Quantification of Gd-BOPTA uptake and biliary excretion from dynamic magnetic resonance imaging in rat livers: model validation with 153Gd-BOPTA.

OBJECTIVES: We sought to develop and validate a pharmacokinetic model allowing description of the magnetic resonance (MR) signal intensity induced by the hepatobiliary contrast agent Gd-BOPTA and to quantify the overall Gd-BOPTA transport in rat liver. MATERIALS AND METHODS: MR signal intensity was recorded during the perfusion of rat livers with Gd-DTPA, an extracellular contrast agent, and Gd-BOPTA, a hepatobiliary contrast agent. Similar experiments were conducted with Gd-labeled contrast agents for quantitative measurement in liver, bile and perfusate. RESULTS: A complete 6-compartment, 8 parameter open model was first developed to describe the pharmacokinetics of the compound based on the radioactivity data analysis. Because perfusate and bile data were not available in MRI experiments, a reduced model (6-compartment, 5 parameters) was considered for the MRI data. The performance of the reduced model was tested using the radioactivity data. The reduced model successfully described the contrast agent amount in the liver and correctly predicted amounts in bile and perfusate. CONCLUSIONS: Pharmacokinetic modeling of MR signal intensity induced by Gd-BOPTA permits quantification of Gd-BOPTA uptake and biliary excretion in rat livers.

Gd-BOPTA transport into rat hepatocytes: pharmacokinetic analysis of dynamic magnetic resonance images using a hollow-fiber bioreactor.

RATIONALE AND OBJECTIVES: To investigate the transport of the hepatobiliary magnetic resonance (MR) imaging contrast agent Gd-BOPTA into rat hepatocytes. MATERIALS AND METHODS: In a MR-compatible hollow-fiber bioreactor containing hepatocytes, MR signal intensity was measured over time during the perfusion of Gd-BOPTA. For comparison, the perfusion of an extracellular contrast agent (Gd-DTPA) was also studied. A compartmental pharmacokinetic model was developed to describe dynamic signal intensity-time curves. RESULTS: The dynamic signal intensity-time curves of the hepatocyte hollow-fiber bioreactor during Gd-BOPTA perfusion were adequately fitted by 2 compartmental models. Modeling permitted to discriminate between the behaviors of the extracellular contrast agent (Gd-DTPA) and the hepatobiliary contrast agent (Gd-BOPTA). It allowed the successfully quantification of the parameters involved in such differences. Gd-BOPTA uptake was saturable at high substrate concentrations. CONCLUSIONS: The transport of Gd-BOPTA into rat hepatocytes was successfully described by compartmental analysis of the signal intensity recorded over time and supported the hypothesis of a transporter-mediated uptake.

Physiologically based pharmacokinetics in drug development and regulatory science: a workshop report (Georgetown University, Washington, DC, May 29-30, 2002).

A 2-day workshop on "Physiologically Based Pharmacokinetics (PBPK) in Drug Development and Regulatory Science" came to a successful conclusion on May 30, 2002, in Washington, DC. More than 120 international participants from the environmental and predominantly pharmaceutical industries, Food and Drug Administration (FDA), and universities attended this workshop, organized by the Center for Drug Development Science, Georgetown University, Washington, DC. The first of its kind specifically devoted to the subject, this intensive workshop, comprising 7 plenary presentations and 10 breakout sessions addressed 2 major objectives: (1) to "define demonstrated and potential contributions of PBPK in drug development and regulatory science," and (2) to "assess current PBPK methodologies with the identification of their limitations and outstanding issues." This report summarizes the presentations and recommendations that emerged from the workshop, while providing key references, software, and PBPK data sources in the appendices. The first day was initially devoted to presentations setting the stage and providing demonstrated applications to date. This was followed by breakout sessions that considered further opportunities and limitations, and which extended into Day 2 to deal with developments in methodologies and tools. Although the primary emphasis was on pharmacokinetics, consideration was also given to its integration specifically with mechanism-based pharmacodynamics.

Time course of clinical response to venlafaxine: relevance of plasma level and chirality.

OBJECTIVE: Early clinical response to antidepressant treatment is an important therapeutic goal, considering the psychological, social and economic consequences of depression. The aim of the present study was to investigate the relationship between the time course of response and the concentration of venlafaxine (V), its active metabolite O-desmethylvenlafaxine (ODV) and enantiomeric ratios V(+)/V(-) and ODV(+)/ODV(-). METHODS: Depressed inpatients ( n=35) received V orally at a fixed 300 mg daily dose. Accepted comedication included clorazepate (maximum 60 mg/day), zopiclone (maximum 15 mg/day) and low-dose trazodone (maximum 200 mg/day). Severity of depression was assessed on days 0, 4, 7, 11, 14, 21 and 28 (Montgomery and Asberg Depression Rating Scale). Blood samples were taken on day 14 and day 28 and submitted to stereoselective determination. All measurements reflected trough steady-state values. First, pattern analysis was used to provide a categorical perspective of clinical response (50% improvement from baseline depression score). Patients displaying non-response, transient response, early persistent response and delayed persistent response were compared with respect to racemic concentrations and enantiomeric ratios. Second, in a dimensional perspective, mixed-effects modelling was used to analyse severity of depression versus time curves with respect to the possible influence of concentrations and enantiomeric ratios. RESULTS: Comparison of patients with and without persistent response did not reveal any significant difference for V, ODV, V+ODV plasma levels or enantiomeric ratios. Persistent response was significantly associated with less frequent pre-study antidepressant medication and less frequent comedication with zopiclone (day 14) and clorazepate (day 28) during the study. Focus on patients with persistent response ( n=19, 54.3%) indicated that early response, first observed before day 14, was associated with significantly higher V+ODV concentration than delayed response (median 725 ng/ml versus 554 ng/ml, P=0.023). No difference was found for pre-study medication or comedication during the study. Shorter time to onset of response was significantly associated with lower V(+)/V(-) enantiomeric ratio (r(s)=0.48, P<0.05). Mixed-effects modelling of depression severity versus time curves in patients with persistent response confirmed that either higher V+ODV plasma level or lower V(+)/V(-) ratio were significantly associated with more rapid decrease of depression score (likelihood ratio tests, P=0.012 and P=0.046, respectively). CONCLUSION: Considering its modest sample size, naturalistic design and limited observation period, the present study provided preliminary indication that earlier clinical response may occur with higher V+ODV plasma level, extending previous dose-response studies. The hypothesis was also raised that exposure to a more potent noradrenergic therapeutic moiety, as reflected by a lower V(+)/V(-) ratio, may be relevant to early improvement of depression.

Therapeutic drug monitoring of olanzapine: the combined effect of age, gender, smoking, and comedication.

Therapeutic drug monitoring (TDM) data for the antipsychotic drug olanzapine were investigated with respect to concentration versus dose relationship, intraindividual versus interindividual variability, and the combined influence of patient characteristics on steady-state concentration. The study included 250 patients, with daily doses ranging from 2.5 to 30 mg. Median concentration to dose ratio was 2.1 (ng/mL)/(mg/d), with 90% of the distribution in a fivefold range. In the first subgroup of patients with two measurements at different doses (n = 21), data were in keeping with linear concentration versus dose relationship. In the second subgroup of patients with repeated measurements at a constant daily dose (n = 40), estimates of within-patient and between-patient variabilities were 30.5% and 49.4%, respectively. In the whole sample, multiple regression analysis of dose-normalized concentration revealed significant effects of time postdose (-18% per 12 hours delay, P < 0.05), age >/=60 years (+27%, P < 0.005), cigarette smoking (-12%, P < 0.05), and comedication with fluvoxamine (+74%, P < 0.001), paroxetine, fluoxetine, or sertraline (considered together, +32%, P < 0.05), venlafaxine (+27%, P < 0.05), and inducers of P450 enzymes (-40%, P < 0.001). The final model included a tendency for higher concentration associated with female gender (+11%, P = 0.07) and accounted for 27% of observed interindividual variability. When considering a worst-case scenario, an elderly, nonsmoking woman prescribed fluvoxamine comedication was predicted to reach a 4.6-fold higher olanzapine concentration than a young male smoker coadministered carbamazepine. The current study suggests that patients characterized by a combination of factors associated with altered metabolism may benefit from olanzapine TDM.

Steady-state concentration of venlafaxine enantiomers: model-based analysis of between-patient variability.

OBJECTIVE: To investigate patients treated for depression with respect to steady-state concentration of venlafaxine enantiomers, to quantify within- and between-subject variability and to study the possible influence of individual characteristics such as gender and age. METHODS: Thirty-five inpatients received venlafaxine orally at a fixed 300-mg daily dose. Blood samples were taken on day 14 and day 28 for therapeutic drug monitoring purposes. All measurements reflected steady-state trough values. In a first stage, plasma concentrations of racemic venlafaxine (V) and O-desmethylvenlafaxine (ODV) were measured using a gas chromatography method. In a second stage, (+)/(-) enantiomeric ratios for V and ODV were determined using a stereoselective capillary electrophoresis method. RESULTS: Interindividual variability was 77% and 33% for concentrations of racemic V and ODV, respectively. Intraindividual variability was below 20% for both compounds. Enantiomeric ratios did not statistically differ from unity, with median (+)/(-) ratios of 1.14 for V and 0.97 for ODV. ODV/V metabolite formation ratios for the (+) and (-) enantiomers did not significantly differ from each other (median values 2.85 and 2.37, respectively). However, reduced ODV/V ratio for the (-) enantiomer was strongly associated with decreased (+)/(-) ratio for V (r(S)=0.71, P<0.001) and increased (+)/(-) ratio for ODV (r(S)=-0.79, P<0.001). In contrast, ODV/V ratio for the (+) enantiomer did not significantly correlate with parent compound (+)/(-) ratio and correlated only weakly with metabolite (+)/(-) ratio (r(S)=-0.38, P<0.05). When compared with males, females displayed a significantly lower ODV/V ratio for the (-) enantiomer (median values 1.42 vs 5.08 on day 14, P<0.05) but not for the (+) enantiomer (median values 2.36 vs 3.27, n.s.). Analysis did not reveal any significant association between ODV/V ratios and age, weight, height, creatinine clearance, smoking or co-medication. A pharmacokinetic model at steady state was developed that postulated two different enzyme systems to contribute to O-desmethylation. ODV(-) formation was supposed to largely depend on a single pathway, possibly impaired in a patient subpopulation. ODV(+) formation was postulated to rely on both pathways to a similar extent. Model predictions were in close agreement with observations in patients. CONCLUSION: Observations, together with model-based simulations, suggested that marked stereoselectivity in a patient subgroup may be related with impairment of O-desmethylation greater for (-) than (+) venlafaxine. This hypothesis requires testing against phenotypic and genotypic characteristics of patients.