Digoxin uptake, receptor heterogeneity, and inotropic response in the isolated rat heart: a comprehensive kinetic model

Hypoxic Stress-Dependent Regulation of Na,K-ATPase in Ischemic Heart Disease

In cardiomyocytes, regular activity of the Na,K-ATPase (NKA) and its Na/K pump activity is essential for maintaining ion gradients, excitability, propagation of action potentials, electro-mechanical coupling, trans-membrane Na⁺ and Ca²⁺ gradients and, thus, contractility. The activity of NKA is impaired in ischemic heart disease and heart failure, which has been attributed to decreased expression of the NKA subunits. Decreased NKA activity leads to intracellular Na⁺ and Ca²⁺ overload, diastolic dysfunction and arrhythmias. One signal likely related to these events is hypoxia, where hypoxia-inducible factors (HIF) play a critical role in the adaptation of cells to low oxygen tension. HIF activity increases in ischemic heart, hypertension, heart failure and cardiac fibrosis; thus, it might contribute to the impaired function of NKA. This review will mainly focus on the regulation of NKA in ischemic heart disease in the context of stressed myocardium and the hypoxia-HIF axis and argue on possible consequences of treatment.

The Coadministration of Levosimendan and Exenatide Offers a Significant Cardioprotective Effect to Isolated Rat Hearts against Ischemia/Reperfusion Injury

(1) Background: The present study aims to investigate the effect of administration of Levosimendan and Exenatide in various concentrations, as well as of the coadministration of those agents in an ischemia–reperfusion injury isolated heart model. (2) Methods: After 30 min of perfusion, the hearts underwent a 30 min period of regional ischemia followed by a 120 min period of reperfusion. All animals were randomly divided into 12 experimental groups of nine animals in each group: (1) Control, (2) Sham, (3) Digox (Negative control, Digoxin 1.67 μg/min), (4) Levo 1 (Levosimendan 0.01 μg/min), (5) Levo 2 (Levosimendan 0.03 μg/mL), (6) Levo 3 (Levosimendan 0.1 μg/min), (7) Levo 4 (Levosimendan 0.3 μg/min), (8) Levo 5 (Levosimendan 1 μg/min), (9) Exen 1 (Exenatide 0.001 μg/min), (10) Exen 2 (Exenatide 0.01 μg/min), (11) Exen 3 (Exenatide 0.1 μg/min) and (12) Combi (Levosimendan 0.1 µg/mL + Exenatide 0.001 μg/min). The hemodynamic parameters were recorded throughout the experiment. Arrhythmias and coronary flow were also evaluated. After every experiment the heart was suitably prepared and infarct size was measured. Markers of myocardial injury were also measured. Finally, oxidative stress was evaluated measuring reactive oxygen species. (3) Results: A dose-dependent improvement of the haemodynamic response was observed after the administration of both Levosimendan and Exenatide. The coadministration of both agents presented an even greater effect, improving the haemodynamic parameters further than the two agents separately. Levosimendan offered an increase of the coronary flow and both agents offered a reduction of arrhythmias. A dose-dependent reduction of the size of myocardial infarction and myocardial injury was observed after administration of Levosimendan and Exenatide. The coadministration of both agents offered a further improving the above parameters. Levosimendan also offered a significant reduction of oxidative stress. (4) Conclusions: The administration of Levosimendan and Exenatide offers a significant benefit by improving the haemodynamic response, increasing the coronary flow and reducing the occurrence of arrhythmias, the size of myocardial injury and myocardial oxidative stress in isolated rat hearts.

Endogenous Cardiotonics: Search And Problems

Medicinal preparations currently used for the treatment of patients with chronic cardiac failure involve those that reduce the heart load (vasodilators, diuretics, beta-blockers, and angiotensin-converting enzyme (ACE) inhibitors). Cardiotonic drugs with the cAMP-dependent mechanism are unsuitable for long-term administration due to the intensification of metabolic processes and an increase in the oxygen demand of the myocardium and all tissues of the body. For many years, digoxin has remained the only preparation enhancing the efficiency of myocardial performance. The detection of digoxin and ouabain in intact animals has initiated a search for other compounds with cardiotonic activity. The review summarizes current data on the effect exerted on the heart performance by endogenous compounds, from simple, such as NO and CO, to steroids, fatty acids, polypeptides, and proteins. Controversial questions and problems with the introduction of scientific achievements into clinical practice are discussed. The results obtained by the authors and their colleagues after many years of studies on the cardiotropic properties of serum lipoproteins are also reported. The experimentally established cardiotonic activity of apoprotein A-1, which is accompanied by a decrease in the relative consumption of oxygen, maybe of great interest.

Multiple myeloma and persistence of drug resistance in the age of novel drugs (Review)

Multiple myeloma (MM) is a mature B cell neoplasm that results in multi-organ failure. The median age of onset, diverse clinical manifestations, heterogeneous survival rate, clonal evolution, intrinsic and acquired drug resistance have impact on the therapeutic management of the disease. Specifically, the emergence of multidrug resistance (MDR) during the course of treatment contributes significantly to treatment failure. The introduction of the immunomodulatory agents and proteasome inhibitors has seen an increase in overall patient survival, however, for the majority of patients, relapse remains inevitable with evidence that these agents, like the conventional chemotherapeutics are also subject to the development of MDR. Clinical management of patients with MM is currently compromised by lack of a suitable procedure to monitor the development of clinical drug resistance in individual patients. The current MM prognostic measures fail to pick the clonotypic tumor cells overexpressing drug efflux pumps, and invasive biopsy is insufficient in detecting sporadic tumors in the skeletal system. This review summarizes the challenges associated with treating the complex disease spectrum of myeloma, with an emphasis on the role of deleterious multidrug resistant clones orchestrating relapse.

Interactions between verapamil and digoxin in Langendorff-perfused rat hearts: the role of inhibition of P-glycoprotein in the heart

P-glycoprotein (P-gp) is expressed in tumour cells as well as normal tissues including heart. Modulation of P-gp transport in vivo may lead to increased drug penetrance to tissues with resulting increases in toxicity. We aimed to investigate the effects of P-gp on the isolated heart by digoxin infusion in the absence and presence of verapamil. The study was performed in Langendorff isolated perfused rat hearts. After a 20 min. stabilisation period with Tyrode Buffer, digoxin (125 μg/5 mL) was infused for 10 min. in the control group (n = 7). The same dose of digoxin was infused during perfusion with verapamil (1 nm) containing Tyrode Buffer (n = 8) in the study group. Outflow concentration and cardiac parameters of digoxin were measured at frequent intervals for 40 min. AUEC((0-40 min)) for left ventricular developed pressure was significantly increased in the presence of verapamil (4260 ± 39.37 mmHg min versus 4607 ± 98.09 mmHg min; 95% CI -587.7 to -105.8; p = 0.0083). The significant increases in left ventricular developed pressure were at 20, 25, 30, 35 and 40 min. AUC((0-40 min)) value for outflow digoxin concentration-time curve was significantly lower in the presence of verapamil. Verapamil increased the positive inotropic effect of digoxin, probably through the inhibition of P-gp, which effluxes digoxin out of cardiac cells.

An improved nonlinear model describing the hepatic pharmacokinetics of digoxin: evidence for two functionally different uptake systems and saturable binding

PURPOSE

To develop a semi-distributed liver model for the evaluation of saturable sinusoidal uptake and binding kinetics of the Oatp1a4 substrate digoxin.

METHODS

In the perfused rat liver, two successive digoxin doses of 42 and 125 microg were administered, and the outflow concentration was determined by LC/MS/MS. [14C]-sucrose was used as vascular reference. The data were analyzed simultaneously by a population approach using sucrose to determine the sinusoidal mixing of digoxin.

RESULTS

The results suggest the existence of a high-affinity, low-capacity system, and a low-affinity, high-capacity system for sinusoidal uptake with apparent Michaelis constants (K(M)) of 0.24 and 332 microg/ml, respectively. Incorporation of saturable sinusoidal binding of digoxin considerably improved the fit, and the parameter estimates were consistent with those of binding to hepatic Na,K-ATPase. Simpler models that neglect the concentration gradient in flow direction failed to describe the outflow data in the high dose range.

CONCLUSION

The semi-distributed liver model with saturable uptake should be useful for a functional characterization of transporters in the in situ rat liver.

Na⁺,K⁺-ATPase as the Target Enzyme for Organic and Inorganic Compounds

This paper gives an overview of the literature data concerning specific and non specific inhibitors of Na⁺,K⁺-ATPase receptor. The immobilization approaches developed to improve the rather low time and temperature stability of Na⁺,K⁺-ATPase, as well to preserve the enzyme properties were overviewed. The functional immobilization of Na⁺,K⁺-ATPase receptor as the target, with preservation of the full functional protein activity and access of various substances to an optimum number of binding sites under controlled conditions in the combination with high sensitive technology for the detection of enzyme activity is the basis for application of this enzyme in medical, pharmaceutical and environmental research.

Cardiac Pharmacokinetics and Inotropic Response of Verapamil in Rats With Endotoxemia

The present study evaluated the effect of endotoxin-induced systemic inflammation on cardiac uptake and negative inotropic response to verapamil in isolated rat hearts. Rats received an i.p. dose of 4 mg/kg Escherichia coli lipopolysaccharide (LPS) or saline. After 5.5 h the outflow concentration-time curve and inotropic response data were measured following a 1.5 nmol dose of [(3)H]-verapamil (infused within 1 min) in Langendorff-perfused hearts and analyzed by pharmacokinetic/pharmacodynamic modeling, where the inotropic effects at individual time points were evaluated in relation to outflow concentrations at corresponding times. Endotoxemia decreased the rate of cardiac verapamil uptake and the maximal negative inotropic effect E(max) to 78% and 55%, respectively, of the values estimated in the control group (p < 0.01). The reduction in E(max) was correlated with the increase in body temperature. With verapamil as a model drug, the results give some information about potential effects of endotoxemia on the cardiac kinetics and dynamics of calcium antagonists.

Mechanistic modeling of digoxin distribution kinetics incorporating slow tissue binding

This study aims to develop a mechanistic pharmacokinetic model that accounts for the kinetics of tissue binding in order to evaluate the effect of slow binding of digoxin to skeletal muscular Na(+)/K(+)-ATPase in humans. The approach is based on a minimal circulatory model with a systemic transit time density function that accounts for vascular mixing, transcapillary permeation and extravascular binding of the drug. The model parameters were estimated using previously published disposition data of digoxin in healthy volunteers and physiological distribution volumes taken from the literature. A time constant of the binding process of 34min was estimated indicating that receptor binding and not permeation clearance is the rate-limiting step of the distribution process. Model simulations suggest that up- or downregulation of sodium pumps, typically observed under physiological or pathophysiological conditions, could be detected with this method. The model allows a quantitative prediction of the effect of changes in skeletal muscular sodium pump activity on plasma levels of digoxin.

Target-mediated drug disposition and dynamics

Nonlinear pharmacokinetics and pharmacodynamics may result from several capacity-limited processes and often represent complicating factors in characterizing the pharmacological properties of drugs. Target-mediated drug disposition (TMDD) corresponds to a special case wherein a significant proportion of a drug (relative to dose) is bound with high affinity to a pharmacological target, such that this interaction is reflected in the pharmacokinetic properties of the drug. Dose-dependent effects on apparent pharmacokinetic parameters may manifest, including the steady-state volume of distribution and total systemic clearance. Although a few small molecular weight compounds have been identified to exhibit TMDD, the incidence of TMDD is likely to increase particularly among emerging biotechnology pharmaceuticals. The goal of this commentary is to describe the basic tenets of TMDD and discuss several mathematical modeling approaches for characterizing this phenomenon. Whereas traditional pharmacokinetic/pharmacodynamic models assume that the amount of the drug-target complex is negligible relative to the total amount of drug in the body, integrated mechanism-based models of TMDD incorporate the binding and stoichiometry of drug-target binding. These models may be utilized to infer the time-course of inaccessible system variables, such as the in vivo density of the drug-target complex, and provide a suitable platform for ascertaining the apparent pharmacodynamic implications of TMDD.

Hazardous to your health: kinetic foundations of risk stratification and therapeutic triage

BACKGROUND

Risk stratification is widely used in the prognostic assessment of patients with a variety of clinical disorders on the unquestioned assumption that the intensity of treatment should be proportionate to the threat of an adverse event over some finite period of time (risk). However, just as the physical trajectory of an object depends on its current magnitude of displacement (velocity) and the concurrent rate of change of that displacement (acceleration), the prognostic trajectory of a patient depends on the current magnitude of risk and the concurrent rate of change of that risk (hazard). Clinical risk stratification nevertheless relies only on the former.

METHODS

We therefore integrated the quantitative assessment of risk and hazard by way of a kinetic model that characterizes the development of an adverse event as a series of exponential state-to-state transitions-from stable to unstable to event. This model serves to shift the clinical emphasis from prognosis (the assessment of risk) to treatment (the improvement in outcome). In this context, treatment is well advised (even in low-risk individuals) when the hazard is large (risk is rising), and is less well advised (even in high-risk individuals) when the hazard is small (risk is stable).

RESULTS

The kinetic model outlined here thereby promises to supersede the superficial practice of risk stratification with a more sophisticated strategy of therapeutic triage that allows one to predict the incremental clinical benefit of alternative treatment strategies.

Mechanism-based modeling of reduced inotropic responsiveness to digoxin in endotoxemic rat hearts

The mechanisms by which endotoxemia affects myocardial contractility and responsiveness to inotropic drugs are not well understood. We examined the positive inotropic effect of digoxin in single-pass Langendorff-perfused hearts from rats after in vivo pretreatment with lipopolysaccharide (LPS, 4 mg/kg, i.p., 4 h before heart isolation). Using a mathematical modeling approach that allows differentiation between effects elicited at the receptor and postreceptor level, we studied uptake, receptor binding and effectuation kinetics after three consecutive digoxin doses (15, 30, and 45 microg) in the absence and presence of the reverse mode Na(+)/Ca(2+) exchange (NCX) inhibitor KB-R7943 (0.1 microM) in perfusate. LPS significantly depressed baseline contractility and the inotropic response to digoxin without affecting its uptake mechanism. Compared with the control group, the slope of the functional receptor occupancy (stimulus)-to-response relationship was reduced by 44% in the LPS group. Model analysis revealed a significant correlation between changes in digoxin action and LPS-induced febrile response: digoxin receptor affinity increased and the response/stimulus ratio decreased with rise in body temperature, respectively. In contrast, the diminished responsiveness to digoxin observed after NCX inhibition in the control group was not further attenuated in the LPS group. These results support the hypothesis that postreceptor events may be responsible for the diminished contractile response to digoxin during endotoxemia.

Down-Regulation of Na+Pump α2Isoform in Isoprenaline-Induced Cardiac Hypertrophy in Rat: Evidence for Increased Receptor Binding Affinity but Reduced Inotropic Potency of Digoxin

Cardiac hypertrophy in rats induces a down-regulation of Na(+),K(+)-ATPase alpha(2) isoform, although its functional consequences are poorly understood. Using a mathematical modeling approach that allows differentiation between effects elicited at the receptor and postreceptor level, we studied uptake, receptor binding kinetics, and positive inotropism of digoxin in single-pass Langendorff-perfused hearts of vehicle- and isoprenaline-pretreated rats (2.4 mg/kg per day over 4 days). Digoxin outflow concentration and left ventricular developed pressure data were measured for three consecutive doses (15, 30, and 45 microg) in the absence and presence of the reverse mode Na(+)/Ca(2+) exchange inhibitor 2-[2-[4-(4-nitrobenzyloxyl-)phenyl]ethyl isothiourea methansulfonate] (KB-R7943) (0.1 microM) in perfusate. In hypertrophied hearts, 1) the amount of alpha(2) receptors was reduced to 52% of control levels; 2) the digoxin binding affinity was increased 12-fold due to a decrease in dissociation rate constants of alpha(1) and alpha(2) receptors, and 3) inotropic responsiveness to digoxin the was attenuated on the stimulus-response level, where the coupling ratio of stimulus to response was reduced to 38% of control values. Only in the lowest dose level (15 microg) was this decrease in inotropic potency counterbalanced by the increase in receptor affinity. The Na(+),K(+)-ATPase isoform shift was not responsible for the diminished inotropic effect of digoxin. Coadministration of KB-R7943 significantly reduced cellular response generation at higher digoxin doses to the same limiting stimulus-response relationship in both the vehicle and isoprenaline group.

Transport of endogenous nucleosides in guinea pig heart

The purpose of this study was to investigate the characteristics of transport of endogenous nucleosides into cardiac tissue from coronary circulation. The study was performed on the isolated perfused guinea pig heart, using the rapid paired tracers single-pass technique. The maximal cellular uptake (Umax) and total cellular uptake (Utot) of adenosine, deoxyadenosine, thymidine, uridine, and cytidine were determined. The cellular uptake of adenosine was significantly higher than the cellular uptake of other studied nucleosides. To elucidate the mechanisms of nucleoside transport, competition studies were performed and the influence of S-(p-nitrobenzyl)-6-thioinosine (NBTI) and sodium ion absence on Umax and Utot was investigated. Self- and cross-inhibition studies indicated the saturable mechanism of nucleosides transport into cardiac tissue and the involvement of different transport mechanisms for purine and pyrimidine nucleosides. The study also showed that both equilibrative-sensitive (es) and sodium-dependent transport were responsible for adenosine and thymidine cellular uptake.Key words: nucleosides, transport, heart.

The alpha 1 isoform of Na,K-ATPase regulates cardiac contractility and functionally interacts and co-localizes with the Na/Ca exchanger in heart

The primary objective of this study was to examine the functional role of the Na,K-ATPase alpha 1 isoform in the regulation of cardiac contractility. Previous studies using knock-out mice showed that the hearts of animals lacking one copy of the alpha 1 or alpha 2 isoform gene exhibit opposite phenotypes. Hearts from alpha 2(+/-) animals are hypercontractile, whereas those of the alpha 1(+/-) animals are hypocontractile. The cardiac phenotype of the alpha 1(+/-) animals was unexpected as other studies suggest that inhibition of either isoform increases contraction. To help resolve this difference, we have used genetically engineered knock-in mice expressing a ouabain-sensitive alpha 1 isoform and a ouabain-resistant alpha 2 isoform of the Na,K-ATPase, and we analyzed cardiac contractility following selective inhibition of the alpha1 isoform by ouabain. Administration of ouabain to these animals and to isolated heart preparations selectively inhibits only the activity of the alpha 1 isoform without affecting the activity of the alpha 2 isoform. Low concentrations of ouabain resulted in positive cardiac inotropy in both isolated hearts and intact animals expressing the modified alpha 1 and alpha 2 isoforms. Pretreatment with 10 microm KB-R7943, which inhibits the reverse mode of the Na/Ca exchanger, abolished the cardiotonic effects of ouabain in isolated wild type and knock-in hearts. Immunoprecipitation analysis demonstrated co-localization of the alpha1 isoform and the Na/Ca exchanger in cardiac sarcolemma. The alpha 1 isoform co-immunoprecipitated with the Na/Ca exchanger and vice versa. These results demonstrate that the alpha 1 isoform regulates cardiac contractility, and that both the alpha 1 and alpha 2 isoforms are functionally and physically coupled with the Na/Ca exchanger in heart.

Systems analysis of digoxin kinetics and inotropic response in the rat heart: effects of calcium and KB-R7943

To gain more insight into the mechanistic processes controlling the kinetics of inotropic response of digoxin in the perfused whole heart, an integrated kinetic model was developed incorporating digoxin uptake, receptor binding (Na(+)-K(+)-ATPase inhibition), and cellular events linking receptor occupation and response. The model was applied to data obtained in the single-pass Langendorff-perfused rat heart for external [Ca(2+)] of 0.5 and 1.5 mM under control conditions and in the presence of the reverse-mode Na(+)/Ca(2+) exchange inhibitor KB-R7943 (0.1 microM) in perfusate. Outflow concentration and left ventricular developed pressure data measured for three consecutive doses (15, 30, and 45 microg) in each heart were analyzed simultaneously. While disposition kinetics of digoxin was determined by interaction with a heterogeneous receptor population consisting of a high-affinity/low-capacity and a low-affinity/high- capacity binding site, response generation was >80% mediated by binding to the high-affinity receptor. Digoxin sensitivity increased at lower external [Ca(2+)] due to higher stimulus amplification. Coadministration of KB-R7943 significantly reduced the positive inotropic effect of digoxin at higher doses (30 and 45 microg) and led to a saturated and delayed receptor occupancy-response relationship in the cellular effectuation model. The results provide further evidence for the functional heterogeneity of the Na(+)-K(+)-ATPase and suggest that in the presence of KB-R7943 a reduction of the Ca(2+) influx rate via the reverse mode Na(+)/Ca(2+) exchanger might become the limiting factor in digoxin response generation.

The alpha2 isoform of Na,K-ATPase mediates ouabain-induced cardiac inotropy in mice

Inhibition of Na,K-ATPase activity by cardiac glycosides is believed to be the major mechanism by which this class of drugs increases heart contractility. However, direct evidence demonstrating this is lacking. Furthermore it is unknown which specific alpha isoform of Na,K-ATPase is responsible for the effect of cardiac glycosides. Several studies also suggest that cardiac glycosides, such as ouabain, function by mechanisms other than inhibition of the Na,K-ATPase. To determine whether Na,K-ATPase, specifically the alpha2 Na,K-ATPase isozyme, mediates ouabain-induced cardiac inotropy, we developed animals expressing a ouabain-insensitive alpha2 isoform of the Na,K-ATPase using Cre-Lox technology and analyzed cardiac contractility after administration of ouabain. The homozygous knock-in animals were born in normal Mendelian ratio and developed normally to adulthood. Analysis of their cardiovascular function demonstrated normal heart function. Cardiac contractility analysis in isolated hearts and in intact animals demonstrated that ouabain-induced cardiac inotropy occurred in hearts from wild type but not from the targeted animals. These results clearly demonstrate that the Na,K-ATPase and specifically the alpha2 Na,K-ATPase isozyme mediates ouabain-induced cardiac contractility in mice.

Simultaneous Modeling of Abciximab Plasma Concentrations and ex Vivo Pharmacodynamics in Patients Undergoing Coronary Angioplasty

An integrated structural pharmacokinetic/pharmacodynamic (PK/PD) model was developed for the glycoprotein IIb/IIIa antagonist abciximab administered to patients undergoing percutaneous transluminal coronary angioplasty. PK/PD data, in the form of plasma abciximab concentrations and ex vivo platelet aggregation in the presence of 20 microM adenosine diphosphate, were obtained from two previously conducted clinical studies. Study 1 consisted of patients who were given abciximab as a single intravenous injection of 0.25 mg/kg (n = 32). Patients in study 2 received an identical bolus dose, followed by a 36-h infusion at 0.125 microg/kg/min (n = 15). The PK component of the final model included drug-receptor binding, nonspecific distribution, and linear systemic clearance, whereas the PD module assumed that ex vivo dynamics were controlled by free plasma drug concentration. Mean PK/PD data from both studies were fitted simultaneously using nonlinear regression. PK profiles from both studies show rapidly decreasing plasma abciximab concentrations at early time points, but with extended terminal disposition phases. The maximum effect (Emax = 83.6%) was achieved rapidly and gradually returned to baseline values, although inhibition could be measured long after abciximab concentrations dropped below the detection limit. The final model well described the resulting PK/PD profiles and allowed for parameter estimation with relatively small coefficients of variation. Simulations were conducted to assess predicted receptor-occupancy and effects of selected parameters on PK/PD profiles. Models such as the one developed in this study demonstrate how drug binding to pharmacological targets may influence the PK of certain drugs and also provide a suitable paradigm for defining the PK/PD relationships of similar compounds.

Receptor-mediated pharmacokinetics and pharmacodynamics of interferon-beta1a in monkeys

A pharmacokinetic/pharmacodynamic (PK/PD) model was developed to simultaneously characterize interferon after i.v. and s.c. dosing at various dose levels. A sequential study in monkeys (n = 18) was conducted, where single doses of 1, 3, and 10 MIU/kg of recombinant-human interferon-beta (IFN-beta) 1a were given i.v. and then s.c. Plasma concentrations of IFN-beta were determined and biphasic neopterin concentrations were used as the pharmacodynamic (PD) endpoint. Multiple dosing also was evaluated by giving 1 MIU/kg s.c. doses once daily for 7 days (n = 3). The integrated model uses target-mediated drug disposition to describe drug elimination by receptor binding and internalization, and well characterizes the observed nonlinear pharmacokinetic (PK) profiles. The s.c. doses exhibited an absorption phase (Tmax = 3 h) and incomplete bioavailability (F = 0.3-0.7). An indirect response model for stimulation of neopterin triphosphate production by activated receptor complex followed by conversion to neopterin was used to jointly model the formation and loss of neopterin with a capacity factor Smax = 23.8. Greater relative neopterin response after s.c. dosing was accounted for by prolonged receptor activation relative to the SC50 value. Repeated daily s.c. dosing produced modestly elevated IFN-beta1a concentrations and neopterin concentrations that were lower than simulated from single-dose modeling. Although several mechanisms could be involved, these phenomena were simply remodeled as down-regulation of Smax and receptors. The PK/PD model for IFN-beta1a depicts receptor binding as a key feature controlling nonlinear elimination, nonstationary kinetics, and neopterin induction in a manner consistent with known processes controlling its disposition and pharmacological effects.