Differential inhibition of cytochrome P450 isoforms by the protease inhibitors, ritonavir, saquinavir and indinavir

Aims To compare the inhibitory potential of the HIV protease inhibitors saquinavir, ritonavir and indinavir against CYP1A2, CYP2C9, CYP2E1 and CYP3A4 catalysed metabolic reactions in human liver microsomes in vitro .
Methods Microsomes from six human livers were utilized in this study. The probe substrates were phenacetin (CYP1A2), tolbutamide (CYP2C9), chlorzoxazone (CYP2E1) and testosterone (CYP3A4). Metabolites were analysed by high performance liquid chromatography. I C ₅₀ (concentration of inhibitor giving 50% decrease in enzyme activity) and, where appropriate, K _i values were calculated.
Results Ritonavir was a very potent inhibitor of CYP3A4 mediated testosterone 6β-hydroxylation (mean K _i=0.019±0.004  μm, mean±s.d.; n =6) and also inhibited tolbutamide hydroxylation (I C ₅₀=4.2±1.3  μm, mean±s.d.; n =6). Inhibition of phenacetin O -deethylation and chlorzoxazone 6-hydroxylation was negligible. Indinavir was an order-of-magnitude less potent in inhibiting CYP3A4 ( K _i=0.17±0.01  μm ) and did not produce appreciable inhibition of the CYP1A2, CYP2C9 or CYP2E1 catalysed reactions. Saquinavir was the least potent CYP3A4 inhibitor ( K _i =2.99±0.87  μm ) and produced some inhibition of CYP2C9 (approximately 50% at 50  μm ).
Conclusions The HIV protease inhibitors have differential effects on CYP isozymes. There is obvious potential for clinically significant drug interactions particularly with ritonavir. Pharmacokinetic drug interaction studies are crucial to gain an overall understanding of the beneficial and potentially harmful effects of this important group of drugs.     

Evidence for involvement of human CYP3A in the 3-hydroxylation of quinine

Aims Our previous studies using in vitro hepatic microsomal preparations suggested that the hepatic metabolism of quinine to form the major metabolite 3-hydroxyquinine is most likely catalysed by human P450 3A (CYP3A). The present study was carried out to investigate the kinetics and to identify and further characterise the human liver CYP isoforms involved in the metabolism of quinine.
Methods In vitro human microsomal techniques were employed.
Results The mean apparent K _m value for 3-hydroxyquinine formation was 83±19 (s.d.)  μm, ranging from 57  μm to 123  μm in microsomes from ten human livers. There was a 6.7-fold variation in V _max values (mean 547±416  pmol min⁻¹  mg⁻¹ ). Quinine 3-hydroxylation was inhibited by the specific CYP3A inhibitors, troleandomycin, midazolam and erythromycin. Inhibitors selective for CYP1A1/2, CYP2D6, CYP2E1, CYP2C9/10 or CYP2C19 had little or no effect on quinine 3-hydroxylation. Using microsomes from a panel of livers, significant correlations were found only between 3-hydroxyquinine activity and other CYP3A activities (caffeine 8-oxidation, omeprazole sulphoxidation, midazolam 1'-hydroxylation and midazolam 4-hydroxylation) and immunoreactive CYP3A content. There were no statistically significant correlations with activities selective for CYP1A2, CYP2C9 and CYP2E1. Competitive inhibition of quinine 3-hydroxylation was observed with a substrate known to be specifically metabolized by human CYP3A, i.e. midazolam, with an apparent K _i value of 11.0  μm.
Conclusions The present results strongly indicate that the conversion of quinine to 3-hydroxyquinine is the major metabolic pathway in human liver in vitro and that the reaction is catalysed by CYP3A isoforms.     

Inhibition of CYP2C9 by selective serotonin reuptake inhibitors in vitro: studies of phenytoin p-hydroxylation

Aims Inhibition of cytochrome P450 (CYP) activity by selective serotonin reuptake inhibitors (SSRIs) has frequently been reported with regard to pathways mediated by CYP2D6, CYP3A4/5, and CYP1A2. Little data exist on the capability of SSRIs to inhibit CYP2C9.
Methods We investigated the effect of SSRIs on p -hydroxylation of phenytoin (PPH), an established index reaction reflecting CYP2C9 activity, in an in vitro assay using liver tissue from six different human donors.
Results In control incubations (without inhibitor), 5-( p -hydroxy-phenyl)-5phenylhydantoin (HPPH) formation rates were: V _max 0.023  nmol min⁻¹  mg⁻¹; K _m 14.3  &mgr;m. Average inhibition constants ( K _i ) differed significantly among the SSRIs, with fluvoxamine having the lowest K _i (6  &mgr;m ) followed by R-fluoxetine (13  &mgr;m ), norfluoxetine (17  &mgr;m ), RS-fluoxetine (19  &mgr;m ), sertraline (33  &mgr;m ), paroxetine (35  &mgr;m ), S-fluoxetine (62  &mgr;m ), and desmethylsertraline (66  &mgr;m ). Thus, assuming comparable molar concentrations at the site of inhibition, fluvoxamine can be expected to have the highest probability of interfering with the metabolism of CYP2C9 substrates. S-fluoxetine is on average a 5 fold weaker CYP2C9 inhibitor than either R-fluoxetine or the racemic mixture.
Conclusions These findings are consistent with published case reports describing SSRI-related increments in plasma phenytoin levels. Because phenytoin has a narrow therapeutic index, plasma levels should be closely monitored when SSRIs are coadministered.     

Metabolic interactions of selected antimalarial and non-antimalarial drugs with the major pathway (3-hydroxylation) of quinine in human liver microsomes

Aims Nine antimalarial (plus two metabolites of proguanil) and twelve non-antimalarial drugs were tested for their possible interaction with CYP3A4-catalysed 3-hydroxylation of quinine by human liver microsomes in vitro .
Methods 3-Hydroxyquinine was assayed in the incubation mixture by an h.p.l.c. method using fluorometric detection. The respective I C ₅₀ values were estimated for the twenty-one drugs and two metabolites of proguanil tested herein.
Results Thirteen drugs exhibited an inhibitory effect on the 3-hydroxylation of quinine. According to the respective mean I C ₅₀ values, the inhibitory rank order of the drugs was: ketoconazole>troleandomycin (TAO, with preincubation)> doxycycline>omeprazole>primaquine>tetracycline=TAO (without preincubation)>nifedipine>erythromycin>verapamil>cimetidine>diltiazem>oleandomycin>hydralazine. Other drugs or metabolites showed little or no inhibition of quinine metabolism (mean I C ₅₀>200 or 500  &mgr;m ). Among the antimalarial drugs, doxycycline showed relatively potent inhibition of quinine 3-hydroxylation with a mean I C ₅₀ value of 17  &mgr;m, followed by primaquine and tetracycline, with mean I C ₅₀ values of 20 and 29  &mgr;m, respectively.
Conclusions When the plasma/serum concentrations possibly attained after their usual therapeutic doses were taken into account, tetracycline, doxycycline, omeprazole, ketoconazole, nifedipine, TAO and erythromycin are likely to be inhibitors of quinine metabolism in patients when the drugs are co-administrated with quinine.     

No effect of short-term omeprazole intake on acenocoumarol pharmacokinetics and pharmacodynamics

Aims To investigate the effect of omeprazole on the pharmacokinetics of R- and S-acenocoumarol and on their combined anticoagulant activity.
Methods Eight healthy male subjects completed a double-blind, randomized, placebo-controlled, two-way cross-over study. Subjects were given either omeprazole 40  mg or placebo once daily for 3 days. On day 2 of each study period, a single 10  mg oral dose of racemic acenocoumarol was administered and venous blood samples were collected for pharmacokinetic and pharmacodynamic assessments. A wash-out period of 2 weeks separated the two study periods.
Results The pharmacokinetics of R- and S-acenocoumarol (AUC 3016±221 and 233±14  ng  ml⁻¹ h, respectively) did not change after omeprazole (AUC 2929±256 and 220±18  ng  ml⁻¹ h, respectively). Anticoagulant activity (INR_max 1.7±0.1) was unaffected by co-administration of omeprazole (INR_max 1.7±0.1).
Conclusions The short-term intake of omeprazole does not affect acenocoumarol pharmacokinetics or pharmacodynamics. These data differ from the results of previous studies on the effect of omeprazole on warfarin, suggesting a different in vivo interaction profile of omeprazole on acenocoumarol than on warfarin. Drug interaction studies with oral anticoagulants should not be restricted to the use of warfarin.     

Inhibition of cytochrome P450 by nefazodone in vitro: studies of dextromethorphan O- and N-demethylation

Nefazodone (NEF), a 5-HT_2A/2C antagonist antidepressant, is extensively metabolized in the human body to hydroxy NEF (OH-NEF), p -hydroxy NEF (pOH-NEF), a dione metabolite, and via cleavage of the molecule to m -chlorophenyl-piperazine (mCPP) and BMY-33604. The latter is further metabolized to BMS-183695-01 (BMSa) and BMS-183562-01 (BMSb). To investigate the potential of NEF and its metabolites to interfere with the metabolism of other drugs, we tested these compounds for their ability to alter dextromethorphan (DMO) O -demethylation to dextrorphan (DOP; an index reaction for CYP2D6) and N -demethylation to 3-methoxy morphinan (MEM, a recently proposed index reaction of CYP3A3/4). The assay was performed in an in vitro system with human liver microsomes from three different donors. NEF, OH-NEF, pOH-NEF, mCPP and BMSb were weak inhibitors of DMO O and N -demethylation, with average K _i values ranging from 18 to 50  μm for DOP formation, and from 21 to >200  μm for MEM formation. The dione metabolite and BMSa did not produce detectable inhibition of either pathway. The findings for DMO O -demethylation, well-established as a CYP2D6-mediated reaction, indicate that NEF and metabolites are weak inhibitors of this reaction, with K _i values at least 100 times higher than fluoxetine ( K _i=0.1  μm±0.09). The implications of results on DMO N -demethylation are not clear. In vivo data, as well as in vitro data based on 'pure' CYP3A3/4 substrates, provide evidence for clinically relevant CYP3A3/4 inhibition by NEF, OH-NEF, and pOH-NEF. Thus, formation of MEM by N -demethylation of DMO may not constitute a suitable index reaction to probe CYP3A3/4 activity.     

(−)-Timolol is a more potent antagonist of the positive inotropic effects of (−)-adrenaline than of those of (−)-noradrenaline in human atrium

1 The affinity of (−)-timolol for β₁- and β₂-adrenoceptors was determined on isolated atrial preparations from patients undergoing open heart surgery. The times for onset and offset of antagonism of the positive inotropic effects of (−)-adrenaline and (−)-noradrenaline by (−)-timolol were measured.
2 The antagonism of the positive inotropic effects of (−)-adrenaline and (−)-noradrenaline by (−)-timolol (0.1–100 nm) was simple competitive in human atrium tissue. The slope of Schild-plots was not significantly different from 1.0 [0.93±0.09 for (−)-adrenaline, 0.97±0.09 for (−)-noradrenaline].
3 The inotropic effects of (−)-adrenaline were antagonized significantly more by each concentration of (−)-timolol than those of (−)-noradrenaline. K _B-values (-log m) were 10.10±0.09 against (−)-adrenaline and 9.43±0.07 against (−)-noradrenaline ( P <0.001).
4 Blocking kinetics of (−)-timolol for the β-adrenoceptor were relatively slow. Half-times for the onset of blockade by 10 times K _B of (−)-timolol were approximately 30  min for both (−)-adrenaline and (−)-noradrenaline; offset times were similar.
5 It is concluded that (−)-timolol has a higher affinity for the β₂-adrenoceptor than for the β₁-adrenoceptor in human atrium. This property may be beneficial clinically in protecting against the β₂-adrenoceptor hypersensitivity induced by cardiac β₁-adrenoceptor blockade, but also explain why severe asthma can occur after administration of very low intra-ocular doses of the drug.     

Pharmacodynamics of oxypurinol after administration of allopurinol to healthy subjects

1   Eight healthy subjects received 50, 100, 300, 600 and 900  mg allopurinol daily for 1 week each, in random order with 1 week separating each treatment period. The pre-dose plasma concentration of oxypurinol, the extent of inhibition of xanthine oxidase, plasma urate concentration and urine urate excretion rate were assessed on the last 2 days of each treatment week.
2   The ratio of 1-methyluric acid (1MU) over 1-methylxanthine (1MX) in the urine, following a dose of 50  mg 1MX infused intravenously over 20  min, was used to measure the inhibition of xanthine oxidase.
3   The steady-state plasma concentration of oxypurinol increased linearly with increasing dose of allopurinol between 50  mg to 600  mg day⁻¹, with a weak indication of saturation at the higher 900  mg day⁻¹ dose rate.
4   The relationships between plasma oxypurinol concentration and xanthine oxidase inhibition (1MU/1MX ratio), plasma urate concentration and urine urate excretion rate were fitted to an inhibition sigmoid E_max model and the C ₅₀ values for oxypurinol were 26.38±4.87, (mean±s.d.) 36.58±8.36 and 24.61±9.08  μm, respectively.
5   1MU/1MX ratio appeared to be a reliable index of xanthine oxidase activity in vivo as the C ₅₀ for oxypurinol observed for 1MU/1MX ratio, plasma urate concentration and urine urate excretion rate were similar.
6   The concentration of oxypurinol required for inhibition of xanthine oxidase, as indicated by C ₅₀, was lower than those often observed in clinical practice.     

Inhibitory effects of antiarrhythmic drugs on phenacetin O-deethylation catalysed by human CYP1A2

Aims The aim of the study was to clarify whether the pharmacokinetic interaction between theophylline and mexiletine is mediated by inhibition of CYP1A2 and to assess the possible interaction potential of other antiarrhythmic drugs with drugs metabolized by CYP1A2.
Methods The inhibitory effects of mexiletine and 10 antiarrhythmic drugs on phenacetin O -deethylation, a marker reaction of CYP1A2, were studied using human liver microsomes and cDNA-expressed CYP1A2.
Results Propafenone and mexiletine inhibited phenacetin O -deethylation with I C ₅₀ values of 29 and 37  μm, respectively. Disopyramide, procainamide and pilsicainide produced negligible inhibition of phenacetin O -deethylation (I C ₅₀>1  mm ). Amiodarone, bepridil, aprindine, lignocaine, flecainide and quinidine inhibited phenacetin O -deethylation in a concentration-dependent manner, although the inhibitory effects were relatively weak with I C ₅₀ values ranging from 86 to 704  μm. Propafenone and mexiletine selectively abolished the high-affinity component of phenacetin O -deethylation in human liver microsomes. In addition, propafenone and mexiletine inhibited phenacetin O -deethylation catalysed by cDNA-expressed CYP1A2.
Conclusions These data suggest that, among the antiarrhythmic drugs studied, propafenone and mexiletine are relatively potent inhibitors of CYP1A2, which may cause a drug-drug interaction with drugs metabolized by CYP1A2.     

Stimulatory as well as inhibitory effects of ethinyloestradiol on the metabolic clearances of propranolol in young women

1   The metabolism of a single 80  mg oral dose of propranolol was determined in nine young women before and after administration of ethinyloestradiol alone (EE₂) or in combination with norethindrone (OC).
2   Whereas the total clearance of propranolol (2713±404  ml min⁻¹ (mean±s.e.mean)) was not significantly altered by either EE₂ (3365±347  ml min⁻¹) or the combined OC (2905±345  ml min⁻¹), significant changes in all three primary metabolic pathways were observed.
3   The clearance through side-chain oxidation decreased from 345±55  ml min⁻¹ to 262±33  ml min⁻¹ after EE₂ ( P <0.05). A similar reduction of cytochrome P450 metabolism by EE₂ has been observed for other drugs.
4   The clearance through glucuronidation increased from 364±61  ml min⁻¹ to 625±117  ml min⁻¹ after EE₂ ( P <0.01). Similar stimulation of glucuronic acid conjugation by EE₂ has also been observed for other drugs.
5   The clearance through ring oxidation increased from 697±109  ml min⁻¹ to 1280±162  ml min⁻¹ after EE₂ ( P <0.01). This observation appears to be a novel finding with EE₂ and cytochrome P450 metabolism.
6   The treatment with OC produced changes in propranolol's metabolic clearances which were qualitatively similar to those generated by EE₂.     

Effect of troleandomycin on the pharmacokinetics of imipramine in Chinese: the role of CYP3A

Aims In vitro data indicate that imipramine (IMI), a widely used tricyclic antidepressant drug, is N -demethylated by several isoforms of cytochrome P450, which include CYP3A4. The aim of this study was to investigate the role of CYP3A in the in vivo N -demethylation of IMI.
Methods Healthy subjects were given troleandomycin (TAO), a selective inhibitor of CYP3A, 250  mg daily for 2 days before a single oral dose of 100  mg IMI was administered.
Results Pretreatment with TAO significantly increased the AUC of IMI by 59% (1971±938 vs 3134±2000  μg l⁻¹  h, 95% confidence interval for difference between means: 218 to 2108  μg  l⁻¹  h, P <0.05) and decreased its oral clearance by 30% (60.9±27.4 vs 42.5±22.7  l h⁻¹, 95% confidence internal for difference between means: 7.2 to 31.7  l h⁻¹, P <0.05).
Conclusions We conclude that CYP3A may play an important role in the in vivo N -demethylation of IMI.     

Effect of food and gender on the pharmacokinetics of tucaresol in healthy volunteers

Aims To investigate the nasal absorption of hydroxocobalamin in 10 healthy elderly adults.
Methods In a cross-over study, blood samples were collected before administration of the drug and after 10, 20, 30, 40, 60, 120, 180 and 240  min. The plasma cobalamin concentration was determined by competitive radioisotope binding technique.
Results The maximal plasma cobalamin concentration ( C _max ) after nasal administration of 750  μg hydroxocobalamin was 1900±900  pmol  l⁻¹ (mean±s.d.). The maximal plasma cobalamin concentration was reached in 35±13  min ( t _max ). The C _max after nasal administration of 1500  μg hydroxocobalamin was 3500±2500  pmol  l⁻¹ with a t _max of 28±16  min. Both the AUC(0,240  min) and AUC(0,00) increased significantly with an increase of the dose from 750  μg to 1500  μg ( P =0.037 and P =0.028, respectively). The nasal spray was well tolerated. No signs of irritation or local sensitivity were noted.
Conclusions The nasal absorption of hydroxocobalamin in healthy elderly adults is rapid, high and well tolerated.     

Clonidine and or adrenaline decrease lignocaine plasma peak concentration after epidural injection

Clonidine is an α₂-adrenoceptor agonist increasingly used in combination with lignocaine for spinal or epidural anaesthesia because of a prolonged analgesic effect. Like adrenaline, it may decrease lignocaine peak concentration ( C _max), thus leading to decreased toxicity. However, the effects of clonidine on resorption of lignocaine into the systemic circulation from the epidural space remain to be established. We studied the pharmacokinetics of lignocaine after epidural injection of lignocaine with or without clonidine, adrenaline and both drugs. Total body clearance and apparent volume of distribution were similar in the four groups, but the maximum observed concentration ( C _max) was markedly increased in the plain solution group as compared with the other groups: (plain lignocaine: 7.15±2.04  μg  ml⁻¹, lignocaine+adrenaline: 3.11±136  μg  ml⁻¹, lignocaine+clonidine: 4.48±1.26 μg  ml⁻¹, lignocaine+adrenaline+clonidine: 4.06±1.42  μg  ml⁻¹ [mean±s.d.]). Our results show that, clonidine decreases lignocaine C _max to the same extent as adrenaline.     

YF476 is a new potent and selective gastrin/cholecystokinin-B receptor antagonist in vitro and in vivo

Background : We newly synthesized YF476 ((R)-1-[2,3-dihydro-2-oxo-1-pivaloylmethyl-5-(2'-pyridyl)-1H-1,4-benzodiazepin-3-yl]-3-(3-methylamino-phenyl)urea) as a gastrin/cholecystokinin-B (CCK-B) receptor antagonist. We investigated the pharmacological profile of YF476 in vitro and in vivo .
Methods : We examined the binding properties of YF476 to the rat brain, cloned canine and cloned human gastrin/CCK-B receptors, and the effect of YF476 on secretagogue-induced gastric acid secretion in rats and Heidenhain pouch dogs.
Results : YF476 replaced the specific binding of [¹²⁵I]CCK-8 to the rat brain, cloned canine and cloned human gastrin/CCK-B receptors, with K _i values of 0.068, 0.62 and 0.19 n M , respectively. The affinity of YF476 for rat brain gastrin/CCK-B receptor was 4100-fold higher than that for rat pancreatic CCK-A receptor. In anaesthetized rats, intravenous YF476 inhibited pentagastrin-induced acid secretion with an ED ₅₀ value of 0.0086 μmol/kg, but did not affect histamine- and bethanechol-induced acid secretion at a dose of 10 μmol/kg. In Heidenhain pouch dogs, intravenous and oral YF476 inhibited pentagastrin-stimulated gastric acid secretion in a dose-dependent manner with ED ₅₀ values of 0.018 and 0.020 μmol/kg, respectively, but did not affect histamine-induced acid secretion.
Conclusion : These results suggest that YF476 is an extremely potent and highly selective gastrin/CCK-B receptor antagonist, and that the gastrin/CCK-B receptor is not involved in histamine- or bethanechol-induced gastric acid secretion in dogs or rats.     

Nitric oxide: an important role in the maintenance of systemic and pulmonary vascular tone in man

Aims The aim of this study was to examine whether nitric oxide (NO) has an important role in maintaining basal vascular tone in normal man by examining the effects of nitric oxide inhibition using N ^G-monomethyl-l-arginine (l-NMMA) on systemic and pulmonary haemodynamics.
Methods Ten normal male volunteers 26±1.6 years were studied on two separate occasions in a double-blind, placebo controlled crossover study. They were randomised to receive either a continuous infusion of l-NMMA (4  mg  kg⁻¹  h⁻¹ ) with a front loaded bolus (4  mg  kg⁻¹ ) or volume matched placebo. Pulsed wave Doppler echocardiography was used to measure cardiac output (CO), mean pulmonary artery pressure (MPAP) and hence systemic vascular resistance (SVR) and total pulmonary vascular resistance (TPR). Measurements were made prior to infusion ( t ₀ ) and after 4, 8, and 12  min ( t ₁, t ₂ and t ₃ ).
Results Infusion of l-NMMA significantly increased mean arterial blood pressure (MAP), SVR and TPR and significantly reduced heart rate (HR), stroke volume (SV) and CO compared to placebo. These effects were observed at t ₁ and persisted during the entire infusion period.
Conclusions These results are consistent with a role for basal nitric oxide generation in the maintenance of basal systemic and pulmonary vascular tone in normal man.     

Carbamazepine treatment induces the CYP3A4 catalysed sulphoxidation of omeprazole, but has no or less effect on hydroxylation via CYP2C19

Aims Omeprazole has been shown previously to be metabolized by the two cytochrome P450 isoforms CYP2C19 (hydroxylation) and CYP3A4 (sulphoxidation). The objective of this study was to test the inducibility of these enzymes by carbamazepine (CBZ).
Methods Omeprazole was given as a single oral dose before and after 3 weeks of treatment of five patients with CBZ (400–600  mg daily).
Results Mean area under the plasma concentration vs time curve (AUC) between 0 and 8  h after drug intake, decreased by about 40% for omeprazole and its hydroxy metabolite and increased for its sulphone metabolite, but the changes were not statistically significant. The ratio of the AUCs of omeprazole and its sulphone, used as an index of CYP3A4 activity, decreased in all patients ( P =0.052), while there was no change in the omeprazole/hydroxyomeprazole AUC ratio used as an index for CYP2C19 activity. There was a significant decrease in the mean ratio of the AUC of the hydroxy and sulphone metabolites from 2.58 to 0.93 ( P =0.046) with a mean difference of 1.79 (95% CI: 0.07 to 3.50) showing that the induction was more pronounced for CYP3A4 than for CYP2C19.
Conclusions CBZ induces CYP3A4, but not, or to a lesser extent, CYP2C19. The induction of the sulphoxidation of omeprazole by CBZ seems to have no major clinical implication.     

Pharmacokinetics of recombinant human interleukin-11 (rhIL-11) in healthy male subjects

Aims To study the pharmacokinetics of recombinant human interleukin-11 (rhIL-11) in healthy male volunteers following subcutaneous (s.c.) and intravenous (i.v.) administration.
Methods RhIL-11 was infused intravenously at 10–50  μg  kg⁻¹ for 1 or 3  h, or administered subcutaneously at 3–50  μg  kg⁻¹ to volunteers. RhIL-11 was also administered at 3  μg  kg⁻¹ s.c. once daily for 7 days. Plasma and urinary concentrations were measured by enzyme-linked immunosorbent assay (ELISA).
Results RhIL-11 showed linear pharmacokinetics after both intravenous infusion and s.c. administration. Comparison of t _1/2 and MRT values after i.v. administration with those after s.c. administration indicated that rhIL-11 pharmacokinetics after s.c. administration were absorption rate-limited. Bioavailability after s.c. administration was about 65%. Since RhIL-11 was not detected in urine after a single 50  μg  kg⁻¹ s.c. dose, rhIL-11 was considered to be eliminated by metabolism. There was no significant change in the pharmacokinetic profile of rhIL-11 following repeated s.c. administration.
Conclusions RhIL-11 demonstrated linear pharmacokinetics at these dose ranges after single and repeated s.c. administration or constant-rate i.v. infusion in healthy volunteers.     

Involvement of CYP1A2 in mexiletine metabolism

Aims Mexiletine has been reported to be hydroxylated by cytochrome P450 2D6 (CYP2D6) in humans. However, the involvement of CYP1A2 in the metabolism of mexiletine has been proposed based on the interaction with theophylline which is mainly metabolized by CYP1A2. The aim of this study was to clarify the role of human CYP1A2 in mexiletine metabolism.
Methods Human CYP isoforms involved in mexiletine metabolism were investigated using microsomes from human liver and B-lymphoblastoid cells expressing human CYPs. The contributions of CYP1A2 and CYP2D6 to mexiletine metabolism were estimated by the relative activity factor (RAF).
Results Mexiletine p - and 2-hydroxylase activities in human liver microsomes were inhibited by ethoxyresorufin and furafylline as well as quinidine. Mexiletine p - and 2-hydroxylase activities in microsomes from nine human livers correlated significantly with bufuralol 1'-hydroxylase activity ( r =0.907, P <0.001 and r =0.886, P <0.01, respectively). Microsomes of B-lymphoblastoid cells expressing human CYP1A2 exhibited lower mexiletine p - and 2-hydroxylase activities than those expressing human CYP2D6. It was estimated by RAF that the major isoform involved in mexiletine metabolism was CYP2D6, and the contribution of CYP1A2 to both mexiletine p - and 2-hydroxylase activities was 7–30% in human liver microsomes. However, the K _m values of the expressed CYP1A2 (∼15  μm ) were almost identical with those of the expressed CYP2D6 (∼22  μm ) and human liver microsomes.
Conclusions Mexiletine is a substrate of CYP1A2. The data obtained in this study suggest that the interaction of mexiletine with theophylline might be due to competitive inhibition of CYP1A2.     

Cyclosporin pharmacokinetics following administration of capsules and Neoral in paediatric patients with lupus nephritis

Aims Neoral is a new microemulsion form of cyclosporin. Pharmacokinetic reports in children are scarce. Therefore, we performed a pharmacokinetic study between Cyclosporin A (CsA) capsules and Neoral in paediatric patients with lupus nephritis.
Methods A single 5  mg  kg⁻¹ dose orally of either CsA capsules or Neoral was given to 10 paediatric patients (serum creatinine<1.5  mg dl⁻¹ ). CsA whole blood levels were measured for 24  h post-dose by h.p.l.c.
Results Neoral had a higher C _max and AUC( C _max: 943±176  ng  ml⁻¹; AUC: 4612±785  ng  ml⁻¹  h) than those of the CsA capsules ( C _max: 697±187  ng  ml⁻¹; AUC: 3483±873  ng  ml⁻¹  h; P <0.05). There was no difference in t _max and t _1/2,z between the two groups.
Conclusions CsA Neoral had improved absorption and bioavailability, which is similar to what is reported in adults. However, interpatient variability still existed. Careful drug monitoring and dose adjustment should be performed during treatment to avoid nephrotoxicity, especially in lupus nephritis.     

1-Methylxanthine derived from caffeine as a pharmacodynamic probe of oxypurinol effect

Aims In the present study we have investigated the use of caffeine, administered in the form of instant coffee, as a prodrug for 1MX to validate the use of the 1MU:1MX ratio following caffeine administration as a pharmacodynamic measure of oxypurinol effect on xanthine oxidase.
Methods Five healthy volunteers took caffeine 75  mg 8 hourly administered as instant coffee over a 7 day period. They were given allopurinol 600  mg on day 4. Urine was collected in 8  h aliquots from day 1–day 7. The ratio of 1-methyluric acid (1MU) to 1-methylxanthuric (1MX) was determined.
Results The relationship between the plasma oxypurinol (the active metabolite of allopurinol) concentration at the midpoint of each caffeine dosage interval and the decrement in the urinary 1MX to 1MU ratio fitted well by a sigmoid E_max model. Mean (±s.d.) values of the oxypurinol E C ₅₀(3.9±1.4  mg  l^{− 1} ), E C ₉₀(8.7±1.8  mg  l^{− 1} ) and the exponent, n (3.0±1.2) were similar to those obtained previously following either the direct administration of 1MX or the use of theophylline as a prodrug for 1MX.
Conclusions These data indicate that the use of caffeine as a source of 1MX could provide a simple and ethically acceptable method for monitoring oxypurinol effect in patients taking allopurinol for the treatment of gout.