Syntheses, antiproliferative activity and theoretical characterization of acitretin-type retinoids with changes in the lipophilic part

A Modular Approach for the Synthesis of Natural and Artificial Terpenoids

Many terpenoids with isoprene unit(s) demonstrating critical biological activities have been isolated and characterized. In this study, we have developed a robust chem-stamp strategy for the construction of the key isoprene unit, which consists of two steps: one-carbon extension of aldehydes to the alkenyl boronates by the boron-Wittig reaction and the rhodium-catalyzed reaction of alkenyl boronates with 2,3-allenols to yield enals. This chem-stamp could readily be applied repeatedly and separately, enabling the modular concise synthesis of many natural and pharmaceutically active terpenoids, including retinal, β-carotene, vitamin A, tretinoin, fenretinide, acitretin, ALRT1550, nigerapyrone C, peretinoin, and lycopene. Owing to the diversified availability of the starting materials, aldehydes and 2,3-allenols, creation of new non-natural terpenoids has been realized from four dimensions: the number of isoprene units, the side chain, and the two terminal groups.

Synthesis of Novel Pyrazolo[3,4-b]pyridines with Affinity for β-Amyloid Plaques

Three novel pyrazolo[3,4-b]pyridines were synthesized via the cyclization of 5-amino-1-phenylpyrazole with the corresponding unsaturated ketone in the catalytic presence of ZrCl4. The ketones were afforded by modifying a stabilized ylide facilitated Wittig reaction in fairly high yields. The novel compounds exhibited exciting photophysical properties with the dimethylamine phenyl-bearing pyrazolopyridine showing exceptionally large Stoke’s shifts. Finally, both the dimethylamino- and the pyrene-substituted compounds demonstrated high and selective binding to amyloid plaques of Alzheimer’s disease (AD) patient brain slices upon fluorescent confocal microscopy observation. These results reveal the potential application of pyrazolo[3,4-b]pyridines in the development of AD amyloid plaque probes of various modalities for AD diagnosis.

Modular Total Synthesis of iso-Archazolids and Archazologs

Full details on the design, development, and successful implementation of suitable synthetic strategies directed toward the total synthesis of iso-archazolids and archazologs are reported. Both a biomimetic and a multistep total synthesis of iso-archazolid B, the most potent and least abundant archazolid, are described. The bioinspired conversion from archazolid B was realized by a high-yielding 1,8-Diazabicyclo[5.4.0]undec-7-ene catalyzed one-step double-bond shift. A highly stereoselective total synthesis was accomplished in 25 steps, involving a sequence of highly stereoselective aldol reactions, an efficient aldol condensation to forge two elaborate fragments, and a challenging ring-closing metathesis macrocyclization with an unusual Stewart-Grubbs catalyst. These strategies proved to be generally useful and could be successfully implemented for the preparation of three novel iso-archazolids as well as five novel archazologs, lacking the thiazole side chain. A wide variety of further archazolids and archazologs may now be targeted for exploration of the promising anticancer potential of these polyketide macrolides.

Synthesis and biological evaluation of novel withangulatin A derivatives as potential anticancer agents

Novel withangulatin A (WA) derivatives were synthesized and evaluated for antiproliferative activity against four human cancer cell lines (U2OS, MDA-MB-231, HepG2, and A549). Among these derivatives, 10 exhibited the most potent antiproliferative activity, with an IC₅₀ value of 74.0 nM against the human breast cancer cell line MDA-MB-231 and potency that was 70-fold that of WA (IC₅₀ = 5.22 µM). Moreover, 10 caused G2-phase cell cycle arrest in a concentration-dependent manner and induced the apoptosis of MDA-MB-231 cells by increasing intracellular reactive oxygen species (ROS). Compound 10 showed a high selectivity index (SI = 267.03) for breast cancer MDA-MB-231 cells. These results suggest that 10 is a promising anticancer agent.

Synthesis of isotopically labeled all-trans retinals for DNP-enhanced solid-state NMR studies of retinylidene proteins

Three all-trans retinals containing multiple ¹³ C labels have been synthesized to enable dynamic nuclear polarization enhanced solid-state magic angle spinning NMR studies of novel microbial retinylidene membrane proteins including proteorhodpsin and channelrhodopsin. The synthetic approaches allowed specific introduction of ¹³ C labels in ring substituents and at different positions in the polyene chain to probe structural features such as ring orientation and interaction of the chromophore with the protein in the ground state and in photointermediates. [10-18-¹³ C₉ ]-All-trans-retinal (1b), [12,15-¹³ C₂ ]-all-trans-retinal (1c), and [14,15-¹³ C₂ ]-all-trans-retinal (1d) were synthesized in in 12, 8, and 7 linear steps from ethyl 2-oxocyclohexanecarboxylate (5) or β-ionone (4), respectively.

Oxygenated N-Acyl Alanine Methyl Esters (NAMEs) from the Marine Bacterium Roseovarius tolerans EL-164

The marine bacterium Roseovarius tolerans EL-164 (Rhodobacteraceae) can produce unique N-acylalanine methyl esters (NAMEs) besides strucutrally related N-acylhomoserine lactones (AHLs), bacterial signaling compounds widespread in the Rhodobacteraceae. The structures of two unprecedented NAMEs carrying a rare terminally oxidized acyl chain are reported here. The compounds (Z)-N-16-hydroxyhexadec-9-enoyl-l-alanine methyl ester (Z9-16-OH-C16:1-NAME, 3) and (Z)-N-15-carboxypentadec-9-enoyl-l-alanine methyl ester (16COOH-C16:1-NAME, 4) were isolated, and the structures were determined by NMR and MS experiments. Both compounds were synthesized to prove assignments and to test their biological activity. Finally, non-natural, structurally related Z9-3-OH-C16:1-NAME (18) was synthesized to investigate the mass spectroscopy of structurally related NAMEs. Compound 3 showed moderate antibacterial activity against microorganisms such as Bacillus, Streptococcus, Micrococcus, or Mucor strains. In contrast to AHLs, quorum-sensing or quorum-quenching activity was not observed.

Synthesis of apo-13- and apo-15-lycopenoids, cleavage products of lycopene that are retinoic acid antagonists

Consumption of the tomato carotenoid, lycopene, has been associated with favorable health benefits. Some of lycopene's biological activity may be due to metabolites resulting from cleavage of the lycopene molecule. Because of their structural similarity to the retinoic acid receptor (RAR) antagonist, β-apo-13-carotenone, the "first half" putative oxidative cleavage products of the symmetrical lycopene have been synthesized. All transformations proceed in moderate to good yield and some with high stereochemical integrity allowing ready access to these otherwise difficult to obtain terpenoids. In particular, the methods described allow ready access to the trans isomers of citral (geranial) and pseudoionone, important flavor and fragrance compounds that are not readily available isomerically pure and are building blocks for many of the longer apolycopenoids. In addition, all of the apo-11, apo-13, and apo-15 lycopenals/lycopenones/lycopenoic acids have been prepared. These compounds have been evaluated for their effect on RAR-induced genes in cultured hepatoma cells and, much like β-apo-13-carotenone, the comparable apo-13-lycopenone and the apo-15-lycopenal behave as RAR antagonists. Furthermore, molecular modeling studies demonstrate that the apo-13-lycopenone efficiently docked into the ligand binding site of RARα. Finally, isothermal titration calorimetry studies reveal that apo-13-lycopenone acts as an antagonist of RAR by inhibiting coactivator recruitment to the receptor.

Trimodal Therapy: Combining Hyperthermia with Repurposed Bexarotene and Ultrasound for Treating Liver Cancer

Repurposing of existing cancer drugs to overcome their physical limitations, such as insolubility, represents an attractive strategy to achieve enhanced therapeutic efficacy and broaden the range of clinical applications. Such an approach also promises to offer substantial cost savings in drug development efforts. Here we repurposed FDA-approved topical agent bexarotene (Targretin), currently in limited use for cutaneous manifestations of T-cell lymphomas, and re-engineer it for use in solid tumor applications by forming self-assembling nanobubbles. Physico-chemical characterization studies of the novel prodrug nanobubbles demonstrated their stability, enhanced target cell internalization capability, and highly controlled release profile in response to application of focused ultrasound energy. Using an in vitro model of hepatocellular carcinoma and an in vivo large animal model of liver ablation, we demonstrate the effectiveness of bexarotene prodrug nanobubbles when used in conjunction with catheter-based ultrasound, thereby highlighting the therapeutic promise of this trimodal approach.

Global optimization-based inference of chemogenomic features from drug-target interactions

MOTIVATION

Gaining insight into chemogenomic drug-target interactions, such as those involving the substructures of synthetic drugs and protein domains, is important in fragment-based drug discovery and drug repositioning. Previous studies evaluated the interactions locally, thereby ignoring the competitive effects of different substructures or domains, but this could lead to high false-positive estimation, calling for a computational method that presents more predictive power.

RESULTS

A statistical model, termed Global optimization-based InFerence of chemogenomic features from drug-Target interactions, or GIFT, is proposed herein to evaluate substructure-domain interactions globally such that all substructure-domain contributions to drug-target interaction are analyzed simultaneously. Combinations of different chemical substructures were included since they may function as one unit. When compared to previous methods, GIFT showed better interpretive performance, and performance for the recovery of drug-target interactions was good. Among 53 known drug-domain interactions, 81% were accurately predicted by GIFT. Eighteen of the top 100 predicted combined substructure-domain interactions had corresponding drug-target structures in the Protein Data Bank database, and 15 out of the 18 had been proved. GIFT was then implemented to predict substructure-domain interactions based on drug repositioning. For example, the anticancer activities of tazarotene, adapalene, acitretin and raloxifene were identified. In summary, GIFT is a global chemogenomic inference approach and offers fresh insight into drug-target interactions.

Preparation and toxicological assessment of functionalized carbon nanotube-polymer hybrids

Novel Carbon Nanotube-Polymer Hybrids were synthesized as potential materials for the development of membranes for water treatment applications in the field of Membrane Bioreactors (MBRs). Due to the toxicological concerns regarding the use of nanomaterials in water treatment as well as the rising demand for safe drinking water to protect public health, we studied the functionalization of MWCNTs and Thin-MWCNTs as to control their properties and increase their ability of embedment into porous anisotropic polymeric membranes. Following the growth of the hydrophilic monomer on the surface of the properly functionalized CNTs, that act as initiator for the controlled radical polymerization (ATRP) of sodium styrene sulfonate (SSNa), the antimicrobial quaternized phosphonium and ammonium salts were attached on CNTs-g-PSSNa through non-covalent bonding. In another approach the covalent attachment of quaternized ammonium polymeric moieties of acrylic acid-vinyl benzyl chloride copolymers with N,N-dimethylhexadecylamine (P(AA12-co-VBCHAM)) on functionalized CNTs has also been attempted. Finally, the toxicological assessment in terms of cell viability and cell morphological changes revealed that surface characteristics play a major role in the biological response of functionalized CNTs.

The EF loop in green proteorhodopsin affects conformation and photocycle dynamics

The proteorhodopsin family consists of retinal proteins of marine bacterial origin with optical properties adjusted to their local environments. For green proteorhodopsin, a highly specific mutation in the EF loop, A178R, has been found to cause a surprisingly large redshift of 20 nm despite its distance from the chromophore. Here, we analyze structural and functional consequences of this EF loop mutation by time-resolved optical spectroscopy and solid-state NMR. We found that the primary photoreaction and the formation of the K-like photo intermediate is almost pH-independent and slower compared to the wild-type, whereas the decay of the K-intermediate is accelerated, suggesting structural changes within the counterion complex upon mutation. The photocycle is significantly elongated mainly due to an enlarged lifetime of late photo intermediates. Multidimensional MAS-NMR reveals mutation-induced chemical shift changes propagating from the EF loop to the chromophore binding pocket, whereas dynamic nuclear polarization-enhanced (13)C-double quantum MAS-NMR has been used to probe directly the retinylidene conformation. Our data show a modified interaction network between chromophore, Schiff base, and counterion complex explaining the altered optical and kinetic properties. In particular, the mutation-induced distorted structure in the EF loop weakens interactions, which help reorienting helix F during the reprotonation step explaining the slower photocycle. These data lead to the conclusion that the EF loop plays an important role in proton uptake from the cytoplasm but our data also reveal a clear interaction pathway between the EF loop and retinal binding pocket, which might be an evolutionary conserved communication pathway in retinal proteins.

Retinoic acid signaling pathways in development and diseases

Retinoids comprise a group of compounds each composed of three basic parts: a trimethylated cyclohexene ring that is a bulky hydrophobic group, a conjugated tetraene side chain that functions as a linker unit, and a polar carbon-oxygen functional group. Biochemical conversion of carotenoid or other retinoids to retinoic acid (RA) is essential for normal regulation of a wide range of biological processes including development, differentiation, proliferation, and apoptosis. Retinoids regulate various physiological outputs by binding to nuclear receptors called retinoic acid receptors (RARs) and retinoid X receptors (RXRs), which themselves are DNA-binding transcriptional regulators. The functional response of RA and their receptors are modulated by a host of coactivators and corepressors. Retinoids are essential in the development and function of several organ systems; however, deregulated retinoid signaling can contribute to serious diseases. Several natural and synthetic retinoids are in clinical use or undergoing trials for treating specific diseases including cancer. In this review, we provide a broad overview on the importance of retinoids in development and various diseases, highlighting various retinoids in the drug discovery process, ranging all the way from retinoid chemistry to clinical uses and imaging.