**4. Nucleoside reverse transcriptase inhibitors (NRTI's)**

Out of the 13 compounds that have been formally approved for the treatment of human immune deficiency virus (HIV) infections, nine are targeted at the viral reverse transcriptase (RT), the other four (saquinavir, ritonavir, indinavir and nelfinavir) being targeted as the viral protease. Of the nine RT inhibitors that have been licensed, six can be considered as nucleoside reverse transcriptase inhibitors (NRTI's) namely zidovudine (AZT) **(1);** Didanosine (ddI) **(2)**; Zalcitabine (ddC) **(3)**; Stavudine (d4T) [4]; Lamivudine (3TC) **(5)** and Abacavir (ABC) **(6).**

*Substituted Azoles as Non-Nucleoside Reverse Transcriptase Inhibitors Activity DOI: http://dx.doi.org/10.5772/intechopen.110758*

Some of these compounds are still in pre clinical stage of development but others have already proceeded to phase I, II or III clinical trial. Most advanced among new RT inhibitors are () FTC (emitricitabine), F-ddA (lodenosine), PMEA (adefovir) and its oral prodrug form bis (POM)-PMEA [adefovir dipivoxil], PMPA (tenofovir) and its oral prodrug form bis (POC)-PMPA (tenofovir disoproxyl fumarate), which can be considered as NRTI's.


#### **Table 1.**

*Side effects of anti-HIV compounds [23, 24].*

The description of the RT inhibitors as anti-HIV agents should be viewed in the broad scope of anti-HIV therapy (1) and therapeutic approaches for intervention with HIV infections (2), and strategies to overcome or prevent the problem of HIV resistance development to anti-HIV agents in general (3) and NNRTI's particular (4).

Nucleoside reverse transcriptase inhibitors (NRTIs) were the first class of drugs as antiretroviral drugs to gain regulatory approval in 1987. Zidovudine (AZT) was the first drug to be licensed for the treatment of HIV infection [22], a mere four years after the identification of HIV as the etiology agent for AIDS (**Table 1**) [23, 24].
