SciCombinator

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Journal: Drug metabolism and disposition: the biological fate of chemicals

170

Carfilzomib, an irreversible proteasome inhibitor, has a favorable safety profile and significant antitumor activity in patients with relapsed and refractory multiple myeloma (MM). Here we summarize the clinical pharmacokinetics (PK), metabolism, and drug-drug interaction (DDI) profile of carfilzomib. The PK of carfilzomib, infused over 2-10 minutes, was evaluated in patients with solid tumors or MM. Metabolites of carfilzomib were characterized in patient plasma and urine samples. In vitro drug metabolism and DDI studies were conducted in human liver microsomes and hepatocytes. A clinical DDI study was conducted in patients with solid tumors to evaluate the effect of carfilzomib on CYP3A activity. Plasma concentrations of carfilzomib declined rapidly and in a biphasic manner after intravenous administration. The systemic half-life was short and the systemic clearance rate was higher than hepatic blood flow. Carfilzomib was cleared largely extrahepatically via peptidase cleavage and epoxide hydrolysis. Cytochrome P450-mediated metabolism played a minor role, suggesting that coadministration of P450 inhibitors or inducers is unlikely to change its PK profile. Carfilzomib showed direct and time-dependent inhibition of CYP3A in human liver microsome preparations and exposure to carfilzomib resulted in reductions in CYP3A and 1A2 gene expression in cultured human hepatocytes. However, administration of carfilzomib did not affect the PK of midazolam in patients with solid tumors, and there were no safety signals indicative of potential drug interactions. We conclude that the rapid systemic clearance and short half-life of carfilzomib limit clinically significant DDI.

Concepts: Pharmacology, Cancer, Liver, Glycogen, Cytochrome P450, Proteasome, Drug metabolism, Hepatocyte

169

Organic anion-transporting polypeptides (OATPs) are multispecific transporters mediating the uptake of endogenous compounds and xenobiotics in tissues that are important for drug absorption and elimination, including the intestine and liver. Silymarin is a popular herbal supplement often used by patients with chronic liver disease; higher oral doses than those customarily used (140 mg three times/day) are being evaluated clinically. The present study examined the effect of silymarin flavonolignans on OATP1B1-, OATP1B3-, and OATP2B1-mediated transport in cell lines stably expressing these transporters, and in human hepatocytes. In overexpressing cell lines, OATP1B1- and OATP1B3-mediated estradiol-17β-glucuronide uptake and OATP2B1-mediated estrone-3-sulfate uptake were inhibited by most of the silymarin flavonolignans investigated. OATP1B1-, OATP1B3-, and OATP2B1-mediated substrate transport was inhibited efficiently by silymarin (IC(50) values of 1.3, 2.2 and 0.3 μM, respectively), silybin A (IC(50) values of 9.7, 2.7 and 4.5 μM, respectively), silybin B (IC(50) values of 8.5, 5.0 and 0.8 μM, respectively), and silychristin (IC(50) values of 9.0, 36.4 and 3.6 μM, respectively). Furthermore, silymarin, silybin A and silybin B (100 μM) significantly inhibited OATP-mediated estradiol-17β-glucuronide and rosuvastatin uptake into human hepatocytes. Calculation of the maximal unbound portal vein concentrations/IC(50) values indicated a low risk for silymarin-drug interactions in hepatic uptake with a customary silymarin dose. The extent of silymarin-drug interactions depends on OATP isoform specificity and concentrations of flavonolignans at the site of drug transport. Clinical investigations that achieve higher concentrations with either increased doses of silymarin or formulations with improved bioavailability may enhance the potential risk of DDIs with OATP substrates.

Concepts: Pharmacology, Liver, Glycogen, Bile, Transport, Hepatocyte, Flavonolignan, Silibinin

169

The effect of rifampicin on the pharmacokinetics of fexofenadine enantiomers was examined in healthy subjects who received fexofenadine alone or with single or multiple doses of rifampicin (600 mg). A single coadministered dose of rifampicin significantly decreased the oral clearance (CL(tot)/F) and renal clearance (CL®) of S- and R-fexofenadine by 76 and 62%, and 73 and 62%, respectively. Even after multiple doses, rifampicin significantly decreased these parameters, although the effect on the CL(tot)/F was slightly blunted. Multiple doses of rifampicin abolished the difference in the CL(tot)/F of fexofenadine enantiomers, whereas the stereoselectivity in the CL® persisted. Rifampicin inhibited the uptake of fexofenadine enantiomers by human hepatocytes via organic anion transporter (OAT) OATP1B3 and its basal-to-apical transport in Caco-2 cells, but not OAT3-mediated or multidrug and toxic compound extrusion 1 (MATE1)-mediated transport. The plasma-unbound fraction of S-fexofenadine was 1.8 times higher than that of R-fexofenadine. The rifampicin-sensitive uptake by hepatocytes was 1.6 times higher for R-fexofenadine, whereas the transport activities by OATP1B3, OAT3, MATE1, or P-glycoprotein were identical for both enantiomers. S-fexofenadine is a more potent human histamine H1 receptor antagonist than R-fexofenadine. In conclusion, rifampicin has multiple interaction sites with fexofenadine, all of which contribute to increasing the area under the curve of fexofenadine when they are given simultaneously, to surpass the effect of the induction of P-glycoprotein elicited by multiple doses.

Concepts: Pharmacology, Human, Receptor, Ligand, Histamine, Receptor antagonist, Inverse agonist, Histamine H1 receptor

168

Sutherlandia frutescens (ST) is a popular medicinal herb widely consumed in Africa by people living with HIV/AIDS. Concomitant use with antiretroviral drugs has generated concerns of herb-drug interaction. This study investigated the inhibitory effects of the crude extracts of ST on the major cytochrome P450 isozymes employing pooled human liver microsomes. Its effect on the metabolic clearance of midazolam using cryopreserved hepatocytes was also monitored. The potential of ST to inhibit human ATP-binding cassette (ABC) transporters (P-gp and BCRP) and the human organic anion transporting polypeptide (OATP1B1 and OATP1B3) activity was assessed using cell lines overexpressing the transporter proteins. ST showed inhibitory potency for CYP1A2 (IC(50) = 41.0 μg/mL), CYP2A6 (IC(50) = 160 μg/mL), CYP2B6 (IC(50) = 20.0 μg/mL), CYP2C8 (IC(50) = 22.4 μg/mL), CYP2C9 (IC(50) = 23.0 μg/mL), CYP2C19 (IC(50) = 35.9 μg/mL) and CYP3A4/5 (IC(50,) = 17.5 μg/mL [with midazolam1'-hydroxylation]; IC(50) = 28.3 μg/mL [with testosterone 6β-hydroxylation]). Time-dependent (irreversible) inhibition by ST was observed for CYP3A4/5 (KI = 296 μg/mL, kinact = 0.063 min(-1)) under the conditions of this study. ST also delays the production of midazolam metabolites in the hepatocytes, decreasing its clearance by 40%. Further, ST inhibited P-gp (IC(50) = 324.8 μg/mL); OATP1B1 (IC(50) = 10.4 μg/mL, and of OATP1B3 (IC(50) = 6.6 μg/mL). The result indicates the potential for HDI between ST and the substrates of the affected enzymes, if sufficient in vivo concentration of ST is attained.

Concepts: AIDS, Metabolism, Enzyme, Liver, Glycogen, Cytochrome P450, Transport, Sutherlandia frutescens

168

The disposition of ertugliflozin (PF-04971729), an orally active selective inhibitor of the sodium-dependent glucose cotransporter 2, was studied after a single 25-mg oral dose of [(14)C]-PF-04971729 to healthy human subjects. Mass balance was achieved with approximately 91% of the administered dose recovered in urine and feces. The total administered radioactivity excreted in feces and urine was 40.9% and 50.2%, respectively. The absorption of PF-04971729 in humans was rapid with a T(max) at ~ 1.0 h. Of the total radioactivity excreted in feces and urine, unchanged PF-04971729 collectively accounted for ~ 35.3% of the dose, suggestive of moderate metabolic elimination in humans. The principal biotransformation pathway involved glucuronidation of the glycoside hydroxyl groups to yield three regioisomeric metabolites M4a, M4b and M4c (~39.3% of the dose in urine) of which M4c was the major regioisomer (~31.7% of the dose). The structure of M4a and M4c were confirmed to be PF-04971729-4-O-β- and -3-O-β-glucuronide, respectively, via comparison of the HPLC retention time and mass spectra with authentic standards. A minor metabolic fate involved oxidation by cytochrome P450 to yield monohydroxylated metabolites M1 and M3 and des-ethyl PF-04971729 (M2), which accounted for ~5.2% of the dose in excreta. In plasma, unchanged PF-04971729 and the corresponding 4-O-β- (M4a) and 3-O-β- (M4c) glucuronides were the principal components, which accounted for 49.9, 12.2 and 24.1% of the circulating radioactivity. Overall, these data suggest that PF-04971729 is well absorbed in humans, and eliminated largely via glucuronidation.

Concepts: Alcohol, Metabolism, Nutrition, Cytochrome P450, Steroid, Pharmacokinetics, Carbohydrate, Excretion

166

Abuse of the stimulant designer drug methylone (methylenedioxymethcathinone) has been documented in most parts of the world. As with many of the new designer drugs that continuously appear in the illicit drug market, little is known about the pharmacokinetics of methylone. Using in vitro studies, CYP2D6 was determined to be the primary enzyme that metabolizes methylone, with minor contributions from CYP1A2, CYP2B6, and CYP2C19. The major metabolite was identified as dihydroxymethcathinone, and the minor metabolites were N-hydroxy-methylone, nor-methylone, and dihydro-methylone. Measuring the formation of the major metabolite, biphasic Michaelis-Menten kinetic parameters were determined: Vmax,1 = 0.046 ± 0.005 (S.E.) nmol/min/mg protein, Km,1 = 19.0 ± 4.2 μM, Vmax,2 = 0.22 ± 0.04 nmol/min/mg protein, and Km,2 = 1953 ± 761 μM; the low-capacity and high-affinity contribution was assigned to the activity of CYP2D6. Additionally, a time-dependent loss of CYP2D6 activity was observed when the enzyme was preincubated with methylone, reaching a maximum rate of inactivation at high methylone concentrations, indicating that methylone is a mechanism-based inhibitor of CYP2D6. The inactivation parameters were determined to be KI = 15.1 ± 3.4 (S.E) μM and kinact = 0.075 ± 0.005 min(-1).

Concepts: Enzyme kinetics, Metabolism, Cytochrome P450, In vitro, Methamphetamine, MDMA, Designer drug, Methylone

165

Mefenamic acid, (MFA), a carboxylic acid-containing nonsteroidal anti-inflammatory drug (NSAID) is metabolized into the chemically-reactive, MFA-1-O-acyl-glucuronide (MFA-1-O-G), MFA-acyl-adenylate (MFA-AMP), and the MFA-S-acyl-CoA (MFA-CoA), all of which are electrophilic and capable of acylating nucleophilic sites on biomolecules. In this study, we investigate the non-enzymatic ability of each MFA acyl-linked metabolite to transacylate amino and thiol functional groups on the acceptor biomolecules glycine (Gly), taurine (Tau), glutathione (GSH), and N-acetylcysteine (NAC). In vitro incubations with each of the MFA acyl-linked metabolites (1 μM) in buffer under physiological conditions with Gly, Tau, GSH, or NAC (10 mM) revealed that MFA-CoA was 11.5- and 19.5-fold more reactive than MFA-AMP towards the acylation of cysteine-sulfhydryl groups of GSH and NAC, respectively. However, MFA-AMP was more reactive towards both Gly and Tau, 17.5-fold more reactive towards the N-acyl-amidation of taurine than its corresponding CoA thioester, while MFA-CoA displayed little reactivity towards glycine. Additionally, MFA-GSH was 5.6- and 108-fold more reactive towards NAC than MFA-CoA and MFA-AMP, respectively. In comparison to MFA-AMP and MFA-CoA, MFA-1-O-G was not significantly reactive towards all four bionucleophiles. MFA-AMP, MFA-CoA, MFA-1-O-G, MFA-GSH, and MFA-Tau were also detected in rat in vitro hepatocyte MFA (100 μM) incubations while MFA-Gly was not. These results demonstrate that MFA-AMP selectively reacts nonenzymatically with the amino functional groups of glycine and lysine, MFA-CoA selectively reacts nonenzymatically with the thiol functional groups of GSH and NAC, and MFA-GSH reacts nonenzymatically with the thiol functional group of GSH, all of which may potentially elicit an idiosyncratic toxicity in vivo.

Concepts: Amino acid, Amine, Functional group, Disulfide bond, Non-steroidal anti-inflammatory drug, Paracetamol, Ibuprofen, Carboxylic acid

164

MMilk thistle (Silybum marianum) extracts, one of the most widely used dietary supplements, contain a mixture of six major flavonolignans (silybin A, silybin B, isosilybin B, isosilybin A, silychristin, and silydianin) and other components. However, the pharmacokinetics of the free individual flavonolignans has only partially been investigated in humans. Further, antioxidant effects of the extract, which may underlie the basis of many therapeutic effects, have not been thoroughly assessed. The present study was to evaluate the pharmacokinetics of the six major flavonolignans in healthy volunteers receiving single doses either one(175 mg), two(350 mg), or three(525 mg) of milk thistle capsule(s) on three separate study visits. Additionally, the steady state pharmacokinetic parameters were determined after the subjects were administered one capsule thrice daily for 28 consecutive days. Our results demonstrated that all six flavonolignans were rapidly absorbed and eliminated. In order of abundance, the exposure to free flavonolignans was greatest for silybin A followed by silybin B, isosilybin B, isosilybin A, silychristin, and silydianin. The systemic exposure to these compounds appeared linear and dose-proportional. The disposition of flavonolignans was stereoselective, as evidenced by the apparent clearance of silybin B, which was significantly greater than silybin A, whereas the apparent clearance of isosilybin B was significantly lower than isosilybin A. The concentrations of urinary 8-epi-prostaglandin F2α, a commonly used biomarker of oxidative status in humans, were considerably decreased in study subjects after a 28-day exposure to the extract (1.3±0.9 versus 0.8±0.9 ng/mg creatinine), but failed to reach statistical significance (P=0.076).

Concepts: Therapeutic effect, Antioxidant, Pharmacokinetics, Silybum marianum, Milk thistle, Flavonolignan, Silibinin, Amanita phalloides

27

5-Methoxy-N,N-dimethyltryptamine (5-MeO-DMT or street name “5-MEO”) is a newer designer drug belonging to a group of naturally occurring indolealkylamines. Our recent study has demonstrated that coadministration of monoamine oxidase A (MAO-A) inhibitor harmaline (5 mg/kg) increases systemic exposure to 5-MeO-DMT (2 mg/kg) and active metabolite bufotenine. This study is aimed at delineating harmaline and 5-MeO-DMT pharmacokinetic (PK) interactions at multiple dose levels, as well as the impact of cytochrome P450 2D6 (CYP2D6) that affects harmaline PK and determines 5-MeO-DMT O-demethylation to produce bufotenine. Our data revealed that inhibition of MAO-A-mediated metabolic elimination by harmaline (2, 5 and 15 mg/kg) led to a sharp increase in systemic and cerebral exposure to 5-MeO-DMT (2 and 10 mg/kg) at all dose combinations. A more pronounced effect on 5-MeO-DMT PK was associated with greater exposure to harmaline in wild-type mice than CYP2D6-humanized (Tg-CYP2D6) mice. Harmaline (5 mg/kg) also increased blood and brain bufotenine concentrations that were generally higher in Tg-CYP2D6 mice. Surprisingly, greater harmaline dose (15 mg/kg) reduced bufotenine levels. The in vivo inhibitory effect of harmaline on CYP2D6-catalyzed bufotenine formation was confirmed by in vitro study using purified CYP2D6. Given these findings, a unified PK model including the inhibition of MAO-A- and CYP2D6-catalyzed 5-MeO-DMT metabolism by harmaline was developed to describe blood harmaline, 5-MeO-DMT and bufotenine PK profiles in both wild-type and Tg-CYP2D6 mouse models. This PK model may be further employed to predict harmaline and 5-MeO-DMT PK interactions at various doses, define the impact of CYP2D6 status, and drive harmaline-5-MeO-DMT pharmacodynamics.

Concepts: Pharmacology, Metabolism, Cytochrome P450, In vitro, Pharmacokinetics, Monoamine oxidase, Monoamine oxidase A, CYP2D6

25

Effective treatments for primary brain tumors and brain metastases represent a major unmet medical need. Targeting the CDK4/6-cyclin D1-Rb-p16/ink4a pathway using a potent CDK4 and CDK6 kinase inhibitor has potential for treating primary CNS tumors such as glioblastoma and some peripheral tumors with high incidence of brain metastases. We compared CNS exposures of two orally bioavailable CDK4 and CDK6 inhibitors: abemaciclib that is currently in advanced clinical development and palbociclib (IBRANCE®, Pfizer), which was recently approved by the U.S. Food and Drug Administration. Abemaciclib antitumor activity was assessed in subcutaneous and orthotopic glioma models alone and in combination with standard of care temozolomide (TMZ). Both inhibitors were substrates for xenobiotic efflux transporters P-glycoprotein and breast cancer resistant protein expressed at the blood-brain barrier (BBB). Brain Kp,uu values were less than 0.2 after equimolar intravenous dose indicative of active efflux, but were (~)10 fold greater for abemaciclib than palbociclib. Kp,uu increased 2.8 and 21 fold, respectively, when similarly dosed in P-gp-deficient mice. Abemaciclib had a brain AUC(0-24 h) Kp,uu of 0.03 in mice and 0.11 in rats following a 30 mg/kg PO dose. Orally-dosed abemaciclib significantly increased survival in a rat orthotopic U87MG xenograft model compared to vehicle-treated animals and efficacy coincided with a dose-dependent increase in unbound plasma and brain exposures in excess of the CDK4 and CDK6 Ki values. Abemaciclib increased survival time of intracranial U87MG tumor-bearing rats similar to TMZ and the combination of abemaciclib and TMZ was additive or greater than additive. These data show abemaciclib crosses the BBB and confirm that both CDK4 and CDK6 inhibitors reach unbound brain levels in rodents that are expected to produce enzyme inhibition, but abemaciclib brain levels are reached more efficiently at presumably lower doses than palbociclib and are potentially on target for a longer period of time.

Concepts: Cancer, Brain, Oncology, Enzyme, Glioma, Brain tumor, Enzyme inhibitor, Glioblastoma multiforme