Hepatitis C has long been a formidable disease with a high death toll and inadequate treatment. However, in the recent years we have seen much more successful treatment options on a backbone of a newly discovered therapeutic molecule called sofosbuvir. 

Sofosbuvir is an antiviral molecule, structurally similar to existing antivirals, but it has proven to be of great benefit to Hepatitis C patients – it almost single-handedly skyrocketed the cure rate from 50% to 95% overnight. This has brought the company who invented brought it to market under the tradename Sovaldi, and later Harvoni, tremendous success. This molecule is currently the best-selling pharmaceutical ingredient in the world, because it plays an essential role in modern Hepatitis C treatment regimen.

How Sofosbuvir Works

Essentially, sofosbuvir molecule works by preventing Hepatitis C virus (HCV) to replicate itself. One of the necessary components for replication is the RNA molecule of the virus. In order to make a copy of RNA for the new virus to form, virus uses an enzyme called RNA polymerase to replicate RNA. Basically, out of 1 existing RNA molecules, and additional RNA is formed in order to be used to construct a new virus and facilitate the propagation of Hepatitis C virus – which for a patient means Hepatitis C will manifest more and more in his or her liver.

Sofosbuvir works by inhibiting enzyme RNA polymerase – the cellular worker that performs the replication process. Being unable to make new viruses due to sofosbuvir mechanism of action, the existing Hepatitis C viruses are at the mercy of patient's immune system which is readily eliminating the virus. Without sofosbuvir, Hepatitis C virus has an ability to rebuilt the lost viruses and build up in the liver. When sofosbuvir, however, is allied with patient's own immune system, there is only a 5% probability that the virus will survive (with appropriate treatment regimen). This is why sofosbuvir has a 95% success rate. 

Sofosbuvir Mechanism of Action

Here is how sofosbuvir molecule looks like in chemical terms. We are about to explain in detail how sofosbuvir works or, in other words, what is sofosbuvir mechanism of action (sofosbuvir MOA).

Sofosbuvir as prodrug

Sofosbuvir is a prodrug – this means that the molecule is chemically changed to a therapeutically useful molecule in a human body. If you look on the structure (on the right); the left part of the molecule remains unchanged while the right part surrounded around P (phosporous atom) will readily change into a therapeutic form with the help of enzymes in the body. In essence, nucleotidyl and nucleoside diphospate kinases are two human enzymes which add phospatic groups to the molecule once the existing groups on P have been cleaved. 

Only when this process and transition from sofosbuvir as prodrug to sofosbuvir as drug is done within the body, the molecule becomes a very effective Hepatitis C treatment.

How does Sofosbuvir enter Hepatitis C virus

The created sofosbuvir drug in the body has to enter Hepatitis C virus in order to be effective in disrupting HCV replication. This is the process that makes sofosbuvir stand out as an effective Hepatitis C therapeutical substance.

It needs to be mentioned that sofosbuvir is not out-of-the-box thinking molecule. Actually, it is very similar to existing antiviral molecules such as acyclovir for example, but one small but very important difference. Every antiviral in this group of therapeutics has to have 3 phosphate groups (Ps in the molecule) to be effective.  Usually, the addition of the first PO _4 ^-3  phosphate group is the hardest and thus slowest, the other two phosphate groups are then readily added. 

Sofosbuvir was designed to marvelously solve this problem by having the first phosphate group already attached to its structure. Thus there is no need for a lengthy and hard process of adding the group. The other two phosphate groups are quickly added to sofosbuvir and the so-called prodrug is now an active drug.  Of additional benefit are also the groups that are bound to the phosphate group of the molecule – these work by masking the negative (-) electric charge of a phosphate group and by that facilitate the entrance of sofosbuvir into the virus. 

 Prior to the discovery of sofosbuvir, a variety of nucleoside analogs had been examined as antihepatitis C treatments, but these exhibited relatively low potency. This low potency arises in part because the enzymatic addition of the first of the three phosphate groups of the triphosphate is slow. The design of sofosbuvir, based on the protide approach, avoids this slow step by building the first phosphate group into the structure of the drug during synthesis. Additional groups are attached to the phosphorus to temporarily mask the two negative charges of the phosphate group, thereby facilitating entry of the drug into the infected cell.[30][31] The NS5B protein is a RNA-dependent RNA polymerase critical for the viral reproduction cycle.