fig. 2). The unsubstituted
diphenyl ester 7a was significantly more active than
FR900098 (po). In contrast, the bis-(2-methyl-phenyl)
ester 7b was significantly less active than FR900098
(po). We interpret these results as indicative of an
increased hydrolytic stability of the ester due to steric
hinderance by the 2-methyl substituent. The most suc-
cessful derivative of this series was the bis-(4-methoxy-
phenyl) ester 7c which was not only much more efficient
than FR900098 (po) but proved to be equivalent to
FR900098 administrated by ip route.
In summary, prodrugs of FR900098 with significantly
increased antimalarial activity were obtained by mask-
ing the polar phosphonate moiety as diaryl esters.
Establishing detailed SARs with regard to the substitu-
tion pattern of the aryl residues remains to be the sub-
ject of further investigation. In addition, the prodrugs
will be evaluated in non-human primates which possess
a more human-like metabolism and can be infected with
human malaria parasites.
Figure 2. Antimalarial efficacy of FR900098 po and ip and of the
prodrugs 7a–c.

A. Reichenberg et al. / Bioorg. Med. Chem. Lett. 11 (2001) 833–835
835
Acknowledgements
Chem. 1962, 655, 59. (d) Berlin, K. D.; Butler, G. B. J. Org.
Chem. 1960, 17, 2006. (e) Satoh, Y.; Powers, C.; Toledo, L. M.;
Kowalski, T. J.; Peters, P. A.; Kimble, E. F. J. Med. Chem.
1995, 38, 68. Typical experimental procedure for the following
steps: 1.02 g (2 mmol) of compound 5a were dissolved in
15 mL of methylenechloride, cooled to 0 ꢁC and 8.55 g
(75 mmol) of trifluoroacetic acid were added dropwise with
stirring. Cooling was maintained for a further 6 h. After the
solvent and excess trifluoroacetic acid were removed by rotary
evaporation, the residue was co-evaporated three times with
each 5 mL portion of chloroform and treated twice with
5 mL pentane. The pentane layer was removed and the resi-
due dried in vacuo to leave 0.78 g (1.85 mmol, 93%) of the
trifluoroacetate 6a. After flushing the flask with argon, 20 mL
of dry ether was added and cooled to 0 ꢁC. 0.37 g (3.7 mmol)
of triethylamine in 5 mL of dry ether were dropped into the
flask via a syringe, followed immediately by dropwise addition
of 0.15 g (1.85 mmol) of acetyl chloride in 5 mL of dry ether.
After stirring for 30 min at 0 ꢁC and 2 h at rt, 30 mL of 1 N
HCl, 2ꢀ20 mL of brine and dried with Na2SO4. Filtration and
The authors would like to thank Dajana Henschker and
Rainer Schmidt for excellent technical assistance and
the Institute of Organic Chemistry of the University of
Gießen for generous support.
References and Notes
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7. (a) 1 to 5a–c were synthesised according to established
procedures: Hewitt, D. G.; Teese, M. W. Aust. J. Chem. 1984,
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evaporation of the solvent yield 0.50 g (71%) of the desired
1
prodrug 7a. HNMR (CDCl /TMSint): d=1.93–2.25 (m, 7H,
3
CH2P, CH2CH2P, CH3); 2.01 (s, 3H, CH3); 3.62–3.84 (m, 2H,
NCH2); 7.02–7.22 and 7.23–7.40 (each m, 10H, Harom.). 13C
NMR (CDCl3/TMSint): d=19.6 (CH2CH2P); 20.3 (CH3); 23.0
(d, JCꢂP=142.3 Hz, CH2P); 47.6 (d, JCꢂP=14.9 Hz, NCH2);
120.5 (d, JCꢂP=3.8 Hz, Carom.); 125.5, 129.9 (Carom.); 150.1 (d,
JCꢂP=9.1 Hz, Cipso); 172.9 (C¼O).
8. FR900098 was dissolved in PBS at 3 mg/mL. Prodrugs 7a–c
were dissolved in DMSO at 150 mg/mL and further diluted in
PBS. Parasitemia was monitored by Giemsa-stained blood
smears. Mice were euthanised when parasitemia was exceeding
app. 40%.