Caracterizaci´on optoelectr´onica de pel´ıculas

Currently, there is a lack of predictably to drug response in CD. As an example, the corticosteroids have been used in the management of CD and UC for more than 70 years118. Data derived from Olmstead County administrative databases revealed that within 30 days of treatment with a corticosteroid, 60% of CD patients will have a complete response to therapy, while 25% will have only a partial response and 15% will have no response118. Similarly, in clinical trials, we see that rates of non-response are as high as 30% and 50% for budesonide and prednisone respectively91, 119. Many groups

have attempted to uncover predictors of glucocorticoid response, but no predictor has made it to large-scale clinical practice120, 121.

Likewise, response rates to biologic agents vary significantly between patients. Ben-Horin et.al. (2014) summarized rates of biologic resistance seen in clinical trials as well as what was reported in select "real-world" case series. The authors found that approximately 40% of clinical trial subjects and 20% of "real-world" patients were resistant to the effects of infliximab or adalimumab122. In a recent editorial by Jean- Francois Colombel, an international expert in IBD management, the author identified the crux of the IBD therapeutic paradigm: "One of the biggest challenges we are now facing [in the management of IBD] is the search for biomarkers predicting efficacy or failure of biologics and small molecule drugs in the hope of personalized therapy." The lack of predictability in drug response suggests a fundamental gap in our knowledge on how IBD impacts drug metabolism and exposure to large and small molecule drugs. Gaining new insight into CD-specific modifications of drug metabolism and response may allow for improved drug efficacy and reduced drug toxicity as well a better understanding of disease pathogenesis.

1.1.8.3

The Effect of Inflammation on Drug Metabolism

The effect of inflammation on the CYP enzymes is well-established. In vitro studies, in vivo murine models and pharmacokinetic analysis in human subjects in an

acute inflammatory state demonstrate that the expression and activity of CYP enzymes

are down-regulated123. Mechanisms of down-regulation have been suggested including inflammation-induced oxidative stress, the effect of inflammatory cytokines (TNF-α, IL-

1, IL-6) or changes in the activation of NRs such as PXR123. This has not been studied in any significant way in the setting of IBD or specifically in CD. One small study

conducted by Sanaee et.al. (2011) evaluated the impact of CD on a patient's systemic exposure to the CYP3A4 substrate verapamil. The authors reported higher verapamil plasma concentrations in the CD cohort versus healthy controls. Plasma concentrations increased with increasing disease activity124. This suggests a dampening effect of CD on CYP3A4 activity and emphasizes the paucity of available data pertaining to CD-related changes in drug metabolism.

1.2 Cytochrome P450

Drug metabolism is often divided into three phases: phase 1 metabolism characterized by reactions such as oxidation, reduction or hydrolysis that increase the hydrophilicity of a compound; phase 2 metabolism characterized by conjugation

reactions that prepare compounds for excretion; and phase 3, where the products of phase 1 or 2 metabolism are recognized by membrane-bound transporters and are transported to the extracellular space125. The reactions that make up phase 1 drug metabolism are catalyzed by a number of enzymes including, but not limited to, the following enzyme families: CYP, monoamine oxidase (MAO) and FAD-containing mono-oxygenase (FMO)125.

The CYP superfamily is one of the most relevant determinants of drug metabolism in humans and is responsible for the metabolism and activation of a vast number of xenobiotics, including many of the drugs used today126. CYP enzymes are

hemoproteins that are able to reversibly catalyze oxidation and reduction reactions using their heme group through the transfer of electrons127. The CYP enzymes are sub- classified into families (1, 2, 3 and 4), sub-families and isoforms on the basis of their shared amino acid structures. In addition to variations in their amino acid sequences, CYP isoforms vary with respect to their catalytic activity and tissue localization126, 127. CYP isoforms found in the hepatic parynchema are likely the most important enzymes for determining the disposition of drugs used in clinical practice today126 (Figure 1.1a).

Figure 1.1a Relative hepatic abundance of the cytochrome P450 enzymes (adapted from Xie et.al. 2009 Chapter 1, pg 4)125

Figure 1.1b Relative intestinal abundance of the cytochrome P450 enzymes (adapted from Paine et.al. 2006)128

1.2.1. Cytochrome P450 3A

The CYP3A subfamily is a key mediator of drug metabolism and disposition in humans129. Overall, its isoforms are highly concentrated in organs essential to first-pass metabolism such as the liver and the intestinal tract and act as a barrier to the systemic exposure of its orally-ingested substrates130. CYP3A accounts for 28% and 80% of the human hepatic and intestinal CYP content respectively (Figure 1.1a, 1.1b) 125, 128. Three functional enzymes have been indentified: CYP3A4, CYP3A5 and CYP3A7. CYP3A4 is the most abundant and comprehensively investigated of the CYP3A isoforms 131. It is responsible for the metabolism of more than 50% of the drugs that are used in clinical practice 132. It is also important for the inactivation of pollutants and environmental chemicals known as xenobiotics as well as the metabolism of endogenous substances such as steroids, sterols, fatty acids and bile acids132. CYP3A4 is localized to the liver, intestinal tract and kidneys, with the highest expression in the liver 131. In the liver, CYP3A4 is localized to the hepatocyte and biliary epithelium, while in the intestinal tract, CYP3A4 is found at the villous tip of individual enterocytes with the highest expression in the jejunum and ileum131, 133-135. CYP3A5 is similarly distributed, but to a lesser degree. Many of the CYP3A4 substrates are shared substrates of CYP3A5 due to their similar amino acid sequences (>85% shared)136. It is often impossible to distinguish between the contributions of either enzyme to a specific substrate's disposition. CYP3A7 is present only in the fetal liver and plays a minimal role in drug metabolism125.

The expression of CYP3A4 is highly variable. It is inducible by a wide selection of compounds including drugs and endogenous substances; however, its inter-individual variability may also be independent of inducers. Studies by Rogers et.al. (2003)137 and

Floyd et.al. (2003)138 have found up to a 10-fold variation in inter-individual clearance of CYP3A substrates in healthy subjects, while other groups cite variation as high as 20- fold139. Different factors influence CYP3A4 expression to a varying degree. This includes inducers and inhibitors such as a wide selection of drugs and endogenous compounds. Signaling pathways, in particular the nuclear receptor superfamily136, 140, 141 as well as sex, age, disease states and the presence or absence of inflammation are also important for determining its activity142.