Using a SNIPER with 4-hydroxy tamoxifen as
the ERa ligand, Okuhira et al. showed successful degradation of ERa and resultant necrotic
death of ERa-expressing MCF-7 breast cancer
cells (28). Another strategy involves treating
cells with two halves of a PROTAC molecule (as
opposed to a larger single compound with less
optimal physiochemical properties) that then self-assembles intracellularly (29). This approach was
used to recruit the E3 ligase cereblon to successfully degrade BRD4 (bromodomain-containing
protein 4) and ERK1/2 (extracellular signal–
regulated kinases 1 and 2), both important targets for cancer therapy.
Similar to the PROTAC technology, hydrophobic tags have been used to hijack the UPS
to degrade proteins of interest. The underlying concept is that partially unfolded or mis-folded proteins expose hydrophobic patches
that are otherwise buried, serving as a recruiting signal for E3 ubiquitin ligases that then
catalyze ubiquitination and subsequent degradation. Researchers have successfully mimicked
this protein unfolding for specific targets by
appending a low–molecular weight hydrophobic tag to the target’s small-molecule ligand;
this results in recruitment of the UPS to degrade
the target protein (30). This strategy has been
applied to degrade the HER3 (human epidermal
growth factor receptor 3) pseudokinase, a currently “undruggable” cancer target, by appending a “greasy tag” to the potent and selective HER3
ligand TX1-85-1 to generate the bifunctional TX2-
121-1 compound (31). The resulting TX2-121-1–
induced HER3 knockdown led to inhibition of
downstream signaling and reduced proliferation
of HER3-dependent cell lines. Gustafson et al.
generated selective androgen receptor degraders
(SARDs) that degrade the AR via hydrophobic tagging (32). SARD279 was shown to be as effective
as enzalutamide (an FDA-approved inhibitor of
AR signaling) in suppressing proliferation of human prostate cancer cells; this drug also suppressed proliferation of enzalutamide-resistant
prostate cancer cells (32).
Protein degradation strategies in
preclinical models of cancer
Within the past 2 years, considerable progress
has been made in the advancement of protein
degradation technology to selectively and effectively degrade key cancer targets. The BET
(bromodomain and extra-terminal) proteins, such
as BRD4, play important roles in the progression of various cancers, including acute myeloid
leukemia (AML), MM, Burkitt’s lymphoma (BL),
ovarian cancer, and prostate cancer (33, 34–39).
To that end, potent small-molecule inhibitors
of BET proteins have been developed and are in
clinical trials (40, 41). The experience to date suggests that the effectiveness of these inhibitors
may be limited by incomplete suppression of the
downstream oncogene c-MYC (39) and a compensatory increase in BRD4 protein levels as a way
to circumvent inhibition (42).
Several labs have reported PROTACs that
degrade the BRD4 protein in cells (33, 42–44).
For example, Lu et al. designed ARV-825, which
degrades BRD4 by engaging the CRBN E3 ligase and incorporates the potent BET inhibitor
OTX015 as a recruiting moiety (42). At subnanomolar concentrations, ARV-825 induced near-complete BRD4 degradation in BL cell lines. It
suppressed c-MYC expression and was a more potent inducer of apoptosis than conventional BET
inhibitors such as JQ1 and OTX015. Winter et al.
designed a PROTAC called dBET1 by appending JQ1 to a phthalimide moiety, which hijacks
the cereblon E3 ubiquitin ligase complex (43).
dBET1 showed greater antiproliferative effects
in AML and lymphoma cells when compared
with JQ1 inhibition. Furthermore, dBET1 inhibited leukemia progression in a mouse xenograft
model of AML. Raina et al.’s ARV-771 compound
recruits BET proteins to the VHL E3 ligase for
degradation at subnanomolar concentrations in
several prostate cancer cell lines. ARV-771 inhibited the proliferation of enzalutamide-resistant
prostate cancer cells and inhibited tumor growth
in a castration-resistant prostate cancer mouse
xenograft model (33). Zengerle et al. designed
a BRD4 PROTAC in which JQ1 is tethered to a
ligand for the VHL E3 ligase and showed that
this compound selectively induces degradation
of BRD4 in cultured cells (44).
The most promising aspect of targeted degradation as a therapeutic strategy may be its potential for targeting proteins for which there
currently is no drug. Undruggable proteins such
as scaffolding proteins, pseudokinases, and transcription factors make up ~80% of the human
proteome; these proteins are neither enzymes nor
receptors and lack an enzymatic activity or functional interaction that can be compromised by
an inhibitor (45). The ability to target any of these
would require the identification of a specific ligand. However, because event-driven protein degradation can be mediated via any binding site on the
surface of the target protein rather than restricted
to a single, identifiable active site (Fig. 2), the
development of simple but potent and selective
ligands may be easier. The targeted degradation
approach to eliminate such proteins has the
potential to render these otherwise-undruggable
proteins pharmaceutically vulnerable.
Targeted degradation may also prove useful
in drug-resistance mechanisms that involve a
compensatory increase in the expression of
inhibited proteins or mutations that result in
the loss of inhibition despite maintained target
engagement. Given the encouraging preclinical
studies on targeted degradation of BET proteins
and the AR, it appears that tools to win this
pharmacological “arms race” may be available.
One possible application is the subgroup of
cetuximab-resistant non–small cell lung cancers
that show increased expression of the epidermal
growth factor receptor, the protein targeted by
cetuximab (46). In this case, substantial loss of
target-protein levels and activity would still be
achieved because of the catalytic nature of the
PROTAC mechanism of action. Finally, although
this Review has focused specifically on appli-
cations in cancer therapy, other disease con-
texts may also benefit from this emerging drug
REFERENCES AND NOTES
1. J. Ferlay et al., Int. J. Cancer 136, E359–E386 (2015).
2. B. A. Teicher, J. E. Tomaszewski, Biochem. Pharmacol. 96, 1–9
3. X. Huang, V. M. Dixit, Cell Res. 26, 484–498 (2016).
4. D. Chauhan et al., Cancer Cell 22, 345–358 (2012).
5. D. S. Johnson, E. Weerapana, B. F. Cravatt, Future Med. Chem.
2, 949–964 (2010).
6. Y. Ye, M. Rape, Nat. Rev. Mol. Cell Biol. 10, 755–764 (2009).
7. R. J. Deshaies, C. A. P. Joazeiro, Annu. Rev. Biochem. 78,
8. J. R. Skaar, J. K. Pagan, M. Pagano, Nat. Rev. Drug Discov. 13,
9. A. Citri et al., EMBO J. 21, 2407–2417 (2002).
10. A. Howell, Endocr. Relat. Cancer 13, 689–706 (2006).
11. T.-D. Zhang et al., Oncogene 20, 7146–7153 (2001).
12. J. Krönke et al., Nature 523, 183–188 (2015).
13. J. Krönke et al., Science 343, 301–305 (2014).
14. L. Mi et al., J. Biol. Chem. 284, 17039–17051 (2009).
15. A. Bill et al., J. Biol. Chem. 289, 11029–11041 (2014).
16. K. M. Sakamoto et al., Proc. Natl. Acad. Sci. U.S.A. 98,
17. K. M. Sakamoto et al., Mol. Cell. Proteomics 2, 1350–1358 (2003).
18. A. Howell, S. J. Howell, D. G. Evans, Cancer Chemother.
Pharmacol. 52 (suppl. 1), 39–44 (2003).
19. Y. Zhou, E. C. Bolton, J. O. Jones, J. Mol. Endocrinol. 54,
20. K. Cyrus, M. Wehenkel, E.-Y. Choi, H. Swanson, K.-B. Kim,
ChemBioChem 11, 1531–1534 (2010).
21. A. R. Schneekloth, M. Pucheault, H. S. Tae, C. M. Crews,
Bioorg. Med. Chem. Lett. 18, 5904–5908 (2008).
22. D. L. Buckley et al., Angew. Chem. Int. Ed. 51, 11463–11467 (2012).
23. D. P. Bondeson et al., Nat. Chem. Biol. 11, 611–617 (2015).
24. A. Lopez-Girona et al., Leukemia 26, 2326–2335 (2012).
25. K. Sekine et al., J. Biol. Chem. 283, 8961–8968 (2008).
26. A. C. Lai et al., Angew. Chem. Int. Ed. 55, 807–810 (2016).
27. Y. Itoh, M. Ishikawa, M. Naito, Y. Hashimoto, J. Am. Chem. Soc.
132, 5820–5826 (2010).
28. K. Okuhira et al., Cancer Sci. 104, 1492–1498 (2013).
29. H. Lebraud, D. J. Wright, C. N. Johnson, T. D. Heightman,
ACS Cent. Sci. 2, 927–934 (2016).
30. T. K. Neklesa et al., Nat. Chem. Biol. 7, 538–543 (2011).
31. T. Xie et al., Nat. Chem. Biol. 10, 1006–1012 (2014).
32. J. L. Gustafson et al., Angew. Chem. Int. Ed. 54, 9659–9662 (2015).
33. K. Raina et al., Proc. Natl. Acad. Sci. U.S.A. 113, 7124–7129 (2016).
34. A. C. Belkina, G. V. Denis, Nat. Rev. Cancer 12, 465–477 (2012).
35. I. A. Asangani et al., Nature 510, 278–282 (2014).
36. J. E. Delmore et al., Cell 146, 904–917 (2011).
37. C. A. French et al., Oncogene 27, 2237–2242 (2008).
38. M. G. Baratta et al., Proc. Natl. Acad. Sci. U.S.A. 112, 232–237 (2015).
39. J. A. Mertz et al., Proc. Natl. Acad. Sci. U.S.A. 108,
40. Z. Cheng et al., Clin. Cancer Res. 19, 1748–1759 (2013).
41. R. K. Prinjha, J. Witherington, K. Lee, Trends Pharmacol. Sci.
33, 146–153 (2012).
42. J. Lu et al., Chem. Biol. 22, 755–763 (2015).
43. G. E. Winter et al., Science 348, 1376–1381 (2015).
44. M. Zengerle, K. H. Chan, A. Ciulli, ACS Chem. Biol. 10,
45. A. P. Russ, S. Lampel, Drug Discov. Today 10, 1607–1610 (2005).
46. D. L. Wheeler et al., Oncogene 27, 3944–3956 (2008).
47. F. Garner, M. Shomali, D. Paquin, C. R. Lyttle, G. Hattersley,
Anticancer Drugs 26, 948–956 (2015).
48. C. Tovar et al., Cancer Res. 73, 2587–2597 (2013).
49. M. Korpal et al., Cancer Discov. 3, 1030–1043 (2013).
C.M.C. is the chief scientific adviser of and is a shareholder in
Arvinas, a biotechnology company focused on developing
protein degradation therapeutics for cancer and other diseases.
C.M.C. is an inventor on patent US7041298 B2 and patent
applications PCT/US2013/040551, PCT/US2013/021136,
EP20150180508, and PCT/US2011/063401 (submitted by Yale
University) and PCT/US2015/025813 (submitted by Arvinas),
which cover targeted protein degradation. C.M.C. acknowledges
support from the Leukemia and Lymphoma Society and the NIH (grant
R35CA197589). C.M.C. also receives research funding from Arvinas.