Drug Metabolism Letters - Volume 3, Issue 2, 2009

Changes in lipids and lopinavir plasma concentrations were examined in 40 HIV-patients exposed to lopinavir/ ritonavir 400/100 mg BID, formulated as tablets and as capsules. Triglycerides and total/HDL-cholesterol ratio were significantly lower with tablets than with capsules. Lopinavir concentrations were higher with tablets than with capsules.

Species and tissue differences in the activity of three major classes of esterases, carboxylesterase (CE), butyrylcholinesterase (BChE) and paraoxonase (PON), were studied. Substantial species differences in activity of these esterases were observed between the mouse, rat, dog monkey and human. Such species differences must be considered when using these preclinical species to optimize the pharmacokinetic properties of ester compounds intended for human use.

To provide a fast and quantitative assessment of CYP inactivation in the drug discovery stage, a mathematical model was derived to calculate enzyme inactivation parameters, kinact and KI, based on experimental data obtained from 2 concentrations of enzyme inactivator. With CYP3A4 inactivators across a range of inactivation potencies, this novel method provided expected rank-ordering of CYP3A4 inactivation. Furthermore, the kinact and KI values obtained in the two-concentration format correlate generally well with the parameters obtained in the six-concentration format.

Myosmine, a minor tobacco alkaloid widely occurring in food products of plant and animal origin, inhibits the conversion of testosterone to estradiol by human aromatase (IC50: 33±2 μM) sevenfold more potent than nicotine (IC50: 223±10 μM) and may have implications for sexual hormone homoeostasis.

From a consideration of first principles for enzymes kinetics, we have employed theoretical methods which enable one to analyse the kinetics of cytochrome P450-mediated reactions which have been based on the known physicochemical principles underlying the majority of chemical or enzymatic reactions. A comparison is made between the correlation equations produced from the QSAR analysis of experimental P450 reaction rate data and those obtained from first principles, where there appears to be a generally satisfactory concordance between the two procedures. In this respect, we have developed expressions based on standard reaction kinetics theory which incorporate the Eyring and Marcus relationships. The analysis of P450-catalyzed reaction rates is elaborated to encompass a treatment of metabolic clearance, and satisfactory correlations are obtained with literature values for both intrinsic clearance and whole body clearance in terms of compound lipophilicity derived from log P data, where P is the octanol/water partition coefficient. The importance of ionization potential as a factor in the overall catalytic turnover of P450-mediated reactions is noted, especially in combination with the lipophilicity parameter, log P.

We investigated the enzyme kinetics of buprenorphine and norbuprenorphine glucuronidation in human liver microsomes and UDP-glucuronosyltransferase (UGT) Supersomes. The involvement of UGT 1A1, 1A3 and 2B7 in buprenorpine and 1A3 in norbuprenorphine glucuronidation were confirmed. Novel involvement of 2B17 with buprenorphine and 1A1 with norbuprenorphine were demonstrated. Scaling of buprenorphine clearance with, or without, correction for the nonspecific microsomal protein binding of buprenorphine (ƒu = 0.42) suggested glucuronidation was a significant route for hepatic clearance of buprenorphine.

Drug candidates with the propensity to induce rat CYP1A1 or 2B1 isoforms are believed to possess a greater tendency to induce hepatic tumors in oncogenicity studies. We have previously published on a manual rat liver slice assay that showed a satisfactory relationship between in vitro CYP2B1 m-RNA induction using real time PCR and the ex vivo pentoxyresorufin O-dealkylase (PROD) activity in liver microsomes prepared from rats treated daily via the oral route for 14 consecutive days with inducers or non-inducers. We now describe this automated in vitro high throughput liver slice technique to screen out drug candidates that are potent rodent CYP1A1 and/or CYP2B1 inducers. A good concordance between in vitro and in vivo data was observed for both CYP1A1 (100 %) and CYP2B1 (90%) isoforms. Automation of key steps has enabled us to increase the annual screening throughput from 200 (manual) to 1500 compounds. The increase in throughput allowed the quick development of structure-induction relationships (SIR's) for multiple drug discovery programs in a facile manner.

Pure silybin was found to be unstable whilst silybin in silymarin was stable in buffers from pH 1.0 to 7.8. The metabolism of silybin was more severe when in its pure form as compared to silybin in silymarin, as tested in a range of biological fluids including plasma, intestinal fluid and liver homogenates. It would appear that components in silymarin have a stabilization effect on its main component silybin.

Anabolic androgenic steroids are the xenobiotic substrates that are metabolized in the body by the protective enzyme systems. Mixed function oxygenase enzymes include a group of enzymes which play an essential role in the metabolism of a broad range of xenobiotics including endogenous and exogenous substrates. Cytochrome P-450, a member of mixed function oxygenase enzymes, plays an important role in oxidative metabolism of drugs and xenobiotics entering human body. Various anabolic steroids are found either to increase or decrease the activity of cytochrome P-450. However, effect of nandrolone decanoate, most commonly abused anabolic steroid, on cytochrome P-450 activity is still fragmentary. In the present study, albino mice were administered intramuscular 2.5 mg of nandrolone decanoate injection at 15 days interval. Cytochrome P-450 activity is determined by following the method of Omura and Sato (1964) in liver and kidney tissues of both normal and experimental groups upto 90 days. Investigation shows a significant (p <0.01) increase of cytochrome P-450 (nmol/mg) activity in liver tissue as compared to that of kidney tissues. A tissue specific and dose specific increase of cytochrome P-450 activity is observed. Mean cytochrome P-450 is found highest in liver tissue on 45th day whereas the activity in kidney tissue is noticed on 90th day of treatment. From the above observation, nandrolone decanoate can be suggested as a potent inducer of cytochrome P-450 activity like other anabolic steroids.

In the early stages of drug discovery, the formation of reactive metabolites is often assessed by co-incubating the drug in liver microsomes with a trapping agent in the presence of NADPH. Our group assessed the capability of commonly used trapping agents to reversibly inhibit major cytochrome P450 (CYP) isoforms. Glutathione and cyanide did not inhibit the enzymes at concentrations up to 10 mM; however methoxylamine did show inhibition, with IC50 values of 0.53 mM for CYP1A2, 4.12 mM for CYP2C9, 2.04 mM for CYP2C19, 9.72 mM for CYP2D6, and 1.26 and >10 mM for CYP3A4/5 (for testosterone and midazolam, respectively, as substrates).