Pharmacokinetics

From Wikipedia, the free encyclopedia

Pharmacokinetics (in Greek: "pharmacon" meaning drug and "kinetikos" meaning putting in motion, the study of time dependency) is a branch of pharmacology dedicated to the determination of the fate of substances administered externally to a living organism. In practice, this discipline is applied mainly to drug substances, though in principle it concerns itself with all manner of compounds ingested or otherwise delivered externally to an organism, such as nutrients, metabolites, hormones, toxins, etc. Pharmacokinetics is often divided into several areas including, but not limited to, the extent and rate of Absorption, Distribution, Metabolism and Excretion. This sometimes is referred to as the ADME scheme.

Absorption is the process of a substance entering the body. Distribution is the dispersion or dissemination of substances throughout the fluids and tissues of the body. Metabolism is the irreversible transformation of substances and its daughter metabolites. Excretion is the elimination of the substances from the body. In rare cases, some drugs irreversibly accumulate in a tissue in the body.

Pharmacokinetics is often studied in conjunction with pharmacodynamics. So while pharmacodynamics explores what a drug does to the body, pharmacokinetics explores what the body does to the drug.

Pharmacokinetics is sometimes abbreviated as "PK".

Contents

[edit] Drug properties that influence its pharmacokinetics

Several drug properties can often influence the pharmacokinetics of many drugs.

[edit] Organ partitioning

[edit] Partition coefficient

The partition or distribution coefficient (KD) is the ratio of concentrations of a compound in the two phases of a mixture of two immiscible solvents at equilibrium.[1]. It can influence the ADME properties (Absorption, Distribution, Metabolism, and Excretion) of the drug. When orally administered drugs are absorbed they must first pass through lipid bilayers in the intestinal epithelium (a process known as transcellular transport). For efficient transport the drug must be hydrophobic enough to partition into the lipid bilayer but not so hydrophobic that once it is in the bilayer it will not partition out again.[2] The partition coefficient is dependent on the hydrophobic or hydrophilic properties of the drug.

[edit] Drug transporters

Drug transporters are transmembrane proteins on the surface of cells and are responsible for facilitating or hindering the intracellular and paracellular transport of nutrients and other substances. Some known drug transporters include Breast Cancer Resistance Protein (BCRP, also known as ABCG2) and human intestinal peptide transporter (hPepT1). BCRP is a type of ATP-binding cassette transporter that can decrease the efficacy of chemotherapeutic agents in breast cancer by exporting agents out of the tumor cells, thus making them resistant to chemotherapy. Valacyclovir uses the hPepT1 transporter to increase the intestinal absorption of valacyclovir compared to acyclovir. Drug transporters can increase or decrease the absorption of drugs into the body as well as limit or facilitate the exposure of certain organs.

[edit] Perfusion

Perfusion or flow of blood to different organs, affects rate of presentation of drugs to different parts of the body and often affects the pharmacokinetics of many drugs. While the partition coefficient can affect the distribution of drugs from the blood stream to the organs, perfusion affects how fast a drug is presented to the organs. Different organs receive a vast supply of drugs while others receive minor amounts. For example, the kidneys receive vast quantities of blood, especially considering the relatively small size of the kidneys. Adipose tissue, on the other hand, receives a minor supply blood. Organs with a rich blood supply would be presented drug at a higher rate than organs with a lower blood supply.

[edit] Analysis

Pharmacokinetic analysis is performed by noncompartmental (model independent) or compartmental methods. Noncompartmental methods estimate the exposure to a drug by estimating the area under the curve of a concentration-time graph. Compartmental methods estimate the concentration-time graph using kinetic models.

[edit] Noncompartmental analysis

Noncompartmental PK analysis is highly dependent on estimation of total drug exposure. Total drug exposure is most often estimated by Area Under the Curve methods, with the trapezoidal rule (numerical differential equations) the most common area estimation method. Due to the dependence of the length of 'x' in the trapezoidal rule, the area estimation is highly dependent on the blood/plasma sampling schedule. That is, the closer your time points are, the closer the trapezoids are to the actual shape of the concentration-time curve.

[edit] Compartmental analysis

Compartmental PK analysis uses kinetic models to describe and predict the concentration-time curve. PK compartmental models are often similar to kinetic models used in other scientific disciplines such as chemical kinetics and thermodynamics. The advantage of compartmental to noncompartmental analysis is the ability to predict the concentration at any time. The disadvantage is the difficulty in developing and validating the proper model. The simplest PK compartmental model is the one-compartmental PK model with IV bolus administration and first-order elimination.

[edit] Bioanalytical methods

Bioanalytical methods are necessary to construct a concentration-time profile. Chemical techniques are employed to measure the concentration of drugs in biological matrix, most often plasma. Proper bioanalytical methods should be selective and sensitive.

[edit] Mass spectrometry

Pharmacokinetics is often studied using mass spectrometry because of the complex nature of the matrix (often blood or urine) and the need for high sensitivity to observe low dose and long time point data. The most common instrumentation used in this application is LC-MS with a triple quadrupole mass spectrometer. Tandem mass spectrometry is usually employed for added specificity. Standard curves and internal standards are used for quantitation of usually a single pharmaceutical in the samples. The samples represent different time points as a pharmaceutical is administered and then metabolized or cleared from the body. Blank or t=0 samples taken before administration are important in determining background and insuring data integrity with such complex sample matrices. Much attention is paid to the linearity of the standard curve; however it is not uncommon to use curve fitting with more complex functions such as quadratics since the response of most mass spectrometers is less than linear across large concentration ranges.[3][4][5]

There is currently considerable interest in the use of very high sensitivity mass spectrometry for microdosing studies, which are seen as a promising alternative to animal experimentation.[6]

[edit] Population pharmacokinetics

Population pharmacokinetics is the study of the sources and correlates of variability in drug concentrations among individuals who are the target patient population receiving clinically relevant doses of a drug of interest.[7][8] Certain patient demographic, pathophysiological, and therapeutical features, such as body weight, excretory and metabolic functions, and the presence of other therapies, can regularly alter dose-concentration relationships. For example, steady-state concentrations of drugs eliminated mostly by the kidney are usually greater in patients suffering from renal failure than they are in patients with normal renal function receiving the same drug dosage. Population pharmacokinetics seeks to identify the measurable pathophysiologic factors that cause changes in the dose-concentration relationship and the extent of these changes so that, if such changes are associated with clinically significant shifts in the therapeutic index, dosage can be appropriately modified. The industry standard software for population pharmacokinetics analysis is NONMEM.[9]

[edit] See also

[edit] References

  1. ^ Leo A, Hansch C, and Elkins D (1971). "Partition coefficients and their uses". Chem Rev 71 (6): 525–616. doi:10.1021/cr60274a001. 
  2. ^ Kubinyi H (1979). "Nonlinear dependence of biological activity on hydrophobic character: the bilinear model". Farmaco [Sci] 34 (3): 248–76. PMID 43264. 
  3. ^ Increasing Speed and Throughput When Using HPLC-MS/MS Systems for Drug Metabolism and Pharmacokinetic Screening, Y. Hsieh and W.A. Korfmacher, Current Drug Metabolism Volume 7, Number 5, 2006, Pp. 479-489
  4. ^ Covey TR, Lee ED, Henion JD. 1986. High-speed liquid chromatography/tandem mass spectrometry for the determination of drugs in biological samples. Anal Chem 58:2453-2460.
  5. ^ Thermospray liquid chromatography/mass spectrometry determination of drugs and their metabolites in biological fluids. Covey TR et al. Anal Chem. 1985 Feb;57(2):474-81
  6. ^ Committee for Medicinal Products for Human Use (CHMP) (2004-06-23). Position Paper on Non-Clinical Studies to Support Clinical Trials with a Single Microdose (en). CPMP/SWP/2599/02 Rev 1. European Medicines Agency, Evaluation of Medicines for Human Use. Retrieved on 2008-06-09.
  7. ^ Sheiner, L.B.; Rosenberg, B., Marathe, V.V. (1977). "Estimation of Population Characteristics of Pharmacokinetic Parameters from Routine Clinical Data". J. Pharmacokin. Biopharm. 5: 445-479. 
  8. ^ Sheiner, L.B.; Beal, S.L., Rosenberg, B. Marathe, V.V. (1979). "Forecasting Individual Pharmacokinetics". Pharmacol. Ther 26: 294-305. 
  9. ^ Beal, S.; Sheiner L.B. (1980). "The NONMEM System". The American Statistician 34: 118-119. 

[edit] External links