Failures In Clinical Trials Biology Essay

Figure 1.7 illustrates the metabolism of vitamin A to its metabolite via aldehyde oxidase (Garattini and Terao, 2012).

1.1.7.2 Reductive transformations

There is evidence to indicate that aldehyde oxidase enzymes can act as reductases in some cases. Nevertheless, these transformations have not been examined as extensively as the oxidative reactions. In vitro investigations have proven that aldehyde oxidase catalyses the reduction of N-oxides. When tested in incubated liver cytosol, from a range of different animals, it was found that imipramine N-oxide was reduced to imipramine (Pryde et al., 2010).

Sulfoxides are also reduced through the catalysing effects of the enzyme. An example of this is sulindac, a non-steroidal anti-inflammatory drug, which is reduced by aldehyde oxidase in guinea pig liver. However, small molecules such as electron donors must be present for this transformation to occur (Pryde et al., 2010).

It has also been noted that nitroguanidine and nitromethylene insecticides can undergo nitro group reductions via aldehyde oxidase in rabbit liver. This results in the formation of the corresponding nitroso or amino metabolites (Garattini and Terao, 2012).

Heterocycle reduction is another type of reductive transformation that aldehyde oxidase enzymes catalyse. This form of reduction is seen with the antipsychotic agent ziprasidone, where the benzisothiazole ring present in this substrate is reduced by the enzyme (Pryde et al., 2010).

Although the evidence for these reductions is well documented for in vitro systems, their relevance in vivo is uncertain. These incubations have to be performed under anaerobic conditions and with the presence of reducing substrates. If these reactions were to occur in vivo, the identities of these substrates would not be known (Garattini and Terao, 2012).

1.1.8 Drug interactions

To prevent the failure of drugs during clinical trials, knowledge of potential drug-drug interactions is important during the drug development process. The first type of drug-drug interaction which can occur is known as the pharmacodynamic effect, which arises when the response to a second drug is changed by the effect of the first drug. The other major category of drug interactions is known as the pharmacokinetic effect, which happens when the exposure of the first administered drug affects the second drug. Although both classes of interactions lead to adverse effects, the pharmacokinetic type is seen much more commonly in patients (Barr and Jones, 2011; Obach et al., 2004).

As with other aspects of aldehyde oxidase, the potential drug-drug interactions have not been investigated in as much detail as other enzymes, such as cytochrome P450. The prediction of such interactions caused by this enzyme is further complicated by the variation in gene expression among different animal species. The number of the drug interactions associated with this enzyme may not be fully established yet, as there are a limited number of drugs which use aldehyde oxidase as their primary metabolism method (Barr and Jones, 2011). To date, only one drug interaction has been identified: the interaction between zaleplon and cimetidine, caused by the inhibition of aldehyde oxidase (Lake et al., 2002). Following a study with human volunteers, it was found that after the administration of an oral dose of 800mg cimetidine, there was a considerable decrease in the clearance of zaleplon (Renwick et al., 2002).

1.1.9 Failures in clinical trials

The failure of drugs in clinical trials is one of the leading reasons for the pharmaceutical industry’s recent interest in aldehyde oxidase. Extensive metabolism of a drug by aldehyde oxidase can lead to a higher clearance than that initially predicted, thus resulting in undesirable pharmacokinetic properties. Table 1.2 includes examples of drugs which are metabolised by aldehyde oxidase that have been discontinued because of clearance or toxicity issues. The systems used to investigate the metabolic clearance properties for these examples involve the use of liver microsomes (LM) which do not fully represent the activity of aldehyde oxidase. They also use extrapolations from studies carried out in animal species which possess a lower aldehyde oxidase activity than humans. Thus, the need for producing a reliable method of predicting in vivo clearance for substrates metabolised by aldehyde oxidase is apparent (Hutzler et al., 2013).

Table 1.2 displays a range of drug candidates metabolised by aldehyde oxidase all of which have been discontinued because of toxicity or clearance issues (Hutzler et al., 2013).

1.2 Cytochrome P450

1.2.1 Background information

The amount of information available regarding the enzyme family cytochrome P450, is substantial especially when compared to that available for aldehyde oxidase. Significantly more research has been carried out in the drug discovery and development area for cytochrome P450. This is largely due to the vast number of drugs which it metabolises.

Cytochrome P450 has various isoforms which in total are accountable for the phase 1 metabolism of approximately 80% of drugs available on the market (Barr and Jones, 2011). The cytochrome P450 enzymes metabolise a large range of xenobiotics due to the broad substrate specificity which they possess (Rosic et al., 2007). In humans, the CYP enzymes belonging to families 1- 4 account for the metabolism of drugs. There are around 30 of these enzymes which are known. It is estimated that around 90% of drug oxidation within humans is carried out by six enzymes (CYP1A2, 2C9, 2C19, 2D6, 2E1 and 3A4/5) (Bibi, 2008).

The increase in reported side effects and drug-drug interactions from substrates which are metabolised via cytochrome P450 enzymes is a problem for pharmaceutical companies producing these drugs. Problems like these can potentially lead to the withdrawal of products from the market thus having costly impacts for those involved (Bibi, 2008). Many patients will be prescribed more than one medication which is metabolised by cytochrome P450, therefore increasing the possibility of an interaction occurring. For this reason, there has been an increase in interest of other methods of metabolism for xenobiotics: metabolism by aldehyde oxidase is an important alternative (Lee et al., 2012).

1.2.2 Aldehyde oxidase and Cytochrome P450

Aldehyde oxidase is not the primary metabolism pathway for many drugs and it often works alongside other enzymes such as cytochrome P450 for xenobiotic metabolism. An example of this is shown by the antidepressant citalopram. This selective serotonin reuptake inhibitor is metabolised to aldehyde intermediates by monoamino oxidase followed by a transformation via cytochrome P450 enzymes. It is then further oxidised by aldehyde oxidase. These two transformations of citalopram are displayed in figure 1.8 (Garattini and Terao, 2012). After establishing that a new drug substrate is metabolised by aldehyde oxidase, it is then important to determine the fraction which this enzyme contributes to the total clearance of a substrate. Knowledge of an enzyme’s contribution to the total metabolic clearance of a compound can aid the design of drugs if this known early in its developmental process. Hydralazine can be used to estimate this fraction due to its selective inhibition of aldehyde oxidase in human hepatocytes (Hutzler et al., 2013).