M. Tandon et al. / Bioorg. Med. Chem. Lett. 14 (2004) 1909–1911
1911
Table 2. Inhibition of MetRS by l-azetidine and l-proline sub-
stituted at R6
R6=H is tolerated since 6a remains the most potent
inhibitor of the set (EfMetRS IC50=730 nM, Table 2).
The most active SaMetRS inhibitor identified in this
SAR exploration was 14a (SaMetRS IC50=18 nM).
The compound had low micromolar activity at
EfMetRS and was devoid of activity at hMetRS.
In conclusion, novel low molecular weight high-
throughput screening hits 1a and b were the starting
point for a three-phase SAR expansion. Successive
improvements in activity against SaMetRS led to 14a,
which had an IC50 of 18 nM against SaMetRS. This was
a 40-fold improvement in IC50 from the original hit 1a.
A 4-fold improvement against the bacterial MetRS
enzyme from the Gram-positive pathogen Enterococuss
faecalis was also obtained for 14a. In addition small
modifications to the structures of 1a and b were shown
to elicit changes in inhibition at the two enzyme targets
and as such may provide additional insight into the
putative binding pockets of MetRS derived from S.
aureus and E. faecalis. Disappointingly compounds
from this series lacked cellular activity and were not
pursued further. However, 14a and the series from
which it was obtained represent a new class of potent
and selective inhibitors of S. aureus methionyl tRNA
synthetase and may provide the basis for the design of
therapeutically useful antibacterials.
Compd R1 R2
R6
n
IC50 (mM)
SaMetRS EfMetRS hMEtRS
10a
10b
11a
11b
12a
12b
13a
13b
14a
14b
15a
15b
Cl
Br
Cl
Br
Cl
Br
Cl
Br
Cl
Br
Cl
Br
Cl
H
Cl
H
Cl
H
Cl
H
Cl
H
Cl
H
Me
Me
Et
2
2
2
2
2
2
2
0.34
1.67
0.18
0.85
0.17
0.76
0.43
0.816.1
6.63
>100
>100
>100
>100
>100
>100
>100
>100
>100
>100
>100
>100
>100
1.28
5.93
99.0
3.11
27.4
Et
nPr
nPr
Ph
Ph
Et
Et 7 10.1
nPr 10.04
nPr 10.087
2 4
810.01
3.51
99
3.52
>100
retained. The screening results for this small array of
compounds are shown in Table 2.
Generally increasing the size of the R6 substituent is
tolerated with all analogues showing comparable or
improved potency against SaMetRS and no activity at
hMetRS. Where Region 2 was set at l-proline both the
2,4-dichlorophenyl and the 4-bromophenyl analogues
are most potent when the R6 substituent is an n-propyl
group 12a, SaMetRS IC50=170 nM and 12b, SaMetRS
IC50=760 nM. However, 13b (R6=Ph) has a compar-
able IC50 to 12b (IC50=810 nM) suggesting that the 4-
bromophenyl analogue is more tolerant of an aromatic
ring at R6 than is the case with corresponding 2,4-
dichlorophenyl analogue (13a, IC50=430 nM). In this
case there is a more than a two-fold loss in activity.
Acknowledgements
We thank Drs. M. Harris for the preparation of the
general screening library and P. Pallai for managerial
support.
References and notes
1. Bush, K.; Macielage, M.; Clancy, J. Emerg. Drugs 2000, 5,
347.
2. Ibba, M.; Soll, D. Annu. Rev. Biochem. 2000, 69, 617.
¨
The most potent SaMetRS inhibitors were found when
Region 2 was the l-azetidine linker. This holds true for
both the 2,4-dichloro- and the 4-bromophenyl analo-
gues. However, the 2,4-dichlorophenyl is present in the
two most potent analogues, 14a and 15a. Both com-
pounds have IC50’s below 50 nM.
3. Bertino, J. S.; Am, J. Health Syst. Pharm. 1997, 54, 2185.
4. Parenti, M. A.; Hatfield, S. M.; Leyden, J. J. Clinical
Pharmacy 1987, 6, 761.
5. Enzymatic inhibition was determined by measuring the
amount of radiolabeled methionine incorporated into the
product, the charged methionyl–tRNA complex. The
enzymatic reaction was run at Km concentrations for ATP
and methionine and a saturating level of 90 mM for
tRNA. A compound’s IC50 was determined by fitting the
results from a 10-point dose–response curve. All results
are the average of at least two measurements.
While none of the analogues in this third set showed
sub-micromolar EfMetRS inhibition the observed SAR
shows that when Region 2 is l-proline an R6 substituent
the size of an ethyl group confers maximal EfMetRS
inhibition (11a, EfMetRS IC50=1.3 mM). However,
when Region 2 is l-azetidine no substituent larger than
6. Behringer, H.; Horst, T. Chem. Ber. 1957, 90, 1398.
7. Benoiton, N. L.; Hudecz, F.; Francis, M. F.; Int, J. Pept.
Prot. Res. 1995, 45, 266.