XL-888

XL-888 is a novel and orally-bioavailable inhibitor of heat shock protein 90 (HSP90) that selectively inhibits HSP90α and HSP90β with values of 50% inhibition concentration IC₅₀ of 22 nM and 44 nM respectively. It also exerts considerably weaker inhibition against a range of other diverse kinases with IC₅₀ more than 3600 nM for all. X-ray crystallographic analysis reveals that the XL-888 binds to HSP90 through the formation of H-bonding between the N-(R)-sec-butylanthranilamide moiety of XL-888 and ASP93 of HSP90. In recent studies, XL-888 has exhibits strong anti-proliferative activities in a panel of tumor cells with values of IC₅₀ ranging from 0.1 nM to 45.5 nM.

Reference

Paraiso KH, Haarberg HE, Wood E, Rebecca VW, Chen YA, Xiang Y, Ribas A, Lo RS, Weber JS, Sondak VK, John JK, Sarnaik AA, Koomen JM, Smalley KS. The HSP90 inhibitor XL888 overcomes BRAF inhibitor resistance mediated through diverse mechanisms. Clin Cancer Res. 2012; 18(9):2502-2514

Bussenius J, Blazey CM, Aay N, Anand NK, Arcalas A, Baik T, Bowles OJ, Buhr CA, Costanzo S, Curtis JK, DeFina SC, Dubenko L, Heuer TS, Huang P, Jaeger C, Joshi A, Kennedy AR, Kim AI, Lara K, Lee J, Li J, Lougheed JC, Ma S, Malek S, Manalo JC, Martini JF, McGrath G, Nicoll M, Nuss JM, Pack M, Peto CJ, Tsang TH, Wang L, Womble SW, Yakes M, Zhang W, Rice KD. Discovery of XL888: a novel tropane-derived small molecule inhibitor of HSP90. Bioorg Med Chem Lett. 2012; 22(17): 5396-5404.

Evaluating melanoma drug response and therapeutic escape with quantitative proteomics.[Pubmed:24760959]

Mol Cell Proteomics. 2014 Jul;13(7):1844-54.

The evolution of cancer therapy into complex regimens with multiple drugs requires novel approaches for the development and evaluation of companion biomarkers. Liquid chromatography-multiple reaction monitoring mass spectrometry (LC-MRM) is a versatile platform for biomarker measurement. In this study, we describe the development and use of the LC-MRM platform to study the adaptive signaling responses of melanoma cells to inhibitors of HSP90 (XL888) and MEK (AZD6244). XL888 had good anti-tumor activity against NRAS mutant melanoma cell lines as well as BRAF mutant cells with acquired resistance to BRAF inhibitors both in vitro and in vivo. LC-MRM analysis showed HSP90 inhibition to be associated with decreased expression of multiple receptor tyrosine kinases, modules in the PI3K/AKT/mammalian target of rapamycin pathway, and the MAPK/CDK4 signaling axis in NRAS mutant melanoma cell lines and the inhibition of PI3K/AKT signaling in BRAF mutant melanoma xenografts with acquired vemurafenib resistance. The LC-MRM approach targeting more than 80 cancer signaling proteins was highly sensitive and could be applied to fine needle aspirates from xenografts and clinical melanoma specimens (using 50 mug of total protein). We further showed MEK inhibition to be associated with signaling through the NFkappaB and WNT signaling pathways, as well as increased receptor tyrosine kinase expression and activation. Validation studies identified PDGF receptor beta signaling as a potential escape mechanism from MEK inhibition, which could be overcome through combined use of AZD6244 and the PDGF receptor inhibitor, crenolanib. Together, our studies show LC-MRM to have unique value as a platform for the systems level understanding of the molecular mechanisms of drug response and therapeutic escape. This work provides the proof-of-principle for the future development of LC-MRM assays for monitoring drug responses in the clinic.

Will Hsp90 inhibitors prove effective in BRAF-mutant melanomas?[Pubmed:22442059]

Clin Cancer Res. 2012 May 1;18(9):2420-2.

The RAF inhibitor vemurafenib has unprecedented activity in BRAF-mutant melanomas, but resistance invariably develops. As Hsp90 is required for the stability of several of the oncoproteins that mediate RAF inhibitor resistance, inhibitors of this cellular chaperone may be effective in patients with intrinsic or acquired resistance to RAF inhibitors.

Discovery of XL888: a novel tropane-derived small molecule inhibitor of HSP90.[Pubmed:22877636]

Bioorg Med Chem Lett. 2012 Sep 1;22(17):5396-404.

With structural guidance, tropane-derived HTS hits were modified to optimize for HSP90 inhibition and a desirable in vivo profile. Through an iterative SAR development process 12i (XL888) was discovered and shown to reduce HSP90 client protein content in PD studies. Furthermore, efficacy experiments performed in a NCI-N87 mouse xenograft model demonstrated tumor regression in some dosing regimens.

Inhibition of Wee1, AKT, and CDK4 underlies the efficacy of the HSP90 inhibitor XL888 in an in vivo model of NRAS-mutant melanoma.[Pubmed:23538902]

Mol Cancer Ther. 2013 Jun;12(6):901-12.

The HSP90 inhibitor XL888 is effective at reversing BRAF inhibitor resistance in melanoma, including that mediated through acquired NRAS mutations. The present study has investigated the mechanism of action of XL888 in NRAS-mutant melanoma. Treatment of NRAS-mutant melanoma cell lines with XL888 led to an inhibition of growth, G2-M phase cell-cycle arrest, and the inhibition of cell survival in three-dimensional spheroid and colony formation assays. In vitro, HSP90 inhibition led to the degradation of ARAF, CRAF, Wee1, Chk1, and cdc2 and was associated with decreased mitogen-activated protein kinase (MAPK), AKT, mTOR, and c-jun NH2 kinase (JNK) signaling. Apoptosis induction was associated with increased BIM expression and a decrease in the expression of the prosurvival protein Mcl-1. The critical role of increased BIM and decreased Mcl-1 expression in the survival of NRAS-mutant melanoma cell lines was shown through siRNA knockdown and overexpression studies. In an animal xenograft model of NRAS-mutant melanoma, XL888 treatment led to reduced tumor growth and apoptosis induction. Important differences in the pattern of client degradation were noted between the in vivo and in vitro studies. In vivo, XL888 treatment led to degradation of CDK4 and Wee1 and the inhibition of AKT/S6 signaling with little or no effect observed upon ARAF, CRAF, or MAPK. Blockade of Wee1, using either siRNA knockdown or the inhibitor MK1775, was associated with significant levels of growth inhibition and apoptosis induction. Together, these studies have identified Wee1 as a key target of XL888, suggesting novel therapeutic strategies for NRAS-mutant melanoma.