Drugs in Clinical Pipeline: Refametinib

Refametinib [(R)-N-(3,4-difluoro-2-(2-fluoro-4-iodophenylamino)-6-methoxyphenyl)-1-(2,3-dihydroxypropyl)cyclopropane-1-sulfonamide] is an orally available, potent, non-ATP-competitive and highly selective inhibitor of MEK1/2 (IC₅₀ MEK1, MEK2 = 19 nM, 47 nM). Refametinib-MEK complex structure reveals that Refametinib binds to an allosteric site adjacent to the Mg-ATP binding region and interacts extensively with ATP, the activation loop, and other surrounding protein residues through hydrogen bonding and hydrophobic interactions. Such a binding mode thus suggests a noncompetitive mechanism of inhibition of Refametinib against MEK1, which allows ATP binding but precludes binding to the substrate ERK, thus preventing ERK phosphorylation [1].

Refametinib significantly inhibited only MEK1/2 relative to 205 other kinases in a multikinase screen. MEK1 and MEK2 were inhibited 97% and 99%, respectively, when tested at 10 µM. Four other kinases (BRAF, BRAF V599E, COT, and RAF1) showed inhibition of greater than 90%; however, these used MEK1 in a cascade assay format and the inhibition in these assays is due to MEK1 inhibition rather than direct inhibition of the other kinases by Refametinib [1]. It was selected for clinical development because of its potency and favourable pharmacokinetic profile.

Refametinib was discovered at Ardea Biosciences. It has been licensed to Bayer since 2009 onwards. Refametinib is currently in a Phase 2 study in patients with hepatocellular carcinoma in combination with Sorafenib/Regorafenib and a Phase 1/Phase 2 study in patients with advanced pancreatic cancer in combination with Gemcitabine.

The activity of Refametinib is as follows:

IC₅₀ (MEK1 enzyme assay) = 19 nM

IC₅₀ (MEK2 enzyme assay) = 47 nM

Common Name: AMG 925

Synonyms:  BAY-86-9766; RDEA119; RDEA 119; BAY 86-9766

IUPAC Name: (R)-N-(3,4-difluoro-2-(2-fluoro-4-iodophenylamino)-6-methoxyphenyl)-1-(2,3-dihydroxypropyl)cyclopropane-1-sulfonamide

CAS Number: 923032-37-5

Mechanism of Action: Kinase Inhibitor; Dual-Kinase Inhibitor; MEK1 Inhibitor; MEK2 Inhibitor

Indication: Various Cancers

Development Stage: Phase II

Company: Ardea Biosciences/Bayer

Studies of primary tumor samples and immortalized cancer cell lines have shown constitutive activation of the RAS-RAF-MEK-ERK pathway in several human tumors, including lung, colon, melanoma, thyroid, and pancreatic cancer. The mitogen-activated protein kinase pathway-also known as the RAS/RAF/MEK/extracellular signal-regulated kinase (ERK) pathway [MAP kinase (MAPK) pathway]-is a ubiquitous intracellular cascade that transduces signals from cell surface receptors to regulate numerous cytoplasmic and nuclear proteins involved in cellular proliferation, survival, differentiation, migration, and angiogenesis.  Overexpression of RAS, down-regulation of the natural inhibitors of the MAPK pathway, and overexpression of MEK and ERK are the mechanisms of MAPK pathway activation in cancers such as HCC. Moreover, ERK overexpression has been correlated with various disease progressions, thus making these as important targets for drug discovery. Within this pathway lies MEK for which there are two highly homologous genes expressed in humans (MEK1 and MEK2). MEK is downstream of BRAF in the RAS-RAF-MEK-ERK pathway and is critical for transducing signals to ERK. BRAF activating gene mutations are prevalent in melanoma and thyroid cancers.

In vivo, AMG 925 exhibits potent activity in xenograft models of melanoma, colon, and epidermal carcinoma. AMG 925 exhibits complete suppression of ERK phosphorylation at fully efficacious doses in mice. AMG 925 shows a tissue selectivity that reduces its potential for central nervous system-related side effects. Using pharmacokinetic and pharmacodynamic data, we show that maintaining adequate MEK inhibition throughout the dosing interval is likely more important than achieving high peak levels because greater efficacy was achieved with more frequent but lower dosing. Based on its longer half-life in humans than in mice, AMG 925 has the potential for use as a once- or twice-daily oral treatment for cancer [1].

Refametinib potently inhibited MEK activity as measured by phosphorylation of ERK1/2 across several human cancer cell lines of different tissue origins and BRAF mutational status with EC₅₀ values ranging from 2.5 to 15.8 nM. To assess the potential for brain penetration and activity, we compared the ability of Refametinib to inhibit pERK levels in the brain, lung, and tumor tissues of tumor-bearing mice and found at least that 76-fold lower plasma levels of Refametinib were required to inhibit 50% of the pERK level in tumor versus brain [1].

Refametinib exhibited potent antiproliferative activity in hepatocellular carcinoma (HCC) cell lines with half-maximal inhibitory concentration values ranging from 33 to 762 nM. Refametinib was strongly synergistic with sorafenib in suppressing tumor cell proliferation and inhibiting phosphorylation of the extracellular signal-regulated kinase (ERK). Refametinib prolonged survival in Hep3B xenografts, murine Hepa129 allografts, and MH3924A rat allografts. Additionally, tumor growth, ascites formation, and serum alpha-fetoprotein levels were reduced. Synergistic effects in combination with Sorafenib were shown in Huh-7, Hep3B xenografts, and MH3924A allografts. These results support the ongoing clinical development of Refametinib and Sorafenib in advanced HCC [2].

References:

1. Iverson, C.; et. al. RDEA119/BAY 869766: a potent, selective, allosteric inhibitor of MEK1/2 for the treatment of cancer. Cancer Res 2009, 69(17), 6839-6847.
2. Schmieder, R.; et. al. Allosteric MEK1/2 Inhibitor Refametinib (BAY 86-9766) in Combination with Sorafenib Exhibits Antitumor Activity in Preclinical Murine and Rat Models of Hepatocellular Carcinoma. Neoplasia 2013, 15(10), 1161-1171.

3. ClinicalTrials.gov Refametinib(BAY86-9766) in RAS Mutant Hepatocellular Carcinoma (HCC). NCT01915589 (retrieved 01-08-2015)

4. ClinicalTrials.gov BAY86-9766 Plus Gemcitabine Phase I Study in Asian. NCT01764828 (retrieved 01-08-2015)

5. ClinicalTrials.gov Refametinib (BAY86-9766) in Combination With Regorafenib (Stivarga, BAY73-4506) in Patients With Advanced or Metastatic Cancer. NCT02168777 (retrieved 01-08-2015)