Pharmacokinetics first-order

Organization of Single-Dose Pharmacokinetic First-Order Absorption Model 239... 

Fig. 9 PK/PD model for the LH suppression and the delay in LH surge following administration of a cetrorelix dose (D). Cc, Cp, and Cg. Concentration of drug in central, peripheral and effect compartments, respectively fca, ku, 21 > ho> kgo - pharmacokinetic first-order rate constants. See text for discussion of the details (modified from [93, 94]).

On some occasions, the body does not behave as a single homogeneous compartment, and multicompartment pharmacokinetics are required to describe the time course of drug concentrations. In other instances certain pharmacokinetic processes may not obey first-order kinetics and saturable or nonlinear models may be required. Additionally, advanced pharmacokinetic analyses require the use of various computer programs, such as those listed on the website http //www.boomer.org/pkin/soft.html. 

III. FIRST-ORDER PHARMACOKINETICS DRUG ELIMINATION FOLLOWING RAPID INTRAVENOUS INJECTION... 

To understand the pharmacokinetic relevance of the proxibarbal-valofan equilibrium, the kinetics and thermodynamics of the reaction were carefully examined in aqueous and biphasic media. The various pseudo-first-order rate constants shown in Fig. 11.19 were determined in the pH range of 6.7 - 8.0... 

Pharmacokinetics is the study of the movement of drug molecules in the body, requiring appropriate differential calculus equahons to study various rates and processes. The rate of elimination of a drug is described as being dependent on, or proporhonal to, the amount of drug remaining to be eliminated, a process that obeys first-order kinetics. The rate of eliminahon can, therefore, be described as... 

C(t) modeled according to two-compartment model with zero-order and first-order absorption Pharmacokinetic/pharmacodynamic relationship modeled using Hill model with first-order absorption. Modeled parameters matched experimental parameters when bicompartmental model with zero-order input was used. Linear PKs, anticlockwise hysteresis loop established for all doses studied. Apomorphine and growth hormone concentration predicted with good accuracy... 

Laplace transformation is particularly useful in pharmacokinetics where a number of series first-order reactions are used to model the kinetics of drug absorption, distribution, metabolism, and excretion. Likewise, the relaxation kinetics of certain multistep chemical and physical processes are well suited for the use of Laplace transforms. 

Pharmacokinetics In healthy adults treated with IV doses of iron sucrose, its iron component exhibits first order kinetics with an elimination half-life of 6 hours, total clearance of 1.2 L/h, non-steady-state apparent volume of distribution of 10 L, and steady-state apparent volume of distribution of 7.9 L. 

Pharmacokinetics Epoetin alfa IV is eliminated via first-order kinetics with a circulating half-life of 4 to 13 hours in patients with CRF. Within the therapeutic dosage range, detectable levels of plasma erythropoietin are maintained for at least 24 hours. After subcutaneous administration of epoetin alfa to patients with CRF,... 

The pharmacokinetic term clearance (CT) best describes the efficiency of the elimination process. Clearance by an elimination organ (e.g., liver, kidney) is defined as the volume of blood, serum, or plasma that is totally cleared of drug per unit time. This term is additive the total body or systemic clearance of a drug is equal to the sum of the clearances by individual eliminating organs. Usually this is represented as the sum of renal and hepatic clearances CT = CT renal -I- CL hepatic. Clearance is constant and independent of serum concentration for drugs that are eliminated by first-order processes, and therefore may be considered proportionally constant between the rate of drug elimination and serum concentration. 

For drugs that follow first-order kinetics, in addition to clearance, the half-life is a useful pharmacokinetic parameter to describe elimination. The elimination half-life (tj/j) is the time required for the concentration of drug to decrease by 50%. In clinical practice, this parameter is referred to as the plasma (or serum) half-life and is usually assessed by measuring the fall of... 

Valproic acid is eliminated by first-order kinetics and has an elimination half-life of 5-20 hours (average, 10.6 hours). Pediatric patients (3 months to 10 years) have a 50% higher clearance of the drug expressed by weight (i.e., mL/min/kg) over the age of 10 years, pharmacokinetic parameters of valproic acid approximate those in adults (Cloyd et al., 1993). Valproic acid is metabolized principally in the liver by (3 (over 40%) and CO oxidation (up to 15%-20%). Thirty through 50% of an administered dose is excreted as glucuron-ide conjugates (Cloyd et al., 1993). 

There is a linear relationship between dose and plasma drug levels (i.e., linear or first-order pharmacokinetics) in normal and ultrarapid metabolizers. In these individuals, the earlier equation can be used to predict the daily dose needed to produce a specific plasma drug level once TDM has been done to estimate the patient s elimination rate. In poor metabolizers, TCAs follow nonlinear pharmacokinetics (i.e., disproportionate increases in plasma drug levels with dose increases) because they lack the CYP 2D6 and must use lower affinity enzymes to metabolize these drugs. 

Chemical clastogenesis and mutagenesis both involve a complex series of processes, including pharmacokinetic mechanisms (uptake, transport, diffusion, excretion), metabolic activation and inactivation, production of DNA lesions and their incomplete repair or misrepair, and steps leading to the subsequent expression of mutations in surviving cells or individuals (Thble 7.1). Each of the steps in these processes might conceivably involve first order kinetics at low doses (e.g., diffusion, MichaeUs-Menten enzyme kinetics) and hence be linear. In principle, therefore, the overall process edso might be linear and without threshold. 

Most drugs have linear pharmacokinetics, i.e. the rate of decrease at any time is directly proportional to the amount present at that time this is known as a first order process. Such a process is termed exponential decay. First order exponential processes for a simple one-compartment system can be expressed mathematically as ... 

The concept of clearance is useful in pharmacokinetics because clearance is usually constant over a wide range of concentrations, provided that ehmination processes are not saturated. Saturation of biotransformation and excretory processes may occur in overdose and toxic okinetic effects should be considered. If a constant fraction of drug is eliminated per unit time, the elimination follows first-order kinetics. However, if a constant amount of drug is eliminated per unit time, the elimination is described by zero-order kinetics. Some drugs, for example, ethanol, exhibit zero-order kinetics at normal or non-intoxicating concentrations. However, for any drug that exhibits first-order kinetics at therapeutic or nontoxic concentrations, once the mechanisms for elimination become saturated, the kinetics become zero order and clearance becomes variable.3... 

A two-compartment open linear model has been described for the pharmacokinetic profile of cocaine after intravenous administration.14 The distribution phase after cocaine administration is rapid and the elimination half-life estimated as 31 to 82 min.14 Cone9 fitted data to a two-compartment model with bolus input and first-order elimination for the intravenous and smoked routes. For the intranasal route, data were fitted to a two-compartment model with first-order absorption and first-order elimination. The average elimination half-life (tx 2 3) was 244 min after intravenous administration, 272 min after smoked administration, and 299 min after intranasal administration. 

Linear pharmacokinetics. For a simple linear pharmacokinetics case, the body can be modeled as a single drug compartment with first-order kinetic elimination—where the dose is administered and drug concentrations are drawn from the same compartment. For an intravenous bolus dose, the expected drug plasma concentration Cp versus time curves are shown in Fig. 1.10. The kinetics for this system are described by Eq. (1.6). The well-known solution to this equation is given by Eq. (1.7), and a linearized version of this solution is given in Eq. (1.8) and shown graphically in Fig. 1.13. 

Factors analogous to those affecting gut absorption also can affect drug distribution and excretion. Any transporters or metabolizing enzymes can be taxed to capacity—which clearly would make the kinetic process nonlinear (see Linear versus Nonlinear Pharmacokinetics ). In order to have linear pharmacokinetics, all components (distribution, metabolism, filtration, active secretion, and active reabsorption) must be reasonably approximated by first-order kinetics for the valid design of controlled release delivery systems. 

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