Promethazine HCl SAR, Physicochemical Properties, IUPAC Nomenclature, Mechanism of action and Uses

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IUPAC nomenclature

Chemical Structure

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Physicochemical Properties
1. Chemical formula: C17H20N2S·HCl
2. Molecular Weight: 320.88 g/mol.
3. Solubility: Promethazine HCl is soluble in water and slightly soluble in alcohol. Its solubility in water makes it suitable for oral and intravenous administration in various pharmaceutical formulations.
4. Melting Point: The melting point of promethazine HCl is approximately 146-147°C (295-297°F).
5. pKa: Promethazine is a weak base. The pKa (acid dissociation constant) of its basic nitrogen atom is around 9.23, indicating that it can be protonated under acidic conditions.
6. Appearance: Promethazine HCl is usually found as a white to faint yellow, crystalline powder. It is odorless and has a slightly bitter taste.
7. Stability: It should be stored in a tightly closed container, protected from light, in a cool, dry place. The drug is stable under normal conditions but may degrade if exposed to extreme heat, moisture, or light.

Structural Activity Relationship (SAR)
1. Aliphatic Side Chain: Promethazine contains an aliphatic side chain attached to the nitrogen atom. This side chain is essential for the molecule’s pharmacological activity, allowing it to interact with specific receptors in the body.
2. Tertiary Amine Group: Promethazine is a tertiary amine, meaning it has three alkyl groups attached to the nitrogen atom. This tertiary amine group is crucial for its interaction with histamine receptors. By acting as an antagonist at the histamine H1 receptor, promethazine blocks the effects of histamine, thereby providing its antihistaminic properties.
3. Phenothiazine Backbone: Promethazine belongs to the phenothiazine class of compounds. The phenothiazine ring structure is fundamental to its antihistaminic and other pharmacological activities. The presence of this scaffold provides promethazine with its unique pharmacological properties.
4. Substituents on the Phenothiazine Ring: Specific substituents on the phenothiazine ring can affect the drug’s potency and selectivity for histamine receptors. Different substitutions can lead to variations in activity and potential side effects.
5. Hydroxyl Group: Promethazine contains a hydroxyl group (-OH) at the C10 position of the phenothiazine ring. This hydroxyl group can influence the drug’s interactions with other molecules and may contribute to its overall pharmacological profile.
6. Aromatic Rings: The presence of multiple aromatic rings in the promethazine molecule contributes to its overall stability and three-dimensional structure, which are important for receptor binding.
7. Substituents on the Aromatic Rings: The specific substituents on the aromatic rings can also influence the drug’s activity and selectivity. Different substituents can lead to variations in receptor affinity and efficacy.
8. Ether Linkage: The presence of ether linkage in the side chain is another structural feature of promethazine. While the exact role of this linkage in the drug’s activity is not specified in the provided information, it is a notable structural aspect.

Mechanism of Action
Promethazine HCl is an antagonist at histamine H1 receptors. Histamine is a chemical released during an allergic reaction, causing symptoms like itching, sneezing, and inflammation. By antagonizing the H1 receptors, promethazine blocks the effects of histamine. This action is responsible for its antihistaminic properties, making it effective in relieving allergy symptoms.

1. Sneezing
2. Runny nose
3. Watery eyes
4. Allergy symptoms
5. Itching
6. Inducing sleep

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