Tetrazolone as an acid bioisostere: Application to marketed drugs containing a carboxylic acid

Article abstract of DOI:10.1039/c6ob01646d

Matched molecular pair analysis was used to evaluate the ability of a tetrazolone group to act as a bioisostere of a carboxylic acid. Compound 7, a tetrazolone of the anti-hypertensive drug, telmisartan 6, was shown to be a potent AT₁ antagonist (K_b = 0.14 nM), with activity comparable to telmisartan itself (K_b = 0.44 nM). Additionally, compound 9, a tetrazolone congener of the marketed anti-cancer agent, bexarotene 8, was shown to be an agonist at the retinoid X receptor alpha (EC₅₀ = 64 nM). Compounds containing a tetrazolone group showed similar microsomal stability and plasma protein binding to marketed acid counterparts, while also reducing the value for clog P. Furthermore, compound 7 displayed an improved rat pharmacokinetic profile cf. telmisartan 6. Taken together, the results demonstrate that a tetrazolone group may serve as a bioisostere for a carboxylic acid.

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Tetrazolone as an acid bioisostere: application to
marketed drugs containing a carboxylic acid†‡
Cite this: DOI: 10.1039/c6ob01646d
Matthew A. J. Duncton,* Ryan B. Murray, Gary Park and Rajinder Singh
Matched molecular pair analysis was used to evaluate the ability of a tetrazolone group to act as a bio-
isostere of a carboxylic acid. Compound 7, a tetrazolone of the anti-hypertensive drug, telmisartan 6, was
shown to be a potent AT1 antagonist (Kb = 0.14 nM), with activity comparable to telmisartan itself
(Kb = 0.44 nM). Additionally, compound 9, a tetrazolone congener of the marketed anti-cancer agent,
bexarotene 8, was shown to be an agonist at the retinoid X receptor alpha (EC50 = 64 nM). Compounds
containing a tetrazolone group showed similar microsomal stability and plasma protein binding to
marketed acid counterparts, while also reducing the value for clog P. Furthermore, compound 7 displayed
an improved rat pharmacokinetic profile cf. telmisartan 6. Taken together, the results demonstrate that a
tetrazolone group may serve as a bioisostere for a carboxylic acid.
Received 1st August 2016,
Accepted 30th August 2016
DOI: 10.1039/c6ob01646d
www.rsc.org/obc
Drugs containing a carboxylic acid (RCO2H), comprise a candidate L-770,644 3,7 suggest that the tetrazolone motif may
significant number of approved medications, and are active have wider potential than is currently recognized. As an exten-
against a diverse-range of biological targets.1 The importance sion to our study of small substituents within medicinal chem-
of carboxylic acids within medicinal chemistry has led to the istry,8 and as a complement to our use of disubstituted
identification of alternative motifs that may be similarly active, tetrazolones as kinase inhibitors,8e–g,9 we sought to explore the
thereby acting as carboxylic acid bioisosteres.2 Such bio- potential of mono-substituted tetrazolones in further detail.
isosteres comprise both “classical” and “non-classical” groups. In this paper, we demonstrate that a tetrazolone group can be
For example, classical acid bioisosteres include sulfonic acids, successfully employed as a carboxylic acid bioisostere.10
phosphonic acids, and sulfonamides, whereas, non-classical Additionally, we show that tetrazolones have similar in vitro
acid bioisosteres include small heterocycles, such as tetrazoles, and in vivo pharmacokinetic characteristics to their acid
oxadiazolones, isoxazolones, and cyclic sulfonimidamides.2–4 counterparts, while reducing the clog P. Together with our
Mono-substituted 1,4-dihydro-5H-tetrazol-5-ones (tetrazolones) accompanying paper describing a one-pot synthesis of tetra-
are another potential non-classical bioisostere of a carboxylic zolones from fully-functionalized precursors,10a,c our results
acid. However, the use of a tetrazolone group has received little indicate that the tetrazolone group is an attractive motif for
attention from the medicinal chemistry community. For medicinal chemists.
example, only one paper has described an acid-to-tetrazolone
We began our examination of the tetrazolone group as a
late-stage
switch in any detail.5 Although pKa and log P data were consist- potential acid bioisostere by undertaking
a
ent with an ability to act as an acid bioisostere, tetrazolone 1 functionalization of acid precursors. For our studies, we chose
was not active when assessed for an ability to lower blood indomethacin 4, telmisartan 6 and bexarotene 8 as representa-
glucose in db/db mice (Fig. 1).5 An under-utilization of tetrazo- tive examples of bioactive acids (Fig. 2). As reported in an
lones within medicinal chemistry is unfortunate, given the accompanying patent and paper, the acids could be converted
structural similarity to the well-known tetrazole group. in to tetrazolone products, in moderate-to-excellent yield, by a
Additionally, the presence of a disubstituted tetrazolone in the one-pot procedure.10a,c We then used a matched molecular
marketed analgesic, alfentanil 2,6 and advanced preclinical pair analysis to compare the tetrazolone products 5 (R007),
7 (R000) and 9 (R006) against their marketed acid counterparts
4, 6 and 8 (Table 1).11 Compound 5, a tetrazolone of the non-
steroidal anti-inflammatory drug indomethacin 4, was inactive
as an inhibitor of cyclooxygenase, with no significant activity
at COX-1, when screened at 1 μM (cf. indomethacin). Although
this result was not encouraging, and illustrates that the
success of an acid-to-tetraozolone switch may be non-predict-
able, and case-dependent, it is important to note that success
Rigel Pharmaceuticals, Inc., 1180 Veterans Boulevard, South San Francisco,
CA 94080, USA. E-mail: mattduncton@yahoo.com, mduncton@rigel.com
†A paper detailing a one-pot synthesis of tetrazolones from fully-functionalized
precursors has been submitted.
‡Electronic supplementary information (ESI) available: Experimental pro-
cedures, analytical data and biological results for compounds 4–9. See DOI:
10.1039/c6ob01646d
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Fig. 1 Non-active glucose-lowering tetrazolone 1, and disubstituted tetrazolones, alfentanil 2, and L-770,644 3.5–7
Fig. 2 Marketed drugs containing a carboxylic acid and their tetrazolone counterparts.10
Table 1 Biological activity, in vitro pharmacokinetic profile, and clog P, for marketed acids and tetrazolone congeners
hCOX-1 (% inhibition hAT1
Compound at 1 μM) IC50 (Kb)
hRXRα
EC50
Half-life human
liver microsomes microsomes
Half-life rat liver Binding to human
plasma protein
clog Pa
4
5
6
7
8
9
88%
37%
—
—
—
—
—
—
—
—
≥45 min
≥45 min
≥45 min
≥45 min
23 min
≥45 min
≥45 min
≥45 min
≥45 min
≥45 min
≥45 min
99.7%
99.9%
99.9%
99.9%
99.9%
99.9%
4.25 0.80
3.85 0.79
6.48 1.19
6.00 1.17
6.90 0.53
5.90 0.51
5.7 nM (0.44 nM)
1.7 nM (0.14 nM)
—
—
—
<10 nM b
—
64 nM b,c ≥45 min
a clog P values calculated using ACD/log P version 11.02.21,22 b EC50 for fluorobexarotene control = 9 nM. c Similar relative efficacy to bexarotene at
maximal tested concentrations (see ESI).
with established bioisosteres, are also non-predictable, and to-date. The results are even more impressive, when it is
dependent on the individual context of use. For example, considered that replacement of the acid in telmisartan, with a
replacing a carboxylic acid with a non-classical bioisostere, tetrazole ring, results in a less-active analog.17 Thus, for the
such as a tetrazole ring, does not always result in retention of case of telmisartan, a tetrazolone serves as a more effective
biological activity,12 as recent experiences with antagonists of bioisostere of a carboxylic acid, than a tetrazole counterpart.
TRPM8 illustrate.13 We then progressed to evaluate telmisartan Finally, we progressed to examine the anti-cancer agent, bexar-
6 and its tetrazolone congener 7, as inhibitors of AT1. We were otene 8, and its tetrazolone congener 9, as RXRα agonists.
more confident that an acid-to-tetrazolone switch may be Tetrazolone 9 was active as an RXRα agonist with an EC50
=
tolerated for this class of compound, since AT1 antagonists 64 nM. Although compound 9 was somewhat less active than
containing both acids, and acid bioisosteres, such as tetra- bexarotene 8 (EC50 < 10 nM), both compounds elicited a similar
zoles, are well-known in the literature.14,15 Additionally, a maximal response (relative efficacy). Indeed, the activity
knowledge of the binding models for AT1 antagonists,16 observed with compound 9 is encouraging, as an ability of a
suggested that a tetrazolone ring could be accommodated tetrazolone to replace an acid, for both agonists (RXRα), and
within the AT1 binding pocket. Gratifyingly, compound 7 was antagonists (AT1), was deemed to be an important milestone for
shown to be a potent antagonist at the AT1 receptor, with a Kb showing a degree of versatility as an acid bioisostere.
value of 0.14 nM (IC50 = 1.7 nM). This compared favourably
We then progressed to examine the pharmacokinetic
with telmisartan 6 in the same experiment (Kb = 0.44 nM; IC50 behaviour of compounds 4 to 9. The pharmacokinetic qualities
= 5.7 nM). The results of this evaluation are significant, since of molecules containing a tetrazolone group have been
they show that a tetrazolone group is capable of functioning as described infrequently. The marketed analgesic, alfentanil 2,
an effective bioisostere for a carboxylic acid. Indeed, com- which contains a disubstituted tetrazolone, has a rapid,
pound 7 is one of the more potent AT1 antagonists described but short duration of action, with a human half-life of
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90–111 min,18 and a half-life in rat of approximately 23 min.19
Interestingly, the pharmacokinetic profile of another
disubstituted tetrazolone, L-770,644 3, was shown to be
superior to imidazolidinone, or imidazolone congeners when
dosed to dogs.7 In contrast to their disubstituted counterparts,
the pharmacokinetic qualities of mono-substituted tetra-
zolones have not been reported. Thus, we initiated studies to
understand the pharmacokinetic behaviour of the tetrazolones
described in Fig. 2. Initially, we examined the microsomal
stability of tetrazolone final compounds, together with their
marketed acid counterparts. As can also be seen from Table 1,
nearly all the compounds showed robust stability with a half-
life >45 min when incubated with either human or rat micro-
somes. This is not a surprising result, given that the starting
points for our studies are established drugs that do not
undergo extensive metabolism. Nevertheless, the observation
that a tetrazolone group did not result in a significant meta-
bolic liability is pleasing. Next, we examined binding to
human and rat plasma protein. Again, results between tetra-
zolones and their parent acids were near identical, with all
compounds showing significant binding to plasma proteins.
In addition to in vitro PK, we also investigated the effect of a
tetrazolone on physicochemical properties by calculating the
octanol/water partition coefficient (clog P) for all compounds
in Table 1.20,21 Replacement of an acid with a tetrazolone
group, resulted in a slight reduction to clog P of 0.4–1.0 log
units.22 The results of our calculations are also consistent with
Table 2 In vivo pharmacokinetics for Telmisartan 6, and its tetrazolone
congener 7, in rata
Compound
6
7
i.v.
Dose (mg kg−1
)
0.79
0.70
Cl (ml min−1 kg−1
)
7.2 1.1
1830 245
1.6 0.1
3.6 0.8
4.5 0.6
2490 249
1.7 0.2
5.4 1.6
AUC0–24 h (ng ml−1 h−1
)
)
Vss (L kg−1
)
Half-life (h)
p.o.
Dose (mg kg−1
)
)
3.95
3.5
Cmax (ng ml−1
506 158
5460 1860
59
1330 515
8420 1660
65
AUC0–24 h (ng ml−1 h−1
F (%)
a Data is the mean value standard deviation.
strategy for reducing clearance. Overall, the in vivo pharmaco-
kinetic profile for compound 7 demonstrated the feasibility of
an acid-to-tetrazolone bioisosteric replacement strategy.
In conclusion, this paper describes the use of a tetrazolone
group as a bioisostere of a carboxylic acid. In favourable
examples, compounds containing a tetrazolone can improve
both the in vitro biological activity and in vivo pharmacokinetic
profile, when compared with a marketed acid counterpart.
Similar to other motifs utilized as a bioisostere of a carboxylic
acid, the success of an acid-to-tetrazolone switch was found to
be case-dependent. Taken with our accompanying paper on
the one-pot synthesis of tetrazolones from acid chlorides,
this work may help facilitate a wider investigation of the
tetrazolone group within medicinal chemistry.
the observations of
a previous study, which showed a
reduction in the measured log P of approximately 0.4 log units
for tetrazolone 1, when compared with an acid counterpart.5
Therefore, an acid-to-tetrazolone switch may result in a small
deflationary outcome for clog P. This finding may be parti-
cularly helpful for medicinal chemists looking to optimize a
bioactive acid, as replacing the acid with a tetrazolone group
Abbreviations
may be used to partially-offset any inflation in clog P during AT1
the optimization process.23
AUC
Given the favourable biological activity, and attractive Cl
Angiotensin receptor subtype 1
Area under the curve
Clearance
in vitro pharmacokinetic profile for compound 7, we also clog P Calculated log of the octanol/water partition
wished to investigate the in vivo pharmacokinetic behaviour of
coefficient
this compound (in rats). Again, we would compare results with Cmax
Maximum/peak concentration
those observed with the parent acid, telmisartan 6, in an COX-1 Cyclooxygenase-1 or prostaglandin-endoperoxide
equivalent experiment. As can be seen from the i.v. pharmaco- synthase 1
kinetic profile in Table 2, tetrazolone showed lower DMPK Drug metabolism and pharmacokinetics
clearance (Cl), and higher exposure (AUC), when compared to
7
F
Bioavailability
telmisartan 6. Compound 7 also had a slightly longer half-life i.v.
than telmisartan (5.4 h for 7 cf. 3.6 h for 6). When admini- log P
stered orally, at 3.5 mg kg−1, tetrazolone 7 showed a signi- PK
ficantly higher Cmax and higher exposure (AUC), than pKa
telmisartan 6 dosed at 3.95 mg kg−1. Thus, when normalizing p.o.
Intravenous
Log of the octanol/water partition coefficient
Pharmacokinetics
Negative log of acid dissociation constant
per os
the exposure to take account of the different doses (AUC per RXRα Retinoid X receptor alpha
dose), the oral exposure for compound 7 was about 1.75 times Vss
higher than for telmisartan 6. The results in Table 2 are
encouraging, as they demonstrate that compounds containing
Steady state volume of distribution.
Acknowledgements
a tetrazolone group can exhibit an attractive pharmacokinetic
profile.24 In particular, our data may suggest that replacement We thank Mark Irving, Van Ybarra and Duayne Tokushige of
of an acid with a tetrazolone group could be a beneficial Rigel, Inc. analytical chemistry for high-resolution mass spec-
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22 Results with another clog P calculator (from Biobyte) gave a
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between acid and tetrazolone counterparts was observed.
See ESI‡ for details.
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24 A referee requested some general comment on the
individual pharmacokinetic parameters and their
significance within drug discovery. This is best
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with the subject matter. For example, see: (a) S. A. Roberts,
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