Question

Given below are two statements, one is labelled as Assertion (A) and other one labelled as Reason (R).
Assertion (A) : All clinically useful local anesthetics are weak bases, and exist in equilibrium between the neutral, non-ionized, lipid soluble, and the ionized (charged), water-soluble form.
Reason (R) : All the clinically important local anesthetics are formulated as acidic solutions of hydrochloride salts which are more highly ionized, and thus water soluble.
In light of the above statements, choose the correct answer from the options given below

• A. Both (A) and (R) are correct and (R) is the correct explanation of (A).
• B. Both (A) and (R) are correct but (R) is NOT the correct explanation of (A).
• C. (A) is true but (R) is false.
• D. (A) is false but (R) is true.

Hint:

Once injected into tissues (which have a physiological pH of ~7.4), the acidic local anesthetic solution is buffered, shifting the equilibrium to increase the non-ionized, lipid-soluble fraction, allowing it to penetrate the nerve sheath.
In infected or inflamed tissues (which are acidic), the drug remains ionized and cannot penetrate the nerve, reducing its anesthetic efficacy.

Answer 1

The Correct Option is B

Step 1: Understanding the Concept:
Local anesthetics prevent the generation and conduction of nerve impulses by blocking sodium channels on the neuronal membrane.
Their pharmacological activity, tissue penetration, and formulation stability are highly dependent on their chemical state and pH.

Step 2: Detailed Explanation:
Let us analyze the validity and relationship of both statements:
- Assertion (A):
Local anesthetics (such as lidocaine, bupivacaine, and procaine) are chemically weak bases.
In solution, they exist in a dynamic equilibrium between the uncharged, non-ionized, lipid-soluble form (which can readily cross lipophilic cell membranes) and the charged, ionized, water-soluble form (which interacts directly with the internal pore of the sodium channel to block it).
Therefore, Assertion (A) is correct.
- Reason (R):
Because the free base form of local anesthetics is highly insoluble in water and unstable when exposed to air, they are formulated commercially as acidic solutions of hydrochloride salts (usually with a pH of 4.0 to 6.0).
In this acidic environment, the weak base is highly ionized, which makes it stable and highly soluble in water inside the vial.
Therefore, Reason (R) is correct.
- Relationship between (A) and (R):
While both statements are factually correct, the reason why they are formulated as acidic hydrochloride salts (R) is to ensure stability and solubility in the vial.
This formulation does not explain the general chemical principle that weak bases exist in a pH-dependent thermodynamic equilibrium between ionized and non-ionized forms in tissue (A).
Rather, this equilibrium is a fundamental property governed by the Henderson-Hasselbalch equation.
Thus, (R) is not the correct explanation of (A).

Step 3: Final Answer:
The correct option is (B).