In vitro evaluation of the effect of linezolid and levofloxacin on Bacillus anthracis toxin production, spore formation and cell growth.

BACKGROUND: Owing to its ability to form spores and toxins, Bacillus anthracis is considered a bioterror agent. Although current therapeutic strategies can be effective, treatment does not prevent sporulation and toxin production. OBJECTIVES: To quantify the combined effect of a protein synthesis inhibitor and a bactericidal agent on B. anthracis toxin production, sporulation and cell growth. METHODS: Susceptibility and synergy titrations were conducted on B. anthracis Sterne and 03-0191 strains using linezolid and levofloxacin. The effect of antibiotic exposure on cell viability was evaluated using a continuous medium replacement model. In vitro static models were used to study the effect of linezolid and levofloxacin on sporulation and toxin production. Spores were quantified using the heat shock method. Toxin was quantified via commercial ELISA. RESULTS: Synergy titrations indicated that the combination was synergistic or indifferent; however, in all models antagonism was observed. In the spore model, linezolid resulted in the lowest sporulation rates, while combination therapy resulted in the highest. In the toxin model, linezolid prevented toxin production altogether. CONCLUSIONS: This study advances our understanding of the effects of combination therapy on B. anthracis infection. Used alone, linezolid therapy abolishes toxin production and reduces sporulation. These results suggest that studies using a step-wise approach using linezolid initially to stop sporulation and toxin production followed by levofloxacin to rapidly kill vegetative B. anthracis can be recommended.

Spermidine/Spermine N1-Acetyltransferase 1 (SAT1)-A Potential Gene Target for Selective Sensitization of Glioblastoma Cells Using an Ionizable Lipid Nanoparticle to Deliver siRNA.

Spermidine/spermine N1-acetyltransferase 1 (SAT1) responsible for cell polyamine catabolism is overexpressed in glioblastoma multiforme (GB). Its role in tumor survival and promoting resistance towards radiation therapy has made it an interesting target for therapy. In this study, we prepared a lipid nanoparticle-based siRNA delivery system (LNP-siSAT1) to selectively knockdown (KD) SAT1 enzyme in a human glioblastoma cell line. The LNP-siSAT1 containing ionizable DODAP lipid was prepared following a microfluidics mixing method and the resulting nanoparticles had a hydrodynamic size of around 80 nm and a neutral surface charge. The LNP-siSAT1 effectively knocked down the SAT1 expression in U251, LN229, and 42MGBA GB cells, and other brain-relevant endothelial (hCMEC/D3), astrocyte (HA) and macrophage (ANA-1) cells at the mRNA and protein levels. SAT1 KD in U251 cells resulted in a 40% loss in cell viability. Furthermore, SAT1 KD in U251, LN229 and 42MGBA cells sensitized them towards radiation and chemotherapy treatments. In contrast, despite similar SAT1 KD in other brain-relevant cells no significant effect on cytotoxic response, either alone or in combination, was observed. A major roadblock for brain therapeutics is their ability to cross the highly restrictive blood-brain barrier (BBB) presented by the brain microcapillary endothelial cells. Here, we used the BBB circumventing approach to enhance the delivery of LNP-siSAT1 across a BBB cell culture model. A cadherin binding peptide (ADTC5) was used to transiently open the BBB tight junctions to promote paracellular diffusion of LNP-siSAT1. These results suggest LNP-siSAT1 may provide a safe and effective method for reducing SAT1 and sensitizing GB cells to radiation and chemotherapeutic agents.

Oseltamivir, an influenza neuraminidase inhibitor drug, does not affect the steady-state pharmacokinetic characteristics of cyclosporine, mycophenolate, or tacrolimus in adult renal transplant patients.

BACKGROUND: An influenza neuraminidase inhibitor drug, oseltamivir (Os) may be prescribed to renal transplant patients to prevent and treat influenza A and B illness. A pharmacokinetic (PK) interaction between Os and immunosuppressive drugs might adversely affect the efficacy and/or toxicity of the latter agents. This study was conducted to determine whether adverse symptoms and acute drug interactions occur during their coadministration. MATERIALS AND METHODS: A randomized, crossover study design was utilized to study the effect of a 75-mg dose of Os on the steady-state PK of cyclosporine A (CyA), mycophenolate mofetil, or tacrolimus (Tac) in a convenience sample of 19 adults with a renal allograft by measurement of total plasma or blood drug concentrations (C(p)) over one 12-hour dose interval. Os PK parameters were determined from its concentrations and those of its metabolite, Os carboxylate, in plasma and urine over 48 hours. RESULTS: Of 19 volunteers, 12 were men, with age (mean ± SD) 46 ± 11 years, weight 83 ± 19 kg, and calculated Cl(creatinine) 64 ± 27 mL/min. Adverse effects were minor and transient. Os did not affect the steady-state C(max), T(max), or area under the concentration versus time curve (AUC) over a 12-hour dose interval of CyA, mycophenolic acid, or Tac or the C(trough) of CyA or mycophenolate but increased the mean C(trough) of Tac by 13%. DISCUSSION: The increase in Tac mean C(trough) during coadministration with Os is not likely clinically important. Os and Os carboxylate PK were similar to those in subjects with native kidneys and similar renal function who have been described in the literature. CONCLUSIONS: These data from a single Os dose study suggest that coadministration is not expected to cause adverse symptoms nor alter the steady-state PK of CyA, mycophenolate mofetil, or Tac in stable adult renal transplant patients with mild renal insufficiency. The data enable a multiple-dose study that reflects clinical practice during influenza exposure and assesses the possibility that chronic exposure to Os might result in a different outcome.

Use of amantadine in the evaluation of response to chemotherapy in lung cancer: a pilot study.

AIM: The assessment of tumor response to therapy is of critical importance as it permits for a prospective end point evaluation and provides a guide to clinicians for making future treatment decisions. However, current practices in early evaluation of chemotherapy are insufficient. Amantadine is a substrate for SSAT-1. The present pilot study tests the hypothesis that SSAT-1 activity within the tumor, as measured by plasma acetylamantadine concentrations, can be used to monitor patient response to therapy. RESULTS: In cases with evidence of disease response, there was a reduction in the plasma acetylamantadine concentration at 4 h by approximately 32%. There was a mean increase of approximately 34% at the 4 h collection in the nonresponders. CONCLUSION: Although large-scale studies are required these findings suggest that the amantadine test could allow for determination of the efficacy of therapeutic interventions earlier, providing an effective test to assess response to treatment and for better management of patients.

Enteric absorption and pharmacokinetics of oseltamivir in critically ill patients with pandemic (H1N1) influenza.

BACKGROUND: Whether the enteric absorption of the neuraminidase inhibitor oseltamivir is impaired in critically ill patients is unknown. We documented the pharmacokinetic profile of oseltamivir in patients admitted to intensive care units (ICUs) with suspected or confirmed pandemic (H1N1) influenza. METHODS: We included 41 patients 18 years of age and older with suspected or confirmed pandemic (H1N1) influenza who were admitted for ventilatory support to nine ICUs in three cities in Canada and Spain. Using tandem mass spectrometry, we assessed plasma levels of oseltamivir free base and its active metabolite carboxylate at baseline (before gastric administration of the drug) and at 2, 4, 6, 9 and 12 hours after the fourth or later dose. RESULTS: Among the 36 patients who did not require dialysis, the median concentration of oseltamivir free base was 10.4 (interquartile range [IQR] 4.8-14.9) microg/L; the median concentration of the carboxylate metabolite was 404 (IQR 257-900) microg/L. The volume of distribution of the carboxylate metabolite did not increase with increasing body weight (R2=0.00, p=0.87). The rate of elimination of oseltamivir carboxylate was modestly correlated with estimations of creatinine clearance (R2=0.27, p<0.001). Drug clearance in the five patients who required continuous renal replacement therapy was about one-sixth that in the 36 patients with relatively normal renal function. INTERPRETATION: Oseltamivir was well absorbed enterically in critically ill patients admitted to the ICU with suspected or confirmed pandemic (H1N1) influenza. The dosage of 75 mg twice daily achieved plasma levels that were comparable to those in ambulatory patients and were far in excess of concentrations required to maximally inhibit neuraminidase activity of the virus. Adjustment of the dosage in patients with renal dysfunction requiring continuous renal replacement therapy is appropriate; adjustment for obesity does not appear to be necessary.

A High-Performing Plasma Metabolite Panel for Early-Stage Lung Cancer Detection.

The objective of this research is to use metabolomic techniques to discover and validate plasma metabolite biomarkers for the diagnosis of early-stage non-small cell lung cancer (NSCLC). The study included plasma samples from 156 patients with biopsy-confirmed NSCLC along with age and gender-matched plasma samples from 60 healthy controls. A fully quantitative targeted mass spectrometry (MS) analysis (targeting 138 metabolites) was performed on all samples. The sample set was split into a discovery set and validation set. Metabolite concentration data, clinical data, and smoking history were used to determine optimal sets of biomarkers and optimal regression models for identifying different stages of NSCLC using the discovery sets. The same biomarkers and regression models were used and assessed on the validation models. Univariate and multivariate statistical analysis identified ß-hydroxybutyric acid, LysoPC 20:3, PC ae C40:6, citric acid, and fumaric acid as being significantly different between healthy controls and stage I/II NSCLC. Robust predictive models with areas under the curve (AUC) > 0.9 were developed and validated using these metabolites and other, easily measured clinical data for detecting different stages of NSCLC. This study successfully identified and validated a simple, high-performing, metabolite-based test for detecting early stage (I/II) NSCLC patients in plasma. While promising, further validation on larger and more diverse cohorts is still required.

Protein kinase inhibition differentially regulates organic cation transport.

Previous studies showed that amantadine transport increased while tetraethylammonium (TEA) transport decreased in kidney tissue from diabetic rats. Changes in transport activity were reversed by exogenous insulin. We hypothesized that this difference in transport regulation is due to differential regulation of different transport systems. Native human embryonic kidney cortex cells (HEK293 cell line) and rat organic cation transporter (rOCT)-transfected cells were used to test the hypothesis. In support of differential regulation, short-term glucose starvation stimulated amantadine transport and inhibited TEA transport, but the effect was bicarbonate-modulated only for amantadine. cAMP analogues inhibited TEA transport while stimulating amantadine transport. This effect was additive to the effect of insulin, and the presence of bicarbonate affected the extent of the change. Our findings indicated that regulation of rOCT 1 and 2 was mediated by transmembrane adenylyl cyclase, and regulation of amantadine transport was mediated by soluble adenylyl cyclase, suggesting that intracellular microdomains of cAMP may be important in determining overall cellular transport for organic cations. Soluble adenylyl cyclase activity is known to be modulated by bicarbonate and lactate. These observations support our hypothesis and reconcile our previous studies demonstrating increased transport affinity for amantadine in the presence of bicarbonate and decreased transport affinity in the presence of lactate.

Use of amantadine as substrate for SSAT-1 activity as a reliable clinical diagnostic assay for breast and lung cancer.

AIM: Spermidine/spermine N1-acetyltransferase (SSAT-1) plays a critical role in cell growth, proliferation and death, and is known to be activated in human cancer cells. Amantadine, a US FDA-approved antiviral drug, is a substrate for SSAT-1 and can be used to indirectly measure SSAT-1 activity because of its conversion to acetylamantadine (AA). This study was undertaken to further validate SSAT-1 activity in breast and lung cancer patients. RESULTS: An increase in the urinary concentration of AA in lung and breast cancer patients was observed. The 0-2 h collection time point was determined to be optimal in revealing significant differences in urinary AA concentration between healthy controls and cancer patients. CONCLUSION: The high urine concentration of AA could be used as a simple and useful test for the detection of breast and lung cancer.

Predictive value and clinical significance of increased SSAT-1 activity in healthy adults.

AIM: Spermidine/spermine N1-acetyltransferase (SSAT-1) regulates cell growth, proliferation and death. Amantadine is converted by SSAT-1 to acetylamantadine (AA). In our earlier studies, although SSAT-1 was activated in patients with cancer, a number of ostensibly healthy adult volunteers had higher than expected AA concentration. This study was therefore undertaken to examine the outlier group. MATERIALS & METHODS: A follow up of urine analysis for AA by liquid chromatography-tandem mass spectrometry as well as clinical assessments and additional blood analyses were conducted. RESULTS: In some of the outlier controls, higher than expected AA concentration was linked to increased serum carcinoembryonic antigen. Clinical and radiographic assessments revealed underlying abnormalities in other cases that could represent premalignant conditions. Hematology tests revealed elevations in white blood cells and platelets, which are markers of inflammation. CONCLUSION: High urine concentration of AA could be used as a simple and useful test for screening of cancer in high-risk populations.

Liquid Biopsy in Lung Cancer Screening: The Contribution of Metabolomics. Results of A Pilot Study.

Background: Lung cancer is the most common cause of cancer-related deaths worldwide. Early diagnosis is crucial to increase the curability chance of the patients. Low dose CT screening can reduce lung cancer mortality, but it is associated with several limitations. Metabolomics is a promising technique for cancer diagnosis due to its ability to provide chemical phenotyping data. The intent of our study was to explore metabolomic effects and profiles of lung cancer patients to determine if metabolic perturbations in the SSAT-1/polyamine pathway can distinguish between healthy participants and lung cancer patients as a diagnostic and treatment monitoring tool. Patients and Methods: Plasma samples were collected as part of the SSAT1 Amantadine Cancer Study. Liquid chromatography-tandem mass spectrometry (LC-MS/MS) was used to identify and quantify metabolite concentrations in lung cancer patient and control samples. Standard statistical analyses were performed to determine whether metabolite concentrations could differentiate between healthy subjects and lung cancer patients, as well as risk prediction modeling applied to determine whether metabolic profiles could provide an indication of cancer progression in later stage patients. Results: A panel consisting of 14 metabolites, which included 6 metabolites in the polyamine pathway, was identified that correctly discriminated lung cancer patients from controls with an area under the curve of 0.97 (95% CI: 0.875-1.0). Conclusion: When used in conjunction with the SSAT-1/polyamine pathway, these metabolites may provide the specificity required for diagnosing lung cancer from other cancer types and could be used as a diagnostic and treatment monitoring tool.

Spermidine/spermine N1-acetyltransferase-1 as a diagnostic biomarker in human cancer.

AIM: SSAT-1 is an enzyme that plays a critical role in cell growth. Amantadine, a FDA-approved antiviral drug, is a substrate for SSAT-1. The utility of amantadine as an agent to demonstrate elevated SSAT-1 activity linked to cancer was conducted. RESULTS: High levels of SSAT-1 expression were measured in tumor human cell lines, and in breast, prostate and lung tumor tissue. An increase in the urinary levels of acetylated amantadine in cancer patients was observed. CONCLUSION: Increases in SSAT-1 contents in tumor tissue could be of value in targeting cancers with high SSAT-1 expression for confirmation/quantification. The high levels of acetylated amantadine could be used as a simple and useful screening test for the presence of cancer.

Theophylline-7ß-d-Ribofuranoside (Theonosine), a New Theophylline Metabolite Generated in Human and Animal Lung Tissue.

While assessing the ability of mammalian lung tissue to metabolize theophylline, a new metabolite was isolated and characterized. The metabolite was produced by the microsomal fraction of lungs from several species, including rat, rabbit, dog, pig, sheep and human tissue. Metabolite production was blocked by boiling the microsomal tissue. This new metabolite, theophylline-7ß-d-ribofuranoside (theonosine), was confirmed by several spectral methods and by comparison to an authentic synthetic compound. Tissue studies from rats, rabbits, dogs, and humans for cofactor involvement demonstrated an absolute requirement for NADP and enhanced metabolite production in the presence of magnesium ion. It remains to be demonstrated whether theonosine may contribute to the known pharmacological effects of theophylline.

NH4+ modulates renal tubule amantadine transport independently of intracellular pH changes.

A bicarbonate-dependent organic cation transporter, unique from rOCT1 and rOCT2, primarily mediates amantadine uptake into renal proximal tubules. We examined whether intracellular pH regulates bicarbonate-dependent amantadine transporter function in these tubules. NH(4)Cl treatment resulted in immediate intracellular alkalinization of tubules for up to 30s followed by gradual acidification that was maximal at 5min. Proximal tubule amantadine uptake was similarly inhibited (60%) by NH(4)Cl during both the early intracellular alkalinization and later acidification phases. Sodium propionate treatment resulted in immediate intracellular acidification of proximal tubules without inhibiting amantadine uptake. NH(4)Cl inhibition of bicarbonate-dependent amantadine uptake was dose-dependent, competitive and sex-dependent. NH(4)Cl, NH(4)NO(3), (NH(4))(2)SO(4) and (NH(4))(2)HPO(4) inhibited amantadine uptake into proximal tubules similarly. NH(4)Cl also stimulated efflux of amantadine and tetraethylammonium from preloaded proximal tubules, suggesting mediation of a facilitated process. These data suggest the potential for direct modulation of organic cation transporters by NH(4)(+) in rat kidney proximal tubules.

Protective effect of minocycline treatment on striatal ischemia.

Minocycline reduces infarct volume measured up to 1 week after focal cerebral ischemia, but it has not been shown that this results in lasting improvement in functional outcome. This study examined behavioral outcome in rats out to 3 weeks after focal ischemia induced by injection of the vasoconstrictor endothelin (ET)-1 (400 pmol in 1 microL of saline) into the striatum. Magnetic resonance imaging confirmed reduced blood flow after administration of ET-1, and was used to determine lesion volumes at 1 and 21 days postischemia. In control rats, intraperitoneal injection of minocycline resulted in plasma levels of 6.6 +/- 2.7 microg mL(-1) between 1 and 8 hours after administration. Based on these results, intraperitoneal minocycline treatment was started either 1 hour before or 3 hours after ET-1 administration, and was repeated daily for 5 days. Outcome, assessed using a composite behavioral deficit score (days 2, 4, 7, 14, and 21) and a test of asymmetric forelimb use (days 7 and 21), was significantly better in both groups of rats treated with minocycline, and the improvement was maintained for the 3-week study period. No differences were found in infarct volumes between groups.

Quantification of cefazolin in serum and adipose tissue by ultra high performance liquid chromatography-Tandem mass spectrometry (UHPLC-MS/MS): application to a pilot study of obese women undergoing cesarean delivery.

Higher doses of cefazolin are required in obese patients for preoperative antibiotic prophylaxis, owing to its low lipophilicity. An ultra high performance liquid chromatography-tandem mass spectrometry method was developed to quantify cefazolin in serum and adipose tissue from 6 obese patients undergoing cesarean delivery, and using stable-isotope labeled cefazolin as an internal standard. The method has a 2µg/g lower limit of quantitation. The concentration in adipose tissue was 3.4±1.6µg/mL, which is less than half of the reported minimum inhibitory concentration of 8µg/mL for cefazolin. Serum cefazolin concentrations were more than 30-fold higher than in adipose tissue.

Adequacy of a vancomycin dosing regimen in patients receiving high-flux hemodialysis.

BACKGROUND: Some investigators have recommended the convenient practice of administering vancomycin doses during the last hour of the hemodialysis treatment. Accepting that a greater amount of vancomycin is lost to dialysis with this recent approach, the objective of this study is to determine the pharmacokinetics of vancomycin and assess the adequacy of this dosing regimen in maintaining therapeutic predialysis concentrations. METHODS: A sampling of 22 consecutive patients administered intradialytic vancomycin, 1 g, intravenously (IV) and maintenance doses of 500 mg during the last hour of high-flux dialysis sessions was studied. A population-modeling program and Bayesian pharmacokinetic analysis were used to identify all global and unique pharmacokinetic parameters of interest based on measured vancomycin predialysis concentrations. RESULTS: For the 22 patients studied, this regimen achieved the targeted predialysis concentration range of 5 to 20 microg/mL for 96% of levels, whereas more narrowly within 5 to 15 microg/mL for 86% of levels. Average amount of vancomycin removed during a standardized 3- to 4-hour dialytic session ranged from 30% +/- 7% to 38% +/- 8%. Average elimination half-life of vancomycin on hemodialysis treatment was 5.4 hours (interquartile range, 5.0 to 5.9 hours). Patients showed an average predialysis plasma concentration of 11 +/- 3 microg/mL for the first 7 days of therapy. CONCLUSION: Our results indicate that intradialytic dosing with vancomycin using a 1-g IV load and 500 mg IV with subsequent high-flux dialysis sessions conveniently maintains adequate predialysis plasma concentrations. The lack of drug accumulation with this regimen provides convincing support for a limited blood sampling approach to plasma concentration determinations.

An evaluation of an optimal sampling strategy for meropenem in febrile neutropenics.

Optimal sampling design with nonparametric population modeling offers the opportunity to determine pharmacokinetic parameters for patients in whom blood sampling is restricted. This approach was compared to a standard individualized modeling method for meropenem pharmacokinetics in febrile neutropenic patients. The population modeling program, nonparametric approach of expectation maximization (NPEM), with a full data set was compared to a sparse data set selected by D-optimal sampling design. The authors demonstrated that the D-optimal sampling strategy, when applied to this clinical population, provided good pharmacokinetic parameter estimates along with their variability. Four individualized and optimally selected sampling time points provided the same parameter estimates as more intensive sampling regimens using traditional and population modeling techniques. The different modeling methods were considerably consistent, except for the estimation of CL(d) with sparse sampling. The findings suggest that D-optimal sparse sampling is a reasonable approach to population pharmacokinetic/pharmacodynamic studies during drug development when limited sampling is necessary.

NMDA receptor antagonists to characterize rat renal organic cation transporter function.

We hypothesized that uncompetitive NMDA glutamate receptor antagonists memantine (2,5-dimethyl-1-adamantanamine), and amino-alkyl-cyclohexane compounds: MRZ 2/579 (1-amino-1,3,3,5,5-pentamethylcyclohexane HCl), MRZ 2/600 (1-amino-1-ethyl-3,3,5,5-tetramethylcyclohexane HCl), and MRZ 2/615 (1-amino-1,3,5,5-tetramethyl-3-ethylcyclohexane HCl), all derivatives of amantadine (1-adamantanamine HCl), would inhibit the energy-dependent uptake of amantadine into rat renal tubules. All compounds displayed a concentration-dependent inhibition of amantadine uptake in the proximal and distal renal tubules. MRZ 2/579 showed a novel distal tubule selectivity of inhibition (P < 0.001). At a therapeutic amantadine concentration, bicarbonate-dependent transporter inhibition selectivity was observed with all compounds (P < 0.05) except MRZ 2/600, the only compound with a sterically bulky group next to the amino group of the cyclohexane ring structure. Steric hindrance around the ionized amino group of the cyclohexane ring appears to prevent bicarbonate-mediated organic cation transport. Furthermore, the distal tubule inhibition selectivity with MRZ 2/579 provides a novel tool to study the relative importance of organic cation transporters (OCTs) in proximal vs. distal renal tubules.

An amantadine hydrochloride dosing program adjusted for renal function during an influenza outbreak in elderly institutionalized patients.

OBJECTIVE: We tested the hypothesis that individualized dosing of amantadine hydrochloride, based upon a patient's creatinine clearance, would maintain efficacy against influenza A infection while reducing adverse reactions to the drug. DESIGN: A prospective cohort study PARTICIPANTS: Residents of two nursing homes with a total population of 301 individuals INTERVENTION: Amantadine hydrochloride was administered prophylactically subsequent to a confirmed influenza A outbreak. The dose was individualized based upon the resident's calculated creatinine clearance. RESULTS: The concentration of amantadine hydrochloride in the circulation at steady-state in patients who had doses adjusted for their estimated creatinine clearance was not different by nursing home or by sex of the resident. The mean concentration was within the 95% CI for the target concentration of 1.6 micromol/L. Side effects were modest and did not require discontinuation of amantadine hydrochloride therapy. Only the presence of concurrent influenza-like illness was significantly associated with adverse events during amantadine hydrochloride therapy. CONCLUSIONS: Adjustment of doses for estimated creatinine clearance is feasible in a long term care facility when amantadine hydrochloride is indicated for influenza A prophylaxis. These data form the basis for a definitive study of amantadine hydrochloride efficacy in patients with reduced renal function. Concurrent influenza-like illness is likely to confound attempts to associate adverse reactions to the administration of amantadine hydrochloride therapy.

Bayesian pharmacokinetic analysis of a gentamicin nomogram in neonates: a retrospective study.

BACKGROUND: Although gentamicin is used extensively within the first week of life for suspected sepsis in neonates, little is known about the performance of gentamicin dosing nomograms in this population. OBJECTIVE: The goal of our study was to retrospectively assess the performance of a gentamicin dosing nomogram in neonates given gentamicin during the first week after birth. METHODS: In this retrospective study, gentamicin therapeutic drug monitoring data were collected during routine clinical care for all neonates who were born in St. Boniface General Hospital (Winnipeg, Manitoba, Canada) between January 1999 and April 2001 and given gentamicin during the first week after birth. We used Bayesian pharmacokinetic analysis to retrospectively assess the performance of our gentamicin dosing nomogram in neonates born at gestation ages <32 weeks, between 32 and 34 weeks, and >34 weeks. Bayesian pharmacokinetic values for parameters within groups were compared and used to explore predicted peak and trough serum gentamicin concentrations based on the institutional dosing nomogram. RESULTS: In a total of 58 neonates, those neonates born at ≤34 weeks' gestation had a weight-normalized apparent volume of gentamicin distribution 1.6 times larger than infants born after 34 weeks' gestation (P<0.001), as identified by Bayesian analysis. Weight-normalized gentamicin clearance was 22% lower in the youngest age category (P<0.01). Only 33% of predicted peak serum gentamicin concentrations were >6 mg/L for neonates born at ≤34 weeks' gestation, whereas 90% were therapeutic in neonates born at >34 weeks' gestation (P<0.001). With the present nomogram, the likelihood of an indication for adjustment of the dosing regimen was 12.4-fold higher (95% CI, 3.5-43.7) for those neonates born at ≤34 weeks' gestation. CONCLUSIONS: These results have important clinical implications with regard to the advisability of determining peak serum gentamicin concentrations in neonates born at ≤34 weeks' gestation. Sampling of peak serum concentrations is indicated in this population to avoid underdosing and potential loss of therapeutic efficacy.