The future of plant drug discovery

Target-directed evolution of novel modulators of the dopamine transporter in Lobelia cardinalis hairy root cultures

The dopamine transporter (DAT) is targeted in substance use disorders (SUDs), and "non-classical"" DAT inhibitors with low abuse potential are therapeutic candidates. Lobinaline, from Lobelia cardinalis, is an atypical DAT inhibitor lead. Chemical synthesis of lobinaline is challenging; thus, "target-directed evolution" was used for lead optimization. A target protein is expressed in plant cells, and a mutant cell population is selected under conditions where target protein functional inhibition confers a survival advantage. Surviving mutants are "mined" for the targeted activity. Applied to a mutant L. cardinalis cell population expressing the human DAT, we identified 20 mutants overproducing DAT inhibitors. Microanalysis prioritized novel lobinaline derivatives, and we first investigated the more water-soluble lobinaline N-oxide. It inhibited rat synaptosomal [3H]DA uptake with an IC₅₀ similar to lobinaline. Against repeated DA microinjections into the rat striatum, lobinaline produced transient DA clearance reductions. In contrast, lobinaline N-oxide prolongingly increased DA peak amplitudes, particularly in the ventral striatum. Lobinaline N-oxide also produced complex changes in post-peak DA clearance inconsistent with simple DAT inhibition. This unusual DAT interaction may prove therapeutically useful for treating SUDs. This study demonstrates the value of target-directed evolution of plant cells for optimizing lead compounds difficult to synthesize chemically.

Nanoharvesting of bioactive materials from living plant cultures using engineered silica nanoparticles

Plant secondary metabolites are valuable therapeutics not readily synthesized by traditional chemistry techniques. Although their enrichment in plant cell cultures is possible following advances in biotechnology, conventional methods of recovery are destructive to the tissues. Nanoharvesting, in which nanoparticles are designed to bind and carry biomolecules out of living cells, offers continuous production of metabolites from plant cultures. Here, nanoharvesting of polyphenolic flavonoids, model plant-derived therapeutics, enriched in Solidago nemoralis hairy root cultures, is performed using engineered mesoporous silica nanoparticles (MSNPs, 165 nm diameter and 950 m2/g surface area) functionalized with both titanium dioxide (TiO2, 425 mg/g particles) for coordination binding sites, and amines (NH2, 145 mg/g particles) to promote cellular internalization. Intracellular uptake and localization of the nanoparticles (in Murashige and Skoog media) in hairy roots were confirmed by tagging the particles with rhodamine B isothiocyanate, incubating the particles with hairy roots, and quenching bulk fluorescence using trypan blue. Nanoharvesting of biologically active flavonoids was demonstrated by observing increased antiradical activity (using 2,2-diphenyl-1-picrylhydrazyl radical scavenging assay) by nanoparticles after exposure to hairy roots (indicating general antioxidant activity), and by the displacement of the radio-ligand [3H]-methyllycaconitine from rat hippocampal nicotinic receptors by solutes recovered from nanoharvested particles (indicating pharmacological activity specific to S. nemoralis flavonoids). Post-nanoharvesting growth suggests that the roots are viable after nanoharvesting, and capable of continued flavonoid synthesis. These observations demonstrate the potential for using engineered nanostructured particles to facilitate continuous isolation of a broad range of biomolecules from living and functioning plant cultures.

Target-directed discovery and production of pharmaceuticals in transgenic mutant plant cells

Plants are a source of complex bioactive compounds, with value as pharmaceuticals, or leads for synthetic modification. Many of these secondary metabolites have evolved as defenses against competing organisms and their pharmaceutical value is "accidental", resulting from homology between target proteins in these competitors, and human molecular therapeutic targets. Here we show that it is possible to use mutation and selection of plant cells to re-direct their "evolution" toward metabolites that interact with the therapeutic target proteins themselves. This is achieved by expressing the human target protein in plant cells, and selecting mutants for survival based on the interaction of their metabolome with this target. This report describes the successful evolution of hairy root cultures of a Lobelia species toward increased biosynthesis of metabolites that inhibit the human dopamine transporter protein. Many of the resulting selected mutants are overproducing the active metabolite found in the wild-type plant, but others overproduce active metabolites that are not readily detectable in non-mutants. This technology can access the whole genomic capability of a plant species to biosynthesize metabolites with a specific target. It has potential value as a novel platform for plant drug discovery and production, or as a means of optimizing the therapeutic value of medicinal plant extracts.

Novel multifunctional pharmacology of lobinaline, the major alkaloid from Lobelia cardinalis

In screening a library of plant extracts from ~1000 species native to the Southeastern United States, Lobelia cardinalis was identified as containing nicotinic acetylcholine receptor (nicAchR) binding activity which was relatively non-selective for the α4β2- and α7-nicAchR subtypes. This nicAchR binding profile is atypical for plant-derived nicAchR ligands, the majority of which are highly selective for α4β2-nicAchRs. Its potential therapeutic relevance is noteworthy since agonism of α4β2- and α7-nicAchRs is associated with anti-inflammatory and neuroprotective properties. Bioassay-guided fractionation of L. cardinalis extracts led to the identification of lobinaline, a complex binitrogenous alkaloid, as the main source of the unique nicAchR binding profile. Purified lobinaline was a potent free radical scavenger, displayed similar binding affinity at α4β2- and α7-nicAchRs, exhibited agonist activity at nicAchRs in SH-SY5Y cells, and inhibited [(3)H]-dopamine (DA) uptake in rat striatal synaptosomes. Lobinaline significantly increased fractional [(3)H] release from superfused rat striatal slices preloaded with [(3)H]-DA, an effect that was inhibited by the non-selective nicAchR antagonist mecamylamine. In vivo electrochemical studies in urethane-anesthetized rats demonstrated that lobinaline locally applied in the striatum significantly prolonged clearance of exogenous DA by the dopamine transporter (DAT). In contrast, lobeline, the most thoroughly investigated Lobelia alkaloid, is an α4β2-nicAchR antagonist, a poor free radical scavenger, and is a less potent DAT inhibitor. These previously unreported multifunctional effects of lobinaline make it of interest as a lead to develop therapeutics for neuropathological disorders that involve free radical generation, cholinergic, and dopaminergic neurotransmission. These include neurodegenerative conditions, such as Parkinson's disease, and drug abuse.

Phytomedicine polypharmacology: Cancer therapy through modulating the tumor microenvironment and oxylipin dynamics

Integrative approaches in cancer therapy have recently been extended beyond the induction of cytotoxicity to controlling the tumor microenvironment and modulating inflammatory cascades and pathways such as lipid mediator biosynthesis and their dynamics. Profiling of important lipid messengers, such as oxylipins, produced as part of the physiological response to pharmacological stimuli, provides a unique opportunity to explore drug pharmacology and the possibilities for molecular management of cancer physiopathology. Whereas single targeted chemotherapeutic drugs commonly lack efficacy and invoke drug resistance and/or adverse effects in cancer patients, traditional herbal medicines are seen as bright prospects for treating complex diseases, such as cancers, in a systematic and holistic manner. Understanding the molecular mechanisms of traditional medicine and its bioactive chemical constituents may aid the modernization of herbal remedies and the discovery of novel phytoagents for cancer management. In this review, systems-based polypharmacology and studies to develop multi-target drugs or leads from phytomedicines and their derived natural products that may overcome the problems of current anti-cancer drugs, are proposed and summarized.

The application of Chinese medicine to novel drug discovery

INTRODUCTION

The apparent productivity crisis in the pharmaceutical industry and the economic and political rise of China have contributed to renewed interest in the application of Chinese medicine for drug discovery.

AREAS COVERED

The author presents an overview of the historical development and basic principles of theory and practice of Chinese herbal medicine, its materia medica and prescription formulas, and discusses the motivation for and rationale of its application to drug discovery. Furthermore, the author distinguishes the five main approaches to drug discovery from Chinese herbal medicine, based on the decreasing amount and detail of historical and clinical Chinese medicine knowledge that informed the research effort.

EXPERT OPINION

Many compounds that have been isolated from the Chinese materia medica exhibit pharmacological activities comparable to pharmaceutical drugs. With the exception of the antimalarial drug artemisinin, however, this knowledge has not led to the successful development of new drugs outside of China. The chance of success in a Chinese medicine-based drug discovery effort will be increased by consideration of the empirical knowledge that has been documented over many centuries in the historical materia medica and prescription literature. Most Chinese medicine-derived compounds affect more than one target and do not correspond to the one compound/one-target drug discovery paradigm. A new frontier is opening up with the development of drugs consisting of combinations of multiple compounds acting on multiple targets under the paradigm of network pharmacology. The ancient practice of combining multiple drugs in prescription formulas can serve as inspirational analogy and a practical guide.

Natural Products Genomics: A novel approach for the discovery of anti-cancer therapeutics

Plants continue to retain some advantages over combinatorial chemistry as sources of novel compounds, for example, they can generate metabolites with a complexity beyond synthetic chemistry. However, this comes with its own problems in production and synthetic modification of these compounds. Natural Products Genomics (NPG) aims to access the plants own genomic capacity to increase yields, and modify complex bioactive metabolites, to alleviate these limitations. NPG uses a combination of gain of function mutagenesis and selection to a) mimic the evolution of novel compounds in plants, and b) to increase yields of known bioactive metabolites. This process is performed rapidly at the cell culture level in large populations of mutants. Two examples demonstrating proof of concept in Nicotiana tabacum (tobacco) and proof of application in the medicinal plant species Catharanthus roseus, are included to illustrate the feasibility of this approach. This biotechnology platform may alter the way in which plant drug discovery is perceived by the pharmaceutical industry, and provides an alternative to combinatorial chemistry for the discovery, modification and production of highly complex bioactive molecules.