Ceohalosporin pharmacology details

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Cephalosporin Pharmacology

Sources: Katzung's Basic and Clinical Pharmacology, 16th Ed.; Lippincott Illustrated Reviews: Pharmacology

1. Structure & Classification

Cephalosporins are semi-synthetic beta-lactam antibiotics derived from 7-aminocephalosporanic acid. The core bicyclic structure consists of a beta-lactam ring fused to a dihydrothiazine ring (vs. the thiazolidine ring in penicillins).

Position 7 (acyl side chain): determines antibacterial spectrum and beta-lactamase stability
Position 3: determines pharmacokinetic profile (half-life, protein binding, route)

Figure: Structural features of cephalosporin antibiotics - the 7-position side chain governs activity, the 3-position governs PK (Lippincott Illustrated Reviews: Pharmacology)

2. Mechanism of Action

Same as all beta-lactams:

Bind penicillin-binding proteins (PBPs) - bacterial enzymes (transpeptidases, carboxypeptidases) involved in cross-linking the peptidoglycan cell wall
Inhibit transpeptidation - the final cross-linking step of peptidoglycan synthesis
Result: structural weakening of the cell wall → bactericidal (cell lysis via autolysins)
Activity is time-dependent: efficacy correlates with time the free drug concentration exceeds the MIC (fT > MIC)

3. Resistance Mechanisms

Three major mechanisms (Lippincott):

Mechanism	Details
Beta-lactamase production	Enzymes hydrolyze the beta-lactam ring. Gram-positives secrete them extracellularly; gram-negatives hydrolyze in the periplasmic space. Includes chromosomal (AmpC, often inducible by 2nd/3rd gen agents) and plasmid-mediated (ESBLs, KPC, MBL) enzymes
Decreased permeability	Reduced or absent outer membrane porins (especially in gram-negatives like P. aeruginosa). Efflux pumps (e.g., in K. pneumoniae) actively remove drug
Altered PBPs	Modified PBPs have lower affinity for beta-lactams. Classic example: MRSA - mecA gene produces PBP2a, which is resistant to most commercially available beta-lactams

4. Generations: Spectrum and Key Agents

First Generation

Drugs: Cefazolin (IV), Cephalexin (PO), Cefadroxil (PO)

Spectrum:

Excellent gram-positive coverage: S. aureus (MSSA), S. epidermidis, S. pneumoniae, S. pyogenes, anaerobic streptococci
Limited gram-negative: E. coli, K. pneumoniae, Proteus mirabilis (the "3 EKPs")
No activity against MRSA, enterococci, Pseudomonas, Enterobacter, Serratia, B. fragilis

Key uses:

Cefazolin: surgical prophylaxis (drug of choice), MSSA bacteremia, soft tissue infections
Cephalexin: oral UTIs, cellulitis, minor skin/soft tissue infections

Dosing (Katzung):

Cephalexin: 0.25-0.5 g PO q.i.d.; peak serum 15-20 mcg/mL
Cefazolin: 0.5-2 g IV q8h; peak serum ~185 mcg/mL after 1 g IV

Second Generation

Drugs: Cefuroxime, Cefaclor, Cefprozil (oral); Cefoxitin, Cefotetan (parenteral cephamycins)

Spectrum:

Retains gram-positive coverage of 1st gen
Extended gram-negative: adds H. influenzae, Moraxella, Klebsiella (including 1st-gen resistant strains), Neisseria
Cephamycins (cefoxitin, cefotetan): add Bacteroides fragilis and anaerobic coverage
None active against enterococci or Pseudomonas

Key uses:

Cefuroxime: respiratory tract infections, sinusitis, otitis media
Cefoxitin/cefotetan: intra-abdominal, pelvic infections (anaerobic coverage); surgical prophylaxis for colorectal procedures
Caution: Do NOT use for Enterobacter infections - chromosomal AmpC beta-lactamase readily selects resistant mutants

Dosing:

Cefuroxime oral: 250-500 mg twice daily
Cefuroxime IV: 75-125 mcg/mL after 1 g infusion

Third Generation

Drugs: Ceftriaxone, Cefotaxime (parenteral); Cefdinir, Cefixime, Cefpodoxime (oral); Ceftazidime (anti-pseudomonal)

Spectrum:

Markedly expanded gram-negative coverage, including Enterobacterales
CSF penetration: therapeutic levels achieved (used for meningitis)
Ceftazidime and ceftizoxime: active against P. aeruginosa
Reduced gram-positive coverage compared to 1st gen
Not active against MRSA, L. monocytogenes, C. difficile, enterococci

Key uses (Katzung):

Ceftriaxone: meningitis (N. meningitidis, S. pneumoniae, H. influenzae), gonorrhea, community-acquired pneumonia, Lyme disease (disseminated), empiric sepsis coverage; unique biliary excretion
Cefotaxime: meningitis, sepsis, serious gram-negative infections; metabolized by liver (useful in renal impairment)
Ceftazidime: Pseudomonas infections (pneumonia, bacteremia)
Oral 3rd gen (cefdinir, cefpodoxime): otitis media, community respiratory infections

Half-life: Ceftriaxone has a long half-life (~8 hours), allowing once-daily dosing

Fourth Generation

Drug: Cefepime (IV/IM only)

Spectrum:

Wide spectrum: gram-positives (streptococci, MSSA - NOT MRSA) + extensive gram-negatives including Enterobacter species and P. aeruginosa
More stable to chromosomal AmpC beta-lactamases than 3rd gen (zwitterionic structure allows rapid outer membrane penetration)
E. coli, K. pneumoniae, P. mirabilis, Enterobacter spp., P. aeruginosa

Key uses: Febrile neutropenia, nosocomial pneumonia, complicated UTIs, Pseudomonas infections, Enterobacter infections where 3rd gen may fail

Advanced Generation (5th Generation)

Drugs: Ceftaroline, Cefiderocol

Ceftaroline

Only beta-lactam in the US with activity against MRSA
Binds PBP2a (the modified PBP that confers MRSA resistance) and PBP of penicillin-resistant S. pneumoniae
Gram-negative coverage similar to ceftriaxone
Gaps: No Pseudomonas, ESBL-producers, Acinetobacter
Indications: complicated skin/soft tissue infections (cSSTI), community-acquired pneumonia (CAP)
Twice-daily dosing limits outpatient use

Cefiderocol (Siderophore Cephalosporin)

Novel mechanism: acts as a siderophore - binds extracellular ferric iron (Fe³+), then enters gram-negative bacteria via the iron uptake (active transport) pathway in addition to passive diffusion through porins
Achieves high intracellular concentrations
Active against multidrug-resistant gram-negatives, including those with KPC, OXA-type, and metallo-beta-lactamases (MBL)
Active against: MDR Enterobacterales, P. aeruginosa, Acinetobacter baumannii
No gram-positive or anaerobic activity
Indications: complicated UTIs (including pyelonephritis), hospital-acquired and ventilator-associated pneumonia
Warning: Associated with increased mortality in some carbapenem-resistant infections in one trial - reserve for limited/no alternative situations
Dose adjustment for augmented renal clearance (may need 4x daily dosing)

5. Pharmacokinetics

Figure: Administration and fate of the cephalosporins - most excreted unchanged in urine; ceftriaxone eliminated in bile (Lippincott Illustrated Reviews: Pharmacology)

Parameter	Details
Route	Most require IV/IM (poor oral absorption). Oral agents: 1st gen (cephalexin), some 2nd gen (cefuroxime, cefaclor), oral 3rd gen (cefdinir, cefixime)
Distribution	Distribute widely into most tissues. Most do NOT penetrate CSF except 3rd gen and cefepime
Elimination	Primarily renal (glomerular filtration + tubular secretion). Probenecid blocks tubular secretion, raising serum levels
Exceptions	Ceftriaxone: biliary elimination (~40%); Cefotaxime: hepatic metabolism to active desacetylcefotaxime
Half-lives	Variable: cephalexin ~1h; cefazolin ~1.8h; ceftriaxone ~8h (once daily dosing possible)
Renal adjustment	Required for most agents when CrCl is impaired (except ceftriaxone). See table below

Dosage adjustment for renal impairment (Katzung):

Drug	Normal dose	CrCl ~50 mL/min	CrCl ~10 mL/min
Cephalexin PO	0.25-0.5 g q6h	50%	25%
Cefazolin IV	0.5-2 g q8h	50%	25%
Ceftriaxone IV	1-2 g q12-24h	100%	100%

6. Adverse Effects

Allergy & Cross-reactivity

Hypersensitivity reactions (anaphylaxis, urticaria, fever, rashes, nephritis, granulocytopenia, hemolytic anemia) - similar to penicillins
Cross-allergenicity with penicillins is low (1-2%) overall - the beta-lactam nucleus differs enough
Cross-reactivity is most common when cephalosporins share identical R-side chains with penicillins (particularly 1st/2nd gen with aminopenicillins)
Patients with penicillin anaphylaxis should NOT receive 1st or 2nd gen cephalosporins; use 3rd/4th gen with caution in a monitored setting (Katzung)

Toxicity

Toxicity	Details
Local irritation	Pain at IM site; thrombophlebitis with IV infusion
Renal toxicity	Interstitial nephritis, tubular necrosis (uncommon)
Hypoprothrombinemia/Bleeding	Agents with methylthiotetrazole (MTT) group - cefotetan is the relevant current US agent. Prevented by vitamin K 10 mg PO twice weekly
Disulfiram-like reaction	MTT-containing agents (cefotetan) cause severe reaction with alcohol - alcohol must be avoided
Diarrhea / C. difficile	Disruption of gut flora; broad-spectrum agents are higher risk

7. Important Coverage Gaps (All Generations)

Cephalosporins as a class are NOT active against:

MRSA (except ceftaroline, cefiderocol has no gram-positive activity either)
Enterococci (inherent resistance)
Listeria monocytogenes
Clostridioides difficile
Atypicals (Mycoplasma, Chlamydia, Legionella - no cell wall)

8. Key Clinical Pearls

Clinical Scenario	Drug of Choice
Surgical prophylaxis	Cefazolin (1st gen)
MSSA bacteremia	Cefazolin (superior to antistaphylococcal PCN in tolerability)
Bacterial meningitis (community)	Ceftriaxone + vancomycin (empiric)
Gonorrhea (uncomplicated)	Ceftriaxone IM
Lyme disease (disseminated/CNS)	Ceftriaxone IV
Pseudomonas infection	Ceftazidime, cefepime, or cefiderocol (MDR)
MRSA infection	Ceftaroline (only beta-lactam option)
Febrile neutropenia	Cefepime
Intra-abdominal (anaerobes)	Cefoxitin or cefotetan (2nd gen cephamycins)
MDR gram-negative (last resort)	Cefiderocol

- Katzung's Basic and Clinical Pharmacology, 16th Ed., pp. 1247-1253 - Lippincott Illustrated Reviews: Pharmacology, pp. 955-962

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