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PHC 222 Part(I) Dr. Huda Al Salem. Lecture (10). How to improve water solubility?. 1- Salt formation. 1- Salt formation. -Salt formation usually improves the water solubility of acidic and basic drugs as the salts of these drugs dissociate in water.

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1 salt formation
1- Salt formation

-Salt formation usually improves the water solubility of acidic and basic drugs as the salts of these drugs dissociate in water.

-The degree of water solubility depends on the structure of the acid or base used to form the salt

1 salt formation1
1- Salt formation

Many GIT disturbances can be treated by ingestion of water-soluble compounds.

Example 1

Tartaric acid >>>>Na K tartrate is used as mild laxative

1 solubility
1- Solubility

- Salt formation is also used to change the taste of drugs to make them more palatable to the patient

Example 2:

Chlorpromazine HClis water soluble but it has a very bitter taste. However, the water insoluble embonate salt is tasteless.. So it can be administered orally in the form of a suspension

how to improve water solubility1

How to improve water solubility?

2- Incorporation of water-solubilizing groups

2 incorporation of water solubilizing groups

2- Incorporation of water-solubilizing groups

a- Type of group introduced

b- Degree of permanency

c- The biological effect of the group

d- Methods of introduction

a type of group introduced
a- Type of group introduced

1- Incorporation of strongly polar alcohol, amine, amide, carboxylic acid, sulphonic acid and phosphorus oxyacid groups give analogues with water solubility higher than those formed by introduction of ether, aldehyde & ketonic functional groups.

2- Introduction of acidic and basic groups give a wide range for dosage forms that increase water solubility.

a type of group introduced1
a- Type of group introduced

3- Zwitterions reduces water solubility.

4- Incorporation of weakly polar groups such as carboxylic acid esters, aryl halides & alkyl halides increases lipid solubility.

b degree of permanency
b- Degree of permanency

1- Groups that are bound directly to the carbon skeleton of the lead compound by C-C, C-O & C-N bonds are attached to by irreversible bonds.

2- Groups that are linked to the lead compound by ester, amide, phosphate & glycosidic links are more likely to be metabolized to reform the parent lead compound.

c the position of the group
C- The position of the group

In order to preserve the type of activity exhibited by the lead, the introduced group should be attached to a part of the structure that is not involved in the drug-receptor interaction (Pharmacophore).

c the biological effect of the group
C- The biological effect of the group

Some groups have certain biological activity..

1- acidic groups exhibit haemolytic properties.

2- aromatic acid groups exhibit anti-inflammatory activity

3- carboxylic acids with an alpha functional group exhibit chelation property.

4- Basic groups have a tendency to change the mode of action

d methods of introduction
d- Methods of introduction

1-Water-solubilising groups may be introduced at any stage in the synthesis of a drug.

2-Many methods involve the use of protecting groups.

3- Groups protected are either the water- solubilizing group or groups already present in the lead structure.

d methods of introduction1
d- Methods of introduction

e.g.:

Acetal protection of a ketone during Reductionof an Ester.

d methods of introduction2
d- Methods of introduction

1- COO- by acylation

Chloramphenicol sodium succinate is supplied as a lyophilized powder which is dissolved only in water when needed & not more than 48 hours

d methods of introduction3
d- Methods of introduction

2- Sulphonic acid groups (-SO3H)

d methods of introduction4
d- Methods of introduction

3- Basic groups

-By alkylation or acylation

-Amide derivatives are usually more stable than esters in aqueous solutions

d methods of introduction5
d- Methods of introduction

4- Polyhydroxy groups

solubility

Solubility

3- Formulation Methods

3 formulation methods

3- Formulation Methods

Cosolvents

Colloidal Solutions

3 formulation methods1
3- Formulation Methods

A-Cosolvents

a second solvent added to the original solvent, in small concentrations, to form a mixture that dissolve the solute.

Requirements

1- Minimal toxic effect

2- Should not affect stability of the drug

Example

Paracetamol elixir in an aqueous solution

is dissolved by the use of mixture of

ethanol & 1,2-dihydroxypropane,

3 formulation methods2
3- Formulation Methods

B-Colloidal Solutions

Prepared by dissolving a high concentration of the drug in an organic solvent that is miscible with water. Then, the concentrated solution is rapidly mixed with an aqueous solution containing a suitable stabilizer which adsorbed on the surface of the colloidal particles.

Ex: Shaving Cream (Foam) (g in L)

Mayonnaise (L in L)

Blood (S in L)