Bioorganic & Medicinal Chemistry Letters
Synthesis and antifungal evaluation of
pentyloxyl-diphenylisoxazoloyl pneumocandins and echinocandins
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Sheo B. Singh , Kithsiri Herath, Jennifer Nielsen Kahn, Paul Mann, George Abruzzo, Mary Motyl
Departments of Medicinal Chemistry and Infectious Disease, Merck Research Laboratories, Merck & Co., P.O. Box 2000, Rahway, NJ 07065, USA
a r t i c l e i n f o
a b s t r a c t
Article history:
Echinocandins and pneumocandins are classes of lipocyclohexapeptides that are broad spectrum anti-
fungal agents. They inhibit fungal specific 1,3-b-glucan synthase activity which is an essential component
of the fungal cell wall. Chemical modifications of these two leads have produced three clinical agents
namely caspofungin, micafungin and anidulafungin. The presence of hydroxy-glutamine versus threonine
and unsaturated linear fatty acid versus branched chain saturated fatty acid differentiate the two classes
of compounds with profound differences in their hemolytic properties. In the current study, we have
replaced the side chain of the cyclohexapeptides with a common aromatic heterocyclic acyl side chain
and compared the biological activities of the cores head-to-head and for the first time demonstrated
the role played by the acyl chain and the hydroxy-glutamine for the antifungal potency.
Ó 2013 Elsevier Ltd. All rights reserved.
Received 6 February 2013
Revised 26 March 2013
Accepted 27 March 2013
Available online 10 April 2013
Keywords:
Caspofungin
Pneumocandins
Echinocandins
Antifungal activity
b-1,3-Glucan synthase inhibitor
Infections caused by opportunistic fungi, Candida spp., Aspergil-
lus spp., and Cryptococcus spp., can be life-threatening particularly
in immunocompromised populations.1 Systemic fungal infections
are treated by three classes of drugs including azoles (e.g., fluco-
nazole)2 polyenes (e.g., amphotericin B)3 and echinocandins (e.g.,
caspofungin (1a), micafungin (1b) and anidulafungin (2a)).4 Caspo-
fungin (1a) was the first of the echinocandins approved by the US
FDA for clinical use in 2001 followed by micafungin (1b) and ani-
dulafungin (2a). These antifungal agents are semisynthetic deriva-
tives of pneumocandins and echinocandins that show broad
spectrum antifungal activity by inhibiting fungal specific b-1,3-glu-
can synthase activity which is essential for fungal cell wall
structure.5
Echinocandins are a class of natural products characterized by
cyclohexapeptide with a long fatty acyl chain. Echinocandin B
was originally isolated in 1974 by Betz et al., and consists of a
2S,3R-threonine and a linoleoyl acyl chain.6,7 Pneumocandins are
members of the echinocandin family of cyclohexapeptides where
threonine is replaced by 3R-hydroxy-2S-glutamine and the linol-
eoyl side chain is replaced by 10R,12S-dimethlymyristoyl chain.
They were discovered in 1980’s.8 Pneumocandin A0 (1c) and pneu-
mocandin B0 (1d) are the two most prominent members of the
pneumocandin family and differ by a methyl group in the proline
moiety. Replacement of the hemiaminal group of pneumocandin
B0 (1d) with an ethylenediamine group and reduction of the
primary amide of the hydroxy glutamine to primary amine gave
caspofungin (1a).9 Micafungin (1b) was prepared by acylation of
FR179642 (decylated cyclohexapeptide nucleus of FR901379) with
an isoxazole based, pentyloxyl-diphenylisoxazoloyl, acyl
chain.10–13 Anidulafungin (2a) was prepared by acylation of
cyclohexapeptide nucleus of echinocandin B with a pentyloxy-
terpenyl-carbonyl group.14–17
Echinocandin B and FR901379 consist of simple (linoleoyl and
palmitoyl) fatty acid side chains which were associated with signif-
icant hemolysis.18 Semisynthetic optimization of echinocandins B
and FR901379 led to the replacement of the side chains with linear
terphenyl and a near linear heteroaromatic acyl chain producing
anidulafungin (2a) and micafungin (1b), respectively, with signifi-
cant attenuation of the hemolytic activity and concomitant
improvement in antifungal spectrum and potency. In contrast,
pneumocandins A0 and B0, consisting of the branched dimethyl
myristate side chain, are not hemolytic. Semisynthetic optimiza-
tion of pneumocandin B0 focused on the core modification leading
to the synthesis of caspofungin (1a) with significant improvement
of antifungal spectrum, potency, solubility and pharmaceutical
properties. It is well known that both side chain and core cyclic
peptide structure contribute to the potency and overall properties
of these compounds.18 The core peptide without the side chain has
no activity and vice versa. Significant literature on the SAR of anti-
fungal activity and hemolysis exists within each series but no SAR
exists across the series with the common acyl side chain. In this re-
port, we have attached the micafungin side chain to the core cyclic
peptides of pneumocandin A0, B0, and echinochandin B and their
antifungal, enzymatic and in vivo activities were evaluated. Details
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Corresponding author.
0960-894X/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved.