Pediatric Population Pharmacokinetic Modeling and Exposure-Response Analysis of Ambrisentan in Pulmonary Arterial Hypertension and Comparison With Adult Data.

This study aimed to develop a population pharmacokinetic (PK) model of ambrisentan in pediatric patients (8 to <18 years) with pulmonary arterial hypertension (PAH) and compare pediatric ambrisentan systemic exposure with previously reported adult data. Association of ambrisentan exposure with efficacy (6-minute walking distance) and safety (adverse events) were exploratory analyses. A population PK model was developed using pediatric PK data. Steady-state systemic exposure metrics were estimated for the pediatric population and compared with previously reported data in adult patients with PAH and healthy subjects. No covariates had a significant effect on PK parameters; therefore, the final covariate model was the same as the base model. The pediatric population PK model was a 2-compartment model including the effect of body weight (allometric scaling), first-order absorption and elimination, and absorption lag time. Steady-state ambrisentan exposure was similar between the pediatric and adult population when accounting for body weight differences. Geometric mean area under the concentration-time curve at steady state in pediatric patients receiving ambrisentan low dose was 3% lower than in the adult population (and similar in both populations receiving high dose). Geometric mean maximum plasma concentration at steady state in pediatric patients receiving low and high doses was 11% and 18% higher, respectively, than in the adult population. There was no apparent association in the pediatric or adult population between ambrisentan exposure and change in 6-minute walking distance or incidence of ambrisentan-related adverse events in pediatric patients. The similar ambrisentan exposure and exposure-response profiles observed in pediatric and adult populations with PAH suggests appropriateness of body-weight-based dosing in the pediatric population with PAH.

Population pharmacokinetic and pharmacodynamic analyses of safinamide in subjects with Parkinson's disease.

Safinamide is an orally administered α-aminoamide derivative with both dopaminergic and non-dopaminergic properties. Nonlinear mixed effects models for population pharmacokinetic (PK) and pharmacokinetic-pharmacodynamic (PKPD) analyses were developed using records from, respectively, 623 and 668 patients belonging to two Phase 3, randomized, placebo-controlled, double-blind efficacy studies. The aim was to estimate safinamide population PK parameters in patients with Parkinson's disease (PD) on stable levodopa therapy, and to develop a model of safinamide effect on the PD phase of normal functioning (ON-time). The final models were internally evaluated using visual predictive checks (VPCs), prediction corrected-VPC, and nonparametric bootstrap analysis. Safinamide profiles were adequately described by a linear one-compartmental model with first-order absorption and elimination. CL/F, Vd/F, and KA (95% confidence interval [CI]) were 4.96 (4.73-5.21) L/h, 166 (158-174) L, and 0.582 (0.335-0.829) h-1, respectively. CL/F and Vd/F increased with body weight, while age, gender, renal function, and exposure to levodopa did not influence safinamide PK. The observed ON-time values were adequately described by a linear model, with time in the study period as dependent variable, and rate of ON-time change and baseline plus offset effect as slope and intercept parameters. Safinamide treatment resulted in an increase in ON-time of 0.73 h (week 4), with further ON-time increase with the same slope as placebo. The increase was not influenced by age, levodopa, or safinamide exposure. The population models adequately describe the population PK of safinamide and safinamide effect on ON-time. No dose adjustments in elderly and mild to moderate renally impaired patients are requested.

In vitro recrudescence of Plasmodium falciparum parasites suppressed to dormant state by atovaquone alone and in combination with proguanil.

We studied the viability of Plasmodium falciparum parasites reappearing in long-term cultures after repetitive exposure to atovaquone and proguanil. Parasites (F32 and FCR3) exposed to 100-5000 nM atovaquone for 96 hours were reduced to <5% of initial parasitaemia but recrudesced after 9-15 days. Also, parasites exposed to 1000 nM atovaquone for 48, 72, 96 and 144 hours recrudesced after 9, 14, 21 and 23 days respectively. Immediately after removal of the drug, only 1-3 schizonts per 10000 red blood cells were found consistently, apparently unable to produce trophozoites and thus, possibly, adopting a "dormant state". Parasites (F32 and FCR3) exposed to 500 nM atovaquone for 72 hours reappeared after 14 days. These recrudescing parasites were then re-exposed and suppressed by atovaquone in three consecutive follow-up experiments. They reappeared after 12, 11 and 9 days respectively. No known point mutations in cytochrome b gene (cytb), associated with atovaquone resistance, were detected in any recrudescing parasites. Finally, parasites (F32) exposed to various concentrations of atovaquone and proguanil in combination for 72 hours reappeared after 9-17 days. The baseline susceptibilities of the parasites to individual drugs were similar before and after recrudescence in all experiments.

Pharmacodynamic interactions among atovaquone, proguanil and cycloguanil against Plasmodium falciparum in vitro.

Synergistic interaction between atovaquone and proguanil has been suggested as the reason for the effectiveness of Malarone. The pharmacodynamic interactions among atovaquone, proguanil and its metabolite cycloguanil were investigated in 4 Plasmodium falciparum parasite strains by culture assays in vitro. The response parameters were determined and 2 statistical methods, log-concentration/response probit method and sum of fractional inhibitory concentrations (sigmaFIC) method, were used to analyse the experimental data. Within therapeutically relevant concentration ratios, the combination of atovaquone and proguanil showed mean sigmaFICs of 0.37 at EC50 (50% effective concentrations) and 0.13 at EC90, indicating high synergism. The combination of atovaquone and cycloguanil yielded corresponding mean sigmaFICs of 3.70 and 2.11, indicating antagonism. The EC50 and EC90 values for proguanil alone were not influenced by RPMI-1640 medium with low concentrations of paraaminobenzoic acid and folic acid (LPLF culture medium), whereas the EC50 and EC90 values for cycloguanil were more than 10 times lower in LPLF medium than in normal RPMI-1640 medium. This confirms the hypothesis that proguanil may act on another target than dihydrofolate reductase. We conclude that the effectiveness of Malarone is due to the synergism between atovaquone and proguanil and may not require the presence of cycloguanil.

Time-dependent pharmacokinetics and drug metabolism of atovaquone plus proguanil (Malarone) when taken as chemoprophylaxis.

OBJECTIVE: To determine the pharmacokinetic profiles of atovaquone (ATO), proguanil (PROG) and its active metabolite cycloguanil (CYCLO) with respect to possible accumulation and kinetic interaction upon repeated dosing with Malarone. METHODS: Thirteen healthy volunteers first received a single dose and then after 1 week, repetitive daily doses of Malarone (one tablet) for 13 days. For analysis of plasma drug concentrations, blood samples were collected at regular intervals over 8 days after a single dose and over 12 days after the last day of multiple dosing. Single-dose and steady-state pharmacokinetic parameters were determined for each individual. Genotyping of the gene coding for CYP2C19, a major enzyme catalyzing PROG metabolism, was performed using polymerase chain reaction, and in vitro enzyme kinetic experiments were carried out to study the possible effect of ATO on the catalytic activities of CYP2C19 and 3A4 using fluorometric assays. RESULTS: For ATO, the ratio of the area under the concentration-time curve (AUC) during the last dose interval to the AUC after the single dose (AUC(0- tau)/AUC(0- infinity)) was found to be 0.90 [95% confidence interval (CI) 0.56, 1.24] indicating absence of undue accumulation. AUC(0- tau), and peak plasma concentration at steady state (C(max,ss)) values were, however, threefold lower than those reported in human immunodeficiency virus-infected subjects after 12 multiple daily doses of 250 mg ATO alone. Four volunteers, with mean CYCLO/PROG AUC(0-tau) of 0.03 (-0.23, 0.09) were classified as poor metaboliser (PM) phenotypes. There was a significant increase in the AUC of PROG at steady state with a PROG AUC(0-tau)/AUC(0-infinity) ratio of 1.38 (1.07, 1.69) in extensive metaboliser (EM) phenotypes. CYCLO/PROG AUC ratios were significantly lower 0.67 (0.54, 0.81) at steady state than that after the first single dose in EM phenotypes. The in vitro kinetic experiments on recombinant enzymes (CYP2C19 and CYP3A4) suggested a possible inhibition of catalytic activity of CYP3A4 by ATO. CONCLUSIONS: There was no unexpected accumulation of ATO following repeated administrations of the combination. In EM phenotypes, PROG elimination was reduced at steady state. Also, at steady state, either the elimination of CYCLO was increased or its formation clearance decreased the latter possibly by inhibition of CYP3A4 by ATO.