When drugs are injected into the muscle, subcutaneous layer absorption still has to take place but it is less dependent on the chemical nature of the drugs since the drugs are absorbed into the circulatory system through the small pores in the capillary walls.
Kinetics of drug action in disease states: towards physiology-based pharmacodynamic (PBPD) models
Distribution is the movement of drugs throughout the body. Determined by the blood flow to the tissues, it is ability of the drug to enter the vasculature system and the ability of the drug to enter the cell if required. The blood stream has the ability to transport relatively insoluble substances. These substances are transferred by binding to the proteins which have a very amphipathic structure.
The hydrophilic group renders the protein soluble in water and the lipophilic compounds are attracted to the lipophilic group and are loosely bound to the protein molecule hence protein bound.
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Most of the drugs travel in the plasma are partly in solution and partly bound to the plasma protein. The bound drug is inactive and the unbound drug is active.
The ratio of bound to the unbound drug varies. Binding is reversible.
Diseased state can cause a problem on the effectiveness of the drug. As globin levels increase with age this factor should be taken into account when treating an elderly person with a basic drug. The protein bound drug is in equilibrium with the free drug. That means that once the free drug enters the target tissue then the protein bound drug will be released to maintain equilibrium. If two drugs bind at the same site of the protein and are administered together then there can be problems.
Warfarin and asprin.. After absorption most drugs are distributed in the blood to the body tissue where they have their effect. The degree to which the drug is likely to accumulate in the tissue is dependent on the lipophilicity and local blood flow to the tissue. Highly perfused organs receive most of the drugs. After the drug is absorbed by the GI tract, it is taken up by the part of the bloodstream called the hepatic portal system. Most of the drugs are absorbed into this system except for the lipids which are absorbed into the lymphatic system and then delivered into the blood by the thoracic duct into the superior vena cava.
The hepatic portal system is designed to take digested foodstuff into the liver where it can be processed, in some cases it is stored before being distributed and it is possible that this may happen to the drug and the drug would be metabolized before reaching the rest of the body. Such drugs that metabolized by the liver are said to have a high hepatic first pass.
Hence drugs with a very high hepatic first pass cannot be given orally. The capillaries in the CNS are different they have pores which are sealed by the connective tissue and hence only small molecules can cross the blood brain barrier and the substances that can cross over have to be very lipophilic in nature. This is sometimes useful as it avoids some drugs from crossing the CNS and causing deleterious effects.
Sometimes the blood brain barrier allows the transport of drugs resulting in unwanted effect for e. It is the process of transformation of a drug within the body to make it more hydrophilic so that it can be excreted out from the body by the kidneys. This needs to be done since drugs and chemicals are foreign substances in our body. If the drug continues to be in the lipohilic state and is going to be filtered by the glomerulus then it will be reabsorbed and remain in the body for prolonged periods.
Hence metabolism deals with making the drug more hydrophilic such that it can be excreted out from the body. In some cases the metabolites can be more active than the drug itself e.
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Some enzymes are highly specific and will breakdown only compounds that they recognize for e. But there are some enzymes such as pepsin which are not specific and will breakdown most soluble proteins into smaller polypeptides or amino acids.
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Binding Kinetics in Drug Discovery | Technology Networks
Effect of infusion rate on phenobarbital concentrations in serum, brain and cerebrospinal fluid of normal rats at onset of loss of righting reflex. Effect of experimental renal dysfunction on phenobarbital concentrations in rats at onset of loss of righting reflex.
Effect of pregnancy on the relationship between phenytoin concentration and antiseizure activity in rats. Effect of pregnancy on phenobarbital concentrations at onset of loss of righting reflex in rats. Acute effect of urea infusion on phenobarbital concentrations in rats at onset of loss of righting reflex. Effect of experimental diabetes on phenobarbital concentrations in rats at onset of loss of righting reflex. Effect of experimental renal dysfunction on the pharmacodynamics of ethanol in rats. Effect of pregnancy on ethanol concentrations at onset of loss of righting reflex in rats.
Effect of experimental fever on phenobarbital concentrations at onset of loss of righting reflex in rats. Pharmacodynamics of the hypnotic effect of salicylamide in rats. Effect of nicotine on the pharmacodynamics and pharmacokinetics of phenobarbital and ethanol in rats.
The results, which were published in 45 papers in the years , showed considerable variation in pharmacodynamics. These initial studies on kinetics of drug action in disease states triggered further experimental research on the relations between pharmacokinetics and pharmacodynamics.
Together with the concepts in Levy's earlier publications "Kinetics of Pharmacologic Effects" Clin Pharmacol Ther 7 3 : , and "Kinetics of pharmacologic effects in man: the anticoagulant action of warfarin" Clin Pharmacol Ther 10 1 : , , they form a significant impulse to the development of physiology-based pharmacodynamic PBPD modeling as novel discipline in the pharmaceutical sciences.
Next it addresses the significance of his research for the evolution of PBPD modeling as a scientific discipline. PBPD models contain specific expressions to characterize in a strictly quantitative manner processes on the causal path between exposure in terms of concentration at the target site and the drug effect in terms of the change in biological function. Pertinent processes on the causal path are: 1 target site distribution, 2 target binding and activation and 3 transduction and homeostatic feedback.
Read Article at publisher's site. How does Europe PMC derive its citations network? Protein Interactions. Protein Families.