Purpose: Despite their preclinical promise, previous MEK inhibitors have shown little benefit for patients. This likely reflects the narrow therapeutic window for MEK inhibitors due to the essential role of the P42/44 MAPK pathway in many nontumor tissues. GSK1120212 is a potent and selective allosteric inhibitor of the MEK1 and MEK2 (MEK1/2) enzymes with promising antitumor activity in a phase I clinical trial (ASCO 2010). Our studies characterize GSK1120212's enzymatic, cellular, and in vivo activities, describing its unusually long circulating half-life.Experimental Design: Enzymatic studies were conducted to determine GSK1120212 inhibition of recombinant MEK, following or preceding RAF kinase activation. Cellular studies examined GSK1120212 inhibition of ERK1 and 2 phosphorylation (p-ERK1/2) as well as MEK1/2 phosphorylation and activation. Further studies explored the sensitivity of cancer cell lines, and drug pharmacokinetics and efficacy in multiple tumor xenograft models.Results: In enzymatic and cellular studies, GSK1120212 inhibits MEK1/2 kinase activity and prevents Raf-dependent MEK phosphorylation (S217 for MEK1), producing prolonged p-ERK1/2 inhibition. Potent cell growth inhibition was evident in most tumor lines with mutant BRAF or Ras. In xenografted tumor models, GSK1120212 orally dosed once daily had a long circulating half-life and sustained suppression of p-ERK1/2 for more than 24 hours; GSK1120212 also reduced tumor Ki67, increased p27Kip1/CDKN1B , and caused tumor growth inhibition in multiple tumor models. The largest antitumor effect was among tumors harboring mutant BRAF or Ras. Conclusions: GSK1120212 combines high potency, selectivity, and long circulating half-life, offering promise for successfully targeting the narrow therapeutic window anticipated for clinical MEK inhibitors.

Background: Fibroblast growth factor receptors (FGFRs) play a role in cell proliferation and survival. Genetic alterations of FGFRs can lead to deregulated activation in various cancers, including squamous cell carcinoma (SCC) of the lung and urothelial bladder cancer. Here, we report on a phase I study of BGJ398 a potent, selective pan-FGFR inhibitor. Methods: Eligible patients (pts; ≥ 18 years of age) had tumors with any FGFR genetic alteration identified by central or local prescreening. Pts received BGJ398 once or twice daily (qd or bid) in 28-day cycles in escalating cohorts. Dose-limiting toxicities (DLTs) were predefined and included both severe events and those resulting in significant dosing delays. Upon determination of maximum tolerated dose (MTD), 3 expansion arms were treated: arm 1 included FGFR1-amplified SCC of the lung (continuous qd dosing); arms 2 and 3 included FGFR-altered solid tumors given qd continuous doses (arm 2) or qd doses on a 3-weeks-on/1-week-off schedule (arm 3). Results: As of September 24, 2013, 94 pts were enrolled (median age 57.5 years). Pts were treated in qd dose cohorts (5-150 mg; n = 90) or a bid dose cohort (50 mg; n = 4). Arms 1 and 3 are ongoing. Of the 82 pts who discontinued, the most common reasons were disease progression (n = 56), consent withdrawal (n = 12), and adverse events (AEs; n = 10). DLTs during dose escalation were grade 3 aminotransferase increases (1 each at 100 and 125 mg), hyperphosphatemia (125 mg), and grade 1 corneal toxicity (150 mg). The 125-mg qd dose was identified as the MTD. Common treatment-emergent AEs (any grade, suspected to be related) at 125 mg qd (n = 41) include hyperphosphatemia (78%), stomatitis (37%), alopecia (32%), decreased appetite (32%), and fatigue (22%) and were generally mild, with stomatitis (7%) the only grade 3/4 AE occurring ≥ 5%. Elevated serum phosphate levels, a pharmacodynamic marker of FGFR pathway inhibition, could be managed through diet, phosphate lowering therapy, and drug interruptions. Preliminary analysis of efficacy data indicates tumor regressions in pts with various FGFR genetic alterations, including 4 of 5 pts with urothelial cell carcinomas (4 of which originated in the bladder) with FGFR3-activating mutations (with tumor reductions ranging from 27% to 48%). Additionally, 1 pt with FGFR1-amplified SCC lung cancer achieved confirmed partial response (PR) while another pt achieved a PR confirmed after 27 days. Tumor reductions were also observed in cholangiocarcinoma with an FGFR2 gene fusion, and FGFR1-amplified breast cancer. Conclusions: BGJ398 had a tolerable safety profile and demonstrated single-agent activity in pts with FGFR-genetically altered solid tumors. The 3-weeks-on/1-week-off schedule has been chosen for future studies based on its improved safety profile. Clinical activity was observed in multiple tumor types, and pts with FGFR3-mutated bladder cancer may be especially sensitive to BGJ398. Citation Format: Lecia V. Sequist, Phillippe Cassier, Andrea Varga, Josep Tabernero, Jan HM Schellens, Jean-Pierre Delord, Patricia LoRusso, D. Ross Camidge, Manuel Hidalgo Medina, Martin Schuler, Mario Campone, G. Gary Tian, Steven Wong, Jesus Corral, Randi Isaacs, Suman K. Sen, Diana Graus Porta, Swarupa G. Kulkarni, Caroline Lefebvre, Jürgen Wolf. Phase I study of BGJ398, a selective pan-FGFR inhibitor in genetically preselected advanced solid tumors. [abstract]. In: Proceedings of the 105th Annual Meeting of the American Association for Cancer Research; 2014 Apr 5-9; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2014;74(19 Suppl):Abstract nr CT326. doi:10.1158/1538-7445.AM2014-CT326

Selective RAF inhibitors have significant activity in patients with metastatic melanoma whose tumors express BRAFV600E. However, not all patients respond equally well to treatment and the duration of response is often limited to less than 6 months. LGX818 was developed with the hypothesis that a more potent inhibitor with excellent pharmacological properties would maximize the degree and duration of patient response. LGX818 is a highly potent RAF inhibitor with selective anti-proliferative and apoptotic activity in cells expressing BRAFV600E. In the A375 (BRAFV600E) human melanoma cell line LGX818 suppresses phospho-ERK (EC50 = 3 nM) leading to potent inhibition of proliferation (EC50 = 4 nM). No significant activity was observed against a panel of 100 kinases (IC50 > 900 nM) and LGX818 did not inhibit proliferation of > 400 cell lines expressing wild-type BRAF. Contributing to the high potency of LGX818 is the extremely slow off-rate from BRAFV600E which is not observed with other RAF inhibitors. In biochemical assays the dissociation half-life was >24 hours which translated into sustained target inhibition in cells following drug wash-out. Single dose PK/PD studies in human melanoma xenograft models (BRAFV600E) indicated that LGX818 treatment at oral doses as low as 6 mg/kg resulted in strong (75%) and sustained (>24 hours) decrease in phospho-MEK, even following clearance of drug from circulation. Decreases in phospho-ERK were consistent with phospho-MEK but markers of downstream transcriptional output (DUSP6 and SPRY4) appeared to provide a more sensitive measure of pathway activation. LGX818 induced tumor regression in multiple BRAF mutant human tumor xenograft models grown in immune compromised mice and rats at doses as low as 1 mg/kg. Consistent with the in vitro data, LGX818 was inactive against BRAF wild-type tumors at doses up to 300 mg/kg bid, with good tolerability and linear increase in exposure. Efficacy was also achieved in a more disease-relevant spontaneous metastatic melanoma and a model of melanoma brain metastasis. LGX818 is a potent and selective RAF kinase inhibitor with unique biochemical properties that contribute to an excellent pharmacological profile. A Phase I clinical trial in patients with BRAF mutant tumors is ongoing. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 103rd Annual Meeting of the American Association for Cancer Research; 2012 Mar 31-Apr 4; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2012;72(8 Suppl):Abstract nr 3790. doi:1538-7445.AM2012-3790

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