Use of a local sensitivity analysis to inform study design based on a mechanistic toxicokinetic model for γ-hydroxybutyric acid

Physiologically based modelling of the pharmacokinetics of three beta-lactam antibiotics after intra-mammary administration in dairy cows

Understanding the pharmacokinetics of intra-mammary antibiotics is important for the prediction of drug residues in milk and for the design of optimal dosage regimens. Unfortunately, compartmental pharmacokinetic models are not valid for this unique system. A minimal physiologically based pharmacokinetic model is presented incorporating the physiology of milk secretion, drug administration at the quarter level, drug absorption and dispersion, drug retention during the inter-milking interval and episodic drug elimination at milking. The primary objective of the study was the development and exploration of a model for major factors controlling drug concentration in milk, rather than generation of rigorously predictive pharmaco-statistical models for any particular drug. This model was implemented in a two-stage approach, using published concentration data for penicillin, cefuroxime, cephapirin and desacetyl-cephapirin in milk of healthy cows. Model simulations evaluated sensitivity and developed predictions of drug residues. The model successfully predicted both drug concentrations and drug residues in milk. The postmilking residual milk volume did not adequately explain antibiotic pharmacokinetics, requiring additional considerations for drug retention. Local sensitivity analysis indicated that increasing the number of quarters treated, the dosage, or the duration of the inter-milking interval prolonged both the persistence of drug residues and the duration that antibiotic concentration exceeded typical minimum inhibitory concentrations. The model was flexible across different beta-lactam drugs as a general description of intra-mammary pharmacokinetics. This model is suitable for the design and analysis of dosage regimens, and could be applied for the prediction of withholding periods when these antibiotic preparations are used off-label. The final model indicates that explicit consideration of the milking regimen is fundamental to the design and interpretation of pharmacokinetic studies of antibiotics in bovine milk.

γ-Hydroxybutyric Acid (GHB) Pharmacokinetics and Pharmacodynamics: Semi-Mechanistic and Physiologically Relevant PK/PD Model

An overdose of γ-hydroxybutyric acid (GHB), a drug of abuse, results in fatality caused by severe respiratory depression. In this study, a semi-mechanistic pharmacokinetic/pharmacodynamic (PK/PD) model was developed to characterize monocarboxylate transporter 1 (MCT1)-mediated transport of GHB, as well as effects of GHB on respiration frequency, for IV doses of 200, 600, and 1500 mg/kg in rats. The proposed PK/PD model for GHB consists of nonlinear metabolism of GHB in the liver, MCT1-mediated renal reabsorption with physiologically relevant concurrent fluid reabsorption, MCT1-mediated uptake into the brain, and direct effects of binding of GHB to GABA_B receptors on the PD parameter, respiration frequency. Michaelis-Menten affinity constants for metabolism, renal reabsorption, and uptake into and efflux from the brain were fixed to the observed in vitro values. The IC ₅₀ value for the effect of GHB on respiration frequency was fixed to a reported value for binding of GHB to GABA_B receptors. All physiological parameters were fixed to the reported values for a 300-g rat. The model successfully captured the GHB PK/PD data and was further validated using the data for a 600-mg/kg dose of GHB after IV bolus administration. Unbound GHB brain ECF/blood partition coefficient (Kp _u,u ) values obtained from the model agreed well with values calculated using experimental ECF concentrations obtained with brain microdialysis, demonstrating the physiological relevance of this model. Sensitivity analysis indicated that the PK/PD model was stable. In conclusion, we developed a semi-mechanistic and physiologically relevant PK/PD model of GHB using in vitro drug-transporter kinetics and in vivo PK/PD data in rats.

Semi-mechanistic kidney model incorporating physiologically-relevant fluid reabsorption and transporter-mediated renal reabsorption: pharmacokinetics of γ-hydroxybutyric acid and L-lactate in rats

This study developed a semi-mechanistic kidney model incorporating physiologically-relevant fluid reabsorption and transporter-mediated active reabsorption. The model was applied to data for the drug of abuse γ-hydroxybutyric acid (GHB), which exhibits monocarboxylate transporter (MCT1/SMCT1)-mediated renal reabsorption. The kidney model consists of various nephron segments--proximal tubules, Loop-of-Henle, distal tubules, and collecting ducts--where the segmental fluid flow rates, volumes, and sequential reabsorption were incorporated as functions of the glomerular filtration rate. The active renal reabsorption was modeled as vectorial transport across proximal tubule cells. In addition, the model included physiological blood, liver, and remainder compartments. The population pharmacokinetic modeling was performed using ADAPT5 for GHB blood concentration-time data and cumulative amount excreted unchanged into urine data (200-1000 mg/kg IV bolus doses) from rats [Felmlee et al (PMID: 20461486)]. Simulations assessed the effects of inhibition (R = [I]/KI = 0-100) of renal reabsorption on systemic exposure (AUC) and renal clearance of GHB. Visual predictive checks and other model diagnostic plots indicated that the model reasonably captured GHB concentrations. Simulations demonstrated that the inhibition of renal reabsorption significantly increased GHB renal clearance and decreased AUC. Model validation was performed using a separate dataset. Furthermore, our model successfully evaluated the pharmacokinetics of L-lactate using data obtained from Morse et al (PMID: 24854892). In conclusion, we developed a semi-mechanistic kidney model that can be used to evaluate transporter-mediated active renal reabsorption of drugs by the kidney.

Semiphysiologically Based Pharmacokinetic Model of Leflunomide Disposition in Rheumatoid Arthritis Patients

A semiphysiologically based pharmacokinetic (semi-PBPK) population model was used to evaluate the influence of enterohepatic recycling and protein binding, as well as the effect of genetic variability in CYP1A2, CYP2C19, and ABCG2, on the large interindividual variability of teriflunomide (active metabolite) concentrations following leflunomide administration in rheumatoid arthritis (RA) patients. The model was developed with total and free teriflunomide concentrations determined in RA patients taking leflunomide, as well as mean teriflunomide concentrations following the administration of leflunomide or teriflunomide extracted from the literature. Once developed, the 15-compartment model was able to predict total and free teriflunomide concentrations and was used to screen demographic and genotypic covariates, of which only fat-free mass and liver function (ALT) improved prediction. This approach effectively evaluated the effects of multiple covariates on both total and free teriflunomide concentrations, which have only been explored previously through simplistic one-compartment models for total teriflunomide.

γ-Hydroxybutyrate (GHB)-induced respiratory depression: combined receptor-transporter inhibition therapy for treatment in GHB overdose

Overdose of γ-hydroxybutyrate (GHB) frequently causes respiratory depression, occasionally resulting in death; however, little is known about the dose-response relationship or effects of potential overdose treatment strategies on GHB-induced respiratory depression. In these studies, the parameters of respiratory rate, tidal volume, and minute volume were measured using whole-body plethysmography in rats administered GHB. Intravenous doses of 200, 600, and 1500 mg/kg were administered to assess the dose-dependent effects of GHB on respiration. To determine the receptors involved in GHB-induced respiratory depression, a specific GABA(B) receptor antagonist, (2S)-(+)-5,5-dimethyl-2-morpholineacetic acid (SCH50911), and a specific GABA(A) receptor antagonist, bicuculline, were administered before GHB. The potential therapeutic strategies of receptor inhibition and monocarboxylate transporter (MCT) inhibition were assessed by inhibitor administration 5 min after GHB. The primary effect of GHB on respiration was a dose-dependent decrease in respiratory rate, accompanied by an increase in tidal volume, resulting in little change in minute volume. Pretreatment with 150 mg/kg SCH50911 completely prevented the decrease in respiratory rate, indicating agonism at GABA(B) receptors to be primarily responsible for GHB-induced respiratory depression. Administration of 50 mg/kg SCH50911 after GHB completely reversed the decrease in respiratory rate; lower doses had partial effects. Administration of the MCT inhibitor l-lactate increased GHB renal and total clearance, also improving respiratory rate. Administration of 5 mg/kg SCH50911 plus l-lactate further improved respiratory rate compared with the same dose of either agent alone, indicating that GABA(B) and MCT inhibitors, alone and in combination, represent potential treatment options for GHB-induced respiratory depression.