Structure-based design, synthesis, and biological evaluation of irreversible human rhinovirus 3C protease inhibitors. Part 7: Structure-activity studies of bicyclic 2-pyridone-containing peptidomimetics

Article abstract of DOI:10.1016/S0960-894X(02)00008-2

The structure-based design, chemical synthesis, and biological evaluation of bicyclic 2-pyridone-containing human rhinovirus (HRV) 3C protease (3CP) inhibitors are described. An optimized compound is shown to exhibit antiviral activity when tested against a variety of HRV serotypes (EC₅₀'s ranging from 0.037 to 0.162 μM).

Full text of DOI:10.1016/S0960-894X(02)00008-2

Bioorganic & Medicinal Chemistry Letters 12 (2002) 733–738
Structure-Based Design, Synthesis, and Biological Evaluation of
Irreversible Human Rhinovirus 3C Protease Inhibitors.
Part 7: Structure–Activity Studies of Bicyclic
2-Pyridone-Containing Peptidomimetics
Peter S. Dragovich,* Thomas J. Prins, Ru Zhou, Theodore O. Johnson,
Edward L. Brown, Fausto C. Maldonado, Shella A. Fuhrman,
Leora S. Zalman, Amy K. Patick, David A. Matthews, Xinjun Hou,
James W. Meador, III, Rose Ann Ferre and Stephen T. Worlandy
Pfizer Global Research and Development-La Jolla/Agouron Pharmaceuticals, Inc., 10777 Science Center Drive,
San Diego, CA 92121-1111, USA
Received 9 October 2001; accepted 12 December 2001
Abstract—The structure-based design, chemical synthesis, and biological evaluation of bicyclic 2-pyridone-containing human rhi-
novirus (HRV) 3Cprotease (3CP) inhibitors are described. An optimized compound is shown to exhibit antiviral activity when
tested against a variety of HRV serotypes (EC50’s ranging from 0.037 to 0.162 mM). # 2002 Elsevier Science Ltd. All rights
reserved.
The human rhinoviruses (HRVs) are members of the
picornavirus family and are the single most significant
cause of the common cold.1,2 The replication of these
viruses is entirely dependent on the proteolytic proces-
sing of a large polyprotein produced by cellular trans-
lation of the viral RNA genome. This processing is
primarily accomplished by the human rhinovirus 3C
protease (3CP),3 a cysteine protease with structural
similarity to the trypsin protein family but possessing
minimal homology to prevalent mammalian enzymes.4
Due to its prominence in the viral replication cycle, 3CP
is an ideal target for the development of novel anti-
rhinoviral agents and multiple examples of 3CP inhibi-
tors have recently appeared in the literature.5 In a
previous report, we described the discovery and devel-
opment of a new class of orally bioavailable 3CP inhi-
bitors that display in vitro antiviral activity against
multiple rhinovirus serotypes.6 These inhibitors are com-
prised of a peptidomimetic 2-pyridone-containing binding
determinant that provides affinity for the target protease
and a Michael acceptor moiety, which irreversibly forms
a covalent adduct with the active site cysteine residue of
the 3Cenzyme (e.g., compound 1, Fig. 1 and Table 1).7
In this report, we describe the further elaboration of
*Corresponding author. Fax: +1-858-622-7998; e-mail: peter.dragovich@
pfizer.com
yPresent address: Anadys Pharmaceuticals, 9050 Camino Santa Fe,
San Diego, CA 92121, USA.
Figure 1. Design of 2-pyridone-containing HRV 3CP inhibitors.
0960-894X/02/$ - see front matter # 2002 Elsevier Science Ltd. All rights reserved.
PII: S0960-894X(02)00008-2

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P.S.Dragovich et al./ Bioorg.Med.Chem.Lett.12 (2002) 733–738
Table 1. Monocyclic and bicyclic 2-pyridone-containing 3Cprotease
inhibitors
mechanism that was divided into two parts as shown
below.
k1
k3
Compd
kobs/[I]
(MÀ1sÀ1 a
ÁÁEcalc
(kcal/mol)b
EC50
(mM)c
CC50
(mM)d
E þ I
EI
!
E À IÃ
À!
À
)
k2
1
2
3
46
5
114,000
26
4540
348
177,000
0.00
16.1
8.7
9.9
ND
0.033
>10
9.0
>10
0.162
50
>10
>100
>10
>10
The inhibitor (I) initially forms a reversible encounter
complex with 3CP (E), which can then undergo a che-
mical reaction (nucleophilic attack by Cys-147) leading
to an irreversible covalent complex (E-I*). The observed
inactivation (kobs/[I]) depends upon both the equili-
brium binding constant (k2/k1) and the chemical rate
(k3) of covalent bond formation.14 For the purposes of
this study, we postulated that (1) the energetics of com-
pound equilibrium binding are determined by non-
covalent molecular recognition and (2) that the
subsequent chemical reaction is dominated by the
intrinsic electrophilicity of the inhibitor and its ability to
adopt a favorable binding orientation for the reaction
to occur. These assumptions are consistent with pre-
vious structure–activity studies of 3CP inhibitors.15,16
We further assumed that the reactive contribution to
kobs/[I] is approximately the same for compounds 1–4
(which contain the same reactive center and adjacent
functional groups) and thus anticipated relative kobs/[I]
values to approximately correspond to relative non-
covalent binding energetics. The computational study
therefore sought to compute relative noncovalent bind-
ing energetics of inhibitors in an effort to rank com-
pounds and guide their optimization.
aInhibition activity against HRV-14 3Cprotease; see ref 22 for assay
method and error.
bCalculated binding energetics of ligand and HRV-2 3C protease
interactions (relative to compound 1); see text for details.
cAntirhinoviral activity versus HRV-14; see ref 22 for assay method
and error.
dCytotoxicity; see ref 22 for assay method and error.
ND=not determined.
such irreversible 3CP inhibitors by the incorporation of
a bicyclic 2-pyridone moiety into the inhibitor design.
Cursory analysis of the HRV-2 3CP-1 X-ray crystal
structure6 suggested that, since no obvious steric clashes
were apparent, the P2-P3 pyridone-Phe portion of 1
could be replaced with some kind of bicyclic entity.8
Such replacement was anticipated to further diversify
the physical and biological properties (e.g., mw, mp,
water solubility, metabolic stability, etc.) of our existing
anti-3CP compounds and presumably thereby improve
our ability to identify such agents suitable for oral
administration. However, many literature examples
exist that demonstrate successful incorporation of
bicyclic peptidomimetic moieties into inhibitors of tryp-
sin-like proteases,9,10 and it was not obvious which of
these (often synthetically complex) possibilities was
most suitable for use in targeting 3CP.
Accordingly, compounds (1–4) were covalently modeled
into a protein binding site constructed from the co-
crystal structure of HRV-2 3CP-1.6 Ab initio partial
atomic charges were computed and assigned to all
ligands and AMBER* charge parameters were assigned
to protein residues. The complexes were energetically
minimized using the Batchmin program of MacroModel
V5.517 with the AMBER* force field and a GB/SA sol-
vation model. Inhibitors and protein binding site resi-
dues were fully flexible and protein residues further
away from the ligand (>5 A) were harmonically con-
strained to their crystallographic positions during the
minimization. Protein–ligand interactions and desolva-
tion energetics were computed from the minimized
complexes (EPLBound). To compute appropriate con-
formational strain energies, inhibitor and protein struc-
tures were independently extracted from the minimized
An initial attempt at identifying bicyclic 3CP inhibitors
was therefore made with the examination of the easily-
prepared compound 2, which incorporated a commer-
cially available bicyclic peptide mimetic.11,12 Unfortu-
nately, this molecule displayed drastically reduced 3CP
inhibition properties relative to the monocyclic 2-pyr-
idone-containing compound 1 and was not an active
antirhinoviral agent in cell culture (Table 1). The precise
reasons for the difference in activities between 1 and 2
were not apparent from simple, non-quantitative mod-
eling experiments conducted with the 3CP X-ray crystal
structure. Although such modeling indicated that 2
would not occupy the S2 3CP peptide binding subsite
filled by the benzyl substituent of 1, this alteration did
not appear to be responsible for the near-total loss in
3CP inhibition activity exhibited by the bicyclic com-
pound. Accordingly, more detailed quantitative mole-
cular modeling studies were performed to better define
and predict the interactions between bicyclic molecules
and the 3CP enzyme.
complexes and further minimized in solvent to obtain
Free
the unbound free states (EL
and EPFree). The ener-
getics of bond making, along with conformational and
configurational entropic terms were assumed to be con-
stant and to cancel within the same series of inhibitors.
The compound binding energetics (relative to inhibitor
1) were then approximated with the following thermo-
dynamic path using the principle of cancellation of
errors.
A summary of the quantitative computational method
employed is outlined below and a more extensive dis-
cussion of the procedure will be published elsewhere.13
The covalent irreversible inactivation of 3CP by
Michael acceptors was modeled based on a kinetic
ÁEL
¼ EPL
À EP
À EL
Bound
Free
Free
calc
ÁÁEcalc ðL1 ! L2Þ ¼ ÁEL2 calc À ÁEL1
calc

P.S.Dragovich et al./ Bioorg.Med.Chem.Lett.12 (2002) 733–738
735
The above computational efforts correctly assessed the
significant difference in anti-3CP activity between inhi-
bitors 1 and 2 (compare ÁÁEcalc values, Table 1).
Importantly, the calculations also indicated that the
interactions between 2 and 3CP could be greatly
improved by changing the sp3 C-3 carbon atom present
in the bicyclic molecule to the sp2 hybridization state.
This alteration was anticipated to better position the
appended Cbz-amino functionality in the S4 3CP bind-
ing subsite. The bicyclic 2-pyridone moiety contained in
structure 3 was eventually identified as a synthetically-
accessible fragment which met the above C-3 sp2 hybri-
dization requirement and compound 3 performed well
relative to 2 in the computational binding energy calcu-
lations (Table 1). The structural similarity between 3
and the known inhibitor 1 was also expected to increase
the probability of favorable 3CP recognition.
containing inhibitor 1, the activity displayed by the
bicyclic molecule was sufficient to justify additional
optimization experiments.
Accordingly, modifications known from our previous
work6 to improve the anti-3CP properties of monocyclic
2-pyridone-containing molecules were incorporated into
the bicyclic inhibitor design. Specifically, simultaneous
introduction of a P1 (S)-g-lactam fragment16 and a P4
amide derived from 5-methylisoxazole-3-carboxylic
acid18 afforded a molecule (5), which displayed greatly
improved 3CP inhibition properties relative to 3 (Table
Table 2. Antirhinoviral activity of 5 versus multiple HRV serotypes
HRV Serotype
EC50 (mM)a
EC90 (mM)a
2
3
9
10
14
16
25
39
87
0.037
0.043
0.121
0.041
0.162
0.072
0.162
0.016
0.058
0.119
0.133
0.257
0.175
0.298
0.303
0.616
0.152
0.110
aCytotoxicity >10 mM in all cases; see ref 22 for assay method and
error.
Figure 2. Crystal structure of 3 complexed with HRV-2 3CP (1.55 A
resolution).
In the event, the bicyclic 2-pyridone-containing com-
pound 3 was prepared and displayed dramatically
improved (>100-fold) 3CP inhibition properties relative
to the previously examined bicyclic molecule 2 (Table
1). Inhibitor 3 also exhibited measurable antirhinoviral
activity in cell culture experiments (Table 1). The
importance of proper conformational constraint impar-
ted by the bicyclic 2-pyridone moiety contained in 3 was
demonstrated with the examination of the monocyclic
inhibitor 4.6 The latter molecule exhibited significantly
poorer anti-3CP properties relative to 3 (Table 1) even
though both compounds were anticipated to contact the
3CP enzyme in roughly the same manner (see X-ray
analysis below). Although compound 3 was not as
potent an anti-3CP agent as the monocyclic 2-pyridone-
Figure 3. Schematic diagram of 3 bound in the HRV-2 3CP active site.
Hydrogen bonds are represented as dashed lines and the residues
which make up the enzyme binding subsites are depicted.

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P.S.Dragovich et al./ Bioorg.Med.Chem.Lett.12 (2002) 733–738
Scheme 1. Reagents and conditions (solid support=Rink amide resin, Tr=CPh3): (a) 3% DBU in CH2Cl2, (b) 1.5 equiv 8, HATU, iPr2NEt, DMF;
(c) 10% collidine in CH2Cl2, Cbz-Cl; (d) 9:1 TFA:CH2Cl2, 45% from 6; (e) 2.0 M HCl in 1,4-dioxane; (f) 1.0 equiv 13, EDC, HOBt, 4-methylmor-
pholine, 60% from 11; (g) 1:1 TFA:CH2Cl2, 5 drops iPr3SiH, 74%.
1). Compound 5 also exhibited potent antirhinoviral
activity in cell culture when tested against a number of
different HRV serotypes (Table 2). Thus, the present
work demonstrates that properly optimized bicyclic 2-
pyridone-containing 3CP inhibitors can function as
potent, broad-spectrum antirhinoviral agents.
bonds (Fig. 3). As expected, the conformationally
mobile side chain of 3CP Ser-128 which was previously
observed to form hydrogen bonds between the P2-P3
amide NH of non-methylated tripeptide-derived 3CP
inhibitors22 was rotated out of the active site by the
bicyclic P2-P3 moiety contained in 3.
The 1.55 A X-ray crystal structure of the covalent
adduct formed between compound 3 and HRV-2
3CP19,20 is shown in Figure 2, and key protein–inhibitor
interactions are illustrated in Figure 3. As expected, the
inhibitor filled a series of shallow grooves on the protein
surface N-terminal to the scissile amide of the enzyme’s
substrate (S1–S4)8 and a covalent bond was observed
between the 3CP active site cysteine residue (Cys-147)
and the b-carbon of the Michael acceptor of 3. A
hydrogen bond between the Michael acceptor ester
moiety and the 3CP Cys-147 amide NH was also pre-
sent in the above complex, but, as with all such 3CP
inhibitors studied to date, it is uncertain whether this
interaction facilitates addition of the cysteine to 3 or
merely arises after the covalent adduct has been formed.
The bicyclic 2-pyridone-containing 3CP inhibitors
described in this study were prepared by two related
synthetic methods and both are illustrated in Scheme 1.
The synthesis of compound 2 was accomplished on solid
support and began with the Fmoc deprotection of resin-
bound entity 618 to afford the corresponding free amine
(7). Subsequent HATU-mediated coupling with the
commercially available bicyclic carboxylic acid 811,12
provided resin-bound intermediate 9. The Fmoc moiety
present in 9 was then replaced with a benzyl carbamate
to give intermediate 10. Cleavage of 10 from the solid
support was effected by exposure to strong acid and the
resulting primary amide (2) was purified by reverse-
phase preparative HPLC. The described solid-phase
synthesis represents a convenient alternative to the
solution-phase methods typically employed in the pre-
paration of 3CP inhibitors and afforded 2 in 45% yield
for the five-step process.
The most significant departure from previously exam-
ined 3CP–ligand structures observed in the above com-
plex was the absence of any appreciable contact between
3 and the S2 binding subsite of the enzyme. Not unex-
pectedly, the 3CP residues that comprise the S2 binding
pocket (Leu-127, Ser-128, and Asn-130 on one side and
His-40 on the other) deviated somewhat from their
observed locations in other 3CP–ligand complexes in
which the S2 subsite was filled. The remaining interac-
tions between the bicyclic inhibitor 3 and 3CP were
essentially identical to those observed in previous crystal
structures of peptide,21 peptidomimetic,21 and mono-
cyclic 2-pyridone-containing 3CP inhibitors.6 These
included the important anti-parallel b-sheet interaction
formed between the pyridone moiety of 3 and Gly-164
along with five additional protein-ligand hydrogen
The preparation of inhibitor 3 began with acidic Boc
deprotection of the known trityl-containing l-glutamine
derivative 11.22 Subsequent carbodiimide-mediated
coupling of the resulting amine salt (12) with the known
bicyclic 2-pyridone moiety 1323 (approximately 60% ee)
afforded intermediate 14 as a single diastereomer in
60% yield after purification by flash column chromato-

P.S.Dragovich et al./ Bioorg.Med.Chem.Lett.12 (2002) 733–738
737
graphy.24 Removal of the trityl protecting group present
in 14 was then accomplished by treatment with tri-
fluoroacetic acid and compound 3 was isolated in 74%
yield after concentration of the reaction mixture, tri-
turation with diethyl ether, and filtration. Inhibitor 5
was synthesized from carboxylic acid 1525 (approxi-
mately 60% ee) and lactam 1626 in a manner completely
analogous to that described above for the preparation
of 3.
S. K.; Chen, L.; Guzman, M. C.; Meador, J. W., III; Ferre, R.
A.; Worland, S. T. J.Med.Chem . accepted for publication.
7. (a) For other examples of Michael acceptor-containing
cysteine protease inhibitors, see: Roush, W. R.; Gwaltney,
S. L., II; Cheng, J.; Scheidt, K. A.; McKerrow, J. H.; Hansell,
E. J.Am.Chem.Soc. 1998, 120, 10994. (b) McGrath, M. E.;
Klaus, J. L.; Barnes, M. G.; Bromme, D. Nat.Struct.Biol.
1997, 4, 105. (c) Bromme, D.; Klaus, J. L.; Okamoto, K.;
Rasnick, D.; Palmer, J. T. Biochem.J. 1996, 315, 85. (d) Pal-
mer, J. T.; Rasnick, D.; Klaus, J. L.; Bromme, D. J.Med.
Chem. 1995, 38, 3193. (e) Liu, S.; Hanzlik, R. P. J.Med.
Chem. 1992, 35, 1067. (f) Hanzlik, R. P.; Thompson, S. A. J.
Med.Chem. 1984, 27, 711.
The studies presented above demonstrate that bicyclic
2-pyridone-containing irreversible inhibitors of the
human rhinovirus 3Cprotease can function as potent,
broad-spectrum antirhinoviral agents. The studies also
illustrate how such bicyclic molecules can be optimized
via a combination of computational methods and crys-
tallographic information.
8. The nomenclature used for describing the individual amino
0
acid residues of a peptide substrate (P2, P1, P1 , P2 , etc.) and
0
0
0
the corresponding enzyme subsites (S2, S1, S1 , S2 , etc.) is
described in: Schechter, I.; Berger, A. Biochem.Biophys.Res.
Commun. 1967, 27, 157.
9. (a) For some recent examples, see: Hanessian, S.; Balaux,
E.; Musil, D.; Olsson, L.-L.; Nilsson, I. Bioorg.Med.Chem.
Lett. 2000, 10, 243. (b) Boatman, P. D.; Ogbu, C. O.; Eguchi,
M.; Kim, H.-O.; Nakanishi, H.; Cao, B.; Shea, J. P.; Kahn, M.
J.Med.Chem. 1999, 42, 1367. (c) Bachand, B.; DiMaio, J.;
Siddiqui, M. A. Bioorg.Med.Chem.Lett. 1999, 9, 913. (d) St.-
Denis, Y.; Augelli-Szafran, C. E.; Bachand, B.; Berryman,
K. A.; DiMaio, J.; Doherty, A. M.; Edmunds, J. J.; Leblond,
L.; Levesque, S.; Narasimhan, L. S.; Penvose-Yi, J. R.; Rubin,
J. R.; Tarazi, M.; Winocour, P. D.; Siddiqui, M. A. Bioorg.
Med.Chem.Lett. 1998, 8, 3193.
Acknowledgements
We are grateful for many helpful discussions through-
out the course of this work with Professor Larry Over-
man and Drs. Benjamin Burke, Robert Kumpf, Steven
Bender, and Siegfried Reich.
10. (a) For recent reviews of peptide mimics, see: Hruby, V. J.;
Balse, P. M. Curr.Med.Chem. 2000, 7, 945. (b) Ripka, A. S.;
Rich, D. H. Curr.Opin.Chem.Biol. 1998, 2, 441. (c) Gante, J.
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donado, F. C.; Fuhrman, S. A.; Zalman, L. S.; Tuntland, T.;
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routinely than the serotype 14-derived protease. The inhibitor-
binding regions of the two enzymes do not differ significantly.

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P.S.Dragovich et al./ Bioorg.Med.Chem.Lett.12 (2002) 733–738
20. Human rhinovirus 3Cprotease of serotype 2 was incu-
bated with a 3-fold molar excess of compound 3 in the pre-
sence of 2% DMSO for 2 h at room temperature followed by
an additional 4 h incubation at 4 ꢀC. The resulting complex
was concentrated to 12 mg/mL and then passed through a 0.45
mM cellulose-acetate filter. Crystals were grown at 13 ꢀCusing
the hanging drop vapor diffusion method in which equal
volumes (3 mL) of the protein/ligand complex and reservoir
solution were layered on plastic coverslips and sealed over
wells filled with 1 mL of reservoir solution containing 0.2 M
CaCl2, 0.1M Tris(HCl) pH 8.5, 25% PEG 4K and 5 mM
man, S. A.; Patick, A. K.; Zalman, L. S.; Hendrickson, T. F.;
Love, R. A.; Prins, T. J.; Marakovits, J. T.; Zhou, R.; Tikhe,
J.; Ford, C. E.; Meador, J. W., III; Ferre, R. A.; Brown, E. L.;
Binford, S. L.; Brothers, M. A.; DeLisle, D. M.; Worland,
S. T. Proc. Natl. Acad. Sci. U.S.A. 1999, 96, 11000.
22. Dragovich, P. S.; Webber, S. E.; Babine, R. E.; Fuhrman,
S. A.; Patick, A. K.; Matthews, D. A.; Lee, C. A.; Reich, S. R.;
Prins, T. J.; Marakovits, J. T.; Littlefield, E. S.; Zhou, R.;
Tikhe, J.; Ford, C. E.; Wallace, M. B.; Meador, J. W., III;
Ferre, R. A.; Brown, E. L.; Binford, S. L.; Harr, J. E. V.;
DeLisle, D. M.; Worland, S. T. J.Med.Chem. 1998, 41, 2806.
23. Dragovich, P. S.; Zhou, R.; Prins, T. J. J.Org.Chem. in
press.
DTT.
A rectangular rod-shaped crystal of approximate
dimensions 0.4ꢁ0.15ꢁ0.15 mm (space group P1, two mole-
cules per asymmetric unit, a=33.02, b=41.98, c=67.79 A,
a=89.99, b=100.94, g=112.20 deg) was prepared for low
temperature data collection by transfer to an artificial mother
liquor solution consisting of 400 mL of the reservoir solution
mixed with 125 mL of glycerol and then flash frozen in a
stream of N2 gas at À170 ꢀC. X-ray diffraction data were col-
lected with a MAR Research 345 mm imaging plate and pro-
cessed with DENZO.27 Diffraction data were 88.9% complete
to a resolution of 1.55 A with Rsym=2.2%. Protein atomic
coordinates from a previously solved isomorphous cocrystal
structure of type 2 3Cprotease were used to initiate rigid-body
refinement in XPLOR28 followed by simulated annealing and
conjugate gradient protocols. Placement of the inhibitor,
addition of ordered solvent, and further refinement proceeded
as described previously.29 The final R factor was 20.9% [39009
reflections with F>2s(F)]. The root-mean-square deviations
from ideal bond lengths and angles were 0.011 A and 2.8 deg,
respectively. The final model consisted of all atoms for resi-
dues 1–180 in both molecules plus 258 water molecules.
24. No attempt was made to quantitate and/or isolate the
other diastereomer of 14 that was presumably also formed.
25. Dragovich, P. S.; Prins, T. J.; Zhou, R.; Johnson, T. O.,
Jr. WO 01/40189 A1; CAN 135:33649.
26. (a) Tian, Q.; Nayyar, N. K.; Babu, S.; Chen, L.; Tao, J.;
Lee, S.; Tibbetts, A.; Moran, T.; Liou, J.; Guo, M.; Kennedy,
T. Tetrahedron Lett. 2001, 42, 6807. (b) Tian, Q.; Nayyar, N.
K.; Babu, S.; Tao, J.; Moran, T. J.; Dagnino, R., Jr.;
Remarchuk, T. P.; Melnick, M. J.; Mitchell, L. J., Jr.; Bender,
S. L. 2001 WO 01/14329, CAN 134:193743.
27. Otwinowski, Z.; Proceedings of the CCP4 Study Weekend,
Sawyer, L.; Isaacs, N.; Baily, S., Eds.; SERCDaresbury
Laboratory: Warrington, 1993; p 55.
28. XPLOR, version 3.1. Brunger, A. T. XPLOR v3.1 Man-
ual; Yale University Press: New Haven, CT, 1992.
29. Webber, S. E.; Okano, K.; Little, T. L.; Reich, S. H.; Xin,
Y.; Fuhrman, S. A.; Matthews, D. A.; Love, R. A.; Hen-
drickson, T. F.; Patick, A. K.; Meador, J. W., III; Ferre, R. A.;
Brown, E. L.; Ford, C. E.; Binford, S. L.; Worland, S. T. J.
Med.Chem. 1998, 41, 2786.
21. Matthews, D. A.; Dragovich, P. S.; Webber, S. E.; Fuhr-