Discovery of 6-Formylpyridyl Urea Derivatives as Potent Reversible-Covalent Fibroblast Growth Factor Receptor 4 Inhibitors with Improved Anti-Hepatocellular Carcinoma Activity

Fibroblast growth factor receptor 4 (FGFR4) has been considered as a potential anticancer target due to FGF19/FGFR4 mediated aberrant signaling in hepatocellular carcinoma (HCC). Several FGFR4 inhibitors have been reported, but none have gained approval. Herein, a series of 5-formyl-pyrrolo[3,2-b]pyridine-3-carboxamides and a series of 6-formylpyridyl ureas were characterized as selective reversible-covalent FGFR4 inhibitors. The representative 6-formylpyridyl urea 8z exhibited excellent potency against FGFR4WT, FGFR4V550L, and FGFR4V550M with IC50 values of 16.3, 12.6, and 57.3 nM, respectively. It also potently suppressed proliferation of Ba/F3 cells driven by FGFR4WT, FGFR4V550L, and FGFR4V550M, and FGFR4-dependent Hep3B and Huh7 HCC cells, with IC50 values of 1.2, 13.5, 64.5, 15.0, and 20.4 nM, respectively. Furthermore, 8z displayed desirable microsomal stability and significant in vivo efficacy in the Huh7 HCC cancer xenograft model in nude mice. The study provides a promising new lead for anticancer drug discovery directed toward overcoming FGFR4 gatekeeper mutation mediated resistance in HCC patients.

Design and Biological Evaluation of Piperazine-Bearing Nitrobenzamide Hypoxia/GDEPT Prodrugs: The Discovery of CP-506

Off-target aerobic activation of PR-104A by human aldo-keto reductase 1C3 (AKR1C3) has confounded the development of this dual hypoxia/gene therapy prodrug. Previous attempts to design prodrugs resistant to AKR1C3 activation have resulted in candidates that require further optimization. Herein we report the evaluation of a lipophilic series of PR-104A analogues in which a piperazine moiety has been introduced to improve drug-like properties. Octanol-water partition coefficients (LogD7.4) spanned >2 orders of magnitude. 2D antiproliferative and 3D multicellular clonogenic assays using isogenic HCT116 and H1299 cells confirmed that all examples were resistant to AKR1C3 metabolism while producing an E. coli NfsA nitroreductase-mediated bystander effect. Prodrugs 16, 17, and 20 demonstrated efficacy in H1299 xenografts where only a minority of tumor cells express NfsA. These prodrugs and their bromo/mesylate counterparts (25-27) were also evaluated for hypoxia-selective cell killing in vitro. These results in conjunction with stability assays recommended prodrug 26 (CP-506) for Phase I/II clinical trial.

Structural Mechanism and Inhibitors Targeting EGFR Exon 20 Insertion (Ex20ins) Mutations

Epidermal growth factor receptor (EGFR) targeted therapy is one of the most important and effective strategies to combat EGFR mutant nonsmall-cell lung cancer (NSCLC). However, a substantial number of patients bearing EGFR exon 20 insertion (Ex20ins) mutations respond poorly to common EGFR targeted therapies. This clinical need remained unmet until recently, when the EGFR Ex20ins mutation inhibitor mobocertinib was approved by the FDA. Despite this progress, the structural mechanisms of EGFR Ex20ins mutation resistance and characterization of inhibitor binding modes have not been systematically summarized. Herein, we analyze the structural mechanisms for ligand binding and resistance and summarize recent developments for the reported inhibitors of EGFR Ex20ins mutations. Furthermore, this Perspective aims to provide insights for the design of the next generation of EGFR Ex20ins inhibitors.

Rational Design of Covalent Kinase Inhibitors by an Integrated Computational Workflow (Kin-Cov)

Covalent kinase inhibitors (CKIs) hold great promise for drug development. However, examples of computationally guided design of CKIs are still scarce. Here, we present an integrated computational workflow (Kin-Cov) for rational design of CKIs. The design of the first covalent leucine-zipper and sterile-α motif kinase (ZAK) inhibitor was presented as an example to showcase the power of computational workflow for CKI design. The two representative compounds, 7 and 8, inhibited ZAK kinase with half-maximal inhibitory concentration (IC₅₀) values of 9.1 and 11.5 nM, respectively. Compound 8 displayed an excellent ZAK target specificity in Kinome profiling against 378 wild-type kinases. Structural biology and cell-based Western blot washout assays validated the irreversible binding characteristics of the compounds. Our study presents a rational approach for the design of CKIs based on the reactivity and accessibility of nucleophilic amino acid residues in a kinase. The workflow is generalizable and can be applied to facilitate CKI-based drug design.

Design, Synthesis, and Biological Evaluation of 5-Formyl-pyrrolo[3,2-b]pyridine-3-carboxamides as New Selective, Potent, and Reversible-Covalent FGFR4 Inhibitors

The FGF19-FGFR4 signaling pathway has been extensively studied as a promising target for the treatment of hepatocellular carcinoma (HCC). Several FGFR4-selective inhibitors have been developed, but none of them receives approval. Additionally, acquired resistance caused by FGFR4 gatekeeper mutations is emerging as a serious limitation for these targeted therapies. Herein, we report a novel series of 5-formyl-pyrrolo[3,2-b]pyridine derivatives as new reversible-covalent inhibitors targeting wild-type and gatekeeper mutant variants of FGFR4 kinase. The representative compound 10z exhibited single-digit nanomolar activity against wild-type FGFR4 and the FGFR4V550L/M mutant variants in biochemical and Ba/F3 cellular assays, while sparing FGFR1/2/3. Furthermore, 10z showed significant antiproliferative activity against Hep3B, JHH-7, and HuH-7 HCC cells with IC50 values of 37, 32, and 94 nM, respectively. MALDI-TOF-MS and X-ray protein crystallography studies were consistent with 10z acting as a reversible-covalent inhibitor of FGFR4, serving as a promising lead compound for further anticancer drug development.

Discovery of Isoform-Selective Akt3 Degraders Overcoming Osimertinib-Induced Resistance in Non-Small Cell Lung Cancer Cells

EGFR inhibitor therapies have brought significant benefit to NSCLC patients. However, all patients gradually progress to acquired resistance via diverse mechanisms. Akt3 overexpression but not Akt1/2 is one of the found molecular events that mediate osimertinib (1) resistance in NSCLC patients. Here, we report 12l as the first bona fide isoform-selective Akt3 degrader which potently induced proteasomal degradation of the target both in vitro and in vivo, whereas its effects on Akt1/2 were minimal. Using 12l as a tool, non-canonical function of Akt3 was validated to contribute greatly to survival of 1-resistant H1975OR NSCLC cells. Degrader 12l potently suppressed the growth of H1975OR as well as several NSCLC cell lines with low nanomolar IC₅₀ values and demonstrated promising in vivo antitumor efficacy in nude mice bearing H1975OR or PC9 NSCLC xenograft models. Selective degradation of Akt3 may be considered as a novel strategy for human cancer therapy.

Bioreductive prodrug PR-104 improves the tumour distribution and titre of the nitroreductase-armed oncolytic adenovirus ONYX-411NTR leading to therapeutic benefit

Advances in the field of cancer immunotherapy have stimulated renewed interest in adenoviruses as oncolytic agents. Clinical experience has shown that oncolytic adenoviruses are safe and well tolerated but possess modest single-agent activity. One approach to improve the potency of oncolytic viruses is to utilise their tumour selectivity to deliver genes encoding prodrug-activating enzymes. These enzymes can convert prodrugs into cytotoxic species within the tumour; however, these cytotoxins can interfere with viral replication and limit utility. In this work, we evaluated the activity of a nitroreductase (NTR)-armed oncolytic adenovirus ONYX-411^NTR in combination with the clinically tested bioreductive prodrug PR-104. Both NTR-expressing cells in vitro and xenografts containing a minor population of NTR-expressing cells were highly sensitive to PR-104. Pharmacologically relevant prodrug exposures did not interfere with ONYX-411^NTR replication in vitro. In vivo, prodrug administration increased virus titre and improved virus distribution within tumour xenografts. Colonisation of tumours with high ONYX-411^NTR titre resulted in NTR expression and prodrug activation. The combination of ONYX-411^NTR with PR-104 was efficacious against HCT116 xenografts, whilst neither prodrug nor virus were active as single agents. This work highlights the potential for future clinical development of NTR-armed oncolytic viruses in combination with bioreductive prodrugs.

Design, Synthesis, and Biological Evaluation of Aminoindazole Derivatives as Highly Selective Covalent Inhibitors of Wild-Type and Gatekeeper Mutant FGFR4

Aberrant FGF19/FGFR4 signaling has been shown to be an oncogenic driver of growth and survival in human hepatocellular carcinoma (HCC) with several pan-FGFR inhibitors and FGFR4-selective inhibitors currently being evaluated in the clinic. However, FGFR4 gatekeeper mutation induced acquired resistance remains an unmet clinical challenge for HCC treatment. Thus, a series of aminoindazole derivatives were designed and synthesized as new irreversible inhibitors of wild-type and gatekeeper mutant FGFR4. One representative compound (7v) exhibited excellent potency against FGFR4, FGFR4^V550L, and FGFR4^V550M with nanomolar activity in both the biochemical and cellular assays while sparing FGFR1/2/3. While compound 7v demonstrated modest in vivo antitumor efficacy in nude mice bearing the Huh-7 xenograft model consistent with its unfavorable pharmacokinetic properties, it provides a promising new starting point for future drug discovery combating FGFR4 gatekeeper mediated resistance in HCC patients.

Tissue Pharmacokinetic Properties and Bystander Potential of Hypoxia-Activated Prodrug CP-506 by Agent-Based Modelling

Hypoxia-activated prodrugs are bioactivated in oxygen-deficient tumour regions and represent a novel strategy to exploit this pharmacological sanctuary for therapeutic gain. The approach relies on the selective metabolism of the prodrug under pathological hypoxia to generate active metabolites with the potential to diffuse throughout the tumour microenvironment and potentiate cell killing by means of a “bystander effect”. In the present study, we investigate the pharmacological properties of the nitrogen mustard prodrug CP-506 in tumour tissues using in silico spatially-resolved pharmacokinetic/pharmacodynamic (SR-PK/PD) modelling. The approach employs a number of experimental model systems to define parameters for the cellular uptake, metabolism and diffusion of both the prodrug and its metabolites. The model predicts rapid uptake of CP-506 to high intracellular concentrations with its long plasma half-life driving tissue diffusion to a penetration depth of 190 µm, deep within hypoxic activating regions. While bioreductive metabolism is restricted to regions of severe pathological hypoxia (<1 µM O₂), its active metabolites show substantial bystander potential with release from the cell of origin into the extracellular space. Model predictions of bystander efficiency were validated using spheroid co-cultures, where the clonogenic killing of metabolically defective “target” cells increased with the proportion of metabolically competent “activator” cells. Our simulations predict a striking bystander efficiency at tissue-like densities with the bis-chloro-mustard amine metabolite (CP-506M-Cl₂) identified as a major diffusible metabolite. Overall, this study shows that CP-506 has favourable pharmacological properties in tumour tissue and supports its ongoing development for use in the treatment of patients with advanced solid malignancies.

Discovery of Cysteine-targeting Covalent Protein Kinase Inhibitors

Small molecule covalent kinase inhibitors (CKIs) have entered a new era in drug discovery, which have the advantage for sustained target inhibition and high selectivity. An increased understanding of binding kinetics of CKIs and discovery of additional irreversible and reversible-covalent cysteine-targeted warheads has inspired the development of this area. Herein, we summarize the major medicinal chemistry strategies employed in the discovery of these representative CKIs, which are categorized by the location of the target cysteine within seven main regions of the kinase: the front region, the glycine rich loop (P-loop), the hinge region, the DFG region, the activation loop (A-loop), the catalytic loop (C-loop), and the remote loop. The emphasis is placed on the design and optimization strategies of CKIs that are generated by addition of a warhead to a reversible lead/inhibitor scaffold. In addition, we address the challenges facing this area of drug discovery.

Selectively Targeting Tumor Hypoxia With the Hypoxia-Activated Prodrug CP-506

Hypoxia-activated prodrugs (HAP) are a promising class of antineoplastic agents that can selectively eliminate hypoxic tumor cells. This study evaluates the hypoxia-selectivity and antitumor activity of CP-506, a DNA alkylating HAP with favorable pharmacologic properties. Stoichiometry of reduction, one-electron affinity, and back-oxidation rate of CP-506 were characterized by fast-reaction radiolytic methods with observed parameters fulfilling requirements for oxygen-sensitive bioactivation. Net reduction, metabolism, and cytotoxicity of CP-506 were maximally inhibited at oxygen concentrations above 1 μmol/L (0.1% O₂). CP-506 demonstrated cytotoxicity selectively in hypoxic 2D and 3D cell cultures with normoxic/anoxic IC₅₀ ratios up to 203. Complete resistance to aerobic (two-electron) metabolism by aldo-keto reductase 1C3 was confirmed through gain-of-function studies while retention of hypoxic (one-electron) bioactivation by various diflavin oxidoreductases was also demonstrated. In vivo, the antitumor effects of CP-506 were selective for hypoxic tumor cells and causally related to tumor oxygenation. CP-506 effectively decreased the hypoxic fraction and inhibited growth of a wide range of hypoxic xenografts. A multivariate regression analysis revealed baseline tumor hypoxia and in vitro sensitivity to CP-506 were significantly correlated with treatment response. Our results demonstrate that CP-506 selectively targets hypoxic tumor cells and has broad antitumor activity. Our data indicate that tumor hypoxia and cellular sensitivity to CP-506 are strong determinants of the antitumor effects of CP-506.

Fourth-Generation Analogues of the Anticancer Peptaibol Culicinin D: Probing the Effects of Hydrophobicity and Halogenation on Cytotoxicity

Preliminary results of the effect of hydrophobicity and halogenation on the cytotoxicity of the anticancer peptaibol culicinin D are reported. Building on previous work, the synthetically challenging (2S,4S,6R)-2-amino-6-hydroxy-4-methyl-8-oxodecanoic acid and (2S,4R)-2-amino-4-methyldecanoic acid building blocks were replaced with derivatives of l-phenylalanine and 2-aminodecanoic acid, respectively. Substitution of (2S,4S,6R)-2-amino-6-hydroxy-4-methyl-8-oxodecanoic acid with l-4,4′-biphenylalanine yielded an analogue that was tenfold more potent than the natural product and was also the most hydrophobic analogue, as judged by an antiproliferative IC50 assay and logD calculations; these results suggest that the potency of culicinin D may be governed by its hydrophobicity. However, the introduction of halogenated moieties into the peptide sequence generated analogues that were similarly potent, although not necessarily hydrophobic. Thus, the parameters regulating the cytotoxicity of culicinin D, and by extension other peptaibols, are multimodal and include both halogenation and hydrophobicity.

Design, Synthesis and In-Vitro Biological Evaluation of Antofine and Tylophorine Prodrugs as Hypoxia-Targeted Anticancer Agents

Phenanthroindolizidines, such as antofine and tylophorine, are a family of natural alkaloids isolated from different species of Asclepiadaceas. They are characterized by interesting biological activities, such as pronounced cytotoxicity against different human cancerous cell lines, including multidrug-resistant examples. Nonetheless, these derivatives are associated with severe neurotoxicity and loss of in vivo activity due to the highly lipophilic nature of the alkaloids. Here, we describe the development of highly polar prodrugs of antofine and tylophorine as hypoxia-targeted prodrugs. The developed quaternary ammonium salts of phenanthroindolizidines showed high chemical and metabolic stability and are predicted to have no penetration through the blood-brain barrier. The designed prodrugs displayed decreased cytotoxicity when tested under normoxic conditions. However, their cytotoxic activity considerably increased when tested under hypoxic conditions.

TANK-binding kinase 1 (TBK1): An emerging therapeutic target for drug discovery

Dysregulation of TANK-binding kinase 1 (TBK1) homeostasis leads to the occurrence and progression of many diseases, such as inflammation, autoimmune diseases, metabolic diseases, and cancer. Therefore, there is a need to develop TBK1 inhibitors as therapeutic agents. In this review, we highlight the diverse biological functions of TBK1 and summarize the promising small-molecule inhibitors of TBK1 that have the potential to be developed as therapeutic candidates.

Tumour Hypoxia-Mediated Immunosuppression: Mechanisms and Therapeutic Approaches to Improve Cancer Immunotherapy

The magnitude of the host immune response can be regulated by either stimulatory or inhibitory immune checkpoint molecules. Receptor-ligand binding between inhibitory molecules is often exploited by tumours to suppress anti-tumour immune responses. Immune checkpoint inhibitors that block these inhibitory interactions can relieve T-cells from negative regulation, and have yielded remarkable activity in the clinic. Despite this success, clinical data reveal that durable responses are limited to a minority of patients and malignancies, indicating the presence of underlying resistance mechanisms. Accumulating evidence suggests that tumour hypoxia, a pervasive feature of many solid cancers, is a critical phenomenon involved in suppressing the anti-tumour immune response generated by checkpoint inhibitors. In this review, we discuss the mechanisms associated with hypoxia-mediate immunosuppression and focus on modulating tumour hypoxia as an approach to improve immunotherapy responsiveness.

Investigation of Covalent Warheads in the Design of 2-Aminopyrimidine-based FGFR4 Inhibitors

Covalent kinase inhibitors are rapidly emerging as a class of therapeutics with clinical benefits. Herein we report a series of selective 2-aminopyrimidine-based fibroblast growth factor receptor 4 (FGFR4) inhibitors exploring different types of cysteine-targeting warheads. The structure-activity relationship study revealed that the chemically tuned warheads α-fluoro acrylamide, vinylsulfonamide, and acetaldehyde amine were suitable as covalent warheads for the design of selective FGFR4 inhibitors. Compounds 6a, 6h, and 6i selectively suppressed FGFR4 enzymatic activity with IC₅₀ values of 53 ± 18, 45 ± 11, and 16 ± 4 nM, respectively, while sparing FGFR1/2/3. X-ray crystal structure and MALDI-TOF studies demonstrated that compound 6h bearing the α-fluoro acrylamide binds to FGFR4 with an irreversible binding mode, whereas compound 6i with an acetaldehyde amine binds to FGFR4 with a reversible covalent mode. 6h and 6i might provide some fundamental structural information for the rational design of new selective FGFR4 inhibitors.

Tarloxotinib Is a Hypoxia-Activated Pan-HER Kinase Inhibitor Active Against a Broad Range of HER-Family Oncogenes

PURPOSE

Approved therapies for EGFR exon 20, ERBB2 mutations, and NRG1 fusions are currently lacking for non-small cell lung cancer and other cancers. Tarloxotinib is a prodrug that harnesses tumor hypoxia to generate high levels of a potent, covalent pan-HER tyrosine kinase inhibitor, tarloxotinib-effector (tarloxotinib-E), within the tumor microenvironment. This tumor-selective delivery mechanism was designed to minimize the dose-limiting toxicities that are characteristic of systemic inhibition of wild-type EGFR.

EXPERIMENTAL DESIGN

Novel and existing patient-derived cell lines and xenografts harboring EGFR exon 20 insertion mutations, ERBB2 mutations and amplification, and NRG1 fusions were tested in vitro and in vivo with tarloxotinib to determine its impact on cancer cell proliferation, apoptosis, and cell signaling.

RESULTS

Tarloxotinib-E inhibited cell signaling and proliferation in patient-derived cancer models in vitro by directly inhibiting phosphorylation and activation of EGFR, HER2, and HER2/HER3 heterodimers. In vivo, tarloxotinib induced tumor regression or growth inhibition in multiple murine xenograft models. Pharmacokinetic analysis confirmed markedly higher levels of tarloxotinib-E in tumor tissue than plasma or skin. Finally, a patient with lung adenocarcinoma harboring an ERBB2 exon 20 p.A775_G776insYVMA mutation demonstrated a dramatic clinical response to tarloxotinib.

CONCLUSIONS

Experimental data with tarloxotinib validate the novel mechanism of action of a hypoxia-activated prodrug in cancer models by concentrating active drug in the tumor versus normal tissue, and this activity can translate into clinical activity in patients.

Small-Molecule Inhibitors Directly Targeting KRAS as Anticancer Therapeutics

KRAS, the most frequently mutated oncogene, plays a predominant role in driving initiation and progression of cancers. Decades of effort to target KRAS using small molecules has been unsuccessful, causing KRAS to be considered an "undruggable" cancer target. However, this view began to change recently, as drug discovery techniques have developed several KRAS G12C allosteric inhibitors that are currently being evaluated in clinical trials. Herein we provide an in-depth analysis of the structure and binding pockets of KRAS, medicinal chemistry optimization processes, and the biological characterization of small-molecule inhibitors that directly target KRAS, including covalent allosteric inhibitors specific for the G12C mutant, GTP-competitive inhibitors targeting the nucleotide-binding site, and protein-protein interaction inhibitors that bind in the switch I/II pocket or the A59 site. Additionally, we propose potential challenges faced by these new classes of KRAS inhibitors under clinical evaluation.

Medicinal Chemistry Strategies for the Development of Kinase Inhibitors Targeting Point Mutations

Clinically acquired resistance to small molecule kinase inhibitors (SMKIs) has become a major "unmet clinical need" in cancer therapy. To date, there are six SMKIs to be approved for the treatment of cancer patients through targeting of clinically acquired resistance caused by on-target mutations. These are mainly focused on the mutant kinases Bcr-Abl T315I, EGFR T790M, and ALK L1196M. Herein, we summarize the major medicinal chemistry strategies employed in the discovery of these representative SMKIs, such as avoiding steric hindrance, making additional interactions with mutated residues, and forming a covalent bond with an active site cysteine to override resistance observed for reversible inhibitors. Additionally, we also briefly describe allosteric kinase inhibitors and proteolysis targeting chimera (PROTAC) as two other potential strategies while addressing future opportunities in this area.

New Promise and Opportunities for Allosteric Kinase Inhibitors

Drugs that function through allosteric inhibition of kinase signaling represent a promising approach for the targeted discovery of therapeutics. The majority of developed allosteric kinase inhibitors are characterized as type III and IV inhibitors that show good kinome selectivity but generally lack the subtype selectivity of same kinase family. Recently allosteric inhibitors have been developed that bind outside the catalytic kinase domain with high selectivity for specific kinase subtypes. Allosteric inhibitors that bind to the pseudokinase domain of pseudokinase or the extracellular domain of receptor tyrosine kinases are reviewed. We also review recent developments in the field of allosteric kinase inhibitors including examples of proteolysis targeting chimeras, and highlight the unique binding modes for each type of inhibitors and address future opportunities in this area.

Alanine scan-guided synthesis and biological evaluation of analogues of culicinin D, a potent anticancer peptaibol

Culicinin D (1), a 10 amino acid peptaibol originally isolated from Culicinomyces clavisporus, exhibits potent activity against a range of cancer cell lines. Building on our previous work exploring the structure-activity relationship (SAR) of the unusual (2S,4S,6R)-AHMOD residue, a series of analogues of culicinin D were prepared to further investigate the SAR of these peptaibols. Alanine scanning of a potent and readily accessible analogue 23 revealed the effect of each residue on antiproliferative activity, and a small panel of analogues were prepared to explore the SAR of the non-natural amino acid residue (2S,4R)-AMD. Results from the alanine scan were used to design an expanded library of culicinin D analogues, leading to the discovery of cyclohexylalanine analogue 52, which exhibited better antiproliferative activity than the natural product 1.

Synthesis and antiproliferative activity of culicinin D analogues containing simplified AHMOD-based residues

Culicinin D is a 10 amino acid peptaibol containing a rare and synthetically challenging (2S,4S,6R)-AHMOD residue, that exhibits potent antiproliferative activity against MDA-MB-468 cells. An SAR study focusing on replacement of the AHMOD residue was undertaken, culminating in the revelation that a 6-hydroxy or 6-keto substituent was essential to retain potent low nanomolar antiproliferative activity.

Engineering Escherichia coli NfsB To Activate a Hypoxia-Resistant Analogue of the PET Probe EF5 To Enable Non-Invasive Imaging during Enzyme Prodrug Therapy

Gene-directed enzyme prodrug therapy (GDEPT) uses tumor-tropic vectors to deliver prodrug-converting enzymes such as nitroreductases specifically to the tumor environment. The nitroreductase NfsB from Escherichia coli (NfsB_Ec) has been a particular focal point for GDEPT and over the past 25 years has been the subject of several engineering studies seeking to improve catalysis of prodrug substrates. To facilitate clinical development, there is also a need to enable effective non-invasive imaging capabilities. SN33623, a 5-nitroimidazole analogue of 2-nitroimidazole hypoxia probe EF5, has potential for PET imaging exogenously delivered nitroreductases without generating confounding background due to tumor hypoxia. However, we show here that SN33623 is a poor substrate for NfsB_Ec. To address this, we used assay-guided sequence and structure analysis to identify two conserved residues that block SN33623 activation in NfsB_Ec and close homologues. Introduction of the rational substitutions F70A and F108Y into NfsB_Ec conferred high levels of SN33623 activity and enabled specific labeling of E. coli expressing the engineered enzyme. Serendipitously, the F70A and F108Y substitutions also substantially improved activity with the anticancer prodrug CB1954 and the 5-nitroimidazole antibiotic prodrug metronidazole, which is a potential biosafety agent for targeted ablation of nitroreductase-expressing vectors.

Rotational Freedom, Steric Hindrance, and Protein Dynamics Explain BLU554 Selectivity for the Hinge Cysteine of FGFR4

Aberration in FGFR4 signaling drives carcinogenesis and progression in a subset of hepatocellular carcinoma (HCC) patients, thereby making FGFR4 an attractive molecular target for this disease. Selective FGFR4 inhibition can be achieved through covalently targeting a poorly conserved cysteine residue in the FGFR4 kinase domain. We report mass spectrometry assays and cocrystal structures of FGFR4 in covalent complex with the clinical candidate BLU554 and with a series of four structurally related inhibitors that define the inherent reactivity and selectivity profile of these molecules. We further reveal the structure of FGFR1 with one of our inhibitors and show that off-target covalent binding can occur through an alternative conformation that supports targeting of a cysteine conserved in all members of the FGFR family. Collectively, we propose that rotational freedom, steric hindrance, and protein dynamics explain the exceptional selectivity profile of BLU554 for targeting FGFR4.

Prototyping kinase inhibitor-cytotoxin anticancer mutual prodrugs activated by tumour hypoxia: A chemical proof of concept study

Amide- and ester-linked kinase inhibitor-cytotoxin conjugates were rationally designed and synthesised as prototype hypoxia-activated anticancer mutual prodrugs. Chemical reduction of an aryl nitro trigger moiety was shown to initiate a spontaneous cyclisation/fragmentation reaction that simultaneously released the kinase inhibitor semaxanib (SU5416) and the amine- or alcohol-linked cytotoxin from the prodrugs. Preliminary cell testing and reduction potential measurements support optimisation of the compounds towards tumour-selective mutual prodrugs.

Discovery and optimization of 1-(1H-indol-1-yl)ethanone derivatives as CBP/EP300 bromodomain inhibitors for the treatment of castration-resistant prostate cancer

The CREB (cAMP responsive element binding protein) binding protein (CBP) and its homolog EP300 have emerged as new therapeutic targets for the treatment of cancer and inflammatory diseases. Here we report the identification, optimization and evaluation of 1-(1H-indol-1-yl)ethanone derivatives as CBP/EP300 inhibitors starting from fragment-based virtual screening (FBVS). A cocrystal structure of the inhibitor (22e) in complex with CBP provides a solid structural basis for further optimization. The most potent compound 32h binds to the CBP bromodomain and has an IC₅₀ value of 0.037 μM in the AlphaScreen assay which was 2 times more potent than the reported CBP bromodomain inhibitor SGC-CBP30 in our hands. 32h also exhibit high selectivity for CBP/EP300 over other bromodomain-containing proteins. Notably, the ester derivative (29h) of compound 32h markedly inhibits cell growth in several prostate cancer cell lines including LNCaP, 22Rv1 and LNCaP derived C4-2B. Compound 29h suppresses the mRNA expression of full length AR (AR-FL), AR target genes and other oncogene in LNCaP cells. 29h also reduces the expression of PSA, the biomarker of prostate cancer. CBP/EP300 inhibitor 29h represents a promising lead compound for the development of new therapeutics for the treatment of castration-resistant prostate cancer.

Abstract A157: Antitumor activity of tarloxotinib, a hypoxia-activated EGFR TKI, in patient-derived lung cancer cell lines harboring EGFR exon 20 insertions

Tarloxotinib (TRLX) is a prodrug that releases an irreversible EGFR/HER2 tyrosine kinase inhibitor (TRLX-TKI) under pathophysiologically hypoxic conditions. Non-small cell lung cancer (NSCLC) has been characterized as a hypoxic disease and approximately 15% of lung adenocarcinomas harbor EGFR mutations. While most EGFR mutations predict for response to several FDA-approved tyrosine kinase inhibitors, in-frame insertions in exon 20 of EGFR are activating mutations in the tyrosine kinase domain that have significantly decreased sensitivity to EGFR inhibitors and currently have no approved targeted therapies. We derived and characterized three human lung adenocarcinoma cell lines with different EGFR exon 20 insertions in order to accelerate development of targeted therapies for this mutation class. Using these novel cell lines, we evaluated tarloxotinib as a therapeutic agent for tumors harboring this type of mutations. We demonstrate that our three patient-derived cell lines: CUTO14 (p.A767_V769dupASV), CUTO17 (p.N771_H773dupNPH), and CUTO18 (p.S768_770dupSVD) are dependent on EGFR for cell proliferation using shRNA-mediated knockdown. Our results show that EGFR exon 20 insertion cell lines are resistant to gefitinib; however, treatment with afatinib or TRLX-TKI reduces cell proliferation and signaling in a similar manner. The IC50 values for the three cell lines were 203nM, 89nM, and 709 nM for afatinib and 208nM, 33nM, and 345nM for TRLX-TKI, respectively. The prodrug form of tarloxitinib has minimal effect on cell proliferation in these models, consistent with the necessity for hypoxia-induced activation (to TRLX-TKI). Importantly, we evaluated the effect of tarloxotinib in vivo using murine xenograft models of CUTO14 and CUTO17. After four weeks of treatment, afatinib did not alter tumor growth compared to untreated tumors, whereas treatment with tarloxotinib induced significant tumor regression. The in vivo data suggest that the activated TKI of tarloxitinib is accumulating to biologically active concentrations in tumors following cleavage of tarlxoxotinib under hypoxic conditions. We conclude that our EGFR exon 20 insertions cell lines represent novel models for the investigation of therapeutic strategies for this mutation class. These cell lines have the ability to develop tumors in vivo and show reduced sensitivity to current EGFR TKIs, mimicking the lack of response in patients with these mutations. Finally, we demonstrate that tarloxotinib can overcome intrinsic EGFR exon 20 mutation resistance to standard EGFR TKIs.

Citation Format: Adriana Estrada-Bernal, Andrea E. Doak, Anh T. Le, Hengbo Zhu, Nan Chen, Shevan Silva, Jeff B. Smaill, Adam V. Patterson, Robert C. Doebele. Antitumor activity of tarloxotinib, a hypoxia-activated EGFR TKI, in patient-derived lung cancer cell lines harboring EGFR exon 20 insertions [abstract]. In: Proceedings of the AACR-NCI-EORTC International Conference: Molecular Targets and Cancer Therapeutics; 2017 Oct 26-30; Philadelphia, PA. Philadelphia (PA): AACR; Mol Cancer Ther 2018;17(1 Suppl):Abstract nr A157.

2-Aminopyrimidine Derivatives as New Selective Fibroblast Growth Factor Receptor 4 (FGFR4) Inhibitors

A series of 2-aminopyrimidine derivatives were designed and synthesized as highly selective FGFR4 inhibitors. One of the most promising compounds 2n tightly bound FGFR4 with a K_d value of 3.3 nM and potently inhibited its enzymatic activity with an IC₅₀ value of 2.6 nM, but completely spared FGFR1/2/3. The compound selectively suppressed proliferation of breast cancer cells harboring dysregulated FGFR4 signaling with an IC₅₀ value of 0.38 μM. Furthermore, 2n exhibited extraordinary target specificity in a Kinome-wide screen against 468 kinases, with S(35) and S(10) selectivity scores of 0.01 and 0.007 at 1.0 μM, respectively.

Rational design of an AKR1C3-resistant analog of PR-104 for enzyme-prodrug therapy

The clinical stage anti-cancer agent PR-104 has potential utility as a cytotoxic prodrug for exogenous bacterial nitroreductases expressed from replicating vector platforms. However substrate selectivity is compromised due to metabolism by the human one- and two-electron oxidoreductases cytochrome P450 oxidoreductase (POR) and aldo-keto reductase 1C3 (AKR1C3). Using rational drug design we developed a novel mono-nitro analog of PR-104A that is essentially free of this off-target activity in vitro and in vivo. Unlike PR-104A, there was no biologically relevant cytotoxicity in cells engineered to express AKR1C3 or POR, under aerobic or anoxic conditions, respectively. We screened this inert prodrug analog, SN34507, against a type I bacterial nitroreductase library and identified E. coli NfsA as an efficient bioactivator using a DNA damage response assay and recombinant enzyme kinetics. Expression of E. coli NfsA in human colorectal cancer cells led to selective cytotoxicity to SN34507 that was associated with cell cycle arrest and generated a robust 'bystander effect' at tissue-like cell densities when only 3% of cells were NfsA positive. Anti-tumor activity of SN35539, the phosphate pre-prodrug of SN34507, was established in 'mixed' tumors harboring a minority of NfsA-positive cells and demonstrated marked tumor control following heterogeneous suicide gene expression. These experiments demonstrate that off-target metabolism of PR-104 can be avoided and identify the suicide gene/prodrug partnership of E. coli NfsA/SN35539 as a promising combination for development in armed vectors.

Advancing Clostridia to Clinical Trial: Past Lessons and Recent Progress

Most solid cancers contain regions of necrotic tissue. The extent of necrosis is associated with poor survival, most likely because it reflects aggressive tumour outgrowth and inflammation. Intravenously injected spores of anaerobic bacteria from the genus Clostridium infiltrate and selectively germinate in these necrotic regions, providing cancer-specific colonisation. The specificity of this system was first demonstrated over 60 years ago and evidence of colonisation has been confirmed in multiple tumour models. The use of "armed" clostridia, such as in Clostridium Directed Enzyme Prodrug Therapy (CDEPT), may help to overcome some of the described deficiencies of using wild-type clostridia for treatment of cancer, such as tumour regrowth from a well-vascularised outer rim of viable cells. Successful preclinical evaluation of a transferable gene that metabolises both clinical stage positron emission tomography (PET) imaging agents (for whole body vector visualisation) as well as chemotherapy prodrugs (for conditional enhancement of efficacy) would be a valuable early step towards the prospect of "armed" clostridia entering clinical evaluation. The ability to target the immunosuppressive hypoxic tumour microenvironment using CDEPT may offer potential for synergy with recently developed immunotherapy strategies. Ultimately, clostridia may be most efficacious when combined with conventional therapies, such as radiotherapy, that sterilise viable aerobic tumour cells.

Abstract A67: Preclinical efficacy of tarloxotinib bromide (TH-4000), a hypoxia-activated EGFR/HER2 inhibitor: rationale for clinical evaluation in EGFR mutant, T790M-negative NSCLC following progression on EGFR-TKI therapy

Tarloxotinib bromide (T) is a prodrug that releases an irreversible EGFR/HER2 inhibitor (T-TKI) under hypoxic conditions. NSCLC is known to be a hypoxic disease and wild type (WT) EGFR is upregulated by multiple hypoxia-driven mechanisms (Curr Pharm Des, 19:907). Mutant EGFR NSCLC is commonly heterozygous and may result in maintenance of WT EGFR signalling (Can Sci, 103:1946; PloS One 8:e54170). Clinical studies indicate NSCLC patients harbouring WT/mut heterozygous EGFR have significantly poorer ORR, PFS and OS on treatment with EGFR-TKI (Can Sci, 99:929). Other mechanisms of resistance to EGFR-TKI include 50-60% with T790M EGFR mutation, 8-13% with HER2 amplification, while 15-20% lack identifiable mutation/amplification events (Nat Rev Clin Onc, 11:473). The combination of cetuximab/afatinib provides an ORR of 25% and PFS of 4.6 months in T790M-negative NSCLC suggesting the persistence of HER signalling plays a role in resistance. However the high proportion of Grade 3/4 toxicity seen with cetuximab/afatinib indicates an opportunity for dose-intensification with an improved therapeutic index (Can Discov, 4:1). In addition, early clinical data on resistance to the 3rd Gen (WT EGFR-sparing) TKI rociletinib, fails to identify further mutations by NGS in some patients and describes reversion to EGFR-WT (T790) status (Can Discov, 5:713). Collectively these data support the hypothesis that WT EGFR heterozygosity may be a mechanism of resistance to current EGFR-TKI.

Current EGFR-TKI lack the therapeutic index to silence WT EGFR signalling in tumors due to on-target skin/GI toxicities (Ann Oncol 18:761). Therefore we sought to examine the potency of T-TKI relative to erlotinib, afatinib and AZD9291 in five human cancer cell lines expressing WT EGFR (H1838, H2073, H1648, H125 and A431). In antiproliferative assays T-TKI was more dose-potent than erlotinib (25- to 110-fold) afatinib (4- to 32-fold) and AZD9291 (120- to 71-fold). This activity correlated with inhibition of WT EGFR phosphorylation and downstream MAPK signalling. We used a prototypic WT EGFR driven xenograft model (A431) to benchmark T activity against each EGFR-TKI by ‘retrotranslation’ of reported plasma exposure for each agent in human subjects back to the xenograft model. Only treatment with clinically relevant doses and schedules of T was associated with tumor regression and durable inhibition of WT EGFR tumor phosphorylation. Consistent with these findings, T treatment can also regress the WT EGFR NSCLC tumor models H125 and H1648, demonstrating T provides the necessary therapeutic index to inhibit WT EGFR in vivo. The transfection of WT EGFR into mutant EGFR NSCLC line PC9 (vs GFP control) conferred TGFα dependent induction of p-EGFR that was supressed by T-TKI but resistant to inhibition by erlotinib, afatinib or AZD9291. This was associated with reduced antiproliferative activity for EGFR-TKIs. Collectively these data indicate T-TKI is a dose-potent inhibitor of WT EGFR signalling and the prodrug T may possess the therapeutic index to silence WT EGFR signalling in xenograft models at plasma exposure levels achieved in a human Ph1 trial. T is under investigation in a Phase 2 clinical trial for EGFR mutant, T790M-negative, NSCLC patients who have progressed on EGFR-TKI (NCT02454842).

Citation Format: Shevan Silva, Victoria Jackson, Christopher Guise, Maria Abbattista, Matthew Bull, Angus Grey, Robert Anderson, Amir Ashoorzadeh, Charles Hart, Tillman Pearce, Adam V. Patterson, Jeff B. Smaill. Preclinical efficacy of tarloxotinib bromide (TH-4000), a hypoxia-activated EGFR/HER2 inhibitor: rationale for clinical evaluation in EGFR mutant, T790M-negative NSCLC following progression on EGFR-TKI therapy. [abstract]. In: Proceedings of the AACR-NCI-EORTC International Conference: Molecular Targets and Cancer Therapeutics; 2015 Nov 5-9; Boston, MA. Philadelphia (PA): AACR; Mol Cancer Ther 2015;14(12 Suppl 2):Abstract nr A67.

Synthesis and cytotoxicity of pyranonaphthoquinone natural product analogues under bioreductive conditions

We have synthesised a focused library of derivatives of natural products containing the pyranonaphthoquinone moiety including the first report of such a scaffold with an appended tetrazole functionality. Examples include kalafungin derivatives as well as analogues of nanaomycin and eleutherin. These compounds were assessed for cytotoxic activation by breast cancer cell lines engineered to express the prototypic human one- and two-electron quinone bioreductive enzymes, NADPH: cytochrome P450 oxidoreductase (POR) and

NAD(P)H

quinoneoxidoreductase 1 (NQO1; DT-diaphorase), respectively. Several compounds were observed to be cytotoxic at sub-micromolar level and a pattern of increased aerobic potency was observed in cells over expressing POR. A subset of analogues was assessed under anoxic conditions, where cytotoxicity was reduced, implicating redox cycling as a major mechanism of toxicity. The substrate specificity for reductive enzymes is relevant to the future design of bioreductive prodrugs to treat cancer.

Toward a high-throughput screening platform for directed evolution of enzymes that activate genotoxic prodrugs

Engineering of enzymes to more efficiently activate genotoxic prodrugs holds great potential for improving anticancer gene or antibody therapies. We report the development of a new, GFP-based, high-throughput screening platform to enable engineering of prodrug-activating enzymes by directed evolution. By fusing an inducible SOS promoter to an engineered GFP reporter gene, we were able to measure levels of DNA damage in intact Escherichia coli and separate cell populations by fluorescence activating cell sorting (FACS). In two FACS iterations, we were able to achieve a 90,000-fold enrichment of a functional prodrug-activating nitroreductase from a null library background.

Bioreductive prodrugs as cancer therapeutics: targeting tumor hypoxia

Hypoxia, a state of low oxygen, is a common feature of solid tumors and is associated with disease progression as well as resistance to radiotherapy and certain chemotherapeutic drugs. Hypoxic regions in tumors, therefore, represent attractive targets for cancer therapy. To date, five distinct classes of bioreactive prodrugs have been developed to target hypoxic cells in solid tumors. These hypoxia-activated prodrugs, including nitro compounds, N-oxides, quinones, and metal complexes, generally share a common mechanism of activation whereby they are reduced by intracellular oxidoreductases in an oxygen-sensitive manner to form cytotoxins. Several examples including PR-104, TH-302, and EO9 are currently undergoing phase II and phase III clinical evaluation. In this review, we discuss the nature of tumor hypoxia as a therapeutic target, focusing on the development of bioreductive prodrugs. We also describe the current knowledge of how each prodrug class is activated and detail the clinical progress of leading examples.

Zinc finger nuclease knock-out of NADPH:cytochrome P450 oxidoreductase (POR) in human tumor cell lines demonstrates that hypoxia-activated prodrugs differ in POR dependence

Hypoxia, a ubiquitous feature of tumors, can be exploited by hypoxia-activated prodrugs (HAP) that are substrates for one-electron reduction in the absence of oxygen. NADPH:cytochrome P450 oxidoreductase (POR) is considered one of the major enzymes responsible, based on studies using purified enzyme or forced overexpression in cell lines. To examine the role of POR in HAP activation at endogenous levels of expression, POR knock-outs were generated in HCT116 and SiHa cells by targeted mutation of exon 8 using zinc finger nucleases. Absolute quantitation by proteotypic peptide mass spectrometry of DNA sequence-confirmed multiallelic mutants demonstrated expression of proteins with residual one-electron reductase activity in some clones and identified two (Hko2 from HCT116 and S2ko1 from SiHa) that were functionally null by multiple criteria. Sensitivities of the clones to 11 HAP (six nitroaromatics, three benzotriazine N-oxides, and two quinones) were compared with wild-type and POR-overexpressing cells. All except the quinones were potentiated by POR overexpression. Knocking out POR had a marked effect on antiproliferative activity of the 5-nitroquinoline SN24349 in both genetic backgrounds after anoxic exposure but little or no effect on activity of most other HAP, including the clinical stage 2-nitroimidazole mustard TH-302, dinitrobenzamide mustard PR-104A, and benzotriazine N-oxide SN30000. Clonogenic cell killing and reductive metabolism of PR-104A and SN30000 under anoxia also showed little change in the POR knock-outs. Thus, although POR expression is a potential biomarker of sensitivity to some HAP, identification of other one-electron reductases responsible for HAP activation is needed for their rational clinical development.

Abstract B278: PR610: A novel hypoxia-selective tyrosine kinase inhibitor in phase I clinical trial

PR610 is a hypoxia-selective irreversible Human Epidermal Growth Factor Receptor (HER) family inhibitor currently in phase I clinical trials in New Zealand and the USA (clinical trial ID NCT01631279). The prodrug PR610 releases the TKI (Tyrosine Kinase Inhibitor) PR610E, a picomolar irreversible inhibitor of EGFR (HER1), under oxygen-limiting conditions typically found in solid tumors. Human neoplastic cell lines exposed to PR610 show an anoxia-selective anti-proliferative response that is associated with G1 arrest and induction of apoptosis arising from inhibition of EGFR auto-phosphorylation and downstream silencing of associated signal transduction pathways. PR610 is optimized for long tumor residency (T½ &gt;2 days); in a preclinical model of erlotinib-resistant NSCLC, a single injection of PR610 produces profound, global shutdown of signal transduction via EGFRT790M/L858R. Tumor PR610E concentrations were above cellular anti-proliferative IC50 concentrations for over 5 days, being more than sufficient to induce apoptosis via “oncogenic shock”.

Pharmacokinetic (PK) studies of PR610 show significant differences in species toxicokinetics. Both the rat and dog preclinical toxicology models display considerable systemic conversion of PR610 to PR610E (6% - 30% and 24% - 29% of prodrug AUC, respectively) with attendant symptoms of EGFR inhibition including acneiform skin rash and diarrhea. In contrast NIH-III nude mice display minimal circulating TKI (PR610E) relative to PR610 (1.5% - 1.7% of AUC) consistent with a substantially improved tolerance as judged by PR610 plasma AUCinf at the maximum tolerated dose (MTD). Human subjects from the phase I clinical trial consistently experience the lowest systemic levels of PR610E (Mean 1.06% ± 0.69%; range 0.34% - 2.7%; n=20) across dose levels ranging from 10 - 150 mg/m2. Notably, a dose of 150 mg/m2 in human subjects produces a PR610 plasma AUCinf equivalent to that measured for 30 mg/kg PR610 in NIH-III mice, a dose that is active in several HER-dependent subcutaneous tumor xenograft models. Collectively, these data indicate that PR610 has the desirable characteristics of a deactivated prodrug in human subjects and preclinical models predict that an active dose range has been reached in the phase I trial. PR610 is a first-in-class hypoxia-selective EGFR/HER2 inhibitor with exciting clinical potential.

Citation Information: Mol Cancer Ther 2013;12(11 Suppl):B278.

Citation Format: Adam V. Patterson, Jagdish Jaiswal, Kendall Carlin, Maria R. Abbattista, Christopher P. Guise, Shevan Silva, Ho Lee, Guo-Liang Lu, Robert F. Anderson, Teresa J. Melink, John C. Gutheil, Jeff B. Smaill. PR610: A novel hypoxia-selective tyrosine kinase inhibitor in phase I clinical trial. [abstract]. In: Proceedings of the AACR-NCI-EORTC International Conference: Molecular Targets and Cancer Therapeutics; 2013 Oct 19-23; Boston, MA. Philadelphia (PA): AACR; Mol Cancer Ther 2013;12(11 Suppl):Abstract nr B278.

Creation and screening of a multi-family bacterial oxidoreductase library to discover novel nitroreductases that efficiently activate the bioreductive prodrugs CB1954 and PR-104A

Two potentially complementary approaches to improve the anti-cancer strategy gene-directed enzyme prodrug therapy (GDEPT) are discovery of more efficient prodrug-activating enzymes, and development of more effective prodrugs. Here we demonstrate the utility of a flexible screening system based on the Escherichia coli SOS response to evaluate novel nitroreductase enzymes and prodrugs in concert. To achieve this, a library of 47 candidate genes representing 11 different oxidoreductase families was created and screened to identify the most efficient activators of two different nitroaromatic prodrugs, CB1954 and PR-104A. The most catalytically efficient nitroreductases were found in the NfsA and NfsB enzyme families, with NfsA homologues generally more active than NfsB. Some members of the AzoR, NemA and MdaB families also exhibited low-level activity with one or both prodrugs. The results of SOS screening in our optimised E. coli reporter strain SOS-R2 were generally predictive of the ability of nitroreductase candidates to sensitise E. coli to CB1954, and of the kcat/Km for each prodrug substrate at a purified protein level. However, we also found that not all nitroreductases express stably in human (HCT-116 colon carcinoma) cells, and that activity at a purified protein level did not necessarily predict activity in stably transfected HCT-116. These results highlight a need for all enzyme-prodrug partners for GDEPT to be assessed in the specific context of the vector and cell line that they are intended to target. Nonetheless, our oxidoreductase library and optimised screens provide valuable tools to identify preferred nitroreductase-prodrug combinations to advance to preclinical evaluation.

Targeted mutagenesis of the Vibrio fischeri flavin reductase FRase I to improve activation of the anticancer prodrug CB1954

Phase I/II cancer gene therapy trials of the Escherichia coli nitroreductase NfsB in partnership with the prodrug CB1954 [5-(aziridin-1-yl)-2,4-dinitrobenzamide] have indicated that CB1954 toxicity is dose-limiting at concentrations far below the enzyme K(M). Here we report that the flavin reductase FRase I from Vibrio fischeri is also a CB1954 nitroreductase, which has a substantially lower apparent K(M) than E. coli NfsB. To enhance the activity of FRase I with CB1954 we used targeted mutagenesis and an E. coli SOS reporter strain to engineer single- and multi-residue variants that possess a substantially reduced apparent K(M) and an increased k(cat)/K(M) relative to the wild type enzyme. In a bacteria-delivered model for enzyme prodrug therapy, the engineered FRase I variants were able to kill human colon carcinoma (HCT-116) cells at significantly lower CB1954 concentrations than wild type FRase I or E. coli NfsB.

Abstract A247: Mechanism of action of the hypoxia-activated irreversible pan-HER inhibitor SN29966

Hypoxia occurs in most human tumors and is associated with disease progression, treatment resistance and poor patient outcome. We have developed the hypoxia-activated prodrug SN29966, designed to release the irreversible pan-HER inhibitor SN29926, following one-electron reduction by hypoxic cells (Smaill et al, Mol Cancer Ther., 2009; 8(12 Suppl), C46). Pharmacokinetic (PK) studies in nude mice bearing A431 tumor xenografts indicated SN29966 has a long tumor half-life (&gt;3 days) and releases SN29926 in tumors. SN29966 demonstrated single agent activity in nude mice bearing A431 and SKOV3 xenografts, inducing striking tumor regressions in both models (Patterson et al, Mol Cancer Ther., 2009; 8(12 Suppl), B76). PR509 and PR610, clinical candidates developed from SN29966, are currently undergoing comparative evaluation with Phase I trials anticipated in early 2012.

The single-agent antitumor activity of SN29966 is arguably counter-intuitive given that it is designed to target hypoxic cells within tumors. This activity may arise from a number of contributing mechanisms including; (i) bioactivity of the unreduced prodrug; (ii) local redistribution of released inhibitor in the tumor; (iii) liver metabolism and circulating inhibitor and (iv) a long tumor half-life allowing for targeting of both chronic and cycling hypoxia. To critically assess the relative contribution of each to the mechanism of action of SN29966 we performed a number of studies. We prepared SN31950, a prodrug of SN29926 designed to be incapable of one-electron fragmentation. In target modulation and anti-proliferative assays SN31950 showed no hypoxia-dependent activity. The murine A431 tumor PK of SN29966 and SN31950 demonstrated that at an equimolar dose (20 μmol/kg, ip), both prodrugs gave comparable tumor exposures (AUC0–72h: SN31950, 50 μmol*h/kg; SN29966, 57 μmol*h/kg). In contrast, the tumor exposure of SN29926 released from each prodrug differed by 40-fold (AUC0–72h: SN29926 from SN31950, 0.3 μmol*h/kg; SN29926 from SN29966, 12 μmol*h/kg). Plasma exposure of each prodrug was comparable, as were levels of SN29926 in plasma (presumed mainly due to hepatic prodrug metabolism). Consistent with the observed lack of inhibitor release in A431 tumors, SN31950 was inactive against A431 tumors in growth delay assays. To confirm the hypoxia-dependent nature of SN29966 inhibitor release in A431 tumors we re-oxygenated tumors in mice breathing 100% oxygen at 2.5 atm in a hyperbaric chamber. Accordingly, mice showed a marked reduction (56%, p&lt;0.001) in the concentration of released inhibitor in tumor relative to air breathing controls, indicating SN29966 tumor metabolism is hypoxia-dependent. Conversely, post-mortem anoxia (2 h, 37 °C) in mice 24 hours after SN29966 administration doubled the concentration of released inhibitor in tumor, indicating SN29966 is bioavailable for metabolism to release inhibitor as de novo hypoxia arises. Collectively the data support the conclusion that the single-agent activity of SN29966 results primarily from hypoxia-dependent tumor metabolism and inhibitor release. Inhibitor bystander effect and exploitation of both chronic and cycling hypoxia, thereby increasing the target cell population, are likely to play important roles. Liver metabolism, circulating inhibitor and activity of unreduced prodrug appear to be less important.

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 A247.

Abstract LB-297: Characterization of novel hypoxia-activated prodrugs of irreversible pan-HER inhibitors

Reversible HER1 (EGFR) inhibitors erlotinib and gefitinib are approved for use in non-small cell lung cancer (NSCLC), particularly demonstrating activity against tumors expressing mutant forms of HER1. Unfortunately, relapse invariably occurs in this patient population, with approximately half of patients having acquired an additional T790M mutation in HER1. Irreversible pan-HER inhibitors (BIBW2992, PF00299804) are under clinical evaluation in the context of NSCLC that has relapsed post-erlotinib/gefitinib treatment. However it is currently unclear whether these agents possess the therapeutic index in man necessary to gain approval in this setting (Regales et al., J. Clin. Invest. 2009, 119:3000-10), prompting the development of HER1T790M mutant-selective irreversible inhibitors (Zhou et al., Nature. 2009, 462: 1070-74).

Hypoxia occurs in most human tumors and is associated with disease progression, resistance to conventional therapies and poor patient outcome. It can however be considered as an exploitable physiological target, as it supports tumor-selective bioreduction of prodrugs. We have developed hypoxia-activated prodrugs of irreversible pan-HER inhibitors as a strategy to broaden their therapeutic index. SN29966 is a prototype nitromethylaryl quaternary (NMQ) ammonium salt prodrug of an irreversible pan-HER inhibitor (SN29926), with masked cellular activity. Radiolytic reduction of SN29966 demonstrated fragmentation of the one-electron adduct to release SN29926 [kfrag 130 ± 10 s-1], and hypoxia-selective metabolism to SN29926 has been confirmed in all cell lines tested.

SN32807 is the lead prodrug of this series, releasing the irreversible pan-HER inhibitor SN32793 in a hypoxia-dependent manner. In a panel of HER1/HER1T790M/HER2-expressing cell lines SN32807 showed hypoxia-dependent inhibition of proliferation (hypoxic/oxic IC50 ratios of 60, 11 and 20 in A431, H1975 and SKOV3 cells, respectively), a property SN32793 lacked (IC50 ratios 0.8-1.6). Tumor growth delay experiments (NIH-III nude mice) in large (500 mm3) HER1T790M-expressing H1975 tumor xenografts showed BIBW2992 (20 mg/kg, p.o. daily) failed to control tumor growth (i.e. progressive disease), with only a modest Tumor Growth Delay value of 35% [TGD % = [(T-C)/C]x100 where T and C are time to 4-fold increase in tumor volume from treatment day-1 for treated and control tumors, respectively]. In contrast, single-agent SN32807 (88 mg/kg; ip, q3dx8) demonstrated complete tumor responses in all animals during the treatment period (TGD value 246%; P&lt;0.0005 vs controls; P&lt;0.0005 vs BIBW-2992; log-rank test). Collectively the data indicates that SN32807 possesses a much improved therapeutic index relative to existing HER-family inhibitors, demonstrating marked activity against a HER1T790M-expressing erlotinib, gefitinib and BIBW2992 resistant tumor xenograft model.

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 LB-297.

Abstract C46: Design and identification of the novel hypoxia-activated irreversible pan-HER inhibitor SN29966

Several irreversible pan-HER inhibitors (HKI-272, BIBW-2992, PF299) are under development and demonstrate encouraging activity against erlotinib resistant non-small cell lung cancer expressing mutant forms of HER1. However, dose limiting toxicities mirror that of erlotinib and are attributed to inhibition of HER1 in gastrointestinal and skin tissues. To introduce additional tumor selectivity to irreversible pan-HER inhibitors and thereby broaden their therapeutic index, we have sought to utilise tumor hypoxia as a physiological target that supports tumor-selective bioreduction. To achieve this we have developed a series of hypoxia-activated prodrugs of the known irreversible pan-HER inhibitor SN29926.

Five nitromethylaryl quaternary ammonium bromide (NMQ) prodrugs were synthesised by quaternizing the tertiary amine of SN29926 with five nitroheterocyclic alpha-methyl bromides. Three further quaternary salts were prepared as Chemical-Biology Tools (CBTs) designed as controls to probe aspects of the mechanism of action of the NMQ prodrugs. The NMQ prodrugs and CBTs were compared, relative to SN29926, in a range of assays to identify a lead compound that (i) is deactivated under oxic conditions with respect to A431 cellular HER1 target modulation and proliferation, (ii) efficiently fragments following one-electron reduction in an oxygen inhibited manner to release the irreversible pan-HER inhibitor, (iii) displays increased anti-proliferative activity against A431 cells under hypoxia.

All of the NMQ/CBT prodrugs were successfully deactivated with respect to inhibition of A431 HER1 autophosphorylation (62- to 201-fold) and proliferation under oxic conditions (12- to 294-fold). Pulse and steady-state radiolysis under nitrogen determined their one-electron reduction potentials (−388 to −493 mV) and demonstrated that only two of the NMQ prodrugs (SN29965, SN29966) efficiently fragment following one-electron reduction (first-order rate constants of 90 and 130 s−1, respectively). SN29966 alone demonstrated significant anti-proliferative activity against A431 cells under hypoxia (hypoxic/oxic IC50 ratio 38). Preliminary growth delay screening of SN29966 and the CBTs in advanced A431 xenografts (∼600 mm3; hypoxic fraction of 32% ± 13%) demonstrated that the CBTs lacked efficacy, while SN29966 induced tumor regressions, with greater than 30 days tumor control using a well tolerated three-dose schedule (133 umol/kg/dose, q4dx3). In summary, prodrug SN29966 is deactivated relative to the parent inhibitor, efficiently fragments following one-electron reduction, is selective against hypoxic A431 cells and has remarkable single-agent activity against hypoxic A431 xenografts using a conservative treatment schedule. We therefore identify SN29966 as a “first-in-class” hypoxia-activated irreversible pan-HER inhibitor that has significant clinical potential.

Citation Information: Mol Cancer Ther 2009;8(12 Suppl):C46.