There are six different classes of anti-retroviral drugs that are currently being used in the treatment of infections caused due to retroviruses like HIV.
1. Nucleoside analogue reverse transcriptase inhibitors (NRTIs): These drugs inhibit the replication of HIV by blocking the enzyme called reverse transcriptase. These drugs get incorporated into the newly synthesized viral genome and function as indirect inhibitors of the enzyme. Some examples include zidovudine (Retrovir), lamivudine (Epivir) didanosine (Videx), zalcitabine (Hivid), stavudine (Zerit) and abacavir (Ziagen).
2. Protease inhibitors (PIs): These drugs function by inhibiting the viral assembly mechanism by blocking an enzyme called Protease. Important among this class include saquinavir (Invirase), ritonavir (Norvir), indinavir (Crixivan), nelfinavir (Viracept), amprenavir (Agenerase), lopinavir, atazanavir (Reyataz) and tipranavir (Aptivus). Darunavir (Prezista)
3. Non-nucleoside reverse transcriptase inhibitors (NNRTIs): These are another important class of retroviral drugs whose function is to bind directly to the reverse transcriptase enzyme and inhibit its function. Reverse transcriptase is important for the replication of viral DNA in HIV. Prominent members among this class include nevirapine (Viramune), delavirdine (Rescriptor), efavirenz (Sustiva) and etravirine (Intelence). These drugs have been approved by FDA for treating HIV infected patients.
4. Nucleotide reverse transcriptase inhibitors (NtRTIs): Mode of action of this class of drugs is quite similar to that of nucleoside analogue reverse transcriptase inhibitors. NtRTIs get incorporated into the viral genome and prevent its replication by inhibiting the reverse transcriptase enzyme. However, these drugs are more potent than NRTIs. The only approved drug in this class, tenofovir (Viread), inhibits both HIV and hepatitis B and appears to be effective in people who are resistant to NRTIs.
5. Fusion inhibitors or Entry inhibitors: This class of drugs is currently the most effective solution against drug-resistant strains of HIV. Fusion inhibitors interfere with the binding, fusion and entry of the viral particle into the host cell. Maraviroc and Enfuvirtide are currently the only available drugs in this class.
6. Integrase inhibitors: These drugs interfere with the activity of a special enzyme called integrase that helps in the integration of viral genome into the DNA of the infected cells. Raltegravir (Isentress) is the only drug in this class to have received FDA approval.