LC-MS-MS determination of nemorubicin (methoxymorpholinyldoxorubicin, PNU-152243A) and its 13-OH metabolite (PNU-155051A) in human plasma

A selective and sensitive liquid chromatography-tandem mass spectrometry (LC-MS-MS) method for quantitative determination of nemorubicin, (PNU-152243A, 3'-deamino-3'[2(S)-methoxy-4-morpholinyl]doxorubicin) hydrocloride and its reduced metabolite PNU-155051 in human plasma has been developed and validated. The method involved solid phase extraction (SPE) in 96-well plates. Plasma samples (0.5 ml plasma, spiked with doxorubicin as internal standard and diluted with 0.5 ml of 0.01 M borate buffer, pH 8.4) were extracted using Oasis HLB SPE material. The elution of PNU-152243, PNU-155051 and of IS was performed with 1 ml of methanol:0.1 M formic acid mixture (90:10, v/v). The organic phase was reduced to dryness under a stream of nitrogen at 20 degrees C and the residue was reconstituted with 0.25 ml of 10 mM ammonium formate buffer pH 4.15:acetonitrile mixture (90:10, v/v). Aliquots of 60 microl of the resulting solution were injected onto the LC-MS-MS system. A Zorbax SB C18 column (2.1 x 150 mm, 3.5 microm) was used to perform the chromatographic analysis. The mobile phase consisted of ammonium formate buffer 10 mM pH 4.15:acetonitrile (73:27, v/v) with a flow-rate of 0.2 ml/min. Detection was achieved by a PE-SCIEX API 3000 with Turbo IonSpray interface, and multiple reaction monitoring (645 --> 321 for PNU-152243, 647 --> 363 for PNU-155051 and 545 --> 345 m/z for doxorubicin) operated in positive ion mode. A weighted linear regression was used to calculate PNU-152243 and PNU-155051 concentrations in QC and unknown samples. Linearity, precision, accuracy and recovery of the method were evaluated over the concentration range of 0.1-5 ng/ml for both compounds. No interference from blank human plasma was observed. The suitability of the method for in vivo samples was assessed by the analysis of samples obtained from patients who had received a single intrahepatic artery dose of PNU-152243A.

Determination of MAG-camptothecin, a new polymer-bound camptothecin derivative, and free camptothecin in dog plasma by HPLC with fluorimetric detection

A high throughput. selective and sensitive high-performance liquid chromatographic (HPLC) method for the determination of a water-soluble polymer-bound Camptothecin conjugate (MAG-CPT) and Camptothecin (CPT) in dog plasma has been developed and validated. The method involved the analysis of free and total CPT (free + polymer-bound). Free CPT (intact lactone plus carboxylate) was extracted from acidified plasma using Oasis SPE material in 96-well plates. For the assay of the total CPT, plasma proteins were first precipitated with methanol in a 96-well plate containing a 10-microm melt blown polypropylene membrane. The methanolic supernatant was separated and collected into a second 96-well plate by simply applying vacuum to the plate. After hydrolysis at pH 9.8 for 18 h and re-acidification, samples were injected directly from the collection plate onto the HPLC system. MAG-CPT concentration was then calculated by subtraction of free from total CPT. The LLOQs of the method were 1.17 ng/ml for free CPT and 103.10 ng/ml (as CPT equivalent) for MAG-CPT using 0.1 and 0.05 ml of plasma, respectively. Linearity, precision, accuracy and recovery of the method were evaluated. The stability of MAG-CPT in plasma alone and after its stabilisation was carefully evaluated. No interference from blank dog, mouse and human plasma was observed. The suitability of the method for in vivo samples was assessed by the analysis of samples obtained from dogs that had received a single and 5-day repeated dose of MAG-CPT.

Effect of rifabutin on ethambutol pharmacokinetics in healthy volunteers

The effect of repeated administration of rifabutin on the pharmacokinetics and metabolism of ethambutol was evaluated in ten healthy volunteers. The subjects received a single oral administration of 1200 mg ethambutol on days 1 and 10 and a single daily oral dose of 300 mg rifabutin from days 3 to 9. No statistically significant difference was found in plasma pharmacokinetics (C(max), t(max), AUC, half-life and MRT) and in the renal clearance, whereas a significant decrease in the amount of unchanged ethambutol excreted in urine was observed. The decrease observed in ethambutol urinary excretion may be accounted for by taking into consideration the variability of the urinary excretion of ethambutol reported in the literature. However, a slight, likely not clinically relevant, induction or activation of kidney alcohol and/or aldehyde dehydrogenase isoenzymes by rifabutin cannot be ruled out at present. Evidence exists in the present study for autoinduction of rifabutin metabolism; this is shown by the lower plasma concentrations obtained 24 h after the seventh dose as compared to the theoretical concentrations.

Pharmacokinetics of reboxetine enantiomers in the dog

Reboxetine, (RS)-2-[(RS)-alpha-(2-ethoxyphenoxy)benzyl]morpholine methanesulphonate, is a racemic compound and consists of a mixture of the (R,R)- and (S,S)-enantiomers. The pharmacokinetics of reboxetine enantiomers were determined in a crossover study in three male beagle dogs. Each animal received the following oral treatments, separated by 1-week washout period: 10 mg/kg reboxetine, 5 mg/kg (R,R)- and 5 mg/kg (S,S)-. Plasma and urinary levels of the reboxetine enantiomers were monitored up to 48 h post-dosing using an enantiospecific HPLC method with fluorimetric detection (LOQ: 1.1 ng/ml in plasma and 5 ng/ml in urine for each enantiomer). After reboxetine administration mean tmax was about 1 h for both enantiomers. Cmax and AUC were about 1.5 times higher for the (R,R)- than for the (S,S)-enantiomer, mean values +/- SD being 704 +/- 330 and 427 +/- 175 ng/ml for Cmax and 2,876 +/- 1,354 and 1,998 +/- 848 ng.h/ml for AUC, respectively. No differences between the (R,R)- and (S,S)-enantiomers were observed in t1/2 (3.9 h). Total recovery of the two enantiomers in urine was similar, the Ae (0-48 h) being 1.3 +/- 0.7 and 1.1 +/- 0.7% of the enantiomer dose for the (R,R)- and the (S,S)-enantiomers, respectively. No marked differences in the main plasma pharmacokinetic parameters were found for either enantiomer on administration of the single enantiomers or reboxetine. No chiral inversion was observed after administration of the separate enantiomers, as already observed in humans.

An investigation of the pharmacokinetics and autoinduction of rifabutin metabolism in mice treated with 10 mg/kg/day six times a week for 8 weeks

Plasma, lung and spleen concentrations of rifabutin have been measured in mice after single and multiple oral administrations of the drug. Rifabutin concentrations in spleen were similar to those measured in plasma, whereas concentrations in lung were higher. No significant autoinduction of rifabutin metabolism was observed.

Stereoselective and species-dependent kinetics of reboxetine in mouse and rat

Reboxetine, (RS)-2-[(RS)-alpha-(2-ethoxyphenoxy)benzyl]morpholine methanesulphonate, is a racemic compound and consists of a mixture of the (R,R)- and (S,S)-enantiomers. In this study, brain and plasma levels of both enantiomers were determined in mice and rats after oral administration of reboxetine at doses (1.1 mg/kg, mouse; 20 mg/kg, rat) twice the respective ED50 values in the antireserpine test. Plasma and brain concentrations of each enantiomer were measured up to 6 h postdosing using an HPLC method with fluorimetric detection after derivatization with a chiral agent (FLEC). In mice and rats, brain and plasma levels of the (R,R)-enantiomer were always higher than those of the (S,S)-enantiomer. After normalization for dose, the mean AUC0-tz values of both the (R,R)- and (S,S)-enantiomers in mouse brain were about 23 and 32 times higher than in rat brain, respectively. In plasma, the corrected mean AUC0-tz values were about 5 (R,R) and 10 (S,S) times higher in mice than in rats. These results provide evidence for the higher bioavailability and/or lower clearance of both enantiomers in mice than in rats, and for a higher penetration of both enantiomers into mouse brain compared to rat brain.