A Prodrug Strategy to Reposition Atovaquone as a Long-Acting Injectable for Malaria Chemoprotection.

Recent malaria drug discovery approaches have been extensively focused on the development of oral, smallmolecule inhibitors for disease treatment whereas parenteral routes of administration have been avoided due to limitations in deploying a shelf-stable injectable even though it could be dosed less frequently. However, an updated target candidate profile from Medicines for Malaria Venture (MMV) and stakeholders have advocated for long-acting injectable chemopreventive agents as an important interventive tool to improve malaria prevention. Here, we present strategies for the development of a long-acting, intramuscular, injectable atovaquone prophylactic therapy. We have generated three prodrug approaches that are contrasted by their differential physiochemical properties and pharmacokinetic profiles: mCBK068, a docosahexaenoic acid ester of atovaquone formulated in sesame oil, mCKX352, a heptanoic acid ester of atovaquone formulated as a solution in sesame oil, and mCBE161, an acetic acid ester of atovaquone formulated as an aqueous suspension. As a result, from a single 20 mg/kg intramuscular injection, mCKX352 and mCBE161 maintain blood plasma exposure of atovaquone above the minimal efficacious concentration for >70 days and >30 days, respectively, in cynomolgus monkeys. The differences in plasma exposure are reflective of the prodrug strategy, which imparts altered chemical properties that ultimately influence aqueous solubility and depot release kinetics. On the strength of the pharmacokinetic and safety profiles, mCBE161 is being advanced as a first-in-class clinical candidate for first-in-human trials.

Positron Emission Tomography Tracer Design of Targeted Synthetic Peptides via 18F-Sydnone Alkyne Cycloaddition.

Chemically synthesized, small peptides that bind with high affinity and specificity to CD8-expressing (CD8+) tumor-infiltrating T cells, yet retain the desirable characteristics of small molecules, hold valuable potential for diagnostic molecular imaging of immune response. Here, we report the development of 18F-labeled peptides targeting human CD8α with nanomolar affinity via the strain-promoted sydnone-alkyne cycloaddition with 4-[18F]fluorophenyl sydnone. The 18F-sydnone is produced in one step, in high radiochemical yield, and the peptide labeling proceeds rapidly. A hydrophilic chemical linker results in a tracer with favorable pharmacokinetic properties and improved image contrast, as demonstrated by in vivo PET imaging studies.

Dearomatization Reactions Using Phthaloyl Peroxide.

A new oxidative dearomatization reaction has been developed using phthaloyl peroxide to chemoselectively install two oxygen-carbon bonds into aromatic precursors. The oxidation reaction proceeds only once; addition of superstoichiometric equivalents of phthaloyl peroxide does not react further with the newly generated 1,3-cyclohexadiene. The reaction has been challenged by the addition of different functional groups and shown to maintain chemoselectivity. Due to the broad reactivity with 1,2-methylenedioxybenzene derivatives, linear free energy correlations were determined and support a mechanism proceeding through diradicals analogous to arene-hydroxylation reactions using phthaloyl peroxide.

Computational and Experimental Studies of Phthaloyl Peroxide-Mediated Hydroxylation of Arenes Yield a More Reactive Derivative, 4,5-Dichlorophthaloyl Peroxide.

The oxidation of arenes by the reagent phthaloyl peroxide provides a new method for the synthesis of phenols. A new, more reactive arene oxidizing reagent, 4,5-dichlorophthaloyl peroxide, computationally predicted and experimentally determined to possess enhanced reactivity, has expanded the scope of the reaction while maintaining a high level of tolerance for diverse functional groups. The reaction proceeds through a novel "reverse-rebound" mechanism with diradical intermediates. Mechanistic insight was achieved through isolation and characterization of minor byproducts, determination of linear free energy correlations, and computational analysis of substituent effects of arenes, each of which provided additional support for the reaction proceeding through the diradical pathway.

Expedited access to vinaxanthone and chemically edited derivatives possessing neuronal regenerative effects through ynone coupling reactions.

The natural product vinaxanthone has demonstrated a remarkable capability to promote nerve growth following injury or transplantation. In rats following total transection of the spinal cord delivery of vinaxanthone enhanced axonal regeneration, remyelination and angiogenesis at the site of injury all leading to an improved reinstatement of motor function. Through the development of a new ynone coupling reaction, chemically edited derivatives of vinaxanthone have been prepared and studied for improved activity. The coupling reaction allows rapid access to new derivatives, wherein n ynone precursors provide n(2) vinaxanthone analogues. These compounds have been tested for their ability to promote neuronal regrowth using laser axotomy, severing axonal connections in Caenorhabditis elegans. This precise microsurgery using C. elegans allows a new in vivo approach for medicinal chemistry based optimization of neuronal growth promoting compounds.

Preparation of phenols by phthaloyl peroxide-mediated oxidation of arenes.

This protocol describes an approach to installing hydroxyls into arenes through the direct replacement of C-H bonds with C-O bonds. This direct oxidation avoids the need to prefunctionalize the substrate, use precious metals, introduce directing groups, or use strong Brønsted or Lewis acids. Phthaloyl peroxide, the sole reagent used for this transformation, can be prepared readily from the commodity chemicals phthaloyl chloride and sodium percarbonate. Phthaloyl peroxide oxidizes a diverse range of arenes, and the reactions that involve its use are characterized by high functional group compatibility, which enables the hydroxylation of simple arenes, advanced synthetic intermediates, natural products and other drug-like molecules forming the corresponding phenolic compounds. Notably, the reaction is operationally straightforward and has no special requirements for the exclusion of oxygen and water. The synthesis of phthaloyl peroxide takes 4 h and the subsequent hydroxylation of mesitylene takes 21 h.

A protocol to generate phthaloyl peroxide in flow for the hydroxylation of arenes.

A flow protocol for the generation of phthaloyl peroxide has been developed. This process directly yields phthaloyl peroxide in high purity (>95%) and can be used to bypass the need to isolate and recrystallize phthaloyl peroxide, improving upon earlier batch procedures. The flow protocol for the formation of phthaloyl peroxide can be combined with arene hydroxylation reactions and provides a method for the consumption of peroxide as it is generated to minimize the accumulation of large quantities of peroxide.

Small-molecule mechanism of action studies in Caenorhabditis elegans.

A general protocol for exogenous small-molecule pull-down experiments with Caenorhabditis elegans is described; it provides a link between small-molecule screens in worms and existing mutant and RNAi technologies, thereby enabling organismal mechanism of action studies for the natural product clovanemagnolol. Forward chemical genetic screens followed by mechanism of action studies with C. elegans, when coupled with genetic validation of identified targets to reproduce the small molecule's phenotypic effects, provide a unique platform for discovering the biological targets of compounds that affect multicellular processes. First, the use of an immobilized FK506 derivative and soluble competition experiments with optimally prepared soluble C. elegans proteome successfully identified interactions with FK506 binding proteins 1 to 6. This approach was used to determine an unknown mechanism of action for clovanemagnolol, a small molecule that promotes axonal branching in both primary neuronal cultures and in vivo in C. elegans. Following the synthesis of an appropriately functionalized solid-phase reagent bearing a clovanemagnolol analogue pull-down experiments employing soluble competition identified kinesin light chain-1 (KLC-1), a protein involved in axonal cargo transport, as a putative target. This was corroborated through the use of mutant worms lacking klc-1 and possessing GFP neuronal labeling, reproducing the axonal branching phenotype induced by the small molecule clovanemagnolol.