Current Drug Metabolism - Volume 8, Issue 4, 2007

Cytochrome P450 (CYP, P450) is the collective term for a superfamily of heme-containing membrane proteins responsible for the metabolism of ∼ 70 - 80 % of clinically used drugs. Besides the liver and other peripheral organs, P450 isoforms are expressed in glial cells and neurons of the brain. To enlighten their function and significance is a topic of high interest, as most of the neuroactive drugs used in therapy today are not only substrates, but also inducers of brain P450s with far reaching consequences. First of all, brain P450s are regulated differentially from those in liver. The availability of the prosthetic heme group appears to be essential for correct membrane insertion and enzymatic functionality of brain P450s. Furthermore, although not contributing to body's overall drug metabolism, brain P450s fulfil particular functions within specific cell types of the brain. In astrocytes of brain's border lines P450 isoforms are expressed at very high level. They form a metabolic barrier regulating drugs' influx, modulate blood-flow regulation, and act as signalling enzymes in inflammation. In neurons, however, P450s apparently have different function. In specified brain regions such as hypothalamus, hippocampus and striatum they provide signalling molecules like steroids and fatty acids necessary for neuronal outgrowth and maintenance. Induction of these P450s by neuroactive drugs can alter steroid hormone signalling directly in drug target cells, which may cause clinically relevant side effects like reproductive disorders and sexual or mental dysfunction. The understanding of brain P450 function appears to be of major interest in long-term drug mediated therapy of neurological diseases.

Despite the introduction of newer drugs, the atypical antipsychotic clozapine remains the most effective drug in psychotic patients who are resistant to treatment with conventional agents. Optimal therapeutic responses to clozapine have been reported with serum concentrations between 350 μg/L and 1000 μg/L. Clozapine is frequently combined with other drugs to enhance efficacy and reduce adverse reactions but pharmacokinetic interactions can have a significant impact on drug response. The majority of the interactions with clozapine are reported to be mediated by cytochrome P450 (CYP) enzymes. CYP1A2 has a major role in the oxidative metabolism of clozapine, with a minor contribution from CYP3A4, and possibly CYP2D6, CYP2C9 and CYP2C19. Interactions mediated by potent CYP1A2 inhibitors (such as fluvoxamine) or inducers (like cigarette smoke) appear to be consistent, predictable and usually clinically significant. There are many case reports of interactions between clozapine and weak CYP1A2 inhibitors or inducers which are also potent inhibitors or inducers of CYP3A4 or CYP2D6. Researchers often explain these observations on the basis of the CYP1A2 involvement. In addition, there are case reports of clinically significant interactions between clozapine and drugs that are not substrates, inhibitors or inducers of CYP1A2. These interactions are difficult to predict and may not be consistent, as reflected by the conflicting literature reports. Further research to elucidate individual differences in clozapine metabolism, with the potential to detect the dominant roles of CYPs other than CYP1A2, may assist us in predicting these interactions.

RLIP76 or Ral binding protein (RalBP-1) was initially cloned as a Ral-effector that was proposed as a link between Ral and Ras pathways. This protein is encoded in humans on chromosome 18p11.3 by a gene with 11 exons and 9 introns and is found ubiquitously from drosophila to humans. RLIP76 displays inhibitory GTPase activity toward Rho/Rac class G-protein cdc42 which is involved in regulation of cytoskeletal organization, lamellipodia, cell migration and apoptosis via Ras. We have recently shown that RLIP76 is also a multispecific transporter of chemotherapeutic agents and glutathione conjugates (GS-E). In human cells RLIP76 accounts for more than two third of the transport activity for GS-E and drugs as opposed to the ABC-transporters including MRP1, which account for less than one third of this activity. Evidence is mounting that RLIP76 is a stress-responsive multi-specific, non-ABC transporter which represents an entirely novel link between stress-inducible G-protein signaling, receptor tyrosine-kinase signaling, endocytosis, heat-shock and stress defense pathways, and transport mediated drug-resistance. The expression of RLIP76 is significantly greater in human cancer cells of diverse origin as compared to the non-malignant cells. Inhibition of RLIP76, using antibodies towards a cell surface epitope, or depletion of RLIP76 using either siRNA or anti-sense phosphorothioate oligonucleotides preferentially causes apoptosis in malignant cells. Administration of RLIP76 antibodies, siRNA, or anti-sense oligonucleotides to mice bearing syngeneic B16 mouse melanoma tumors causes rapid and complete regression of tumors. Studies summarized in this review strongly suggest that RLIP76 is a logical target for clinical intervention of not only multi-drug resistance but also for diseases resulting from oxidative stress.

The extracts from the roots of Salvia miltiorrhiza Bunge (Danshen) are widely and traditionally used in the treatment of angina pectoris, acute myocardial infarct, hyperlipidemia and stroke in China and other Asian countries. In this study, we have investigated the role of P-glycoprotein (P-gp) in the intestinal absorption of tanshinone IIA (TSA), a major active constituent of Danshen, using several in vitro and in vivo models. The oral bioavailability of TSA was about 2.9-3.4% in rats, with non-linear pharmacokinetics when its dosage increased. In a single pass rat intestinal perfusion model, the permeability coefficients (Papp) based on TSA disappearance from the luminal perfusates (Plumen) were 6.2- to 7.2-fold higher (P < 0.01) than those based on drug appearance in mesenteric venous blood (Pblood). The Pblood, but not Plumen, was significantly increased when co-perfused with verapamil, or quinidine (both P-gp inhibitors). The uptake and efflux of TSA in confluent Caco-2 cells were significantly altered in the presence of verapamil, quinidine, MK-571, or probenecid. The transport of TSA across Caco-2 monolayers was pH-, temperature- and ATP-dependent. Furthermore, the transport from the apical (AP) to basolateral (BL) side of the Caco-2 monolayers was 3.3- to 8.5-fold lower than that from the BL to AP side, but such a polarized transport was attenuated by co-incubated verapamil or quinidine. A polarized transport was also observed in the control MDCKII cells and more apparent in MDR1-MDCKII monolayers, with the Papp values of TSA in the BL-AP direction being 7- to 9-fold higher in MDR1-MDCKII monolayers than those in the control MDCKII cells. Moreover, TSA significantly inhibited P-gp-mediated transport of digoxin in P-gp-overexpressing membrane vesicles with an IC50 of 2.6 μM, but stimulated vanadate-sensitive P-gp ATPase activity with estimated Km and Vmax values of 10.70 ± 0.69 μM and 67.65 ± 1.31 nmol/min/mg protein, respectively. TSA was extensively metabolized to tanshinone IIB (TSB), and two other oxidative metabolites in rat liver microsomes, but the formation rate of TSB in rat intestinal microsomes was only about 1/10 of that in liver microsomes. These findings indicate that TSA is a substrate and reversing agent for P-gp; and P-gp-mediated efflux of TSA into the gut lumen and the first-pass metabolism contribute to the low oral bioavailability. Further studies are needed to explore the role of other drug transporters and first-pass metabolism in the low bioavailability of TSA.

Drug transporters, including efflux transporters (the ATP binding cassette (ABC) proteins) and uptake transporters (the solute carrier proteins (SLC)), have an important impact on drug disposition, efficacy, drug-drug interactions and toxicity. Identification of the interactions of chemical scaffolds with transporters at the early stages of drug development can assist in the optimization and selection of new drug candidates. In this review, we discuss current in vitro and in vivo models used to investigate the interactions between drugs and transporters such as P-gp, MRP, BCRP, BSEP, OAT, OATP, OCT, NTCP, PEPT1/2 and NT. In vitro models including cell-based, cellfree, and yeast systems as well as in vivo models such as genetic knockout, gene deficient and chemical knockout animals are discussed and compared. The applications, throughput, advantages and limitations of each model are also addressed in this review.Drug transporters, including efflux transporters (the ATP binding cassette (ABC) proteins) and uptake transporters (the solute carrier proteins (SLC)), have an important impact on drug disposition, efficacy, drug-drug interactions and toxicity. Identification of the interactions of chemical scaffolds with transporters at the early stages of drug development can assist in the optimization and selection of new drug candidates. In this review, we discuss current in vitro and in vivo models used to investigate the interactions between drugs and transporters such as P-gp, MRP, BCRP, BSEP, OAT, OATP, OCT, NTCP, PEPT1/2 and NT. In vitro models including cell-based, cellfree, and yeast systems as well as in vivo models such as genetic knockout, gene deficient and chemical knockout animals are discussed and compared. The applications, throughput, advantages and limitations of each model are also addressed in this review.

Cryptotanshinone (CTS), a major constituent from the roots of Salvia miltiorrhiza (Danshen), is widely used in the treatment of coronary heart disease, stroke and less commonly Alzheimer's disease. Our recent study indicates that CTS is a substrate for Pglycoprotein (PgP/MDR1/ABCB1). This study has investigated the nature of the brain distribution of CTS across the brain-blood barrier (BBB) using several in vitro and in vivo rodent models. A polarized transport of CTS was found in rat primary microvascular endothelial cell (RBMVEC) monolayers, with facilitated efflux from the abluminal side to luminal side. Addition of a PgP (e.g. verapamil and quinidine) or multi-drug resistance protein 1/2 (MRP1/2) inhibitor (e.g. probenecid and MK-571) in both luminal and abluminal sides attenuated the polarized transport. In a bilateral in situ brain perfusion model, the uptake of CTS into the cerebrum increased from 0.52 ± 0.1% at 1 min to 11.13 ± 2.36 ml/100 g tissue at 30 min and was significantly greater than that of sucrose. Co-perfusion of a PgP/MDR1 (e.g. verapamil) or MRP1/2 inhibitor (e.g. probenecid) significantly increased the brain distribution of CTS by 35.1-163.6%. The brain levels of CTS were only about 21% of those in plasma, and were significantly increased when coadministered with verapamil or probenecid in rats. The brain levels of CTS in rats subjected to middle cerebral artery occlusion and rats treated with quinolinic acid (a neurotoxin) were about 2- to 2.5-fold higher than the control rats. Moreover, the brain levels in mdr1a(-/-) and mrp1(-/-) mice were 10.9- and 1.5-fold higher than those in the wild-type mice, respectively. Taken collectively, these findings indicate that PgP and Mrp1 limit the brain penetration of CTS in rodents, suggesting a possible role of PgP and MRP1 in limiting the brain penetration of CTS in patients and causing drug resistance to Danshen therapy and interactions with conventional drugs that are substrates of PgP and MRP1. Further studies are needed to explore the role of other drug transporters in restricting the brain penetration of CTS and the clinical relevance.

Bacterial infection is a frequent event in renal transplant recipients and often requires the use of antimicrobial agents. In this paper it is reported an evidence of pharmacokinetic interaction between clarithromycin and sirolimus in a kidney transplanted woman, suffering from pulmonary infection sustained by a bacterial pathogen, in particular Hemophilus Influenzae. In the present case report, the concomitant administration of clarithromycin and sirolimus determined impressive increase of sirolimus trough blood concentrations from 6.2 up to 54 ng/mL and this increase was associated with an acute impairment of renal function, almost completely reversed upon both drugs discontinuation. This drug-drug interaction is due to a likely inhibition of activity of both cytochrome P450 3A4 and P-glycoprotein. Although this interaction could be predicted, it represents the first reported clinical evidence.

Epigenetic modifications are reversible chromatin rearrangements that in normal cells modulate gene expression, without changing DNA sequence. Alterations of this equilibrium, mainly affecting the two interdependent mechanisms of DNA methylation and histone acetylation, are frequently involved in the genesis of cancer. The histone code, regulating gene expression, is constituted by the combination of different acetylated lysine residues of histones. In neoplastic cells, the abundance of deacetylated histones is usually associated with DNA hypermethylation and gene silencing. Several compounds, known to have in vitro antineoplastic activity, have been eventually shown to act as histone deacetylase inhibitors. Thus, HDAC inhibitors have been successfully introduced in clinical trials as antitumour agents. They are classified according to their chemical structures and are endowed with different specificity and affinity for the HDACs of classes 1, 2, 4. Among HDAC inhibitors, the most potent are the hydroxamic acid derivatives, like SAHA, which has been recently approved for therapy of cutaneous T-cell lymphomas. Other classes of HDAC inhibitors are short chain fatty acids (SCFA), benzamides, epoxyketone and non-epoxyketone containing cyclic tetrapeptides, and hybrid molecules. SCFA, although widely used (especially valproic acid) and clinically efficacious, have weak HDAC inhibition constants. Benzamides, like MS-275, and cyclic peptides, like depsipeptide, have been studied in numerous clinical trials and demonstrated low toxicity and activity in solid and haematological neoplasms. HDAC inhibitors are also potent radiation sensitizers. Their future in oncology may thus be based on their activity as single agents and on their synergy with the hypomethylating drugs and with chemo- and radiotherapeutics.

To potentiate the response of acute myeloid leukemia (AML) to TRAIL cytotoxicity, we have adopted a strategy of combining nutlin-3, a potent non-genotoxic activator of the p53 pathway, with recombinant TRAIL. The rationale for using such a combination was that deletions and/or mutations of the p53 gene occur in only 5-10% of AML and that TRAIL and nutlin-3 activate the extrinsic and intrinsic pathways of apoptosis, respectively. TRAIL induced a rapid increase of apoptosis when added to OCI M4-type and MOLM M5- type AML cells, carrying a wild-type p53, as well as to NB4 M3-type AML, carrying a mutated p53. On the other hand, the small molecule activator of the p53 pathway nutlin-3 induced p53 accumulation, cell cycle arrest and a slow progressive increase of apoptosis in OCI and MOLM but not in NB4. Of note, nutlin-3 up-regulated the surface expression of TRAIL-R2 and synergized with TRAIL in inducing apoptosis in OCI and MOLM as well as in primary M4-type and M5-type AML blasts, but not in NB4 cells. Moreover, while nutlin- 3 up-regulated the expression of cyclin dependent kinase inhibitor p21, a p53-target gene mediating cell cycle block and showing antiapoptotic activity, the simultaneous addition of TRAIL plus nutlin-3 induced the caspase-dependent cleavage of p21. The relevance of p21 down-regulation for sensitizing AML cells to apoptosis was underscored in knocking-down experiments with small interfering RNAs. Our data suggest that the combined treatment of nutlin-3 plus TRAIL might offer a novel therapeutic strategy for AML.