This article is available online at http://dmd.aspetjournals.org ABSTRACT:Oxycodone undergoes N-demethylation to noroxycodone and Odemethylation to oxymorphone. The cytochrome P450 (P450) isoforms capable of mediating the oxidation of oxycodone to oxymorphone and noroxycodone were identified using a panel of recombinant human P450s. CYP3A4 and CYP3A5 displayed the highest activity for oxycodone N-demethylation; intrinsic clearance for CYP3A5 was slightly higher than that for CYP3A4. CYP2D6 had the highest activity for O-demethylation. Multienzyme, Michaelis-Menten kinetics were observed for both oxidative reactions in microsomes prepared from five human livers. Inhibition with ketoconazole showed that CYP3A is the high affinity enzyme for oxycodone N-demethylation; ketoconazole inhibited >90% of noroxycodone formation at low substrate concentrations. CYP3A-mediated noroxycodone formation exhibited a mean K m of 600 ؎ 119 M and a V max that ranged from 716 to 14523 pmol/mg/min. Contribution from the low affinity enzyme(s) did not exceed 8% of total intrinsic clearance for N-demethylation. Quinidine inhibition showed that CYP2D6 is the high affinity enzyme for O-demethylation with a mean K m of 130 ؎ 33 M and a V max that ranged from 89 to 356 pmol/mg/min. Activity of the low affinity enzyme(s) accounted for 10 to 26% of total intrinsic clearance for O-demethylation. On average, the total intrinsic clearance for noroxycodone formation was 8 times greater than that for oxymorphone formation across the five liver microsomal preparations (10.5 l/min/mg versus 1.5 l/min/mg). Experiments with human intestinal mucosal microsomes indicated lower N-demethylation activity (20-50%) compared with liver microsomes and negligible O-demethylation activity, which predict a minimal contribution of intestinal mucosa in the first-pass oxidative metabolism of oxycodone.

Review: The need for a fiber with the heat resistance of asbestos and the stiffness of glass led to the development of Kevlar which has a molecular orientation parallel to the fiber axis (see figure). Brian Phillips et al. trace the many faceted course of the development of Kevlar from the initial scientific discovery to the opening of a 45 million lb/yr production plant.

The pharmacokinetics of cyclophosphamide (CY) and its metabolites hydroxycyclophosphamide and carboxyethylphosphoramide mustard were determined in 75 patients receiving targeted oral busulfan followed by i.v. CY ((T)BU/CY) and in 147 patients receiving i.v. CY followed by total body irradiation (CY/TBI) in preparation for hematopoietic cell transplantation (HCT). In the (T)BU/CY patients only, the association of the pharmacokinetic data with liver toxicity, relapse, and survival was evaluated. CY was infused at 60 mg/kg/day over 1 or 2 hours on 2 consecutive days; the majority of patients had BU levels targeted to a steady state plasma concentration (Css) of 800-900 ng/mL. Systemic exposure (i.e., area under the concentration-time curve [AUC]) of CY, hydroxycyclophosphamide, and carboxyethylphosphoramide mustard was measured. Liver toxicity was assessed as the development of hepatic sinusoidal obstruction syndrome (SOS). CY metabolism was highly variable and age dependent. (T)BU/CY-treated patients had lower AUC(CY) (P < .0001), higher AUC(HCY) (P < .0001), and higher AUC(CEPM) (P = .15) than CY/TBI-conditioned patients. Among patients receiving (T)BU/CY, 17 (23%) developed SOS, and there were no statistically significant associations between the AUC of CY or its metabolites and SOS, nonrelapse mortality, relapse, or survival (all P >.15). In conclusion, CY exhibits conditioning-regimen dependent pharmacokinetics and pharmacodynamics, suggesting that lowering CY doses is unlikely to improve outcomes to (T)BU/CY. Alternative strategies, such as administering i.v. busulfan or CY before BU, should be explored.

This study investigates the efficacy and safety of personalized cyclophosphamide (CY) dosing in 50 patients receiving CY with total body irradiation (TBI). Participants received CY 45 mg/kg with subsequent therapeutic drug monitoring with Bayesian parameter estimation to personalize the second CY dose to a target area under the curve for carboxyethylphosphoramide mustard (a reporter for CY-derived toxins) and for hydroxycyclophosphamide (to ensure engraftment). The mean second CY dose was 66 mg/kg; the total dose ranged from 45–145 mg/kg. After completion of this phase II study, we compared participants’ clinical outcomes to those of concurrent controls (N=100) who received TBI with standard CY doses of 120 mg/kg. Patients receiving personalized CY dosing had significantly lower post-conditioning peak total serum bilirubin (p=0.03); a 38% reduction in the hazard of acute kidney injury (p=0.03); and similar non-relapse and overall survival (p=0.70 and 0.63, respectively) despite lower doses of CY in most patients.

The findings indicate that the spinal pharmacokinetics of these drugs are complex and, in some ways, counterintuitive. Also, the bioavailability of opioids in the cerebrospinal fluid and epidural space is determined primarily by their hydrophobicity, with less hydrophobic drugs having greater bioavailability.