Articles

Special Report on Natural Products: Mining metabolites

Rebirth of an industry requires reinvention
Written byRandall C Willis
| 17 min read

When trying to contemplate natural products research, the first images that may come to mind are possibly from the media: Sean Connery striding across his hut in the Amazonian rainforest to check the latest LC/MS data (a la the film Medicine Man) or video of a snorkeler hunting a tropical reef for a poisonous snail (from a source like National Geographic perhaps).

Wade through the pages of the Journal of Natural Products and you can be forgiven for assuming that efforts to convert environmental metabolites into medicines have never been stronger.

Yet, for many large pharmaceutical companies, the revolutionary discoveries like Selman Waksman and streptomycin are seen as moments of past glory. They are events to be commemorated, as with the 2015 Nobel Prize for Medicine for the discovery of the anti-malarials avermectins and artemisinin as wonderful serendipities.

“The great discoveries of the past were made come-by-chance or by happenstance,” concurs Nathan Magarvey, McMaster University researcher and Adapsyn Bioscience founder. “They didn't occur in any kind of directedness.”

“There's a beauty in that, there's a serendipity in it,” he continues. “But it does not lend itself to a clean-headed think about what to do and when and how.”

Risk averse

“Natural product drug discovery is a highly risky business venture,” explains Russell Kerr, University of PEI researcher and founder of Nautilus Biosciences. “It's also a highly important venture.”

“There are many, many success stories, but given the risk involved in getting a drug lead through this very lengthy, very expensive process, Big Pharma is shying away from engaging in the early steps of drug discovery,” he adds.

For Andy Haigh, Adapsyn CEO, it comes down to the notion of productivity.

“You can get a hit,” he explains. “Then you have to deconvolute it. Spend months working on things only to say, hey, I've discovered this thing and it's been seen before or it's not that interesting or the chemistry is not that good or it's not drug-like.”

Despite the diversity of secondary metabolites that could be probed, he continues, only a small proportion have the potential to be developed as drugs.

“And at the same time, you saw combinatorial chemistry and DNA-encoded libraries [DELs] and all these other things, which were that much more predictable or easier to work with,” Haigh recalls.

By the 1990s, industry had grown weary of rediscovering the same molecules over and over, whether that was due to the metabolite being the most highly expressed or its bioactivity overwhelming signals from less active constituents.

Where was the chemical diversity that everyone had been promised?

“Major pharmaceutical companies diminished or even terminated their [natural products] programs and shifted their focus to target-based drug discovery, which relies on HTS [high-throughput screening] of synthetic chemical libraries,” explained Wayne State University’s Ping Dou and colleagues in a 2019 editorial. “Unlike phenotype screens, target-based approaches are based on the validated understanding of the interaction of tested compounds with the designated targets and thus are considered to have higher hitting rates ... Unfortunately, this strategic shift correlated with the overall reduction in novel lead compounds in the drug development pipeline and the substantial decline in new drug approval.”

Not everyone has given up on the natural product space, however. New technologies and approaches are allowing researchers to tap deeper into the elusive chemical diversity, offering a glimmer of a new generation of transformative discovery.

“We're not trying to reinvent the modality of natural product, but we're trying to really improve the discovery process by looking at capturing all the biosynthetic diversity out of the environment,” says Brad Hover, vice president of Discovery Research and Platform Technology at Lodo Therapeutics. “And then also improve the production process by looking at things such as the tractability for both discovery and the scale-up of those molecules.”

Old school, new effort

Rather than reinvent the harvest-extract-analyze wheel, some researchers like Barry O’Keefe and colleagues at the National Cancer Institute (NCI) and Leidos Biomedical Research are focused on finding ways to improve the wheel.

As they explained, the NCI Program for Natural Product Discovery (NPNPD) has a mandate to facilitate sample prefractionation and develop integrated resources for rapid isolation and structure identification of biologically active natural products. In early 2020, they described an automated fractionation process using solid-phase extraction followed by HPLC.

“In an HTS context, this workflow is able to process 500 primary hits and provide in excess of 10,000 wells containing dried-down pure and semi-purified compounds back to the screening lab in a two-week window at a quantity that yields a sufficient amount of subfractions for multiple secondary assays,” the authors highlighted.

The researchers then analyzed active wells with a combination of NMR, LC/MS, and FTIR, comparing the spectroscopic and spectrometric data against in-house and commercial databases to identify the biologically active components and permit dereplication of known compounds.

“Using this approach on all of the hits, we found that most subfractions contained pure or semi-purified compounds in quantities that enabled collection of high-quality analytical data and subsequent rapid dereplication of known compounds,” the researchers noted. “In total, we dereplicated the structures of 28 known biologically active natural products as well as identified subfractions that contained new natural products.”

They were also able to identify and isolate moderately active compounds that might have been missed in crude extracts or whose activity might be masked by more active components.

In other ways, natural products research is evolving with the convergence of newer technologies that are allowing researchers to explore new questions.

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