DOI: 10.1111/exd.12400
Commentary
www.wileyonlinelibrary.com/journal/EXD
Melanoma never says die Nikolas K. Haass1,2,3 and Udo Schumacher4 1
The University of Queensland, The University of Queensland Diamantina Institute, Translational Research Institute, Brisbane, Qld, Australia; 2The Centenary Institute, Newtown, NSW, Australia; 3Discipline of Dermatology, University of Sydney, Camperdown, NSW, Australia; 4Institute of Anatomy and Experimental Morphology, University Hospital Hamburg-Eppendorf, Hamburg, Germany Correspondence: Nikolas K. Haass, The University of Queensland Diamantina Institute, Translational Research Institute, 37 Kent St, Woolloongabba, Qld 4102, Australia, Tel.: +61-7-3443-7087, Fax: +61-7-3443-6966, e-mail: [email protected] Abstract: Drug resistance in melanoma is commonly attributed to ineffective apoptotic pathways. Targeting apoptosis regulators is thus considered a promising approach to sensitizing melanoma to therapy. In the previous issue of Experimental Dermatology, Pl€ otz and Eberle discuss the role that apoptosis plays in melanoma progression and drug resistance and the utility of apoptosisinducing BH3-mimetics as targeted therapy. There are a number of compounds in clinical development and the field seems close to
translating recent findings into benefits for patients with melanoma. Thus, this viewpoint is timely and achieves a valuable summary of the current state of apoptosis-inducing therapy of melanoma.
Melanoma cells never say die: Firstly, malignant cells, including melanoma cells, are often selected for apoptosis deficiency, and secondly, defective apoptosis pathways are one barrier to effective systemic treatment of melanoma (1). Melanoma is a deadly skin cancer mainly because of its propensity to metastasize early and its resistance to conventional chemotherapeutic agents. Considering the latter, chemotherapeutic agents either initially inhibit cell proliferation and consequently initiate apoptosis, or they primarily induce apoptosis directly. Both mechanisms, however, require the uptake of the agents into the cell. Targeted BRAF inhibition has been a very successful new approach to treat melanoma; however, resistance invariably develops, making the search for augmentation of the therapy necessary. Strengthening the pro-apoptotic signalling cascade may therefore be a rewarding way to induce apoptosis in melanoma cells. BH3-only proteins represent a class of proteins which are pro-apoptotic by neutralizing anti-apoptotic Bcl-2 proteins (Fig. 1a). The balance between pro- and anti-apoptotic proteins thus determines the fate of a cell. The large and yet increasing body of literature focusing on targeting the intrinsic apoptosis pathway as a strategy for melanoma therapy indeed supports the importance of this field of research (2). Pl€ otz and Eberle (3) propose in their excellent viewpoint two major strategies to target this pathway, namely gene therapy with adenoviral vectors which induce expression of pro-apoptotic genes (4) or small molecules which act as BH3-mimetics. From the BH3mimetics, it is hoped that they prevent resistance to or increase susceptibility for kinase inhibitors (3). The development of BH3- or SMAC-mimetics – antagonists of inhibitor of apoptosis proteins (IAPs) – has helped us to understand the mechanisms of apoptotic resistance (2). SMAC-mimetic studies show that both XIAP and the cIAPs must be targeted to effectively induce apoptosis of cancer cells (2). The highly studied BH3-mimetic ABT-737 imitates BAD and therefore inhibits BCL-2, BCL-w and BCL-xL (5). We have shown that single agent treatment with ABT-737 is not sufficient as melanoma therapy but that inhibition of MCL-1, or induction of its antagonist NOXA, strongly sensitize melanoma cells to ABT-737 in vitro (6) (Fig. 1b). In accordance with this, and based on the fact
that BIM can interact with and suppress all anti-apoptotic BCL-2 proteins simultaneously and that BIM is regulated through a number of signalling pathways, Pl€ otz and Eberle (3) propose as a central issue in their viewpoint that a BH3-mimetic specific for BIM may represent an even better strategy for melanoma therapy (Fig. 1c). Importantly, the two most prominent pro-survival pathways in melanoma, the MAPK and PI3K/AKT pathways, do not only drive uncontrolled cell proliferation but also apoptosis resistance: Activated ERK1/2 interferes with binding of BIM to MCL-1 and BCLXL (7), targets BIM for proteasomal degradation (8) and thus inhibits BIM-induced apoptosis. Conversely, MAPK pathway inhibition promotes apoptosis in melanoma by up-regulating BIM and PUMA, activating BMF and down-regulating the anti-apoptotic protein MCL-1 (9). Loss of PTEN contributes to intrinsic BRAF inhibitor resistance via suppression of BIM-mediated apoptosis (10). In our hands, combination of BRAF inhibitors with ABT-737 killed melanoma cells synergistically in vitro, dependent on induction of BIM and down-regulation of MCL-1 (11). The BH3-mimetics obatoclax (BCL-2 and MCL-1, lower affinity to BCL-XL, BCL-w and BFL-1/A1) and prodigiosin (MCL-1) also inhibit mTORC1 and mTORC2 preventing full activation of AKT (12). Further, we have shown that inhibition of PI3K/AKT signalling enhances the cytotoxic effect of MAPK pathway inhibition (13). This provides a rationale for combining BH3-mimetics with MAPK pathway inhibitors. A promising regimen for patients with melanoma harbouring a BRAFV600E mutation would be a BH3-mimetic combined with a selective BRAF inhibitor and/or a MEK inhibitor. Indeed, a currently recruiting trial will study the side effects and best dose of a threefold combination of small molecule inhibitors: dabrafenib (BRAF), trametinib (MEK) and navitoclax (BCL-2, BCL-w and BCL-xL) (ClinicalTrials.gov). Theoretically, a BIMmimetic drug (Fig. 1c), as proposed by Pl€ otz and Eberle, would work independently of the BRAF status and would therefore provide a therapeutic approach to both BRAF-mutant and BRAF-wildtype melanomas. We propose that addition of novel BH3-mimetics to current drug regimens and new combinations are avenues worth exploring for melanoma therapy.
ª 2014 John Wiley & Sons A/S. Published by John Wiley & Sons Ltd Experimental Dermatology, 2014, 23, 471–472
Key words: BH3-mimetic – drug resistance – intrinsic apoptosis pathway – melanoma therapy – SMAC-mimetic
Accepted for publication 27 March 2014
471
Commentary
PUMA
(a)
BCL-2
BMF
BCL-XL
BAD
BIM
BID
BCL-w
HRK
BFL-1/A1
MCL-1
BIK
NOXA
BV
(b) NOXA induction
ABT-737 ABT-263
O2, nutrients BCL-2
BCL-XL
BCL-w
BFL-1/A1
MCL-1
Drugs BAX
BAK
Interstitial fluid pressure
Mitochondrion Cytochrome C
(c)
BAX
BCL-2
BCL-XL
BCL-w
BAK
BFL-1/A1
MCL-1
BIM mimetic
Figure 1. Mimicking BH3-only pro-apoptotic proteins. (a) BH3-only pro-apoptotic proteins (orange) have distinct specificities for anti-apoptotic proteins (blue). (b) ABT-737 and ABT-263 (navitoclax) mimic BAD inhibiting BCL-2, BCL-w and BCL-xL. Induction of NOXA, which inhibits BFL-1/A1 and MCL-1, sensitizes melanoma cells to ABT-737, resulting in inhibition of all five anti-apoptotic proteins expressed in melanoma (blue) and leading to apoptosis (skull) (6). (c) A BIM-mimetic, which would simultaneously inhibit all five anti-apoptotic proteins expressed in melanoma (blue), may represent an even better strategy for melanoma therapy (3).
Novel molecular signatures can help guide both melanoma diagnosis and therapy selection (14). The promising results from targeting the intrinsic apoptosis pathway for melanoma therapy urge identification of prognostic markers. While BCL-2 expression levels were not prognostic for melanoma survival (15), we and others showed that sensitivity to BH3-mimetics is determined predominantly by expression of MCL-1 and its antagonist NOXA (6,16). We therefore propose that MCL-1 and NOXA should be measured in future trials. However, a sense of caution is needed. A subpopulation of melanoma cells expresses transmembrane transporters of the ATP-binding cassette (ABC family). This family of proteins is the largest family of transmembrane transporters in mammals, and a member of particular interest in melanoma is the ABCB5 transporter (17). Depending on their substrate specificity, they can rapidly clear the cytoplasm of low molecular weight small molecules. This mechanism of drug resistance is not targeted by these BH3-mimetic molecules and may therefore represent a future challenge for successful therapies.
Figure 2. Drug penetration into a solid tumor. Illustration of the gradients in oxygen/nutrients and drug concentration. Anticancer drugs have limited distribution from blood vessels (BV) in solid tumors, which limits their effectiveness. This can be explained in part by the oxygen/nutrient gradient which (i) might slow down or arrest the cell cycle and (ii) might lead to a hypoxic, acidic microenvironment. In either condition, many drugs are less active. (iii) Distant cells might have limited drug access due to high interstitial fluid pressure compared with the adjacent normal tissue, lack of convection, drug metabolism and binding (18–20).
Another obstacle is represented by the interstitial fluid pressure (18). This phenomenon is due to the fact that tumors are devoid of functional lymphatic vessels. Hence, the interstitial fluid normally absorbed by lymphatic vessels accumulates within the tumor and builds pressure. This in turn represents a hindrance for the penetration of diagnostic and therapeutic antibodies and likely also small molecule drugs into the tumor (Fig. 2) (19). We propose that optimization of current antimelanoma treatments, including the induction of apoptosis by BH3- or SMAC-mimetics, needs to be supported by reduction in the interstitial fluid pressure and hence optimal therapies for melanoma may be further way than currently hoped for.
Acknowledgements N.K.H. is a Cameron Fellow of the Melanoma and Skin Cancer Research Institute, Australia, and a Sydney Medical School Foundation Fellow. N.K.H. also acknowledges contributing grant support from the Cancer Council NSW (RG 09-08, RG 13-06), Cancer Australia/Cure Cancer Australia Foundation (570778), Cancer Institute New South Wales (08/RFG/1-27) and the National Health and Medical Research Council Australia (1003637). We would also like to thank Dr. John D. Allen, Garvan Institute of Medical Research, Sydney, and Dr. Crystal A. Tonnessen, The University of Queensland Diamantina Institute, Brisbane, for many fruitful discussions.
Conflict of Interest The authors have declared no conflict of interest.
References 1 Soengas M S, Lowe S W. Oncogene 2003: 22: 3138–3151. 2 Mohana-Kumaran N, Hill D S, Allen J D et al. Pigment Cell Melanoma Res 2014: 27: 525–539. €tz M, Eberle J. Exp Dermatol 2014: 23: 375– 3 Plo 378. 4 Eberle J, Fecker L F, Hossini A M et al. Exp Dermatol 2008: 17: 1–11. 5 Oltersdorf T, Elmore S W, Shoemaker A R et al. Nature 2005: 435: 677–681. 6 Lucas K M, Mohana-Kumaran N, Lau D et al. Clin Cancer Res 2012: 18: 783–795. 7 Ewings K E, Wiggins C M, Cook S J. Cell Cycle 2007: 6: 2236–2240.
472
8 Boisvert-Adamo K, Aplin A E. Oncogene 2008: 27: 3301–3312. 9 Berger A, Quast S A, Plotz M et al. J Invest Dermatol 2014: 134: 430–440. 10 Paraiso K H, Xiang Y, Rebecca V W et al. Cancer Res 2011: 71: 2750–2760. 11 Wroblewski D, Mijatov B, Mohana-Kumaran N et al. Carcinogenesis 2013: 34: 237–247. 12 Espona-Fiedler M, Soto-Cerrato V, Hosseini A et al. Biochem Pharmacol 2012: 83: 489–496. 13 Smalley K S, Haass N K, Brafford P A et al. Mol Cancer Ther 2006: 5: 1136–1144. 14 Haass N K, Smalley K S. Mol Diagn Ther 2009: 13: 283–296.
15 Espindola M B, Corleta O C. World J Surg Oncol 2008: 6: 65. 16 van Delft M F, Wei A H, Mason K D et al. Cancer Cell 2006: 10: 389–399. 17 Lee N, Barthel S R, Schatton T. Lab Invest 2014: 94: 13–30. 18 Hompland T, Ellingsen C, Galappathi K et al. BMC Cancer 2014: 14: 92. 19 Heine M, Freund B, Nielsen P et al. PLoS ONE 2012: 7: e36258. 20 Minchinton A I, Tannock I F. Nat Rev Cancer 2006: 6: 583–592.
ª 2014 John Wiley & Sons A/S. Published by John Wiley & Sons Ltd Experimental Dermatology, 2014, 23, 471–472
Commentary
www.wileyonlinelibrary.com/journal/EXD
Melanoma never says die Nikolas K. Haass1,2,3 and Udo Schumacher4 1
The University of Queensland, The University of Queensland Diamantina Institute, Translational Research Institute, Brisbane, Qld, Australia; 2The Centenary Institute, Newtown, NSW, Australia; 3Discipline of Dermatology, University of Sydney, Camperdown, NSW, Australia; 4Institute of Anatomy and Experimental Morphology, University Hospital Hamburg-Eppendorf, Hamburg, Germany Correspondence: Nikolas K. Haass, The University of Queensland Diamantina Institute, Translational Research Institute, 37 Kent St, Woolloongabba, Qld 4102, Australia, Tel.: +61-7-3443-7087, Fax: +61-7-3443-6966, e-mail: [email protected] Abstract: Drug resistance in melanoma is commonly attributed to ineffective apoptotic pathways. Targeting apoptosis regulators is thus considered a promising approach to sensitizing melanoma to therapy. In the previous issue of Experimental Dermatology, Pl€ otz and Eberle discuss the role that apoptosis plays in melanoma progression and drug resistance and the utility of apoptosisinducing BH3-mimetics as targeted therapy. There are a number of compounds in clinical development and the field seems close to
translating recent findings into benefits for patients with melanoma. Thus, this viewpoint is timely and achieves a valuable summary of the current state of apoptosis-inducing therapy of melanoma.
Melanoma cells never say die: Firstly, malignant cells, including melanoma cells, are often selected for apoptosis deficiency, and secondly, defective apoptosis pathways are one barrier to effective systemic treatment of melanoma (1). Melanoma is a deadly skin cancer mainly because of its propensity to metastasize early and its resistance to conventional chemotherapeutic agents. Considering the latter, chemotherapeutic agents either initially inhibit cell proliferation and consequently initiate apoptosis, or they primarily induce apoptosis directly. Both mechanisms, however, require the uptake of the agents into the cell. Targeted BRAF inhibition has been a very successful new approach to treat melanoma; however, resistance invariably develops, making the search for augmentation of the therapy necessary. Strengthening the pro-apoptotic signalling cascade may therefore be a rewarding way to induce apoptosis in melanoma cells. BH3-only proteins represent a class of proteins which are pro-apoptotic by neutralizing anti-apoptotic Bcl-2 proteins (Fig. 1a). The balance between pro- and anti-apoptotic proteins thus determines the fate of a cell. The large and yet increasing body of literature focusing on targeting the intrinsic apoptosis pathway as a strategy for melanoma therapy indeed supports the importance of this field of research (2). Pl€ otz and Eberle (3) propose in their excellent viewpoint two major strategies to target this pathway, namely gene therapy with adenoviral vectors which induce expression of pro-apoptotic genes (4) or small molecules which act as BH3-mimetics. From the BH3mimetics, it is hoped that they prevent resistance to or increase susceptibility for kinase inhibitors (3). The development of BH3- or SMAC-mimetics – antagonists of inhibitor of apoptosis proteins (IAPs) – has helped us to understand the mechanisms of apoptotic resistance (2). SMAC-mimetic studies show that both XIAP and the cIAPs must be targeted to effectively induce apoptosis of cancer cells (2). The highly studied BH3-mimetic ABT-737 imitates BAD and therefore inhibits BCL-2, BCL-w and BCL-xL (5). We have shown that single agent treatment with ABT-737 is not sufficient as melanoma therapy but that inhibition of MCL-1, or induction of its antagonist NOXA, strongly sensitize melanoma cells to ABT-737 in vitro (6) (Fig. 1b). In accordance with this, and based on the fact
that BIM can interact with and suppress all anti-apoptotic BCL-2 proteins simultaneously and that BIM is regulated through a number of signalling pathways, Pl€ otz and Eberle (3) propose as a central issue in their viewpoint that a BH3-mimetic specific for BIM may represent an even better strategy for melanoma therapy (Fig. 1c). Importantly, the two most prominent pro-survival pathways in melanoma, the MAPK and PI3K/AKT pathways, do not only drive uncontrolled cell proliferation but also apoptosis resistance: Activated ERK1/2 interferes with binding of BIM to MCL-1 and BCLXL (7), targets BIM for proteasomal degradation (8) and thus inhibits BIM-induced apoptosis. Conversely, MAPK pathway inhibition promotes apoptosis in melanoma by up-regulating BIM and PUMA, activating BMF and down-regulating the anti-apoptotic protein MCL-1 (9). Loss of PTEN contributes to intrinsic BRAF inhibitor resistance via suppression of BIM-mediated apoptosis (10). In our hands, combination of BRAF inhibitors with ABT-737 killed melanoma cells synergistically in vitro, dependent on induction of BIM and down-regulation of MCL-1 (11). The BH3-mimetics obatoclax (BCL-2 and MCL-1, lower affinity to BCL-XL, BCL-w and BFL-1/A1) and prodigiosin (MCL-1) also inhibit mTORC1 and mTORC2 preventing full activation of AKT (12). Further, we have shown that inhibition of PI3K/AKT signalling enhances the cytotoxic effect of MAPK pathway inhibition (13). This provides a rationale for combining BH3-mimetics with MAPK pathway inhibitors. A promising regimen for patients with melanoma harbouring a BRAFV600E mutation would be a BH3-mimetic combined with a selective BRAF inhibitor and/or a MEK inhibitor. Indeed, a currently recruiting trial will study the side effects and best dose of a threefold combination of small molecule inhibitors: dabrafenib (BRAF), trametinib (MEK) and navitoclax (BCL-2, BCL-w and BCL-xL) (ClinicalTrials.gov). Theoretically, a BIMmimetic drug (Fig. 1c), as proposed by Pl€ otz and Eberle, would work independently of the BRAF status and would therefore provide a therapeutic approach to both BRAF-mutant and BRAF-wildtype melanomas. We propose that addition of novel BH3-mimetics to current drug regimens and new combinations are avenues worth exploring for melanoma therapy.
ª 2014 John Wiley & Sons A/S. Published by John Wiley & Sons Ltd Experimental Dermatology, 2014, 23, 471–472
Key words: BH3-mimetic – drug resistance – intrinsic apoptosis pathway – melanoma therapy – SMAC-mimetic
Accepted for publication 27 March 2014
471
Commentary
PUMA
(a)
BCL-2
BMF
BCL-XL
BAD
BIM
BID
BCL-w
HRK
BFL-1/A1
MCL-1
BIK
NOXA
BV
(b) NOXA induction
ABT-737 ABT-263
O2, nutrients BCL-2
BCL-XL
BCL-w
BFL-1/A1
MCL-1
Drugs BAX
BAK
Interstitial fluid pressure
Mitochondrion Cytochrome C
(c)
BAX
BCL-2
BCL-XL
BCL-w
BAK
BFL-1/A1
MCL-1
BIM mimetic
Figure 1. Mimicking BH3-only pro-apoptotic proteins. (a) BH3-only pro-apoptotic proteins (orange) have distinct specificities for anti-apoptotic proteins (blue). (b) ABT-737 and ABT-263 (navitoclax) mimic BAD inhibiting BCL-2, BCL-w and BCL-xL. Induction of NOXA, which inhibits BFL-1/A1 and MCL-1, sensitizes melanoma cells to ABT-737, resulting in inhibition of all five anti-apoptotic proteins expressed in melanoma (blue) and leading to apoptosis (skull) (6). (c) A BIM-mimetic, which would simultaneously inhibit all five anti-apoptotic proteins expressed in melanoma (blue), may represent an even better strategy for melanoma therapy (3).
Novel molecular signatures can help guide both melanoma diagnosis and therapy selection (14). The promising results from targeting the intrinsic apoptosis pathway for melanoma therapy urge identification of prognostic markers. While BCL-2 expression levels were not prognostic for melanoma survival (15), we and others showed that sensitivity to BH3-mimetics is determined predominantly by expression of MCL-1 and its antagonist NOXA (6,16). We therefore propose that MCL-1 and NOXA should be measured in future trials. However, a sense of caution is needed. A subpopulation of melanoma cells expresses transmembrane transporters of the ATP-binding cassette (ABC family). This family of proteins is the largest family of transmembrane transporters in mammals, and a member of particular interest in melanoma is the ABCB5 transporter (17). Depending on their substrate specificity, they can rapidly clear the cytoplasm of low molecular weight small molecules. This mechanism of drug resistance is not targeted by these BH3-mimetic molecules and may therefore represent a future challenge for successful therapies.
Figure 2. Drug penetration into a solid tumor. Illustration of the gradients in oxygen/nutrients and drug concentration. Anticancer drugs have limited distribution from blood vessels (BV) in solid tumors, which limits their effectiveness. This can be explained in part by the oxygen/nutrient gradient which (i) might slow down or arrest the cell cycle and (ii) might lead to a hypoxic, acidic microenvironment. In either condition, many drugs are less active. (iii) Distant cells might have limited drug access due to high interstitial fluid pressure compared with the adjacent normal tissue, lack of convection, drug metabolism and binding (18–20).
Another obstacle is represented by the interstitial fluid pressure (18). This phenomenon is due to the fact that tumors are devoid of functional lymphatic vessels. Hence, the interstitial fluid normally absorbed by lymphatic vessels accumulates within the tumor and builds pressure. This in turn represents a hindrance for the penetration of diagnostic and therapeutic antibodies and likely also small molecule drugs into the tumor (Fig. 2) (19). We propose that optimization of current antimelanoma treatments, including the induction of apoptosis by BH3- or SMAC-mimetics, needs to be supported by reduction in the interstitial fluid pressure and hence optimal therapies for melanoma may be further way than currently hoped for.
Acknowledgements N.K.H. is a Cameron Fellow of the Melanoma and Skin Cancer Research Institute, Australia, and a Sydney Medical School Foundation Fellow. N.K.H. also acknowledges contributing grant support from the Cancer Council NSW (RG 09-08, RG 13-06), Cancer Australia/Cure Cancer Australia Foundation (570778), Cancer Institute New South Wales (08/RFG/1-27) and the National Health and Medical Research Council Australia (1003637). We would also like to thank Dr. John D. Allen, Garvan Institute of Medical Research, Sydney, and Dr. Crystal A. Tonnessen, The University of Queensland Diamantina Institute, Brisbane, for many fruitful discussions.
Conflict of Interest The authors have declared no conflict of interest.
References 1 Soengas M S, Lowe S W. Oncogene 2003: 22: 3138–3151. 2 Mohana-Kumaran N, Hill D S, Allen J D et al. Pigment Cell Melanoma Res 2014: 27: 525–539. €tz M, Eberle J. Exp Dermatol 2014: 23: 375– 3 Plo 378. 4 Eberle J, Fecker L F, Hossini A M et al. Exp Dermatol 2008: 17: 1–11. 5 Oltersdorf T, Elmore S W, Shoemaker A R et al. Nature 2005: 435: 677–681. 6 Lucas K M, Mohana-Kumaran N, Lau D et al. Clin Cancer Res 2012: 18: 783–795. 7 Ewings K E, Wiggins C M, Cook S J. Cell Cycle 2007: 6: 2236–2240.
472
8 Boisvert-Adamo K, Aplin A E. Oncogene 2008: 27: 3301–3312. 9 Berger A, Quast S A, Plotz M et al. J Invest Dermatol 2014: 134: 430–440. 10 Paraiso K H, Xiang Y, Rebecca V W et al. Cancer Res 2011: 71: 2750–2760. 11 Wroblewski D, Mijatov B, Mohana-Kumaran N et al. Carcinogenesis 2013: 34: 237–247. 12 Espona-Fiedler M, Soto-Cerrato V, Hosseini A et al. Biochem Pharmacol 2012: 83: 489–496. 13 Smalley K S, Haass N K, Brafford P A et al. Mol Cancer Ther 2006: 5: 1136–1144. 14 Haass N K, Smalley K S. Mol Diagn Ther 2009: 13: 283–296.
15 Espindola M B, Corleta O C. World J Surg Oncol 2008: 6: 65. 16 van Delft M F, Wei A H, Mason K D et al. Cancer Cell 2006: 10: 389–399. 17 Lee N, Barthel S R, Schatton T. Lab Invest 2014: 94: 13–30. 18 Hompland T, Ellingsen C, Galappathi K et al. BMC Cancer 2014: 14: 92. 19 Heine M, Freund B, Nielsen P et al. PLoS ONE 2012: 7: e36258. 20 Minchinton A I, Tannock I F. Nat Rev Cancer 2006: 6: 583–592.
ª 2014 John Wiley & Sons A/S. Published by John Wiley & Sons Ltd Experimental Dermatology, 2014, 23, 471–472
