Pharmacokinetic/pharmacodynamic analysis of tacrolimus-induced QT prolongation in guinea pigs

Recently, several reports of clinical cases of QT prolongation and torsades de pointes, associated with the use of tacrolimus (FK506), have come to light. We have previously demonstrated FK506-induced QT prolongation in guinea pigs [Minematsu T., et al., Life Sci., 65, PL197-PL202 (1999)]. We now examined the relationship between QTc prolongation and the pharmacokinetics of FK506 in guinea pigs, in order to evaluate the arrhythmogenicity of FK506 when compared with that of quinidine sulfate (QND). Thus, dose-response relationships for FK506 (0.01 or 0.1 mg/h/kg) or QND (30 mg/h/kg) were investigated during and after intravenous infusion and also following intravenous bolus administration of FK506 (0.2 mg/kg). The dose-response relationship between plasma drug concentration and QTc prolongation for FK506 and QND were subsequently analyzed using an effect compartment model. The pharmacodynamic parameters thus obtained were as follows: kE0 2.72 x 10(-4) (min-1), Emax 27.1 (ms), EC50 0.376 (ng/ml) for FK506; and kE0 0.148 (min-1), K 8.41 (ms.ml/microgram) for QND. The anti-clockwise hysteresis observed for FK506-induced QT prolongation was successfully analyzed by the present pharmacokinetic/pharmacodynamic model, which may provide a rational basis for developing a clinical dosing regimen to avoid possible QT prolongation induced by FK506.     

Increased tumor cisplatin levels in heated tumors in mice after administration of thermosensitive, large unilamellar vesicles encapsulating cisplatin

Hyperthermia (HT)-dependent cisplatin (CDDP) release and tumor CDDP level increase after the administration of thermosensitive, large unilamellar vesicles (LUVs: LUV-1 and LUV-2) and a thermosensitive, small unilamellar vesicle (SUV: SUV-1) were examined in comparison with those following administration of a non-thermosensitive LUV (LUV-3) and a CDDP solution (Sol) in tumor bearing mice. The LUV-1 and LUV-2 released CDDP at a faster rate than SUV-1 when incubated in saline at temperatures between 41 and 44 degrees C. The blood CDDP levels after liposome administration were higher than those after Sol administration. The systemic clearance of LUV-2 was slightly larger than those of the other liposomes. The tumor CDDP levels after thermosensitive liposome administration were increased in response to HT in comparison to LUV-3 or Sol. The increased ratio for LUV-1 was the largest. The ratio of the area under the tumor CDDP level versus time curve (AUC) for LUV-1 + HT to the AUC for Sol + HT was approximately 5. The results indicate that (1) the tumor-CDDP level increase after thermosensitive liposome administration is due to CDDP release from the liposome in the blood at or adjacent to the heated tumor, (2) the increase is highly dependent on the heat sensitivity and systemic stability of the liposome, and (3) LUV, such as LUV-1, exhibit higher heat sensitivity and larger, targeted drug delivery efficiency than SUV.     

Effects of CYP2D6 genotypes on plasma concentrations of risperidone and enantiomers of 9-hydroxyrisperidone in Japanese patients with schizophrenia

It has been shown that risperidone (+)-9-hydroxylation is enantioselectively catalyzed by the polymorphic CYP2D6 in human liver. This study aimed to examine the effect of CYP2D6 genotype on (+)-9-hydroxylation of risperidone in schizophrenic patients. Subjects were 38 Japanese schizophrenic inpatients receiving 6 mg/day of risperidone. Plasma concentrations of risperidone and (+)- and (-)-9-hydroxyrisperidone at steady state were quantified using LC/MS/MS and HPLC with alpha 1 acid-AGP chiral column, respectively. The CYP2D6*5(*5) and *10 alleles were identified using polymerase chain reaction (PCR) methods. Twenty patients had no mutated allele, 14 had one mutated allele, and 4 had two mutated alleles. There were significant differences in the steady-state plasma concentrations of risperidone (ANOVA; p < 0.0001) among the three genotype groups, while the CYP2D6 genotype did not affect the steady-state plasma concentrations of (+)-9-hydroxyrisperidone (p = 0.314) or (-)-9-hydroxyrisperidone (p = 0.957). The concentration ratio of risperidone to 9-hydroxyrisperidone was strongly dependent on the CYP2D6 genotypes. This study suggests that CYP2D6 activity strongly influences the steady-state plasma concentrations of risperidone and risperidone/9-hydroxyrisperidone concentration ratios but is unlikely to determine enantio-selectivity in the steady-state plasma concentrations of 9-hydroxyrisperidone in the clinical situation.     

Transport of benzylpenicillin by the rat choroid plexus in vitro

To characterize the transport system by which benzylpenicillin, an organic anion, is accumulated by the isolated rat choroid plexus, kinetic analysis of benzylpenicillin transport was performed. Accumulation of benzylpenicillin was against an electrochemical potential gradient via a saturable process (Km = 58 microM and Vmax = 84 nmol/ml of tissue per min) that was inhibited by sulfhydryl reagents (p-hydroxymercuribenzoate and N-ethylmaleimide), metabolic inhibitors (KCN and 2,4-dinitrophenol) and anion exchange inhibitors (4-acetamide-4'-isothiocyanatostilbene-2,2'-disulfonic acid and 4,4'-diisothiocyanatostilbene-2,2'-disulfonic acid), but is major transport system did not require the inward Na+ gradient. Organic anions, such as 5-hydroxyindoleacetic acid, homovanillic acid, p-aminohippuric acid and probenecid inhibited the accumulation of benzylpenicillin, whereas dipeptides did not affect it. Kinetic analysis of the accumulation of benzylpenicillin suggests that both phenoxymethylpenicillin and cefpiramide are also transported via the benzylpenicillin transport system. Other penicillin and cephalosporin derivatives inhibited the accumulation of benzylpenicillin with different affinities. Penicillin derivatives without dissociating groups in the side chain had the higher affinity for the benzylpenicillin transport system than other beta-lactam antibiotics did. Among penicillin derivatives examined, a good correlation (r = 0.92) was observed between the lipophilicity and the affinity for the benzylpenicillin transport system, whereas no correlation was observed among the cephalosporin derivatives. These findings suggest that the major transport system of benzylpenicillin in the rat choroid plexus is via a carrier-mediated active anion transport process which is distinct from that of dipeptides, and does not require the inward Na+ gradient.(ABSTRACT TRUNCATED AT 250 WORDS)     

Recurrent intraoral herpes simplex virus infection

A 48-year-old female had primary herpetic gingivostomatitis, followed by recurrent intraoral herpes simplex virus (HSV) disease; HSV isolates were obtained from the swabs of primary and recurrent lesions; restriction endonuclease cleavage analysis of the viral DNAs extracted from Vero cells infected with the HSV isolates according to the method of Hirt was carried out. The viral DNAs were cleaved by restriction endonucleases such as BamHI, KpnI and SalI and resolved by agarose gel electrophoresis, followed by staining with ethidium bromide. Consequently, their cleavage patterns were very similar to one another and were identified as HSV type 1. From these findings, it can be concluded that primary and recurrent lesions of this case are caused by the same virus.     

Pharmacokinetic analysis of the disposition of valproate in pregnant rats.

Kinetic analysis of valproate (VPA) disposition in pregnant rats was performed. A dose-dependent saturable plasma elimination was observed in both the control and pregnant rats. Saturation was more remarkable in the pregnant rats and this was assumed to be due to the decreased hepatic extraction by metabolism. Pharmacokinetic parameters were calculated using a two-compartment open model with a Michaelis-Menten-type elimination process from the central compartment. In the pregnant rats, the calculated values of Km and Vmax decreased significantly, by one-half of those of the control rats, whereas no significant difference was observed in distribution rate constants (k12, k21) and volumes of distribution (V1, Vdss) between two groups of rats. Furthermore, the hepatic extraction of VPA in the control and pregnant rats was investigated in order to explain nonlinear pharmacokinetics of VPA. The values of the hepatic extraction ratio in the control and pregnant rats were 39.9 and 22.4% at the steady state concentration of approximately 20 micrograms/ml, respectively. The extraction ratios in the pregnant rats were lower than those of the control rats. This fact may be one of the reasons why the elimination of VPA in the pregnant rats is more susceptible to saturation.     

Studies on diameter distribution of axons in the optic pathway in human glaucoma

The distribution of diameters of axons and somas at the level of the optic nerve, lamina cribrosa and retina were compared between human glaucomatous eye and normal control eyes. At the level of lamina scleralis of the glaucomatous eye, there was a tendency for large nerve fibers to be lost in the arcuate nerve fiber area particularly in the inferior arcuate nerve fiber area, where the corresponding visual field defect was detected. The percentages of large axons and somas in the arcuate nerve fiber area decreased in the glaucomatous eye both at the level of the optic nerve, retina but most of all at the level of lamina cribrosa. These results suggest that the mode of optic nerve damage in early glaucoma is characterized by the selective damage of large nerve fibers, which is most severe at the lamina cribrosa.     

Estimation of drug absorption rates using a deconvolution method with nonequal sampling times

A method affording direct estimation of the drug absorption rate from blood level data using arbitrary time intervals has been derived based on the staircase input principle. In the derivation, the drug was assumed to follow linear kinetics where the plasma concentration of the drug after an impulse input is expressed by a multiexponential function. Drug absorption was assumed to occur at a constant rate during each subsequent sampling interval. The absorption rate profiles obtained by the method using several numerical examples were expressed as a set of rectangular pulses. Divergence in the profiles reflected blood sampling measurement errors rather than errors due to the deconvolution. Smoothing of the rate profiles by calculating the mean of the absorption rates between adjacent time intervals gave realistic results. Absorption rate profiles for theophylline obtained by the method using published data gave information on the initiation and termination of the absorption as well as the extent of absorption from the dosage form.