Pharmacology studies at the 2nd IAS conference
Ross Hewitt, HIVandHepatitis.com
The 2nd IAS Conference in Paris featured a session on pharmacology that included data on drug transporters and drug metabolism enzymes, the impact of race, gender and co-infections on pharmacology, gender differences in saquinavir concentrations, pregnancy and nelfinavir, and more.
- Drug transporters and drug metabolism enzymes
- Impact of gender, ethnicity and co-infections on antiretroviral pharmacology
- Gender differences in saquinavir concentrations
- Plasma nelfinavir concentrations are significantly lower in pregnancy
- Multidrug resistance transporter in placentas from HIV infected mothers
- MDR and CYP polymorphisms in South Africa
Drug transporters and drug metabolism enzymes
Dr Charles Flexner from Baltimore led the session off by stating that pharmacologists view the body as one big drug metabolising entity.  However, this approach does not answer all of the questions that face us. Greater understanding of drug metabolism has come with the discovery of xenobiotic response elements (XREs).
These are genetic binding sites for drug-drug transporter complexes that then induce transcription and ultimately protein synthesis of drug metabolising enzymes. Xenobiotics are foreign substances that our bodies recognise and then attempt to detoxify, treating them as if they were dangerous. An example is induction of cytochrome P450 enzymes (CYP 450) by protease inhibitors (PIs).
How is expression of CYP 450 regulated? At first it was thought that increased transcription was responsible. Drugs bound to a pregnane X receptor (PXR) bind to a retinoid X receptor, which then bind to the transcription XRE. Cell lines that we use to predict drug interactions may not express PXR and thus may not be helpful.
XREs are not only linked to CYP 450 3A4 but also linked to CYP 2D6, GST gene, MDR1 (the P-gp gene), and OATP – a variety of enzymes important for drug transport and metabolism. These have evolved as a result of encountering toxic substances in nature in an effort to survive. This explains how one drug can upregulate many enzyme systems. There are other mediators of pathways such as vitamin D, and bile acids. These provide additional complexity and redundancy in eliminating foreign substances
- Amprenavir (APV) is a moderate CYP 3A4 inhibitor and inducer, ritonavir (RTV) is a major inhibitor and moderate inducer.
- Lopinavir (LPV) may have some effects as well.
- APV and LPV/RTV (Kaletra) have a significant but unpredicted drug interaction. APV trough concentrations increased 4.6 fold versus APV alone, while LPV and ritonavir concentrations decreased by 38% when given in standard doses together. Why does this happen?
- APV induces clearance of RTV. Less RTV means less inhibition of LPV clearance. Possible fixes: give more RTV or increase the LPV/RTV dose.
- Increasing ritonavir did result in a better viral load response in one study.
- One study at this meeting showed that giving five LPV/RTV capsules or three LPV/RTV capsules + two extra RTV capsules twice daily both gave higher LPV levels than standard LPV/RTV. 
Some PIs may interfere with RTV’s interaction with PXRs was shown with saquinavir (SQV). Because SQV is not an inducer, the interaction would not have been predicted on knowledge of CYP 3A4 alone.
RTV inhibits rifampin induction of PXR/XRE binding. In the absence of ritonavir, a substantial reduction with rifampin alone occurs. When adding RTV, a reduction in the rifampin effect occurs. St Johns wort has been shown to bind and activate PXR, and explains why it is a CYP 3A4 inducer.
Can RTV be given once a day (steady state elimination half-life of three hours)? He studied RTV and SQV given one daily dose every 48 hours. If you give SQV first, four hours before RTV you get very low SQV levels; however, 48 hours later, after the last RTV 400mg dose, SQV concentrations increase 20 to 30 fold. This appears to be a mechanism that provides a rationale for once daily dosing of boosted PIs.
Finally, there are polymorphisms in these systems, the XREs as well as the genes themselves, that may be the target of future pharmacogenomics studies.
Impact of gender, ethnicity and co-infections on antiretroviral pharmacology
Professor David Back from Liverpool summarised the data on three factors that may influence HIV pharmacology. There is a balance between environmental factors and our own host genetics that interact to create a steady drug level.  In patients, PI and other levels can vary quite widely for the same dose. For example, with nelfinavir (NFV) we see a large range of concentrations, and a marked inter-individual variability exists.
Gender differences do exist. Bioavailability is better in females. Males and females have differences in protein binding, distribution of lipophilic and nonlipophilic drugs and glomerular excretion (kidney function). In a Thai study of SQV pharmacokinetics (PK), on average men had a 12 kg higher body weight, with higher SQV concentrations seen with lower weights. There is also a hint of increased LPV concentrations in women. Within a cohort in Liverpool monitored with therapeutic drug monitoring (TDM), women have higher concentrations as a population then men. Atazanavir (ATV) concentrations are also increased by 20% in women. Women also have 20% higher nevirapine (NVP) and 30% higher efavirenz (EFV) concentrations than men in more than one cohort data set. There is an independent effect of gender above and beyond weight.
Pregnancy has effects on drug concentrations as well. In the third trimester, lower indinavir (IDV) concentrations have been observed. SQV levels in pregnancy in the third trimester were also lower than predicted. TDM might be useful in pregnancy to adjust levels, especially as the pregnancy progresses.
Ethnicity encompasses genetic and environment factors. There are ethnic differences in drug metabolising enzymes. Poor metabolisers using CYP 2C19 have been in observed in Chinese subjects to a much greater extent (13% versus 3%) for other Asian ethnicities. CYP 3A4 has also had ethnic differences definitively identified due to genetic polymorphism. Differences have been shown for the MDR1 genotypes T/T versus C/T versus C/C, a difference between Caucasians and Africans exists. Differences were also seen between Malaysians, Chinese and Asian Indians. Black subjects had lower levels than whites in a US study of LPV. There is also a trend of lower clearance of NVP in Blacks. It is impossible to generalise and we need individual drug data to adequately guide treatment and dosing decisions.
Herbal medicines can potentially interact with antiretrovirals, such as aloe vera, cat’s claw, echinacea, garlic, milk thistle, St John’s wort, and vitamin E, all have potential interactions with HIV medications and antiretrovirals.
Viral co-infection also plays a role in drug metabolism and elimination. Overall, hepatitis C virus (HCV) infected patients had a 50% increase in exposure to NFV, while HCV patients with cirrhosis had a 3-fold increase in exposure. NVP PK also suggest high levels in association with hepatotoxicity. Clearly pharmacologic issues need to be applied to optimise therapy.
Gender differences in saquinavir concentrations
Dr Courtney Flexner from Minneapolis reported gender differences seen in ACTG 359.  This was a salvage study that paired SQV with RTV or NFV and combined the dual PI with delavirdine (DLV) or adefovir (ADV) or both. The study was conducted over 24 weeks with an optional 24-week extension. Patient demographics were 53% white, 27% black, 17% Hispanic. SQV median exposures and trough concentrations were higher with RTV and with NFV.
However, unexpectedly ADV decreased SQV levels. Overall, female concentrations were about 25% higher than male concentrations. Week 16 virologic response was associated with higher drug exposure and women also had a greater response rate, reaching undetectable viral loads almost twice as often as men. When accounting for levels, gender dropouts and the levels remain significant predictors of response.
The interaction of SQV and ADV appears to be similar to the atazanavir and tenofovir interaction that has recently been reported. There was a contribution of body weight to higher concentrations with lower weight leading to higher concentrations, however, even when this is taken into account, sex differences still occur. This might be explained by men having a higher concentration of P-gp, the drug transporter, and thus they are pumping the drug out of the cells at a higher rate.
Plasma nelfinavir concentrations are significantly lower in pregnancy
Amsterdam researchers studied NFV PK in pregnancy.  Pregnancy induces changes, such as induction of liver metabolism, increase in total water and fat, glucocorticoid levels increase and increased gastrointestinal transit time. Low NFV concentrations have been associated with higher risk of virologic failure. They compared 27 pregnant women with 48 nonpregnant women. Pregnant women were younger in age. Levels were measured during all trimesters. Nonpregnant women had slightly less viral load suppression and slightly higher incidence of hepatitis C (15 versus 0%). NFV levels were 65% lower in the pregnant women. All potential confounders could not be accounted for. Protein binding may change as well. Luckily, the low levels of NFV did not result in either virologic failure or infection in their newborns; however, this finding should raise concern regarding the use of NFV in pregnancy and the potential need for TDM while doing so.
Multidrug resistance transporter in placentas from HIV infected mothers
French investigators studied drug transporter p-glycoprotein (P-gp) in the placenta.  P-gp is located on the maternal side of placental cells. They measured MDR1 transcripts (a surrogate for P-gp levels) in the placentas of 28 HIV-negative and 24 HIV-positive women. They isolated chorionic villi from placentas immediately after giving birth. A 3-fold increase in MDR1 expression was seen compared to uninfected women. The increased P-gp levels might result in lower foetal exposure and allow mother to child transmission. This information provides more support for measuring drug levels in pregnant women.
MDR and CYP polymorphisms in South Africa
Researchers from Durban studied genetic differences in South Africa.  P-gp is encoded for by the MDR-1 gene. It traverses the cell membrane, allowing it to pump drugs out of the cell. MDR-1 gene can be measured with the SNP at exon 26 (C3435T) polymorphism and it is associated with differential expression of P-gp. P-gp expression is greater in the CC genotype, less in TT and in between in CT group.
CC genotype patients have lower intracellular levels of drugs. CYP 3A4 1B polymorphisms have been linked to decreased metabolism. They studied African, Indian and Caucasian patients from South Africa. Africans had higher CC genotype incidence, Indians had higher CT genotype and Caucasians had higher TT genotype.
These results suggest that antiretroviral drug levels, and thus successful outcomes, could differ in these different ethnic populations.
Unless stated otherwise, references are to the programme and abstracts of the 2nd IAS Conference on HIV Pathogenesis and Treatment, 13-16 July 2003, Paris.
- Flexner CW. State of the art talk. The new biology of drug transporters and drug metabolising enzymes: implications for clinical practice. Abstract not available.
- Flexner C, Chiu YL, Foit C et al. Steady-state pharmacokinetics (PK) and short-term virologic response of two lopinavir/ritonavir (LPV/r) high-dose regimens in multiple antiretroviral (ARV)-experienced subjects. Abstract 843.
- Back D. State of the art talk. The impact of gender, ethnicity and co-infections on antiretroviral pharmacokinetics. Abstract not available
- Fletcher CV, Jiang H, Brundage RC et al. Sex differences in virological response and saquinavir (SQV) pharmacology in ACTG 359. Abstract 128.
- Wit F, Nellen J, Bergshoeff A, et al. Plasma nelfinavir concentrations are significantly lower in pregnancy.. Abstract 129.
- Camus M, Gil S, Delomenie C et al. Expression and localisation of the multidrug resistant transporter in placentas from HIV-infected mothers. Abstract 130.
- Chelule PK, Mosam A, Gordon M et al. Preparing for HIV-1 therapy in South Africa: will host polymorphisms in MDR1 and CYP3A4 influence therapeutic outcome? Abstract 131.
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