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A 55-year-old man is brought to the local hospital emergency department by ambulance. His wife reports that he had been in his normal state of health until 3 days ago when he developed a fever and a productive cough. During the last 24 hours he has complained of a headache and is increasingly confused. His wife reports that his medical history is significant only for hypertension, for which he takes hydrochlorothiazide and lisinopril, and that he is allergic to amoxicillin. She says that he developed a rash many years ago when prescribed amoxicillin for bronchitis. In the emergency department, the man is febrile (38.7°C [101.7°F]), hypotensive (90/54 mm Hg), tachypneic (36/min), and tachycardic (110/min). He has no signs of meningismus but is oriented only to person. A stat chest x-ray shows a left lower lung consolidation consistent with pneumonia. The plan is to start empiric antibiotics and perform a lumbar puncture to rule out bacterial meningitis. What antibiotic regimen should be started to treat both pneumonia and meningitis? Does the history of amoxicillin rash affect the antibiotic choice? Why or why not?

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Penicillins

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The penicillins share features of chemistry, mechanism of action, pharmacology, and immunologic characteristics with cephalosporins, monobactams, carbapenems, and β-lactamase inhibitors. All are β-lactam compounds, so named because of their four-membered lactam ring.

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Chemistry

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All penicillins have the basic structure shown in Figure 43–1. A thiazolidine ring (A) is attached to a β-lactam ring (B) that carries a secondary amino group (RNH–). Substituents (R; examples shown in Figure 43–2) can be attached to the amino group. Structural integrity of the 6-aminopenicillanic acid nucleus (rings A plus B) is essential for the biologic activity of these compounds. Hydrolysis of the β-lactam ring by bacterial β-lactamases yields penicilloic acid, which lacks antibacterial activity.

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Figure 43–1
Graphic Jump Location

Core structures of four β-lactam antibiotic families. The ring marked B in each structure is the β-lactam ring. The penicillins are susceptible to bacterial metabolism and inactivation by amidases and lactamases at the points shown. Note that the carba-penems have a different stereochemical configuration in the lactam ring that imparts resistance to most common β lactamases. Substituents for the penicillin and cephalosporin families are shown in Figures 43–2 and 43–6, respectively.

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Figure 43–2
Graphic Jump Location

Side chains of some penicillins (R groups of Figure 43–1).

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Classification
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Substituents of the 6-aminopenicillanic acid moiety determine the essential pharmacologic and antibacterial properties of the resulting molecules. Penicillins can be assigned to one of three groups (below). Within each of these groups are compounds that are relatively stable to gastric acid and suitable for oral administration, eg, penicillin V, dicloxacillin, and amoxicillin. The side chains of some representatives of each group ...

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