Levofloxacin Population Pharmacokinetics in South African Children Treated for Multidrug-Resistant Tuberculosis

Levofloxacin is increasingly used in the treatment of multidrug-resistant tuberculosis (MDR-TB). There are limited pediatric pharmacokinetic data to inform dose selection for children. Children routinely receiving levofloxacin (250-mg adult tablets) for MDR-TB prophylaxis or disease in Cape Town, South Africa, underwent pharmacokinetic sampling following receipt of a dose of 15 or 20 mg/kg of body weight given as a whole or crushed tablet(s) orally or via a nasogastric tube. Pharmacokinetic parameters were estimated using nonlinear mixed-effects modeling. Model-based simulations were performed to estimate the doses across weight bands that would achieve adult exposures with 750-mg once-daily dosing. One hundred nine children were included. The median age was 2.1 years (range, 0.3 to 8.7 years), and the median weight was 12 kg (range, 6 to 22 kg). Levofloxacin followed 2-compartment kinetics with first-order elimination and absorption with a lag time. After inclusion of allometric scaling, the model characterized the age-driven maturation of clearance (CL), with the effect reaching 50% of that at maturity at about 2 months after birth and 100% of that at maturity by 2 years of age. CL in a typical child (weight, 12 kg; age, 2 years) was 4.7 liters/h. HIV infection reduced CL by 16%. By use of the adult 250-mg formulation, levofloxacin exposures were substantially lower than those reported in adults receiving a similar dose on a milligram-per-kilogram basis. To achieve adult-equivalent exposures at a 750-mg daily dose, higher levofloxacin pediatric doses of from 18 mg/kg/day for younger children with weights of 3 to 4 kg (due to immature clearance) to 40 mg/kg/day for older children may be required. The doses of levofloxacin currently recommended for the treatment of MDR-TB in children result in exposures considerably lower than those in adults. The effects of different formulations and formulation manipulation require further investigation. We recommend age- and weight-banded doses of 250-mg tablets of the adult formulation most likely to achieve target concentrations for prospective evaluation.

A Target-Mediated Drug Disposition Model to Explain Nonlinear Pharmacokinetics of the 11β-Hydroxysteroid Dehydrogenase Type 1 Inhibitor SPI-62 in Healthy Adults

SPI‐62 is a selective and potent small‐molecule inhibitor of 11β‐hydroxysteroid dehydrogenase type 1 (HSD‐1). SPI‐62 has demonstrated substantial and complex nonlinear pharmacokinetics (PK) in humans that is characterized by unusually low plasma exposure at low doses, dose‐dependent volume of distribution, nonlinear PK following the first dose, and dose‐proportional PK at steady state, as well as unusually high accumulation ratios at low doses. The most likely explanation for the observed nonlinearity of SPI‐62 is the saturable binding of SPI‐62 to its pharmacological target HSD‐1, a phenomenon known as target‐mediated drug disposition (TMDD). Because of the nonlinear and complex PK of SPI‐62, the relationship among SPI‐62 dose, exposure, and response is no longer intuitive and consequently dose selection can be challenging. To facilitate dose selection and clinical trial design, in the current study population PK analysis was performed to characterize SPI‐62 dose‐exposure relationship in humans quantitatively. SPI‐62 PK was best characterized by a 2‐compartment TMDD model with 3 transit absorption compartments. The model was successfully established to explain the substantial and unusual nonlinear PK of SPI‐62 in humans, and it provided adequate fitting for both single‐ and multiple‐dose data. Our modeling work has provided a strong foundation for dose selection in future SPI‐62 clinical trials.

Exploring the Pharmacokinetics of Phenoxymethylpenicillin (Penicillin-V) in Adults: A Healthy Volunteer Study

This healthy volunteer study aimed to explore phenoxymethylpenicillin (penicillin-V) pharmacokinetics (PK) to support the planning of large dosing studies in adults. Volunteers were dosed with penicillin-V at steady state. Total and unbound penicillin-V serum concentrations were determined, and a base population PK model was fitted to the data.

Novel Approach for the Bioequivalence Assessment of Topical Cream Formulations: Model-Based Analysis of Tape Stripping Data Correctly Concludes BE and BIE

PURPOSE

The purpose of this study was (a) to suggest a novel dermatopharmacokinetic (DPK) approach from which pharmacokinetic parameters relevant to the bioequivalence (BE) assessment of a topical formulation can be deduced while circumventing the need for numerous measurements and assumptions, and (b) to investigate whether this approach enables the correct conclusion of BE and bioinequivalence (BIE).

METHODS

Bioequivalent and bioinequivalent formulations of acyclovir were compared versus a reference product (Zovirax®). Tape Stripping was conducted at only one dose duration during the uptake phase to generate drug content in stratum corneum versus time profiles, each time point corresponding to one stripped layer. Nonlinear mixed effect modeling (ADAPT5®) (MLEM algorithm) was used to fit the DPK data and to estimate the rate (K_in) and extent (F_S) of drug absorption/input into the skin. Results were evaluated using the average BE approach.

RESULTS

Estimated exposure metrics were within the usual BE limits for the bioequivalent formulation (F_S: 102.4 [90%CI: 97.5-107.7]; K_in: 94.2 [90%CI: 83.7-106.0]), but outside those limits for the bioinequivalent formulation (F_S: 43.4 [90%CI: 27.9-67.6]; K_in: 54.5 [90%CI: 36.6-81.1]).

CONCLUSIONS

The proposed novel DPK approach was shown to be successful, robust and applicable to assess BE and BIE correctly between topical formulations.

Population Pharmacokinetic Modeling Of On-Demand And Surgical Use Of Nonacog Beta Pegol (N9-GP) And rFIXFc Based Upon The paradigm 7 Comparative Pharmacokinetic Study

Aim/objective

Understanding pharmacokinetic (PK) differences between standard and extended half-life (EHL) products is important, particularly for factor IX (FIX), where differences are more significant than for factor VIII. Two single-dose PK trials showed N9-GP achieves higher FIX levels and greater area-under-the-curve than pdFIX, rFIX, and rFIXFc through higher recovery and longer terminal half-life. In paradigm 7, N9-GP demonstrated consistently favorable PK characteristics compared with rFIXFc. Collins et al explored population PK differences between N9-GP and pdFIX/rFIX based upon paradigm 1 data. This analysis uses population PK models based upon the paradigm 7 trial.

Methods

15 patients (21-65 years) with hemophilia B received single 50-IU/kg doses of N9-GP and rFIXFc ≥21 days apart. A population PK model developed from single-dose PK profiles simulated plasma FIX activity following dosing for surgery and on-demand treatment of bleeds. Simulations explored doses and frequencies required to sustain target World Federation of Hemophilia (WFH) factor activity levels.

Results

PK profiles of N9-GP and rFIXFc were described by one- and three-compartment models, respectively. Simulations predicted significantly reduced dosing frequency and consumption for N9-GP than rFIXFc. For severe bleeds, a single N9-GP dose (80 IU/kg) is sufficient to maintain WFH-recommended FIX levels, whereas multiple rFIXFc doses are required. For surgery, redosing in the first week with N9-GP is modeled at day 6 vs rFIXFc dosing at 6, 30, 54, 78, and 126 hrs. For life-threatening bleeds, N9-GP is required at days 0, 3, 6, 13, and 18 vs rFIXFc redosing after 6 hrs with 10 additional doses at 24-, 48-, and 72 hr intervals.

Conclusion

PK modeling approaches based upon direct comparative studies offer insights into PK differences between EHL FIX products. Model simulations show N9-GP may allow on-demand treatment and perioperative management with 55-75% fewer injections and 65-74% lower overall factor concentrate consumption than rFIXFc.

Population pharmacokinetics of penicillin G: insights into increased clearance at low concentrations to guide development of improved long-acting formulations for syphilis and prevention of rheumatic fever

Although benzylpenicillin (penicillin G) is listed by the World Health Organization as an Essential Medicine, dose optimization is a persistent challenge, especially for long-acting intramuscular formulations. Maintaining sustained antibiotic exposure at target concentrations is crucial for secondary chemoprophylaxis of rheumatic heart disease and treatment of syphilis. This study compared the pharmacokinetic profile of continuous low-dose benzylpenicillin infusions with a standard-dose bolus and evaluated which renal function marker (serum creatinine, cystatin C, or combined e-glomerular filtration rate [eGFR]) best predicted clearance. Healthy adult volunteers received a single 600 mg IV benzylpenicillin bolus followed by randomization to continuous infusions targeting steady-state concentrations of 3, 6, 9, 12, or 20 ng/mL. Plasma benzylpenicillin concentrations were measured by liquid chromatography-mass spectrometry. Population pharmacokinetic analysis was performed using NONMEM by incorporating both bolus and infusion data, and various GFR estimations were evaluated as covariates for clearance. Data from 72 participants were analyzed, including 504 bolus and 389 continuous infusion samples. A two-compartment model improved fit when the ratio of central volume of distribution between bolus and low-dose infusion was incorporated, and clearance differences at steady state plasma concentration of 3 ng/mL were accounted for. Of the GFR estimations, cystatin C-based eGFR significantly enhanced model fit compared with creatinine-based equations. Benzylpenicillin pharmacokinetics at very low concentrations demonstrated both a higher volume of distribution and increased clearance. Cystatin C-based eGFR may more accurately predict benzylpenicillin clearance, enabling precision dosing for long-acting preparations used for treatment of syphilis and prevention of rheumatic fever.

Characterizing the Nonlinear Pharmacokinetics and Pharmacodynamics of BI 187004, an 11β-Hydroxysteroid Dehydrogenase Type 1 Inhibitor, in Humans by a Target-Mediated Drug Disposition Model

BI 187004, a selective small-molecule inhibitor of 11β-hydroxysteroid dehydrogenase-1 (11β-HSD1), displayed complex nonlinear pharmacokinetics (PK) in humans. Following nine single oral doses, BI 187004 exhibited nonlinear PK at low doses and linear PK at higher doses. Notably, substantial hepatic 11β-HSD1 inhibition (50%) was detected in a very low-dose group, achieving a consistent 70% hepatic enzyme inhibition in subsequent ascending doses without any dose-dependent effects. The unusual PK and PD profiles of BI 187004 suggest the presence of pharmacological target-mediated drug disposition (TMDD), arising from the saturable binding of BI 187004 compound to its high-affinity and low-capacity target 11β-HSD1. The non-intuitive dose, exposure, and response relationship for BI 187004 pose a significant challenge in rational dose selection. This study aimed to construct a TMDD model to explain the complex nonlinear PK behavior and underscore the importance of recognizing TMDD in this small-molecule compound. Among the various models explored, the best model was a two-compartment TMDD model with three transit absorption components. The final model provides insights into 11β-HSD1 binding-related parameters for BI 187004, including the total amount of 11β-HSD1 in the liver (estimated to be 8000 nmol), the second order association rate constant (estimated to be 0.102 nM-1h-1), and the first-order dissociation rate constant (estimated to be 0.11 h-1). Our final population PK model successfully characterized the intricate nonlinear PK of BI 187004 across a wide dose range. This modeling work serves as a valuable reference for the rational selection of the dose regimens for BI 187004's future clinical trials.