By David N. Leff
Whether trying to dam a tumor's blood supply at the source or head off a neurotransmitter at the synapse, when it comes to drug discovery, the name of the game is inhibition.
Pharmaceutical and biotech players screen libraries of oligonucleotides against target substances, looking for promising inhibitory compounds — ideally, orally available small molecules.
One week ago today, in London, drug development chemist Wolfgang Pieken told a select meeting of his industrial and academic peers about a novel technology for synthesizing potential therapeutic compounds. The international symposium, convened by the Ciba Foundation, discussed the agenda: "Oligonucleotides as Therapeutic Agents."
In his talk, Pieken, director of process chemistry at NeXstar Pharmaceuticals Inc., in Boulder, Colo., described his invention of PASS — Product Anchored Sequential Synthesis.
To appreciate PASS, Pieken told BioWorld Today, one must look at the evolution of solid-state and solution-state oligonucleotide (oligo) synthesis.
"The way oligos are made currently," he explained, "is by the very elegant solid-state synthesis pioneered by Marvin Carruthers at the University of Colorado in the 1980s. It works extremely well in a small-scale lab setting. But," he said, "most companies scale it up, so over time it becomes grossly out of sync with a normal, predictable process chemical scale-up.
"Our whole approach at NeXstar was how to rectify that situation," Pieken said.
As a point of departure, he and his research and development team turned to solution-based synthesis, an attractive alternative to solid state, but with drawbacks of its own. "You now have to handle very laborious work-up steps that make for very costly intermediates."
How PASS Pulls Product From Mix
PASS avoids this and other problems, Pieken told the London conference, "by allowing us to selectively pull the product from the solution-phase mixture. In reality," he observed, "it has all kinds of other stuff in it — the activators, solvents, everything we want to get rid of."
He went on: "So now what we do is add a novel chemical entity that we have developed * we call it 'tag X' — run it over the resin, and selectively retain the product that we want, based on the ability of that group X to react with the resin. Then we break the bond between it and tag X, which releases the product, and gets us the starting material for the next addition cycle."
"This group X," Pieken added, "is new to the industry. It's something others have been trying to achieve for a long time. And that's exactly what our peers in London realized immediately, and were excited about."
PASS, Pieken pointed out, "is strictly a downstream technology that solves a manufacturing problem. It does not impact drug discovery per se."
That impact comes from NeXstar's flagship process, SELEX — which the company's founder and chief scientific officer, Larry Gold, began developing three decades ago.
SELEX stands for Systematic Evolution of Ligands by Exponential Enrichment. Gold brings its story up to date in a paper appearing in the current Proceedings of the National Academy of Sciences (PNAS), dated Jan. 7, 1997. Its title: "From oligonucleotide shapes to genomic SELEX: Novel biological regulatory loops."
Pieken made the point that "Oligos are prime candidates for us as therapeutics, because of the elegance of our SELEX technology. But at the end of the day, it has to compete with other technologies."
SELEX is a technology where Gold realized that a large library of random oligos really encodes a rich repertoire of unique and very complex three-dimensional nucleotide shapes. It's a way to enrich a molecule * an aptomer — that exerts a specific pharmaceutical action." (See BioWorld Today, Dec. 20, 1996, p. 2.)
Pieken observed: "Larry realized that you could use those shapes as keys to inhibit the locks of different proteins."
Such a protein is vascular endothelial growth factor (VEGF), which promotes angiogenesis — the network of blood vessels that supply nutrients to a growing tumor.
"In recent months," NeXstar scientists have seen in vivo efficacy in several animal models of VEGF inhibiting oligos."
The 100-Trillion Oligo Solution Solution
To find these candidate cancer-blocking compounds, they created a library of 1013 (100 trillion) — random-sequence oligos, and queried the teeming flask with potential drug target molecules.
"In conventional combinatorial chemistry," Pieken remarked, "you would never find them."
The team applied an unconventional trick: They regenerated the library and rescreened it several times. "By doing that in those 100 trillion molecules," he said, " there were only maybe 10 or 100 that did what they wanted them to. They found a number of motifs represented in the winning molecules; not a single answer, but families of answers."
NeXstar's president and CEO, Patrick Mahaffy, told BioWorld Today: "Using PASS and SELEX, we are studying several different compounds, for oncological and anti-inflammatory indications, in a range of animal models, from rodents to chimps."
As for the cost-efficiency imperative, "Ultimately," Mahaffy said, "we'll be able to make oligo-based products at meaningfully under $1,000 a gram. The amount of that meaningful advantage will be determined by technological advances over the next couple of years.
"The thing that will drive the savings far beyond where you could ever go in solid-state synthesis," he added, "is the fact that product purification is built into the steps of the synthesis, rather than having to do a dramatic purification at the end of the process, and giving up a huge amount of your product through that purification."
Mahaffy concluded: "PASS will be providing the majority of compounds that we use to test in animal models, certainly by the end of the year. And we intend to be in the clinic with those oligos next year." *
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