Anti-HIV drugs

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How HIV treatment works
The different classes of anti-HIV drugs
Combination therapy
Anti-HIV drugs currently available

How HIV treatment works

HIV is a type of virus known as a retrovirus. Retroviruses use RNA as their genetic code as opposed to DNA which is found in normal viruses and, of course, human cells. The drugs used to treat HIV are therefore called anti-retrovirals. They are also known simply as anti-HIV drugs.

Once HIV gets inside the white blood cells in your immune system that it targets, known as CD4 cells, it will begin to make copies of itself (it will begin to replicate). This replication process involves a series of different stages, and HIV needs certain enzymes for it to be able to complete three of these stages. Most (but not all) anti-HIV drugs have been created to target these enzymes and stop them from helping HIV to make copies of itself. These anti-HIV drugs are said to inhibit these enzymes.

If you inhibit one of these enzymes then you also stop that stage of the replication process. Without one of the stages of replication, the whole process falls down and HIV cannot then make more copies of itself inside that CD4 cell.

Currently, there are three enzymes that anti-HIV drugs target.

The first enzyme is called Reverse Transcriptase. This enzyme helps HIV convert its RNA (its genetic code) into viral DNA inside a CD4 cell at the start of the replication process. This process is known as 'reverse transcription'.

The second enzyme is called Protease, and this helps the HIV inside a CD4 cell divide up into new HIV copies at the end of the replication process.

The third enzyme is called Integrase, and this enables the viral DNA produced after the 'reverse transcription' process to be incorporated into the CD4 cell's own DNA. This process is known as 'integration'.

The different classes of anti-HIV drugs

Anti-HIV drugs are divided into different classes. These different classes of drugs do different things to stop HIV replicating.

There are six different classes currently available:

Nucleoside Reverse Transcriptase Inhibitors – these were the first group of drugs developed, and are abbreviated to ‘NRTIs’ and commonly known as ‘Nukes’. As the name suggests these inhibit the enzyme Reverse Transcriptase.

Nucleotide Reverse Transcriptase Inhibitors – a much newer class of drugs, and similar to the NRTIs above in that they inhibit Reverse Transcriptase. These are abbreviated to ‘NtRTIs’.

Both of these groups of drugs inhibit reverse transcriptase in a similar way, however they have different chemical structures referred to as either ‘nucleoside’ or ‘nucleotide’.

Non-Nucleoside Reverse Transcriptase Inhibitors – the third class of drugs that inhibit Reverse Transcriptase, but in a different way from the two classes above. These are abbreviated to ‘NNRTIs’ and commonly known as ‘Non-nukes’. As with the two classes above, ‘non-nucleoside’ refers to the chemical structure of the drugs in this class.

Protease Inhibitors – as the name suggests, this class of drugs inhibits the enzyme Protease. These are abbreviated to ‘PIs’.

Entry Inhibitors (includes Fusion Inhibitors and CCR5 Antagonists). Unlike the four classes above, Entry Inhibitors don't actually inhibit an enzyme. Instead they stop HIV from binding (or fusing) with, and then entering, a CD4 cell in the first place. These drugs can be abbreviated to ‘EIs’.

Integrase Inhibitors – this newest class of drugs inhibits the enzyme Integrase. These can be abbreviated to ‘IIs’.

Combination therapy

Treatment for HIV is known as ‘HAART’, which stands for Highly Active Anti-Retroviral Therapy. Usually, at least three drugs from two or more classes of anti-HIV drugs are used together in combination which is why HIV treatment is also sometimes called combination therapy.

The most common combinations of anti-HIV drugs consist of two different drugs from the NRTI / NtRTI classes, then a third drug from either the NNRTI or the Protease Inhibitor class (although Protease Inhibitors are often boosted by a fourth drug, which we talk about in the anti HIV-drugs currently available section below). The two drugs from the NRTI / NtRTI classes are often referred to as your treatment backbone.

These days, most people don’t have to take the drugs more than twice a day, and there are some combinations available that only need to be taken once a day.

Anti-HIV drugs currently available

Below are some tables showing the different anti-HIV drugs currently available. The drugs in each class usually have more than one name. The main names listed in the tables below are the ones most commonly used in the UK. As well as these names, each drug has a brand name given by the drug company that make it. Some drugs, especially those in the NRTI and NtRTI classes, have yet another name. These other names are more commonly used in the US than the names we use in the UK. The dosages given here are the standard dosages prescribed. However, dosages can vary; for instance if you weigh less than 60kg, dosages of certain drugs will probably be lower.

NRTIs

Name (brand name, other name)	Standard dosage	Food restrictions
AZT (Retrovir, zidovudine)	1 x 250mg tablet twice a day	None
3TC (Epivir, lamivudine)	1 x 300mg tablet once a day, or 1 x 150mg tablet twice a day	None
ddI EC (Videx EC, didanosine)	1 x 400mg capsule once a day, or 1 x 200mg capsule twice a day	No food or drink except water for 2 hours before and 1 hour after taking ddI
d4T (Zerit, stavudine)	1 x 40mg capsule twice a day	None
Abacavir (Ziagen)	1 x 300mg tablet twice a day, or 2 x 300mg tablets once a day	None
FTC (Emtriva, emtricitabine)	1 x 200mg capsule once a day	None

NtRTIs

Picture (not to scale)	Name (brand name, other name)	Standard dosage	Food restrictions
	Tenofovir (Viread)	1 x 200mg tablet once a day	None

Protease Inhibitors (PIs)

In most cases these days, protease inhibitors are taken with a small dose of another protease inhibitor called ritonavir (brand name Norvir). The ritonavir acts to boost the levels of the main protease inhibitor in the blood, helping to ensure that levels don’t fall too low to allow HIV to become more active again. Protease inhibitors taken with ritonavir in this way are therefore referred to as ‘boosted PIs’. Ritonavir is almost always only used as a booster, and rarely prescribed as the only protease inhibitor in an anti-HIV drug combination.

Name (brand name)	Standard dosage	Food restrictions
Lopinavir/ritonavir (Kaletra)	Two 200mg lopinavir + 50mg ritonavir tablets twice a day	None
Indinavir (Crixivan) + ritonavir	Two 400mg indinavir capsules + one 100mg ritonavir capsule twice a day	Take at least one hour before or two hours after food
Nelfinavir (Viracept)	Two 625mg or five 250mg tablets twice a day	Take with food
Saquinavir (Invirase) + ritonavir	Two x 500mg saquinavir tables + one 100mg ritonavir capsule twice a day	Take within 2 hours of a meal
Atazanavir (Reyataz) + ritonavir	Two 150mg atazanavir capsules + one 100mg ritonavir capsule once a day	Take with food
Fosamprenavir (Telzir) + ritonavir	One 700mg Fosamprenavir tablet + one 100mg ritonavir capsule twice a day	None
Tipranavir (Aptivus) + ritonavir	Two 250mg tipranavir capsules + two 100mg ritonavir capsules twice a day	Take with food
Darunavir (Prezista) + ritonavir	Two 300mg darunavir capsules + one 100mg ritonavir capsule twice a day	Take with food

NNRTIs

Picture (not to scale)	Name (brand name)	Standard dosage	Food restrictions
	Efavirenz (Sustiva)	One 600mg tablet once a day	None
	Nevirapine (Viramune)	Two 200mg tablets once a day, or one 200mg tablet twice a day	None

Entry Inhibitors

Picture (not to scale)	Name (brand name, other name)	Standard dosage	Food restrictions
	T-20 (Fuzeon, enfuvirtide – a ‘Fusion Inhibitor’)	90mg T-20 powder dissolved in vial of water and injected under the skin twice a day	None
	Maraviroc (Celsentri – a ‘CCR5 Antagonist’)	One 300mg tablet twice a day except: one 150mg tablet twice a day when taken in combination with all boosted PIs except fosamprenavir and tipranavir; two 300mg tablets twice a day when taken with efavirenz	None

Integrase Inhibitors

Picture (not to scale)	Name (brand name)	Standard dosage	Food restrictions
	Raltegravir (Isentress)	One 400mg tablet twice a day	None

Fixed Dose Combinations

Fixed dose combinations contain two or more different anti-HIV drugs combined into just one pill. They are known as ‘fixed dose’ since doctors cannot alter the dosage of any of the drugs contained in these pills. These pills have been developed to reduce the number of pills that people taking anti-HIV drugs have to take.

Name (combined drugs)	Standard dosage	Food restrictions
Combivir (AZT + 3TC)	One tablet containing 300mg AZT + 150mg 3TC twice a day	None
Kivexa (abacavir + 3TC)	One tablet containing 600mg abacavir + 300mg 3TC once a day	None
Truvada (tenofovir + FTC)	One tablet containing 300mg tenofovir + 200mg FTC once a day	None
Trizivir (AZT + 3TC + abacavir)	One tablet containing 300mg AZT + 150mg 3TC + 300mg abacavir twice a day	None
Atripla (tenofovir + FTC + efavirenz)	One tablet containing 300mg tenofovir + 200mg FTC + 600mg efavirenz once a day	Take on empty stomach

You can find more information about the specific anti-HIV drugs you are taking, or thinking of taking, on NAM’s website, Aidsmap. Visit the section on drugs used by people with HIV.