Studies of drug disposition in hemodialysis patients : impact of genetics, inflammation and vitamin D

Abstract

Chronic kidney disease (CKD) patients are at a high risk for drug adverse effects due to accumulation of drugs, which normally are excreted via the kidneys. It is believed that drugs that are metabolized by the liver are safe to prescribe in normal doses to end-stage renal disease (ESRD) patients. However, emerging data show that kidney failure itself can affect enzymes and drug transporters. Thus, the concentration of many drugs that normally are metabolized by the liver increases significantly in CKD patients. A common feature of the uremic phenotype that may affect drug metabolism is persistent inflammation. Inflammation has been shown to reduce the activity of both drug metabolizing enzymes and transporters. The increased risk of drug-drug interaction implicates the necessity of safer and individualized adjusted drug dosing methods. Factors that are important I this matter need to be identified. We have studied the impact of genetic polymorphism, inflammation and vitamin D on drug metabolism in prevalent hemodialysis patients.

In paper I we studied the influence of three factors on drug disposition: genetic polymorphism, impaired renal excretion of drug metabolites and the possible elimination by hemodialysis (HD), using codeine as a test substance. Based on the genotyping of three CYP2D6 polymorphisms in 228 HD patients, 9 extensive metabolizers (EMs) and 2 poor metabolizers (PMs) were given a single oral dose of 50 mg codeine phosphate. Plasma levels of its metabolites codeine-6-glucuronide (C6G), morphine-3-glucuronide (M3G) and morphine-6-glucuronide (M6G) were determined after 2, 4, 6, 8 and 24 hours (beginning of the HD session), respectively and again after 4 hours of HD (28 hours). Our results showed that the formation of the codeine metabolites M3G and M6G was dependent on the CYP2D6 genotype as previously shown in healthy subjects. The elimination of the glucuronides in these patients was absent until the dialysis was performed. Our data suggest that CYP genotypes need to be taken into consideration, when drugs metabolized by polymorphic CYPs are prescribed in HD patients.

In Papers II and III, we studied the effect of inflammation on cytochrome P450 3A4 (CYP3A4) activity using alprazolam and quinine as test drugs, both known substrates of CYP3A4. Twenty-six prevalent HD patients were included in study II and 44 prevalent HD patients in study III. Each patient received a single dose of the test drug and concentration of the test drugs and their metabolites were measured at the beginning of the next HD session. Inflammatory markers were followed prior to the drug test. The ratio of unconjugated alprazolam/4-hydroxyalprazolam and 4β-OH-cholesterol/cholesterol (paper II) and ratio of quinine/3-OH-quinine and 4β-OH-cholesterol/cholesterol (paper III) were used as surrogate markers of CYP3A4 activity. In both studies, we found significant correlation between inflammation and CYP3A4 expressed as alprazolam/4-hydroxyalprazolam and quinine/3-OH-quinine respectively, but not with 4β-OH-cholesterol/cholesterol. Our results suggest that inflammation down-regulate the activity of CYP3A4. Further studies are needed to confirm this finding and to assess the extent and duration of the effect of inflammation. CYP3A4 metabolizes almost 50% of all drugs currently used in health care. Thus, if persistent inflammation affects CYP activities this will have important clinical impact by increasing the risk of drug-drug interaction and adverse side effects.

In paper IV we studied the impact of 25-OH-cholecalciferol on CYP3A4 activity. Eight prevalent hemodialysis (HD) patients completed the study. The concentration of 25-OH-vitamin D3 was measured at the start of the study and subsequently once a month. Subjects were given a daily dose of 800-1600 IU of 25-OH-vitamin D3 until its concentration was close to 75 nmol/L. A single dose of 100 mg quinine was given to each subject. Concentration of quinine and its metabolite 3-OH-quinine were measured 12 hours after drug intake at the beginning of the next dialysis. Ratio of quinine/3-OH-quinine and 4β-OH-cholesterol/cholesterol were used as surrogate markers for CYP3A4 activity. Concentrations of inflammatory markers were measured at the beginning and at the end of the study. Our results show no significant association between 25-OH-vitamin D and CYP3A4 activity expressed as either quinine/3-OH-quinine or 4β-OH-cholesterol/cholesterol. Our finding suggests that short term supplementation of 25-OH-vitamin D3 does not affect CYP activity.