Enzyme-catalyzed processes of first-pass hepatic and intestinal drug extraction

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Abstract

Oral bioavailability of pharmacologically effective drugs is often limited by first-pass biotransformation. In humans, both hepatic and intestinal enzymes can catalyze the metabolism of a drug as it transits between the gastrointestinal lumen and systemic blood for the first time. Although a spectrum of drug biotransformations can occur during first-pass, the most common are oxidations catalyzed by cytochromes P450. It is the isozymes CYP2D6, CYP3A4, CYP1A2, CYP2C9 and CYP2C19 that are most often implicated in first-pass drug elimination. For any given substrate, enzyme specificity, enzyme content, substrate binding affinity and sensitivity to irreversible catalytic events all play a role in determining the overall efficiency, or intrinsic clearance, of elimination. Several models have been proposed over the past twenty-five years that mathematically describe the process of drug extraction across the liver. The most widely used, the well-stirred model, has also been considered for depiction of first-pass drug elimination across the intestinal wall. With these models it has been possible to examine sources of interindividual variability in drug bioavailability including, variable constitutive enzyme expression (both genetic and environmentally determined), enzyme induction by drugs, disease and diet, and intrinsic or acquired differences in plasma protein binding and organ blood flow (food and drug effects). In recent years, the most common application of hepatic clearance models has been the determination of maximum organ availability of a drug from in vitro derived estimates of intrinsic metabolic clearance. The relative success of the in vitro–in vivo approach for both low and highly extracted drugs has led to a broader use by the drug industry for a priori predictions as part of the drug selection process. A considerable degree of effort has also been focused on gut wall first-pass metabolism. Important pathways of intestinal Phase II first-pass metabolism include the sulfation of terbutaline and isoproterenol and glucuronidation of morphine and labetalol. It is also clear that some of the substrates for CYP3A4 (e.g., cyclosporine, midazolam, nifedipine, verapamil and saquinavir) undergo significant metabolic extraction by the gut wall. For example, the first-pass extraction of midazolam by the intestinal mucosa appears, on average, to be comparable to extraction by the liver. However, many other CYP3A substrates do not appear susceptible to a gut wall first-pass, possibly because of enzyme saturation during first-pass or a limited intrinsic metabolic clearance. Both direct biochemical and indirect in vivo clearance data suggest significant inter-individual variability in gut wall CYP3A-dependent metabolism. The source of this constitutive variability is largely unknown. Because of their unique anatomical location, enzymes of the gut wall may represent an important and highly sensitive site of metabolically-based interactions for orally administered drugs. Again, interindividual variability may make it impossible to predict the likelihood of an interaction in any given patient. Hopefully, though, newer models for studying human gut wall metabolic extraction will provide the means to predict the average extraction ratio and maximum first-pass availability of a putative substrate, or the range of possible inhibitory or inductive changes for a putative inhibitor/inducer.

Introduction

It has long been recognized that some drug molecules are much less effective when administered by mouth than when given parenterally. A number of causes for this phenomenon can be considered, including instability of the drug molecule in the gastrointestinal environment, incomplete release of drug from the dosage form, poor intestinal permeability, and inadequate drug concentrations in blood resulting from a delayed or erratic rate of entry into the body. It has also become clear that some orally administered drugs display low systemic availability, and diminished efficacy, because of extensive pre-systemic or first-pass metabolism. While factors that limit drug release in the gastrointestinal lumen can be overcome with proper drug formulation, first-pass metabolism is an unavoidable obstacle to the achievement of optimal bioavailability. Indeed, for some drugs, sic., lidocaine and fentanil opioids, it effectively precludes oral drug therapy. For others, such as the new HIV protease inhibitor, saquinavir, and the immune suppressant, tacrolimus, a mean oral bioavailability of <20% was accepted in the absence of better alternatives. Unfortunately, a metabolic barrier that limits oral bioavailability often brings with it the problem of significant inter-individual variability in systemic blood concentrations of drug as a consequence of variability in metabolic enzyme expression. Understanding the biochemical and physiologic basis for variability in the extent of first-pass drug metabolism is an essential step in optimizing oral drug therapy. This chapter will review general principles of drug clearance by the two principal presystemic eliminating organs, the liver and the small intestine.

Drug metabolizing enzymes in the liver and gut mucosa are well situated to limit the systemic exposure to foreign molecules that have been absorbed from the gastrointestinal lumen. It can be argued that this evolutionary development provides an advantage to herbivorous or omnivorous animal species since numerous molecules, such as the alkaloids, with profound and sometimes adverse pharmacological effects are found in plant food sources. Any foreign molecule that is absorbed into the capillary beds of the small and large intestine must pass through the liver via the hepatic portal vein before access to the rest of the body is achieved (Fig. 1). Only those that are absorbed into the lymphatic system or distal rectum effectively bypass the liver. The most optimal site for drug absorption is across the villi of the proximal small intestine. Columnar epithelia that form the surface barrier to the gastrointestinal lumen (Fig. 2) contain relatively high amounts of oxidative, conjugative and hydrolytic drug metabolizing enzymes. Foreign molecules which gain access to the intestinal capillary bed by diffusion or transport across the lumenal plasma membrane of the most mature enterocytes must pass though this intracellular enzymatic barrier. Thus, if enzyme activities are sufficiently high, first-pass metabolism at the mucosal epithelium can approach 100% extraction efficiency. Similarly, if a foreign molecule moves readily from the hepatic sinusoid into hepatocytes, metabolism during first passage through the liver can also approach 100% extraction efficiency.

The architecture of the gastrointestinal mucosa is exquisitely designed for the task of xenobiotic absorption after oral intake. Drug absorption at this site can be described as a sequence of thermodynamically driven events. Disintegration of the dosage form and dissolution of drug is generally controlled by formulation but can be affected by peristaltic movement, lumenal pH, the release of bile salts and the presence of food. For most drugs (excluding some sustained release formulations), absorption occurs predominately within the duodenum and jejunum. Absorption across the gut wall is mediated by either transcellular and paracellular mechanisms. That is, drug can either diffuse or be transported across the apical and basolateral plasma membranes of villous epithelial cells or it can move between cells by passive diffusion. Studies with Caco-2 cell monolayer cultures —a human colorectal cell line— suggest that transcellular absorption predominates for most lipophilic drugs, whereas polar, hydrophilic compounds are taken up via a paracellular mechanism 1, 2, 3, 4.

Whether the transcellular or paracellular mechanism is operative, the rate of passive drug absorption across the apical epithelium will be dictated, according to Fick's law, by the magnitude of the lumenal–vascular concentration gradient and the permeability of drug through the unstirred water layer at the surface boundary of the lumen–epithelium and through the epithelial apical and basolateral plasma membranes. Movement of drug into the villous epithelium positions it for intracellular, enzyme-catalyzed metabolism. In addition, for many peptide or peptide-based drugs, extracellular, enzyme-catalyzed degradation can also occur, within the epithelial brush border and unstirred water layer. Drug in the intracellular or intercellular space will continue to diffuse along a concentration gradient into the interstitial space found between the epithelial basement membrane and capillary endothelium, and subsequently diffuse across a `leaky' endothelium for delivery into capillary blood. Mucosal capillary blood flow drains into the superior and inferior mesenteric veins which converge to form the hepatic portal vein.

The hepatic portal vein branches within the liver to form the hepatic acinus, a complex capillary bed intimately associated with cells of the liver, including parenchymal cells, the primary site of hepatic drug metabolism. From the perspective of first-pass drug delivery, a hepatic portal–parenchymal cell concentration gradient drives the diffusion of drug across the vascular endothelium, the Space of Disse, and finally, the sinusoidal plasma membrane of the hepatocyte. Diffusion of drug into the hepatocyte competes with bulk flow of blood that removes drug from the sinusoidal space into venules that ultimately form the hepatic vein. The uptake of drug into parenchymal cells is not obligatory during first-pass through the liver. However, drug that enters the parenchymal cell is subject to a wide variety of metabolic and excretory processes. Under conditions when metabolism is rapid and diffusion from blood to the intracellular enzyme active site is not rate-limiting, metabolism acts to maintain a concentration gradient and promote the extraction of drug from the vascular compartment.

Section snippets

Liver enzymes

The drug metabolizing capacity of the liver is impressive. Cytochromes P450, glucuronosyl transferases, sulfotransferases and all other drug metabolizing enzymes are found in abundance [5]. From the standpoint of first-pass metabolism, the family of cytochrome P450 enzymes represents the most important of the hepatic enzymes. It has been estimated that the entire endoplasmic reticulum of the liver, which comprises approximately 5% of body weight (∼1500 g for adults), contains approximately

Enzymology of the gastrointestinal tract

Many of the drug metabolizing enzymes found in human liver have also been detected within the mucosal epithelium of the gastrointestinal tract. These include cytochromes P450 62, 68, 69, 70, 71, 72, 73, glucuronosyl transferases 74, 75, 76, 77, 78, 79, sulfotransferases 75, 80, 81, 82, N-acetyl transferase 78, 83, glutathione S-transferases 70, 76, esterases 78, 83, epoxide hydrolase 69, 70 and alcohol dehydrogenase 84, 85, 86. Many of these enzymes have been implicated in the metabolism of

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