Penetration of cefprozil into middle ear fluid of patients with otitis media

Evaluation of Antimicrobial Treatments in Children with Acute Otitis Media in Spain: A Pharmacokinetic-Pharmacodynamic (PK/PD) Approach

Pharmacokinetic/pharmacodynamic (PK/PD) principles are priceless tools for evaluating the effectiveness of different antimicrobial treatments for different infections. However, very few studies deal with pediatric dosages and take into account the unbound drug serum levels. Our study is focused on the most frequent antibiotic dosing schedules used in Spain for the treatment of acute otitis media (AOM) in children, where high rates of penicillin and macrolide resistance exist among pneumococcal isolates. Pharmacokinetic parameters of antibiotics in children where obtained from the literature. The minimum inhibitory concentrations (MIC90) of antibiotics for pediatric strains of Streptococcus pneumoniae and Haemophilus influenzae were obtained from the SAUCE 2 project. Only ceftriaxone (50 mg/kg single intramuscular dose) and high doses of co-amoxiclav (27-33 mg/kg q8h) provided adequate efficacy indexes (tss(%)>MIC) for both S. pneumoniae and H. influenzae in AOM in children. These results are consistent with MEF (medium ear fluid) levels obtained from the literature. Our results confirm the utility of serum unbound levels to predict efficacy of antibiotics in children with AOM.

Antimicrobial stewardship and the role of pharmacokinetics-pharmacodynamics in the modern antibiotic era

Antimicrobial stewardship, a term coined by Dale Gerding, is defined as the optimal selection, dose, and duration of an antimicrobial that results in the best clinical outcome for the treatment or prevention of infection, with minimal toxicity to the patient and minimal impact on subsequent resistance development. Methods to promote and ensure good antimicrobial stewardship have been implemented and studied, and have typically provided tangible benefits in terms of a reduction in overall or targeted antimicrobial usage and resistance emergence. Although most of the programmatic antimicrobial stewardship efforts have been conducted in acute care inpatient settings, some strategies usually involving education have been evaluated in the outpatient venue. In this review, we shall discuss issues related to why antimicrobial stewardship is of particular importance in the modern antibiotic era. In addition, general pharmacokinetic-pharmacodynamic (PK-PD) concepts will be reviewed and specific PK-PD analyses that support the optimal selection, dosing, and duration of therapy for beta-lactam antimicrobials will be provided.

Pharmacokinetics of cefprozil in plasma and middle ear fluid: in children undergoing treatment for acute otitis media

BACKGROUND

Despite the wide-scale use of cefprozil for acute otitis media (AOM), there are only limited data available regarding the pharmacokinetic profile of this agent in the pediatric population.

OBJECTIVE

To characterize the plasma and middle ear fluid (MEF) pharmacokinetic profile of cefprozil in pediatric patients with AOM.

METHODS

Pharmacokinetic sampling was obtained as part of a phase IV, multicenter, open-label study of children with AOM receiving cefprozil suspension 15 mg/kg twice daily. A single blood sample was obtained 4-6 days after the initiation of cefprozil therapy and a simultaneous MEF sample was obtained by tympanocentesis when clinically indicated. Cefprozil concentrations in both matrices were determined using a validated high-performance liquid chromatography methodology. A composite profile of cefprozil concentration data in each matrix was constructed and values for the pharmacokinetic parameters were obtained using conventional modeling techniques.

RESULTS

Plasma concentrations were obtained in 53 children aged 6-48 months. In this population the maximum concentration (C(max)) in plasma was 9.18 microg/mL, the time to C(max) (t(max)) was 1.5 hours, and the terminal elimination half-life (t((1/2))(beta)) was 0.98 hours. Simultaneous plasma and MEF concentration data were available in 22 children. In this subset the C(max) in plasma was 8.2 microg/mL, the t(max) was 1.9 hours, and the t((1/2))(beta) was 1.02 hours; the corresponding MEF C(max) was 2.4 microg/mL, the t(max) was 3.5 hours, and the t((1/2))(beta) was 1.23 hours. Cefprozil MEF penetration as assessed using the ratio of the area under the concentration-time curves from the two matrices was 28%. Moreover, concentrations in MEF approximated 1 microg/mL 6 hours' post-dose.

CONCLUSIONS

The plasma profile of cefprozil in the current analysis is consistent with previously reported values in children receiving the 15 mg/kg twice daily dose. MEF penetration and the duration of drug exposure at the site of infection support the clinical utility of this agent for organisms with minimum inhibitory concentrations (MIC) of < or =1 microg/mL. However, these results also predict higher clinical failure when using this dose of cefprozil against penicillin-non-susceptible Streptococcus pneumoniae or Haemophilus influenzae because of typically higher MIC values for these organisms.

Pharmacodynamics of cefprozil against Haemophilus influenzae in an in vitro pharmacodynamic model

An in vitro pharmacodynamic model was used to determine the pharmacokinetic-pharmacodynamic (PK-PD) measure and magnitude most strongly related to cefprozil activity against Haemophilus influenzae. Using 3 clinical isolates of H. influenzae, a series of dose-fractionation studies were conducted, simulating cefprozil pediatric pharmacokinetics. The studies were designed to deliver a range of free cefprozil AUC(24) given once daily, twice daily, and by continuous infusion (CI). Drug effect, characterized by computing the log(10) ratio of the area under the 24-h bacterial colony-forming unit (CFU) (AUC(CFU)) curve to drug-free control, was fit to a Hill-type model for 3 PK-PD measures of activity: AUC(24)/MIC, C(max)/MIC, and %T > MIC. Once daily regimens provided much less activity than twice daily or CI regimens. AUC(24)/MIC and %T > MIC characterized the data well, whereas C(max)/MIC did not. Based on the PK-PD model results, for cefprozil twice daily, 50% and 80% of maximum drug effect (E(max)) was achieved at a %T > MIC of approximately 52% and 75%, respectively. A 2-log(10) reduction in log(10) ratio would require free drug %T > MIC of 58% or AUC(24)/MIC of 86. Bacteriostasis was achieved at a %T > MIC and an AUC(24)/MIC of approximately 25% and 30%, respectively. An in vitro pharmacodynamic model was able to characterize the PK-PD of cefprozil against H. influenzae. Consistent with limited clinical data, a minimum %T > MIC of 40% to 50% would be suggested to achieve in vivo activity in otitis media, with maximal activity at approximately 70%T > MIC.

Efficacy and tolerability assessment of cefprozil in children with acute otitis media

Young children contract as many as six to eight upper respiratory tract viral infections per year, and these infections frequently lead to secondary bacterial infections such as acute otitis media and sinusitis. Cefprozil is an orally active third generation cephalosporin which has demonstrated activity against the gram-positive organisms Streptococcus pyogenes, pneumoniae and agalactiae and against methicilin-susceptible Staphylococcus aureus. Cefprozil is also active against various gram-ves and certain anaerobic organisms, and is stable to hydrolysis by a number of b-lactamases. Present study is an effort to study the efficacy and safety of cefprozil in children with acute otitis media. Three hundred and thirty four children aged 6 months through 12 years with clinical symptoms and tympanic membrane signs of AOM received cefprozil 30 mg/kg/day in two divided doses per day for 10 days. Clinically, 96.6% patients were cured, 2.4% improved and there was failure of therapy in 1% of the patients. There was no need for any rescue medication and any change in antibiotic in any patient. A satisfactory bacteriological outcome was (i.e. cure, presumed cure, and cure plus reinfection with a different pathogen) was achieved in 95% of patients. In conclusion, cefprozil is a well tolerated and effective drug for acute otitis media in children. Moreover, its expanded spectrum of activity, ability to achieve adequate concentrations in tissues, suitability for twice-daily dosing, and proven tolerability suggest that it is a better alternative to agents conventionally used in acute otitis media.

Penetration of cefprozil to middle ear effusion in children with chronic otitis media with effusion

BACKGROUND

Because of chronic otitis media with effusion (COME) demonstrates pathogenic bacteria, treatment with appropriate antibiotic is reasonable.

OBJECTIVE

We determined the penetration of cefprozil into the middle ear effusion (MEE) in children with COME.

MATERIALS AND METHODS

25 patients 2-13 years of age with COME were eligible for study. After the single dose of 15 mg/kg patients were assigned to have MEE, and serum samples were obtained during ventilation tube insertion at 0.5, 2, 3, 5, or 6 h after administration of the dose. The concentration of cefprozil was measured using validated high performance liquid chromatography method.

RESULTS

The mean concentrations of cefprozil in the MEE ranged from 0.4 to 4.4 microg/ml. The penetration into MEE was rapid and effective. Cefprozil in the MEE was maintained at a greater level than MIC 90 in S. pneumoniae for at least 6 h after administration of 15 mg/kg.

CONCLUSION

Cefprozil penetrated well into the MEE in children with COME.

Cefprozil: a review

Cefprozil is a novel third generation, broad-spectrum oral cephalosporin with activity against a spectrum of aerobic gram-negative and positive bacteria, as well as certain anaerobes. The beta-lactamase stability of cefprozil may exceed that of other oral cephalosporins for some important pathogens. Cefprozil may be a suitable alternative to several other commonly used beta-lactams and cephalosporins in the treatment of mild to moderate upper and lower respiratory tract infections including sinusitis, otitis media, pharyngitis/tonsillitis, secondary bacterial infection of acute bronchitis, and acute bacterial exacerbations of chronic bronchitis, and skin and skin structure infections in children. Available data indicate the safety of cefprozil in both pediatric and adult population.

Otitis media: the chinchilla model

Streptococcus pneumoniae infection and disease have been modeled in several animal species including infant and adult mice, infant and adult rats, infant Rhesus monkeys, and adolescent and adult chinchillas. Most are models of sepsis arising from intravenous or intraperitoneal inoculation of bacteria, and a few were designed to study disease arising from intranasal infection. Chinchillas provide the only animal model of middle ear pneumococcal infection in which the disease can be produced by very small inocula injected into the middle ear (ME) or intranasally, and in which the disease remains localized to the ME in most cases. This model, developed at the University of Minnesota in 1975, has been used to study pneumococcal pathogenesis at a mucosal site, immunogenicity and efficacy of pneumococcal capsular polysaccharide (PS) vaccine antigens, and the kinetics and efficacy of antimicrobial drugs. Pathogenesis experiments in the chinchilla model have revealed variation in ME virulence among different pneumococcal serotypes, enhancement of ME infection during concurrent intranasal influenza A virus infections, and natural resolution of pneumococcal otitis media (OM) without intervention. Research has explored the relative contribution of pneumococcal and host products to ME inflammation. Pneumococcal cell wall components and pneumolysin have been studied in the model. Host inflammatory responses studied in the chinchilla ME include polymorphonuclear leukocyte oxidative products, hydrolytic enzymes, cytokine and eicosanoid metabolites, and ME epithelial cell adhesion and mucous glycoprotein production. Both clinical (tympanic membrane appearance) and histopathology (ME, Eustachian tube, inner ear) endpoints can be quantified. Immunologic and inflammatory studies have been facilitated by the production of affinity-purified antichinchilla immunoglobulin G (IgG), IgM, and secretory IgA polyclonal antibody reagents, and the identification of cross-reactivity between human and chinchilla cytokines, and between guinea pig and chinchilla C3. Alteration of ME mucosa by pneumococcal neuraminidase and alteration of ME epithelial cell (MEEC) surface carbohydrates during intranasal pneumococcal infection have been demonstrated. Pathogenesis studies have been aided by cultured chinchilla MEEC systems, in which the ability of platelet activating factor and interleukin (IL)-1 beta to stimulate epithelial mucous glycoprotein synthesis has recently been demonstrated. Because chronic OM with effusion is characterized by presence of large amounts of mucous glycoprotein in the ME, pneumococcus may have an important role in both acute and chronic ME disease. Both unconjugated PS and PS-protein-conjugated vaccines are immunogenic after intramuscular administration without adjuvant in chinchillas. Passive protection studies with human hyperimmune immunoglobulin demonstrated that anti-PS IgG alone is capable of protecting the chinchilla ME from direct ME challenge with pneumococci. Active PS immunization studies demonstrated protection following direct ME and intranasal pneumococcal challenge with and without concurrent influenza A virus infection. An attenuated influenza A virus vaccine also showed protection for pneumococcal OM. Antimicrobial treatment of acute OM has been based almost exclusively on empirical drug use and clinical trials without a foundation of ME pharmacokinetics. Studies in the chinchilla model have started to bring a rational basis to drug selection and dosing. Microassays have been developed using high-pressure liquid chromatography for many relevant drugs. Studies have explored the in vivo ME response in pneumococcal OM to antimicrobial drugs at supra- and sub-minimum inhibitory concentration (MIC), the effect of concurrent influenza A virus infection on ME drug penetration, and the effect of treatment on sensorineural hearing loss produced by pneumococcal OM.

Antimicrobial therapy of acute otitis media: review of treatment recommendations

OBJECTIVE

This paper reviews 3 previously published articles that provided recommendations for antimicrobial therapy of acute otitis media (AOM) and combines them to provide revised recommendations.

BACKGROUND

AOM is one of the most common pediatric infections requiring a prescription for an antimicrobial agent. The optimal approach to treatment of AOM requires early, efficacious, and practical therapy. Several experts and organizations have developed recommendations for the management of AOM, but the number of these may overwhelm the busy primary care practitioner. A MEDLINE search of the pediatric and infectious disease literature on AOM treatment recommendations was used to select 3 representative, previously published articles for this review. When selecting an agent, physicians should consider in vitro activity, particularly against drug-resistant Streptococcus pneumoniae; pharmacokinetics; adverse events; palatability of the suspension; and cost. In addition, physicians' clinical experience is an important determinant.

CONCLUSIONS

Amoxicillin is recommended as the first-line agent to treat uncomplicated AOM. For clinical treatment failures after 3 days of amoxicillin, recommended antimicrobial agents include oral amoxicillin/clavulanate, cefuroxime axetil, cefprozil, cefpodoxime proxetil, and intramuscular (i.m.) ceftriaxone. I.m. ceftriaxone should be reserved for severe cases or patients in whom noncompliance is expected. Tympanocentesis for identification of pathogens and susceptibility to antimicrobial agents is recommended for selection of third-line agents.

In vitro activity and pharmacodynamics of oral beta-lactam antibiotics against Streptococcus pneumoniae from southeast Missouri

STUDY OBJECTIVES

To determine the frequency of reduced susceptibility to penicillin, and to compare the in vitro activity and pharmacodynamics of oral beta-lactam antibiotics against clinical isolates of Streptococcus pneumoniae from southeast Missouri.

SETTING

Cape Girardeau, Missouri (population 35,500). Interventions. Minimum inhibitory concentrations (MICs) were determined for penicillin, amoxicillin, amoxicillin-clavulanic acid, cefprozil, cefuroxime, cefpodoxime, cefaclor, and loracarbef by E test for 108 isolates of S. pneumoniae. The MIC50, MIC90, and percentage susceptibility were calculated for each agent. Pharmacokinetic variables were obtained from the literature, and serum concentration-time profiles were simulated for a 25-kg child taking pediatric dosages commonly administered to treat otitis media. The average time above MIC (T > MIC) was calculated as percentage of the dosing interval using free concentrations and the MIC for each individual isolate. Analysis of variance (Scheffe post hoc test) was used to determine differences among agents for in vitro activity and T > MIC (level of significance, p<0.05).

MEASUREMENTS AND MAIN RESULTS

The frequency of penicillin-nonsusceptible S. pneumoniae was 28.7% (31/108). For 25 penicillin-intermediate isolates, amoxicillin and amoxicillin-clavulanic acid were significantly more active than cefprozil, cefaclor, and loracarbef. The T > MIC for amoxicillin and amoxicillin-clavulanic acid, simulated at 13.3 mg/kg every 8 hours, was significantly longer than that for all other beta-lactams.

CONCLUSION

Amoxicillin and amoxicillin-clavulanic acid have superior in vitro activity and longer T > MIC for penicillin-intermediate isolates than the other oral beta-lactams.

Treatment of upper and lower respiratory tract infections: clinical trials with cefprozil

The oral second generation cephalosporin cefprozil has a broad spectrum microbiologic profile, with good in vitro activity against respiratory pathogens; 90% or more of Streptococcus pneumoniae, Haemophilus influenzae and Moraxella catarrhalis isolates are susceptible to cefprozil. Clinical trials of cefprozil have consistently demonstrated good clinical success rates in upper and lower respiratory tract infections, including otitis media, sinusitis, pharyngitis/ tonsillitis and acute bacterial exacerbations of chronic bronchitis. Most recently cefprozil has demonstrated success in children with recurrent and persistent acute otitis media. Data from clinical trials including more than 4000 children and adults have shown that cefprozil is well-tolerated. The most common adverse events associated with cefprozil are gastrointestinal disturbances (i.e. diarrhea and nausea). In two patient satisfaction surveys (pediatric and adult), cefprozil was cited for having a low incidence of side effects and was rated by children as having a pleasing taste. These data indicate that cefprozil is a practical therapeutic choice for the treatment of upper and lower respiratory tract infections.

Amoxicillin middle ear fluid penetration and pharmacokinetics in children with acute otitis media

BACKGROUND

Acute otitis media (AOM) is a common childhood infectious disease. The efficacy of antibiotic dosing regimens is usually assessed by antibiotic plasma pharmacokinetics or middle ear fluid (MEF) concentration at one or two time points. Viral coinfection in AOM reduced antibacterial efficacy of antibiotics.

OBJECTIVE

To determine amoxicillin MEF penetration and pharmacokinetics in bacterial and combined bacterial and viral AOM.

METHODS

Thirty-four children with AOM were enrolled, and MEF was collected by tympanocentesis for bacterial culture and viral studies. Nasal wash and venous blood were also obtained for viral culture and serologic studies, respectively. Subjects were treated with amoxicillin 40 mg/kg/day orally, divided in equal doses every 8 h. During the second visit (48 to 72 h later) the subjects, with the regular morning amoxicillin dose withheld, were given an oral amoxicillin dose of 25 mg/kg. Thereafter two blood samples and one MEF sample by tympanocentesis were collected from each child at selected times between 0.5 and 4.0 h after dosing for bacterial and viral studies and amoxicillin concentration determination by high performance liquid chromatography.

RESULTS

Eleven (37%) children had only bacterial infection, 6 (20%) had viral infection only, 6 (20%) had both bacterial and viral infections and in 7 (23%) neither bacterial nor viral pathogens were recovered. MEF bacterial culture was positive in 23 of 40 ears (57.5%) before treatment with amoxicillin (40 mg/kg/day) and was still positive in 4 of 38 ears (10.5%) after 2 to 3 days of treatment. Amoxicillin plasma concentration reached its peak at 1.0 to 1.5 h after a 25-mg/kg oral dose. The estimated MEF concentration peak occurred 3.0 h after the dose with MEF concentrations ranging from undetectable to 20.6 microg/ml and a mean of approximately 9.5 microg/ml. Geometric mean amoxicillin concentrations were lowest in virus-infected children (2.7 microg/ml), nearly the same in culture-negative samples from children without viral infection (2.9 microg/ml), higher in children with combined bacterial and viral infection (4.1 microg/ml) and highest in children with bacterial-only infection (5.7 microg/ml).

CONCLUSIONS

MEF amoxicillin penetration tended to be lower in children with viral infection. The current amoxicillin dosing recommendation of 40 mg/kg/day in three divided dose is inadequate to effectively eradicate resistant Streptococcus pneumoniae, particularly during viral coinfection. A dosing regimen of 75 to 90 mg/kg/day is recommended for AOM.

Cefprozil treatment of persistent and recurrent acute otitis media

OBJECTIVE

We identified the pathogens causing persistent and recurrent acute otitis media (AOM) and the clinical efficacy of cefprozil as treatment.

STUDY DESIGN

This was a noncomparative, open label multicenter trial. Children ages 6 months to 12 years with signs and symptoms of AOM and evidence of middle ear effusion, as confirmed by pneumatic otoscopy or tympanometry, underwent tympanocentesis and subsequent treatment with cefprozil (15 mg/kg given twice daily) for 10 days. Patients with recurrent otitis media or failure of previous antibiotic therapy or prophylaxis were particularly sought for the study.

RESULTS

Two hundred sixty-two (99%) of 265 enrolled children were considered evaluable. The median age of the study group was 1 year. Ninety-eight (37%) of the children had a history (within 30 days) of prior antibiotic use. Ninety-seven (37%) met our definition of recurrent AOM, 48 (18%) met our definition of persistent AOM and 132 (50%) children had 3 or more previous episodes of acute otitis media within 12 months before study. Eighty-two (31%) of the enrollment tympanocentesis had no growth, 150 (57%) had a single bacterial pathogen and 29 (11%) had multiple bacterial pathogens. Of the 93 Streptococcus pneumoniae pretreatment isolates, 50 (54%) were penicillin-susceptible, 12 (13%) were penicillin-intermediate resistant and 31 (33%) were penicillin-resistant. Of the 75 Haemophilus influenzae pretreatment isolates, 42 (56%) produced beta-lactamase as did 4 (27%) of the 15 Moraxella catarrhalis strains. A satisfactory clinical response by pathogen was found in 75% (70 of 93) with S. pneumoniae, 75% (56 of 75) with H. influenzae and 93% (13 of 14) with M. catarrhalis; the response with single pathogen infections was higher than those with multiple pathogens (118 of 150 (78%) and 17 of 29 (59%), respectively; P = 0.03). The response for patients with isolates of S. pneumoniae that were penicillin-susceptible, -intermediate or -resistant were 39 of 50 (78%), 11 of 12 (92%) and 21 of 31 (68%), respectively. Older children had a satisfactory clinical outcome more frequently than younger children (P < 0.001), and the response to therapy varied for persistent, recurrent and recently untreated AOM (P < 0.01).

CONCLUSION

Persistent and recurrent AOM involves the same pathogens as recently untreated AOM but bacteria with reduced antibiotic susceptibility may be more frequently present. This noncomparative study suggests that cefprozil 30 mg/kg/day given in two divided doses for 10 days may be effective in the treatment of children with persistent and recurrent AOM.

Using antibiotic concentrations in middle ear fluid to predict potential clinical efficacy

BACKGROUND

The effectiveness of an antibiotic in eradicating an infection within an anatomic compartment is related to both its availability (penetration) and the susceptibility of the causative pathogen. Antimicrobial penetration can be assessed by measuring concentrations in serum or tissue. However, to assess antibiotic effectiveness in the treatment of otitis media, both antibiotic concentrations and pathogen profiles in middle ear fluid (MEF) should be considered. Tympanocentesis is required to access fluid in the middle ear compartment. This paper considers data on (MEF) antibiotic concentrations obtained with this procedure.

METHODS

Data on MEF antibiotic concentrations were considered in relation to the antibiotic susceptibility profiles of the more common otitis media pathogens.

RESULTS

Amoxicillin reaches MEF concentrations that exceed MICs for penicillin-susceptible and penicillin relatively resistant Streptococcus pneumoniae, but not for beta-lactamase-producing Haemophilus influenzae. Several beta-lactamase-stable drugs, e.g. cefixime and amoxicillin/clavulanate, however, achieve MEF concentrations that easily exceed their respective MIC90s for both beta-lactamase-producing and nonproducing H. influenzae but at ordinary doses may not exceed MIC90s for penicillin relatively resistant S. pneumoniae.

CONCLUSION

Ongoing observation of organisms and their antibiotic resistance patterns in patients failing initial therapy of acute otitis media indicate that stability to beta-lactamase remains an essential quality for effective second line therapies. However, when possible tympanocentesis with culture is the ideal method of targeting specific therapy for patients failing multiple consecutive antibiotic regimens.

Bactericidal activities of cefprozil, penicillin, cefaclor, cefixime, and loracarbef against penicillin-susceptible and -resistant Streptococcus pneumoniae in an in vitro pharmacodynamic infection model

We examined the bactericidal activities of penicillin, cefprozil, cefixime, cefaclor, and loracarbef against three clinical isolates of Streptococcus pneumoniae which were susceptible, moderately susceptible, and resistant to penicillin. An in vitro two-compartment glass infection model was used to simulate human pharmacokinetics in the presence of bacteria. Also, changes in organism susceptibility and development of resistant subpopulations were evaluated. Simulated pediatric dosage regimens and target peak concentrations in the central compartment were as follows: penicillin V-potassium, 26 mg/kg of body weight every 6 h (q6h) and 14 micrograms/ml; cefaclor, 13.4 mg/kg q8h and 16 micrograms/ml; loracarbef, 15 mg/kg q12h and 19 micrograms/ml; cefprozil, 15 mg/kg q12h and 11 micrograms/ml; and cefixime, 8mg/kg q24h and 4 micrograms/ml. Targeted half-lives of each agent were 1 h for penicillin, cefaclor, and loracarbef; 1.3 h for cefprozil; and 3.5 h for cefixime. Growth controls were performed at two different pump rates, 0.8 and 2.0 ml/min (half-lives = 3.5 and 1 h, respectively). Each isolate demonstrated autolysis at the lower rate which was attributed to a decreased supply of fresh nutrients available to the organisms in the infection compartment. Against the susceptible isolate, the time to 99.9% killing was statistically significant between penicillin V-potassium and both cefaclor and cefixime (P < 0.029). Loracarbef never achieved a 99.9% reduction in the inoculum. At 48 h penicillin, cefprozil, and cefaclor were equivalent in extent of killing. Against the intermediately resistant isolate, cefprozil was superior to all other regimens with respect to rate of killing (P < 0.013) and extent of killing at 24 h (P < 0.0003). At 48 h penicillin, cefprozil, and cefaclor were equivalent in extent of killing. All of the regimens exhibited inferior activity against this penicillin-resistant isolate. A 99.9% kill was never obtained with any of the regimens, nor was there an appreciable decrease in the colony counts. In conclusion, it appears that cefprozil, penicillin, and cefaclor are effective therapies against sensitive and even intermediately sensitive isolates of S. pneumoniae. However, none of the oral therapies appear to be of any benefit against penicillin-resistant isolates. The in vitro model may be an effective tool in evaluating other multiple-dose therapies against this fastidious organism, since the continual supply of fresh medium maintains the viability of S. pneumoniae with minimal stationary-phase autolysis.