Step by Step
NRTI
Nucleoside RT inhibitors — chain terminators
NRTIs (tenofovir, emtricitabine, abacavir) work as chain terminators — they lack the 3'-OH group needed to continue building the viral DNA chain, so once incorporated, reverse transcription simply stops.
NNRTI
Non-nucleoside RT inhibitors — a different binding approach
NNRTIs (efavirenz, nevirapine) also target reverse transcriptase, but bind it allosterically (at a separate site) rather than being incorporated into the DNA chain itself.
PI
Protease inhibitors — blocking viral maturation
Protease inhibitors (ritonavir, atazanavir) block the cleavage of viral polyproteins into their functional pieces, resulting in immature, non-infectious virions.
INSTI
Integrase inhibitors — the preferred first-line class
Integrase strand transfer inhibitors (dolutegravir, raltegravir) block the viral DNA from integrating into the host cell's genome. Dolutegravir in particular is currently a preferred first-line agent. Standard treatment combines drugs from multiple classes into a three-drug regimen (ART/HAART) specifically to prevent resistance.
A newly diagnosed HIV patient is started on a three-drug regimen combining an integrase inhibitor with two NRTIs — a standard first-line approach that hits HIV replication at multiple distinct steps simultaneously, making it much harder for the virus to develop resistance to the entire regimen at once.
Applied Walkthrough
1
A patient newly diagnosed with HIV is started on treatment, and the regimen includes dolutegravir alongside two other drugs from a different class.
2
Ask: why not just use a single, highly effective drug rather than combining three? Because HIV mutates extremely rapidly, and a single drug — no matter how effective initially — will eventually select for resistant viral mutants if used alone.
3
Combining drugs that target different steps of the replication cycle (here, an integrase inhibitor plus two chain-terminating NRTIs) means the virus would need to simultaneously develop resistance mutations against multiple, mechanistically unrelated drugs at once — a far less likely event than developing resistance to just one.
4
This is exactly why standard HIV treatment (ART/HAART) is built around combination therapy from the start, rather than being escalated to combination therapy only after a single drug fails, the way antibiotic therapy sometimes is for other infections.
Exam Application
Exams test matching each of the four drug classes (NRTI, NNRTI, PI, INSTI) to its specific point of action in HIV's replication cycle, understanding NRTIs' chain-termination mechanism specifically (no 3'-OH group), and why standard treatment always uses a multi-drug combination regimen rather than single-drug therapy.
⚠ Common Trap
The most common trap is confusing NRTIs and NNRTIs, since both target reverse transcriptase but through completely different mechanisms — NRTIs are incorporated directly into the growing DNA chain and terminate it, while NNRTIs bind allosterically at a separate site without being incorporated at all.
✓ Quick Self-Check
1. How do NRTIs stop HIV replication?
They act as chain terminators — lacking a 3'-OH group, they stop viral DNA chain elongation once incorporated during reverse transcription.
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2. How do NNRTIs differ mechanistically from NRTIs, even though both target reverse transcriptase?
NNRTIs bind reverse transcriptase allosterically at a separate site, rather than being incorporated into the DNA chain the way NRTIs are.
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3. What do protease inhibitors block, and what's the result?
They block cleavage of viral polyproteins into functional pieces, resulting in immature, non-infectious virions.
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4. What do integrase strand transfer inhibitors block, and which drug in this class is a preferred first-line agent?
They block viral DNA from integrating into the host genome; dolutegravir is a preferred first-line agent.
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5. Why is HIV treatment always given as a multi-drug combination rather than a single drug?
Because HIV mutates rapidly, and single-drug therapy would quickly select for resistant mutants; combining drugs targeting different replication steps makes simultaneous resistance to the entire regimen far less likely.
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