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This chapter will be most useful after having a basic understanding of the material in Chapter 52, Sulfonamides, Trimethoprim-Sulfamethoxazole, Quinolones, and Agents for Urinary Tract Infections in Goodman & Gilman’s The Pharmacological Basis of Therapeutics, 12th Edition. In addition to the material presented here, the 12th Edition contains:

  • Structural formulas for each of the drugs in this chapter, in addition to the figures reproduced here

  • Table 52-2 which is a compilation of the structural formulas for selected quinolones and fluoroquinolones


  • Understand the mechanism of action of sulfonamide drugs.

  • Identify the various sulfonamide drugs and categorize them according to their absorption from the gastrointestinal (GI) tract.

  • Identify the therapeutic uses and untoward effects of sulfonamide drugs including trimethoprim-sulfamethoxazole.

  • Describe the therapeutic uses, mechanisms of action, and toxicities of quinolone antibiotic drugs.

  • Identify the uses and limitations of antiseptic and analgesic drugs for the treatment of urinary tract infections.


Mafenide (SULFAMYLON) Methenamine

Nitrofurantoin (FURADANTIN, MICROBID, others)

Phenazopyridine (PYRIDIUM, others)

Quinolones (norfloxacin [NOROXIN, others], ofloxacin, [FLOXIN, others], ciprofloxacin [CIPRO, others], moxifloxacin [AVELOX])

Silver sulfadiazine (SILVADENE, others)



Sulfadoxine (FANSIDAR)


Sulfasalazine (AZULFADINE, others)


Trimethoprim-sulfamethoxazole (BACTRIM, SEPTRA, others)


Resistance to sulfonamides is the consequence of altered enzymatic constitution of the bacterial cell characterized by:

  • A lower affinity of dihydropteroate synthesis enzymes.

  • Decreased bacterial permeability or active efflux of the drug.

  • An alternative metabolic pathway for synthesis of an essential metabolite.

  • Increased production of an essential metabolite or drug antagonist.


Resistance to quinolines may develop during therapy via mutations in the bacterial chromosomal genes encoding DNA gyrase or topoisomerase IV (see Figure 38-3), or by active transport of the drug out of the bacteria.

The mechanism of action of sulfonamides is shown in Figure 38-2 and the resistance to sulfonamides is described in the side bar BACTERIAL RESISTANCE TO SULFONAMIDES

The mechanism of action of quinolines is shown in Figure 38-3 and the resistance to quinolines is described in the side bar BACTERIAL RESISTANCE TO QUINOLINES.

CASE 38-1

A 56-year-old woman presents with symptoms of her second urinary tract infection within 2 months. She has no fever and her white blood cell count is not elevated. A previous culture showed Escherichia coli, and she responded well to the combination of trimethoprim-sulfamethoxazole. After obtaining the appropriate cultures and sensitivities you decide to treat her again with this combination.

  1. What is unique about the mechanism of action of this combination that makes it effective in treating bacterial infections?

    The sulfonamides can be classified on the basis of the rapidity with which they are absorbed and excreted (see Table 38-1). The structural formulas of selected members of this class are shown ...

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