Non-cryogenic ultra-low field MRI of wrist-forearm area.

Ultra-low field (ULF) MRI as an alternative to high field MRI can find some niche applications where high field is a liability. Previously we demonstrated hand images with a non-cryogenic ULF MRI system, but such a system was restrictive to the size of the imaging objects. We have modified the previous setup to increase the imaging volume and demonstrate the image of human hand near the wrist area. One goal for the demonstration is the evaluation of quality of larger bone structure to project image quality to other parts of extremities, such as elbows, shoulders, and knees. We found that after 12 min of acquisition, the image quality was quite satisfactory. To achieve this image quality, several problems were solved that appeared in the new system. The increase in the imaging volume size led to an increase in transient time and various measures were taken to reduce this time. We also explored a method of overcoming the artifacts and image quality reduction arising from field drifts present in the system due to heating of the coils. We believe that our results can be useful for evaluation of diagnostic capability of non-cryogenic ULF MRI of extremities and other parts of the body. The system can be also applied to image animals and tissues.

Carbamazepine and oxcarbazepine decrease phenytoin metabolism through inhibition of CYP2C19.

Multiple studies suggest that phenytoin concentrations increase with CBZ co-medication. This study evaluated the hypothesis that CBZ and/or its major metabolite (CBZE) inhibit CYP2C19-mediated phenytoin metabolism using human liver microsomes and cDNA-expressed CYP2C19. Oxcarbazepine (OXC), and its 10-monohydroxy metabolite (MHD) were also evaluated. CBZ and MHD inhibited CYP2C19-mediated phenytoin metabolism at therapeutic concentrations. Thus, administration of CBZ and OXC with CYP2C19 substrates with narrow therapeutic ranges should be done cautiously.

Influence of stiripentol on cytochrome P450-mediated metabolic pathways in humans: in vitro and in vivo comparison and calculation of in vivo inhibition constants.

OBJECTIVE: The spectrum of cytochrome P450 inhibition of stiripentol, a new anticonvulsant, was characterized in vitro and in vivo. METHODS: Stiripentol was incubated in vitro with (R)-warfarin, coumarin, (S)-warfarin, (S)-mephenytoin, bufuralol, p-nitrophenol, and carbamazepine as probes for CYPs 1A2, 2A6, 2C9, 2C19, 2D6, 2E1, and 3A4, respectively. Caffeine demethylation and the 6 beta-hydroxycortisol/cortisol ratio were monitored in vivo before and after 14 days of treatment with stiripentol as measures of CYP1A2 and CYP3A4 activity, and dextromethorphan O- and N-demethylation were used to measure CYP2D6 and CYP3A4 activity, respectively. In vivo inhibition constants for CYP3A4 were calculated with use of data that previously documented the interaction between stripentol and carbamazepine. RESULTS: In vitro, stiripentol inhibited CYPs 1A2, 2C9, 2C19, 2D6, and 3A4, with inhibition constant values at or slightly higher than therapeutic (total) concentrations of stiripentol, but it did not inhibit CYPs 2A6 and 2E1 even at tenfold therapeutic concentrations. In vivo inhibition of caffeine demethylation and dextromethorphan N-demethylation were consistent with inhibition of CYP1A2 and CYP3A4, respectively. The 6 beta-hydroxycortisol/cortisol ratio did not provide a reliable index of CYP3A4 inhibition. Inhibition of CYP2D6-mediated O-demethylation was not observed in vivo. With use of carbamazepine, in vivo inhibition constants for CYP3A4 ranged between 12 and 35 mumol/L, whereas the corresponding in vitro value was 80 mumol/L. CONCLUSIONS: Stiripentol appears to inhibit several CYP450 enzymes in vitro and in vivo. In vivo inhibition constants show that stiripentol inhibition of CYP3A4 is linearly related to plasma concentration in patients with epilepsy.

Formation of (R)-8-hydroxywarfarin in human liver microsomes. A new metabolic marker for the (S)-mephenytoin hydroxylase, P4502C19.

Kinetic studies demonstrate that two forms of human liver cytochrome P450 are responsible for the formation of (R)-8-hydroxywarfarin: a low-affinity enzyme (KM approximately 1.5 mM), previously identified as P4501A2; and a high-affinity enzyme (KM = 330 microM), now identified as P4502C19 on the basis of the following evidence. In crossover inhibition studies with P4501A2-depleted human liver microsomes between (R)-warfarin and (S)-mephenytoin, reciprocal competitive inhibition was observed. Apparent KM values for (S)-mephenytoin-4'-hydroxylation (52-67 microM) were similar to the determined Ki values (58-62 microM) for (S)-mephenytoin inhibition of (R)-8-hydroxywarfarin formation. Similarly, the apparent KM for (R)-warfarin 8-hydroxylation in furafylline-pretreated microsomes (KM = 289-395 microM) was comparable with the Ki values (280-360 microM) for (R)-warfarin inhibition of (S)-4'-hydroxymephenytoin formation. Inhibition studies with tranylcypromine, a known inhibitor of (S)-mephenytoin hydroxylase activity, and either substrate in three different microsomal preparations yielded nearly identical inhibitory constants: Ki = 8.7 +/- 1.6 microM for inhibition of (S)-4'-hydroxymephenytoin formation and 8.8 +/- 2.5 microM for inhibition of (R)-8-hydroxywarfarin formation. In addition, fluconazole, a potent inhibitor of (R)-warfarin 8-hydroxylation, Ki = 2 microM, was found to inhibit (S)-mephenytoin hydroxylation with an identical Ki (2 microM). Finally, a strong correlation between (S)-mephenytoin 4-hydroxylation and (R)-warfarin 8-hydroxylation activities in furafylline-pretreated microsomes was demonstrated in 14 human liver microsomal preparations (r2 = 0.97).

Human liver carbamazepine metabolism. Role of CYP3A4 and CYP2C8 in 10,11-epoxide formation.

A number of drugs inhibit the metabolism of carbamazepine catalyzed by cytochrome P450, sometimes resulting in carbamazepine intoxication. However, there is little information available concerning the identity of the specific isoforms of P450 responsible for the metabolism of this drug. This study addressed the role of CYP3A4 in the formation of carbamazepine-10,11-epoxide, the major metabolite of carbamazepine. Results of the study showed that: (1) purified CYP3A4 catalyzed 10,11-epoxidation; (2) cDNA-expressed CYP3A4 catalyzed 10,11-epoxidation (Vmax = 1730 pmol/min/nmol P450, Km = 442 microM); (3) the rate of 10,11-epoxidation correlated with CYP3A4 content in microsomes from sixteen human livers (r2 = 0.57, P < 0.001); (4) triacetyloleandomycin and anti-CYP3A4 IgG reduced 10,11-epoxidation to 31 +/- 6% (sixteen livers) and 43 +/- 2% (four livers) of control rates, respectively; and (5) microsomal 10,11-epoxidation but not phenol formation was activated 2- to 3-fold by alpha-naphthoflavone and progesterone and by carbamazepine itself (substrate activation). These findings indicate that CYP3A4 is the principal catalyst of 10,11-epoxide formation in human liver. Experiments utilizing a panel of P450 isoform selective inhibitors also suggested a minor involvement of CYP2C8 in liver microsomal 10,11-epoxidation. Epoxidation by CYP2C8 was confirmed in incubations of carbamazepine with cDNA-expressed CYP2C8. The role of CYP3A4 in the major pathway of carbamazepine elimination is consistent with the number of inhibitory drug interactions associated with its clinical use, interactions that result from a perturbation of CYP3A4 catalytic activity.