Comparison of the Kinase Profile of Midostaurin (Rydapt) with That of Its Predominant Metabolites and the Potential Relevance of Some Newly Identified Targets to Leukemia Therapy.

The multitargeted protein kinase inhibitor midostaurin is approved for the treatment of both newly diagnosed FLT3-mutated acute myeloid leukemia (AML) and KIT-driven advanced systemic mastocytosis. AML is a heterogeneous malignancy, and investigational drugs targeting FLT3 have shown disparate effects in patients with FLT3-mutated AML, probably as a result of their inhibiting different targets and pathways at the administered doses. However, the efficacy and side effects of drugs do not just reflect the biochemical and pharmacodynamic properties of the parent compound but are often comprised of complex cooperative effects between the properties of the parent and active metabolites. Following chronic dosing, two midostaurin metabolites attain steady-state plasma trough levels greater than that of the parent drug. In this study, we characterized these metabolites and determined their profiles as kinase inhibitors using radiometric transphosphorylation assays. Like midostaurin, the metabolites potently inhibit mutant forms of FLT3 and KIT and several additional kinases that either are directly involved in the deregulated signaling pathways or have been implicated as playing a role in AML via stromal support, such as IGF1R, LYN, PDPK1, RET, SYK, TRKA, and VEGFR2. Consequently, a complex interplay between the kinase activities of midostaurin and its metabolites is likely to contribute to the efficacy of midostaurin in AML and helps to engender the distinctive effects of the drug compared to those of other FLT3 inhibitors in this malignancy.

Increasing metabolic stability via the deuterium kinetic isotope effect: An example from a proline-amide-urea aminothiazole series of phosphatidylinositol-3 kinase alpha inhibitors.

In vitro metabolic identification studies with a PI3K-α inhibitor lead molecule 1 identified a single predominant site of oxidative metabolism to be occurring within a tert.butyl moiety. Modification of the tert.butyl group within the lead molecule 1, to the corresponding d9-tert.butyl analogue 2, led to an increase in both the in vitro and in vivo metabolic stability. This increase in metabolic stability resulted in a 2-fold increase in the oral bioavailability measured in the rat, and a 3-fold increase in potency in a chronic in vivo study in the mouse, for 2 when compared to 1.

Identification and optimisation of 4,5-dihydrobenzo[1,2-d:3,4-d]bisthiazole and 4,5-dihydrothiazolo[4,5-h]quinazoline series of selective phosphatidylinositol-3 kinase alpha inhibitors.

A cyclisation within a 4',5-bisthiazole (S)-proline-amide-urea series of selective PI3Kα inhibitors led to a novel 4,5-dihydrobenzo[1,2-d:3,4-d]bisthiazole tricyclic sub-series. The synthesis and optimisation of this 4,5-dihydrobenzo[1,2-d:3,4-d]bisthiazole sub-series and the expansion to a related tricyclic 4,5-dihydrothiazolo[4,5-h]quinazoline sub-series are described. From this work analogues including 11, 12, 19 and 23 were identified as potent and selective PI3Kα inhibitor in vivo tool compounds.

Identification and optimisation of a 4',5-bisthiazole series of selective phosphatidylinositol-3 kinase alpha inhibitors.

Exploring the affinity-pocket binding moiety of a 2-aminothiazole (S)-proline-amide-urea series of selective PI3Kα inhibitors using a parallel-synthesis approach led to the identification of a novel 4',5-bisthiazole sub-series. The synthesis and optimisation of both the affinity pocket and (S)-proline amide moieties within this 4',5-bisthiazole sub-series are described. From this work a number of analogues, including 14 (A66) and 24, were identified as potent and selective PI3Kα inhibitor in vitro tool compounds.

Discovery of NVP-BYL719 a potent and selective phosphatidylinositol-3 kinase alpha inhibitor selected for clinical evaluation.

Phosphatidylinositol-3-kinase α (PI3Kα) is a therapeutic target of high interest in anticancer drug research. On the basis of a binding model rationalizing the high selectivity and potency of a particular series of 2-aminothiazole compounds in inhibiting PI3Kα, a medicinal chemistry program has led to the discovery of the clinical candidate NVP-BYL719.

Identification of NVP-CLR457 as an Orally Bioavailable Non-CNS-Penetrant pan-Class IA Phosphoinositol-3-Kinase Inhibitor.

Balanced pan-class I phosphoinositide 3-kinase inhibition as an approach to cancer treatment offers the prospect of treating a broad range of tumor types and/or a way to achieve greater efficacy with a single inhibitor. Taking buparlisib as the starting point, the balanced pan-class I PI3K inhibitor 40 (NVP-CLR457) was identified with what was considered to be a best-in-class profile. Key to the optimization to achieve this profile was eliminating a microtubule stabilizing off-target activity, balancing the pan-class I PI3K inhibition profile, minimizing CNS penetration, and developing an amorphous solid dispersion formulation. A rationale for the poor tolerability profile of 40 in a clinical study is discussed.

The RET kinase inhibitor NVP-AST487 blocks growth and calcitonin gene expression through distinct mechanisms in medullary thyroid cancer cells.

The RET kinase has emerged as a promising target for the therapy of medullary thyroid cancers (MTC) and of a subset of papillary thyroid cancers. NVP-AST487, a N,N'-diphenyl urea with an IC(50) of 0.88 mumol/L on RET kinase, inhibited RET autophosphorylation and activation of downstream effectors, and potently inhibited the growth of human thyroid cancer cell lines with activating mutations of RET but not of lines without RET mutations. NVP-AST487 induced a dose-dependent growth inhibition of xenografts of NIH3T3 cells expressing oncogenic RET, and of the MTC cell line TT in nude mice. MTCs secrete calcitonin, a useful indicator of tumor burden. Human plasma calcitonin levels derived from the TT cell xenografts were inhibited shortly after treatment, when tumor volume was still unchanged, indicating that the effects of RET kinase inhibition on calcitonin secretion were temporally dissociated from its tumor-inhibitory properties. Accordingly, NVP-AST487 inhibited calcitonin gene expression in vitro in TT cells, in part, through decreased gene transcription. These data point to a previously unknown physiologic role of RET signaling on calcitonin gene expression. Indeed, the RET ligands persephin and GDNF robustly stimulated calcitonin mRNA, which was blocked by pretreatment with NVP-AST487. Antagonists of RET kinase activity in patients with MTC may result in effects on plasma calcitonin that are either disproportionate or dissociated from the effects on tumor burden, because RET kinase mediates a physiologic pathway controlling calcitonin secretion. The role of traditional tumor biomarkers may need to be reassessed as targeted therapies designed against oncoproteins with key roles in pathogenesis are implemented.

Entry into a new class of protein kinase inhibitors by pseudo ring design.

A pyrimidin-4-yl-urea motif forming a pseudo ring by intramolecular hydrogen bonding has been designed to mimic the pyrido[2,3-d]pyrimidin-7-one core structure of a well-established class of protein kinase inhibitors. Potent inhibition of a number of protein kinases was obtained with the first prototype compound synthesized to probe the design concept.

Hypoxia modulation and radiosensitization by the novel dual EGFR and VEGFR inhibitor AEE788 in spontaneous and related allograft tumor models.

Concomitant inhibition of ErbB1/2- and VEGF receptor-signaling synergizes when used in combination with DNA-damaging agents. Here, we investigated for the first time the combined treatment modality of the novel dual specific receptor tyrosine kinase inhibitor AEE788 with ionizing radiation and analyzed treatment-induced end points in situ as indicators for a potential sensitizing mechanism. Furthermore, we assessed tumor hypoxia in response to different antiangiogenic and antiproliferative treatment modalities. The combined treatment effect was investigated in a spontaneously growing mammary carcinoma model and against Her-2/neu-overexpressing mammary carcinoma allografts. In tumor allografts derived from murine mammary carcinoma cells of mouse mammary tumor virus/c-neu transgenic mice, a minimal treatment regimen with AEE788 and fractionated irradiation resulted in an at least additive tumor response. Treatment response in the corresponding spontaneous tumor model strongly exceeded the response induced in the isogenic allografts. Treatment-induced changes of tumor proliferation, apoptosis, and microvessel density were similar in the two tumor models. Treatment with AEE788 alone or in combination with IR strongly improved tumor oxygenation in both tumor models as determined by the detection of endogenous and exogenous markers of tumor hypoxia. Specific inhibition of the VEGF-receptor tyrosine kinase versus Erb1/2-receptor tyrosine kinase indicated that it is the antiproliferative and not the antiangiogenic potency of AEE788 that mediates the hypoxia-reducing effect of this dual kinase-specific inhibitor. Overall, we show that concomitant inhibition of ErbB- and VEGF-receptor signaling by AEE788, in combination with ionizing radiation, is a promising treatment approach, especially in hypoxic, oncogenic ErbB-driven tumors.

Discovery of Potent, Selective, and Structurally Novel Dot1L Inhibitors by a Fragment Linking Approach.

Misdirected catalytic activity of histone methyltransferase Dot1L is believed to be causative for a subset of highly aggressive acute leukemias. Targeting the catalytic domain of Dot1L represents a potential therapeutic approach for these leukemias. In the context of a comprehensive Dot1L hit finding strategy, a knowledge-based virtual screen of the Dot1L SAM binding pocket led to the discovery of 2, a non-nucleoside fragment mimicking key interactions of SAM bound to Dot1L. Fragment linking of 2 and 3, an induced back pocket binder identified in earlier studies, followed by careful ligand optimization led to the identification of 7, a highly potent, selective and structurally novel Dot1L inhibitor.

AEE788: a dual family epidermal growth factor receptor/ErbB2 and vascular endothelial growth factor receptor tyrosine kinase inhibitor with antitumor and antiangiogenic activity.

Aberrant epidermal growth factor receptor (EGFR) and ErbB2 expression are associated with advanced disease and poor patient prognosis in many tumor types (breast, lung, ovarian, prostate, glioma, gastric, and squamous carcinoma of head and neck). In addition, a constitutively active EGFR type III deletion mutant has been identified in non-small cell lung cancer, glioblastomas, and breast tumors. Hence, members of the EGFR family are viewed as promising therapeutic targets in the fight against cancer. In a similar vein, vascular endothelial growth factor (VEGF) receptor kinases are also promising targets in terms of an antiangiogenic treatment strategy. AEE788, obtained by optimization of the 7H-pyrrolo[2,3-d]pyrimidine lead scaffold, is a potent combined inhibitor of both epidermal growth factor (EGF) and VEGF receptor tyrosine kinase family members on the isolated enzyme level and in cellular systems. At the enzyme level, AEE788 inhibited EGFR and VEGF receptor tyrosine kinases in the nm range (IC(50)s: EGFR 2 nm, ErbB2 6 nm, KDR 77 nm, and Flt-1 59 nm). In cells, growth factor-induced EGFR and ErbB2 phosphorylation was also efficiently inhibited (IC(50)s: 11 and 220 nm, respectively). AEE788 demonstrated antiproliferative activity against a range of EGFR and ErbB2-overexpressing cell lines (including EGFRvIII-dependent lines) and inhibited the proliferation of epidermal growth factor- and VEGF-stimulated human umbilical vein endothelial cells. These properties, combined with a favorable pharmacokinetic profile, were associated with a potent antitumor activity in a number of animal models of cancer, including tumors that overexpress EGFR and or ErbB2. Oral administration of AEE788 to tumor-bearing mice resulted in high and persistent compound levels in tumor tissue. Moreover, AEE788 efficiently inhibited growth factor-induced EGFR and ErbB2 phosphorylation in tumors for >72 h, a phenomenon correlating with the antitumor efficacy of intermittent treatment schedules. Strikingly, AEE788 also inhibited VEGF-induced angiogenesis in a murine implant model. Antiangiogenic activity was also apparent by measurement of tumor vascular permeability and interstitial leakage space using dynamic contrast enhanced magnetic resonance imaging methodology. Taken together, these data indicate that AEE788 has potential as an anticancer agent targeting deregulated tumor cell proliferation as well as angiogenic parameters. Consequently, AEE788 is currently in Phase I clinical trials in oncology.

Discovery of Novel Dot1L Inhibitors through a Structure-Based Fragmentation Approach.

Oncogenic MLL fusion proteins aberrantly recruit Dot1L, a histone methyltransferase, to ectopic loci, leading to local hypermethylation of H3K79 and misexpression of HoxA genes driving MLL-rearranged leukemias. Inhibition of the methyltransferase activity of Dot1L in this setting is predicted to reverse aberrant H3K79 methylation, leading to repression of leukemogenic genes and tumor growth inhibition. In the context of our Dot1L drug discovery program, high-throughput screening led to the identification of 2, a weak Dot1L inhibitor with an unprecedented, induced pocket binding mode. A medicinal chemistry campaign, strongly guided by structure-based consideration and ligand-based morphing, enabled the discovery of 12 and 13, potent, selective, and structurally completely novel Dot1L inhibitors.

Protein kinases as targets for anticancer agents: from inhibitors to useful drugs.

Many components of mitogenic signaling pathways in normal and neoplastic cells have been identified, including the large family of protein kinases, which function as components of signal transduction pathways, playing a central role in diverse biological processes, such as control of cell growth, metabolism, differentiation, and apoptosis. The development of selective protein kinase inhibitors that can block or modulate diseases caused by abnormalities in these signaling pathways is widely considered a promising approach for drug development. Because of their deregulation in human cancers, protein kinases, such as Bcr-Abl, those in the epidermal growth factor-receptor (HER) family, the cell cycle regulating kinases such as the cyclin-dependent kinases, as well as the vascular endothelial growth factor-receptor kinases involved in the neo-vascularization of tumors, are among the protein kinases considered as prime targets for the development of selective inhibitors. These drug-discovery efforts have generated inhibitors and low-molecular weight therapeutics directed against the ATP-binding site of various protein kinases that are in various stages of development (up to Phase II/III clinical trials). Three examples of inhibitors of protein kinases are reviewed, including low-molecular weight compounds targeting the cell cycle kinases; a potent and selective inhibitor of the HER1/HER2 receptor tyrosine kinase, the pyrollopyrimidine PKI166; and the 2-phenyl-aminopyrimidine STI571 (Glivec(R), Gleevec) a targeted drug therapy directed toward Bcr-Abl, the key player in chronic leukemia (CML). Some members of the HER family of receptor tyrosine kinases, in particular HER1 and HER2, have been found to be overexpressed in a variety of human tumors, suggesting that inhibition of HER signaling would be a viable antiproliferative strategy. The pyrrolo-pyrimidine PKI166 was developed as an HER1/HER2 inhibitor with potent in vitro antiproliferative and in vivo antitumor activity. Based upon its clear association with disease, the Bcr-Abl tyrosine kinase in CML represents the ideal target to validate the clinical utility of protein kinase inhibitors as therapeutic agents. In a preclinical model, STI571 (Glivec(R), Gleevec) showed potent in vitro and in vivo antitumor activity that was selective for Abl, c-Kit, and the platelet-derived growth factor-receptor. Phase I/II studies demonstrated that STI571 is well tolerated, and that it showed promising hematological and cytogenetic responses in CML and clinical responses in the c-Kit-driven gastrointestinal tumors.

Characterization of the novel and specific PI3Kα inhibitor NVP-BYL719 and development of the patient stratification strategy for clinical trials.

Somatic PIK3CA mutations are frequently found in solid tumors, raising the hypothesis that selective inhibition of PI3Kα may have robust efficacy in PIK3CA-mutant cancers while sparing patients the side-effects associated with broader inhibition of the class I phosphoinositide 3-kinase (PI3K) family. Here, we report the biologic properties of the 2-aminothiazole derivative NVP-BYL719, a selective inhibitor of PI3Kα and its most common oncogenic mutant forms. The compound selectivity combined with excellent drug-like properties translates to dose- and time-dependent inhibition of PI3Kα signaling in vivo, resulting in robust therapeutic efficacy and tolerability in PIK3CA-dependent tumors. Novel targeted therapeutics such as NVP-BYL719, designed to modulate aberrant functions elicited by cancer-specific genetic alterations upon which the disease depends, require well-defined patient stratification strategies in order to maximize their therapeutic impact and benefit for the patients. Here, we also describe the application of the Cancer Cell Line Encyclopedia as a preclinical platform to refine the patient stratification strategy for NVP-BYL719 and found that PIK3CA mutation was the foremost positive predictor of sensitivity while revealing additional positive and negative associations such as PIK3CA amplification and PTEN mutation, respectively. These patient selection determinants are being assayed in the ongoing NVP-BYL719 clinical trials.