Amongst the antiretrovirals, the greatest degree of variability in blood levels is seen with protease inhibitors (PIs). Research into drug absorption shows that PI levels vary between individuals and many people have drug levels outside the normal range (Gatti; Starr; see Key research on testing for drug levels in Anti-HIV therapy: Testing drug levels). In addition, high drug levels are associated with more severe side-effects, while low drug levels are associated with poor viral load response.

Processing protease inhibitors

PIs are processed (metabolised) into inactive products relatively quickly, hence the need for frequent dosing. Their metabolism is dependent on the liver, involving a 'pathway' called the cytochrome P450 system, which is responsible for processing many other drugs and nutrients. Interactions between these different substances can affect the speed at which they are metabolised, causing blood levels to rise or fall.

The activity of P450 is itself variable - some people are rapid P450 metabolisers and others are slow, and this produces variation between individuals in PI levels in the blood. Tests designed to distinguish between rapid and slow metabolisers, one example being an erythromycin breath test, have so far been unable to predict response effectively.

There are gender differences in drug metabolism too. Women have been shown to metabolise saquinavir more slowly than men, resulting in 50% higher blood levels (AUC), due perhaps to lower body weight (although gender was also shown to have an independent effect on saquinavir levels in this study (Brundage 2002) (see Gender differences in drug levels for further information on this topic).

Also, researchers from Ireland have reported that intracellular levels (i.e. within cell levels) of the PIs may not correspond with blood plasma levels, and that intracellular levels may explain viral suppression in the presence of low drug concentrations in the plasma (Hennessy). This finding opens the way for further research into the intracellular pharmacokinetics of PIs.

For discussion of PI levels in the cerebrospinal fluid, the brain, and the genital fluids, see Treating HIV in the brain and other compartments in Anti-HIV therapy: Choosing your treatment strategy.

Effect of low protease inhibitor levels

A poor response to treatment may be due, in part, to low drug levels. A study presented at the 1999 European Conference on Clinical Aspects and Treatment of HIV-Infection found that non-responders were significantly more likely to have sub-optimal drug levels than responders (Urban 1999). This has subsequently been confirmed by other studies (see Key research on testing for drug levels).

This type of research has led to an interest in boosting PI levels by the use of low dose ritonavir which slows the clearance of other PIs from the body.

Effect of high protease inhibitor levels

However, there is a down-side to higher drug levels. While they have a stronger antiviral effect, high drug levels also cause more side-effects and lead to treatment discontinuations. One study, for example, showed that higher peak and trough levels of ritonavir and nelfinavir were predictive of toxicity as well as viral suppression (Marzolini). Another study found that peak and trough drug levels of ritonavir were significantly higher in people with side-effects, compared to people without side-effects (Gatti 1999). High levels of indinavir have also been associated with more side-effects and a higher failure rate (Burger 2003). Women, in particular, may be at risk of increased side-effects due to high drug levels. See Key research on testing for drug levels in Anti-HIV therapy: Testing drug levels for detailed summaries of relevant research.

Poly-glycoprotein

Another factor is growing in importance in our understanding of variations in drug absorption, particularly of PIs. Poly-glycoprotein, or P-gp, is a protein found on the surface of cells in the gut, the blood brain barrier, kidneys, liver and around one in ten CD4 cells, the immune cells which HIV targets. Dubbed the cellular vacuum cleaner, it flushes drugs out of cells and back into the gut.

In terms of HIV pharmacology, P-gp seems to impact on levels of the protease inhibitors found in cells. While some PI levels are lowered by P-gp, nelfinavir, saquinavir and ritonavir all inhibit P-gp. Drugs which induce P-gp (such as St Johns wort) in turn lower intracellular levels of the protease inhibitors. There is preliminary evidence that inhibiting P-gp can increase intracellular PI levels (Kim).

There is also evidence that P-gp plays an important role in preventing PIs from penetrating the brain and fetus, thus creating possible sanctuary sites where HIV can remain largely unchecked. Boosting saquinavir with low-dose ritonavir failed to inhibit P-gp activity in clearing drug from these sites (Huisman 2001).

A review of the Swiss HIV Cohort found that patients with a genetic polymorphism (natural variation) associated with lower plasma drug concentrations paradoxically had better CD4 cell responses six months after starting HAART. MDR-1 TT genotype was associated with an average 257 cell increase from baseline, compared with increases of 157 cells and 121 cells for the other genotypes. MDR-1 TT genotype was associated with the lowest levels of p-glycoprotein, suggesting that lower p-glycoprotein production may facilitate the entry of antiretrovirals into some compartments, particularly lymphocytes. The MDR-1 TT genotype is found in about a quarter of Caucasians, but is considerably rarer in people of African origin (Fellay).

Drug concentrations were also analysed according to genotype for particular cytochrome p450 isoenzymes (CYP3A4, CYP3A5, CYP2D6 and CYP2C19). These isoenzymes are involved in the metabolism of protease inhibitors, so differences between individuals in the capacity of the enzyme to break down the drug might have an effect on response to treatment.

No relationship between cytochrome p450 genotypes and drug concentrations was found except in CYP2D6, where a genetic pattern consistent with slow metabolism of drugs was associated with higher nelfinavir and efavirenz levels. Nor was cytochrome p450 genotype associated with response to treatment.

MRP is another protein which flushes protease inhibitors from cells.

Both P-gp and MRP are expressed by T cells. Expression is regulated by a particular gene which has variable expression in humans; there is evidence that Caucasians are more likely to express P-gp than black Africans (Back). Experts do not currently know whether HIV interferes with these proteins but there is some evidence that P-gp plays a role in HIV binding and entry (Flexner 2001). One recent study found lower expression of P-gp among HIV-infected individuals (Meaden).

People with high levels of alpha 1-acid glycoprotein (AGP) may have reduced antiviral effectiveness, especially if drug-resistant virus is present. AGP is a protein present in plasma which is elevated in people with high viral load. AGP binds protease inhibitors and other drugs and prevents them from acting against free virus in the plasma. AGP is also elevated in individuals with higher body weight and people of African origin.

When assessing the minimum concentration necessary to inhibit viral replication, its important to take into account the effect of protein binding. However, different research groups and drug companies have chosen different ways to do this, which makes it difficult to compare measurements like trough levels and minimum concentrations between studies.

For references, see Measuring protease inhibitor levels in Anti-HIV therapy: Testing drug levels.

References

See Key research on testing for drug levels for references.