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DOI: 10.1002/cmdc.201300573
Structure-Based Design of Novel Human Toll-like
Receptor 8 Agonists
[
a]
[a]
[b]
[b]
Hari Prasad Kokatla, Diptesh Sil, Hiromi Tanji, Umeharu Ohto,
[a]
[a]
[b]
[a]
Subbalakshmi S. Malladi, Lauren M. Fox, Toshiyoki Shimizu,* and Sunil A. David*
Toll-like receptor (TLR)-8 agonists activate adaptive immune re-
sponses by inducing robust production of T helper 1-polarizing
cytokines, suggesting that TLR8-active compounds might be
promising candidate vaccine adjuvants. Recently, a C2-butyl
furo[2,3-c]quinoline was reported with purely TLR8 agonistic
activity. This compound was successfully co-crystallized with
the human TLR8 ectodomain, and the co-crystal structure re-
vealed ligand-induced reorganization of the binding pocket of
TLR8. The loss of a key hydrogen bond between the oxygen
atom of the furanyl ring of the agonist and Thr574 in TLR8
suggested that the furan ring is dispensable. Employing a dis-
connection strategy, 3- and 4-substituted aminoquinolines
were investigated. Focused structure-based ligand design stud-
ies led to the identification of 3-pentyl-quinoline-2-amine as
a novel, structurally simple, and highly potent human TLR8-
specific agonist (EC =0.2 mm). Preliminary evaluation of this
volves recognition not only of specific antigenic determinants,
but also of certain characteristics or patterns common on infec-
[2]
tious agents but absent from the host.” Janeway’s seminal
ideas of nonclonal recognition of nonself by pattern recogni-
tion receptors (PRRs) served as the foundation for the discov-
[3]
ery of the sensors of the innate immune system. Among the
well-studied of such PRRs initiating innate immune afferent
[4]
signals are the Toll-like receptors (TLRs). There are ten func-
tional TLRs encoded in the human genome. TLRs -3, -7, -8 and
[5]
-9 function within the endolysosomal compartment. The acti-
vation of TLRs by their cognate ligands leads to production of
inflammatory cytokines, and upregulation of major histocom-
patibility complex molecules and co-stimulatory signals in anti-
gen-presenting cells, as well as activating natural killer cells
[6]
(innate immune response). These events lead to the priming
of na ¨ı ve lymphocytes and subsequent induction and amplifica-
tion of antigen-specific T-, and B-cell effector functions (adap-
50
compound in ex vivo human blood assay systems revealed
that it retains prominent cytokine-inducing activity. Together,
these results indicate the suitability of this compound as
a novel vaccine adjuvant, warranting further investigation.
[7]
tive immune responses).
Our recent efforts on evaluating small-molecule agonists of
[8]
TLR8 are primarily aimed at examining such compounds as
potential vaccine adjuvants. TLR8 is expressed in myeloid den-
dritic cells, monocytes, and monocyte-derived dendritic cells.
Engagement by TLR8 agonists evokes a dominant proinflam-
matory cytokine profile, including tumor necrosis factor-
The immune system protects the host from infectious agents
by first recognizing the presence of the infectious organism
and then responding rapidly and appropriately to contain and
eliminate the threat. The mobilization of adaptive immune re-
sponses involving T-, and B-lymphocytic effector functions are
exquisitely pathogen-specific but relatively slow, requiring days
[9]
a (TNF-a), interleukin (IL)-12, and IL-18, and appear unique in
markedly upregulating the production of Th1-polarizing cyto-
[10]
kines TNF-a and IL-12 in neonatal antigen-presenting cells.
These data, taken together, suggest that TLR8 agonists could
be useful as adjuvants for enhancing immune responses in
[
1]
or weeks. The enormous diversity of infectious organisms
and their short generation times led Charles Janeway to postu-
late that “the immune system has evolved specifically to recog-
nize and respond to infectious microorganisms, and that this in-
[11]
newborns.
A prerequisite for the careful evaluation of TLR8 agonists as
potential vaccine adjuvants is the characterization of pure
TLR8 agonists with negligible TLR7 activity, for almost all
known agonists of TLR8, typified by certain imidazoquinolines
[
a] Dr. H. P. Kokatla, Dr. D. Sil, S. S. Malladi, L. M. Fox, Prof. S. A. David
Department of Medicinal Chemistry, University of Kansas
Multidisciplinary Research Building, Room 320D
[12]
[8b,13]
and thiazoloquinolines, such as CL097 (2) and CL075 (1),
respectively (Figure 1), and the 2-aminobenzazepine VTX-
[14]
2
030 Becker Drive, Lawrence KS 66047 (USA)
2337, display mixed TLR7/TLR8 agonism. TLR8-biased agonis-
tic properties have been described for a novel 2-aminobenza-
E-mail: sdavid@ku.edu
[15]
[
b] Dr. H. Tanji, Dr. U. Ohto, Prof. T. Shimizu
Graduate School of Pharmaceutical Sciences
Faculty of Pharmaceutical Sciences, University of Tokyo
zepine derivative (VTX-294), whose complete structure has
not been disclosed. We recently reported pure TLR8 agonistic
activity in a C2-butyl furo[2,3-c]quinoline (3) with IL-12 and IL-
18 induction profiles, and yet without IFN-a inducing proper-
7
-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033 (Japan)
and
[8a]
Core Research for Evolutional Science & Technology (CREST)
Japan Science & Technology Agency (JST)
K’s Gobancho 6F, 7, Gobancho, Chiyoda-ku, Tokyo 102-0076 (Japan)
E-mail: shimizu@mol.f.u-tokyo.ac.jp
ties, confirming its selectivity for human TLR8. Crystal struc-
tures of the ectodomain of human TLR8 in complex with
mixed TLR7/TLR8-agonistic thiazoloquinolines and imidazoqui-
[16]
nolines (including 1 and 2) allowed a rationalization of our
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/cmdc.201300573.
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2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ChemMedChem 2014, 9, 719 – 723 719