Phenobarbital absorption

Dodson WE, Rust RS. Phenobarbital Absorption, distribution, and excretions In Levy R, Mattson R, Meldrum B, eds. Antiepileptic Drugs, 4th ed. New York, Raven Press, 1995 379-387. 

Dispositional antagonism occurs when one drug alters the pharmacokinetics (absorption, distribution, biotransformation, or excretion) of a second drug so that less of the active compound reaches the target tissue. Tor example, phenobarbital induces the biotransformation of warfarin, reducing its anticoagulant activity... 

Although transfer of drugs across the intestinal wall can occur by facilitated transport, active transport, en-docytosis, and filtration, the predominant process for most drugs is diffusion. Thus, the pK of the drug and the pH of the intestinal fluid (pH 5) will strongly influence the rate of drug absorption. While weak acids like phenobarbital (pK 7.4) can be absorbed from the stomach, they are more readily absorbed from the small intestine because of the latter s extensive surface area. 

The principal precaution with use of the bile acid resins is the possibility of impaired absorption of other drugs given orally at the same time. Cholestyramine and colestipol can bind many other drugs, such as digitoxin, phenobarbital, chlorothiazide, and warfarin, and delay or prevent their absorption. For this reason, other drugs should always be taken at least 1 hour before or 4 to 6 hours after the resin. The resins can also decease absorption of fat-soluble vitamins. 

Conversely, certain drugs modify the effectiveness or side effects of aspirin. Phenobarbital, occasionally used for seizures, induces liver enzymes that increase the metabolism and excretion of aspirin, (3-adrenoceptorblocking drugs, such as propranolol, and decrease the antiinflammatory effects of aspirin, whereas reserpine decreases its analgesic effects. Antacids decrease the absorption of aspirin. Alcohol consumption in combination with aspirin increases the latter s ulcerogenic effects. 

Activated charcoai Piroxicam, theophylline phenobarbital Increased absorption... 

Phenobarbital Enhances phasic GABAa receptor responses reduces excitatory synaptic responses Nearly complete absorption not significantly bound to plasma proteins peak concentrations in Vi to 4 h no active metabolites tjy2 varies from 75 to 125 h Generalized tonic-clonic seizures, partial seizures, myoclonic seizures, generalized seizures, neonatal seizures, status epilepticus Toxicity Sedation, cognitive issues, ataxia, hyperactivity Interactions Valproate, carbamazepine, felbamate, phenytoin, cyclosporine, felodipine, lamotrigine, nifedipine, nimodipine, steroids, theophylline, verapamil, others... 

Cholecalciferol [Vitamin D3] (Delta D) [Vitamin/Dietary Supplement] Uses Dietary supl to Rx vit D deficiency Action T Intestinal Ca2+ absorption Dose 400-1000 Int Units/d PO Caution [A (D doses above the RDA), +] Contra T Ca2+, hypervitaminosis, allergy Disp Tabs SE Vit D tox Interactions T Risk of arrhythmias w/ cardiac glycosides X effects w/ cholestyramine, colestipol, mineral oil, orlistat, phenobarbital, phenytoin EMS Can cause vit D tox (Tin serum Ca2+ weakness, AMS, Gl upset and cardiac arrhythmias) OD May cause T risk of vitD tox give IV fluids... 

Iwaoku, R., et al. 1982. Enhanced absorption of phenobarbital from suppositories containing phenobarbital-(3-cyclodextrin inclusion complex. Chem Pharm Bull 30 1416. 

Fusari and coworkers3 have developed a similar method for the assay of phenytoin and phenobarbital in dosage forms. They have used Silanized Celite 545 as the support material, 25% amyl alcohol in chloroform as the stationary phase and 0.05M borate buffer of pH 9.5 as the mobile phase. Phenobarbital was eluted with the mobile phase and phenytoin eluted with absolute ethanol. Ultraviolet absorption was used to quantitate the substances. 

Figure 4. Conjugated diene absorption in the liver of rats fed a CS or CD diet and the same diets containing 0.06% phenobarbital (CSPHB, CDPHB) for 2 weeks. The difference spectrum is in the inset.

Changes in the erythrocytes osmotic resistance were not observed. Adsorption of total plasma proteins on modified MC was lower than 12 %, but it was about 60 - 70 % on unmodified particles. Table 6 summarizes the results obtained of MC sorption efficiency to substances of different molecular mass in donor plasma. The sorption mechanism of low and middle molecular weight substances (phenobarbital and cyanocobalamin) on iron-carbon and restored-iron MC is apparently connected with absorption of molecules into the sorbent s pores. Iron-carbon composites have a more porous structure than restored-iron, therefore the... 

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