Sensitive determination of oseltamivir and oseltamivir carboxylate in plasma, urine, cerebrospinal fluid and brain by liquid chromatography-tandem mass spectrometry.

This manuscript describes the determination of oseltamivir (OP) and oseltamivir carboxylate (OC) in rat plasma, cerebrospinal fluid (CSF) and brain and in human plasma and urine using liquid chromatography coupled to tandem mass spectrometry. Threefold deuterated OP and OC served as internal standards. Protein precipitation with perchloric acid was followed by on-line solid-phase extraction and gradient separation on a reversed-phase column. After electrospray ionization, the compounds were detected in positive ion selected reaction monitoring (SRM) mode. Run time was 3.6 min. The lower limits of quantification (LLOQ) were 0.1 ng/mL in rat plasma and CSF, 0.5 ng/g in brain and 1 ng/mL in human plasma and urine. Inter-day and intra-day precisions and inaccuracies in rat matrices were below 10.2% and 13.9% (below 19.0% at LLOQ), respectively. Intra-assay precisions and inaccuracies in human matrices were below 11.7% and 8.9%, respectively. The recoveries were close to 100%, and no significant matrix effect was observed. The method was successfully applied to rat study samples.

Similarity in pharmacokinetics of oseltamivir and oseltamivir carboxylate in Japanese and Caucasian subjects.

The pharmacokinetics of oseltamivir and oseltamivir carboxylate in healthy Japanese (n = 14) and Caucasian (n = 14) males were compared. Subjects in each ethnic group were randomized to twice-daily oral oseltamivir 75 mg, 150 mg, or placebo for 13 doses. Oseltamivir was well tolerated across doses and ethnic groups. Oseltamivir was rapidly absorbed and hydrolyzed to oseltamivir carboxylate in all subjects. The mean plasma concentration-time profiles for oseltamivir and oseltamivir carboxylate were similar in Japanese and Caucasian subjects. At steady state, there was no evidence of any ethnic difference in the individual AUC(0-12) values for oseltamivir or oseltamivir carboxylate. Despite a significant difference in group mean body weight (approximately 20 kg) between the Japanese and Caucasian subjects, there was no evidence that dose-adjusted AUC(0-12) and C(max) for oseltamivir carboxylate were affected by body weight or ethnicity. Day 7 trough concentrations (C(min)) for oseltamivir carboxylate markedly exceeded the IC(50) (50% inhibitory concentration) against influenza A and B isolates. In conclusion, the results of this study support the use of the same dose regimens of oseltamivir in both Caucasian and Japanese subjects because of similarity in pharmacokinetics.

Development and validation of a liquid chromatographic-tandem mass spectrometric method for determination of oseltamivir and its metabolite oseltamivir carboxylate in plasma, saliva and urine.

A bioanalytical method for the analysis of oseltamivir (OP) and its metabolite oseltamivir carboxylate (OC) in human plasma, saliva and urine using off-line solid-phase extraction and liquid chromatography coupled to positive tandem mass spectroscopy has been developed and validated. OP and OC were analysed on a ZIC-HILIC column (50 mm x 2.1 mm) using a mobile phase gradient containing acetonitrile-ammonium acetate buffer (pH 3.5; 10mM) at a flow rate of 500 microL/min. The method was validated according to published FDA guidelines and showed excellent performance. The lower limit of quantification for OP was determined to be 1, 1 and 5 ng/mL for plasma, saliva and urine, respectively and for OC was 10, 10 and 30 ng/mL for plasma, saliva and urine, respectively. The upper limit of quantification for OP was determined to be 600, 300 and 1500 ng/mL for plasma, saliva and urine, respectively and for OC was 10,000, 10,000 and 30,000 ng/mL for plasma, saliva and urine, respectively. The within-day and between-day precisions expressed as R.S.D., were lower than 5% at all tested concentrations for all matrices and below 12% at the lower limit of quantification. Validation of over-curve samples ensured that it would be possible with dilution if samples went outside the calibration range. Matrix effects were thoroughly evaluated both graphically and quantitatively. No matrix effects were detected for OP or OC in plasma or saliva. Residues from the urine matrix (most likely salts) caused some ion suppression for both OP and its deuterated internal standard but had no effect on OC or its deuterated internal standard. The suppression did not affect the quantification of OP.

The novel carboxylesterase 1 variant c.662A>G may decrease the bioactivation of oseltamivir in humans.

BACKGROUND: Human carboxylesterase 1 (CES1) is a serine esterase that hydrolyses various exogenous and endogenous compounds including oseltamivir, a prodrug used to treat influenza. A novel CES1 c.662A>G single nucleotide polymorphism (SNP) was predicted to decrease CES1 enzymatic activity in an in silico analysis. This study evaluated the effect of the c.662A>G SNP on the pharmacokinetics (PK) of oseltamivir in humans. METHODS: A single oral dose of oseltamivir at 75 mg was administered to 20 healthy subjects, 8 heterozygous c.662A>G carriers (c.662AG) and 12 non-carriers (c.662AA). The concentrations of oseltamivir and its active metabolite, oseltamivir carboxylate, were measured in plasma and urine using a validated liquid chromatography-tandem mass spectrometry (LC-MS/MS) method. The PK parameters were calculated using a noncompartmental method. The geometric mean ratios (GMR, c.662AG to c.662AA) of the PK parameters and their 90% confidence intervals (CI) were calculated. RESULTS: The systemic exposure to oseltamivir, as assessed by the AUC0-48h of oseltamivir, was increased by 10% in c.662AG subjects, whereas the AUC0-48h of oseltamivir carboxylate was 5% lower in c.662AG subjects. The GMR and 90% CI of the metabolic ratio (AUC0-48h, Oseltamivir carboxylate/AUC0-48h, Oseltamivir) was 0.87 (0.66-1.14). The amount of unchanged oseltamivir excreted in the urine was increased by 15% in subjects with the c.662AG genotype. CONCLUSIONS: This result suggests that CES1 enzymatic activity may be decreased in these heterozygous allele carriers, although further studies are warranted to investigate the clinical implications of this genetic variation on CES1 substrate drugs. TRIAL REGISTRATION: ClinicalTtrials.gov NCT01902342.

Pharmacokinetics of oseltamivir in young and very elderly subjects.

BACKGROUND: Pharmacokinetic studies of oseltamivir in very elderly patients (> or = 80 y) have not previously been performed. OBJECTIVE: To compare the pharmacokinetics of oseltamivir and the active carboxylate metabolite in healthy young and very elderly Japanese subjects. METHODS: Young (20-35 y, fasting, n = 7) and very elderly subjects (> or = 80 y, fed, n = 5) were enrolled in single-center studies and received a single oral dose of oseltamivir 75 mg. Plasma and urine samples were collected (24 h) for pharmacokinetic analysis, and safety was assessed. RESULTS: The time to maximum plasma concentration (tmax) for oseltamivir was delayed in the very elderly compared with the young subjects (2.30 vs 0.71 h, respectively). Furthermore, oseltamivir maximum plasma concentration (Cmax) and AUC(inf) were 52% and 80% higher, respectively, in the very elderly compared with the young subjects. Oral clearance was 45% lower in elderly patients, possibly due to the effects of administration of oseltamivir with a meal. For the active metabolite, oseltamivir carboxylate, Cmax and AUC(inf) values were, respectively, 22% and 91% higher in the very elderly subjects than in the young subjects, while oral clearance was 50% lower in the elderly population. The increased exposure of the active metabolite is likely to correlate with an age-related decline in renal function. For both oseltamivir and the active metabolite, there was large interpatient variability in the Cmax values. The data reported here indicate that oseltamivir would be effective in both of these populations, as trough concentrations for the active metabolite at 12 and 24 hours exceeded the 50% inhibitory concentration against the neuraminidase of influenza A and B isolates by more than 50-fold. Oseltamivir was well tolerated in both groups. CONCLUSIONS: Exposures (AUC(inf)) to both the parent drug and active metabolite were increased by more than 80% in the small number of very elderly subjects presented here. However, oseltamivir was well tolerated by these subjects.

Understanding the within-host dynamics of influenza A virus: from theory to clinical implications.

Mathematical models have provided important insights into acute viral dynamics within individual patients. In this paper, we study the simplest target cell-limited models to investigate the within-host dynamics of influenza A virus infection in humans. Despite the biological simplicity of the models, we show how these can be used to understand the severity of the infection and the key attributes of possible immunotherapy and antiviral drugs for the treatment of infection at different times post infection. Through an analytic approach, we derive and estimate simple summary biological quantities that can provide novel insights into the infection dynamics and the definition of clinical endpoints. We focus on nine quantities, including the area under the viral load curve, peak viral load, the time to peak viral load and the level of cell death due to infection. Using Markov chain Monte Carlo methods, we fitted the models to data collected from 12 untreated volunteers who participated in two clinical studies that tested the antiviral drugs oseltamivir and zanamivir. Based on the results, we also discuss various difficulties in deriving precise estimates of the parameters, even in the very simple models considered, when experimental data are limited to viral load measures and/or there is a limited number of viral load measurements post infection.

Effect of milk on the pharmacokinetics of oseltamivir in healthy volunteers.

We previously showed that oseltamivir, a prodrug of the influenza virus neuraminidase inhibitor Ro 64-0802, is a substrate of proton-coupled oligopeptide transporter (PEPT1), and its intestinal absorption in rats is markedly inhibited by administration with milk. To investigate the importance of PEPT1 for oseltamivir absorption in humans, and the characteristics of the drug-milk interaction, a crossover clinical study was conducted in healthy volunteers, who received 75 mg of oseltamivir with 400 mL of water or milk. Milk significantly reduced the maximum plasma concentration (C(max) ) and the area under the plasma concentration-time curve from 0 to 2 h (AUC(0-2) ) of both oseltamivir and Ro 64-0802 (oseltamivir, 68.9% and 34.5%; Ro 64-0802, 69.5% and 14.2%, respectively, vs. water), but had no significant effect on the apparent terminal half-life (t(1/2) ) or AUC(0-∞) . Urinary recovery of oseltamivir and Ro 64-0802 was significantly reduced to 77.5% of the control by milk. The early reduction of oseltamivir absorption might be through the PEPT1 inhibition by milk peptides. However, the extent of interaction in humans was limited as compared with that in rats, possibly because of species difference in the PEPT1 expression and its contribution. This might be the first report suggesting the clinical drug-food interaction via PEPT1.