PHYSICOCHEMICAL PROPERTIES OF DRUGS INFLUENCING BIOLOGICAL ACTIONS

PHYSICOCHEMICAL PROPERTIES OF DRUGS INFLUENCING BIOLOGICAL ACTIONS (1) I- OBJECTIVES To recognize the role of organic functional groups to drug’s physichochemical properties To explain h ow physichochemical properties influence biological actions of drugs II- PROCEDURE Case Analysis A long-distance truck driver comes into the pharmacy complaining of seasonal allergies. He asks you to recommend an agent that will act as an antihistamine but that will not cause drowsiness. He regularly takes TUMS for indigestion because of the bad food that he eats while on the road. Refer from the given structures of drugs below. 1. Identify the functional groups present in Cetirizine and Clemastin, and evaluate the effect of each functional group on the ability of the drug to cross lipophilic membranes (e.g., blood-brain barrier). Based on your assessment of each agent's ability to cross the blood-brain barrier (and, therefore, potentially cause drowsiness), provide a rationale for whether the truck driver should be taking Cetirizine or Clemastin. Drug Functional Groups Hydrophilic/ Hydrophobic Characteristics Effect on absorption Effect on ability to cross Blood-Brain Barrier Cetirizine Aromatic Hydrocarbon Hydrophobic Halogenated Aromatic Hydrocarbon Hydrophobic Tertiary Amine Hydrophilic Ether Hydrophilic Carboxylic acid Hydrophilic Hydrocarbon Hydrophobic Clemastine Aromatic Hydrocarbon Hydrophobic Halogenated Aromatic Hydrocarbon Hydrophobic Tertiary Amine Hydrophilic Ether Hydrophilic Hydrocarbon Hydrophobic Ether Carboxylic Acid Tertiary Amines Aliphatic Hydrocarbons Aromatic Hydrocarbon Halogenated Aromatic Hydrocarbon Ether DF Aromatic Hydrocarbon Tertiary Amines Halogenated Aromatic Hydrocarbon Aliphatic Hydrocarbons INFLUENCING Cetirizine is a second-generation H1-antagonist and is purported to be nonsedating. Clemastine is a first-generation H1-antagonist and is considered to be a sedating antihistamine. Based on the structure evaluation process, both cetirizine and clemastine contain several hydrophobic functional groups that would facilitate their crossing the blood-brain barrier. Both molecules contain an ionizable amine that will be predominantly ionized in the plasma. A key structural difference between these drug molecules is the presence of a carboxylic acid in Cetirizine. This functional group is hydrophilic and will be predominantly ionized in the plasma and therefore, may limit the extent of absorption across the blood-brain barrier. In order to limit the degree of drowsiness, it would be appropriate to recommend Cetirizine (Zyrtec) for this patient. 2. Patanol is sold as an aqueous solution of the hydrochloride salt. Modify the structure above to show the appropriate salt form of this agent. This agent is applied to the eye to relieve itching associated with allergies. Describe why this agent is soluble in water and what properties make it able to be absorbed into the membranes that surround the eye. Olopatadine hydrochloride is water soluble 3. Consider the structural features of Zyrtec and Tavist. In which compartment (stomach [pH 1] or intestine [pH 6–7]) will each of these two drugs be best absorbed? Cetirizine: In both compartments there will be at least one ionized functional group. In the intestine roughly 50% of the time there will be two functional groups ionized, which will limit the extent of absorption from this site. Cetirizine is probably absorbed from both sites but is probably 4. TUMS neutralizes stomach acid to pH 3.5. Based on your answer to question 3, determine whether the truck driver will get the full antihistaminergic effect if he takes his antihistamine at the same time that he takes his TUMS. Provide a rationale for your answer. At this pH the carboxylic acid will become ionized and the extent of absorptiON