Abstract 4758: Disruption of CRAF-mediated MEK activation is required for effective MEK inhibition in KRAS mutant tumors

MEK inhibitors are clinically active in BRAF V600E melanomas, but only marginally so in KRAS-mutant tumors. While MEK inhibitor treatment resulted in sustained inhibition of ERK phopshorylation in BRAF V600E tumors, it was associated with a rebound in ERK phosphorylation after prolonged exposure in KRAS mutant tumors. To understand this phenomenon, we performed an RNAi screen in a KRAS-mutant model and found that CRAF knockdown enhanced the antiproliferative effects of MEK inhibition both in vivo and in vitro. In agreement with previous findings, MEK inhibitor treatment in KRAS mutant cells resulted in CRAF reactivation and induction of MEK phosphorylation. However, MEK activated by CRAF was less susceptible to traditional MEK inhibitors, such as PD0325901, compared to when activated by BRAF V600E. Consistent with this observation, the rebound in ERK phosphorylation following MEK inhibitor treatment in KRAS mutant tumors was dependent on intact CRAF expression. Furthermore, traditional MEK inhibitors induced RAF-MEK complexes in KRAS mutant models, and disrupting these complexes enhanced the inhibition of CRAF-dependent ERK signaling. In an effort to identify more effective compounds we found that newer MEK inhibitors, such as trametinib and CH5126766, do not only target the catalytic activity of MEK, but in addition, they also impair its reactivation by CRAF. This occurs through two distinct mechanisms: trametinib disrupts RAF-MEK complex formation, as evidenced by co-immunoprecipitation of endogenous proteins and surface plasmon resonance assays with purified enzymes. Instead, CH512676 induces the interaction of MEK with RAF, yet it prevents the phosphorylation of MEK in this complex. To elucidate the mechanism responsible for the latter property, we determined the ternary structure of CH512676-bound MEK1. CH5126766 interacts with Asn 221 and Ser 222 residues in MEK1 to displace the activation segment and disrupt RAF-mediated phosphorylation. These data provide a blueprint for developing better MEK inhibitors that effectively inhibit ERK signaling in KRAS mutant tumors.

Citation Format: Piro Lito, Anna Saborowski, Jingyin Yue, Martha Solomon, Eric Joseph, Christof Fellman, Kazuhiro Ohara, Kenji Morikami, Takaaki Miura, Christine Lucas, Nobuya Ishii, Scott Lowe, Neal Rosen. Disruption of CRAF-mediated MEK activation is required for effective MEK inhibition in KRAS mutant 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 4758. doi:10.1158/1538-7445.AM2014-4758

Analytical validation of a quantitative reverse transcriptase polymerase chain reaction assay for evaluation of T-cell targeted immunosuppressive therapy in the dog

Cyclosporine is an immunosuppressive agent that inhibits T-cell function by decreasing production of cytokines such as interleukin-2 (IL-2) and interferon-γ(IFN-γ). In dogs, there is currently no reliable analytical method for determining effective cyclosporine dosages in individual patients. Our laboratory has developed a quantitative reverse transcriptase polymerase chain reaction (RT-qPCR) assay that measures IL-2 and IFN-γ gene expression, with the goal of quantifying immunosuppression in dogs treated with cyclosporine. This study focuses on analytical validation of our assay, and on the effects of sample storage conditions on cyclosporine-exposed samples. Heparinized whole blood collected from healthy adult dogs was exposed to a typical post-treatment blood concentration for cyclosporine(500 ng/mL) for 1 h, and then stored for 0, 24, and 48 h at both room temperature and 4 ◦C.The study was then repeated using a cyclosporine concentration of 75 ng/mL, with sample storage for 0, 24, and 48 h at 4 ◦C. Cytokine gene expression was measured using RT-qPCR,and assay efficiency and inter- and intra-assay variability were determined. Storage for upto 24 h at room temperature, and up to 48 h at 4 ◦C, did not significantly alter results compared to samples that were processed immediately. Validation studies showed our assay to be highly efficient and reproducible and robust enough to be feasible under standard practice submission conditions.

Pharmacokinetic evaluation of oral dantrolene in the dog

The pharmacokinetics of dantrolene and its active metabolite, 5-hydroxydantrolene, after a single oral dose of either 5 or 10 mg/kg of dantrolene was determined. The effects of exposure to dantrolene and 5-hydroxydantrolene on activated whole-blood gene expression of the cytokines interleukin-2 (IL-2) and interferon-γ (IFN-γ) were also investigated. When dantrolene was administered at a 5 mg/kg dose, peak plasma concentration (Cmax ) was 0.43 μg/mL, terminal half-life (t1/2 ) was 1.26 h, and area under the time-concentration curve (AUC) was 3.87 μg·h/mL. For the 10 mg/kg dose, Cmax was 0.65 μg/mL, t1/2 was 1.21 h, and AUC was 5.94 μg·h/mL. For all calculated parameters, however, there were large standard deviations and wide ranges noted between and within individual dogs: t1/2 , for example, ranged from 0.43 to 6.93 h, Cmax ratios ranged from 1.05 to 3.39, and relative bioavailability (rF) values ranged from 0.02 to 1.56. While activated whole-blood expression of IL-2 and IFN-γ as measured by qRT-PCR was markedly suppressed following exposure to very high concentrations (30 and 50 μg/mL, respectively) of both dantrolene and 5-hydroxydantrolene, biologically and therapeutically relevant suppression of cytokine expression did not occur at the much lower drug concentrations achieved with oral dantrolene dosing.

An integrated approach to dissecting oncogene addiction implicates a Myb-coordinated self-renewal program as essential for leukemia maintenance

Although human cancers have complex genotypes and are genomically unstable, they often remain dependent on the continued presence of single-driver mutations-a phenomenon dubbed "oncogene addiction." Such dependencies have been demonstrated in mouse models, where conditional expression systems have revealed that oncogenes able to initiate cancer are often required for tumor maintenance and progression, thus validating the pathways they control as therapeutic targets. Here, we implement an integrative approach that combines genetically defined mouse models, transcriptional profiling, and a novel inducible RNAi platform to characterize cellular programs that underlie addiction to MLL-AF9-a fusion oncoprotein involved in aggressive forms of acute myeloid leukemia (AML). We show that MLL-AF9 contributes to leukemia maintenance by enforcing a Myb-coordinated program of aberrant self-renewal involving genes linked to leukemia stem cell potential and poor prognosis in human AML. Accordingly, partial and transient Myb suppression precisely phenocopies MLL-AF9 withdrawal and eradicates aggressive AML in vivo without preventing normal myelopoiesis, indicating that strategies to inhibit Myb-dependent aberrant self-renewal programs hold promise as effective and cancer-specific therapeutics. Together, our results identify Myb as a critical mediator of oncogene addiction in AML, delineate relevant Myb target genes that are amenable to pharmacologic inhibition, and establish a general approach for dissecting oncogene addiction in vivo.