In treating ear infections with antibiotics, topical delivery has a number of advantages over systemic delivery. These advantages include the following:
A vastly higher concentration of antibiotic can be delivered to the site of infection.
Medications delivered topically have no systemic effects.
Topical delivery allows alteration of the local microenvironment.
Ototopical medications are usually less expensive than comparable systemic medications.
A 0.3% antibiotic solution contains 3000 mcg/mL of antibiotic. For comparison, consider the following commonly accepted middle ear concentrations after oral administration of antibiotics.
Amoxicillin at 90 mg/kg (6-10 mcg/mL)
Cefuroxime at 500-mg dose (2-4 mcg/mL)
Cefpodoxime at 200-mg dose (1-2 mcg/mL)
Clarithromycin at 500-mg dose (2-5 mcg/mL)
Even parenteral ceftriaxone, which can deliver an exceptionally high middle ear fluid level (35 mcg/mL), cannot begin to compete with the concentration of antibiotic delivered into the middle ear by topical delivery.
These high concentrations are pharmacodynamically important for antibiotics known to have a concentration-dependent mechanism of action. Aminoglycosides and quinolones are both concentration-dependent drugs. Consequently, bacteriocidal activity and lethality of bacteriocidal kill is progressively enhanced by the extent to which the delivered concentration exceeds the minimal inhibitory concentration (MIC). Although the MIC of ciprofloxacin for Pseudomonas is reported to be as high as 256 mcg/mL, this level is not the norm. End-point MICs rarely exceed 64 mcg/mL, even for highly resistant Pseudomonas.
Consequently, the concentration of delivered antibiotic, when topical administration is used, is always well above the MIC of the relevant organism. This makes the emergence of bacterial resistance extremely improbable. The possibility for the emergence of resistance seems to be vastly lower when topical routes of administration are used as compared with drugs that are administered systemically.
An important consequence of the high concentration of antibiotics delivered when topical preparations are used is the recognition that MICs reported by clinical laboratories become irrelevant. The clinical laboratory determination of resistance is based entirely on the drug level that can be achieved by systemic administration. A Pseudomonas organism with an MIC of 8 mcg/mL for ciprofloxacin is considered resistant. Clearly, however, the same organisms are rapidly killed by 0.3% topical solution containing 3000 mcg/mL.
Characteristic of topical delivery systems is the absence of systemic effects, which is an advantage if no systemic effect is required. Systemic toxicity requires systemic exposure. Consequently, systemic reaction to topical antibiotics is extremely uncommon. Because no appreciable systemic delivery of topically administered agents occurs, the normal flora in the respiratory and gastrointestinal tracts is not exposed to antibiotics. Topical administration, therefore, avoids selecting for resistant organisms in the upper aerodigestive tract, a common mechanism by which resistant organisms develop and propagate. The absence of systemic delivery is another reason that resistance to topical administration occurs with less frequency than resistance to systemically administered agents.
The use of topical agents allows for the simultaneous modification of the local microenvironment. The pH of the external auditory canal, for example, is normally slightly acidic. The administration of an antibiotic in an acidic drop helps restore and fortify this normal host defense mechanism.
Ototopical medications are generally less expensive than systemic medications. Although the newer quinolone antibiotic drops are more expensive than some systemic antibiotics, such as amoxicillin, the quinolone drops are much less expensive than systemic quinolones. To compare the cost of amoxicillin, trimethoprim-sulfamethoxazole, or erythromycin with these ototopical preparations is not appropriate. These systemic antibiotics have a poor spectrum of activity against otic pathogens.