First-order processes passive diffusion

If it is assumed that dmg absorption proceeds by passive diffusion of the dmg, it can he considered to be a first-order process. Thus the rate of absorption is proportional to the concentration, C, of dmg remaining at the injection site ... 

As one may observe from equation (2) that absorption by passive diffusion is nothing but a first-order process, hence the rate of drug absorption is directly proportional to the concentration of drug at the absorption site. In other words, the larger the concentration of drug, the faster is the rate of absorption. At any time after the administration of the drug, the percentage of the dose absorbed remains the same irrespective of the dose administered. 

Because absorption by passive diffusion is a first-order process, the rate of absorption (dCtdt in Eq. 9.2) is directly proportionai to the concentration at the site of absorption (C. ). The greater the... 

Generally, when a first-order process and passive diffusion are applicable, as the concentration of drug in the body (serum, plasma) increases, so does its rate of elimination clearance, however, remains independent of the dose administered. 

In this question, plasma concentration versus time data is provided following the administration of two different doses of a drug (cinoxacin Cinobac). Because of the assumption of the first-order process and passive diffusion, one would expect the plasma concentration of a drug at any time to be directly proportional to the dose administered however, the fundamental pharmacokinetic parameters of a dmg will remain unaffected by the administered dose. We plotted... 

Please note that peak plasma concentration is always directly proportional to the administered dose (assuming the first-order process and passive diffusion are operative) of a drug. Therefore, following the administration of 250 mg and 750 mg doses of the same drug via the same formulation, the same dosage form and the same route of administration, plasma concentrations of 14.76pgmL and 44.28pgmL, respectively, will result. 

The elimination of the drug follows a first-order process and passive diffusion. 

Linear pharmacokinetics applies that is, the rate process obeys passive diffusion and first-order elimination kinetics (please review first-order process). 

Please note the values of A and B are directly proportional to the dose administered since the empirical constants A and B have concentration units and the drug is assumed to follow the first-order process (i.e. concentration-independent kinetics) and passive diffusion. All the rate constants involved in a two-compartment model, therefore, will have units consistent with the first-order process. 

Passive diffusion is a first-order rate process. Is this statement true or false ... 

The statement is true. Passive diffusion is a first-order rate process as it is dependent on the concentration of the chemical. In contrast, active transport is a zero-order process as it is not dependent on the concentration. 

First-order input (11). First-order kinetic input (II) delivers drug at a rate proportional to the concentration gradient driving the transfer of drug movement. A classic example of a first-order kinetic process is the passive diffusion of drug across a homogeneous barrier. The differential equation describing first-order kinetic behavior is shown in Eq. (1.13) ... 

The first step in this direction is to correlate the permeability values obtained in the Caco-2 cell lines (apical-basolateral direction, Pay) with the fraction of dose absorbed in vivo in rat For this proposal eight fluoroquinolones were assayed and the results found with Caco-2 cell lines were compared with those obtained in vivo in rat In the Caco-2 cell lines the permeability of the quinolones was evaluated at different initial concentration, in order to test for no linearities in the absorption process. For some of them, it was observed that a secretion system worked in the opposite direction to passive diffusion for this reason the permeability value used for the correlations, in the case of secretion, was the one obtained at the highest concentration of the quinolone, which corresponds to saturation of the secretion process. 

The permeation of most drugs through cellular membranes is by the process of passive diffusion, a nonsaturable process that follows first-order kinetics. Concentration gradient and lipid solubility of the drug are important determinants of the rate of diffusion. Only a few drug molecules are substrates for active transport processes (eg, tubular secretion of beta-lactam antibiotics) these are saturable at high concentrations. Only very small ions (eg, Li+) or drugs (eg, ethanol) may penetrate biomembranes via aqueous pores. 

After administering a dose, the change in drug concentration in the body with time can be described mathematically by various equations, most of which incorporate exponential terms (i.e. e or e ). This suggests that ADME processes are "first order" in nature at therapeutic doses and, therefore, drug transfer in the body is possibly mediated by "passive diffusion." Therefore, there is a directly proportional relationship between the observed plasma concentration and/or the amount of drug eliminated in the urine and the administered dose... 

The pharmacokinetic parameters-elimination half life, elimination rate constant, the apparent volume of distribution and the systemic, renal and metabolic clearances (Cls, Clr, and Clm, respectively) for a dmg are always independent of the dose administered as long as the drug follows a first-order elimination process and passive diffusion. 

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