A nonlabeled method to evaluate cortisol production rate by modeling plasma CBG-free cortisol disposition

Effect of Protein Binding on Exposure of Unbound and Total Mycophenolic Acid: A Population Pharmacokinetic Analysis in Chinese Adult Kidney Transplant Recipients

OBJECTIVES

The population pharmacokinetic (popPK) characteristics of total mycophenolic acid (tMPA) have been investigated in various ethnic populations. However, investigations of popPK of unbound MPA (uMPA) are few. Thus, a popPK analysis was performed to: (1) characterize the PK of uMPA and tMPA and its 7-O-mycophenolic acid glucuronide (MPAG) metabolite in kidney transplant patients cotreated with cyclosporine (CsA), and (2) identify the clinically significant covariates that explain variability in the dose-exposure relationship.

METHODS

A total of 740 uMPA, 741 tMPA, and 734 total MPAG (tMPAG) concentration-time data from 58 Chinese kidney transplant patients receiving MPA in combination with CsA were analyzed using NONMEM® software with the stochastic approximation expectation maximization (SAEM) followed by the important sampling (IMP) method. The influence of covariates was tested using a stepwise procedure.

RESULTS

The PK of uMPA and unbound MPAG (uMPAG) were characterized by a two- and one-compartment model with first-order elimination, respectively. A linear protein binding model was used to link uMPA and tMPA. Apparent clearance (CL/F) and central volume of distribution (VC/F) of uMPA (CLuMPA/F and VCuMPA/F, respectively) and protein binding rate constant (k B) were estimated to be 851 L/h [relative standard error (RSE), 7.1%], 718 L (18.5%) and 53.4/h (2.3%), respectively. For uMPAG, the population values (RSE) of CL/F (CLuMPAG) and VC/F (VCuMPAG/F) were 5.71 L/h (4.4%) and 29.9 L (7.7%), respectively. Between-subject variability (BSVs) on CLuMPA/F, VCuMPA/F, CLuMPAG/F, and VCuMPAG/F were 51.0, 80.0, 31.8 and 48.4%, respectively, whereas residual unexplained variability (RUVs) for uMPA, tMPA, and uMPAG were 47.0, 45.9, and 22.0%, respectively. Significant relationships were found between k B and serum albumin (ALB) and between CLuMPAG/F and glomerular filtration rate (GFR). Additionally, model-based simulation showed that changes in ALB concentrations substantially affected tMPA but not uMPA exposure.

CONCLUSIONS

The established model adequately described the popPK characteristics of the uMPA, tMPA, and MPAG. The estimated CLuMPA/F and unbound fraction of MPA (FUMPA) in Chinese kidney transplant recipients cotreated with CsA were comparable to those published previously in Caucasians. We recommend monitoring uMPA instead of tMPA to optimize mycophenolate mofetil (MMF) dosing for patients with lower ALB levels.

Prediction of Free from Total Mycophenolic Acid Concentrations in Stable Renal Transplant Patients: A Population-Based Approach

BACKGROUND

A population pharmacokinetic (PK) protein-binding model was developed to (1) predict free mycophenolic acid (fMPA) based on total MPA (tMPA) concentrations in renal transplant patients, to establish the therapeutic range of fMPA through pharmacokinetic-pharmacodynamic studies; and (2) provide a guideline for dosing mycophenolate mofetil (MMF).

METHODS

Full PK profiles of 56 patients (from five different occasions) during the first year after transplantation who were treated with oral MMF and cyclosporine, or macrolides (either tacrolimus or sirolimus), were analysed. fMPA protein-binding was modelled using nonlinear mixed effects modelling (NONMEM). The influence of physiological factors and coadministered immunosupressant was studied.

RESULTS

A two-compartment model with first-order absorption and elimination, linear protein binding and enterohepatic circulation (EHC) best described the PK of MPA. Different recycling rate constants were considered depending on the coadministered immunosuppressant. The protein-binding rate constant (K_B [relative standard error, RSE%]) increased nonlinearly with renal function according to K _B = 43.1 (3.13)·(CL_CR/59.51)^0.394(10.66) h^-1. Furthermore, fMPA plasma clearance, given by clearance of the free mycophenolic acid (CL_fMPA), CL_fMPA = 410 (RSE%3.00)·(1+CsA·0.594 (22.39)) L/h, was 59.4% greater in cyclosporine-treated patients than in macrolide-treated patients, leading to lower MPA exposures. External evaluation proved acceptable area under the plasma concentration-time curve and trough concentration predictions.

CONCLUSIONS

A reliable protein-binding population PK model was developed for prediction of fMPA or tMPA from each other and for dose guiding in stable renal transplant recipients.

Predicting Cortisol Exposure from Paediatric Hydrocortisone Formulation Using a Semi-Mechanistic Pharmacokinetic Model Established in Healthy Adults

BACKGROUND AND OBJECTIVE

Optimisation of hydrocortisone replacement therapy in children is challenging as there is currently no licensed formulation and dose in Europe for children under 6 years of age. In addition, hydrocortisone has non-linear pharmacokinetics caused by saturable plasma protein binding. A paediatric hydrocortisone formulation, Infacort^® oral hydrocortisone granules with taste masking, has therefore been developed. The objective of this study was to establish a population pharmacokinetic model based on studies in healthy adult volunteers to predict hydrocortisone exposure in paediatric patients with adrenal insufficiency.

METHODS

Cortisol and binding protein concentrations were evaluated in the absence and presence of dexamethasone in healthy volunteers (n = 30). Dexamethasone was used to suppress endogenous cortisol concentrations prior to and after single doses of 0.5, 2, 5 and 10 mg of Infacort^® or 20 mg of Infacort^®/hydrocortisone tablet/hydrocortisone intravenously. A plasma protein binding model was established using unbound and total cortisol concentrations, and sequentially integrated into the pharmacokinetic model.

RESULTS

Both specific (non-linear) and non-specific (linear) protein binding were included in the cortisol binding model. A two-compartment disposition model with saturable absorption and constant endogenous cortisol baseline (Baseline _cort,15.5 nmol/L) described the data accurately. The predicted cortisol exposure for a given dose varied considerably within a small body weight range in individuals weighing <20 kg.

CONCLUSIONS

Our semi-mechanistic population pharmacokinetic model for hydrocortisone captures the complex pharmacokinetics of hydrocortisone in a simplified but comprehensive framework. The predicted cortisol exposure indicated the importance of defining an accurate hydrocortisone dose to mimic physiological concentrations for neonates and infants weighing <20 kg. EudraCT number: 2013-000260-28, 2013-000259-42.

Characterization of Cortisol Secretion Rate in Secondary Adrenal Insufficiency

Context:

In secondary adrenal insufficiency (SAI), chronic deficiency of adrenocorticotropin (ACTH) is believed to result in secondary changes in adrenocortical function, causing an altered dose-response relationship between ACTH concentration and cortisol secretion rate (CSR).

Objective:

We sought to characterize maximal cortisol secretion rate (CSR_max) and free cortisol half-life in patients with SAI, compare results with those of age-matched healthy controls, and examine the influence of predictor variables on ACTH-stimulated cortisol concentrations.

Design:

CSR_max was estimated from ACTH_1-24 (250 μg)_–stimulated cortisol time-concentration data. Estimates for CSR_max and free cortisol half-life were obtained for both dexamethasone (DEX) and placebo pretreatment conditions for all subjects.

Setting:

Single academic medical center.

Patients:

Patients with SAI (n = 10) compared with age-matched healthy controls (n = 21).

Interventions:

The order of DEX vs placebo pretreatment was randomized and double-blind. Cortisol concentrations were obtained at baseline and at intervals for 120 minutes after ACTH_1-24.

Main Outcome Measures:

CSR_max and free cortisol half-life were obtained by numerical modeling analysis. Predictors of stimulated cortisol concentrations were evaluated using a multivariate model.

Results:

CSR_max was significantly (P < 0.001) reduced in patients with SAI compared with controls for both placebo (0.17 ± 0.09 vs 0.46 ± 0.14 nM/s) and DEX (0.18 ± 0.13 vs 0.43 ± 0.13 nM/s) conditions. Significant predictors of ACTH_1-24–stimulated total cortisol concentrations included CSR_max, free cortisol half-life, and baseline total cortisol, corticosteroid-binding globulin, and albumin concentrations (all P < 0.05).

Conclusions:

Our finding of significantly decreased CSR_max confirms that SAI is associated with alterations in the CSR-ACTH dose-response curve. Decreased CSR_max contributes importantly to the laboratory diagnosis of SAI.

Pharmacokinetic/pharmacodynamic modeling of benazepril and benazeprilat after administration of intravenous and oral doses of benazepril in healthy horses

Pharmacokinetic and pharmacodynamic (PK/PD) properties of the angiotensin-converting enzyme inhibitor (ACEI) benazeprilat have not been evaluated in horses. This study was designed to establish PK profiles for benazepril and benazeprilat after intravenous (IV) and oral (PO) administration of benazepril using a PK/PD model. This study also aims to determine the effects of benazeprilat on serum angiotensin converting enzyme (ACE), selecting the most appropriate dose that suppresses ACE activity. Six healthy horses in a crossover design received IV benazepril at 0.50mg/kg and PO at doses 0 (placebo), 0.25, 0.50 and 1.00mg/kg. Blood pressures (BP) were measured and blood samples were obtained at different times in order to measure serum drug concentrations and serum ACE activity, using liquid chromatography-tandem mass spectrometry (LC-MS/MS) and spectrophotometry, respectively. Systemic bioavailability of benazeprilat after PO benazepril was 3-4%. Maximum ACE inhibitions from baseline were 99.63% (IV benazepril), 6.77% (placebo) and 78.91%, 85.74% and 89.51% (for the three PO benazepril doses). Significant differences in BP were not found. Although oral availability was low, benazeprilat 1.00mg/kg, reached sufficient serum concentrations to induce long lasting serum ACE inhibitions (between 88 and 50%) for the first 48h. Additional research on benazepril administration in equine patients is indicated.

Reversible increase in maximal cortisol secretion rate in septic shock

OBJECTIVE

Cortisol clearance is reduced in sepsis and may contribute to the development of impaired adrenocortical function that is thought to contribute to the pathophysiology of critical illness-related corticosteroid insufficiency. We sought to assess adrenocortical function using computer-assisted numerical modeling methodology to characterize and compare maximal cortisol secretion rate and free cortisol half-life in septic shock, sepsis, and healthy control subjects.

DESIGN

Post hoc analysis of previously published total cortisol, free cortisol, corticosteroid-binding globulin, and albumin concentration data.

SETTING

Single academic medical center.

PATIENTS

Subjects included septic shock (n = 45), sepsis (n = 25), and healthy controls (n = 10).

INTERVENTIONS

I.v. cosyntropin (250 μg).

MEASUREMENTS AND MAIN RESULTS

Solutions for maximal cortisol secretion rate and free cortisol half-life were obtained by least squares solution of simultaneous, nonlinear differential equations that account for free cortisol appearance and elimination as well as reversible binding to corticosteroid-binding globulin and albumin. Maximal cortisol secretion rate was significantly greater in septic shock (0.83 nM/s [0.44, 1.58 nM/s] reported as median [lower quartile, upper quartile]) compared with sepsis (0.51 nM/s [0.36, 0.62 nM/s]; p = 0.007) and controls (0.49 nM/s [0.42, 0.62 nM/s]; p = 0.04). The variance of maximal cortisol secretion rate in septic shock was also greater than that of sepsis or control groups (F test, p < 0.001). Free cortisol half-life was significantly increased in septic shock (4.6 min [2.2, 6.3 min]) and sepsis (3.0 min [2.3, 4.8 min] when compared with controls (2.0 min [1.2, 2.6 min]) (both p < 0.004).

CONCLUSIONS

Results obtained by numerical modeling are consistent with comparable measures obtained by the gold standard stable isotope dilution method. Septic shock is associated with generally not only higher levels but also greater variance of maximal cortisol secretion rate when compared with control and sepsis groups. Additional studies would be needed to determine whether assessment of cortisol kinetic parameters such as maximal cortisol secretion rate and free cortisol half-life is useful in the diagnosis or management of critical illness-related corticosteroid insufficiency.

Increased whole-body and sustained liver cortisol regeneration by 11beta-hydroxysteroid dehydrogenase type 1 in obese men with type 2 diabetes provides a target for enzyme inhibition

OBJECTIVE

The cortisol-regenerating enzyme 11β-hydroxysteroid dehydrogenase type 1 (11β-HSD1) amplifies glucocorticoid levels in liver and adipose tissue. 11β-HSD1 inhibitors are being developed to treat type 2 diabetes. In obesity, 11β-HSD1 is increased in adipose tissue but decreased in liver. The benefits of pharmacological inhibition may be reduced if hepatic 11β-HSD1 is similarly decreased in obese patients with type 2 diabetes. To examine this, we quantified in vivo whole-body, splanchnic, and hepatic 11β-HSD1 activity in obese type 2 diabetic subjects.

RESEARCH DESIGN AND METHODS

Ten obese men with type 2 diabetes and seven normal-weight control subjects were infused with 9,11,12,12-[(2)H](4)cortisol (40%) and cortisol (60%) at 1.74 mg/h. Adrenal cortisol secretion was suppressed with dexamethasone. Samples were obtained from the hepatic vein and an arterialized hand vein at steady state and after oral administration of cortisone (5 mg) to estimate whole-body and liver 11β-HSD1 activity using tracer dilution.

RESULTS

In obese type 2 diabetic subjects, the appearance rate of 9,12,12-[(2)H](3)cortisol in arterialized blood was increased (35 ± 2 vs. 29 ± 1 nmol/min, P < 0.05), splanchnic 9,12,12-[(2)H](3)cortisol production was not reduced (29 ± 6 vs. 29 ± 6 nmol/min), and cortisol appearance in the hepatic vein after oral cortisone was unchanged.

CONCLUSIONS

Whole-body 11β-HSD1 activity is increased in obese men with type 2 diabetes, whereas liver 11β-HSD1 activity is sustained, unlike in euglycemic obesity. This supports the concept that inhibitors of 11β-HSD1 are likely to be most effective in obese type 2 diabetic subjects.

Pharmacokinetics and pharmacodynamics of single rising intravenous doses (0.5 mg-10 mg) and determination of absolute bioavailability of the dipeptidyl peptidase-4 inhibitor linagliptin (BI 1356) in healthy male subjects

BACKGROUND AND OBJECTIVES

Linagliptin (BI 1356) is a highly specific inhibitor of dipeptidyl peptidase (DPP)-4, which is currently in phase III clinical development for the treatment of type 2 diabetes mellitus. Linagliptin exhibits nonlinear pharmacokinetics after oral administration, which are mainly related to concentration-dependent binding of linagliptin to its target, DPP-4. The objectives of the study were to investigate the pharmacokinetics and pharmacodynamics after intravenous administration of linagliptin and to determine its absolute bioavailability (F).

SUBJECTS AND METHODS

This was a single rising-dose, randomized, four-group, placebo-controlled, single-blind (within dose groups) study. Thirty-six healthy men aged 18-50 years were enrolled and randomized into four sequential treatment groups. Group 1 received linagliptin 0.5 mg intravenously, group 2 received 2.5 mg intravenously and group 4 received 10 mg intravenously. In group 3, subjects underwent a two-way randomized crossover, receiving 5 mg intravenously and a 10 mg oral tablet. Linagliptin concentrations in plasma and urine, as well as plasma DPP-4 activity, were determined by validated assays. Noncompartmental analysis and population pharmacokinetic modelling were performed.

RESULTS

Linagliptin showed nonlinear pharmacokinetics after intravenous infusion of 0.5-10 mg, with a less than dose-proportional increase in exposure. Noncompartmental parameters were calculated on the basis of total (i.e. bound and unbound) plasma concentrations. The total clearance value was low and increased with dose from 2.51 to 14.3 L/h. The apparent steady-state volume of distribution (V(ss)) increased with dose from 380 to 1540 L. Renal excretion of the unchanged parent compound increased with increasing plasma concentrations from 2.72% in the 0.5 mg dose group to 23.0% in the 10 mg dose group. The terminal elimination half-life was comparable across dose groups (126-139 hours). Because of the nonlinear pharmacokinetics, the standard approach of comparing the area under the plasma concentration-time curve (AUC) after oral administration with the AUC after intravenous administration led to dose-dependent estimates of the absolute bioavailability. Therefore, a population pharmacokinetic model was developed, accounting for the concentration-dependent protein binding of linagliptin to its target enzyme, DPP-4. The model-derived estimates of the V(ss) and clearance of linagliptin not bound to DPP-4 were 402.2 L and 26.9 L/h, respectively. The absolute bioavailability was estimated to be about 30% for the linagliptin 10 mg tablet.

CONCLUSION

The nonlinear pharmacokinetic characteristics and the pharmacokinetic/pharmacodynamic relationship of linagliptin were independent of the mode of administration (intravenous or oral). Because of the nonlinear pharmacokinetics, the standard approach of comparing the AUC after oral administration with the AUC after intravenous administration was inappropriate to determine the absolute bioavailability of linagliptin. By a modelling approach, the absolute bioavailability of the 10 mg linagliptin tablet was estimated to be about 30%.

Impact of target-mediated drug disposition on Linagliptin pharmacokinetics and DPP-4 inhibition in type 2 diabetic patients

The pharmacokinetics of the novel dipeptidyl-peptidase 4 (DPP-4) inhibitor linagliptin is nonlinear. Based on in vitro experiments, concentration-dependent binding to DPP-4 is the most likely cause for the nonlinearity. Population pharmacokinetic/pharmacodynamic modeling was performed using linagliptin plasma concentrations and plasma DPP-4 activities from 2 phase 2a studies. In these studies, type 2 diabetic patients received either 1, 2.5, 5, or 10 mg of linagliptin once daily over 12 days (study 1) or 2.5, 5, or 10 mg of linagliptin once daily over 28 days (study 2). The modeling results supported the hypothesis that linagliptin exhibits target-mediated drug disposition. The linagliptin plasma concentrations were best described by a 2-compartment model including concentration-dependent protein binding in the central and peripheral compartment. The plasma DPP-4 activity was included in the model in a semi-mechanistic way by relating it to the model-calculated plasma DPP-4 occupancy with linagliptin. The target binding has a major impact on linagliptin pharmacokinetics. Although unbound linagliptin is cleared efficiently (CL/F 220 L/h), the concentration-dependent binding is responsible for the long terminal half-life (approximatelly 120 hours) of linagliptin and its nonlinear pharmacokinetics. The model allowed a comprehensive understanding of the impact of target-mediated drug disposition and provides a useful tool to support clinical development.

Quantifying nonlinear interactions within the hypothalamo-pituitary-adrenal axis in the conscious horse

Cortisol is an important mediator of physiological stress responses. Hypothalamic CRH and arginine vasopressin (AVP) and pituitary ACTH, in addition to hypothalamic and pituitary cortisol feedback, regulate cortisol secretion. Importantly, joint interactions among the four, rather than the signal of any one hormone, govern this life-preserving axis. Quantifying in vivo strength of such joint interactions has been difficult, especially without direct injection of cortisol, CRH, AVP, or ACTH. The goal of the present research was to estimate these joint feedback and feedforward interactions in vivo in the conscious horse during low-cortisol and hypoglycemic stress. Pituitary venous sampling of ACTH, CRH, and AVP was performed every 0.5-1 min and jugular venous sampling of cortisol every 15-20 min. Estimation of hypothalamic dynamics revealed that: 1) hypocortisolemia amplifies CRH and AVP secretion, when mean (slow) and rate-adjusted (rapid) cortisol feedback concentrations decrease by 0-25%; and 2) reduced peptide feedback augments CRH and AVP secretion, when CRH and AVP secretion each decreases by 0-25 and 50% of its respective maximum. Thus, low-cortisol feedback enhances CRH outflow in part by relieving CRH's autoinhibition. Estimation of pituitary dynamics disclosed that: 1) endogenous CRH and AVP synergize in evoking ACTH secretion, and 2) hypocortisolemia potentiates individual and conjoint stimulation of ACTH secretion by CRH and AVP. Formulations such as the present one should have application to evaluating other complex endocrine dynamics.

Motivations and methods for analyzing pulsatile hormone secretion

Endocrine glands communicate with remote target cells via a mixture of continuous and intermittent signal exchange. Continuous signaling allows slowly varying control, whereas intermittency permits large rapid adjustments. The control systems that mediate such homeostatic corrections operate in a species-, gender-, age-, and context-selective fashion. Significant progress has been made in understanding mechanisms of adaptive interglandular signaling in vivo. Principal goals are to understand the physiological origins, significance, and mechanisms of pulsatile hormone secretion. Key analytical issues are: 1) to quantify the number, size, shape, and uniformity of pulses, nonpulsatile (basal) secretion, and elimination kinetics; 2) to evaluate regulation of the axis as a whole; and 3) to reconstruct dose-response interactions without disrupting hormone connections. This review will focus on the motivations driving and the methodologies used for such analyses.

Pharmacokinetics and pharmacokinetic/pharmacodynamic relationships for angiotensin-converting enzyme inhibitors

The pharmacokinetic (PK) properties and the pharmacokinetic/pharmacodynamic (PK/PD) relationships for the angiotensin-converting enzyme (ACE) inhibitors (ACEIs), such as enalaprilat, benazeprilat, imidaprilat and ramiprilat, differ from those of conventional drugs. This is because of their immediate and saturable binding to an ACE pool which is partly circulating (and contributing to the measured plasma concentration), and partly noncirculating (tissular), being anchored to the endothelium of vessels and not measurable by the analytical technique. A physiologically based model is required to allow appropriate interpretation of the different phases of the disposition curve of ACEI. The protracted terminal phase observed for all ACEIs is not a conventional elimination phase but a phase dependent on ACEI dissociation from ACE. In contrast, the phase which reflects ACEI elimination (and which is interpreted as a distribution phase for a conventional drug) has a short half-life, thus explaining the absence of drug accumulation during repeated dosing and mild kidney failure. ACE inhibition is the surrogate endpoint generally selected for establishing a PK/PD relationship and for simulating dosage regimen scenarios in order to decide on the appropriate dosage regimen for ACEIs.

Pharmacokinetic/pharmacodynamic modelling of the disposition and effect of benazepril and benazeprilat in cats

The disposition and effect of benazepril and its active metabolite, benazeprilat, were evaluated in cats using a pharmacokinetic/pharmacodynamic model. Cats received single 1 mg/kg doses of intravenous 14C-benazeprilat and oral 14C-benazepril.HCl, and single and repeat (eight daily) oral administrations of 0.25, 0.5 and 1.0 mg/kg nonlabelled benazepril.HCl. The pharmacokinetic endpoints were plasma concentrations of benazepril and benazeprilat, and recovery of radioactivity in faeces and urine. The pharmacodynamic endpoint was plasma angiotensin-converting enzyme (ACE) activity. Benazeprilat data were fitted to an equation corresponding to a single-compartment model with a volume equal to the blood space (Vc = 0.093 L/kg). Within this space, benazeprilat was bound nonlinearly to ACE, which was mainly tissular (89.4%) rather than circulating (10.6%). Free benazeprilat was eliminated quickly from the central compartment (t1/2 approximately 1.0 h; Cl approximately 0.125 L/kg/h), elimination being principally biliary ( approximately 85%) rather than urinary ( approximately 15%). Nevertheless, inhibition of ACE was long-lasting (t1/2 16-23 h) due to high affinity binding of benazeprilat to ACE (Kd approximately 3.5 mmol/L, IC50 approximately 4.3 mmol/L). Simulations using the model predict a lack of proportionality between dose of benazepril, plasma benazeprilat concentrations and effect due to the nonlinear binding of benazeprilat to ACE. For example, increasing the dose of benazepril (e.g. above 0.125 mg/kg q24 h) produced only small incremental inhibition of ACE (either peak effect or duration of action).