Antibacterial activity of apalcillin (PC-904) against gram-negative bacilli, especially ampicillin-, carbenicillin-, and gentamicin-resistant clinical isolates

Apalcillin [PC-904]: spectrum of activity and beta-lactamase hydrolysis/inhibition

Apalcillin is a Pseudomonas-active penicillin with a broad spectrum of antibacterial activity similar to that of piperacillin, except for the greater potency of apalcillin against Acinetobacter spp. and Pseudomonas aeruginosa. Studies with 846 isolates representative of the common bacterial pathogens compared apalcillin to piperacillin, azlocillin, mezlocillin, carbenicillin, ticarcillin, cefotaxime, and cefoperazone. Apalcillin and piperacillin were both active against all 13 species of the Enterobacteriaceae that were tested (MIC 50s less than or equal to 8.0 micrograms/ml) but some strains were resistant to both penicillins. Apalcillin was active against Pseudomonas aeruginosa and Acinetobacter spp. (MIC 50 less than 2.0 micrograms/ml). Like other penicillins, apalcillin was not effective against beta-lactamase-producing Staphylococcus spp., Haemophilus spp. or Neisseria gonorrhoeae. Rates of hydrolysis by six beta-lactamase preparations from gram-negative bacilli were determined, comparing apalcillin, piperacillin, azlocillin, ticarcillin, ampicillin and dicloxacillin to benzyl penicillin. Apalcillin and ticarcillin were more resistant than piperacillin and azlocillin to hydrolysis by the PSE-2 enzyme from P. aeruginosa. As did many other penicillins, apalcillin inhibited the Type 1 beta-lactamase that is produced by Enterobacter cloacae. The other enzymes were inhibitory only in very high concentrations.

Interpretive criteria and tentative quality control limits for apalcillin disk susceptibility tests

In vitro studies with 661 bacterial isolates were performed to establish interpretive criteria. In addition, a nine-laboratory study was performed to establish quality control limits for tests with 100-micrograms apalcillin disks and to confirm testing criteria for tests with 100-micrograms piperacillin disks. The two drugs were very similar, and nearly identical criteria were recommended for interpretation and for control of the two types of disks. Neither disk is recommended for testing Staphylococcus spp. or Haemophilus spp.; with other microorganisms, zone size limits of less than or equal to 13 mm (resistant) and greater than or equal to 18 mm (susceptible) are proposed for tests with 100-micrograms apalcillin disks.

In vitro activity of apalcillin compared with those of piperacillin and carbenicillin against 6,797 bacterial isolates from four separate medical centers

Quantitative susceptibility tests were performed in four separate medical centers, in which apalcillin was compared with piperacillin and carbenicillin. Data from tests of 6,797 isolates confirmed that apalcillin and piperacillin had nearly identical spectra of activity but that apalcillin was significantly more active against Pseudomonas aeruginosa (MIC required to inhibit 90% of strains, 2.0 versus 64 micrograms/ml) and Acinetobacter calcoaceticus subsp. anitratus (MIC required to inhibit 90% of strains, 2.0 versus 16 micrograms/ml). Against 166 anaerobic bacterial isolates, apalcillin demonstrated in vitro activity.

Comparative pharmacokinetics of apalcillin and piperacillin

The pharmacokinetics of apalcillin and piperacillin, each administered intravenously as a single 2-g dose, were compared in 10 volunteers in a randomized study of crossover design using bioassay and high-pressure liquid chromatographic procedures. The concentrations of both penicillins in serum were determined over a period of 12 h and in urine over 24 h. Concentrations of apalcillin and piperacillin at the end of the 15-min infusion were similar; however, at 8 h, concentrations of piperacillin were below measurable levels, whereas concentrations of apalcillin were still measurable at 10 h. Pharmacokinetic parameters were calculated according to a two-compartment open model. The area under the curve and the half-life for apalcillin were larger than for piperacillin. On the other hand, renal clearance of piperacillin was substantially greater than that of apalcillin. Of the apalcillin excreted via the kidneys, approximately one-fifth was eliminated as two microbiologically inactive penicilloic acid derivatives. The nonrenal clearance of apalcillin was 79% of total clearance. Binding of apalcillin to serum protein was almost twice that of piperacillin.

A microbiological evaluation of apalcillin

The in vitro activity of apalcillin was tested against 350 clinical isolates of Escherichia coli, Enterobacter, Klebsiella, Proteus, Pseudomonas aeruginosa and Streptococcus faecalis. Using a minimal inhibitory concentration of 16 mg/l as the breakpoint, only 19.7% of the strains were resistant to apalcillin. A regression analysis demonstrated that it is possible to test bacteria for sensitivity with a 20 micrograms apalcillin disc. The results on Mueller-Hinton agar are very similar to those on Iso-Sensitest medium. Like other ureido penicillins, apalcillin is sensitive to most beta-lactamases; it is effective against ampicillin-resistant strains since it penetrates the outer membrane of gram-negative bacteria well and is highly effective against target proteins. Strains producing high amounts of beta-lactamases do become resistant to apalcillin.

In vitro activity of apalcillin compared with that of other new penicillins and anti-Pseudomonas cephalosporins

Apalcillin, a naphthydridine derivative of ampicillin, was compared with ticarcillin, azlocillin, mezlocillin, piperacillin, cefotaxime, and cefoperazone against gram-negative and gram-positive bacterial isolates and with cefsulodin and tobramycin against Pseudomonas aeruginosa. The minimal concentrations of apalcillin at which 50 and 90% of hospital isolates of Escherichia coli were inhibited were similar to those of mezlocillin and piperacillin (1.6 and 100 micrograms/ml, respectively). Apalcillin had minimal inhibitory concentrations similar to those of piperacillin against Citrobacter freundii and Citrobacter diversus. Against Klebsiella, apalcillin inhibited 50% of organisms at a concentration of 6.3 micrograms/ml, similar to piperacillin. The activity of apalcillin against Enterobacter (E. aerogenes, E. cloacae, and E. agglomerans) was similar to that of mezlocillin and piperacillin and greater than that of ticarcillin. The activity of apalcillin against Proteus mirabilis was similar to that of the other agents, as was its activity against indole-positive Proteus and Providencia. Only 40% of Serratia were inhibited at an apalcillin concentration of 25 micrograms/ml. Apalcillin was as active as piperacillin but twofold less active than cefoxitin or moxalactam against Bacteroides fragilis. It was as active as piperacillin, cefoperazone, and cefsulodin against P. aeruginosa (apalcillin inhibited 90% of organisms at a concentration of 25 mg/ml). There was an inoculum effect and a difference in the minimal inhibitory concentration and minimal bactericidal concentration with beta-lactamase strains. Apalcillin was hydrolyzed by plasmid beta-lactamase but not as well by cephalosporinases.

Comparative bactericidal effects of azlocillin and ticarcillin against Pseudomonas aeruginosa

Azlocillin was relatively ineffective against actively growing cultures of Pseudomonas aeruginosa in tests of bacteriolytic and bactericidal activity in which ticarcillin demonstrated pronounced bactericidal effects over a wide range of concentrations. Microscopic observation showed that azlocillin generally induced the formation of filamentous cells of P. aeruginosa which lysed only slowly, but ticarcillin caused the production of spheroplasts and subsequent rapid lysis. During the course of the bactericidal tests, azlocillin was inactivated, presumably by the beta-lactamase produced by P. aeruginosa, and the filamentous cells resumed normal cell division and growth. In contrast, there was no loss of ticarcillin activity, and there was no evidence of resumption of growth of P. aeruginosa in the presence of ticarcillin. These results suggest that the different bactericidal effects demonstrated by azlocillin and ticarcillin against P. aeruginosa are related primarily to dose-related differences in inhibition of cell wall synthesis and secondarily to the instability of azlocillin to pseudomonal beta-lactamase.

In vitro antimicrobial activity of cefotaxime, a new cephalosporin

Cefotaxime, a new semisynthetic cephalosporin derivative, showed a broad spectrum of antibacterial activity against clinically isolated strains of gram-positive and gram-negative bacteria. This cephalosporin was slightly less active than cefazolin against Staphylococcus aureus but 4 to 300 times as active as carbenicillin against gram-negative organisms, including Pseudomonas aeruginosa, Pseudomonas cepacia, Enterobacter cloacae, and Serratia marcescens. Cefotaxime was the most active compound against members of the Enterobacteriaceae and 20- to 100-fold more active than cefoxitin against the indole-positive Proteus group. The minimal bactericidal concentrations of the compound were identical to, or two times higher than, the minimal inhibitory concentrations against Escherichia coli and P. aeruginosa and four times higher against S. marcescens. A reduction of inoculum size decreased greatly the minimal inhibitory and bactericidal concentrations of cefotaxime against E. coli P. aeruginosa, and S. marcescens. The antibiotic was very stable to penicillinase and cephalosporinase produced by gram-negative bacteria, including Proteus vulgaris.

In vitro antibacterial activity of AM-715, a new nalidixic acid analog

AM-715 [1-ethyl-6-fluoro-1,4-dihydro-4-oxo-7-(1-piperazinyl)-3-quinolinecarboxylic acid] is a new nalidixic acid analog. AM-715 has a broad spectrum of antibacterial activity against gram-positive and gram-negative bacteria. The antibacterial activity of AM-715 was greater than those of pipemidic acid and nalidixic acid. AM-715 had higher antibacterial activity against Pseudomonas aeruginosa than did gentamicin. Most nalidixic acid-resistant bacteria were susceptible to AM-715, and cross-resistance was not observed between AM-715 and various antibiotics. The minimal concentration of AM-715 required to inhibit the growth of 75% of the total number of clinical isolates was as follows: Escherichia coli, 0.04 mug/ml; Klebsiella pneumoniae, 0.1 mug/ml; Serratia marcescens, 0.88 mug/ml; Enterobacter spp., 0.076 mug/ml; Staphylococcus aureus, 1.10 mug/ml; P. aeruginosa, 0.38 mug/ml; and nalidixic acid-resistant strains of gram-negative bacteria, 0.62 mug/ml. AM-715 at minimal inhibitory concentrations or at slightly higher concentrations had bactericidal activity against various species of bacteria. The effect of inoculum sizes on minimal inhibitory concentrations and minimal bactericidal concentrations of AM-715 against gram-negative bacteria was smaller than on those of pipemidic acid and nalidixic acid. The dose-response curve of AM-715 indicated a steep gradient, and the 50% inhibited doses of AM-715 were 0.014 mug/ml against E. coli ML4707 and 0.21 mug/ml against P. aeruginosa NC-5.

Comparative antibacterial activity of azlocillin, mezlocillin, carbenicillin and ticarcillin and relative stability to beta-lactamases of pseudomonas aeruginosa and klebsiella aerogenes

The antibacterial activities of two ureidopenicillins, azlocillin and mezlocillin, were compared with those of the alpha-carboxypenicillins, carbenicillin and ticarcillin, against a large number of gram-positive and gram-negative bacteria. All four penicillins were active against a wide range of bacteria including Pseudomonas aeruginosa, but there were differences in the antibacterial spectra and in the antibacterial effects demonstrated by the two classes of penicillins. In particular, the minimum inhibitory concentrations of azlocillin and mezlocillin against Klebsiella aerogenes and against P. aeruginosa were greatly influenced by the size of bacterial inoculum tested whereas there was no significant inoculum effect with carbenicillin and ticarcillin. In stability tests, the ureidopenicillins were inactivated rapidly by the beta-lactamases of K. aerogenes and P. aeruginosa whereas the alpha-carboxypenicillins were stable. It seems probable that the inoculum effect seen with azlocillin and mezlocillin in antibacterial tests with K. aerogenes and P. aeruginosa is associated with the instability of the compounds to the beta-lactamases of these bacteria.

New antipseudomonal penicillin, PC-904: affinity to penicillin-binding proteins and inhibition of the enzyme cross-linking peptidoglycan

The mechanism of action of a new antipseudomonal penicillin, PC-904, was studied with respect to its binding affinities to penicillin-binding proteins (PBPs) and its inhibitory activities on cross-linking enzymes of peptidoglycan synthesis in vitro. PC-904 showed especially high affinity (compared with that of penicillin G) to Escherichia coli PBP-3. It also had high affinities to PBP-2 and -1Bs and low affinities to PBP-1A, -4, -5, and -6. Similar results were obtained with Pseudomonas aeruginosa, in which this antibiotic showed very high affinity (compared with that of penicillin G) to PBP-3, -1A (presumably corresponding to E. coli PBP-1Bs), and -2; there was especially high affinity to PBP-3 and much less affinity to PBP-1B (presumably corresponding to E. coli PBP-1A). These results are compatible with morphological observations that at concentrations near its minimal inhibitory concentration or less, this antibiotic induced the formation of filamentous cells of E. coli and P. aeruginosa. At higher concentrations or after prolonged incubation, it induced lysis of the cells. The remarkably high affinity of PC-904 to pseudomonal PBP-3, -1A, and -2 may partly explain the potent antipseudomonal activity of this antibiotic. In E. coli, the concentration of PC-904 required to inhibit the cross-linking reaction in enzymatic peptidoglycan synthesis, presumably carried out by PBP-1Bs, was as low as the inhibitory concentrations of penicillin G, ampicillin, and carbenicillin.