Phototoxicity─Medicinal Chemistry Strategies for Risk Mitigation in Drug Discovery.

Phototoxicity is a common safety concern encountered by project teams in pharmaceutical research and has the potential to stop progression of an otherwise promising candidate molecule. This perspective aims to provide an overview of the approaches toward mitigation of phototoxicity that medicinal chemists have taken during the lead optimization phase in the context of regulatory standards for photosafety evaluation. Various strategies are laid out based on available literature examples in order to highlight how structural modification can be utilized toward successful mitigation of a phototoxicity liability. A proposed flowchart is presented as a guidance tool to be used by the practicing medicinal chemist when facing a phototoxicity risk. The description of available tools to consider in the drug design process will include an overview of the evolution of in silico methods and their application as well as structure alerts for consideration as potential phototoxicophores.

Discovery of a novel, highly potent and orally bioavailable pyrrolidinone indole series of irreversible Myeloperoxidase (MPO) inhibitors.

Myeloperoxidase (MPO) is a heme-containing peroxidase from phagocytic cells, which plays an important role in the innate immune response. The primary anti-microbial function of MPO is achieved by catalyzing the oxidation of halides by hydrogen peroxide (H2O2). Upon activation of phagocytes, MPO activity is detectable in both phagosomes and extracellularly, where it can remain or transcytose into interstitial compartments. Activated MPO leads to oxidative stress and tissue damage in many inflammatory states, including cardiovascular disease. Starting from a low molecular weight (LMW) high throughput screening (HTS) hit, here we report the discovery of a novel pyrrolidinone indole (IN-4) as a highly potent MPO inhibitor. This compound displays similar in vitro potency across peroxidation, plasma and NETosis assays. In a dilution/dialysis study, <5% of the original MPO activity was detected post-incubation of MPO with IN-4, suggesting irreversible enzyme inhibition. A fast MPO inactivation rate (kinact/Ki) and low partition ratio (k3/k4) make IN-4 kinetic properties attractive for an MPO inhibitor. This compound also displays significant selectivity over the closely related thyroid peroxidase (TPO), and is selective for extracellular MPO over intracellular (neutrophil) MPO. Moreover, IN-4 shows good exposure, low clearance and high oral bioavailability in mice, rats and dogs. The high in vitro MPO activity and high oral exposure observed with IN-4 result in a dose-dependent inhibition of MPO activity in three mouse models of inflammation. In conclusion, IN-4 is a novel, potent, mechanism-based and selective MPO inhibitor, which may be used as superior therapeutic agent to treat multiple inflammatory conditions, including cardiovascular disease.

Discovery of a novel indole pharmacophore for the irreversible inhibition of myeloperoxidase (MPO).

Myeloperoxidase (MPO) activity and subsequent generation of hypochlorous acid has been associated with the killing of host-invading microorganisms (e.g. bacteria, viruses, and fungi). However, during oxidative stress, high MPO activity can damage host tissue and is linked to several chronic inflammatory conditions. Herein, we describe the development of a novel biaryl, indole-pyrazole series of irreversible mechanism-based inhibitors of MPO. Derived from an indole-containing high-throughput screen hit, optimization efforts resulted in potent and selective 6-substituted indoles with good oral bioavailability and in vivo activity.

Successful Strategies for Mitigation of a Preclinical Signal for Phototoxicity in a DGAT1 Inhibitor.

Diacylglycerol O-acyltransferase 1 (DGAT1) inhibitor Pradigastat (1) was shown to be effective at decreasing postprandial triglyceride levels in a patient population with familial chylomicronemia syndrome (FCS). Although pradigastat does not cause photosensitization in humans at the high clinical dose of 40 mg, a positive signal was observed in preclinical models of phototoxicity. Herein, we describe a preclinical phototoxicity mitigation strategy for diarylamine containing molecules utilizing the introduction of an amide or suitable heterocyclic function. This strategy led to the development of two second-generation compounds with low risk of phototoxicity, disparate exposure profiles, and comparable efficacy to 1 in a rodent lipid bolus model for post-prandial plasma triglycerides.

Discovery of 1-((6-Aminopyridin-3-yl)Methyl)-3-(4-Bromophenyl)Urea as a Potent, Irreversible Myeloperoxidase Inhibitor.

Myeloperoxidase (MPO) is a leukocyte-derived redox enzyme that has been linked to oxidative stress and damage in many inflammatory states, including cardiovascular disease. We have discovered aminopyridines that are potent mechanism-based inhibitors of MPO, with significant selectivity over the closely related thyroid peroxidase. 1-((6-Aminopyridin-3-yl)methyl)-3-(4-bromophenyl)urea (Aminopyridine 2) inhibited MPO in human plasma and blocked MPO-dependent vasomotor dysfunction ex vivo in rat aortic rings. Aminopyridine 2 also showed high oral bioavailability and inhibited MPO activity in vivo in a mouse model of peritonitis. Aminopyridine 2 could effectively be administered as a food admixture, making it an important tool for assessing the relative importance of MPO in preclinical models of chronic inflammatory disease.

High and dry? Comparing active dry EEG electrodes to active and passive wet electrodes.

Dry electrodes are becoming popular for both lab-based and consumer-level electrophysiological-recording technologies because they better afford the ability to move traditional lab-based research into the real world. It is unclear, however, how dry electrodes compare in data quality to traditional electrodes. The current study compared three EEG electrode types: (a) passive-wet electrodes with no onboard amplification, (b) actively amplified, wet electrodes with moderate impedance levels, and low impedance levels, and (c) active-dry electrodes with very high impedance. Participants completed a classic P3 auditory oddball task to elicit characteristic EEG signatures and event-related potentials (ERPs). Across the three electrode types, we compared single-trial noise, average ERPs, scalp topographies, ERP noise, and ERP statistical power as a function of number of trials. We extended past work showing active electrodes' insensitivity to moderate levels of interelectrode impedance when compared to passive electrodes in the same amplifier. Importantly, the new dry electrode system could reliably measure EEG spectra and ERP components comparable to traditional electrode types. As expected, however, dry active electrodes with very high interelectrode impedance exhibited marked increases in single-trial and average noise levels, which decreased statistical power, requiring more trials to detect significant effects. This power decrease must be considered as a trade-off with the ease of application and long-term use. The current results help set constraints on experimental design with novel dry electrodes, and provide important evidence needed to measure brain activity in novel settings and situations.

Consecutively reconstructing absorption and scattering distributions in turbid media with multiple-illumination photoacoustic tomography.

We propose a theoretical framework for consecutively reconstructing absorption and scattering distributions in turbid soft tissue in an iterative manner. This approach takes advantage of the stability of a recently reported least-squares fixed-point iterative method for reconstructing an optical absorption coefficient map to iteratively update estimates of absorption and scattering for each iteration. Simulations demonstrate that this method converges to an accurate estimate of the optical properties within only a small number of iterations and is robust to noise at realistic signal-to-noise levels.

An "aprotic" Tamao oxidation/syn-selective tautomerization reaction for the efficient synthesis of the C1-C9 fragment of fludelone.

An efficient synthesis of the C(1)-C(9) fragment of fludelone has been developed. The key step is a tandem silylformylation-crotylsilylation/Tamao oxidation sequence that establishes the C(5) ketone, the C(6), C(7), and C(8) stereocenters, and the C(9) alkene in a single operation from a readily accessed starting material. The stereochemical outcome at C(6) depends critically on the development of an "aprotic" Tamao oxidation, which leads to a reversal in the intrinsic diastereoselectivity observed using "standard" Tamao oxidation conditions.

Toward more "ideal" polyketide natural product synthesis: a step-economical synthesis of zincophorin methyl ester.

A highly efficient and step-economical synthesis of zincophorin methyl ester has been achieved. The unprecedented step economy of this zincophorin synthesis is principally due to an application of the tandem silylformylation-crotylsilylation/Tamao oxidation-diastereoselective tautomerization reaction, which achieves in a single step what would typically require a significant multistep sequence.

Enamides and enesulfonamides as nucleophiles: formation of complex ring systems through a platinum(II)-catalyzed addition/Friedel-Crafts pathway.

Cyclic enamine derivatives (enesulfonamides and enamides) tethered to an 1-arylalkynyl fragment undergo a platinum(II)-catalyzed tandem alkyne addition/Friedel-Crafts ring closure to form nitrogen-containing polycyclic structures. Regioselectivity in the initial addition of the enesulfonamide or enamide nucleophile to the platinum(II)-alkyne complex is important. Electron-rich arenes and heterocycles led to the formation of products resulting from an initial 6-endo cyclization. Twenty-three examples of this process are presented.

Platinum(II)-catalyzed cyclizations forming quaternary carbon centers, using enesulfonamides, enecarbamates, or enamides as nucleophiles.

Cyclic enesulfonamides, enecarbamates, or enamides tethered to an alkyne cyclize readily with use of platinum(II) chloride. This reaction generates quaternary-substituted carbon centers within simple spiro-fused or more complex tri- and tetracyclic heterocyclic ring systems. The yields for this process range from 50% to 83%. [reaction: see text].

Pyrrole synthesis catalyzed by AgOTf or cationic Au(I) complexes.

Either silver trifluoromethanesulfonate or a mixture of gold(I) chloride, silver trifluoromethanesulfonate, and triphenylphosphine catalyze the formation of pyrroles from substituted beta-alkynyl ketones and amines. The reactions proceed by using 5 mol % of catalyst with yields of isolated pyrroles ranging from 13% to 92%. Sixteen examples are used to compare the effectiveness of each catalyst.

An expeditious, high-yielding construction of the food aroma compounds 6-acetyl-1,2,3,4-tetrahydropyridine and 2-acetyl-1-pyrroline.

[reaction: see text] The key compound responsible for the aroma of bread, 6-acetyl-1,2,3,4-tetrahydropyridine (1), has been constructed in an efficient three-step procedure from 2-piperidone in an overall yield of 56%. Compound 1 was liberated in the final step under basic conditions. A related synthetic route produced 2-acetyl-1-pyrroline (2), the principal component of cooked rice, in 10% overall yield.

Pt(II) or Ag(I) salt catalyzed cycloisomerizations and tandem cycloadditions forming functionalized azacyclic arrays.

[reaction: see text] Cyclic ene-N-p-toluenesulfonamides tethered to an electron-deficient alkyne undergo cycloisomerizations readily under the influence of catalytic Pt(II) salts (PtCl2 or [dppbPtmu-OH]2(BF4)2) or AgOTf. Yields for this process range from 47% to 99%. The resulting functionalized 2-azahydrindans can be reacted further using the Diels-Alder reaction. Tandem cycloisomerization-cycloaddition reactions in one pot generate highly functionalized 1-azadecalin ring systems in a highly stereocontrolled manner.

A modular and concise total synthesis of (+/-)-daurichromenic acid and analogues.

A modular and concise total synthesis of (+/-)-daurichromenic acid has been accomplished in four steps from ethyl acetoacetate, ethyl crotonate, and trans,trans-farnesal. A series of analogues of this natural product, which has potent anti-HIV activity, were also prepared from ethyl or methyl acetoacetate and a series of readily available alpha,beta-unsaturated esters and aldehydes.