Acridones Are Highly Potent Inhibitors of Toxoplasma gondii Tachyzoites

Acridone derivatives, which have been shown to have in vitro and in vivo activity against Plasmodium spp, inhibit Toxoplasma gondii proliferation at picomolar concentrations. Using enzymatic assays, we show that acridones inhibit both T. gondii cytochrome bc₁ and dihydroorotate dehydrogenase and identify acridones that bind preferentially to the Q_i site of cytochrome bc₁. We identify acridones that have efficacy in a murine model of systemic toxoplasmosis. Acridones have potent activity against T. gondii and represent a promising new class of preclinical compounds.

Total Synthesis and Antimalarial Activity of 2-(p-Hydroxybenzyl)-prodigiosins, Isoheptylprodigiosin, and Geometric Isomers of Tambjamine MYP1 Isolated from Marine Bacteria

Highly efficient and straightforward synthetic routes toward the first total synthesis of 2-(p-hydroxybenzyl)-prodigiosins (2-5), isoheptylprodigiosin (6), and geometric isomers of tambjamine MYP1 ((E/Z)-7) have been developed. The crucial steps involved in these synthetic routes are the construction of methoxy-bipyrrole-carboxaldehydes (MBCs) and a 20-membered macrocyclic core and a regioselective demethylation of MBC analogues. These new synthetic routes enabled us to generate several natural prodiginines 24-27 in larger quantity. All of the synthesized natural products exhibited potent asexual blood-stage antiplasmodial activity at low nanomolar concentrations against a panel of Plasmodium falciparum parasites, with a great therapeutic index. Notably, prodiginines 6 and 24-27 provided curative in vivo efficacy against erythrocytic Plasmodium yoelii at 25 mg/kg × 4 days via oral route in a murine model. No overt clinical toxicity or behavioral change was observed in any mice treated with prodiginines and tambjamines.

Lead Optimization of Second-Generation Acridones as Broad-Spectrum Antimalarials

The global impact of malaria remains staggering despite extensive efforts to eradicate the disease. With increasing drug resistance and the absence of a clinically available vaccine, there is an urgent need for novel, affordable, and safe drugs for prevention and treatment of malaria. Previously, we described a novel antimalarial acridone chemotype that is potent against both blood-stage and liver-stage malaria parasites. Here, we describe an optimization process that has produced a second-generation acridone series with significant improvements in efficacy, metabolic stability, pharmacokinetics, and safety profiles. These findings highlight the therapeutic potential of dual-stage targeting acridones as novel drug candidates for further preclinical development.

Discovery and Structural Optimization of Acridones as Broad-Spectrum Antimalarials

Malaria remains one of the deadliest diseases in the world today. Novel chemoprophylactic and chemotherapeutic antimalarials are needed to support the renewed eradication agenda. We have discovered a novel antimalarial acridone chemotype with dual-stage activity against both liver-stage and blood-stage malaria. Several lead compounds generated from structural optimization of a large library of novel acridones exhibit efficacy in the following systems: (1) picomolar inhibition of in vitro Plasmodium falciparum blood-stage growth against multidrug-resistant parasites; (2) curative efficacy after oral administration in an erythrocytic Plasmodium yoelii murine malaria model; (3) prevention of in vitro Plasmodium berghei sporozoite-induced development in human hepatocytes; and (4) protection of in vivo P. berghei sporozoite-induced infection in mice. This study offers the first account of liver-stage antimalarial activity in an acridone chemotype. Details of the design, chemistry, structure-activity relationships, safety, metabolic/pharmacokinetic studies, and mechanistic investigation are presented herein.

Boron Trifluoride Etherate Promoted Microwave-Assisted Synthesis of Antimalarial Acridones

A microwave-assisted, rapid and efficient method using boron trifluoride etherate (BF₃.Et₂O) for the synthesis of acridones, via an intramolecular acylation of N-phenylanthranilic acid derivatives, has been developed. The reaction proceeds under solvent-free conditions, tolerates a wide range of functional groups, and provides rapid access to a range of acridones in good to excellent yields. Several of the synthesized acridones exhibited potent antimalarial activities against CQ sensitive and multi-drug resistant (MDR) parasites.

A Case of Vanishing Metastatic Mass: Right Atrial Mass in the Setting of Primary Epithelioid Hemangioendothelioma of the Spine

Epithelioid hemangioendothelioma (EHE) is a rare low-grade vascular neoplasm that is characterized as intermediate between benign hemangioma and high-grade angiosarcoma affecting 1 in 1,000,000 people worldwide. It has been described throughout the body with lung, liver, skin, and bone being the most frequent sites. Primary EHE of the spine has been reported in 56 cases so far with no correlation of age and sex. Our case highlights a rare clinical presentation, etiopathogenesis, diagnosis, and treatment of EHE of the spine with metastasis to the right atrium. This is the first documented case of EHE of the spine with metastatic spread to the heart treated with bevacizumab leading to resolution of the heart metastatic mass. Further studies are warranted to develop a treatment formula for this rare tumor, to consider combination chemotherapy and new adjuvant targeted immunotherapies to prevent progression of disease.

MarH, a Bifunctional Enzyme Involved in the Condensation and Hydroxylation Steps of the Marineosin Biosynthetic Pathway

A novel bifunctional enzyme, MarH, has been identified, and its key functional role in the marineosin biosynthesis successfully probed. MarH catalyzes (1) a condensation step between 4-methoxy-2,2'-bipyrrole-5-carboxaldehyde (MBC) and 2-undecylpyrrole (UP) to form undecylprodiginine (UPG) and (2) hydroxylation of the alkyl chain of UPG to form the (S)-23-hydroxyundecylprodiginine (HUPG), which is essential for MarG catalyzed bicyclization toward the formation of an unusual spiro-tetrahydropyran-aminal ring of marineosins. The final enigmatic steps in the marineosin biosynthesis have now been deciphered.

Stenotrophomonas maltophilia OleC-Catalyzed ATP-Dependent Formation of Long-Chain Z-Olefins from 2-Alkyl-3-hydroxyalkanoic Acids

The bacterial pathway of olefin biosynthesis starts with OleA catalyzed "head-to-head" condensation of two CoA-activated long-chain fatty acids to generate (R)-2-alkyl-3-ketoalkanoic acids. A subsequent OleD-catalyzed reduction generates (2R,3S)-2-alkyl-3-hydroxyalkanoic acids. We now show that the final step in the pathway is an OleC-catalyzed ATP-dependent decarboxylative dehydration to form the corresponding Z olefins. Higher kcat /Km values were seen for substrates with longer alkyl chains. All four stereoisomers of 2-hexyl-3-hydroxydecanoic acid were shown to be substrates, and GC-MS and NMR analyses confirmed that the product in each case was (Z)-pentadec-7-ene. LC-MS analysis supported the formation of AMP adduct as an intermediate. The enzymatic and stereochemical course of olefin biosynthesis from long-chain fatty acids by OleA, OleD and OleC is now established.

Stereospecific synthesis of 23-hydroxyundecylprodiginines and analogues and conversion to antimalarial premarineosins via a Rieske oxygenase catalyzed bicyclization

Facile and highly efficient synthetic routes for the synthesis of (S)- and (R)-23-hydroxyundecylprodiginines ((23S)-2, and (23R)-2), 23-ketoundecylprodiginine (3), and deuterium-labeled 23-hydroxyundecylprodiginine ([23-d]-2) have been developed. We demonstrated a novel Rieske oxygenase MarG catalyzed stereoselective bicyclization of (23S)-2 to premarineosin A (4), a key step in the tailoring process of the biosynthesis of marineosins, using a marG heterologous expression system. The synthesis of various A-C-ring functionalized prodiginines 32-41 was achieved to investigate the substrate promiscuity of MarG. The two analogues 32 and 33 exhibit antimalarial and cytotoxic activities stronger than those of the marineosin intermediate 2, against Plasmodium falciparum strains (CQ(S)-D6, CQ(R)-Dd2, and 7G8) and hepatocellular HepG2 cancer cell line, respectively. Feeding of 34-36 to Streptomyces venezuelae expressing marG led to production of novel premarineosins, paving a way for the production of marineosin analogues via a combinatorial synthetic/biosynthetic approach. This study presents the first example of oxidative bicyclization mediated by a Rieske oxygenase.

Elucidation of final steps of the marineosins biosynthetic pathway through identification and characterization of the corresponding gene cluster

The marine Streptomyces sp. CNQ-617 produces two diastereomers, marineosins A and B. These are structurally related to alkyl prodiginines, but with a more complex cyclization and an unusual spiroaminal skeleton. We report the identification of the mar biosynthetic gene cluster and demonstrate production of marineosins through heterologous expression in a S. venezuelae host named JND2. The mar cluster shares the same gene organization and has high homology to the genes of the red cluster (which directs the biosynthesis of undecylprodiginine) but contains an additional gene, named marA. Replacement of marA in the JND2 strain leads to the accumulation of premarineosin, which is identical to marineosin with the exception that the middle pyrrole (Ring B) has not been reduced. The final step of the marineosin pathway is thus a MarA catalyzed reduction of this ring. Replacement of marG (a homologue of redG that directs undecylprodiginine cyclization to give streptorubin B) in the JND2 strain leads to the loss of all spiroaminal products and the accumulation of 23-hydroxyundecylprodiginine and a shunt product, 23-ketoundecylprodiginine. MarG thus catalyzes the penultimate step of the marineosin pathway catalyzing conversion of 23-hydroxyundecylprodiginine to premarineosin. The preceding steps of the biosynthetic marineosin pathway likely mirror that in the red-directed biosynthetic process, with the exception of the introduction of the hydroxyl functionality required for spiroaminal formation. This work presents the first experimentally supported scheme for biosynthesis of marineosin and provides a new biologically active molecule, premarineosin.