International Journal of Molecular Sciences Review Pharmaceutical Excipients and Drug Metabolism: A Mini-Review Rahul Patel, James Barker and Amr ElShaer * Drug Discovery, Delivery and Patient Care (DDDPC), School of Life Sciences, Pharmacy and Chemistry, Kingston University, Kingston upon Thames, Surrey KT1 2EE, UK; [email protected] (R.P.); [email protected] (J.B.) * Correspondence: [email protected]; Tel.: +44-(0)20-8417-7416 Received: 7 October 2020; Accepted: 20 October 2020; Published: 3 November 2020 Abstract: Conclusions from previously reported articles have revealed that many commonly used pharmaceutical excipients, known to be pharmacologically inert, show effects on drug transporters and/or metabolic enzymes. Thus, the pharmacokinetics (absorption, distribution, metabolism and elimination) of active pharmaceutical ingredients are possibly altered because of their transport and metabolism modulation from the incorporated excipients. The aim of this review is to present studies on the interaction of various commonly-used excipients on pre-systemic metabolism by CYP450 enzymes. Excipients such as surfactants, polymers, fatty acids and solvents are discussed. Based on all the reported outcomes, the most potent inhibitors were found to be surfactants and the least effective were organic solvents. However, there are many factors that can influence the inhibition of CYP450, for instance type of excipient, concentration of excipient, type of CYP450 isoenzyme, incubation condition, etc. Such evidence will be very useful in dosage form design, so that the right formulation can be designed to maximize drug bioavailability, especially for poorly bioavailable drugs. Keywords: Pharmaceutical excipients; metabolism; cytochrome P450 1. Introduction The most popular route for drug delivery is oral administration because of pain avoidance, ease of ingestion, patient compliance and versatility of drug candidates. Moreover, the manufacturing for oral drug delivery systems is less expensive as the production process is simple and there are no requirements for sterile conditions [1]. The growth rate of the oral drug delivery market between 2010 and 2017 was 10.3% [2]. Despite all the benefits of oral delivery, poor bioavailability of oral formulations is a limiting factor that can alter the efficacy and therapeutic effect [3]. Various factors are contributing to low oral bioavailability including physiological factor, high gastric emptying time, the effect of food, intestinal barrier and enzymatic degradation of drugs (Table1). First-pass metabolism is one of the key factors responsible for poor bioavailability. The extensive metabolism of drugs prior to reaching the systemic circulation is known as the first-pass metabolism. After oral administration, the drug is absorbed by the gastrointestinal tract (GIT) and transported to the liver through the portal veins. Then, the drug is metabolized in the liver before reaching systemic circulation, resulting in a low available concentration at the intended target site (Figure1). Due to insu fficient plasma concentrations, the bioavailability of the drug is significantly reduced and therefore a high dose of the drug is required [4]. Int. J. Mol. Sci. 2020, 21, 8224; doi:10.3390/ijms21218224 www.mdpi.com/journal/ijms Int. J. Mol. Sci. 2020, 21, 8224 2 of 21 Table 1. Examples of drugs with very poor bioavailability with reported reasons. Pharmacological Bioavailability Drugs Reasons References Class (%) Poor solubility and Alendronate Bisphosphonates 0.59–0.78 [5–7] absorption Atorvastatin Statins 14 P-gp and CYP450 activities [8–10] Dopamine receptor Bromocriptine 5–10 Extensive first-pass effect [11–13] agonists Poor solubility and Clodronate Bisphosphonates 1 [5–7] absorption Intestinal and hepatic Cytarabine Antimetabolites 20 [14] first-pass D2 receptor Domperidone 15 Gut and liver first-pass [15] antagonists Anthracycline Hepatic and intestinal Doxorubicin 5 [16] antibiotics metabolism Budesonide Corticosteroids 11 Hepatic first-pass effect [17] Poor solubility and Etidronate Bisphosphonates 5 [6,7,13] absorption Calcium channel Felodipine 15 P-gp and CYP450 activities [17] blockers Calcium channel Isradipine 15 P-gp and CYP450 activities [18] blockers Fluvastatin Statins 20 P-gp and CYP450 activities [8–10] Calcium Channel Nimodipine 13 P-gp and CYP450 activities [19] blockers Hyoscine Antispasmodics 20 Hepatic metabolism [20] Dissociative Hepatic and intestinal Ketamine 20 [21] anesthetics metabolism Lovastatin Statins <5 P-gp and CYP450 activities [8–10] Morphine Opioids 20–33 Gut and liver first-pass [22] Acetylcholinesterase Pyridostigmine 14 Poor absorption [23] inhibitors Extensive first-pass but Naloxone Opioid antagonists 2–10 [24] 90% absorption First-pass, enterohepatic Naltrexone Opiate antagonists 5–40 [10] recycling Poor solubility and Pamidronate Bisphosphonates 1 [5–7] absorption Pravastatin Statins 17–34 P-gp and CYP450 activities [8–10] Intestinal and hepatic Prochlorperazine Phenothiazines 20 [25] first-pass Poor solubility and Risedronate Bisphosphonates <1 [5–7] absorption Monoamine Selegiline oxidase type B 20 Extensive first-pass [26] inhibitors Simvastatin Statins 5–48 P-gp and CYP450 activities [8–10] Serotonin receptor Sumatriptan 20 Hepatic first-pass [27] agonists Cholinesterase Tacrine 10–30 Hepatic first-pass [28] inhibitors Adrenergic Extensive first-pass and Terbutaline 9–21 [29] receptor agonists poor absorption Lidocaine Local anesthetics 3 Hepatic first-pass effect [30] Poor solubility and Tiludronate Bisphosphonates 6 [5–7] absorption Int. J. Mol. Sci. 2020, 21, x FOR PEER REVIEW 3 of 21 Intestinal and Prochlorperazine Phenothiazines 20 [25] hepatic first-pass Poor solubility Risedronate Bisphosphonates <1 [5–7] and absorption Monoamine Extensive first- Selegiline oxidase type B 20 [26] pass inhibitors P-gp and Simvastatin Statins 5–48 [8–10] CYP450 activities Serotonin Hepatic first- Sumatriptan 20 [27] receptor agonists pass Cholinesterase Hepatic first- Tacrine 10–30 [28] inhibitors pass Extensive first- Adrenergic Terbutaline 9–21 pass and poor [29] receptor agonists absorption Hepatic first- Lidocaine Local anesthetics 3 [30] pass effect Poor solubility Tiludronate Bisphosphonates 6 [5–7] Int. J. Mol. Sci. 2020, 21, 8224 and absorption 3 of 21 Figure 1. Schematic diagram depicting the route of poor bioavailability after oral administration of Figurethe drugs. 1. Schematic diagram depicting the route of poor bioavailability after oral administration of the drugs. The principle enzymes responsible for first-pass metabolism or biotransformation of drug moleculesThe principle are cytochrome enzymes P450respons (CYP450).ible forCYP450 first-pass includes metabolism a superfamily or biotransformation of hemeproteins of whichdrug is moleculescategorized are cytochrome into families P450 and (CYP450). subfamilies CYP450 based includes on amino a superfamily acid sequence of homologyhemeproteins [31 ].which They is are categorizedallotted a familyinto families number and (for subfamilies instance, CYP1 based or on CYP2) amino followed acid sequence by a subfamily homology letter [31]. (e.g., TheyCYP1A are or allottCYP2Ced a )family and are number distinguished (for instance, by a numberCYP1 or for CYP2) the individualfollowed by enzyme a subfamily or isoforms letter (e.g., (e.g., CYP1ACYP1A2 or or CYP2CCYP2C19) and)[ are31]. distinguished According to by Coller a number et al., thefor the most individual commonly enzyme prescribed or isoforms drugs in (e.g., the UnitedCYP1A2 States or CYP2C19(USA) are) [31]. reported According to be to metabolized Coller et al., by the CYP1, most CYP2 commonly and CYP3 pres families.cribed drugs The most in the common United enzymesStates (USA)responsible are reported for oxidation to be metabolized of approximately by CYP1, 79% CYP2 of and these CYP3 drugs families. are CYP2C9, The most CYP2D6, common CYP2C19 enzymes and CYP3A4/5 [32]. 2. Traditional Strategies to Overcome Pre-Systemic Metabolism Several novel, as well as traditional, methods are being investigated to circumvent drug metabolism. Prodrugs, enzyme inhibitors, polymeric excipients, self-emulsifying drug delivery systems (SEDDS) and liposomes have been investigated over the years as inhibitors to metabolic enzymes. 2.1. Prodrug Approaches Prodrug is one of the common approaches investigated, according to the literature. Approximately 10% of the drugs in the pharmaceutical market are categorized as prodrugs and one third of small molecular weight (Mw) drugs are classified as prodrugs [33–38]. Prodrugs are pharmacologically inert substances which undergo conversion, once inside the body, to release the active ingredient for its therapeutic effect [39,40]. Prodrugs successfully overcome the drug physicochemical and biopharmaceutical obstacles, thus enhancing their pharmacokinetic properties such as oral absorption and metabolism [41,42]. 2.2. Enzyme Inhibitors Another promising strategy to reduce the pre-systemic metabolism is the co-administration of CYP450 inhibitors with orally administered drugs [43]. Probably, the best-known enzyme inhibitor is Int. J. Mol. Sci. 2020, 21, 8224 4 of 21 grapefruit juice, which significantly improves the oral bioavailability of many drugs [44,45]. Food–drug interaction is a common occurrence and can be significant when the drug’s pharmacokinetics are altered. The classic example is the interaction between grapefruit juice and felodipine. Felodipine is known to undergo high pre-systemic metabolism resulting in very low absolute bioavailability