Nereus Pharmaceuticals CEO Kobi M. Sethna likes to point out that the basic chemistry of the world’s best-selling drug, Pfizer’s cholesterol-lowering Lipitor, ultimately came from a microbe found somewhere in Japan. He should know, having worked for Pfizer. Lipitor brings Pfizer around $12 billion in revenue each year—more than 10 times the amount required to qualify as a “blockbuster” drug.

Pfizer’s cholesterol-lowering Lipitor,

Mr. Sethna is now looking in bluer pastures for his new company. San Diego-based Nereus is, Mr. Sethna says, the only company competely committed to searching for new drugs in marine microbes. Earlier this month, Nereus received $18.3 million as part of a preferred stock financing round led by HBM BioVentures and HBM BioCapital, bringing its total to $80.6 million since 2000.

Nereus’ business model may sound like an untraditional—and highly speculative—plan. But the company does have scientific rationale on its side, and to some degree, precedent. Hunting for medicines in nature, a practice known as bioprospecting, was common before the age of biotech. A pink flower from Madagascar—the rosy periwinkle, which Eli Lilly researchers found in 1958 with the help of a local shaman—contains the compounds that treat childhood leukemia. And last April, San Diego-based Amylin received U.S. regulatory approval for diabetes drug Byetta, found in the saliva of the Gila monster desert lizard.

Replenishing Returns

On land, most experts agree that the low-hanging medicinal fruit has been picked, and bioprospectors are seeing diminishing returns. But the marine realm is a different story. Other companies have tried, but ran into difficulties when they attempted to grow marine microbes in the lab. Nereus, however, has a secret weapon: the knowledge of University of California, San Diego, professor William Fenical, who has spent 15 years figuring out how to nurture marine organisms once they’re taken out of the ocean.

The ocean likely holds more potential medicines than land: Because life evolved in the oceans, marine organisms constitute a much greater range of evolution’s four-billion-year-old chemical experiment.

But Nereus may find it difficult to convince pharmaceutical R&D execs who see bioprospecting as passé. “There is tremendous in-built bias,” says Forward Ventures’ Standish Fleming, who first convinced Mr. Sethna to join Nereus.

The company is building value like a normal pharma startup, by pushing its two lead compounds into clinical trials, which Mr. Sethna expects to start in the second quarter of this year. One of these experimental drugs came from a marine fungus. It aims to treat tumors by interfering with their internal blood supply, something that could potentially complement highly profitable drugs such as Genentech’s Avastin, which works by cutting the blood supply to the tumor at its edge. Nereus’ other main product inhibits a molecular complex that breaks down proteins, some of which are linked to cancer.

But long term, the company’s real value is probably in its enormous discovery resources. “The tip of the iceberg doesn’t even describe it,” says Mr. Fleming. “We have the whole ocean to draw upon, and we’ve taken a couple of cups of water out of it.”