Oseltamivir can be metabolized by the human body into a neuraminidase inhibitor (NI), thereby inhibiting the formation of new virus particles, reducing its spread in the human body, and alleviating flu symptoms. Today's article will share with you the development history of this blockbuster drug.

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The Development History of Tamiflu
Oseltamivir
Influenza (referred to as flu) is an acute respiratory infection caused by influenza virus,
and it is also a highly contagious and fast spreading disease.
Oseltamivir, as the only oral neuraminidase inhibitor drug in the world, has long
been recognized as one of the most effective influenza drugs in the world.
Oseltamivir can be metabolized by the human body into a neuraminidase inhibitor (NI),
thereby inhibiting the formation of new virus particles, reducing its spread in the
human body, and alleviating flu symptoms.
Today's article will share with you the development history of this blockbuster drug.
Design Principles of Anti-Influenza Targeted Drugs
There are three types of human influenza viruses: A, B, and C, which are the
pathogens of influenza (flu).
The influenza A virus was successfully isolated in 1933. The antigenicity of influenza
A virus is prone to mutate, causing many worldwide pandemics. For example, in the
pandemic from 1918 to 1919, at least 20 to 40 million people worldwide died of
influenza.
Influenza B virus was acquired in 1940. Influenza B virus is also relatively pathogenic
to humans, but people have not found that influenza B virus has caused a worldwide
pandemic.
Influenza C virus was not successfully isolated until 1949. Influenza C virus only
causes unobvious or mild upper respiratory tract infections in humans and rarely
causes epidemics.

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Image Source: https://www.cdc.gov/flu/about/viruses/types.htm
The structure of influenza virus includes three parts: envelope, matrix protein and core.
There are three proteins on the surface of influenza A virus particles: M2 ion channel
protein, hemagglutinin (HA) and neuraminidase (NA). The three proteins M2, HA and
NA play an important role in the virus life cycle, and the three proteins have naturally
become important targets for fighting influenza viruses.
Among them, NA is a glycoprotein composed of four subunits. The most
characterized function of NA is its action as a sialidase enzyme. The enzyme helps
viruses to be released after budding from the plasma membrane of a host cell.
Influenza virus membranes contain two glycoproteins: hemagglutinin and
neuraminidase. While the hemagglutinin on the surface of the virion is needed for
infection, its presence inhibits the release of the particle after budding. Viral
neuraminidase cleaves terminal neuraminic acid (also called sialic acid) residues
from glycan structures on the surface of the infected cell. This promotes the release of
progeny viruses and the spread of the virus from the host cell to uninfected
surrounding cells. Neuraminidase also cleaves sialic acid residues from viral proteins,
preventing aggregation of viruses.
According to the theoretical evidence, the researchers speculate that the mechanism
of neuraminidase is that the substrate undergoes hydrolysis of the a-glycosidic bond
under the action of neuraminidase to generate ion intermediate 1, and then water
molecules attack 1, and continue the subsequent reaction process.

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The catalytic mechanism of neuraminidase
Therefore, the design principle of Neuraminidase inhibitors (NAIs) is to simulate the
ionic configuration of intermediate 1 in the transition state of sialic acid, interfere with
the NA catalytic reaction process, and then exert an antiviral effect.
The First Anti-Flu Neuraminidase Inhibitors
(NAIs)-Zanamivir
As mentioned earlier, the design principle of neuraminidase inhibitors is to mimic the
ionic configuration of intermediate 1 in the transition state of sialic acid.
The first influenza virus neuraminidase inhibitor based on substrate design came into
being, which is substrate analog 2.
Molecular structure of compound 1 and 2
However, although the substrate analogue 2 shows certain neuraminidase inhibitory
activity in vitro, it has no specific selectivity for influenza virus and has no activity in
animal models of influenza infection. After that, the researchers began to make simple
structural modifications to 2 in an attempt to find more preferred compounds. However,
the activity of the new derivatives in vivo is still not ideal, and all the sialic acid analogs
have no higher inhibitory activity against influenza virus neuraminidase than the
substrate analog 2.
In view of this, the von Itzstein research team started from the crystal structure of
neuraminidase and conducted a structure-based drug design system study.
According to research, it is found that replacing the C4 hydroxyl group in the substrate
analog 2 with an amino group can further enhance the strength of the salt bridge
formed by the new compound and the Glu119 residue, thereby improving its activity.

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Further analysis showed that a larger group could be accommodated in the active site.
4-amino-4-deoxy-Neu5Ac2en (compound 3) and 4-deoxy-4-guanidino-Neu5Ac2en
(compound 4) were synthesized and proved to be competitive inhibitors for viral
neuraminidase and significantly inhibited both A and B influenza replication in
vitro and in vivo. 4-deoxy-4-guanidino-Neu5Ac2en showed not only to be the better
inhibitor but also showed considerable lower affinity for other isoforms of
neuraminidase. For these reasons, 4-deoxy-4-guanidino-Neu5Ac2en was selected as
the main drug candidate under the name Zanamivir. High polar nature and rapid
excretion contribute to the drug's low bioavailability and rapid elimination.
Molecular structure of compound 3 and 4
Gilead Develops Oral Antiviral Drugs - Oseltamivir
In 1992, Norbert Bischofberger of Gilead Sciences discovered at an ICAAC annual
meeting that the zanamivir being developed by GlaxoSmithKline may become a real
antiviral drug, but its chemical structure is not suitable for oral administration. .
Although scientists at Biota and GlaxoSmithKline feel that the inhaled method may be
more advantageous because the virus mainly affects the patient's respiratory system,
Bischofberger believes that patients may prefer oral administration when faced with
both oral and inhaled administration methods.
So he decided to devote himself to the development of oral anti-flu drugs. Soon after
the meeting, a 10-person scientific research team vigorously invested in the research
and development of the anti-fluid drug project at the headquarters of Gilead.
Initially, the molecular design of oseltamivir was very similar to zanamivir. Gilead
scientists used transition state intermediate 1 and substrate analog 2 as the initial
research objects. In order to facilitate structural modification and transformation, the
researchers replaced the dihydropyran ring skeleton in the substrate analog 2 with a
carbon ring.
At the same time, Intermediate 1 is a phosphonium salt containing a double bond.
Based on the principle of isosteres, the researchers decided to replace the

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dihydropyran ring with cycloalkenes to obtain compounds 5 and 6.
The structure-activity relationship study of compounds 5 and 6 found that the position
of the double bond in the carbocyclic compound is very critical to the activity of the
compound. Compound 5 showed obvious inhibitory activity on neuraminidase, and
finally compound 5 was further studied.
Gilead researchers conducted a large number of technical demonstrations in the early
stage, and finally decided to further modify the alkyl part of the alkoxy substituent in
the seed compound 5. The C1 carboxyl group, C4 acetylamino group, and C5 amino
group are temporarily retained, and only the C3 substituent is further modified. The
researchers synthesized a large number of R group derivatives and substituted the R
group with 2-ethylpropyl to obtain compound 14. The inhibitory activity against
influenza A and B viruses increased suddenly, and the IC value reached the level of
nmol/L.
Continue to replace the R group, compounds with better activity can be obtained, but
these compounds are either effective against only one of influenza A and B viruses, or
the structure is more complex than compound 14 while with equivalent activity.
Therefore, Gilead identified the R group as 2-ethylpropyl and numbered compound 14
as GS4071.
Unfortunately, although GS4071 has excellent in vitro anti-A and B NA activity, in vivo
activity (mice) studies have found that its bioavailability is low and cannot be
developed into an oral drug. Gilead scientists quickly found a solution, by esterifying
compound GS4071 to make a prodrug, and then change its absorption performance.

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Carboxyl modification of GS4071
After a lot of systematic research, researchers finally selected GS4104 as the prodrug
of GS4071, which has the most comprehensive performance. At this point, the
compound structure of oseltamivir was finally determined.
Commercialization of Oseltamivir
In 1996, Roche Pharmaceuticals obtained the right to use oseltamivir and began
clinical research.
In 1998, Oseltamivir obtained a compound patent in the United States.
In 1999, oseltamivir was the first to be approved for listing in Switzerland under the
trade name "Tamiflu". In the same year, oseltamivir was approved for listing in the
United States.
Huateng Pharma, as a professional manufacturer of pharmaceutical APIs and
intermediates, provides Tamiflu API and intermediates with full-scale production. We
have a 34000m2 product base and a 5000m2 R&D lab that can meet the requirement
of worldwide clients.
Related articles:
[1] The Basic Things We Have To Know About Influenza Virus
[2] Xofluza vs. Tamiflu: Differences Between The Antiviral Flu Treatments
[3] Baloxavir Marboxil for Uncomplicated Influenza: Mechanism of Action & Side Effects