4.1. Comparing Parent Drugs to Drug Metabolites
All retrieved drugs and metabolites data are shown in Table 1
. Metabolic oxidation of diclofenac (DCL) gives the unsTable 4’-OH-DCL and 5-OH-DCL, which induce hepatoxicity and can be metabolised further. This is in accordance with the finding that the drug metabolites have lower oxidation potentials than diclofenac. 5-OH-DCL forms a GSH conjugate without NADPH, but 4′-OH-DCL relies on NADPH, which demonstrates that 5-OH-DCL is liable to auto-oxidation [25
Clozapine undergoes first pass metabolism in the liver, mainly catalysed by CYP1A2. Clozapine increases interactions with other drugs, which induces severe adverse effects [74
]. It is further metabolised to give Norclozapine and Clozapine N
-oxide. The redox activities of Clozapine and its metabolite (Norclozapine) are similar [27
Flupirtine metabolism gives glucuronides and mercapturic acid derivatives (via the carbamate), which are unstable and toxic. A reactive diamine radical intermediate is formed which is toxic to cells and genes [75
]. Further metabolism affords D13223. D13223 is a major active metabolite of Flupirtine and is further metabolised. Compared to Flupirtine, D13223 is easily oxidised, but the reduction is more difficult [76
It was found that drug metabolites had lower OP vlaues than the parent drug. For example, 4′-OH-DCL and 5-OH-DCL, Norclozapine and D13223 have lower OP values than their parent drugs Diclofenac, Clozapine and Flupirtine, respectively (Table 1
). Furthermore, these metabolites were more toxic than the parent drug. Among the drug metabolites generated from the same parent drug, metabolites with lower OP are highly toxic compared to others (i.e., 4′-OH-DCL is more toxic than 5-OH-DCL).
4.2. Drug Metabolism Pathways
Oxidation potential (OP) can inform the ability of the compounds to generate reactive oxygen species (ROS), and therefore the oxidative stress induction [77
]. More than one measurement of OP value was collected for some compounds (i.e., Paracetamol, Diclofenac, Trimethoprim, Clozapine and Flupirtine) where available. Secondly, the half-life is an important parameter to assess drug stability alongside hepatic clearance. A less stable drug is intensively metabolised, and both half-life and clearance rate are responsible for drug elimination [78
]. A drug metabolite with a longer half-life is the major (most stable) product, although it is not always safe, as it implies drugs will stay in the body for an extended period before being removed. Hepatoxicity is induced because of the reactive intermediate induced by the metabolism processes. In most cases, metabolites are responsible for toxicity due to bioactivation [79
Although the plot appeared to be random before standardising OP (Figure 1
a), a stronger correlation was seen for the standardised (std.) OP (Figure 1
b). As shown in Figure 1
a, the peak of half-life was seen at 250–500 mV. This newly identified inverse correlation trend, whereby a lower OP has a longer half-life and vice versa, implies that a compound with a lower OP is more biologically stable, rather than reactive. However, increasing the biological stability of a drug implies metabolites (with a lower OP) may stay in the body longer.
A multitude of factors affect a drug’s pharmacology, including stability and toxicity. Clearly, predicting all the drug’s interactions in the body from one factor such as OP is not possible, however a rapid screen of new drug entities’ (or during hit-to-lead development) OP values would give insight into the much later biological behaviour of the compound under study, and provide new knowledge to guide the intelligent design of improved drug molecules.
Due to the limited number of drugs that have both a reported OP and half-life in the literature, a statistical analysis to determine if this inversely proportional trend between OP and half-life is valid would not be relevant.
We therefore looked at the respective drug metabolism pathways for further insight. Promazine is a metabolite of Chlorpromazine [80
]. More than 80% of Promethazine is absorbed in the body, and metabolism is via first-pass (liver), glucuronidation and sulfation [81
Acebutolol is metabolised in the liver to give hepatoxic (active) metabolites such as diacetolol, hydroxylamine and ‘auto-oxidised’ metabolites, which are responsible for toxicity [82
]. Hydrolysis forms an arylamine, and the CYP450 oxidation of arylamine induces toxicity [82
The organometallic 1,1′-ferrocene dimethanol is not actively metabolised.
Warfarin is metabolised by the liver in a regio- and stereo-selective manner. Oxidation gives hydroxywarfarin which accounts for up to 85% of warfarin’s metabolites [83
- and R
-warfarin are metabolised by different CYP450 isoforms, including CYP2C19 and CYP2C8 (with CYP2C19), respectively [83
-warfarin has a shorter half-life (mean 32 h) than R
-warfarin (mean 58 h) [30
The metabolism of Paracetamol is responsible for both its therapeutic effects and toxicity (hepatoxicity) [84
]. Reactive metabolites such as the quinone are rapidly formed. Biotransformation is mainly a detoxification processes, but N
-benzoquinone imine (NAPQI) is responsible for hepatocyte cell death due to GSH’ depletion’ [85
Only 10–20% of Trimethoprim is metabolised in the liver [86
]. Children have a rapid clearance rate, three times greater than adults [87
Amodiaquine is metabolised in the liver and neutrophils to give a metabolite (menaquinone) [88
]. Accumulating metabolites increases free haem level-induced toxicity [89
Caffeine is metabolised in the liver to give Theobromine, Paraxanthine (major) and Theophylline [90
]. The cardiovascular, respiratory, renal and nervous systems are affected by caffeine intake [91
The reaction mechanisms for Carisoprodol have not been entirely elucidated [49
]. CYP2C19 is responsible for hepatic biotransformation [92
It can be seen that the available drugs in the literature that have both oxidation potential and stability measurements (half-life) have a wide range of different metabolic pathways, thus permitting confidence in the inverse relationship between measured OP and drug half-life.
4.3. New Drug OP Measurements and Stability Inference
Next, we applied the knowledge gained from the data-mining experiment to previously unreported bioactive molecules to expand the data set and test the finding of an inverse relationship between OP and half-life. An under-represented drug class in the original data set is the short half-life drugs.
Propanidid was selected as a short-acting phenylacetate general anaesthetic, which has been withdrawn from the market due to its side effect of causing anaphylactic reactions. The primary metabolite for Propanidid is (4-(2-[diethylamino]-2-oxoethoxy)-3-methoxy-benzeneacetic acid (DOMA) [93
]. To the best of our knowledge, no studies in the literature have reported any electrochemical data for propanidid.
Entacapone is used in the treatment of Parkinson’s disease in combination with other drugs. The primary metabolite is the glucuronide formed in the liver, and the remaining 5% is converted to the Z
]. Several electrochemical methods have been reported in the literature for the detection of the metabolites [95
Lidocaine is a local anaesthetic and is one of the most studied drugs for both the detection and isolation of a drug metabolite using electrochemical methods [98
]. The major metabolite of lidocaine is the N
We found that OP has an inverse relationship with the drug’s stability in the body (half-life), with 1450 mV, 1250 mV and 1150 mV correlating to 5.9 min [65
], 24 min [66
] and 87–108 min [67
], for propanidid, entacapone and lidocaine, respectively.