Highly antimalaria-active artemisinin derivatives: Biological activity does not correlate with chemical reactivity

Article abstract of DOI:10.1002/anie.200352343

New leads for antimalarial drugs? 10-Alkylaminoartemisinin derivatives like that shown display in vivo antimalarial activities greater than those of the parent artemisinin. Previous conclusions regarding the chemical basis for the antimalarial activity of these compounds must be reconsidered.

Full text of DOI:10.1002/anie.200352343

Angewandte
Chemie
most potent and rapidly acting antimalarial drugs.[1–3] Never-
theless, artemether (3) and artesunate (4) have short terminal
half-lives, a consequence of facile metabolism to DHA,[4]
a
highly neurotoxic compound.[5–8] Therefore, there is an urgent
need to develop practical routes to new derivatives that are
more stable and do not undergo metabolism to DHA.[2,3,9]
Antimalarial Drugs
Highly Antimalaria-Active Artemisinin
Derivatives: Biological Activity Does Not
Correlate with Chemical Reactivity**
Richard K. Haynes,* Wing-Yan Ho, Ho-Wai Chan,
Burkhard Fugmann, Jörg Stetter, Simon L. Croft,
Livia Vivas, Wallace Peters, and Brian L. Robinson
The compounds act on all stages of the intraerythrocytic
parasite, induce ultrastructural changes in parasite mem-
branes,[10,11] and affect membrane and other proteins.[12] The
bioactive agents are held to be C-centered radicals, generated
by scission of the peroxide to give an alkoxyl radical, followed
by abstraction of a hydrogen atom at C4 and cleavage of the
Artemisinin (1, qinghaosu), and the derivatives dihydroarte-
misinin (DHA, 2), artemether (3), and artesunate (4) are the
[*] Prof. Dr. R. K. Haynes, W.-Y. Ho, Dr. H.-W. Chan
Department of Chemistry
À
C3 C4 bond (Scheme 1). Peroxide scission is induced either
Hong Kong University of Science and Technology
Clear Water Bay, Kowloon, Hong Kong (P.R. China)
Fax: (+852)2358-1594
E-mail: haynes@ust.hk
Dr. B. Fugmann+
Bayer AG Central Research
Head, Medicinal Chemistry
51368 Leverkusen (Germany)
Prof. Dr. J. Stetter++
Bayer AG Central Research
Chemistry for Life Sciences
51368 Leverkusen (Germany)
Scheme 1. Proposed formation of C-centered radicals from artemisinin (1) by fer-
rous heme [FeIIPPIX] or exogenous FeII. FeIII ꢀferric heme or FeIII. Detailed discus-
sions are given in refs.[25,27].
Dr. S. L. Croft, Dr. L. Vivas
Department of Infectious and Tropical Diseases
London School of Hygiene and Tropical Medicine
Keppel Street, London WC1E 7HT (UK)
by heme within the parasite food vacuole or by non-heme,
“exogenous” iron. In both cases, iron must be in the + ii
state.[13,14] The C-centered radicals are held to alkylate
undefined sensitive biomolecules in the parasite.[15–21] As
artemisinin (1) reacts with heme to give covalent adducts,[22,23]
a correlation has been drawn between this type of reaction
and antimalarial activity for trioxanes in general.[24] For
exogenous iron, trapping experiments with cysteine are also
interpreted in terms of C-centered radicals.[25] On the basis of
the hypothesis,[26] “rational design” in the synthesis of new
analogues[15] and modular antimalarial drugs[27] has been
claimed.
Prof. Dr. W. Peters, B. L. Robinson
Centre for Tropical Antiprotozoal Chemotherapy
Northwick Park Institute for Medical Research
Block Y, Watford Road, Harrow, Middlesex HA1 3UJ (UK)
[+] Current affiliation:
Bayer AG, BBS, Science and Technology
51368 Leverkusen (Germany)
++] Current affiliation:
[
Bayer AG, Corporate Development, Head of Innovation
51368 Leverkusen (Germany)
[**] This work was supported by grants from Bayer AG (Leverkusen,
Germany) and by the Government of the HKSAR University Grants
Council (grant no. HKUST 6091/02P).
We are preparing new artemisinin derivatives in a
program in which we address the problems of metabolism
to DHA and of neurotoxicity, and seek to improve efficacy
through the application of pharmacokinetic parameters
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
Angew. Chem. Int. Ed. 2004, 43, 1381 –1385
DOI: 10.1002/anie.200352343
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Table 1: In vivo screens against the chloroquine (CQ)-sensitive P. berghei N strain and the CQ-resistant
P. yoelii NS strain according to the Peters four-day test; mice treated daily subcutaneously (sc) or orally
(po) from day of infection (D0) through D +3; results (ED90 [mgkgÀ1]) evaluated from parasite counts in
peripheral blood on D+4.
designed to enhance solubility and
adsorption of the neutral drug.[2]
We now report that some of our
compounds display very high activ-
ity in vivo against the malaria
parasite, and they thereby provide
an opportunity to test the current
hypothesis for the mode of action.
A solution of the DHA trime-
Cmpd.
P. berghei
ED90
P. berghei
ED90
P. berghei
artesunate
index[a]
P. yoelii
ED90
P. yoelii
P. yoelii
artesunate
index[a]
sc
ED90
[mgkgÀ1: sc]
[mgkgÀ1: po]
[mgkgÀ1: sc]
[mgkgÀ1: po]
sc
po
4
17
18
19
20
22
4.6
9.3
3.5
2.4
1.5
1.3
5.0
1.0
3.2
5.9
10.0
25.6
4.0
1.0
42.0
22.0
0.85
0.52
1.25
1.08
–
–
3.0
2.0
1.84
–
1.0
1.9
49.4
81.0
33.6
39.6
1.45
0.78
0.46
0.18
1.16
2.66
3.88
6.2
7.15
1.86
thylsilyl ether 5 in dichlorome-
thane was treated with bromotri-
methylsilane[28] and then with an
excess of a primary or secondary
alkylamine to give exclusively the
10a-alkylamino derivatives 6–20
(Scheme 2). Arylamino deriva-
[a] ED90(4)/ED90(derivative).
tives, for example, 21, may also be prepared.[29] The reaction
proceeds by means of nucleophilic displacement by the amine
on the unstable 10b (axial) bromide intermediate, which is
given in Table 1.[30] For comparative purposes, data acquired
in the same screens for artesunate (4), the most active of the
artemisinin derivatives currently in clinical use, and the aryl
derivative 22 are also included.[28] For P. berghei, the amino
derivatives are 3–26 times more active than artesunate when
administered subcutaneously (sc), and 3–7 times more active
when administered orally (or). For P. yoelii, the compounds
are 2–81 times more active by the sc route (Table 1). The
activities of compounds 18–20 in the murine malaria models
appear to be superior to those of any other peroxide-
containing antimalarial compound, either derived from
artemisinin or of the synthetic trioxane-type, ever recorded.
Thus, we have identified a new class of artemisinin derivatives
with the potential for further development.
detectable in reaction mixtures by H NMR spectroscopy.[28]
1
Whilst yields are moderate (up to 60%), the method is
straightforward and applicable to primary and secondary
alkylamines, and primary aromatic amines.
Results of in vivo screens for compounds 17–20 conducted
against chloroquine(CQ)-sensitive P. berghei and CQ-resis-
tant P. yoelii in mice according to the Peters' four-day test are
In order to evaluate the correlation between the antima-
larial efficacy of these compounds with their reactivity
towards ferrous heme or FeIII, we also include the hydrolyti-
cally stable compounds 22 and 23. The former possesses very
good in vivo activities, especially against CQ-resistant P. yoelii
(Table 1), and the latter, unlike artemisinin (1) and DHA (2),
does not interact at all, for example, by complex formation,
with ferric heme, and thereby does not interfere in formation
of b-hematin or hemozoin, the heme dimer.[31] In comparison
with artemisinin (1), compound 23 possesses superior in vitro
activity against P. falciparum and is approximately equipotent
in vivo against P. berghei.[32]
The heme experiments were carried out as previously
described.[13] The artemisinin derivative, heme (ferriproto-
porphyrin IX chloride), and l-cysteine (1 equiv of each) were
stirred in buffered solution of deionized water/acetonitrile
(1:1, pH 7.4) under nitrogen in the dark for 24 h.[33] After
extraction with ether and dichloromethane, and then filtra-
Scheme 2. Preparation of C10 aminoalkyl and aminoaryl derivatives
6–21 of DHA (2). a) 1. SiMe3Br, CH2Cl2, 08C, 45 min; 2. alkylamine
(2.0 equiv), 08C to room temperature, 12 h, then NH4Cl/H2O.
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tion through Celite, the crude product mixture was examined
1
by H NMR spectroscopy with an internal standard (1,3,5-
trimethoxybenzene) to determine the amounts of the
unreacted derivative. The signal for H12 (for numbering,
see 1), a singlet which appears away from other signals in the
spectra of these compounds, was used as the reference signal.
The major products are presumed to be heme adducts of the
artemisinin derivatives, which are insoluble in organic sol-
vents;[13] no attempt was made to isolate them. The heme
adduct from artemisinin has been thoroughly character-
ized.[23] For the amino derivatives 18 and 19, the most
significant by-product was DHA (2); very little of any other
product was obtained for the other compounds (Table 2). The
most striking observation is that compounds 22 and 23 do not
undergo significant decomposition.
Table 2: Reaction of artemisinin derivatives with ferriprotoporphyrin IX
chloride/l-cysteine (each 1 equiv) in buffered MeCN/H2O (pH 7.4)
during 24 h at 238C.
the poor reactivity of the first two compounds towards FeII
Cmpd.
Unreacted cmpd. [%]
Other products (%)
arises because the basic piperazinyl groups prevent the iron
from entering into the redox chemistry associated with
cleavage of the peroxide (cf. Scheme 1) through modulation
of the pH of the aqueous reaction medium. However, that the
FeII in fact remains catalytically active was established by
treatment of equimolar mixtures of compounds 19 and 23
with iron(ii) sulfate under the foregoing conditions. Each
compound in the mixture behaved essentially as it did alone;
that is, FeII remains catalytically active in inducing decom-
position of 23, whereas 19 was still unreactive (Table 3).
Reactions with compound 23 were also run in the presence of
2-naphthalenethiol and iron(ii) sulfate at different concen-
trations (0.005, 0.3, and 1.0 equiv). This did not greatly affect
yields or ratios of products, although
18
19
20
22
23
18
62
67
>99
>99
2 (6), 24 (6), 25 (4)
2 (18), 25 (2.5)
not determined
not determined
27 + 30 (<1)
Next, the reactivity of the derivatives towards FeII was
assessed (Table 3). Iron(ii) sulfate in aqueous MeCN was
used, in line with established practice in this type of
Table 3: Reaction of artemisinin derivatives with FeII sulfate in 1:1
MeCN/water during 24 h at 238C.
the unstable adduct 32 was obtained in
variable, but low, yields.[37] Its forma-
Artemisinin FeSO4·7H2O Unreacted
derivative
Other products (%)
[equiv]
derivative [%]
tion belies the elaborate explana-
tion[25] put forward for formation of
18
0.3
1.0
0.3
1.0
0.3
96
63
97
78
0
24 (3), 25 (1)
24 (25), 25 (12)
24 (2), 25 (1)
24 (16), 25 (6)
26 (1), 27 (41), 28 (51), 29
(5), 31 (1.5)
cysteine adducts in related experi-
ments.
In summary, we have developed a
straightforward route to highly anti-
19
23
malaria-active artemisinin derivatives, which will be amena-
ble for development of selected lead compounds. We also
show that the hydrolytically stable compounds 22 and 23 are
inert to ferrous heme, whereas under essentially the same
conditions, compound 23 is completely decomposed by free
FeII. In further contrast with the reactivity of 23, the highly
antimalaria-active alkylamino derivatives 18 and 19 are
relatively unreactive towards free FeII. Their reactions in
the presence of ferrous heme appears to depend partially on
the nature of the amino group at C10. In any event, the
formation of the hydrolysis product, DHA (2), in the heme
reactions indicates activation of the piperazinyl substituent by
complexation with heme-FeII or -FeIII to displacement by
water in the reaction medium takes place. In the reactions
involving free FeII, it is possible that the piperazine forms a
complex with FeII, and the amine–iron complex blocks access
of further iron to the peroxide bridge.
1.0
0.3
1.0
0
26 (<1), 27 (46), 28 (45), 29
(6), 31 (2.0)
19 + 23
(1 equiv)
19: 96, 23: 4 24 (3), 25 (1), 26 (6), 27
(43), 28 (41), 29 (6), 31 (0)
19: 89, 23: 0 24 (7), 25 (4), 26 (13), 27
(41), 28 (41), 29 (5), 31 (0)
study.[20,21,25,34] The artemisinin derivative with iron(ii) sulfate
in 1:1 deionized water/MeCN was stirred at 238C for 24 h.
The mixture was extracted with dichloromethane, and the
products (24–31) were isolated by chromatography. Identity
was secured by spectroscopic characterization and by com-
parison with literature data.[35,36]
Notably, the highly antimalarial-active compounds 18 and
19 displayed poor reactivity towards FeII. In contrast, com-
pound 23 was completely decomposed. It may be argued that
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These results, coupled with those of earlier work,[13,31] and
Ser. B 1999, 354, 739 – 749; World Health Organization, Trans. R.
Soc. Trop. Med. Hyg. 2000, 94, Supplement 1, 36 – 37.
[2]R. K. Haynes, Curr. Opin. Infect. Dis. 2001, 14, 719 – 726.
[3]Y. Li, Y.-L. Wu, Curr. Med. Chem. 2003, 10, 2197 – 2231.
[4]V. Navaratnam, S. M. Mansor, N. W. Sit, J. Grace, Q. G. Li, P.
Olliaro, Clin. Pharmacokinet. 2000, 39, 255 – 270.
[5]R. F. Genovese, D. B. Newman, T. G. Brewer, Pharmacol.
Biochem. Behav. 2000, 67, 37 – 44.
[6]A. Nontprasert, S. Pukrittayakamee, M. Nosten-Bertrand, S.
Vanijanonta, N. J. White, Am. J. Trop. Med. Hyg. 2000, 62, 409 –
412.
[7]G. Schmuck, R. K. Haynes, Neurotoxic. Res. 2000, 2, 37 – 49.
[8]G. Schmuck, E. Roehrdanz, R. K. Haynes, R. Kahl, Antimicrob.
Agents Chemother. 2002, 46, 821 – 827.
[9]F. Chorki, F. Grellepois, B. Crousse, V. D. Hoang, N. Van Hung,
D. Bonnet-Delpon, J. P. Begue, Org. Lett. 2002, 4, 757 – 759.
[10]D. S. Ellis, Z. L. Li, H. M. Gu, W. Peters, B. L. Robinson, G.
Tovey, D. C. Warhurst, Ann. Trop. Med. Parasitol. 1985, 79, 367 –
374.
[11]P.-Q. Chen, J. Yuan, Q.-Y. Du, Zhongguo Yaoli Xuebao 2000, 21,
234 – 238 [Chem. Abstr. 2000, 132, 317662, 2000:190391].
[12]W. Asawamahasadka, I. Ittarat, Y. M. Pu, H. Ziffer, S. R.
Meshnick, Antimicrob. Agents Chemother. 1994, 38, 1854 –
1858; J. Bhisutthibhan, X.-Q. Pan, P. A. Hossler, D. J. Walker,
C. A. Yowell, J. Carlton, J. B. Dame, S. R. Meshnick, J. Biol.
Chem. 1998, 273, 16192 – 16198; T. Akompong, J. VanWye, N.
Ghori, K. Haldar, Mol. Biochem. Parasitol. 1999, 101, 71 – 79.
[13]R. K. Haynes, S. C. Vonwiller, Tetrahedron Lett. 1996, 37, 253 –
256.
other arguments (see below), seem now to exclude the
possibility that an activation pathway involving heme-FeII
underpins antimalarial activity of artemisinin derivatives. In
the case of free, that is, non-heme or “exogenous” FeII, others
have sought to demonstrate a correlation between antima-
larial activity and reactivity under the reaction conditions
described herein.[20,21,25,34,38] However, as no such correlation
exists for our compounds,[39] the use of such a probe to
establish the chemical basis for antimalarial activity in general
has to be treated with considerable caution, especially as so
little is known of the biological environment in which the
artemisinin derivative purportedly reacts with the iron. Given
that the hypothesis involving “activation” of artemisinins by
FeII, and the migration and reaction of the resulting C-
centered radicals with sensitive biomolecules has its genesis in
such studies,[15] the hypothesis itself must also be brought into
question.
We have noted previously that several antimalaria-active
trioxanes cannot obviously generate C-centered radicals
according to the formalism of Scheme 1.[14,18,40] Whilst many
peroxides are susceptible to reductive cleavage by ferrous
iron,[41] a large number also display at best feeble antimalarial
activity,[36,42] and further, the FeIII formed by oxidation of FeII
by the peroxide (cf. Scheme 1) is capable of oxidizing the C-
centered radicals to carbocations.[40,43,44] Artemisinin deriva-
tives do not inhibit but rather are potent inducers of PhaseI-
metabolizing P450 (CYP) enzymes, in which the heme
hydroxylates the periphery of the molecule syn to the
peroxide without interfering with it.[2] Further, the seco-C4
radical (Scheme 1) held to be produced in the reaction with
ferrous heme clearly cannot be trapped by the l-cysteine
(present work), or other thiols,[13] used to generate ferrous
heme; clearly, if the C-centered seco-C4 is formed at all, it
reacts exclusively with the proximate heme. In general,
artemisinin and derivatives affect intraerythrocytic stages of
the parasite in which catabolism of hemoglobin to produce
heme does not take place, namely tiny rings and gametocytes,
and further, locate to parasite membranes, not the food
vacuole containing the heme.[10,45] Overall, the weight of
evidence suggests that binding of the artemisinin to a specific
target is involved, as we have pointed out previously.[2,18,31]
The peroxide itself may be activated concomitantly with, or
subsequent to, binding to generate either hydroperoxide, or
derived electrophilic oxygenating agent, or oxygen-centered
radical capable of inducing irreversible inhibition.[2,18,31,46]
[14]R. K. Haynes, S. C. Vonwiller, Tetrahedron Lett. 1996, 37, 257 –
260.
[15]See inter alia G. H. Posner, C. H. Oh, J. Am. Chem. Soc., 1992,
114, 8328 – 8329; G. H. Posner, C. H. Oh, D. Wang, L. Gerena,
W. K. Milhous, S. R. Meshnick, W. Asawamahasadka, J. Med.
Chem. 1994, 37, 1256 – 1258; G. H. Posner, J. N. Cumming, P.
Ploypradith, C. H. Oh, J. Am. Chem. Soc. 1995, 117, 5885 – 5886;
G. H. Posner, S. B. Park, L. Gonzalez, D. Wang, J. N. Cumming,
D. Klinedinst, T. A. Shapiro, M. D. Bachi, J. Am. Chem. Soc.
1996, 118, 3537 – 3538; P. M. O'Neill, A. Miller, L. P. D. Bishop,
S. Hindley, J. L. Maggs, S. A. Ward, S. M. Roberts, F. Schein-
mann, A. V. Stachulski, G. H. Posner, B. K. Park, J. Med. Chem.
2001, 44, 58 – 68.
[16]S. R. Meshnick, T. E. Taylor, S. Kamchonwongpaisan, Microbiol.
Rev. 1996, 60, 301 – 315.
[17]S. R. Meshnick, . C. W. Jefford, G. H. Posner, M. A. Avery, W.
Peters, Parasitol. Today 1996, 12, 79 – 82; J. N. Cumming, P.
Ploypradith, G. H. Posner, Adv. Pharmacol. 1997, 37, 253 – 297;
G. H. Posner, J. N. Cumming, M. Krasavin, Biomed. Chem. 2000,
289 – 309; G. H. Posner, M. Krasavin, M. McCutchen, P.
Ploypradith, J. P. Maxwell, J. S. Elias, M. H. Parker in Antima-
larial Chemotherapy (Ed.: P. J. Rosenthal), Humana Press, 2001,
pp. 255 – 263.
[18]P. Olliaro, R. K. Haynes, B. Meunier, Y. Yuthavong, Trends
Parasitol. 2001, 17, 122 – 126.
[19]S. R. Meshnick, Int. J. Parasitol. 2002, 32, 1655 – 1660.
[20]W.-M. Wu, Y.-K. Wu, Y.-L. Wu, Z.-J. Yao, C.-M. Zhou, Y. Li, F.
Shan, J. Am. Chem. Soc. 1998, 120, 3316 – 3325.
Received: July 9, 2003 [Z52343]
Published Online: February 16, 2004
Keywords: heme proteins · iron · medicinal chemistry ·
peroxides · radicals
[21]D.-Y. Wang, Y.-K. Wu, Y.-L. Wu, Y. Li, F. Shan, J. Chem. Soc.
Perkin Trans. 1 1999, 1827 – 1831.
[22]Y. L. Hong, Y. Z. Yang, S. R. Meshnick, Mol. Biochem. Parasitol.
1994, 63, 121 – 128.
.
[23]A. Robert, J. Cazelles, B. Meunier, Angew. Chem. 2001, 113,
2008 – 2011; Angew. Chem. Int. Ed. 2001, 40, 1954 – 1957.
[24]A. Robert, B. Meunier, Chem. Eur. J. 1998, 4, 1287 – 1296; F.
Zouhiri, D. Desmale, J. D'Angelo, J. Mahuteau, C. Riche, F.
Gay, L. CicØron, Tetrahedron Lett. 1998, 39, 2897 – 2906; F.
Zouhiri, D. Desma le, J. D'Angelo, J. Mahuteau, C. Riche, F.
[1]Y. Li, Y.-L. Wu, Med. Trop. 1998, 58, Supplement 3, 9 – 12, and
references therein; P. I. Trigg, Econ. Med. Plant Res. 1989, 3, 20 –
55; T. T. Hien, N. J. White, Lancet 1993, 341, 603 – 608; F. Nosten,
T. T. Hien, N. J. White, Med. Trop. 1998, 58, 45 – 49; M. A.
van Agtmael, T. A. Eggelte, C. J. van Boxtel, Trends Pharmacol.
Sci. 1999, 20, 199 – 205; N. J. White, Philos. Trans. R. Soc. London
1384
ꢀ 2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
Angew. Chem. Int. Ed. 2004, 43, 1381 –1385

Angewandte
Chemie
Gay, L. CicØron, Eur. J. Org. Chem. 1998, 2897 – 2906; O. Provot,
B. Camuzat-Dedenis, M. Hamzaoui, H. Moskowitz, J. Mayr-
argue, A. Robert, J. Cazelles, B. Meunier, F. Zouhiri, D.
Desmale, J. D'Angelo, J. Mahuteau, F. Gay, L. CicØron, Eur.
J. Org. Chem. 1999, 1935 – 1938.
3.06 (m, 2H), 2.55–2.39 (m, 2H), 1.89 (s, 3H), 1.81–1.77 (m, 1H),
1.69–1.52 (m, 2H), 1.53–1.04 (m, 6H), 0.96–0.94 (d, J = 6.3 Hz,
3H), 0.78–0.76 ppm (d, J = 7.2 Hz, 3H).
[38]In ref. [21,]it is reported that stereoisomeric naphthyl-10-deoxo-
10-dihydroartemisinin derivatives differing in configuration at
C9 and C10 react at markedly different rates with FeSO4 in
aqueous MeCN; the less reactive isomer in this case displays
very low activity against P. falciparum in vitro.
[39]A referee contends that differences in reactivity may be due to
different solubilities of the artemisinin derivatives in the reaction
medium (1:1 MeCN/H2O). In this respect, compound 20 is
observed to be poorly soluble, but it is indeed decomposed by
FeII (0.3 equiv) under the standard conditions (Table 3) to leave
unreacted 20 (36%) and provide products 24 (13%) and 25
(6%), in addition to large amounts of as yet uncharacterized
products bearing the morpholino substituent. Further, the
contrasting reactivities of the benzylpiperazino compound 18
and 10-deoxo compound 23 towards each of heme-FeII and -FeII
under essentially identical reaction conditions (Tables 2 and 3)
are not in key with a solubility factor.
[40]R. K. Haynes, H.-O. Pai, A. Voerste, Tetrahedron Lett. 1999, 40,
4715 – 4718.
[41]For reviews and examples, see Organic Peroxides (Ed.: W.
Ando), Wiley, Chichester, 1992, chap. 3, pp. 174–178; Organic
Peroxides (Ed.: W. Ando), Wiley, Chichester, 1992, chap. 6,
pp. 295–304.
[42]G. Rucker, E. Breitmaier, D. Manns, W. Maier, A. Marek, B.
Heinzmann, K. Heiden, S. Seggeweis, Arch. Pharm. 1997, 330,
12 – 16.
[43]W. Herz, R. C. Ligroin, J. A. Turner, J. F. Blount, J. Org. Chem.
1977, 42, 1885.
[25]Y.-K. Wu, Z.-Y. Wu, Y.-L. Wu, Angew. Chem. 1999, 38, 2730 –
2733; Angew. Chem. Int. Ed. 1999, 38, 2580 – 2582; Y.-K. Wu,
Z.-Y. Yue, H.-H. Liu, Helv. Chim. Acta 2001, 84, 928 – 932; Y.-K.
Wu, Acc. Chem. Res. 2002, 35, 255 – 259.
[26]In ref. [19,] the statement is made to the effect that “19 other
research groups in 11 countries have corroborated the impor-
tance of heme in the activation of artemisinin and its deriva-
tives”.
[27]O. Dechy-Cabaret, F. Benoit-Vical, A. Robert, B. Meunier,
ChemBioChem 2000, 1, 281 – 283; A. Robert, O. Dechy-Cabaret,
J. Cazelles, B. Meunier, Acc. Chem. Res. 2002, 35, 167 – 174.
[28]R. K. Haynes, H. W. Chan, M.-K. Cheung, S.-K. Chung, W.-L.
Lam, H.-W. Tsang, A. Voerste, I. D. Williams, Eur. J. Org. Chem.
2003, 2098 – 2114.
[29]Preparation of 10 a-substituted alkylamino derivatives has not
been previously reported. For preparation of the 10-arylamino
derivatives, see Y.-H. Yang, Y. Li, Y.-L Shi, J.-DYang, B.-A. Wu,
Bioorg. Med. Chem. Lett. 1995, 5, 1791 – 1794; for an indirect
preparation of the 10-arylamino derivatives bearing bromine at
C9, see A. J. Lin, L.-Q. Li, D. L. Klayman, C. F. George, J. L.
Flippen-Anderson, J. Med. Chem. 1990, 33, 2610 – 2614. Whilst
these compounds display good efficacy in vitro, they have no
significant activity in vivo.
[30]W. Peters, B. L. Robinson in Handbook of Animal Models of
Infection (Eds.: O. Zak, M. Sande), Academic Press, London,
1999, chap. 92, Section VI, pp. 756–771.
[44]A. M. Szpilman, E. E. Korshin, R. Hoos, G. H. Posner, M. D.
Bachi, J. Org. Chem. 2001, 66, 6531 – 6540.
[45]F. ter Kuile, N. J. White, P. H. Holloway, G. Pasvol, S. Krishna,
Exp. Parasitol. 1993, 76, 85 – 95.
[31]R. K. Haynes, D. Monti, D. Taramelli, N. Basilico, S. Parapini, P.
Olliaro, Antimicrob. Agents Chemother. 2003, 47, 1175; R. K.
Haynes, D. Monti, D. Taramelli, N. Basilico, S. Parapini, P.
Olliaro, Antimicrob. Agents Chemother. 2003, 47, 2712 – 2713.
[32]In vitro IC 50 [ngmLÀ1]values against W2 (chloroquine-resistant)
and D6 strains of P. falciparum: 23: 0.58, 0.15; artemisinin (1):
2.33, 1.21; in vivo against P. berghei in mice, 23 single dose D + 3,
640 mgkgÀ1 5/5 cures: M. Jung, X. Li, D. A. Bustos, H. N.
El Sohly, J. D. McChesney, W. K. Milhous, J. Med. Chem. 1990,
33, 1516 – 1518; in vivo against P. berghei in mice, 23 daily dose
60 mgkgÀ1, 100% cure rate, artemisinin (1) 96% cure rate: B.
Ye, G.-F. Wu, X.-Q. Jiao, Acta Pharm. Sin. 1991, 26, 228 – 230.
[33]The reactivity of artemisinin ( 1) under these conditions has been
established; see ref. [13].
[46]After submission of the manuscript, it was reported that
artemisinin (1) exerts its antimalarial activity through irrever-
sible inhibition of the Ca2+-dependent Plasmodium falciparum
ATPase (PfATP6), as expressed in Xenopus oocytes: U.
Eckstein-Ludwig, R. J. Webb, I. D. A. Van Goethem, J. M.
East, A. G. Lee, M. Kimura, P. M. O'Neill, P. G. Bray, S. A.
Ward, S. Krishna, Nature 2003, 424, 957 – 961. The enzyme is a
transmembrane protein associated with the parasite endoplas-
mic reticulum. This beautiful work convincingly demonstrates
the irrelevance of the heme-activation idea. Most importantly,
the identification of the target will now allow for a rational
examination of the mechanism of action of this remarkable class
of antimalarial drug and the precise role, if any, played by iron in
artemisinin activation at the binding site.
[34]W.-M. Wu, Z.-J. Yao, Y.-L. Wu, K. Jiang, Y.-F. Wang, H.-B. Chen,
F. Shan, Y. Li, Chem. Commun. 1996, 2213 – 2214; D.-Y. Wang,
Y.-L. Wu, Y. Wu, J. Liang, Y. Li, J. Chem. Soc. Perkin Trans. 1
2001, 605 – 609.
[35]For compounds 24, 27,and 28, see ref. [20]; for compounds 25, 29,
and 30, see A. J. Lin, A. D. Theoharides, D. L. Klayman,
Tetrahedron 1986, 42, 2181 – 2184; for compound 26, see
ref. [36]and M. Jung, X. Li, D. A. Bustos, H. N. ElSohly, J. D.
McChesney, W. K. Milhous, J. Med. Chem. 1990, 33, 1516 – 1518;
for compound 31, see Y.-M. Pu, H. Yeh, H. Ziffer, Heterocycles
1993, 36, 2099 – 2107. Detailed explanations have been presented
previously for the formation of the products; see foregoing
references, especially refs. [13,14,20,25,36].
[36]Treatment of compound 23 with anhydrous FeBr2 in THF rapidly
provides products 26 (8%) and 29 (79%): M. A. Avery, P.-C.
Fan, J. M. Karle, J. D. Bonk, R. Miller, D. K. Goins, J. Med.
Chem. 1996, 39, 1885 – 1897.
[37]Compound 32 is easily detected by thin-layer chromatography;
however, it is admixed with approximately 15% of another
adduct, possibly the C1-naphthyl-linked compound; oil,
1H NMR spectrum (300 MHz, CDCl3): d = 8.16–7.36 (m, 7H),
5.97 (s, 1H), 3.68–3.60 (m, 1H), 3.48 (t, J = 11.7 Hz, 1H), 3.11–
Angew. Chem. Int. Ed. 2004, 43, 1381 –1385
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