Pharmacological target-mediated drug disposition (TMDD) represents a special source of nonlinear pharmacokinetics, and its occurrence in large-molecule compounds has been well recognized because numerous protein drugs have been reported to have TMDD due to specific binding to their pharmacological targets. Although TMDD can also happen in small-molecule compounds, it has been largely overlooked. In this mini-review, we summarize the occurrence of TMDD that we discovered recently in a series of small-molecule soluble epoxide hydrolase (sEH) inhibitors. Our journey started with an accidental discovery of target-mediated kinetics of 1-(1-propanoylpiperidin-4-yl)-3-[4-(trifluoromethoxy)phenyl]urea (TPPU), a potent sEH inhibitor, in a pilot clinical study. To confirm what we observed in humans, we conducted a series of mechanism experiments in animals, including pharmacokinetic experiments using sEH knockout mice as well as in vivo displacement experiments with co-administration of another potent sEH inhibitor. Our mechanism studies confirmed that the TMDD of TPPU is due to its pharmacological target sEH. We further expanded our evaluation to various other sEH inhibitors and found that TMDD is a class effect of this group of small-molecule sEH inhibitors. In addition to summarizing the occurrence of TMDD in sEH inhibitors, in this mini-review we also highlighted the importance of recognizing TMDD of small-molecule compounds and its impact in clinical development as well as using pharmacometric modeling in facilitating quantitative understanding of TMDD.

1-(1-propanoylpiperidin-4-yl)-3-[4-(trifluoromethoxy)phenyl]urea (TPPU) and 1-(4-trifluoro-methoxy-phenyl)-3-(1-cyclopropanecarbonyl-piperidin-4-yl)-urea (TCPU) are potent inhibitors of soluble epoxide hydrolase (sEH) that have much better efficacy in relieving nociceptive response than the Food and Drug Administration-approved drug gabapentin in a rodent model of diabetic neuropathy. Experiments conducted in sEH knockout mice or with coadministration of a potent sEH displacer demonstrated that the pharmacokinetics of TPPU and TCPU were influenced by the specific binding to their pharmacologic target sEH, a phenomenon known as target-mediated drug disposition (TMDD). To quantitatively characterize the complex pharmacokinetics of TPPU and TCPU and gain better understanding on their target occupancy, population pharmacokinetics analysis using a nonlinear mixed-effect modeling approach was performed in the current study. The final model was a novel simultaneous TMDD interaction model, in which TPPU and TCPU compete for sEH, with TCPU binding to an additional unknown target pool with larger capacity that we refer to as a refractory pool. The total amount of sEH enzyme in mice was predicted to be 16.2 nmol, which is consistent with the experimental value of 10 nmol. The dissociate rate constants of TPPU and TCPU were predicted to be 2.24 and 2.67 hours^-1, respectively, which is close to the values obtained from in vitro experiments. Our simulation result predicted that 90% of the sEH will be occupied shortly after a low dose of 0.3 mg/kg TPPU administration, with ≥40% of sEH remaining to be bound with TPPU for at least 7 days. Further efficacy experiments are warranted to confirm the predicted target occupancy. SIGNIFICANCE STATEMENT: Although target-mediated drug disposition (TMDD) models have been well documented, most of them were established in a single compound scenario. Our novel model represents the first TMDD interaction model for two small-molecule compounds competing for the same pharmacological target.

Epoxide hydrolases (EHs) are enzymes that play roles in metabolizing xenobiotic epoxides from the environment, and in regulating lipid signaling molecules, such as juvenile hormones in insects and epoxy fatty acids in mammals. In this study we fed mosquitoes with an epoxide hydrolase inhibitor AUDA during artificial blood feeding, and we found the inhibitor increased the concentration of epoxy fatty acids in the midgut of female mosquitoes. We also observed ingestion of AUDA triggered early expression of defensin A, cecropin A and cecropin B2 at 6 h after blood feeding. The expression of cecropin B1 and gambicin were not changed more than two fold compared to controls. The changes in gene expression were transient possibly because more than 99% of the inhibitor was metabolized or excreted at 42 h after being ingested. The ingestion of AUDA also affected the growth of bacteria colonizing in the midgut, but did not affect mosquito longevity, fecundity and fertility in our laboratory conditions. When spiked into the blood, EpOMEs and DiHOMEs were as effective as the inhibitor AUDA in reducing the bacterial load in the midgut, while EETs rescued the effects of AUDA. Our data suggest that epoxy fatty acids from host blood are immune response regulators metabolized by epoxide hydrolases in the midgut of female mosquitoes, inhibition of which causes transient changes in immune responses, and affects growth of microbes in the midgut.

COX metabolites of 8,9-EET, previously observed as potent mitogenic lipid mediators, were synthesized for the first time by using two synthetic approaches. These synthetic materials allow for structural confirmation of COX metabolites of 8,9-EET and further study of their biological roles.

Epoxyeicosatrienoic acids (EETs) are produced by cytochrome P450 epoxygenases from arachidonic acid, and their rapid metabolism is mainly through soluble epoxide hydrolase (sEH). EETs exert vasodilatory, anti-inflammatory, anti-apoptotic, and pro-angiogenic effects. Administration of sEH inhibitors before or at the onset of stroke is protective, but the effects of post-treatment at reperfusion, when inflammation is augmented, has not been as well studied. We tested the hypothesis that 1-Trifluoromethoxyphenyl-3-(1-propionylpiperidin-4-yl)urea (TPPU), a potent and highly selective sEH inhibitor, suppresses inflammation and protects the brain when administered at reperfusion. Vehicle or 1 mg/kg TPPU was administered at reperfusion after 90 minutes of focal ischemia and again 24 hours later. Protein expression and activity of sEH increased after reperfusion and activity was decreased by TPPU administration. TPPU decreased infarct volume by 50%, reduced neurologic deficits and improved performance on sensorimotor tasks. Furthermore, TPPU significantly lowered the mRNA expression of interleukin-1beta by 3.5-fold and tumor necrosis factor-alpha by 2.2-fold, increased transforming growth factor-beta mRNA by 1.8-fold, and augmented immunostaining of vascular endothelial growth factor in peri-infarct cortex. Thus, inhibition of sEH at reperfusion significantly reduces infarction and improves sensorimotor function, possibly by suppressing early proinflammatory cytokines and promoting reparative cytokines and growth factors.

Cytochrome P450 (CYP) monooxygenases epoxidize the omega-3 polyunsaturated fatty acid (PUFA) docosahexaenoic acid into novel epoxydocosapentaenoic acids (EDPs) that have multiple biological actions. The present study determined the ability of the most abundant EDP regioisomer, 19,20-EDP to reduce kidney injury in an experimental unilateral ureteral obstruction (UUO) renal fibrosis mouse model. Mice with UUO developed kidney tubular injury and interstitial fibrosis. UUO mice had elevated kidney hydroxyproline content and five-times greater collagen positive fibrotic area than sham control mice. 19,20-EDP treatment to UUO mice for 10 days reduced renal fibrosis with a 40%-50% reduction in collagen positive area and hydroxyproline content. There was a six-fold increase in kidney α-smooth muscle actin (α-SMA) positive area in UUO mice compared to sham control mice, and 19,20-EDP treatment to UUO mice decreased α-SMA immunopositive area by 60%. UUO mice demonstrated renal epithelial-to-mesenchymal transition (EMT) with reduced expression of the epithelial marker E-cadherin and elevated expression of multiple mesenchymal markers (FSP-1, α-SMA, and desmin). Interestingly, 19,20-EDP treatment reduced renal EMT in UUO by decreasing mesenchymal and increasing epithelial marker expression. Overall, we demonstrate that a novel omega-3 fatty acid metabolite 19,20-EDP, prevents UUO-induced renal fibrosis in mice by reducing renal EMT.

Soluble epoxide hydrolase (sEH), a novel therapeutic target for neuropathic pain, is a largely cytosolic enzyme that degrades epoxy-fatty acids (EpFAs), an important class of lipid signaling molecules. Many inhibitors of sEH have been reported, and to date, the 1,3-disubstituted urea has the highest affinity reported for the sEH among the central pharmacophores evaluated. An earlier somewhat water soluble sEH inhibitor taken to the clinic for blood pressure control had mediocre potency (both affinity and kinetics) and a short in vivo half-life. We undertook a study to overcome these difficulties, but the sEH inhibitors carrying a 1,3-disubstituted urea often suffer poor physical properties that hinder their formulation. In this report, we described new strategies to improve the physical properties of sEH inhibitors with a 1,3-disubstituted urea while maintaining their potency and drug-target residence time (a complementary in vitro parameter) against sEH. To our surprise, we identified two structural modifications that substantially improve the potency and physical properties of sEH inhibitors carrying a 1,3-disubstituted urea pharmacophore. Such improvements will greatly facilitate the movement of sEH inhibitors to the clinic.

Arachidonic acid (ARA) is metabolized by cyclooxygenase (COX) and cytochrome P450 to produce proangiogenic metabolites. Specifically, epoxyeicosatrienoic acids (EETs) produced from the P450 pathway are angiogenic, inducing cancer tumor growth. A previous study showed that inhibiting soluble epoxide hydrolase (sEH) increased EET concentration and mildly promoted tumor growth. However, inhibiting both sEH and COX led to a dramatic decrease in tumor growth, suggesting that the contribution of EETs to angiogenesis and subsequent tumor growth may be attributed to downstream metabolites formed by COX. This study explores the fate of EETs with COX, the angiogenic activity of the primary metabolites formed, and their subsequent hydrolysis by sEH and microsomal EH. Three EET regioisomers were found to be substrates for COX, based on oxygen consumption and product formation. EET substrate preference for both COX-1 and COX-2 were estimated as 8,9-EET > 5,6-EET > 11,12-EET, whereas 14,15-EET was inactive. The structure of two major products formed from 8,9-EET in this COX pathway were confirmed by chemical synthesis: ct-8,9-epoxy-11-hydroxy-eicosatrienoic acid (ct-8,9-E-11-HET) and ct-8,9-epoxy-15-hydroxy-eicosatrienoic acid (ct-8,9-E-15-HET). ct-8,9-E-11-HET and ct-8,9-E-15-HET are further metabolized by sEH, with ct-8,9-E-11-HET being hydrolyzed much more slowly. Using an s.c. Matrigel assay, we showed that ct-8,9-E-11-HET is proangiogenic, whereas ct-8,9-E-15-HET is not active. This study identifies a functional link between EETs and COX and identifies ct-8,9-E-11-HET as an angiogenic lipid, suggesting a physiological role for COX metabolites of EETs.

A series of inhibitors of the soluble epoxide hydrolase (sEH) containing two urea groups has been developed. Inhibition potency of the described compounds ranges from 2.0 μM to 0.4 nM. 1,6-(Hexamethylene)bis[(adamant-1-yl)urea] (3b) was found to be a potent slow tight binding inhibitor (IC50=0.5 nM) with a strong binding to sEH (Ki=3.1 nM) and a moderately long residence time on the enzyme (koff=1.05 × 10(-3) s(-1); t1/2=11 min).

Pseudomonas aeruginosa secretes an epoxide hydrolase with catalytic activity that triggers degradation of the cystic fibrosis transmembrane conductance regulator (CFTR) and perturbs other host defense networks. Targets of this CFTR inhibitory factor (Cif) are largely unknown, but include an epoxy-fatty acid. In this class of signaling molecules, chirality can be an important determinant of physiological output and potency. Here we explore the active-site chemistry of this two-step α/β-hydrolase and its implications for an emerging class of virulence enzymes. In combination with hydrolysis data, crystal structures of 15 trapped hydroxyalkyl-enzyme intermediates reveal the stereochemical basis of Cif's substrate specificity, as well as its regioisomeric and enantiomeric preferences. The structures also reveal distinct sets of conformational changes that enable the active site to expand dramatically in two directions, accommodating a surprising array of potential physiological epoxide targets. These new substrates may contribute to Cif's diverse effects in vivo, and thus to the success of P. aeruginosa and other pathogens during infection.