System approaches in pharmacokinetics: II. Applications

Pharmacokinetic differentiation of drug candidates using system analysis and physiological-based modelling. Comparison of C.E.R.A. and erythropoietin

Evaluation of the pharmacokinetics (PKs) in a proper physiological context is paramount to elucidate the factors that may improve a drug's PK properties. Using modern system analysis-based physiological modelling principles, this work applies a novel kinetic analysis framework to a PK comparison of two erythropoietically active drugs, C.E.R.A. (continuous erythropoietin receptor activator) and recombinant human erythropoietin (Epo), aimed at elucidating the main factors responsible for the substantial PK differences seen. The evaluation according to the new model is compared with a compartmental model analysis. Sheep (n = 7 for Epo; n = 8 for C.E.R.A.) received intravenous bolus injections of Epo and C.E.R.A. Baseline and 20–30 blood samples per injection were assayed by radioimmunoassay. Fundamental physiologically based PK building block principles were introduced, proceeding to the construction of a general PK model and several sub-models from which a final PK model was selected based on information theoretical principles. The compartmental comparison analysis use a two-compartment model with central Michaelis-Menten elimination. Several lines of evidence support the hypothesis that the desirable slow elimination of C.E.R.A. relative to Epo is mainly caused by a smaller recirculation extraction fraction, which appears more influential on the elimination kinetics than the mean circulation transit time. The compartmental analysis demonstrates large differences in several PK parameters that contribute to C.E.R.A.'s slower elimination, consistent with the recirculation model analysis. It is hypothesized that C.E.R.A.'s smaller recirculatory extraction fraction is due to a reduced receptor-mediated elimination, consistent with in-vitro measurements where C.E.R.A. shows Epo-receptor binding with a lower association constant and a larger dissociation constant.

Pharmacokinetic differentiation of drug candidates using system analysis and physiological-based modelling. Comparison of C.E.R.A. and erythropoietin

Evaluation of the pharmacokinetics (PKs) in a proper physiological context is paramount to elucidate the factors that may improve a drug's PK properties. Using modern system analysis-based physiological modelling principles, this work applies a novel kinetic analysis framework to a PK comparison of two erythropoietically active drugs, C.E.R.A. (continuous erythropoietin receptor activator) and recombinant human erythropoietin (Epo), aimed at elucidating the main factors responsible for the substantial PK differences seen. The evaluation according to the new model is compared with a compartmental model analysis. Sheep (n = 7 for Epo; n = 8 for C.E.R.A.) received intravenous bolus injections of Epo and C.E.R.A. Baseline and 20-30 blood samples per injection were assayed by radioimmunoassay. Fundamental physiologically based PK building block principles were introduced, proceeding to the construction of a general PK model and several sub-models from which a final PK model was selected based on information theoretical principles. The compartmental comparison analysis use a two-compartment model with central Michaelis-Menten elimination. Several lines of evidence support the hypothesis that the desirable slow elimination of C.E.R.A. relative to Epo is mainly caused by a smaller recirculation extraction fraction, which appears more influential on the elimination kinetics than the mean circulation transit time. The compartmental analysis demonstrates large differences in several PK parameters that contribute to C.E.R.A.'s slower elimination, consistent with the recirculation model analysis. It is hypothesized that C.E.R.A.'s smaller recirculatory extraction fraction is due to a reduced receptor-mediated elimination, consistent with in-vitro measurements where C.E.R.A. shows Epo-receptor binding with a lower association constant and a larger dissociation constant.

Lessons learned from the past: a guide for the future of clinical pharmacology in the 21st century

This article is a documentary of the history of the American College of Clinical Pharmacology (ACCP) and the Journal of Clinical Pharmacology. The history of the college and of the journal is presented as summarized by a few of the many leaders who played key roles in the growth of the profession of Clinical Pharmacology. Together, the college, the journal, and all clinical pharmacologists working in academia, industry, CROs, or government in many different subspecialty areas of the discipline contribute to the advancement of clinical pharmacology, the development of new drugs, and to an improved quality of life for mankind. Achieving leadership in health care in an era of change requires actions to be dynamic and flexible. Leaders must be capable of self-development and self-education. Leaders must examine challenges from top to bottom and build on the leadership foundations of vision, courage, and knowledge. Strong leaders are needed for the future of clinical pharmacology to address the rapidly changing environment for health care givers and the challenges faced by those working in drug development or training new leaders. One lesson from the past, from the professional life of Harry Gold, is that it is very important to convey the excitement of the field of clinical pharmacology and to pass on this excitement and knowledge base to those currently leading the educational process necessary to keep clinical pharmacologists in the forefront of the medical arena of today and tomorrow. The college became an instrument for change in the field of clinical pharmacology. Forward thinking efforts of the college did not allow stagnation. All founding leaders of the college were possessed of a dream of "what could be and what should be." Two points should be emphasized: the importance of teaching teachers how to teach and the importance of forming national networks, such as the college, to concentrate on the role of teaching students. Today's students are the leaders of tomorrow. The training of any given student in the field of clinical pharmacology continues over many years and requires the effort of many qualified mentors. The ACCP continues to prepare for the changing demands of a new millennium. The college responded to three challenges: (1) utilization of new technologies consistent with contemporary trends; (2) identification of new member services; and (3) dissemination of a position paper defining the philosophical foundations of the college. The basic tenant of the college remains the same today as it has been over the years: provide a multidisciplinary educational environment to have a broad appeal to the membership interested in clinical pharmacology.

Pharmacokinetics and safety of lamotrigine (Lamictal) in patients with epilepsy

In a double-blind parallel study, patients with epilepsy on stable regimen of antiepileptic drugs (AEDs) were given lamotrigine (8 pts) or placebo (3 pts). Patients were sequentially dosed with 100, 200 and 300 mg/day given as a b.i.d. regimen. After steady state was achieved, timed plasma lamotrigine levels were obtained post dose. No medical, psychogenic, neurologic, or hematologic changes were observed and no subjective effects were detected as a result of treatment with lamotrigine. No changes in heart rhythm or blood pressure were observed related to lamotrigine. Pharmacokinetic parameters were calculated using 1-compartment and non-compartment models. The results were similar using both models. Area under the plasma concentration vs. time curves increased linearly with dose. Mean half life (13.5 h), volume of distribution (1.36 l/kg) and clearance (1.27 ml/min/kg) were similar to previously reported results and did not change with increasing dose. These findings indicate that lamotrigine pharmacokinetics can be described by the 1-compartment model, has linear kinetics, and does not induce its own metabolism in patients on concomitant AEDs.

Stepwise determination of multicompartment disposition and absorption parameters from extravascular concentration-time data. Application to mesoridazine, flurbiprofen, flunarizine, labetalol, and diazepam

When disposition is monoexponential, extravascular concentration-time (C, t) data yield both disposition and absorption parameters, the latter via the Wagner-Nelson method or deconvolution which are equivalent. Classically, when disposition is multiexponential, disposition parameters are obtained from intravenous administration and absorption data are obtained from extravascular C, t data via the Loo-Riegelman or Exact Loo-Riegelman methods or via deconvolution. Thus, in multiexponential disposition one assumes no intrasubject variation in disposition, a hypothesis that has not been proven for most drugs. Based on the classical two- and three-compartment open models with central compartment elimination, and using postabsorptive extravascular C, t data only, we have developed four equations to estimate k10 when disposition is biexponential and two other equations to estimate k10 when disposition is triexponential. The other disposition rate constants are readily obtained without intravenous data. We have analyzed extravascular data of flurbiprofen (12 sets), mesoridazine (20 sets), flunarizine (5 sets), labetalol (9 sets), and diazepam (4 sets). In the case of diazepam intravenous C, t data were also available for analysis. After disposition parameters had been estimated from the extravascular data the Exact Loo-Riegelman method with the Proost modification was applied to the absorptive extravascular data to obtain AT/VP as a function of time. These latter data for each subject and each drug studied were found to be fitted by a function indicating either simple first-order absorption, two consecutive first-order processes, or zero-order absorption. After absorption and disposition parameters had been estimated, for each set of extravascular data analyzed, a reconstruction trend line through the original C, t data was made. The new methods allow testing of the hypothesis of constancy of disposition with any given drug. There is also a need for new methods of analysis since the majority of drugs have no marketed intravenous formulation, hence the classical methods cannot be applied.

Mean time parameters in pharmacokinetics. Definition, computation and clinical implications (Part II)

Part I of this article, which appeared in the previous issue of the Journal, covered the following topics: fundamental definitions, general mean time parameter relationships and mean time parameters of classical compartmental systems. It also offered a number of examples to clarify and illustrate the various concepts.

System approaches in pharmacokinetics: I. Basic concepts

System approaches mathematically describe a general property of a pharmacokinetic system without modeling in specific terms the kinetic processes responsible for the general property considered. Definitions, basic concepts, kinetic basis, and rationale for system approaches as well as advantages and disadvantages of system approaches are presented and discussed. It appears that the system approaches may offer certain advantages over more traditional methods for many practical applications.