Low level of antioxidant capacity biomarkers but not target overexpression predicts vulnerability to ROS-inducing drugs

Despite a strong rationale for why cancer cells are susceptible to redox-targeting drugs, such drugs often face tumor resistance or dose-limiting toxicity in preclinical and clinical studies. An important reason is the lack of specific biomarkers to better select susceptible cancer entities and stratify patients. Using a large panel of lung cancer cell lines, we identified a set of "antioxidant-capacity" biomarkers (ACB), which were tightly repressed, partly by STAT3 and STAT5A/B in sensitive cells, rendering them susceptible to multiple redox-targeting and ferroptosis-inducing drugs. Contrary to expectation, constitutively low ACB expression was not associated with an increased steady state level of reactive oxygen species (ROS) but a high level of nitric oxide, which is required to sustain high replication rates. Using ACBs, we identified cancer entities with a high percentage of patients with favorable ACB expression pattern, making it likely that more responders to ROS-inducing drugs could be stratified for clinical trials.

Inclusion Complexes of Gold(I)-Dithiocarbamates with β-Cyclodextrin: A Journey from Drug Repurposing towards Drug Discovery

The gold(I)-dithiocarbamate (dtc) complex [Au(N,N-diethyl)dtc]2 was identified as the active cytotoxic agent in the combination treatment of sodium aurothiomalate and disulfiram on a panel of cancer cell lines. In addition to demonstrating pronounced differential cytotoxicity to these cell lines, the gold complex showed no cross-resistance in therapy-surviving cancer cells. In the course of a medicinal chemistry campaign on this class of poorly soluble gold(I)-dtc complexes, >35 derivatives were synthesized and X-ray crystallography was used to examine structural aspects of the dtc moiety. A group of hydroxy-substituted complexes has an improved solubility profile, and it was found that these complexes form 2 : 1 host-guest inclusion complexes with β-cyclodextrin (CD), exhibiting a rarely observed "tail-to-tail" arrangement of the CD cones. Formulation of a hydroxy-substituted gold(I)-dtc complex with excess sulfobutylether-β-CD prevents the induction of mitochondrial reactive oxygen species, which is a major burden in the development of metallodrugs.

Structural Basis for the Selective Inhibition of HDAC10, the Cytosolic Polyamine Deacetylase

The cytosolic class IIb histone deacetylase HDAC10 is an emerging target for drug design. As an inducer of autophagy, its selective inhibition suppresses the autophagic response that otherwise attenuates the efficacy of cytotoxic cancer chemotherapy drugs. HDAC10 is a zinc-dependent polyamine deacetylase exhibiting maximal catalytic activity against N⁸-acetylspermidine. As revealed in the structure of Danio rerio (zebrafish) HDAC10, two conserved structural motifs direct this narrow substrate specificity: a 3₁₀ helix containing the P(E,A)CE motif that sterically constricts the active site and an electrostatic "gatekeeper," E274, that confers selectivity for cationic polyamine substrates. To accelerate drug design efforts targeting human HDAC10, we now report the preparation of "humanized" zebrafish HDAC10 in which two amino acid substitutions, A24E and D94A, yield an active site contour more similar to that of human HDAC10. X-ray crystal structures of this HDAC10 variant complexed with Tubastatin A and indole analogues bearing pendant tertiary amines reveal that inhibitors capable of hydrogen bonding with gatekeeper E274 exhibit high affinity and selectivity for HDAC10 over HDAC6 (the other class IIb isozyme). Moreover, these structures reveal that the P(E,A)CE motif helix can shift by up to 2 Å to accommodate the binding of bulky inhibitors. Thus, slender polyamine-like inhibitor structures are not exclusively required for selective, high affinity binding to HDAC10. Indeed, the flexibility of the P(E,A)CE motif helix could conceivably enable the binding of certain protein substrates.

Design and Synthesis of Dihydroxamic Acids as HDAC6/8/10 Inhibitors

We report the synthesis and evaluation of a class of selective multitarget agents for the inhibition of HDAC6, HDAC8, and HDAC10. The concept for this study grew out of a structural analysis of the two selective inhibitors Tubastatin A (HDAC6/10) and PCI-34051 (HDAC8), which we recognized share the same N-benzylindole core. Hybridization of the two inhibitor structures resulted in dihydroxamic acids with benzyl-indole and -indazole core motifs. These substances exhibit potent activity against HDAC6, HDAC8, and HDAC10, while retaining selectivity over HDAC1, HDAC2, and HDAC3. The best substance inhibited the viability of the SK-N-BE(2)C neuroblastoma cell line with an IC50 value similar to a combination treatment with Tubastatin A and PCI-34051. This compound class establishes a proof of concept for such hybrid molecules and could serve as a starting point for the further development of enhanced HDAC6/8/10 inhibitors.

Selective Inhibition of Histone Deacetylase 10: Hydrogen Bonding to the Gatekeeper Residue is Implicated

The discovery of isozyme-selective histone deacetylase (HDAC) inhibitors is critical for understanding the biological functions of individual HDACs and for validating HDACs as drug targets. The isozyme HDAC10 contributes to chemotherapy resistance and has recently been described to be a polyamine deacetylase, but no studies toward selective HDAC10 inhibitors have been published. Using two complementary assays, we found Tubastatin A, an HDAC6 inhibitor, to potently bind HDAC10. We synthesized Tubastatin A derivatives and found that a basic amine in the cap group was required for strong HDAC10 binding. HDAC10 inhibitors mimicked knockdown by causing dose-dependent accumulation of acidic vesicles in a neuroblastoma cell line. Furthermore, docking into human HDAC10 homology models indicated that a hydrogen bond between a cap group nitrogen and the gatekeeper residue Glu272 was responsible for potent HDAC10 binding. Taken together, our data provide an optimal platform for the development of HDAC10-selective inhibitors, as exemplified with the Tubastatin A scaffold.

Mechanistic Study of Belinostat Oral Absorption From Spray-Dried Dispersions

Spray-dried dispersions (SDDs) are an important technology for enhancing the oral bioavailability of poorly water-soluble drugs. To design an effective oral SDD formulation, the key rate-determining step(s) for oral drug absorption must be understood. This work combined in vivo and in vitro tests with in silico modeling to identify the rate-determining steps for oral absorption of belinostat SDDs made with 3 different polymers (PVP K30, PVP VA64, and HPMCAS-M). The goal was developing a belinostat SDD formulation that maximizes oral bioavailability (ideally matching the performance of a belinostat oral solution) and defining critical performance attributes for formulation optimization. The in vivo pharmacokinetic study with beagle dogs demonstrated that 1 of the 3 SDDs (PVP K30 SDD) matched the performance of the oral solution. In vitro data coupled with in silico modeling elucidated differences among the SDDs and supported the hypothesis that absorption of belinostat in the small intestine from the other 2 SDDs (PVP VA64 and HPMCAS-M) may be limited by dissolution rate or reduced drug activity (maximum concentration) in the presence of polymer. It was concluded that drug concentration in the stomach before emptying into the proximal intestine is a key factor for maximizing in vivo performance.

Lipophilic salts of poorly soluble compounds to enable high-dose lipidic SEDDS formulations in drug discovery

Self-emulsifying drug delivery systems (SEDDS) have been used to solubilize poorly water-soluble drugs to improve exposure in high-dose pharmacokinetic (PK) and toxicokinetic (TK) studies. However, the absorbable dose is often limited by drug solubility in the lipidic SEDDS vehicle. This study focuses on increasing solubility and drug loading of ionizable drugs in SEDDS vehicles using lipophilic counterions to prepare lipophilic salts of drugs. SEDDS formulations of two lipophilic salts-atazanavir-2-naphthalene sulfonic acid (ATV-2-NSA) and atazanavir-dioctyl sulfosuccinic acid (ATV-Doc)-were characterized and their performance compared to atazanavir (ATV) free base formulated as an aqueous crystalline suspension, an organic solution, and a SEDDS suspension, using in vitro, in vivo, and in silico methods. ATV-2-NSA exhibited ∼6-fold increased solubility in a SEDDS vehicle, allowing emulsion dosing at 12mg/mL. In rat PK studies at 60mg/kg, the ATV-2-NSA SEDDS emulsion had comparable exposure to the free-base solution, but with less variability, and had better exposure at high dose than aqueous suspensions of ATV free base. Trends in dose-dependent exposure for various formulations were consistent with GastroPlus™ modeling. Results suggest use of lipophilic salts is a valuable approach for delivering poorly soluble compounds at high doses in Discovery.

Spiroepoxytriazoles Are Fumagillin-like Irreversible Inhibitors of MetAP2 with Potent Cellular Activity

Methionine aminopeptidases (MetAPs) are responsible for the cotranslational cleavage of initiator methionines from nascent proteins. The MetAP2 subtype is up-regulated in many cancers, and selective inhibition of MetAP2 suppresses both vascularization and growth of tumors in animal models. The natural product fumagillin is a selective and potent irreversible inhibitor of MetAP2, and semisynthetic derivatives of fumagillin have shown promise in clinical studies for the treatment of cancer, and, more recently, for obesity. Further development of fumagillin derivatives has been complicated, however, by their generally poor pharmacokinetics. In an attempt to overcome these limitations, we developed an easily diversifiable synthesis of a novel class of MetAP2 inhibitors that were designed to mimic fumagillin's molecular scaffold but have improved pharmacological profiles. These substances were found to be potent and selective inhibitors of MetAP2, as demonstrated in biochemical enzymatic assays against three MetAP isoforms. Inhibitors with the same relative and absolute stereoconfiguration as fumagillin displayed significantly higher activity than their diastereomeric and enantiomeric isomers. X-ray crystallographic analysis revealed that the inhibitors covalently modify His231 in the MetAP2 active site via ring-opening of a spiroepoxide. Biochemically active substances inhibited the growth of endothelial cells and a MetAP2-sensitive cancer cell line, while closely related inactive isomers had little effect on the proliferation of either cell type. These effects correlated with altered N-terminal processing of the protein 14-3-3-γ. Finally, selected substances were found to have improved stabilities in mouse plasma and microsomes relative to the clinically investigated fumagillin derivative beloranib.

Cu(II)-selective bispidine-dye conjugates

The substitution of tetradentate bispidine ligands with rhodamine and cyanine dye molecules, coupled to an amine donor, forming an amide as potential fifth donor, is described. Bispidines are known to lead to very stable Cu(II) complexes, and the coordination to Cu(II) was expected to efficiently quench the fluorescence of dye molecules. However, at physiological pH the amide is not coordinated, as shown by titration experiments and crystallographic structural data of three possible isomers of these complexes. This may be due to the specific cavity shape of bispidines and the Jahn-Teller lability of the Cu(II) center. While Cu(II) coordination in aqueous solution leads to efficient fluorescence quenching, experiments show that the complex stabilities are not large enough for Cu(II) sensing in biological media, and possibilities are discussed, how this may be achieved by optimized bispidine-dye conjugates.

Polymeric nanoparticles for increased oral bioavailability and rapid absorption using celecoxib as a model of a low-solubility, high-permeability drug

Purpose

To demonstrate drug/polymer nanoparticles can increase the rate and extent of oral absorption of a low-solubility, high-permeability drug.

Methods

Amorphous drug/polymer nanoparticles containing celecoxib were prepared using ethyl cellulose and either sodium caseinate or bile salt. Nanoparticles were characterized using dynamic light scattering, transmission and scanning electron microscopy, and differential scanning calorimetry. Drug release and resuspension studies were performed using high-performance liquid chromatography. Pharmacokinetic studies were performed in dogs and humans.

Results

A physical model is presented describing the nanoparticle state of matter and release performance. Nanoparticles dosed orally in aqueous suspensions provided higher systemic exposure and faster attainment of peak plasma concentrations than commercial capsules, with median time to maximum drug concentration (Tmax) of 0.75 h in humans for nanoparticles vs. 3 h for commercial capsules. Nanoparticles released celecoxib rapidly and provided higher dissolved-drug concentrations than micronized crystalline drug. Nanoparticle suspensions are stable for several days and can be spray-dried to form dry powders that resuspend in water.

Conclusions

Drug/polymer nanoparticles are well suited for providing rapid oral absorption and increased bioavailability of BCS Class II drugs.