Pharmacokinetic/pharmacodynamic modeling and simulation to determine effective dosage regimens for doripenem

Computational pharmacology of rifampin in mice: an application to dose optimization with conflicting objectives in tuberculosis treatment

Dose selection for rifampin in the treatment of active pulmonary tuberculosis (TB) illustrates some of the challenges for dose optimization within multidrug therapies. Rifampin-based anti-TB regimens are often combined with antiretroviral therapies to treat human immunodeficiency virus (HIV) coinfection. The potent cytochrome P450 (CYP) enzyme inducing properties of rifampin give rise to significant drug-drug interactions, the minimization of which by limiting the dose, conflicts with the maximization of bacterial killing by increasing the dose. Such multiple and conflicting objectives lead to a set of trade-off optimal solutions for dose optimization rather than a single best solution. Here, we combine pharmacokinetic/pharmacodynamic (PK/PD) modeling with multiobjective optimization to quantitatively explore trade-offs between therapeutic and adverse effects of optimal dosing for the example of rifampin in TB-infected mice. The PK/PD model describes rifampin concentrations in plasma and liver following oral administration together with hepatic CYP enzyme induction and bacterial killing kinetics. We include optimization objectives descriptive of antimicrobial efficacy, CYP-mediated drug-drug interactions, and drug exposure-dependent toxicity. Results show non-conventional dosing scenarios that allow for increased efficacy relative to uniform dosing without increasing drug-drug interactions. Additionally, we find currently employed dosages for rifampin to be nearly optimal with respect to trade-offs between efficacy and toxicity. While limited by the accuracy and applicability of the PK/PD model, these results provide an avenue for experimental investigation of complex dose optimization problems. This method can be extended to include additional drugs and optimization objectives, and may provide a useful tool for individualized medicine.

Applications of pharmacometrics in the clinical development and pharmacotherapy of anti-infectives

With the increased emergence of anti-infective resistance in recent years, much focus has recently been drawn to the development of new anti-infectives and the optimization of treatment regimens and combination therapies for established antimicrobials. In this context, the field of pharmacometrics using quantitative numerical modeling and simulation techniques has in recent years emerged as an invaluable tool in the pharmaceutical industry, academia and regulatory agencies to facilitate the integration of preclinical and clinical development data and to provide a scientifically based framework for rational dosage regimen design and treatment optimization. This review highlights the usefulness of pharmacometric analyses in anti-infective drug development and applied pharmacotherapy with select examples.

A paradigm shift in drug development for treatment of rare multidrug-resistant gram-negative pathogens

Multidrug-resistant (MDR) gram-negative pathogens pose a major threat to patients worldwide. Although the organisms remain relatively uncommon overall, their incidence is steadily increasing with associated increases in mortality and pharmacoeconomic impact. As evidenced by the dearth of new products in the pipeline or in clinical use, the conventional paradigm for the development of drugs against such pathogens is generally ineffectual. We advocate the need for a shift in the current paradigm and propose innovative development programs that involve implementation of a graduated approval process. The initial phase of the proposed regulatory paradigm includes early approval of a new drug based on a robust nonrandomized study, buttressed by data from concurrent controls and a pharmacokinetic-pharmacodynamic package generated from nonclinical studies. The postapproval commitment phase will include a randomized controlled trial, when disease prevalence permits, as well as continued assessment of risks and benefits under "real world" settings.

Penetration of doripenem into skeletal muscle and subcutaneous adipose tissue in healthy volunteers

Sufficient antibiotic concentrations at the infection site are a prerequisite for good bacterial killing. This study was performed to determine pharmacokinetics of doripenem in soft tissues and saliva. Six healthy male volunteers received a single intravenous infusion of 500 mg doripenem over 1 h. The concentrations of doripenem were measured over 8 h in saliva, plasma, and extracellular space fluid of skeletal muscle and subcutaneous adipose tissue employing in vivo microdialysis. Unbound drug concentrations were determined using ultra-high-performance liquid chromatography-tandem mass spectrometry. Maximum concentrations of doripenem were 15.3 ± 6.0 mg/liter in plasma, 9.9 ± 2.3 mg/liter in subcutaneous adipose tissue, 6.6 ± 2.9 mg/liter in skeletal muscle, and 0.5 ± 0.2 mg/liter in saliva. Areas under the concentration-time curve (AUC) from 0 to infinity were 26.3 ± 10.1, 20.4 ± 3.8, 12.8 ± 3.0, and 1.0 ± 0.5 mg · h/liter in plasma, adipose tissue, skeletal muscle, and saliva, respectively. Ratios of AUC in adipose tissue, skeletal muscle, and saliva to those in plasma were 0.84 ± 0.28, 0.53 ± 0.19, and 0.04 ± 0.03, respectively. In all six volunteers, a threshold of ≥40% for "time above MIC," an index indicative of good antimicrobial activity, was exceeded in adipose tissue for MICs of ≤2 mg/liter and in skeletal muscle for MICs ≤1.5 mg/liter. Doripenem penetrates well into interstitial space fluid of skeletal muscle and adipose tissue, suggesting good antimicrobial activity in infected soft tissues, whereas it is detectable in relatively low concentrations in saliva.

Population pharmacokinetics of fluconazole after administration of fosfluconazole and fluconazole in critically ill patients

WHAT IS KNOWN AND OBJECTIVE

Fluconazole is an antifungal agent that is commonly used to treat patients with serious systemic fungal infections in intensive care units. Fosfluconazole is a phosphate prodrug of fluconazole, which was developed to reduce the volume of fluid required to administer fluconazole by intravenous injection. The objective of this study was to characterize the pharmacokinetics of the antifungal fluconazole after the intravenous administration of the prodrug fosfluconazole or fluconazole in critically ill patients with serious systemic fungal infections, by population pharmacokinetic analysis using the nonmem software package.

METHODS

Clinical biochemical data including serum fluconazole levels were obtained from 57 patients treated in the intensive care unit along with two naïve pooled patients gleaned from previous reports. The pharmacokinetic model of fluconazole was estimated using a one-compartment model. The probability that the area under the concentration-time curve is higher than 800 μg h/mL was determined by simulation.

RESULTS

It was assumed that all the administered fosfluconazole was converted to fluconazole with an estimated fosfluconazole-fluconazole conversion rate constant of 2·05/h. The significant covariates for clearance for fluconazole (CL) and volume of distribution for fluconazole (Vd) were resulted in creatinine clearance (CLcr) and body weight (BW), respectively, in the final pharmacokinetic model equations: CL (L/h) = 0·799 × [CLcr (mL/min)/92·7](0·685) and Vd (L) = 48·1 × [BW (kg)/65](1·40) , where the interpatient variabilities in CL and Vd and the intrapatient variability were 44·8%, 79·7% and 19·8%, respectively. On the basis of the results of the Monte Carlo simulation, the probabilities of target attainment were 60%, 26% and 11% for 400 mg/day administration as fluconazole equivalent at CLcr values of 40, 70 and 100 mL/min, respectively.

WHAT IS NEW AND CONCLUSION

The present population pharmacokinetic analysis strongly indicates that fosfluconazole (and fluconazole) dosage should be optimized in terms of CLcr in critically ill patients.