P. Zheng et al. / Bioorg. Med. Chem. Lett. 21 (2011) 3435–3438
3437
H
MeO
MeO
MeO
MeO
MeO
MeO
MeO
MeO
NH2
MeO
MeO
N
EtO2C
86%
CO2Et
P2O5
73%
H2, PtO2
86%
CO2Et
CO2Et
70%
N
NH
N
O
CO2Et
11
12
CO2Et
CO2Et
CO2Et
13
14
15
MeO
MeO
MeO
MeO
MeO
MeO
MeO
MeO
R3
NaH,
DMF, rt or 50 oC
10% HCl
NaOEt
52%
N
X
NaBH4
N
N
O
N
O
H
CO2Et
R3
R3
R3
CO2Et
16
O
OH
18a R3: cyclobutyl, 58%
18b R3: cyclopentyl, 40%
18c R3: cyclohexyl, 53%
17a X: Br, R3: cyclobutyl, 27%
17b X: I, R3: cyclopentyl, 71%
17c X: Br, R3: cyclohexyl, 15%
4c R3: cyclobutyl, 75%
4d R3: cyclopentyl, 59%
4e R3: cyclohexyl, 72%
Scheme 3. The synthesis of 3-alkyl-DTBZ derivatives (4c–4e).
Acknowledgments
This work was supported by a NIH Grant DK-081342. We thank
Patrick J. Carroll for the acquisition of the X-ray data of 3c, and Zhi-
Ping Zhuang for helpful discussion.
Supplementary data
Supplementary data (Experimental procedures, characteriza-
tion data, as well as biological studies. This material is available
free of charge via the Internet) associated with this article can be
References and notes
Figure 2. X-ray structure of 3-cyclobutyl methyl DTBZ (4c).
1. Zheng, G.; Dwoskin, L. P.; Crooks, P. A. The AAPS Journal 2006, 8, E682.
2. Erickson, J. D.; Eiden, L. E.; Hoffman, B. J. Proc. Natl. Acad. Sci. U.S.A. 1992, 89,
10993.
3. Liu, Y.; Peter, D.; Roghani, A.; Schuldiner, S.; Prive, G. G.; Eisenberg, D.; Brecha,
N.; Brecha, R. H. Cell 1992, 70, 539.
Table 1
Inhibition constants of new ligands on
binding to VMAT2 in rat striatal homogenates
[
125I]-iodovinyl-TBZ or 18F](+)-FP-DTBZ
[
4. Erickson, J.; Eiden, L. J. Neurochem. 1993, 61, 2314.
5. Brossi, A.; Lindlar, H.; Walter, M.; Schnider, O. Helv. Chim. Acta 1958, 41, 119.
6. Hayden, M. R.; Leavitt, B. R.; Yasothan, U.; Kirkpatrick, P. Nat. Rev. Drug Disc.
2009, 8, 17.
7. Bohnen, N. I.; Albin, R. L.; Koeppe, R. A.; Wernette, K. A.; Kilbourn, M. R. J. Cereb.
Blood Flow Metab. 2006, 26, 1198.
Entry
Compound
Kia (nM)
1
2
3
4
5
6
7
8
(+)-DTBZ, 2
(+)-FP-DTBZ, 3
1.03 0.11
0.10 0.0110c
253 6.0b
5.98 0.23
26.9 3.43
227 19.0
137 35.0
22.4 1.25b
4a
4b
4c
4d
4e
8. Kilbourn, M. R. Nucl. Med. Biol. 1997, 24, 615.
9. Koeppe, R. A.; Frey, K. A.; Vander Borght, T. M.; Karlamangla, A.; Jewett, D. M.;
Lee, L. C.; Kilbourn, M. R.; Kuhl, D. E. J. Cereb. Blood Flow Metab. 1996, 16, 1288.
10. FP-DTBZ as pancreas imaging agent, see: (a) Goswami, R.; Ponde, D. E.; Kung,
M.-P.; Hou, C.; Kilbourn, M. R.; Kung, H. F. Nucl. Med. Biol. 2006, 33, 685; (b)
Kilbourn, M. R.; Hockley, B.; Lee, L.; Hou, C.; Goswami, R.; Ponde, D. E.; Kung,
M.-P.; Kung, H. F. Nucl. Med. Biol. 2007, 34, 233; (c) Kung, M.-P.; Hou, C.;
Goswami, R.; Ponde, D. E.; Kilbourn, M. R.; Kung, H. F. Nucl. Med. Biol. 2007, 34,
239; (d) Okamura, N.; Villemagne, V. L.; Drago, J.; Pejoska, S.; Dhamija, R. K.;
Mulligan, R. S.; Ellis, J. R.; Ackermann, U.; O’Keefe, G.; Jones, G.; Kung, H. F.;
Pontecorvo, M. J.; Skovronsky, D.; Rowe, C. C. J. Nucl. Med. 2010, 51, 223; FP-
DTBZ as pancreas imaging agent, see: (e) Kung, H. F.; Lieberman, B. P.; Zhuang,
Z.-P.; Oya, S.; Kung, M.-P.; Choi, S. R.; Ploessl, K.; Blankemeyer, E.; Hou, C.;
Skovronsky, D.; Kilbourn, M. Nucl. Med. Biol. 2008, 35, 825; (f) Kung, M.-P.; Hou,
C.; Lieberman, B. P.; Oya, S.; Ponde, D. E.; Blankemeyer, E.; Skovronsky, D.;
Kilbourn, M. R.; Kung, H. F. J. Nucl. Med. 2008, 49, 1171; (g) Tsao, H.-H.; Lin, K.-
J.; Juang, J.-H.; Skovronsky, D. M.; Yen, T.-C.; Wey, S.-P.; Kung, M.-P. Nucl. Med.
Biol. 2010, 37, 413.
11. Amino TBZ, see: (a) Canney, D. J.; Kung, M.-P.; Kung, H. F. Nucl. Med. Biol. 1995,
22, 527; (b) Zhu, L.; Liu, J.; Kung, H. F. Bioorg. Med. Chem. Lett. 2009, 19, 5026.
12. Iodovinyl TBZ, see: (a) Canney, D. J.; Guo, Y. Z.; Kung, M.-P.; Kung, H. F. J.
Labelled Compd. Rad. 1993, 33, 355; (b) Kung, M.-P.; Canney, D. J.; Frederick, D.;
Zhuang, Z.; Billings, J. J.; Kung, H. F. Synapse 1994, 18, 225.
13. Racemic synthesis, see: (a) Openshaw, H. T.; Whittaker, N. J. Chem. Soc. 1963,
1449; (b) Yu, Q.; Luo, W.; Deschamps, J.; Holloway, H. W.; Kopajtic, T.; Katz, J.
L.; Brossi, A.; Greig, N. H. A. C. S. Med. Chem. Lett. 2010, 1, 105; Asymmetric
synthesis, see: (c) Rishel, M. J.; Amarasinghe, K. K. D.; Dinn, S. R.; Johnson, B. F. J.
Org. Chem. 2009, 74, 4001; (d) Paek, S.-M.; Kim, N.-J.; Shin, D.; Jung, J.-K.; Jung,
J.-W.; Chang, D.-J.; Moon, H.; Suh, Y.-G. Chem. Eur. J. 2010, 16, 4623.
14. Johnson, A. W.; Markham, E.; Price, R. Org. Syn. 1962, 42, 75.
15. Ayed, T. B.; Amri, H. Synthetic Commun. 1995, 25, 3813.
10a
a
Kd value of 8.2 nM for [125I]-iodovinyl-TBZ was used for the calculation based on
the reported Ki value (see Ref. 12b). The results are the mean S.E.M. of three
independent measurements done in duplicates. [125I]-iodovinyl-TBZ was used as
radioligand, unless otherwise stated.
b
[
18F](+)-FP-DTBZ was used.
When the isobutyl side chain was replaced with cycloalkyl
methyl group, the binding affinity generally decreased (Table 1, en-
tries 5–7). Cyclobutyl substrate 4c showed better affinity than
cyclopentyl 4d and cyclohexyl 4e. Interestingly, cyclohexyl 4e
exhibited slightly better affinity than cyclopentyl derivative 4d.
In summary, we have synthesized a new series of 3-alkyl DTBZ
derivatives. These new ligands were tested in vitro for their po-
tency to inhibit the binding of [125I]-iodovinyl-TBZ or [18F](+)-FP-
DTBZ in rat striatal membrane homogenates. These compounds
exhibited moderate to good binding affinity to VMAT2 binding
sites in the brain. We have demonstrated that the hydrophobic
binding pocket of vesicular monoamine transporter 2 (VMAT2) is
highly sensitive to steric size of the side chain at the C-3 position.
Further structure–activity relationship studies of DTBZ derivatives
are currently being pursued in our laboratory.
16. See the Supplementary data for the details.
17. Typical procedure for the synthesis of 8b: A solution of dihydroisoquinoline
hydrochloride salt (9.3 g, 40.9 mmol) and methyl vinyl ketone 7b (6.1 g,
49.1 mmol) in absolute EtOH (41 mL) was stirred at 90 °C for 48 h. The solvent