As a drug passes through the renal tubules, it will be either reabsorbed or eliminated in the urine. Which path it takes depends on the pKa of the drug and the pH of the urine. The term pKa is used to describe the strength of acids (the lower the pKa, the stronger the acid). The urine pH can range from 4.5 to 8.0.
The majority of drugs are weak organic acids or bases. For example, barbiturates are weak acids, whereas amphetamines and opiates are weak bases. In an aqueous environment, acids and bases exist either in an unionized form or in an ionized form. The following reactions demonstrate how a weak acid or base is converted to its ionized form.
Weak acid: RCOOH ↔ RCOO- + H+
Weak base: RNH2 + H+ ↔ RNH3+
Since an ionized form is charged (by definition), it will not easily cross a nonpolar lipid membrane. Thus, it is important to recognize the potential of the drug to ionize in order to predict its solubility and the degree to which it can be reabsorbed. The degree of ionization is determined by the drug’s pKa and the pH of its environment. Weak acids and bases are 50% ionized and 50% unionized when the surrounding pH equals the drug’s pKa. At 2 pH units above or below the pKa of the drug, nearly 100% of the drug is ionized or unionized.
