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Making Drugs Survive Longer in Blood

Longer-lasting drugs could mean more effective treatments.
December 14, 2009

Taking a hint from natural antibiotics, a startup spun out of Stanford University is developing a way to chemically alter existing drugs to dramatically improve their half-life.

Hiding drugs: In this human blood smear, a modified protease inhibitor (shown in green) is sequestered inside a white blood cell. The cell’s DNA is shown in blue. The drug slowly leaches out into the plasma, greatly extending the drug’s half-life.

Researchers at Amplyx Pharmaceuticals decorate drug compounds with molecules designed to bind to specific proteins within cells, as well as binding to the drug’s treatment target. By sequestering the drugs within cells, the researchers hope to protect them from the body’s efforts to destroy them. So far, the company has developed long-lasting versions of a protease inhibitor to fight HIV, as well as the antibiotic carbapenem. Amplyx is now developing new versions of a number of drugs that fight infection, and aims to test them in clinical trials within the next two years.

An effective drug needs to have a relatively long half-life in the bloodstream so that it has time to get to its target before metabolizing enzymes in the liver break it down. Other researchers and biotechnology companies are attempting to deal with this problem by enveloping drugs in nanoparticles or other materials that slow their breakdown. Amplyx is instead altering the molecules themselves.

“We’re hoping this will be quite generally useful,” says Gerald Crabtree, a biologist at Stanford, in whose lab the research originated. “One of the things I like about this approach is the combinatorial nature of the synthesis of the molecules.” Researchers have developed a number of stabilizing structures–those that bind to intracellular proteins–as well as a number of “linkers,” which connect the stabilizers to the drug.

Amplyx’s technology emerged from the study of a handful of large-molecule drugs, such as the immunosuppressant rapamycin, that are derived from microbes. Large molecules often make poor drugs, in part because the body quickly metabolizes them–but despite this, drugs such as rapamycin actually last longer in the blood. Crabtree and his team discovered that rapamycin, originally derived from bacteria in the soil on Easter Island, and related compounds, works well because it attaches to a group of proteins within cells called FK506 binding proteins.

“We wondered if we could generalize this to a way of increasing stability for many drugs with problematic pharmacology, which can make them unusable or inconvenient to take,” says Crabtree. “Some drugs have to be taken five times per day, which in turn leads to patient compliance issues.”

To create longer-lasting drugs, the researchers generated chemical structures that mimic those found in rapamycin and are designed to bind to the FK506 binding proteins inside the cell. Researchers then attached the structures to the drug chemically. “Red blood cells have a lot of the binding proteins, so the drug binds tightly to them and slowly leaches out of blood cells,” says Jason Gestwicki, a biologist at the University of Michigan who is collaborating with Amplyx.

In a proof of principle experiment published earlier this year, Gestwicki and collaborators at Amplyx and Stanford created a long-lasting version of a protease inhibitor used to treat HIV. These drugs typically have a short half-life because they are easily broken down by liver enzymes. So they must be given with a second drug, ritonavir, that inhibits the function of those metabolizing enzymes. However, because the same enzymes are needed to metabolize other foreign molecules in the body, ritonavir can have toxic side effects. “This raises the possibility of avoiding the use of ritonavir for HIV proteases,” says Mitchell Mutz, chief scientific officer at Amplyx.

“It looks quite promising,” says Daryl Drummond, senior director of liposome discovery at Merrimack Pharmaceuticals, in San Francisco, of the protease inhibitor research. “This is more or less a slow release and sustained delivery system.” However, Drummond says it’s not yet clear how broadly applicable the technology will be or how attaching these structures to drugs will affect their toxicity.

Amplyx will initially focus on anti-infective drugs, many of which have short half-lives. The startup is partnering with an unnamed pharmaceutical company and has already developed a new version of the antibiotic carbapenem. That drug must be infused three to four times a day, making it expensive to administer. “Attaching our molecule to help the antibiotic bind to FKBP protein, we were able to show it helps increase its time in circulation,” says Mutz.

While pharmaceutical companies routinely screen and modify candidate molecules to improve half-lives and other pharmacological properties, Crabtree says Amplyx’s approach is much more directed. “We can work through it in a way that is more predictable than semirandom modifications that are used to improve half-life and stability,” he says.

Gestwicki says he hopes that studying biologically derived drugs might provide additional clues to better drug design. “Maybe natural products have types of chemical functionalities that give them improved pharmacological characteristics,” he says. “Maybe we can study them and figure out what those features are and install them into synthetic molecules.”

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