Drug movement- absorption and distribution

1. Absorption- drug is absorbed from site of administration, entry into the plasma
2. Distribution- drug leaves bloodstream and is distributed into interstitial and intracellular fluids
3. Metabolism- drug transformation by metabolism - liver and other tissues
4. Excretion- drug and/drug metabolites excreted in urine, faeces or bile

If the right dosage of a drug is administered through an appropriate route, it should give the required therapeutic effects. If it is lower in dose or through a different route, it may offer subpar benefits. However, a dosage too high may also make us susceptible to the toxic effects of the drug.

The cell membranes are barriers between the aqueous compartments of the body. The plasma membrane separates the extracellular 'compartment' from the intracellular 'compartment'

<ol> <li>Passive diffusion</li> <li>Facilitated diffusion</li> <li>Active transport</li> <li>Endocytosis</li> </ol> Signup and view all the answers

<p>Small molecules move directly through the lipid or through aqueous pores formed by aquaporins that transverse the lipid bilayer. Many lipid soluble drugs cross cell membranes in this way.</p> Signup and view all the answers

<p>Small molecules move via specialised carrier proteins that bind the drug on one side of the bind molecule on one side of the membrane then change conformation and release on the other side. It does not require energy, but it does require a concentration gradient.</p> Signup and view all the answers

<p>Small molecules move via specialised carrier proteins. It requires energy and can move the drug molecules across the concentration gradient.</p> Signup and view all the answers

<p>Endocytosis through pinocytosis is a process in which there is an invagination of a part of the membrane. The drug is encased in a small vesicle then 'released' inside the cell.</p> Signup and view all the answers

<p>Nonpolar or lipid-soluble materials pass through plasma membranes more easily than polar materials, allowing a faster rate of diffusion.</p> Signup and view all the answers

<p>Carrier mediated drug transport including facilitated diffusion and active transport has principle sites of action:-</p> <ol> <li>Blood brain barrier</li> <li>Gastrointestinal tract</li> <li>Placenta</li> <li>Renal tubule</li> <li>Biliary tract</li> </ol> Signup and view all the answers

<p>Carrier mediated transport is important for some drugs that are chemically related to endogenous substances such as neurotransmitters.</p> Signup and view all the answers

<p>pKa is the pH at which 50% of the drug is ionised and 50% is un-ionised.</p> Signup and view all the answers

<p>It depends on both the pKa of the drug and the local pH</p> Signup and view all the answers

<p>Weak acids and weak bases are well absorbed but strong acids (pKa&lt;3) and strong bases (pKa&gt;10) are poorly absorbed.</p> Signup and view all the answers

<p>Acidic drugs become less ionised in an acid environment. Basic drugs become less ionised in a basic environment.</p> Signup and view all the answers

<p>Lower the pKa, the more acidic it becomes. And, higher the pKa, the more basic it is.</p> Signup and view all the answers

<p>The more the drug is in its un-ionized form, the more likely it is to be lipid-soluble and transferred by passive diffusion through the membrane.</p> Signup and view all the answers

<p>Accumulation</p> Signup and view all the answers

<p>Good absorption</p> Signup and view all the answers

<p>The apparent volume of distribution (Vd) describes the extent to which a drug partitions between the plasma and tissue compartments. For a drug with equal potency, a drug with high Vd will require a higher dosage than a drug with low Vd.</p> Signup and view all the answers

<p>The hydrophilic or ionised drugs have a lower Vd while lipophilic or unionised drugs have a higher Vd</p> Signup and view all the answers

<ol> <li>Plasma protein binding reduces the availability of the drug for diffusion to the drug target organ.</li> <li>It may reduce the transport of the drug to the non vascular compartments</li> <li>High protein binding can slow drug elimination by preventing metabolism in the liver and glomerular filtration.</li> </ol> Signup and view all the answers

<p>Drug that does not exhibit high plasma protein binding easily diffuses across the membrane making it easier for the target organ to associate with the drug. This decreases the drug's plasma concentration but increases Vd. And vice versa.</p> Signup and view all the answers

<p>Vd=Dose/[Drug]plasma conc.</p> Signup and view all the answers