Enzalutamide is rapidly absorbed, with a C
max reached after 1–2 h (Table 1) . N-desmethyl enzalutamide (M2) and the carboxylic enzalutamide (M1) are formed slowly, with a median C
max in plasma reached after 132 and 96 h, respectively . For enzalutamide, the absolute bioavailability is unknown since no intravenous formulation of this agent is available; however, based on data from the mass balance study, approximately 84 % of enzalutamide appears to be absorbed after oral administration . In order to improve the solubility of this BCS class II substance (indicating high permeability and low solubility), the formulation of a soft capsule with enzalutamide dissolved in caprylocaproyl macrogolglycerides (CCMG) was developed . A moderate food effect was observed on the rate of enzalutamide absorption but not on the extent of absorption . Dose-proportional pharmacokinetics were observed from 40 to 360 mg .
Enzalutamide is extensively distributed into tissues and is highly protein bound (approximately 97–98 %), which results in an apparent oral distribution volume (Vd/F) of approximately 110 L . The active metabolite N-desmethyl enzalutamide is equally protein bound (95 %) . In the mass balance and biotransformation study, the overall whole blood to plasma ratio was 0.55, which indicates little binding or distribution to red blood cells . Partitioning to the brain was evaluated in rats , and penetration in human brain tissue still needs to be explored.
As confirmed in a drug interaction study, in vitro studies indicated that enzalutamide is mainly metabolized by CYP2C8, with minor CYP3A4/5 involvement . In a 14C-enzalutamide mass balance study, a total of seven metabolites were identified . The two main metabolites in circulation are the active N-desmethyl enzalutamide (M2), which in vitro is equally active as enzalutamide, and the inactive carboxylic acid metabolite (M1) (Fig. 2) [15, 20]. The mean steady-state C
trough concentrations are similar for enzalutamide and its active metabolite N-desmethyl enzalutamide, and therefore both substances contribute to pharmacological activity. The inactive carboxylic metabolite accounts for approximately 75 % of the exposure [15, 20]. The proposed pathway to form M2 is via M6 and M1, through CYP2C8 and CYP3A4 metabolism (Fig. 2) . N-desmethyl enzalutamide (M2) is metabolized by carboxylesterase 1 to the carboxyl metabolite (M1). No CYP enzymes involved in further metabolism were identified .
Renal elimination is the major route of excretion. Seventy-one percent of the total dose was recovered in urine, primarily as the carboxyl metabolite (approximately 63 %), and only a trace amount of unchanged parent and metabolites were found in urine and feces . In a post hoc population pharmacokinetic analysis based on pre-existing renal function, no difference in clearance for mild and moderate renal impairment was observed  (Table 2). No data are available on pharmacokinetic changes in patients with severe renal impairment.
The influence of mild, moderate, and severe hepatic impairment was evaluated. Enzalutamide and N-desmethyl enzalutamide exposure increased approximately 14 % in patients with mild (Child–Pugh A) and moderate (Child–Pugh B) liver impairment , while in severe hepatic impairment (Child–Pugh C), exposure increased 34 %. Nevertheless, in patients with severe hepatic impairment, the elimination half-life of enzalutamide was increased twofold . Considering the marginal effect on exposure, an adjusted starting dose is not required in patients with mild/moderate hepatic impairment (Table 2) [15, 20, 30].
As mentioned previously, enzalutamide is primarily metabolized by CYP2C8 and also, to a lesser extent, CYP3A4 (Table 3). Therefore, an interaction study was conducted to investigate the effect of gemfibrozil, a strong CYP2C8 inhibitor, on the sum exposure of enzalutamide and N-desmethyl enzalutamide, which increased 2.2-fold . The strong CYP3A4 inhibitor itraconazole increased the sum exposure 1.3-fold . An interaction study with the inducer rifampin showed that exposure to enzalutamide decreased by 66 %, and exposure to N-desmethyl enzalutamide increased 15 %. Consequently, the sum exposure decreased only 37 %, which could be explained by the major effect of rifampicin on enzalutamide metabolism (CYP3A4 and CYP2C8), with only limited effect on N-desmethyl enzalutamide, which is mainly metabolized by carboxylesterase [15, 20] (Fig. 1) [ClinicalTrials.gov identifier NCT02138799). Table 4 provides a summary of drug–drug interactions that affect enzalutamide exposure.
Enzalutamide strongly induces CYP3A4 and moderately induces CYP2C19 and CYP2C9. A decrease in exposure of 86, 70, and 56 %, respectively, was observed for the CYP3A4 substrate midazolam, the CYP2C19 substrate omeprazole, and the CYP2C9 substrate S-warfarin when coadministered with enzalutamide . The drug label warns for coadministration of drugs that are substrates of these enzymes, with a narrow therapeutic index . In contrary to the in vitro observation, the induction of CYP2C8 was deemed not clinically significant when the effect of enzalutamide on pioglitazone exposure, a CYP2C8 substrate, was evaluated . Gibbons et al. reported that the pregnane X receptor (PXR) might also be involved as exposure to the hydroxyl metabolites of the tested substrates decreased while elevated exposures were expected. Induction of the UDP glucuronosyltransferase 1 polypeptide A1 (UGT1A1) via PXR might explain this phenomenon since many metabolites are glucuronidated via UGT1A1 .
Finally, in vitro studies suggested that enzalutamide is an inhibitor of CYP2B6, CYP1A2, and CYP2D6 . Unexpectedly, in a clinical study with CYP1A2 and CYP2D6 probes, the exposure of the CYP1A2 substrate caffeine was reduced by 11 %. Exposure of dextromethorphan, a CYP2D6 substrate, was also reduced by 31 % due to enzalutamide coadministration. In this study, the metabolites of dextromethorphan were elevated. It was suggested that the induction of CYP3A4 and UGT might contribute to the reduced exposure of dextromethorphan (ClinicalTrials.gov identifier NCT02225093). An additional explanation is that enzalutamide is not a CYP2D6 (or CYP1A2) inhibitor in vivo, but a weak CYP2D6 inducer. Lastly, no data are available on the effect of enzalutamide on CYP2B6 substrates in vivo. On the other hand, this might be of less relevance considering the limited number of CYP2B6 substrates used in clinical practice.
In conclusion, strong CYP2C8 inhibitors and strong CYP3A4 inducers have a clinically relevant effect on exposure of enzalutamide, and enzalutamide dose adjustments are indicated when they are combined. The dosage of enzalutamide should be reduced to 80 mg once daily in combination with strong CYP2C8 inhibitors [15, 20]; in combination with strong CYP3A4 inducers, the FDA and EMA approved conflicting labeling. Both regulatory authorities advise against the use of enzalutamide with strong CYP3A inducers. In contrast to the EMA drug label, the drug label approved by the FDA suggests dose elevation to 240 mg once daily when avoidance is undesirable, although this is not based on clinical data [15, 20]. In our opinion, it could be possible to combine enzalutamide with strong CYP3A inducers when the dose of enzalutamide is elevated to 240 mg once daily and drug levels can be measured. In our opinion, drugs that are a substrate of CYP3A4 should be avoided in combination with enzalutamide. Concomitant use of drugs that are substrate to CYP2C19, CYP2C9, or CYP2D6 might also require dose adjustments due to the gradual loss of efficacy in combination with enzalutamide. In our opinion, coadministered substrates of CYP2C19 and CYP2C9 should be avoided, especially when they have a narrow therapeutic index. If dosing is necessary, awareness of this phenomenon is warranted and doses of substrates should be elevated based on efficacy. CYP2D6 substrates will be moderately less active, and substrates with a narrow therapeutic index should be avoided when possible (Table 5). Examples of drugs that are potentially subject to drug–drug interactions with enzalutamide are provided in Table 6.
Enzalutamide is not a substrate of P-gp or the breast cancer resistance protein (BCRP). In in vitro experiments, enzalutamide and N-desmethyl enzalutamide were shown to be inhibitors of P-gp, and the carboxyl metabolite was not an inhibitor or a substrate of P-gp . In the drug–drug interaction study, Gibbons et al. hypothesized that the P-gp transporter may be induced via induction of PXR, while in vitro studies do not support this hypothesis . Furthermore, inhibition of multidrug resistance-associated protein 2 (MRP2), BCRP and OATP1B1 could not be excluded based on in vitro work . The emulsifier CCMG that is used to improve the bioavailability of enzalutamide is known to inhibit P-gp in vitro . In the drug interaction trial of Gibbons et al., a placebo with this emulsifier was used as a comparator, therefore the effect of CCMG could not be determined . Thus far, no clinical studies have been conducted to confirm transporter-mediated drug–drug interactions. Nevertheless, the drug label approved by the EMA states that drugs that are substrates of P-gp should be used with caution .
No formal study has yet been conducted to investigate the influence of patient characteristics on exposure of enzalutamide and N-desmethyl enzalutamide. For registration purposes, a post hoc population pharmacokinetic analysis (healthy volunteers and patients) was conducted in which no significant influence of covariates was identified . The effect of severe renal impairment on the pharmacokinetics of enzalutamide has not been studied. Caution regarding the effects of the enzyme induction of enzalutamide is especially warranted when coadministered with antiepileptic drugs as this may result in lower pharmacokinetic exposure and loss of seizure control . Ethnicity (Asian vs. Caucasian) does not appear to affect the pharmacokinetics of enzalutamide .
Steady-state concentrations of enzalutamide are reached after approximately 1 month. For enzalutamide, no exposure–response relationship was identified for overall survival when administered at 160 mg. This was expected since interpatient variability in enzalutamide and N-desmethyl enzalutamide exposure (AUC, C
min and C
max) is low (≤30 %). In a retrospective analysis, efficacy was similar in the four different exposure quartiles based on steady-state C
trough , which may imply that patients in the lower quartile are treated equally effectively.
In a phase I study, more patients had seizures from dose levels higher than 240 mg/day  Due to this finding and the high incidence of grade 3 fatigue, the dose was lowered before phase III trials started. In the phase III trials, patients with a risk or history of seizures were excluded; however, 0.9 % of patients experienced seizures . An analysis of the association between exposure and new-onset seizures was precluded by the low incidence of seizures in this selected subgroup and the limited variability in the pharmacokinetics of the drug. . A postmarketing safety trial was requested in order to assess the risk of seizure in patients who were excluded from the randomized clinical trial .