Streptomycin sulphate loaded solid lipid nanoparticles show enhanced uptake in macrophage, lower MIC in Mycobacterium and improved oral bioavailability.

Present study addresses the challenge of incorporating hydrophilic streptomycin sulphate (STRS; log P -6.4) with high dose (1 g/day) into a lipid matrix of SLNs. Cold high-pressure homogenization technique used for SLN preparation achieved 30% drug loading and 51.17 ± 0.95% entrapment efficiency. Polyethylene glycol 600 as a supporting-surfactant assigned small size (218.1 ± 15.46 nm) and mucus-penetrating property. It was conceived to administer STRS-SLNs orally rather than intramuscularly. STRS-SLNs remained stable on incubation for varying times in SGF or SIF. STRS-SLNs were extensively characterised for microscopic (TEM and AFM), thermal (DSC), diffraction (XRD) and spectroscopic (NMR and FTIR) properties and showed zero-order controlled release. Enhanced (60 times) intracellular uptake was observed in THP-1 and Pgp expressing LoVo and DLD-1 cell lines, using fluorescein-SLNs. Presence of SLNs in LoVo cells was also revealed by TEM studies. STRS-SLNs showed 3 times reduction in MIC against Mycobacterium tuberculosis H37RV (256182) in comparison to free STRS. It also showed better activity against both M. bovis BCG and Mycobacterium tuberculosis H37RV (272994) in comparison to free STRS. Cytotoxicity and acute toxicity studies (OECD 425 guidelines) confirmed in vitro and in vivo safety of STRS-SLNs. Single-dose oral pharmacokinetic studies in rat plasma using validated LCMS/MS technique or the microbioassay showed significant oral absorption and bioavailability (160% - 710% increase than free drug).

Development of a simple and rapid HPLC-MS/MS method for quantification of streptomycin in mice and its application to plasma pharmacokinetic studies.

Streptomycin was the first discovered aminoglycoside antibiotic. It has been widely applied in veterinary medicine for the prevention and treatment of bacterial infection. However, the current detection methods are not satisfactory in terms of sensitivity and sample process, which makes them unsuitable for a pharmacokinetic study. A high-performance liquid chromatography-mass spectrometric method employing positive electrospray ionization was developed and validated for the determination of streptomycin concentration in mice plasma. A simple protein precipitation method was utilized to extract streptomycin as well as the internal standard (kanamycin) from mouse plasma. This assay method was validated in terms of specificity, sensitivity, precision, accuracy and recovery. This method was applied to a pharmacokinetic study in mice following intramuscular administration of 200 mg/kg streptomycin. The lower limit of quantification of the developed assay method for streptomycin was 10 ng/mL. The intra-day and inter-day precision was evaluated with the coefficient of variations <14.3%, whereas the mean accuracy ranged from 87.0 to 105.0%. The samples were stable under the experimental conditions. The present method provides a robust, fast and sensitive analytical approach for the quantification of streptomycin in mouse plasma and has been successfully applied to a pharmacokinetic study in mice.

Kasugamycin potentiates rifampicin and limits emergence of resistance in Mycobacterium tuberculosis by specifically decreasing mycobacterial mistranslation.

Most bacteria use an indirect pathway to generate aminoacylated glutamine and/or asparagine tRNAs. Clinical isolates of Mycobacterium tuberculosis with increased rates of error in gene translation (mistranslation) involving the indirect tRNA-aminoacylation pathway have increased tolerance to the first-line antibiotic rifampicin. Here, we identify that the aminoglycoside kasugamycin can specifically decrease mistranslation due to the indirect tRNA pathway. Kasugamycin but not the aminoglycoside streptomycin, can limit emergence of rifampicin resistance in vitro and increases mycobacterial susceptibility to rifampicin both in vitro and in a murine model of infection. Moreover, despite parenteral administration of kasugamycin being unable to achieve the in vitro minimum inhibitory concentration, kasugamycin alone was able to significantly restrict growth of Mycobacterium tuberculosis in mice. These data suggest that pharmacologically reducing mistranslation may be a novel mechanism for targeting bacterial adaptation.

Continuous hypoxia reduces the concentration of streptomycin in the blood.

BACKGROUND: A high incidence and mortality of plague in the past two decades occurred in the Qinghai-Tibet Plateau, China. High dose streptomycin (6-8 g/d) remained the first practical strategy for controlling the progressive, vicious clinical circumstances for patients with pneumonic plague in the Plateau, as opposed to the routine dosage recommended by the World Health Organization. To investigate whether patients with pneumonic plague truly required a large dosage of streptomycin in the hypoxic environment of the Tibetan Plateau, we investigated the hypothesis that hypoxic environment would change the pharmacokinetics of streptomycin in vivo. METHODS: (1) We retrospectively analyzed the data of pneumonic plague patients administered streptomycin from January 1, 2000 to December 31, 2012 in these areas, which came from the database of the Qinghai Center for Disease Control; and (2) We used a persistent hypoxia chamber to simulate the plateau hypoxic environment and fed Sprague Dawley rats in the chambers for one month. Then, we continuously administered hypoxic rats a single loading dose (200 mg/kg) of streptomycin and analyzed its concentrations by high performance liquid chromatography. The pharmacokinetic profiles were analyzed using a non-compartmental method in the Phoenix WinNonlin program. RESULTS: (1) There were 32 cases of patients with pneumonic plague in the past two decades totally and 9 of them died (all-cause mortality 28.125%, 9/32), including 7 cases died of delayed diagnosis without treatment of streptomycin, and the only 2 patients received normal dose of streptomycin. (2) The pharmacokinetic behaviors of streptomycin were different between the hypoxic and normal rats. Administration in a hypoxic state resulted in 74.81% and 29.28% decreases in maximum plasma concentration and area under the concentration-time curve from time zero to infinity compared with those values under normal condition for streptomycin. CONCLUSIONS: These results indicated that hypoxic condition could significantly decrease the absorption rate and extent of streptomycin. Therefore, patients with pneumonic plague require higher doses of streptomycin to maintain effective drug concentrations in Qing Hai and the Tibetan Plateau.

Chitosan conjugation enables intracellular bacteria susceptible to aminoglycoside antibiotic.

Most chronic infections are difficult to eradicate because bacteria capable of surviving in host-infected cells may be protected from the killing actions of antibiotics, leading to therapy failures and disease relapses. Here we demonstrated that covalent-coupling chitosan to streptomycin significantly improved intracellular bactericidal capacity towards multiple organisms within phagocytic or nonphagocytic cells. Structure-activity relationship investigations indicated that antibiotic contents, molecular size and positive charges of the conjugate were the key to retain this intracellular bactericidal activity. Mechanistic insight demonstrated the conjugate was capable to target and eliminate endocytic or endosomal escaped bacteria through facilitating the direct contact between the antibiotic and intracellular organism. In vivo acute infection models indicated that compared to equal dose of the antibiotic, chitosan-streptomycin (C-S) conjugate and especially the human serum album binding chitosan-streptomycin conjugate (HCS) complex formed by human serum album and C-S conjugate greatly decreased the bacteria burden in the spleen and liver in both wild type and immuno-suppressive mice. Furthermore, the HCS complex remarkably reduced mortality of infected TLR2 deficient mice, mimicking immune-compromised persons who were more susceptible to bacterial infections. These findings might open up a new avenue to combat intracellular bacterial infection by aminoglycosides antibiotics at a lower effective dose.

Pharmacokinetics of Second-Line Antituberculosis Drugs after Multiple Administrations in Healthy Volunteers.

Therapeutic drug monitoring (TDM) of second-line antituberculosis drugs would allow for optimal individualized dosage adjustments and improve drug safety and therapeutic outcomes. To evaluate the pharmacokinetic (PK) characteristics of clinically relevant, multidrug treatment regimens and to improve the feasibility of TDM, we conducted an open-label, multiple-dosing study with 16 healthy subjects who were divided into two groups. Cycloserine (250 mg), p-aminosalicylic acid (PAS) (5.28 g), and prothionamide (250 mg) twice daily and pyrazinamide (1,500 mg) once daily were administered to both groups. Additionally, levofloxacin (750 mg) and streptomycin (1 g) once daily were administered to group 1 and moxifloxacin (400 mg) and kanamycin (1 g) once daily were administered to group 2. Blood samples for PK analysis were collected up to 24 h following the 5 days of drug administration. The PK parameters, including the maximum plasma concentration (Cmax) and the area under the plasma concentration-time curve during a dosing interval at steady state (AUCτ), were evaluated. The correlations between the PK parameters and the concentrations at each time point were analyzed. The mean Cmax and AUCτ, respectively, for each drug were as follows: cycloserine, 24.9 mg/liter and 242.3 mg · h/liter; PAS, 65.9 mg/liter and 326.5 mg · h/liter; prothionamide, 5.3 mg/liter and 22.1 mg · h/liter; levofloxacin, 6.6 mg/liter and 64.4 mg · h/liter; moxifloxacin, 4.7 mg/liter and 54.2 mg · h/liter; streptomycin, 42.0 mg/liter and 196.7 mg · h/liter; kanamycin, 34.5 mg/liter and 153.5 mg · h/liter. The results indicated that sampling at 1, 2.5, and 6 h postdosing is needed for TDM when all seven drugs are administered concomitantly. This study indicates that PK characteristics must be considered when prescribing optimal treatments for patients. (This study has been registered at ClinicalTrials.gov under registration no. NCT02128308.).

Synthesis, characterization, controlled release, and antibacterial studies of a novel streptomycin chitosan magnetic nanoantibiotic.

This study describes the preparation, characterization, and controlled release of a streptomycin-chitosan-magnetic nanoparticle-based antibiotic in an effort to improve the treatment of bacterial infections. Specifically, chitosan-magnetic nanoparticles were synthesized by an incorporation method and were characterized by Fourier transform infrared spectroscopy, X-ray diffraction, thermogravimetric analysis, and vibrating sample magnetometry. Streptomycin was incorporated into the nanoparticles to form a streptomycin-coated chitosan-magnetic nanoparticle (Strep-CS-MNP) nanocomposite. The release profiles showed an initially fast release, which became slower as time progressed. The percentage of drug released after 350 minutes was around 100%, and the best fit mathematical model for drug release was the pseudo-second order model. The Strep-CS-MNP nanocomposite showed enhanced antibacterial activity against methicillin-resistant Staphylococcus aureus. This study forms a significant basis for further investigation of the Strep-CS-MNP nanocomposite in the treatment of various bacterial infections.

Intranasal delivery of streptomycin sulfate (STRS) loaded solid lipid nanoparticles to brain and blood.

Factors like unreliable and poor oral absorption, including an active Pgp-efflux point towards a compromised oral bioavailability (BA) of streptomycin sulfate (STRS). Latter instigates its parenteral use (i.m.) only. Furthermore, its chronic use leads to serious side effects like nephrotoxicity and ototoxicity. In the present study, we propose to develop streptomycin sulfate (STRS) loaded solid lipid nanoparticles (STRS-SLNs) for non-invasive intranasal (IN) delivery. STRS-SLNs were prepared using patented nanocolloidal aqueous dispersion technique (Indian Patent application 3093/DEL/2012). Small particle size (140.1±7.0 nm) and significant entrapment efficiency (54.83±2.1%) was achieved. Biodistribution studies using (99m)Tc showed a 3.15 and 11.0 times higher concentrations in the brain and blood of mice, respectively, on IN administration of STRS-SLNs in comparison to free (F)-STRS. Lower concentrations (3.3 times) in kidneys implicate lower nephrotoxicity. Similarly a 12 and 4 times lower levels of drug in liver and spleen, respectively upon administration of STRS-SLNs as compared to F-STRS also indicate its lesser accumulation in these reticuloendothelial system organs. Lipophillic enclosure imparted to STRS, coupled with small particle size, and its purported ability to inhibit Pgp-efflux due to the presence of tween 80, is considered to be responsible for a better BA shown by STRS upon incorporation into SLNs. This is predicted to result in an effective treatment of all types of tuberculosis including cerebral tuberculosis as indicated by high relative distribution to brain in comparison to free-STRS.

Chemiluminescence determination of streptomycin in pharmaceutical preparation and its application to pharmacokinetic study by a flow injection analysis assembly.

A novel and rapid method for the determination of streptomycin has been established by chemiluminescence (CL) based on significant intensity enhancement of streptomycin on the weak CL of N-bromosuccinimide (NBS) and eosin in alkaline medium. The method is simple, rapid and effective to determine streptomycin in the range of 8.0×10(-9)-1.0×10(-6)gmL(-1) with a determination limit of 2.25×10(-9)gmL(-1). The relative standard deviation is 1.95% for the determination of 2.0×10(-7)gmL(-1) streptomycin (n=11). The pharmacokinetics of streptomycin in plasma of rat coincides with the two-compartment open model. The T1/2α, T1/2ß, CL/F, AUC(0-t), MRT, Tmax and Cmax were 18.83±1.24min, 82.14±3.07min, 0.0026±0.0011Lkg(-1)min(-1), 36044.50±105.02mgmin(-1)L(-1), 92.29±8.21min, 21.63±1.26min and 375.61±8.50µgmL(-1), respectively. There was no significant difference between the results obtained by CL and HPLC. The FI-CL method can be used to determine streptomycin in pharmaceutical preparation and biological samples. The established method is simple, rapid and sensitive without expensive instruments. The possible enhancement mechanism was also investigated.

Randomized pharmacokinetic and pharmacodynamic comparison of fluoroquinolones for tuberculous meningitis.

Tuberculous meningitis (TBM) is the most lethal form of tuberculosis, and new treatments that improve outcomes are required. We randomly assigned adults with TBM to treatment with standard antituberculosis treatment alone or in combination with ciprofloxacin (750 mg/12 h), levofloxacin (500 mg/12 h), or gatifloxacin (400 mg/24 h) for the first 60 days of therapy. Fluoroquinolone concentrations were measured with plasma and cerebrospinal fluid (CSF) specimens taken at predetermined, randomly assigned times throughout treatment. We aimed to describe the pharmacokinetics of each fluoroquinolone during TBM treatment and evaluate the relationship between drug exposure and clinical response over 270 days of therapy (Controlled Trials number ISRCTN07062956). Sixty-one patients with TBM were randomly assigned to treatment with no fluoroquinolone (n = 15), ciprofloxacin (n = 16), levofloxacin (n = 15), or gatifloxacin (n = 15). Cerebrospinal fluid penetration, measured by the ratio of the plasma area under the concentration-time curve from 0 to 24 h (AUC(0-24)) to the cerebrospinal fluid AUC(0-24), was greater for levofloxacin (median, 0.74; range, 0.58 to 1.03) than for gatifloxacin (median, 0.48; range, 0.47 to 0.50) or ciprofloxacin (median, 0.26; range, 0.11 to 0.77). Univariable and multivariable analyses of fluoroquinolone exposure against a range of different treatment responses revealed worse outcomes among patients with lower and higher plasma and CSF exposures than for patients with intermediate exposures (a U-shaped exposure-response). TBM patients most likely to benefit from fluoroquinolone therapy were identified, along with exposure-response relationships associated with improved outcomes. Fluoroquinolones add antituberculosis activity to the standard treatment regimen, but to improve outcomes of TBM, they must be started early, before the onset of coma.

Nanoparticle-based oral drug delivery system for an injectable antibiotic - streptomycin. Evaluation in a murine tuberculosis model.

BACKGROUND: To develop an oral drug delivery system for an injectable antibiotic, streptomycin. METHODS: Poly-lactide-co-glycolide (PLG) nanoparticles encapsulating streptomycin were prepared by the multiple emulsion technique and administered orally to mice for biodistribution and chemotherapeutic studies. RESULTS: The mean particle size was 153.12 nm with 32.12 +/- 4.08% drug encapsulation and 14.28 +/- 2.83% drug loading. Streptomycin levels were maintained for 4 days in the plasma and for 7 days in the organs following a single oral administration of PLG nanoparticles. There was a 21-fold increase in the relative bioavailability of PLG-encapsulated streptomycin compared with intramuscular free drug. In Mycobacterium tuberculosis H(37)Rv infected mice, eight doses of the oral streptomycin formulation administered weekly were comparable to 24 intramuscular injections of free streptomycin. Further, the nanoparticle formulation did not result in nephrotoxicity as assessed on a biochemical basis. CONCLUSION: Our results suggest that nanoencapsulation might be useful for developing a suitable oral dosage form for streptomycin and perhaps for other antibiotics that are otherwise injectable.

Impact of resistance selection and mutant growth fitness on the relative efficacies of streptomycin and levofloxacin for plague therapy.

Yersinia pestis, the bacterium that causes plague, is a potential agent of biowarfare and bioterrorism. The aminoglycoside antibiotic streptomycin is the gold standard for treatment. However, this recommendation is based on scant animal and clinical data. We used an in vitro pharmacodynamic infection model to compare the efficacies of 10-day regimens of streptomycin versus the fluoroquinolone antibiotic levofloxacin for the treatment of Y. pestis infection and to evaluate for emergence of resistance. The human serum concentration-time profiles for standard clinical regimens of 1 g of streptomycin given every 12 h and 500 mg of levofloxacin given every 24 h were simulated. The growth fitness of drug-resistant mutants was examined in neutropenic and immunocompetent mouse thigh infection models. In the in vitro infection system, untreated bacteria grew from 10(7) to 10(10) CFU/ml. Streptomycin therapy caused a 10(5) CFU/ml reduction in the number of bacteria over 24 h, followed by regrowth with streptomycin-resistant mutants. Levofloxacin resulted in a 10(7) CFU/ml reduction in the number of bacteria within 12 h, ultimately sterilizing the culture without resistance selection. In both the normal and neutropenic mouse infection models, streptomycin-resistant and wild-type strains were equally fit. However, 90% of levofloxacin-resistant isolates, cultured from the control in vitro infection arm, did not proliferate in the mouse models. Thus, the fluoroquinolone antibiotic levofloxacin was superior to streptomycin in our in vitro infection model. The majority of levofloxacin-resistant mutants were less fit than streptomycin-resistant and wild-type Y. pestis.

Aminoglycosides are captured from both periplasm and cytoplasm by the AcrD multidrug efflux transporter of Escherichia coli.

To understand better the mechanisms of resistance-nodulation-division (RND)-type multidrug efflux pumps, we examined the Escherichia coli AcrD pump, whose typical substrates, aminoglycosides, are not expected to diffuse spontaneously across the lipid bilayer. The hexahistidine-tagged AcrD protein was purified and reconstituted into unilamellar proteoliposomes. Its activity was measured by the proton flux accompanying substrate transport. When the interior of the proteoliposomes was acidified, the addition of aminoglycosides to the external medium stimulated proton efflux and the intravesicular accumulation of radiolabeled gentamicin, suggesting that aminoglycosides can be captured and transported from the external medium in this system (corresponding to cytosol). This activity required the presence of AcrA within the proteoliposomes. Interestingly, the increase in proton efflux also occurred when aminoglycosides were present only in the intravesicular space. This result suggested that AcrD can also capture aminoglycosides from the periplasm to extrude them into the medium in intact cells, acting as a "periplasmic vacuum cleaner."

Coenzyme Q1-catalyzed luminol chemiluminescent assay for rapid antimicrobial susceptibility testing of Mycobacterium bovis.

Coenzyme Q1 is herein proposed as the best catalyst among coenzymes Q and vitamins K for quinone-catalyzed luminol chemiluminescent assays applied to rapid determination of viability or rapid antimicrobial susceptibility tests of Mycobacterium bovis. Luminol chemiluminescence intensity (LCI) was determined 10 min after the incubation of M. bovis with coenzyme Q1, and was proportional to CFU (colony-forming unit)/ml in the range of 9,000 to 2,250,000. LCI depended on the the production of the superoxide anion (O2-) rather than H2O2 during a 10-min incubation of M. bovis with coenzyme Q1, as superoxide dismutase reduced LCI more effectively than catalase. The minimal inhibitory concentrations (MICs) of 10 kinds of antituberculous agents estimated on the basis of decrease in LCI after one or two days' cultivation were in good agreement with MICs determined by turbidity analysis, which requires upwards of 1 week to complete.

A study of streptomycin blood level information of patients undergoing hemodialysis.

Pharmacokinetic feature of streptomycin (SM) was investigated before and during hemodialysis (HD) in four patients with renal failure undergoing HD. SM concentrations were assayed by using TDX SM KIT (DAINABOT). Patients received 10 mg/kg of SM by intramuscular injection before HD. Pharmacokinetic parameters of SM intramuscular injection before HD were k(a)=2.38+/-0.53 h(-1); k(e)=0.0130+/-0.0025 h(-1); V(d)=0.313+/-0.026l/kg and t(1/2)=55.6+/-10.4 h. The maximum concentration (C(max)) of SM was observed at about 2 h after the SM administration and the mean serum concentration of SM was 30.4 microg/ml; even 4 h after the SM injection, the concentration still remained in a range over 30 microg/ml. The data suggest that a possible toxicity might have appeared in the patients. During the hemodialysis an average t(1/2) value was 3.32 h. This value is close to the value of a healthy person. The k(e) value of patient A during the hemodialysis became 24 times as large as that observed before the hemodialysis. On the average it was 17 times as large as that observed before the hemodialysis. Thus, it was found that the values of pharmacokinetics parameters such as k(e) and t(1/2) during the hemodialysis were similar to those of a healthy person, although there are some variations.

Population pharmacokinetics of intravenous and intramuscular streptomycin in patients with tuberculosis.

STUDY OBJECTIVES: To determine population pharmacokinetic parameters of streptomycin after administration of multiple intramuscular and intravenous doses. DESIGN: Prospective, unblinded clinical study. SETTING: Two medical centers in Denver, Colorado. PATIENTS: Thirty patients with tuberculosis. INTERVENTION: Patients received multiple doses of streptomycin as part of their tuberculosis treatment. They received concurrent drugs based on in vitro susceptibility data. MEASUREMENTS AND MAIN RESULTS: Serum samples were collected over a 10-hour period and assayed by validated high-performance liquid chromatography Concentration-time data were analyzed using population methods. Streptomycin concentrations increased linearly with increasing intravenous doses. The intramuscular doses did not produce as linear a relationship, presumably because of variability in rates of and, potentially, completeness of absorption. Streptomycin elimination decreased with declining renal function. Higher, intermittent doses were well tolerated and appeared to maximize the peak concentration:minimal inhibitory concentration ratio. CONCLUSION: Overall, pharmacokinetic parameters of streptomycin were comparable with those previously published for streptomycin and other aminoglycosides. Higher, intermittent doses maximize pharmacodynamic parameter estimates and might have advantages for treatment of tuberculosis.

Failure of oily chloramphenicol depot injection to treat plague in a murine model.

Effective low-cost single-dose therapy would be invaluable in treating human plague. The efficacy of single- or two-dose injections of oily chloramphenicol (OCm) was compared with that of standard multiple injections of reference drugs (streptomycin or chloramphenicol) in a murine plague model. A single injection of OCm was ineffective. Two doses cleared bacteraemia and limited bacterial growth in the mouse spleen but were less effective in reducing mortality than standard therapy. However, because of the marked pharmacokinetic differences between mice and humans, the failure of depot injection of OCm in murine plague treatment is not indicative of its ineffectiveness in human plague.

Resistência a drogas e produçäo de bacteriocinas em linhagens de Escherichia coli e Enterobacter isoladas de indivíduos hospitalizados / Drug resistance and colicin production by Escherichia coli and Enterobacter isolated from hospitalized humans

Escherichia coli e Enterobacter foram isolados de grupos de indivíduos hospitalizados (H), recém-hospitalizados (P), sendo estudados quanto ao perfil de resistência às drogas ampicilina, cefalotina, cloranfenicol, estreptomicina, tetraciclina, gentamicina, canamicina e ao bicloreto de mercúrio, por meio da técnica de diluiçäo em meio sólido, e quanto à produçäo de colicinas. A resistência aos antibióticos betalactâmicos foi maior nas amostras isoladas dos grupos de portadores näo-hospitalizados (P) (modelo ampicilina-cefalosporina), em especial para o gênero Enterobacter. Por outro lado, a freqüencia das amostras colicinogênicas descresceu no grupo H em comparaçäo com amostras colicinogênicas decresceu no grupo H em comparaçäo com as amostras do grupo P. É provável que, em ambientes seletivos pela presença de elevadas concentraçöes de antibióticos, as linhagens portadoras de fenótipos sejam selecionadas e a colicinogênese, deslocada ou substituída, sendo mantida em ambientes näo-seletivos, devido à competitividade as linhagens de uma mesma espécie

Antibiotic treatment of experimental pneumonic plague in mice.

A mouse model was developed to evaluate the efficacy of antibiotic treatment of pneumonic plague; streptomycin was compared to antibiotics with which there is little or no clinical experience. Infection was induced by inhalation of aerosolized Yersinia pestis organisms. Antibiotics were administered by intraperitoneal injection every 6 hours for 5 days, at doses that produced levels of drug in serum comparable to those observed in humans treated for other serious infections. These studies compared in vitro to in vivo activity and evaluated the efficacy of antibiotics started at different times after exposure. Early treatment (started 24 h after challenge, when 0 of 10 mice tested had positive blood cultures) with netilmicin, ciprofloxacin, ofloxacin, ceftriaxone, ceftazidime, aztreonam, ampicillin, and rifampin (but not cefazolin, cefotetan, or ceftizoxime) demonstrated efficacy comparable to streptomycin. Late treatment (started 42 h after exposure, when five of five mice tested had positive blood cultures) with netilmicin, ciprofloxacin, ofloxacin, and a high dose (20 mg/kg of body weight every 6 h) of gentamicin produced survival rates comparable to that with streptomycin, while all of the beta-lactam antibiotics (cefazolin, cefotetan, ceftriaxone, ceftazidime, aztreonam, and ampicillin) and rifampin were significantly inferior to streptomycin. In fact, all groups of mice treated late with beta-lactam antibiotics experienced accelerated mortality rates compared to normal-saline-treated control mice. These studies indicate that netilmicin, gentamicin, ciprofloxacin, and ofloxacin may be alternatives for the treatment of pneumonic plague in humans. However, the beta-lactam antibiotics are not recommended, based upon poor efficacy in this mouse model of pneumonic plague, particularly when pneumonic plague may be associated with bacteremia.