Symptoms of an influenza viral infection were described by Hippocrates 2400 years ago. In recent recorded history there have been pandemics such as the 1918-1920 Spanish Flu and the more recent 2009 Swine Flu. These influenza pandemics killed millions of people worldwide.
In recent modern medical times, there were only two medications indicated to treat an influenza infection, amantadine, and rimantadine. This all changed in 1999 with the introduction of Tamiflu (oseltamivir).
Virologists had discovered a viral enzyme called neuraminidase. This enzyme is used by many strains of viruses. It was thought if this enzyme could be blocked then it would lessen the impact of viral infections by inhibiting viral replication. The virus uses neuraminidase to break open cell walls releasing viral particles that have replicated inside the host cell using the host cell RNA and DNA.
The first neuraminidase inhibitor to reach the market was Relenza (zanamivir). Ironically, this drug was not recommended for approval by the FDA advisory committee. However, the FDA leadership overruled the committee’s disapproval and approved it anyway, based on the results of a single clinical trial that showed positive results. Other clinical trials had shown no efficacy over placebo.
Relenza did validate that neuraminidase was a viable target for antiviral medications. The problem with Relenza was its method of administration. It was inhaled as a powder, which proved to be inconvenient and cumbersome. It had very poor oral physicochemical properties.
The scientists at Gilead became interested in neuraminidase inhibitors and soon developed GS-4701 a neuraminidase inhibitor that has a 3700-fold increase in neuraminidase inhibition. This molecule had a different enough structure to be eligible for a separate patent. However, GS-4701 did not show much greater oral bioavailability than Relenza.
What finally occurred at Gilead was the creation of Tamiflu, which is a prodrug. A prodrug is a drug that is metabolized in the body to release the active compound. When Tamiflu is ingested orally it is metabolized to the GS-4701 molecule that has the neuraminidase inhibition activity.
This is a very good example of using prodrugs to bring to market important pharmaceuticals that can be administered more conveniently, thus creating more compliance and effectiveness.
Source: Basic Principles of Drug Discovery and Development, Benjamin Blass
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