Current Drug Metabolism - Volume 4, Issue 3, 2003

The hepatobiliary system and the kidneys are the main routes by which drugs and their metabolites leave the body. Compounds that are mainly excreted into bile in general have relatively high molecular weights, are amphipathic and highly bound to plasma proteins. In contrast, compounds that are predominantly excreted into urine have relatively low molecular weights, are more hydrophilic and generally less protein bound. The first step in drug elimination in liver and kidney is uptake into hepatocytes or into proximal tubular cells. The substrate specificity and affinity of the uptake carriers expressed at the basolateral membranes of hepatocytes and proximal tubular cells could therefore play an important role for the determination of the main elimination route of a compound. This review discusses the tissue distribution, substrate specificity, transport mechanism, and regulation of the members of the organic anion transporting polypeptide (Oatp / OATP) superfamily (solute carrier family SLC21A) and the SLC22A family containing transporters for organic cations (OCTs) and organic anions (OATs). The Oatps / OATPs are mainly important for the hepatic uptake of large amphipathic organic anions, organic cations and uncharged substrates, whereas OCTs and OATs mediate uptake of predominantly small organic cations and anions in liver and kidney.

In developing a drug delivery strategy, issues of absorption, distribution, metabolism, and elimination must be considered. The eye presents unique opportunities and challenges when it comes to the delivery of pharmaceuticals, and is most accessible to the application of topical medications. While absorption by this route is inefficient, there are few side effects.While it has been assumed that topically applied drugs penetrated into the intraocular environment through the cornea, this is currently being reassessed. More recent investigations have shown that the conjunctival route of entry plays an important role in the penetration of drugs into the anterior segment. Furthermore, topically applied drugs have been shown to have access to the sclera from the conjunctiva. As such, it is conceivable that such drugs could find their way to the posterior segment. Data suggest that the sclera is readily permeable to even large molecular weight compounds (∼150 kD).The recent finding that topically applied nepafenac inhibited choroidal and retinal neovascularization by decreasing the production of VEGF, as well as our data showing that even a large molecular weight peptide like insulin can accumulate in the retina and optic nerve after topical application, supports the contention that topically applied drugs can not only reach the posterior segment, but that they can also be therapeutic. Finally, the implications of our findings that topically applied insulin also accumulates in the contralateral eye as well as in the central nervous system are discussed.

Tamoxifen, a non-steroidal antiestrogen, is the class representative of a group of drugs that include toremifene, droloxifene and idoxifene. Tamoxifen has been successfully used worldwide as adjuvant therapy in the treatment of women with breast cancer. However, such therapy results in a slightly increased risk of endometrial cancers. Lifetime exposure of rats to high doses of tamoxifen results in a high incidence of liver tumors. Tamoxifen itself is not genotoxic but is activated in the liver to α-hydroxytamoxifen. This is further conjugated to form the sulfate ester as the putative reactive intermediate. Studies with recombinant human CYPs show only CYP3A4 is able to catalyze the formation of α- hydroxytamoxifen and the irreversible binding of [14C]tamoxifen to DNA. CYP3A4 and CYP2D6 convert tamoxifen to Ndesmethyltamoxifen. The formation 4-hydroxytamoxifen is catalyzed by CYP2D6 and at a much lower level by CYP2C19. In women, detoxication of α-hydroxytamoxifen via a stable glucuronide occurs at a rate in the order of 100 fold higher than in rats whereas rates of sulfation are 3 fold lower than in rats. These factors, together with the low dose of tamoxifen used therapeutically in women, indicates a minimum risk of liver cancers. Results from 32P-postlabeling and accelerator mass spectrometry suggest that low levels of uterine DNA binding does occur but this is probably too low to play a role in uterine tumor development and it is more likely to be the estrogen agonist action of this class of drug that is the most important factor in tumor development in humans.

Gallic acid and its structurally related compounds are found widely distributed in fruits and plants. Gallic acid, and its catechin derivatives are also present as one of the main phenolic components of both black and green tea. Esters of gallic acid have a diverse range of industrial uses, as antioxidants in food, in cosmetics and in the pharmaceutical industry. In addition, gallic acid is employed as a source material for inks, paints and colour developers. Studies utilising these compounds have found them to possess many potential therapeutic properties including anti-cancer and antimicrobial properties.In this review, studies of the effects of gallic acid, its esters, and gallic acid catechin derivatives on Phase I and Phase II enzymes are examined. Many published reports of the effects of the in vitro effects of gallic acid and its derivatives on drug metabolising enzymes concern effects directly on substrate (generally drug or mutagen) metabolism or indirectly through observed effects in Ames tests. In the case of the Ames test an antimutagenic effect may be observed through inhibition of CYP activation of indirectly acting mutagens and / or by scavenging of metabolically generated mutagenic electrophiles.There has been considerable interest in the in vivo effects of the gallate esters because of their incorporation into foodstuffs as antioxidants and in the catechin gallates with their potential role as chemoprotective agents. Principally an induction of Phase II enzymes has been observed however more recent studies using HepG2 cells and primary cultures of human hepatocytes provide evidence for the overall complexity of actions of individual components versus complex mixtures, such as those in food.Further systematic studies of mechanisms of induction and inhibition of drug metabolising enzymes by this group of compounds are warranted in the light of their distribution and consequent ingestion, current uses and suggested therapeutic potential. However, it must be noted that numerous constituents of foodstuffs have been found to be potent modulators of xenobiotic metabolism and the net human health effects may depend on concentrations of individual components and individual genetic makeup.

The UDP glucuronosyltransferase (UGT) content of cells and tissues is a major determinant of our response to those chemicals that are primarily eliminated by conjugation with glucuronic acid. There are marked interindividual differences in the content of UGTs in the liver and other organs. The mechanisms that lead to these differences are unknown but are most likely the result of differential UGT gene expression. Several transcription factors involved in the regulation of UGT genes have been identified. These include factors such as Hepatocyte Nuclear Factor 1, CAAT-Enhancer Binding Protein , Octamer transcription Factor 1 and Pbx2, which appear to control the constitutive levels of UGTs in tissues and organs. In addition, UGT gene expression is also modulated by hormones, drugs and other foreign chemicals through the action of proteins that bind and / or sense the presence of these chemicals. These proteins include the Ah receptor, members of the nuclear receptor superfamily, such as CAR and PXR and transcription factors that respond to stress.