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Animal Models of Infection for the Study of Antibiotic Pharmacodynamics 91(nonneutropenic) animals, most of the experience with this model has been obtainedin neutropenic mice (38). Most investigators induce neutropenia by the administrationof 150 and 100 mg/kg of cyclophosphamide on days 0 and 3, respectively.This results in severe neutropenia by day 4, which is sustained over at least a 24- to48-hour period (38). Inocula should be prepared in a suitable medium and allowedto grow into log phase. After dilution, 0.1 mL of bacterial suspension (10 5 –10 6 CFU)is injected into each thigh while the animals are under light anesthesia (e.g.,isoflurane, sodium pentobarbital, ether). For some experimental designs, a differentorganism may be injected in the contralateral thigh (e.g., when studying resistancemechanisms in isogenic strains), or mixtures of organisms can be used. Theorganisms are usually allowed to grow for two hours prior to the start of treatment.Treatment is given in multiple dosing regimens over 24 hours on the basis of thePKs of the drug and PK–PD parameters being tested. After 24 hours of treatment,the animals are euthanized and their thighs are removed, homogenized in sterilesaline, and serially diluted and cultured on a suitable medium to perform colonycounts. In the event of agents with long half-lives (e.g., azithromycin), thighhomogenates can be assayed for the drug to exclude the possibility of drugcarryover interfering with the results, or a substance to inactivate drug (e.g., b-lactamase) may be added. The CFU/thigh determined for each dosing regimen canbe quantified, and relationships to drug exposure fit to a suitable PD model.Pneumonia ModelsThe pneumonia model described below is a mouse model; however, there are anumber of different models in rats (21,39), hamsters (40), or rabbits (41). Organismsused in pneumonia models include, but are not limited to, Streptococcuspneumoniae, K. pneumoniae, P. aeruginosa, S. aureus, Pseudomonas mirabilis, Fecal coli,Legionella pneumophila, and Aspergillus fumigatus. Adjuvants are often used toincrease virulence. In the mouse model, most Swiss albino mice of either sex canbe used.Depending upon the organism, the mice may or may not need to be renderedneutropenic. Inocula should be prepared in a suitable medium and allowed togrow into log phase. After dilution, the animals are infected by intranasal instillationof 0.05 mL of bacterial suspension (10 6 –10 7 CFU) while the animals are underanesthesia (isoflourane, sodium pentobarbital, ether, etc.). Treatment is given inmultiple dosing regimens for up to two days starting 24 hours after infection. After24 to 48 hours of treatment, the animals are killed and their lungs are removed,homogenized, serially 10-fold diluted, and then cultured on a suitable medium toperform colony counts. The CFU/lungs determined for each dosing regimen canthen be fit to a suitable PD model. The advantages of this model are that theprolonged endpoint (24–48 hours) allows for several cycles of bacterial growth.Using this model, Woodnutt and Berry (21) found that the efficacy ofamoxicillin–clavulanate (a combination of the two drugs) was best described bypercentage time above the MIC, with the maximum bactericidal effect beingachieved at a T > MIC of 35% to 40%.Pyelonephritis/Kidney Infection ModelsKidney infection models for study of antifungal activity have been described inboth mice (24) and rats (42). Organisms used in kidney infection models include,but are not limited to, Bacterium faecalis, F. coli, S. aureus, Candida albicans, andA. fumigatus.