- Email: [email protected]
The Broad Stroke of Hsp90 Inhibitors: Painting over the RAF Inhibitor Paradox
COMMENTARY
Erikson, 2014), affect response to PLK inhibitors. Other issues need further evaluation before these findings can be translated from bench to bedside. For example, while PLK1 inhibitors seem to have a manageable safety profile (Gjertsen and Schoffski, 2015), the tolerability of MEK/ PLK1i combinations remains to be determined. In addition, identification of predictive biomarkers of responses to MEK/PLK1i will be valuable. Certainly, assessing the efficacy of this combination in the context of a functional immune system would be absolutely necessary. Answering these questions would provide valuable information to advance this promising combination and to provide rational and effective treatment options for patients with NRAS-mutant melanoma. CONFLICT OF INTEREST
The authors state no conflict of interest.
ACKNOWLEDGMENTS Work in our laboratory is supported by NIH/NCI grants K01 CA175269, PO1 CA114046, P30 CA010815, the American Cancer Society, the V Foundation for Cancer Research, and the Melanoma Research Alliance.
REFERENCES Ascierto PA, Schadendorf D, Berking C et al. (2013) MEK162 for patients with advanced melanoma harbouring NRAS or Val600 BRAF mutations: a non-randomised, openlabel phase 2 study. Lancet Oncol 14: 249–56 Burd CE, Liu W, Huynh MV et al. (2014) Mutationspecific RAS oncogenicity explains NRAS codon 61 selection in melanoma. Cancer Discov 4:1418–29 Gjertsen BT, Schoffski P (2015) Discovery and development of the Polo-like kinase inhibitor volasertib in cancer therapy. Leukemia 29: 11–9 Kneisel L, Strebhardt K, Bernd A et al. (2002) Expression of polo-like kinase (PLK1) in thin melanomas: a novel marker of metastatic disease. J Cutan Pathol 29:354–8 Kwong LN, Costello JC, Liu H et al. (2012) Oncogenic NRAS signaling differentially regulates survival and proliferation in melanoma. Nat Med 18:1503–10 Luo J, Emanuele MJ, Li D et al. (2009) A genomewide RNAi screen identifies multiple synthetic lethal interactions with the Ras oncogene. Cell 137:835–48 Posch C, Cholewa BD, Vujic I et al. (2015) Combined Inhibition of MEK and Plk1 has Synergistic Anti-Tumor Activity in NRAS Mutant Melanoma. J Invest Dermatol 135: 2475–83
Posch C, Moslehi H, Feeney L et al. (2013) Combined targeting of MEK and PI3K/mTOR effector pathways is necessary to effectively inhibit NRAS mutant melanoma in vitro and in vivo. Proc Natl Acad Sci USA 110: 4015–20 Prior IA, Lewis PD, Mattos C (2012) A comprehensive survey of Ras mutations in cancer. Cancer Res 72:2457–67 Schmit TL, Zhong W, Setaluri V et al. (2009) Targeted depletion of Polo-like kinase (Plk) 1 through lentiviral shRNA or a small-molecule inhibitor causes mitotic catastrophe and induction of apoptosis in human melanoma cells. J Invest Dermatol 129:2843–53
Sosman JA, Kittaneh M, Lolkema MP (2014) A phase 1b/2 study of LEE011 in combination with binimetinib (MEK162) in patients with NRAS-mutant melanoma: Early encouraging clinical activity. J Clin Oncol 32:5s Strebhardt K (2010) Multifaceted polo-like kinases: drug targets and antitargets for cancer therapy. Nat Rev Drug Discov 9:643–60 Thomas NE, Alexander A et al. (2015) Association between NRAS and BRAF mutational status and melanoma-specific survival among patients with higher-risk primary melanoma. JAMA Oncol 1:359–68 Yim H, Erikson RL (2014) Plk1-targeted therapies in TP53- or RAS-mutated cancer. Mutat Res Rev Mutat Res 761:36–9
See related letter to the editor on pg 2542
The Broad Stroke of Hsp90 Inhibitors: Painting over the RAF Inhibitor Paradox Michael J. Vido1,2 and Andrew E. Aplin1,2 The novel Hsp90 inhibitor XL888 is undergoing clinical investigation for use in conjunction with the rapidly accelerated fibrosarcoma (RAF) kinase inhibitor vemurafenib to treat unresectable melanoma. The addition of XL888 to current regimens may serve an additional purpose by blocking the RAF inhibitor paradox. Such activity could reduce adverse events in patients and provide a biomarker for the successful inhibition of Hsp90 target proteins. Journal of Investigative Dermatology (2015) 135, 2355–2357. doi:10.1038/jid.2015.239
In this issue of Journal of Investigative Dermatology, Phadke et al. (2015) report results from a phase I dose-escalation study of XL888, a novel Hsp90 inhibitor, in combination with the rapidly accelerated fibrosarcoma (RAF) kinase inhibitor vemurafenib in patients with unresectable BRAF-mutant melanoma (NCT01657591). Fifty percent of patients with melanomas harbor an activating valine to glutamic acid substitution (V600E) in the serine/threonine kinase BRAF that signals through the extracellular signal-regulated kinase 1/2 (ERK1/2) pathway. Despite initial efficacy, BRAF inhibitor monotherapy and newer BRAF/
mitogen-activated protein kinase kinase (MEK) inhibitor combination therapy inevitably yield to therapy resistance. Mechanisms driving resistance include the acquisition of NRAS mutations, expression of BRAF splice variants, amplification of BRAF V600E, and the upregulation of receptor tyrosine kinases leading to ERK and AKT pathway activation (Hartsough et al., 2014). The use of Hsp90 inhibitors in conjunction with vemurafenib is being pursued to provide a broad stroke of inhibition against these resistant pathways. Here the authors report a second potential clinical benefit, namely, the reduction of paradoxical
1
Department of Cancer Biology, Thomas Jefferson University, Philadelphia, Pennsylvania, USA and 2Sidney Kimmel Cancer Center; Thomas Jefferson University, Philadelphia, Pennsylvania, USA Correspondence: Michael Vido, Department of Cancer Biology and Sidney Kimmel Cancer Center, Thomas Jefferson University, Philadelphia, Pennsylvania 19107, USA. E-mail: [email protected]
www.jidonline.org 2355
COMMENTARY
Clinical Implications ●
The RAF inhibitors paradoxically cause hyperplastic lesions in melanoma patients.
●
Treatment with Hsp90 inhibitors may block this event and reduce its frequency.
●
Reduction of paradoxical signaling may serve as a biomarker for successful Hsp90 inhibition.
ERK1/2 activation that occurs as a result of treatment with first-generation RAF inhibitors. In addition to limited durable treatment responses, vemurafenib and other firstgeneration RAF inhibitors display paradoxically activating ERK1/2 signaling in BRAF wild-type cells. As reviewed by Gibney et al. (2013) this activation occurs via enhanced dimerization of RAF monomers, either through the relief of BRAF autoinhibition or through conformational changes that stabilize protein–protein interaction when a single RAF monomer is bound to an inhibitor. Ultimately, dimerization with and activation of CRAF enables phosphorylation of downstream MEK and ERK1/2 targets (Poulikakos et al., 2010). In patients, this paradoxical activation causes the generation of a number of hyperproliferative cutaneous lesions, including squamous cell carcinomas, keratocanthomas, papillary lesions, and verrucous keratosis, as well as nevi and new primary melanomas. These lesions have been treated by excision and no patients with metastases have been encountered; however, the potential for malignancies at other lessaccessible locations remains a concern. The observation that rare cases of colonic adenomas, gastric polyps, and a single case of myelomonocytic leukemia have been encountered during RAF inhibition highlights the potential for paradoxical ERK1/2 activation in other tissues (Gibney et al., 2013). The Hsp90 chaperone protein binds to target proteins to support refolding and thereby can promote cell survival during times of stress. Many proteins integral to melanoma progression and therapy resistance represent Hsp90 “clients”, including mutant BRAF (but not wildtype BRAF), CRAF, COT, PDGFR, IGF1R, and AKT; it is through these interactions that Hsp90 exhibits pro-tumorigenic
activity (Grbovic et al., 2006). Firstgeneration Hsp90 inhibitors derived from the natural geldanamycin product studied within the last decade have demonstrated promising preclinical results but have ultimately failed to garner the Food and Drug Administration (FDA) approval. In human melanoma cell lines, the Hsp90 inhibitor 17-allylamino17-demethoxygeldanamycin (17-AAG) induced ubiquitination and proteasomemediated degradation of BRAF V600E and caused BRAF V600E degradation and growth inhibition in SK-MEL-28 xenografts (Grbovic et al., 2006). 17AAG progressed to phase I and phase II clinical trials and was implemented at the highest tolerated dose (450 mg m − 2 intravenous once weekly). Significant increases in Hsp70 protein levels at a posttreatment biopsy (median 44 hours) compared with pretreatment biopsy indicated successful Hsp90 inhibition; however, BRAF and CRAF levels remained unchanged, as did phospho-ERK1/2, a measurement of ERK1/2 pathway activity (Solit et al., 2008). These findings contrasted with the phase I results that showed reductions in client proteins at 24 hours (Banerji et al., 2005). Taken together with the measured increase in Hsp70 in the phase II studies, this finding suggests that the biologic effect of 17AAG was short lived and that more potent Hsp90 inhibitors capable of chronic dosing could improve treatment efficacy. The FDA approval of targeted therapies with diverse resistance mechanisms has rekindled interest in Hsp90 inhibitors. 17-AAG required bioreduction to a hydroquinone 17-AAGH2 to elicit its full effects, and the expression of P-glycoprotein or loss of NAD(P)H dehydrogenase, quinone 1 (NQO1), could prevent this metabolic event. XL888 is a novel non-benzoquinone,
2356 Journal of Investigative Dermatology (2015), Volume 135
ATP-competitive inhibitor of Hsp90 (Catalanotti and Solit, 2012). Inhibition of Hsp90 with XL888 leads to proteolytic degradation of these proteins in multiple melanoma models. Preclinical data have shown that such inhibitors can effectively block ERK1/2 signaling in RAF inhibitor–resistant cell lines (Paraiso et al., 2012) and in combination with vemurafenib can delay the emergence of resistance in xenograft models (Smyth et al., 2014). Whether XL888 will be hampered by the same limitations as 17-AAG remains to be seen. This first report of phase I results provides some optimism regarding the ability of XL888 to successfully destabilize Hsp90 client proteins. Following 24 weeks of vemurafenib and XL888 treatment (35, 45, 90, and 135 mg twice per week by mouth for four cohorts, respectively) the authors report an overall reduction in the number of hyperproliferative/neoplastic skin lesions. Notably, the cohort treated with the highest concentration of XL888 experienced two verruca vulgaris lesions among six patients and no squamous cell carcinomas, keratocanthomas, or new primary melanomas. In mutant NRAS/ wild-type BRAF mutant cell lines, XL888 is capable of reversing the paradoxical activation of ERK1/2 induced by vemurafenib at concentrations higher than 100 nM. Mechanistically, Phadke et al. show that treatment with 300 nM XL888 reduces expression of CRAF, a critical mediator of the paradox effect (Poulikakos et al., 2010; Gibney et al., 2013). Inhibition of the paradox effect is undoubtedly important in the context of RAF inhibition. In addition to requiring an increased number of follow-up procedures to remove cutaneous lesions, the activation of ERK1/2 signaling can also lead to other non-cutaneous lesions that are difficult to identify (Gibney et al., 2013). FDA-approved RAF and MEK inhibitor combination therapies now provide this benefit, and it remains to be seen whether XL888 therapy achieves this aim in a statistically significant fashion. Perhaps more critically, these findings point to the possibility that the reduction in CRAF levels could represent a biomarker for successful Hsp90 therapy in conjunction with RAF inhibition. Despite the common use of
COMMENTARY
increased Hsp70 expression as a surrogate for successful Hsp90 inhibition, patients receiving Hsp90 inhibition often demonstrate varied expression levels compared with Hsp70 (Catalanotti and Solit, 2012), and, as observed with 17-AAG, client protein, destabilization and treatment effects may not correlate with Hsp70 induction (Solit et al., 2008). As investigation continues with XL888, validation of such a biomarker may provide a more robust measure of clinically relevant Hsp90 inhibition and favorable patient response. Ultimately, additional data are needed to understand whether XL888 is eliciting the desired effect on Hsp90 client proteins. Preclinical data point to the possibility that XL888 can inhibit the diverse modes of resistance encountered with RAF inhibition and that combination therapy with vemurafenib can delay the time to relapse. Further testing of XL888 efficacy will come in the form of pre/posttreatment biopsies that directly measure the effects of Hsp90 inhibition on client protein
expression and ERK1/2 pathway activation. This phase I trial was insufficiently powered to demonstrate changes in hyperproliferative lesions that are statistically useful. However, the promising results presented in this article suggest that there is indeed an inhibitory effect. Quantifying these lesions will remain a focus in an upcoming phase II clinical trial testing XL888 in conjunction with combined RAF and MEK inhibitors. Only with this added clinical data will XL888 be spared the fate of 17-AAG and other first-generation Hsp90 inhibitors. CONFLICT OF INTEREST
The authors state no conflict of interest.
REFERENCES Banerji U, O’Donnell A, Scurr M et al. (2005) Phase I pharmacokinetic and pharmacodynamic study of 17-allylamino, 17-demethoxygeldanamycin in patients with advanced malignancies. Clin Oncol 23:4152–61 Catalanotti F, Solit DB (2012) Will Hsp90 inhibitors prove effective in BRAF-mutant melanomas? Clin Cancer Res 18:2420–2 Gibney GT, Messina JL, Fedorenko IV et al. (2013) Paradoxical oncogenesis—the long-term
effects of BRAF inhibition in melanoma. Nat Rev Clin Oncol 10:390–9 Grbovic OM, Basso AD, Sawai A et al. (2006) V600E B-Raf requires the Hsp90 chaperone for stability and is degraded in response to Hsp90 inhibitors. Proc Natl Acad Sci USA 103:57–62 Hartsough E, Shao Y, Aplin AE (2014) Resistance to RAF inhibitors revisited. J Invest Dermatol 134: 319–25 Paraiso KHT, Haarberg HE, Wood E et al. (2012) The HSP90 inhibitor XL888 overcomes BRAF inhibitor resistance mediated through diverse mechanisms. Clin Cancer Res 18:2502–14 Phadke M, Gibney GT, Rich CJ et al. (2015) XL888 limits vemurafenib-induced proliferative skin events by suppressing paradoxical MAPK activation. J Invest Dermatol 135: 2542–4 Poulikakos PI, Zhang C, Bollag G et al. (2010) RAF inhibitors transactivate RAF dimers and ERK signalling in cells with wild-type BRAF. Nature 464:427–30 Solit DB, Osman I, Polsky D et al. (2008) Phase II trial of 17-allylamino-17-demethoxygeldanamycin in patients with metastatic melanoma. Clin Cancer Res 14:8302–7 Smyth T, Paraiso KHT, Hearn K et al. (2014) Inhibition of HSP90 by AT13387 delays the emergence of resistance to BRAF inhibitors and overcomes resistance to dual BRAF and MEK inhibition in melanoma models. Mol Cancer Ther 13:2793–804
www.jidonline.org 2357
Erikson, 2014), affect response to PLK inhibitors. Other issues need further evaluation before these findings can be translated from bench to bedside. For example, while PLK1 inhibitors seem to have a manageable safety profile (Gjertsen and Schoffski, 2015), the tolerability of MEK/ PLK1i combinations remains to be determined. In addition, identification of predictive biomarkers of responses to MEK/PLK1i will be valuable. Certainly, assessing the efficacy of this combination in the context of a functional immune system would be absolutely necessary. Answering these questions would provide valuable information to advance this promising combination and to provide rational and effective treatment options for patients with NRAS-mutant melanoma. CONFLICT OF INTEREST
The authors state no conflict of interest.
ACKNOWLEDGMENTS Work in our laboratory is supported by NIH/NCI grants K01 CA175269, PO1 CA114046, P30 CA010815, the American Cancer Society, the V Foundation for Cancer Research, and the Melanoma Research Alliance.
REFERENCES Ascierto PA, Schadendorf D, Berking C et al. (2013) MEK162 for patients with advanced melanoma harbouring NRAS or Val600 BRAF mutations: a non-randomised, openlabel phase 2 study. Lancet Oncol 14: 249–56 Burd CE, Liu W, Huynh MV et al. (2014) Mutationspecific RAS oncogenicity explains NRAS codon 61 selection in melanoma. Cancer Discov 4:1418–29 Gjertsen BT, Schoffski P (2015) Discovery and development of the Polo-like kinase inhibitor volasertib in cancer therapy. Leukemia 29: 11–9 Kneisel L, Strebhardt K, Bernd A et al. (2002) Expression of polo-like kinase (PLK1) in thin melanomas: a novel marker of metastatic disease. J Cutan Pathol 29:354–8 Kwong LN, Costello JC, Liu H et al. (2012) Oncogenic NRAS signaling differentially regulates survival and proliferation in melanoma. Nat Med 18:1503–10 Luo J, Emanuele MJ, Li D et al. (2009) A genomewide RNAi screen identifies multiple synthetic lethal interactions with the Ras oncogene. Cell 137:835–48 Posch C, Cholewa BD, Vujic I et al. (2015) Combined Inhibition of MEK and Plk1 has Synergistic Anti-Tumor Activity in NRAS Mutant Melanoma. J Invest Dermatol 135: 2475–83
Posch C, Moslehi H, Feeney L et al. (2013) Combined targeting of MEK and PI3K/mTOR effector pathways is necessary to effectively inhibit NRAS mutant melanoma in vitro and in vivo. Proc Natl Acad Sci USA 110: 4015–20 Prior IA, Lewis PD, Mattos C (2012) A comprehensive survey of Ras mutations in cancer. Cancer Res 72:2457–67 Schmit TL, Zhong W, Setaluri V et al. (2009) Targeted depletion of Polo-like kinase (Plk) 1 through lentiviral shRNA or a small-molecule inhibitor causes mitotic catastrophe and induction of apoptosis in human melanoma cells. J Invest Dermatol 129:2843–53
Sosman JA, Kittaneh M, Lolkema MP (2014) A phase 1b/2 study of LEE011 in combination with binimetinib (MEK162) in patients with NRAS-mutant melanoma: Early encouraging clinical activity. J Clin Oncol 32:5s Strebhardt K (2010) Multifaceted polo-like kinases: drug targets and antitargets for cancer therapy. Nat Rev Drug Discov 9:643–60 Thomas NE, Alexander A et al. (2015) Association between NRAS and BRAF mutational status and melanoma-specific survival among patients with higher-risk primary melanoma. JAMA Oncol 1:359–68 Yim H, Erikson RL (2014) Plk1-targeted therapies in TP53- or RAS-mutated cancer. Mutat Res Rev Mutat Res 761:36–9
See related letter to the editor on pg 2542
The Broad Stroke of Hsp90 Inhibitors: Painting over the RAF Inhibitor Paradox Michael J. Vido1,2 and Andrew E. Aplin1,2 The novel Hsp90 inhibitor XL888 is undergoing clinical investigation for use in conjunction with the rapidly accelerated fibrosarcoma (RAF) kinase inhibitor vemurafenib to treat unresectable melanoma. The addition of XL888 to current regimens may serve an additional purpose by blocking the RAF inhibitor paradox. Such activity could reduce adverse events in patients and provide a biomarker for the successful inhibition of Hsp90 target proteins. Journal of Investigative Dermatology (2015) 135, 2355–2357. doi:10.1038/jid.2015.239
In this issue of Journal of Investigative Dermatology, Phadke et al. (2015) report results from a phase I dose-escalation study of XL888, a novel Hsp90 inhibitor, in combination with the rapidly accelerated fibrosarcoma (RAF) kinase inhibitor vemurafenib in patients with unresectable BRAF-mutant melanoma (NCT01657591). Fifty percent of patients with melanomas harbor an activating valine to glutamic acid substitution (V600E) in the serine/threonine kinase BRAF that signals through the extracellular signal-regulated kinase 1/2 (ERK1/2) pathway. Despite initial efficacy, BRAF inhibitor monotherapy and newer BRAF/
mitogen-activated protein kinase kinase (MEK) inhibitor combination therapy inevitably yield to therapy resistance. Mechanisms driving resistance include the acquisition of NRAS mutations, expression of BRAF splice variants, amplification of BRAF V600E, and the upregulation of receptor tyrosine kinases leading to ERK and AKT pathway activation (Hartsough et al., 2014). The use of Hsp90 inhibitors in conjunction with vemurafenib is being pursued to provide a broad stroke of inhibition against these resistant pathways. Here the authors report a second potential clinical benefit, namely, the reduction of paradoxical
1
Department of Cancer Biology, Thomas Jefferson University, Philadelphia, Pennsylvania, USA and 2Sidney Kimmel Cancer Center; Thomas Jefferson University, Philadelphia, Pennsylvania, USA Correspondence: Michael Vido, Department of Cancer Biology and Sidney Kimmel Cancer Center, Thomas Jefferson University, Philadelphia, Pennsylvania 19107, USA. E-mail: [email protected]
www.jidonline.org 2355
COMMENTARY
Clinical Implications ●
The RAF inhibitors paradoxically cause hyperplastic lesions in melanoma patients.
●
Treatment with Hsp90 inhibitors may block this event and reduce its frequency.
●
Reduction of paradoxical signaling may serve as a biomarker for successful Hsp90 inhibition.
ERK1/2 activation that occurs as a result of treatment with first-generation RAF inhibitors. In addition to limited durable treatment responses, vemurafenib and other firstgeneration RAF inhibitors display paradoxically activating ERK1/2 signaling in BRAF wild-type cells. As reviewed by Gibney et al. (2013) this activation occurs via enhanced dimerization of RAF monomers, either through the relief of BRAF autoinhibition or through conformational changes that stabilize protein–protein interaction when a single RAF monomer is bound to an inhibitor. Ultimately, dimerization with and activation of CRAF enables phosphorylation of downstream MEK and ERK1/2 targets (Poulikakos et al., 2010). In patients, this paradoxical activation causes the generation of a number of hyperproliferative cutaneous lesions, including squamous cell carcinomas, keratocanthomas, papillary lesions, and verrucous keratosis, as well as nevi and new primary melanomas. These lesions have been treated by excision and no patients with metastases have been encountered; however, the potential for malignancies at other lessaccessible locations remains a concern. The observation that rare cases of colonic adenomas, gastric polyps, and a single case of myelomonocytic leukemia have been encountered during RAF inhibition highlights the potential for paradoxical ERK1/2 activation in other tissues (Gibney et al., 2013). The Hsp90 chaperone protein binds to target proteins to support refolding and thereby can promote cell survival during times of stress. Many proteins integral to melanoma progression and therapy resistance represent Hsp90 “clients”, including mutant BRAF (but not wildtype BRAF), CRAF, COT, PDGFR, IGF1R, and AKT; it is through these interactions that Hsp90 exhibits pro-tumorigenic
activity (Grbovic et al., 2006). Firstgeneration Hsp90 inhibitors derived from the natural geldanamycin product studied within the last decade have demonstrated promising preclinical results but have ultimately failed to garner the Food and Drug Administration (FDA) approval. In human melanoma cell lines, the Hsp90 inhibitor 17-allylamino17-demethoxygeldanamycin (17-AAG) induced ubiquitination and proteasomemediated degradation of BRAF V600E and caused BRAF V600E degradation and growth inhibition in SK-MEL-28 xenografts (Grbovic et al., 2006). 17AAG progressed to phase I and phase II clinical trials and was implemented at the highest tolerated dose (450 mg m − 2 intravenous once weekly). Significant increases in Hsp70 protein levels at a posttreatment biopsy (median 44 hours) compared with pretreatment biopsy indicated successful Hsp90 inhibition; however, BRAF and CRAF levels remained unchanged, as did phospho-ERK1/2, a measurement of ERK1/2 pathway activity (Solit et al., 2008). These findings contrasted with the phase I results that showed reductions in client proteins at 24 hours (Banerji et al., 2005). Taken together with the measured increase in Hsp70 in the phase II studies, this finding suggests that the biologic effect of 17AAG was short lived and that more potent Hsp90 inhibitors capable of chronic dosing could improve treatment efficacy. The FDA approval of targeted therapies with diverse resistance mechanisms has rekindled interest in Hsp90 inhibitors. 17-AAG required bioreduction to a hydroquinone 17-AAGH2 to elicit its full effects, and the expression of P-glycoprotein or loss of NAD(P)H dehydrogenase, quinone 1 (NQO1), could prevent this metabolic event. XL888 is a novel non-benzoquinone,
2356 Journal of Investigative Dermatology (2015), Volume 135
ATP-competitive inhibitor of Hsp90 (Catalanotti and Solit, 2012). Inhibition of Hsp90 with XL888 leads to proteolytic degradation of these proteins in multiple melanoma models. Preclinical data have shown that such inhibitors can effectively block ERK1/2 signaling in RAF inhibitor–resistant cell lines (Paraiso et al., 2012) and in combination with vemurafenib can delay the emergence of resistance in xenograft models (Smyth et al., 2014). Whether XL888 will be hampered by the same limitations as 17-AAG remains to be seen. This first report of phase I results provides some optimism regarding the ability of XL888 to successfully destabilize Hsp90 client proteins. Following 24 weeks of vemurafenib and XL888 treatment (35, 45, 90, and 135 mg twice per week by mouth for four cohorts, respectively) the authors report an overall reduction in the number of hyperproliferative/neoplastic skin lesions. Notably, the cohort treated with the highest concentration of XL888 experienced two verruca vulgaris lesions among six patients and no squamous cell carcinomas, keratocanthomas, or new primary melanomas. In mutant NRAS/ wild-type BRAF mutant cell lines, XL888 is capable of reversing the paradoxical activation of ERK1/2 induced by vemurafenib at concentrations higher than 100 nM. Mechanistically, Phadke et al. show that treatment with 300 nM XL888 reduces expression of CRAF, a critical mediator of the paradox effect (Poulikakos et al., 2010; Gibney et al., 2013). Inhibition of the paradox effect is undoubtedly important in the context of RAF inhibition. In addition to requiring an increased number of follow-up procedures to remove cutaneous lesions, the activation of ERK1/2 signaling can also lead to other non-cutaneous lesions that are difficult to identify (Gibney et al., 2013). FDA-approved RAF and MEK inhibitor combination therapies now provide this benefit, and it remains to be seen whether XL888 therapy achieves this aim in a statistically significant fashion. Perhaps more critically, these findings point to the possibility that the reduction in CRAF levels could represent a biomarker for successful Hsp90 therapy in conjunction with RAF inhibition. Despite the common use of
COMMENTARY
increased Hsp70 expression as a surrogate for successful Hsp90 inhibition, patients receiving Hsp90 inhibition often demonstrate varied expression levels compared with Hsp70 (Catalanotti and Solit, 2012), and, as observed with 17-AAG, client protein, destabilization and treatment effects may not correlate with Hsp70 induction (Solit et al., 2008). As investigation continues with XL888, validation of such a biomarker may provide a more robust measure of clinically relevant Hsp90 inhibition and favorable patient response. Ultimately, additional data are needed to understand whether XL888 is eliciting the desired effect on Hsp90 client proteins. Preclinical data point to the possibility that XL888 can inhibit the diverse modes of resistance encountered with RAF inhibition and that combination therapy with vemurafenib can delay the time to relapse. Further testing of XL888 efficacy will come in the form of pre/posttreatment biopsies that directly measure the effects of Hsp90 inhibition on client protein
expression and ERK1/2 pathway activation. This phase I trial was insufficiently powered to demonstrate changes in hyperproliferative lesions that are statistically useful. However, the promising results presented in this article suggest that there is indeed an inhibitory effect. Quantifying these lesions will remain a focus in an upcoming phase II clinical trial testing XL888 in conjunction with combined RAF and MEK inhibitors. Only with this added clinical data will XL888 be spared the fate of 17-AAG and other first-generation Hsp90 inhibitors. CONFLICT OF INTEREST
The authors state no conflict of interest.
REFERENCES Banerji U, O’Donnell A, Scurr M et al. (2005) Phase I pharmacokinetic and pharmacodynamic study of 17-allylamino, 17-demethoxygeldanamycin in patients with advanced malignancies. Clin Oncol 23:4152–61 Catalanotti F, Solit DB (2012) Will Hsp90 inhibitors prove effective in BRAF-mutant melanomas? Clin Cancer Res 18:2420–2 Gibney GT, Messina JL, Fedorenko IV et al. (2013) Paradoxical oncogenesis—the long-term
effects of BRAF inhibition in melanoma. Nat Rev Clin Oncol 10:390–9 Grbovic OM, Basso AD, Sawai A et al. (2006) V600E B-Raf requires the Hsp90 chaperone for stability and is degraded in response to Hsp90 inhibitors. Proc Natl Acad Sci USA 103:57–62 Hartsough E, Shao Y, Aplin AE (2014) Resistance to RAF inhibitors revisited. J Invest Dermatol 134: 319–25 Paraiso KHT, Haarberg HE, Wood E et al. (2012) The HSP90 inhibitor XL888 overcomes BRAF inhibitor resistance mediated through diverse mechanisms. Clin Cancer Res 18:2502–14 Phadke M, Gibney GT, Rich CJ et al. (2015) XL888 limits vemurafenib-induced proliferative skin events by suppressing paradoxical MAPK activation. J Invest Dermatol 135: 2542–4 Poulikakos PI, Zhang C, Bollag G et al. (2010) RAF inhibitors transactivate RAF dimers and ERK signalling in cells with wild-type BRAF. Nature 464:427–30 Solit DB, Osman I, Polsky D et al. (2008) Phase II trial of 17-allylamino-17-demethoxygeldanamycin in patients with metastatic melanoma. Clin Cancer Res 14:8302–7 Smyth T, Paraiso KHT, Hearn K et al. (2014) Inhibition of HSP90 by AT13387 delays the emergence of resistance to BRAF inhibitors and overcomes resistance to dual BRAF and MEK inhibition in melanoma models. Mol Cancer Ther 13:2793–804
www.jidonline.org 2357