The CONTIN algorithm and its application to determine the size distribution of microgel suspensions.

We review a powerful regularization method, known as CONTIN, for obtaining the size distribution of colloidal suspensions from dynamic light scattering data. We show that together with the so-called L-curve criterion for selecting the optimal regularization parameter, the method correctly describes the average size and size distribution of microgel suspensions independently characterized using small-angle neutron scattering. In contrast, we find that when using the default regularization process, where the regularizer is selected via the "probability to reject" method, the results are not as satisfactory.

Neural correlates of adherence to extended-release naltrexone pharmacotherapy in heroin dependence.

Injectable extended-release naltrexone (XRNTX) presents an effective therapeutic strategy for opioid addiction, however its utility could be hampered by poor adherence. To gain a better insight into this phenomenon, we utilized blood oxygenation level-dependent functional magnetic resonance imaging (fMRI) in conjunction with a validated cue-induced craving procedure to examine neural correlates of XRNTX adherence. We operationalized treatment adherence as the number of monthly XRNTX injections (range: 0-3) administered to a group of fully detoxified heroin-dependent subjects (n=32). Additional outcomes included urine toxicology screening and self-reported tobacco use. The presented heroin-related visual cues reliably elicited heroin craving in all tested subjects. Nine, five, three and 15 of the participants, respectively, received zero, one, two and three XRNTX injections, predicted by the individual baseline fMRI signal change in response to the cues in the medial prefrontal cortex, a brain region involved in inhibitory self-control and emotional appraisal. The incidence of opioid-positive urines during the XRNTX therapy was low and remained about half the pre-treatment rate after the XRNTX ended. During the treatment, cigarette smoking behaviors followed patterns of opioid use, while cocaine consumption was increased with reductions in opioid use. The present data support the hypothesis that medial prefrontal cortex functions are involved in adherence to opioid antagonist therapy. A potential role of concurrent non-opioid addictive substances consumption during the XRNTX pharmacotherapy warrants further investigation. Our findings set the stage for further bio-behavioral investigations of the mechanisms of relapse prevention in opioid dependence.

Form factor of pNIPAM microgels in overpacked states.

We study the form factor of thermoresponsive microgels based on poly(N-isopropylacrylamide) at high generalized volume fractions, ζ, where the particles must shrink or interpenetrate to fit into the available space. Small-angle neutron scattering with contrast matching techniques is used to determine the particle form factor. We find that the particle size is constant up to a volume fraction roughly between random close packing and space filling. Beyond this point, the particle size decreases with increasing particle concentration; this decrease is found to occur with little interpenetration. Noteworthily, the suspensions remain liquid-like for ζ larger than 1, emphasizing the importance of particle softness in determining suspension behavior.

Induction of DNA damage and caspase-independent programmed cell death by vitamin E.

Vitamin E comprises 8 functionally unique isoforms and may be a suitable candidate for the adjuvant treatment of prostate cancer. In this study, we examined the ability of 2 vitamin E isoforms [α-tocotrienol (γ-TT) and δ-tocotrienol (δ-TT)] and 4 synthetic derivatives [γ- and δ-tocotrienol succinate (γ-TS, δ-TS), α-tocopheryl polyethylene glycol succinate (TPGS), and α-tocopheryl polyethylene glycol ether (TPGS-e)] of vitamin E to induce cell death in AR- (DU145 and PC-3) and AR+ (LNCaP) prostate cancer cell lines. Our results show that δ-TT and TPGS-e are the most effective isoform and synthetic derivative, respectively, of all compounds examined. Overall, the results of our study suggest that isoforms and synthetic derivatives of vitamin E have the potency to trigger both caspase-dependent and -independent DNA damage and dominant caspase-independent programmed cell death. The capacity of vitamin E to trigger caspase-independent programmed cell death suggests that it may be useful in the chemotherapy of prostate cancer since it may prevent the tumor resistance commonly associated with the use of classical chemotherapeutic agents that trigger caspase-dependent programmed cell death.

Biochemical and molecular analysis of carboxylesterase-mediated hydrolysis of cocaine and heroin.

BACKGROUND AND PURPOSE: Carboxylesterases (CEs) metabolize a wide range of xenobiotic substrates including heroin, cocaine, meperidine and the anticancer agent CPT-11. In this study, we have purified to homogeneity human liver and intestinal CEs and compared their ability with hydrolyse heroin, cocaine and CPT-11. EXPERIMENTAL APPROACH: The hydrolysis of heroin and cocaine by recombinant human CEs was evaluated and the kinetic parameters determined. In addition, microsomal samples prepared from these tissues were subjected to chromatographic separation, and substrate hydrolysis and amounts of different CEs were determined. KEY RESULTS: In contrast to previous reports, cocaine was not hydrolysed by the human liver CE, hCE1 (CES1), either as highly active recombinant protein or as CEs isolated from human liver or intestinal extracts. These results correlated well with computer-assisted molecular modelling studies that suggested that hydrolysis of cocaine by hCE1 (CES1), would be unlikely to occur. However, cocaine, heroin and CPT-11 were all substrates for the intestinal CE, hiCE (CES2), as determined using both the recombinant protein and the tissue fractions. Again, these data were in agreement with the modelling results. CONCLUSIONS AND IMPLICATIONS: These results indicate that the human liver CE is unlikely to play a role in the metabolism of cocaine and that hydrolysis of this substrate by this class of enzymes is via the human intestinal protein hiCE (CES2). In addition, because no enzyme inhibition is observed at high cocaine concentrations, potentially this route of hydrolysis is important in individuals who overdose on this agent.

Encephalopathy with combined lithium-risperidone administration.

OBJECTIVE: Lithium-neuroleptics induced encephalopathy is a rare drug interaction. Here I am reporting a patient who developed reversible encepatholopathy with lithium-risperidone combination. METHOD: A single case report. RESULT: A patient of bipolar disorder, who presented with manic symptoms with psychotic feature, started with a combination of lithium and risperidone. Within few days, the patient developed encepatholopathy, which reversed upon discontinuation of lithium and risperidone. CONCLUSION: Combining lithium and neuroleptic is useful in treatment of bipolar disorder. However, encepatholapthy can be anticipated to result when lithium is used with high potency anti-psychotic such as haloperidol and risperidone and there are baseline EEG abnormalities.

An improved human carboxylesterase for enzyme/prodrug therapy with CPT-11.

CPT-11 is a potent antitumor agent that is activated by carboxylesterases (CE) and intracellular expression of CEs that can activate the drug results in increased cytotoxicity to the drug. As activation of CPT-11 (irinotecan-7-ethyl-10-[4-(1-piperidino)-1-piperidino]carbonyloxycamptothecin) by human CEs is relatively inefficient, we have developed enzyme/prodrug therapy approaches based on the CE/CPT-11 combination using a rabbit liver CE (rCE). However, the in vivo application of this technology may be hampered by the development of an immune response to rCE. Therefore, we have developed a mutant human CE (hCE1m6), based on the human liver CE hCE1, that can activate CPT-11 approximately 70-fold more efficiently than the wild-type protein and can be expressed at high levels in mammalian cells. Indeed, adenoviral-mediated delivery of hCE1m6 with human tumor cells resulted in up to a 670-fold reduction in the IC(50) value for CPT-11, as compared to cells transduced with vector control virus. Furthermore, xenograft studies with human tumors expressing hCE1m6 confirm the ability of this enzyme to activate CPT-11 in vivo and induce antitumor activity. We propose that this enzyme should likely be less immunogenic than rCE and would be suitable for the in vivo application of CE/CPT-11 enzyme/prodrug therapy.

Effects of innervation state on Hsp25 content and phosphorylation in inactive rat plantaris muscles.

AIM: Previous reports suggest a role for neuromuscular activity levels and/or connectivity in modulating Hsp25 expression and phosphorylation (pHsp25) in skeletal muscles. However, pHsp25 has only been studied in denervated muscles and/or muscles exposed to high levels of residual neuromuscular activity. Spinal cord isolation (SI) provides a model in which the muscle is exposed to nearly complete inactivity with maintenance of the nerve-muscle connection. To parcel out the roles of innervation state and activity-independent neural factors, we compared Hsp25 and pHsp25 in the plantaris of control (Con), SI, and denervated (Den, inactivity without neural connectivity) rats. METHODS: Hsp25 and pHsp25 protein levels (soluble and insoluble fractions) were measured with Western blot analysis after 1, 3, 8, 14, or 28 days of SI or Den. pHsp25 was normalized to non-pHsp25 at each time point. RESULTS: Hsp25 was unchanged (days 1, 3 and 14) or increased (days 8 and 28) in the soluble fraction, and decreased (day 1) or increased (days 3, 8 and 14) in the insoluble fraction in Den compared with Con rats. pHsp25 was reduced after 1 and 28 days of Den, but near control levels on days 3, 8, and 14 in the soluble fraction. In the insoluble fraction, pHsp25 levels were lower in Den than Con rats on all days. In both fractions, Hsp25 was lower in SI than Con rats. pHsp25 levels were lower in the soluble fraction and higher in the insoluble fraction in SI than Con rats. CONCLUSION: These results suggest that an intact innervation, even in the absence of muscle activation and/or loading, is critical for Hsp25 phosphorylation in the insoluble fraction. However, the time-dependent decrease in Hsp25 with SI suggests a role for minimal levels of muscle activation and/or loading in maintaining Hsp25 expression during sustained inactivity.

Alterations in bilateral force judgment following strenuous eccentric exercise.

This study investigated the effects of strenuous eccentric exercise on bilateral force-matching ability. After unilaterally performing 50 maximal eccentric elbow actions, participants were evaluated for bilateral force-matching ability. Participants were asked to match a reference force held by the control (CNT) arm with their exercised (EXD) arm. The experimental condition was then reversed. Constant error (bias) of the EXD arm was increased through 5 days postexercise, when it underestimated the CNT force in all trials. In contrast, an overestimation of the EXD force by the CNT arm resulted in an increased constant and variable error (variability around the bias) through 5 days postexercise, when the EXD arm served as the force reference. Strenuous eccentric exercise severely compromises bilateral force-matching ability, regardless of whether the EXD attempted to match the reference force or served as the force reference, indicating central or peripheral alterations to force judgment.

Pharmacokinetic/pharmacodynamic predictors of time to clinical resolution in patients with acute bacterial exacerbations of chronic bronchitis treated with a fluoroquinolone.

Forty nine subjects with acute bacterial exacerbations of chronic bronchitis (ABECB) treated with grepafloxacin were evaluated for parameters predictive of clinical outcome. Signs and symptoms associated with ABECB were serially collected and evaluated for changes. Coughs per day, sputum volume and the percentage of sputum neutrophils were associated with clinical outcome. A by groups analysis, based on clinical success was performed using Cox regression analysis to determine factors associated with time to clinical success and time to reduction in sputum volume, coughs per day and sputum neutrophil percent. Factors evaluated included AUIC (AUC/MIC), isolate species, years and type of underlying lung disease, alcohol use, smoking history and number of ABECB within the previous 12 months. AUIC<276 (mg h/l)/mg/l (P<0.03) and or the presence of mild bronchiectasis (P<0.01) were associated with longer time to clinical success. In addition a relationship was found between AUIC>212 (mg h/l)/mg/l (P<0.01) and AUIC>576 (mg h/l)/mg/l (P<0.02) and decreasing days to sputum volume reduction and coughs per day, respectively. A diagnosis of mild bronchiectasis prolonged the time to reduce coughs per day (P<0.03) and neutrophil percentage (P<0.01). Patients with mild bronchiectasis were found to have an increase in the time to clinical success, coughs per day improvement and sputum neutrophil percent improvement. AUIC is an important PK/PD parameter predictive of successful outcome in ABECB, even in subjects with mild bronchiectasis. Grepafloxicin has been withdrawn from sale since these studies were carried out. This work is published to illustrate the relationship between pharmacodynamics and clinical efficacy and the use of AUIC as a valuable predictive parameter for fluoroquinolones.

Evaluation of the effect of performance monitoring and feedback on care process, utilization, and outcome.

OBJECTIVE: We evaluated a program of performance measurement and monitoring by assessing care process, utilization of services, and outcomes. RESEARCH DESIGN AND METHODS: Information on 63,264 diabetic individuals who were continuously enrolled as members of Kaiser Permanente Southern California from 1 January 1994 to 31 December 1997 was used to evaluate the program. Time trends in testing for glycemic test and control and screening for dyslipidemia, use of lipid-lowering drugs, and microalbuminuria were evaluated as measures of care process. Time trends in hospitalization, outpatient appointments, prescriptions, and laboratory tests were evaluated as measures of utilization. Outcomes were hospitalization for myocardial infarction, ischemic stroke, and lower-limb amputation. RESULTS: Between 1994 and 1997, improvements were evident in the process measures. The mean number of hospitalizations and the mean and median number of outpatients visits did not change. The mean number of laboratory tests increased from 13.2 in 1994 to 23.6 in 1997. The mean number of prescriptions for any medication increased from 19.7 to 24.3. Hospitalization rates for myocardial infarction did not change, but rates increased for ischemic stroke and lower-limb amputation. CONCLUSIONS: Our findings suggest that measurement and monitoring of clinical performance can bring about modest improvements in measures of the processes of care in the absence of financial incentives, centrally driven interventions, and specialty care for all patients. In our setting, process improvements were associated with higher utilization of laboratory services and more prescriptions without an immediate return in terms of lower hospital utilization.

Comparison of the abilities of grepafloxacin and clarithromycin to eradicate potential bacterial pathogens from the sputa of patients with chronic bronchitis: influence of pharmacokinetic and pharmacodynamic variables.

A randomized open-label study was conducted to compare the pharmacokinetics and pharmacodynamics of grepafloxacin with those of clarithromycin in patients with chronic bronchitis whose sputa were colonized with potential bacterial pathogens. Patients received oral grepafloxacin 400 mg od for 10 days (n = 15) or oral clarithromycin 500 mg bd for 10 days (n = 10). Sputum samples were collected before the first dose, 1, 4 and 8 h after a dose on day 1 and then before a dose on days 2, 3, 5, 7 and 10 to determine the time to eradication (T(erad)) of the potential bacterial pathogens. Blood samples for measurement of grepafloxacin or clarithromycin and 14-hydroxyclarithromycin concentrations were obtained before a dose and 1, 2, 4, 8 and 12 h after doses on days 1 and 5. The area under the inhibitory serum concentration-time curve over 24 h (AUIC(24)), peak serum concentration:MIC ratio (C(max):MIC) and the percentage of the dosing interval during which the serum concentration exceeded the MIC (%tau >MIC) were calculated and serum inhibitory titres (SITs) were determined. Haemophilus spp. were the predominant potential bacterial pathogens and were recovered from the sputa of 24 patients. Strains of Streptococcus pneumoniae were isolated from two patients in the grepafloxacin group and a strain of Moraxella catarrhalis was isolated from one patient in the clarithromycin group. Haemophilus spp. isolates were eradicated from the sputa of 13 of 14 (93%) patients given grepafloxacin, but from only two of 10 (20%) patients given clarithromycin (P < 0.05). In the other eight (80%) patients who received clarithromycin, the sputum cultures remained positive throughout the 10 day course. Grepafloxacin eliminated potential bacterial pathogens more quickly than clarithromycin (median T(erad) 4 h versus 76 h). The S. pneumoniae strains were eradicated by grepafloxacin within 4 h and the single M. catarrhalis strain was eradicated by clarithromycin within 1 h. The greater efficacy of grepafloxacin, compared with that of clarithromycin, in terms of the incidence and speed of eradication of the Haemophilus spp. isolates, was associated with higher median values of AUIC(24) (169 SIT(-1)*h versus 8.1 SIT(-1)*h), C(max):MIC ratio (23.6 versus 0.7) and %tau >MIC (100% versus 0%). A Hill-type model adequately described the relationship between the percentage probability of eradicating potential bacterial pathogens from sputa and the plasma grepafloxacin concentration.

Pharmacodynamic modeling of risk factors for ciprofloxacin resistance in Pseudomonas aeruginosa.

OBJECTIVE: To determine risk factors for ciprofloxacin resistance in Pseudomonas aeruginosa. METHODS: Patients with cultures (any site) positive for P aeruginosa, susceptible to ciprofloxacin, between January 1993 and December 1996 were identified using a computerized database. Factors predictive of emergence of ciprofloxacin resistance in P aeruginosa strains isolated from the same cultured site, within 21 days of the initial culture, were determined. Factors considered included length of stay prior to initial P aeruginosa culture, isolation site, initial minimum inhibitory concentration (MIC), antibiotic area under the 24-hour concentration curve (AUC24), total area under the 24-hour inhibitory concentration curve ([AUIC24] AUC24/MIC summed for all active drugs), antibiotic(s) used as dichotomous variables (yes/no), and use of monotherapy or combination therapy. RESULTS: Of 635 patients, 43 (7%) subsequently had ciprofloxacin-resistant P aeruginosa isolated. Four significantly differing patient groups were identified: group 1, P aeruginosa isolates from all sites other than the respiratory tract, treated with any drugs; group 2, respiratory tract isolates treated with drugs other than ciprofloxacin; group 3, respiratory tract isolates treated with ciprofloxacin at AUIC24 >110 (microg x h/mL)/microg/mL; and group 4, respiratory tract isolates treated with ciprofloxacin at AUIC24 < or =110 (microg x h/mL)/microg/mL. The observed percentage resistant was a continuous function of prior length of stay in all four groups. Respiratory tract isolates had higher rates of ciprofloxacin resistance (12%) than isolates from other infection sites (4%). Respiratory tract isolates exposed to ciprofloxacin at AUIC24 < or =110 (microg x h/mL)/microg/mL had the highest resistance (17%). At AUIC24 >110 (microg x h/mL)/microg/mL, resistance was decreased to 11%, a rate similar to that seen in respiratory isolates not exposed to ciprofloxacin (7%). CONCLUSIONS: Application of pharmacokinetic and pharmacodynamic principles to dosing of ciprofloxacin may reduce the risk of ciprofloxacin resistance to the level seen in isolates exposed to other agents.

Potential role of pharmacokinetics, pharmacodynamics, and computerized databases in controlling bacterial resistance.

Bacterial resistance to antibiotics continues to be a problem, in spite of increased knowledge of resistance mechanisms. Due to the multifactorial nature of bacterial resistance, studies that evaluate the association between antimicrobial exposure and emergence of resistance may fail to find a relationship unless other factors, in particular the association between patient-pathogen pharmacokinetics (PK) and pharmacodynamics (PD) and the emergence of bacterial resistance, are evaluated as well. It has been hypothesized that, in conjunction with good infection control practices, cycling of antimicrobial agents may prove to be effective in reducing resistance emergence. There is some indication that there may be a relationship between the level of antibiotic exposure and the probability of emergence of bacterial resistance. As shown in our companion article in this supplement, factors associated with ciprofloxacin resistance in Pseudomonas aeruginosa included increased length of stay prior to isolation, exposure to ciprofloxacin, and respiratory tract site of bacterial isolation. However, in patients who received ciprofloxacin therapy, when exposure was at an area under the 24-hour inhibitory concentration curve (AUIC24)>110 (microg x h/mL)/microg/mL, resistance was decreased to 11%, a rate similar to that seen in respiratory isolates not exposed to ciprofloxacin (7%). While the length of time the patient spends in the hospital and the site of infection cannot be controlled, by using PK and PD principles for dosing of ciprofloxacin, the emergence of ciprofloxacin resistance in P aeruginosa may be reduced. Prospective antibiotic-cycling studies may help to determine not only the impact of antibiotic cycling on the institution's antibiogram but also, through the use of PK and PD principles, may help to determine appropriate dosing schedules for antibiotics in order to reduce the probability of emergence of bacterial resistance.

Establishment of the circumferential actin filament network is a prerequisite for localization of the cadherin-catenin complex in epithelial cells.

With the adhesion molecules, the actin cytoskeleton controls cell-cell and cell-substrate interactions and participates in transmembrane signaling. The relationships between actin and adhesion complexes at the sites of adhesion have been well documented. Here we investigate by a series of studies whether a relationship exists between actin organization and the localization and function of the components of the cadherin-catenin complex (CCC) that participates in the cell-cell adherens junction. Reversible actin depolymerization reversibly affects the peripheral distribution of CCCs. Mutations in adenovirus E1A and the small GTPase rac1, but not Ha-ras, disrupt the circumferential, cortical actin filament (CAF) network and the targeting of CCC components to the cell surface. Disruption of actin stress fibers or microtubules does not interfere with CCC localization and function. Constitutive loss of the apical cortical actin ring results in epithelial cells in which components of the CCCs are found only in intracellular vesicles and never at the surface. A kinetic analysis of the de novo appearance of the CAF network and the CCCs at the cell surface was also conducted. When F-actin was dissolved, surface CCC components were internalized. Reestablishment of CAFs required about 4 h, during which time E-cadherin and alpha-catenin were found first in a juxtanuclear location and then in intracellular vesicles or post-Golgi carriers, similar to what was observed in cells expressing mutant E1A or rac1. Thus, disruption of preexisting CCCs resulted in their internalization and recycling to the Golgi. It was only after the regeneration of the filamentous actin ring beneath the cell surface that peripheral localization of CCCs was observed. A similar result was observed with dominant negative rac1. These data suggest that the status of cortical actin is assessed and transduced and thereby regulates the transport and delivery of cadherin and catenins to the cell surface.

Creatine kinase release and clearance using MM variants following repeated bouts of eccentric exercise.

PURPOSE: This study investigated the release and clearance of plasma CK-MM (muscle) isoforms following two bouts of eccentric exercise to determine whether enhanced enzyme clearance could in part explain the blunted creatine kinase (CK) response to a second bout of exercise. METHODS: Nonweight trained college-aged male subjects performed both bouts of 50 forced lengthening contractions of the forearm flexor muscles separated by 6 d either with the same arm (CON; N = 4) or with one arm followed by the contralateral arm (EXP; N = 4). Range of motion, arm circumference, maximal isometric strength, perceived muscle soreness, total CK (TCK), and MM variants were assessed. Each MM isoform was measured as a percentage of TCK activity and in absolute activity (IU.L-1) following isoelectric separation and densitometry. The MM1:MM3 ratio, calculated from absolute concentrations, was used to indicate periods of release and clearance. RESULTS: Non-CK criterion measures indicated that similar damage occurred in both arms for EXP (P > 0.05), whereas CON exhibited a blunted response on bout 2 (P < 0.01). Postbout 1, TCK peaked at 96 h for CON (3530 +/- 927 IU.L-1) and EXP (6683 +/- 433 IU.L-1) (P < 0.01). Postbout 2, CON TCK showed no additional increase; however, a second rise in EXP TCK reached its highest point by day 5 (3602 +/- 1190 IU.L-1). Expectedly, %MM1 and the MM1: MM3 ratio were increased after bout 1 in both groups (P < 0.01). New CK release was observed postbout 2 in both groups as indicated by an increase in %MM1 (P < 0.01), despite no increase in TCK after bout 2 for CON and a smaller CK response for EXP. CONCLUSION: Accelerated clearance of CK seems to be one factor contributing to the blunted response of this enzyme following a repeated bout of exercise.

Genesis of methicillin-resistant Staphylococcus aureus (MRSA), how treatment of MRSA infections has selected for vancomycin-resistant Enterococcus faecium, and the importance of antibiotic management and infection control.

We extensively studied the epidemiology and time course of endemic methicillin-resistant Staphylococcus aureus (MRSA) in the Millard Fillmore Hospital, a 600-bed teaching hospital in Buffalo. The changeover from methicillin-susceptible S. aureus to MRSA begins on the first hospital day, when patients are given cefazolin as presurgical prophylaxis. Under selective antibiotic pressure, colonizing flora change within 24 to 48 hours. For patients remaining hospitalized, subsequent courses of third-generation cephalosporins further select and amplify the colonizing MRSA population. Therefore, managing antibiotic selective pressure might be essential. Other strategies include attention to dosing, so that serum concentrations of drug exceed the minimum inhibitory concentration, and antibiotic cycling. Although there are some promising new antibiotics on the horizon, it is necessary to deal with many resistance patterns by using the combined strategies of infection control and antibiotic management.

Legal implications of treating HIV patients.

A patient's HIV status often has significant impact on a physician's willingness and ability to provide quality medical care. A physician may face substantial penalties for refusing to treat a patient who has HIV. Even the referral of a patient with HIV to an HIV specialist may constitute a discriminatory act. The law provides certain guidelines that, if followed, may keep a physician out of the courtroom.

Pharmacodynamic evaluation of factors associated with the development of bacterial resistance in acutely ill patients during therapy.

The selection of bacterial resistance was examined in relationship to antibiotic pharmacokinetics (PK) and organism MICs in the patients from four nosocomial lower respiratory tract infection clinical trials. The evaluable database included 107 acutely ill patients, 128 pathogens, and five antimicrobial regimens. Antimicrobial pharmacokinetics were characterized by using serum concentrations, and culture and sensitivity tests were performed daily on tracheal aspirates to examine resistance. Pharmacodynamic (PD) models were developed to identify factors associated with the probability of developing bacterial resistance. Overall, in 32 of 128 (25%) initially susceptible cases resistance developed during therapy. An initial univariate screen and a classification and regression tree analysis identified the ratio of the area under the concentration-time curve from 0 to 24 h to the MIC (AUC[0-24]/MIC) as a significant predictor of the development of resistance (P < 0.001). The final PK/PD model, a variant of the Hill equation, demonstrated that the probability of developing resistance during therapy increased significantly when antimicrobial exposure was at an AUC[0-24]/MIC ratio of less than 100. This relationship was observed across all treatments and within all organism groupings, with the exception of beta-lactamase-producing gram-negative organisms (consistent with type I beta-lactamase producers) treated with beta-lactam monotherapy. Combination therapy resulted in much lower rates of resistance than monotherapy, probably because all of the combination regimens examined had an AUC[0-24]/MIC ratio in excess of 100. In summary, the selection of antimicrobial resistance appears to be strongly associated with suboptimal antimicrobial exposure, defined as an AUC[0-24]MIC ratio of less than 100.