PHYSIO-CHEMICAL PROPERTIES OF THE DRUG. It is the interaction of the drug with its environment

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1 PYSIO-CEMICAL PROPERTIES OF TE DRUG It is the interaction of the drug with its environment

2 Reducing the complexity Biological process in drug action Dissolution of drug in gastrointestinal fluids Absorption from small intestine Underlying physical chemistry Energy of dissolution; lipophilicity & crystal packing Diffusion rate, membrane partition coefficient Physical chemistry model Solubility in buffer, acid or base logp, logd, polar surface area, hydrogen bond counts, MWt Blood protein binding Binding affinity to blood proteins e.g. albumin Plasma protein binding, logp and logd Distribution of compound in tissues Binding affinity to cellular membranes logp, acid or base

3 What must a drug do other than bind? An oral drug must be able to: bladder kidneys bile duct BBB dissolve survive a range of ps (1.5 to 8.0) survive intestinal bacteria cross membranes survive liver metabolism avoid active transport to bile avoid excretion by kidneys partition into target organ avoid partition into undesired places (e.g. brain, foetus) liver

4 The p-partition ypothesis on Drug Absorption This theory provides a basic framework for understanding of drug absorption from the GIT and drug transport across the biological membrane. The principle points of this theory are: 1. The GIT and other biological membranes act as lipid barriers. 2. The un-ionized form of the acidic or basic drug is preferentially absorbed. 3. Most drugs are absorbed by passive diffusion. 4. The rate of drug absorption and the amount of drug absorbed are related to its oil-water partition coefficient, the more lipophilic the drug, the faster is its absorption. 5. Weak acidic and neutral drugs may be absorbed from stomach but basic drugs are not.

5 Ionisation Ionisation = protonation or deprotonation resulting in charged molecules About 85% of marketed drugs contain functional groups that are ionised to some extent at physiological p (p 1.5 8). The acidity or basicity of a compound plays a major role in controlling: Absorption and transport to site of action Solubility, bioavailability, absorption and cell penetration, plasma binding, volume of distribution Binding of a compound at its site of action un-ionised form involved in hydrogen bonding ionised form influences strength of salt bridges or -bonds Elimination of compound Biliary and renal excretion CYP P450 metabolism

6 IONIZATION (pka) Ionized form - gives good water solubility need for good binding of drug to receptor. Un-ionized form - helps to cross the cell membranes. Equal ratio ionized : unionized = better pharmacokinetic and pharmacodynamic properties of the drug. Ionization and p at Absorption site The fraction of the drug existing in its un-ionized form in a solution is a function of both the dissociation constant and the p of the solution at the absorption site.

7 IONIZATION of DRUGS Drug Absorption & Transport Depends on: Drug solubility Partition coefficient Ionization Drug Transport is a Compromise Between: Increased 2O solubility of ionized form Superior passage of unionized (undissociated)

8 IONIZATION of DRUGS IN GENERAL: drugs pass through membranes in undissociated form but act as ions, if possible pka range of 6 8 seems most favorable (for passive transport)

9 ow does p vary in the body? Fluid p Aqueous humour 7.2 Blood 7.4 Colon 5-8 Duodenum (fasting) Duodenum (fed) Saliva 6.4 Small intestine 6.5 Stomach (fasting) Stomach (fed) 3-7 Sweat 5.4 Urine So the same compound will be ionised to different extents in different parts of the body. This means that, for example, basic compounds will not be so well absorbed in the stomach than acidic compounds since it is generally the unionised form of the drug which diffuses into the blood stream.

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11 CALCULATION OF IONIZATION Ionizable drugs (weak acids & bases) do so, depending upon: Dissociation constant (pka) p of the environment

12 WEAKLY ACIDIC DRUGS A + 2 O Ka 3 O acid 1 base 2 conjugate acid 2 + A conjugate base 1 [ 3 O] [A] Ka = [A] pka = p + log [A] enderson-asselbalch [A]

13 For Ka an acid: Ionisation constants The equilibrium between un-ionised and ionised forms is defined by the acidity constant Ka or pka = -log10 K a A + + A Ka = [+][A-] [A] 100 % ionised = (pKa - p) For a base: B + K a + + B Ka = [+][B] [B+] 100 % ionised = (p - pka) When an acid or base is 50% ionised: p = pka

14 WEAKLY ACIDIC DRUGS pka = p + log [A] [A] % ionized = [ionized] [ ionized] + [ unionized] x 100 % ionized = antilog (pka - p) % ionized = (pka - p)

15 WEAKLY BASIC DRUGS Autoprotolysis Constant of Water (Kw) 2 2 O Kw 3 O + O Kw = [ 3 O] [O] [ 2 O] pkw = pka + pkb Kw = Ka x Kb 2 Kw = [ 3 O] [O] use this equation to define the Kb term on next slide

16 WEAKLY BASIC DRUGS B + 2 O Kb B + O base 1 acid 2 conjugate acid 1 conjugate base 2 Kb = % ionized = [B] [O] [B] antilog (p - pka) pka = p % ionized = log (p - [B] [B] pka)

17 WEAKLY ACIDIC DRUGS % ionized = (pka - p) pka p % ionized / = 100/1.01 = 99.01% / = 100/1.1 = 90.91% = 50% /1+101 = 100/11 = 9.09% /1+102 = 100/101 = 0.99%

18 WEAKLY BASIC DRUGS % ionized = (p - pka) pka p % ionized /1+102 = 100/101 = 0.99% /1+101 = 100/11 = 9.09% = 50% / = 100/1.1 = 90.91% / = 100/1.01= 99.01%

19 IONIZATION SUMMARY Remember For an acid drug, the smaller the pka, the stronger the acid For a basic drug, the larger the pka (i.e. the smaller the pkb), the stronger the base

20 IONIZATION SUMMARY A useful relationship pkw = pka + pkb Acid strength may be expressed as Ka or Kb of its conjugate base. Ka of an acid may be calculated if Kb is known. Stronger the acid, the weaker its conjugate base.

21 knowing pka and p allows determination of % ionization Acidic groups 100% ionized when p is 2 units above pka Basic groups 100% ionized when p is 2 units below pka

22 Acidic drugs that are highly ionized at p 7.4 Drug pka Penicillin G 2.76 ASA 3.49 All > 99% ionized Sulfisoxazole 5.00

23 Basic drugs that are highly ionized at p 7.4 Drug pka Atropine 9.65 Procaine 8.80 Chlorcyclizine 8.15 All > 85% ionized

24 Groups on receptors may also be highly ionized at p 7.4 Acidic groups pka aspartic acid 3.7 glutamic acid 4.30 phosphoryl 1.00 All > 99% ionized

25 Groups on receptors may also be highly ionized at p 7.4 Basic groups pka arginine 12.5 lysine 10.5 glutamine 9.10 All > 98% ionized

26 IONIZATION Examples propranolol O OC 2 CC 2 NCMe 2 pka = 9.45 What does this pka refer to? i.e. is there an acidic functional group? Is drug acidic, basic, amphoteric? B + 2 O Kb B + O base 1 acid 2 conjugate acid 1 conjugate base 2 What is the pkb? 4.55

27 IONIZATION Examples sulfasalazine What do the pka s refer to? N pka = 2.4, 9.7, 11.8 COO NSO 2 N N O i.e. are there acidic and/or basic functional groups? A + 2 O Ka 3 O + A acid 1 base 2 conjugate acid 2 conjugate base 1

28 IONIZATION of POLYPROTIC DRUGS Polyprotic acids donate >1 proton Each dissociation stage has an equilibrium expression and therefore pka. Et O Ph N O N O pka1 7.4 pka2 ~ It becomes progressively more difficult to donate protons phenobarbital

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31 Methamphetamine, with a pka of 9.87, is dissolved in a solution at p 7.87:

32 Diethylbarbituric has an unionized :B form, and an ionized B: form. It has a pka of 8.0, and is dissolved in fluid with a p of 7:

33 Change of the ionization state will affect: 1. Movement from aqueous phase to lipid upon crossing a membrane 2. Movement from aqueous phase to hydrophobic binding pocket 3. Movement from aqueous phase to location adjacent to a charged or polar residue in an active site 4. In order to elicit a pharmacological effect, drugs must be sufficiently soluble in water to be absorbed and distributed throughout the body. They must also have sufficient lipophilicity to be able to pass through biological membranes.

34 Water Solubility and ydrogen Bonding A stronger and important form of chemical bonding is the dipole-dipole bond, specific example of which is the hydrogen bond. A dipole results from the unequal sharing of a pair of electrons making up a covalent bond. This occurs when the two atoms making up the covalent bond differ significantly in electro-negativity. N... O O... S R ydrogen Bond ydrogen bonding of an amine to water and a thiol to water

35 Water has a dipole moment, due to the degree bond angle, and the pull of electronegative oxygen on the attached hydrogens. This induced polarity gives water a higher boiling point and melting point than other hydrides (e.g. -S-, hydrogen sulfide, is a gas at room temperature). This dipole also allows water to hydrogen bond, and in pure water, it -bonds to itself, forming a lattice (network).

36 Ionized molecules carry charge and favor interaction with water dipoles making these molecules water-soluble. Other molecules, e.g., glucose, are not charged, but, have an uneven electron density and are thus polar molecules that interact with water dipoles and are freely soluble in water. This association with water molecules makes these watersoluble compounds less soluble in oils, fat, and lipid. These types of molecules are said to be hydrophilic (water-liking). In contrast, nonpolar and noncharged molecules tend to be much more lipid-soluble or hydrophobic (water-hating) or lipophilic (lipid-liking).

37 Lipophilicity Lipophilicity ( fat-liking ) is the most important physical property of a drug in relation to its absorption, distribution, potency, and elimination. Lipophilicity is often an important factor in all of the following, which include both biological and physicochemical properties: Solubility Absorption Plasma protein binding Metabolic clearance Volume of distribution Enzyme / receptor binding Biliary and renal clearance CNS penetration Storage in tissues Bioavailability Toxicity

38 The hydrophobic effect This is entropy driven (remember δg = δ TδS). ydrophobic molecules are encouraged to associate with each other in water. Placing a non-polar surface into water disturbs network of water-water hydrogen bonds. This causes a reorientation of the network of hydrogen bonds to give fewer, but stronger, water-water -bonds close to the non-polar surface. Water molecules close to a non-polar surface consequently exhibit much greater orientational ordering and hence lower entropy than bulk water. Molecular interactions why don t oil and water mix? O O O O O O O O O O O O O O

39 The hydrophobic effect This principle also applies to the physical properties of drug molecules. If a compound is too lipophilic, it may be insoluble in aqueous media (e.g. gastrointestinal fluid or blood) bind too strongly to plasma proteins and therefore the free blood concentration will be too low to produce the desired effect distribute into lipid bilayers and be unable to reach the inside of the cell Conversely, if the compound is too polar, it may not be absorbed through the gut wall due to lack of membrane solubility. So it is important that the lipophilicity of a potential drug molecule is correct. ow can we measure this?

40 Partition Coefficient (Lipid/Water Partition Coefficient) Ø It is a means of expressing a drug's solubility is lipid versus water. A drug is added to a two-phase solution of oil (or other organic solvent like 1-octanol) and water, mixed, and the concentration of drug in the organic and water phases determined. The ratio of the two phases reflects the relative lipid/water solubility. ow does one determine a drug s partition coefficient? 1. Add drug 2. Equilibrate 3. Determine [Drug] org and [Drug] aqu Organic phase Aqueous phase 4. Calculate K org/aqu Mathematically, K org/aqu = [Drug] org [Drug] aqu

41 Lipid-Water Partition Coefficient The ratio of the concentration of the drug in two immiscible phases: a nonpolar liquid or organic solvent (representing the membrane); and an aqueous buffer, p 7.4 (representing the plasma)

42 Lipid-Water Partition Coefficient The higher the lipid/water p.c. the greater the rate of transfer across the membrane polarity of a drug, by increasing ionization will the lipid/ water p.c. polarity of a drug, suppression of ionization will the lipid/ water p.c.

43 A drug s partition coefficient, Korg/aqu is an index of the drug s lipophilicity. Log P = 1 means 10:1 Organic:Aqueous Log P = 0 means 1:1 Organic:Aqueous Log P = -1 means 1:10 Organic:Aqueous In general, assuming passive absorption Optimum CNS penetration around Log P = 2 +/- 0.7 Optimum Oral absorption around Log P = 1.8 Optimum Intestinal absorption Log P =1.35 Optimum Colonic absorption Log P = 1.32 Optimum Sub lingual absorption Log P = 5.5 Optimum Percutaneous Log P = 2.6 (& low mw)

44 What else does logp affect? logp Binding to enzyme / receptor Aqueous solubility Binding to P450 metabolising enzymes So log P needs to be optimised Absorption through membrane Binding to blood / tissue proteins less drug free to act Binding to herg heart ion channel -cardiotoxicity risk

45 The partition ratio of a given drug will determine its solubility in plasma, its ability to traverse cell membranes, and which tissues it will reach. Drugs must have some aqueous solubility since this is essential for absorption through membranes, and for the production of an adequate concentration at the site-of-action. A balance between hydrophilicity and lipohilicity is necessary. This must be taken into account when chemically modifying a drug for optimal activity.

46 The Ferguson Principle

47 The Ferguson Principle ü ü ü Ferguson s suggested in 1939, the potency of drug for nonspecific drugs is determined by its thermodynamic activity. it states that the concentration of drug is directly proportional to its activity. Concentration of drug can be measured by either Molarity (or) Partial pressure. The relationship between physicochemical properties and drug action Theoretical representations The hypothesis states that, the higher the partition ratio P, the higher the pharmacological effect. ü Ferguson Constant is represented by X where:

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49 Ø igh thermodynamic activity means that the activity of the drug is based on its physicochemical properties only, such as in a gaseous anesthetic. Such drugs are known as non-specific agents. Ø Low thermodynamic activity means that the activity of the drug is based on its structure rather than physicochemical properties. Ø Agents in this category are called specific agents, and their activity at low concentrations infers that they have a specific receptor.