Salbutamol (structure below, pKa 9.3):

Question

BlackTom AI · Accepted Answer

The prompt presents a question about Salbutamol with a given pKa of 9.3 and asks us to consider its membrane permeability.

First, consider the chemistry: Salbutamol contains a tertiary amine with a pKa around 9.3. At physiological pH (~7.4), the amine is predominantly protonated, so the molecule exists mainly in its ionized (BH+) form rather than the neutral (B) form. According to the Henderson–Hasselbalch principle for bases, the ratio BH+/B at pH 7.4 is about 10^(pKa−pH) ≈ 10^(9.3−7.4) ≈ 79, meaning the ionized form dominates by a large margin. This strongly affects membrane permeability, since charged species cross lipid membranes far less readily than neutral molecules.

Option analysis:
- If an option says that Salbutamol penetrates membranes readily when ionized: this would be incorrect. The ionized form does not cross membranes easily, so this statement contradicts fundamental pharmacokinetic principles about ionization and permeability.

- If an option asserts that Salbutamol crosses membranes primarily in the unionized form: this would be more accurate, but still incomplete without acknowledging that at physiological pH the unionized fraction is small (about 1–2%). The premise would be technically possible but not representative of typical in-body behavior given its pKa.

- If an option claims that permeability is independent of ionization state: this would be incorrect, because ionization dramatically alters lipophilicity and membrane passage, so permeability is strongly pH- and ionization-dependent.

- If an option suggests Salbutamol’s effectiveness or absorption relates to routes that bypass this limitation (e.g., inhalation delivering drug to lungs where local absorption differs): this would be contextually relevant, but it does not contradict the core principle that membrane crossing is less favorable in the ionized form.

In summary, the key takeaway is that Salbutamol, with a pKa of 9.3, exists mainly in the ionized form at physiological pH, and ionized forms cross membranes less readily. This underpins why systemic absorption via certain routes is limited and why inhaled administration is particularly advantageous for targeting the lungs.

PCOL2021 2025 Module 2 practice Quiz

Salbutamol (structure below, pKa 9.3):

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