Penicillins

Chapter 59


Penicillins






INDICATIONS


See Table 59-1 for specific indications.



TABLE 59-1


Penicillin Indications With Dosage and Administration Recommendations



















































































































Drug Bacteria Site/Disease Dosage (oral)
penicillin G benzathine (IM only) Group A streptococcus Tonsil, URI, skin, soft tissue Adult: 1.2 million units as a single dose
Child (<60 lb): 300,000-600,000 units as a single dose
  T. pallidum Syphilis (venereal and congenital; not indicated as a single dose for neurosyphilis but can be given weekly × 3 weeks following IV treatment) Adult, child >12 years: 2.4 million units as a single dose
Child <2 years old: 50,000 units/kg as a single dose
penicillin V Streptococcus URI, scarlet fever, erysipelas Adult and child >12 years: 125-250 mg q6-8h × 10 days
  Streptococcus Pharyngitis in children Child <12 years old: 25-50 mg/kg/day divided q6h × 10 days
  Pneumococcus URI, otitis media 250-500 mg q6-8h until afebrile × 2 days minimum
  Staphylococcus
Group A streptococcus		 Skin and soft tissue
Prevention of recurrence following rheumatic fever		 250-500 mg q6-8h
250 mg bid continuous
dicloxacillin Penicillinase-resistant Staphylococcus Mild to moderate infection Adult: 125 mg q6h
Child: 12.5 mg/kg/day in divided doses q6h
    Severe infection Adult: 125-500 mg q6h
Child: 25 mg/kg/day divided doses q6h
oxacillin Penicillinase-resistant Staphylococcus Mild infection Adult: 500 mg q4-6h
Child <40 kg: 50 mg/kg/day in divided doses q6h
    Severe infection (following parenteral therapy) Adult: 1 g q4-6h
Child <40 kg: 100 mg/kg/day in divided doses q4-6h
ampicillin H. influenzae, Staphylococcus, S. pneumoniae, spp. Shigella, E. coli, Salmonella, P. mirabilis, enterococci N. gonorrhoeae URI, soft tissue
GI, GU infections		 ≤20 kg: 50 mg/kg/day in divided doses q6-8h
>20 kg: 250 mg q6h
Adult, child >20 kg: 500 mg q6h
Child ≤20 kg: 100 mg/kg/day q6h 3.5 g as a single dose plus 1 g probenecid
amoxicillin ENT: Streptococcus α- and β-hemolytic, S. pneumoniae, Staphylococcus spp, H. influenzae Mild/moderate Adult, child >40 kg: 500 mg q12h or 250 mg q8h
Child >3 mo and <40 kg: 25 mg/kg/day in divided doses q12h or 20 mg/kg/day in divided doses q8h
    Severe 875 mg q12h or 500 mg q8h
45 mg/kg/day in divided doses q12h or 40 mg/kg/day in divided doses q8h
  Skin: Streptococcus spp, α- and β-hemolytic, Staphylococcus spp, E. coli Mild/moderate 500 mg q12h or 250 mg q8h
25 mg/kg/day in divided doses q12h or 20 mg/kg/day in divided doses q8h
    Severe Adult, child >40 kg: 875 mg q12h or 500 mg q8h
Child >3 mo and <40 kg: 45 mg/kg/day in divided doses q12h or 40 mg/kg/day in divided doses q8h
  GU tract: E. coli, P. mirabilis, Enterococcus faecalis Mild/moderate Adult: 500 mg q12h or 250 mg q8h
Child: 25 mg/kg/day in divided doses q12h or 20 mg/kg/day in divided doses q8h
Severe, Adult, child >40 kg: 875 mg q12h or 500 mg q8h
Severe, Child: 45 mg/kg/day in divided doses q12h or 40 mg/kg/day in divided doses q8h
  Streptococcus spp, α- and β-hemolytic, S. pneumococcus, Staphylococcus spp, H. influenzae Lower respiratory tract (mild/moderate/severe) Adult: 875 mg q12h or 500 mg q8h
Child >3 mo and <40 kg: 45 mg/kg/day in divided doses q12h or 40 mg/kg/day in divided doses q8h
  N. gonorrhoeae Gonorrhea Adult: 3 g as single oral dose
Prepubertal children >2 yr: 50 mg/kg with probenecid 25 mg/kg as single dose
amoxicillin and potassium clavulanate H. influenzae, M. catarrhalis Otitis media, sinusitis Adult, child >40 kg: 500 mg q12h or 250 mg q8h
Child <3 mo: 30 mg/kg/day divided q12h (duration otitis media 10 days)
Child >3 mo: 200 mg/5 ml suspension: 25 mg/kg/day divided q8h; 125 mg/5 ml suspension: 20 mg/kg/day divided q8h
  H. influenzae, M. catarrhalis Severe infection, lower respiratory tract Adult: 875 mg q12h or 500 mg q8h
Child >3 mo: 200 mg/5 ml suspension: 45 mg/kg/day divided q8h; 125 mg/5 ml suspension: 40 mg/kg/day divided q8h
Augmentin XR H. influenzae, M. catarrhalis, H. parainfluenzae, K. pneumoniae or methicillin-susceptible S. aureus, S. pneumoniae Pneumonia, sinusitis 4000/250 mg/day given as two tablets q12h × 10 days; sinusitis, × 7 to 10 days for pneumonia


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Sir Alexander Fleming, a British scientist, discovered penicillin in 1928, and the medicine has been available since the 1940s. Since that time, penicillins have remained an extremely important class of antibiotics for treating most gram-positive bacteria; some penicillins can treat gram-negative bacteria. Penicillins are considered the safest antibiotics. Important penicillins for the primary care provider include penicillin VK, amoxicillin, amoxicillin/clavulanate, and dicloxacillin. These generally are indicated in the treatment of mild to moderately severe infections caused by penicillin-sensitive microorganisms. They are used extensively for empirical therapy and for treatment determined by culture. Different penicillins exhibit different antimicrobial activity. This chapter focuses on the outpatient use of oral penicillins. Nafcillin is available as IV treatment only and is not discussed further. The carboxypenicillins and the ureidopenicillins are active against a large number of infectious diseases. These generally are given IV in a hospital setting and are not discussed further. Other β-lactam drugs, the carbapenems (e.g., ertapenem, imipenem, and meropenem), are similar in chemical structure to the penicillins. These drugs are used in hospitals and are given IM or IV for severe infections; these also will not be discussed further.



Therapeutic Overview


Allergy


Before prescribing any penicillin, check for drug allergy. The incidence of penicillin allergy is unknown but is probably between 5% and 10%. Life-threatening anaphylaxis is estimated at 0.01% to 0.05%. A patient’s report of penicillin allergy correlates poorly with a positive skin test. A patient with a negative skin test has a very low probability of an allergic reaction to penicillin. Among patients with a positive skin test for penicillin, less than 5% will have an immediate reaction to a cephalosporin. Current recommendations are that if a penicillin skin test is positive, the patient should avoid β-lactam antimicrobials or should be considered for penicillin or cephalosporin desensitization. Cross reactivity to carbapenems is also possible; therefore, a carbapenem may not be a reasonable choice for penicillin-allergic patients.



Resistance


Microbial resistance to the penicillins, now common, has become a major limitation to their use. Resistance of bacteria to penicillins and other β-lactam antibiotics occurs through three mechanisms. The most important mechanism involves bacteria-producing β-lactamase, which breaks down the β-lactam ring and renders the penicillin inactive. Staphylococcus aureus is one of the most important of these organisms. Methicillin-resistant Staphylococcus aureus (MRSA) is now resistant to most antibiotics and is very difficult to treat. Methicillin is no longer marketed in the United States because of resistance and toxicity. Inhibitors of the β-lactamases such as clavulanic acid, sulbactam, and tazobactam often are used in combination with certain penicillins, to prevent their inactivation.


The second mechanism of resistance is diminished permeability of the drug through the bacterial outer cell membrane. The antibiotic is unable to penetrate the cell wall, especially of gram-negative bacteria. The last mechanism involves decreased binding of the drug at its target sites on the inner bacterial membrane.

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Jul 22, 2016 | Posted by in PHARMACY | Comments Off on Penicillins

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