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Pharmacodynamics (PD) is the study of the biochemical and physiologic effects of drugs (especially pharmaceutical drugs). The effects can include those manifested within animals (including humans), microorganisms , or combinations of organisms (for example, infection ).
PK/PD relationships can be described by simple equations such as linear model, Emax model or sigmoid Emax model. [5] However, if a delay is observed between the drug administration and the drug effect, a temporal dissociation needs to be taken into account and more complex models exist: [6] [7] Direct vs Indirect link PK/PD models
The Hill equation reflects the occupancy of macromolecules: the fraction that is saturated or bound by the ligand. [1] [2] [nb 1] This equation is formally equivalent to the Langmuir isotherm. [3] Conversely, the Hill equation proper reflects the cellular or tissue response to the ligand: the physiological output of the system, such as muscle ...
The Hill equation can be used to describe dose–response relationships, for example ion channel-open-probability vs. ligand concentration. [9] Dose is usually in milligrams, micrograms, or grams per kilogram of body-weight for oral exposures or milligrams per cubic meter of ambient air for inhalation exposures. Other dose units include moles ...
The model equations follow the principles of mass transport, fluid dynamics, and biochemistry in order to simulate the fate of a substance in the body. [9] Compartments are usually defined by grouping organs or tissues with similar blood perfusion rate and lipid content (i.e. organs for which chemicals' concentration vs. time profiles will be similar).
The absorption rate constant K a is a value used in pharmacokinetics to describe the rate at which a drug enters into the system. It is expressed in units of time −1. [1] The K a is related to the absorption half-life (t 1/2a) per the following equation: K a = ln(2) / t 1/2a.
The solution of this differential equation is useful in calculating the concentration after the administration of a single dose of drug via IV bolus injection: = C t is concentration after time t; C 0 is the initial concentration (t=0) K is the elimination rate constant
For an example, one might consider the hypothetical drug foosporin. Suppose it has a long lifetime in the body, and only ten percent of it is cleared from the blood each day by the liver and kidneys. Suppose also that the drug works best when the total amount in the body is exactly one gram.