Topics of pharmacodynamics

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).

Pharmacodynamics and pharmacokinetics are the main branches of pharmacology, being itself a topic of biology interested in the study of the interactions of both endogenous and exogenous chemical substances with living organisms.

In particular, pharmacodynamics is the study of how a drug affects an organism, whereas pharmacokinetics is the study of how the organism affects the drug. Both together influence dosing, benefit, and adverse effects. Pharmacodynamics is sometimes abbreviated as PD and pharmacokinetics as PK, especially in combined reference (for example, when speaking of PK/PD models).

Pharmacodynamics places particular emphasis on dose–response relationships, that is, the relationships between drug concentration and effect.^[1] One dominant example is drug-receptor interactions as modeled by

${\displaystyle {\ce {L + R <=> LR}}}$

where L, R, and LR represent ligand (drug), receptor, and ligand-receptor complex concentrations, respectively. This equation represents a simplified model of reaction dynamics that can be studied mathematically through tools such as free energy maps.

Basics

There are four principal protein targets with which drugs can interact:

• Enzymes – (e.g. neostigmine and acetyl cholinesterase)
  • Inhibitors
  • Inducers
  • Activators
• Membrane carriers – [Reuptake vs Efflux] (e.g. tricyclic antidepressants and catecholamine uptake-1)
  • Enhancer (RE)
  • Inhibitor (RI)
  • Releaser (RA)
• Ion channels – (e.g. nimodipine and voltage-gated Ca²⁺ channels)
  • Blocker
  • Opener
• Receptor – (e.g. Listed in table below)
  • Agonists can be full, partial or inverse.
  • Antagonists can be competitive, non-competitive, or uncompetive.
  • Allosteric modulator can have 3 effects within a receptor. One is its capability or incapability to activate a receptor (2 possibilities). The other two are agonist affinity and efficacy. They may be increased, decreased or unaffected (3 and 3 possibilities).

NMBD = neuromuscular blocking drugs; NMDA = N-methyl-d-aspartate; EGF = epidermal growth factor.^[3]

Effects on the body

The majority of drugs either

There are 7 main drug actions:^[4]

• stimulating action through direct receptor agonism and downstream effects
• depressing action through direct receptor agonism and downstream effects (ex.: inverse agonist)
• blocking/antagonizing action (as with silent antagonists), the drug binds the receptor but does not activate it
• stabilizing action, the drug seems to act neither as a stimulant or as a depressant (ex.: some drugs possess receptor activity that allows them to stabilize general receptor activation, like buprenorphine in opioid dependent individuals or aripiprazole in schizophrenia, all depending on the dose and the recipient)
• exchanging/replacing substances or accumulating them to form a reserve (ex.: glycogen storage)
• direct beneficial chemical reaction as in free radical scavenging
• direct harmful chemical reaction which might result in damage or destruction of the cells, through induced toxic or lethal damage (cytotoxicity or irritation)

Some molecular mechanisms of pharmacological agents

Receptor binding and effect

The binding of ligands (drug) to receptors is governed by the law of mass action which relates the large-scale status to the rate of numerous molecular processes. The rates of formation and un-formation can be used to determine the equilibrium concentration of bound receptors. The equilibrium dissociation constant is defined by:

${\displaystyle {\ce {L + R <=> LR}}}$                      ${\displaystyle K_{d}={\frac {[L][R]}{[LR]}}}$

where L=ligand, R=receptor, square brackets [] denote concentration. The fraction of bound receptors is

${\displaystyle {p}_{LR}={\frac {[LR]}{[R]+[LR]}}={\frac {1}{1+{\frac {K_{d}}{[L]}}}}}$

Where ${\displaystyle {p}_{LR}}$ is the fraction of receptor bound by the ligand.

This expression is one way to consider the effect of a drug, in which the response is related to the fraction of bound receptors (see: Hill equation). The fraction of bound receptors is known as occupancy. The relationship between occupancy and pharmacological response is usually non-linear. This explains the so-called receptor reserve phenomenon i.e. the concentration producing 50% occupancy is typically higher than the concentration producing 50% of maximum response. More precisely, receptor reserve refers to a phenomenon whereby stimulation of only a fraction of the whole receptor population apparently elicits the maximal effect achievable in a particular tissue.

The simplest interpretation of receptor reserve is that it is a model that states there are excess receptors on the cell surface than what is necessary for full effect. Taking a more sophisticated approach, receptor reserve is an integrative measure of the response-inducing capacity of an agonist (in some receptor models it is termed intrinsic efficacy or intrinsic activity) and of the signal amplification capacity of the corresponding receptor (and its downstream signaling pathways). Thus, the existence (and magnitude) of receptor reserve depends on the agonist (efficacy), tissue (signal amplification ability) and measured effect (pathways activated to cause signal amplification). As receptor reserve is very sensitive to agonist's intrinsic efficacy, it is usually defined only for full (high-efficacy) agonists.^[7][8][9]

Often the response is determined as a function of log[L] to consider many orders of magnitude of concentration. However, there is no biological or physical theory that relates effects to the log of concentration. It is just convenient for graphing purposes. It is useful to note that 50% of the receptors are bound when [L]=K_d .

The graph shown represents the conc-response for two hypothetical receptor agonists, plotted in a semi-log fashion. The curve toward the left represents a higher potency (potency arrow does not indicate direction of increase) since lower concentrations are needed for a given response. The effect increases as a function of concentration.

Multicellular pharmacodynamics

The concept of pharmacodynamics has been expanded to include Multicellular Pharmacodynamics (MCPD). MCPD is the study of the static and dynamic properties and relationships between a set of drugs and a dynamic and diverse multicellular four-dimensional organization. It is the study of the workings of a drug on a minimal multicellular system (mMCS), both in vivo and in silico. Networked Multicellular Pharmacodynamics (Net-MCPD) further extends the concept of MCPD to model regulatory genomic networks together with signal transduction pathways, as part of a complex of interacting components in the cell.^[10]

Toxicodynamics

Pharmacokinetics and pharmacodynamics are termed toxicokinetics and toxicodynamics in the field of ecotoxicology. Here, the focus is on toxic effects on a wide range of organisms. The corresponding models are called toxicokinetic-toxicodynamic models.^[11]

See also

• Mechanism of action
• Dose-response relationship
• Pharmacokinetics
• ADME
• Antimicrobial pharmacodynamics
• Pharmaceutical company
• Schild regression

References

External links

Wikimedia Commons has media related to Pharmacodynamics.

• Vijay. (2003) Predictive software for drug design and development. Pharmaceutical Development and Regulation 1 ((3)), 159–168.
• Werner, E., In silico multicellular systems biology and minimal genomes, DDT vol 8, no 24, pp 1121–1127, Dec 2003. (Introduces the concepts MCPD and Net-MCPD)
• Dr. David W. A. Bourne, OU College of Pharmacy Pharmacokinetic and Pharmacodynamic Resources.
• Introduction to Pharmacokinetics and Pharmacodynamics (PDF)