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Despite the clinical success of RAF inhibitors in BRAF-mutated melanomas, attempts to target RAF kinases in the context of RAS-driven or otherwise RAF wild-type tumours have not only been ineffective, but RAF inhibitors appear to aggravate tumorigenesis in these settings. Subsequent preclinical investigation has revealed several regulatory mechanisms, feedback pathways and unexpected enzymatic quirks in the MAPK pathway, which may explain this paradox. In this review, we cover the various proposed molecular mechanisms for the RAF paradox, the clinical consequences and strategies to overcome it.

Following the discovery of NVP-BEZ235, our first dual pan-PI3K/mTOR clinical compound, we sought to identify additional phosphoinositide 3-kinase (PI3K) inhibitors from different chemical classes with a different selectivity profile. The key to achieve these objectives was to couple a structure-based design approach with intensive pharmacologic evaluation of selected compounds during the medicinal chemistry optimization process. Here, we report on the biologic characterization of the 2-morpholino pyrimidine derivative pan-PI3K inhibitor NVP-BKM120. This compound inhibits all four class I PI3K isoforms in biochemical assays with at least 50-fold selectivity against other protein kinases. The compound is also active against the most common somatic PI3Kα mutations but does not significantly inhibit the related class III (Vps34) and class IV (mTOR, DNA-PK) PI3K kinases. Consistent with its mechanism of action, NVP-BKM120 decreases the cellular levels of p-Akt in mechanistic models and relevant tumor cell lines, as well as downstream effectors in a concentration-dependent and pathway-specific manner. Tested in a panel of 353 cell lines, NVP-BKM120 exhibited preferential inhibition of tumor cells bearing PIK3CA mutations, in contrast to either KRAS or PTEN mutant models. NVP-BKM120 shows dose-dependent in vivo pharmacodynamic activity as measured by significant inhibition of p-Akt and tumor growth inhibition in mechanistic xenograft models. NVP-BKM120 behaves synergistically when combined with either targeted agents such as MEK or HER2 inhibitors or with cytotoxic agents such as docetaxel or temozolomide. The pharmacological, biologic, and preclinical safety profile of NVP-BKM120 supports its clinical development and the compound is undergoing phase II clinical trials in patients with cancer. Mol Cancer Ther; 11(2); 317–28. ©2011 AACR.

Tumor growth, tumor angiogenesis, and vascular endothelial growth factor (VEGF)–specific angiogenesis are all enhanced in β3-integrin–null mice. Furthermore, endothelial cells isolated from β3-null mice show elevated levels of Flk1 (VEGF receptor 2) expression, suggesting that β3-integrin can control the amplitude of VEGF responses by controlling Flk1 levels or activity. We now show that Flk1 signaling is required for the enhanced tumor growth and angiogenesis seen in β3-null mice. Moreover, β3-null endothelial cells exhibit enhanced migration and proliferation in response to VEGF in vitro, and this phenotype requires Flk1 signaling. Upon VEGF stimulation, β3-null endothelial cells exhibit higher levels of phosphorylated Flk1 and extracellular-related kinases 1 and 2 than wild-type endothelial cells. Furthermore, signaling via ERK1/2 is required to mediate the elevated responses to VEGF observed in β3-null endothelial cells and aortic rings in vitro. These data confirm that VEGF signaling via Flk1 is enhanced in β3-integrin–deficient mice and suggests that this increase may mediate the enhanced angiogenesis and tumor growth observed in these mice in vivo.

The poor prognosis associated with Non Small Cell Lung Cancers (NSCLC) harboring KRAS mutations (∼20-30% frequency) has highlighted the need for RAS targeted therapeutics in oncology for decades. However, attempts to target RAS directly using small molecule inhibitors have historically proven unsuccessful. Additionally, pre-clinical data repeatedly demonstrates that KRAS mutated cancer cells show variable responses to inhibition of downstream effectors, RAF, MEK and PI3K, suggesting that proliferation and survival may be driven through additional pathways and that the molecular mechanisms through which KRAS drives oncogenesis have yet to be fully elucidated. Moreover, parallel pathways activated in human lung cancer that might cooperate with RAS to maintain a transformed state, likewise remain poorly defined. By screening a panel of KRAS mutant NSCLCs for functional dependence on KRAS by shRNA knockdown, we have identified cell lines that require KRAS for proliferation and suppression of apoptosis. In an effort to identify the signaling pathways that promote cellular proliferation and survival in KRAS-mutant lung cancers we have conducted an unbiased cellular screen to identify shRNAs that mitigate dependence on the KRAS oncoprotein. Using this method we have identified molecular pathways that appear to cooperate with the KRAS oncogene to promote tumorogenesis and contribute to the distinct proliferative and apoptotic phenotypes of KRAS dependent NSCLC cell lines. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 101st Annual Meeting of the American Association for Cancer Research; 2010 Apr 17-21; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2010;70(8 Suppl):Abstract nr 5752.

The PI3K/Akt/mTor signaling pathway plays an important role in controlling cell growth, proliferation and survival. Through various mechanisms, the pathway is frequently dysregulated in human cancers, suggesting the use of PI3K inhibitors as novel targeted anticancer therapeutic agents. To this end, substantial drug discovery efforts have been devoted both in pharmaceutical companies and in academia to identify and develop therapeutic agents able to specifically down regulate PI3K or other components of this pathway in tumors cells. Following the discovery of NVP-BEZ235, our first dual pan-PI3K/mTOR clinical compound, we sought to identify additional PI3K inhibitors from different chemical classes with more stringent selectivity profiles. The key to achieve these objectives was to pursue a structure-based design approach coupled with intensive pharmacological evaluation of selected compounds during the medicinal chemistry optimization process. Here we report on the biological characterization of the pan-PI3K pyrimidine-derived inhibitor NVP-BKM120. This compound inhibits all four Class I PI3K isoforms (IC50 values in the 35 to 248 nM range) with at least 50-fold selectivity (compared to p110α) towards protein kinases. The compound is also active against the most common somatic PI3Kα mutations (H1047R, E542K and E545K). NVP-BKM120 does not significantly inhibit the related Class III (Vps34) and Class IV (mTOR, DNA-PK) PI3K kinases. Consistent with its mechanism of action, NVP-BKM120 decreases the cellular levels of p-Akt in mechanistic and relevant tumor cell lines (e.g., IC50 for S473P-Akt in Rat1-p110α cells of 93 nM). This biological activity correlates with inhibition of various Akt downstream signaling pathway components, and with its anti-proliferative activity. Thus, the compound demonstrates significant, concentration dependent cell growth inhibition and induction of apoptosis in a variety of tumor cancer cells, particularly for those harboring p110α mutants and/or over-expressing erbB2. In addition, NVP-BKM120 demonstrates significant, dose dependent in vivo pharmacodynamic activity as measured by inhibition of p-Akt in relevant xenograft models. The pharmacological, biological and preclinical safety profile of NVP-BKM120 supports its clinical development and the compound is currently undergoing Phase 1/II clinical trials in cancer patients. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 101st Annual Meeting of the American Association for Cancer Research; 2010 Apr 17-21; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2010;70(8 Suppl):Abstract nr 4498.

A substantial number of epidemiological and experimental studies support an important role for PI3K in the biology of human cancer. The activation of PI3K, and its downstream effectors, has been clearly validated as an essential step for the initiation and maintenance of the tumorigenic phenotype. Parallel to the clinical development of our dual pan-PI3K/mTOR modulators (e.g., NVP-BEZ235), we continued our drug discovery activities to identify PI3K inhibitors with distinct biological and pharmacological profiles. Following a structure-based design strategy, we have identified a new clinical candidate, NVP-BKM120. This 2,6-dimorpholino pyrimidine derivative is a potent pan-PI3K (e.g., IC50= 35 nM, p110α) that does not significantly inhibit other protein or lipid kinases (e.g., IC50= 4.6 μM, mTOR). The compound exhibits potent antiproliferative activity against a broad panel of tumour cell lines by specifically blocking the biological function of PI3K signaling components (e.g. IC50 = 93 nM S473P-Akt in Rat1-p110α cells). The activity of this pan-PI3K inhibitor in cellular settings translates well in in vivo models of human cancer. NVP-BKM120 shows good oral bioavailability in preclinical species and demonstrated significant antitumor activity (growth inhibition or regression) at tolerated doses in xenografts of diverse cancer lineage in mice and rat models. Consistent with its mechanism of action, transient increases in plasma insulin and glucose levels are observed in these studies. Analyses of tumor tissues after acute dosing or at the end of efficacy studies showed a good correlation between compound exposure, PI3K pathway blockade (reduction in P-Akt levels) and antitumor activity. Dose dependent tumor growth delay or regression is also observed in primary tumor models, and, as demonstrated previously with our dual pan-PI3K/mTOR inhibitor NVP-BEZ235, in vivo synergistic activity is observed when NVP-BKM120 is combined with a MEK inhibitor in K-Ras human cancer models. NVP-BKM120 is currently undergoing Phase I/II human clinical trials for the treatment of solid tumors and hematological malignancies. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 101st Annual Meeting of the American Association for Cancer Research; 2010 Apr 17-21; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2010;70(8 Suppl):Abstract nr 4497.

ATP competitive inhibitors of the BRAF(V600E) oncogene paradoxically activate downstream signaling in cells bearing wild-type BRAF (BRAF(WT)). In this study, we investigate the biochemical mechanism of wild-type RAF (RAF(WT)) activation by multiple catalytic inhibitors using kinetic analysis of purified BRAF(V600E) and RAF(WT) enzymes. We show that activation of RAF(WT) is ATP dependent and directly linked to RAF kinase activity. These data support a mechanism involving inhibitory autophosphorylation of RAF's phosphate-binding loop that, when disrupted either through pharmacologic or genetic alterations, results in activation of RAF and the mitogen-activated protein kinase (MAPK) pathway. This mechanism accounts not only for compound-mediated activation of the MAPK pathway in BRAF(WT) cells but also offers a biochemical mechanism for BRAF oncogenesis.