Discovery of a potent, orally active, nonsteroidal androgen receptor agonist: 4-Ethyl-1,2,3,4-tetrahydro-6-(trifluoromethyl)-8-pyridono[5,6-g]- quinoline (LG121071) [3]

Full text of DOI:10.1021/jm9806648

210
J . Med. Chem. 1999, 42, 210-212
Sch em e 1a
Discover y of a P oten t, Or a lly Active,
Non ster oid a l An d r ogen Recep tor
Agon ist: 4-Eth yl-1,2,3,4-tetr a h yd r o-6-
(tr iflu or om eth yl)-8-p yr id on o[5,6-g]-
qu in olin e (LG121071)
Lawrence G. Hamann,*,† Neelakandha S. Mani,†
Robert L. Davis,† Xiao-Ning Wang,‡
Keith B. Marschke,§ and Todd K. J ones†,|
Departments of Medicinal Chemistry, Endocrine Research,
and New Leads Discovery, Ligand Pharmaceuticals, Inc.,
10275 Science Center Drive, San Diego, California 92121.
a
(a) LDA, MeI, THF, -78 °C, 98%; (b) NiCl2‚6H2O, NaBH4,
MeOH, 0 °C-rt, 96%; (c) HNO3, H2SO4, -10 °C, 10 min, 91%; (d)
H2, 10% Pd/C, EtOH/EtOAc, rt, 16 h, 94%; (e) ethyl 4,4,4-
trifluoroacetoacetate, ZnCl2, EtOH, reflux, 8 h, 82%.
Received November 24, 1998
In tr od u ction . Deficiencies in circulating levels of the
androgens testosterone (T, 1a ) and dihydrotestosterone
(DHT, 1b) in hypogonadal men can be compensated for
by administration of exogenous androgens,1,2 which
have proven efficacious in hormone replacement therapy,
abrogating age-related deterioration of muscle and
bone,3 and regulating plasma lipids.4 Cancer cachexia,5
male contraception,6 and performance enhancement7
have also been investigated as clinical targets of andro-
gen therapy.8
Efforts to identify more receptor- and tissue-selective
compounds which might avoid steroid-related side ef-
fects and toxicities have shifted focus away from steroid
structural templates. Though representatives of several
structural classes have been developed or are currently
undergoing late-stage preclinical development as human
androgen receptor (hAR) antagonists,10 efforts to dis-
cover and develop nonsteroidal AR agonists have been
few.11 To date, no known nonsteroidal AR agonists have
been reported to exhibit activity in vivo. In the course
of our investigations into the structure-activity rela-
tionships of dihydroquinoline-based AR antagonists
such as 3,12 we had the opportunity to examine the effect
of removal of 2,2-dialkyl substitution. It had previously
been noted that substitution at this position played a
critical role in driving the receptor into a transcription-
ally inactive conformation and that lack of geminal
substitution (as in 2-monoalkyl-substituted or 2,2-
dihydro analogues) greatly impacted the transcriptional
competency of the AR-ligand complex. It was more
specifically observed that tetrahydro analogue
8
(LG121071) exhibited tighter binding affinity than the
substituted analogues and scored as a full agonist in
cotransfection assays, which led us to investigate the
ability of 8 to exhibit AR agonist activity in a classic
animal model.
The beneficial effects of administered steroidal an-
drogens are often overshadowed by their rapid metabolic
conversion to DHT by 5R-reductase and to estrogens by
aromatase, resulting in side effects. Circumventing this
metabolism through alternate routes of administration,
such as intramuscular injection or transdermal patch
applied to the scrotal skin,9 and attempts to improve
oral half-life of T using long-chain alkyl esters as
prodrugs have met with limited success.10 Alkylation
of androgens at C-17, as in methyltestosterone (1c) and
fluoxymesterone (2), has been observed to slow hepatic
metabolism, allowing oral administration, but subse-
quent liver toxicity limits their use for chronic admin-
istration.
Ch em istr y. Our previously reported efforts at con-
struction of 2,2-dialkyl-substituted pyridonoquinolines
involved sequential annulation of each terminal ring
about a central core through cyclization strategies.13
Synthesis of 1,2,3,4-tetrahydro-8-pyridono[5,6-g]quino-
lines was achieved in a more efficient manner from a
single annulation onto an existing quinoline core after
appropriate functionalization (Scheme 1).14 The requi-
site intermediate 4-ethyl-1,2,3,4-tetrahydroquinoline
(6)15 was synthesized in a two-step sequence starting
from commercially available lepidine (4). Treatment of
4 with LDA16 and trapping of the resultant anion with
iodomethane furnished 4-ethylquinoline (5)17 in excel-
lent yield (98%). Reduction of the quinoline ring using
* Address correspondence to this author. Phone: (619) 550-7618.
Fax: 619-550-7249. E-mail: lhamann@ligand.com.
† Department of Medicinal Chemistry.
18
NaBH4-NiCl2 afforded the required 4-ethyltetrahy-
droquinoline 6 in 96% yield. Standard nitration and
subsequent reduction by catalytic hydrogenation pro-
vided diamine 7, which smoothly underwent Knorr
cyclization19 (ethyl 4,4,4-trifluoroacetoacetate, ZnCl2,
‡ Department of Endocrine Research.
§ Department of New Leads Discovery.
| Present address: Ontogen Corp., 2325 Camino Vida Roble, Carls-
bad, CA 92009.
10.1021/jm9806648 CCC: $18.00 © 1999 American Chemical Society
Published on Web 01/08/1999

Communications to the Editor
J ournal of Medicinal Chemistry, 1999, Vol. 42, No. 2 211
F igu r e 2. Suppression of luteinizing hormone (LH) in cas-
trated (Cx) mature rats by compound 8 (20 mg/kg, po) or
testosterone propionate (TP) (1 mg/kg, sc) daily for 2 weeks
(n ) 4 for all groups). All values represent the mean serum
LH concentrations ( SEM.
F igu r e 1. hAR agonist dose response of compound 8 in
cotransfected CV-1 cells. Values represent mean ( SEM of at
least triplicate determinations.
Ta ble 1. hAR Agonist and Antagonist Activity in
Cotransfected CV-1 Cells and Binding Affinities for hAR in
Transiently Transfected COS-1 Cellsa
exogenous androgens.22 Compound 8 was examined in
this established model of androgen action to assess its
ability to suppress castration-induced elevation of serum
LH after oral administration (Figure 2).
hAR agonist
activity
hAR antagonist
activity
hAR binding
Resu lts a n d Discu ssion . Compound 8 stimulates
reporter gene expression in a concentration-dependent
manner in the cotransfection assay, with a potency and
efficacy equivalent to that of DHT (Figure 1). A slight
antagonist response is observed only at the highest
concentration (10 µM) in the cellular background used
for this assay. In vivo, testosterone propionate admin-
istered subcutaneously at a dose of 1 mg/kg completely
blocked the effects of castration, restoring serum LH
levels to that of the intact control animals. Compound
8 at a dose of 20 mg/kg administered orally was also
fully efficacious at suppressing the castration-induced
elevation of LH in the male rat.
Con clu sion . The data shown for LG121071 (8)
represent the discovery of the first known orally active,
nonsteroidal AR agonist. This finding, together with
earlier reports from these laboratories,23 provides fur-
ther support for a drug discovery approach targeting
both agonists and antagonists of sex steroid hormone
receptors diverging from a common pharmacophore.
Compounds based on this novel template are the
subjects of continued investigations toward development
of therapeutically useful AR agonists with desirable
tissue selectivity and avoiding structure-based side
effects associated with compounds derived from steroi-
dal templates.
b
b
EC50
efficacyc
(%)
IC50
efficacyc
(%)
Kib
(nM)
compd
(nM)
(nM)
1b
2
8
5 ( 1
100 ( 0
nad
na
3 ( 1
4 ( 1
17 ( 3
0.3 ( 0.1 128 ( 12
4 ( 1
100 ( 7 7481 ( 1655 36 ( 10
a
Cotransfection assay experiment values represent at least
b
triplicate determinations. Values represent mean ( SEM. EC50
values represent the concentration of ligand required to give half-
maximal activation; IC50 values represent the concentration of
ligand required to give half-maximal inhibition of DHT at its EC50
.
c Efficacies were compared to that of dihydrotestosterone (100%).
d
Not active; defined as efficacy < 20%, potency > 10 000 nM.
EtOH, reflux) to afford 4-ethyl-1,2,3,4-tetrahydro-6-
(trifluoromethyl)-8-pyridono[5,6-g]quinoline (8).
In Vitr o a n d in Vivo Biologica l Activity. 1.
Cotr a n sfection a n d Bin d in g Assa ys. The AR agonist
activities of 8 as well as that of the known AR agonists
1b and 2 were studied experimentally in a cellular
background through both ligand-dependent stimulation
of reporter gene (luciferase) induction using the cotrans-
fection assay20 (Figure 1) and a whole-cell receptor
binding assay (Table 1). Also included in Table 1 are
data for the compound and standards tested in cotrans-
fection assays using hAR in the antagonist mode in the
presence of DHT at its EC50. Activities on other IRs
including human progesterone receptor (hPR-B), human
glucocorticoid receptor (hGR), human mineralocorticoid
receptor (hMR), and human estrogen receptor (hER)
were also determined, and there was found to be no
agonist or antagonist response induced by compound
8.21
Ack n ow led gm en t. We thank Dr. William Schrader
for helpful discussions in the preparation of this manu-
script and the Department of New Leads Discovery for
performing in vitro assays.
2. Tw o-Week LH Su p p r ession Assa y in Ra ts. The
hypothalamic-pituitary axis in male rats and men
functions as a feedback loop to regulate circulating
levels of endogenous steroid (T, DHT) and gonadotropins
[luteinizing hormone (LH), follicle-stimulating hormone
(FSH)]. Castration causes a dramatic increase in secre-
tion of pituitary LH, and restoration of androgen control
and subsequently restoration of LH to normal physi-
ologic levels can be achieved by administration of
Su p p or tin g In for m a tion Ava ila ble: Synthetic proce-
dures and chemical characterization data for compounds 5-8
and biological assay methods. This material is available free
of charge via the Internet at http://pubs.acs.org.
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