Posaconazole Is Effective as Salvage Therapy in Zygomycosis: A Retrospective Summary of 91 Cases

To evaluate the activity of posaconazole for treatment of zygomycosis, a disease for which therapeutic options are limited, we conducted a retrospective study including 91 patients with zygomycosis (proven zygomycosis, 69 patients; probable zygomycosis, 22 patients). Patients had infection that was refractory to prior antifungal treatment (n=81) or were intolerant of such treatment (n=10) and participated in the compassionate-use posaconazole (800 mg/day) program. The rate of success (i.e., either complete or partial response) at 12 weeks after treatment initiation was 60%, and 21% of patients had stable disease. The overall high success and survival rates reported here provide encouraging data regarding posaconazole as an alternative therapy for zygomycosis.

Hydroxylated analogues of the orally active broad spectrum antifungal, Sch 51048 (1), and the discovery of posaconazole [Sch 56592; 2 or (S,S)-5]

As part of a detailed study, the syntheses, biological activities, and pharmacokinetic properties of hydroxylated analogues of the previously described broad spectrum antifungal agents, Sch 51048 (1), Sch 50001 (3), and Sch 50002 (4), are described. Based on an overall superior profile, one of the alcohols, Sch 56592 (2), was selected for clinical studies.

Activity of posaconazole in the treatment of central nervous system fungal infections

OBJECTIVES

A multinational, multicentre, open-label clinical trial was conducted to evaluate the safety and efficacy of posaconazole, an extended-spectrum triazole antifungal agent, in subjects with invasive fungal infections who had refractory disease or who were intolerant of standard antifungal therapy. In this subanalysis, we report on those subjects in this trial who had a fungal infection that involved the CNS.

METHODS

Subjects received posaconazole oral suspension 800 mg/day in divided doses for up to 1 year; however, subjects could receive additional therapy as part of a treatment-use extension protocol. A blinded, third-party data review committee determined subject eligibility and outcome.

RESULTS

Of the 330 subjects who enrolled in the study, 53 had infections of the CNS, of which 39 were considered evaluable for efficacy. Most had refractory disease (37 of 39) and underlying HIV infection (29 of 39). Twenty-nine subjects had cryptococcal infections, and 10 had infections caused by other fungal pathogens [Aspergillus spp. (four), Pseudallescheria boydii (two), Coccidioides immitis (one), Histoplasma capsulatum (one), Ramichloridium mackenziei (one), and Apophysomyces elegans plus a Basidiomycetes sp. (one)]. Successful outcomes were observed in 14 of 29 (48%) subjects with cryptococcal meningitis and five of 10 (50%) subjects with CNS infections due to other fungal pathogens. Posaconazole was well tolerated.

CONCLUSIONS

These data suggest that posaconazole, as an oral medication, has clinical activity against fungal infections of the CNS and may provide a valuable alternative to parenteral therapy in patients failing existing antifungal agents.

Posaconazole is a potent inhibitor of sterol 14alpha-demethylation in yeasts and molds

Posaconazole (POS; SCH 56592) is a novel triazole that is active against a wide variety of fungi, including fluconazole-resistant Candida albicans isolates and fungi that are inherently less susceptible to approved azoles, such as Candida glabrata. In this study, we compared the effects of POS, itraconazole (ITZ), fluconazole (FLZ), and voriconazole (VOR) on sterol biosynthesis in strains of C. albicans (both azole-sensitive and azole-resistant strains), C. glabrata, Aspergillus fumigatus, and Aspergillus flavus. Following exposure to azoles, nonsaponifiable sterols were extracted and resolved by liquid chromatography and sterol identity was confirmed by mass spectroscopy. Ergosterol was the major sterol in all but one of the strains; C. glabrata strain C110 synthesized an unusual sterol in place of ergosterol. Exposure to POS led to a decrease in the total sterol content of all the strains tested. The decrease was accompanied by the accumulation of 14alpha-methylated sterols, supporting the contention that POS inhibits the cytochrome P450 14alpha-demethylase enzyme. The degree of sterol inhibition was dependent on both dose and the susceptibility of the strain tested. POS retained activity against C. albicans isolates with mutated forms of the 14alpha-demethylase that rendered these strains resistant to FLZ, ITZ, and VOR. In addition, POS was a more potent inhibitor of sterol synthesis in A. fumigatus and A. flavus than either ITZ or VOR.

Changes in susceptibility to posaconazole in clinical isolates of Candida albicans

OBJECTIVES

To characterize the molecular mechanisms responsible for reduced susceptibility to azoles in Candida albicans clinical isolates.

MATERIALS AND METHODS

Seven sequential C. albicans isolates were cultured from an AIDS patient treated with posaconazole for refractory oropharyngeal candidiasis. Expression levels of the CDR1, CDR2 and MDR1 genes, encoding efflux pumps previously implicated in azole resistance, and ERG11, encoding the azole target site, were monitored using northern blot and real-time PCR. The ERG11 genes from all seven isolates were sequenced.

RESULTS

The seven closely related isolates exhibited significant decreases in susceptibility to fluconazole (MIC >or= 32 mg/L) and voriconazole (MIC >or= 2 mg/L) and progressive decreases in susceptibility to both posaconazole (isolates 1-4 MIC 0.25 mg/L, isolates 5-7 MIC 2 mg/L) and itraconazole (isolates 1-4 MIC 1 mg/L, isolates 5-7 MIC > 8 mg/L). None of the isolates exhibited any significant changes in the expression levels of ERG11 or the efflux pump genes. All seven isolates had multiple mutations in ERG11; isolates one through four each had five missense mutations; four of the resultant amino acid changes were previously associated with azole resistance. The fifth isolate had an additional novel mutation in one copy of ERG11, resulting in a Pro-230 to Leu substitution. This mutation was present in both ERG11 genes in the last two isolates. Select ERG11 genes were expressed in Saccharomyces cerevisiae, the ERG11 allele with all six mutations conferred the highest level of posaconazole resistance.

CONCLUSIONS

Multiple mutations in ERG11 are required to confer decreased susceptibility to posaconazole.

Mutations in Aspergillus fumigatus resulting in reduced susceptibility to posaconazole appear to be restricted to a single amino acid in the cytochrome P450 14alpha-demethylase

To better understand the molecular basis of posaconazole (POS) resistance in Aspergillus fumigatus, resistant laboratory isolates were selected. Spontaneous mutants arose at a frequency of 1 in 10(8) and fell into two susceptibility groups, moderately resistant and highly resistant. Azole resistance in A. fumigatus was previously associated with decreased drug accumulation. We therefore analyzed the mutants for changes in levels of transcripts of genes encoding efflux pumps (mdr1 and mdr2) and/or alterations in accumulation of [(14)C]POS. No changes in either pump expression or drug accumulation were detected. Similarly, there was no change in expression of cyp51A or cyp51B, which encode the presumed target site for POS, cytochrome P450 14alpha-demethylase. DNA sequencing revealed that each resistant isolate carried a single point mutation in residue 54 of cyp51A. Mutations at the same locus were identified in three clinical A. fumigatus isolates exhibiting reduced POS susceptibility but not in susceptible clinical strains. To verify that these mutations were responsible for the resistance phenotype, we introduced them into the chromosome of a POS-susceptible A. fumigatus strain under the control of the glyceraldehyde phosphate dehydrogenase promoter. The transformants exhibited reductions in susceptibility to POS comparable to those exhibited by the original mutants, confirming that point mutations in the cyp51A gene in A. fumigatus can confer reduced susceptibility to POS.

The use of direct cDNA selection to rapidly and effectively identify genes in the fungus Aspergillus fumigatus

Aspergillus fumigatus is one of the causes of invasive lung disease in immunocompromised individuals. To rapidly identify genes in this fungus, including potential targets for chemotherapy, diagnostics, and vaccine development, we constructed cDNA libraries. We began with non-normalized libraries, then to improve this approach we constructed a normalized cDNA library using direct cDNA selection. Normalization resulted in a reduction of the frequency of clones with highly expressed genes and an enrichment of underrepresented cDNAs. Expressed sequence tags generated from both the original and the normalized libraries were compared with the genomes of Saccharomyces cerevisiae, Schizosaccharomyces pombe, and Candida albicans, indicating that a large proportion of A. fumigatus genes do not have orthologs in these fungal species. This method allowed the expeditious identification of genes in a fungal pathogen. The same approach can be applied to other human or plant pathogens to rapidly identify genes without the need for genomic sequence information.

EmtA, a rRNA methyltransferase conferring high-level evernimicin resistance

Enterococcus faecium strain 9631355 was isolated from animal sources on the basis of its resistance to the growth promotant avilamycin. The strain also exhibited high-level resistance to evernimicin, a drug undergoing evaluation as a therapeutic agent in humans. Ribosomes from strain 9631355 exhibited a dramatic reduction in evernimicin binding, shown by both cell-free translation assays and direct-binding assays. The resistance determinant was cloned from strain 9631355; sequence alignments suggested it was a methyltransferase and therefore it was designated emtA for evernimicin methyltransferase. Evernimicin resistance was transmissible and emtA was localized to a plasmid-borne insertion element. Purified EmtA methylated 50S subunits from an evernimicin-sensitive strain 30-fold more efficiently than those from a resistant strain. Reverse transcription identified a pause site that was unique to the 23S rRNA extracted from resistant ribosomes. The pause corresponded to methylation of residue G2470 (Escherichia coli numbering). RNA footprinting revealed that G2470 is located within the evernimicin-binding site on the ribosome, thus providing an explanation for the reduced binding of the drug to methylated ribosomes.

Genetic footprinting in bacteria

In vivo genetic footprinting was developed in the yeast Saccharomyces cerevisiae to simultaneously assess the importance of thousands of genes for the fitness of the cell under any growth condition. We have developed in vivo genetic footprinting for Escherichia coli, a model bacterium and pathogen. We further demonstrate the utility of this technology for rapidly discovering genes that affect the fitness of E. coli under a variety of growth conditions. The definitive features of this system include a conditionally regulated Tn10 transposase with relaxed sequence specificity and a conditionally regulated replicon for the vector containing the transposase and mini-Tn10 transposon with an outwardly oriented promoter. This system results in a high frequency of randomly distributed transposon insertions, eliminating the need for the selection of a population containing transposon insertions, stringent suppression of transposon mutagenesis, and few polar effects. Successful footprints have been achieved for most genes longer than 400 bp, including genes located in operons. In addition, the ability of recombinant proteins to complement mutagenized hosts has been evaluated by genetic footprinting using a bacteriophage lambda transposon delivery system.

In vitro Gram-positive antimicrobial activity of evernimicin (SCH 27899), a novel oligosaccharide, compared with other antimicrobials: a multicentre international trial

The antimicrobial activity of evernimicin (formerly SCH 27899), a novel oligosaccharide antimicrobial of the everninomicin class, was evaluated against four groups of Gram-positive pathogens: (i) Streptococcus pneumoniae (n = 1452); (ii) methicillin- or oxacillin-resistant Staphylococcus aureus (MRSA) and coagulase-negative staphylococci (MR-CoNS; n = 1427); (iii) enterococci (n = 1517); and (iv) non-pneumococcal streptococci (n = 1388), using the Etest method at each study centre throughout Eastern and Western Europe, Scandinavia, South Africa, Turkey and North America. Comparative MICs were determined for a variety of reference compounds, including vancomycin, quinupristin/dalfopristin, chloramphenicol, penicillin, ampicillin, oxacillin, ceftriaxone and ciprofloxacin. Evernimicin was highly active against all strains tested, with MIC90 values < or = 1.0 mg/L, ranging from 0.047 mg/L against S. pneumoniae to 1.0 mg/L against MRSA/MR-CoNS and enterococci. Compared with the reference agents, the MIC90 of evernimicin were lower against all species. Against MRSA and MR-CoNS the MIC90s of evernimicin, quinupristin/dalfopristin and vancomycin (the three most active agents) were 1.0, 1.5 and 3.0 mg/L, respectively. Against all species tested, the relative activities and spectra of these agents were: evernimicin > vancomycin > quinupristin/dalfopristin. The Etest proved to be reliable and reproducible, despite occasional interpretive difficulties caused by observer inexperience. Quality control results were excellent among the 33 participant sites. The results of this in vitro, multicentre, multinational study demonstrate that evernimicin possesses high antimicrobial activity against Gram-positive organisms that compares favourably with established antibacterial treatments and newer agents such as quinupristin/dalfopristin. Further clinical investigations of everninomicin class compounds appear warranted.

Effects of mutations in ribosomal protein L16 on susceptibility and accumulation of evernimicin

Chemical mutagenesis of Staphylococcus aureus RN450 generated two strains that displayed a stable reduction (30- to 60-fold) in susceptibility to evernimicin. Cell-free translation reactions demonstrated that the resistance determinant was located in the ribosomal fraction. Compared to ribosomes isolated from a wild-type strain, ribosomes from the mutant strains displayed an 8- to 10-fold reduction in affinity for [(14)C]evernimicin. In contrast, the mutants displayed no alteration in either binding affinity or in vitro susceptibility to erythromycin. Exponential cultures of the mutant strains accumulated significantly less [(14)C]evernimicin than the wild-type strain, suggesting that accumulation is dependent on the high affinity that evernimicin displays for its binding site. Sequencing rplP (encodes ribosomal protein L16) in the mutant strains revealed a single base change in each strain, which resulted in a substitution of either cysteine or histidine for arginine at residue 51. Introduction of a multicopy plasmid carrying wild-type rplP into the mutant strains restored sensitivity to evernimicin, confirming that the alterations in rplP were responsible for the change in susceptibility. Overexpression of the mutant alleles in S. aureus RN450 had no effect on susceptibility to evernimicin, demonstrating that susceptibility is dominant over resistance.

Evernimicin (SCH27899) inhibits a novel ribosome target site: analysis of 23S ribosomal DNA mutants

Spontaneous mutants of susceptible clinical and laboratory isolates of Streptococcus pneumoniae exhibiting reduced susceptibility to evernimicin (SCH27899; MIC, 0.5 to 4.0 mg/liter) were selected on plates containing evernimicin. Four isolates that did not harbor mutations in rplP (which encodes ribosomal protein L16) were further analyzed. Whole chromosomal DNA or PCR products of the 23S ribosomal DNA (rDNA) operons from these mutants could be used to transform the susceptible S. pneumoniae strain R6 to resistance at frequencies of 10(-5) and 10(-4), respectively, rates 10- to 100-fold lower than that for a single-allele chromosomal marker. The transformants appeared slowly (48 to 72 h) on selective medium, and primary transformants passaged on nonselective medium produced single colonies that displayed heterogeneous susceptibilities to evernimicin. A single passage on selective medium of colonies derived from a single primary transformant homogenized the resistance phenotype. Sequence analysis of the 23S rDNA and rRNA from the resistant mutants revealed single, unique mutations in each isolate at the equivalent Escherichia coli positions 2469 (A --> C), 2480 (C --> T), 2535 (G --> A), and 2536 (G --> C). The mutations map within two different stems of the peptidyltransferase region of domain V. Because multiple copies of rDNA are present in the chromosome, gene conversion between mutant and wild-type 23S rDNA alleles may be necessary for stable resistance. Additionally, none of the characterized mutants showed cross-resistance to any of a spectrum of protein synthesis inhibitors, suggesting that the target site of evernimicin may be unique.

In vitro and in vivo activities of SCH 56592 (posaconazole), a new triazole antifungal agent, against Aspergillus and Candida

SCH 56592 (posaconazole), a new triazole antifungal agent, was tested in vitro, and its activity was compared to that of itraconazole against 39 Aspergillus strains and to that of fluconazole against 275 Candida and 9 Cryptococcus strains. The SCH 56592 MICs for Aspergillus ranged from </=0.002 to 0.5 microg/ml, and those of itraconazole ranged from </=0.008 to 1 microg/ml. The SCH 56592 MICs for Candida and Cryptococcus strains ranged from </=0. 004 to 16 microg/ml, and those of fluconazole ranged from </=0.062 to >64 microg/ml. SCH 56592 showed excellent activity against Aspergillus fumigatus and Aspergillus flavus in a pulmonary mouse infection model. When administered therapeutically, the 50% protective doses (PD(50)s) of SCH 56592 ranged from 3.6 to 29.9 mg/kg of body weight, while the PD(50)s of SCH 56592 administered prophylactically ranged from 0.9 to 9.0 mg/kg; itraconazole administered prophylactically was ineffective (PD(50)s, >75 mg/kg). SCH 56592 was also very efficacious against fluconazole-susceptible, -susceptible dose-dependent, or -resistant Candida albicans strains in immunocompetent or immunocompromised mouse models of systemic infection. The PD(50)s of SCH 56592 administered therapeutically ranged from 0.04 to 15.6 mg/kg, while the PD(50)s of SCH 56592 administered prophylactically ranged from 1.5 to 19.4 mg/kg. SCH 56592 has excellent potential for therapy against serious Aspergillus or Candida infections.

Efficacy of SCH27899 in an animal model of Legionnaires' disease using immunocompromised A/J mice

The efficacy of SCH27899, a new everninomicin antibiotic, against replicative Legionella pneumophila lung infections in an immunocompromised host was evaluated using a murine model of Legionnaires' disease. A/J mice were immunocompromised with cortisone acetate and inoculated intratracheally with L. pneumophila serogroup 1 (10(5) CFU per mouse). At 24 h postinoculation, mice were administered either SCH27899 (6 to 60 mg/kg [MPK] intravenously) or a placebo once daily for 5 days, and mortality and intrapulmonary growth of L. pneumophila were assessed. In the absence of SCH27899, there was 100% mortality in L. pneumophila-infected mice, with exponential intrapulmonary growth of the bacteria. In contrast, administration of SCH27899 at a dose of > or =30 MPK resulted in > or =90% survival of infected mice, which was associated with inhibition of intrapulmonary growth of L. pneumophila. In subsequent studies, the efficacy of SCH27899 was compared to ofloxacin (OFX) and azithromycin (AZI). Administration of SCH27899, OFX, or AZI at a dose of > or =30 MPK once daily for 5 days resulted in > or =85% survival of infected mice and inhibition of intrapulmonary growth of the bacteria. However, L. pneumophila CFU were recovered in lung homogenates following cessation of therapy with all three antibiotics. These studies demonstrate that SCH27899 effectively prevents fatal replicative L. pneumophila lung infection in immunocompromised A/J mice by inhibition of intrapulmonary growth of the bacteria. However, in this murine model of pulmonary legionellosis, SCH27899, like OFX and AZI, was bacteriostatic.

Evernimicin binds exclusively to the 50S ribosomal subunit and inhibits translation in cell-free systems derived from both gram-positive and gram-negative bacteria

Evernimicin (SCH 27899) is a new antibiotic with activity against a wide spectrum of gram-positive bacteria and activity against some gram-negative bacteria. Previous metabolic labeling studies indicated that evernimicin specifically inhibited protein synthesis in Staphylococcus aureus. Using a susceptible Escherichia coli strain, we demonstrated that evernimicin also inhibited protein synthesis in E. coli. In cell-free translation assays with extracts from either E. coli or S. aureus, evernimicin had a 50% inhibitory concentration of approximately 125 nM. In contrast, cell-free systems derived from wheat germ and rabbit reticulocytes were inhibited only by very high levels of evernimicin. Evernimicin did not promote transcript misreading. [(14)C]evernimicin specifically bound to the 50S subunit from E. coli. Nonlinear regression analysis of binding data generated with 70S ribosomes from E. coli and S. aureus and 50S subunits from E. coli returned dissociation constants of 84, 86, and 160 nM, respectively. In binding experiments, performed in the presence of excess quantities of a selection of antibiotics known to bind to the 50S subunit, only the structurally similar drug avilamycin blocked binding of [(14)C]evernimicin to ribosomes.

Mutations in ribosomal protein L16 conferring reduced susceptibility to evernimicin (SCH27899): implications for mechanism of action

A clinical isolate of Streptococcus pneumoniae (SP#5) that showed decreased susceptibility to evernimicin (MIC, 1.5 microgram/ml) was investigated. A 4,255-bp EcoRI fragment cloned from SP#5 was identified by its ability to transform evernimicin-susceptible S. pneumoniae R6 (MIC, 0.03 microgram/ml) such that the evernimicin MIC was 1.5 microgram/ml. Nucleotide sequence analysis of this fragment revealed that it contained portions of the S10-spc ribosomal protein operons. The nucleotide sequences of resistant and susceptible isolates were compared, and a point mutation (thymine to guanine) that causes an Ile52-Ser substitution in ribosomal protein L16 was identified. The role of this mutation in decreasing susceptibility to evernimicin was confirmed by direct transformation of the altered L16 gene. The presence of the L16 mutation in the resistant strain suggests that evernimicin is an inhibitor of protein synthesis. This was confirmed by inhibition studies using radiolabeled substrates, which showed that the addition of evernimicin at sub-MIC levels resulted in a rapid decrease in the incorporation of radiolabeled isoleucine in a susceptible isolate (SP#3) but was much less effective against SP#5. The incorporation of isoleucine showed a linear response to the dose level of evernimicin. The incorporation of other classes of labeled substrates was unaffected or much delayed, indicating that these were secondary effects.

Synthesis and antibacterial activity of 2-alkoxy penems

The phosphite mediated Oxalimide cyclization reaction was extended to 4-dithiocarbonates of N-oxalyl-2-azetidinones to synthesize 2-alkoxy penems 3. In general, the in vitro antibacterial potency of compounds 3 was weak compared to the highly potent 2-alkylthiopenems 2.

Cloning and characterization of an aminoglycoside 6'-N-acetyltransferase gene from Citrobacter freundii which confers an altered resistance profile

A novel gene encoding a 6'-N-aminoglycoside acetyltransferase, aac(6')-In, has been cloned and sequenced from Citrobacter freundii 13996-19, a clinical isolate from Venezuela. This gene mediates resistance to amikacin, 2'-N-ethylnetilmicin, isepamicin, kanamycin, netilmicin, and tobramycin. The aac(6')-In gene is 573 nucleotides in length and encodes a putative protein of 190 amino acids. AAC(6')-In is most closely related to AAC(6')-Im and AAC(6')-Ie, demonstrating 64.4% and 62.3% similarity, respectively, at the protein level, suggesting these proteins share a common ancestor. The aac(6')-In flanking sequences demonstrated homology to integron- and transposon-related elements which are often found associated with resistance determinants. Hybridization studies performed with an intragenic probe specific for aac(6')-In indicate that this gene is prevalent within Venezuela but has not been observed outside of the country. Furthermore, the aac(6)-In gene was found in 10 different species of gram-negative bacteria.

The most frequent aminoglycoside resistance mechanisms--changes with time and geographic area: a reflection of aminoglycoside usage patterns? Aminoglycoside Resistance Study Groups

The aminoglycoside resistance mechanisms revealed by two surveys in Europe and other countries have been compared to those revealed in earlier studies. Mechanisms have become more complex in all bacterial groups. In Providencia, Serratia, Pseudomonas, Acinetobacter, and Staphylococcus species isolates, genus-specific mechanisms were very common, and it was not possible to see differences between different geographic areas. In other Enterobacteriaceae, the increasing complexity of mechanisms was most often caused by combinations of gentamicin-modifying enzymes with AAC(6')-I, which acetylates amikacin but not gentamicin. The occurrence of these combinations varied by geographical region and among hospitals. The frequency of these combinations correlated with aminoglycoside usage in either the geographical regions or in individual hospitals. These broad-spectrum combinations occurred most frequently in Citrobacter, Enterobacter, and Klebsiella species but also occurred in Escherichia, Morganella, Proteus, Salmonella, and Shigella species. Often the only clinically available aminoglycoside that retained its normal activity was isepamicin.

Cloning and characterization of a 3-N-aminoglycoside acetyltransferase gene, aac(3)-Ib, from Pseudomonas aeruginosa

A novel gene encoding an aminoglycoside 3-N-acetyltransferase, which confers resistance to gentamicin, astromicin, and sisomicin, was cloned from Pseudomonas aeruginosa Stone 130. Its sequence was determined and found to show considerable similarity to an aac(3)-I gene previously cloned from R plasmids from Enterobacter, Pseudomonas, and Serratia spp. We have designated the genes from the R plasmids and this work aac(3)-Ia and aac(3)-Ib, respectively. The two aac(3)-I genes share 74% nucleotide identity, and their deduced protein products are 88% similar. These data suggest that the genes derive from a common ancestor. Homology between the flanking sequences of both aac(3)-I genes and other resistance determinants known to reside in integron environments was also observed. Intragenic probes specific for either aac(3)-Ia or aac(3)-Ib were used in hybridization studies with a series of gentamicin-, astromicin-, and sisomicin-resistant clinical isolates. Of 59 clinical isolates tested, no isolates hybridized with both probes, 30 (51%) hybridized with the aac(3)-Ia probe, 12 (20%) hybridized with the aac(3)-Ib probe, and 17 (29%) did not hybridize with either probe. These data suggest the existence of at least one other aac(3)-I gene.

The changing nature of aminoglycoside resistance mechanisms and the role of isepamicin--a new broad-spectrum aminoglycoside. The Aminoglycoside Resistance Study Groups

Aminoglycoside resistance mechanisms from recent studies were compared with those found in earlier studies in the USA and Europe for three pathogen groups. Among Citrobacter-Enterobacter-Klebsiella, four single mechanisms (AAc(3)-II, AAC(3)-I, ANT(2")-I and AAC(6')-I were found in all studies, but the most recent studies showed a significant increase in combinations of AAC(6')-I with the other common mechanisms. Since AAC(6')-I confers resistance to tobramycin, netilmicin and amikacin, combinations of it with the other gentamicin modifying enzymes conferred broad-spectrum resistance to all clinically available aminoglycosides except isepamicin. Similar changes occurred in Escherichia-Morganella-Proteus-Salmonella-Shigella except that the frequency of combinations was much lower and two additional single mechanisms - AAC(3)-IV and permeability - were also found frequently. Among aminoglycoside-resistant Pseudomonas, three mechanisms, AAC(6')-II, ANT(2")-I and permeability, were always common and remained common. However, combinations of the three mechanisms with each other and with other mechanisms were more common in the recent surveys. Different genes which produce different proteins with the same aminoglycoside-modifying activity are now known. The results of hybridisation studies with two aac(3)-I, 2 aac(6')-II and 4 aac(6')-I gene probes are presented. The most commonly occurring genes were: aac(3)-Ia, aac(3)-IIa, aac(6')-IIa, aac(6')-Ib and, in Serratia, aac(6')-Ic. The activity of isepamicin against amikacin resistant strain which produce AAC(6')-I can be related to differences in the structure of these two similar aminoglycosides at Position 3". Amikacin may form a stable complex with AAC(6')-I enzymes via binding interaction at Position 3 and 3". Isepamicin, which has a secondary amino group at Position 3", may only be able to interact at Position 3 and enzyme-isepamicin complexes are likely to be less stable.

Cloning and characterization of KNR4, a yeast gene involved in (1,3)-beta-glucan synthesis

k9 killer toxin from Hansenula mrakii was used to select a number of resistant mutants from Saccharomyces cerevisiae. Preliminary biochemical and genetic studies showed that some of them acquired structural defects in the cell wall. One of these mutants, the knr4-1 mutant, displays a number of cell wall defects, including osmotic sensitivity; sensitivity to cercosporamide, a known antifungal agent; and resistance to Zymolyase, a (1,3)-beta-glucanase. We report here the isolation and analysis of the KNR4 gene. DNA sequence analysis revealed an uninterrupted open reading frame which contains five potential start codons. The longest coding template encodes a protein of 505 amino acids with a calculated molecular mass of 57,044 Da. A data base search revealed 100% identity with a nuclear protein, SMI1p. Disruption of the KNR4 locus does not result in cell death; however, it leads to reduced levels of both (1,3)-beta-glucan synthase activity and (1,3)-beta-glucan content in the cell wall. The gene was mapped to the right arm of chromosome VII.

Analysis of the aac(3)-VIa gene encoding a novel 3-N-acetyltransferase

Biochemical analysis (G. A. Papanicolaou, R. S. Hare, R. Mierzwa, and G. H. Miller, abstr. 152, Program Abstr. 29th Intersci. Conf. Antimicrob. Agents Chemother., 1989) demonstrated the presence of a novel 3-N-acetyltransferase in Enterobacter cloacae 88020217. This organism was resistant to gentamicin, and the MIC of 2'-N-ethylnetilmicin for it was fourfold lower than that of 6'-N-ethylnetilmicin, a resistance pattern which suggested 2'-acetylating activity. However, high-pressure liquid chromatography analysis demonstrated that the enzyme acetylated sisomicin in the 3 position. We have cloned the structural gene for this enzyme from a large (> 70-kb) conjugative plasmid present in E. cloacae. Subcloning experiments have localized the aac(3)-VIa gene to a 2.1-kb Sau3A fragment. The deduced AAC(3)-VIa protein showed 48% amino acid identity to the AAC(3)-IIa protein and 39% identity to the AAC(3)-VII protein. Examination of the 5'-flanking sequences demonstrated that the aac(3)-VIa gene was located 167 bp downstream of the aadA1 gene and was present in an integron. In addition, the aac(3)-VIa gene is also downstream of a 59-base element often seen in an integron environment. Primer extension analysis has identified a promoter for the aac(3)-VIa gene downstream of both the aadA1 gene and a 59-base element.

Nucleotide sequence analysis and DNA hybridization studies of the ant(4')-IIa gene from Pseudomonas aeruginosa

The ant(4')-IIa gene was previously cloned from Pseudomonas aeruginosa on a 1.6-kb DNA fragment (G. A. Jacoby, M. J. Blaser, P. Santanam, H. Hächler, F. H. Kayser, R. S. Hare, and G. H. Miller, Antimicrob. Agents Chemother. 34:2381-2386, 1990). In the current study, the ant(4')-IIa gene was localized by gamma-delta mutagenesis. A region of approximately 600 nucleotides which contained the ant(4')-IIa gene was identified, and DNA sequence analysis revealed two overlapping open reading frames (ORFs) within this region. Northern (RNA) blot analysis demonstrated expression of both ORFs in P. aeruginosa; therefore, site-directed mutagenesis was used to identify the ORF which encodes the ant(4')-IIa gene. No homology was found between ant(4')-IIa and ant(4')-Ia DNA sequences. Hybridization experiments confirmed that the ant(4')-Ia probe hybridized only to gram-positive presumptive ANT(4')-I strains and that the ant(4')-IIa probe hybridized only to gram-negative strains presumed to carry ANT(4')-II. Seven gram-negative strains which had been classified as having ANT(4')-II resistance profiles did not hybridize with probes for either ant(4')-Ia or ant(4')-IIa, suggesting that at least one additional ant(4') gene may exist. The predicted amino-terminal sequences of the ANT(4')-Ia and ANT(4')-IIa proteins showed significant sequence similarity between residues 38 and 63 of the ANT(4')-Ia protein and residues 26 and 51 of the ANT(4')-IIa protein.

Molecular genetics of aminoglycoside resistance genes and familial relationships of the aminoglycoside-modifying enzymes

The three classes of enzymes which inactivate aminoglycosides and lead to bacterial resistance are reviewed. DNA hybridization studies have shown that different genes can encode aminoglycoside-modifying enzymes with identical resistance profiles. Comparisons of the amino acid sequences of 49 aminoglycoside-modifying enzymes have revealed new insights into the evolution and relatedness of these proteins. A preliminary assessment of the amino acids which may be important in binding aminoglycosides was obtained from these data and from the results of mutational analysis of several of the genes encoding aminoglycoside-modifying enzymes. Recent studies have demonstrated that aminoglycoside resistance can emerge as a result of alterations in the regulation of normally quiescent cellular genes or as a result of acquiring genes which may have originated from aminoglycoside-producing organisms or from other resistant organisms. Dissemination of these genes is aided by a variety of genetic elements including integrons, transposons, and broad-host-range plasmids. As knowledge of the molecular structure of these enzymes increases, progress can be made in our understanding of how resistance to new aminoglycosides emerges.

Sch 39304, a new antifungal agent: oral and topical treatment of vaginal and superficial infections

Sch 39304 is a new broad spectrum triazole antifungal agent that is active, orally and topically, against superficial Trichophyton mentagrophytes and vaginal Candida albicans infections. Sch 39304 was compared to fluconazole (FLZ) in a T. mentagrophytes infection model in guinea pigs. Following topical administration, Sch 39304 (0.125%, twice daily, 10 days), was 5-8-fold more effective than FLZ, based on culture and lesion score results. Following oral administration, Sch 39304 (2.5 mg kg-1, once daily, 10 days) produced a dramatic reduction in lesion scores and was 20-fold more active than FLZ; however, due to the length of time it takes for the drugs to reach the infected area of the skin and eradicate the infections, most animals remained culture positive with both drugs. Sch 39304 was also compared with FLZ in a vaginal C. albicans infection in hamsters. Following oral administration (4 days), Sch 39304 (1.6 mg kg-1), cured all hamsters and was 4-fold more active than FLZ. In addition, Sch 39304 as a single oral dose (10 mg kg-1) also cured all hamsters. When treatment was intravaginal (8 days), Sch 39304 was again more active than FLZ (2-fold), and also micronazole (8-fold), with 100% of the hamsters cured at concentrations as low at 0.025%.

Cloning and DNA sequence analysis of an aac(3)-Vb gene from Serratia marcescens

The AAC(3)-V resistance mechanism is characterized by high-level resistance to the aminoglycosides gentamicin, netilmicin, 2'-N-ethylnetilmicin, and 6'-N-ethylnetilmicin and moderate resistance levels to tobramycin. Serratia marcescens 82041944 contains an AA(3)-V resistance mechanism as determined from aminoglycoside resistance profiles. This strain, however, does not exhibit hybridization with a probe derived from the previously cloned aac(3)-Va gene, (R. Allmansberger, B. Bräu, and W. Piepersberg, Mol. Gen. Genet. 198:514-520, 1985). High-pressure liquid chromatography analysis of the acetylation products of sisomicin carried out by extracts of S. marcescens 82041944 have demonstrated the presence of an AAC(3) enzyme. We have cloned the gene encoding this acetyltransferase and have designated it aac(3)-Vb. Nucleotide sequence comparisons show that the aac(3)-Va and aac(3)-Vb genes are 72% identical. The predicted AAC(3)-Vb protein is 28,782 Da. Comparisons of the deduced amino acid sequences show 75% identity and 84% similarity between the AAC(3)-Va and AAC(3)-Vb proteins. The use of a DNA fragment internal to the aac(3)-Vb as a hybridization probe demonstrated that the aac(3)-Vb gene is very rare in clinical isolates possessing an AAC(3)-V mechanism.

Characterization of the chromosomal aac(6')-Ic gene from Serratia marcescens

The DNA sequence of the chromosomal aac(6')-Ic gene from Serratia marcescens, which had been previously cloned (H. M. Champion, P. M. Bennett, D. A. Lewis, and D. S. Reeves, J. Antimicrob. Chemother. 22:587-596, 1988) was determined. High-pressure liquid chromatographic analysis of extracts prepared from Escherichia coli carrying the chromosomal aac(6')-Ic gene on a plasmid confirmed the presence of 6'-N-acetyltransferase activity in this strain, which was suggested by the aminoglycoside resistance profile. DNA sequence analysis of the cloned 2,057-bp PstI fragment revealed several regions of homology to previously characterized sequences from GenBank, including the rpoD and tRNA-2 genes of E. coli. Subcloning experiments confirmed the coding sequence of the aac(6')-Ic gene to be at positions 1554 to 1992. The predicted amino acid sequence of the AAC(6')-Ic protein suggested that it was the third member of a family of AAC(6') proteins which included a coding region identified between the aadB and aadA genes of Tn4000 and an AAC(6') protein encoded by pUO490, which was isolated from Enterobacter cloacae. Primer extension analysis suggested that the -35 region of the aac(6')-Ic promoter overlapped a large palindromic sequence which may be involved in the regulation of the aac(6')-Ic gene. Hybridization experiments utilizing a restriction fragment from the aac(6')-Ic gene showed that all S. marcescens organisms carried this gene whether or not the AAC(6')-I resistance profile was expressed. Organisms other than Serratia spp. did not hybridize to this probe.

Genetic analysis of bacterial acetyltransferases: identification of amino acids determining the specificities of the aminoglycoside 6'-N-acetyltransferase Ib and IIa proteins

The aminoglycoside 6'-N-acetyltransferase [AAC(6')-I] and AAC(6')-II enzymes represent a class of bacterial proteins capable of acetylating tobramycin, netilmicin, and 2'-N-ethylnetilmicin. However, an important difference exists in their abilities to modify amikacin and gentamicin. The AAC(6')-I enzymes are capable of modifying amikacin. In contrast, the AAC(6')-II enzymes are capable of modifying gentamicin. Nucleotide sequence comparison of the aac(6')-Ib gene and the aac(6')-IIa gene showed 74% sequence identity (K. J. Shaw, C. A. Cramer, M. Rizzo, R. Mierzwa, K. Gewain, G. H. Miller, and R. S. Hare, Antimicrob. Agents Chemother. 33:2052-2062, 1989). Comparison of the deduced protein sequences showed 76% identity and 82% amino acid similarity. A genetic analysis of these two proteins was initiated to determine which amino acids were responsible for the differences in specificity. Results of domain exchanges, which created hybrid AAC(6') proteins, indicated that amino acids in the carboxy half of the proteins were largely responsible for determining specificity. Mutations shifting the specificity of the AAC(6')-Ib protein to that of the AAC(6')-IIa protein (i.e., gentamicin resistance and amikacin sensitivity) have been isolated. DNA sequence analysis of four independent isolates revealed base changes causing the same amino acid substitution, a leucine to serine, at position 119. Interestingly, this serine occurs naturally at the same position in the AAC(6')-IIa protein. Oligonucleotide-directed mutagenesis was used to construct the corresponding amino acid change, a serine to leucine, in the AAC(6')-IIa protein. This change resulted in the conversion of the AAC(6')-IIa substrate specificity to that of AAC(6')-Ib. Analysis of additional amino acid substitutions within this region of AAC(6')-Ib support the model that we have identified an aminoglycoside binding domain of these proteins.

Revision of standards for adjusting the cation content of Mueller-Hinton broth for testing susceptibility of Pseudomonas aeruginosa to aminoglycosides

A multilaboratory study was undertaken to reassess the amount of calcium and magnesium that should be added to Mueller-Hinton broth when testing Pseudomonas aeruginosa against amikacin, gentamicin, isepamicin, netilmicin, and tobramycin. To achieve parity with agar dilution tests, cation-adjusted broth should contain 20 to 25 mg of calcium and 10 to 12.5 mg of magnesium per liter rather than the 50- and 25-mg/liter supplements recommended previously. For quality control of tests with contemporary media, MIC control limits should be adjusted by lowering the current MIC limits by at least 1 doubling-dilution interval.

In vitro and in vivo activities of SCH 42427, the active enantiomer of the antifungal agent SCH 39304

SCH 39304, a new triazole antifungal agent, is a 50:50 racemic mixture of two enantiomers, SCH 42427 and SCH 42426. The activities of these three compounds were compared in a series of in vitro and in vivo experiments. SCH 42427 was twofold more active in vitro against a variety of yeasts and dermatophytes than SCH 39304, while SCH 42426 was inactive (MICs greater than 64 micrograms/ml). In a systemic Candida albicans infection in mice, SCH 42427 administered orally (p.o.) (50% protective dose [PD50], 0.17 mg/kg of body weight; 50% effective dose, [ED50], 0.47 mg/kg) had greater efficacy than SCH 39304 (PD50, 0.21 mg/kg; ED50, 0.62 mg/kg) and SCH 42426 (greater than 100 mg/kg for PD50 and ED50). In a pulmonary Aspergillus flavus infection in mice, SCH 42427 p.o. (PD50, 13 mg/kg) was also more effective than SCH 39304 (18 mg/kg) and SCH 42426 (greater than 250 mg/kg). In a C. albicans vaginal infection in hamsters, SCH 42427 p.o. (ED50, 3.5 mg/kg) was more active than SCH 39304 (8.5 mg/kg) and SCH 42426 (320 mg/kg). Following topical administration, against a Trichophyton mentagrophytes infection in guinea pigs, SCH 42427 was about 2-fold more active than SCH 39304 and about 100-fold more active than SCH 42426. These and other results indicated that SCH 42427 is the active enantiomer, responsible for all the antifungal activity observed with SCH 39304.

Comparison of SCH 39304, fluconazole, and ketoconazole for treatment of systemic infections in mice

SCH 39304 was compared with fluconazole and ketoconazole in a systemic Candida albicans infection in mice (10(6) CFU per mouse). Results were based on survival rates and CFU in kidneys following once-daily oral treatment of 2, 5, or 10 days duration. In normal mice, SCH 39304 (dose to reduce kidney counts by 4 log units, 0.5 mg/kg of body weight) was 3 and 200 times more active than fluconazole and ketoconazole, respectively. In immunocompromised mice (gamma irradiation, 600 rads), SCH 39304 (dose to reduce kidney counts by 4 log units, 1.3 mg/kg) was 35 and greater than 100 times more active than fluconazole and ketoconazole, respectively. In normal mice, when the infecting inoculum varied from 10(5) to 10(7) CFU, only a fivefold increase in the dose to reduce kidney counts by 4 log units was observed with SCH 39304. Excellent protection was also seen when mice were treated with a single oral dose of SCH 39304 up to 24 h prior to infection with C. albicans. Studies in a systemic C. albicans infection model indicated that SCH 39304 is equally efficacious following either oral or intravenous administration. In a systemic Aspergillus flavus infection, mice treated with SCH 39304 (5 mg/kg) survived twice as long (16 days) as those treated with fluconazole (50 mg/kg) or controls did.

Correlation between aminoglycoside resistance profiles and DNA hybridization of clinical isolates

DNA hybridization data and aminoglycoside resistance profiles (AGRPs) were determined for 4,088 clinical isolates from three studies (United States, Belgium, and Argentina). The correlation between susceptibility profiles and hybridization results was determined with nine DNA probes. For each of the seven aminoglycoside resistance profiles which we were able to test, the data suggested at least two distinct genes could encode enzymes which lead to identical resistance profiles. Furthermore, the DNA hybridization data showed that individual strains carried up to six unique aminoglycoside resistance genes. DNA hybridization revealed interesting differences in the frequencies of these genes by organism and by country.

Appearance of amikacin and tobramycin resistance due to 4'-aminoglycoside nucleotidyltransferase [ANT(4')-II] in gram-negative pathogens

Following the use of amikacin as the principal aminoglycoside at a Denver hospital, amikacin resistance appeared first in Pseudomonas aeruginosa and then in Escherichia coli, Klebsiella pneumoniae, and other enteric organisms from debilitated and compromised patients who had spent time in intensive care units and who had been treated with multiple antibiotics, usually including amikacin. In a P. aeruginosa isolate, resistance to amikacin and tobramycin was transferable by the IncP-2 plasmid pMG77, while in E. coli and K. pneumoniae resistance was carried by the transmissible plasmids pMG220, pMG221, and pMG222 belonging to the IncM group. Isolates and transconjugants produced an enzyme with adenyltransferase activity with substrates having a 4'-hydroxyl group, such as amikacin, kanamycin, neomycin, Sch 21768, isepamicin (Sch 21420), or tobramycin, but not with aminoglycosides lacking this target, such as dibekacin, netilmicin, sisomicin, or gentamicin C components. Genes encoding the 4'-aminoglycoside nucleotidyltransferase [ANT(4')] activity were cloned from pMG77, pMG221, and pMG222. A DNA probe prepared from the ANT(4') found in P. aeruginosa hybridized with the ANT(4') determinant found in E. coli. A probe for the ANT(4') from Staphylococcal spp., which differs in its modification of substrates, like dibekacin, that have a 4"- but not a 4'-hydroxyl group, failed to hybridize with the gram-negative ANT(4') determinant, which consequently has been termed ANT(4')-II.

Microbiological and pharmacokinetic studies of acyl demycinosyltylosin and related tylosin derivatives

A series of tylosins and acyl derivatives of 23-O-demycinosyltylosin (DMT) were initially tested for in vitro antibacterial activity and serum levels in squirrel monkeys (po) and mice (iv). Overall, the DMT compounds were more active in vitro than the tylosins. Two tetraacylated DMTs, Sch 37644 and Sch 38646, were selected from the initial studies for further evaluation and compared to erythromycin and A-56268 (6-O-methyl erythromycin). Sch 37644 and Sch 38646 were 2 to 8-fold less potent in vitro against Gram-positive bacteria than erythromycin and A-56268. In squirrel monkeys, Sch 37644 (AUC, 19.7 micrograms.hour ml) and A-56268 (21.6 micrograms.hour/ml) had similar serum levels following po administration of 20 mg/kg, while Sch 38646 (11.8 micrograms.hour/ml) and erythromycin (1.5 micrograms.hour/ml) had lower levels. In mice administered 200 mg/kg orally, Sch 37644 (AUC, 19.4 micrograms.hour/ml) and Sch 38646 (15.4 micrograms.hour/ml) had higher serum levels than erythromycin (5.7 micrograms.hour/ml). A-56268 was the most active po macrolide in mouse protection studies (PD50S) against Staphylococci and Streptococci, while Sch 37644 and Sch 38646 were similar to erythromycin.

Resistance to aminoglycoside antibiotics in gram-negative bacilli and staphylococci isolated from blood. Report from a European collaborative study. The ESGAR Study Group (European Study Group on Antibiotic Resistance)

The incidence of resistance to gentamicin, tobramycin, amikacin and netilmicin was determined by the microdilution method in Mueller-Hinton broth among blood culture isolates consecutively collected in 37 laboratories in 14 European countries. The distribution of bacteria was similar in each laboratory, Escherichia coli and staphylococci predominating. Resistance levels varied between laboratories but they were higher to all four antibiotics in Southern Europe than in Central and Northern Europe. Aminoglycoside resistance was usually associated with production of aminoglycoside-modifying enzymes, ANT(2"), AAC (3)-V, AAC (6')-I predominating in Gram-negative bacilli and APH (2") + AAC (6') and ANT (4')-I in staphylococci.

Isolation, characterization, and DNA sequence analysis of an AAC(6')-II gene from Pseudomonas aeruginosa

The gene encoding a 6'-N-acetyltransferase, AAC(6')-II, was cloned from Pseudomonas aeruginosa plasmid pSCH884. This gene mediates resistance to gentamicin, tobramycin, and netilmicin but not amikacin or isepamicin. The DNA sequence of the gene and flanking regions was determined. The 5'- and 3'-flanking sequences showed near identity to sequences found abutting a variety of different genes encoding resistance determinants. It is likely that the current structure arose by the integration of the 572-base-pair sequence containing the AAC(6')-II gene into a Tn21-related sequence at the recombinational hot spot, AAAGTT. We have compared the sequence of the AAC(6')-II gene to genes of other 6'-N-acetyltransferases. An AAC(6')-Ib protein (encoded by the aacA4 gene; G. Tran Van Nhieu and E. Collatz, J. Bacteriol. 169:5708-5714, 1987) that results in resistance to amikacin but not gentamicin was found to share 82% sequence similarity with the AAC(6')-II protein. We speculate that these two genes arose from a common ancestor and that the processes of selection and dissemination have led to the observed differences in the spectrum of aminoglycoside resistance.

Use of plasmid analysis and determination of aminoglycoside-modifying enzymes to characterize isolates from an outbreak of methicillin-resistant Staphylococcus aureus

We compared disk susceptibility, plasmid analysis, aminoglycoside resistance patterns, and DNA hybridization for their usefulness in characterizing isolates from a hospital outbreak of methicillin-resistant Staphylococcus aureus. Fifteen isolates were susceptible (group 1) and 28 were resistant (group 2) to gentamicin. A total of 15 of 15 (100%) group 1 and 22 of 28 (79%) group 2 isolates carried a 21.5-megadalton plasmid. All group 2 isolates and none of the group 1 isolates possessed a 33-megadalton plasmid. Aminoglycoside resistance pattern determinations revealed the presence of the ANT(4')-I enzyme (aminoglycoside 4' adenyltransferase) in all group 1 isolates but was unable to demonstrate presence of this enzyme in group 2 organisms. The APH(2") + AAC(6')-II enzyme (aminoglycoside 2" phosphotransferase plus 6' acetyltransferase) was found in all of the group 2 isolates but in none of the group 1 isolates. Use of DNA hybridization revealed the presence of the ANT(4')-I enzyme in both groups (group 1, 14 of 15; group 2, 26 of 28). In this hospital outbreak, we found good correlation between disk susceptibility, plasmid profile, aminoglycoside resistance patterns, and DNA hybridization results. It was difficult to predict the presence of the ANT(4')-I enzyme in the presence of the bifunctional [APH(2") + AAC(6')-II] enzyme by the aminoglycoside resistance pattern method because of overlap of the substrate profile.

Influence of cation supplements on activity of netilmicin against Pseudomonas aeruginosa in vitro and in vivo

In vitro studies were performed with 74 Pseudomonas aeruginosa isolates which were collected during a multicenter trial. The isolates were obtained from 70 patients who had been treated with netilmicin as the only antipseudomonal antibiotic. Clinically, 83% of the patients were cured or improved, and 64% of the Pseudomonas isolates were eliminated by chemotherapy. The 74 clinical isolates and 38 additional isolates with known mechanisms of aminoglycoside resistance were tested in three separate laboratories by disk diffusion methods and by microdilution tests with three broth media (Mueller-Hinton broth with full, half, and no cation supplements). Isolates that responded to netilmicin therapy and those that failed to respond were all susceptible by the disk test, and most were susceptible by microdilution tests with unsupplemented broth. However, over half of the clinical isolates appeared to be resistant when cations were added to the broth medium. Strains capable of producing enzymes that inactivate netilmicin were resistant by all methods tested. Broth dilution and agar dilution results were most comparable when half of the recommended cation supplements was added to Mueller-Hinton broth. Further consideration should be given to reducing the concentration of cations that are added to Mueller-Hinton broth when netilmicin susceptibility tests are being performed. However, additional studies with other aminoglycosides are needed before appropriate testing conditions can be standardized.

In vitro and in vivo characterization of novel 8-methoxy derivatives of chlortetracycline

The in vitro activities of three new 8-methoxychlortetracyclines, Sch 36969, 33256 and 34164 were compared to tetracycline, minocycline and doxycycline. Against aerobic Gram-negative rods Sch 36969 had a geometric mean MIC (GMM) of 4.2 micrograms/ml, about 8-fold more potent than Sch 33256, and similar to all the other compounds. Sch 36969 also had good activity against methicillin-resistant (GMM, 0.21 micrograms/ml) and -susceptible Staphylococci (GMM, 0.14 micrograms/ml), Streptococci (GMM, 0.06 micrograms/ml), and most anaerobic bacteria (GMM, less than 0.5 micrograms/ml). In general, Sch 36969 was similar to, or more potent than, all the other compounds tested. Serum levels of Sch 36969 in squirrel monkeys were 4-fold lower (AUC, 4.5 micrograms.hours/ml) than those of chlortetracycline (AUC, 16.1 micrograms.hours/ml). In mouse protection tests (PD50s) against various strains of bacteria, Sch 36969 was similar in activity to tetracycline, but up to 6-fold less active than chlortetracycline. The structure activity relationships for these new chlortetracyclines are described.

Netilmicin disk susceptibility tests: effect of cations on the MIC correlates

When testing Pseudomonas aeruginosa against netilmicin, MICs were markedly affected by the concentration of cations added to the test medium. A susceptible disk test result (zone greater than or equal to 15 mm) corresponded to MIC less than or equal to 4.0 micrograms/ml in unsupplemented broth, less than or equal to 12 micrograms/ml in broth with half the usual amount of cations and less than or equal to 32 micrograms/ml in broth with the recommended concentration of cations. Tests with 30 micrograms netilmicin disks best predicted susceptibility as determined by MICs in broth without added cations. When the MICs were determined in cation supplemented broth, the number of interpretive discrepancies increased to an unacceptably high level.

Effect of NaCl supplementation of Mueller-Hinton broth on susceptibility of staphylococci to aminoglycosides

The addition of NaCl to cation-supplemented Mueller-Hinton broth increased the MICs of gentamicin, amikacin, and netilmicin for coagulase-positive and -negative staphylococci in a concentration-dependent manner. At 2% NaCl, geometric mean MICs were elevated 14.1- to 25.6-fold. Mueller-Hinton broth containing added NaCl should not be used for testing the susceptibility of staphylococci to aminoglycosides.

Comparison of aminoglycoside resistance patterns in Japan, Formosa, and Korea, Chile, and the United States

The resistance mechanisms of more than 2,000 aminoglycoside-resistant gram-negative aerobic bacteria were estimated by a method that assigned a biochemical mechanism based on susceptibility to selected aminoglycosides. Strains from hospitals in Japan, Formosa, and Korea (the Far East) were compared with strains from Chile and the United States. Of the strains from Chile, 90% had an aminoglycoside resistance pattern indicative of the 3-N-acetyltransferase [AAC(3)-V] enzyme. Of the strains from the Far East, 78% had susceptibility patterns suggesting the presence of AAC(6') enzymes. In contrast, strains from the United States had a wider variety of resistance mechanisms including 2''-O-adenylyltidyltransferase [ANT(2'')], AAC(3), AAC(6'), and AAC(2'). Reflecting these differences in resistance patterns, the frequencies of resistance to gentamicin, tobramycin, dibekacin, and amikacin in strains from the United States were different from those in strains from the Far East. These differences seem to be correlated with different aminoglycoside usage in the two regions. In the United States, where gentamicin was the most widely used aminoglycoside, 92% of the strains were resistant to gentamicin, 81% were resistant to dibekacin, and 8.8% were resistant to amikacin. In the Far East, dibekacin and kanamycin were widely used in the past and more recently amikacin has been frequently used. Of the strains from this region, 99% were resistant to dibekacin, 85% were resistant to gentamicin, and 35% were resistant to amikacin.

Evaluation of the in-vivo efficacy of Sch 34343

Sch 34343 showed a linear dose response (with respect to AUCs) in mice following both intravenous and subcutaneous administration. It was 100% bioavailable following subcutaneous administration. Peak serum levels, AUCs, beta-phase half-life and recovery of Sch 34343 from the urine of mice indicated that it was similar to cephalothin and cefamandole. In experimental mouse infections, against Gram-negative strains, Sch 34343 was more active than cephalothin, equal to or more active than cefamandole and cefoxitin, but less active than latamoxef (moxalactam) and cefotaxime following single or multiple dose therapy. It was the most active compound against Staphylococcus. Sch 34343 was equally active against strains sensitive to beta-lactams and strains producing beta-lactamases. In an anaerobic abscess model in mice, Sch 34343 was more active than cefoxitin and clindamycin against Bacteroides fragilis. In Escherichia coli meningitis in rabbits, it cured rabbits with a single intravenous dose of 50 mg/kg.

In-vitro activity of Sch 34343 against Gram-negative bacteria producing characterized beta-lactamases

The activity of Sch 34343 against 13 strains producing large amounts of characterized beta-lactamases was compared with that of imipenem, latamoxef (moxalactam), aztreonam and other third-generation cephalosporins. Sch 34343, like imipenem, was active against all strains, including many resistant to all other beta-lactams. MICs of Sch 34343 determined for 16 different inocula were rarely increased even at very high inocula. Sch 34343 was rapidly bactericidal against Escherichia coli TEM-2, Enterobacter agglomerans (with an induced beta-lactamase) and two strains of Bacteroides fragilis with highly active cephalosporinases. Like cefoxitin, Sch 34343 was only slowly inactivated by concentrated crude penicillinases which inactivated cefotaxime within 1 h. Sch 34343 was even more stable to cephalosporinases than was cefoxitin. Stability of the antibiotics to the different beta-lactamases was also determined by pre-incubating them with dilutions of the beta-lactamases before determination of MICs against E. coli 25922. Very large amounts of all enzymes were required to increase the MICs significantly for Sch 34343 and imipenem. These results indicate the good stability of Sch 34343 to beta-lactamases.

Modification of interpretive breakpoints for netilmicin disk susceptibility tests with Pseudomonas aeruginosa

Regression analysis of data correlating 30-micrograms netilmicin disk zone diameters with microdilution MICs, obtained by testing close-interval dilution steps, was performed with 77 selected strains of Pseudomonas aeruginosa, each tested in three independent laboratories. A zone of greater than or equal to 15 mm correlated with an MIC of less than or equal to 12 micrograms/ml (susceptible), and a zone of less than or equal to 12 mm correlated with an MIC of greater than 16 micrograms/ml (resistant). Additional disk tests were performed with 256 strains having known resistance mechanisms and 280 susceptible strains: the majority were appropriately categorized by these interpretive zone standards. The previously recommended standards of greater than or equal to 17 mm (MIC, less than or equal to 8.0 micrograms/ml) for the susceptible category inappropriately placed a significant number of truly susceptible P. aeruginosa strains in the intermediate category.