Pediatric Extrapolation Approach for U.S. Food and Drug Administration Approval of Brexpiprazole in Patients Aged 13 to 17 Years with Schizophrenia.

A pharmacokinetic (PK) bridging approach was successfully employed to support the dosing regimen and approval of brexpiprazole in pediatric patients aged 13-17 years with schizophrenia. Brexpiprazole was approved in 2015 for the treatment of schizophrenia and the adjunctive treatment of major depressive disorder in adults based on efficacy and safety data from clinical trials. On January 13, 2020, the US Food and Drug Administration issued a general advice letter to sponsors highlighting the acceptance of efficacy extrapolation of certain atypical antipsychotics from adult patients to pediatric patients considering the similarity in disease and exposure-response relationships. Brexpiprazole is the first atypical antipsychotic approved in pediatrics using this approach. The PK data available from pediatric patients aged 13-17 years have shown high variability due to the limited number of PK evaluable subjects, which limits a robust estimation of differences between adult and pediatric patients. The PK model-based approach was thus utilized to evaluate the appropriateness of the dosing regimen by comparing PK exposures in pediatric patients aged 13-17 years with exposures achieved in adults at the approved doses. In addition to exposure matching, safety data from a long-term open-label clinical study in pediatric patients informed the safety profile in pediatric patients. This report illustrates the potential of leveraging previously collected efficacy, safety, and PK data in adult patients to make a regulatory decision in pediatric patients for the indication of schizophrenia.

Application of Population Pharmacokinetic Modeling, Exposure-Response Analysis, and Classification and Regression Tree Analysis to Support Dosage Regimen and Therapeutic Drug Monitoring of Plazomicin in Complicated Urinary Tract Infection Patients with Renal Impairment.

In 2018, the FDA approved plazomicin for the treatment of complicated urinary tract infections (cUTI) including pyelonephritis in adult patients with limited or no alternative treatment options. The objective of this article is to provide the scientific rationales behind the recommended dosage regimen and therapeutic drug monitoring (TDM) of plazomicin in cUTI patients with renal impairment. A previous population pharmacokinetic (PK) model was used to evaluate the dosage regimen in cUTI patients with different degrees of renal impairment. The exposure-response analysis was conducted to identify the relationship between plazomicin exposure and nephrotoxicity incidence in cUTI patients with renal impairment. Classification and regression tree (CART) analysis was utilized to assess the TDM strategy. The receiver operating characteristics curve was plotted to compare two TDM thresholds in cUTI patients with renal impairment. The analyses suggested that dose reduction is necessary for cUTI patients with moderate or severe renal impairment. TDM should be implemented for cUTI patients with mild, moderate, or severe renal impairment to reduce the risk of nephrotoxicity. The trough concentration of 3 µg/mL is a reasonable TDM threshold to reduce the nephrotoxicity incidence while maintaining efficacy in cUTI patients with renal impairment. The application of population PK modeling, exposure-response analysis, and CART analysis allowed for the evaluation of a dosage regimen and TDM strategy for plazomicin in cUTI patients with renal impairment. Our study demonstrates the utility of pharmacometrics and statistical approaches to inform a dosage regimen and TDM strategy for drugs with narrow therapeutic windows.

Model-Informed Drug Development in Pediatric Dose Selection.

Model-informed drug development (MIDD) has been a powerful and efficient tool applied widely in pediatric drug development due to its ability to integrate and leverage existing knowledge from different sources to narrow knowledge gaps. The dose selection is the most common MIDD application in regulatory submission related to pediatric drug development. This article aims to give an overview of the 3 broad categories of use of MIDD in pediatric dose selection: leveraging from adults to pediatric patients, leveraging from animals to pediatric patients, and integrating mechanism in infants and neonates. Population pharmacokinetic analyses with allometric scaling can reasonably predict the clearance in pediatric patients aged >5 years. A mechanistic-based approach, such as physiologically based pharmacokinetic accounting for ontogeny, or an allometric model with age-dependent exponent, can be applied to select the dose in pediatric patients aged ≤2 years. The exposure-response relationship from adults or from other drugs in the same class may be useful in aiding the pediatric dose selection and benefit-risk assessment. Increasing application and understanding of use of MIDD have contributed greatly to several policy developments in the pediatric field. With the increasing efforts of MIDD under the Prescription Drug User Fee Act VI, bigger impacts of MIDD approaches in pediatric dose selection can be expected. Due to the complexity of model-based analyses, early engagement between drug developers and regulatory agencies to discuss MIDD issues is highly encouraged, as it is expected to increase the efficiency and reduce the uncertainty.

Evaluation of Hemodialysis Effect on Pharmacokinetics of Meropenem/Vaborbactam in End-Stage Renal Disease Patients Using Modeling and Simulation.

The objectives of this study were to evaluate the effect of hemodialysis (HD) on the pharmacokinetics (PK) of meropenem/vaborbactam, an approved beta-lactam/beta-lactamase inhibitor combination, and provide the rationale for the recommended timing of meropenem/vaborbactam administration relative to HD in end-stage renal disease (ESRD) patients. Population PK models were developed separately for meropenem and vaborbactam in subjects with normal renal function and different degrees of renal impairment, including those receiving HD. Simulations were performed to evaluate the exposure of meropenem and vaborbactam in ESRD patients who received a fixed dose of 0.5 g/0.5 g meropenem/vaborbactam every 12 hours as a 3-hour intravenous infusion under various drug administration schedules relative to HD. The probability of target attainment (PTA) analyses were conducted with pharmacokinetic/pharmacodynamic (PK/PD) targets of meropenem and vaborbactam. Simulations showed that HD reduces the accumulation of vaborbactam, but the exposure of vaborbactam is still above the PK/PD target regardless of whether meropenem/vaborbactam is administered predialysis or postdialysis. For meropenem, drug infusion completed right prior to initiation of HD may substantially reduce exposure leading to poor PTA results. In contrast, drug infusion completed at least 2 hours prior to initiation of HD is not predicted to result in efficacy loss based on PTA analysis. The results of simulation indicate that meropenem/vaborbactam infusion completed at least 2 hours prior to initiation of HD or administered immediately after the end of HD can avoid potential efficacy loss in ESRD patients.

PA5001 gene involves in swimming motility and biofilm formation in Pseudomonas aeruginosa.

Pseudomonas aeruginosa is a nosocomial human pathogen causing infections in immunocompromised patients. To explore new genes involved in P. aeruginosa swimming motility, Mu transposon mutagenesis library was screened for isolates with altered swimming motility. Eleven nonmobile mutants were identified. Sequence analysis shows the nonmobile phenotype of one isolate was attributed to the inactivation of PA5001 gene. PA5001 knockout mutant based on the PAK lab strain also displayed comparable phenotypes suggesting the universal gene function regardless of strain. Exotic PA5001 gene fragment provided on expressing plasmid was capable of storing nonmobile phenotype of PA5001 mutant, suggesting the functional involvement of PA5001 gene on bacterial swimming. Impact of PA5001 inactivation on biofilm formation was examined, as adhesion and interaction during biofilm formation is highly dependent of bacterial mobility. The result shows that normal architecture of biofilm was disrupted in the mutant. Complementing by exotic PA5001 gene fragment resulted in the restoration of biofilm phenotype. Our results provide evidences suggesting the functional participation of PA5001 gene in bacterial mobility and biofilm formation. The critical function by PA5001 in bacterial motility and biofilm might serve as hint for the novel target for the treatment of chronic infections caused by P. aeruginosa.

FDA Approval Summary: Tagraxofusp-erzs For Treatment of Blastic Plasmacytoid Dendritic Cell Neoplasm.

Tagraxofusp-erzs (Elzonris, Stemline) is a cytotoxin that targets CD123-expressing cells. On December 21, 2018, FDA approved tagraxofusp-erzs for the treatment of blastic plasmacytoid dendritic cell neoplasms (BPDCN) in adult and pediatric patients 2 years and older. Approval was based on the response rate in a single-arm trial, Study STML-401-0114; the pivotal cohort included 13 patients with treatment-naïve BPDCN who received tagraxofusp-erzs monotherapy. The complete response or clinical complete response (CR/CRc) rate in the pivotal cohort was 54% (95% CI: 25%-81%), and the median duration of CR/CRc was not reached with a median follow-up of 11.5 months (range: 0.2-12.7). In a separate exploratory cohort, a CR/CRc was achieved by 2 (13%) patients with R/R BPDCN. Safety was assessed in 94 patients with myeloid neoplasms treated with tagraxofusp-erzs at the approved dose and schedule. The major toxicity was capillary leak syndrome (CLS), which occurred in 55% of patients and was fatal in 2%. Hepatotoxicity and hypersensitivity reactions were reported in 88% and 46% of patients, respectively. Other common (≥30%) adverse reactions were nausea, fatigue, peripheral edema, pyrexia, and weight increase. A high proportion of patients (85%) developed neutralizing antidrug antibodies. Tagraxofusp-erzs is the first FDA-approved treatment for BPDCN.

Role of Model-Informed Drug Development in Pediatric Drug Development, Regulatory Evaluation, and Labeling.

The unique challenges in pediatric drug development require efficient and innovative tools. Model-informed drug development (MIDD) offers many powerful tools that have been frequently applied in pediatric drug development. MIDD refers to the application of quantitative models to integrate and leverage existing knowledge to bridge knowledge gaps and facilitate development and decision-making processes. This article discusses the current practices and visions of applying MIDD in pediatric drug development, regulatory evaluation, and labeling, with detailed examples. The application of MIDD in pediatric drug development can be broadly classified into 3 categories: leveraging knowledge for bridging the gap, dose selection and optimization, and informing clinical trial design. In particular, MIDD can provide evidence for the assumption of exposure-response similarity in bridging existing knowledge from reference to target population, support the dose selection and optimization based on the "exposure-matching" principle in the pediatric population, and increase the efficiency and success rate of pediatric trials. In addition, the role of physiologically based pharmacokinetics in drug-drug interaction in children and adolescents and in utilizing ontogeny data to predict pharmacokinetics in neonates and infants has also been illustrated. Moving forward, MIDD should be incorporated into all pediatric drug development programs at every stage to inform clinical trial design and dose selection, with both its strengths and limitations clearly laid out. The accumulated experience and knowledge of MIDD has and will continue to drive regulatory policy development and refinement, which will ultimately improve the consistency and efficiency of pediatric drug development.

FDA Approval Summary: Glasdegib for Newly Diagnosed Acute Myeloid Leukemia.

On November 21, 2018, the FDA approved glasdegib (Daurismo; Pfizer), a small-molecule Hedgehog inhibitor, in combination with low-dose cytarabine (LDAC) for treatment of newly diagnosed acute myeloid leukemia (AML) in adults ≥ 75 years or with comorbidities that preclude use of intensive induction chemotherapy. Evidence of clinical benefit came from Study BRIGHT AML 1003, a randomized trial comparing glasdegib+LDAC with LDAC alone for treatment of newly diagnosed AML in 115 patients either ≥ 75 years old or ≥ 55 years old with preexisting comorbidities. Efficacy was established by improved overall survival (OS) with the combination compared with LDAC alone (HR, 0.46; 95% confidence interval, 0.30-0.71; one-sided stratified log-rank P = 0.0002). Median OS was 8.3 months with the combination and 4.3 months with LDAC alone. Common adverse reactions included cytopenias, fatigue, hemorrhage, febrile neutropenia, musculoskeletal pain, nausea, edema, dyspnea, decreased appetite, dysgeusia, mucositis, constipation, and rash. The label includes a boxed warning for embryo-fetal toxicity and a warning for QT interval prolongation. There is a limitation of use for patients with moderate-to-severe hepatic and severe renal impairment; trials studying glasdegib in these patient populations are required as a condition of this approval.See related commentary by Fathi, p. 6015.

Clinical Pharmacokinetics and Pharmacodynamics of Ceftazidime-Avibactam Combination: A Model-Informed Strategy for its Clinical Development.

Avibactam is a non-ß-lactam, ß-lactamase inhibitor of the diazabicyclooctane class that covalently acylates its ß-lactamase targets, encompassing extended spectrum of activities that cover serine ß-lactamases but not metallo-ß-lactamases. Ceftazidime and avibactam have complementary pharmacokinetic (PK) profiles. Both drugs have a half-life of approximately 2 h, making them suitable to be combined in a fixed-dose combination ratio of 4:1 (ceftazidime:avibactam). Renal clearance is the primary elimination pathway of both ceftazidime and avibactam, and dose adjustment is required in patients with moderate and severe renal impairment. Population PK models of ceftazidime and avibactam were developed separately and incorporated body weight, disease state, ethnicity, and renal function (creatinine clearance) as covariates of clearance and volume of distribution. The clinical dosing regimen of ceftazidime/avibactam combination was determined from population PK model simulations in the patient population for dosing regimens that can achieve sufficient joint probability of target attainment for ceftazidime minimum inhibitory concentration (MIC) of 8 mg/L at a fixed 4 mg/L avibactam concentration (MIC ≤ 8/4 mg/L); 8 mg/L is the breakpoint of ceftazidime in Enterobacteriaceae and Pseudomonas aeruginosa for the target pharmacodynamic indices of ceftazidime and avibactam of 50% time at which the free ceftazidime concentration is above the MIC (fT > MIC) and 50% time at which the free avibactam is above a threshold concentration of 1 mg/L (fT > CT). Whereas the static index approach does not take into account the changing potency of ceftazidime in the presence of changing avibactam concentration, a mathematical model based on kill-curve kinetics was utilized to validate the dose selection in humans. The clinical dosing regimen of 2/0.5 g ceftazidime/avibactam administered every 8 h as a 2-h intravenous infusion in patients with normal renal function, with dose adjustment in renal impairment, demonstrated statistical non-inferiority to carbapenem in phase III studies on the treatment of complicated intra-abdominal infection, complicated urinary tract infection, and nosocomial pneumonia, including ceftazidime non-susceptible Gram-negative pathogens. The success of the phase III studies validated the dose selection and exposure target that were associated with efficacy based on a model-informed approach.

Dosage Considerations for Canakinumab in Children With Periodic Fever Syndromes.

Periodic fever syndromes are a group of rare diseases with a highly variable onset, yet limited treatment options are available for children at an early age. Canakinumab has been approved to treat patients with cryopyrin-associated periodic syndrome, a periodic fever syndrome, and systemic juvenile systemic arthritis, with age cutoffs of 4 years and 2 years, respectively. In 2016, the US Food and Drug Administration (FDA) approved canakinumab, without an age restriction, for the treatment of three conditions of periodic fever syndromes, including familial Mediterranean fever, hyperimmunoglobulin D syndrome/mevalonate kinase deficiency, and tumor necrosis factor receptor-associated periodic syndrome. This review discusses the pharmacokinetic (PK), efficacy, safety, and exposure-response relationship of canakinumab and provides the rationale for dosage recommendation in children younger than 2 years of age with the three conditions of periodic fever syndromes. The approval of canakinumab for these pediatric patients addresses a critical unmet medical need.

A mathematical model-based analysis of the time-kill kinetics of ceftazidime/avibactam against Pseudomonas aeruginosa.

Objectives: To characterize quantitatively the effect of avibactam in potentiating ceftazidime against MDR Pseudomonas aeruginosa by developing a mathematical model to describe the bacterial response to constant concentration time-kill information and validating it using both constant and time-varying concentration-effect data from in vitro and in vivo infection systems. Methods: The time course of the bacterial population dynamics in the presence of static concentrations of ceftazidime and avibactam was modelled using a two-state pharmacokinetic/pharmacodynamic (PK/PD) model, consisting of active and resting states, to account for bactericidal activities, bacteria-mediated ceftazidime degradation and inhibition of degradation by avibactam. Ceftazidime's effect on the bacterial population was described as an enhancement of the death rate of the active population, with the effect of avibactam being to increase ceftazidime potency. Model validation was performed by comparing simulated time courses of bacterial responses with those from in vitro and in vivo experimental exposures of ceftazidime and avibactam that represented those predicted in an average patient dosed with 2 g/0.5 g ceftazidime/avibactam administered every 8 h as 2 h infusions. Results: The two-state model successfully described the bacterial population dynamics, ceftazidime degradation and its inhibition by avibactam. For external validation, the model correctly predicted the bacterial response of P. aeruginosa isolates evaluated in in vitro hollow-fibre and in vivo neutropenic mouse thigh and lung infection models. Conclusions: The PK/PD model and modelled strains successfully replicated the spread in activity when compared with a large selection of P. aeruginosa strains reported in the literature.

Potentiation of ceftazidime by avibactam against ß-lactam-resistant Pseudomonas aeruginosa in an in vitro infection model.

Objectives: This study evaluated the in vitro pharmacodynamics of combinations of ceftazidime and the non-ß-lactam ß-lactamase inhibitor, avibactam, against ceftazidime-, piperacillin/tazobactam- and meropenem-multiresistant Pseudomonas aeruginosa by a quantitative time-kill method. Methods: MICs of ceftazidime plus 0-16 mg/L avibactam were determined against eight isolates of P. aeruginosa . Single-compartment, 24 h time-kill kinetics were investigated for three isolates at 0-16 mg/L avibactam with ceftazidime at 0.25-4-fold the MIC as measured at the respective avibactam concentration. Ceftazidime and avibactam concentrations were measured by LC-MS/MS during the time-kill kinetic studies to evaluate drug degradation. Results: Avibactam alone displayed no antimicrobial activity. MICs of ceftazidime decreased by 8-16-fold in the presence of avibactam at 4 mg/L. The changes in log 10 cfu/mL at both the 10 h and 24 h timepoints (versus 0 h) revealed bacterial killing at ≥1-fold MIC. Significantly higher concentrations of ceftazidime alone, as compared with those of ceftazidime in combination, were required to produce any given kill. Without avibactam, ceftazidime degradation was significant (defined as degradation t 1/2 < 24 h), with as little as 19% ± 18% of the original concentration remaining at 8 h for the most resistant strain. In combination with avibactam, ceftazidime degradation at ≥ 1-fold MIC was negligible. Conclusion: The addition of avibactam protected ceftazidime from degradation in a dose-dependent manner and restored its cidal and static activity at concentrations in combination well below the MIC of ceftazidime alone.

Osimertinib for the Treatment of Metastatic EGFR T790M Mutation-Positive Non-Small Cell Lung Cancer.

On November 13, 2015, the FDA granted accelerated approval to osimertinib (TAGRISSO; AstraZeneca), a breakthrough therapy-designated drug for the treatment of patients with metastatic EGFR T790M mutation-positive non-small cell lung cancer, as detected by an FDA-approved test, with progression on or after EGFR tyrosine kinase inhibitor therapy. Approval was based on durable tumor response rates in two single-arm, multicenter trials: the dose extension cohort of a first-in-human trial (FIH; AURA extension; n = 201) and a fixed-dose, activity-estimating trial (AURA2; n = 210). Osimertinib was administered at 80 mg orally once daily. The objective response rates (ORR) per blinded independent committee review were 57% [95% confidence interval (CI), 50-64) in AURA extension and 61% (95% CI, 54-68) in AURA2. Median duration of response (DOR) could not be estimated. Supportive efficacy data from 63 patients in the dose-finding part of the FIH trial demonstrated an ORR of 51% (95% CI, 38-64), with a median DOR of 12.4 months. Common adverse events (AE) evaluated in 411 patients included diarrhea (42%), rash (41%), dry skin (31%), and nail toxicity (25%). Grade 3 to 4 AEs occurred in 28% of patients, and 5.6% discontinued treatment due to AEs. Clin Cancer Res; 23(9); 2131-5. ©2016 AACR.

In vitro pharmacokinetics/pharmacodynamics of the combination of avibactam and aztreonam against MDR organisms.

OBJECTIVES: The combination of aztreonam and avibactam has been proposed for the treatment of infections caused by metallo-ß-lactamase-producing Gram-negative organisms, given the stability of aztreonam against metallo-ß-lactamases plus the broad coverage of avibactam against AmpC ß-lactamases and ESBLs. This study aimed to evaluate the efficacy of the combination against four clinical isolates with defined but diverse ß-lactamase profiles. METHODS: The MICs of aztreonam were determined without and with avibactam (1, 2, 4, 8 and 16 mg/L). Using the MIC values, the static time-kill kinetic studies were designed to encompass aztreonam concentrations of 0.25, 0.5, 1, 2 and 4 times the MIC at the respective avibactam concentrations from 0 to 8 mg/L. Aztreonam and avibactam concentrations were determined by LC-MS/MS during the course of the time-kill kinetic studies to evaluate whether avibactam protects aztreonam from degradation. RESULTS: Three of the four isolates had aztreonam MICs ≥128 mg/L in monotherapy. Dramatically increasing susceptibility associated with a decrease in aztreonam MIC was observed with increasing avibactam concentration. Against all isolates, the combinations resulted in greater killing with a much lower dose requirement for aztreonam. The resulting changes in base-10 logarithm of cfu/mL at both the 10 h and 24 h references (versus 0 h) were synergistic. In contrast, a significantly higher concentration of aztreonam in the monotherapy was required to produce the same kill as that in the combination therapy, due to rapid aztreonam degradation in two isolates. CONCLUSIONS: The aztreonam/avibactam combination protects aztreonam from hydrolysis and provides synergy in antimicrobial activity against multiple ß-lactamase-expressing strains with a wide MIC range.

Gentamicin dosing strategy in patients with end-stage renal disease receiving haemodialysis: evaluation using a semi-mechanistic pharmacokinetic/pharmacodynamic model.

OBJECTIVES: Gentamicin is widely used in end-stage renal disease (ESRD) patients for the treatment of infections. The goal of this study was to find the most reasonable dosing regimen for gentamicin in ESRD patients receiving haemodialysis. METHODS: The in vitro antimicrobial activity of gentamicin was evaluated by static and dynamic time-kill experiments against three bacterial strains of MSSA, MRSA and Pseudomonas aeruginosa. A semi-mechanistic pharmacokinetic/pharmacodynamic (PK/PD) model was established afterwards, allowing the characterization of the antibacterial effect of gentamicin in the human body. The model was utilized to assess dosing regimens of gentamicin in ESRD patients receiving haemodialysis, taking both efficacy and safety into account. RESULTS: The PK/PD model was capable of describing the bacterial response to gentamicin exposure in all three strains. Simulation based on the PK/PD model showed that pre-dialysis and post-dialysis dosing would bring comparable benefit to the ESRD patient regardless of whether the PK/PD target (fCmax/MIC >8-fold) was achieved, while the post-dialysis dosing resulted in a significantly lower trough concentration. The result of simulated dose fractionation demonstrated that both fCmax/MIC and fAUC(0-24)/MIC are strong predictors of drug effectiveness, but the PK/PD model would provide a more precise prediction of antibacterial activity as well as valuable information on dose selection in ESRD patients receiving haemodialysis. CONCLUSIONS: Our study supports the original FDA label with regard to the dosing regimen of gentamicin in ESRD patients, which offers adequate clinical benefit as well as an acceptable safety profile.

Pharmacokinetics and pharmacodynamics in antibiotic dose optimization.

INTRODUCTION: Identifying the optimized dosing regimen and algorithm is critical in the development of antibiotics. Suboptimal regimens and inappropriate choice of drug give rise to drug-resistant bacteria which have limited the therapeutic utility of many commercially available antimicrobial agents. Strategies to optimize therapy of antimicrobial candidates to speed up the development process are urgently needed. AREAS COVERED: We examined pharmacokinetics and pharmacodynamics of antimicrobial agents with modeling and simulation approaches. The approach that is based on minimum inhibitory concentration to evaluate antimicrobial dosing strategy is widely utilized in drug development. The modeling approach utilizing information from time-kill kinetic studies is a tool that can provide more information on the time-course of bacterial response to a particular dosing regimen. Animal studies of dosing regimens that mimic human pharmacokinetics are another option to evaluate antimicrobial efficacy. Empirical, semi-mechanistic and mechanistic models of bacterial dynamics and development of drug resistance in response to drug therapy are discussed. EXPERT OPINION: Both theories and applications of these approaches provide an overall understanding of how the tools can streamline drug development process and help make crucial decisions. Many opportunities and potentials are presented to incorporate more rigorous integration of PK-PD modeling approaches even at preclinical stage to extrapolate to clinical settings, thus enabling successful trials and optimizing dosing strategies in relevant populations where the drug is mostly used.

Theory and Application of Microdialysis in Pharmacokinetic Studies.

The major aim of current pharmacokinetic studies is to investigate the drug absorption, disposition, metabolism and excretion in animals and humans. The time courses of plasma concentrations are usually characterized and linked to the pharmacodynamic effect to evaluate drug efficacy. However, under certain circumstance, site of action is located in tissues rather than in circulation, which requires direct measurement of tissue concentrations instead of plasma concentrations. Microdialysis is one of those techniques that have been demonstrated to be feasible for measurement of free drug concentration in many tissues, and is capable of being applied in most pharmacokinetic studies to enhance drug efficacy and reduce the unnecessary side effects. This paper reviews this technique from the perspective of theoretical background including the history of development, basic principle, components and their functions. Moreover, the relative recovery of microdialysis and the key factors are discussed, followed by calibration methods available to reduce the systemic bias. In addition, microdialysis is compared with conventional tissue sampling technique for superiorities and limitations. At last, the application of microdialysis in pharmacokinetic study is summarized, especially in assessment of drug efficacy and safety in different tissues. So far, microdialysis is a valuable tool in drug development and will play an unique role in a variety of pharmacokinetic and pharmacodynamic studies in future.

A novel regulator PA5022 (aefA) is involved in swimming motility, biofilm formation and elastase activity of Pseudomonas aeruginosa.

Pseudomonas aeruginosa is an opportunistic pathogen contributing to a range of nosocomial infections. To identify new genes involved in P. aeruginosa swimming motility, an important mechanism of pathogenesis, mutants with altered swimming motility patterns from Mu transposon mutagenesis library of the P. aeruginosa clinical strain PA68 were isolated and characterized. We identified a mutant with transposon inactivation of PA5022 has completely abolished its swimming motility while still possesses a normal terminal flagellum according to electronic microscopy analysis. Microscopic examination revealed that the PA5022 mutant forms thicker biofilms compared to the PA68 wild-type strain and is impaired in its elastase activity. To exclude the possibility of genetic diversity in affecting gene functions among different strains, we constructed a PA5022 knock out mutant based on the PAK lab strain. The PAKΔPA5022 has similar phenotypes to the PA5022 (PA5022::Mu) mutant of PA68 strain. Furthermore, transcriptional fusion assays were carried out to investigate the regulatory mechanism of PA5022 by using the PlasI-lacZ, PrhlI-lacZ, PrpoN-lacZ, PrpoS-lacZ, PqscR-lacZ, PvqsR-lacZ fusions. ß-Galactosidase activity assays indicated that the expression of the vqsR, lasI and rhlI promotors was reduced in the PA5022 mutant compared to the PA68 wild-type. Our study showed that PA5022 links swimming motility and quorum sensing, which might be an important regulator for the pathogenesis of P. aeruginosa.

Evaluation of in vitro synergy between vertilmicin and ceftazidime against Pseudomonas aeruginosa using a semi-mechanistic pharmacokinetic/pharmacodynamic model.

The aim of this study was to develop a semi-mechanistic pharmacokinetic/pharmacodynamic (PK/PD) model to evaluate the in vitro synergy between vertilmicin and ceftazidime against Pseudomonas aeruginosa. The in vitro antimicrobial activity of vertilmicin alone was initially assessed by static and dynamic time-kill experiments against three bacterial strains, including MSSA, MRSA and P. aeruginosa. The combined killing effect with ceftazidime was then evaluated in a static time-kill study against P. aeruginosa. Vertilmicin displayed a concentration-dependent killing effect against the three bacterial strains, and its short half-life may possibly have a dramatic impact on antimicrobial activities. A two-compartment pharmacodynamic model consisting of drug-susceptible and -resistant compartments was developed to characterise the relationship between drug exposure and bacterial response for the time-kill curves from both monotherapy and combination therapy. Loewe additivity was incorporated into the pharmacodynamic model to describe the drug-drug interactive effect in the combination therapy. For monotherapy, the estimated EC50 of the dynamic time-kill study against each strain was close to its MIC but was higher than that of the static time-kill study. The EC50 of combination therapy was estimated at 2.67 mg/L compared with 4.54 mg/L in monotherapy, indicating an enhanced bactericidal capacity. The drug-drug interactive effect was not significantly synergistic but highly varied at each specific combination. Potential synergistic combinations could be screened using PK/PD modelling and simulation. These results demonstrated that PK/PD modelling provides an innovative approach to assist dose selection of combination vertilmicin and ceftazidime for future clinical study design.

Application of pharmacokinetic/pharmacodynamic modelling and simulation for the prediction of target attainment of ceftobiprole against meticillin-resistant Staphylococcus aureus using minimum inhibitory concentration and time-kill curve based approaches.

The purpose of this report was to compare two different methods for dose optimisation of antimicrobials. The probability of target attainment (PTA) was calculated using Monte Carlo simulation to predict the PK/PD target of fT>MIC or modelling and simulation of time-kill curve data. Ceftobiprole, the paradigm compound, activity against two MRSA strains was determined, ATCC 33591 (MIC=2mg/L) and a clinical isolate (MIC=1mg/L). A two-subpopulation model accounting for drug degradation during the experiment adequately fit the time-kill curve data (concentration range 0.25-16× MIC). The PTA was calculated for plasma, skeletal muscle and subcutaneous adipose tissue based on data from a microdialysis study in healthy volunteers. A two-compartment model with distribution factors to account for differences between free serum and tissue interstitial space fluid concentration appropriately fit the pharmacokinetic data. Pharmacodynamic endpoints of fT>MIC of 30% or 40% and 1- or 2-log kill were used. The PTA was >90% in all tissues based on the PK/PD endpoint of fT>MIC >40%. The PTAs based on a 1- or 2-log kill from the time-kill experiments were lower than those calculated based on fT>MIC. The PTA of a 1-log kill was >90% for both MRSA isolates for plasma and skeletal muscle but was slightly below 90% for subcutaneous adipose tissue (both isolates ca. 88%). The results support a dosing regimen of 500mg three times daily as a 2-h intravenous infusion. This dose should be confirmed as additional pharmacokinetic data from various patient populations become available.