Urea pharmacokinetics

Engineering Aspects of Hemodialysis. Engineering interest in hemodialysis is concentrated on the optimization of the hemodialysis membrane (4,41), the dependency of solute removal on membrane and device characteristics (14,15), and quantitation of hemodialysis therapy through urea pharmacokinetics (42—44). 

Urea Pharmacokinetics. Pharmacokinetics summarizes the relationships between solute generation, solute removal, and concentration in a patient s blood stream. In the context of hemodialysis, this analysis is most readily appHed to urea, which has, as a consequence, become a surrogate for other uremic toxins in the quantitation of therapy and in attempts to describe its adequacy. In the simplest case, a patient is assumed to have no residual renal function. Urea is generated from the breakdown of dietary protein, accumulates in a single pool equivalent to the patient s fluid volume, and is removed uniformly from that pool during hemodialysis. A mass balance around the patient yields the following differential equation ... 

Cyclic ureas incorporating a 1,3-diazepinone skeleton continue to be of pharmacological interest. The Ai V-disubstituted system 89 has been prepared and shown to be a potent inhibitor of factor Xa in vitro and to have an improved pharmacokinetic profile in the rabbit. The binding model for this series of compounds was confirmed by an X-ray crystallographic analysis of one analogue in the series <00BMCL301>. 

For a drug that is to be developed for a disease that occurs mainly in the elderly, it is often advisable to evaluate tolerability and pharmacokinetics in healthy elderly volunteers before clinical trials in the patient population. Dosage may need to be reduced and particular care taken when the kidney is the major organ of elimination, which should be established in the healthy young before administration to the elderly. It should be remembered that the GFR in the healthy elderly with normal plasma creatinine and urea is generally much lower than that in the young. One reason why healthy elderl/ studies have... 

The first and third routes are important in relation to pharmacokinetic mechanisms. The aqueous pores are too small in diameter for diffusion of most drugs and toxicant, although important for movement of water and small polar molecules (e.g., urea). Pinocytosis is important for some macromolecules (e.g., insulin crossing the blood-brain barrier). 

Pharmacokinetics Methenamine is orally administered. In addition to formaldehyde, ammonium ion is produced in the bladder. Because the liver rapidly metabolizes ammonia to form urea, methenamine is contraindicated in patients with hepatic insufficiency, in which elevated levels of circulating ammonium ions would be toxic to the CNS. Methenamine is distributed throughout the body fluids, but no decomposition of the drug occurs at pH 7.4 thus, systemic toxicity does not occur. The drug is eliminated in the urine. 

Because pharmacokinetics comprises the first few chapters of this book and figures prominently in subsequent chapters, we will pause here to introduce the clinically most important concept in pharmacokinetics the concept of clearance. In 1929, Moller et al. (22) observed that, above a urine flow rate of 2 mL/min, the rate of urea excretion by the kidneys is proportional to the amount of urea in a constant volume of blood. They introduced the term clearance to describe this constant and defined urea clearance as the volume of blood that one minute s excretion serves to clear of urea. Since then, creatinine clearance has... 

The hemodynamic basis for these changes in CLg was investigated subsequently in a dog model (13). Urea and inulin were used as probes and were injected simultaneously 2 hours before dialysis. The pharmacokinetic model shown in Figure 6.2 was used for... 

Allometric scaling may be used to answer a variety of questions relevant to the design of preclinical studies of intraperitoneal pharmacokinetics. Figure 30.3 shows the variation of PA, which is approximately equivalent to peritoneal clearance if Cpc Cpi, for urea and inulin in rats, rabbits, dogs, and humans (14). The slope of the urea line is 0.74 while the slope of the inulin line is 0.62. The average of these is 0.68, or approximately two-thirds as might be expected for peritoneal surface area. This would imply that the intrinsic membrane permeability, P, is similar among... 

Pyridoglutethimide therefore seems a potentially useful aromatase inhibitor in vitro. In vivo, it reduces tumour growth in rats treated with nitrosomethyl-urea [77]. Preliminary studies have been made of its metabolism and pharmacokinetics in rats and rabbits [78]. It has a half-life of 6 h in rats and 16.4 h in rabbits, and in both species the sole metabolite is the A-oxide. Limited data from a human subject who received a single oral dose of 50 mg suggest that... 

Kemia have disclosed a series of bis-aniline urea-based derivatives of the Pfizer tropane series induding 31 which showed good antiviral activity HIV-1 (EC90 6nM) and encouraging pharmacokinetics in the rat P 52%, To.s 7.2 h) [126]. 

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