First Aid USMLE Step 1: Pharmacology (Complete)

First Aid USMLE Step 1: Pharmacology
(Complete)

Penicillin G (IV and IM)

Penicillin M (oral) Correct Answer: *MOA:*

*β-lactam*: a structural analog of D-ala-D-ala (a normal component of cell walls) which binds and
inactivates bacterial transpeptidases (penicillin-binding protein, PBPs) to block peptidoglycan cross
linking



*Clinical use:*

Bactericidal for Gram-⊕ cocci/rods, Gram-⊖ cocci (Neisseria), and spirochetes



Mostly used for Gram-⊕ organisms (S. pneumoniae, S. pyogenes, Actinomyces). Also used for Gram-⊖
cocci (mainly N. meningitidis) and spirochetes (namely T. pallidum). Bactericidal for Gram-⊕ cocci,
Gram-⊕ rods, Gram-⊖ cocci, and spirochetes. Penicillinase sensitive.



*Toxicity:*

Hypersensitivity reactions (type II), hemolytic anemia



*MOR:*

Penicilinase (a type of β-lactamase) in bacteria cleaves



Oxacillin

Nafcillin

Dicloxacillin Correct Answer: Penicillinase-resistant penicillins

*MOA:*

,Same as penicillin.

*Narrow spectrum*; penicillinase resistant because bulky R group blocks access of β-lactamase to β-
lactam ring.



*Clinical use:*

Bactericidal, narrow spectrum: *S. aureus* (except MRSA; resistant due to altered penicillin-binding
protein target site).



*Toxicity:*

Hypersensitivity reactions (type II), interstitial nephritis



"Use *naf* (*naf*cillin) for staph." Nafcillin has penetration to CNS



Aminopenicillins:

Ampicillin

Amoxicillin Correct Answer: Penicillinase-sensitive penicillins

*MOA:*

Same as penicillin.

Wider spectrum with better activity against Gram-⊖ bugs; penicillinase sensitive.

Administer with clavulanic acid to protect against destruction by β-lactamase.



*Clinical use:*

Bactericidal for Gram-⊕ cocci/rods, Gram-⊖ cocci, spirochetes; Extended-spectrum penicillin: *H.*
influenzae, *H.* pylori, *E.* coli, *L*isteria monocytogenes, *P*roteus mirabilis, *S*almonella,
*S*higella, enterococci. *HHELPSS*



*Toxicity:*

,Hypersensitivity reactions (type II), ampicillin rash (often with EBV infection- reaction-not an allergy),
pseudomembranous colitis



*MOR:*

β-lactamase activity



Ticarcillin

Piperacillin Correct Answer: Ureidopenicillins: "Antipseudomonals"

*MOA:*

Same as penicillin. Extended spectrum.



*Clinical use:*

*Pseudomonas spp.* and Gram-⊖ rods

Hospital-acquired pneumonias (Klebsiella, H. influenzae, Enterobacter)

Susceptible to penicillinase; use with β-lactamase inhibitors.



*Toxicity:*

Hypersensitivity reactions (type II)



Clavulanic Acid

Sulbactam

Tazobactam Correct Answer: *MOA:*

β-lactamase inhibitors



*Clinical use:*

, Often added to penicillin antibiotics (aminopenicillins, antipseudomonals) to protect the antibiotic from
destruction by β-lactamase (penicillinase).



"*CAST*" - *C*lavulanic *A*cid, *S*ulbactam, *T*azobactam



Cefadroxil

Cephalexin

Cephradine (PO)

Cefazolin (IV)

Cefazolin

Cephalexin Correct Answer: *1st Generation Cephalosporins*

*MOA:*

Same as that of penicillins



*Clinical use:*

Gram-⊕ cocci: streptococci, methicillin-susceptible staphylococci

Community-acquired strains of enteric Gram-⊖ rods, *P*roteus spp., *E. c*oli, *K*lebsiella spp.
(*PEcK*)

Cefazolin: used prior to surgery to prevent S. aureus wound infections.



*Toxicity:*

Hypersensitivity reactions, autoimmune hemolytic anemia, disulfiram-like reaction, vitamin K deficiency.
Exhibit cross-reactivity with penicillins. Potentiate the nephrotoxicity of aminoglycosides.



*MOR:*

Structural change in penicillin-binding proteins (transpeptidases).