Background: Sixty-percent of melanomas express mutant BRAF oncoproteins, which result in constitutive activation of the Rasmitogen activated protein kinase (MAPK) pathway and thus abnormal growth and ultimately a malignant phenotype. It has also been demonstrated that many melanomas are dependent on the MAPK pathway for survival and proliferation. MEK is a seronine/threonine kinase that is part of the BRAF downstream signaling cascade and is responsible for the phosphorylation of ERK1/2. Interestingly, ERK1/2 appear to be the sole target of the MEK protein kinase. Without ERK1/2 nuclear translocation, the inappropriate nuclear signaling responsible for the malignant phenotype is inhibited. The development of BRAF inhibitors has provided an exciting new treatment option for BRAF mutated melanoma, but unfortunately many patients have relapsed due to downstream alternative pathway activation. Investigational drug TAK-733 (Millennium Pharmaceuticals, Inc) is a potent, novel, selective, non-ATP competitive, allosteric inhibitor of MEK 1/2. With molecular inhibition downstream of BRAF, CRAF, COT, or NRAS mutations, tumor growth would theoretically be slowed or suppressed altogether. We explored the preclinical efficacy of TAK-733 in both in vitro and in vivo models of melanoma. Based on these translational results, TAK-733 demonstrates promising pre-clinical data supportive of further clinical development.

Methods & Results: 36 melanoma cell lines were exposed to TAK-733 and the IC50 was determined using the SRB method. The cell lines were then classified as sensitive (S) or resistant (R) based on an IC50<0.01uM or IC50>0.1uM, respectively. BRAF status was obtained on all cell lines and did not correlate with responsiveness to TAK-733. Immunoblotting for effector proteins was performed on both S and R cell lines after 72 hrs of treatment with TAK-733. Phospho-ERK was suppressed in both S and R cell lines, confirming TAK-733-mediated inhibition of MEK 1/2. The evidence of MEK inhibition in the relatively resistant cell lines suggests other escape pathways are also contributing to melanoma survival and proliferation. There was no evidence of up-regulation of known MAPK-associated alternative pathways involving PI3 kinase or phospho-S6. Ten patient-derived melanoma tumors (5 V600E BRAF mutant, 2 wild-type BRAF, 3 unknown) were implanted into nude mice and treated daily via oral gavage with TAK-733 10mg/kg or 25mg/kg daily. The tumors were measured three times weekly and tumor growth inhibition (TGI) was analyzed. Nine explants, regardless of BRAF status, showed statistically significant TGI, ranging 87–97%, when compared to controls. One of the ten explants (BRAF V600E mutant) was resistant to TAK-733 with a TGI of 23%. Additionally, in one sensitive explant (BRAF wild type), treatment with TAK-733 was stopped, re-growth to 1533 mm3 was allowed, and subsequent re-treatment with TAK-733 induced tumor shrinkage to <50mm3. The compound was well-tolerated and no significant weight loss or lethality was observed.

Conclusion: These data demonstrate that TAK-733 exhibited robust tumor growth inhibition and regression in human melanoma cell lines and human melanoma explant models in mice. Based on these results, TAK-733 demonstrates promising preclinical data supportive of further clinical development in melanoma.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the AACR-NCI-EORTC International Conference: Molecular Targets and Cancer Therapeutics; 2011 Nov 12-16; San Francisco, CA. Philadelphia (PA): AACR; Mol Cancer Ther 2011;10(11 Suppl):Abstract nr C182.