1. HOW DRUGS ACT: GENERAL PRINCIPLES

2. Learning Objective
At the end of this session the student should be able to identify some important general principles underlying the interaction of drugs with living systems

3. Introduction Pharmacology can be defined as:
The study of the manner in which the functions of living systems is affected by chemical agents
Knowledge of the normal & abnormal functioning of the body is necessary

4. Introduction… Pharmacology comprises two broad divisions, which are:
1. Pharmacodynamics-the biological and therapeutic effects of drugs
2. Pharmacokinetics-the absorption, distribution, metabolism and excretion of drugs

5. The distinction can be put crudely thus
Pharmacodynamics is what drugs do to the body
while
Pharmacokinetics is what the body does to the drugs

6. Introduction…
It is self evident that knowledge of pharmacodynamics is essential to the choice of drug therapy.
But the well-chosen drug may fail (or be poisonous) because too little or too much is present at the site of action for too short or too long a time.
Drug therapy can fail for pharmacokinetic as well as for pharmacodynamic reasons.

7. Pharmacodynamics
Understanding the mechanisms of drug action is not only an objective of the pharmacologist who seeks to develop better drugs , but also permits a more intelligent use of medicines

8. Pharmacodynamics…
Consider the treatment of hypertension or asthma for e.g.-
Using combinations of drugs with the same mode of action will not only provide additive therapeutic effect but also additive adverse effects.
Selection of combinations of drugs having different modes of action will also provide additive therapeutic efficacy and reduce the risk of additive adverse effects

9. BINDING OF DRUG MOLECULES TO CELLS

10. How do drugs act/work?
Body functions are mediated through control systems that involve receptors, enzymes, carrier molecules and specialized macromolecules such as DNA.
Most drugs act by altering the body’s control systems

11. Majority of drugs act by binding to cells
To produce a biological response drug molecules must exert some chemical influence on one or more constituents of the cell OR
Drug molecules must get very close to these constituent cellular molecules for their function to be altered

12. molecules in the organism greatly outnumber the drug molecules and if the drug molecules were merely distributed at random, the chance of interaction with any particular class of cellular molecule would be negligible.
Pharmacological effects, therefore, require in general, the non-uniform distribution of the drug molecule within the body or tissue, which is the same as saying that:

13. Drug molecules must be “bound” to particular constituents of cells and tissues in order to produce an effect
Paul Ehrlich summed it up as follows: “corpora non agunt nisi fixata” - (in this context, ‘a drug will not work unless it is bound’).

14. Exceptions to Ehrlich’s dictum:
A few drugs act by simple mechanisms related to their chemical or physical properties, without being bound to any tissue constituent e.g.:
EDTA (EthyleneDiamineTetraAcetate) is a metal chelating agent with high affinity for Pb2+. It is used for treatment of lead intoxication
Antacids such as Mg(OH)2 & Al(OH)3 are bases and act by neutralizing acid after oral administration
Mannitol: an osmotic diuretic, biologically inert, does not penetrate into cells. Given IV it is filtered in the glomerulus but not reabsorbed. - Diuresis

15. The major aim of pharmacological research is:
To understand the nature of these binding sites
To understand the mechanism by which the association of a drug molecule with a binding site leads to a physiological/biological response

16. Most drugs produce their effects by binding, in the first instance, to protein molecules-often called targets

17. What are these binding sites?
Mainly proteins
The only exception to protein as target sites is DNA (site of action for some anticancer drugs, some antimicrobials, mutagenic & carcinogenic agents)

18. Protein targets for drug binding
Four kinds of regulatory protein are commonly involved as primary drug targets, namely:
Enzymes
Ion channels
Carrier molecules
Receptors
NB: there are still many drugs whose binding sites are still unknown

19. A few other types of protein are known to function as drug targets.
Many drugs are known to bind (in addition to their primary targets) to plasma protein ( see in cell proliferation & aptosis) as well as to cellular constituents, without producing any obvious physiological effect
In general most drugs act on one or other of the four types of protein listed .
(The mechanisms by which such binding leads to cellular responses will be discussed later)

20. a) ENZYMES
In common with all catalysts, enzymes speed up a reaction by providing an alternative route of lower activation energy.
They do this by the reactant molecule called the substrate and holding it in a favorable orientation for the reaction

21. a) ENZYMES… There are many drugs which act by targeting enzymes:
Act as substrate analogues which competitively inhibit the enzyme
(a) reversibly e.g. neostigmine which inhibits acetyl cholinesterase
(b) irreversibly e.g inhibition of cyclo-oxygenase by aspirin

22. Act as false substrates
- drug act as a natural substrate
- undergo chemical transformation to form an abnormal product
- inhibit the normal metabolic pathway
e.g.: - fluorouracil (anticancer) replaces uracil as an intermediate in purine biosynthesis
- it can not be converted into thymidylate
- blocks DNA synthesis and hence cell division

23. Activation of drugs by enzymes
- some drugs require enzymic degradation to convert them to the active form
- such drugs are called prodrugs
e.g.: Proguanil cycloguanil

24. b) ION CHANNELS
These are pores which are situated on the cell membrane
On the outer part of the pores are situated ions:
- K+ (potassium channels)
- Ca2+ (Calcium channels)
- Na+ (sodium channels)
- Cl- (chloride channels)

25. Some ion channels are directly linked to a receptor and open only when the receptor is occupied by an agonist (Ligand-gated ion channels)
Some are direct targets of drug action
- the simplest type of interaction is physical blockade of the ion channel by a drug:
e.g.: (1)the blockade of the voltage gated sodium channels by local anesthetics
(2) the blockade of sodium entry into renal tubular cells by the diuretic amiloride

26. Channel function can also be modulated by drugs which bind to accessory sites on the channel protein
The binding of such drugs influence the gating of the channel
examples
Dihydropyridines (vasodilators)
Ca2+ channels open in response to depolarization of the cell membrane
The binding of dihydropyridines to the channel may inhibit or facilitate the opening depending on the structure of the dihydropyridine

27. GABA receptor/chloride-channel complex
GABA-Gamma Amino Butyric Acid (natural substrate)
Stimulation of the receptor by GABA opens the associated Cl- channel leading to inhibition of neurotransmission in the CNS
Benzodiazepines/barbiturates - act on sites which are different to the GABA binding site
Facilitate opening of the Cl- channel by GABA

28. c) CARRIER MOLECULES
Ions and small molecules (glucose) are not sufficiently lipid soluble to cross the cell membrane and get into the cells
Transport into the cells requires carriers
Types of carriers:
-for transport of glucose & amino acids in the gastro-intestinal tract
-for transport of ions & organic molecules by renal tubules
- for transport of Na+, Ca2+ ions out of the cell etc.
These carriers belong to a family of very well defined transporter systems

29. d) RECEPTORS Are recognition sites for drugs
A receptor produces an effect only when a particular drug is bound to it; otherwise it is functionally inert
Agonists: drugs which activate receptors
Antagonists: (a) drugs which on their
own do not activate receptors
(b) displace agonists from their receptors and reduce their biological
effect

31. Receptors (2)
Receptors form a key part of the system of chemical communication that all multi-cellular organisms use to coordinate the activities of their cells and organs
The term receptor is, therefore, reserved for interactions of the regulatory type, where a ligand (chemical molecule) may function as an agonist or antagonist
It is limited to molecules/structures which have a physiological regulatory function

32. Drug specificity
For a chemical agent/drug to be useful as a therapeutic or scientific tool it must act selectively on particular cells and tissues i.e. it must show a high degree of binding-site specificity
Conversely: proteins that function as drug targets generally show a high degree of ligand specificity
They will recognize only ligands of a certain precise type and ignore closely related molecules

33. Drug specificity

34. NO DRUGS ARE COMPLETELY SPECIFIC IN THEIR ACTIONS
In many cases, if you increase the dose, a drug will affect targets other than the principal one, and lead to side effects

35. Emphasis is that No drugs acts with complete specificity…
Thus tricyclic antidepressant drugs act by blocking monoamine transporters but are notorious for producing side effects (e.g. dry mouth) related to their ability to block various receptors.
In general, the lower the potency of a drug, and the higher the dose needed, the more likely it is that sites of action other than the primary one will assume significance.
In clinical terms, this is often associated with the appearance of unwanted side effects, of which no drug is free.

36. Terminologies
Dose: is a specified quantity of a therapeutic agent, such as a drug or medicine, prescribed to be taken at one time or at stated intervals. It has dimension mass (e.g units ng, ug, mg, gm, pmole,nmole,umole)
A concentration of a drug is an amount per unit volume and has the dimension mass/volume (e.g. units ng/ml, umole/L)
People and animals are given doses of drugs and concentrations are achieved in bodily fluids (see Drug Disposition and metabolism)

37. Potency
Potency of a drug is a measure of the dilution in which it causes a specified effect; OR the amount (weight) of drug in relation to its effect
thus a drug that evokes the specified effect when present in great dilution (i.e small concentration) is said to be highly potent.

38. Efficacy
Efficacy is the capacity of a drug to produce an effect and refers to the maximum such effect
E.g. if drug A can produce a therapeutic effect that cannot be obtained with drug B, however much of drug B is given, then drug A has the higher therapeutic efficacy
Differences in therapeutic efficacy are of great clinical importance

39. Sensitivity
Sensitivity is the tissue or target equivalent of potency. In other words potency is the property of the drug that describes how little of it is needed. Sensitivity is the property of the responding system that describes the concentration at which it responds to a drug. The units of measurement are the same in both cases (e.g nmole/L)

40. Selectivity
Selectivity is the central theme of pharmacology and is the phenomenon that allows drugs to be useful.
No drug is absolutely specific-that is produces only one (desirable) effect at very high potency.
Selectivity is the term used to describe the ability of a given dose or concentration of a drug to produce one effect rather than another.

42. Dose response curves
A pharmacological response is a function of the dose/concentration
The relationship between dose & response is represented graphically by dose-response curves

43. Quantal vs graded responses
There are two types of responses:
(a) quantal or ‘all-or-none responses
(b) graded responses
Quantal responses:
- analgesia for headache
-digitalis to stop heart fibrillations
- sleep or lethal dose for anaesthetics

44. Quantal vs graded responses (2)
Responses are represented as cumulative percentage of subjects exhibiting a defined effect
Quantal relationships can be defined for both toxic & therapeutic effects
This allows the calculation of therapeutic index
A safe drug has a large therapeutic index.

45. Quantal effects. A set of data obtained after administration of increasing doses of a drug to a group of patients, and observation of the minimum dose at which each patient responded with the desired outcome. The results have been plotted as a histogram, and fit with a gaussian curve. μ = mean response; σ = standard deviation.

47. Therapeutic Index
When the dose of a drug is increased progressively, the response in the patient usually rises to a maximum beyond which further increases in dose do not elicit a greater effect but induce only unwanted effects.
This is because a drug does not have a single dose-response curve, but a different curve for each action, so that new and unwanted actions are recruited if dose is increased after the maximum therapeutic effect has been achieved

48. Therapeutic index
Ehrlich introduced the concept of the therapeutic index as the maximum tolerated dose divided by the minimum curative dose, but since there are no single such doses that apply to all individuals, the index is never calculated in this way.

49. Therapeutic index…
The index can be calculated for animals by using the ratio:
Median lethal dose
Median effective dose OR
LD50/ED50 = 400/100 = 4
i.e. the dose that is lethal to 50% of animals LD50 divided by the dose that has the desired effect in 50% of animals (ED50)

50. Therapeutic Index…
Similarly, a dose that has some unwanted effect in 50% of humans (e.g. a specified increase in heart rate say, in the case of an adrenoceptor agonist bronchodilator) can be related to that which is effective in 50% (e.g. a specified decrease in airways resistance) although in practice such information is not available for many drugs.
The TI embody a concept that is fundamental in comparing the usefulness of one drug with another, namely safety in relation to efficacy

51. The relationship between the dose-response relationships for producing therapeutic and toxic side effects. The Therapeutic Index (TI) is defined as the ratio of the TD50/ED50.

52. (b) Graded responses
Responses are often described as a percentage of maximal response
A linear concentration scale yields a rectangular hyperbolic curve
A log scale yields a sigmoid curve

54. Practice Problem 1
A 55-year-old woman with hypertension is to be treated with a vasodilator drug. Drugs X and Y have the same mechanism of action. Drug X in a dose of 5 mg produces the same decrease in blood pressure as 500 mg of drug Y. Which of the following statements best describes these results?
(A) Drug Y is less efficacious than drug X
(B) Drug X is about 100 times more potent than drug Y
(C) Toxicity of drug X is less than that of drug Y
(D) Drug X has a wider therapeutic window than drug Y
(E) Drug X will have a shorter duration of action than drug Y because less of drug X is present over the time course of drug action

55. Practice Problem 2
Graded and quantal dose-response curves are being used for evaluation of a new antiasthmatic drug in the animal laboratory and in clinical trials. Which of the following statements best describes quantal dose-response curves?
(A) More precisely quantitated than graded dose-response
curves
(B) Obtainable from the study of intact subjects but not
from isolated tissue preparations
(C) Used to determine the maximal efficacy of the drug
(D) Used to determine the statistical variation (standard
deviation) of the maximal response to the drug
(E) Used to determine the variation in sensitivity of subjects
to the drug

56. Quantitative aspects of Drug-Receptor Interactions
Here we present some aspects of receptor theory which is based on applying the Law of Mass action to the drug-receptor interaction and which has served well as a framework for interpreting a large body of quantitative experimental data

57. The mechanism of drug-receptor interaction
The 1st step in drug action, on a specific receptor, is the formation of a reversible drug-receptor complex;
The reaction being governed by the law of mass action.
The Law of mass action states that the rate of a chemical reaction is proportional to the product of the concentrations of reactants) can be applied to this reaction

58. The mechanism of drug-receptor interaction (2)
Suppose that a piece of tissue (heart muscle or uterine smooth muscle) contains a total number of receptors, NT, for an agonist such as adrenaline, A.
When the tissue is exposed to the agonist (A) at a conc. XA and allowed to come to equilibrium, a certain number, NA of the receptors will be occupied and
The number of vacant receptors will be reduced to NT – NA
Normally the number of adrenaline molecules applied to the tissue in solution greatly exceeds NT, so that the binding reaction does not generally reduce XA

59. The mechanism of drug-receptor interaction (3)
The magnitude of the response produced by adrenaline XA will be related to the number of receptors occupied, so it is useful to consider that quantitative relationship is predicted between NA and XA
The reaction can be represented by:
Drug + free receptor =drug/receptor complex or
K+1
A R = AR or
K-1
(XA) (NT – NA) = (NA)

61. The mechanism of drug-receptor interaction (4)

62. Two important terms of dose-response curves
Potency:
The location of the concentration curve along the concentration axis
It is related to the dose of a drug required to produce a given effect
Efficacy:
The magnitude of effect that can be produced by a drug
Maximal efficacy is reflected in the plateau of the dose-response curve

63. Drug A is more potent than drug B, but both show the same efficacy
Drug C and D shows lower potency and lower efficacy than A or B

64. Dose-response relationships for four agonists that vary in efficacy
Dose-response relationships for four agonists that vary in efficacy. Each drug has essentially the same EC50 value (equi-potent), but differ in terms of the maximum response they can produce at high concentrations that saturate all available receptor sites. Drug A has a relative efficacy that is 2 times than Drug C, and ~100 times more than Drug D.

65. B = agonist +competitive antagonist
Bind reversibly at the agonist binding site
Its effect can be overcome by increasing concentration of agonist
Non-competitive antagonist
Bind irreversibly to agonist binding site
The effect is equivalent to removing receptors from the system
A = agonist alone
B = agonist +competitive antagonist
C = Agonist + non-competitive antagonist

66. Competitive antagonism

67. Non-competitive antagonism

68. Partial agonists
Agonists can be divided into two classes: Partial & Full agonist
A partial agonist is a drug that displays efficacy that is intermediate between that of an agonist and an antagonist
Partial agonist produce a lower response at full receptor occupancy than full agonists (Refer to next slide)
Partial agonists also produce concentration –effect curves that resemble those observed with full agonists

69. Mechanism of action of partial agonist
The allosteric theory of drug action
This theory assumes that a receptor exists in two states, the active state (Ra) and inactive state (Ri)
Both forms are capable of binding a drug but with different affinities
These two forms exist in an equilibrium, but, while at rest Ri dominates

71. Ra Ri
D:Ra D:Ri
When an agonist binds to the receptor equilibrium shifts to Ra
A full agonist will completely drive the receptor to the active state

72. A partial agonist is incapable of driving the equilibrium to the active state
An antagonist has equal affinity for Ra & Ri
-it does not change the equilibrium of the resting state
-it only interferes with the action of agonist by occupying the binding sites
If a compound has a higher affinity for Ri than Ra, it is called an inverse agonist

76. Receptor classification
Based on pharmacological criteria
classified on the basis of the effects of particular drugs
The Direct measurement of ligand binding to receptors has allowed many new receptor subtypes to be defined; subtypes that could not easily be distinguished by studies of drug effects.

77. Molecular cloning which has revealed the amino acid sequence of many receptors
Analysis of the biochemical pathways that are linked to receptor activation provides yet another basis of classification
All these have led to a lot of confusion in receptor classification

78. Responding to this growing confusion the International Union of Pharmacological Sciences (IUPHAR)
Has set up various expert working groups to produce agreed receptor classifications for the major receptor types, taking into account the pharmacological, molecular & biochemical information available
A useful summary of known receptors is published annually (Trends in Pharmacological Sciences, Receptor Supplement)
A comprehensive IUPHAR database of known receptor classes is available (see

79. Spare Receptors
Stephenson (1956), studying the actions of acetylcholine analogues in isolated tissues, found that many full agonists were capable of eliciting maximal responses at very low occupancies, often less than
Such systems may be said to possess spare receptors, or a receptor reserve.
This is common with drugs that elicit smooth muscle contraction.

80. Spare Receptors (2)
It is less for other types of receptor-mediated response (secretion, smooth muscle relaxation or cardiac stimulation, where the effect is more nearly proportional to receptor occupancy)
Thus biological response, can be reached with a lower concentration of hormone or neurotransmitter than would be the case if fewer receptors were provided.
Economy of hormone or transmitter secretion is thus achieved at the expense of providing more receptors

81. Drug Antagonism And Synergism
Apart from Competitive antagonism, the effect of one drug can be reduced in the presence of another in other ways.
These are mechanisms includes:
Chemical antagonism
Pharmacokinetic antagonism
Block of receptor-effector linkage
Physiological antagonism

82. Chemical Antagonism
It refers to the uncommon situation where the two substances combine in solution; as a result, the effect of the active drug is lost.
Eg 1→ chelating agent dimercaprol that bind to heavy metals and thus reduce their toxicity
Eg 2 → the neutralising antibody infliximab which has an anti-inflammatory action due to its ability to sequester the inflammatory cytokine, tumour necrosis factor

83. Pharmacokinetic Antagonism
PK antagonism occurs when:
The rate of elimination of the active drug is increased (e.g. the anticoagulant effect of warfarin is ↓ when given together with phenobarbital,
The rate of absorption of the active drug from the Git is reduced,
The rate of renal excretion may be increased.

84. Block Of Receptor-effector Linkage
When an agonist bind to a receptor, the chain of events that leads to the production of a response by the agonist which may include influx of ions into the cells may be blocked by some drugs.
For example, drugs such as verapamil and nifedipine prevent the influx of Ca2+ through the cell membrane and thus block non-specifically the contraction of smooth muscle produced by other drugs.

85. Block Of Receptor-effector Linkage (2)

86. Physiological Antagonism
Physiological antagonism occurs when two drugs whose opposing actions in the body tend to cancel each other.
For example, histamine acts on receptors of the parietal cells of the gastric mucosa to stimulate acid secretion, while omeprazole blocks this effect by inhibiting the proton pump;

87. Practice Problem
A study was carried out in isolated, perfused animal hearts.In the absence of other drugs, pindolol, a β-adrenoceptor ligand, caused an increase in heart rate. In the presence of highly effective β stimulants, however, pindolol caused a dose-dependent, reversible decrease in heart rate. Which of the following expressions best describes pindolol?
(A) A chemical antagonist
(B) An irreversible antagonist
(C) A partial agonist
(D) A physiologic antagonist
(E) A spare receptor agonist

88. Desensitisation And Tachyphylaxis (1)
Desensitisation and tachyphylaxis are synonymous terms used to describe the decrease of drug effect gradually when given continuously or repeatedly.
This happens in the course of a few minutes.
The term tolerance is conventionally used to describe a more gradual decrease in responsiveness to a drug, taking days or weeks to develop
Drug resistance is a term used to describe the loss of effectiveness of antimicrobial or antitumour drugs

89. Desensitization and Tachyphylaxis (2)
Different mechanisms may account for Desensitization such as:
change in receptors
translocation of receptors
exhaustion of mediators
increased metabolic degradation of the drug
physiological adaptation
active extrusion of drug from cells (mainly relevant in cancer chemotherapy

90. Change in receptors
Desensitization mostly occurs in receptors directly coupled to ion channels.
It can occurs at the neuromuscular junction as the result of a conformational change in the receptor, resulting in tight binding of the agonist molecule without the opening of the ionic channel.
Desensitization of ion channels can also be caused by phosphorylation of intracellular regions of the receptor protein which is a second, slower mechanism.

91. Change in Receptors (2)
Phosphorylation of the receptor interferes with its ability to activate second messenger cascades, although it can still bind the agonist molecule.
This type of desensitization usually takes a few minutes to develop, and recovers at a similar rate when the agonist is removed.

92. Translocation of Receptors
Prolonged exposure to agonists often results in a gradual decrease in the number of receptors expressed on the cell surface, as a result of internalisation of the receptors.
This is shown for β-adrenoceptors whereby the number of β-adrenoceptors can fall to about 10% of normal in 8 h in the presence of a low concentration of isoprenaline, and recovery takes several days.

93. Translocation of Receptors (2)
The internalised receptors are taken into the cell by endocytosis of patches of the membrane, a process that also depends on receptor phosphorylation.
This type of adaptation is common for hormone receptors and has obvious relevance to the effects produced when drugs are given for extended periods.

94. Exhaustion Of Mediators
In some cases, desensitisation is associated with depletion of an essential intermediate substance.
Drugs such as amphetamine, which acts by releasing amines from nerve terminals show marked tachyphylaxis because the amine stores become depleted

95. Altered Drug Metabolism
Tolerance to some drugs, for example barbiturates and ethanol occurs partly because repeated administration of the same dose produces a progressively lower plasma concentration, because of increased metabolic degradation.
On the other hand, the pronounced tolerance to nitrovasodilators results mainly from decreased metabolism, which reduces the release of the active mediator, nitric oxide.

96. Physiological Adaptation
Decrease of a drug's effect may occur because it is nullified by a homeostatic response.
For example, the blood pressure-lowering effect of thiazide diuretics is limited because of a gradual activation of the renin-angiotensin system
Such homeostatic mechanisms are very common, and if they occur slowly the result will be a gradually developing tolerance.

97. Physiological adaptation (2)
It is a common experience that many side effects of drugs, such as nausea or sleepiness, tend to subside even though drug administration is continued.
Assumption is that some kind of physiological adaptation is occurring, presumably associated with altered gene expression resulting in changes in the levels of various regulatory molecules,
However little is known about the mechanisms involved

98. The Nature Of Drug Effects
So far we have focused mainly on the consequences of receptor activation.
Later, the receptors and their linkage to effects at the cellular level will be described.
It is important to consider that drugs in a therapeutic context, generally lead to secondary, delayed effects, which are often highly relevant in a clinical situation in relation to both therapeutic efficacy and harmful effects

99. Nature of Drug Effects (2)
For example, activation of a β-adrenoceptor in the heart causes rapid changes in the functioning of the heart muscle, but also slower (minutes to hours) changes in the functional state of the receptors (e.g. desensitisation), and even slower (hours to days) changes in gene expression that produce long-term changes (e.g. hypertrophy) in cardiac structure and function.

100. Nature of drug effects (3)
Similarly, antidepressant drugs, which have immediate effects on transmitter metabolism in the brain take weeks to produce therapeutic benefit. Opioids produce an immediate analgesic effect but, after a time, tolerance and dependence ensue, and in some cases long-term addiction.

101. Nature of Drug Effects (4)
In these and many other examples, the nature of the intervening mechanism is unclear, although as a general rule any long-term phenotypic change necessarily involves alterations of gene expression.
Drugs are often used to treat chronic conditions, and understanding long-term as well as acute drug effects is becoming increasingly important.
Our focus should be on both short-term physiological responses, as well as delayed effects.

102. Early and late responses to drugs
Early and late responses to drugs. Many drugs act directly on their targets (left-hand arrow) to produce a rapid physiological response. If this is maintained, it is likely to cause changes in gene expression that give rise to delayed effects. Some drugs (right-hand arrow) have their primary action on gene expression, producing delayed physiological responses. Drugs can also work by both pathways. Note the bidirectional interaction between gene expression and response